Synthesis, structure and some catalytic applications of platinum complexes with terphenyl phosphine ligands

UNIVERSIDAD DE SEVILLA CONSEJO SUPERIOR DE INVESTIGACIONES CIENTÍFICAS Departamento de Química Inorgánica Instituto de Investigaciones Químicas Synthesis, Structure and Some Catalytic Applications of Platinum Complexes with Terphenyl Phosphine Ligands Laura Ortega Moreno Tesis Doctoral Sevilla 2016 Synthesis, Structure and Some Catalytic Applications of Platinum Complexes with Terphenyl Phosphine Ligands Por Laura Ortega Moreno Trabajo presentado para aspirer al Título de Doctora en Química Sevilla, 2016 Laura Ortega Moreno Directores de Tesis: Riccardo Peloso Ernesto Carmona Guzmán Profesor Ayudante de Doctor Catedrático Química Inorganica Química Inorganica Universidad Sevilla Universidad de Sevilla A la madre que me ciclometaló A mi padre A mi hermano ACKNOWLEDGEMENTS We gratefully acknowledge the following contributions to this Thesis:  195Pt NMR studies by Prof. Juan Forniés, University of Zaragoza (Spain).  Support on catalytic assays by Dr. Andrés Suárez, Consejo Superior de Investigaciones Científicas (Sevilla).  X-ray diffraction studies by Dr. Celia Maya, University of Sevilla.  Support on DFT calculations and NMR experiments by Joaquín López- Serrano, University of Sevilla. Se acaba la etapa del doctorado para mí. Muchos dicen que es una de las etapas de la vida más bonitas y entrañables que recordar. Si echo la vista atrás, a mí no me cabe ninguna duda, puesto que no solo ha sido un aprendizaje a nivel académico o profesional sino también a nivel personal durante los últimos 4 años. He pulido mi carácter, a veces un tanto brusco, y he aprendido a controlar mis impulsos, a pensar dos veces antes de abrir la boca y a empatizar con el resto de personas que me rodean. Todo esto se lo debo agradecer a todas y cada una de las maravillosas personas que he conocido aquí en Sevilla y que de una manera u otra son participes de esta Tesis. En primer lugar me gustaría plasmar mi agradecimiento al Prof. Ernesto Carmona, uno de mis directores de Tesis, puesto que fue él el que sin apenas conocerme me ofreció la posibilidad de empezar un nuevo proyecto de investigación en su grupo. Le agradezco no solo la confianza depositada en mí, sino también las palabras de ánimo y cariño que siempre he escuchado de él. Le agradezco también el tiempo dedicado a esta tesis puesto que siempre tenía una nueva idea o enfoque para la química que he desarrollado, demostrando una vez más su dedicación absoluta a la investigación. Por todo ello, GRACIAS Ernesto! A la tarea de director de tesis se sumó algo más tarde Riccardo Peloso. A él le agradezco todos los consejos tanto fuera como dentro del laboratorio y estando siempre con una sonrisa en la cara e irradiando felicidad y positivismo. Ha sabido compaginar los esfuerzos en la investigación en el laboratorio con las recompensas fuera de él. Siempre me voy a acordar de las noches de charlas con rissotto en tu casa y las correcciones de la Tesis mordisqueadas por su conejo. Eres grande y único. Gracias Ric! Dimethyl Complexes [PtMe2(PMe2Ar’)] and Mixed Methyl Chloride Species [PtMe(Cl)(PMe2ArDipp2)] 72 The Diphenyl Complex [PtPh2(PMe2ArDipp2)] 80 Silyl Hydride Complexes 82 Recapitulation of Structural Data Pertaining Neutral Pt(II) Complexes with Bidentate Phosphine Coordination 89 I.2.2.2. Forcing Monodentate P-Coordination of the Phosphine. Reactivity Towards Lewis Bases 92 The Allyl Platinum(II) Complex [Pt(3-C3H5)Br(PMe2ArDipp2)] 98 I.2.2.3. Cationic Unsaturated Pt(II) Complexes that Exhibit Bidentate (1-P, 1-arene) Phosphine Coordination 101 · Cationic Monochloride and Related · Dicationic Complexes 101 · Cationic Complexes with Platinum to Carbon Sigma Bonds. Methyl and Phenyl Derivatives 114 · Reactivity of cis-[PtMe(PMe2ArDipp2)]+ (20·S) towards dihydrogen 134 · The Phenyl Cationic Complex [PtPh(py)(PMe2ArDipp2)][BArF] (22) 139 · Reactivity of cis-[PtMe(S)(PMe2ArDipp2)]+ (20·S) towards cis-[PtMe2(PMe2ArDipp2)] (8) 141 I.2.2.4. Reactivity of the Cationic Platinum(II) Methyl Complex [PtMe(PMe2ArDipp2)(S)]+ (20·S) towards C2H4 and C2H4. A Combined Experimental and Computational Study. 146 I.2.2.5. Some Catalytic studies 167 Hydroarylation of alkynes 167 Hydroamination of terminal alkynes 170 I.3. EXPERIMENTAL SECTION 173 I.3.1. General Considerations 173 I.3.2. Synthesis and Characterisation of the New Complexes Obtained 174 I.3.3. Kinetic Studies on the Cyclometallation of Complex 2 243 I.3.4. Reaction of complexes 2 and 3 with [H(Et2O)2][BArF] 244 I.3.5. Characterisation of detected intermediates in the formation of complex 24 244 I.3.6. Reaction of 20·S with H2 and D2 246 I.3.7. Computational Details 246 I.3.8. 1D Selective EXSY Experiments 248 I.3.9. General Method for Catalytic Hydroarylation of Alkynes 256 I.3.10. General Method for Catalytic Hydroamination of Alkynes 256 I.4. BIBLIOGRAPHY 257 Chapter II II.1. PREFACE 273 II.2. INTRODUCTION 277 II.3. RESULTS AND DISCUSSION 289 II.3.1. Synthesis of Pt(0)-olefin Complexes 289 II.3.2. Reactivity of Pt(0)-olefin Complexes 312 II.3.2.1. Reactivity towards CO 312 II.3.2.2.Reactivity towards Protonation 315 II.3.2.3. Oxidative Addition of Allylbromides 319 II.3.3. Catalytic Hydrosilylation of Alkynes 322 II.4. EXPERIMENTAL SECTION 329 II.4.1. General Considerations 329 II.4.2. Synthesis and Characterisation of the New Complexes Obtained 330 II.4.3. General Method for Catalytic Hydrosilylation of Alkynes 347 II.5. BIBLIOGRAPHY 349 Conclusiones 355 1 ABBREVIATIONS Me methyl, -CH3 Et ethyl, -CH2CH3 iPr iso-propyl, CH(CH3)2 tBu tert-butyl, -C(CH3)3 Ph phenyl, -C6H5 Dipp 2,6-diisopropylphenyl, 2,6-iPr2C6H3 Tipp 2,4,6-triisopropylphenyl, 2,4,6-iPr3C6H2 Xyl xylyl, 2,6-(CH3)2C6H3 Ar’ aryl ArXyl2 2,6-Xyl2C6H3 ArDipp2 2,6-Dipp2C6H3 ArTipp2 2,6-Tipp2C6H3 THF tetrahydrofuran, C4H8O Et2O diethyl ether, CH3CH2OCH2CH3 CH3CN acetonitrile NHC N-heterocyclic carbene L 2 electron donor ligand  ligand hapticity  number of atoms of carbon directly bound to a metal center  infrared vibrational frequency (cm-1) h hours min minutes equiv. equivalents 2 atm atmospheres Keq equilibrium constant k rate constant t1/2 half-life Anal. Calc. analysis calculated Exp. Experimental dn number of deuterium atoms in a molecule g grams mmol millimol mL millilitre cm centimetre Å Angstrom º degree C Celsius K Kelvin M molar concentration Ref. reference p page vol volume ORTEP crystallographic representation (Oak Ridge Thermal Ellipsoid Program) IR infrared eelectron HRMS high resolution mass spectrometry ESI electrospray 3 NMR Abbreviations NMR Nuclear Magnetic Resonance  chemical shift ppm parts per million NOESY Nuclear Overhauser Enhancement Spectroscopy COSY 1H-1H correlation spectroscopy HSQC Correlation spectroscopy (Heteronuclear Single Quantum Coherence) HMBC Correlation spectroscopy (Heteronuclear Multiple Bond Correlation) S singlet d doublet t triplet q quartet setp septet m multiplet o ortho m meta p para br broad nJAB coupling constant between A and B nuclei separated by n bonds Hz Hertz 4 Consideraciones Generales 5 CONSIDERACIONES GENERALES La investigación científica tiene como misión fundamental generar nuevo conocimiento, en las fronteras de la ciencia actual, que conduzca a una mejor comprensión de la naturaleza y que además contribuya a solucionar las necesidades presentes y futuras de la sociedad. La investigación química, en particular, se enfrenta en nuestro tiempo a grandes retos, puesto que su actuación se extienda a múltiples y muy variadas parcelas de la vida ordinaria, en las que debe desarrollar procesos y tecnologías sostenibles, es decir, transformaciones limpiar y eficaces, de bajo consumo energético que produzcan ínfimas cantidades de residuos. Así, la investigación química presente no se circunscribe a áreas estrictamente relacionadas con esta disciplina, a saber, a la síntesis química inorgánica u orgánica y a las aplicaciones directas de estas moléculas o materiales, sino que se adentra en campos como la agricultura, la cosmética, los productos farmacéuticos, los nuevos materiales de importantes aplicaciones prácticas, o la búsqueda de fuentes alternativas de energía, entre otros transcendentes desafíos. Una de las grandes campos de actuación de la Química moderna es la química organometálica, que se centra en el estudio del metal-carbono, y que por ello se sitúa en la divisoria, en la región fronteriza, de los dos grandes territorios clásicos de esta ciencia, la química inorgánica y la orgánica. Aunque la disciplina es en rigor mucho más antigua, el Consideraciones Generales 6 desarrollo real de la química organometálica que propició su extraordinario avance hasta alcanzar la posición preferente que la caracteriza en nuestros días, se produjo en los primeros años de la década de 1950, y estuvo fuertemente influido por los trabajos pioneros de las escuelas de E.O. Fischer y G. Wilkinson, como se reconoció en el Premio Nobel de Química de 1973, concedido a estos dos grandes investigadores ―for their pioneering work, performed independently, on the chemistry of the organometallic, so called sándwich compounds‖. En fechas recientes (2009), un muy destacado representante de la química organometálica del último cuarto del siglo XX, el profesor Helmut Werner de la Universidad de Würzburg (Alemania), ha publicado una rigurosa, al tiempo que amena, monografía, de lectura muy recomendable, que ilustra el desarrollo y la evaluación de la química organometálica durante el pasado siglo hasta nuestros días, y su influencia en otras áreas de la Química y en la industria química: ―Landmarks in Organo-Transition Metal Chemistry, A Personal View‖ (Springer Science, 2009). En la actualidad se conocen infinidad de compuestos organometálicos de naturaleza muy diversa, muchos de los cuales exhiben reactividad química sin precedentes, que en no pocos casos ha llevado al desarrollo de procesos industriales que, en última instancia, han contribuido de forma notoria a mejorar la calidad de nuestra vida. Existen asimismo múltiples aplicaciones de los compuestos organometálicos en procesos relacionados con la vida, en la denominada química bioinorgánica, o en su variante bioorganometálica, como agentes antitumorales o con otros fines biológicos, así como en el desarrollo de nuevos materiales y en otras importantes áreas de la química actual. Su influencia en el desarrollo de la catálisis homogénea ha sido de tal transcendencia que tres grandes campos de esta disciplina han sido Consideraciones Generales 7 distinguidos por The Royal Swedish Academy of Sciences con la concesión de otros tantos Premios Nobel: - 2001 a W. S. Knowles, R. Noyori and K. B. Sharples, ―for their work on chirally catalysed hydrogenation and oxidation reactions‖ - 2005 a Y. Chauvin, R. H. Grubbs and R. R. Schrock, ―for the development of the metathesis method in organic synthesis‖ - 2010 a R. F. Heck, E. Negishi and A. Suzuki, ―for palladiumcatalysed cross coupling in organic synthesis‖ En esta memoria se presentan y discuten resultados que por su naturaleza se encuadran en las líneas de investigación que desarrolla el grupo de Química Organometálica y Catálisis Homogénea del Instituto de Investigaciones Químicas (Centro mixto Universidad de Sevilla – Consejo Superior de Investigaciones Científicas), y en particular en la que se concierne al estudio de la activación de moléculas pequeñas y de enlaces C—H mediante complejos de metales de la mena del platino, considerándose en esta Tesis exclusivamente este metal noble. Como se analiza con el debido detalle en secciones posteriores, la atención investigadora se concentra en compuestos organometálicos de platino que contienen ligandos auxiliares de tipo fosfina terciaria con gran impedimento estérico, como resultado de la presencia de un radical terfenilo muy voluminoso unido directamente al átomo de fósforo. Estos ligandos permiten estabilizar estructuras complejas de bajo número de coordinación y como consecuencia con una importante insaturación electrónica. Además de los propios complejos de platino se describen los pertinentes estudios de reactividad, y los de caracterización estructural, estos últimos por aplicación de técnicas espectroscópicas (RMN de 1H, 13C y 31P, mono y bidimensionales) o de difracción de rayos X, que se han Summary of Compounds 14 CHAPTER I (cont.) Summary of Compounds 15 CHAPTER I (cont.) Summary of Compounds 16 CHAPTER II Abstract 17 ABSTRACT Chapter I contains synthetic, structural and reactivity studies on Pt(II) complexes bearing the bulky dimethylterphenylphosphines PMe2ArXyl2 (L1), PMe2ArDipp2 (L2) or PMe2ArTipp2 (L3) (see page 13 for the structural formulae of these phosphines). It comprises two main sections. The first is dedicated to compounds of the xylyl substituted terphenylphosphine (L1). Despite the bulkiness of this ligand, bis(phosphine) Pt(II) complexes could be synthesised. Since the ligand contains -C-H bonds, cyclometallation reactions could be achieved to form seven-membered platinacycles. Kinetic studies on the cyclometallation reaction were also developed. Two different Pt(II) precursors were used in this section, namely cis-[PtCl2(COD)] and cis- [PtMe2(SMe2)2]. The newly prepared complexes included bis(chloride) and bis(methyl derivatives with different molecular geometry, i.e. trans- [PtCl2(PMe2ArXyl2)2] and cis-[PtMe2(PMe2ArXyl2)2], as well as some cyclometallated complexes. The protonation reactions of some of these platinum organometallics were investigated by variable temperature NMR spectroscopy. The second section of Chapter I describes a variety of Pt(II) complexes of ligands L2 and L3. These ligands are able to stabilize lowcoordinated species that feature formally a 14-electron count. In order to counterbalance the metal coordinative and electronic unsaturation, these compounds feature a relatively weak Pt···Carene interaction with the ipso carbon atom of a flanking aryl ring of the terphenyl group of the phosphine. Abstract 18 A series of neutral Pt(II) complexes were characterized by common techniques (NMR spectroscopy and single crystal X-ray diffraction) for a variety of monoanionic ligands, such as Me-, Cl-, H- and SiR3-. New compounds in this section comprised the methyl derivatives cis- [PtMe2(PMe2Ar’)] of ligands L2 and L3, as well as the chloro-methyl species cis-[Pt(Me)Cl(PMe2ArDipp2)] and some hydride-silyl complexes, cis-[Pt(H)(SiR3)(PMe2Ar’)], among others. The secondary interaction offered by the phosphine was easily replaced by various Lewis bases alike CO or PR3. In addition, cationic complexes were also investigated. The important steric protection provided by the bulky terphenylphosphines allowed abstraction of one or the two chloride ligands of the complex cis- [PtCl2(PMe2ArDipp2)] with formation of mono- and bis-cationic complexes with weakly bound molecules of poorly coordinating solvents completing the metal coordination sphere. Some of the Pt(II) cations investigated were generated by protonation of cis-[PtMe2(PMe2ArDipp2)], among them the dication cis-[Pt(PMe2ArDipp2)(S)2]2+, isolated as the bis(triflimidate) salt (NTf2-). This complex contains only one firmly bound  2-P,C phosphine ligand and two molecules of poor donor solvents like dichloromethane, diethylether or water (adventitious), and may be therefore viewed as a source of the [Pt(PMe2ArDipp2)]2+ fragment. Another, in our opinion interesting complex that was fully characterised in this work is the monocation cis-[PtMe(S)(PMe2ArDipp2)]+ (the analogous PMe2ArTipp2 derivative was also studied although only for comparative structural analysis). We studied the reactivity of PMe2ArDipp2 complex towards some small inorganic and organic molecules. Specifically, the reaction with C2H2 yielded the four-coordinate allyl complex [Pt(3- C3H5)(PMe2ArDipp2)]+ through a reactive vinylidene intermediate, as demonstrated by isotopic labelling studies using C2D2. Computational Abstract 19 studies on this reaction and on the related process involving C2H4 were also developed. To complete this section, we deemed appropriate some catalytic studies using these cations as catalyst precursors. The reactions embraced a variety of alkyne hydroarylations, along with the hydroamination of some terminal alkynes. Chapter II is dedicated to the study of Pt(0) complexes of the terphenylphosphine ligands L2 and L3. Since it is well known that Pt(0)- PR3 complexes are one of the most important families of organometallic catalysts, it was though mandatory disclosing the structural and reactivity properties of Pt(0) derivatives of these bulky phosphines, including some catalytic applications. The core of these investigations are Pt(0)-PMe2Ar’-olefin complexes of the types [Pt(C2H4)n(PMe2ARDipp2)], where n = 1 or 2. Since the mono(ethylene) complexes feature relatively poor thermal stability, the analogous complexes [Pt(CH2=CHtBu)(PMe2Ar’)] of the bulkier 3,3- dimethylbut-1-ene olefin were prepared and characterized. As discussed in a forthcoming section of this Thesis, these complexes were generated by two different methods. The first consisted in the Zn powder reduction of the corresponding cis-[PtCl2(PMe2Ar’)] precursor in the presence of the alkene, while the second involved reductive elimination of HSiEt3 from the hydride.silyl predecessor induced by the olefin. A chelating alkene such as bis(allyl)ether, (CH2=CHCH2)2O, as also utilised successfully to generate complexes of this kind. Besides the structural characterization of the new compounds, some reactivity studies were carried out, in particular the reaction with CO, that yielded an interesting triplatinum cluster, namely [Pt3(μ- CO)3(PMe2ArDipp2)3], demonstrating that the Pt(0)-PMe2Ar’-olefin complexes are a useful source of the reactive [Pt(PMe2Ar’)] fragment. Low Abstract 20 temperature protonation studies made evident the formation of cis- [Pt(H)(olefin)(PMe2Ar’]+ species, which rearranged by migratory insertion of the olefin into the Pt—H bond and behave as precatalysts for olefin dimerization reactions. In addition, the Pt(0)-PMe2Ar’ complexes were shown to undergo oxidative addition reactions. This reactivity was demonstrated with the formation of platinum(II) allyl complexes when complex [Pt(C2H4)2(PMe2ArDipp2)] was treated with BrCH2C(R)=CH2 (R = H, Me). In the same way as in Chapter I, Chapter II ends with catalytic studies based on some of the new Pt(0) complexes as precatalysts for the hydrosilylation of terminal alkynes. As discussed later, the hydrosilylation of alkyl-substituted terminal alkynes was accomplished with high activity and E-selectivity, while for arylacetylenes a strong dependence of the catalytic activity with electronic effects on the para-substituents was disclosed. CHAPTER I Pt(II) complexes 23 I.1. INTRODUCTION I.1.1. A brief historical preface This Doctoral Thesis is devoted to the study of platinum complexes that contain coordinated tertiary phosphine ligands. Doubtless, this is a classical area of research in modern Inorganic and Organometallic Chemistry, and as such has been intensively investigated in the past decades. Therefore, progress over the years has been collected in many textbooks and specialised monographs, as well as in countless review articles, and in readily accessible encyclopaedia like Comprehensive Coordination Chemistry 1 and Comprehensive Organometallic Chemistry. 2 The first editions of these collective works were published in 1987 and 1982, respectively. Despite this extensive literature coverage, we believe that a brief historical survey on phosphine ligands and their metal complexes with emphasis placed on platinum is appropriate. 1 Comprehensive Coordination Chemistry, 2nd ed. (Eds.: J. A. McCleverty, T. J. Meyer), Elsevier, 2004. First edition published in 1987 2 Comprehensive Organometallic Chemistry, 3rd ed. (Ed.: R. H. Crabtree), Elsevier, 2007. First edition published in 1982. Pt(II) complexes 31 I.1.2. Introductory comments As noted in the previous section, phosphines and related P(III) phosphorus ligands, PR3, are one of the most widely used classes of ligands in coordination and organometallic chemistry, as they usually form inert and stable bonds with the majority of the transition metals. They are particularly useful because their electronic and steric properties can be altered in a systematic and predictable way by varying the R groups, which do not have to be necessarily identical, but can range within a variety of inorganic and organic radicals such as hydrogen, halide, hydrocarbyl, alkoxy, amide, etc. Phosphine ligands are able to stabilize a broad range of metal oxidation states and are of exceptional industrial interest because of their ability to promote diverse catalytic reactions in combination with transition metals. Indeed, their crucial role in the stabilization of reactions intermediates is well-known. 15 The introduction of elements of chirality in the molecular structure of mono or polydentate phosphines and related 15 a) M. Bochmann, Organometallics and Catalysis. An Introduction., Oxford University Press, Oxford, UK, 2015; b) R. H. Crabtree, The Organometallic Chemistry of the Transition Metals, 6th ed., John Wiley & Sons, Inc., Hoboken, 2014; c) J. Hartwig, Organotransition Metal Chemistry: From Bonding to Catalysis, Sausalito, 2010; d) R. H. Crabtree, J. Organomet. Chem. 2005, 690, 5451-5457; e) C. A. Tolman, Chem. Rev. 1977, 77, 313-348. Chapter I 32 ligands, and their application in asymmetric catalysis represented one of the landmark discoveries of the chemical community in the last century. 16 A key feature of PR3 ligands is that electronic properties can be easily varied without any major change in steric features (e.g., by moving from PBu3 to P(OiPr)3), and vice versa (i.e. upon changing PPh3 by P(otolyl)3). While the variation of the donor-acceptor abilities of the ligand can perturb oxidative and reductive catalytic steps favouring higher or lower oxidation states of the metals, the steric bulk is expected to stabilize low-coordination-number species, which are known to be particularly relevant in catalysis. 17 In recent years bulky phosphines with unprecedented properties, such as those reported by Beller 18 , Buchwald 19 , Fu 20 , and Hartwig 21 have been widely utilised for the synthesis of transition metal complexes and the development of challenging catalytic processes (Figure 2). 16 a) R. Noyori, Angew. Chem. Int. Ed. 2002, 41, 2008-2022; b) W. S. Knowles, Angew. Chem. Int. Ed. 2002, 41, 1998-2007. 17 a) J. Halpern, Inorg. Chim. Acta 1981, 50, 11-19; b) C. P. Casey, G. T. Whiteker, M. G. Melville, L. M. Petrovich, J. A. Gavney, D. R. Powell, J. Am. Chem. Soc. 1992, 114, 5535-5543; c) L. H. Shultz, D. J. Tempel, M. Brookhart, J. Am. Chem. Soc. 2001, 123, 11539-11555; d) Z. Liu, E. Somsook, C. B. White, K. A. Rosaaen, C. R. Landis, J. Am. Chem. Soc. 2001, 123, 11193-11207. 18 A. Zapf, A. Ehrentraut, M. Beller, Angew. Chem. Int. Ed. 2000, 39, 4153-4155. 19 a) J. P. Wolfe, R. A. Singer, B. H. Yang, S. L. Buchwald, J. Am. Chem. Soc. 1999, 121, 9550-9561; b) A. Aranyos, D. W. Old, A. Kiyomori, J. P. Wolfe, J. P. Sadighi, S. L. Buchwald, J. Am. Chem. Soc. 1999, 121, 4369-4378; c) J. P. Wolfe, S. Wagaw, J.-F. Marcoux, S. L. Buchwald, Acc. Chem. Res. 1998, 31, 805-818; d) R. Martin, S. L. Buchwald, Acc. Chem. Res. 2008, 41, 1461. 20 A. F. Littke, G. C. Fu, Angew. Chem. Int. Ed. 2002, 41, 4176-4211. 21 a) Q. Shelby, N. Kataoka, G. Mann, J. Hartwig, J. Am. Chem. Soc. 2000, 122, 10718- 10719; b) G. Mann, C. Incarvito, A. L. Rheingold, J. F. Hartwig, J. Am. Chem. Soc. 1999, 121, 3224-3225; c) J. F. Hartwig, Angew. Chem. Int. Ed. 1998, 37, 2046-2067. Pt(II) complexes 33 In particular, the dialkylbiaryl phosphines reported by Buchwald have acquired great importance and are amply used in a variety of catalytic organic transformations mediated by transition metals. Notably, the Pdcatalysed arylation of nitrogen nucleophiles using this ligand type has undergone considerable development, which has been driven by innovations in ligand design and by optimization of reaction conditions. As a result, the reactions can often be employed with complex substrates and low catalyst loadings (Scheme 1).19, 22 Scheme 1: Pd-catalysed amination of aryl halides using dialkylbiaryl phosphines. 22 a) D. S. Surry, S. L. Buchwald, Chem. Sci. 2011, 2; b) T. J. Maimone, P. J. Milner, T. Kinzel, Y. Zhang, M. K. Takase, S. L. Buchwald, J. Am. Chem. Soc. 2011, 133, 18106- 18109; c) B. P. Fors, D. A. Watson, M. R. Biscoe, S. L. Buchwald, J. Am. Chem. Soc. 2008, 130, 13552-13554; d) T. E. Barder, S. L. Buchwald, J. Am. Chem. Soc. 2007, 129, 12003-12010; e) T. E. Barder, S. D. Walker, J. R. Martinelli, S. L. Buchwald, J. Am. Chem. Soc. 2005, 127, 4685-4696. Figure 2. Some representative phosphines studied by Buchwald, Beller, Fu, Hartwig and their co-workers. Chapter I 34 In a Perspective article,22a Surry and Buchwald have recently summarized the key variables that affect the success of this reaction. Basically, the structures of the amine and the electrophile determine the choice of the essential reaction parameters of ligand, palladium precursor, base and solvent. Since this Thesis concentrates attention on platinum complexes of related dialkylterphenyl phosphines we deem proper summarizing the influence of the phosphine in a given palladium catalysed amination reaction. The key feature is doubtless the structure of the ligand (the one represented in Figure 2 is named SPhos). The original studies made use of ligands DavePhos and JohnPhos for amination reactions,22a but a major breakthrough came with the discovery of XPhos and RuPhos (Figure 3), which yielded improved reactivity in a variety of demanding amination Figure 3. Some dialkylbiaryl phosphines (top) and Pd(II) catalyst precursors (bottom) for amination reactions (R = Cy or tBu; Ar‘ = biaryl; adapted from Reference 22. Pt(II) complexes 35 reactions. More recently, BrettPhos (Figure 3) was introduced, and appears to date as the most generally useful ligand for the arylation of primary amines, whereas RuPhos is the phosphine of choice for the arylation of secondary amines.22a Monophosphine Pd(0) complexes of this type are highly reactive towards oxidative addition. In accordance with this observation, for simple substrates the ideal metal to ligand ratio is 1:1. Often an extra equivalent of the ligand is, however, needed to stabilize the catalyst in highly demanding transformations that require long reaction times or when a high TON (Turn-Over Number) is desirable. It should be noted that due to the bulkiness of Buchwald‘s phosphines, even in the presence of a protective extra equivalent of the ligand the monophosphine species can still be dominant in solution. Moreover, biarylphosphines are able to stabilise lowcoordinate palladium centres by means of weak bonding interactions usually involving the ipso carbon atom of the pending aryl ring.19d,22 The efficiency with which the active catalyst [Pd(PR2Ar‘)] is formed is also a pivotal factor. In this regard, we point out that cyclometallated amine complexes (N^C) of the type represented in Figure 3 have proven ideal to generate [Pd(PR2Ar‘)] under reaction the conditions.22a Some of these precatalysts are commercially available, but they are not possible for all PR2Ar‘ ligands. Bulky phosphines with aryl substituents often act as hemilabile ligands, 23 as the coordination to the metal can occur through the phosphine and also through the weakly coordinating -system of the arene. The arene interaction can lead to 6-binding or to lower hapticity such as 1 or 2- coordination. As a result, the substituents of the phosphine provide steric 23 a) P. Braunstein, F. Naud, Angew. Chem. Int. Ed. 2001, 40, 680-699; b) H. Grützmacher, Angew. Chem. Int. Ed. 2008, 47, 1814-1818; c) H. Werner, Dalton Trans. 2003, 3829-3837; d) H. Werner, G. Canepa, K. Ilg, J. Wolf, Organometallics 2000, 19, 4756-4766. Chapter I 36 shielding, protecting the phosphorus and metal atoms towards oxidation, and contribute to the stabilization of unsaturated reaction intermediates. There are examples of such stabilization in late transition metal complexes of biarylphosphines, not only on palladium,22b-e but also on Rh, Ir, Pt and Au complexes. 24 Besides phosphines of the types represented in Figure 1, a diversity of mono- and polydentate phosphine ligands have been developed in recent years, and their late transition metal complexes investigated in numerous stoichiometric and catalytic transformations. Reference 25 collects some representative work with leading citations to different aspects of this field of research. In addition, it is pertinent to mention some recent, interesting advancements in the use of electron-poor phosphine ligands, which find ample use in the chemistry of Pt(II) and Au(I), and in their applications in catalysis. 25 A somewhat classical strategy for the synthesis of P-donors with overall acceptor properties implies the use of perfluoro-alkyl or -aryl substituents on phosphorus. Ligands of this kind encompass monodentate phosphines (for instance PR(C2F5)2, R = alkyl or aryl radical), bidentate donors like (C2F5)2PCH2CH2P(C2F5)2 (dfepe), and pincer ligands alike 1,3- C6H4(CH2P(CF3)2)2, 1,3-C6H4(CH2P(C6F5)2)2, and related molecules. 26 , 27 24 a) A. R. O‘Connor, W. Kaminsky, B. C. Chan, D. M. Heinekey, K. I. Goldberg, Organometallics 2013, 32, 4016-4019; b) A. R. O‘Connor, W. Kaminsky, D. M. Heinekey, K. I. Goldberg, Organometallics 2011, 30, 2105-2116; c) A. DeAngelis, V. W. Shurtleff, O. Dmitrenko, J. M. Fox, J. Am. Chem. Soc. 2011, 133, 1650-1653; d) A. Grirrane, H. Garcia, A. Corma, E. Álvarez, ACS Catal. 2011, 1, 1647-1653; e) Y. Wang, K. Ji, S. Lan, L. Zhang, Angew. Chem. Int. Ed. 2012, 51, 1915-1918; f) E. Herrero- Gómez, C. Nieto-Oberhuber, S. López, J. Benet-Buchholz, A. M. Echavarren, Angew. Chem. Int. Ed. 2006, 45, 5455-5459. 25 a) R. J. Lundgren, M. Stradiotto, Aldrichimica Acta 2012, 45, 59-65; b) M. Albrecht, M. M. Lindner, Dalton Trans. 2011, 40, 8733-8744; c) C. A. Fleckenstein, H. Plenio, Chem. Soc. Rev. 2010, 39, 694-711; d) A. J. Kendall, C. A. Salazar, P. F. Martino, D. R. Tyler, Organometallics 2014, 33, 6171-6178; e) J. Keller, C. Schlierf, C. Nolte, P. Mayer, B. F. Straub, Synthesis 2006, 2006, 354-365. 26 a) J. J. Adams, A. Lau, N. Arulsamy, D. M. Roddick, Inorg. Chem. 2007, 46, 11328- 11334; b) S. Basu, N. Arulsamy, D. M. Roddick, Organometallics 2008, 27, 3659-3665; Pt(II) complexes 37 The electron-poor nature of their cationic platinum (II) complexes is attested by the high ῡ(CO) values recorded, for instance around 2170 cm-1 in [PtMe(CO)(dfepe)]+, 2150 cm-1 for trans-[PtMe(CO)(PMe(C2F5)2)2] and around 2110 and 2150 cm-1 in the analogous derivatives of the above mentioned fluorinated pincer ligands.26,27 Recently, an alternative plan of action was put into practice. The new approach envisioned the use of cationic rather than formally neutral radical substituents bonded to the phosphorus atom and led to a variety of carbeniophosphanes embracing P+,P+-chelating bis(imidazoliophosphine) and related P+CP+ pincer ligands. 28 Recent work from the group of Alcarazo 29 has resulted in phosphenium cations of the types represented in Figure 4. The new ligands include some pyridinio-dialkyl or -diaryl phosphines,29b as well as mono-, bis- and tris- (dialkylamino)cyclopropenium substituted phosphines. It is remarkable that the bis(dialkylamino)cyclopropenium disubstituted phosphines have electron-donor properties comparable to those of P(CF3)3.29c It has also been found that in the [PtCl3(L)][ClO4]2 complex of the tricationic phosphine, the LPt -donation is lower than the Pt(II)L -back- donation, that is, the P-ligand removes net electron density from the metal, thereby enhancing the intrinsic -acidity of Pt(II).29d c) J. J. Adams, N. Arulsamy, D. M. Roddick, Organometallics 2009, 28, 1148-1157; d) B. Thapaliya, S. Debnath, N. Arulsamy, D. M. Roddick, Organometallics 2015, 34, 4018- 4022. 27 B. G. Anderson, J. L. Spencer, Chem. Eur. J. 2014, 20, 6421-6432. 28 a) C. Barthes, C. Lepetit, Y. Canac, C. Duhayon, D. Zargarian, R. Chauvin, Inorg. Chem. 2013, 52, 48-58; b) Y. Canac, C. Maaliki, I. Abdellah, R. Chauvin, New J. Chem. 2012, 36, 17-27. 29 a) Á. Kozma, T. Deden, J. Carreras, C. Wille, J. Petuškova, J. Rust, M. Alcarazo, Chem. Eur. J. 2014, 20, 2208-2214; b) H. Tinnermann, C. Wille, M. Alcarazo, Angew. Chem. Int. Ed. 2014, 53, 8732-8736; c) J. Carreras, G. Gopakumar, L. Gu, A. Gimeno, P. Linowski, J. Petuškova, W. Thiel, M. Alcarazo, J. Am. Chem. Soc. 2013, 135, 18815- 18823; d) J. Carreras, M. Patil, W. Thiel, M. Alcarazo, J. Am. Chem. Soc. 2012, 134, 16753-16758; e) J. Petuškova, M. Patil, S. Holle, C. W. Lehmann, W. Thiel, M. Alcarazo, J. Am. Chem. Soc. 2011, 133, 20758-20760; f) J. J. Weigand, K.-O. Feldmann, F. D. Henne, J. Am. Chem. Soc. 2010, 132, 16321-16323. Chapter I 38 Figure 4. Some newly designed mono and polycationic phosphorus donor ligands (See Reference 29). Our research group has had a long-standing interest in transition metal complexes of tertiary phosphines and related P-donor ligands. 30 , 31 In recent years, terphenyl groups have been employed to stabilize unusual dimolybdenum complexes that contain MoMo quadruple bonds. 32 Bearing in mind the extraordinary practical importance of Buchwald‘s dialkylbiaryl phosphines, we thought it worthwhile to explore the chemistry of the even bulkier terphenylphosphine ligands, PR2Ar‘ (see Figure 5; Ar‘ represents a terphenyl substituted radical) for which there is a scarcity of information in the chemical literature.11, 33 This chapter deals 30 See for example: a) E. Carmona, F. Gonzalez, M. L. Poveda, J. L. Atwood, R. D. Rogers, J. Chem. Soc., Dalton Trans. 1980, 2108-2116; b) E. Carmona, F. Gonzalez, M. L. Poveda, J. L. Atwood, R. D. Rogers, J. Chem. Soc., Dalton Trans. 1981, 777-782. 31 See for example: a) E. Carmona, J. M. Marin, M. L. Poveda, J. L. Atwood, R. D. Rogers, J. Am. Chem. Soc. 1983, 105, 3014-3022; b) R. Alvarez, E. Carmona, M. L. Poveda, R. Sanchez-Delgado, J. Am. Chem. Soc. 1984, 106, 2731-2732. 32 a) M. Carrasco, I. Mendoza, E. Álvarez, A. Grirrane, C. Maya, R. Peloso, A. Rodríguez, A. Falceto, S. Álvarez, E. Carmona, Chem. Eur. J. 2015, 21, 410-421; b) M. Carrasco, I. Mendoza, M. Faust, J. López-Serrano, R. Peloso, A. Rodríguez, E. Álvarez, C. Maya, P. P. Power, E. Carmona, J. Am. Chem. Soc. 2014, 136, 9173-9180. 33 a) D. V. Partyka, M. P. Washington, J. B. Updegraff Iii, X. Chen, C. D. Incarvito, A. L. Rheingold, J. D. Protasiewicz, J. Organomet. Chem. 2009, 694, 1441-1446; b) R. C. Smith, R. A. Woloszynek, W. Chen, T. Ren, J. D. Protasiewicz, Tetrahedron Lett. 2004, 45, 8327-8330; c) B. Buster, A. A. Diaz, T. Graham, R. Khan, M. A. Khan, D. R. Powell, Pt(II) complexes 39 with the synthesis and reactivity of a variety of Pt(II) complexes stabilized with the terphenylphosphines PMe2ArXyl2, PMe2ArDipp2, and PMe2ArTipp2 (ArXyl2 = C6H3-2,6-(C6H3-2,6-Me2)2; ArDipp2 = C6H3-2,6-(C6H3-2,6-iPr2)2; ArTipp2 = C6H3-2,6-(C6H2-2,4,6-iPr3)2. Their catalytic activities for hydroamination and hydroarylation of alkynes have also been tested. Figure 5. Dimethylterphenylphosphines utilised in this work. Although four-coordinate square-planar geometries, with a formal count of 16 electrons, are absolutely dominant in isolated Pt(II) compounds, three-coordinate-14 electron Pt(II) complexes are believed to be key intermediates in a number of platinum-mediated organometallic transformations. Most of these unsaturated Pt(II) complexes feature a T- shaped coordination geometry, although a rare example of Y-shaped complex has also been documented. 34 In spite of the very small number of three-coordinate Pt(II) complexes that have been isolated and characterized R. J. Wehmschulte, Inorg. Chim. Acta 2009, 362, 3465-3474; d) L. Ortega-Moreno, R. Peloso, C. Maya, A. Suarez, E. Carmona, Chem. Commun. 2015, 51, 17008-17011; e) M. F. Espada, J. Campos, J. López-Serrano, M. L. Poveda, E. Carmona, Angew. Chem. Int. Ed. 2015, 54, 15379-15384. 34 G. Berthon-Gelloz, B. de Bruin, B. Tinant, I. E. Markó, Angew. Chem. Int. Ed. 2009, 48, 3161-3164. Chapter I 46 subsequent methylation of the resulting dihalo terphenylphosphines with methylmagnesium bromide (Scheme 5). This ligand is a moderately airstable colourless solid and it was completely characterized by NMR spectroscopy.11 Scheme 5. Synthesis of PMe2ArXyl2 (L1) I.2.1.1. Bis(phosphine) Pt(II) Complexes The phosphine PMe2ArXyl2, L1, reacted cleanly (toluene, 110 ºC) with the Pt precursor cis-[PtCl2(COD)] (COD = 1,5-cyclooctadiene), in a 2:1 molar ratio to generate complex 1 in high yield (Scheme 6). As can be seen, compound 1 resulted from the substitution of the neutral COD ligand by two molecules of L1 and concomitant isomerisation to the trans isomer. Pt(II) complexes 47 The isomerisation is probably driven by the steric bulk of the terphenyl phosphine ligand, coupled with the low trans-influence of the chloride group. 38 Curiously, no evidence for the formation of the mono(phosphine) adduct was found, and consequently, the optimized reaction conditions for the preparation of complex 1 required the already stated 1:2 ratio of Pt:PR2Ar‘. Scheme 6. Synthesis of trans-[PtCl2(PMe2ArXyl2)2], 1. Complex 1 is an air-stable colourless solid, highly soluble in common organic solvents such as benzene, toluene, chloroform and dichloromethane. Figure 7 shows the aliphatic region of the HSQC 1H-13C NMR spectrum (300 MHz, CDCl3, 25 ºC) of complex 1. The eight equivalent methyl groups of the xylyl moieties give rise to a singlet at 2.07 ppm in the 1H NMR spectrum of 1. The equivalency of these groups can be ascribed either to the molecular symmetry of the complex or to dynamic processes involving the phosphine ligand facile rotation around Car-Car and Car-P bonds. In addition, the methyl groups directly bound to the P atom appear as a pseudotriplet in the 1H and 13C{1H} NMR spectra, due to 38 a) T. G. Appleton, M. A. Bennett, Inorg. Chem. 1978, 17, 738-747; b) E. M. Shustorovich, M. A. Porai-Koshits, Y. A. Buslaev, Coord. Chem. Rev. 1975, 17, 1-98; c) K. F. Purcell, J. C. Kotz, in Inorg. Chem., W. B. Saunders Co., Philadelphia, 1977, Chapter 13. Chapter I 48 virtual coupling to the 31P nuclei, 39 clearly demonstrating the trans arrangement of the ligands in this complex. The 31P{1H} resonance of complex 1 appears as a singlet at -17.3 ppm (1JPPt = 2555 Hz) with a shift to higher frequencies of 23 ppm with respect to the free phosphine ligand. Figure 7. Aliphatic region of the HSQC 1H-13C NMR spectrum of complex 1 (300 MHz, CDCl3, 25 ºC). The phosphine PMe2ArXyl2, L1, also reacted with the organometallic Pt(II) precursor formed by the mixture of [Pt(Me)2(-SMe2)]2 and Pt(Me)2(SMe2)2, 40 (CH2Cl2, 25ºC) in a 2:1, P:Pt molar ratio, to afford cleanly the neutral mononuclear complex cis-[PtMe2(PMe2ArXyl2)2], 2 (Scheme 7). 39 a) R. H. Crabtree, The Organometallic Chemistry of the Transition Metals, Vol. Chapter 10, 1st ed., John Wiley & Sons, Inc., 1988; b) R. K. Harris, Can. J. Chem. 1964, 42, 2275- 2281; c) A. Ault, J. Chem. Educ. 1970, 47, 812; d) D. A. Redfield, L. W. Cary, J. H. Nelson, Inorg. Chem. 1975, 14, 50-59. 40 J. D. Scott, R. J. Puddephatt, Organometallics 1983, 2, 1643-1648. Pt(II) complexes 49 Scheme 7: Synthesis of complex 2. By contrast with the analogous dichloride derivative 1, the dimethyl complex 2 features the typical cis ligand arrangement found in complexes of this kind,41 despite the bulkiness of the PMe2ArXyl2 ligands. Since the van der Waals radius of chlorine is similar, if somewhat smaller, to that of the methyl group (1.80 and 2.0 Å, respectively),42 it seems probable that observed geometry is a consequence of the strong -donor character, and consequently high trans-influence of the methyl groups,9,38 which tend to avoid each other in opposite coordination positions. The 1H NMR data for compound 2 are typical of square-planar cis-[PtMe2(PR3)2] complexes.41 The platinum bonded methyl groups are equivalent and give rise to a 31P- coupled multiplet centred at 0.0 ppm (Figure 8), with 195Pt satellites of 71 Hz. The corresponding 13C{1H} resonance is located at  1.3 with apparent JCP coupling constants of 105 and 9 Hz, and a one-bond 13C-195Pt coupling constant of 610 Hz. No attempts were made to compute these spectra.41a The 31P{1H} spectrum is a sharp singlet placed at -5.3 ppm, with 1JPPt = 1773 Hz. All these parameters are analogous to those reported previously 41 See for example: a) R. J. Goodfellow, M. J. Hardy, B. F. Taylor, J. Chem. Soc., Dalton Trans. 1973, 2450-2453; b) C. M. Haar, S. P. Nolan, W. J. Marshall, K. G. Moloy, A. Prock, W. P. Giering, Organometallics 1999, 18, 474-479. 42 L. Pauling, The Nature of the Chemical Bond, 3rd. ed., Cornell University Press, Cornell University, New York, 1960. Chapter I 50 for similar complexes. Figure 8: 1H NMR spectrum of complex 2 (300 MHz, CDCl3, 25 ºC). The preparation of complexes 1 and 2 motivated us to search for metallacyclic species derived from these compounds. The historical importance of platinacycles was briefly commented in Section I.1.1 of this Thesis. It was then noted that the fundamental work of Shaw and coworkers demonstrated that steric hindrance, and the feasibility of -C—H metallation leading to five-membered rings, considerably facilitated the formation of platinacycles.10 On these grounds, the o-tolyl-substituted phosphines, PPh(o-tolyl)2, PtBu(o-tolyl)2 and PtBu2(o-tolyl) proved particularly useful. In pursuit of such structures, a large number of cyclometallated complexes were prepared in the subsequent decades, 43 43 For some early review articles, see: a) G. W. Parshall, Acc. Chem. Res. 1970, 3, 139- 144; b) J. Dehand, M. Pfeffer, Coord. Chem. Rev. 1976, 18, 327-352; c) M. I. Bruce, Angew. Chem. Int. Ed. Engl. 1977, 16, 73-86; d) I. Omae, Coord. Chem. Rev. 1980, 32, 235-271. Pt(II) complexes 51 encompassing a variety of metals and P^C, P^N, P^O, P^S, etc., bidentate ligands.15d,35f,44-49 4445464748 - 49 Recent work from our group focused on Pt(II) complexes of the bulky phosphines PiPr2(Xyl) and PMe(Xyl)2, permitted the synthesis of five-membered bis-metallacycles and the study of their chemical reactivity.36 Milstein and co-workers reported six-membered platinacycles derived from the aryl-monophosphine 2- (diisopropylphosphino)isodurene.45 However, internal metallation of the terphenylphosphine PMe2ArXyl2, L1, is not expected to be a facile reaction, since generation of the purported seven-member metallacycle would need the activation of a-C—H bond. In accordance with this hypothesis, while PiPr2(Xyl) and PMe(Xyl)2 reacted with [PtCl2(COD)] in the presence of 44 a) A. D. Ryabov, Chem. Rev. 1990, 90, 403-424; b) M. E. van der Boom, D. Milstein, Chem. Rev. 2003, 103, 1759-1792; c) M. Albrecht, Chem. Rev. 2010, 110, 576-623; d) I. Omae, Cyclometalation Reactions, Springer, Japan, 2014; e) Y.-F. Han, G.-X. Jin, Chem. Soc. Rev. 2014, 43, 2799-2823; f) F. Mohr, S. H. Privér, S. K. Bhargava, M. A. Bennett, Coord. Chem. Rev. 2006, 250, 1851-1888. 45 a) M. E. van der Boom, S.-Y. Liou, L. J. W. Shimon, Y. Ben-David, D. Milstein, Organometallics 1996, 15, 2562-2568; b) M. E. van der Boom, J. Ott, D. Milstein, Organometallics 1998, 17, 4263-4266; c) M. Montag, G. Leitus, L. J. W. Shimon, Y. Ben-David, D. Milstein, Chem. Eur. J. 2007, 13, 9043-9055. 46 a) J. Forniés, A. Martín, R. Navarro, V. Sicilia, P. Villarroya, Organometallics 1996, 15, 1826-1833; b) M. Sano, Y. Nakamura, J. Chem. Soc., Dalton Trans. 1991, 417-424. 47 a) R. Romeo, M. R. Plutino, A. Romeo, Helv. Chim. Acta 2005, 88, 507-522; b) A. Marrone, N. Re, R. Romeo, Organometallics 2008, 27, 2215-2222. 48 a) W. Baratta, E. Herdtweck, P. Rigo, Angew. Chem. Int. Ed. 1999, 38, 1629-1631; b) W. Baratta, E. Herdtweck, P. Martinuzzi, P. Rigo, Organometallics 2001, 20, 305-308; c) W. Baratta, C. Mealli, E. Herdtweck, A. Ienco, S. A. Mason, P. Rigo, J. Am. Chem. Soc. 2004, 126, 5549-5562; d) W. Baratta, A. Del Zotto, G. Esposito, A. Sechi, M. Toniutti, E. Zangrando, P. Rigo, Organometallics 2004, 23, 6264-6272; e) W. Baratta, M. Ballico, A. Del Zotto, E. Zangrando, P. Rigo, Chem. Eur. J. 2007, 13, 6701-6709. 49 a) A. Zucca, G. L. Petretto, S. Stoccoro, M. A. Cinellu, G. Minghetti, M. Manassero, C. Manassero, L. Male, A. Albinati, Organometallics 2006, 25, 2253-2265; b) A. Zucca, G. L. Petretto, S. Stoccoro, M. A. Cinellu, M. Manassero, C. Manassero, G. Minghetti, Organometallics 2009, 28, 2150-2159; c) G. L. Petretto, J. P. Rourke, L. Maidich, S. Stoccoro, M. A. Cinellu, G. Minghetti, G. J. Clarkson, A. Zucca, Organometallics 2012, 31, 2971-2977; d) L. Maidich, G. Zuri, S. Stoccoro, M. A. Cinellu, M. Masia, A. Zucca, Organometallics 2013, 32, 438-448; e) L. Maidich, G. Zuri, S. Stoccoro, M. A. Cinellu, A. Zucca, Dalton Trans. 2014, 43, 14806-14815; f) A. Zucca, L. Maidich, L. Canu, G. L. Petretto, S. Stoccoro, M. A. Cinellu, G. J. Clarkson, J. P. Rourke, Chem. Eur. J. 2014, 20, 5501-5510. Chapter I 52 K2CO3 to afford the expected platinacycles,11 the reaction of PMe2ArXyl2 under otherwise identical conditions resulted in an intractable mixture of products. Use of isolated complex 1 and a diversity of bases (K2CO3, KOtBu, 2,2,6,6-tetramethylpiperidine) made no improvement. At variance with these observations, the dimethyl complex 2 underwent cyclometallation under mild conditions to originate the desired sevenmember platinacycle 3 (Scheme 8), accompanied by liberation of methane. This result was not unforeseen since it has long been known that the existence of Pt-Me bonds (in general Pt-alkyl bonds) facilitates considerably the cyclometallation reaction.45,47 Scheme 8: Synthesis of complex 3. Compound 3 was characterized by microanalysis, solution NMR spectroscopy and X-ray crystallography. As expected, and as a consequence of the lower symmetry of 3 compared to 2, five singlets due to the benzylic CH3 protons appear in the 1H NMR spectrum of complex 3 between ca.  1.90 and 2.45 ppm (Figure 9). Pt(II) complexes 53 In addition, the platinum-bound methylene protons are diastereotopic and resonate at 2.81 and 2.93 ppm as deceptively simple doublets of doublets, with apparent JPH coupling constants of 19.5 and 8.8 Hz, also showing Pt satellites with a large 2JHPt coupling constant of about 100 Hz. As regards the 13C{1H} spectrum of 3, the methylene carbon nucleus gives also rise to a seemingly simple doublet of doublets, with apparent 13C-31P coupling constants of 95 and 6 Hz, 1JPtC = 549 Hz. The inequivalency of the two phosphorus atoms is reflected in the 31P{1H} spectrum of complex 3, which consists of two doublets centred at -7.9 and -4.3 ppm with a small 31P-31P coupling of 6 Hz, and 195Pt-31P constants of 1748 Hz and 1985 Hz, respectively, the latter two in the range normally expected for complexes of this type. Figure 9: 1H NMR spectrum of complex 3 (300 MHz, C6D6, 25 ºC). Chapter I 54 As anticipated, the solid structure of compound 3 was also confirmed by single crystal X-ray diffraction studies (Figure 10). The Pt(II) atom is fourcoordinate and lies in a distorted square-planar environment. The angle between the two planes defined by atoms C49, Pt1, and C1, and P1, Pt1, and P2, is actually of 10.88º. This notable distortion is reasonable to ascribe to the crowded coordination environment and to the rigidity of the 7-membered platinacycle. Figure 10. ORTEP view of complex 3; hydrogen atoms are excluded for clarity and thermal ellipsoids are set at 50% level of probability. Selected bond distances (Å) and angles(º): Pt1P1 2.3000(7), Pt1C1 2.107(3), Pt1P2 2.2943(7), Pt1C49 2.126(3), P1Pt1C1 86.05(8), P1Pt1P2 103.62(2), P2Pt1C49 86.29(7), C49Pt1C1 84.6(1). The mechanism of the cycloplatination reaction was thoroughly investigated by the group of Romeo and Re with the aid of extensive experimental kinetic studies, complemented with computational work.47 The results of these investigations, based on the complex cis- [PtMe2(dmso)(P(o-tolyl)3)], supported a mechanism initiated by reversible dissociation of dmso and formation of a three-coordinate, T-shaped species. We monitored the cyclometallation reaction that converts complex Pt(II) complexes 55 2 into 3 by 1H and 31P{1H} NMR spectroscopy at 55 ºC, using C6D6 as the solvent (Figure 11). Similarly to Romeo and Re‘s results, our monitoring of the cycloplatination reaction revealed the presence of only the starting (bottom spectra in Figures 11a and 11b) and the final cyclometallated complexes (top spectra) with no evidence for the formation of detectable amounts of any intermediate species. As shown in Figures 11 and 12, the reaction went to completion in about 90 minutes at 55 ºC (ca. six halflives) and followed first-order kinetics. An Eyring representation of kinetic data measured at 40, 45, 50, 55, and 60 ºC (Figure 12 bottom) yielded Hǂ and Sǂ values of 31±1 kcal·mol-1 and 22±4 cal·K-1·mol-1, with Gǂ 25±3 kcal·mol-1 (at 25 ºC). Chapter I 62 Scheme 10. Formation of the PPh3 complexes 5. The cis geometry proposed for this complex is definitely deduced from its 31P{1H} NMR spectrum, that contains two doublets at  -7.8 and 23.8 ppm, with an unquestionably cis 2JPP coupling of only 7 Hz. The two signals are escorted by 195Pt satellites of 1978 and 1886 Hz, respectively, in agreement with their phosphorus atoms being part of trans-P—Pt—C arrangements. By similarity with what was observed for the also cyclometallated complexes 3 and 4, the platinum bonded methylene protons are diastereotopic and give rise to 31P- and 195Pt-coupled multiplets located at 2.59 and 2.31 ppm (Figure 16). The corresponding 13C{1H} signal appears as a first-order doublet of doublets at 33.6 ppm, with trans and cis couplings to the 31P nuclei of 89 and 5 Hz, respectively. Pt(II) complexes 63 Figure 16: Aliphatic region of the 1H NMR spectrum of complex cis-5 (500 MHz, CD2Cl2, 25 ºC). I.2.1.3. Protonation of Complexes 2 and 3 To investigate the possibility of generating three-coordinate T-shaped Pt(II) cationic species stabilized by ligand L1, we studied the protonation of complexes 2 and 3 with [H(Et2O)2][BArF] ([BArF] = [B(3,5- (CF3)2C6H3)4]), which may accurately be viewed as a non-coordinating acid. It is worth recalling that this is a common synthetic method for the production of low-coordinated species that feature agostic interactions. Moreover, a similar procedure was utilised by Goldberg, Brookhart and coworkers for the successful low-temperature characterisation of the first transition metal methane complex. 51 51 W. H. Bernskoetter, C. K. Schauer, K. I. Goldberg, M. Brookhart, Science 2009, 326, 553-556. Chapter I 64 The protonation of complex 2 by [H(Et2O)2][BArF] (1 equiv) was brought about at -80 ºC (CD2Cl2) and the progress of the reaction monitored by 1H and 31P{1H} NMR spectroscopy. Figure 17 contains variable temperature 31P{1H} spectra in the temperature interval from -80 º to 50 ºC. At the lowest temperature studied, a mixture of two complexes appears to exist. The major one contains two doublets centred at -8.4 and 24.9 ppm that correspond to an AX spin system with a cis-AX coupling of 11 Hz. The higher frequency signal features JPPt = 4760 Hz, whereas for the other, the one bond 31P-195Pt coupling constant amounts 2200 Hz. We suggest that these parameters correspond to the formally three coordinate complex A (see Figure 17) in which the phosphorus atom responsible for the -8.4 resonance is trans to a weakly bound molecule of solvent (not represented in Figure 17), whereas the other is trans to the methyl group. The value of 2200 Hz found the 31P-195Pt coupling for this phosphorus nucleus is comparable to others found for trans-P—Pt—Calkyl units reported in this Thesis. In turn, the minor species appears as a singlet at - 5.8 ppm and features a 31P-195Pt coupling of 2980 Hz, typical for trans-P— Pt—P units (structure B in Figure 17). As represented in Figure 17, at 0 ºC the trans complex B became the major product at the expense of the cis isomer, most probably to avoid steric repulsions between the phosphine ligands of the latter. No other significant change was detected, although upon warming at 25 ºC two new structures C and D appeared with accompanying loss of A and B. We propose that the new compounds result from facile cycloplatination of one of the PMe2ArXyl2 ligands (vide supra), with liberation of CH4 (detected by 1H NMR). Complex C is proposed to contain a trans distribution of the non-equivalent phosphorus nuclei, as hinted by the observation of an AB quartet, characterized by A = 2.0 ppm, B = 17.2 ppm and 2JAB = 386 Hz along with 2JPAPt and 2JPBPt values of 3119 and 3078 Hz, respectively. Pt(II) complexes 65 Figure 17.Variable temperature 31P{1H} NMR spectra recorded for the protonation of complex 2 with [H(Et2O)2][BArF] (161.9 MHz, CD2Cl2). The fourth platinum coordination position is proposed to be occupied by a molecule of CD2Cl2. As for D, a cis arrangement of the phosphorus atoms is considered, as insinuated by the observation of two doublets with  -9.6 and 21.6 ppm, and 2JPP equal to 14 Hz. These 31P resonances are further characterised by 1JPPt couplings of 4831 and 2087 Hz, in accordance with the proposed structure. Besides signals due to the cis and trans isomers, a singlet at -14.4 Chapter I 66 ppm is observed for an unidentified product. Heating at 50 ºC for 22 h results in full consumption of the initial reaction products, complexes A and B, with no other relevant variation relative to the room temperature spectrum. Similar intermediates and final products were also expected to form in the protonation of complex 3. Interestingly, this species contains two different Pt-C bonds and therefore there are two alternative positions for the proton to attack, that is the cyclometallated Pt-CH2 linkage and the Pt- Me bond. In fact, the 31P{1H} NMR spectrum of the crude reaction mixture that emanates from the protonation of complex 3 at low temperatures (-80 ºC) reveals the formation of the four possible structures that could derive from H+ addition at the two Pt-C sites, followed by cis-to-trans isomerisation of the corresponding T-shaped intermediates. Upon increasing the temperature to room temperature, the mixture evolves to afford as the main products of the protonation reaction the cyclometallated isomers detected for compound 2, that is, complexes C and D, characterised by resonances at 21.6 and -9.6 ppm (2JPP = 14 Hz) for the cis isomer, and at 17.2 and 2.0 ppm (2JPP = 386 Hz, 1JPPt = 3078 and 3119 Hz, respectively) for the trans one (Figure 18). In agreement with these observations, the protonation of complexes 2 and 3 can be proposed to occur as summarized in Scheme 11. Pt(II) complexes 67 Figure 18: Variable temperature 31P{1H} NMR spectra of the protonation of complex 3 with [H(Et2O)2][BArF] (129.5 MHz, CD2Cl2). Scheme 11. Proposed product manifold for the protonation of complexes 2 and 3 with [H(Et2O)2][BArF]. As in Figure 17, the fourth platinum coordination site in structures A-D is proposed to be occupied by a molecule of CD2Cl2. Chapter I 68 I.2.2. Pt(II) Complexes of the Terphenylphosphines PMe2ArDipp2 and PMe2ArTipp2 In this section new results concerning the synthesis, characterisation, reactivity and catalytic properties of neutral and cationic complexes of Pt(II) containing the terphenylphosphine ligands PMe2ArDipp2 (L2), and PMe2ArTipp2 (L3), are described (Figure 19). Figure 19: Dimethylterphenylphosphines L2 and L3. The synthetic routes used for the preparation of L2 and L3 are analogous to that employed for L1. The two phosphines are pale yellow solids, soluble in common organic solvents such as pentane, benzene, or dichloromethane, and exhibit good stability towards air, particularly in the solid state. The majority of the new compounds reported contain ligand L2, although some derivatives of L3 were also prepared for comparative purposes. As noted above, some are neutral species, but others are cationic, with one or two positive charges on the Pt(II) centre. Some of the complexes are coordination compounds in a classical sense (e.g. halide- or hydride-phosphine complexes) whereas others are truly organometallic, actually hydrocarbyl species, comprising methyl, phenyl and allyl Platinum (II) Complexes 69 derivatives. In many of the complexes the phosphine ligand binds to platinum in a bidentate fashion, through the phosphorus atom and the ipso carbon atom of one of the aryl substituents of the terphenyl group, while in others monodentate P-coordination was imposed by action of other Lewis bases. I.2.2.1. Neutral Platinum(II) Complexes with Bidentate (1-P, 1- arene) Phosphine Coordination Dichloro Complexes Phosphines L2 and L3 reacted cleanly (toluene, 80 ºC) with one equivalent of the commercially available Pt precursor PtCl2, to generate complexes 6 and 7 in high yield (Scheme 12), that contain cis chloride ligands. Scheme 12. Synthesis of the complexes [PtCl2(PMe2Ar‘)], 6 and 7. Formally in complexes 6 and 7 there are only three ligands in the metal coordination sphere and therefore they might be viewed as T-shaped 14-electron species. Notwithstanding, to counterbalance this unsaturation, Chapter I 70 the phosphine acts as a bidentate ligand with a 1-P, 1-arene coordination mode, as revealed by X-ray studies to be discussed later. Because of this unusual aryl phosphine binding, the symmetry of the terphenyl group changes compared with that of the free ligand, and this variation becomes evident by NMR spectroscopy. In the 1H NMR spectra of both complexes (Figure 20), four different doublet resonances for the methyl groups of the iPr substituents at the 2,6-aryl positions are detected and, in the case of complex 7, two additional doublets for the 4-iPr methyl groups are also observed. These spectroscopic data reveal the non-equivalent nature of the flanking aryl rings. The 31P{1H} NMR resonances and 1JPPt values for both complexes are very similar (10.9 ppm and 3189 Hz for 6 and 10.6 ppm and 3226 Hz for 7). Figure 20. The 1H NMR spectra of complexes 6 and 7 (CD2Cl2, 300 MHz (top), 400 MHz (bottom), 25ºC) in the chemical shift range ca. 0.80 - 3.50 ppm. Platinum (II) Complexes 71 The molecular structure of complex 6 was determined in the solid state by single crystal X-ray diffraction studies (Figure 21). In accordance with the NMR data already analysed, the phosphine acts as a bidentate 1- P, 1-Cipso ligand with Pt—P and Pt—Cipso contacts of 2.215(1) and 2.228(5) Å, respectively. The latter distance denotes significant Pt—Carene bonding interaction. 52 It is, however, longer than the Pt(II)-olefinic carbon bonds in Zeise‘s salt (roughly 2.16 Å) 53 , in accordance with the partial unsaturated character of these complexes, which can be envisioned as a source of [PtCl2(PMe2Ar‘)] fragments. Congruent with these considerations the Pt1A-Cl1 bond trans to the Carene (C1 in Figure 21) at 2.350(1) Å is shorter than the Pt1A-Cl2 bond placed trans with respect to the phosphine ligand (2.401(1) Å).9,38 Figure 21: ORTEP view of complex 6; hydrogen atoms are excluded for clarity and thermal ellipsoids are set at 50 % level probability. Selected bond distances (Å) and angles (º): Pt1A-P1 2.215(1), Pt1A-Cl1 2.350(1), Pt1A-Cl2 2.401(1), Pt1A-C1 2.228(5), P1-Pt1A-Cl1 85.66(4), Cl1-Pt1A-Cl2 87.01(4), Cl2-Pt1A-C1 104.6(1), C1-Pt1A-P1 82.8(1). 52 A. Falceto, E. Carmona, S. Alvarez, Organometallics 2014, 33, 6660-6668. 53 M. Black, R. H. B. Mais, P. G. Owston, Acta Crystall. B 1969, 25, 1753-1759. Chapter I 78 Compound 10 results also from a comproportion reaction between dichloride 6 and dimethyl 8, but the most convenient synthetic procedure for the isolation of analytically pure samples of the mixed methyl-chloride complex 10 consists in the protonation of the dimethyl derivative 8 with a 1 M ether solution of HCl, according to Scheme 16. Two different isomers could form in this reaction but protonation seemed to occur selectively at the Pt-Me bond trans to the phosphine ligand, affording complex 10 quantitatively (by 1H NMR), with elimination of CH4. A detailed mechanistic study of this reaction was not attempted. Scheme 16. Synthesis of complex 10. Complex 10 was fully characterized by solution NMR spectroscopy studies and by X-ray crystallography. Once more, structural data are consistent with a bidentate  1-P,  1-Carene coordination mode of the phosphine ligand, both in solution and in the solid state. The mutually cis distribution of the methyl and phosphine ligands is unambiguously inferred from 1H and 13C{1H} NMR data. Thus, the Pt—Me linkage is responsible for a doublet 1H resonance (Figure 26) at  0.87 ppm (2JPtH = 86, 3JPH = 4.3 Hz) and for a 13C{1H} signal at -11.3 ppm, that exhibits 1JPtC and 2JPC couplings of 771 and 6 Hz, respectively. The 31P{1H} NMR spectrum is the expected singlet, characterized by 0.4 ppm and 1JPPt = 4300 Hz. Platinum (II) Complexes 79 Although the magnitude of this coupling appears unusually large in comparison with other trans-P—Pt—Cl linkages studied in this work, it is comparable to the 4179 Hz value found for 1JPPt trans to chloride in the somewhat related compound cis-[Pt(Me)Cl(PEt3)2]. 56 Figure 26. Aliphatic area of 1H NMR spectrum of complex 10 (300MHz, CDCl3, 25 ºC). With the identification by X-ray studies of the structure of complex 10 (Figure 27), we complete in this Thesis an interesting set of cis- [Pt(X)(Y)(PMe2ArDipp2)] structures, X and Y being methyl and/or chloride ligands, namely cis-[PtCl2(PMe2ArDipp2)] (6), cis-[PtMe2(PMe2ArDipp2)] (8), and cis-[Pt(Me)Cl(PMe2ArDipp2)] (10). The solid state structure of the molecules 6, 8, and 10 are similar, although they reveal the variations expected upon changing the chloride ligands by methyl groups. For 56 F. H. Allen, A. Pidcock, J. Chem. Soc., A 1968, 2700-2704. Chapter I 80 instance, in the present complex 10, the Pt—P bond (2.215(1) Å) is shorter than in 8 (2.248(4) Å), while the Pt—Me bond remains essentially invariant at roughly 2.05 Å. The Pt···Car interaction in 10 appears to be somewhat longer than in 8 (ca. 2.37 vs. 2.33 Å), a variation that finds no simple justification on electronic grounds. Figure 27. ORTEP view of the complex 10; hydrogen atoms are excluded for clarity and thermal ellipsoids are set at 50 % level of probability. Selected bond distances (Å) and angles (º): Pt1-P1 2.215(1), Pt1-C33 2.053(5), Pt1-Cl1 2.280(2), Pt1-C19 2.368(4), P1-Pt1-C33 88.8(1), C33-Pt1-Cl1 84.1 (1), Cl1-Pt1-C19 105.4(1), C19-Pt1-P1 87.77(9). The Diphenyl Complex cis-[Pt(Ph)2(PMe2ArDipp2)] The preparative procedure utilised for the isolation of the dimethyl complex 8 was successfully applied to obtain the analogous bis(phenyl) complex 11. Thus, as depicted in Scheme 17, the terphenylphosphine L2 Platinum (II) Complexes 81 reacted cleanly (CH2Cl2, 25 ºC) with one equivalent (with reference to the P:Pt ratio) of the mixture formed by [Pt(Ph)2(-SMe2)]2 and [Pt(Ph)2(SMe2)2] 57 , to generate complex 11, in wich the phosphine also exhibits a  1-P,  1-Carene coordination mode. Scheme 17. Synthesis of complex 11. Figure 29 shows the 1H and 1H{31P} NMR spectra (300 MHz, CD2Cl2, 25 ºC) of the new complex in the region from ca. 0.7 to 2.7 ppm. Once more, the methyl groups of the phosphine iso-propyl substituents appear as four doublets at 0.98, 1.01, 1.29 and 1.84 ppm, in accord with the existence of a Pt···Carene interaction. The 31P{1H} resonance of this complex appears as a singlet at 12.8 ppm (1JPPt = 1930 Hz). 57 M.-E. Moret, P. Chen, Organometallics 2008, 27, 4903-4916. Chapter I 82 Figure 29. Partial 1H (bottom) and 1H{31P} (top) NMR spectra of complex 11 (300 MHz, CD2Cl2, 25 ºC). Silyl Hydride Complexes Transition-metal silyl chemistry has experienced an enormous growth over the past decades. 58 This is due mainly to the influential chemical role of hydrosilanes, SiR4-nHn (n = 1-3), which represent one of the most important families of reagents in organic synthesis, 59 as they are able to add catalytically to C—C, C—O and C—N multiple bonds, in the well-known, widely utilised hydrosilylation reaction. 60 , 61 Platinum 58 J. Y. Corey, J. Braddock-Wilking, Chem. Rev. 1999, 99, 175-292. 59 F. Alonso, I. P. Beletskaya, M. Yus, Chem. Rev. 2002, 102, 4009-4092. 60 a) B. Marciniec, Silicon Chemistry 2002, 1, 155-174; b) A. K. Roy, in Adv. Organomet. Chem., Vol. Volume 55 (Eds.: A. F. H. Robert West, J. F. Mark), Academic Press, 2007, 1-59; c) E. Calimano, T. D. Tilley, J. Am. Chem. Soc. 2009, 131, 11161-11173. 61 a) R. Malacea, R. Poli, E. Manoury, Coord. Chem. Rev. 2010, 254, 729-752; b) R. H. Morris, Chem. Soc. Rev. 2009, 38, 2282-2291. Platinum (II) Complexes 83 complexes are overwhelmingly preferred as catalysts. 62 , 63 Two of them, namely the Speir catalyst, H2PtCl6/iPrOH, and the Kardstedt catalyst, [Pt2(CH2=CHSi(Me)2OSi(Me)2CH=CH2)3] have found ample use in the silicone industry.62 We therefore considered appropriate to study the formation of platinum(II)-silyl complexes and envisioned that the dimethyl derivatives cis-[PtMe2(PMe2Ar‘)] (8) and (9) could be suitable starting materials yielding silyl complexes by means of a combination of oxidative addition and reductive elimination reactions. In this section, the synthesis and structural characterisation of a series of hydride silyl complexes of composition cis- [Pt(H)(SiR3)(PMe2Ar‘)] is reported. As for other complexes already considered, the terphenylphosphine ligand, PMe2ArDipp2 or PMe2ArTipp2, acts as bidentate, coordinating to Pt(II) through the phosphorus atom and through an ipso carbon atom of a lateral terphenyl aromatic ring. Further platinum-silyl chemistry, this time catalytic involving hydrosilylation reactions, will be discussed in an ensuing section devoted to the chemistry of Pt(0) complexes of these terphenylphosphines. When complex 8 was treated with 2 mol-equiv of the tertiary silanes HSiR3, in benzene at room temperature (Scheme 18), the neutral hydride silyl species cis-[Pt(H)(SiR3)(PMe2ArDipp2)] (12) were obtained (SiR3 = SiEt3 12a, SiPh3 12b, SiMePh2 12c). The triethylsilyl complex cis- [Pt(H)(SiEt3)(PMe2ArTipp2)] (13), was also prepared by a similar procedure (Scheme 19). 62 For leading work and references see: I. E. Markó, S. Stérin, O. Buisine, G. Mignani, P. Branlard, B. Tinant, J. Declercq, Science 2002, 298, 204-206. 63 See for example: a) D. A. Rooke, E. M. Ferreira, Angew. Chem. Int. Ed. 2012, 51, 3225-3230; b) S. E. Parker, J. Börgel, T. Ritter, J. Am. Chem. Soc. 2014, 136, 4857-4860. Chapter I 84 The newly prepared hydride silyl complexes 12a-12c and 13 were fully characterized with the aid of conventional analytical and spectroscopic techniques. The SiEt3 derivative 12a, was additionally investigated by X-ray crystallography. Before pertinent structural data are explained and interpreted, it is worth mentioning that the formation of the hydride silyl derivatives from the cis-[PtMe2(PMe2Ar‘)] precursors took Scheme 18. Synthesis of complexes 12a, 12b and 12c. Scheme 19. Synthesis of complex 13. Platinum (II) Complexes 85 place with concurrent elimination of CH4 and MeSiR3, as convincingly indicated by 1H NMR studies of the reaction leading to complex 12a (Figure 30). One may speculate with the reaction being triggered by oxidative addition of HSiEt3 to cis-[PtMe2(PMe2ArDipp2)], followed by reductive elimination of CH4 (expected to be kinetically favoured relative to MeSiEt3 elimination) to yield an undetected mixed methyl silyl platinum(II) complex. Somewhat analogous cis-[Pt(Me)(SiPh3)(PR3)2] complexes are known to eliminate readily MeSiPh3 in solution. 64 Therefore, the unobserved methyl silyl intermediate could experience reductive elimination of MeSiEt3, followed by oxidative addition of HSiEt3 to the resulting Pt(0) complex. The last step constitutes a well-known procedure for the synthesis of Pt(II) hydride silyl complexes. For example, Duckett, Perutz and others, reported recently a series of platinum hydride silyl complexes cis-[Pt(H)(SiR2R‘)(PCy3)2] that were obtained by reaction of [Pt(PCy3)2] with the appropriate silane, HSiR2R‘. 65 64 a) F. Ozawa, T. Hikida, T. Hayashi, J. Am. Chem. Soc. 1994, 116, 2844-2849; b) F. Ozawa, J. Organomet. Chem. 2000, 611, 332-342; c) F. Ozawa, T. Hikida, K. Hasebe, T. Mori, Organometallics 1998, 17, 1018-1024; d) K. Hasebe, J. Kamite, T. Mori, H. Katayama, F. Ozawa, Organometallics 2000, 19, 2022-2030. 65 D. Chan, S. B. Duckett, S. L. Heath, I. G. Khazal, R. N. Perutz, S. Sabo-Etienne, P. L. Timmins, Organometallics 2004, 23, 5744-5756. Chapter I 86 At room temperature the 1H NMR spectra of the four hydride silyl complexes contain some broad signals that witness the non-rigidity of their structures. The solution dynamics concerns, however, only with signals due to the terphenylphosphine substituents, clearly pointing out the interchange of the flanking aryl rings. Figure 31 contains variable temperature (-60 to 25 ºC) 1H NMR data for complex 12a in the aliphatic and hydride region of the spectra (from ca. 2.7 to -2.7 ppm). With reference, for the sake of simplicity, to the methine protons of the isopropyl substituents two septets are discerned at -60 ºC, with  2.29 and 2.40 ppm, that convert into a broad unresolved signal at 0 ºC and a poorly resolved septet at 25 ºC. Using the slow-exchange approximation, a value of 102 s-1 can be estimated for the rate of the exchange at the coalescence temperature (ca. -10 ºC), which leads to a Gǂ value of 12.9 kcal·mol-1. Figure 30: Aliphatic region of 1H NMR spectra of the reaction of complex 8 with HSiEt3 at different temperatures (400 MHz, C6D6) Platinum (II) Complexes 87 The hydride resonance appears in the four complexes in the rather narrow chemical shift range from -1.7 to -1.2 ppm, and it is characterized by a 2JHP coupling to the trans 31P nucleus of ca. 170 Hz and by 1JHPt of ca. 1000 Hz. The 31P{1H} NMR spectra of compounds 12a-c shows a singlet at 23-25 ppm, with 1JPPt coupling constants comprised between 2348 and 2544 Hz. All these NMR parameters compare well with those already reported for analogous complexes.65 For instance, complex cis-[Pt(H)(SiEt3)(PCy3)2] features a hydride signal at  -3.6 ppm, with 2JHP (trans) = 146 and 1JHPt = 823 Hz. The 31P nucleus trans to the hydride exhibits 1JPPt = 2721 Hz, whereas for that trans to SiEt3, the corresponding coupling is significantly smaller, 1368 Hz. Figure 31. Variable temperature aliphatic area of 1H NMR spectra of complex 12a (CD2Cl2, 400 MHz). Crystals of complex 12a suitable for X-ray studies were obtained by cooling its solutions in diethylether at -20 ºC. Figure 32 shows an Chapter I 94 of the coordination of the new ligand, that frees the Pt···Carene interaction of 6. The one-bond 31P-195Pt coupling in 6·CO is, however, comparable to that of 6 (2970 vs. 3190 Hz) and therefore a cis geometry is proposed for this compound (Scheme 20). Despite the larger cone angle of PPh3 relative to CO (145º vs. ca 95º)15e a cis structure is also suggested in Scheme 20 for the triphenylphosphine adduct 6·PPh3. This proposition is unmistakingly confirmed by the two-bond 31P-31P coupling of 13 Hz observed in the 31P{1H} NMR spectrum of 6·PPh3 (doublets at 8.3 (PPh3) and -20.9 ppm, with 1JPPt = 3857 and 3566 Hz, respectively). It is, nonetheless, interesting to note that while in complex 6·CO the two flanking aryl rings of the terphenylphosphine substituent are equivalent and give rise to only one septet and two doublets for the methane and methyl protons of the four isopropyl substituents (Figure 35), in adduct 6·PPh3 two septets ( 3.51 and 2.22) and four doublets (between approximately 1.7 and 0.65 ppm) are recorded. A reasonable explanation for this difference is that the bulkiness of PPh3 impedes free rotation around P-Car and Car-Car bonds within the terphenylphosphine ligand. Platinum (II) Complexes 95 Figure 35: Aliphatic region of the 1H NMR spectra of adducts 6·CO (top) and 6·PPh3 (bottom) (300 MHz, CD2Cl2, 25 ºC). By analogy with previous finding on the parent dichloride complex cis-[PtCl2(PMe2ArDipp2)] (6), we expected the 1-arene interaction in compounds 8 and 10 to be labile, and consequently easily substituted by a Lewis base such as CO or another phosphine ligand. In this way we generated the corresponding adducts 8·CO and 8·PMe3, and 10·CO and 10·PPh3 respectively. As can be seen in Scheme 21, 8·CO and 8·PMe3 possess cis methyl groups. Similarly, in 10·CO and 10·PPh3 the anionic ligands Me- and Cloccupy adjacent coordination sites. Chapter I 96 Scheme 21. Synthesis of adducts 8·CO, 8·PMe3, 10·CO and 10·PPh3. The two carbonyl and the two phosphine aducts represented in Scheme 21 were characterized in solution by multinuclear 1D and 2D NMR studies. The molecular structure of 8·CO was also elucidated by single crystal X-ray diffraction studies. For the two carbonyl complexes, the Pt—CO resonance appears in the 13C{1H} NMR spectrum in the proximity of 180 ppm as a doublet, with a 2JCP value of 5 Hz, indicating that the carbonyl ligand simply replaces the Pt···Car interaction of 8 and places itself in a cis position with respect to the phosphine ligand. The 31P{1H} resonances of the two complexes are the expected singlets, with chemical shifts -13.1 (8·CO) and -18.2 ppm (10·CO), and 1JPPt coupling constants of 1603 and 3938 Hz, respectively, the latter in accordance with the proposed trans-P—Pt—Me and trans-P—Pt—Cl arrangements. Other NMR data for these compounds are collected in the Experimental Section and are also in agreement with the proposed structures. It is worth remarking that the two complexes feature strong IR bands in the carbonyl region, characterised by ῡ(CO) values of 2031 and 2092 cm-1, respectively. If one recalls that for the parent [PtCl2(CO)(PMe2ArDipp2)] complex 6·CO, Platinum (II) Complexes 97 ῡ(CO) appears at 2114 cm-1, it becomes evident that replacement of one or the two chloride ligands in 6 by electron-rich methyl groups increases the electron density at the Pt(II) centre thereby augmenting -back donation to the strong -acceptor carbon monoxide ligand. In these two compounds as well as in the phosphine adducts, the two Dipp substituents of the terphenylphosphine become equivalent and give rise to only one septet and two doublets for, respectively, the methine and methyl protons of the iso-propyl substituents. This can be seen in Figure 36 that contains the 1H NMR spectrum of complex 8·PMe3 in the aliphatic region, from ca. -0.3 to 3.1 ppm (bottom spectrum). The top representation corresponds to the 31P-decoupled proton NMR spectrum. Figure 36. Aliphatic area of 1H NMR spectrum (bottom) and 1H{31P} NMR spectrum (top) of complex 8·PMe3 (400 MHz, CD2Cl2, 25 ºC). As commented above, complex 8·CO was further characterized by X-ray diffraction analysis (Figure 37). Its molecular structure confirms the Chapter I 98 1-P coordination mode of the phosphine. The Pt1-C33 bond trans to CO has a distance of approximately 2.08 Å, that is comparable, if slightly longer, to the Pt—Me bond trans to the secondary Car interaction in complexes 8 and 10 (ca. 2.05 Å). In turn, the Pt—Me bond trans to phosphorus, with a length of 2.096(5) Å (Pt1-C34 in Figure 37) is very similar to the analogous bond in compound 8 (2.108(2) Å). Figure 37. ORTEP view of the complex 8·CO; hydrogen atoms are excluded for clarity and thermal ellipsoids are set at 50 % level of probability. Selected bond distances (Å) and angles (º): Pt1-P1 2.319(1), Pt1-C33 2.076(5), Pt1-C34 2.096(5), Pt1-C35 1.891(5), P1-Pt1-C33 87.1(1), C33-Pt1-C34 86.6 (2), C34-Pt1- C35 88.6(2), C35-Pt1-P1 97.8(1). The Allyl Platinum(II) Complex [Pt(3-C3H5)Br(PMe2ArDipp2)] Transition metal allyl complexes have been studied extensively and have a long history.15a-c, 70 A classical route to compounds of this kind involves allylation of a metal halide precursor, employing a lithium or Grignard-allyl reagent. An alternative method is based on the oxidative 70 C. Elschenbroich, Organometallics, 3rd ed., 2006. Platinum (II) Complexes 99 addition of an allyl transfer reagent, for instant an allyl halide, to suitable Pt(0) complex precursors. 71 These two procedures have been utilised in this Thesis. Here, in Chapter I, we describe the synthesis of complex [Pt(3- C3H5)Br(PMe2ArDipp2)] (14), by reaction of cis-[PtCl2(PMe2ArDipp2)] (6) with Mg(C3H5)Br, while in Chapter II the oxidative addition of CH2=CHCH2Br to [Pt(C2H4)2(PMe2ArDipp2)] will be reported. In addition, in a coming section of this chapter, formation of the cationic allyl platinum(II) complex [Pt(3-C3H5)(PMe2ArDipp2)]+ (24) will be discussed as resulting from a C—C bond forming reaction between cis- [PtMe(S)(PMe2ArDipp2)] (20·S) and C2H2, through the intermediacy of a vinylidene species (See I.2.2.4 Section, page 147) Scheme 22. Synthesis of complex 14. As depicted in Scheme 22, treatment of the dichloride complex 6 with an small excess of Mg(C3H5)Br afforded selectively and cleanly the new bromo-allyl complex 14, with monodentate P-coordinated terphenylphosphine ligand. The new compound was isolated as a yellow solid, soluble in common organic solvents like CH2Cl2, C6H6 or C4H8O, and was fully characterised by NMR spectroscopy. Figure 38 shows its 1H 71 J. Forniés, E. Lalinde, in Comprehensive Organometallic Chemistry III (Ed.: R. H. Crabtree), Elsevier, Oxford, 2007, 8.09, 611-673. Chapter I 100 NMR spectrum in the region 4.6-0.9 ppm, approximately. Consistent with the propsed non-symmetric structure, the two P-bonded methyl groups, PMe2Ar‘, are diastereotopic and yield two doublets at 1.47 and 1.35 ppm, with the same 2JHP = 10.5 Hz, and somewhat different 3JHPt couplings of 46.0 and 39.5 Hz, respectively. The Dipp2 rings of the phosphine yield two partially superimposed septets centred at 2.85 and 2.80 ppm plus four doublets (4.17, 3.91, 2.54 and 2.44 ppm). These resonances are attributed to the four iso-propyl substituents of the aromatic rings, which become equivalent two by two, probable as a consequence of fast rotation around the aryl conecting C—C bonds. The five allylic protons are non-equivalent. They originate signals at 1.34 (masked by isopropylic methyl resonances), 2.44, 2.54, 3.91, and 4.17 ppm. In the 13C{1H} NMR spectrum, the 13C allylic nuclei resonate at 108.3 (1JCPt = 47 Hz), 64.3 (2JCP = 35 Hz, 1JCPt = 53 Hz) and 44.7 (1JCPt = 259 Hz) ppm. Finally, the 31P{1H} NMR resonance is the expected singlet at -10.0 ppm, with a 31P-195Pt coupling constant of 4370 Hz. All of these NMR data are in agreement with those reported for other allyl complexes of platinum(II).54a Platinum (II) Complexes 101 Figure 38. Aliphatic area of 1H{31P} (top) and 1H (bottom) NMR of complex 14 (400 MHz, CD2Cl2, 25 ºC). I.2.2.3. Cationic Unsaturated Platinum(II) Complexes that Exhibit Bidentate (1-P, 1-arene) Phosphine Coordination Cationic Monochloride and Related Dicationic Complexes As mentioned briefly earlier, one of the objectives of this Thesis was the generation of low-coordinate, unsaturated Pt(II) complexes, taking advantage of the large steric requirements of terphenylphosphines and of the protection they provide to metals bonded to a small number of ligands. We therefore decided to perform the reactions of the neutral dichloride complexes cis-[PtCl2(PMe2Ar‘)] 6 and 7, with Ag[SbF6] or Na[BArF], as chloride abstracting reagents. The corresponding reactions were carried out in CH2Cl2 and occurred quantitatively by NMR spectroscopy, to afford the Chapter I 102 expected cationic complexes [PtCl(PMe2Ar‘)(S)]+, in which a molecule of the solvent (S=CH2Cl2) occupies the place of the leaving chloride ligand.67 The newly formed species provided, however, very broad NMR signals, undoubtedly due to dynamic processes facilitated by the lability of the coordinated molecule of CH2Cl2. Treatment of the crude reaction products with acetonitrile neatly generated the corresponding adducts, 15·NCMe and 16·NCMe, that were isolated as pale yellow solids (Scheme 23). Formally the new compounds could be envisioned as twocoordinate cationic [PtCl(PMe2Ar‘)]+ species, with one of the lateral terphenyl rings partly offsetting the coordination and electronic unsaturation of the Pt(II) ion by means of a secondary Pt···Car interaction, and a molecule of CH2Cl2 or NCMe completing the distorted tetracoordinate geometry. The analysis of the NMR data does not sustain an unequivocal structural proposal although it firmly buttresses the trans- Scheme 23: Synthesis of complexes 15·NCMe and 16·NCMe. Platinum (II) Complexes 103 P—Pt—Cl arrangement represented in Scheme 24 for the NCMe complexes 15·NCMe and 16·NCMe. In fact, the anticipated 1-P, 1-C bidentate bonding of the phosphine is strongly supported by the inequivalency of the flanking aryl rings. In the 1H NMR spectrum of 15·NCMe, they give rise to two septets and four doublets for the iPr substituents (Figure 39, bottom), whereas in that of 16·NCMe four CHMe2 septets and six CHMe2 doublets are clearly discerned (Figure 39, top spectrum). The 31P{1H} NMR spectrum of these compounds shows the expected singlet at 7.5 (15·NCMe) and 7.7 ppm (16·NCMe). These resonances are accompanied by 195Pt satellites that lead to 1JPPt values of 3335 and 3395 Hz, respectively, which are not very different from the ca. 3200 Hz value found in the parent compounds 6 and 7 (vide supra). Since in the also cationic NCMe adduct [PtMe(PMe2ArDipp2)(NCMe)]+, to be discussed in a forthcoming section, that features trans phosphine and NCMe ligands, the corresponding coupling amounts 4304 Hz, the trans-P- Pt-Cl ligand distribution shown in Scheme 23 is proposed for these compounds. In agreement with this proposal, we additionally note that the above 1JPPt values are very similar to those reported for cis-[PtCl2(PR3)2] complexes, which were found to vary between ca 3500 and 3640 Hz.54c Chapter I 110 Figure 42: Aliphatic area of variable temperature 1H NMR spectrum of compound 19 (CH2Cl2, 400 MHz). Figure 42 shows variable temperature 1H NMR spectra of complex 19 in the chemical shift range ca. 6.0-0.5 ppm. Although the spectrum is rather complicated, particularly in the region 2.0-1.0 ppm, and therefore little informative, the observation of two septets (2.26 and 2.42 ppm) for the iPr methine protons is in agreement with the foreseen bidentate coordination of the phosphine ligand. Furthermore, in the room temperature spectrum there are two rather broad resonances with  1.89 and 3.76 ppm that can be attributed to the CH2 and CH2O protons of tetrahydrofuran. These signals have relative intensities that denote a THF:PMe2Ar‘ molar ratio of ca. 2.5:3 and suggest fast exchange between bonded and free THF at this temperature. Upon cooling, the resonance at 3.76 becomes broader (See Figure 42, spectrum recorded at 0 ºC) and at - Platinum (II) Complexes 111 20 ºC splits, giving rise to an unresolved signal at  3.79 due to free THF 81 and to two broad peaks with  3.67 and 3.44 ppm, which can be tentatively attributed to the diastereotopic CH2O protons of a coordinated molecule of THF. Lowering the temperature to -40 ºC intensifies these changes (Figure 42). A similar variation can be detected for the 1.89 ppm THF resonance, although the existence in this region of the spectrum of other resonances makes the assignment less accurate. Finally, and perhaps somewhat speculatively, the broad 1H resonance discernible in the room temperature 1H NMR spectrum at ca. 4.86 ppm may be due to the presence of water. Upon cooling it converts into a broad singlet at 5.09 ppm (-20 ºC) which splits into two resonances centred at 5.07 and 4.52 ppm. A reasonable, if somewhat adventurous, explanation is that small amounts of adventitious water compete with THF for one of the two available coordination sites at the dicationic metal centre of the [Pt(PMe2ArDipp2)]2+ unit. Besides these uncertainties, the possibility of NTf2- coordination adds further complications (vide infra). To complete the discussion of the NMR data, two additional comments sould be made. At -40 ºC, the 19F NMR spectrum of compound 19 recorded in CD2Cl2 contains a sharp singlet at ca. -79 ppm and a very broad resonance that is nearly hidden under the base line, centred at about - 70 ppm. Following Bergman and Tilley80b the first may be attributed to the free, dissociated counteranion, while the second is due to platinum-bonded NTf2-, that is, to a Pt-NTf2 linkage. Thus, in addition to the intricate solution dynamics due to the lability of the coordinated molecules of H2O and THF, NTf2- coordination and dissociation adds further complexity. Secondly, in accordance with the above 1H and 19F NMR data, the 31P{1H} spectrum of complex 19, recorded at room temperature in a solution in 81 G. R. Fulmer, A. J. M. Miller, N. H. Sherden, H. E. Gottlieb, A. Nudelman, B. M. Stoltz, J. E. Bercaw, K. I. Goldberg, Organometallics 2010, 29, 2176-2179. Chapter I 112 CD2Cl2 is a broad signal centred at ca. 13.7 ppm. Cooling at 0 ºC converts this resonance into a well-defined singlet with essentially the same chemical shift (Figure 43) whereas additional cooling first at -20 ºC and then at -40 ºC, originates two singlet resonances of similar relative intensities. It is evident from all these data that at low temperatures two main species exist in CD2Cl2 solutions and it seems that one of them may contain a bound NTf2- counteranion, while the other could consist of the true [Pt(PMe2ArDipp2)(S)2]2+ dications, with non-bonded NTf2- counteranions. As for the nature of S, water, tetrahydrofuran and perhaps CD2Cl2 may compete for the vacant coordination sites in the two closely related isomeric complex salts. Figure 43: Variable temperature 31P{1H}NMR spectra of complex 19 (CD2Cl2, 161.9 MHz) The above conjectures find strong support in the crystallographic analysis of single crystals of this compound. The solid-state structure of its Platinum (II) Complexes 113 molecules was determined at -100 ºC and can be observed in Figure 44 in the form of ORTEP representation. The complex exhibits the expected distorted square-planar geometry, but the coordination polygon is made of the terphenylphosphine, which binds to the platinum dication in a  1-P, 2- arene fashion, and of two actually poor donor ligands, viz a molecule of tetrahydrofuran and a molecule of water. Surprisingly, complex 19 represents the first structurally characterized Pt(II) complex that contains coordinated molecules of H2O and THF. The Pt—O distances are 2.072(4) Å for the Pt—OH2 bond and 2.118(4) Å for the Pt—OC4H8 bond. The Pt—OC4H8 distance is significantly shorter than in trans-[Pt(H)(THF)(PiPr3)2]+ (2.224(7) Å)67 and comparable to the 2.142(4) Å value found in a related Pt(II) complex of a chelating Ph2PCH2C4H7O ligand that incorporates a molecule of tetrahydrofuran. 82 The Pt—OH2 bond is also comparatively short. For instance, in some cationic aryl complexes trans-[Pt(Ar)(H2O)(PR3)2]+, the Pt—OH2 distance was found in the 2.175-2.190 Å range, 83 while in a recently reported [Pt(PCP)(H2O)]+ complex26c it amounts 2.145(2) Å. By contrast to other PMe2Ar‘ complexes described in this Thesis, complex 19 features an 2 (rather than 1) bonding interaction with one of the lateral aryl rings of the terphenylphosphine ligand (see Figure 44 for appropriate Pt—C distances). Moreover, this interaction can be defined as strong if Pt—Car separations are used as criterium for robustness. Thus, The Pt1- C27 distance is 2.168(6) Å. Not only is this contact the shortest of the Pt···Car interactions described in this Thesis, besides, it is comparable to the olefinic Pt—C bonds in Zeise‘s salt (ca. 2.16 Å).53 As already mentioned, there is another Pt···Car distance which can be judged as 82 N. W. Alcock, A. W. G. Platt, P. G. Pringle, J. Chem. Soc., Dalton Trans. 1989, 2069- 2072. 83 a) Y. K. Kryschenko, S. R. Seidel, D. C. Muddiman, A. I. Nepomuceno, P. J. Stang, J. Am. Chem. Soc. 2003, 125, 9647-9652; b) M. Ito, M. Ebihara, T. Kawamura, Inorg. Chim. Acta 1994, 218, 199-202. Chapter I 114 bonding,52 namely Pt1-C8 at 2.480(6) Å, which as shown in Figure 44 involves one of the ortho carbon atoms of the interacting aryl ring. Figure 44: ORTEP view of the complex 19; selected hydrogen atoms and NTf2 anions are excluded for clarity and thermal ellipsoids are set to 50% level probability. Selected bond distances (Å) and angles (º): Pt1-P1 2.206(1), Pt1-C7 2.168(6), Pt1-C8 2.480(6), Pt1-O1 2.072(4), Pt1-O2 2.118(4), O1-Pt1-O2 82.0(2), O2-Pt1-C7 102.8(2), C7-Pt1-P1 83.8 (2), P1-Pt1-O1 91.2(1). Cationic Complexes with Platinum to Carbon Sigma Bonds. Methyl and Phenyl Derivatives As stated in preceding sections of this Thesis, cationic organometallic complexes are reactive species that find diverse applications in catalysis, spanning C-C bond formation, enantioselective olefin hydrogenation, olefin polymerisation and alkane C-H bond Platinum (II) Complexes 115 activation reactions.29d, 84 - 86 Low-coordinate cationic complexes of Pt(II) that contain platinum to carbon sigma bonds such as alkyls and aryls are of special importance, for they combine the existence of readily accessible coordination sites and enchanced electrophilicity of the positively charged metal, with the intrinsic reactivity of the M-C -bond.35e,35f,72a, 87 - 89 As a natural extension of the chemistry described in the foregoing sections, we considered of interest to carry out the protonation of cis- [PtMe2(PMe2ArDipp2)] (8) with [H(Et2O)2][BArF]. With this reactivity we targeted at the synthesis of the cationic Pt(II) complexes cis- [PtMe(S)(PMe2ArDipp2)]+ (20·S), expecting  2-P,C phosphine coordination and therefore the existence of labile Pt···Car and Pt···S interactions, the latter involving a molecule of a poorly coordinating solvent, like H2O, Et2O, THF, etc. The objective of this work was the study of the reactivity of such cationic species 20·S towards some small inorganic and organic molecules. In particular, we aimed as a major goal to ascertain the 84 a) S. D. Ittel, L. K. Johnson, M. Brookhart, Chem. Rev. 2000, 100, 1169-1204; b) M. Bochmann, Organometallics 2010, 29, 4711-4740; c) D. S. McGuinness, Chem. Rev. 2011, 111, 2321-2341. 85 a) J. J. Verendel, O. Pàmies, M. Diéguez, P. G. Andersson, Chem. Rev. 2014, 114, 2130-2169; b) G. S. Chen, J. A. Labinger, J. E. Bercaw, PNAS 2007, 104, 6915-6920. 86 a) R. Dorel, A. M. Echavarren, Chem. Rev. 2015, 115, 9028-9072; b) I. Braun, A. M. Asiri, A. S. K. Hashmi, ACS Catal. 2013, 3, 1902-1907; c) C. Hahn, M. Miranda, N. P. B. Chittineni, T. A. Pinion, R. Perez, Organometallics 2014, 33, 3040-3050. 87 a) R. Romeo, Comments Inorg. Chem. 2002, 23, 79-100; b) R. Romeo, G. D'Amico, E. Sicilia, N. Russo, S. Rizzato, J. Am. Chem. Soc. 2007, 129, 5744-5755; c) R. J. Puddephatt, Coord. Chem. Rev. 2001, 219–221, 157-185; d) E. M. Prokopchuk, R. J. Puddephatt, Organometallics 2003, 22, 787-796. 88 a) L. Johansson, M. Tilset, J. A. Labinger, J. E. Bercaw, J. Am. Chem. Soc. 2000, 122, 10846-10855; b) J. S. Owen, J. A. Labinger, J. E. Bercaw, J. Am. Chem. Soc. 2006, 128, 2005-2016; c) A. E. Shilov, G. B. Shul'pin, Chem. Rev. 1997, 97, 2879-2932; d) U. Fekl, K. I. Goldberg, in Adv. Inorg. Chem., Vol. Volume 54, Academic Press, 2003, pp. 259- 320, 89 a) S. H. Crosby, G. J. Clarkson, J. P. Rourke, Organometallics 2011, 30, 3603-3609; b) O. Rivada-Wheelaghan, M. Roselló-Merino, M. A. Ortuño, P. Vidossich, E. Gutiérrez- Puebla, A. Lledós, S. Conejero, Inorg. Chem. 2014, 53, 4257-4268; c) O. Rivada- Wheelaghan, M. Roselló-Merino, J. Díez, C. Maya, J. López-Serrano, S. Conejero, Organometallics 2014, 33, 5944-5947; d) J. L. Spencer, G. S. Mhinzi, J. Chem. Soc., Dalton Trans. 1995, 3819-3824. Chapter I 116 coordination of C2H2 and C2H4 to these Pt(II) cations and to unveil the ensuing C-C bond forming reactivity that might arise between the existing Pt-Me bond and the coordinated, unsaturated hydrocarbons. In general, cationic Pt(II) complexes are -acidic systems that catalyse efficiently (along with Au(I) analogues) a good many electrophilic activations of alkenes and alkynes.14,75i Carbene complexes, including the intriguing subclass of metal-vinylidenes, 90 , 91 are often invoked as active intermediates, but in most instances there is no direct evidence for their involvement, and proposed mechanistic paths rely mainly on reaction outcomes and computational data. In fact, while carbene and vinylidene complexes of W, Ru, Os, Rh, and Ir have been widely investigated,90,91 little is known about platinum analogues.11,36, 92 In accordance with these arguments, we investigated by experimental and computational methods the possible implication as reactive intermediates of the cationic carbene structures [Pt=C(H)Me]+ and [Pt=C=CH2]+, resulting from tautomerisation of C2H4 and C2H2, respectively, upon coordination to the Pt(II) centre of 20·S. Indeed, it was found that the use C2D2 provided unequivocal evidence for the participation of a platinum-vinylidene intermediate in the reaction of 20·S with acetylene. 90 a) J. Silvestre, R. Hoffmann, Helv. Chim. Acta 1985, 68, 1461-1506; b) M. I. Bruce, Chem. Rev. 1991, 91, 197-257; c) C. Bruneau, P. H. Dixneuf, Angew. Chem. Int. Ed. 2006, 45, 2176-2203. 91 a) J. M. Lynam, Chem. Eur. J. 2010, 16, 8238-8247; b) O. J. S. Pickup, I. Khazal, E. J. Smith, A. C. Whitwood, J. M. Lynam, K. Bolaky, T. C. King, B. W. Rawe, N. Fey, Organometallics 2014, 33, 1751-1761; c) M. Jiménez-Tenorio, M. C. Puerta, P. Valerga, M. A. Ortuño, G. Ujaque, A. Lledós, Inorg. Chem. 2013, 52, 8919-8932; d) H. Werner, Coord. Chem. Rev. 2004, 248, 1693-1702. 92 For early work on mononuclear Pt(II) vinylidenes see: a) U. Belluco, R. Bertani, F. Meneghetti, R. A. Michelin, M. Mozzon, J. Organomet. Chem. 1999, 583, 131-135; b) U. Belluco, R. Bertani, R. A. Michelin, M. Mozzon, J. Organomet. Chem. 2000, 600, 37-55; c) M. Rashidi, R. J. Puddephatt, J. Am. Chem. Soc. 1986, 108, 7111-7112. Platinum (II) Complexes 117 To complete this section, the protonation resctions of cis- [PtMe2PMe2ArTipp2)] (9), and cis-[PtPh2PMe2ArDipp2)] (11) were also studied. Low–temperature protonation (CD2Cl2, -80 ºC) of complex cis- [PtMe2(PMe2ArDipp2)] (8) with [H(OEt2)2][BArF] ([BArF] = [B(3,5- (CF3)2C6H3)4]) did not permit detection of a transient cationic Pt(II) -CH4 complex.51 Instead liberation of CH4 was observed with quantitative formation (by 1H and 31P{1H} NMR spectroscopy) of the cationic complex [PtMe(S)(PMe2ArDipp2)]+ (20·S). As represented in Scheme 26, protonation occurred formally at the Pt—Me bond trans to the P atom and generated a coordination vacancy that was probably occupied by a labile molecule of solvent present in the reaction medium (CH2Cl2, OEt2 or adventitious water). No unequivocal spectroscopic evidence was obtained for solvent coordination (see below) although as discussed later, the X-ray diffraction analysis of a crystalline sample of 20·S obtained following work-up of the reaction mixture revealed the presence of a molecule of adventitious water directly ligated to the metal centre. Scheme 26. Synthesis of complex 20·S. Chapter I 118 The NMR data recorded for this complex are in agreement with the structure proposed in Scheme 26. Figure 45 contains the low frequency range of its 1H NMR spectrum in the  region 3.7-0.5 ppm. Once again, the observation of two septets between 2.30 and 2.55 ppm and of four doublets, two of them accidentally with the same chemical shift, between ca. 1.0 and 1.6 ppm, attributed respectively to the methine and methyl protons of the terphenyl iso-propyl substituents, supports bidentate 2-P, 1-C coordination of the phosphine ligand. Besides a doublet resonance at 1.55 ppm assigned to the PMe2Ar‘ protons (2JHP = 10 Hz, 3JHPt = 70 Hz), the spectrum in Figure 45 contains a slightly broad unresolved signal at 0.76 ppm due to the platinum-bound methyl group along with a triplet at 1.18 and a quartet at 3.55 ppm, which might be due to a coordinated molecule of Et2O. We note, however, that the differences in these chemical shift values with those of free, uncoordinated diethylether, are almost negligible. In the 13C{1H} NMR spectrum there is a slightly broad Pt—Me singlet at -6.9 ppm with unresolved 13C-31P coupling, that shows 195Pt satellites yielding a 1JCPt value of 769 Hz. The 31P{1H} NMR spectrum recorded at 25 ºC is a broad resonance at -0.5 ppm, probably as a consequence of rapid exchange of the trans solvent molecule in the NMR time scale. This signal is characterized by a large 1JPPt value of 4882 Hz that attests the weakness of the Pt—S bond and the very small transinfluence exerted by S. Platinum (II) Complexes 119 In addition to facile exchange of the coordinated solvent molecule (vide infra), complex 20·S exhibits in solution a dynamic behaviour that involves the exchange of the flanking aryl ring of the phosphine terphenyl substituents. Figure 46 collects the variations experienced by the resonances due to the CH protons of the iso-propyl substituents when the temperature is varied between 25 and 115 ºC. The spectra were recorded in C2D2Cl4. At room temperature two septets at 1.64 and 1.75 ppm can be clearly distinguished. These resonances broaden at higher temperatures and coalesce at about 100 ºC, to merge into a broad resonance at 105 ºC and yield a discernible septet centred at 1.78 ppm. The calculated rate constant for the exchange at the coalescence temperature is 87.1 s-1, with an associated activation energy, Gǂ of 18.7 kcal·mol-1. Figure 45. Aliphatic region of 1H NMR spectrum of complex 20·S (300 MHz, CD2Cl2, 25 ºC). Chapter I 126 Figure 50. 19F NMR spectrum of complex 20·S-NTf2 at 25 ºC and -30 ºC (376.31 MHz, CD2Cl2). Scheme 29: Equilibrium in solution between cationic and neutral complexes due to triflimide coordination. Single crystals of 20 S-NTf2 were obtained when concentrated solutions of the complex in a mixture of dichloromethane and pentane were cooled at -20 ºC for three days. Resolution of the X-ray data collected at -100 ºC led to the rather intriguing observation that, once again, adventious water, in rivalry with large concentrations of CH2Cl2, dominated the contest to occupy the fourth platinum coordination site of Platinum (II) Complexes 127 cation [PtMe(PMe2ArDipp2)]+ (Figure 51). Therefore, the single crystals of 22·S-NTf2 utilised for the X-ray diffraction analysis had composition cis- [PtMe(S)(PMe2ArDipp2)][NTf2], the complex cation being chemically identical to that found in the structure of 20·S (Figure 47). Metrical parameters for the new compounds are consequently very similar (Figure 51). Figure 51. ORTEP view of the cation of complex 20·H2O-NTf2; NTf2 anion and hydrogen atoms are excluded for clarity and thermal ellipsoids are set at 50 % level probability. Selected bond distances (Å) and angles(º): P1-Pt1 2.1821(7), Pt1-C1 2.046(3), Pt1-O1 2.129(2), Pt1-C10 2.354(2), P1-Pt1-C189.29(9), C1-Pt1- O1 85.9(1), O1-Pt1-C10 101.16(8), C10-Pt1-P1 83.53(5). Other Cationic Methyl Complexes Several additional cationic complexes of the type ‗cis- [PtMe(L)(PMe2ArDipp2)]+ were also prepared either by displacement of the weakly bonded molecule of the solvent of 20·S (Scheme 30), by protonation of the neutral complexes 8·CO and 8·PMe3 (Scheme 31) or by other procedures discussed later. The substitution of the molecule of S in 20·S occurred readily at room temperature in the presence of an excess of the Lewis base (L = THF, NCMe and CO) to afford the expected adducts, as shown in Scheme 30. Chapter I 128 Referring to the acetonitrile/adduct 20·NCMe as a representative example, the 1H NMR spectrum contains sharp, well-resolved resonances in accordance with the proposed structure, which is rigid under these conditions (Figure 52). This structure implies, once more, bidentate coordination of the terphenylphosphine, and gives rise to 31P{1H} resonances in the range from approximately -4 to -15 ppm for all the complexes studied, with 1JPPt values between 3230 and 4640 Hz. Scheme 30. Synthesis of adducts 20·L. Platinum (II) Complexes 129 Protonation of the dimethyl complexes cis- [PtMe2(L)(PMe2ArDipp2)], L = CO, 8·CO and PMe3, 8·PMe3 also yielded the expected cationic adducts, 8·CO+ and 8·PMe3+, respectively (Scheme 31). Figure 52. 1H NMR spectrum of adduct 20·CNMe (500 MHz, CD2Cl2, 25 ºC). Chapter I 130 Scheme 31. Protonation of adducts 8·PMe3 and 8·CO with [H(Et2O)2][BArF]. The synthesis of 8·CO+ was of interest as a test for the -acidity of the [PtMe(PMe2ArDipp2)]+ cationic fragment. The compound was isolated as a pale yellow solid, and features an IR absorption at 2127 cm-1 due to the stretching vibration of the coordinated molecule of CO. This ῡ(CO) value is in the upper part of the region expected for [Pt(II)—CO]+ entities11,68a,27 and is close to the 2143 cm-1 wavenumber of free CO, albeit slightly smaller. For comparison, we note that for the parent complex cis- [PtMe2(CO)(PMe2ArDipp2)] (8·CO), the CO stretching appears at significantly lower wavenumber, at 2030 cm-1. Somewhat unexpectedly, low-temperature (-30 ºC) protonation of cis-[PtMe2(PMe2ArDipp2)(PMe3)] (8·PMe3) led to an almost 1:1 mixture of the monomethyl cations 8·PMe3+ (Scheme 31). Although we have not investigated the mechanism of this reaction, it seems probable that protonation leads to a three-coordinate, T-shaped intermediate [PtMe(PMe2ArDipp2)(PMe3)]+ that undergoes fast isomerisation between the two possible structures, namely with trans-P—Pt—P‘ and trans-P—Pt— Platinum (II) Complexes 131 Me linkages (see Section I.2.1.3, page 63), prior to 1 coordination of one of the terphenyl phosphine pendant aryl rings. In the 1H NMR spectrum of the mixture of cis- and trans- 8·PMe3+, two low-frequency resonances are recorded with  -1.04 and 0.69 ppm, that exhibit very different 2JHPt coupling constants of 29.5 and 82.5 Hz, respectively, as expected for methyl groups trans to phosphine and to a weak Car donor ligand. The corresponding 13C{1H} signals appear at 27.5 and -6.2 ppm, for the cis and trans isomers, respectively. 31P{1H} NMR spectroscopy of this solution mixture of cis and trans-8·PMe3+ isomers proved most informative (Figure 53). Thus two almost first-order doublets were observed for each complex, one with a reduced 2JPP constant of 13 Hz, and the other with a much larger 31P-31P coupling constant of 413 Hz, corresponding respectively to the cis and trans isomeric structures. Figure 53. 31P{1H} NMR spectrum of isomers cis- and trans-8·PMe3+ (CD2Cl2, 161.9 MHz, 25 ºC). Chapter I 132 We succeeded in separating these isomers by slow diffusion of hexane in a dichloromethane solution of the two compounds at -23 ºC, which allowed isolation of small amounts of the cis isomer as a crystalline solid. The solid state molecular structure of cis-8·PMe3+ was determined by single crystal X-ray diffraction studies (Figure 54). This analysis revealed a significant difference of nearly 0.08 Å in the lengths of the two Pt—P bonds (see Figure 54 for the metrical parameters), in agreement with the poor trans influence that exerts the 1-arene interaction, compared to the methyl ligand. Figure 54: ORTEP view of the cation of complex cis-8·PMe3+; BArF anion and hydrogen atoms are excluded for clarity and thermal ellipsoids are set at 50 % level probability. Selected bond distances (Å) and angles(º): P1-Pt1 2.3090(7), P2-Pt1 2.2307(6), C1-Pt1 2.159(3), C25-Pt1 2.353(2), P1-Pt1-P2 98.33(2), P2- Pt1-C1 81.08(6), C1-Pt1-C25 100.37(8), C25-Pt1-P1 80.31(6). To complete this discussion on cis-[PtMe(L)(PMe2ArDipp2)]+ complexes it should be noted that they could be alternatively generated by other procedures commonly used for the synthesis of such compounds (Scheme 32). Thus, treatment of dimethyl complex 8 with [CPh3][PF6] in the presence of NCMe, provided adduct 20·NCMe, isolated as the PF6- Platinum (II) Complexes 133 salt, with elimination of MeCPh3. Similarly, chloride abstraction with AgSbF6 from cis-[PtMe(Cl)(PMe2ArDipp2)] (10), using CH2Cl2 as reaction solvent, resulted one more time in the crystallization of the aqua complex cis-[PtMe(H2O)(PMe2ArDipp2)][SbF6], isolated on this occasion as the SbF6- salt. Scheme 32. Alternative synthetic methods to form unsaturated methyl cationic complexes Finally, the dimethyl platinum complex of the PMe2ArTipp2 ligand cis-[PtMe2(PMe2ArTipp2)] (9), was also protonated with [H(Et2O)2][BArF] in the presence of CO and NCMe (Scheme 33). The reactions proceeded quantitatively by 1H NMR and yielded adducts 21·CO and 21·NCMe, which were fully characterized by spectroscopy. Infrared and NMR data Chapter I 134 are similar to those recorded for the analogous PMe2ArDipp2 complexes (see Experimental Section). Scheme 33. Synthesis of cationic adducts 21·CO and 21·CNMe derived from PMe2ArTipp2. Reactivity of cis-[Pt(Me)(S)(PMe2ArDipp2]+ (20·S) towards Dihydrogen To assess further the electrophilicity of the platinum(II) centre of the title complex, we studied its reactivity toward H2.67, 93 No new complexes were detected by 1H and 31P{1H} NMR spectroscopy between - 80 and -20 ºC, and upon warming at room temperature methane liberation was observed by 1H NMR. Extensive descomposition of the Pt(II) complex 20·S took also place, as evidenced by formation of a finelly divided black solid, probably platinum metal. It seems likely that H2 replaced the weakly bonded solvent molecule, forming an undetected cationic -H2 complex, [PtMe(H2)(PMe2ArDipp2)]+. This species then underwent intramolecular heterolysis of the Pt—Me bond with elimination of methane and formed an 93 a) S. S. Stahl, J. A. Labinger, J. E. Bercaw, Inorg. Chem. 1998, 37, 2422-2431; b) K. Q. Almeida Lenero, Y. Guari, P. C. J. Kamer, P. W. N. M. van Leeuwen, B. Donnadieu, S. Sabo-Etienne, B. Chaudret, M. Lutz, A. L. Spek, Dalton Trans. 2013, 42, 6495-6512; c) D. G. Gusev, J. U. Notheis, J. R. Rambo, B. E. Hauger, O. Eisenstein, K. G. Caulton, J. Am. Chem. Soc. 1994, 116, 7409-7410. Platinum (II) Complexes 135 also undetected cationic hydride, [PtH(S)(PMe2ArDipp2)]+, which presumably decomposed under the reaction conditions. In fact, as discussed in Chapter II of this Thesis, related cationic hydrides generated by protonation of Pt(0)-PMe2ArDipp2 complexes are unstable and decompose readily under conditions similar to those above. Hydrogenolysis of metal-alkyl bonds in an important reaction for both early and late transition metal alkyls. 94 For late transition metal systems, the hydrogenolysis of a M-Me bond may occur through two possible pathways, as represented in Scheme 34 for a [PtII]—Me linkage. Route a implies oxidative addition of H2 to form a Pt(IV) methyl bis(hydride) intermediate that renders the final Pt(II) hydride product by reductive elimination of CH4. By contrast, path b involves the formation of a -H2 complex, conversion to a -CH4 complex and dissociation of methane. The latter reaction route, in which there is no higher-oxidation- state intermediate, has been termed by Perutz and Sabo-Etienne - complex-assisted metathesis (-CAM). 95 94 a) J. Campos, S. Kundu, D. R. Pahls, M. Brookhart, E. Carmona, T. R. Cundari, J. Am. Chem. Soc. 2013, 135, 1217-1220; b) J. R. Webb, C. Munro-Leighton, A. W. Pierpont, J. T. Gurkin, T. B. Gunnoe, T. R. Cundari, M. Sabat, J. L. Petersen, P. D. Boyle, Inorg. Chem. 2011, 50, 4195-4211; c) D. Stern, M. Sabat, T. J. Marks, J. Am. Chem. Soc. 1990, 112, 9558-9575. 95 R. N. Perutz, S. Sabo-Etienne, Angew. Chem. Int. Ed. 2007, 46, 2578-2592. Chapter I 238 33.4 (s, CH-Dipp), 31.9 (s, CH-Dipp), 26.5 (s, Me-Dipp), 25.4 (s, Me- Dipp), 24.4 (s, Me-Dipp), 21.8 (s, Me-Dipp), 13.3 (d, 1JCP = 45 Hz, PMe2) ppm. 31P{1H} NMR (202.4 MHz, 25 ºC, CD2Cl2): 4.6 (s, 1JPPt = 3757 Hz) ppm. Platinum (II) Complexes 239 Synthesis and Characterisation of [{PtMe(PMe2ArDipp2)}2(- OH)][BArF], 23 In a young NMR tube, a solid mixture of [PtMe2(PMe2ArDipp2)], 8, (20 mg, 0.03 mmol) and [PtMe(PMe2ArDipp2)(solvent)][BArF], 20·S, (47 mg, 0.03 mmol) was dissolved in CD2Cl2 (0.5 mL) under nitrogen at room temperature. After removing all volatiles under vacuum the compound was isolated as a pale yellow solid. HR-MS (ESI) m/z Calcd. for M+: 1353.60. Expt.: 1353.59. 1H NMR (500 MHz, 25 ºC, CD2Cl2): 7.73 (s, 8 H, o-BArF), 7.57 (s, 4 H, p-BArF), 7.53 (t, 2 H, 3JHH = 7.8 Hz, p-Dipp), 7.50 (t, 2 H, 3JHH =7.8 Hz, p- C6H3), 7.46 (t, 2 H, 3JHH =7.8 Hz, p-Dipp), 7.30 (d, 4 H, 3JHH = 7.8 Hz, m- Dipp), 7.27 (d, 2 H, 3JHH = 7.8 Hz, m-Dipp), 7.22 (dd, 2 H, 3JHH = 7.5 Hz, 4JHP = 2.3 Hz, m-C6H3), 7.05 (d, 2 H, 3JHH =7.8 Hz, m-Dipp), 6.86 (dd, 2 H, 3JHH = 7.7 Hz, 4JHP = 1.5 Hz, m-C6H3), 2.43 (hept, 2 H, 3JHH = 6.8 Hz, CH-Dipp), 2.37 (hept, 2 H,3JHH = 6.7 Hz, CH-Dipp), 2.35 (hept, 2 H, 3JHH = 6.7 Hz, CH-Dipp), 2.33 (hept, 2 H, 3JHH = 6.8 Hz, CH-Dipp), 1.72 (d, 6 H, 3JHH = 6.9 Hz, Me-Dipp), 1.34 (d, 12 H, 2JHP = 11.0 Hz, 3JHPt = 50.0 Hz, PMe2), 1.34 (d, 6 H, 3JHH = 6.8 Hz, Me-Dipp), 1.31 (d, 6 H, 3JHH = 6.7 Hz, Me-Dipp), 1.26 (d, 6 H, 3JHH = 6.8 Hz, Me-Dipp), 1.07 (d, 6 H, 3JHH = 6.8 Hz, Me-Dipp), 1.04 (d, 6 H, 3JHH = 6.7 Hz, Me-Dipp), 0.97 (d, 6 H, 3JHH = 6.6 Hz, Me-Dipp), 0.89 (d, 6 H, 3JHH = 6.9 Hz, Me-Dipp), 0.50 (d, 6 H, 3JHP = 2.7 Hz, 2JHPt = 78.0 Hz, Pt-Me), 2.35 (t br, 1 H, 2JHPt = 23.5 Hz, 3JHP = 1.5 Hz, -OH) ppm. 13C{1H} NMR (125.7 MHz, 25 ºC, CD2Cl2): 162.4 (q, Cipso-BArF), 149.3 (s, o-Dipp), 147.6 (s, o-Dipp), 147.6 (s, o-Dipp), 146.8 (s, o-C6H3), 146.6 (s, o-C6H3), 146.4 (s, o-Dipp), 144.7 (s, o-C6H3), 135.4 (s, m-BArF), 133.6 (d, 3JCP = 7 Hz, m-C6H3), 133.9 (s, p-Dipp), 132.9 (s, p-Dipp), 131.5 (s, m- Chapter I 240 C6H3), 131.4 (s, m-C6H3), 131.3 (s, p-C6H3), 131.3 (s, p-C6H3), 130.3 (s, p- Dipp), 124.5 (s, m-Dipp), 123. 7 (s, m-Dipp), 123.6 (s, m-Dipp), 123.5 (s, m-Dipp), 118.1 (hept, 4JCF = 4 Hz, p-BArF), 33.3 (s, CH-Dipp), 32.9 (s, CH-Dipp), 31.9 (s, CH-Dipp), 31.8 (s, CH-Dipp), 26.6 (s, Me-Dipp), 26.4 (s, Me-Dipp), 24.9 (s, Me-Dipp), 24.1 (s, Me-Dipp), 21.9 (s, Me-Dipp), 21.7 (s, Me-Dipp), 14.5 (d, 1JCP = 44 Hz, PMe2), 14.1 (d, 1JCP = 45 Hz, PMe2), -0.8 (t, 2JCP = 3 Hz, Pt-Me) ppm. 31P{1H} NMR (202.4 MHz, 25 ºC, CD2Cl2):  -3.2 (s, 1JPPt = 4270 Hz) ppm. Platinum (II) Complexes 241 Synthesis and Characterisation of [Pt(ƞ3- C3H5)(PMe2ArDipp2)][BArF], 24 1) The cationic methyl complex 20·S (50 mg, 0.032 mmol) was dissolved in CD2Cl2 (0.5 mL) under nitrogen at room temperature. The system was degassed (3 freeze-pump-thaw cycles) and charged with acetylene (2 bar). After 6 hours, the product formed with quantitative spectroscopic yield. After removing all volatiles under vaccum, the product was obtained as a pale yellow solid. 2) This sintethic procedure was performed in the absence of light. A solid mixture of the allyl bromide complex [Pt(PMe2ArDipp2)(3-C3H5)Br], 14, (55 mg, 0.07 mmol) and AgSbF6 (24 mg, 0.07 mmol) was dissolved in dichloromethane (c.a. 3 mL) at room temperature. After 30 min stirring, the suspension was filtered and all voltiles were removing under vacuum, obtaining the product as a pale yellow solid. (56 mg, 85 %). Anal. Calc. for C67H60BF24PPt: C, 51.6; H, 3.9. Found: C, 51.4; H, 4.2. 1H NMR (500 MHz, 25 ºC, CD2Cl2): 7.73 (s, 8 H, o-BArF), 7.65 (t, 1 H, 3JHH = 7.5 Hz, p-Dipp), 7.62 (t, 1 H, 3JHH = 7.6 Hz, p-C6H3), 7.57 (s, 4 H, p-BArF), 7.51 (t, 1 H, 3JHH = 7.8 Hz, p-Dipp), 7.37 (dd, 1 H, 3JHH = 7.6 Hz, 4JHP = 2 Hz, m-C6H3), 7.34 (d, 2 H, 3JHH = 7.4 Hz, m-Dipp), 7.33 (d, 2 H, 3JHH = 7.0 Hz, m-Dipp), 6.86 (dd, 1 H, 3JHH = 7.6 Hz, 4JHP = 2.0 Hz, m- C6H3), 5.08 (tt, 1 H, 3JHH = 6.7 Hz, 3JHH = 12.7 Hz, HE), 3.22 (m, 1 H, 2JHH = 2.2 Hz, 3JHH = 6.7 Hz, 3JHP = 3 Hz, HA), 3.14 (dd, 1 H, 3JHH = 13.4 Hz, 3JHP = 8.1 Hz, HD), 2.38 (hept, 2 H, 3JHH = 6.3 Hz, CH-Dipp), 2.31 (hept, 2 H, 3JHH = 6.3 Hz, CH-Dipp), 2.08 (d br, 1 H, 3JHH = 13 Hz, HB), 1.84 (br, 1 H, 3JHH = 7.5 Hz, 3JHP = 7.0 Hz, HC), 1.61 (d, 3 H, 2JHP = 10.9 Hz, 3JHPt = Chapter I 242 49.2 Hz, PMe), 1.48 (d, 3 H, 2JHP = 11.1 Hz, 3JHPt = 52.6 Hz, PMe), 1.32 (d, 6 H, 3JHH = 6.9 Hz, Me-Dipp), 1.29 (d, 3 H, 3JHH = 6.8 Hz, Me-Dipp), 1.28 (d, 3 H, 3JHH = 7.1 Hz, Me-Dipp), 1.09 (d, 3 H, 3JHH = 6.8 Hz, Me- Dipp), 1.04 (d, 3 H, 3JHH = 6.8 Hz, Me-Dipp), 1.01 (d, 3 H, 3JHH = 6.9 Hz, Me-Dipp), 0.95 (d, 3 H, 3JHH = 6.8 Hz, Me-Dipp) ppm. 13C{1H} NMR (125.7 MHz, 25 ºC, CD2Cl2): 162.4 (q, m-BArF), 151.8 (s), 149.4 (s), 147.6 (s), 147.5 (s), 147.0 (s), 145.6 (s), 145.3 (s), 144.6 (s), 134.4 (d, 3JCP = 6 Hz, m-C6H3), 133.3 (d, 4JCP = 3 Hz, p-C6H3), 132.4 (s, m-Dipp), 132.3 (s, m-C6H3), 130.9 (s, p-Dipp), 129.5 (q, Cipso-BArF), 128.3 (s, p-Dipp), 123.8 (s, m-Dipp), 118.1 (q, o-BArF), 115.1 (br, 1JCPt = 23 Hz, CH2-CH-CH2), 94.5 (d, 1JCP = 28 Hz, CCD), 43.8 (S, 1JCPt = 259 Hz, CAB), 34.2 (s, CH-Dipp), 33.9 (s, CH-Dipp), 31.9 (s, CH-Dipp), 31.8 (s, CH- Dipp), 26.6 (s, Me-Dipp), 26.5 (s, Me-Dipp), 25.4 (s, Me-Dipp), 24.6 (s, Me-Dipp), 24.4 (s, Me-Dipp), 24.2 (s, Me-Dipp), 21.8 (s, Me-Dipp), 21.7 (s, Me-Dipp), 17.3 (d, 1JCP = 38 Hz, PMe),16.5 (d, 1JCP = 39 Hz, PMe) ppm. 31P{1H} NMR (202.4 MHz, 25 ºC, CD2Cl2): 18.6 (s, 1JPPt = 4078 Hz) ppm. Platinum (II) Complexes 243 I.3.3. Kinetic Studies on the Cyclometallation of Complex 2 In a typical experiment, a pure sample of complex 2 was dissolved in benzene-d6, introduced into the NMR device, and heated at the desired temperature for about 2-3 hours, during which time the reaction was followed by 31P{1H} and 1H NMR. The experiment was repeated at 40, 45, 50, 55, and 60 °C. The activation parameters ΔS‡ and ΔH‡ were calculated by means of an Eyring plot (temperature dependence of the rate constant). Figure 75: Time depending conversion of 2 (blue) into 3 (red) at 55 ºC in C6D6. Molar ratio was calculated based on the integrals of both species in the 1H NMR spectra. 0 10 20 30 40 50 60 70 80 90 100 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Molar ratio Time (min) Chapter I 244 Figure 76: Eyring plot for the intramolecular cyclometallation reaction of 2. I.3.4. Reaction of complexes 2 and 3 with [H(Et2O)2][BArF] A solid mixture of the complex (2 or 3) (0.017 mmol) and [H(Et2O)2][BArF] (0.02 mmol) was placed in a Young NMR tube and dissolved in CD2Cl2 (0.5 mL) at -80 ºC. The reaction was followed by 31P and 1H NMR at different temperatures. Once at room temperature, the reaction mixture was heated in an oil bath at 50 ºC. I.3.5. Characterisation of detected intermediates in the formation of complex 24 In Young NMR tube complex 20·S (50 mg) was dissolved in CD2Cl2 (0.5 mL) under N2. The system was degassed (3 freeze-pump-thaw cycles) and bubbled with C2H2 at -80 ºC for 1 min. The mixture of both detected intermediates (A: acetylene adduct and B: vinilydene derivative) were characterized at -20 ºC in solution by NMR spectroscopy. y = -15673x + 34,848 R² = 0,994 -16 -15 -14 -13 -12 -11 -10 0,00295 0,003 0,00305 0,0031 0,00315 0,0032 0,00325 Ln(K/T) 1/T Platinum (II) Complexes 245 Intermediate A 1H NMR (400 MHz, 20 ºC, CD2Cl2):  3.08 (d, 3JHP = 4.6 Hz, 2JHPt = 50.0 Hz, 2 H, C2H2), 0.60 (d, 3JHP = 3.8 Hz, 2JHPt = 76 Hz, 3 H, Pt-CH3) ppm. 13C NMR (100.6 MHz, 20 ºC, CD2Cl2):  77.7 (dd, 1JCH = 263.0 Hz, 2JCH = 44.3 Hz, 2JCP = 11.5 Hz, C2H2), 3.2 (qd, 1JCH = 140 Hz, 2JCP = 4.7 Hz, Pt-CH3) ppm. 31P{1H} NMR (161.9 MHz, 20 ºC, CD2Cl2):  11.1 (s, 1JPPt = 4296 Hz) ppm. Chapter I 246 Intermediate B 1H NMR (400 MHz, 20 ºC, CD2Cl2):  2.21 (s, 2 H, Pt-CCH2), 0.44 (s, 2JCPt = 74 Hz, 3 H, Pt-CH3) ppm. 13C NMR (100.6 MHz, 20 ºC, CD2Cl2):  221.2 (s, -PtCCH2), 33.7 (t, -PtCCH2)), -6.6 (qd, 1JCH = 135.0 Hz, 2JCP = 5.5 Hz, Pt-CH3) ppm. 31P{1H} NMR (161.9 MHz, 20 ºC, CD2Cl2):  -4.2 (s, 1JPPt = 4615 Hz) ppm. I.3.6. Reaction of 20·S with H2 and D2 In a Young NMR tube complex 20·S (50 mg) was dissolved in CD2Cl2 (0.5 mL). The system was degassed (3 freeze-pump-thaw cycles) and charged with H2 or D2 (2 atm) at room temperature. The reaction was followed by 1H and 31P NMR spectroscopies. I.3.7. Computational Details Calculations were carried out at the DFT level with Gaussian 09 suite of programs, 118 with Truhlar‘s hybrid meta-GGA functional M06. 119 The Pt 118 M. J. Frisch, G. W. Trucks, G. E. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Platinum (II) Complexes 247 atom was represented by the Stuttgart/Dresden Effective Core Potential and the associated basis set as implemented in Gaussian 09 (SDD). 120 The remaining H, C, O and P atoms were represented by means of the 6- 31G(d,p) basis set. 121 The geometries for all species described were optimized in the gas phase without symmetry restrictions. Frequency calculations were performed on the optimized structures at the same level of theory to characterize the stationary points, as well as for the calculation of gas-phase enthalpies (H), entropies (S) and Gibbs energies (G). The nature of the intermediates connected by a transition state was determined by Intrinsic Reaction Coordinate (IRC) calculations or by perturbing the transition states along the TS coordinate and optimizing to a minimum. The solvent effects (dichloromethane) were modelled with the SMD109 continuum model by single point calculations on gas phase-optimized geometries and free optimizations in solution. The relative free energies in solution were calculated according to: Gsolution = Esolution + (Ggas-Egas), were Esolution is the electronic energy plus the solvent entropy. 122 The delocalization energies are estimated as follows Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. 119 Y. Zhao, D. Truhlar, Theor. Chim. Acta 2008, 120, 215. 120 D. Andrae, U. H., M. Dolg, H. Stoll, H. Preul, Theor. Chim. Acc. 1990, 123. 121 a) W. J. Hehre, R. Ditchfield, J. A. Pople, J. Phys. Chem. 1972, 56, 2257; b) P. C. Hariharan, J. A. Pople, Theor. Chim. Acta. 1973, 28, 213; c) M. M. Francl, W. J. Pietro, W. J. Hehre, J. S. Binkley, M. S. Gordon, D. J. Defrees, J. A. Pople, J. Chem. Phys. 1982, 77, 3654. 122 A. A. C. Braga, G. Ujaque, F. Maseras, Organometallics 2006, 25, 3647-3658. Chapter I 254 -35 ºC (238 K) -30 ºC (243 K) 0 20 40 60 80 100 120 0 0,2 0,4 0,6 0,8 1 Percentage (%) Mixing Time (s) A B 0 10 20 30 40 50 60 70 80 90 100 0 0,2 0,4 0,6 0,8 1 Percentage (%) Mixing time (s) A B Platinum (II) Complexes 255 -25 ºC (248 K) -20 ºC (253 K) Figures 83-87. Representation of the peak intensities (percentage) as a function of the exchange or reaction time. The rate constants determined as described before were used to calculate the activation parameters. The resulting representation of the Eyring equation is illustrated in Figure 88. 0 20 40 60 80 100 0 0,2 0,4 0,6 0,8 1 Percentage (%) Mixing time (s) A B 0 10 20 30 40 50 60 70 80 90 100 0 0,2 0,4 0,6 0,8 1 Percentage (%) Mixing time (s) A B Chapter I 256 Figure 88: Eyring representation of the exchange rate constants obtained from the 1D Selective EXSY experiments for acethylene exchange on 22·C2H2 complex. I.3.9. General Method for Catalytic Hydroarylation of Alkynes In a typical experiment, a Screw-cap NMR tube was charged with a mixture containing the alkyne (0.17 mmol), the indole (0.17 mmol), and 5 mol% of the catalytic mixture composed by 6 and Na[BArF] (2 eq with respect to Pt). The mixture was dissolved with CD2Cl2 (0.5 mL) and heated in an oil bathat 55 ºC for a period of 24 h. The reaction progress was monitored by 1H NMR spectroscopy. I.3.10. General Method for Catalytic Hydroamination of Alkynes In a typical experiment, a Screw-cap NMR tube was charged with a mixture containing the alkyne (0.13 mmol), the amine (0.13 mmol), 1,3,5- tris(ter-butyl)benzene (0.01 mmol, internal standard) and 0.5 mol% of 20·S. The mixture was dissolved with CD2Cl2 (0.5mL) and heated in an oil bath at 60 ºC. The reaction progress was monitored by 1H NMR spectroscopy. y = -7714,4x + 25,929 R² = 0,9989 -7 -6,5 -6 -5,5 -5 -4,5 -4 -3,5 -3 0,0039 0,00395 0,004 0,00405 0,0041 0,00415 0,0042 0,00425 Platinum (II) Complexes 257 I.4. BIBLIOGRAPHY 1. Comprehensive Coordination Chemistry, 2nd ed. (Eds.: J. A. McCleverty, T. J. Meyer), Elsevier, 2004. 2. Comprehensive Organometallic Chemistry, 3rd ed. (Ed.: R. H. Crabtree), Elsevier, 2007. 3. a) See Historical Introduction by J. Chatt in Chapter 1 of reference 3b. b) L. H. Pignolet, Homogeneous Catalysis with Metal Phosphine Complexes, Ed. 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Figure 1: Dimethylterphenylphosphines L2 and L3. The chemistry of Pt(0)—PR3 complexes is one more chapter of the coordination and organometallic chemistry of this element, that is Chapter II 274 historically relevant, for, as recalled in the general introduction that will follow this prologue, Pt—PR3 complexes were among the first transition metal compounds in which the metal centre attained the zero oxidation state (leaving aside binary carbonyls such as [Ni(CO)4] or [Fe(CO)5], vide infra). The first Pt(0)—PR3 complexes (and the Pd(0) analogues) were prepared by Malatesta and coworkers, 1 and included [Pt(EPh3)4] (E = P, As), [Pt(PPh3)3] and some mixed phosphine-carbonyl species such as [Pt(PPh3)3(CO)] and [Pt(PPh3)2(CO)2], among other complexes.1b At the time, chemists were, however, reluctant to accept the concept that metals in the zero oxidation state could form stable molecular compounds. Thus, shortly afterwards, a Nature publication by Chopoorian, Lewis and Nyholm, proposed re-formulation of Malatesta‘s Pt(0)—PR3 complexes as metal hydrides. 2 As remarked in the following section, in 1963 Malatesta and Ugo convincingly proved the real nature of their complexes. 3 The general Introduction presented in the following section of this Thesis highlights the current importance of Pt(0)—PR3 and related complexes, in particular, their capacity to act as a source of coordinative and electronically unsaturated [Pt(L)] and [PtL2] fragments, capable to activate a variety of element-element bonds in stoichiometric and catalytic reactions. The interest shown by our research group in the last few years in the synthesis and coordination chemistry of terphenylphosphines PR2Ar‘, 4 where Ar‘ represents a terphenyl substituent of exceptional protecting steric features, like for instance Dipp and Tipp in Figure 1, led us to initiate 1 a) L. Malatesta, M. Angoletta, J. Chem. Soc. 1957, 1186-1188; b) L. Malatesta, C. Cariello, J. Chem. Soc. 1958, 2323-2328. 2 J. A. Chopoorian, J. Lewis, R. S. Nyholm, Nature 1961, 190, 528-529. 3 L. Malatesta, R. Ugo, J. Chem. Soc. 1963, 2080-2082. 4 L. Ortega-Moreno, M. F. Espada, J. J. Moreno, C. Navarro-Gilabert, J. Campos, S. Conejero, J. López-Serrano, C. Maya, R. Peloso, E. Carmona, Polyhedron 2016, Accepted. Pt(0) complexes 275 the study of Pt(0) complexes stabilised by coordination to these bulky phosphines. It is worth anticipating that at variance with very bulky trialkylphosphine ligands, like PCy3 or PtBu3, that form bis(phosphine), [Pt(PR3)2] compounds (see Section II.3.1) only mono(phosphine) platinum(0) complexes of PMe2ArDipp2 and PMe2ArTipp2 could be obtained in this work. In fact, the [Pt(PMe2Ar‘)(olefin)n] (n = 1,2) compounds described in this Thesis have been found to act as a source of the formally mono-coordinate, 12-electron fragments [Pt(PMe2Ar‘)]. This chapter describes our first results in this area. It concentrated attention on Pt—PR2Ar‘—olefin complexes and in some stoichiometric and catalytic reactivity studies. Other closely related areas of research that nowadays attract considerable attention, such as the structure and reaction properties of Pt—PMe2Ar‘—L species derived from alkynes, N- heterocyclic carbenes, or bulk isocyanides, could not be considered within the time framework of this Thesis, and will hopefully be the subject of future work from our laboratories. Pt(0) complexes 277 II.2. INTRODUCTION Even after the correct structure of transition metal carbonyls such as Ni(CO)4 or Fe(CO)5 had been recognised, they were not considered as derivatives of M(0) metals because prior to 1942 molecular complexes of zerovalent transition metals were considered unlikely. In 1942, Eastes and Burgess reported the synthesis of the alkalymetal cyanonickelates MI4Ni(CN)4 and MI2Ni(CN)3 but while for the latter they suggested that nickel appeared to be monovalent, no explanation of the state of valence of nickel in the former was offered. 5 One year later, Burbage and Fernelius reported the synthesis of a zerovalent compound of palladium, K4[Pd(CN)4] by reduction of K2[Pd(CN)4] by potassium in liquid ammonia. 6 These observations and the fact that K4[Ni(CN)4] could be obtained from elemental nickel and KCN, in a reaction that occurred without a change in the oxidation state of the atoms involved, led chemists 5 J. W. Eastes, W. M. Burgess, J. Am. Chem. Soc. 1942, 64, 1187-1189. 6 J. J. Burbage, W. C. Fernelius, J. Am. Chem. Soc. 1943, 65, 1484-1486. Chapter II 278 to consider the possibility that a metal in a complex might be in the oxidation state of zero. 7 The importance of metal-to-ligand -back donation in complexes of metals in a zero oxidation state was soon recognized although, it required many years of intense research to gather unambiguous experimental results in favour of such a bond.7, 8 In fact, the most common ligands which allowed the isolation of metals in low oxidations states (see Table 1) did not include water, ammonia, halide ions and others commonly found in Werner complexes. These somewhat unusual at the time, Lewis bases were called -acceptor or -acid ligands and incorporated conspicuously phosphine and other P-donor ligands.8 Table 1: Most common ligands in compounds of metals in low oxidations (Adapted from Ref 3b) CO carbon monoxide CN- cyanide ion C2R- acetylide ion C5H5- cyclopentadienide ion RCOR alkylcarboxycarbenes RCNR2 alkyldialkylaminocarbenes C6H6 benzene C4H4 cyclobutadiene C3H5 allyl C2R2 alkynes C2R4 alkenes CnH(2n-2) dienes CnH(2n-2m) polyenes C6H4O2 para-benzoquinona N2 nitrogen 7 a) Part 1 (pp 1-67) of Ref 7b; b) L. Malatesta, S. Cenini, in Zerovalent Compounds of Metals (Eds.: P. M. Maitlis, F. G. A. Stone, R. West), Academic Press Inc., London, 1974, 8 J. G. Verkade, Coord. Chem. Rev. 1972, 9, 1-106. Pt(0) complexes 285 researchers found that complexes of type B containing the bis(allyl) ether diolefin ligand was highly active for the regioselective hydrosilylation of terminal and internal alkynes.20c Figure 3: Molecular structure of selected compounds from Ref. 20c and 23. Platinum(0) complexes are also relevant as models for the Pdcatalysed ethane hydromethoxycarbonylation reaction that converts C2H4, CO and MeOH into an acrylic ester, CH3CH2C(O)OMe, in the Lucite Process. 23 The commercialised Pd-catalyst contains the diphosphine ligand C, first reported Shaw in 1976. 24 Spencer et al. later reported the first examples of -agostic of Pt-CH2CH3 complexes by protonation of the [Pt(P^P)(C2H4)] complex of ligand C. 25 The proposed mechanism for the Pt-catalysed formation of CH3CH2C(O)OMe from C2H4, MeOH and CO is shown in Scheme 3, and includes two non-productive exocyclic equilibrium implicating cationic Pt(II) carbonyl complexes. 23 S. J. K. Forrest, P. G. Pringle, H. A. Sparkes, D. F. Wass, Dalton Trans. 2014, 43, 16335-16344. 24 C. J. Moulton, B. L. Shaw, J. Chem. Soc., Chem. Commun. 1976, 365-366. 25 a) N. Carr, B. J. Dunne, A. G. Orpen, J. L. Spencer, J. Chem. Soc., Chem. Commun. 1988, 926-928; b) L. Mole, J. L. Spencer, N. Carr, A. G. Orpen, Organometallics 1991, 10, 49-52. Chapter II 286 Scheme 3: Outline of the mechanism of Pt-catalysed hydromethoxycarbonylation of ethane including two exocyclic equilibria. Recent work from Pringle, Wass et al.23 has provided evidence for the rapid formaton and interconversion of the monoad dicarbonylplatinum(0) complexes 2 and 3 of Scheme X, that contain ligands C and D (denoted with a and b, respectively in Scheme 4) Furthermore, they demonstrated the feasibily of cationic Pt(II) carbonylhydrides of type 5, with the structural characterisation by X-ray crystallography of complex 5a of ligand C above.23 Pt(0) complexes 287 Scheme 4: Reaction scheme from Ref 23. Pt(0) complexes 289 II.3. RESULTS AND DISCUSSION II.3.1. Synthesis of Pt(0)-olefin complexes A widely used procedure for the synthesis of Pt(0)-PR3-olefin complexes consists in reductive processes that make use of readily available Pt(II) precursors. A variety of reducing agents may be used, including alkali metals, hydrazine, sodium borohydride and others.9, 26 We found that platinum(II) complexes cis-[PtCl2(PMe2Ar‘)] (Ar‘ = ArDipp2, 6; ArTipp2, 7 from Chapter I) could be reduced by Na, K, sodium amalgam, etc. under an atmosphere of C2H4 (1.5 bar) to afford the desired complexes [Pt(C2H4)2(PMe2Ar‘)], that contained two coordinated molecules of the olefin. From an experimental point of view, Zn powder proved to be the most convenient reductant (Scheme 5) and yielded complexes 1 and 2 after stirring the reaction mixture at room temperature for about 14 h, employing THF as the solvent. As an alternative procedure, compounds 1 and 2 could also be obtained from the corresponding hydride silyl derivatives, 12a and 13, respectively, by formal reductive elimination of HSiEt3 induced by C2H4 (2 bar). 26 J. Forniés, E. Lalinde, in Comprehensive Organometallic Chemistry III (Ed.: R. H. Crabtree), Elsevier, Oxford, 2007, pp. 611-673, Chapter II 290 Scheme 5. Synthesis of the bis(ethylene) Pt(0) complexes 1 and 2. Compounds 1 and 2 were isolated as air-sensitive brown solids and were characterized by microanalysis and NMR spectroscopy in solution. Additionally, the solid-state molecular structure of 1 was determined by X- ray diffraction. Before discussing their structural characterisation, some comments on their formation from the corresponding hydride-silyl precursors 12a and 13, respectively appear pertinent. Although we did not consider necessary studying in detail the mechanism of the reaction, some comments on its course are appropriate, however speculative they might appear. A reasonable reaction path could involve ethylene substitution of Pt(0) complexes 291 the highly labile Pt···Car interaction in complex 13a, which is trans to the high trans-effect –SiEt3 ligand. 27 As represented in Scheme 6a, this could be followed by migratory insertion reactivity, to afford a transient platinum(II)-ethyl-silyl-phosphine intermediate, stabilised either by a - agostic ethyl (not shown) or an 1-Car interaction. Such a sterically congested intermediate would render the observed [Pt(C2H4)2(PMe2ArDipp2)] and SiEt4 products, by reductive elimination of the silane induced by excess of ethylene. Figure 4. Aliphatic zone of 1H NMR spectrum of the reaction between complex 12a (Chapter I) and C2H4 (2 bar) on the moment of reaction (bottom, C6D6, 25 ºC, 300 MHz) and after c.a. 12 hours (top, C6D6, 25 ºC, 400 MHz). 27 a) T. G. Appleton, H. C. Clark, L. E. Manzer, Coord. Chem. Rev. 1973, 10, 335-422; b) F. R. Hartley, Chem. Soc. Rev. 1973, 2, 163-179; c) L. J. Manojlovic-Muir, K. W. Muir, Inorg. Chim. Acta 1974, 10, 47-49. Chapter II 292 1H NMR monitoring of this transformation (Figure 4, t = 0) demonstrated, however, that complex [Pt(C2H4)2(PMe2ArDipp2)] (1) and HSiEt3 formed (top of Scheme 6b) at the very early stages of the reaction (mixing of the reactants in an NMR tube and intermediate recording of the 1H NMR spectrum) and that the hydrosilane slowly converted into SiEt4, probably by C2H4 hydrosilylation catalysed by complex 1 (top spectrum in Figure 4, recorded after 12 hours). Scheme 6: Possible mechanistic routes for the conversion of complex 12a and C2H4 into complex 1 plus SiEt4 depicted in the bottom reaction of Scheme 5. See text for details. Pt(0) complexes 293 Although the bis(ethylene) complex 1 formed cleanly under the reaction conditions, that included the use of an excess of C2H4, as represented in Scheme 7, solutions of isolated complex 1 maintained under an atmosphere of dinitrogen slowly underwent C2H4 dissociation with formation of the apparently two coordinate monoethylene complex [Pt(C2H4)(PMe2ArDipp2)] (1’). In solid state, ethylene dissociation from 1 was very slow under ambient conditions and required around three months under a N2 or Ar atmosphere (inside the globe-box). At 50 ºC under high vacuum (10-4 torr) several days were needed for the conversion of 1 into 1’. Considerable decomposition took, however, place during the heating process. Convincing evidence for the dynamic equilibrium between complexes 1 and 1’ was provided by quantitative conversion of solution equilibrium mixtures of the two compounds into exclusively the bis(ethylene) adduct 1 upon treatment with excess C2H4. This permitted isolation of this complex in analytically pure form and as single crystals suitable for X-ray studies. Scheme 7: Equilibrium between mono- and bisethylene adducts, 1’ and 1 respectively. Even though two-coordinate Pt(0) complexes are known, stable compounds, contain bulky PR3, NHC or related ligands (see II.2. Introduction). Internal alkynes with bulky substituents also give rise to Chapter II 294 [Pt(alkyne)2] complexes with thermal stability depending markedly upon the size of the R-C≡C-R R groups.11d To our knowledge, complexes of the [Pt(PR3)(alkene)] are presently unknown, and accordingly the experimental evidence gained for the existence of compound 1’ that will be completed later in this section prompted us to explore the possibility of preparing a stable, formally two coordinate Pt(0)-phosphine-alkene complex, with the introduction of a sterically demanding substituent in the olefin molecular framework. Such a substituent would be expected to impede the coordination of a second molecule of the olefin, thereby forcing the desired two-ligand structure. Tert-butylethylene (tbe) was the olefin of choice and led selectively and cleanly to the targeted complex 3 and 4, with the structures suggested in Scheme 8. Similarly to the related C2H4 complexes, the new compounds could be obtained by: (i) Zn powder reduction of the PtCl2-precursors 6 and 7 or, alternatively, (ii) reductive elimination reactions of the hydridosilyls 12a and 13 promoted by the olefin. Additionally (Scheme 9), the PMe2ArDipp2 derivative 3 was also prepared from complex 1 (or from 1/1’ mixtures) by action of an excess of tbe. Pt(0) complexes 301 Similar variations were recorded in the 13C{1H} NMR spectrum (Figure 6, bottom), where the room temperature singlet at 43.2 ppm converted into two singlets at 41.1 and 44.8 ppm. Of note is the remarkable similarity between the activation energy determined for complex 1 and the values of ca. 13 kcal·mol-1 reported for both [Pt(C2H4)2(PCy3)]30a and [Pt(C2H4)2(PMe3)]. This resemblance hints that steric factors are not implicated. Room temperature NMR spectra of mixtures of the bis- and monoolefin complexes 1 and 1’ revealed the presence of the two types of coordinated alkene molecules. For example, in the 1H NMR spectrum, besides the 2.35 ppm signal of 1 there is another resonance at 2.20 ppm (2JHPt = 68 Hz) assigned to the mono-ethylene complex 1’. This signal has an associated 13C{1H} resonance at 42.5 ppm (2JCPt = 270 Hz). In the 31P{1H} NMR spectrum (Figure 7), the molecules of 1’ provide a resonance at 24.1 ppm, further characterised by a 1JPPt coupling constant of 4333 Hz. These data are strikingly similar to those recorded for the structurally characterised tbe complex 3 (23.9 ppm, 4457 Hz) and therefore support strongly the bidentate 1-P, -arene coordination proposed for 1’ in Scheme 7. Chapter II 302 Figure 7: 31P{1H} NMR spectrum of a solution mixture of 1 and 1’ (C6D6, 161.9 MHz, 25 ºC). The CH2=CHtBu complexes 3 and 4 (Scheme 8) exhibit NMR parameters which are markedly different from those recorded for the bis(ethylene) compounds 1 and 2. The dissimilarities are mostly associated with resonances due to the terphenylphosphine ligand and denote its bidentate coordination through the P and a Car donor atom. Figure 8 depicts the room temperature 1H NMR spectrum of 3, revealing the presence of two sets of signals for the flanking aryl rings of the terphenyl substituent, is in agreement with the interaction of one of them with the metal centre. Likewise, the presence of a substituent on the coordinated alkene reduces further the symmetry of this species, removing the symmetry plane, as demonstrated by the diastereotopic nature of the methyl groups directly bound to the P atom, which resonate with chemical shifts of 1.45 and 1.38 ppm for complex 3, and 1.45 and 1.50 ppm for complex 4. Nevertheless, fast rotation at room temperature along the PPt bond of the ligand, probably with temporary disengagement of the weak Pt···Car interaction, makes isochronus the isopropyl protons of the same ring (Figure 9). The Pt(0) complexes 303 13C{1H} resonances for the olefinic nuclei of complex 3 appear at 78.0 (1JCPt = 460 Hz, 2JCP = 4 Hz) and 45.6 (2JCP = 38 Hz) ppm. As this complex has a low coordination number, the 1JPPt value (4457 Hz) found in 31P{1H} NMR spectra is significantly larger than those found for complexes 1 and 2 (3265 Hz). As mentioned before, the 31P nucleus of 3 resonates at 23.9 ppm, and features a one-bond coupling constant to the 195Pt nucleus of 4457 Hz. Figure 8. Aliphatic area of 1H NMR spectrum of complex 3 (400 MHz, C6D6, 25 ºC). Chapter II 304 Figure 9: Fluxional behaviour in solution at room temperature of complexes 3 and 4. Variable temperature NMR studies shed more light on the fluxional behaviour of 3 and 4 in solution. As observed in Figure 10 for complex 3, the 1H NMR resonances due to the phosphine ligand experience relevant changes. For example, at room temperature only one broad resonance for all methine CHMe2 protons of the iso-propyl substituents is observed, whereas at -40 ºC four signals are clearly detected, with chemical shifts 1.84, 1.93, 2.41 and 2.42 ppm. The analysis of the resonances pertaining to the two Dipp phosphine substituents in complex 3 indicates a fluxional process that exchanges the two rings. An approximate G‡ value of 12.6 kcal·mol1 can be estimated for this rearrangement from the calculated rate at the coalescence temperature (ca. 0 °C). The temperature dependence of these resonances can be ascribed to a fast rotation around the PCaryl bond (Scheme 11) that switches the coordinated and the free flanking aryl substituents. Pt(0) complexes 305 Figure 10. Variable temperature 1H NMR (left: aromatic area, right: aliphatic region) of complex 3 (CD2Cl2, 400 MHz). Scheme 11. Proposed mechanism for the fast exchange of the flanking aryl rings of complex 3 (C6D6 at 25 °C). The X-ray studies carried out with complex 3 demonstrate the existence of two diastereomers, A and B, in the crystal lattice (see below). In solution, however, only one stereomer of 3 exists between -80 and 25 ºC. It seems probable that the two stereomers exist in solution at 25 ºC in a fast equilibrium on the laboratory time scale that favours strongly one of them. Unfortunately, NOESY experiments did not permit discriminating Chapter II 306 the solution structure. To close this discussion, it should be remarked that the observation for the olefinic 1H and 13C{1H} resonances of coupling to the 31P and 195Pt nuclei between -80 and 25 ºC rules out any dissociative mechanism for the observed site exchanges. The platinum(0) allyl ether complex 5 (Scheme 10) features the expected monodentate P-coordination of the PMe2ArDipp2 ligand. Unenivocal proof for this type of ligation is provided by NMR data. For instance the 31P{1H} NMR spectrum is a singlet at -18.7 ppm, with 195Pt satellites that afford a 1JPPt value of 3532 Hz. These parameters are strikingly similar to those recorded for the structurally characterised [Pt(C2H4)2(PMe2ArDipp2)] complex 1 (-16.2 ppm, 3265 Hz) and markedly different from data characteristic for [Pt(CH2=CHtBu)(PMe2ArDipp2)] (3), which contains a bidentate , 2-P,C-bound phosphine ligand (23.9 ppm, 4457 Hz). Besides, the 1H NMR spectrum of compound 5 (Figure 11), contains one septet and two doublets for the CHMe2 and CHMe2 protons of the terphenyl iso-propyl substituents, along with one doublet for the two methyl groups bonded to phosphorus. Regarding the olefinic termini, the two are chemically equivalent but the three protons of each alkene unit are non-equivalent and give rise to corresponding multiplets centred at 4.02 (2JHPt = 23.6 Hz), 2.66 and 1.81 ppm. The observed many-line pattern results from 1H-1H and 1H-31P couplings and is further complicated by platinum satellites. Naturally, it simplifies considerably upon 31P- decoupling (see Figure 12). The calculated NMR parameters are collected in the Experimental Section. Pt(0) complexes 307 Figure 11. 1H NMR spectra of complex 5 (400 MHz, CD2Cl2, 25 ºC). Figure 12: Selected areas of 1H (bottom) and 1H{31P} (top) NMR spectra of compound 5 (CD2Cl2, 25 ºC, 400MHz). Chapter II 308 The solid-state molecular structures of two of the newly reported Pt(0)-PMe2Ar‘-olefin complexes, 1 and 3, were determined by X-ray crystallography and are presented in the form of ORTEP views in Figures 13 and 14. Complex 1 is an example of a three-ligand species, i.e. PMe2ArDipp2 plus two molecules of C2H4, whereas complex 3 is a case of a two-ligand derivative, for in addition to the phosphine it accommodates only one molecule of the alkene CH2=CHtBu. Although bis(phosphine)-, bis(NHC)-, bis(alkyne)- and mixed phosphine-NHC platinum(0) complexes are known (see section II.2. Introduction), to our knowledge no [Pt(PR3)(olefin)] complexes have, so far, been reported. In complex 1 (Figure 13) the Pt(0) centre lies in a distorted trigonal planar coordination environment, the three coordination positions being occupied by two C=C double bonds and the phosphorus atom of the phosphine ligand. Notice that, in spite of the high steric demands of the PMe2Ar‘ ligand, the coordination angle between the two ethylene ligands at 132.7(1)º is significantly larger than the two P-Pt-alkene centroid angles of 113.8 (1) and 113.22(8)º, probably to minimize C2H4···C2H4 contacts that would arise from the disposition of the four carbon atoms in the coordination plane. Notwithstanding the presence of two p-acceptor olefin ligands, the coordinated C=C bonds are relatively long, 1.425(5) (C1—C2) and 1.415(4) Å (C3—C4), only slightly shorter than the C=C bond of the single olefin ligand in [Pt(C2H4)(PPh3)2] (1.434(13) Å)28 and [Pt(C2H4)(PCy3)2] (1.440(7) Å) 33 The four Pt—C bonds are nearly identical , as they cluster in the very narrow range 2.115(3)-2.121(2) Å. They are slightly longer than in the above PPh3 derivative (2.106(8) and 2.116(9) Å)28, but somewhat shorter than in the PCy3 complex analogue (2.137(1) Å).33 Comparison of the metric parameters found for 1 with 33 H. C. Clarkt, G. Ferguson, M. J. Hampden-Smith, B. Kaitner, H. Ruegger, Polyhedron 1988, 7, 1349-1353. Pt(0) complexes 309 those reported for [Pt(C2H4)(C2F4)(PCy3)]30b is also appropriate. The Pt—P bond length of 2.2866(6) Å in 1 is significantly shorter that in the mixed C2H4/C2F4 complex, viz.: 2.343(2) Å. In the latter complex the coordinated CH2=CH2 and CF2=CF2 bonds have lengths of 1.35(2) and 1.45(2) Å, respectively, these differences reflecting the higher  donor and * acceptor capacity of C2F4 relative to C2H4. Accordingly, the Pt—C(F) separations (ca. 2.03 Å) are shorter than the Pt—C(H) ones of ca. 2.17 Å.30b Back bonding to the * orbital of the C2H4 becomes reduced by the presence of the better -acceptor C2F4 and this variation is also manifested in the reduction in the barrier to C2H4 rotation by approximately 3 kcal·mol-1 relative to the bis(ethylene) analogue [Pt(C2H4)2(PCy3)] (9.8 vs. 12.8 kcal·mol-1).30a In complex 1 back donation from the Pt(0) centre is divided between two C2H4 ligands, leading therefore, as already discussed, to shorter Pt—C(H) bonds (ca. 2.12 Å), longer C=C bonds (ca. 1.42 Å) and higher rotation barriers (13.4 kcal·mol-1). Figure 13. ORTEP view of the complex 1; selected hydrogen atoms are excluded for clarity and thermal ellipsoids are set to 50% level probability. Selected bond distances(Å) and angles(º): Pt1-P1 2.2866(6), Pt1-C1 2.117(4), Pt1-C2 2.115(3), Pt1-C3 2.121(2), Pt1-C4 2.115(3), C1-C2 1.425(5), C3-C4 1.415(4), P1-Pt1- centroid(C1-C2) 113.8(1), P1-Pt1-centroid(C3-C4) 113.22(8), centroid(C1-C2)- Pt1-centroid(C3-C4) 132.7(1). Chapter II 310 The X-ray of [Pt(CH2=CHtBu)(PMe2ArDipp2)] (3) is presented in Figure 14. All Pt(0)L2 compounds listed in the Cambridge Structural Database (CSD) contain phosphine or NHC ligands. 34 Furthermore, only a handful of transition metal complexes of tert-butylethylene have been characterised by X-ray crystallography. 35 They include a Pt(II) derivative35d but no Pt(0) species. The X-ray data collected for single crystals of 3 revealed the presence of two diastereomers in the crystal lattice. Figure 14 contains data for the molecules arbitrarily designated as A and B. Molecules A approach a two-coordinate geometry with a Pt—P bond that is shorter than in the trigonal planar complex 1 (2.193(2) vs. 2.2866(6) Å) and it is in the lower end of the range of the Pt-PR3 distances for compounds listed in the CSD.34 Olefin coordination in 3 is characterised by nearly identical Pt—C bonds (ca. 2.08 Å, av. value), once more shorter than in 1 (roughly 2.12 Å) and by a C—C bond of length 1.40(1) Å, comparable to that found in other CH2=CHtBu transition metal complexes.35 However, at variance with the linearity expected for a two coordinated complex, the angle subtended by the phosphorus atom and olefin centroid at platinum is of nearly 142º. This deviation is caused by the existence of a weak Pt···Carene interaction with C27 (2.314(4) Å) that counterbalances in part the coordinative and electronic unsaturation of the Pt(0) centre. Since the next shortest platinum contact (to C32) spans 2.627(6) Å, the Pt···Carene electronic interaction can be defined as 1. 36 In molecules B, the prochiral olefin binds to platinum through the other face. Pt—C distances within the Pt—C2H3tBu linkage are nonetheless similar, 34 Cambridge Structural Database (Version 5.36). F. Allen, O. Kennard, Chemical Design Automation News 1993, 8, 31-37. 35 a) E. Molinos, S. K. Brayshaw, G. Kociok-Kohn, A. S. Weller, Dalton Trans. 2007, 4829-4844; b) S. Gatard, C. Guo, B. M. Foxman, O. V. Ozerov, Organometallics 2007, 26, 6066-6075; c) D. L. Coombs, S. Aldridge, A. Rossin, C. Jones, D. J. Willock, Organometallics 2004, 23, 2911-2926; d) U. Fekl, W. Kaminsky, K. I. Goldberg, J. Am. Chem. Soc. 2003, 125, 15286-15287. 36 A. Falceto, E. Carmona, S. Alvarez, Organometallics 2014, 33, 6660-6668. Pt(0) complexes 317 Figure 18. 1H NMR spectra of complex 8 (500 MHz, CD2Cl2, -50 ºC). On warming the reaction mixture to room temperature, the coordinated alkene molecule inserts into the Pt-H bond to form the corresponding three coordinate 14-electron alkyl cationic complexes. Interestingly, the tbe derivative converts regioselectively into the less hindered alkyl species. Both complexes show fluxional behaviour in solution and were characterized by NMR spectroscopy as the corresponding pyridine adducts, 9 and 10 (Scheme 13). The molecular structure of 10 has been determined by X-ray diffraction studies. Again, NMR studies in solution of the pyridine adducts 9 and 10 are in agreement with the presence of an interaction between the metal centre and the terphenyl fragment which partially compensate the unsaturation of these organometallic species. Although the Pt—CH2 proton resonances cannot be clearly observed because they overlap with other aliphatic signals, the methyl protons of the ethyl fragment resonates as a Chapter II 318 triplet at 0.53 ppm showing a 1H-195Pt coupling constant value of 90.1 Hz (Figure 19). In the aromatic area of the spectra, diagnostic signals of coordinated pyridine molecule are detected at 8.52, 7.86 and 7.43 ppm. The carbon atom directly bound to the platinum centre resonates at 10.6 ppm (1JCPt = 776 Hz) for 9 and at 7.0 ppm for 10 in the 13C{1H} spectra, while 13C nucleus in  position appears at 44.5 ppm (2JCPt = 37 Hz) and 15.3 ppm for 9 and 10, respectively. Figure 19. 1H NMR spectra of adduct 9 (400 MHz, CD2Cl2, 25 ºC). The solid state structure of complex 10 was confirmed by single crystal X-ray diffraction studies. The molecular structure of the cationic complex of 10 is depicted in Figure 20. The square-planar coordination environment of the metal is constituted by two neutral and one anionic ligands, which formally provide 5 electrons to the covalent electron-count. The fourth coordination site is occupied by a weak bonding interaction with the ipso-carbon atom of the lateral ring of the terphenyl group with a Pt(0) complexes 319 Pt-C distance of 2.387(6) Å. The alkyl ligand is in the trans position with respect to the secondary interaction, possibly due to the minor trans influence comparing with the P atom. Figure 20. ORTEP view of the cation of complex 10; hydrogen and BArF atoms are excluded for clarity and thermal ellipsoids are set to 50% level probability. Selected bond distances (Å) and angles (º): Pt1-P1 2.214(2), Pt1-C1 2.080(6), Pt1-N1 2.094(7), Pt1-C32 2.387(6), P1-Pt1-C1 89.1(1), C1-Pt1-N1 83.8(2), N1- Pt1-C32 105.4(2), C32-Pt1-P1 81.7(1). II.3.2.3. Oxidative addition of allylbromides Treatment of complex 1 with the allylhalides, 3-bromopropene and 3-bromo-2-methylpropene, yielded the neutral 3-allyl complexes 11 and 12, respectively, resulting from the oxidative addition of the Br—C bond to the Pt(0) centre (Scheme 14). Complex 11 was alternatively obtained by treatment of cis-[Pt(PMe2ArDipp2)Cl2] with allylmagnesiumbromide (Chapter I). Chapter II 320 Scheme 14. Synthesis of complexes 11 and 12. Both compounds were characterized by microanalysis and NMR spectroscopy. As expected, in these 16-electron complexes the phosphine acts as a 1-P ligand. The existence of only two CH isopropylic resonances in their 1H NMR spectra is in accordance with a fast rotation of the Pt-P bond or the P-arene bond in a molecule which lacks a symmetry plane, considering the coordination of the allyl ligand is asymmetric with respect to the coordination plane. As mentioned above the coordination of the allyl fragment is 3, as evidenced by 1H and 13C{1H} resonances showing coupling with 195Pt and 31P nuclei (Figure 21). For complex 11 the 13C{1H} allylic resonances appear at 108.3, 64.3 and 44.7 ppm showing coupling constants with the 195Pt nucleus from 47 Hz to 259 Hz. The 31P nucleus resonate at c.a. 10 ppm with 1JPPt values of 4370 and 4241 Hz for complexes 11 and 12, respectively. Pt(0) complexes 321 Figure 21. 1H{31P} NMR spectrum (top) and 1H NMR spectrum (bottom) of complex 11 (CD2Cl2, 400 MHz, 25 ºC). Chapter II 322 II.3.3. Catalytic Hydrosilylation of Alkynes The transition-metal catalysed hydrosilylation of unsaturated C-C bonds provides a simple and atom-economical method for the preparation of organosilicon compounds 39 . For example, the addition of hydrosilanes to alkynes yields alkenylsilanes, which are versatile reagents in organic synthesis 40 . Also, the hydrosilylation of alkynes is a useful reaction for the synthesis of silicon polymers. The major concern in this transformation is selectivity. Hydrosilylation of terminal alkynes may give a mixture of three isomeric alkenylsilanes, namely, the -(E), -(Z) and -alkenyl silanes depicted in Scheme 15. Scheme 15: Catalytic Hydrosilylation of terminal Alkynes. Classical hydrosilylation Pt catalysts such as Speier (H2PtCl6) and Karstedt (Pt2(dvds)3, dvds = [(H2C=CH)Me2Si]O) complexes provide high catalytic activity, although low-to-moderate selectivity towards the  -(E) isomer and formation of platinum colloids is commonly observed 41 . 39 a) B. Marciniec, Hydrosilylation. Advances in Silicon Science. , Springer, 2009; b) A. K. Roy, Adv. Organomet. Chem. 2008, 55, 1-59. 40 a) E. Langkopf, D. Schinzer, Chem. Rev. 1995, 95, 1375-1408; b) I. Fleming, A. Barbero, D. Walter, Chem. Rev. 1997, 97, 2063-2192; c) T. A. Blumenkopf, L. E. Overman, Chem. Rev. 1986, 86, 857-873; d) G. R. Jones, Y. Landais, Tetrahedron 1996, 52, 7599-7662; e) T. Hiyama, E. Shirakawa, Top. Curr. Chem. 2002, 219, 61-85; f) I. Fleming, J. Dunogues, R. Smithers, Org. React. 1989, 37, 57-575; g) S. E. Denmark, M. H. Ober, Aldrichimica Acta 2003, 36, 75-85; h) S. E. Denmark, R. F. Sweis, Acc. Chem. Res. 2002, 35, 835-846. 41 a) J. L. Speier, in Adv. Organomet. Chem., Vol. Volume 17 (Eds.: F. G. A. Stone, W. Robert), Academic Press, 1979, pp. 407-447, ; b) M. P. Doyle, K. G. High, C. L. Nesloney, T. W. Clayton, J. Lin, Organometallics 1991, 10, 1225-1226; c) L. N. Lewis, K. G. Sy, G. L. Bryant, P. E. Donahue, Organometallics 1991, 10, 3750-3759. Pt(0) complexes 323 Following seminal studies by Stone and Tsipis 42 , platinum catalysts based on bulky (highly basic) phosphines 43 , as well as other 2-electron neutral donors such as N-heterocyclic carbenes11d,20c, 44 - 45 , were tested in the hydrosilylation of alkynes leading to improved catalytic systems for the regioselective synthesis of E-vinylsilanes. Based on these precedents, we examined the catalytic behaviour of our platinum(0) complex 3 in the addition of hydrosilanes to terminal alkynes (Scheme 16). Scheme 16: Hydrosilylation of phenylacetylene with HSiEt3 (see Table 1 for reaction conditions). Initial catalytic tests were performed using the benchmark reaction of phenylacetylene with triethylsilane (Table 1). Scheme 16 shows that complex 3 attained high selectivity towards the  -(E) vinylsilane, accompanied by minor amounts of the  isomer, using 0.5 mol% of catalyst in different solvents (entries 1-3). Higher catalytic activities were 42 a) M. Green, J. L. Spencer, F. G. A. Stone, C. A. Tsipis, J. Chem. Soc., Dalton Trans. 1977, 1525-1529; b) C. A. Tsipis, J. Organomet. Chem. 1980, 187, 427-446. 43 a) P. J. Murphy, J. L. Spencer, G. Procter, Tetrahedron Lett. 1990, 31, 1051-1054; b) T. Kyoko, M. Tatsuya, O. Yoshio, H. Tamejiro, Tetrahedron Lett. 1993, 34, 8263-8266; c) S. E. Denmark, Z. Wang, Org. Lett. 2001, 3, 1073-1076; d) K. Itami, K. Mitsudo, A. Nishino, J.-i. Yoshida, J. Org. Chem. 2002, 67, 2645-2652; e) M. Blug, X.-F. Le Goff, N. Mézailles, P. Le Floch, Organometallics 2009, 28, 2360-2362; f) A. Hamze, O. Provot, J.- D. Brion, M. Alami, Tetrahedron Lett. 2008, 49, 2429-2431; g) W. Wu, C.-J. Li, Chem. Commun. 2003, 1668-1669; h) H. Aneetha, W. Wu, J. G. Verkade, Organometallics 2005, 24, 2590-2596. 44 a) G. De Bo, G. Berthon-Gelloz, B. Tinant, I. E. Markó, Organometallics 2006, 25, 1881-1890; b) G. F. Silbestri, J. C. Flores, E. de Jesús, Organometallics 2012, 31, 3355- 3360. 45 S. Dierick, E. Vercruysse, G. Berthon-Gelloz, I. E. Markó, Chem. Eur. J. 2015, 21, 17073-17078. Chapter II 324 observed in CD3CN and C6D6, although a better selectivity in the latter solvent was found. Interestingly, at 80 oC catalyst loading could be lowered to 0.1 mol% without erosion of the selectivity, thus allowing for a minimum TOF of 4000 h-1 (entry 4). Also, this catalytic precursor provided a higher (E)-selectivity than the more encumbered complex 4 (entry 5). However, use of lower loadings of complex 3 significantly retarded the reaction and gave poorer E-selectivity (entry 6). This effect may be ascribed to the formation in the presence of a large excess of alkyne of less selective catalytic species lacking a coordinated phosphine11d,20c. In fact, when the same reaction was carried out in the presence of 1.5 additional equivalents of the terphenyl phosphine the selectivity was reestablished (entry 7). Also, it should be noted that addition of Hg to the reaction did not affect the catalytic activity and selectivity of the catalyst, in agreement with the occurrence of a homogeneously catalyzed process 46 - 47 . 46 Formation of nanoparticles has previously been observed for the unmodified Karstedt catalyst: a) L. N. Lewis, N. Lewis, J. Am. Chem. Soc. 1986, 108, 7228-7231; b) L. N. Lewis, J. Am. Chem. Soc. 1990, 112, 5998-6004; c) L. N. Lewis, R. J. Uriarte, Organometallics 1990, 9, 621-625; d) L. N. Lewis, R. J. Uriarte, N. Lewis, J. Catal. 1991, 127, 67-74. 47 For selected examples of heterogeneous Pt catalysts: a) M. Chauhan, B. J. Hauck, L. P. Keller, P. Boudjouk, J. Organomet. Chem. 2002, 645, 1-13; b) F. Alonso, R. Buitrago, Y. Moglie, J. Ruiz-Martínez, A. Sepúlveda-Escribano, M. Yus, J. Organomet. Chem. 2011, 696, 368-372; c) R. Cano, M. Yus, D. J. Ramón, ACS Catal. 2012, 2, 1070-1078. Pt(0) complexes 325 Table 1. Catalytic hydrosilylation of phenylacetylene with Et3SiH.a Entry Cat. (mol%) Solvent T (oC) Conv. (%) % E selectivity 1 3 (0.5) CD2Cl2 50 78 (2 h) 98 2b 3 (0.1) CD3CN 50 >99 (0.5 h) 84 3 3 (0.5) C6D6 50 98 (2 h) 98 4 3 (0.1) C6D6 80 99 (0.25 h) 97 5 4 (0.1) C6D6 80 99 (1 h) 88 6 3 (0.02) C6D6 80 99 (4.5 h) 82 7c 3 (0.02) C6D6 80 42 (45 h) 98 aReactions were carried out using 1.35 mmol of phenylacetylene, 1.35 mmol of Et3SiH and 0.07 mmol of hexamethylbenzene as internal standard. [Phenylacetylene] = 2.7 M. Conversion and selectivity were determined by 1H NMR spectroscopy. Only formation of  -(E) and  isomers were observed. bConversion and selectivity were determined by GC-MS. cIn the presence of 1.5 equivalents of terphenylphosphine with respect to complex 3. Next, other hydrosilanes were studied in the hydrosilylation of phenylacetylene under the previous reaction conditions (Table 2). For example, complex 3 completed the reaction of phenylacetylene with HSiPhMe2 in less than 15 min with an E-selectivity of 91% (entry 1). Also, the reaction of a dihydrosilane, H2SiPh2, with phenylacetylene in a 1:1 ratio produced the corresponding E-monovinylsilane with a notable level of selectivity, whereas the use of two equivalents of alkyne yielded the (E,E)-divinylsilane with 80% selectivity (entries 2 and 3). Since alkenylalcoxysilanes and –siloxanes are interesting substrates for Hiyama C-C couplings,40g-h the catalytic activity of 3 in the hydrosilylation with HSi(OEt)3 and HSi(OTMS)3 was studied (entries 4 and 5). These silicon derivatives added to phenylacetylene to yield the  -(E)-isomers with 83 Chapter II 326 and 77% selectivity respectively, although a lower catalytic activity was found. Finally, the reaction of HSiCl3 with phenylacetylene was tested, providing the corresponding  -(E)-vinylsilane with 88% selectivity (entry 6). Table 2. Catalytic hydrosilylation of phenylacetylene with silanes catalysed by 3.a Entry Silane Conv. (%) % E selectivity 1 HSiPhMe2 99 (0.5 h) 91 2 H2SiPh2 99 (0.5 h) 70[b] 3 H2SiPh2 99 (1.25 h) 80[c] 4 HSi(OEt)3 99 (5 h) 83 5 HSi(OTMS)3 87 (22 h) 77 6 HSiCl3 99 (4 h) 88 a Reactions were carried out at 80 ºC in C6D6 using 1.35 mmol of phenylacetylene, 1.35 mmol of silane, 0.07 mmol of hexamethylbenzene (internal standard) and 0.1 mol% of 3. [Phenylacetylene] = 2.7 M. Conversion and selectivity were determined by 1H NMR spectroscopy. Only formation of  - (E) and  isomers were observed. b Referred to the silane:alkyne 1:1 addition product. c A silane/alkyne = 2 ratio was employed. E selectivity refers to the (E,E)-divinylsilane. As an additional assay, a series of alkynes were hydrosilylated with HSiEt3 in the presence of complex 3 (Table 3). In the case of arylacetylenes, a significant dependence of the catalytic activity with the electronic effects of the para-substituents was evidenced (entries 1 and 2). Thus, the reaction rate of the (p-bromophenyl)acetylene was significantly slower than that of the phenyl- and (p-methoxyphenyl)acetylene. On the other hand, hydrosilylation of alkyl-substituted terminal alkynes was accomplished with high activity and E-selectivity (entries 3 and 4), whereas a notable selectivity was obtained in the addition of HSiEt3 to Pt(0) complexes 333 3JHH = 6.5 Hz, Me-Dipp), 0.91 (d, 3 H, 3JHH = 7.7 Hz, Me-Dipp), 0.89 (d, 3 H, 3JHH = 7.4 Hz, Me-Dipp), 0.77 (m, 7 H, Me-Dipp and HA), 0.70 (s, 9 H, tBu), 0.28 (t, 1 H, 3JHP = 9.3 Hz, 3JHH = 9.3Hz, 2JHPt = 28 Hz, HB) ppm. 13C{1H} NMR (96.6 MHz, 40 ºC, CD2Cl2) : 147.4 (d, 2JCP = 41 Hz, o- C6H3), 147.1 (s, o-Dipp), 146.9 (s, o-Dipp), 144.8 (s, o-C6H3), 143.9 (d, 1JCP = 33 Hz, Cipso-C6H3), 139.1 (s, o-Dipp‘), 137.7 (s, Cipso-Dipp), 131.8 (s, m-C6H3), 131.4 (s, m-C6H3), 128.9 (s, p-Dipp), 128.7 (s, p-C6H3), 125.9 (s, o-Dipp‘), 124.2 (s, m-Dipp‘), 122.6 (s, m-Dipp), 122.5 (s, m-Dipp), 121.5 (s, m-Dipp‘), 121.0 (s, p-Dipp‘), 117.0 (s, Cipso-Dipp‘), 78.0 (d, 1JCPt = 460 Hz, 2JCP = 4 Hz, CH), 45.6 (d, 2JCP = 38 Hz, CAB), 34.7 (s, (Me)3C), 31.6 (s, 2JCPt = 63 Hz, (Me)3C), 33.0 (s, CH-Dipp), 32.1 (s, CH-Dipp), 31.1 (s, CH-Dipp), 31.0 (s, CH-Dipp), 26.2 (s, Me-Dipp), 26.1 (s, Me-Dipp), 24.7 (s, Me-Dipp), 24.6 (s, Me-Dipp), 23.8 (s, Me-Dipp), 22.2 (s, Me- Dipp), 21.4 (s, Me-Dipp), 21.4 (s, Me-Dipp), 18.25 (d, 1JCP = 26 Hz, PMe), 17.6 (d, 1JCP =26 Hz, PMe) ppm. 31P{1H} NMR (161.9 MHz, 25 ºC, C6D6):  23.9 (s, 1JPPt = 4457 Hz) ppm. 31P{1H} NMR (161.9 MHz, 40 ºC, CD2Cl2):  23.7 (s, 1JPPt = 4414 Hz) ppm. Chapter II 334 Synthesis and Characterisation of [Pt(CH2=CHtBu)(PMe2ArTipp2)], 4 To a toluene solution (10 mL) of complex [PtMe2(PMe2ArTipp2)] (10 from Chapter I) (0.4 g, 0.52 mmol) SiHEt3 (0.175 mL, 1.04 mmol) was added at room temperature. After one hour 3,3-dimethyl-1-butene was added (3.4 mL, 26 mmol) and the reaction mixture was stirred overnight. The volatiles were removed under high vacuum (10-4 bar) at 50 °C for several hours, yielding the compound as a dark brown solid ( 265 mg, 62 %). Anal. Calc. for C44H67PPt: C, 64.3; H, 8.2. Found: C, 63.9; H, 8.0. 1H NMR (500 MHz, 25 ºC, C6D6) : 7.27 (br, 2 H, m-Tipp‘), 7.18 (s, 2 H, m-Tipp), 6.87-7.00 (m, 3 H, C6H3), 3.86 (ddd, 1 H, 3JHH = 9.8 Hz, 3JHH = 11.7 Hz, 3JHP = 2.9 Hz, 2JHPt = 110.5 Hz, HC), 2.87 (sept, 2H, p-CH-Tipp), 2.47 (br, 4 H, o-CH-Tipp), 1.50 (d, 3 H, 2JHP = 7.0 Hz, PMe), 1.44 (d, 3 H, 2JHP = 7.6 Hz, PMe), 1.39 (br, 6 H, o-Me-Tipp), 1.34 (d, 12 H, 3JHH = 6.8 Hz, p-Me-Tipp), 1.31 (d, 6 H, 3JHH = 6.9 Hz, o-Me-Tipp), 1.25 (m, 1 H, HB), 1.12 (s, 9 H, tBu), 1.01 (d, 6 H, 3JHH = 6.6 Hz, o-Me-Tipp), 0.97 ( d, 6 H, 3JHH = 6.6 Hz, o-Me-Tipp), 0.75 (t, 1 H, 3JHH = 9.4 Hz, HA) ppm. 13C{1H} NMR (125.72 MHz, 25 ºC, C6D6) : 147.0 (br), 146.9 (s), 145.3 (d, 1JCP = 32 Hz, Cipso-C6H3), 132.3 (s, m-C6H3), 132.3 (s, m-C6H3), 128.7 (d, 4JCP = 2 Hz, p-C6H3), 121.3 (br, m-Tipp), 120.7 (s, m-Tipp), 78.5 (d, 2JCP = 5 Hz, 1JCPt = 468 Hz, CHC), 44.2 (d. 2JCP = 39 Hz, CHAHB), 35.1 (d.3JCP = 2 Hz, C(Me)3), 34.7 (s, p-CH-Tipp), 32.4 (s, C(CH3)3), 32.1 (br, o-CH-Tipp), 25.5 (br, o-Me-Tipp and p-Me-Tipp), 22.6 (br, o-Me-Tipp), 18.9 (d, 1JCP = 26 Hz, PMe), 18.2 (d, 1JCP = 25 Hz, PMe) ppm. 31P{1H} NMR (202.4 MHz, 25 ºC, C6D6): 24.0 (s, 1JPPt = 4419 Hz) ppm. Pt(0) complexes 335 Synthesis and Characterisation of [Pt(PMe2ArDipp2)(CH2=CH-CH2)2O], 5 [Pt(C2H4)2(PMe2ArDipp2)], 1, (20 mg, 0.03 mmol) was dissolved in THF (2 mL). Protected from light, allyl ether (5 μL, 1.5 eq) was added at room temperature and the reaction mixture was stirred overnight. All volatiles were removed under vacuum. The complex was washed with pentane (3 x 1 mL) at 80ºC obtaining the complex as a dark brown solid. Yield: 17 mg (78%). Anal. Calc. for C38H53OPPt: C, 61.3; H, 7.0. Found: C, 61.2; H, 7.2. 1H NMR (400 MHz, 25 ºC, CD2Cl2):  7.35 (td, 1 H, 3JHH = 7.6 Hz, 5JHP = 1.5 Hz, p-C6H3), 7.29 (t, 2 H, 3JHH = 7.7, p-Dipp), 7.12 (d, 4 H, 3JHH = 7.8 Hz, m-Dipp), 7.09 (dd, 2 H, 3JHH = 7.7 Hz, 4JHP = 2.6 Hz, m-C6H3), 4.02 (ddd, 2 H, 4JHH = 12.3 Hz, 3JHH= 3.6 Hz, 3JHP = 6.5 Hz, 2JHPt = 23.6 Hz, Ha), 2.74 (sept, 4 H, 3JHH = 6.8 Hz, CH-Dipp), 2.66 (m, 2 H, 3JHH = 3.4 Hz, 3JHH = 8.0 Hz, 2JHPt = 22Hz, Hc), 1.81 (m, 2 H, 3JHH = 11.5 Hz, 2JHPt = 11.6 Hz, Hb), 1.60 (ddd, 2 H, 3JHH = 8.2 Hz, 4JHH = 1 Hz, 4JHP = 6 Hz, 3JHPt = 54.8 Hz, CH2-O), 1.25 (d, 12 H, 3JHH = 6.9 Hz, Me-Dipp), 1.23 (d, 6 H, 2JHP = 7.9 Hz, 3JHPt = 17.4 Hz, PMe2),1.21 (dd, 2 H, 3JHH = 11.4 Hz, 4JHP = 7.8 Hz, 3JHPt = 36.6 Hz, CH2-O), 0.97 (d, 12 H, 3JHH = 6.7 Hz, Me-Dipp) ppm. 13C{1H} NMR (96.6 MHz, 25 ºC, CD2Cl2): 147.3 (s, o-Dipp), 143.4 (d, 2JCP = 9 Hz, o-C6H3), 141.2 (s, Cipso-Dipp), 133.8 (d, 3JCP = 6 Hz, m-C6H3), 128.6 (s, p-Dipp), 127.2 (s, p-C6H3), 123.1 (s, m-Dipp), 70.0 (d, 2JPC = 15 Hz, =CH), 56.4 (d, 2JPC = 5 Hz, CH2=), 37.0 (s, CH2-O), 31.5 (s, CH- Dipp), 25.9 (s, Me-Dipp), 23.0 (s, Me-Dipp), 21.3 (d, 1JCP = 31 Hz, 2JCPt = 42 Hz, PMe2) ppm. 31P{1H} NMR (161.9 MHz, 25 ºC, CD2Cl2):  18.7 (s, 1JPPt = 3532 Hz) ppm. Chapter II 336 Synthesis and Characterisation of [Pt(PMe2ArDipp2)(-CO)]3, 6 [Pt(C2H4)2(PMe2ArDipp2)], 1, (0.1 g, 0.14 mmol) was dissolved in C6H6 (5 mL). CO was bubbled through the solution for 5 minutes at room temperature causing the product to precipitate as an orange solid, which was filtered and washed with hexane. Yield: 68 mg (71%). Anal. Calc. for C99H129O3P3Pt3: C, 58.1; H, 6.4. Found: C, 58.4; H, 6.8. IR (Nujol): ῡCOcm 1H NMR (500 MHz, 10 ºC, CD2Cl2): 7.28 (t, 3 H, 3JHH = 7.4 Hz, p- C6H3), 7.26 (t, 6 H, 3JHH = 7.8 Hz, p-Dipp), 7.08 (d, 6 H, 3JHH = 6.8 Hz, m- C6H3), 7.07 (d, 6 H, 3JHH = 6.8 Hz, m-Dipp), 7.01 (d, 6 H, 3JHH = 7.8 Hz, m-Dipp), 2.93 (sept, 6 H, 3JHH = 6.6 Hz, CH-Dipp), 2.46 (sept, 6 H, 3JHH = 6.7 Hz, CH-Dipp), 1.21 (d, 18 H, 3JHH = 6.5 Hz, Me-Dipp), 0.94 (br, 18 H, PMe2), 0.81 (d, 18 H, 3JHH = 6.5Hz, Me-Dipp), 0.77 (d, 18 H, 3JHH = 6.5 Hz, Me-Dipp), 0.31 (d, 18 H, 3JHH = 6.5 Hz, Me-Dipp) ppm. 13C{1H} NMR (125.72 MHz, 10 ºC, CD2Cl2): 182.6 (s, CO), 147.6 (s, Cipso-Dipp), 146.7 (s, Cipso-Dipp), 142.8 (m, o-C6H3), 140.3 (s, o-Dipp), 135.5 (m, Cipso-C6H3), 131.8 (s, m-C6H3), 128.3 (p-Dipp), 126.9 (p-C6H3), 123.2 (s, m-Dipp), 122.2 (s, m-Dipp), 30.8 (s, CH-Dipp), 30.0 (s, CH- Dipp), 25.6 (s, Me-Dipp), 25.3 (s, Me-Dipp), 24.3 (s, Me-Dipp), 22.0 (s, Me-Dipp), 18.2 (m, PMe2) ppm. 31P{1H} NMR (121.44 MHz, 25 ºC, CD2Cl2): 41.2 (s, 2JPPt = 504 Hz, 3JPP = 72 Hz, 1JPPt = 4440 Hz) ppm. Pt(0) complexes 337 Synthesis and Characterisation of [PtH(C2H4)(PMe2ArDipp2)][BArF], 7 In a Young NMR tube complex [Pt(C2H4)2(PMe2ArDipp2)], 1 (50 mg, 0.07 mmol) and [H(Et2O)2][BArF] (74.2 mg, 0.07 mmol) were dissolved in CD2Cl2 (0.7 mL) at 80 °C. The reaction was monitored by 1H and 31P{1H} NMR spectroscopy and the product was characterized at 40 °C with a quantitative spectroscopic yield. 1H NMR (500 MHz, 50 ºC, CD2Cl2):-19.18 ( s, 1 H, 1JHPt = 1668.9 Hz, Pt-H), 0.99 (d, 6 H, 3JHH = 6.7 Hz, Me-Dipp), 1.01 (d, 6 H, 3JHH = 6.7 Hz, Me-Dipp), 1.28 (d, 6 H, 3JHH = 6.7 Hz, Me-Dipp), 1.31 (d, 6 H, 3JHH = 6.7 Hz, Me-Dipp), 1.63 (d, 6 H, 2JHP = 11.5 Hz, 3JHPt = 72.4 Hz, PMe2), 2.26 (sept, 2 H, 3JHH = 6.7 Hz, CH-Dipp), 2.33 (sept, 2 H, 3JHH = 6.8 Hz, CH- Dipp), 3.95 (s br, 4 H, 2JHPt = 35.0 Hz, C2H4), 6.92 (m, 1 H, Har), 7.15-7.55 (m, 8 H, Har) ppm. 13C{1H} NMR (96.6 MHz, -40 ºC, CD2Cl2):147.4 (s), 146.9 (d, JCP = 21 Hz), 133.5 (s), 133.1 (s), 132.2 (s), 131.7 (d, JCP = 16 Hz), 130.3 (s), 125.9 (s), 125.6 (s), 123.3 (s), 120.5 (s), 96.4 (d, 2JCP = 10 Hz, C2H4), 32.9 (s, CH-Dipp), 31.4 (s, CH-Dipp), 26.1 (s, Me-Dipp), 24.0 (s, Me-Dipp), 23.8 (s, Me-Dipp), 21.2 (s, Me-Dipp), 18.0 (d, 1JCP = 43.2 Hz, PMe2) ppm. 31P{1H} NMR (161.9 MHz, -40 ºC, CD2Cl2):  11.0 (s, 1JPPt = 3679 Hz) ppm. Chapter II 338 Synthesis and Characterisation of [PtH(CH2=CHBut)(PMe2ArDipp2)][BArF], 8 In a Young NMR tube complex [Pt(CH2=CHtBu)(PMe2ArDipp2)] (30 mg, 0.04 mmol) and [H(Et2O)2][BArF] (41.2 mg, 0.04 mmol) were dissolved in CD2Cl2 (0.7 mL) at 80 °C. The reaction was monitored by 1H and 31P{1H} NMR spectroscopy and the product was characterized at 50 °C with a quantitative spectroscopic yield. 1H NMR (500 MHz, 50 ºC, CD2Cl2): 7.76 (s, 8 H, o- BArF), 7.60-7.66 (m, 2 H, Har), 7.57 (s, 4 H, p-BArF), 7.52 (d, 1 H, 3JHH = 7.6 Hz, Har), 7.49 (t, 1 H, 3JHH = 6.7 Hz, Har), 7.43 (d, 1 H, 3JHH = 7.8 Hz, Har), 7.31-7.38 (m, 2 H, Har), 7.29 (d, 1 H, 3JHH = 7.2 Hz, Har), 6.87 (d, 1 H, 3JHH = 7.0 Hz, Har), 4.91 (m, 1 H, CH-alkene), 3.66 (d, 1 H, 3JHH = 15.9 Hz, CH2), 2.37 (sept, 1 H, 3JHH = 6.5 Hz, CH-Dipp), 2.35 (sept, 1 H, 3JHH = 6.5 Hz, CH- Dipp), 2.22 (sept, 1 H, 3JHH = 6.5 Hz, CH-Dipp), 2.19 (sept, 1 H, 3JHH = 6.5 Hz, CH-Dipp), 2.04 (dd, 1 H, 3JHH = 8.8 Hz, 3JHP = 4.3 Hz, CH2), 1.63 (d, 3 H, 2JHP = 11.5 Hz, PMe), 1.54 (d, 3 H, 2JHP = 11.5 Hz, PMe), 1.36 (d, 3 H, 3JHH = 6.9 Hz, Me-Dipp), 1.27 (d, 3 H, 3JHH = 6.6 Hz, Me-Dipp), 1.25 (d, 3 H, 3JHH = 6.9 Hz, Me-Dipp), 1.21 (d, 3 H, 3JHH = 6.9 Hz, Me-Dipp), 1.10 (d, 3 H, 3JHH = 6.6 Hz, Me-Dipp), 1.03 (d, 3 H, 3JHH = 6.5 Hz, Me- Dipp), 0.95 (s, 9 H, But), 0.79 (d, 3 H, 3JHH = 7.0 Hz, Me-Dipp), 0.93 (d, 3 H, 3JHH = 6.4 Hz, Me-Dipp), 19.35 (d, 1 H, 2JHP = 6.6 Hz, 1JHPt = 1657 Hz, Pt-H) ppm. 13C{1H} NMR (125.7 MHz, 50ºC, CD2Cl2) : 161.8 (q, Cipso-BArF), 134.7 (s, o-BArF), 128.7 (q, CF3- BArF), 117.6 (s, p- BArF), 89.9 (d, 2JCP = 12 Hz, CH2), 33.1 (s, CH-Dipp), 36.9 (s, CMe3), 32.8 (s, CH-Dipp), 31.4 (s, CH-Dipp), 31.3 (s, CH-Dipp), 28.7 (s, CMe3), 26.3 (s, Me-Dipp), 25.9 (s, Me-Dipp), 25.5 (s, Me-Dipp), 24.6 (s, Me-Dipp), 23.5 (s, Me-Dipp), 22.1 (s, Me-Dipp), 21.2 (s, Me-Dipp), 21.1 (s, Me-Dipp), 18.5 (d, 1JCP = 44 Hz, PMe), 17.6 (d, 1JCP = 42 Hz, PMe) ppm. Pt(0) complexes 339 31P{1H} NMR (202.4 MHz, 50ºC, CD2Cl2):9.5 (s, 1JPPt = 3595 Hz) ppm. Chapter II 340 Synthesis and Characterisation of [PtEt(Py)(PMe2ArDipp2)][BArF], 9 A solution of [H(Et2O)2][BArF] (0.214 mg, 0.21 mmol) in dichloromethane (ca. 5 mL) at 80°C was added to a solution of [Pt(C2H4)2(PMe2ArDipp2)], 1, (150 mg, 0.21 mmol) in the same solvent (ca. 5 mL). The reaction mixture was allowed to warm slowly to the room temperature while stirring overnight. Pyridine (0.1 mL) was added to the reaction mixture and all volatiles were removed under vacuum, obtaining the product as a yellow solid. For further purification, this complex can be crystallized by slow diffusion of pentane into a CH2Cl2 solution at 20 ºC (1:1 by vol). Yield: 245 mg (72%). Anal. Calc. for C39H53NPPt: C, 61.5; H, 7.0. Found: C, 61.3; H, 7.2. 1H NMR (500 MHz, 25 ºC, CD2Cl2): 8.52 (br, 1 H, p-Py), 7.86 (br, 2 H, o-Py), 7.74 (s, 8 H, o-BArF), 7.57 (s, 4 H, p-BArF), 7.55 (t, 1 H, p-C6H3), 7.52 (t, 1 H, 3JHH = 7.8 Hz, p-Dipp), 7.43 (t, 2 H, 3JHH = 6.9 Hz, m-Py), 7.35 (d, 2 H, 3JHH = 8.0 Hz, m-Dipp), 7.32 (dd, 1 H, 3JHH = 7.6 Hz, 4JHP = 2.7 Hz, m-C6H3), 7.02 (d, 2 H, 3JHH = 7.9 Hz, m-Dipp‘), 6.81 (dd, 1 H, 3JHH = 7.6 Hz, 4JHP = 2.7 Hz, m-C6H3), 6.52 (t, 1 H, 3JHH = 7.8 Hz, p-Dipp‘), 2.48 (sept, 1 H, 3JHH = 6.7 Hz, CH-Dipp), 2.43 (sept, 1 H, 3JHH = 6.7 Hz, CH-Dipp), 1.69 (d, 6 H, 3JHH = 7.1 Hz, Me-Dipp), 1.47 (d, 6 H, 2JHP = 11.0 H, 3JHPt = 54.6 Hz, PMe2), 1.35 (d, 6 H, 3JHH = 6.8 Hz, Me-Dipp), 1.29 (m, 2 H, CH3-CH2-Pt), 1.05 (d, 6 H, 3JHH = 6.7 Hz, CH3-Dipp), 1.01 (d, 6 H, 3JHH = 7.0 Hz, CH3-Dipp), 0.53 (t, 3 H, 3JHH = 7.4 Hz, 3JHPt = 90.1 Hz, CH3-CH2-Pt) ppm. 13C{1H} NMR (125.7 MHz, 25 ºC, CD2Cl2): 161.7 (q, 1JCB = 50 Hz, Cipso-BArF), 148.4 (s, o-Dipp‘), 147.0 (s, o-Dipp), 145.4 (s, o-C6H3), 145.2 (s, o-C6H3), 144.7 (s, Cipso-C6H3), 135.7 (d, 3JCP = 6 Hz, Cipso-Dipp), 134.8 (s, o-BArF), 133.4 (d, 3JCP = 7 Hz, m-C6H3), 132.4 (s, p-Dipp‘), 131.4 (d, 4JCP = 3 Hz, p-C6H3), 131.0 (d, 3JCP = 13 Hz, m-C6H3), 130.0 (s, p-Dipp), Pt(0) complexes 341 128.9 (qq, CF3-BArF), 125.9 (s, m-Py), 124.5 (s, m-Dipp‘), 123.2 (s, Cipso- Dipp‘), 123.2 (s, m-Dipp), 120.5 (s, m-BArF), 117.4 (sept, 3JHF = 4 Hz, p- BArF), 32.7 (s, CH-Dipp), 31.3 (s, CH-Dipp), 25.9 (s, Me-Dipp), 24.1 (s, Me-Dipp), 23.9 (s, Me-Dipp), 21.2 (s, Me-Dipp), 15.3 (s, Pt-CH2-CH3), 12.3 (d, 1JCP = 43 Hz, PMe2), 7.0 (d, 2JCP = 4 Hz, Pt-CH2-CH3) ppm. 31P{1H} NMR (202.4 MHz, 25 ºC, CD2Cl2): 1.0 (s, 1JPPt = 4005 Hz) ppm. Chapter II 342 Synthesis and Characterisation of [Pt(CH2-CH2-But)Py(PMe2ArDipp2)][BArF], 10 In a Young NMR tube complex [Pt(CH2=CHBut)(PMe2ArDipp2)] (50 mg, 0.068 mmol) and [H(Et2O)2][BArF] (75.5 mg, 0.074 mmol) were dissolved in CH2Cl2 (3 mL) at 80 °C. The reaction mixture was allowed to warm up slowly to the room temperature while stirring overnight. Pyridine (0.1 mL) was added to the resulting solution and all volatiles were removed under vacuum obtaining the product as a brown solid. Yield: 79 mg (70 %). Anal. Calc. for C75H73BF24NPPt·CH2Cl2: C, 51.68; H, 4.28. Found: C, 51.2; H, 4.4. 1H NMR (400 MHz, 25 ºC, CD2Cl2):8.56 (s br, 1 H, p-Py), 7.99 (t br, 1 H, 3JHH = 7.3 Hz, p-C6H3), 7.88 (m, 2 H, o-Py), 7.77 (s, 8 H, o-BArF), 7.59 (s, 4 H, p-BArF), 7.56 (dd, 1 H, 3JHH = 7.5 Hz, 4JHP = 1.8 Hz, m-C6H3), 7.51 (t, 1 H, 3JHH = 7.8 Hz, p-Dipp‘), 7.41 (t, 2 H, 3JHH = 6.8 Hz, m-Py), 7.35 (d, 2 H, 3JHH = 7.8 Hz, m-Dipp‘), 7.04(d, 2 H, 3JHH = 7.8 Hz, m-Dipp), 6.83 (dd, 1 H, 3JHH = 7.7 Hz, 4JHP = 2.8 Hz, m-C6H3), 6.54 (t, 1 H, 3JHH = 7.8 Hz, p-Dipp), 2.49 (d, 2 H, 3JHH = 7.0 Hz, CH-Dipp), 2.45 (d, 2 H, 3JHH = 7.0 Hz, CH-Dipp), 1.69 (d, 6 H, 3JHH = 7.0 Hz, Me-Dipp), 1.50 (d, 6 H, 2JHP = 10.9 Hz, 3JHPt = 52.2 Hz, PMe2), 1.36 (d, 6 H, 3JHH = 6.8 Hz, Me- Dipp), 1.32 (m, 2 H, Pt-CH2), 1.06 (d, 6 H, 3JHH = 6.5 Hz, Me-Dipp), 1.01 (d, 6 H, 3JHH = 6.7 Hz, Me-Dipp), 0.86 (m, 2 H, CH2-tBu), 0.50 (s, 9 H, tBu) ppm. 13C{1H} NMR (96.6 MHz, 25 ºC, CD2Cl2):10.6 (d, 2JCP = 4 Hz, 1JCPt = 776 Hz, Pt-CH2), 13.0 (d, 1JCP = 43 Hz, 2JCPt = 52 Hz, PMe2), 21.8 (s, Me- Dipp), 24.5 (s, Me-Dipp), 24.7 (s, Me-Dipp), 26.6 (s, Me-Dipp), 28.8 (s, CMe3), 31.6 (s, CMe3), 32.0 (s, CH-Dipp), 33.8 (s, CH-Dipp), 44.5 (s, 2JCPt = 37 Hz, CH2-tBu), 123.8 (s, m-Dipp), 125.0 (s, o-C6H3), 125.1 (s, m- Pt(0) complexes 349 II.5. 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Organomet. Chem. 2011, 696, 368-372; c) R. Cano, M. Yus, D. J. Ramón, ACS Catal. 2012, 2, 1070-1078. 48. For other examples of electronic effects on selectivity: D. A. Rooke, E. M. Ferreira, Angew. Chem. Int. Ed. 2012, 51, 3225-3230. Conclusiones 355 CONCLUSIONES 1. Las características estructurales dilucidadas en este trabajo para los complejos de Pt(II) y Pt(0) que se han investigado y los estudios de reactividad química con ellos desarrollados, permiten concluir que los ligandos fosfina de tipo PMe2Ar’, donde Ar’ simboliza un radical terfenilo voluminoso, tienen excelentes propiedades de coordinación, fruto de su elevada capacidad para donar densidad electrónica, y permiten además estabilizar estructuras de alta insaturación, tanto electrónica como de coordinación, gracias de una parte a su gran tamaño, y de otra a su capacidad para establecer interacciones electrónicas secundarias, relativamente débiles, mediante el sistema aromático de uno de los anillos laterales del grupo terfenilo. 2. Las fosfinas de terfenilo más voluminosas utilizadas (PMe2ArDipp2, L2 y PMe2ArTipp2, L3) son capaces de estabilizar por sí solas complejos tricoordinados de Pt(II), [PtX2(L)] o [Pt(X)(Y)L] donde X e Y representan ligandos monoaniónicos como CH3-, Ph-, H-, SiR3-, y Cl-, gracias a una interacción secundaria Pt···Car cuya fuerza depende de la influencia-trans del ligando X o Y que ocupa la posición de coordinación opuesta, como se infiere de modo inequívoco de los estudios estructurales y de reactividad realizados. Uno, o los dos ligandos aniónicos, X e Y, de estos complejos se pueden eliminar de estas estructuras, originando especies mono y dicatiónicas, [Pt(X)(L)]+ y [Pt(L)]2+, respectivamente, que presentan una elevada electrofilia, como consecuencia de su carácter insaturado. Conclusiones 356 3. Una de tales especies, cis-[PtMe(S)(PMe2ArDipp2)]+ (S = H2O, Et2O u otra molécula de escasa capacidad donadora) se escogió como modelo para desarrollar estudios de reactividad química que corroboran la anterior hipótesis acerca de su marcada electrofilia. De este modo, se encontró que al reemplazar S por CO la banda correspondiente a la vibración ῡ(CO) aparece a 2127 cm-1, valor próximo al de esta molécula libre, y que al reaccionar con H2 se originan complejos catiónicos -H2 y -CH4, que aunque inestables, perviven el tiempo suficiente para experimentar intercambios isotópicos de protio y de deuterio con el grupo metilo coordinado, Pt—CH3. Muy en especial, se ha demostrado de forma concluyente que el catión [PtMe(PMe2ArDipp2)]+ posee capacidad de inducir la tautomería de una molécula de acetileno coordinada, [Pt—C2H2]+, a vinilideno, [Pt=C=CH2]+, y la consiguiente formación de un nuevo enlace C—C con el grupo metilo coordinado. 4. En la química del platino en estado de oxidación cero, las fosfinas de terfenilo han permitido aislar complejos de composición [Pt(olefina)2(PMe2Ar’)] y [Pt(olefina)(PMe2Ar’)], asimismo insaturados en su coordinación y en sus requerimientos electrónicos, los cuales se comportan en disolución como fuentes del fragmento [Pt(PMe2Ar’)], de gran interés por su capacidad para inducir la activación de enlaces H-X o C-X. Dicho fragmento se genera, por ejemplo, en la reacción de los complejos anteriores con CO, que origina un clúster trinuclear [Pt3(μ-CO)3(PMe2Ar’)3]. Conclusiones 357 5. Además de una notable actividad química en reacciones estequiométricas de diversa naturaleza, los nuevos complejos de Pt(0) y Pt(II) de estos ligandos terfenilfosfina poseen una destacada actividad catalítica en reacciones de hidrosililación, hidroaminación o hidroarilación de alquinos. En particular, el complejo [Pt(CH2=CHtBu)(PMe2ArDipp2)] cataliza la hidrosililación de alquinos terminales como el fenilacetileno con conversiones próximas al 100% y elevada selectividad hacia la formación del correspondiente -E-alquenilsilano, en condiciones de temperaturas moderadas (50-80 ºC) y con bajas concentraciones de catalizador (0.02-0.1 mol%).