New Phosphonite Ligands with High Steric Demand and Low Basicity: Synthesis, Structural Properties and Cyclometalated Complexes of Pt(II)

Citation: Alcaide, M.M.; Pugliesi, M.; Álvarez, E.; López-Serrano, J.; Peloso, R. New Phosphonite Ligands with High Steric Demand and Low Basicity: Synthesis, Structural Properties and Cyclometalated Complexes of Pt(II). Inorganics 2022, 10, 109. https://doi.org/10.3390/ inorganics10080109 Academic Editors: Rainer Winter and J. Derek Woollins Received: 4 July 2022 Accepted: 26 July 2022 Published: 29 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). inorganics Article New Phosphonite Ligands with High Steric Demand and Low Basicity: Synthesis, Structural Properties and Cyclometalated Complexes of Pt(II) María M. Alcaide 1, Matteo Pugliesi 2, Eleuterio Álvarez 1, Joaquín López-Serrano 1,* and Riccardo Peloso 1,* 1Instituto de Investigaciones Químicas (IIQ), Departamento de Química Inorgánica and Centro de Innovación en Química Avanzada (ORFEO−CINQA), Consejo Superior de Investigaciones Científicas (CSIC), Universidad de Sevilla, 41092 Sevilla, Spain; [email protected] (M.M.A.); [email protected] (E.Á.) 2Dipartimento di Chimica e Chimica Industriale, Universitádi Pisa, Via G. Moruzzi 13, I-56124 Pisa, Italy; [email protected].it *Correspondence: [email protected] (J.L.-S.); [email protected] (R.P.) Abstract: Two phosphonite ligands bearing the highly sterically demanding 2,6-bis (2,6-dimethylphen- yl)phenyl group (Ar Xyl2 ), PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 , were prepared from the parent dihalophosphines PAr Xyl2 X 2 (X = Cl, Br) and the corresponding phenols, 4-nitrophenol and 4-nitro-2,6-dimethylphenol, respectively. DFT methods were used to examine their structural features and to determine three steric descriptors, namely the Tolman cone angle, the percentage of buried volume, and the percentage of the coordination sphere protected by the ligand. A comparison with the related terphenyl phosphines is also provided. Reactions of PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 with several Pt(II) precursors were investigated, revealing a high tendency of both phosphonites to undergo C-H activation processes and generate five- or six-membered cyclometalated structures. The coordination chemistry of the new ligands was explored with isolation, among others, of three carbonyl complexes, 1-3·CO , and the triphenylphosphine adduct 3·PPh3 . X-ray diffraction methods permitted the determination of the solid-state structures of the mononuclear methyl carbonyl complex 1·CO , the dinuclear chloride-bridged complex 2 and the doubly cyclometalated complex 3·SMe2 , including the conformations adopted by the ligands upon coordination. All of the new compounds were characterized by multinuclear NMR spectroscopy in solution. Keywords: P(III) ligands; phosphonites; platinum complexes; cyclometalated structures; DFT; bulky ligands 1. Introduction In spite of the long and rich history of phosphorus donor ligands in coordination chemistry [ 1 – 7 ], the development of new molecules containing trivalent phosphorus atoms capable of stabilizing diverse types of metal complexes [ 8 – 13 ], supramolecular structures [ 14 ], and nanoparticles [ 11 , 15 , 16 ] is still an area of intense research activity. Electronic and steric properties of this class of Lewis bases can easily be modulated with the introduction of atoms or molecular fragments of the desired characteristics directly at the phosphorus atom, or in selected positions of the organic groups that are usually bonded to it. In addition to the widespread use of P(III) ligands in catalytic processes [5,6], bulky phosphines and related mono- or polydentate ligands have permitted the stabilization of particularly elusive metal complexes, such as the first σ -methane complexes of Rh(I) characterized by Brookhart and collaborators taking advantage of the steric protections provided by the hybrid bis (phosphinite)-pyridine ligand, 2,6-(tBu 2 PO) 2 C 5 H 3 N [ 17 ]. In recent years, our research group described the synthesis of a number of highly steric demanding tertiary phosphines bearing terphenyl groups [ 18 , 19 ] and disclosed a variety of stoichiometric [ 20 – 25 ] and Inorganics 2022,10, 109. https://doi.org/10.3390/inorganics10080109 https://www.mdpi.com/journal/inorganics Inorganics 2022,10, 109 2 of 16 catalytic reactions [ 21 , 26 – 28 ] involving their complexes with late transition metals. Closely related to our findings, Nicasio and coworkers successfully employed dialkyl terphenyl phosphines to stabilize rare examples of 2-coordinate complexes of group 10 metals in the zero-oxidation state [29]. In contrast to the increasing data available on terphenyl phosphines and their applications, terphenyl-substituted phosphite and phosphinite have not been explored yet, with the sole exception of a ferrocene-based terphenyl phosphonite reported recently by Breher, together with a gold(I) complex used in hydroamination catalysis [ 30 ]. In this context, we became interested in the preparation of terphenyl phosphonites of the general formula Ar 0 P(OR) 2 (Ar 0 =m-terphenyl) with special emphasis on the development of sterically hindered π -accepting ligands. The conjunction of these characteristics is expected to be valuable for the stabilization of coordinatively unsaturated metal complexes in low-oxidation states [ 21 , 29 , 31 ] and, in general, in enhancing the electrophilicity of their coordination compounds. In the present work, we report on the synthesis and characterization of two phosphonite ligands bearing the bulky Ar Xyl2 fragment, PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 , and the electron-poor 4-nitrophenoxy and 4-nitro-2,6-dimethylphenoxy group, respectively, together with a theoretical evaluation of their steric properties. Moreover, we describe their reactions with several Pt(II) precursors, which give access to a series of mononuclear and dinuclear cyclometalated complexes. 2. Results 2.1. Synthesis and Properties of Terphenyl Phosphonites The synthesis of the phosphonite ligands PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 was selectively accomplished with high yields by reacting the parent dihalo phosphines PAr Xyl2 X 2 (X = Cl, Br; Ar Xyl2 = 2,6-bis(2,6-dimethylphenyl)phenyl) with 4-nitrophenol and 4-nitro-2,6-dimethylphenol, respectively, in the presence of triethylamine (Scheme 1) [ 30 , 32 ]. Both compounds were isolated as yellow solid materials and feature high solubility in common organic solvents, including saturated hydrocarbons. Although they can be stored indefinitely under nitrogen or argon without decomposition and can be exposed to the air for a short time, phosphonites PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 are slowly hydrolyzed by moisture both in solution and in the solid state, affording the corresponding monosubstituted phosphinic acids [33]. Inorganics 2022, 10, 109 2 of 16 demanding tertiary phosphines bearing terphenyl groups [18,19] and disclosed a variety of stoichiometric [20–25] and catalytic reactions [21,26–28] involving their complexes with late transition metals. Closely related to our findings, Nicasio and coworkers successfully employed dialkyl terphenyl phosphines to stabilize rare examples of 2-coordinate complexes of group 10 metals in the zero-oxidation state [29]. In contrast to the increasing data available on terphenyl phosphines and their applications, terphenyl-substituted phosphite and phosphinite have not been explored yet, with the sole exception of a ferrocene-based terphenyl phosphonite reported recently by Breher, together with a gold(I) complex used in hydroamination catalysis [30]. In this context, we became interested in the preparation of terphenyl phosphonites of the general formula Ar’P(OR)2 (Ar’ = m-terphenyl) with special emphasis on the development of sterically hindered -accepting ligands. The conjunction of these characteristics is expected to be valuable for the stabilization of coordinatively unsaturated metal complexes in lowoxidation states [21,29,31] and, in general, in enhancing the electrophilicity of their coordination compounds. In the present work, we report on the synthesis and characterization of two phosphonite ligands bearing the bulky ArXyl2 fragment, PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2, and the electron-poor 4-nitrophenoxy and 4-nitro-2,6-dimethylphe- noxy group, respectively, together with a theoretical evaluation of their steric properties. Moreover, we describe their reactions with several Pt(II) precursors, which give access to a series of mononuclear and dinuclear cyclometalated complexes. 2. Results 2.1. Synthesis and Properties of Terphenyl Phosphonites The synthesis of the phosphonite ligands PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 was selectively accomplished with high yields by reacting the parent dihalo phosphines PArXyl2X2 (X = Cl, Br; ArXyl2 = 2,6-bis(2,6-dimethylphenyl)phenyl) with 4-nitrophenol and 4- nitro-2,6-dimethylphenol, respectively, in the presence of triethylamine (Scheme 1) [30,32]. Both compounds were isolated as yellow solid materials and feature high solubility in common organic solvents, including saturated hydrocarbons. Although they can be stored indefinitely under nitrogen or argon without decomposition and can be exposed to the air for a short time, phosphonites PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 are slowly hydrolyzed by moisture both in solution and in the solid state, affording the corresponding monosubstituted phosphinic acids [33]. Scheme 1. Synthesis of the terphenyl phosphonite ligands, PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2. Phosphonites PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 were characterized by multinuclear NMR spectroscopy. Their 31P{1H} NMR spectra in CDCl3 consist of a unique singlet at ca. 165 ppm, in the expected range for this type of P(III) compounds [34]. In the 1H NMR of both compounds, the terphenyl fragment gives rise to a pattern of signals that Scheme 1. Synthesis of the terphenyl phosphonite ligands, PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2. Phosphonites PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 were characterized by multinuclear NMR spectroscopy. Their 31 P{ 1 H} NMR spectra in CDCl 3 consist of a unique singlet at ca. 165 ppm, in the expected range for this type of P(III) compounds [ 34 ]. In the 1 H NMR of both compounds, the terphenyl fragment gives rise to a pattern of signals that can be rationalized by an apparent C 2v symmetry with the C 2 axis containing the P—C Inorganics 2022,10, 109 3 of 16 bond. So, only one resonance at ca. 2.1 ppm is observed for the four methyl groups of the xylyl fragments and only two sets of signals for the aromatic protons. On the other hand, whereas the ortho and meta protons of the OPhNO2 groups in PArXyl2(OPhNO2)2 only originate two doublets as in the free 4-nitrophenol, the introduction of two methyl groups at the ortho positions impedes the rotation of the phenoxy moieties on the NMR time scale, which results in two clearly differentiated signals for the benzylic protons of the OPhNO2,Me group. Schematic stereo views of the molecular structures of the two phosphonite ligands with rotations accounting for the apparent symmetry in solution are shown in Figure 1. Inorganics 2022, 10, 109 3 of 16 can be rationalized by an apparent C 2v symmetry with the C 2 axis containing the P—C bond. So, only one resonance at ca. 2.1 ppm is observed for the four methyl groups of the xylyl fragments and only two sets of signals for the aromatic protons. On the other hand, whereas the ortho and meta protons of the OPh NO 2 groups in PAr Xyl 2(OPh NO 2) 2 only originate two doublets as in the free 4-nitrophenol, the introduction of two methyl groups at the ortho positions impedes the rotation of the phenoxy moieties on the NMR time scale, which results in two clearly differentiated signals for the benzylic protons of the OPh NO 2 ,Me group. Schematic stereo views of the molecular structures of the two phosphonite ligands with rotations accounting for the apparent symmetry in solution are shown in Figure 1. Figure 1. Schematic views of the 3D structures of PAr Xyl 2 (OPh NO 2 ) 2 and PAr Xyl 2 (OPh NO 2 ,Me ) 2 . Curved arrows indicate fast rotations which account for the apparent symmetry observed in NMR experiments. In order to compare the conformation adopted by these phosphonites with those of the related terphenyl phosphines, the gas-phase structures of both phosphonites were calculated by DFT methods (ωB97XD/6-31g(d,p) level; see below) and are shown in Figure 2. Figure 2. Views of the calculated gas-phase structures of PAr Xyl 2 (OPh NO 2 ) 2 (left) and PAr Xyl 2 (OPh NO 2 ,Me ) 2 (right) (C: grey; N: purple; O: red; P: orange). Conformations C and A relate to previously reported discussions on terphenyl phosphines ligands [18,19,26]. The two planes defining the dihedral angle are that containing the central ring of the terphenyl group and that containing the P—C bond and the axis of the P lone pair. Figure 1. Schematic views of the 3D structures of PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 . Curved arrows indicate fast rotations which account for the apparent symmetry observed in NMR experiments. In order to compare the conformation adopted by these phosphonites with those of the related terphenyl phosphines, the gas-phase structures of both phosphonites were calculated by DFT methods ( ω B97XD/6-31g(d,p) level; see below) and are shown in Figure 2. Inorganics 2022, 10, 109 3 of 16 can be rationalized by an apparent C 2v symmetry with the C 2 axis containing the P—C bond. So, only one resonance at ca. 2.1 ppm is observed for the four methyl groups of the xylyl fragments and only two sets of signals for the aromatic protons. On the other hand, whereas the ortho and meta protons of the OPh NO 2 groups in PAr Xyl 2(OPh NO 2) 2 only originate two doublets as in the free 4-nitrophenol, the introduction of two methyl groups at the ortho positions impedes the rotation of the phenoxy moieties on the NMR time scale, which results in two clearly differentiated signals for the benzylic protons of the OPh NO 2 ,Me group. Schematic stereo views of the molecular structures of the two phosphonite ligands with rotations accounting for the apparent symmetry in solution are shown in Figure 1. Figure 1. Schematic views of the 3D structures of PAr Xyl 2 (OPh NO 2 ) 2 and PAr Xyl 2 (OPh NO 2 ,Me ) 2 . Curved arrows indicate fast rotations which account for the apparent symmetry observed in NMR experiments. In order to compare the conformation adopted by these phosphonites with those of the related terphenyl phosphines, the gas-phase structures of both phosphonites were calculated by DFT methods (ωB97XD/6-31g(d,p) level; see below) and are shown in Figure 2. Figure 2. Views of the calculated gas-phase structures of PAr Xyl 2 (OPh NO 2 ) 2 (left) and PAr Xyl 2 (OPh NO 2 ,Me ) 2 (right) (C: grey; N: purple; O: red; P: orange). Conformations C and A relate to previously reported discussions on terphenyl phosphines ligands [18,19,26]. The two planes defining the dihedral angle are that containing the central ring of the terphenyl group and that containing the P—C bond and the axis of the P lone pair. Figure 2. Views of the calculated gas-phase structures of PAr Xyl2 (OPh NO2 ) 2 ( left ) and PAr Xyl2 (OPh NO2,Me ) 2 ( right ) (C: grey; N: purple; O: red; P: orange). Conformations C and A relate to previously reported discussions on terphenyl phosphines ligands [ 18 , 19 , 26 ]. The two planes defining the dihedral angle are that containing the central ring of the terphenyl group and that containing the P—C bond and the axis of the P lone pair. Curiously, the bulkier phosphonite, PArXyl2(OPhNO2,Me)2 , shows the same conformation adopted by the least sterically demanding terphenyl phosphines of the series that we have recently examined [ 18 ], namely PMe 2 Ar Dtbp2 (Ar Dtbp2 = 2,6-(3,5-tBu 2 C 6 H 3 )C 6 H 3 ) and PMe 2 Ar Ph2 (Ar Ph2 = 2,6-Ph 2 C 6 H 3 ), the two featuring P-Me bonds and no ring substitution Inorganics 2022,10, 109 4 of 16 at positions 2 and 6 of the flanking rings. Conversely, the less hindered phosphonite, PArXyl2(OPhNO2)2 , adopts the conformation displayed by highly steric demanding phosphines, such as PiPr 2 Ar Xyl2 and PAr Xyl2 Cy 2 . These findings show that the introduction of OAr substituents on the P atom significantly changes not only the electron-donating properties of this class of ligands, but also their conformations. It is important to mention that for both phosphonites’ ligands, the calculated C-O-P angles are much larger than expected for a sp 3 oxygen atom and span from ca. 120 ◦ to ca. 150 ◦ , the latter in PArXyl2(OPhNO2,Me)2 , thus providing significant relief to the intramolecular steric repulsions between the substituted phenoxy fragments and xylyl rings. To provide a detailed description of the steric properties of PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 , we decided to calculate three steric parameters: the Tolman cone angle (TCA), the percentage of buried volume (%Vbur), and the percentage of the coordination sphere protected by the ligand (%G). For the sake of comparison, a less hindered phosphonite, PAr Xyl2 (OMe) 2 , was also examined using its DFT-optimized geometry ( Table 1 ). Table 1. %V bur , TCA y %G values for the phosphonites studied in this work and for some related phosphine ligands. Ligand %Vbur TCA(◦) %G Ref PArXyl2(OPhNO2)259.7 218.1 57.2 this work PArXyl2(OPhNO2,Me)255.3 226.5 68.2 this work PArXyl2(OMe)249.1 210.8 50.3 this work PArXyl2Me238.6 153.9 38.8 [18] PArXyl2(c-C6H11)245.2 187.3 47.1 [18] PArDtbp2(c-C5H9)256.7 212.4 46.2 [18] It is pertinent to compare the steric parameters calculated for the phosphonites of this work to those of related terphenyl phosphines, which have recently been reported by some of us. On the basis of the following observations and data: • The highest TCA value among the phosphine series is 212.4 ◦ , for PAr Dtbp2 (c-C 5 H 9 ) 2 , in which the flanking rings of the terphenyl moiety are functionalized with two bulky tert-butyl groups at the meta positions and two cyclopentyl groups are bound to the P atom; • The TCA values for PAr Xyl2 R 2 phosphines, which bear the same terphenyl group as the phosphonites of this work, are in the range 153.9–187.3, with the highest value for the highly steric demanding R = cyclohexyl; •PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 have TCA values of 218.1 and 226.5, respectively, i.e., significantly higher than those of the related PR2ArXyl2; • The TCA increases by ca. 60 ◦ from the dimethyl terphenyl phosphine PAr Xyl2 Me 2 to the dimethoxy phosphonite PArXyl2(OMe)2; • Data reported for a series of PR 3 and P(OR) 3 ligands indicate that, in most cases, the TCA is reduced slighlty when passing from a phosphane to the related phosphite, e.g., from PPh3(TCAtetrehedral = 165.8◦) to P(OPh)3(TCAtetrehedral = 152.9◦). We could conclude that the introduction of substituted phenoxy groups at the phosphorus atom, in conjunction with the terphenyl group, significantly increases the cone angles of this class of ligands, which is likely a consequence of the flexibility of the POC angle (vide supra). So, the introduction of O spacers between the P atom and the R groups relieves the repulsion of the latter with the Xyl rings but drives them closer to the metal centre, thus increasing the TCA. Accordingly, terphenyl phosphonites feature higher %V bur values than the related phosphines and provide, therefore, steric protection closer to the metal center. Finally, when comparing the characteristics of PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 , it can be noted that the introduction of four methyl groups at the ortho positions of the phenoxyde rings results in increased TCA and %G values and a slightly reduced %V bur value. As a final comment about the quantification of the steric properties of terphenyl-substituted Inorganics 2022,10, 109 5 of 16 P(III) ligands, it is convenient to stress that the method used for this purpose does not consider that, upon coordination, the conformation of the ligand, and particularly the orientation of the P lone pair, may change with parallel variation in the steric descriptors. 2.2. Synthesis and Characterization of Cyclometalated Complexes of Pt(II) with PAr Xyl2 (OPh NO2 ) 2 and PArXyl2(OPhNO2,Me)2 Platinum complexes bearing terphenyl phosphines have recently proved to possess an interesting structural chemistry, unprecedented reactivity, and also attractive catalytic properties. On these grounds, we decided to undertake analogous studies with the new phosphonite ligands of this work, which were expected to enhance the electrophilic character of the metal center owing to their reduced electron-donating ability, while maintaining the high steric protection provided by the terphenyl group. Inour firstexperiments, typical platinum(II) dichloride precursors, namely PtCl 2 (DMSO) 2 , PtCl 2 (1,5-cyclooctadiene), and PtCl 2 (SMe 2 ) 2 , were treated at room temperature with equimolar amounts of ligands PArXyl2(OPhNO2)2 or PArXyl2(OPhNO2,Me)2 but no reaction was observed, either at higher temperatures (up to 100 ◦ C) or using more polar organic solvents. We thought that the low trans-effect of the chloride was not sufficient to promote the substitution of the neutral ligands of the precursor with the bulky and poorly donating phosphonites. Consequently, the more reactive dinuclear precursor [PtMe 2 ( µ -SMe 2 )] 2 was assayed in a 1:2 reaction with PArXyl2(OPhNO2)2 at room temperature in dichloromethane (Scheme 2). Analysis of the reaction mixture by 31 P{ 1 H} NMR spectroscopy revealed the quantitative and selective formation of a new Pt complex after a few hours, as inferred by the disappearance of the singlet resonance of the ligand at ca. 166 ppm and the formation of a new one with satellites at 169.3 (s, 1 J PPt = 2550 Hz). The new complex was isolated as a colorless, air-stable, solid material and characterized by 1 H and 13 C{ 1 H} NMR, which allowed us to propose the cyclometalated structure 1·SMe2depicted in Scheme 2. Inorganics 2022, 10, 109 5 of 16 %Vbur values than the related phosphines and provide, therefore, steric protection closer to the metal center. Finally, when comparing the characteristics of PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2, it can be noted that the introduction of four methyl groups at the ortho positions of the phenoxyde rings results in increased TCA and %G values and a slightly reduced %Vbur value. As a final comment about the quantification of the steric properties of terphenyl-substituted P(III) ligands, it is convenient to stress that the method used for this purpose does not consider that, upon coordination, the conformation of the ligand, and particularly the orientation of the P lone pair, may change with parallel variation in the steric descriptors. 2.2. Synthesis and Characterization of Cyclometalated Complexes of Pt(II) with PArXyl2(OPhNO2)2 and PArXyl2(OPhNO2,Me)2 Platinum complexes bearing terphenyl phosphines have recently proved to possess an interesting structural chemistry, unprecedented reactivity, and also attractive catalytic properties. On these grounds, we decided to undertake analogous studies with the new phosphonite ligands of this work, which were expected to enhance the electrophilic character of the metal center owing to their reduced electron-donating ability, while maintaining the high steric protection provided by the terphenyl group. In our first experiments, typical platinum(II) dichloride precursors, namely PtCl2(DMSO)2, PtCl2(1,5-cyclooctadiene), and PtCl2(SMe2)2, were treated at room temperature with equimolar amounts of ligands PArXyl2(OPhNO2)2 or PArXyl2(OPhNO2,Me)2 but no reaction was observed, either at higher temperatures (up to 100 °C) or using more polar organic solvents. We thought that the low trans-effect of the chloride was not sufficient to promote the substitution of the neutral ligands of the precursor with the bulky and poorly donating phosphonites. Consequently, the more reactive dinuclear precursor [PtMe2(μ- SMe2)]2 was assayed in a 1:2 reaction with PArXyl2(OPhNO2)2 at room temperature in dichloromethane (Scheme 2). Analysis of the reaction mixture by 31P{1H} NMR spectroscopy revealed the quantitative and selective formation of a new Pt complex after a few hours, as inferred by the disappearance of the singlet resonance of the ligand at ca. 166 ppm and the formation of a new one with satellites at 169.3 (s, 1JPPt = 2550 Hz). The new complex was isolated as a colorless, air-stable, solid material and characterized by 1H and 13C{1H} NMR, which allowed us to propose the cyclometalated structure 1∙SMe2 depicted in Scheme 2. Scheme 2. Synthesis of complexes 1 and 2. Scheme 2. Synthesis of complexes 1and 2. Particularly informative were the following NMR data: (a) 1 H and 13 C{ 1 H} NMR spectra of 1·SMe2 contain one single Pt-CH 3 signal, at 0.43 ( 3 J HP = 8.6 Hz, 2JHPt = 65.1 Hz ) and 8.9 ( 1 J CPt = 145 Hz) ppm, respectively; (b) a singlet with satellites at 2.06 ppm ( 3JHPt = 31.1 Hz ), which integrates for six H nuclei, consistent with a Pt-bonded dimethylsulfide molecule; (c) a doublet resonance with satellites at 8.22 ppm ( 4 J HP = 2.3 Hz, 3JHPt = 78.6 Hz ) that can be assigned to a proton at the meta position of an ortho-cyclometalated Ph NO2 ring. Interestingly, in agreement with the interpretation of the NMR data for Inorganics 2022,10, 109 6 of 16 PArXyl2(OPhNO2)2 , the four methyl groups of the terphenyl ring in 1·SMe2 give rise to two different signals in the corresponding NMR spectra, with a decrease in the apparent symmetry of the Ar Xyl2 moiety (from C 2v to C 2 ) due to the metalation of one of the two Ph NO2 groups. This observation is consistent with the fast rotation around the P—C bond on the NMR time scale, as in the free ligand. To confirm that 1·SMe2 formed with the concomitant elimination of methane, the synthesis was performed in CD 2 Cl 2 in a sealed NMR tube. Indeed, a singlet resonance at 0.21 ppm was detected in the 1 H NMR spectrum of the reaction mixture after a few hours and assigned to CH4in dichloromethane solution according to the literature [34]. The synthesis of the carbonyl complex 1·CO was straightforward by exposing dichloromethane solutions of 1·SMe2 to 1 atm of CO at room temperature. Although 1·CO could be isolated as a pure solid material by the fast elimination of the volatiles from the reaction mixture under reduced pressure, the appearance of a dark solid material indicated that some decomposition took place after 20–30 min under vacuum. The IR spectrum of 1·CO in CH 2 Cl 2 solution shows a strong absorption at 2077 cm −1 , which is in the expected range for Pt(II) carbonyl complexes. X-ray diffraction analyses permitted ascertaining the molecular structure of 1·CO in the solid state (Figure 3). The platinum atom lies in a distorted square planar environment with the phosphonite ligand adopting a B conformation ( C-C-P-Pt dihedral angle of ca. 40 ◦ ). The smallest bond angle at the metal center, 80.86(6) ◦ , corresponds to the five-member ring originated by the cyclometallation of one of the two Ph NO2 rings, whereas the largest one (ca. 100 ◦ ) is that comprised by the CO ligand and the P atom and is likely influenced by the steric hindrance of the phosphorated ligand. The trend in the three different Pt-C bond distances parallels the hybridization of the C atom involved in the bond, i.e., Pt-Csp < Pt-Csp2< Pt-Csp3. Inorganics 2022, 10, 109 6 of 16 Particularly informative were the following NMR data: (a) 1H and 13C{1H} NMR spectra of 1∙SMe2 contain one single Pt-CH3 signal, at 0.43 (3JHP = 8.6 Hz, 2JHPt = 65.1 Hz) and 8.9 (1JCPt = 145 Hz) ppm, respectively; (b) a singlet with satellites at 2.06 ppm (3JHPt = 31.1 Hz), which integrates for six H nuclei, consistent with a Pt-bonded dimethylsulfide molecule; (c) a doublet resonance with satellites at 8.22 ppm (4JHP = 2.3 Hz, 3JHPt = 78.6 Hz) that can be assigned to a proton at the meta position of an ortho-cyclometalated PhNO2 ring. Interestingly, in agreement with the interpretation of the NMR data for PArXyl2(OPhNO2)2, the four methyl groups of the terphenyl ring in 1∙SMe2 give rise to two different signals in the corresponding NMR spectra, with a decrease in the apparent symmetry of the ArXyl2 moiety (from C2v to C2) due to the metalation of one of the two PhNO2 groups. This observation is consistent with the fast rotation around the P—C bond on the NMR time scale, as in the free ligand. To confirm that 1∙SMe2 formed with the concomitant elimination of methane, the synthesis was performed in CD2Cl2 in a sealed NMR tube. Indeed, a singlet resonance at 0.21 ppm was detected in the 1H NMR spectrum of the reaction mixture after a few hours and assigned to CH4 in dichloromethane solution according to the literature [34]. The synthesis of the carbonyl complex 1∙CO was straightforward by exposing dichloromethane solutions of 1∙SMe2 to 1 atm of CO at room temperature. Although 1∙CO could be isolated as a pure solid material by the fast elimination of the volatiles from the reaction mixture under reduced pressure, the appearance of a dark solid material indicated that some decomposition took place after 20–30 min under vacuum. The IR spectrum of 1∙CO in CH2Cl2 solution shows a strong absorption at 2077 cm−1, which is in the expected range for Pt(II) carbonyl complexes. X-ray diffraction analyses permitted ascertaining the molecular structure of 1∙CO in the solid state (Figure 3). The platinum atom lies in a distorted square planar environment with the phosphonite ligand adopting a B conformation (C-C-P-Pt dihedral angle of ca. 40°). The smallest bond angle at the metal center, 80.86(6)°, corresponds to the five-member ring originated by the cyclometallation of one of the two PhNO2 rings, whereas the largest one (ca. 100°) is that comprised by the CO ligand and the P atom and is likely influenced by the steric hindrance of the phosphorated ligand. The trend in the three different Pt-C bond distances parallels the hybridization of the C atom involved in the bond, i.e., Pt-Csp < Pt-Csp2 < Pt-Csp3. Figure 3. ORTEP view of the X-ray molecular structure of complex 1∙CO, with ellipsoids at the 50% probability level and H atoms omitted for clarity (left); schematic view of the B conformation adopted by the ligand PArXyl2(OPhNO2)2 (right) [18,26]. Selected bond distances (Å) and angles (°): Pt1-P1 2.2643(8), Pt1-C1 2.054(2), Pt1-C2 2.110(3), Pt1-C3 1.901(2), C3-O1 1.126(3); P1-Pt1-C1 80.86(6); C1-Pt1-C2 90.78(9); C2-Pt1-C3 88.06(1); C3-Pt1-P1 100.41(8). Figure 3. ORTEP view of the X-ray molecular structure of complex 1·CO , with ellipsoids at the 50% probability level and H atoms omitted for clarity ( left ); schematic view of the B conformation adopted by the ligand PArXyl2(OPhNO2)2 ( right ) [ 18 , 26 ]. Selected bond distances (Å) and angles ( ◦ ): Pt1-P1 2.2643(8), Pt1-C1 2.054(2), Pt1-C2 2.110(3), Pt1-C3 1.901(2), C3-O1 1.126(3); P1-Pt1-C1 80.86(6); C1-Pt1-C2 90.78(9); C2-Pt1-C3 88.06(1); C3-Pt1-P1 100.41(8). Our previous studies on terphenyl phosphine complexes of platinum proved that, in some cases, these ligands can be forced to adopt a κ2 -P,C coordination mode in which one of the C atoms of a xylyl group is involved in a secondary interaction with the metal center [18–22] . So, we envisaged that a complex of this type could be prepared by reacting PtCl 2 with the phosphonites reported here. Accordingly, PArXyl2(OPhNO2)2 was treated with equimolar amounts of PtCl 2 in toluene at 100 ◦ C (higher reaction temperatures are required because PtCl 2 is an insoluble polymeric material). Nevertheless, after 72 h, the ligand was quantitatively converted into the dinuclear platinum complex, 2 , which again resulted from the ortho-metallation of a Ph NO2 ring and elimination of HCl. Its characterization by NMR spectroscopy does not deserve any specific comment other than the dramatic Inorganics 2022,10, 109 7 of 16 increase in the 1 J PPt coupling constant with respect to complexes 1 (6222 Hz for 2 ; 2550 and 2193 Hz for 1·SMe2 and 1·CO , respectively), as a consequence of the low trans influence of the bridging chloride ligands compared to the strong σ-donor methyl ligand [35]. The molecular structure of complex 2 was determined by X-ray diffraction studies and is represented in Figure 4(left) together with schematic views of the coordination planes and the angle they form and with the binary axis C 2 (top right) and the conformation adopted by the phosphonite ligand (bottom right). Inorganics 2022, 10, 109 7 of 16 Our previous studies on terphenyl phosphine complexes of platinum proved that, in some cases, these ligands can be forced to adopt a κ 2-P,C coordination mode in which one of the C atoms of a xylyl group is involved in a secondary interaction with the metal center [18–22]. So, we envisaged that a complex of this type could be prepared by reacting PtCl2 with the phosphonites reported here. Accordingly, PArXyl2(OPhNO2)2 was treated with equimolar amounts of PtCl2 in toluene at 100 °C (higher reaction temperatures are required because PtCl2 is an insoluble polymeric material). Nevertheless, after 72 h, the lig- and was quantitatively converted into the dinuclear platinum complex, 2, which again resulted from the ortho-metallation of a PhNO2 ring and elimination of HCl. Its characterization by NMR spectroscopy does not deserve any specific comment other than the dramatic increase in the 1JPPt coupling constant with respect to complexes 1 (6222 Hz for 2; 2550 and 2193 Hz for 1∙SMe2 and 1∙CO, respectively), as a consequence of the low trans influence of the bridging chloride ligands compared to the strong σ-donor methyl ligand [35]. The molecular structure of complex 2 was determined by X-ray diffraction studies and is represented in Figure 4 (left) together with schematic views of the coordination planes and the angle they form and with the binary axis C2 (top right) and the conformation adopted by the phosphonite ligand (bottom right). Figure 4. ORTEP view of the X-ray molecular structure of complex 2, with ellipsoids at the 50% probability level and H atoms omitted for clarity (left); schematic view of the coordination planes in 2 with the angle they form and the binary axis C2 (top right); schematic view of the B conformation adopted by the ligand PArXyl2(OPhNO2)2 (bottom right) [18,26]. Selected bond distances (Å) and angles (°): Pt1-P1 2.146(1), Pt1-C1 1.974(4), Pt1-Cl1 2.410(1), Pt1-Cl1′ 2.398(1); P1-Pt1-C1 82.6(1); C1-Pt1- Cl1′ 95.4(1); Cl-Pt1-Cl’ 82.89(4); Cl1-Pt1-P1 99.10(4); Pt1-Cl1-Pt1′ 82.60(4). The asymmetric unit contains half a molecule of 2, the second one being generated by a C2 axis that forms an angle of ca. 30° with the coordination plane around the metal center, giving rise to a chiral species. The two coordination planes form an angle of ca. 60° (Figure 4, top right). Similarly to complex 1∙CO, the metalated phosphonite ligands adopt a B conformation with a C-C-P-Pt dihedral angle of ca. 50°. Both the Pt-P and Pt-C bond distances are significantly shorter in 2 than in the carbonyl compound, in accordance with the lower trans influence of the bridging chlorides compared to Me and CO. The dinuclear complex 2 was easily converted into the mononuclear carbonyl chloride complex 2∙CO by treatment with CO at room temperature (Scheme 2). The Figure 4. ORTEP view of the X-ray molecular structure of complex 2 , with ellipsoids at the 50% probability level and H atoms omitted for clarity ( left ); schematic view of the coordination planes in 2 with the angle they form and the binary axis C 2 (top right ); schematic view of the B conformation adopted by the ligand PArXyl2(OPhNO2)2 (bottom right) [ 18 , 26 ]. Selected bond distances (Å) and angles ( ◦ ): Pt1-P1 2.146(1), Pt1-C1 1.974(4), Pt1-Cl1 2.410(1), Pt1-Cl1 0 2.398(1); P1-Pt1-C1 82.6(1); C1-Pt1-Cl1095.4(1); Cl-Pt1-Cl082.89(4); Cl1-Pt1-P1 99.10(4); Pt1-Cl1-Pt1082.60(4). The asymmetric unit contains half a molecule of 2 , the second one being generated by a C 2 axis that forms an angle of ca. 30 ◦ with the coordination plane around the metal center, giving rise to a chiral species. The two coordination planes form an angle of ca. 60 ◦ (Figure 4, top right). Similarly to complex 1·CO , the metalated phosphonite ligands adopt a B conformation with a C-C-P-Pt dihedral angle of ca. 50 ◦ . Both the Pt-P and Pt-C bond distances are significantly shorter in 2 than in the carbonyl compound, in accordance with the lower trans influence of the bridging chlorides compared to Me and CO. The dinuclear complex 2 was easily converted into the mononuclear carbonyl chloride complex 2·CO by treatment with CO at room temperature (Scheme 2). The configuration proposed in Scheme 2for 2·CO , i.e., a trans distribution of the P donor and CO ligand around the platinum center, is based on the higher values of the CO stretching vibration of 2·CO compared to 1·CO (2110 vs. 2077 cm −1 , respectively), in accordance with the presence of a π -acidic, poorly σ -donating phosphonite trans to the carbonyl ligand in 2·CO. Moreover, the 1 J PPt coupling of 4758 Hz is similar to that of the carbonyl complex 3·CO (5023 Hz, vide infra), in which the P(III) and the CO ligands occupy mutually trans positions, and much lower than that of the chloride complex 2 (6222 Hz), in which the P donor is trans to a chloride ligand. As mentioned in the Introduction, the main goal of our work on new bulky phosphonite ligands was the stabilization of homoleptic two-coordinate Pt(0) complexes. However, the reactions reported in Scheme 2demonstrate that PArXyl2(OPhNO2)2 easily undergoes ortho-cyclometallation, which makes difficult any further reduction of the resulting complex. The use of the ligand PArXyl2(OPhNO2,Me)2 , with the four ortho positions of the Inorganics 2022,10, 109 8 of 16 4-nitrophenoxy rings occupied by methyl groups, was expected to prevent cyclometallation reactions and to provide more steric protection to the metal center. So, we treated the same starting materials shown Scheme 2, i.e., PtCl 2 and [PtMe 2 ( µ -SMe 2 )] 2 , with PArXyl2(OPhNO2,Me)2 at room temperature, but no reaction took place, likely as a consequence of the combined effects of low basicity and high steric demand of the phosphonite. While no coordination to platinum was detected using PtCl 2 as the starting material, even at higher temperatures (up to 100 ◦ C), with [PtMe 2 ( µ -SMe 2 )] 2 , a new complex formed when heating 1:2 mixtures of [PtMe 2 ( µ -SMe 2 )] 2 and PArXyl2(OPhNO2,Me)2 in THF at 60 ◦ C, as revealed by the presence of a singlet at 165.6 ppm ( 1 J PPt of 6185 Hz) in the 31 P{ 1 H} NMR spectrum of the crude. The slow conversion of the starting materials was completed after ca. 48 h. The new substance was isolated as a colorless, air-stable solid, characterized by NMR spectroscopy and X-ray diffraction analyses, and identified as the bis-cyclometalated Pt(II) complex, 3·SMe2 (Scheme 3), which is the result of C-H activation of two methyl groups of the OPh NO2,Me rings, elimination of methane, and eventual formation of two Pt-Csp3bonds in a trans disposition. Inorganics 2022, 10, 109 8 of 16 configuration proposed in Scheme 2 for 2∙CO, i.e., a trans distribution of the P donor and CO ligand around the platinum center, is based on the higher values of the CO stretching vibration of 2∙CO compared to 1∙CO (2110 vs. 2077 cm−1, respectively), in accordance with the presence of a π-acidic, poorly σ-donating phosphonite trans to the carbonyl ligand in 2∙CO. Moreover, the 1JPPt coupling of 4758 Hz is similar to that of the carbonyl complex 3∙CO (5023 Hz, vide infra), in which the P(III) and the CO ligands occupy mutually trans positions, and much lower than that of the chloride complex 2 (6222 Hz), in which the P donor is trans to a chloride ligand. As mentioned in the Introduction, the main goal of our work on new bulky phosphonite ligands was the stabilization of homoleptic two-coordinate Pt(0) complexes. However, the reactions reported in Scheme 2 demonstrate that PArXyl2(OPhNO2)2 easily undergoes ortho-cyclometallation, which makes difficult any further reduction of the resulting complex. The use of the ligand PArXyl2(OPhNO2,Me)2, with the four ortho positions of the 4- nitrophenoxy rings occupied by methyl groups, was expected to prevent cyclometallation reactions and to provide more steric protection to the metal center. So, we treated the same starting materials shown Scheme 2, i.e., PtCl2 and [PtMe2(μ-SMe2)]2, with PArXyl2(OPhNO2,Me)2 at room temperature, but no reaction took place, likely as a consequence of the combined effects of low basicity and high steric demand of the phosphonite. While no coordination to platinum was detected using PtCl2 as the starting material, even at higher temperatures (up to 100 °C), with [PtMe2(𝜇-SMe2)]2, a new complex formed when heating 1:2 mixtures of [PtMe2(μ-SMe2)]2 and PArXyl2(OPhNO2,Me)2 in THF at 60 °C, as revealed by the presence of a singlet at 165.6 ppm (1JPPt of 6185 Hz) in the 31P{1H} NMR spectrum of the crude. The slow conversion of the starting materials was completed after ca. 48 h. The new substance was isolated as a colorless, air-stable solid, characterized by NMR spectroscopy and X-ray diffraction analyses, and identified as the bis-cyclometalated Pt(II) complex, 3∙SMe2 (Scheme 3), which is the result of C-H activation of two methyl groups of the OPhNO2,Me rings, elimination of methane, and eventual formation of two Pt- Csp3 bonds in a trans disposition. Scheme 3. Synthesis of complexes 3. The most relevant NMR data for 3∙SMe2 are listed below: • The four methyl groups of the xylyl rings give rise to one singlet at 2.10 ppm and one singlet at 21.5 ppm in the 1H and 13C{1H} NMR spectra, respectively, in accordance with fast rotation around the P—C bond on the NMR time-scale, as in the free phosphonite ligands; • The two metalated OPhNO2,Me fragments are equivalent, as shown by the presence of only one resonance for the CH3 groups in both the 1H- and 13C{1H} NMR spectra; • In the 1H NMR spectrum, the four diasterotopic methylene protons are coupled with the 31P nucleus, thus constituting first-order AMX spin systems, which give rise to a doublet (2JHH = 11.5 Hz, 3JHP ≈ 0 Hz) and a triplet (2JHH ≈ 3JHP = 11.5 Hz), both with 195Pt satellites; Scheme 3. Synthesis of complexes 3. The most relevant NMR data for 3·SMe2are listed below: • The four methyl groups of the xylyl rings give rise to one singlet at 2.10 ppm and one singlet at 21.5 ppm in the 1 H and 13 C{ 1 H} NMR spectra, respectively, in accordance with fast rotation around the P—C bond on the NMR time-scale, as in the free phosphonite ligands; • The two metalated OPh NO2,Me fragments are equivalent, as shown by the presence of only one resonance for the CH3groups in both the 1H- and 13C{1H} NMR spectra; • In the 1 H NMR spectrum, the four diasterotopic methylene protons are coupled with the 31 P nucleus, thus constituting first-order AMX spin systems, which give rise to a doublet ( 2 J HH = 11.5 Hz, 3 J HP ≈ 0 Hz) and a triplet ( 2 J HH ≈3 J HP = 11.5 Hz), both with 195Pt satellites; • The six protons of the coordinated dimethylsulfide resonate as a doublet at 2.34 ppm with 4JHP = 4.0 Hz and 3JHPt = 36.4 Hz. The molecular structure of 3·SMe2 is represented in Figure 5together with a schematic view of the conformation adopted by the phosphonite ligand, again of the B type with a C-C-P-Pt dihedral angle of approximately 40◦. The coordination geometry is best described as slightly distorted square planar, with the metal center lying at a distance of ca. 0.13 Å above the midplane defined by the four donor atoms. The two Pt—C bond distances are significantly different (ca. 0.03 Å), likely as a consequence of the different conformations adopted by the two six-membered rings to which they pertain. Such conformations are, in turn, influenced by the dihedral angle shown in Figure 6, which forces one of the two P—O bonds to a quasi-eclipsed conformation with the Pt—C2 bond (see Figure 6). Inorganics 2022,10, 109 9 of 16 Inorganics 2022, 10, 109 9 of 16 • The six protons of the coordinated dimethylsulfide resonate as a doublet at 2.34 ppm with 4JHP = 4.0 Hz and 3JHPt = 36.4 Hz. The molecular structure of 3∙SMe2 is represented in Figure 5 together with a schematic view of the conformation adopted by the phosphonite ligand, again of the B type with a C-C-P-Pt dihedral angle of approximately 40°. Figure 5. ORTEP view of the X-ray molecular structure of complex 3∙SMe2, with ellipsoids at the 50% probability level and H atoms omitted for clarity (left); schematic view of the B conformation adopted by the ligand PArXyl2(OPhNO2,Me)2 (right) [17,25]. Selected bond distances (Å) and angles (°): Pt1-P1 2.1526(9), Pt1-C1 2.155(3), Pt1-C2 2.120(3), Pt1-S1 2.339(1); P1-Pt1-C1 84.18(7); C1-Pt1-S1 86.74(8); S1-Pt1-C2 99.09(9); C2-Pt1-P1 89.23(8); C1-Pt1-C2 169(1). The coordination geometry is best described as slightly distorted square planar, with the metal center lying at a distance of ca. 0.13 Å above the midplane defined by the four donor atoms. The two Pt—C bond distances are significantly different (ca. 0.03 Å), likely as a consequence of the different conformations adopted by the two six-membered rings to which they pertain. Such conformations are, in turn, influenced by the dihedral angle shown in Figure 6, which forces one of the two P—O bonds to a quasi-eclipsed conformation with the Pt—C2 bond (see Figure 6). Figure 6. Schematic view along the Pt—P bond of the conformations adopted by the two cyclometalated 6-membered rings together with the values of the O-P-Pt-C dihedral angles. The carbonyl complex 3∙CO was selectively obtained by exposing dichloromethane solutions of the parent dimethylsulfide adduct to 1 atm of CO (Scheme 3). The unexpectedly high value of 2140 cm−1 found for the C≡O stretching vibration in the IR spectrum of Figure 5. ORTEP view of the X-ray molecular structure of complex 3·SMe2 , with ellipsoids at the 50% probability level and H atoms omitted for clarity ( left ); schematic view of the B conformation adopted by the ligand PArXyl2(OPhNO2,Me)2 ( right ) [ 17 , 25 ]. Selected bond distances (Å) and angles ( ◦ ): Pt1-P1 2.1526(9), Pt1-C1 2.155(3), Pt1-C2 2.120(3), Pt1-S1 2.339(1); P1-Pt1-C1 84.18(7); C1-Pt1-S1 86.74(8); S1-Pt1-C2 99.09(9); C2-Pt1-P1 89.23(8); C1-Pt1-C2 169(1). Inorganics 2022, 10, 109 9 of 16 • The six protons of the coordinated dimethylsulfide resonate as a doublet at 2.34 ppm with 4JHP = 4.0 Hz and 3JHPt = 36.4 Hz. The molecular structure of 3∙SMe2 is represented in Figure 5 together with a schematic view of the conformation adopted by the phosphonite ligand, again of the B type with a C-C-P-Pt dihedral angle of approximately 40°. Figure 5. ORTEP view of the X-ray molecular structure of complex 3∙SMe2, with ellipsoids at the 50% probability level and H atoms omitted for clarity (left); schematic view of the B conformation adopted by the ligand PArXyl2(OPhNO2,Me)2 (right) [17,25]. Selected bond distances (Å) and angles (°): Pt1-P1 2.1526(9), Pt1-C1 2.155(3), Pt1-C2 2.120(3), Pt1-S1 2.339(1); P1-Pt1-C1 84.18(7); C1-Pt1-S1 86.74(8); S1-Pt1-C2 99.09(9); C2-Pt1-P1 89.23(8); C1-Pt1-C2 169(1). The coordination geometry is best described as slightly distorted square planar, with the metal center lying at a distance of ca. 0.13 Å above the midplane defined by the four donor atoms. The two Pt—C bond distances are significantly different (ca. 0.03 Å), likely as a consequence of the different conformations adopted by the two six-membered rings to which they pertain. Such conformations are, in turn, influenced by the dihedral angle shown in Figure 6, which forces one of the two P—O bonds to a quasi-eclipsed conformation with the Pt—C2 bond (see Figure 6). Figure 6. Schematic view along the Pt—P bond of the conformations adopted by the two cyclometalated 6-membered rings together with the values of the O-P-Pt-C dihedral angles. The carbonyl complex 3∙CO was selectively obtained by exposing dichloromethane solutions of the parent dimethylsulfide adduct to 1 atm of CO (Scheme 3). The unexpectedly high value of 2140 cm−1 found for the C≡O stretching vibration in the IR spectrum of Figure 6. Schematic view along the Pt—P bond of the conformations adopted by the two cyclometalated 6-membered rings together with the values of the O-P-Pt-C dihedral angles. The carbonyl complex 3·CO was selectively obtained by exposing dichloromethane solutions of the parent dimethylsulfide adduct to 1 atm of CO (Scheme 3). The unexpectedly high value of 2140 cm −1 found for the C ≡ O stretching vibration in the IR spectrum of 3·CO in CH 2 Cl 2 substantially matches that of free CO (2143 cm −1 ), suggesting an almost negligible contribution of π -backdonation to the Pt—CO bonding, and exceeds by 70 cm −1 that found for 1·CO . This strong difference observed for two apparently similar platinum(II) carbonyls can reasonably be ascribed to the different electronic properties of the ligand trans to CO in each complex: a strongly σ -donating aryl ligand in 1·CO and a P-bonded electron-poor phosphonite in 3·CO , albeit in a dianionic form. In contrast to what was observed for 3·SMe2 , the 1 H NMR spectrum of 3·CO shows two well-separated doublets of doublets with 195 Pt satellites for the AMX spin system of the two equivalent methylene groups at 2.55 and 1.48 ppm (Figure 7). The dimethylsulfide ligand was also easily displaced from 3·SMe2 by triphenylphosphine, affording the air-stable complex 3·PPh3 , whose 31 P{ 1 H} NMR spectrum consists of two doublets at 180.8 (phosphonite) and 24.8 ppm (phosphane) with a large 2 J PP coupling constant of 528 Hz, in agreement with the mutual trans disposition of the two 31 P nuclei. A significantly larger 31 P- 195 Pt coupling is observed for the phosphonite (4875 Hz) than for the phosphine (3101 Hz) [ 36 ]. Regarding the 1 H NMR of 3·PPh3 , the two equivalent Pt-bonded methylene groups give rise to a doublet of doublets and a triplet of doublets, Inorganics 2022,10, 109 16 of 16 46. Bruker. SAINT APEX2 2007; Bruker AXS Inc.: Madison, WI, USA, 2007. 47. Sheldrick, G.M. SADABS, Programs for Scaling and Absorption Correction of Area Detector Data 1997; University of Göttingen: Göttingen, Germany, 1997. 48. Burla, M.C.; Camalli, M.; Carrozzini, B.; Cascarano, G.L.; Giacovazzo, C.; Polidori, G.; Spagna, R. 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