The coordination chemistry of the neutral tris-2-pyridyl silicon ligand [PhSi(6-Me-2-py)3]

Dalton Trans ARTICLE This journal is © The Royal Society of Chemistry 20xx Dalton Trans., 2018, 00, 1-3 | 1 Please do not adjust margins Please do not adjust margins a. Chemistry Department. Cambridge University, Lensfield Road, Cambridge CB2 1EW (U.K.). Email: [email protected] b. Anorganisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270, 69120 Heidelberg (Germany) c. GIR MIOMeT-IU Cinquima-Química Inorgánica, Facultad de Ciencias, Campus Miguel, Delibes, Universidad de Valladolid 47011 Valladolid, Spain. E-mail: [email protected]. Electronic Supplementary Information (ESI) available: NMR spectroscopic and X- ray characterisation. CCDC: 1833558–1833563. DOI: 10.1039/x0xx00000x Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/ The Coordination Chemistry of the Neutral Tris-2-pyridyl Silicon Ligand [PhSi(6-Me-2-py)3] Alex J. Plajer,a Annie L. Colebatch,a Markus Enders,b Álvaro García-Romero,c Andrew D. Bond,a Raúl García-Rodríguez,c,* and Dominic S. Wright a,* Difficulties in the preparation of neutral ligands of the type [RSi(2-py)3] (where 2-py is an unfunctionalised 2-pyridyl ring unit) have thwarted efforts to expand the coordination chemistry of ligands of this type. However, simply switching the pyridyl substituents to 6-methyl-pyridyl groups (6-Me-2-py) in the current paper has allowed smooth, high-yielding access to the [PhSi(6-Me-2-py)3] ligand (1), and the first exploration of its coordination chemistry with transition metals. The synthesis, single-crystal X-ray structures and solution dynamics of the new complexes [{PhSi(6-Me-2-py)3}CuCH3CN][PF6], [{PhSi(6-Me-2-py)3}CuCH3CN][CuCl2], [{PhSi(6-Me-2-py)3}FeCl2], [{PhSi(6-Me-2-py)3}Mo(CO)3] and [{PhSi(6-Me-2-py)3}CoCl2] are reported. The paramagnetic Fe2+ and Co2+ complexes show strongly shifted NMR resonances for the coordinated pyridyl units due to large Fermi-contact shifts. However, magnetic anisotropy also leads to considerable pseudo-contact shifts so that both contributions have to be included in the paramagnetic NMR analysis. 1. Introduction C3-symmetric, tripodal ligands are used extensively as auxiliary ligands in single-site transition metal homogeneous catalysis and biomimetic systems.1-6 One of the major classes of these are tris-pyrazolyl borates (Figure 1a), which combine ease of synthesis with the ability to tune the steric and electronic character of the donor set by the introduction of electrondonating or electron-accepting substituents into the pyrazolyl ring units.7 An emerging but less well developed strategy for tuning ligands is the substitution of the bridgehead atom itself for other main group elements, with the potential not only for tuning the electronic character of the ligand but also the ligand bite in a systematic way. Our interest in this area has been in the tris-2-pyridyl family of ligands. The majority of studies in the past three decades have concerned neutral frameworks containing lighter, non-metallic bridgehead atoms Y(2-py’)3 (Y = CR, COR, CH, N, P, P=O; 2-py’ = an unsubstituted or substituted 2-pyridyl group) (e.g., Figure 1b).8 More recently, however, attention has turned to the effects of incorporating more metallic Group 13,9 1410,11 and 1512 bridgeheads.13 These isoelectronic metallic relatives now span almost the entire range of p-block elements, from anionic aluminate ligands (e.g., inset to Figure 1, A ),9 through to the heaviest counterpart containing a BiIII bridgehead (inset to Figure 1, C ).12 We have shown recently that changing the bridgehead atom can have significant effects on both the coordination chemistry and reactivity of tris(2-pyridyl) ligands. For example, the increasing Lewis acidity of the elements as Group 15 is descended can impact dramatically on the ligand coordination mode as well as the  -donor character of the ligand frameworks, introducing the potential for modulation of the structures and catalytic activity of metal complexes.12 Heavier Group 14 ligands of this type can be divided into two classes, neutral element-IV ligands of the type [RE(2- py’)3]10 and element-II anions of the type [E(2-py’)3]-.11 Although Zeckert and coworkers have developed the coordination chemistry of the latter in a number of key studies, there is currently a surprising lack of structural data available on the element-IV oxidation state ligands [RE(2- py)3] . 10 Although there are reports of the coordination and supramolecular chemistry of bis(2-py) and tris(4-py) Si(IV) ligands14 and some early reports of the synthesis of tris(2-py) ligands,15 there are no structurally characterised examples of complexes of Ge(IV) tris(2-pyridyl) ligands [RGe(2-py’)3] and only one example for the Si(IV) ligands [RSi(2-py’)3], [MeSi(2- ARTICLE Journal Name 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins py)3LiX (X = Cl/Br), containing the neutral [MeSi(2-py)3] ligand (2-py = unsubstituted 2-pyridyl) (inset to Figure 1, B ).10c In the current paper we set out with the simple aim of expanding knowledge of the coordination chemistry of Si(IV) tris(2-pyridyl) ligands. Entry into this system was facilitated by the synthesis of the new Si(IV) ligand [PhSi(6-Me-2-py)3] which was then used to obtain a range of (the first) transition metal complexes containing this type of donor. The synthesis, singlecrystal X-ray structures and selected solution dynamics of the new complexes [{PhSi(6-Me-2-py)3}CuCH3CN][PF6], [{PhSi(6- Me-2-py)3}CuCH3CN][CuCl2], [{PhSi(6-Me-2-py)3}CoCl2], [{PhSi(6-Me-2-py)3}FeCl2] and [{PhSi(6-Me-2-py)3}Mo(CO)3] are described. Figure 1 a) An unsubstituted tris-pyrazolyl-borate anion, b) the family of neutral unsubstituted tris(2-pyridyl) ligands containing non-metal bridgeheads. The inset shows three previously reported examples of tris(2-pyridyl) ligands spanning the range of more metallic bridgehead arrangements. 2. Results and Discussion In previous work, we have found that the main obstacle to developing this area is the low yielding synthesis of unsubstituted tris(2-pyridyl) ligands from 2-lithio-pyridine and RSiCl3. As a result, the only coordination compound obtained so far in this area has been [MeSi(2-py)3LiCl], which was isolated in low yield (Scheme 1a).10c, However, a clue to a way around this problem is provided by an early study which reported that the 6-bromo-substituted Si(IV) ligand [MeSi(6- Br-2-py)3] can be obtained in 54% yield from the reaction of MeSiCl3 and lithiated 2,6-dibromo-pyridine.15a We recently showed that 6-methyl substitution at the pyridyl substituents not only leads to cleaner lithiation of the corresponding 2- bromo-pyridine, but also stabilises the tris(2-pyridyl) products themselves by suppressing the reductive elimination of bipyridine. Thus, for example, whereas in situ reaction of unsubstituted 2-lithio-py with BiCl3 or SbCl3 could not be used to prepare the Bi(2-py)3 or Sb(2-py)3 ligands, the reaction involving 6-Me-2-py gave Bi(6-Me-2-py)3 and Sb(6-Me-2-py)3 in good yields.12 In the current study, we were able to obtain the new Si(IV) ligand [PhSi(6-Me-2-py)3] ( 1 ) in high yield (82%) from the reaction of PhSiCl3 with 6-Me-2-Li-py in thf (Scheme 1b), providing gram-quantities for the further investigation of coordination chemistry. In contrast, the reaction between unmethylated 2-lithio-pyrdine and PhSiCl3 yields a mixture of products, as is apparent from the 1H NMR spectrum of the crude reaction mixture. In addition, in the case of 1 we were able to remove all LiCl/LiBr byproducts by extraction of the crude reaction mixture with toluene (the previously reported complex [MeSi(2-py)3]LiCl0.8Br0.2 being crystallized from thf).10c Elemental analysis confirms that unlike the previously reported Si(IV) system, no lithium halide coordination occurs for 1 in crystalline samples. This is further confirmed by the single-crystal X-ray structure, which shows a C3-symmetric molecular arrangement in the solid state in which the pyridyl- N atoms are orientated towards the Si(IV) bridgehead atom (Figure 2a). The absence of LiX (X= Cl, Br) coordination in 1 presents a technical advantage (in addition to the high yield of the ligand) because there is no need to separate LiX from reaction products after transfer of the ligand to other metal centres. It can be noted also that although 1 is prepared under inert-atmosphere conditions, it is in fact air-stable and only hygroscopic. Molecules of 1 are paired up in the lattice, forming a ‘sextuple embrace’ in which the three pyridyl rings of each molecule are interdigitate (Figure 2b).16 Scheme 1 a) Synthesis of the previous ligand [MeSi(2-py) 3 ], as the LiX complex (X = 0.2Br/0.8Cl) and b) the closely related synthesis of [PhSi(6-Me-2-py) 3 ] ligand ( 1 ). a) b) Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3 Please do not adjust margins Please do not adjust margins Figure 2 a) Molecular structure of the ligand 1 showing displacement ellipsoids at 50% probability, with H atoms omitted; b) the ‘sextuple embrace’ of two molecules in the lattice. Selected bond lengths (Å) and angles ( o ): Si–C phenyl 1.879(2), Si–C pyridyl range 1.882(2)-1.886(2), C pyridyl -Si-C pyridyl range 107.25(8)- 110.18(8), Si-C pyridyl -N range 114.15(13)-114.61(14). Colour key, Si (yellow), nitrogen (blue). Coordination studies of 1 with a range of transition metal salts and organometallics were undertaken. The overall results of these studies are shown in Scheme 2. Scheme 2 The coordination of ligand 1 to various transition metals in the current study. Yields of crystalline complexes 2, 31%; 3, 70%; 4, 65%; 5, 21%. The new complexes [{PhSi(6-Me-2-py)3}CuCH3CN]PF6 ( 2 ), [{PhSi(6-Me-2-py)3}CoCl2] ( 3 ), [{PhSi(6-Me-2-py)3}FeCl2] ( 4 ) and [{PhSi(6-Me-2-py)3}Mo(CO)3] ( 5 ) were characterised by elemental (C, H, N) analysis and (where appropriate) 1H and 13C NMR spectroscopy. The single-crystal X-ray structures were also obtained (see Table 1, ESI). The reaction of [Cu(CH3CN)4]PF6 with 1 in CH3CN at room temperature gives the crystalline complex [{PhSi(6-Me-2- py)3}CuCH3CN]PF6 ( 2 ) in 31% isolated yield. The C3-symmetric coordination of CuI is apparent from the changes in the chemical shifts of the 1H and 13C NMR spectra for 2 compared to 1 and the presence of only one 6-Me-2-py environment. The single-crystal X-ray structure of 2 shows that it is an ionseparated complex containing [{PhSi(6-Me-2-py)3}CuCH3CN]+ cations (Figure 3) and PF6  anions. The tris-coordination of all three of the N-atoms of the pyridyl rings to various metal centres is common for the tris-pyridyl family of ligands and the previously reported ion-separated complexes [{RC(6-Me-2- py)3}CuCH3CN]PF6 (R = H, Me), containing C-bridged pyridyl ligands, are closely related to 2 (containing C-bridged analogues of ligand 1 , with the same 6-Me-2-py substituents).17 There are noticeable effects in changing the bridgehead atom from the smaller C- to the larger Si-atom. In particular, the Cu-Npyridyl bonds in the cation of 1 [2.061(4)- 2.121(4) Å] are on average longer than those in the [{RC(6-Me- 2-py)3}CuCH3CN]+ cations [1.993(8)-2.088(7) Å].16 There is also a large expansion of the internal Npyridyl-Cu-Npyridyl coordination angles from 90.23(9)-91.8(3)o in the [{RC(6-Me-2- py)3}CuCH3CN]+ cations16 to 97.65(16)-100.65(16)o in the cation of 1 . Both of these changes can be traced to the increase in the bridgehead Si-C bond length in the Si analogue. Figure 3 Structure of the [{PhSi(6-Me-2-py) 3 }Cu CH3CN ] + cation of 2 showing displacement ellipsoids at 50% probability, with H atoms omitted. Selected bond lengths (Å) and angles (o): Si–Cphenyl 1.871(5), Si–Cpyridyl range 1.888(5)-1.892(5), Cu- Npyridyl range 2.061(4)-2.121(4), Cu-NMeCN 1.936(4), Cpyridyl-Si-Cpyridyl range 104.0(2)- 110.0(2), Si-Cpyridyl-N range 114.0(3)-117.4(3), Npyridyl-Cu-Npyridyl 97.65(16)-100.65(16). Colour key, Cu (red), Si (yellow), N (blue). Interestingly, the 1 : 1 stoichiometric reaction of 1 with CuIICl2 in CH3CN initially forms a green solution characteristic of CuII before slowly turning yellow at room temperature. The yellow all-CuI complex [{PhSi(6-Me-2-py)3}CuICH3CN]+CuICl2- is the only solid product that could be isolated (Scheme 3). Since this was obtained in low yield it was only characterised by single-crystal X-ray crystallography (see ESI). The reduction of CuII in the presence of metal or semi-metal bridged tris-pyridyl ligand frameworks has been seen before and is probably coupled to the reductive elimination of 6,6’-di-methyl- bipyridine, although the precise mechanism involved is not certain. For example, the attempted coordination of the SnIV tris-pyridyl ligand [nBuSn(2-py)3] to CuII results in a CuI complex.10a This redox instability contrasts with C-bridged trispyridyl ligands which can be transferred intact to the Cu2+ cation without breakdown of the ligand framework,17 e.g., as in the case of the complex [{HC(6-Me-2-py)3}CuBr2].18 The 1 : 1 stoichiometric reactions of CoCl2 and FeCl2 with 1 give clean transfer of the transition metal ions into the corresponding complexes [{PhSi(6-Me-2-py)3}CoCl2] ( 3 ) (70%) and [{PhSi(6-Me-2-py)3}FeCl2] ( 4 ) (65%), respectively. ARTICLE Journal Name 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Scheme 3 Formation of the all-Cu I complex from 1 and Cu II Cl 2 . For the structure of the complex see the ESI. Satisfactory elemental analyses were obtained for both compounds. However, room-temperature 1H spectroscopy proved unhelpful in their characterisation owing to the paramagnetic nature of the high-spin d6 and d7 electronic configurations of the transition metal ions and the presence of a fluxional process (described later). Unambiguous characterisation was made by single-crystal X-ray diffraction. Both complexes are isostructural (Figure 4), consisting of molecules in which the tris-pyridyl ligand 1 adopts a bidentate coordination mode in which only two of the three N-atoms are bonded to the transition metal ions. Figure 4 Molecular structure of [{PhSi(6-Me-2-py) 3 }CoCl 2 ] ( 3 ); [{PhSi(6-Me-2- py) 3 }FeCl 2 ] ( 4 ) is isostructural in the solid state. Displacement ellipsoids are shown at 50% probability and H atoms are omitted. Selected bond lengths (Å) and angles (o): 3, Si–Cphenyl 1.866(2), Si–Cpyridyl range 1.872(2)-1.885(2), Co-Npyridyl range 2.0505(18)-2.0589(19), Co-Cl 2.2339(7)-2.2623(6), Cpyridyl-Si-Cpyridyl range 105.67(11)- 115.78(10), Si-Cpyridyl-N range 119.81(17)-122.80(16) (coordinating pyridyl groups), Si- Cpyridyl-N 113.26(17) (non-coordinating pyridyl group), N-Co-N 110.47(7), Cl-Co-Cl 118.50(3). 4, Si–Cphenyl 1.866(3), Si–Cpyridyl range 1.874(3)-1.884(3), Fe-Npyridyl range 2.106(2)-2.118(2), Fe-Cl range 2.2486(8)-2.2885(7), Cpyridyl-Si-Cpyridyl range 105.60(12)- 117.09(10), Si-Cpyridyl-N range 119.90(18)-122.11(17) (coordinating pyridyl groups), Si- Cpyridyl-N 113.57(18) (non-coordinating pyridyl group), N-Fe-N 108.88(8), Cl-Fe-Cl 124.58(3). Colour key, Si (yellow), N (blue), Co (light blue), Cl (green). There are no closely related analogues of 3 and 4 containing C-bridged tris-pyridyl ligands, the closest relatives being the 2 : 1 ‘sandwich’ complexes [{MeC(2-py)3}2M]2+ (M = CoII,FeII).19 However, the bis-coordination of two of the N- atoms of ligand 1 found in both complexes is the same as that observed for the isoelectronic P(6-Me-2-py)3 ligand in the complex [{P(6-Me-2-py)3}FeCl2], which has a very similar structural arrangement.20 The reasons behind this biscoordination of the metal cations in 3 and 4 , rather than triscoordination in the potential alternative ionisation isomer [{P(6-Me-2-py)3}FeCl]+[Cl]-, are likely to stem from the combined effects of the presence of sterically constraining 6- Me substituents (which also make the formation of 2 : 1 sandwich complexes unfavourable with small metal cations) and the greater strength of M-Cl bonds compared to M-N bonds. The metal-N bond lengths in 3 and 4 are as expected for tetrahedral Co2+ and Fe2+ cations.21 As noted before, the 1H NMR spectra of 3 and 4 at 298K were uninformative due to unusually broad resonances, even for paramagnetic complexes of CoII and FeII. However, reducing the temperature of solutions of 3 and 4 in toluene results in significant sharpening of the 1H resonances for both compounds. The sharpening of the spectra strongly indicates that a dynamic (fluxional) process is occurring. In the case of 3 , gradual sharpening of the signals is observed between 353 and 253K to show eventually the same 2 : 1 desymmetrization of the 6-Me-2-py substituents that is present in the solid-state structure. At the same time, the line-widths of the Phresonances remain unchanged in this temperature interval, showing that fluctionality of the pyridyl groups alone is responsible for the observed effect. Additional information is obtained from the temperature-dependent behaviour of 4 which shows similar behaviour as 3 between 230 and 298K. However, increasing the temperature above 298K also results in sharpening of the 6-Me- and 2-py resonances into a C3- symmetric arrangement containing only one 6-Me-2-py environment. This behaviour is consistent with an intramolecular fluxional process involving precession of the bis-coordinating pyridyl groups in both complexes (Scheme 4). Scheme 4 Precession of the pyridyl groups, as a potential explanation for the fluxional behaviour of 3 and 4 (M = Co, Fe). The low-temperature (230 K) 1H NMR signals of the coordinated pyridyl groups in 3 and 4 can be assigned using DFT calculated spin-densities.22 However, the correlation of the calculated NMR shifts using Fermi-contact and orbital shifts alone is not satisfactory (Figure S9, ESI). EXSY (2-D Exchange) NMR spectroscopy allows the assignment of H2 of non-coordinated pyridyl at +12 ppm (exchange peak with H2c at  –28 ppm). This atom is seven bonds away from the paramagnetic centre and therefore no Fermi-contact shift contributes to the observed NMR value, only orbital and pseudocontact shifts. Using the molecular structure and a magnetic axis that bisects the N-Fe-N angle gives an axial magnetic anisotropy (  ax) of 7 x 10-32m3 for 4 (see Figure S11, ESI). In a similar way  ax was determined for 3 (8 x 10-32m3). Including both Fermi-contact and pseudo-contact shifts leads to a much better agreement with the experimental 1H NMR spectrum (Figure 5). The anisotropy of 4 compares well with a known 4-coordinate Fe2+ complex 23 whereas in octahedral Co2+ compounds much larger anisotropies compared to 3 have been observed.24 Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5 Please do not adjust margins Please do not adjust margins Figure 5 Correlation of experimental with calculated 1 H NMR shifts of 3 and 4 (at 230 K), considering orbital, Fermi-contact and pseudo-contact shifts. H xc and H xnc denotes atoms of coordinated and non-coordinated pyridyl units, respectively.For the numbering scheme see Figure 7 The 1 : 1 stoichiometric reaction of Mo(CO)6 with 1 in MeCN at reflux produces the complex [{PhSi(6-Me-2- py)3}Mo(CO)3] ( 5 ) in 21% yield after crystallisation from CH2Cl2 (as the CH2Cl2 mono-solvate). As in the case of diamagnetic 1 , significant changes in the chemical shifts in the 1H and 13C NMR spectra provided an initial indication of the formation of the complex. The C3-symmetric, tridentate coordination mode of the [PhSi(6-Me-2-py)3] is also obvious from the presence of only one 2-py environment in the 1H NMR spectrum. The solidstate structure of 5 shows the expected tris-coordination of 1 to a Mo(CO)3 unit within its molecular arrangement (Figure 6). This is similar to that of the previously reported complex [{nBuSn(2-py)3}Mo(CO)3], containing a Sn-bridged 2-pyridyl ligand.10b The IR spectrum of solid 5 shows two CO stretching bands at 1893 and 1750 cm-1. This can be compared to the solid-state IR spectra reported previously for [{HC(2-py)3}Mo(CO)3] (1897 and 1769 cm-1)25 and [{nBuSn(2-py)3}Mo(CO)3] (1900 and (averaged) 1763 cm-1).10b Unfortunately it is impossible on this basis to delineate the effect of the electronegativity of the bridgehead atom from the additional effect of the electrondonating 6-Me groups present in 5 , since the decrease in electronegativity going down Group 14 and the electrondonating Me substituents should both result in greater  - donor character. This said, it appears that 5 has similar  - donor/  -acceptor properties to the [HC(2-py)3] and [nBuSn(2- py)3] ligands. Figure 6 Molecular structure of [{PhSi(6-Me-2-py) 3 }Mo(CO) 3 ] ( 5 ) in the solvate 5 .CH 2 Cl 2 , showing displacement ellipsoids at 50% probability. H-atoms and the CH 2 Cl 2 lattice solvent are omitted. Selected bond lengths (Å) and angles (o): Si–Cphenyl 1.876(3), Si–Cpyridyl range 1.870(3)-1.874(3), Mo-Npyridyl range 2.343(2)-2.424(2), Mo-C range 1.921(3)-1.933(3), Cpyridyl-Si-Cpyridyl range 103.96(13)-112.88(13), Si-Cpyridyl-N range 115.9(2)-120.4(2), Npyridyl-Mo-Npyridyl 86.87(8)-90.05(8), C-Mo-C range 79.91(13)- 86.28(13). Colour key, Mo (green), Si (yellow), N (blue), O (red). Conclusions In conclusion, substitution at the 6-position of the pyridyl ring units makes the synthesis of the resulting [PhSi(6-Me-2-py)] ligand much more amenable and produces usable amounts for further coordination studies. This has allowed access to the first examples of transition metal complexes of this type of SiIV tris-pyridyl ligand. Synthetic studies show that, while these ligands function similarly to their C-bridged relatives, they nonetheless exhibit some redox instability, depending on the metal coordinated; a characteristic of related SnIV tris-pyridyl ligands. This study forms part of our on-going work designed to investigate how changing the main group bridgehead atoms can be used to modulate the reactivity of tris-pyridyl and related tripodal ligand arrangements, in particular. Future studies will explore the applications of ligands of this type in a range of catalytic reactions. Acknowledgements We thank The Leverhulme Trust (Grant for DSW and RG-R, postdoctoral funding for ALC), The Spanish MINECO-AEI and The EU (ESF) for a Ramon y Cajal contract (RG-R, RYC-2015- 19035) and The Cambridge Trust (Vice Chancellor Scholarship for AJP). Conflicts of Interest There are no conflicts of interest. H4c H2nc H1c H2c H3c H7 H4c H1c H2c H3c H7 R² = 0,9654 R² = 0,9912 -30 030 60 90 -30-101030507090  calc [ppm]  exp [ppm] Fe Co ARTICLE Journal Name 6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Experimental Section General Experimental Techniques All experiments were carried out on a Schlenk-line under a nitrogen atmosphere or with the aid of a N2-filled glove box (Saffron type α). MeCN and CH2Cl2 were dried over CaH2. THf and toluene were distilled over Na/benzophenone. 6-Methyl- 2-bromo-pyridine was distilled over CaH2 and stored over 4 Å molecular sieves. PhSiCl3 was acquired from Aldrich Chemical Company and distilled prior to use. 1H, 13C {1 H} and 31P{1H} NMR spectra were recorded on a Bruker Avance 400 QNP or Bruker Avance 500 MHz cryo spectrometer. All spectra were recorded in d8-toluene or CD3CN with SiMe4 (1H) or the toluene Me-peak as external and internal standards. Unambiguous assignments of NMR resonances were made on the basis of 2D NMR experiments (1H-1H COSY, 1H-1H NOESY, 1H-13C HMQC and 1H-13C HMBC). Figure 7 shows the labelling scheme for NMR assignments used throughout the Experimental Section. Elemental analysis was obtained using a Perkin Elmer 240 Elemental Analyser. Figure 7 Showing the labelling scheme used for NMR spectra in the following characterisation and in Figure 5 in the main text. X-ray Crystallographic Studies. Data were collected for 1 , 2 , 3 , 4 and 5 on a Bruker D8 QUEST Photon-100 diffractometer with an Incoatec I  S Cu microfocus source. The temperature was held at 180(2) K using an Oxford Cryosystems N2 cryostat. Crystals were mounted directly from solution using perfluorohydrocarbon oil to prevent atmospheric oxidation, hydrolysis and solvent loss. Further details of data collection and refinements can be found in the ESI (Table S1). CCDC: 1833561 ( 1 ), 1833563 ( 2 ), 1833559 ( 3 ), 1833560 ( 4 ), 1833562 ( 5. CH2Cl2), 1833558 ([{PhSi(6-Me-2- py)3}CuICH3CN]+CuICl2-). Synthesis of compounds 1–5. Synthesis of 1: 2-Bromo-6-methyl pyridine (2.28 ml, 20 mmol) was dissolved in 40 ml of thf. To this nBuLi (12.5 ml, 20 mmol, 1.6 M in hexanes) was added dropwise at −78 °C. The resulting dark orange solution was stirred for 3 h at −78 °C. PhSiCl3 (1.4 g, 6.66 mmol) in 5 ml of thf was added dropwise to the dark red lithiated species. The resulting pale brown mixture was allowed to warm to room temperature. After stirring overnight, a dark brown solution with a light brown precipitate was formed. All volatiles were removed under vacuum and the resulting solid residue was extracted with 40 ml of warm toluene. The suspension was filtered through Celite to yield a clear-brown solution which was concentrated under vacuum until the precipitation of a white solid was observed, which was redissolved by gentle heating. Storage overnight at -15 °C yielded 1 as colourless needles suitable for X-ray crystallography. The product was isolated by filtration and storage of the mother liquor yielded a second crop of crystals. Combined yield 2.10 g (4.61 mmol, 82%). Elemental analysis (%) calcd. for 1: C 75.5, H 6.1, N 11.0 found: C 74.5, H 5.9, N 10.5. 1H NMR (25°C, d8-toluene, 500.12 MHz): δ(ppm) = 8.20 (d, J = 7.0 Hz, 2H, H-5), 7.86 (d, J = 7.4 Hz, 3H, H-1), 7.24 (m, 3H, H-6, H-7), 7.11 (t, J = 7.7 Hz, 3H,H-2), 6.68 (d, J = 7.8 Hz, 3H, H-3), 2.40 (s, 9H, H-4). 13C{1H} NMR (25 °C, d8–toluene, 125.78 MHz): δ(ppm) = 162.7 (C-1), 157.9 (C-2) , 136.7 (C-4), 134.3 (C-7), 133.6 (C-3), 129.8 (C-9), 129.0 (C-5) , 127.3 (C-8), 122.2 (C-10), 24.0 (C-6). Synthesis of 2 , 3 and 4 : A Schlenk tube was charged with (6-Me- py)3SiPh (300 mg, 0.78 mmol, 1 eq.) and 1 equivalent of the transition metal source [Cu(MeCN)4PF6 (314 mg, 0.78 mmol) for 2, CoCl2 (100mg, 0.78 mmol, 1 eq) for 3, (100 mg, 0.78 mmol) for 4 in a glove-box. The Schlenk tube was transferred to a vacuum line and 25 ml of CH3CN was added. The resulting solution (yellow for 2, green for 4 and 5) was stirred at room temperature overnight and then concentrated under vacuum until the precipitation of a solid was observed (yellow for 2 and 4, blue for 3). The solid was redissolved by gentle heating. Storage overnight at -15 °C yielded crystals of 2 (yellow), 3 (blue) or 4 (yellow) suitable for X-ray crystallography which were isolated by filtration. For 2: Yield 152 mg (0.24 mmol, 31 %). Elemental analysis (%) calcd. for 2: C 49.5, H 4.2, H 8.9, found: C 49.2, H 4.2, N 9.1. 1H NMR (25 °C, CD3CN, 500.12 MHz): δ(ppm) = 8.08 (d, J = 6.9 Hz, 2H, H-5), 7.78 (t, J = 7.5 Hz, 1H, H-7), 7.72 (m, 5H, H-6, H-2), 7.60 (d, J = 7.5 Hz, 3H, H-1), 7.40 (d, J = 7.8 Hz, 3H, H-3), 2.83 (s, 9H, H-4), 1.99 (s, 3H, Acetonitrile). 31P NMR (25 °C, CD3CN, 202.48 MHz): δ(ppm) = -144.6 (hpt, 1JPF = 705.69 Hz, PF6). 13C{1H} NMR (25 °C, CD3CN, 125.78 MHz): δ(ppm) = 160.2 (C-1), 158.6 (C-2) , 148.9 (C-4), 136.7 (C-7), 136.1 (C-3), 131.6 (C-9), 130.2 (C-5) , 128.9 (C-8), 125.3 (C-10), 24.7 (C-6). For 3: Yield 210 mg (0.55 mmol, 70%). Elemental analysis (%) calcd. for 2: C 49.5, H 4.2, H 8.9, found: C 49.2, H 4.2, N 9.1. For 4: Yield 195mg (0.50 mmol, 65%). Elemental analysis (%) calcd. for 4: C 56.7, H 4.5, N 8.3, found C 55.3, H 4.5, N 8.8. For details of the paramagnetic 1H NMR studies of 3 and 4, see the ESI. Synthesis of 5; A Schlenk tube was charged with PhSi(6-Me-py)3 (500 mg, 1.30 mmol) and Mo(CO)6 (343 mg, 1.30 mmol, 1 eq) inside a N2-filled glove box. The Schlenk tube was transferred to a vacuum line and 25 ml of CH3CN was added. The resulting solution was brought to reflux overnight during which the colour changed to brown/red. The solvent was removed and the solid brown residue was dissolved in 25 ml of dichloromethane and stirred for 2 hours. The solution was concentrated under vacuum until the precipitation of a red solid was observed, which was redissolved by gentle heating. Storage overnight at -15°C yielded yellow crystals of 5•CH2Cl2 suitable for X-ray crystallography which were isolated by filtration. Isolation of this solvate under vacuum (1 bar) results in loss of ca. 0.5 CH2Cl2 molecules per molecular unit, to give a final product with the formula 5•0.5CH2Cl2. Yield 150mg (0.27 mmol, 21%). Elemental analysis (%) calcd. for 5•0.5CH2Cl2 C54.7, H 4.0, N 7.0; found, C 54.9, H 4.1, N 8.0. IR (Solid), CO stretch: 1893 cm-1 and 1750 cm-1. 1H NMR (25 °C, 500 MHz, CDCl3), δ[ppm] = 8.06 (d, J = 6.8 Hz, 2H, H-5), 7.72 (m, 3H, H-6, H-7), 7.50 (m, 6H, H-1, H-2), 7.27 (d, J = 8.9 Hz, 3H, H-3), 3.34 (s, 9H, H-4) (CH2Cl2 also present at 5.3 (s)). 13C NMR (25 °C, 125.78 MHz, CDCl3), δ[ppm] = 228.16 (CO), 166.03 (C-1), 159.33 (C-2) , 137.01 (C-4), 134.35 (C-7), 131.56 (C-3), 130.13 (C-9), 129.21 (C-5), 128.44 (C-8), 126.24 (C-10), 29.04 (C-6). References 1. P. Hofmann, L. H. Gade, Molecular Catalysis: Structure and Functional Design, WILEY VCH Verlag, Weinheim Germany, 2014. 2. C. Moberg, Angew. Chemie Int. Ed., 1998, 37, 248. 3. C. Vogel, F. W. Heinemann, J. Sutter, C. Anthon, K. Meyer, Angew. Chemie Int. Ed., 2008, 47, 2681. 4. C. Dro, S. Bellemin-Laponnaz, R. Welter, L. H. Gade, Angew. Chemie Int. Ed., 2004, 43, 4479. 5. N. L. Lampland, M. Hovey, D. Mukherjee, A. D. 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