Molecular Structures of the Silicon Pyridine-2-(thi)olates Me 3 Si(pyX), Me 2 Si(pyX) 2 and Ph 2 Si(pyX) 2 (py = 2-Pyridyl, X = O, S), and Their Intra- and Intermolecular Ligand Exchange in Solution

: A series of pyridine-2-olates (pyO) and pyridine-2-thiolates (pyS) of silicon was studied in solid state and in solution. The crystal structures of Me 3 Si(pyO) ( 1a ), Me 3 Si(pyS) ( 1b ), Me 2 Si(pyO) 2 ( 2a ), Me 2 Si(pyS) 2 ( 2b ), Ph 2 Si(pyO) 2 ( 3a ) and Ph 2 Si(pyS) 2 ( 3b ) were determined by X-ray diffraction. For that purpose, crystals of the (at room temperature) liquid compounds 1a and 1b were grown in a capillary on the diffractometer. Compounds 1a , 1b , 2a , 2b and 3a feature tetracoordinate silicon atoms in the solid state, whereas 3b gave rise to a series of four crystal structures in which the Si atoms of this compound are hexacoordinate. Two isomers ( 3b 1 with all- cis arrangement of the C 2 N 2 S 2 donor atoms in P 1, and 3b 2 with trans S-Si-S axis in P 2 1 / n ) formed individual crystal batches, which allowed for their individual 29 Si NMR spectroscopic study in the solid state (the determination of their chemical shift anisotropy tensors). Furthermore, the structures of a less stable modiﬁcation of 3b 2 (in C 2/ c ) as well as a toluene solvate 3b 2 (toluene) (in P 1) were determined. In CDCl 3 , the equimolar solutions of the corresponding pairs of pyO and pyS compounds ( 2a / 2b and 3a / 3b ) showed substituent scrambling with the formation of the products Me 2 Si(pyO)(pyS) ( 2c ) and Ph 2 Si(pyO)(pyS) ( 3c ), respectively, as minor components in the respective substituent exchange equilibrium.


General Considerations
Starting materials 2-hydroxypyridine and 2-mercaptopyridine (ABCR, Karlsruhe, Germany, 98%) were used as received without further purification. Dichlorodimethylsilane and dichlorodiphenylsilane were distilled prior to use. Triethylamine was distilled from sodium/benzophenone and stored under argon atmosphere. THF, toluene and n-hexane were distilled from sodium/benzophenone prior to use. CDCl 3 (Deutero, Kastellaun, Germany, 99.8%), toluene-d 8 (ARMAR, Leipzig, Germany, 99.5%) and n-pentane (Th.Geyer, Renningen, Germany, >99%) were stored over activated molecular sieves (3 Å) for at least 7 days and used without further purification. All reactions were carried out under an atmosphere of dry argon utilizing standard Schlenk techniques. Solution NMR spectra ( 1 H, 13 C, 29 Si) (cf. Figures S1-S12 in the supporting information) were recorded on Bruker Avance III 500 MHz and Bruker Nanobay 400 MHz spectrometers. 1 H, 13 C and 29 Si chemical shifts are referenced to Me 4 Si (0 ppm) as internal reference. 1 H and 13 C NMR signals were assigned in accord with mutual coupling patterns (in case of 1 H) and according to the shifts of corresponding 1 H or 13 C NMR signals in related compounds MeSi(pyO) 3 [10], PhP(pyO) 2 [21], P(pyS) 3 , PhSb(pyS) 2 and Ph 2 Sb(pyS) [22]. The 29 Si (CP/MAS) NMR spectra of compounds 3b 1 and 3b 2 (modification 1) (cf. Figures S13 and S14 in the supporting information) were recorded on a Bruker Avance 400 WB spectrometer using 7 mm zirconia (ZrO 2 ) rotors with KelF-inserts at an MAS frequency of υ spin = 900 Hz. The determination of the chemical shift anisotropy (CSA) tensor principal components from the spinning sideband spectra was carried out with the SOLA module contained in the Bruker software package TOPSPIN. Principal components δ 11 , δ 22 and δ 33 as well as span Ω and skew κ are reported according to the Herzfeld-Berger notation [23,24]. Elemental analyses were performed on an Elementar Vario MICRO cube. For single-crystal X-ray diffraction analyses, crystals were selected under an inert oil and mounted on a glass capillary (which was coated with silicone grease). In case of the (at room temperature) liquid compounds 1a and 1b, we adapted the method we had previously applied for the crystal structure determination of some liquid chlorosilanes [25]: Some drops of the compound were transferred (via syringe) into a glass capillary (diameter 1.0 mm), which was then flame sealed. This glass capillary was then mounted on a magnetic sample holder for Stoe goniometer heads and then placed under a stream of cold nitrogen at a fixed kappa angle of 45 • . The sample was cooled until crystallization commenced, whereupon the sample was heated until its melting point to leave a tiny spot of crystalline material behind in the coldest part of the capillary. Thereafter the sample was slowly cooled to allow for the slow growth of larger singlecrystalline domains. One of the domains adjacent to the initial crystallite was then moved towards the center of the cold stream, and heating/melting followed by slow cooling was repeated. After these two cycles the domains adjacent to the seed crystal were of sufficient quality for single-crystal X-ray diffraction. Thus, the upper domain was centered and used for data collection. Diffraction data were collected on a Stoe IPDS-2/2T diffractometer (STOE, Darmstadt, Germany) using Mo Kα-radiation. Data integration and absorption correction were performed with the STOE software XArea and XShape, respectively. The structures were solved by direct methods using SHELXS-97 or SHELXT and refined with the full-matrix least-squares methods of F 2 against all reflections with SHELXL-2014/7 or SHELXL-2018/3 [26][27][28][29][30]. All non-hydrogen atoms were anisotropically refined. Hydrogen atoms were isotropically refined in idealized position (riding model). For the details of data collection and refinement (including the use of SQUEEZE in the refinement of the structure of 3b 2 (toluene) see Appendix A. For the phase analyses of compounds 3b 1 and 3b 2 via X-ray powder diffraction at room temperature, a large single crystal of the respective compound (ca. 1.2 mm diameter) was mounted on a glass capillary and was embedded in a drop of epoxy resin. Upon unit cell determination (using some single-crystal X-ray diffraction frames), the powder diffraction diagram of this compound was simulated using a Gandolfi-scan (180 • omega scan in combination with 6000 phi-rotations). For comparison, powdered bulk material of 3b 1 and 3b 2 (the samples which were used for solid state 29 Si NMR investigations) was transferred into glass capillaries (diameter 1.0 mm), and their X-ray powder diffraction diagrams were recorded using the same Gandolfi-scan parameters on the same instrument (cf. Figure S17 in the supporting information). Powder diffraction data were converted using PowDLL converter [31]. The graphics of molecular structures were generated with ORTEP-3 [32,33] and POV-Ray 3.7 [34]. CCDC 2181113 (3a), 2181114 (2a), 2181115 (3b 2 (toluene)), 2181116 (1a), 2181117 (1b), 2181118 (2b), 2181119 (3b 2 (modif. 1)), 2181120 (3b 2 (modif. 2)) and 2181121 (3b 1 ) contain the supplementary crystal data for this article. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/ (accessed on 22 June 2022).
The geometry optimizations were carried out with ORCA 5.0.3 [35] using the restricted PBE0 functional with a relativistically recontracted Karlsruhe basis set ZORA-def2-TZVPP [36,37] for all atoms, scalar relativistic ZORA Hamiltonian [38,39], atom-pairwise dispersion correction with the Becke-Johnson damping scheme (D3BJ) [40,41] and COSMO solvation (chloroform). Calculations were started from the molecular structures obtained by single-crystal X-ray diffraction analysis and isomers were generated therefrom. Numerical frequency calculations were performed to prove convergence at the local minimum after geometry optimization and to obtain the Gibbs free energy (293.15 K). Relaxed energy scans were performed in 10 • steps by an increase of the Si-X-C 2 -N (X = O, S) torsion angle in the range from 0-180 • . A transition state search was performed with nudged elastic band calculations with 11 steps on 1a. A numerical frequency calculation from the structure with the highest total energy level did confirm the transition state was reached. The transition state search for 1b was started from this geometry by changing oxygen to sulphur. A numerical frequency calculation was performed on this geometry and the result was used as initial Hessian for the transition state search. A numerical frequency calculation was performed on the final structure confirming the transition state was reached. Single-point calculations were performed with restricted B2T-PLYP functional [42] with a relativistically recontracted Karlsruhe basis set ZORA-def2-TZVPP [36,37] for all atoms and utilizing the AutoAux generation procedure [43], scalar relativistic ZORA Hamiltonian [38,39] and COSMO solvation (chloroform).
The H-atom positions of 3b 1 and 3b 2 (of the molecular structures obtained by singlecrystal X-ray diffraction analyses) were optimized with ORCA 5.0.3. The geometry of tetrametylsilane (TMS) was optimized as described above. NMR chemical shifts were calculated with ADF-2019.303 [44] and referenced to TMS using an all-electron basis set TZ2P for all atoms [45] and scalar relativistic ZORA Hamiltonian [38,39]. As functionals PBE and PBE0 were used. Graphics were generated using Chemcraft [46].

Syntheses and Characterization
Samples of the trimethylsilyl derivatives of 2-hydroxypyridine (1a [47]) and of 2-mercaptopyridine (1b [48]) were prepared according to literature protocols and were available from previous investigations in our group.
Compound 2a (Me 2 Si(pyO) 2 , C 12 H 14 N 2 O 2 Si). A Schlenk flask was charged with magnetic stirring bar and 2-hydroxypyridine (3.00 g, 31.5 mmol), then evacuated and set under Ar atmosphere prior to adding THF (50 mL) and triethylamine (3.78 g, 37.4 mmol). The resultant mixture was stirred at room temperature, and dichlorodimethylsilane (2.06 g, 15.9 mmol) was added dropwise via syringe through a septum. Upon the completed addition of silane, stirring was continued for 30 min, whereupon the mixture was stored at 5 • C overnight. Thereafter, the triethylamine hydrochloride precipitate was removed by filtration and washed with THF (15 mL). From the combined filtrate and washings, the solvent was removed under reduced pressure (condensation into a cold trap) to afford a colorless oil. This crude product was dissolved into THF (4 mL), then n-pentane (4 mL) was added, and this solution was stored at 5 • C for 2 h. Within this time, some residual triethylamine hydrochloride precipitated, which was removed by filtration of the cold solution. From the clear filtrate, the volatiles were removed under reduced pressure, and the clear colorless oily residue was then dissolved into n-pentane (5 mL) and stored at −24 • C for one week to afford colorless crystals of 2a. The Schlenk flask was then stored in an ice/ethanol bath (ca. −10 • C), and from the solid product the supernatant solution was removed by decantation, followed by washing with n-pentane (2 × 2 mL) and drying in vacuum. Yield: 2.60 g (10.55 mmol, 67%). Elemental analysis for C 12  Compound 2b (Me 2 Si(pyS) 2 , C 12 H 14 N 2 S 2 Si). A Schlenk flask was charged with magnetic stirring bar and 2-mercaptopyridine (4.10 g, 36.8 mmol), then evacuated and set under Ar atmosphere prior to adding THF (50 mL) and triethylamine (4.22 g, 41.6 mmol). The resultant mixture was stirred at room temperature, and dichlorodimethylsilane (2.44 g, 19.0 mmol) was added dropwise via syringe through a septum. Upon completed addition of silane, stirring was continued for 30 min, whereupon the mixture was stored at 5 • C overnight. Thereafter, the triethylamine hydrochloride precipitate was removed by filtration and washed with THF (20 mL). From the combined filtrate and washings, the solvent was removed under reduced pressure (condensation into a cold trap) to afford an orange oil. This crude product was dissolved into THF (4 mL), then n-pentane (4 mL) was added, and this solution was stored at 5 • C for 2 h. Within this time, some residual triethylamine hydrochloride precipitated, which was removed by filtration of the cold solution. From the clear filtrate, the volatiles were removed under reduced pressure, and the clear orange oily residue was then dissolved into toluene (1 mL) and n-pentane (4 mL) and stored at −24 • C for one week to afford orange crystals of 2b. The Schlenk flask was then stored in an ice/ethanol bath (ca. −10 • C), and from the solid product the supernatant solution was removed by decantation, followed by washing with n-pentane (2 × 2 mL) and drying in vacuum. Yield: 3.95 g (14.2 mmol, 77%). Elemental analysis for C 12  Compound 3a (Ph 2 Si(pyO) 2 , C 22 H 18 N 2 O 2 Si). A Schlenk flask was charged with magnetic stirring bar and 2-hydroxypyridine (2.50 g, 26.3 mmol), then evacuated and set under Ar atmosphere prior to adding THF (60 mL) and triethylamine (3.24 g, 32.2 mmol). The resultant mixture was stirred in an ice bath, and dichlorodiphenylsilane (3.46 g, 13.7 mmol) was added dropwise via syringe through a septum. Upon the completed addition of silane, stirring at 0 • C was continued for 60 min, whereupon the triethylamine hydrochloride precipitate was removed by filtration and washed with THF (20 mL). From the combined filtrate and washings, the solvent was removed under reduced pressure (condensation into a cold trap) to afford a white residue. This crude product was recrystallized in THF (1.5 mL). From the solid product the supernatant solution was removed by decantation, followed by washing with 8 mL of a mixture of n-pentane and THF (1:2 v/v) and drying in vacuum. Yield: 1.60 g (4.31 mmol, 31%). Elemental analysis for C 22   Compound 3b (Ph 2 Si(pyS) 2 , C 22 H 18 N 2 S 2 Si). A Schlenk flask was charged with magnetic stirring bar and 2-mercaptopyridine (3.00 g, 27.0 mmol), then evacuated and set under Ar atmosphere prior to adding THF (45 mL) and triethylamine (2.95 g, 29.2 mmol). The resultant mixture was stirred at room temperature, and dichlorodiphenylsilane (3.37 g, 13.3 mmol) was added dropwise via syringe through a septum. Upon the completed addition of silane, stirring was continued for 60 min, whereupon the suspension was stored in an ice bath for 30 min prior to filtration of the triethylamine hydrochloride precipitate. The latter was washed with THF (10 mL). From the combined filtrate and washings, the solvent was removed under reduced pressure (condensation into a cold trap) to afford a yellow residue. This crude product was recrystallized in toluene (6 mL). From the solid product the supernatant solution was removed by decantation, followed by washing with 1 mL of toluene and drying in vacuum. Yield: 4.15 g (10.3 mmol, 77%). The coarse crystalline compound thus obtained consists of isomer 3b 2 (modification 1) (this batch was employed for X-ray single-crystal and powder-diffraction analyses as well as the 29   Isomer 3b 1 was obtained following a similar protocol: A Schlenk flask was charged with magnetic stirring bar and 2-mercaptopyridine (2.00 g, 18.0 mmol), then evacuated and set under Ar atmosphere prior to adding THF (30 mL) and triethylamine (2.09 g, 20.7 mmol). The resultant mixture was stirred at room temperature, and dichlorodiphenylsilane (2.39 g, 9.40 mmol) was added dropwise via syringe through a septum. Upon the completed addition of silane, stirring was continued for 12 h, whereupon the suspension was stored at 5 • C for 4 h prior to filtration of the triethylamine hydrochloride precipitate. The latter was washed with THF (15 mL). From the combined filtrate and washings, the solvent was removed under reduced pressure (condensation into a cold trap) to afford a yellow residue. This crude product was recrystallized in toluene (3 mL). For crystallization, the solution was stored at 5 • C. To the solid product (with its supernatant) 5 mL of toluene were added, and then the solution was removed by filtration, followed by drying of the product in vacuum. Yield: 2.52 g (6.30 mmol, 70%). The product was identified as 3b 1 by single-crystal X-ray diffraction. The filtrate was heated to ca. 60 • C, and n-hexane (2 mL) was added, whereupon this clear solution was placed in a fridge (5 • C) for further crystallization. Needles of the toluene solvate 3b 2 (toluene) formed, which were suitable for single-crystal X-ray diffraction analysis. Within a few hours of storage (in the presence of the mother liquor), those crystals transformed into a fine crystalline powder.
The recrystallization of isomer 3b 2 in THF afforded crystals of 3b 1 , too. A sample of 1.2 g of 3b 2 (modification 1, as obtained from the abovementioned synthesis) was dissolved into 3.2 mL of hot THF to afford a clear yellow solution. Upon cooling to room temperature, n-pentane (2.5 mL) was added to the solution with stirring. The still clear yellow solution was placed in a fridge (5 • C), whereupon thin long plates of 3b 2 (modification 2) formed. From this batch, a crystal was taken for single-crystal X-ray diffraction analysis of 3b 2 (modification 2). Within few hours (still in the mother liquor and stored in the fridge), the thin plates 3b 2 (modification 2) transformed into a fine-crystalline powder. From this product, the supernatant was removed with a syringe, and the solid residue was again recrystallized from THF. This time, THF was slowly added to the hot solution to avoid the complete dissolution of the starting material, to retain some seed crystals. During cooling to room temperature crystal growth commenced, and the crystalline material thus obtained was identified as 3b 1 (this sample was employed for X-ray single-crystal and powder-diffraction analyses as well as the 29 Si solid state NMR analyses of 3b 1 ).

Crystallographic Analyses of 1a and 1b
Selected parameters of unit cell, data collection and structure refinement of 1a and 1b are summarized in Appendix A (Table A1). Compound 1a ( Figure 2) crystallized in the orthorhombic space group Pbca with two molecules in the asymmetric unit. Compound 1b ( Figure 2) crystallized in the monoclinic space group P2 1 /m. The asymmetric unit comprises one half of the molecule because of its location on a crystallographic bisecting plane. In both compounds the pyridine N atom approaches the Si atom from far distance (ca. 3.0 Å), which is much longer than the sum of their covalent radii (1.82 Å [49]) but still shorter than the sum of van der Waals radii (3.65 Å [50]). Thus, it can be regarded as a remote [4+1]-coordination. The effects of Si-hypercoordination on the Si coordination sphere, however, are marginal; the lengthening of the N···Si trans-bond (with respect to the other Si-C bonds in the same molecule) lies between 3σ and 6σ, and the sum of idealized equatorial bond angles with respect to the N-Si-C axis (332.7 • and 332.0 • for the two molecules in the structure of 1a, 335.5 • for 1b) still reflects the close to tetrahedral Si coordination sphere. Furthermore, the geometry parameter τ 4 [51] (as a measure for the distortion of the tetrahedral coordination sphere) is still very close to 1 (0.96 and 0.97 for the two independent molecules of 1a, 0.96 for 1b). Additionally, the C-X-Si (X = O, S) angles in compounds 1a and 1b are even wider than corresponding angles in aryl-X-SiMe 3 (11), N1-Si1-C8 150.10(9). The asymmetric unit of 1b comprises one half of the Me 3 Si(pyS) molecule, the atoms of which (except from methyl group C6) are located on a crystallographically imposed bisecting plane (symmetry operation *: x, 0.5−y, z). Selected interatomic distances (Å) and angles (deg.) of 1b: Si1-S1 2.1627(6), Si1-C6 1.8581(15), Si1-C7 1.867(2), Si1···N1 3.0454(17), Si1-S1-C1 103.13(6), N1-Si1-C7 160.21 (8).
Our crystallographic analysis of 1b revises the original assumption of an Si-N bound thiopyridone group in this compound [9]. This assumption had been founded on the 29 Si NMR shift of 1b, which was very similar to the 29 Si NMR shift of trimethylsilylmethimazole, a compound which features an Si-N bound thiourea motif [54]. For insights into the energetic differences of isomers and conformers of compounds 1a and 1b, we performed quantum chemical calculations (cf. Section 3.1.2).
The different crystal packings of these (on the molecular level) structurally related compounds 1a and 1b can be attributed to the hard vs. soft nature of the Si-bound chalcogen atom. Whereas the closer proximity of O1 (and O2 of the second molecule in the asymmetric unit of 1a) is determined by C-H···O contacts to the pyridine rings of adjacent molecules, close contacts of S1 and CH 3 groups are found in the packing of 1b.

Computational Analyses of Isomers of 1a and 1b
The molecular conformations of 1a and 1b in their crystal structures are posing two fundamental questions, which will be addressed here: (1) For the isomers with Si-X connectivity (X = O, S), as encountered in the crystals, is the conformation with remote N···Si coordination favorable for the isolated molecule or driven by crystal packing? (2) How favored is the Si-X over the Si-N binding mode, depending on X = O or S? As in Me 3 Si-X-Aryl derivatives [52,53] the torsion of the Si-X (X = O, S) bond about the X-C bond out of the aryl plane indicates potentially favorable alternative molecular conformations for 1a and 1b, we performed potential energy surface (PES) scans, in which the Si-X-C-N dihedral angle was varied in steps of 10 degrees, followed by relaxation of the remaining bonds and angles of the respective molecule. Figure 3 illustrates the energetic relation of the conformers thus obtained. In both cases, the conformer with Si-X-C-N dihedral angle of 0 degrees was most favorable. Noteworthy, no local energetic minimum was encountered in the PES scans for dihedral angles around 90 degrees, only the decreasing slope of the graphs hints at some stabilizing effects through the lowered steric repulsion in those conformers. However, as the torsion approaches 180 degrees, a local minimum (for 1a) or at least a plateau (for 1b) was encountered. Regardless of the atom X (O vs. S), the 180 degree torsion about the X-C bond is accompanied by an energy difference of ca. 3 kcal·mol −1 . Thus, for further investigation with the full optimization of the molecular conformation, two conformers of the Si-X-bound isomers were considered in addition to the respective isomer with Si-N bond. The full optimization of the two conformers of Si-X-bound compounds 1a and 1b essentially confirmed their energetic difference (in Gibbs free energy at 293 K) of ca. 3 kcal·mol −1 . The alternative isomer of 1a and 1b with Si-N bound pyridine derivative was found to be less stable by 6.1 and 3.2 kcal·mol −1 , respectively ( Figure 4). Thus, in dynamic equilibria these isomers may still play a minor role. Especially in case of 1b, the transition state for the interconversion of the two isomers into one another (13.9 kcal·mol −1 ) is feasible at ambient temperature. The greater energetic difference in case of the isomers of 1a is in accord with the oxophilicity of silicon, and the higher energetic barrier posed by the transition state (19.9 kcal·mol −1 ) indicates a pronounced kinetic hindrance for the formation of isomer 1a N as well. Selected parameters of unit cell, data collection and structure refinement of 2a and 2b are summarized in Appendix A (Table A1). Compounds 2a (in space group P2 1 /c) and 2b (in space group P1) both feature two crystallographically independent but conformationally very similar molecules in the asymmetric unit. For each of them, a representative example is shown in Figure 5. As found for 1a and 1b (cf. Section 3.1.1), compounds 2a and 2b feature essentially tetracoordinate Si atoms, the coordination spheres of which can be described as a remote [4+2]-coordination. Once again, Si···N contacts with interatomic separations around 3.0 Å complement the tetrahedral coordination sphere, and the Si-O-C-and Si-S-C angles in 2a and 2b, respectively, are similar to those in 1a and 1b, respectively. With respect to the geometry parameter τ 4 [51], the distortion of the almost tetrahedral coordination sphere of 2a (0.96 and 0.94 for the two independent molecules) is less pronounced than for 2b (0.91 for both of the two independent molecules).
Besides those similarities, the presence of two pyX groups in compounds 2a and 2b gives rise to the formation of different conformers within the portfolio of [4+2]-coordination setups. Figure 6a illustrates the three conformations, which differ in the set of axial arrangements: Conformer 1 features two N···Si-C axes (i.e., the pyridine N atoms are capping tetrahedral faces opposite to an Si-C bond). This conformation is encountered in the solid state structure of 2a. Conformer 2 features two N···Si-X axes (i.e., the pyridine N atoms are capping tetrahedral faces opposite to the Si-X bonds, X being the anchor atom of an adjacent pyX moiety). This conformation is encountered in the solid state structure of 2b. Last but not least, conformation 3 with an N···Si-X and an N···Si-C axis should be feasible, but has not been encountered within these crystal structures, however. As the capping of tetrahedral faces trans to rather electronegative atoms X should be favorable because of N→Si lone pair donation into a lower lying σ*(Si-X) molecular orbital, the conformation 2 (as encountered with 2b) was rather expected, whereas the presence of conformer 1 in the structure of 2a was rather surprising. In dimethylsilicon carbamates Me 2 Si(-O(O=)C-NRR') 2 it is a common feature that the carbonyl O atoms are capping tetrahedral faces trans to Si-O bonds [55][56][57][58]. Therefore, we will have a closer look at the energetic differences between the conformers of compounds 2 in Section 3.2.3.  (pyO) 2 (2a) and (b) Me 2 Si(pyS) 2 (2b) in the crystal with thermal displacement ellipsoids at the 50% probability level and atom labels. For the sake of clarity, the H atoms are omitted from the graphic. In both cases, the asymmetric unit comprises two individual (but conformationally very similar) molecules. For each compound, only one of them is depicted as a representative example. Selected interatomic distances (Å) and angles (deg.) of 2a: (12), N2-Si1-C12 152.29 (14). Selected interatomic distances (Å) and angles (deg.) of 2b: Si1-S1 2.1708 (5)  The different crystal packings of this set of (on the molecular level) structurally related compounds 2a and 2b can again be attributed to the hard vs. soft nature of the Si-bound chalcogen atom. Whereas the closer proximity of the O atoms of 2a is determined by C-H···O contacts to pyridine rings of adjacent molecules (such as in 1a), C-H···S contacts (also to pyridine rings of adjacent molecules) give rise to two different arrangements of sulfur atoms in the packing of 2b (as schematically represented in Figure 6b), in which the interatomic S···S-separations are in the upper range of the sum of the van der Waals radii [50].
With respect to the poor tendency of forming hypercoordinate Si-compounds with SiMe 2 moieties, the rather poor Lewis acidity of the Si atom (because of the rather electron releasing nature of alkyl groups) seems to play a partial role only, and the need for forming a (rather strained) four-membered chelate poses another obstacle. Using the N-oxide of the pyO ligand, which forms five-membered (O,O')-chelates, Kraft  Even though the characterization of the molecular structures of the compounds of the type Me 2 Si(pyX) 2 has been missing in the literature so far, compound Me 2 Si(pyO) 2 (2a) had been mentioned in a patent already [60], but it was proposed to feature hexacoordinate Si with chelating pyO ligands. Our current study complements the knowledge of the small library of silicon compounds which carry either pyO or pyS groups. With view on their potential use as ligands in transition metal coordination chemistry, combinations of hard and soft donor atoms within the same molecule may become of interest in terms of ambiphilic ligand properties. Thus, we also address the fundamental question as to the accessibility of compounds of the type R 2 Si(pyO)(pyS), in this particular case compound Me 2 Si(pyO)(pyS) (2c).
As the computational analyses in Section 3.1.2 indicated a rather high mobility of Si-bound pyS groups (including Si-N/Si-S bond exchange), we expected easy substituent scrambling in the (pyO)(pyS)-silane system, similar to the substituent scrambling encountered with mixtures of methimazolylsilanes and chlorosilanes [54]. In an NMR-scale experiment, the equimolar solutions of 2a in CDCl 3 (0.88 M) and 2b in CDCl 3 (0.88 M) were prepared, and the 29 Si NMR spectra of these stock solution as well as of a mixture of equal volume fractions of these solutions were recorded (Figure 7). Whereas the individual stock solutions of 2a and 2b gave rise to only one 29 Si NMR signal (at 0.7 ppm and 18.6 ppm, respectively), the solution of the mixture gave rise to three signals of similar intensity (at 0.7 ppm, 4.3 ppm and 18.6 ppm). The additional signal at 4.3 ppm can be assigned to the product of substituent scrambling (2c). In contrast to a statistical distribution and intensity ratio 2a:2b:2c of 1:1:2 (according to the equilibrium shown in Figure 7), compound 2c is underrepresented, the equilibrium side of starting materials 2a and 2b favored. (See also Figure S15 in the supporting information.) This finding will be underlined by results presented in the computational Section 3.2.3, and it points to potential problems associated with attempts of synthesizing organosilicon compounds, which bear both pyO and pyS groups within one molecule. They may be likely to undergo substituent scrambling in solution in a dynamic equilibrium, thus hampering the isolation of the desired product.

Computational Analyses of Conformers of 2a, 2b and 2c
The molecular conformations of compounds 2a, 2b and 2c, which correspond to the conformations shown in Figure 6, were optimized at the PBE0 D3BJ ZORA-def2-TZVPP level of theory, and their relative Gibbs free energies were estimated for 293 K (Figure 8). In case of compound 2c, two different conformers with (Si-X)(Si-C) trans-capping had to be considered because of the mixed substitution pattern, which gives rise to conformers with (Si-S)(Si-C) and (Si-O)(Si-C) trans-capping.   ) 2b', 2b" and 2c". For 2a, 2b and 2c, the superscript indices indicate the conformer in which the N atoms are capping tetrahedral faces trans to the superscript indexed atoms. Labels of the other isomers are based on related capping or on the relative S-Si-S arrangement within an octahedral coordination sphere (cf. Figures S27-S59 and Tables S9-S41 in the supporting information).
For compound 2a, the three conformers are very similar in Gibbs free energy, thus explaining as to why conformer 1 (2a CC ) could crystallize, as it is only 0.4 kcal·mol −1 less stable than the energetic minimum conformer (2a OO ). In case of compound 2b, the conformers 2b SS and 2b SC (with an energetic difference of only 0.1 kcal·mol −1 ) represent the minima, well in accord with the experimentally encountered conformation 2 (2b SS ) in the solid state. In case of compound 2c, conformer 2 (2c SO ) should dominate as the alternative configurations are at least 1.5 kcal·mol −1 higher in energy. The comparison of the total energies (in Hartree) associated with the Gibbs free energy values of the couples of energetic minimum conformers 2a OO + 2b SC vs. 2 2c SO implies that the formation of two molecules of 2c out of 2a+2b would be exergonic (with −0.45 kcal·mol −1 ). The accessibility of further conformers low in energy (2a CC and 2b SS ), however, may explain as to why formation 2c is less favored in the dynamic equilibrium observed by NMR spectroscopy (cf. Figure 7).
As the computational analysis of compound 1b revealed that the isomerization of the pyS ligand into its Si-N bound form (cf. Section 3.1.2) may be associated with rather low energy difference, we considered isomers with Si-N bound pyS and/or pyO group(s) in the computational analyses of compounds 2 as well. The set of molecular structures derived therefrom, as well as their energies, can be found in the supporting information. For compound 2a the isomers with at least one Si-N bound pyO group are noticeably less stable (relative Gibbs free energy +5.6 kcal·mol −1 or more with respect to 2a OO ). For compound 2b, three further isomers relatively low in energy were found upon changing one (2b' SC +1.1 kcal·mol −1 ) or two pyS groups into Si-N binding mode (2b" SSiScis +1.8 kcal·mol −1 and 2b" SSiStrans +1.9 kcal·mol −1 ). The Si coordination sphere of the former resembles a trigonal bipyramid with S-Si-S axis, the latter two are related to two isomers with rather octahedral Si coordination sphere and cis-disposed methyl groups (with cis-disposed S-Si-S bonds in one and trans-situation in the other). These isomers indicate that, in a dynamic equilibrium, Si hypercoordination may play a role in solutions of compound 2b. In fact, with respect to 2a (∆δ = +0.3 ppm) the 29 Si NMR shift of compound 2b is more responsive to change of solvent and exhibits a slight upfield shift of the signal when using toluene-d 8 instead of CDCl 3 as NMR solvent (∆δ = −0.6 ppm). However, this effect may also originate from varying contributions of other conformers and is not unequivocally speaking for the influence of Si hypercoordination. For compound 2c, two isomers low in energy with Si-N bound pyS group were encountered (2c" OC +1.2 kcal·mol −1 and 2b" NO +0.8 kcal·mol −1 ), both exhibiting a rather trigonal-bipyramidal Si coordination sphere with O-Si-S axis.

Crystallographic Analyses of 3a and 3b
Selected parameters of unit cell, data collection and structure refinement of 3a are summarized in Appendix A (Table A1). Compound 3a (Figure 9) crystallized in the triclinic space group P1, and the asymmetric unit consists of three individual molecules of 3a, which adopt the same conformation. That is, their Si atoms are tetracoordinate and the N atoms cap the tetrahedral faces trans to the Si-C bonds (as in 2a). Additionally, on all three sites the same motif of disorder is present (cf. Figure 9b). That is, the formal site exchange of one pyO group and one phenyl group, while the remaining pair of pyO and Ph groups essentially remains on their places and just responds to the slightly different spatial demand of the neighboring groups. Even though this disorder lowers the precision of bond lengths and angles, the rather high occupancies of the predominant parts allowed for their refinement without restraints (restraints were applied to the low occupancy parts only) and the data obtained give a reasonable impression of the molecular conformations. Selected data of molecule 1 will be discussed as a representative example. Even though the geometry parameter τ 4 [51] hints at rather weak distortion (0.94) as it accounts for the deviations of angles from the tetrahedral angle regardless of atom type, the distortion of the tetrahedral Si coordination sphere is both slightly more pronounced than in compound 2a and, moreover, the deviations from tetrahedral angle are counter-VSEPR; i.e., the C11-Si1-C17 angle, which should be somewhat wider than 109.5 • according to VSEPR, is 108.88(13) • , whereas the O1-Si1-O2 (113.31(11) • ) angle is widened. This kind of deformation can be rationalized by the type of capping of tetrahedral faces: as N1 approaches Si1 trans to C17, the tetrahedral face Si1(O1,O2,C11) will be widened, thus widening the O1-Si1-O2 angle and compressing the C11-Si1-C17 angle. Analogously, as N2 approaches Si1 trans to C11, the tetrahedral face Si1(O1,O2,C17) will be widened, thus widening the O1-Si1-O2 angle again and compressing the C11-Si1-C17 angle again. The intramolecular Si···N contacts (separations Si1···N1 2.966(3) Å, Si1···N2 2.936(4) Å) are similar to those in 2a, however. Thus, the lowered steric demand of the phenyl groups (with respect to the methyl groups in 2a), which arises from their ability to adapt to steric requirements by torsion about the Si-C bond, is expected to be the origin of the pronounced distortion of the tetrahedral Si coordination sphere in 3a. The Si1-O1-C1 and Si1-O2-C6 angles (126.7(2) • and 125.6(3) • , respectively) are also similar to the corresponding angles in 2a. Thus, the remote N→Si coordination in the diphenylsilicon compound 3a is a similar to the situation in the methylsilicon compounds 1a, 2a and MeSi(pyO) 3 [10] as well as in compound Si(pyO) 4 [10]. The crystal structure of 3a exhibits an interesting feature in its molecular packing. As shown in Figure 10, the orientations of the three individual molecules of the asymmetric unit of this packing are almost related to one another by local (not crystallographic) symmetry elements. While molecules 2 and 3 reflect inversion symmetry, the relative positions of molecules 1 and 3 indicate a glide plane perpendicular to the longest axis. Therefore, a test structure solution in the centrosymmetric space group P1 delivered a model with two molecules in the unit cell (which correspond to the pair of molecules 2 and 3), while data integration in a monoclinic setting (which was possible because of two angles of the unit cell being close to 90 • ) and a test structure solution in the non-centrosymmetric space group Pc delivered another model with two molecules in the unit cell (which correspond to the pair of molecules 1 and 3). In both cases (models in P1 and in Pc), the models failed to describe the missing molecule 1 and 2, respectively. In case of the analogous pyridine-2-thiolato compounds, the switch from SiMe 2 (in 2b) to SiPh 2 (in 3b) gave rise to noticeable changes in the coordination chemistry. While the Si atom of 2b is tetracoordinate even in the solid state, compound 3b exhibits hexacoordinate Si atoms in the solid. Several attempts at crystallizing compound 3b delivered (so far) four different crystalline solids, all of which feature 3b as hexacoordinate Si complex. An interesting aspect about the different crystal structures determined therefrom is the individual characterization of two different coordination isomers (3b 1 and 3b 2 ). Figure 11 shows the molecular structures of 3b 1 and 3b 2 (the latter in the solvent free modification 1, in space group P2 1 /n). As the other two crystal structures comprise isomer 3b 2 , too (in a solvate 3b 2 (toluene) and in a less stable solvent-free modification 3b 2 in C2/c), the latter two structures are contained in this publication as Supplementary Materials. The molecular conformation of 3b 2 in its three different crystal structures is essentially alike, and bond lengths and also angles are very similar. For example, the S-Si-S angles range from 156.82(3) • (in modification 1) to 157.89(6) • (in modification 2), C-Si-C angles from 99.43 (8) • (in modification 1) to 100.64 (9) • (in toluene solvate), N-Si-N angles from 81.02(7) • (in toluene solvate) to 82.31(6) • (in modification 1). Therefore, we just point out that the crystallographically determined density of 3b 2 in modification 2 in C2/c (1.29 g·cm −1 at 200 K) is noticeable lower than in modification 1 in P2 1 /n (1.36 g·cm −1 at 180 K), which is in accord with the stabilization of the modification 1 by closer packing.

NMR-Spectroscopic Study of Substituent Scrambling of Ph 2 Si(pyO) 2 (3a) and
Ph 2 Si(pyS) 2 (3b) with Formation of Ph 2 Si(pyO)(pyS) (3c) As ligand scrambling between 2a and 2b in CDCl 3 solution gave rise to the formation of compound 2c, we also studied the system of diphenylsilicon compounds 3a and 3b in CDCl 3 solution in an analogous manner (cf. Figure 12). As expected, the solution of compound 3a gives rise to a 29 Si NMR signal at −33.2 ppm, characteristic of a tetracoordinate Si compound. In CDCl 3 solution, the Si atoms of compounds 3b are also tetracoordinate, the 29 Si NMR signal (at −18.2 ppm) is in the typical region of tetracoordinate Si compounds, too. The equimolar mixture of these stock solutions also produces a third signal (at −13.9 ppm), which can be assigned to compound 3c, the product of substituent scrambling (see also Figure S16 in the supporting information.) In case of the equilibrium mixture of the diphenylsilicon compounds, the product of substituent scrambling (3c) is even more underrepresented than in the case of the corresponding system of dimethylsilicon compounds. We attribute the further shift of the equilibrium to the side of the starting materials 3a+3b to the stabilization of 3b by the chelation of its Si atom. Even though the 29 Si solution NMR signal of 3b is speaking for predominant silicon tetracoordination, the isomers with hexacoordinate Si atom should still serve a small contribution within a dynamic coordination equilibrium. Figure 12. Stack plot of 29 Si{ 1 H} NMR spectra of (from bottom to top) a 0.38 M solution of 3a in CDCl 3 (blue trace), a 0.38 M solution of 3b in CDCl 3 (red trace) and of an equimolar mixture of these stock solutions (green trace) as well as a reaction scheme and signal assignment. We attribute the signal broadening for 3b to dynamic effects in the Si coordination sphere (cf. Section 3.2.3, the computational analyses of conformers and isomers of compound 2b).

NMR Spectroscopic and Computational Analyses of 29 Si Chemical Shift Anisotropy
Tensors of 3b 1 and 3b 2 (Two Isomers of Ph 2 Si(pyS) 2 (3b)) The crystallization of the individual isomers 3b 1 and 3b 2 , both of which feature hexacoordinate Si atoms, allowed to study their 29 Si NMR chemical shift anisotropy (CSA) by solid state NMR spectroscopy. Their individual CSA tensors should be influenced by the different donor atom arrangements in the Si coordination sphere. Therefore, two batches of 3b (one of which contained isomer 3b 1 and the other one contained 3b 2 ) were checked for the clear predominance of the isomer under investigation by (1) homogeneous appearance of crystal shapes, (2) multiple crystal picking and unit cell determination and (3) the grinding of a representative sample for X-ray powder diffraction. Even though we cannot guarantee the absolute absence of the respective other isomer, the samples are "phase pure" for the individual isomer of 3b within the boundaries of the powder XRD diagrams recorded, and thus, the solid state NMR spectra should represent the CSA features of the respective isomer.
The two samples produced 29 Si CP/MAS NMR spectra with essentially the same isotropic chemical shift (δ iso = −127 ppm). Using high spinning rates, thus recording only the isotropic shift signal, one would not be able to distinguish between the two isomers in this case. Spinning side band spectra, however, reveal the different nature of the two individual samples (Figure 13). To confirm the assignment of the different CSA tensors to the respective isomers of 3b, their CSA tensors were calculated (based on the atomic coordinates of a single molecule of the respective crystal structure, i.e., 3b 1 and modification 1 of 3b 2 ). The computational and experimental results are collated in Table 1. In principle, the computational data reflect the features of the experimentally determined CSA tensors in a satisfactory manner. (In addition to general experimental errors, both in data extraction and analysis from CP/MAS NMR experiments as well as deviations caused by the computational method applied, the computational analysis of a single molecule vs. experimental analysis of molecules within the crystal packing can be expected to give rise to some deviations).   29 Si CSA tensor characteristics (isotropic chemical shift δ iso , principal components δ 11 , δ 22 , δ 33 , span Ω and skew κ) of 3b 1 and of modification 1 of 3b 2 determined from solid state NMR spectra ("exp", cf. Figure 13) and calculated at the PBE TZ2P and PBE0 TZ2P level of theory. Interestingly, in compound 3b 1 one principal component is particularly well shielded (positive skew of the CSA tensor), a feature which has also been found with hexacoordinate diorganosilicon complexes with a salen-type ligand (with SiO 2 N 2 C 2 coordination sphere and trans-C-Si-C axis) [61] and with paddlewheel-shaped complexes with SiN 4 ClE (E = Ge, Sn) coordination spheres (N 4 equatorial plane and Cl-Si-E axis) [62]. In contrast, the CSA tensor of 3b 2 features one considerably less shielded component (i.e., negative skew). This feature has been encountered with the pyridine and bipyridine complexes of dichlorosilane [6,63]. Figure 14 illustrates the directions of the principal components of the 29 Si CSA tensor with respect to the Si coordination sphere for compounds 3b 1 and 3b 2 . As the Si coordination spheres are highly distorted, far from octahedral, they exhibit hardly any idealized symmetry axes, which should coincide with a particular principal axis of the CSA tensor. In compound 3b 1 , the particularly well shielded direction (33) is close to perpendicular to the idealized Si,N,N-plane of the molecule, thus (with some offset) pointing along the direction of a C-Si-S axis. The two principal components of lower shielding are located close to within this plane. In compound 3b 2 , one component (11) is particularly less shielded. The molecule nearly exhibits C 2 symmetry (the idealized two-fold axis passing through and bisecting the N-N-, S-S-and C-C-axes), and from the principal components the direction of component (11) is closest to this idealized axis, but nonetheless exhibits a noticeable offset therefrom.

Conclusions
With this contribution, we give some first insights into the Si coordination chemistry of tri-and diorganosilicon compounds with pyridine-2-(thi)olato substituents. The combination of the latter (which are potentially four-membered chelators) with rather electron releasing hydrocarbyl substituents at silicon (Me) lead to compounds with tetracoordinate Si atom even in the solid state. In all cases (Me 3 Si(pyO) (1a), Me 3 Si(pyS) (1b), Me 2 Si(pyO) 2 (2a), Me 2 Si(pyS) 2 (2b)) the pyridine derivative is Si-O or Si-S bound, and the pyridine nitrogen atoms establish Si···N contacts with separations of ca. 3 Å. Whereas this Si tetracoordination motif can be found for other silicon pyridine-2-olates (such as Ph 2 Si(pyO) 2 (3a), as reported herein), the corresponding thiolate (Ph 2 Si(pyS) 2 (3b)) features Si hexacoordination in the solid state and Si tetracoordination in chloroform solution. It thus represents a link between tetracoordinate Si compounds, such as 2b, and hexacoordinate Si compounds, such as the previously reported complex MeClSi(pyS) 2 [8], which retains Si hexacoordination in solution. Isomerization in solution (in dynamic equilibria) gave rise to the crystallization of two individual isomers of 3b (3b 1 and 3b 2 ), which allowed the study of their individual molecular features by X-ray crystallography and 29 Si solid state NMR spectroscopy.
The solutions to the combinations of the corresponding silicon pyridine-2-olates and -thiolates (2a + 2b, 3a + 3b) undergo substituent scrambling. In those two systems, the products with mixed substituent patterns (R 2 Si(pyO)(pyS), 2c R = Me, 3c R = Ph) are underrepresented. As shown by computational analyses, the mixed substitution pattern does not provide any significant energetic sink to foster their formation. In turn, compounds of the type R 2 Si(pyO)(pyS) can be expected to undergo substituent scrambling with the formation of the respective homoleptic compounds (with respect to PyO and pyS). As a practical consequence thereof, the use of such compounds in transition metal coordination chemistry might be highly limited. Transition metal compounds of pyridine-2-olato silanes are currently under investigation with respect to their use as catalysts [12][13][14][15]. Catalyst tuning by the partial replacement of pyO by pyS bridges, if relevant for future studies, may thus be highly challenging.
Supplementary Materials: The following Supporting Information can be downloaded at: https: //www.mdpi.com/article/10.3390/cryst12081054/s1. Crystallographic data for the compounds reported in this paper (in CIF format) and a document containing graphics of the 1 H, 13 C and 29 Si NMR spectra of compounds 2a, 2b, 3a and 3b; X-ray powder diffraction diagrams of 3b 1 and 3b 2 (modification 1); graphical representations of the Si coordination polyhedra of compounds 3b 1 and 3b 2 ; data sets (consisting of molecular graphic, atomic coordinates and total energies) of the optimized molecular structures of isomers/conformers of 1a, 1b, 2a, 2b and 2c as well as of the transition state for the Si-O/Si-N and Si-S/Si-N isomerization of 1a and 1b, respectively. Figure S1: 1H NMR spectrum of 2a in CDCl3 (full spectrum and magnified inset of group of signals); Figure  S2: 13C{1H} NMR spectrum of 2a in CDCl3; Figure S3: 29Si{1H} NMR spectrum of 2a in CDCl3; Figure S4: 1H NMR spectrum of 2b in CDCl3 (full spectrum and magnified insets of group of signals); Figure S5: 13C{1H} NMR spectrum of 2b in CDCl3; Figure S6: 29Si{1H} NMR spectrum of 2b in CDCl3; Figure S7: 1H NMR spectrum of 3a in CDCl3 (full spectrum and magnified insets of group of signals); Figure S8: 13C{1H} NMR spectrum of 3a in CDCl3; Figure S9: 29Si{1H} NMR spectrum of 3a in CDCl3; Figure S10: 1H NMR spectrum of 3b in CDCl3 (full spectrum and magnified inset of group of signals); Figure S11: 13C{1H} NMR spectrum of 3b in CDCl3; Figure S12: 29Si{1H} NMR spectrum of 3b in CDCl3; Figure S13: 29Si CP/MAS NMR spectrum of 3b1 (MAS frequency 900 Hz, spinning side bands are asterisked) showing the experimental spectrum (top, blue trace) and the spinning side band spectrum simulated from the CSA tensor derived therefrom (bottom, red trace); Figure S14: 29 Si CP/MAS NMR spectrum of 3b 2 (MAS frequency 900 Hz, spinning side bands are asterisked) showing the experimental spectrum (top, blue trace) and the spinning side band spectrum simulated from the CSA tensor derived therefrom (bottom, red trace); Figure S15: 29 Si{ 1 H} NMR spectrum of an equimolar mixture of 2a and 2b in CDCl 3 (with integrals of the signal intensities of 2a, 2b and 2c); Figure S16: 29 Si{ 1 H} NMR spectrum of an equimolar mixture of 3a and 3b in CDCl 3 (with integrals of the signal intensities of 3a, 3b and 3c); Figure S17: X-ray powder diffraction patterns of compounds 3b 1 and 3b 2 (modification 1) recorded with Mo Kα radiation using Gandolfi scans: (a) Diffraction pattern recorded from a powdered bulk sample (blue trace) and from a single crystal (red trace) of compound 3b 1 : (b) Diffraction pattern recorded from a powdered bulk sample (blue trace) and from a single crystal (red trace) of modification 1 of compound 3b 2 : (c) Superposition (lowangle fraction) of the diffraction patterns recorded from powdered bulk samples of compound 3b 1 (blue trace) and of modification 1 of compound 3b 2 (red trace); Figure S18: Graphical representations of the Si coordination polyhedra of compounds 3b 1 (a) and 3b 2 (modification 1) (b) with atom labels (bottom) and with some additional atoms attached for a better image of the polyhedron within the molecule (top): Thermal displacement ellipsoids are shown at the 50% probability level; Figure S19: Optimized molecular structure of 1a O0 ; Figure S20: Optimized molecular structure of 1a N ; Figure S21: Optimized molecular structure of 1a O180 ; Figure S22: Optimized molecular structure of 1a TS ; Figure S23: Optimized molecular structure of 1b S0 ; Figure S24: Optimized molecular structure of 1b N ; Figure S25: Optimized molecular structure of 1b S180 ; Figure S26: Optimized molecular structure of 1b TS ; Figure S27: Optimized molecular structure of 2a OO ; Figure S28: Optimized molecular structure of 2a OC ; Figure S29: Optimized molecular structure of 2a CC ; Figure S30: Optimized molecular structure of 2a NO ; Figure S31: Optimized molecular structure of 2a OC ; Figure S32: Optimized molecular structure of 2a CC ; Figure S33: Optimized molecular structure of 2a NC ; Figure S34: Optimized molecular structure of 2a NN ; Figure S35: Optimized molecular structure of 2b SS ; Figure S36: Optimized molecular structure of 2b SC ; Figure S37: Optimized molecular structure of 2b CC ; Figure S38: Optimized molecular structure of 2b NS ; Figure S39: Optimized molecular structure of 2b SC ; Figure S40: Optimized molecular structure of 2b CC ; Figure S41: Optimized molecular structure of 2b NC ; Figure S42: Optimized molecular structure of 2b NN ; Figure S43: Optimized molecular structure of 2b SSiScis ; Figure S44: Optimized molecular structure of 2b SSiStrans ; Figure S45: Optimized molecular structure of 2c SO ; Figure S46: Optimized molecular structure of 2c OC ; Figure S47: Optimized molecular structure of 2c SC ; Figure S48: Optimized molecular structure of 2c CC ; Figure S49: Optimized molecular structure of 2c NS ; Figure S50: Optimized molecular structure of 2c SC ; Figure S51: Optimized molecular structure of 2c CC ; Figure S52: Optimized molecular structure of 2c NC ; Figure S53: Optimized molecular structure of 2c NO ; Figure S54: Optimized molecular structure of 2c OC ; Figure S55: Optimized molecular structure of 2c CC ; Figure S56: Optimized molecular structure of 2c NC ; Figure S57: Optimized molecular structure of 2c NN ; Figure S58: Optimized molecular structure of 2c OSiCtrans ; Figure S59: Optimized molecular structure of 2c SSiCtrans ; Table S1: Atomic coordinates for optimized structure of 1a O0 ;  Table S17: Atomic coordinates for optimized structure of 2b SS ; Table S18: Atomic coordinates for optimized structure of 2b SC ; Table S19: Atomic coordinates for optimized structure of 2b CC ; Table S20: Atomic coordinates for optimized structure of 2b NS ; Table S21: Atomic coordinates for optimized structure of 2b SC ; Table S22: Atomic coordinates for optimized structure of 2b CC ; Table S23: Atomic coordinates for optimized structure of 2b NC ; Table S24: Atomic coordinates for optimized structure of 2b NN ; Table S25: Atomic coordinates for optimized structure of 2b SSiScis : Table S26: Atomic coordinates for optimized structure of 2b SSiStrans ; Table S27: Atomic coordinates for optimized structure of 2c SO ; Table S28: Atomic coordinates for optimized structure of 2c OC ; Table S29: Atomic coordinates for optimized structure of 2c SC ; Table S30: Atomic coordinates for optimized structure of 2c CC ; Table S31: Atomic coordinates for optimized structure of 2c NS ; Table S32: Atomic coordinates for optimized structure of 2c SC ; Table S33: Atomic coordinates for optimized structure of 2c CC ; Table S34: Atomic coordinates for optimized structure of 2c NC ; Table S35: Atomic coordinates for optimized structure of 2c NO ; Table S36: Atomic coordinates for optimized structure of 2c OC ; Table S37: Atomic coordinates for optimized structure of 2c CC ; Table S38: Atomic coordinates for optimized structure of 2c NC ;  Table A2. Crystallographic data from data collection and refinement for 3b 1 , 3b 2 (two modifications) and 3b 2 (toluene).