The Hexacoordinate Si Complex SiCl 4 (4-Azidopyridine) 2 —Crystallographic Characterization of Two Conformers and Probing the Inﬂuence of SiCl 4 -Complexation on a Click Reaction with Phenylacetylene

: 4-Azidopyridine ( 1 ) and SiCl 4 react with the formation of the hexacoordinate silicon complex SiCl 4 (4-azidopyridine) 2 ( 2 ). Upon dissolving in warm chloroform, the complex dissociates into the constituents 1 and SiCl 4 and forms back upon cooling. Depending on the cooling, two different crystalline modiﬁcations of 2 were obtained, which feature two different trans -conformers. Slow cooling to room temperature afforded conformer 2 (cid:48) , which features coplanar pyridine rings. Rapid cooling to − 39 ◦ C afforded crystals of conformer 2 (cid:48)(cid:48) , in which the planes of the pyridine ligands are nearly orthogonal to one another. Whereas 2 (cid:48) resembles the molecular arrangement of various other known Si X 4 (pyridine) 2 ( X = halide) complexes, 2 (cid:48)(cid:48) represents the ﬁrst crystallographically conﬁrmed example of a Si X 4 (pyridine) 2 complex in this conformation. Conformers 2 (cid:48) and 2 (cid:48)(cid:48) were studied with 13 C and 29 Si solid state NMR spectroscopy. Their differences in 29 Si chemical shift anisotropy, as well as energetic differences, were further investigated with computational analyses. In spite of the similar stabilities of the two conformers as isolated molecules, the crystal packing of 2 (cid:48)(cid:48) is less stable, and its crystallization is interpreted as a kinetically controlled effect of seed formation.


Adduct Formation (4-Azidopyridine + SiCl4)
The reaction of 4-azidopyridine (1) and SiCl4 (in approximately 2:1 molar ratio) in chloroform at room temperature with stirring proceeded with the formation of a clear solution, from which a solid crystallized in the course of the following hours.This solid dissolved upon heating, and upon cooling, the product crystallized again.As both compound 1 and SiCl4 are liquids at room temperature, the solid state of the new product indicated the formation of an adduct (such as SiCl4(4-azidopyridine)2), and elemental analyses (C,H,N) were in accordance with this composition.Solution state NMR spectroscopy (of a sample dissolved in CDCl3), however, pointed at the co-existence of the starting materials.The 1 H and 13 C{ 1 H} spectra essentially represented those of 1, and the 29 Si{ 1 H} NMR spectrum exclusively exhibited a signal at ca. −19 ppm, characteristic of SiCl4 with its tetracoordinate Si atom (cf.Appendix A incl.Scheme A1 and Figure A1).Thus, dissolution and crystallization of this product proceed with dissociation and formation of the adduct SiCl4(4-azidopyridine)2.In the course of exploring different crystallization conditions for improved yield, we noticed that both storage of the product solution at room temperature and in a fridge (ca.6 °C) afforded a coarse-crystalline product (block-like crystals), whereas instant transfer of the solution into a freezer (ca.−39 °C) gave rise to the formation of a fine-crystalline product with needle-shaped crystals.In both cases, crystals suitable for single-crystal X-ray diffraction analysis were found in the batches, and their analyses proved the formation of two different modifications of the product SiCl4(4-azidopyridine)2 (2) (Figures 2 and 3, Tables 1 and A1).Also, the crystallographic analyses confirmed the formation of the predicted Lewis acid-base adduct with a hexacoordinate Si atom.As the two crystalline modifications accommodate two different conformers of this compound, both the modifications and their respective conformers will be denoted as

Adduct Formation (4-Azidopyridine + SiCl 4 )
The reaction of 4-azidopyridine (1) and SiCl 4 (in approximately 2:1 molar ratio) in chloroform at room temperature with stirring proceeded with the formation of a clear solution, from which a solid crystallized in the course of the following hours.This solid dissolved upon heating, and upon cooling, the product crystallized again.As both compound 1 and SiCl 4 are liquids at room temperature, the solid state of the new product indicated the formation of an adduct (such as SiCl 4 (4-azidopyridine) 2 ), and elemental analyses (C,H,N) were in accordance with this composition.Solution state NMR spectroscopy (of a sample dissolved in CDCl 3 ), however, pointed at the co-existence of the starting materials.The 1 H and 13 C{ 1 H} spectra essentially represented those of 1, and the 29 Si{ 1 H} NMR spectrum exclusively exhibited a signal at ca. −19 ppm, characteristic of SiCl 4 with its tetracoordinate Si atom (cf.Appendix A incl. and Figure A1).Thus, dissolution and crystallization of this product proceed with dissociation and formation of the adduct SiCl 4 (4-azidopyridine) 2 .
In the course of exploring different crystallization conditions for improved yield, we noticed that both storage of the product solution at room temperature and in a fridge (ca.6 • C) afforded a coarse-crystalline product (block-like crystals), whereas instant transfer of the solution into a freezer (ca.−39 • C) gave rise to the formation of a fine-crystalline product with needle-shaped crystals.In both cases, crystals suitable for single-crystal X-ray diffraction analysis were found in the batches, and their analyses proved the formation of two different modifications of the product SiCl 4 (4-azidopyridine) 2 (2) (Figures 2 and 3, Tables 1 and A1).Also, the crystallographic analyses confirmed the formation of the predicted Lewis acid-base adduct with a hexacoordinate Si atom.As the two crystalline modifications accommodate two different conformers of this compound, both the modifications and their respective conformers will be denoted as 2 (for the variety obtained at a higher temperature) and 2 (for the variety obtained at a lower temperature) in the following.2′ (for the variety obtained at a higher temperature) and 2″ (for the variety obtained at a lower temperature) in the following.2′ (for the variety obtained at a higher temperature) and 2″ (for the variety obtained at a lower temperature) in the following.In principle, both 2 and 2 feature octahedral trans-SiCl 4 N 2 coordination spheres, and their Si-Cl and Si-N bond lengths (Table 1) are similar to one another.Furthermore, these bonds are similar to those of the archetype complex SiCl 4 (pyridine) 2 [13], which exhibits Si-Cl and Si-N bond lengths of 2.183(4) and 1.976(9) Å, respectively.This allows the conclusion that the 4-azido substituent (vs.H atom) has only marginal influence on the pyridine-N→Si coordination in the solid state.The azido groups themselves exhibit the same bond lengths in both crystal structures, but the C-N-N angle to and the N-N-N angle of the almost linear azido group as well as its variable torsion out of the pyridine plane (demonstrated by the C2-C3-N2-N3 dihedral angle) indicate its capability to adapt to different requirements of crystal packing.The influence of the packing on the N 3 moieties was probed with Raman spectroscopy.Both 2 (at 2114 cm −1 ) and 2 (at 2126 cm −1 ) give rise to absorption bands characteristic of the N 3 stretch mode (cf.bands at 2130 cm −1 [36], at 2096 cm −1 [37] and at 2135, 2093 cm −1 [38] reported for the IR spectra of neat 4-azidopyridine).Whereas the overall appearance of the Raman spectra of 2 and 2 is very similar (cf.Figures S1 and S2 in the Supporting Information), this band of the N 3 stretch allows us to distinguish the two modifications.The difference in wave numbers (by 12 cm −1 ) must be attributed to various differences between the two modifications' azido groups' environments: In addition to the C-N-N angle and C-C-N-N torsion angle (cf.Table 1), the N 3 groups are involved in different H contacts.In Note: For these contacts, no s.u.s are given because they are below 0.01 for the non-H-atom separations, and the H atom positions have not been refined (positioning of H atoms in the structure model in idealized positions).
In both modifications 2 and 2 , the Si atoms of the silicon complexes are located in a special position (in 2 on a center of inversion, in 2 on a two-fold axis), which also has the consequence that the asymmetric unit is constituted by a half molecule of this complex in each case.Whereas in the former, the crystallographic symmetry element implies a constraint on the N-Si-N angle of 180 • (and parallel arrangement of the pyridine rings), the latter liberates this parameter.Nonetheless, also in 2 , the N-Si-N angle is close to 180 • .The space fill model in Figure 3 demonstrates that the N-Si-N coordination is also stabilized by two bifurcated Cl•••H•••Cl contacts between the pyridine H atoms in 2,6-position and Cl atoms of two alternating quadrants of the square-shaped SiCl 4 plane.These interactions on the two opposite sides of the SiCl 4 plane stabilize the N→Si coordination perpendicular to the SiCl 4 plane and thus the essentially linear angle.The striking difference between 2 and 2 , however, is the dihedral angle between the pyridine planes (ca.0 • and ca.90 • , respectively).In 2 , the opposing pyridine H atoms establish bifurcated Cl•••H•••Cl contacts within the same quadrants of the SiCl 4 plane (we may call it an eclipsed conformation), whereas in 2 the quadrants are alternating (in a staggered conformation).Interestingly, inspection of the crystallographically characterized silicon tetrahalide pyridine adducts available in the Cambridge Structure Database (a search in CSD 2023 version 5.44) revealed the exclusive encountering of the eclipsed conformation.In SiCl 4 (pyridine) 2 [13], which is located on a crystallographically imposed bisecting plane (in space group C2/m), the pyridine rings are forced to coplanarity and, because of near square-planar SiCl 4 -arrangement, C-N-Si-Cl torsion angles must be close to 45 • , but even in case of crystal structures devoid of the mirror symmetry these torsion angles are close to 45 • (in 2 they are in the range 44.3(2) • -46.9(1) • ), and the C-N-N-C torsion angles are merely 2.1(3) • .Even though in complex SiF 4 (4-phenylpyridine) 2 [12] the dihedral angle between the pyridine planes deviates markedly from 0 • (C-N-N-C torsion angles are 34.9(3)• and 35.9(3) • , and C-N-Si-F torsion angles of 26.7(4) • and 27.4(4) • are encountered) the F•••H contacts of opposing pyridine ligands are still within the same quadrants of the SiF 4 plane.As an isolated example, only in the case of SiCl 2 H 2 complexes, where the dihedral angle of pyridines deviates markedly from planarity in SiCl 2 H 2 (4-ethylpyridine) 2 (C-N-N-C torsion angles are 39.1(1) • and 41.4(1) • ) [17], the pyridine 2,6-H-atoms of opposing pyridine ligands are pointing into alternating quadrants of the SiCl 2 H 2 plane (with C-N-Si-H torsion angles of 19.5(1) • and 20.7(1) • ). Figure 4 gives a schematic impression of the conformations encountered with most of the Si(halide) 4 (pyridine) 2 complexes (e.g., in the parent complex SiCl 4 (pyridine) 2 [13]), as well as with SiF 4 (4-phenylpyridine) 2 [12], SiCl 2 H 2 (4-ethylpyridine) 2 [17]   For the isolated molecule of compound 2, a Potential Energy Surface Scan (PES) was performed for the rotation of one pyridine ring about the N-Si-N axis (variation in the C-N-N-C dihedral angle in increments of 10° followed by partial relaxation of the remaining parameters, starting from the fully optimized molecular structure of 2′).Close to local minima, full optimization was allowed for analysis of the local energetic minimum.The diagram in Figure 5 shows the energetic development of the conformational variation for a 180° rotation.In addition to the conformations at 0° and 180° (which are basically distinguished by their relative orientations of the remote azido groups), the conformation at a dihedral angle of 90° (staggered conformation corresponding to 2″) exhibits an energetic minimum of essentially the same depth (the energetic difference of 0.13 kcal mol −1 can be interpreted as meaningless).This is in accordance with an older study of D2h and D2d symmetric conformers of trans-SiCl4(pyridine)2.These conformers were shown to exhibit essentially the same thermodynamic characteristics (enthalpy and entropy of dissociation into SiCl4 and pyridine) [39].Thus, the packing efficiency might be the predominant parameter.Analysis of the Hirshfeld surface (HS) of 2′ indicates a greater number and higher intensity of close intermolecular contacts with respect to the HS of 2″ (Appendix C, Fig- ures A2-A4).For an energetic analysis of the difference in packing, DFT methods were applied.The molecules of 2′ and 2″ were optimized in their periodic crystal environment (with full optimization of the molecular conformation and optimization of the unit cell parameters allowed, as the analysis aimed at an energetic comparison at 293 K rather than a comparison at the temperature of crystal structure determination (which was 200 K).The analysis revealed 2′ in its crystal environment to be more stable than 2″ in its packing For the isolated molecule of compound 2, a Potential Energy Surface Scan (PES) was performed for the rotation of one pyridine ring about the N-Si-N axis (variation in the C-N-N-C dihedral angle in increments of 10 • followed by partial relaxation of the remaining parameters, starting from the fully optimized molecular structure of 2 ).Close to local minima, full optimization was allowed for analysis of the local energetic minimum.The diagram in Figure 5 shows the energetic development of the conformational variation for a 180 • rotation.In addition to the conformations at 0 • and 180 • (which are basically distinguished by their relative orientations of the remote azido groups), the conformation at a dihedral angle of 90 • (staggered conformation corresponding to 2 ) exhibits an energetic minimum of essentially the same depth (the energetic difference of 0.13 kcal mol −1 can be interpreted as meaningless).This is in accordance with an older study of D 2h and D 2d symmetric conformers of trans-SiCl 4 (pyridine) 2 .These conformers were shown to exhibit essentially the same thermodynamic characteristics (enthalpy and entropy of dissociation into SiCl 4 and pyridine) [39].Thus, the packing efficiency might be the predominant parameter.Analysis of the Hirshfeld surface (HS) of 2 indicates a greater number and higher intensity of close intermolecular contacts with respect to the HS of 2 (Appendix C, Figures A2-A4).For an energetic analysis of the difference in packing, DFT methods were applied.The molecules of 2 and 2 were optimized in their periodic crystal environment (with full optimization of the molecular conformation and optimization of the unit cell parameters allowed, as the analysis aimed at an energetic comparison at 293 K rather than a comparison at the temperature of crystal structure determination (which was 200 K).The analysis revealed 2 in its crystal environment to be more stable than 2 in its packing by 8.5 kcal mol −1 with respect to 1 mol of each kind of molecule in its respective crystal environment.In regard to the experimental data, this computational finding is in accordance with the different molecular volumes occupied in their crystal packings:  From the above results, we can conclude that conformers 2′ and 2″ (as isolated molecules in chloroform solvent environment) should coexist in solution.However, 2′ (in its crystalline arrangement) represents the thermodynamically more favored modification, and 2″ (in its crystalline form) must be a result of kinetics.Apparently, the arrangement of 2″ in the solid allows for easier crystal seed formation upon cooling, thus enabling isolation of crystalline 2″ by crystallization at −39 °C, whereas crystallization at room temperature either inhibits the formation of seeds of 2″ or causes their rapid dissolution and eventually yields crystals of the more stable modification and conformer 2′.
In addition to the crystallization at low temperatures, the accessibility of 2″ depends on the solvent used.Syntheses of 2 in toluene, both by gas phase diffusion of SiCl4 into a toluene solution of 1 at room temperature and by adding 1 to a toluene solution of SiCl4 at −78 °C with stirring, resulted in the formation of 2′.In the former case, large crystals were obtained, and the determination of the unit cell confirmed the presence of modification 2′.In the latter case, a fine powder was obtained, and 13 C CP/MAS NMR spectroscopy thereof confirmed the formation of 2′ (vide infra).
The availability of the two conformers 2′ and 2″ in their solid forms allowed for comparative investigation of their 13 C and 29 Si NMR properties in the solid state.Each of the 13 C NMR spectra (Figure 6  From the above results, we can conclude that conformers 2 and 2 (as isolated molecules in chloroform solvent environment) should coexist in solution.However, 2 (in its crystalline arrangement) represents the thermodynamically more favored modification, and 2 (in its crystalline form) must be a result of kinetics.Apparently, the arrangement of 2 in the solid allows for easier crystal seed formation upon cooling, thus enabling isolation of crystalline 2 by crystallization at −39 • C, whereas crystallization at room temperature either inhibits the formation of seeds of 2 or causes their rapid dissolution and eventually yields crystals of the more stable modification and conformer 2 .
In addition to the crystallization at low temperatures, the accessibility of 2 depends on the solvent used.Syntheses of 2 in toluene, both by gas phase diffusion of SiCl 4 into a toluene solution of 1 at room temperature and by adding 1 to a toluene solution of SiCl 4 at −78 • C with stirring, resulted in the formation of 2 .In the former case, large crystals were obtained, and the determination of the unit cell confirmed the presence of modification 2 .In the latter case, a fine powder was obtained, and 13 C CP/MAS NMR spectroscopy thereof confirmed the formation of 2 (vide infra).
The availability of the two conformers 2 and 2 in their solid forms allowed for comparative investigation of their 13 C and 29 Si NMR properties in the solid state.Each of the 13 C NMR spectra (Figure 6) features five signals, a pattern which is in accordance with the five crystallographically independent pyridine C atoms in each of the crystal structures 2 and 2 .Each of the two spectra exhibits two signals of the rather well-shielded C atoms in 3,5-position of the pyridine ring as well as three noticeably more downfield located signals for the C atoms in 2,6-and 4-position, the latter being the most downfield shifted signal.(The assignment of the latter 13 C NMR signal to the 4-position was made by cross-polarization magic angle spinning (CP/MAS) spectra of variable CP contact times.Whereas contact times of τ = 2 ms were sufficient for polarization transfer to all different kinds of 13 C atoms, noticeably shorter contact times (τ = 100 µs) only allowed for efficient polarization transfer to the 13 C sites involved in a C-H bond.)The absence of the signals of 2 in the spectrum of 2 (and vice versa) indicates the clear predominance of each of the crystalline modifications in the respective sample.The latter finding is essential for the evaluation of the 29 Si solid-state NMR spectra (Figure 7).In spite of the different molecular conformations, 2 (δ iso = −177.4)and 2 (δ iso = −177.8)exhibit practically the same isotropic 29 Si NMR shift in the solid state and, as a consequence, the spinning side bands are located in corresponding positions for spectra recorded at the same MAS frequency.Determination of the principal values of the chemical shift anisotropy (CSA) tensor from the spinning side band spectra relies on the prerequisite that the side band pattern is produced by nuclei with one CSA tensor rather than being a superposition of two different side band spectra in unknown ratio.The CSA tensor data extracted from the spectra in Figure 7a,b are listed in Table 2.These data are complemented by the corresponding values calculated for the molecules of 2 and 2 (the atomic coordinates of which had been optimized in their crystal environment, and referencing of their calculated 29 Si NMR shifts was performed with tetramethylsilane as well as with SiCl 2 (oxinate) 2 [40]).
Inorganics 2023, 11, x FOR PEER REVIEW 8 of 23 contact times of τ = 2 ms were sufficient for polarization transfer to all different kinds of 13 C atoms, noticeably shorter contact times (τ = 100 µs) only allowed for efficient polarization transfer to the 13 C sites involved in a C-H bond.)The absence of the signals of 2′ in the spectrum of 2″ (and vice versa) indicates the clear predominance of each of the crystalline modifications in the respective sample.The latter finding is essential for the evaluation of the 29 Si solid-state NMR spectra (Figure 7).In spite of the different molecular conformations, 2′ (δiso = −177.4)and 2″ (δiso = −177.8)exhibit practically the same isotropic 29 Si NMR shift in the solid state and, as a consequence, the spinning side bands are located in corresponding positions for spectra recorded at the same MAS frequency.Determination of the principal values of the chemical shift anisotropy (CSA) tensor from the spinning side band spectra relies on the prerequisite that the side band pattern is produced by nuclei with one CSA tensor rather than being a superposition of two different side band spectra in unknown ratio.The CSA tensor data extracted from the spectra in Figure 7a,b are listed in Table 2.These data are complemented by the corresponding values calculated for the molecules of 2′ and 2″ (the atomic coordinates of which had been optimized in their crystal environment, and referencing of their calculated 29 Si NMR shifts was performed with tetramethylsilane as well as with SiCl2(oxinate)2 [40]).The features of the CSA tensors (the order of the principal components δ11, δ22, and δ33, as well as span Ω and skew κ) are reported according to the Herzfeld-Berger notation [41,42].The calculated CSA tensors represent the features of the experimentally determined data of the two modifications 2′ and 2″ very well, especially the narrow span (with its feature Ω(2′) > Ω(2″)) and the noticeable difference in the skew are reflected effectively.With respect to the chemical shift scale, however, the calculated data are systematically downfield shifted.This may, in part, originate from referencing data calculated for compounds optimized in crystal environment against data calculated for an isolated molecule, TMS (cf.entries calc-ref1).Therefore, the molecule SiCl2(oxinate)2 [40] was employed as an alternative reference for this calculation (cf.entries calc-ref2).In addition to being a solid, this compound features hexacoordinate Si, as well as some N and Cl atoms, in its coordination sphere.This approach improved the fit of the CSA tensor on the chemical shift scale to some extent.For both 2′ and 2″, the greatest deviations between experimental and calculated CSA tensors are found for principal component δ33, which points along the N-Si-N axis.The orientations of the CSA principal components within the molecules are visualized in Figures S14 and S15 in the Supporting Information.With respect to CSA tensors of related pyridine adducts ("pyridine" may represent a variety of 4-substituted  pyridines) of dichlorosilane [17] and trichlorosilane [19], the isotropic chemical shifts of the SiCl4 adducts 2′ and 2″ are more upfield, which reflects a trend δiso(SiCl2H2("pyridine")2) > δiso(SiCl3H("pyridine")2) > δiso(SiCl4("pyridine")2).For both the pyridine adducts of dichlorosilane [17] and trichlorosilane [19], the most shielded direction was shown to point along the N-Si-N axis, with CSA tensor principal values δ33 of ca.−200 ppm (e.g., −210 ppm for SiCl2H2(4-tBu-pyridine)2 [17] and −202 ppm for SiCl3H(4-tBu-pyridine)2 [19]).This feature is reflected by compounds 2′ and 2″, both of which exhibit very similar values for δ33 (−197 ppm) and imply a trend of decreasing shielding in this direction with an increase in Cl (instead of H) substitution at the Si atom.Principal axes ( 11) and ( 22), which are located in the SiCl4 plane, are influenced by the different relative conformations of the pyridine ligands.Whereas in 2′ the pyridine rings are nearly coplanar and thus can exert simultaneous in-plane and perpendicular-to-plane shielding effects, the angular dependence of the shielding in 2″ is always determined by both kinds of effects because of the torsion of the pyridine rings, causing an approaching of δ11 and δ22 to one another.The upfield shift of principal value δ11 (by ca. 10 ppm), which results therefrom, can be seen as the major cause of the reduced span Ω (by ca. 10 ppm) for the CSA tensor of 2″.(This is also reflected by the graphic in Figure S16.)Moreover, the mutual approaching of δ11 and δ22 to one another eventually causes a noticeable change in the skew κ.This is furthermore supported by the CSA tensor characteristics calculated for the optimized conformer of SiCl4(4-azidopyridine)2 at a dihedral angle of 90° between the pyridine ligands (cf. the conformation, which represents the local minimum at 90° of the PES analysis in Figure 5).Entry 2″ (calc-ref1-90°opt) in Table 2 shows that this closer approach to a D2d symmetric conformer (merely the remote N3 groups violate this symmetry) should eventually result in an axial CSA tensor with identical values for δ11 and δ22 as well as a skew κ = 1 resulting therefrom.an MAS frequency of 1 kHz.The spinning side bands are marked with an asterisk (*).In the overlay (c), the intensities of the two spectra were adjusted to the side band at −165 ppm, which has a similar relative intensity in both spectra (in both cases, its integral represents ca.15% of the sum of integrals of the isotropic signal and the two adjacent side bands).
Table 2. 29 Si CSA tensor characteristics (isotropic chemical shift δ iso , principal components δ 11 , δ 22 , δ 33 , span Ω and skew κ) of 2 and 2 determined from solid-state NMR spectra (cf. Figure 7) as indicated with suffix (exp) as well as the corresponding data calculated for these molecules upon optimization of their atomic coordinates in the crystal environment, indicated with suffix (calc-ref1) for referencing against the isotropic chemical shift calculated for tetramethylsilane (TMS) set at δ = 0 ppm, and with suffix (calc-ref2) for referencing against the isotropic chemical shift calculated for the optimized molecular structure of SiCl 2 (oxinate) 2 in its crystal structure set at δ = −158.7 ppm [40].The entries 2 (calc-ref1-0 • opt) and 2 (calc-ref1-90 • opt) contain the corresponding CSA tensor characteristics calculated for the eclipsed and staggered, respectively, local minimum molecular conformation of 2 and 2 (cf.entries at 0 • and 90 • , respectively, in Figure 5) upon full optimization of the molecular conformation in chloroform solvent environment (COSMO model) and referenced against TMS.The features of the CSA tensors (the order of the 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 [41,42].The calculated CSA tensors represent the features of the experimentally determined data of the two modifications 2 and 2 very well, especially the narrow span (with its feature Ω(2 ) > Ω(2 )) and the noticeable difference in the skew are reflected effectively.With respect to the chemical shift scale, however, the calculated data are systematically downfield shifted.This may, in part, originate from referencing data calculated for compounds optimized in crystal environment against data calculated for an isolated molecule, TMS (cf.entries calc-ref1).Therefore, the molecule SiCl 2 (oxinate) 2 [40] was employed as an alternative reference for this calculation (cf.entries calc-ref2).In addition to being a solid, this compound features hexacoordinate Si, as well as some N and Cl atoms, in its coordination sphere.This approach improved the fit of the CSA tensor on the chemical shift scale to some extent.For both 2 and 2 , the greatest deviations between experimental and calculated CSA tensors are found for principal component δ 33 , which points along the N-Si-N axis.The orientations of the CSA principal components within the molecules are visualized in Figures S14 and S15 in the Supporting Information.With respect to CSA tensors of related pyridine adducts ("pyridine" may represent a variety of 4-substituted pyridines) of dichlorosilane [17] and trichlorosilane [19], the isotropic chemical shifts of the SiCl 4 adducts 2 and 2 are more upfield, which reflects a trend δ iso (SiCl 2 H 2 ("pyridine") 2 ) > δ iso (SiCl 3 H("pyridine") 2 ) > δ iso (SiCl 4 ("pyridine") 2 ).For both the pyridine adducts of dichlorosilane [17] and trichlorosilane [19], the most shielded direction was shown to point along the N-Si-N axis, with CSA tensor principal values δ 33 of ca.−200 ppm (e.g., −210 ppm for SiCl 2 H 2 (4-tBu-pyridine) 2 [17] and −202 ppm for SiCl 3 H(4-tBu-pyridine) 2 [19]).This feature is reflected by compounds 2 and 2 , both of which exhibit very similar values for δ 33 (−197 ppm) and imply a trend of decreasing shielding in this direction with an increase in Cl (instead of H) substitution at the Si atom.Principal axes ( 11) and ( 22), which are located in the SiCl 4 plane, are influenced by the different relative conformations of the pyridine ligands.Whereas in 2 the pyridine rings are nearly coplanar and thus can exert simultaneous in-plane and perpendicular-to-plane shielding effects, the angular dependence of the shielding in 2 is always determined by both kinds of effects because of the torsion of the pyridine rings, causing an approaching of δ 11 and δ 22 to one another.The upfield shift of principal value δ 11 (by ca. 10 ppm), which results therefrom, can be seen as the major cause of the reduced span Ω (by ca. 10 ppm) for the CSA tensor of 2 .(This is also reflected by the graphic in Figure S16.)Moreover, the mutual approaching of δ 11 and δ 22 to one another eventually causes a noticeable change in the skew κ.This is furthermore supported by the CSA tensor characteristics calculated for the optimized conformer of SiCl 4 (4-azidopyridine) 2 at a dihedral angle of 90 • between the pyridine ligands (cf. the conformation, which represents the local minimum at 90 • of the PES analysis in Figure 5).Entry 2 (calc-ref1-90 • opt) in Table 2 shows that this closer approach to a D 2d symmetric conformer (merely the remote N 3 groups violate this symmetry) should eventually result in an axial CSA tensor with identical values for δ 11 and δ 22 as well as a skew κ = 1 resulting therefrom.

Click Reaction of 4-Azidopyridine and Phenylacetylene
The azido group, which has been a spectator substituent only in Section 2.1.,offers an intriguing entry into further chemistry.Whereas the decomposition of azides with formation of nitrenes opens various routes of further syntheses [43,44], the intact azido group can be utilized in (3+2)-cycloadditions with unsaturated groups such as alkynes [45][46][47] or nitriles [48].As nitriles may represent competitor Lewis bases (some examples of complexes of pronounced Lewis acidic silanes and nitrile ligands even allowed for their crystallographic investigation [22,49,50]), we chose an alkyne (i.e., phenylacetylene) to investigate its (3+2)-cycloaddition with 4-azidopyridine and with the SiCl 4 adduct thereof.(3+2)-cycloadditions of azides and alkynes are facile, especially when carried out in protic solvents (e.g., in tBuOH/water [46]) and when catalyzed by transition metals or some of their complexes [51]) which contributed to their classification as "Click reactions".Aiming at investigations of related cycloadditions of alkynes and silicon complexes such as 2, it is clear that both protic solvents and transition metal catalysts need to be avoided.The former likely cause solvolysis of, e.g., Si-Cl bonds and the latter may bind to the pyridine ligand and thus compete with Si complex formation.Thus, we explored the thermal addition of 4-azidopyridine and phenylacetylene in rather innocent hydrocarbyl solvents (toluene and p-xylene).Upon cooling to room temperature, the reaction products crystallized because of the poor solubility in the solvents used.Both isomers, the 1,4-and the 1,5-substituted triazole 1,4-3 and 1,5-3, respectively, formed, as indicated in Scheme 1.The total weight of the isolated product provided information on the degree of conversion, and the different crystal shapes (1,4-3 formed thin plates, 1,5-3 formed compact blocks) allowed for crystal picking and, thus, for the determination of their individual crystal structures by single-crystal X-ray diffraction (Table A1, Figure A5).With respect to the latter, graphics of the molecules (Figures S8 and S9) and tables with selected bond lengths and angles (Tables S1 and S2) are provided in the Supporting Information.Furthermore, we point out that the structure of 1,5-3 (with respect to the unit cell parameters) is related to that of 4-phenyl-3-(4-pyridyl)-4H-1,2,4-triazole [52], which also has essentially the same molecular shape as compound 1,5-3.The structure of 1,4-3 is related to that of 3,6-bis(pyridin-4-yl)pyridazine [53].These molecules also have similar molecular shapes.However, in this case, they share a similar reduced unit cell only (1,4-3 crystallized in a triclinic cell, the latter in the monoclinic space group type C2/c, which is possible because of the higher symmetry of the molecule).NMR spectroscopic data of these two isomers are available in the literature [54].They allowed for signal assignment in the spectra of the samples obtained and for the determination of the isomer ratio by integration of selected 1 H NMR signals.For that purpose, the samples obtained along the series of varied solvents and reaction times were finely ground before a representative sample was dissolved in CDCl 3 for 1 H NMR spectroscopic determination of the isomer ratio.Table 3 contains the results of that series.In principle, in this solvent environment and in the absence of any catalyst, the (3+2)-cycloaddition of 4-azidopyridine and phenylacetylene is rather slow and proceeds in the course of some hours at temperatures above 100 • C. As expected, the reaction rate is enhanced by elevated temperatures (reflux temperature of p-xylene rather than toluene).The ratio of isomers in the isolated product, however, is hardly altered.The isomer ratio was determined from the ratio of1 H NMR signal integrals of the protons shown in Figure 8. 1 The yield reported in this table is the yield of the solid product obtained upon storage of the reaction mixture for one day at room temperature.
However, in this case, they share a similar reduced unit cell only (1,4-3 crystallized in a triclinic cell, the latter in the monoclinic space group type C2/c, which is possible because of the higher symmetry of the molecule).NMR spectroscopic data of these two isomers are available in the literature [54].They allowed for signal assignment in the spectra of the samples obtained and for the determination of the isomer ratio by integration of selected 1 H NMR signals.For that purpose, the samples obtained along the series of varied solvents and reaction times were finely ground before a representative sample was dissolved in CDCl3 for 1 H NMR spectroscopic determination of the isomer ratio.Table 3 contains the results of that series.In principle, in this solvent environment and in the absence of any catalyst, the (3+2)-cycloaddition of 4-azidopyridine and phenylacetylene is rather slow and proceeds in the course of some hours at temperatures above 100 °C.As expected, the reaction rate is enhanced by elevated temperatures (reflux temperature of p-xylene rather than toluene).The ratio of isomers in the isolated product, however, is hardly altered.The isomer ratio was determined from the ratio of 1 H NMR signal integrals of the protons shown in Figure 8.
Table 3. Results of (3+2)-cycloaddition reactions of 1 and excess phenylacetylene (4.6 mmol) in 5 mL of the respective solvent.As compound 2 exhibits very poor solubility in toluene and p-xylene, we probed the (3+2)-cycloaddition of 1 and phenylacetylene as well as the related reaction of 2 and phenylacetylene in chloroform at 60 °C (Table 4).For the purpose of 1 H NMR spectroscopic analysis of the resultant solution, CDCl3 was used.The relevant parts of the 1 H NMR spectra, which were used for the evaluation of the results, are shown in Figure 9. Entries 1 and 2 of Table 4 (spectra (a) and (b), respectively, in Figure 9) contain the results of CDCl3 solutions of comparable molar concentrations of 4-azidopyridine moieties and phenylacetylene.In spite of the slightly higher concentration of starting materials for entry 2, the conversion is merely one-third with respect to the same reaction carried out with a solution of the SiCl4 adduct 2. Furthermore, the presence of SiCl4 in the solution (entry 1) As compound 2 exhibits very poor solubility in toluene and p-xylene, we probed the (3+2)-cycloaddition of 1 and phenylacetylene as well as the related reaction of 2 and phenylacetylene in chloroform at 60 • C (Table 4).For the purpose of 1 H NMR spectroscopic analysis of the resultant solution, CDCl 3 was used.The relevant parts of the 1 H NMR spectra, which were used for the evaluation of the results, are shown in Figure 9. Entries 1 and 2 of Table 4 (spectra (a) and (b), respectively, in Figure 9) contain the results of CDCl 3 solutions of comparable molar concentrations of 4-azidopyridine moieties and phenylacetylene.In spite of the slightly higher concentration of starting materials for entry 2, the conversion is merely one-third with respect to the same reaction carried out with a solution of the SiCl 4 adduct 2. Furthermore, the presence of SiCl 4 in the solution (entry 1) enhances the selectivity of the (3+2)-cycloaddition in favor of triazole isomer 1,5-3 (an isomer ratio 1,4-3:1,5-3 of ca.1:6 was encountered).Higher concentrations of starting materials (1 and phenylacetylene, cf.Table 4 entry 3) may enhance the conversion of the reaction within this time frame, but the product ratio (ca.1:3) remains essentially the same and, furthermore, rather similar to the (1:2)-ratio observed for this non-catalyzed reaction when performed in toluene or p-xylene (cf.Table 3).enhances the selectivity of the (3+2)-cycloaddition in favor of triazole isomer 1,5-3 (an isomer ratio 1,4-3:1,5-3 of ca.1:6 was encountered).Higher concentrations of starting materials (1 and phenylacetylene, cf.Table 4 entry 3) may enhance the conversion of the reaction within this time frame, but the product ratio (ca.1:3) remains essentially the same and, furthermore, rather similar to the (1:2)-ratio observed for this non-catalyzed reaction when performed in toluene or p-xylene (cf.Table 3).

General Considerations
Caution: Some azides may be highly explosive, and safety precautions were taken for the preparation of organic azides such as 4-azidopyridine [38].Therefore, in addition to precautions such as working behind a blast shield and preparation of 4-azidopyridine in an apparatus open to the atmosphere, avoiding direct sunlight and contact with metals, the preparation of this starting material was limited to the 5 g scale.Furthermore, even though we did not encounter any hints at the hazardous explosive nature of SiCl4 adducts of 4-azidopyridine, we applied similar precautions (albeit working under an inert atmosphere) for the further conversions of this compound, too.In the case of solid reaction products (such as 2), excessive heating of the pure compound or of the undissolved solid in dispersions was avoided.

General Considerations
Caution: Some azides may be highly explosive, and safety precautions were taken for the preparation of organic azides such as 4-azidopyridine [38].Therefore, in addition to precautions such as working behind a blast shield and preparation of 4-azidopyridine in an apparatus open to the atmosphere, avoiding direct sunlight and contact with metals, the preparation of this starting material was limited to the 5 g scale.Furthermore, even though we did not encounter any hints at the hazardous explosive nature of SiCl 4 adducts of 4-azidopyridine, we applied similar precautions (albeit working under an inert atmosphere) for the further conversions of this compound, too.In the case of solid reaction products (such as 2), excessive heating of the pure compound or of the undissolved solid in dispersions was avoided.
The geometry optimizations of isolated molecules were carried out with ORCA 5.0.3 [65] using the restricted PBE0 functional with a relativistically recontracted Karlsruhe basis set ZORA-def2-TZVPP [66,67] for all atoms, scalar relativistic ZORA Hamiltonian [68,69], atom-pairwise dispersion correction with the Becke-Johnson damping scheme (D3BJ) [70,71] and COSMO solvation (chloroform).Calculations were started from the molecular structures obtained by single-crystal X-ray diffraction analysis.Numerical frequency calculations were performed to prove convergence at the local minimum after geometry optimization and to obtain the Gibbs free energy (293.15K).The geometry of tetramethylsilane (TMS) was optimized as described above.Relaxed energy scans were performed in 10 • steps by an increase in the C-N-N-C torsion angle in the range from 0 to 180 • .The crystal structures of 2 , 2 , and SiCl 2 (oxinate) 2 were optimized with TUR-BOMOLE rev.V7-7-1 [72] using the restricted PBE functional with a basis set pob-TZVP and RI-J auxiliary basis set for all atoms.A grid size of m4 and atom-pairwise dispersion correction with the Becke-Johnson damping scheme (D3BJ) [70,71] were used.During the periodic boundary optimization, the redundant coordinates and cell parameters were optimized.NMR chemical shifts were calculated with ADF-2019.303[73] and referenced as described in the main text using a PBE function and an all-electron basis set TZ2P for all atoms [74] and spin-orbit relativistic ZORA Hamiltonian.Graphics were generated using Chemcraft [75].
Route B: Under a dry argon atmosphere, a Schlenk flask was charged with toluene (5 mL) and 1 (0.294 g, 2.45 mmol).Via glass pipe, this flask was connected to a second Schlenk flask, which contained SiCl 4 (0.533 g, 3.14 mmol).This setup was stored at room temperature to allow for the diffusion of SiCl 4 into the toluene solution of 1 via the gas phase.In the course of some days, crystals of 2 formed (confirmed by determination of the unit cell parameters by single-crystal X-ray diffraction).
Route C: Under a dry argon atmosphere, a Schlenk flask was charged with a magnetic stirring bar, toluene (15 mL), and SiCl 4 (0.537 g, 3.16 mmol), and the mixture was stirred at −78 • C (in a dry ice/isopropanol bath).To the stirred mixture, 1 (0.447 g, 3.72 mmol) was added dropwise, whereupon precipitation of a white solid commenced.Stirring at −78 • C was continued for 1 h, followed by stirring in an ice bath for 30 min, whereupon the cold dispersion was filtered.The solid product was washed with 3 × 1.5 mL of cold toluene (cooled in an ice bath) and dried in a vacuum.Yield: 0.70 g (1.71 mmol, 92%) of 2 . 13C CP/MAS NMR spectroscopy confirmed the formation of modification 2 (cf. Figure S7 in the Supporting Information).
Compounds 1,4-3 and 1,5-3 (C 13 H 10 N 4 ).For the series of experiments listed in Table 3: Under a dry argon atmosphere, into a Schlenk tube with a magnetic stirring bar were added 5 mL of the respective solvent (toluene or p-xylene), phenylacetylene (0.50 mL, ca.4.6 mmol) and the respective amount of 1 (ca.3.2-3.9mmol).This solution was then placed in an oil bath at the temperature listed in Table 3 (110 • C for toluene solutions, 140 • C for p-xylene solutions) and stirred at this temperature for 0.5 or 2 h.Thereafter, the oil bath was removed, and the Schlenk tube was stored undisturbed for cooling to room temperature and for crystallization of the product.After 1 d, the crystalline product was separated from the supernatant by filtration, washing with the respective solvent (2 mL), and drying in a vacuum.The yields are given in Table 3, and the sets of 1 H and 13 C NMR spectra are similar to one another.As an example, see Figure 8.The positions of the signals are in accordance with those reported in the literature [54].For a representative sample, an elemental analysis was performed, calculated for C 13 H 10 N 4 (222.25 g•mol −1 ): C, 70.26%; H, 4.54%; N, 25.21%; found C, 70.69%; H, 4.21%; N, 25.06%.

Conclusions
4-Azidopyridine forms adducts with SiCl 4 (SiCl 4 (4-azidopyridine) 2 2).Investigation of these compounds gave rise to the first crystallographically characterized example of an idealized D 2d symmetric silane-pyridine adduct 2 (i.e., the staggered configuration of the trans-disposed pyridine ligands at the hexacoordinate Si atom).Furthermore, the    1 The data set of compound 1,4-3 was collected from a twinned crystal (1 0 0, 0.137 −1 0, 0.853 0 and the structure was refined as a two-domain twin using a HKLF5 format data set, which contai isolated reflections of the predominant domain and overlapping reflections.The batch scale fa (twin population) refined to 0.472(3). 2 The ShelXL2018/03 refinement output of this HKLF5 t refinement did not report Rint of the data set.The CheckCIF structure factor report lists Rint = 0.0

Figure 1 .
Figure 1.Generic representation of some different classes of silicon complexes with pyridine ligands (neutral complexes A and cationic complexes C) and with bipyridyl or phenanthroline type ligands (neutral complexes B and cationic complexes D).The Si-bound groups X and the ligand-bound substituents R are specified in the text for particular examples.

Figure 1 .
Figure 1.Generic representation of some different classes of silicon complexes with pyridine ligands (neutral complexes A and cationic complexes C) and with bipyridyl or phenanthroline type ligands (neutral complexes B and cationic complexes D).The Si-bound groups X and the ligand-bound substituents R are specified in the text for particular examples.

Figure 2 .
Figure 2. Molecular structure of (a) 2′ with displacement ellipsoids at the 50% probability level and labels of selected non-hydrogen atoms, supplemented by (b) a ball-and-stick model view along the N-Si-N axis of 2′ (with azido groups omitted for clarity).Sections (c) and (d) show the corresponding ellipsoid and ball-and-stick plots, respectively, for 2″.The symmetry indicators * and ** at some atom labels refer to the inversion at Si1 in 2′ (−x + 1, −y, −z + 1) and the two-fold axis through Cl1-Si1-Cl3 in 2″ (−x, y, −z + 1/2), respectively.

Figure 2 .
Figure 2. Molecular structure of (a) 2 with displacement ellipsoids at the 50% probability level and labels of selected non-hydrogen atoms, supplemented by (b) a ball-and-stick model view along the N-Si-N axis of 2 (with azido groups omitted for clarity).Sections (c) and (d) show the corresponding ellipsoid and ball-and-stick plots, respectively, for 2 .The symmetry indicators * and ** at some atom labels refer to the inversion at Si1 in 2 (−x + 1, −y, −z + 1) and the two-fold axis through Cl1-Si1-Cl3 in 2 (−x, y, −z + 1/2), respectively.

Figure 2 .
Figure 2. Molecular structure of (a) 2′ with displacement ellipsoids at the 50% probability level and labels of selected non-hydrogen atoms, supplemented by (b) a ball-and-stick model view along the N-Si-N axis of 2′ (with azido groups omitted for clarity).Sections (c) and (d) show the corresponding ellipsoid and ball-and-stick plots, respectively, for 2″.The symmetry indicators * and ** at some atom labels refer to the inversion at Si1 in 2′ (−x + 1, −y, −z + 1) and the two-fold axis through Cl1-Si1-Cl3 in 2″ (−x, y, −z + 1/2), respectively.
and the ultimate staggered conformation in 2 .The latter features C-N-N-C torsion angles of 85.0(1) • and 88.3(1) • , as well as C-N-Si-Cl torsion angles in the range of 42.6(1) • -47.9(1) • .This exposes 2 as the first example of a real staggered conformation within the class of octahedral pyridine-silane adducts.As the intramolecular interactions (one N→Si dative bond and two Cl•••H•••Cl contacts per pyridine ligand) are essentially the same in both conformers 2 and 2 , we analyzed the energetic difference between the two varieties (conformers and modifications) with the aid of computational methods to find an answer as to why the eclipsed conformation may dominate the crystallographically encountered molecular shapes.Inorganics 2023, 11, x FOR PEER REVIEW 6 of 23 opposing pyridine ligands are still within the same quadrants of the SiF4 plane.As an isolated example, only in the case of SiCl2H2 complexes, where the dihedral angle of pyridines deviates markedly from planarity in SiCl2H2(4-ethylpyridine)2 (C-N-N-C torsion angles are 39.1(1)° and 41.4(1)°) [17], the pyridine 2,6-H-atoms of opposing pyridine ligands are pointing into alternating quadrants of the SiCl2H2 plane (with C-N-Si-H torsion angles of 19.5(1)° and 20.7(1)°).Figure 4 gives a schematic impression of the conformations encountered with most of the Si(halide)4(pyridine)2 complexes (e.g., in the parent complex SiCl4(pyridine)2 [13]), as well as with SiF4(4-phenylpyridine)2 [12], SiCl2H2(4-ethylpyridine)2 [17] and the ultimate staggered conformation in 2″.The latter features C-N-N-C torsion angles of 85.0(1)° and 88.3(1)°, as well as C-N-Si-Cl torsion angles in the range of 42.6(1)°-47.9(1)°.This exposes 2″ as the first example of a real staggered conformation within the class of octahedral pyridine-silane adducts.As the intramolecular interactions (one N→Si dative bond and two Cl•••H•••Cl contacts per pyridine ligand) are essentially the same in both conformers 2′ and 2″, we analyzed the energetic difference between the two varieties (conformers and modifications) with the aid of computational methods to find an answer as to why the eclipsed conformation may dominate the crystallographically encountered molecular shapes.

Whereas in 2
one molecule of the complex requires a volume of 416 Å 3 , the volume per molecule of 393 Å 3 indicates a much more efficient packing for 2 .Inorganics 2023, 11, x FOR PEER REVIEW 7 of 23 by 8.5 kcal mol −1 with respect to 1 mol of each kind of molecule in its respective crystal environment.In regard to the experimental data, this computational finding is in accordance with the different molecular volumes occupied in their crystal packings: Whereas in 2″ one molecule of the complex requires a volume of 416 Å 3 , the volume per molecule of 393 Å 3 indicates a much more efficient packing for 2′.

Figure 5 .
Figure 5. Energetic development of a Potential Energy Surface Scan (PES) for the torsion of a pyridine ligand about the N-Si-N axis in increments of 10° (followed by partial relaxation), starting from the molecular conformation of 2′.
) features five signals, a pattern which is in accordance with the five crystallographically independent pyridine C atoms in each of the crystal structures 2′ and 2″.Each of the two spectra exhibits two signals of the rather well-shielded C atoms in 3,5-position of the pyridine ring as well as three noticeably more downfield located signals for the C atoms in 2,6-and 4-position, the latter being the most downfield shifted signal.(The assignment of the latter 13 C NMR signal to the 4-position was made by cross-polarization magic angle spinning (CP/MAS) spectra of variable CP contact times.Whereas

Figure 5 .
Figure 5. Energetic development of a Potential Energy Surface Scan (PES) for the torsion of a pyridine ligand about the N-Si-N axis in increments of 10 • (followed by partial relaxation), starting from the molecular conformation of 2 .

Figure 6 .
Figure 6. 13 C CP/MAS NMR spectra of (a) 2″ and (b) 2′ recorded at a MAS frequency of 10 kHz and with a CP contact time of τ = 2 ms.The graphic shows the signals at the isotropic chemical shifts with signal assignment.(In accordance with the crystal structures, cf. Figure 2, in both solids, the C atom positions 2,6 as well as 3,5 are chemically not equivalent.).

Figure 6 .
Figure 6. 13 C CP/MAS NMR spectra of (a) 2 and (b) 2 recorded at a MAS frequency of 10 kHz and with a CP contact time of τ = 2 ms.The graphic shows the signals at the isotropic chemical shifts with signal assignment.(In accordance with the crystal structures, cf. Figure 2, in both solids, the C atom positions 2,6 as well as 3,5 are chemically not equivalent).

Figure 7 .
Figure7.29 Si CP/MAS NMR spectra of (a) 2″, (b) 2′, and (c) an overlay of both spectra, recorded at an MAS frequency of 1 kHz.The spinning side bands are marked with an asterisk (*).In the overlay (c), the intensities of the two spectra were adjusted to the side band at −165 ppm, which has a similar

Figure 8 . 1 H
Figure 8. 1 H NMR signals of the mixture of 1,4-3 and 1,5-3 obtained by reaction of 4-azidopyridine and phenylacetylene in p-xylene at 140 °C for 0.5 h (with signals used for determination of isomer ratio and a sketch with a signal assignment to the protons of origin).

Figure 8 .
Figure 8. 1 H NMR signals of the mixture of 1,4-3 and 1,5-3 obtained by reaction of 4-azidopyridine and phenylacetylene in p-xylene at 140 • C for 0.5 h (with signals used for determination of isomer ratio and a sketch with a signal assignment to the protons of origin).

Figure 9 .
Figure 9. Selected section of the 1 H NMR spectra used for determination of the data shown in Table 4. Subfigures (a), (b), and (c) correspond to entries 1, 2, and 3, respectively, in Table 4.The signal assignments H a and H b (as well as H b′ ) are the same as in Figure 8.The additional signals, denoted with P and S, are the parent signals of the 2,6 protons of remaining starting material 4-azidopyridine and their 1 J( 13 C 1 H) satellites, respectively.Because of the severe superposition of signal H b' with satellites, signals H a and H b were used for the determination of the isomer ratio.The slightly different individual chemical shifts are attributed to their dependence on the concentration of the various species in the solution (e.g., solvent, 4-azidopyridine, phenylacetylene) and on the presence vs. absence of SiCl4 in these solutions.

Figure 9 .
Figure 9. Selected section of the 1 H NMR spectra used for determination of the data shown in Table 4. Subfigures (a), (b), and (c) correspond to entries 1, 2, and 3, respectively, in Table 4.The signal assignments H a and H b (as well as H b ) are the same as in Figure 8.The additional signals, denoted with P and S, are the parent signals of the 2,6 protons of remaining starting material 4-azidopyridine and their 1 J( 13 C 1 H) satellites, respectively.Because of the severe superposition of signal H b' with satellites, signals H a and H b were used for the determination of the isomer ratio.The slightly different individual chemical shifts are attributed to their dependence on the concentration of the various species in the solution (e.g., solvent, 4-azidopyridine, phenylacetylene) and on the presence vs. absence of SiCl 4 in these solutions.

Scheme A1 .
Scheme A1.Dissociation and formation of 2 in solution.Scheme A1.Dissociation and formation of 2 in solution.

Figure A3 .
Figure A3.Fingerprint plots (de vs. di) of the Hirshfeld surface analyses (performed with CrystalExplorer version 21.5, revision 608bb32 [84]) of (a) the crystal structure of 2′ and (b) the crystal structure of 2″.In 2′, the intermolecular contacts are established at shorter interatomic separations in general (the features of the map are shifted to shorter de, di values), and pronounced interactions are found with H•••N-and H•••Cl-contacts.In the structure of 2″, some shorter N•••N-and N•••Cl-contacts are characteristic of the packing (features inside the red circle in Figure3b), which are not encountered with 2′ (cf. the red circle in Figure3a).

Figure A4 .
Figure A4.Percentage distribution (with respect to the area on the Hirshfeld surface) of the intermolecular contacts of 2′ and 2″.

Figure A3 .
Figure A3.Fingerprint plots (d e vs. d i ) of the Hirshfeld surface analyses (performed with CrystalExplorer version 21.5, revision 608bb32 [84]) of (a) the crystal structure of 2 and (b) the crystal structure of 2 .In 2 , the intermolecular contacts are established at shorter interatomic separations in general (the features of the map are shifted to shorter d e , d i values), and pronounced interactions are found with H•••N-and H•••Cl-contacts.In the structure of 2 , some shorter N•••N-and N•••Cl-contacts are characteristic of the packing (features inside the red circle in Figure3b), which are not encountered with 2 (cf. the red circle in Figure3a).

FOR PEER REVIEW 19 of 23 Figure A3 .
Figure A3.Fingerprint plots (de vs. di) of the Hirshfeld surface analyses (performed with CrystalExplorer version 21.5, revision 608bb32 [84]) of (a) the crystal structure of 2′ and (b) the crystal structure of 2″.In 2′, the intermolecular contacts are established at shorter interatomic separations in general (the features of the map are shifted to shorter de, di values), and pronounced interactions are found with H•••N-and H•••Cl-contacts.In the structure of 2″, some shorter N•••N-and N•••Cl-contacts are characteristic of the packing (features inside the red circle in Figure3b), which are not encountered with 2′ (cf. the red circle in Figure3a).

Figure A4 .
Figure A4.Percentage distribution (with respect to the area on the Hirshfeld surface) of the intermolecular contacts of 2′ and 2″.

Figure A4 .
Figure A4.Percentage distribution (with respect to the area on the Hirshfeld surface) of the intermolecular contacts of 2 and 2 .