Lewis Acid-Base Adducts of α-Amino Acid-Derived Silaheterocycles and N-Methylimidazole

In chloroform solution, the reaction of bis(tert-butylamino)dimethylsilane ((tBuNH)2SiMe2) and an α-amino acid (α-amino isobutyric acid, H2Aib; D-phenylglycine, H2Phg; L-valine, H2Val) in the presence of N-methylimidazole (NMI) gave rise to the formation of the pentacoordinate silicon complexes (Aib)SiMe2-NMI, (Phg)SiMe2-NMI and (Val)SiMe2-NMI, respectively. Therein, the amino acid building block was a di-anionic bidentate chelator at the silicon atom. In solution, the complexes were involved in rapid coordination–dissociation equilibria between the pentacoordinate Si complex (e.g., (Aib)SiMe2-NMI) and its constituents NMI and a five-membered silaheterocycle (e.g., (Aib)SiMe2), as shown by 29Si NMR spectroscopy. The energetics of the Lewis acid-base adduct formation and the competing solvation of the NMI molecule by chloroform were assessed with the aid of computational methods. In CDCl3 solution, deuteration of the silaheterocycle NH group proceeded rapidly, with more than 50% conversion within two days. Upon cooling to −44 °C, the chloroform solvates of the adducts (Aib)SiMe2-NMI and (Phg)SiMe2-NMI crystallized from their parent solutions and allowed for their single-crystal X-ray diffraction analyses. In both cases, the Si atom was situated in a distorted trigonal bipyramidal coordination sphere with equatorial Si–C bonds and an equatorial Si–N bond (the one of the silaheterocycle). The axial positions were occupied by a carboxylate O atom of the silaheterocycle and the NMI ligand’s donor-N-atom.


Introduction
The chemistry of hypercoordinate silicon compounds (i.e., compounds in which the Si atom has a coordination number greater than four) have attracted scientists' interest for more than two centuries.In 1812, Davy described the synthesis of the ammonia adduct SiF 4 (NH 3 ) 2 [1].Even though ammonia remained a scarcely encountered ligand in silicon coordination chemistry, various other N-donor molecules were found capable of binding to silanes with electronegative substituents and thus enhancing the Si coordination number, e.g., in the adduct SiCl 4 (pyridine) 2 [2].The use of chelating ligands enhanced the variety of N-donors ligands, which thus formed penta-or hexacoordinate Si-compounds, and various reviews (e.g., [3][4][5][6][7][8]) provide insights into the wealth of silicon coordination chemistry with additional N-donors or with various other ligands.In this context, α-amino acids represent a class of chelators with N-donor functionality, which should be suitable to bind to silicon in a chelating manner (with the formation of five-membered rings, a common motif in Si-coordination chemistry), to enhance the Si coordination number.In some classes of oligodentate O,N-chelators the α-amino acid motif is contained, e.g., in silatranes with a carboxylate moiety (such as I and II [9,10], Figure 1) and in amino acid-derived Schiff base complexes (such as III and IV [11,12]).As many α-amino acids themselves are available from the natural chiral pool, it is noteworthy that only a comparatively small number of silicon complexes with their mono-or di-anionic α-amino acid ligands have been crystallographically characterized so far (compounds V-XVII) [13][14][15][16].
Molecules 2023, 28, x FOR PEER REVIEW 2 of 20 and in amino acid-derived Schiff base complexes (such as III and IV [11,12]).As many αamino acids themselves are available from the natural chiral pool, it is noteworthy that only a comparatively small number of silicon complexes with their mono-or di-anionic α-amino acid ligands have been crystallographically characterized so far (compounds V-XVII) [13][14][15][16].In this rather limited portfolio, our recently reported compound XVII [16] takes on a prominent role as it represents the first crystallographically characterized ammonia adduct of an organosilicon compound (and, moreover, the first adduct of an α-amino acid derived silaheterocycle with a neutral Lewis base).So far, there is no crystallographic evidence for the isolation and structural characterization of silacycles with di-anionic chelators of the α-amino acid type which feature tetracoordinate silicon.Neither simple silacycles of type XVIII nor related spirocycles can be found in the CSD (a search in CSD 2023 version 5.44).Nonetheless, compounds such as (Phe)SiMe2 [17] or (His)SiMe2 [18] (with Phe and His being the di-anions of phenylalanine and histidine, respectively) were reported in the literature more than two decades ago as intermediates in reactions such as amide formation [17] and the functionalization of a trityl type resin [18].Ammonia adduct XVII decomposes when dissolved in chloroform with dissociation of the Si-NH3 bond In this rather limited portfolio, our recently reported compound XVII [16] takes on a prominent role as it represents the first crystallographically characterized ammonia adduct of an organosilicon compound (and, moreover, the first adduct of an α-amino acid derived silaheterocycle with a neutral Lewis base).So far, there is no crystallographic evidence for the isolation and structural characterization of silacycles with di-anionic chelators of the α-amino acid type which feature tetracoordinate silicon.Neither simple silacycles of type XVIII nor related spirocycles can be found in the CSD (a search in CSD 2023 version 5.44).Nonetheless, compounds such as (Phe)SiMe 2 [17] or (His)SiMe 2 [18] (with Phe and His being the di-anions of phenylalanine and histidine, respectively) were reported in the literature more than two decades ago as intermediates in reactions such as amide formation [17] and the functionalization of a trityl type resin [18].Ammonia adduct XVII decomposes when dissolved in chloroform with dissociation of the Si-NH 3 bond and formation of a wealth of decomposition products with tetracoordinate Si atoms [16].Thus, we conclude that this adduct formation, which enhances the Si coordination num-ber, stabilizes these particular five-membered silacycles and, vice versa, this stabilizing effect gave rise to the formation of such a rare ammonia adduct of an organosilicon compound.As the addition of stoichiometric amounts of ammonia to a chloroform solution is hard to accomplish, and the removal of solvent during sample preparation may be accompanied by loss of this volatile ligand, adduct formation with an alternative N-donor molecule may be considered for related studies.N-methylimidazole (NMI) represents such an alternative.In principle, NMI is known as a good N-donor moiety in Si coordination chemistry.With a variety of silicon halides, it induces ionic dissociation of Si-X (X = Cl, Br) bonds with formation of cationic Si-complexes.Structurally characterized examples thereof are known for compounds with tetracoordinate Si: [Me 3 Si(NMI)] + X − (X = Cl [19], Br [20]) [22], Et 2 [23], -(CH 2 ) 5 - [24]), with pentacoordinate Si: [R 2 Si(NMI) 3 ] 2+ 2Br − (R 2 = Et,H [23], Ph 2 [23]) and with hexacoordinate Si: [26], H 2 [26], -(CH 2 ) 4 - [27]).Furthermore, some non-ionic hexacoordinate Si-NMI-complexes have been crystallographically confirmed (Si(o-C 6 H 4 O 2 ) 2 (NMI) 2 [28], XIX [29]), and an adduct of the composition MeSiCl 3 (NMI) 2 is mentioned in the literature [30].In addition to just binding to Si (as in the latter cases) or activating Si-X bonds (as in the former cases), NMI was found to activate Si-Si bonds and thus catalyze the cleavage of a variety of disilanes with formation of oligosilanes [30,31], and it stabilized a low-coordinate transition metal-silicon complex (XX [32]).Because of this prominent tendency of adduct formation with silicon compounds, NMI became our N-donor of choice for the following study.

Compounds Overview
The previously reported compound XVII formed in a reaction of hexamethylcyclotrisilazane and L-valine [16].The N-donor ligand of this adduct (i.e., ammonia) was the leaving group of the SiMe 2 -containing starting material (Scheme 1).In order to avoid the formation of ammonia-stabilized silaheterocycles as a potential competitive reaction, in favor of adduct formation with the target N-donor molecule NMI, we engaged a starting material with a sterically more demanding leaving group (tBuNH 2 ), i.e., bis(tert-butylamino)dimethylsilane (Scheme 2).In chloroform solution, the three amino acids under investigation (α-aminoisobutyric acid and formation of a wealth of decomposition products with tetracoordinate Si atoms [16].Thus, we conclude that this adduct formation, which enhances the Si coordination number, stabilizes these particular five-membered silacycles and, vice versa, this stabilizing effect gave rise to the formation of such a rare ammonia adduct of an organosilicon compound.As the addition of stoichiometric amounts of ammonia to a chloroform solution is hard to accomplish, and the removal of solvent during sample preparation may be accompanied by loss of this volatile ligand, adduct formation with an alternative N-donor molecule may be considered for related studies.N-methylimidazole (NMI) represents such an alternative.In principle, NMI is known as a good N-donor moiety in Si coordination chemistry.With a variety of silicon halides, it induces ionic dissociation of Si-X (X = Cl, Br) bonds with formation of cationic Si-complexes.
In addition to just binding to Si (as in the latter cases) or activating Si-X bonds (as in the former cases), NMI was found to activate Si-Si bonds and thus catalyze the cleavage of a variety of disilanes with formation of oligosilanes [30,31], and it stabilized a lowcoordinate transition metal-silicon complex (XX [32]).Because of this prominent tendency of adduct formation with silicon compounds, NMI became our N-donor of choice for the following study.

Compounds Overview
The previously reported compound XVII formed in a reaction of hexamethylcyclotrisilazane and L-valine [16].The N-donor ligand of this adduct (i.e., ammonia) was the leaving group of the SiMe2-containing starting material (Scheme 1).In order to avoid the formation of ammonia-stabilized silaheterocycles as a potential competitive reaction, in favor of adduct formation with the target N-donor molecule NMI, we engaged a starting material with a sterically more demanding leaving group (tBuNH2), i.e., bis(tert-butylamino)dimethylsilane (Scheme 2).In chloroform solution, the three amino acids under investigation (α-aminoisobutyric acid H2Aib, D-phenylglycine H2Phg and L-valine H2Val) formed the respective target compound, i.e., the pentacoordinate Si complex (Aib)SiMe2-NMI, (Phg)SiMe2-NMI and (Val)SiMe2-NMI, respectively (cf.Section 2.3, NMR spectroscopic characterization).In two cases ((Aib)SiMe2-NMI and (Phg)SiMe2-NMI) we succeeded in obtaining these adducts as crystalline solids at −44 °C.Under these conditions, they crystallized as the chloroform solvates (Aib)SiMe2-NMI • CHCl3 and (Phg)SiMe2-NMI • 2CHCl3, respectively, and their molecular structures were determined by single-crystal X-ray diffraction analysis.Scheme 1. Reaction of the formation of compound XVII [16].Scheme 1. Reaction of the formation of compound XVII [16].Scheme 2. Syntheses of the compounds under investigation.

Single-Crystal X-ray Diffraction
The solvates (Aib)SiMe2-NMI • CHCl3 and (Phg)SiMe2-NMI • 2CHCl3, which crystallized upon storage of the reaction mixture at −44 °C, could be isolated from the supernatant by decantation while cold, but these solids formed a melt at room temperature.In the case of (Phg)SiMe2-NMI • 2CHCl3, which crystallized in compact blocks, the use of an ice-cooled Petri dish was sufficient for crystal preparation for X-ray diffraction analysis.In the case of (Aib)SiMe2-NMI • CHCl3, which formed thin needles, the solid was allowed to melt at room temperature.A small amount of this melt was transferred into a glass capillary, which was then sealed and mounted on the goniometer.Upon slow cooling on the diffractometer, crystals of this solvate formed again, and a single-crystalline needle was located inside the capillary, which was suitable for X-ray diffraction analysis.The molecular structures of these NMI adducts are shown in Figure 2; selected bond lengths and angles are listed in Table 1, and selected parameters of the data collection and structure refinement are listed in Appendix A, Table A1.Scheme 2. Syntheses of the compounds under investigation.

Single-Crystal X-ray Diffraction
The solvates (Aib)SiMe 2 -NMI • CHCl 3 and (Phg)SiMe 2 -NMI • 2CHCl 3 , which crystallized upon storage of the reaction mixture at −44 • C, could be isolated from the supernatant by decantation while cold, but these solids formed a melt at room temperature.In the case of (Phg)SiMe 2 -NMI • 2CHCl 3 , which crystallized in compact blocks, the use of an ice-cooled Petri dish was sufficient for crystal preparation for X-ray diffraction analysis.In the case of (Aib)SiMe 2 -NMI • CHCl 3 , which formed thin needles, the solid was allowed to melt at room temperature.A small amount of this melt was transferred into a glass capillary, which was then sealed and mounted on the goniometer.Upon slow cooling on the diffractometer, crystals of this solvate formed again, and a single-crystalline needle was located inside the capillary, which was suitable for X-ray diffraction analysis.The molecular structures of these NMI adducts are shown in Figure 2; selected bond lengths and angles are listed in Table 1, and selected parameters of the data collection and structure refinement are listed in Appendix A, Table A1.In compounds (Aib)SiMe2-NMI and (Phg)SiMe2-NMI, the Si atom is accommodated in a distorted trigonal bipyramidal coordination sphere, which is set up by the Si-O bond and the formally dative Si-N bond to the additional N-donor ligand (NMI) in axial positions, while the Si-C bonds and the formally covalent Si-N bond to the di-anionic chelator reside in equatorial positions.Among the two NMI adducts and, moreover, with respect to the ammonia adduct XVII, the corresponding bond lengths of these molecules are similar to one another.Most noteworthy, even the Si1-N2 bonds to the additional Ndonor ligand are in a very narrow range in spite of the different donor molecules and their N atoms' hybridization (sp 2 in NMI, sp 3 in ammonia).Corresponding angles in the Si atoms' coordination spheres are also within very narrow ranges, and the geometry index τ5 [33], which is derived therefrom, clearly points at the predominance of the trigonal bipyramidal shape (over square pyramidal) to similar degrees in the three cases.The deviation of the axial angle from linearity (by ca.8-9 deg.) originates only in part from the bite angle of the amino acid di-anion (which is ca. 5 deg.below the right angle).The systematically "too small" C-Si-C angle (with respect to VSEPR) and the resultant  In compounds (Aib)SiMe 2 -NMI and (Phg)SiMe 2 -NMI, the Si atom is accommodated in a distorted trigonal bipyramidal coordination sphere, which is set up by the Si-O bond and the formally dative Si-N bond to the additional N-donor ligand (NMI) in axial positions, while the Si-C bonds and the formally covalent Si-N bond to the di-anionic chelator reside in equatorial positions.Among the two NMI adducts and, moreover, with respect to the ammonia adduct XVII, the corresponding bond lengths of these molecules are similar to one another.Most noteworthy, even the Si1-N2 bonds to the additional N-donor ligand are in a very narrow range in spite of the different donor molecules and their N atoms' hybridization (sp 2 in NMI, sp 3 in ammonia).Corresponding angles in the Si atoms' coordination spheres are also within very narrow ranges, and the geometry index τ 5 [33], which is derived therefrom, clearly points at the predominance of the trigonal bipyramidal shape (over square pyramidal) to similar degrees in the three cases.The deviation of the axial angle from linearity (by ca.8-9 deg.) originates only in part from the bite angle of the amino acid di-anion (which is ca. 5 deg.below the right angle).The systematically "too small" C-Si-C angle (with respect to VSEPR) and the resultant repulsion of the N-donor ligand from the SiMe 2 moiety can be regarded as making another contribution.Within the equatorial arrangement of one Si-N and two Si-C bonds, the latter (because of the lower electronegativity of C vs. N) should exert pronounced repulsion.
Molecules 2023, 28, 7816 6 of 19 Thus, it is a noteworthy observation that the Si coordination spheres of the adducts listed in Table 1 exhibit C-Si-C angles well below 120 • .We interpret this "too small" C-Si-C angle as a result of pronounced "equatoriophilicity". Whereas widening of the C-Si-C angle would agree with VSEPR and Bent's rule [34] (i.e., the central atom's bonds to the substituents with lower electronegativity feature pronounced s-orbital contributions, and thus favor a transition from sp 3 toward sp 2 or even sp hybrid orbitals), it would also shift the Si-C bonds into more axial positions and result in their being more trans-disposed to one another.Thus, we interpret the rather narrow C-Si-C angle as a result of avoidance of a trans-C-Si-C arrangement, which would shift the character of the Si-C bonds from two 2e2c-bonds to a 4e3c-bond system.A search in the CSD (CSD 2023 version 5.44) revealed that most of the pentacoordinate dimethylsilicon compounds of the type Me 2 Si(X,X ,X ) (with X,X ,X being atoms from the group N,O,F,Cl) exhibit C-Si-C angles below or close to 120 • .Only some representants with a pronounced electron deficient Si atom (substituents with very high electronegativity and/or cationic complexes) exhibit C-Si-C angles well above 120 The Si1-N2 bond lengths (to the NMI molecule) in (Aib)SiMe 2 -NMI and (Phg)SiMe 2 -NMI are slightly longer than the corresponding bond in the neutral hexacoordinate Si complex XIX (1.962(1) Å) [29] as well as in the ionic complex [SiPhH(NMI) 4 ] 2+ (1.959(4)-1.970(4)Å) [23], and markedly longer than in the ionic hexacoordinate Si-complex [SiCl 2 (NMI) 4 ] 2+ , in which Si-N bond lengths are found in the range 1.876(3)-1.927(4)Å [26].In related cationic Si complexes with a pentacoordinate Si atom, the axial Si-N(NMI) bonds are markedly longer than the equatorial ones, e.g., in [SiEtH(NMI) 3 ] 2+ (1.979(2), 1.952(2) Å vs. 1.817(2)Å) [23].Whereas the axial Si-N(NMI) bond lengths are similar to those in hexacoordinate Si complexes, the equatorial ones are closer to Si-N(NMI) bonds in related cationic complexes with a tetracoordinate Si atom, e.g., in [SiEt 2 (NMI) 2 ] 2+ (1.796(2) and 1.798(2) Å) [23].With respect to neutral pentacoordinate Si-complexes with NMI as an N-donor ligand, compounds in (Aib)SiMe 2 -NMI and (Phg)SiMe 2 -NMI are the first crystallographically characterized representatives.Some related motifs involve chelate-bound NMI derivatives in axial positions at the pentacoordinate Si atom (compounds XXI [37], XXII [38], XXIII [39], Figure 3).Comparison reveals that the Si-N(NMI) bond lengths of (Aib)SiMe 2 -NMI and (Phg)SiMe 2 -NMI are similar to the corresponding bonds in compounds XXI and XXII, which also feature trans-disposed O-atoms.The partial ionic dissociation of the Si-Cl bond in compound XXIII [39] can be interpreted as a contributor to the shorter Si-N bond, in accordance with the short Si-N(NMI) bonds in cationic hypercoordinate Si-NMI-complexes.
repulsion of the N-donor ligand from the SiMe2 moiety can be regarded as making another contribution.Within the equatorial arrangement of one Si-N and two Si-C bonds, the latter (because of the lower electronegativity of C vs. N) should exert pronounced repulsion.Thus, it is a noteworthy observation that the Si coordination spheres of the adducts listed in Table 1 exhibit C-Si-C angles well below 120°.We interpret this "too small" C-Si-C angle as a result of pronounced "equatoriophilicity". Whereas widening of the C-Si-C angle would agree with VSEPR and Bent's rule [34] (i.e., the central atom´s bonds to the substituents with lower electronegativity feature pronounced s-orbital contributions, and thus favor a transition from sp 3 toward sp 2 or even sp hybrid orbitals), it would also shift the Si-C bonds into more axial positions and result in their being more trans-disposed to one another.Thus, we interpret the rather narrow C-Si-C angle as a result of avoidance of a trans-C-Si-C arrangement, which would shift the character of the Si-C bonds from two 2e2c-bonds to a 4e3c-bond system.A search in the CSD (CSD 2023 version 5.44) revealed that most of the pentacoordinate dimethylsilicon compounds of the type Me2Si(X,X′,X″) (with X,X′,X″ being atoms from the group N,O,F,Cl) exhibit C-Si-C angles below or close to 120°.Only some representants with a pronounced electron deficient Si atom (substituents with very high electronegativity and/or cationic complexes) exhibit C-Si-C angles well above 120°: [Me2SiF3] − 124.4° [35], [Me2Si(OTf)(bipy)] + (OTf = trifluoromethanesulfonate, bipy = 2,2′-bipyridyl) 128.5° [36] and [Me2Si(NMI)3] 2+ 130.8° [19].

NMR Spectroscopic Analyses
As mentioned above, only two of the three adducts under investigation could be isolated as solids for single-crystal X-ray diffraction analyses.Isolation of the solids on a preparative scale was hampered by the good solubility of the compounds in the solvent used and their decomposition in the absence of NMI.Cold decantation and washing with chloroform afforded rather small amounts of these solids, which melted below room temperature and were not useful for solid state NMR spectroscopic characterization.In CDCl3 solution these compounds decomposed, as indicated by their 29 Si{ 1 H} NMR spectra, which exhibited many signals in addition to a broader signal, which can be assigned to the NMI adduct (Amac)SiMe2-NMI (Amac = Aib, Phg).(For these spectra, see Figures S1  and S2).Therefore, we characterized these compounds in their synthesis solution.For that purpose, the syntheses were performed on NMR scale in CDCl3 as the solvent.Table 2 lists the amounts of starting materials used in these experiments.Figure 5 shows the highfield section of the 29 Si{ 1 H} INEPT NMR spectra of the solutions (Amac)SiMe2-NMI-4 (Amac = Aib, Phg, Val).These signals, which represent the main product of each solution (for the full spectra, see Figure S13 in the supporting information), emerge at 29 Si NMR shifts

NMR Spectroscopic Analyses
As mentioned above, only two of the three adducts under investigation could be isolated as solids for single-crystal X-ray diffraction analyses.Isolation of the solids on a preparative scale was hampered by the good solubility of the compounds in the solvent used and their decomposition in the absence of NMI.Cold decantation and washing with chloroform afforded rather small amounts of these solids, which melted below room temperature and were not useful for solid state NMR spectroscopic characterization.In CDCl 3 solution these compounds decomposed, as indicated by their 29 Si{ 1 H} NMR spectra, which exhibited many signals in addition to a broader signal, which can be assigned to the NMI adduct (Amac)SiMe 2 -NMI (Amac = Aib, Phg).(For these spectra, see Figures S1  and S2).Therefore, we characterized these compounds in their synthesis solution.For that purpose, the syntheses were performed on NMR scale in CDCl 3 as the solvent.Table 2 lists the amounts of starting materials used in these experiments.Figure 5 shows the highfield section of the 29 Si{ 1 H} INEPT NMR spectra of the solutions (Amac)SiMe 2 -NMI-4 (Amac = Aib, Phg, Val).These signals, which represent the main product of each solution (for the full spectra, see Figure S13 in the supporting information), emerge at 29 Si NMR shifts characteristic of SiMe 2 -compounds with pentacoordinate silicon atoms.For example, compound XVII has a 29 Si NMR shift of −75.5 ppm in the solid state [16], and compound XXI produces a signal at −59.2 ppm [37].The position of the signal strongly depends on the stoichiometry of the starting materials used and on the degree of dilution, as shown for the series of (Aib)SiMe 2 -NMI batches in Figure 6.Thus, we conclude that these NMI adducts are part of dynamic equilibria, which involve compounds with tetracoordinate and compounds with a pentacoordinate Si atom (as indicated in Scheme 2).Both lowered NMI concentration (cf.batch (Aib)SiMe 2 -NMI-2) and lowered total concentration (cf. the diluted batch (Aib)SiMe 2 -NMI-4-dil) shift the equilibrium toward Si-N(NMI) bond dissociation and enhanced concentration of the compound with a tetracoordinate Si atom.characteristic of SiMe2-compounds with pentacoordinate silicon atoms.For example, compound XVII has a 29 Si NMR shift of −75.5 ppm in the solid state [16], and compound XXI produces a signal at −59.2 ppm [37].The position of the signal strongly depends on the stoichiometry of the starting materials used and on the degree of dilution, as shown for the series of (Aib)SiMe2-NMI batches in Figure 6.Thus, we conclude that these NMI adducts are part of dynamic equilibria, which involve compounds with tetracoordinate and compounds with a pentacoordinate Si atom (as indicated in Scheme 2).Both lowered NMI concentration (cf.batch (Aib)SiMe2-NMI-2) and lowered total concentration (cf. the diluted batch (Aib)SiMe2-NMI-4-dil) shift the equilibrium toward Si-N(NMI) bond dissociation and enhanced concentration of the compound with a tetracoordinate Si atom.2).The spectra are internally referenced to SiMe4 at 0 ppm.2).The spectra are internally referenced to SiMe 4 at 0 ppm.
In the spectra in Figures 5 and 6, a small additional peak emerges slightly upfield shifted relative to each main signal.Time dependent 29 Si NMR spectroscopic monitoring of sample (Aib)SiMe 2 -NMI-4 (Figure 7a) revealed that this peak became the predominant signal in the course of two days.Simultaneously, the 13 C NMR signal of this compound's α-C atom (Figure 7b) exhibited a similar development of a new peak, and the solvent signal (Figure 7c) indicated the conversion of CDCl 3 into CHCl 3 .Thus, the respective new signals in Figure 7a,b are assigned to the N-deuterated compound (Aib)SiMe 2 -NMI-4.
In Figure 7a, it is evident that the positions of the signals exhibit slight variations.A temperature-dependent study of solution (Aib)SiMe 2 -NMI-4 (spectra recorded at 15, 20, and 25 • C) revealed the strong temperature dependence of these 29 Si NMR signals' shifts and of the position of the equilibrium of NMI adduct formation and dissociation (Figure 8).Thus, we interpret the 29 Si NMR shift variations in Figure 7a as an effect of the temperature variations in the NMR laboratory in the course of this time-dependent study.2).The spectra are internally referenced to SiMe4 at 0 ppm.
In the spectra in Figures 5 and 6, a small additional peak emerges slightly upfield shifted relative to each main signal.Time dependent 29 Si NMR spectroscopic monitoring of sample (Aib)SiMe2-NMI-4 (Figure 7a) revealed that this peak became the predominant signal in the course of two days.Simultaneously, the 13 C NMR signal of this compound´s α-C atom (Figure 7b) exhibited a similar development of a new peak, and the solvent signal (Figure 7c) indicated the conversion of CDCl3 into CHCl3.Thus, the respective new signals in Figure 7a,b are assigned to the N-deuterated compound (Aib)SiMe2-NMI-4.In Figure 7a, it is evident that the positions of the signals exhibit slight variations.A temperature-dependent study of solution (Aib)SiMe2-NMI-4 (spectra recorded at 15, 20, and 25 °C) revealed the strong temperature dependence of these 29 Si NMR signals´ shifts and of the position of the equilibrium of NMI adduct formation and dissociation (Figure 8).Thus, we interpret the 29 Si NMR shift variations in Figure 7a as an effect of the temperature variations in the NMR laboratory in the course of this time-dependent study.2).The spectra are internally referenced to SiMe4 at 0 ppm.
In the spectra in Figures 5 and 6, a small additional peak emerges slightly upfield shifted relative to each main signal.Time dependent 29 Si NMR spectroscopic monitoring of sample (Aib)SiMe2-NMI-4 (Figure 7a) revealed that this peak became the predominant signal in the course of two days.Simultaneously, the 13 C NMR signal of this compound´s α-C atom (Figure 7b) exhibited a similar development of a new peak, and the solvent signal (Figure 7c) indicated the conversion of CDCl3 into CHCl3.Thus, the respective new signals in Figure 7a,b are assigned to the N-deuterated compound (Aib)SiMe2-NMI-4.In Figure 7a, it is evident that the positions of the signals exhibit slight variations.A temperature-dependent study of solution (Aib)SiMe2-NMI-4 (spectra recorded at 15, 20, and 25 °C) revealed the strong temperature dependence of these 29 Si NMR signals´ shifts and of the position of the equilibrium of NMI adduct formation and dissociation (Figure 8).Thus, we interpret the 29 Si NMR shift variations in Figure 7a as an effect of the temperature variations in the NMR laboratory in the course of this time-dependent study.As the NMR shifts of these compounds (both 29 Si shifts as well as 1 H and 13 C NMR shifts) are highly dependent on concentration and temperature and, moreover, the solutions used in this study also contained the byproduct tBuNH 2 in stoichiometric amounts, we do not report the NMR shifts observed as characteristic fingerprint features of these complexes.Instead, the sets of 1 H, 13   As the NMR shifts of these compounds (both 29 Si shifts as well as 1 H and 13 C NMR shifts) are highly dependent on concentration and temperature and, moreover, the solutions used in this study also contained the byproduct tBuNH2 in stoichiometric amounts, we do not report the NMR shifts observed as characteristic fingerprint features of these complexes.Instead, the sets of 1 H, 13

Calculation of 29 Si NMR Shifts
As the solution NMR experiments (cf.Section 2.3) have indicated, the species (Amac)SiMe2 with a tetracoordinate Si atom (Amac = generic expression for the di-anion of the respective α-amino acid used) and NMI are in a dynamic equilibrium with the NMI adduct (Amac)SiMe2-NMI (with a pentacoordinate Si atom).As the tetracoordinate Sicompounds (Amac)SiMe2 are not stable (according to our previous experience with the decomposition of compound XVII in chloroform solution), and the compounds of the type (Amac)SiMe2-NMI of the current study were not accessible for solid state NMR investigations, we evaluated the 29 Si NMR shifts of those species by computational methods.The knowledge of these boundaries (i.e., of the expected 29 Si NMR shifts of the individual compounds with a tetra-and pentacoordinate Si atom) allows us to evaluate the position of the dynamic equilibrium under the conditions of this study.The 29 Si NMR signal observed (at δ 29 Siexp) appears at the weighted chemical shift between the shifts of the two contributors (Amac)SiMe2 and (Amac)SiMe2-NMI: δ 29 Si(observed) = δ 29 Si((Amac)SiMe2)•X((Amac)SiMe2) + δ 29 Si((Amac)SiMe2-NMI)•X((Amac)SiMe2-NMI), with X being the respective molar fractions.For that purpose, the three species (Amac)SiMe2-CHCl3 and their respective NMI adducts (Amac)SiMe2-NMI-CHCl3 as well as reference molecule SiMe4 were optimized at the same level of theory within a continuous solvent environment model (COSMO) for chloroform (We decided to use the CHCl3 solvated representatives for this purpose because further calculations, cf.Section 2.4.2, have shown that solvation of the carbonyl O atom by CHCl3 is energetically even more important than NMI-coordination at the Si atom).The calculated 29 Si NMR shifts as

Calculation of 29 Si NMR Shifts
As the solution NMR experiments (cf.Section 2.3) have indicated, the species (Amac)SiMe 2 with a tetracoordinate Si atom (Amac = generic expression for the di-anion of the respective α-amino acid used) and NMI are in a dynamic equilibrium with the NMI adduct (Amac)SiMe 2 -NMI (with a pentacoordinate Si atom).As the tetracoordinate Si-compounds (Amac)SiMe 2 are not stable (according to our previous experience with the decomposition of compound XVII in chloroform solution), and the compounds of the type (Amac)SiMe 2 -NMI of the current study were not accessible for solid state NMR investigations, we evaluated the 29 Si NMR shifts of those species by computational methods.The knowledge of these boundaries (i.e., of the expected 29 Si NMR shifts of the individual compounds with a tetra-and pentacoordinate Si atom) allows us to evaluate the position of the dynamic equilibrium under the conditions of this study.The 29 Si NMR signal observed (at δ 29 Si exp ) appears at the weighted chemical shift between the shifts of the two contributors (Amac)SiMe 2 and (Amac)SiMe 2 -NMI: δ 29 Si(observed) = δ 29 Si((Amac)SiMe 2 )•X((Amac)SiMe 2 ) + δ 29 Si((Amac)SiMe 2 -NMI)•X((Amac)SiMe 2 -NMI), with X being the respective molar fractions.For that purpose, the three species (Amac)SiMe 2 -CHCl 3 and their respective NMI adducts (Amac)SiMe 2 -NMI-CHCl 3 as well as reference molecule SiMe 4 were optimized at the same level of theory within a continuous solvent environment model (COSMO) for chloroform (We decided to use the CHCl 3 solvated representatives for this purpose because further calculations, cf.Section 2.4.2, have shown that solvation of the carbonyl O atom by CHCl 3 is energetically even more important than NMI-coordination at the Si atom).The calculated 29 Si NMR shifts as well as the experimentally observed data are collated in Table 3.The combination of these experimentally and computationally obtained 29 Si NMR shifts indicates that in the three samples under investigation, only 85-90% of the silicon compound are NMI-complexes and, in spite of the large excess of NMI used (four equivalents), more than 10% of the tetracoordinate Si-compound is contained in the equilibrium mixture.  2Si NMR shifts of species (Amac)SiMe 2 -CHCl 3 and (Amac)SiMe 2 -NMI-CHCl 3 ), experimentally observed chemical shifts of equilibrium mixtures (cf. Figure 5) and estimated molar fraction X of the pentacoordinate Si complex (Amac)SiMe 2 -NMI-CHCl 3 ) in this equilibrium.

(Amac)
δ 29 Si calc ((Amac)SiMe 2 -CHCl 3 ) The experimental data are based on in situ monitoring of samples which contain rather high concentrations of the sample material (and thus the solvent used must be regarded as a stoichiometric reactant used in excess rather than having a very large excess of solvent).Furthermore, the samples contain another component (tBuNH 2 ), which also has hydrogen bridge donor and acceptor qualities, which may thus compete with CHCl 3 and NMI in hydrogen bond-forming reactions.Therefore, we investigated the energetics of some competitive donor-acceptor interactions in simplified systems [(Amac)SiMe 2 + NMI + CHCl 3 ] with the aid of computational methods (Section 2.4.2).

Evaluation of Structural Effects of Coordination and CHCl 3 Solvation
For the current study, we investigated the systems [(Amac)SiMe 2 + NMI + CHCl 3 ] as diluted solutions in excess solvent.For that purpose, the molecules of the silicon-containing species, the free NMI molecule, and the individual CHCl 3 molecule (which was used for explicit solvation of particular hydrogen bond acceptor sites) as well as their adducts were optimized in a continuous solvent environment model (COSMO).The optimized molecular structures of the optimized silicon compounds (Scheme 3) as well as those of NMI and CHCl 3 are shown in the supporting information (Figures S14-S28).Some selected structural features of the four different types of silicon compounds are listed in Tables 4 and 5.
well as the experimentally observed data are collated in Table 3.The combination of these experimentally and computationally obtained 29 Si NMR shifts indicates that in the three samples under investigation, only 85-90% of the silicon compound are NMI-complexes and, in spite of the large excess of NMI used (four equivalents), more than 10% of the tetracoordinate Si-compound is contained in the equilibrium mixture.Table 3. Calculated 29 Si NMR shifts of species (Amac)SiMe2-CHCl3 and (Amac)SiMe2-NMI-CHCl3), experimentally observed chemical shifts of equilibrium mixtures (cf. Figure 5) and estimated molar fraction X of the pentacoordinate Si complex (Amac)SiMe2-NMI-CHCl3) in this equilibrium.

(Amac)
δ 29 Sicalc((Amac)SiMe2-CHCl3) The experimental data are based on in situ monitoring of samples which contain rather high concentrations of the sample material (and thus the solvent used must be regarded as a stoichiometric reactant used in excess rather than having a very large excess of solvent).Furthermore, the samples contain another component (tBuNH2), which also has hydrogen bridge donor and acceptor qualities, which may thus compete with CHCl3 and NMI in hydrogen bond-forming reactions.Therefore, we investigated the energetics of some competitive donor-acceptor interactions in simplified systems [(Amac)SiMe2 + NMI + CHCl3] with the aid of computational methods (Section 2.4.2).

Evaluation of Structural Effects of NMI Coordination and CHCl3 Solvation
For the current study, we investigated the systems [(Amac)SiMe2 + NMI + CHCl3] as diluted solutions in excess solvent.For that purpose, the molecules of the siliconcontaining species, the free NMI molecule, and the individual CHCl3 molecule (which was used for explicit solvation of particular hydrogen bond acceptor sites) as well as their adducts were optimized in a continuous solvent environment model (COSMO).The optimized molecular structures of the optimized silicon compounds (Scheme 3) as well as those of NMI and CHCl3 are shown in the supporting information (Figures S14-S28).Some selected structural features of the four different types of silicon compounds are listed in Tables 4 and 5. Scheme 3. Atom numbering scheme for the comparison of selected bond lengths in the optimized molecular structures of (Amac)SiMe2-NMI and (Amac)SiMe2 as well as their chloroform solvates (Amac)SiMe2-NMI-CHCl3 and (Amac)SiMe2-CHCl3 for Amac = Aib, Phg, Val (cf.Tables 4 and 5).Scheme 3. Atom numbering scheme for the comparison of selected bond lengths in the optimized molecular structures of (Amac)SiMe 2 -NMI and (Amac)SiMe 2 as well as their chloroform solvates (Amac)SiMe 2 -NMI-CHCl 3 and (Amac)SiMe 2 -CHCl 3 for Amac = Aib, Phg, Val (cf.Tables 4 and 5).

Table 4.
Comparison of selected bond lengths (Å) in the optimized molecular structures of the tetracoordinate Si-compounds (Amac)SiMe 2 and their chloroform solvates (Amac)SiMe 2 -CHCl 3 ) for Amac = Aib, Phg, Val.(Ø∆ is the average bond length change upon solvation with chloroform.For labels of relevant atoms, cf.Scheme 3).Both Tables 4 and 5 mirror the effect of chloroform solvation of the carbonyl O atom on the bond lengths in the COO-motif and the Si coordination sphere in their last column.The Ø∆ listed there are the average values (from the three sets of couples of compounds) of bond lengthening (+) or shortening (−) upon chloroform solvation.In the tetracoordinate Sicompounds (Amac)SiMe 2 (Table 4) The formation reactions of the NMI adducts (Amac)SiMe 2 -NMI out of the respective heterocycle (Amac)SiMe 2 and NMI are predicted to be exothermal (up to −11.9 kcal•mol −1 ) but endergonic at room temperature (up to 4.0 kcal•mol −1 ).The formation of the Hbonded solvates (Amac)SiMe 2 -CHCl 3 is less exothermal (up to −5.6 kcal•mol −1 ) and more endergonic (up to 8.3 kcal•mol −1 ) than NMI-Si-complex formation.Moreover, this solvate formation is energetically similar to NMI-CHCl 3 -solvation (exothermal, −5.1 kcal•mol −1 , endergonic, 7.0 kcal•mol −1 ).Thus, coordination of NMI to the Si atom of (Amac)SiMe 2 should be slightly favored over the formation of an H-bonded adduct with the solvent.
Note: The predicted endergonic character of the aforementioned reactions may originate from the assumption of isolated molecules (in a polar environment, COSMO), which is comparable to entropy effects in gas phase (a "polar gas phase", though).However, experiments were carried out in the condensed phase.All of these reactions have in common a decrease of the number of molecules by 1 mol (from starting materials to products).Thus, the overestimation of endergonic character could be described as the effect of the loss of entropy of vaporization for 1 mol of molecules, which should be close to 6 kcal•mol −1 at 293.15 K for molecules without specific interactions (according to Trouton's rule [40]).Taking this into consideration, the corrected Gibbs free energies should be closer to 0 kcal•mol −1 (slightly exergonic for the formation of (Amac)SiMe 2 -NMI, and, if still endergonic, only slightly endergonic for the formation of chloroform solvates (Amac)SiMe 2 -CHCl 3 and NMI-CHCl 3 .) As in excess chloroform (used as the solvent), the non-solvated species (Amac)SiMe 2 and free NMI appear unlikely (in the latter case, experimental thermochemical studies revealed the exothermal dissolution of NMI in chloroform [41]); we consider the solvates (Amac)SiMe 2 -CHCl 3 and NMI-CHCl 3 as more relevant starting materials involved in further reactions such as the formation of compounds (Amac)SiMe 2 -NMI-CHCl 3 .Therefore, instead of the formation of (Amac)SiMe 2 -NMI-CHCl 3 out of the three individual components and (Amac)SiMe 2 + NMI + CHCl 3 (associated with changes in energy up to −18.6 kcal•mol −1 and in Gibbs free energy up to 10.9 kcal•mol −1 , which can thus be expected to be slightly exergonic upon correction by ca. 12 kcal•mol −1 , i.e., the number of molecules changed by 2 mol in this case), we regard the reaction (Amac)SiMe 2 -CHCl 3 + NMI-CHCl 3 → (Amac)SiMe 2 -NMI-CHCl 3 + CHCl 3 as more relevant.The changes in energy and Gibbs free energy associated with this reaction (in Table 6 with the suffix "eff.", up to −7.9 and to −5.1 kcal•mol −1 , respectively) are slightly exothermal and exergonic, respectively, and should reflect the thermodynamic foundation for the association-dissociation equilibria in these solutions in a more reasonable way.Nonetheless, this prediction is associated with a too-exergonic Gibbs free energy.As the solution 29 Si NMR experiments have shown that both tetra-and pentacoordinate Si-complexes must coexist in an equilibrium in concentrations of the same order of magnitude, it should be closer to 0 kcal•mol −1 .We attribute the deviations between the experiment and computational study to the more complex system in the real solutions.The latter involve further components (such as tBuNH 2 ), which may contribute to entropy in a statistical manner, and they involve further possibilities of specific interactions (such as hydrogen bonding of other H-bond donor and acceptor moieties), which have not been considered in this study.For example, even in case of the simple molecule NMI, further solvation with chloroform (which exceeds the 1:1 adduct formation) may be considered to contribute.Rakipov et al. attributed the pronounced exothermal reaction of NMI and chloroform to the 1:2 solvate formation in chloroform-rich solution [41].
Apart from the deviation of the exergonic character of the reactions from the expected range of close to 0 kcal•mol −1 , the energy values listed in Table 6 give rise to the conclusion that these systems exhibit cooperative adduct formation.The simultaneous binding of both chloroform (to the carbonyl O atom) and NMI (to the Si atom) at the (Amac)SiMe 2 moiety with the formation of (Amac)SiMe 2 -NMI-CHCl 3 is thermodynamically favored over the individual binding of NMI and CHCl 3 with formation of (Amac)SiMe 2 -NMI and (Amac)SiMe 2 -CHCl 3 .This is reflected in Table 6 by the entries with the suffix "coop.",which represent the energetics of the reaction (Amac)SiMe 2 -NMI + (Amac)SiMe 2 -CHCl 3 → (Amac)SiMe 2 -NMI-CHCl 3 + (Amac)SiMe 2 .In addition to the cooperative effect of the binding of NMI and CHCl 3 to the (Amac)SiMe 2 moiety on the molecular structures of (Amac)SiMe 2 -NMI-CHCl 3 (shortening of Cl 3 C-H•••O H-bonds by NMI-coordination at Si, and shortening of Si-N(NMI) bonds by chloroform solvation of the carbonyl O atom), this mutual binding makes a thermodynamic contribution to the stabilization of these pentacoordinate silicon complexes.

General Considerations
The starting material (tBuNH) 2 SiMe 2 was prepared according to a published method [42].The amino acids α-aminoisobutyric acid (Roth, Karlsruhe, Germany, ≥97%), D-phenylglycine, and L-valine (Merck, Darmstadt, Germany, ≥99%) were commercially available and were used without further purification.(Note: For the chiral amino acids, phenylglycine, and valine, the choice of the respective enantiomer was made by the availability of the starting materials from previous studies.)N-Methylimidazole (Sigma-Aldrich, Steinheim, Germany, ≥99%), chloroform, stabilized with amylenes (Honeywell, Seelze, Germany, ≥99.5%) and CDCl 3 (Deutero, Kastellaun, Germany, 99.8%) 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-S13 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 reported relative to Me 4 Si (0 ppm) as internal reference.(Note: The referencing of 1 H and 13 C chemical shifts against solvent signals is not useful in these systems which contain large amounts of strong hydrogen bond acceptors, e.g., NMI.Such components may influence the 1 H and 13 C chemical shift of chloroform noticeably.For instance, in a 1 H NMR spectrum of a solution of NMI in TMS-containing CDCl 3 , which was recorded for a purity check of the NMI used, the residual CHCl 3 signal emerged at 7.37 ppm rather than at 7.26 ppm in neat CDCl 3 .)For single-crystal X-ray diffraction analysis of (Phg)SiMe 2 -NMI • 2CHCl 3 a crystal was selected under an inert oil on an ice-cooled Petri dish, mounted on a glass capillary and instantly moved to the cold nitrogen stream of the diffractometer.In the case of (Aib)SiMe 2 -NMI • CHCl 3 , the compound was allowed to melt at room temperature, whereupon a small amount of the melt was transferred into a glass capillary, which was then sealed, mounted on the goniometer, and slowly cooled to allow for the re-crystallization of the compound inside the capillary on the diffractometer.(The crystallization procedure was similar to our approach of crystallizing chlorosilanes for X-ray diffraction analyses [43].)Diffraction data were collected on a Stoe IPDS-2 diffractometer (STOE, Darmstadt, Germany) using Mo Kα-radiation.Data integration and absorption correction were performed with the STOE software XArea (version 1.75) and XShape (version 2.17), respectively.The structures were solved using SHELXT [44] and refined with the full-matrix least-squares methods of F 2 against all reflections with SHELXL-2018/3 [45,46].All non-hydrogen atoms were anisotropically refined, and hydrogen atoms were isotropically refined in an idealized position (riding model).For details of data collection and refinement see Appendix A, Table A1.Graphics of molecular structures were generated with ORTEP-3 [47,48] and POV-Ray 3.7 [49].
The geometry optimizations were carried out with ORCA 5.0.3 [50] using the restricted PBE0 functional with relativistically recontracted Karlsruhe basis sets ZORA-def2-TZVPP [51,52] for all atoms, the scalar relativistic ZORA Hamiltonian [53,54], atompairwise dispersion correction with the Becke-Johnson damping scheme (D3BJ) [55,56], and COSMO solvation (CHCl 3 , ε = 4.8, rsolv = 3.17).Very-TightSCF and slowconv options were applied and the DEFGRID3 was used with a radial integration accuracy of 10 for Si for all calculations.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).NMR calculations were carried out with ORCA 5.0.3 at the same level of theory as mentioned above.Graphics were generated using ChemCraft [57].

Synthesis and Characterization
Compound (Aib)SiMe 2 -NMI: In a small Schlenk tube, equipped with a magnetic stirring bar, α-amino isobutyric acid (H 2 Aib, 295 mg, 2.86 mmol) was evacuated and then set under an argon atmosphere.Upon addition of chloroform (2 mL) and NMI (0.99 g, 12.0 mmol) the mixture was cooled in an ice bath.To the cold mixture, the aminosilane (tBuNH) 2 SiMe 2 (623 mg, 3.08 mmol) was added with a syringe, and stirring at 0 • C was continued for 6 h.(During that time, the amino acid had dissolved, and a clear solution was obtained.)Thereafter, the solution was immediately placed in a freezer (at −44 • C), where crystalline needles of the product formed over the course of some days.Upon the transfer of the Schlenk tubes into a cold bath (n-decane, cooled with dry ice, ca.−30 • C), the supernatant was removed with a syringe; the crystals were washed with a small amount of cold chloroform (ca. 1 mL, also cooled to −30 • C prior to use) and briefly dried in vacuum while cold.The product was then stored in a small ice bath, which was allowed to attain room temperature over the course of 1 h.Melting of the crystalline product was observed at ca. 15 • C water bath temperature.Yield: 464 mg, 1.29 mmol, 45% (yield is based on the composition (Aib)SiMe 2 -NMI • CHCl 3 , as found in the single-crystal X-ray diffraction analysis.According to 29 Si NMR spectroscopy, the compound undergoes decomposition when dissolved in CDCl 3 (cf.Figure S1 in the supporting information).
For further characterization of compounds (Amac)SiMe 2 -NMI (Amac = Aib, Phg, Val), solution samples were synthesized in CDCl 3 and investigated with 1 H, 13 C{ 1 H} and 29 Si{ 1 H} INEPT NMR spectroscopy in situ.The amounts of starting materials used are listed in Table 2.The same synthesis procedure (as listed above for (Aib)SiMe 2 -NMI) applies.For NMR spectroscopic data of these samples, see Figures S3-S13 in the supporting information.

Conclusions
To date, the solvates (Aib)SiMe 2 -NMI • CHCl 3 and (Phg)SiMe 2 -NMI • 2CHCl 3 represent the first crystallographically characterized neutral Si-NMI-adducts with a pentacoordinate silicon atom.In chloroform solution, compounds of the type (Amac)SiMe 2 -NMI (Amac = di-anionic moiety of an α-amino acid, in our study Amac = Aib, Phg, Val) are involved in association-dissociation equilibria.As none of the tetracoordinate silicon compounds of type (Amac)SiRR' (Amac = any di-anionic moiety of an α-amino acid, R,R' = any kind of substituent) has been characterized crystallographically yet (according to a search in CSD 2023 version 5.44), and literature reports allow the conclusion that these species are relevant as reaction intermediates [17,18], the herein reported NMI adducts indicate adduct formation with a Lewis base as a tool to stabilize these compounds and to isolate them, which may become useful for their deliberate application as starting materials in further reactions.(In this regard, the SiMe 2 functionalization as a special kind of silylation may be of particular interest for amide formation.Recent studies have shown that methyltrimethoxysilane can be useful as a tool in direct amidation of carboxylic acids [58], and triaryl silanols may act as catalysts in amidation reactions [59]).Our crystallographic and computational investigation of chloroform solvates of these NMI adducts highlights the role of the presence of an H-bond donor at the carbonyl atom of the α-amino acid moiety.It enhances the binding of the additional Lewis base to the Si atom and thus aids the Lewis base stabilization of compounds (Amac)SiRR'.Both NMI and chloroform bind to (Amac)SiMe 2 (Amac = Aib, Phg, Val) in a cooperative manner.This general principle (Lewis base and H-bond donor binding to Si and carbonyl-O atom respectively) is also true for the previously reported ammonia adduct (Val)SiMe 2 -NH 3 (compound XVII), in which the Lewis base (NH 3 ) also serves as an H-bond donor to an adjacent molecule in the solid state.The current study serves as a pioneer: The principle of stabilization and isolation of compounds (Amac)SiRR' by mutual stabilization with lone pair donor atoms at Si and H-bond donors at the carbonyl group is yet to be explored, and various potential fields of application for the compounds derived therefrom can be found in organic syntheses.

Figure 1 .
Figure 1.Examples of compounds which feature α-amino acid motifs in silatranes (I,II) and in Schiff base complexes of silicon (III,IV), overview of crystallographically characterized Si-compounds with mono-and/or dianionic α-amino acid anions as O,N-bidentate chelators (V-XVII).(Related silacyclic motifs with tetracoordinate Si atom, XVIII, with R,R′,R″ = H, hydrocarbyl, have not yet been confirmed crystallographically.) Selected examples of NMI adducts of silicon compounds: XIX and XX.For clarity reasons, formally dative bonds (to additional charge neutral lone pair donors) are drawn as dashed lines.

Figure 1 .
Figure 1.Examples of compounds which feature α-amino acid motifs in silatranes (I,II) and in Schiff base complexes of silicon (III,IV), overview of crystallographically characterized Si-compounds with mono-and/or dianionic α-amino acid anions as O,N-bidentate chelators (V-XVII).(Related silacyclic motifs with tetracoordinate Si atom, XVIII, with R,R ,R = H, hydrocarbyl, have not yet been confirmed crystallographically.) Selected examples of NMI adducts of silicon compounds: XIX and XX.For clarity reasons, formally dative bonds (to additional charge neutral lone pair donors) are drawn as dashed lines.

Figure 3 .
Figure 3. Examples of crystallographically characterized neutral pentacoordinate Si-compounds which feature a chelate-bound NMI derivative (NMI′) as an N-donor ligand in the trigonal bipyramidal Si-coordination sphere.Their Si-N(NMI′) bond lengths are given in Å.

Figure 3 .
Figure 3. Examples of crystallographically characterized neutral pentacoordinate Si-compounds which feature a chelate-bound NMI derivative (NMI ) as an N-donor ligand in the trigonal bipyramidal Si-coordination sphere.Their Si-N(NMI ) bond lengths are given in Å.In contrast to compound XVII, which crystallizes from chloroform without the inclusion of solvent of crystallization, both (Aib)SiMe 2 -NMI and (Phg)SiMe 2 -NMI crystallize as chloroform solvates, and in both cases the solvate molecules establish Cl 3 C-H•••O contacts with the carbonyl O atom O2 of the amino acid moiety (Figure4).Furthermore, in both

Table 2 .1
Amounts of starting materials (mg, mmol) used for the in situ characterization of compounds (Aib)SiMe 2 -NMI, (Phg)SiMe 2 -NMI and (Val)SiMe 2 -NMI in CDCl 3 solution.(For each batch, 2.0 mL of CDCl 3 were used).With respect to the 1:1 stoichiometry of the reaction of the respective amino acid and (tBuNH) 2 SiMe 2 , as shown in Scheme 2, slight excess of the silane (tBuNH) 2 SiMe 2 was used to achieve complete conversion of the amino acid (which has poor solubility in chloroform) within a few hours. 2 A sample of 0.3 mL of batch (Aib)SiMe 2 -NMI-4 was diluted with 0.3 mL of CDCl 3 .

33 1
With respect to the 1:1 stoichiometry of the reaction of the respective amino acid and (tBuNH)2SiMe2, as shown in Scheme 2, slight excess of the silane (tBuNH)2SiMe2 was used to achieve complete conversion of the amino acid (which has poor solubility in chloroform) within a few hours.2A sample of 0.3 mL of batch (Aib)SiMe2-NMI-4 was diluted with 0.3 mL of CDCl3.

Figure 7 .
Figure 7. Selected parts of (a) the 29 Si{ 1 H} INEPT and (b,c) 13 C{ 1 H} NMR spectra of product solution (Aib)SiMe2-NMI-4 at the indicated times of storage at room temperature.The spectra are internally referenced to SiMe4 at 0 ppm.

Figure 7 .
Figure 7. Selected parts of (a) the 29 Si{ 1 H} INEPT and (b,c) 13 C{ 1 H} NMR spectra of product solution (Aib)SiMe2-NMI-4 at the indicated times of storage at room temperature.The spectra are internally referenced to SiMe4 at 0 ppm.

Figure 7 .
Figure 7. Selected parts of (a) the 29 Si{ 1 H} INEPT and (b,c) 13 C{ 1 H} NMR spectra of product solution (Aib)SiMe 2 -NMI-4 at the indicated times of storage at room temperature.The spectra are internally referenced to SiMe 4 at 0 ppm.
, solvation of the carbonyl O atom results in Cl 3 C-H•••O hydrogen bonds of ca.2.05-2.11Å H•••O separation.This causes a slight lengthening of the C=O bond (by 0.004 Å), shortening of the C-O bond (by 0.006 Å), and slight lengthening of the Si-O bond (by 0.004 Å).The effects on other bonds are less pronounced.In the corresponding NMI adducts with a pentacoordinate Si atom (Table 5), solvation of the carbonyl O atom is more pronounced (Cl 3 C-H•••O hydrogen bonds of ca.1.97-2.02Å H•••O separation).Even though the effect on the bond lengths in the carboxyl group is similar to that of the corresponding NMI-free tetracoordinate Si-compounds, the Si-O bond lengthening is noticeable more pronounced (0.011 Å).This Si-O bond lengthening is accompanied by an even more pronounced Si-N(NMI) bond shortening (by 0.015 Å).These observations indicate cooperative "solvation" of compounds (Amac)SiMe 2 by chloroform and NMI.While CHCl 3 solvation induces pronounced Si-N(NMI) bond shortening, the presence of NMI at the Si atom allows for more pronounced solvation of the carbonyl O atom (shorter Cl 3 C-H•••O hydrogen bonds).In this regard, this comparison is in accordance with the discussion of the molecular structures of solvates (Aib)SiMe 2 -NMI • CHCl 3 and (Phg)SiMe 2 -NMI • 2CHCl 3 , which deliver experimental support for longer Si-O and shorter Si-N(NMI) bonds in case of enhanced chloroform solvation of the carbonyl O atom.
1Because of the different individual labels of the Si-bound C atoms, a generic "C" is used here, and groups of relevant bond lengths or angles are listed without differentiation between the two Si-bound C atoms.

Table 5 .
Comparison of selected atom distances (Å) in the optimized molecular structures of the pentacoordinate Si-compounds (Amac)SiMe 2 -NMI and their chloroform solvates (