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.