Lewis Acid-Base Adducts of α-Amino Isobutyric Acid-Derived Silaheterocycles and Amines.

The 1:1 stoichiometric reactions of α-amino isobutyric acid (HAib) and diaminosilanes of the type Si'(N) (SiMe(imidazol-1-yl), SiMe(NHPr), and Si'(pyrrolidin-1-yl) with ,' = Me,Me, Me,H, Me,Vi, and Et,Et) afforded the pentacoordinate silicon complexes (Aib)Si'(HN) with the release of one equivalent of HN. Single-crystal X-ray diffraction analyses confirmed the coordination of the N-donor Lewis base (i.e., imidazole, -propylamine, and pyrrolidine, respectively) in an axial position of the distorted trigonal-bipyramidal Si-coordination sphere, to the carboxylate O atom of the Si-chelating Aib-dianion. The N-H moieties of the adduct-forming Lewis bases are involved in N-H⋯O hydrogen bonds with carboxylate groups of adjacent complex molecules, thus supporting the supramolecular structures of these adducts. The equatorially bound NH group of the Aib-dianion is involved in N-H⋯O hydrogen bonds in most cases, and it gives rise to residual dipolar coupling of the N nucleus with its directly bound atoms C and Si, thus causing characteristic shapes of both the Si and C NMR signals of these two atoms in the solid-state spectra. In contrast to the adduct-formation reactions, the analogous conversion of HAib and SiMe(NHBu) did not afford an amine adduct. Instead, a second equivalent of HAib entered the reaction, and the ionic silicon complex [BuNH][(Aib)SiMe] was obtained and characterized by crystallography and solution NMR spectroscopy.