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

The 1:1 stoichiometric reactions of α-amino isobutyric acid (H 2 Aib) and diaminosilanes of the type Si RR ′(N R 1 R 2 ) 2 (SiMe 2 (imidazol-1-yl) 2 , SiMe 2 (NH n Pr) 2 , and Si RR ′(pyrrolidin-1-yl) 2 with R , R ′ = Me,Me, Me,H, Me,Vi, and Et,Et) afforded the pentacoordinate silicon complexes (Aib)Si RR ′(HN R 1 R 2 ) with the release of one equivalent of HN R 1 R 2 . Single-crystal X-ray diffraction analyses confirmed the coordination of the N-donor Lewis base (i.e., imidazole, n -propylamine, and pyrrolidine, respectively) in an axial position of the distorted trigonal-bipyramidal Si-coordination sphere, trans 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 14 N nucleus with its directly bound atoms C and Si, thus causing characteristic shapes of both the 29 Si and 13 C NMR signals of these two atoms in the solid-state spectra. In contrast to the adduct-formation reactions, the analogous conversion of H 2 Aib and SiMe 2 (NH t Bu) 2 did not afford an amine adduct. Instead, a second equivalent of H 2 Aib entered the reaction, and the ionic silicon complex [ t BuNH 3 ] + [(Aib) 2 SiMe] − was obtained and characterized by crystallography and solution NMR spectroscopy.