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

The 1:1 stoichiometric reactions of α-amino isobutyric acid (H₂Aib) and diaminosilanes of the type SiRR'(NR¹R²)₂ (SiMe₂(imidazol-1-yl)₂, SiMe₂(NHnPr)₂, and SiRR'(pyrrolidin-1-yl)₂ with R,R' = Me,Me, Me,H, Me,Vi, and Et,Et) afforded the pentacoordinate silicon complexes (Aib)SiRR'(HNR¹R²) with the release of one equivalent of HNR¹R². 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 ¹⁴N nucleus with its directly bound atoms C and Si, thus causing characteristic shapes of both the ²⁹Si and ¹³C NMR signals of these two atoms in the solid-state spectra. In contrast to the adduct-formation reactions, the analogous conversion of H₂Aib and SiMe₂(NHtBu)₂ did not afford an amine adduct. Instead, a second equivalent of H₂Aib entered the reaction, and the ionic silicon complex [tBuNH₃]⁺[(Aib)₂SiMe]^- was obtained and characterized by crystallography and solution NMR spectroscopy.