Valinate and SiMe2 – An interesting couple in pentacoordinate Si-complexes: Templated generation of the dipeptide val-val and formation of an organosilicon-ammonia-adduct

Abstract

Reaction of dichlorodimethylsilane and L-valin in molar ratio 1:2 (with addition of a supporting base, Et3N) afforded a complex mixture of products, from which the pentacoordinate silicon complex (val-val)SiMe2 (1) could be isolated and identified by single-crystal X-ray diffraction analysis. This complex features the di-anion of dipeptide valinylvalin as a tridentate (N,N‘,O)-ligand. Using a stochiometric ratio of starting materials adjusted to the formation of 1 (the use of dichlorodimethylsilane and L-valin in molar ratio 1:1, the additional silane serving as a water trap to foster peptide condensation) allowed for the preparation of this compound in moderate yield. Using 1,1,3,3,5,5-hexamethylcyclotrisilazane, which may serve both as a source of SiMe2 moieties and deprotonator for valin, afforded another pentacoordinate Si-complex, i.e., (val)SiMe2(NH3) (2). In addition to the chelating di-anion of valin its Si atom carries an ammonia molecule as an axial ligand within a trigonal-bipyramidal coordination sphere, as confirmed by single-crystal XRD. For both solids 1 and 229Si CP/MAS NMR spectra were recorded and the chemical shift anisotropy tensors were determined therefrom.