Reversible Neutral Dissociation of the N−Si Dative Bond in Hexacoordinate Hydrido Complexes of Silicon

Abstract

Hexacoordinate silicon dichelates with hydrido and methyl monodentate ligands were prepared by transsilylation. The temperature-dependent 29Si NMR spectra of toluene-d8 or CDCl3 solutions provide evidence for a reversible, neutral dissociation of the dative N−Si bond, accompanied by a shift of the 29Si resonance from high field (hexacoordinate) to lower field (pentacoordinate) with increasing temperature. When a phenyl group is attached directly to silicon (replacing methyl), the N−Si dissociation is prevented, presumably due to electron withdrawal by the phenyl. Two of the silicon complexes were characterized by crystallographic analysis. Quantum chemical calculations at the RB3LYP/6-31G(d) level confirm the stability of an N−Si dissociated species (1.03 kcal mol−1 lower in energy than the hexacoordinate dichelate) and rule out the alternative possibility of an O−Si zwitterionic dissociation, thus supporting the mechanism suggested by the experimental results.