Probing the non-innocent nature of an amino-functionalised β-diketiminate ligand in silylene/iminosilane systems

Abstract

Electron-rich β-diketiminate ligands, featuring amino groups at the backbone β positions (“N-nacnac” ligands) have been employed in the synthesis of a range of silylene (Si^II) complexes of the type (N-nacnac)SiX (where X = H, Cl, N(SiMe₃)₂, P(SiMe₃)₂ and Si(SiMe₃)₃). A combination of experimental and quantum chemical approaches reveals (i) that in all cases rearrangement to give an aza-butadienyl Si^IV imide featuring a contracted five-membered heterocycle is thermodynamically favourable (and experimentally viable); (ii) that the kinetic lability of systems of the type (N-nacnac)SiX varies markedly as a function of X, such that compounds of this type can be isolated under ambient conditions for X = Cl and P(SiMe₃)₂, but not for X = H, N(SiMe₃)₂ and Si(SiMe₃)₃; and (iii) that the ring contraction process is most facile for systems bearing strongly electron-donating and sterically less encumbered X groups, since these allow most ready access to a transition state accessed via intramolecular nucleophilic attack by the Si^II centre at the β-carbon position of the N-nacnac ligand backbone.