Catalytic Dehydrocoupling of Amine-Boranes using Cationic Zirconium(IV)-Phosphine Frustrated Lewis Pairs

A series of novel, intramolecular Zr(IV)/P frustrated Lewis pairs (FLPs) based on cationic zirconocene fragments with a variety of ancillary cyclopentadienyl and 2-phosphinoaryloxide (-O(C6H4)PR2, R = Bu-t and 3,5-CF3(C6H3)) ligands are reported and their activity as catalysts for the dehydrocoupling of dimethylamine-borane (Me2NH center dot BH3) assessed. The FLP system [(C9H7)(2)ZrO(C6H4)(PBu2)-Bu-t]-[B(C6F5)(4)] is shown to give unprecedented turnover frequencies (TOF) for a catalyst based on a group 4 metal (TOF >= 600 h(-1)), while also proving to be the most efficient FLP catalyst reported to date. The mechanism of this reaction has been probed using analogous intermolecular Zr(W)/P FLPs, permitting deconvolution of the reactions taking place at both the Lewis acidic and basic sites. Elucidation of this mechanism revealed an interesting cooperative two-cycle process where one cycle is FLP mediated and the other, a redistribution of a linear diborazane intermediate, relies solely on the presence of a Zr(IV) Lewis acid.