Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation

Abstract

Metal–metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (M^δ+⋯M′^δ−) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N–H and B–H bonds of the substrate via the synergetic action of Lewis acidic (M⁺) and basic hydride (M–H) metal species derived from the same mononuclear complex (M–Br). It was also demonstrated that this system generated in situ from the air-stable Mn(I) complex fac-[(CO)₃(bis(NHC))MnBr] and NaBPh₄ shows high activity for H₂ production from several substrates (Me₂NHBH₃, tBuNH₂BH₃, MeNH₂BH₃, NH₃BH₃) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me₂NHBH₃ (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.