Phosphine-NHC Manganese Hydrogenation Catalyst Exhibiting a Non-Classical Metal-Ligand Cooperative H2 Activation Mode

Deprotonation of the Mn-I NHC-phosphine complex fac-[MnBr(CO)(3)(k(2)P,C-Ph2PCH2NHC)] (2) under a H-2 atmosphere readily gives the hydride fac-[MnH(CO)(3)(k(2)P,C-Ph2PCH2NHC)] (3) via the intermediacy of the highly reactive 18-e NHC-phosphinomethanide complex fac-[Mn-(CO)(3)(k(3)P,C,C-Ph2PCHNHC)] (6a). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16-e mangana-substituted phosphonium ylide complex fac-[Mn(CO)(3)(k(2)P,C-Ph2P=CHNHC)] (6b) as key intermediate able to activate H-2 via a non-classical mode of metal-ligand cooperation implying a formal lambda(5)-P-lambda(3)-P phosphorus valence change. Complex 2 is shown to be one of the most efficient pre-catalysts for ketone hydrogenation in the Mn-I series reported to date (TON up to 6200).