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

Abstract

Deprotonation of the Mn^I NHC-phosphine complex fac-[MnBr(CO)₃ (κ² P,C-Ph₂ PCH₂ NHC)] (2) under a H₂ atmosphere readily gives the hydride fac-[MnH(CO)₃ (κ² P,C-Ph₂ PCH₂ NHC)] (3) via the intermediacy of the highly reactive 18-e NHC-phosphinomethanide complex fac-[Mn(CO)₃ (κ³ P,C,C-Ph₂ PCHNHC)] (6 a). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16-e mangana-substituted phosphonium ylide complex fac-[Mn(CO)₃ (κ² P,C-Ph₂ P=CHNHC)] (6 b) as key intermediate able to activate H₂ via a non-classical mode of metal-ligand cooperation implying a formal λ⁵ -P-λ³ -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).

Keywords: DFT calculations; N-heterocyclic carbenes; manganese; metal-ligand cooperation; phosphonium ylides.