BIFUNCTIONAL AMINOPHOPSHINE MN(I) AND FE(II) COMPLEXES: SYNTHESIS AND APPLICATION IN TRANSFER HYDROGENATION CATALYSIS.

Abstract

Aminophsophine PNH complexes of Mn(I) and Fe(II) were synthesized and fully characterized. The prepared complexes were subjected to catalytic transfer hydrogenation of nitriles to amines using ammonia borane as a hydrogen source. Compared to manganese, an iron-based system, (PNH)FeBr2, showed a superior catalytic activity, representing a rare example of an iron catalyst for transfer hydrogenation catalysis beyond the reduction of carbonyl compounds (aldehydes and ketones), and the first example of an iron catalyst for selective transfer hydrogenation of nitriles to primary amines. Using 5 mol% of (PNH)FeBr2, various aromatic and aliphatic nitriles were efficiently converted to their primary amine derivatives under mild conditions (24 hours at 60 °C). Based on a series of control experiments and density functional theory calculations, the (PNH)FeBr2-catalyzed transfer hydrogenation of nitriles was proposed to proceed via a metal-ligand cooperative pathway enabled by the proton transfer from the secondary amine moiety of the PNH ligand. Moreover, the aminophosphine Mn(I) complex (PNH)Mn(Br)(CO)3 together with a phosphine-free Mn(II) derivative, (NNHO)MnBr2, were tested in the reduction of carboxamides (N, N-dimethylformamide and benzamide) with ammonia borane, affording the products of the amide C=O and C-N bond cleavage, respectively. Finally, catalytic activities of Mn(I) systems with the aminophoshine PNH ligand and the pyridine-substituted PNHN ligand were evaluated in the transfer hydrogenation of alkynes with ammonia borane. These studies could serve as grounds for further developments of Mn-based catalysts for semi-transfer hydrogenation of alkynes to alkenes. An important advantage of the developed systems is that neither addition of the pressurized hydrogen gas nor the use of a base activator is required, and the reactions are initiated by ammonia borane.