SYNTHESIS OF BIFUNCTIONAL MANGANESE COMPLEXES AND THEIR APPLICATION IN TRANSFER HYDROGENATION OF NITRILES

Abstract

A series of bifunctional Mn(0), Mn(I), and Mn(II) complexes bearing polydentate ligands with secondary amino functionality were prepared, characterized, and tested as pre-catalysts in transfer hydrogenation of nitriles to amines using ammonia borane as a reducing agent in the presence of 5 mol% of Mn(I) or Mn(II) pre-catalysts, aromatic and aliphatic substrates were converted to either primary or secondary amines 24 h at 80 ⁰C. Primary amine products were isolated as their hydrochloride salts, obtained by the treatment of the reaction mixtures with HCl. Transfer hydrogenation reactions were found to be highly sensitive to the choice of solvent (benzene vs. isopropanol) and the nature of the catalyst used. The superior catalytic activity in transfer hydrogenation of nitriles to primary amines was demonstrated for Mn(I) complexes bearing tridentate PNHN of bidentate PNH ligands. Moreover, high turnovers of nitriles to primary amines have been also demonstrated for the readily accessible phosphine-free NNHO Mn(II) complex. Catalytic reduction reactions were proposed to proceed via a metal-ligand cooperative mechanism and do not require the use of pressurized hydrogen gas and base additives for catalyst activation.