High activities of nickel (II) complexes containing phosphorus-nitrogen ligands in hydrogen transfer reaction of imines using formic acid as a renewable hydrogen source

The catalytic reduction of imines to obtain secondary amines is an important route in the synthesis of molecules of biological or pharmaceutical interest. A series of new nickel (II) complexes bearing phosphorous-nitrogen (P,N) or phosphorous-nitrogen-phosphorous (P,N,P) ligands were characterized and studied as catalysts in hydrogen transfer reaction of imines, using formic acid as a hydrogen source, while avoiding the use of molecular hydrogen (H₂). The complexes, of general formula NiCl₂(L), [Ni(L)₂](CF₃SO₃)₂ (in which L is a P,N ligand) and [NiCl(L)]Cl (in the case of a tridentate PNP ligand), were active catalysts in the imine hydrogen, with turnover frequencies (TOF) up to 373 h⁻¹ with NiCl₂L1 complex (L1 = 2-(diphenylphosphinoamino) pyridine) and N-benzylideneaniline as substrate, while a [Ni(L)₂](CF₃SO₃)₂ complex bearing the same P,N ligand achieved a slightly lower activity (TOF = 320 h⁻¹). Catalyst recyclability was also studied, achieving near full conversion after four cycles, with a total substrate/catalyst ratio of 1000:1.