Applications of imino-pyridine Ni(II) complexes as catalysts in the transfer hydrogenation of ketones

Abstract

Five imino-pyridine Ni(II) complexes: [{Ni(L1)Cl2}2] Ni1; [{Ni(L2)Cl2}2] Ni2; [{Ni(L3)Cl2}2] Ni3; [{Ni(L4)Cl2}2] Ni4 and [Ni(L5)2Cl2] Ni5 derived from ligands 2,6-diisopropyl-N-[(pyridin-2-yl) methylene] aniline (L1); 2,6-diisopropyl-N-[(pyridin-2-yl) ethylidene]aniline (L2); 2,6-dimethyl-N-[(pyridin-2-yl) methylene] aniline (L3); 2,6-dimethyl-N-[(pyridin-2-yl) ethylidene] aniline (L4) and N-[(pyridin-2-yl) methylene] aniline (L5) were evaluated as catalysts in the transfer hydrogenation of ketones. The Ni(II) complexes demonstrated moderate catalytic activities giving a turnover number (TON) of up to 126 at catalyst loading of 0.5 mol%. The structure of the complexes and nature of ketone substrate influenced the catalytic activities of the complexes. Deactivation studies using mercury and sub-stoichiometric poisoning experiments pointed to the presence of both Ni(0) nanoparticles and Ni(II) homogeneous as the active species.