Dipyrromethane-diphosphine: the effect of substituents on the formation of nickel complexes and on their performance in the transfer hydrogenation of ketones.

Three dipyrromethane-diphosphine ligands containing phenyl (L1H2), ethyl (L2H2) and cyclohexyl (L3H2) groups at their positions and their nickel complexes were synthesized and structurally characterized. Treatment of PhC(CHN)-1,9-(CHPPh) (L1H2) with [NiCl(DME)] gave complex [NiCl(κ-,-L1H2)] . Conversely, the analogous reactions of L2H2 and L3H2 with [NiCl(DME)] showed a mixture of products containing both a pyrrolide nitrogen coordinated complex of type [Ni(κ-,,,-L)] without an exogenous base and a chelated complex of type . In addition, all three ligands react with [NiCl(DME)] in the presence of a strong base to give a complex of type . Furthermore, a novel binuclear Ni(0) complex bearing L1H2 was characterized by X-ray crystallography. Both complexes and (0.5 mol% of loading) catalyze the transfer hydrogenation of a series of aromatic and aliphatic ketones (20 substrates) to their corresponding secondary alcohols using PrOH as a hydrogen source in the presence of KOH at 100 °C in 6 h. The kinetic trace of the catalytic reaction shows that the -phenyl substituted diphosphine coordinated nickel complexes perform better than the other two ligand coordinated nickel complexes.