Dipyrromethane-diphosphine: the effect of meso 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 meso positions and their nickel complexes were synthesized and structurally characterized. Treatment of Ph 2 C(C 4 H 3 N) 2 -1,9-(CH 2 PPh 2 ) 2 (L1H2) with [NiCl 2 (DME)] gave complex [NiCl 2 (κ 2 - P , P -L1H2)] 2a . Conversely, the analogous reactions of L2H2 and L3H2 with [NiCl 2 (DME)] showed a mixture of products containing both a pyrrolide nitrogen coordinated complex of type [Ni(κ 4 - P , N , N , P -L)] 3 without an exogenous base and a chelated complex of type 2a . In addition, all three ligands react with [NiCl 2 (DME)] in the presence of a strong base to give a complex of type 3 . Furthermore, a novel binuclear Ni(0) complex bearing L1H2 was characterized by X-ray crystallography. Both complexes 2a and 3 (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 i 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 meso -phenyl substituted diphosphine coordinated nickel complexes perform better than the other two ligand coordinated nickel complexes.