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 Ph₂C(C₄H₃N)₂-1,9-(CH₂PPh₂)₂ (L1H2) with [NiCl₂(DME)] gave complex [NiCl₂(κ²-P,P-L1H2)] 2a. Conversely, the analogous reactions of L2H2 and L3H2 with [NiCl₂(DME)] showed a mixture of products containing both a pyrrolide nitrogen coordinated complex of type [Ni(κ⁴-P,N,N,P-L)] 3 without an exogenous base and a chelated complex of type 2a. In addition, all three ligands react with [NiCl₂(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 ⁱPrOH 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. [ABSTRACT FROM AUTHOR]