Dipyrromethane-diphosphines: nickel and palladium complexes as catalysts for the selective hydroboration of CO2 to methoxyborane under mild conditions with mechanistic study.

The dipyrromethane-diphosphine ligands [R2C(C4H3N)2-1,9-(CH2PPh2)2] represented as L1H2, L2H2 and L3H2 containing phenyl, ethyl and cyclohexyl groups, react with [Pd(COD)Cl2] to give complexes of the type [PdCl2(L1H2-κ2-P,P)] 3, [Pd(L1-κ4-P,N,N,P)] 4a, [Pd(L2-κ4-P,N,N,P)] 4b and [Pd(L3-κ4-P,N,N,P)] 4c which were characterized by both spectroscopic and single crystal X-ray diffraction methods. The coordination mode of ligand κ2 or κ4 can readily be identified by UV-vis spectrum. The catalytic applications of these palladium complexes and the reported analogous nickel complexes were investigated in the hydroboration of CO2 using 9-BBN. Methoxyborane was selectively obtained in a quantitative yield with a TON of 200 from the reduction of CO2 (1 atm) in the presence of 0.5 mol% of palladium complex 4a at 25 °C. Mechanistic studies established the formation of Ni and Pd hydrides by the cleavage of the B-H bond across the palladium atom and ligand framework, and CO2 insertion followed by further reductions to give methoxyborane via its acetal derivative.