Hydrogenation of CO2 in Water Using a Bis(diphosphine) Ni-H Complex

The water-soluble Ni bis(diphosphine) complex [NiL2](BF4)(2) (L = 1,2-[bis(dimethoxypropyl)phosphino]ethane and the corresponding hydride, [HNiL2]BF4, were synthesized and characterized. These complexes were specifically designed for CO2 hydrogenation. For HNiL2+, the hydricity (Delta G(H-)(o)) was determined to be 23.2(3) kcal/mol in aqueous solution. On the basis of the hydricity of formate, 24.1 kcal/mol, the transfer of a hydride from HNiL2+ to CO2 to produce formate is favorable by 1 kcal/mol. Starting from either NiL22+ or HNiL2+ in water, catalytic hydrogenation of CO2 was observed with NaHCO3 (0.8 M) as the only additive. A maximum turnover frequency of [4.0(5)] x 10(-1) h(-1) was observed at 80 degrees C and 34 atm of a 1:1 mixture of CO2 and H-2. This report demonstrates the use of a homogeneous first-row transition-metal catalyst for CO2 hydrogenation in water using NaHCO3 as an inexpensive, readily available base.