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 (ΔG°_H^–) 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)] × 10^–1 h^–1 was observed at 80 °C and 34 atm of a 1:1 mixture of CO₂ and H₂. 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.