Abstract
In nature, hydrogenase enzymes catalyze H2 production from water with the turnover frequency (TOF) of 9000 s-1. In this context, studies on the water-splitting reactions with use of earth-abundant metal complexes have attracted great attention, and some nickel complexes have recently been reported to show efficient catalysis in hydrogen production. Dubois et al. demonstrated that a nickel(II) bis(diphosphine) complex with tertiary amines serves as an electrocatalyst for H2 production with the TOF of 106,000 s-1. The pendant amine was proposed to act as a proton relay promoting the nickel hydride formation together with H-H bond fixation, mimicking the reaction field of [FeFe] hydrogenase. In addition, visible-light-driven hydrogen production in the presence of the analogous complex is also promoted with 2700 turnover numbers (TONs) after 150 h, as reported by Holland et al. On the other hand, Eisenberg et al. showed that a biomimetic pyridinethiolate nickel(II) complex gives 5500 TONs over 40 h, employing the photocatalytic H2-evolving system. In the active form of the complex, coordinated pyridine is suggested to dissociate from nickel center to serve as a proton shuttle instead. These results indicate that a nickel(II) compound has considerable potential to become an alternative to platinum catalyst in the future.
