The effect of oxidation state of metal on electrochemical and photochemical driven hydrogen evolution catalyzed by nickel complexes of maleonitriledithiolate ligands

Two nickel-based catalysts, TBA2[NiII(mnt)2] 1 and TBA[NiIII(mnt)2] 2 are formed by the reactions of NiCl2·6H2O, Na2(mnt), and tetra-n-butylammoniumbromide (TBABr) without or with iodine (I2) (mnt = 2,2-dicyanoethylene-1,1-dithiolate), respectively. Under an overpotential (OP) of 941.6 mV, complexes 1 and 2 can electrocatalyze hydrogen evolution from acetic acid with a turnover frequency (TOF) of 36.3 and 148.31 mol H2/mol catalyst/h, respectively. Under blue light (λmax = 469 nm), 1 or 2, together with CdS nanorods (CdS NRs) as a photosensitizer, and ascorbic acid (H2A) as a sacrificial electron donor, can produce hydrogen with a turnover number (TON) of 7760 and 17,570 moles of H2 per mole of catalyst during 35 h irradiation, respectively. The average value of apparent quantum yield (AQY) is ∼9.2% (complex 1) and 28% (complex 2), respectively. The results indicate that the nickel(III) complex has a better active activity for hydrogen production than the nickel(II) species.