Thiolato-bridged cyclopentadienylnickel dimeric complexes have been prepared and found to be efficient and robust proton reduction electrocatalysts using acetic acid as the proton source. From cyclic voltammetry studies, moderate overpotentials of around 0.6 V and i c / i p values from 7.8 to 12.2 have been determined for 20 equiv of acetic acid at a scan rate of 100 mV/s. A turnover number of around 7 has been determined for each of the nickel complexes. The thiolato substituent of the complex does not appear to influence the catalysis significantly. Each of the nickel complexes acts as a robust homogeneous catalyst that could sustain continuous proton reduction for hours. On the basis of the experimental data, an electrochemical− chemical−electrochemical−chemical mechanism describing the catalytic process has been proposed as well.

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Manganese tricarbonyl diimine complexes bearing pyridine and imidazole ligands have been prepared as electrocatalysts for proton reduction using acetic acid as the proton source. The electron-donor ability of the diimine ligand is found to play an important role in determining the efficiency of the electrocatalysts with [MnBr(pybz)(CO) 3 ] (pybz = 2-(2-pyridyl)benzimidazole) exhibiting the lowest overpotential (0.28 V) toward proton reduction. The [Mn(pybz)(CO) 3 (MeCN)] + cationic complex prepared via debromination of [MnBr(pybz)(CO) 3 ] by a silver salt has also been shown to catalyze proton reduction upon its electrochemical reduction. A neutral complex [Mn(pyridine-benzimidazolate)(CO) 3 (MeCN)], which can be synthesized by reacting [MnBr(pybz)(CO) 3 ] with a strong base, has been detected using IR-SEC (infrared spectroelectrochemistry) as an intermediate species in the catalytic process. Using [MnBr(pybz)(CO) 3 ] as the model electrocatalyst, we have carried out density functional calculations to propose a proton reduction mechanism consistent with our experimental observations.

Four copper(I) complexes [Cu(MeCN) 4 ]PF 6 (1), [Cu(dppe) 2 ]PF 6 (2), [Cu(bpy) 2 ]PF 6 (3) and [Cu(bpy)(dppe)]PF 6 (4) have been used as electrocatalysts in the generation of dihydrogen from proton reduction. Low overpotentials of 0.4 V have been measured using cyclic voltammetry when 1 and 3 were used as the catalysts in the presence of acetic acid as the proton source in acetonitrile. In contrast, a higher overpotential is observed for 4 while 2 is unable to catalyse the process. A mechanism for proton reduction has been proposed.