Homogeneous catalytic reduction of polyoxometalate by hydrogen gas with a hydrogenase model complex

The homogeneous catalytic reduction of a polyoxometalate (POM) by hydrogen gas in aqueous media was investigated for the first time by using a [NiRu] hydrogenase model complex (I) under very mild conditions. By bubbling hydrogen gas into the buffer solution containing I and the Dawson-type POM (II_ox), the color of the solution turned from pale yellow to dark blue, suggesting the reduction of II_ox. The catalytic and kinetic studies revealed that I acted as an efficient catalyst to yield one-electron-reduced Dawson-type POM (II_red) with a low energy barrier for activating dihydrogen and reducing II_ox via a hydride complex of I. The process for the one-electron reduction of II_ox was confirmed by UV-vis spectroscopy, controlled potential electrolysis, and X-ray photoelectron spectroscopy. POM II_red could stably store protons and electrons and release them by addition of oxidants, demonstrating that POMs acted as redox active mediators for transporting protons and electrons from hydrogen gas to acceptors. The recycle study showed that II_ox and II_red could be reduced and oxidized by hydrogen and oxygen gases, respectively, at least five times with >99% yield of reduced species, showing a durable system for extracting protons and electrons from hydrogen gas.