Iron-only hydrogenases: a functional artificial H-cluster

Converting protons to hydrogen at an electrode and its reverse getting electricity from hydrogen are reactions of fundamental importance to a hydrogen economy. Platinum is currently the best catalyst for both obtaining hydrogen as a fuel from water by electrolysis and using hydrogen to produce electricity by oxidising it in fuel cells. It is an expensive metal and in the long term is likely unsustainable with respect to demand from an expanding hydrogen economy . Issues relating to security of supply have also been raised. Nature carries out the same reactions using hydrogenase enzymes to catalyse the production or use of hydrogen in bacteria and algae . The machinery at the heart of one of these enzymes is a cluster of iron and sulfur atoms (abundant elements) known as the H-cluster. We are addressing the question 'can we make new materials based on nature's blueprint to replace platinum in fuel cells?' So far we and others have shown that structures closely similar to the catalytic centre of the enzyme can be synthesised. However these have proved poor catalysts, in part because the artifical structures have an 'extra' molecule of carbon monoxide attached to one of the iron atom and this 'blocks' its reaction with hydrogen or protons. This project sets out to build a functioning H-cluster. This will both help us to develop our understanding of how the biological catalysis works and may provide lead materials of technological relevance.

The active site of the iron-only hydrogenases,the H-cluster, is comprised of a diiron unit linked to a 4F4S cluster by a cysteinyl bridge.The aim of the project is to construct a functional artificial H-cluster. Specifically, a subsite-cubane assembly capable of catalysing the reversible oxidation of molecular hydrogen / reduction of protons. Ideally the system should operate at diffusion controlled rates at potentials not far removed from the equilibrium potential of the H+/H2 couple at moderate pH. The construction of such an assembly will provide a means of probing the electronic interplay and mechanistic roles of the conjoined subsite and cubane components which comprises the biological H-cluster. This will contribute both to understanding how the iron-only hydrogenases work and to the design of new materials for catalysing this technologically important interconversion. The approach will be to build on recent work where we have shown that by using active thioesters it is possible to link diiron subsites to cubane clusters and so construct the FeS framework of the H-cluster.