Credit: © 2008 Wiley

The large-scale production of hydrogen, with its burgeoning use in fuel cells, is of great interest given the current energy situation. In particular, hydrogenases — enzymes that catalyse the reversible oxidation of hydrogen — that contain metal clusters such as di-iron carbonyl complexes as active sites could inspire the design of catalysts not based on expensive precious metals. However, no hydrides of these di-iron complexes have been isolated, and the reactivity of such complexes towards hydrogen, and hydrogen-rich substrates in general, is not well understood.

Now, Luca De Gioia, Thomas Rauchfuss and co-workers, in a collaboration between the University of Milan-Biococca and the University of Illinois, have used an organosilane — a hydrogen-rich material — as a substrate model to reveal1 how hydrogen interacts with these di-iron complexes. Theoretical calculations showed that the initial stages of the reactions of a di-iron carbonyl complex with both substrates were similar. Removal of a carbonyl group from the substrate, for example by photolysis, is necessary before it can be added to the unsaturated metal centre.

The resulting cluster undergoes an intramolecular oxidative addition, where one hydrogen atom separates from the substrate to coordinate directly to the iron centre of the cluster. The cleavage of the organosilane is easier than that of hydrogen, but for both substrates the presence of electron-donor ligands, such as a phosphine, on the non-coordinated iron centre is crucial for this step. The hydride ligand then forms a bridge between the two metals. This pathway, in good agreement with spectroscopic data, was further confirmed when details of the structures of the organosilane complexes were obtained by crystallography.