An unexpected leading role for [Fe2(CO)6(μ-pdt)] in our understanding of [FeFe]-H2ases and the search for clean hydrogen production

When Ziegler and Korav serendipitously prepared [Fe2(CO)6(μ-pdt)] in 1979 they could not have anticipated that some 40 years later it would be a mainstay of attempts to understand and replicate the workings of a 4-billion-year-old class of enzyme, [FeFe]-H2ases, mimicking the function of which has become a major societal goal, namely the clean production of hydrogen. This review traces this journey from chance discovery to innovative exploitation that has brought organometallic chemistry close to the Origins of Life. Much focus is on the extensive substitution chemistry, which can be used to tune both the steric and electronic nature of the diiron centre. This provides access to complexes that are close mimics of the six different identified states of [FeFe]-H2ases and has allowed chemists and biochemists to piece together a viable overall catalytic cycle. This work has also led to the development of many complexes, including [Fe2(CO)6(μ-pdt)] itself, that are (reasonably) efficient proton-reduction catalysts, but finding derivatives that are active for hydrogen oxidation, a key process in a hydrogen fuel cell, remains a significant challenge.