Assignment of molecular structures to the electrochemical reduction products of diiron compounds related to [Fe-Fe] hydrogenase: a combined experimental and density functional theory study

The reduction chemistry of (mu-bridge)[Fe(CO)3]2 [bridge = propane-1,3-dithiolate (1) and ethane-1,2-dithiolate (2)] is punctuated by the formation of distinct products, resulting in a marked difference in CO inhibition of electrocatalytic proton reduction. The products formed following reduction of 2 have been examined by a range of electrochemical, spectroelectrochemical, and spectroscopic approaches. Density functional theory has allowed assessment of the relative energies of the structures proposed for the reduction products and agreement between the calculated spectra (IR and NMR) and bond distances with the experimental spectra and EXAFS-derived structural parameters. For 1 and 2, one-electron reduction is accompanied by dimerization, but the structure, stability, and reaction with CO of the dimer is different in the two cases, and this is responsible for the different CO inhibition response for electrocatalytic proton reduction. Calculations of the alternate structures of the two-electron, one-proton reduced forms of 2 show that the isomers with terminally bound hydrides are unlikely to play a significant role in the chemistry of these species. The hydride-transfer chemistry of the 1B species is more reasonably attributed to a hydride-bridged form. The combination of experimental and computational results provides a solid foundation for the interpretation of the reduction chemistry of dithiolate-bridged diiron compounds, and this will underpin translation of the diiron subsite of the [FeFe] hydrogenase H cluster into an abiological context.