Insights into the two-electron reductive process of [FeFe]H2ase biomimetics. Cyclic voltammetry and DFT investigation on chelates control of redox properties of [Fe2(CO)4(κ2-chelate)(µ-dithiolate)]

The electrochemical reduction of complexes [Fe2(CO)4(k2-phen)(μ-xdt)] (phen = 1,10-phenanthroline ; xdt = pdt (1), adtiPr (2)) in MeCN-[Bu4N][PF6] 0.2 M is described as a two-reduction process. DFT calculations show that 1 and its reduced form 1- display metal- and phenanthroline-centered frontier orbitals (LUMO and SOMO) indicating the non-innocence of the phenanthroline ligand. Two energetically close geometries were found for the doubly reduced species suggesting an intriguing influence of the phenanthroline ligand leading to the cleavage of a Fe-S bond as proposed generally for this type of complex or retaining the electron density and avoiding Fe-S cleavage. Extension of calculations to other complexes with edt, adtiPr bridge and even virtual species [Fe2(CO)4(k2-phen)(μ-adtR)] (R = CH(CF3)2, H)  or [Fe2(CO)4(k2-phen)(μ-pdtR)] (R = CH(CF3)2, iPr) showed that the relative stability between both two-electron-reduced isomers depends on the nature of the bridge and the possibility to establish a remote anagostic interaction between the iron centre {Fe(CO)3} and the group carried by the bridged-head atom of the dithiolate group.