Bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics [Fe(μ-Rodt)(CO)(κ-diphosphine)] (R = Ph and H) with chelating diphosphines.

In order to develop an attractive generation of bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics with chelating diphosphines, two new series of asymmetrically diphosphine-substituted diiron model complexes [Fe(μ-Rodt)(CO)(κ-diphosphine)] (3-5) with bulky Phodt bridge and their reference counterparts (6-8) with common odt bridge were obtained from the MeNO-assisted substitutions of diiron hexacarbonyl precursors [Fe(μ-Rodt)(CO)] (Rodt = (SCHR)O, R = Ph (1) and H (2)) with different diphosphines such as (PhP)NBn (labelled PNP, Bn = benzyl), (PhPCH)NBn (PCNCP), and (PhPCH)CH (DPPP)), respectively. All the as-prepared complexes have been characterized by elemental analysis, IR plus NMR spectroscopies, and particularly by X-ray crystallography for 3-8. It is interesting to note that complexes 3 and 6 chelating by small bite-angle PNP diphosphine have the favorable dibasal isomer whereas analogues 4, 5 and 7, 8 chelating by flexible backbone PCNCP or DPPP ligands possess the main apical-basal isomer in solution or in the solid state. Further, the electrochemical properties of two pairs of representative complexes 3, 6 and 5, 8 are explored and compared by cyclic voltammetry (CV) in the absence and presence of trifluoroacetic acid (CFCOH) as proton source, indicating that the complete protonations of 3, 6 and 5, 8 with higher concentration of CFCOH lead to two new catalytic waves for the electrocatalytic proton reduction to hydrogen (H).