Bulky oxadithiolate-bridged [FeFe]‑hydrogenase mimics [Fe2(μ-R2odt)(CO)4(κ2-diphosphine)] (R = Ph and H) with chelating diphosphines

Abstract

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 [Fe₂(μ-R₂odt)(CO)₄(κ²-diphosphine)] (3-5) with bulky Ph₂odt bridge and their reference counterparts (6-8) with common odt bridge were obtained from the Me₃NO-assisted substitutions of diiron hexacarbonyl precursors [Fe₂(μ-R₂odt)(CO)₆] (R₂odt = (SCHR)₂O, R = Ph (1) and H (2)) with different diphosphines such as (Ph₂P)₂NBn (labelled PN^BnP, Bn = benzyl), (Ph₂PCH₂)₂NBn (PCN^BnCP), and (Ph₂PCH₂)₂CH₂ (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 PN^BnP diphosphine have the favorable dibasal isomer whereas analogues 4, 5 and 7, 8 chelating by flexible backbone PCN^BnCP 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 (CF₃CO₂H) as proton source, indicating that the complete protonations of 3, 6 and 5, 8 with higher concentration of CF₃CO₂H lead to two new catalytic waves for the electrocatalytic proton reduction to hydrogen (H₂).

Keywords: Bulky dithiolate bridges; Electrochemical proton reduction; Structure; Synthesis; [FeFe]‑hydrogenase mimics.