Effect of the S-to-S bridge on the redox properties and H2 activation performance of diiron complexes related to the [FeFe]-hydrogenase active site.

Three biomimetic models of the [FeFe]-hydrogenase active site, namely diiron dithiolates of [(μ-edt){Fe(CO)3}{Fe(CO)(κ2-PNP)}] (1, edt = ethane-1,2-dithiolate, PNP = Ph2PCH2N(nPr)CH2PPh2), [(μ-bdtMe){Fe(CO)3}{Fe(CO)(κ2-PNP)}] (2, bdtMe = 4-methylbenzene-1,2-dithiolate), and [(μ-adtBn){Fe(CO)3}{Fe(CO)(κ2-PNP)}] (3, adtBn = N-benzyl-2-azapropane-1,3-dithiolate), were prepared and structurally characterized. These complexes feature the same PNP ligand but different S-to-S bridges. Influence of the S-to-S bridge on the electrochemical properties and chemical oxidation reactivity of 1-3 was studied by cyclic voltammetry and by in situ IR spectroscopy. The results reveal that the S-to-S bridge has a considerable effect on the oxidation reactivity of 1-3 and on the stability of in situ generated single-electron oxidized complexes, [1]+, [2]+, and [3]+. The performances of [1]+ and [2]+ for H2 activation were explored in the presence of a mild chemical oxidant, while rapid decomposition of [3]+ thwarted the further study of this complex. Gratifyingly, 1 was found to be catalytically active, although in a low turnover number, for H2 oxidation in the presence of excess mild oxidant and a proton trapper under 1 atm H2 at room temperature.