Synthesis and Electrocatalytic Activity of [FeFe]-Hydrogenase Model Complexes with Non-Innocent Chelating Nitrogen-Donor Ligands

To probe the influence of redox non-innocent ligands on a well-known class of [FeFe]-hydrogenase models, three new asymmetrically disubstituted diiron complexes of the general formula (µ-SRS)[Fe(CO)₃][Fe(CO)(N-N)] {SRS = propane-1,3-dithiolate (pdt) or benzene-1,2-dithiolate (bdt), and N-N = 2,2′-bipyridine (bipy) or 2,2′-bipyrimidine (bpym)} have been synthesized from their parent hexacarbonyls and characterized. The new complexes, (µ-pdt)Fe₂(CO)₄(κ²-bpym) (2), (µ-bdt)Fe₂(CO)₄(κ²-bipy) (3), and (µ-bdt)Fe₂(CO)₄(κ²-bpym) (4), were fully characterized by spectroscopic and electrochemical techniques, and the results are compared to those of a related complex, (µ-pdt)Fe₂(CO)₄(κ²-bipy) (1). The crystal structures of 2–4 show that, in each complex, the two iron units are in an eclipsed orientation, and the N-N ligand lies in the basal plane. IR spectra and electrochemical analyses indicate that electron density at the iron centers decreases in the order 1 > 2 > 3 > 4. Furthermore, compound 2 undergoes a ligand-centered reduction at the same potential that the hexacarbonyl precursor undergoes its first reduction. However, unlike the 2,2′-bipy derivatives 1 and 3, the 2,2′-bpym complexes 2 and 4 are not effective catalysts for electrochemical proton reduction from acetic acid.