Diiron hexacarbonyl complexes bearing naphthalene-1,8-dithiolate bridge moiety as mimics of the sub-unit of [FeFe]-hydrogenase: synthesis, characterisation and electrochemical investigations

Abstract

Eight diiron hexacarbonyl complexes bearing a 1,8-dithionaphthalenyl bridging linkage as mimics of the diiron subunit of [FeFe]-hydrogenase are reported. Reaction of Fe₃(CO)₁₂ with naphtha[1,8-cd][1,2]dithiole-n-carbaldehyde (n = 2: 2a or 4: 2b) gave two complexes, [Fe₂(μ-S)₂R(CO)₆] (–SRS– = n-formylnaphthalene-1,8-bis(thiolate), n = 2: 3a, 4: 3b), which were further used as precursors to prepare six other complexes by manipulating the formyl groups. Converting the corresponding formyl group into a hydroxymethyl group (CH₂OH) led to complexes 4a and 4b. Their reactions with halobutanoyl chloride formed complexes 5a and 5b (halo group = Cl), and 6a and 6b (halo group = Br), respectively. Among the complexes, 3a, 3b, 4b, 5b, and 6a were crystallographically analysed. Electrochemical investigations into these complexes revealed that the formyl group exerts profound electronic influence on the electrochemistry, and thus catalysis of proton reduction, due to its involvement in the conjugation of the bridging linkage. DFT calculations indicate that the formyl group influences the electrochemistry and catalysis by significantly altering the composition of the LUMOs.