The influence of a peripheral functional group of diiron hexacarbonyl complexes on their electrochemistry and electrocatalytic reduction of proton

Abstract

Three diiron hexacarbonyl complexes (2, 3 and 4) with naphthalene-1,8-bis(thiolate) skeleton as their bridging linkages are reported. For comparison, the protonated form of complex 4 was also prepared (4H+). They bear respectively a functional group on the naphthalene ring at the position 2, i.e. −CH2OH (2), −COOH (3), −CH2N(Et)2 (4) and −CH2NH+(Et)2 (4H+). Complex 2 was derived from the direct reduction of its precursor bearing aldehyde group (−CHO, 1) by NaBH4 while complexes 3 and 4 were routinely synthesized by reacting Fe3(CO)12 with ligands L2 and L3, respectively, which were derived from ligand L1, naphtho[1,8-cd][1,2]dithiole-3-carbaldehyde. These complexes were fully characterized and complexes 3 and 4 were analyzed using X-ray single crystal diffraction. Electrochemistry of these complexes was also investigated by cyclic voltammetry. The carboxylic acid of complex 3 shows significant influence on the second reduction due to the acid group involving reaction with the reduced species. Both infrared spectral data and the first reduction potentials of the complexes suggest that these functional groups exert hardly electronic influence on the metal center. However, the functional groups which can carry proton (−COOH and −CH2N+H(Et)2) can ease the kinetics of the catalysis of proton reduction via probably PCET (proton-coupled electron transfer) mechanism. These proton carriers can also improve catalytic efficiency by acting a proton relay during the catalysis as suggested by the linear plots of peak current against acid concentration for the three complexes.