Diiron Proton Reduction Catalysts Possessing Electron-Rich and Electron-Poor Naphthalene-1,8-dithiolate Ligands

The diiron complexes [Fe-2(CO)(6)(1,8-S-2-2,4,5,7-Cl4C10H2)] (3), [Fe-2(CO)(6)(1,8-S2C10H6)] (4), and [Fe-2(CO)(6)(1,8-S-2-2,7-tBu(2)C(10)H(4))] (5) were synthesized in moderate yield from naphthalene-1,8-dithiols and [Fe-3(CO)(12)]. They were characterized by IR, H-1 and C-13 NMR spectroscopy and 3 and 4 were characterized by X-ray crystallography. They contain a butterfly Fe2S2 core with Fe-Fe distances of 2.529(1) (3) and 2.506(.1) angstrom (4). Cyclic voltammetry (CH2Cl2/nBu(4)PF(6)) revealed two one-electron reductions, at potentials more positive than those for the related [Fe-2(CO)(6)(S(CH2)(3)S)] (1), and one one-electron oxidation. The first reduction of the electron poor 3 (-1.60 V vs Fe/Fe+) is 240 mV more positive than the first reduction of the electron rich 5 (-1.84 V). Electrochemical studies revealed that 3-5 facilitate the proton reduction of p-toluenesulfonic acid. The rates of catalysis, with a 95% confidence limit, are 2.4 +/- 0.8 (1), 5.2 +/- 0.8 (3), 3.1 +/- 1.1 (4), 2.9 +/- 0.6 (5), and 4.5 +/- 0.2 h(-1) for the related [Fe-2(CO)(6)(1,2-S2C6H4)]. The rates were determined by bulk electrolysis at -.1.70 V (Fc/Fc(+)), which correspond to an overpotential of 1.05 V for p-toluene-sulfonic acid.