New Tetracobalt Cluster Compounds for Electrocatalytic Proton Reduction: Syntheses, Structures, and Reactivity

Reaction of Co-2(CO)(8) and 1,3-propanedithiol in a 1: 1 molar ratio in toluene affords a novel tetracobalt complex, [(mu(2)-pdt)(2)(mu(3)-S)Co-4(CO)(6)] (pdt=-SCH2CH2CH2S-, 1), which possesses some of the structural features of the active site of [FeFe]-hydrogenase. Carbonyl displacement reaction of complex 1 in the presence of mono-or diphosphine ligands leads to the formation of [(mu(2)-pdt)(2)(mu(3)-S)Co-4(CO)(5)(PCy3)] (2) and [(mu(2)-pdt)(2)(mu(3)-S)Co-4(CO)(4)(L)] [L= Ph2PCH=CHPPh2, 3; Ph2PCH2N(Ph)CH2PPh2, 4; Ph-2 PCH2N(iPr)CH2PPh2, 5]. Complexes 1-5 have been fully characterized by spectroscopy and single-crystal X-ray diffraction studies. Cyclic voltammetry has revealed that complexes 1-5 show a reversible first reduction wave and are active for electrocatalytic proton reduction in the presence of CF3COOH. Protonation reactions have been monitored by P-31 and H-1 NMR and infrared spectroscopies, which revealed the formation of different protonated species. The mono-reduced species of 1-5 have been spectroscopically characterized by EPR and spectro-electroinfrared techniques.