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

Reaction of Co₂(CO)₈ and 1,3‐propanedithiol in a 1:1 molar ratio in toluene affords a novel tetracobalt complex, [(μ₂‐pdt)₂(μ₃‐S)Co₄(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 [(μ₂‐pdt)₂(μ₃‐S)Co₄(CO)₅(PCy₃)] ( 2 ) and [(μ₂‐pdt)₂(μ₃‐S)Co₄(CO)₄(L)] [L=Ph₂PCH?CHPPh₂, 3 ; Ph₂PCH₂N(Ph)CH₂PPh₂, 4 ; Ph₂PCH₂N(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 ³¹P and ¹H 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‐electro‐infrared techniques.