[FeFe]-hydrogenase synthetic mimics based on peri-substituted dichalcogenides

Eight dithiolato-, diselenolato-, and mixed S,Se-Fe₂(CO)₆ complexes based on peri-substituted naphthalene and phenanthrene dichalcogenides are prepared by oxidative insertion of Fe₃(CO)₁₂ into the dichalcogen bonds of 2,7-dimethoxynaphtho[1,8-cd][1,2]dithiole, three naphtho[1,8-cd][1,2]diselenoles, two naphtho[1,8-cd][1,2]thiaselenoles, phenanthro[1,10-cd][1,2]dithiole, and phenanthro[1,10-cd][1,2]diselenole. Complexes are characterized by ¹H, ¹³C NMR, UV/vis, and IR spectroscopy and by X-ray analysis. The effect of replacing sulfur with selenium, incorporating electron-donating groups (2,7-di-tert-butyl, 2,7-dimethoxy) on the naphthalene ring system, and changing the degree of conjugation in the aromatic backbone (naphthalene vs phenanthrene) on the reduction potential is evaluated by cyclic voltammetry. The electrocatalytic activity of these [FeFe]-hydrogenase synthetic mimics for proton reduction in the presence of increasing concentrations of p-TsOH is investigated. Diselenolate-based [FeFe]-complexes show enhanced catalytic activity for proton reduction compared with their sulfur congeners.