Synthesis and electrochemical properties of tetranuclear di-μ-oxo-bis[di-μ-phenolatodiiron(III)] complexes

A series of tetrairon(III) complexes of di-μ-oxo-bis[di-μ- phenolatodiiron(III)] core, [{Fe₂(L)(L'′)}₂(μ-O) ₂](PF₆)₂ (1-11), where L^2- is the phenol-based macrocyclic compartmental ligand and (L′)- is acetate, monochloroacetate, dichloroacetate, trifluoroacetate, pentafluoropropionate, benzoate, p-nitrobenzoate, pentafluorobenzoate, diethyl phosphate, diphenyl phosphate, or diethyldithiophosphate for 1-11, respectively, were synthesized. X-ray crystallographic studies for 4, 7, and 9 indicated that two {Fe ₂(L)(L′)} units were connected by two oxo bridges to afford a tetranuclear di-μ.-oxo-bis[di-μ.-phenolatodiiron(III)] core. Cyclic voltammograms of 1-11 in acetonitrile showed a quasi-reversible or irreversible reduction wave between -332 and -620 mV (vs. Ag/Ag⁺), which were attributed to a two-electron process involving the Fe₄ ^{III,III,III,III}/Fe₄^{II,II,III,III}. The Fe ₄^{III,III,III,III}/Fe₄^{II,II,III,III} potential shifted positive with a decrease in the electron-donating ability of the end-cap ligand, and a good linear correlation was found between the potentials and the pK_a values of HL′. The Fe₄ ^{II,II,III,III} species were generated in acetonitrile by electrochemical reduction and showed broad intervalence transition bands in the range of 900-1500 nm.