Mössbauer and electrochemical studies on Fe3MoS4 and Fe3WS4 cubane-like cluster dimers

Abstract

⁵⁷ Fe Mössbauer parameters of several [Fe₆M₂S₈(SR)₉]^3–(where M = Mo or W) complexes and [Fe₆W₂S₈(OMe)₃(SPh)₆]^3– have been obtained at 4.2, 77, 195, and 293 K. These are seen to be very similar to those obtained for [Fe₄S₄(SR)₄]^2– complexes and, on the basis of the isomer shift, each iron atom is considered to have a net oxidation state of ca. 2.5, thus implying that each molybdenum or tungsten atom has an oxidation state between 3 and 4. Electrochemical reduction of the trianions to the 4–, 5–, 6– and, in some instances, 7– species has been monitored. The first of these reductions occurs at a potential virtually identical to that of the corresponding [Fe₄S₄(SR)₄]^2– complex, the second reduction occurs at a potential ca. 200 mV more negative. These two reductions generally appear to be reversible for most of the complexes in acetonitrile solution. However, rapid scan, staircase cyclic voltammetry showed that only [Fe₆M₂S₈(SEt)₉]^3–(M = Mo or W) in acetonitrile solution approach good electrochemical reversibility.