Diiron Complexes with Pendant Phenol Group(s) as Mimics of the Diiron Subunit of [FeFe]-Hydrogenase: Synthesis, Characterisation, and Electrochemical Investigation

Four diiron hexacarbonyl complexes, [Fe(2)(mu-SCH(2)-o-C(6)H(4)OMe)(2)(CO)(6)] (4a), [Fe(2){mu-SCH(2)-o,m-C(6)H(3)(OMe)(2)}(2)(CO)(6)] (4b), [Fe(2){mu-SCH(2)-o,o'-C(6)H(3)(CO(2)Me)(OMe)}(2)(CO)(6)] (4c) and the demethylated form of complex 4a, [Fe(2)(mu-SCH(2)-o-C(6)H(4)OH)(2)(CO)(6)] (5a), were synthesised and fully characterised. Complexes 4b and 4c were also structurally analysed. Electrochemical investigations revealed that the integrity of the bridging linkages of the examined diiron complexes significantly affect their reduction reversibility and catalysis through a coupled chemical reaction in a unique ECE mechanism, widely adopted by complexes with the core [Fe(2)(CO)4-6}. Demethylation of complexes 4a and 1Me, [Fe(2)(mu-SCH(2))(2)CMe(CH(2)-o-C(6)H(4)OMe)(CO)(6)], by BBr(3) led to complexes (5a and 1H, [Fe(2)(mu-SCH(2))(2)CMe(CH(2)-o-C(6)H(4)OH)(CO)(6)]) with pendant phenol group( s), a weak acid. Deprotonation of the two complexes produced the pendant phenolate, which instantly intramolecularly substitutes the bound CO to yield species of the coordination form Fe(I)-OR (R = phenolic moiety). Electrochemical investigation revealed that the pendant phenol groups in complexes 1H and 5a do not seem to improve their catalytic efficiency in proton reduction in the medium acetic acid/dichloromethane.