Synthesis, Structures, and Electrochemical Properties of Multinuclear Iron-Sulfur Complexes Related to [FeFe]-Hydrogenases

To extend a rarely-studied family of multinuclear [FeFe]-hydrogenase mimics for catalytic proton reduction into hydrogen (H2), we carried out the esterification reactions of bridgehead-carboxyl-having diiron azadithiolato precursor [Fe2(CO)6{(μ-SCH2)2N(C6H4CH2CO2H)}] (1) and three primary alcohols (CH3CH2OH, (CH3)2C(CH2OH)2 and CH3C(CH2OH)3 to afford three new fully-esterified products [Fe2(CO)6{(μ-SCH2)2N(C6H4CH2C[O])}]nR (n = 1, R = OCH2CH3 for diiron 2; n = 2, R = (OCH2)2C(CH3)2 for tetrairon 3; and n = 3, R = (OCH2)3C(CH3) for hexairon 5) and two new partly-esterified products [Fe2(CO)6{(μ-SCH2)2N(C6H4CH2C[O])}]nR (n = 1, R = (OCH2)C(CH3)2(CH2OH) for diiron 4 and n = 2, R = (OCH2)2C(CH3)(CH2OH) for tetrairon 6). Meanwhile, the fully-esterified iron-sulfur complexes 3 and 5 can be obtained in good yield from the complete esterification of their hydoxyl-bearing counterparts 4 and 6, respectively. All the as-obtained new complexes 2–6 are characterized by means of elemental analysis as well as various spectroscopies, and for 2 by X-ray crystallography. Further electrochemical investigation of homolog complexes 2, 3 and 5 is performed without and with acetic acid (HOAc) through cyclic voltammetry (CV), exhibiting that hexairon complex 5 has a greatest turnover frequency and a lower overpotential for electrocatalytic H2 evolution among them.