Electrochemical and theoretical investigations of the reduction of [Fe2(CO)5L{µ-SCH2XCH2S}] complexes related to [FeFe] hydrogenase

Abstract

The complexes [Fe2(CO)6{μ-SCH2N(R)CH2S}] (R = CH2CH2OCH3, 1a; R = iPr, 1b) and [Fe2(CO)6(μ-pdt)] 2 (pdt = S(CH2)3S) are structural analogues of the [2Fe]H subsite of [FeFe]H2ases. Electrochemical investigation of 1 and 2 in MeCN–[NBu4][PF6] under Ar and under CO has demonstrated that the reduction can be resolved into two one-electron transfer steps by using fast scan cyclic voltammetry. At slow scan rates the reduction of 1 tends towards a two-electron process owing to the fast disproportionation of the anion, while the two-electron reduction of 2 is clearly favoured in the presence of CO. Substitution of a CO ligand in 2 by a N-heterocyclic carbene results in the destabilisation of the anion. Thus, in MeCN–, thf- or CH2Cl2–[NBu4][PF6], the electrochemical reduction of Fe2(CO)5LNHC(μ-pdt)] 3 (LNHC = 1,3-bis(methyl)-imidazol-2-ylidene, 3a; 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene, 3b) occurs in a single-step, two-electron process at moderate scan rates; under appropriate conditions this process can be separated into two one-electron steps. Density Functional Theory calculations successfully rationalize the effects of the S-to-S linkage on the electrochemistry of the complexes.