Electrochemical and theoretical investigations of the reduction of [Fe-2(CO)(5)L{mu-SCH2XCH2S}] complexes related to [FeFe] hydrogenase

The complexes [Fe-2(CO)(6){mu-SCH2N(R)CH2S}] (R = CH2CH2OCH3, 1a; R= Pr-i, 1b) and [Fe-2(CO)(6)(mu-pdt)] 2 (pdt = S(CH2)(3)S) are structural analogues of the [2Fe](H) subsite of [FeFe]H(2)ases. 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 Fe-2(CO)(5)L-NHC(mu-pdt)] 3 (L-NHC = 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