The influence of net charge on the redox potentials of Fe4S4 cubane-type clusters in aprotic solvents

The recent demonstration of the existence of the [Fe4S4]0 oxidation state in the iron protein of nitrogenase raises the issue of stabilization of reduced states of Fe4S4 clusters in proteins and synthetic compounds. In thiolate-ligated clusters, potentials of the redox couples [Fe4S4(SR)4]2−/3− have been shown to scale linearly with substituent (R) constants. In this work, the combined result of inductive and electrostatic effects on potentials has been examined by use of the previously reported clusters with R=p-C6H4NO2, C6F5 and new clusters having R=CH2CH2NMe3 +, p-C6H4NR3 +, and p-C6H4CH2NR3 +. The crystal structure of (Bu4N)2[Fe4S4(SC6H4-p-NO2)4] reveals a tetragonally compressed [Fe4S4]2+ core. Synthesis of salts of the thiol cations p-HSC6H4NR3 + and p-HSC6H4CH2NR3 + are described. The clusters [Fe4S4(SR)4]2+ and the [1:3] site-differentiated species [Fe4S4(LS3)(SR)]1− were prepared by ligand substitution reactions. Potentials (vs. SCE) and potential shifts relative to appropriate reference clusters for 17 clusters are reported in aprotic solvents. Substantial positive potential shifts can be achieved with the use of strongly electron-withdrawing monoanionic and neutral thiolate ligands. The two least negative [Fe4S4]2+/1+ potentials are found for the couples [Fe4S4(SC6H4-p-NO2)4]2−/3− (−0.65 V) and [Fe4S4(SC6H4-p-NMe3)4]2+/1+ (−0.73 V) in DMF. Also reported are [Fe4S4]1+/0 potentials for seven clusters, all of which are ≲−1.5 V and do not correspond to strictly chemically reversible reactions. The redox behavior of the cluster in the iron protein of nitrogenase, with reported reductions at −0.31 and −0.46 V (vs. SHE), is contrasted with synthetic clusters in aqueous or non-aqueous media. None of the latter reduces at comparable potentials and the difference between first and second potentials is always much larger. Synthesis of the [Fe4S4]0 state is a remaining goal in the synthetic analogue chemistry of biological iron–sulfur clusters.