Tuning the oxidative power of free iron-sulfur clusters.

The gas-phase reactions between a series of di-iron sulfur clusters Fe2Sx+ (x = 1-3) and the small alkenes C2H4, C3H6, and C4H8 have been investigated by means of Fourier-transform ion-cyclotron resonance mass spectrometry. For all studied alkenes, the reaction efficiency is found to increase in the order Fe2S+ < Fe2S2+ < Fe2S3+. In particular, Fe2S+ and Fe2S2+ only form simple association products, whereas the sulfur-rich Fe2S3+ is able to dehydrogenate propene and 2-butene via desulfurization of the cluster and formation of H2S. This indicates an increased propensity to induce oxidation reactions, i.e. oxidative power, of Fe2S3+ that is attributed to an increased formal oxidation state of the iron atoms. Furthermore, the ability of Fe2S3+ to activate and dissociate the C-H bonds of the alkenes is observed to increase with increasing size of the alkene and thus correlates with the alkene ionization energy.