Tuning the oxidative power of free iron-sulfur clusters.

The gas-phase reactions between a series of di-iron sulfur clusters Fe2 Sx + (x = 1-3) and the small alkenes C2 H4 , C3 H6 , and C4 H8 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 Fe2 S+ < Fe2 S2 + < Fe2 S3 + . In particular, Fe2 S+ and Fe2 S2 + only form simple association products, whereas the sulfur-rich Fe2 S3 + is able to dehydrogenate propene and 2-butene via desulfurization of the cluster and formation of H2 S. This indicates an increased propensity to induce oxidation reactions, i.e. oxidative power, of Fe2 S3 + that is attributed to an increased formal oxidation state of the iron atoms. Furthermore, the ability of Fe2 S3 + 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.