Tuning the oxidative power of free iron-sulfur clusters

The gas-phase reactions between a series of di-iron sulfur clusters Fe₂S_x⁺ (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.