Oxygen Sensitivity of Free Nonligated Iron–Sulfur Clusters

Abstract

In view of the potential functionalization of iron–sulfur clusters as biomimetic catalysts, the oxygen sensitivity of these particles plays an important role. To gain fundamental insights into the interaction of iron–sulfur clusters with molecular oxygen, we employ free nonligated FexSx+ (x = 2–6) clusters, that is, isolated iron–sulfur clusters without any additional ligands typically found in protein bound clusters, as well-defined model systems. Pressure-dependent gas phase ion trap experiments performed under multicollision conditions in conjunction with first-principles density functional theory simulations reveal the degradation of Fe2S2+ via activation of O2 to a superoxo species and subsequent thermodynamically favorable but kinetically hampered sequential exchange of the cluster sulfur atoms with oxygen atoms. A considerably faster exchange of multiple sulfur atoms is observed for Fe3S3+, Fe5S5+, and Fe6S6+ in agreement with a kinetically and thermodynamically favored reaction mechanism, which involves the concerted exchange of two sulfur atoms of Fe3S3+. In marked contrast, Fe4S4+ appears to be oxygen-tolerant and S/O exchange is not observed, which is attributed to an intrinsic weaker cluster–oxygen interaction.