Thermal stability of iron-sulfur clusters

Abstract

The thermal decomposition of free cationic iron-sulfur clusters Fe_xS_y⁺ (x = 0-7, y = 0-9) is investigated by collisional post-heating in the temperature range between 300 and 1000 K. With increasing temperature the preferential formation of stoichiometric Fe_xS_y⁺ (y = x) or near stoichiometric Fe_xS_y⁺ (y = x ± 1) clusters is observed. In particular, Fe₄S₄⁺ represents the most abundant product up to 600 K, Fe₃S₃⁺ and Fe₃S₂⁺ are preferably formed between 600 K and 800 K, and Fe₂S₂⁺ clearly dominates the cluster distribution above 800 K. These temperature dependent fragment distributions suggest a sequential fragmentation mechanism, which involves the loss of sulfur and iron atoms as well as FeS units, and indicate the particular stability of Fe₂S₂⁺. The potential fragmentation pathways are discussed based on first principles calculations and a mechanism involving the isomerization of the cluster prior to fragmentation is proposed. The fragmentation behavior of the iron-sulfur clusters is in marked contrast to the previously reported thermal dissociation of analogous iron-oxide clusters, which resulted in the release of O₂ molecules only, without loss of metal atoms and without any tendency to form particular prominent and stable Fe_xO_y⁺ clusters at high temperatures.