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.