Combined versus individual labilising effects of H+, Na+ and nucleophile on catalysed substitution reactions: studies on [Fe4S4X4]2– (X = Cl or PhS)†

Abstract

The reactions between [Fe₄S₄X₄]^2– (X = PhS or Cl) and Et₂NCS₂^– to form [Fe₄S₄X₂(S₂CNEt₂)₂]^2– have been studied in MeCN. The kinetics are consistent with a dissociative mechanism under all conditions. The addition of Na⁺ led to an increase in rate for [Fe₄S₄(SPh)₄]^2– and analysis of the kinetics indicates that a single Na⁺ binds and labilises the cluster. Comparison is drawn with the established effect of H⁺ on the lability of this cluster. The presence of a thiolate ligand is necessary to bind Na⁺ since the reaction between [Fe₄S₄Cl₄]^2– and Et₂NCS₂^– is unaffected by the addition of Na⁺. The addition of acid to [{Fe₄S₄(SPh)₄}Na]^– further accelerates the rate of substitution. Quantitative analysis shows that the combined labilising effect of Na⁺ and H⁺ is no more than that expected from the individual labilisation afforded by each cation. Similar analyses show the same is true for H⁺ and nucleophile in acid-catalysed associative substitution mechanisms, and two H⁺ in the acid-catalysed dissociative mechanisms of Fe–S-based clusters. The generality of these observations is discussed.