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DOI: 10.1039/A807769J (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1999, 119-126

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

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Richard A. Henderson

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.

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