The rates of binding protons and substrates to [Fe4S4Cl4]2−

Abstract

The kinetics of the substitution reaction between [Fe₄S₄Cl₄]²⁻ and Bu^tNC to form [Fe₄S₄Cl₂(CNBu^t)₆], in the presence of [H₂N(CH₂)₃CH₂]⁺, [NHEt₃]⁺ or [lutH]⁺ (lut = 2,6-dimethylpyridine) have been studied. With the weakest acid, [H₂N(CH₂)₃CH₂]⁺ (pK_a = 21.5), Bu^tNC binds before the transfer of the proton. Analysis of the kinetics yields the rate constant for the binding of Bu^tNC to [Fe₄S₄Cl₄]²⁻ (k = 2.1 ± 0.5 × 10³ dm³ mol⁻¹ s⁻¹). With the stronger acids, [NHEt₃]⁺ (pK_a = 18.5) and [lutH]⁺ (pK_a = 15.4), two protons bind to the cluster. Earlier work indicated that for a thermodynamically favourable proton-transfer reaction, the rate of the first proton transfer to [Fe₄S₄Cl₄]²⁻ is at least 10⁴ times slower than the diffusion-controlled limit (i.e.k = ca. 1 × 10⁶ dm³ mol⁻¹ s⁻¹). Studies presented herein on the reactions between [Fe₄S₄Cl₄]²⁻ and Bu^tNC or Br⁻ indicate that the rate of the second proton transfer is about 10⁷ times slower than the diffusion-controlled limit (i.e.k = ca. 1 × 10³ dm³ mol⁻¹ s⁻¹). One consequence of these slow rates of proton transfer to [Fe₄S₄Cl₄]²⁻ is that the rate of binding Bu^tNC or Br⁻ to the cluster is slower than the initial proton transfer, but faster than the second one. The results of these studies are compared with those reported earlier for the reaction of [Fe₄S₄Cl₄]²⁻ with PhSH in the presence of [lutH]⁺, where the poor nucleophilicity of the thiol results in PhSH binding to the cluster only after diprotonation. The possible relevance of these studies to the mechanisms of substrate binding and transformation at both synthetic and natural Fe–S clusters is discussed.