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The rates of binding protons and substrates to [Fe4S4Cl4](2-)

Lookup NU author(s): Adrain John Dunford, Professor Richard Henderson


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The kinetics of the substitution reaction between [Fe4S4Cl4](2-) and (BuNC)-N-t to form [Fe4S4Cl2 (CNBut)(6)], in the presence of [H2N(CH2)(3)CH2](+), [NHEt3](+) or [lutH](+) (lut = 2,6-dimethylpyridine) have been studied. With the weakest acid, [H2N(CH2)(3)CH2](+) (pK(a) = 21.5), (BuNC)-N-t binds before the transfer of the proton. Analysis of the kinetics yields the rate constant for the binding of (BuNC)-N-t to [Fe4S4Cl4](2-) (k = 2.1 +/- 0.5 x 10(3) dm(3) 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 10(4) times slower than the diffusion-controlled limit (i.e. k = ca. 1 x 10(6) dm(3) mol(-1) s(-1)). Studies presented herein on the reactions between [Fe4S4Cl4](2-) and (BuNC)-N-t or Br- indicate that the rate of the second proton transfer is about 10(7) times slower than the diffusion-controlled limit (i.e. k = ca. 1 x 10(3) dm(3) mol(-1) s(-1)). One consequence of these slow rates of proton transfer to [Fe4S4Cl4](2-) is that the rate of binding (BuNC)-N-t 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.

Publication metadata

Author(s): Dunford AJ, Henderson RA

Publication type: Article

Publication status: Published

Journal: Journal of the Chemical Society: Dalton Transactions

Year: 2002

Issue: 14

Pages: 2837-2842

ISSN (print): 1472-7773

ISSN (electronic): 1364-5447

Publisher: Royal Society of Chemistry


DOI: 10.1039/b202850f


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