Enforced slow protonation of [Fe₄S₄Cl₄]^2– and the maximum rate of protonation of the cluster core †

Abstract

Kinetic studies on the reaction of PhS^– with [Fe₄S₄Cl₄]^2– (pK_a = 18.8) in the presence of the weak acid [[upper bond 1 start]NH₂(CH₂)₃C[upper bond 1 end]H₂]⁺ in MeCN showed that the mechanism involves initial, rate-limiting, binding of PhS^– to the cluster, followed by protonation of the cluster core (presumably a µ₃-S), then dissociation of chloride. This is different to the sequence of elementary reactions established with the stronger acid, [NHEt₃]⁺ (pK_a = 18.5), where protonation precedes binding of the thiol. Quantitative comparison of these two systems reveals that the literature value of pK_a = 19.6 for [[upper bond 1 start]NH₂(CH₂)₃C[upper bond 1 end]H₂]⁺ is inconsistent with our kinetic results and that pK_a = 21.5 is more appropriate, both in this and other systems. The kinetic data show that the rate of protonation of the cluster core falls in the range 2 × 10⁵ ≤ k ≤ 4.8 × 10⁶ dm³ mol^–1 s^–1, for a thermodynamically favourable reaction. The reasons why this protonation is 10⁴–10⁵ times slower than the diffusion-controlled limit are discussed.

References

1. R. H. Holm, P. Kennepohl and E. I. Solomon, Chem. Rev., 1996, 96, 2239 and refs. therein.
2. R. H. Holm, Adv. Inorg. Chem., 1992, 38, 1 and refs. therein.
3. J.-F. You and R. H. Holm, Inorg. Chem., 1992, 31, 2166 and refs. therein.
4. R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Dalton Trans., 1998, 1731 and refs. therein.
5. V. R. Almeida, C. A. Gormal, K. L. C. Grönberg, R. A. Henderson, K. E. Oglieve and B. E. Smith, Inorg. Chim. Acta, 1999, 291, 212 and refs. therein.
6. K. Izutsu, Acid–Base Dissociation Constants in Dipolar Aprotic Solvents, Blackwell Scientific, Oxford, 1990.
7. G. Cauquis, A. Deronzier, D. Serve and E. Vieil, J. Electroanal. Chem. Interfacial Electrochem., 1975, 60, 205.
8. R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Chem Commun., 1994, 377.
9. G. B. Wong, M. A. Bobrik and R. H. Holm, Inorg. Chem., 1978, 17, 578.
10. B. A. Averill, T. Herskovitz, R. H. Holm and J. A. Ibers, J. Am. Chem. Soc., 1973, 95, 3523.
11. R. E. Palermo, P. P. Power and R. H. Holm, Inorg. Chem., 1982, 21, 173.
12. J. R. Dilworth, R. A. Henderson, P. Dahlstrom, T. Nicholson and J. A. Zubieta, J. Chem. Soc., Dalton Trans., 1987, 529.
13. R. A. Henderson, J. Chem. Soc., Dalton Trans., 1982, 917.
14. R. G. Wilkins, Kinetics and Mechanism of Reactions of Transition Metal Complexes, 2nd edn., VCH, Weinheim, 1991, p. 5.
15. J. H. Espenson, Chemical Kinetics and Reaction Mechanisms, McGraw-Hill, New York, 1981, p. 72.
16. J. F. Coetzee and G. R. Padmanabhan, J. Am. Chem. Soc., 1965, 87, 5005.
17. J. F. Coetzee, Prog. Phys. Org. Chem., 1967, 4, 45 and refs. therein.
18. R. A. Henderson and K. E. Oglieve, J. Chem. Soc., Dalton Trans., 1993, 1467.
19. K. L. C. Grönberg and R. A. Henderson, J. Chem. Soc., Dalton Trans., 1996, 3667 and refs. therein.
20. K.-T. Smith, M. Tilset, S. S. Kristjánsdóttir and J. R. Norton, Inorg. Chem., 1995, 34, 6497.
21. R. P. Bell, The Proton in Chemistry, 2nd edn., Chapman Hall, London, 1973, p. 195 and refs. therein.
22. K. W. Kramarz and J. R. Norton, Prog. Inorg. Chem., 1994, 42, 1 and refs. therein.
23. R. A. Henderson, Angew. Chem., 1996, 35, 946 and refs. therein.
24. J. G. Reynolds, C. L. Coyle and R. H. Holm, J. Am. Chem. Soc., 1980, 102, 4350 and refs. therein.
25. J. M. Carroll and J. R. Norton, J. Am. Chem. Soc., 1992, 114, 8744 and refs. therein.
26. J. Hirst, J. L. C. Duff, G. N. L. Jameson, M. A. Kemper, B. K. Burgess and F. A. Armstrong, J. Am. Chem. Soc., 1998, 120, 7085 and refs. therein.