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 [NH₂(CH₂)₃CH₂]⁺ 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 [NH₂(CH₂)₃CH₂]⁺ 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.