Binding small molecules and ions to [Fe4S4Cl4]2− modulates rate of protonation of the cluster

Abstract

The mechanism of the acid-catalyzed substitution reaction of the terminal chloro-ligands in [Fe₄S₄Cl₄]²⁻ by PhS⁻ in the presence of NHBuⁿ₃⁺ involves rate-limiting proton transfer from NHBuⁿ₃⁺ to the cluster (k₀ = 490 ± 20 dm³ mol⁻¹ s⁻¹). A variety of small molecules and ions (L = substrate = Cl⁻, Br⁻, I⁻, RNHNH₂ (R = Me or Ph), Me₂NNH₂, HCN, NCS⁻, N₃⁻, Bu^tNC or pyridine) bind to [Fe₄S₄Cl₄]²⁻ and this affects the rate of subsequent protonation of [Fe₄S₄Cl₄(L)]ⁿ⁻. Where the kinetics allow, the equilibrium constants for the substrates binding to [Fe₄S₄Cl₄]²⁻ (K^L) and the rates of proton transfer from NHBuⁿ₃⁺ to [Fe₄S₄Cl₄(L)]ⁿ⁻ (k^L₀) have been determined. The results indicate the following general features. (i) Bound substrates increase the rate of protonation of the cluster, but the rate increase is modest (k^L₀/k₀ = 1.6 to ≥72). (ii) When K^L is small, so is k^L₀/k₀. (iii) Binding substrates which are good σ-donors or good π-acceptors lead to the largest k^L₀/k₀. This behaviour is discussed in terms of the recent proposal that protonation of [Fe₄S₄Cl₄]²⁻ at a μ₃-S, is coupled to concomitant Fe–(μ₃-SH) bond elongation/cleavage.