Binding small molecules and ions to [Fe4S4Cl4]2− modulates rate of protonation of the cluster†
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−, ButNC 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− (KL) and the rates of proton transfer from NHBun3+ to [Fe4S4Cl4(L)]n− (kL0) 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 (kL0/k0 = 1.6 to ≥72). (ii) When KL is small, so is kL0/k0. (iii) Binding substrates which are good σ-donors or good π-acceptors lead to the largest kL0/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.