Binding small molecules and ions to [Fe4S4O2]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 NHBu(n)3(+) involves rate-limiting proton transfer from NHBu(n)3(+) to the cluster (k(o) = 490 +/- 20 dm(3) mol(-1) s(-1)). A variety of small molecules and ions (L = substrate = Cl-, Br-, I-, RNHNH2 (R = Me or Ph), Me2NNH2, HCN, NCS-, N-3(-), (BuNC)-N-t or pyridine) bind to [Fe4S4Cl4](2-) and this affects the rate of subsequent protonation of [Fe4S4Cl(L)](n-). Where the kinetics allow, the equilibrium constants for the substrates binding to [Fe5S4Cl](2-) (K-L) and the rates of proton transfer from NHBu(n)3(+) to [Fe-4 S4Cl4(L)r(k(0)(L)) 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(o)(L)/k(o) = 1.6 to >72). (ii) When K-L is small, so is k(o)(L)/k(o). (iii) Binding substrates which are good n-donors or good pi-acceptors lead to the largest k(0)(L)/k(0). This behaviour is discussed in terms of the recent proposal that protonation of [Fe4S4Cl4](2-) at a mu(3)-S, is coupled to concomitant Fe-(mu(3)-SH) bond elongation/cleavage.