Binding nucleophiles to [Fe4Y4Cl4]2- (Y = S or se) can increase or suppress the rate of proton transfer to the cluster

In the proton transfer reactions between [Fe4Y4Cl4](2-) (Y = S or Se) and [pyrH](+) (pyr = pyrrolidine) in the presence of a variety of nucleophiles (L = I-, Br-, PhS-, EtS- or ButNC), initial binding of the nucleophile can occur to generate [Fe4Y4Cl4(L)](n-). The subsequent rate of proton transfer depends markedly on the nature of L. Stopped-flow kinetic studies show that proton transfer from [pyrH](+) to [Fe4Y4Cl4](2-) {(s)k(4) = (2.1 +/- 0.5) x 10(4) dm(3) mol(-1) s(-1); (Se)k(4) = (8.0 +/- 0.5) x 10(3) dm(3) mol(-1) s(-1)} is increased by prior binding of L = PhS- or BuINC to form [Fe4Y4Cl4(L)](n-) (sK(7)(L) similar to 1 x 10(6) dm(3) mol(-1) s(-1)), but prior binding of L = I-, Br-, or EtS- to the clusters inhibits the rate of proton transfer (e.g. (S)k(7)(1) = (6.0 +/- 0.8) x 10(2) dm(3) mol(-1) s(-1); (Se)k7(1) = (4.5 +/- 0.5) x 10(2) dm(3) mol(-1) s(-1)). This behavior is correlated with the bonding characteristics of L and the effect this has on bond length reorganization within the cluster upon proton transfer.