Fe4S4 clusters functionalized with molecular receptor ligands

Cubane-type Fe4S4 clusters have been functionalized with the concave dithiol ligands LX(yl)2H and L(AC)2H, derived from a diphenylglucoluril-based receptor molecule. In the monomeric cluster compounds [Fe4S4(L-XYl)(2)] (PPh4)(2) (cluster A) and [Fe4S4(L-AC)(2)] (PPh4)(2) (cluster B), an Fe4S4 core is combined with receptor sites for alkali metal ions and aromatic guest molecules. Molecular modelling studies show that the Fe4S4 core in compound B is tightly encapsulated by its two dithiol ligands, whereas this isnot the case for compound A. However, unlike the clusters in High-Potential Iron-sulfur Proteins (HiPIP's), the Fe4S4 core in B is still accessible to solvent molecules, as has been established by solution electrochemical studies. Both cluster compounds bind alkali metal ions and undergo anodic shifts in their 2(-)/3(-) reduction potentials upon binding of these ions. Electrochemical titrations indicate the complexation of four alkali metal ions per cluster compound. Binding by cluster B takes place preferentially at the exterior of the receptor ligands, whereas in the case of cluster A the ions also bind to the interior of the receptor. The more open structure of A allows the binding of dimethyl-paraquat 5 [K-ass = (5.6 +/- 0.6) x 10(3) m(-1)] to this cluster compound. On complexation, the first reduction potential of the guest molecule shifts in the cathodic direction, whereas the reduction potential of the cluster remains unaffected. This observation can be rationalized by assuming that the twofold positively charged guest molecule binds between the aromatic side-walls of the receptor Ligands, whereas the one-electron reduced, singly charged positive species is not bound.