Fe4S4 clusters functionalized with molecular receptor ligands

Cubane-type Fe4S4 clusters have been functionalized with the concave dithio l ligands LX(yl)2H and L(AC)2H, derived from a diphenylglucoluril-based rec eptor 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 i s combined with receptor sites for alkali metal ions and aromatic guest mol ecules. 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-su lfur Proteins (HiPIP's), the Fe4S4 core in B is still accessible to solvent molecules, as has been established by solution electrochemical studies. Bo th cluster compounds bind alkali metal ions and undergo anodic shifts in th eir 2(-)/3(-) reduction potentials upon binding of these ions. Electrochemi cal titrations indicate the complexation of four alkali metal ions per clus ter compound. Binding by cluster B takes place preferentially at the exteri or 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 t he 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 th e guest molecule shifts in the cathodic direction, whereas the reduction po tential of the cluster remains unaffected. This observation can be rational ized by assuming that the twofold positively charged guest molecule binds b etween the aromatic side-walls of the receptor Ligands, whereas the one-ele ctron reduced, singly charged positive species is not bound.