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.