The [1:3] site-differentiated cubane-type clusters
[Fe4Q4(LS3)Cl]2-
(Q = S, Se; LS3 =
1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzenate(3−))
undergo substitution reactions at the unique iron site
with a variety of ligands including thiolates, phenolates, cyclic
triamines and a trisulfide, imidazoles, and tertiary
phosphines. Reactions are readily followed because of the extreme
sensitivity of isotropically shifted resonances
to the nature of ligand L‘ in the product clusters
[Fe4Q4(LS3)L‘]2-,-.
Isotropic shifts and redox potentials are
reported for over 40 clusters, including many cluster pairs differing
only in the core chalcogenide atom. In this
way, comparative properties of sulfide and selenide clusters can be
elicited. It is shown that the larger isotropic
shifts consistently observed for selenide clusters and dominantly
contact in nature arise from larger magnetic
susceptibilities, which indicate a lesser extent of antiferromagnetic
coupling. It is further demonstrated that,
without exception, redox potentials of selenide clusters are more
positive than those of sulfide clusters, usually
by 20−60 mV, at parity of ligation. The difference in potentials
of ca. 300 mV between the
[Fe4S4]2+,+
couples
of
[Fe4S4(LS3)(SEt)]2-
and
[Fe4S4(LS3)(Im)]-
(Im = imidazole) clusters is the best available estimate of
the
intrinsic potential difference between protein-bound
[Fe4S4(Cys-S)4]
and
[Fe4S4(Cys-S)3(His-N)]
clusters. The
ligand LS3 undergoes spontaneous transfer between
[Fe4S4]2+,
[Fe4Se4]2+, and
[MoFe3S4]3+ cores in
reactions
whose equilibrium constants are near the statistical value. The
ligand binding affinity order PhS- > PhO-
>
CN- ≫ Cl- for
[Fe4Q4]2+ cores was
established. When taken together with earlier results from this
laboratory,
a comprehensive picture of ligand binding to single iron sites in
[Fe4Q4]2+ clusters
emerges.