Redox Chemistry of Noninnocent Quinones Annulated to 2Fe2S Cores

Noninnocent
ligands that are electronically coupled to active catalytic
sites can influence the redox behavior of the catalysts. A series
of (μ-dithiolato)­Fe2(CO)6 complexes, in
which the sulfur atoms of the μ-dithiolato ligand are bridged
by 5-substituted (Me, OMe, Cl, t-Bu)-1,4-benzoquinones,
1,4-naphthoquinone, or 1,4-anthraquinone, have been synthesized and
characterized. In addition, the bis-phosphine complex derived from
the 1,4-naphthoquinone-bridged complex has also been prepared and
characterized. Cyclic voltammetry of these complexes shows two reversible
one-electron reductions at potentials 0.2 to 0.5 V less negative than
their corresponding parent quinones. In the presence of acetic acid
two-electron reductions of the complexes result in conversion of the
quinones to hydroquinone moieties. EPR spectroscopic and computational
studies of the one-electron-reduced complexes show electron delocalization
from the semiquinones to the 2Fe2S moieties, thereby revealing the
“noninnocent” behavior of these ligands with these catalysts.