Deprotonation in Mixed-Valent Diiron(II,III) Complexes with Aniline or Benzimidazole Ligands

We
have previously investigated cis/trans isomerization processes in
phenoxido-bridged mixed-valent FeIIFeIII complexes
that contain either one aniline or one anilide ligand. In this work,
we compare the properties of similar complexes bearing one terminal
protic ligand, either aniline or 1H-benzimidazole.
Whatever the ligand, 1H NMR spectroscopy clearly evidences
that the complexes are present in CH3CN as a mixture of
cis- and trans-isomers in a close to 1:1 ratio. We show here that
addition of NEt3 indeed allows the deprotonation of these
ligands, the resulting complexes bearing either anilide or benzimidazolide
that are coordinated to the ferric site. The latter are singular examples
of a high-spin ferric ion coordinated to a benzimidazolide ligand.
Whereas the trans-isomer of the anilide complex is the overwhelming
species, benzimidazolide species are mixtures of cis- and trans-isomers
in equal proportions. Moreover, cyclic voltammametry studies show
that FeIIIFeIII complexes with 1H-benzimidazole are more stable than their aniline counterparts, whereas
the reverse is observed for the deprotonated species.