NHC Versus Pyridine: How “Teeth” Change the Redox Behavior of Iron(II) Complexes

A series
of octahedral iron­(II) complexes with tetradentate NHC/pyridine
hybrid ligands containing up to three pyridyl units was designed to
study the influence of NHC and pyridine donors on the electronic structure
of the metal center. Structural analysis of the iron complexes by
NMR spectroscopy and single-crystal X-ray diffraction reveals different
coordination modes of the ligand depending on the linkage of the different
donor moieties. The oxidation potentials of all complexes correlate
linearly with the number of NHC moieties coordinated to iron, as shown
by cyclic voltammetry. The influence, although minor, of structural
properties on the oxidation potential and (in one case) the influence
of the oxidation state of the coordination geometry of the hybrid
ligand are also demonstrated.