Neutral Bis(α-iminopyridine)metal Complexes of the First-Row Transition Ions (Cr, Mn, Fe, Co, Ni, Zn) and Their Monocationic Analogues: Mixed Valency Involving a Redox Noninnocent Ligand System

A series of bis(α-iminopyridine)metal complexes featuring the first-row transition ions (Cr, Mn,
Fe, Co, Ni, and Zn) is presented. It is shown that these ligands are redox noninnocent and their paramagnetic
π radical monoanionic forms can exist in coordination complexes. Based on spectroscopic and structural
characterizations, the neutral complexes are best described as possessing a divalent metal center and
two monoanionic π radicals of the α-iminopyridine. The neutral M(L•)2 compounds undergo ligand-centered,
one-electron oxidations generating a second series, [(Lx)2M(THF)][B(ArF)4] [where Lx represents either the
neutral α-iminopyridine (L)0 and/or its reduced π radical anion (L•)-]. The cationic series comprise mostly
mixed-valent complexes, wherein the two ligands have formally different redox states, (L)0 and (L•)-, and
the two ligands may be electronically linked by the bridging metal atom. Experimentally, the cationic Fe
and Co complexes exhibit Robin−Day Class III behavior (fully delocalized), whereas the cationic Zn, Cr,
and Mn complexes belong to Class I (localized) as shown by X-ray crystallography and UV−vis
spectroscopy. The delocalization versus localization of the ligand radical is determined only by the nature
of the metal linker. The cationic nickel complex is exceptional in this series in that it does not exhibit any
ligand mixed valency. Instead, its electronic structure is consistent with two neutral ligands (L)0 and a
monovalent metal center or [(L)2Ni(THF)][B(ArF)4]. Finally, an unusual spin equilibrium for Fe(II), between
high spin and intermediate spin (SFe = 2 ↔ SFe = 1), is described for the complex [(L•)(L)Fe(THF)][B(ArF)4],
which consequently is characterized by the overall spin equilibrium (Stot = 3/2 ↔ Stot = 1/2). The two different
spin states for Fe(II) have been characterized using variable temperature X-ray crystallography, EPR
spectroscopy, zero-field and applied-field Mössbauer spectroscopy, and magnetic susceptibility measurements. Complementary DFT studies of all the complexes have been performed, and the calculations support
the proposed electronic structures.