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

摘要

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, [(L~x)_2M(THF)][B(Ar_F)_4] [where L~x represents either the neutral a-iminopyridine (L)~0 and/or its reduced n 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(Ar_F)_4]. Finally, an unusual spin equilibrium for Fe(Ⅱ), between high spin and intermediate spin (S_(Fe) = 2 reversible S_(Fe) = 1), is described for the complex [(L~·)(L)Fe(THF)][B(Ar_F)_4], which consequently is characterized by the overall spin equilibrium (S_(tot) = 3/2 reversible S_(tot) = 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 Mossbauer spectroscopy, and magnetic susceptibility measurements. Complementary DFT studies of all the complexes have been performed, and the calculations support the proposed electronic structures.