Neutral, Cationic, and Anionic Low-Spin Iron(III) Complexes Stabilized by Amidophenolate and Iminobenzosemiquinonate Radical in N,N,O Ligands

A brownish-black complex [FeIII(L)2] (1) (S = 0), supported by two tridentate redox-active azo-appended o-amidophenolates [H2L = 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol], has been synthesized and structurally characterized. In CH2Cl2 1 displays two oxidative and two reductive 1e– redox processes at E1/2 values of 0.48 and 1.06 V and −0.42 and −1.48 V vs SCE, respectively. The one-electron oxidized form [1]+ isolated as a green solid [FeIII(L)2]­[BF4] (2) (S = 1/2) has been structurally characterized. Isolation of a dark ink-blue one-electron reduced form [1]− has also been achieved [CoIII(η5-C10H15)2]­[FeIII(L)2] (3) (S = 1/2). Mössbauer spectral parameters unequivocally establish that 1 is a low-spin (LS) FeIII complex. Careful analysis of Mössbauer spectral data of 2 and 3 at 200 and 80 K reveal that each complex has a major LS FeIII and a minor LS FeII component (redox isomers): [FeIII{(LISQ)−•}2]+ and [FeII{(LIBQ)0}­{(LISQ)−•}]+ (2) and [FeIII{(LAP)2–}2]− and [FeII{(LISQ)−•}­{(LAP)2–}]− (3). Notably, for both at 8 K mainly the major component exists. Broken-Symmetry (BS) Density Functional Theory (DFT) calculations at the B3LYP level reveals that in 1 the unpaired electron of LS FeIII is strongly antiferromagnetically coupled with a π-radical of o-iminobenzosemiquinonate­(1−) (LISQ)−• form of the ligand, delocalized over two ligands providing 3– charge (X-ray structure). DFT calculations reveal that the unpaired electron in 2 is due to (LISQ)−• [LS FeIII (SFe = 1/2) is strongly antiferromagnetically coupled to one of the (LISQ)−• radicals (Srad = 1/2)] and 3 is primarily a LS FeIII complex, supported by two o-amidophenolate­(2−) ligands. Time-Dependent-DFT calculations shed light on the origin of UV–vis–NIR spectral absorptions for 1–3. The collective consideration of Mössbauer, variable-temperature (77–298 K) electron paramagnetic resonance (EPR), and absorption spectral behavior at 298 K, and DFT results reveals that in 2 and 3 the valence-tautomerism is operative in the temperature range 80–300 K.