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

Abstract

A brownish-black complex [Fe-III(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 E-1/2 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 [Fe-III(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 [Co-III(eta(5)-C10H15)(2)][Fe-III(L)(2)] (3) (S = 1/2). Mossbauer spectral parameters unequivocally establish that 1 is a low-spin (LS) Fe-III complex. Careful analysis of Mossbauer spectral data of 2 and 3 at 200 and 80 K reveal that each complex has a major LS Fe-III and a minor LS Fe-II component (redox isomers): Fe-III{(L-ISQ)(-center dot)}(2) and Fe-II{(L-IBQ)(0)}{(L-ISQ)(-center dot)} (2) and Fe-III{(L-AP)(2-)}(2) and Fe-II{(L-ISQ)(-center dot)}{(L-AP)(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 Fe-III is strongly antiferromagnetically coupled with a pi-radical of o-iminobenzosemiquinonate(1-) (L-ISQ)(-center dot) 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 (L-ISQ)(-center dot) [LS Fe-III (S-Fe = 1/2) is strongly coupled to one of the (L-ISQ)(-center dot) radicals (S-rad = 1/2)] and 3 is primarily a LS Fe-III 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 Mossbauer, 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.