Six-coordinate [CoIII(L)2]z (z = 1−, 0, 1+) complexes of an azo-appended o-aminophenolate in amidate(2−) and iminosemiquinonate π-radical (1−) redox-levels: the existence of valence-tautomerism

Abstract

Aerobic reaction of the ligand H₂L¹, 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol, CoCl₂·6H₂O and Et₃N in MeOH under refluxing conditions produces, after work-up and recrystallization, black crystals of [Co(L¹)₂] (1). When examined by cyclic voltammetry, 1 displays in CH₂Cl₂ three one-electron redox responses: two oxidative, E¹_1/2 = 0.30 V (peak-to-peak separation, ΔE_p = 100 mV) and E²_1/2 = 1.04 V (ΔE_p = 120 mV), and one reductive E_1/2 = −0.27 V (ΔE_p = 120 mV) vs. SCE. Consequently, 1 is chemically oxidized by 1 equiv. of [Fe^III(η⁵-C₅H₅)₂][PF₆], affording the isolation of deep purple crystals of [Co(L¹)₂][PF₆]·2CH₂Cl₂ (2), and one-electron reduction with [Co^II(η⁵-C₅H₅)₂] yielded bluish-black crystals of [Co^III(η⁵-C₅H₅)₂][Co(L¹)₂]·MeCN (3). A solid sample of 1 exhibits temperature-independent (50–300 K) magnetism, revealing the presence of a free radical (S = 1/2), which exhibits an isotropic EPR signal (g = 2.003) at 298 K and at 77 K an eight-line feature characteristic of hyperfine-interaction of the radical with the Co (I = 7/2) nucleus. Based on X-ray structural parameters of 1–3 at 100 K, magnetic and EPR spectral behaviour of 1, and variable-temperature (233–313 K) ¹H NMR spectral features of 1–3 and ¹³C NMR spectra at 298 K of 2 and 3 in CDCl₃ point to the electronic structure of the complexes as either [Co^III{(L^AP)²⁻}{(L^ISQ)}˙⁻] or [Co^III{(L¹)₂}˙³⁻] (delocalized nature favours the latter description) (1), [Co^III{(L^ISQ)˙⁻}₂][PF₆]·2CH₂Cl₂ (2) and [Co^III(η⁵-C₅H₅)₂][Co^III{(L^AP)²⁻}₂]·MeCN (3) [(L^AP)²⁻ and (L^ISQ)˙⁻ represent the redox-level of coordinated ligands o-amidophenolate(2−) ion and o-iminobenzosemiquinonate(1−) π-radical ion, respectively]. Notably, all the observed redox processes are ligand-centred. To the best of our knowledge, this is the first time that six-coordinate complexes of a common tridentate o-aminophenolate-based ligand have been structurally characterized for the parent 1, its monocation 2 and the monoanion 3 counterparts. Temperature-dependent ¹H NMR spectra reveal the existence of valence-tautomeric equilibria in 1–3. Density Functional Theory (DFT) calculations at the B3LYP-level of theory corroborate the electronic structural assignment of 1–3 from experimental data. The origins of the observed UV–VIS–NIR absorptions for 1–3 have been assigned, based on time-dependent (TD)-DFT calculations.