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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: 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 workup 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, E1/21 = 0.30 V (peak-to-peak separation, ΔEp = 100 mV) and E1/22 = 1.04 V (ΔEp = 120 mV), and a reductive E1/2 = –0.27 V (ΔEp = 120 mV) vs. SCE. Consequently, 1 is chemically oxidized by 1 equiv. of [FeIII(η5-C5H5)2][PF6] affording isolation of deep purple crystals of [Co(L1)2][PF6].2CH2Cl2 (2) and one-electron reduction with [CoII(η5-C5H5)2] yielded bluish-black crystals of [CoIII(η5-C5H5)2][Co(L1)2].MeCN (3). Solid sample of 1 exhibits temperature-independent (40–300 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 Co (I = 7/2) nucleus. Based on X-ray structural parameters of 13 at 100 K, magnetic and EPR spectral behaviour of 1, and variable-temperature (233–313 K) 1H NMR spectral feature of 13 and 13C NMR spectra at 298 K of 2 and 3 in CDCl3 point to the electronic structure of the complexes as either [CoIII{(LAP)2–)}{(LISQ}•−] or [CoIII{(L1)2}•3–] (delocalized nature favours latter description) (1), [CoIII{(LISQ)•−}2][PF6].2CH2Cl2 (2) and [CoIII(η5-C5H5)2] [CoIII{(LAP)2–}2].MeCN (3) [(LAP)2– and (LISQ)•− represent 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, it is for 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 13. Density Functional Theory (DFT) calculations at the B3LYP-level of theory corroborate the electronic structural assignment of 13 from experimental data. The origin of observed UV−VIS−NIR absorptions for 13 have been assigned, based on time-dependent (TD)-DFT calculations.

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Publication details

The article was received on 09 Aug 2018, accepted on 01 Nov 2018 and first published on 02 Nov 2018


Article type: Paper
DOI: 10.1039/C8DT03257B
Citation: Dalton Trans., 2018, Accepted Manuscript
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    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: existence of valence-tautomerism

    A. Rajput, A. K. Sharma, S. K. Barman, F. Lloret and R. Mukherjee, Dalton Trans., 2018, Accepted Manuscript , DOI: 10.1039/C8DT03257B

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