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Issue 13, 2013
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Stabilization of oxidovanadium(IV) by organic radicals

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o-Imino-p-R′-benzosemiquinone anion radical (LR′IS˙) complexes of oxidovanadium(IV) of type [(L1R−)(VO2+)(LR′IS˙)] (R = H, R′ = H, 1; R = H, R′ = –CMe3, 2; R = –CMe3, R′ = H, 3 and R = –CMe3, R′ = –CMe3, 4) incorporating the redox-innocent tridentate NNO-donor L1R− ligands (L1RH = 2,4-di-R-6-{(2-(pyridin-2-yl)hydrazono)methyl}phenol) were isolated and substantiated by elemental analyses, IR, mass, NMR and UV-vis spectra including the single crystal X-ray structure determinations. The V–Ophenolato (cis to the V[double bond, length as m-dash]O) lengths spanning 1.905(3)–1.9355(15) Å in 1–4 are consistent with the coordination to the [VO]2+ state. The V–OIS (trans to the V[double bond, length as m-dash]O) lengths, 2.1505(17)–2.1869(15) Å, in 1–4 are longer due to the trans influence of the V[double bond, length as m-dash]O bond. The V–NIS lengths, 1.906(3)–1.924(2) Å, in 1–4 are comparatively shorter due to the higher affinity of the paramagnetic [VO]2+ ion towards the LR′IS˙ anion radicals. Density functional theory (DFT) calculations using B3LYP, B3PW91 and PBE1PBE functionals on 1 and 2 authenticated that the closed shell singlet (CSS) solutions (dianionic o-amido-p-R′-phenolates (LR′AP2−) coordinated to VO3+, Type I) of 1–4 are unstable with respect to the open shell singlet (OSS) perturbations. Broken symmetry, BS (1,1) Ms = 0 (LR′IS˙ coordinated to the VO2+ ion, Type III) solutions of 1–4 are stable and reproduce the experimental bond parameters. Frozen glasses EPR spectra of [1–4]+ ions (e.g. g|| = 1.948, g = 1.978, A|| = 184 (22 G), A = 62(15 G) for [2]+) and unrestricted DFT calculations on [1]+, [2]+, [1] and [2] ions using doublet spin state elucidated that the reversible anodic waves at [0.15–0.31] V of 1–4 complexes are due to the oxidation of LR′IS˙ generating [(L1R−)(VO2+)(LR′IQ)]+ complexes (LR′IQ = o-imino-p-R′-benzoquinone) coordinated to the [VO]2+ ion (Type V) while the irreversible cathodic waves at −[1.08–1.49] V are due to the formation of unstable [(L1R−)(VO2+)(LR′AP2−)] complexes (Type II). The second anodic waves at [0.76–0.89] V are assigned to a [VO]3+–[VO]2+ couple affording diamagnetic [(L1R−)(VO3+)(LR′IQ)]2+, [1–4]2+ complexes (Type VI) which are identified by UV-vis spectra, DFT and time dependent (TD) DFT calculations. Spectro-electrochemical measurements and TD DFT calculations on 1 and 2 disclosed that lower energy electronic absorption bands of 1–4 are due to the LMCT and CSS-OSS perturbation which disappear in [1–4]+ ions. [1–4]+ absorb at 600–650 nm due to d–d transitions and MLCT which are absent in VO3+ complexes, [1–4]2+.

Graphical abstract: Stabilization of oxidovanadium(iv) by organic radicals

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The article was received on 12 Nov 2012, accepted on 07 Jan 2013 and first published on 07 Jan 2013

Article type: Paper
DOI: 10.1039/C2DT32693K
Citation: Dalton Trans., 2013,42, 4586-4601
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    Stabilization of oxidovanadium(IV) by organic radicals

    S. Kundu, S. Maity, A. N. Maity, S. Ke and P. Ghosh, Dalton Trans., 2013, 42, 4586
    DOI: 10.1039/C2DT32693K

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