Stabilization of oxidovanadium(iv) by organic radicals

Abstract

o-Imino-p-R′-benzosemiquinone anion radical (L^R′_IS˙⁻) complexes of oxidovanadium(IV) of type [(L₁^R−)(VO²⁺)(L^R′_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 L₁^R− ligands (L₁^RH = 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–O_phenolato (cis to the VO) lengths spanning 1.905(3)–1.9355(15) Å in 1–4 are consistent with the coordination to the [VO]²⁺ state. The V–O_IS (trans to the VO) lengths, 2.1505(17)–2.1869(15) Å, in 1–4 are longer due to the trans influence of the VO bond. The V–N_IS 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 L^R′_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 (L^R′_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) M_s = 0 (L^R′_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 L^R′_IS˙⁻ generating [(L₁^R−)(VO²⁺)(L^R′_IQ)]⁺ complexes (L^R′_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 [(L₁^R−)(VO²⁺)(L^R′_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 [(L₁^R−)(VO³⁺)(L^R′_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⁺.