Stabilisation of a μ2-Oxido Bridge Dinuclear VO3+ Core by Incorporating Redox-Active Tridentate (NNO)-Donor Ligands

Two diamagnetic dinuclear μ2-oxido bridged oxovanadium(V) complexes, [ $${\text{V}}_{2}^{{\text{V}}}$$ O4(L1)2] (I), (L1H = (E)-1-(pyridin-2-yldiazenyl)naphthalen-2-ol, and [ $${\text{V}}_{2}^{{\text{V}}}$$ O4(L2)2] (II) L2H = (E)-4-methyl-2-(thiazol-2-yldiazenyl)phenol) have been synthesised by using different “redox-active” tridentate NNO-donor ligands and VO(acac)2. Both complexes have been well characterised by FT-IR, UV-vis, NMR, ESI-MS spectroscopy, and cyclic voltammetry data analysis. Geometries of I and II are confirmed by the single-crystal X-ray crystallography. Complexes have two symmetrical oxovanadium cores (in anti-angular configurations) held by two μ-O bridging ions. The bond angle between the central VO3+ unit and the oxido bridges V(1)−O(2)−V(1A) is 101.9(3)° and 100.11(12)°, respectively in I and II. In complex II, vanadium centres are separated from each other by 3.0672 (12) Å, whereas it is slightly higher in complex I (3.162 Å). The unsymmetrical bridge bond lengths, {V(1)–O(2) and V(1)–O(2A)}, are 1.660(6) and 2.371(7) in I, whereas 1.663(3) and 2.302(3) Å in II. Both complexes exhibit significant absorption peaks in the visible region due to (LMCT) ligand-to-metal as well as (ILCT) intra-ligand charge transfer. The cyclic voltammetry study shows the stepwise electron reduction for both I and II. The first reduction at –0.70 V is due to ( $${{{\text{L}}_{{{\text{azo}}}}^{ - }} \mathord{\left/ {\vphantom {{{\text{L}}_{{{\text{azo}}}}^{ - }} {{\text{L}}_{{{\text{azo}}}}^{{\bullet 2 - }}}}} \right. \kern-0em} {{\text{L}}_{{{\text{azo}}}}^{{\bullet 2 - }}}}$$ ) (L = L1 (I) and L = L2 (II)), whereas the second reduction occurs at around –1.30 V and may be due to VV/VIV redox couple. Chemically generated in-situ mono-reduced analogues I– and II– produce single-line isotropic EPR spectra with ‘g’ values of 2.003 and 2.006, respectively. These signals indicate that organic radicals are present in the systems. Nevertheless, no further equivalent hyperfine splitting (HFS) of the magnetic nuclei of organic radicals was detected in any of the reduced species despite varying the concentration, sweep range, and modulation parameters. The Mulliken spin density plots of I– and EPR spectra authenticate that mono-reduced analogues can be defined as azo-anion radical-coupled oxovanadium(V) of types [ $${\text{V}}_{2}^{{\text{V}}}$$ O4 $$({\text{L}}_{1}^{ - })_{2}^{{\bullet - }}$$ ] (I–) and [ $${\text{V}}_{2}^{{\text{V}}}$$ O4 $$({\text{L}}_{2}^{ - })_{2}^{{\bullet - }}$$ ] (II–).