Family of Mixed-Valence Oxovanadium(IV/V) Dinuclear Entities Incorporating N₄O₃-Coordinating Heptadentate Ligands: Synthesis, Structure, and EPR Spectra

Dinuclear (VIVVV) oxophenoxovanadates of general formula [V2O3L] have been synthesized in excellent yields by reacting bis(acetylacetonato)oxovanadium(IV) with H3L in a 2:1 ratio in acetone under an N2 atmosphere. Here L3- is the deprotonated form of 2,6-bis[{{(2-hydroxybenzyl)(N‘,N‘-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol (H3L1), 2,6-bis[{{(5-methyl-2-hydroxybenzyl)(N‘,N‘-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol (H3L2), 2,6-bis[{{(5-tert-butyl-2-hydroxybenzyl)(N‘,N‘-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol (H3L3), 2,6-bis[{{(5-chloro-2-hydroxybenzyl)(N‘,N‘-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol(H3L4), 2,6-bis[{{(5-bromo-2-hydroxybenzyl)(N‘N‘-(dimethylamino)ethyl)}amino}methyl]-4-methylphenol (H3L5), or 2,6-bis[{{(5-methoxy-2-hydroxybenzyl)(N‘N‘-(dimethylamino)ethyl)}methyl]-4-methylphenol (H3L6). In [V2O3L1], both the metal atoms have distorted octahedral geometry. The relative disposition of two terminal VO groups in the complex is essentially cis. The OV···VO torsion angle is 24.6(2)°. The V−Ooxo−V and V−Ophenoxo−V angles are 117.5(4) and 93.4(3)°, respectively. The V···V bond distance is 3.173(5) Å. X-ray crystallography, IR, UV−vis, and 1H and 51V NMR measurements show that the mixed-valence complexes contain two indistinguishable vanadium atoms (type III). The thermal ellipsoids of O2, O4, C10, C14, and C15 also suggests a type III complex in the solid state. EPR spectra of solid complexes at 77 K display a single line indicating the localization of the odd electron (3dxy1). Valence localization at 77 K is also consistent with the 51V hyperfine structure of the axial EPR spectra (3dxy1 ground state) of the complexes in frozen (77 K) dichloromethane solution: S = 1/2, g∥ ∼ 1.94, g⊥∼ 1.98, A∥ ∼ 166 × 10-4 cm-1, and A⊥ ∼ 68 × 10-4 cm-1. In contrast isotropic room-temperature solution spectra of the family have 15 hyperfine lines (giso ∼ 1.974 and Aiso ∼ 50 × 10-4 cm-1) revealing that the unpaired electron is delocalized between the metal centers. Crystal data for the [V2O3L1]·CH2Cl2 complex are as follows: chemical formula, C32H43O6N4Cl2V2; crystal system, monoclinic; space group, C2/c; a = 18.461(4), b = 17.230(3), c = 13.700(3) Å; β = 117.88(3)°; Z = 8.