Mixed-Oxidation Divanadium(IV,V) Compound with Ligand Asymmetry: Electronic and Molecular Structure in Solution and in the Solid State

Reaction of [VIVOL1(Im)] (H2L1 = S-methyl-3-((2-hydroxyphenyl)methyl)dithiocarbazate) with [VVOL(OCH3)]
allows isolation of (ImH)[L1OV-(μ-O)-VOL] complexes 2 (H2L = H2L2 = S-methyl-3-((5-bromo-2-hydroxyphenyl)methyl)dithiocarbazate) and 3 (H2L = H2L1), one of which (2) has ligand asymmetry not previously known
in this type of complex. In the solid state, (ImH)[L1OV-(μ-O)-VOL2] (2) provides an example of a divanadium(IV,V) compound with a syn angular [V2O3]3+ core structure that exhibits crystallographically imposed mirror
symmetry due to static disorder. Crystals of 2 are orthorhombic, space group Pnma, with a = 10.740(2) Å, b =
18.912(4) Å, c = 17.163(4) Å, and Z = 4. In toluene at room temperature, both 2 and 3 have 8-line EPR spectra,
characteristic of trapped-valence structure. When acetonitrile is added to these solutions, the spectra reveal 15-line features with asymmetric distortions that smooth out with the lowering of temperature. This probably has its
origin in a solvent-dependent equilibrium involving two magnetically inequivalent structural forms of the
divanadium(IV,V) compound, with syn angular and anti linear structures of the [V2O3]3+ core. Variable temperatures
(298−220 K) 51V NMR spectroscopic studies in solution also support this view. In acetonitrile, both 2 and 3
exhibit an intervalence transfer band in the near-IR region at ca. 970 nm (ε, 1600 and 1480 M-1 cm-1 for 2 and
3, respectively) and they undergo one-electron reversible oxidation at ca. 0.40 V (vs SCE) due to the VIVVV/VVVV couple.