Mixed-oxidation divanadium(IV,V) compound with ligand asymmetry: Electronic and molecular structure in solution and in the solid state

Reaction of [(VOL1)-O-IV(Im)] (H2L1 = S-methyl-3-((2-hydroxyphenyl)methyl)dithiocarbazate) with [(VOL)-O-V(OCH3)] allows isolation of (ImH)[(LOV)-O-1-(mu-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)[(LOV)-O-1-(mu-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) Angstrom, b = 18.912(4) Angstrom, c = 17.163(4) Angstrom, 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) V-51 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 (epsilon, 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 (VVV)-V-IV/(VVV)-V-V couple.