We report the structural characterization of vanadyl acetylacetonate in imidazolium room temperature ionic liquids—bbimNTf2, bmimNTf2, C3OmimNTf2, bm2imNTf2, bmimPF6, bmimOTf, bmimBF4, bmimMeCO2, bmimMeSO4, bmimMe2PO4 and bmimN(CN)2—and organic solvents. The complex was characterized by visible electronic (Vis) and EPR spectroscopies. VO(acac)2 shows solvatochromism in the selected ionic liquids and behaves as in organic solvents, evidencing coordination of the ionic liquid anion in the solvents with higher coordinating ability. The Lewis basicity order obtained for the IL anions was: PF6− < NTf2− < OTf− ≈ MeCO2− < MeSO4− < BF4− ≈ N(CN)2− < Me2PO4−. The solvent effect on the spectroscopic data was tentatively examined using linear solvation energy relationships based on the Kamlet–Taft solvent scale (α, β and π*), however no suitable correlation was found with all data. The EPR characterization showed the presence of two isomers in bmimOTf, bmimMeCO2 and bmimMe2PO4, suggesting coordination of the ionic liquid anions in both equatorial and axial positions. The full geometry optimization of cis-/trans-VO(acac)2(OTf)− and cis-/trans-VO(acac)2(OTf)(mmim) structures was done at the B3P86/6-31G* level of theory. The calculations confirm that the anion OTf− is able to coordinate to VO(acac)2 with the trans isomer being more stable than the cis by 4.8 kcal mol−1.
