Density functional theory (DFT) computational modeling [B3LYP*/6-311++G(d,p)] of a series of potentially valence tautomeric (VT) dinuclear 2:1 adducts of CoII bis-acetylacetonate, bis-trifluoroacetylacetonate and bis-hexafluoroacetylacetonate with redox-active tetradentate di-o-iminoquinone ligands has been performed. The significant energy preference of the low-spin electromeric forms of the complexes on the basis of CoII bis-acetylacetonate points to a low probability of VT in these compounds. Electron-withdrawing CF3-groups in the diketonate moiety of the mixed-ligand complexes promote narrowing of the energy gaps between the electronic states, which allows one-step VT rearrangements to be expected in the adducts of CoII bis-trifluoromethylacetylacetonate. The most suitable energy parameters for the occurrence of two-step VT conversion (stability of the adduct with respect to dissociation into the components, small relative energies of electromeric forms and thermally achievable energy barriers to intramolecular electron transfers estimated as minimum energy crossing points) are found for the adduct of di-o-iminoquinone with CoII bis-trifluoroacetylacetonates comprising a diphenylene linker.
Keywords: Cobalt complexes; DFT calculations; Di-o-iminoquinone ligands; Spin-forbidden reactions; Valence tautomerism.