Computational design of valence tautomeric adducts of Co(II) diketonates with redox-active o-benzoquinone ligands.

A new concept for the structural design of valence tautomeric (VT) metal complexes involving the formation of stable adducts of a tetracoordinate transition metal complex with a suitable bidentate redox-active ligand has been computationally studied using the DFT B3LYP*/6-311++G(d,p) method. The calculations, performed on a series of adducts of Co(II) diketonates with o-benzoquinone and its mono- and diimines, showed that the mixed-ligand complexes of bis-(hexafluoroacetylacetonate) Co(II) with o-benzoquinone, o-benzoquinone imine and o-benzoquinone diimine satisfy the whole set of necessary conditions to be met by compounds exhibiting VT behaviour (stability of the adduct with respect to dissociation into the components, energy preference of the low-spin electronic state and thermally achievable energy barrier to intramolecular electron transfer determining the intrinsic mechanism of VT rearrangements). These compounds can be regarded as a feasible synthetic target of a broad series of mixed-ligand VT complexes.