Temperature-Induced Solid-State Valence Tautomeric Interconversion in Two Cobalt−Schiff Base Diquinone Complexes

The mixed-ligand complexes [CoIII(tpy)(Cat-N-SQ)]Y and [NiII(tpy)(Cat-N-BQ)]PF6 (tpy = 2,2‘:6‘,2‘ ‘-terpyridine;
Cat-N-BQ, Cat-N-SQ = mononegative and radical dinegative Schiff base diquinone ligand; Y = PF6, BPh4) were
prepared. Structural and spectroscopic data support the different charge distribution of the two compounds. The
temperature-dependent electronic and spectral properties of solutions containing the [CoIII(tpy)(Cat-N-SQ)]+ suggest
that this compound undergoes a thermally driven valence tautomeric interconversion to [CoII(tpy)(Cat-N-BQ)]+ complex,
the metal ion being in high-spin configuration. The comparison of the electrochemical properties of the cobalt and
nickel derivatives supports the observed behavior. The same interconversion process was found to occur also in
the solid state with a significant higher Tc value than in solution. It was found that the previously reported
[CoIII(Cat-N-BQ)(Cat-N-SQ)] shows a similar behavior. The large difference between the interconversion Tc in the
solid state and in solution is suggested to come from the entropy changes associated with the modifications of
vibronic interactions.