Valence tautomerism interconversion triggers transition to stable charge distribution in solid polymeric cobalt–polyoxolene complexes

Abstract

The reaction of 3,3,3′,3′-tetramethyl-5,5′,6,6′-tetrahydroxy-1,1′spiro-bis(indane), L, with cobalt salts in the presence of diazine ligands affords polymeric derivatives of general formula [CoL(diazine)]_∞·nH₂O, L being coordinated as a mixed-valence SQ-Cat species. The bipiridyl and o-phenantroline derivatives have been characterised by means of magnetic, EPR and calorimetric measurements. The characterisation showed that both systems are obtained in a metastable charge distribution, presumably trapped due to the class I character of the mixed valence form of the ligand. The occurrence of entropy-driven valence tautomeric interconversion induces the transition to the most stable charge distribution. The photomagnetic characterisation evidenced that valence tautomeric process can be photoinduced and that the lifetime of the photoinduced metastable state, obtained in low yield, is of ca. 2 × 10⁵ s at 9 K.