Putting the Squeeze on Valence Tautomerism in Cobalt-Dioxolene Complexes

Two isostructural cobalt complexes, [Co(Mentpa)(dbdiox)]+ (Mentpa = tris(2-pyridylmethyl)amine where n = 2 or 3, corresponding to successive methylation of the 6-position of the pyridine rings, dbdiox = 3,5-di-tert-butyl-1,2-dioxolene), were studied using single-crystal X-ray diffraction, and supported by density functional theory (DFT) calculations. The less sterically hindered dimethylated complex exhibits two-step thermally-induced interconversion between the high-spin CoII-seminquinonate and low-spin CoIII-catecholate forms (valence tautometrism) at 155 and 95 K due to the presence of two symmetry-independent complexes. In contrast, the more sterically hindered trimethylated complex does not display thermal valence tautomerism. Both complexes exhibit unique behaviour under high pressure. The dimethylated species undergoes gradual, one-step valence tautomerism in both symmetry-independent complexes concurrently between 0.43 GPa and 1.30 GPa. In the trimethylated species, pressure is sufficient to overcome steric hindrance, leading to one-step valence tautomerism between 2.60 GPa and 3.10 GPa; the first record of pressure-induced valence tautomerism in a thermally inactive complex.