Toward Controlling the Solid State Valence Tautomeric Interconversion Character by Solvation

Crystals of [Co(diox) 2­(4-NO2-py)2] (1) and [Co(diox) 2­(4-CN-py)2] (2) where diox are the o-dioxolene 3,5-di-t-butylsemiquinonate (SQ•–) and/or 3,5-di-t-butylcatecholate (Cat2–) ions, 4-NO2-py is 4-nitro-pyridine, 4-CN-py is 4-cyano-pyridine, are among the few known crystals presenting both thermally induced and photoinduced ls-[M+3(SQ•–)­(Cat2–)] ↔ hs-[M2+(SQ•–)2] valence tautomeric interconversion (VTI). In 2, the thermal-induced VTI is cooperative, characterizing an abrupt conversion, and in 1 it is noncooperative. In this work, crystals of [Co(diox) 2­(4-NO2-py)2]­·benzene (1BZ), [Co(diox) 2­(4-NO2-py)2]­·toluene (1TL), [Co(diox) 2­(4-CN-py)2]­·benzene (2BZ), and [Co(diox) 2­(4-CN-py)2]­·toluene (2TL) have been prepared and analyzed by single crystal X-ray diffraction in order to investigate how solvation modulates thermally induced VTI. Crystallographic data were also successfully used together with the two-state equilibrium equation to estimate ΔH° and ΔS° VTI thermodynamic parameters. The solvate crystals, like the nonsolvated ones, present essentially reversible thermally induced VTI. The 1TL crystal presents the same monoclinic symmetry and the same intermolecular hydrogen-bonded network of 1, and both present a noncooperative thermal-induced VTI. The 1BZ crystal has triclinic symmetry and presents a cooperative VTI with a thermal hysteresis of ∼30 K. In contrast to 2, thermally induced VTI in 2BZ and 2TL is noncooperative despite the fact that 2, 2BZ, and 2TL crystals exhibit the same monoclinic symmetry and the same intermolecular hydrogen-bonded network. In 2BZ and 2TL benzene and toluene molecules as well as the t-butyl groups of the o-dioxolene molecules convert gradually from being dynamically disordered at about 300 K to a static disorder state below 150 K. The layer separation distance of interacting [Co(diox) 2­(4-X-py)2], X = CN and NO2, molecules in all solvate crystals is ∼15 Å, whereas in 2, which presents cooperative VTI, it is ∼12 Å. An order–disorder component might account for the stabilization of the metastable hs-Co2+ state in 2BZ and in 2TL, but no disorder was found in the 1TL crystals. Therefore, the lack of cooperativity in the thermally induced VTI in these crystals seems to be due to the large distance between the layers of interacting molecules. Cooperativity in the VTI of 1BZ crystal is likely to be related with the unique molecular bond scheme network that connects neighboring active [Co(diox) 2­(4-NO2-py)2] molecules