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Pancake-bonding of semiquinone radicals under variable temperature and pressure conditions

The effects of temperature (100–370 K) and pressure (0–6 GPa) on the nonlocalized two-electron multicentric covalent bonds (‘pancake bonding’) in closely bound radical dimers were studied using single-crystal X-ray diffraction on a 4-cyano-N-methylpyridinium salt of 5, 6-dichloro-2, 3-dicyanosemiquinone radical anion (DDQ) as the sample compound. On cooling, the anisotropic structural compression was accompanied by continuous changes in molecular stacking ; the discontinuities in the changes in volume and b and c cell parameters suggest that a phase transition occurs between 210 and 240 K. At a pressure of 2.55 GPa, distances between radical dimers shortened to 2.9 A °, which corresponds to distances observed in extended π-bonded polymers. Increasing pressure further to 6 GPa reduced the interplanar separation of the radicals to 2.75 A °. This may indicate that the covalent component of the interaction significantly increased, in accordance with the results of DFT calculations reported elsewhere [Molčanov et al. (2019), Cryst. Growth Des. 19, 391–402].

multicentric two-electron bonding ; pancake bonding ; semiquinone radicals ; high pressure ; low temperature

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