Stacks of Equidistant 5,6-Dichloro-2,3-dicyanosemiquinone (DDQ) Radicals under Variable-Temperature and High-Pressure Conditions

The effects of temperature (80-400 K) and pressure (up to 4.67 GPa) variations on stacks of equidistant semiquinone radicals were studied by single-crystal X-ray diffraction using a salt of 2-amino-5-methyl-N-methylpyridinium (MMA) and 5,6-dichloro-2,3-dicyanosemiquinone radical anion (DDQ). A temperature-driven phase transformation occurs in the temperature range 150-240 K. The nature of stacking changes upon cooling: equidistant radicals in the room-temperature phase are re-arranged into two-electron/multicenter (2 e/mc) bonded dimers of radicals at low temperature. The phase transition proceeds via a pair-exchange dynamic mechanism. By application of pressure, the interplanar separation in the stacks is reduced by 0.37 angstrom, from 3.25 angstrom at ambient pressure to 2.88 angstrom at 4.67(3) GPa. This may indicate that the covalent component of the interaction considerably increases, consistent with previously performed density functional theory (DFT) calculations.