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

Abstract

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 Å, from 3.25 Å at ambient pressure to 2.88 Å 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.