Structural Diversity and Bistability in Salts of Organic Radical-Ions Driven by Crystal-Lattice Solvent Loss

The reactivity of the radical-cation 1Me+• with cyanocarbon radical anions TCNE–• and TCNQ–• was studied in nitrile solvents (RCN; R = Me, Et, n-Pr, i-Pr, Ph). The reaction of 1Me[OTf] with [K(18c6)][TCNE] in MeCN yielded two polymorphs of 1Me[TCNE], featuring hetero-π-pairs. In contrast, the reaction of 1Me[OTf] with [K(18c6)][TCNQ] in RCN (R = n-Pr, i-Pr, Ph) produced the mixed salt [1Me]4[OTf]2[TCNQ]2·TCNQ containing neutral and charged TCNQ units due to solvent-induced electron transfer. Using 1Me[SbF6] with [K(18c6)][TCNQ] yielded the solvated polymorphs 1Me[TCNQ]·0.25RCN (R = n-Pr, i-Pr) and 1Me[TCNQ]·0.5RCN (R = n-Pr, Ph) as well as desolvated 1Me[TCNQ] as a minor side product characterized by X-ray diffraction methods. Thermogravimetric analyses revealed facile solvent loss with powder X-ray diffraction experiments showing single-crystal-to-single-crystal transformations of some of the solvates into paramagnetic high temperature 1Me[TCNQ]. Differential scanning calorimetry and infrared spectroscopy indicated a reversible first-order transition between high temperature 1Me[TCNQ] (discrete TCNQ–• radicals) and diamagnetic low temperature 1Me[TCNQ] (TCNQ σ-dimers) states near room temperature.