Heteroradical Salts Based on Paramagnetic Derivatives of Difurazanopyrazine and N-Alkylpyridinium Nitronyl Nitroxides

The effective approach to the synthesis of heteroradical organic salts based on the paramagnetic difurazanopyrazine derivative and pyridinium-substituted nitronyl nitroxides has been proposed. It was demonstrated that the use of [NN^p/mR]I iodides as a cation-radical source in the reaction with NaL(H₂O)₃ (where L is the anion-radical of bis(1,2,5-oxadiazolo)[3,4-b:3′,4′-e]pyrazine) results in reducing L and the formation of solid solutions [NN^pMe](L)_x(HL)_1–x·H₂O containing both the paramagnetic anion L and its reduced form anion HL. It was found that the use of [NN^p/mR]NO₃ nitrates allows to avoid the reduction of L, and heteroradical salts [NN^p/mR]L can be easily obtained. A single crystal XRD study showed that the solid phases of [NN^p/mR]L salts form crystals formed by quartets of anions and cations {NN^p/mR···L···L···NN^p/mR}. Within the quartets, the distances between the anion-radical planes of L are ∼3.3 Å, which ensures the presence of antiferromagnetic exchange interactions between them and determines the magnetic properties, in general. It was found that in the [NN^mMe]L·H₂O structure, alternating cationic and anionic radicals are ferromagnetically coupled and form magnetic chains, which are linked into layers by O_H2O···N_L hydrogen bonds. The [NN^p/mR]L heteroradical salts represent a new type of paramagnet based on organic fragments with fixed charge and spin states.