Coexistence of Alternating Ferromagnetic and Antiferromagnetic Intermolecular Interactions in Organic Compounds. Synthesis, Structure, Thermal Stability, and Magnetic Properties of 2,4-Hexadiynylenedioxybis[2-(p-phenylene)-4,4,5,5- tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl] Diradical

Abstract

A crystalline phase of the title diradical has been prepared and characterized by X-ray diffraction, IR, UV-vis, and EPR spectroscopies, and magnetic susceptibility measurements. This phase belongs to the C2/c space group [a = 16.57(2) {angstrom}, b = 16.116(2) {angstrom}, c = 13.10(1) {angstrom}, {beta} = 123.05(4){degrees}, V = 2931(4) {angstrom}{sup 3}, Z = 4, d{sub calc} = 1.30 g cm{sup {minus}3}, T = 21{degrees}C, R{sub u} = 0.092, R{sub w} = 0.116]. The molecular structure of the diradical is characterized by an asymmetrical Z-shaped conformation. The most relevant features observed in the molecular packing are the large interdiacetylene separations - the shortest one is 8.285 {angstrom}-and the alternation in the characteristics of the intermolecular contacts between the radical side groups of the DA; which are jointed by hydrogen bonds between the oxygen atoms of NO groups and aromatic hydrogen atoms. On the basis of accepted structural criteria, this solid-state structure should not support a single-crystal topochemical polymerization and, accordingly, the UV-induced polymerization is not achieved. Thermal treatment, however, turns the crystals from blue to dark brown. Thermal analyses under nitrogen, performed with DSC and TGA techniques, reveal an explosive and complex decomposition, at temperatures higher than 90{degrees}C, with anmore » evolution of gaseous NO (GC-MS) and a destruction of most of the radical centers of diradical molecules, as demonstrated by EPR and magnetic measurements. The study of the temperature dependence of the EPR signals of very diluted solutions of diradical 1 shows that it has a thermally modulated intramolecular exchange interaction due to the flexibility of the spacers joining the two radical centers and, furthermore, that when this diradical adopts a rigid conformation the two radical moieties are magnetically isolated (J{sup intra}/k {approximately} 0 K). 44 refs., 12 figs., 3 tabs.« less