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 (original) (raw)

1994, Chemistry of Materials

A crystalline phase of the 2,4-hexadiynylenedioxybis[2-(p-phenylene)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-l-oxyl] diradical, 1, has been prepared and characterized by X-ray diffraction, IR, W-vis, and EPR spectroscopies, and magnetic susceptibilit measurements. This phase belongs to the C21c space group [a = 16.57(2) A, b = 16.116(2) i, c = 13.10(1) A, / 3 = 123.05(4)", V = 2931(4) A3, 2 = 4, dcalc = 1.30 g ~m-~, T = 21 "C, R, = 0.092, R, = 0.1161. 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 A-and the alternation in the characteristics of the intermolecular contacts between the radical side groups of the DA; which are joined 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 W-induced polymerization was 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 "C, with an 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 (Jintra/k-0 K). EPR studies on polycrystalline samples of diradical 1 provide evidences for significant intermolecular exchange couplings between radical side groups of neighbor diradicals. The magnetic susceptibility of 1 suggests the simultaneous presence of dominant antiferromagnetic interactions together with very weak ferromagnetic ones; in agreement with the observed alternation of structural characteristics and the solid state EPR spectrum. This magnetic behavior is quantitatively described by a linear Heisenberg chain of S = l/2 spins with alternating F-AF intermolecular interactions of Jminter/k =-3.9 K and J F~~~~~/~-+1.2 K. The presence of ferromagnetic interactions in 1 is attributed t o the substituted diacetylenic unit, which, by structural reasons, obstructs the natural tendency of radical centers to interact only in an antiferromagnetic fashion. Finally, a reinterpretation of the magnetic data of other related organic compounds showing alternating F-AF interactions is presented, and the resulting exchange parameters correlated with the structural features of the solids.