A new polymorph of 4,4′-(butadiyne-1,4-diyl)bis(2,2,6,6-tetramethyl-4-hydroxypiperidin-1-oxyl), 1, has been prepared. Both phases have been characterized by X-ray diffraction, infrared and Raman spec¬troscopies, and magnetic susceptibility. The α-phase belongs to the Pccn space group [a = 19.049 (4) Å, b = 16.107 (4) Å, and c = 14.113 (3) Å,V = 4330 (3) Å3, Z = 8, T = −115 °C, Ru = 7.0%, Rw = 8.0%] and the β-phase belongs to the Pca21 space group [a = 14.265 (1) Å, b = 8.079 (3) Å, and c = 18.865 (2) Å, V = 2174.1 Å3, Z = 4, T = −100 °C, Ru = 4.8%, Rw = 5.0%]. The molecular structures of both phases are essentially equivalent with the average C≡C, CC–CC, and NO distances of 1.200, 1.385, and 1.288 Å, respectively. The hydrogen bonding OH…ON (or O…O) interactions [i.e., 1.70 (2.834) and 1.72 Å (2.757 Å) for the α-phase and 1.85 (2.804) and 1.84 Å (2.788 Å) for the β-phase] are the only significant intermolecular interactions. Intense vNO and vC≡C (Raman) vibrations occur at 1341 and 2236 cm−1, respectively, for both phases. A pleated sheet and helical, hydrogen-bonded solid-state motifs are observed for the α- and β-phases, respectively. On the basis of accepted structural criteria, both solid-state structures should not support single-crystal topochemical polymerization and UV, electron beam, and γ-ray induced polymerization has not been achieved for either phase. Thermal treatment, however, turns the crystals black. Thermogravimetric analysis under nitrogen reveals an explosive decomposition at ~140 °C for both phases. Thermal de¬composition monitored by FTIR reveals the loss of the nitroxyl group, destruction of the ring system, and formation of small molecules and a residue more complex than that expected for a simple polymerization. The SQUID magnetic susceptibility of both phases obey the Curie–Weiss expression with θ = ~−1.8 K. The effective moment is ~2.45 μB per molecule, which is consistent with two independent S = 1/2 spins per molecule. Analysis of the exchange narrowing of the ESR line of α- and β-phases led to an intermonomer exchange of J ~ 0.165 K (0.115 cm−1). Application of the Weiss molecular field model for the measured |θ| of ~ 1.8 K leads to an estimate of ~ 0.155 K (0.108 cm−1) for the intermonomer exchange J, in very good agreement with the estimate derived from ESR. Upon thermal treatment (~20 h at 80–100 °C) of either phase, a resultant black material forms that exhibit a reduced susceptibility corresponding to <1 μB per monomer and nearly zero Curie–Weiss temperature, θ. EPR studies of the thermal degradation products show a complex pattern characteristic of isolated S = 1/2 spins interacting with the S = 1 nitrogen nuclear spin and broadened by the dipole interactions with the nearby proton spins. Study of the temperature-dependent EPR of dilute solutions of 1 confirms this analysis and suggests that the majority of the residual spins remaining after heat treatment are monoradicals. The temperature dependence of the intensity of triplet EPR observed for the heat-treated samples suggests the presence of a triplet-exchange interaction of Jtriplet ~ 10 K. The static and dynamic magnetic data of the thermal degradation products do not provide any evidence for significant 3-D ferromagnetism or even magnetic coupling and indicate the presence of only very weak intradimer ferromagnetic (triplet) coupling. © 1990, American Chemical Society. All rights reserved.
