S = 1 Tetraazacyclophane Diradical Dication with Robust Stability: A Case of Low-Temperature One-Dimensional Antiferromagnetic Chain

One-dimensional (1D) spin-1 (S = 1) chain of organic radicals with low local magnetic anisotropy may provide a better understanding of the low-dimensional magnetism. We report solid-state studies, including single crystal X-ray crystallography, of air-stable tetraazacyclophane diradical dication salt 1 ^2·2+ ·2[Al(OC(CF ₃ ) ₂ CH ₃ ) ₄ ] ^- with a triplet ground state (ΔE _ST ≈ 0.5 kcal mol ^-1 ). The magnetic behavior for 1 ^2·2+ at low temperature is best modeled by 1D spin S = 1 Heisenberg chain with intrachain antiferromagnetic coupling of J′/k = -5.4 K, which is associated with the interaryl C···C contacts, including π-π interactions. Zero-field splitting value, |D/hc| ≈ 5.6 × 10 ^-3 cm ^-1 , for 1 ^2·2+ is rather small; thus, the 1D chains are characterized by the high degree of isotropicity |D/2J′| ≈ 7.5 × 10 ^-4 . The diradical dication salt possesses extraordinary stability with onset of decomposition at temperature of about 180 °C (∼450 K), based on thermogravimetric analysis and EPR spectroscopy.