X-ray Crystallography and Magnetic Studies of a Stable Macrocyclic Tetranitroxide. Intramolecular Dimer of Nitroxides in a Constrained Geometry of the Upper Rim of Calix[4]arene

Synthesis, crystallography, and magnetic characterization of a stable macrocyclic tetranitroxide 1, a calix[4]arene, which is functionalized with four tert-butylnitroxides at the upper rim, is described. In solution, 1 has a 4-fold symmetric fixed cone conformation on the NMR time scale and small, but nonnegligible, exchange interactions between the radicals (30 K > |J/k| ≫1.8 mK). In the solid state, dimerization of one diagonal pair of nitroxides leads to a pinched cone conformation for 1 with strong intradimer antiferromagnetic coupling with |J/k| = 200−300 K (singlet−triplet energy gap, ΔEST ≈ 1 kcal/mol).