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, ΔE_ST ≈ 1 kcal/mol).