The Synthesis, Crystal Structure of 5,17-Di-(N-p-tert-butyl-N-hydro -xyamino)-11,23-di-bromo-25,26,27,28-tetramethoxycalix [4]arene and ESR Studies of Its Oxide

5,17-Di-(N-p-tert-butyl-N-hydroxyamino)-11,23-di-bromo-25,26,27,28- tetramethoxycalix[4]arene (II), which is a precursor of nitroxide radical, is synthesized by selective halogen-lithium exchange reaction of 5, 11, 17, 23-tetrabromo-tetramethoxycalix[4]arene (I) with n-BuLi and 2-methyl-2-nitrosopropane (MNP). Compound II is characterized by IR, ¹H NMR, mass spectrum and elementary analysis. The crystal structure of compound II has been determined. Crystals are monoclinic, space group C2/c (No. 15) with a = 3.225 5 (7) nm, b = 1.439 4 (3) nm, c = 2.175 3 (5) nm, β = 110.210 (9)°, V = 9.478 (3) nm³ and Z = 8, with final residuals R = 0.073 8 and R_w = 0.160 3. According to X-Ray analysis compound II is presented in a flattened partial cone conformation, which is less common for calix[4]arenes in solid. The two opposite phenyl rings bearing hydroxyamino groups are in an syn orientation and each molecule exhibits two intramolecular hydrogen bonds, N⋯H-O, which can also be deduced from IR and NMR spectra. The bond length of N⋯H is about 0.2nm and bond angle of N⋯H-O is about 150°. Compound II in solution is easily oxidized to a stable nitroxide biradical and electron spin resonance (ESR) observation reveals that there exists magnetic dipole-dipole interaction and weak spin exchange interaction between the two unpaired electrons in the molecule.