Abstract:

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 ^lH 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 0.2 nm 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.

Key words: calix[4]arene, synthesis, crystal structure, electron spin resonance