Syntheses, structures, and properties of quinone-bridged calix[6]arenes

The quinone-bridged calix[6]arenes 1 and 2, carrying a 1,4-benzoquinone moiety in the cavity of a calixarene macrocycle, were synthesized from the corresponding p-methoxyphenol derivatives. X-ray crystallography revealed the Structures of 1 and 2, in which the quinone moiety is located inside the cone-shaped calix[6]arene framework and surrounded by the lower-rim substituents. Their H-1 NMR spectra showed upfield shifts of the protons on the bridging unit in comparison with reference compounds without the calixarene framework, in accordance with their crystal structures. The cyclic voltammograms of 1 and 2 showed two waves, the second ones being considerably broadened. Their reduction potentials were found to be negatively shifted in comparison with those of the reference compounds. These shifts were explained in terms of the nonbonded interaction between the ether oxygen atoms of the calixarene moiety and the quinone moiety, as well as the sterically restricted access of the counter cation to the semiquinone anion radical. Reactions of 1 with hydroxylamine and phenylhydrazine afforded the corresponding hydroquinone along with the quinone monoimine derivatives, indicating that electron transfer from the amines proceeds quickly, although the negatively shifted reduction potentials of 1 are apparently unfavorable for the electron transfer process.