A NEW CHELATING CYCLOPHANE AND ITS COMPLEXATION WITH NI2- SPECTROSCOPIC PROPERTIES AND ALLOSTERISM VIA RING CONTRACTION(, CU2+, AND ZN2+ )

A condensation reaction between ethylenediaminetetraacetic dianhydride and p-xylenediamine gave a new chelating cyclophane, is(carboxymethyl )-2,5,8,11,20,23,26,29-octaaza[12. 12]paracyclophane, abbreviated as ( 32edtaxan)H-4, which has three types of electron-donor groups, i.e., a mine, carboxylate, and amide groups. The formation of the cyclophane h as been confirmed by a single-crystal X-ray analysis of its Zn2+ compl ex, [Zn-2(32edtaxan)].7.5H(2)O, which crystallized in the monoclinic s pace group P2(1)/c with a = 19.818(1) Angstrom, b = 13.169(1) Angstrom , c = 18.134(1) Angstrom, beta = 104.491(6)degrees, and Z = 4. Each cy clophane molecule coordinates two Zn2+ ions and results in the formati on of a binuclear chelate molecule. The coordination geometry around e ach metal ion is distorted octahedral, the donor atoms being two carbo xylate oxygen atoms, two amine nitrogen atoms, and two amide oxygen at oms. The new cyclophane exhibited a well-defined fluorescence band at 290 nm with 210 nn excitation. The emission intensity was markedly inc reased in the Zn2+ complex, in which the coordination of Zn2+ ions inc reases the rigidity of the cyclophane leading to a high fluorescence q uantum yield. When the cyclophane was coordinated to Cu2+ ions, the mo lar absorptivity of a pi-pi transition band observed at 260 nm was in creased by a factor of about 10. Such a large spectral change was not observed for the Zn2+ and Ni2+ complexes. In the CU2+ complex, the two phenyl rings of the cyclophane are expected to be brought closer, as a result of the coordination of deprotonated amide nitrogens to the ce ntral metal ion. This allosterism via ring contraction is responsible for the novel behavior of the absorption spectrum. The emission band o f the cyclophane was weakened by coordination of copper and nickel as a result of fluorescence quenching caused by a photoinduced electron t ransfer.