A New Chelating Cyclophane and Its Complexation with Ni2+, Cu2+, and Zn2+: Spectroscopic Properties and Allosterism via Ring Contraction

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Abstract
A condensation reaction between ethylenediaminetetraacetic dianhydride and p-xylenediamine gave a new chelating cyclophane, 3,10,21,28-tetraoxo-5,8,23,26-tetrakis(carboxymethyl)-2,5,8,11,20,23,26,29-octaaza[12.12]paracyclophane, abbreviated as (32edtaxan)H4, which has three types of electron-donor groups, i.e., amine, carboxylate, and amide groups. The formation of the cyclophane has been confirmed by a single-crystal X-ray analysis of its Zn2+ complex, [Zn2(32edtaxan)]·7.5H2O, which crystallized in the monoclinic space group P21/c with a = 19.818(1) Å, b = 13.169(1) Å, c = 18.134(1) Å, β = 104.491(6)°, and Z = 4. Each cyclophane molecule coordinates two Zn2+ ions and results in the formation of a binuclear chelate molecule. The coordination geometry around each metal ion is distorted octahedral, the donor atoms being two carboxylate oxygen atoms, two amine nitrogen atoms, and two amide oxygen atoms. The new cyclophane exhibited a well-defined fluorescence band at 290 nm with 210 nm excitation. The emission intensity was markedly increased in the Zn2+ complex, in which the coordination of Zn2+ ions increases the rigidity of the cyclophane leading to a high fluorescence quantum yield. When the cyclophane was coordinated to Cu2+ ions, the molar absorptivity of a π−π* transition band observed at 260 nm was increased 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 central metal ion. This allosterism via ring contraction is responsible for the novel behavior of the absorption spectrum. The emission band of the cyclophane was weakened by coordination of copper and nickel as a result of fluorescence quenching caused by a photo-induced electron transfer.

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