Two macrocyclic pentaaza compounds containing pyridine evaluated as novel chelating agents in copper(II) and nickel(II) overload


Two pentaaza macrocycles containing pyridine in the backbone, namely 3,6,9,12,18-pentaazabicyclo[12.3.1] octadeca-1(18),14,16-triene ([15]pyN(5)), and 3,6,10,13,19-pentaazabicyclo[13.3.1]nonadeca-1(19),15,17-triene ([16]pyN(5)), were synthesized in good yields. The acid-base behaviour of these compounds was studied by potentiometry at 298.2 K in aqueous solution and ionic strength 0.10 M in KNO3. The protonation sequence of [15]pyN(5) was investigated by H-1 NMR titration that also allowed the determination of protonation constants in D2O. Binding studies of the two ligands with Ca2+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb2+ metal ions were performed under the same experimental conditions. The results showed that all the complexes formed with the 15-membered ligand, particularly those of Cu2+ and especially Ni2+, are thermodynamically more stable than with the larger macrocycle. Cyclic voltammetric data showed that the copper(II) complexes of the two macrocycles exhibited analogous behaviour, with a single quasi-reversible one-electron transfer reduction process assigned to the Cu(II)/Cu(I) couple. The UV-visible-near IR spectroscopic and magnetic moment data of the nickel(II) complexes in solution indicated a tetragonal distorted coordination geometry for the metal centre. X-band EPR spectra of the copper(II) complexes are consistent with distorted square pyramidal geometries. The crystal structure of [Cu([15]pyN(5))](2+) determined by X-ray diffraction showed the copper(II) centre coordinated to all five macrocyclic nitrogen donors in a distorted square pyramidal environment. (C) 2010 Elsevier Inc. All rights reserved.




Biochemistry & Molecular Biology; Chemistry


Fernandes, AS; Cabral, MF; Costa, J; Castro, M; Delgado, R; Drew, MGB; Felix, V

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The authors acknowledge the financial support from Fundacao para a Ciencia e a Tecnologia (FCT), with co-participation of the European Community fund FEDER (project no. PTDC/QUI/67175/2006). The authors wish to thank the Elemental Analysis Service Unit of ITQB-UNL for providing analytical data. A.S.F. acknowledges Fundacao para a Ciencia e a Tecnologia, Portugal, for the financial support (PhD grant SFRH/BD/28773/2006). We also thank the EPSRC (U.K.) and the University of Reading for funds for the diffractometer.

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