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RESEARCH PRODUCT

Equilibrium and Kinetic Properties of Cu II Cyclophane Complexes: The Effect of Changes in the Macrocyclic Cavity Caused by Changes in the Substitution at the Aromatic Ring

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The o-B232, m-B232 and p-B232 cyclophanes result from attaching the terminal amine groups of 1,4,8,11-tetraazaundecane (232) to the benzylic carbons of the corresponding o-, m- or p-xylanes. The cavity size of these cyclophanes changes moderately as a consequence of the substitution at the aromatic ring. The effects caused by these changes on the equilibrium constants for protonation and CuII complex formation of the cyclophanes are analyzed and compared with those of the noncyclic 232 polyamine. All three cyclophanes form mononuclear complexes, but only o-B232 is able to coordinate to CuII through the four amine groups simultaneously, whereas m-B232 and p-B232 can only use three nitrogen donors. The larger cavity and higher flexibility of the latter two cyclophanes allow them to form stable dinuclear CuII complexes. The kinetics of the acid-promoted dissociation of CuII from its mono- and dinuclear complexes has also been studied and the results indicate the existence of large differences in the kinetic properties for the complexes with these three closely related ligands. Whereas the mononuclear m-B232 and p-B232 complexes decompose with very similar kinetic behaviour to that of the Cu(232)2+ complex, the lability of the Cu–N bonds is largely reduced in the Cu(o-B232)2+ complex, and this leads to slower decomposition and even to a different rate law for this complex. The dinuclear complexes are not formed with o-B232, and very different decomposition kinetics is observed for the complexes formed by m-B232 and p-B232. In the case of m-B232, the two metal ions are situated very close to each other, which causes a rapid release of one of them upon reaction with acid. In contrast, the larger size of p-B232 allows it to accommodate two metal ions without important electrostatic or steric interactions so that they are released with statistically controlled kinetics.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)