Ligand exchange reactions between the pyridine-containing macrocycle 3,7,10,16-tetraazabicyclo[10,3,1]-hexadeca-1,(16),12,14,-triene, pyaneN4, and the Cu(II) complexes of diglycine (GG) and triglycine (GGG) have been investigated in aqueous solution. The kinetics were studied by stopped-flow techniques at 25°C and I = 0.2 M (KNO3). A single kinetic step was observed at the d-d transition band (max = 580 nm) for the Cu(II) macrocycle. All kinetic measurements were carried out under pseudo first-order conditions in the pH range 5.0-6.2 with the ligand concentration at least 10-fold excess. The specific rate constant, ki, for the two systems studied describes the reaction between the 1 : 1 Cu(II)-polyglycine complex and the monoprotonated form, LH+, of the macrocycle. Cu(II)-GGG complexes exchange the metal more rapidly than the corresponding Cu(II)-GG complexes: where k1 = 9.1 × 105 and 5.8 × 106 M−1 s−1 and k2 = 2.5 × 105 and 1.3 × 106 M−1 s−1 for the GG and GGG systems, respectively. The reaction mechanisms of these reactions are discussed. Formation constants for the ternary systems Cu-GG-pyaneN4 and Cu-GGG-pyaneN4 are evaluated by pH-electrode titrations in aqueous solution at 25°C and I = 0.2 M (KNO3). Attempts to separate the solid ternary compounds from the blue perchlorate solution mixture of Cu(II), pyaneN4 and GG (or GGG) were unsuccessful. Instead, red plate-shaped crystals separated from the blue solution. The crystal structure of the red compound [Cu(pyaneN4)](ClO4)2 was determined.
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Document Type: Research Article
Chemistry Department University of Aden Aden-Crater PO Box 4848 Yemen
Chemistry Department Kuwait University PO Box 5969 Safat 13060 Kuwait
Gulf University for Science and Technology PO Box 29623 Safat 13157 Kuwait
Publication date: May 1, 2004