Reactions of py[13]aneN₄ with the copper(II) complexes of diglycine and triglycine

Ligand exchange reactions between the pyridine-containing macrocycle 3,7,10,16-tetraazabicyclo[10,3,1]-hexadeca-1,(16),12,14,-triene, py[13]aneN₄, 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, k_i, 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-py[13]aneN₄ and Cu-GGG-py[13]aneN₄ 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), py[13]aneN₄ and GG (or GGG) were unsuccessful. Instead, red plate-shaped crystals separated from the blue solution. The crystal structure of the red compound [Cu(py[13]aneN₄)](ClO₄)₂ was determined.