Equilibria and kinetics of copper(II) complex formation of a linear and of 13–15-membered macrocyclic dioxo-tetra-amines

Abstract

Ligating properties of a linear (L⁸) and of 13–15-membered cyclic dioxo-tetra-amines (L⁵–L⁷)(which possess two internal amide bonds) have been investigated with reference to structurally relevant tetra-amines (L¹–L⁴) and the tripeptides glycylglycylglycine (L⁹) and glycylglycylhistidine (L¹⁰). The pH-metric titrations show that ionization of the two hydrogens (most likely at the two amide nitrogens) with concurrent 1 : 1 complex ([MH_–2L]) formation with Cu^II occurs at pH < 7. The complex species [ML]²⁺ and [MH_–1L]⁺ are not found in the equilibria or kinetic studies. The macrocyclic effect and the cation–ring size selectivity observed for tetra-amines L¹–L⁴ are retained for the dioxo-tetra-amines, as illustrated by logarithmic values of the cumulative formation constants K_CuH–2L(=[CuH _–2L][H⁺]²/[Cu^II][L]) of –2.2, 1.0, –4.5, and –5.1 (at I 0.2 mol dm^–3 and 25 °C) for L⁵, L⁶, L⁷ and L⁸, respectively. The stability of the 14-membered L⁶ complex surpasses that of the L¹⁰ complex. The kinetics have been measured for the [CuH_–2L] formation (L = L⁵ and L⁶) in acetate buffers (4.8 < pH < 5.9 at I 0.2 mol dm^–3 and 25 °C). The rate of reaction is expressed as k_L[Cu(O₂CMe)⁺][L]+k_HL[Cu(O₂CMe)⁺][HL⁺] for L⁵(where k_L= 2.0 × 10⁷ and k_HL= 9.5 × 10² dm³ mol^–1s^–1) and as k_HL[Cu(O₂CMe)^–][HL⁺] for L⁶(where k_HL= 3.1 × 10³ dm³ mol^–1s^–1).