Effects of ring size on thermodynamic and kinetic properties of copper(II)-diaminodiamide complexes in aqueous solution

Abstract

In order to study the effects of ring size on the thermodynamic and kinetic properties of copper(II)–diaminodiamide complexes, a new quadridentate ligand, N,N′- bis(carbamoylmethyl)-trimethylenediamine (L¹³¹), was synthesized. The log K values for the protonation constants of the ligand at 25.0 °C and 0.1 mol dm^–3 NaClO₄ are 8.22 and 6.33. The log K value for the formation constant of [Cu(L¹³¹)]²⁺ is 13.68, and the log K values for the deprotonation constants of [Cu(L¹³¹)]²⁺ and [Cu(H_–1L¹³¹)]⁺ are –6.94 and –8.25, respectively. The complexation kinetics of copper(II) with this ligand has been studied under the same conditions using the stopped-flow technique. The resolved formation rate constants (in 0.10 mol dm^–3 NaClO₄ at 25.0 °C) for copper(II) reacting with the unprotonated and monoprotonated ligand, L¹³¹ and [HL¹³¹]⁺, are 1.88 × 10⁹ and 1.79 × 10⁶ dm³ mol^–1 s^–1 respectively. Two possible pathways for the complexation reaction are discussed.