Kinetics and mechanism of the complexation reaction of copper(II) with diazadioximes

The protonation constants of four diazadioximes, 3,3,8,8-tetramethyl-4,7-diazadecane-2,9-dione dioxime (L¹), 3,3,9,9-tetramethyl-4,8-diazaundecane-2,10-dione dioxime (L²), 3,3,6,6,9,9-hexamethyl-4,8-diazaundecane-2,10-dione dioxime (L³), and 3,3,10,10-tetramethyl-4,9-diazadodecane-2,11-dione dioxime (L⁴), have been determined potentiometrically in 0.10 moldm^-3 NaClO₄ at 25.0±0.1°C. The formation of their copper(II) complexes and the deprotonation reactions at the two oxime sites of these complexes have been investigated quantitatively by the potentiometric technique and by the measurement of their electronic spectra. The kinetics of complexation obtained by using the stopped-flow technique indicate that the first copper(II)-amino nitrogen bond formation between copper(II) and the unprotonated ligands is the rate-determining step and that the proton loss is the rate-limiting step in the reaction of copper(II) with the monoprotonated ligand. The thermodynamic, spectroscopic, and kinetic data are discussed in relation to a combination of steric, inductive, and ring-strain effects.