Thermodynamic and kinetic study of copper(II) complexes with N -methylene(phenylphosphinic acid) derivatives of cyclen and cyclam

Equilibria in the Cu 2+ –H 4 L 1 and Cu 2+ –H 4 L 2 systems, where H 4 L 1 is 1,4,7,10-tetraaza-cyclododecane-1,4,7,10-tetrayl-tetramethylene-tetrakis(phenylphosphinic acid) and H 4 L 2 is 1,4,8,11-tetraaza-cyclotetradecane-1,4,8,11-tetrayl-tetramethylene-tetrakis(phenylphosphinic acid), were investigated by glass electrode potentiometry at 25°C ( I =0.1 mol dm −3 KNO 3 ). A simple chemical model with the metal:ligand molar ratio 1:1 was found in the systems. The presence of main species, [CuL 1 ] 2− (log β =20.37(4)) and [CuL 2 ] 2− (log β =17.19(2)), was also confirmed by MALDI-TOF/MS. The dissociation kinetics of the complexes was followed by spectrophotometry and a mechanism of the dissociation was proposed. Activation parameters (activation enthalpy and entropy) of the dissociation were estimated. For the Cu 2+ –H 4 L 1 system, the complex dissociation starts after protonation of the phosphinic pendant arms and its mechanism is similar to the decomplexation of [Cu(cyclen)] 2+. The Cu 2+ complex with H 4 L 2 is kinetically much less stable. The proton transfer from the phosphinic pendant arm to the azacycle plays a significant role in the reaction mechanism of both the complexes.