The stabilities and formation kinetics of some macrocycles with copper(II): crystal structures of some pendant arm macrocycles

Kinetics of complex formation and stability constants of tetra-(2-hydroxpropyl) substituted cyclam (L-3) and cyclen (L-4) with copper(II) have been studied in aqueous solution at room temperature. These data are compared to the corresponding parent compounds (cyclam L-1 and cyclen L-2) in an attempt to define the effect of pendant arm upon kinetics and stability constants of the complexes. The kinetics were observed by stopped-flow measurements followed at multiwavelengths. These ligands were chosen to furnish information concerning effect of pendant groups and cavity size on the kinetics and stability of the complexes. Stopped-flow and spectrophotometric titration techniques were used for evaluation of the kinetics and stability constants, respectively. The apparent rate constants increase as CuL3 > CuL4 > CuL1 > CuL2. Activation parameters and stability constants of the complexes were estimated. The effect of cavity size on the rate of reaction can be observed in CuL3 > CuL4 and CuL1 > CuL2 and the effect of pendant groups in CuL3 > CuL1 and CuL4 > CuL2. Mechanism of the complex formation reaction is proposed. The enhanced stability of the copper(II) complexes formed with L-1 and L-2 macrocyclic ligands is compared to those formed with analogous pendant arm species.