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

Abstract

Kinetics of complex formation and stability constants of tetra-(2-hydroxpropyl) substituted cyclam (L³) and cyclen (L⁴) with copper(II) have been studied in aqueous solution at room temperature. These data are compared to the corresponding parent compounds (cyclam L¹ and cyclen L²) 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 L¹ and L² macrocyclic ligands is compared to those formed with analogous pendant arm species.

Keywords:

• Kinetics
• Stability
• Pendant arm
• Cavity size
• Stopped-flow
• Spectroscopic titration

Acknowledgments

Financial support is received from the Scientific and Technological Research Council of Turkeys research program 1001. Grant for 104T389 is gratefully acknowledged.