Unusual variability of isomers in copper(II) complexes with 1,8-bis(2-hydroxybenzyl)-cyclam

Abstract

Copper isotopes and their complexes are intensively studied due to their high potential for the application in radiodiagnosis and radiotherapy. Here we study the Cu^II complex of 1,8-bis(2-hydroxybenzyl)-cyclam (H₂L) that forms unexpected versatility of isomers differing in the mutual orientation of substituents on cyclam nitrogen atoms, protonation of phenolate pendant, and ligand denticity. The interconversion of isomers is rather slow, which made the isolation, identification and investigation of some of the individual species possible. The most stable and the most common form is the hexacoordinated trans-III isomer. However, several other forms were also observed in solution in the course of HPLC, UV-VIS and electrochemical measurements. The isomers present in solution were identified by comparison with the solid-state structures solved by X-ray diffraction analysis on single crystals and with the help of theoretical calculations. The phenolate pendant is coordinated both in the protonated and deprotonated state, however, the coordination in axial position of the hexacoordinated trans-III complex is weak, especially in its protonated state. On the other hand, Cu^II ion is pentacoordinated in the isomer cis-V with only one phenolate strongly coordinated in the basal plane of the distorted tetragonal pyramid. The computational data showed that the phenolate groups might form strong intraligand hydrogen bonds competitive to the metal-phenolate bonds, stabilizing the structure of the complex. In addition, theoretical calculations revealed that several geometries are energetically close to the optimal one, which indicates possible dynamic behaviour of the complex in the solution.