Capillary electrophoretic separation and kinetic study of inert copper(II) complexes of 1,8-bis(methylphosphonate) derivative of cyclam

The behaviour of two structural isomers of the copper(II) complex of 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid) (H4dipon), penta- and hexacoordinated species, was studied by capillary electrophoresis. The separation and determination of both the isomers with the same molecular weight and charge and the free H4dipon ligand were achieved in 75 mM disodium tetraborate aqueous solution (pH 9.5) as background electrolyte and, simultaneously, magnitudes of their effective mobilities were determined. The influence of both variables (pH and buffer composition/concentration) on the separation was established and the principle of the separation was proposed. The method of separation was applied to the study of the reaction mechanism of the isomerization. It was proved that the reaction can be explained by several mechanisms with the same fit through experimental data. The first step of the isomerization reaction proceeds relatively slowly and is the rate-determining step (rds) of overall isomerization reaction. Since no intermediate was detected during the reaction, it was concluded that the next step(s) is/are much faster than the rds (at least 10 times). The magnitudes of the rate constant at two different temperatures (k80 °C (1.23 ± 0.09) × 10−3 min−1, k90 °C (2.45 ± 0.06) × 10−3 min−1) and activation energy (Ea = 72 ± 8 kJ mol−1), enthalpy (ΔH# = 69 ± 8 kJ mol−1) and entropy (ΔS# = −106 ± 23 J K−1 mol−1) were determined.

Graphical abstractPentacoodinated pc-[Cu(L)]2− and octahedral hx-[Cu(L)]2− complexes and the free ligand dianion (H2L)2− were successfully separated by means of CE. The method was used for evaluation of isomerization kinetics of the pentacoordinated isomer to the octahedral species. The isomerization was proved to be a highly energetically inconvenient process.Figure optionsDownload full-size imageDownload as PowerPoint slide