Study of the cyclen derivative 2-[1,4,7,10-tetraazacyclododecan-1-yl]-ethanethiol and its complexation behaviour towards d-transition metal ions

The compound 2-[1,4,7,10-tetraazacyclododecan-1-yl]-ethanethiol (L2) has been synthesized and characterized by multinuclear NMR spectroscopy and mass spectrometry. Its thiol-protected precursor L1 has also been isolated and characterized, including by X-ray structural analysis. The protonation constants of L2 were determined by potentiometric methods at 25.0 °C and 0.10 mol dm−3 KNO3 ionic strength. 13C NMR studies and 2D NMR spectra recorded at different pD values have been used to analyse its protonation scheme. Stability constants of L2 with Cu2+, Zn2+ and Cd2+ were also determined by potentiometry, and the Zn(II) and Cu(II) complexes were studied in solution by NMR, UV–Vis, and EPR spectroscopies. The pM values (pH 7.4) calculated for the metal complexes of L2 are higher than the corresponding values found for cyclen and cyclam, but the selectivity of L2 for Cu2+ is low.

Graphical abstractThe compound 2-[1,4,7,10-tetraazacyclododecan-1-yl]-ethanethiol (L2) has been synthesized and fully characterized. The protonation constants of L2 as well as its stability constants with Cu2+, Zn2+ and Cd2+ were determined by potentiometric methods. Zn(II)/L2 and Cu(II)/L2 complexes were studied by NMR, UV–Vis, and EPR spectroscopies. The pM values calculated for the metal complexes of L2, at physiological pH, are higher than the pM values found for the corresponding complexes with cyclen or cyclam, but L2 is less selective for copper.Figure optionsDownload full-size imageDownload as PowerPoint slide