Complexes of 1-thia-4,7,10-triazacyclododecane ([12]aneN3S): Synthesis,structure and stability constants

The 12-membered macrocyclic ligand 1-thia-4,7,10-triazacyclododecane (12]aneN₃S) has been synthesised, although upon crystallization from acetonitrile a product in which carbon dioxide had added to one secondary amine in the macrocyclic ring (H12]aneN₃S–CO₂·H₂O) was isolated and subsequently characterised by X-ray crystallography. The protonation constants for 12]aneN₃S and stability constants with Zn(II), Pb(II), Cd(II) and Cu(II) have been determined either potentiometrically or spectrophotometrically in aqueous solution, and compared with those measured or reported for the ligands 1-oxa-4,7,10-triazacyclododecane (12]aneN₃O) and 1,4,7,10-tetraazacyclododecane (12]aneN₄). The magnitudes of the stability constants are consistent with trends observed previously for macrocyclic ligands as secondary amine donors are replaced with oxygen and thioether donors although the stability constant for the Hg(12]aneN₄)]²⁺ complex has been estimated from an NMR experiment to be at least three orders of magnitude larger than reported previously. Zinc(II), mercury(II), lead(II), copper(II) and nickel(II) complexes of 12]aneN₃S have been isolated and characterised by X-ray crystallography. In the case of copper(II), two complexes Cu(12]aneN₃S)(H₂O)](ClO₄)₂ and Cu₂(12]aneN₃S)₂(OH)₂](ClO₄)₂ were isolated, depending on the conditions employed. Molecular mechanics calculations have been employed to investigate the relative metal ion size preferences of the 3333], asym-2424] and sym-2424] conformation isomers. The calculations predict that the asym-2424] conformer is most stable for M–N bond lengths in the range 2.00–2.25 Å whilst for the larger metal ions the 3333] conformer is dominant. The disorder seen in the structure of the Zn(12]aneN₃S)(NO₃)]⁺ complex is also explained by the calculations.