Synthesis, Kinetic Study and Molecular Orbital Investigation of Cadmium( II ), Mercury( II ) and Lead( II ) Complexes with the Mixed Pendant‐Arm Macrocyclic Ligand 1,7‐Bis(carboxymethyl)‐4,10‐bis(1‐methylimidazol‐2‐ylmethyl)‐1,4,7,10‐tetraazacyclododecane

The neutral [ML 6 ]·3H 2 O complexes (M = Cd II , Hg II and Pb II ) formed with the potentially octadentate ligand 1,7‐bis(carboxymethyl)‐4,10‐bis(1‐methylimidazol‐2‐ylmethyl)‐1,4,7,10‐tetraazacyclododecane (H 2 L 6 ) in its anionic deprotonated form, which bears two pairs of different dangling groups in trans positions on the cyclen skeleton, have been synthesized. Their low‐temperature limiting NMR spectra ( 1 H and 13 C) in D 2 O show that both the dangling groups and the ethylenic moieties of the macrocycle ring are in a fixed conformation in all complexes at 273 K. These conditions are relaxed at higher temperatures, yielding a dynamic behaviour that proceeds through both ring inversion and rearrangement of the pendant arms. The kinetic parameters for ring inversion have been obtained from the temperature‐dependent 13 C NMR spectra. Cd: k (298 K) = 236 s −1 ; Δ H ‡ = 72.9 ± 0.6 kJ·mol −1 ; Δ S ‡ = 45 ± 2 J K −1 ·mol −1 ; Hg: k (298 K) = 292 s −1 ; Δ H ‡ = 68 ± 3 kJ·mol −1 ; Δ S ‡ = 29 ± 10 J K −1 ·mol −1 ; Pb: k (298 K) = 122 s −1 ; Δ H ‡  = 65 ± 2 kJ·mol −1 ; Δ S ‡ = 14 ± 5 J K −1 ·mol −1 . In the absence of experimental structure determinations, some insight into the geometries has been obtained by quantum‐mechanical calculations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)