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 [ML6].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 (H2L6) 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 (H-1 and C-13) in D2O 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 C-13 NMR spectra. Cd: k(298 K) = 236 S-1; DeltaHdouble dagger = 72.9 +/- 0.6 kJ.mol(-1); DeltaSdouble dagger = 45 +/- 2 J K-1.mol(-1); Hg: k(298 K) = 292 s(-1); DeltaHdouble dagger = 68 +/- 3 kJ.mol(-1); DeltaSdouble dagger = 29 +/- 10 J K-1.mol(-1); Pb: k(298 K) = 122 S-1; DeltaHdouble dagger = 65 +/- 2 kJ.mol(-1); DeltaSdouble dagger = 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. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)