| Abstract
| - The lead(II) binding properties in water of one primary (R = H) and two tertiary (R = Me and Et) N-carbamoylmethyl-substituted 1,4,8,11-tetraazacyclotetradecane macrocycles are presented. Thermodynamic and kinetic parameters revealed the crucial role played by intramolecular hydrogen bonds in determining the ligand's basicities, lead affinities, and reactivities. Stopped-flow kinetic measurements enabled unraveling the formation and proton-assisted dissociation processes that proceed in a single rate-limiting step. An Eigen−Winkler mechanism is suggested for lead binding.
- En route toward the development of hybrid organic−inorganic extracting materials incorporating lead-selective chelatorsand their implementation in water purification processes, the lead(II) binding properties of three N-carbamoylmethyl-substituted 1,4,8,11-tetraazacyclotetradecanes (cyclams) have been fully investigated by spectroscopic (IR, UV−vis, MALDI-TOF MS, 1H and 13C NMR), X-ray crystallographic, potentiometric, and kinetic methods. Solution NMRstudies revealed that the Pb2+ ion is entrapped in a molecular cage constituted by the four macrocyclic nitrogenand four amidic oxygen atoms. Protonation and lead binding constants determined in aqueous solution were shownto be linearly dependent, so that all three derivatives possess a similar affinity at any pH value. Thermodynamicand kinetic parameters revealed the crucial role played by the intramolecular hydrogen bonds also evidenced inthe crystal structure of the tetraacetamide derivative L1, which involve the lone pair of each macrocyclic tertiaryamine and one amidic hydrogen atom belonging to the appended arm. In contrast to L1, the absence of suchintramolecular interactions for N-(dimethyl)carbamoylmethyl- and N-(diethyl)carbamoylmethyl-substituted cyclams(L2 and L3, respectively) accounts for the 2−3 orders of magnitude enhancement of their proton and lead bindingaffinities. Stopped-flow kinetic measurements enabled unraveling the formation process of the three lead(II) complexesthat proceeds in a single rate-limiting step according to the Eigen−Winkler mechanism, while the apparent rateconstants were found to increase in the order L3< L2 ≪ L1 as a consequence of the more acidic character of L1.A common proton-assisted dissociation mechanism has been found for the three lead(II) complexes, which involvesthe rapid formation of a protonated, six-coordinate intermediate followed by either a unimolecular decomposition ora bimolecular attack of a second hydronium ion.
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