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New Insights into the Complexation of Lead(II) by 1,4,7,10‐Tetrakis(carbamoylmethyl)‐1,4,7,10‐tetraazacyclododecane (DOTAM): Structural, Thermodynamic, and Kinetic Studies
Author(s) -
Cuenot François,
Meyer Michel,
Espinosa Enrique,
Bucaille Arnaud,
Burgat Romain,
Guilard Roger,
MarichalWestrich Claire
Publication year - 2008
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.200700819
Subject(s) - chemistry , cyclen , cyclam , lone pair , coordination sphere , amide , crystallography , ligand (biochemistry) , hydrogen bond , crystal structure , intramolecular force , coordination complex , stereochemistry , aqueous solution , metal , molecule , organic chemistry , biochemistry , receptor
The lead(II) coordination properties of the tetrapodal ligand DOTAM [1,4,7,10‐tetrakis(carbamoylmethyl)‐1,4,7,10‐tetraaza‐cyclododecane] have been investigated both in the solid state and in solution in order to ascertain the stereoactivity of the lone pair and to rationalize the structural effects of a cyclen‐based scaffold on the metal uptake kinetics. The crystal structure of the free base shows that the pendant acetamide groups are not equivalent: two are folded over the macrocycle and maintained by an intramolecular hydrogen bond involving an amide hydrogen atom and a neighboring tertiary amine of the cyclen ring, while the other two are extended and point away from the macrocyclic cavity. The spontaneous reaction between Pb 2+ and DOTAM, even under mild acidic conditions, leads to a mononuclear complex. The crystal structure of [Pb(DOTAM)](NO 3 ) 2 · 3.5H 2 O reveals that the eight‐coordinate metal cation is trapped inside the core of the ligand, interacting with the four cyclen nitrogen and the four amide oxygen atoms. The helical layout of the folded arms leaves no significant gap in the coordination sphere of Pb 2+ , thus leading to a holodirected structure that is characteristic of a stereochemically inactive 6s 2 electronic lone pair. While the coordination scheme is maintained in solution, variable temperature NMR studies enabled characterization of the dynamics related to the inversion of configuration of the amide substituents. Finally, the lead(II)‐binding mechanism in aqueous solution has been investigated over a wide p[H] range (1–7) by means of classical and stopped‐flow spectrophotometry. The complexation reaction proceeds in a single rate‐limiting step according to an Eigen–Winkler mechanism. (© WILEY‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)