Supramolecular Recognition of Heteropairs of Lanthanide Ions: A Step toward Self-Assembled Bifunctional Probes

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• 作者：Nicolas André ; Thomas B. Jensen ; Rosario Scopelliti ; Daniel Imbert ; Mourad Elhabiri ; Gé ; rard Hopfgartner ; Claude Piguet ; and Jean-Claude G. B&uuml ; nzli
• 刊名：Inorganic Chemistry
• 出版年：2004
• 出版时间：January 26, 2004
• 年：2004
• 卷：43
• 期：2
• 页码：515 - 529
• 全文大小：287K
• 年卷期：v.43,no.2(January 26, 2004)
• ISSN：1520-510X

文摘

Three unsymmetrical ditopic hexadentate ligands coded for the recognition of trivalent lanthanide ions have beensynthesized, L^AB, L^AC, and L^BC, where A represents a benzimidazole-pyridine-benzimidazole coordination unit, Ba benzimidazole-pyridine-carboxamide one, and C a benzimidazole-pyridine-carboxylic acid moiety. Understoichiometric 2:3 (Ln:L) conditions, these ligands self-assemble with lanthanide ions to yield triple-stranded bimetallichelicates having a sizable stability in acetonitrile: log ges/gifchars/beta2.gif" BORDER=0 ALIGN="middle">₂₃ values for Eu are equal to 23.9 ± 0.5 (L^AB), 23.3 ± 0.7(deprotonated L^AC), and 29.8 ± 0.5 (deprotonated L^BC). The crystal structure of the EuEu helicate with L^AB shows9-coordinate metal ions and an HHH (H stands for head) configuration of the helically wrapped ligand strands. Inthe presence of equimolar quantities of Ln and Ln' ions, L^AB displays a remarkable predisposition to form HHH-heterobimetallic edifices, as proved both in the solid state by the crystal structures of the LaEu, LaTb, PrEr, andPrLu helicates and in solution by NMR spectroscopy. In all cases, the benzimidazole-pyridine-carboxamide unitsof the three ligands are bound to the smaller lanthanide ion, a fact further ascertained by high-resolution luminescencedata on LaEu and by ¹H NMR. Analysis of the lanthanide-induced ¹H NMR shifts and of the spin-lattice relaxationtimes of the [LnLu(L^AB)₃]⁶⁺ series (Ln = Ce, Pr, Nd, Sm, Eu) demonstrates the isostructural nature of the complexesin solution and that the crystal structure of LaTb is a good model for the solution structure. The selectivity of L^ABfor heteropairs of Ln^III ions increases with increasing difference in ionic radius, resulting in 70% of the heterobimetallicspecies for ges/gifchars/Delta.gif" BORDER=0 >r_i = 0.1 Å and up to 90% for LaLu (ges/gifchars/Delta.gif" BORDER=0 >r_i = 0.18 Å), and corresponding to ges/gifchars/Delta.gif" BORDER=0 >(ges/gifchars/Delta.gif" BORDER=0 >G) in the range 3-10kJ·mol^-1. The origins of this stabilization are discussed in terms of the donor properties of the coordinating unitsand of the preferential formation of HHH isomers.