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The first self-assembled trimetallic lanthanide helicates driven by positive cooperativity.
Sébastien Floquet1, Nadjet Ouali, Bernard Bocquet
1Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, 30 quai E. Ansermet, Switzerland.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|April 17, 2003
Summary
Lanthanide complexes with ligand L7 form stable triple-stranded helicates, driven by cooperative self-assembly. Luminescence and NMR studies reveal distinct electronic properties and structural dynamics of metal sites within these lanthanide helicates.
Area of Science:
- Coordination Chemistry
- Supramolecular Chemistry
- Lanthanide Chemistry
Background:
- Lanthanide(III) ions are crucial in developing functional materials due to their unique photophysical and magnetic properties.
- Self-assembly of polynuclear lanthanide complexes offers pathways to create intricate supramolecular architectures with tunable properties.
- Understanding the structural and electronic interplay between multiple lanthanide centers in complexes is key to designing advanced materials.
Purpose of the Study:
- To synthesize and characterize novel lanthanide(III) complexes with a segmental tris-tridentate ligand (L7).
- To investigate the self-assembly behavior and structural characteristics of the resulting trimetallic triple-stranded helicates.
- To explore the photophysical properties and solution-state dynamics of these lanthanide helicate complexes.
Main Methods:
- Reaction of lanthanide(III) salts with ligand L7 in acetonitrile.
- Formation constant determination and Scatchard analysis for self-assembly.
- X-ray crystallography for structural elucidation of the europium complex.
- Photophysical measurements (luminescence spectroscopy) and paramagnetic NMR spectroscopy with Ligand-Induced Shift (LIS) analysis.
Main Results:
- Formation of dinuclear [Ln(2)(L7)(3)](6+) and trimetallic [Ln(3)(L7)(3)](9+) complexes.
- Self-assembly of trimetallic helicates is cooperative and exhibits negligible size-discrimination across the lanthanide series.
- X-ray structure reveals a rigid triple-helical framework with Ln ions at ~9 Å separation, coordinated in pseudo-trigonal prismatic arrangements.
- Luminescence studies show dominant emission from terminal Eu(III) sites due to a low-lying LMCT state affecting the central site.
- NMR data confirm the triple helical structure in solution and suggest a relaxed geometry for the central metal site compared to terminal sites.
Conclusions:
- The ligand L7 effectively promotes the formation of stable, cooperative self-assembled trimetallic lanthanide triple-stranded helicates.
- Distinct electronic properties exist between terminal and central metal sites within the helicate, influencing luminescence.
- The triple helical structure is maintained in solution, although subtle geometric differences exist between central and terminal metal coordination environments.