Abstract
Controlled formation of desired lanthanide supramolecular complexes is challenging because of the difficulties in predicting coordination geometry, as well as a labile coordination number. Herein, we explore the effect of ionic radii and linker length on supramolecular species formation. A helicate-to-tetrahedron transformation occurred between [Ln2L13] and [Ln4L16] (Ln = La, Sm, Eu, Gd, Tb and Lu). For six lanthanide ions, the unfavored tetrahedron [La4L16] can only be observed in a concentrated mixture with the helicate [La2L13] where no pure [La4L16] species was isolated via crystallization. For Sm, Eu, Gd, Tb, the [Ln4L16] supramolecular tetrahedron can be isolated via crystallization from diisopropyl ether. A similar result was also observed for Lu, but the tetrahedral structure was found to be relatively stable and transformed back to [Lu2L13] much slower upon dissolution. No tetrahedron formation was observed with L3 giving rise to only [Ln2L33] species, in which L3 contains a longer and more flexible linker compared with that of L1. Results show that the supramolecular transformation in these systems is governed by both the ionic radii as well as the ligand design. Special focus is on both [Eu2L13] and [Eu4L16] which form chiral entities and exhibit interesting circular polarized luminescence.
Highlights
Controlled formation of desired lanthanide supramolecular complexes is challenging because of the difficulties in predicting coordination geometry, as well as a labile coordination number
As supramolecular architectures are selfassembled by spontaneous assembly of noncovalent interactions[14], typically, some unwanted thermodynamically stable clusters are formed
The use of C2-symmetrical bis-bidentate ligands is ideal for preparing M2L3 helicates
Summary
Controlled formation of desired lanthanide supramolecular complexes is challenging because of the difficulties in predicting coordination geometry, as well as a labile coordination number. Results show that the supramolecular transformation in these systems is governed by both the ionic radii as well as the ligand design. 1234567890():,; Self-assembly of various supramolecular architectures has received phenomenal attention over recent years. This is primarily due to the diversity in the range of applications for the resultant materials such as catalysts[1,2], luminescent probes[3,4], and magnetic materials[5]. Two chelating groups of the C2-symmetrical bis-bidentate ligand must be parallel and the offset of the two chelating group in the ligand should be small so that the C3 and C2 axis can approach 90° to form a helicate[17,18]. Since entropy favors the lower stoichiometry M2L3 complex[15], the metal binding units of the ligand should be offset from one another so that the helicate formation is disfavored[17]
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