ISSN:
0947-6539
Keywords:
esters
;
helical structures
;
lanthanides
;
luminescence
;
tridentate ligands
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The ligand diethyl pyridine-2, 6-dicarboxylate (L5) reacts with LnIII in acetonitrile to successively give the complexes [Ln(L5)i]3+ (Ln = La to Lu, i = 1-3). Spectroscopic investigations (ES-MS, UV/Vis, NMR) show that the 1:3 complexes [Ln(L5)3]3+ have poor stability in solution and exist as a mixture of rapidly interconverting conformers. Variable-temperature NMR data show that the helical P→M interconversion and dynamic on-off equilibria of the ester side arms both control the observed average structure in solution. Contrary to similar lanthanide building blocks possessing benzimidazole or carboxamide side arms, [Eu(L5)3]3+ has a sizable quantum yield in anhydrous acetonitrile; this has been attributed to an improved ligand → EuIII energy transfer resulting from a good energetic match between the ligand- and metal-centered excited states. Pure 1:3 complexes cannot be isolated in the solid state, but crystalline 1:2 complexes [Ln(L5)2](TfO)3.nH2O have been prepared. The X-ray crystal structure of [Eu(L5)2(TfO)2(OH2)]TfO (1) reveals two meridionally tricoordinated ligands L5, but the long Eu-O(ester) bonds imply only weak interactions between the carbonyl groups of the ester side arms and EuIII, providing a limited protection of the metallic site. The photophysical studies show that nonacoordinate EuIII in 1 binds an additional water molecule to give a decacoordinate complex in the solid state, thus confirming the accessibility of the metallic site for further complexation.
Additional Material:
7 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/chem.19970031015
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