Lanthanide complexes of a new sterically hindered potentially hexadentate podand ligand based on a tris(pyrazolyl)borate core; crystal structures, solution structures and luminescence properties
| Autor: | Zoe R. Reeves, Michael D. Ward, Jon A. McCleverty, Karen L. V. Mann, John C. Jeffery, Nicola Armaroli, Francesco Barigelletti |
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| Rok vydání: | 1999 |
| Předmět: | |
| Zdroj: | Journal of the Chemical Society, Dalton Transactions. :349-356 |
| ISSN: | 1364-5447 0300-9246 |
| DOI: | 10.1039/a808724e |
| Popis: | The new podand ligand hydrotris[3-(6-methyl)pyridin-2-ylpyrazol-1-yl]borate [L1]– was prepared which contains three bidentate pyrazolyl/pyridine arms attached to a {BH}– head-group. This ligand differs from an earlier ligand hydrotris[3-(2-pyridyl)pyrazol-1-yl]borate [L2]– by the presence of methyl groups attached to the C6 positions of the pyridyl rings, which would interfere with each other sterically if the ligand co-ordinated in a fully hexadentate manner. Instead, crystallographic analysis of the complexes [M(L1)(NO3)2(H2O)] (M = Eu, Tb or Gd) showed that partial dissociation of the podand occurs to relieve this potential steric problem: either one or two of the pyridyl groups are not co-ordinated, such that [L1]– is penta- or tetra-dentate, but instead are involved in intramolecular N· · ·H–O hydrogen-bonding interactions with the co-ordinated water molecule. The presence of both structural forms in single crystals of the gadolinium and europium complexes shows that interconversion between them in solution must be facile. Variable-temperature 1H NMR spectra of the diamagnetic lanthanum(III) analogue shows that, whereas all three ligand arms are equivalent on the NMR timescale at high temperatures, at –80 °C there is mirror symmetry in the complex such that two arms are equivalent and the third is different from the other two; this is consistent with the crystalline form in which [L1]– is tetradentate with two pendant pyridyl arms, which has pseudo-mirror symmetry. Luminescence studies showed that whereas the ligand-based luminescence is retained in the gadolinium(III) complex, in the europium(III) and terbium(III) complexes the ligand-centred emission is quenched by ligand-to-metal energy transfer, resulting in the usual metal-centred emission spectra. The intensity of the emission from the europium(III) and terbium(III) complexes of [L1]– is substantially reduced compared to the emission from the analogous complexes [M(L2)(NO3)2] (M = Eu or Tb) which we ascribe to the sterically induced poorer co-ordination of the podand ligand, resulting in (i) less efficient ligand-to-metal energy transfer, and (ii) co-ordination of labile solvent molecules (H2O) to the metal centres. |
| Databáze: | OpenAIRE |
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