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

Abstract

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.

References

1. D. Parker and J. A. G. Williams, J. Chem. Soc., Dalton Trans., 1996, 3613.
2. S. Aime, M. Botta, M. Fasano and E. Terreno, Chem. Soc. Rev., 1998, 27, 19.
3. N. Sabbatini, M. Guardigli and J.-M. Lehn, Coord. Chem. Rev., 1993, 123, 201.
4. N. Sabbatini, M. Guardigli and I. Manet, in Handbook on The Physics and Chemistry of Rare Earths, eds. K. A. Gschneidner Jr. and L. Eyring, Elsevier, Amsterdam, 1996, vol. 23.
5. B. Alpha, R. Ballardini, V. Balzani, J.-M. Lehn, S. Perathoner and N. Sabbatini, Photochem. Photobiol., 1990, 52, 299.
6. L. Prodi, M. Maestri, R. Ziessel and V. Balzani, Inorg. Chem., 1991, 30, 3798.
7. S. L. Wu and DeW. Horrocks Jr., J. Chem. Soc., Dalton Trans., 1997, 1497.
8. A. P. de Silva, H. Q. N. Gunaratne, T. E. Rice and S. Steward, Chem. Commun., 1997, 1891.
9. M. P. O. Wolbers, F. C. J. M. van Veggel, B. H. M. Snellink-Ruël, J. W. Hofstraat, F. A. J. Geurts and D. N. Reinhoudt, J. Am. Chem. Soc., 1997, 119, 138.
10. N. Sabbatini, A. Casnati, C. Fischer, R. Girardini, M. Guardigli, I. Manet, G. Sarti and R. Ungaro, Inorg. Chim. Acta, 1996, 252, 19; G. Ulrich, R. Ziessel, I. Manet, M. Guardigli, N. Sabbatini, F. Fraternali and G. Wipff, Chem. Eur J., 1997, 3, 1815.
11. N. Martin, J.-C. G. Bünzli, V. McKee, C. Piguet and G. Hopfgartner, Inorg. Chem., 1998, 37, 577.
12. M. Lata, H. Takalo, V.-M. Mukkala and J. Kankare, Inorg. Chim. Acta, 1998, 267, 63.
13. P. L. Jones, A. J. Amoroso, J. C. Jeffery, J. A. McCleverty, E. Psillakis, L. H. Rees and M. D. Ward, Inorg. Chem., 1997, 36, 10.
14. N. Armaroli, V. Balzani, F. Barigelletti, M. D. Ward and J. A. McCleverty, Chem. Phys. Lett., 1997, 276, 435.
15. D. Pubanz, G. González, D. H. Powell and A. E. Merbach, Inorg. Chem., 1995, 34, 4447.
16. Z. Wang, J. Reibenspies, R. J. Motekaitis and A. E. Martell, J. Chem. Soc., Dalton Trans., 1995, 1511.
17. J. E. Parks, B. E. Wagner and R. H. Holm, J. Organomet. Chem., 1973, 56, 53.
18. SADABS, A program for absorption correction with the Siemens SMART area-detector system, G. M. Sheldrick, University of Göttingen, 1996.
19. SHELXTL 5.03 program system, Siemens Analytical X-Ray Instruments, Madison, WI, 1995.
20. J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 93, 2841.
21. K. Nakamaru, Bull. Soc. Chem. Jpn., 1982, 5, 2697.
22. S. R. Meech and D. J. Philips, J. Photochem., 1983, 23, 193.
23. Y. Lin and S. A. Lang, Jr., J. Heterocycl. Chem., 1977, 14, 345.
24. A. Novak, Struct. Bonding (Berlin), 1974, 18, 177.
25. D. A. Bardwell, J. C. Jeffery, P. L. Jones, J. A. McCleverty and M. D. Ward, J. Chem. Soc., Dalton Trans., 1995, 2921.
26. R. Reisfeld and C. K. Jørgensen, Lasers and Excited States of Rare Earths, Springer, Berlin, 1977.
27. W. D. Horrocks and D. R. Sudnick, Acc. Chem. Res., 1981, 14, 384.