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

The new podand ligand hydrotris[3-(6-methyl)pyridin-2-ylpyrazol-1-yl]borate [L-1](-) was prepared which contains three bidentate pyrazolyl/pyridine ar ms attached to a {BH}(-) head-group. This ligand differs from an earlier li gand hydrotris[3-(2-pyridyl)pyrazol-1-yl]borate [L-2](-) by the presence of methyl groups attached to the C-6 positions of the pyridyl rings, which wo uld interfere with each other sterically if the ligand co-ordinated in a fu lly hexadentate manner. Instead, crystallographic analysis of the complexes [M(L-1)(NO3)(2)(H2O)] (M = Eu, Tb or Cd) showed that partial dissociation of the podand occurs to relieve this potential steric problem: either one o r two of the pyridyl groups are not co-ordinated, such that [L-1](-) is pen ta- or tetra-dentate, but instead are involved in intramolecular N ... H-O hydrogen-bonding interactions with the co-ordinated water molecule. The pre sence of both structural forms in single crystals of the gadolinium and eur opium complexes shows that interconversion between them in solution must be facile. Variable-temperature H-1 NMR spectra of the diamagnetic lanthanum( III) analogue shows that, whereas all three ligand arms are equivalent on t he NMR timescale at high temperatures, at -80 degrees C there is mirror sym metry in the complex such that two arms are equivalent and the third is dif ferent from the other two; this is consistent with the crystalline form in which [L-1](-) is tetradentate with two pendant pyridyl arms, which has pse udo-mirror symmetry. Luminescence studies showed that whereas the ligand-ba sed luminescence is retained in the gadolinium(III) complex, in the europiu m(III) and terbium(III) complexes the ligand-centred emission is quenched b y ligand-to-metal energy transfer, resulting in the usual metal-centred emi ssion spectra. The intensity of the emission from the europium(III) and ter bium(III) complexes of [L-1](-) is substantially reduced compared to the em ission from the analogous complexes [M(L-2)(NO3)(2)] (M = Eu or Tb) which w e ascribe to the sterically induced poorer co-ordination of the podand liga nd, resulting in (i) less efficient ligand-to-metal energy transfer, and (i i) co-ordination of labile solvent molecules (H2O) to the metal centres.