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.