Structural and near-IR photophysical studies on ternary lanthanide complexes containing poly(pyrazolyl)borate and 1,3-diketonate ligands

Abstract

The ligands tris[3-(2-pyridyl)pyrazol-1-yl]hydroborate (L¹, potentially hexadentate) and bis[3-(2-pyridyl)pyrazol-1-yl]dihydroborate (L², potentially tetradentate) have been used to prepare ternary lanthanide complexes in which the remaining ligands are dibenzoylmethane anions (dbm). [Eu(L¹)(dbm)₂] is eight-coordinate, with L¹ acting only as a tetradentate chelate (with one potentially bidentate arm pendant) and two bidentate dbm ligands. [Nd(L¹)(dbm)₂] was also prepared but on recrystallisation some of it rearranged to [Nd(L¹)₂][Nd(dbm)₄], which contains a twelve-coordinate [Nd(L¹)₂]⁺ cation (two interleaved hexadentate podand ligands) and the eight-coordinate anion [Nd(dbm)₄]⁻ which, uniquely amongst eight-coordinate complexes having four diketonate ligands, has a square prismatic structure with near-perfect O₈ cubic coordination. Formation of this sterically unfavourable geometry is assumed to arise from favourable packing with the pseudo-spherical cation. The isostructural series of complexes [Ln(L²)(dbm)₂] (Ln = Pr, Nd, Eu, Gd, Tb, Er, Yb) was also prepared and all members structurally characterised; again the metal ions are eight-coordinate, from one tetradentate ligand L² and two bidentate dbm ligands. Photophysical studies on the complexes with Ln = Pr, Nd, Er, and Yb were carried out; all show the near-IR luminescence characteristic of these metal ions, with longer lifetimes in CD₃OD than in CH₃OH. For [Yb(L²)(dbm)₂], two species with different luminescence lifetimes were observed in CH₃OH solution, corresponding to species with zero or one coordinated solvent molecules, in slow exchange on the luminescence timescale. For [Nd(L²)(dbm)₂] a single average solvation number of 0.7 was observed in MeOH. For [Pr(L²)(dbm)₂] a range of emission lines in the visible and NIR regions was detected; time-resolved measurements show a particularly high susceptibility to quenching by solvent CH and OH oscillators.