A solid-state study of eight-coordinate lanthanide(iii) complexes (Ln = Eu, Gd, Tb, Dy) with 1-hydroxy-2-pyridinone

Abstract

Four lanthanide complexes of 1-hydroxy-2-pyridinone (HOPO) have been synthesised from aqueous solution at pH ≈ 7 and their structures and luminescent properties have been established. This ligand coordinates to lanthanide(III) ions as an anionic bidentate chelate (1-oxy-2-pyridinonate, OPO⁻) and crystallographic analyses show the formation of two structural types with different stoichiometries. In the first part of this series (Eu, Gd) the structural type consists of dimer units Na₂[Ln₂(OPO)₈]·9H₂O, crystallising in the orthorhombic system, space group P2₁2₁2₁. The immediate coordination sphere around Ln is occupied by the eight oxygen atoms of four bidentate OPO⁻ ligands. These complexes display the Na⁺ ions associated with the complex anion via interactions with bridging CO and NO groups of the ligands, inducing a polymeric structure. In the second part of the series (Tb, Dy) the structures obtained are [Ln(OPO)₃(H₂O)₂]·H₂O, crystallising in triclinic system, space group P. The Tb(III) and Dy(III) ions are also eight-coordinated but two water molecules occupy the coordination sphere instead of one ligand OPO⁻. The Dy(III), Eu(III) and Tb(III) complexes display in the solid state and at room temperature a photoluminescence that is achieved by an indirect excitation process (antenna effect). The terbium(III) complex exhibits intense sensitized green emission, which is temperature independent.