Lanthanoid containing phosphotungstates: the syntheses, crystal structure, electrochemistry, photoluminescence and magnetic properties

Abstract

A series of ten mononuclear lanthanoid containing phosphotungstate clusters were synthesized by the reaction of the dilacunary ligand [P₂W₁₉O₆₉(H₂O)]¹⁴⁻ with Ln(NO₃)₃·xH₂O in a potassium acetate buffer (pH 4.8), leading to the formation of Peacock and Weakley type dimers [Ln(PW₁₁O₃₉)₂]¹¹⁻ (Ln = Pr³⁺ (1), Nd³⁺ (2), Eu³⁺ (3), Gd³⁺ (4), Tb³⁺ (5), Dy³⁺ (6), Ho³⁺ (7), Er³⁺ (8), Tm³⁺ (9), and Yb³⁺ (10)). All isostructural molecular clusters were structurally characterized using various analytical techniques like FT-IR, ³¹P NMR spectroscopy, photoluminescence and solid state UV/vis, electrochemistry, magnetism and single crystal X-ray diffraction. The compounds crystallize as mixed potassium/caesium or potassium salts K₁₁[Ln(PW₁₁O₃₉)₂]·xH₂O (Ln = Pr³⁺ (1a), Nd³⁺ (2a), Eu³⁺ (3a), Gd³⁺ (4a), Tb³⁺ (5a), Dy³⁺ (6a), Ho³⁺ (7a), Er³⁺ (8a), and Tm³⁺ (9a), in the monoclinic crystal system, space group P2₁/c (14) with the exception of the Gd(4a) and Ho(7a) complexes which crystallize in the triclinic system, space group P. The prominent effect of the paramagnetic lanthanoid ion along the group on the monolacunary Keggin unit (α-PW₁₁O₃₉) was illustrated by FT-IR and ³¹P NMR spectroscopy. This is attributed to its small size and high charge density (lanthanoid contraction). The photoluminescence of the molecular clusters Eu(3a), Dy(6a) and Ho(7a) was studied at room temperature at different excitation wavelengths and shows interesting photochromic properties. The voltammetric patterns associated with the W-centers in the cluster series have enough distinct features to allow us to establish the relative basicity of the reduced polyoxometalates: {PW11} > Tm(9a) > Eu(3a) ≈ Tb(5a) ≈ Ho(7a) > Gd(4a) ≈ Dy(6a) ≈ Er(8a). Among the lanthanoid cations, just the Eu³⁺ centre turned out to be electroactive, as expected. The magnetic measurement of Gd(4a) shows the paramagnetic behaviour of the material.