Octahedral lanthanide clusters containing a central PO43− anion: structural, luminescent, magnetic and relaxometric properties

Abstract

Lanthanide clusters with good stability and intriguing physical properties are attractive in many fields. By reacting 9-anthracenylphosphonic acid (AnPO₃H₂) and lanthanide nitrates under solvothermal conditions, we obtained a series of hexanuclear lanthanide phosphonate cages [H₃O][Ln₆(PO₄)(AnPO₃)₈(DMF)₆]·2DMF·H₂O (Ln₆, Ln = Nd^III, Eu^III, Gd^III, Dy^III, Ho^III, Er^III, Yb^III). Within the cluster, the six Ln atoms form an octahedron and its eight faces are covered by phosphonate groups. The in situ generated phosphate anion resides inside the cage and binds to the six Ln atoms via its four oxygen atoms. Photoluminescence studies show that Nd₆, Er₆ and Yb₆ can emit near-infrared (NIR) luminescence due to the energy transfer from the anthracene ligand to the lanthanide ions. Magnetic studies reveal the magnetocaloric effect of Gd₆ with an entropy change (−ΔS_m) of 25.92 J kg⁻¹ K⁻¹ at 2.5 K and ΔH = 0–7 T. The possibility of using Gd₆ as a contrast agent for magnetic resonance imaging was also explored with longitudinal (r₁) and transverse (r₂) relaxivities of 5.68 mM⁻¹ s⁻¹ per Gd and 158.11 mM⁻¹ s⁻¹ per Gd, respectively.