Octahedral lanthanide clusters containing a central PO₄^3- anion: structural, luminescent, magnetic and relaxometric properties

Lanthanide clusters with good stability and intriguing physical properties are attractive in many fields. By reacting 9-anthracenylphosphonic acid (AnPO(3)H(2)) and lanthanide nitrates under solvothermal conditions, we obtained a series of hexanuclear lanthanide phosphonate cages [H3O][Ln(6)(PO4) (AnPO(3))(8)(DMF)(6)]center dot 2DMF center dot H2O (Ln(6), 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 Nd-6, Er-6 and Yb-6 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 Gd-6 with an entropy change (-Delta S-m) of 25.92 J kg(-1) K-1 at 2.5 K and Delta H = 0-7 T. The possibility of using Gd-6 as a contrast agent for magnetic resonance imaging was also explored with longitudinal (r(1)) and transverse (r(2)) relaxivities of 5.68 mM(-1) s(-1) per Gd and 158.11 mM(-1) s(-1) per Gd, respectively.