Pentanuclear Lanthanide Mono-organophosphates: Synthesis, Structure, and Magnetism

Research on rare-earth phosphates has recently received substantial interest because of their unique physical and chemical properties. In recent years, because of their low solubility, research interest has been built on developing methodologies to prepare nanostructures and grow single crystals of inorganic rare-earth phosphates. The chemistry of rare-earth organophosphates, however, is still at a latent stage. Contrary to the traditional hydrothermal route, we report rare examples of discrete pentanuclear lanthanide(III) organophosphate clusters assembled from a sterically encumbered monoester of phosphoric acid under mild reaction conditions. Single-crystal X-ray analysis revealed that all of the compounds possess a similar core structure, [Ln₅(μ₃-OH)(dipp)₆(NO₃)_x(CH₃OH)_y(H₂O)_z]²⁺ [Ln = Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6), Ho (7), Er (8), Tm (9); dipp = 2,6-diisopropylphenylphosphate], where the anionic charge balance is maintained by the presence of chelating nitrate anions (in the case of 9, x = 0), protonated tmeda, or dipp^2- ligands. The vacant coordination sites on the metal ions are satisfied by coordinated methanol or water molecules. The core structure of these clusters is built on a [Ln₃(μ₃-OH)(dipp)₆] triangle where the phosphate ligands bridge to two further Ln(III) ions. The complexes display lanthanide contraction along the series, with Ln(III) ions displaying different coordination environments/geometries as we move along the series. All of the compounds have been characterized by both analytical and spectroscopic techniques. Magnetic studies revealed the presence of weak antiferromagnetic exchange through the bridging μ₃-hydroxo moiety and organophosphate groups for the {Gd^III₅} analogue, with a significant magnetic entropy change (25.8 J kg^-1 K^-1, ΔH = 7 T). The anisotropic complexes reveal an absence of slow relaxation of magnetization, except for Nd (1), Dy (6), and Er (8), which show slow relaxation in an applied DC field.