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 dipp2– 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 {GdIII5} 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.