Synthetic Control to Achieve Lanthanide(III)/Pyrimidine-4,6-dicarboxylate Compounds by Preventing Oxalate Formation: Structural, Magnetic, and Luminescent Properties

Control over the synthetic conditions in many metal/diazinedicarboxylato
systems is crucial to prevent oxalate formation, since dicarboxylato
ligands easily undergo degradation in the presence of metal salts.
We report here an efficient route to obtain oxalato-free compounds
for the lanthanide/pyrimidine-4,6-dicarboxylato (pmdc) system on the
basis of the reaction temperature and nonacidic pH or oxygen free
atmosphere. Two different crystal architectures have been obtained:
{[Ln­(μ-pmdc)1.5(H2O)3]·xH2O}n (1-Ln) and {[Ln2(μ4-pmdc)2(μ-pmdc)­(H2O)2]·H2O}n (2-Ln) with Ln­(III) = La–Yb, except Pm.
Both crystal structures are built from distorted two-dimensional honeycomb
networks based on the recurrent double chelating mode established
by the pmdc. In compounds 1-Ln, the tricapped trigonal
prismatic coordination environment of the lanthanides is completed
by three water molecules, precluding a further increase in the dimensionality.
Crystallization water molecules are arranged in the interlamellar
space, giving rise to highly flexible supramolecular clusters that
are responsible for the modulation found in compound 1-Gd. Two of the coordinated water molecules are replaced by nonchelating
carboxylate oxygen atoms of pmdc ligands in compounds 2-Ln, joining the metal–organic layers together and thus providing
a compact three-dimensional network. The crystal structure of the
compounds is governed by the competition between two opposing factors:
the ionic size and the reaction temperature. The lanthanide contraction
rejects the sterically hindered coordination geometries whereas high-temperature
entropy driven desolvation pathway favors the release of solvent molecules
leading to more compact frameworks. The characteristic luminescence
of the Nd, Eu, and Tb centers is improved when moving from 1-Ln to 2-Ln compounds as a consequence of the decrease
of the O–H oscillators. The magnetic properties of the compounds
are dominated by the spin–orbit coupling and the ligand field
perturbation, the exchange coupling being almost negligible.