Two- and Three-Dimensional Networks of Gadolinium(III) with Dicarboxylate Ligands: Synthesis, Crystal Structure, and Magnetic Properties

Four gadolinium(III) complexes with dicarboxylate ligands of formulas [Gd2(mal)3(H2O)5]n·2nH2O (1), [Gd2(mal)3(H2O)6]n (2), [NaGd(mal)(ox)(H2O)3]n (3), and [Gd2(ox)3(H2O)6]n·2.5nH2O (4) (mal = malonate; ox = oxalate) have
been prepared, and their magnetic properties have been investigated as a function of the temperature. The structures
of 1−3 have been determined by X-ray diffraction methods. The crystal structure of 4 was already known, and it
is made of hexagonal layers of Gd atoms that are bridged by bis-bidentate oxalate. Compound 1 is isostructural
with the europium(III) malonate complex [Eu2(mal)3(H2O)5]n·2nH2O, whose structure was reported elsewhere. The
Gd atoms in 1 define a two-dimensional network where a terminal bidentate and bridging bidentate/bis-monodentate
and tris-bidentate coordination modes of malonate occur. Compound 2 has a three-dimensional structure with a
structural phase transition at 226 K, which involves a change of the space group from I2/a to Ia. Although its
structure at room temperature was already known, that below 226 K was not. Pairs of Gd atoms with a double
oxo−carboxylate bridge occur in both phases, and the main differences between both structures deal with the Gd
environment and the H-bond pattern. 3 is also a three-dimensional compound, and it was obtained by reacting
Gd(III) ions with malonic acid in a silica gel medium. Oxalic acid results as an oxidized product of the malonic acid,
and single crystals of the heteroleptic complex were produced. The Gd atoms in 3 are connected through bis-bidentate oxalate and carboxylate−malonate bridges in the anti−anti and anti−syn coordination modes. Compounds
1 and 2 exhibit weak but significant ferromagnetic couplings between the Gd(III) ions through the single (1) and
double (2) oxo−carboxylate bridges, whereas antiferromagnetic interactions across the bis-bidentate oxalate account
for the overall antiferromagnetic behavior observed in 3 and 4.