Tetranuclear Lanthanide(III) Complexes with a Zigzag Topology from the Use of Pyridine-2,6-dimethanol: Synthetic, Structural, Spectroscopic, Magnetic and Photoluminescence Studies

Reaction between
Ln­(NO3)3·xH2O (x = 5 or 6) and the potentially tridentate (N,O,O)
chelating/bridging ligand pyridine-2,6-dimethanol (pdmH2), in the presence of base NEt3, affords a family of isostructural
tetranuclear [LnIII4(NO3)2(pdmH)6(pdmH2)2]­(NO3)4 (LnIII = EuIII, GdIII, TbIII, DyIII, HoIII, ErIII,
YbIII) complexes with a rare zigzag topology. All complexes
contain a [Ln4(μ-OR)6]6+ core
with bridging ligation provided by the alkoxido arms of six η1:η1:η2:μ pdmH– groups. The LnIII ions are eight coordinate
with distorted geometries. Direct current magnetic susceptibility
studies revealed predominant weak antiferromagnetic exchange interactions
between the metal centers, which were quantified in the case of isotropic
GdIII4 to give J = −0.09(1)
cm–1 and g = 2.00(1). The observation
of out-of-phase (χ″M) ac susceptibility signals
suggested that the DyIII4 analogue might be
a molecular nanomagnet. Solid-state photoluminescence studies showed
that the EuIII4 and TbIII4 compounds exhibit intense, sharp, and narrow emission bands in the
red and green visible regions, respectively, which arise from the
characteristic 5D0 → 7FJ and 5D4 → 7FJ transitions. The combined results
demonstrate the ability of pdmH2 ligand to yield homometallic
4f clusters with interesting magnetic and optical properties.