Rectangle versus Square Oxalate-Connective Tetralanthanide Cluster Anchored in Lacunary Lindqvist Isopolytungstates: Syntheses, Structures, and Properties

Two types of unique oxalate-connective
lanthanide-substituted isopolyoxotungstates,
Na10[Ln2(C2O4)­(H2O)4(OH)­W4O16]2·30H2O (1) and K4Na16[Ln­(C2O4)­W5O18]4·60H2O (2) (Ln = EuIII, HoIII, ErIII, or TbIII), have been synthesized under
conventional aqueous solution conditions and structurally characterized
by elemental analyses, IR spectra, single-crystal X-ray diffraction,
and thermogravimetric analyses. It should be pointed out that the
utilization of different alkaline cations leads to the formation of
two structural types. When only Na+ ions are present in
the system, type 1 was obtained, while when Na+ and K+ ions are used, type 2 was found.
Complex 1 is a double-oxalate-bridging di-Ln substituted
Lindqvist dimer with a rectangle tetra-Ln cluster, whereas 2 is a single-oxalate-connective mono-LnIII Lindqvist tetramer
with square tetra-Ln cluster. As far as we know, such di-Ln substituted
Lindqvist fragment in 1 is observed for the first time.
Moreover, 2 represents the first organic–inorganic
hybrid square Ln-substituted isopolyoxotungstate. The solid-state
luminescent properties of 1-Eu, 1-Tb, 2-Eu, and 2-Tb have been measured. 1-Eu and 2-Eu display intense, sharp, and narrow emission
bands in the orange visible region that originate from the characteristic 5D0 → 7FJ transitions, and their fluorescence lifetimes are 1.18 and
1.20 ms, respectively. 1-Tb and 2-Tb exhibit
green photoluminescence mainly derived from 5D4 → 7F5 transitions. The decay behavior
of 1-Tb can be fitted to a biexponential function with
lifetimes of τ1 = 0.43 ms and τ2 = 1.25 ms, whereas the decay behavior of 2-Tb can be
fitted to single exponential function with the lifetime of 1.03 ms.
Magnetic susceptibilities of 1 and 2 have
been measured, and the decline of χMT upon cooling for 1 and 2 is mostly related
to the progressive thermal depopulation of the excited state of Ln
cations.