Keggin-Type Tridecanuclear Europium-Oxo Nanocluster Protected by Silsesquioxanes

The
Keggin structure, known as an archetypical polyoxometalate
(POM) structural motif, has widened up our fundamental understanding
of the growth behavior of natural metal oxide minerals. However, devising
a strategy for synthesis of such species consisting of pure-metal
ions remains a great challenge. Different from those six-coordinated
early transition metal ions in Keggin POMs, lanthanide prefers higher
coordination numbers, making the construction of lanthanide-Keggin
structure more difficult. Here, we report the unprecedented luminescent
Keggin-type Eu13 nanocluster constructed through a facile
solvothermal method using the high-symmetry macrocyclic tetrasilane
(Ph4Si4O8)4– as
the capping ligand. This protocol demonstrates an important proof
of concept for assembly of metallasilsesquioxanes. The structure shows
a rare polyoxometalate-Eu-oxo-core@SiO2-shell@organic-shell
(Eu13) consisting of a central EuO8 cube,
surrounded by 12 edge-sharing EuO7 monocapped trigonal
prisms. It is the highest-nuclearity lanthanide–silsesquioxane
nanocluster. Electrospray ionization mass spectrometry reveals that Eu13 is stable in solution except for some labile terminal
dimethylformamide on the ligand shell, which endows Eu13 with a high catalytic activity for CO2 cycloaddition
reaction with small epoxides under ambient conditions. Eu13 also exhibits intense red luminescence upon visible light excitation
(466 and 535 nm). The photophysical and photoelectric properties of
the drop-casting film of Eu13 indicate its great potential
in optoelectric device applications.