Metal-oxo clusters offer an opportunity to assemble inorganic and metal-organic frameworks (MOFs) by a controlled building-block approach, which led to the revolutionary discoveries of zeolites and MOFs. Polyoxometalate clusters are soluble in water, but more challenging to link into frameworks; the inert oxo-caps that provide solubility are resistant to replacement or further connectivity. We demonstrate how the unique directional bonding and varying basicity of the decaniobate ([Nb10 ]) oxo-caps can be exploited to build 1D, 2D, and 3D inorganic frameworks. In nine structures, A+ (A=Li, Na, K, Rb and Cs), AE2+ (AE=Ca, Sr, Ba) and Mn2+ demonstrate that the dimensionality of the obtained material is controlled by cation charge and size. Increased cation charge decreases selectivity for oxo-site bonding, leading to higher dimensional linking. Larger cation radii also decreases bonding selectivity, yielding higher dimensional materials. Ion-exchange studies of the A+ -Nb10 family shows exclusive selectivity for Cs+ over other alkalis, which is important for radioactive Cs removal and sequestration.
Keywords: basicity; directional bonding; ion exchange; polyoxometalates; polyoxoniobates.
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