Directional Bonding in Decaniobate Inorganic Frameworks.

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 ([Nb ]) oxo-caps can be exploited to build 1D, 2D, and 3D inorganic frameworks. In nine structures, A (A=Li, Na, K, Rb and Cs), AE (AE=Ca, Sr, Ba) and Mn 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 -Nb family shows exclusive selectivity for Cs over other alkalis, which is important for radioactive Cs removal and sequestration.