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Cagey structures add a new dimension to silicophosphates

Adamantane-like molecule could become a favorite porous material for a range of applications

by Stephen K. Ritter
September 11, 2017 | A version of this story appeared in Volume 95, Issue 36

Silicophosphates are network compounds with Si–O–P linkages that form highly porous materials with enormous surface area. These materials, known as xerogels, are of interest for their possible use in making ion-conducting films for sensors, catalyst supports, biocompatible materials such as dental cement, and storage materials for confining nuclear waste. But because silicophosphates are made from moisture-sensitive starting materials and tend to be moisture sensitive themselves, their reproducible synthesis has been a challenge and only a few examples are known. Jiri Pinkas and colleagues at Masaryk University in the Czech Republic have been perfecting condensation reactions of acetoxysilanes and trimethylsilyl phosphates under anhydrous conditions to make molecular silicophosphates. In its latest work, the Pinkas group has built an adamantane-like cage molecule that has not been seen before (Inorg. Chem. 2017, DOI: 10.1021/acs.inorgchem.7b01572). When the researchers treated the silicophosphate with the Lewis acid B(C6F5)3, they found that boron coordinates to the phosphate groups to form two new compounds. One compound has three borane-phosphate pairs, but one of its P=O groups remains free and is inverted to the center of the cage, giving rise to threefold symmetry—the molecule is chiral with two enantiomers. The second compound undergoes reorganization of the Si–O–P linkages to form yet another new but smaller silicophosphate cage. The researchers anticipate that ongoing study of the synthesis, structure, and bonding of the silicophosphates will lead to additional design ideas for functional materials.


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