Organosilicon compounds (siloxanes) underlie many materials widely used in various fields of human activity, from the aerospace industry to medicine and personal care products. This is due to their resistance to large temperature changes (from –100oC to 500oC and more), water-repellent properties, the ability to pass gases and biocompatibility.

In addition, siloxanes can potentially be used as part of metal-organic frameworks. These are new-generation porous materials that consist of two types of building blocks: organic molecules of various structures and metal ions. By combining such blocks, it is possible to obtain materials with pores of different sizes and other characteristics. Metal-organic frameworks can be used for gas separation, storage and delivery of medicines, absorption of oils and dyes, acceleration of reactions (as catalysts) and other purposes. The use of water-repellent and thermally stable siloxanes instead of organic compounds as blocks may make it possible to transfer these properties to final materials.

Researchers from the A.N. Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences (Moscow) and colleagues have synthesized metal-organic frameworks based on symmetrical organosilicon acid for the first time. This molecule is interesting because it is based on a ring of silicon and oxygen, with four acid groups on one side and four organosilicon groups with water—repellent properties on the other. This makes it possible to form bonds with metal ions necessary for the construction of metal-organic frameworks, and to impart water-repellent properties to the resulting compounds.

"We have obtained metal-organic frameworks based on this acid and various metal ions. One of the important results was the production of a copper-containing material with a unique set of properties. It turned out that simple mixing of alcoholic solutions of copper salt and organosilicon acid, as well as subsequent drying, make it possible to obtain a turquoise powder. This porous material has a very low density (100 times lower than that of water), and therefore can be characterized as a metal-organic aerogel," says Sergey Kutumov, a participant in the project supported by an RNF grant, a junior researcher at INEOS RAS.

The resulting aerogel is stable at high temperatures and also easily changes color to blue when heated to 80°C. This is due to the fact that the material loses water molecules, which are normally "trapped" next to copper ions. If, after heating, the aerogel is placed in a humid environment, then the reverse color change to turquoise occurs. This property is convenient to use for visual detection of moisture, for example, in packages for materials sensitive to it, including electronic components. In addition, the resulting aerogel can serve as a reusable catalyst (accelerator) for various chemical reactions.

"In the future, we plan to continue research on metal-organic frameworks based on organosilicon compounds. New materials will be obtained and their properties investigated, as well as solutions for finer tuning of their characteristics will be developed. We hope that this will increase the efficiency of our materials and allow us to move on to solving larger—scale applied problems, such as industrial wastewater treatment or moisture collection in arid conditions," says Ashot Arzumanyan, PhD, Head of the Laboratory of Functional Organoelement Compounds at INEOS RAS, head of the project supported by the Russian Science Foundation grant.

The results of the study, supported by a grant from the Russian Science Foundation (RSF), are published in the journal Organic Chemistry Frontiers.