Crystalline and Amorphous MOFs Based on an Amphiphilic Cyclic Stereoregular p-Carboxyphenylsiloxane: Synthesis, Structures and Properties

Abstract

The use of the organosiloxane compound, all-cis-tetra(p-carboxyphenyl)-cyclotetrasiloxane (A) as a ligand for the construction of MOFs was studied. A has a stereoregular cyclic structure and amphiphilic nature due to 4 hydrophilic carboxyl and 4 hydrophobic trimethylsiloxy groups attached to the opposite sides of the cyclosiloxane. In spite of the well-defined configuration of A, the flexible organosiloxane skeleton allows the ligand to adopt various conformations, as illustrated by obtaining a large family of crystalline MOFs (10 examples). The structural features of these organosiloxane MOFs were studied in depth using a combination of analytic physical methods (SCXRD, PXRD, MAS SSNMR, XAS, SEM, TEM, etc.) using Cu-containing MOF (B) as an example. Then, using acquired understanding, amorphous Cu-containing MOFs, with metal-organic aerogel (MOA) being the brightest example, were obtained. Its structure was confirmed by comparison with the well-defined structure of MOF B and other related products. MOA has a very low density (0.01 g/mL) and a moderate specific surface area, as well as being highly hydrophobic, chemically and thermally stable. For example, MOA could be used as an active and recyclable catalyst for C–N cross-coupling and aerobic oxidation. In addition, it can be evenly dispersed in a hydrophobic matrix (PDMS), allowing it to overcome challenges in obtaining homogeneous and isotropic monoliths. Due to the mobile organosiloxane skeleton of the ligand А, it is possible to obtain breathing and stimulus-responsive materials, e.g., repeatedly recyclable (at least 60 times) moisture indicators. Another important advantage is the resource- and energy efficiency of both A (in just two key synthetic steps under mild conditions and using inexpensive and commercially available reagents) and A-based MOFs (at room temperature, 1 atm, air) synthesis. These results demonstrate the high potential of stereoregular cyclic organosiloxane ligands for designing CP and MOF structures and the creation of a new-generation of stable, multifunctional, and smart hybrid materials.