Breathing layered metal–organic frameworks based on 1,3-bis(imidazolyl)propane

Metal–organic frameworks are a class of porous materials that are widely used in various fields of science, such as gas sorption and separation, sensing, catalysis, and others.

Currently, most MOFs are created on the basis of hard aromatic carboxylic acids or N-donor ligands. However, ligands with flexible aliphatic bridges that provide conformational mobility to the ligand are promising. This mobility can be transferred to the MOF, or at least the flexibility of such a ligand can lead to its variability and atypical structure. Such structures may exhibit unusual gas adsorption properties.

Layered MOFs have a special feature—their layers are not bound by rigid ligands, so they can "breathe", that is, they can move relative to each other. In the initial state, a MOF may be a low-porosity or non-porous compound, but at gas pressure, it begins to show a "gate-opening" effect, which can lead to the selective adsorption of gases.

Our work is devoted to the study of layered MOFs based on 1,3-bis(imidazolyl)propane (bip) containing a -C3H6-alkyl group. Due to its conformational mobility, bip acts as a bridging ligand that connects two metal atoms into one secondary building block (SBU). Secondary building blocks M (bip)₂²⁺ (M = Cd, Cu) form a layered mobile structure with the help of functionalized terephthalic acids (bdc-NO₂).

The sorption characteristics of these compounds are of great interest to the research community. As part of our work, we investigated the adsorption of industrially important hydrocarbons such as methane, ethane, ethylene, acetylene, propane, and propylene at different temperatures. Using the example of C₃-hydrocarbons, we show an anomalous dependence of the amount of adsorbed gas on temperature. Usually, as the temperature increases, the volume of sorbed gas decreases. In the case of the studied compounds, an inverse, more complex dependence is observed.