Tuning the Structural Flexibility for Multi-Responsive Gas Sorption in Isonicotinate-Based Metal-Organic Frameworks

Flexible metal-organic frameworks (MOFs) are of high interest as smart programmable materials for gas sorption due to their unique structural changes triggered by external stimuli. Owing to this property, which leads to opportunities such as maximizing deliverable gas capacity, flexible MOFs sometimes offer more advantages in sorption applications compared to their more rigid counterparts. Herein, we elucidate the effect of transition metal identity of a series of isonicotinate-based flexible MOFs, M(4-PyC)₂ [M═Mg, Mn, and Cu; 4-PyC = 4-pyridine carboxylic acid] on the structural dynamic response to different gases (C₂H₄, C₂H₆, Xe, Kr, and SO₂). Isotherms at different temperatures show that C₂H₄, C₂H₆, and Xe can form sufficiently strong interactions with both Mg(4-PyC)₂ and Mn(4-PyC)₂ frameworks resulting in gate-opening behavior due to the rotation of the linker's pyridine ring, while Kr cannot induce this phenomenon for the two MOFs under the measured conditions. In contrast, the gate-opening behavior occurs for Cu(4-PyC)₂ solely in the presence of C₂H₄, and no other measured gas, due to the open metal sites of Cu centers. In comparison, SO₂, a strong polar molecule, triggers the gate-opening effect in all three MOFs. Interestingly, a shape memory effect is observed for Cu(4-PyC)₂ during the second SO₂ sorption cycle. When comparing the different gate-opening pressures of each gas, we observed that the structural flexibility of the three frameworks is highly sensitive to the chemical hardness of the Lewis acidic metal ions (Mg²⁺ > Mn²⁺ > Cu²⁺). As a result, the gate opening behavior is observed at lower pressures for the MOFs containing weaker M-N bonds (harder metal ions), with the exception of Cu(4-PyC)₂ toward C₂H₄. These observations reveal that different transition metals can be used to finely control the structural flexibility of the frameworks.