Large Differences in Carbon Dioxide and Water Sorption Capabilities in a System of Closely Related Isoreticular Cd(II)-based Mixed-Ligand Metal-Organic Frameworks.

Abstract

We report the synthesis of two isoreticular, mixed-ligand metal-organic frameworks (MOFs), [Cd(μ2-mia)(μ2-bpe)]n·n(DMF)0.5·n(H2O)0.5 (1) and [Cd(μ2-nia)(μ2-bpee)]n·nDMF (2), where mia = 5-methoxyisophthalate, nia = 5-nitroisophthalate, bpe = 1,2-bis(4-pyridyl)ethane, bpee = 1,2-bis(4-pyridyl)ethylene, and DMF = N,N'-dimethylformamide. Variable-temperature powder X-ray diffraction studies confirmed that both MOFs remain crystalline with activation at high temperatures. Variable-temperature single-crystal X-ray diffraction studies were performed on 1, 2, and a previously published, isoreticular structure, [Cd(μ2-nia)(μ2-bpe)1.5]n·nDMF0.84 (3). These studies show that upon desolvation that monocrystallinity is retained to significantly higher temperatures for 2 and 3 when compared to 1 for which only a partially desolvated crystal structure could be obtained. Carbon dioxide sorption is negligible for 1 at 195 and 298 K, while it is higher for 2 than 3 at 298 K and reversed at 195 K. Water vapor sorption increases in the order 1, 2, and 3. On the contrary, water liquid sorption was significantly higher for 1 when compared to 2. The variable-temperature structures of the (partially) desolvated forms of 1, 2, and 3 give some insight into the reasons for the remarkably different gas, vapor, and liquid sorption properties.