Increase of network hydrophilicity from sql to lvt supramolecular isomers of Cu-MOFs with the bifunctional 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoate linker

Abstract

The bifunctional linker 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (H₂mpba) was used for the synthesis of new (square lattice) sql 2D metal–organic frameworks (MOFs) [Cu(Hmpba)₂]·L (L = DMF or ACN) in a solvent-mixture of dimethylformamide/water and acetonitrile/water. These sql 2D MOFs are supramolecular isomers of the lvt 3D network [Cu(Hmpba)₂]·4MeOH·1H₂O (lvt-MeOH) that was synthesized previously by Richardson and co-workers. All these frameworks are potentially porous structures with solvent molecules included in the channels of the as synthesized materials. After activation all three materials showed good CO₂ adsorption capacity, demonstrated here for lvt-MeOH for the first time, with a saturation uptake of 113 cm³ g⁻¹ (lvt-MeOH-act.), 111 cm³ g⁻¹ (sql-DMF-act.) and 90 cm³ g⁻¹ (sql-ACN-act.) at 195 K. The flexibility of the lvt-MeOH-act. network is evidenced by a gate-opening effect seen in the CO₂ measurement at 195 K and under gravimetric high-pressure CO₂ adsorption. According to the water and ethanol sorption measurements the new sql frameworks can be categorized as hydrophobic materials in contrast to the hydrophilic lvt framework. In the lvt-MeOH structure the crystal solvent can be replaced with water to yield the structurally authenticated water-only network lvt-H₂O containing 3D arrays of S₄-symmetric (H₂O)₂₀ clusters.