Construction and selective gas adsorption properties of two heteroSBU MOFs based on unsymmetrical tetracarboxylate linkers

Abstract

By employing the strategy of positional isomerism, we designed and synthesized two unsymmetrical biphenyl tetracarboxylate ligands, and used them to successfully construct two novel copper-based MOFs. Structural determinations show that the resulting MOFs are three-dimensional networks, each incorporating two different kinds of discrete copper-carboxylate clusters, including the typical dicopper Cu₂(COO)₄(H₂O)₂ paddlewheel, as inorganic SBUs, and they display distinctly different topologies, highlighting the significant effect of ligand desymmetrization. ZJNU-9 is a neutral coordination network composed of one typical dicopper paddlewheel and a new dicopper cluster Cu₂(COO)₄(DMF)₂(H₂O)₂ connected by the organic ligands, while ZJNU-10 is based on one typical dicopper paddlewheel and a new tetracopper cluster Cu₄(COO)₄(OH)₂Cl(H₂O)₄ linked by the organic ligands to a cationic framework containing three types of cages. The two novel copper-containing clusters discovered in the two compounds greatly enrich the diversity and complexity of inorganic copper-carboxylate SBUs and thus MOF structures. Furthermore, a systematical investigation of their selective adsorption properties with respect to C₂H₂, CO₂, and CH₄ through pure-component isotherm measurements and IAST predictions revealed their promising potential for C₂H₂ recovery and CO₂ removal from CH₄ under ambient conditions. This work represents a rare example of homometallic pure-carboxylate hetero-SBU MOFs exhibiting highly selective adsorption of C₂H₂ and CO₂ over CH₄.