Synthesis and structural characterisation of CuII-based MOFs constructed by combining functionalised 1,4-bis(1H-imidazol-1-Yl)benzene ligands with copper sulfate†

Anion-pillared metal–organic frameworks (APMOFs) are a class of coordination polymers in which divalent anions connect two adjacent layers of a 2D network generated by neutral ligands and cationic metal centres. This class of MOFs is commonly composed by materials built using pyridine-based ligands and octahedral fluorinated anions as linear pillars. Only recently, the use of nitrogen based five-membered ring ligands and/or tetrahedral anions, such as sulfate, has been reported. The combination of 1,1′-(2-(trifluoromethyl)-1,4-phenylene)bis(1H-imidazole) (bibCF₃) ligand with copper(II) sulfate generates a 2D MOF (UdP-20) of minimal formula [CuSO₄(bibCF₃)_1.5]·3H₂O. UdP-20 possesses a square lattice (sql) topology in which the copper sulfate dimer molecular building block acts as 4-connected nodes and exhibits flexibility, transitioning to a closed pore phase (UdP-20-cp) upon heating. The use of different functionalised bib ligands allowed us to obtain three new examples of APMOFs: UdP-21 with 1,1′-(2-chloro-1,4-phenylene)bis(1H-imidazole) (bibCl), UdP-22 with 1,1′-(2-methyl-1,4-phenylene)bis(1H-imidazole) (bibMe) and UdP-23 with 1,1′-(2-methoxy-1,4-phenylene)bis(1H-imidazole) bibOMe. All these materials possess 2D layers connected to each other by bridging sulfate anions leading to an overall 3D anion pillared framework. All the herein reported materials were characterised by elemental analysis, thermogravimetric analysis, and powder and single crystal X-ray diffraction. Moreover, their CO₂ adsorption behaviour was investigated, revealing results in alignment with what is already reported in the literature for similar materials in the case of UdP-20, while UdP-21, UdP-22 and UdP-23 exhibited moderate CO₂ adsorption, due to transition to denser phases upon activation.