An anionic metal–organic framework based on angular tetracarboxylic acid and a mononuclear copper ion for selective gas adsorption

Abstract

An anionic metal–organic framework [CuL]·2(CH₃)₂NH₂·DMF (ZJNU-55, L = 5,5′-(pyrimidin-2-methyl-4,6-diyl)diisophthalate) was synthesized by a solvothermal reaction of a custom-designed angular diisophthalate and CuCl₂·2H₂O, and structurally characterized by single-crystal X-ray diffraction. Notably, although the ligand bears a close resemblance to the previously reported diisophthalate used to construct PCN-307, the resultant MOF's structure is totally different from that of PCN-307. The framework exhibits a mononuclear Cu(COO)₄ inorganic secondary building unit, instead of the typical dinuclear Cu₂(COO)₄ paddlewheel unit. The combination of Cu(COO)₄ with the deprotonated organic ligand results in a three-dimensional network structure featuring helical nanotube channels along the crystallographic c axis. TOPOS software analyses indicate that two alternative simplifications based on the organic linkers can produce a 4-connected binodal net with the Schläfli symbol (4·6²·8³)(4·6⁴·8) or a (3,4)-connected 3-nodal network with the Schläfli symbol (6·8·9)₂(6·8³·9²), which have been rarely observed in MOF chemistry. Furthermore, its gas adsorptive properties were systematically investigated. The activated ZJNU-55a exhibits highly selective adsorption of C₂H₂ and CO₂ over CH₄ with the adsorption selectivities of 39.2–64.9 and 13.1–29.2 at 298 K, respectively. As far as we are aware, this is a rare example of a MOF constructed from mononuclear Cu(COO)₄ for selective gas adsorption. This work also demonstrates that subtle ligand modification can drastically affect the structures and properties of the resultant MOF.