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