Construction of metal-organic frameworks with unsaturated Cu sites for efficient and fast reduction of nitroaromatics: A combined experimental and theoretical study

Metal-organic frameworks (MOFs) functionalized with open metal sites (OMSs) have received widespread attention in various applications due to their fascinating electronic properties and unique interactions with guest molecules. However, rational tailoring of the coordination environment of metal nodes during the synthesis of MOFs remains a great challenge due to their tendency of saturated coordination with linkers. Herein, we reported the construction of three new MOFs featuring unsaturated Cu(II) sites, namely [Cu(HCOO)(pzta)]n (HL-1), {[Cu(PTA)0.5(pzta)(H2O)]·2H2O}n (HL-2) and [Cu(NA)0.5(pzta)]n (HL-3) (Hpzta = 3-pyrazinyl-1,2,4-triazole; PTA = terephthalic acid; NA = 1,4-naphthalene dicarboxylic acid), based on the mixed-linker strategy via specific selection of versatile Hpzta ligand and carboxylate ligands. Remarkably, the obtained MOFs exhibited excellent activity and good recyclability for the catalytic reduction of nitroaromatics under mild conditions (25°C and 1 atm). In particular, the complete conversion of 4-nitrophenol (4-NP) took only 30 s on HL-2, reaching a record-high TOF value compared with previously reported metal catalysts. The combined experimental and theoretical studies on HL-2 revealed that the open Cu site with positive-charged nature could improve the adsorption and subsequent electron transport between the substrates, and was responsible for the outstanding performance. This work shined lights on the further enhancement of performance for MOFs through rational OMSs construction.