Metal-Organic Framework-Based Selective Molecular Recognition of Organic Amines and Fixation of CO2 into Cyclic Carbonates

Synthesis and structural depiction of two new metal-organic frameworks (MOFs), namely, [{Zn(L)(oba)}center dot 4H(2)O](alpha) (Zn-MOF-1) and [{Cd-1/2(L)(1/2)(nipa)(1/2)(H2O)(1/2)}center dot(DMF)(1/2)(H2O)](alpha) (Cd-MOF-2) (where L = N-2,N-6-di(pyridin-4-yl)naphthalene-2,6-dicarboxamide, 4,4'-H(2)oba = 4,4'-oxybisbenzoic acid, and 5-H(2)nipa = 5-nitroisophthalic acid) are reported. Both Zn-MOF-1 and Cd-MOF-2 have been prepared by reacting ligand L and coligand 4,4'-H(2)oba or 5-H(2)nipa with the respective dihydrates of Zn(OAc)(2) and Cd(OAc)(2) (OAc = acetate). Crystal structure X-ray analysis discloses that Zn-MOF-1 displays an overall 2D -> 3D interpenetrated framework structure. The topological analysis by ToposPro suggests a (4)-connected uninodal sql topology with a point symbol of {4(4).6(2)} having 2D + 2D parallel polycatenation. However, crystal packing of Cd-MOF-2 adapted a porous framework architecture and was topologically simplified as (3,4)-connected binodal 2D net. In addition, both Zn-MOF-1 and Cd-MOF-2 were proved to be multifunctional materials for the recognition of organic amines and in the fixation of CO2 to cyclic carbonates. Remarkably, Zn-MOF-1 and Cd-MOF-2 showed very good fluorescence stability in aqueous media and have shown 98 and 97% quenching efficiencies, respectively, for 4-aminobenzoic acid (4-ABA), among all of the researched amines. The mechanistic study of organic amines recognition proposed that fluorescence quenching happened mainly through hydrogen-bonding and pi-pi stacking interactions. Additionally, cycloaddition of CO2 to epoxide in the presence of Zn-MOF-1 and Cd-MOF-2 afforded up to 96% of cyclic carbonate within 24 h. Both Zn-MOF-1 and Cd-MOF-2 exhibited recyclability for up to five cycles without noticing an appreciable loss in their sensing or catalytic efficiency.

Automated summary

This study synthesized and described two new metal-organic frameworks (MOFs) and investigated their structures and properties. The results showed that both MOFs exhibited multifunctionality, being able to recognize organic amines and fix CO2 into cyclic carbonates. Additionally, they displayed good fluorescence stability in aqueous media and high quenching efficiency for certain amine compounds. The mechanism of amine recognition was proposed to involve hydrogen bonding and pi-pi stacking interactions. Furthermore, both MOFs showed high catalytic efficiency in the cycloaddition reaction and were recyclable for multiple cycles without significant loss of their sensing or catalytic efficiency.