Metal-Organic Framework-Based Selective Molecular Recognition of Organic Amines and Fixation of CO 2 into Cyclic Carbonates.

Synthesis and structural depiction of two new metal-organic frameworks (MOFs), namely, [{Zn( L )(oba)}·4H 2 O] α (Zn-MOF -1 ) and [{Cd 1/2 ( L ) 1/2 (nipa) 1/2 (H 2 O) 1/2 }·(DMF) 1/2 (H 2 O)] α (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 CO 2 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 π-π stacking interactions. Additionally, cycloaddition of CO 2 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.