Interpenetrated Metal–Organic Frameworks of Cobalt(II): Structural Diversity, Selective Capture, and Conversion of CO₂

Abstract

Three new metal–organic frameworks (MOFs) of Co(II), [{Co(muco)(bpa)(2H2O)}·2H2O]n (1), [{Co(muco)(azopy)}·H2O]n (2), and [{Co(muco)(4bpdp)}·2DMF·H2O]n (3) (where, muco = trans, trans-muconate dianion, bpa = 1,2-bis(4-pyridyl)ethane, azopy = 4,4′-bisazobipyridine, and 4bpdb = 1,4- bis(4-pyridyl)-2,3-diaza-1,3-butadiene) have been synthesized and structurally characterized by single-crystal X-ray diffraction analysis. Structural analysis revealed that MOF1 exhibits an interesting 3-fold interpenetrated three-dimensional (3D) pillar-layered framework structure with 4-c, {66} topology, while 2 and 3 possess a 2-fold interpenetrated 3D pillar-layered framework structure with 7-c, {3∧3.4∧13.5∧4.6} net topology. Gas adsorption study of the compounds shows that 1 exhibits selective adsorption of CO2 over other (H2, N2, and Ar) gases with an uptake higher than those of 2 and 3. Further, the metal coordinated water molecules in MOF1 can be reversibly removed by high temperature treatment to generate a dehydrated framework of 1 composed of highly unsaturated Lewis acidic Co(II) ions projected in the one-dimensional pore channels. Interestingly, the activated framework of 1 acts as an efficient recyclable catalyst for heterogeneous cycloaddition of carbon dioxide with epoxides to generate cyclic carbonates with high yield and selectivity even at mild conditions (1 atm of CO2). Furthermore, the catalyst can be easily recycled and reused for four successive cycles without significant loss of catalytic activity and structural rigidity. Thus, the influence of N,N′-donor spacers on the structure, topology, and the functionality of MOFs has been presented.