Construction of 3-Fold-Interpenetrated Three-Dimensional Metal–Organic Frameworks of Nickel(II) for Highly Efficient Capture and Conversion of Carbon Dioxide

Abstract

A series of three new isostructural metal-organic frameworks (MOFs) of nickel(II), [{Ni(muco)(bpa)(2H₂O)}·2H₂O] (1), [{Ni(muco)(bpe)(2H₂O)}·2.5H₂O] (2), and [{Ni(muco)(azopy)(2H₂O)}·2H₂O] (3) [where muco = trans,trans-muconate dianion, bpa = 1,2-bis(4-pyridyl)ethane, bpe = 1,2-bis(4-pyridyl)ethylene, and azopy = 4,4'-bis(azobipyridine)], have been synthesized and characterized by single-crystal X-ray diffraction analysis and other physicochemical methods. Compounds 1-3 exhibit an interesting 3-fold-interpenetrated three-dimensional pillar-layered framework structure constituted of 4-coordinating (4-c) Ni^II nodes with {6⁶}-neb net topology. Remarkably, in spite of 3-fold interpenetration, the structures possess one-dimensional channels with dimensions of ∼8.05 × 5.25 Ų. Gas (N₂, Ar, H₂, and CO₂) adsorption studies of compounds 2 and 3 revealed selective adsorption properties for CO₂ over other gases. In all three structures, the 4-c Ni^II node has two coordinated H₂O molecules that can be reversibly removed by high-temperature treatment to generate a dehydrated framework composed of highly unsaturated, Lewis acidic Ni^II ions. Further, the activated compounds of 1-3 act as efficient recyclable catalysts for heterogeneous cycloaddition of CO₂ with styrene oxide, resulting in cyclic carbonate with high conversion and selectivity. Interestingly, the cycloaddition reactions of CO₂ with bulky epoxides show a decrease in the activity with an increase in the alkyl chain length of the substrate due to confinement of the pore size of the MOF. The high catalytic efficiency and size-dependent selectivity for smaller epoxides show the potential utility of 1 as a promising heterogeneous catalyst for the cycloaddition of CO₂. Furthermore, the catalyst can be easily separated and reused for several cycles without significant reduction in the catalytic activity as well as structural rigidity. Compounds 1-3 represent rare examples of interpenetrated MOFs exhibiting selective storage and conversion of CO₂ at mild conditions.