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)- (2H 2 O)} · 2H 2 O] ( 1 ), [{Ni(muco)(bpe)(2H 2 O)} · 2.5H 2 O] ( 2 ), and [{Ni(muco)(azopy)(2H 2 O)} · 2H 2 O] ( 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 di ff raction 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 {6 6 }-neb net topology. Remarkably, in spite of 3-fold interpenetration, the structures possess one-dimensional channels with dimensions of ∼ 8.05 × 5.25 Å 2 . Gas (N 2 , Ar, H 2 , and CO 2 ) adsorption studies of compounds 2 and 3 revealed selective adsorption properties for CO 2 over other gases. In all three structures, the 4-c Ni II node has two coordinated H 2 O 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 e ffi cient recyclable catalysts for heterogeneous cycloaddition of CO 2 with styrene oxide, resulting in cyclic carbonate with high conversion and selectivity. Interestingly, the cycloaddition reactions of CO 2 with bulky epoxides show a decrease in the activity with an increase in the alkyl chain length of the substrate due to con fi nement of the pore size of the MOF. The high catalytic e ffi ciency and size-dependent selectivity for smaller epoxides show the potential utility of 1 as a promising heterogeneous catalyst for the cycloaddition of CO 2 . Furthermore, the catalyst can be easily separated and reused for several cycles without signi fi cant 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 CO 2 at mild conditions.