A Ni(salen)-Based Metal-Organic Framework: Synthesis, Structure, and Catalytic Performance for CO2 Cycloaddition with Epoxides

A three-dimensional (3D) chiral metal-organic framework [Cd-2{Ni(salen)}(DMF)(3)]4 center dot DMF center dot 7H(2)O (1) based on a new enantiopure tetracarboxyl-functionalized metallosalen Ni(H(4)salen) {where H(6)salen is (R,R)-N,N-bis[3-tert-butyl-5-(3,5-dicarboxybenzyl)salicylidene]-1,2-diphenylethylenediame} was synthesized and characterized by infrared spectroscopy, thermogravimetric analysis, nitrogen and carbon dioxide adsorption, and powder and single-crystal X-ray diffractions. In 1, the dinuclear Cd-2 cluster [Cd-2(COO)(4)(DMF)(3)] as a node is cross-linked by four isophthalate groups on the salen ligands, forming a 2D lamellar structure, which are further linked by Ni(salen) into the 3D network with a 1D open channel (ca. 7.0 x 8.0 angstrom(2)) along the a axis. On account of its porosity, Lewis acid sites, and moderate uptake for CO2, 1 can be used as an efficient heterogeneous catalyst for the CO2 cycloaddition with epoxides under relatively mild conditions. Moreover, the bulky epoxide shows a decrease in activity with an increase in the alkyl chain length of the substrate as a result of the confinement effect of 1, showing size-dependent selectivity.