Construction of Acylamide-functionalized MOFs for efficient catalysis on the conversion of CO2

By employing a C3-symmetric ligand, 3,3′,3′'-[1,3,5-benzenetriyltris- (carbonylimino)]tris-benzoic acid (H3L), three metal-organic frameworks formulated as {Na4[Zn3(L)2(µ3-OH)(µ2-OH)3]·3DMF}n (1), {[Ca2(L)(MeOH)2(DMF)3]·Cl}n (2) and [Sr2(L)(H2O)2(DMF)Cl]n (3) have been solvothermally synthesized and structurally characterized by IR, PXRD and X-ray single crystal diffraction analyses. Complex 1 features a 2D twofold interpenetrated structure in which the [Zn3(µ3-OH)(µ2-OH)3(CO2)3O3] subunit acts as a 6-connected node and the L3− ligand as a 3-connected node, exhibiting a binodal (3, 6)-connected topology with a {43}2{43.612} point symbol. Complex 2 shows a 2D double-layered structure, exhibiting a (3,6)-connected {43}2{46.66.83} topology based on a centrosymmetric tetranuclear [Ca4(CO2)6] subunit as a 6-connected node and the L3− ligand as a 3-connected node. Complex 3 exhibits an extended 3D framework with a {46}2{412.612.84} topology in which the centrosymmetric {Sr4(CO2)6 [(O^O)acylamide]2} subunit performs as an eight connected node and the L3− ligand as an unusual 4-connected node. Complex 1 is explored well as a catalyst for the cycloaddition reaction of CO2 with epoxides into cyclic carbonates under the reaction conditions of 70°C, 1 bar atmosphere and free solvent. The efficient catalytic ability of 1 perhaps attributes to the high density of Lewis acidic sites of Zn2+ and Lewis basic -NH- groups in 1. Beside the basic function, the -NH- group in 1 can also perform as hydrogen bond donor (HBD) for the activation of epoxide in the catalytic process. A plausible mechanism for synergistic catalysis is proposed for the cycloaddition reaction.