Carbon Dioxide (CO2) Fixation: Linearly Bridged Zn2 Paddlewheel Nodes by CO2 in a Metal-Organic Framework.

When the reaction of zinc nitrate with 4',4‴,4‴″,4‴‴'-(ethene-1,1,2,2-tetrayl)tetrakis[(1,1'-biphenyl-3-carboxylic acid)] (H4tmpe) in dimethylformamide (DMF) under hydrothermal condition is performed in air or carbon dioxide (CO2), [Zn4(tmpe)2(H2O)2(μ2-CO2)]·8DMF·18H2O (1) crystallizes out. However, if it is in dioxygen, argon, or carbon monoxide, [Zn2(tmpe)(DMF)]·2DMF·8H2O (2) is the product. Both compounds are chemically stable coordination polymers. 1 contains zinc carboxylate paddlewheels as nodes linearly bridged by CO2 into two interpenetrating lattices, and 2 has an infinite single framework formed by a tetranuclear node. 1 is the second example containing the linear CO2-bridged paddlewheel node. Interestingly, CO2 fixation in a μ2-η2O,O bridging mode is observed in 1, which is rarely characterized structurally and has been confirmed using IR and gas chromatography analysis. The stability of 1 is further verified by density functional theory calculations, which found an energy minimum with a Zn-O═C angle of 180°. Both compounds display strong emission around 490 nm and excited-state lifetimes around 2.4 ns.