Effect of pillar modules and their stoichiometry in 3D porous frameworks of Zn(II) with [Fe(CN)6]3-: high CO2/N2 and CO2/CH4 selectivity.

We report the synthesis, single-crystal structural characterization, and selective gas adsorption properties of three new 3D metal-organic frameworks of Zn(II), {[Zn3(bipy)3(H2O)2][Fe(CN)6]2·2(bipy)·3H2O}n (1), {[Zn3(bipy)][Fe(CN)6]2·(C2H5OH)·H2O}n (2), and {[Zn3(azpy)2(H2O)2][Fe(CN)6]2·4H2O}n (3) (bipy = 4,4'-bipyridyl and azpy = 4,4'-azobipyridyl), bridged by Fe(CN)6 and exobidentate pyridyl-based linkers. Compounds 1-3 have been successfully isolated by varying the organic linkers (bipy and azpy) and their ratios during the synthesis at RT. Frameworks 1 and 3 feature a biporous-type network. At 195 K, compounds 1-3 selectively adsorb CO2 and completely exclude other small molecules, such as N2, Ar, O2, and CH4. Additionally, we have also tested the CO2 uptake capacity of 1 and 3 at ambient temperatures. By using the isotherms measured at 273 and 293 K, we have calculated the isosteric heat of CO2 adsorption, which turned out to be 35.84 and 35.53 kJ mol(-1) for 1 and 3, respectively. Furthermore, a reasonably high heat of H2 adsorption (7.97 kJ mol(-1) for 1 and 7.73 kJ mol(-1) for 3) at low temperatures suggests strong interaction of H2 molecules with the unsaturated Zn(II) metal sites and as well as with the pore surface. Frameworks 1 and 3 show high selectivity to CO2 over N2 and CH4 at 273 K, as calculated based on the IAST model. The high values of ΔH(CO2) and ΔH(H2) stem from the preferential electrostatic interaction of CO2 with the unsaturated metal sites, pendent nitrogen atoms of Fe(CN)6, and π-electron cloud of bipyridine aromatic rings as understood from first-principles density functional theory based calculations.