Ag2[Cr3O(OOCC2H5)6(H2O)3]2[alpha-SiW12O40] [1] is a nonporous flexible ionic crystal composed of 2D-layers of polyoxometalates ([alpha-SiW12O40](4-)) and macrocations ([Cr3O(OOCC2H5)6(H2O)3](+)) stacking along the b-axis. The silver ions are located in the vicinity of the oxygen atoms of the polyoxometalates. The sorption amounts of small unsaturated hydrocarbons such as ethylene, propylene, n-butene, acetylene, and methyl acetylene into 1 are comparable to or larger than 1.0 mol mol(-1) and large hystereses are observed, while those of paraffins and larger unsaturated hydrocarbons are smaller than the adsorption on the external surface (<0.2 mol mol(-1)). Fine crystals of 1 exhibit ethylene/ethane and propylene/propane sorption ratios over 100 at 298 K and 100 kPa, and the values are larger by 1 order of magnitude among those reported. The results of sorption kinetics, in situ IR spectroscopy, single crystal X-ray crystallography, and in situ powder XRD studies show that small unsaturated hydrocarbons penetrate into the solid bulk of 1 through the pi-complexation with Ag(+). The sorption property of 1 is successfully applied to the collection of ethylene from the gas mixture of ethane and ethylene.

The design and syntheses of porous materials such as zeolites and metal-organic frameworks (MOFs) are areas of intense research because of their unique properties in gas storage, separation, and heterogeneous catalysis.[1] Crystalline microporous zeolites show shape-selective adsorption properties because the sizes of the channel apertures formed by the covalently bonded [TO 4 ] (T= Si, Al, P, Ti, etc.) and/or [MO 6 ] (M = V, Mn, Mo, etc.) units can be controlled.

The design and syntheses of porous materials such as zeolites and metal-organic frameworks (MOFs) are areas of intense research because of their unique properties in gas storage, separation, and heterogeneous catalysis. [1] Crystalline microporous zeolites show shape-selective adsorption properties because the sizes of the channel apertures formed by the covalently bonded [TO 4 ] (T= Si, Al, P, Ti, etc.) and/or [MO 6 ] (M = V, Mn, Mo, etc.) units can be controlled. [1e,l-n] In contrast, the pore sizes of MOFs, which are constructed from coordinatively bonded metal ions and organic ligands, can be controlled by the lengths and functional groups of organic ligands. [1j,k] In particular, the use of aromatic units induces p-p interactions [2] and enables the rational control of the complexation of the building units. [3] In addition, the presence of the aromatic moieties means that the resulting framework would show unique structural and guest-sorption properties. For example, the guest sorption in a Cu II dihydroxybenzoate/4,4'-bipyridine MOF [3c] causes the gliding of p-p-stacked building units, which leads to the change in the framework structure, and a Co II benzenetricarboxylate MOF [3e] separates the aromatic units from the aliphatic hydrocarbons.Trinuclear metal carboxylates have often been used as building units for the construction of MOFs, [1i, 4] which mostly contain dicarboxylates as bridging ligands to connect the trinuclear metal units, and do not show p-p interactions among the building units. [1i, 4a-c] The use of aromatic units as terminal ligands of trinuclear metal carboxylates would induce p-p interactions, which lead to the unique structural and guest sorption properties. Based on these considerations, we have used pyridine as a terminal ligand of the trinuclear metal carboxylate macrocation [Cr 3 O(OOCH) 6 (pyridine) 3 ] + in this work. Ionic crystals normally possess densely packed crystal lattices because of the strong and isotropic coulombic interaction, and thus are nonporous.