Highly selective CO(2) capture in flexible 3D coordination polymer networks.

Carbon dioxide capture has been in the center of interests in the scientific community in recent years because of the implications for global warming, and the development of efficient methods for capturing CO2 from industrial flue gas has become an important issue. It has been revealed that coordination polymer networks (CPNs) with channels or pores can be applied in gas storage, 2] gas separation, ion exchange, and selective adsorption of organic or inorganic molecules. 5–7] It has been reported that large amounts of CO2 can be adsorbed in some CPNs, for example 20 wt% at 195 K and 1 bar in [{Cu(pyrdc)(bpp)}2]n (pyrdc = pyridine2,3-dicarboxylate, bpp = 1,3-bis(4-pyridyl)propane), 16 wt % at 298 K and 50 atm in [Cu(dhbc)2(4,4’-bpy)] (dhbc = 2,5-dihydroxybenzoate, bpy = bipyridine), 114 wt% at 195 K and 1 bar in SNU-6, 150 wt % at 298 K and 42 bar in MOF-177, and 176 wt % at 303 K and 50 bar in MIL-101c. However, the selective capture of CO2, in particular at ambient temperature and pressure, from industrial emission streams that contain other gases such as N2, CH4, and H2O still remains challenging. [1c,10, 11] Our approach to selectively capturing CO2 with porous materials is based on the construction of highly flexible 3D coordination polymer networks whose channels or pores open and close depending on the gas type. Compared to N2 and H2, we expected that CO2 would interact with the host network more efficiently because of its quadrupole moment ( 1.4 10 39 C m) and open up channels that are closed for other gases. Our design strategy for flexible 3D networks is to use 2D grids formed from square-planar Ni macrocyclic complexes as linear linkers 6, 7a,b, 12] and 1,1’-biphenyl-3,3’,5,5’tetracarboxylate (bptc ) as a square organic building block, and then to connect the 2D grids with highly flexible alkyl pillars by utilizing alkyl-bridged Ni bismacrocyclic complexes such as [Ni2L ](ClO4)4 (A) and [Ni2L ](ClO4)4·8 H2O (B, Scheme 1). We previously prepared a 2D pillared bilayer network that behaves like a sponge from other Ni bismacrocyclic complexes and 1,3,5-benzenetricarboxylate. The formation of either 2D or 3D pillared network depends on how the bismacrocyclic complex connects 2D planes (Chart S1 in the Supporting Information), which is affected by the pore size of the 2D layer and the steric hindrance between the pillars. Herein, we report two flexible 3D coordination polymer networks, [(Ni2L )(bptc)]·6H2O·3DEF (1, DEF = N,N-diethylformamide) and [(Ni2L )(bptc)]·14H2O (2), which exhibit highly selective CO2 adsorption over N2, H2, and CH4 gases as well as thermal stability up to 300 8C and air and water stability. The CO2 adsorption isotherms of 1 and 2 show gate opening and closing phenomena as well as hysteretic desorption, which allow efficient CO2 capture, storage, and sensing. Compounds 1 and 2 are the first 3D pillared networks assembled from Ni bismacrocyclic complexes. The self-assembly of A and H4bptc in DEF/H2O/TEA (2:3:0.16, v/v; TEA = triethylamine) yielded violet crystals of 1. The self-assembly of B and Na4bptc in DEF/H2O (1:4, v/v) afforded 2. Compounds 1 and 2 are insoluble in water and common organic solvents such as MeOH, EtOH, MeCN, chloroform, acetone, toluene, dimethylformamide, and dimethylsulfoxide. In the X-ray crystal structure of 1 (Figure 1), each Ni macrocyclic unit of A is coordinated by two bptc ligand at the trans positions, and each bptc ligand binds four Ni ions belonging to four different bismacrocyclic complexes to construct 2D grids extending parallel to the ab plane. The 2D grid generates rhombic cavities with effective sizes of 3.32 8.14 , each of which involves four Ni macrocyclic units and four bptc units. The layer is not completely flat, as Scheme 1. a) Alkyl-bridged Ni bismacrocyclic complexes and H4bptc. b) Design strategies for construction of 3D networks. Bismacrocyclic complexes located upward and downward with respect to a 2D plane are indicated by the different colors (pink and orange).