Mixed-Ligand Metal-Organic Frameworks with Large Pores: Gas Sorption Properties and Single-Crystal-to-Single-Crystal Transformation on Guest Exchange

Two isomorphous 3D metalorganic frameworks, {[Cu-2(BPnDC)(2)(bpy)]center dot 8DMF center dot 6H20}(n) (1) and {[Zn-2- (BPnDC)(2)(dabco)]center dot 13DMF center dot 3H(2)O}(n) (2), have been prepared by the solvothermal reactions of benzophenone 4,4'-dicarboxylic acid (H(2)BPnDC) with Cu(NO3)(2)center dot 2.5 H2O and 4,4'-bipyridine (bpy), and with Zn(NO3)(2)center dot 6H(2)O and 4-diazabicyclo[2.2.2]octane (dabco), respectively. Compounds 1. and 2 are composed of paddle-wheel {M-2- (O2CR)(4)} cluster units, and they generate 2D channels with two different large pores (effective size of larger pore: 18.2 angstrom for 1, 11.4 angstrom for 2). The framework structure of desolvated solid, [Cu-2(BPnDC)(2)(bpy)](n) (SNU-6; SNU=Seoul National University), is the same as that of 1, as evidenced by powder X-ray diffraction patterns. SNU-6 exhibits high permanent porosity (1.05 cm(3)g(-1)) with high Langmuir surface area (2910 m(2)g(-1)). It shows high H, gas storage capacity (1.68 wt % at 77 K and 1. atm; 4.87 wt % (excess) and 10.0 wt % (total) at 77 K and 70 bar) with high isosteric heat (7,74 W mol(-1)) of H-2 adsorption as well as high CO, adsorption capability (113.8 wt % at -195 K and.1 atm). Compound 2 undergoes a single-crystal-to-single-crystal transformation on guest exchange with n-hexane to provide [[Zn-2(BPnDC)(2)(dabco)]center dot 6 (n-hexane)center dot 3 H2O}(n) (2(hexane)). The transformation involves dynamic motion of tile molecular components in the crystal, mainly a bending motion of the square planes of the paddle-wheel units resulting from rotational rearrangement of phenyl rings and carboxylate planes of BPnDC(2-)..