Structural variation in Zn(II) coordination polymers built with a semi-rigid tetracarboxylate and different pyridine linkers: synthesis and selective CO2 adsorption studies

In an effort towards the rational design of porous MOFs with a functionalized channel surface, 3,3',5,5'-tetracarboxydiphenylmethane (H4L1) has been used in combination with two different bipyridine ligands of similar lengths as linkers, and Zn(II) ions as nodes. Under solvothermal conditions, two Zn(II) coordination polymers, {[Zn(H2L1)(L-2)]center dot DMF center dot 2H(2)O}(n) (1) and {[Zn-2(L-1)(L-3)(DMF)(2)]center dot DMF center dot 4H(2)O}(n) (2) (DMF = dimethyl formamide, L-2 = 3,6-di-pyridin-4-yl-[1,2,4,5]tetrazine, L-3 = 4,4'-bispyridylphenyl) are formed in moderate yields. The obvious kink in the central methylene spacer of H4L1 induces either C-2v or C-s symmetry in the ligand, allowing different architectures in the resulting frameworks. Single crystal X-ray analysis shows that compound 1 is a one-dimensional (1D) double chain architecture with rhombus voids, linked by Zn-2(CO2)(4) paddle-wheel secondary building units (SBUs). The tetrazine and pyridine moieties of the co-ligand and free carboxylic acid groups are lined along the voids of the framework. Compound 2, on the other hand, crystallizes as an infinite two-dimensional corrugated sheet structure, where individual sheets are stacked in -ABAB- patterns along the crystallographic b-axis. Thermogravimetric analysis (TGA) and variable temperature powder X-ray diffraction (VTPXRD) studies reveal high thermal stability for 1 but 2 collapses soon after desolvation. The desolvated framework 1' shows selective CO2 adsorption over N-2, H-2, and CH4 at 273 K, with an isosteric heat of CO2 adsorption of 21.3 kJ mol(-1), suggesting an interaction of the CO2 molecules with the channel walls.