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

Abstract

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)(L₂)]·DMF·2H₂O}_n (1) and {[Zn₂(L₁)(L₃)(DMF)₂]·DMF·4H₂O}_n (2) (DMF = dimethyl formamide, L₂ = 3,6-di-pyridin-4-yl-[1,2,4,5]tetrazine, L₃ = 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 Zn₂(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 N₂, H₂, 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.