Two-Step Adsorption on Jungle-Gym-Type Porous Coordination Polymers: Dependence on Hydrogen-Bonding Capability of Adsorbates, Ligand-Substituent Effect, and Temperature

A preliminary study of isopropanol (IPA) adsorption/desorption isotherms on a jungle-gym-type porous coordination polymer, [Zn₂(bdc)₂(dabco)]_n (1, H₂bdc = 1,4-benzenedicarboxylic acid, dabco =1,4-diazabicyclo[2.2.2]octane), showed unambiguous two-step profiles via a highly shrunk intermediate framework. The results of adsorption measurements on 1, using probing gas molecules of alcohol (MeOH and EtOH) for the size effect and Me₂CO for the influence of hydrogen bonding, show that alcohol adsorption isotherms are gradual two-step profiles, whereas the Me₂CO isotherm is a typical type-I isotherm, indicating that a two-step adsorption/desorption is involved with hydrogen bonds. To further clarify these characteristic adsorption/desorption behaviors, selecting nitroterephthalate (bdc-NO₂), bromoterephthalate (bdc-Br), and 2,5-dichloroterephthalate (bdc-Cl₂) as substituted dicarboxylate ligands, isomorphous jungle-gym-type porous coordination polymers, {[Zn₂(bdc-NO₂)₂(dabco)]·solvents}_n (2 ⊃ solvents), {[Zn₂(bdc-Br)₂(dabco)]·solvents}_n (3 ⊃ solvents), and {[Zn₂(bdc-Cl₂)₂(dabco)]·solvents}_n (4 ⊃ solvents), were synthesized and characterized by single-crystal X-ray analyses. Thermal gravimetry, X-ray powder diffraction, and N₂ adsorption at 77 K measurements reveal that [Zn₂(bdc-NO₂)₂(dabco)]_n (2), [Zn₂(bdc-Br)₂(dabco)]_n (3), and [Zn₂(bdc-Cl₂)₂(dabco)]_n (4) maintain their frameworks without guest molecules with Brunauer−Emmett−Teller (BET) surface areas of 1568 (2), 1292 (3), and 1216 (4) m² g⁻¹. As found in results of MeOH, EtOH, IPA, and Me₂CO adsorption/desorption on 2−4, only MeOH adsorption on 2 shows an obvious two-step profile. Considering the substituent effects and adsorbate sizes, the hydrogen bonds, which are triggers for two-step adsorption, are formed between adsorbates and carboxylate groups at the corners in the pores, inducing wide pores to become narrow pores. Interestingly, such a two-step MeOH adsorption on 2 depends on the temperature, attributed to the small free-energy difference (ΔF_host) between the two guest-free forms, wide and narrow pores.