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 (2solvents), {[Zn2(bdc-Br)2(dabco)]·solvents}n (3solvents), and {[Zn2(bdc-Cl2)2(dabco)]·solvents}n (4solvents), 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 24, 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 (ΔFhost) between the two guest-free forms, wide and narrow pores.