Two-Step Adsorption on Jungle-Gym-Type Porous Coordination Polymers: Dependence on Hydrogen-Bonding Capability of Adsorbates, Ligand-Substituent Effect, and Temperature UemuraKazuhiro YamasakiYukari OnishiFumiaki KitaHidetoshi EbiharaMasahiro 2010 A preliminary study of isopropanol (IPA) adsorption/desorption isotherms on a jungle-gym-type porous coordination polymer, [Zn<sub>2</sub>(bdc)<sub>2</sub>(dabco)]<sub><i>n</i></sub> (<b>1</b>, H<sub>2</sub>bdc = 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 <b>1</b>, using probing gas molecules of alcohol (MeOH and EtOH) for the size effect and Me<sub>2</sub>CO for the influence of hydrogen bonding, show that alcohol adsorption isotherms are gradual two-step profiles, whereas the Me<sub>2</sub>CO 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-NO<sub>2</sub>), bromoterephthalate (bdc-Br), and 2,5-dichloroterephthalate (bdc-Cl<sub>2</sub>) as substituted dicarboxylate ligands, isomorphous jungle-gym-type porous coordination polymers, {[Zn<sub>2</sub>(bdc-NO<sub>2</sub>)<sub>2</sub>(dabco)]·<i>solvents</i>}<sub><i>n</i></sub> (<b>2</b> ⊃ <i>solvents</i>), {[Zn<sub>2</sub>(bdc-Br)<sub>2</sub>(dabco)]·<i>solvents</i>}<sub><i>n</i></sub> (<b>3</b> ⊃ <i>solvents</i>), and {[Zn<sub>2</sub>(bdc-Cl<sub>2</sub>)<sub>2</sub>(dabco)]·<i>solvents</i>}<sub><i>n</i></sub> (<b>4</b> ⊃ <i>solvents</i>), were synthesized and characterized by single-crystal X-ray analyses. Thermal gravimetry, X-ray powder diffraction, and N<sub>2</sub> adsorption at 77 K measurements reveal that [Zn<sub>2</sub>(bdc-NO<sub>2</sub>)<sub>2</sub>(dabco)]<sub><i>n</i></sub> (<b>2</b>), [Zn<sub>2</sub>(bdc-Br)<sub>2</sub>(dabco)]<sub><i>n</i></sub> (<b>3</b>), and [Zn<sub>2</sub>(bdc-Cl<sub>2</sub>)<sub>2</sub>(dabco)]<sub><i>n</i></sub> (<b>4</b>) maintain their frameworks without guest molecules with Brunauer−Emmett−Teller (BET) surface areas of 1568 (<b>2</b>), 1292 (<b>3</b>), and 1216 (<b>4</b>) m<sup>2</sup> g<sup>−1</sup>. As found in results of MeOH, EtOH, IPA, and Me<sub>2</sub>CO adsorption/desorption on <b>2</b>−<b>4</b>, only MeOH adsorption on <b>2</b> 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 <b>2</b> depends on the temperature, attributed to the small free-energy difference (Δ<i>F</i><sub>host</sub>) between the two guest-free forms, wide and narrow pores.