Cooperative Effect of Unsheltered Amide Groups on CO₂ Adsorption Inside Open-Ended Channels of a Zinc(II)–Organic Framework

Abstract

A unique spatial arrangement of amide groups for CO₂ adsorption is found in the open-ended channels of a zinc­(II)–organic framework {[Zn₄(BDC)₄(BPDA)₄]·5DMF·3H₂O}_<i>n</i> (<b>1</b>, BDC = 1,4-benzyl dicarboxylate, BPDA = <i>N,N′</i>-bis­(4-pyridinyl)-1,4-benzenedicarboxamide). Compound <b>1</b> consists of 4⁴-<b>sql</b> [Zn₄(BDC)₄] sheets that are further pillared by a long linker of BPDA and forms a 3D porous framework with an α-Po 4¹²·6³ topology. Remarkably, the unsheltered amide groups in <b>1</b> provide a positive cooperative effect on the adsorption of CO₂ molecules, as shown by the significant increase in the CO₂ adsorption enthalpy with increasing CO₂ uptake. At ambient condition, a 1:1 ratio of active amide sites to CO₂ molecules was observed. In addition, compound <b>1</b> favors capture of CO₂ over N₂. DFT calculations provided rationale for the intriguing 1:1 ratio of amide sorption sites to CO₂ molecules and revealed that the nanochamber of compound <b>1</b> permits the slipped-parallel arrangement of CO₂ molecules, an arrangement found in crystal and gas-phase CO₂ dimer