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

A unique spatial arrangement of amide groups for CO2 adsorption is found in the open-ended channels of a zinc(II) organic framework {[Zn-4(BDC)(4)(BPDA)(4)]center dot 5DMF center dot 3H(2)O}(n) (1, BDC = 1,4-benzyl dicarboxylate, BPDA = N,N'-bis(4-pyridinyl)-1,4-benzenedi-carboxamide). Compound 1 consists of 4(4)-sql [Zn-4(BDC)(4)] sheets that are further pillared by a long linker of BPDA and forms a 3D porous framework with an alpha-Po 4(12).6(3) topology. Remarkably, the unsheltered amide groups in 1 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 1 favors capture of CO2 over N-2. DFT calculations provided rationale for the intriguing 1:1 ratio of amide sorption sites to CO2 molecules and revealed that the nanochamber of compound 1 permits the slipped-parallel arrangement of CO2 molecules, an arrangement found in crystal and gas-phase CO2 dimer.