Three-Dimensional Metal–Organic Framework with Highly Polar Pore Surface: H₂ and CO₂ Storage Characteristics

A three-dimensional (3D) pillared-layer metal–organic framework, [Cd­(bipy)_0.5(Himdc)]­(DMF)]_<i>n</i> (<b>1</b>), (bipy =4,4′-bipyridine and Himdc = 4,5-imidazoledicarboxylate) has been synthesized and structurally characterized. The highly rigid and stable framework contains a 3D channel structure with highly polar pore surfaces decorated with pendant oxygen atoms of the Himdc linkers. The desolvated framework [Cd­(bipy)_0.5(Himdc)]_<i>n</i> (<b>1</b>′) is found to exhibit permanent porosity with high H₂ and CO₂ storage capacities. Two H₂ molecules occluded per unit formula of <b>1</b>′ and the corresponding heat of H₂ adsorption (Δ<i>H</i>_H2) is about ∼9.0 kJ/mol. The high value of Δ<i>H</i>_H2 stems from the preferential electrostatic interaction of H₂ with the pendent oxygen atoms of Himdc and aromatic bipy linkers as determined from first-principles density functional theory (DFT) based calculations. Similarly, DFT studies indicate CO₂ to preferentially interact electrostatically (C^δ+···O^δ‑) with the uncoordinated pendent oxygen of Himdc. It also interacts with bipy through C–H···O bonding, thus rationalizing the high heat (Δ<i>H</i>_CO2 ∼ 35.4 kJ/mol) of CO₂ uptake. Our work unveiled that better H₂ or CO₂ storage materials can be developed through the immobilization of reactive hetero atoms (O, N) at the pore surfaces in a metal–organic framework