Solid-State Gas Adsorption Studies with Discrete Palladium(II) [Pd 2 (L) 4 ] 4+ Cages.

The need for effective CO ₂ capture systems remains high, and due to their tunability, metallosupramolecular architectures are an attractive option for gas sorption. While the use of extended metal organic frameworks for gas adsorption has been extensively explored, the exploitation of discrete metallocage architectures to bind gases remains in its infancy. Herein the solid state gas adsorption properties of a series of [Pd ₂ (L) ₄ ] ⁴⁺ lantern shaped coordination cages (L = variants of 2,6-bis(pyridin-3-ylethynyl)pyridine), which had solvent accessible internal cavities suitable for gas binding, have been investigated. The cages showed little interaction with dinitrogen gas but were able to take up CO ₂ . The best performing cage reversibly sorbed 1.4 mol CO ₂ per mol cage at 298 K, and 2.3 mol CO ₂ per mol cage at 258 K (1 bar). The enthalpy of binding was calculated to be 25-35 kJ mol ^-1 , across the number of equivalents bound, while DFT calculations on the CO ₂ binding in the cage gave ΔE for the cage-CO ₂ interaction of 23-28 kJ mol ^-1 , across the same range. DFT modelling suggested that the binding mode is a hydrogen bond between the carbonyl oxygen of CO ₂ and the internally directed hydrogen atoms of the cage.
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