Borderline microporous–ultramicroporous palladium(ii) coordination polymer networks. Effect of pore functionalisation on gas adsorption properties

Abstract

Pd^II coordination polymer networks of the type [Pd(μ-X-pymo-N¹,N³)₂·(H₂O)_m]_n (X = H, 2_HHH; F, 2_FFF; Br, 2_BrBrBr; I, 2_III), containing the X-pymo (X-Hpymo = 5-X-2-hydroxypyrimidine) ligands, have been obtained by refluxing aqueous suspensions of the monomeric trans-[Pd(X-Hpymo-N¹)₂Cl₂] (1_HH-1_II) precursors with NaOH in a 1 : 1 ratio. While 2_BrBrBr and 2_III have been recovered as amorphous materials, the microcrystalline species 2_HHH and 2_FFF, in their hydrated and anhydrous forms, have been structurally characterised by ab initio XRPD methods: square planar Pd^II ions, symmetrically connected by exobidentate N¹,N³-X-pymo bridges, give rise to neutral sodalite zeotypic frameworks (with H₂O guest molecules eventually included in the pores). The thermal properties and porosity have been determined for all of the 2_XXX systems. The H₂, N₂ and CO₂ adsorption isotherms showed that the 2_HHH and 2_FFF materials possess permanent porosity, as opposed to the 2_BrBrBr and 2_III ones. Moreover, the isomorphic incorporation of fluorine in the framework causes important modifications in the gas-adsorption isotherms; stronger interactions at very low pressures (Henry's law region) appear in all of the probes studied (N₂, CO₂ and H₂). Compounds 2_HHH and 2_FFF possess high H₂ adsorption capacities at 77 K with respective values of 1.3 and 1.15 weight percentage, and storage densities of about 0.018 kg H₂ L⁻¹, which are combined with large isosteric adsorption heats (8–9 kJ mol⁻¹).