Borderline microporous-ultramicroporous palladium(II) coordination polymers. effect of pore functionalization on gas adsorption properties

Pd(II) coordination polymer networks of the type [Pd(mu-X-pymo-N1,N3)2*(H2O)m]n (X = H, 2H; F, 2F; Br, 2Br; I, 2I), 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] (1H-1I) precursors with NaOH in a 1 : 1 ratio. While 2Br and 2I have been recovered as amorphous materials, the microcrystalline species 2H and 2F, in their hydrated and anhydrous forms, have been structurally characterised by ab initio XRPD methods: square planar PdII 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 2X systems. The H2, N2 and CO2 adsorption isotherms showed that the 2H and 2F materials possess permanent porosity, as opposed to the 2Br and 2I 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 2H and 2F 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).