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

Pd-II coordination polymer networks of the type [Pd(mu-X-pymo-N-1,N-3)(2)center dot(H2O)(m)](n) (X = H, 2(H); F, 2(F); Br, 2(Br); I, 2(I)), 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-N-1)(2)Cl-2] (1(H)-1(I)) precursors with NaOH in a 1 : 1 ratio. While 2(Br) and 2(I) have been recovered as amorphous materials, the microcrystalline species 2(H) and 2(F), in their hydrated and anhydrous forms, have been structurally characterised by ab initio XRPD methods: square planar PdII ions, symmetrically connected by exobidentate N-1,N-3-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(X) systems. The H-2, N-2 and CO2 adsorption isotherms showed that the 2(H) and 2(F) materials possess permanent porosity, as opposed to the 2(Br) and 2(I) 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 (N-2, CO2 and H-2). Compounds 2(H) and 2(F) possess high H-2 adsorption capacities at 77 K with respective values of 1.3 and 1.15 weight percentage, and storage densities of about 0.018 kg H-2 L-1, which are combined with large isosteric adsorption heats (8-9 kJ mol(-1)).