The role of sulfonate groups and hydrogen bonding in the proton conductivity of two coordination networks.

The proton conductivity of two coordination networks, [Mg(HO)(HL)]·HO and [Pb(HL)]·HO (HL = (HOPCH)-NCH-CH-SOH), is investigated by AC impedance spectroscopy. Both materials contain the same phosphonato-sulfonate linker molecule, but have clearly different crystal structures, which has a strong effect on proton conductivity. In the Mg-based coordination network, dangling sulfonate groups are part of an extended hydrogen bonding network, facilitating a "proton hopping" with low activation energy; the material shows a moderate proton conductivity. In the Pb-based metal-organic framework, in contrast, no extended hydrogen bonding occurs, as the sulfonate groups coordinate to Pb, without forming hydrogen bonds; the proton conductivity is much lower in this material.