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

The proton conductivity of two coordination networks, [Mg(H(2)O)(2)(H(3)L)]·H(2)O and [Pb(2)(HL)]·H(2)O (H(5)L = (H(2)O(3)PCH(2))(2)-NCH(2)-C(6)H(4)-SO(3)H), 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(2+), without forming hydrogen bonds; the proton conductivity is much lower in this material.