Effect of alkaline-earth metal ion radius on the coordination number and proton conductivity of isomorphous MOFs

Differences in metal ions in isomorphous MOFs for modulating proton conductivity are an interesting and meaningful topic. In order to investigate how differences in the radii of homologous metal ions affect the amount of coordinated water in MOFs and control the continuity of the hydrogen bonding network, the monocarboxylic tetrazolium organic ligand HL (5-(2-pyridyl)tetrazole-2-acetic acid) was used to construct two isomeric MOFs M (L)2(H2O)2 (M = Mg: MOF-Mg; M = Ca: MOF-Ca), which have only two coordinated waters per metal and discontinuous hydrogen bonding hopping paths. The same ligand was used to construct two isomeric MOFs [M2(L)4(H2O)6]center dot 2H2O (M = Sr: MOF-Sr; M = Ba: MOF-Ba), which have three coordinated waters per metal and a continuous hydrogen-bonding hopping path, resulting in a high proton conductivity of 1.28 x 10-3 S cm-1 at 65 degrees C and 98% RH for MOF-Ba which is over 4000 times that of MOF-Ca. The proton conductivity mechanism was calculated to be a Grotthuss mechanism and verified by the deuterium-hydrogen isotopic effects. Moreover, the proton conductivity was maintained for more than 72 h without any obvious decrease.