Effects of Secondary Anion on Proton Conduction in a Flexible Cationic Phosphonate Metal-Organic Framework

Four new phosphonate MOFs were prepared with cationic dimethylbipiperdinium units: [La2L1.5AcO2Br•3.25H2O], α-PCMOF-21-Br; [La2L1.5AcOCl2•5.25H2O], α-PCMOF-21-Cl; [LaLAcOBr0.5•4.93H2O(HBr)1.11], β-PCMOF-21-Br; and [LaLAcO0.5Cl•5.42H2O(HCl)1.79], β-PCMOF-21-Cl. All frameworks have the same La phosphonate network structure but differ in the secondary anions (acetate, bromide, chloride), both coordinated and free. All frameworks showed the abil-ity to dehydrate reversibly. Different phases result from very subtle differences in preparation, specifically, the degree of hydration of the ligand impacts the product phase even though syntheses are carried out in water. The alpha phases show a flexible structure by powder X-ray diffraction. The beta phases contain a reproducible stoichiometry of free lig-and in the pores that both locks and partially fills the open structure. AC Impedance analysis was performed to study proton conductivity. All compounds, except for β-PCMOF-21-Cl, conduct better than 10-3 S cm-1 a...