Effects of Secondary Anions on Proton Conduction in a Flexible Cationic Phosphonate Metal–Organic Framework

Four new phosphonate MOFs were prepared with cationic dimethylbipiperdinium units: [La2(H2L)1.5(AcO)2Br·3.25H2O], α-PCMOF-21-Br; [La2(H2L)1.5(AcO)Cl2·5.25H2O], α-PCMOF-21-Cl; [La(H2L) (AcO)Br0.5·4.93H2O(HBr)1.11], β-PCMOF-21-Br; and [La(H2L) (AcO)0.5Cl·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 ability 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 ligands in the pores that both locks and partially fills the open structure. Alternating current impedance analysis was performed to study proton conductivity. All compounds, except for β-PCMOF-21-Cl, conduct better than 10–3 S cm–1 at 85 °C and 95% RH. The trends show that the alpha phases conduct better than the partially pore-blocked beta phases and also that the bromide structures conduct better than the chlorides. To further study the role of the anion, 35Cl and 81Br solid-state NMR was performed to elucidate dynamics. These studies also showed the ability of anions to be volatilized from the pores, and TGA–MS confirmed the loss of HX species. Impedance analysis showed a clear decrease in proton conductivity after the loss of the halides, more pronounced in the bromide-containing structures.