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.