Tuning Intrinsic and Extrinsic Proton Conduction in Metal–Organic Frameworks by the Lanthanide Contraction

Seven
isomorphous lanthanide metal–organic frameworks in
the PCMOF-5 family, [Ln­(H5L)­(H2O)n]­(H2O) (L = 1,2,4,5-tetrakis­(phosphonomethyl)­benzene,
Ln = La, Ce, Pr, Nd, Sm, Eu, Gd) have been synthesized and characterized.
This family contains 1-D water-filled channels lined with free hydrogen
phosphonate groups and gives a very low activation energy pathway
for proton transfer. The lanthanide contraction was employed to systematically
vary the unit cell dimensions and tune the proton conducting pathways.
LeBail fitting of the crystalline series shows that the crystallographic a-axis, along the channel, can be varied in increments less
than 0.02 Å correspondingly shortening the proton transfer pathway.
The proton conductivities for the La and Pr complexes were roughly
an order of magnitude higher than other members of the series (10–3 S cm–1 versus 10–4 S cm–1). Single crystal structures of the high
and low conducting members of the series (La, Pr for high and Ce for
low) affirm the structural similarities extend beyond the unit cell
parameters to positions of free acid groups and included water molecules.
Scanning electron microscopy reveals marked differences in particle
size of the different members of the Ln series owing to lattice strain
effects induced by changing the lanthanide. Notably, the high conducting
La and Pr complexes have the largest particle sizes. This result contradicts
any notion that degradation of the MOF at grain boundaries is enabling
the observed conductivity as proton conduction dominated by extrinsic
pathways would be enabled by small particles (i.e., the La and Pr
complexes would be the worst conductors). Proton conductivity measurements
of a ball milled sample of the La complex corroborate this result.