Tuning Proton Conductivity in Alkali Metal Phosphonocarboxylates by Cation Size-Induced and Water-Facilitated Proton Transfer Pathways

The structural and functional chemistry of a family of alkali-metal ions with racemic R, S -hydroxyphosphonoacetate ( M-HPAA; M = Li, Na, K, Cs) are reported. Crystal structures were determined by X-ray data (Li +, powder diffraction following an ab initio methodology; Na +, K +, Cs +, single crystal). A gradual increase in dimensionality directly proportional to the alkali ionic radius was observed. [Li 3 (OOCCH(OH)PO 3 )(H 2 O) 4 ]·H 2 O ( Li-HPAA ) shows a 1D framework built up by Li-ligand “slabs” with Li + in three different coordination environments (4-, 5-, and 6-coordinated). Na-HPAA, Na 2 (OOCCH(OH)PO 3 H)(H 2 O) 4, exhibits a pillared layered “house of cards” structure, while K-HPAA, K 2 (OOCCH(OH)PO 3 H)(H 2 O) 2, and Cs-HPAA, Cs(HOOCCH(OH)PO 3 H), typically present intricate 3D frameworks. Strong hydrogen-bonded networks are created even if no water is present, as is the case in Cs-HPAA . As a result, all compounds show proton conductivity in the range 3.5 × 10 –5 S cm –1 ( Cs-HPAA ) to 5.6 × 10 –3 S cm –1 ( Na-HPAA ) at 98% RH and T = 24 °C. Differences in proton conduction mechanisms, Grothuss (Na + and Cs + ) or vehicular (Li + and K + ), are attributed to the different roles played by water molecules and/or proton transfer pathways between phosphonate and carboxylate groups of the ligand HPAA. Upon slow crystallization, partial enrichment in the S enantiomer of the ligand is observed for Na-HPAA, while the Cs-HPAA is a chiral compound containing only the S enantiomer.