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)PO3)(H2O)(4)] center dot H2O (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)PO3H)(H2O)(4), exhibits a pillared layered house of cards structure, while K-HPAA, K-2(OOCCH(OH)PO3H)(H2O)(2), and Cs-HPAA, Cs(HOOCCH(OH)PO3H), 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 x 10(-5) S cm(-1) (Cs-HPAA) to 5.6 x 10(-3) S cm(-1) (Na-HPAA) at 98% RH and T = 24 degrees 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.