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

Abstract

The structural and functional chemistry of a family of alkali-metal ions with racemic R,Shydroxyphosphonoacetate (M-HPAA; M = Li, Na, K, Cs) are reported. Crystal structures were determined by X-ray powder diffraction (Li + ), following an ab initio methodology, or by single crystal (Na + , K + , Cs + ). A gradual increase in dimensionality directly proportional to the alkali ionic radius was observed. [Li 3 (ΟOCCH(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 (ΟOCCH(OH)PO 3 H)(H 2 O) 4 , exhibits a pillared layered "house of cards" structure, while K-HPAA, K 2 (ΟOCCH(OH)PO 3 H)(H 2 O) 2 , and Cs-HPAA, Cs(HΟOCCH(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.

Platonic Relationships in Metal Phosphonate Chemistry: Ionic Metal Phosphonates

“…The metal cation is bound by aqua ligands (solvent), and occasionally, by other, externally-added ligands (4,4'-bpy in this case for Ni-bpy-EDPA). As reported before, there is a correlation between the ionic radius of the metal ion and the bond distances between the metal and the ligand atoms coordinated to the metal center [2,[56][57][58]. Since there is no direct metal-phosphonate bonding in the compounds reported herein, the only correlations that can be drawn are those for metal-Owater bonds.…”

Magnesium doped Gallium Phosphonates Ga1–xMgx[H3+x(O3PCH2)3N] (x = 0, 0.20) and the Influence on Proton Conductivity

“…To the best of our knowledge, only few systematic studies on the influence of the central metal atom on the proton conductivity have been published. [8e], Bazaga‐Garcia et al exchanged the metal atoms (Li + , Na + , K + , Cs + ) in a R , S ‐hydroxyphosphonoacetate (HPAA 2– ) based CP. The overall structure of [Na 2 (OOCCH(OH)PO 3 H)(H 2 O) 4 ] (Na‐HPAA) can be described as a pillared structure.…”

“…Their intrinsic proton conductivities increase with increasing size of the metal cations: Al 3+ (67.5 pm) < Ga 3+ (76.0 pm) < Fe 3+ (high spin) (78.5 pm) and reaches a maximum value for the iron compound at 5.2 × 10 –4 S · cm –1 at 90 °C. [10b]…”

Oriented design and synthesis of water-stable heterometallic metal–organic frameworks by bridging ligands containing hydrophobic (–CH3) substituents

“…This chain can well maintain the thermo-stability in a lower skeletal density. Some frameworks that have analogous features were reported, but it was still very lack of the research in this field [29][30][31].…”

“…This chain can well maintain the thermo-stability in a lower skeletal density. Some frameworks that have analogous features were reported, but it was still very lack of the research in this field [29][30][31].In addition, it is conducive to enhance the stability of the frameworks by importing hydrophobic substituents such as -CH 3 , -OCH 3 . The steric-hinerance effect and hydrophobicity of the substituents can insulate the unsaturated coordinated metal ions, and improve the stability of the building-blocks, thereby enhancing the stability of the whole frameworks.…”