A homochiral tartrate-bridged dinuclear chromium(III) complex anion with a resonance-assisted hydrogen bond for proton conduction.

The synthesis of a homochiral building block based on L-tartrate-chromium(III) complex anions is reported. The dinuclear complex anion, which contains two bridging L-tartrate ligands and one aromatic N-donor ligand coordinated to chromium(III) ions, exhibits a boat conformation in which intramolecular resonance-assisted hydrogen bonding is present. The sodium L-tartrate-chromium(III) compound with the formula Na[Cr2(bpy)2(L-tart)2H]·9H2O (1) crystallizes from a methanol-water solution as a high water content material in the monoclinic space group P2. The as-synthesized compound is only stable at high relative humidity and undergoes structural transformations during drying, which are accompanied by water loss. However, these transformations are reversible and upon wetting, the material returns to its high water content structure. Based on a combination of experimental techniques (PXRD, in situ ATR-FTIR and EPR spectroscopy), the structure of the complex anions appears to be insensitive to the humidity variable processes (wetting and drying). Due to the presence of several hydrogen acceptor and donor groups in the L-tartrate-chromium(III) complex anion, we investigated the proton transport properties of a sodium L-tartrate-chromium(III) compound by impedance spectroscopy under dry and wet conditions at different temperatures. Since the relative humidity affects the structural transformations in this system, it also has a large influence on the proton conductivity, which varies by up to four orders of magnitude depending on the degree of hydration. These results confirm that the proton conductivity can be tuned in flexible structures in which non-covalent interactions determine the crystal packing.

Interfacial Water Molecules as Agents for Phase Change Control and Proton Conductivity Enhancement in the Ammonium Vanadyl Tartrate System.

This study demonstrates the reversible structural transformation, single-crystal-to-single-crystal, of the ammonium vanadyl (L-tartrate) complex salt from the hydrate phase to the anhydrous phase. The transformation can be initiated by stimuli, such as temperature, humidity, or vacuum conditions. The hydrate and anhydrous phases exhibit a tetragonal structure (P41212), with marked differences in hydrogen bonding due to the presence or absence of one water molecule per asymmetric unit. The intricate relationship between crystal packing and intermolecular interactions in the hydrate phase was investigated by crystallographic charge density analysis revealing, at the molecular level, the reasons for the observed 5 orders of magnitude higher proton conductivity of the hydrate phase compared to that of the anhydrous phase. To gain further insight into the processes occurring at the surfaces of grain boundaries and the proton transfer mechanisms in this system, rehydration of the complex salt was carried out by using D2O instead of H2O and monitored by in situ ATR-FTIR spectroscopy. The results highlight the critical role of interfacial water molecules in driving structural transformations and influencing proton conductivity.