Chiral Supramolecular Proton Conductors: Harnessing\nHighly Charged Zirconium-Amino Acid Oxo-Clusters

Incorporation of amino acid capping\nmolecules (alanine (Ala), methionine\n(Met), phenylalanine (Phe), tryptophan (Trp), tyrosine (Tyr), and\nvaline (Val)) in their zwitterionic form into archetypal [Zr<sub>6</sub>(μ<sub>3</sub>-O)<sub>4</sub>(μ<sub>3</sub>–OH)<sub>4</sub>]<sup>12+</sup> clusters creates supramolecular frameworks\nin which the assembly of these highly charged discrete units with\nchloride counterions provides a unique combination of porosity, chirality,\nand proton conductivity. The supramolecular frameworks assembled from\nthese cluster entities (i.e., <b>ZrAla</b>, <b>ZrMet</b>, <b>ZrPhe</b>, <b>ZrTrp</b>, <b>ZrTyr</b>, and <b>ZrVal</b>) are based on the counterbalancing of the cationic hexanuclear\nentities by chloride anions. The resulting structures provide porous\nstructures (except <b>ZrVal</b>) with variability of chemical\nand structural stability based on their supramolecular interactions.\nAmong these compounds, <b>ZrPhe</b> and <b>ZrVal</b> remain\nstable due to the presence of a double chelation-like interaction\ninvolving four hydrogen bonds formed between a chloride anion, two\nammonium groups, and two coordinated water molecules from two adjacent\nhexanuclear units. The presence of multiple acidic proton positions\nand strong hydrogen-bond donor/acceptor groups on the water channels\ngives rise to easy proton conduction pathways within the structures.\nIn fact, <b>ZrPhe</b> and <b>ZrVal</b>, together with <b>ZrTyr</b>, exhibit high proton conductivity values with varying\ndependencies on the atmospheric humidity. Finally, the correlation\nbetween proton conduction and porosity is discussed.