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

Abstract

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 (P4 1 2 1 2), 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 D 2 O instead of H 2 O 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.

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

“…The positions of the bands related to the asymmetric stretching vibrations of the carbonyl group [ ν as (CO)] in the initial phase appear at 1622 and 1600 cm −1 , and in the dry phase, they shift to 1634 and 1608 cm −1 , respectively. 18,19,43 The symmetric stretching vibration of the carbonyl group [ ν s (CO)] shifts from 1353 cm −1 in the initial phase to 1348 cm −1 in the dry phase. The vibrations in the bpy molecule are less sensitive to intermolecular hydrogen bonding and remain at their original frequencies; e.g.…”

“…The presence of molecular adsorbed water layers has been experimentally confirmed by spectroscopic and impedance methods on silicon oxide and ceramic materials, 44,45 and recently in a metal–organic system composed of ammonium cations and a vanadium( iv )- l -tartrate complex. 18 In a metal–organic system under humid conditions (RH 75%), the generation of so-called liquid-like layers of adsorbed water leads to a one order of magnitude increase in proton conductivity. 18 In compound 1 , due to the very large number of water molecules in the structure, it is difficult to distinguish the contributions of ice-like and liquid-like water layers by ATR-FTIR spectroscopy, but impedance spectroscopy clearly indicates their formation, which is reflected in the difference in conductivity measured at RH 60 and 75% (Fig.…”

“…18 In a metal–organic system under humid conditions (RH 75%), the generation of so-called liquid-like layers of adsorbed water leads to a one order of magnitude increase in proton conductivity. 18 In compound 1 , due to the very large number of water molecules in the structure, it is difficult to distinguish the contributions of ice-like and liquid-like water layers by ATR-FTIR spectroscopy, but impedance spectroscopy clearly indicates their formation, which is reflected in the difference in conductivity measured at RH 60 and 75% (Fig. 4a).…”

“…4a). Similar to the vanadium( iv )- l -tartrate compound, 18 in compound 1 water molecules also serve a dual purpose by both enhancing proton conduction and facilitating the structural transformation from dry to fully hydrated phases.…”

“…The flexibility of structures containing naturally occurring carboxylic acids, such as l -tartaric acid, has been demonstrated in several transition metal complexes that undergo reversible solid-state transformation upon exposure to moisture. 14–19 l -Tartaric acid serves several purposes as a ligand in coordination chemistry: it coordinates, chelates or bridges the metal centres, can be pH-adjusted to form different ionic species (−4 ≤ ionic charge ≤ 0) and contains sufficient functional groups to participate in non-covalent interactions that stabilize the structure. In most of the structures found in the Cambridge Structural Database (CSD), 20 the l -tartrate ligand bridges the transition metal centres, leading either to extended polymeric structures, as in a series of isomorphous compounds {[M( NN )( l -tart-H 2 )]· x H 2 O} n (M 2+ = Mn, Co, Cu, and Zn; NN = 2,2′-bipyridine, 1,10-phenanthroline; l -tart-H 2 = dianion of l -tartaric acid; x = 5, 6), 21–26 or to isolated dinuclear complexes of the formula [{M( NN )} 2 ( l -tart-H 2 )] (M 2+ = Pd and Pt).…”