Proton transport in oxalate compounds of iron(iii) containing (alkyl)ammonium cations: the influence of the density of hydrogen bonds on conductivity

Abstract

The proton conduction behaviour of novel oxalate-based compounds of iron(III) of different nuclearity with various ammonium components, (NH₄)₂[Fe(H₂O)Cl₃(C₂O₄)]·H₂O (1), {[NH(CH₃)₂(C₂H₅)][FeCl₂(C₂O₄)]·H₂O}_n (2) and {[N(CH₃)(C₂H₅)₃][FeCl₂(C₂O₄)]}_n (3), was investigated by impedance spectroscopy in addition to characterization by single-crystal and powder X-ray diffraction and infrared spectroscopy. The discrete mononuclear compound 1 exhibits a very high proton conductivity of 2.17 × 10⁻³ (Ω cm)⁻¹ at 25 °C and 74% relative humidity (RH), with σ_DC increased by nearly seven orders of magnitude. The three-dimensional (3D) hydrogen-bonding network of NH₄⁺ cations and water molecules (crystal and coordinated) as hydrogen-bond donors and the bidentate oxalate ligand and chloride ions of the anionic moiety [Fe(H₂O)Cl₃(C₂O₄)]²⁻ as hydrogen-bond acceptors plays a crucial role in proton transport; the density of the formed hydrogen-bonds positively impacts the mobility of protons in the nonporous structure of 1. The one-dimensional (1D) compounds 2 and 3, both containing infinite anionic zig-zag chains [FeCl₂(C₂O₄)]_nⁿ⁻ and cations (CH₃)₂(C₂H₅)NH⁺ (2) or (CH₃)(C₂H₅)₃N⁺ (3), showed an increase in conductivity of six and five orders of magnitude, reaching values of 2.00 × 10⁻⁴ (Ω cm)⁻¹ and 9.17 × 10⁻⁶ (Ω cm)⁻¹, respectively, at 25 °C and 93% RH. Compared to compound 3, compound 2 has hydrogen-bonded protonated (CH₃)₂(C₂H₅)NH⁺ cations that exhibit greater hydrophilicity, which is closely related to the affinity for water molecules that significantly affect proton transport and conductivity.