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High proton conductivity of the homo- and heterometallic oxalate-based compounds of different dimensionality

Recently, proton conductivity of the metal-organic frameworks (MOFs) has been considered as a new functionality, mainly due to their crystallinity, high porosity, designability and tunability in structure and properties, and potential application in solid-state electrolytes. The simplest method to introduce proton carriers is to include a counterion such as hydronium (H3O+), ammonium [NH4+, (CH3)2NH2+, ...] or an anion (SO42−), resulting in charged compounds. The counterions form the hydrogen-bonding arrays with the guest water or other constituents of compound, creating proton-conducting pathways.1, 2 A very important role in the design and synthesis of multifunctional materials belongs to the oxalate moiety, C2O42−, due to its various possibilities of coordination to metal centers and its ability to mediate electronic effects between paramagnetic metal ions. The proton conductive materials must have good water and chemical durability under the influence of humidity and temperature, and in general, oxalate-based systems have regular structures and stable frameworks. In addition, the oxygen atoms of the oxalate group can construct complex hydrogen-bonded networks, which are more than suitable for proton conduction. Although high proton conductivity is often found in two-dimensional (2D) and three-dimensional (3D) oxalate-based compounds, the low-dimensional structures can have excellent proton conductivity properties via the hydrogen-bonding chain because a low void space can be advantageous for proton hopping.2, 3 We explored the proton conduction behaviour of three oxalate-based compounds of different nuclearity and dimensionality, prepared and characterized in our group: homometallic discrete mononuclear (NH4)2[Fe(H2O)Cl3(C2O4)]·H2O (1) and one-dimensional (1D) {; ; [NH(CH3)(C2H5)2][FeCl2(C2O4)]·H2O}; ; n (2), and 2D heterometallic {; ; [NH(CH3)2 (C2H5)]8[Mn4Cl4Cr4(C2O4)12]}; ; n (3). The proton conductivity of 1, 2 and 3 at 298 K increases with increasing relative humidity (RH), achieving values of 2.17 × 10−3 (Ω cm)−1 at 74% RH, 2.70 × 10−4 (Ω cm)−1 at 93% RH and 1.6 × 10–3 (Ω cm)–1 at 90% RH, respectively. From the obtained values, it can be confirmed and concluded that the amount of protonated proton carriers acting as proton donors, the number of non-protonated sites acting as proton acceptors, and the efficient proton transport pathway made of H-bonding network are extremely important for obtaining materials with high conductivity, rather than their dimensional arrangement. References (1) X.-X. Xie, Y.-C. Yang, B.-H. Dou, Z.-F. Li, G. Li, Coord. Chem. Rev. 2020, 404, 213100. (2) D-W. ; Lim, H. Kitagaw, Chem. Soc. Rev. 2021, 50, 6349-6368. (3) S. Burazer, K. Molčano, A. Šantić, T. Klase, E. Wenger, D. Pajić, Z. Jagličić, J. Popović, M. Jurić, Materials 2021, 14, 5543. Acknowledgement This work has been funded by the Croatian Science Foundation under project number IP-2019-04-5742.

Oxalate-Based Compounds ; Alkyl-Ammonium Cations ; Proton Conductivity ; H-Bonding Network

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