High proton conductivity in 2D oxalate [MnIICrIII] polymer with alkyl ammonium cations and its oxide- related usage

It is well known that the oxalate group, C2O42–, used in the preparation of metal-organic coordination compounds, is one of the most versatile ligands with different coordination modes towards metal centres which likewise has ability to mediate magnetic interactions between paramagnetic metal ions. One of the synthetic strategy for preparation of (hetero)polynuclear species is "building block chemistry" in which a molecular anionic ligand, very often the tris(oxalato)metalate anion, [MIII(C2O4)3]3−, is used as a ligand towards other metal cations. The topology of these compounds is controlled by a templating counterion. Combining the intrinsic properties of the host, especially the magnetic ones, with additional functionalities originating from the selected guest molecules, very interesting multifunctional properties can be obtained. Proton conductivity as a new functionality of metal-organic compounds can be achieved by incorporating a counterion such as hydronium (H3O+), ammonium [NH4+, (CH3)2NH2+, ...] or an anion (SO42−) into the anionic network. The counterions form the hydrogen bonds with the guest water or other components of compound and create proton-conducting pathways consisting of hydrogen bonding networks [1]. In addition, heterometallic complexes as single-source precursors provide simplified synthetic routes through one-step thermal decomposition to form mixed metal oxide materials. The advantage of a solid phase transition is the retention of the elemental composition defined by the molecular precursor with only a loss of volatile decomposition products, which allows excellent stoichiometric control of the intermetallic ratio in the oxide products [2]. Motivated by our previous results [1], we investigated the proton conduction behaviour of the novel oxalate-bridged coordination polymer {;[NH(CH3)2(C2H5)]8[Mn4Cl4Cr4(C2O4)12]};n (1) obtained using an aqueous solution of [NH(CH3)2(C2H5)]3[Cr(C2O4)3] as a building block in reaction with Mn2+ ions. Compound 1, consisting of 2D anionic [Mn4Cl4Cr4(C2O4)12)]n8n– networks and hydrogen-bonded [NH(CH3)2(C2H5)]+ cations between these networks, exhibits remarkable humidity-sensing properties and very high proton conductivity at room temperature [1.60 × 10-3 (·cm)-1 at 298 K under 90% relative humidity]. Moreover, this heterometallic compound was tested as a single molecular precursor for the preparation of MnCrO4 oxide in one step, due to its suitable metal ratio. The oxalate group, C2O42−, easily decomposes at low temperatures into gaseous CO2 and CO, and thus the oxalate‐based solids can serve as suitable precursors for the preparation of mixed metal oxides. In addition to powder and single crystal X-ray diffraction, the obtained compound has been characterized by thermal analysis, IR and impedance spectroscopy.