Water-assisted proton conductivity and a magnetic study of heterotrinuclear oxalate-bridged compounds: molecular precursors for the Mn2CrO4 spinel

Abstract

Novel oxalate-bridged heterotrinuclear complexes [A][Mn₂Cr(bpy)₂(H₂O)₂Cl₂(C₂O₄)₃] (A = (CH₃)₂(C₂H₅)NH⁺ (1) and (CH₃)(C₂H₅)₂NH⁺ (2); bpy = 2,2′-bipyridine) were synthesized using an aqueous solution of [A]₃[Cr(C₂O₄)₃] as a building block in reaction with Mn²⁺ ions and with the addition of the N-donor ligand bipyridine. The isostructural heterometallic complex salts were characterized by single-crystal and powder X-ray diffraction, infrared and impedance spectroscopy, thermal analysis and magnetization measurements. The trinuclear anion [{Mn(bpy)(H₂O)Cl(μ-C₂O₄)}₂Cr(C₂O₄)]⁻ consists of two [Mn(bpy)(H₂O)Cl]⁺ units bridged by the [Cr(C₂O₄)₃]³⁻ anion, which acts as a bidentate ligand towards each of the manganese atoms. The anions are hydrogen bonded to each other via coordinated chloride anions, water molecules and oxygen oxalate atoms, resulting in two-dimensional (2D) hydrogen bonding layers. Compounds exhibit water-assisted proton conductivity behaviour, which was investigated at different temperatures and relative humidities (RH). At 25 °C, an increase in RH from 60% to 93% resulted in an obvious proton conducting switch from 9.1 × 10⁻¹¹ to 5.6 × 10⁻⁵ S cm⁻¹ for 1 and from 7.4 × 10⁻¹⁰ to 1.8 × 10⁻⁶ S cm⁻¹ for 2, corresponding to high on/off ratios of about 10⁶ for 1 and 10⁴ for 2. In situ powder X-ray diffraction (PXRD) analysis showed that unit cell parameters of compounds 1 and 2 slightly increase when exposed to humid conditions. This confirmed that incorporation of water molecules into structures with pores and voids causes the proton conductivity switching phenomenon. Magnetic susceptibility measurements indicate a ferromagnetic interaction between Cr³⁺ and Mn²⁺ ions bridged by the bis(bidentate) oxalate group. The prepared compounds 1 and 2 were explored as single-source precursors for the formation of spinel oxide by their thermal treatment. With increasing temperature, the spinel composition changed according to the formula Mn_1+xCr_2–xO₄ (0 ≤ x ≤ 1), where x = 0.7 at 500 °C and x = 1 at 900 °C when ^tet[Mn^II]^oct[Mn^IIICr^III]O₄ is formed. The (micro)structure, morphology, and optical properties of spinel Mn₂CrO₄ were characterized by PXRD, scanning electron microscopy and UV-Vis diffuse reflectance spectroscopy. The photocatalytic activity of this oxide in degradation of the methylene blue dye under Vis irradiation without and with the support of hydrogen peroxide was further investigated.