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Controlled incorporation of alkyl ammonium cations into oxalate-bridged compounds [MnIICrIII] by N- ligand: A precursor for mixed oxide

Multifunctional properties of the metal-organic coordination compounds can be achieved by combining the intrinsic properties of the host, especially the magnetic ones, with an additional functionalities originating from the selected guest molecules. The oxalate group, C2O42–, has proven to be one of the most versatile ligands used in the preparation of these systems. Due to its different coordination modes towards the metal centres, as well as its ability to mediate magnetic interactions between paramagnetic metal ions, a large number of oxalate-based transition metal species of different dimensionality have been synthesized and characterized, many of them having tunable magnetic frameworks. Most of the oxalate-bridged systems described to date have been obtained by the "complex-as-ligand" approach ; a building block, the tris(oxalato)metalate [MIII(C2O4)3]3− anion, is used as a ligand toward other metal ions. The topology of these compounds is controlled by a templating counterion.1 Proton conductivity has recently been considered as a new functionality of metal-organic compounds ; the simplest method to introduce proton carriers is to incorporate a counterion such as hydronium (H3O+), ammonium [NH4+, (CH3)2NH2+, ...] or an anion (SO42−), resulting in the charged compounds. The counterions form the hydrogen bonds with the guest water or other components of compound, forming proton-conducting pathways consisting of hydrogen bonding networks. The oxygen atoms of the oxalate group may also be involved in the formation of hydrogen bonds, which makes oxalate systems suitable for electrical property research in addition to their their high crystallinity and high stability to water.2, 3 Further, the use of heterometallic complexes or frameworks as single-source precursors provides 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 – allowing excellent stoichiometric control of the intermetallic ratio in the oxide products.4 To achieve our ultimate goal – the exploration of complexes with remarkable structural, magnetic and electrical properties1, 2 three novel heterometallic oxalate-bridged compounds were obtained from the reaction A3[CrIII(C2O4)3] [A = (C2H5)2(CH3)NH+ or (C2H5)(CH3)2NH+], as ligand towards manganise(II) ions and N-ligand [2, 2'- bipyridine (bpy) or 2, 2' ; 6', 2''-terpyridine (terpy)]: trinuclear [NH(CH3)(C2H5)2][{;Mn(bpy) (H2O)Cl};2Cr(C2O4)3] (1a) and [NH(CH3)2(C2H5)] [{;Mn(bpy)(H2O)Cl};2Cr(C2O4)3] (1b), and dinuclear [terpyH][Mn(H2O)2(terpy)Cr(C2O4)3]·4H2O (2). Unlike compounds 1a and 1b, which were obtained with a smaller bidentate 2, 2'-bypiridine ligand, the use of a bulky tridentate N-donor ligand terpyridine does not allow the incorporation of an alkyl ammonium cation into the structure of 2. This 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 preparation of mixed metal oxides.1, 4 In addition to the powder and single crystal X-ray diffraction, obtained compounds were characterized by thermal analysis, IR and impedance spectroscopy.

coordination compounds, oxalate group, crystal structure, proton conductivity, mix metal oxide

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