Use of [CoCr] complexes with alkyl-substituted ammonium cations for the preparation of the spinel oxide Co 2 CrO 4 Ana Lozančić, Krešimir Molčanov, Sanja Renka, Sanja Burazer, Lidija Androš Dubraja, Marijana Jurić Ruđer Bošković Institute, Bijenička cesta 54, Zagreb, Croatia A very important role in the design and synthesis of multifunctional materials is played by the oxalate moiety, C 2 O 4 2− , due to its various possibilities of coordination to metal centers and its ability to mediate electronic effects between paramagnetic metal ions. The synthetic strategy for preparing (hetero)polynuclear oxalate species is "building block chemistry", in which a tris(oxalato)metalate anion, [M III (C 2 O 4 ) 3 ] 3− , is used as a ligand towards other metal cations. The topology of these compounds is controlled by templating counterions, and by combining the intrinsic properties of the host, especially magnetic, with additional functionalities from the selected guest molecules, very interesting multifunctional properties can be obtained. Proton conductivity has recently been considered as a new functionality of oxalate-based compounds;the simplest method to introduce proton carriers is to incorporate a counterion such as hydronium (H 3 O + ), ammonium [NH 4 + , (CH 3 ) 2 NH 2 + ,...], resulting in the charged compounds. The counterions form hydrogen bonds with the guest water or other components of compound, forming proton-conducting pathways consisting of hydrogen bonding networks [1,2]. In addition, heterometallic complexes are used as single-source precursors, providing simplified synthetic routes for the formation of mixed metal oxide materials by thermal decomposition in one-step. 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 [3]. In order to simultaneously incorporate alkyl-ammonium counterions as proton carriers and overcome problems in the synthesis, we employed a procedure in which an aqueous solution of [Cr(C 2 O 4 ) 3 ] 3− containing (CH 3 ) 2 (C 2 H 5 ) NH + or (CH 3 )(C 2 H 5 ) 2 NH + is used as a building block in reactions with cobalt(II) ions and 2,2';6',2''-terpyridine (terpy). Two heterometallic compounds with mononuclear species, [NH(CH 3 ) 2 (C 2 H 5 )][Cr(terpy) 2 ][CoCl 4 ] 2 ( 1 ) and [NH(CH 3 )(C 2 H 5 ) 2 ][Cr(terpy) 2 ][CoCl 4 ] 2 ( 2 ) were formed; surprisingly, decomposition of the tris(oxalato)chromate(III) anion occurred during the crystallization process, and no oxalate-based compounds were formed. Due to the incorporation of the alkyl-substituted ammonium cations into the crystal structures, the proton conductivity of 1 and 2 was investigated. Based on the appropriate metal ratio, the obtained compounds were used to prepare the spinel oxide Co 2 CrO 4 by thermal treatment. We focused on the structural, optical and photocatalytic properties of the nanocrystalline product prepared by this route, successfully bypassing long‐term heating at high temperatures and repeated grinding. References 1. L. Molčanov, P. Šenjug, D. Barišić, D. Pajić, K. Molčanov, M. Jurić. Oxalate-based [Cu II Cr III ] coordination compounds affected by the tridentate ligand, simple anion, and reactant ratio: structural and magnetic features. Dalton Trans . 2022, 51, 16292- 16306. 2. S. Burazer, K.Molčano, A. Šantić, T. Klase, E. Wenger, D. Pajić, Z. Jagličić, J. Popović, M. Jurić, Humidity-Sensing Properties of an 1D Antiferromagnetic Oxalate-Bridged Coordination Polymer of Iron(III) and Its Temperature-Induced Structural Flexibility. Materials 2021, 14 , 5543. 3. L. Molčanov, L. Androš Dubraja, M. Vrankić, M. Jurić, A 3D oxalate-bridged [Cu II Fe II ] coordination polymer as molecular precursor for CuFe 2 O 4 spinel – photocatalytic features. J. Am. Ceram. Soc . 2023, 106, 2997-3008.
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