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Structural, Electrical, and Magnetic Versatility of the Oxalate-Based [CuFe] Compounds Containing 2,2′:6′,2″-Terpyridine: Anion-Directed Synthesis

The heterodimetallic [CuFe] compounds [CuII4(terpy)4Cl5][FeIII(C2O4)3]·10H2O (1 ; terpy = 2, 2′:6′, 2′′-terpyridine), [CuII2(H2O)2(terpy)2(C2O4)][CuIIFeIII(CH3OH) (terpy)(C2O4)3]2 (2), and {; ; ; [Cu2IIFeIII(H2O) (terpy)2(C2O4)7/2]·6H2O}; ; ; n (3) were obtained using building block approach, from reaction of aqueous solution of [Fe(C2O4)3]3– and a methanol solution containing Cu2+ ions and terpy by the layering technique. Interestingly, by changing only the anion of the starting salt of copper(II), Cu(NO3)2·3H2O instead of CuCl2·2H2O, an unexpected change in the type of bridge, oxalate (2 and 3) versus chloride (1), was achieved, thus affecting the overall structural architecture. Two polymorphs of 3D coordination polymer [CuIIFeII2(H2O)(terpy)(C2O4)3]n (4), crystallizing in the triclinic (a) and monoclinic (b) space groups, were formed hydrothermally, depending on whether CuCl2·2H2O or Cu(NO3)2·3H2O was added to the water, besides K3[Fe(C2O4)3]·3H2O and terpy, respectively. Under hydrothermal conditions iron(III) from initial building block is reduced to the divalent state, creating 2D honeycomb [FeII2(C2O4)3]n2n– layers, which are bridged by [Cu(H2O)(terpy)]2+ cations. Compounds were investigated by single-crystal X- ray diffraction, IR, and impedance spectroscopies, magnetization measurements, and density functional theory (DFT) calculations. In compounds 1 and 2, 0D magnetism is observed, with 1 having a ground-state spin of 1 due to different interactions through chloride bridges of Cu2+ ions in tetramer [CuII4(terpy)4Cl5]3+ and 2 showing strong antiferromagnetic coupling of Cu2+ ions mediated by oxalate ligand in [CuII2(H2O)2(terpy)2(C2O4)]2+ and weak ones between Cu2+ and Fe3+ ions through oxalate bridge in [CuIIFeIII(CH3OH)(terpy)(C2O4)3]−. Polymer 4 exhibits antiferromagnetic phase transition at 25 K: The [FeII2(C2O4)3]n2n– layers are antiferromagnetically ordered, and a small amount of interlayer interaction is transferred through [Cu(H2O)(terpy)]2+ cations via Oox–Cu–Oox bridges. Additionally, compounds 1 and 2 are electrical insulators, while 4a and 4b show proton conductivity.

coordination polymers ; oxalate-bridged ; crystal structure ; electrical property ; magnetic property

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