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

The heterodimetallic [CuFe] compounds [CuII 4 (terpy)4 Cl5 ][FeIII (C2 O4 )3 ]·10H2 O ( 1 ;terpy = 2,2':6',2''-terpyridine), [CuII 2 (H2 O)2 (terpy)2 (C2 O4 )][CuII FeIII (CH3 OH)(terpy)(C2 O4 )3 ]2 ( 2 ), and {[Cu2 II FeIII (H2 O)(terpy)2 (C2 O4 )7/2 ]·6H2 O} n ( 3 ) were obtained using building block approach, from reaction of aqueous solution of [Fe(C2 O4 )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 ·3H2 O instead of CuCl2 ·2H2 O, 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 [CuII FeII 2 (H2 O)(terpy)(C2 O4 )3 ] n ( 4 ), crystallizing in the triclinic ( a ) and monoclinic ( b ) space groups, were formed hydrothermally, depending on whether CuCl2 ·2H2 O or Cu(NO3 )2 ·3H2 O was added to the water, besides K3 [Fe(C2 O4 )3 ]·3H2 O and terpy, respectively. Under hydrothermal conditions iron(III) from initial building block is reduced to the divalent state, creating 2D honeycomb [FeII 2 (C2 O4 )3 ] n 2 n - layers, which are bridged by [Cu(H2 O)(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 [CuII 4 (terpy)4 Cl5 ]3+ and 2 showing strong antiferromagnetic coupling of Cu2+ ions mediated by oxalate ligand in [CuII 2 (H2 O)2 (terpy)2 (C2 O4 )]2+ and weak ones between Cu2+ and Fe3+ ions through oxalate bridge in [CuII FeIII (CH3 OH)(terpy)(C2 O4 )3 ]- . Polymer 4 exhibits antiferromagnetic phase transition at 25 K: The [FeII 2 (C2 O4 )3 ] n 2 n - layers are antiferromagnetically ordered, and a small amount of interlayer interaction is transferred through [Cu(H2 O)(terpy)]2+ cations via Oox -Cu-Oox bridges. Additionally, compounds 1 and 2 are electrical insulators, while 4a and 4b show proton conductivity.