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Anion-Directed Synthesis of Oxalate-Based [CuM] (M = Cr or Fe) Complexes Containing a Tridentate Ligand: Structural and Magnetic Properties

Heteropolynuclear metal complexes have been extensively studied for their ability to form various architectures and topologies, from discrete polynuclear metal compounds to polymeric one- (1D), two- (2D) or threedimensional (3D) assemblies, featuring a range of distinct magnetic properties. In recent decades, the chemistry of oxalate-containing complexes has become an active area of research – the oxalate moiety, C2O42−, acts as a linker between metal centres with various possibilities of bridiging modes and has the ability to mediate magnetic interactions between paramagnetic metal centres. Stable mononuclear anionic oxalate complexes such as tris(oxalato)metalate anions, [MIII(C2O4)3]3− (MIII = Mn, Cr, Fe, V), are often used as ligands toward another metal ion for the preparation of extended multifunctional systems. Also, the introduction of organic ligands containing N- donors into the metal–oxalate systems also influences the nuclearity of the metal centres and additionally stabilizes their solid-state structures. Applying the layering technique, crystals of the tetranuclear chloride-bridged compounds [Cu4(terpy)4Cl5][M(C2O4)3]·nH2O M = Cr3+ ; n = 9 (1) and M = Fe3+ n = 10 (2)] grew from the reaction of an aqueous solution of K3[Cr(C2O4)3]·3H2O or K3[Fe(C2O4)3]·3H2O, respectively, methanol solution containing CuCl2·2H2O and a tridentate ligand 2, 2′:6′, 2′′- terpyridine (terpy). When Cu(NO3)2·3H2O was used instead of the chloride salt, unexpected change in type of the bridge, oxalate versus chloride, was achieved and crystals of the 1D oxalate-bridged coordination polymers {; ; [Cu4Cr2III(H2O)2(terpy)4(C2O4)7]·10H2O}; ; n (3) and {; ; [CuII2FeIII(H2O)(terpy)2(C2O4)7/2]·6H2O}; ; n (4) together with compound [CuII2(H2O)2(terpy)2(C2O4)] [CuIIFeIII(CH3OH)(terpy)(C2O4)3]2 (5), respectively, have been formed. Further, two polymorphs of 3D coordination polymer [CuIIFeII2(H2O)(terpy)(C2O4)3]n (6a and 6b), 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. Compounds 1 and 2 exhibit a ground-state spin of 1, which is due to antiferromagnetic and ferromagnetic interactions of Cu2+ ions across the chloride bridges in the tetramers ; ferromagnetic coupling transferred through the oxalate bridge was found between Cu2+ ions in 3, as well as strong antiferromagnetic coupling of the Cu2+ ions mediated by the oxalate ligand in cation moiety, and weak ones between Cu2+ and Fe3+ ions through oxalate bridge in the anion of compound 5. Polymer 6 exhibits an antiferromagnetic phase transition at 25 K

structural and magnetic properties ; oxalate-based ; anion-directed synthesis

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