Magneto-structural correlations of cyclo-tetravanadates functionalized with mixed-ligand copper(ii) complexes

Abstract

Tetravanadate [V₄O₁₂]⁴⁻ anion, one of the cyclic polyoxovanadates formed by the condensation of VO₄ units, was used as the bidentate bridging ligand to design bimetallic materials from its functionalization with copper(II) complexes based on α-amino acids and typical aromatic ligands. The complexes {[Cu(Orn)(phen)]₂[μ₂-V₄O₁₂]}·6H₂O (1), {[Cu(Lys)(bipy)]₄[μ₂-V₄O₁₂]·14H₂O}_n (2) and {[Cu(Gly)(phen)]₂[μ₂-V₄O₁₂]} [Cu(Gly)(phen)(H₂O)]₂·10H₂O (3) (Orn = ornithine, phen = 1,10-phenanthroline, Lys = lysine, bipy = 2,2′-bipyridine, Gly = glycine) were prepared under non-hydrothermal conditions. These compounds were obtained at basic pH values (close to 9), which facilitated the formation of tetravanadate anion (from the precursor NH₄VO₃) and its stabilization with the metal complexes. All compounds crystallized in the triclinic system and showed an interesting structural and magnetic diversity, despite the fact that the components that form them are very similar. Although in compounds 1 and 3 the copper atoms are separated by the inorganic [V₄O₁₂]⁴⁻ anion by about 10 Å, the magnetic properties suggest antiferromagnetic interactions, which are explained through the intermolecular Cu⋯Cu interactions and the magnetic exchange pathways between the COO⁻ groups and the magnetic centres. The most striking feature of compound 2 was the formation of a 1D polymeric chain constructed with [V₄O₁₂]⁴⁻ units separated by [Cu(Lys)(bipy]₂⁴⁺ dinuclear bridges. The intramolecular Cu⋯Cu separation in this compound was 5.2 Å, and the magnetic properties suggest a canted antiferromagnetic ordering at low applied magnetic fields.