Oxalate-based [CuIICrIII] coordination compounds affected by the tridentate ligand, simple anion, and reactant ratio: structural and magnetic features

Abstract

Seven novel oxalate-based [Cu^IICr^III] compounds: [Cu₄(terpy)₄Cl₅][Cr(C₂O₄)₃]·9H₂O (1; terpy = 2,2′:6′,2′′-terpyridine), {[Cr₂Cu₄(H₂O)₂(terpy)₄(C₂O₄)₇]·10H₂O}_n (2), [Cr₂Cu₄(H₂O)₂(terpy)₄(C₂O₄)₇]·12H₂O (3), [Cu(H₂O)₃(terpy)][CrCu(H₂O)(terpy)(C₂O₄)₃]₂·9H₂O (4), [Cu(H₂O)(terpy)(NO₃)][CrCu(H₂O)(terpy)(C₂O₄)₃]·6H₂O (5), [CrCu₂(terpy)₂(C₂O₄)₃(NO₃)]·1.5H₂O·CH₃OH (6) and [Cr₂Cu₄(H₂O)₄(terpy)₄(C₂O₄)₆][Cr₂Cu₂(terpy)₂(C₂O₄)₆]·9H₂O·CH₃OH (7) were obtained from the reaction of an aqueous solution of the building block [Cr(C₂O₄)₃]³⁻ and a methanol solution containing Cu²⁺ ions and terpyridine ligand by the layering technique. Interestingly, changing only the anion of the starting salt of copper(II), NO₃⁻ instead of Cl⁻, resulted in an unexpected modification in the bridge type, namely oxalate (compounds 2–7) versus chloride (compound 1). During the crystallization process in the test tube, the partial decomposition of the tris(oxalato)chromate(III) anion leads to the release of the oxalate ligand from the coordination sphere of chromium(III). Consequently, this oxalate ligand is coordinated to copper(II) ions in the reaction mixture, resulting in the oxalate-bridged cationic moieties of copper(II) ions [(terpy)Cu(μ-C₂O₄)Cu(terpy)]²⁺ of 2 and 3. Compounds 4–7 were formed in the same test tube using identical components as for 2 and 3, but in a different ratio; during preparation, the starting material did not decompose and retained its original role as a building block. The compounds were studied by single-crystal X-ray diffraction, IR spectroscopy, magnetization measurements and density functional theory (DFT) calculations. Compound 1 exhibits a ground-state spin of 1 due to antiferromagnetic and ferromagnetic interactions of Cu²⁺ ions across the chloride bridges in the tetramer; ferromagnetic coupling transferred through the oxalate bridge was found between Cu²⁺ ions in compound 2 and between Cu²⁺ and Cr³⁺ in compounds 4 and 5. Since compound 3 is considered to be a very similar fragment of compound 2, a ferromagnetic interaction between two Cu²⁺ ions bridged by a bis(bidentate) oxalate group is also expected. The performed calculations for compound 7 indicate that the main interaction is ferromagnetic.