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Seven novel oxalate-based [CuIICrIII] 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.

Three heterometallic oxo-bridged compounds, [Cr2(phen)4(μ-O)4Nb2(C2O4)4]·2H2O (1 ; phen = 1, 10-phenanthroline), [Cr2(terpy)2(H2O)2(μ-O)4Nb2(C2O4)4]·4H2O (2 ; terpy = 2, 2′ ; 6′, 2′′-terpyridine) and [Cr(terpy)(C2O4)(H2O)][Cr2(terpy)2(C2O4)2(μ-O)2Nb(C2O4)2]·3H2O (3), have been synthesized using a building block approach and characterized by IR spectroscopy, single-crystal and powder X-ray diffraction, magnetization measurements, and DFT calculations. The molecular structures of 1 and 2, crystallizing in P42212 and P21/n space groups, respectively, contain a square-shaped {; ; ; ; CrIII2(μ-O)4NbV2}; ; ; ; unit, while that of complex salt 3 (P[1 with combining macron] space group) consists of a mononuclear cation containing CrIII and trinuclear anions in which two CrIII ions are bridged by a –O–NbV–O– fragment. Besides hydrogen-bonding patterns resulting in a 1D- or 3D-supramolecular arrangement in 1–3, an unusual intermolecular contact has been noticed between parallel oxalate moieties occurring due to the electrostatic attraction of electron-rich carbonyl oxygen and severely electron-depleted carbon atoms in the crystal packing of 2. The antiferromagnetic coupling observed in all three compounds, determined from magnetization measurements (J = −13.51(2), −8.41(1) and −7.44(4) cm−1 for 1, 2 and 3, respectively) and confirmed by DFT calculations, originates from two CrIII ions with spin 3/2 interacting through diamagnetic –O–NbV–O– bridge(s).

The reaction of bis(phenanthroline)metal(II) cations (M = Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) with bis(oxalato)chromium(III) anions in a water/ethanol solution gives rise to a series of compounds with oxalate-bridged cations, [{; ; M(phen)2}; ; 2(μ-C2O4)][Cr(phen)(C2O4)2]2·4H2O [Mn2Cr2 (1), Co2Cr2 (2), Ni2Cr2 (3), Cu2Cr2 (4) and Zn2Cr2 (5)]. Their structural analysis reveals that all the prepared compounds crystallize in the triclinic system, space group P[1 with combining macron], having similar unit cell parameters, molecular structures and crystal packing features. All metal centres in 1–5 are octahedrally coordinated: M2+ in homodinuclear cations are coordinated with two phen molecules and one bridging oxalate ligand ; Cr3+ in anions is coordinated with one phen ligand and two bidentate oxalate groups. The copper atom in Cu2Cr2 (4) exhibits a Jahn–Teller-distorted octahedral coordination. Owing to the considerable number of pyridyl groups present in 1–5 (from phen ligands) the crystal packing of cations and anions is driven by stacking interactions appearing in offset-face-to-face (OFF) and edge-to-face (EF) orientations. The hydrogen bonds between the anions and water molecules of crystallization form 1D ladder-like motifs. In addition to the single crystal X-ray diffraction studies, the characterization of the new complexes was accomplished by means of IR and UV/Vis spectroscopy and magnetization measurements on a SQUID magnetometer. The temperature dependence of magnetic susceptibility reveals different magnetic super exchange interactions taking place in homodinuclear oxalate-bridged cations depending on the transition metal centre (Mn2+, Co2+, Ni2+ and Cu2+). Oxalate ligands mediate the ferromagnetic coupling of Cu2+ metal cations in Cu2Cr2 (4), whereas in Mn2Cr2 (1), Co2Cr2 (2) and Ni2Cr2 (3), antiferromagnetic interactions are observed between Mn2+, Co2+ and Ni2+ cations, respectively. Also, relatively large zero-field splitting parameters for Cr3+ ions (from mononuclear anions), D ≈ 1 cm−1, were observed.