Relatively strong intramolecular antiferromagnetic coupling in a neutral CrIII2NbV2 heterobimetallic molecular square

Magnetic and Electrical Behaviors of the Homo- and Heterometallic 1D and 3D Coordination Polymers Based on the Partial Decomposition of the [Cr(C2O4)3]3- Building Block

One-dimensional (1D) oxalate-bridged homometallic {[Mn(bpy)(C₂O₄)]·1.5H₂O}_n (1) (bpy = 2,2’-bipyridine) and heterodimetallic {[CrCu₃(bpy)₃(CH₃OH)(H₂O)(C₂O₄)₄][Cu(bpy)Cr(C₂O₄)₃]·CH₂Cl₂·CH₃OH·H₂O}_n (2) coordination polymers, as well as the three-dimensional (3D) heterotrimetallic {[CaCr₂Cu₂(phen)₄(C₂O₄)₆]·4CH₃CN·2H₂O}_n (3) (1,10-phenanthroline) network, have been synthesized by a building block approach using a layering technique, and characterized by single-crystal X-ray diffraction, infrared (IR) and impedance spectroscopies and magnetization measurements. During the crystallization process partial decomposition of the tris(oxalato)chromate(III) happened and 1D polymers 1 and 2 were formed. The antiferromagnetic interactions between the manganese(II) ions were mediated by oxalate ligands in the chain [Mn(bpy)(C₂O₄)]_n of 1, with intra-chain super-exchange interaction 𝐽 = (−3.134 ± 0.004) K; magnetic interaction between neighbouring chains is negligible making this system closer than other known Mn-chains to the ideal 1D Heisenberg antiferromagnet. Compound 2 comprises a 1D coordination anion [Cu(bpy)Cr(C₂O₄)₃]_nⁿ⁻ (Cr2–Cu4) with alternating [Cr(C₂O₄)₃]³⁻ and [Cu(bpy)]²⁺ units mutually bridged through the oxalate group. Another chain (Cr1–Cu3) is similar, but involves a homodinuclear unit [Cu(bpy)(H₂O)(µ-C₂O₄)Cu(bpy)(CH₃OH)]²⁺ (Cu1–Cu2) coordinated as a pendant group to a terminal oxalate oxygen. Magnetic measurements showed that the Cu1−Cu2 cationic unit is a strongly coupled antiferromagnetic dimer, independent from the other magnetic ions within ferromagnetic chains Cr1–Cu3 and Cr2–Cu4. A 3D polymer {[CaCr₂Cu₂(phen)₄(C₂O₄)₆]·4CH₃CN·2H₂O}_n (3) comprising three different metal centers (Ca²⁺, Cr³⁺ and Cu²⁺) oxalate-bridged, contains Ca²⁺ atoms as nodes connected with four Cr³⁺ atoms through oxalate ligands. The network thus formed can be reduced to an underlying graph of diamondoid (dia) or (6⁶) topology. Magnetization of 3 shows the ferromagnetic oxalate-bridged dimers [Cu^IICr^III], whose mutual interaction could possibly originate through the spin polarization of Ca²⁺ orbitals. Compounds 1 and 3 exhibit lower electrical conductivity at room temperature (RT) in comparison to compound 2.

From a square core to square opening: structural diversity and magnetic properties of the oxo-bridged [CrIIINbV] complexes

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 {Cr(μ-O)₄Nb} unit, while that of complex salt 3 (P1[combining macron] space group) consists of a mononuclear cation containing Cr^III and trinuclear anions in which two Cr^III ions are bridged by a -O-Nb^V-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 Cr^III ions with spin 3/2 interacting through diamagnetic -O-Nb^V-O- bridge(s).

Towards oxalate-bridged iron(ii), cobalt(ii), nickel(ii) and zinc(ii) complexes through oxotris(oxalato)niobate(v): an open air non-oxidizing synthetic route

Oxalate release by [Nb^VO(C₂O₄)₃]^3– is a suitable strategy to prepare oxalate-bridged dimetal(ii) complexes that exhibit antiferromagnetic interactions.

Experimental and Theoretical Investigation of the Anti-Ferromagnetic Coupling of CrIII Ions through Diamagnetic -O-NbV-O- Bridges

The synthesis and properties of a novel hetero-tetranuclear compound [Cr₂(bpy)₄(μ-O)₄Nb₂(C₂O₄)₄]·3H₂O (1; bpy = 2,2'-bipyridine), investigated by single-crystal X-ray diffraction, magnetization measurements, IR, UV/visible spectroscopy, electron paramagnetic resonance (EPR; X- and Q-bands and high-field), and density functional theory (DFT) calculations, are reported. Crystal structure of 1 (orthorhombic Pcab space group) consists of a square-shaped macrocyclic {Cr₂(μ-O)₄Nb₂} core in which Cr^III and Nb^V ions are alternately bridged by oxo ions and three uncoordinated water molecules. The intramolecular Cr^III···Cr^III distances through the -O-Nb^V-O- bridges are 7.410(2) and 7.419(2) Å, while diagonal separation is 5.406(2) Å. The temperature dependence of magnetization M(T) evidences an anti-ferromagnetic ground state, which originates from a magnetic interaction between two Cr^III ions of spin 3/2 through two triatomic -O-Nb^V-O- diamagnetic bridges. A spin Hamiltonian appropriate for polynuclear isolated magnetic units was used. The best-fitting curve for this model is obtained with the parameters g_Cr = 1.992(3), J = -12.77(5) cm^-1, and |D| = 0.17(4) cm^-1. The Cr^III···Cr^III dimer model is confirmed by EPR spectra, which exhibit a pronounced change of their shape around the temperature corresponding to the intradimer coupling J. The EPR spectra simulations and DFT calculations reveal the presence of a single-ion anisotropy that is close to being uniaxial, D = -0.31 cm^-1 and E = 0.024 cm^-1.