Experimental and Theoretical Study of the Antisymmetric Magnetic Behavior of Copper inverse-9-Metallacrown-3 Compounds

Use of PhPyCNO⁻/X⁻ “blends” (PhPyCNOH = phenyl 2-pyridyl ketoxime; X⁻ = OH⁻, alkanoato, ClO₄⁻) in copper chemistry yielded trinuclear clusters that have been characterized as inverse-9-metallacrown-3 compounds and accommodate one or two guest ligands. The magnetic behavior showed a large antiferromagnetic interaction and a discrepancy between the low-temperature magnetic behavior observed experimentally and that predicted from a magnetic model. The discrepancy between the Brillouin curve and the experimental result provides clear evidence of the influence of the antisymmetric interaction. Introducing the antisymmetric terms derived from the fit of the susceptibility data into the magnetization formula caused the simulated curve to become nearly superimposable on the experimental one. The EPR data indicated that the compound [Cu₃(PhPyCNO)₃(μ₃-OH)(2,4,5-T)₂] (1), where 2,4,5-T is 2,4,5-trichlorophenoxyacetate, has isosceles or lower magnetic symmetry (δ ≠ 0), that antisymmetric exchange is important (G ≠ 0), and that ΔE > hν. The structures of the complexes 1 and [Cu₃(PhPyCNO)₃(μ₃-OH)(H₂O)(ClO₄)₂] (2) were determined using single-crystal X-ray crystallography. Theoretical calculations based on density functional theory were performed using the full crystal structures of 1, 2, [Cu₃(PhPyCNO)₃(OH)(CH₃OH)₂(ClO₄)₂] (3), and [Cu₃(PhPyCNO)₃(μ₃-OMe)(Cl)(ClO₄)] (4). The geometries of the model compounds [Cu₃(κ³N,N,O-HNCHCHNO)₃(μ₃-OH)(μ₂-HCOO)(HCOO)] (5), [Cu₃(κ³N,N,O-HNCHCHNO)₃(μ₂-HCOO)(HCOO)]⁺ (6), [Cu₃(κ³N,N,O-HNCHCHNO)₃(μ₃-O)]⁺ (7), and [Cu₃(κ³N,N,O-HNCHCHNO)₃]³⁺ (8) were optimized at the same level of theory for both the doublet and quartet states, and vibrational analysis indicated that the resulting equilibrium geometries corresponded to minima on the potential energy surfaces. Both e_g and t_2g magnetic orbitals seem to contribute to the magnetic exchange coupling. The latter contribution, although less important, might be due to overlap of the t_2g orbitals with the p-type orbitals of the central triply bridging oxide ligand, thereby affecting its displacement from the Cu₃ plane and contributing to the antiferromagnetic coupling. The crucial role of the triply bridging oxide (μ₃-O) ligand on the antiferromagnetic exchange coupling between the three Cu(II) magnetic centers is further evidenced by the excellent linear correlation of the coupling constant J with the distance of the μ₃-O ligand from the centroid of the Cu₃ triangle.