Trinuclear Cu^II complexes containing peripheral ketonic oxygen bridges and a μ₃-OH core: Syntheses, crystal structures, spectroscopic and magnetic properties

Four new trinuclear copper(II) complexes, [(CuL¹) ₃(μ₃-OH)](ClO₄)₂·H ₂O (1), [(CuL²)₃(μ₃-OH)](ClO ₄)₂ (2), [(CuL³)₃-(μ₃- OH)](ClO₄)₂·H₂O (3), and [(CuL ⁴)₃(μ₃-OH)](ClO₄) ₂·H₂O (4), where HL¹ = 8-amino-4,7,7-trimethyl-5-azaoct-3-en-2-one, HL² = 7-amino-4-methyl-5-azaoct-3-en-2-one, HL³ = 7-(ethylamino)-4-methyl- 5-azahept-3-en-2-one, and HL⁴ = 4-methyl-7-(methylamino)-5-azahept-3- en-2-one, have been derived from the four tridentate Schiff bases (HL ¹, HL², HL³, and HL⁴) and structurally characterized by X-ray crystallography. For all compounds, the cationic part is trinuclear with a Cu₃OH core held by three carbonyl oxygen bridges between each pair of copper(II) atoms. The copper atoms are five-coordinate with a distorted square-pyramidal geometry; the equatorial plane consists of the bridging oxygen atom of the central OH group together with three atoms (N, N, O) from one ligand whereas an oxygen atom of a second ligand occupies the axial position. Magnetic measurements have been performed in the 2-300 K temperature range. The experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = -J(S₁S ₂+S₂S₃+S₁S₃) yielding as best-fit parameters: J = -66.7 and g = 2.19 for 1, J = -36.6 and g = 2.20 for 2, J = -24.5 and g = 2.20 for 3, and J = -14.9 and g = 2.05 for 4. EPR spectra at low temperature show the existence of spin frustration in complexes 3 and 4, but it has not been possible to carry out calculations of the antisymmetric exchange parameter, G, from magnetic data. In frozen methanolic solution, at 4 K, hyperfine splitting in all complexes and spin frustration in complex 4 seem to be confirmed.