Trinuclear Cu^II complexes containing peripheral ketonic oxygen bridges and a &#956;<sub>3</sub >-OH core: syntheses, crystal structures, spectroscopic and magnetic properties

Four new trinuclear copper (II) complexes, [(CuL1)3(&#956;3-OH)](ClO4)2•H2O (1), [(CuL2)3 (&#956;3-OH)](ClO4)2 (2), [(CuL3)3(&#956;3-OH)](ClO4)2•H2O (3) and [(CuL4)3(&#956;3-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 (HL1, 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(S1S2+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