RESUMEN
An unusual and unique conformation of a paddle wheel type binuclear copper(II) complex containing acetate and acetamido ligands, {Cu2(µ2-O2CCH3)4}(OCNH2CH3) (1), was obtained by solvothermal synthesis. The structural characterization of this compound shows that the apical (acetamido) ligands are disposed at a 62° dihedral angle, generating a special conformation as a consequence of the synthetic method used. This conformation has not been reported in other paddle wheel copper(II) tetraacetate compounds. Electron paramagnetic resonance (EPR) spectra of powder samples of (1) were obtained at 9.5 and 33.8 GHz, while single crystal spectra were obtained at 33.8 GHz with a B0 applied in three orthogonal planes. The fit of the single crystal experimental data allowed gave g⥠= 2.345 ± 0.003, and g⥠= 2.057 ± 0.005. The angular variation of the EPR line allows evaluation of the fine structure of (1), giving D = -0.337 ± 0.002 cm(-1) and E = -0.005 ± 0.001 cm(-1). The line width angular dependence, used together with the Anderson model and Kubo-Tomita theory, permitted the interdimer interaction to be evaluated as |J'| = (0.051 ± 0.002) cm(-1). Using the powder spectral temperature dependence it was possible to evaluate the intradinuclear exchange coupling constan J0 as -101 ± 2 cm(-1), which is considerably lower than that reported for other analogous copper(II) tetraacetate paddle wheel compounds (Cu(II)-PW), showing the remarkable effect of the conformation of the terminal ligands on the magnetic interaction.
RESUMEN
EPR measurements have been performed on single crystals of [Cu(L-tyrosine)(2)](∞) at 33.8 GHz and at room temperature. The EPR spectra display partially resolved EPR lines for most orientations of the magnetic field in the ab plane, and only one resonance for orientations close to the crystal axes, while only a single line is observed along any direction in the ca and cb crystal planes. This behavior is a result of the selective collapse of the resonances corresponding to the four copper sites in the unit cell produced by the exchange interactions between the copper ions. The magnitudes of the exchange interactions between the copper ions were evaluated from the angular variation of the line width and the collapse of the EPR lines. The value |J(AD)/k(B)|=0.8 K between neighboring copper atoms at 4.942 Å is assigned to a syn-anti equatorial-apical carboxylate bridge with a total bond length of 6.822 Å, while the small value |J(AB)/k(B)|=0.004 K is assigned to a long bridge of 11 atoms with a total bond length of 19.186 Å, that includes one resonance assisted hydrogen bond (RAHB). This finding is discussed in terms of values obtained for similar paths in other model compounds and in proteins.