Antisymmetric Exchange
in Triangular Tricopper(II) Complexes: Correlation among Structural,
Magnetic, and Electron Paramagnetic Resonance Parameters
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Abstract
Two new trinuclear copper(II) complexes, [Cu<sub>3</sub>(μ<sub>3</sub>-OH)(daat)(Hdat)<sub>2</sub>(ClO<sub>4</sub>)<sub>2</sub>(H<sub>2</sub>O)<sub>3</sub>](ClO<sub>4</sub>)<sub>2</sub>·2H<sub>2</sub>O (<b>1</b>) and [Cu<sub>3</sub>(μ<sub>3</sub>-OH)(aaat)<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>](ClO<sub>4</sub>)<sub>2</sub>·3H<sub>2</sub>O (<b>2</b>) (daat = 3,5-diacetylamino-1,2,4-triazolate,
Hdat = 3,5-diamino-1,2,4-triazole, and aaat = 3-acetylamino-5-amino-1,2,4-triazolate),
have been prepared from 1,2,4-triazole derivatives and structurally
characterized by X-ray crystallography. The structures of <b>1</b> and <b>2</b> consist of cationic trinuclear copper(II) complexes
with a Cu<sub>3</sub>OH core held by three <i>N</i>,<i>N</i>-triazole bridges between each pair of copper(II) atoms.
The copper atoms are five-coordinate with distorted square-pyramidal
geometries. The magnetic properties of <b>1</b> and <b>2</b> and those of five other related 1,2,4-triazolato tricopper(II) complexes
with the same triangular structure (<b>3–7</b>) (whose
crystal structures were already reported) have been investigated in
the temperature range of 1.9–300 K. The formulas of <b>3–7</b> are [Cu<sub>3</sub>(μ<sub>3</sub>-OH)(aaat)<sub>3</sub>(H<sub>2</sub>O)<sub>3</sub>](NO<sub>3</sub>)<sub>2</sub>·H<sub>2</sub>O (<b>3</b>), {[Cu<sub>3</sub>(μ<sub>3</sub>-OH)(aat)<sub>3</sub>(μ<sub>3</sub>-SO<sub>4</sub>)]·6H<sub>2</sub>O}<sub><i>n</i></sub> (<b>4</b>), and [Cu<sub>3</sub>(μ<sub>3</sub>-OH)(aat)<sub>3</sub>A(H<sub>2</sub>O)<sub>2</sub>]A·<i>x</i>H<sub>2</sub>O [A = NO<sub>3</sub><sup>–</sup> (<b>5</b>), CF<sub>3</sub>SO<sub>3</sub><sup>–</sup> (<b>6</b>), or ClO<sub>4</sub><sup>–</sup> (<b>7</b>); <i>x</i> = 0 or 2] (aat =3-acetylamino-1,2,4-triazolate). The magnetic
and electron paramagnetic resonance (EPR) data have been analyzed
by using the following isotropic and antisymmetric exchange Hamiltonian: <i>H</i> = –<i>J</i>[<i>S</i><sub>1</sub><i>S</i><sub>2</sub> + <i>S</i><sub>2</sub><i>S</i><sub>3</sub>] – <i>j</i>[<i>S</i><sub>1</sub><i>S</i><sub>3</sub>] + <i>G</i>[<i>S</i><sub>1</sub> × <i>S</i><sub>2</sub> + <i>S</i><sub>2</sub> × <i>S</i><sub>3</sub> + <i>S</i><sub>1</sub> × <i>S</i><sub>3</sub>]. <b>1</b>–<b>7</b> exhibit strong antiferromagnetic coupling
(values for both –<i>J</i> and –<i>j</i> in the range of 210–142 cm<sup>–1</sup>) and antisymmetric
exchange (<i>G</i> varying from to 27 to 36 cm<sup>–1</sup>). At low temperatures, their EPR spectra display high-field (<i>g</i> < 2.0) signals indicating that the triangles present
symmetry lower than equilateral and that the antisymmetric exchange
is operative. A magneto-structural study showing a lineal correlation
between the Cu–O–Cu angle of the Cu<sub>3</sub>OH core
and the isotropic exchange parameters (<i>J</i> and <i>j</i>) has been conducted. Moreover, a model based on Moriya’s
theory that allows the prediction of the occurrence of antisymmetric
exchange in the tricopper(II) triangles, via analysis of the overlap
between the ground and excited states of the local Cu(II) ions, has
been proposed. In addition, analytical expressions for evaluating
both the isotropic and antisymmetric exchange parameters from the
experimental magnetic susceptibility data of triangular complexes
with local spins (<i>S</i>) of <sup>1</sup>/<sub>2</sub>, <sup>3</sup>/<sub>2</sub>, or <sup>5</sup>/<sub>2</sub> have been
purposely derived. Finally, the magnetic and EPR results of this work
are discussed and compared with those of other tricopper(II) triangles
reported in the literature
