Antisymmetric Exchange in Triangular Tricopper(II) Complexes: Correlation among Structural, Magnetic, and Electron Paramagnetic Resonance Parameters

Two new trinuclear copper­(II) complexes, [Cu₃(μ₃-OH)­(daat)­(Hdat)₂(ClO₄)₂(H₂O)₃]­(ClO₄)₂·2H₂O (<b>1</b>) and [Cu₃(μ₃-OH)­(aaat)₃(H₂O)₃]­(ClO₄)₂·3H₂O (<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 Cu₃OH 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 [Cu₃(μ₃-OH)­(aaat)₃(H₂O)₃]­(NO₃)₂·H₂O (<b>3</b>), {[Cu₃(μ₃-OH)­(aat)₃(μ₃-SO₄)]·6H₂O}_<i>n</i> (<b>4</b>), and [Cu₃(μ₃-OH)­(aat)₃A­(H₂O)₂]­A·<i>x</i>H₂O [A = NO₃^– (<b>5</b>), CF₃SO₃^– (<b>6</b>), or ClO₄^– (<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>₁<i>S</i>₂ + <i>S</i>₂<i>S</i>₃] – <i>j</i>[<i>S</i>₁<i>S</i>₃] + <i>G</i>[<i>S</i>₁ × <i>S</i>₂ + <i>S</i>₂ × <i>S</i>₃ + <i>S</i>₁ × <i>S</i>₃]. <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^–1) and antisymmetric exchange (<i>G</i> varying from to 27 to 36 cm^–1). 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 Cu₃OH 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 ¹/₂, ³/₂, or ⁵/₂ 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

Nonanuclear Spin-Crossover Complex Containing Iron(II) and Iron(III) Based on a 2,6-Bis(pyrazol-1-yl)pyridine Ligand Functionalized with a Carboxylate Group

The synthesis and magnetostructural characterization of [Fe^III₃(μ₃-O)­(H₂O)₃[Fe^II(bppCOOH)­(bppCOO)]₆]­(ClO₄)₁₃·(CH₃)₂CO)₆·(solvate) (<b>2</b>) are reported. This compound is obtained as a secondary product during synthesis of the mononuclear complex [Fe^II(bppCOOH)₂]­(ClO₄)₂ (<b>1</b>). The single-crystal X-ray diffraction structure of <b>2</b> shows that it contains the nonanuclear cluster of the formula [Fe^III₃(μ₃-O)­(H₂O)₃[Fe^II(bppCOOH)­(bppCOO)]₆]¹³⁺, which is formed by a central Fe^III₃O core coordinated to six partially deprotonated [Fe^II(bppCOOH)­(bppCOO)]⁺ complexes. Raman spectroscopy studies on single crystals of <b>1</b> and <b>2</b> have been performed to elucidate the spin and oxidation states of iron in <b>2</b>. These studies and magnetic characterization indicate that most of the iron­(II) complexes of <b>2</b> remain in the low-spin (LS) state and present a gradual and incomplete spin crossover above 300 K. On the other hand, the Fe^III trimer shows the expected antiferromagnetic behavior. From the structural point of view, <b>2</b> represents the first example in which bppCOO^– acts as a bridging ligand, thus forming a polynuclear magnetic complex

Nonanuclear Spin-Crossover Complex Containing Iron(II) and Iron(III) Based on a 2,6-Bis(pyrazol-1-yl)pyridine Ligand Functionalized with a Carboxylate Group

The synthesis and magnetostructural characterization of [Fe^III₃(μ₃-O)­(H₂O)₃[Fe^II(bppCOOH)­(bppCOO)]₆]­(ClO₄)₁₃·(CH₃)₂CO)₆·(solvate) (<b>2</b>) are reported. This compound is obtained as a secondary product during synthesis of the mononuclear complex [Fe^II(bppCOOH)₂]­(ClO₄)₂ (<b>1</b>). The single-crystal X-ray diffraction structure of <b>2</b> shows that it contains the nonanuclear cluster of the formula [Fe^III₃(μ₃-O)­(H₂O)₃[Fe^II(bppCOOH)­(bppCOO)]₆]¹³⁺, which is formed by a central Fe^III₃O core coordinated to six partially deprotonated [Fe^II(bppCOOH)­(bppCOO)]⁺ complexes. Raman spectroscopy studies on single crystals of <b>1</b> and <b>2</b> have been performed to elucidate the spin and oxidation states of iron in <b>2</b>. These studies and magnetic characterization indicate that most of the iron­(II) complexes of <b>2</b> remain in the low-spin (LS) state and present a gradual and incomplete spin crossover above 300 K. On the other hand, the Fe^III trimer shows the expected antiferromagnetic behavior. From the structural point of view, <b>2</b> represents the first example in which bppCOO^– acts as a bridging ligand, thus forming a polynuclear magnetic complex

Nonanuclear Spin-Crossover Complex Containing Iron(II) and Iron(III) Based on a 2,6-Bis(pyrazol-1-yl)pyridine Ligand Functionalized with a Carboxylate Group

The synthesis and magnetostructural characterization of [Fe^III₃(μ₃-O)­(H₂O)₃[Fe^II(bppCOOH)­(bppCOO)]₆]­(ClO₄)₁₃·(CH₃)₂CO)₆·(solvate) (<b>2</b>) are reported. This compound is obtained as a secondary product during synthesis of the mononuclear complex [Fe^II(bppCOOH)₂]­(ClO₄)₂ (<b>1</b>). The single-crystal X-ray diffraction structure of <b>2</b> shows that it contains the nonanuclear cluster of the formula [Fe^III₃(μ₃-O)­(H₂O)₃[Fe^II(bppCOOH)­(bppCOO)]₆]¹³⁺, which is formed by a central Fe^III₃O core coordinated to six partially deprotonated [Fe^II(bppCOOH)­(bppCOO)]⁺ complexes. Raman spectroscopy studies on single crystals of <b>1</b> and <b>2</b> have been performed to elucidate the spin and oxidation states of iron in <b>2</b>. These studies and magnetic characterization indicate that most of the iron­(II) complexes of <b>2</b> remain in the low-spin (LS) state and present a gradual and incomplete spin crossover above 300 K. On the other hand, the Fe^III trimer shows the expected antiferromagnetic behavior. From the structural point of view, <b>2</b> represents the first example in which bppCOO^– acts as a bridging ligand, thus forming a polynuclear magnetic complex