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

Two Novel Ternary Dicopper(II) μ‑Guanazole Complexes with Aromatic Amines Strongly Activated by Quantum Dots for DNA Cleavage

Two novel (μ-guanazole)-bridged binuclear copper­(II) complexes with 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy), [Cu₂(μ-<i>N</i>2,<i>N</i>4-Hdatrz)­(phen)₂(H₂O)­(NO₃)₄] (<b>1</b>) and [Cu₂(μ-<i>N</i>1,<i>N</i>2-datrz)₂(μ-OH₂)­(bipy)₂]­(ClO₄)₂ (<b>2</b>) (Hdatrz = 3,5-diamino-1,2,4-triazole = guanazole), have been prepared and characterized by X-ray diffraction, spectroscopy, and susceptibility measurements. Compounds <b>1</b> and <b>2</b> differ in the aromatic amine, which acts as a coligand, and in the Cu···Cu′-bridging system. Compound <b>1</b>, which contains two mono-bridged copper ions, represents the first example of a discrete Cu–(<i>N</i>C<i>N</i>-trz)–Cu′ complex. Compound <b>2</b>, with two triply bridged copper ions, is one of the few compounds featuring a Cu–[(<i>NN</i>-trz)₂ + (<i>O-</i>aquo)]–Cu′ unit. Both compounds display antiferromagnetic coupling but of different magnitude: <i>J</i> (μ_2,4-triazole) = −52 cm^–1 for <b>1</b> and <i>J</i> (μ_1,2-triazolate) = −115 cm^–1 for <b>2</b>. The DNA binding and cleavage properties of the two compounds have been investigated. Fluorescence, viscosimetry, and thermal denaturation studies reveal that both complexes have high affinity for DNA (<b>1</b> > <b>2</b>) and that only <b>1</b> acts as an intercalator. In the presence of a reducing agent like 3-mercaptopropionic acid, <b>1</b> produces significant oxidative DNA cleavage, whereas <b>2</b> is inactive. However, in the presence of very small quantities of micelles filled with core–shell CdSe-ZnS quantum dots (15 nM), <b>1</b> and <b>2</b> are considerably more active and become highly efficient nucleases as a result of the different possible mechanisms for promoting cooperative catalysis (metal–metal, metal–hydrogen bonding, metal–intercalation, and metal–nanoparticle). Electrophoresis DNA-cleavage inhibition experiments, X-ray photoelectron spectroscopy studies, and fluorescence ethidium bromide displacement assays reveal that in these novel nucleases the QDs act as redox-active protein-like nanoparticle structures that bind to the DNA and deliver electrons to the copper­(II) centers for the generation of Cu­(I) and reactive oxygen species

Two Novel Ternary Dicopper(II) μ‑Guanazole Complexes with Aromatic Amines Strongly Activated by Quantum Dots for DNA Cleavage

Two novel (μ-guanazole)-bridged binuclear copper­(II) complexes with 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy), [Cu₂(μ-<i>N</i>2,<i>N</i>4-Hdatrz)­(phen)₂(H₂O)­(NO₃)₄] (<b>1</b>) and [Cu₂(μ-<i>N</i>1,<i>N</i>2-datrz)₂(μ-OH₂)­(bipy)₂]­(ClO₄)₂ (<b>2</b>) (Hdatrz = 3,5-diamino-1,2,4-triazole = guanazole), have been prepared and characterized by X-ray diffraction, spectroscopy, and susceptibility measurements. Compounds <b>1</b> and <b>2</b> differ in the aromatic amine, which acts as a coligand, and in the Cu···Cu′-bridging system. Compound <b>1</b>, which contains two mono-bridged copper ions, represents the first example of a discrete Cu–(<i>N</i>C<i>N</i>-trz)–Cu′ complex. Compound <b>2</b>, with two triply bridged copper ions, is one of the few compounds featuring a Cu–[(<i>NN</i>-trz)₂ + (<i>O-</i>aquo)]–Cu′ unit. Both compounds display antiferromagnetic coupling but of different magnitude: <i>J</i> (μ_2,4-triazole) = −52 cm^–1 for <b>1</b> and <i>J</i> (μ_1,2-triazolate) = −115 cm^–1 for <b>2</b>. The DNA binding and cleavage properties of the two compounds have been investigated. Fluorescence, viscosimetry, and thermal denaturation studies reveal that both complexes have high affinity for DNA (<b>1</b> > <b>2</b>) and that only <b>1</b> acts as an intercalator. In the presence of a reducing agent like 3-mercaptopropionic acid, <b>1</b> produces significant oxidative DNA cleavage, whereas <b>2</b> is inactive. However, in the presence of very small quantities of micelles filled with core–shell CdSe-ZnS quantum dots (15 nM), <b>1</b> and <b>2</b> are considerably more active and become highly efficient nucleases as a result of the different possible mechanisms for promoting cooperative catalysis (metal–metal, metal–hydrogen bonding, metal–intercalation, and metal–nanoparticle). Electrophoresis DNA-cleavage inhibition experiments, X-ray photoelectron spectroscopy studies, and fluorescence ethidium bromide displacement assays reveal that in these novel nucleases the QDs act as redox-active protein-like nanoparticle structures that bind to the DNA and deliver electrons to the copper­(II) centers for the generation of Cu­(I) and reactive oxygen species

Two Novel Ternary Dicopper(II) μ‑Guanazole Complexes with Aromatic Amines Strongly Activated by Quantum Dots for DNA Cleavage

Two novel (μ-guanazole)-bridged binuclear copper­(II) complexes with 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy), [Cu₂(μ-<i>N</i>2,<i>N</i>4-Hdatrz)­(phen)₂(H₂O)­(NO₃)₄] (<b>1</b>) and [Cu₂(μ-<i>N</i>1,<i>N</i>2-datrz)₂(μ-OH₂)­(bipy)₂]­(ClO₄)₂ (<b>2</b>) (Hdatrz = 3,5-diamino-1,2,4-triazole = guanazole), have been prepared and characterized by X-ray diffraction, spectroscopy, and susceptibility measurements. Compounds <b>1</b> and <b>2</b> differ in the aromatic amine, which acts as a coligand, and in the Cu···Cu′-bridging system. Compound <b>1</b>, which contains two mono-bridged copper ions, represents the first example of a discrete Cu–(<i>N</i>C<i>N</i>-trz)–Cu′ complex. Compound <b>2</b>, with two triply bridged copper ions, is one of the few compounds featuring a Cu–[(<i>NN</i>-trz)₂ + (<i>O-</i>aquo)]–Cu′ unit. Both compounds display antiferromagnetic coupling but of different magnitude: <i>J</i> (μ_2,4-triazole) = −52 cm^–1 for <b>1</b> and <i>J</i> (μ_1,2-triazolate) = −115 cm^–1 for <b>2</b>. The DNA binding and cleavage properties of the two compounds have been investigated. Fluorescence, viscosimetry, and thermal denaturation studies reveal that both complexes have high affinity for DNA (<b>1</b> > <b>2</b>) and that only <b>1</b> acts as an intercalator. In the presence of a reducing agent like 3-mercaptopropionic acid, <b>1</b> produces significant oxidative DNA cleavage, whereas <b>2</b> is inactive. However, in the presence of very small quantities of micelles filled with core–shell CdSe-ZnS quantum dots (15 nM), <b>1</b> and <b>2</b> are considerably more active and become highly efficient nucleases as a result of the different possible mechanisms for promoting cooperative catalysis (metal–metal, metal–hydrogen bonding, metal–intercalation, and metal–nanoparticle). Electrophoresis DNA-cleavage inhibition experiments, X-ray photoelectron spectroscopy studies, and fluorescence ethidium bromide displacement assays reveal that in these novel nucleases the QDs act as redox-active protein-like nanoparticle structures that bind to the DNA and deliver electrons to the copper­(II) centers for the generation of Cu­(I) and reactive oxygen species