Solvate-Dependent Spin Crossover and Exchange in Cobalt(II) Oxazolidine Nitroxide Chelates

Two oxazolidine nitroxide complexes of cobalt­(II), [Co^II(L^•)₂]­(B­(C₆F₅)₄)₂·CH₂Cl₂ (<b>1</b>) and [Co^II(L^•)₂]­(B­(C₆F₅)₄)₂·2Et₂O (<b>2</b>), where, L^• is the tridentate chelator 4,4-dimethyl-2,2-bis­(2-pyridyl)­oxazolidine <i>N</i>-oxide, have been investigated by crystallographic, magnetic, reflectivity, and theoretical (DFT) methods. This work follows on from a related study on [Co^II(L^•)₂]­(NO₃)₂ (<b>3</b>), a multifunctional complex that simultaneously displays magnetic exchange, spin crossover, and single molecule magnetic features. Changing the anion and the nature of solvation in the present crystalline species leads to significant differences, not only between <b>1</b> and <b>2</b> but also in comparison to <b>3</b>. Structural data at 123 and 273 K, in combination with magnetic data, show that at lower temperatures <b>1</b> displays low-spin Co­(II)-to-radical exchange with differences in fitted <i>J</i> values in comparison to DFT (broken symmetry) calculated <i>J</i> values ascribed to the sensitive influence of a tilt angle (θ) formed between the Co­(d_<i>z</i>²) and the <i>trans</i>-oriented O atoms of the NO radical moieties in L^•. Spin crossover in <b>1</b> is evident at higher temperatures, probably influenced by the solvate molecules and crystal packing arrangement. Complex <b>2</b> remains in the high-spin Co­(II) state between 2 and 350 K and undergoes antiferromagnetic exchange between Co–radical and radical–radical centers, but it is difficult to quantify. Calculations of the magnetic orbitals, eigenvalue plots, and the spin densities at the Co and radical sites in <b>1</b> and <b>2</b> have yielded satisfying details on the mechanism of metal–radical and radical–radical exchange, the radical spins being in π*_NO orbitals

Synthesis and applications of copillar[5]arene dithiols

<p>A novel copillar[4+1]arene incorporating alkylthiol substituents was synthesised in three steps and structurally characterised as the first example of a pillar[n]arene to incorporate two terminal thiols on the same aromatic ring. The macrocycle was attached to gold electrodes through a standard dipping technique. Cyclic voltammetry demonstrated selectivity for Li⁺ over other alkali metal cations. The copillar[4+1]arene was also used as a capping agent in the preparation of 3 nm gold nanoparticles.</p

Observation of Ferromagnetic Exchange, Spin Crossover, Reductively Induced Oxidation, and Field-Induced Slow Magnetic Relaxation in Monomeric Cobalt Nitroxides

The reaction of [Co^II(NO₃)₂]·6H₂O with the nitroxide radical, 4-dimethyl-2,2-di­(2-pyridyl) oxazolidine-<i>N</i>-oxide (L^•), produces the mononuclear transition-metal complex [Co^II(L^•)₂]­(NO₃)₂ (<b>1</b>), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co²⁺ ion with both ligands in the neutral radical form (L^•) forming a linear L^•···Co­(II)···L^• arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical–radical intramolecular ferromagnetic interactions stabilizing a <i>S</i> = ³/₂ ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that <b>1</b> has a positive <i>D</i> value and nonzero <i>E</i> values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*_NO orbitals with a small orbital overlap leading to radical–radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical–radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co^III(L^–)₂]­(BPh₄) (<b>2</b>) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on <b>1</b> and <b>2</b> revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues

Observation of Ferromagnetic Exchange, Spin Crossover, Reductively Induced Oxidation, and Field-Induced Slow Magnetic Relaxation in Monomeric Cobalt Nitroxides

The reaction of [Co^II(NO₃)₂]·6H₂O with the nitroxide radical, 4-dimethyl-2,2-di­(2-pyridyl) oxazolidine-<i>N</i>-oxide (L^•), produces the mononuclear transition-metal complex [Co^II(L^•)₂]­(NO₃)₂ (<b>1</b>), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co²⁺ ion with both ligands in the neutral radical form (L^•) forming a linear L^•···Co­(II)···L^• arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical–radical intramolecular ferromagnetic interactions stabilizing a <i>S</i> = ³/₂ ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that <b>1</b> has a positive <i>D</i> value and nonzero <i>E</i> values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*_NO orbitals with a small orbital overlap leading to radical–radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical–radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co^III(L^–)₂]­(BPh₄) (<b>2</b>) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on <b>1</b> and <b>2</b> revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues