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

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• 作者：Ian A. Gass ; Subrata Tewary ; Ayman Nafady ; Nicholas. F. Chilton ; Christopher J. Gartshore ; Mousa Asadi ; David W. Lupton ; Boujemaa Moubaraki ; Alan M. Bond ; John F. Boas ; Si-Xuan Guo ; Gopalan Rajaraman ; Keith S. Murray
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：July 1, 2013
• 年：2013
• 卷：52
• 期：13
• 页码：7557-7572
• 全文大小：765K
• 年卷期：v.52,no.13(July 1, 2013)
• ISSN：1520-510X

文摘

The reaction of [Co^II(NO₃)₂]路6H₂O with the nitroxide radical, 4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L^{鈥?/sup>), produces the mononuclear transition-metal complex [CoII(L鈥?/sup>)₂](NO₃)₂ (1), 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鈥?/sup>) forming a linear L鈥?/sup>路路路Co(II)路路路L鈥?/sup> arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical鈥搑adical intramolecular ferromagnetic interactions stabilizing a S = 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 1 has a positive D value and nonzero E values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical 蟺*_NO orbitals with a small orbital overlap leading to radical鈥搑adical 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鈥搑adical interaction is computed to be large and ferromagnetic. The monocationic complex [CoIII(L鈥?/sup>)₂](BPh₄) (2) 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 1 and 2 revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues.}