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

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*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

59 Citations (Scopus)

Abstract

The reaction of [Co II (NO 3) 2]·6H 2O with thenitroxide radical, 4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L•), produces the mononuclear transition-metalcomplex [Co II (L•) 2 ](NO 3) 2 (1), which has been investigatedusing temperature-dependent magnetic susceptibility, electronparamagnetic resonance (EPR) spectroscopy, electrochemis-try, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibilitymeasurements and X-ray diffraction (XRD) analysis reveal acentral low-spin octahedral Co 2+ ion with both ligands in theneutral radical form (L•) forming a linear L•···Co(II)···L•arrangement. This shows a host of interesting magneticproperties including strong cobalt-radical and radical−radical intramolecular ferromagnetic interactions stabilizing a S = 3/ 2ground 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 thepossibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radicalπ*NO orbitals with a small orbital overlap leading to radical−radical ferromagnetic interactions while the cobalt-radical interactionis computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagneticcobalt-radical interaction, whereas the radical−radical interaction is computed to be large and ferromagnetic. The monocationiccomplex [Co III (L−)2 ](BPh 4) (2) is formed by a rare, reductively induced oxidation of the Co center and has been fullycharacterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemicalstudies on 1 and 2 revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with thatof the Fe analogues
Original languageEnglish
Pages (from-to)7557-7572
Number of pages16
JournalInorganic Chemistry
Volume52
Issue number13
DOIs
Publication statusPublished - 18 Jul 2013
Externally publishedYes

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