title: Observation of ferromagnetic exchange, spin crossover, reductively induced oxidation, and field-induced slow magnetic relaxation in monomeric cobalt nitroxides.
abstract: The reaction of [Co(II)(NO3)2]·6H2O with the nitroxide radical, 4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition-metal complex [Co(II)(L(•))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 Co(2+) 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 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 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) (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.
authors.affiliation: School of Chemistry, Monash University, Clayton, Victoria 3800, Australia.
authors.forename: Ian A
authors.initials: IA
authors.lastname: Gass
authors.forename: Subrata
authors.initials: S
authors.lastname: Tewary
authors.forename: Ayman
authors.initials: A
authors.lastname: Nafady
authors.forename: Nicholas F
authors.initials: NF
authors.lastname: Chilton
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Gartshore
authors.forename: Mousa
authors.initials: M
authors.lastname: Asadi
authors.forename: David W
authors.initials: DW
authors.lastname: Lupton
authors.forename: Boujemaa
authors.initials: B
authors.lastname: Moubaraki
authors.forename: Alan M
authors.initials: AM
authors.lastname: Bond
authors.forename: John F
authors.initials: JF
authors.lastname: Boas
authors.forename: Si-Xuan
authors.initials: SX
authors.lastname: Guo
authors.forename: Gopalan
authors.initials: G
authors.lastname: Rajaraman
authors.forename: Keith S
authors.initials: KS
authors.lastname: Murray