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

The reaction of [CoII(NO3)2]·6H2O with the nitroxide radical, 4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L•), produces the mononuclear transition-metal complex [CoII(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 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 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...