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 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 [CoIII(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.