Observation of ferromagnetic exchange, spin crossover, reductively induced oxidation, and field-induced slow magnetic relaxation in monomeric cobalt nitroxides

Abstract

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 πNO 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. © 2013 American Chemical Society.