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 (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