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.

A New Method for the Synthesis of Heterospin Complexes

“…In [Co5(Piv)4L were never recorded. This is fully consistent with the data of [23][24][25][26][27], where the authors also used the metals capable of changing the oxidation level.…”

“…When the products of the interactions of metals with dinitroxides were studied, unusual compounds were isolated, in which one of the nitroxyl groups was reduced to the hydroxylamine anion during the reaction [24,25,34]. The oxidation of the transition metal induced by the reduction of one of the coordinated nitroxides was described in [26,27]. A specific copper(II)-nitroxide complex was described, which has a nontrivial structural peculiarity: it contains both the coordinated O atoms of nitroxide and those of the corresponding hydroxylamine anion [35].…”

“…Hydrogen atoms were refined isotropically with the use of geometrical constraints. Since the solvent molecules in {Co5} and {Ni3} complexes could not be modeled properly, they were squeezed out with PLATON [27,39]. All calculations were performed with the Bruker SHELXTL (Version 6.14) and SHELXL (Version 2014/6) program packages [40].…”

Radical ligand-containing single-molecule magnets

“…Recently, the ligand 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-Noxyl (dmpoNO) [107], which bears two pyridyl donors, was treated with divalent metal salts in a 2:1 ratio to afford the compounds [(dmpoNO) 2 M II ] 2+ (M = Co (1; Fig. 2), Fe, Cu) [107][108][109]. Among these complexes, the cobalt analog 1 is intriguing not only due to the observation of spin crossover but also to its single-molecule magnet behavior.…”

Synthesis, crystal structure and magnetism of two cobalt(II) complexes with imino and nitronyl nitroxides

“…By using this approach, various highly interesting species such as molecular-based ferromagnets [5][6][7], single-chain magnets (SCMs) [8][9][10][11][12][13], single-molecule magnets (SMMs) [14][15][16][17][18][19] and molecule spin transition species [20][21][22][23] have been achieved so far. However, most of the work performed in this area has involved in copper(II), nickel(II), manganese(II) or lanthanide compounds, and the reported cobalt(II)-radical compounds are currently very scarce [24][25][26].…”