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.
Two oxazolidine nitroxide complexes of cobalt(II), [Co(II)(L(•))2](B(C6F5)4)2·CH2Cl2 (1) and [Co(II)(L(•))2](B(C6F5)4)2·2Et2O (2), where, L(•) is the tridentate chelator 4,4-dimethyl-2,2-bis(2-pyridyl)oxazolidine N-oxide, have been investigated by crystallographic, magnetic, reflectivity, and theoretical (DFT) methods. This work follows on from a related study on [Co(II)(L(•))2](NO3)2 (3), a multifunctional complex that simultaneously displays magnetic exchange, spin crossover, and single molecule magnetic features. Changing the anion and the nature of solvation in the present crystalline species leads to significant differences, not only between 1 and 2 but also in comparison to 3. Structural data at 123 and 273 K, in combination with magnetic data, show that at lower temperatures 1 displays low-spin Co(II)-to-radical exchange with differences in fitted J values in comparison to DFT (broken symmetry) calculated J values ascribed to the sensitive influence of a tilt angle (θ) formed between the Co(dz(2)) and the trans-oriented O atoms of the NO radical moieties in L(•). Spin crossover in 1 is evident at higher temperatures, probably influenced by the solvate molecules and crystal packing arrangement. Complex 2 remains in the high-spin Co(II) state between 2 and 350 K and undergoes antiferromagnetic exchange between Co-radical and radical-radical centers, but it is difficult to quantify. Calculations of the magnetic orbitals, eigenvalue plots, and the spin densities at the Co and radical sites in 1 and 2 have yielded satisfying details on the mechanism of metal-radical and radical-radical exchange, the radical spins being in π*NO orbitals.
The mononuclear oxazolidine nitroxide complex [Mn II(L)2](ClO4)2 (1) (L ,4-dimethyl-2,2-di(2-pyridyl)oxazolidine N-oxide) has been synthesized and investigated using single-crystal X-ray diffraction, variable-temperature magnetic susceptibility measurements, and electrochemistry. The structural analysis reveals bond lengths compatible with a linear L –MnII–L arrangement where the ligands are in the neutral ligand form and the central MnII ion is high spin (S = 5/2). Although analysis of the variable-temperature magnetic susceptibility data suggests a strong antiferromagnetic metal– radical interaction, the radical–radical intramolecular interaction could not be determined unambiguously from such fits. The resultant isolated S = 3/2 ground state is confirmed by low-temperature magnetization versus field measurements. Electrochemical studies reveal similar square schemes and redox intermediates to the previously reported analogues [FeII(L)2][BF4]2 and [CoII(L )2][NO3]2
The reaction of [FeII(L.)2](BF4)2 with Li2TCNQF4 results in the formation of [FeIII(L−)2][TCNQF4.−] (1) where L. is the radical ligand, 4,4‐dimethyl‐2,2‐di(2‐pyridyl)oxazolidine‐N‐oxide and TCNQF4 is 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane. This has been characterised by X‐ray diffraction, Raman and Fourier transform infrared (FTIR) spectroscopy, variable‐temperature magnetic susceptibility, Mössbauer spectroscopy and electrochemistry. X‐ray diffraction studies, magnetic susceptibility measurements and Raman and FTIR spectroscopy suggest the presence of low‐spin FeIII ions, the anionic form (L−) of the ligand and the anionic radical form of TCNQF4; viz. TCNQF4.−. Li2TCNQF4 reduces the [FeII(L.)2]2+ dication, which undergoes a reductively induced oxidation to form the [FeIII(L−)2]+ monocation resulting in the formation of [FeIII(L−)2][TCNQF4.−] (1), the electrochemistry of which revealed four well‐separated, diffusion‐controlled, one‐electron, reversible processes. Mössbauer spectroscopy and electrochemical measurements suggest the presence of a minor second species, likely to be [FeII(L.)2][TCNQF42−].
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