Hexahalorhenate(iv) salts of metal oxazolidine nitroxides

Abstract

Eight coordination compounds of formulae [Fe^II(L˙)₂][Re^IVCl₆] (1a), [Fe^II(L˙)₂][Re^IVBr₆] (1b), [Co^II(L˙)₂][Re^IVCl₆]·CH₃CN (2a), [Co^II(L˙)₂][Re^IVBr₆] (2b), [Ni^II(L˙)(CH₃CN)₃][Re^IVCl₆]·CH₃CN (3a), [Ni^II(L˙)(CH₃CN)₃][Re^IVBr₆]·3CH₃CN (3b), [Cu^II(L˙)₂][Re^IVCl₆] (4a) and [Cu^II(L˙)₂][Re^IVBr₆] (4b), where L˙ is the aminoxyl radical chelating ligand, 4,4′-dimethyl-2,2′-di(2-pyridyl)oxazolidine-N-oxide, have been synthesised. Structural and magnetic studies reveal metal–radical intramolecular antiferromagnetic interactions in the [M^II(L˙)₂]²⁺ cations in the iron, cobalt and copper based compounds (1a, 1b, 2a, 2b, 4a and 4b) with the central metal ion low-spin in the case of iron (1a and 1b) and a gradual, cobalt based, spin-crossover transition present in 2a and 2b. The nickel based compounds, 3a and 3b, were analysed in the dried form (3a(dried) and 3b(dried)) and directly in acetonitrile (3a(solvated) and 3b(solvated)). Microanalysis and IR spectroscopy on 3a(dried) and 3b(dried) suggest that the dried samples are best formulated as [Ni^II(L˙)(H₂O)₃][Re^IVX₆], where X = Cl (3a(dried)) and Br (3b(dried)). All forms of 3a and 3b exhibit cationic metal–radical ferromagnetic interactions resulting in S = 3/2 ground states. In addition, 3a(dried) exhibits spin-canting behaviour with an ordering temperature of 2.7 K, an open hysteresis loop with a coercive field H_c = 580 Oe, and a remanent magnetisation M_r = 0.21μ_B, resulting in a canting angle of ∼1.8°. In contrast, 3b(dried) shows no spin-canting behaviour; a maximum in χ_Mvs. T at T = 3 K suggesting long-range antiferromagnetic ordering. 3a(solvated) and 3b(solvated) show no indication of long-range magnetic ordering, unlike 4a and 4b where anomalies are evident in the low-temperature magnetic susceptibility measurements.