Complexes of CuII with nitroxides and their magnetochemical behavior

The synthesis, structures, and magnetochemical data for the heterospin chain polymer complexes Cu(hfac)₂L^All and Cu(hfac)₂L^Bu·0.5Solv, where hfac is the hexafluoroacetylacetonate anion, L^All and L^Bu are 2-(1-allyl-1H-pyrazol-4-yl)- and 2-(1-butyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyls, respectively, Solv is isopropylbenzene or tert-butylbenzene, were described. These polycrystalline solids were found to exhibit similar magnetic anomalies. Thus, the effective magnetic moment first decreases and then increases with temperature, resulting in the specific minimum in the curve μ_eff(T). The study of the magneto-structural correlations showed that the appearance of the minimum is attributed to different factors. For Cu(hfac)₂L^All, this is the phase transition accompanied by the structural rearrangement of the exchange cluster >N—·O—Cu^II—O·—N<, which leads to a change in the energy of the exchange interaction between the unpaired electrons of the paramagnetic centers. By contrast, for Cu(hfac)₂L^Bu·0.5PrⁱPh and Cu(hfac)₂L^Bu·0.5Bu^tPh, the appearance of the minimum is a consequence of the coexistence of exchange interaction energies with opposite signs, while the structure of the solid phase remains unchanged.