An unusual temperature dependence of the EPR parameters of copper(II) and oxovanadium(IV) bis(acetylacetonate) complexes

A series of X-band electron paramagnetic resonance (EPR) spectra were obtained in the temperature range 120–280K at 10K intervals for the bis(acetylacetonate) complexes of copper(II) and oxovanadium(IV) cations in dimethylformamide. The magnetic parameters and molecular rotational correlation time determined from simulations of the spectra revealed an unexpected temperature dependence of the isotropic hyperfine coupling constant .Aiso/. A small, but non-negligible, abrupt increase in Aiso is observed for both complexes at a temperature (215 K) corresponding to the change in molecular motional state from the rigid limit to slow motion. The change is approximately the same for both the Cu(II) and vanadyl complexes (ca 2 104 cm1/. No temperature dependence is observed in the isotropic g-factor for either complex within the experimental error. It is proposed that the data can be explained in terms of a change in the degree of distortion at the metal site coincident with the change in motional state and induced by a solvent (dimethylformamide) phase transition. Calculation of the bonding parameters shows that the in-plane -bonding is more covalent in the vanadyl complex. Comparison with data from the literature reveals the importance of the solvent in determining the type of temperature dependence displayed by the magnetic parameters.