The single-crystal electron spin resonance spectrum of N,N,N′,N′-tetramethylethylenediaminedium aquabis(malonato)oxovanadate(IV) dihydrate, a weak two-dimensional exchanging system

Abstract

Single-crystal e.s.r. spectra of the title compound [H₂tmen][VO(mal)₂(H₂O)]·2H₂O over the temperature range 310–4.2 K are reported. The crystal structure shows the presence of two-dimensional layers of anions in the crystal, with all VO vectors perpendicular to these layers. The e.s.r. spectra with the applied magnetic field parallel to the VO direction are compatible with a weak isotropic exchange interaction, J₀, within this layer, and have been simulated on this basis. It is shown that J₀ varies almost linearly in magnitude, and changes sign, with temperature, being ca.–200 G (ferromagnetic) at 300 K, changing sign at ca. 190 K, and ca.+290 G (antiferromagnetic) at 4.2 K. Possible mechanisms for the isotropic exchange, and reasons for its variation with temperature, are discussed.