Interactions in Cu(II)Cu(II), VO(II)VO(II) and Cu(II)VO(II) pairs through oxalato bridging ligand

Abstract

The goal of this paper was to compare the interaction in Cu(II)Cu(II), VO(II)VO(II) and Cu(II)VO(II) pairs through the same oxalato bridging ligand. For this, two new bimetallic complexes were synthesized, namely acacVO(C 2O 4)VOacac·4H 2O noted [VOVO] and tmenCu(C 2O 4)VO(C 2O 4)·3H 2O noted [CuVO], with acac = acetylacetonato and tmen = N,N,N′,N′-tetramethylethylenediamine. The [CuCu] compound of formula [tmen(H 2O)Cu(C 2O 4)Cu(H 2O)tmen] (ClO 4) 2·1.25H 2O has already been described and its crystal structure solved by X-ray diffraction. The singlet-triplet energy gaps arising from the intramolecular interaction, determined from the magnetic data, are −385.4 cm −1 in [CuCu], −5.75 cm −1 in [VOVO] and ∣J∣ < 1 cm −1 in [CuVO]. The EPR spectrum of this last compound, however, shows a transition in a triplet state with a singlet-triplet energy gap larger, in absolute value, than the incident quantum (∼0.3 cm −1 in X-band). To obtain this result, the spectrum was compared to those of the monomeric species tmenCu(C 2O 4)·4H 2O and (NH 4) 2VO(C 2O 4) 2·2H 2O. The magnitude of the singlet-triplet gaps J was rationalized within the framework of an orbital model developed in our group. [CuVO] is a new heterobimetallic compound in which the interaction is expected to be purely ferromagnetic owing to the strict orthogonality of the magnetic orbitals. Finally, an explanation of the absence of zero field splitting in the triplet state of [CuCu] was proposed. It was shown that the anisotropic exchange interaction in [CuCu] might be considered as being proportional to the isotropic exchange interaction in [CuVO].