Synthesis, characterisation and magnetic properties of octahedral chromium(iii) compounds with six C-donor ligands

摘要

The homoleptic, square pyramidal organochromium(iii) compound [NBu 4]2[Cr(C6F5)5] (1) reacts with excess organic isocyanides, CNR [R = tBu, 2, 6-dimethylphenyl (Xy)], under dissociation of the apical C6F5 ligand to give the more saturated, singly charged complexes [NBu4][trans- Cr(C6F5)4(CNR)2] [R = tBu (2), Xy (3)], containing six monodentate C-donor ligands. These compounds exhibit an axially distorted octahedral structure (single-crystal X-ray diffraction) with the four C6F5 groups defining the equatorial positions and the CNR ligands occupying the axial ones. Compounds 2 and 3 both behave as spin quartet species (S = 3/2) at microscopic level (EPR spectroscopy), their macroscopic magnetic properties depending upon the nature of the terminal R group, as established by magnetisation measurements. When the R substituent is the saturated alkyl group tBu, the compound (2) behaves as a simple paramagnet, with no magnetic interaction between individual CrIII centres along the whole temperature range measured (1.8-265 K). By contrast, a weak antiferromagnetic interaction is detected for compound 3 at low temperature with TN = 0.19(1) K. Since the closest intermetallic distances are similar in the crystals of 2·CH2Cl2 and 3·1.75CH2Cl2 (ca. 1.1 nm), we conclude that the insaturation of the aromatic Xy group together with the extended intermolecular π-π stacking interactions between Xy rings observed in the crystal lattice of 3·1.75CH2Cl2 (centroid-to-centroid distance: 0.35 nm) favour magnetic interaction between the individual magnetic centres.