Mixed-valent diruthenium long-chain carboxylates. 2. Magnetic properties

The magnetic properties of several mixed-valent diruthenium long-chain carboxylates of general formula Ru-2(RCO2)(4)X (X = Cl, DOS, or RCO2, DOS = dodecyl sulfate, and RCO2 = linear aliphatic carboxylate or dialkoxy- or trialkoxybenzoate) were studied in the temperature range 6-400 K. All of the compounds exhibit a strong zero-field splitting (D = ca. 75 cm(-1)), independent of the nature of the axial anion X or of the equatorial substituent R. In the X = RCO2 series an intermolecular antiferromagnetic (AF) interaction zJ = ca. -2 cm(-1) was found, whereas in the case of an X = DOS analogue, this interaction is very weak (-0.2 cm(-1)). The X = Cl series shows three distinct types of interdimer magnetic exchange: very weak, moderate (\zJ\ approximate to 2 cm(-1)), or strong (\zJ\ > 10 cm(-1)). One representative complex in this series, Ru-2(C4H9CO2)(4)Cl, has been structurally characterized by X-ray crystallography. Crystal data: tetragonal system, space group <I(4)over bar>2d, a = 14.137(3) Angstrom, c = 26.246(5) Angstrom, and Z = 8. Examination of the structures and magnetic behaviors suggests that the AF exchange in this series correlates with the Ru-Cl-Ru intermolecular angle; a qualitative explanation in terms of overlap of magnetic orbitals is proposed. Magnetic susceptibility measurements in the columnar mesophase of the mesomorphic congeners indicate that no significant structural change occurs at the crystal-liquid crystal transition.