Magnetic and spectroscopic properties of some heterotrinuclear basic acetates of chromium(III), iron(III), and divalent metal ions

Reaction of Fe3+, M2+, and acetate ions in aqueous solution gives [FeIII2MIIO(MeCO2)6(H2O)3]·3H2O (M = Mg, Mn, Co, Ni, or Zn), which on crystallisation from pyridine (py) is converted into [FeIII2MIIO(MeCO2)6(py)3]. Reaction of chromium(II) acetate with metal(II) acetate in pyridine in the presence of air gives [CrIII2MIIO(MeCO2)6(py)3](M = Mg, Co, or Ni), and with [FeIII2MIIO-(MeCO2)6(py)3] in pyridine inthe absence of air gives [CrIIIFeIIIMnIIO(MeCO2)6(py)3](M = Mn or Fe) and [CrIII2FeIIIO(MeCO2)6(py)3]. The compounds with co-ordinated pyridine have been examined crystallographically and magnetically. The Cr2Mg, Cr2Ni, Fe2Ni, and CrFeNi compounds crystallise in the monoclinic system, space group Cc or C2/c, Z= 4, but the other seven compounds are rhombohedral and isomorphous with [M3O(MeCO2)6(py)3]·py (M = Mn or Fe), indicating that the three metal atoms are crystallographically equivalent as a result of disorder in the molecular orientation. Powder magnetic susceptibilities for the Cr2Co and Cr2Ni compounds(4.2–60 K), for the Fe2Mn and Fe2Ni compounds (4.2–295 K), and for the Fe2Ni(aquo) and Fe2Mg compounds (80–295 K) have been fitted by use of an exchange Hamiltonian to yield values of the exchange parameters J for the Cr–Cr, Fe–Fe, Cr–Ni, Fe–Ni, and Fe–Mn interactions, which are discussed in terms of superexchange mechanisms. The values of JFeFe in the Fe2Mg, Fe2Mn, and Fe2Ni compounds and of JCrCr in the Cr2Co and Cr2Ni compounds are approximately twice their values in the cations [MIII3O(MeCO2)6(H2O)3]+(M = Cr or Fe), indicating that the µ3-O atom provides the main super-exchange pathway. Diffuse-reflectance and solution spectra (6 000–40 000 cm–1) of the compounds have been recorded at room temperature and are discussed in terms of the ligand-field model; a band in the spectra of the FeIII2FeII and CrIIIFeIIIFeIII compounds at ca. 7 000 cm–1 is assigned to intervalence transfer. The room-temperature spectra of [M3O(MeCO2)6L3]Cl (M3= CrIII3, CrIII2FeIII, CrIIIFeIII2 or FeIII3; L = H2O or py) have also been obtained, and intense absorption bands at ca. 19 000 and 26 000 cm–1 are tentatively assigned to simultaneous (Cr3+, Fe3+) double excitations.