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

Abstract

Reaction of Fe³⁺, M²⁺, and acetate ions in aqueous solution gives [Fe^III₂M^IIO(MeCO₂)₆(H₂O)₃]·3H₂O (M = Mg, Mn, Co, Ni, or Zn), which on crystallisation from pyridine (py) is converted into [Fe^III₂M^IIO(MeCO₂)₆(py)₃]. Reaction of chromium(II) acetate with metal(II) acetate in pyridine in the presence of air gives [Cr^III₂M^IIO(MeCO₂)₆(py)₃](M = Mg, Co, or Ni), and with [Fe^III₂M^IIO-(MeCO₂)₆(py)₃] in pyridine inthe absence of air gives [Cr^IIIFe^IIIMn^IIO(MeCO₂)₆(py)₃](M = Mn or Fe) and [Cr^III₂Fe^IIIO(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 J_FeFe in the Fe₂Mg, Fe₂Mn, and Fe₂Ni compounds and of J_CrCr in the Cr₂Co and Cr₂Ni compounds are approximately twice their values in the cations [M^III₃O(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 Fe^III₂Fe^II and Cr^IIIFe^IIIFe^III compounds at ca. 7 000 cm^–1 is assigned to intervalence transfer. The room-temperature spectra of [M₃O(MeCO₂)₆L₃]Cl (M₃= Cr^III₃, Cr^III₂Fe^III, Cr^IIIFe^III₂ or Fe^III₃; 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.