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Synthesis, Characterization, and Magnetic Studies of Two Novel Isostructural Pentanuclear Iron(II) Complexes

1997; American Chemical Society; Volume: 36; Issue: 1 Linguagem: Inglês

10.1021/ic960783i

1520-510X

Susanna Herold, Stephen J. Lippard,

Metal complexes synthesis and properties

The synthesis, X-ray structural characterization, and Mössbauer and magnetic properties of the two pentanuclear complexes [Fe5(μ3-F)2(XDK)2(L)4(O2CPh)4], where L = pyridine (py) or N-methylimidazole (N-MeIm), are described. Reaction of [Fe(H2O)6](BF4)2, the deprotonated dinucleating dicarboxylate ligand XDK, H2XDK = m-xylylenediamine bis(Kemp's triacid)imide, benzoate, and a nitrogen donor ligand, py or N-MeIm, afforded [Fe5(μ3-F)2(XDK)2(py)4(O2CPh)4] (1) and [Fe5(μ3-F)2(XDK)2(N-MeIm)4(O2CPh)4] (2), respectively. Complex 1·4CH2Cl2 crystallizes in the triclinic space group P1̄, with a = 13.1474(8) Å, b = 15.081(1) Å, c = 16.560(1) Å, α = 98.807(6)°, β = 111.242(6)°, γ = 93.723(5)°, V = 2998.4(4) Å3, and Z = 1 (R = 0.084, Rw = 0.084), and complex 2·2Et2O crystallizes in the monoclinic space group P21/n, with a = 20.641(3) Å, b = 13.845(2) Å, c = 21.170(4) Å, β = 102.56(1)°, V = 5905(2) Å3, and Z = 2 (R = 0.066, Rw = 0.082). In both complexes, the five iron atoms occupy the corners of two equivalent, nearly equilateral triangles with one vertex in common, which is situated on a crystallographic inversion center. A triply-bridging fluoride ion is located in the center of each triangle, raised slightly (∼0.18 Å) above the plane formed by the three iron atoms. The coordination spheres of the four outer iron ions are trigonal bipyramidal, whereas the central iron displays an octahedral coordination geometry. The inequivalence of the iron atoms is reflected by the presence of two overlapping quadrupole doublets, with an approximate intensity ratio of 4:1, in the Mössbauer spectra of the two complexes (δ1 = 1.25 mm/s, δ2 = 1.30 mm/s, ΔEQ1 = 2.80 mm/s, and ΔEQ2 = 2.06 mm/s for 1 and δ1 = 1.32 mm/s, δ2 = 1.42 mm/s, ΔEQ1 = 3.14 mm/s, and ΔEQ2 = 2.85 mm/s for 2). Conductivity and Mössbauer measurements of methanol solutions of 1 suggested partial dissociation of the complex, probably of the benzoate ligands. Magnetic susceptibility data for both compounds 1 and 2 at 5000 G in the temperature range 4−300 K revealed a decrease in the effective moment with decreasing temperature. The molar magnetic susceptibility versus temperature plots could be fit with a simplified theoretical model obtained by considering two different exchange pathways, consistent with the symmetry of the molecule.