Synthesis, characterization, and magnetic studies of two novel isostructural pentanuclear iron(II) complexes

The synthesis, X-ray structural characterization, and Mossbauer and magnetic properties of the two pentanuclear complexes [Fe-5(mu(3)-F)(2)(XDK)(2)(L)(4)(O(2)CPh)(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, H(2)XDK = m-xylylenediamine bis(Kemp's triacid)imide, benzoate, and a nitrogen donor ligand, py or N-MeIm, afforded [Fe-5(mu(3)-F)(2)(XDK)(2)(py)(4)(O(2)CPh)(4)] (1) and [Fe-5(mu(3)-F)(2)(XDK)(2)(N-MeIm)(4)(O(2)CPh)(4)] (2), respectively. Complex 1 . 4CH(2)Cl(2) crystallizes in the triclinic space group <P(1)over bar>, with a = 13.1474(8) Angstrom, b = 15.081(1) Angstrom, c = 16.560(1) Angstrom, alpha = 98.807(6)degrees, beta = 111.242(6)degrees, gamma = 93.723(5)degrees, V = 2998.4(4) Angstrom(3), and Z = 1 (R = 0.084, R(W) = 0.084), and complex 2 . 2Et(2)O crystallizes in the monoclinic space group P2(1)/n, a = 20.641(3) Angstrom, b = 13.845(2) Angstrom, c = 21.170(4) Angstrom, beta = 102.56(1)degrees, V = 5905(2) Angstrom(3), and Z = 2 (R = 0.066, R(W) = 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 (similar to 0.18 Angstrom) 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 Mossbauer spectra of the two complexes (delta(1) = 1.25 mm/s, delta(2) = 1.30 mm/s, Delta E(Q1) = 2.80 mm/s, and Delta E(Q2) = 2.06 mm/s for 1 and delta(1) = 1.32 mm/s, delta(2) = 1.42 mm/s, Delta E(Q1) = 3.14 mm/s, and Delta E(Q2) = 2.85 mm/s for 2). Conductivity and Mossbauer measurements of methanol solutions of 1 suggested partial dissociation of th 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.