POLYIRON(III) - ALKOXO CLUSTERS - A NOVEL TRINUCLEAR COMPLEX AND ITS RELEVANCE TO THE EXTENDED LATTICES OF IRON-OXIDES AND HYDROXIDES

The synthesis, solid-state structure and magnetic characterization of the complex KFe3(OCH3)(7)(dbm)(3) . 4CH(3)OH (I) (Hdbm = dibenzoylmethane) are reported. Complex I readily assembles in methanolic solutions of iron(III) chloride and Hdbm in the presence of an excess of potassium methoxide; it crystallizes in the triclinic space group <P(1)over bar> with unit-cell parameters a 13.044(1) Angstrom, b = 14.174(1) Angstrom, c = 18.657(2) Angstrom, alpha = 97.94(1)degrees, beta = 104.13(1)degrees, gamma = 114.12(1)degrees, V = 2940.6(6) Angstrom(3), and Z = 2 at 190 K. Refinement of 10076 reflections with 705 parameters yielded R = 0.046 and R(w) = 0.051. The solid-state molecular structure of I consists of a triangular array of iron(III) ions connected by a triply bridging methoxide and three mu(2)-methoxide ligands. The oxygen donors of a monodentate methoxide and of a chelating dbm ligand complete the coordination sphere of each metal ion. The resulting mononegative Fe-3(mu(3)-OCH3)(mu(2)-OCH3)(3)(OCH3)(3)(dbm)(3) moiety coordinates the K counterion through the oxygens of the mu(2)-OCH3 ligands. The oxygen atoms in the core of I are arranged in two essentially parallel layers and display a closest-packing which is found in iron oxides and hydroxides. The high-spin iron(III) ions are antiferromagnetically coupled with an S = 1/2 spin ground state. Assumption of a C-2v point-group symmetry for the cluster leads to a satisfactory reproduction of the observed magnetic behavior with g = 2.0 and either J = 10.6(1), J' = 15.3(2) cm(-1) or J = 12.9(2), J' = 9.7(1) cm(-1), where the spin-only Heisenberg Hamiltonian is defined as H = Sigma(i)>(j)J(ij)S(i) . S-j and J' is the unique coupling constant. The relative contributions of geometrical distortions and of antisymmetric exchange to the splitting of the 4-fold (2)E electronic ground state of an idealized system with trigonal symmetry have been estimated from X-band EPR spectra recorded on powdered samples.