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 KFe3(OMe)7(dbm)3·4MeOH (I; Hdbm = dibenzoylmethane) are reported. I readily assembles in methanolic solns. of Fe(III) chloride and Hdbm in the presence of an excess of K methoxide; it crystallizes in the triclinic space group P-1 with a = 13.044(1), b = 14.174(1), c = 18.657(2) Å, α = 97.94(1), β = 104.13(1), γ = 114.12(1)°, and Z = 2 at 190 K, R = 0.046 and Rw = 0.051. The solid-state mol. structure of I consists of a triangular array of Fe(III) ions connected by a triply bridging methoxide and three μ2-methoxide ligands. The O donors of a monodentate methoxide and of a chelating dbm ligand complete the coordination sphere of each metal ion. The resulting mononeg. Fe3(μ3-OMe)(μ2-OMe)3(OMe)3(dbm)3 moiety coordinates the K counterion through the oxygens of the μ2-OMe ligands. The O atoms in the core of I are arranged in two essentially parallel layers and display a closest-packing which is found in Fe oxides and hydroxides. The high-spin Fe(III) ions are antiferromagnetically coupled with an S = 1/2 spin ground state. Assumption of a C2v point-group symmetry for the cluster leads to a satisfactory reprodn. of the obsd. 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 = .sum.i>jJijSi·Sj and J' is the unique coupling const. The relative contributions of geometrical distortions and of antisym. exchange to the splitting of the 4-fold 2E electronic ground state of an idealized system with trigonal symmetry were estd. from X-band EPR spectra recorded on powd. samples.