A Hexairon(III) Complex with Three Nonplanar η2-μ4-Peroxo Ligands Bridging Two Basic Iron Acetate Units

The novel η2-μ4-peroxo hexairon(III) complex, [Fe6(O)2(O2)3(OAc)9]-, was prepared as its [Fe3O(OAc)6(H2O)3]+ salt by addition of H2O2 to basic iron acetate in the presence of NaOCH3. The compound [Fe3O(OAc)6(H2O)3][Fe6(O)2(O2)3(OAc)9]·8H2O crystallizes in the hexagonal space group, P63/m with unit cell parameters of a = 14.276(5), c = 18.935(3) Å, V = 3342.0(1) Å3, and Z = 2; the structure was solved and refined to a final residual of R = 0.044. The anionic component of the salt contains two trinuclear {Fe3O}7+ units arranged in a face-to-face manner. The resulting trigonal prism has η2-μ4-peroxo ligands bridging each rectangular face and η2-μ2-acetates bridging each edge of the rectangular and triangular faces. Two distinguishing structural features are an elongated O−O bond distance of 1.472(9) Å for the bridging peroxo ligand and significant displacement of this group by 1.05 Å out of the plane of the four iron atoms that it links. The νO-O stretching mode is readily observed by Raman spectroscopy at 844 cm-1, as in other peroxo-bridged polyiron(III) complexes, but the UV−visible spectrum lacks the distinctive charge transfer band in the 500−800 nm region found in most other ferric peroxo species. Mössbauer isomer shift and quadrupole splitting parameters at 4.2 K are δ = 0.51 and 0.53 mm/s and ΔEq = 1.16 and 0.79 mm/s for the cation and anionic iron clusters, respectively. Asymmetric line broadening indicated the onset of slow spin relaxation at the lowest temperatures (4.2 K). Variable temperature magnetic susceptibility data measured on a solid sample of [Fe3O(OAc)6(H2O)3][Fe6(O)2(O2)3(OAc)9] revealed antiferromagnetic coupling within the {Fe3O}7+ units (J ∼ −30 to −50 cm-1) and weak ferromagnetic coupling (J ∼ 1 cm-1) through the η2-μ4-peroxo bridges, where ℋ = −2J|Si·Sj|.