A HEXAIRON(III) COMPLEX WITH 3 NONPLANAR ETA(2)-MU(4)-PEROXO LIGANDS BRIDGING 2 BASIC IRON ACETATE UNITS

The novel eta(2)-mu(4)-peroxo hexairon(III) complex, [Fe-6(O)(2)(O)(23 )(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 e3O(OAc)(6)-(H2O)(3)][Fe-6(O)(2)(O-2)(3)(OAc)(9)]. 8H(2)O cry stallizes in the hexagonal space group, P6(3)/m with unit cell paramet ers of a=14.276(5), c=18.935(3) Angstrom, V=3342.0(1) Angstrom(3), 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 pr ism has eta(2)-mu(4)-peroxo ligands bridging each rectangular face and eta(2)-mu(2)-acetates bridging each edge of the rectangular and trian gular faces. Two distinguishing structural features are an elongated O -O bond distance of 1.472(9) Angstrom for the bridging peroxo ligand a rid significant displacemeat of this group by 1.05 Angstrom out of the plane of the four iron atoms that it links. The v(0-0) stretching mod e is readily observed by Raman spectroscopy at 844 cm(-1), as in other peroxo-bridged polyiron(III) complexes, but the UV-visible spectrum l acks the distinctive charge transfer band in the 500-800 Mn region fou nd in most other ferric peroxo species. Mossbauer isomer shift and qua drupole splitting parameters at 4.2 K are delta=0.51 and 0.53 mm/s and Delta E(q)=1.16 and 0.79 mm/s for the cation and anionic iron cluster s, respectively. Asymmetric line broadening indicated the onset of slo w spin relaxation at the lowest temperatures (4.2 K). Variable tempera ture magnetic susceptibility data measured on a solid sample of [Fe3O( OAC)(6)(H2O)(3)][Fe-6(O)(2)(O-2)(3)(OAC)(9)] revealed antiferromagneti c coupling within the {Fe3O}(7+) units (J similar to-30 to -50 cm(-1)) and weak ferromagnetic coupling (J similar to 1 cm(-1)) through the e ta(2)-mu(4)-peroxo bridges, where H=-2J/S-i . S-j/.