A TRINUCLEAR, OXO-CENTERED MIXED-VALENCE IRON COMPLEX WITH UNPRECEDENTED CARBOXYLATE COORDINATION - [FE3O(O2CCH3)6(TACN)].2CHCL3

The trinuclear, oxo-centered mixed-valence complex [Fe3O(O2CCH3)6(TACN)].2CHCl3 (TACN = 1,4,7-triazacyclononane) was synthesized by air oxidation of ferrous acetate in the presence of TACN. The structure of the compound, determined by X-ray crystallography at 195 K, contains the well-known triangular {Fe3O}6+ core. Only one pair of adjacent Fe atoms is bridged by two bidentate acetate ligands, however, whereas the other two pairs are bridged by only one such ligand. The resulting isomer is unique among the extensive family of [M(III)2M(II)O(O2CR)6L3] complexes. The terminal positions in the distorted octahedral coordination spheres of two of the three iron atoms are completed each by a rare bidentate acetate ligand, while the third iron atom is coordinated by a tridentate TACN ligand. Analysis of the iron-ligand bond lengths at each of the three metal atoms comprising the isosceles triiron(II,III,III) triangle reveals that [Fe3O(O2CCH3)6(TACN)].2CHCl3 is a localized mixed-valence compound at 195 K. The Fe-(mu-oxo) bond distances are 1.862 (4) and 1.867 (4) angstrom to the ferric ions and 1.998 (4) angstrom to the ferrous ion. Interestingly, the TACN ligand is coordinated to one of the trivalent iron centers, resulting in an asymmetric complex. The existence of this valence isomer is rationalized on the basis of intramolecular hydrogen bonding between the NH protons of the TACN ligand and the bidentate terminal acetate ligands, which seems to be optimized in the observed structure. The infrared spectrum shows resolved bands arising from the nu-asym(OCO) and nu-sym(OCO) vibrations of the bridging and terminal acetate ligands. Mossbauer spectroscopic studies of the compound reveal two resolved quadrupole doublets due to Fe(II) and Fe(III) at all temperatures in the range 4.2-295 K, confirming the valence-localized classification of the compound. Magnetic susceptibility measurements of [Fe3O(O2CCH3)6(TACN)].2CHCl3 from 2 to 300 K were well fit by a model having only two spin-exchange coupling constants J13 = J23 = -16.5 (4) cm-1 along the edges of the isosceles triangle joining the Fe(II)/Fe(III) pairs and J12 = -41.8 (10) cm-1 along the edge linking the two Fe(III) centers. The net antiferromagnetic coupling leads to an S(T) = 1 ground state.