Mixed-Valent {Fe^IV(μ-O)(μ-carboxylato)₂Fe^III}³⁺ Core

The symmetrically ligated complexes 1, 2, and 3 with a (μ-oxo)bis(μ-acetato)diferric core can be one-electron oxidized electrochemically or chemically with aminyl radical cations [•NR3][SbCl6] in acetonitrile yielding complexes which contain the mixed-valent {(μ-oxo)bis(μ-acetato)iron(IV)iron(III)}3+ core: [([9]aneN3)2FeIII2(μ-O)(μ-CH3CO2)2](ClO4)2 (1(ClO4)2), [(Me3[9]aneN3)2FeIII2(μ-O)(μ-CH3CO2)2](PF6)2 (2(PF6)2), and [(tpb)2FeIII2(μ-O)(μ-CH3CO2)2] (3) where ([9]aneN3) is the neutral triamine 1,4,7-triazacyclononane and (Me3[9]aneN3) is its tris-N-methylated derivative, and (tpb)- is the monoanion trispyrazolylborate. The asymmetrically ligated complex [(Me3[9]aneN3)FeIII(μ-O)(μ-CH3CO2)2FeIII(tpb)](PF6) (4(PF6)) and its one-electron oxidized form [4ox]2+ have also been prepared. Finally, the known heterodinuclear species [(Me3[9]aneN3)CrIII(μ-O)(μ-CH3CO2)2Fe([9]aneN3)](PF6)2 (5(PF6)2) can also be one-electron oxidized yielding [5ox]3+ containing an iron(IV) ion. The structure of 4(PF6)·0.5CH3CN·0.25(C2H5)2O has been determined by X-ray crystallography and that of [5ox]2+ by Fe K-edge EXAFS-spectroscopy (FeIV−Ooxo: 1.69(1) Å; FeIV−Ocarboxylato 1.93(3) Å, FeIV−N 2.00(2) Å) contrasting the data for 5 (FeIII−Ooxo 1.80 Å; FeIII−Ocarboxylato 2.05 Å, Fe−N 2.20 Å). [5ox]2+ has an St = 1/2 ground state whereas all complexes containing the mixed-valent {FeIV(μ-O)(μ-CH3CO2)2FeIII}3+ core have an St = 3/2 ground state. Mössbauer spectra of the oxidized forms of complexes clearly show the presence of low spin FeIV ions (isomer shift ∼0.02 mm s-1, quadrupole splitting ∼1.4 mm s-1 at 80 K), whereas the high spin FeIII ion exhibits δ ≈ 0.46 mm s-1 and ΔEQ ≈ 0.5 mm s-1. Mössbauer, EPR spectral and structural parameters have been calculated by density functional theoretical methods at the BP86 and B3LYP levels. The exchange coupling constant, J, for diiron complexes with the mixed-valent FeIV−FeIII core (H = −2J S1·S2; S1 = 5/2; S2 = 1) has been calculated to be −88 cm-1 (intramolecular antiferromagnetic coupling) and for the reduced diferric form of −75 cm-1 in reasonable agreement with experiment (J = −120 cm-1).