Single Crystal EPR Studies of [enH2][{Fe(HEDTA)}2O]·6H2O [en = Ethylenediamine; HEDTA = N-Hydroxyethylethylenediaminetriacetate(3−)]

Single-crystal EPR studies of the Fe−O−Fe dimer, [enH2][{Fe(HEDTA)}2O]·6H2O, have been performed at both X- and Q-band frequencies and room temperature. Resonances arising from the S‘ = 1, 2, and 3 states have been observed in the powder spectrum at Q-band frequency and analysis by spectral simulation has led to the zero-field splitting parameters: |D 1| = 1.950 cm-1, |E 1| = 0.650 cm-1, |D 2| = 0.150 cm-1, |E 2| = 0.0195 cm-1, |D 3| = 0.570 cm-1, and |E 3| = 0.000 cm-1. In addition, analysis of three orthogonal planes of single-crystal EPR data at both X- and Q-band frequencies has allowed the orientation of the zero-field splitting tensor with respect to the molecular geometry in the dominant (S‘ = 2) state to be determined. The largest principal value of the D tensor (D zz ) is found to lie approximately perpendicular to the plane of the Fe−O−Fe bridge; the nonlinear bridging angle is 165°. The results obtained in this study are compared with those from a number of other studies. A pattern for the orientations of the zero-field splitting parameters in the dominant spin state (S‘ = 2) of the Fe−O−Fe dimers, so far studied by single-crystal EPR spectroscopy, emerges.