Figure 1. γ-FeOOH structure: a double chain of edge-sharing Fe(O,OH)6 octahedra: iron atoms (hidden) are located in the centers of octahedra, oxygen atoms are in vertexes of octahedra (in red), hydrogen centers are in light pink. Dashed lines show the hydrogen bonding between layers of the hydroxide.
The abstraction of proton and electron from vertex monocoordinated hydroxyl group forms the ferryl group FeIV=O which can then relatively easily (with a barrier of 12 kcal/mol) couple lattice oxygen at three-coordinated position to form peroxide species. Further release of molecular oxygen from this peroxide species faces no any substantial barriers.[8] However, far more active terminal oxo center can appear at the second step of proton-electron pair withdrawal via the following steps.[8]Electrophilic external water molecule attack on the ferryl group can result in the appearance of two hydroxyls instead of ferryl group:
H2O + FeIV=O → HO-FeIV-OH.
The removal of electron-proton pair from one of these hydroxyls forms another bare terminal oxygen of the oxyl or ferryl type at the same iron cation:
HO-FeIV-OH – (H+,e) → HO-FeIV-O• ,
or
HO-FeIV-OH – (H+,e) → HO-FeV=O.
So-formed group possessing oxyl configuration HO-FeIV-O• can be apparently quite reactive toward the O-O coupling process due to high value of β-spin density on it.
With these reasons in mind, the model reactive center was chosen to be above mentioned cubane cluster having terminal oxo and hydroxo ligands at the corner (Figure 2). The total electron spin projection of this cluster is set to 9 on the base of the following data: for previously considered tetramer Fe4O4(OH)4 the lowest total energy corresponds to maximal total spin of 10 meaning ferromagnetic coupling of metal centers with spin of 5/2.[8] Removal of first proton-electron pair from hydroxyl group to form terminal ferryl oxo center decreases total spin to 19/2. Hydroxylation of ferryl center to form the HO-FeIV-OH moiety followed by the second proton-electron pair removal from hydroxyl group at FeIV further decreases total spin to 9. Formally, the electron configuration for obtained iron core is most likely FeV(S=3/2)FeIII(S=5/2)FeIII(S=5/2)FeIII(S=5/2) or FeIV(S=2)FeIII(S=5/2)FeIII(S=5/2)FeIII(S=5/2) with the reactive iron Fe1 being in competing FeV (in ferryl group) or FeIV (in oxyl group) state corresponding formally to S=0 or S=1/2 on terminal oxo site, respectively.