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.