Figure 3. Energy diagram for
nucleophilic attack of water molecule on oxyl oxygen in
HO-FeIV-O• moiety to form Fe-OOH at the Fe1 site : (a)
initial complex containing adsorbed water molecule, (b) transition-state
complex, and (c) resulting hydroxylated complex. Integer numbers in
energy diagram show the total energy differences (in kcal/mol).
Hydroxylation
Terminal oxyl oxygen disappears under the electrophilic water attack
since the oxygen is exposed to the proton moiety of the water molecule.
The only way to arrange such an approach of water molecule is to adsorb
it first on the neighboring metal cation (Figure 4a) to avoid undesired
interaction with hydroxyl group on Fe1. When oxyl oxygen in the
HO-FeIV-O• moiety abstracts proton from adsorbed water
molecule, two hydroxyl groups appear on metal sites (Figure 4c), rather
than the Fe-OH moiety and free OH radical as it might be expected for
the water oxidation route by analogy with the abstraction of hydrogen
from methane on oxyl oxygen producing methyl
radical.[15] Therefore, the low barrier process (9
kcal/mol) has to be assigned to the dissociative adsorption or
hydroxylation.
The same is true for electrophilic water attack on ferryl oxygen (Figure
5). For the latter case, the abstraction of hydrogen by ferryl oxygen to
form two hydroxyl anions goes through even a lower barrier of 4 kcal/mol
(Figure 5). This might be explained by more nucleophilic nature of
ferryl oxygen preferable for proton abstraction. Nucleophilicity of
ferryl and oxyl oxo sites can be compared by means of the difference
between the energy of their calculated 1s(O) level. The latter is 1.2 eV
lower than the former, which means less negatively charged oxyl oxygen
(Table S1). This explanation is in line with intuitive picture of the
oxyl oxygen appearance via “back” transfer of electron from from
ferryl-state oxygen to iron center. Mulliken charges on oxyl and ferryl
oxygen agree surprisingly good with the core level energies.
Although the hydroxylation of oxyl oxygen in the
HO-FeIV-O• group and neighboring
FeIII center has to form two chemically equivalent
sites having two hydroxo ligands (Figure 4c), their spin density (and so
the oxidation state) remains different at all stages of process (Figure
4). However, a crucial change takes place for the Fe2 site despite the
fact that is not directly involved in the process. The spin density at
Fe2 drops from 4.21 to 3.40. Taking into account that energies of 1s
level for the iron centers with spins 3.34 (Fe1) and 3.40 (Fe2) (Figure
4c) are equal within 0.01 eV (Table S1), one might guess that the
oxidation state of Fe2 is 4+ coinciding with that for Fe1 (Figure 4a).
What is interesting, initial oxyl containing cubane has oxidation states
Fe4(IV,III,III,III) (Figure 4a). Therefore, the
hydroxylation changes this configuration to
Fe4(IV,IV,III,III) (Figure 4c) implying delocalization
of spin over the oxo bridges. This result reveals unusual effect that
the dissociative adsorption of water (which normally proceeds without
any electron transfer) on cluster affects the oxidation states of
connected iron centers. This effect is certainly connected with the
partial disruption of cubane structure as seen from the elongation of
one of edges by almost 1 Å (Figure 4c). In case of hydroxylation of the
ferryl-oxo cubane the oxidation scheme is
Fe4(IV,IV,III,III) from the beginning at each steps of
the process (Figure 5).
Worthwhile noting that the account of solvation does not change much the
structure and relative energies of above given process of water
dissociation. For the processes (in both oxyl and ferryl cases) modeled
without such account, the barrier is only 1-2 kcal/mol larger than that
for the model with solvation (Figure S4-S5).