Figure 5. Water electrophilic
attack on ferryl HO-FeV=O site: (a) initial complex
containing adsorbed water molecule, (b) transition-state complex, and
(c) resulting hydroxylated complex.
4. Conclusions
In the present work the cubane cluster
OFe4(μ-O)4(OH)5 was used
to model the O-O coupling versus hydroxylation of the reactive terminal
oxo center on the iron-containing oxyhydroxides by means of DFT with
solvent account. The following results are obtained.
- The water nucleophilic attack on the oxyl oxygen to form the OOH group
proceeds with a remarkably low barrier of 11 kcal/mol.
- The water electrophilic attack on the reactive corner site results in
an easy hydroxylation of both types of oxo sites
(HO-FeIV-O• and HO-FeV=O).
- The activation barrier for hydroxylation of oxyl oxygen is predicted
to be 9 kcal/mol, while in the case of ferryl oxygen the barrier is as
low as 4 kcal/mol.
The barrier estimations allow one to conclude that the O-O coupling and
hydroxylation and are equally probable. From two forms of terminal oxo
center the ferryl one is preferred for hydroxylation and is less
preferred for O-O coupling. Therefore, the hydroxylation in fact
enhances selectivity of the O-O coupling on the oxyl-oxygen centers.