3. Results and discussion
Oxidation of water
True O-O coupling takes place in case of the water molecule nucleophilic
attack on the terminal oxyl oxygen. The process starts from the
formation of hydrogen bonds between water molecule and the OH ligand of
reactive iron center (Figure 3a). Two simultaneous steps take place:i ) transfer of proton from water to hydroxyl ligand at the Fe1
site and ii ) coupling of remaining OH group with terminal oxyl
oxygen. In transition state of this process the terminal oxygen changes
its spin density from -0.83 to 0.23 reflecting a change of electron
configuration of the HO-Fe-O moiety (Figure 3b). Correspondingly, water
oxygen acquires a distinct negative spin density of -0.55 becoming a
radical-like species (Figure 3b). This apparently facilitates the
electron pairing to form closed shell in forming the OOH group at
reactive iron Fe1.
The barrier of 11 kcal/mol for splitting of water oxo center at the oxyl
site looks amazingly low in comparison with barriers of 22-43 kcal/mol
for direct coupling of oxo centers on neighboring iron centers as found
in our previous work (Figure 5-7 in ref. [8]). Especially
interesting is the comparison with the OOH group formation (with a
barrier of 18 kcal/mol) at the same site without participation of
external water molecule (Figure 4 in ref. [8]). Present work reveals
an effect of upcoming water molecule, which catalyzes this process
dropping the barrier by 7 kcal/mol. Before water splitting an initial
combination of oxidation states for cluster with oxyl oxygen is formally
Fe4(IV,III,III,III) (Figure 3a). Under the water
molecule attack it becomes Fe4(II,III,III,III) as seen
from dropping of spin density on Fe1 from 3.40 to 2.83 (Figures 3a, c)
and corresponding rising of the core energy ε1s(Fe) by
2.2 eV (Table S1) implying substantial “back” transfer of electron
density from oxo and hydroxo ligands into iron center.
One might suspect that a low barrier for the O-O coupling obtained for
five-coordinated reactive iron site Fe1 is an artifact resulting from
the absence of sixth ligand. To clarify that an additional water
molecule was attached to Fe1 to form six-coordinated iron center of
cubane (Figure S2a) and the process has been modeled again. It appears
that the water ligand remains untouched by the process. Therefore, it is
not surprising that the barrier of the O-O coupling decreases by only 2
kcal/mol, in fact coinciding with the results for coordinatively
unsaturated iron site. This reveals the evidence that the actual proton
transfer between upcoming water molecule and hydroxo group and
simultaneous O-OH coupling is governed mostly by the electron state of
Fe1 and the terminal oxyl oxygen. Moreover, the barrier seems to be
determined by the ability of reactive iron Fe1 cation and its immediate
three oxo neighbors (connecting Fe1 to other metal sites) to adopt
formally two electrons from terminal oxo center and hydroxo groups
becoming OOH and water ligands. Therefore, water solvation could not
bring noticeable contribution into this process.