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.