Unraveling the Mechanism of Water Oxidation Catalyzed by Nonheme Iron Complexes

Density functional theory (DFT) is employed to: 1)propose a viable catalytic cycle consistent with our experimental results for the mechanism of chemically driven (Ce-IV) O-2 generation from water, mediated by nonheme iron complexes; and 2)to unravel the role of the ligand on the nonheme iron catalyst in the water oxidation reaction activity. To this end, the key features of the water oxidation catalytic cycle for the highly active complexes [Fe(OTf)(2)(Pytacn)] (Pytacn: 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane; OTf: CF3SO3) (1) and [Fe(OTf)(2)(mep)] (mep: N,N-bis(2-pyridylmethyl)-N,N-dimethyl ethane-1,2-diamine) (2) as well as for the catalytically inactive [Fe(OTf)(2)(tmc)] (tmc: N,N,N,N-tetramethylcyclam) (3) and [Fe(NCCH3)((Py2CH)-Py-Me-tacn)](OTf)(2) ((Py2CH)-Py-Me-tacn: N-(dipyridin-2-yl)methyl)-N,N-dimethyl-1,4,7-triazacyclononane) (4) were analyzed. The DFT computed catalytic cycle establishes that the resting state under catalytic conditions is a [Fe-IV(O)(OH2)(LN4)](2+) species (in which LN4=Pytacn or mep) and the rate-determining step is the OO bond-formation event. This is nicely supported by the remarkable agreement between the experimental (G=17.6 +/- 1.6kcalmol(-1)) and theoretical (G=18.9kcalmol(-1)) activation parameters obtained for complex 1. The OO bond formation is performed by an iron(V) intermediate [Fe-V(O)(OH)(LN4)](2+) containing a cis-Fe-V(O)(OH) unit. Under catalytic conditions (Ce-IV, pH0.8) the high oxidation state Fe-V is only thermodynamically accessible through a proton-coupled electron-transfer (PCET) process from the cis-[Fe-IV(O)(OH2)(LN4)](2+) resting state. Formation of the [Fe-V(O)(LN4)](3+) species is thermodynamically inaccessible for complexes 3 and 4. Our results also show that the cis-labile coordinative sites in iron complexes have a beneficial key role in the OO bond-formation process. This is due to the cis-OH ligand in the cis-Fe-V(O)(OH) intermediate that can act as internal base, accepting a proton concomitant to the OO bond-formation reaction. Interplay between redox potentials to achieve the high oxidation state ((FeO)-O-V) and the activation energy barrier for the following OO bond formation appears to be feasible through manipulation of the coordination environment of the iron site. This control may have a crucial role in the future development of water oxidation catalysts based on iron.