Unraveling the mechanism of water oxidation catalyzed by nonheme iron complexes

Text Complet
UnravelingMechanism.pdf closed access
Sol·licita còpia a l'autor de l'article
En omplir aquest formulari esteu demanant una còpia de l'article dipositat al repositori institucional (DUGiDocs) al seu autor o a l'autor principal de l'article. Serà el mateix autor qui decideixi lliurar una còpia del document a qui ho sol•liciti si ho creu convenient. En tot cas, la Biblioteca de la UdG no intervé en aquest procés ja que no està autoritzada a facilitar articles quan aquests són d'accés restringit.
Density functional theory (DFT) is employed to: 1)propose a viable catalytic cycle consistent with our experimental results for the mechanism of chemically driven (CeIV) O2 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)(MePy 2CH-tacn)](OTf)2 (MePy2CH-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 [FeIV(O)(OH 2)(LN4)]2+ species (in which LN 4=Pytacn or mep) and the rate-determining step is the O-O bond-formation event. This is nicely supported by the remarkable agreement between the experimental (ΔG≠=17.6±1.6kcalmol -1) and theoretical (ΔG≠=18.9kcalmol -1) activation parameters obtained for complex 1. The O-O bond formation is performed by an iron(V) intermediate [FeV(O)(OH)(LN 4)]2+ containing a cis-FeV(O)(OH) unit. Under catalytic conditions (CeIV, pH0.8) the high oxidation state Fe V is only thermodynamically accessible through a proton-coupled electron-transfer (PCET) process from the cis-[FeIV(O)(OH 2)(LN4)]2+ resting state. Formation of the [FeV(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 O-O 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 O-O bond-formation reaction. Interplay between redox potentials to achieve the high oxidation state (FeVO) and the activation energy barrier for the following O-O 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. Give me a high 5! The full DFT catalytic cycle for the water oxidation mediated by nonheme iron complexes has allowed the identification of the key steps in this reaction as: 1)FeIV/Fe V, and 2)the energy barrier for the O-O bond formation (see figure). This may have a crucial role in the future development of water oxidation catalysts based on iron ​
​Tots els drets reservats