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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 (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

We thank European Research Foundation for project FP7-PEOPLE-2010-ERG-268445 (J.L.-F.) and ERC-2009-StG-239910 (M. C.), MICINN for projects CTQ2009-08464 (M. C.) and CTQ2011-23156/BQU (J.M.L.), for a Ramon y Cajal con-tract (J.L.-F.) and Generalitat de Catalunya for an ICREA Academia Award and project 2009SGR637, and the RahuCat is acknowledged by a generous gift of tritosylTACN

© Chemistry - A European Journal, 2014, vol. 20, p. 5696-5707

Wiley-VCH Verlag

Author: Acuña-Parés, Ferran
Codolà Duch, Zoel
Costas Salgueiro, Miquel
Luis Luis, Josep Maria
Lloret Fillol, Julio
Date: 2014
Abstract: 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
We thank European Research Foundation for project FP7-PEOPLE-2010-ERG-268445 (J.L.-F.) and ERC-2009-StG-239910 (M. C.), MICINN for projects CTQ2009-08464 (M. C.) and CTQ2011-23156/BQU (J.M.L.), for a Ramon y Cajal con-tract (J.L.-F.) and Generalitat de Catalunya for an ICREA Academia Award and project 2009SGR637, and the RahuCat is acknowledged by a generous gift of tritosylTACN
Format: application/pdf
ISSN: 0947-6539 (versió paper)
1521-3765 (versió electrònica)
Document access: http://hdl.handle.net/10256/10210
Language: eng
Publisher: Wiley-VCH Verlag
Collection: MICINN/PN 2010-2012/CTQ2009-08464
MICINN/PN 2012-2014/CTQ2011-23156
AGAUR/2009-2014/2009 SGR-637
Reproducció digital del document publicat a: http://dx.doi.org/10.1002/chem.201304367
Articles publicats (D-Q)
info:eu-repo/grantAgreement/EC/FP7/268445
info:eu-repo/grantAgreement/EC/FP7/239910
Is part of: © Chemistry - A European Journal, 2014, vol. 20, p. 5696-5707
Rights: Tots els drets reservats
Subject: Oxidació
Oxidation
Catàlisi homogènia
Homogeneous catalysis
Funcional de densitat, Teoria del
Density functionals
Lligands
Ligands
Mecanismes de reacció (Química)
Reaction mechanisms (Chemistry)
Reacció d’oxidació-reducció
Oxidation-reduction reaction
Title: Unraveling the mechanism of water oxidation catalyzed by nonheme iron complexes
Type: info:eu-repo/semantics/article
Repository: DUGiDocs

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