Unraveling the Mechanism of Water Oxidation Catalyzed by Nonheme Iron Complexes

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 (Ce(IV) ) 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: CF3 SO3 () ) (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 )((Me) Py2 CH-tacn)](OTf)2 ((Me) Py2 CH-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 OO bond-formation event. This is nicely supported by the remarkable agreement between the experimental (ΔG(≠) =17.6±1.6 kcal mol(-1) ) and theoretical (ΔG(≠) =18.9 kcal mol(-1) ) activation parameters obtained for complex 1. The OO 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) , pH 0.8) 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 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 (Fe(V) O) 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.

Evidence for an oxygen evolving iron–oxo–cerium intermediate in iron-catalysed water oxidation

“…3). Taking into consideration the Nernst equation, the redox potential for the Fe III -OH 2 /Fe IV (O) couple under the low pH reaction conditions is estimated at ~1.4 V versus NHE, which matches that obtained in recent DFT calculations 22 .…”

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

“…[117] However,t he barrier for this step was not calculated. In contrast, Lloret-Fillol [114] proposed that the OÀOb ond formationh ad to take place in the Fe V oxidation state through as tepwise homolytic mechanism (Figure 14). First, ap roton is transferred from water to the Fe V -boundh ydroxide,c oupled with electron transfer from water to the metal center.T his leads to the generation of ar eactive hydroxyl radical, which then reacts with the oxo moiety to give an Fe III -peroxide intermediate.…”

“…The use of Fe IV = O(OH) for OÀOb ond formation was also considered. [115] However,t he associated barrier was more than 30 kcal mol À1 ,w hich ruled out this possibility.…”

“…[49] At etradentatel igand could, in principle, leave two empty coordination sites, and it was discovered that the iron complexes with two available cis sites were efficient in water oxidation, but ineffective with two available trans sites. The water oxidation mechanism of ar epresentative complex, [Fe(Pytacn)] (Pytacn = 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane;F igure 14), was then subjected to extensive DFT studies by the same group, [114][115][116] and also by Kasapbasi and Whangbo. [117] Kasapbasia nd Whangbo suggested OÀO bond formation through nucleophilic attack of water on the Fe IV =O(OH) species.…”

Introducing a closed system approach for the investigation of chemical steps involving proton and electron transfer; as illustrated by a copper-based water oxidation catalyst

“…These two approaches have emerged from within the natural photosynthesis community, 62,63 and have also gained traction within the artificial photosynthetic field. 12,32,64,65 It is therefore apt to compare the two. Geometries of the intermediates were optimised and the most energetically favourable multiplicity for each intermediate determined as shown in the SI (Table S3).…”