Determining the Fate of a Non-Heme Iron Oxidation Catalyst Under Illumination, Oxygen, and Acid

Esarey, Samuel L.; Holland, Joel C.; Bartlett, Bart M.

doi:10.1021/acs.inorgchem.6b01538

Cited by 11 publications

(9 citation statements)

References 65 publications

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“…In other words, the ligand remains ligated to the iron center and free ligand never accumulates in solution, despite the highly acidic conditions of the reaction mixtures. The high stability of 2α under acidic conditions is especially notable in comparison with its ferrous precursor 1α . This contrasting behavior can be understood by considering the respective labile and inert character of the high-spin ( S = 2) Fe(II) and low-spin ( S = 1) Fe(IV) complexes.…”

Section: Resultsmentioning

confidence: 99%

“…6a,e In acidic aqueous solution, in the absence of an oxidant, it also undergoes slow hydrolytic decomposition, a behavior that can be traced to the lability of the high spin ferrous center. 7 On the other hand, the isomerically related 1β is poorly active with chemical oxidants and it is very rapidly degraded under acidic conditions. The reaction mechanism proposed for these catalysts operating in acidic media with chemical oxidants such as CAN is shown in figure 1 and entails subsequent oxidation of the ferrous complexes, forming Fe IV (O) species which constitute the resting state during catalysis.…”

Section: Introductionmentioning

confidence: 99%

“…The pair of catalysts α-[Fe(OTf) 2 (mcp)] ( 1α ; mcp = N , N ′-dimethyl- N , N ′-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine, OTf = trifluoromethanesulfonate anion) and β-[Fe(OTf) 2 (mcp)] ( 1β ) are interesting because, being topological isomers, they exhibit very important differences in their catalytic activity. , 1α is a particularly efficient WO catalyst when chemical oxidants such as cerium ammonium nitrate (CAN) and NaIO 4 are employed under acidic conditions, but it rapidly degrades under photochemical and or basic conditions. , In acidic aqueous solution, in the absence of an oxidant, it also undergoes slow hydrolytic decomposition, a behavior that can be traced to the lability of the high-spin ferrous center . On the other hand, the isomerically related 1β is poorly active with chemical oxidants and it is very rapidly degraded under acidic conditions.…”

Section: Introductionmentioning

confidence: 99%

“…6a,e In acidic aqueous solution, in the absence of an oxidant, it also undergoes slow hydrolytic decomposition, a behavior that can be traced to the lability of the high-spin ferrous center. 7 On the other hand, the isomerically related 1β is poorly active with chemical oxidants and it is very rapidly degraded under acidic conditions.…”

Section: ■ Introductionmentioning

confidence: 99%

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Design of Iron Coordination Complexes as Highly Active Homogenous Water Oxidation Catalysts by Deuteration of Oxidation-Sensitive Sites

et al. 2018

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The nature of the oxidizing species in water oxidation reactions with chemical oxidants catalyzed by α-[Fe(OTf)2(mcp)] (mcp = N, N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine, OTf = trifluoromethanesulfonate anion), (1α) and β-[Fe(OTf)2(mcp)], (1β) has been investigated. Mössbauer spectroscopy provides definitive evidence that 1α and 1β generate oxoiron(IV) species as the resting state. Decomposition paths of the catalysts have been investigated by identifying and quantifying ligand fragments that form upon degradation. This analysis correlates the water oxidation activity of 1α and 1β with stability against oxidative damage of the ligand via aliphatic C-H oxidation. The site of degradation and the relative stability against oxidative degradation is shown to be dependent on the topology of the catalyst. Furthermore, the mechanisms of catalyst degradation have been rationalized by computational analyses, which also explain why the topology of the catalyst enforces different oxidation sensitive sites. This information has served for creating catalysts where sensitive C-H bonds have been replaced by C-D bonds. Deuterated analogs, D4-α-[Fe(OTf)2(mcp)] (D4-1α), D4-β-[Fe(OTf)2(mcp)] (D4-1β) and D6-β-[Fe(OTf)2(mcp)] (D6-1β) were prepared, and their catalytic activity has been studied. D4-1α proves to be an extraordinarily active and efficient catalyst (up to 91% of O2 yield); it exhibits initial reaction rates identical to its protio analogue, but it is substantially more robust towards oxidative degradation and yields more than 3400 TON (n(O2)/n(Fe)). Altogether evidence that the water oxidation catalytic activity is performed by a well-defined coordination complex and not by iron oxides formed after oxidative degradation of the ligands.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Design of Iron Coordination Complexes as Highly Active Homogenous Water Oxidation Catalysts by Deuteration of Oxidation-Sensitive Sites

et al. 2018

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“…Detection of in situ ‐generated nanoparticles can be very challenging. Hence, publications on molecular WOCs generally include multiple experiments to prove that the catalyst activity stems from a molecular species [44,52–54] . In the context of our ongoing research to explore novel first‐row metal‐based WOCs, [55] we were interested in developing novel nickel‐based catalysts.…”

Section: Methodsmentioning

confidence: 99%

Potential‐ and Buffer‐Dependent Catalyst Decomposition during Nickel‐Based Water Oxidation Catalysis

Hessels

Detz

et al. 2020

ChemSusChem

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The production of hydrogen by water electrolysis benefits from the development of water oxidation catalysts. This development process can be aided by the postulation of design rules for catalytic systems. The analysis of the reactivity of molecular complexes can be complicated by their decomposition under catalytic conditions into nanoparticles that may also be active. Such a misinterpretation can lead to incorrect design rules. In this study, the nickel‐based water oxidation catalyst [NiII(meso‐L)](ClO4)2, which was previously thought to operate as a molecular catalyst, is found to decompose to form a NiOx layer in a pH 7.0 phosphate buffer under prolonged catalytic conditions, as indicated by controlled potential electrolysis, electrochemical quartz crystal microbalance, and X‐ray photoelectron spectroscopy measurements. Interestingly, the formed NiOx layer desorbs from the surface of the electrode under less anodic potentials. Therefore, no nickel species can be detected on the electrode after electrolysis. Catalyst decomposition is strongly dependent on the pH and buffer, as there is no indication of NiOx layer formation at pH 6.5 in phosphate buffer nor in a pH 7.0 acetate buffer. Under these conditions, the activity stems from a molecular species, but currents are much lower. This study demonstrates the importance of in situ characterization methods for catalyst decomposition and metal oxide layer formation, and previously proposed design elements for nickel‐based catalysts need to be revised.

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Iridium Water Oxidation Catalysts Based on Pyridine‐Carbene Alkyl‐Substituted Ligands

et al. 2019

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Iridium complexes bearing pyridine triazolylidene ligands with variable steric hindrance, derived by the presence of an R group (R=H, Me, Et, nPr, iPr, Bu, and Oct) onto the N1‐nitrogen, have been synthesized, fully characterized and tested as water oxidation catalysts (WOCs), using chemical sacrificial oxidants (CAN and NaIO4) or in photocatalytic experiments ([Ru(bpy)3]Cl2 as phosensitizer and Na2S2O8 as an electron acceptor). The catalytic activity is barely affected by the nature of R when WO is driven by NaIO4 (1 min−1<TOF<14 min−1) or light/[Ru(bpy)3]Cl2/Na2S2O8 (TOF≈0.15 min−1); on the contrary, a remarkable effect is observed with CAN. Particularly, in the latter case, complexes with R=H and Me exhibit similar activity (10 min−1<TOF<20 min−1), whereas all other complexes (R=Et, Pr, i‐Pr, Bu, and Oct) are significantly more active and exhibit comparable TOFs in the range 40–130 min−1. Thus, a marked discontinuity in performances occurs when passing from R=Me to R=Et. This can be hardly explained solely based on the previously proposed hypothesis, suggesting that an increased R‐dimension might favor the association of iridium complexes leading to the formation of highly active multimetallic species. An additional, more specific effect has to be present. It appears plausible that the steric hindrance introduced in close proximity of the iridium center, when R≥Et, hampers transfer of the hydroperoxo or peroxo moiety from iridium active species to cerium, a process that could slow down kinetics of O2 evolution.

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Determining the Fate of a Non-Heme Iron Oxidation Catalyst Under Illumination, Oxygen, and Acid

Cited by 11 publications

References 65 publications

Design of Iron Coordination Complexes as Highly Active Homogenous Water Oxidation Catalysts by Deuteration of Oxidation-Sensitive Sites

Design of Iron Coordination Complexes as Highly Active Homogenous Water Oxidation Catalysts by Deuteration of Oxidation-Sensitive Sites

Potential‐ and Buffer‐Dependent Catalyst Decomposition during Nickel‐Based Water Oxidation Catalysis

Iridium Water Oxidation Catalysts Based on Pyridine‐Carbene Alkyl‐Substituted Ligands

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