Controlling product selectivity during dioxygen reduction with Mn complexes using pendent proton donor relays and added base

Cook, Emma N.; Courter, Ian M.; Dickie, Diane A.; Machan, Charles W.

doi:10.1039/d3sc02611f

Cited by 3 publications

(3 citation statements)

References 49 publications

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“…This can be rationalized by the second quinol in 2 serving as an additional proton relay group; similar effects have been documented for other small molecule electrocatalysts for the ORR. 21,30,39,68 The additional quinol in 2 also hastens intramolecular H + transfer�which enables some electrocatalytic O 2 reduction in the absence of a buffered proton source�as evidenced by the higher intra-k H + rate constant (Table 3). With 5 and 6, we likewise found nonzero intercepts and evidence of intramolecular H + transfer.…”

Section: E Ementioning

confidence: 99%

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Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O₂ to H₂O at High Rates and Low Overpotentials

Obisesan,

Parvin,

Tao

et al. 2024

Inorg. Chem.

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We prepare iron(II) and iron(III) complexes with polydentate ligands that contain quinols, which can act as electron proton transfer mediators. Although the iron(II) complex with N-(2,5-dihydroxybenzyl)-N,N′,N′-tris(2-pyridinylmethyl)-1,2-ethanediamine (H 2 qp1) is inactive as an electrocatalyst, iron complexes with N,N′-bis(2,5-) were found to be much more active and more selective for water production than a previously reported cobalt-H 2 qp1 electrocatalyst while operating at low overpotentials. The catalysts with H 2 qp3 can enter the catalytic cycle as either Fe(II) or Fe(III) species; entering the cycle through Fe(III) lowers the effective overpotential. On the basis of their TOF 0 values, the successful iron−quinol complexes are better electrocatalysts for oxygen reduction than previously reported iron-porphyrin compounds, with the Fe(III)−H 2 qp3 arguably being the best homogeneous electrocatalyst for this reaction. With iron, the quinol-for-phenol substitution shifts the product selectivity from H 2 O 2 to water with little impact on the overpotential, but unlike cobalt, this substitution also greatly improves the activity, as assessed by TOF max , by hastening the protonation and oxygen binding steps. The addition of a second quinol further enhances the activity and selectivity for water but modestly increases the effective overpotential.

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Section: E Ementioning

confidence: 99%

“…First-row transition metal complexes with nonporphyrinic ligands have also been investigated as catalysts for ORR. − These tend to function at lower overpotentials but generally yield H 2 O 2 as the major product. In addition, most of these ORR reactions have also been studied using chemical reductants rather than a current.…”

Section: Introductionmentioning

confidence: 99%

Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O₂ to H₂O at High Rates and Low Overpotentials

Obisesan,

Parvin,

Tao

et al. 2024

Inorg. Chem.

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“…[1][2][3][7][8][9] Substantial work has been done to better understand reactivity of dioxygen (O2) at molecular transition metal centers, including assessing the structure-function parameters which control selectivity and activity. [1][2][3][10][11][12][13][14] The instability of many organic molecules toward the reactive oxygen species (ROSs) that are intermediates of the ORR has slowed their development as catalysts relative to transition metal complexes. However, there have been a few reports on homogeneous ORR mediated by organic molecules, including methyl viologen 15 and substituted methylacridinium salts.…”

Section: Introductionmentioning

confidence: 99%

Acid Strength Effects on Off-Cycle Dimerization During Metal-Free Catalytic Dioxygen Reduction

Cook,

Flaxman,

Reid

et al. 2024

Preprint

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Development of earth-abundant catalysts for the reduction of dioxygen (ORR) is essential for the development of alternative industrial processes and energy sources. Here, we report a transition metal-free diium organocatalyst (Ph2Phen2+) for the ORR via an outer-sphere mechanism. The ORR performance of this compound was studied in acetonitrile solution under both electrochemical conditions and spectrochemical conditions, using halogenated acetic acid derivatives spanning a pKa range of 12.65 to 20.3. Interestingly, it was found that under electrochemical conditions, an off-cycle dimer species forms via an inner-sphere reaction due to the relatively high concentration of Ph2Phen•+ in the reaction-diffusion layer. However, under spectrochemical conditions, strong acids were able to rapidly protonate O2•– en route to disproportionation, avoiding the off-cycle dimeric species, whereas weaker acids were found to be rate-limited by the dimer equilibrium. Together, these results provide insight into the mechanisms of ORR by organic-based, metal-free catalysts, suggesting that balancing redox activity and unsaturated character of these molecules with respect to the pKa of intermediates can enable tuning analogous to transition metal-based systems.

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Design of Cr-Based Molecular Electrocatalyst Systems for the CO₂ Reduction Reaction

Moberg,

Machan

2024

Acc. Chem. Res.

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Metrics & MoreArticle Recommendations CONSPECTUS: Human influence on the climate system was recently summarized by the sixth Intergovernmental Panel on Climate Change (IPCC) Assessment Report, which noted that global surface temperatures have increased more rapidly in the last 50 years than in any other 50-year period in the last 2000 years. Elevated global surface temperatures have had detrimental impacts, including more frequent and intense extreme weather patterns like flooding, wildfires, and droughts. In order to limit greenhouse gas emissions, various climate change policies, like emissions trading schemes and carbon taxes, have been implemented in many countries. The most prevalent anthropogenic greenhouse gas emitted is carbon dioxide (CO 2 ), which accounted for 80% of all U.S. greenhouse gas emissions in 2022. The reduction of CO 2 through the use of homogeneous electrocatalysts generally follows a two-electron/twoproton pathway to produce either carbon monoxide (CO) with water (H 2 O) as a coproduct or formic acid (HCOOH). These reduced carbon species are relevant to industrial applications: the Fischer−Tropsch process uses CO and H 2 to produce fuels and commodity chemicals, while HCOOH is an energy dense carrier for fuel cells and useful synthetic reagent. Electrochemically reducing CO 2 to value-added products is a potential way to address its steadily increasing atmospheric concentrations while supplanting the use of nonrenewable petrochemical reserves through the generation of new carbon-based resources. The selective electrochemical reduction of CO 2 (CO 2 RR) by homogeneous catalyst systems was initially achieved with late (and sometimes costly) transition metal active sites, leading the field to conclude that transition metal complexes based on metals earlier in the periodic table, like chromium (Cr), were nonprivileged for the CO 2 RR. However, metals early in the table have sufficient reducing power to mediate the CO 2 RR and therefore could be selective in the correct coordination environment. This Account describes our efforts to develop and optimize novel Cr-based CO 2 RR catalyst systems through redox-active ligand modification strategies and the use of redox mediators (RMs). RMs are redoxactive molecules which can participate cocatalytically during an electrochemical reaction, transferring electrons�often accompanied by protons�to a catalytic active site. Through mechanistic and computational work, we have found that ligand-based redox activity is key to controlling the intrinsic selectivity of these Cr compounds for CO 2 activation. Ligand-based redox activity is also essential for developing cocatalytic systems, since it enables through-space interactions with reduced RMs containing redox-active planar aromatic groups, allowing charge transfer to occur within the catalyst assembly. Following a summary of our work, we offer a perspective on the possibilities for future development of catalytic and cocatalytic systems with early transition metals for small molecule activation.

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Controlling product selectivity during dioxygen reduction with Mn complexes using pendent proton donor relays and added base

Abstract: The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous...

Cited by 3 publications

References 49 publications

Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O₂ to H₂O at High Rates and Low Overpotentials

Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O₂ to H₂O at High Rates and Low Overpotentials

Acid Strength Effects on Off-Cycle Dimerization During Metal-Free Catalytic Dioxygen Reduction

Design of Cr-Based Molecular Electrocatalyst Systems for the CO₂ Reduction Reaction

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Controlling product selectivity during dioxygen reduction with Mn complexes using pendent proton donor relays and added base

Abstract: The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous...

Cited by 3 publications

References 49 publications

Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O2 to H2O at High Rates and Low Overpotentials

Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O2 to H2O at High Rates and Low Overpotentials

Acid Strength Effects on Off-Cycle Dimerization During Metal-Free Catalytic Dioxygen Reduction

Design of Cr-Based Molecular Electrocatalyst Systems for the CO2 Reduction Reaction

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Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O₂ to H₂O at High Rates and Low Overpotentials

Installing Quinol Proton/Electron Mediators onto Non-Heme Iron Complexes Enables Them to Electrocatalytically Reduce O₂ to H₂O at High Rates and Low Overpotentials

Design of Cr-Based Molecular Electrocatalyst Systems for the CO₂ Reduction Reaction