Proton-coupled electron transfer kinetics for the hydrogen evolution reaction of hangman porphyrins

Roubelakis, Manolis M.; Bediako, D. Kwabena; Dogutan, Dilek K.; Nocera, Daniel G.

doi:10.1039/c2ee21123h

Cited by 163 publications

(170 citation statements)

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“…1, have been investigated as HER electrocatalysts (20,21). Experimental (22) and theoretical (23) examination of the key PCET step within the HER mechanism of the cobalt "hangman" complex (the cobalt analog of [1-H] in Fig. 1) revealed a sequential ET-PT mechanism, with an experimentally measured PT rate constant k PT = 8.5 × 10 6 s −1 (22).…”

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confidence: 99%

“…Experimental (22) and theoretical (23) examination of the key PCET step within the HER mechanism of the cobalt "hangman" complex (the cobalt analog of [1-H] in Fig. 1) revealed a sequential ET-PT mechanism, with an experimentally measured PT rate constant k PT = 8.5 × 10 6 s −1 (22). In the proposed mechanism, the formally Co(I) is reduced to a Co("0.5") complex in which the unpaired electron is shared between the metal and the ligands, breaking the aromaticity of the porphyrin ring.…”

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Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism

Solis

Maher

Dogutan

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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147

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The development of more effective energy conversion processes is critical for global energy sustainability. The design of molecular electrocatalysts for the hydrogen evolution reaction is an important component of these efforts. Proton-coupled electron transfer (PCET) reactions, in which electron transfer is coupled to proton transfer, play an important role in these processes and can be enhanced by incorporating proton relays into the molecular electrocatalysts. Herein nickel porphyrin electrocatalysts with and without an internal proton relay are investigated to elucidate the hydrogen evolution mechanisms and thereby enable the design of more effective catalysts. Density functional theory calculations indicate that electrochemical reduction leads to dearomatization of the porphyrin conjugated system, thereby favoring protonation at the meso carbon of the porphyrin ring to produce a phlorin intermediate. A key step in the proposed mechanisms is a thermodynamically favorable PCET reaction composed of intramolecular electron transfer from the nickel to the porphyrin and proton transfer from a carboxylic acid hanging group or an external acid to the meso carbon of the porphyrin. The C-H bond of the active phlorin acts similarly to the more traditional metal-hydride by reacting with acid to produce H 2 . Support for the theoretically predicted mechanism is provided by the agreement between simulated and experimental cyclic voltammograms in weak and strong acid and by the detection of a phlorin intermediate through spectroelectrochemical measurements. These results suggest that phlorin species have the potential to perform unique chemistry that could prove useful in designing more effective electrocatalysts.electrocatalysis | metalloporphyrin | proton transfer | dearomatization D irect solar-to-fuel processes are important components of global energy sustainability efforts (1, 2). Such processes include the hydrogen evolution reaction (HER), oxidation of water to oxygen, and reduction of CO 2 to hydrocarbons (3, 4). Protoncoupled electron transfer (PCET), which is generally defined in terms of coupling between electron transfer (ET) and proton transfer (PT) reactions, is essential to all of these processes. PCET can be classified as occurring via either a sequential or a concerted mechanism (5, 6). The mechanism is determined to be sequential rather than concerted if a stable intermediate associated with initial ET or PT can be identified. This distinction is not rigorous, however, because the identification of a stable intermediate may depend on the experimental approach or the level of theory, as well as the lifetime of the intermediate. Regardless of the specific mechanism, the coupling of ET and PT plays a significant role in a wide range of energy conversion processes (7-11). Moreover, the coupling of ET and PT can be enhanced by incorporating proton relays into molecular catalysts, exploiting the proximal positioning of the proton donor and acceptor (12-16). Recognition and characterization of successful PCET ...

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confidence: 99%

Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism

Solis

Maher

Dogutan

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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147

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“…61 Catalysts used for oxygen production: [Rh III (dmbpy) 2 Cl 2 + ] where dmbpy = 4,4′-dimethyl-2,2′-bipyridine or Rh , 62 (2,5-dpp)PdCl 2 with 2,5-di(pyridine-2-yl)pyrazine or Pd1 , 63 (2,6-dpp)PdCl 2 with 2,6-di(pyridine-2-yl)pyrazine or Pd2 , 64 [Co III (CR)Cl 2 ] + where CR = 2,12-dimethyl-3,7,11,17-tetra-azabicyclo (11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) or Co3 , 65 and 5-[4-(5-carboxyl-2,7-di-tert-butyl-9,9-dimethylxanthene)]-10,15,20-tris(pentafluoro-phenyl)porphyrinatocobalt(I) or Co4 . 66 …”

Section: Resultsmentioning

confidence: 99%

Computational characterization of competing energy and electron transfer states in bimetallic donor-acceptor systems for photocatalytic conversion

Fredin

Persson

2016

The Journal of Chemical Physics

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The rapidly growing interest in photocatalytic systems for direct solar fuel production, such as hydrogen generation from water splitting is grounded in the unique opportunity to achieve charge separation in molecular systems provided by electron transfer processes. In general, both photoinduced and catalytic processes involve complicated dynamics that depend on both structural and electronic effects. Here the excited state landscape of metal centered light harvester-catalyst pairs is explored using density functional theory (DFT) calculations. In weakly bound systems, the interplay between structural and electronic factors involved can be constructed from the various mononuclear relaxed excited states. For this study, supramolecular states of electron transfer and excitation energy transfer character have been constructed from constituent full optimizations of multiple charge/spin states for a set of three Ru-based light harvesters and nine transition metal catalysts (based on Ru, Rh, Re, Pd, and Co) in terms of energy, structure, and electronic properties. The complete set of combined charge-spin states for each donor-acceptor system provides information about the competition of excited state energy transfer states with the catalytically active electron transfer states, enabling the identification of the most promising candidates for photocatalytic applications from this perspective.

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“…Nocera and coworkers showed that cobalt(II) hangman porphyrins can catalyze proton reduction with less overpotential and weaker acids than their standard porphyrin cousins (Fig. 13d) [85,86]. Both features are thought to be a result of the enhanced proton donation by the carboxylic acid of the hangman substituent.…”

Section: Mononuclear Cobalt Proton Reduction Catalystsmentioning

confidence: 90%

Biomimetic Complexes for Production of Dihydrogen and Reduction of CO2

Jennings

Laureanti

Jones

2015

Homo- And Heterobimetallic Complexes in Catalysis

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The active sites of several bioenergetically important metalloenzymes that perform multielectron redox reactions feature heterobimetallic complexes. Herein, we review recent understanding of the structure and mechanisms of hydrogenases, formate dehydrogenases, and carbon monoxide dehydrogenases. Then we evaluate progress toward creating functional, small-molecule complexes that reproduce the activities of these active sites. Particular emphasis is placed on comparing catalytic properties including turnover number, turnover frequency, required overpotential, and catalyst stability. Opportunities and challenges for future work are also considered.

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Proton-coupled electron transfer kinetics for the hydrogen evolution reaction of hangman porphyrins

Cited by 163 publications

References 26 publications

Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism

Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism

Computational characterization of competing energy and electron transfer states in bimetallic donor-acceptor systems for photocatalytic conversion

Biomimetic Complexes for Production of Dihydrogen and Reduction of CO2

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