Proton Relay Research Articles

Value‐Added Cascade Synthesis Mediated by Paired‐Electrolysis Using an Ultrathin Microenvironment‐Inbuilt Metalized Covalent Organic Framework Heterojunction

AbstractEnergy‐saving and value‐added management in advanced catalysis is highly desirable but is challenged by the limitations of multifunctional catalysts and catalytic modules. Herein, an azo‐linked phthalocyanine‐porphyrin covalent organic framework (COF) with the ultrathin layered nanostructure grown on carbon nanotubes (NiPc‐azo‐H2Pp@CNTs) has been designed and synthesized, which can serve as a highly active and stable bifunctional heterojunction electrocatalyst for selective paired‐electrosynthesis through coupling anodic iodide oxidation reaction and cathodic CO2 conversion. Particularly, the inbuilt local microenvironment conferred by the dihydroporphyrin moieties in COF can act as a proton reservoir to promote the proton relay at the heterojunction interface during the electrocatalytic process. Moreover, through the cascade construction of dual electrocatalytic/organocatalytic modules, the cathode‐generated CO can be further converted to dimethyl carbonate with a yield of 6.21 mmol L−1 h−1, while the anode‐produced iodine can be derived into iodoform on the hundred‐milligram scale. It is worth noting that the value‐added cascade synthesis mediated by paired‐electrolysis using the distinctive and high‐powered electrocatalysts will help advance the sustainable development of industrial intelligent manufacturing.

Mechanistic Insights into Electrocatalytic Hydrogen Evolution by an Exceptionally Stable Cobalt Complex.

Co(aPPy) is one of the most stable and active molecular first-row transition-metal catalysts for proton reduction reported to date. Understanding the origin of its high performance via mechanistic studies could aid in developing even better catalysts. In this work, the catalytic mechanism of Co(aPPy) was electrochemically probed, in both organic solvents and water. We found that different mechanisms can occur depending on the solvent and the acidity of the medium. In organic solvent with a strong acid as the proton source, catalysis initiates directly after a single-electron reduction of CoII to CoI, whereas in the presence of a weaker acid, the cobalt center needs to be reduced twice before catalysis occurs. In the aqueous phase, we found drastically different electrochemical behavior, where the Co(aPPy) complex was found to be a precatalyst to a different electrocatalytic species. We propose that in this active catalyst, the pyridine ring has dissociated and acts as a proton relay at pH ≤ 5, which opens up a fast protonation pathway of the CoI intermediate and results in a high catalytic activity. Furthermore, we determined with constant potential bulk electrolysis that the catalyst is most stable at pH 3. The catalyst thus functions optimally at low pH in an aqueous environment, where the pyridine acts as a proton shuttle and where the high acidity also prevents catalyst deactivation.

Substitution of both histidines in the active site of yeast alcohol dehydrogenase 1 exposes underlying pH dependencies

Histidine residues 44 and 48 in yeast alcohol dehydrogenase (ADH) bind to the coenzymes NAD(H) and contribute to catalysis. The individual H44R and H48Q substitutions alter the kinetics and pH dependencies, and now the roles of other ionizable groups in the enzyme were studied in the doubly substituted H44R/H48Q ADH. The substitutions make the enzyme more resistant to inactivation by diethyl pyrocarbonate, modestly improve affinity for coenzymes, and substantially decrease catalytic efficiencies for ethanol oxidation and acetaldehyde reduction. The pH dependencies for several kinetic parameters are shifted from pK values for wild-type ADH of 7.3–8.1 to values for H44R/H48Q ADH of 8.0–9.6, and are assigned to the water or alcohol bound to the catalytic zinc. It appears that the rate of binding of NAD+ is electrostatically favored with zinc-hydroxide whereas binding of NADH is faster with neutral zinc-water. The pH dependencies of catalytic efficiencies (V/EtKm) for ethanol oxidation and acetaldehyde reduction are similarly controlled by deprotonation and protonation, respectively. The substitutions make an enzyme that resembles the homologous horse liver H51Q ADH, which has Arg-47 and Gln-51 and exhibits similar pK values. In the wild-type ADHs, it appears that His-48 (or His-51) in the proton relay systems linked to the catalytic zinc ligands modulate catalytic efficiencies.

CO2 Reduction by an Iron(I) Porphyrinate System: Effect of Hydrogen Bonding on the Second Coordination Sphere.

Transforming CO2 into valuable materials is an important reaction in catalysis, especially because CO2 concentrations in the atmosphere have been growing steadily due to extensive fossil fuel usage. From an environmental perspective, reduction of CO2 to valuable materials should be catalyzed by an environmentally benign catalyst and avoid the use of heavy transition-metal ions. In this work, we present a computational study into a novel iron(I) porphyrin catalyst for CO2 reduction, namely, with a tetraphenylporphyrin ligand and analogues. In particular, we investigated iron(I) tetraphenylporphyrin with one of the meso-phenyl groups substituted with o-urea, p-urea, or o-2-amide groups. These substituents can provide hydrogen-bonding interactions in the second coordination sphere with bound ligands and assist with proton relay. Furthermore, our studies investigated bicarbonate and phenol as stabilizers and proton donors in the reaction mechanism. Potential energy landscapes for double protonation of iron(I) porphyrinate with bound CO2 are reported. The work shows that the bicarbonate bridges the urea/amide groups to the CO2 and iron center and provides a tight bonding pattern with strong hydrogen-bonding interactions that facilitates easy proton delivery and reduction of CO2. Specifically, bicarbonate provides a low-energy proton shuttle mechanism to form CO and water efficiently. Furthermore, the o-urea group locks bicarbonate and CO2 in a tight orientation and helps with ideal proton transfer, while there is more mobility and lesser stability with an o-amide group in that position instead. Our calculations show that the o-urea group leads to reduction in proton-transfer barriers, in line with experimental observation. We then applied electric-field-effect calculations to estimate the environmental effects on the two proton-transfer steps in the reaction. These calculations describe the perturbations that enhance the driving forces for the proton-transfer steps and have been used to make predictions about how the catalysts can be further engineered for more enhanced CO2 reduction processes.

Elementary Reactions in the Functional Triads of the Blue-Light Photoreceptor BLUF Domain.

The blue light using the flavin (BLUF) domain is one of the smallest photoreceptors in nature, which consists of a unique bidirectional electron-coupled proton relay process in its photoactivation reaction cycle. This perspective summarizes our recent efforts in dissecting the photocycle into three elementary processes, including proton-coupled electron transfer (PCET), proton rocking, and proton relay. Using ultrafast spectroscopy, we have determined the temporal sequence, rates, kinetic isotope effects (KIEs), and concertedness of these elementary steps. Our findings provide important implications for illuminating the photoactivation mechanism of the BLUF domain and suggest an engineering platform to characterize intricate reactions involving proton motions that are ubiquitous in nonphotosensitive protein machines.

Co(iii) corroles with pendant amidophenol and amidopyridine as proton-relay arms to facilitate the electrocatalytic hydrogen evolution reaction

It is known that the efficiency of the hydrogen evolution reaction (HER) can be promoted by the presence of proton relay groups neighboring a catalytic metal center.

Mechanisms of hydrogen evolution by six-coordinate cobalt complexes: a density functional study on the role of a redox-active pyridinyl-substituted diaminotriazine benzamidine ligand as a proton relay.

The hydrogen evolution reaction is an important process for energy storage. The six-coordinate cobalt complex [CoIII(L1-)(LH)]2+ (LH = N-(4-amino-6-(pyridin-2-yl)-1,3,5-triazin-2-yl)benzamidine) was found to catalyze photocatalytic hydrogen evolution. In this work, we performed density functional calculations to obtain the reduction potentials and the proton-transfer free energy of possible intermediates to determine the preferred pathways for proton reduction. The mechanism involves the metal-based reduction of Co(III) to Co(II) before the protonation at the amidinate N on the pyridinyl-substituted diaminotriazine benzamidinate ligand L1- to form [CoII(LH)(LH)]2+. Essentially, the subsequent electron transfer is not metal-based reduction, but rather ligand-based reduction to form [CoII(LH)(LH˙1-)]1+. Through a proton-coupled electron transfer process, the cobalt hydride [CoIIH(LH)(LH2˙)]1+ is formed as the key intermediate for hydrogen evolution. As the cobalt hydride complex is coordinatively saturated, a structural change is required when the hydride on Co is coupled with the proton on pyridine. Notably, the redox-active nature of the ligand results in the low acidity of the protonated pyridine moiety of LH2˙, which impedes its function as a proton relay. Our findings suggest that separating the proton relay fragment from the electron reservoir fragment of the redox-active ligand is preferred for fully utilizing both features in catalytic H2 evolution.

Exploring the effect of a pendent amine group poised over the secondary coordination sphere of a cobalt complex on the electrocatalytic hydrogen evolution reaction.

A CoIII complex (2) of a bispyridine-dioxime ligand (H2LNMe2) containing a tertiary amine group in the proximity of the Co center is synthesized and characterized. One of the oxime protons of the ligand is deprotonated, and the amine group remains protonated in the solid-state structure of the CoII complex (2a). The acid-base properties of 2 showed pKa values of 5.9, 8.4, and 9.6, which are assigned to the dissociation of two consecutive oxime protons and amine protons, respectively. The electrocatalytic proton reduction of 2 was investigated in an aqueous phosphate buffer solution (PBS), revealing a catalytic hydrogen evolution reaction (HER) at an Ecat/2 of -1.01 V vs. the SHE, with an overpotential of 673 mV and a kobs value of 2.6 × 103 s-1 at pH 7. For comparison, the HER of the Co complex (1) lacking the tert-amine group at the secondary sphere was investigated in PBS, which showed a kobs of 1.3 × 103 s-1 and an overpotential of 577 mV. At pH 4, however, 2 revealed a ∼3 times higher kobs value than 1, which suggests that the protonated amine group likely works as a proton relay site. Notably, no significant change in the reaction rate was observed at different pH values for 1, implying that oxime protons may not be involved in the intramolecular proton-coupled electron transfer reaction in the HER. The kobs values for Co complexes at pH 7.0 are significantly higher than those of the [Co(dmgH)2(pyridine)(Cl)] complex, implying that the primary coordination sphere around 1 or 2 enhances the HER and offers better catalyst stability in acidic buffer solutions.

Solvent isotope and mutagenesis studies on the proton relay system in yeast alcohol dehydrogenase 1

Alcohol dehydrogenase catalyzes the reversible transfer of a hydride directly from an alcohol to the nicotinamide ring of NAD+ to form an aldehyde and NADH, and the proton from the alcohol probably is transferred through a hydrogen-bonded system to the imidazole of His-48. Studies of the pH dependencies, and solvent and substrate isotope effects on the wild-type and the enzyme with His-48 substituted with Gln-48 were used to demonstrate a role for the proton relay system. The H48Q substitution increases affinities for NAD+ and NADH by ∼2-fold, suggesting that the overall protein structure is maintained. In contrast, catalytic efficiencies (V/Km) on ethanol and acetaldehyde and affinity for 2,2,2-trifluoroethanol are decreased by about 10-fold. The pH dependencies for catalytic efficiencies on ethanol and acetaldehyde (log V/Km versus pH), show pK values of about 7.5 for wild-type enzyme, but ethanol oxidation by H48Q ADH is essentially linear over the pH range from 5.5 to 9.2 with a slope of 0.47. Steady-state kinetics and substrate isotope effects suggest that the kinetic mechanism of H48Q ADH has become partly random for oxidation of ethanol. Both wild-type and H48Q ADHs have pH-independent isotope effects for oxidation (V1/Kb) of 1-butanol/1-butanol-d9 of 4, suggesting that hydride transfer is a major rate-limiting step. The pH dependence for butanol oxidation by wild type ADH shows a wavy profile over the pH range from pH 6 to 10, with a ∼2.3-fold larger V1/Kb in D2O than in H2O, an “inverse” isotope effect. The substrate isotope effect of 4 is not altered by the solvent isotope effect, suggesting concerted proton/hydride transfer. The solvent isotope effect can be explained by a ground state with a water bound to the catalytic zinc in the enzyme-NAD+ complex, and a transition state that resembles a complex with NADH and aldehyde. In contrast, the H48Q enzyme has a diminished inverse solvent isotope effect of ∼1.3 and an essentially linear pH dependence with a slope of log V1/Kb against pH of 0.49 for oxidation of 1-butanol, which together are consistent with a transition state where hydroxide ion directly accepts a proton from the 2′-hydroxyl group of the nicotinamide ribose in the proton relay system in the enzyme-NAD+-alcohol complex. The results support a catalytic role for His-48 in the proton relay system.

Oxygen Vacancy-Originated Inorganic Proton Relay for the Oxygen Evolution Reaction: Fast Proton Hopping via La-Doped Praseodymium Carbonate Hydroxide

Oxygen Vacancy-Originated Inorganic Proton Relay for the Oxygen Evolution Reaction: Fast Proton Hopping via La-Doped Praseodymium Carbonate Hydroxide

Proton Relay for the Rate Enhancement of Electrochemical Hydrogen Reactions at Heterogeneous Interfaces.

Proton transfer is critically important to many electrocatalytic reactions, and directed proton delivery could open new avenues for the design of electrocatalysts. However, although this approach has been successful in molecular electrocatalysis, proton transfer has not received the same attention in heterogeneous electrocatalyst design. Here, we report that a metal oxide proton relay can be built within heterogeneous electrocatalyst architectures and improves the kinetics of electrochemical hydrogen evolution and oxidation reactions. The volcano-type relationship between activity enhancement and pKa of amine additives confirms this improvement; we observe maximum rate enhancement when the pKa of a proton relay matches the pH of the electrolyte solution. Density-functional-theory-based reactivity studies reveal a decreased proton transfer energy barrier with a metal oxide proton relay. These findings demonstrate the possibility of controlling the proton delivery and enhancing the reaction kinetics by tuning the chemical properties and structures at heterogeneous interfaces.

What Can be Learned from Proton Relays in Photosystem II toward Complete Biomimicking of Oxygen Evolution Reaction Electrocatalysis?

AbstractHow to put together a comprehensive water oxidation design is the long‐term problem for the generation of sustainable fuels. In nature, plants are equipped with a complex mechanism (the Calvin cycle) in the stroma that, over the billions of years, efficiently takes CO2 and converts it to sugar. This reductive process needs electrons from an oxygen‐evolving complex (OEC) in the core of photosystem II, where a water oxidation reaction (WOR) releases molecular oxygen. The initial function of proton relays in natural WOR happens when Tyrosine Yz is oxidized by light harvested P680 through a proton‐coupled electron transfer reaction. The second and main one, is composed of water‐included polypeptide channels from and to CaMn4O5. These channels were created utilizing hydrogen‐bonded networks (HBN). HBN is crucial in providing water substrate and removing the released protons to prevent the accumulation of protons and preserve the stability of the catalytic center. This concept summarizes the challenges in employing biomimicry to create artificial leaves from the viewpoint of proton relays, with the main purpose being providing structural functionality to assist the quick proton hopping.

Copper‐Sulfur‐Nitrogen Cluster Providing a Local Proton for Efficient Carbon Dioxide Photoreduction

AbstractAtomically precise Cu clusters are highly desirable as catalysts for CO2 reduction reaction (CO2RR), and they provide an appropriate model platform for elaborating their structure–activity relationship. However, an efficient overall photocatalytic CO2RR with H2O using assembled Cu‐cluster aggregates as single component photocatalyst has not been reported. Herein, we report a stable crystalline Cu−S−N cluster photocatalyst with local protonated N−H groups (denoted as Cu6−NH). The catalyst exhibits suitable photocatalytic redox potentials, high structural stability, active catalytic species, and a narrow band gap, which account for its outstanding photocatalytic CO2RR performance under visible light, with ≈100 % selectivity for CO evolution. Remarkably, systematic isostructural Cu‐cluster control experiments, in situ infrared spectroscopy, and density functional theory calculations revealed that the protonated pyrimidine N atoms in the Cu6−NH cluster act as a proton relay station, providing a local proton during the photocatalytic CO2RR. This efficiently lowers the energy barrier for the formation of the *COOH intermediate, which is the rate‐limiting step, efficiently enhancing the photocatalytic performance. This work lays the foundation for the development of atomically precise metal‐cluster‐based photocatalysts.

Copper-Sulfur-Nitrogen Cluster Providing a Local Proton for Efficient Carbon Dioxide Photoreduction.

Atomically precise Cu clusters are highly desirable as catalysts for CO2 reduction reaction (CO2 RR), and they provide an appropriate model platform for elaborating their structure-activity relationship. However, an efficient overall photocatalytic CO2 RR with H2 O using assembled Cu-cluster aggregates as single component photocatalyst has not been reported. Herein, we report a stable crystalline Cu-S-N cluster photocatalyst with local protonated N-H groups (denoted as Cu6 -NH). The catalyst exhibits suitable photocatalytic redox potentials, high structural stability, active catalytic species, and a narrow band gap, which account for its outstanding photocatalytic CO2 RR performance under visible light, with ≈100 % selectivity for CO evolution. Remarkably, systematic isostructural Cu-cluster control experiments, in situ infrared spectroscopy, and density functional theory calculations revealed that the protonated pyrimidine N atoms in the Cu6 -NH cluster act as a proton relay station, providing a local proton during the photocatalytic CO2 RR. This efficiently lowers the energy barrier for the formation of the *COOH intermediate, which is the rate-limiting step, efficiently enhancing the photocatalytic performance. This work lays the foundation for the development of atomically precise metal-cluster-based photocatalysts.

Facile Proton Relay in the Triazole Framework to Promote the Electrocatalytic Hydrogen Evolution Reaction

Facile Proton Relay in the Triazole Framework to Promote the Electrocatalytic Hydrogen Evolution Reaction

Thermally Activated Geometrical Regioselective E→Z Isomerization-Enabled Cascade Sequences of Conjugated Dienals: Experimental and DFT Studies.

The geometrical regioselective E→Z isomerization of a conjugated alkene under thermal activation pose a challenge due to microscopic reversibility. Herein we report that such reversibility issues can be circumvented by integrating E→Z isomerization with subsequent cyclization cascade, particularly in the absence of commonly employed light, acids, or metal-catalysts. Thus, linearly conjugated dienals in a mixture of toluene-alcohol (2 : 1) solvents or only with alcohol at 60-70 °C can be converted to γ-alkoxybutenolides in moderate to good yields. The intermediary 2Z,4E-isomer can be isolated, which includes the first example of isolating the regioselective isomerization product under thermal conditions. Density functional theory (DFT) studies have been employed to shed light on the feasibility of geometrical alkene isomerization and ensuing cascade sequences. It has been observed that the regioselective 2E,4E→2Z,4E isomerization of dienal is a thermodynamically facile (ΔG <0) process. Structural elucidation further reveals that the presence of a certain charge transfer and a non-covalent interaction may be the primary reasons for the enhanced stability of the 2Z,4E-isomer. The thermodynamic plausibility of the subsequent cascade reaction from the Z-isomer to the anticipated product in the presence of a polar protic solvent (here MeOH) is also explicated. Out of the two probable pathways, the "hemiacetal pathway" involving a relay proton transfer is kinetically more feasible due to the diminished activation barrier than the "conjugate addition pathway".

Highly Efficient Intramolecular Proton Transfer in p-Aminobenzoic Acid by a Single Ammonia Molecule as a Vehicle.

Proton transfer is classified into two mechanisms: the Grotthuss (proton-relay) and vehicle mechanisms. It has been well studied on gas-phase proton transfer by a proton relay involving multiple molecules. However, a vehicle mechanism in which a single molecule transports a proton has rarely been reported. Here, we have obtained clear evidence that the proton transfers efficiently between the two protonation sites in protonated p-aminobenzoic acid (PABA·H+) by a single ammonia molecule as a vehicle. The gaseous PABA·H+ ions were reacted with NH3 or ND3 under single-collision conditions in a cold ion trap, and the proton-transferred ions were identified by cryogenic ion mobility-mass spectrometry. A reaction intermediate PABA·H+·NH3 was also detected for the first time. The reaction pathway search calculations and ab initio molecular dynamics simulations supported the present experimental finding that intramolecular proton transfer occurs very efficiently by the vehicle mechanism.

Mapping the Ultrafast Mechanistic Pathways of Co Photocatalysts in Pure Water through Time-resolved X-ray Spectroscopy.

Nanosecond time-resolved X-ray (tr-XAS) and optical transient absorption spectroscopy (OTA) are applied to study 3 multimolecular photocatalytic systems with [Ru(bpy)3]2+photoabsorber, ascorbic acid electron donor and Co catalysts with methylene (1), hydroxomethylene (2) and methyl (3) amine substituents in pure water. OTA and tr-XAS of 1 and 2 show that the favored catalytic pathway involves reductive quenching of the excited photosensitizer and electron transfer to the catalyst to form a CoII square pyramidal intermediate with a bonded aqua molecule followed by a CoI square planar derivative that decays within ~8 µs. By contrast, a CoI square pyramidal intermediate with a longer decay lifetime of ~35µs is formed from an analogous CoII geometry for 3 in H2O. These results highlight the protonation of CoI to form the elusive hydride species to be the rate limiting step and show that the catalytic rate can be enhanced through hydrogen containing pendant amines that act as H-H bond formation proton relays.

Deciphering Reversible Homogeneous Catalysis of the Electrochemical H2 Evolution and Oxidation: Role of Proton Relays and Local Concentration Effects**

AbstractNickel bisdiphosphine complexes bearing pendant amines form a unique series of catalysts (so‐called DuBois’ catalysts) capable of bidirectional/reversible electrocatalytic oxidation and production of dihydrogen. This unique behaviour is directly linked to the presence of proton relays installed close to the metal center. We report here for the arginine derivative [Ni(P2CyN2Arg)2]6+ on a mechanistic model and its kinetic treatment that may apply to all DuBois’ catalysts and show that it allows for a good fit of experimental data measured at different pH values, catalyst concentrations and partial hydrogen pressures. The bidirectionality of catalysis results from balanced equilibria related to hydrogen uptake/evolution on one side and (metal)‐hydride installation/capture on the other side, both controlled by concentration effects resulting from the presence of proton relays and connected by two square schemes corresponding to proton‐coupled electron transfer processes. We show that the catalytic bias is controlled by the kinetic of the H2 uptake/evolution step. Reversibility does not require that the energy landscape be flat, with redox transitions occurring at potentials up to 250 mV away for the equilibrium potential, although such large deviations from a flat energy landscape can negatively impacts the rate of catalysis when coupled with slow interfacial electron transfer kinetics.

Deciphering Reversible Homogeneous Catalysis of the Electrochemical H2 Evolution and Oxidation: Role of Proton Relays and Local Concentration Effects.

Nickel bisdiphosphine complexes bearing pendant amines form a unique series of catalysts (so-called DuBois' catalysts) capable of bidirectional/reversible electrocatalytic oxidation and production of dihydrogen. This unique behaviour is directly linked to the presence of proton relays installed close to the metal center. We report here for the arginine derivative [Ni(P2 Cy N2 Arg )2 ]6+ on a mechanistic model and its kinetic treatment that may apply to all DuBois' catalysts and show that it allows for a good fit of experimental data measured at different pH values, catalyst concentrations and partial hydrogen pressures. The bidirectionality of catalysis results from balanced equilibria related to hydrogen uptake/evolution on one side and (metal)-hydride installation/capture on the other side, both controlled by concentration effects resulting from the presence of proton relays and connected by two square schemes corresponding to proton-coupled electron transfer processes. We show that the catalytic bias is controlled by the kinetic of the H2 uptake/evolution step. Reversibility does not require that the energy landscape be flat, with redox transitions occurring at potentials up to 250 mV away for the equilibrium potential, although such large deviations from a flat energy landscape can negatively impacts the rate of catalysis when coupled with slow interfacial electron transfer kinetics.