PCET Process Research Articles

Hydrogen-bond-promoted native lignin degradation via PCET process enabled by visible light

Hydrogen-bond-promoted native lignin degradation via PCET process enabled by visible light

Proton transfer mediator for boosting the current density of biomass electrooxidation to the ampere level

Phosphate can be employed as a hydrogen transfer mediator to accelerate the PCET process of HMF dehydrogenation, and Ru, as a high valence metal, can reduce the band gap and improve the charge transfer efficiency.

Proton-Coupled Electron Transfer at the Surface of Polyoxovanadate-Alkoxide Clusters.

ConspectusProton-coupled electron transfer (PCET) is a fundamental process involved in all areas of chemistry, with relevance to biological transformations, catalysis, and emergent energy storage and conversion technologies. Early observations of PCET were reported by Meyer and co-workers in 1981 while investigating the proton dependence of reduction of a molecular ruthenium oxo complex. Since that time, this conceptual framework has grown to encompass an enormous scope of charge transfer and compensation reactions. In this Account, we will discuss ongoing efforts in the Matson Laboratory to understand the fundamental thermodynamics and kinetics of PCET processes at the surface of a series of Lindqvist-type polyoxovanadate clusters. This project aims to provide atomistic resolution of net H atom uptake and transfer at the surfaces of transition-metal oxide materials.First, we discuss our efforts aimed at understanding PCET at metal oxide surfaces using the Lindqvist-type polyoxovanadate-alkoxide (POV-alkoxide) cluster [nBu4N]2[V6O13(TRIOLNO2)2]. These clusters reversibly bind H atom equivalents at bridging oxide sites, mirroring the proposed uptake and release of e-/H+ pairs at transition-metal oxide surfaces. Summarized results include the measurement of bond dissociation free energies of surface hydroxide moieties (BDFE(O-H)) as well as mechanistic analyses that verify concerted proton electron transfer as the operative pathway for PCET at the surface of POV-alkoxide clusters.Next, we discuss net proton and H atom uptake at the surface of reduced variants of the Lindqvist-type POV-alkoxide cluster, [V6O7(OR)12]n (R = Me, Et; n = -2, -1, 0, + 1). In the case of these low-valent POV-alkoxide clusters, nucleophilic bridging sites are kinetically inhibited by functionalization of the cluster surface with organic ligands. This molecular modification enables site-selectivity in proton and H atom uptake to terminal oxide sites. The impact of reaction site and cluster electronics on reaction driving force of PCET is explored, with core electron density playing a critical role in dictating thermodynamics of H atom uptake and transfer. Additional work described here contrasts the kinetics of PCET at terminal oxide sites to the reactivity observed at bridging oxides in POV-alkoxide clusters.Overall, this Account summarizes our foundational knowledge regarding the assessment of PCET reactivity at the surfaces of molecular metal oxides. Drawing analogies between POV-alkoxide clusters and nanoscopic metal oxide materials provide design principles for the advancement of materials applications with atomic precision. These complexes are additionally highlighted as tunable redox mediators in their own right; our studies demonstrate how cluster surface reactivities can be optimized by modifying electronic structure and surface functionalities.

Hydrogen Atom Transfer for C(sp3)−H Functionalization Enabled by Photoredox/Thianthrene/Methanol Synergistic Catalysis

AbstractA synergetic catalytic system of organophotocatalyst 4CzIPN, thianthrene, and methanol was developed for visible‐light‐induced Giese reactions of various alkanes and alkenes, upon using alkoxy radicals as hydrogen atom transfer reagents. The reaction can be performed under mild conditions to provide the alkylated products in moderate to excellent yields, avoiding the use of any strong oxidants and transition metal reagents. Different from the known LMCT and PCET processes, the key to success of this approach is the in situ thianthrenation of methanol by thianthrene radical cation to trigger the generation of methoxy radical.

How Doping Affects the Activity of the Aluminum Oxide Support.

The performance of heteronuclear clusters [AlXO3 ]+ (X=Al, AlO4 , AlMg2 O2 , AlZnO, AlAu2 , Mg, Y, VO, NbO, TaO) in activating methane has been explored by a combination of high-level quantum calculations with reported and supplementary gas-phase experiments. With different dopants in [AlXO3 ]+ , the mechanism, reactivity and selectivity towards methane activation varies accordingly. The classic HAT competes with PCET, depending on the composition of intramolecular interactions. Although the existence of terminal oxygen radical is beneficial for classic HAT, the Alt -C interaction in the [AlXO3 ]+ clusters as enhanced by the strongly electronegative doping groups (X=Al, AlZnO, Mg, Zn, VO, NbO, TaO) favors the PCET process, facilitating C-H bond breaking. In addition, with different dopants, the destiny of the split methyl group varies accordingly. While strong interaction between Alt and CH3 results in the formation of the Alt -C bond, dopants with variable valance may promote the formation of deep-oxidation products like formaldehyde. It has been discussed in detail how to regulate the activity and selectivity of the active center of the catalyst via rational doping.

Electron transfer catalysis mediated by 3d complexes of redox non-innocent ligands possessing an azo function: a perspective.

It was first reported almost two decades ago that ligands with azo functions are capable of accepting electron(s) upon coordination to produce azo-anion radical complexes, thereby exhibiting redox non-innocence. Over the past two decades, there have been numerous reports of such complexes along with their structures and diverse characteristics. The ability of a coordinated azo function to accept one or more electron(s), thereby acting as an electron reservoir, is currently employed to carry out electron transfer catalysis since they can undergo redox transformation at mild potentials due to the presence of energetically accessible energy levels. The cooperative involvement of redox non-innocent ligand(s) containing an azo group and the coordinated metal centre can adjust and modulate the Lewis acidity of the latter through selective ligand-centred redox events, thereby manipulating the capacity of the metal centre to bind to the substrate. We have summarized the list of first row transition metal complexes of iron, cobalt, nickel, copper and zinc with redox non-innocent ligands incorporating an azo function that have been exploited as electron transfer catalysts to effectuate sustainable synthesis of a wide variety of useful chemicals. These include ketazines, pyrimidines, benzothiazole, benzoxazoles, N-acyl hydrazones, quinazoline-4(3)H-ones, C-3 alkylated indoles, N-alkylated anilines and N-alkylated heteroamines. The reaction pathways, as demonstrated by catalytic loops, reveal that the azo function of a coordinated ligand can act as an electron sink in the initial steps to bring about alcohol oxidation and thereafter, they serve as an electron pool to produce the final products either via HAT or PCET processes.

Selective cleavage of Cα–Cβbonds in lignin models using a bifunctional pyridinium photocatalystviaa PCET process

A pyridinium-based photocatalyst possessing both redox-active and hydrogen acceptor sites has been developed for the conversion of lignin models to afford benzaldehyde and phenyl formate as the main products through selective cleavage of C–C bonds.

A Cobalt(III)−Hydroxo Complex Bearing a Pentadentate Amidate Ligand as an Electrocatalyst for Water Oxidation

AbstractA Co(III)−hydroxo complex, [CoIII(dpaq)OH]ClO4 (1‐OH) bearing a pentadentate ligand, H‐dpaq, (H‐dpaq=(2‐[bis(pyridin‐2‐ylmethyl)]amino‐N‐quinolin‐8‐yl‐acetamidate]) catalyses water oxidation in mildly alkaline medium (pH 8.0) at a potential of 1.4 VNHE with an average Turn‐Over‐Frequency (TOFmax) of 2.8×104 s−1 and faradaic efficiency of 88 %. Post‐electrolysis characterization of the electrode rules out the formation of any heterogeneous electroactive species. Electrochemical results and theoretical calculations confirm the occurrence of both metal and ligand centered PCET processes during anodic scanning. The resulting formally Co(V)−oxo/oxyl intermediate undergoes water nucleophilic attack to install the O−O bond. The role of axial ligand in water oxidation by Co(III)−dpaq system has been examined by comparing the reactivity of the Co‐hydroxide complex (1‐OH) with that of its chloride‐ligated counterpart, [CoIII(dpaq)Cl]Cl (1‐Cl). The results confirm the ability of the Co‐dpaq complexes to bind water/or water derived ligands over chloride or non‐aqueous solvents. The interplay of ligand redox non‐innocence and σ‐donating ability of the N5‐carboxamido ligand helps to store oxidizing equivalents and triggers O−O bond formation.

On thermodynamics of electron, proton and PCET processes of catechol, hydroquinone and resorcinol – Consequences for redox properties of polyphenolic compounds

• Systematic DFT study of hydroxybenzene and their semiquinone and quinone forms. • Acidity constants are predicted. • Pourbaix diagrams are evaluated, and redox properties are discussed. • Alternative reaction pathways for radical scavenging activities are investigated. The hydroxyl group acidities represent a characteristic feature of p-hydroquinone, catechol and resorcinol in their electric neutral and charged states. The transformation of dihydroxybenzenes to semiquinone followed by semiquinone to quinone forms can proceed through different proton-coupled electron transfer steps as demonstrated by the Density functional theory calculations combined with solvent continuum model for water environment. These steps were also analysed using the Pourbaix diagram (electrochemical potential vs. pH) diagrams, constructed from the theoretically predicted p K a values, and theoretically estimated electrochemical standard potentials. The obtained results enable the identification of alternative reaction pathways of the radical scavenging abilities of dihydroxybenzenes or polyphenols occurring in redox reactions and biological processes.

Heteroleptic Copper(I)-Based Complexes Incorporating BINAP and π-Extended Diimines: Synthesis, Catalysis and Biological Applications.

A series of copper-based photocatalysts of the type Cu(NN)(BINAP)BF4 were synthesized bearing π-extended diimine ligands. Their behavior in several photocatalytic processes were evaluated and revealed acceptable levels of activity in an SET process, but negligible activity in PCET or ET processes. Suitable activity in ET processes could be restored through modification of the ligand. The BINAP-derived complexes were then evaluated for activity against triple-negative breast cancer cell lines. Controls indicated that copper complexes, and not their ligands, were responsible for activity. Encouraging activity was displayed by a homoleptic complex Cu(dppz)2BF4.

Tuning Oxygen Reduction Catalysis of Dinuclear Cobalt Polypyridyl Complexes by the Bridging Structure.

The four-electron oxygen reduction reaction (4e--ORR) is the mainstay in chemical energy conversion. Elucidation of factors influencing the catalyst's reaction rate and selectivity is important in the development of more active catalysts of 4e--ORR. In this study, we investigated chemical and electrochemical 4e--ORR catalyzed by Co2(μ-O2) complexes bridged by xanthene (1) and anthracene (3) and by a Co2(OH)2 complex bridged by anthraquinone (2). In the chemical ORR using Fe(CpMe)2 as a reductant in acidic PhCN, we found that 1 showed the highest initial turnover frequency (TOFinit = 6.8 × 102 s-1) and selectivity for 4e--ORR (96%) in three complexes. The detailed kinetic analyses have revealed that the rate-determining steps (RDSs) in the catalytic cycles of 1-3 have the O2 addition to [CoII2(OH2)2]4+ as an intermediate in common. In the only case that complex 1 was used as a catalyst, kcat depended on proton concentration because the reaction rate of the O2 addition to [CoII2(OH2)2]4+ was so fast as compared to that of the concerted PCET process of 1. Through X-ray, Raman, and electrochemical analyses and stoichiometric reactions, we found the face-to-face structure of 1 characterized by a slightly flexible xanthene was advantageous in capturing O2 and stabilizing the Co2(μ-O2) structure, thus increasing both the reaction rate and selectivity for 4e--ORR.

Effectual electrocatalytic proton and water reduction by CuII terpyridine scaffolds

In this paper, three Cu(II) complexes [{(OAc)2Cu(3py-tpy)}2Cu(OAc)2(H2O)2] (1a), {[Cu(4py-tpy)(OAc)]Cl}n (2a) and [Cu(Ph-tpy)(OAc)2] (3a) have been successfully employed for electrochemical hydrogen production in both organic and acidic aqueous medium (3py-tpy = 4′-(pyridin-3-yl)-2,2′:6′,2″-terpyridine; 4py-tpy = 4′-(pyridin-4-yl)-2,2′:6′,2″-terpyridine; Ph-tpy = 4′-phenyl-2,2′:6′,2″-terpyridine). All the complexes exhibit efficient catalytic activity for proton reduction in 95:5 (v/v) DMF/H2O using acetic acid as a proton source. Among all the three complexes, 1a shows the highest TOF value of 1473 s−1. The complexes show similar acid–base equilibria, and pKa for all the complexes are found to be 4.8, 4.6, and 4.3 respectively, for 1a, 2a, and 3a. The catalysts generate the aqua complex, through the substitution of the axial ligand. The aqua complex undergoes deprotonation to generate the corresponding hydroxo complex, i.e., [CuL(OAc)(H2O)]+ ⇆ [CuL(OAc)(OH)] + H+ (where L indicates 3py-tpy, or 4py-tpy or Ph-tpy). The complexes remain stable in acidic conditions at low pH and exhibit very high catalytic activity. Among all these complexes 3a shows the higher catalytic activity for water reduction and TOF value of 810 mol of H2 h−1 (mole of catalyst)−1. The presence of PCET process was noticed in case of proton reduction, which generates [Cu0L(OAc)(OH2)] from [CuIIL(OAc)(OH)], followed by protonation to generate the CuII‒H intermediate species. The CuII‒H in presence of H2O revert into [CuL(OAc)(OH)]. During water reduction in an acidic aqueous medium of pH 1.62, the [CuIIL(OAc)(H2O)]+ undergoes 2e− reduction to generate [Cu0L(OAc)(OH2)]−. The [Cu0L(OAc)(OH2)] interacts with H+ to generate CuII‒H intermediate species. The CuII‒H in the presence of H3O+ evolves H2 and revert to [CuIIL(OAc)(H2O)]+.

Evaluating heteroleptic copper(I)-based complexes bearing π-extended diimines in different photocatalytic processes

A series of 12 new copper-based photocatalysts of the type Cu(N^N)(P^P)BF4 were synthesized bearing π-extended diimine ligands. The complexes have red shifted absorptions and larger extinction coefficients than complexes prepared with a parent diimine, dmp. The complexes were evaluated for their ability to promote three different photochemical transformations. Although the complexes were inactive in a reductive PCET process, the complexes afforded good yields in both SET and ET processes. Interestingly, homoleptic copper-complexes derived from the π-extended diimines were significantly more active in SET processes than analogous complexes with simpler diimines.

Tuning proton-coupled electron transfer by crystal orientation for efficient water oxidization on double perovskite oxides

Developing highly efficient and cost-effective oxygen evolution reaction (OER) electrocatalysts is critical for many energy devices. While regulating the proton-coupled electron transfer (PCET) process via introducing additive into the system has been reported effective in promoting OER activity, controlling the PCET process by tuning the intrinsic material properties remains a challenging task. In this work, we take double perovskite oxide PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) as a model system to demonstrate enhancing OER activity through the promotion of PCET by tuning the crystal orientation and correlated proton diffusion. OER kinetics on PBSCF thin films with (100), (110), and (111) orientation, deposited on single crystal LaAlO3 substrates, were investigated using electrochemical measurements, density functional theory (DFT) calculations, and synchrotron-based near ambient X-ray photoelectron spectroscopy. The results clearly show that the OER activity and the ease of deprotonation depend on orientation and follow the order of (100) > (110) > (111). Correlated with OER activity, proton diffusion is found to be the fastest in the (100) film, followed by (110) and (111) films. Our results point out a way of boosting PCET and OER activity, which can also be successfully applied to a wide range of crucial applications in green energy and environment.

Radical-Mediated Strategies for the Functionalization of Alkenes with Diazo Compounds.

One of the most common reactions of diazo compounds with alkenes is cyclopropanation, which occurs through metal carbene or free carbene intermediates. Alternative functionalization of alkenes with diazo compounds is limited, and a methodology for the addition of the elements of Z-CHR2 (with Z = H or heteroatom, and CHR2 originates from N2═CR2) across a carbon-carbon double bond has not been reported. Here we report a novel reaction of diazo compounds utilizing a radical-mediated addition strategy to achieve difunctionalization of diverse alkenes. Diazo compounds are transformed to carbon radicals with a photocatalyst or an iron catalyst through PCET processes. The carbon radical selectively adds to diverse alkenes, delivering new carbon radical species, and then forms products through hydroalkylation by thiol-assisted hydrogen atom transfer (HAT), or forms azidoalkylation products through an iron catalytic cycle. These two processes are highly complementary, proceed under mild reaction conditions, and show high functional group tolerance. Furthermore, both transformations are successfully performed on a gram-scale, and diverse γ-amino esters, γ-amino alcohols, and complex spirolactams are easily prepared with commercially available reagents. Mechanistic studies reveal the plausible pathways that link the two processes and explain the unique advantages of each.

Highly Polarized Coumarin Derivatives Revisited: Solvent-Controlled Competition Between Proton-Coupled Electron Transfer and Twisted Intramolecular Charge Transfer.

Linking a polarized coumarin unit with an aromatic substituent via an amide bridge results in weak electronic coupling that affects the intramolecular electron-transfer (ET) process. As a result of this, interesting solvent-dependent photophysical properties can be observed. In polar solvents, electron transfer in coumarin derivatives of this type induces a mutual twist of the electron-donating and -accepting molecular units (TICT process) that facilitates radiationless decay processes (internal conversion). In the dyad with the strongest intramolecular hydrogen bond, the planar form is stabilized, such that twisting can only occur in highly polar solvents, whereas a fast proton-coupled electron-transfer (PCET process) occurs in nonpolar n-alkanes. The kPCET rate constant decreases linearly with the energy of the fluorescence maximum in different solvents. This observation can be explained in terms of competition between electron- and proton-transfer from a highly polarized (ca. 15 D) and fluorescent locally excited (1 LE) state to a much less polarized (ca. 4 D) charge-transfer (1 CT) state, a unique occurrence. Photophysical measurements performed for a family of related coumarin dyads, together with results of quantum-chemical computations, give insight into the mechanism of the ET process, which is followed by either a TICT or a PCET process. Our results reveal that dielectric solvation of the excited state slows down the PCET process, even in nonpolar solvents.

Does the photoredox reaction affect the photorelease of anthraquinone protected benzaldehyde? A time-resolved spectroscopic study.

In this study, combined time-resolved spectroscopies of femtosecond transient absorption, nanosecond transient absorption, and nanosecond time-resolved resonance Raman spectroscopies as well as DFT calculations were performed to unravel the photorelease reaction mechanism of anthraquinon-2-ylethyl-1,2-diol protected carbonyl compound (Aqe-diol-PPG). It was shown that the triplet state of Aqe-diol-PPG underwent a PCET process to form a xylylene intermediate. After the C-O bond cleavage, a diradical intermediate was generated, which further released the protected benzaldehyde with the by-product Aqe-diol. A comparison work suggested that Aqe-diol underwent a further photoredox reaction in aqueous solutions.

Ligand Noninnocence in Nickel Porphyrins: Nickel Isobacteriochlorin Formation under Hydrogen Evolution Conditions.

An electron-deficient nickel porphyrin complex undergoes facile ring reduction to form a nickel isobacteriochlorin complex under hydrogen evolution conditions. Spectroscopic experiments indicate that the reduced nickel porphyrin undergoes subsequent reduction and protonation to form a phlorin anion rather than a metal hydride, demonstrating that the key initial proton-coupled electron transfer step is directed toward the ligand versus the metal. The phlorin anion facilely converts to the isobacteriochlorin in the presence of two-electron and three-proton equivalents. Cyclic voltammetry (CV) and spectroscopic experiments reveal that the four-electron, four-proton electrochemical reduction of nickel porphyrin to isobacteriochlorin occurs promptly in the presence of the strong proton donor tosic acid, followed by hydrogen evolution reaction (HER) catalysis at slightly more negative potentials. CVs of independently synthesized Ni isobacteriochlorin show a catalytic HER at the same potentials as those observed for the HER in CVs of the Ni porphyrin. We find that, under strongly acidic conditions, the HER catalysis arises from conversion of the Ni isobacteriochlorin into a nickel-containing, catalytically active electrode-adsorbed species. These results show that Ni porphyrin converts to Ni isobacteriochlorin under HER catalysis conditions via a ligand-based PCET process and that it is the isobacteriochlorin complex which gives rise to an active HER catalysis.

Electrocatalytic water oxidation studies of a tetranuclear Cu(ii) complex with cubane-like core Cu4(μ3-O)4

A bio-inspired cubane-like tetranuclear cluster [Cu4(pdmH)4(OAc)2](NO3)2·3H2O can electrocatalyze water oxidation under aqueous alkaline conditions through a PCET process.

Consecutive ligand-based PCET processes affording a doubly reduced nickel pyrazinedithiolate which transforms into a metal hydride required to evolve H2

Our DFT results demonstrate that hydrogen evolution from water catalyzed by a nickel pyradinedithiolate (dcpdt) molecular hydrogen evolution catalyst [NiII(dcpdt)2]2- proceeds via the formation of a square-planar nickel(ii) hydride intermediate which is given by unprecedented structural transformation of a doubly reduced triply protonated species [NiII(dcpdtH2)(dcpdtH)]-, afforded as a result of two consecutive ligand-based reductions of [NiII(dcpdt)(dcpdtH)]- through proton-coupled electron transfer (PCET) pathways.