Proton Shuttling Research Articles

One-Step Esterification of Phosphoric, Phosphonic and Phosphinic Acids with Organosilicates: Phosphorus Chemical Recycling of Sewage Waste.

Global concerns regarding the depletion and strategic importance of phosphorus resources have increased demand for the recovery and recycling. However, waste-derived phosphorus compounds, primarily as chemically inert phosphoric acid or its salts, present a challenge to their direct conversion into high-value chemicals. We aimed to develop an innovative technology that utilizes the large quantities of sewage waste, bypasses the use of white phosphorus, and enables esterification of phosphoric acid to produce widely applicable phosphate triesters. Tetraalkyl orthosilicates emerged as highly effective reagents for the direct triple esterification of 85% phosphoric acid, as well as the esterification of organophosphinic and phosphonic acids. Furthermore, we achieved esterification of recovered phosphoric acid with tetraalkyl orthosilicate, thus pioneering a recycling pathway from sewage waste to valuable phosphorus chemicals. Experimental and theoretical investigations revealed a novel mechanism, wherein tetraalkyl orthosilicates facilitate multimolecular aggregation to achieve alkyl transfer from tetraalkylorthosilicate to phosphoric acid via multiple proton shuttling.

Investigating proton shuttling and electrochemical mechanisms of amines in integrated CO2 capture and utilization

Carbon capture and utilization (CCU) technologies present a promising solution for converting CO2 emissions into valuable products. Here we show how amines, such as monoethanolamine (MEA) and 2-amino-2-methyl-1-propanol (AMP), influence the electrochemical CO2 reduction process in an integrated CCU system. Using in situ spectroscopic techniques, we identify the key roles of carbamate bond strength, proton shuttling, and amine structure in dictating reaction pathways on copper (Cu) and lead (Pb) electrodes. Our findings demonstrate that on Cu electrodes, surface blockage by ammonium species impedes CO₂ reduction, whereas on Pb electrodes, proton shuttling enhances the production of hydrocarbon products. This study provides additional insights into optimizing CCU systems by tailoring the choice of amines and electrode materials, advancing the selective conversion of CO₂ into valuable chemicals.

Mitochondrial uncoupling caused by a wide variety of protonophores is differently sensitive to carboxyatractyloside in rat heart and liver mitochondria

Mitochondrial uncoupling by small-molecule protonophores is generally accepted to proceed via transmembrane proton shuttling. The idea of facilitating this process by the adenine nucleotide translocase ANT originated primarily from the partial reversal of the DNP-induced mitochondrial uncoupling by the ANT inhibitor carboxyatractyloside (CATR). Recently, the sensitivity to CATR was also observed for the action of such potent OxPhos uncouplers as BAM15, SF6847, FCCP and niclosamide. Here, we report measurements of the CATR effect on the activity of a large number of conventional and novel uncouplers in isolated mammalian mitochondria. Despite the broad variety of chemical structures, CATR attenuated the uncoupling efficacy of all the anionic protonophores in rat heart mitochondria with high abundance of ANT, whereas the effect was much less pronounced or even absent, e.g. for SF6847, in rat liver mitochondria with low ANT content. The fact that the uncoupling action is tissue specific for a broad spectrum of anionic protonophores is highlighted here for the first time. Only with the cationic uncoupler ellipticine and the channel-forming peptide gramicidin A, no sensitivity to CATR was found even in rat heart mitochondria. By contrast, with the recently described ester-stabilized ylidic protonophores [Kirsanov et al. Bioelectrochemistry 2023], the stimulating effect of CATR was discovered both in liver and heart mitochondria.

Modulating Hydrogen Shuttling in Ammonia by Neutral and Cationic Boron‐Containing Frustrated Lewis Pairs (FLPs)

AbstractXanthene‐backbone FLPs featuring secondary borane functions −B(ArX)H (where ArX=C6F5 (ArF) or C6Cl5 (ArCl)) have been targeted through reactions of the dihydroboranes Me2S ⋅ BArXH2 with [4,5‐xanth(PR2)Li]2 (R=Ph, iPr), and investigated in the synthesis of related cationic systems via hydride abstraction. The reactivity of these systems (both cationic and charge neutral) with ammonia have been probed, with a view to probing the potential for proton shuttling via N−H bond ‘activation.’ We find that in the case of four‐coordinate boron systems (cationic or change neutral), the N−H linkage remains intact, supported by a NH⋅⋅⋅P hydrogen bond which is worth up to 17 kcal mol−1 thermodynamically, and enabled by planarization of the flexible xanthene scaffold. For cationic three coordinate systems, N‐to‐P proton transfer is viable, driven by the ability of the boron centre to stabilise the [NH2]− conjugate base through N‐to‐B π bonding. This proton transfer can be shown to be reversible in the presence of excess ammonia, depending on the nature of the B‐bound ArX group. It is viable in the case of C6F5 substituents, but is prevented by the more sterically encumbering and secondary donor‐stabilising capabilities of the C6Cl5 substituent.

Modulating Hydrogen Shuttling in Ammonia by Neutral and Cationic Boron-Containing Frustrated Lewis Pairs (FLPs).

Xanthene-backbone FLPs featuring secondary borane functions -B(ArX)H (where ArX=C6F5 (ArF) or C6Cl5 (ArCl)) have been targeted through reactions of the dihydroboranes Me2S ⋅ BArXH2 with [4,5-xanth(PR2)Li]2 (R=Ph, iPr), and investigated in the synthesis of related cationic systems via hydride abstraction. The reactivity of these systems (both cationic and charge neutral) with ammonia have been probed, with a view to probing the potential for proton shuttling via N-H bond 'activation.' We find that in the case of four-coordinate boron systems (cationic or change neutral), the N-H linkage remains intact, supported by a NH⋅⋅⋅P hydrogen bond which is worth up to 17 kcal mol-1 thermodynamically, and enabled by planarization of the flexible xanthene scaffold. For cationic three coordinate systems, N-to-P proton transfer is viable, driven by the ability of the boron centre to stabilise the [NH2]- conjugate base through N-to-B π bonding. This proton transfer can be shown to be reversible in the presence of excess ammonia, depending on the nature of the B-bound ArX group. It is viable in the case of C6F5 substituents, but is prevented by the more sterically encumbering and secondary donor-stabilising capabilities of the C6Cl5 substituent.

Bioinspired Diiron Complex with Proton Shuttling and Redox-Active Ligand for Electrocatalytic Hydrogen Evolution.

A μ-oxo diiron complex, featuring the pyridine-2,6-dicarboxamide-based thiazoline-derived redox-active ligand, H2L (H2L = N2,N6-bis(4,5-dihydrothiazol-2-yl)pyridine-2,6-dicarboxamide), was synthesized and thoroughly characterized. [FeIII-(μ-O)-FeIII] showed electrocatalytic hydrogen evolution reaction activity in the presence of different organic acids of varying pKa values in dimethylformamide. Through electrochemical analysis, we found that [FeIII-(μ-O)-FeIII] is a precatalyst that undergoes concerted two-electron reduction to generate an active catalyst. Fourier transform infrared spectrum of reduced species and density functional theory (DFT) investigation indicate that the active catalyst contains a bridged hydroxo unit which serves as a local proton source for the Fe(III) hydride intermediate to release H2. We propose that in this active catalyst, the thiazolinium moiety acts as a proton-transferring group. Additionally, under sufficiently strong acidic conditions, bridged oxygen gets protonated before two-electron reduction. In the presence of exogenous acids of varying strengths, it displays electro-assisted catalytic response at a distinct applied potential. Stepwise electron-transfer and protonation reactions on the metal center and the ligand were studied through DFT to understand the thermodynamically favorable pathways. An ECEC or EECC mechanism is proposed depending on the acid strength and applied potential.

The Mechanism of Rh(I)-Catalyzed Coupling of Benzotriazoles and Allenes Revisited: Substrate Inhibition, Proton Shuttling, and the Role of Cationic vs Neutral Species.

Direct coupling of benzotriazole to unsaturated substrates such as allenes represents an atom-efficient method for the construction of biologically and pharmaceutically interesting functional structures. In this work, the mechanism of the N2-selective Rh complex-catalyzed coupling of benzotriazoles to allenes was investigated in depth using a combination of experimental and theoretical techniques. Substrate coordination, inhibition, and catalyst deactivation was probed in reactions of the neutral and cationic catalyst precursors [Rh(μ-Cl)(DPEPhos)]2 and [Rh(DPEPhos)(MeOH)2]+ with benzotriazole and allene, giving coordination, or coupling of the substrates. Formation of a rhodacycle, formed by unprecedented 1,2-coupling of allenes, is responsible for catalyst deactivation. Experimental and computational data suggest that cationic species, formed either by abstraction of the chloride ligand or used directly, are relevant for catalysis. Isomerization of benzotriazole and cleavage of its N-H bond are suggested to occur by counteranion-assisted proton shuttling. This contrasts with a previously proposed scenario in which oxidative N-H addition at Rh is one of the key steps. Based on the mechanistic analysis, the catalytic coupling reaction could be optimized, leading to lower reaction temperature and shorter reaction times compared to the literature.

Vanadium Alkylidyne Initiated Cyclic Polymer Synthesis: The Importance of a Deprotiovanadacyclobutadiene Moiety.

Reported is the catalytic cyclic polymer synthesis by a 3d transition metal complex: a V(V) alkylidyne, [(dBDI)V≡CtBu(OEt2)] (1-OEt2), supported by the deprotonated β-diketiminate dBDI2- (dBDI2- = ArNC(CH3)CHC(CH2)NAr, Ar = 2,6-iPr2C6H3). Complex 1-OEt2 is a precatalyst for the polymerization of phenylacetylene (PhCCH) to give cyclic poly(phenylacetylene) (c-PPA), whereas its precursor, complex [(BDI)V≡CtBu(OTf)] (2-OTf; BDI- = [ArNC(CH3)]2CH, Ar = 2,6-iPr2C6H3, OTf = OSO2CF3), and the zwitterion [((C6F5)3B-dBDI)V≡CtBu(OEt2)] (3-OEt2) exhibit low catalytic activity despite having a neopentylidyne ligand. Cyclic polymer topologies were verified by size-exclusion chromatography (SEC) and intrinsic viscosity studies. A component of the mechanism of the cyclic polymerization reaction was probed by isolation and full characterization of 4- and 6-membered metallacycles as model intermediates. Metallacyclobutadiene (MCBD) and deprotiometallacyclobutadiene (dMCBD) complexes (dBDI)V[C(tBu)C(H)C(tBu)] (4-tBu) and (BDI)V[C(tBu)CC(Mes)] (5-Mes), respectively, were synthesized upon reaction with bulkier alkynes, tBu- (tBuCCH) and Mes-acetylene (MesCCH), with 1-OEt2. Furthermore, the reaction of the conjugate acid of 1-OEt2, [(BDI)V≡CtBu(OTf)] (2-OTf), with the conjugated base of phenylacetylene, lithium phenylacetylide (LiCCPh), yields the doubly deprotio-metallacycle complex, [Li(THF)4]{(BDI)V[C(Ph)CC(tBu)CC(Ph)]} (6). Protonation of the doubly deprotio-metallacycle complex 6 yields 6-H+, a catalytically active species toward the polymerization of PhCCH, for which the polymers were also confirmed to be cyclic by SEC studies. Computational mechanistic studies complement the experimental observations and provide insight into the mechanism of cyclic polymer growth. The noninnocence of the supporting dBDI2- ligand and its role in proton shuttling to generate deprotiometallacyclobutadiene (dMCBD) complexes that proposedly culminate in the formation of catalytically active V(III) species are also discussed. This work demonstrates how a dMCBD moiety can react with terminal alkynes to form cyclic polyalkynes.

A mechanistic study reveals the roles of the ligand and substrate in the Pd-catalyzed C–H bond hydroxylation of a carboxylic acid with molecular oxygen

DFT studies reveal critical factors influencing the reactivity of Pd-catalyzed C–H bond hydroxylation with O2: (1) substrate axial coordination; (2) proton shuttling by the ligand; and (3) dimeric palladium species formation.

Quantum tunnelling effect in the cis-trans isomerization of uranyl tetrahydroxide.

The role of quantum tunnelling (QT) in the proton transfer kinetics of the uranyl tetrahydroxide (UTH, [UO2(OH)4]2-) cis to trans isomerization was computationally studied under three possible reaction pathways. The first pathway involved a direct proton transfer from the hydroxide ligand to the oxo atom. In the other two pathways, one or two water molecules were added to the second sphere. The first H2O, bound by hydrogen bonds to the ligands, acts as a bridge enabling a proton shuttling, a concerted hopping of a proton from the hydroxide to the oxo atom similar to the Grotthuss mechanism. In the third pathway, the second water molecule does not participate in the H-transfer chain, but works as an anchor for the first water molecule, limiting its movement and therefore enhancing the QT. Since experimentally the reaction occurs in water, the first two pathways (no water or one H2O) serve only as models of the gas phase behaviour, while the third pathway will always be thermodynamically and kinetically preferred. The effects were investigated in the gas phase as well as in a continuum aqueous model, including the H/D Kinetic Isotope Effect (KIE). The results indicate that at very low temperatures, QT is the only mechanism that permits the reaction kinetics, consistent with the large computed KIE. At higher temperatures, thermally activated tunnelling competes with the classical crossing over the potential barrier.

Structure-Function Analysis of the Biotechnologically Important Cytochrome P450 107 (CYP107) Enzyme Family.

Cytochrome P450 monooxygenases (CYPs; P450s) are a superfamily of heme-containing enzymes that are recognized for their vast substrate range and oxidative multifunctionality. CYP107 family members perform hydroxylation and epoxidation processes, producing a variety of biotechnologically useful secondary metabolites. Despite their biotechnological importance, a thorough examination of CYP107 protein structures regarding active site cavity dynamics and key amino acids interacting with bound ligands has yet to be undertaken. To address this research knowledge gap, 44 CYP107 crystal structures were investigated in this study. We demonstrate that the CYP107 active site cavity is very flexible, with ligand binding reducing the volume of the active site in some situations and increasing volume size in other instances. Polar interactions between the substrate and active site residues result in crucial salt bridges and the formation of proton shuttling pathways. Hydrophobic interactions, however, anchor the substrate within the active site. The amino acid residues within the binding pocket influence substrate orientation and anchoring, determining the position of the hydroxylation site and hence direct CYP107's catalytic activity. Additionally, the amino acid dynamics within and around the binding pocket determine CYP107's multifunctionality. This study serves as a reference for understanding the structure-function analysis of CYP107 family members precisely and the structure-function analysis of P450 enzymes in general. Finally, this work will aid in the genetic engineering of CYP107 enzymes to produce novel molecules of biotechnological interest.

The synthesis and characterization of a bidentate complex of 1,1′-(1,3-phenylene)bis(3-butylimidazolium) pincer proligand with molybdenum – A non-pincer binding mode

The synthesis of the bidentate Mo complex tetrachlorido[6-(3′-butylimidazolium-1′-yl)-2-(3″-butylimidazol-1″-yl-2″-idene-κC2)phenyl-κC1]molybdenum(IV) 3 was carried out using the metalation followed by transmetalation methodology. The transmetalation process led to a bidentate complex after the reaction of DCM with HNMe2 formed an acidic ammonium ion that protonated the bidentate complex. Optimization of the synthetic methodology provided the tetrachlorido[6-(3′-butylimidazolium-1′-yl)-2-(3″-butylimidazol-1″-yl-2″-idene-κC2)phenyl-κC1]molybdenum(IV) complex in high yield. The crystal structure of the bidentate complex, 3, is reported herein. Attempts to avoid the acidic reaction conditions with different solvents or starting materials produced the bis-ligated Mo complex bis[2,6-bis(3′-butylimidazol-1′-yl-2′-idene-κC2)phenyl-κC1]molybdenum(IV) dichloride based on MS analysis. Electronic and coordinative unsaturation in the resulting bidentate complex, 3, open new possibilities for coordination of incoming substrates while also allowing access to the pincer-like motif via oxidative addition. Access to the interconversion of a NHC to/from imidazolium opens new avenues of non-innocent ligand pathways for proton shuttling in reactions such as nitrogen reduction to ammonia.

Organic-inorganic photocatalyst with S-scheme heterojunction based proton shuttle strategy for efficient dye degradation

The organic–inorganic S-scheme heterojunction with proton shuttling ability has been proposed to enhance the treatment of organic dyes. A PS-PANI@ZnO photocatalyst combining cellulose, phytic acid and polyaniline was prepared to address the issues of difficult excitation and fast recombination of photogenerated electrons and carriers in common inorganic semiconductor photocatalysts. PS-PANI(100)@ZnO could achieve a high degradation rate (> 99%) of MO and RhB in 160 and 140 min, respectively. Moreover, the catalytic activity remained high under alkaline conditions, and the degradation rates could be maintained above 98% after five cycles. The specific surface area of ZnO was increased from 107.79 m2/g to 112.54 m2/g after modification, in which phytic acid could provide more active sites for the reduction of O2 to •O2−. In addition, the PS-PANI can effectively reduce the band gap of ZnO. PS-PANI(50,100,150,200)@ZnO were 3.148, 3.133, 3.068, and 3.100 eV, respectively. The economic cost analysis of the prepared catalysts showed that the cost was approximately 37% higher while the degradation effect was enhanced by 4 times. Meanwhile, environmental toxicity tests showed that the treated wastewater had low toxicity for mung bean seed growth.

Proton Shuttling by Polyaniline of High Brønsted Basicity for Improved Electrocatalytic Ethylene Production from CO2.

Hybrid organic/inorganic composites with the organic phase tailored to modulate local chemical environment at the Cu surface arise as an enchanting category of catalysts for electrocatalytic CO2 reduction reaction (CO2 RR). A fundamental understanding on how the organics of different functionality, polarity, and hydrophobicity affect the reaction path is, however, still lacking to guide rational catalyst design. Herein, polypyrrole (PPy) and polyaniline (PANI) manifesting different Brønsted basicity are compared for their regulatory roles on the CO2 RR pathways regarding *CO coverage, proton source and interfacial polarity. Concerted efforts from in situ IR, Raman and operando modelling unveil that at the PPy/Cu interface with limited *CO coverage, hydridic *H produced by the Volmer step favors the carbon hydrogenation of *CO to form *CHO through a Tafel process; Whereas at the PANI/Cu interface with concentrated CO2 and high *CO coverage, protonic H+ shuttled through the benzenoid -NH- protonates the oxygen of *CO, yielding *COH for asymmetric coupling with nearby *CO to form *OCCOH under favored energetics. As a result of the tailored chemical environment, the restructured PANI/Cu composite demonstrates a high partial current density of 0.41 A cm-2 at a maximal Faraday efficiency of 67.5 % for ethylene production, ranking among states of the art.

Proton Shuttling by Polyaniline of High Brønsted Basicity for Improved Electrocatalytic Ethylene Production from CO2

AbstractHybrid organic/inorganic composites with the organic phase tailored to modulate local chemical environment at the Cu surface arise as an enchanting category of catalysts for electrocatalytic CO2 reduction reaction (CO2RR). A fundamental understanding on how the organics of different functionality, polarity, and hydrophobicity affect the reaction path is, however, still lacking to guide rational catalyst design. Herein, polypyrrole (PPy) and polyaniline (PANI) manifesting different Brønsted basicity are compared for their regulatory roles on the CO2RR pathways regarding *CO coverage, proton source and interfacial polarity. Concerted efforts from in situ IR, Raman and operando modelling unveil that at the PPy/Cu interface with limited *CO coverage, hydridic *H produced by the Volmer step favors the carbon hydrogenation of *CO to form *CHO through a Tafel process; Whereas at the PANI/Cu interface with concentrated CO2 and high *CO coverage, protonic H+ shuttled through the benzenoid ‐NH‐ protonates the oxygen of *CO, yielding *COH for asymmetric coupling with nearby *CO to form *OCCOH under favored energetics. As a result of the tailored chemical environment, the restructured PANI/Cu composite demonstrates a high partial current density of 0.41 A cm−2 at a maximal Faraday efficiency of 67.5 % for ethylene production, ranking among states of the art.

A comprehensive review on electrochemical green ammonia synthesis: From conventional to distinctive strategies for efficient nitrogen fixation

Ammonia (NH3) is an excellent transition fuel of green hydrogen and a future contender in the energy market. However, industrial NH3 production currently depends on the unsustainable and energy-intensive Haber–Bosch process. Hence, developing a viable and efficient method to produce NH3 economically has become a new challenge for researchers as its demand surges with each passing decade. Ammonia production from renewable energy sources can power the globe without emitting carbon. The electrochemical method of NH3 has emerged as an appealing strategy for sustainable NH3 production under mild conditions. However, this approach presents several challenges related to activating the highly stable N≡N bond, choice of electrolytes, proton source, etc. This study explores electrochemical methods with distinctive approaches that have been reported, which include redox-mediated processes, lithium cycling electrification, integrated plasma technology, phosphonium proton shuttling, and more. A comprehensive analysis of the underlying principles, experimental parameters, challenges, and potential applications are discussed for each method. It also assesses the recent electrocatalyst advancements employed for effective N2 fixation and their corresponding electrocatalytic performance. Finally, it emphasises the ongoing research efforts to overcome barriers and enable the widespread adoption of renewable grid energy systems for powering N2 gas electrolysis in green NH3 production.

Energetics and dynamics of the proton shuttle of carbonic anhydrase II

Human carbonic anhydrase II catalyzes the reversible reaction of carbon dioxide and water to form bicarbonate and a proton. His64-mediated proton shuttling between the active site and the bulk solvent is rate limiting. Here we investigate the protonation behavior of His64 as well as its structural and dynamic features in a pH dependent way. We derive two pKa values for His64, 6.25 and 7.60, that we were able to assign to its inward and outward conformation. Furthermore, we show that His64 exists in both conformations equally, independent of pH. Both conformations display an equal distribution of their two neutral tautomeric states. The life time of each conformation is short and both states display high flexibility within their orientation. Therefore, His64 is never static, but rather poised to change conformation. These findings support an energetic, dynamic and solution ensemble-based framework for the high enzymatic activity of human carbonic anhydrase II.

Copper Complex Catalyzed Two-Electron and Proton Shuttle Mechanism of O-O Bond Formation from DFT-Based Metadynamics Simulations.

We performed first-principles metadynamics simulations to explore the mechanistic pathway of oxygen-oxygen bond formation catalyzed by cis-bis(hydroxo) and cis-(hydroxo)oxo copper complexes. The ligands of considered complexes involve modified bipyridine ligands with oxo and hydroxo groups on 6, 6' positions. The study focuses on the kinetics and thermodynamics of the oxygen-oxygen bond formation. The individual migration of the proton to the hydroxyl group and hydroxide to the oxo and hydroxo moieties of the complexes was examined. The proton transfer requires more kinetic barrier than the hydroxide migration. The nature of the electronic density was analyzed with the help of spin population analysis. The molecular orbitals and natural orbital analysis were carried out to examine the nature of the orbitals involved in the oxygen-oxygen bond formation. The σ*(dx2-y2-px) molecular orbital of the Cu-O or Cu-OH bond overlaps with the pz orbital of the hydroxide ion in forming the oxygen-oxygen bond. The two-electron two-centered (2e--2C) bond is observed in the oxygen-oxygen bond formation. In the oxidation process, these ligands stabilize the electron density from the water or hydroxide ion. These redox-active ligands also help stabilize the formed hydrogen peroxide or peroxide complexes.

Hydroxypyridine/Pyridone Interconversions within Ruthenium Complexes and Their Application in the Catalytic Hydrogenation of CO2

Reaction of a new ligand 6-DiPPon (6-diisopropylphosphino-2-pyridone) with 0.5 equiv of [RuCl2(p-cymene)]2 resulted in the formation of a mixture of [RuCl2(p-cymene)(κ1-P-6-DiPPon)]2 (1) and [RuCl(p-cymene)(κ2-P,N-6-DiPPin)]Cl ([2]Cl) (where 6-DiPPin = 6-diisopropylphosphino-2-hydroxypyridine). The ratio between the two products can be controlled by the nature of the solvent. The similar reaction between 6-DiPPon and [RuCl2(p-cymene)]2 in the presence of AgOTf and Na[BArF24] (where BArF24 = [{3,5-(CF3)2C6H3}4B]-) resulted in the formation of the complexes [RuCl(p-cymene)(κ2-P,N-6-DiPPin)]OTf, ([2]OTf) and [RuCl(p-cymene)(κ2-P,N-6-DiPPin)]BArF24 ([2]BArF24), respectively. Reactions between complex [2]Cl, [2]OTf, or [2]BArF24 and a base (either DBU or NaOMe) resulted in the deprotonation of the hydroxyl functional group to form a novel neutral orange-colored dearomatized complex, 3. The identity of complex 3 was confirmed as [RuCl(p-cymene)(κ2-P,N-6-DiPPon*)], where 6-DiPPon* is the anionic species (6-diisopropylphosphino-2-oxo-pyridinide), which contains the deprotonated moiety. The new 6-DiPPon ligand and its corresponding air stable half-sandwich derivative ruthenium complexes 1, [2]OTf, [2]BArF24, and 3 were all isolated in good yields and fully characterized by spectroscopic and analytical methods. The interconversions between the neutral and anionic forms of the ligands 6-DiPPon, 6-DiPPin, and 6-DiPPon* offer the potential for novel secondary sphere interactions and proton shuttling reactivity. The consequences for this have been explored in the activation of H2 and the subsequent catalytic hydrogenations of CO2 into formate salts in the presence of a base.

Immobilization of a Molecular Copper Complex and a Carboxylate-Terminated Cocatalyst on a Metal Oxide Electrode for Enhanced Electrocatalytic Oxygen Reduction

Positioning basic/acidic groups near a molecular catalyst has become a popular strategy to modulate proton-coupled electron transfer reactions in energy conversion and storage. However, the detailed understanding of the real role played by basic/acidic groups remains lacking. Here, we report the immobilization of a copper(II) dipicolylamine catalyst bearing a phosphonic anchor (CuL) on a metal oxide electrode to investigate the activity of oxygen reduction reaction (ORR) in aqueous solution. The catalytic current of ORR by the CuL catalyst was significantly enhanced by three times as a carboxylate-terminated ligand was integrated onto the metal oxide surfaces to act as a cocatalyst. The CuL-coated metal oxide electrode with a carboxylate-terminated cocatalyst exhibits turnover frequency values higher than the one with an amine-terminated cocatalyst by 2.5-fold and the CuL-only case by 9.6-fold. Extensive experimental and theoretical studies reveal that a hydrogen bond formed between the proximal carboxylate of the cocatalyst and a Cu-OOH intermediate generated via protonation of a Cu-O2 adduct by the bulk buffer solution that aids O–O bond cleavage, thereby promoting the four-electron four-proton reduction of dioxygen into water reaching a Faradaic efficiency of 97.6%. Our finding is in stark contrast to the typical assignment on the role of basic groups near a catalyst that features proton shuttling to facilitate protonation of a metal-O2 intermediate forming a metal-OOH intermediate. This work offers a facile strategy to enhance simultaneously the activity and selectivity of surface-supported non-precious metal electrocatalysts that are important toward realizing alternative energy conversion schemes.