Deprotonation Step Research Articles

Achieving High OER Performance by Tuning the Co/Mn Content in Prussian Blue Analogues.

The need for efficient, economical, and clean energy systems is increasing, and as a result, interest in water-splitting techniques to produce green hydrogen is also increasing. However, the sluggish kinetics of the oxygen evolution reaction (OER) hinders the practical application and widespread use of water-splitting technologies; therefore, to address this challenge, it is essential to develop cost-effective and efficient OER catalysts. In this work, we have synthesized an inexpensive and tunable FeCoMn Prussian blue analogue (PBAs) as an efficient OER catalyst via a straightforward process. The ratio of the Co and Mn to optimize the electrochemical performance, and as a result, the FeCo0.41Mn0.42 PBA catalyst demonstrated the best electrochemical performance (260/304 mV overpotential at 10/50 mA cm-2, a low Tafel slope of 48 mV dec-1 and a good stability of 72 h at 10 mA cm-2). Additionally, X-ray absorption spectroscopy (XAS) measurements and density functional theory (DFT) calculations suggest that the FeCo0.41Mn0.42 PBA possesses the optimized electronic density distribution at the active site (Co), and the doping of Mn and Fe can not only increase the electricity conductivity but also activate the critical H2O deprotonation step.

Study on Titanate perovskites with different morphologies for oxygen-evolution reaction catalysts

Metal exsolution from perovskite oxides has been used to produce well-designed electro-catalysts where metal nanoparticles are homogeneously distributed on the surface of perovskites. Our previous works revealed that the metal exsolution concept is effective not only for high-temperature system such as solid oxide fuel/electrolysis cells but also for alkaline water electrolysis at near-ambient temperatures. In this study, we prepare cuboidal and spherical Ni-doped CaTiO3 to investigate the effects of the different perovskite morphologies on Ni exsolution and catalytic activity for OER. Ni ions seem to be incorporated into the A-site of CaTiO3 with structural OH ions via solvothermal route, and the stoichiometry of spheres and cubes were confirmed as (Ca0.82Ni0.04)Ti1O2.63(OH)0.46 and (Ca0.90Ni0.04)Ti1O2.88(OH)0.12, respectively. The spheres show random distribution of Ni nanoparticles along grain boundaries, whereas Ni nanoparticles are ex-solved along the edges in the cubes. With the small amount of Ni contents (∼4 mol %), geometric current density of each cube and sphere exhibits comparable OER activities of ∼10 mA cm−2 and ∼7 mA cm−2 at 1.7 V to previously reported catalysts such as transition metals or perovskites. We suppose from Tafel slope that different A-site and oxygen stoichiometry of the CaTiO3 may affect deprotonation step during OER in terms of proton conduction.

Mechanistic and thermodynamic studies on directing group-assisted copper-catalyzed α-C(sp2)−H amination of aryl amines with alkylamines

In mechanistic, three possible pathways of α-C(sp[2])−H functionalization amination of picolinamide with morpholine catalyzed by Cu(OAc)2/O2 in p-xylene were studied. Through theoretical calculations, it was found that the pathway involving the radical-cation intermediates produced by the subsequent single electron transfer (SET) was proved to be the most favorable. Then, we also discussed the solvation and substituent effects of three key steps, including N − H bond deprotonation, C − N bond formation rate-determining step and C − H bond deprotonation step, so as to fully understand the mechanism of directing group-assisted copper-catalyzed α-C − H functionalization of aniline and the screening of substrates and solvents. In thermodynamic, the SET process plays a crucial role in the entire catalytic cycle. Therefore, we further studied the reduction potential (Ered) values of Cu(II) radical-cation intermediates involved in the SET process, reflecting the occurrence trend of SET from the thermodynamic point of view, and studied the substituent effects and structure-activity relationships on Ered values of Cu(II) complexes derivatives formed by different Cu(Ⅱ) catalysts, different directing groups, different alkylamines and aromatic amines substrates. It is also found that there are many quantitative structure-activity relationships between Ered and the singly occupied molecular orbital (SOMO) energy, Hammett substituent constant σ+ and other structural parameters.

Designing neighboring-site activation of single atom via tunnel ions for boosting acidic oxygen evolution

Realizing an efficient turnover frequency in the acidic oxygen evolution reaction by modifying the reaction configuration is crucial in designing high-performance single-atom catalysts. Here, we report a “single atom–double site” concept, which involves an activatable inert manganese atom redox chemistry in a single-atom Ru-Mn dual-site platform with tunnel Ni ions as the trigger. In contrast to conventional single-atom catalysts, the proposed configuration allows direct intramolecular oxygen coupling driven by the Ni ions intercalation effect, bypassing the secondary deprotonation step instead of the kinetically sluggish adsorbate evolution mechanism. The strong bonding of Ni ions activates the inert manganese terminal groups and inhibits the cross-site disproportionation process inherent in the Mn scaffolding, which is crucial to ensure the dual-site platform. As a result, the single-atom Ru-Ni-Mn octahedral molecular sieves catalyst delivers a low overpotential, adequate mass activity and good stability.

Oxidation of Ammonia Catalyzed by a Molecular Iron Complex: Translating Chemical Catalysis to Mediated Electrocatalysis.

Ammonia is a promising candidate in the quest for sustainable, clean energy. With its capacity to serve as an energy carrier, the oxidation of ammonia opens avenues for carbon-neutral approaches to address worldwide growing energy needs. We report the catalytic chemical oxidation of ammonia by an Earth-abundant transition metal complex, trans-[LFeII(MeCN)2][PF6]2, where L is a macrocyclic ligand bearing four N-heterocyclic carbene (NHC) donors. Using triarylaminium radical cations in MeCN, up to 182 turnovers of N2 per Fe were obtained from chemical catalysis with an extremely low loading of the Fe catalyst (0.043 mM, 0.004 mol % catalyst). This chemical catalysis was successfully transitioned to mediated electrocatalysis for the oxidation of ammonia. Molecular electrocatalysis by the Fe catalyst and the mediator (p-MeOC6H4)3N exhibited a catalytic half-wave potential (Ecat/2) of 0.18 V vs [Cp2Fe]+/0 in MeCN, and achieved 9.3 turnovers of N2 at an applied potential of 0.20 V vs [Cp2Fe]+/0 at -20 °C in controlled-potential electrolysis, with a Faradaic efficiency of 75 %. Based on computational results, the catalyst undergoes sequential oxidation and deprotonation steps to form [LFeIV(NH2)2]2+, and thereafter bimetallic coupling to form an N-N bond.

Oxidation of Ammonia Catalyzed by a Molecular Iron Complex: Translating Chemical Catalysis to Mediated Electrocatalysis

Ammonia is a promising candidate in the quest for sustainable, clean energy. With its capacity to serve as an energy carrier, the oxidation of ammonia opens avenues for carbon‐neutral approaches to address worldwide growing energy needs. We report the catalytic chemical oxidation of ammonia by an Earth‐abundant transition metal complex, trans‐[LFeII(MeCN)2][PF6]2, where L is a macrocyclic ligand bearing four N‐heterocyclic carbene (NHC) donors. Using triarylaminium radical cations in MeCN, up to 182 turnovers of N2 per Fe were obtained from chemical catalysis with an extremely low loading of the Fe catalyst (0.043 mM, 0.004 mol % catalyst). This chemical catalysis was successfully transitioned to mediated electrocatalysis for the oxidation of ammonia. Molecular electrocatalysis by the Fe catalyst and the mediator (p‐MeOC6H4)3N exhibited a catalytic half‐wave potential (Ecat/2) of 0.18 V vs [Cp2Fe]+/0 in MeCN, and achieved 9.3 turnovers of N2 at an applied potential of 0.20 V vs [Cp2Fe]+/0 at −20 °C in controlled‐potential electrolysis, with a Faradaic efficiency of 75%. Based on computational results, the catalyst undergoes sequential oxidation and deprotonation steps to form [LFeIV(NH2)2]2+, and thereafter bimetallic coupling to form an N−N bond.

A Comprehensive Computational Study on the Thermodynamics and Kinetics of Tetrahydrobiopterin Regeneration Process.

One of the most crucial enzymatic cofactors in the human body is tetrahydrobiopterin, which is acquired through biological synthesis and self-regeneration. During this regenerative process, it undergoes oxidation, deprotonation, further oxidation, and subsequent deprotonation, resulting in the formation of quinonoid-dihydrobiopterin, which then undergoes tautomerization to yield dihydrobiopterin. This study presents the thermodynamic and kinetic properties associated with each stage of the regeneration process using theoretical calculations. The redox potentials for oxidation steps and the pKa values for deprotonation steps are determined employing the Born-Haber cycle and the direct change of free energy in implicit solvent models. The redox metabolites are characterized and confirmed from their calculated absorption spectra using the time-dependent density functional theory method. For the tautomerization steps, an IRC calculation is executed, and rate constants are computed using Eyring's Transition State Theory (TST). The tunnelling probability of the H atom during the tautomerization process is incorporated using Wigner's tunnelling correction in the calculation of the rate constant. Notably, we identify the N3 atom as the most probable deprotonation site for H3B+ and predict its geometry based on our calculations. Furthermore, we elucidate the spectral properties of intermediates involved in the regeneration process, highlighting key electronic transitions responsible for their excitations. Our results indicate that each step of tautomerization occurs along vibrational bending modes. We have observed that these tautomerization processes have high activation energies by optimising transition states. Additionally, considering tunnelling correction can significantly affect the reaction rates associated with these processes. These results provide a comprehensive understanding of the thermodynamics and kinetics of the regeneration process of tetrahydrobiopterin, which will help in the modulation of its biological activity.

Catalyst‐free Chloroamination Cyclization Cascade with Sodium Hypochlorite: from N‐(Pentenyl)sulfonylamides to 2‐(1‐Chloromethyl)pyrrolidines

AbstractA mild and catalyst‐free protocol to obtain several 2‐chloromethylpyrrolidines by the use of a commercial solution of sodium hypochlorite is here reported, without the need for a light source. The choice of the solvent revealed to be crucial in the success of the reaction. Mechanistic studies, both experimental and computational, confirmed a radical mechanism, where the deprotonation step, followed by the oxidation of a N‐centered anion to the corresponding radical, allowed 2‐chloromethylpyrrolidines, by a very fast cyclization, to be obtained. Sodium hypochlorite plays the role of both the oxidant and the chlorine source.

Mechanism of Z-Selective Allylic Functionalization via Thianthrenium Salts.

A detailed mechanistic study of the Z-selective allylic functionalization via thianthrenium salts is presented. Kinetic analyses, deuterium labeling experiments, and computational methods are used to rationalize the observed reactivity and selectivity. We find that the reaction proceeds via a rate-determining and stereodetermining allylic deprotonation of an alkenylthianthrenium species. The Z-configuration of the resultant allylic ylide is translated into the Z-allylic amine product through a sequence of subsequent fast and irreversible steps: protonation to form a Z-allylic thianthrenium electrophile and then regioselective substitution by the nucleophile. In the stereodetermining deprotonation step, computational studies identified a series of stabilizing nonbonding interactions in the Z-alkene-forming transition state that contribute to the stereoselectivity.

Tailoring atomically local electric field of NiFe layered double hydroxides with Ag dopants to boost oxygen evolution kinetics

The demand for clean energy sources has driven focus towards advanced electrochemical systems. However, the sluggish kinetics of the oxygen evolution reaction (OER) constrain the energy conversion efficiency of relevant devices. Herein, a one-step method is reported to grow oxygen vacancies (Vo) rich NiFeAg layered double hydroxides nanoclusters on carbon cloth (Vo-NiFeAg-LDH/CC) for serving as the self-supporting electrode to catalyze OER. The OER performance of Vo-NiFeAg-LDH/CC has been remarkably enhanced through Ag and Vo co-modification compared with pristine NiFe-LDH, achieving a low Tafel slope of 49.7 mV dec−1 in 1 m KOH solution. Additionally, the current density of Vo-NiFeAg-LDH/CC is 3.23 times higher than that of the state-of-art IrO2 at 2 V under an alkaline flow electrolyzer setup. Theoretical calculations and experimental results collectively demonstrate that Ag dopant and Vo strengthen the O* adsorption with active sites, further promoting the deprotonation step from OH* to O* and accelerating the catalytic reaction. In a word, this work clarifies the structural correlation and synergistic mechanism of Ag dopant and Vo, providing valuable insights for the rational design of catalyst for renewable energy applications.

2-Pyridone-Directed CuII-Catalyzed General Method of C(sp2)-H Activation for C-S, C-Se, and C-N Cross-Coupling: Easy Access to Aryl Thioethers, Selenide Ethers, and Sulfonamides and DFT Study.

We have demonstrated N-substituted 2-pyridones as an N,O-directing group for selective C(sp2)-H-activated thiolation, selenylation, and sulfonamidation of ortho C-H bonds of benzamides. This method utilizes a cost-effective Cu(II)-salt catalyst instead of precious metal catalysts, achieving high yields, including gram-scale synthesis and excellent functional group tolerance. We applied this protocol to access 30 different compounds with high yields, demonstrating thiolation of fluorine-substituted benzamides as well. Density functional theory (DFT) calculations support the mechanism, including acetate-supported concerted metalation deprotonation (CMD) steps and the unique role of dimethyl sulfoxide (DMSO) solvent. The facile synthesis of pharmaceutically important sulfonamides and other compounds highlights the method's potential in chemistry and medicinal chemistry.

In Situ Probing the Structure Change and Interaction of Interfacial Water and Hydroxyl Intermediates on Ni(OH)2 Surface over Water Splitting.

There is growing acknowledgment that the properties of the electrochemical interfaces play an increasingly pivotal role in improving the performance of the hydrogen evolution reaction (HER). Here, we present, for the first time, direct dynamic spectral evidence illustrating the impact of the interaction between interfacial water molecules and adsorbed hydroxyl species (OHad) on the HER properties of Ni(OH)2 using Au/core-Ni(OH)2/shell nanoparticle-enhanced Raman spectroscopy. Notably, our findings highlight that the interaction between OHad and interfacial water molecules promotes the formation of weakly hydrogen-bonded water, fostering an environment conducive to improving the HER performance. Furthermore, the participation of OHad in the reaction is substantiated by the observed deprotonation step of Au@2 nm Ni(OH)2 during the HER process. This phenomenon is corroborated by the phase transition of Ni(OH)2 to NiO, as verified through Raman and X-ray photoelectron spectroscopy. The significant redshift in the OH-stretching frequency of water molecules during the phase transition confirms that surface OHad disrupts the hydrogen-bond network of interfacial water molecules. Through manipulation of the shell thickness of Au@Ni(OH)2, we additionally validate the interaction between OHad and interfacial water molecules. In summary, our insights emphasize the potential of electrochemical interfacial engineering as a potent approach to enhance electrocatalytic performance.

Why Does Monoamine Oxidase (MAO) Catalyze the Oxidation of Some Tetrahydropyridines?

Results pertaining to the mechanism of the oxidation of the tertiary amine 1-methyl-4-(1-methyl-1-H-pyrrol-2-yl)-1,2,3,6-tetrahydropyridine (MMTP, a close analog of the Parkinsonism inducing compound MPTP) by 3-methyllumiflavin (3MLF), a chemical model for the FAD cofactor of monoamine oxidase, are reported. MMTP and related compounds are among the few tertiary amines that are monoamine oxidase B (MAO-B) substrates. The MMTP/3MLF reaction is catalytic in the presence of O2 and the results under anaerobic conditions strongly suggest the involvement of radical intermediates, consistent with a single electron transfer mechanism. These observations support a new hypothesis to explain the MAO-catalyzed oxidations of amines. In general, electron transfer is thermodynamically unfavorable, and as a result, most 1° and 2° amines react via one of the currently accepted polar pathways. Steric constraints prevent 3° amines from reacting via a polar pathway. Those select 3° amines that are MAO substrates possess certain structural features (e. g., a C-H bond that is α- both to nitrogen and a C=C) that dramatically lower the pKa of the corresponding radical cation. Consequently, the thermodynamically unfavorable electron transfer equilibrium is driven towards products by an extremely favorable deprotonation step in the context of Le Chatelier's principle.

Direct C-H Hydroxylation of N-Heteroarenes and Benzenes via Base-Catalyzed Halogen Transfer.

Hydroxylated (hetero)arenes are valued in many industries as both key constituents of end products and diversifiable synthetic building blocks. Accordingly, the development of reactions that complement and address the limitations of existing methods for the introduction of aromatic hydroxyl groups is an important goal. To this end, we apply base-catalyzed halogen transfer (X-transfer) to enable the direct C-H hydroxylation of mildly acidic N-heteroarenes and benzenes. This protocol employs an alkoxide base to catalyze X-transfer from sacrificial 2-halothiophene oxidants to aryl substrates, forming SNAr-active intermediates that undergo nucleophilic hydroxylation. Key to this process is the use of 2-phenylethanol as an inexpensive hydroxide surrogate that, after aromatic substitution and rapid elimination, provides the hydroxylated arene and styrene byproduct. Use of simple 2-halothiophenes allows for C-H hydroxylation of 6-membered N-heteroarenes and 1,3-azole derivatives, while a rationally designed 2-halobenzothiophene oxidant extends the scope to electron-deficient benzene substrates. Mechanistic studies indicate that aromatic X-transfer is reversible, suggesting that the deprotonation, halogenation, and substitution steps operate in synergy, manifesting in unique selectivity trends that are not necessarily dependent on the most acidic aryl position. The utility of this method is further demonstrated through streamlined target molecule syntheses, examples of regioselectivity that contrast alternative C-H hydroxylation methods, and the scalable recycling of the thiophene oxidants.

Kinetic study on CO2 absorption by aqueous secondary amine + tertiary amine blends: Kinetic model and effect of the chain length of tertiary amine

To study the reaction kinetic of CO2 absorption in aqueous secondary amine + tertiary amine blends, 1-Diethylamino-2-propanol (DEA2P) and 1-Dimethylamino-2-propanol (DMA2P) were selected to form aqueous amine blended solutions with 2-(Ethylamino) ethanol (EAE), respectively. The reaction kinetics of CO2 absorption in aqueous EAE/DEA2P solutions and EAE/DMA2P solutions were explored using the stopped-flow apparatus, and the reaction mechanism was then explored by Density Functional Theory (DFT) calculations. Based on the zwitterion mechanism and the base-catalyzed hydration mechanism, experimental reaction kinetics models were established. It was found that in secondary amine + tertiary amine blends, tertiary amines played the key role of the proton acceptor in step of zwitterion deprotonation. With the increase of the chain length of tertiary amine, the CO2 absorption rate increased. Through analysis, it was found that the main reason for the increased CO2 absorption rate was the presence of H-bond or the stronger proton affinity of N atom.

The Resorcinarene Hexameric Capsule as a Supramolecular Photoacid to Trigger Olefin Hydroarylation in Confined Space.

The self-assembled resorcinarene capsule C6 shows remarkable photoacidity upon light irradiation, which is here exploited to catalyze olefin hydroarylation reactions in confined space. An experimental pKa* value range of -3.3--2.8 was estimated for the photo-excited hexameric capsule C6 *, and consequently an increase in acidity of 8.8 log units was observed with respect to its ground state (pKa=5.5-6.0). This makes the hexameric capsule the first example of a self-assembled supramolecular photoacid. The photoacid C6 * can catalyze hydroarylation reaction of olefins with aromatic substrates inside its cavity, while no reaction occurred between them in the absence of irradiation and/or capsule. DFT calculations corroborated a mechanism in which the photoacidity of C6 * plays a crucial role in the protonation step of the aromatic substrate. A further proton transfer to olefin with a concomitant C-C bond formation and a final deprotonation step lead to product releasing.

Functional and Mechanistic Characterization of the 4,5-diepi-Isoishwarane Synthase from the Liverwort Radula lindenbergiana.

The microbial type sesquiterpene synthase RlMTPSL4 from the liverwort Radula lindenbergiana was investigated for its products, showing the formation of several sesquiterpene hydrocarbons. The main product was structurally characterized as the new compound 4,5-diepi-isoishwarane, while the side products included the known hydrocarbons germacrene A, α-selinene, eremophilene and 4,5-diepi-aristolochene. The cyclization mechanism towards 4,5-diepi-isoishwarane catalyzed by RlMTPSL4 was investigated through isotopic labeling experiments, revealing the stereochemical course for the deprotonation step to the neutral intermediate germacrene A, a reprotonation for its further cyclization, and a 1,2-hydride shift along the cascade. The absolute configuration of 4,5-diepi-isoishwarane was determined using a stereoselective deuteration approach, revealing an absolute configuration typically observed for a microbial type sesquiterpene.

Carboxylate-Catalyzed C-Silylation of Terminal Alkynes.

A carboxylate-catalyzed, metal-free C-silylation protocol for terminal alkynes is reported using a quaternary ammonium pivalate as the catalyst and commercially available N,O-bis(silyl)acetamides as silylating agents. The reaction proceeds under mild conditions, tolerates a range of functionalities, and enables concomitant O- or N-silylation of acidic OH or NH groups. A Hammett ρ value of +1.4 ± 0.1 obtained for para-substituted 2-arylalkynes is consistent with the proposed catalytic cycle involving a turnover-determining deprotonation step.

QM/MM Study of the Catalytic Mechanism and Substrate Specificity of the Aromatic Substrate C-Methyltransferase Fur6.

In the field of medical chemistry and other organic chemistry, introducing a methyl group into a designed position has been difficult to achieve. However, owing to the vigorous developments in the field of enzymology, methyltransferases are considered potential tools for addressing this problem. Within the methyltransferase family, Fur6 catalyzes the methylation of C3 of 1,2,4,5,7-pentahydroxynaphthalene (PHN) using S-adenosyl-l-methionine (SAM) as the methyl donor. Here, we report the catalytic mechanism and substrate specificity of Fur6 based on computational studies. Our molecular dynamics (MD) simulation studies reveal the reactive form of PHN and its interactions with the enzyme. Our hybrid quantum mechanics/molecular mechanics (QM/MM) calculations suggest the reaction pathway of the methyl transfer step in which the energy barrier is 8.6 kcal mol-1. Our free-energy calculations with a polarizable continuum model (PCM) indicate that the final deprotonation step of the methylated intermediate occurs after it is ejected into the water solvent from the active center pocket of Fur6. Additionally, our studies on the protonation states, the highest occupied molecular orbital (HOMOs), and the energy barriers of the methylation reaction for the analogs of PHN demonstrate the mechanism of the specificity to PHN. Our study provides valuable insights into Fur6 chemistry, contributing to a deeper understanding of molecular mechanisms and offering an opportunity to engineer the enzyme to achieve high yields of the desired product(s).

Comparative Solution Equilibrium Studies on Anticancer Estradiol-Based Conjugates and Their Copper Complexes

Steroids are often considered valuable molecular tools for the development of anticancer agents with improved pharmacological properties. Conjugation of metal chelating moieties with a lipophilic sterane backbone is a viable option to obtain novel anticancer compounds. In this work, two estradiol-based hybrid molecules (PMA-E2 and DMA-E2) with an (N,N,O) binding motif and their Cu(II) complexes were developed. The lipophilicity, solubility, and acid-base properties of the novel ligands were determined by the combined use of UV-visible spectrophotometry, pH-potentiometry, and 1H NMR spectroscopy. The solution speciation and redox activity of the Cu(II) complexes were also investigated by means of UV-visible and electron paramagnetic resonance spectroscopy. Two structurally analogous ligands (PMAP and DMAP) were also included in the studies for better interpretation of the solution chemical data obtained. Three pKa values were determined for all ligands, revealing the order of the deprotonation steps: pyridinium-NH+ or NH(CH3)2+, secondary NH2+, and OH. The dimethylamine derivatives (DMA-E2, DMAP) are found in their H2L+ forms in solution at pH 7.4, whereas the fraction of the neutral HL species is significant (34–37%) in the case of the pyridine nitrogen-containing derivatives (PMA-E2, PMAP). Both estradiol derivatives were moderately cytotoxic in human breast (MCF-7) and colon adenocarcinoma (Colo-205) cells (IC50 = 30–63 μM). They form highly stable complexes with Cu(II) ions capable of oxidizing ascorbate and glutathione. These Cu(II) complexes are somewhat more cytotoxic (IC50 = 15–45 μM) than their corresponding ligands and show a better selectivity profile.