Proton Transfer Agent Research Articles

Redox-Active Carboranyl Diphosphine as an Electron and Proton Transfer Agent.

In this work, we report the first example of the PCET reactivity for a boron cluster compound, the zwitterionic nido-carboranyl diphosphonium derivative 7-P(H)tBu2-10-P(H)iPr2-nido-C2B10H10. This main-group reagent efficiently transfers two electrons and two protons to quinones to yield hydroquinones and regenerate a neutral closo-carboranyl diphosphine, 1-PtBu2-2-PiPr2-closo-C2B10H10. As we have previously reported the conversion of this closo-carboranyl diphosphine into the zwitterionic nido- derivative upon reaction with main group hydrides, the transformation reported herein represents a complete synthetic cycle for the metal-free reduction of quinones, with the redox-active carboranyl diphosphine scaffold acting as a mediator. The proposed mechanism of this reduction, based on pKa determination, electrochemical studies, and kinetic isotope effect determination, involves the electron transfer from the nido- cluster to the quinone coupled with the delivery of protons.

Reversible Capture Mechanism of CO2 as a Zn(II)-Methylcarbonate.

In this study, we elucidate the reaction mechanism for capturing CO2 with the ZnL1(MeOH) complex (L1=diacetyl-2-(4-methyl-3-thiosemicarbazone)-3-(2-hydrazinatopyridine)) in a methanol solution, using density functional theory calculations. One pathway involves the protonation of ZnL1(MeOH) by methylcarbonic acid, followed by ligand exchange of MeOH with MeOCO2 -. An alternative mechanism suggests a tautomerization between ZnL1(MeOH) and Zn(HL1)(OMe), followed by CO2 insertion. The latter pathway is energetically more favorable than the former and more complex than initially proposed. In fact, we unveiled that the solvent catalyzes tautomerization, as one explicit methanol molecule acts as a proton transfer agent. Then, Zn(HL1)(OMe) captures CO2, yielding a methylcarbonate bound to the metal center. The final step involves a rearrangement that leads to the cleavage of the Zn-O(Me)(COO) bond and the formation of a new Zn-O(COOMe) bond, along with the rotation of the methylcarbonate group. We consider an additional mechanism that combines tautomerization and ligand exchange but is endergonic and requires a high activation barrier for the ligand exchange. Furthermore, we evaluate the ligand basicity through the pKa calculated values of the Zn(II) complexes, the effects of varying the ligand from 4-methyl-thiosemicarbazone to 4-ethyl (L2), 4-phenethyl (L3), and 4-benzyl (L4) derivatives, and reversibility of the reaction in an argon environment.

Role of H2O in Catalytic Conversion of C1 Molecules.

Due to their role in controlling global climate change, the selective conversion of C1 molecules such as CH4, CO, and CO2 has attracted widespread attention. Typically, H2O competes with the reactant molecules to adsorb on the active sites and therefore inhibits the reaction or causes catalyst deactivation. However, H2O can also participate in the catalytic conversion of C1 molecules as a reactant or a promoter. Herein, we provide a perspective on recent progress in the mechanistic studies of H2O-mediated conversion of C1 molecules. We aim to provide an in-depth and systematic understanding of H2O as a promoter, a proton-transfer agent, an oxidant, a direct source of hydrogen or oxygen, and its influence on the catalytic activity, selectivity, and stability. We also summarize strategies for modifying catalysts or catalytic microenvironments by chemical or physical means to optimize the positive effects and minimize the negative effects of H2O on the reactions of C1 molecules. Finally, we discuss challenges and opportunities in catalyst design, characterization techniques, and theoretical modeling of the H2O-mediated catalytic conversion of C1 molecules.

DFT study on mechanism of acetylene hydroamination catalyzed by metal chloride

Hydroamination of alkynes and ammonia is a promising route to synthesize enamines. In order to explore the mechanism of the hydroamination between the carbon-carbon unsaturated bond and ammonia, the pathway for hydroamination of acetylene with ammonia in gas phase and solvent media are investigated using DFT method. Without catalysts, the reaction is prohibited due to the extremely high energy barrier of ammonia nucleophilic addition. With metal chloride catalysts (i.e., ZnCl2, AlCl3, CuCl2), the whole catalytic cycle is facile to proceed via formation of the more stable co-adorption complex C2H2-ZnCl2-NH3, C2H2-AlCl3-NH3 or C2H2-CuCl2-NH3, ammonia nucleophilic addition, the first proton transfer, the particular metal chloride migration and the second proton transfer step. The excessive ammonia molecules not only act as the nucleophile, but also an efficient proton transfer agent, substantially facilitating two proton transfer steps. In gas phase, ZnCl2 and AlCl3 finally leads to imine product, whereas CuCl2 produces enamine product due to high energy barrier of the second proton transfer step. In acetone and toluene solvents, the second proton transfer is drastically enhanced and the final product for all catalysts is imine. Thus, the metal chloride catalytic hydroamination of acetylene with ammonia is kinetically favorable.

In Situ Spectroelectrochemical Detection of Oxygen Evolution Reaction Intermediates with a Carboxylated Graphene-MnO2 Electrocatalyst.

In electrocatalyst-assisted water splitting, the oxygen evolution reaction (OER) imposes a performance limit due to the formation of different catalyst-bound intermediates and the scaling relationship of their adsorption energies. To break this scaling relationship in OER, a bifunctional mechanism was proposed recently, in which the energetically demanding step of forming the *OOH intermediate, through the attack of a water molecule on the oxo unit (*O, with * representing a reactive metal center), is facilitated by proton transfer to the second catalytic site. This mechanism was supported theoretically but so far by only very few experiments with a proton-transfer agent in basic media. However, active metal-containing catalysts could be destroyed in alkaline media, raising questions on practical applications. To date, this mechanism still lacks a systematic spectroscopic support by observing the short-lived and limited amount of reactive intermediates. Here, we report an operando Raman spectroscopic observation of the OER intermediates in neutral media, for the first time, via a bifunctional mechanism using a carboxylated graphene-MnO2 (represented by Gr-C-MnO2) electrocatalyst. The formation of the Mn-OOH intermediate after the attack of a water molecule on the Mn═O complex is followed by a proton transfer from Mn-OOH to the functionalized carboxylates. The role of the functionalized carboxylates to improve the catalytic efficiency was further confirmed by both pH-dependent and isotope (H/D)-labeling experiments. Furthermore, with a unique strategy of using a hybrid aqueous/nonaqueous electrolyte, the OER was alleviated, allowing sufficient Mn-OH and Mn-OOH intermediates for in situ Raman spectroscopic observation.

The many roles of solvent in homogeneous catalysis - The reductive amination showcase

In the global effort to fight climate change, the design and development of novel procedures for complex chemical multi-step reactions is essential for the transformation of chemical industry. The transformation of chemical production processes from petrochemicals towards renewable, sustainable feedstock and the identification of green solvent candidates require a careful assessment of the effects of solvent on thermodynamics and kinetics. As a prime example for the many roles of solvent in chemical catalysis, the reaction mechanism of the homogeneous rhodium-catalyzed reductive amination of 1-undecanal from plant oils with diethylamine forming the long-chain tertiary N,N-diethylundecylamine is investigated. The many roles of solvent during the course of a chemical reactions become apparent when direct substrate-solvent and catalyst-solvent interactions are considered and their polarization effects. Hydrogen bond forming solvent molecules promote the enamine intermediate formation in terms of thermodynamics and kinetics by actively participating as proton transfer agents but a de-solvation penalty for polar groups compromises the overall pathway. The sophisticated bidentate phosphine (SulfoXantPhos)RhH reducing catalyst controls the regioselectivity of the reaction by dedicated ligand-substrate interactions. Its activity is critically dependent on the strength of solvent coordination. The effect of solvent on the reaction rate becomes apparent from a solvent screening of the transition state of the rate-determining step and give a perspective on solvent control of rate constants in this complex multi-step reaction. Only in presence of an appropriate solvent, the calculated Gibbs free potential energy surface becomes shallow and flat and delivers thermodynamic and kinetic parameters in good agreement with experiment.

Relayed hyperpolarization from para-hydrogen improves the NMR detectability of alcohols.

The detection of alcohols by magnetic resonance techniques is important for their characterization and the monitoring of chemical change. Hyperpolarization processes can make previously inpractical measurements, such as the determination of low concentration intermediates, possible. Here, we investigate the SABRE-Relay method in order to define its key characteristics and improve the resulting 1H NMR signal gains which subsequently approach 103 per proton. We identify optimal amine proton transfer agents for SABRE-Relay and show how catalyst structure influences the outcome. The breadth of the method is revealed by expansion to more complex alcohols and the polarization of heteronuclei.

Cooperative Multifunctional Organocatalysts for Ambient Conversion of Carbon Dioxide into Cyclic Carbonates

A series of pincer-type compounds possessing an N-heterocyclic carbene precursor and a carboxyl group as proton transfer agent were synthesized and used as organocatalysts for the cycloaddition of epoxides with CO2. In this context, we have demonstrated the high activity of these one-component organocatalysts in the CO2 transformation to cyclic carbonates under ambient conditions (room temperature, 1 bar of CO2). The catalytic potential of these multifunctional organocatalysts on challenging internal epoxides is particularly deserving of mention because organocatalysts that are able to mediate the cycloaddition reaction of internal epoxides with CO2 under mild conditions remain scarce. The intramolecular synergistic activation mechanism was elucidated by control experiments and DFT calculations.

Conformational control of cofactors in nature: The effect of methoxy group orientation on the electronic structure of ubisemiquinone

Ubiquinone is the key electron and proton transfer agent in biology. Its mechanism involves the formation of its intermediate one-electron reduced form, the ubisemiquinone radical. This is formed in a protein-bound form which permits the semiquinone to vary its electronic and redox properties. This can be achieved by hydrogen bonding acceptance by one or both oxygen atoms or as we now propose by restricted orientations for the methoxy groups of the headgroup. We show how the orientation of the two methoxy groups of the quinone headgroup affects the electronic structure of the semiquinone form and demonstrate a large dependence of the ubisemiquinone spin density distribution on the orientation each methoxy group takes with respect to the headgroup ring plane. This is shown to significantly modify associated hyperfine couplings which in turn needs to be accounted for in interpreting experimental values in vivo. The study uncovers the key potential role the methoxy group orientation can play in controlling the electronic structure and spin density of ubisemiquinone and provides an electronic-level insight into the variation in electron affinity and redox potential of ubiquinone as a function of the methoxy orientation. Taken together with the already known influence of cofactor conformation on heme and chlorophyll electronic structure, it reveals a more widespread role for cofactor conformational control of electronic structure and associated electron transfer in biology.

Small Molecule Activation by Constrained Phosphorus Compounds: Insights from Theory.

An exciting new development in main group chemistry has been the use of a constrained, "flat", phosphorus-based complex to mediate in reactions such as the dehydrogenation of ammonia borane (AB), and the activation of the N-H bond in primary amines. Its importance is based on the fact that it shows that main group compounds, when properly designed, can be as effective as transition metal complexes for doing significant chemical transformations. What the current computational study, employing density functional theory (DFT), reveals is that a common, general mechanism exists that accounts for the behavior of the flat phosphorus compound in the different reactions that have been experimentally reported to date. This mechanism, which involves the mediation by a base as a proton transfer agent, is simpler and energetically more favorable than the previous mechanisms that have been proposed for the same reactions in the literature. It is likely that the knowledge gained from the current work about the chemical behavior of this phosphorus compound can be utilized to design new constrained phosphorus-based compounds.

Investigation of nanocomposite membranes based on crosslinked poly(vinyl alcohol)–sulfosuccinic acid ester and hexagonal boron nitride

Hexagonal boron nitride (hBN) has received a great deal of attention from the scientific community owing to its distinctive mechanical properties, chemical stability, electrical features, and thermal stability. This work reports the preparation and characterization of novel anhydrous proton-conducting nanocomposite membranes based on poly(vinyl alcohol) (PVA), sulfosuccinic acid (SSA), and hBN. The proton conductivities of composite membranes were examined as a function of hBN composition and temperature. SSA was used as a sulfonating agent to obtain a crosslinked structure. hBN was incorporated into the PVA–SSA matrix as a proton transfer agent and nanofiller additive. The obtained membranes were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), water uptake, methanol permeability, and impedance spectroscopy. The semicrystalline nature of the composites was confirmed by X-ray investigation. The PVA–SSA–15hBN showed a maximum proton conductivity of 0.028 S cm−1 at 150 °C under anhydrous conditions.

MIMICKING THE SECONDARY SPHERE OF HOMOPROTOCATECHUATE DIOXYGENASE (HPCD) ENZYME ACTIVE SITE; A THEORETICAL STUDY

Fe-HPCD is a non-heme extradiol enzyme that operates in the oxidative ring opening pathways of aromatic compounds.[1,2] Recent studies showed that the proton transfer to afford 3 or 4 from 0 (Figure 1) is assisted by an His200 residue that is lying in the secondary sphere of HPCD active site.[3,4] Thus, the main role of His200 is acting as a Bronsted base. Herein, we present a Density Functional Theory analysis of the energetics of a modified first coordination shell with the objective of mimicking the role of His200 by incorporation of various functional groups on one of the imidazole rings (Figure 1). The aim is to create a proton transfer agent that is adequate in terms of thermodynamic and kinetic parameters in comparison with the native protein environment. Figure 1 shows the reaction mechanism and possible functional groups capable to act as a proton shuttle.

Probing the Mechanism of the Asymmetric Aminolysis of meso-Epoxides Catalyzed by a Proline-Based N,N′-Dioxide-Indium Tris(triflate) Complex

An exploration of the mechanism of the aminolysis of meso-epoxides catalyzed by the proline-based N,N′-dioxide-indium tris(triflate) complex was performed using control experiments, UV-Vis spectroscopy, 1H NMR, electrospray ionization mass spectrometry (ESI-MS) and scanning electron microscopy (SEM). Control experiments disclosed that the ligand-to-indium tris(triflate) ratio, the catalyst loading, the concentration, and the presence of 4 A MS have dramatic effects on this reaction with regard to both the yield and the enantioselectivity. Combined with control experiments, UV-Vis spectroscopy, 1H NMR, ESI-MS and SEM analyses revealed that molecular sieves perform multiple functions in the catalysis. A plausible molecular sieves-assisted reaction pathway was proposed. In this pathway, molecular sieves perform (i) as desiccant to in situ dry the reaction system, (ii) as proton transfer agent to accelerate the catalysis, and (iii) as counter ion source to preserve the electroneutrality of the transition states. Besides, the generality of the substrate scope was further explored; excellent yields (up to 99%) and enantioselectivities (up to 99% ee) were obtained.

The synthesis and characterization of anhydrous proton conducting membranes based on sulfonated poly(vinyl alcohol) and imidazole

Membranes with high anhydrous proton conducting properties have attracted remarkable interest as polymer electrolyte membrane fuel cell (PEMFC) at intermediate temperature (100–200 °C). In this study, a new anhydrous proton conducting membrane based on poly(vinyl alcohol) (PVA), sulfosuccinic acid (SSA), and imidazole (Im) at various stoichiometric ratios was prepared. Sulfosuccinic acid was used as sulfonating agent to form a crosslinked structure and as proton source. Im was intercalated into the crosslinked PVA–SSA matrix and used as a proton transfer agent in the absence of humidity. The proton conductivities of membranes were investigated as a function of blend composition, SSA composition, and operating temperature. For characterizations, the synthesized membranes were analyzed by means of FT-IR, TGA, DSC, mechanical analysis, methanol permeability and impedance measurements for proton conductivity. The thermal decomposition of the PVA–SSA–Im hybrid membranes occurred at about 200 °C. Differential scanning calorimetry (DSC) results illustrated the homogeneity of the blends. The conductivities of PVA–SSA–Im hybrid membranes increased with the increasing temperature, SSA content and Im content. In the absence of humidity, the proton conductivity of PVA–SSA–Im (S1) was 1.4 × 10 −3 at 140 °C and the conductivity of PVA–SSA–Im (M1) was 1.7 × 10 −4 at 140 °C. Mechanical analysis showed that the storage modulus increases with increasing the crosslink density.

An Alternative Mechanism for the 1,4-Asymmetric Induction in the Stereoselective Addition of (R)-Pantolactone to 2-Phenylpropylketene

An alternative mechanism for the 1,4-assymmetric induction in the stereoselective addition of (R)-pantolactone to 2-phenylpropylketene was theoretically investigated. A mechanism involving an intermolecular proton transfer assisted by two amine molecules was proposed, which considered the active participation of the dimethylethylamine and its ion as proton transfer agents. In the first step, a neutral dimethylethylamine interacts with the seven-membered ring of the enol intermediate. A specific acid-base interaction is established between the hydroxyl group of the enol and the nitrogen atom of the dimethylethylamine. The neutral dimethylethylamine is basic enough to remove the proton. Another protonated dimethylethylamine is considered to donate its proton to the C=C double bond to give the desired products. The stereochemical outcome was defined by the way that the neutral and protonated dimethylethylamine approached to the enol. The diastereoisomeric ratio found is in good agreement with experiments [for (S, R) and (R, R) it is 99:1].

Heterolytic cleavage of ammonia N–H bond by bifunctional activation in silica-grafted single site Ta(V) imido amido surface complex. Importance of the outer sphere NH3 assistance

Ammonia N–H bond is cleaved at room temperature by the silica–supported tantalum imido amido complex [(≡SiO)2Ta(NH)(–NH2)], 2, if excess ammonia is present, but requires 150 °C to achieve the same reaction if only one equivalent NH3 is added to 2. MAS solid-state 15N NMR and in situ IR spectroscopic studies of the reaction of either 15N or 2H labeled ammonia with 2 show that initial coordination of the ammonia is followed by scrambling of either 15N or 2H among ammonia, amido and imido groups. Density functional theory (DFT) calculations with a cluster model [{(μ-O)[(H3SiO)2SiO]2}Ta(NH)(–NH2)(NH3)], 2q·NH3, show that the intramolecular H transfer from Ta–NH2 to TaNH is ruled out, but the H transfers from the coordinated ammonia to the amido and imido groups have accessible energy barriers. The energy barrier for the ammonia N–H activation by the Ta-amido group is energetically preferred relative to the Ta-imido group. The importance of excess NH3 for getting full isotope scrambling is rationalized by an outer sphere assistance of ammonia acting as proton transfer agent, which equalizes the energy barriers for H transfer from coordinated ammonia to the amido and imido groups. In contrast, additional coordinated ammonia does not favor significantly the H transfer. These results rationalize the experimental conditions used.

Photopromoted Ru-Catalyzed Asymmetric Aerobic Sulfide Oxidation and Epoxidation Using Water as a Proton Transfer Mediator

Ru(NO)-salen complexes were found to catalyze asymmetric aerobic oxygen atom transfer reactions such as sulfide oxidation and epoxidation in the presence of water under visible light irradiation at room temperature. Oxidation of sulfides including alkyl aryl sulfides and 2-substituted 1,3-dithianes using complex 2 as the catalyst proceeded with moderate to high enantioselectivity of up to 98% ee, and epoxidation of conjugated olefins using complex 3 as the catalyst proceeded with good to high enantioselectivity of 76-92% ee. Unlike biological oxygen atom transfer reactions that need a proton and electron transfer system, this aerobic oxygen atom transfer reaction requires neither such a system nor a sacrificial reductant. Although the mechanism of this oxidation has not been completely clarified, some experimental results support the notion that an aqua ligand coordinated with the ruthenium ion serves as a proton transfer agent for the oxygen activation process, and it is recycled and used as the proton transfer mediator during the process. Thus, we have achieved catalytic asymmetric oxygen atom transfer reaction using molecular oxygen that can be carried out under ambient conditions.

Reaction Mechanism of the Gold(I)-Catalyzed Addition of Phenols to Olefins: A Concerted Process Accelerated by Phenol and Water

The reaction mechanism of the gold(I)-phosphine-catalyzed addition of phenols to olefins was analyzed by means of theoretical methods combined with polarizable continuum models. Several mechanistic pathways for the reaction were considered and evaluated. The most favorable one includes the ligand substitution of triflate by the alkene in the catalytically active R3PAuOTf species and subsequent nucleophile attack of phenol on the activated double bond concomitantly with the proton transfer from the OH group to the other carbon atom. The energy barrier for this concerted transition state diminishes dramatically when a proton transfer agent is present. These species are acting as proton shuttles in the proton transfer step. Noteworthy, the proton transfer in the phenol addition, both for the PhOH- or H2O-assisted processes, is a concerted step.

Ion Chemistry of VX Surrogates and Ion Energetics Properties of VX: New Suggestions for VX Chemical Ionization Mass Spectrometry Detection

Room temperature rate constants and product ion branching ratios have been measured for the reactions of numerous positive and negative ions with VX chemical warfare agent surrogates representing the amine (triethylamine) and organophosphonate (diethyl methythiomethylphosphonate (DEMTMP)) portions of VX. The measurements have been supplemented by theoretical calculations of the proton affinity, fluoride affinity, and ionization potential of VX and the simulants. The results show that many proton transfer reactions are rapid and that the proton affinity of VX is near the top of the scale. Many proton transfer agents should detect VX selectively and sensitively in chemical ionization mass spectrometers. Charge transfer with NO(+) should also be sensitive and selective since the ionization potential of VX is small. The surrogate studies confirm these trends. Limits of detection for commercial and research grade CIMS instruments are estimated at 80 pptv and 5 ppqv, respectively.

Microplasma Discharge Ionization Source for Ambient Mass Spectrometry

In this paper, we demonstrate the first use of a microplasma ionization source for ambient mass spectrometry. This device is a robust, easy-to-operate microhollow discharge that enables ambient direct analysis of gaseous, liquid, and solid-phase samples with minimum requirements in terms of operating power and high purity gas consumption. The initial performance of the microplasma device has been evaluated by ionizing samples containing dimethyl sulfoxide (DMSO), dimethylformamide (DMF), methyl salicylate, caffeine, l-leucine, l-histidine, loratadine, ibuprofen, acetaminophen, acetylsalicylic acid, and cocaine in various forms. These molecules are diverse in nature, but almost all have relatively high proton affinities. Thus, the major species observed in all obtained mass spectra corresponded to protonated molecules. Though these microplasmas are known to produce significant densities of metastable species and electrons with mean energies greater than several electronvolt, minimal fragmentation was observed. Background spectra showed prominent signals corresponding to H(+)(H(2)O)(2) ions and a distinct lack of H(3)O(+). Small water cluster ions are likely the dominant proton transfer agents, giving rise to mass spectral data very similar to that obtained using other plasma-based ambient ionization techniques. The simplicity, low cost, low power, low rate of gas consumption, and possibility of being batch-fabricated, makes these microplasma devices attractive candidates as ion sources for miniaturized mass spectrometry and other field detection applications.