Electric Field Catalytic Reactions: Comparison
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Please note this is a comparison between Version 2 by Jason Zhu and Version 1 by Zhibing Tan.

The role of catalysis in controlling chemical reactions is crucial. As an important external stimulus regulatory tool, electric field (EF) catalysis enables further possibilities for chemical reaction regulation. To date, the regulation mechanism of electric fields and electrons on chemical reactions has been modeled. The electric field at the single-molecule electronic scale provides a powerful theoretical weapon to explore the dynamics of individual chemical reactions. The combination of electric fields and single-molecule electronic techniques not only uncovers new principles but also results in the regulation of chemical reactions at the single-molecule scale. 

  • electric field catalysis
  • single-molecule electronic techniques
  • chemical reactions

1. Introduction

Catalysis can regulate the rate of chemical reactions and selectively synthesize products, which is one of the most important methods of controlling chemical reactions. In addition to using catalysts, external stimuli, including light [1[1][2][3],2,3], heat [4[4][5],5], electrochemical stimuli [6[6][7][8],7,8], electric fields [9,10,11][9][10][11] and magnetic fields [12], etc., are also typically used to excite catalytic reactions. Due to the flexible adjustment of parameters, such as field strength and direction, using an electric field to catalyze chemical reactions has unique advantages [13].
In the past two decades, single-molecule measurement techniques and data analysis methods have developed rapidly, allowing users to obtain precise and stable conductance distribution signals [14,15,16][14][15][16]. These techniques were gradually used to explore the physical and chemical properties of chemical changes [17], such as trapping reaction intermediates and transition states [18,19,20][18][19][20] or analyzing the thermodynamic and dynamic properties of a reaction [21,22,23][21][22][23].
The single-molecule break junction electrical characterization method uses a precise displacement control technique to control the continuous opening and closing of two tiny electrodes, forming a nano-spacer that matches the molecule during the opening of the electrodes, and the anchoring groups on the molecule are connected to the electrodes to form a circuit. Relying on current detection devices with ultra-high sensitivity in the external circuit to monitor the current in real time, a large number of conductance distance curves are obtained and then statistically analyzed to obtain accurate molecular conductance [39][24]. Many studies have proved that it is not difficult to add a strong electric field to this single-molecule technology [21,25,40][21][25][26]. The advantages of the instrument include technical support for studying the mechanism of nanoscale chemical reactions catalyzed by electric fields, as well as the possibility of the synthesis of complex organic materials and the formation of multifunctional molecular devices.

2. Diels–Alder Reactions

The Diels–Alder (DA) reaction is an important type of reaction in organic chemistry, and it is widely used in basic organic synthesis [48][27], the production of fine chemicals [49][28], and other fields. The reaction uses a 1,4-cycloaddition reaction of a diene reagent with a conjugated diene to form a six-element cyclic alkene. At room temperature, it is difficult for the reaction to proceed spontaneously, and a Lewis acid is generally used as the catalyst [50][29]. Charged radicals can also regulate the reaction process, though the reaction is less efficient [51,52,53][30][31][32]. Therefore, a more efficient means of regulation needs to be developed. Shaik and colleagues once predicted that the barrier of some Diels–Alder reactions would be influenced by the external electric field (EEF) [54][33]. Thus, the EEF holds promise in regulation of DA reaction progression.
In 2016, the Coote and Diez-Perez groups were the first teams to experimentally prove that an electric field could accelerate the progress of non-redox bond formation [17]. The researchers measured the conductance of a Diels–Alder reaction between a diene (a furan) and dienophile (a norbornylogous bridge, (±)-NB, tricyclo[4.2.1.02,5]non-7-ene-3,4-dimethanethiol) using a scanning tunneling microscopy break-junction (STM-BJ) technique. The reaction produced four structurally different DA products, each of which had two diastereoisomers with the furan substituent located on the left or right (red diastereomer) side of the molecule. In the negative bias case, the potential for the formation of the red isomer will decrease with increasing field strength; the probability of detecting the reaction product is significantly higher and positively correlated with the applied bias voltage. However, in the case of positive bias voltage, the formation potential of the blue isomer is independent of the field strength, and the probability of product generation decreases. Since the dienes are electron-rich and the amphiphiles lack electrons, the negatively biased EEF makes the transition state conform to the lowest potential barrier by stabilizing the charge resonance.
In addition, the STM break-junction “flicker” experiment showed that the conductance of the molecular bridge is constant under positive voltage bias. Conversely, under the negative bias, the product formation increased 4.4-fold from 4.2 to 18.6%. It was found that the molecular configuration was significantly affected by the negative bias through quantum calculation. The negative bias energy significantly reduces the reaction barrier. When an electric field is applied to electrostatically stabilize these covalent compounds, a small charge separates the resonance, resulting in the overall stabilization of the molecule or transition state.
Previous studies demonstrated that electric fields affect the rate of the DA reaction and tune the relative orientation between the oriented external electric field (OEEF) and the reaction axis for selective electrostatic catalysis of multistage reactions. In 2021, the Guo group and their collaborators reported the precise temporal trajectory and detailed DA reaction pathway directly observed on in situ label-free graphene-based single-molecule devices using precise single-molecule detection [55][34]. Chen et al. first designed and synthesized conjugated molecules with maleimide as the functional center and modified with amino groups at the ends, which were attached between graphene point electrodes with carbonyl functional groups via amide bonds to construct stable single-molecule devices. The single-molecule devices were jointly characterized by their self-developed ultra-high spatial and temporal resolution photoelectric integrated detection system with electrical and optical dual-mode methods, providing the first direct experimental evidence of synchronization of the reaction. They first demonstrated the accepted mechanism of synergistic reactions and captured the charge–transfer complex salts that passed through the corresponding key intermediates before generating products with an endo or exo conformation. Next, by recording the precise time trajectories and detailed reaction paths through a single-molecule electrical detection platform, they revealed a new mechanism for a second stepwise (via ampholytic intermediates) addition reaction. After clarification of the studied mechanism, the synergistic or stepwise DA reactions were regulated by varying the bias and temperature.
A new mechanism for electric field-catalyzed Diels–Alder addition reactions was revealed by combining experiment and theory: a strong electric field (~109 V/m) was applied, which decreased the potential energy of the key intermediate in the stepwise reaction (the ampholytic intermediate that only adds one bond), thereby greatly enhancing its stability. This electric field catalytic effect allowed the discovery of the stepwise reaction pathways that had never been observed experimentally in the system, adding to the conventional knowledge of Diels–Alder addition reactions. The researchers also performed a careful analysis of the thermodynamics and kinetics of these processes, which led to the establishment of a new approach to the regulation of intermediate lifetimes, as well as chemical reaction pathways, via electric fields. Mejía et al. employed molecular dynamics (MD) coupled to quantum transport simulations alone to perform theoretical calculations of DA reactions at the single-molecule scale [56][35]. The calculations show that the DA reaction activation energy is dominated by entropy and that the chemical reactions are thermodynamically influenced. The construction of molecular junctions can also affect the reaction transition states and, thus, modulate the reaction paths of DA reaction occurrence, further providing important theoretical support for single-molecule junctions to monitor antichemical reactions. This observation provides unlimited possibilities for understanding many mechanisms that are difficult to decipher in organic chemistry and lays the foundation for the regulation of chemical reactions and the evolution of the life sciences.
Electric fields can also regulate the direction of the chemical reaction. Hu et al. reported the first example of in situ label-free single-molecule detection of thermally reversible DA reactions using a scanning tunneling microscopy break junction (STM-BJ) technique equipped with a thermocouple mounted under a substrate with feedback control [44][36]. The thermally reversible DA reaction using anthracene-2,6-diamine (AnAm) as the diene and fullerene C60. A homemade STM-BJ coupled with a feedback-controlled thermocouple was used to capture individual molecules. The initial and terminated conductance states of the reaction-based single-molecule junctions can be reversibly switched in situ between two different temperature levels. After testing using different bias voltages, the reaction rate constant (k) calculated via conductance was 1.083 × 10−2 min−1, which is significantly higher than the k values obtained using UV-vis measurements (7.49 × 10−3 min−1) for the forward reaction. For the reverse reaction, the two reaction rate constants are similar (5.0 × 10−5 and 5.4 × 10−5 mM L−1 min−1). In addition, impressively, quantitative statistical analysis of the single-molecule reaction kinetics showed that the directed external electric field selectively accelerates the forward DA reaction by a factor of more than three, while not affecting the reverse reaction. The theoretical calculations indicated that the applied electric field stabilizes the dipole in the transition state, thus reducing the energy potential barrier for the forward reaction. Thus, the reaction kinetics suggest a significant selective acceleration effect of EEF on the forward direction of this DA reaction.
Subsequently, the Diels–Alder class of reactions was investigated using single-molecule techniques by Huang et al. [45][37]. The selective electrostatic catalysis using OEEFs in different processes of the cascade reaction was investigated by tuning the relative orientation between the oriented external electric field (OEEF) and the reaction axis on a single-molecule scale using the MCBJ method. A two-step cascade reaction at room temperature was chosen, in which compound A [3,6-di(4-pyridyl)-1,2,4,5-tetrazine] undergoes an anti-electron-demand Diels–Alder (iEDDA) reaction with dihydrofuran to form b, followed by an aromatization reaction to form c. This Diels–Alder reaction has a reaction axis (a→b) orthogonal to the orientation of the OEEFs, while the subsequent aromatization process (b→c) exhibits a non-orthogonal configuration between the reaction axis and the OEEFs, which provides an experimental platform for evaluating electric field-induced selective catalysis of chemical reactions. The reaction kinetics of this two-cascade reaction was determined via single-molecule conductance monitoring using the MCBJ technique.The changes in the characteristic conductance peaks in the conductivity statistics plot all visualize the course of the reaction. Experiments at the nanoscale reactor revealed that if the applied electric field is perpendicular to the reaction axis, the electric field has no effect on the chemical reaction; if the electric field has a component in the direction of the reaction axis, the electric field can increase the reaction rate by more than one order of magnitude, and the electric field plays a catalytic role in promoting the reaction. The intermediate state of the chemical reaction pathway was confirmed via single-molecule device electrical transport simulations; the results of transition state calculations showed that the directed electric field can effectively stabilize the transition state of the chemical reaction, thus reducing the reaction energy barrier [57,58,59][38][39][40]. Therefore, the application of electric fields can provide a new opportunity to tune the reaction rate and selectivity of chemical reactions for efficient chemical synthesis and future green chemistry.

3. Cleavage Reactions

In the 1970s, Pocker et al. pioneered theoretical models showing that electrostatic power can catalyze the formation or breaking of chemical bonds [60,61][41][42]. For non-polar covalent bonds, the molecule exhibits a resonantly stable state. When the electric field is present, it changes the stabilization of the covalent bond, reduces the dissociation energy of the molecule, and causes molecular cleavage. Initially, the electric field is provided by the charged functional groups [62,63][43][44]. However, the electrostatic effect is more obvious in the gas phase, and it is greatly influenced by the solubility in the liquid phase. Exploring the influence of applied electric fields on molecular cracking is conducive to expanding the research scope of electrostatic catalysis mechanisms, and is of great significance to the regulation and application of electrostatic catalysis.
In 2018, Michelle L. Coote et al. explored the ability of electric fields to rapidly undergo irreversible alkoxyamine cleavage (C-O bond breaking). At room temperature and under different magnitudes of bias stimulation, bridging alkoxyamine molecules between STM gold tips and gold substrates were used to demonstrate that alkoxyamines can undergo irreversible alkoxyamine cleavage (C-O bond breaking) [41][45]. At low deviations (<100 mV), only the parent alkoxylamine molecule (1 × 10−5 G0) is present in the system. However, between 100 and 200 mV, a mixture of nitrogen oxides and parent alkoxylamines is present. Above 200 mV, only nitrogen oxide radicals are detected. Nitrogen oxide has a known affinity for gold surfaces, and the same conductivity characteristics were observed in control experiments using standard nitrogen oxide (4-amino-TEMPO) solutions, confirming that the nitrogen oxide radicals are, indeed, products of OEEF-catalyzed (C-O) alkoxyamine bond breakage. The role of electric fields in promoting the homolysis of alkoxyamines is explained via quantum chemical calculations. The reaction spectra in the different intensities of electric fields are aligned along the N-O bond axis, which indicates that the homolysis of alkoxyamines can be promoted by the electric field up to 35 kJ mol−1. The effect of this energy barrier reduction is consistent with the expected p-nitro radical (N-O• ↔ N•-O+−) and the stabilization of charge separation resonances, conclusively demonstrating that the electrostatic environment in the solution catalyzes the formation of radicals through cleavage, offering the prospect of electrostatic catalytic decomposition, instead of triggering catalysis.
Recently, the Latha research group systematically quantified chemical rate enhancement using an electric field to clarify the catalytic effect of EEFs [42][46]. Using the STM-BJ technique, they found that the electric field could catalyze the homogeneous cleavage of the radical initiator 4-(methylthio)benzoic peroxyanhydride in the absence of co-initiators or photochemical activators. The conductance changed over time. It was also demonstrated that the reaction rate in the electric field was affected by the solvent and increased linearly with the dielectric constant of the solvent.
To quantify the effect of bias on molecular catalysis, this research group used the HPLC peak area integral to obtain the curve of the molecular concentration of the reactant and product over time. With a bias imposed at 100 mV, the half-life of 1 is t1/2 = 63.0 min, matching the formation rate of 2, thus suggesting that 1 is selectively transformed to 2 as the chemical reaction proceeds. Meanwhile, the chemical reaction rate varies significantly under different biases. At a zero bias, the half-life is increased nearly 6-fold from 100 mV, suggesting that gold also catalyzes the reaction, albeit to a small extent. At a 10 mV bias, the half-life is increased 3-fold compared to the 100 mV half-life.

4. Coupling Reaction of Transition Metal Complexes

A permanent electric field at the electrode–solution interface has also been shown to affect chemical transformations. Computational studies have shown that the oxidative addition of aryl halides at the palladium center can be affected by an external electric field [64][47]. Latha’s group used scanning tunneling microscopy break junctions (STM-BJs) to demonstrate for the first time that the reactivity of a kinetically inert transition metal complex can be induced by applying an external electric field to affect the coupling reaction [43][48]. A large electric field was applied to the molecular solution in the region between the STM tip and the substrate, and a mixture of the nickel (0) olefin complex Ni (COD)(DQ) and the iodinated aromatic compound was placed in 1,2,4-trichlorobenzene (TCB) for conductivity measurement. A new significant peak in the conductance value was observed, indicating the generation of two new molecular species with gold-linked groups in solution. In contrast, no conductivity peak was observed in the absence of an applied electric field, indicating that the applied electric field promotes the homogeneous coupling of iodinated aryl groups (4-iodothioanisole). It was shown that only in the presence of such an electric field did the nickel complexes undergo aryl iodide coupling chemistry at room temperature. Bias modulation and solvent selection are both strategies used to control the intensity of the local electric field and, subsequently, the extent of organic transformation. The modulation of organometallic coupling in a nanojunction environment by local electric fields highlights the importance of electric field effects in reaction chemistry, and it provides a new strategy to modulate organometallic reactivity.

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