TOF-determining Transition State Research Articles

DFT studies on the reaction mechanism of Ru(II)-catalyzed the C[sbnd]H activation of aromatic amide and alkylation of non-active olefins

Density functional theory were performed to investigate the ruthenium-catalyzed CH activation of aromatic amides and alkylation of 1-hexene. Our findings indicate that the plausible catalytic cycles encompass CH activation, 1-hexene insertion, protonation, catalyst recovery, and the final product formation. Our calculations confirm that the [SbF6]− fragment remains positioned between ligand p-cymene and reactant aromatic amide(1a). Given that the reactant 1-hexene(2a) presents two potential reaction sites, a detailed discussion of the reaction branching point, resulting in two distinct products, is provided. The modified energy span model deduced that the turnover frequency (TOF) determining transition state (TDTS) and intermediate (TDI) correspond to the CC coupling transition states (TS5a or TS5b) and initial point, respectively. Considering the concentration effect of reactant 2a, the calculated apparent free-energy barriers for these two reaction pathways are 30.3 and 31.9 kcal mol−1. This difference offers a rational explanation for the observed experimental product ratio.

Catalytic Nitrous Oxide Reduction with H2 Mediated by Pincer Ir Complexes.

Reduction of nitrous oxide (N2O) with H2 to N2 and water is an attractive process for the decomposition of this greenhouse gas to environmentally benign species. Herein, a series of iridium complexes based on proton-responsive pincer ligands (1-4) are shown to catalyze the hydrogenation of N2O under mild conditions (2 bar H2/N2O (1:1), 30 °C). Among the tested catalysts, the Ir complex 4, based on a lutidine-derived CNP pincer ligand having nonequivalent phosphine and N-heterocyclic carbene (NHC) side donors, gave rise to the highest catalytic activity (turnover frequency (TOF) = 11.9 h-1 at 30 °C, and 16.4 h-1 at 55 °C). Insights into the reaction mechanism with 4 have been obtained through NMR spectroscopy. Thus, reaction of 4 with N2O in tetrahydrofuran-d8 (THF-d8) initially produces deprotonated (at the NHC arm) species 5NHC, which readily reacts with H2 to regenerate the trihydride complex 4. However, prolonged exposure of 4 to N2O for 6 h yields the dinitrogen Ir(I) complex 7P, having a deprotonated (at the P-arm) pincer ligand. Complex 7P is a poor catalytic precursor in the N2O hydrogenation, pointing out to the formation of 7P as a catalyst deactivation pathway. Moreover, when the reaction of 4 with N2O is carried out in wet THF-d8, formation of a new species, which has been assigned to the hydroxo species 8, is observed. Finally, taking into account the experimental results, density functional theory (DFT) calculations were performed to get information on the catalytic cycle steps. Calculations are in agreement with 4 as the TOF-determining intermediate (TDI) and the transfer of an apical hydrido ligand to the terminal nitrogen atom of N2O as the TOF-determining transition state (TDTS), with very similar reaction rates for the mechanisms involving either the NHC- or the P-CH2 pincer methylene linkers.

An Electronic Structure Study of the Conversion from 1,2‐Diphenylacetylene to (E)‐1,2‐Diphenylethene Using a Bidentate Ru(II)−NC Catalyst

AbstractThe {(C5H4N)(C6H4)}RuCl(CO)(PPh3)2 catalyst has been shown to be highly efficient and selective for semi‐hydrogenation reactions with a wide range of internal alkynes containing electron‐withdrawing and electron‐donating groups to E‐alkenes. In this work, the reaction mechanism previously proposed by experimentalists to the formation of (E)‐1,2‐diphenylethene from 1,2‐diphenylacetylene (Organometallics, 2020, 39, 862) is evaluated by using the Density Functional Theory. Our calculations show the existence of four new intermediates to avoid steric hindrance issues and to explain the releasing of the (Z)‐1,2‐diphenylethene and (E)‐1,2‐diphenylethene products at the end of the first and second cycles, respectively. Besides, by using the energetic span model, the turnover frequency (TOF)‐determining transition state (TDTS) and intermediate (TDI) are found in the first cycle, providing an energetic span for the catalytic cycle of 16.8 kcal/mol at 383.15 K. This result assures that the reaction can really proceed by this mechanism.

DFT study on the ruthenium-catalyzed decarbonylative annulation of an alkyne with a six-membered hydroxychromone via C–H/C–C activation

Both steady-state approximation and the modified energy span model have been employed to determine the TOF-determining transition state and TOF-determining intermediate for the title catalytic reaction.

Reaction Path Determination of Rhodium(I)-Catalyzed C-H Alkylation of N-8-Aminoquinolinyl Aromatic Amides with Maleimides.

The rhodium(I)-catalyzed reaction of N-8-aminoquinolinyl aromatic amides with maleimides results in C-H alkylation at the ortho position of the amide. The reaction path and formation of the alkylation product with density functional theory (DFT) calculations were done. The detailed computational study showed that the reaction proceeds in the following steps: (I) deprotonation of the NH amide proton, (II) oxidative addition of the ortho C-H bond, (III) migratory insertion of the maleimide, (IV) reductive elimination with the C-C bond formation, and (V) protonation. The energetic span model showed that the turnover frequency (TOF)-determining transition state (TDTS) is the oxidative addition, while the TOF-determining intermediate (TDI) is the formation of an Rh(I)-complex after N-H deprotonation. It was also found that the change in the oxidation number of the Rh catalyst is a key determinant of the reaction path.

Mechanistic Study of Tungsten Bipyridyl Tetracarbonyl Electrocatalysts for CO2 Fixation: Exploring the Roles of Explicit Proton Sources and Substituent Effects.

Tungsten bipyridyl tetracarbonyl complexes were shown to reduce CO2 to CO in acetonitrile [Chem. Sci., 2014, 5, 1894-1900]. Here, we employ density functional theory (DFT) calculations to investigate the electronic structure and reactivity of a series of tungsten electrocatalysts, [W(bpy-R)(CO)4] (where R = H, CH3, tBu, OCH3, CF3, and CN), for the CO2 reduction reaction (CO2RR). Our proposed mechanism suggests that initial reduction of the starting material by two electrons is required to access the active catalyst upon CO dissociation, which is slightly endergonic, consistent with the slow product release observed experimentally. The doubly reduced species, which has a closed-shell singlet ground state, can bind CO2 via an η2-CO2 binding mode to yield the metallocarboxylate intermediate. Based on the energy span model, CO2 addition is the TOF-determining transition state (TDTS) in the presence of water as the proton source. Different substituents at the 4,4'-positions of the bipyridine ligand in [W(bpy-R)(CO)4] (R = H, CH3, tBu, OCH3, CF3, and CN) were considered to comprehend the substituent effects for CO2RR. DFT results show that electron-withdrawing substituents, such as CN and CF3, do not yield efficient CO2 reduction catalysts due to the necessity of forming high energy intermediates for the protonation steps, resulting in low TOFs and high overpotentials. Among electron-donating groups, the parent compound and tert-butyl substituted complex are the most active catalysts for CO2RR due to higher TOFs at low overpotentials. Overall, based on the energy span model and theoretical Tafel plots, our computational approach provides quantitative information for designing CO2 reduction electrocatalysts.

Computational Study for CO2-to-CO Conversion over Proton Reduction Using [Re[bpyMe(Im-R)](CO)3Cl]+ (R = Me, Me2, and Me4) Electrocatalysts and Comparison with Manganese Analogues

The Nippe group has previously reported a series of imidazolium-functionalized rhenium bipyridyl tricarbonyl electrocatalysts, [Re[bpyMe(Im-R)](CO)3Cl]+ (R = Me and Me2), for CO2-to-CO conversion using H2O as the proton source [Sung, S.; Kumar, D., Electrocatalytic CO2 Reduction by Imidazolium-Functionalized Molecular Catalysts. J. Am. Chem. Soc. 2017, 139, 40, 13993−13996. 10.1021/jacs.7b07709]. These compounds feature charged imidazolium ligands in the secondary coordination sphere and exhibit higher catalytic activities as compared to the Lehn catalyst [Re(bpy)(CO)3Cl] (where bpy = 2,2′-bipyridine). However, the reaction mechanism for the CO2 reduction reaction (CO2RR) over the competing hydrogen evolution reaction (HER) is unclear. Here, we employ density functional theory (DFT) and restricted active space self-consistent field (RASSCF) methods to study the selectivity for CO2 fixation using [Re[bpyMe(ImMe)](CO)3Cl]+ (1+) in water and compare its reactivity to [Re[bpyMe(ImMe2)](CO)3Cl]+ (2+) and [Re[bpyMe(ImMe4)](CO)3Cl]+ (3+). Our results reveal that the turnover frequency (TOF) for CO2RR using 1+ is 4 orders of magnitude higher than for proton reduction, consistent with controlled potential electrolysis (CPE) experiments in which CO was the only detectable reduction product. The imidazolium moiety in the secondary coordination sphere stabilizes the metallocarboxylate species and assists the C–O cleavage through intermolecular hydrogen-bonding stabilizations. Furthermore, our calculations imply that the strongest hydrogen-bonding interactions at the C2 position in 1+ contribute to the faster reaction rate observed experimentally with respect to 2+. More significantly, the use of the energy span model demonstrates that the turnover frequency-determining transition state (TDTS) corresponds to the formation of the Re–CO2 adduct, contrasting with manganese analogues in which the C–O bond cleavage step is the TDTS. We attribute this distinction based on the electronic structures of doubly reduced active catalysts. Indeed, RASSCF calculations indicate that rhenium compounds are best described as a rhenium(I) coupled with a doubly reduced bipyridine ligand, [ReI[bpyMe(ImMe)2–](CO)3]0. In contrast, manganese analogues feature a metal center in a formal zero oxidation state antiferromagnetically coupled with an unpaired electron on the bpy, [Mn0[bpyMe(ImMe)•–](CO)3]0.

Substituent effects and mechanism studies in CO2 transformation to benzoxazinone derivatives as worthwhile N‐containing heterocycles: Insight from Density functional theory

AbstractThe substituted arynes, an imine compound, and atmospheric CO2 have been considered as the starting materials in a theoretical study of a CO2 transformation to N‐containing heterocycles. Functional groups show substantial effects on the studied mechanisms. The obtained results show that Mechanism A, in which the imine compound is added to arynes, is the most probable mechanism. Kinetic studies are performed by the energetic span model. The transition stats of Step 1 and the corresponding intermediates are the turnover‐frequency determining transition state (TDTS) and turnover‐frequency determining intermediate (TDI), respectively. Also, electron localization function (ELF) analyses reveal that the electron densities of developing monosynaptic basins on the carbon atom (V[C]) at the transition state show a good linear correlation with the calculated energy values of TDTS. Electron‐withdrawing and electron‐releasing characters of the substituents affect the TDTS energy values.

Theoretical Exploration of Copper-Catalyzed Mechanisms of Cope-Type Hydroamination of Cyclopropene and Oxime

The mechanism of copper-catalyzed Cope-type hydroamination of cyclopropene and oxime using density functional theory (DFT) are carefully calculated. An energetically feasible metalla-retro-Cope transition state is proposed for the enantioselectivity of the final product and discussed through the non-covalent interaction (NCI) analysis. Also, the turnover frequency TOF-determining transition state (TDTS) is considered, which covers an energetic span (δE) with 16.5 kcal/mol. In the overall catalytic cycle, the dominant weak interactions play essential roles in stabilizing both the enantioselectivity-determining transition state and the TDTS structure.

DFT study on selective autocatalyzed [formula omitted]-alkylation of ketones with alcohols

Base-promoted transition-metal-free α-alkylation of ketones with alcohols is an efficient, practical, and green method for the formation of new CC bonds. Here, we present a DFT study on NaOH/KOH-promoted selective autocatalyzed α-alkylation of ketones with alcohols. For the production of alkylated alcohol, the disodium alkoxide species Na1D is the mediator, and the transformation involves four stages: (I) aldehyde formation; (II) aldol condensation; (III) chalcone reduction to generate an alkylated ketone and an aldehyde; and (IV) reduction of the alkylated ketone to produce an alkylated alcohol. Stage I only occurs at the beginning of the reaction, and the subsequent reaction proceeds via the catalytic cycle cyc-Na involving stages II, III, and IV. The energetic span and the turnover frequency (TOF) of cyc-Na are 35.2 kcal/mol and 5.7×10-7 1/s, respectively. For the production of alkylated ketone, the dipotassium alkoxide species K1D is the mediator, and the transformation includes dioxygen-assisted oxidation of alkylated alcohol (stage V) and the above four stages. Stage I only takes place initially, too, but the subsequent reaction has two selectable channels. The main channel is the catalytic cycle cyc-K consisting of stages II and III. The energetic span and the TOF of cyc-K are 29.5 kcal/mol and 8.8×10-5 1/s, respectively. The side channel involves stages II, III, IV, and V, in which the apparent activation energy (AAE) is 34.1 kcal/mol. The calculations present the TOF-determining intermediates and the TOF-determining transition state, and uncover the essence of autocatalysis and the origins of the product selectivity. One important finding is the alkylated ketone selectivity may be enhanced by adding a small quantity of aldehyde corresponding to the alcohol reactant and lowering the reaction temperature.

Theoretical investigation on acetylene cyclotrimerization catalysed by TiO2 and Ti

AbstractThe reaction mechanisms for acetylene cyclotrimerization using TiO2 and Ti catalysts were studied by the density functional theory (DFT) method. Interestingly, the reaction catalysed by TiO2 occurs on the potential energy surface (PES) with a singlet state. For the same reaction catalysed by Ti, spin‐orbit coupling (SOC) calculations were performed to discuss spin inversion between the triplet and singlet PESs. The chance that an electron hops in the vicinity of the minimum‐energy crossing point (MECP) was verified regarding the Landau‐Zener model. The possibilities of single ( ) and double ( ) at MECP1 (SOC = 253.38 cm−1) are about 0.35 and 0.46, respectively. The energetic span model developed by Kozuch was applied in the above reactions. The turnover frequency (TOF)‐determining transition state (TDTS) and TOF‐determining intermediate (TDI) were verified. The TOF value indicates that Ti is a more active catalyst compared with TiO2 in C2H2 cyclotrimerization.

Formic Acid Dehydrogenation by Ruthenium Catalyst – Computational and Kinetic Analysis with the Energy Span Model

The ruthenium (cis‐RuCl2(DPPM)2) based catalytic dehydrogenation reaction of formic acid in the presence of an amine base in a biphasic system experimentally tested by Treigerman and Sasson (ChemistrySelect 2017, 2, 5816) was studied computationally to ascertain its mechanism. The energy span model was applied on the double‐hybrid DFT computed energy profile to comprehend its kinetics. The catalytic network includes three possible interconnected cycles depending on the ancillary ligands, going through decarboxylation, protonation and H2 release. The dihydride cycle proves to be the most efficient after pre‐activation steps coming from the other cycles. The turnover frequency (TOF) determining intermediate (TDI) is the formatohydride species, while the TOF determining transition state (TDTS) corresponds to a formate decarboxylation. Herein we include the effect of reactants concentrations to the energy span model, which proved to be essential to comprehend the experimental ESI‐MS results and to propose a more accurate mechanism.

Mechanism of Aerobic Alcohol Oxidation Mediated by Water‐Soluble CuII‐TEMPO Catalyst in Water: A Density Functional Theory Study

AbstractThe catalytic mechanism for the aerobic alcohol oxidation in the alkaline water solution catalyzed by a CuII/2,2,6,6‐tetramethylpiperidinyl‐1‐oxy (TEMPO) catalyst system, ([(L)CuII(Hen)(H2O)] (H3L: 3‐(5‐Chloro‐2‐hydroxy‐3‐sulfophenylhydrazo) pentane‐2,4‐dione; en: Ethylenediamine), is presented by density functional theory (DFT) calculations. Four pathways (path A, path B, path C and path D) are presented. Our calculations demonstrate that path A is the favourable pathway and zwitterionic property of the catalyst is not likely affect the efficiency of alcohol oxidation. In path A, the catalytic cycle consists of catalyst activation, substrate oxidation and catalyst regeneration parts. The calculated turn‐over frequency (TOF=3.89 h−1) is in line with the experimental result (TOF=5.40 h−1). It is also found that the H atom migrates from alkoxide to the oxygen atom of TEMPO in the TOF determining transition state (TDTS).

A theoretical study on La-activated bicyclo-oligomerization of acetylene to form naphthalene in gas phase using density functional theory (DFT)

In a recent experimental research, the formation of naphthalene has been demonstrated by La-mediated acetylene bicyclo-oligomerization in the gas phase, and this is the first report of metal-activated acetylene bicyclo-oligomerization to form the naphthalene. In this work, the complete reaction mechanism has been systematically analyzed on the doublet potential energy surface by employing density functional theory (DFT), the results showed that the computational results were consistent with experimental dates. Among them, two possible reaction pathways were identified: (1) LaC4H2 is formed by a second addition of acetylene molecule to LaC2H2 followed by dehydrogenation (path (a)). (2) First, dehydrogenation of LaC2H2, followed by the addition of a second acetylene molecule (path (b)), we found that the optimum pathway was path (a). According to the thermodynamic point of view, the reaction is highly exothermic and favorable. In addition, sequential acetylene additions coupled with dehydrogenation showed that the bicycle-oligomerization reaction can occur. For further analysis of the observed kinetic behavior, the energetic span model was utilized and confirmed the TOF-determining transition state (TDTS) and TOF-determining intermediate (TDI) of the overall reaction. Finally, the optimum path was found and demonstrated.

Mechanistic Study of the Direct Intramolecular Allylic Amination Reaction Catalyzed by Palladium(II)

DFT calculations (PBE1PBE/6-31G(d,p), Def2-TZVPPD) were performed to study the intramolecular C–H amination of an unsaturated carbamate catalyzed by [Pd(LL)(OAc)2] (2), where LL is the bis(sulfoxide) ligand PhS(O)(CH2)2S(O)Ph. The coordination takes place by an associative path over a trigonal-bipyramidal transition state. The LL ligand undergoes a coordination shift from κ2S,S to κ1S, leaving an open position for binding of the substrate (C═C). In the next step, the C–H activation, the transition state for the hydrogen abstraction from the substrate to form the σ-allyl complex has an energy of 124.0 kJ mol–1, which is the highest energy in the whole mechanism (TOF-determining transition state). The σ-allyl converts easily in the π-allyl, the acetic acid molecule leaving the coordination sphere. The remaining acetate receives the second hydrogen from the NH group, while the newly formed acetic acid molecule is replaced by the pendant arm of the LL ligand, and the cyclization takes place (nucleophilic atta...

Insights into the Mechanistic Role of Diphenylphosphine Selenide, Diphenylphosphine, and Primary Amines in the Formation of CdSe Monomers.

The formation mechanism of CdSe monomers from the reaction of cadmium oleate (Cd(OA)2) and SePPh2H in the presence of HPPh2 and RNH2 was studied systematically at the M06//B3LYP/6-31++G(d,p),SDD level in 1-octadecene solution. Herein, SePPh2H, HPPh2, and RNH2 act as hydrogen/proton donors with a decreased capacity, leading to the release of oleic acid (RCOOH). The longer the radius of the coordinated atom is, the larger the size of the cyclic transition state is, which lowers the activation strain and the Gibbs free energy of activation for the release of RCOOH. From the resulting RCOOCdSe-PPh2, for the formation of Ph2P-CdSe-PPh2 (G), SePPh2H acts as a catalyst, in which the turnover frequency determining transition state (TDTS) is characteristic of the Se-P bond cleavage. For the formation of RHN-CdSe-PPh2 (H), SePPh2H also serves as a catalyst, in which the TDTS is representative of the N-H bond cleavage. For the formation of Ph2PSe-CdSe-NHR (I), HPPh2 behaves as a catalyst, in which the TDTS is typical of the Se-P and N-H bond cleavage. The rate constants increase as kI < kH < kG, which is in good agreement with our previous experimental observations reported. The present study brings insight into the use of additives such as HPPh2 and RNH2 to synthesize colloidal quantum dots.

DFT and Kinetic Monte Carlo Study of TMS-Substituted Ruthenium Vinyl Carbenes: Key Intermediates for Stereoselective Cyclizations

Mechanistic pathways for the cyclization of 1,5-alkynylacetal with N2CHTMS in the presence of Cp- and Cp*RuCl(cod) to afford (Z)- and (E)-(trimethylsilyl)vinyl spiroacetals have been calculated. Calculations show the presence of three conformers in equilibrium for the initially formed ruthenium carbenes. Differences in the stabilities and reactivities of the conformers, depending on the use of a Cp or Cp* ruthenium catalyst, are responsible for the favorable active reaction pathways in each case, even though the geometry of the resulting product is the same regardless of the catalyst used. Kinetic Monte Carlo (KMC) simulations with rate coefficients, including tunneling probabilities for the hydride transfer step, were used to model the evolution of reactants, intermediates, and products for all calculated pathways. It was shown that one path is almost exclusively active for each catalyst. Finally, the energetic span model of Kozuch and Shaik was used to calculate the energetic span (δE), the TOF-determining transition state (TDTS), the TOF-determining intermediate (TDI), and the TOF value for each of the feasible mechanistic pathways.

Theoretical study of water gas shift reaction on Cu n Ni (n = 1–12) clusters

Density functional theory has been used to study the water-gas-shift reaction on CunNi (n = 1–12) clusters. The reaction mechanism of carboxyl has been examined. Using the energetic span model (ESM), we find that the turnover frequency-determining transition state (TDTS) is the carboxyl dissociation (COOH → CO2 + H) for CunNi (n = 1–12) and the turnover frequency-determining intermediate (TDI) is the co-adsorptions of CO and H2O on CunNi clusters for CunNi (n = 2–6, 11, 12). When it comes to CunNi (n = 1, 7–10), the turnover frequency-determining intermediate (TDI) is the co-adsorption of OH and H on CunNi clusters. Our calculation shows that the Cu8Ni cluster with the highest value of the calculated turnover frequence (TOF) exhibits high catalytic activity towards water gas shift reaction.

DFT Study on Mechanism of N-Alkylation of Amino Derivatives with Primary Alcohols Catalyzed by Copper(II) Acetate

DFT calculations have been carried out to study the mechanism of Cu(AcO)2-catalyzed N-alkylation of amino derivatives with primary alcohols. The calculations indicate that tBuOK is necessary for the generation of the active catalyst from Cu(AcO)2 and that the catalytic cycle involves three sequential steps: (1) Cu-catalyzed alcohol oxidation to give the corresponding aldehyde and copper hydride, (2) aldehyde-amine condensation to generate an imine, (3) imine reduction to yield the expected N-alkylation secondary amine product and to regenerate the active catalyst. Based on the comparison of different reaction pathways, we conclude that the outer-sphere hydrogen transfer in a stepwise manner is the most favorable pathway for both alcohol oxidation and imine reduction. Thermodynamically, alcohol oxidation and imine formation are all uphill, but imine reduction is downhill significantly, which is the driving force for the catalytic transformation. Using the energetic span model, we find that the turnover frequency-determining transition state (TDTS) and the turnover frequency-determining intermediate (TDI) are the hydride transfer transition state for imine reduction and the active catalyst, respectively. The calculated turnover frequency (TOF) roughly agrees with the experimental observation and, therefore, further supports the validity of the proposed hydrogen transfer mechanism.

Theoretical investigation for the cycle reaction of methane oxidation catalyzed by FeO&amp;lt;sup&amp;gt;+&amp;lt;/sup&amp;gt; in the gas phase

The mechanism of the spin-forbidden reaction of CH4 with O2 catalyzed by FeO+ (6Σ+, 4Π+) on both quartet and sextet potential energy surfaces has been investigated at the B3LYP level of theory. Six competitive reaction pathways for the catalytic reaction and Path 6 are likely in competition with other pathways from energetic viewpoint. There are 6 crossing points in Path 6 and the crossing points 1 and 3 are very important. The minimum energy crossing points (MECP) of corresponding crossing points are optimized. The potential energy surfaces and the possible spin inversion processes are discussed by means of spin-orbit coupling (SOC) and probability of intersystem crossing (ISC) calculations. The SOC of MECP1 is 947.58 cm-1, and MECP3 is 1372.51 cm-1. The probability of ISC are 0.535 and 0.590 respectively. In order to understand the competitive reaction mechanism involving H atom transfer, the activation strain model was adopted for a simple analysis of the competitive reaction. The energetic span ( δE ) model coined by Kozuch is applied in this cycle. X TOF (degree of TOF control) of all of transition state and intermediate are calculated, and the turnover frequency (TOF)-determining transition state (TDTS) and TOF-determining intermediate (TDI) are confirmed. In this catalytic reaction, the TOF is 5.6716×10-42 s-1.