The reaction mechanism underlying the hypergolic reaction of pure monomethylhydrazine (MMH) with 1-chloro-1,1-dinitro-2-(N-chloroamidino)ethane (CDNCE) was theoretically investigated with the density functional theory method. We identified two key atomistic level factors that affect ignition delay: (1) exothermicity for the formation of aerosol mCDNCE·nMMH complexes (m, n = 1, 2). The most cost-effective form was found to be 2CDNCE·MMH with the highest energy release (releasing energy: 23.4 kcal/mol), indicating that the oxidizer-rich form is favorable. These complexes contributed the most to heat gathering and temperature increases in the system at the beginning of all reactions. (2) For the initial reaction of MMH with CDNCE, the SN2 mechanism was preferred. The activation barrier of the primary reactions was calculated to be 27.4 kcal/mol, which is also the rate-limiting step of this path. Because the rate of formation of NO2 was four orders of magnitude lower than the SN2 reaction at room temperature, the effect of MMH with NO2 was less significant at temperatures below 800 K. Thus, we consider the ignition reaction of MMH with CDNCE to be well characterized.

DFT methods have been used to study the reaction mechanism of 1-phenylethanol with benzyl alcohol catalyzed by ferrocenecarboxaldehyde/NaOH. The structures of intermediates and transition states, and the exchange of electronic density are computed in detail. In general, the catalyzed reaction is consists of three steps: hydride transfer step with the electron transfer, crossing-aldol condensation step, and the reduction step. Hydride transfer is the speed control step with the highest energy barrier (about 32 kcal/mol). Our calculation results are fundamentally coincident with the experimental detections, and manifest the crossing-coupling reaction occurs through a reliable mechanism.In this paper, we investigated the mechanism of 1-phenylethanol β-alkylation with benzyl alcohol using DFT methods at the M062x level. In the reaction, the ferrocenecarboxaldehyde is the catalyst and p-xylene is chosen as the solvent, which is consistent with the experimental conditions. The calculated results show that the entire catalytic cycle includes three successive steps: (1) hydride transfer; (2) cross-aldol condensation; and (3) reduction. The iron catalyst slips the Cp ring to maintain its 18e stable structure in the reaction process. The present investigation is in good agreement with the experiment results.

DFT methods and the energetic span model have been used to study the mechanism of the N-alkylation of amines with alcohols catalyzed by the PdCl2/dppe/LiOH system (dppe = 1,2-bis(diphenylphosphino)ethane). The energetic results indicate that the most favorable pathway is the inner-sphere hydrogen transfer pathway, which consists of initiation of the three-coordinated active alkoxide complex [Pd(PhCH2O)(dppe)]+ (Int4i) and the catalytic cycle CC1. Initiation of Int4i includes two sequential steps: (i) generation of the three-coordinated active species [Pd(OH)(dppe)]+ and [Pd(PhNH)(dppe)]+ and (ii) PhCH2OH deprotonation with the aid of [Pd(PhNH)(dppe)]+ to afford Int4i. Catalytic cycle CC1 includes three sequential steps: (i) β-H elimination of Int4i to generate benzaldehyde and the Pd hydride species [PdH(dppe)]+, (ii) condensation of benzaldehyde with aniline to give the imine, and (iii) imine reduction to supply the amine product and to regenerate Int4i. The calculated turnover frequencies (TOFs) support that CC1 is the most favorable, although it is inhibited by the reverse process of PhCH2OH deprotonation catalyzed by [Pd(PhNH)(dppe)]+. By calculating the degree of TOF control, we identify the TOF-determining intermediate (TDI) and the TOF-determining transition state (TDTS) in CC1, and find that all the influential intermediates are the off-cycle LiCl2–-coordinated complexes in the overall reaction pathway, which leads us to conclude that LiCl2– is the TOF-affecting key species. Our additional calculations show that the TOF may be improved by the addition of AgOTf or AgBF4, which can scavenge the Cl– and supply the weak ligand OTf– or BF4–. Hopefully, these results are useful for further catalyst development.Keywords: alcohols; DFT methods; hydrogen autotransfer mechanism; N-alkylation of amines; palladium catalyst; the energetic span model

DFT calculations have been performed to study the mechanism of the N-alkylation of amines with alcohols catalyzed by [Cp*IrCl2]2 (Cp* = η5-C5Me5) in the presence of K2CO3. The energetic results show that this N-alkylation reaction proceeds via the hydrogen autotransfer mechanism and the catalytic cycle includes three sequential stages: (1) alcohol oxidation to produce aldehyde, (2) aldehyde–amine condensation to form an imine and (3) imine reduction to afford the secondary amine product. For stages 1 and 3, the most favorable pathways are the inner-sphere hydrogen transfer pathway under the catalysis of Cp*Ir(NHPh)Cl (C) and the inner-sphere hydrogen transfer pathway with KHCO3 as the proton donor. Thermodynamically, both stages 1 and 2 are endergonic, but stage 3 is highly exergonic. Thus stage 3 is the driving force for the catalytic cycle. The energetic span model has also been used to assess the catalytic cycle, and it is found that the turnover frequency-determining intermediate (TDI) and the turnover frequency-determining transition state (TDTS) are the 18e complex Cp*Ir(κ2-CO3K)Cl (A) and the transition state BC-TS5i for β-H elimination, respectively. The calculated turnover frequency (TOF), 4.68 h−1, agrees with the experimentally determined TOF and, therefore, provides strong support for the proposed catalytic cycle.

A complete reaction mechanism for interconversion between hydrogen and formic acid catalyzed by [C,N] cyclometallated organoiridium complex [IrIII(Cp*)(4-(1H-pyrazol-1-yl-κN2)benzoic acid-κC3)(H2O)]2·SO4, i.e. [Ir-1]2·SO4, has been revealed by density functional theory (DFT) calculations. For both the hydrogen storage catalytic cycle I and hydrogen evolution catalytic cycle II, the detailed reaction profiles with the key transition states and intermediates are revealed. Catalytic cycle I shows that the dihydrogen heterolysis facilitated by OH− gives the considerable stable iridium hydride intermediate M-4, followed by an outer-sphere hydrogen transfer to afford a metal–formate complex M-6. Upon the increasing of pH, catalytic cycle II occurs via the generation of the metal–formate complex M-7, followed by the outer-sphere β-H elimination to form a metal–hydride complex M-9, which is subsequently protonated by the hydrated proton H3O+ to afford dihydrogen. The decomposition of bicarbonate and the β-hydride elimination of formate are believed to be the rate-determining steps for cycle I and II, respectively. The acid–base equilibrium between the hydroxy and oxyanion form on the catalyst [C,N] ligand has a considerable influence on the catalytic hydrogen transfer. Our studies are in good agreement with experimental results. Remarkably, the new theoretically designed low-cost cobalt(III) complex, as a promising catalyst, exhibits catalytic activity for the interconversion between hydrogen and formic acid.

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.Keywords: amino derivatives; Cu(AcO)2; DFT calculations; hydrogen transfer mechanism; N-alkylation reaction; primary alcohols; tBuOK

Developing efficient dehydrogenation is critical to understanding organic hydride hydrogen storage. The catalytic mechanism of the pH-dependent acceptorless-alcohol-dehydrogenation in aqueous solution catalyzed by a novel [C,N] cyclometalated Cp*Ir-complex, [IrIII(Cp*)-(4-(1H-pyrazol-1-yl-κN2)benzoic acid-κC3)(H2O)]2SO4, has been investigated using density functional theory (DFT) with the M06 dispersion-corrected functional. Using water as the solvent with liberation of dihydrogen represents a safe and clean process for such oxidations. The overall catalytic cycle has been fully characterized. The pre-catalyst AIr first reacts with the ethanol in basic solution to generate an active hydride complex DIrvia an inner-sphere mechanism, involving the hemi-decoordination of [C,N] ligand followed by the β-H elimination. Subsequently, the complex DIr interacts with the protons in acid solution to generate H2 molecules, which is a downhill process nearly without an energy barrier. The present theoretical results have shown that both the hydroxyl in basic solution and the proton in acidic solution play a crucial role in promoting the whole catalytic cycle. Therefore, our results theoretically demonstrated a significant dependence of the reaction system studied on pH value. The present study also predicts that the FIr (at the first triplet excited state, T1) formed from DIr under laser excitation can catalyze the dehydrogenation of ethanol. Remarkably, the replacement of Ir by Ru may yield an efficient catalyst in the present system.

Using cinchona alkaloid-derived primary amine as catalyst and benzoic acid as co-catalyst, Michael-type addition reactions between enolizable carbonyl compounds and nitroalkenes have been extensively studied; however, our understanding of the mechanism is far from complete. In this paper, a theoretical study is presented for the Michael addition reaction between trans-1-nitro-2-phenylethylene and 2-methylpropionaldehyde catalyzed by 9-epi-QDA and benzoic acid. By performing DFT and ab initio calculations, we have identified a detailed mechanism. The calculations indicated that four continuous steps are involved in the overall reaction: (1) the formation of an iminium intermediate, (2) an addition reaction between the iminium and trans-1-nitro-2-phenylethylene, (3) the proton transfer process, and (4) hydrolysis and regeneration of the catalyst. The rate-determining step is the second proton transfer from the amine group to β-carbon of trans-1-nitro-2-phenylethylene, and the enantioselectivity is also controlled by this step. The calculated results provide a general model that explains the mechanism and enantioselectivity of the title reaction.

A range of novel octahedral iron(IV)–nitrido complexes with the TMC ligand (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) in the equatorial plane and one axial ligand trans to the nitrido have been designed theoretically, and a systematic comparative study of their geometries, electronic properties, and reactivities in hydrogen atom abstraction reactions regarding the iron(IV)–oxo and −sulfido counterparts has been performed using density-functional theory methods. Further, the relative importance of the axial ligands on the reactivity of the iron(IV)–nitrido systems is probed by sampling the reactions of CH4 with [FeIV═N(TMC)(Lax)]n+, (Lax = none, CH3CN, CF3CO2–, N3–, Cl–, NC–, and SR–). As we find, one hydrogen atom is abstracted from the methane by the iron(IV)–nitrido species, leading to an FeIII(N)–H moiety together with a carbon radical, similar to the cases by the iron(IV)–oxo and −sulfido compounds. DFT calculations show that, unlike the well-known iron(IV)–oxo species with the S = 1 ground state where two-state reactivity (TSR) was postulated to involve, the iron(IV)–nitrido and −sulfido complexes stabilize in a high-spin (S = 2) quintet ground state, and they appear to proceed on the single-state reactivity via a dominantly and energetically favorable low-lying quintet spin surface in the H-abstraction reaction that enjoys the exchange-enhanced reactivity. It is further demonstrated that the iron(IV)–nitrido complexes are capable of hydroxylating C–H bond of methane, and potential reactivities as good as the iron(IV)–oxo and −sulfido species have been observed. Additionally, analysis of the axial ligand effect reveals that the reactivity of iron(IV)–nitrido oxidants in the quintet state toward C–H bond activation enhances as the electron-donating ability of the axial ligand weakens.

The triplet δ-mechanism different from the previously reported ones, i.e., the π-channel with the unoccupied π*xz/yz (FeO) orbital and the σ-channel involving the unoccupied α-spin Fe-σ*z2 orbital, has been theoretically described for the methane hydroxylation by [FeIV = O(TMC)(SR)]+ and its derivative [FeIV = O(TMC)(OH)]+ complex for the first time, and we have undertaken a detailed DFT study on the nature of this state by probing its geometry, electronic property and reactivity in comparison to all other possibilities. DFT calculations indicate that the electron transfer for the 3δ-channel from the σC–H orbital of the substrate to the final acceptor σ*x2−y2 orbital of the catalyst occurs through a complex mechanism, which is initiated by the original α-spin electron transfer from the π* orbital of the catalyst to the σ*x2−y2 orbital, where the α-spin electron from the σC–H orbital of the substrate shifts to the just empty α-spin π* orbital of the catalyst via the O-px/y based π*xz/yz-orbital concomitantly. It is also found that the electron-donating ability of the axial ligand could influence the reaction channels, evident by the distinction that the electron-deficient F− and CF3CO2− ligands react via the 3σ-channel, whereas the electron-rich SR− and OH− ligands proceed by the 3δ-channel. With respect to reactivity, the 3δ-pathway has a comparable barrier to the 3π and 5π-pathways, which may offer a new approach for the specific control of C–H bond activation by the iron(IV)-oxo species.

Comprehensive density functional theory computations on substrate hydroxylation by a range of nonheme iron(IV)–oxo model systems [FeIV(O)(NH3)4L]+ (where L = CF3CO2−, F−, Cl−, N3−, NCS−, NC−, OH−) have been investigated to establish the effects of axial ligands with different degrees of electron donor ability on the reactivity of the distinct reaction channels. The results show that the electron-pushing capability of the axial ligand can exert a considerable influence on the different reaction channels. The σ-pathway reactivity decreases as the electron-donating ability of the axial ligand strengthens, while the π-pathway reactivity follows an opposite trend. Moreover, the apparently antielectrophilic trend observed for the energy gap between the triplet π- and quintet σ-channel (ΔG(T–Q)) stems from the fact that the reaction reactivity can be fine-controlled by the interplay between the exchange-stabilization benefiting from the 5TSH relative to the 3TSH by most nonheme enzymes and the destabilization effect of the orbital by the anionic axial ligand. When the former counteracts the latter, the quintet σ-pathway will be more effective than the other alternatives. Nevertheless, when the dramatic destabilization effect of the orbital by a strong binding axial σ-donor ligand like OH− counteracts but does not override the exchange-stabilization, the barrier in the quintet σ-pathway will remain identical to the triplet π-pathway barrier. Indeed, the axial ligand does not change the intrinsic reaction mechanism in its respective pathway; however, it can affect the energy barriers of different reaction channels for C–H activation. As such, the tuning of the reactivity of the different reaction channels can be realised by increasing/decreasing the electron pushing ability.

Comprehensive density functional theory computations on substrate hydroxylation by a range of nonheme iron(IV)–oxo model systems [FeIV(O)(NH3)4L]+ (where L = CF3CO2−, F−, Cl−, N3−, NCS−, NC−, OH−) have been investigated to establish the effects of axial ligands with different degrees of electron donor ability on the reactivity of the distinct reaction channels. The results show that the electron-pushing capability of the axial ligand can exert a considerable influence on the different reaction channels. The σ-pathway reactivity decreases as the electron-donating ability of the axial ligand strengthens, while the π-pathway reactivity follows an opposite trend. Moreover, the apparently antielectrophilic trend observed for the energy gap between the triplet π- and quintet σ-channel (ΔG(T–Q)) stems from the fact that the reaction reactivity can be fine-controlled by the interplay between the exchange-stabilization benefiting from the 5TSH relative to the 3TSH by most nonheme enzymes and the destabilization effect of the orbital by the anionic axial ligand. When the former counteracts the latter, the quintet σ-pathway will be more effective than the other alternatives. Nevertheless, when the dramatic destabilization effect of the orbital by a strong binding axial σ-donor ligand like OH− counteracts but does not override the exchange-stabilization, the barrier in the quintet σ-pathway will remain identical to the triplet π-pathway barrier. Indeed, the axial ligand does not change the intrinsic reaction mechanism in its respective pathway; however, it can affect the energy barriers of different reaction channels for C–H activation. As such, the tuning of the reactivity of the different reaction channels can be realised by increasing/decreasing the electron pushing ability.

The triplet δ-mechanism different from the previously reported ones, i.e., the π-channel with the unoccupied π*xz/yz (FeO) orbital and the σ-channel involving the unoccupied α-spin Fe-σ*z2 orbital, has been theoretically described for the methane hydroxylation by [FeIV = O(TMC)(SR)]+ and its derivative [FeIV = O(TMC)(OH)]+ complex for the first time, and we have undertaken a detailed DFT study on the nature of this state by probing its geometry, electronic property and reactivity in comparison to all other possibilities. DFT calculations indicate that the electron transfer for the 3δ-channel from the σC–H orbital of the substrate to the final acceptor σ*x2−y2 orbital of the catalyst occurs through a complex mechanism, which is initiated by the original α-spin electron transfer from the π* orbital of the catalyst to the σ*x2−y2 orbital, where the α-spin electron from the σC–H orbital of the substrate shifts to the just empty α-spin π* orbital of the catalyst via the O-px/y based π*xz/yz-orbital concomitantly. It is also found that the electron-donating ability of the axial ligand could influence the reaction channels, evident by the distinction that the electron-deficient F− and CF3CO2− ligands react via the 3σ-channel, whereas the electron-rich SR− and OH− ligands proceed by the 3δ-channel. With respect to reactivity, the 3δ-pathway has a comparable barrier to the 3π and 5π-pathways, which may offer a new approach for the specific control of C–H bond activation by the iron(IV)-oxo species.