Lewis Acid Promoted Hydrogenation of CO2 and HCOOâ by Amine

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Lewis Acid Promoted Hydrogenation of CO and HCOO by Amine Boranes: Mechanistic Insight from Computational Approach Lisa Roy, Boyli Ghosh, and Ankan Paul J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b03843 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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The Journal of Physical Chemistry

Lewis Acid Promoted Hydrogenation of CO2 and HCOO- by Amine Boranes: Mechanistic Insight from Computational Approach Lisa Roy†,* Boyli Ghosh and Ankan Paul* Raman Centre for Atomic Molecular and Optical Sciences, Indian Association for the Cultivation of Science, 2A and 2B, Raja S. C. Mullick Road, Jadavpur, Kolkata - 700032, India.

ABSTRACT: We employ quantum chemical calculations to study the hydrogenation of carbon dioxide by amine-boranes, NMe3BH3 (Me3AB) and NH3BH3 (AB) weakly bonded to a bulkier Lewis acid, Al(C6F5)3 (LA). Additionally, computations has also been conducted to elucidate the mechanism of hydrogenation of carbon dioxide by

Me3

AB while captured between one Lewis

Base (P(o-tol3), (LB)) and two Lewis acids, Al(C6F5)3. In agreement to the experiments, our computational study predicts that hydride transfer to conjugated HCO2‒, generated in the reaction of

Me3

AB-LA with CO2, is not feasible. This is in contrast to the potential hydrogenation of

bound HCO2H, developed in the reduction of CO2 with AB-LA, to further reduced species like H2C(OH)2. However, the FLP-trapped CO2 effortlessly undergoes three hydride (H‒) transfers from Me3AB to produce a CH3O‒ derivative. DFT calculations reveal that the preference for a H‒ abstraction by an intrinsically anionic formate moiety is specifically dependent on the electrophilicity of the 2 e‒ reduced carbon centre, which in particular is controlled by the

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electron-withdrawing capability of the associated substituents on the oxygen. These theoretical predictions are justified by Frontier Molecular Orbitals and Molecular Electrostatic Potential plots. The global electrophicility index which is a balance of electron affinity and hardness reveals that the electrophilicity of the formate species undergoing hydrogenation is twice the electrophilicity of the ones where hydrogenation is not feasible. The computed activation energies at M06-2X/6-31++G(d,p) closely predicts the observed reactivity. In addition, the possibility of a dissociative channel of the Frustrated Lewis Pair trapped CO2 system has been ruled out based on predominantly high endergonicity. Knowledge of the underlying principle of these reactions would be helpful in recruiting appropriate Lewis acids/amine boranes for effective reduction of CO2 and its hydrogenated forms in a catalytic fashion.

Introduction Unrelenting CO2 emission as an outcome of industrial revolution has proven to be one of the potential threats to the greenhouse effect in the earth’s atmosphere.1,2 Efforts are being channelized to reduce concentration of CO2 in the atmosphere through CO2 sequestration3 or its incorporation in the chemical industry as C-1 feedstock.4,5 Synthesis of value-added fine chemicals like carbonates and carboxylic acid derivatives, resultant of CO2 coupling reactions, are extensively considered due to low-cost and abundant supply of CO2.6-10 Furthermore methods are conceptualized for the hydrogenation of CO2 to benign usable chemicals like formic acid 11,12 or synthesis of methanol.13-17 Reduction of carbon dioxide to methanol is believed to be a promising route to achieve the carbon dioxide recycling process. Methanol is touted as a wellsuited raw material for synthetic hydrocarbons as well as an alternative energy resource.13-17 Unfortunately, in carbon dioxide carbon achieves its highest oxidation state (+4) which makes it a thermodynamically stable and kinetically inert compound. These factors are responsible for the


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limited chemical reactivity of CO2 which poses a major challenge in the development of efficient CO2 conversion processes.

Figure 1. Optimized geometry of intermediate 1 and 1a. Color code: C (black), H (white), Al (pink), P (orange), O (red), F (cyan), Cl (light green). Bond distances reported are in units of Å. The past few decades has witnessed a fervent surge in the development of catalysts for CO2 fixation and reduction. In parallel to heterogeneous catalysts, homogeneous transition-metal complexes have been reported for the reduction of CO2.18-27 Although, initial focus had been devoted to utilize noble metals,11,28 recent progress has been made in using first-row transition metals29 for the synthesis of formate and its derivatives. Notably transition metal free (or even metal free) examples to selectively functionalize CO2 are rare.30-37 Main-group mediated conversion of carbon dioxide into methanol is an attractive substitute for the fossil-fuel based energy crisis. Organocatalysts consisting of Lewis acids like aluminium species,38,39 silyl cations,40 organoscandium catalysts

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or ambiphilic systems comprising of boranes and

phosphanes42-45 are extensively utilized towards CO2 reduction. One such relevant and


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interesting study has been put forth by Stephan and co-workers where CO2 is trapped in P/Al based Frustrated Lewis acid base Pair (FLP) system and transformed to methanol derivative on hydrogenation with ammonia borane (NH3BH3, AB).34 It is noteworthy that such "metal-free" strategies provide a new straightforward techniques to transient capture and fixation of CO2.46-48 Recent experimental effort from the Stephan group has shown that a FLP.CO2 complex consisting of P(o-tol)3C(OAl(C6F5)3)2 (1, Figure 1) undergoes stoichiometric reduction to a formate derivative on exposure to Me3NBH3 (Me3AB) and further reduction to methoxy derivative under harsher conditions.49 Interestingly the authors also claim that reduction of carbon dioxide is possible in absence of Lewis base (P(o-tol)3, LB) with the intermediacy of an amine borane/alane adduct, NMe3BH3‒Al(C6F5)3, 2 and NH3BH3‒Al(C6F5)3, 2AB.49 Strong Lewis acids and bases used in classical FLP systems provide remarkable complexation of CO2. Earlier Musgrave et al. has propounded the beneficiary role of a Lewis base to increase the concentration of CO2 in the FLP.CO2 complex.50 It is therefore intriguing to evaluate the chemistry behind the reduction of carbon dioxide by 2, in absence of LB. Also the fact that altering the amine borane from Me3AB to AB changes the final product of hydrogenation makes it more intriguing, indicating that the amine boranes hydrogenate CO2 through different channels. In this report we therefore analyze the different reaction schemes undertaken for hydrogenation of CO2 by

Me3

AB and AB, in presence of Al(C6F5)3 (LA), and try to rationalize them from the

context of Frontier Molecular Orbitals, Molecular Electrostatic Potential and Global Electrophilicity Index. Additionally it was argued by the experimental group that the strongly complexed FLP-CO2 system (1, Figure 1) gets dissociated to of

Me3

Me3

AB-LA adduct on introduction

AB.49 Nevertheless, the entropic disfavor associated with release and recapture of CO2

should be considered which indicates possibility of different reaction mechanisms, apart from the


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dissociative pathway. In our previous report we elucidated how B-H activation of AB inititates the reduction of CO2 trapped by FLP (1a, Figure 1).51 In conjunction to our earlier mechanistic prediction, in our present article therefore we have explored the mechanistic intricacies of complex 1 towards

Me3

AB, involving a direct hydride attack by

Me3

AB.51 In an attempt to

correlate the hydrogenation of HCO2‒ complexed with different Lewis acids, generated in the reactions of 2 with CO2 or 1 with

Me3

AB, we predict that electrophilicity of the formate carbon

decides the possibility of further hydrogenation which in turn is governed by the substituents on the adjacent oxygen. A fundamental understanding of the unique reactivity of carbon dioxide and the underlying mechanism leading to the FLP-trapped and

Me3

AB-LA bound CO2 reduction

would provide valuable insights for development of improved Lewis acids/bases for CO2 reduction.

Computational Details Calculations were performed on the full structures of all the reported compounds and the quantum chemically identified species by using density functional theory within the Gaussian09 suite of programs. Geometries were obtained through optimization in gas phase without any symmetric or geometric constraints employing the M06-2X functional with the standard Pople 631++G(d,p) basis set. The M06-2X functional52 was developed specifically to encounter weak non-bonding interactions and has been found to correctly describe the mechanism of addition reactions of Frustrated Lewis Pair.53,54 Harmonic vibrational frequencies were calculated in the gas phase at the same level of theory to identify intermediates with no imaginary frequency and transition states (TS) with only one imaginary frequency. The vibrational data were used to relax the geometry of each transition state towards the reactants and products to confirm its nature. At


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the optimum geometry, a single point calculation was done in the solvent phase (bromobenzene solvent; ε=5.3954) employing the conductor-like polarizable continuum solvent model CPCM55,56 with M06-2X, ωB97x-D,57 and B3LYP-GD3/6-31++G(d,p)58,59 to correctly represent the dispersion effects. To incorporate empirically the D3 version of Grimme’s dispersion correction with the original D3 damping function within Gaussian09 D.01 version, the keyword “EmpiricalDispersion=GD3" has been used along with the functional, B3LYP. Furthermore, important intermediates and transition states corresponding to the rate determining barriers were re-optimized at ωB97x-D/6-31++G(d,p) and solvent effect was accounted with single point calculations with the respective functionals in conjunction to 6-31++G(d,p) and 6-311++G(d,p) basis sets. Mulliken charge analyses were performed at the M06-2X/6-31++G(d,p) level to assign atomic charges. Molecular electrostatic potential (MEP) plot provides a reasonable understanding of the positive and negative fragments of a molecule and is therefore a good marker for assessing the molecular reactivity. Calculation of MEP was also done at M06-2X/631++G(d,p) with the optimized geometry at the same level of theory. Global chemical reactivity parameter called electrophilicity index (ω) has been computed from combination of HOMO and LUMO values to determine the electrophilic power of various formate/formic acid species. It is defined as: ω = µ2/2η (1) where µ = - [IP+EA]/2 and η = [IP-EA]/2; IP = Ionization potential ≈ EHOMO and EA = Electron Affinity ≈ -ELUMO. Details about these computations have been provided in the supporting information.

Gibbs free energies and enthalpies were calculated within the harmonic approximation for vibrational frequencies. Gas phase enthalpies and Gibbs free energies are computed by adding enthalpic corrections and Gibbs free energy corrections respectively to the total energies at 298.15 K and 1 atm. However the Sackur-Tetrode equation used to obtain the entropy


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contributions in gas phase calculations employs the ideal gas model. Adding this entropy contribution to the solvent-phase single-point energies overestimates the total entropy of the molecule because the damping of translational and rotational degrees of freedom in solvent phase gets ignored. To compensate such a situation, solvent phase free energies were estimated at 298 K and 1 atm by using the solvent phase entropies, which were in turn obtained by empirically scaling gas-phase entropies by a factor of 0.5. However, for CO2(g) we have taken the gas phase entropy (1.0 scaling factor applied to entropy obtained from gas phase quantum chemical model). This empirical approach for obtaining solvent phase entropic corrections based on computations of gas phase entropies according to the ideal gas model has been widely used in other density functional studies of reaction mechanisms.60-64 It must be noted that application of the correction scheme does not overrule our proposed reaction mechanism. For validation we have provided enthalpic values in the profiles. ∆G indicates relative free energy and ∆H is the enthalpy in gas or solvent phase as stated. In the following text the reaction energies and barriers are discussed in terms of solvent phase free energies at the M06-2X/6-31++G(d,p) level, if not mentioned otherwise.

Results and Discussion To understand the origin of differences in the formation of different products in the reactions of CO2 with either amineborane/alane adduct or amine-borane/FLP, we first investigated the probable mechanistic scheme and thereby constructed the overall potential energy surface. This is followed by extensive analysis of the Frontier Molecular Orbitals generated from DFT calculations, charge analysis and electrophilicity parameter calculations.


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Reduction of carbon dioxide with Me3AB-LA adduct

Scheme 1. Proposed mechanism for the reduction of carbon dioxide to a trapped formate (4 and 5) by Me3AB-LA (2). Reduction of the generated formate by 2 is not found. The adduct (2) between

Me3

AB and LA is characterized by a conventional Lewis acid-base

complex between the hydridic hydrogen (B-H) of amine borane (Me3AB) and the Al centre of LA (Scheme1) forming a 3c-2e B-H-Al bond, reminiscent of σ-complexes between B-H and transition metals.65 The computational structural data shows good agreement with the


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Figure 2. Solvent phase free energy profile diagram and optimized geometries of the intermediates and transition states for the reduction of carbon dioxide with 2. Enthalpy values are given in parenthesis. Color code: C (black), H (white), Al (pink), O (red), B (yellow), N (dark green), F (cyan). Bond distances reported are in units of Å. experimentally reported values in the solid state.49 The optimized structure of 2 reveals an elongated B-H bond at 1.26 Å which is larger than the other B-H bond distances (~1.20 Å). Also the bridging hydrogen atom is at a distance of 1.76 Å (Figure 2) from the aluminum atom of LA. Due to the presence of this donor-acceptor interaction among B-H-Al triad, 2 is a very stable species with binding energy of -20 kcal/mol.49 Hydrogenation of carbon dioxide with 2 is initiated with the formation of a typical van der Walls complex 3 (Scheme 1), which is 6.8 kcal/mol higher in free energy. In this complex the carbon dioxide fragment is linear and is strategically placed between the Al centre and the nucleophilic hydrogen atom. In addition to B-


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H-Al 3c-2e coordination mode, complex 3 display two Lewis acid-base interactions: H(-)‒C and Al‒O. Similar intermediates displaying van der Waals interaction between CO2 and Lewis pair, (Me3C6H2)2P-CH2CH2-B(C6F5)2 has been earlier reported by Grimme and co-workers.66 The reduction of CO2 to bound HCO2‒ takes place sequentially with initial CO2 activation followed by the formate rearrangement as shown in Scheme 1 and Figure 2. In the transition state located for direct hydride transfer from B-H (of

Me3

AB) to the carbon (of CO2), CO2 binds

to Al(C6F5)3 in an η1-OCO mode with a O-Al distance of 1.92 Å. This type of weak intermolecular contacts between the O atoms of CO2 and a neighboring acceptor atom is typical in the coordination of CO2 to synthetic transition metal catalysts as well as bio-inspired complexes.67,68 For example in [NiFe]-CODHs the CO2 binds to Fe at a Fe-O distance of 2.05 Å.67 Related coordination of O to metals or Lewis acids is believed to facilitate attack by hydride to CO2. Compared to the unbound CO2, LA-coordinated CO2 display a significantly bent O-C-O angle of 151°, with one of the C-O(Al) bonds being elongated to 1.21 Å. The transition state geometry leads to a linear B‒H‒C arrangement with the B‒H bond distance being slightly elongated to 1.28 Å and C-H distance is equal to 1.54 Å. The calculated reaction energy profile predicts the free energy barrier (∆G‡S) for this hydride transfer (HT, computed Mulliken charge on B‒H is -0.166 e‒) to be 16.6 kcal mol-1 (see Figure 2) and the corresponding enthalpic barrier (∆H‡S) to be 9.1 kcal mol-1. ∆H‡S for TS1 at MP2/6-311++G(d,p) is 15.4 kcal mol-1 which is higher than an analogous B-H activation (of AB) to AlCl3-trapped CO2 earlier studied by Musgrave et al.50 The comparatively higher barrier for B-H activation by CO2 in the present work may be accredited to the geometric constraints around the C center due to the bulky Al(C6F5)3 group and the methyl substituents of Me3AB. This can be accounted by higher TS strain energy overcoming intermolecular interaction and has been discussed in details the supporting


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information (SI2). However, unlike typical four-membered hydroboration transition state as predicted by DiMare and others for reduction of aldehydes and ketones,69,

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in the present

mechanistic exploration we observe only a direct HT to the LA trapped CO2. The resultant intermediate is an asymmetrical formate species with B-O and Al-O covalent linkages to the same oxygen center. This is represented by intermediate 4 in the energy profile diagram (Figure 2). The reaction is exergonic by -17.3 kcal mol-1. In order to attain uniform disposition of the Lewis acids [NMe3BH2+ and Al(C6F5)3] around the HCO2‒ moiety, migration of LA from the bridging oxygen to the terminal oxygen is potent to generate the experimentally observed formate derivative 5. This is marked by crossing the transition state, TS2, at an intrinsic free energy activation barrier of 8.8 kcal mol-1 (Figure 2). The formate rebound step is significantly exergonic by -29.5 kcal mol-1. To be noted that geometrical isomer of intermediate 5 (i.e. 5') where the LA coordinates from the “back-face” of the formate has an equivalent energy as the one reported in the main text (see SI9 for details). In order to have an in-depth analysis of the efficiency of the catalytic channels in the reaction profile shown in Figure 2, the energetic span model (ESM) has been utilized which is a well-verified and simplified approach to determine the “speed” of individual chemical steps of a process characterized by several barriers and transient intermediates. The details of this concept have been discussed elsewhere70 and have been documented in brief in the supporting information (SI7). In short, we have determined the turnover frequency (TOF) of TS1 and TS2 by identifying appropriate turnover frequency determining transition state (TDTS) and intermediate (TDI). Our calculated TOF of CO2 activation via TS1 = 22.4 h-1 while that for formate rebound in TS2 = 98.3 h-1. The TOF ratio of TS2:TS1 ~ 4.4:1 shows that indeed CO2 activation is a rather difficult step as compared to the subsequent processes which has also been mentioned in previous accounts.71,72 The reverse


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reaction from 5 to yield CO2 and intermediate 2 has a calculated free energy barrier of ~ 46.0 kcal mol-1, which is consistent with the experimental observations of the substantial yield of the formate moiety (5) and suggests the irreversibility of the reaction at the experimental condition.49 Recently it has been shown that an ambiphilic system consisting of a β-diketiminato gallium complex is capable of activating H2 and thereby selective hydrogenation of CO2 to a methanol derivative.73 Interestingly, X-ray crystallography reveals that the formate bound gallium intermediate has one of the oxygen coordinated to the Lewis acidic metal species,73 in close proximity to the formate species theoretically delineated in this report. Next we tried to characterize HT of the experimentally observed formate derivative, 5, or the theoretically proposed unsymmetrical formate derivative, 4, in presence of intermediate 2. We identified intermediates 4a and 5a respectively where the incoming hydride (of

Me3

AB-LA) adduct is

sterically congested by the existing LA moieties coordinated to the formates. We found that the bulky Al(C6F5)3 groups on the HCO2- and

Me3

AB-LA sterically repel each other to a significant

extent to drift the donor-acceptor fragments apart (to a distance as large as 4.8 Å). An attempt to bring B-H in close proximity to the formate carbon through linear transit scan (SI8) resulted in asymptotic rise of the energy providing highly unstable (∆E > 30.0 kcal mol-1) trapped CH2O2 intermediates. Thereby despite several attempts we were unsuccessful in locating any feasible transition state or any probable post-transition state intermediate for hydrogenation of 4 and 5, where the B-H activation of Me3AB takes place to hydrogenate the formate directly bound to LA and NMe3BH2+. The reason for this behavior of intermediate 4 and 5 is the rather poor electrophilicity of sp2 C atom in the Al-O(B)C(H)O‒/Al-OC(H)O-B‒ moieties which has been rationalized in a later section.


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Reduction of carbon dioxide with AB-LA adduct

Scheme 2. Proposed mechanism for the reduction of carbon dioxide to a bound di-alcohol by AB-LA.


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Figure 3. Solvent phase free energy profile diagram and optimized geometries of the intermediates and transition states for the reduction of carbon dioxide with 2AB. Enthalpy values are given in parenthesis. Color code: C (black), H (white), Al (pink), O (red), B (yellow), N (dark green), F (cyan). Bond distances reported are in units of Å.


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Stephan et. al. has provided evidence (1H NMR analysis) in support of methanol and formic acid generation, in addition to a formate fragment, in a reaction involving 2AB with isotope labeled CO2.49 The reported higher reactivity of 2AB towards CO2 as compared to 2 intrigued us to identify the possible intermediates and transition states in the reaction of the former (Scheme 2). Like adduct 2, adduct 2AB also display a strong Lewis acid-base interaction between B-H and Al. In the presence of CO2, a van der Waals complex, 3AB is generated; the formation of which is endothermic by 5.7 kcal mol-1 in terms of free energy (Figure 3). The optimized geometry of 3AB is very similar to 3 with a weak non-bonding interaction between O and Al. As AB has an N-H proton, unlike

Me3

AB, one may fathom a concerted B-H and N-H attack to the C and O of CO2

respectively. However, initially we have found a LA triggered HT from AB to CO2 analogous to the predicted reduction of LA-OCO by

Me3

AB.50,51 We have located TS1AB where the B-H

hydride of AB is elongated to 1.25 Å from 1.18 Å in the ground state, with C—H distance of 1.65 Å (Figure 3). Akin to the HT in TS1, TS1AB shows a B-H activation from AB (Mulliken charge on B-H = -1.14 e‒) to CO2 at a free energy activation barrier of 18.2 kcal mol-1. TS1AB is followed by intermediate 4AB, the formation of which is thermo-neutral with regard to the ∆GS of the separated reactants. The optimized geometry of 4AB shows that the C-O bond distances are significantly elongated, while one N-H proton has a hydrogen bond with the terminal oxo (O2‒) group. We have found that the proton transfer (PT) can easily take place from N-H → OC(H)O‒ through TS2AB followed by release of NH2BH2 into the reaction medium and development of a formic acid derivative, 5AB. This indicates that our proposed mechanistic conduit and the related energetics are in agreement to the identification of a formic acid by experiments.49 Analyzing the nearly barrierless PT it can be presumed that protonation to oxygen from NH3BH2+ is facilitated in an urge to form the dehydrogenated product NH2BH2. It is known that NH2BH2 participate in


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oligomerization to develop into oligomers like (NH2BH2)n, lowering the total energy of the system.51,74 The transformation of 4AB to 5AB is also a thermo-neutral process in terms of ∆GS. Subsequently, we tried to evaluate the possibility of reduction of the bound HCOOH in presence of a second equivalent of 2AB. Our DFT calculations predict a concerted HT/PT where the B-H basic hydrogen (δ‒) of AB is transferred to the C center and the N-H acidic hydrogen (δ+) to the O center which is directly bound to the Al of Al(C6F5)3 (Scheme 2). This is represented by TS3AB in the free energy profile diagram of Figure 3 and entails a barrier of ~ 21 kcal mol-1. The possibility of simultaneous HT/PT to bound HCOOH with hydrogenation of the OH‒ group was found to be improbable due to involvement of a high activation barrier of > 30 kcal mol-1 along this reaction pathway (not shown in Figure 3). This is quite obvious from the perspective that the cleavage of the highly polarized C-O bond would entail high activation energy. TS3AB develops into a di-alcohol (7AB) in the forward direction which is a slightly endothermic species. Our calculations therefore suggest that binding of LA to HCOOH is favorable to lower the overall barrier for transformation of carbon dioxide to di-alcohol derivative. 7AB has potential sites for B-H and N-H attack, mechanism of which is not dealt in the present article. We presume that it would also involve a concerted HT/PT to the di-alcohol to generate methanol and water. Interestingly, our thorough investigation of the pathways revealed the involvement of the acidic N-H hydrogen of AB in the reduction of the formic acid derivative which makes the distinction with

Me3

AB quite clear. In fact we explored a similar case of PT preceding hydrogenation of

formate carbon in our previous effort.51


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Reduction of FLP-trapped carbon dioxide with Me3AB In our previous endeavor, we had computationally studied the reduction of FLP-trapped CO2 to methanol by AB.51 Our mechanistic study and the associated energy profile indicated plausibility of initial B-H activation followed by N-H activation for the reduction of trapped CO2, with concerted HT/PT for the last reduction step. Computational work by Musgrave et. al also suggested a HT from AB to the FLP bound CO2.50 However, Ménard and Stephan in their recent article claim that the initial reaction between 1 and Me3AB involves dissociation of the CO2 bound complex into the starting materials as shown in Scheme 3.49 Although the formation of the ion pair 11 following this principle is a slightly exoergic process, the initial de-complexation of 1

Scheme 3. Proposed mechanism for the dissociation of complex 1 in presence of

Me3

AB and

thereby recapture of CO2 to produce the formate species 5 and its corresponding ion pair 11 involving another

Me3

AB-LA adduct as shown in Reference 22. Relative free energy values (in

kcal mol-1) computed with M06-2X(CPCM)/6-31++G(d,p) are denoted by red (italics) numbers.


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Scheme 4. Proposed mechanism for three reductions of FLP trapped CO2 (1) to methoxy derivative by Me3AB.


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into components like 2, LB, LA and free CO2 molecule involves a prohibitively high free energy of 30 kcal mol-1 (similar free energy is obtained at ωB97XD/6-31++G(d,p)). This shows that releasing CO2 into the medium and then recapture of it is a thermodynamically unfavorable process. On the contrary direct B-H attack by AB has been shown to involve reasonable energy barriers and driving force which are in agreement to the experiments.49-51,71 This prompted us to identify the probable low barrier channel that might be conducive in the formation of 10 and the methanol derivative as observed by experimental techniques.49 We hypothesize that the mechanism identified for the reaction of FLP activated CO2 employing AlCl3 as the LA and PMes3 as the LB (1a) along with AB as the hydrogen source in our previous effort may be extended to the reaction of 1 with Me3AB.51 An analogous hydride migration B‒H → C (observed in previous instances like TS1 and TS1AB) would trigger the overall transformation of FLP-trapped CO2 to methanol derivative.50,51 In fact the geometry of the predicted transition state for B-H promoted (of Me3AB) reduction of 1 namely TS3 (Scheme 4) is similar to the HT from AB to the FLP-trapped CO2 (1a) studied earlier.50,51 The potential energy surface in Figure 4 of the stoichiometric reduction of FLPtrapped CO2 reveals that such a direct hydrogen transfer through TS3 ensues at a calculated barrier of 13.3 kcal mol-1, which is accessible at the experimental conditions and appropriately represents the reduction of trapped carbon dioxide to a formate species (7) (Scheme 4). From the optimized geometry of TS3 (Figure 4) we see that the activation of the trapped carbon dioxide by Me3

AB leads to a slight lengthening of P-C distance from 1.89 Å in complex 1 to 1.97 Å in the

TS. Additionally, we observe lengthening of the B···H and C···H bonds structurally comparable to the one predicted by Musgrave et al. with MP2//B97-D for the reduction of 1a with AB.50 In contrast, a free CO2 molecule undergoes


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Figure 4. Solvent phase free energy profile diagram and optimized geometries of the important intermediates and transition states for the reduction of 1 with Me3AB. Enthalpy values are given in parenthesis. Color code: C (black), H (white), Al (pink), O (red), B (yellow), N (dark green), F (cyan). Bond distances reported are in units of Å.


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hydroboration by a

Me3

AB species, and the resulting adduct is a formate derivative, H-CO2-

BH2NMe3, quite close to intermediate 5 (without the Al(C6F5)3 group). The corresponding transition state (TSaMe3AB, see SI6) is free energetically 57.8 kcal mol-1 above the starting materials, thus emphasizing the positive role of the FLP/LA systems on strong binding and activation of carbon dioxide. Contrary to HT mechanism followed by

Me3

AB, the uncatalyzed

reduction of CO2 by AB is channelized through concerted HT/PT from B-H and N-H to the carbon and oxygen of CO2 (TSaAB, see SI6) respectively with a ∆G‡S = 23.3 kcal mol-1 at M062X/6-31++G(d,p) level of theory. Zimmerman et. al has shown that an analogous reduction of free CO2 to CH2(OH)2 by AB occurs at a predicted free energy cost of 27 kcal mol-1 in THF employing CCSD(T) calculations.71 Furthermore, recent calculations by Li and others on the reaction of AB and CO2 utilizing the automated reaction finding program75 at CCSD(T)/aug-ccpVTZ level of theory indicates that the concerted proton and hydride transfer to CO2 from AB to form formic acid also occurs at the highest theoretically determined barrier.72 Interestingly the reduction of complex 1 with AB, however, undergoes a HT pathway marked by a paltry barrier of 6.1 kcal mol-1 which is in consensus to the observation that 1 is converted to the methoxy products in a brisk with sterically less crowded amine boranes like AB.49 An analogous initial reduction of FLP-CO2 complex 1a with AB is predicted to occur at a similar low barrier (6.5 kcal mol-1) by M06-2X calculations which is significantly lower than that predicted with B3LYP (~ 15 kcal mol-1).51 This observation furthers supports that B3LYP overestimates the activation barrier, indicated by Musgrave50 and Grimme76 previously and signifies the positive impact of inclusion of dispersion correction in analogous reaction mechanism study. This also validates the employment of M06-2X for optimization of these large systems which involve significant noncovalent interactions.


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Scheme 5. Conversion of intermediate 10 to the ion pair 11 via TS5. After the initial HT from Me3AB to 1 along the reaction path an intermediate 7 is located which corresponds to the dissociation of the P(o-tol)3 from the reduced (LA)2•HCO2‒ with a P····C distance of ~5.9 Å which is slightly exoergic with respect to intermediate 8 where the LB is removed completely. The optimized structure of 8 reveals non-covalent interaction between the NMe3-BH2+ cationic species with a C6F5 ring. Earlier IR spectroscopic analysis and ab initio calculations have confirmed identification of similar van der Waals complexes between carbonyl fluoride (COF2) and borontrifluoride (BF3).77 Intermediate 8 readily converts into the more stable adduct 9 (∆G(s)=-20.2 kcal mol-1),78 following a nucleophilic attack by the nearest formate oxygen onto the electrophilic B center of the insitu formed NMe3-BH2+. This involves a four membered transition state (TS4, ∆G‡S = 3.3 kcal mol-1, ∆H‡S = 11.2 kcal mol-1). 9 is an asymmetrically substituted HCO2‒ species where coordination of NMe3BH2+ to oxygen results in an increase in the O‒Al bond distances as compared to the ion pair 8 (see Figure 4). Along with the hydrogenation (from an incoming

Me3

AB) of the generated HCO2‒ in 9, we have also

investigated a possible hydride attack on the B atom of NMe3-BH2+ coordinated to oxygen. The upcoming sections would shed light on the details of the mechanistic approach undertaken.


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Introduction of

Me3

AB close to 9 (that is forming intermediate 10) is an exothermic process

(∆HS = -18.8 kcal mol-1 for 9 vs. -23.0 kcal mol-1 for 10). B-H bond activation of the approaching Me3

AB by the oxygen coordinated NMe3-BH2+ group leads to the rupture of the existing B-O

bond through TS5 at a free energy cost of 12.7 kcal mol-1 and an equivalent enthalpic cost. Formation of the symmetrical [NMe3BH2-H-BH2NMe3]+ cation is rather favorable which is reflected in the stability of the subsequent ion pair, 11 (Scheme 5), which has a ∆∆G(s)= -9.6 kcal mol-1 and ∆∆H(s) = -14.1 kcal mol-1 with regard to intermediate 9, in consensus to the experimental findings and enhances the reliability of our predicted mechanism. It must be mentioned over here that alike intermediate 4 and 5, attempts were made to identify HT from BH of Me3AB to formate C of 9 which failed. Alternatively, the B-H hydride of an incoming Me3AB in intermediate 10 is activated by the electrophilic formate carbon center. This is characterized by TS6 in which reduction of the formate carbon takes place at ∆G‡S = 6.6 kcal mol-1 to generate intermediate 12 (∆GS = -23.3 kcal mol-1, ∆HS = -28.0 kcal mol-1). After removal of hydrogen from Me3AB by HCO2‒ another NMe3BH2+ fragment is formed which has a weak interaction with the phenyl rings. The presence of a strong Lewis donor in the form of P(o-tol)3 in the medium indicate the possibility of formation of an adduct [P(o-tol)3–BH2NMe3]+, an experimentally observed entity along with development of an acetal derivative, 13.49 Formation of this Lewis acid-base adduct lowers the total energy significantly (∆GS= -37.4 kcal mol-1, ∆HS = -41.6 kcal mol-1 with regard to the initial separated reactants). The reduction pathway then continues with another Me3AB group attacking the acetal carbon in 14. A partial “acetal hydrolysis” is predicted for the transformation of 14 to 15 via transition state TS7 (see Figure 4). For all the reactions studied so far, we see that the

Me3

AB undergoes only B-H activation to trigger nucleophilic

substitution at B centers coordinated to oxygen or reduction of potential electrophilic carbon


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Page 24 of 52

centers through hydridic hydrogen transfer. This is understandable since the amine borane is devoid of any protic hydrogen. Unlike our previous study where concerted HT/PT from AB to an acetal derivative elicit "partial acetal hydrolysis", the present study highlights the sufficiency of a B-H hydride activation (of

Me3

AB) for cleavage of the C—O bond (Scheme 4).51 The optimized

geometry of TS7 obtained at M06-2X/6-31++G(d,p) level of theory reveals elongated C─O and C─H(B) bond distances at 2.75 Å and 1.74 Å respectively (Figure 4). This is attributed to polarization of the C atom resulting from coordination of the incoming B-H bond to C. However, unlike our previous endeavor where the initial HT to CO2 captured in FLP was found to be the RDS,51 here TS7 is predicted to be the rate determining step with a free energy of activation ~ 26 kcal mol-1. This indicates that 13 has a lower hydride affinity than 1. This also suggests that such hydride transfer leading to methoxy derivative 15 is accessible and favored at slightly elevated temperature (50—60°C), concurrent with the experimental observation for the reduction of formate to methoxy derivative.49 The current mechanistic approach undertaken for the stoichiometric reduction of 1 with

Me3

AB, following the probable low barrier conduits is

comparable to the reduction of 1 with AB, differing in the last reduction step which was earlier predicted to follow simultaneous two hydrogen transfer.51 Criteria for reduction of formate/formic acid derivatives In the current study we have computationally identified five formate intermediates, 4, 5, 9 and the ion pair 11, and one formic acid species, 5AB. It was interesting to find that only intermediate 5AB and 9 could undergo a second hydrogen abstraction from an approaching AB or

Me3

AB

respectively. Analysis of the Lowest Unoccupied Molecular Orbitals (LUMOs) corresponding to these intermediates as shown in Figure 5 display no significant contribution from the formate carbon center other than intermediate 9 and the HCO2H coordinated to a LA (5AB). This is


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probably due to curtailing of electrophilicity of the carbon center of a trapped formate. However for intermediate 9 it is observed that one of the oxygen centers is attached to a BH2NMe3+ fragment and a Al(C6F5)3 moiety. This is suggestive of the fact that a quenching of electron density around the carbon center is required which

Figure 5. Lowest Unoccupied Molecular Orbital (LUMO) of intermediate 4, 5, 11, 9, and 5AB. Apart from 9 and 5AB, the LUMO is devoid of any contribution from the C-centered orbitals in the intermediates. This displays the possibility of transfer of coupled two electrons/one proton, i.e. a hydride transfer to the target C. becomes possible in 9 since one of the oxygen centers are bonded to two Lewis acids. Interestingly, intermediate 5AB which has a proton and a Al(C6F5)3 group attached to each oxygen centers has significant contribution to the LUMO from the C 2p orbitals (Figure 5). To have a closer analysis of the selectivity we have mapped the Molecular Electrostatic Potential (MEP) of


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these five species. The MEP plot in Figure 6 reveals distribution of negative electrostatic potential (red coloured isosurface) on the flourinated phenyl rings and positive electrostatic potential (blue colored isosurface) on the NMe3BH2+ moeity. The formate carbon is in the borderline region (pale yellow colored isosurface) except for intermediate 9. Also the HCO2H group in 5AB is positively charged. The MEP mapping clearly shows that the formate carbon center in 9 and the formic acid species in 5AB is enveloped by a positive potential (Figure 6) and explains the reason for inclination towards acceptance of a partially negatively charged hydride from an incoming Me3AB/AB. Furthermore, this observation correlates our earlier

Figure 6. Molecular electrostatic potential surface in 4, 5, 11, 9, and 5AB. Value of isosurface = 0.02 a.u.. The color spectrum ranges from negative (red) to positive (blue) electrostatic potential within ± 1.000e-3 a.u.


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Table 1. Calculated global reactivity descriptors at M06-2X/6-31++G(d,p) in e.V Hardness (η)

Chemical potential (µ)

Global electrophilicity (ω)

4

2.20

-4.23

4.05

5

1.90

-4.23

4.71

11

1.89

-4.19

4.63

9

1.27

-5.09

10.20

5AB

1.38

-4.90

8.69

reported principle for the FLP-trapped-CO2 (1a) reduction where we have shown that a hydrogen bonding between a proton of NH3BH2+ and oxygen of formate is required prior to the second reduction process.51 To gain quantitative insights for the chemical reactivity and selectivity of the reduced species, density functional calculations were conducted to deduce reactivity parameters like hardness (η), chemical potential (µ) and global electrophilicity (ω). It is interesting to find that energy derived quantities like chemical hardness for 4 is higher than 9 by nearly 1 eV as found in Table 1. However, explicit plausibility of reactivity is provided by electrophilicity marker calculated from equation (1) (see SI1 for details) and provides a reasonable trend of chemical reactivity towards HT. To be noted that the above mentioned scheme yields global electrophilicity and hence reactivity trend as follows: 9 > 5AB > 5 ≈ 11 > 4. It is evident from the quantities (in Table 1) that 9 and 5AB are at least twice as prone to accommodate negatively charged incoming hydrogen as 5, 11 or 4. This is very interesting because apart from 5AB, all of the species consist of the HCOO- moiety with differential substituents and different degrees of strength of the Lewis acids (like Al(C6F5)3, BH2NMe3+ etc.) on the oxygen atoms. However, the


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Page 28 of 52

arrangement of the Lewis acids around the two oxygen centers is different which governs hydride affinity. Also the barrier for HT to 9 (via TS5, 12.7 kcal mol-1) and 5AB (via TS3AB, 21.2 kcal mol-1) is correlated to their predicted electrophilicity index, i.e., 10.2 and 8.69 eV respectively. A 1:1 combination of (like 4 & 5). But a 1:2 pairing of

Me3

Me3

AB and LA generates formates with low electrophilicity

AB and LA preferably forms a high electrophilic formate

moiety (like 9). Nevertheless, formation of ion pairs like 11 are inevitable is presence of excess Me3

AB. Since TS5 has 6 kcal mol-1 higher barrier than TS6 it can be argued that kinetically HT

would be preferred over ion-pair generation. This observation therefore provides a better quantified reasoning of the effect of substituents and the need to choose appropriate Lewis acids for incessant reaction and higher chemical yield.

Conclusion In summary, we have conducted a detailed computational analysis on reduction of carbon dioxide with Lewis acid/amine-borane adduct and reduction with simple amine-borane while trapped in a Frustrated Lewis Pair. Our computational mechanistic study has elucidated how the hydride rich amine borane

Me3

AB facilitates reduction of the CO2 fragment both in presence of

LA and the LB-LA pair. The overall transformation of 1 to methoxy derivative involves three hydride (partially negatively charged hydrogen) transfer consistent with the requirement of three equivalents of

Me3

AB in the experimental set up.49 A Frustrated Lewis Pair trapped carbon

dioxide reduction with amine-borane involves transfer of hydrogen from the borane part (of Me3

AB) to the carbon centre (of trapped CO2). This is in contrast to the hypothesis put forth

earlier that all the fragments of the FLP-CO2 system separates from the complex 1 in presence of an incoming NMe3BH3 to form the [Me3AB—Al(C6F5)3] adduct.49 We find that such a dissociative process would be unattainable under ambient conditions (∆Gs=30.0 kcal mol-1) and


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hence the basic principle of reduction of carbon dioxide in complexes 1 and 1a would be similar, that is an initial HT takes place from amine-borane to the CO2. Furthermore, the salient feature which enables hydrogenation of the formate group in 9 and formic acid in 5AB is the quenching of the electron donating capability of oxygen, thus enhancing the electrophilic character of the carbon center and enabling it susceptible to nucleophilic attack. This behavior of the formate species (4 and 5) formed after reduction of CO2 with amine-borane/alane adduct indicates that the need to incorporate higher excess of LA in addition to LA/Me3AB adduct for appropriate combination to carbon dioxide and its complete transformation. Although, LA/AB is sufficient for proper reduction of CO2, this procedure is plagued by the formation of aminoborane oligomers.74 Thus the selection of Lewis acids and bases being a tunable property for reduction of CO2, we therefore, anticipate that the mechanistic principles depicted in this article would be helpful in preparing more effective complexes to achieve catalytic reduction of CO2. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. Additional computational details, Prediction of Activation Strain Energy, Generation of anion intermediate 11, Comparison of DFT Computed Reaction Free Energies and Reaction Barriers, Atomic Partial Charges from the NBO calculations, Optimized geometry of TSaMe3AB and TSaAB, Energetic Span Model, Relaxed Surface Scans, Thermodynamics of Formates, XYZ Coordinates of important Intermediates and Transition States and References. AUTHOR INFORMATION Corresponding Author *Lisa Roy. E-mail: [email protected]; *Ankan Paul, E-Mail: [email protected]


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Present Addresses †

Department of Molecular Theory and Spectroscopy, Max Planck Institute for Chemical Energy

Conversion, Stiftstr. 34–36, D-45470 Muelheim an der Ruhr, Germany. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT L.R. would like to thank the Council of Scientific and Industrial Research (India) for Junior and Senior Research Fellowships. B.G. acknowledges a Junior Research Fellowship of the Inspire Fellowship Scheme of Department of Science and Technology (India). REFERENCES [1] Pachauri, K. R.; Reisinger, A. IPCC, 2007: Climate Change 2007: Synthesis Report, Contribution Of Working Groups I, II And III To The Fourth Assessment Report Of The Intergovernmental Panel On Climate Change. IPCC, Geneva, Switzerland, 2007, 104. [2] Solomon, S.; Plattner, G.-K.; Knutti, R.; Friedlingstein, P. Irreversible Climate Change Due To Carbon Dioxide Emissions. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 1704–1709. [3] Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. High-Throughput Synthesis Of Zeolitic Imidazolate Frameworks And Application To CO2 Capture. Science 2008, 319, 939-943.


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TOC Graphic


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Figure 1. Optimized geometry of intermediate 1 and 1a. Color code: C (black), H (white), Al (pink), P (orange), O (red), F (cyan), Cl (light green). Bond distances reported are in units of Å. 222x126mm (150 x 150 DPI)



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Scheme 1. Proposed mechanism for the reduction of carbon dioxide to a trapped formate (4 and 5) by Me3AB-LA (2). Reduction of the generated formate by 2 is not found. 283x286mm (150 x 150 DPI)


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Figure 2. Solvent phase free energy profile diagram and optimized geometries of the intermediates and transition states for the reduction of carbon dioxide with 2. Enthalpy values are given in parenthesis. Color code: C (black), H (white), Al (pink), O (red), B (yellow), N (dark green), F (cyan). Bond distances reported are in units of Å. 280x176mm (150 x 150 DPI)



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Scheme 2. Proposed mechanism for the reduction of carbon dioxide to a bound di-alcohol by AB-LA. 271x302mm (150 x 150 DPI)


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Figure 3. Solvent phase free energy profile diagram and optimized geometries of the intermediates and transition states for the reduction of carbon dioxide with 2AB. Enthalpy values are given in parenthesis. Color code: C (black), H (white), Al (pink), O (red), B (yellow), N (dark green), F (cyan). Bond distances reported are in units of Å. 483x286mm (150 x 150 DPI)



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Scheme 3. Proposed mechanism for the dissociation of complex 1 in presence of Me3AB and thereby recapture of CO2 to produce the formate species 5 and its corresponding ion pair 11 involving another Me3AB-LA adduct as shown in Reference 22. Relative free energy values (in kcal mol-1) computed with M06-2X(CPCM)/6-31++G(d,p) are denoted by red (italics) numbers. 255x153mm (150 x 150 DPI)


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Scheme 4. Proposed mechanism for three reductions of FLP trapped CO2 (1) to methoxy derivative by Me3AB. 350x406mm (150 x 150 DPI)



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Figure 4. Solvent phase free energy profile diagram and optimized geometries of the important intermediates and transition states for the reduction of 1 with Me3AB. Enthalpy values are given in parenthesis. Color code: C (black), H (white), Al (pink), O (red), B (yellow), N (dark green), F (cyan). Bond distances reported are in units of Å. 434x256mm (150 x 150 DPI)


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Scheme 5. Conversion of intermediate 10 to the ion pair 11 via TS5. 296x106mm (150 x 150 DPI)



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Figure 5. Lowest Unoccupied Molecular Orbital (LUMO) of intermediate 4, 5, 11, 9, and 5AB. Apart from 9 and 5AB, the LUMO is devoid of any contribution from the C-centered orbitals in the intermediates. This displays the possibility of transfer of coupled two electrons/one proton, i.e. a hydride transfer to the target C 279x194mm (150 x 150 DPI)


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Figure 6. Molecular electrostatic potential surface in 4, 5, 11, 9, and 5AB. Value of isosurface = 0.02 a.u.. The color spectrum ranges from negative (red) to positive (blue) electrostatic potential within ± 1.000e-3 a.u 342x251mm (150 x 150 DPI)



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TOC 85x56mm (150 x 150 DPI)


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Lewis Acid Promoted Hydrogenation of CO2 and HCOOâ by Amine

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