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Catalytic Hydrogenation of CO2 by Fe-Complexes Containing Pendant Amines: Role of Water and Base Kuber Singh Rawat, Arup Mahata, and Biswarup Pathak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09333 • Publication Date (Web): 14 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016

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The Journal of Physical Chemistry

Catalytic Hydrogenation of CO2 by Fe-Complexes Containing Pendant Amines: Role of Water and Base Kuber Singh Rawat, † Arup Mahata, † Biswarup Pathak †,#,* †

Discipline of Chemistry and #Discipline of Metallurgy Engineering and Material Science, Indian

Institute of Technology (IIT) Indore, Indore. Madhya Pradesh 453552, India Email: [email protected] ; Telephone: +91-731-2438-772.

Abstract: The role of the outer sphere ligand is reported to be very crucial toward the hydrogenation of CO2. We have investigated a series of Fe-complexes with flexible pendant amines and predicted their potentials for CO2 hydrogenation to formic acid using density functional theory calculations.

Among

the

modelled

Fe-complexes,

the

Fe-complex

[Cp Fe(P N )] containing the electron-withdrawing Cp ligand is the most active catalyst with a total free energy barrier of only 14.9 kcal/mol. Besides, we find that the rate determining steps (heterolytic cleavage and hydride transfer) of CO2 hydrogenation are

improved significantly when catalysed by the pendant amine based [Cp Fe(P N )] complex. The roles of water and base found to be very crucial for the whole catalytic cycle. The ligand-assisted pathway is very favourable toward one of the rate-determining steps and therefore such flexible outer sphere ligands are crucial for the designing of catalyst for CO2 hydrogenation. 1. Introduction: The catalytic conversion of CO2 into useful products (HCOOH, CH3OH, CO, and CH4) is highly desirable as CO2 is a promising feedstock for one–carbon building block based organic products.1 1 ACS Paragon Plus Environment


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One promising way for energy storage and transportation is to convert gas into liquid, which is then converted back in times for low renewable electricity production. 2-4 Hydrogenation of CO2 to energetically richer molecules like CO, CH3OH, and HCOOH is one possible way to store energy and having the additional advantage of closing the carbon cycle.5-10 Among all the products, formic acid (HCOOH) has more advantages compared to others due to its nontoxicity and ease of transportation. Besides, it remains liquid under ambient conditions, which is very helpful for the direct formic acid fuel cell (DFAFC) applications.11 Therefore, the development of a molecular catalyst for the inter-conversion of chemical to electrical energy has attracted an increasing attention due to their flexible nature. The catalytic properties of a molecular catalyst can be tuned by changing the nature of the ligands.12 Over the last three decades, many highly active homogeneous transition metal-based catalysts have been experimentally13-19 and theoretically 20-23 reported for the catalytic conversion of CO2 to formates and formic acid. However, in all these reports metals are directly involved in the catalytic process, whereas ligands act as a spectator. Recently, the role of the outer sphere ligand found to be very crucial for the CO2 hydrogenation reaction. Hazari and co-workers developed a series of Ir(III)PNP pincer (PNP = 2,6-bis(di-alkylphosphinomethyl)pyridine) complexes for the CO2 hydrogenation reaction where the secondary coordination sphere interactions facilitate the CO2 insertion reaction.24 Milstein and co-workers reported the activation of CO2 by the Ru-PNP complex [Ru(PNPtBu)(H)(CO)] via metal-ligand cooperation mechanism.25 Recently, Pidko and co-workers reported the Ru-PNP catalyst for CO2 hydrogenation reaction with a TOF of 1892000 h−1 132°C, where the metal-ligand facilities the cooperative CO2 hydrogenation. In fact, this is one of the highest activity reported to date for the CO2 hydrogenation to formate.26-28 Later, Pidko and co-workers reported a lutidine derived bis-N-heterocyclic carbene (NHC) ruthenium CNC−pincer

2 ACS Paragon Plus Environment

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complex for the metal−ligand based cooperative CO2 hydrogenation.29-30 In another study, Milstein and co-workers reported a family of iron complexes with pyrazine-based pincer ligands for the CO2 hydrogenation.31 Some of the earlier studies reported that the PNP ligand does not actively participate toward the heterolytic H2 cleavage and thus an external base is required for the heterolytic H2 cleavage.20, 32 Thereby, the metal-ligand cooperation mechanism does not play an active role in the rate-determining steps; heterolytic H2 cleavage and hydride transfer steps. Therefore a strong donor is required within the ligand sphere, which can act as a pendant base to promote the H2 cleavage. Bernskoetter and co-workers reported a family of iron (II) carbonyl hydride complexes supported by a PNP based ligand containing a secondary/tertiary amine, which show remarkable CO2 hydrogenation activity based on the metal-ligand cooperation mechanism.33 Himeda et al. reported a water soluble Cp*Ir(6,6′-R2-bpy)(OH2)]SO4 (bpy = 2,2′-bipyridine, R = OH) complex for the CO2 hydrogenation and suggested that the hydroxyl groups at 6,6′ positions on 2,2′-bipyridine enhance the hydrogenation rate remarkably while compared to the hydroxyl groups at 4,4′ position. The hydroxyl groups at 6,6′ positions act as pendant base and facilitates the hetereolytic H2 cleavage. 34 Thereby, the role of the outer sphere ligand toward any catalytic reaction is very important and demands for extensive theoretical investigations to get more insights into the whole catalytic cycle.35-38 Recently, Hou et al. reported a detailed computational study on the Co, Rh and Ir bipyridine complexes using hydroxyl substitutions at 6,6′ positions on the bipyridine ligand and concluded that the heterolytic H2 cleavage is the rate-determining step.39 Similarly, Yang et al. computationally designed a series of iron complexes with acylmethylpyridinol based PNP ligands and demonstrated that the acylmethylpyridinol ligand acts as a pendant base and promotes the heterolytic H2 cleavage by lowering the activation barrier.40 In another study, Yang et al.



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investigated the pendant amines based Fe-complexes (PtBu2NtBu2)FeH(CO)2R, similar to the active site structure of [FeFe]-hydrogenase for the CO2 hydrogenation to methanol.41 In the CO2 hydrogenation reaction, the heterolytic H2 cleavage and hydride transfer are the rate-determining steps depending on the reaction conditions.22, 23, 35, 39, 42 However in most of these reports, the nature of the pendant base is rigid and therefore not enough flexible to facilitate the heterolytic H2 cleavage. So, it is important to have a flexible pendant base to promote the heterolytic H2 cleavage. Recently, Bullock and co-workers reported a family of Fe, Mn, Ni and Co metal-based complexes with flexible pendant amines that improve the heterolytic H2 cleavage dramatically. 4350

The significance of the pendant amines is its close proximity to the vacant site of the metal

center. The heterolytic H2 cleavage and the transfer of protons between the metal center and solution (through a proton channel by forming a M−Hδ−···Hδ+ −N moiety) are the key features for such improved activity.51 Further, they exhibited that the addition of water molecules improves the H2 oxidation activity significantly. 52-53 Therefore, inspired by all these findings, a detailed mechanism for the hydrogenation of CO2 to formic acid is predicted and explored through density functional theory (DFT) calculations. We have designed the homogeneous earth-abundant Fe metal-based catalysts for CO2 hydrogenation reaction with flexible pendant amines. A series of Fe-complexes are modelled by tuning the electronic properties of the Cp ligand. The roles of water and external base are systematically studied toward the proton abstraction mechanism. Besides, the roles of the pendant amines toward the heterolytic H2 cleavage and the hydride transfer mechanism are investigated in detail for designing an efficient outer sphere ligand based molecular catalyst for CO2 hydrogenation reaction.


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2. Computational Details: All the calculations are performed with the Gaussian 09 package54 using the hybrid exchangecorrelation functional PBE1PBE.55 The high reliability of this functional has been demonstrated by previous benchmark studies on various systems.26-27, 56-57 The full electron 6-311G(d,p) basis set58-59 is used for non-metals (C, H, O, N and P) and LANL2DZ basis set is used for iron.60-61 All the structures are optimized in THF solvent using the polarizable continuum model (PCM) model62 as implemented in G09 and frequency calculations confirm that the structures are minimum energy structures in the potential energy surfaces. Further the accuracy of the basis sets have been tested on some of the selected reaction (heterolytic cleavage and hydride transfer) steps by using a larger triple-zeta + polarization quality basis set Def2-TZVPP for Fe63 and 6311+G(d,p) basis set for the non-metals (C, H, O, N and P). Similarly, the accuracy of the functional is tested using M06 functional

64

for those selected steps. The calculated activation

barriers using different functional (M06) and basis set [Def2-TZVPP for Fe and 6-311+G(d,p) for light elements] agree well (Table S1) within the range of 1-2 kcal/mol as obtained by using the standard methodology. We have optimized all the three spin states (singlet, triplet and quintet) for all the intermediates and we find that the singlet is the ground state for most of the intermediates

except

for [Cp Fe(P N )] ,

[CpFe(P N )] ,

and

[Cp Fe(P N )] , where triplet is the ground state. Furthermore, we have used different functional such as UB3LYP65-67 functional to verify our results and we find that our calculated results using PBE1PBE are in good agreement with UB3LYP level of theory. Thus, there will be a transition state crossing the potential energy surface of these two states. However, the reaction will proceed from the singlet state. Thus, we have used the energy corresponds to singlet state for all the complexes.



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Harmonic vibrational frequencies are calculated to confirm the local minimum energy structures. The transition states are characterized by only one imaginary vibrational frequency, corresponding to the eigenvector along the reaction path. The reaction free energies (∆G) are calculated from the energy difference between the final state and intial using the results of the normal-mode analysis within the ideal gas approximation at a pressure of 1 atm and temperatures of 298 K, whereas free enegy barries (∆ ‡ ) are calculated from the energy difference between the transition state and initial state strcutures. Zero-point energy and entropy contributions are included in our free energy calculations (at 298.15 K). IRC calculations have been performed to confirm the transition state structure. The proton affinity of the iron complexes has been studied by calculating their pKa values in THF solvent (see details in the Supporting Information).68

3. Results and Discussion: Bullock and co-workers have synthesized a series of pendant amine based Fe(II) complexes and

reported a pendent amine based Fe-complex [Cp Fe(P N )Cl] as one of the most efficient catalysts for H2 oxidation.44 Besides, they concluded that the heterolytic H2 cleavage and Fe-H bond formation are very much favourable when catalyzed by pendant amines based complexes. In another report, they demonstrated a more active Fe catalyst with a C5F4N ligand in the Cp ring for H2 oxidation.45 However, we feel that such an electron deficient ligand CpC5F4N may not be good enough for hydride transfer reaction.23,69-70 Thus, inspired by these findings, we believe that the pendant amines based catalyst with a C6F5 ligand in the Cp ring could be a very promising catalyst for CO2 hydrogenation reaction as the heterolytic H2 cleavage and metalhydride bond formation are the two most important steps in the CO2 hydrogenation reaction. Bullock and co-workers demonstrated that the role of the Cp ligand is very important toward H2


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oxidation reaction. Keeping this in mind, we have modelled a series of pendant amines based iron(II) complexes by changing the electronic property of the Cp ligand: (i) 1

!"

i. e. [Cp Fe(P N )] , (ii) 1-Cp i.e. [CpFe(P N )] , and (iii) % &

'(" i. e. [Cp Fe(P N )] (Figure 1). We have chosen the P N ligand in

place of P N because Bullock and coworkers71 reported that the catalytic activity toward

heterolytic H2 cleavage increases when the P N is substituted by a P N ligand. All these ligands satisfy the 18-electron rule in the complexes. Here, we have systematically investigated the role of the pendant amine toward the CO2 hydrogenation reaction. The catalytic investigations are divided into two parts. In the beginning, the heterolytic H2 cleavage is discussed followed by hydride transfer mechanism.

Figure 1. Fe(II) complexes considered for CO2 hydrogenation reaction.

3.1 Heterolytic H2 Cleavage: Scheme 1 shows the predicted catalytic cycle for the heterolytic H2 cleavage, which is initiated either by the pendent amine or by the external base 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU). Figure 2 and 3 show the corresponding transition states structures and free energy profile. Intermediate 1 has a vacant site (Scheme 1) for coordination, where H2 molecule can bind for the formation of a Fe-H2 σ-complex 2.The possibility of DBU binding at the vacant site of the



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complex is ruled out due to the bulky size of tert-butyl groups and the electron withdrawing nature of the Cp ring. Similarly, experimental report shows that the water favours binding to the vacant site of metal centre over H2 binding.44-45 However such binding depends on the presence of different phosphorus substituents such as phenyl and t-butyl. Phenyl substituent favours the water binding, whereas t-butyl favours the H2 binding.52 Thus, we have not considered water and DBU binding to the vacant site of the metal complex. The formations of Metal-H2 σ-complexes are calculated to be exergonic by -5.5, -4.0 and -0.4 kcal/mol for 2-

!"

, 2-Cp, and 2-'(" ,

respectively. Therefore, the presence of an electron-withdrawing group favours the H2-adduct formation. Then the catalyst would initiate the heterolytic H2 cleavage for the formation of a metal-hydride complex. However, it requires a base to initiate the heterolytic H2 cleavage (Scheme 1). Earlier studies reported that they have used an external base for the heterolytic H2 cleavage (2→4, Scheme 1). However, the heterolytic H2 cleavage step can be initiated through an outer sphere ligand interactions (2→3, Scheme 1) followed by a proton abstraction (3→4) using an external base. Here, the pendent amines present in the outer coordination sphere assist the heterolytic H2 cleavage (2→3).

Scheme 1: Predicted catalytic steps for the heterolytic H2 cleavage.


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The heterolytic cleavage initiated by a pendant amine forms intermediate 3 (Fe−Hδ−···Hδ+−N) via the transition state TS1 (Figure 2). The Hδ−···Hδ+ distances are 1.42, 1.44 and 1.36 Å in 3

!"

, 3-Cp, and 3-'(" complexes, respectively. The free energy barriers (Figure 2) for the

heterolytic H2 cleavage are 3.0, 6.9 and 4.6 kcal/mol for 2-

!"

, 2-Cp, and 2-'(" ,

respectively. Furthermore, we have calculated H2 activation barrier (2→3) when assisted by a water molecule. The calculated water assisted H2 activation barriers are 19.8, 22.5 and 18.9 kcal/mol when catalyzed by 2-

!"

, 2-Cp, and 2-'(" complexes, respectively. Thus, the

calculated activation barriers are higher than the pendant amine assisted H2 activation barriers of 3.0, 6.9 and 4.6 kcal/mol for 2-

!"

, 2-Cp, and 2-'(" , respectively. It indicates that water

dose not play important role toward H2 activation. However, the heterolytic H2 cleavage is very much favourable in the presence of an outer sphere pendant amine ligand and the activation barrier is lowest in the presence of an electron-withdrawing Cp ligand. Once 3 is formed, the proton abstraction (Hδ+) is very important to complete the catalytic cycle (Scheme 1). Thereby, an external base DBU is modelled for our proton abstraction study. DBU is considered as it is reported to be an efficient base for the CO2 hydrogenation reaction.[26-27,30] So, DBU can directly abstract the proton either from 2 or from 3 for the formation of 4 and intermediates with DBU are given in supporting Information. The activation barriers (TS2; Figure 2) for the direct proton abstraction (2→4) are 17.3, 18.7 and 21.6 kcal/mol for 2-

!"

2-Cp, and 2-'(" , respectively. However, we could not locate the TS for the direct proton abstraction from 3. The proton abstraction from 3 might be very difficult from such complexes as the proton is hindered due to the presence of the bulky groups (tBu) in the complex. [52-53] This could be the reason for high activation barrier for the direct proton abstraction. However, Bullock and co-workers reported that the addition of water accelerates the proton relays between



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the Hδ+−N and solvent through a base.

[52-53]

Therefore, we have investigated the proton

abstraction mechanism with the help of a water molecule through the transition state TS3 (Figure 2). The free energy barriers (3→4) for the water mediate proton abstraction are 2.9, 6.6 and 4.9 kcal/mol for 3-

!"

, 3-Cp, and 3-'(" , respectively. The proton abstraction trend (from 2

and 3) can be explained based on the proton affinity of the metal complexes. Our calculated pKa values for 2 are 11.5, 17.2 and 16.8 for 2- that 2-

!"

!"

, 2-Cp and 2-'(" , respectively, suggesting

has a lowest proton affinity among all the three iron complexes. Therefore, the

proton abstraction is easier from 2- pKa value is lower in 3-

!"

!"

than 2-'(" and 2-Cp, respectively. Similarly, the

(10.3) than in 3-'(" (14.3) and 3-Cp (14.8), respectively. This

indicates that the electron-withdrawing group (-C6F5) in the Cp ring favours the heterolytic H2 cleavage and proton abstraction steps, which is very much consistent with previous experimental reports.44-45, 50 Hence, the water mediate proton abstraction is much more favourable for all the three complexes. This could be due to the flexibility nature of the pendant amine.51 Further, the heterolytic H2 cleavage followed by hydride transfer is very much favourable for the Fe-complex containing an electron-withdrawing

!"

ligand with an overall reaction free energy of -8.4

kcal/mol. More importantly, the combination of an outer sphere pendant amine ligand with an electron-withdrawing

!"

ligand facilitates the whole process significantly and such

combination deserves attention for further studies.


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Figure 2: Optimized transition state structures (TS1, TS2 and TS3) of Scheme 1. Here (a)

!"

, (b) Cp, and (c) '(" are the different types of Cp ligands in the Fe-complexes. Bond

lengths are in Å. White, skyblue, blue, red, lime green, green and orange colours denote the hydrogen, carbon, nitrogen, oxygen, fluorine, phosphorous and iron atom respectively.



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Figure 3: Reaction free energy profile for the heterolytic H2 cleavage and proton abstraction in Fe-complexes (Scheme 1).

3.2 Electronic Effect on Heterolytic H2 Cleavage: As we all know, electronic factor plays a major role for the designing of active catalysts. Thus, the electronic effects are investigated (Figure 4) based on the position of frontier molecular orbitals (HOMO and LUMO) as we change the electronic nature of the Cp ring. We find that the HOMO is highly stabilized in the presence of an electron withdrawing C6F5 ligand (1-

!"

),

whereas least stabilized in the presence of an electron donating ligand (1-'(" ). Further, we find that the HOMO−LUMO gap is maximum in 1-Cp (5.28 eV) and minimum in 1- 12 ACS Paragon Plus Environment

!"

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(4.54 eV). This indicates that 1-

!"

complex is more active than 1-Cp and 1-'(" toward

heterolytic H2 cleavage. As LUMO of iron complex would interact with the HOMO of H2 molecule, therefore the position of the LUMO is important for the heterolytic H2 cleavage. The LUMO is most stabilized in the presence of an electron withdrawing ligand (1-

!"

), whereas

least stabilized in the presence of an electron donating ligand. Therefore, the HOMO−LUMO gap and the position of LUMO favors H2 cleavage in 1-

!"

than the other complexes (1-Cp

and 1-'(" ). Furthermore, the dz2 orbital (Figure 4) represents the LUMO of the metal complex, which behaves as an acceptor and ready to accept σ-electrons from the H2 molecule.

Figure 4. Molecular orbitals of 1-'(" , 1-Cp and 1-

!"

complexes. The orbital energies are

in eV.



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3.3 CO2 Hydrogenation Further, the hydride transfer mechanism is investigated, as the pendant amine plays an important role for the heterolytic H2 cleavage, which is the first important step for CO2 hydrogenation reaction. Thus, after the H2 heterolytic cleavage, the incoming CO2 molecule can react with the metal-hydride the formation of formic acid. Scheme 2 shows the predicted catalytic cycle for the hydrogenation of CO2 to formic acid. Figure 5 and Figure 6 show the related transition state structures and free energy profile, respectively. We have considered both the intermediates 3 and 4 could react with an incoming CO2 molecule though the formation of 4 is highly favourable in the presence of water and base molecules. In case of 3, the CO2 molecule can interact with the hydridic (M−Hδ−) and protic (Hδ+ −N) hydrogens present in the complex. So here we have studied whether such a metal-ligand cooperation mechanism is helpful for the formation of a formic acid in the pendant amines based complexes or not?

Scheme 2: Predicted catalytic pathways for the CO2 hydrogenation.


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The calculated free energy barriers (Figure 6) for the formation of 5 are 23.3, 22.0 and 16.9 kcal/mol for 3-

!"

, 3-Cp, and 3-'(" , respectively. Therefore, the activation barrier is

significantly high for such metal-ligand cooperation mechanism. Once 5 is formed, the formic acid will be released for the formation of 1 and the free energies for this step (5→1) are -9.5, -9.8 and -13.6 kcal/mol for 5-

!"

, 5-Cp, and 5-'(" , respectively. Besides, the reaction free

energy profile suggests (Figure 6a) that the overall reaction free energy is high (7.3 kcal/mol) for the outer spheres ligand based CO2 hydrogenation reaction. Therefore, we predict that this outer sphere mechanism is not favourable for the hydride transfer mechanism in the pedant amine based CO2 hydrogenation reaction. On the other hand, CO2 can react with 4 for the formation of a H-bound metal formate [Fe-(η1HCOO] (6) via the transition state TS5 (Figure 5). Here, the overall reaction free energy (0.2 kcal/mol) suggests that the reaction might be proceeding through the following intermediates (4→6→1 and 4→7→1). The free energy berries for the formation of 6 are 14.9, 12.7 and 11.4 kcal/mol for 4-

!"

, 4-Cp, and 4&'(" , respectively. Consequently, the H-bound formate

would dissociate into 1 to complete the catalytic cycle. The reaction free energies for the formate dissociation are exergonic by -6.1, -13.3 and -11.8 kcal/mol for 6-

!"

, 6-Cp, and 6-'(" ,

respectively. Alternatively, the H-bound formate 6 can rearrange itself to a O-bound metal formate intermediate 7 (Fe-(η1-OCHO) through the TS6 (Figure 6). This would be followed by the dissociation of O-bound formate. The calculated free energy barriers for the formation of Obound formate are 12.2, 7.3 and 10.4 kcal/mol for 6-

!"

, 6-Cp, and 6-'(" , respectively.

The associated free energy changes for this step are exergonic by -9.9, -11.2 and -9.8 kcal/mol for 6-

!"

, 6-Cp, and 6-'(" , respectively. The reaction free energies for the formate

dissociation from 7 are 3.8, -2.1 and -2.0 kcal/mol for 7-

!"

, 7-Cp, and 7-'(" respectively.



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Although our reaction free energy and activation barrier values suggest that there is a possibility for the formation of O-bound formate, however this reaction is not favourable as our free energy profile shows that the reaction has to go through the high energy (Figure 6b) states. Therefore, we find that the dissociation (6→ →1) of H-bound formate ion is favourable over the formation of O-bound formate (6→ →7).

Hence, our overall reaction free energy profile (Figure 6a) suggests that the formation of formic acid is not favourable via outer sphere ligand mechanism. However, the reaction is favourable via intermediate 4.


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Figure 5: Optimized transition state structures (TS4, TS5 and TS6) for Scheme 2. Here (a)

!"

, (b) Cp, and (c) '(" are the different types of Cp ligands in the Fe-complexes. Bond

lengths are in Å. White, skyblue, blue, red, lime green, green and orange colours denote the hydrogen, carbon, nitrogen, oxygen, fluorine, phosphorous and iron atom respectively.



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(a)

)

+

(b)

)

+

Figure 6: Free energy profile for scheme 2 for the hydrogenation of CO2 to formic acid.


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Further, we have done molecular orbital (Figure 7) analysis for 3 and 4 to understand the hydride transfer mechanism. We predict that the Fe-H σ bonding orbital will interact with the π* orbital of the CO2 for the hydride transfer mechanism. However, the highest lying occupied molecular orbitals (HOMO, HOMO-1, and HOMO-2) of the complexes have different orbital symmetries and thus they do not represent the Fe-H σ bonding orbital. Besides, the low-lying molecular orbitals do represent the Fe-H σ bonding. On the other hand, the LUMO of CO2 is a π* orbital with 1.33 eV of energy. Thus, the LUMO of CO2 is electrophilic in nature and ready to accept the hydride from Fe-H complexes. On the other hand, the Fe-H σ orbital energies are 8.24 (HOMO-7), -8.12 (HOMO-6) and -7.91 eV (HOMO-6) in 3-

!"

, 3-Cp, and 3-'(" ,

respectively. Therefore, the energy differences between the LUMO of CO2 and HOMO of Fe-H are 9.57, 9.45 and 9.24 eV in 3-

!"

, 3-Cp, and 3-'(" , respectively. Thereby, the electron

donating Cp ligand favours the hydride transfer mechanism. This is very much in consistent with our calculated activation barriers for the hydride transfer process. The energy differences between the LUMO of CO2 and Fe-H σ bond are 8.66, 8.53 and 8.31 eV in 4-

!"

, 4-Cp, and

4-'(" , respectively and the associated free energy barriers for hydride transfer are 14.9, 12.7 and 11.4 kcal/mol, respectively. Thereby, the energy differences are less in 4 compared to in 3. So, our detailed molecular orbital studies and calculated energy barriers confirm that 4 favours hydride transfer over 3. Thus, we predict that the hydride transfer mechanism is favourable due to the presence of the electron-donating group (Me) in the Cp ring. This is opposite to the heterolytic H2 cleavage, where electron-withdrawing group favours the heterolytic H2 cleavage.



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Figure 7: The energy difference (in eV) between the Fe-H σ orbitals and π* orbital (LUMO) of the CO2 molecule.

Thereby, the hydride transfer step is favourable in the presence of an electron donating ligand. It indicates that the electron-donating group present in the Cp ring increases the electron density at the metal centre, which in turn increases the hydricity (hydride donation nature) of the Fe-H bond. In the whole catalytic cycle, the hydride transfer rates are slower than the heterolytic cleavage and thus hydride transfer will be the rate-determining step (RDS). Further, in our free energy profile shows that if the reaction proceeds through 1→2→3→4→6→1 intermediates, then the CO2 hydrogenation reaction is highly favourable and the heterolytic H2 cleavage and hydride transfer are no longer a rate determining step in the presence of an electron withdrawing

!"

ligand. Therefore, [Cp Fe(P N )] is the most active catalyst for CO2

hydrogenation among all the proposed Fe-complexes, with a total free energy barrier of 14.9 kcal/mol. Previous computational studies on CO2 hydrogenation reaction catalysed by the Fe complexes with a pendant base (acylmethylpyridinol) also reported a barrier of 23.9 kcal/mol for 20 ACS Paragon Plus Environment

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CO2 hydrogenation.

40

So in our present study, the hydride transfer barrier of 14.9 kcal/mol

indicates that this complex can be a promising catalyst for efficient conversion of CO2 to formic acid. Based on our thermodynamics, here we propose the most favourable pathway (Scheme 3) and

mechanism

for

the

CO2

hydrogenation

reaction

when

catalyzed

by

the

[Cp Fe(P N )] complex.

HOH + HDBU + TS3

tBu

P P

tBu

Fe tBu

H

Bu

4

N

Fe t

t

C

Bu

6F 5

H

Bu

N

CO2

TS 5

P P

tBu

N t

tBu

H2O + DBU

F5 C6

N

+

H

3 TS1

N

Bu

P P

Fe tBu

C6F5

H

O C

N tBu

tBu

O

6

N

Fe t

Bu

N tBu

F5 C6

tBu

P P

H

tBu

+

H

t

+ BU HD

O2 + HC BU HD

2

tBu t

Bu N

P P

Fe t

Bu

N tBu

H2 F5 C6

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


+

1

Scheme 3: Proposed reaction mechanism for the CO2 hydrogenation by an iron catalyst containing pendant amines and Cp ligand.



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4. Steric vs. Electronic: The intrinsic reactivity of the Fe-complexes can be understood from the reaction enthalpies (activation enthalpy barriers), whereas the role of solvent and steric factor can be understood from the entropy part. For this, we have compared the changes in reaction enthalpies (enthalpy barriers) and reaction free energies (activation free energy barriers) for some important CO2 hydrogenation reaction steps. As we know that the change in enthalpy depends upon the electronic factor, whereas the change in entropy depends upon the steric/solvation factor. Table 1 shows that the reaction enthalpies and reaction enthalpy barriers are lower than the reaction free energies and reaction free energy barriers for the heterolytic H2 cleavage and hydride transfer reaction steps. This suggests that the complexes are highly active though the solvent and steric factors play important role towards their catalytic activity.

Table 1. Reaction Enthalpies (∆H), Reaction Enthalpy Barriers (∆H ‡ ), Reaction Free energies (∆G), Reaction Free Energy Barriers (∆G ‡ ) for Important Heterolytic H2 Cleavage and Hydride Transfer Steps. All energies are in kcal/mol. Complexes

TS2

TS4

(for H2 cleavage)

(for

TS5

hydride

transfer

catalyzed by 3)

when (for hydride transfer when catalyzed by 4)

∆H (∆G)

∆H ‡ (∆G ‡ ) ∆H (∆G)

∆H ‡ (∆G ‡ )

∆H (∆G)

∆H ‡ (∆G ‡ )

-1.2 (-2.9)

5.2 (17.3)

8.9 (20.6)

12.3 (23.3)

3.6(14.7)

2.9 (14.9)

1-Cp

3.5 (4.9)

4.9 (18.7)

5.8 (17.7)

10.8 (22.0)

1.6 (12.6)

2.9 (12.7)

1-'("

4.4 (4.3)

7.4 (21.6)

5.0 (17.9)

5.2 (16.9)

-3.4 (8.1)

-0.7 (11.4)

1-

!"

5. Conclusions: 22 ACS Paragon Plus Environment

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Inspired by the Bullock and co-workers findings toward H2 oxidation, we have proposed a series of Fe-complexes with pendant amines as the outer sphere ligands for CO2 hydrogenation reactions. A systematic study is performed by dividing the CO2 hydrogenation catalytic cycle into two parts: (i) the heterolytic H2 cleavage and (ii) the hydride transfer step. The roles of the outer sphere pendant amine ligand toward the heterolytic H2 cleavage and the hydride transfer step are investigated in greater detail. Our results demonstrate that the heterolytic H2 cleavage step improves dramatically due to the presence of the outer sphere pendant amine and the

activation barrier is lowest when catalyzed by [Cp Fe(P N )] complex containing the electron with-drawing

!"

ligand. More importantly, the water and DBU mediated proton

abstraction is almost a barrierless process while comparing to the DBU mediated proton abstraction step. This is very much in consistent with previous experimental findings. Hence, the role of water is very crucial for such pendant amine based hydrogen oxidation reaction and the use of an expensive strong base can be avoided due to this. Further, the hydride transfer step is calculated to be slowest and thus the rate determining step for the CO2 hydrogenation reaction.

However, among the proposed complexes, [Cp Fe(P N )] is the most active one with a total free energy barrier of 14.9 kcal/mol for the hydride transfer mechanism. We find that the rate determining steps (heterolytic cleavage and hydride transfer) of CO2 hydrogenation are

improved significantly when catalysed by the pendant amine based [Cp Fe(P N )] complex. However, the hydride transfer is the rate determining step as the barrier for this step is the highest one among them. Thus we predict that our findings can be very instrumental for designing the outer sphere pendant amine ligand based catalyst for the CO2 hydrogenation reaction.



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* Supporting Information: Full Gaussian reference, accuracy of basis sets and functional, calculations of pKa values, scheme and reaction free energy of all possible intermediates for the heterolytic H2 cleavage, intrinsic reaction coordinate (IRC) and optimized coordinates of all intermediates and transition states.

Acknowledgments: We thank IIT Indore for the lab and computing facilities. This work was supported by DST-SERB [Grant number: EMR/2015/002057], New Delhi. K.S.R. and A.M thank UGC and MHRD, respectively for the research fellowship.

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