Role of Water, CO2, and Noninnocent Ligands in the CO2

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Role of Water, CO2, and Noninnocent Ligands in the CO2 Hydrogenation to Formate by an Ir(III) PNP Pincer Catalyst Evaluated by Static-DFT and ab Initio Molecular Dynamics under Reaction Conditions C. S. Praveen,*,†,‡ Aleix Comas-Vives,‡ Christophe Copéret,‡ and J. VandeVondele† †

Nanoscale Simulations, Department of Materials, ETH Zurich Wolfgang-Pauli-Straße 27, 8093 Zurich, Switzerland Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog Weg 1-5, 8093 Zurich, Switzerland

‡

S Supporting Information *

ABSTRACT: Reaction pathways relevant to the CO2 hydrogenation to formate using a homogeneous IrIIIH3(PNP) pincer catalyst have been investigated using static DFT calculations and ab initio molecular dynamics simulations under reaction conditions. The role of a base, solvent, and the noninnocent ligand are demonstrated. Static DFT calculations with an implicit solvent highlight the importance of selecting a highly polar solvent. The insertion of CO2 and regeneration of the catalyst are identified as the key steps in the reaction mechanism. Unlike in the static DFT calculations, the AIMD simulations show that an additional CO2 molecule and explicit water molecules can expedite the direct cleavage of the H2 coordinated to the metal center to regenerate the catalyst. In addition, the AIMD simulations indicate that the pyridine ring does not participate in the hydrogenation mechanism, while the α-carbon at the pyridine ring is very acidic and participates in off-cycle reactions.

1. INTRODUCTION The rapid increase in global energy demands resulted in the excessive use of fossil fuels.1,2 The combustion of such nonrenewable energy sources has expedited the unsustainable increase in anthropogenic emissions of CO2.3 Hence, reduction and especially the reuse of CO2 has drawn significant attention in academia, since CO2 could be viewed as a C1 building block instead of industrial waste or part of the energy cycle.4 However, the high thermodynamic stability and the inertness of CO2 requires highly active catalysts and co-reactants such as H2, methanol, and epoxides.5 Ideally, catalytic hydrogenation of CO2 can offer pathways to various products, such as CO, methane, methanol, formic acid, dimethyl ether (DME), dimethylformamide (DMF), carbonates, and carboxylic acids,6−11 to name but a few. Of them, formic acid is of particular interest, because it can be used as a chemical feedstock or directly as a fuel.12−14 In addition, the demand for formic acid is forecasted to increase in the near future owing to its excellent potential as an H2 carrier.15−17 Several homogeneous transition-metal-based catalysts show high activity in the hydrogenation of CO2 to formate in the presence of additives such as bases including buffers or amines to yield the corresponding formate salts. In 1970, Kohnle et al.18 reported the formation of formamides from CO2, amines, and H2 in the presence of homogeneous transition-metal complexes. Decades later, several Rh phosphine complexes were reported to promote CO2 hydrogenation to formate at © XXXX American Chemical Society

room temperature and at a relatively low CO2/H2 pressure (40 bar) with turnover numbers (TONs) of up to 3400.19,20 In 2002, Jessop and co-workers screened more than 40 Ru phosphine species in order to find the optimal ligand to promote the catalyst solubility in supercritical carbon dioxide (scCO2), wherein [RuCl2(OAc)(PMe3)4] was the most active with a turnover frequency (TOF) of up to 95000 h−1.21 In the CO2 hydrogenation to formate the Ru pincer complex Ru(PNN)CO(H) (PNN = 6-(di-tert-butylphosphinomethylene)-2-(N,N-diethylaminomethyl)-1,6-dihydropyridine) was used in the presence of base, yielding a TON of up to 23000 and TOF of 2200 h−1.22,23 In addition to the existence of a vast library of Ru catalysts, several Ir catalysts have also proven to be very effective in CO2 hydrogenation.24−32 In particular, Nozaki and co-workers set a milestone in the conversion of CO2 to potassium formate (HCOOK) with a highly efficient (PNP)IrH3 pincer trihydride complex (designated as Ir(III) from here onward).24,33 The significant TON of 3.5 × 106 and TOF of 150000 h−1 was accomplished in an aqueous KOH solution at 120 °C and 6 MPa (H2:CO2 = 1:1). This is still the highest TON obtained with homogeneous catalytic systems so far, even though other efficient approaches have been developed afterwards.28−32 Clearly, the choice of ligand plays an important role in the promotion of the catalytic Received: October 12, 2017

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DOI: 10.1021/acs.organomet.7b00761 Organometallics XXXX, XXX, XXX−XXX

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Organometallics activity. In the phosphine family, pincer phosphine ligands have been demonstrated to outperform other types of phosphine ligands. It has been proposed that this difference probably originates from their noninnocent behavior in the catalytic cycle. In their original work, Nozaki et al.24,33 noticed a significant temperature and pressure effect for the catalytic mechanism with a reduced activity at ambient pressure and room temperature. They concluded that the reaction rate also depended on the strength of the base as well as the presence of a polar solvent. For a better understanding of the role of ligands in the reaction mechanism and the rate-determining steps, an atomistic insight into the catalytic system is needed. Theoretical calculations can provide systematic information regarding the detailed structures and energetics associated with all the intermediates and transition states to unravel the reaction pathways and the rate-determining steps. Density functional theory (DFT) calculations have proven to be very effective in predicting relative energies of different stable intermediates and transition states at relatively low cost. However, such static calculations are not always capable of unambiguously answering the preference of competing pathways. On the other hand, by using ab initio molecular dynamics (AIMD) simulations, a detailed understanding of the chemistry of the transformation events can be gained by generating reactive trajectories. If the time scale is reachable, such simulations can be useful tools for the discrimination of competing pathways leading toward an understanding and design of more effective catalysts. Several static DFT calculations have been reported on the catalytic mechanism of Nozaki’s original Ir(III) pincer catalyst33−37 in the CO2 hydrogenation to formate. However, the role of the PNP pincer ligand in the regeneration of the catalytic mechanism is still being debated.33−38 It was proposed that regeneration of the catalyst via the H2 cleavage with the participation of the pyridine ring was the most favorable step.24 Later on, it was suggested36 that H2 cleavage assisted by a base is preferred over that involving the pyridine ring by 20 kcal mol−1. Finally, it has been recently suggested that both mechanisms, i.e. with and without the involvement of the pyridine ring, could operate concurrently.33 In the present paper we aim at providing a deeper insight into the role of the solvent, ligands, and base with combined static DFT and AIMD simulations in the CO2 hydrogenation to formate by means of the Ir(III) pincer catalyst.

explore the effect of solvent, solvent corrections were included via single-point calculations using a Poisson solver based implicit solvent method as implemented recently in CP2K.44 It has been reported that45 different quantum mechanical theories can predict arguably different reaction mechanisms when they are used with continuum solvation methods. However, recent reports46 show that smaller scale CO2 hydrogenation systems under explicit solvent modeling have been found to be largely independent of the level of theory employed when using explicit solvent modeling instead of a continuum solvation model. The AIMD simulations were carried out within the framework of spin-polarized density functional theory, employing a Perdew− Burke−Ernzerhof (PBE)47 exchange correlation functional at the GGA level in order to perform longer trajectory runs and at the same time avoid the highly expensive hybrid calculations. Within the AIMD simulations, the long-range dispersion interactions were explicitly treated by Grimme’s DFT-D3 dispersion correction.48

2. COMPUTATIONAL DETAILS

Since the CO2 hydrogenation is often carried out in aqueous media, all of the relative energies provided in the text correspond to the solvent corrections obtained with a dielectric constant of 80, unless otherwise specified. The corresponding values calculated in the gas phase are given in parentheses. 3.0.1. Insertion of CO2 to the Metal−Hydride Bond and Dissociation of Formate Species. In the pristine Ir(III) pincer catalyst (complex 1 shown in Figure 1a), the trans hydride displays significantly longer Ir−H bonds (Ir−H bond length 1.67 Å) in comparison with the cis hydride (Ir−H bond length 1.59 Å), consistent with the strong trans effect of hydride ligands. Insertion of CO2 into the coordination sphere of the catalyst costs only −0.95 (1.41) kcal mol−1 with respect to 1 + CO2. The reference energy is hence referenced to the electronic energy of the CO2-coordinated complex 2, which is aligned to be 0. As a first step in the catalytic mechanism, we have evaluated the reaction of CO2 with one of the trans hydrides to generate a formate species that binds to the metal via its hydrogen atom. The calculated energy barrier for the

3. RESULTS OF STATIC DFT CALCULATIONS As a prelude to the AIMD simulations, it is necessary to identify the key reaction steps and the role of each species in the reaction mechanism using static DFT calculations as a reference. Therefore, we have obtained all of the possible reaction mechanisms for CO2 hydrogenation using the Ir(III) pincer catalyst in the gas phase as well as with several implicit solvents. The implicit solvents were selected by choosing dielectric constants close to nine commonly used solvents, as shown in Table 1. Short descriptions of the key observations are described hereafter, and comparisons to the previous static DFT calculations are provided. Table 1. Solvents with Dielectric Constants Close to the Dielectric Constant Values of the Implicit Solvents Used in the Present Study

DFT calculations were carried out in the mixed Gaussian plane wave scheme as implemented in the CP2K code.39,40 The B3LYP hybrid functional was employed to calculate the exchange correlation energy for the static calculations. The TZVP basis set41 in combination with Goedecker−Teter−Hutter pseudopotentials42 were used with a plane wave cutoff energy of 500 Ry. The relative energies calculated within the B3LYP scheme (without the dispersion correction) are sufficient to reproduce the trend in the reaction profile and reaction barriers previously reported (see the Supporting Information for more details).33−37 Zero-point energy (ZPE) corrections were not included in the total energies, as they are computationally very demanding in the condensed phase. However, we have also calculated the free energies by reoptimizing the structures with the Gaussian code (see the Supporting Information for more information) also using the B3LYP functional. We note that the inclusion of free energies does not make any significant difference in the trends observed in the energy profiles using electronic energy. Therefore, hereafter we have used only the electronic energies for the discussion of the evaluated pathways. Transition state energies were obtained using a climbing image nudged elastic band (CI-NEB) method43 using CP2K. To B

dielectric constant (ϵ)

similar solvent

2 4 8 16 24 32 48 64 80

scCO2/toluene/triethylamene diethyl ether/chloroform tetrahydrofuran isobutyl alcohol ethanol methanol dimethyl sulfoxide propylene carbonate water

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Figure 1. (a) Reaction mechanism of the formate formation and catalyst regeneration through H2 cleavage followed by rearomatization of the pyridine ring. The mechanism is represented as cycle 1 in the text. Insertion and removal of species are represented by blue and red arrows, respectively. The dash-dotted black arrows in 6 and 9 represent the proton transfer. Color coding of the atoms is displayed inside the circle. (b) Solvent-corrected reaction profile for cycle 1. Blue and red horizontal bars show the corresponding relative electronic energies in the gas phase and in the presence of water as the solvent (implicit solvent with dielectric constant 80).

formate formation, TS2/3, is 4.6 (7.8) kcal mol−1. The formate binds to the Ir with a binding energy of only 2.7 (0.23) kcal mol−1 with respect to the free species in the gas phase. In addition, the resulting complex 3 is 1.9 (7.6) kcal mol−1 higher in energy with respect to 2. Therefore, regeneration of the CO2 species is energetically feasible. When the formate reorganizes to bind to the metal atom via one of its oxygen atoms (η1 coordination), the resulting dihydridoformatoiridium complex 4 is lower in energy than the initial complex by 8.6 (6.7) kcal mol−1. This observation is in agreement with the mechanism proposed by Nozaki et al.33 The barrier for the reorganization

of the formate, TS3/4, is 9.9 (14.6) kcal mol−1. From the reaction profile displayed in Figure 1b, it is clear that, as the dielectric constant increases, the barrier for the CO2 insertion and formate reorganization are considerably reduced. In addition, the stability of the resulting formatoiridium complex 3 increases with an increase in the dielectric constant, while the dielectric constant has only very little effect on the stability of complex 4. To proceed with the catalytic cycle, the formate species has to be decoordinated from complex 3 or 4. This is very energy demanding in the gas phase due to a huge barrier (>90 kcal C

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Organometallics mol−1) in both cases. However, in the presence of a polar solvent such as water, the formate removal is uphill by only 2.8 and 13.3 kcal mol−1, respectively, from complexes 3 and 4. Therefore, the resulting complex 5 is very likely to be formed via 2 → TS2/3 → 3 → 5. The cationic complex 5 is 4.7 kcal mol−1 higher in energy in comparison to the initial complex 2, while complex 5 is considerably destabilized for any other solvent with a dielectric constant of less than 16. This result supports the requirement of a highly polar solvent not only to initiate the CO2 reduction mechanism but also to stabilize the cationic complex. 3.0.2. Role of the Pincer Ligand and a Strong Base in Regenerating the Catalyst. After the formate leaves the coordination sphere of the metal center, the initial metal complex can be regenerated by following different routes depending on the species (OH−, H2, H2O) coordinated to the Ir metal center in the cationic complex 5. When the neutral complex 6 is formed due to the coordination of OH− to the metal center, the pincer ligand plays a key role in regenerating the catalyst. The ligand transfers one of its methylene (α-carbon) protons to the OH− ion bound at the metal center to form the water-coordinated complex 7, through TS6/7, which has a barrier of 17.4 (15.0) kcal mol−1. This deprotonation mechanism leads to dearomatization of the pyridine and leaves the α-carbon unsaturated. Eliminating water from complex 7 is only 4.8 (8.8) kcal mol−1 uphill to yield 8. Now the catalytic cycle can be closed by inserting an H2 molecule (in η2 coordination) at the metal center 9, followed by a heterolytic cleavage of the H2 molecule to rearomatize the pyridine ring through TS9/1. However, the barrier for this reaction is calculated to be 31.8 (31.6) kcal mol−1, which hinders the reaction to proceed via 5 → 6 → TS6/7 → 7 → 8 → 9→ TS9/1 → 1 by involving the pincer ligand. In addition, we observed that the choice of implicit solvent has very little or no effect on this part of the reaction mechanism. See Figure 1a for a pictorial representation of the full reaction mechanism and Figure 1b for the complete reaction profile. When an H2 molecule is coordinated to the metal center (in η2 coordination), the resulting cationic complex 10 is highly unstable in the gas phase and the stability increases with an increase in the dielectric constant. From complex 10, the initial catalyst can be regenerated in the presence of a strong base with and without the involvement of the pincer ligand. As shown in Figure 2, the OH− ion can cleave the H2 molecule via two different mechanisms. In the first mechanism, (represented as cycle 2 in Figure 3a), the OH− base attacks the pyridine ring of the ligand and spontaneously forms (barrierless) a water molecule by acceptance of a methylene proton and simultaneous dearomatization of the pyridine ring. The generated water molecule interacts weakly with the deprotonated α-carbon, resulting in complex 11, and is more stable in the gas phase than in the solvents. Thereafter, the H2O molecule returns the proton (methylene proton) to saturate the α-carbon through TS11/12 and thus the pyridine ring becomes rearomatized. Concurrently, the leftover hydroxyl ion cleaves the H2 at the metal center and forms a water molecule by accepting one of the resulting protons. Unlike the case in the first reaction cycle (cycle 1), here the H2 cleavage is initiated by a single-step dearomatization and rearomatization process initiated by the base. This reaction, however, has to overcome an energy barrier of 17.5 (19.4) kcal mol−1 (TS11/12). The resulting watercoordinated complex 12 is −16.7 (−14.6) kcal mol−1 lower in

Figure 2. Scheme representing the regeneration of the catalyst by the cleavage of H2 coordinated to complex 10. Cycles 2 and 3 represent the regeneration mechanism with and without the involvement of the pyridine ring, respectively. The color coding of the atoms is same as in Figure 1. The protons relevant in the corresponding mechanism are highlighted using orange and dark blue. The proton transfer is indicated using solid curved arrows. The isopropyl groups are omitted from the figure for better visibility.

energy than 11 and is the lowest energy complex in the whole catalytic cycle. Finally, the original pincer complex 1 could be regenerated by eliminating the water molecule with an energy cost equal to only 2.9 (7.0) kcal mol−1. A complete reaction profile of this path is given in Figure 3b. In the second mechanism, i.e., without the involvement of the pyridine ring (designated as cycle 3 in Figure 4a), the reaction could proceed by directly attacking the H2 in complex 10 by the OH− ion. The OH− ion spontaneously cleaves the H2 and forms a water molecule, binding very weakly to complex 1. The resulting complex 13 has the same energy as 12 in cycle 2 (Figure 4b). Therefore, eliminating a water molecule from 13 to regenerate 1 is only 2.9 (7.0) kcal mol−1 uphill. This reaction mechanism has the lowest energy barrier of all and is therefore very likely to dominate. Recently, Yang36 has reported a similar minimum energy pathway, but with an enthalpy barrier for the H2 cleavage. This result contradicts what has been obtained in earlier calculations, with a calculated energy barrier of 18 kcal mol−1. This difference probably arises from the fact that previous optimizations were performed using a solvation model, while we use gas-phase optimized structures, in which a proton is directly transferred to OH− due to spontaneous cleavage of H2. Note, however, that during the AIMD simulations (as we shall see later) including additional water molecules, i.e. using explicit model solvation, the OH− group still spontaneously attacks the coordinated H2 complex, generating Ir−H and H2O (vide infra). A complete reaction profile for this path is given in Figure 4b. 3.0.3. Effect of Explicit Water Molecules in the Reaction Mechanism. In the presence of explicit water molecules, the cationic complex 5 could be saturated by an H2O molecule to yield the H2O-coordinated complex 14, as shown in Figure 5. The coordination of H2O into complex 5 is thermodynamically favorable by 12.5 (9.0) kcal mol−1. The relative energy of the cationic complex 14 is −4.3 (88.1) kcal mol−1 with respect to 2. This implies the high instability of the H2O-coordinated complex in the gas phase. Moreover, 14 is 4.3 kcal mol−1 higher in energy than the H2-coordinated complex 10 in the gas phase ((83.8) kcal mol−1). On the other hand, in the presence of solvent, complex 14 and dihydridoformatoiridium complex 4 D

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Figure 3. (a) Reaction mechanism of the formate formation and catalyst regeneration through OH−-assisted H2 cleavage involving the pyridine ring. The mechanism is represented as cycle 2 in the text. The dash-dotted black arrows in 11 and 12 represent the proton transfer. (b) Reaction profile for cycle 2. The color coding in the figure is same as in Figure 1.

differ in energy by only 4.3 kcal mol−1, in favor of the latter. This suggests a possible H2O/HCOO− equilibrium in the presence of a solvent. Therefore, it is very unlikely that an H2O molecule is stably coordinated to the Ir center both in the gas phase and in a solvent. The highest barrier observed so far is for the H2 cleavage mechanism (TS9/1, cf. cycle 1; TS11/12, cf. cycle 2). Therefore, one to three water molecules were explicitly added to complex 9 and the reaction barrier (corresponding to TS 9/1 in cycle 1) was recalculated for each case to check whether or not the inclusion of additional water molecules has any effect on this

mechanism. The corresponding reaction profiles are displayed in Figure 6. It is clear that the addition of water molecules considerably reduces the reaction barrier. Previous results have shown that the solvent can assist the H2 cleavage.49−51 The cleavage of H2 is initiated by hydrogen bonds, as highlighted in Figure 6 (hydrogen bonds are indicated by dashed purple lines). Already the presence of a single water molecule reduces the reaction barrier to 14.8 (20.8) kcal mol−1, which is almost half of the original barrier (Figure 6a). The single H2O molecule exchanges one of its protons to the α carbon to rearomatize the pyridine ring and at the same time regains a E

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Figure 4. (a) Reaction mechanism of the formate formation and catalyst regeneration through spontaneous H2 cleavage by OH− without the involvement of the pyridine ring. The mechanism is represented as cycle 3 in the text. (b) Reaction profile for cycle 3. The color coding in the figure is as same as in Figure 1.

are added to the system, the reaction could proceed in two ways. In the first case (Figure 6b), the water molecule close to the metal center takes one proton by cleaving the H2 molecule and simultaneously initiates a cyclic proton transfer involving the second water molecule to rearomatize the pyridine ring, resulting in two water molecules interacting with 1. This mechanism is very similar to that proposed by Nozaki et al.,33 although with the observation of an intermediate state. In the second case (Figure 6c), only one water molecule participates in the H2 cleavage and rearomatization mechanism, though the presence of a second water molecule hydrogen-bonded to the first H2O molecule further slightly reduces the barrier and the resulting complex is much more stable. The calculated barrier for the H2 cleavage in this situation is 13.2 (16.8) kcal mol−1,

Figure 5. Optimized structure of a water molecule coordinated to the unsaturated metal center in the cationic complex 5 to yield complex 14. The isopropyl groups are omitted from the figure for better visibility.

proton by cleaving the H2 coordinated to the Ir center to form a water coordinated to complex 1. When two water molecules F

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4. MOLECULAR DYNAMICS SIMULATIONS To gain detailed insight into the reaction mechanisms obtained using static DFT calculations, we performed a series of ab initio molecular dynamics (AIMD) simulations under the reaction conditions using a canonical ensemble. The pincer catalyst was introduced into a cubic box of side 20 Å containing a mixture of H2 and CO2 in the ratio of 3:1 (43 H2 and 14 CO2 molecules) at a pressure of 400 bar. Initially the whole system was equilibrated for 25 ps with a target temperature of 450 K using a GLE thermostat. A snapshot of the corresponding starting simulation cell is represented in Figure 7. Starting from the

Figure 7. A 20 Å cubic box showing the initial AIMD simulation cell equilibrated using a GLE thermostat at 450 K. The color coding of the atoms is same as in Figure 1.

equilibrated geometry, several AIMD simulations were performed for at least 25 ps each to unravel the atomistic details of the reactions. The key observations from the simulations are presented below. 4.1. Interaction of CO2 with Pincer Catalyst and the Nature of Formates. First, a few CO2 molecules were placed very close to the trans hydrides at the metal center and a simulation was run for 25 ps. No formates were formed during the simulation. This indicates a rather high barrier for establishing a hydride transfer to the linear CO2 molecule for its activation, in agreement with the description of a high barrier in the gas phase for the formation of the formates presented in section 3. In a second simulation, one of the trans hydrides was brought closer to an artificially bent CO2 molecule by stretching the Ir−H bond. This, however, yielded a linear CO2 molecule and the pincer complex 1 back within 1 ps after the simulation began (see Figure S4 in the Supporting Information). This further supports the assumption of the presence of a high barrier for formate formation and the possible instability of the formate species. In a third simulation, a formate species was first generated by pulling away one of the trans hydrides and by manually establishing a bond to CO2 by reducing the O−C−O bond angle. The formate thus generated was kept far away from the metal center. During the simulation, the formate oriented itself toward the metal center, established a bond to the metal center via one of its oxygen atoms (η1 coordination), and remained stable for an entire 25 ps. This strongly supports the possible self-reorganization of the formate and its stability described in cycle 1. Thereafter, the formate was placed in the proximity of the pyridine ring. As seen from the time scale representation of the relevant distances in Figure 8, the formate molecule was converted to a formic acid

Figure 6. Effect of water molecules in the regeneration of the catalyst 1 in the presence of one (a), two (b, c), and three (d) water molecules, respectively. Hydrogen bonds are highlighted by using dashed purple lines. The solvent-corrected relative electronic energies corresponding to the initial (IS), transition (TS), and final states (FS) are given. The values given in parentheses show the corresponding gas-phase values. The color coding of the atoms is same as in Figure 1.

which is the lowest of all. Remarkably, adding a third water molecule (Figure 6d) did not result in any significant reduction in the barrier. However, when the Gibbs free energy corrections are included, the results show that the preferred pathway is that involving one water molecule since the assistance by two or three water molecules has a high entropic cost (see the Supporting Information). Markedly, the mechanisms described in Figure 6b−d are very similar for the H2 cleavage mechanism in TS11/12. Therefore, a similar reduction in the barrier for TS11/12 is expected in the presence of explicit water molecules. The importance of the water molecule in accelerating the H2 cleavage mechanism is thus clear. Our results suggest that the pyridine-based PNP ligand does not play a key role in the reaction mechanism in the absence of water molecules. However, in the presence of explicit water molecules, the PNP pincer ligand could be active in the regeneration of the catalyst (5 → 10 → 11 → TS11/12 → 12 → 1) due to a considerable reduction in the H2 cleavage barrier in the presence of water molecules. Nevertheless, from the static DFT calculations we found that the most feasible pathway is the regeneration of the catalyst by the base-assisted direct cleavage of H2 at the metal center, i.e., 5 → 10 → 13 → 1, as reported in Figure 4b. G

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Figure 8. Time evolution (in ps) of selected bond distances (in Å) in the AIMD simulation showing the formation of formic acid by depronated dearomatization at the pyridine ring (top panel). An enlargement of the relevant transformation window is shown (bottom left). A scheme showing the mechanism is displayed in the bottom right panel. The corresponding distances in the graph are color coded and numbered in the schematic representation. Note that solid and dashed lines indicate bonding and nonbonding situations, respectively.

molecule by scavenging one proton from the α-carbon of the pyridine ring by a deprotonative dearomatization process after 20 ps of the simulation. Afterward, we did not observe any mechanism leading to the rearomatization of the pyridine ring. Thereafter, to explore the effect of a base in the reaction mechanism, a formal KOH (in the form of a K+ and OH− ions) molecule was added to the simulation cell. The OH− ion attacked one of the closest CO2 molecules in the unit cell and formed a bicarbonate by transferring the hydroxyl species (see Figure S5 in the Supporting Information) to the CO2. This was always the case whenever the KOH molecule came in the proximity of a CO2 molecule before interacting with the catalyst or the formate species. The interaction of the base with the catalyst and its role in regenerating the catalyst are further analyzed below. 4.2. Catalyst Regeneration in the Presence of OH− and Water Molecules after Removing HCOOK. In this set of simulations, the potassium formate salt (HCOOK) was removed from the simulation cell, while the hydroxyl ion was kept. In addition, three water molecules were introduced into the neutral cell and four different simulations were performed for 25 ps each with the same computational setup. A schematic representation of the initial configurations of all four simulations in this set are shown in Figure 9. In the first simulation, the Ir center was kept unsaturated (Figure 9a). Immediately after the simulation began, the α-carbon at the pyridine ring initiated a proton transfer to the hydroxyl ion and formed a water molecule. Initially, this water molecule was weakly bound to the metal center and afterward hydrogen bonds were established to the nearby water molecules (see Figure S6 in the Supporting Information) and detached from the metal center. In the second simulation, the initial system was kept as it is and a water molecule placed at the vacant metal center (Figure 9b). The initial water molecule, coordinated to the metal center, desorbed immediately, which further corroborates our observation of the instability of the H2O molecule at the metal center. Soon after, the hydroxyl ion was converted to a water molecule with the same mechanism as described above by deprotonation of the α-carbon (see Figure S7 in the Supporting Information). Therefore, it is very clear

Figure 9. Scheme representing the initial configurations of the MD simulations described in section 4.2. H2 and CO2 are omitted from the schematic representation.

that that the proton transfer from the α-carbon to the OH−, followed by dearomatization of the pyridine ring, is a very probable step due to the highly acidic nature of this carbon atom. In the third simulation, the hydroxyl ion was placed at the metal center (Figure 9c), which remained very stable and established hydrogen bonds to the nearby water molecule. On the other hand, we have not observed any proton transfer from the α-carbon of the pyridine ring to the hydroxyl ion, presumably because of a high barrier. This suggests that the proton transfer to the OH− while it is coordinated to the metal center via TS6/7 (cf. Figure 1b) is highly unlikely to be initiated. In the fourth set of calculations, the vacant metal center was saturated with an H2 molecule (Figure 9d). Here, the reaction proceeded with a two-step mechanism. In the first step the hydroxyl ion attacked one of the CO2 molecules and formed a bicarbonate species, as shown in Figure 10. In the second step, H

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Figure 10. Time evolution (in ps) of selected bond distances (in Å) in the AIMD simulation showing the formation of bicarbonate (top left) and its subsequent hydroxylation to carbonic acid (top right). Enlargements of the relevant transformation windows are shown in the middle panel. A scheme showing the mechanism is displayed in the bottom panels. The corresponding distances in the graph are color coded and numbered in the schematic representation.

after several picoseconds, the bicarbonate species was converted to a carbonic acid molecule by cleaving the dihydrogen molecule at the metal center (in η2 coordination). This gave the original pincer complex 1 back, which again indicates that the pyridine ring remained inactive in regenerating the catalyst. Thus, we can conclude that a second CO2 could also participate in the catalyst regeneration, as the regeneration of the catalyst via a (CO2 + OH−) → bicarbonate → carbonic acid route is also feasible. 4.3. Catalyst Regeneration in the Presence of KOH and Water Molecules after Removing HCOO. Within these simulations, only the formate species is removed from the metal center and the KOH species and three explicit water molecules are kept. The whole system remains positively charged, and five different simulations were performed with the starting configurations shown in Figure 11. In the first case, the Ir center remains unsaturated (Figure 11a). After the calculation was run for a few picoseconds, the base attacked the metal center and formed a water molecule by taking one of the remaining hydrides from the metal center. The atom of the K+ cation binds very weakly to the metal center along with a water molecule, and the metal center remains saturated (see Figure S8 in the Supporting Information). In a second simulation, initially the vacant Ir center was coordinated with a water molecule (Figure 11b). The water molecule desorbed immediately and was replaced by a hydroxyl ion. This shows a rapid OH−/H2O equilibrium established in the catalytic system. In the third simulation, the metal center was saturated with an H2 molecule (Figure 11c). In this case, after a few picoseconds, the H2 at the metal center

Figure 11. Scheme representing the initial configurations of the MD simulations described in section 4.3. H2 and CO2 are omitted from the schematic representation.

(in η2 coordination) was spontaneously cleaved by the base and a water molecule was formed. As a result of the H2 cleavage at the metal center, the initial pincer complex was regenerated. This is very much in agreement with the reaction described in cycle 3 (Figure 12a). In a subsequent calculation, a water molecule was placed in the proximity of KOH. In this case, the water transferred a I

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Organometallics

Figure 12. (a) Time evolution (in ps) of selected bond distances (in Å) in the AIMD simulation showing (a) the spontaneous splitting of dihydrogen at the metal center by the base to regenerate the catalyst (top panel). An enlargement of the relevant transformation windows is shown in the bottom left panel. A scheme showing the mechanism is displayed in the bottom right panel. (b) Time evolution (in ps) of the formation of water at the metal center due to the interaction of the hydroxyl ion with one of the trans hydrides and the simultaneous splitting of dihydrogen (top panel). An enlargement of the relevant transformation windows is shown in the bottom left panel. The corresponding distances in the graph are color coded and numbered in the schematic representation.

proton to the hydroxyl ion bound to the K+ cation and the hydroxyl ion was converted to water coordinated very weakly to the K+ cation. The hydroxyl ion, generated from the initial water molecule placed close to KOH, spontaneously cleaved the H2 coordinated to the Ir center, and it converted to a water molecule by accepting a proton. Hence, the original Ir complex is regenerated. This mechanism is very similar to the cyclic proton exchange as shown in Figure 6c, although the pyridine ring was not involved. In a final set of calculations with the same setup, we observed that the hydroxyl ion formed a water molecule by attacking a hydrogen atom at the Ir center. At the same time the H2 at the Ir center was spontaneously cleaved and the pincer catalyst was regenerated, as displayed in Figure 12b. Therefore, these sets of AIMD simulations indicate that the catalyst regeneration through a direct cleavage of H2 by OH− is the most favorable mechanism of all. In addition, bicarbonate or

water molecules could act as intermediates for the cleavage mechanism as described above.

5. CONCLUSIONS Static DFT calculations as well as ab initio molecular dynamics within periodic boundary conditions were performed to identify key steps in the reaction pathways of the CO2 hydrogenation to formate using the Ir(III) PNP pincer complex. Static DFT calculations show that the solvent, first considered by using an implicit solvent model with a broad range of dielectric constants, plays a major role in stabilizing the intermediates along the catalytic cycle. The insertion of CO2 into the metal hydride and the regeneration of the catalyst by heterolytic cleavage of H 2 coordinated to the metal center (η 2 coordination) are the two key steps in the reaction mechanism (Scheme 1). While the formation of the formate is the most energy demanding, the coordination of H2 on Ir, requiring the decoordination of the formate anion ligand, requires a very J

DOI: 10.1021/acs.organomet.7b00761 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Scheme 1. Observations from AIMD Simulations: (a) CO2 Insertion into the Ir−H Bond and the Removal of Formate Species; (b) Regeneration of the Catalyst by H2 Cleavage Mechanism without Involving the Pyridine Ring; (c) H2 Cleavage Mechanism Involving the Pyridine Ring

the OH− base to produce a formic acid or water molecule, respectively, due to the highly acidic nature of the α-carbon proton. Such off-cycle reactions could be detrimental to the catalytic activity, pointing to possible strategies to improve the design of the current catalyst in order to shut down such unproductive pathways. Further experimental investigations are needed in order to fully understand the role of the ligand and to further tune the catalyst and ligand structure. Therefore, the AIMD simulations bring new insights into the reaction mechanisms such as the formation of formic acid by proton transfer of the pyridine ring to formate, the generation of bicarbonates from CO2 and OH−, and the subsequent formation of carbonic acid by proton transfer to bicarbonate from the H2 coordinated to the Ir center. The simulations clearly suggest that the bicarbonates could act as a proton acceptor ultimately transforming OH− to H2O to regenerate the catalyst. Overall, AIMD shows that the ligand does not participate in the CO2 hydrogenation cycle however, it does participate in off-cycle reactions (H transfer).

polar solvent, in particular H2O, as it generates a cationic species. The next step involves the cleavage of the H2 molecule, which is achieved with the help of a base and may or may not involve the pyridine ring of the pincer ligand. Of the various possibilities, the most efficient pathway involves the regeneration of the catalyst via spontaneous (barrierless) splitting of the H2 assisted by the hydroxyl ion (OH−) to generate a H2O molecule, without involvement of the pyridine ring. The mechanism involving the pyridine ring can take place in the presence of an explicit water molecule included in the calculation but is more energy demanding. The extensive set of AIMD simulations under reaction conditions, which explores the free energy surface, reveals that the CO2 insertion does not take place spontaneously since it is an activated process. These simulations also show that the reorganization of formate is feasible and leads to a configuration of this species that is stable for at least 25 ps. In the AIMD simulations with a CO2:H2 ratio of 3:1 with the presence of KOH and a few water molecules, the heterolytic cleavage of H2 assisted by the base without involving the pyridine ring was also observed, as a key catalyst regeneration step. This can occur with OH− or alternatively with bicarbonate (Scheme 1) generated in situ, which supports our observation of a barrierless splitting of H2 by OH− from static DFT calculations, also when explicit water molecules are included in the simulation and dynamic effects. The ligand does not participate in the catalytic cycle per se, in opposition to what has been previously proposed. Indeed, a previous study suggested that the regeneration of the catalyst via H2 cleavage took place with the participation of the pyridine ring.24,33 Nevertheless, the ligand still plays a role and is involved in offcycle reactions, such as the transfer of a proton to formate or to

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00761.

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Additional AIMD schemes (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for C.S.P.: [email protected]. K

DOI: 10.1021/acs.organomet.7b00761 Organometallics XXXX, XXX, XXX−XXX

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Organometallics ORCID

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C. S. Praveen: 0000-0001-9721-7895 Aleix Comas-Vives: 0000-0002-7002-1582 Christophe Copéret: 0000-0001-9660-3890 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the SINERGIA project (SNF Project No. CRSII2_154448) for funding and CSCS production projects CH5 and S629 for the computational time at the Swiss national computing center. A.C.-V. acknowledges financial support from the Holcim Foundation. C.S.P. acknowledges Hung-Kun Lo for helpful discussions.

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DOI: 10.1021/acs.organomet.7b00761 Organometallics XXXX, XXX, XXX−XXX