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Article

Substituent’s Effects of PNP Ligands in Ru(II)-Catalyzed CO2 Hydrogenation to Formate: Theoretical Analysis Considering Steric Hindrance and Promotion of Hydrogen Bonding

by
Xiangyang Feng
,
Jun Li
* and
Zhuhong Yang
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(7), 760; https://doi.org/10.3390/catal12070760
Submission received: 18 May 2022 / Revised: 27 June 2022 / Accepted: 5 July 2022 / Published: 8 July 2022
(This article belongs to the Special Issue Catalysts with New Formulations/Structures: Synthesis and Applications)
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Abstract

:
This paper investigates the effects of substituents in PNP-type ruthenium complexes in the catalytic hydrogenation of CO2 to formate using the DFT method. Six groups were considered as substituents linked to the P atom of the PNP ligand: hydrogen, methyl, iso-propyl, tert-butyl, cyclopentyl, and cyclohexyl. The substituent effects were analyzed from the perspectives of steric hindrance and promotion of hydrogen bonding. With the joint functions of steric hindrance and hydrogen bonding promotion during the CO2 coordination step, hydride addition step, and HCOO− rotation step, these groups exhibited very different substituent effects. The results showed that the methyl group was the most favorable substituent when the solvent’s effects were not included, as it formed hydrogen bonding with relatively weak steric hindrance. The second favorable substituent was the iso-propyl group, while the tert-butyl group was the most unfavorable one, due to remarkable steric hindrance. When the substituent was cyclopentyl or cyclohexyl, the complex provided a wider open space for the reaction compared with the tert-butyl-substituted complex, because cyclopentyl and cyclohexyl are cyclic groups. Therefore, the principle for choosing the substituent in PNP-type complexes allowing the design of highly efficient catalysts for CO2 hydrogenation indicates that more hydrogen atoms but wider open space are ideal. In addition, the substituent’s effects can be markedly impacted by the solvent used.

1. Introduction

The large amount of CO2 emissions is generally considered as a major factor leading to global warming. To reduce carbon emissions and mitigate the greenhouse effect, negative carbon emission technologies (NETs) are required [1]. Catalytic CO2 hydrogenation has been studied for decades and is a negative emission technology with high atomic utilization rate, which is greatly important for carbon neutrality [2,3,4,5]. This technique applies CO2 as a feedstock to produce other chemicals, such as formic acid, methanol, etc. Due to a wide range of uses of formic acid in agriculture and industries and its potential use as a hydrogen storage material, the conversion of CO2 to formic acid or formate has been widely investigated in recent years [6,7,8,9,10,11,12].
In 2009, Nozaki and coworkers [13] developed Ir(III) trihydride complexes with the PNP (PNP = 2,6-(di-iso-propylphosphinomethyl)-pyridine) pincer ligand to catalyze the hydrogenation of CO2 to formate and achieved a turnover frequency (TOF) of 150,000 h−1 and a turnover number (TON) as high as 3,500,000 at 200 °C and 5.0 MPa. In their experiments, they tested two groups on P atoms in the PNP ligand (Scheme 1) and found that the iso-propyl (iPr) group can result in higher activity than the tert-butyl (tBu) group. They thought that this was probably the result of the higher solubility of the iPr-substituted PNP complex. They observed a similar result in their subsequent study using Ru(II) complexes with iPr or tBu groups [14]. Recently, Huang and coworkers [15] synthesized tBu-substituted and cyclopentyl- (cPe) substituted PN3P-type Ir(III) trihydride complexes. When they reported the catalytic performance of these complexes in the hydrogenation of CO2, they mentioned large differences in the yield, TON, and TOF between the cPe-substituted complexes and the tBu-substituted complexes. Moreover, Bernskoetter and coworkers [16] used iPr-substituted and cyclohexyl- (Cy) substituted PNP-type (PNP = NH{CH2CH2(PR2)}2) Fe(II) complexes (Scheme 1) for CO2 hydrogenation to formate and also found different catalytic activities of these complexes. Other transition metals, such as Ru and Rh, have been demonstrated as highly efficient catalyst as well for the hydrogenation of CO2 to formate or formic acid [17,18,19,20,21,22]. For instance, Pidko and coworkers [23] developed a PNP-type Ru(II) dihydride complex ([Ru(PNP)(CO)H2], i.e., PNP = 2,6-(di-tert-butylphosphinomethyl)-pyridine) that increased the TOF to 1,892,000 h−1 at lower temperature and pressure (132 °C and 4.0 MPa).
To elucidate the reaction mechanism of CO2 hydrogenation to formate catalyzed by these PNP-type Ir or Ru complexes, a number of theoretical studies have been conducted, focused on understanding the rate-limiting step [24,25], the relation between the mechanism of CO2 insertion and the property of the metal−hydride bond [26,27], the impact of metal−ligand cooperation [28,29], the trans influence of boryl ligands [30], the role of base, solvent, and noninnocent ligands [31], and so on [32,33,34,35]. Corminboeuf and coworkers [36], lately, analyzed in theory the pincer ligand effects by constructing various PNP, PNN, or NNN pincer ligands. However, although different substitutes in the pincer ligands were used and different catalytic performances were observed in the experiments of CO2 hydrogenation with Ir or Ru complexes, there is no theoretical study yet addressing this topic. In addition, to save computational resources, some work used small groups to represent bigger groups that were used in the experiments. For example, Ahlquist [37] replaced the iPr groups by H atoms to investigate the Nozaki’s system, and Schmeier et al. [38] simplified the iPr groups as Me when they studied the mechanism of CO2 reduction with Ir-PNP pincer complexes. The aim of this work was to figure out the effects of different substitute groups of P atoms in the PNP ligand in the hydrogenation of CO2 to formate using Ru-PNP pincer complexes by density functional theory calculations.
We considered two influencing factors associated with different substitutes of the PNP ligand, i.e., steric hindrance and the promotion of hydrogen bonding. CO2 insertion into the Ru−H bond to produce formate consists of (1) CO2 coordination, (2) hydride direct addition to CO2, and (3) HCOO− rotation [26,27]. Large substitutes will block the coordination of CO2 and the rotation of the HCOO− moiety. Therefore, the first effect is steric hindrance, which is negative for the production of formate. Steric effects have been discussed in the hydrogenation of CO2 to formic acid on Ru(II) bidentate phosphine complexes and Ru(II) polypyridyl complexes [39,40]. On the other hand, more and stronger hydrogen bonding may form between large substitutes and CO2. Hydrogen bonding can strengthen the coordination of CO2 and accelerate the direct addition of hydride to CO2 [27,41,42,43,44], which is positive for the total reaction. In this work, we choose Me, iPr, tBu, cPe, Cy groups (Scheme 2) and the hydrogen atom as substituents in PNP ligands to study the substituent effects in Ru(II)-catalyzed hydrogenation of CO2 to formate and to discern the role of steric hindrance and the promotion of hydrogen bonding.

2. Results and Discussion

The process of CO2 hydrogenation to formate on Ru-PNP complex includes (1) CO2 coordination, (2) hydride addition to CO2, and (3) HCOO− rotation, as shown in Scheme 3. Therefore, we will discuss the substituent effects in three steps, analyzing the role of steric hindrance and hydrogen bonding in gas phase. Finally, we will discuss the impact of different solvents on the substituent effects. The complexes were named A, B, C, D, E, F, according to the groups attached to the P atoms in the PNP ligand (H, Me, iPr, tBu, cPe, and Cy group, respectively). The optimized geometries of these complexes are shown in Figure S1 in the Supporting Information.

2.1. Substituent’s Effects on CO2 Coordination

The first step of CO2 insertion into the Ru−H bond to produce formate is CO2 coordination. The optimized geometries of CO2 coordination on each complex are shown in Figure 1. The binding energy and binding free energy for CO2 coordination are reported in Table 1. The calculated percent buried volume (%Vbur) for a PR2 group on the PNP ligand are listed in Table 2, and the parameters of selected hydrogen bonding are collected in Table 3. When the substituent in the PNP ligand was H, CO2 was fixed with a distance of 2.674 Å between the C atom of CO2 and a hydride on the Ru center to form complex A-2. At the same time, hydrogen bonding formed between the O atom of CO2 and an H atom on the PNP arm at a distance of 2.566 Å. The binding energy was −11.2 kJ/mol, and the binding free energy was 14.2 kJ/mol. When the substituent was Me, the binding energy was −13.0 kJ/mol and the binding free energy was 12.4 kJ/mol. Therefore, CO2 coordination in complex B-2 was stronger than that in A-2. The %Vbur value of PH2 in A-2 was 17.7, and that of PMe2 in B-2 was 20.1, indicating that steric hindrance in B-2 was stronger. However, in B-2, hydrogen bonding formed between the O atoms of CO2 and three H atoms on the PNP arm at a distance of 2.612 Å, 2.841 Å, 2.849 Å, respectively. More hydrogen bonds make CO2 fixation more stable. When the substituent was iPr, hydrogen bonding formed between the O atoms of CO2 and four H atoms on the PNP arm at a distance of 2.611 Å, 2.645 Å, 2.924 Å, 2.983 Å, respectively. Although more hydrogen bonds formed, the binding energy (−12.5 kJ/mol) in complex C-2 was close to that in B-2. In C-2, the distance between CO2 and the hydride on the Ru center was 2.704 Å, longer than that (2.628 Å) in B-2. The %Vbur value of P(iPr)2 in C-2 was 24.5, larger than that of PMe2 in B-2. Steric hindrance became stronger in C-2, but the strength of steric hindrance was balanced by the effect of hydrogen bonding. When the substituent was tBu, steric hindrance became more remarkable, since the binding energy was lower, being −10.0 kJ/mol, and the distance between CO2 and the hydride on the Ru center was much higher, being 3.019 Å. And the %Vbur value of P(tBu)2 in D-2 was 27.5, being bigger than the values of PH2, PMe2, and P(iPr)2. Although hydrogen bonding formed between the O atoms of CO2 and four H atoms on the PNP arm in complex D-2, similar to the situation in C-2, the strength of steric hindrance in D-2 was much greater than the effect of hydrogen bonding.
The cPe and Cy substituents are cyclic groups. When the substituent was cPe, the distance between CO2 and the hydride on the Ru center was 2.705 Å, and the binding energy of CO2 coordination was −12.6 kJ/mol. When the substituent was Cy, the distance between CO2 and the hydride on the Ru center was 2.850 Å, and the binding energy was −14.9 kJ/mol. The binding of CO2 in complex E-2 or F-2 was stronger than that in D-2, because there was a wider open space for CO2 coordination in E-2 and F-2, despite an increased number of C atoms. The %Vbur value of P(cPe)2 in E-2 was 24.2, and that of P(Cy)2 in F-2 was 24.4, both lower than that of P(tBu)2 in D-2. In addition, hydrogen bonding formed between the O atoms of CO2 and three H atoms on the PNP arm in E-2 at a distance of 2.644 Å, 2.670 Å, 2.736 Å, respectively. However, in F-2 hydrogen bonding formed between the O atoms of CO2 and four H atoms on the PNP arm at a distance of 2.504 Å, 2.553 Å, 2.803 Å, 3.009 Å, respectively. Therefore, CO2 fixation in F-2 was stronger than in E-2.

2.2. Substituent’s Effects on Hydride Addition to CO2

After CO2 is fixed, direct addition of hydride to CO2 will take place. The free energy profiles for this process on six Ru–PNP complexes with different substituents are shown in Figure 2, and the optimized geometries of all transition states involved are presented in Figure 3. The parameters of selected hydrogen bonds are reported in Table 4. The optimized geometries of the products are shown in Figure S2 in the Supporting Information.
CO2 is an inert molecule, and thus only highly active hydride can be directly added to CO2 [26,27]. However, if CO2 is pre-activated by hydrogen bonding, this process will be promoted, because hydrogen bonding weakly changes the electron distribution in CO2 [27,41,42,43,44]. When the substituent was H, this step was endothermic by 38.0 kJ/mol. The free energy of the transition state was higher by 34.6 kJ/mol than that of the initial state A-2, but it was lower than that of the corresponding final state A-3. It means there was no activation barrier for this simple hydride transfer step. When the substituent was Me, this step was endothermic by 30.8 kJ/mol, without an activation barrier. The lower energy indicates the promotion of hydrogen bonding. When the substituent was iPr, this step was endothermic by 26.7 kJ/mol and had an activation barrier of 27.7 kJ/mol. The help of hydrogen bonding to hydride addition to CO2 was more significant. However, when the substituent was tBu, this step was endothermic by 40.9 kJ/mol, with an activation barrier of 42.3 kJ/mol. The change in activation barrier was great. As mentioned above, the effect of steric hindrance in D-2 was stronger than the effect of hydrogen bonding. In D-2, the distance between the hydride and the C atom of CO2 was 3.019 Å. This long distance made hydride transfer more difficult. Furthermore, the O−C−O bond angle of CO2 in D-2 was 178.2°, larger than in other complexes, which led to a weakest pre-activation of CO2. These factors result in a highest activation barrier for hydride addition to CO2 on the tBu-substituted complex.
When the substituent was the cPe cyclic group, this step was endothermic by 28.5 kJ/mol and had an activation barrier of 31.3 kJ/mol. When the substituent was Cy, this step was endothermic by 25.0 kJ/mol, with an activation barrier of 27.3 kJ/mol. The activation barriers required were both significantly lower than that on the tBu-substituted complex, because the cPe or Cy group are characterized by a wider open space compared to the tBu group, despite an increased number of C atoms. It should be mentioned that hydrogen bonding formed between the O atoms of CO2 and five H atoms on the PNP arm at a distance of 2.342 Å, 2.474 Å, 2.616 Å, 2.640 Å, 3.005 Å, respectively. It is obvious that the increase of hydrogen bonds made hydride addition to CO2 more facile. The situation on Cy-substituted complex was similar.

2.3. Substituent’s Effects on HCOO− Rotation

The following step is the rotation of the HCOO− moiety to produce a more stable formate complex. The free energy profiles for this step on six Ru-PNP complexes with different substituents are shown in Figure 4, and the optimized geometries of all transition states involved are exhibited in Figure 5. The parameters of selected hydrogen bonds are reported in Table 5. The optimized geometries of the products are shown in Figure S3 in the Supporting Information.
For the HCOO− rotation process, a wider space is favorable. Thus, the effect of steric hindrance is more obvious. As for hydrogen bonding, it is always favorable for CO2 coordination and helpful for hydride addition to CO2, but is not definitely beneficial for the rotation of the HCOO− moiety. When the substituent was H, this step was exothermic by 68.8 kJ/mol and had an activation barrier of 37.9 kJ/mol. When the substituent was Me, this step was exothermic by 62.0 kJ/mol and had an activation barrier of 31.1 kJ/mol. The decrease in the activation barrier can be thought of as the promotion of hydrogen bonding. When the substituent was iPr, this step was exothermic by 59.9 kJ/mol, with an activation barrier of 36.4 kJ/mol. Steric hindrance should be considered in the iPr-substituted complex, as mentioned above. It can be known that the effect of steric hindrance suppresses the effect of hydrogen bonding, and thus the activation barrier increases. For the hydride addition step, when the substituent was altered from Me to iPr, the barrier decreased. However, the barrier for HCOO− rotation increased, indicating a stronger influence of steric hindrance than hydrogen bonding on HCOO− rotation. When the substituent was tBu, this step was exothermic by 58.1 kJ/mol, with an activation barrier of 56.0 kJ/mol. The barrier increased greatly, due to the extremely strong effect of steric hindrance.
When the substituent was cPe, this step was exothermic by 56.5 kJ/mol, with an activation barrier of 39.4 kJ/mol. When the substituent was Cy, this step was exothermic by 54.6 kJ/mol, with an activation barrier of 41.4 kJ/mol. In these cases, the activation barriers were both lower than that on the tBu-substituted complex, because the cPe or Cy groups provided a wider space for HCOO− rotation. It should be mentioned that the direction of HCOO− rotation in A-TS3/4, B-TS3/4, and C-TS3/4 was opposite to that in D-TS3/4, E-TS3/4, and F-TS3/4, as shown in Figure 5, because of limited space in the last three complexes.

2.4. Substituent’s Effects on Total Reaction

We will now examine the substituent’s effects on the total reaction of CO2 hydrogenation to formate. The free energy profiles for the total reaction on six Ru-PNP complexes with different substituents are shown in Figure 6 and Figure 7. The total barriers for the hydrogenation of CO2 to formate on each Ru-PNP complex are also reported in Table 6. On the H-substituted complex, the whole reaction was exothermic by 16.6 kJ/mol and had a total barrier of 90.0 kJ/mol. On the Me-substituted complex, the whole reaction was exothermic by 18.8 kJ/mol, and the total barrier was 74.3 kJ/mol. On the Me-substituted or H-substituted complex, steric hindrance was weak. The decrease in total barrier was due to the effect of hydrogen bonding. As for the iPr-substituted complex, the whole reaction was exothermic by 16.2 kJ/mol, with a total barrier of 80.2 kJ/mol. For the iPr-substituted complex, steric hindrance should be seriously considered. Steric hindrance and the promotion of hydrogen bonding play their roles at the same time. In the CO2 coordination step, steric hindrance was balanced by the effect of hydrogen bonding. In the hydride addition step, steric hindrance did not influence and hydrogen bonding facilitated the addition of hydride to CO2. In the HCOO− rotation step, the effect of steric hindrance was significant, while the effect of hydrogen bonding was minor. With the joint effects of steric hindrance and hydrogen bonding, the total barrier was slightly increased. When tBu was used as the substituent, the whole reaction was exothermic by 6.4 kJ/mol, with a total barrier of 107.7 kJ/mol. This total barrier was the highest measured for the six Ru-PNP complexes. In the CO2 coordination step, the hydride addition step, and the HCOO− rotation step, steric hindrance was so significant that the effect of hydrogen bonding was suppressed. This example suggests that a substitute with strong steric hindrance is usually unfavorable for CO2 hydrogenation to formate. However, a cyclic group can provide a wider open space for the reaction, although its number of C atoms is high. As mentioned above, the %Vbur value of P(cPe)2 was 24.2, and that of P(Cy)2 was 24.4, both lower than that of P(tBu)2 (27.5) and very close to that of P(iPr)2 (24.5). On the cPe-substituted complex, the whole reaction was exothermic by 10.5 kJ/mol and had a total barrier of 85.4 kJ/mol. On the Cy-substituted complex, the whole reaction was exothermic by 12.2 kJ/mol, with a total barrier of 83.8 kJ/mol. The total barriers in the last two cases were lower than that on the tBu-substituted complex, indicating that cPe or Cy can be better substituents than tBu. It can be deduced from these examples that an appropriate substituent in the PNP ligand should be chosen on the basis of the principle that more hydrogen atoms for forming hydrogen bonds but a wider open space for reducing steric hindrance are ideal.
When simulating an experimental work, large groups are usually reduced to smaller groups to simplify and save computational resources. However, our results revealed that if the tBu or iPr group was replaced by the H or Me group, the difference was so significant that it could not be neglected. Especially, when substituting tBu with Me, the difference in total barrier for CO2 hydrogenation measured on tBu-substituted complex and on Me-substituted complex was 33.4 kJ/mol. The Me group was the most favorable, while the tBu group was the most unfavorable.

2.5. Impact of Solvents on Substituent’s Effects

Nozaki observed that the iPr substituent can lead to higher catalytic activity than the tBu substituent on Ru [14] or Ir [13] complexes. Our results calculated in gas phase agree with these previous experimental finding. However, Huang and coworkers found that the tBu-substituted PN3P-type Ir complex was more active than the cPe-substituted Ir complex [15]. Bernskoetter and coworkers reported that the Cy-substituted PNP-type Fe complex was slightly more active than the iPr-substituted Fe complex [16]. These findings are in contrast with our theoretical prediction mentioned above. We noticed that Huang’s and Bernskoetter’s experiments were performed in THF solvent, whereas our computational data were calculated in gas phase. Although the metal center and the PNP ligand were both different from those used in this work, we are aware of the importance of solvent’s effects. We performed the calculations in THF solution and aqueous phase. The total barriers for CO2 hydrogenation to formate on the six Ru-PNP complexes in aqueous phase and THF solution are reported in Table 6.
Nozaki’s experiments took place in aqueous phase. Our calculation results in aqueous phase showed that the total barrier for CO2 hydrogenation to formate on the iPr-substituted Ru complex was 61.9 kJ/mol, and that on the tBu-substituted Ru complex was 67.6 kJ/mol. It means that the iPr substituent was more favorable than the tBu substituent in aqueous phase, still consistent with Nozaki’s work. In THF solution, the total barrier for CO2 hydrogenation to formate on the iPr-substituted Ru complex was 66.5 kJ/mol, and that on the Cy-substituted Ru complex was 64.3 kJ/mol. The activity associated with Cy was slightly higher than that brought by iPr. It is interesting that the order of catalytic activity associated with Cy and iPr shifted, compared with that in gas phase. This can explain Bernskoetter’s results. As for the comparison between tBu and cPe, the total barrier for CO2 hydrogenation on the tBu-substituted Ru complex was 76.0 kJ/mol, and that on the cPe-substituted Ru complex was 61.8 kJ/mol in THF solution. Therefore, the tBu substituent was not better than the cPe substituent for this Ru-PNP complex. Our computational data cannot explain Huang’s conclusion. Very recently, Beller and co-workers tested the activity of a series of PNP-type (PNP = NH{CH2CH2(PR2)}2) Mn complexes in the hydrogenation of CO2 to formate in THF solution and found that almost no formate could be detected when the tBu-substituted Mn complex was used [45]. The impact of the solvent on the substituent’s effects in this reaction is very complicated. Solvents’ effects can change the order of catalytic activity of different substituents. We will examine this topic in detail in another study.

3. Computational Methods

The geometries of all species were optimized by using the B3LYP hybrid functional [46] with the Gaussian 09 program [47]. For Ru, the Stuttgart−Dresden pseudopotential-basis set (SDD) [48] was applied and supplemented with two sets of f functions and a set of g functions [49]. For other main group elements, the Dunning cc-pVDZ [50] basis set was applied. Frequency calculations were performed to determine the minimum or transition state of each stationary point and obtain the thermochemical properties of all species. Gibbs free energies were computed at 298.15 K and 1 atm. All transition states were verified through intrinsic reaction coordinate (IRC) calculation. The binding energy was defined as the change of enthalpy during CO2 coordination and calculated by: ∆H = H(PNPRu−CO2) − H(PNPRu) − H(CO2). Here, H(PNPRu) is the enthalpy of the Ru–PNP complex, and H(PNPRu−CO2) is the enthalpy of the association complex after CO2 coordination. The calculation of the binding free energy is similar. To consider the impact of the functional with dispersion corrections, we tested the B3LYP-D3(BJ) functional [51]. The results are presented in Table S1 and Figure S4 in the Supporting Information as a reference. When the reaction was simulated in aqueous phase or THF solution, the solvent effects were approximated with the polarizable continuum model (PCM) [52,53], especially the integral equation formalism model (IEF-PCM) [54,55]. All structural diagrams in this paper were drawn using the VMD software [56]. The %Vbur values of the PR2 group in the PNP ligand were obtained with the SambVca 2.1 program developed by Cavallo and co-workers [57]. Cartesian coordinates for optimized geometries of all the species in gas phase are collected in Table S2 in the Supporting Information.

4. Conclusions

In this work, we investigated the effects of substituents in the PNP ligand on Ru–PNP complexes in the hydrogenation of CO2 to formate. Six groups were examined as substituents linked to the P atom of the PNP ligand: hydrogen, methyl, iso-propyl, tert-butyl, cyclopentyl, and cyclohexyl. The substituent’s effects were discussed in detail from the perspectives of steric hindrance and promotion of hydrogen bonding. In the presence of both steric hindrance and hydrogen bonding promotion in each step involved in CO2 insertion into the Ru−H bond (CO2 coordination step, hydride addition step, and HCOO− rotation step), these groups exerted very different effects. The results revealed that the methyl group was the most favorable substituent when the solvent’s effects were not included, as it allowed hydrogen bonding with relatively weak steric hindrance. On the Me-substituted complex, the whole reaction was exothermic by 18.8 kJ/mol, and the total barrier was 74.3 kJ/mol. The next favorable group was the iso-propyl group. On the iPr-substituted complex, the total barrier was 80.2 kJ/mol. The tert-butyl substituent was the most unfavorable, due to remarkable steric hindrance. On the tBu-substituted complex, the total barrier was as high as 107.7 kJ/mol. It is interesting that the cyclopentyl or cyclohexyl substituent was more favorable than the tert-butyl group, because the cPe-substituted or Cy-substituted complex provided a wider open space for the reaction, compared with the commonly used tBu-substituted complex. This indicates the possibility of using cyclopentyl and cyclohexyl as suitable substituents in the Ru-PNP complex for the hydrogenation of CO2. A principle for choosing the substituent in the design of highly efficient catalysts for CO2 hydrogenation is the following: more hydrogen atoms to form hydrogen bonding but a wider open space for reducing steric hindrance are ideal. In addition, the substituent’s effects were shown to be markedly impacted by the solvent. The solvent’s effects can change the order of catalytic activity of different substituents.
The results also indicate that when the tert-butyl or iso-propyl group was simplified to a methyl group in theoretical calculations, the systematic error was not negligible.

Supplementary Materials

The following are available online at: https://www.mdpi.com/article/10.3390/catal12070760/s1, Figure S1: Optimized geometries of six Ru–PNP complexes with different substituents; Figure S2: Optimized geometries of the products in the hydride addition step on the six Ru–PNP complexes; Figure S3: Optimized geometries of the products in the HCOO− rotation step on the six Ru–PNP complexes; Table S1: Binding energy and binding free energy for CO2 coordination on Ru–PNP complexes with different substituents calculated with the B3LYP-D3(BJ) functional; Figure S4. Free energy profiles for the total reaction of CO2 hydrogenation on R-substituted (R = H, Me, iPr, tBu) complexes calculated with the B3LYP-D3(BJ) functional; Table S2: Cartesian coordinates for optimized geometries of all the species in gas phase.

Author Contributions

Conceptualization, J.L.; methodology, J.L.; investigation, X.F.; writing—original draft preparation, X.F.; writing—review and editing, J.L.; supervision, J.L.; funding acquisition, J.L. and Z.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 21776123 and the Project for Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Data Availability Statement

The data presented in this study are available in the Supporting Information.

Acknowledgments

We are thankful to the High-Performance Computing Center of Nanjing Tech University for supporting the computational resources.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00760 sch001 550
Scheme 1. Structure of PNP pincer complexes.
Scheme 1. Structure of PNP pincer complexes.
Catalysts 12 00760 sch001
Catalysts 12 00760 sch002 550
Scheme 2. Diagram of the substituent groups in the PNP ligand.
Scheme 2. Diagram of the substituent groups in the PNP ligand.
Catalysts 12 00760 sch002
Catalysts 12 00760 sch003 550
Scheme 3. Reaction process of CO2 hydrogenation to formate on the Ru-PNP complex. R presents the substituent group in the PNP ligand (H, Me, iPr, tBu, cPe, and Cy group).
Scheme 3. Reaction process of CO2 hydrogenation to formate on the Ru-PNP complex. R presents the substituent group in the PNP ligand (H, Me, iPr, tBu, cPe, and Cy group).
Catalysts 12 00760 sch003
Catalysts 12 00760 g001 550
Figure 1. Optimized geometries of CO2 coordination on six Ru–PNP complexes with different substituents. Pink balls: Ru; orange: P; red: O; blue: N; cyan: C; white: H; The distances are in Å, and the O−C−O bond angles are in degree (°). The red dotted lines represent hydrogen bonds, and the number indicates the distance of O···H (Å). The detailed parameters of these hydrogen bonds are listed in Table 3.
Figure 1. Optimized geometries of CO2 coordination on six Ru–PNP complexes with different substituents. Pink balls: Ru; orange: P; red: O; blue: N; cyan: C; white: H; The distances are in Å, and the O−C−O bond angles are in degree (°). The red dotted lines represent hydrogen bonds, and the number indicates the distance of O···H (Å). The detailed parameters of these hydrogen bonds are listed in Table 3.
Catalysts 12 00760 g001
Catalysts 12 00760 g002 550
Figure 2. Free energy profile for hydride addition to CO2 on six Ru–PNP complexes with different substituents.
Figure 2. Free energy profile for hydride addition to CO2 on six Ru–PNP complexes with different substituents.
Catalysts 12 00760 g002
Catalysts 12 00760 g003 550
Figure 3. Optimized geometries of the transition state TS2/3 in hydride addition to CO2. Pink balls: Ru; orange: P; red: O; blue: N; cyan: C; white: H; The distances are in Å, and the O−C−O bond angles are in degree (°). The red dotted lines represent hydrogen bonds, and the number indicates the distance of O···H (Å). The detailed parameters of these hydrogen bonds are listed in Table 4.
Figure 3. Optimized geometries of the transition state TS2/3 in hydride addition to CO2. Pink balls: Ru; orange: P; red: O; blue: N; cyan: C; white: H; The distances are in Å, and the O−C−O bond angles are in degree (°). The red dotted lines represent hydrogen bonds, and the number indicates the distance of O···H (Å). The detailed parameters of these hydrogen bonds are listed in Table 4.
Catalysts 12 00760 g003
Catalysts 12 00760 g004 550
Figure 4. Free energy profile for HCOO− rotation on six Ru-PNP complexes with different substituents.
Figure 4. Free energy profile for HCOO− rotation on six Ru-PNP complexes with different substituents.
Catalysts 12 00760 g004
Catalysts 12 00760 g005 550
Figure 5. Optimized geometries of the transition state TS3/4 in HCOO− rotation. The atom colors are the same as above. The distances are in Å, and the O−C−O bond angles are in degree (°). The red dotted lines represent hydrogen bonding, and the number indicates the distance of O···H (Å). The detailed parameters of these hydrogen bonds are listed in Table 5.
Figure 5. Optimized geometries of the transition state TS3/4 in HCOO− rotation. The atom colors are the same as above. The distances are in Å, and the O−C−O bond angles are in degree (°). The red dotted lines represent hydrogen bonding, and the number indicates the distance of O···H (Å). The detailed parameters of these hydrogen bonds are listed in Table 5.
Catalysts 12 00760 g005
Catalysts 12 00760 g006 550
Figure 6. Free energy profiles of the total reaction of CO2 hydrogenation on R-substituted (R = H, Me, iPr, tBu) complexes.
Figure 6. Free energy profiles of the total reaction of CO2 hydrogenation on R-substituted (R = H, Me, iPr, tBu) complexes.
Catalysts 12 00760 g006
Catalysts 12 00760 g007 550
Figure 7. Free energy profiles of the total reaction of CO2 hydrogenation on R-substituted (R = Me, tBu, cPe, Cy) complexes.
Figure 7. Free energy profiles of the total reaction of CO2 hydrogenation on R-substituted (R = Me, tBu, cPe, Cy) complexes.
Catalysts 12 00760 g007
Table
Table 1. Binding energy and binding free energy for CO2 coordination on Ru-PNP complexes with different substituents.
Table 1. Binding energy and binding free energy for CO2 coordination on Ru-PNP complexes with different substituents.
Complex CodeSubstituent TypeBinding Energy (kJ/mol)Binding Free Energy (kJ/mol)
AHydrogen (H)−11.214.2
BMethyl (Me)−13.012.4
Ciso-Propyl (iPr)−12.517.1
Dtert-Butyl (tBu)−10.010.8
ECyclopentyl (cPe)−12.617.6
FCyclohexyl (Cy)−14.917.4
Table
Table 2. Value of %Vbur for the PR2 group in the PNP ligand.
Table 2. Value of %Vbur for the PR2 group in the PNP ligand.
PR2 Group%Vbur for Sphere Radius at 3.5 Å
PH217.7
PMe220.1
P(iPr)224.5
P(tBu)227.5
P(cPe)224.2
P(Cy)224.4
Table
Table 3. Parameters of hydrogen bonds in the optimized geometries of CO2 coordination on Ru–PNP complexes.
Table 3. Parameters of hydrogen bonds in the optimized geometries of CO2 coordination on Ru–PNP complexes.
Serial
Number		Distance of
O···H		Angel of
O···H−C		Serial
Number		Distance of
O···H		Angel of
O···H−C
(1)2.566146.8(11)3.029119.3
(2)2.612155.8(12)3.048112.3
(3)2.841142.2(13)2.644153.1
(4)2.849145.6(14)2.670157.8
(5)2.611165.3(15)2.736132.4
(6)2.645157.0(16)2.504161.6
(7)2.924139.8(17)2.553161.5
(8)2.983135.0(18)2.803151.6
(9)2.670160.9(19)3.009123.7
(10)2.687152.5
Table
Table 4. Parameters of hydrogen bonding in the optimized geometries of the transition state TS2/3 during hydride addition to CO2.
Table 4. Parameters of hydrogen bonding in the optimized geometries of the transition state TS2/3 during hydride addition to CO2.
Serial
Number		Distance of
O···H		Angel of
O···H−C		Serial
Number		Distance of
O···H		Angel of
O···H−C
(1)2.332141.3(12)2.678140.0
(2)2.356147.2(13)2.342147.9
(3)2.474141.9(14)2.474148.9
(4)2.474143.1(15)2.616137.4
(5)2.319145.0(16)2.640126.5
(6)2.425153.5(17)3.005133.7
(7)2.492149.8(18)2.363146.3
(8)2.714141.2(19)2.489148.5
(9)2.384134.9(20)2.507155.1
(10)2.452151.2(21)2.596140.5
(11)2.672162.1(22)3.053113.0
Table
Table 5. Parameters of hydrogen bonds in the optimized geometries of the transition state TS3/4 in HCOO− rotation.
Table 5. Parameters of hydrogen bonds in the optimized geometries of the transition state TS3/4 in HCOO− rotation.
Serial
Number		Distance of
O···H		Angel of
O···H−C		Serial
Number		Distance of
O···H		Angel of
O···H−C
(1)2.092136.1(10)2.161152.5
(2)2.167140.5(11)2.189172.1
(3)2.207140.6(12)2.113146.1
(4)2.222138.8(13)2.169147.4
(5)2.136151.6(14)2.494144.1
(6)2.161142.6(15)2.753130.8
(7)2.236142.8(16)2.105146.5
(8)2.437137.7(17)2.173146.8
(9)2.054152.9(18)2.300163.1
Table
Table 6. Total barriers for CO2 hydrogenation to formate on six Ru-PNP complexes with different substituents in gas phase, aqueous phase, and THF solution.
Table 6. Total barriers for CO2 hydrogenation to formate on six Ru-PNP complexes with different substituents in gas phase, aqueous phase, and THF solution.
Complex
Code		Substituent
Type		Total Barrier (kJ/mol)
In Gas PhaseIn Aqueous PhaseIn THF Solution
AH90.074.678.3
BMe74.364.065.8
CiPr80.261.966.5
DtBu107.767.676.0
EcPe85.461.261.8
FCy83.860.864.3
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Feng, X.; Li, J.; Yang, Z. Substituent’s Effects of PNP Ligands in Ru(II)-Catalyzed CO2 Hydrogenation to Formate: Theoretical Analysis Considering Steric Hindrance and Promotion of Hydrogen Bonding. Catalysts 2022, 12, 760. https://doi.org/10.3390/catal12070760

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Feng X, Li J, Yang Z. Substituent’s Effects of PNP Ligands in Ru(II)-Catalyzed CO2 Hydrogenation to Formate: Theoretical Analysis Considering Steric Hindrance and Promotion of Hydrogen Bonding. Catalysts. 2022; 12(7):760. https://doi.org/10.3390/catal12070760

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Feng, Xiangyang, Jun Li, and Zhuhong Yang. 2022. "Substituent’s Effects of PNP Ligands in Ru(II)-Catalyzed CO2 Hydrogenation to Formate: Theoretical Analysis Considering Steric Hindrance and Promotion of Hydrogen Bonding" Catalysts 12, no. 7: 760. https://doi.org/10.3390/catal12070760

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