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Article

Theoretical Study of Reversible Hydrogenation of CO2 to Formate Catalyzed by Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P Complexes: The Effect of the Transition Metal Center

by
Lingqiang Meng
,
Lihua Yao
and
Jun Li
*
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 2024, 14(7), 440; https://doi.org/10.3390/catal14070440
Submission received: 15 June 2024 / Revised: 5 July 2024 / Accepted: 6 July 2024 / Published: 9 July 2024
(This article belongs to the Special Issue Catalysis for Selective Hydrogenation of CO and CO2, 2nd Edition)
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Abstract

:
In 2022, Beller and coworkers achieved the reversible hydrogenation of CO2 to formic acid using a Mn(I)–PN5P complex with excellent activity and reusability of the catalyst. To understand the detailed mechanism for the reversible hydrogen release–storage process, especially the effects of the transition metal center in this process, we employed DFT calculations according to which Ru(II) and Fe(II) are considered as two alternatives to the Mn(I) center. Our computational results showed that the production of formic acid from CO2 hydrogenation is not thermodynamically favorable. The reversible hydrogen release–storage process actually occurs between CO2/H2 and formate rather than formic acid. Moreover, Mn(I) might not be a unique active metal for the reversible hydrogenation of CO2 to formate; Ru(II) would be a better option.

1. Introduction

The significant consumption of fossil fuels has led to an increase in atmospheric CO2 concentrations from 317 ppm in 1960 to 428 ppm in 2024 [1,2]. Although CO2 poses various environmental problems, it is also a relatively abundant and non-toxic carbon source that can be used to produce value-added chemicals such as methanol and formic acid [3]. Hydrogen energy is currently attracting considerable attention as a renewable and clean energy. However, the transportation and storage of hydrogen are problematic due to its physical and explosive properties in oxygen-containing mixtures. CO2 hydrogenation to formic acid, which can convert hydrogen into a liquid organic hydrogen carrier, presents a promising solution to this problem [4,5]. Therefore, in recent years, extensive research has been performed to improve the hydrogenation of CO2 to formic acid or formate and the subsequent dehydrogenation of formic acid [6,7,8,9].
For CO2 hydrogenation, Inoue et al. conducted a preliminary study on the hydrogenation of CO2 to formate using transition metal catalysts [10]. Since then, the application of transition metal homogeneous catalysts in CO2 hydrogenation has gained significant attention, though catalytic efficiency remains a challenge [11,12,13,14]. Until 2009, Nozaki et al. achieved efficient catalytic CO2 hydrogenation to formate using an Ir–PNP (PNP = 2,6-(di-iso-propylphosphinomethyl)-pyridine) complex, with a turnover number (TON) of 3500,000 and a turnover frequency (TOF) of 120,000 h−1 at 200 °C and 5.0 MPa [15]. In 2014, Pidko et al. synthesized a Ru–PNP complex by substituting Ir with Ru and used this complex to catalyze the hydrogenation of CO2 to formate (TOF = 1,100,000 h−1 at 120 °C and 40 bar with DMF as the solvent and DBU as the base) [16]. Other Ru-based complexes have also been shown to be highly efficient catalysts for the hydrogenation of CO2 to formic acid (in ionic liquids) or formates (in the presence of organic bases) [17,18,19]. For non–precious metals, Milstein et al. synthesized an Fe-based complex, (tBuPNP)Fe(H)2(CO), which demonstrated catalytic activity comparable to that of precious metals, Ir and Ru, achieving low-pressure (10 bar) conversion to formate [20]. Then, a series of other Fe(II) carbonyl hydride complexes and Fe–PN3P (PN3P = 2,6-diaminopyridine scaffold) complexes were used for CO2 hydrogenation [21,22]. In 2017, Gonsalvi et al. reported, for the first time, the synthesis of two Mn(I)-based complexes, Mn(PN3PNHiPr)(H)(CO)2 and Mn(PN3PNMeiPr)(H)(CO)2, and achieved a maximum conversion of 10,000 and a yield of more than 99% at 80 °C and 80 bar [23].
Several studies have explored the dehydrogenation of formic acid [24,25]. For precious metals, Nozaki et al. catalyzed the dehydrogenation of formic acid (FA) using Ir–PNP complexes to obtain a TOF of 120,000 h−1 [26]. In 2014, Pidko et al. used the Ru–PNP complex, which demonstrated excellent stability and the highest FA dehydrogenation reaction rate in DMF. In the presence of triethylamine (Et3N), the maximum TOF was 257,000 h−1, and the maximum TON was 326,500 [16]. Huang et al. achieved better results using the Ru–PN3P complex in dimethyl sulfoxide solvent [27]. For non-precious metal complexes, Milstein et al. used the Fe–PNP complex for FA dehydrogenation and obtained a TON of 100,000 and a TOF of 417 h−1 at 40 °C [28]. Other Fe-based complexes also show high catalytic activity in formic acid dehydrogenation reactions [29,30]. Boncella et al. used an aliphatic Mn–PNP complex for FA dehydrogenation without a base, achieving a TON of 190 [31].
Although various noble and non-precious metal complexes have been used for CO2 hydrogenation and FA dehydrogenation, these processes were carried out under different conditions. In 2022, Beller and coworkers achieved the reversible hydrogenation of CO2 to formic acid by using a Mn(I)–PN5P complex in mixed solution (H2O:THF = 1:1) with excellent activity and the reusability of the catalyst (CO2 hydrogenation: TON = 230,000; FA dehydrogenation: TON = 29,000) [32]. This is the first time this result was achieved under the same reaction conditions (additives and solvents).
For the reaction mechanism for CO2 hydrogenation, some theoretical studies have been performed [33,34,35]. In 2014, Pidko et al. showed that the intermediate (Ru–OCHO) after the rotation of formate had the lowest energy based on theoretical calculations [36]. But the mechanism proposed by Pathak et al. using aliphatic Mn–PNP complexes showed that formic acid is formed directly without the rotation of formate [37]. And in 2022, when Batista et al. suggested the mechanism for CO2 hydrogenation to formic acid, they considered the rotation of formate but did not study this intermediate further [38]. In Beller’s paper, the authors proposed a mechanism for CO2 hydrogenation and FA dehydrogenation where formic acid is formed directly from formate [32]. However, previous work has shown that the formate intermediate is so stable that the formation of formic acid directly from formate is energetically unfavorable. Therefore, the detailed mechanism for the reversible hydrogenation of CO2 to FA is still in doubt [39,40,41].
In addition, the Ru–based complexes are the most frequently used for CO2 hydrogenation and FA dehydrogenation. And among non-precious metals, the Fe-based complexes exhibited excellent catalytic efficiency. However, Beller et al. accomplished the reversible hydrogenation of CO2 to FA for the first time using the Mn(I) complex. In order to make clear whether the Mn(I) complex is a unique catalyst for this reversible reaction, we compared the catalytic activity of the Fe(II) and Ru(II) complexes with the Mn(I) complex, as shown in Scheme 1.

2. Results and Discussion

Three complexes, Mn(I)–PN5P, Fe(II)–PN5P, and Ru(II)–PN5P, were considered for the theoretical study of the formation of formic acid and the regeneration of the catalyst throughout the catalytic cycle. In addition, the mechanism for the reversible operation of hydrogen storage and release was further discussed. All calculations in this paper were performed in mixed solutions (H2O:THF = 1:1).

2.1. The Metal Centers’ Effects on the Formation of Formic Acid from CO2 Hydrogenation

The reaction mechanism for CO2 hydrogenation to formic acid or formate has been extensively studied. The whole process includes the coordination of CO2, the direct addition of hydride to CO2, and the rotation of formate [42,43,44]. Herein, we propose a complete catalytic cycle for the hydrogenation of CO2 to formic acid, as shown in Scheme 2. Initially, CO2 interacts with the hydride on the metal center of complex 1 to form complex 2. Secondly, the hydride can be added directly to CO2 to form complex 3. The formate moiety then rotates itself to form a very stable intermediate complex 4. The rotated formate attracts a proton from the arm of the PN5P ligand to form complex 5, which leads to the dearomatization of the PN5P ligand. Subsequently, after FA is released, complex 6 is yielded. The final step, 6→1, is the regeneration of the catalyst. The optimized geometries of all species in CO2 hydrogenation to formic acid catalyzed by the Ru(II)–PN5P complex are shown in Figure 1. The optimized geometries of all species involved under the Fe(II)–PN5P and Mn(I)–PN5P complexes are provided in the Supporting Information (Figures S1 and S2).
The first step in the formation of FA is CO2 coordination. CO2 interacts with the hydride of the M–H bond on complex 1 to form complex 2. The distances between CO2 and the hydride on the Ru, Fe, and Mn center are 2.951 Å, 3.039 Å, and 3.354 Å, respectively. The O–C–O bond angles are 178.8°, 179.0°, and 179.7°, respectively. The Gibbs free energy changes in CO2 coordination to Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P complexes are 2.4 kcal/mol, 2.4 kcal/mol, and 2.2 kcal/mol, respectively. After CO2 is fixed, the hydride can be directly added to CO2 to form complex 3. The formed C−H bond distances in complex 3 with the Ru, Fe, and Mn centers are 1.216 Å, 1.214 Å, and 1.219 Å, respectively. The O–C–O bond angles are 135.1°, 134.9°, and 135.5°, respectively. For the Ru(II)–PN5P complex, the activation energy for this step is 3.5 kcal/mol. For the Fe(II)–PN5P and Mn(I)–PN5P complexes, the activation energies are 3.1 kcal/mol and 4.3 kcal/mol, respectively. The following step is the rotation of the HCOO− moiety to form a more stable formate complex 4. For the Ru(II)–PN5P complex, this step is exergonic by 10.2 kcal/mol with an activation energy of 2.8 kcal/mol. For the Fe(II)–PN5P complex, this step is exergonic by 12.9 kcal/mol with an activation energy of 3.0 kcal/mol. For the Mn(I)–PN5P complex, this step is exergonic by 7.6 kcal/mol with an activation energy of 8.7 kcal/mol. From complexes 4 to 5, the formation of FA is endergonic by 14.7 kcal/mol and has an activation energy of 13.7 kcal/mol on the Ru(II)–PN5P complex. On the Fe(II)–PN5P complex, this step is endergonic by 15.7 kcal/mol with an activation energy of 14.3 kcal/mol. On the Mn(I)–PN5P complex, this step is endergonic by 10.8 kcal/mol with an activation energy of 10.0 kcal/mol. Finally, formic acid is removed from complex 5, and the free energy changes for this step are 5.3 kcal/mol, 6.8 kcal/mol, and 10.0 kcal/mol, respectively.
As shown in Figure 2, for the overall process of CO2 hydrogenation to FA, the highest barrier occurs at the stage from 4 to 6. For the Ru(II)–PN5P complex, it is 20.0 kcal/mol. And, for the Fe(II)–PN5P and Mn(I)–PN5P complex, it is 22.5 kcal/mol and 20.8 kcal/mol, respectively. These values indicate that the Ru(II)–PN5P complex exhibits the best catalytic activity, but the Mn(I)–PN5P complex is comparable to the Ru(II)–PN5P complex; the activity of the Fe(II)–PN5P complex is the worst. The reverse process of CO2 hydrogenation to FA is just the dehydrogenation of FA. Thus, 6→5→4→3→2→1 is the pathway for FA dehydrogenation. For FA dehydrogenation, the highest overall barrier occurs at step 4→TS3/4. It is 13.0 kcal/mol, 15.9 kcal/mol, and 16.3 kcal/mol on the Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P complexes, respectively, suggesting that the order of catalytic activity for FA dehydrogenation is Ru(II)–PN5P > Fe(II)–PN5P > Mn(I)–PN5P.
The global minimum of the total free energy profile corresponds to complex 4, the rotated formate species, with relative values of –8.8 kcal/mol, –12.0 kcal/mol, and –11.1 kcal/mol, respectively. At the same time, the relative values of FA + 6 are 11.2 kcal/mol, 10.5 kcal/mol, and 9.7 kcal/mol, respectively. This means that the formation of FA is energetically unfavorable; if FA is not removed from the reaction system, the final product will be complex 4 rather than complex 6 and FA.

2.2. The Metal Centers’ Effects on the Regeneration of Catalysts

The regeneration of catalysts can proceed down two primary routes: (1) direct H2 cleavage after its coordination on complex 6, which is actually a process of proton transfer; (2) water molecules in solution can accelerate the process as a bridge for proton transfer, as shown in Scheme 3. Firstly, H2 is bound to the metal center in complex 6 to form complex 7. Then, H2 is broken on complex 7 to regenerate complex 1 by direct cleavage (route 1). Route 2 shows the cleavage of H2 with the help of a water bridge. The optimized geometries of selected species in the regeneration process on the Ru(II)–PN5P complex are illustrated in Figure 3. The optimized geometries of selected species on the Fe(II)–PN5P and Mn(I)–PN5P complexes during the regeneration process are given in Figures S3 and S4.
As shown in Figure 4, for the regeneration of catalysts, the first step is H2 coordination. The bond distance of free H2 molecules is 0.762 Å, while the distance increases to 0.841 Å, 0.829 Å, and 0.815 Å, respectively, after bonding on Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P complexes. This step is endergonic by 3.1 kcal/mol, 3.9 kcal/mol, and 8.2 kcal/mol, respectively, for the Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P complexes. In route 1, the direct cleavage of H2 on the Ru(II)–PN5P complex is exergonic by 5.4 kcal/mol with an activation energy of 31.2 kcal/mol. On the Fe(II)–PN5P complex, this step is exergonic by 5.6 kcal/mol with an activation energy of 30.0 kcal/mol. On the Mn(I)–PN5P complex, this step is exergonic by 9.1 kcal/mol with an activation energy of 24.3 kcal/mol. For the overall process of catalyst regeneration, the apparent activation barrier is 34.3 kcal/mol, 33.9 kcal/mol, and 32.5 kcal/mol, respectively. In route 2, with the help of a water bridge, the activation barrier is lowered to 22.8 kcal/mol, 20.5 kcal/mol, and 18.7 kcal/mol, respectively.

2.3. The Metal Centers’ Effects on Hydrogen Storage and Release

In Beller’s experiment, the H2 storage–release cycle started from the dehydrogenation of FA, catalyzed by complex 6 at 90 °C, in which the pressure was carefully decreased to release H2. Then, CO2 was reloaded under high pressure (20 bar), followed by fulfilling H2 (80 bar) at 85 °C. After the H2 storage step, H2 release took place when the pressure decreased to start a new cycle. According to our computational study mentioned above, the pathway for FA dehydrogenation is 6→5→4→3→2→1→9→8→7→6, and the pathway for CO2 hydrogenation is 6→7→8→9→1→2→3→4. This is the first cycle, and the end of this cycle is complex 4 as the stable-rotated formate complex. When the pressure decreases, CO2 is released first, and then H2 is released (4→3→2→1→9→8→7→6). The H2 storage operation is the reverse process 6→7→8→9→1→2→3→4. This provides the details of the second cycle. We find that the H2 storage in the second cycle is the same as that in the first cycle, but the H2 release is different. In the first cycle, H2 is released from FA dehydrogenation (6→5→4→3→2→1→9→8→7→6); however, in the second cycle, H2 is released from formate dehydrogenation (4→3→2→1→9→8→7→6). All of the subsequent H2 storage–release cycles occur as the second cycle. The free energy profile for the H2 storage–release cycle is presented in Figure 5. The apparent activation barriers (6→TS8/9) for H2 storage on the Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P complexes are 18.7 kcal/mol, 20.5 kcal/mol, and 22.8 kcal/mol, respectively, as collected in Table 1. And for the reverse H2 release, the apparent activation barriers (9→TS8/9) for H2 release on the Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P complexes are 24.4 kcal/mol, 25.7 kcal/mol, and 27.5 kcal/mol, respectively. It can be seen that the Mn(I)–PN5P complex is not the most highly active catalyst even for H2 storage or for H2 release; the Ru(II)–PN5P complex may have the best catalytic efficiency, and the second-best option is the Fe(II)–PN5P complex.

3. Computational Methods

All calculations were performed with the Gaussian 09 program [45]. Geometry optimization and frequency analyses were carried out using the B3LYP hybrid functional [46]. To obtain more accurate single-point energy, the B3LYP–D3(BJ) functional with dispersion corrections was further applied [47]. For Ru, Fe, and Mn, the Stuttgart–Dresden pseudopotential basis set (SDD) was employed, which was supplemented by two sets of f functions and a set of g functions [48]. And the Dunning cc–pVDZ [49] basis set was applied for the other main group elements. Frequency analyses were performed to determine the minimum or transition state at each stationary point and to obtain the thermochemical properties of all species. Gibbs free energy was calculated at 298.15 K and 1 atm. All transition states were verified by intrinsic reaction coordinate (IRC) calculations. In order to keep in accord with the experimental conditions, we modeled the mixed solvent (H2O:THF = 1:1) in this paper. To deal with the solvent effects of the mixed solution (H2O:THF = 1:1), the polarizable continuum model (PCM) [50,51] was applied by defining the static dielectric constants (eps = 42.8905) and the dynamic dielectric constant (epsinf = 1.875937) of the solvent, respectively. All structural plots in this paper were drawn using VMD software (version 1.9.2, 29 December 2014) [52].

4. Conclusions

In this study, the reversible hydrogenation of CO2 was investigated by theoretical calculations. For the process of CO2 hydrogenation to FA, the highest barrier occurs at stages 4 to 6. This means that the formation of FA is energetically unfavorable; if FA is not removed from the reaction system, the final product will be complex 4 rather than complex 6 and FA. The Ru(II)–PN5P complex exhibits the best catalytic activity, but the Mn(I)–PN5P complex is comparable, and the Fe(II)–PN5P complex is the worst. For FA dehydrogenation, the highest overall barrier occurs at step 4→TS3/4. And the order of catalytic activity is Ru(II)–PN5P > Fe(II)–PN5P > Mn(I)–PN5P. During catalyst regeneration, the activation barrier is lowered with the help of a water bridge.
We also further investigated the mechanism of H2 storage and release. We found that H2 storage in the second cycle was the same as that in the first cycle, but H2 release was different. In the first cycle, H2 is released from FA dehydrogenation. However, in the second cycle, H2 is released from formate dehydrogenation. All of the subsequent H2 storage–release cycles occur as the second cycle. In the process of H2 storage and release, it is the formate rather than formic acid that participates in the reversible cycle. The Mn(I)–PN5P complex is not the most highly active catalyst; the Ru(II)–PN5P complex may have the best catalytic efficiency, and the second-best option is the Fe(II)–PN5P complex. The calculations demonstrate that the Mn(I)–PN5P catalyst does not have a special presence, while Ru(II)–PN5P is more favorable. This study provides a theoretical basis for experimental research by expanding the range of catalysts through theoretical studies.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14070440/s1. Figure S1. The optimized geometries of all species in CO2 hydrogenation to formic acid catalyzed by the Fe(II)–PN5P complex; Figure S2. The optimized geometries of selected species in the regeneration of Fe(II)–PN5P complexes; Figure S3. The optimized geometries of all species in CO2 hydrogenation to formic acid catalyzed by the Mn(I)–PN5P complex; Figure S4. The optimized geometries of selected species in the regeneration of Mn(I)–PN5P complexes; and Table S1. Cartesian coordinates for optimized geometries of all the species of the three complexes in the mixed solution (H2O:THF = 1:1).

Author Contributions

Conceptualization, J.L.; methodology, J.L.; investigation, L.M.; writing—original draft preparation, L.M.; writing—review and editing, J.L. and L.Y.; supervision, J.L.; funding acquisition, J.L. 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

Data are contained within the article and Supplementary Materials.

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 conflicts of interest.

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Catalysts 14 00440 sch001
Scheme 1. Mn(I)–PN5P, Fe(II)–PN5P, and Ru(II)–PN5P complexes considered in this work (P = PiPr2; R = NH–C3H5).
Scheme 1. Mn(I)–PN5P, Fe(II)–PN5P, and Ru(II)–PN5P complexes considered in this work (P = PiPr2; R = NH–C3H5).
Catalysts 14 00440 sch001
Catalysts 14 00440 sch002
Scheme 2. Proposed mechanism for CO2 hydrogenation to formic acid (P = PiPr2, R1 = NH–C3H5; M = Ru, Fe, R2 = H; M = Mn, R2 = CO).
Scheme 2. Proposed mechanism for CO2 hydrogenation to formic acid (P = PiPr2, R1 = NH–C3H5; M = Ru, Fe, R2 = H; M = Mn, R2 = CO).
Catalysts 14 00440 sch002
Catalysts 14 00440 g001
Figure 1. The optimized geometries of all species in CO2 hydrogenation to formic acid catalyzed by the Ru(II)–PN5P complex. The R1 groups in TS4/5 and complex 5 are omitted for clarity. The bond distances are in angstrom (Å), and the bond angles are in degrees (◦). (Ru: pink; C: cyan; P: orange; O: red; N: blue; H: white).
Figure 1. The optimized geometries of all species in CO2 hydrogenation to formic acid catalyzed by the Ru(II)–PN5P complex. The R1 groups in TS4/5 and complex 5 are omitted for clarity. The bond distances are in angstrom (Å), and the bond angles are in degrees (◦). (Ru: pink; C: cyan; P: orange; O: red; N: blue; H: white).
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Figure 2. Free energy profile for CO2 hydrogenation to formic acid catalyzed by Mn(I)–PN5P, Fe(II)–PN5P, and Ru(II)–PN5P complexes.
Figure 2. Free energy profile for CO2 hydrogenation to formic acid catalyzed by Mn(I)–PN5P, Fe(II)–PN5P, and Ru(II)–PN5P complexes.
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Catalysts 14 00440 sch003
Scheme 3. The pathway for the regeneration of catalysts (P = PiPr2, R1 = NH–C3H5; M = Ru, Fe, R2 = H; M = Mn, R2 = CO).
Scheme 3. The pathway for the regeneration of catalysts (P = PiPr2, R1 = NH–C3H5; M = Ru, Fe, R2 = H; M = Mn, R2 = CO).
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Catalysts 14 00440 g003
Figure 3. The optimized geometries of selected species in the regeneration of the Ru(II)–PN5P complex. The R1 groups in the TS7/1 and TS8/9 are omitted for clarity. The distances are angstrom (Å) (Ru: pink; C: cyan; P: orange; O: red; N: blue; H: white).
Figure 3. The optimized geometries of selected species in the regeneration of the Ru(II)–PN5P complex. The R1 groups in the TS7/1 and TS8/9 are omitted for clarity. The distances are angstrom (Å) (Ru: pink; C: cyan; P: orange; O: red; N: blue; H: white).
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Figure 4. Free energy profile for the regeneration of Mn(I)–PN5P, Fe(II)–PN5P, and Ru(II)–PN5P complexes.
Figure 4. Free energy profile for the regeneration of Mn(I)–PN5P, Fe(II)–PN5P, and Ru(II)–PN5P complexes.
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Catalysts 14 00440 g005
Figure 5. Free energy profile for the reversible hydrogen storage–release catalyzed by Mn(I)–PN5P, Fe(II)–PN5P, and Ru(II)–PN5P complexes.
Figure 5. Free energy profile for the reversible hydrogen storage–release catalyzed by Mn(I)–PN5P, Fe(II)–PN5P, and Ru(II)–PN5P complexes.
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Table 1. The activation energy for the reversible hydrogen storage–release catalyzed by the three complexes.
Table 1. The activation energy for the reversible hydrogen storage–release catalyzed by the three complexes.
Activation Energy (kcal/mol)Ru(II)–PN5PFe(II)–PN5PMn(I)–PN5P
CO2 hydrogenation to formate (H2 storage)18.720.522.8
Dehydrogenation of formate (H2 release)24.425.727.5
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Meng, L.; Yao, L.; Li, J. Theoretical Study of Reversible Hydrogenation of CO2 to Formate Catalyzed by Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P Complexes: The Effect of the Transition Metal Center. Catalysts 2024, 14, 440. https://doi.org/10.3390/catal14070440

AMA Style

Meng L, Yao L, Li J. Theoretical Study of Reversible Hydrogenation of CO2 to Formate Catalyzed by Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P Complexes: The Effect of the Transition Metal Center. Catalysts. 2024; 14(7):440. https://doi.org/10.3390/catal14070440

Chicago/Turabian Style

Meng, Lingqiang, Lihua Yao, and Jun Li. 2024. "Theoretical Study of Reversible Hydrogenation of CO2 to Formate Catalyzed by Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P Complexes: The Effect of the Transition Metal Center" Catalysts 14, no. 7: 440. https://doi.org/10.3390/catal14070440

APA Style

Meng, L., Yao, L., & Li, J. (2024). Theoretical Study of Reversible Hydrogenation of CO2 to Formate Catalyzed by Ru(II)–PN5P, Fe(II)–PN5P, and Mn(I)–PN5P Complexes: The Effect of the Transition Metal Center. Catalysts, 14(7), 440. https://doi.org/10.3390/catal14070440

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