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Article

Trans Influence of Boryl Ligands in CO2 Hydrogenation on Ruthenium Complexes: Theoretical Prediction of Highly Active Catalysts for CO2 Reduction

by
Tian Liu
,
Zhangyong Liu
,
Lipeng Tang
,
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 2021, 11(11), 1356; https://doi.org/10.3390/catal11111356
Submission received: 31 March 2021 / Revised: 26 October 2021 / Accepted: 3 November 2021 / Published: 12 November 2021
(This article belongs to the Special Issue Carbon Dioxide Utilization: From Homogeneous to Heterogeneous Catalysis)
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Abstract

:
In this work, we study the trans influence of boryl ligands and other commonly used non-boryl ligands in order to search for a more active catalyst than the ruthenium dihydride complex Ru(PNP)(CO)H2 for the hydrogenation of CO2. The theoretical calculation results show that only the B ligands exhibit a stronger trans influence than the hydride ligand and are along increasing order of trans influence as follows: –H < –BBr2 < –BCl2 ≈ –B(OCH)2 < –Bcat < –B(OCH2)2 ≈ –B(OH)2 < –Bpin < –B(NHCH2)2 < –B(OCH3)2 < –B(CH3)2 < –BH2. The computed activation free energy for the direct hydride addition to CO2 and the NBO analysis of the property of the Ru–H bond indicate that the activity of the hydride can be enhanced by the strong trans influence of the B ligands through the change in the Ru–H bond property. The function of the strong trans influence of B ligands is to decrease the d orbital component of Ru in the Ru–H bond. The design of a more active catalyst than the Ru(PNP)(CO)H2 complex is possible.

Graphical Abstract

1. Introduction

The development of high-efficiency catalysts for CO2 hydrogenation to formic acid or other carbohydrates is still attracting significant attention, as it can address the increasingly severe environmental problems and energy crisis [1,2,3,4,5,6,7]. One of the feasible methods is the homogeneous hydrogenation of CO2 using noble metal catalysts based on Ir, Ru, and Rh [8,9,10,11,12,13,14]. In 2009, Nozaki and co-workers reported an impressive iridium catalyst (Ir(PNP)H3, PNP = 2,6-(diisopropylphosphinomethyl)-pyridine) for CO2 hydrogenation to formate [8]. The turnover frequency (TOF) and turnover number (TON) reached 1.5 × 105 h−1 and 3.5 × 106, respectively, at 200 °C and 5.0 MPa. In 2014, Pidko and co-workers achieved this reaction with a Ru(PNP)(CO)H2 catalyst and greatly increased the TOF to 1.89 × 106 h−1 at a lower temperature and pressure (132 °C and 4.0 MPa) [13]. The catalytic mechanisms for the CO2 hydrogenation on these catalysts have been intensively studied [15,16,17,18,19,20]. It is believed that these catalysts are so active and stable because the hydride on these catalysts can be directly added to CO2, leading to CO2 reduction, shown as route I in Scheme 1. Another route for CO2 hydrogenation is via the precoordination of CO2 to the metal center followed by hydride transfer to CO2 (route II). In this route, CO2 should be precoordinated to the metal center before its reaction with the hydride, but CO2 cannot compete with other molecules in the system to occupy the active site, owing to the weak coordination ability of CO2. In a word, a highly active hydride is the key to the design of high-efficiency catalysts for the hydrogenation of CO2.
However, not all hydrides on transition metal complexes are active enough for the direct addition to CO2. When we studied Nozaki’s Ir(PNP)H3 catalyst, we found that there exist two types of Ir–H bond, and only the hydride on the Ir–H bond formed from the mixing of the sd2 orbital of Ir with the 1s orbital of H is active enough [17]. Then, we checked several Ru complexes and concluded that, if the Ru–H bond is formed from the mixing of the sd3 orbital of Ru with the 1s orbital of H, the hydride is not active for the direct addition to CO2 [21]. It only can take place for the hydride contained by a Ru–H bond formed from the mixing of the sd2 orbital of Ru with the 1s orbital of H, the same as the situation on the Ir complex. Moreover, we noticed that, if less d orbital component is involved in the formation of Ru–H bond, the hydride will be more active. Inspired by these findings, we thought the property of the Ru–H bond could be influenced by its trans ligand because this ligand shares the same orbital of Ru combined with the hydride. We tested two boryl ligands (Bcat and Bpin, cat = catecholate, pin = pinacolate) as the trans ligand to the hydride, and found that the activation barriers for the direct hydride addition to CO2 on the Ru(PNP)(CO)HBcat and Ru(PNP)(CO)HBpin complexes are both lower than that on the Ru(PNP)(CO)H2 complex [22]. Further, we explored the whole pathway for the hydrogenation of CO2 to formate. The advantage of the Ru(PNP)(CO)HBcat or Ru(PNP)(CO)HBpin complex is more remarkable; the overall activation free energy for CO2 hydrogenation to formate on the complexes Ru(PNP)(CO)H2, Ru(PNP)(CO)HBcat, and Ru(PNP)(CO)HBpin were calculated to be 18.2, 16.4, and 13.0 kcal·mol−1, respectively.
In this paper, we would like to examine the trans influence of more boryl ligands, together with that of various commonly used non-boryl ligands, in the direct addition of hydride to CO2. It is shown only the B ligands exhibit a stronger trans influence than the hydride. We then calculated the activation free energy for the addition of hydride to CO2 on the Ru complexes with several selected trans B ligands, in comparison with that on the Ru(PNP)(CO)H2 complex. The properties of Ru–H bonds on these complexes were studied with natural bond orbital (NBO) analysis. It is indicated that the activity of the hydride can be systematically enhanced by the strong trans influence of the B ligands through the modification of the Ru–H bond property. In addition, we studied the case on another Ru dihydride complex trans-[Ru(dmpe)2H2] (dmpe = Me2PCH2CH2PMe2) [23]. The trans influence of B ligands on this complex is not as significant as that on the Ru(PNP)(CO)H2 complex. We will discuss this in the main text.

2. Results and Discussion

2.1. Trans Influence of Non-Boryl and Boryl Ligands

The bond distance of the Ru–H bond is an index for the trans influence of the trans ligand, as well as an index for the activity of the hydride on the Ru–H bond. In general, the longer the distance of the Ru–H bond, the stronger the trans influence of the ligand trans to this Ru–H bond, the weaker the Ru–H bond, and the more active the hydride on it [24]. We first examined various commonly used non-boryl ligands, and compared the computed distance of the Ru–H bond on each complex with that on the Ru(PNP)(CO)H2 complex, in which two hydrides are trans to each other. The results are collected in Table 1. The optimized geometries of all the species are given in the Supplementary Materials. It is shown that the order of increasing trans influence of non-boryl ligands is as follows: –NO3 ≈ –Br < –Cl < –SCN < –ONO ≈ –F < –OMe < –OH ≈ –NO2 < –NH2 < –CN < –SiH3 < –Ph < –Me < –Et < –H. Greif et al. studied the trans influence of several ligands on a square-planar Pt complex and provided a sequence of increasing trans influence as follows: –NO3 < –ONO < –Cl < –Br < –SCN < –NO2 < –CN < –Ph < –Me [25]. Sajith and Suresh quantified the trans influence of various ligands on a hypervalent iodine complex and found a sequence of increasing trans influence as follows: –NO3 < –F < –Cl < –OH < –Br < –OMe < –NO2 < –NH2 < –Me < –Ph < –Et [26]. Our result is in reasonable agreement with the above two results. However, there is an inverse trans influence of –Br < –Cl < –F in this case. The inverse trans influence of the –Br, –Cl, and –F ligand was observed by the experimental and theoretical studies [27,28]. All the Ru–H bonds on the complexes with these trans non-boryl ligands are shorter than that on the Ru(PNP)(CO)H2 complex, thus we deduce that the hydride on these complexes will be less active for CO2 hydrogenation than that on the Ru(PNP)(CO)H2 complex.
Some studies in the literature have reported that the boryl ligands have a very strong trans influence [29,30,31,32,33]. Thus, we examined the trans influence of some usual boryl ligands. The computed distances of Ru–H bonds on these complexes with trans B ligands are collected in Table 1. The diagrams of some boryl ligands are given in Chart 1. Their optimized geometries are presented in the Supporting Information. It can be seen that the B ligands actually have a stronger trans influence than the hydride, judged by the longer distance of the Ru–H bond. Bcat and Bpin ligand are among the most popular B ligands found in transition metal complexes, thus we tested them in our previous work [22]. However, the trans influences of Bcat and Bpin ligands are not the strongest; the BH2, B(CH3)2, and B(OCH3)2 ligands show a stronger trans influence than the Bcat and Bpin ligands. The total order of increasing trans influence of boryl ligands considered here is as follows: –H < –BBr2 < –BCl2 ≈ –B(OCH)2 < –Bcat < –B(OCH2)2 ≈ –B(OH)2 < –Bpin < –B(NHCH2)2 < –B(OCH3)2 < –B(CH3)2 < –BH2. Zhu and Lin investigated the trans influence of boryl ligands on a square-planar Pt complex and reported a sequence of increasing trans influence as follows: –H < –BBr2 ≈ –BCl2 ≈ –Bcat ≈ –B(OCH)2 < –B(OCH2)2 < –Bpin < –B(NHCH2)2 < –BH2 < –B(CH3)2 [29]. In that case, B(CH3)2 shows the strongest trans influence.

2.2. Activity of the Hydride under Trans Influence

As the B ligands exhibit a stronger trans influence than the hydride, we expected to confirm the activity of the hydride under the trans influence of B ligands. For this purpose, we calculated the activation free energy for the addition of hydride to CO2 on the Ru PNP-type complexes with four selected trans B ligands. The activation barrier for the direct hydride addition to CO2 on the Ru(PNP)(CO)H2 complex was also computed for comparison. The results are collected in Table 2. To show this information more clearly, we drew a picture of the free energy profile for the direct hydride addition to CO2 on the Ru(PNP)(CO)H2 complex and four Ru complexes with trans B ligands, presented as Figure 1. The optimized geometries of all the species involved in Figure 1 are provided in Figure 2 and in the Supporting Information. When CO2 approaches the hydride on the Ru(PNP)(CO)H2 complex, an association complex (IS) will be formed, in which CO2 is fixed by the interaction between CO2 with the hydride and the hydrogen bonding of CO2 with the methyl group H atoms on P atoms. The angle of CO2 is 176.5°, indicating that CO2 is just weakly activated. After the addition of hydride to CO2, an H-bound formate complex (FS) is formed. In this state, the angle of OCO is decreased to 138.0°, indicating that CO2 is already reduced. This step is endergonic by 6.7 kcal mol−1 and is involved with an activation free energy of 7.0 kcal mol−1. It should be mentioned that the whole process of CO2 insertion into the Ru–H bond to form a formate complex (Scheme 1) is exergonic, but the step of hydride addition to CO2 is always an endergonic step; the less endergonic the step of hydride addition to CO2, the more favorable the formation of the formate complex, by lowering the energetics of the whole catalytic cycle [15,16,17,18,19,34]. When CO2 approaches the hydride on the Ru complex with Bcat, Bpin, B(NHCH2)2, or B(OCH3)2 as trans ligand, a similar mechanism will occur. The angle of CO2 in the association complexes with these trans B ligands is 176.8°, 176.6°, 176.3°, and 176.4°, respectively, very close to that on the Ru(PNP)(CO)H2 complex, but the angle of OCO in the final state is 137.3°, 136.3°, 136.1°, and 135.9°, respectively, less than that on the Ru(PNP)(CO)H2 complex. The activation barrier for the hydride addition to CO2 is 5.9, 5.6, 5.2, and 4.9 kcal·mol−1, respectively. These values are lower than that on the Ru(PNP)(CO)H2 complex and along a decreasing order in accordance with the increasing order of trans influence of these B ligands. Moreover, these steps are endergonic by 5.3, 4.5, 3.6, and 3.1 kcal mol−1, respectively, indicating that the formed formate complexes are more thermodynamically stable. The addition of hydride to CO2 is the first and the key step for the hydrogenation of CO2. If the whole pathway for the CO2 hydrogenation to formate is explored, the advantage will be more remarkable, as we reported on the Ru(PNP)(CO)HBcat and Ru(PNP)(CO)HBpin complexes [22].
To confirm the origin of the trans influence of B ligand, we investigated the properties of the Ru–H bonds in the above complexes with NBO analysis. The bonding parameters are collected in Table 2. As reported in our previous work [21], if less d orbital component of Ru is involved in the formation of the Ru–H bond, the hydride will be more active for direct addition to CO2. The ligand trans to the hydride is an important factor influencing the Ru–H bond property, because this ligand shares the same orbital of Ru with the hydride. The Ru–H bond in the Ru(PNP)(CO)H2 complex is a σ bond formed via the mixing of the sd2.11 hybrid orbital of Ru with the 1 s orbital of H. The Ru–H bond in the complex with Bcat, Bpin, B(NHCH2)2, or B(OCH3)2 trans ligand is also a σ bond, but the d orbital component of Ru–H bond in these complexes is 1.83, 1.80, 1.76, and 1.63, respectively, in a decreasing order that is also in accordance with the increasing order of trans influence of these B ligands. Consequently, the B ligands can modify the property of the Ru–H bond to improve the activity of the hydride. The essence of the strong trans influence of B ligands is to decrease the d orbital component of Ru in the Ru–H bond.

2.3. Trans Influence of B Ligands on Trans-[Ru(dmpe)2HX] Complexes

The trans-[Ru(dmpe)2H2] dihydride complex is another highly active complex in that the direct addition of hydride to CO2 can occur as well, as reported by Baiker and co-workers [23]. Trans-[Ru(dmpe)2H2] is one of two isomers of Ru(dmpe)2H2, but cis-[Ru(dmpe)2H2] is not that active. We wondered about the trans influence of B ligands on this complex. Thus, we selected four B ligands as trans ligands to substitute one hydride of trans-[Ru(dmpe)2H2]. We optimized their geometries and calculated the activation free energy for hydride addition to CO2 on these complexes. The free energy profile for this step on the trans-[Ru(dmpe)2HX] complexes (X may be hydride or B ligand) is presented in Figure 3. The optimized geometries of all the species involved above are provided in the Supporting Information. On trans-[Ru(dmpe)2H2], the addition of hydride to CO2 is endergonic by 2.5 kcal mol−1, with an activation free energy of 3.6 kcal mol−1. This activation barrier is even lower than that on Ru(PNP)(CO)H2. This is because the assistance of hydrogen bonding for the fixation and the activation of CO2 in this system is more significant than that on Ru(PNP)(CO)H2. On trans-[Ru(dmpe)2H2], CO2 can be fixed and assisted by at least four hydrogen bonds, but on Ru(PNP)(CO)H2, there are only two hydrogen bonds. However, one may notice that the activation barrier for this step on the complexes with Bcat, Bpin, B(CH3)2, or BH2 as trans ligand is not lower than that on trans-[Ru(dmpe)2H2]. The value on trans-[Ru(dmpe)2HBpin] is even as high as 5.8 kcal mol−1. It is usually believed that the help of hydrogen bonding is positive for the hydrogenation of CO2 and some big groups on the complexes can provide more hydrogen atoms for the formation of hydrogen bonding. However, the big groups might bring greater steric hindrance on the other hand. Considering that the interaction of CO2 with the hydride is very weak, the steric hindrance of big groups might offset the assistance of hydrogen bonding. In this sense, the effects of big groups for the hydrogenation of CO2 are not always positive. Anyway, the formation of the H-bound formate complexes is thermodynamically favorable compared with that on trans-[Ru(dmpe)2H2].

3. Computational Methods

All calculations were implemented with the Gaussian 09 programs [35]. The B3LYP hybrid functional [36] was used. For Ru, the Stuttgart–Dresden basis set-relativistic effective core potential (RECP) combination [37], supplemented with two sets of f functions and a set of g functions [38], was applied. For the P, O, N, C, B, or H element, the Dunning cc-pVDZ basis set [39] was applied. Each stationary point was confirmed as a minimum or transition state by calculating the vibrational frequency, and all transition states were verified by intrinsic reaction coordinate (IRC) calculations. The Ru–H bond properties were characterized using NBO analysis with NBO 5.9 version [40]. For the Ru PNP-type complexes, the groups on P atoms are methyl groups. Gibbs free energies of all species were computed at 298.15 K and 1 atm.

4. Conclusions

For the purpose of the design of high-activity catalysts for the hydrogenation of CO2, we have studied the trans influence of boryl ligands and various commonly used non-boryl ligands. Our calculation results show that, on the Ru PNP-type dihydride complex, the order of increasing trans influence of non-boryl ligands is as follows: –NO3 ≈ –Br < –Cl < –SCN < –ONO ≈ –F < –OMe < –OH ≈ –NO2 < –NH2 < –CN < –SiH3 < –Ph < –Me < –Et < –H and the sequence of increasing trans influence of boryl ligands is as follows: –H < –BBr2 < –BCl2 ≈ –B(OCH)2 < –Bcat < –B(OCH2)2 ≈ –B(OH)2 < –Bpin < –B(NHCH2)2 < –B(OCH3)2 < –B(CH3)2 < –BH2. Only the B ligands exhibit a stronger trans influence than the hydride ligand. The computed activation free energy for the hydride addition to CO2 and the NBO analysis of the property of the Ru–H bond indicate that the activity of the hydride can be systematically enhanced by the strong trans influence of the B ligands through the modification of the Ru–H bond property. The purpose of the strong trans influence of B ligands is to decrease the d orbital component of Ru in the Ru–H bond. In addition, we investigated the case on another Ru dihydride complex trans-[Ru(dmpe)2H2]. In this case, B ligands fail to lower the activation barrier for the hydride addition to CO2. We think it is because the large groups on this kind of complex bring hydrogen bonding that is positive for the activation of CO2, and at the same time form steric hindrance that is negative for the approaching of CO2 to the hydride. Nevertheless, the design of a more active catalyst than the dihydride complex is possible.
Our next work is to probe the property of the Ru–B bond and the factors that can influence the Ru–B bond property, and then summarize some rules for the design of boryl-ligand-containing complexes with high activity for the hydrogenation of CO2.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11111356/s1, Figure S1: Optimized geometries of the species involved in the addition of hydride to CO2 on trans-[Ru(dmpe)2HX] complexes; Table S1: Cartesian coordinates for the optimized geometries of all the species.

Author Contributions

Conceptualization, J.L.; methodology, J.L.; investigation, T.L., Z.L. and L.T.; writing—original draft preparation, T.L. and Z.L.; 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).

Acknowledgments

We thank 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 11 01356 sch001 550
Scheme 1. Two routes for CO2 insertion into the M–H bond.
Scheme 1. Two routes for CO2 insertion into the M–H bond.
Catalysts 11 01356 sch001
Catalysts 11 01356 ch001 550
Chart 1. Diagram of some boryl ligands.
Chart 1. Diagram of some boryl ligands.
Catalysts 11 01356 ch001
Catalysts 11 01356 g001 550
Figure 1. Free energy profile for the addition of hydride to CO2 on the Ru PNP-type complex with different trans ligands. The substituent on phosphorus is omitted for clarity.
Figure 1. Free energy profile for the addition of hydride to CO2 on the Ru PNP-type complex with different trans ligands. The substituent on phosphorus is omitted for clarity.
Catalysts 11 01356 g001
Catalysts 11 01356 g002 550
Figure 2. Optimized geometries of the species involved in the addition of hydride to CO2 on selected Ru PNP-type complexes. Bond distances are given in angstrom (Å) and bond angles are in degree (°) (green balls: Ru; violet: P; red: O; blue: N; grey: C; orange: B; white: H).
Figure 2. Optimized geometries of the species involved in the addition of hydride to CO2 on selected Ru PNP-type complexes. Bond distances are given in angstrom (Å) and bond angles are in degree (°) (green balls: Ru; violet: P; red: O; blue: N; grey: C; orange: B; white: H).
Catalysts 11 01356 g002
Catalysts 11 01356 g003 550
Figure 3. Free energy profile for hydride addition to CO2 on trans-[Ru(dmpe)2HX] complexes (X may be hydride or B ligand). The substituent on phosphorus is omitted for clarity.
Figure 3. Free energy profile for hydride addition to CO2 on trans-[Ru(dmpe)2HX] complexes (X may be hydride or B ligand). The substituent on phosphorus is omitted for clarity.
Catalysts 11 01356 g003
Table
Table 1. Bond distance of the Ru–H bond on the Ru PNP-type complex with different trans ligands.
Table 1. Bond distance of the Ru–H bond on the Ru PNP-type complex with different trans ligands.
Trans LigandDistance of Ru–H Bond (Å)Trans LigandDistance of Ru–H Bond (Å)
–H1.697–H1.697
–Et1.683–BBr21.711
–Me1.682–BCl21.713
–Ph1.678–B(OCH)21.713
–SiH31.676–Bcat1.714
–CN1.662–B(OCH2)21.715
–NH21.656–B(OH)21.715
–NO21.637–Bpin1.716
–OH1.637–B(NHCH2)21.717
–OMe1.636–B(OCH3)21.720
–F1.621–B(CH3)21.726
–ONO1.621–BH21.739
–SCN1.616
–Cl1.611
–Br1.609
–NO31.609
Table
Table 2. Bond distance of the Ru–H bond, bonding parameter of the Ru–H bond, and activation free energy for hydride addition to CO2 on the Ru PNP-type complex with different trans ligands.
Table 2. Bond distance of the Ru–H bond, bonding parameter of the Ru–H bond, and activation free energy for hydride addition to CO2 on the Ru PNP-type complex with different trans ligands.
Trans LigandDistance of Ru–H Bond (Å)Coefficients/Hybrids of Ru–H Bond OrbitalActivation Free Energy for Hydride Addition to CO2 (kcal mol−1)
–H1.6970.706 Ru(sd2.11) + 0.709 H(s)7.0
–Bcat1.7140.702 Ru(sd1.83) + 0.712 H(s)5.9
–Bpin1.7160.699 Ru(sd1.80) + 0.715 H(s)5.6
–B(NHCH2)21.7170.699 Ru(sd1.76) + 0.715 H(s)5.2
–B(OCH3)21.7200.686 Ru(sd1.63) + 0.728 H(s)4.9
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Liu, T.; Liu, Z.; Tang, L.; Li, J.; Yang, Z. Trans Influence of Boryl Ligands in CO2 Hydrogenation on Ruthenium Complexes: Theoretical Prediction of Highly Active Catalysts for CO2 Reduction. Catalysts 2021, 11, 1356. https://doi.org/10.3390/catal11111356

AMA Style

Liu T, Liu Z, Tang L, Li J, Yang Z. Trans Influence of Boryl Ligands in CO2 Hydrogenation on Ruthenium Complexes: Theoretical Prediction of Highly Active Catalysts for CO2 Reduction. Catalysts. 2021; 11(11):1356. https://doi.org/10.3390/catal11111356

Chicago/Turabian Style

Liu, Tian, Zhangyong Liu, Lipeng Tang, Jun Li, and Zhuhong Yang. 2021. "Trans Influence of Boryl Ligands in CO2 Hydrogenation on Ruthenium Complexes: Theoretical Prediction of Highly Active Catalysts for CO2 Reduction" Catalysts 11, no. 11: 1356. https://doi.org/10.3390/catal11111356

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