Simple Ligand Modifications to Modulate the Activity of RutheniumCatalysts for CO2 Hydrogenation: Trans Influence of Boryl Ligands and Nature of Ru―H Bond

LIU Tian , LI Jun ,＊, LIU Weijia , ZHU Yudan ,＊, LU Xiaohua

1 College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University,Nanjing 210009, Jiangsu Province, P. R. China.

2 Nanjing Boiler and Pressure Vessel Inspection Institute, Nanjing 210019, Jiangsu Province, P. R. China.

Abstract: The development of efficient catalysts for the hydrogenation of CO2 to formic acid (FA) or formate has attracted significant interest as it can address the increasingly severe energy crisis and environmental problems. One of the most efficient methods to transform CO2 to FA is catalytic homogeneous hydrogenation using noble metal catalysts based on Ir, Ru, and Rh. In our previous work, we demonstrated that the activity of CO2 hydrogenation via direct addition of hydride to CO2 on Ir(III) and Ru(II) complexes was determined by the nature of the metal-hydride bond. These complexes could react with the highly stable CO2 molecule because they contain the same distinct metal-hydride bond formed from the mixing of the sd2 hybrid orbital of metal with the 1s orbital of H, and evidently, this property can be influenced by the trans ligand. Since boryl ligands exhibit a strong trans influence, we proposed that introducing such ligands may enhance the activity of the Ru―H bond by weakening it as a result of the trans influence. In this work, we designed two potential catalysts, namely, Ru-PNP-HBcat and Ru-PNPHBpin, which were based on the Ru(PNP)(CO)H2 (PNP = 2,6-bis(dialkylphosphinomethyl)pyridine) complex, and computationally investigated their reactivity toward CO2 hydrogenation. Bcat and Bpin (cat = catecholate, pin = pinacolate)are among the most popular boryl ligands in transition metal boryl complexes and have been widely applied in catalytic reactions. Our optimization results revealed that the complexes modified by boryl ligands possessed a longer Ru―H bond.Similarly, natural bond orbital (NBO) charge analysis indicated that the nucleophilic character of the hydride in Ru-PNPHBcat and Ru-PNP-HBpin was higher as compared to that in Ru-PNP-H2. NBO analysis of the nature of Ru―H bond indicated that these complexes also followed the law of the bonding of Ru―H bond proved in the previous works (Bull.Chem. Soc. Jpn. 2011, 84 (10), 1039; Bull. Chem. Soc. Jpn. 2016, 89 (8), 905), and the d orbital contribution of the Ru atom in Ru-PNP-HBcat and Ru-PNP-HBpin was smaller than that in Ru-PNP-H2. Consequently, the Ru-PNP-HBcat and Ru-PNP-HBpin complexes were more active than Ru-PNP-H2 for the direct hydride addition to CO2 because of the lower activation energy barrier, i.e., from 29.3 kJ·mol?1 down to 24.7 and 23.4 kJ·mol?1, respectively. In order to further verify our proposed catalyst-design strategy for CO2 hydrogenation, the free energy barriers of the complete pathway for the hydrogenation of CO2 to formate catalyzed by complexes Ru-PNP-H2, Ru-PNP-HBcat, and Ru-PNP-HBpin were calculated to be 76.2, 67.8, and 54.4 kJ·mol?1, respectively, indicating the highest activity of Ru-PNP-HBpin. Thus, the reactivity of Ru catalysts for CO2 hydrogenation could be tailored by the strong trans influence of the boryl ligands and the nature of the Ru―H bond.

Key Words: CO2 hydrogenation; Ru complex; Boryl ligand; Trans influence; Ru―H bond

1 Introduction

The carbon dioxide emissions have increased owing to rising energy consumption. The catalytic hydrogenation of CO2into fuels and useful chemicals, especially for formic acid (FA) or formate, holds potential to address this problem. On one hand,hydrogenation of CO2to produce FA1–7could mitigate the CO2in atmosphere. On the other hand, FA can serve as the most promising hydrogen storage media and can be directly used for formic acid fuel cell1,8. In addition, FA and its salts are widely used in the textile, leather, dye industry and other industrial areas9.It is well known that CO2 is an attractive C1 feedstock with the advantages of abundance, non-flammability and nontoxicity, but its inherent stability still poses a big challenge to chemists10,which determines the key issue in the process of CO2transformation is activation of CO2.

Homogeneous hydrogenation of CO2to formic acid by noble metal catalysts has become one of the most efficient pathways11–16.In 2009, Tanaka et al.14reported an impressive pincer-supported iridium trihydride catalyst Ir(III)-PNP for CO2hydrogenation to formate, the turnover frequencies (TOFs) and turnover number(TON) achieved 150000 h?1and 3500000 at 200 °C and 5000000 Pa. Compared to iridium catalyst, ruthenium catalysts are more common in CO2 hydrogenation. More importantly, the ruthenium element is much less expensive. Currently, a significant breakthrough was achieved by Filonenko et al. by combination of complex Ru(PNP)(CO)H2and strong DBU base15,17. It allows TOFs as high as 1892000 h?1at the lower temperature and pressure (132 °C and 4000000 Pa). Previously,we found that they are so active to catalyze highly stable CO2because they contain same distinct metal-hydride bond formed from the mixing of the sd2hybrid orbital of metal with the 1s orbital of H18. In addition, this unique nature of metal-hydride bond also plays vital role in CO2activation in other ruthenium catalysts, such as trans-Ru(dmpe)2H2, cis-Ru(dmpe)2H2, (dmpe =Me2PCH2CH2PMe2), and cis-Ru(PMe3)3(H2O)H219. A distinct feature of them is that the hydrido complex is capable of transferring a hydride to CO2, which makes the CO2 activation is possible. Even though these studies have achieved extraordinary efficiency, the harsh reaction conditions have to be carried out. Thus, the design and development of high-efficiency catalysts that can operate under milder conditions is highly desirable.

Developing a highly efficient catalyst need to accelerate the process of hydride transferring, since this process is postulated as key issue in CO2activation. The comparison of trans-Ru(dmpe)2H2and cis-Ru(dmpe)2H2may give us some enlightenment. The similarity between the two is that they are in the same ligand environment. Differently, the trans ligand of hydride of trans-Ru(dmpe)2H2 is another hydride, the counterpart of cis-Ru(dmpe)2H2is phosphine ligand. The difference of Ru―H bond properties and activation barriers of two isomers might relate to the ligand trans to the hydride. Our earlier study also showed that the ligand trans to hydride is an important factor to influence the bond distance and nature of Ru―H bond of Ru(PNP)(CO)H219. Such a phenomenon can be attributed to trans influence of ligand opposite to hydride, which means that a ligand can influence some properties of its trans ligand, such as bond distance, the vibrational frequency, force constant and a battery of other parameters20. Therefore, the activity of the hydride on a Ru(II) complex might be adjusted by the change of the ligand trans to the hydride. Schmeier and Langer’s group21,22also consciously controlled this factor to optimize catalytic performance and designed active pincer complexes for CO2hydrogenation containing two hydrides trans to each other. However, they have already thought hydride is the most active trans ligand and have not explored the nature of metal-hydride bond. These studies inspire us to explore stronger trans influence than hydride ligand trans to reacting hydride to weaken Ru―H bond and apply it to CO2hydrogenation reaction.

Boryl ligands are known to its strong trans influence in transition-metal boryl complexes23,24. Besides, boryl ligands have shown to alter the electronic property of transition metals markedly25. Lin and Zhu have investigated the trans influence of boryl ligands via density functional theory (DFT) calculations on square-planar platinum(II) complexes, and presented that trans influence of boryl ligands are stronger than hydride group20.In addition, bis-phosphino-boryl (PBP) pincer transition-metal complexes were synthesized26, and were applied in the field of catalysis. For instance, Peters et al.27have synthesized cobalt(I)-N2complex supported by PBP ligand and explored its catalytic performance on catalytic olefin hydrogenation as well as amineborane dehydrogenation. He also presented the synthesis of boryl-Ni complex (tBuPBP)NiH, which reacts with CO2due to trans influence engendered by boryl ligand to give formate complex28.

In continuation of our work on ruthenium catalysts and combined with strong trans influence of boryl ligands, we would like to design more efficient catalysts based on Ru(II) complex Ru(PNP)(CO)H2for CO2hydrogenation. Designing more efficient ruthenium catalysts, in other words in this system,designing more efficient hydride that attack to CO2. Given strong trans influence of boryl ligands, we would like to take an easy coordinating form between ruthenium and boryl ligands into consideration, and position the boryl ligands trans to hydride in ruthenium complexes so as to exert its strong trans influence. Therefore, it will be interesting to study the role of boryl ligand for changing character of its trans hydride, which in turn might be helpful for CO2hydrogenation reaction. To this end, two boryl ligands are employed, and the corresponding complexes are presented in Fig. 1 through substituting one of hydrides in Ru-PNP-H2with Bcat and Bpin ligands, named Ru-PNP-HBcat and Ru-PNP-HBpin. Bcat and Bpin ligands are among the most popular boryl ligands in transition metal boryl complexes and widely applied in catalysis reaction29,30. In this paper, we have computationally examined the activities of two complexes for the hydrogenation of CO2using DFT, and compared the reactivity with their parent complex Ru-PNP-H2.Since the key step in hydrogenation of CO2 is insertion of CO2 into the active metal-hydride bond31,32, so as to Ru-PNP-H2.Furthermore, the step of formato ligand isomerization is also vital to CO2transformation since it may be more difficult than hydride transfer18, and the barriers of CO2transformation to formate were also calculated in this work.

2 Computational details

DFT calculations were implemented using the Gaussian 09 suite of programs33. The geometry optimization has employed the B3LYP exchange-correlation functional34. The choice of the B3LYP functional is based on its successful application in describing the ruthenium complexes13,35,36. The Stuttgart-Dresden basis set-relativistic effective core potential (RECP)combination, which was supplemented with two sets of f functions and a set of g functions37, was used for Ru atom. The Dunning cc-pVDZ basis set was used for P, O, N, C, B and H atom38. Same basis sets were employed for these atoms in our earlier works18,19,39and others40–42.

In order to maintain consistency with the previous work19, all geometry optimizations were carried out in vacuum. To verify the validity of geometry optimization of complexes, the comparison with related complexes is necessary, and found that they are consistent with experimentally reported Ru complexes43–45.For the sake of identifying all the stationary points as minima(zero imaginary frequency) or transition states (one imaginary frequency) and obtaining ZPE corrections, frequency calculation were carried out on all the optimized structures at the same level of theory used for the optimization step. Intrinsic reaction coordinate (IRC) calculations were adopted to confirm the connection of the transition states and two relevant minima. The properties and WBI of Ru―H bond were characterized by natural bond orbital (NBO) analysis46, and carried out on the structures of some intercepted stationary points. For all ruthenium complexes, the tert-butyl groups of P atom were replaced by methyl groups, this simplification also carried out in others paper to decrease computational cost21. Gibbs free energies of all species were computed at 298.15 K and 101300 Pa.

3 Results and discussion

Fig. 1 Optimized geometries and schematic drawings of the structures of Ru(II) complexes generated from Ru-PNP-H2 with modified ligands.Bond distances are in nanometer (nm). H atoms on methyl group are omitted for clarity. Green balls: Ru; violet: P; red: O; pink: B; blue: N; grey:C; white: H; orange: methyl group, color online.

In order to present more straightforward trans influence of boryl ligand, a series of parameters of hydride ligand are considered as the metric of trans influence and catalytic efficiency. The hydride ligand is in the same position as boryl ligands and commonly used in pincer complex for homogeneous CO2hydrogenation. In this work, we would explore boryl ligands how to exert its strong trans influence in key structural parameters, the nature of Ru―H bond and modulate the activity toward CO2hydrogenation compared to hydride ligand at an atomic level. These Ru complexes (Fig. 1) are modeled by changing the ligand trans to reacting hydride. Ru-PNP-H2 have two hydrides trans to each other, one of the axial hydride ligands of complex Ru-PNP-H2is substituted by a Bcat and Bpin ligand to model complex Ru-PNP-HBcat and Ru-PNP-HBpin. The reserved hydride ligand is considered as reacting hydride to form ruthenium hydrido complex, and what discussed are based on these complexes.

3.1 Atomic and electronic structure of Ru(II) complex

The optimized geometries and schematic drawings of the structures of Ru-PNP-H2, Ru-PNP-HBcat and Ru-PNP-HBpin are shown in Fig. 1. Some key structural parameters are listed in Table 1. First of all, distance of metal-hydride bond play an important role in hydride transferring process. Ru―H bond distance of Ru-PNP-H2 is 0.1697 nm, and the Ru―H bonddistances of Ru-PNP-HBcat and Ru-PNP-HBpin are 0.1714 and 0.1716 nm, respectively. The distances of Ru―H bonds on the two complexes Ru-PNP-HBcat and Ru-PNP-HBpin are larger than that of Ru-PNP-H2, illustrate that the boryl ligands exert strong trans influence to stretch the Ru―H bond. Furthermore,the bond length of Ru―B are 0.2140 and 0.2170 nm in Ru-PNPHBcat and Ru-PNP-HBpin, indicate that the bond length of Ru―B is influenced by its substituents in the B atom. The tendency of distance of Ru―B bonds is similar to Pt―B bonds of square-planar platinum(II) boryl complexes47. The angle of three atoms in trans position of three complexes are 175.7°,176.0°, 177.7°, respectively. This trend implies that boryl ligands are more prone to locate trans position of hydride, and the magnitude of the trend is in the order of Bpin ＞ Bcat ＞ H. Finally,the relative position of boryl ligands is appealing. The dihedral angle of P1-P2-O1-O2 in Ru-PNP-HBcat and Ru-PNP-HBpin are 62.0° and 35.2°, and are not ideal octahedral geometry. The smaller dihedral angle in Ru-PNP-HBpin might result from the bulky nature of Bpin ligand. The NBO charge analysis is done to understand the charge variation when varied the trans ligand of hydride. Table 2 listed NBO charges of various ligands, the stretching frequency and WBI of Ru―H bond. When we changed the trans ligand of hydride, the PNP and CO ligands areacting as donators. In addition, introducing boryl ligands also increases electron density at the metal center, and the electron density of metal center is one of influencing factor related to the barrier for hydride transfer48. More importantly, the NBO charge on the reacting hydride becomes more negative as we change boryl ligands with different trans influence. The nucleophilicity of the hydride can be estimated by NBO charge,the more negative charge, and the more nucleophilic. From this perspective, the natural charge on hydride of Ru-PNP-H2 is?0.245e, the natural charge on hydride are ?0.270e and ?0.276e for Ru-PNP-HBcat and Ru-PNP-HBpin, respectively. Boryl ligands make the natural charge on hydride more negative,indicates that the character of nucleophilicity of hydride are increased in Ru-PNP-HBcat and Ru-PNP-HBpin, we attribute a strongly donating trans ligand of Bcat and Bpin20. Similar to bond distance of Ru―H, stretching frequencies of Ru―H bonds in complexes modified by Bcat and Bpin (1658, 1644 cm?1) are significantly smaller than that of Ru-PNP-H2(1853 cm?1),demonstrate that a weaker Ru―H bond in Ru-PNP-HBcat and Ru-PNP-HBpin than that of Ru-PNP-H2. Moreover, Wiberg bond indices (WBI) also support the relatively weak interaction between hydride and the central Ru atom in Ru-PNP-HBcat and Ru-PNP-HBpin. The WBI value for the Ru―H bond of 0.8620 in Ru-PNP-H2is higher than those of Ru-PNP-HBcat and Ru-PNP-HBpin with 0.8372, 0.8370, respectively. In the present case, the bond distance, NBO charge on hydride, stretching frequency and WBI of Ru―H bond all indicate that the hydride in Ru-PNP-HBcat and Ru-PNP-HBpin should be more favorable for hydrogenation of CO2, and more detailed theoretical study of CO2hydrogenation mediated by them is required to confirm our suppose.

Table 1 Structural comparison among three Ru(II) complexes

Table 2 Summed up NBO charges on the ligands and metal, stretching frequency and WBI of Ru―H bond of three complexes.

3.2 The nature of Ru―H bond of Ru(II) complex

A longer, weaker Ru―H bond, higher electron density of Ru,and more nucleophilic hydride of Ru-PNP-HBcat and Ru-PNPHBpin indicate that the hydride of them are more prone to nucleophilic attack to CO2than Ru-PNP-H2. We turn to calculate the activation barrier for the hydrogenation of CO2via direct addition of hydride to CO2. And the nature of Ru―H bond was explored, since it is the nature of the Ru―H bond that determines the activity of hydride addition to CO2. New properties of Ru―H bonds were investigated based on NBO analysis, and the bonding parameters of these Ru―H bonds were collected in Table 3. We remark that the property of Ru―H bond of Ru-PNP-H2is slightly different from our earlier paper because tert-butyl groups of P atoms were replaced by methylgroups in this work. Compared to hydride ligand, trans influence of boryl ligands on the nature of Ru―H bond is consistent to their influence of Ru―H bond distance, stretching frequency,and natural charge of hydride. The properties of Ru–H bonds in Ru-PNP-HBcat, Ru-PNP-HBpin are formed via the mixing of the sd1.83hybrid orbital of Ru with the 1s orbital of H and the sd1.80hybrid orbital of Ru with the 1s orbital of H, respectively.The d orbital contribution of Ru atom in Ru-PNP-HBcat and Ru-PNP-HBpin are less than that of Ru-PNP-H2 formed from sd2.11hybrid orbital of Ru with the 1s orbital of H. For activity of CO2 activation, Ru-PNP-HBcat and Ru-PNP-HBpin make the activation energies for hydride addition to CO2from 29.3 kJ·mol?1drop to 24.7 and 23.4 kJ·mol?1, respectively, and the Gibbs free energy change of direct hydride transfer (1 → 2) to the C atoms of the CO2 are calculated from 29.7 kJ·mol?1decline to 22.6 and 18.8 kJ·mol?1, respectively. Thus, if the trans ligand alters the property of Ru―H bond with less d orbital components than hydride, its activation energy and Gibbs free energy change for hydride direct addition to CO2would decrease to some extent. The longer Ru―H bond, the less d orbital component, and the lower activation barrier, as shown in Table 3, which is consistent with our previous results19. The complexes Ru-PNP-HBcat and Ru-PNP-HBpin would be more active than complex Ru-PNP-H2for the step of direct hydride addition to CO2due to lower activation energy and larger Ru―H bond distance. Combined with the properties of Ru―H bond and activation barriers, the enhancement of activity of direct hydride addition to CO2 for Ru-PNP-HBcat and Ru-PNP-HBpin might originates from the less d orbital component of Ru atom, which leads to the ease of transferring hydride to CO2. This conjecture is supported by the WBI value of Ru―H bonds, the lower d orbital components are involved in the formation of the Ru―H bond, corresponding to the relatively weak interaction between hydride and Ru.

Table 3 Bond distance, bonding parameter of Ru―H bond, activation energy for hydride (E a) addition to CO2 on three complexes.

3.3 Hydrogenation process of CO2 to formate on Ru(II) complex

Besides the step of hydride direct addition to CO2, we also have investigated the detailed whole pathway for the hydrogenation of CO2 to formate on the new systems, Ru-PNPHBcat and Ru-PNP-HBpin, and compared the activity with their parent complex Ru-PNP-H2. The whole pathway for the hydrogenation of CO2to formate are composed of below several steps: 1) CO2interacts with the Ru(II) complex; 2) hydride transfer from the Ru hydrido complex to CO2; 3) the rotation of the formato ligand, Scheme 1. In earlier work, our attention was focused on the activation energy barrier of hydride addition to CO2due to this process is determined by the nature of Ru―H bond. In the present case, our work presented that not only the nature of Ru―H bond correlates with the hydride transferring process, but also the trans ligand of hydride relates to catalytic activity in the whole pathway for the hydrogenation of CO2 to formate. Furthermore, some study49demonstrated that the process of CO2insertion into Ru―H bond via direct hydride abstraction pathway is a sequentially uphill step. Therefore, the whole process of hydrogenation of CO2 to formate should be paid attention and the overall barriers are used to measure the catalytic efficiency of these complexes.

Scheme 1 The hydrogenation of CO2 to formate on three complexes (X = H, Bcat, Bpin).

The calculated free energy profile is presented in Fig. 2 and the optimized structures of key intermediates and transition states are shown in Fig. 3. All catalysts and CO2were considered as the starting point of energy profiles. Firstly, complexes Ru-PNP-H2 and Ru-PNP-HBcat interact with CO2 and form an octahedral precursor complex 1, 1-Bcat. This process is an endergonic step, which are slightly uphill by 14.7 and 18.0 kJ·mol?1, respectively. In the precursor complex 1 and 1-Bcat,the Ru―H bonds distances maintain to 0.1695 and 0.1714 nm,and the distance of H and CO2 are far to 0.2629 and 0.2690 nm,respectively. This behavior resembles to that found in the case of Ir(III) trihydride complex that did present a CO2directly attacked to the nucleophilic hydride18. Similarly, the bond distances of Ru―H bond and Ru―B bond positioned trans to reacting hydride also slightly shorten to 0.1694 and 0.2133 nm,and this tendency is similar to other study50. Furthermore, CO2 is weakly activated because its bond angle changes from linear to 176.5°, 176.8°, respectively. The angle of CO2, distance between C and H atoms, and distance between Ru and H atoms are listed in Table 4 as the hydride addition to CO2proceeds.Next step is hydride migrate to CO2to form H-bound formate complexes (2, 2-Bcat), the formation of H-bound formate complex requires a transition state TS1-2. This step is key to whole process, the calculated free energies barriers are 29.3 and 24.7 kJ·mol?1for Ru-PNP-H2and Ru-PNP-HBcat, respectively.Our calculation results indicate that Ru-PNP-HBcat catalyst has the lower activation barrier for hydride transfer to CO2. In formate complexes (2, 2-Bcat), the distance of Ru―H bond becomes longer are accompany with the approach of C atom and H atom. The distance between C atom and H atom are 0.1280,0.1266 nm, and the distances of the Ru―H bond are 0.1857 and 0.1901 nm, respectively, indicate that the breaking of Ru―H bond and the formation of C―H bond. The CO2molecules also change from linear to bent fashion and reach to 137.3° in 2-Bcat,which is greater deformation than counterpart complex 2.Subsequently, we investigated the next step of catalytic cycle,namely, the rotation of the formato ligand to form the O-bound formate complex 3 and 3-Bcat owing to H-bound formate complex is not stable. This step is more difficult than forward step, and the step of formation of formate is a successive uphill process. This step is exergonic by 17.2 kJ·mol–1with an activation energy barrier of 31.8 kJ·mol–1for Ru-PNP-H2. For Ru-PNPHBcat, it is more exergonic by 1.7 kJ·mol?1with a less activation energy of 27.2 kJ·mol–1. The series of pivotal thermodynamic and kinetic data (Gibbs free energy change, activation barrier)illustrate that the model complex Ru-PNP-HBcat is more efficient than the extraordinarily active catalyst Ru-PNP-H2. The optimized transition state structures (TS2-3, TS2-3-Bcat) of rotation of the formato ligand are shown in Fig. S1, the broken Ru―H bond continuously stretch to 0.2413 and 0.2501 nm,respectively. In addition, the two C―H bonds both shorten to as much as 0.1160 nm. The ultimate O-bound formate complexes 3, 3-Bcat are stable, the C―H bonds both shorten to 0.1122 nm in formato ligand, and the distance between Ru and O atoms are 0.2230 and 0.2260 nm, respectively.

Fig. 2 Free energy profiles for the key steps of CO2 hydrogenation by three Ru(II) complexes.Schematic drawings of the structures for the species involved are provided for clarity.

Fig. 3 Optimized geometries of the species involved in the direct addition of hydride to CO2 on the three Ru(II) complexes.Bond distances are in nanometer (nm) and bond angles are in degree (°). H atoms on methyl group are omitted for clarity. Green balls: Ru; violet: P; red: O;pink: B; blue: N; gray: C; white: H; orange: methyl groups, color online.

Table 4 Angle of CO2, distance between C and H atoms, distance between Ru and H atoms for hydride addition to CO2 on three complexes.

For complex Ru-PNP-HBpin, its reaction pathway is similar to the case for Ru-PNP-HBcat. First of all, CO2interacts with hydride of Ru-PNP-HBpin to form association complex 1-Bpin.It is worth noting that this association complex 1-Bpin is less endergonic than complex 1 by 0.4 kJ·mol?1, while in 1-Bcat is more endergonic by 3.3 kJ·mol?1. In the association complex 1-Bpin, the Ru―H bond elongated to 0.1717 nm, and the distance between hydride and C atom of CO2is 0.2674 nm. The angel of CO2 is 176.6°, which is weakly activated comparable to complex 1 and 1-Bcat. Next step is hydride transfer to electron-deficient C atom to form H-bound formate complex (1-Bpin), transition state is found in this process. The calculated activation barrier is 23.4 kJ·mol?1, which have a moderate reduction in comparison to Ru-PNP-H2(29.3 kJ·mol?1). In addition, its Gibbs free energy change also have a moderate decreases by 11.3 and 7.5 kJ·mol?1compared to Ru-PNP-H2 and Ru-PNP-HBcat, respectively. The distance between Ru atom and H atom of formed H-bound formate complex 2-Bpin has a longer with 0.1931 nm compared to complex 2 (0.1857 nm), 2-Bcat (0.1901 nm). Besides, the angel of CO2in H-bound formate complex (2-Bpin) is 136.3°,indicates that CO2is polarized completely due to electrons have been transferred into CO218. For the rotation of the formato ligand of the H-bound formate complex (2-Bpin) to form O-bound formate complex (3-Bpin), this step is exothermic by 24.7 kJ·mol?1and has an activation barrier of 21.4 kJ·mol?1. In transition state structures of TS2-3-Bpin, the distance between Ru and H atoms continuously elongates to 0.2562 nm, which is longer than that of TS2-3 and TS2-3-Bcat (Fig. S1).Furthermore, the distance between Ru and O atoms in the O-bound formate complexes is 0.2281 nm due to strong trans influence of Bpin ligand, which is larger than those of 3 and 3-Bcat. Surprisingly, in this model catalyst, rotation of the formato ligand is easier than hydride transfer to CO2. The overall free energy barrier is 54.4 kJ·mol?1, which leads to decreases of 21.8 kJ·mol?1in the overall barrier compared to Ru-PNP-H2. And the results illustrate that a simple ligand modification imparts changes in the overall activation barrier for the CO2hydrogenation on complex Ru-PNP-HBpin.

The process of hydrogenation of CO2to formate is just half catalytic cycle of production of FA, and we focus on this process because CO2addition to hydride is the rate-determining step calculated by Filonenko et al.17. They predicted the total activation energy barrier is 67 kJ·mol?1, which is in good agreement with our result. But they ignored the step of formato ligand isomerization, and this step also studied in other ruthenium complexes13,35,51. We can see from the energy profile in Fig. 2 shown above: all the complexes proceed via direct abstraction hydride mechanism for hydrogenation of CO2 to formate. The overall activation barriers for CO2 hydrogenation to formate on complexes Ru-PNP-H2, Ru-PNP-HBcat, and Ru-PNP-HBpin are 76.2, 67.8, and 54.4 kJ·mol?1, respectively. Ru-PNP-HBcat and Ru-PNP-HBpin that contain boryl ligand exerted strong trans influence both have lower activation energy of whole pathway for the hydrogenation of CO2to formate than Ru-PNP-H2. The complex Ru-PNP-HBpin could be a very efficient catalyst for the hydrogenation of CO2under mild condition. The free energy barriers of hydrogenation of CO2are influenced by ligand trans to reacting hydride significantly, and an appropriate adjustment of the functional groups trans to reacting hydride could greatly improve the catalytic activity.

4 Conclusions

In conclusion, our current calculations performed by detailed DFT calculations verify our proposed catalyst-design strategy for CO2hydrogenation. Ru-PNP-HBcat and Ru-PNP-HBpin are proposed based on strong trans influence of boryl ligands and examined the activities of two complexes for the hydrogenation of CO2. From perspective of related structural, electronic parameters and the nature of Ru―H bond, the introduction of boryl ligand increase the nucleophilic character of hydride, and make the hydride involved in a Ru―H bond have less d orbital components than that of Ru-PNP-H2. As a consequence, the step of direct addition of hydride to CO2 for Ru-PNP-HBcat and Ru-PNP-HBpin have lower activation barriers compared to Ru-PNP-H2. Moreover, introducing strongly trans-influence boryl ligands trans to reacting hydride is found to speed up the overall process of hydrogenation of CO2to formate. The overall activation barriers for CO2hydrogenation on Ru-PNP-HBcat,Ru-PNP-HBpin are 67.8, 54.4 kJ·mol–1, which are 8.4 and 21.8 kJ·mol–1lower than the reaction catalyzed by Ru-PNP-H2,respectively. Therefore, the reactivity of ruthenium complexes can be fine-tuned by changing trans ligand of active hydride to influence the nature of Ru―H bond. Further design of non-noble metal catalysts based on strong trans influence of boryl ligands and more in depth reasons for their excellent performance in CO2hydrogenation are underway.

Acknowledgment: We are grateful to the High Performance Computing Center of Nanjing Tech University for supporting the computational resources.

Supporting Information: available free of charge via the internet at http://www.whxb.pku.edu.cn.