Citation: LIU Tian, LI Jun, LIU Weijia, ZHU Yudan, LU Xiaohua. Simple Ligand Modifications to Modulate the Activity of Ruthenium Catalysts for CO₂ Hydrogenation: Trans Influence of Boryl Ligands and Nature of Ru―H Bond[J]. Acta Physico-Chimica Sinica, ;2018, 34(10): 1097-1105. doi: 10.3866/PKU.WHXB201712131 shu

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

LIU Tian ¹ ,
LI Jun ^1,, ,
LIU Weijia ² ,
ZHU Yudan ^1,, ,
LU Xiaohua ¹

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

Corresponding author: LI Jun, lijun@njtech.edu.cn ZHU Yudan, ydzhu@njtech.edu.cn
Received Date: 15 November 2017
Revised Date: 8 December 2017
Accepted Date: 8 December 2018
Available Online: 13 October 2017
Fund Project: Research Fund for the Doctoral Program of Higher Education of China 20123221120015National Natural Science Foundation of China 91334202National Key Basic Research Development Program of China (973) 2013CB733501Natural Science Foundation of Jiangsu Province BK2012421National Key Basic Research Development Program of China (973) 2013CB733505The project was supported by National Key Basic Research Development Program of China (973) (2013CB733505, 2013CB733501), National Natural Science Foundation of China (91334202), Natural Science Foundation of Jiangsu Province (BK2012421), Research Fund for the Doctoral Program of Higher Education of China (20123221120015), Project for Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD)

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-PNP-HBpin, 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-PNP-HBcat 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.
- CO₂ hydrogenation,
- Ru complex,
- Boryl ligand,
- Trans influence,
- Ru―H bond
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Simple Ligand Modifications to Modulate the Activity of Ruthenium Catalysts for CO2 Hydrogenation: Trans Influence of Boryl Ligands and Nature of Ru―H Bond

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通讯作者: 陈斌, bchen63@163.com

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