Acta Phys. -Chim. Sin. ›› 2018, Vol. 34 ›› Issue (10): 1097-1105.doi: 10.3866/PKU.WHXB201712131

Special Issue: 材料科学的分子模拟

• ARTICLE • Previous Articles     Next Articles

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

Tian LIU1,Jun LI1,*(),Weijia LIU2,Yudan ZHU1,*(),Xiaohua LU1   

  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
  • Received:2017-11-15 Published:2018-04-13
  • Contact: Jun LI,Yudan ZHU E-mail:lijun@njtech.edu.cn;ydzhu@njtech.edu.cn
  • Supported by:
    National Key Basic Research Development Program of China (973)(2013CB733505);National Key Basic Research Development Program of China (973)(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)

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-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.

Key words: CO2 hydrogenation, Ru complex, Boryl ligand, Trans influence, Ru―H bond

MSC2000: 

  • O643