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物理化学学报  2018, Vol. 34 Issue (10): 1097-1105    DOI: 10.3866/PKU.WHXB201712131
所属专题: 材料科学的分子模拟
论文     
简单的配体改变调控钌配合物催化还原CO2的活性:硼基配体的对位效应和Ru―H键的性质
刘甜1,黎军1,*(),刘维佳2,朱育丹1,*(),陆小华1
1 南京工业大学化工学院、材料化学工程国家重点实验室,南京 210009
2 南京市锅炉压力容器检验研究院,南京 210019
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 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
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摘要:

开发高效的催化剂用于催化还原CO2转化为甲酸和它的盐类已经成为研究的热点,是因为将CO2转化为C1产物不仅可以解决CO2的含量升高带来的环境问题,还可以解决化石能源燃烧日趋严重的问题。贵金属配合物催化CO2转化为甲酸和甲酸盐类是目前这类反应最有效的方式,尤其是Ru、Ir和Rh等贵金属。我们之前的研究结果表明Ir(Ⅲ), Ru(Ⅱ)类配合物催化还原CO2转化为甲酸盐的活性是由配合物Ru―H键的成键性质决定的。它们能高活性的催化CO2是由于它们都含有同一种特点的Ru―H键,是由Ru的sd2杂化轨道和H的1s轨道杂化而成的,而且这一特点可以被活性氢的对位配体显著影响。鉴于硼基配体具有强的对位效应,我们基于高活性的均相催化剂Ru(PNP)(CO)H2 (PNP = 2, 6-二(二叔丁基磷甲基)-吡啶)设计了Ru-PNP-HBcat和Ru-PNP-HBpin,并计算了二者催化还原CO2的活性。Bcat和Bpin配体是实验上常用的硼基配体。我们的计算结果表明Ru-PNP-HBcat和Ru-PNP-HBpin有比Ru-PNP-H2更长的Ru―H键、亲核性更强的活性氢,其Ru―H键中的Ru原子的d轨道杂化成分的贡献也比Ru-PNP-H2的更少。相应地Ru-PNP-HBcat和Ru-PNP-HBpin活化CO2的能垒比Ru-PNP-H2低。而且Ru-PNP-H2、Ru-PNP-HBcat和Ru-PNP-HBpin催化CO2转化为甲酸盐的能垒分别为76.2、67.8、54.4 kJ∙mol-1,表明Ru-PNP-HBpin具有最高的催化活性。因此,钌配合物催化还原CO2的活性可由硼基配体强的对位效应和Ru―H键的成键性质来调控。

关键词: CO2还原钌配合物硼基配体对位效应Ru―H键    
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
收稿日期: 2017-11-15 出版日期: 2017-12-13
中图分类号:  O643  
基金资助: 国家重点基础研究发展计划(973)(2013CB733505);国家重点基础研究发展计划(973)(2013CB733501);国家自然科学基金(91334202);江苏省自然科学基金(BK2012421);国家教育部高等学校博士点基金(20123221120015);江苏省高校优势学科建设工程资助项目
通讯作者: 黎军,朱育丹     E-mail: lijun@njtech.edu.cn;ydzhu@njtech.edu.cn
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引用本文:

刘甜,黎军,刘维佳,朱育丹,陆小华. 简单的配体改变调控钌配合物催化还原CO2的活性:硼基配体的对位效应和Ru―H键的性质[J]. 物理化学学报, 2018, 34(10): 1097-1105.

Tian LIU,Jun LI,Weijia LIU,Yudan ZHU,Xiaohua LU. Simple Ligand Modifications to Modulate the Activity of Ruthenium Catalysts for CO2 Hydrogenation: Trans Influence of Boryl Ligands and Nature of Ru―H Bond. Acta Physico-Chimica Sinca, 2018, 34(10): 1097-1105.

链接本文:

http://www.whxb.pku.edu.cn/CN/10.3866/PKU.WHXB201712131        http://www.whxb.pku.edu.cn/CN/Y2018/V34/I10/1097

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.
Ru-H1/nm Ru-H2 or Ru-B/nm H1-Ru-H2 or H1-Ru-B/(°) P1-P2-O1-O2/(°)
Ru-PNP-H2 0.1697 0.1697 175.7 ?
Ru-PNP-HBcat 0.1714 0.2140 176.0 62.0
Ru-PNP-HBpin 0.1716 0.2170 177.7 35.2
Table 1  Structural comparison among three Ru(Ⅱ) complexes
Trans ligand NBO charge/e Stretching frequency of Ru―H bond/cm?1 WBI of Ru―H bond
Ru H1 H2/BX CO PNP
H? ?0.552 ?0.245 ?0.245 0.113 0.930 1853 0.8620
Bcat? ?0.607 ?0.270 ?0.278 0.148 1.007 1658 0.8372
Bpin? ?0.598 ?0.276 ?0.236 0.132 0.978 1644 0.8370
Table 2  Summed up NBO charges on the ligands and metal, stretching frequency and WBI of Ru―H bond of three complexes.
Trans ligand Ru―H bond distance/nm Coefficients/hybrids of Ru―H bond orbital Ea/(kJ·mol?1)
H? 0.1697 0.706Ru(sd2.11) + 0.709H(s) 29.3
Bcat? 0.1714 0.702Ru(sd1.83) + 0.712H(s) 24.7
Bpin? 0.1716 0.699Ru(sd1.80) + 0.715H(s) 23.4
Table 3  Bond distance, bonding parameter of Ru―H bond, activation energy for hydride (Ea) addition to CO2 on three complexes.
Scheme 1  The hydrogenation of CO2 to formate on three complexes (X = H, Bcat, Bpin).
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.
Trans ligand 1 TS1-2 2
∠O―C―O C…H Ru…H ∠O―C―O C…H Ru…H ∠O―C―O C…H Ru…H
H? 176.8 0.2629 0.1695 153.1 0.1680 0.1748 138.0 0.1280 0.1857
Bcat? 176.8 0.2690 0.1714 153.0 0.1654 0.1763 137.3 0.1266 0.1901
Bpin? 176.5 0.2674 0.1717 154.7 0.1752 0.1766 136.3 0.1252 0.1931
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.
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