CO2 Hydrogenation and Formic Acid Dehydrogenation Using Ir Catalysts with Amide-Based Ligands
CO2 Hydrogenation and Formic Acid Dehydrogenation Using Ir Catalysts with Amide-Based Ligands
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DOI：
10.1021/acs.organomet.9b00809
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发表时间：
2020-05-11
期刊：
影响因子：
2.8
通讯作者：
Himeda, Yuichiro
中科院分区：
文献类型：
--
作者：
Kanega, Ryoichi;Ertem, Mehmed Z.;Himeda, Yuichiro
制备了一系列含酰胺基配体的Ir催化剂，并假设配位阴离子氮原子和金属中心附近的质子响应性OH基团的强供电子能力将提高催化剂的CO2加氢和甲酸脱氢活性。在环境条件（25 ℃，0.1 MPa H-2/CO2（v/v = 1/1））和稍微苛刻的条件（50 ℃，1.0 MPa H-2/CO2）下，在碱性水溶液中，用选定的催化剂连同氘动力学同位素效应（KIEs）一起研究了配体结构的修饰对CO2氢化的催化活性的影响。Cp*Ir（L12）（H2O）HSO 4（L12 = 6-羟基-N-苯基吡啶酰胺酸酯）具有阴离子配位N原子和在第二配位层中的OH基团，基于反应的初始1小时，其表现出198 h（-1）的转换频率（TOF）。据我们所知，该TOF是在碱性水溶液中环境条件下报道的最高值。然而，Cp*Ir（L10）（H2O）HSO 4（L10 =（4-hydroxy-N-methylpicolinamidate））在环境条件下的长期CO2氢化中表现更好（348 h后TON高达14700，[Ir] = 10 μ M，最终甲酸盐浓度为0.643 M，[Ir] = 250 μ M）。此外，在三种不同的条件（pH 1.6、2.3和3.5）下检测FA脱氢的催化活性。由于其不稳定性，在任何这些条件下的Cp*Ir（L12）（H2O）HSO 4配合物与不含邻位OH的吡啶氨基甲酸酯催化剂相比活性较低。不含OH基团的配合物Cp*Ir（L 8）（H2O）HSO 4（L 8 = N-苯基吡啶氨基甲酸酯）在60 ℃下具有高的飞行时间（118 000 h（-1））。理论计算进行了研究的催化机理，并提出了一个逐步的机制，为CO2加氢和FA脱氢反应。[Cp*Ir（L3）（H2O）] HSO 4（L3 = picolinamidate）的密度泛函理论计算和[Cp*Ir（L7）（H）]中心点H2O（L7 = N-methylpicolinamidate）配合物的X射线结构表明，随着水溶液pH值的降低，配合物的构象由N，N配位转变为N，O配位.
A series of Ir catalysts bearing amide-based ligands generated by a deprotonated amide moiety was prepared with the hypotheses that the strong electron-donating ability of the coordinated anionic nitrogen atom and the proton-responsive OH group near the metal center will improve the catalytic activity for CO2 hydrogenation and formic acid (FA) dehydrogenation. The effects of the modifications of the ligand architecture on the catalytic activity were investigated for CO2 hydrogenation at ambient conditions (25 degrees C with 0.1 MPa H-2/CO2 (v/v = 1/1)) and under slightly harsher conditions (50 degrees C with 1.0 MPa H-2/CO2) in basic aqueous solutions together with deuterium kinetic isotope effects (KIEs) with selected catalysts. Cp*Ir(L12)(H2O)HSO4 (L12 = 6-hydroxy-N-phenylpicolinamidate) that has an anionic coordinating N atom and an OH group in the second coordination sphere, exhibits a turnover frequency (TOF) of 198 h(-1) based on the initial 1 h of reaction. This TOF which, to the best of our knowledge, is the highest value ever reported under ambient conditions in basic aqueous solutions. However, Cp*Ir(L10)(H2O)HSO4 (L10 = (4-hydroxy-N-methylpicolinamidate) performs better in long-term CO2 hydrogenation (up to a TON of 14 700 with [Ir] = 10 mu M after 348 h and the final formate concentration of 0.643 M with [Ir] = 250 mu M) at ambient conditions. Further, the catalytic activity for FA dehydrogenation was examined under three different conditions (pH 1.6, 2.3, and 3.5). The Cp*Ir(L12)(H2O)HSO4 complex in any of these conditions is less active compared to the picolinamidate catalysts without ortho-OH, owing to its instability. The complex without OH group, Cp*Ir(L8)(H2O)HSO4 (L8 = N-phenyl-picolinamidate), exhibits a high TOF (up to 118 000 h(-1)) at 60 degrees C. Theoretical calculations were performed to examine the catalytic mechanism, and a step-by-step mechanism has been proposed for both CO2 hydrogenation and FA dehydrogenation reactions. Density functional theory calculations of [Cp*Ir(L3)(H2O)]HSO4 (L3 = picolinamidate) and the X-ray structure of the [Cp*Ir(L7)(H)]center dot H2O (L7 = N-methylpicolinamidate) complex imply a pH-dependent conformational change from N,N coordination to N,O coordination upon lowering the pH of the aqueous solution.