Curley Julia B, Hert Clayton, Bernskoetter Wesley H, Hazari Nilay, Mercado Brandon Q
The Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States.
The Department of Chemistry, The University of Missouri, Columbia, Missouri 65211, United States.
Inorg Chem. 2022 Jan 10;61(1):643-656. doi: 10.1021/acs.inorgchem.1c03372. Epub 2021 Dec 25.
A novel pincer ligand, PNP [PhN(CHCHPPr)], which is an analogue of the versatile MACHO ligand, PNP [HN(CHCHPPr)], was synthesized and characterized. The ligand was coordinated to ruthenium, and a series of hydride-containing complexes were isolated and characterized by NMR and IR spectroscopies, as well as X-ray diffraction. Comparisons to previously published analogues ligated by PNP and PNP [CHN(CHCHPPr)] illustrate that there are large changes in the coordination chemistry that occur when the nitrogen substituent of the pincer ligand is altered. For example, ruthenium hydrides supported by the PNP ligand always form the syn isomer (where syn/anti refer to the relative orientation of the group on nitrogen and the hydride ligand on ruthenium), whereas complexes supported by PNP form the anti isomer and complexes supported by PNP form a mixture of syn and anti isomers. We evaluated the impact of the nitrogen substituent of the pincer ligand in catalysis by comparing a series of PNP (R = H, Me, Ph)-ligated ruthenium hydride complexes as catalysts for formic acid dehydrogenation and carbon dioxide (CO) hydrogenation to formate. The PNP-ligated species is the most active for formic acid dehydrogenation, and mechanistic studies suggest that this is likely because there are kinetic advantages for catalysts that operate via the syn isomer. In CO hydrogenation, the PNP-ligated species is again the most active under our optimal conditions, and we report some of the highest turnover frequencies for homogeneous catalysts. Experimental and theoretical insights into the turnover-limiting step of catalysis provide a basis for the observed trends in catalytic activity. Additionally, the stability of our complexes enabled us to detect a previously unobserved autocatalytic effect involving the base that is added to drive the reaction. Overall, by modifying the nitrogen substituent on the MACHO ligand, we have developed highly active catalysts for formic acid dehydrogenation and CO hydrogenation and also provided a framework for future catalyst development.
合成并表征了一种新型钳形配体PNP [PhN(CHCHPPr)]，它是通用的MACHO配体PNP [HN(CHCHPPr)]的类似物。该配体与钌配位，分离得到了一系列含氢配合物，并通过核磁共振光谱、红外光谱以及X射线衍射对其进行了表征。与先前发表的由PNP和PNP [CHN(CHCHPPr)]连接的类似物进行比较表明，当钳形配体的氮取代基发生变化时，配位化学会发生很大变化。例如，由PNP配体支撑的钌氢化物总是形成顺式异构体（其中顺式/反式是指氮上的基团与钌上的氢化物配体的相对取向），而由PNP支撑的配合物形成反式异构体，由PNP支撑的配合物形成顺式和反式异构体的混合物。通过比较一系列PNP（R = H、Me、Ph）连接的钌氢化物配合物作为甲酸脱氢和二氧化碳（CO）加氢生成甲酸盐的催化剂，我们评估了钳形配体的氮取代基对催化作用的影响。PNP连接的物种对甲酸脱氢最具活性，机理研究表明这可能是因为通过顺式异构体操作的催化剂具有动力学优势。在CO加氢反应中，PNP连接的物种在我们的最佳条件下再次最具活性，并且我们报道了一些均相催化剂的最高周转频率。对催化作用的周转限制步骤的实验和理论见解为观察到的催化活性趋势提供了基础。此外，我们配合物的稳定性使我们能够检测到一种以前未观察到涉及添加以驱动反应的碱的自催化作用。总体而言，通过修饰MACHO配体上的氮取代基，我们开发了用于甲酸脱氢和CO加氢的高活性催化剂，并为未来的催化剂开发提供了框架。
