Control of Catalyst Isomers Using an N-Phenyl-Substituted RN((CH2CH2PPr2)-Pr-i)(2) Pincer Ligand in CO2 Hydrogenation and Formic Acid Dehydrogenation

A novel pincer ligand,(PNP)-P- iPr-P-Ph [PhN-((CH2CH2PPr2)-Pr-i)(2)], which is an analogue of the versatile MACHO ligand, (PNP)-P-iPr-P-H [HN((CH2CH2PPr2)-Pr-i)(2)], 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)-P-iPr-P-H and (PNP)-P-iPr-P-Me [CH3N((CH2CH2PPr2)-Pr-i)(2)] 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)-P-iPr-P-Ph 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)-P-iPr-P-H form the anti isomer and complexes supported by (PNP)-P-iPr-P-Me 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)-P-iPr-P-R (R = H, Me, Ph)-ligated ruthenium hydride complexes as catalysts for formic acid dehydrogenation and carbon dioxide (CO2) hydrogenation to formate. The (PNP)-P-iPr-P-Ph-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 CO2 hydrogenation, the (PNP)-P-iPr-P-Ph-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 CO2 hydrogenation and also provided a framework for future catalyst development.