Control of Catalyst Isomers Using an N-Phenyl-Substituted RN(CH₂CH₂PⁱPr₂)₂ Pincer Ligand in CO₂ Hydrogenation and Formic Acid Dehydrogenation

Abstract

2 ) 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 iPr PN H P and iPr PN Me P [CH 3 N(CH 2 CH 2 P i Pr 2 ) 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 iPr PN Ph P 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 iPr PN H P form the anti isomer and complexes supported by iPr PN Me P 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 iPr PN R P (R = H, Me, Ph)-ligated ruthenium hydride complexes as catalysts for formic acid dehydrogenation and carbon dioxide (CO 2 ) hydrogenation to formate. The iPr PN Ph P-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 2 hydrogenation, the iPr PN Ph P-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 2 hydrogenation and also provided a framework for future catalyst development.

Iron, Cobalt, and Nickel Complexes Supported by a iPrPNPhP Pincer Ligand

“…The Fe–P (2.465(2) and 2.460(2) Å) and Fe–Cl (2.245(2) and 2.246(2) Å) bond lengths of 1 are shorter than the corresponding bond distances in ( iPr PN H P)­FeCl 2 ( 1-H ) and ( iPr PN Me P)­FeCl 2 ( 1-Me ), as expected for a complex with a lower coordination number. , The P–Fe–P bond angle is 109.61(8)°, as opposed to the approximately linear angle observed when R PN R’ P ligands bind in a tridentate fashion. The structure of 1 suggests that the nitrogen atom of the pincer ligand is a poorer σ-donor in iPr PN Ph P than in its N–H and N–Me analogues as observed for Ru, although steric factors could also cause the decoordination of the nitrogen donor.…”

“…4d,17 The P−Fe−P bond angle is 109.61(8)°, as opposed to the approximately linear angle observed when R PN R' P ligands bind in a tridentate fashion. The structure of 1 suggests that the nitrogen atom of the pincer ligand is a poorer σ-donor in iPr PN Ph P than in its N−H and N−Me analogues as observed for Ru, 14 although steric factors could also cause the decoordination of the nitrogen donor. 57 Fe Mossbauer spectroscopy was performed on a powder sample of 1 to gain further insight into its electronic structure (Figure 3a).…”

“…We recently reported the synthesis of the novel iPr PN Ph P ( iPr PN Ph P = PhN(CH 2 CH 2 P i Pr 2 ) 2 ) ligand, which contains a phenyl substituent on the nitrogen donor (Figure 1b), and prepared a series of Ru hydride complexes ligated by iPr PN Ph P. 14 N bond lengths in comparison to their iPr PN Me P and iPr PN H P congeners, indicating that the nitrogen ligand is a weaker σdonor. The relatively large steric bulk of the phenyl group also resulted in the synthesis of a formate complex where the formate ligand is oriented anti to the N-phenyl moiety.…”

“…(a) Previous investigations of R PN R ’P complexes. (b) Recently reported Ru iPr PN Ph P complexes . (c) Some of the novel iPr PN Ph P-supported Fe, Co, and Ni complexes synthesized and characterized in this work.…”

“…We recently reported the synthesis of the novel iPr PN Ph P ( iPr PN Ph P = PhN­(CH 2 CH 2 P i Pr 2 ) 2 ) ligand, which contains a phenyl substituent on the nitrogen donor (Figure b), and prepared a series of Ru hydride complexes ligated by iPr PN Ph P . These complexes exhibit significantly longer Ru–N bond lengths in comparison to their iPr PN Me P and iPr PN H P congeners, indicating that the nitrogen ligand is a weaker σ-donor.…”