Use of ligand steric properties to control the thermodynamics and kinetics of oxidative addition and reductive elimination with pincer-​ligated Rh complexes

Abstract

Reductive elimination from transition metal complexes is a common pathway to form new bonds in many catalytic processes. Thus, controlling the energetics of reductive elimination is key to several catalytic processes. A series of (^RPNP)RhCl complexes (^RPNP = 2,6-bis{(di-R-phosphino)methyl}pyridine, R = tert-Bu, iso-Pr, mesityl or phenyl) were synthesized with different size of the substituents on the ligated phosphines. These phosphine substituents introduce different "axial" steric around the Rh center. The effect of the sterics of the substituents on reductive elimination from Rh(III) as well as the reverse reaction, oxidative addn., were evaluated. The hypothesis, suggested by initial computational modeling, is that sterically bulky ligands will bias the thermodn. of Rh(III/I) redox processes toward Rh(I) (relative to ligands with small substituents), and that the thermodn. will also provide more rapid reductive elimination reactions from Rh(III). Exptl. studies of oxidative addn. on the series of (PNP)Rh(I) complexes using iodomethane or dichloromethane were consistent with this hypothesis. Kinetic studies for oxidn. addn. of MeI were completed to probe the detailed energetics of the reactions and to compare with computational results.