Study of axial steric effects on reductive elimination from (PNP)RhIII complexes

Abstract

Reductive elimination reactions from transition metal complexes are a common pathway for the formation of new bonds in many catalytic processes. (^RPNP)RhCl complexes (^RPNP = 2,6-bis{(di-R-phosphino)methyl}pyridine, R = tert-Bu, cyclohexyl, mesityl and phenyl) are synthesized and used to evaluate the "axial" steric effect, which is due to the size of the alkyl or aryl substituents on the phosphine, on reductive elimination of Rh(III) as well as the reverse oxidative addn. reaction. According to the computational modeling, there is a 5 kcal/mol increase in the energy of transition state for reductive elimination of methanol from [Rh(Me)(OH)(OH_2)]^+ when the phosphine substituent is changed from Me to tert-Bu. The calcns. are consistent with the hypothesis that ligands with enhanced steric bulk favor the Rh(I) oxidn. State compared to smaller ligands. In exptl. studies, oxidative addn. reactions to Rh(I) involving chloroform, dichloromethane, and iodomethane followed the predictions based on computational modeling. The oxidative addn. of CHCl_3 to (^(Cy)PNP)RhCl and (^(tBu)PNP)RhCl reveals pseudo-first order kinetics. In addn., a fast equil. between (^(tBu)PNP)Rh(I) and (^(tBu)PNP)Rh(III) complexes was obsd. while iodomethane was utilized as the oxidizing agent, which indicates that the energy barrier of the reductive elimination is effectively reduced compared to when chloroform and dichloromethane were used.