Substituent effects on reductive elimination from disubstituted aryl hydride complexes: Mechanistic and thermodynamic considerations

Article abstract of DOI:10.1021/om00012a038

The complexes (C₅Me₅)Rh(PMe₃)(3,5-C₆H ₃R₂)H for R = C(CH₃)₃, CH(CH₃)₂, Si(CH₃)₃, CH₃, and CF₃ (1c-g) have been prepared both by irradiation of (C₅Me₅)Rh(PMe₃)(C₂H₄) in neat arene and by thermolysis of (C₅Me₅)Rh(PMe₃)(Ph)H (1a) in neat arene. Quenching the hydride species with CHBr₃ allowed isolation of the corresponding bromide complexes (C₅Me₅)Rh(PMe₃)(3,5-C₆H ₃R₂)Br (2c-g). Rates of reductive elimination of arene for the series of disubstituted aryl hydride complexes were measured at various temperatures and activation parameters ΔH? and ΔS? obtained and compared with those of (C₅Me₅)-Rh(PMe₃)(Ph)H (1a) and (C₅Me₅)Rh(PMe₃)(tolyl)H (1b). ΔH? values range from +35 to +18 kcal/mol, and ΔS? values range from +16.4 to -19 cal/mol K. Laser flash photolysis experiments using (C₅Me₅)Rh(PMe₃)(C₂H₄) in neat toluene at various temperatures allowed the determination of activation parameters ΔH? and ΔS? for intramolecular C-H bond oxidative addition of the η²-arene complex 3b. Equilibrium measurements allowed determination of ΔG° values for several of the disubstituted aryl hydride complexes versus the parent phenyl hydride complex. A kinetic isotope effect of k_H/k_D = 1.0 ± 0.1 was measured for the reaction of (C₅Me₅)Rh(PMe₃)(C₂H₄) with a 1:1 mixture of 5-deuterio-1,3-di-tert-butylbenzene and 1,3-di-tert-butylbenzene. The equilibrium isotope effect for the same reaction K_eq = 2.27(1) favors the aryl hydride and free deuterated arene.