SUBSTITUENT EFFECTS ON REDUCTIVE ELIMINATION FROM DISUBSTITUTED ARYL HYDRIDE COMPLEXES - MECHANISTIC AND THERMODYNAMIC CONSIDERATIONS

The complexes (C(5)Me(5))Rh(PMe(3))(3,5-C(6)H(3)R(2))H for R = C(CH3)(3), CH(CH3)(2), Si(CH3)(3), CH3, and CF3 (1c-g) have been prepared both by irradiation of (C(5)Me(5))Rh(PMe(3))(C2H4) in neat arene and by thermolysis of (C(5)Me(5))Rh(PMe(3))(Ph)H (1a) in neat arene, Quenching the hydride species with CHBr3 allowed isolation of the corresponding bromide complexes (C(5)Me(5))Rh(PMe(3))(3,5-C(6)H(3)R(2))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 Delta H-double dagger and Delta S-double dagger obtained and compared with those of (C(5)Me(5))-Rh(PMe(3))(Ph)H (1a) and (C(5)Me(5))Rh(PMe(3))(tolyl)H (1b). Delta H-double dagger values range from +35 to +18 kcal/mol, and Delta S-double dagger values range from +16.4 to -19 cal/mol K. Laser flash photolysis experiments using (C(5)Me(5))Rh(PMe(3))(C2H4) in neat toluene at various temperatures allowed the determination of activation parameters Delta H-double dagger and Delta S-double dagger for intramolecular C-H bond oxidative addition of the eta(2)-arene complex 3b. Equilibrium measurements allowed determination of Delta G degrees values for several of the disubstituted aryl hydride complexes versus the parent phenyl hydride complex. A kinetic isotope effect of h(H)/h(D) = 1.0 +/- 0.1 was measured for the reaction of (C(5)Me(5))Rh(PMe(3))(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.