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 bot h by irradiation of (C(5)Me(5))Rh(PMe(3))(C2H4) in neat arene and by t hermolysis 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). R ates of reductive elimination of arene for the series of disubstituted aryl hydride complexes were measured at various temperatures and acti vation parameters Delta H-double dagger and Delta S-double dagger obta ined 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 o f the eta(2)-arene complex 3b. Equilibrium measurements allowed determ ination of Delta G degrees values for several of the disubstituted ary l hydride complexes versus the parent phenyl hydride complex. A kineti c isotope effect of h(H)/h(D) = 1.0 +/- 0.1 was measured for the react ion 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 i sotope effect for the same reaction K-eq = 2.27(1) favors the aryl hyd ride and free deuterated arene.