BETA-MIGRATORY INSERTION REACTIONS OF (ETA-5-C5R'5)RH(L)(C2H4)R+BF4- (R' = H, CH3 R = H, C2H5 L = P(OME)3, PME3) - COMPARISON OF THE ENERGETICS OF HYDRIDE VERSUS ALKYL MIGRATION

Protonation of the rhodium ethylene complexes C5R5(L)Rh(C2H4), 1a,b and 2a,b (1 R = CH3, 2 R = H, a L = P(OMe)3, b L = PMe3), with HBF4.Me2O in CH2Cl2 yields the four corresponding ethylene hydride complexes C5R5(L)Rh(C2H4)H+, 3a,b and 4a,b. NMR techniques were used to measure the rates of migratory insertion for these species. Values for the free energies of activation were found to be 12.2 (3a), 12.1 (3b), 15.0 (4a), and 15.0 (4b) kcal/mol. Eyring plots based on rate constants measured over the -20 to +20-degrees-C range established DELTAS(double dagger) = 0 for these migratory insertions. Treatment of 3a,b and 4a,b with excess ethylene gave the ethyl ethylene complexes C5R5(L)RhCH2CH3(C2H4)+, 7a,b and 8a,b, whose structures were established by H-1 and C-13 NMR spectroscopy. These complexes function as ethylene dimerization catalysts; NMR spectroscopic analysis of the catalytic systems shows that 7a,b and 8a,b are the catalyst resting states. The turnover-limiting step is the beta-migratory insertion reaction of the ethyl ethylene complexes. The DELTAG(double dagger) values for beta-migratory insertion calculated from turnover rates were found to be 22.4 (7a), 23.4 (7b), 23.4 (8a), and 24.7 (8b) kcal/mol. The DELTADELTAG(double dagger) values for H versus CH2CH3 migration for these systems thus lie in the range of 8.5-11 kcal/mol, which corresponds to relative migratory aptitudes k(H)/k(Et) of 10(6)-10(8) at 23-degrees-C.