COMPETITIVE REDUCTIVE ELIMINATION OF C-H AND SI-H BONDS FROM IRH(SIHPH(2))(MES)(CO)(DPPE) - COMPARISON OF THE KINETIC ACTIVATION PARAMETERSFOR REDUCTIVE ELIMINATIONS IN IR-III(DPPE) COMPLEXES

The complex IrH(SiHPh(2))(mes)(CO)(dppe) (1; mes = 2,4,6-trimethylphen yl; dppe = bis(diphenylphosphino)ethane) undergoes competing irreversi ble mesitylene and reversible diphenylsilane reductive eliminations in benzene, leading to the known Ir complexes Ir(mes)(CO)(dppe) and IrH( SiHPh(2))(2)(CO)(dppe) (2). The kinetics for the C-H vs Si-H competiti ve reductive eliminations have been studied and indicate that both rea ctions follow simple first-order kinetics, For mesitylene reductive el imination, Delta H-double dagger = 21.8 +/- 0.3 kcal mol(-1), Delta S- double dagger = -5.0 +/- 1.1 eu, and Delta G(298)(double dagger) = 23. 3 +/- 0.4 kcal mol(-1), while for diphenylsilane reductive elimination , Delta H-double dagger = 15.6 +/- 0.5 kcal mol(-1), Delta S-double da gger = -25.2 +/- 1.1 eu, and Delta G(298)(double dagger) = 23.1 +/- 0. 6 kcal mol(-1). A striking similarity exists among the kinetic paramet ers for diphenylsilane reductive elimination from 1 and for H-2 reduct ive eliminations from IrH2(C2H5)(CO)(dppe) (5) and IrH2(C(O)C2H5)(CO)( dppe) (6). To determine if a kinetic similarity for silane and H-2 red uctive eliminations-exists among Ir-III (dppe) complexes, the kinetic parameters for triethylsilane reductive elimination from IrH2(Si(C2H5) (3))(CO)(dppe) (7) and H-2 reductive elimination from IrH2(SiHPh(2))(C O)(dppe) (8) were determined. For triethylsilane reductive elimination from 7, Delta H-double dagger = 29.2 +/- 0.3 kcal mol(-1), Delta S-do uble dagger = 12.8 +/- 0.9 eu, and Delta G(298)(double dagger) = 25.4 +/- 0.4 kcal mol(-1). For H-2 reductive elimination from 8, Delta H-do uble dagger = 25.2 +/- 0.7 kcal mol(-1), Delta S-double dagger = -4.2 +/- 2.1 eu, and Delta G(298)(double dagger) = 26.4 +/- 0.9 kcal mol(-1 ). The negative entropies of activation for Si-H and H-H reductive eli minations are rationalized in terms of sigma-complex formation prior t o dissociation.