Reductive elimination of methane from methyl hydride halfsandwich phosphane complexes of the Group 9 metals has been investigated by DFT calculations on the model system [CpM(PH3)(CH3)(H)] (M=Co, Rh, Ir). For each metal, the unsaturated product has a triplet ground state; thus, spin crossover occurs during the reaction. All relevant stationary points on the two potential energy surfaces (PES)and the minimum energy crossing point (MECP)w ere optimized. Spin crossover occurs very near the s- CH4 complex local minimum for the Co system, whereas the heavier Rh and Ir systems remain in the singlet state until the CH4 molecule is almost completely expelled from the metal coordination sphere. No local s-CH4 minimum was found for the Ir system. The energetic profiles agree with the nonexistence of the Co III methyl hydride complex and with the greater thermal stability of the Ir complex relative to the Rh complex. Reductive elimination of methane from the related oxidized complexes [CpM(PH3)(CH3)(H)]+ (M=Rh, Ir)proceeds entirely on the spin doublet PES, because the 15-electron [CpM(PH3)]+ products have a doublet ground state. This process is thermodynamically favored by about 25 kcalmol_1 relative to the corresponding neutral system. It is essentially barrierless for the Rh system and has a relatively small barrier (ca. 7.5 kcalmol_1)for the Ir system. In both cases, the reaction involves a s- CH4 intermediate. Reductive elimination of ethane from [CpM(PH3)- (CH3)2]+ (M=Rh, Ir)shows a similar thermodynamic profile, but is kinetically quite different from methane elimination from [CpM(PH3)(CH3)(H)]+: the reductive elimination barrier is much greater and does not involve a scomplex intermediate. The large difference in the calculated activation barriers (ca. 12.0 and ca. 30.5 kcalmol_1 for the Rh and Ir systems, respectively) agrees with the experimental observation, for related systems, of oxidatively induced ethane elimination when M= Rh, whereas the related Ir systems prefer to decompose by alternative pathways.
|Autori:||A. PETIT; P. RICHARD; CACELLI I; R. POLI|
|Titolo:||A two-state computational investigation of the methane C-H and ethane C-C oxidative addition to [CpM(PH$_3$]n+ (M=Co,Rh,Ir; n=0,1)|
|Anno del prodotto:||2006|
|Appare nelle tipologie:||1.1 Articolo in rivista|