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In this paper, a theoretical investigation of bond activations of PhSiH3, catalyzed by Cp2SmH, is proposed by examination of the energy profiles of these reactions. The formation of the minor and major products observed experimentally is rationalized. The experimentally proposed two-step mechanism, starting with a Si−C activation to give Cp2SmPh as an intermediate, is confirmed. The second step involves reaction of this intermediate with the Si−H bond of PhSiH3to give Ph2SiH2and regenerate the samarium hydride. The relative free enthalpy barriers of these two steps are in agreement with the lack of observation of any intermediate during the catalytic reaction. In addition to the main experimentally proposed mechanism, a second mechanism that involves two successive Si−H activations is proposed in order to account for the formation of byproducts. Theoretically, these two reactions are demonstrated to occur competitively, which explains the formation of secondary products arising from redistribution and dehydrocoupling. [ABSTRACT FROM AUTHOR]

The reaction of various actinide and transition metal bisalkyl complexes with pyridine N-oxide has been investigated at the DFT level for M = Zr, Th, and U. Rather than the expected oxygen transfer reaction, an ortho C−H activation is observed experimentally for Th and U and is explained theoretically. The oxygen transfer reaction implies the formation of ethane by coupling two methyl groups; however, the transition state lies very high in energy due to the electrostatic repulsion between the two negatively charged methyl groups. In contrast, C−H activation leads to a six-membered-ring transition state at low energy with a perfect alternation of charges. The latter is found to be kinetically accessible for Th and U but not for Zr, in agreement with the experimental observations. This is related to the ability of Th and U to hybridize their unoccupied 5f orbitals with the 6d in order to form a covalent M−Me bond, leading to strong M−Me→σ*(C−H) back-donation at the transition state. Thorium is thus behaving more like an actinide that can use the 5f orbitals for bonding than as a transition metal. [ABSTRACT FROM AUTHOR]

The coordination of the four model amino−carbene ligands A−Dto SmCl3has been theoretically investigated. Strong coordination energies were predicted for all carbenes (G(25 °C) < −35 kcal mol-1for the monoadducts). The nature of the Sm−carbene bonds was studied by molecular orbital and natural bond orbital (NBO) analyses. No evidence for significant carbene-to-Sm -donation or Cl-to-carbene back-donation was observed. The strong Sm−carbene bonds, culminating in the abnormal NHC species D, can thus be essentially attributed to carbene-to-Sm -donation. [ABSTRACT FROM AUTHOR]

The monomeric metallocenecerium hydride, Cp'2CeH (Cp' = 1,2,4-tri-tert-butylcyclopentadienyl), reacts instantaneously with CH3F, but slower with CH2F2, to give Cp'2CeF and CH4 in each case, a net H for F exchange reaction. The hydride reacts very slowly with CHF3, and not at all with CF4, to give Cp'2- CeF, H2, and 1,2,4- and 1,3,5-tri-tert-butylbenzene. The substituted benzenes are postulated to result from trapping of a fluorocarbene fragment derived by α-fluoride abstraction from Cp'2CeCF3. The fluoroalkyl, Cp'2CeCF3, is generated by reaction of Cp'2CeH and Me3SiCF3 or by reaction of the metallacycle, ((Cp')-(Me3C)2C5H2C(Me2)CH2]Ce, with CHF3, and its existence is inferred from the products of decomposition, which are Cp'2CeF, the isomeric tri-teit-butylbenzenes and in the case of Me3SiCF3, Me3SiH. The fluoroalkyls, Cp'2CeCH2F and Cp'2CeCHF2, generated from the metallacycle and CH3F and CH2F2, respectively, are also inferred by their decomposition products, which are Cp'2CeF, CH2, and CHF, respectively, which are trapped. DFT(B3PW91) calculations have been carried out to examine several reaction paths that involve CH and CF bond activation. The calculations show that the CH activation by Cp2CeH proceeds with a low barrier. The carbene ejection and trapping by H2 is the rate-determining step, and the barrier parallels that found for reaction of H2 with CH2, CHF, and CF2. The barrier of the rate-determining step is raised as the number of fluorines increases, while that of the CH activation path is lowered as the number of fluorines increases, which parallels the acidity. [ABSTRACT FROM AUTHOR]