A Two‐State Computational Investigation of Methane CH and Ethane CC Oxidative Addition to [CpM(PH₃)]ⁿ⁺ (M=Co, Rh, Ir; n=0, 1)

Abstract

Reductive elimination of methane from methyl hydride half-sandwich phosphane complexes of the Group 9 metals has been investigated by DFT calculations on the model system [CpM(PH(3))(CH(3))(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) were optimized. Spin crossover occurs very near the sigma-CH(4) complex local minimum for the Co system, whereas the heavier Rh and Ir systems remain in the singlet state until the CH(4) molecule is almost completely expelled from the metal coordination sphere. No local sigma-CH(4) 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(PH(3))(CH(3))(H)](+) (M = Rh, Ir) proceeds entirely on the spin doublet PES, because the 15-electron [CpM(PH(3))](+) products have a doublet ground state. This process is thermodynamically favored by about 25 kcal mol(-1) relative to the corresponding neutral system. It is essentially barrierless for the Rh system and has a relatively small barrier (ca. 7.5 kcal mol(-1)) for the Ir system. In both cases, the reaction involves a sigma-CH(4) intermediate. Reductive elimination of ethane from [CpM(PH(3))(CH(3))(2)](+) (M = Rh, Ir) shows a similar thermodynamic profile, but is kinetically quite different from methane elimination from [CpM(PH(3))(CH(3))(H)](+): the reductive elimination barrier is much greater and does not involve a sigma-complex intermediate. The large difference in the calculated activation barriers (ca. 12.0 and ca. 30.5 kcal mol(-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.

Transition-Metal Hydride Radical Cations

“…Reductive elimination of methane from the model system [CpM­(PH 3 ) (CH 3 )­(H)] (M = Rh, Ir) has been investigated by DFT calculations . As expected, reductive elimination becomes more favorable upon one-electron oxidation.…”

“…140 Reductive elimination of methane from the model system [CpM(PH 3 ) (CH 3 )(H)] (M = Rh, Ir) has been investigated by DFT calculations. 141 As expected, reductive elimination becomes more favorable upon one-electron oxidation. Oxidation of Cp*IrH(PPh 3 ) (CH 3 ) with two equivalents of Cp 2 Fe + in acetonitrile is accompanied by intramolecular CH 4 elimination.…”

Group 9 Metallacyclopentadienes as Key Intermediates in [2+2+2] Alkyne Cyclotrimerizations. Insight from Activation Strain Analyses

“…For example, they form in olefin metathesis reactions, [1] and in alkyne cyclotrimerizations, [2][3][4][5] or they are unwanted products resulting from ortho-metalation reactions. They are important reactive intermediates in catalytic processes.…”

Photochemistry of CpMn(CO)3 and Related Derivatives: Spectroscopic Observation of Singlet and Triplet CpMn(CO)2

“…To achieve a shorter lifetime, the barrier to spin crossover must be lower for Cp*Mn­(CO) 2 ( 2c ) than for CpMn­(CO) 2 ( 2a ). This can arise, for example, from the triplet surface for Cp*Mn­(CO) 2 being higher in energy and roughly parallel to the triplet surface for CpMn­(CO) 2 and the singlet surfaces for the two species being approximately the same (Figure ). , A detailed description of the spin crossover barriers will require additional computational and theoretical analyses, which are beyond the scope of our present study.…”

“…In contrast, there is less breadth in the literature regarding 16-electron organometallic species with open-shell electronic states (singlet or triplet). The role of spin multiplicity in organometallic chemistry remains a topic of current interest. − In this article, we describe matrix-isolation studies concerning the family of triplet species (η 5 -L)­Mn­(CO) 2 ( 3 2a – d ) obtained upon photolysis of (η 5 -L)­Mn­(CO) 3 ( 1a – d ) (L = C 5 H 5 (Cp), C 5 H 4 Me (Cp′), C 5 Me 5 (Cp*), C 9 H 7 (Ind, indenyl)) (Scheme ).…”