Kinetics and Mechanism of Reductive Elimination of C−H Bonds from (μ-H)₃Ru₃(μ₃-CX)(CO)₉ Revisited:  CO Associative, CO Independent, or CO Dissociative?

Abstract

The kinetics of the isomerizations of (μ-H)3Ru3(μ3-CCO2Me)(CO)9 to (μ-H)2Ru3(μ3-η2-CHCO2Me)(CO)9 and of (μ-H)3Ru3(μ3-CSEt)(CO)9 to (μ-H)Ru3(μ3-η2-CH2SEt)(CO)9, including the measurements of activation volumes, are reported. An earlier study of reductive elimination of CH3X from H3Ru3(μ3-CX)(CO)9 found that the reaction was promoted by CO for X = Ph, Cl, and Et and was unaffected by CO pressure for X = CO2Me. The activation volume ΔV ⧧ for the intramolecular isomerization of (μ-H)3Ru3(μ3-CCO2Me)(CO)9 to (μ-H)2Ru3(μ3-η2-CHCO2Me)(CO)9 was determined to be −0.3(0.7) cm3/mol at 57.0 °C. For the isomerization of (μ-H)3Ru3(μ3-CSEt)(CO)9 to (μ-H)Ru3(μ3-η2-CH2SEt)(CO)9, the rate is inhibited by CO; activation parameters (ΔH ⧧=121(3) kJ/mol, ΔS ⧧= +36(10) J/(mol K), ΔV ⧧= +22.0(1.4) cm3/mol) are consistent with a mechanism involving reversible CO dissociation prior to the rate-determining step but following an intramolecular rearrangement. The change in mechanism of reductive elimination of a C−H bond from CO associative to CO independent to CO dissociative is due to anchimeric assistance by the methylidyne substituent. These results may have relevance to C−H formation occurring on metal surfaces.

Activation and Reaction Volumes in Solution. 3

“…The trinuclear ruthenium carbonyl complexes isomerize in decane (entries 868-870) by different mechanisms depending on the composition of the other ligands. 256 For example the compound in entry 868 has a near zero ∆V q and ∆S q , consistent with an intramolecular process in which there are no ap-preciable changes in bond lengths upon forming the transition state or there is compensation for any changes. Other possible mechanisms are ruled out because they would either generate distinctly positive (ligand dissociation) or distinctly negative (CO association or bimolecular hydride transfer) volumes of activation.…”

“…The trinuclear ruthenium carbonyl complexes isomerize in decane (entries 868−870) by different mechanisms depending on the composition of the other ligands . For example the compound in entry 868 has a near zero Δ V ⧧ and Δ S ⧧ , consistent with an intramolecular process in which there are no appreciable changes in bond lengths upon forming the transition state or there is compensation for any changes.…”

Oxidative Addition of Group 14 Hydrides to an Unsaturated Metal Cluster. Kinetics of Addition of HER3 (ER3 = SiEt3, SiPh3, GeBu3, SnBu3, SnPh3) to H2Os3(CO)10

“…The mechanisms of reductive elimination of C−H bonds from saturated clusters and intramolecular oxidative additions have been examined by several groups ,, Oxidative addition of the agostic C−H bond of HOs 3 (CO) 10 (μ-η 2 -HCH 2 ), yielding (μ-H) 2 Os 3 (CO) 10 (μ-CH 2 ), is very rapid, with a rate constant of 1 × 10 -3 s -1 at 14 °C . The corresponding ethyl to ethylidene conversion has a rate constant of 1.6 × 10 -4 s -1 at −10 °C 19b.…”

“…The corresponding ethyl to ethylidene conversion has a rate constant of 1.6 × 10 -4 s -1 at −10 °C 19b. Intermediates containing agostic bonds have been proposed in the reductive elimination of C−H bonds from triruthenium clusters …”