Carbon-hydrogen activation of cycloalkanes by cyclopentadienylcarbonylrhodium-A lifetime enigma

Amanda L. Pitts, Alisdair Wriglesworth, Xue Zhong Sun, James A. Calladine, Snežana D. Zarić, Michael W. George, Michael B. Hall

Research output: Journal PublicationArticlepeer-review

27 Citations (Scopus)

Abstract

Carbon-hydrogen bond activation reactions of four cycloalkanes (C 5H10, C6H12, C7H 14, and C8H16) by the Cp′Rh(CO) fragments (Cp′ = η5-C5H5 (Cp) or η5-C5Me5 (Cp*)) were modeled theoretically by combining density functional and coupled cluster theories, and their reaction rates were measured by fast time-resolved infrared spectroscopy. The reaction has two steps, starting with the formation of a σ-complex intermediate, followed by oxidative addition of the C-H bond by the rhodium. A range of σ-complex stabilities among the electronically unique C-H bonds in a cycloalkane were calculated and are related to the individual strengths of the C-H bond's interactions with the Rh fragment and the steric repulsion that is incurred upon forming the specific σ-complex. The unexpectedly large increase in the lifetimes of the σ-complexes from cyclohexane to cycloheptane was predicted to be due to the large range of stabilities of the different σ-complexes found for cycloheptane. The reaction lifetimes were simulated with two mechanisms, with and without migrations among the different σ-complexes, to determine if ring migrations prior to C-H activation were influencing the rate. Both mechanisms predicted similar lifetimes for cyclopentane, cyclohexane, and, to a lesser extent, cycloheptane, suggesting ring migrations do not have a large impact on the rate of C-H activation for these cycloalkanes. For cyclooctane, the inclusion of ring migrations in the reaction mechanism led to a more accurate prediction of the lifetime, indicating that ring migrations did have an effect on the rate of C-H activation for this alkane, and that migration among the σ-complexes is faster than the C-H activation for this larger cycloalkane.

Original languageEnglish
Pages (from-to)8614-8625
Number of pages12
JournalJournal of the American Chemical Society
Volume136
Issue number24
DOIs
Publication statusPublished - 18 Jun 2014
Externally publishedYes

ASJC Scopus subject areas

  • Catalysis
  • Chemistry (all)
  • Biochemistry
  • Colloid and Surface Chemistry

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