Kinetic and mechanistic studies of carbon-to-metal hydrogen atom transfer involving os-centered radicals: Evidence for tunneling

Anna Lewandowska-Andralojc, David C. Grills, Jie Zhang, R. Morris Bullock, Akira Miyazawa, Yuji Kawanishi, Etsuko Fujita

Research output: Contribution to journalArticle

11 Citations (Scopus)

Abstract

We have investigated the kinetics of novel carbon-to-metal hydrogen atom transfer reactions, in which homolytic cleavage of a C-H bond is accomplished by a single metal-centered radical. Time-resolved IR spectroscopic measurements revealed efficient hydrogen atom transfer from xanthene, 9,10-dihydroanthracene, and 1,4-cyclohexadiene to Cp(CO)2Os and (η5-iPr4C5H)(CO) 2Os radicals, formed by photoinduced homolysis of the corresponding osmium dimers. The rate constants for hydrogen abstraction from these hydrocarbons are in the range 1.5 × 105 M -1 s-1 to 1.7 × 107 M-1 s -1 at 25 C. For the first time, kinetic isotope effects for carbon-to-metal hydrogen atom transfer were determined. Large primary deuterium kinetic isotope effects of 13.4 ± 1.0 and 16.8 ± 1.4 were observed for the hydrogen abstraction from xanthene to form Cp(CO)2OsH and (η5-iPr4C5H)(CO)2OsH, respectively, at 25 C. Temperature-dependent measurements of the kinetic isotope effects over a 60 C temperature range were carried out to obtain the difference in activation energies (ED-EH) and the pre-exponential factor ratio (AH/AD). For hydrogen atom transfer from xanthene to (η5-iPr4C 5H)(CO)2Os, the (ED-E H) = 3.3 ± 0.2 kcal mol-1 and AH/A D = 0.06 ± 0.02 values suggest a quantum mechanical tunneling mechanism.

Original languageEnglish
Pages (from-to)3572-3578
Number of pages7
JournalJournal of the American Chemical Society
Volume136
Issue number9
DOIs
Publication statusPublished - Mar 5 2014

ASJC Scopus subject areas

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

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