Carbon-to-metal hydrogen atom transfer: Direct observation using time-resolved infrared spectroscopy

Jie Zhang, David Grills, Kuo Wei Huang, Etsuko Fujita, R Morris Bullock

Research output: Contribution to journalArticle

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Abstract

We report the direct spectroscopic observation of hydrogen atom transfer reactions from carbon to metals, in which homolytic cleavage of a C-H bond is accomplished at a single metal center. Laser flash photolysis (355 nm) of a solution of [Cp(CO)2Os]2 leads to homolysis of the Os-Os bond and formation of the osmium-centered radical, Cp(CO)2Os, as observed by time-resolved infrared (TRIR) spectroscopy. DFT computations on Cp(CO)2Os support this assignment. Continuous photolysis (λ > 300 nm) of [Cp(CO)2Os]2 in the presence of excess 1,4-cyclohexadiene produces the osmium hydride Cp(CO)2OsH. The kinetics of this carbon-to-metal hydrogen atom transfer were examined by TRIR spectroscopy. The second-order rate constant for hydrogen atom transfer from 1,4-cyclohexadiene to Cp(CO)2Os in hexane at 23 °C is kH = (2.1 ± 0.2) × 106 M-1 s-1. The pKa of Cp(CO)2OsH was determined as 32.7 in CH3CN, and use of a thermochemical cycle provided an estimated lower limit of 82 kcal/mol for the Os-H bond dissociation energy, indicating that it is an exceptionally strong M-H bond. Photolysis of [Tp(CO)2Os]2 (Tp = hydridotris(pyrazolyl)borate) results in carbon-to-metal hydrogen atom transfers from even stronger C-H bonds (THF or toluene) and produces Tp(CO)2OsH.

Original languageEnglish
Pages (from-to)15684-15685
Number of pages2
JournalJournal of the American Chemical Society
Volume127
Issue number45
DOIs
Publication statusPublished - Nov 16 2005

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Photolysis
Hydrogen
Infrared spectroscopy
Spectrum Analysis
Carbon
Metals
Observation
Osmium
Atoms
Borates
Toluene
Hexanes
Lasers
Hexane
Discrete Fourier transforms
Hydrides
Rate constants
1,4-cyclohexadiene
Kinetics

ASJC Scopus subject areas

  • Chemistry(all)

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Carbon-to-metal hydrogen atom transfer : Direct observation using time-resolved infrared spectroscopy. / Zhang, Jie; Grills, David; Huang, Kuo Wei; Fujita, Etsuko; Bullock, R Morris.

In: Journal of the American Chemical Society, Vol. 127, No. 45, 16.11.2005, p. 15684-15685.

Research output: Contribution to journalArticle

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AB - We report the direct spectroscopic observation of hydrogen atom transfer reactions from carbon to metals, in which homolytic cleavage of a C-H bond is accomplished at a single metal center. Laser flash photolysis (355 nm) of a solution of [Cp(CO)2Os]2 leads to homolysis of the Os-Os bond and formation of the osmium-centered radical, Cp(CO)2Os•, as observed by time-resolved infrared (TRIR) spectroscopy. DFT computations on Cp(CO)2Os• support this assignment. Continuous photolysis (λ > 300 nm) of [Cp(CO)2Os]2 in the presence of excess 1,4-cyclohexadiene produces the osmium hydride Cp(CO)2OsH. The kinetics of this carbon-to-metal hydrogen atom transfer were examined by TRIR spectroscopy. The second-order rate constant for hydrogen atom transfer from 1,4-cyclohexadiene to Cp(CO)2Os• in hexane at 23 °C is kH = (2.1 ± 0.2) × 106 M-1 s-1. The pKa of Cp(CO)2OsH was determined as 32.7 in CH3CN, and use of a thermochemical cycle provided an estimated lower limit of 82 kcal/mol for the Os-H bond dissociation energy, indicating that it is an exceptionally strong M-H bond. Photolysis of [Tp(CO)2Os]2 (Tp = hydridotris(pyrazolyl)borate) results in carbon-to-metal hydrogen atom transfers from even stronger C-H bonds (THF or toluene) and produces Tp(CO)2OsH.

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