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 language | English |
|---|---|
| Pages (from-to) | 15684-15685 |
| Number of pages | 2 |
| Journal | Journal of the American Chemical Society |
| Volume | 127 |
| Issue number | 45 |
| DOIs | |
| Publication status | Published - Nov 16 2005 |
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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 journal › Article
}
TY - JOUR
T1 - Carbon-to-metal hydrogen atom transfer
T2 - Direct observation using time-resolved infrared spectroscopy
AU - Zhang, Jie
AU - Grills, David
AU - Huang, Kuo Wei
AU - Fujita, Etsuko
AU - Bullock, R Morris
PY - 2005/11/16
Y1 - 2005/11/16
N2 - 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.
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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UR - http://www.scopus.com/inward/citedby.url?scp=27844469470&partnerID=8YFLogxK
U2 - 10.1021/ja0555724
DO - 10.1021/ja0555724
M3 - Article
C2 - 16277493
AN - SCOPUS:27844469470
VL - 127
SP - 15684
EP - 15685
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 45
ER -