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

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 (lambda > 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 degrees C is kH = (2.1 +/- 0.2) x 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.