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 (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 k_H = (2.1 ± 0.2) × 10⁶ M^-1 s^-1. The pK_a 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.