Kinetic and Mechanistic Studies of Carbon-to-Metal Hydrogen Atom Transfer Involving Os-Centered Radicals: Evidence for Tunneling

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- i Pr4C5H)(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- i Pr4C5H)(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 (E D – E H) and the pre-exponential factor ratio (A H/A D). For hydrogen atom transfer from xanthene to (η5- i Pr4C5H)(CO)2Os•, the (E D – E H) = 3.3 ± 0.2 kcal mol–1 and A H/A D = 0.06 ± 0.02 values suggest a quantum mechanical tunneling mechanism.