Comparison of hydrogen atom abstraction rates of terminal and bridging hydrides in triosmium clusters: Absolute abstraction rate constants for benzyl radical

Absolute rate constants for hydrogen atom abstraction by benzyl radical from Os₃(μ-H)₂ (CO)₉PPh₃ (1), Os₃(μ-H)(H)(CO)₁₀PPh₃ (2), Os₃(μ-H)(CO₉) (μ₃-η²-C₉H₆N) (3), Os₃(μ-H)(CO₉)- (μ-η²-C₉H₆N)(PPh₃) (4), and Os₃(μ-H)(CO₁₀)(μ-η²- C₉H₆N) (5) were determined in benzene by competition of the abstraction reaction with the self-termination of benzyl radical. Thus, experimental values of k_abs/k_t^1/2 were combined with rate constants for self-termination of benzyl radical in benzene from the expression ln(2k_t/M^-1 s^-1 = 27.23 - 2952.4/RT), RT in cal/mol, to give absolute rate constants for abstraction, k_abs: for Os₃(μ-H)₂(CO)₉PPh₃ (1) in benzene, log(k_abs/M^-1 s^-1) = 9.40 ± 0.30) - (8.11 ± 0.47)/θ; for Os₃ (μ-H)(H)(CO)₁₀PPh₃ (2) (log(k_abs/M^-1 s^-1) = (8.08 ± 0.33) - (4.32 ± 0.1)/θ; for Os₃(μ-H)(CO)₉ (μ₃-η²-C₉H₆N) (3) log(k_abs/M^-1 s^-1) = (10.1 ± 2) - (10.5 ± 3)/θ; and for Os₃(μ-H)(CO₉)(μ-η²-C₉ H₆N) (PPh₃) (5) log(k_abs/M^-1 s^-1) = (7.0 ± 0.27) - (4.25 ± 0.41)/θ, θ = 2.303RT kcal/mol. The terminal hydride on the Os₃ cluster 2 is about 10 times more reactive than the bridging hydride in 1. The results show that while μ-H bridging retards the rate of hydrogen abstraction relative to terminal hydrogen, the bridging hydrogen remains appreciably reactive in the μ-H form. In fact, one of the fastest rates observed was for the bridging hydride in 4, Os₃(μ-H)(CO₁₀) (μ-η²-C₉H₆N). The 293 K rate constant for hydrogen atom abstraction from this electron-rich cluster, 5 ± 2 × 10⁴ M^-1 s^-1, is almost as fast as that for the terminal hydrogen atom cluster, 2, Os₃(μ-H)(H)(CO)₁₀PPh₃, k_abs(298 K) = 8.2 × 10⁴ M^-1 s^-1. The rate constant for hydrogen atom abstraction by benzyl radical from the Os₃ clusters appears to increase with electron-rich osmium clusters and decrease with increasing steric bulk of the ligands.