| Abstrakt: |
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 kabs/kt1/2 were combined with rate constants for self-termination of benzyl radical in benzene from the expression ln(2kt/M-1 s-1 = 27.23 − 2952.4/RT), RT in cal/mol, to give absolute rate constants for abstraction, kabs: for Os3(μ-H)2(CO)9PPh3 (1) in benzene, log(kabs/M-1 s-1) = (9.40 ± 0.30) − (8.11 ± 0.47)/θ; for Os3(μ-H)(H)(CO)10PPh3 (2) log(kabs/M-1 s-1) = (8.08 ± 0.33) − (4.32 ± 0.1)/θ; for Os3(μ-H)(CO)9(μ3-η2-C9H6N) (3) log(kabs/M-1 s-1) = (10.1 ± 2) − (10.5 ± 3)/θ; and for Os3(μ-H)(CO9)(μ-η2-C9H6N)(PPh3) (5) log(kabs/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, kabs(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. |
