Web Release Date: May 8,
Activation of the Sulfhydryl Group by Mo Centers: Kinetics of
Reaction of Benzyl Radical with a Binuclear Mo(
-SH)Mo
Complex and with Arene and Alkane Thiols
Contribution from The Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352
Received February 6, 2004
Abstract:
This paper provides evidence from kinetic experiments and electronic structure calculations of
a significantly reduced S-H bond strength in the Mo(-SH)Mo function in the homogeneous catalyst model,
CpMo(-S)2(-SH)2MoCp (1, Cp = 
5-cyclopentadienyl). The reactivity of 1 was explored by determination
of a rate expression for hydrogen atom abstraction by benzyl radical from 1 (log(kabs/M-1 s-1) = (9.07 ±
0.38) - (3.62 ± 0.58)/
) for comparison with expressions for CH3(CH2)7SH, log(kabs/M-1 s-1) = (7.88 ±
0.35) - (4.64 ± 0.54)/, and for 2-mercaptonaphthalene, log(kabs/M-1 s-1) = (8.21 ± 0.17) - (4.24 ±
0.26)/ ( = 2.303RT kcal/mol, 2
error). The rate constant for hydrogen atom abstraction at 298 K by
benzyl radical from 1 is 2 orders of magnitude greater than that from 1-octanethiol, resulting from the
predicted (DFT) S-H bond strength of 1 of 73 kcal/mol. The radical CpMo(-S)3(-SH)MoCp, 2, is revealed,
from the properties of slow self-reaction, and exclusive cross-combination with reactive benzyl radical, to
be a persistent free radical.
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