Activation of the sulfhydryl group by Mo centers: Kinetics of reaction of benzyl radical with a binuclear Mo(μ-SH)Mo complex and with arene and alkane thiols

This paper provides evidence from kinetic experiments and electronic structure calculations of a significantly reduced S-H bond strength in the Mo(μ-SH)Mo function in the homogeneous catalyst model, CpMo(μ-S) ₂(μ-SH)₂MoCp (1, Cp = η ⁵-cyclopentadienyl). The reactivity of 1 was explored by determination of a rate expression for hydrogen atom abstraction by benzyl radical from 1 (log(k_abs/M^-1 s^-1) = (9.07 ± 0.38) - (3.62 ± 0.58)/θ) for comparison with expressions for CH₃(CH₂)₇SH, log(k_abs/M ^-1 s^-1) = (7.88 ± 0.35) - (4.64 ± 0.54)/θ, and for 2-mercaptonaphthalene, log(k_abs/M ^-1 s^-1) = (8.21 ± 0.17) - (4.24 ± 0.26)/θ (θ = 2.303RT kcal/mol, 2σ 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(μ-S) ₃(μ-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.