Formation and reactivity of a persistent radical in a dinuclear molybdenum complex

Aaron M. Appel, Sun Jane Lee, James A. Franz, Daniel L. DuBois, M. Rakowski DuBois, Jerome C. Birnbaum, Brendan Twamley

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Abstract

The reactivity of the S-H bond in Cp*Mo(μ-S)2(μ-SMe) (μ-SH)MoCp* (S4MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical Cp*Mo(μ-S) 3(μ-SMe)MoCp* (S4Me•), as well as reaction of hydrogen with the radical-dimer equilibrium to reform the S-H complex. From the temperature dependent rate data for the abstraction of hydrogen atom by benzyl radical, ΔH and ΔS were determined to be 1.54 ± 0.25 kcal/mol and -25.5 ± 0.8 cal/mol K, respectively, giving kabs = 1.3 × 106 M-1 s-1 at 25°C. In steady state abstraction kinetic experiments, the exclusive radical termination product of the Mo2S4 core was found to be the benzyl cross-termination product, Cp*Mo(μ-S) 2(μ-SMe)(μ-SBz)MoCp* (S4MeBz), consistent with the Fischer-Ingold persistent radical effect. S4Me• was found to reversibly dimerize by formation of a weak bridging disulfide bond to form the tetranuclear complex (Cp*Mo(μ-S)2(μ-SMe)MoCp*) 2(μ-S2) ((S4Me)2). The radical-dimer equilibrium constant has been determined to be 5.7 × 10 4 ± 2.1 × 104 M-1 from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 × 103 s-1 at 25°C, based on variable temperature 1H NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 × 107 M-1 s-1 in toluene at room temperature, based on the dimer dissociation rate constant and the equilibrium constant for dimerization. Structures are presented for (S 4Me)2, S4MeBz, and the cationic Cp*Mo(μ-S2)(μ-S)(μ-SMe)MoCp*(OTf) (S 4Me+), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo 2S4 core is presented.

Original languageEnglish
Pages (from-to)8940-8951
Number of pages12
JournalJournal of the American Chemical Society
Volume130
Issue number28
DOIs
Publication statusPublished - Jul 16 2008

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ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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