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

The reactivity of the S-H bond in Cp*Mo(mu-S)(2)(mu-SMe)(mu-SH)MoCp* (S4MeH) has been explored by determination of kinetics of hydrogen atom abstraction to form the radical CP*MO(mu-S)(3)(mu-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, Delta H-not equal and AS: were determined to be 1.54 +/- 0.25 kcal/mol and -25.5 +/- 0.8 cal/mol K, respectively, giving k(abs) = 1.3 x 10(6) M-1 s(-1) at 25 degrees 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(mu-S)(2)(mu-SMe)(mu-SBz)MoCp* (S(4)MeBz), 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(mu-S)(2) mu-SMe)MoCP*)(2)(mu-S-2) ((S4Me)(2)). The radical-dimer equilibrium constant has been determined to be 5.7 x 10(4) +/- 2.1 x 10(4) M-1 from EPR data. The rate constant for dissociation of the dimer was found to be 1.1 x 10(3) s(-1) at 25 degrees C, based on variable temperature H-1 NMR data. The rate constant for dimerization of the radical has been estimated to be 6.5 x 10(7) 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 (S4Me)(2), S(4)MeBz, and the cationic CP*Mo(mu-S-2)(mu-S)(mu-SMe)MoCp*(OTf) (S4Me+), a precursor of the radical and the alkylated derivatives. Evidence for a radical addition/elimination pathway at an Mo2S4 core is presented.