PNNL

Abstract

The reactivity of the S-H bond in Cp*Mo(? S)2(? SMe)(? SH)MoCp* has been explored by hydrogen atom abstraction to form the radical Cp*Mo(? S)3(? SMe)MoCp*, as well as hydrogen addition to the radical to reform the S-H complex. The rate expression for abstraction of hydrogen atom by benzyl radical has been determined, giving log(kabs/M 1 s 1) = (2.41 ? 0.37)/(2.303RT) + (7.88 ? 0.25) with kabs = 1.3 x 106 M 1 s 1 at 25?C. The radical termination product of the Mo2S4 core was found to be the benzyl cross-termination product, Cp*Mo(? S)2(? SMe)(? SBz)MoCp*. Structures are presented for this complex as well as the cationic Cp*Mo(? S2)(? S)(? SMe)MoCp*(OTf), a precursor of the radical and the alkylated derivatives. The radical undergoes reversible dimerization to form the tetranuclear complex (Cp*Mo(? S)2(? SMe)MoCp*)2(? S2), as shown by the solid state structure. In solution, the radical and the dimer are in equilibrium, and the equilibrium constant for dimerization has been estimated from magnetic susceptibility and EPR data. The rate of dissociation of the dimer has been determined to be 640 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 ?1x107 M-1 s-1, based on the dimer dissociation rate constant and the lower limit of the equilibrium constant for dimerization.