Computational Insights into the Mechanisms of H2 Activation and H2/D2 Isotope Exchange by Dimolybdenum Tetrasulfide Complexes

The mechanisms for H-2 activation by [Cp*Mo](2)(mu-S)(2)(mu-S-2) (1-a, Cp* = pentamethylcyclopentadienyl) and its reaction product [Cp*Mo](2)(mu-S)(2)(mu-SH)(2) (2) have been investigated by DFT methods. The reaction of 1-a involves the homolytic addition of H-2 to its mu-S ligands, followed by the cleavage of the S-S bond of the mu-S-2 ligand in a subsequent step. Complex 2 can adopt five conformations that only differ in the stereochemistry of the mu-SH and mu-S ligands; although an isomer with adjacent mu-S ligands (2-a) is formed initially, it then isomerises into the experimentally observed 2-d. This species promotes H/D scrambling in H-2/D-2 mixtures, and the mechanism of the process has also been studied. Notably, all of the computed pathways for the addition of D-2 to 2-d present prohibitive barriers; instead, only those isomers with adjacent mu-S ligands are able to react further. The homolytic activation of D-2 by these leads to isomers of [Cp2Mo2(mu-SH)(2)(mu-SD)(2)], the interconversion of which is the rate-determining step.