Mechanistic impact of water addition to Sml2:: Consequences in the ground and transition state

The mechanistic impact of water addition to Sml(2) on the ground state and rate-limiting transition state structures in the reduction of benzyl bromide was determined using UV-vis spectroscopy, cyclic voltammetry, vapor pressure osmommetry, and stopped-flow spectrophotometric studies. The results obtained from these studies show that, upon addition of water, Sml(2) in THF (or DME) becomes partially water-solvated by displacing metal-coordinated solvent. Further addition of water displaces remaining bound solvent and induces a monomer-dimer equilibrium of the Sml(2)-water complex. Concomitant with this process, a thermodynamically more powerful reductant is created. Rate studies on the reduction of benzyl bromide by Sml(2)-water are consistent with reaction occurring through a dimeric transition state with the assembly of the activated complex requiring an equivalent of water at low concentrations but not at higher concentrations. The mechanistic complexity of the Sml(2)-water system shows that simple empirical models describing the role of water in Sml(2)-mediated reductions are likely to contain a high degree of uncertainty.