Kinetic
and DFT Studies on the Mechanism of C–S
Bond Formation by Alkyne Addition to the [Mo<sub>3</sub>S<sub>4</sub>(H<sub>2</sub>O)<sub>9</sub>]<sup>4+</sup> Cluster
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Abstract
Reaction
of [Mo<sub>3</sub>(μ<sub>3</sub>-S)(μ-S)<sub>3</sub>]
clusters with alkynes usually leads to formation of two C–S
bonds between the alkyne and two of the bridging sulfides. The resulting
compounds contain a bridging alkenedithiolate ligand, and the metal
centers appear to play a passive role despite reactions at those sites
being well illustrated for this kind of cluster. A detailed study
including kinetic measurements and DFT calculations has been carried
out to understand the mechanism of reaction of the [Mo<sub>3</sub>(μ<sub>3</sub>-S)(μ-S)<sub>3</sub>(H<sub>2</sub>O)<sub>9</sub>]<sup>4+</sup> (<b>1</b>) cluster with two different
alkynes, 2-butyne-1,4-diol and acetylenedicarboxylic acid. Stopped-flow
experiments indicate that the reaction involves the appearance in
a single kinetic step of a band at 855 or 875 nm, depending on the
alkyne used, a position typical of clusters with two C–S bonds.
The effects of the concentrations of the reagents, the acidity, and
the reaction medium on the rate of reaction have been analyzed. DFT
and TD-DFT calculations provide information on the nature of the product
formed, its electronic spectrum and the energy profile for the reaction.
The structure of the transition state indicates that the alkyne approaches
the cluster in a lateral way and both C–S bonds are formed
simultaneously