Carbon–Hydrogen Bond Activation in Bis(2,6-dimethylbenzenethiolato)­tris(trimethylphosphine)ruthenium(II): Ligand Dances and Solvent Transformations

2015-07-13T00:00:00Z (GMT) by Amanda L. Pitts Michael B. Hall
Density functional theory (DFT) calculations are used to predict the mechanism for the intramolecular carbon–hydrogen bond activation of an ortho methyl group on the Ru<sup>II</sup>(SC<sub>6</sub>H<sub>3</sub>Me<sub>2</sub>-2,6-κ<sup>1</sup>S)<sub>2</sub>(PMe<sub>3</sub>)<sub>3</sub> complex to form the cycloruthenated product <i>cis</i>-Ru­[SC<sub>6</sub>H<sub>3</sub>-(2-CH<sub>2</sub>)­(6-Me)-κ<sup>2</sup>S<sub>2</sub>C]­(PMe<sub>3</sub>)<sub>4</sub> and HSC<sub>6</sub>H<sub>3</sub>Me<sub>2</sub>-2,6 in the presence of PMe<sub>3</sub>. The DFT calculations also show how changing the solvent from benzene to methanol prevents C–H activation and results in the unactivated six-coordinate product Ru­(SC<sub>6</sub>H<sub>3</sub>Me<sub>2</sub>-2,6-κ<sup>1</sup>S)<sub>2</sub>(PMe<sub>3</sub>)<sub>4</sub> in 100% yield. The reactant was determined to have two plausible σ-bond metathesis pathways in which to react, one for each of the two thiolate ligands. The steps in both mechanisms were influenced by the electronic interactions between the sulfur lone pairs and the Ru 4d orbitals and the steric repulsion between the methyl groups on the five ligands in such a way that the methyl group in the SAr (Ar = SC<sub>6</sub>H<sub>3</sub>Me<sub>2</sub>-2,6) ligand closest to the Ru pirouettes away to activate the other methyl group. The equatorial pathway was calculated to be the lower energy mechanism and, therefore, the dominant pathway for the overall reaction. The difference between reaction mediums was predicted, by both implicit and explicit solvation modeling, to be a result of the polarity and binding of methanol, which transforms the geometry of the reactant from a less polar distorted trigonal-bipyramidal geometry to a more polar distorted square-pyramidal geometry. This change in geometry favors the more rapid addition of a fourth PMe<sub>3</sub> ligand to the more open coordination site, which prevents the C–H activation.