Alkene Epoxidation Catalysts [Ru(pdc)(tpy)] and [Ru(pdc)(pybox)] Revisited: Revealing a Unique Ru^IVO Structure from a Dimethyl Sulfoxide Coordinating Complex

The X-ray crystal structure of a dimethyl sulfoxide (DMSO) coordinating complex [RuII(κ2-pdc)­(tpy)­(DMSO)] (H2pdc = 2,6-pyridyl dicarboxylic acid and tpy = 2,2′:6′,2″-terpyridine) led to the discovery of a unique RuIVO configuration for the Ru-pybox (pybox = pyridine-bis­(oxazoline) ligands) epoxidation catalyst by theoretical calculations. On the basis of this structure, a detailed theoretical study was conducted on the alkene epoxidation reaction using ruthenium-based epoxidation catalysts. It was found that the process of H2O2 coordination proceeded via an associative path in which one carboxylate detached. The following H2O-elimination step was found to be facilitated by the detached carboxylate group. The resulting RuIVO rearranges to the species trans-2a-oxo, in which one carboxylate group is situated over the tpy ring; the trans-2a-oxo was found to have the lowest activation free energies toward alkene epoxidation. These results demonstrated the importance of the hemilabile properties of the pdc2– ligand for the Ru-pdc alkene epoxidation catalysts.