Exclusive C-C oxidative addition in a rhodium thiophosphoryl pincer complex and computational evidence for an ν ³-C-C-H agostic intermediate

The room-temperature reaction between the Rh(I) precursor [Rh(COE) ₂(acetone) ₂]BF ₄ (COE = cyclooctene) and a new thiophosphoryl-based SCS pincer ligand leads to oxidative addition of an sp ²-sp ³ C-C bond as the only observed outcome, despite the presence of accessible sp ³ C-H bonds. A DFT study reveals that the chemistry of the SCS system is controlled by π repulsion between occupied rhodium d orbitals and the lone-pair electrons on the two sulfur atoms. This repulsion gives rise to the thermodynamic selectivity for C-C over C-H cleavage, as it is attributed to the higher electronegativity of a methyl versus hydride ligand, thereby allowing more effective release of excessive π electron density. It is also demonstrated that the observed C-C and unobserved C-H cleavage pathways originate from a common intermediate that features a novel ν ³-C-C-H agostic interaction. The COE ligand is shown to play an important role by greatly stabilizing this intermediate, making it the only available entry point to both reaction pathways.