Revisiting C–C and C–H Bond Activation in Rhodium Pincer Complexes: Thermodynamics and Kinetics Involving a Common Agostic Intermediate

The experimental investigation of C–H and C–C bond activation in transition-metal complexes of pincer ligands, wherein such reactions occur intramolecularly, has provided fundamental mechanistic insights into these important transformations. This has gone hand-in-hand with computational studies, which enabled the elucidation of reaction intermediates that were experimentally unobserved. Earlier studies of this kind have taught us that C–H and C–C activation in pincer systems is preceded by weak interactions with the metal center, namely, η2-C–H agostic bonding leads to C–H cleavage, whereas bonding with the arene moiety, in a κ1 or η2 manner, results in C–C cleavage. In the present work, we revisit previously studied C–H and C–C activation reactions involving the Rh­(I) precursor [Rh­(olefin)2(solvent)2]­BF4 and PCP-, PCN-, PCO-, POCOP-, and SCS-type pincer ligands. By employing a basis set that is larger than formerly used, and accounting for dispersion interactions, we show that each of these pincer systems features a single olefin-stabilized η3-C–C–H agostic intermediate as a common gateway to both C–C and C–H cleavage. Our computed reaction profiles correlate well with the experimentally observed behavior of the examined pincer systems and allow us to assess the individual roles of the pincer and ancillary ligands, as well as solvent polarity.