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.