Synthesis, Characterization, and Theoretical Investigation of a Transition State Analogue for Proton Transfer during C–H Activation by a Rhodium-Pincer Complex

A heterobimetallic
rhodium-pincer complex bearing a phenylzinc
ligand was synthesized and characterized by multinuclear NMR, COSY,
NOESY, and X-ray crystallography. The crystal structure of this complex
shows that it possesses a bridging Rh–Zn–C fragment
with a geometry similar to the Rh–H–C fragment in a
proposed transition state for metal to ligand proton transfer during
redox-neutral C–H activation with dearomatized rhodium pincer
complexes. Bonding analysis indicates that these fragments are isolobal,
suggesting that the transition state analogue models not only the
structure but also the bonding interactions that underlie metal–ligand
cooperativity in the C–H activation transition state. The similarity
of the transition state and its analogue prompted re-evaluation of
the relevant rate equations to determine the relative contributions
of viable proton transfer pathways. Parallel analysis of the transition
state and its isolobal analogue thus serves as a bridge between theory
and experiment that is rarely available in studies of bonding in transition
states.