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.