Vibrational Motions Make Significant Contributions to Sequential Methyl C-H Activations in an Organometallic Complex

Abstract

[(Pentamethylcyclopentadienyl)Rh(III)(bipyridine)(chloride)]+ (Cp*Rh-Cl) undergoes sequential deuteriation of its 15 Cp∗ CH groups in polar deuterated solvents. Vibrational spectra of H14-Cp*Rh-Cl and D14-Cp*Rh-Cl were captured via inelastic neutron spectroscopy (INS) and assigned using density functional theory (DFT) phonon calculations. These calculations were precisely weighted to the spectrometer's neutronic response. The Cp∗ ring behaves as a moving carousel, bringing each CH3 close to the Rh-OH/D center where proton abstraction occurs. Vibrations relevant for carousel movement and proximal positioning for H transfer were identified. DFT modeling uncovered changes in vibrations along the reaction path, involving a Rh(I)-fulvene intermediate. Vibronic energy contributions are large across the entire transition. Remarkably, they amount to over a 400-fold increase in the proton transfer rate. The inclusion of vibrational degrees of freedom could be applied more widely to catalysts and molecular machines to harness the energetics of these vibrations and increase their effective rates of operation.