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 H 14 –Cp*Rh–Cl and D 14 -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.

Neutron spectroscopy as a method for classical force-field parameterization: Past methods, present successes and future challenges

“…Here we see a mix of intra-molecular and inter-molecular vibrations, methyl rotations, and phonons. This area of the spectrum elucidates much of the bonding properties of the system [29,30], but gives limited information on the delocalised motion of each atom. For these less localised motions, we must use the technique of Quasi-elastic Neutron Spectroscopy (QENS).…”

Kinetics and mechanism of sequential ring methyl C–H activation in cyclopentadienyl rhodium(iii) complexes

“…12) for proton transfer between Rh-OH hydroxide and Cp X methyl groups for complex 1, calculated for the models involving a methanol molecule compared to those obtained for the model assuming purely intramolecular (no methanol molecule involved). 15…”

“…This has yet to be achieved for previously reported molecular machines. 28,29 Our initial vibronic studies 15 have revealed that vibrations are crucial for the reaction pathways, particularly, the conversion of local vibrations into translational motion for progression along the reaction coordinate, and for weakening bonds in the transition state, so lowering the vibrational energy.…”

“…1 ) and its deuterated D 15 -Cp* analogue D 15 -1 . 15 The vibrations were fully assigned using Density Functional Theory (DFT) phonon calculations. 15 The vibrational analysis revealed that the Cp* ring behaves like a moving carousel, and brings each methyl proton close to the Rh–OH/D centre where proton abstraction occurs ( Scheme 1 ).…”

The Unlocking of High-Pressure Science with Broadband Neutron Spectroscopy at the ISIS Pulsed Neutron & Muon Source

“…In addition, the past decade has witnessed a very rapid increase in the quantity and quality of combined in-silico and INS studies, giving unprecedented clarity to a range of chemical phenomena at the atomic scale [4,5,6]. Some examples include the understanding of phase transitions [7,8,9], quantifying the role of vibrations to the kinetics in catalytic processes [10] and gas absorption in porous media [11].…”