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.
Highlights
ABSTRACT: [(Pentamethylcyclopentadienyl)Rh(III)(bipyridine)(chloride)]+ (Cp*Rh−Cl) undergoes sequential deuteriation of its 15 Cp* CH groups in polar deuterated solvents
This is possible in some organometallic complexes through the polarization effects of the metal.[1−3] An example is the methyl group on the pentamethylcyclopentadienyl (Cp*) ring in the Rh(III) coordination complex [(η5Cp*)Rh(bipyridine)(chloride)]+ (Cp*Rh−Cl), a half-sandwich pseudo-octahedral structure with three of the coordination sites occupied by a π-bonded Cp* and the other three sites by an N,N-chelated bipyridyl ligand and a monodentate chlorido ligand, Figure 1.4
Electronic effects explain some of the features of the reaction pathway for these deuteriation reactions, we sought to determine whether vibrational dynamics of individual atoms play a significant role in tuning the mechanism for this methyl activation and deuteration
Summary
ABSTRACT: [(Pentamethylcyclopentadienyl)Rh(III)(bipyridine)(chloride)]+ (Cp*Rh−Cl) undergoes sequential deuteriation of its 15 Cp* CH groups in polar deuterated solvents. We substantiated the involvement of the fulvene intermediate by trapping it in Diels−Alder cycloaddition reactions with conjugated dienes such as isoprene and linoleic acid.[4] Additional interest in this class of compounds arises from their anticancer activity and ability to act as transfer hydrogenation catalysts.[5,6] electronic effects explain some of the features of the reaction pathway for these deuteriation reactions, we sought to determine whether vibrational dynamics of individual atoms play a significant role in tuning the mechanism for this methyl activation and deuteration The atoms in this organometallic complex are involved as groups of two or more atoms in a range of dynamic motions, usually characterized by vibrational spectroscopy as stretching, bending, and deformation modes, such as wagging, twisting, scissoring, and rocking.[7] These vibrations are crucial for reaction pathways,[8] the conversion of local vibrations into a translational motion along the reaction path. Received: November 2, 2020 Accepted: December 15, 2020 Published: January 4, 2021
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