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We describe the first comparative data on metal-mediated C-H activation and functionalization reactions at two-carbon-atom legacy refrigerants and DFT analyses on the resulting organometallic products. We reveal that, depending on the degree of fluorination in the refrigerant, C-H activation could lead either to stable M(H)Rf products or to a M(fluoroolefin) complex. The first example of a metal-mediated dehydrofluorination of R-143a is described, resulting from a β-fluoride elimination reaction of a putative [Ir(H)(CH2CF3)] intermediate and the loss of hydrogen fluoride. This work also reports the first examples of metal-mediated activation of R-125 (CF3CF2H) and R-134a (CF3CFH2) where the direct C-H activation products can be observed spectroscopically and, in the case of R-125, structurally characterized. The stability of the [Ir(H)(Rf)] complexes is notable, given that the direct products of C-H activation of non-chelating alkanes at transition metals remains relatively rare. The clean formation of isolable [Ir(H)(Rf)] complexes is expected to facilitate studies of migratory insertion reactions in these species, which bodes well for efforts to repurpose high-global-warming-potential legacy refrigerants. Finally, DFT calculations provide insight into how the degree of fluorination affects C=C bond lengths and energies of propellor-like rotations in the coordinated fluoroolefins, the relative free energies of fluoroolefin binding, and the role of observed CH···F contacts in the calculated structures.
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Mechanistic molecular motion of transition-metal mediated β-hydrogen transfer: quasiclassical trajectories reveal dynamically ballistic, dynamically unrelaxed, two step, and concerted mechanisms
The transfer of a β-hydrogen from a metal-alkyl group to ethylene is a fundamental organometallic transformation. Previously proposed mechanisms for this transformation involve either a two-step β-hydrogen elimination and migratory insertion sequence with a metal hydride intermediate or a one-step concerted pathway. Here, we report density functional theory (DFT) quasiclassical direct dynamics trajectories that reveal new dynamical mechanisms for the β-hydrogen transfer of [Cp*Rh III (Et)(ethylene)] + . Despite the DFT energy landscape showing a two-step mechanism with a Rh–H intermediate, quasiclassical trajectories commencing from the β-hydrogen elimination transition state revealed complete dynamical skipping of this intermediate. The skipping occurred either extremely fast (typically <100 femtoseconds (fs)) through a dynamically ballistic mechanism or slower through a dynamically unrelaxed mechanism. Consistent with trajectories begun at the transition state, all trajectories initiated at the Rh–H intermediate show continuation along the reaction coordinate. All of these trajectory outcomes are consistent with the Rh–H intermediate <1 kcal mol −1 stabilized relative to the β-hydrogen elimination and migratory insertion transition states. For Co, which on the energy landscape is a one-step concerted mechanism, trajectories showed extremely fast traversing of the transition-state zone (<50 fs), and this concerted mechanism is dynamically different than the Rh ballistic mechanism. In contrast to Rh, for Ir, in addition to dynamically ballistic and unrelaxed mechanisms, trajectories also stopped at the Ir–H intermediate. This is consistent with an Ir–H intermediate that is stabilized by ∼3 kcal mol −1 relative to the β-hydrogen elimination and migratory insertion transition states. Overall, comparison of Rh to Co and Ir provides understanding of the relationship between the energy surface shape and resulting dynamical mechanisms of an organometallic transformation.
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- Award ID(s):
- 1952420
- PAR ID:
- 10245288
- Date Published:
- Journal Name:
- Dalton Transactions
- Volume:
- 49
- Issue:
- 23
- ISSN:
- 1477-9226
- Page Range / eLocation ID:
- 7747 to 7757
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1039/D0DT01687J
