Solid-State Confinement Effects in Selective exo-H/D Exchange in the Rhodium σ-Norbornane Complex [(Cy2PCH2CH2PCy2)Rh(η22-C7H12)][BArF4]

Tobias Krämer*, F. Mark Chadwick, Stuart A. Macgregor, Andrew S. Weller

*Corresponding author for this work

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Abstract

Density functional theory calculations modelling selective exo-H/D exchange observed in the Rh σ-alkane complex [(Cy2PCH2CH2PCy2)Rh(η22-endo-NBA)][BArF4], [1-NBA][BArF4], are reported, where ArF=3,5-C6H3(CF3)2 and NBA=norbornane, C7H12. Two models were considered 1) an isolated molecular cation, [1-NBA]+ and 2) a full model in which [1-NBA][BArF4] is treated in the solid state through periodic DFT. After an initial endo-exo rearrangement, both models predict H/D exchange to proceed through D2 addition and oxidative cleavage followed by a rate-limiting C−H activation of the norbornane through a σ-CAM step to form a [1-Rh(D)(η2-HD)(norbornyl)]+ intermediate. HD rotation followed by a σ-CAM C−D bond formation, HD reductive coupling and HD loss then complete the H/D exchange process. exo-H/D exchange is facilitated by a supporting agostic interaction and is consistently more accessible kinetically than the potentially competing endo-H/D exchange (isolated cation: ΔGexo=+15.9 kcal/mol, ΔGendo=+18.4 kcal/mol; solid state: ΔGexo=+22.1 kcal/mol, ΔGendo=+25.1 kcal/mol). The solid-state environment has a significant impact on the computed energetics, with barriers increasing by ca. 7 kcal/mol, while only the solid-state model correctly predicts the endo-bound NBA complex to be the resting state of the system. These outcomes reflect solid-state confinement effects within the pocket occupied by the [1-NBA]+ cation and defined by the pseudo-octahedral array of neighbouring [BArF4] anions. The asymmetry of the solid-state environment also requires a second H/D exchange pathway to be defined to account for reaction at all four exo-C−H bonds. These entail slightly higher barriers (ΔGexo=. +24.8 kcal/mol, ΔGendo=+27.5 kcal/mol) but retain a distinct preference for exo- over endo-H/D exchange.

Original languageEnglish
Article numbere202200154
JournalHelvetica Chimica Acta
Volume106
Issue number2
Early online date31 Jan 2023
DOIs
Publication statusPublished - Feb 2023

Keywords

  • confinement
  • C−H activation
  • density functional theory calculations
  • H/D exchange
  • mechanism
  • periodic-DFT
  • rhodium
  • σ-alkane complexes

ASJC Scopus subject areas

  • Catalysis
  • Biochemistry
  • Drug Discovery
  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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