Solid‐State Confinement Effects in Selective exo‐H/D Exchange in the Rhodium σ‐Norbornane Complex [(Cy₂PCH₂CH₂PCy₂)Rh(η²:η²‐C₇H₁₂)][BAr^F₄]

Abstract

Density functional theory calculations modelling selective exo‐H/D exchange observed in the Rh σ‐alkane complex [(Cy2PCH2CH2PCy2)Rh(η2:η2‐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: ΔG≠exo=+15.9 kcal/mol, ΔG≠endo=+18.4 kcal/mol; solid state: ΔG≠exo=+22.1 kcal/mol, ΔG≠endo=+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 (ΔG≠exo= +24.8 kcal/mol, ΔG≠endo=+27.5 kcal/mol) but retain a distinct preference for exo‐ over endo‐H/D exchange.

Solid/Gas In Crystallo Reactivity of an Ir(I) Methylidene Complex

“…A similar reaction profile was computed for the isolated molecular cation, albeit with variations of ≲3 kcal/mol in individual stationary points that indicate some impact of the solid-state environment on reactivity . Experimentally, addition of NH 3 (1.2 bara) to [1][BAr F 4 ] in a C 6 H 5 F solution at 20 °C resulted in the rapid formation of [5][BAr F 4 ] , which was then converted into [4][BAr F 4 ] .…”