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

Density functional theory calculations modelling selective exo-H/D exchange observed in the Rh σ-alkane complex [(Cy₂PCH₂CH₂PCy₂)Rh(η²:η²-endo-NBA)][BAr^F₄], [1-NBA][BAr^F₄], are reported, where Ar^F=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][BAr^F₄] 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 [BAr^F₄]⁻ 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.