Modulation of σ-Alkane Interactions in [Rh(L₂)(alkane)]⁺ Solid-State Molecular Organometallic (SMOM) Systems by Variation of the Chelating Phosphine and Alkane: Access to η²,η²-σ-Alkane Rh(I), η¹-σ-Alkane Rh(III) Complexes, and Alkane Encapsulation

Solid/gas single-crystal to single-crystal (SC-SC) hydrogenation of appropriate diene precursors forms the corresponding σ-alkane complexes [Rh(Cy₂P(CH₂)_nPCy₂)(L)][BAr^F ₄] (n = 3, 4) and [RhH(Cy₂P(CH₂)₂(CH)(CH₂)₂PCy₂)(L)][BAr^F ₄] (n = 5, L = norbornane, NBA; cyclooctane, COA). Their structures, as determined by single-crystal X-ray diffraction, have cations exhibiting Rh···H-C σ-interactions which are modulated by both the chelating ligand and the identity of the alkane, while all sit in an octahedral anion microenvironment. These range from chelating η²,η² Rh···H-C (e.g., [Rh(Cy₂P(CH₂)_nPCy₂)(η²η²-NBA)][BAr^F ₄], n = 3 and 4), through to more weakly bound η¹ Rh···H-C in which C-H activation of the chelate backbone has also occurred (e.g., [RhH(Cy₂P(CH₂)₂(CH)(CH₂)₂PCy₂)(η¹-COA)][BAr^F ₄]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the supporting anion microenvironment, [Rh(Cy₂P(CH₂)₃PCy₂)][COÅBAr^F ₄], in which the metal center instead forms two intramolecular agostic η¹ Rh···H-C interactions with the phosphine cyclohexyl groups. CH₂Cl₂ adducts formed by displacement of the η¹-alkanes in solution (n = 5; L = NBA, COA), [RhH(Cy₂P(CH₂)₂(CH)(CH₂)₂PCy₂)(κ¹-ClCH₂Cl)][BAr^F ₄], are characterized crystallographically. Analyses via periodic DFT, QTAIM, NBO, and NCI calculations, alongside variable temperature solid-state NMR spectroscopy, provide snapshots marking the onset of Rh···alkane interactions along a C-H activation trajectory. These are negligible in [Rh(Cy₂P(CH₂)₃PCy₂)][COÅBAr^F ₄]; in [RhH(Cy₂P(CH₂)₂(CH)(CH₂)₂PCy₂)(η¹-COA)][BAr^F ₄], σ_C-H → Rh σ-donation is supported by Rh → σ∗_C-H "pregostic" donation, and in [Rh(Cy₂P(CH₂)_nPCy₂)(η²η²-NBA)][BAr^F ₄] (n = 2-4), σ-donation dominates, supported by classical Rh(dπ) → σ∗_C-H π-back-donation. Dispersive interactions with the [BAr^F ₄]^- anions and Cy substituents further stabilize the alkanes within the binding pocket.