Modulation of σ-Alkane Interactions in [Rh(L)(alkane)] Solid-State Molecular Organometallic (SMOM) Systems by Variation of the Chelating Phosphine and Alkane: Access to η,η-σ-Alkane Rh(I), η-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(CyP(CH) PCy)(L)][BAr] ( n = 3, 4) and [ RhH(CyP(CH)( CH)(CH)PCy)(L)][BAr] ( 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 η,η Rh···H-C (e.g., [Rh(CyP(CH) PCy)(ηη-NBA)][BAr], n = 3 and 4), through to more weakly bound η Rh···H-C in which C-H activation of the chelate backbone has also occurred (e.g., [ RhH(CyP(CH)( CH)(CH)PCy)(η-COA)][BAr]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the supporting anion microenvironment, [Rh(CyP(CH)PCy)][COA⊂BAr], in which the metal center instead forms two intramolecular agostic η Rh···H-C interactions with the phosphine cyclohexyl groups. CHCl adducts formed by displacement of the η-alkanes in solution ( n = 5; L = NBA, COA), [ RhH(CyP(CH)( CH)(CH)PCy)(κ-ClCHCl)][BAr], 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(CyP(CH)PCy)][COA⊂BAr]; in [ RhH(CyP(CH)( CH)(CH)PCy)(η-COA)][BAr], σ → Rh σ-donation is supported by Rh → σ* "pregostic" donation, and in [Rh(CyP(CH) PCy)(ηη-NBA)][BAr] ( n = 2-4), σ-donation dominates, supported by classical Rh(dπ) → σ* π-back-donation. Dispersive interactions with the [BAr] anions and Cy substituents further stabilize the alkanes within the binding pocket.