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)]­[BArF4] (n = 3, 4) and [RhH­(Cy2P­(CH2)2(CH)­(CH2)2PCy2)­(L)]­[BArF4] (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)]­[BArF4], 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)]­[BArF4]) 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)]­[COA⊂BArF4], 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)]­[BArF4], 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)]­[COA⊂BArF4]; in [RhH­(Cy2P­(CH2)2(CH)­(CH2)2­PCy2)­(η1-COA)]­[BArF4], σC–H → Rh σ-donation is supported by Rh → σ*C–H “pregostic” donation, and in [Rh­(Cy2P­(CH2)nPCy2)­(η2η2-NBA)]­[BArF4] (n = 2–4), σ-donation dominates, supported by classical Rh­(dπ) → σ*C–H π-back-donation. Dispersive interactions with the [BArF4]− anions and Cy substituents further stabilize the alkanes within the binding pocket.