Modulation of sigma-Alkane Interactions in [Rh(L-2)(alkane)(+) Solid-State Molecular Organometallic (SMOM) Systems by Variation of the Chelating Phosphine and Alkane: Access to eta(2), eta(2)-sigma-Alkane Rh(I), eta(1)-sigma-Alkane Rh(III) Complexes, and Alkane Encapsulation

Solid/gas single-crystal to single-crystal (SC-SC) hydrogenation of appropriate diene precursors forms the corresponding a-alkane complexes [Rh(Cy2P(CH2)(n)PCY2)(L)]-[BAr4F] (n = 3, 4) and <([RhH(Cy2P)(CH2)(2)(CH))over bar>-(CH2)(2)PCY2)(L)][BAr4F] (n = 5, L = norbornane, NBA; cyclooctane, COA). Their structures, as determined by single crystal X-ray diffraction, have cations exhibiting Rh center dot center dot center dot H-C sigma-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 eta(2),eta(2) Rh center dot center dot center dot H-C (e.g., [Rh(Cy2P(CH2)(n)PCY2)(eta(2)eta(2)-NBA)][BAr4F], n = 3 and 4), through to more weakly bound eta(1) Rh center dot center dot center dot H-C in which C-H activation of the chelate backbone has also occurred (e.g., <([RhH(Cy2P(CH2)(2)(CH))over bar>(CH2)(2)PCy2)(eta(1)-COA)]-[BAr4F]) 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)(3)PCy2)][COACBAr(4)(F)], in which the metal center instead forms two intramolecular agostic eta(1) Rh center dot center dot center dot H-C interactions with the phosphine cyclohexyl groups. CH2Cl2 adducts formed by displacement of the eta(1)-alkanes in solution (n = 5; L = NBA, COA), <([RhH(Cy2P(CH2)(2)(CH))over bar>(CH2)(2)PCY2)(K-1-ClCH2Cl)][BAr4F], 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 center dot center dot center dot alkane interactions along a C-H activation trajectory. These are negligible in [Rh(Cy2P(CH2)(3)PCy2)][COACBAr(4)(F)]; in <([RhH((CyP)-P-2(CH2)(2)(CH))over bar>(CH2)(2)PCy2)(eta(1)-COA)][BAr4F], sigma*(C-H) -> Rh sigma-donation is supported by Rh -> sigma*(C-H) pregostic donation, and in [Rh(Cy2P(CH2)(n)PCy2)(eta(2)eta(2)-NBA)][BAr4F] (n = 2-4), sigma-donation dominates, supported by classical Rh(d pi) -> sigma*(C-H) it-back-donation. Dispersive interactions with the [BAr4F]-anions and Cy substituents further stabilize the alkanes within the binding pocket.