Solid-state synthesis and characterization of σ-alkane complexes, [Rh(L₂)(η²,η²-C₇H₁₂)][BAr^F₄] (L₂ = bidentate chelating phosphine)

The use of solid/gas and single-crystal to single-crystal synthetic routes is reported for the synthesis and characterization of a number of σ-alkane complexes: [Rh(R₂P(CH₂)_nPR₂)(η²,η²-C₇H₁₂)][BAr^F₄]; R = Cy, n = 2; R = ⁱPr, n = 2,3; Ar = 3,5-C₆H₃(CF₃)₂. These norbornane adducts are formed by simple hydrogenation of the corresponding norbornadiene precursor in the solid state. For R = Cy (n = 2), the resulting complex is remarkably stable (months at 298 K), allowing for full characterization using single-crystal X-ray diffraction. The solid-state structure shows no disorder, and the structural metrics can be accurately determined, while the ¹H chemical shifts of the Rh⋯H-C motif can be determined using solid-state NMR spectroscopy. DFT calculations show that the bonding between the metal fragment and the alkane can be best characterized as a three-center, two-electron interaction, of which σ_CH → Rh donation is the major component. The other alkane complexes exhibit solid-state ³¹P NMR data consistent with their formation, but they are now much less persistent at 298 K and ultimately give the corresponding zwitterions in which [BAr^F₄]^- coordinates and NBA is lost. The solid-state structures, as determined by X-ray crystallography, for all these [BAr^F₄]^- adducts are reported. DFT calculations suggest that the molecular zwitterions within these structures are all significantly more stable than their corresponding σ-alkane cations, suggesting that the solid-state motif has a strong influence on their observed relative stabilities.