Solid-State Synthesis and Characterization of σ-Alkane Complexes, [Rh(L2) (η2,η2-C7H12)][BAr4F] (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 sigma-alkane complexes: [Rh(R2P(CH2)(n)PR2)(eta(2),eta(2)-C7H12)][BAr4F]; R = Cy, n = 2; R = Pr-i, 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 center dot center dot center dot 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 sigma(CH) -> Rh donation is the major component. The other alkane complexes exhibit solid-state P-31 NMR data consistent with their formation, but they are now much less persistent at 298 K and ultimately give the corresponding zwitterions in which [BAr4F](-) coordinates and NBA is lost. The solid-state structures, as determined by X-ray crystallography, for all these [BAr4F](-) adducts are reported. DFT calculations suggest that the molecular zwitterions within these structures are all significantly more stable than their corresponding sigma-alkane cations, suggesting that the solid-state motif has a strong influence on their observed relative stabilities.