C−C Bond Activation of a Cyclopropyl Phosphine: Isolation and Reactivity of a Tetrameric Rhodacyclobutane

Reaction of the functionalized phosphine PtBu2CH2(C3H5) with [Rh(C8H14)2Cl]2 (C8H14 = cis-cyclooctene) resulted in selective C−C bond activation of the cyclopropyl group and formation of a tetrameric rhodacyclobutane, [RhCl(κ3-PtBu2CH2CH(CH2)2)]4, which was characterized in the solid state by X-ray crystallography. This complex acts as a latent source of the [Rh(κ3-PtBu2CH2CH(CH2)2)]+ fragment, forming a range of new complexes by salt metathesis with NaCp, [RhCp(κ3-PtBu2CH2CH(CH2)2)], chloride abstraction in the presence of arenes, [Rh(η6-arene)(κ3-PtBu2CH2CH(CH2)2)][BArF4] [arene = C6H5F, C6H3Me3; ArF = 3,5-C6H3(CF3)2], or fragmentation by addition of phosphine ligands, trans-[RhCl(PR3)(κ3-PtBu2CH2CH(CH2)2)] (R = iBu, Cy). In contrast, reaction with carbon monoxide results in the reductive elimination of the tethered cyclopropane, demonstrating reversible C−C bond activation, and formation of cis-[RhCl(CO)2(PtBuCH2(C3H5))]. These complexes were characterized in solution by NMR spectroscopy and in the solid state by X-ray diffraction. Chloride abstraction from [RhCl(PiBu3)(κ3-PtBu2CH2CH(CH2)2)] resulted in the regioselective isomerization of the tethered cyclopropane to a terminal alkene, viz., the formally 14-electron complex [Rh(PiBu3)(κ3-PtBu2CH2CH2CHCH2)][BArF4], notable for the presence of a strong agostic interaction with a phosphine iBu substituent, apparent both from the solid-state structure and in solution by 1H NMR spectroscopy. Addition of hydrogen to this complex resulted in hydrogenation of the alkene and formation of [Rh(H)2(PiBu3)(PtBu2nBu)][BArF4]. These steps overall correspond to selective C−C bond functionalization (hydrogenation) of the tethered cyclopropane and are of direct significance to a related catalytic process [J. Am. Chem. Soc. 2003, 125, 886] and, more generally, to the rhodium-mediated transformation of alkanes.