Synthesis, Characterization, and Reactivity of Arene-Stabilized Rhodium Complexes

The synthesis and characterization of (COD)Rh(I) and (NBD)Rh(I) (COD = cyclooctadiene; NBD = norbornadiene) chloride complexes containing the 2-(dicyclohexylphosphino)biphenyl (PCy2biPh) ligand are reported. Abstraction of the halide with Na(BArF)4 yields cationic Rh(I) complexes [(NBD)Rh(PCy2biPh)][B(ArF)4] (2) and [(COD)Rh(PCy2biPh)][B(ArF)4] (7) (ArF = 3,5-bis(trifluoromethyl)phenyl). In complex 2, the pendent arene of the ligand is coordinated in an η2-fashion to rhodium. Complex 7 exists in two configurations that were characterized by low-temperature NMR spectroscopy. One structure is analogous to 2 with η2-coordination of the arene, and the other exhibits η6-coordination. These structures interconvert on the NMR time scale at room temperature. Addition of H2 to complex 2 yields the Rh(III) dihydride complex [(PCy2biPh)RhH2][B(ArF)4] (5), while the addition of H2 to 7 generates the Rh(I) olefin complex [(COE)Rh(PCy2biPh)][B(ArF)4] (8). In both 5 and 8, the pendent arene of the ligand is bound η6 to Rh. Benzene hydrogenation to cyclohexane using 2 as a catalyst precursor is described. Poisoning experiments indicate that heterogeneous rhodium is likely to be the active catalyst in this arene hydrogenation reaction.