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 (PCy(2)biPli) ligand are reported. Abstraction of the halide with Na(BArF)(4) yields cationic Rh(:0 complexes [(NBD)Rh(PCy(2)biPh)][B(Ar-F)(4)] (2) and [(COD)Rh(PCy(2)biPh)][B(Ar-F)(4)] (7) (Ar-F = 3,5-bis(trifluoromethyl)phenyl). In complex 2, the pendent arene of the ligand is coordinated in an eta(2)-fashion to rhodium. Complex 7 exists in two configurations that were characterized by low-temperature NMR spectroscopy. One structure is analogous to 2 with eta(2)-coordination of the arene, and the other exhibits eta(6)-coordination. These structures interconvert on the NMR time scale at room temperature. Addition of H-2 to complex 2 yields the Rh (III) dihychide complex [(PCy(2)biPh)RhH2][B(Ar-F)(4)] (5), while the addition of H-2 to 7 generates the Rh(I) olefin complex [(COE)Rh(PCy(2)biPh)][B(Ar-F)(4)] (8). In both 5 and 8, the pendent arene of the ligand is bound eta(6) 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.