Mechanistic Studies of Styrene Production from Benzene and Ethylene Using [(η²-C₂H₄)₂Rh(μ-OAc)]₂as Catalyst Precursor: Identification of a Bis-Rh^IMono-Cu^IIComplex As the Catalyst

We report a combined experimental and computational study focused on the mechanism of oxidative conversion of benzene and ethylene to styrene using [(η2-C2H4)2Rh(μ-OAc)]2 as the catalyst precursor in the presence of Cu(OPiv)2 (OPiv = pivalate). Using [(η2-C2H4)2Rh(μ-OAc)]2 as the catalyst precursor, ∼411 turnovers of styrene are observed after 1 h, giving an apparent turnover frequency of ∼0.11 s-1 (calculated assuming the binuclear structure is maintained in the active catalyst). We identify the catalyst resting state to be [(η2-C2H4)2RhI(μ-OPiv)2]2(μ-Cu), which is a heterotrinuclear molecular complex in which a central CuII atom bridges two Rh moieties. At high Rh concentration in the presence of Cu(OPiv)2 and pivalic acid (HOPiv), the trinuclear complex [(η2-C2H4)2RhI(μ-OPiv)2]2(μ-Cu) converts to the binuclear Rh(II) complex [(HOPiv)RhII(μ-OPiv)2]2, which has been identified by 1H NMR spectroscopy and single crystal X-ray diffraction. The binuclear Rh(II) [(HOPiv)RhII(μ-OPiv)2]2 is not a catalyst for styrene production, but under catalytic conditions [(HOPiv)RhII(μ-OPiv)2]2 can be partially converted to the active catalyst, the Rh-Cu-Rh complex [(η2-C2H4)2RhI(μ-OPiv)2]2(μ-Cu), following an induction period of ∼6 h. Using quantum chemical calculations, we sampled possible mononuclear and binuclear Rh species, finding that the binuclear Rh(II) [(HOPiv)RhII(μ-OPiv)2]2 paddle-wheel is a low energy global minimum, which is consistent with experimental observations that [(HOPiv)RhII(μ-OPiv)2]2 is not a catalyst for styrene formation. Further, we investigated the mechanism of styrene production starting from [(η2-C2H4)2RhI(μ-OAc)2]2(μ-Cu), [(η2-C2H4)2Rh(μ-OAc)]2, and (η2-C2H4)2Rh(κ2-OAc). For all reaction pathways studied, the predicted activation barriers for styrene formation from [(η2-C2H4)2Rh(μ-OAc)]2 and (η2-C2H4)2Rh(κ2-OAc) are too high compared to experimental kinetics. In contrast, the overall activation barrier for styrene formation predicted by DFT from the Rh-Cu-Rh complex [(η2-C2H4)2RhI(μ-OPiv)2]2(μ-Cu) is in agreement with experimentally determined rates of catalysis. Based on these results, we conclude that incorporation of Cu(II) into the active Rh-Cu-Rh catalyst reduces the activation barrier for benzene C-H activation, O-H reductive elimination, and ethylene insertion into the Rh-Ph bond.