Mechanistic Studies of Single-Step Styrene Production Catalyzed by Rh Complexes with Diimine Ligands: An Evaluation of the Role of Ligands and Induction Period

Studies of catalytic benzene alkenylation using different diimine ligated Rh­(I) acetate complexes and Cu­(OAc)2 as the oxidant revealed statistically identical results in terms of activity and product selectivity. Under ethylene pressure, two representative diimine ligated rhodium­(I) acetate complexes were demonstrated to exchange the diimine ligand with ethylene rapidly to form [Rh­(μ-OAc)­(η2-C2H4)2]2 and free diimine. Thus, it was concluded that diimine ligands are not likely coordinated to the active Rh catalysts under catalytic conditions. At 150 °C under catalytic conditions using commercial Cu­(OAc)2 as the oxidant, [Rh­(μ-OAc)­(η2-C2H4)2]2 undergoes rapid decomposition to form catalytically inactive and insoluble Rh species, followed by gradual dissolution of the insoluble Rh to form the soluble Rh, which is active for styrene production. Thus, the observed induction period under some conditions is likely due to the formation of insoluble Rh (rapid), followed by redissolution of the Rh (slow). The Rh decomposition process can be suppressed and the catalytically active Rh species maintained by using soluble Cu­(II) oxidants or Cu­(OAc)2 that has been preheated. In such cases, an induction period is not observed.