Mechanistic Studies of Ethylene Hydrophenylation Catalyzed by Bipyridyl Pt(II) Complexes

Cationic platinum(II) complexes [(tbpy)Pt(Ph)(L)]+ [tbpy =4,4′-di-tert-butyl-2,2′-bipyridyl; L = THF, NC5F5, or NCMe] catalyze the hydrophenylation of ethylene to generate ethylbenzene and isomers of diethylbenzene. Using ethylene as the limiting reagent, an 89% yield of alkyl arene products is achieved after 4 h at 120 °C. Catalyst efficiency for ethylene hydrophenylation is diminished only slightly under aerobic conditions. Mechanistic studies support a reaction pathway that involves ethylene coordination to Pt(II), insertion of ethylene into the Pt–phenyl bond, and subsequent metal-mediated benzene C–H activation. Studies of stoichiometric benzene (C6H6 or C6D6) C–H/C–D activation by [(tbpy)Pt(Ph-dn)(THF)]+ (n = 0 or 5) indicate a kH/kD = 1.4(1), while comparative rates of ethylene hydrophenylation using C6H6 and C6D6 reveal kH/kD = 1.8(4) for the overall catalytic reaction. DFT calculations suggest that the transition state for benzene C–H activation is the highest energy species along the catalytic cycle. In CD2Cl2, [(tbpy)Pt(Ph)(THF)][BAr′4] [Ar′ = 3,5-bis(trifluoromethyl)phenyl] reacts with ethylene to generate [(tbpy)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] with kobs = 1.05(4) × 10–3 s–1 (23 °C, [C2H4] = 0.10(1) M). In the catalytic hydrophenylation of ethylene, substantial amounts of diethylbenzenes are produced, and experimental studies suggest that the selectivity for the monoalkylated arene is diminished due to a second aromatic C–H activation competing with ethylbenzene dissociation.