Mechanistic Investigation of Catalytic Carbon−Carbon Bond Activation and Formation by Platinum and Palladium Phosphine Complexes

The complexes Pt(PEt 3 ) 3 and Pd(PEt 3 ) 3 cleave the C−C bond of biphenylene to give (PEt 3 ) 2 Pt(2,2‘-biphenyl), 1, and (PEt 3 ) 2 Pd(2,2‘-biphenyl), respectively. Heating (PEt 3 ) 2 Pt(2,2‘-biphenyl) in the presence of biphenylene leads to C−C cleavage of a second biphenylene to give (PEt 3 ) 2 Pt(2,2‘-tetraphenyl), 2, via a Pt(IV) intermediate. 2 reductively eliminates tetraphenylene at 115 °C. At 120 °C the reaction is catalytic; Pt(PEt 3 ) 3 or 1 converts biphenylene to tetraphenylene. The intermediates in the catalytic cycle have been identified, and 1 and 2 have been characterized by X-ray analysis. Under catalytic conditions 1 and 2 approach steady-state concentrations. Kinetic analysis reveals that the steady-state concentration ratio, resting state species, and overall rate of catalysis, k obs, depend on the ratio of biphenylene to PEt 3 . This observation is consistent with loss of PEt 3 from 1, resulting in the 14-electron species (PEt 3 )Pt(2,2‘-biphenyl), I . At 130 °C, I coordinates to PEt 3 approximately 130 times faster than it activates the C−C bond of biphenylene. The complex (depe)Pt(2,2‘-biphenyl), 7 (depe = bis(diethylphosphino)ethane), does not cleave the C−C bond of biphenylene. Compound 2 is also capable of activating the C−H bonds of benzene and biphenylene to give trans -(PEt 3 ) 2 Pt(α-biphenyl)(phenyl), 5, and trans -(PEt 3 ) 2 Pt(α-biphenyl)(α-biphenylenyl), 6, respectively. Compounds 5 and 6 have been characterized by X-ray analysis. Substitution of Pd for Pt results in more rapid catalysis; (PEt 3 ) 2 Pd(2,2‘-biphenyl) is a very efficient catalyst (20 turnovers/h at 120 °C).