Mechanistic investigation of catalytic carbon-carbon bond activation and formation by platinum and palladium phosphine complexes

The complexes Pt(PEt3)(3) and Pd(PEt3)(3) cleave the C-C bond of biphenylene to give (PEt3)(2)Pt(2,2'-biphenyl), 1, and (PEt3)(2)Pd(2,2'-biphenyl), respectively. Heating (PEt3)(2)Pt(2,2'-biphenyl) in the presence of biphenylene leads to C-C cleavage of a second biphenylene to give (PEt3)(2)Pt(2,2'-tetraphenyl), 2, via a Pt(IV) intermediate. 2 reductively eliminates tetraphenylene at 115 degrees C. At 120 degrees C the reaction is catalytic; Pt(PEt3)(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 PEt3. This observation is consistent with loss of PEt3 from 1, resulting in the 14-electron species (PEt3)Pt(2,2'-biphenyl), I. At 130 degrees C, I coordinates to PEt3 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-(PEt3)(2)Pt(alpha-biphenyl)(phenyl), 5, and trans-(PEt3)(2)Pt(alpha-biphenyl)(alpha-biphenyl 6, respectively. Compounds 5 and 6 have been characterized by X-ray analysis. Substitution of Pd for Pt results in more rapid catalysis; (PEt3)(2)Pd(2,2'-biphenyl) is a very efficient catalyst (20 turnovers/h at 120 degrees C).