Catalytic hydrogenolysis of biphenylene with platinum, palladium, and nickel phosphine complexes

The catalytic hydrogenolysis of biphenylene was carried out using Pt, Pd, and Ni phosphine complexes under an atmosphere of H-2 between 56 and 120 degrees C. The Pt species Pt(PEt3)(3), (PEt3)(2)Pt(2,2'-biphenyl), 1, trans-(PEt3)(2)PtH2, and trans-(PEt3)(2)Pt(alpha-biphenyl)H, 4, were all viable catalysts. The resting state species in each case was complex 4. At 80 degrees C under an atmosphere of H-2, 4 reductively eliminates biphenyl and forms trans-(PEt3)(2)PtH3, Free PEt3 inhibits the rate of reductive elimination from 4 and the overall rate of hydrogenolysis. The novel Pt(IV) dihydride trans,cis-(PEt3)(2)Pt(2,2'-biphenyl)H-2 was synthesized and characterized by X-ray analysis. traits, cis-(PEt3)(2)Pt(2,2'-biphenyl)H-2 undergoes unimolecular reductive elimination to give 2. On the basis of these results a catalytic cycle is proposed. A mixture of [(dippe)PtH](2) and (dippe)PtH2 (dippe = bis(diisopropylphosphino)ethane) was also capable of catalyzing the hydrogenolysis of biphenylene under an atmosphere of H-2 at 120 degrees C. The rate of hydrogenolysis increases as the concentration of biphenylene, H-2, and (dippe)PtH2 increases. These observations are consistent with the C-C bond activation of biphenylene occurring via (dippe)PtH2, not [(dippe)Pt-0]. The rate of catalytic hydrogenolysis was not affected by the length of the chelating phosphine bridge. The Ni complex [(dippe)NiH](2) was the most efficient catalyst for the hydrogenolysis of biphenylene (16 turnovers/day at 56 degrees C. The resting state species was (dippe)Ni(2,2'-biphenyl).