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 H2 between 56 and 120 °C. The Pt species Pt(PEt3)3, (PEt3)2Pt(2,2′-biphenyl), 1, trans-(PEt3)2PtH2, and trans-(PEt3)2Pt(R-biphenyl)H, 4, were all viable catalysts. The resting state species in each case was complex 4. At 80 °C under an atmosphereofH2,4reductivelyeliminatesbiphenylandformstrans-(PEt3)2PtH2. FreePEt3 inhibitstherateofreductiveeliminationfrom4andtheoverallrateofhydrogenolysis. The novel Pt(IV) dihydride trans,cis-(PEt3)2Pt(2,2′-biphenyl)H2 was synthesized and characterized by X-ray analysis. trans,cis-(PEt3)2Pt(2,2′-biphenyl)H2 undergoes unimolecular reductive eliminationtogive2. Onthebasisoftheseresultsacatalyticcycleisproposed. Amixture of [(dippe)PtH]2 and (dippe)PtH2 (dippe ) bis(diisopropylphosphino)ethane) was also capable of catalyzing the hydrogenolysis of biphenylene under an atmosphere of H2 at 120 °C. The rate of hydrogenolysis increases as the concentration of biphenylene, H2, and (dippe)PtH2 increases. These observations are consistent with the C-C bond activation of biphenylene occurring via (dippe)PtH2, not [(dippe)Pt0]. 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 °C). The resting state species was (dippe)Ni(2,2′-biphenyl).