Catalytic Hydrogenolysis of Biphenylene with Platinum, Palladium, and Nickel Phosphine Complexes

Abstract

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(α-biphenyl)H, 4, were all viable catalysts. The resting state species in each case was complex 4. At 80 °C under an atmosphere of H2, 4 reductively eliminates biphenyl and forms trans-(PEt3)2PtH2. Free PEt3 inhibits the rate of reductive elimination from 4 and the overall rate of hydrogenolysis. 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 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 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).