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 °C. The Pt species Pt(PEt 3 ) 3, (PEt 3 ) 2 Pt(2,2‘-biphenyl), 1, trans -(PEt 3 ) 2 PtH 2, and trans -(PEt 3 ) 2 Pt(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 H 2, 4 reductively eliminates biphenyl and forms trans -(PEt 3 ) 2 PtH 2 . Free PEt 3 inhibits the rate of reductive elimination from 4 and the overall rate of hydrogenolysis. The novel Pt(IV) dihydride trans,cis -(PEt 3 ) 2 Pt(2,2‘-biphenyl)H 2 was synthesized and characterized by X-ray analysis. trans,cis -(PEt 3 ) 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)PtH 2 (dippe = bis(diisopropylphosphino)ethane) was also capable of catalyzing the hydrogenolysis of biphenylene under an atmosphere of H 2 at 120 °C. The rate of hydrogenolysis increases as the concentration of biphenylene, H 2, and (dippe)PtH 2 increases. These observations are consistent with the C−C bond activation of biphenylene occurring via (dippe)PtH 2, 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 °C). The resting state species was (dippe)Ni(2,2‘-biphenyl).