Contrasting Reactivity of Fluoropyridines at Palladium and Platinum:  C−F Oxidative Addition at Palladium, P−C and C−F Activation at Platinum

Abstract

The divergent behavior of palladium(0) and platinum(0) is revealed in the reactivity of [M(PR 3 ) 2 ] (M = Pd or Pt; R = Cy or i Pr) toward pentafluoropyridine and 2,3,5,6-tetrafluoropyridine . The palladium complexes react with pentafluoropyridine at 100 °C to yield the fluoride complexes trans -[Pd(F)(4-C 5 NF 4 )(PR 3 ) 2 ]. They do not react with 2,3,5,6-tetrafluoropyridine. The reaction of platinum(0) complexes [Pt(PR 3 ) 2 ] with pentafluoropyridine in THF at ambient temperature yields trans- [Pt(R)(4-C 5 NF 4 )(PR 3 )(PFR 2 )] complexes, whereas the reaction of [Pt(PCy 3 ) 2 ] with 2,3,5,6-tetrafluoropyridine results in C−H activation to form cis -[Pt(H)(4-C 5 NF 4 )(PCy 3 ) 2 ]; this complex may be converted to the trans isomer by photolysis. The c is -hydride also forms during the reaction of [Pt(PCy 3 ) 2 ] with C 5 NF 5 in hexane. These reactions also contrast with earlier studies of the reactivity of the same substrates toward {Ni(PEt 3 ) 2 }, which yield [Ni(F)(2-C 5 NF 5 )(PEt 3 ) 2 ] with pentafluoropyridine and [Ni(F)(2-C 5 NF 4 H)(PEt 3 ) 2 ] with tetrafluoropyridine. Thus palladium has different regioselectivity from nickel and is the least reactive. Platinum is capable of both C−F and C−H activation and is alone in the triad in undergoing rearrangement to the alkyl complex with the fluorophosphine ligand. Mechanisms for the rearrangement are proposed. The platinum dihydride complex trans -[Pt(H) 2 (PR 3 ) 2 ] reacts with pentafluoropyridine at room temperature, yielding a 1:1:1 mixture of trans -[PtH(FHF)(PR 3 ) 2 ], trans -[Pt(H)(4-C 5 NF 4 )(PR 3 ) 2 ], and trans- [Pt(R)(4-C 5 NF 4 )(PR 3 )(PFR 2 )]. Crystal structures are reported for trans -[Pd(F)(4-C 5 NF 4 )(PCy 3 ) 2 ]·H 2 O·C 6 H 6, trans -[Pd(F)(4-C 5 NF 4 )(P i Pr 3 ) 2 ], trans- [Pt(C 6 H 11 )(4-C 5 NF 4 )(PCy 3 )(PFCy 2 )]· CH 2 Cl 2, and cis -[Pt(H)(4-C 5 NF 4 )(PCy 3 ) 2 ].