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

摘要

The divergent behavior of palladium(O) and platinum(O) is revealed in the reactivity of [M(PR_3)_2] (M = Pd or Pt;R = Cy or 'Pr) toward pentafluoropyridine and 2,3,5,6-tetrafluoropyridine.The palladium complexes react with pentafluoropyridine at 100 degC to yield the fluoride complexes trans-[Pd(F)(4-C_5NF_4)(PR_3)_2].They do not react with 2,3,5,6-tetrafluoropyridine.The reaction of platinum(O) complexes [Pt(PR_3)_2] with pentafluoropyridine in THF at ambient temperature yields trans-[Pt(R)(4-C_5NF_4)(PR_3)(PFR_2)] complexes,whereas the reaction of [Pt(PCys)_2] with 2,3,5,6-tetrafluoropyridine results in C-H activation to form cis-[Pt(H)(4-C_5NF_4)(PCy_3)_2];this complex may be converted to the trans isomer by photolysis.The cis-hydride also forms during the reaction of [Pt(PCy_3)_2] with C_5NF_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_5NF5)(PEt_3)_2] with pentafluoropyridine and [Ni(F)-(2-C_5NF_4H)(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_5NF_4)(PR_3)_2],and trans-[Pt(R)(4-C_5NF_4)(PR_3)(PFR_2)].Crystal structures are reported for trans-[Pd(F)(4-C_5NF_4)-(PCy_3)_2]-H_20-C_6H6^rans-[Pd(F)(4-C_5NF_4)(PPR_3)_2],fraAis-[Pt(C_6Hii)(4-C_5NF_4)(PCy_3)(PFCy2)]-CH_2Cl_2,and cis-[Pt(H)(4-C_5NF_4)(PCy_3)_2].