Electrophilic Platinum Complexes: Methyl Transfer Reactions and Catalytic Reductive Elimination of Ethane from a Tetramethylplatinum(IV) Complex

Abstract

Methyl ligand transfer reactions occur easily between tetramethylplatinum(IV) complexes [PtMe4(LL)] [LL = 4,4‘-di-tert-butyl-2,2‘-bipyridine (bu2bpy) (1), N,N,N‘,N‘-tetramethylethylenediamine (tmeda) (2), bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (Ar2NN) (3), 1,2-bis(diphenylphosphino)ethane (dppe) (4)] and electrophilic platinum complexes fac-[PtMe3(SO3CF3)(LL)] [LL = bu2bpy (6a), tmeda (7a), Ar2NN (8a)] or [PtMe(SO3CF3)(dppe)] (10a). It is proposed that the complexes 6a, 7a, 8a, or 10a undergo initial dissociation of the triflate ligand to give an electrophilic platinum cation which attacks one of the mutually trans methylplatinum ligands of [PtMe4(LL)]. The reaction of [PtMe4(LL)] [LL = bu2bpy (1), tmeda (2), Ar2NN (3)] with fac-[PtMe3(SO3CF3)(L‘L‘)] [L‘L‘ = bu2bpy (6a), tmeda (7a), Ar2NN (8a)] gives an equilibrium mixture between the reactants and fac-[PtMe3(SO3CF3)(LL)] and [PtMe4(L‘L‘)], in which [PtMe4(LL)] is favored in the same order as the π-accepting properties of the LL ligand (Ar2NN ≫ bu2bpy > tmeda). The reaction of [PtMe4(dppe)] (4) with 6a, 7a, 8a, or 10a results in rapid room-temperature catalytic, C−C bond reductive elimination to give C2H6 and [PtMe2(dppe)] (5). It is proposed that the actual reductive elimination step occurs from the five-coordinate species [PtMe3(dppe)]+, formed by methyl ligand transfer from 4 to the electrophilic platinum complex (6a, 7a, 8a or 10a). The triflato complexes 6a−10a are in rapid equilibrium with the cationic aqua complexes fac-[PtMe3(OH2)(LL)]SO3CF3 [LL = bu2bpy (6b), tmeda (7b), Ar2NN (8b), dppe (9b)] and [PtMe(OH2)(dppe)]SO3CF3 (10b), respectively, thus confirming lability of the triflate ligands.