Reactivity of electrophilic palladium alkyl cations stabilized by electron-rich chelating diphosphine ligands. Evidence for dinuclear intermediates and the formation of a dinuclear mixed-valence methyl cation‡

Abstract

The reactivity of the electron-rich palladium alkyl cation, [Pd(dippe)R]⁺BAr₄^– (dippe = 1,2-bis(diisopropylphosphino)ethane; R = η³-CH₂Ph or CH₃; BAr₄ = {B[3,5-(F₃C)₂C₆H₃]₄}) with a variety of small molecules is reported. Although the benzyl cation is unreactive towards carbon monoxide and dihydrogen, the corresponding methyl cation, [Pd(dippe)Me(s)]⁺ (s = Et₂O, THF or o-dichlorobenzene) reacts rapidly with H₂ to produce the dihydride-bridged dimer {[(dippe)Pd]₂(µ-H)₂}²⁺, and with CO to produce the dinuclear mixed-valence, cationic complex [Pd(dippe)(µ-CO)Pd(dippe)Me][BAr₄]. In addition, the methyl cation can abstract alkyl groups from neutral dialkyl complexes; thus, the addition of [Pd(dippe)Me(s)]⁺ to Pd(dippe)(CH₂Ph)₂ results in the formation of the methyl benzyl derivative Pd(dippe)Me(CH₂Ph) and the cationic benzyl cation [Pd(dippe)(η³-CH₂Ph)]⁺. Methyl group interchange is also observed for the reaction of the methyl cation with the neutral dimethyl; when [Pd(dippe)Me(s)]⁺ is mixed with Pd(dippe)(¹³CH₃)₂, the carbon-13 label is immediately scrambled to the cation. These exchange reactions are suggested to occur via dinuclear intermediates. The mixed-valence dinuclear species mentioned above has been investigated in some detail; mechanistic studies have indicated that the addition of CO to [Pd(dippe)Me(s)]⁺ probably proceeds via simple substitution of the solvent by CO to generate the expected mononuclear methyl carbonyl cation [Pd(dippe)Me(CO)]⁺, followed by migratory insertion to give the acetyl carbonyl derivative [Pd(dippe)(COMe)(CO)]⁺. The final product is the dinuclear mixed-valence species [Pd(dippe)(µ-CO)Pd(dippe)Me][BAr₄], which is accompanied by the formation of acetone (Me₂CO) and the dicarbonyl dication [Pd(dippe)(CO)₂]²⁺. Presumably, methyl transfer occurs at some stage from the methyl cation to generate the methyl–acetyl complex, Pd(dippe)Me(COMe); reductive elimination of acetone under CO from the methyl–acetyl complex produces the Pd⁰ complex Pd(dippe)CO which then reacts with the starting methyl cation to generate the dinuclear mixed-valence species. Addition of CO to the mixed-valence species does not result in formation of mononuclear complexes, rather 1 equivalent of CO adds to form a new dinuclear complex with two bridging COs.