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.