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
Md. Fryzuk et al., 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, Journal of the Chemical Society. Dalton transactions, (12), 1998, pp. 2007-2016
The reactivity of the electron-rich palladium alkyl cation, [Pd(dippe)
R]+BAr4- (dippe = 1,2-bis(diisopropylphosphino)ethane; R = eta(3)-CH2P
h or CH3;BAr4 = {B[3,5-(F3C)(2)C6H3](4)})with a variety of small molec
ules is reported. Although the benzyl cation is unreactive towards car
bon monoxide and dihydrogen, the corresponding methyl cation, [Pd(dipp
e)Me(s)](+) (s = Et2O, THF or o-dichlorobenzene) reacts rapidly with H
-2 to produce the dihydride-bridged dimer {[(dippe)Pd](2)(mu-H)(2)}(2), and with CO to produce the dinuclear mixed-valence, cationic comple
x [Pd(dippe)(mu-CO)Pd(dippe)Me][BAr4]. In addition, the methyl cation
can abstract alkyl groups from neutral dialkyl complexes; thus, the ad
dition of [Pd(dippe)Me(s)](+) to Pd(dippe)(CH2Ph)(2) results in the fo
rmation of the methyl benzyl derivative Pd(dippe)Me(CH2Ph) and the cat
ionic benzyl cation [Pd(dippe)(eta(3)-CH2Ph)](+). Methyl group interch
ange is also observed for the reaction of the methyl cation with the n
eutral dimethyl; when [Pd(dippe)Me(s)](+) is mixed with Pd(dippe)((CH3
)-C-13)(2), the carbon-13 label is immediately scrambled to the cation
. These exchange reactions are suggested to occur via dinuclear interm
ediates. The mixed-valence dinuclear species mentioned above has been
investigated in some detail; mechanistic studies have indicated that t
he 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 in
sertion to give the acetyl carbonyl derivative [Pd(dippe)(COMe)(CO)](). The final product is the dinuclear mixed-valence species [Pd(dippe)
(mu-Co)Pd(dippe)Me][BAr4], which is accompanied by the formation of ac
etone (Me2CO) and the dicarbonyl dication [Pd(dippe)(CO)(2)](2+). Pres
umably, methyl transfer occurs at some stage from the methyl cation to
generate the methyl-acetyl complex, Pd(dippe)Me(COMe); reductive elim
ination of acetone under CO from the methyl-acetyl complex produces th
e Pd-0 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 mononucl
ear complexes, rather 1 equivalent of CO adds to form a new dinuclear
complex with two bridging COs.