New carboalkoxybis(triphenylphosphine)palladium(II) cationic complexes: Synthesis, characterization, reactivity and role in the catalytic hydrocarboalkoxylation of ethene. X-ray structure of trans-[Pd(COOMe)(TsO)(PPh3)2]·2CHCl3

Article abstract of DOI:10.1016/j.molcata.2008.10.002

The cationic complexes trans-[Pd(COOR)(H₂O)(PPh₃)₂](TsO) have been synthesised by reacting cis-[Pd(H₂O)₂(PPh₃)₂](TsO)₂·2H₂O with CO in ROH (R = Me and Et), practically under room conditions, or by methathetical exchange of trans-[Pd(COOMe)Cl(PPh₃)₂] with Ag(TsO) (R = n-Pr, iso-Pr, n-Bu, iso-Bu, sec-Bu). They have been characterised by IR, ¹H NMR and ³¹P NMR spectroscopies. The X-ray investigation of trans-[Pd(COOMe)(TsO)(PPh₃)₂] reveals that the palladium center is surrounded in a virtually square planar environment realized by two PPh₃ trans to each other, the carbon atom of the carbomethoxy ligand and an oxygen atom of the p-toluensulfonate anion, with two crystallization molecules of CHCl₃. The Pd-O-S angle, 151.9 (3)°, is very wide, probably due to the interaction of one CHCl₃ molecule with the complex inner core. The carbomethoxy derivatives react with R′OH yielding the corresponding R′ carboalkoxy derivative (R′ = Et, n-Pr and iso-Pr); ethene does not insert into the Pd-COOMe bond; decarbomethoxylation occurs when treated with TsOH/H₂O in MeOH at 50 °C. All the carboalkoxy are precursors for the catalytic carboalkoxylation of ethene if used in combination of PPh₃ and TsOH, better in the presence of some water. Experimental evidences are more in favor of the so-called "hydride" mechanism rather than the "carbomethoxy" mechanism.