Synthesis, characterization and X-ray structure of trans-[Pd(COOCH3)(H2O)(PPh3)2](TsO), a possible intermediate in the catalytic hydroesterification of olefins (TsO= p-toluenesulfonate)

The complex trans-[Pd(COOCH3)(H2O)(PPh3)2](TsO) (I) has been synthesized by reacting trans-[Pd(COOCH3)Cl(PPh3)2] with AgTsO in methanol. It has been characterized by IR, 1H and 31P NMR spectroscopy. Crystals of trans-[Pd(COOCH3)(H2O)(PPh3)2](TsO)·(CH3OH) (II) have been obtained by re-crystallization of I in methanol. The structure of the complex has been determined by X-ray analysis. It shows a slightly distorted square planar conformation around the central palladium. The coordinated water molecule and the clathrated methanol form a contact with the uncoordinated TsO- anion, suggesting hydrogen bond interaction. Since I is a possible intermediate in the catalytic hydroesterification of olefins, its catalytic activity in the hydroesterification of ethylene has been tested at 100 °C under 45 atm of a 1/1 mixture of ethylene and CO in methanol as solvent, also in the presence of PPh3 and TsOH. Without addition of PPh3 and TsOH, I affords to traces of methylpropionate together with Pd(0) complexes and Pd metal. The same results are obtained when complex I is tested in the presence of PPh3, except that in this case formation of palladium metal is avoided. Using the system I/PPh3/TsOH=1/6/8, a TOF (mol of ester/mol Pd*h) of 1800 h-1 is obtained. We propose that the role of the acid TsOH is to favor the formation of a Pd–hydride intermediate and/or to reactivate the Pd(0) species, stabilized by the excess of PPh3, via an oxidative addition of the acid. Since the acid does not favor the formation of Pd-alkoxy species we suggest that complex I plays only a minor role in catalysis and that this occurs via a Pd-hydride species.