Synthesis, characterization and X-ray structure of trans-[Pd(COOCH 3)(H 2O)(PPh 3) 2](TsO), a possible intermediate in the catalytic hydroesterification of olefins (TsO= p-toluenesulfonate)

Abstract

The complex trans-[Pd(COOCH 3)(H 2O)(PPh 3) 2](TsO) ( I) has been synthesized by reacting trans-[Pd(COOCH 3)Cl(PPh 3) 2] with AgTsO in methanol. It has been characterized by IR, 1H and 31P NMR spectroscopy. Crystals of trans-[Pd(COOCH 3)(H 2O)(PPh 3) 2](TsO)·(CH 3OH) ( 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 PPh 3 and TsOH. Without addition of PPh 3 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 PPh 3, except that in this case formation of palladium metal is avoided. Using the system I/PPh 3/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 PPh 3, 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.