Hard ligands as donors to soft metals. Part 3. Cationic bis(solvent) complexes of palladium(II); cations for catalysis

Abstract

Treatment of [Pd(dppe)Cl2], dppe = Ph2PCH2CH2PPh2, with 2 equivalents of a silver(I) salt in the presence of a solvent ligand (solvent) leads to the formation of [Pd(dppe)(solvent)2]Y2 where Y = ClO4 or BF4. In this way stable complexes with solvent = pyridine, O-bonded dimethyl sulphoxide, dimethylformamide, benzonitrile, o-toluonitrile, acetone, or water as well as (solvent)2= 1,6-dicyanohexane or o-phthalonitrile have been isolated and characterised. Complexes with solvent = methanol, ethanol, acetonitrile, or tetrahydrofuran are stable in solution; on attempted isolation the ethanol complex deposits palladium metal, the methanol and tetrahydrofuran perchlorate complexes yield the bis(perchlorato)-complexes [Pd(dppe)(OClO3)2], and the acetonitrile complex yields mainly [Pd(dppe)(NCMe)2][ClO4]2 with a small amount of perchlorate-co-ordinated complex. The co-ordinated perchlorate ligands are readily displaced in solution by any of the solvent ligands; the bis(acetone) complex is very sensitive to moisture yielding [Pd(dppe)(OH2)2][ClO4]2 on exposure to air. Attempts to displace acetone from [Pd(dppe)(OCMe2)2]2+ and perchlorate from [Pd(dppe)(OClO3)2] by carbon monoxide failed. The solvent molecules dichloromethane and benzene do not form palladium(II) complexes of the present type; treatment of [Pd(dppe)Cl2] with 2 equivalents of silver(I) perchlorate in dichloromethane–benzene yields [Pd(dppe)(OClO3)2]. Preliminary investigation of the catalytic abilities of these palladium(II) complexes has shown that [Pd(dppe)(OCMe2)2][ClO4]2 catalyses the hydrogenation of styrene to ethylbenzene at atmospheric pressure and 30 °C, which is remarkably high activity for palladium(II).