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).