Reactions of co-ordinated ligands. Part XII. The preparation and thermal rearrangement of η1-but-3-enyl complexes of palladium(II): the crystal and molecular structure of trans-chloro-η1-[3-chloro-cis-l,2-bis-(methoxycarbonyl)but-3-enyl]bis(pyridine)palladium(II)

Abstract

The reaction of the η³-but-3-enyl complexes [Pd(X)CH(CO₂Me)CR¹CHR¹](X = Cl, hexafluoroacetylacetonate; R¹= Cl or H, R²= H or alkoxy) with pytidine or cycle-octa-1, 5-diene leads to the formation of thermally stable η¹-but-3-enyl complexes of Pd^II. A single-crystal X-ray diffraction study forms the basis of a discussion as to the origins of the stability of these complexes. Crystals of [PdCl(py)₂{CH(CO₂Me)·CH(CO₂Me)·C(Cl):CH₂}] are triclinic. P, with four molecules in a unit cell of dimensions: a= 14.794(8), b= 11.599(8), c= 13.131 (8)Å, α= 89.53(5), β= 113.04(4), γ= 91.50(5)°. The structure has been elucidated by conventional heavy-atom methods from 5 397 diffracted intensities measured on a four-circle diffractometer, and refined to R 0.075. The asymmetric unit thus comprises two crystallographically distinct molecules, and while these do not differ fundamentally in either their geometry or their stereochemistry at the two chiral centres, the ligands adopt different equilibrium positions as a result of free rotation around each ligand axis. The important conclusions of the structure determination are that (i) the two chiral centres have opposite configurations (one R, and one S), (ii) the groups on the two chiral centres are in a staggered configuration relative to one another, (iii) the hydrogen atoms lie trans to one another. Because the space group is centrosymmetric, the crystals comprise equal numbers of R–S and S–R molecules. Reaction of the η¹-but-3-enyl palladium(II) complexes in refluxing benzene affords π-allylic complexes. The stereochemistry of these is discussed in terms of their ¹H n.m.r. spectra and the mechanism of their formation.