The trans influence of F, Cl, Br and I ligands in a series of square-planar Pd(II) complexes. Relative affinities of halide anions for the metal centre in trans-[(Ph 3P) 2Pd(Ph)X

Abstract

Single crystal X-ray diffraction studies of trans-[(Ph 3P) 2Pd(Ph)X] (X = F ( 1), Cl ( 2), Br ( 3), and I ( 4) were carried out. The four structures split in two isostructural and isomorphous groups, namely orthorhombic for 1 and 2 (space group Pbca, Z = 8) and triclinic for 3 and 4 (space group P-1, Z = 2). According to the PdC bond length, the trans influence of X within these pairs follows the trend Cl>F and 1>Br. However, the trans influence of Cl is slightly stronger than that of Br. Both structural and 13C NMR studies revealed that electron-donating effects of (Ph 3P) 2PdX increase along the series X=I<Br<Cl<F, in accord with the previously reported ‘halogen anomaly’ phenomenon. Relative affinities of X − for the Pd centre in [(Ph 3P) 2Pd(Ph)] − were studied by 31P NMR in rigorously anhydrous CH 2Cl 2 solutions, and equilibrium constants and ΔG values were obtained for all possible combinations. The sequence F − > Cl − > Br − > I − is characteristic of halide preference for the Pd complexes. Dissolving 1 and PPN Cl in dry CH 2Cl 2 resulted in the release of ‘naked’ F − which fluorinated the solvent smoothly to give a mixture of CH 2ClF and CH 2F 2 in high yield. When chloroform was used instead of CH 2Cl 2, dichlorocarbene was generated slowly, forming the corresponding cyclopropane in the presence of styrene. All observations were rationalized successfully in terms of the filled/filled effect and push/pull interactions.