Palladium(II) complexes of chiral tridentate nitrogen pybox ligands

Article abstract of DOI:10.1016/0022-328X(95)05759-I

A series of mono-and di-cationic palladium(II) complexes containing different chiral tridentate nitrogen ligands, pybox, have been prepared [pybox = 2,6-bis[4′-(S)-ⁱPr (or Ph, or Bz or p-EtOC₆H₄)oxazoline-2′-y1]pyridine (1-4), respectively]. The molecular structures for two of these, [Pd(CH₃CN)(2)](BF₄)₂ (6) and [Pd(PPh₃)(3)](BF₄)₂ (21g), have been determined by X-ray diffraction and show no major steric hindrance in the fourth coordination position. In connection with the aldol reaction of CNCH₂CO₂Me with PhCHO, several new isonitrile Pd_II complexes have also been prepared. It is shown that, under catalytic conditions, the chiral tridentate pybox ligand is completely displaced, thus explaining its failure as a chiral auxiliary. Preparative details for a series of chiral Pd(L)(3)ⁿ⁺(BF₄)_n (21) complexes [L = 4-methylpyridine, 2,6-dimethylpyridine, 4-methyl aniline, H₂NCH₂CH(OMe)₂, H₂NCH₂CH₂OH, H₂N(CH₂)₅CH₃, N^-₃, HCO^-₂, Cl^-] are given, as are preparative details for some model Pd^II acetonitrile complexes with chiral phosphorus and nitrogen chelating ligands. For 6, i.e. PdC₂₅H₂₂N₄O₂B₂F₈, the crystals are monoclinic with space group P2₁ (No. 4), a = 13.582(6) A, b = 13.826(6) A, c = 14.667(6) A, β = 97.28(3)°°, V = 2732(2) A³, Z = 4. For 21g, i.e. C₄₃H₃₈B₂F₈N₃O₂P₂Pd, the crystals are orthorhombic with space group, P2₁2₁2₁, a = 10.616(4) A, b = 16.774(2) A, c = 23.086(4) A, V = 4111(3) A³, Z = 4.