Synthesis, structure and reactivity of palladium(ii) complexes of chiral N-heterocyclic carbene–imine and –amine hybrid ligands

Abstract

The synthesis and structures of chiral N-heterocyclic carbene (NHC)–N-donor complexes of silver(I) and palladium(II) are reported. The X-ray structure of an NHC–imine silver(I) complex [(^nPrCN^CHPh)AgBr] (2) exhibits an Ag₂Br₂ dimer motif where the imine group is not coordinated to the silver atom. Reaction between 2 and [PdCl₂(MeCN)₂] gives the palladium(II) complex [(κ²-^nPrCN^CHPh)PdCl₂] (3) that contains a chelating NHC–imine ligand as shown by single-crystal X-ray diffraction. Slow hydrolysis of related complexes [(κ²-^nPrCN^CHPh)PdCl₂] (3) and [(κ²-^(Ph)₂CHCN^CHPh)PdCl₂] (4) using triethylammonium chloride and water lead to the precipitation of single crystals of insoluble NHC–amino palladium(II) complexes [(κ²-^nPrCN^H₂)PdCl₂] (6) and [(κ²-^(Ph)₂CHCN^H₂)PdCl₂] (7), respectively. In the solid state, complexes 6 and 7 both exhibit intermolecular hydrogen bonding between chlorine and an amino–hydrogen atom resulting in an infinite chain structure. Substitution of an amino hydrogen for an ethyl group gives the soluble complex [(κ²-^iPrCN^(H)Et)PdCl₂] (12). Reaction between two equivalents of 2 and [PdCl₂(MeCN)₂] gives the di-NHC complex [(κ¹-^nPrCN^CHPh)₂PdCl₂] (5) that does not contain a coordinated imine as shown by single crystal X-ray diffraction. Conproportionation between 5 and an equivalent of [PdCl₂(MeCN)₂] to 3 does not occur at temperatures up to 100 °C in CD₃CN.