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

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 [((CNCHPh)-C-nPr)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 [(kappa(2)-(CNCHPh)-C-nPr)PdCl2] (3) that contains a chelating NHC-imine ligand as shown by single-crystal X-ray diffraction. Slow hydrolysis of related complexes [(kappa(2)-(CNCHPh)-C-nPr)PdCl2] (3) and [(kappa(2)-(CNCHPh)-C-(Ph)2CH)PdCl2] (4) using triethylammonium chloride and water lead to the precipitation of single crystals of insoluble NHC-amino palladium(II) complexes [(kappa(2)-(CNH2)-C-nPr)PdCl2] (6) and [(kappa(2)-(CNH2)-C-(Ph)2CH)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 [(kappa(2)-(CN(H)Et)-C-iPr)PdCl2] (12). Reaction between two equivalents of 2 and [PdCl2(MeCN)(2)] gives the di-NHC complex [(kappa(1)-(CNCHPh)-C-nPr)(2)PdCl2] (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 degreesC in CD3CN.