Synthesis and Structure of Multinuclear Pd(II) Complexes Bridged by Phosphide or Azide Ligands

The synthesis and structure of phosphide- and azide-bridged multinuclear Pd(II) complexes bearing phosphine ligands [PdX(μ-X')(PR 3 )] n (X = Cl and N 3 ; X' = PR 2 ' and N 3 ; n = 2 and 4) are reported. The oxidative addition of R 2 ' PCl to Pd(PMe 3 ) 2 furnished the phosphide-bridged dinuclear Pd(II) complexes [PdCl(μ-PR 2 ' )(PMe 3 )] 2 [R' = i Pr ( 1a ) and Cy ( 1b )]. However, the oxidative addition of ( o -tolyl) 2 PCl to Pd(PMe 3 ) 2 produced a nonbridged mononuclear Pd(II) complex with the bis( o -tolyl)phosphinic ligand, trans -[Pd(PMe 3 ) 2 {P(O)( o -tolyl) 2 }] ( 2 ), via oxidation of the phosphinyl ligand. The reaction of the chloride-bridged dinuclear Pd(II) complexes [PdCl(μ-Cl)(PR 3 )] 2 [PR 3 = PEt 3 ( 3a ) and PPhMe 2 ( 3b )] with NaN 3 afforded the azide-bridged dinuclear and tetranuclear Pd(II) complexes [Pd(N 3 )(μ-N 3 )(PEt 3 )] 2 ( 4 ) and [Pd(N 3 )(μ-N 3 )(PPhMe 2 )] 4 ( 5 ). Comparisons of the X-ray structures of 4 and 5 show that the square-planar molecular geometry of the Pd(II) centers of 4 are more distorted than those of 5 . Density functional theory calculations suggest that the tetranuclear eight-membered ring structure like 5 is more stable than the dinuclear four-membered ring structure like 4 in the gas phase in both PEt 3 and PPhMe 2 systems. However, because the relative energy difference between the four-membered and eight-membered ring structures is small in the PEt 3 system with smaller steric hindrance compared with PPhMe 2 , it is assumed that this difference is compensated by the crystal packing energy, and the dinuclear four-membered ring complex 4 is actually obtained.