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(mu-X')(PR3)](n) (X = CI and N-3; X' = PR2' and N-3; n = 2 and 4) are reported. The oxidative addition of R-2'PCI to Pd(PMe3)(2) furnished the phosphide-bridged dinuclear Pd(II) complexes [PdCI(mu-PR2')(PMe3)](2) [R' = 'Pr (1a) and Cy (1b)]. However, the oxidative addition of (o-tolyl)(2)PCI to Pd(PMe3)(2) produced a nonbridged mononuclear Pd(II) complex with the bis(o-tolyl)phosphinic ligand, trans[Pd(PMe3)(2){P(O)(o-tolyl)(2)}] (2), via oxidation of the phosphinyl ligand. The reaction of the chloride-bridged dinuclear Pd(II) complexes [PdCI(mu-Cl)(PR3)](2) [PR3 = PE3 (3a) and PPhMe2 (3b)] with NaN, afforded the azide-bridged dinuclear and tetranuclear Pd(II) complexes [Pd(N-3)(mu-N-3)(PEt3)](2) (4) and [Pd(N-3)(mu-N-3)(PPhMe2)](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 PEt3 and PPhMe2 systems. However, because the relative energy difference between the four-membered and eight-membered ring structures is small in the PEt3 system with smaller steric hindrance compared with PPhMe2, it is assumed that this difference is compensated by the crystal packing energy, and the dinuclear four-membered ring complex 4 is actually obtained.