The
synthesis and structure of phosphide- and azide-bridged multinuclear
Pd(II) complexes bearing phosphine ligands [PdX(μ-X′)(PR3)]
n
(X = Cl and N3;
X′ = PR2
′ and N3; n = 2 and 4) are reported. The
oxidative addition of R2
′PCl to Pd(PMe3)2 furnished the
phosphide-bridged dinuclear Pd(II) complexes [PdCl(μ-PR2
′)(PMe3)]2 [R′ =
i
Pr
(1a) and Cy (1b)]. However, the oxidative
addition of (o-tolyl)2PCl 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 [PdCl(μ-Cl)(PR3)]2 [PR3 = PEt3 (3a) and PPhMe2 (3b)] with NaN3 afforded
the azide-bridged dinuclear and tetranuclear Pd(II) complexes [Pd(N3)(μ-N3)(PEt3)]2 (4) and [Pd(N3)(μ-N3)(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.