Reactivity of (Ferrocenylmethyl)phosphine toward Palladium and Platinum Chlorides. X-ray Structure of [Pd(PH₂CH₂Fc)Cl(μ-PHCH₂Fc)]₄ (Fc = Ferrocenyl), a Unique Complex Containing a Pd₄P₄ Cycle

The reaction of 2 equiv of the air-stable primary phosphine (ferrocenylmethyl)phosphine (PH₂CH₂Fc, <b>1</b>) with [Pd(cod)Cl₂] (Fc = ferrocenyl; cod = 1,5-cyclooctadiene) at 298 K gave the phosphanido-bridged Pd(II) tetramer [Pd(PH₂CH₂Fc)Cl(μ-PHCH₂Fc)]₄ (<b>2</b>), which shows an unprecedented arrangement of four Pd atoms embedded in an eight-membered Pd₄P₄ ring. An X-ray diffraction study showed that <b>2</b> crystallizes in the triclinic space group <i>P</i>1̄ with <i>a</i> = 17.607(7) Å, <i>b</i> = 17.944(7) Å, <i>c</i> = 18.792(7) Å, α = 107.120(12)°, β = 96.344(13)°, γ = 117.087(15)°. Each molecule contains four palladium atoms in a distorted square-planar coordination formed by one chlorine and three phosphorus atoms. Two of the latter belong to bridging primary phosphanides and the remaining one is contributed by a terminal PH₂CH₂Fc ligand. The coordination environments of neighboring metal centers adopt an almost perpendicular mutual orientation. The reaction of 2 equiv of <b>1</b> with [Pt(cod)Cl₂] at 323 K yielded the analogous Pt(II) tetramer of formula [Pt(PH₂CH₂Fc)Cl(μ-PHCH₂Fc)]₄ (<b>3</b>), which was fully characterized by multinuclear and dynamic NMR, IR, and elemental analyses. Single-crystal X-ray diffraction on <b>3</b> confirmed the tetranuclear arrangement in the solid state, but orientational disorder of the molecule precludes a more detailed discussion of the structure. Low-temperature NMR experiments in CD₂Cl₂ showed the presence of two slowly interconnecting conformers. Reaction of <b>1</b> and [M(cod)Cl₂] (M = Pd or Pt) at lower temperatures (273 K for Pd, 295 K for Pt) in dichloromethane allowed the detection in solution of the mononuclear species <i>cis</i>-[M(PH₂CH₂Fc)₂Cl₂] (M = Pd, <b>4</b>; M = Pt, <b>5</b>) which, upon heating, transformed into the tetramers <b>2</b> and <b>3</b>, respectively. Solid samples of <b>4</b> and <b>5</b> could be isolated after workup at low temperature and were characterized by conventional spectroscopic methods.