Tetrakis(triphenylphosphine)palladium(0)


Tetrakispalladium is the chemical compound , often abbreviated Pd4, or rarely PdP4. It is a bright yellow crystalline solid that becomes brown upon decomposition in air.

Structure and properties

The four phosphorus atoms are at the corners of a tetrahedron surrounding the palladium center. This structure is typical for four-coordinate 18 e− complexes. The corresponding complexes Ni4 and Pt4 are also well known. Such complexes reversibly dissociate PPh3 ligands in solution, so reactions attributed to Pd4 often in fact arise from Pd3 or even Pd2.

Preparation

Tetrakispalladium was first prepared by Lamberto Malatesta et al. in the 1950s by reduction of sodium chloropalladate with hydrazine in the presence of the phosphine. It is commercially available, but can be prepared in two steps from Pd precursors:
Both steps may be carried out in a one-pot reaction, without isolating and purifying the PdCl22 intermediate. Reductants other than hydrazine can be employed. The compound is sensitive to air, but can be purified by washing with methanol to give the desired yellow powder. It is usually stored cold under argon.

Applications

Pd4 is widely used as a catalyst for palladium-catalyzed coupling reactions. Prominent applications include the Heck reaction, Suzuki coupling, Stille coupling, Sonogashira coupling, and Negishi coupling. These processes begin with two successive ligand dissociations followed by the oxidative addition of an aryl halide to the Pd center: