Palladium forms a variety of ionic, coordination, and organopalladium compounds, typically with oxidation state Pd0 or Pd2+. Palladium(III) compounds have also been reported. Palladium compounds are frequently used as catalysts in cross-coupling reactions such as the Sonogashira coupling and Suzuki reaction.
Most ionic compounds of palladium involve the Pd2+ oxidation state. Palladium(II) chloride is a starting point in the synthesis of other palladium compounds and complexes. Palladium(II) acetate plus triphenylphosphine is used as a catalyst in organic synthesis.[1]
Coordination compounds of palladium contain ligands coordinated to a central Pd0 or Pd2+ center. They are typically synthesized by adding ligands to an ionic palladium compound. For example, acetonitrile, benzonitrile, or triphenylphosphine may be coordinated to palladium(II) chloride to form bis(acetonitrile)palladium dichloride, bis(benzonitrile)palladium dichloride, or bis(triphenylphosphine)palladium chloride, respectively. Many other more exotic ligands form a large variety of palladium-phosphine catalysts, such as 1,1'-bis(diphenylphosphino)ferrocene (dppf) to form [1,1'-bis(diphenylphosphino)ferrocene]palladium(II) dichloride .
Another precursor to coordination compounds of palladium is sodium tetrachloropalladate, to which dibenzylideneacetone (dba) and acetylacetonate may be coordinated to form tris(dibenzylideneacetone)dipalladium(0) [2] and palladium(II) bis(acetylacetonate), respectively.
Bis(triphenylphosphine)palladium chloride, which contains palladium as Pd2+, may be reduced using hydrazine in the presence of triphenylphosphine to form tetrakis(triphenylphosphine)palladium(0), which contains Pd0.[3]
See main article: Organopalladium chemistry.
Both ionic and coordination palladium compounds are frequently used to catalyze cross-coupling reactions. The catalytic ability is due to palladium's ability to switch between the Pd0 and Pd2+ oxidation states. An organic compound adds across Pd0 to form an organic Pd2+ complex (oxidative addition). After transmetalation with an organometallic compound, two organic ligands to Pd2+ may exit the palladium complex and combine, forming a coupling product and regenerating Pd0 (reductive elimination).[1]
For the Suzuki reaction, commonly used catalysts include , , , as well as plus triphenylphosphine .[1] A large variety of phosphine-based ligands may be used in palladium-phosphine catalysts. Bulky, electron-rich ligands such as tris(2,4,6-trimethoxyphenyl)phosphine result in catalysts that are more reactive in the oxidative addition step[1] and can catalyze the coupling of aryl chlorides, which are typically unreactive.[4]