Palladium compounds
Template:Short description 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.
Ionic compounds
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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.[1] Palladium(II) acetate plus triphenylphosphine is used as a catalyst in organic synthesis.[2]
Coordination compounds
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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 (Template:Chem2) to form bis(acetonitrile)palladium dichloride (Template:Chem2), bis(benzonitrile)palladium dichloride (Template:Chem2), or bis(triphenylphosphine)palladium chloride (Template:Chem2),[1] 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 (Template:Chem2).
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) (Template:Chem2)[3] 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) (Template:Chem2), which contains Pd0.[4]
Organopalladium compounds
Catalysis
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).[2]
For the Suzuki reaction, commonly used catalysts include [[Tetrakis(triphenylphosphine)palladium(0)|Template:Chem2]], [[Bis(triphenylphosphine)palladium chloride|Template:Chem2]],[1] [[(1,1'-Bis(diphenylphosphino)ferrocene)palladium(II) dichloride|Template:Chem2]], as well as [[Palladium(II) acetate|Template:Chem2]] plus triphenylphosphine (Template:Chem2).[2] 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[2] and can catalyze the coupling of aryl chlorides, which are typically unreactive.[5]