Allylpalladium(II) chloride dimer (APC) is a chemical compound with the formula [([[hapticity|η]]3-C3H5)PdCl]2. This yellow air-stable compound is an important catalyst used in organic synthesis.[1] It is one of the most widely used transition metal allyl complexes.
The compound has a dimeric structure that is centrosymmetric. Each allyl group lies in a plane at an angle of about 111.5° to the square formed by the palladium and carbon atoms, and the Pd–C distances are all equal. Its unit cell is monoclinic.[2]
The compound is prepared by purging carbon monoxide through a methanolic aqueous solution of sodium tetrachloropalladate (prepared from palladium(II) chloride and sodium chloride), and allyl chloride.[1]
2 Na2PdCl4 + 2 CH2=CHCH2Cl + 2 CO + 2 H2O → [(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)PdCl]2 + 4 NaCl + 2 CO2 + 4 HClAnother method is the reaction of propene with palladium(II) trifluoroacetate, followed by ion exchange with chloride:
2 (CF3COO)2Pd + 2 CH2=CHCH3 → [(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Pd(CF<sub>3</sub>COO)]2
[(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)Pd(CF<sub>3</sub>COO)]2 + 2 Cl− → [(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)PdCl]2 + 2 CF3COO−
APC reacts with sources of cyclopentadienyl anion to give the corresponding 18e− complex cyclopentadienyl allyl palladium:
[(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)PdCl]2 + 2 NaC5H5 → 2 [(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)Pd(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)] + 2 NaCl
The dimer reacts with a variety of Lewis bases (:B) to form adducts (η3-C3H5)PdCl:B. Its reaction with pyridine and the corresponding enthalpy are:
1/2 [(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)PdCl]2 + :NC5H5 → (η3-C3H5)PdCl:NC5H5 ΔH=−30.1 kJ.mol−1This enthalpy corresponds to the enthalpy change for a reaction forming one mole of the product, (η3-C3H5)PdCl:NC5H5, from the acid dimer.The dissociation energy for the Pd dimer, which is an energy contribution prior to reaction with the donor,
[(η<sup>3</sup>-C<sub>3</sub>H<sub>5</sub>)PdCl]2 → 2 (η3-C3H5)PdClhas been determined by the ECW model to be 28 kJ.mol−1.
APC catalyzes many organic reactions, such as cross-coupling, nucleophilic addition to dienes, and decomposition of diazo compounds to reactive carbenes. It is also a useful precursor of other Pd catalysts.