Palladium(II) acetate

Palladium(II) acetate is a chemical compound of palladium described by the formula Pd(O₂CCH₃)₂ or Pd(OAc)₂. It is considered more reactive than the related platinum. It is soluble in many organic solvents.

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Structure

Palladium(II) acetate is trimeric, consisting of an equilateral triangle of Pd atoms each pair of which is bridged with two acetate groups in a butterfly conformation. Each metal atom achieves approximate square planar co-ordination.^[1]

Preparation

It can be prepared in two steps from palladium sponge, hot glacial acetic acid, and nitric acid via the intermediacy of the dinitrate:^[2]

Pd + 2 HNO₃ → Pd(NO₃)₂ + H₂

Pd(NO₃)₂ + 2 CH₃COOH → Pd(O₂CCH₃)₂ + 2 HNO₃

An excess of palladium sponge is used to insure that all HNO₃ is consumed.

Uses

Palladium acetate is a catalyst for many organic reactions by combining with many common classes of organic compounds such as alkenes, dienes, and alkyl, aryl, and vinyl halides to form reactive adducts. Alkenes and π-allyl coordination to palladium(II) acetate involves sigma-type donation from the pi orbital of the alkene or π-allyl with concomitant pi-backbonding into an empty pi* orbital on the alkene or π-allyl. The greater the sigma donation to the metal is, the greater the pi-backbonding. The greater the pi-backbonding is, the greater the reduction in the bond order of the alkene or π-allyl.^[3] Reduction of the alkenes or π-allyl by coordination to palladium(II) acetate reverses the reactivity of the organic ligand allowing them to undergo reactions with nucleophiles rather than electrophiles.^[4]

Examples of palladium(II) acetate catalyzed reactions are:

• Vinylation: An example is the Heck reaction
• Rearrangement of Acyclic Dienes: An example is the Cope reaction
• Carbonylation reaction: carboxylation of aryl halides and benzyl chloride derivatives in ionic liquid media (l-butyl-3-methylimidazolium hexafluorophosphate, tetrafluoroborate, and Aliquat®336 )
• Reductive amination of aldehydes or ketones using potassium formate.^[5]
• Wacker process: the oxidation of ethylene in water to form acetaldehyde (precursor to poly(vinyl acetate), a common glue).
• Buchwald-Hartwig synthesis of aryl amines from aryl halides/pseudohalides and primary or secondary amines.^[6]

Pd(O₂CCH₃)₂ converts aryl bromides into aryltrimethylsilanes, an important functional group in many organic compounds including the fungicide "Latitude".

RC₆H₄Br + Si₂(CH₃)₆ → RC₆H₄Si(CH₃)₃ + Si(CH₃)₃Br

Pd(O₂CCH₃)₂ is compatible with the electronic properties of aryl bromides, and unlike other methods of synthesis, this method does not require high pressure equipment.^[7]

Precursor to other Pd compounds

Palladium acetate is used to produce other palladium (II) compounds. For example, phenylpalladium acetate, used to isomerize allylic alcohols to aldehydes, is prepared by the following reaction:

Hg(C₆H₅)(CH₃COO) + Pd(CH₃COO)₂ → Pd(C₆H₅)(O₂CCH₃) + Hg(O₂CCH₃)₂^[8]

Palladium(II) acetate reacts with acetylacetone to produce Pd(acac)₂, a precursor to Pd(0).^[1]

Light or heat reduce palladium acetate to give thin layers of palladium and can produce nanowires and colloids.^[2]

References

1. ^ Skapski, A C., and M. L. Smart. "The Crystal Structure of Trimeric Palladium(II) Acetate." Chemical Communications 1970 658-659.
2. ^ ^{a b} Bakhmutov, V. I.,; Berry, J. F.; Cotton, F. A.; Ibragimov, S.; Murillo, C. A. "Non-Trivial Behavior of Palladium (II) Acetate." Dalton Transactions 2005: 1989-1992.
   "High Purity Homogeneous Catalyst." Engelhard. Sept. 2005. Engelhard Corp. 24 Feb. 2006. .
3. ^ Toreki, R. "Allyl Ligands." The Organometallic HyperTextBook. 20 Nov. 2003. Chemglass. 01 Apr. 2006 .
4. ^ Suggs, J W. "Palladium: Organometallic Chemistry." Encyclopedia of Inorganic Chemistry. Ed. R B. King. 8 vols. Chichester: Wiley, 1994.
5. ^ Basu, B., Satadru J., Mosharef H. B., and Pralay D. "A Simple Protocol for the Direct Reductive Amination of Aldehydes and Ketones Using Potassium Formate and Catalytic Palladium Acetate." Synlett 4 (2003): 555-557.
6. ^ Buchwald-Hartwig Cross Coupling Reaction.
7. ^ Gooben, L J. "Research Area "New Pd-Catalyzed Cross-Coupling Reactions"" 28 Feb. 2006 .
8. ^ Heck, R F. "Aldehydes from Allylic Alchohols and Phenylpalladium Acetate: 2-Methyl-3-Phenylpropionaldehyde" Ed. R. A. Clement and R. E. Benson. 2005. 28 Feb. 2006 .

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