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Catalytic Properties of Palladium

Introduction

Palladium was discovered by William Hyde Wollaston in 1803. Wollaston isolated the newly discovered element by dissolving platinum in a solution of hydrochloric acid and nitric acid. This process created a residue that was subsequently subjected to a series of chemical reactions yielding palladium cyanide. Finally, the palladium cyanide was heated to isolate the pure palladium metal. Palladium has many industrial applications due to its catalytic properties. Palladium catalysts are used in the production of fuel cells, catalytic converters in automobiles, and the synthesis of industrial chemicals.

Properties

Palladium posses unique properties that make it suitable for specific industrial applications. The predicted ground-state electron configuration for palladium is [Ar] 5s²4d⁸. The two electrons in the 5s orbital are higher in energy than if they were in the 4d orbital. The electrons from palladium’s 5s orbital fill the 4d orbital, resulting in the lower energy state. This causes the ground-state electron configuration of palladium to be [Ar] 4d¹⁰. The oxidation state of palladium is critical to palladium catalysis. During the oxidation of palladium, electrons are displaced from the 4d orbital and not the 5s orbital. Common oxidation states of palladium include Pd(0), Pd(II), and Pd(IV).

Dehydrogenation

Palladium is commonly used as a catalyst in the reduction of carbon-carbon bonds., , The reduction of alkenes takes place at room temperature using hydrogen gas at atmospheric pressure. This reaction is thermodynamically favorable at room temperature, because the alkane product is lower in energy than the alkene. The heat released in this reaction is specifically referred to as the “Heat of Hydrogenation.” Subjecting the reactants to the palladium lowers the activation energy for the reaction driving it forward. Diatomic hydrogen molecules adsorb to the surface of the palladium, and the reduction reaction takes place.

Figure 1. Reaction Mechanism for the Palladium-Catalyzed Reduction of an Alkene

Figure 1 shows the reaction mechanism for the palladium-catalyzed reduction of an alkene. The first step shows the adsorption of the H₂ to the surface of the palladium. After the adsorption of H₂, the alkene is adsorbed to the surface of the palladium. Alkenes will specifically bind to the interstitial lattice sites on the surface of palladium. This interaction is facilitated by asymmetrical electron-cloud distributions in palladium and the incoming alkene.

Figure 2 shows the interaction of the atomic orbitals of palladium and H₂. Palladium donates d π-orbital electrons to the sigma anti-bonding orbitals in H₂. This destabilizes the bond between constituent atoms of H₂ causing them to break and form new bonds to palladium. Hydrogen is then added across the double bond via syn-addition. Syn-addition results from the addition of hydrogen atoms along the plane of the alkene's π-bonds. In sum, the orbital interactions depicted in Figure 2 demonstrate inherent qualities of palladium that make it an ideal catalyst for the reduction of alkenes at room temperature.

Palladium-Catalyzed Cross Coupling

The cross coupling reaction mechanism was described by Suzuki et al. as a three-step reaction involving three distinct intermediates. The three reactions are characterized as an oxidative addition, transmetalation, followed by a reductive elimination.

Geometry of Palladium

Palladium catalytic complexes are commonly used in cross-coupling reactions as part of methodologies for the formation of carbon-carbon and carbon-halogen bonds. The geometry of palladium contributes to many different aspects of the cross-coupling mechanism. , During the reaction mechanism, Pd(0) is reduced to form Pd(II). The geometry of Pd(0) is tetrahedral, while the geometry of Pd(II) is square planar. This altered geometry changes the potential interactions between the orbitals of palladium and reactive species. These effects are considered when designing methodologies involving palladium.

Ligand Effects

Ligands are coordinately bound to palladium to form a catalytic complex. The choice of ligand and the number of ligands can be altered in designing experimental methodologies.

Figure 3. Palladium-Ligand Molcular Orbital Diagram

Figure 3 shows the molecular orbital energy diagram of the bonds between palladium and PR₃ which specifically represents a trisubstituted phosphate molecule. In the diagram, PR₃ is ligated to palladium forming a coordination bond. The symmetry of the molecular orbitals depicted in the diagram is a critical factor in characterizing the bond between the ligand and palladium. The sigma-symmetry bond forms between the filled sigma-orbital on the ligand and the empty pi-orbital of palladium. The pi-symmetry bond forms between the filled d-orbital of Palladium and the sigma anti-bonding orbital of the ligand. This bonding interaction is termed, synergic, as the two bonding symmetries strengthen one another. These interactions between the orbitals of palladium and the ligand are considered when designing a palladium catalytic complex.