Draft:Triarylphosphinegold complexes

• Comment: Also, much of this article overlaps with well documented aspects of trialkylphosphine ligands and organometallic reactions. You should remove these and add more properties that is notable for the compound itself, like its catalytic properties. Pygos (talk) 10:01, 24 August 2024 (UTC)

• Comment: This article requires additional citations. Pygos (talk) 09:59, 24 August 2024 (UTC)

• Comment: Large chunks are unsourced, and when I spot-checked the sources they at least sometimes do not support the adjacent content. Is this an LLM dump? DMacks (talk) 09:54, 24 August 2024 (UTC)

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Triarylphosphine–gold complexes have a phosphine ligand where the phosphorus atom is bonded to three aryl groups, such as phenyl rings. These complexes stand out in organometallic chemistry because of their remarkable stability, easy synthesis, and the ability to fine-tune their electronic and steric properties by modifying the aryl groups. This versatility makes triarylphosphine–gold complexes particularly attractive for use in catalysis, medicinal chemistry, and material science.

Chemical structure and bonding

Ligand structure

Triarylphosphine ligands are a class of organophosphorus compounds where a phosphorus atom is bonded to three aryl groups, such as phenyl (C₆H₅), tolyl (C₆H₄CH₃), or other substituted aromatic rings. The general formula for a triarylphosphine ligand is PAr₃, where "Ar" represents an aryl group.

• Aryl groups influence: Phenyl groups (as in triphenylphosphine, PPh₃) offer a balance between electronic donation through their σ-bonding abilities and moderate steric hindrance around the phosphorus center. Tolyl groups, with their methyl substituents, add extra steric bulk and slightly increase the electron-donating capability due to the inductive effect of the methyl group.
• Electronic effects: The electronic properties of the aryl groups can be modified. We can introduce electron-donating groups or electron-withdrawing groups on the aromatic rings. Electron-donating groups like methoxy (-OCH₃) increase the electron density on the phosphorus atom. This enhances the ability of the phosphorus to donate electrons to the metal center. On the other hand, electron-withdrawing groups like nitro (-NO₂) decrease the electron density on the phosphorus. This makes the ligand a weaker donor.
• Steric effects: The size and shape of the aryl groups control the steric effects. Bulky aryl groups, such as those with ortho-substituents, can create significant steric hindrance around the metal center. This steric hindrance can influence the coordination geometry and reactivity of the gold complex. The steric bulk can be used strategically to stabilize specific coordination environments or to prevent unwanted side reactions.

Gold coordination

Gold(I) complexes

• Linear Coordination Geometry: In gold(I) complexes, the gold center typically exhibits a linear coordination geometry, where the gold atom is bonded to two ligands in a straight line. For example, in a complex like (Ph₃P)AuCl, the gold atom is linearly coordinated to the phosphorus atom of the triarylphosphine ligand and the chloride ion: ${\ce {(Ph3P)AuCl}}$.
• Rationale for Linearity: The linear geometry of gold(I) complexes is due to the d¹⁰ electron configuration. This configuration favors a two-coordinate, 180° arrangement. This is because there is little ligand-ligand repulsion, and the coordination number is low. The linearity is a characteristic feature of gold(I) complexes. It is also an important factor in their stability and reactivity.

Gold(III) Complexes

• Square Planar Coordination Geometry: Gold(III) complexes often adopt a square planar geometry, where the gold center is bonded to four ligands arranged in a plane. For example, in a complex like (Ph₃P)AuCl₃, the gold(III) center is coordinated to three chloride ions and one phosphorus atom in a square planar arrangement: ${\ce {(Ph3P)AuCl3}}$.
• Rationale for Square Planarity: The square planar geometry is preferred for gold(III) compounds. This is because the d⁸ electron configuration of gold(III) stabilizes the four-coordinate arrangement. Many other d⁸ transition metal complexes, such as those of platinum(II) and palladium(II), also have this square planar geometry.

Electronic properties

The electronic characteristics of the gold-phosphine bond are crucial in determining the behavior and reactivity of triarylphosphinegold complexes. The bond interaction involves both σ-donation and π-backbonding.

• σ-Donation: The phosphorus atom has a lone pair of electrons. This lone pair is donated to the gold center. This forms a strong sigma bond. The strength of this sigma bond depends on the electron-donating ability of the aryl groups attached to the phosphorus. Electron-donating groups enhance the sigma donation. This strengthens the bond. Electron-withdrawing groups have the opposite effect. They weaken the bond.
• π-Backbonding: The gold center's filled d-orbitals donate electron density to the empty π* orbitals of the phosphorus ligand. This interaction is generally weak in gold(I) complexes due to the d¹⁰ configuration. However, this interaction can be more significant in gold(III) complexes, although it is still less prominent than σ-donation.
• Bonding Implications: The balance between σ-donation and π-backbonding affects the bond strength and the reactivity of the gold center. Stronger σ-donation leads to more stable complexes, but it may also reduce reactivity. Chemists can fine-tune the reactivity of the complex by adjusting the aryl groups on the phosphine ligand. This allows them to use the complex for various catalytic and chemical applications.

Synthesis

Synthetic methods

The synthesis of triarylphosphine–gold complexes can be done through different methods. These methods allow to adjust the properties and reactivity of the resulting complexes. Some methods: -

Reaction of Gold Salts with Triarylphosphines

Direct reaction between a gold(I) salt, like gold(I) chloride (AuCl), and a triarylphosphine ligand, usually triphenylphosphine (PPh₃). This method is popular because it is simple, efficient, and easy to scale up. The reaction involves a ligand substitution process, where the chloride ligand on the gold center is replaced by the triarylphosphine ligand. The general reaction can be represented as

AuCl + PPh₃ → (Ph₃P)AuCl.

This reaction happens easily under gentle conditions, usually at room temperature, and usually produces the wanted complex in high purity. The resulting compound, (Ph₃P)AuCl, is a linear, two-coordinate gold(I) complex. This is a common structure for gold(I) phosphine complexes. This method is particularly useful for preparing gold(I) complexes that are stable and can be easily handled under standard laboratory conditions. These complexes can be used as starting materials for further modifications or they can be directly used in catalytic processes.

Oxidative addition reactions

The oxidation happens in the presence of a dihalide or another oxidizing agent. These applications are different from gold(I) complexes. In oxidative addition, the gold(I) center is oxidized to gold(III) while simultaneously adding two ligands, typically halides, to form a complex like (Ph₃P)AuX₂:

Au(I) + X₂ + PPh₃ → (Ph₃P)AuX₂

Here, X represents a halide, such as chloride or bromide. This process often requires an oxidizing agent, such as molecular halogens (Cl₂, Br₂), or peroxides, to facilitate the oxidation from Au(I) to Au(III). Gold(III) complexes synthesized via oxidative addition are particularly useful in catalytic reactions that require higher oxidation states, such as cross-coupling reactions or oxidative transformations.

Ligand exchange reactions

Ligand exchange reactions involve the substitution of one or more ligands in a pre-existing gold complex. During this process, the ligands are replaced with a triarylphosphine ligand. This method allows modifing the ligand environment around the gold center. A typical ligand exchange reaction involves a gold(I) or gold(III) complex with labile ligands, such as halides, being treated with a triarylphosphine:

(AuCl)₂ + 2PPh₃ → 2(Ph₃P)AuCl

In this reaction, the triarylphosphine ligand replaces one or more of the original ligands on the gold center. This method is highly versatile, allowing for the introduction of various triarylphosphines, each conferring different steric and electronic properties to the gold complex. Ligand exchange reactions are widely used to prepare complexes with specific properties, such as increased catalytic activity, improved stability, or enhanced solubility.

Reaction conditions

Solvents

he choice of solvent is very important when making triarylphosphine–gold complexes. Common solvents used include dichloromethane (DCM), tetrahydrofuran (THF), toluene, and acetonitrile. These solvents are chosen because they can dissolve both the gold precursors and the triarylphosphine ligands. This allows the reactants to mix well and react efficiently.

Temperature

Most reactions involving the synthesis of triarylphosphine–gold complexes happen at room temperature or slightly higher temperatures, usually between 20 and 60 °C. This temperature range is enough to finish the reaction without decomposing sensitive reactants or products. The gold-phosphine bond is generally stable when heated, but at higher temperatures, there is a risk of the ligand falling apart or the complex breaking down. Therefore, it is important to carefully control the reaction temperature, especially when working with fragile or highly reactive ligands. The gold-phosphine bond is generally stable when heated, but at higher temperatures, there is a risk of the ligand falling apart or the complex breaking down. Therefore, it is important to carefully control the reaction temperature, especially when working with fragile or highly reactive ligands.

Catalysts

Many syntheses of triarylphosphine–gold complexes do not require catalysts. Oxidative addition reactions often do require catalysts. In these cases, an oxidizing agent can be used to facilitate the oxidation of gold(I) to gold(III). The oxidizing agents used can be chlorine gas, bromine, or peroxides. The choice of oxidizing agent or catalyst depends on the desired oxidation state of the gold center and the specific reaction pathway. For example, chlorine gas might be used for direct halogenation. Other oxidants might be preferred for more controlled or selective reactions.

Purification

After the reaction is complete, the crude product often needs to be purified to remove any unreacted starting materials, by-products, or solvents. Common purification techniques include recrystallization, column chromatography, and washing with non-polar solvents.

References

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Categories:

• Inorganic chemistry
• Organometallic chemistry
• Coordination chemistry
• Homogeneous catalysis
• Gold catalysis
• Ligand design
• Chemical reactions