Gold Compounds

Gold as a Catalyst

Before the 1980s, gold was considered to have limited catalytic activity. However, advancements led by pioneers such as F. Dean Toste and Alois Fürstner have elevated gold to a key role in transition metal catalysis. Gold-mediated catalysis (sometimes referred to as pi-acid catalysis) usually relies on phosphine-ligated gold(I) complexes and has recently emerged as potent catalysts for forming C–C bonds, capable of executing various reactions under mild conditions, including cyclopropanations, enyne isomerizations, Rautenstrauch rearrangements, ene reactions, and ring expansions. The catalyst system typically involves a phosphine gold(I) chloride complex, combined with a silver salt co-catalyst, to generate the active species in situ.

Gold also has transcended its ornamental role to become a catalyst of remarkable importance in pharmaceutical processes. Gold catalysts play a pivotal role in synthesizing pharmaceutical intermediates, elevating the efficiency of drug manufacturing.

Gold catalysis has been shown to be a particularly powerful synthetic tool when paired with organocatalysis. The synergy between gold complexes and organocatalysts showcases remarkable efficiency, promoting diverse reactions from carbonyl additions to cycloadditions. This catalytic prowess not only accelerates reaction rates but also enables the synthesis of complex drug intermediates with exquisite selectivity. As pharmaceutical research advances, binary catalytic systems using both gold and organocatalysts continue to redefine synthetic strategies, paving the way for more streamlined and sustainable drug development processes.

Gold chloride

Gold(III) chloride, a compound formed by the combination of gold and chlorine, exhibits a monoclinic structure in nature. It exists in two forms: hydrated and anhydrous. Both forms are hygroscopic and light-sensitive solids. Gold(III) chloride is a Lewis acid and reacts with HCl to form HAuCl4.

It serves as a catalyst in organic synthesis, facilitating the creation of complex molecular structures essential for pharmaceutical advancements. Additionally, its antimicrobial properties contribute to research on novel antibiotics, addressing drug-resistant infections.

Furthermore, the thermal decomposition of gold(III) chloride produces aurous chloride, also known as gold(I) chloride. It has a tetragonal crystal structure and is slightly soluble in water. Gold(I) chloride serves as a catalyst with amines to conduct synergistic catalysis in the functionalization of aldehydes to form alkynyl aldehyde and the allenyl aldehyde via α-alkynylation and α-allenylation.

Gold chloride trihydrate is a crystalline compound combining gold, chlorine, and water molecules. It serves as a reagent in analytical chemistry, assisting in the identification and quantification of substances, and is also used in the synthesis of various gold compounds. Additionally, it plays a crucial role in the electroplating process, enabling the deposition of gold onto other metal surfaces. Moreover, gold(III) chloride trihydrate is used as a critical precursor for the synthesis of Au NPs using different methods. For example, HAuCl4 is used in Turkevich method to synthesize 20 nm particles. The Brust-Schiffrin method was developed to control the size and low dispersity of Au NPs using the HAuCl4 solution.

Gold Nanoparticles

Colloidal nanoparticles, known as gold nanoparticles (AuNPs), exhibit diverse surface functions along with excellent thermo-mechanical properties, a high surface area, and low toxicity. Gold nanoparticles are commonly produced in a liquid medium by reducing chloroauric acid. After dissolving the acid, it is swiftly mixed with a reducing agent. This process leads to the reduction of Au3+ ions to neutral gold atoms. As more of these gold atoms are generated, the solution becomes supersaturated, and subsequently, sub-nanometer-sized particles of gold start to precipitate.

Due to their spherical structure, large surface-to-volume ratio, and excellent biocompatibility, gold nanoparticles are widely utilized in biomedical applications, including electrochemical sensor-based diagnostics and drug delivery. They are also used to detect biomarkers in the diagnosis of heart diseases, cancers, and infectious agents. Gold nanoparticles are also common in lateral flow immunoassays, with a common household example being the home pregnancy test. Additionally, they allow conjugation with therapeutic agents due to their huge surface area-to-volume ratio. Gold nanoparticles can generate heat when exposed to light between 700 and 800 nm. This property allows them to destroy specific tumors. When light is applied to a tumor containing gold nanoparticles, they quickly heat up, killing the tumor cells. This treatment is called hyperthermia therapy.

Gold nanoparticles are also used in resonance scattering dark-field microscopy for detecting microbial cells and their metabolites, bio-imaging tumor cells, and identifying receptors on their surface. They are also utilized in the study of endocytosis. Additionally, DNA-coated gold nanoparticles are injected into plant cells and embryos to ensure the penetration and modification of genetic material, enhancing the functionality of plant plastids.

Gold Nanoparticles are used as catalysts in a variety of organic transformations. Solid-supported Au NPs can be highly active catalysts for CO oxidation and heterogeneous catalysis. They can be used for organic reactions such as oxidation/reduction and C-C coupling reactions.

Gold Nanorods

Gold nanorods (AuNRs) exhibit a rod-like structure, featuring unique optical properties with a strong absorption band in the visible spectrum. Easily tunable across various wavelengths, gold nanorods are extensively applied in biological settings for sensors, photothermal therapy, and imaging devices. Leveraging size and shape-dependent quantum effects, these nanoparticles demonstrate distinctive surface plasmon resonance absorption, scattering, fluorescence, and photothermal characteristics, making them suitable for diverse applications such as catalysis, chemical sensing, biosensing, cellular and bioimaging, drug and gene delivery, and photothermal therapy. Their fluorescent labeling enhances emission in fluorophores, transforming them into dual mode nanoprobe agents for combined drug delivery and bioimaging applications.