Aryl Diazonium Salts and Related Compounds

This first chapter gives a general overview of the surface modification by diazonium salts: the different methods that permit to attach aryl groups to surfaces, the characterization of the bond between the surface and the organic film, the mechanism of the grafting reaction, the structure of the organic film, the different groups that can functionalize the substrates, the different emerging substrates. In each case, we emphasize the most recent results and we refer the reader to the different chapters for additional information. In a final section, we describe an original method that permits, starting from a specially designed diazonium salt, grafting of a wide variety of different molecules such as, for example, organic halides, acetonitrile, methylamine. This method is based on atom abstraction and could apply to large range of molecules.

This chapter provides data on the qualitative and quantitative relationships between the structures of diazonium salts Ar(Het)N₂⁺X⁻ and their stability and safety. The effects of diazonium cation structure and the nature of the anion on stability are discussed. Examples of stable and safe triazenes as surrogates of diazonium salts are given. Unconventional methods for producing diazonium salts under low acidity conditions are also discussed.

In aqueous acid solution and in mixed alcohol-water solvents ([H₃O⁺] > 10^–2 M), in the dark and in the absence of reductants, the spontaneous decomposition of aryldiazonium, ArN₂⁺, salts proceeds through borderline S_N1 (D_N + A_N) -S_N2 mechanisms. The rate constant values depend strongly on the nature of the substituents attached to the aromatic ring of ArN₂⁺ and, for those with electron-withdrawing substituents, on solution composition. The product distribution is proportional to the composition of the solvation shell of the ipso carbon, which reflects the composition of the water/cosolvent mixture. However, upon decreasing moderately the acidity, reactions involving the formation of diazohydroxides, ArN₂OH, diazoethers, ArN₂OR, and diazoates, ArN₂O⁻, become competitive and may even be the main decomposition pathway. The stability of ArN₂OH, ArN₂OR, and ArN₂O⁻ species (which may coexist with ArN₂⁺ in solution) is intimately related to the Z-E (syn-anti, cis-trans) isomerization of the O-adducts, so that they may undergo further reactions when they are components of a Lewis acid-base equilibrium, or undergo homolytic scission to produce homolytic reduction products. In this book chapter, we aim to provide the reader with a practical and (hopefully) useful view of the complex chemistry of ArN₂⁺ in aqueous and mixed alcohol-water solutions, mainly covering the kinetics and mechanisms of the reactions. In a last section, we introduce some analytical methods for the determination of diazonium salts and their degradation products.

The chapter is dedicated to the modern trends in surface modification by iodonium salts as an alternative to the common diazonium chemistry. The iodonium salts with higher reduction potential commonly are not prone to the spontaneous reaction with surfaces that provide unique opportunities for the precise control of the grafting process. This chapter will include general information about the structure, reactivity, and preparation of iodonium salts, as well as a full overview of surface functionalization using iodonium salts.

Electroreduction of diazonium salts for the grafting of organic entities onto a wide variety of substrates has been extensively developed to generate modified surfaces for a large range of applications. The major issue with this approach lies in the lack of control of the grafting mechanism, which leads, in most of cases, to disorganized and loosely packed multilayers. Over the past decade, several strategies have been proposed to control the layer growth, and ideally stop at the monolayer stage. This chapter describes and compares the most relevant approaches, whether playing on the steric or kinetic aspects of the grafting technique.

The atomistic modeling aspects of the mechanism of diazonium salt grafting on different materials including metals, carbon-based materials, silicone and phosphorus-based materials are discussed in this chapter. These concern the dissociation, the grafting and spectroscopic signatures of diazonium salts. The aryl group is grafted favorably on different materials like metals, carbon-based nanomaterial’s, silicone and phosphorus two-dimensional materials. In all cases the adsorption was found exothermic allowing to non-covalent, covalent or ionic bonds formation. The spectroscopic analysis allows to depict the characteristic of aryl grafting concerning the stretching modes N–N and C–N and bending C–N–N shifting toward low frequencies.

After a summary of the chronology of early studies on surface modification using diazonium salt presented for all types of sp² carbonaceous materials of macro, micro, and nano size, this chapter reviews the last decade's advances in the field of the carbon nanotube functionalization via diazonium salt chemistry. The different strategies of carbon nanotube functionalization based on aryl diazonium salt are then overviewed. The key parameters determining the selectivity of the diazonium salt functionalization reaction of single-wall carbon nanotubes (SWCNT) are discussed and more particularly the importance of their Fermi level as well as the substituents of the aryl diazonium. This chapter then focuses on the optical properties, including photoluminescence and electrical properties of the nanotubes specifically provided by the surface modification of CNTs via diazonium salt, and concludes with the presentation of some applications including catalytic platforms, nanocomposites, energy conversion, and (bio) chemical sensing.

The production of graphene with controlled properties and structure is one of the most challenging aspects for a chemist. Covalent functionalization is one of the common approaches to obtain well-defined and robust modification of carbon materials. Different protocols have been proposed for carrying out this functionalization step. However, aryl diazonium salts chemistry should be highlighted due to its efficiency and simplicity. In this book chapter we focus on the modification of carbon materials with sp² hybridization (graphite and graphene) by using aryl diazonium salts. The on-surface chemistry of diazonium salts on model substrates is explored with a focus on the attempts that have been done to improve the fundamental knowledge about the aryl-carbon interface. Recent developments include control of the structure and the spatial distribution of the aryl moieties on the surface. Finally, the expansion of the protocols to bulk dispersions of graphene and the advantages for the mass production and development of applications based on this material are highlighted.

The synthesis of aryldiazonium tetrachloroaurate(III) salts [X–4–C₆H₄N≡N]AuCl₄ (X=F, Cl, Br, I, CN, NO₂, COOH, bisaniline, triazine-based dendrimers, C₈F₁₇, C₆H₁₃) is reported by the protonation of anilines with chloroauric acid in acetonitrile followed by one-electron oxidation using nitrosonium salt [NO]X (X=tetrafluoroborate, hexafluorophosphate). X-ray crystal structure of X=CN, NO₂, COOH, C₈F₁₇, C₆H₁₃ from acetonitrile or water (X=COOH) displayed [–N≡N]⁺ bond distance typical of a triple bond. Electrochemical reduction of the salts showed low or even positive potential values versus silver/silver chloride reference electrode. The robust gold-aryl nanoparticles, termed organometallic nanoparticles, were constructed using green and mild chemical reduction routes of the diazonium gold(III) salts for forensic, environmental, and nanomedicine engineering applications. Specifically, the salts were used in, for example, the development of latent fingerprints on copper and nickel coins, formation of gold-silver alloys, gold-aryl core-tine oxide shell structures, formation of protein and amino acid bioconjugates, and electrochemical synthesis of gold-aryl nanoparticles stabilized with polyaniline.

The surface engineering of polymers is still in development in order to modulate their properties to meet the requirements of the targeted applications. Lignocellulosic and agro-waste materials are increasingly studied as alternative to conventional ones employed for pollution remediation but also to develop original composites. In both cases, the surface of the material could be modified by coating or by covalent grafting. The diazonium chemistry has been successfully employed to modify the surface of carbon and metallic materials but its application to organic surfaces is still incipient. This chapter provides an overview of the recent developments in diazonium chemistry to modify polymer and biomass surfaces and its application in biosensor design, catalysis, and pollution removal.

This chapter provides an overview of the various surface modification strategies based on the use of aryl diazonium salts for the functionalization of plasmonic nanoparticles. Aryl diazonium salts appear as a valuable alternative to commonly used thiol self-assembled monolayers due to several advantages, including the formation of robust interfacial metal-surface bonds, simple synthesis, and surface grafting protocols, a large range of functional groups, and the possibility to form multilayers around the nanoparticle surface. Owing to these outstanding features, the combination of aryl diazonium salts and plasmonic nanoparticles is receiving increasing attention for potential applications in various fields such as nanosensors, bioimaging, environment, forensic science, and antimicrobial materials. This chapter describes the chemical processes involved in the surface modification of plasmonic nanoparticles by diazonium salts, the resulting nanohybrids, and the targeted applications.

This chapter describes the advantages of diazonium electroreduction for generating molecular junctions and the main electronic functions obtained using such systems. It is an updated version of a review article entitled “Electrochemistry Does the Impossible: Robust and Reliable Large Area Molecular Junction” published in “Current Opinion in Electrochemistry” in 2018 [1]. Part of this chapter is reproduced with permission from Ref. (Lacroix in Curr Opin Electrochem 7:153–160, 2018) Copyright 2018 Elsevier.

Since their first report in 2012, calix[4]arene tetradiazonium derivatives have experienced a growing interest. They now represent a favored method to obtain robust post-functionalizable monolayers with controlled composition on a wealth of surfaces (conductive, semi-conductive, or insulating, as well on large surfaces as on nanomaterials). These compounds are easily synthesized and handled and, so far, have been used to functionalize surfaces for applications in (bio)sensing, catalysis, as well as for the development of hydrophobic or antifouling coatings. This chapter describes the current synthetic methods, applications, and limitations of these polydiazonium salts and discusses the potential of the field.

Diazonium chemistry has been widely adopted for numerous biomedical applications such as implant materials, tissue engineering scaffolds, and drug delivery. The versatility of substrate materials that can be modified by diazonium chemistry and the wide range of functional groups that can be introduced on the surface are among the advantages of diazonium chemistry over other surface modification techniques. Indeed, it provides a versatile tool to change the surface properties of biomaterials to control their wettability, biocompatibility, protein adsorption, and immune response. Diazonium chemistry is also useful as a linker to attach biomolecules or polymeric layer on biomaterial surfaces. In this chapter, we provide an up-to-date review of the use of diazonium chemistry in biomaterials used in biomedical applications.

We summarize the existing knowledge on the use of diazonium salts as a new generation of surface modifier and coupling agents to functionalize substrates and nanostructures for applications in photo/catalysis. More specifically, we tackle arylation of carbon allotropes to immobilize organometallic nanocatalyst and monometallic and bimetallic nanoparticles. We also discuss the role of arylation or arylation in situ polymerization sequential steps to tune and enhance the photocatalytic performances of TiO₂ (active under UV) and RuO₂-TiO₂ mixed oxide photocatalysts (active under sunlight). This chapter clearly stresses the important role of diazonium salts in controlling the interfacial and catalytic performances of heterogeneous and immobilized catalysts.

This chapter discusses the use of aryl diazonium salts in the activation of cationic and free radical polymerizations which can be triggered by specific responses to light, with possible combination of additives. Diazonium salts can be activated with additives in a thermodynamically favorable way owing to their ease of reduction. The current literature clearly indicates that these salts would find wider application in the near future in polymer science.

Polymer adhesion to modified surfaces is a hot, ever-progressing topic. It concerns the attachment of prefabricated polymers or polymers grown by in-situ polymerization. Herein, we summarize the recent progress achieved in 2012–2021 in polymer adhesion to arylated surfaces bearing functional groups able to initiate polymerization or to favorably interact with prefabricated or precipitating (pre)polymers. We focus first on radical polymerization techniques with emphasis on UV or sunlight-triggered polymerization processes. The latter enables obtaining patterned polymer coatings. We also discuss the design of imprinted polymer and antibacterial coatings grown on arylated flat, flexible, or particulate surfaces, as well the making of adhesive layers of vinylic polymers bearing redox groups in their repeat units. Diazonium salts permit also attach “clickable” groups for click polymerization or for making layered sol-gel coatings. In a second important section, we demonstrate that diazonium salts are unique coupling agents for obtaining adhesive conjugated polymer layers of major importance in the design of flexible electrochemical sensors or in the development of electronic materials. The Chapter finishes with eye-catchy new trends in surface-confined polymerization such as plasmon-triggered nitroxide mediated polymerization, and the design of covalently bonded biopolymer to alloy surface for corrosion control applications in simulated Dead Sea water.

In this chapter, the utilization of diazonium chemistry for surface modification of plasmon-active surface(s) and the creation of analyte-targeted SERS substrates is discussed. Such substrates are able to selectively entrap and recognize the targeted analyte from solution by SERS measurement, ensuring its detection at very low concentrations even at dominant background of interfering molecules. The stability of diazonium-created coatings allows performing several steps of plasmonic surface modification and their subsequent utilization in various, often challenging conditions. In the first part of the chapter, the main principle of SERS method and functional SERS substrates is described. In the second part, the preparation and diazonium-functionalization of plasmon-active grating surface aimed at selective SERS detection are discussed. In the last part of the chapter, several examples of diazonium chemistry utilization for SERS-based recognition of enantiomers, environmental contaminants, dangerous compounds, and disease markers are given.

Aryldiazonium salt chemistry, as a versatile surface decoration and functionalization method, has been intensively employed in construction of sensing interface. The para-position (–R) of aryldiazonium salt bearing different functional moieties endows the sensing surface with different purposes. Typically, bioreceptor linkage groups (e.g. –COOH, –NH₂) are essential, and inert groups (e.g. –H, –CH₃)-based spacer and antifouling moieties can be incorporated according to the actual requirement. Introduction of molecular wires, or nanomaterials (e.g. carbon based or noble metal based) can further enhance signal and/or electron transfer helping to increase the sensitivity. This chapter focuses on the design of the sensing interface by applying the aryldiazonium salt surface chemistry with the format of forming single/mixed aryldiazonium salt layers, and their translations into biosensing of different target analytes.

This chapter summarizes the knowledge of filler modification with diazonium salts, with a view to designing high-performance reinforced polymer composites. Arylation of fillers serves for attaching prepolymers or for triggering polymerization processes. The level of modification is molecular or macromolecular in nature. Both arylated and macromole-modified fillers served to design reinforced polymer composites with unique mechanical and dielectric properties. One important feature is the rheology of formulations prior to cure. Interfacial chemical compositions could be investigated with surface-specific techniques such as XPS and ToF–SIMS. Handpicked cases concern the design of reinforced polymers with silica, zeolite, clays, and carbon allotropes. We demonstrate that fine control over the surface chemical composition of fillers has profound effect on mechanical, thermal, viscoelastic, and electrical properties of end polymer composites.

This chapter will provide insight into the use of aryl diazonium salts to functionalize carbon fiber. Examining the production of carbon fiber and its current industry applications. The methods of analysis of functionalized carbon fiber; mechanical tests to evaluate the structural integrity and strength of the fiber will also be covered. Moreover, the chemical analysis that can be used to confirm successful surface grafting has occurred. The literature reports of aryl diazonium salt grafting to carbon fiber over the past decade are explored, examining approaches of functionalization and the increased diversity of applications these chemistries provide.

Chemical grafting of organic molecules by the diazonium chemistry on various materials used in electrochemical technologies has been shown to lead to significant improvement of the electrochemical performance of lithium-ion batteries, electrochemical capacitors, biofuel cells, and microbial fuel cells. For all these applications, the search of new materials is going at a very fast pace with the aim to increase their performance and increase the likelihood of applications in practical devices. The chemical modification of materials may not be considered to lead to real new materials. Nonetheless, appropriate surface modification may give rise to materials with improved stability that could reach a level that would be sufficient to find a specific application.

In this chapter, we will show how it is possible to modify the properties of material surfaces, thanks to the chemistry of aromatic diazonium salts applied to their functionalization. Overall, it is sufficient to play with the fragility of the C–N bond between the aromatic and the diazonium (–N₂⁺) group. This bond breaks during a reduction reaction and leads to the formation of an extremely reactive aryl radical, which can react with a surface and form a covalently grafted coating by successive additions or initiate a radical polymerization reaction. This diazonium salt chemistry is simple, inexpensive, works in water and does not require energy input. It can be adapted to many substrates and is thus particularly suitable for industrial developments. We will see through the three following examples how to take advantage of these reactions to confer new properties to the surface of materials as varied as those encountered to biocompatibilize surgical implants (example 1), to make a robust optical sensor (example 2) or to treat light alloys for aeronautics (example 3).