**1. Introduction**

Aryldiazonium, ArN+2 , salts have been profusely employed in organic chemistry for years [1–3]. The chemistry of these compounds has been originally carried out in aqueous solutions [4], but chemists went further and developed new synthetic methods that use reactions of diazonium salts in organic solvents [5–8]. In recent years, their chemistry has been exploited in the synthesis of unnatural amino acids [5], inpalladium-catalyzed cross-coupling reactions [7,9,10] and, for example, as nitrogen-based Lewis acids [6,11].

Aryldiazonium ions are inherently reactive molecules that undergo a wide variety of chemical transformations that can be carried out under milder conditions, often at ambient temperature and pH levels [1,2,5,12–14]. Their reactivity is frequently dominated by the loss of the N2 moiety, leading to the formation of heterolytic products, such as phenols, haloderivatives, and ethers. Scheme 1 shows some of the most common reactions of aryldiazonium ions, including the formation of diazoethers (O-coupling), azo dye (Ccoupling) reactions, and their attachments to surfaces and nanoparticles.

In the last decades, after the pioneering work by J. Pinson et al. [15], the reductive properties of aryldiazonium salts have been exploited as a very potent method for surface functionalization [16–20]. The method is relatively simple, easy to process, is fast, and can be employed to functionalize massive surfaces that are either flat, or are nanomaterials of various shapes and sizes [17]. The formed surfaces are robust and resistant to heat, chemical degradation, ultrasonication, and, most importantly, the grafting of surfaces is a useful and practical approach that can be applied to a variety of conducting and insulating substrates.

**Citation:** Losada-Barreiro, S.; Bravo-Díaz, C. Effects of Electrolytes on the Dediazoniation of Aryldiazonium Ions in Acidic MeOH/H2O Mixtures. *Compounds* **2022**, *2*, 54–67. https://doi.org/ 10.3390/compounds2010005

Academic Editors: Nuno Basílio and Maria Lurdes Santos Cristiano

Received: 12 December 2021 Accepted: 9 February 2022 Published: 15 February 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Briefly, the grafting procedure comprises the formation of highly reactive aryl radicals that attack the surface and that are generated, for example, through the electrochemical reduction of ArN+2 . Variations of the method include the reaction of generated radicals with already-grafted aryl species, leading to the formation of multilayer films [17,20]. Scheme 1B illustrates the use of ArN+2for the grafting of both spherical nanoparticles and flat surfaces.

**Scheme 1.** (**A**) Some common reactions of aryldiazonium ions in solution. (**B**) Illustrative examples of the use of aryldiazonium ions as reagents to functionalize surfaces.

Aryldiazonium ions can be easily prepared from their anilines both in situ and in the solid state [1,21], and most of them can be easily handled under ambient conditions. Nevertheless, the stability of ArN+2 depends strongly on the nature of substituents R and counterions X<sup>−</sup>. For example, aryldiazonium chlorides are highly unstable and can be explosive above 0 ◦C, as well as aryldiazonium perchlorates [12]. Nowadays, most synthetically prepared aryldiazonium salts are formulated as tetrafluoroborates because of the high stability and availability of the BF<sup>−</sup>4 anion [3,12].

In an aqueous solution, in the dark, and in the absence of reducing species, ArN+2 undergoes a rich chemistry, comprising aromatic *ipso*-substitutions, *N*-terminal addition reactions, and O-coupling reactions, among others. However, the fate of the reactions strongly depends on the acidity of the solution. For instance, we showed, in previous dediazoniation studies [2,14,22–24] that, in an aqueous acid solution and in mixed alcohol–water solvents ([H3O+] > 10−<sup>2</sup> M), in the dark and in the absence of reductants, the spontaneous decomposition of aryldiazonium ArN+2 salts proceeds through a SN1-type heterolytic mechanism, where a highly reactive aryl cation is formed, reacting with nucleophiles present in their solvation shell (dissociation + addition mechanism, DN + AN, Scheme 2). Recent dediazoniation research suggests, however, that ArN+2 may decompose heterolytically through borderline SN1–SN2 mechanisms. Surprisingly, upon moderately decreasing the acidity, reactions involving the formation of diazohydroxides, ArN2OH, diazoethers, ArN2OR, and diazoates, ArN2O<sup>−</sup>, become competitive and may even constitute the main decomposition pathway [2].

In this work, we analyze the effects of the solvent composition and of added electrolytes (NaCl, LiCl, and LiClO4) on the esolvolytic dediazoniation of 2-, 3-, and 4-methylbenzenediazonium ions (2MBD, 3MBD, and 4MBD, respectively) in acidic MeOH/H2O mixtures (20% and 99.5%). The aim of the manuscript is two-fold: (1) to complement previous kinetic studies that were focused on the effects of electrolytes on short-lived

carbocations, particularly on the effects of the solvent composition on the solvation shell of the *ipso* carbon of aryldiazonium ions, and (2) to investigate if the addition of electrolytes may modify the composition of the solvation shell of aryl carbocations.

**Scheme 2.** Illustrative representations of the (**A**) ionic or heterolytic SN1 (DN + AN) dediazoniation mechanism in the presence of various nucleophiles comprising the formation of an ion–molecule pair that traps (with very low selectivity) nucleophiles present in its solvation shell. (**B**) SN2 dediazoniation mechanism. In both cases, heterolytic products are formed.

For this purpose, we determined the dediazoniation rate constants for ArN+2 loss and for product formation, as well as the product distribution at various solvent compositions. Reported thermodynamic and kinetic data, obtained upon the solvolysis of ArN+2 in a number of alcohol–water mixtures (MeOH, EtOH, 2,2,2-trifluoroethanol, BuOH), are consistent with the SN1 mechanism; that is, the rate-determining formation of a highly reactive aryl cation that traps the nucleophiles in its solvation shell [14,22–29]. The major dediazoniation products are the corresponding methylphenyl ethers and cresols, and the equal amounts of products that are produced at water molar fractions of 0.34–0.36, suggesting that the aryldiazonium ions undergo preferential solvation by water, and that the preferential solvation around the *ipso* carbon reflects the experimental product yield obtained [2,14,22].

The rates of the reactions of stable carbocations and aryldiazonium ions with water, alcohols, and anions have been described in terms of the Ritchie's equation log(*k*/*k*0) = *N*+, based on the assumption that the relative reactivities of two nucleophiles are controlled by the differences in their *N*+ values, which are considered independent of the electrophilicities of the reaction partners. Later, Mayr et al. [30–32] demonstrated that the rates of these reactions can be described in terms of their electrophilicity and nucleophilicity parameters, log(*k*) = *s*(*E* + *N*), where s is a nucleophile-specific parameter. A theoretical interpretation of the physical meaning of s has been published and a comprehensive list of N and E parameters can be found elsewhere [33]. The development of nucleophilicity scales has intrigued chemists for years, and Grunwald and Winstein proposed a relationship for the solvolyses of SN1 reactions on the basis of the solvent-ionizing power *Y* and a substratespecific parameter *m* whose value was one for *t*-butyl chloride [34–36], log (*k*/*k*W) = *mY*. This equation holds for SN1 reactions, where the nucleophilic participation of the solvent in the rate-determining step is negligible [2,35].

In this work, we are particularly interested in analyzing the effects of ClO<sup>−</sup>4 ions on solvolytic dediazoniations. Aryldiazonium perchlorates are particularly effective for the Heck palladium-catalyzed arylation of olefins when reactions are carried out in alcohol– water mixtures because of the high yields obtained and because they are cheaper reagents than those with other counterions [37]. However, cautions must be taken, because some aryl diazonium perchlorates have been reported to be explosive when prepared in the solid state [38,39]. Conversely, they can be conveniently generated in situ [37]. Perchlorate is a commercially available anion that forms salts with many cations (NH<sup>+</sup>4 , Li+, Na+, K+, etc.). Probably the most common form of perchlorate includes ammonium perchlorate (frequently used as a solid rocket oxidant and as ignition source in fireworks) and potassium perchlorate (used in road flares and in air bag inflation systems). Other commercial perchlorate counterions include H+, Li+, Na+, K+, NH+4 , Al3+, and N2H<sup>+</sup>5 . Perchlorate is also formed in laboratory waste as a byproduct of perchloric acid. It is a very poor complexing

agent, similar to other weak anions, such as tetrafluoroborate or trifluoromethanesulfonates (triflate, CF3SO− 3 ), making it very useful in metal cation chemistry [40]. The high solubility in both aqueous and non-aqueous media, together with the highly delocalized monovalent charge over the four oxygen atoms and its large volume, allows it to be widely used to adjust ionic strength in kinetic experiments. From the thermodynamic point of view, perchlorates are expected to be powerful oxidizers (*E*<sup>0</sup> = −1.229 V), but the stability of ClO− 4 in solution is governed by kinetics and not thermodynamics, and, therefore, they do not easily oxidize [40,41].

#### **2. Materials and Methods**
