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Boronate Synthesis Essay

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1. Introduction

Boronic acids (R-B(OH)2) are a family of molecules which present a wide range of applications [1]. Their main use is as reagents in the Miyaura-Suzuki coupling reaction (2010 Nobel Prize), for the formation of C-C bonds. Their reactivity is also of great interest for other fields [2,3,4,5,6], like in organocatalysis [2] and for the development of sensors (detection of carbohydrates, H2O2, fluoride anions, etc.) [3]. It has also been shown that some boronic acids can play the role of enzyme inhibitors [6], as is the case for bortezomib (Velcade®), an anticancer molecule used for the treatment of multiple myeloma (Figure 1).

Benzoxaboroles are cyclic derivatives of boronic acids [7]. While the first synthesis of a benzoxaborole dates back to 1957, it is only in 2006 that the research community regained interest in these molecules, due to their capacity to bind to sugars at physiological pH, moreover with a higher affinity than the corresponding boronic acids [8]. This property has since been widely exploited, notably in the biomedical field, where a variety of benzoxaborole-based drugs have been developed, with antifungal, anti-inflammatory, antimicrobial, antiprotozoal, antiviral or anticancer properties. In 2014, the first benzoxaborole-based drug, tavaborole (AN2690—Figure 1), received FDA approval for the treatment of onychomycosis [9].

In addition to applications in molecular chemistry, boronic acids and benzoxaboroles are also increasingly used in materials chemistry [1,7,10,11]. In most cases, the organoboron function is exposed at the periphery of the material so that its intrinsic reactivity can be exploited to provide a given property to the material (e.g., capacity to bind to diols). This is for example the case for polymers developed for the detection of dopamine [12], for chromatographic columns designed for the separation of sugars [11,13], and for mesoporous silica nanoparticles allowing the controlled release of insulin [14]. Another possibility which has been looked into consists of using the boronic acids themselves as building blocks for the preparation of functional (nano)materials. For example, covalent organic frameworks (COFs) have been synthesized from boronic acid precursors, in view of gas storage and molecular electronics applications [10,15].

In contrast to boronic acids and benzoxaboroles, their anionic counterparts, boronates and benzoxaborolates, had up until recently hardly been looked into as possible building blocks for materials applications. This is surprising considering the large number of studies involving other organic oxo-anions like phosphonates (R-PO2(OH), R-PO32−) and carboxylates (R-COO) which can be found in the literature, where they serve as building blocks for the preparation of coordination networks or Metal Organic Frameworks (MOFs), and more generally hybrid organic-inorganic materials. The purpose of this article is thus to highlight the work performed over the past five years regarding the preparation of the first materials based on boronate and benzoxaborolate units, in which the boron is in a tetrahedral configuration (Figure 2).

In the first part, the intrinsic coordination properties of boronate and benzoxaborolate anions will be described, by looking at the crystal structures of phases prepared starting from very simple boronic acids and benzoxaboroles. In the second part, the spectroscopic signatures of boronates and benzoxaborolates in these materials will be highlighted, as they are of importance for the study of more complex hybrid organic-inorganic materials, for which, in some cases, no crystal structure is available. Finally, in the third part, the importance of boronates/benzoxaborolates for the preparation of new families of materials will be underscored, and emerging applications of (nano)materials involving these anions will be highlighted.

2. Crystal Structures Involving Simple Boronates and Benzoxaborolates

2.1. Acid-Base Properties of Boronic Acids and Benzoxaboroles

In water, boronic acids and benzoxaboroles behave as Lewis acids. Their conjugate bases are the boronate (also called tri-hydroxyborate) and benzoxaborolate anions, in which the boron is in a tetrahedral environment (Figure 2). The tetrahedral nature of the boronate anion was elucidated in 1959 [16], but it is only in 2006 that a crystal structure involving such a boronate anion was described for the first time [17].

The pKa of the boronic acid/boronate couple is generally >8, and depends on the nature of the organic chain linked to the boron [1]. For phenylboronic acid (C6H5-B(OH)2), the pKa is ~8.9 (and thus similar to that of boric acid B(OH)3), while for methylboronic acid it is ~ 10.4 [1,18]. The pKa of the benzoxaborole/benzoxaborolate couple is ~7.3 due to the cyclic form of the organoboron function. For benzoxaboroles, pKa values vary depending on the nature of the substituents on the aromatic ring, as well as on the methylene group [19]. It is worth noting that the pKa values of boronic acids and their derivatives are globally higher compared to those of other organic acids such as carboxylic and phosphonic (Table 1).

In boronates and benzoxaborolates, the negative charge is shared between the three oxygen atoms. This is what makes these anions attractive for coordination chemistry applications, as they can potentially play the role of tridentate ligands with respect to metal cations. Examples along this line are provided in the following sub-sections.

2.2. Boronate-Based Crystal Structures with Alkaline-Earth Metals

A series of investigations aiming at describing the intrinsic coordination properties of the R-B(OH)3 anions in materials have been reported [22,23,24]. Boronic acids bearing non-coordinating R- chains were used for this purpose (R- = C6H5-, C4H9-, C8H17-), in order to ensure that the materials would form only based on the metal-ligand interactions. Reactions were carried out in water (or in water-ethanol mixtures), by adding a solution of a metal cation into a solution of the boronate (Figure 3), in order to precipitate a crystalline phase (e.g., a coordination polymer). Due to the high pKa values of the boronic acid/boronate couples, alkaline-earth metal cations were selected for the precipitation. Indeed, these ions should not lead to the competitive formation of metal-hydroxide precipitates during the reaction, in contrast with most other metal ions [20].

Materials were isolated in most cases as microcrystalline powders [22,23,24]. Scanning electron microscopy (SEM) analyses showed that in many cases, the crystallites had a sheet-like morphology, as illustrated in Figure 4 for the Sr-phenylboronate monohydrate and Sr-butylboronate phases. Structure determination was made possible by (i) recording X-ray diffraction powder patterns using synchrotron radiation; (ii) performing a series of additional characterization (notably by IR and solid state NMR); (iii) modeling the structures computationally, notably to position the hydrogen atoms and to compute the NMR parameters. All materials were found to present a layered arrangement. More specifically, they were found to be made of planes of metal cations interconnected by boronate ligands, with the organic chains facing each other in the interlayer space (Figure 4).

Based on the different crystal structures solved, the coordination modes of the boronates could be determined. These anions were found to play the role of bridging ligands, by coordination to two to three distinct alkaline-earth metals (Figure 5). This demonstrates that boronates, in which the boron atom is in a tetrahedral geometry, can serve as building blocks for the construction of coordination networks. Another possibility, not developed in this review but which further underscores the versatility of boronate ligands, concerns the use of the deprotonated boronate forms (in which the boron is in a planar environment), to construct hybrid organic-inorganic materials [26].

2.3. Benzoxaborolate-Based Crystal Structures with Alkaline-Earth Metals

The synthetic strategy used in the case of benzoxaborolates is the same as for the boronates. The simplest benzoxaborolate (Figure 2b) was precipitated using an aqueous solution of alkaline earth metal, the reaction being carried out at room temperature, under reflux, or using hydrothermal conditions (Figure 6) [27]. In the vast majority of cases, microcrystalline precipitates were obtained, except for two phases, for which single crystals suitable for X-ray diffraction analyses were isolated.

To date, the structures of three phases have been solved: two involving Mg2+ and one Ca2+ [27]. Although these structures have an overall layered organization, as shown by powder X-ray diffraction analyses, the benzoxaborolates here play two different roles: either they bridge different metal cations, or they simply serve as counter-anions, as illustrated in Figure 7 for the Mg-benzoxaborolate decahydrate phase.

Based on the crystal structures solved, several coordination modes to Mg2+ and Ca2+ have been identified for benzoxaborolates (Figure 8). Clearly, a very large diversity of binding modes is present, the benzoxaborolate anion being bound to 1 to 3 different metal cations, with both the OH groups and the oxygen atom of the cycle serving as binding sites. Interestingly, in the case of the Ca phase, a deprotonated anion was also observed (Figure 8