Coordination Networks Based on Boronate and ... - MDPI

crystals Review

Coordination Networks Based on Boronate and Benzoxaborolate Ligands Saad Sene 1 , Marie Alix Pizzoccaro 1 , Joris Vezzani 1 , Marc Reinholdt 1 , Philippe Gaveau 1 , Dorothée Berthomieu 1 , Sylvie Bégu 1 , Christel Gervais 2 , Christian Bonhomme 2 , Guillaume Renaudin 3,4 , Adel Mesbah 5 , Arie van der Lee 6 , Mark E. Smith 7,8 and Danielle Laurencin 1, * 1

2

3 4 5 6 7 8

*

Institut Charles Gerhardt de Montpellier, UMR 5253, CNRS UM ENSCM, Montpellier 34095, France; [email protected] (S.S.); [email protected] (M.A.P.); [email protected] (J.V.); [email protected] (M.R.); [email protected] (P.G.); [email protected] (D.B.); [email protected] (S.B.) Laboratoire de Chimie de la Matière Condensée de Paris, UMR 7574, Sorbonne Universités, UPMC Univ Paris 06, CNRS, Collège de France, Paris 75252, France; [email protected] (C.G.); [email protected] (C.B.) Université Clermont Auvergne, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000 Clermont-Ferrand, France; [email protected] CNRS, UMR 6296, ICCF, F-63178 Aubière, France Institut de Chimie Séparative de Marcoule, UMR 5257, CNRS CEA UM ENSCM, Bagnols-sur-Cèze 30207, France; [email protected] Institut Européen des Membranes, CNRS-UMR 5635, UM, Montpellier 34095, France; [email protected] Vice-Chancellor’s Office, University House, Lancaster University, Lancaster LA1 4YW, UK; [email protected] Magnetic Resonance Centre, Department of Physics, University of Warwick, Coventry CV4 7HS, UK Correspondence: [email protected]; Tel.: +33-4-6714-3802

Academic Editors: Umit B. Demirci, Philippe Miele and Pascal G. Yot Received: 29 March 2016; Accepted: 27 April 2016; Published: 2 May 2016

Abstract: Despite the extensive range of investigations on boronic acids (R-B(OH)2 ), some aspects of their reactivity still need to be explored. This is the case for the coordination chemistry of boronate anions (R-B(OH)3 ´ ), which has only recently been started to be studied. The purpose of this review is to summarize some of the key features of boronate ligands (and of their cyclic derivatives, benzoxaborolates) in materials: (i) coordination properties; (ii) spectroscopic signatures; and (iii) emerging applications. Keywords: boronate; tri-hydroxyborate; benzoxaborolate; boronic acid; benzoxaborole; coordination polymer; solid state NMR; IR spectroscopy; crystallography; DFT

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 [2–6], like in organocatalysis [2] and for the development of sensors (detection of carbohydrates, H2 O2 , 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 Crystals 2016, 6, 48; doi:10.3390/cryst6050048

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Benzoxaboroles are cyclic derivatives of boronic acids [7]. While the first synthesis of a

these molecules, due dates to their to2006 sugars pH, moreover with in a higher benzoxaborole backcapacity to 1957, ittois bind only in that at thephysiological research community regained interest these molecules, due to their boronic capacity toacids bind to sugars physiological moreover a higher affinity than the corresponding [8]. Thisat property haspH, since been with widely exploited, than the corresponding boronic acidsof [8].benzoxaborole-based This property has sincedrugs been widely exploited, notably affinity in the biomedical field, where a variety have been developed, 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. with antifungal, anti-inflammatory, antimicrobial, antiprotozoal, antiviral or anticancer properties. In 2014, the first benzoxaborole-based drug, tavaborole (AN2690—Figure 1), received FDA approval In 2014, the first benzoxaborole-based drug, tavaborole (AN2690—Figure 1), received FDA approval for the treatment of onychomycosis [9].[9]. for the treatment of onychomycosis

bortezomib

tavaborole (AN2690) OH B O F

Figure 1.Figure Representation of ofthe two organoboron commercialized for their 1. Representation thestructures structures ofoftwo organoboron drugs drugs commercialized for their anticancer (left) andantifungal antifungal (right) anti-cancer (left) and (right)properties. properties. In addition to applications in molecular chemistry, boronic acids and benzoxaboroles are also

In addition to used applications in molecular chemistry, acids benzoxaboroles increasingly in materials chemistry [1,7,10,11]. In boronic most cases, the and organoboron function isare 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 property to theofmaterial (e.g., capacity to diols). This is for the casetofor exposedgiven at the periphery the material so thattoitsbind intrinsic reactivity canexample be exploited provide for the detection of dopamine [12], for designed a given polymers propertydeveloped to the material (e.g., capacity to bind to chromatographic diols). This is columns for example theforcase for the separation of sugars [11,13], and for mesoporous silica nanoparticles allowing the controlled polymers developed for the detection of dopamine [12], for chromatographic columns designed for the release of insulin [14]. Another possibility which has been looked into consists of using the boronic separation of sugars [11,13], and for mesoporous silica nanoparticles allowing the controlled release acids themselves as building blocks for the preparation of functional (nano)materials. For example, of insulin [14]. Another possibility which been lookedfrom intoboronic consists using theinboronic covalent organic frameworks (COFs) havehas been synthesized acidofprecursors, view of acids themselves as building blocks for the preparation of [10,15]. functional (nano)materials. For example, covalent gas storage and molecular electronics applications In contrast to boronic benzoxaboroles, their anionic counterparts,inboronates andstorage organic frameworks (COFs) have acids been and synthesized from boronic acid precursors, view of gas benzoxaborolates, had up until recently hardly been looked into as possible building blocks for and molecular electronics applications [10,15]. materials applications. This is surprising considering the large number of studies involving other In contrast to boronic acids and benzoxaboroles, their anionic counterparts, boronates and organic oxo-anions like phosphonates (R-PO2(OH)−,R-PO32−) and carboxylates (R-COO−) which can benzoxaborolates, had up until recently hardly been looked into possible of building blocks for be found in the literature, where they serve as building blocks for theas preparation coordination materials applications. is surprising considering large number of studies involving other networks or MetalThis Organic Frameworks (MOFs), andthe more generally hybrid organic-inorganic ´ , R-PO ´ ) which 2´ )performed Thelike purpose of this article(R-PO is thus 2to(OH) highlight the work over the past five years organic materials. oxo-anions phosphonates and carboxylates (R-COO 3 regarding theliterature, preparation of thethey first serve materials based on blocks boronatefor and in can be found in the where as building thebenzoxaborolate preparation ofunits, coordination which the boron is in a tetrahedral configuration (Figure 2). networks or Metal Organic Frameworks (MOFs), and more generally hybrid organic-inorganic In the first part, the intrinsic coordination properties of boronate and benzoxaborolate anions materials. The purpose of this article is thus to highlight the work performed over the past five years will be described, by looking at the crystal structures of phases prepared starting from very simple regarding the preparation of the first materials based onthe boronate and benzoxaborolate units,and in which boronic acids and benzoxaboroles. In the second part, spectroscopic signatures of boronates the boron is in a tetrahedral configuration (Figure 2). benzoxaborolates in these materials will be highlighted, as they are of importance for the study of more complex organic-inorganic materials, for which, some cases, crystal structure is anions In the first part, hybrid the intrinsic coordination properties of in boronate andnobenzoxaborolate available. Finally, in the third part, the importance of boronates/benzoxaborolates for thefrom preparation will be described, by looking at the crystal structures of phases prepared starting very simple of new families of materials will be underscored, and emerging applications of (nano)materials boronic acids and benzoxaboroles. In the second part, the spectroscopic signatures of boronates and involving these anions will be highlighted. 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.

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boronic acid

benzoxaborole

boronate

benzoxaborolate

(a)

(b)

Figure 2. Representation of the structures of boronic acids/boronates (a), and benzoxaboroles/

Figure 2. Representation of the structures of boronic acids/boronates (a); and benzoxaboroles/ benzoxaborolates (b). Only the tetrahedral-boron forms of boronate/benzoxaborolate anions are benzoxaborolates (b). Only the tetrahedral-boron forms of boronate/benzoxaborolate anions are shown here. shown here. 2. Crystal Structures Involving Simple Boronates and Benzoxaborolates

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 2.1. Acid-Base Properties of Boronic Acids and Benzoxaboroles boronate (also called tri-hydroxyborate) and benzoxaborolate anions, in which the boron is in a In water, boronic acids and 2). benzoxaboroles behave as Lewis acids. conjugate tetrahedral environment (Figure The tetrahedral nature of the boronate anion Their was elucidated in bases are the boronate (also called tri-hydroxyborate) and benzoxaborolate anions, in which the 1959 [16], but it is only in 2006 that a crystal structure involving such a boronate anion was describedboron is in for the firstenvironment time [17]. a tetrahedral (Figure 2). The tetrahedral nature of the boronate anion was elucidated The pK of is the boronic acid/boronate couple is generally > 8, and such depends on the nature the described in 1959 [16], buta it only in 2006 that a crystal structure involving a boronate anionofwas organic chain linked to the boron [1]. For phenylboronic acid (C6H5-B(OH)2), the pKa is ~ 8.9 (and thus for the first time [17]. similar to that of boric acid B(OH)3), while for methylboronic acid it is ~ 10.4 [1,18]. The pKa of the The pKa of the boronic acid/boronate >8, depends onfunction. the nature of the benzoxaborole/benzoxaborolate couple is ~7.3couple due to is thegenerally cyclic form of and the organoboron organic chain linked to the boron [1]. For phenylboronic acid (C H -B(OH) ), the pK a is ~8.9 (and thus 6 substituents 5 2 on the aromatic For benzoxaboroles, pKa values vary depending on the nature of the similar B(OH)group for methylboronic ita is ~ 10.4 [1,18]. acids The pKa of the ring,toasthat wellof asboric on theacid methylene [19]. It is worth noting that acid the pK values of boronic 3 ), while and their derivatives are globallycouple higher compared to those of other organic suchorganoboron as carboxylic function. benzoxaborole/benzoxaborolate is ~7.3 due to the cyclic formacids of the and phosphonic (Table For benzoxaboroles, pK 1). values vary depending on the nature of the substituents on the aromatic ring, a

as well as on the methylene group [19]. It is worth noting that the pKa values of boronic acids and Table 1. pKa values (in water, at 25 °C) of simple boronic acids/benzoxaboroles, in comparison to other their derivatives are globally higher compared to those of other organic acids such as carboxylic and organic acids. phosphonic (Table 1). Acid-Base Couple pKa Reference C6H5-B(OH)2/C6H5-B(OH)3− ~8.9 [1] Table 1. pKCH (in water, at 25 ˝ C) of simple in comparison to other a values 3-B(OH)2/CH3-B(OH)3− 10.4 boronic acids/benzoxaboroles, [18] organic acids. − 1 C7H6BO(OH)/C7H6BO(OH)2 ~7.3 [19] CH3COOH/CH3COO− 4.8 [20] pKa [21] Reference CH3-PO(OH)2/CHAcid-Base 3-PO2(OH)- Couple 2.2 −/CH3-PO32− CH3-PO2(OH) 7.5 ´ C H -B(OH) /C H -B(OH) ~8.9 [21] [1] 1

6

5

2

3

2

6

5

3

This entryCH corresponds the benzoxaborole/benzoxaborolate -B(OH)to/CH -B(OH) ´ 10.4 couple (Figure [18] 2b). 3

3

´1

[19] the three oxygen C7benzoxaborolates, H6 BO(OH)/C7 H6the BO(OH) 2 In boronates and negative charge ~7.3 is shared between ´ CHthese 4.8 chemistry[20] 3 COOH/CH 3 COO for coordination atoms. This is what makes anions attractive applications, as they ´ CHrole -PO2 (OH) 2.2metal cations.[21] 3 -PO(OH) 2 /CH3ligands can potentially play the of tridentate with respect to Examples along this ´ /CH -PO 2´ 7.5 [21] 3 -PO2 (OH) 3 3 line are provided in theCH following sub-sections. 1

This entry corresponds to the benzoxaborole/benzoxaborolate couple (Figure 2b).

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.

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

2.2. Boronate-Based Crystal Structures with 4 of 13 aiming atAlkaline-Earth describing Metals the intrinsic coordination properties of the ´ series ofininvestigations aiming describing the intrinsic properties of the R-B(OH)3A anions materials have beenatreported [22–24]. Boroniccoordination acids bearing non-coordinating 2.2. Boronate-Based Crystal Structures with MetalsBoronic acids bearing non-coordinating R-B(OH) 3− used anionsfor in materials have been R- chains were this purpose (R= Alkaline-Earth Creported H9 -, C8 H17 -), in order to ensure that the materials 6 H5 -, C4 [22–24]. R-form chains wereof used for (R-at = Cdescribing 6H5-, C4H9-, C8H 17-), in order to ensure that theinmaterials A series investigations aiming the intrinsic coordination properties of the(or in would only based on this the purpose metal-ligand interactions. Reactions were carried out water − would form only based on the metal-ligand interactions. Reactions were carried out in water (or in 3), R-B(OH)3 mixtures), anions in materials have been reported [22–24]. Boronic bearing non-coordinating water-ethanol by adding a solution of a metal cation into aacids solution of the boronate (Figure water-ethanol mixtures), by adding a solution of a metal cation into a solution of the boronate R- chains were usedafor this purpose (R- =(e.g., C6H5a-, coordination C4H9-, C8H17-), polymer). in order to ensure materials in order to precipitate crystalline phase Due tothat thethe high pKa values (Figureform 3), inonly order to precipitate a crystalline phase (e.g.,Reactions a coordination polymer). to the(or high would based on the metal-ligand interactions. were carried outDue in water in of the boronic acid/boronate couples, alkaline-earth metal cations were selected for the precipitation. pK a values of the boronic acid/boronate couples, alkaline-earth metal cations were selected for the water-ethanol mixtures), by adding a solution of a metal cation into a solution of the boronate Indeed, these ions Indeed, should not lead the competitive of metal-hydroxide precipitates during precipitation. these ionstoashould not lead toformation the competitive formation of metal-hydroxide (Figure 3), in order to precipitate crystalline phase (e.g., a coordination polymer). Due to the high the reaction, in contrast with most other metal ions [20]. precipitates during the reaction, in contrast with most other metal ionscations [20]. were selected for the pK a values of the boronic acid/boronate couples, alkaline-earth metal Crystals 2016,of 6, 48investigations A series

precipitation. Indeed, these ions should not lead to the competitive formation of metal-hydroxide H2O or in contrast with most other metal ions [20]. precipitates during the reaction, 2+ EtOH/H 2O (1:1)

0.5 eq M

(aq)

or 1H eq2ONaOH EtOH/H 2O (1:1)

RT or reflux 2+ 0.5 eqtoM (aq) 30 min 2 days

1 eq NaOH

R- = C6H5-, C4H9-, C8H17-

RT or reflux 30 to 2 Sr, days Mmin = Ca, Ba

R- = C6H5-, C4H9-, C8H17-

M = Ca, Sr, Ba

M [R-B(OH)3]2 .xH2O(s) microcrystalline M [R-B(OH) 3]2 .xH2O(s) powder

microcrystalline powder Figure 3. Reaction scheme formationof of metal metal boronate structures by precipitation. Figure 3. Reaction scheme forfor thethe formation boronatecrystalline crystalline structures by precipitation.

Materials were isolated in most cases as microcrystalline powders [22–24]. Scanning electron

Figure 3. Reaction scheme for the formation of metal boronate crystalline structures by precipitation. Materials isolated most cases microcrystalline powders Scanning electron microscopywere (SEM) analysesinshowed that inas many cases, the crystallites had a[22–24]. sheet-like morphology, microscopy (SEM) analyses showed that in many cases, the crystallites had a sheet-like morphology, as illustrated Figure 4 forin the Sr-phenylboronate monohydrate and[22–24]. Sr-butylboronate phases. Materials in were isolated most cases as microcrystalline powders Scanning electron as illustrated in(SEM) Figure 4 for the Sr-phenylboronate monohydrate and Sr-butylboronate phases. Structure determination wasshowed made possible by (i)cases, recording X-ray diffraction powder patterns using microscopy analyses that in many the crystallites had a sheet-like morphology, synchrotron radiation, (ii) performing a series of additional characterization (notably by IR and solid Structure determination was made possible by (i) recording X-ray diffraction powder patterns as illustrated in Figure 4 for the Sr-phenylboronate monohydrate and Sr-butylboronate phases.using state NMR), (iii) modeling structures computationally, notably to position the hydrogen synchrotron radiation; (ii) performing a series characterization (notably by IRatoms and solid Structure determination wasthe made possible byof(i)additional recording X-ray diffraction powder patterns using and to compute the NMR parameters. All materials were found to present a layered arrangement. (ii)the performing a series of additional characterization (notably byhydrogen IR and solidatoms statesynchrotron NMR); (iii)radiation, modeling structures computationally, notably to position the MoreNMR), specifically, they werethe found to be made of planes of metal cations interconnected by boronate state (iii) structures to position the hydrogen atoms and to compute the modeling NMR parameters. Allcomputationally, materials werenotably found to present a layered arrangement. ligands, with the organic chains facing each other in the interlayer space (Figure 4). 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 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).

ligands, with the organic chains facing each other in the interlayer space (Figure 4).

Figure 4. SEM images and crystal structures of Sr(C6H5-B(OH)3)2.H2O (left) and Sr(C4H9-B(OH)3)2 (right). Adapted with permission from [22] (copyright American Chemical Society, 2011), [24] (copyright Wiley-VCH Verlag 2013) and [25]of (copyright Actualité 2014). Figure 4. SEM images andand crystal structures Sr(C (left) and Sr(C H9 -B(OH) Figure 4. SEM images crystal structures of Sr(C 6H -B(OH)33))2Chimique, .H 2O (left) and Sr(C 4H94-B(OH) 3)2 6H 5 5-B(OH) 2 .H 2O 3 )2 (right). Adapted permission [22] (copyright American Chemical Society, [24] (right). Adapted with with permission from from [22] (copyright American Chemical Society, 2011),2011), [24] (copyright (copyright Wiley-VCH Verlag and [25]Actualité (copyrightChimique, Actualité Chimique, Wiley-VCH Verlag 2013) and [25]2013) (copyright 2014). 2014).

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Based on2016, the6, different crystal structures solved, the coordination modes of the boronates Crystals 48 5 of 13 could Based on the different crystal structures solved, the coordination modes of the boronates could be be determined. These foundtotoplay play of bridging ligands, by coordination determined. Theseanions anionswere were found thethe rolerole of bridging ligands, by coordination to two toto two Based on the different crystal structures solved, the coordination modes of the boronates could to threethree distinct alkaline-earth 5).This Thisdemonstrates demonstrates boronates, in which the boron distinct alkaline-earthmetals metals (Figure (Figure 5). thatthat boronates, in which the boron be determined. These anions were found to play the role of bridging ligands, by coordination to two atom isatom in aistetrahedral geometry, can blocksforfor construction of coordination in a tetrahedral geometry, canserve serveas as building building blocks thethe construction of coordination to three distinct alkaline-earth metals (Figure 5). This demonstrates that boronates, in which the boron networks. Another possibility, not developed in this review but which further underscores the networks. possibility, not developed in this review but further underscores the atomAnother is in a tetrahedral geometry, can serve as building blocks for thewhich construction of coordination versatility of boronate ligands, concerns the use of the deprotonated boronate forms (in which the versatility of boronate concerns the use in of this the review deprotonated boronate forms (in which networks. Anotherligands, possibility, not developed but which further underscores the the boron is in a planar environment), to construct hybrid organic-inorganic materials [26]. boron isversatility in a planar environment), construct hybrid organic-inorganic materials of boronate ligands, to concerns the use of the deprotonated boronate forms [26]. (in which the boron is in a planar environment), to construct hybrid organic-inorganic materials [26]. C H B M O

C H B M O

Figure 5. Coordination modes of phenyl and butylboronate ligands with respect to alkaline earth

5. Coordination modes of phenyland andbutylboronate butylboronate ligands with respect to alkaline earth earth Figure Figure 5. Coordination modes of phenyl ligands with respect to alkaline metals (M = Ca, Sr) [22–24]. For the octylboronate ligand, the same binding mode as for metals (M = Ca, Sr) [22–24]. For the octylboronate ligand, the same binding mode as for the the metals butylboronate (M = Ca, Sr) [22–24]. For the octylboronate ligand, the same binding mode as for the butylboronatewas wasobserved observed[24]. [24]. Adapted Adapted with with permission permission from from [22] [22] (copyright (copyright American AmericanChemical Chemical butylboronate was observed [24]. Adapted withVerlag permissionand from [22]and (copyright American Chemical Society, Society, 2011), 2011), [23,24] [23,24] (copyright (copyright Wiley-VCH Wiley-VCH Verlag 2013 2013 and 2015) 2015) and [25] [25] (copyright (copyright Actualité Actualité 2014). (copyright Wiley-VCH Verlag 2013 and 2015) and [25] (copyright Actualité Society,Chimique, 2011), [23,24] Chimique, 2014). Chimique, 2014). 2.3. 2.3. Benzoxaborolate-Based Benzoxaborolate-Based Crystal Crystal Structures Structureswith withAlkaline-Earth Alkaline-EarthMetals Metals

2.3. Benzoxaborolate-Based Crystal Structures with Alkaline-Earth Metals The strategy used thecase case benzoxaborolates is the same as the for boronates. the boronates. The synthetic synthetic strategy used ininthe ofof benzoxaborolates is the same as for The The simplest benzoxaborolate (Figure 2b) was precipitated using an aqueous solution of alkaline

simplest benzoxaborolate (Figure 2b) wasofprecipitated using an aqueous solution of the alkaline earth The The synthetic strategy used in the case benzoxaborolates is the same as for boronates. earth the reaction being carried room temperature,under underreflux, reflux, or or using using hydrothermal metal,metal, the reaction being carried outout at at room temperature, hydrothermal simplest benzoxaborolate (Figure 2b) was precipitated using an aqueous solution of alkaline earth conditions conditions (Figure (Figure 6) 6) [27]. [27]. In In the the vast vast majority majority of of cases, cases, microcrystalline microcrystalline precipitates precipitates were were obtained, obtained, metal, except the reaction being for carried atcrystals room suitable temperature, under reflux, or using hydrothermal for single diffraction analyses were except fortwo twophases, phases, forwhich whichout single crystals suitablefor forX-ray X-ray diffraction analyses wereisolated. isolated. 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. 1 eq NaOH H2O/EtOH (1/1) or H2O 1 eq NaOH

H2O/EtOH (1/1) or H2O

0.5 eq M2+(aq) RT or reflux or 0.5autoclave eq M2+(aq)

« metal benzoxaborolate »

« metal benzoxaborolate »

RT=orCa, reflux M Sr or autoclave

Figure Reactionscheme schemefor forthe theformation formationofofmetal metal benzoxaborolate crystalline structures Figure 6. 6. Reaction benzoxaborolate crystalline structures by by precipitation. precipitation.

M = Ca, Sr

2+2+and 2+ [27]. To Todate, date,the thestructures structuresofofthree threephases phaseshave havebeen beensolved: solved:two twoinvolving involvingMg Mg andone oneCa Ca2+ Figure 6. Reaction scheme for the formation of metal benzoxaborolate crystalline structures by Although have an overall layered organization, as shown powder diffraction Althoughthese thesestructures structures have an overall layered organization, asbyshown byX-ray powder X-ray precipitation. analyses, benzoxaborolates here play here two play different roles: either bridge different metal diffractionthe analyses, the benzoxaborolates two different roles:they either they bridge different cations, or they simply as counter-anions, as illustratedasinillustrated Figure 7 forinthe Mg-benzoxaborolate metal cations, or theyserve simply serve as counter-anions, Figure 7 for the Mg2+ and one Ca2+ [27]. Todecahydrate date, the structures of three phases have been solved: two involving Mg phase. benzoxaborolate decahydrate phase.

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 Mgbenzoxaborolate decahydrate phase.

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a

c b a

C H B Mg O

c

C H B Mg O

2+2+ Figure 7.Figure Coordination environment of Mg ininthe Mg-benzoxaborolate decahydrate phase, showing the Figure7.7. Coordination Coordinationenvironment environmentof ofMg Mg2+ in the Mg-benzoxaborolate Mg-benzoxaborolate decahydrate decahydrate phase, phase,showing showing that metal isislinked 66water molecules, benzoxaborolate anions balance the that the metal is linked to 6totowater molecules, while the benzoxaborolate anions the that the thecation metal cation cation linked water molecules,while whilethe the benzoxaborolate anions balancebalance the charge [27]. charge [27]. charge [27].

2+

2+

Based on the crystal structures solved, several coordination modes to to MgMgand2+and have2+ been Based structures solved, several coordination modes Ca Based on the crystalcrystal structures solved, several coordination modes to Mg Ca and Cahave have been identified for benzoxaborolates (Figure 8). Clearly, a very alarge of binding modes ismodes present, been identified for benzoxaborolates (Figure 8). Clearly, verydiversity large diversity of binding is identified for benzoxaborolates (Figure 8). Clearly, a 1very large diversity of binding modes is present, the benzoxaborolate anion being bound 1 to 3todifferent metal cations, with both theboth OHthe groups present, the benzoxaborolate anion beingto bound to 3 different metal cations, with OH the benzoxaborolate being bound to as 1serving to 3 different metal cations, with and theand oxygen atom of the cycle serving binding Interestingly, in the case of both the case Cathe phase, groups theanion oxygen atom of the cycle assites. binding sites. Interestingly, in the ofOH thea groups deprotonated anion was also observed (Figure 8, dashed orange line). Forming such aForming deprotonated and the oxygen atom of the cycle serving as binding sites. Interestingly, in the case of thesuch Ca phase, a Ca phase, a deprotonated anion was also observed (Figure 8, dashed orange line). species was somewhat unexpected given that reactions had been carried out in water, and that so far a deprotonated species was somewhat unexpected given that reactions had been carried out in water, deprotonated anion was also observed (Figure 8, dashed orange line). Forming such a deprotonated deprotonated had only been observed whenobserved working when in anhydrous conditions and that so far benzoxaborolates deprotonated benzoxaborolates had only had been working in anhydrous species was somewhat unexpected given that reactions been carried out in water, and[28]. that so far It is interesting tointeresting notice that in thisthat Ca-benzoxaborolate structure, in which both both planar and conditions [28]. It is to notice in this Ca-benzoxaborolate structure, in which planar deprotonated benzoxaborolates had only been observed when workingare inall anhydrous conditions [28]. tetrahedral benzoxaborolate anions are present [27],[27], the Ca…O essentially the same, and tetrahedral benzoxaborolate anions are present the Ca .distances . . O distances are all essentially the It is interesting to notice that in this Ca-benzoxaborolate structure, in which both planar and meaning that the Ca…O distance is not directly on the protonation state of the oxygen in same, meaning that the Ca . . . O distance is notdependant directly dependant on the protonation state of the thebenzoxaborolate benzoxaborolate. tetrahedral anions are present [27], the Ca…O distances are all essentially the same, oxygen in the benzoxaborolate. meaning that the Ca…O distance is not directly dependant on the protonation state of the oxygen in the benzoxaborolate. 2+

2+

Figure Figure8.8.Coordination Coordination modes modes of ofthe thesimplest simplestbenzoxaborolate benzoxaborolate ligand ligand with with respect respectto toalkaline alkalineearth earth metals (M = Mg, Ca). The deprotonated benzoxaborolate found in the Ca structure is outlined metals (M = Mg, Ca). The deprotonated benzoxaborolate found in the Ca structure is outlined in in dashed dashedorange. orange.Adapted Adaptedwith withpermission permissionfrom from[27] [27](copyright (copyrightAmerican AmericanChemical ChemicalSociety, Society,2015). 2015).

3. Boronate and and Benzoxaborolate Benzoxaborolate Spectroscopic Spectroscopic Signatures Signatures in in Materials Materials 3. Boronate

Figure 8. Coordination modes of the simplest benzoxaborolate ligand with respect to alkaline earth 3.1. Solid metals = State Mg, NMR Ca). The deprotonated benzoxaborolate found in the Ca structure is outlined in 3.1.(M Solid State NMR dashed orange. with permission from [27] (copyright Chemical Society, 2015). Each of Adapted the materials presented in the previous section wasAmerican characterized by multinuclear solid

Each of the materials presented in the previous section was characterized by multinuclear solid state characterizationisisuseful usefulnot notonly onlytotovalidate validate the structural models proposed on state NMR. NMR. Such Such characterization the structural models proposed on the the basis of the X-ray diffraction analyses, but also to establish the NMR signatures of boronate 3. Boronate and Benzoxaborolate Spectroscopic Signatures in Materials basis of the X-ray diffraction analyses, but also to establish the NMR signatures of boronate and and benzoxaborolate anions in solids, canbe then be helpful theofstudy more complex benzoxaborolate anions in solids, whichwhich can then helpful for the for study moreof complex organicorganic-inorganic materials. materials. 3.1. Solid inorganic State NMR 1 13 C, 11 B, Among 1H, 13 Amongthe thedifferent differentNMR-active NMR-activenuclei nucleipresent presentininboronates boronatesand andbenzoxaborolates benzoxaborolates( (H, C, 11B, 17 O), boron-11 is the most straightforward to look at by solid state NMR. It is a spin-3/2 quadrupolar Each of the materials presented in the previous section was characterized by multinuclear 17O), boron-11 is the most straightforward to look at by solid state NMR. It is a spin-3/2 quadrupolar solid 11 atoms depending nucleus high (~80%), to state NMR. Suchof is useful notwhich onlyallows to validate thedistinguish structural11Bmodels proposed on the nucleus ofcharacterization highnatural naturalabundance abundance (~80%), which allows toclearly clearly distinguish B atoms depending 11 on whether they are in aaplanar or tetrahedral environment [29]. The magic angle spinning (MAS) 11B on whether they are in planar or tetrahedral environment [29]. The B magic angle spinning (MAS) basis of the X-ray diffraction analyses, but also to establish the NMR signatures of boronate and solid state NMR spectra of some of the alkaline-earth metal boronate and benzoxaborolate structures solid state NMR spectra of some of the alkaline-earth metal boronate and benzoxaborolate structures benzoxaborolate anions in9. solids, which can then be helpful for thefor study of more complex are spectra areare obtained thethe organoboron anions, inorganicareshown shownininFigure Figure 9.InInboth bothcases, cases,very verydifferent different spectra obtained for organoboron anions, inorganiccomparison materials. to the corresponding boronic acidacid and and benzoxaborole forms, both both in terms of chemical shift in comparison to the corresponding boronic benzoxaborole forms, in terms of chemical

1H, 13C, 11B, Among nuclei present in boronates and benzoxaborolates shift the (δisodifferent ), and of NMR-active quadrupolar parameters (CQ and ηQ). For example, for boronic acids ( and 17O), boron-11 is the most look at by and solid state NMR. ItCisQ ~a 1.1–1.5 spin-3/2 quadrupolar benzoxaboroles, CQ ~straightforward 2.8–3.3 MHz, whiletofor boronates benzoxaborolates, MHz. 11 nucleus of high natural abundance (~80%), which allows to clearly distinguish B atoms depending on whether they are in a planar or tetrahedral environment [29]. The 11B magic angle spinning (MAS)

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(δiso ), and of quadrupolar parameters (CQ and ηQ ). For example, for boronic acids and benzoxaboroles, CQ ~ 2.8–3.3 MHz, Crystals 2016, 6, 48while for boronates and benzoxaborolates, CQ ~ 1.1–1.5 MHz. 7 of 13 Concerning boronates, the 11 B lineshape depends on the nature of the organic chain linked to Concerning boronates, the 11B lineshape depends on the nature of the organic chain linked to the the boron, and on the coordination environment of the anion. In cases when several coordination boron, and on the coordination environment of the anion. In cases when several coordination environments are present in a crystal structure, these can be resolved using 2-dimensional experiments environments are present in a crystal structure, these can be resolved using 2-dimensional like theexperiments multiple-quantum magic angle spinning (MQMAS [23]. Concerning like the multiple-quantum magic angle spinning [29,30]) (MQMASexperiment [29,30]) experiment [23]. benzoxaborolates, different signatures observed for thefor tetrahedral benzoxaborolate Concerning benzoxaborolates, differentare signatures are observed the tetrahedral benzoxaborolate anions anionson depending on their local environment in the crystalstructure. structure. Moreover, the deprotonated depending their local environment in the crystal Moreover, the deprotonated benzoxaborolate anion identified in the Ca-benzoxaborolate phase (in which the boron is in a planar benzoxaborolate anion identified in the Ca-benzoxaborolate phase (in which the boron is in a planar configuration—see Figure 8) has a completely different 11B NMR signature, with δiso and CQ values configuration—see Figure 8) has a completely different 11 B NMR signature, with δiso and CQ values more similar to those of the benzoxaborole, but with a different asymmetry parameter ηQ [27]. more similar to those of the benzoxaborole, but with a different asymmetry parameter ηQ [27].

(a)

(b)

Figure 9. 11B solid state NMR spectra of alkaline-earth metal boronates (a) and benzoxaborolates (b),

Figure as 9. 11 B solid state NMR spectra of alkaline-earth metal boronates (a) and benzoxaborolates (b), well as for the corresponding boronic acid and benzoxaborole. The spectra shown here were as well recorded as for theunder corresponding boronic acidmagnetic and benzoxaborole. spectraacids, shown MAS at two different fields (9.4 T The for boronic vs.here 14.1were T forrecorded under MAS at two different magnetic fields (9.4 T for boronic acids, vs. 14.1 T for benzoxaboroles), benzoxaboroles), and present second order quadrupolar lineshapes. Adapted with permission from [27] (copyright Chemical Society, 2015). and present second American order quadrupolar lineshapes. Adapted with permission from [27] (copyright American Chemical Society, 2015).

C solid state NMR experiments have also been carried out on the different materials. In the case of the butylboronate phases, they were found to be particularly useful to validate the structural 13 C solid state NMR experiments have out onperformed the different model proposed by X-ray diffraction [23]. 1Halso solidbeen state carried NMR analyses, using materials. either fast In the conditions and/or with 1they H homonuclear decoupling sequences (e.g., DUMBOdecoupling case of MAS the butylboronate phases, were found to be particularly useful to validate the structural 1 under mind-boggling optimization [31]), were shown to be informative on the number of strong model proposed by X-ray diffraction [23]. H solid state NMR analyses, performed using Heither fast bonds present in the materials, to complement well the studies performed using infra-red (IR) MAS conditions and/or with 1 H and homonuclear decoupling sequences (e.g., DUMBOdecoupling spectroscopy [22–24]. Finally, it should be noted that 17O solid state NMR experiments have not been under mind-boggling optimization [31]), were shown to be informative on the number of strong reported on these phases so far, due to its very unfavorable NMR properties (very low natural H-bonds present in the materials, and to complement well the studies performed using infra-red abundance and low resonance frequency) [29,32]. 17 O solid state NMR experiments have not (IR) spectroscopy [22–24]. Finally, it should be noted that Another way to gain insight into the structure of coordination networks involving boronate and benzoxaborolate ligands consists of studying thevery NMRunfavorable signatures of the metal ions to which theylow are natural been reported on these phases so far, due to its NMR properties (very 25Mg, 43Ca bound. Most of the crystalline structures mentioned above have been characterized using abundance and low resonance frequency) [29,32]. and 87Sr solid state NMR, as illustrated in Figure 10. Alkaline earth metals are very challenging for Another way to gain insight into the structure of coordination networks involving boronate and NMR, due to their low resonance frequency, low natural abundance, and/or large quadrupole benzoxaborolate ligands consists of studying the [33–35]. NMR signatures ions toNMR which they 43Ca, and 87Sr moment, and their isotopic enrichment is costly Nevertheless,of25the Mg, metal 13

are bound. Most of the crystalline structures mentioned above have been characterized using 25 Mg, 43 Ca and 87 Sr solid state NMR, as illustrated in Figure 10. Alkaline earth metals are very challenging for NMR, due to their low resonance frequency, low natural abundance, and/or large quadrupole

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moment, and their isotopic enrichment is costly [33–35]. Nevertheless, 25 Mg, 43 Ca, and 87 Sr NMR spectra spectra can can provide provide important important clues clues concerning concerning the the local local environments environments of of the the metals. metals. In In particular, particular, 43 δδiso ( Ca) is highly informative about the local structure around calcium, and it was shown that it iso(43Ca) is highly informative about the local structure around calcium, and it was shown that it 2+ could could be be used used as as aa very very sensitive sensitive probe probe for forpositioning positioning the the boronate boronate O-H O-H groups groupsaround aroundthe theCa Ca2+,, using using aa combined combined experimental-computational experimental-computationalapproach approachbased basedon onadvanced advancedDFT DFTcalculations calculations[22]. [22].

25 25 43 Ca43and 87 Sr solid Figure metal boronates/ Figure 10. 10.Natural Naturalabundance abundanceMg, Mg, Ca and 87Sr state solidNMR statespectra NMRofspectra of metal benzoxaborolates, recorded at 14.1 or 20 T. The experimental conditions for each spectrum can spectrum be found boronates/benzoxaborolates, recorded at 14.1 or 20 T. The experimental conditions for each in the publications. publications. Adapted withAdapted permission [24] (copyright 2015), can becorresponding found in the corresponding withfrom permission from [24]Wiley-VCH, (copyright Wileyand [27] (copyright American Chemical Society, 2015). VCH, 2015), and [27] (copyright American Chemical Society, 2015).

3.2. 3.2. Infrared Infrared Spectroscopy Spectroscopy Within Withinthe the crystal crystal structures structures of of alkaline-earth alkaline-earth metal metal boronates boronates and and benzoxaborolates, benzoxaborolates, other other weak weak interactions toto thethe metal-ligand ones. In particular, hydrogen-bonds playplay a keya interactionsare arepresent presentininaddition addition metal-ligand ones. In particular, hydrogen-bonds role. TheyThey can be identified easilyeasily by infrared spectroscopy, leading to broad ~3500 key role. can be identified by infrared spectroscopy, leading to bands broad between bands between ´ 1 −1 and 2900 The. The other OHOH groups, which areare notnot involved inin any ~3500 andcm 2900.cm other groups, which involved anyH-bonding, H-bonding,lead leadto tosharper sharper bands bands at at higher higher frequencies, frequencies, as asillustrated illustratedin inFigure Figure11 11for fortwo twophenylboronate phenylboronatephases. phases. To Togo gofurther further in in the the interpretation interpretation of of these these IR IR spectra, spectra, the the O-H O-H stretching stretching modes modes were were calculated calculated by from the the crystal crystal structures structuresininwhich whichthe theprotons protonshad had been positioned using by DFT, DFT, starting from been positioned using an an “NMR-crystallography” type approach [22–24].AsAsshown shownininFigure Figure 11, 11, although small shifts “NMR-crystallography” type of of approach [22–24]. shifts remain remain between between experimental experimental and and calculated calculated values, values, DFT DFT calculations calculations can can nevertheless nevertheless be be useful useful to to identify identify some some of of the the stretching stretching bands, bands, such such as as those those belonging belonging to to the the bridging bridging water water molecules molecules in in the the case .H22OO phase phase [24]: [24]: the the O-H O-H stretching stretching modes modes of water appear as a broad case of of the the Sr(C Sr(C66H H55-B(OH)33.H broad resonance resonance beneath beneath the the sharper sharper C-H C-Hstretching stretchingfrequencies frequenciesof ofthe thephenyl phenylgroup. group. Overall, Overall, the the studies studies performed performed on on the the alkaline-earth alkaline-earth metal metal boronates boronates and and benzoxaborolates benzoxaborolates have have shown that the use of multinuclear solid state NMR and infra-red spectroscopy, in conjunction with DFT shown that the use of multinuclear solid state NMR and infra-red spectroscopy, in conjunction with calculations, are essential to fully to grasp thegrasp structure of the coordination networks. This is particularly DFT calculations, are essential fully the structure of the coordination networks. This is true when trying establish thetoH-bond network in structures for which no single-crystal X-ray particularly true to when trying establish the H-bond network in structures for which no diffraction data were available. Understanding in detail the structure of boronate-based coordination single-crystal X-ray diffraction data were available. Understanding in detail the structure of networks can becoordination of importance to fully can rationalize their properties, hence combining advanced boronate-based networks be of importance to fullyand rationalize their properties, and spectroscopic analyses with advanced computational methods is crucial. hence combining advanced spectroscopic analyses with advanced computational methods is crucial.

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Ca(C6H5-B(OH)3)2 Experimental spectrum

Calculated O-H stretching frequencies 3600

3400

3200

3000

Sr(C6H5-B(OH)3)2.H2O Experimental spectrum

Calculated O-H stretching frequencies 3600

3400 3200 σ(cm-1)

3000

Figure11. 11.Portion Portion of ofthe theexperimental experimentalIR IRspectra spectraof ofthe theCa-phenylboronate Ca-phenylboronateand andSr-phenylboronate Sr-phenylboronate Figure monohydrate phases, phases, and andrepresentation representation of ofthe theDFT-calculated DFT-calculatedO-H O-Hstretching stretchingfrequencies frequenciesfor forthese these monohydrate structures. The dashed green arrow points to the O-H bands identified as belonging to the water structures. The dashed green arrow points to the O-H bands identified as belonging to the water molecules (beneath (beneath the the aromatic aromatic C-H C-Hstretching stretchingfrequencies). frequencies). Adapted Adapted with with permission permission from from [23] [23] molecules (copyright Wiley-VCH, 2013), and [24] (copyright Wiley-VCH, 2015). (copyright Wiley-VCH, 2013), and [24] (copyright Wiley-VCH, 2015).

4. Emerging 4. Emerging Applications Applications of of Materials Materials Involving Involving Boronates Boronates and and Benzoxaborolates Benzoxaborolates 4.1. New New Functional Functional Coordination CoordinationPolymers PolymersBased Basedon onBoronates Boronates 4.1. 4.1.1. 4.1.1. Boronates Boronatesas asaaNew NewClass Classof ofLigands Ligands In Inorder orderto todemonstrate demonstratethe theutility utilityof of boronate boronateligands ligandsfor forthe the construction constructionof of novel novel coordination coordination networks, properties were were compared comparedtotothose thoseofofother othertridentate tridentateligands, ligands, such networks, their their coordination properties such as as phosphonates. Phosphonates are indeed already used for the preparation phosphonates. Phosphonates are indeed already widelywidely used for the preparation of MOFs of andMOFs other and other hybrid materials [36]. Four Ca-phosphonate phasesthus weresynthesized, thus synthesized, hybrid materials [36]. Four Ca-phosphonate phases were using using either either phenylphosphonate or butylphosphonate ligands: Ca(C (OH)62H , Ca(C H5 -PO )¨ 2H42H O, phenylphosphonate or butylphosphonate ligands: Ca(C6H 5-PO (OH) 2,2Ca(C 5-PO36 )·2H 2O, 3Ca(C 962H 5 -PO 2+, whether to Ca(C H -PO (OH)) , Ca(C H -PO )¨ H O [37]. The mode of binding of these phosphonates PO2(OH)) 2 , Ca(C 4 H 9 -PO 3 )·H 2 O [37]. The mode of binding of these phosphonates to Ca in 4 9 2 2 4 9 3 2 2+ , whether in their simply deprotonated− form R-PO (OH)´ or their fully deprotonated Ca their simply deprotonated form R-PO2(OH) or their fully deprotonated form R-PO32−, wereform then 2 2 ´ ´ ) and butylboronate − R-PO , were thenofcompared to those of (C the6Hphenylboronate (C6 H5 -B(OH)3(C compared to those the phenylboronate 5-B(OH)3 ) and butylboronate 4H9-B(OH)3−) anions. 3 (C ) anions. Figurechain 12, for(phenyl each organic chainthe (phenyl or butyl), the modes As4 Hshown in3 ´Figure 12,As forshown each in organic or butyl), modes of binding of the 9 -B(OH) of binding of the phosphonates differed fromboronates, those of the boronates, the state protonation phosphonates differed from those of the whatever the whatever protonation of the state of the phosphonate. Similar conclusions could also from be drawn from the comparison of the phosphonate. Similar conclusions could also be drawn the comparison of the strontium strontium Sr(C6 H -B(OH) ) ¨ H O and Sr(C H -PO (OH)) [22]. Overall, these comparisons structuresstructures Sr(C6H5-B(OH) 3)25·H 2O and Sr(C 6 H 5 -PO 2 (OH)) 2 [22]. Overall, these comparisons underscore 3 2 2 6 5 2 2 underscore theand difference and complementarity boronates with respect to common ligands like the difference complementarity of boronates of with respect to common ligands like phosphonates, phosphonates, andfact point to the fact that boronates as a of new class ofblocks building for and point to the that boronates can act as acan newactclass building for blocks preparing preparing coordination polymers. coordination polymers.

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Boronates

Phosphonates

C6H5-B(OH)3-

C6H5-PO2(OH) -

C6H5-PO32-

C4H9-PO2(OH) -

C4H9-PO32-

C H B Ca O P

C4H9-B(OH)3-

Figure 12. Comparison of of thethe coordination observedfor forphosphonates phosphonates boronates Figure 12. Comparison coordination modes modes observed andand boronates with with respect to calcium in different crystalstructures. structures. Adapted from [22] [22] (copyright respect to calcium in different crystal Adaptedwith withpermission permission from (copyright American Chemical Society, [23,24] (copyright Wiley-VCH Verlag 20132015), and 2015), [25] American Chemical Society, 2011),2011), [23,24] (copyright Wiley-VCH Verlag 2013 and [25] (copyright (copyright Actualité Chimique, 2014), and [37] (copyright Royal Society of Chemistry 2013). Actualité Chimique, 2014), and [37] (copyright Royal Society of Chemistry 2013).

4.1.2. Structures Based on Mixed Boronate/Carboxylate Ligands

4.1.2. Structures Based on Mixed Boronate/Carboxylate Ligands Recently, Guillou and coworkers demonstrated the usefulness of boronate type ligands for

Recently, Guillou and coworkers demonstrated usefulnesscoordination of boronate type ligands preparing functional molecular materials [38]. A seriesthe of crystalline polymers were for preparing functional materials [38]. A series crystalline coordination polymers were synthesized using molecular a mixed carboxylate-boronate ligand of (prepared from 4-carboxyphenylboronic synthesized using a mixed from 4-carboxyphenylboronic acid), and lanthanide ionscarboxylate-boronate ((Pr3+−Nd3+, Sm3+−Lu3+, ligand and Y3+(prepared ). They were characterized by various 3+ ´Nd3+ and 3+ ´Lu3+ , and including X-ray solid NMRwere (11B, 13characterized C and 89Y NMR). acid),techniques and lanthanide ions ((Prdiffraction , Smmultinuclear Y3+state ). They byThe various 11 13 89 luminescence and magnetic properties of the compounds were explored. Some of the materials were techniques including X-ray diffraction and multinuclear solid state NMR ( B, C and Y NMR). found to haveand temperature-dependent properties, is interesting formaterials the The luminescence magnetic properties luminescent of the compounds werewhich explored. Some of the development of molecular thermometers. Moreover, the magnetic properties of the Yb3+ derivative were found to have temperature-dependent luminescent properties, which is interesting for the were promising, as it was found to exhibit a single molecular magnet behavior. Overall, this shows development of molecular thermometers. Moreover, the magnetic properties of the Yb3+ derivative that the way in which boronates coordinate to metal ions and thereby lead to new coordination were networks promising, as it was found to exhibit a single molecular magnet behavior. Overall, this shows that in comparison to more conventional ligands like carboxylates and phosphonates is highly the way in which boronates to metal ions and thereby lead to new coordination networks attractive to prepare novelcoordinate functional materials. in comparison to more conventional ligands like carboxylates and phosphonates is highly attractive to 4.2. novel Hybridfunctional Materials Involving Benzoxaborolates: Towards New Ways of Formulating Benzoxaborole Drugs prepare materials. With the increasing number of benzoxaborole-containing drugs under study, the possibility of formulating them in a wide range of common biomaterials needs to be explored. In particular, the fast expansion of nanomedicine hasbenzoxaborole-containing led to the development and use of aunder varietystudy, of nanoparticles for of With the increasing number of drugs the possibility drug-delivery [39–41]. this context, we have recently looked into the formulation of the formulating them applications in a wide range of In common biomaterials needs to be explored. In particular, simple benzoxaboroles (including the antifungal drug AN2690) in two materials already studied in fast expansion of nanomedicine has led to the development and use of a variety of nanoparticles for the pharmaceutical context: an inorganic nanomaterial (Mg-Al layered double hydroxide - LDH) and drug-delivery applications [39–41]. In this context, we have recently looked into the formulation of a biopolymer (poly-L-lactic acid - PLLA) [27,42]. In the former case, the idea was to intercalate the simple benzoxaboroles the antifungal drug AN2690) inthe two materials already studied benzoxaborole taken(including in its anionic form (benzoxaborolate) between positively charged sheets of in the pharmaceutical context: an inorganic nanomaterial (Mg-Al layered double hydroxide—LDH) and the LDH, while in the later case, the benzoxaborole was directly incorporated in the PLLA film.

4.2. Hybrid Materials Involving Benzoxaborolates: Towards New Ways of Formulating Benzoxaborole Drugs

a biopolymer (poly-L-lactic acid—PLLA) [27,42]. In the former case, the idea was to intercalate the benzoxaborole taken in its anionic form (benzoxaborolate) between the positively charged sheets of the LDH, while in the later case, the benzoxaborole was directly incorporated in the PLLA film.

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Concerning the LDH-based materials, several different characterization techniques were applied, in particular multinuclear solid state NMR (11 B, 13 C, 1 H, 27 Al, 25 Mg, 19 F) [27]. Interpretations of the 11 B NMR spectra were made on the basis of the data which had been obtained for the crystalline alkaline-earth benzoxaborolate phases mentionned in section 2.3 and in Figure 9b. It was shown that the intercalation of the organoboron molecules was successful, and that both the benzoxaborole and benzoxaborolate forms could be present in the interlayer space. Moreover, it was found that the materials need to be stored at low temperature to avoid any degradation of the molecules in the interlayer space and to prevent any further evolutions of their layered organization towards second staging structures. Finally, release kinetics were studied in a simulated physiological fluid, showing a rapid release of the intercalated molecules due to exchange of the benzoxaborolates with the phosphates present in the medium [27]. Subsequently, it was possible to tune these release kinetics by preparing PLLA-LDH composite materials, which were obtained by dispersion of the intercalated LDH particles in a PLLA film [42]. Overall, these studies are the first to look into the structure of more complex hybrid organic-inorganic (nano)materials involving benzoxaborolates, and important conclusions regarding the choice of the biomaterial for benzoxaborole drug formulations could be reached. However, it should be noted that investigations are still in progress to understand better the mode of intercalation of benzoxaborolates with respect to inorganic materials like LDH, using combined experimental-computational approaches. 5. Conclusions In this review, we have summarized some of the key results reported in recent years on crystalline structures involving boronate and benzoxaborolate anions, by emphasizing their coordination properties and spectroscopic signatures. Although the number of crystalline materials involving these anions is still scarce, the few examples reported to date underscore the true potential of these anions for the preparation of new coordination networks (with applications in magnetism or luminescence for example), or for the search of new means to formulate organoboron drugs. Among the future directions to explore, the elaboration of nanoversions of the boronate-based coordination networks will also need to be looked into, because this should help expand their range of properties, as already shown in the literature for other families of coordination networks [43,44]. Acknowledgments: This review mainly summarizes results which were obtained through studies supported by the ANR (ANR JCJC “BOROMAT”), the 7th European framework program (Marie Curie ERG 239206), and a CNRS PICS project (QMAT-NMR). Access to large scale facilities was an important part of this work: (1) 850 MHz NMR spectrometer in Warwick (funded by the Birmingham Science City Advanced Materials projects with support from Advantage West Midlands (AWM) and part funded by the European Regional Development Fund (ERDF)); (2) synchrotron beamlines for powder X-ray diffraction (Soleil—project 20140477; PSI-SLS—project 20120147; (3) supercomputers for DFT calculations (GENCI-IDRIS Grant 097535, and CINES project x2015087394). Author Contributions: All authors contributed to the experiments and discussions presented in this review. Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

References 1. 2. 3. 4.

Hall, D.G. Boronic Acids: Preparation and Applications in Organic Synthesis Medicine and Materials, 2nd ed.; Wiley-VCH: Weinheim, Germany, 2011. Zheng, H.; Hall, D.G. Boronic acid catalysis: An atom-economical platform for direct activation and functionalization of carboxylic acids and alcohols. Aldrichim. Acta 2014, 47, 41–51. Sun, X.; Zhai, W.; Fossey, J.S.; James, T.D. Boronic acids for fluorescence imaging of carbohydrates. Chem. Commun. 2016, 52, 3456–3469. [CrossRef] [PubMed] Brooks, W.L.A.; Summerlin, B.S. Synthesis and applications of boronic acid-containing polymers: From materials to medicine. Chem. Rev. 2016, 116, 1375–1397. [CrossRef] [PubMed]

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5. 6. 7. 8. 9. 10. 11. 12. 13.

14.

15. 16. 17.

18. 19. 20. 21. 22.

23.

24.

25. 26.

12 of 13

Kubo, Y.; Nishiyabu, R.; James, T.D. Hierarchical supramolecules and organization using boronic acid building blocks. Chem. Comm. 2015, 51, 2005–2020. [CrossRef] [PubMed] Smoum, R.; Rubinstein, A.; Dembitsky, V.M.; Srebnik, M. Boron containing compounds as protease inhibitors. Chem. Rev. 2012, 112, 4156–4220. [CrossRef] [PubMed] Adamczyk-Wo´zniak, A.; Borys, K.M.; Sporzynski, ´ A. Recent developments in the chemistry and biological applications of benzoxaboroles. Chem. Rev. 2015, 115, 5224–5247. [CrossRef] [PubMed] Dowlut, M.; Hall, D.G. An improved class of sugar-binding boronic acids, soluble and capable of complexing glycosides in neutral water. J. Am. Chem. Soc. 2006, 128, 4226–4227. [CrossRef] [PubMed] Anacor Pharmaceuticals. Available online: www.anacor.com (accessed on 28 April 2016). Clair, S.; Abel, M.; Porte, L. Growth of boronic acid based two-dimensional covalent networks on a metal surface under ultrahigh vacuum. Chem. Commun. 2014, 50, 9627–9635. [CrossRef] [PubMed] Li, D.; Chen, Y.; Liu, Z. Boronate affinity materials for separation and molecular recognition: Structure, properties and applications. Chem. Soc. Rev. 2015, 44, 8097–8123. [CrossRef] [PubMed] Cambre, J.N.; Sumerlin, B.S. Biomedical applications of boronic acid polymers. Polymer 2011, 52, 4631–4643. [CrossRef] Li, H.; Wang, H.; Liu, Y.; Liu, Z. A benzoboroxole-functionalized monolithic column for the selective enrichment and separation of cis-diol containing biomolecules. Chem. Commun. 2012, 48, 4115–4117. [CrossRef] [PubMed] Zhao, Y.; Trewyn, B.G.; Slowing, I.I.; Lin, V.S.-Y. Mesoporous silica nanoparticle-based double drug delivery system for glucose-responsive controlled release of insulin and cyclic AMP. J. Am. Chem. Soc. 2009, 131, 8398–8400. [CrossRef] [PubMed] Ding, S.-Y.; Wang, W. Covalent organic frameworks (COFs): From design to applications. Chem. Soc. Rev. 2013, 42, 548–568. [CrossRef] [PubMed] Lorand, J.P.; Edwards, J.O. Polyol complexes and structure of the benzeneboronate ion. J. Org. Chem. 1959, 24, 769–774. [CrossRef] Cammidge, A.N.; Goddard, V.H.M.; Gopee, H.; Harrison, N.L.; Hughes, D.L.; Schubert, C.J.; Sutton, B.M.; Watts, G.L.; Whitehead, A.J. Aryl trihydroxyborates: Easily isolated discrete species convenient for direct application in coupling reactions. Org. Lett. 2006, 8, 4071–4074. [CrossRef] [PubMed] Babcock, L.; Pizer, R. Dynamics of boron acid complexation reactions. Formation of 1:1 boron acid-ligand complexes. Inorg Chem. 1980, 19, 56–61. [CrossRef] Tomsho, J.W.; Pal, A.; Hall, D.G.; Benkovic, S.J. Ring structure and aromatic substituent effects on the pKa of the benzoxaborole pharmacophore. ACS Med. Chem Lett. 2012, 3, 48–52. [CrossRef] [PubMed] Haynes, W.M. Handbook of Chemistry and Physics, 92nd ed.; CEC Press: Boca Raton, FL, USA, 2011–2012. Popov, K.; Rönkkömäki, H.; Lajunen, L.H. Critical evaluation of the stability constants of phosphonic acids. Pure Appl. Chem. 2001, 73, 1641–1677. [CrossRef] Reinholdt, M.; Croissant, J.; Di Carlo, L.; Granier, D.; Gaveau, P.; Bégu, S.; Devoisselle, J.-M.; Mutin, H.; Smith, M.E.; Bonhomme, C.; et al. Synthesis and characterization of crystalline structures based on phenylboronate ligands bound to alkaline earth cations. Inorg. Chem. 2011, 50, 7802–7810. [CrossRef] [PubMed] Sene, S.; Reinholdt, M.; Renaudin, G.; Berthomieu, D.; Zicovich-Wilson, C.M.; Gervais, C.; Gaveau, P.; Bonhomme, C.; Filinchuk, Y.; Smith, M.E.; et al. Boronate ligands in materials: Determining their local environment by using a combination of IR/solid-state NMR spectroscopies and DFT calculations. Chem. Eur. J. 2013, 19, 880–891. [CrossRef] [PubMed] Berthomieu, D.; Gervais, C.; Renaudin, G.; Reinholdt, M.; Sene, S.; Smith, M.E.; Bonhomme, C.; Laurencin, D. Coordination polymers based on alkylboronate ligands: Synthesis, characterization, and computational modeling. Eur. J. Inorg. Chem. 2015, 2015, 1182–1191. [CrossRef] Laurencin, D. Les acides boroniques et les boronates, des briques élémentaires pour la construction de matériaux. Actual. Chim. 2014, 391, 23–31. Wang, S.; Alekseev, E.V.; Miller, H.M.; Depmeier, W.; Albrecht-Schmitt, T.E. Boronic acid flux synthesis and crystal growth of uranium and neptunium boronates and borates: A low-temperature route to the first neptunium(V) borate. Inorg. Chem. 2010, 49, 9755–9757. [CrossRef] [PubMed]

Crystals 2016, 6, 48

27.

28.

29. 30. 31. 32. 33. 34. 35.

36.

37.

38.

39. 40. 41.

42.

43. 44.

13 of 13

Sene, S.; Bégu, S.; Gervais, C.; Renaudin, G.; Mesbah, A.; Smith, M.E.; Mutin, P.H.; van der Lee, A.; Nedelec, J.-M.; Bonhomme, C.; et al. Intercalation of benzoxaborolate anions in layered double hydroxides: Toward hybrid formulations for benzoxaborole drugs. Chem. Mater. 2015, 27, 1242–1254. [CrossRef] Ja´skowska, E.; Justyniak, I.; Cyranski, ´ M.K.; Adamczyk-Wo´zniak, A.; Sporzynski, ´ A.; Zygadło-Monikowska, E.; Ziemkowska, W. Benzoxaborolate ligands in group 13 metal complexes. J. Organomet. Chem. 2013, 732, 8–14. [CrossRef] MacKenzie, K.J.D.; Smith, M.E. Multinuclear Solid State NMR of Inorganic Materials; Pergamon Elsevier: Oxford, UK, 2002. Frydman, L.; Harwood, L.J.S. Isotropic spectra of half-integer quadrupolar spins from bidimensional magic-angle spinning NMR. J. Am. Chem. Soc. 1995, 117, 5367–5368. [CrossRef] Sakellariou, D.; Lesage, A.; Hodgkinson, P.; Emsley, L. Homonuclear dipolar decoupling in solid-state NMR using continuous phase modulation. Chem. Phys. Lett. 2000, 319, 253–260. [CrossRef] Ashbrook, S.E.; Smith, M.E. Solid state O-17 NMR—An introduction to the background principles and applications to inorganic materials. Chem. Soc. Rev. 2006, 35, 718–735. [CrossRef] [PubMed] Freitas, J.C.C.; Smith, M.E. Recent Advances in solid-state Mg-25 NMR spectroscopy. Ann. Rep. NMR Spectrosc. 2012, 75, 25–114. Laurencin, D.; Smith, M.E. Development of 43 Ca solid state NMR spectroscopy as a probe of local structure in inorganic and molecular materials. Prog. Nucl. Magn. Reson. Spectrosc. 2013, 68, 1–40. [CrossRef] [PubMed] Bonhomme, C.; Gervais, C.; Folliet, N.; Pourpoint, F.; Coelho Diogo, C.; Lao, J.; Jallot, E.; Lacroix, J.; Nedelec, J.-M.; Iuga, D.; et al. 87 Sr solid-state NMR as a structurally sensitive tool for the investigation of materials: Antiosteoporotic pharmaceuticals and bioactive glasses. J. Am. Chem. Soc. 2012, 134, 12611–12628. [CrossRef] [PubMed] Guerrero, G.; Alauzun, J.G.; Granier, M.; Laurencin, D.; Mutin, P.H. Phosphonate coupling molecules for the control of surface/interface properties and the synthesis of nanomaterials. Dalton Trans. 2013, 42, 12569–12585. [CrossRef] [PubMed] Sene, S.; Bouchevreau, B.; Martineau, C.; Gervais, C.; Bonhomme, C.; Gaveau, P.; Mauri, F.; Bégu, S.; Mutin, P.H.; Smith, M.E.; et al. Structural study of calcium phosphonates: A combined synchrotron powder diffraction, solid-state NMR and first-principle calculations approach. CrystEngComm 2013, 15, 8763–8775. [CrossRef] Fan, X.; Freslon, S.; Daiguebonne, C.; Le Pollès, L.; Calvez, G.; Bernot, K.; Yi, X.; Huang, G.; Guillou, O.A. Family of lanthanide-based coordination polymers with boronic acid as ligand. Inorg. Chem. 2015, 54, 5534–5546. [CrossRef] [PubMed] Couvreur, P. Nanoparticles in drug delivery: Past, present and future. Adv. Drug Deliv. Rev. 2013, 65, 21–23. [CrossRef] Argyo, C.; Weiss, V.; Bräuchle, C.; Bein, T. Multifunctional mesoporous silica nanoparticles as a universal platform for drug delivery. Chem. Mater. 2014, 26, 435–451. [CrossRef] Zhang, K.; Xu, Z.P.; Lu, J.; Tang, Z.Y.; Zhao, H.J.; Good, D.A.; Wei, M.Q. Potential for layered double hydroxides-based, innovative drug delivery systems. Int. J. Mol. Sci. 2014, 15, 7409–7428. [CrossRef] [PubMed] Sene, S.; McLane, J.; Schaub, N.; Bégu, S.; Mutin, P.H.; Ligon, L.; Gilbert, R.; Laurencin, D. Formulation of benzoxaborole drugs in PLLA: From materials preparation to in vitro release kinetics and cellular assays. J. Mater. Chem. B 2016, 4, 257–272. [CrossRef] Spokoyny, A.M.; Kim, D.; Sumrein, A.; Mirkin, C.A. Infinite coordination polymer nano- and microparticle structures. Chem. Soc. Rev. 2009, 38, 1218–1227. [CrossRef] [PubMed] Lin, W.; Rieter, W.J.; Taylor, K.M.L. Modular synthesis of functional nanoscale coordination polymers. Angew. Chem. Int. Ed. 2009, 48, 650–658. [CrossRef] [PubMed] © 2016 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 (http://creativecommons.org/licenses/by/4.0/).