Structural Induction via Solvent Variation in Assemblies of

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Structural Induction via Solvent Variation in Assemblies of Triphenylboroxine and PiperazinePotential Application as SelfAssembly Molecular Sponge Aaron Torres-Huerta,† Miriam de Jesús Velásquez-Hernández,† Diego Martínez-Otero,† Herbert Höpfl,‡ and Vojtech Jancik*,† †

Centro Conjunto de Investigación en Química Sustentable UAEM-UNAM, Carretera Toluca-Atlacomulco, km. 14.5, Toluca, México, 50200, México ‡ Centro de Investigaciones Químicas, Instituto de Investigación en Ciencias Básicas y Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Cuernavaca, Morelos, 62209, México S Supporting Information *

ABSTRACT: This study examined the direct effect of solvent on the chemical composition and structure of supramolecular assemblies formed from triphenylboroxine ((PhBO)3) and piperazine (ppz) through N→B bonds. Oxygen-containing solvents with a molecular size smaller than 4.1 Å produce 1D polymeric structures (1:1 boroxine/piperazine) of compositions {(PhBO)3(ppz)}n·nTHF (1a· THF) and {(PhBO)3(ppz)}n·nAcetone (1a·Acetone), in which the boroxine B3O3 rings are linked through N→B bonded piperazine molecules in a cis-conformation. In both cases, a pseudocavity is generated between two polymer chains, which is occupied by a solvent molecule interacting through bifurcated N−H···O···H−N hydrogen bonds with one of the chains. In contrast, oxygen-based solvents with a size larger than 6.3 Å give rise to discrete 2:1 assemblies, {(PhBO)3}2(ppz)·2Ethyl acetate (2·AcOEt) and {(PhBO)3}2(ppz)·2Pentanone (2·Pentanone), with the piperazine molecule bridging two B3O3 rings and interacting with two solvent molecules via N−H···O hydrogen bonds. In chloroform or dichloromethane 2:3 adducts, {(PhBO)3}2(ppz)3·4CHCl3 (3·CHCl3) and {(PhBO)3}2(ppz)3·2.09CH2Cl2 (3·CH2Cl2), were obtained, with N−H···N interactions formed between the piperazine molecules directing the crystal lattice. Finally, unlike with THF and acetone, the presence of two coordination sites in dioxane gives rise to a 1D polymeric 1:1 clathrate-type assembly with trans-conformation, {(PhBO)3(ppz)}n·3.5nDioxane (1b·Dioxane). In accordance with the structural characterization, the thermogravimetric analysis of compounds 1−2 evidenced relatively high decomposition (solvent elimination) temperatures for the inclusion complexes derived from oxygen-containing solvents (Tpeak = 76.4 to 145.4 °C). On the contrary, solvates based on halogenated solvents (3·CHCl3 and 3·CH2Cl2) or 1,4-dioxane started to decompose already at room temperature. In view of potential applications for the storage and structural characterization of volatile or highly reactive reagents, a final inclusion experiment was carried out with racemic 1,2-epoxybutane. As expected, the resulting N→B bonded inclusion complex exhibited a 1:1 cis-polymeric structure, in which the guest molecules were bonded by bifurcated Npip−H···Oepoxy···H−Npip hydrogen bonds. conditions.15−18 Currently, one of the main remaining challenges in crystal engineering is the development of effective synthetic procedures to achieve the desired target structure and properties of the coordination polymer. Even though a plethora of self-assembly systems have been studied, the processes involved in the formation of molecular aggregates and supramolecular synthons to consolidate the crystal structure are often not clear. This fact is evidenced by phenomena such as supramolecular isomerism,19,20 polymorphism,21,22 and solvatomorphism.23 Important factors that influence the

1. INTRODUCTION Nowadays, there is growing interest in the design and synthesis of macrocycles, molecular cages, coordination polymers, and three-dimensional systems using self-assembly processes.1−4 The success of the self-assembly approach lies in its proficiency to form periodic systems with high dimensionality using a minimum number of simple building units. This reduces the number of reaction steps and increases yields.5−8 Additionally, systems based on coordinative bonds present the advantage of possible self-healing due to the reversibility of the construction processes favoring the thermodynamic product.9−14 Besides, the relatively low thermodynamic stability of coordinative bonds increases the structural diversity of the products obtained from a set of building blocks upon changing the reaction © XXXX American Chemical Society

Received: December 16, 2016 Revised: March 27, 2017

A

DOI: 10.1021/acs.cgd.6b01845 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Scheme 1. Solvent Effect on the N→B Adduct Formation between Triphenylboroxine and Piperazine, Giving 1:1 (1a·THF, 1a· Acetone, and 1b·Dioxane), 2:1 (2·AcOEt and 2·Pentanone), and 2:3 (3·CH2Cl2 and 3·CHCl3) Adducts

account, the study of the susceptibility of these assemblies toward the properties of the solvent is relevant, mainly because the lability of the N→B bond in the corresponding adducts is well-known.39 Considering the aforementioned reasons, we have decided to study assemblies generated between triphenylboroxine ((PhBO)3) and piperazine (ppz). This is because triphenylboroxine contains electron-deficient sp2-hybridized boron atoms with a vacant p orbital and the phenyl rings can be involved in C−H···π, π···π, and C−H···X interactions (where X = Cl, O, N). On the other hand, piperazine is capable of donating the electron lone pairs of the two nitrogen atoms, and is also a suitable candidate to generate N−H···X hydrogen bonds (where X = O, N), which play an important role in supramolecular chemistry due to their directionality.84−87 Herein, we explored the solvent-induced variations of the structure and stoichiometry of the supramolecular assemblies formed between triphenylboroxine and piperazine. Even though assemblies exhibiting supramolecular isomerism caused by the variation of the solvent have been reported,31,32 to the best of our knowledge, this is the first report on boroxine-amine assemblies, in which the solvent has a direct effect on the stoichiometric ratio of the product.

supramolecular arrangement during the crystallization process are the solvent, temperature, pressure, and pH.24−30 In this context, the solvent plays an important role due to the formation of noncovalent interactions with the reagents during the formation of the coordination polymer, as it is filling the cavities and voids in the crystal lattice, serving as a template, and leading to tunable host−guest interactions.31,32 Therefore, it is important to gather additional information that helps to understand and control the impact of the solvent during the supramolecular assembly process. Derivatives of boronic acids have been widely used as building blocks in the development of ordered structures to generate macrocycles, cages, calixarene systems, or two- and three-dimensional networks.33−41 Boroxines are anhydrides of organoboronic acids and contain a heterocyclic B3O3 ring and have been also successfully applied as building blocks for macrocycles, molecular cages, polymers, and three-dimensional networks,42−50 with applications in methane storage,51 electronic52,53 and optical devices,54 and as a source of aryl and vinyl groups.55,56 Additionally, they can easily form supramolecular systems due to their ability to generate N→B coordinative bonds with organic amines.57 Although the boroxine B3O3 ring is theoretically capable of coordinating up to three nitrogen atoms, it is well-known that the most stable adducts involve the interaction with only one nitrogen atom per ring,58−64 as reflected in the number of structures reported with this stoichiometric composition.65−73 In contrast, triarylboroxine species carrying aryl rings with amine substituents in the ortho position represent the few examples, in which a B3O3 ring is coordinated to more than one nitrogen atom.74−78 The number of 1:2 boroxine/amine adducts is limited and 1:3 adducts with independent organic amines are currently unknown.79−83 With the interest in organoboron compounds and their applications in supramolecular chemistry taken into

2. RESULTS AND DISCUSSION 2.1. Preparation and Spectroscopic Characterization. The aim of this work was to study in depth the susceptibility of triphenylboroxine-piperazine N→B adducts toward the characteristics and properties of solvents with varying hydrogen bonding interaction capacities and size. For a systematic study, acetone and tetrahydrofuran (THF) were selected for the presence of an oxygen atom with different hybridization (sp3 vs sp2). Ethyl acetate and 3-pentanone allowed analysis of the changes induced by the increase in the size of the solvent. B



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adduct formation is further indicated by the absence of signals for uncoordinated piperazine (1H and 13C RMN CH2 δ 2.84 and 47.3 ppm; 1H RMN N−H δ 1.66 ppm). In the 11B NMR spectra of adducts 1−3, only one broad signal was observed in the range of δ 18.8−21.9 ppm (λ1/2 = 18−22 Hz), despite the presence of two different chemical environments for the boron atoms (coordinated and uncoordinated); however, these 11B NMR values differ considerably from that measured for uncoordinated triphenylboroxine (δ 29.4 ppm, Figures S19− S37 in the SI). The presence of only one signal for the three boron atoms in 1−3 can be attributed to a fast dynamic exchange process involving probably mainly the 1:1 N→B adduct. The N→B bond strength depends on the electronic nature of the substituent attached to the boron atom and the basicity of the nitrogen atom from the base. For N→B polymers based on boronic acid esters, association constants of up to 6 × 105 M−1 have been reported;39 however, for N→B adducts with boroxines, significantly lower association constants are expected. Additionally, in compounds 1−3, the 1H and 13C NMR spectra revealed the presence of lattice solvent, with integrals of the 1H NMR signals consistent with the stoichiometry determined by X-ray diffraction analysis (vide infra). However, for compounds 1b and 3, the amount of lattice 1,4-dioxane and CHCl3/CH2Cl2 could be determined properly only, if the sample for the 1H NMR study was prepared from crystals freshly removed from the mother liquor. If dried samples of 1b and 3 were used, the signals associated with the solvent were absent or had only a fraction of the expected intensity. The stoichiometry of the remaining N→B adducts, 1:1 (1a·THF, 1a·Acetone, and 1b·Dioxane), and 2:1 (2·AcOEt and 2· Pentanone), was also confirmed in solution by integration of the signals corresponding to triphenylboroxine and piperazine in the 1H NMR spectra. For compounds 1a·THF, 1a·Acetone, 2·AcOEt, and 2· Pentanone, phase purity of the bulk samples was established by comparison of the experimental PXRD patterns with those simulated from the single-crystal X-ray diffraction data (Figures S7−S13 in the SI). Nevertheless, substantial loss of crystallinity was observed in the case of 1b·Dioxane, 3·CHCl3, and 3· CH2Cl2 due to the fast loss of the lattice solvent. This is in agreement with the results obtained from elemental analysis, the 1H and 13C NMR spectroscopic studies, and the thermogravimetric analysis (vide infra and Figures S38−S44 in the SI). 2.2. Single-Crystal X-ray Diffraction Analysis. N→B Adducts Formed from Solvents Containing Oxygen. Singlecrystal X-ray diffraction analysis corroborated the importance of the solvent effect on the molecular and crystal structures resulting from the self-assembly of triphenylboroxine and piperazine. In the case of the adducts obtained from THF and acetone (1a·THF, 1a·Acetone), 1D polymeric chains assembled through N→B coordinative bonds with a 1:1 boroxine/piperazine stoichiometry were formed. Compounds 1a·THF and 1a·Acetone crystallized in the monoclinic P21/c space group with one boroxine, two halves of piperazine, and a solvent molecule in the asymmetric unit. Although 1a·THF and 1a·Acetone contain different solvent molecules, they can be considered isomorphous, as in both cases the solvent molecules fill the voids generated between two neighboring 1D polymeric chains. The variations in the a, b, and c distances (0.35 Å), and in the β angle (1.2°), can be explained by the slightly different size of the pseudocavity due to different molecular volumes of

Dichloromethane (CH2Cl2) and chloroform (CHCl3) were used due to their ability to generate weaker N−H···Cl and C− H···Cl interactions.88−90 Finally, 1,4-dioxane was utilized to explore the effect of the presence of two donor sites. The results show that the molecular size and hydrogen bonding characteristics of the solvent determine the composition, molecular structure, and conformation, giving four different molecular and polymeric arrangements formed between triphenylboroxine and piperazine. All compounds exhibit N→ B bonds, but have different boroxine:piperazine stoichiometries, 1:1 (1a·THF, 1a·Acetone, and 1b·Dioxane), 2:1 (2·AcOEt and 2·Pentanone), and 2:3 (3·CHCl3 and 3·CH2Cl2). Moreover, in the 1:1 coordination polymers, two different stereoisomers have been observed (cis and trans), as illustrated in Scheme 1. For the synthesis of the different N→B adducts, phenylboronic acid was placed in a vial with the corresponding crystallization solvent and the solution was stirred for 4 h. Subsequently, piperazine was added in a 1:1 stoichiometry and the reaction mixture was stirred overnight. Upon completing the reaction, the solvent was removed under reduced pressure to obtain white crystalline solids in high yields. Depending on the solvent used for the reaction, N→B adducts with (PhBO)3:ppz ratios of 1:1 (cis-1a·THF, 1a·Acetone, and trans-1b·Dioxane), 2:1 (2·AcOEt and 2·Pentanone), and 2:3 (3·CHCl3 and 3·CH2Cl2) were obtained. For all compounds crystals suitable for single-crystal X-ray diffraction analysis could be grown by slow solvent evaporation. To optimize the reaction economy, for the 2:1 and 2:3 adducts, the stoichiometry of the components was adjusted in further syntheses. The products were characterized by IR spectroscopy; 1H, 13C, and 11B NMR spectroscopy; mass spectrometry; elemental analysis; powder and single crystal X-ray diffraction analysis; and thermogravimetric analysis. For the sake of clarity, 1−3 will be used when referring to the whole set of compounds (1a·THF, 1a·Acetone, 1b·Dioxane, 2·AcOEt, 2·Pentanone, 3·CHCl3, and 3·CH2Cl2). The formation and existence of the N→B coordinative bond in solution was first examined by 1H and 13C NMR spectroscopy in CDCl3, showing similar chemical shifts for compounds 1−3 (see Table S4 in the Supporting Information (SI)). In the 1H NMR spectra, the signals associated with the N−H and −CH2− protons of piperazine were found in the range of δ 1.56−2.49 ppm and δ 2.95−3.12 ppm, respectively. The signals of the boroxine-phenyl groups were observed in the range of δ 7.40−7.41 ppm for the meta and para hydrogens, while the signals for the ortho hydrogens appeared in the δ 7.98−8.03 ppm interval. In the 13C NMR spectra of adducts 1− 3, the signals for the Cp, Cm, Co, and Ci carbon atoms gave signals in intervals of δ 128.9−130.0 ppm, δ 127.5−127.7 ppm, δ 133.9−134.1 ppm, and δ 136.9−137.8 ppm, respectively. Similarly, the signals for the methylene groups of piperazine were observed in the range of δ 42.8−45.7 ppm. These chemical shifts are significantly different from those observed for uncoordinated triphenylboroxine in the same solvent, which in the 1H NMR spectrum gives three sets of signals for Hm, Hp, and Ho at δ 7.52, 7.61, and 8.27 ppm and in the 13C NMR spectrum four signals for Cp, Cm, Co, and Ci at δ 132.8, 128.1, 135.8, and 130.3 ppm, respectively (Figures S14−S18 in the SI). Thus, the signal of the ipso-carbon atom Ci in 1−3 is lowfield-shifted by 6.6−7.5 ppm, which can be attributed to the elimination of the π-electron density delocalization in the O2BCPh segment upon the formation of N→B adducts with tetrahedral instead of trigonal-planar boron atoms. N→B C



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Table 1. Selected Crystallographic Data for Compounds 1a·THF, 1a·Acetone, 2·AcOEt, and 2·Pentanone chemical formula fw (g·mol−1) crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z T/K λ/Å μ/mm−1 ρcalcd /g·cm−3 F(000) crystal size/mm3 θ Range/° Limiting indices

Reflections collected Reflections unique (Rint) No. of data/restraints/parameters Goodness of fit on F2 R1,a wR2b (I > 2σ(I)) R1,a wR2b (all data) Residual electron density/e·Å−3 CCDC a

1a·THF

1a·Acetone

2·AcOEt

2·Pentanone

C26H33B3N2O4 469.97 Monoclinic P21/c 12.0207(3) 12.0001(3) 16.7352(4) 90 95.4752(9) 90 2403.03(10) 4 100(2) 1.54178 0.673 1.299 1000 0.227 × 0.093 × 0.088 3.694−70.452 −14 ≤ h ≤ 14 0 ≤ k ≤ 14 0 ≤ l ≤ 20 4604 4604 (0.0279) 4604/3/323 1.037 0.0323, 0.0813 0.0335, 0.0822 0.283/−0.227 1522046

C25H31B3N2O4 455.95 Monoclinic P21/c 11.9216(3) 11.8499(3) 17.0871(4) 90 96.4920(13) 90 2398.41(10) 4 100(2) 0.71073 0.083 1.263 968 0.233 × 0.184 × 0.152 2.096−26.372 −14 ≤ h ≤ 14 −14 ≤ k ≤ 14 −20 ≤ l ≤ 21 30985 4891(0.0393) 4891/2/316 1.078 0.0442, 0.1083 0.0526, 0.1135 0.420/−0.233 1522047

C48H56B6N2O10 885.80 Monoclinic P21/c 11.8970(6) 11.8073(6) 16.7738(8) 90 92.5130(9) 90 2354.0(2) 2 100(2) 0.71073 0.084 1.250 936 0.376 × 0.328 × 0.285 1.713−27.446 −15 ≤ h ≤ 15 −15 ≤ k ≤ 15 −21 ≤ l ≤ 21 51098 5372(0.0250) 5372/1/303 1.074 0.0389, 0.1039 0.0417, 0.1060 0.382/−0.282 1522048

C50H60B6N2O8 881.86 Triclinic P1̅ 9.8575(3) 10.7724(3) 12.2700(3) 91.4044(5) 101.5224(5) 109.2973(5) 1199.16(6) 1 100(2) 0.71073 0.079 1.221 468 0.555 × 0.318 × 0.239 2.013−27.515 −12 ≤ h ≤ 12 −13 ≤ k ≤ 13 −15 ≤ l ≤ 15 22376 5499(0.0253) 5499/1/303 1.035 0.0399, 0.1037 0.0447, 0.1075 0.414/−0.219 1522049

R1 = ∑∥F0| − | Fc∥/∑|F0|. bwR2 = [∑w(F02 − Fc2)2/∑(F02)2]1/2.

The molecular structures of compounds 1a·THF and 1a· Acetone comprise zigzag-type 1D polymeric chains that extend parallel to the a axis (Figure 1), generated by N→B bond formation between triphenylboroxine and piperazine. Each boroxine interacts with two piperazine molecules, which exhibit mutual cis-orientation (i.e., coordination from the same direction with respect to the boroxine plane). The N→B bond distances in 1a·THF (1.702 and 1.707 Å) are practically identical to those in 1a·Acetone (1.694 and 1.699 Å). For comparison, N→B bond distances for monoadducts (ArBO)3L and 1:2 adducts (PhBO)3·2L with Ar = aryl and L = aliphatic or aromatic N-donor ligand have values in the range of 1.61−1.67 Å and 1.64−1.74 Å, respectively.65−73,79−83,91 The coordination geometry around the sp3 boron atoms in 1a·THF and 1a· Acetone corresponds to a distorted tetrahedron with bond angles in the range of 101.3−115.7°. The third noncoordinated boron atom has a trigonal-planar geometry with angles between 117.6° and 122.9°. The B−O−B−O torsion angles in the B3O3 ring of 1a·THF and 1a·Acetone vary from −6.6° to +7.8° and −7.6° to +9.9°, respectively. The propagation of neighboring chains in compounds 1a· THF and 1a·Acetone generates pseudocavities, which are filled by the corresponding solvent (Figure 2). In both cases, the oxygen atom of the solvent is involved in the formation of asymmetric N−H···O···H−N bifurcated hydrogen bonds with D···A distances; DHA, and N···O···N angles of 2.932/2.977 Å; 174.4/178.6°, and 82.5° for 1a·THF, and 2.993/3.018 Å; 169.5/174.7°, and 84.9° for 1a·Acetone, respectively. Fur-

the guest solvent. These structures are also solvatomorphs of the {(PhBO)3(ppz)}n·nDMF adduct reported recently.91 In contrast, the structures obtained from ethyl acetate and 3pentanone (2·AcOEt and 2·Pentanone) correspond to discrete 2:1 adducts and crystallized in the triclinic and monoclinic P21/ c and P1̅ space groups, respectively, with a boroxine, a solvent, and half of a piperazine molecule in the asymmetric unit. The arrangements obtained from CHCl3 and CH2Cl2 correspond to 2:3 adducts (3·CH2Cl2 and 3·CHCl3), which crystallized in the triclinic and monoclinic P1̅ and P2 1 /c space groups, respectively. The asymmetric unit in 3·CH2Cl2 contains two boroxine molecules coordinated each to one and a half molecules of piperazine and 2.09 molecules of dichloromethane (see Experimental Section). In the case of 3·CHCl3, the asymmetric unit contains one boroxine molecule coordinated to one and a half molecule of piperazine and two chloroform molecules. Finally, 1,4-dioxane gives rise to a 1D polymeric chain with 1:1 (boroxine:ppz) ratio, in which the N→B bonds exhibit, in contrast to 1a·THF and 1a·Acetone, mutual transconformation (1b·Dioxane). This compound crystallized in the triclinic P1̅ space group with a boroxine molecule coordinated to two halves of piperazine and 3.5 molecules of 1,4-dioxane in the asymmetric unit. In this case, the dioxane molecules are involved in multiple mutual C−H···O interactions (solvent− solvent interactions) and N−H···O interactions with the N−H group of the polymer (solvate−solute interactions), generating a clathrate-type system (vide infra). The crystallographic and refinement data are summarized in Tables 1 and 2. D



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Table 2. Selected Crystallographic Data for Compounds 3·CHCl3, 3·CH2Cl2, and 1b·Dioxane chemical formula fw (g·mol−1) crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z T/K λ/Å μ/mm−1 ρcalcd /g·cm−3 F(000) crystal size/mm3 θ Range/° Limiting indices

Reflections collected Reflections unique (Rint) No. of data/restraints/parameters Goodness of fit on F2 R1,a wR2b (I > 2σ(I)) R1,a wR2b (all data) Residual electron density/e·Å−3 CCDC a

3·CH2Cl2

3·CHCl3

1b·Dioxane

C50.09H64.18B6Cl4.18N6O6 1059.19 Triclinic P1̅ 13.6365(3) 13.7262(4) 17.9091(5) 95.5363(14) 110.0810(12) 115.0885(12) 2733.45(13) 2 100(2) 1.54178 2.466 1.287 1111 0.220 × 0.118 × 0.054 2.740−70.386 −16 ≤ h ≤ 16 −16 ≤ k ≤ 16 −21 ≤ l ≤ 21 41185 10077(0.0242) 10077/679/798 1.021 0.0382, 0.0986 0.0423, 0.1021 0.564/−0.483 1522050

C52H64B6Cl12N6O6 1359.35 Monoclinic P21/c 12.6278(5) 15.1548(6) 18.0108(7) 90 110.3007(7) 90 3232.7(2) 2 100(2) 0.71073 0.565 1.397 1400 0.298 × 0.292 × 0.083 1.719−27.445 −16 ≤ h ≤ 16 −19 ≤ k ≤ 19 −23 ≤ l ≤ 23 47234 7393(0.0335) 7393/168/416 1.073 0.0339, 0.0878 0.0412, 0.0925 0.405/−0.415 1522051

C36H53B3N2O10 706.23 Triclinic P1̅ 11.2934(3) 11.3986(3) 15.1483(4) 75.7921(10) 77.5641(10) 87.6803(10) 1845.87(9) 2 100(2) 0.71073 0.090 1.271 756 0.365 × 0.154 × 0.142 1.843−27.444 −14 ≤ h ≤ 14 −14 ≤ k ≤ 14 −19 ≤ l ≤ 19 34252 8428 (0.0286) 8428/3/466 1.024 0.0438, 0.1070 0.0518, 0.1120 0.464/−0.259 1522052

R1 = ∑∥F0| − | Fc∥/∑|F0|. bwR2 = [∑w(F02 − Fc2)2/∑(F02)2]1/2.

N→B bond distances of 1.660 and 1.646 Å for 2·AcOEt and 2· Pentanone, respectively. These distances are approximately 0.05 Å shorter than those in 1a·THF and 1a·Acetone. In both compounds, the B−O bond distances are very similar to those for the four-coordinate boron atom B1. The average value for the B−O bonds around B(1) is 1.461(1) Å, which is significantly larger than the B−O bonds at the B(2) and B(3) atoms (av. 1.37 Å). The distorted tetrahedral sp3 boron atom has bond angles in the range of 102.0−113.3°, while the trigonal-planar boron atoms have bond angles spanning 117.6− 121.4°. The torsion angles in the B3O3 ring have values from 0.5° to 20°. When compared to 1a·THF and 1a·Acetone, an important additional distinctive feature of 2·AcOEt and 2· Pentanone is the different coordination mode of the solvent: ethyl acetate and 3-pentanone are interacting with only one amine group via a N−H···O hydrogen bond. Attempts to obtain polymeric structures in the presence of ethyl acetate or 3-pentanone by varying the 1:1 stoichiometry of the reagents were unsuccessful and the only products were the 2:1 N→B adducts. In 2·Pentanone, neighboring adducts are connected through C(11)−H(11)···π interactions to give 1D assemblies running along the a axis (Figure 4a,b), whereas in 2·AcOEt only weak van der Waals interactions between the N→B adducts having alternating orientation were observed (Figure 4c). In both cases, the solvent molecules are located in the cavities formed between the N→B complexes. Similar 2:1 adducts {(PhBO)3L(PhBO)3} have been reported for L = 4,4′bipyridine,82 1,2-di(4-pyridyl)ethylene,82 hexamethylenetetr-

Figure 1. Fragment of the polymeric chains in compounds (a) 1a· THF and (b) 1a·Acetone, showing bifurcated hydrogen-bonding interactions with the lattice solvent.

thermore, the methylene groups of tetrahydrofuran and the methyl groups of acetone present several C−H···π type interactions with the surrounding phenyl rings of triphenylboroxine. Therefore, the directional bifurcated N−H···O···H−N hydrogen-bonding, as well as the weaker C−H···π interactions induced by the solvent molecules, are crucial for stabilizing the observed cis-configuration (for gas-phase molecules considered as less stable when compared to the trans-configuration).60 In the 2:1 adducts formed from ethyl acetate and 3pentanone (2·AcOEt and 2·Pentanone), the triphenylboroxine entities are linked in a trans-conformation by a piperazine molecule through the formation of two N→B coordinative bonds (Figure 3). In both structures, only one boron atom from each B3O3 ring is bonded to the piperazine molecule with E



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Figure 2. Fragment of the crystal lattice showing the interaction of two {(PhBO)3(ppz)}n polymer chains with the lattice solvent in 1a·THF and 1a· Acetone.

described structures, which were formed from solvents carrying oxygen atoms as hydrogen bond acceptors. The main differences consist in the different stoichiometric ratio between triphenylboroxine and piperazine and the absence of solvent− amine interactions. In 3·CHCl3, two different N→B bond distances were identified, N(1)−B(1) 1.685(2) Å and N(2)−B(3) 1.677(2) Å. The B−O bond distances around the sp3 boron atoms have values in the range of 1.462 to 1.425 Å and are longer than those around the sp2-hybridized boron atom (1.357 and 1.360 Å). While the sp3 boron atoms B(1) and B(3) exhibit distorted tetrahedral coordination geometries with bond angles in the range 101.8−115.6°, the remaining uncoordinated B(2) atom has trigonal-planar geometry with bond angles in the interval of 118.3−122.9°. The B−O−B−O torsion angles in the B3O3 ring have values in the range from −11.3 to +13.1°. Additionally, the N(1)−H(1) proton is involved in an intermolecular N(1)− H(1)···N(3) hydrogen bond, whereas the proton of the N(2)− H(2) moiety does not participate in any secondary interaction. Two chloroform molecules were found in the asymmetric unit, and are involved in C−H···Cl and Cl3C−H···π interactions [C(3)−H(3)···Cl, 2.830 Å, 127.8°; C(17)−H(17)···Cl(4), 2.872 Å, 122.4°; C(16)−H(16)···Cl5, 2.864 Å, 132.1°] [C(25)−H(25)···π, 2.349 Å and C(26)−H(26)···π, 2.473 Å; distance to centroid]. As mentioned earlier, the unit cell of compound 3·CH2Cl2 contains two crystallographically independent molecules, one of them presenting a disorder of the B(1)O(1)B(2)O(2)B(3)O(3) boroxine ring and the C(7)− C(12) phenyl group. This disorder is also reflected in the two positions observed for a nearby dichloromethane molecule and

Figure 3. Molecular structures of compounds (a) 2·AcOEt and (b) 2· Pentanone, showing hydrogen bonds between the solvent molecules and the N−H groups of piperazine.

amine, 81 1,4-diazabicyclo[2.2.2]octane, 58 p-phenylenediamine,58 and trans-1,4-cyclohexanediamine.91 N→B Adducts Formed from Halogenated Solvents. When exploring the effect of chloroform and dichloromethane in 3· CHCl3 and 3·CH2Cl2 on the crystal arrangement, in both cases the boroxine:piperazine ratio observed in the N→B adducts was 2:3. To the best of our knowledge, these derivatives are the first examples of boroxine−diamine complexes with such a stoichiometric ratio (Figure 5). The molecular structures of compounds 3·CHCl3 and 3·CH2Cl2 can be related to the 2:1 complexes found in 2·AcOEt and 2·Pentanone with the difference that an additional piperazine molecule is attached to each boroxine ring (compare Figures 3 and 5). Therefore, the geometry of an additional boron atom in each triphenylboroxine entity changed from trigonal planar to distorted tetrahedral. The molecular structures obtained from the chlorinated solvents clearly contrast with the previously F



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Figure 4. (a) C−H···π interactions in 2·Pentanone. (b) Supramolecular arrangement found in 2·Pentanone. (c) Supramolecular arrangement observed in 2·AcOEt. Hydrogen atoms that are not participating in secondary interactions were omitted for clarity.

Figure 5. (a) Molecular structure for compound 3·CHCl3. (b) Crystallographically independent molecules found within the asymmetric unit of compound 3·CH2Cl2. Carbon-bound hydrogen atoms, except those of the solvent, were omitted for clarity.

is accompanied by the presence of a partially occupied (9%) third molecule of the solvent. The N→B bonds involving the terminal piperazine molecules (N(1)−B(1) 1.686(2) Å and N(4)−B(4) 1.712(2) Å) are practically identical to those formed by the bridging piperazine (N(3)−B(2) 1.701(3) Å and N(6)−B(5) 1.687(2) Å). The bond angles around the sp3 and sp2 boron atoms vary from 98.6° to 123.0° and 112.1° to 128.3°, respectively. The torsion angles in the B3O3 ring without disorder present values in the range of −23.5° to +18.5°. Furthermore, 3·CH2Cl2 contains two different types of hydrogen bonds, i.e., strong N−H···N interactions [N(3)− H(3)···N(5), 3.026 Å, 172.0°; N(6)−H(6)···N(2), 2.984 Å,

174.2°] and weaker N−H···OB contacts [N(2)−H(2)···O(6), 3.080 Å, 142.8°; N(5)−H(5)···O(3), 3.374 Å, 144.8°]. Additionally, multiple C−H···π interactions between the Bphenyl rings are present. Also in this case, the solvent molecules are involved only in several C−H···Cl and Cl2C−H···π type interactions. Despite the crystallographic differences and the lattice solvent, the molecular arrangements in compounds 3· CHCl3 and 3·CH2Cl2 are similar and are determined by the N−H···N interactions of the amine groups of the piperazine molecules (Figure 6). N→B Adducts Formed from a Solvent Containing Two Oxygen Atoms. In view of the previously described results, we G



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Figure 6. Crystalline arrangement observed for compounds (a) 3·CHCl3 and (b) 3·CH2Cl2. Part of the hydrogen atoms were omitted for clarity.

Figure 7. (a) Molecular structure of compound 1b·Dioxane and interaction with dioxane molecules I and II. (b) Polymer−dioxane(I) and dioxane(I)−dioxane(I) interactions. (c) Lateral view of two polymeric chains of compound 1b·Dioxane with space filling presentation of dioxane molecules I and III. Dioxane molecules II and IV and part of the hydrogen atoms were omitted for clarity.

decided to also explore the effect of a solvent having two coordination sites on the self-assembly between triphenylboroxine and piperazine, as it can participate in more hydrogen bonds and thus organize the crystal structure in a distinct manner. In the presence of 1,4-dioxane, a 1D polymeric N→B adduct (1b·Dioxane) with a 1:1 stoichiometric ratio and transconformation of the boroxine-piperazine chain was formed (Figure 7a). This conformation is distinct to the cisconformation observed in 1a·THF and 1a·Acetone. Moreover, the asymmetric unit contains three and a half crystallo-

graphically independent dioxane molecules [I, O(4)(C4H8)O(5); II, O(6)(C4H8)O(7); III, O(8)(C4H8)O(9); and IV, O(10)(C4H8)O(10A)], of which the molecule containing O(10) is located on an inversion center. Overall, the crystal lattice of 1b·Dioxane can be described as a 3D clathrate, in which the boroxine-piperazine polymeric chains are located in the cavities. The N(1)−B(1) and N(2)− B(3) bond lengths of 1.679 and 1.685 Å are slightly shorter than the values for the N→B bonds in the cis polymers in 1a· THF and 1a·Acetone. The bond angles around the sp3 boron H



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Figure 8. Hydrogen bonding interactions between dioxane molecules I and III along the a axis in 1b·Dioxane (left). View of the ab plane of the clathrate type 3D network formed by the dioxane molecules. The polymer chains running parallel to the c axis have been omitted for clarity (right).

Figure 9. (a,b) Dimensions of the pseudocavity present in compounds 1a·THF, 1a·Acetone, and 1a·DMF; top view and centroid···centroid distances between the aromatic rings forming part of the pseudocavity walls. (c) Hydrogen···hydrogen distance between terminal atoms in each of the solvent molecules. (From left to right: DMF, THF, acetone, ethyl acetate, 3-pentanone and dioxane).

H(25)···O(9) interactions with O(9) from the dioxane molecule III with a D···A distance of 3.392 Å and a DHA angle of 153.4°. O(9) is also involved in a second dioxanedioxane C(33)−H(33)···O(9) interaction with a D···A distance of 3.280 Å and DHA angle of 123.6° and multiple C−H···π and C−H···O interactions with the polymeric chain (Figure 7c). The interactions between molecules I and III give rise to a continuous zigzag chain of dioxane molecules parallel to the a axis (Figure 8a). In the case of dioxane IV, only a weak C− H···π type interaction was observed. Thus, the 3D network of the dioxane molecules is generated via the formation of a total of six crystallographically independent C−H···O secondary interactions (Figure 8b). Table S2 in the SI contains the details

atoms B1 and B2 have values in the range 101.6−115.3° and 103.9−115.6°, respectively, while the values for the sp2 boron atom B(3) are between 117.6° and 123.2°. The B3O3 ring presents a high degree of planarity as reflected by rather small B−O−B−O torsion angles with values spanning from −5.0° to 5.4°. In the polymeric chain, the N−H protons interact with dioxane molecules I and II via two hydrogen bonds, N(1)− H(1)···O(4) and N(2)−H(2)···O(6), with D···A distances of 2.866 and 2.995 Å, and DHA angles of 172.1° and 163.9°, respectively. The second oxygen atom in molecule I forms weak dioxane-dioxane C(24)−H(24)···O(5) and C(34)−H(34)··· O(5) interactions with D···A distances of 3.366 and 3.501 Å, and DHA angles of 112.4° and 166.8° (Figure 7b). Furthermore, one CH2 hydrogen is involved in C(25)− I



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Figure 10. N→B, D−H, and H···A bond distances, perspective of the B3O3 rings, and relevant interactions with the solvent molecules for compounds 1−3.

polymer−solvent interactions (Figure 9c). Besides the requirement of a hydrogen bond acceptor atom in the solvent guest, the importance of the solvent size rather than the difference in the polarity for the composition and topology of the assembly is illustrated by the absence of a direct relationship between the polarity of the solvent and the generated structure (DMF, 0.39; acetone, 0.35; 3-pentanone, 0.26; ethyl acetate, 0.23; THF, 0.21).92 In the case of the assemblies with CHCl3 and CH2Cl2, the absence of functions capable of forming strong hydrogen bonds favors the formation of 2:3 adducts, wherein the terminal piperazine molecules substitute the solvent and coordinate to neighboring molecules via the N−H protons of the central piperazine molecule bridging the two boroxine rings. In the case of 1,4-dioxane, the stability of the trans polymer chain is induced by the presence of a second coordination site in the solvent molecule, which enables the formation of several C− H···O interactions. Despite the small size of dioxane (3.95 Å), the cis-conformation is disfavored due to the structural rigidity of the polymer chains that limits the formation of C−H···O interactions to the second oxygen atom of the dioxane molecule thereby decreasing the stability of the final network. Moreover, compounds 1−3 exhibit significant differences in the characteristics of the R2N−H···O hydrogen bonds, the N→ B coordinative bond and the torsion angles within the B3O3 rings, which are summarized in Figure 10. In the cis-polymeric systems 1a·DMF, 1a·Acetone, and 1a·THF with twofold N→ B-coordinated B3O3 rings, the bifurcated hydrogen bonds are almost symmetrical with N···O distances in the range of 2.891− 3.018 Å and N···O···N angles between 82.8° and 86.3°. Interestingly, from 1a·DMF, 1a·Acetone to 1a·THF the increase of 0.1 Å in the D···A distance is associated with an increase of 0.2 Å in the N→B distance (Tables S2 and S3 in the SI). The N···O or N···N donor−acceptor distances in the compounds having trans-conformation (1b·Dioxane, 3·CHCl3, and 3·CH2Cl2) are comparable to those observed in the cispolymers. For 1b·Dioxane, 3·CHCl3, 3·CH2Cl2(A), and 3· CH2Cl2(B) no significant variations are observed for the N→B bond distances of approximately 1.69 Å. The boroxine monoadducts 2·AcOEt and 2·Pentanone have N→B and N···O distances of 1.660/2.832 Å and 1.646/2.918 Å,

for the hydrogen bonds in 1−3, while selected bond distances and angles are summarized in Table S3 in the SI. Influence of the Structural Features of the Solvent on the Boroxine-Piperazine Assembly. To unravel the solvent effect on the boroxine/piperazine stoichiometry and the resulting geometry of compounds 1−3, the structures of the 1:1 polymers (cis, trans), as well as 2:1 and 2:3 adducts, were related to the structural characteristics of the solvent used during the preparation. For this purpose, the previously reported 1:1 (boroxine:piperazine) cis-polymeric structure obtained from DMF (1b·DMF) is also included.91 Compounds 1b·THF, 1b·Acetone, and 1b·DMF can be considered isomorphous (omitting the difference in the solvent), because the variations in the cell parameters a, b, c, and the β angle are less than 0.10, 0.15, 0.42 Å, and 1.55°, respectively. In all three cases, the role of the solvent consisted in the stabilization of the cis-conformation through the formation of bifurcated hydrogen bonds and C−H···π interactions with the aromatic rings of the triphenylboroxine fragments, by filling the voids formed between the polymeric chains. Considering the rigidity of the crystal lattice in the cis polymer and the minimal variation of the cell parameters between the three solvates, the dimensions of the pseudocavity formed in the polymeric systems become relevant (Figure 9a). Particularly, the space created by four aromatic rings from the triphenylboroxine with a quadrangular shape and distances between the centroids of the rings of 5.91− 6.24 Å × 5.99−6.07 Å × 5.79−6.08 Å × 5.98−6.02 Å seem to be the limiting factor (Figure 9b). Therefore, the cis-polymer system is only generated when small solvents (maximum 4 Å in size) such as DMF, THF, and acetone are used. To try to determine if there is a minimum size of the solvent required to generate the cis-polymer, water was used as the reaction medium. However, we were not able to identify any homogeneous product, mainly due to the poor solubility of the reagents in water. Larger solvents such as ethyl acetate and 3-pentanone favor the formation of 2:1 adducts. Ethyl acetate and 3-pentanone have lengths of approximately 6.31 and 6.65 Å, respectively, which is significantly larger than the space available in the pseudocavity in the cis polymer, disfavoring the C−H···π secondary interactions relevant for the stabilization of the mutual orientation of the polymeric chains and the J



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the preparation of the samples and stabilization of the equipment prior to the measurement, thus excluding analysis of weight losses as the initial composition is not exactly known. However, DSC and the qualitative TGA analyses are still possible. For compounds 1a·THF and 1a·Acetone, loss of the solvent was observed in the temperature range of approximately 110− 160 °C, showing an almost continuous transition to the elimination of piperazine. On the contrary, for 2·AcOEt and 2· Pentanone the solvent loss is clearly separated from the loss of the diamine and occurs at temperature ranges of 90−130 °C for 2·AcOEt and 110−160 °C for 2·Pentanone, respectively. In the case of the 2:3 adducts (3·CHCl3 and 3·CH2Cl2), the thermograms corroborated the partial or total absence of CHCl3 and CH2Cl2 in the crystal lattice, indicating low stability of the inclusion complex. Interestingly, the decomposition process of the resulting solids initiates at different temperatures (Tpeak = 121.0 °C for 3·CHCl3; Tpeak = 180.4 °C for 3· CH2Cl2), showing that the phases formed after solvent evaporation are different. This is also evidenced by the PXRD patterns given in Figures S11 and S12. For 1b·dioxane, the decomposition of the 1,4-dioxane solvate occurred in three steps at Tpeak = 76.4, 126.1, and 139.8 °C with a continuous transition to the elimination of piperazine. With exception of 3·CHCl3 and 3·CH2Cl2, after the loss of solvent, the boroxine-piperazine residue decomposes in two steps, although the transition is mostly continuous. The first in the range of approximately 250−300 °C and the second in the range of 300 to 400 °C. For 3·CHCl3 and 3·CH2Cl2, the decomposition of the N→B adducts initiates at significantly lower temperatures (vide supra), which can be explained by the presence of monocoordinated (terminal) piperazine molecules in these 2:3 adducts. The larger number of N-donor atoms reduces the interconnectivity via N→B bonds between the boroxine and diamine connectors, giving lower decomposition temperatures. The similar shape of the thermographs for 1a·THF, 1a· Acetone, 2·AcOEt, 2·Pentanone, and 1b·dioxane at temperatures above 250 °C indicates that at this stage probably the same solid state phase is present. The residual weight at temperatures above 400 °C can be attributed to the formation of boron oxides and carbides.

respectively. In this case, the increase in the D···A distance is accompanied by a reduction in the N→B bond length, contrary to what is observed in the cis-polymers. Concerning the B3O3 ring, the values for the torsion angles in the trans-polymer 2· Dioxane are in the range −5.0° to 5.4°, corresponding to an almost planar boroxine. The cis-polymers 1·DMF (−7.2° to 7.2°), 1·THF (−6.6° to 7.8°), and 1·Acetone (−7.6° to 9.9°) show larger deviations from planarity, which further increase for the compounds with trans-conformation (3·CHCl3, −11.3° to 13.1°; 3·CH2Cl2, −23.5° to 18.5°), and the 2:1 adducts (2· AcOEt, −9.0° to 12.6°; 2·Pentanone, −13.5° to 20°). For comparison, a structural overlay of the trans-configured N→B adducts in 3·CH2Cl2 (both crystallographically independent molecules), in 3·CHCl3, and in a fragment of the chain present in 1b·Dioxane is depicted in Figure 11. Tables S2 and S3 in the SI contain a comparison of selected bond lengths and a list of the hydrogen bonds for compounds 1−3.

Figure 11. Comparison of the molecular structures or a fragment of the N→B adducts with trans-configuration by means of an overlap projection: (a) 3·CH2Cl2 (A) (dark blue), (b) 3·CH2Cl2 (B) (light blue), (c) 3·CHCl3 (gold), and (d) 1b·Dioxane (green).

2.3. Thermogravimetric Analysis. In order to provide some more insight into the physical properties of the solvent inclusion compounds studied herein, the relative thermal stability of the solvent inclusion compounds 1−3 was studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Figures 12 and S38−S44 in the SI). For all compounds, a partial loss of the solvent was observed during

Figure 12. TGA traces of compounds 1−3. K



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2.4. Formation of an Inclusion Complex with a Volatile Epoxide and Possible Applications. In the final section of this research project we explored whether the formation of inclusion complexes with boroxine-piperazine adducts could be used for practical purposes. Because the cispolymer 1a revealed the strongest interaction with oxygen donor solvents (N−H···O···H−N bond formation), its use as a self-assembly molecular sponge for the encapsulation of small molecules that are liquid at ambient temperature and whose solid-state structure is not known was explored. A compound fulfilling the requirements and having the appropriate size is 1,2-epoxybutane. It has a melting point of −129.5 °C, is rather volatile (boiling point of 63 °C), irritant, and toxic,93 and therefore quite problematic to crystallize in a pure form. Thus, racemic 1,2-epoxybutane was combined with (PhBO)3 and piperazine in 1:1 stoichiometry. Although a chemical reaction between piperazine and the 1,2-epoxybutane leading to polymer formation was observed, it was possible to also isolate crystals suitable for X-ray structural analysis, which confirmed the formation of 1a·1,2-Epoxybutane (space group P21/c, Figure 13, Tables S1−S3 in the SI). Both enantiomers of 1,2-

3. CONCLUSIONS In summary, the structural characterization of a series of solvent inclusion compounds based on triphenylboroxine and piperazine enabled a systematic study of the effect of the structural and chemical characteristics of the solvent on the composition and topology of the N→B adducts. Although from the literature it is known that the thermodynamically most stable boroxine:amine adducts carry only one amine molecule per B3O3 ring (monoadducts), the formation of 1:1 twofold coordinated polymeric systems in the thermodynamically less stable cis-conformation was observed in the presence of oxygencontaining solvent molecules having molecule sizes smaller than 4.1 Å (THF, Acetone, DMF). Bonding through the formation of a strong, bifurcated N−H···O···H−N hydrogen bonding pattern, which is accomplished by secondary C−H···π contacts with the boroxine backbone of the polymeric N→B adduct chains are considered to be the main reasons for the formation of the cis-polymer topology. The use of oxygen-containing solvents with a single coordination site and a size larger than 6.3 Å favored the formation of discrete 2:1 adducts, in which the solvent molecules were bound by N−H···O hydrogen bonds, but occupied voids between neighboring molecules. On the contrary, in solvents forming weak non-covalent interactions, such as dichloromethane or chloroform, so far unknown 2:3 adducts were formed. With the twofold oxygen donor solvent 1,4-dioxane, a 3D crystalline clathrate-like network formed, in which the 1:1 N→B adducts of polymeric composition and trans-conformation were located within the channels formed by the dioxane molecules. The structural variety of the compounds synthesized herein allowed analysis of the structural variations in the products, such as the bond lengths and angles around the sp2 and sp3 boron atoms, the N→B bond distances, as well as the conformational changes caused by the coordination of one or two nitrogen atoms to the B3O3 ring. The structural variations found between the triphenylboroxine:piperazine assemblies as a result of the variation of the guest solvate helps one understand, predict, and design future structures resulting from the self-assembly between boroxines and amines. These effects can also be extrapolated to other systems with similar chemical and structural characteristics and probably even hydrogen-bonded assemblies with strong primary hydrogen bonding interactions. Concerning applications, N→B adducts are easily accessible low-weight materials stable in the ambient, and, therefore, may be employed for the storage and structural characterization of volatile, highly reactive, or unstable compounds under normal ambient conditions.

Figure 13. Fragment of the polymeric chains in compound 1a·1,2Epoxybutane (top). Hydrogen···hydrogen distance between neighboring methylene moieties in the 1,2-epoxybutane guest (bottom).

epoxybutane were found to be present in the cavity in an approximate 0.85:0.15 proportion as indicated by the refinement of the atom occupancies. However, due to the presence of an inversion center in the unit cell, the symmetry related cavity will contain the opposite ratio of the enantiomers and thus the ratio of the enantiomers in the unit cell is 1:1. The bond lengths and angles of the boroxine and piperazine components within 1a·1,2-Epoxybutane are comparable to those of 1a· THF, 1a·DMF, and 1a·acetone (Tables S2 and S3 in the SI). Unfortunately, further analytical characterization of 1a·1,2Epoxybutane was hampered by the presence of a significant amount of polymeric impurities in the bulk sample as can be seen from the 1H NMR spectrum (Figure S37 in the SI).

4. EXPERIMENTAL SECTION X-ray Diffraction Analysis. Powder X-ray diffraction patterns were measured on a Bruker D8 Advance X-ray diffractometer equipped with a LynxEye detector using Cu Kα radiation (λ = 1.5406 Å; monochromator: germanium). Single crystals of all compounds were mounted on nylon loops and placed in the cold nitrogen stream (100 K) inside a Bruker APEX DUO diffractometer equipped with an Apex II CCD detector. Frames were collected using omega scans and integrated with SAINT.94 Multiscan absorption correction (SADABS) was applied.94 The structures were solved by using direct methods (SHELXT)95 and refined using full-matrix least-squares on F2 with SHELXL96 using the ShelXle GUI.97 Weighted R factors, Rw, and all goodness-of-fit indicators are based on F2. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms of the C−H bonds were placed in idealized positions, whereas the hydrogen atoms from the NH moieties were localized from the difference electron density map; their L



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position was refined with Uiso tied to the parent atom with distance restraints (DFIX or SADI). The disordered groups and solvent molecules (3·CH2Cl2, 1 × B3O3, 1 × Ph, 2 × CH2Cl2; 3·CHCl3, 1 × CHCl3, 1a·1,2-Epoxybutane, 1,2-epoxybutane molecule) were refined using geometry (SADI, SAME) and Uij restraints (SIMU, RIGU) implemented in SHELXL.95 Compound 1b·THF was refined as a nonmerohedral twin with the twin law 1 0 0 0 1̅ 0 0̅.267 0 1̅ and proportion of the domains 75.6:26.4%. Molecular graphics were prepared using ShelXle,97 DIAMOND,98 GRETEP,99 POV-RAY,100 and GIMP.101 CCDC-1522046−1522052 and 1534809 contain the supplementary crystallographic data for this paper. Copies of the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/const/ retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)1223−336−033; e-mail: [email protected]). General Remarks. Phenylboronic acid and piperazine were purchased from Aldrich. While phenylboronic acid was recrystallized from water prior to use, piperazine was used as received. Reagent grade solvents were used in the reactions without further purification. NMR spectroscopic data were recorded on Bruker Avance III 300 MHz and Varian NMR Systems 500 MHz spectrometers and referenced to residual signals of the deuterated solvent for 1H and 13 C nuclei. Mass spectra (EI-MS) of all compounds contained only peak of the triphenylboroxine at m/z 312 and are thus not reported. Elemental analyses (C, H, N) were determined on an Elementar MicroVario Cube analyzer. FT-IR spectra were recorded on a Bruker ALPHA FTIR spectrometer using the ATR technique with a diamond window in the range of 500−4000 cm−1. Melting points were measured on a Büchi B-540 melting point apparatus. DSC/TG measurements were carried out in the temperature range of 20−450 °C on a Netzsch STA 449 F3 Jupiter equipment using a heating rate of 10 °C/min. The measurements were performed with a constant flow of nitrogen gas (50 mL/min), using 5 mm aluminum crucibles. The Savitzky-Golay smoothing algorithm was employed for the TG and DSC curves. General Synthesis of Compounds 1−3. Phenylboronic acid (300 mg, 2.46 mmol) was placed in a 10 mL vial and a corresponding solvent (2 mL) was added. The solution was stirred for 4 h to promote the formation of the triphenylboroxine, and thereafter, piperazine (69 mg, 0.82 mmol) was added. The reaction mixture was stirred at ambient temperature for 12 h and the solvent was removed under reduced pressure to obtain white crystalline solid. After identification of the products and their boroxine:piperazine ratio, the initial 1:1 stoichiometry (boroxine:piperazine; 1 equiv of triphenylboroxine corresponds to 3 equiv of phenylboronic acid) was adjusted to phenylboronic acid (300 mg, 2.46 mmol) and piperazine (104 mg, 1.23 mmol) (2:3) for compounds 2·AcOEt and 2·Pentanone and phenylboronic acid (300 mg, 2.46 mmol) and piperazine (35 mg, 0.41 mmol) (2:1) for 3·CH2Cl2 and 3·CHCl3. {(PhBO)3(ppz)}n·nTHF (1a·THF). Yield: 80% (0.311 g, 0.66 mmol); M.P. 139−141 °C. Elemental analysis (%) Calcd for C22H25B3N2O3· C4H8O (469.98 g·mol−1): C 66.44, H 7.08, N 5.96; Found: C 65.91, H 6.96, N 5.77. FT-IR (ATR) ṽ 3139 (w, N−H), 1359 (m, N→B), 1311 (m, B−O), 707 (s, B3O3 out of plane) cm−1. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 1.85 (m, 4H, CH2 from THF), 2.27 (s, 2H, NH), 2.97 (s, 8H, CH2 from ppz), 3.75 (s, 4H, CH2O from THF), 7.41 (s, 9H, p, m, H of Ar), 8.01 (s, 6H, o, H of Ar) ppm. 13C{1H} NMR (75.57 MHz, CDCl3, 25 °C) 25.7 (CH2 from THF), 44.7 (CH2 from ppz), 68.1 (CH2O from THF), 127.6, 129.9, 134.1, 137.7 (p, m, o, i, C of Ar) ppm. 11B NMR (96.25 MHz, CDCl3, 25 °C) δ 18.8 ppm. {(PhBO)3(ppz)}n·nAcetone (1a·Acetone). Yield: 92% (0.345 g, 0.76 mmol); M.P. 161−162 °C. Elemental analysis (%) Calcd for C22H25B3N2O3·C3H6O (455.96 g·mol−1): C 65.85, H 6.85, N 6.14; Found: C 65.74, H 6.71, N 5.73. FT-IR (ATR) ṽ 3204, 3160 (w, N− H), 1699 (m, CO from acetone), 1358 (m, N→B), 1308 (m, B− O), 700 (s, B3O3 out of plane) cm−1. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 2.14 (s, 6H, CH3 from acetone), 2.45 (s, 2H, NH), 2.96 (s, 8H, CH2 from ppz), 7.41 (s, 9H, p, m, H of Ar), 8.00 (s, 6H, o, H of Ar) ppm. 13C{1H} NMR (75.57 MHz, CDCl3, 25 °C) 31.0 (CH3 from

acetone), 45.0 (CH2 from ppz), 127.6, 129.9, 134.1, 137.7 (p, m, o, i, C of Ar) ppm. 11B NMR (96.25 MHz, CDCl3, 25 °C) δ 20.2 ppm. {(PhBO)3}2(ppz)·2Ethyl acetate (2·AcOEt). Yield: 87% (0.318 g, 0.36 mmol); M.P. 242−243 °C. Elemental analysis (%) Calcd for C40H40B6N2O6·2C4H8O2 (887.85 g·mol−1): C 65.08, H 6.37, N 3.16; Found: C 64.97, H 6.42, N 3.07. FT-IR (ATR) ṽ 3104 (w, N−H), 1689 (m, CO from acetone), 1278 (m, N→B), 1248 (s, B−O), 702 (s, B3O3 out of plane) cm−1. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 1.25 (t, 6H, CH3 from ethyl acetate), 1.56 (s, 2H, NH), 2.03 (s, 6H, OCCH3 from ethyl acetate), 3.11 (s, 8H, CH2 from ppz), 4.09 (q, 4H, CH2O from ethyl acetate) 7.41 (s, 18H, p, m, H of Ar), 8.01 (s, 12H, o, H of Ar) ppm. 13C{1H} NMR (75.57 MHz, CDCl3, 25 °C) 14.1 (CH3 from ethyl acetate), 21.1 (OCCH3 from ethyl acetate), 42.8 (CH2 from ppz), 60.7 (CH2O from ethyl acetate), 127.6, 130.0, 134.1, 137.0 (p, m, o, i, C of Ar), 171.8 (CO from ethyl acetate) ppm. 11B NMR (96.25 MHz, CDCl3, 25 °C) δ 21.4 ppm. {(PhBO)3}2(ppz)·2Pentanone (2·Pentanone). Yield: 81% (0.294 g, 0.33 mmol); M.P. 240−241 °C. Elemental analysis (%) Calcd for C40H40B6N2O6·2C5H10O (881.89 g·mol−1): C 68.10, H 6.86, N 3.18; Found: C 67.62, H 6.86, N 3.38. FT-IR (ATR) ṽ 3162 (w, N−H), 1697 (m, CO from acetone), 1278 (m, N→B), 1256 (m, B−O), 699 (s, B3O3 out of plane) cm−1. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 1.03 (t, 12H, CH3 from 3-pentanone), 2.40 (q, 8H, CH2 from 3pentanone), 3.12 (s, 8H, CH2 from ppz), 7.40 (s, 18H, p, m, H of Ar), 7.97 (s, 12H, o, H of Ar) ppm. 13C{1H} NMR (75.57 MHz, CDCl3, 25 °C) 7.9 (CH3 from 3-pentanone), 35.4 (CH2 from 3-pentanone), 42.9 (CH2 from ppz), 127.6, 129.9, 134.0, 137.2 (p, m, o, i, C of Ar), 214.0 (CO from 3-pentanone) ppm. 11B NMR (96.25 MHz, CDCl3, 25 °C) δ 21.9 ppm. {(PhBO)3}2(ppz)3 (3). Yield: 94% (0.340 g, 0.39 mmol); M.P. 173− 175 °C. Elemental analysis (%) Calcd for C48H60B6N6O6 (881.89 g· mol−1): C 65.37, H 6.86, N 9.53; Found: C 65.19, H 6.81, N 9.14. FTIR (ATR) ṽ 3279, 3230 (w, N−H), 1356 (m, N→B), 1302 (m, B−O), 703 (s, B3O3 out of plane) cm−1. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 1.81 (s, 6H, NH), 2.95 (s, 24H, CH2 from ppz), 7.41 (s, 18H, p, m, H of Ar), 8.03 (s, 12H, o, H of Ar) ppm. 13C{1H} NMR (75.57 MHz, CDCl3, 25 °C) 45.8 (CH2 from ppz), 127.6, 129.8, 134.1, 137.6 (p, m, o, i, C of Ar) ppm. 11B NMR (96.25 MHz, CDCl3, 25 °C) δ 19.3 ppm. {(PhBO)3(ppz)}n·3.5nDioxane (1b·Dioxane). Yield: 78% (0.310 g, 0.64 mmol); M.P. 237−238 °C. Elemental analysis (%) Calcd for C22H25B3N2O3·C4H8O2 (485.98 g·mol−1): C 64.26, H 6.84, N 5.76; Found: C 64.35, H 6.83, N 5.84. FT-IR (ATR) ṽ 1305 (m, N→B), 1253 (m, B−O), 704 (s, B3O3 out of plane) cm−1. 1H NMR (300.53 MHz, CDCl3, 25 °C) δ 2.49 (s, 2H, NH), 2.95 (s, 8H, CH2 from ppz), 3.69 (s, 24H, CH2 from 1,4-dioxane), 7.40 (s, 9H, p, m, H of Ar), 8.01 (s, 6H, o, H of Ar) ppm. 13C{1H} NMR (75.57 MHz, CDCl3, 25 °C) 45.0 (CH2 from ppz), 67.2 (CH2 from 1,4-dioxane), 127.6, 129.9, 134.1, 137.8 (p, m, o, i, C of Ar) ppm. 11B NMR (96.25 MHz, CDCl3, 25 °C) δ 19.8 ppm.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01845. Molecular structures with thermal ellipsoids; experimental and calculated PXRD patterns; 1H, 13C, and, 11B NMR spectra; thermogravimetric analyses (TGA−DSC) of compounds 1−3, as well as NMR spectra for uncoordinated (PhBO)3. Additionally, tables with selected bond lengths, torsion angles, geometric parameters, and selected 1H, 13C, and 11B NMR chemical shifts for compounds 1−3 (PDF) Accession Codes

CCDC 1522046−1522052 and 1534809 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ M



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cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +52 (55) 56-16-22-17. ORCID

Herbert Höpfl: 0000-0002-4027-0131 Vojtech Jancik: 0000-0002-1007-1764 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Dirección General de Asuntos del Personal Académico from UNAM (PAPIIT Grant IN205115) and CONACyT (Grant No. 179348). A. Núñez Pineda, L. Triana Cruz, N. Zavala Segovia, and U. Hernández Balderas are acknowledged for technical assistance. M.J.V.-H. is grateful to CONACyT for a Ph.D. scholarship (Grant No. 262860), while A.T.-H. acknowledges DGAPA-UNAM for postdoctoral fellowship.

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