Structural Induction via Solvent Variation in Assemblies of Triphenylboroxine and Piperazine - Potential Application as Self-Assembly Molecular Sponge

Article abstract of DOI:10.1021/acs.cgd.6b01845

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 ? 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 ? 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 (T_peak = 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 N_pip-H···O_epoxy···H-N_pip hydrogen bonds.

Full text of DOI:10.1021/acs.cgd.6b01845

Article
pubs.acs.org/crystal
Structural Induction via Solvent Variation in Assemblies of
Triphenylboroxine and PiperazinePotential Application as Self-
Assembly Molecular Sponge
Aaron Torres-Huerta,† Miriam de Jesus Velasquez-Hernandez,† Diego Martínez-Otero,† Herbert Hopfl,‡
́
́
́
̈
and Vojtech Jancik*,†
†Centro Conjunto de Investigacion
Mexico, 50200, Mexico
‡Centro de Investigaciones Químicas, Instituto de Investigacion
Morelos, Av. Universidad 1001, Cuernavaca, Morelos, 62209, Mex
́
en Química Sustentable UAEM-UNAM, Carretera Toluca-Atlacomulco, km. 14.5, Toluca,
́
́
́
́ ́
en Ciencias Basicas y Aplicadas, Universidad Autonoma del Estado de
́
ico
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
1. INTRODUCTION
Nowadays, there is growing interest in the design and synthesis
synthetic procedures to achieve the desired target structure and
of macrocycles, molecular cages, coordination polymers, and
three-dimensional systems using self-assembly processes.1−4
properties of the coordination polymer. Even though a plethora
of self-assembly systems have been studied, the processes
The success of the self-assembly approach lies in its proficiency
involved in the formation of molecular aggregates and
to form periodic systems with high dimensionality using a
supramolecular synthons to consolidate the crystal structure
minimum number of simple building units. This reduces the
number of reaction steps and increases yields.5−8 Additionally,
are often not clear. This fact is evidenced by phenomena such
as supramolecular isomerism,19,20 polymorphism,21,22 and
systems based on coordinative bonds present the advantage of
solvatomorphism.23 Important factors that influence the
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
Received: December 16, 2016
Revised: March 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.6b01845
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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
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, triarylborox-
ine 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
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 triphenylbor-
oxine 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.
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 charac-
teristics 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
DOI: 10.1021/acs.cgd.6b01845
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Article
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.
adduct formation is further indicated by the absence of signals
for uncoordinated piperazine (1H and 13C RMN CH2 δ 2.84
1
and 47.3 ppm; H 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 con-
stants are expected.
For the synthesis of the different N→B adducts, phenyl-
boronic 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
1
Additionally, in compounds 1−3, the H 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
1
sample for the H 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
1
were characterized by IR spectroscopy; H, 13C, and 11B NMR
spectroscopy; mass spectrometry; elemental analysis; powder
and single crystal X-ray diffraction analysis; and thermogravi-
metric 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
1
in the H 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. Single-
crystal 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
1
(SI)). In the H 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 low-
field-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
DOI: 10.1021/acs.cgd.6b01845
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Article
Table 1. Selected Crystallographic Data for Compounds 1a·THF, 1a·Acetone, 2·AcOEt, and 2·Pentanone
1a·THF
1a·Acetone
2·AcOEt
2·Pentanone
C50H60B6N2O8
chemical formula
fw (g·mol−1
C26H33B3N2O4
469.97
C25H31B3N2O4
455.95
C48H56B6N2O10
885.80
)
881.86
crystal system
space group
a/Å
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
Triclinic
P1
̅
12.0207(3)
12.0001(3)
16.7352(4)
90
11.9216(3)
11.8499(3)
17.0871(4)
90
11.8970(6)
11.8073(6)
16.7738(8)
90
9.8575(3)
10.7724(3)
12.2700(3)
91.4044(5)
101.5224(5)
109.2973(5)
1199.16(6)
1
b/Å
c/Å
α/deg
β/deg
95.4752(9)
90
96.4920(13)
90
92.5130(9)
90
γ/deg
V/Å3
2403.03(10)
4
2398.41(10)
4
2354.0(2)
2
Z
T/K
100(2)
100(2)
100(2)
100(2)
λ/Å
1.54178
0.71073
0.71073
0.71073
0.079
μ/mm−1
ρcalcd /g·cm−3
F(000)
0.673
0.083
0.084
1.299
1.263
1.250
1.221
1000
968
936
468
crystal size/mm3
θ Range/°
Limiting indices
0.227 × 0.093 × 0.088
3.694−70.452
−14 ≤ h ≤ 14
0 ≤ k ≤ 14
0 ≤ l ≤ 20
4604
0.233 × 0.184 × 0.152
2.096−26.372
−14 ≤ h ≤ 14
−14 ≤ k ≤ 14
−20 ≤ l ≤ 21
30985
0.376 × 0.328 × 0.285
1.713−27.446
−15 ≤ h ≤ 15
−15 ≤ k ≤ 15
−21 ≤ l ≤ 21
51098
0.555 × 0.318 × 0.239
2.013−27.515
−12 ≤ h ≤ 12
−13 ≤ k ≤ 13
−15 ≤ l ≤ 15
22376
Reflections collected
Reflections unique (Rint)
No. of data/restraints/parameters
Goodness of fit on F2
4604 (0.0279)
4604/3/323
1.037
4891(0.0393)
4891/2/316
1.078
5372(0.0250)
5372/1/303
1.074
5499(0.0253)
5499/1/303
1.035
a
b
R1, wR2 (I > 2σ(I))
0.0323, 0.0813
0.0335, 0.0822
0.283/−0.227
0.0442, 0.1083
0.0526, 0.1135
0.420/−0.233
1522047
0.0389, 0.1039
0.0417, 0.1060
0.382/−0.282
1522048
0.0399, 0.1037
0.0447, 0.1075
0.414/−0.219
1522049
a
b
R1, wR2 (all data)
Residual electron density/e·Å−3
CCDC
1522046
a
b
2
2
R1 = ∑∥F0| − | Fc∥/∑|F0|. wR2 = [∑w(F0 − Fc2)2/∑(F0 )2]1/2
.
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 3-
pentanone (2·AcOEt and 2·Pentanone) correspond to discrete
2:1 adducts and crystallized in the triclinic and monoclinic P21/
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-
c and P1 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 P1 and P2 /c space groups,
̅
1
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 trans-
conformation (1b·Dioxane). This compound crystallized in the
triclinic P1 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
DOI: 10.1021/acs.cgd.6b01845
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Article
Table 2. Selected Crystallographic Data for Compounds 3·CHCl3, 3·CH2Cl2, and 1b·Dioxane
3·CH2Cl2
3·CHCl3
1b·Dioxane
chemical formula
fw (g·mol−1
crystal system
space group
a/Å
C
50.09H64.18B6Cl4.18N6O6
C52H64B6Cl12N6O6
1359.35
C36H53B3N2O10
706.23
)
1059.19
Triclinic
Monoclinic
P21/c
Triclinic
P1
P1
̅
̅
13.6365(3)
13.7262(4)
17.9091(5)
95.5363(14)
110.0810(12)
115.0885(12)
2733.45(13)
2
12.6278(5)
15.1548(6)
18.0108(7)
90
11.2934(3)
11.3986(3)
15.1483(4)
75.7921(10)
77.5641(10)
87.6803(10)
1845.87(9)
2
b/Å
c/Å
α/deg
β/deg
110.3007(7)
90
γ/deg
V/Å3
3232.7(2)
2
Z
T/K
100(2)
100(2)
100(2)
λ/Å
1.54178
0.71073
0.71073
0.090
μ/mm−1
ρcalcd /g·cm−3
F(000)
2.466
0.565
1.287
1.397
1.271
1111
1400
756
crystal size/mm3
θ Range/°
Limiting indices
0.220 × 0.118 × 0.054
2.740−70.386
−16 ≤ h ≤ 16
−16 ≤ k ≤ 16
−21 ≤ l ≤ 21
41185
0.298 × 0.292 × 0.083
1.719−27.445
−16 ≤ h ≤ 16
−19 ≤ k ≤ 19
−23 ≤ l ≤ 23
47234
0.365 × 0.154 × 0.142
1.843−27.444
−14 ≤ h ≤ 14
−14 ≤ k ≤ 14
−19 ≤ l ≤ 19
34252
Reflections collected
Reflections unique (Rint)
No. of data/restraints/parameters
Goodness of fit on F2
10077(0.0242)
10077/679/798
1.021
7393(0.0335)
7393/168/416
1.073
8428 (0.0286)
8428/3/466
1.024
a
b
R1, wR2 (I > 2σ(I))
0.0382, 0.0986
0.0423, 0.1021
0.564/−0.483
0.0339, 0.0878
0.0412, 0.0925
0.405/−0.415
1522051
0.0438, 0.1070
0.0518, 0.1120
0.464/−0.259
1522052
a
b
R1, wR2 (all data)
Residual electron density/e·Å−3
CCDC
1522050
a
b
2
2
R1 = ∑∥F0| − | Fc∥/∑|F0|. wR2 = [∑w(F0 − Fc2)2/∑(F0 )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 triphenylbor-
oxine. 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 3-
pentanone (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
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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
Figure 3. Molecular structures of compounds (a) 2·AcOEt and (b) 2·
Pentanone, showing hydrogen bonds between the solvent molecules
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 inde-
pendent 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
and the N−H groups of piperazine.
amine,81 1,4-diazabicyclo[2.2.2]octane,58 p-phenylenedi-
amine,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 triphenylbor-
oxine entity changed from trigonal planar to distorted
tetrahedral. The molecular structures obtained from the
chlorinated solvents clearly contrast with the previously
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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 B-
phenyl 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
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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 triphenylbor-
oxine 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 trans-
conformation of the boroxine-piperazine chain was formed
(Figure 7a). This conformation is distinct to the cis-
conformation 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
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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).
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)−
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 dioxane-
dioxane 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
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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.
for the hydrogen bonds in 1−3, while selected bond distances
and angles are summarized in Table S3 in the SI.
polymer−solvent interactions (Figure 9c). Besides the require-
ment 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 cis-
polymers. 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,
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
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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.
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 temper-
atures 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.
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.
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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 cis-
polymer 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 piper-
azine 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 oxygen-
containing 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 hydro-
gen 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·1,2-
Epoxybutane (top). Hydrogen···hydrogen distance between neighbor-
ing 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 refine-
ment 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·1,2-
Epoxybutane was hampered by the presence of a significant
amount of polymeric impurities in the bulk sample as can be
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
1
seen from the H NMR spectrum (Figure S37 in the SI).
L
DOI: 10.1021/acs.cgd.6b01845
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
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 non-
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) v 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.
merohedral twin with the twin law 1 0 0 0 1 0 0.267 0 1 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: deposit@ccdc.cam.ac.uk).
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
{(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) v 3162 (w, N−H),
̃
1697 (m, CO from acetone), 1278 (m, N→B), 1256 (m, B−O),
1
699 (s, B3O3 out of plane) cm−1. H NMR (300.53 MHz, CDCl3, 25
1
referenced to residual signals of the deuterated solvent for H and
°C) δ 1.03 (t, 12H, CH3 from 3-pentanone), 2.40 (q, 8H, CH2 from 3-
pentanone), 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. FT-
13C 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 Buchi 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.
IR (ATR) v 3279, 3230 (w, N−H), 1356 (m, N→B), 1302 (m, B−O),
̃
1
703 (s, B3O3 out of plane) cm−1. H 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.
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·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) v 1305 (m, N→B),
̃
1
1253 (m, B−O), 704 (s, B3O3 out of plane) cm−1. H 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.
{(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
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.6b01845.
■
S
6.96, N 5.77. FT-IR (ATR) v 3139 (w, N−H), 1359 (m, N→B), 1311
̃
1
(m, B−O), 707 (s, B3O3 out of plane) cm−1. H 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;
Molecular structures with thermal ellipsoids; experimen-
tal and calculated PXRD patterns; H, 13C, and, 11B
1
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)
Found: C 65.74, H 6.71, N 5.73. FT-IR (ATR) v 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
Accession Codes
CCDC 1522046−1522052 and 1534809 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
M
DOI: 10.1021/acs.cgd.6b01845
Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design
Article
(23) du Plessis, M.; Barbour, L. J. Dalton Trans. 2012, 41, 3895−
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
3898.
́
(24) Forster, P. M.; Burbank, A. R.; Livage, C.; Ferey, G.; Cheetham,
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Huang, W. J. Phys. Chem. C 2008, 112, 8649−8653.
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Growth Des. 2011, 11, 5182−5187.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: vjancik@unam.mx. Fax: +52 (55) 56-16-22-17.
ORCID
Herbert Hopfl: 0000-0002-4027-0131
̈
(28) Nagarkar, S. S.; Chaudhari, A. K.; Ghosh, S. K. Cryst. Growth
Des. 2012, 12, 572−576.
Vojtech Jancik: 0000-0002-1007-1764
(29) Li, S.; Li, Q.; Zhou, J.; Wang, R.; Jiang, Z.; Wang, K.; Xu, D.;
Liu, J.; Liu, B.; Zou, G.; Zou, B. J. Phys. Chem. B 2012, 116, 3092−
3098.
Notes
The authors declare no competing financial interest.
(30) Teixeira, R. S.; Veiga, F. J. B.; Oliveira, R. S.; Jones, S. A.; Silva,
S. M. C.; Carvalho, R. A.; Valente, A. J. M. Int. J. Pharm. 2014, 466,
349−358.
ACKNOWLEDGMENTS
■
This work was supported by Direccion
́
General de Asuntos del
(31) Wu, P.; Chen, J.; Yin, P.; Xiao, Z.; Zhang, J.; Bayaguud, A.; Wei,
Y. Polyhedron 2013, 52, 1344−1348.
Personal Academ
and CONACyT (Grant No. 179348). A. Nunez Pineda, L.
́
ico from UNAM (PAPIIT Grant IN205115)
́
̃
(32) Patil, R. S.; Drachnik, A. M.; Kumari, H.; Barnes, C. L.;
Deakyne, C. A.; Atwood, J. L. Cryst. Growth Des. 2015, 15, 2781−2786.
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Triana Cruz, N. Zavala Segovia, and U. Hernandez 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 fellow-
ship.
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DOI: 10.1021/acs.cgd.6b01845
Cryst. Growth Des. XXXX, XXX, XXX−XXX