Precise Steric Control over 2D versus 3D Self-Assembly of Antimony(III) Alkoxide Cages through Strong Secondary Bonding Interactions

Article abstract of DOI:10.1021/acs.inorgchem.7b01049

Antimony(III) alkoxide cages were designed as building blocks for predictable supramolecular self-assembly. Supramolecular synthons featuring two Sb···O secondary bonding interactions (SBIs), each SBI stronger than 30 kJ/mol, were used to drive the formation of the supramolecular architectures. Judicious choice of pendant groups provided predictable control over the formation of self-assembled 3D columnar helices, which crystallized with hollow morphologies, or a self-assembled 2D bilayer. The Sb-O stretching frequency provides a spectroscopic signature of Sb···O SBI formation.

Full text of DOI:10.1021/acs.inorgchem.7b01049

Article
pubs.acs.org/IC
Precise Steric Control over 2D versus 3D Self-Assembly of
Antimony(III) Alkoxide Cages through Strong Secondary Bonding
Interactions
Shiva Moaven, Jingze Yu, Jason Yasin, Daniel K. Unruh, and Anthony F. Cozzolino*
Department of Chemistry and Biochemistry, Texas Tech University, Box 41061, Lubbock, Texas 79409-1061, United States
*
S Supporting Information
ABSTRACT: Antimony(III) alkoxide cages were designed as
building blocks for predictable supramolecular self-assembly.
Supramolecular synthons featuring two Sb···O secondary bonding
interactions (SBIs), each SBI stronger than 30 kJ/mol, were used to
drive the formation of the supramolecular architectures. Judicious
choice of pendant groups provided predictable control over the
formation of self-assembled 3D columnar helices, which crystallized
with hollow morphologies, or a self-assembled 2D bilayer. The Sb−
O stretching frequency provides a spectroscopic signature of Sb···O
SBI formation.
22
23
INTRODUCTION
the structures of Sb(OMe)3 and Sb(O C H )OMe as a
2 6 4
■
result of Sb···O SBIs. Other alkoxides, however, do not form
these same structures as a result of sterics and rotational
flexibility of the alkoxide. Noting this, we have designed a rigid
antimony alkoxide cage (Scheme 1) that is anticipated to
Nature has utilized self-assembly of a fairly limited number of
building blocks to achieve great diversity. This has inspired
chemists to follow suit and push the limits of self-assembly
through rational design of novel building blocks to achieve
predictable architectures. Hydrogen bonding has been the
predominant tool of the supramolecular chemist. This is likely
due to the prevalence of the hydrogen bond in natural systems
which act as a source of inspiration. Nature also uses other
weak interactions to direct self-assembly including the
secondary bonding interactions (SBIs) of the heavy p-block
Scheme 1
1
−3
4,5
elements,
the halogens in particular. While the halogen
SBIs have certainly emerged as a useful supramolecular
6
−13
tool,
those of their neighbors on the periodic table have
facilitate three SBIs at the antimony center with one opposite
to each primary bond which can lead to the predictable self-
assembly of 2D bilayers when they arrange in an antiparallel
fashion. It is notable that this type of triol has been used to
many benefits from a design point of view. The strength of
these SBIs is maximized with a polar primary bond. Polar Te(II
or IV) and Sb(III) bonds (to N or O) are far more synthetically
14
accessible than the equivalent bonds with I(I). Furthermore,
while iodine facilitates one SBI, Te(II) or Te(IV) can each
facilitate two SBIs, and Sb(III) can facilitate three. Despite
these apparent advantages, predictable self-assembly has been
far more limited with antimony or tellurium although some
24−31
support many different metal clusters;
however, with
phosphorus and arsenic, the anticipated caged structure is
32
obtained.
RESULTS AND DISCUSSION
Preliminary DFT Evaluation of Design. DFT calculations
were used to probe the validity of this design and shed light on
the possible interactions that could occur between pairs of
these molecules. Initial geometry optimizations of the parent
15−18
■
exciting examples are emerging.
The mineral senarmontite, Sb O , highlights the propensity
3
4
of antimony compounds to undergo homomolecular self-
assembly through SBIs and provides inspiration for designing
antimony compounds that self-assemble in predictable arrange-
ments. The structure is composed of self-assembled bilayers
that are further assembled into a 3D network by additional Sb···
O SBIs. The strength of the Sb SBIs is highlighted when
contrasting the melting point of senarmontite with the lighter
molecule, 1 (R = H), yielded a C symmetrical molecule that is
3
similar to that of analogous phosphites that have been
19,33−37
structurally characterized
but with a slight twist in the
1
9
20
21
homologues (23, 312, and 656 °C for P, As, and Sb,
Received: April 25, 2017
respectively). Similar self-assembled bilayers can be found in
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01049
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
cage to accommodate the larger size of Sb compared to that of
P. The Sb−O bond lengths are 1.983 Å, comparable to those of
3
8
39
Sb(ODmp)3 and Sb(ODpp) (Dmp = 2,6-dimethylphenyl,
3
Dpp = 2,6-diphenylphenyl) which are both monomeric in the
solid state (Table 1). Unlike the Lewis basic phosphites, these
Table 1. Sb−O and Sb ···O Distances from Crystal Data of
1
Monomeric and Self-Assembled Antimony Alkoxides and
Aryloxides
a
compound
Sb(ODmp)3
Sb −On
Sb ···On
ref
38
1
1
1.967(2)
NA
1
1
.968(2)
.976(2)
NA
NA
Sb(ODpp)3
1.975(2)
NA
39
22
1
1
.975(2)
.975(2)
NA
NA
b
Sb(OMe)3
2.003(9)
2.565
2.886
3.003
2.635
2.357
3.495
NA
2
1
.012(8)
.986(6)
Figure 1. Fukui function integrated from above (top) and the
electrostatic potential (bottom) from two different angles (left and
right, 180° rotation) mapped onto the electron density surface (0.01
a.u. isosurfaces). Blue represents regions of nucleophilic susceptibility
and negative potential, respectively.
b
Sb(O C H )OMe
2.00(1)
23
2
6
4
2
2
.06(1)
.01(1)
tBuOSb-Calix[5]arene
c
1.982(2)
49
1
1
.995(4)
.906(3)
NA
NA
2
3
4
1.990(2)
2.805
2.703
2.668
3.073
2.720
2.591
2.597
3.398
2.421
this work
this work
this work
2
2
.001(2)
.004(2)
1.991(2)
2
1
.015(2)
.987(2)
2.033(1)
1
2
.977(1)
.071(1)
a
These distances correspond to the SBI distances that are opposite to
b
the corresponding Sb−O primary bond. The standard deviations are
too large to allow for a conclusion to be drawn from this structure.
c
(
μ2−5,11,17,23,29-penta-t-butylcalix(5)arene-35-benzyloxy-
1,32,33,34-tetrolato)-t-butoxy-dimethyl-antimony−silicon.
3
antimony compounds are anticipated to be Lewis acidic at the
pnictogen. To confirm this, the electrostatic potential and
+
40−42
Fukui function integrated from above, f (r),
can be
Figure 2. Line drawing of 1 and DFT-optimized dimers 1 (∥), Δ-
2
mapped onto the electron density surfaces (Figure 1). The
surfaces illustrate that there will be complementary covalent
and electrostatic contributions to the SBIs that can occur
opposite to each of the primary bonds. The electrostatic
potential also illustrates the obvious fact that the oxygen atoms
can serve as Lewis basic/nucleophilic sites. This demonstrates
the ambiphilic nature of these molecules and suggests that they
will succumb to homomolecular self-assembly as often observed
in the crystal structures of heavy p-block element containing
1
2
(⊥), and Λ-1 (⊥).
2
ring (Figure 2). The energy of dimerization (ΔE ) is −81.2 kJ/
D
mol. The SBIs are each 2.550 Å, which is 70.8% of the sum of
3
,45
the van der Waals radii (3.6 Å).
Similar SBI distances are
22
observed in the structures of Sb(OMe) and Sb(O C H )-
2
6
4
23
OMe, both of which form self-assembled polymeric structures
in the solid state. The other two supramolecular conformations,
Δ-1 (⊥) and Λ-1 (⊥), were nonsuperimposable mirror images
2
2
4
3
molecules.
of each other and therefore had equal dimerization energies
(−92.1 kJ/mol). The pairs of molecules were oriented
orthogonally (⊥) to form a different virtual four-membered
ring with two Sb−O single bonds and two Sb···O SBIs. The
SBIs are each 2.615 Å, which is 72.5% of the sum of the van der
Waals radii. The ∼10 kJ/mol difference in energy between the
two types of supramolecular synthons results from a less
For self-assembly to be truly predictable, there must be only
one way in which two molecules can come together. DFT was
used to model the energetics of possible dimer arrangements,
12. Various starting geometries (Figure S3) led to three
different conformations (Figures 2 and S4) that had a modest
difference (10.9 kJ/mol) in binding energies (Table 3). One
conformation, 1 (∥), has both molecules in an antiparallel (∥)
favorable reorganization energy (E ) for the antiparallel dimer
and a more favorable dispersion energy (EDisp) for the
orthogonal dimer. The origin of the difference in dispersion
2
R
43,44
alignment with a supramolecular synthon
comprised of two
Sb−O and two Sb···O SBIs forming a virtual four-membered
B
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Inorganic Chemistry
Article
4
6,47
energy is likely the result of the closer contact between the
atoms in the cages in the orthogonal dimer; this difference
should disappear in a solid-state structure where each molecule
will be fully surrounded by other molecules. Both have almost
identical binding energies (the energy required to cleave the
two molecules apart without allowing them to relax), suggesting
that the supramolecular synthons in both cases are equally
stabilized by SBIs. A fourth arrangement that only had a single
and alkoxides.
This shift in the IR frequency is the result of
the population of the Sb−O σ* orbitals by the lone pair of
electrons from the O atoms on a neighboring molecule. This
weakens the Sb−O primary bonds which results in a shift to
lower vibrational frequencies.
The DFT calculated structures show that the primary bond
between the Sb and O atoms becomes elongated from 1.983 to
2.003 Å in response to the formation of an opposing SBI. A
search of the Cambridge Crystallographic Database Centre
(CCDC); 892 156 entries) revealed some monomeric and
self-assembled structures that contain Sb(OR)3 molecules.
There are three reported crystal structures that have
monomeric compounds. In each case, self-assembly was
prevented by steric hindrance. Table 1 reports the primary
bond distances and corresponding secondary bond distances (if
applicable), and the values appear consistent with the state of
self-assembly.
SBI, 1 (1-SBI), was optimized and found to have half of the
2
48
dimerization energy as compared to those of the others. It is,
therefore, energetically unfavorable for a single SBI to prevail
over the double interactions found in the supramolecular
synthons of 1 (∥), Δ-1 (⊥), and Λ-1 (⊥).
2
2
2
A combination of the perpendicular and antiparallel
arrangements can be expected to lead to a 3D structure. If,
however, the orthogonal supramolecular conformation can be
prevented, then a 2D bilayer might be anticipated based on a
grid of antiparallel arranged molecules. Geometry optimizations
of a tetramer (1 (∥)) and nonamer (1 (∥)) of 1 in antiparallel
Synthesis and Characterization of Compounds.
Compound 3 has been previously prepared by the treatment
of the alcohol with Sb O but only elemental analysis and a lack
4
9
conformations (Figure 3) were run to assess the likelihood of
2
3
of any O−H stretching vibration was provided as character-
50
ization. In this study, an alternative approach to prepare these
compounds was used. Three tripodal antimony(III) alkoxides,
2
−4, were prepared by treating a THF solution of the alcohol
51
with a THF solution of antimony(III) tert-butoxide (Scheme
). Compounds 2−4 immediately precipitated as white solids
2
Scheme 2. Synthesis of Compounds 2−4
Figure 3. Geometry optimized tetramer 1 (∥) (left) and nonamer
4
1
9
(∥) (right).
formation of a 2D structure. Comparing the energy of each
and were recovered and washed with fresh THF to yield a
crude material that was pure according to the H NMR
spectrum but was sublimed prior to further characterization.
This method should act as a general gateway to this entire class
of compounds.
supramolecular synthon (E ) in 1 (∥) and 1 (∥) with those of
1
S
4
9
the other dimers shows that they are anticipated to be stable
(
see Table 3) and suggests that formation of the 2D structure is
likely if the perpendicular conformation can be prevented.
The calculated vibrational frequencies show that there is a
marked shift in the Sb−O stretching frequency upon self-
To confirm the formation of antimony(III) alkoxide cages
2
−4, several spectroscopic methods were applied. The most
−1
−1
assembly (from 559 cm in 1 to 511 cm in 1 , Table 2). This
2
recognizable indication for the synthesis of the products 2−4 is
shift in Sb−O frequency has been used previously as a
spectroscopic signature of self-assembly with antimony aryl-
1
the absence of the hydroxyl hydrogens by H NMR
spectroscopy (4.15−4.20 ppm in 5−7) and the lack of an
O−H stretch/bend in the IR spectrum. In addition,
replacement of the hydroxyl hydrogens by antimony results
in a characteristic shift in the cage methylene hydrogen
Table 2. Experimental and Calculated ν
Frequencies
Sb−O
−1
(cm )
1
resonances by 0.70−0.75 ppm to higher frequency in the H
DFT dimer
NMR spectrum. A similar shift to higher frequency (9.80−
13
DFT
monomer
Sb···O SBIs in solid
10.40 ppm) is observed in the C NMR spectra of 2−4 for the
cage methylene carbon atoms. The most definitive spectro-
scopic signature associated with the formation of the desired
structure is the new sharp Sb−O asymmetric stretch that can be
exp.
∥
⊥(Δ/Λ)
state?
4
6
Sb(OMe)3
457
555
491
571
454
492
yes
no
4
6
Sb(OEt)3
5
64
−1
detected around 500 cm in the FTIR spectrum of compounds
t
Sb(O Bu)3
587
566
559
N/A
510
no
2−4 (Table 2). Combustion analysis is consistent with the
proposed formulas and proves the purity of these compounds.
In an effort to determine the stability of these compounds
toward hydrolysis, deuterated water was added to the NMR
samples (Figures S12−S14), and the quantitative formation of
1
512
2
3
4
499
501
503
yes
yes
yes
C
DOI: 10.1021/acs.inorgchem.7b01049
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Inorganic Chemistry
Article
the free alcohols was observed. The sensitivity was lessened for
solid state samples; only partial hydrolysis (Figures S15−S17)
was observed in solid samples of 2−4 that were left exposed to
1
ambient conditions for 45 h. Using H NMR integrations, 40%
of compounds 2 and 3 and 60% of compound 4 were
decomposed.
Characterization of Solid-State Supramolecular Be-
havior. Evidence of self-assembly was sought next. For reasons
explained above, vibrational spectroscopy was anticipated to be
the most telling given the expected lower energy vibrational
frequencies upon self-assembly predicted by DFT. Table 2
shows the relevant experimental vibrational modes. Included in
the table are the experimental frequencies for three antimony-
(III) alkoxides that are known to be either monomeric or self-
assembled along with DFT calculated frequencies of the
monomers and dimers. Comparing the vibrational frequencies
of antimony(III) methoxide, ethoxide, and tert-butoxide shows
not only that the methoxide is self-assembled but also that
more bulky alkoxides result in only monomeric compounds.
Here the DFT calculations and experimental data are in good
agreement. This encouraged us to compare the Sb−O
stretching frequencies of compounds 2−4 with the DFT
calculated vibration frequencies of the monomer and dimers of
−1
compound 1. Frequencies of 511−512 cm were determined
−
1
for the dimers as contrasted with 559 cm in the monomer.
The experimental data for compounds 2−4 reveals frequencies
around 500 cm− which is consistent with self-assembly in the
solid state.
Figure 4. Ball and stick representation of crystal structures of 2 (top)
and 3 (bottom). Left: repeating units; right: self-assembled supra-
molecular structure.
1
X-ray quality single crystals of 2 and 3 could be grown either
from a saturated DMSO solution or by sublimation.
Compound 2 crystallized in a trigonal unit cell with the R3
space group. As predicted, each molecule is interacting with
three other molecules: one antiparallel and two orthogonal.
Both enantiomers of the orthogonal arrangement can be
observed, and they are related by an inversion center. The two
types of supramolecular synthons control the self-assembly of
the entire crystal. The SBI distances within each supramolecular
synthon are 2.703(2) and 2.805(2) Å for the orthogonal pair
and 2.668(2) Å for each SBI in the antiparallel pair. The
primary bonds are elongated to an average distance of 2.015(2)
Å which is slightly longer than the Sb−O bonds in monomeric
antimony(III) alkoxides (see Table 1) due to presence of the
SBIs. The resulting supramolecular structure is a columnar helix
where the alkyl chains (R) attached to the cages are pointing
into the open channel of the helices (Figure 4). As shown in
Figure 4, there is a void space within the hexagonal columns
that is sufficiently large that the channel should be able to
accommodate an additional carbon on the R group of this
molecule. Accordingly, the crystal structure of compound 3
shows an almost identical supramolecular arrangement with a
̅
trigonal unit cell and an R3 space group. Here, the tails
completely fill in the void space in the center of the helical
column (Figure 4). Compared to compound 2, the distances
between the Sb and O atoms in the antiparallel supramolecular
synthons have each increased by 11.76% to 3.078(2) Å. Since
the antiparallel supramolecular synthons control the diameter
of the channel, this lengthening serves to accommodate the
larger tails. The SBI distances in the orthogonal supramolecular
synthons decreased (compared to that of 2) to 2.720(2) and
PXRD analysis of the bulk materials revealed that the bulk
crystalline phase matches that of the single crystals.
Interestingly, compounds 2 and 3 crystallized with hollow
morphologies by both sublimation and through cooling of a
saturated DMSO solution. Figure 5 illustrates an example of a
̅
Figure 5. Micrograph of single crystal of 3 illustrating hollow
morphology. Micrograph was captured with crystal immersed in
Paratone N oil. The bubble is of the gas from the interior escaping
from the crystalline tube.
single crystal of 3 that is releasing trapped nitrogen into the
surrounding oil upon immersion of the tube. Such tubular
crystals have been previously observed in various classes of
5
2−56
materials including small organics,
plexes,
coordination com-
and binary selenides. Typically, they are formed
by either the dissolution of the core with simultaneous
57−59
60
5
2,56
recrystallization of the outer walls
or by crystal growth
61
around a templating crystallite. Given that the tubular crystals
of 2 and 3 can be grown by sublimation, it is likely that they are
formed by templating rather than dissolution/recrystallization.
Crystals of both 2 and 3 of up to 1 cm in length with an outer
diameter of 1 mm and an inner diameter of 0.4 mm could
routinely be grown by sublimation.
2.591(2) Å. Although the distance in the orthogonal supra-
molecular synthons became shorter, the average distances in
the supramolecular synthons end up being larger than that in 2.
Given that the ethyl group in 3 completely fills the void
space, it seemed likely that the addition of another carbon to
D
DOI: 10.1021/acs.inorgchem.7b01049
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Inorganic Chemistry
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the R group would prevent the orthogonal supramolecular
synthon and force only the antiparallel alignments to produce a
Table 3. DFT Calculated Energetics of Self-Assembly (kJ/
mol)
2D bilayer. Single X-ray quality crystals of 4, grown from
a
a
a
a
b
ΔED
ΔEB
ΔER
ΔEDisp
ΔES
DMSO, confirmed this prediction. Compound 4 crystallizes in
the Pbca space group. Unlike our initial predictions of a 3-fold
symmetrical supramolecular arrangement (calculated tetramer
Δ-1 (⊥)
−92.1
−92.1
−81.2
−46.2
−62.7
−62.8
−62.3
−23.8
11.7
11.7
15.4
2.3
−51.7
−51.7
−44.1
−29.8
−51.0
−51.0
−46.9
−21.5
−47.0
−47.2
−44.6
−48.4
−23.1
−32.1
−21.4
−22.1
2
Λ-1 (⊥)
2
12
(∥)
14
(∥) and nonamer 1 (∥), Figure 3), each molecule is
9
12
(1 SBI)
interacting with four different molecules in antiparallel
14
(∥)
arrangements. As is shown in Figure 6, the supramolecular
19(∥)
22
22
32
32
42
42
(∥)
(⊥)
(∥)
(⊥)
(∥)′
(∥)″
−76.9
−87.4
−42.4
−70.6
−46.7
−51.4
−51.2
−54.3
−23.9
−35.0
−25.2
−32.7
6.7
5.8
−40.3
−47.9
−22.5
−46.0
−30.0
−36.2
0.8
2.8
3.8
10.6
a
ΔE
ΔE
= ΔEB(inding) + ΔE
= (ΔE
+ ΔEBSSE + ΔEDisp(ersion).
D(imerization)
R(eorganization)
b
+ ΔER(eorganization))/no. of supra-
S(upramolecular synthon)
B(inding)
Figure 6. Ball and stick representation of crystal structures of 4. Left:
repeating units with omitted hydrogen atoms; right: self-assembled
supramolecular structure depicting bilayer motif.
molecular synthons.
the crystal structure of 4 with those of 1−3, especially 4 (∥)″,
2
synthons are noncentrosymmetric which leads to large
variations (up to 0.9 Å) in the Sb···O SBI distances. The
distances are 2.421(1) and 3.398(1) Å in one antiparallel dimer
reveals significantly weaker SBIs. The weakening of the
individual SBIs is offset by the increase in the number of
SBIs around each Sb. A calculation of the SBI energy on a per
antimony basis reveals that the repeat unit in each supra-
molecular structure is stabilized by approximately the same
(
∥)′ and 2.597(1) and 3.427(1) Å in the other (∥)″.
Since the crystal structures of 2 and 3 only contain Sb···O
SBIs as structure directing interactions and they were observed
to sublime without melting at atmospheric pressure, the heat of
amount: −125.8, −91.8, and −98.1 kJ/mol for 2−4,
Sb
respectively. The values for 2 and 3 are in line with those
obtained from the DSC measurements. DFT calculations
appear to overestimate the value for 2 and underestimate that
of 3. These discrepancies could be the result of the dispersion
interactions between the neighboring hydrocarbons in the
structure that are not accounted for in these calculations, which
would be stabilizing, and the collective small distortions to the
molecular structures necessary for efficient packing but are
overall destabilizing. Collectively, these crystal packing forces
can account for the differences between experiment and theory.
Applying the same calculation to the hypothetical repeat unit
of the 2D layer illustrated by 1 (using the ΔE from 1 (∥))
sublimation (ΔH ) should be a direct measure of the strength
sub
of the SBIs. DSC was used to obtain a value of ΔH (256 °C)
sub
=
−101 ± 4 kJ/mol for 2 and ΔH (253 °C) = −101 ± 6 kJ/
sub
mol for 3, suggesting a similar average SBI strength within both
structures. Given that each antimony participates in 3 SBIs, the
average Sb···O SBI strength can be estimated as 34 kJ/mol. The
same process was not applied to 4 as it contains a mixture of
Sb···O SBIs within the 2D bilayer and other weak interactions
between the bilayers. To provide perspective, the average
hydrogen bond enthalpy in water or hydrogen peroxide from
62
the sublimation enthalpy is ∼21 kJ/mol. In the A−T
9
D
2
Watson−Crick base pair, it is 25.3 kJ/mol, and for the GC
reveals a stabilizing energy of −121.8 kJ/mol which is in the
Sb
63
base pair, it is 29.3 kJ/mol. The average free energy of
same range as the previous compounds. This suggests that the
judicious choice of the R group may allow the selection of a 3-
fold symmetrical bilayer.
binding for an I···N SBI in a dimer with three points of
1
1
attachment is ∼7 kJ/mol in a cyclohexane solution and 5.1
SBI
kJ/mol
in a benzene/acetone/methanol solution for a
SBI
8
supramolecular capsule with 4 I···N SBIs.
CONCLUSIONS
■
DFT calculations were carried out on dimers of 2−4 to
provide insight into the energetics of self-assembly in the
different structures and corroborate the DSC measurements.
The R groups on each molecule were replaced with hydrogen
atoms since they were not anticipated to have a significant
electronic effect on the energetics of dimer formation. In order
to mimic the steric effect of the R group on the supramolecular
synthon, the SbO, SbSb, and OO distances within each
supramolecular synthon were constrained during the optimiza-
tion (see Figure S2) to the distances that are observed in the
DFT calculations were used to predict the modes of self-
assembly of novel antimony(III) alkoxide cages through Sb···O
SBIs. Two different arrangements of a strong supramolecular
synthon were modeled. Sterics were used to select between
these and control the self-assembly of these molecules. Smaller
groups selected for self-assembly of 3D columnar helices while
larger groups selected for 2D bilayers. Infrared spectroscopy
could be used to probe self-assembly. These building blocks can
therefore be applied as a tool for predictable self-assembly, and
the SBI formation can be monitored spectroscopically gating
the application of these types of systems to self-assembly in
solution.
crystal structures of 2−4. The ΔE (Table 3) within 2 are
D
comparable to those of 1, which confirms the perfect fit of 2 in
the 3D structure. The additional carbon in the tail of 3 results
in a lower ΔE as compared to those of 1 and 2 by about 25
METHODS AND EXPERIMENTAL PROCEDURES
D
■
kJ/mol. This weakening of the supramolecular synthon is
consistent with the lengthening of the SBIs to accommodate
the larger tail in the center of the helical column in the crystal
General Methods. The starting materials antimony(III) tri-
chloride (99%, Strem Chemicals), potassium tert-butoxide (+98%,
Acros Organics), 1,1,1-trimethylol ethane (98%, AlfaAesar), 1,1,1-
trimethylolpropane (99%, Merck), potassium hydroxide (85%, Fisher
structure. Comparing the ΔE of the two different dimers in
D
E
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Inorganic Chemistry
Article
Chemical), pentaldehyde (99%, Aldrich), and formaldehyde (37%
solution in water, Fisher) were used as purchased. Ethanol (Absolute
ACS-grade, Pharmco-Aaper), chloroform (ACS-grade, Macron Fine
Chemicals), and acetonitrile (99.5%, Sigma-Aldrich) were used
without further purification. Anhydrous tetrahydrofuran (THF) was
obtained by passing HPLC-grade THF over a bed of activated
molecular sieves in a commercial (LC Technologies Solutions Inc.)
solvent purification system (SPS). Deuterated solvents, purchased
from Cambridge Isotopes Laboratory, were degassed using three
freeze−pump−thaw cycles before being transferred onto freshly
activated molecular sieves. After sitting on the sieves for 2 days, they
were again transferred onto freshly activated sieves for storage. Air-
Hydrolytic Stability Tests of 2−4. To test the stability of the
produced caged antimony alkoxide two different approaches were
used: solution state and solid state. For the solution state, small
amounts (1−3 mg) of 2−4 were dissolved in ∼0.7 mL of dry degassed
DMSO-d inside the glovebox. Then, 0.05 mL of D O was injected
through the septum cap inside the NMR tube, and the H NMR
spectrum of each compound was collected. In this method, all
compounds were completely hydrolyzed. In solid state, powders of 2−
4 were left outside the glovebox under ambient conditions for 45 h.
6
2
1
After the reported amount of time, the powder of each compound was
1
dissolved in dry degassed DMSO-d
, and the H NMR spectrum was
6
collected for each compound.
1,1,1-Trimethylolbutane, (HOCH ) CC H (7). The synthesis was
2 3 3 7
65
sensitive manipulations were performed in an N -purged inert
2
atmosphere box (LC Technology Solutions Inc.). All NMR spectra
were collected at 60 °C on a JEOL ECS 400 MHz NMR spectrometer.
All IR spectra were obtained using a Nicolet iS 5 FT-IR spectrometer
equipped with a Specac Di Quest ATR accessory, and CHN analysis
were obtained either on-site with a PerkinElmer 2400 Series II
CHNS/O analyzer or from Midwest Microlabs Inc. DSC analysis was
done using SHIMADZU instrument model DSC60 with TA-60WS
thermal analyzer. Samples were crimped and prepared under N2
atmosphere inside aluminum cuvettes. The test was done under
nitrogen atmosphere with a flow rate of 100 mL/min against an empty
aluminum cuvette as reference.
based on a method reported by Weibull and Matell. A mixture of
pentanal (8.6 g, 0.1 mol) and aqueous formaldehyde (37%, 65 g; 0.8
mol) were added to a round-bottomed flask. Then, 200 mL of a 1:1
ethanol and water solution was added to dissolve the mixture. The
mixture was cooled down with an ice−water bath and stirred. An
ethanol−water (1:1) solution of potassium hydroxide (5.6 g in 100
mL, 1 M) was added to the cold aldehyde mixture during 1 h while
stirring. After the addition of the alkali solution, the mixture was
heated overnight at 50 °C. Ethanol was removed from the mixture, and
product was extracted with diethyl ether. The ether was evaporated
and the resulting mixture was distilled under vacuum. The resulting
residue (7) was recrystallized once from chloroform and a second time
Synthesis. Antimony(III) tert-Butoxide, Sb(OC(CH ) ) . This
3
3 3
1
preparation was adapted from the previously reported synthesis of
from acetonitrile. Yield: 1.5 g, 10%. Mp 94−98 °C. H NMR (400
64
bismuth(III) tert-butoxide. All manipulations were performed under
a nitrogen gas atmosphere. Potassium tert-butoxide (16.8 g, 150
mmol) was dissolved in 150 mL of THF, and the solution was then
MHz, DMSO-d
6
, δ): 0.83 (t, J = 8 Hz, 3H, CH
2
CH
2
CH ), 1.12 (t, J =
3
4
Hz, 2H, CH CH CH ), 1.25 (m, 2H, CH CH CH ), 3.26 (d, J = 8
2
2
3
2
2
3
1
3
1
Hz, 6H, (HOCH ) C), 4.19 (t, J = 4 Hz, 3H, (HOCH ) C). C{ H}
2
3
2 3
NMR (100.6 MHz, DMSO-d , δ): 15.71, 16.24, 32.30, 44.02, 62.77.
cooled down to −78 °C. SbCl (11.4 g, 50.0 mmol) was dissolved in
6
3
−1
FTIR (ATR, cm ) 3198.37 (sb), 2924.68 (m); 1011.53 (s)). Anal.
Calcd for C H O : C, 56.73; H, 10.88. Found: C, 56.61; H, 11.11).
5
0 mL of THF and added slowly to the potassium tert-butoxide
solution with stirring. After the addition was finished, the reaction
mixture was allowed to warm up to room temperature and was stirred
for an additional 24 h. The reaction mixture was taken to dryness
under vacuum. Hexanes (200 mL) were added to the residue, and the
suspension was centrifuged to separate the solids from the solution.
The solution was decanted, and the solvent was removed to yield an
7
16
3
Computational Method. Calculations were performed using the
ORCA 3.0 quantum chemistry program package from the develop-
66
ment team at the Max Planck Institute for Bioinorganic Chemistry.
The LDA and GGA functionals employed were those of Perdew and
6
7
Wang (PW-LDA, PW91). In addition, all calculations were carried
6
8,69
1
out using zero-order regular approximation (ZORA).
For all
oily colorless liquid (14.3 g, 83%). H NMR (400 MHz, C D , δ): 1.36
6
6
70,71
13
1
calulations the def2-TZV(pp) basis sets were used for all atoms.
(
(
1
s, 27H, C(CH ) ). C{ H} NMR (100.6 MHz, C D , δ): 34.06
C(CH ) ), 74.78 (C(CH ) ). FTIR (ATR, cm ) 2968.60 (vs),
171.91 (vs), 587.44 (s).
Antimony(III) Alkoxide Cages Sb(OCH ) CC H (n = 1, 2, and
n 2n+1
3
3
6
6
−1
Spin-restricted Kohn−Sham determinants were chosen to describe the
closed shell wave functions, employing the RI approximation and the
tight SCF convergence criteria provided by ORCA. Basis set
superposition errors (BSSE) were corrected for using the method of
3
3
3 3
2
3
3
) (2−4). Antimony(III) tert-butoxide (0.341 g, 1 mmol) was dissolved
7
2
in 5 mL of THF, and equimolar THF solution of the triol was added
while stirring at 22 °C under nitrogen atmosphere. Upon mixing, a
white precipitate was immediately observed. The mixture was allowed
to stir overnight. The mixture was filtered and rinsed three times with
anhydrous THF. The collected white solid was stored under nitrogen.
Boys and Bernardi. London dispersion forces were calculated using
73
Grimme’s D3 correction in conjunction with the BP86 func-
6
7,74
tional.
Analytical frequencies calculations performed on all low
energy conformations of unconstrained molecules revealed no negative
frequencies.
To estimate the energetics from the crystals, the R group of each
molecule was replaced by a hydrogen atom, and the distances between
the atoms within each synthon in the dimers were constrained to the
distances from the crystal structures (see Figure S1). All other
distances and angles could freely refine. As these structures will
necessarily not be at a minimum, the frequency calculations (for
obtaining zero-point energy, enthalpy and entropy) were not
performed on these structures.
Characterization Data for 2. Yield: 154 mg, 64.5%. Mp 330−334
1
°
(
(
(
C (inside sealed capillary tube under nitrogen atmosphere). H NMR
400 MHz, DMSO-d , δ): 0.44 (s, 3H, CH ), 3.96 (s, 6H,
6
3
13
1
SbOCH ) C). C{ H} NMR (100.6 MHz, DMSO-d , δ): 18.53
2
3
6
CH ), 39.16 (SbOCH ) C), 74.54 ((SbOCH ) C). FTIR (ATR,
3
2
3
2 3
−
1
cm ) 2842.10 (s), 1031.82 (s), 497.59 (s). Anal. Calcd for
C H O Sb: C, 25.14; H, 3.80. Found: C, 24.86; H, 3.66).
5
9
3
Characterization Data for 3. Yield: 167 mg, 66.0%. Mp 292−294
1
Energetics in the manuscript were reported according to the
following equations:
°
(
C (inside sealed capillary tube under nitrogen atmosphere). H NMR
400 MHz, DMSO-d , δ0.69 (t, J = 8 Hz, 3H, CH CH ), 0.90 (q, J = 8
6
2
3
1
3
1
Hz, 2H, CH CH ), 3.98 (s, 6H, (SbOCH ) C). C{ H} NMR (100.6
2
3
2 3
ΔED(imerization) = ΔEB(inding) + ΔER(eorganization) + ΔEBSSE
MHz, DMSO-d , δ): 7.26 (CH CH ), 25.75 (CH CH ), 40.47
6
2
3
2
3
−1
(
(
4
SbOCH ) C), 72.45 ((SbOCH ) C). FTIR (ATR, cm ) 2849.84
2
3
2 3
+
ΔEDisp(ersion)
(1)
(2)
s), 1048.35 (s); 501.07 (s). Anal. Calcd for C H O Sb: C, 28.49; H,
6
11
3
.38. Found: C, 28.29; H, 4.15.
Characterization Data for 4. Yield: 266.9 mg, 70.4%. Mp 242−244
ΔEB(inding) + ΔER(eorganization)
( )
1
ΔES(upramolecular synthon) =
°
(
C (inside sealed capillary tube under nitrogen atmosphere). H NMR
no. of supramolecular synthons
400 MHz, DMSO-d , δ): 0.81 (m, 5H, CH CH CH ), 1.11 (td, J = 8
6
2
2
3
1
3
1
Hz, CH CH CH ), 3.99 (s, 6H, (SbOCH ) C. C{ H} NMR (100.6
Here, ΔED is the stabilization energy upon dimerization of two
2
2
3
2 3
MHz, DMSO-d , δ): 15.05, 15.65, 35.83, 40.88, 72.69. FTIR (ATR,
molecules, ΔE is the energetic cost of breaking the SBIs without
6
B
−
1
cm ) 2838.94 (s), 1045.01 (s), 502.18 (s). Anal. Calcd for
C H O Sb: C, 31.50; H, 4.91. Found: C, 31.27; H, 4.45.
allowing the monomers to relax to their gas phase minimum, ΔE is
R
the relaxation or reorganization energy (the energy difference between
7
13
3
F
DOI: 10.1021/acs.inorgchem.7b01049
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the monomers in the dimer geometry and the gas phase geometry of
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the monomer), ΔE
is the correction for the basis set superposition
error using the counterpoise method of Boys and Bernardi, and
accounts for the attractive London forces between the two
BSSE
72
(4) Scholfield, M. R.; Zanden, C. M. V.; Carter, M.; Ho, P. S.
Halogen bonding (X-bonding): A biological perspective. Protein Sci.
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ΔE
Disp(ersion)
molecules. In order to compare the extended structures, the energy per
supramolecular synthon (ΔE ) was determined according to eq 2.
(5) Auffinger, P.; Hays, F. A.; Westhof, E.; Ho, P. S. Halogen bonds
in biological molecules. Proc. Natl. Acad. Sci. U. S. A. 2004, 101,
S
CCDC Searches. Two CCDC (892 156 entries, May 2017)
searches were performed in this work. The first was to find instances
of antimony(III) alkoxides and aryloxides that either self-assembled or
were monomeric in he solid state. The search was performed by
constructing a fragment that contained one tricoordinate antimony
bound to three dicoordinate oxygen atoms by single bonds. Each
oxygen was singly bonded with one carbon atom. Instances with
carboxylate groups bond to the antimony or intra- and hetero-
molecular interactions were manually excluded from this search. The
second search was to count the number of structurally reported
molecules containing polar bonds of N or O with Te(II or IV),
Sb(III), and I(I). Separate searches for compounds containing at least
one single bond between each aforementioned elements was
performed while limiting the coordination number of each heavy
element to its valence. The search resulted in 321 compounds for
1
6789−16794.
6) Fanfrlík, J.; Kolar
Cousido-Siah, A.; Mitschler, A.; Rezac
(
́
,
̌
M.; Kamlar, M.; Hurny,
́
D.; Ruiz, F. X.;
̌
́
,
̌
J.; Munusamy, E.; Lepsík, M.;
̌
et al. Modulation of Aldose Reductase Inhibition by Halogen Bond
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2
(
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(
Thiel, W.; Sanchez-Garcia, E.; Sander, W. Switching the Spin State of
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1689−1697.
II/IV
III
Te −N/O, 264 compounds for Sb −N/O, and 44 compounds for
I
I −N/O.
(10) Jungbauer, S. H.; Schindler, S.; Herdtweck, E.; Keller, S.; Huber,
S. M. Multiple Multidentate Halogen Bonding in Solution, in the Solid
State, and in the (Calculated) Gas Phase. Chem. - Eur. J. 2015, 21,
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01049.
■
*
S
1
(
3625−13636.
11) Jungbauer, S. H.; Bulfield, D.; Kniep, F.; Lehmann, C. W.;
Herdtweck, E.; Huber, S. M. Toward Molecular Recognition: Three-
Point Halogen Bonding in the Solid State and in Solution. J. Am.
Chem. Soc. 2014, 136, 16740−16743.
Crystal data, NMR spectra, FTIR spectra, DSC traces,
Cartesian coordinates, and DFT calculated energies
(12) Massena, C. J.; Wageling, N. B.; Decato, D. A.; Martin
Rodriguez, E.; Rose, A. M.; Berryman, O. B. A Halogen-Bond-Induced
Triple Helicate Encapsulates Iodide. Angew. Chem., Int. Ed. 2016, 55,
12398−12402.
(
PDF)
Accession Codes
CCDC 1544159−1544161 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing 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.
(13) Cao, D.; Hong, M.; Blackburn, A. K.; Liu, Z.; Holcroft, J. M.;
Stoddart, J. F. Two-point halogen bonding between 3,6-dihalopyr-
omellitic diimides. Chem. Sci. 2014, 5, 4242−4248.
(14) A search through CCDC for polar bonds (to N or O) of Te(II/
IV) and Sb(III) results of 321 and 264 hits, respectively. The same
search for iodine only gave 44 hits.
(
15) Cozzolino, A. F.; Whitfield, P. S.; Vargas-Baca, I. Supramolecular
Chromotropism of the Crystalline Phases of 4,5,6,7-Tetrafluorobenzo-
,1,3-telluradiazole. J. Am. Chem. Soc. 2010, 132, 17265−17270.
16) Garrett, G. E.; Gibson, G. L.; Straus, R. N.; Seferos, D. S.;
AUTHOR INFORMATION
Corresponding Author
E-mail: anthony.f.cozzolino@ttu.edu. Fax: +1 806 742 1289.
Tel.: +1 806 834 1832.
■
*
2
(
Taylor, M. S. Chalcogen Bonding in Solution: Interactions of
Benzotelluradiazoles with Anionic and Uncharged Lewis Bases. J.
Am. Chem. Soc. 2015, 137, 4126−4133.
ORCID
(
17) Garrett, G. E.; Carrera, E. I.; Seferos, D. S.; Taylor, M. S. Anion
recognition by a bidentate chalcogen bond donor. Chem. Commun.
016, 52, 9881−9884.
18) Ho, P. C.; Szydlowski, P.; Sinclair, J.; Elder, P. J. W.; Ku
Anthony F. Cozzolino: 0000-0002-1100-0829
Funding
The present work was supported by the Robert A. Welch
Foundation (D-1838, USA), Texas Tech University, and the
National Science Foundation (NMR instrument grant CHE-
2
(
̈
bel, J.;
Gendy, C.; Lee, L. M.; Jenkins, H.; Britten, J. F.; Morim, D. R.; Vargas-
Baca, I. Supramolecular macrocycles reversibly assembled by Te···O
chalcogen bonding. Nat. Commun. 2016, 7, 11299−11309.
1048553). J. Yu was funded through the Welch Summer
Scholar Program.
Notes
The authors declare no competing financial interest.
(19) Jansen, M.; Moebs, M. Structural investigations on solid
tetraphosphorus hexaoxide. Inorg. Chem. 1984, 23, 4486−4488.
(20) Ballirano, P.; Maras, A. Refinement of the crystal structure of
arsenolite, AS O . Z. Kristallogr. - New Cryst. Struct. 2002, 217, 177−
2
3
1
(
78.
21) Whitten, A. E.; Dittrich, B.; Spackman, M. A.; Turner, P.;
ACKNOWLEDGMENTS
We are grateful to the Hope-Weeks group at Texas Tech for
assistance with DSC measurements.
■
Brown, T. C. Charge density analysis of two polymorphs of
antimony(III) oxide. Dalton Trans. 2004, 23−29.
(22) Ensinger, U.; Schwarz, W.; Schrutz, B.; Sommer, K.; Schmidt, A.
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