Single-crystal-to-single-crystal transformation of the desolvation of a cyclotriveratrylene-acetonitrile inclusion complex: Via a gating mechanism with subsequent polymorphism

Article abstract of DOI:10.1039/c6ce01078d

A cyclotriveratrelyne-acetonitrile inclusion complex is shown to undergo a single-crystal-to-single-crystal transformation upon desolvation to an intermediate phase and it is furthermore demonstrated that the preparation conditions may introduce polymorphism of the apohost. We characterised the as-synthesized and desolvated polymorphs using single crystal (polymorph 1) and powder X-ray diffraction, thermal analysis and gas sorption. Successive data collections on the as-synthesized crystal at 50 °C show that the rotation of a single, unique methoxy group of the host molecule may allow evacuation of solvent molecules from the seemingly non-porous crystal.

Full text of DOI:10.1039/c6ce01078d

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ARTICLE
Single-crystal-to-single-crystal transformation of the desolvation
of a cyclotriveratrylene-acetonitrile inclusion complex via a gating
mechanism with subsequent polymorphism
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
DOI: 10.1039/x0xx00000x
Clive L. Oliver * and Richard M. Payne
www.rsc.org/
A cyclotriveratrelyne-acetonitrile inclusion complex is shown to undergo a single-crystal-to-single-crystal transformation
upon desolvation to an intermediate phase and it is furthermore demonstrated that the preparation conditions may
introduce polymorphism of the apohost. We characterised the as-synthesized and desolvated polymorphs using single
crystal (polymorph 1) and powder X-ray diffraction, thermal analysis and gas sorption. Successive data collections on the
as-synthesized crystal at 50°C on the same crystal show that the rotation of a single, unique methoxy group of the host
molecules may allow evacuation of solvent molecules from the seemingly non-porous crystal.
Introduction
Cyclotriveratrylene (CTV), a cyclic trimer of veratrole, acts as a
bowl-shaped, molecular host in its crown conformation, which
is capable of binding ions or small organic/organometallic
1
-3
molecules. CTV (Scheme 1) has a triangular pyramid shape
with the aromatic rings forming three faces of the pyramid and
4
-7
the methylene groups positioned at the apices.
CTV is
related to calix[n]arenes (where n is most commonly 4, 6 or 8)
but is less common in terms of the variety of inclusion
1
,8
complexes formed and their diversity of applications.
Molecules based on a CTV scaffold are common and have been
used in a variety of applications in both soft materials and
1
,9
supramolecular chemistry. These include being used as
constituents of discrete, cage-like, metallo-supramolecular
1
assemblies, such as a self-entangled Solomon’s cube and
0
1
1-12
assemblies of triply interlocking cage-catenane species,
been performed on conjugate microporous polymers and
0-31
These
3
and coordination polymers. CTV and its analogues have also porous organic polymers which incorporate CTV.
been utilized as constituents in self-assembled and have been shown to display modest hydrogen uptake of 0.5-
2
-
coordination supramolecular systems, and as precursors for 0.8 % wt at 77 K and ~1 bar with surface areas of 200-350 m g
1
,13-19
1
cavitands and cryptophanes,
organometallic and organic guests.
where they bind smaller
0-22
Pd3L2 metallo-cryptophane cages with CTV-type L ligands
.
2
Their capability to host which are stabilized by a bis-N-heterocyclic carbene as an
guests such as xenon, lanthanides and C60 and anionic C70 auxiliary cis-protecting ligand, have been shown to take up 1,2-
dimers has led to the growth of functionalized CTV derivatives dichlorobenzene or iodine via a single-crystal-to-single-crystal
3
2
for biomedical applications such as the distribution of (SCSC) process. . SCSC transformations allow the
fullerenes and their use in MRI-based diagnostic techniques.
2
3-
determination of crystal structures of materials after physical
To date, very little work has been done on porous structures events such as desolvation/resolvation or polymorphic
2
9
incorporating CTV or functionalized CTV. Porosity studies have transformations and may thus yield important insights into
mechanisms of transformation which may help in the design of
3
3
new materials. It is still a relatively rare phenomenon,
however increasing examples are being reported, especially
for metal-organic frameworks (MOFs), including in our own
3
4-35
laboratory.
The higher prevalence for MOFs may be due to
the greater degree of robustness of the crystal structure
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afforded by coordination bonds, nevertheless, examples of Hyper HAD colour video camera and captured using the Soft
purely organic systems undergoing SCSC transformations in Imaging System program analySIS. Differential scanning
3
6-37
porosity studies have been reported.
Herein we present, calorimetry (DSC) was performed using a DSC Q200
the synthesis of a CTV-acetonitrile inclusion compound, which instrument. The sample (~4 mg) was weighed directly into a
is seemingly non-porous due to its isolated cavities. The pierced aluminium pan, on an analytical balance.
inclusion compound can be desolvated into two different Thermogravimetric analysis (TGA) was performed using a TGA
polymorphs, depending on the desolvation conditions. The Q500 instrument. The sample (~3.7 mg) was weighed directly
solvated and desolvated forms have been characterized by X- into open aluminium crucibles. During the DSC and TGA
ray diffraction, thermal analysis and gas sorption.
experiments the sample holder was continuously purged with
a dry nitrogen flow of 60 mL/min, whilst a heating rate of 10
°
C/min was employed.
Experimental
Materials
Results & Discussion
Formaldehyde solution (> 34.5 wt %) and veratrole (>99 %)
were purchased from Sigma Aldrich and used as received. CTV 1a crystallized from a 1:1 v/v mixture of acetonitrile and
3
8
was synthesized via a literature method.
chloroform. Single crystal X-ray structure analyses show that
the asymmetric unit (ASU) of 1a consists of two CTV molecules
and one acetonitrile molecule (chloroform was excluded from
Crystal growth
CTV (60 mg) was dissolved in 1:1 v/v acetonitrile:chloroform (5 the crystal structure but was added for solubilisation of
1). The
mL) with stirring. Slow evaporation of the solvents leads to the two unique CTV molecules play two different structural roles
appearance of crystals after 7 – 10 days.
in the crystal structure with one forming centrosymmetric,
dimeric capsules which encapsulate two acetonitrile molecules
Single crystal X-ray diffraction analysis and structure per dimer (Fig. 1a), whilst the second self-includes to form
determination columns of stacked CTV molecules (Fig. 1b). The dimers and
Single crystals of suitable quality were selected and mounted stacked columns are arranged in 2D layers in the bc plane
on a cryoloop in oil. Data collections were carried out on a which stack along the a-axis (Fig. 1c). No classical hydrogen
Bruker DUO APEX II CCD diffractometer using graphite bonds are present in the structure of 1a, however five unique
monochromated MoKα (λ = 0.71073 Å) radiation with the C-H⋅⋅⋅O hydrogen bonds seem to play an important role in
crystal cooled to 173 K or 153 K using an Oxford Cryostream- host-solvent and host-host interactions (Table S1). The
7
00. Data reduction and cell refinement were performed using centrosymmetric dimer is indirectly held together by six host-
3
9
SAINT-Plus. The X-ray diffraction data were scaled and solvent C-H⋅⋅⋅O/C-H···N hydrogen bonds. Four of these
corrected for absorption effects using SADABS. The structure hydrogen bonds (C25A-H25A⋅⋅⋅N1S and C26A-H26A⋅⋅⋅N1S)
4
0
4
1
was solved by direct methods, implemented in SHELXS-97.
involve the acetonitrile nitrogen atom acting as a bifurcated
4
Refinement, via the OLEX2 interface, proceeded by the full- hydrogen bond acceptor for host methyl hydrogen bond
2
2
matrix least-squares method, based on F values against all donors, whilst the remaining two hydrogen bonds (C2S-
reflections as implemented in SHELXL-2014/7 with anisotropic H2SC⋅⋅⋅O5A) involve the host methoxy oxygen atom acting as
displacement parameters for all non-H atoms. X-Seed and hydrogen bond acceptor for a methyl hydrogen bond donor of
4
1
MERCURY were used for generating high-quality images using the solvent molecule. In addition, the acetonitrile molecule
4
3-45
POV-RAY.
PLATON was used to determine hydrogen also forms two unique C-H⋅⋅⋅π interactions with the dimer CTV
bonding parameters as well as, together with MSROLL, void molecule (Table S1). Subjecting crystals of 1a to a dynamic
4
6-47
spaces using specified probe radii.
vacuum at 90 °C for 24 h resulted in the desolvated form 2a, a
process which occurred via a SCSC transformation. The
asymmetric unit of 2a consists of two crystallographically
Powder X-ray diffraction
Powder X-ray diffraction (PXRD) data were collected on a independent CTV molecules, as was the case for 1a, however,
Bruker D8 Advance X-ray diffractometer with copper radiation the absence of appreciable electron density in the difference
(CuKα, λ = 1.5406 Å) at 30 kV and 40 mA. Each sample was Fourier map indicated the absence of acetonitrile. This was
scanned between 4 and 40° 2θ with a step size of 0.02°. confirmed by thermogravimetric analysis, whilst the SQUEEZE
MERCURY was used to calculate PXRD patterns from single routine in PLATON indicated a residual electron density of only
crystal data for comparison with experimentally obtained 6 electrons per unit cell. The void space for 2a (Fig. 2d)
PXRD patterns.
calculated by PLATON (using a probe radius of 1.2 Å) is 4.6%
per unit cell and is largely due to the cavity of the dimer, with a
much smaller portion of this space (0.8% per unit cell) due to
Thermal analysis
Hot-stage microscopy (HSM) was performed using a Nikon the space between the columns of stacked CTV molecules. This
SMZ-10 stereoscopic microscope fitted with Linkam 4.6% void space is smaller than the 7.2% potential void space
a
THMS600 hot stage and a Linkam TP92 control unit. Samples calculated for 1a if the acetonitrile molecules are artificially
were placed under silicone oil on a cover slip and heated at 10 removed, which indicates that structural changes occurred
°
C/min. Thermal events were monitored with a Sony Digital upon
solvent
removal.
The
2
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ARTICLE
(a)
(b)
b
a
(c)
(d)
(e)
(f)
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Table 1 Crystal data and refinement parameters for 1a, 2a and 3a
Journal Name
1a
2a
Empirical formula
C
28
H
31.5
N
0.5
O
6
27 30 6
C H O
Formula weight
471.04
173.15
450.51
153.15
Temperature/K
Crystal system
monoclinic
P2 /c
monoclinic
P2 /c
Space group
1
1
a/Å
b/Å
c/Å
α/°
β/°
γ/°
18.7531(8)
25.0061(11)
10.5276(5)
90
18.606(2)
24.551(3)
10.4027(13)
90
91.2590(10)
90
90.819(3)
90
3
Volume/Å
4935.6(4)
8
4751.5(10)
8
Z
3
ρ
calcg/cm
1.268
1.26
-
1
μ/mm
0.089
0.088
F(000)
2008
1920
3
Crystal size/mm
Radiation
0.30 × 0.20 × 0.10
MoKα (λ = 0.71073 Å)
2.714 to 59.034
0.6 × 0.33 × 0.24
MoKα (λ = 0.71073 Å)
2.746 to 52.944
2
Θ range for data collection/°
Index ranges
-25 ≤ h ≤ 25, -34 ≤ k ≤ 32, -14 ≤ l ≤ 14
112892
-23 ≤ h ≤ 22, -30 ≤ k ≤ 30, -12 ≤ l ≤ 13
Reflections collected
Independent reflections
Data/restraints/parameters
48505
13782 [Rint = 0.0792, Rsigma = 0.0468]
13782/0/635
9790 [Rint = 0.0362, Rsigma = 0.0310]
9790/0/607
1.027
2
Goodness-of-fit on F
1.023
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å
R
R
1
1
= 0.0476, wR
= 0.0925, wR
2
2
= 0.1117
= 0.1322
R
R
1
1
= 0.0444, wR
= 0.0696, wR
2
2
= 0.1005
= 0.1122
-
3
0.36/-0.25
0.68/-0.38
voids are isolated in 2a (as well as in 1a with the solvent O1A-C7A points towards the interior of the dimer cavity which
molecules artificially removed) and it is reasonable to suggest is allowed by the absence of the acetonitrile solvent molecule
that the 1a to 2a transition involves evacuation of the (Figs 1e and 1f). In 1a two unique C-H⋅⋅⋅O hydrogen bonds
acetonitrile molecules along the ~10 Å axis with at least one (C16B-H16D···O1A and C25B-H25E···O5A) serve to link the two
unique rotatable methoxy groups acting as ‘gates’. We show unique CTV molecules so that 1D, zigzag strands of connected
that this is plausible in a simple fashion by taking one methoxy CTV molecules of both types are formed along the ~25 Å axis
group group of a dimer CTV molecule in 1a (O6A-C26A) and (Fig. 2c). In 2a, CTV molecules of a dimer are now directly
artificially rotating it by 75° about the C20A-O6A bond so that connected to each other by one unique C-H···O hydrogen bond
C26A points away from the dimer cavity. This resulted in the (C8A-H8AB···O5A), indicative of the lateral shift of these
void space of two neighbouring cavities connecting (Fig S1). molecules. A second unique C-H···O hydrogen bond (C7B-
The hypothesis is also supported by the fact that the two H7BB···O6A) connects the dimer to a ‘stack’ CTV molecule on
corresponding methoxy groups in the actual structures are both sides of the dimer. Unlike in 1a, these ‘stack’ CTV
found in significantly different orientations in 1a and 2a as molecules are not connected to a second dimer resulting in
indicated by the torsion angles C26A-O6A-C20A-C19A and C7A- hydrogen-bonded tetramers instead (Figs. 2d and S2). These
O1A-C1AS-C2A of -170.0(1)
°
and -80.4(1)
°
for 1a and 2a
,
tetramers which are related to each other by the
1
respectively, indicating that this methoxy group is rotated in crystallographic 2 axis along the b-direction (Fig. 2d).
the crystal structures. In 2a the corresponding methoxy group
4
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~
25 Å
(
a)
(b)
~18 Å
(
c)
(d)
Figure 2 Packing diagrams of CTV molecules in (a) 1a and (b) 2a with ‘stacked’ CTV molecules shown in space-filling mode. (c) CTV
molecules in 1a and (d) 2a connected by C-H···O hydrogen bonding shown in space-filling mode and the same colour. All views are down
the ~10 Å axis.
longer a single crystal as indicated by the drastic loss of
Variable temperature X-ray diffraction studies
The solvated form 1a, could also be prepared by gentle experimental conditions are very different from how 2a was
grinding of CTV in the presence of acetonitrile (without produced initially (90 C under vacuum vs the flow of
chloroform) for approximately 20 minutes. Variable continuous dry air over the crystal), we adjusted our approach
temperature PXRD experiments (VT-PXRD) performed on 1a by doing successive data collections at 50 C on the same
prepared in this manner, over the temperature range 30–210 crystal (2 hours required for a complete data set). We then
C indicated that the sample is crystalline over the entire completely solved and refined the host structure and
temperature range of analysis (Fig. 3). The 30 C PXRD pattern subjected each structure to a SQUEEZE analysis to ascertain
intensity of the diffraction spots. Keeping in mind that the
°
,
°
°
°
agrees closely with the calculated PXRD pattern from the
single crystal structure of 1a, indicating that this method of
producing 1a was successful. At 100
.9 and 9.2 degrees 2 are reduced and the pattern matches
that of the calculated 2a pattern more closely. From 130-200
C the gradual appearance of peaks at 8.2 and 9.1 degrees 2
indicates that 2a transforms to another polymorph 3a.
°C the three peaks at 8.4,
8
θ
°
θ
We also performed in situ variable-temperature SCXRD
experiments on 1a in order to investigate the possibility of
following the SCSC transformation to 2a and subsequently 3a
in situ. Heating was achieved by the flow of hot dry air over
the crystal using the diffractometer’s cryostream. At first we
collected unit cell parameters at temperatures from 25°-
10 C. It was clear that after 100 C that the crystal was no
Figure 3 VT-PXRD patterns in the range 30-200°C compared
with the calculated patterns of 1a and 2a
1
°
°
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2
1°C
110°C
120°C
250°C
Figure 5 Hot stage microscope photographs of compound 1a under
silicone oil at various temperatures (bubbles indicate the release of
solvent)
Figure 4 Occupancy of acetonitrile and C26A position after
successive in situ 2 hour SCXRD data collections at 50°C. Note
that acetonitrile occupancy was calculated from a SQUEEZE
electron count per unit cell analysis and the C26 occupancy was
refined in SHELXL-2014.
the electron count per unit cell for each data collection. We
present these results in Figure 4 and Table S2. Successive data
collections show a continuous reduction in the electron count
per unit cell indicative of the loss of the acetonitrile from the
crystal. The count after the 24 hours (12 data collections) was
1
8 electrons per unit cell which is slightly more than the 6
electrons per unit cell calculated for the structure of the crystal
subjected to a temperature of 90 under vacuum for 24 hours.
Furthermore, after 6 hours the position of C26A had to be crystals on the DSC equipment to 130
Figure 6 DSC and TGA thermograms of 1a
°
°
C and subjecting them
modelled over two positions (second position labelled C27A) to PXRD analysis (Fig S4). The experimental conditions for the
indicating dynamic rotation of this methyl group. The electron VT-PXRD were less ‘harsh’ by slower heating of 1a by recording
density of C27A points towards the interior of the dimer cavity patterns at 20-25
as in the case of 2a in Fig. 1f) as this space becomes available allowed 2a to observed. The TGA thermogram of 1a (Fig. 6)
to allow a degree of self-inclusion. The fact that the rotation shows a single-step mass loss of 4.12% (calculated 4.39%) in
away from the dimer cavity is not observed, as suggested in the temperature range 30-150 C corresponding to one
°C intervals from room temperature which
(
°
Fig. S1 for the cavities to ‘connect’, indicates that this rotation acetonitrile molecule per asymmetric unit.
is short-lived compared to the rotation towards the interior of
the cavity. Nevertheless, the observed disorder supports a Gas sorption studies
‘
gating’ hypothesis for movement of molecules in and out of Forms 2a and 3a (preparation and confirmation in Fig S4) were
the structure. The in situ SCXRD experiments prove that the subjected to gas sorption analyses from vacuum up to 700-800
transformation from 1a to 2a can occur via a SCSC mm Hg absolute pressure. Nitrogen sorption at 77 K was
transformation under the appropriate conditions. We could negligible for both forms, which we suspect is due to nitrogen
not show that this could be the case for the transition from 2a not being able to penetrate the structures at these modest
to 3a and could therefore not determine the structure of the pressures. Carbon dioxide sorption was significant for the
latter. However, we suggest that 3a is probably a higher- pressure and temperature (273K) of analysis (Fig. 7). The
3
-1
density form than 2a by virtue of a greater degree of self- sorption of 9.47 cm (STP) g at 783 mm Hg for 2a corresponds
inclusion.
to ~0.4 CO
occurs despite an unobstructed path for CO
indicating that CO is able to induce rotation of the methyl
to induce
indication of their desolvation as bubbles are formed in the structural changes in many seemingly non-porous materials,
silicone oil upon heating. The first bubble appears at 60
C, often selectively, has been attributed to its quadrupole
with more vigorous bubbling occurring at 110
C accompanied moment. However, in a recent study combining in situ PXRD
2
molecules per asymmetric unit. This sorption
2
sorption
Thermal analysis
2
HSM photographs (Fig. 5) of crystals of 1a allow visual groups for sorption to occur. The ability of CO
2
°
°
by the crystals becoming opaque which is an indication of the and SCXRD experiments with computational studies, weak van
loss of monocrystallinity. DSC analysis (Fig. 6) show a broad der Waals interaction were also shown to be important for a
4
8
endotherm, with a shoulder, in the temperature range 100- metal-organic framework. The transformation of 2a to 3a is
20 C, representing desolvation and
concomitant from a single crystal to a polycrystalline material (as confirmed
1
°
rearrangement of the host molecules to form 3a. In a separate by in situ SCXRD and VT-PXRD experiments) and therefore
experiment, this rearrangement was confirmed by heating would involve an increase in surface area and therefore a
6
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5.
H. Erdtman, R. Ryhage, F. Haglid, Acta Chem. Scand., 1964, 18
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.
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3
9.
1
1
2
Figure 7 CO sorption isotherms of 2a and 3a at 273 K.
1
relative increase in sorption compared to if it was via an SCSC
transformation. However, we believe that 3a is probably a
higher-density form of 2a and therefore this factor negates the
effect of the increase in surface area. Another interesting
phenomenon was that for both 2a and 3a hysteresis was
present indicating that this material is able to hold on to
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,
,
1
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1
4. M. J. Hardie, R. Ahmad, C.J. Sumby, New J. Chem., 2005, 29
,
,
,
Conclusions
1231-1240.
We have isolated a new CTV inclusion compound with
acetonitrile, 1a, obtained from a mixture of acetonitrile and
chloroform. The CTV molecules are found to be present as
dimers, which include acetonitrile, and self-included molecules
which stack into columns. The solvate 1a could be desolvated
into polymorphs 2a and 3a, with the transition to 2a occurring
1
5. M.J. Hardie, C.L. Raston, Angew. Chem., Int. Ed., 2000, 39
3835-3839.
16. S.T. Mough, J.C. Goeltz, K.T Holman, Angew. Chem., Int. Ed.
2004, 43, 5631-5635.
via
a
single-crystal-to-single-crystal transformation. The
17. A. Arduini, F. Calzavacca, D. Demuru, A. Pochini, A. Secchi, J.
Org. Chem., 2004, 69, 1386-1388.
desolvation of 1a probably involves cooperative rotation of the
methoxy methyl groups of the dimer CTV molecules with the
columns of stacked CTV molecules imparting stability on the
structure so that monocrystallinity is retained. Carbon dioxide
gas sorption analysis show appreciable sorption for 2a despite
the modest pressures employed and the modest void space
calculated for 2a, whilst 3a also shows carbon dioxide sorption
indicating that the proposed ‘closed’ structure is able to ‘open
up’ on exposure to this gas.
1
1
2
2
8. C.J. Sumby, J. Fisher, T.J. Prior, M.J. Hardie, Chem. – Eur. J.
,
2006, 12, 2945-2959.
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Acknowledgements
CLO and RMP thank the National Research Foundation of
South Africa and the University of Cape Town for financial
support.
1
740.
22. R. Ahmad, M.J. Hardie, Supramol. Chem., 2006, 18, 29-38.
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