Clathrate tetraldehyde cavitand: Single-crystal structure and NMR study

Article abstract of DOI:10.1080/10610278.2015.1102261

The tetraldehyde cavitand, C₁₀₀H₈₈O₂₀, was synthesised as the penultimate stage of a procedure to obtain cavitand-capped porphyrins. The complete structural characterisation was performed by single-crystal X-ray diffraction studies, NMR and IR spectroscopy. The inclusion phenomena of the solvent molecules were confirmed using NMR spectroscopy. The structure is the first example of a clathrate having an aromatic tetraldehyde cavitand grown as a single crystal with acetonitrile at the lower rim of the molecule and methanol at the upper rim of the molecule. (Figure Presented).

Full text of DOI:10.1080/10610278.2015.1102261

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Clathrate tetraldehyde cavitand: single-crystal
structure and NMR study
Pramod B. Pansuriya, Hitesh M. Parekh, Glenn E. M. Maguire & Holger B.
Friedrich
To cite this article: Pramod B. Pansuriya, Hitesh M. Parekh, Glenn E. M. Maguire & Holger
B. Friedrich (2016) Clathrate tetraldehyde cavitand: single-crystal structure and NMR study,
Supramolecular Chemistry, 28:3-4, 329-334, DOI: 10.1080/10610278.2015.1102261
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Date: 16 March 2016, At: 01:28

Supramolecular chemiStry, 2016
Vol. 28, NoS. 3–4, 329–334
http://dx.doi.org/10.1080/10610278.2015.1102261
Clathrate tetraldehyde cavitand: single-crystal structure and NMR study
Pramod B. Pansuriya, Hitesh M. Parekh, Glenn E. M. Maguire and Holger B. Friedrich
School of chemistry and physics, university of KwaZulu-Natal, Durban, South africa
ABSTRACT
ARTICLE HISTORY
received 5 July 2015
accepted 25 September 2015
The tetraldehyde cavitand, C100H88O20, was synthesised as the penultimate stage of a procedure to
obtain cavitand-capped porphyrins. The complete structural characterisation was performed by
single-crystal X-ray diffraction studies, NMR and IR spectroscopy. The inclusion phenomena of the
solvent molecules were confirmed using NMR spectroscopy. The structure is the first example of a
clathrate having an aromatic tetraldehyde cavitand grown as a single crystal with acetonitrile at the
lower rim of the molecule and methanol at the upper rim of the molecule.
KEYWORDS
tetraldehyde cavitand;
molecular recognition;
crystal structure; Nmr; tGa
1. Introduction
non-covalent but intimate contact of the host–guest sys-
tem can render changes in physical and/or chemical prop-
erties of the guest as well as the host. Hemicarcerand and
carceplex materials have inspired a number of researchers
to prepare a wide range of building blocks having host–
guest binding properties (4). As a result, novel chemistry
methods have been developed in order to improve the
size selectivity of molecular containers. However, typi-
cally these procedures involve multiple organic synthetic
The ‘molecular recognition’ concept (1) has led to an ever
growing field of molecular containers having expanded
inner cavities capable of accommodating sizable or mul-
tiple guests. This concept has been applied to fields such
as separation science, drug delivery systems and molec-
ular sensing and recognition (2). In particular, the host–
guest complex can enhance the chemical stability of the
guest and the subsequent release of products (3). The
CONTACT holger B. Friedrich
© 2015 taylor & Francis
friedric@ukzn.ac.za

330
P. B. PANsuRIyA ET Al.
steps, purification steps which yield target hosts in very
low yield. In an attempt to improve this situation and
broaden the field, new synthesises and approaches have
been attempted including metal–ligand self-assembly
and hydrogen-bonding self-assembly (5). Efforts have
also been made to modify the head groups of the upper
rim of the cavitands to enhance the total volume of the
cavity using suzuki cross-coupling, Heck coupling or by
bridging the resorcinarene hydroxyl groups with differ-
ent functionalised derivatives (6). In particular, Williamson
ether synthesis has been applied (4, 7).
Herein, we report on the crystal structure of a clath-
rate tetraldehyde cavitand, and further characterisation
of this deepened tetraldehyde cavitand, which was syn-
thesised as the penultimate stage in the synthesis of a
cavitand-capped porphyrin.
Compound 1 has all four aromatic aldehyde groups in
a similar arrangement at the upper rim, 2 exhibits two
different orientations of these groups: in one orientation,
two adjacent aldehyde groups appear upright and the
remaining two appear splayed, perpendicular to the first
two (Figure 1).
Compound 1 is a methanol-acetonitrile clathrate, while
each of the cavitands for 2 contained one ethyl acetate
molecule which was highly disordered and was removed
using SQUEEZE (9) in final refinement. The present tetralde-
hyde 1 has methanol at the centre of the upper rim bound
with four C–O–π interactions (Table 2, supplementary
material). In the lower rim, the acetonitrile molecule is
bound with four C–H–N short contacts (where C–H–N:
3.717 Å, C–H: 0.987 Å, H–N: 2.73 Å, C–H–N: 175.29°). The
methanol oxygen and acetonitrile nitrogen atoms are
exactly opposite to each other. The distance between the
methanol carbon and the acetonitrile carbon is 4.16 Å. The
asymmetric units of the tetraldehyde through the 001 axes
show the arrangement of the head group and the metha-
nol guest in the upper rim, whilst acetonitrile is the guest
in the lower rim.
In the packing arrangement (Figure s1, supplementary
material), the cavitand 1 demonstrates intermolecular
contacts via C–H–O interactions (Table 2, supplementary
material) in a head-to-head arrangement. The head-to-
tail interactions occur via the aromatic C–H of the feet
to the head oxygen and C–H–π interactions (Table 2,
supplementary material), whilst the head-to-head inter-
actions occur via the aldehyde oxygen to aldehyde C–H
and ether oxygen to aldehyde C–H. The ether oxygen to
aldehyde oxygen close contacts keep the upper rim bent
and downwards. In this scenario, one can see that in the
2. Result and discussion
The structure is the first example of a clathrate tetralde-
hyde cavitand having an aromatic aldehyde at the upper
rim of the molecule. Disordered methanol, as well as ace-
tonitrile, co-crystallised as guests (scheme 1).
In this study, we compare (scheme 2) our novel struc-
ture (1) to a compound reported by us previously having
the same‘head groups’with a different spacer group and
‘feet’(8). Both of these tetraldehyde cavitands have been
synthesised at the penultimate stage of the synthesis
of cavitand-capped porphyrins. The clathrate tetralde-
hyde cavitand 1 is completely soluble in ethyl acetate,
methylene chloride, chloroform, acetonitrile, and THF
and is insoluble in methanol and ethanol. The X-ray data
are summarised in Table 1 (supplementary material).
Figure 1. (colour online) asymetric unit of tetraldehyde cavitand 1 and 2.

suPRAMOlECulAR CHEMIsTRy
331
head-to-feet and feet-to-head arrangements, guest sol-
vents have opposite functional groups to each other. The
C–H–π interactions (Table 2, supplementary material)
also play significant roles to shape the flower structure
of the tetraldehyde cavitand. The reported tetraldehyde
2 fashioned head-to-tail arrangements via C–H–O and
C–H–π interactions, the splayed aldehyde arms make lay-
ers and are organised in a stair-like structure, whilst the
aldehyde groups perpendicular to the splayed arms make
a zigzag-layered structure. The π–π interactions (Table 3,
supplementary material) in the tetraldehyde cavitand 1
extend vertically through the a-axis and make a layer-like
structure.
10.26 ppm and the peak split in the presence of methanol.
In addition, the protons of the ethyl bridge, as well as the
protons located in the lower rim ethyl moiety shifted from
4.33 to 4.29, 4.72 to 4.69, 5.64 to 5.62, 2.42 to 2.41 and
2.61 to 2.58 ppm, respectively. These upfield shifts are in
agreement with the transformation of hydrogen-bonding
established in the cavitand.
A more significant change was observed for protons
located in the lower rim ethyl moiety when acetonitrile
alone was recognised, where linkers may be between
the nitrile group and the lower rim ethyl groups, which
occurred at 2.39–2.46 and 2.59–2.63 ppm in the free
cavitand and at 2.45–2.47 and 2.58–2.61 ppm in the ace-
tonitrile clathrate. This unusual peak shift is rationalised
by the unique position of the protons. Noticeable is that
there is no peak splitting observed for only acetonitrile
recognition.
1
The recognition of solvent was confirmed by H NMR
studies carried out in deuterated chloroform. In this sol-
vent, nearly all the signals of the cavitand underwent
upfield shifts and peak splitting in the presence of meth-
anol and methanol with acetonitrile (Figure 2). Moreover,
the most remarkable shifts were observed in the presence
of methanol for protons ArH 7.80–7.75 and 7.52–7.47 ppm.
The singlet of the aldehyde group shifted from 10.31 to
Thermogravimetric analysis (Figure 3) for 1 shows that
weight loss in the range of 26–176 °C is 4.32%, correspond
-
ing to the loss of one acetonitrile (2.43%) and one metha-
nol (1.90%) molecules (calc. 4.34%), indicating that none
Figure 2. (colour online) Nmr of 1 in cDcl3 (1), 1 with methanol (2), 1 with acetonitrile (3) and 1 with methanol and acetonitrile (4).
of the crystallised solvent loss occurred at room temper-
ature. On raising the temperature, slowly decomposition
started and continuous weight loss was observed up to
600 °C. The decomposition took place in two steps. In the
range of 176–425 °C, a 47.5% mass loss occurred, which is
also further divided into two steps and one can observe
this in the DTG trace. This may be oxidative degradation
or formation of different reactive species during this step.
The second decomposition of 47.6% takes place in the
temperature range of 425–555 °C and is probably due to
the destruction of the complete molecule, leaving behind
a negligible amount of unreactive ash mass.
Figure 3. (colour online) DSc and tG-DtG of 1.

332
P. B. PANsuRIyA ET Al.
with methanol overnight, filtered and washed with
methanol to obtain a pure white compound. (0.74 g, 93%
yield), Mp 106–108 °C. (Figures s2–s4 in supplementary
3. Conclusion
The clathrate tetraldehyde cavitand 1 has a deep enough
cavity to act as host, is completely soluble in ethyl acetate,
methylene chloride, chloroform, acetonitrile and THF and
is insoluble in methanol and ethanol. The structure of the
cavitand was confirmed by single-crystal X-ray analysis.
The crystal analysis gave insight into the host–guest nature
of these molecules and the stabilisation of solid state struc-
tures via C–H…O, C–H…π, C–O…π and π–π interactions in
the packing arrangement. The NMR solution phase results
were complimented by the single-crystal structure anal-
ysis. The TG-DTG results support that no guest release
occurred at room temperature.
1
material) H NMR (400 MHz, CDCl3, ppm): δ = 2.39–2.46
(m, 8H, CH2CH2Ph), 2.59–2.63 (m, 8H, CH2CH2Ph), 4.33
(m, 20H, inner of OCH2O and OCH2CH2O), 4.72 (t, 4H, CH),
5.64 (d, 4H, outer of OCH2O), 6.80 (s, 4H, Cavitand Ar–H),
6.99 (m, 4H, Ar–H), 7.16 (m, 4H, Ar–H), 7.18–7.24 (m, 20H,
CH2CH2Ph), 7.52 (td, 4H, Ar–H), 7.80 (dd, 4H, Ar–H), 10.31 (s,
4H, CHO). 13C NMR (100 MHz, CDCl3, ppm): δ 189.9, 161.3,
148.4, 144.3, 141.9, 138.9, 136.0, 128.2, 128.0, 126.1, 125.1,
121.0, 114.4, 112.9, 99.4, 71.5, 68.3, 37.1, 34.5, 32.3. FT-IR/
ATR, cm−1: 2939, 2870, 1683, 1599, 1480, 1455, 1438, 1315,
1286, 1188, 1155, 1103, 1018, 980, 938, 831, 752, 697, 654,
587. Anal. Calcd. for C100H88O20: C, 74.61; H, 5.51. Found: C,
74.73; H, 5.57%.
4. Experimental
4.1. Synthesis of tetraldehyde cavitand
4.2. Materials and methods
To the stirring solution of oven-dried (100 °C) K2CO3
(0.69 g, 5.00 mmol) in dry dimethylformamide (40 ml), the
tetrolcavitand (0.50 g, 0.490 mmol) (4, 10) was added and
stirred until completely dissolved. To the resulting solu-
tion, 2-(2-bromoethoxy)benzaldehyde (0.90 g, 3.93 mmol)
was added and the reaction mixture gently heated at 55 °C
for three days. During this period, the solution became
cloudy and a precipitate started to form on the sides of the
reaction flask. The reaction mixture was cooled to room
temperature and filtered. The filtrate was collected and
evaporated under vacuum. The oily residue was stirred
All chemicals were purchased from Aldrich. The chemical
reagents were laboratory grade and unless stated other-
wise used without further purification. The tetraldehyde
cavitand was synthesised from the tetrolcavitand as per
the reported method (10). The IR spectra were recorded
using a PerkinElmer universal ATR spectrometer. NMR
spectra were recorded employing Bruker Avance 400 &
600 MHz instruments with CDCl3 as the solvent. The DsC
and TG-DTG experiments were conducted using a sDT
Q600 V20.9 (TA Instruments, usA) equipped with universal
Scheme 1. Synthesis of tetraldehyde cavitand.
OH C
CHO
OHC
CHO
O
O
O
O
O
O
OHC
CHO
OHC
CHO
O
O
O
O
O
O
O
H
O
O
O
OO
O
O
O
O
O
O
O
O
H
H
H
H
H
R
R
H
H
R
R
R
R
R
R
1
R: CH2CH2Ph ( )
R: CH3 (2)
Scheme 2. tetraldehdye cavitands.

suPRAMOlECulAR CHEMIsTRy
333
Analysis 2000 software (version 4.5A). The tetraldehyde
cavitand was accurately weighed into crimped aluminium
pans and subjected to a thermal scan from room temper-
ature to 600 °C at a heating rate of 10.0 °C/min.
Acknowledgements
We gratefully acknowledge financial support by the DsT-Na-
tional Research Foundation, Centre of Excellence in Catalysis,
c*change. Dr Hitesh M. Parekh is thankful the university of Kwa-
Zulu-Natal for postdoctoral fellowship. Dr Pramod B. Pansuriya
would like to thank the DsT-National Research Foundation,
Centre of Excellence in Catalysis, c*change for a postdoctoral
fellowship.
4.3. Data collection and refinement
single-crystal X-ray diffraction data were collected on a
Bruker KAPPA APEX II DuO diffractometer using graph-
ite-monochromated Mo-Kα radiation (λ = 0.71073 Å).
Data collection was carried out at 173(2) K. Temperature
was controlled by an Oxford Cryostream cooling system
(Oxford Cryostat). Cell refinement and data reduction
were performed using the program sAINT (11). The data
were scaled and absorption correction performed using
sADABs (11). The structure was solved by direct methods
using sHElXs-97 and refined by full-matrix least-squares
methods based on F2 using sHElXl-97 (12). The program
Olex2 was used to prepare molecular graphic images (13).
All hydrogen atoms, except those of the solvent molecules
(acetonitrile and methanol), were placed in idealised posi-
tions and refined in riding models with uiso assigned the
values to be 1.2 or 1.5 times those of their parent atoms
and the constraint distances of C–H ranging from 0.95 to
1.00 Å. The hydrogen atoms of the solvent molecules are
excluded from the structure model, due to the fact that the
solvent molecules are situated on special positions with
four fold symmetry. The anisotropic displacement param-
eters (ADP) of most atoms were restrained. The atoms C7A,
C7B, C8, O4, O5, C9 to C15 were all refined with site occu-
pancy factors of 0.50. C7A and C7B were constrained at the
same position with the same ADP. There is a solvent acces-
sible void of 148 Å3 per unit cell, which could not be mod-
elled as discrete atomic sites, probably due to disorder. The
program PlATON sQuEEZE was employed to calculate the
contribution to the diffraction from the missing solvent
molecules and it produced a set of partial-solvent-free dif-
fraction intensities (9). This set of intensities was used for
final refinements. sQuEEZE estimated a total count of 86
electrons per unit cell contributed by the missing solvents,
which were excluded in the formula and subsequent cal-
culation of molecular weight, density etc. (9).
Disclosure statement
No potential conflict of interest was reported by the authors.
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