Correlating Solution- and Solid-State Structures of Conformationally Flexible Resorcinarenes: Significance of a Sulfonyl Group in Intramolecular Self-Inclusion

Article abstract of DOI:10.1002/chem.201905211

The synthesis of tetramethoxyresorcinarene podands bearing p-toluene arms connected by -SO₃- (1) and -CH₂O- (2) linkers is presented herein. In the solid state, the resorcinarene podand 1 forms an intramolecular self-inclusion complex with the pendant p-toluene group of a podand arm, whereas the resorcinarene podand 2 does not show self-inclusion. The conformations of the flexible resorcinarene podands in solution were investigated by variable-temperature experiments using 1D and 2D NMR spectroscopic techniques as well as by computational methods, including a conformational search and subsequent DFT optimisation of representative structures. The ¹H NMR spectra of 1 and 2 at room temperature show a single set of proton signals that are in agreement with C_4v symmetry. At low temperatures, the molecules exist as a mixture of boat conformations featuring slow exchange on the NMR timescale. Energy barriers (ΔG^≠₂₉₈) of 55.5 and 52.0 kJ mol^?1 were calculated for the boat-to-boat exchange of 1 and 2, respectively. The results of the ROESY experiments performed at 193 K and computational modelling suggest that in solution the resorcinarene podand 1 adopts a similar conformation to that present in its crystal structure, whereas podand 2 populates a more versatile range of conformations in solution.

Full text of DOI:10.1002/chem.201905211

A Journal of
Accepted Article
Title: Correlating solution and solid state structures of conformationally
flexible resorcinarenes – the significance of a sulfonyl group in
intramolecular self-inclusion
Authors: Małgorzata Pamuła, Maija Nissinen, and Kaisa Helttunen
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Correlating solution and solid state structures of
conformationally flexible resorcinarenes – the significance of a
sulfonyl group in intramolecular self-inclusion
Małgorzata Pamuła,[a] Maija Nissinen[a] and Kaisa Helttunen*[a]
Abstract: In this study, the synthesis of tetramethoxy resorcinarene
podands bearing p-toluene arms connected by -SO3- (1) and -CH2O-
(2) linkers is presented. In the solid state, the resorcinarene podand
1 forms an intramolecular self-inclusion complex of the p-toluene
group, whereas the resorcinarene podand 2 did not show self-
inclusion. The conformation of the flexible resorcinarene podands in
solution was investigated by 1D and 2D NMR spectroscopic
techniques using variable temperature experiments, as well as, with
computational methods including conformational search and
subsequent density functional theory (DFT) optimization for
representative structures. The 1H NMR spectra of 1 and 2 at room
due to their unique properties of self-healing,[13] stimuli-
responsiveness[14] and shape memory.[15]
Intermolecular self-inclusion is most often detected in crystal
structures. Self-included dimers have been obtained in several
crystal structures of calixarene macrocycles, such as deep
cavitands,[16] thiacalix[4]arenes,[17] and N-alkyl resorcinarene
halides.[18,19] In addition, there are few examples of crystalline
calixarenes featuring chains with head-to-tail intermolecular self-
inclusion behaviour.[20,21] In contrast, intramolecular self-inclusion
requires 180° degree back folding of a part of the compound,
typically a covalent host-guest conjugate or a macrocycle
equipped with conformationally flexible functional groups.
However, crystal structures of intramolecular self-inclusion
monomers are relatively rare.[6,22,23]
Control of intermolecular self-inclusion is important for the
preparation of supramolecular materials and self-assembled
structures. Few examples of pillararenes show a concentration-
dependent transition from intramolecular to intermolecular self-
inclusion in solution, which is utilized for reversible self-assembly
of dimers or supramolecular polymers.[6,24] An interesting
example of self-inclusion complexation in the solid state is a
pillar[5]arene derivative, which breaks when dissolved in a
temperature show
a single set of proton signals that are in
agreement with C4v-symmetry. At low temperatures the molecules
exist as a mixture of boat conformations featuring slow exchange on
the chemical shift time scale. An energy barrier (DG‡298) of 55.5
kJ/mol and 52.0 kJ/mol was calculated for the boat-to-boat exchange
of 1 and 2, respectively. The results of the ROESY experiments
performed at 193 K and computational modelling suggest that in
solution the resorcinarene podand 1 adopts similar conformation to
that present in its crystal structure, whereas podand 2 populates
more versatile range of conformations in solution.
solvent of small molecular size but remains intact in a larger
[25]
molecular size solvent.
Similarly, a pillar[5]arene based
pseudo[1]rotaxane that forms a self-inclusion complex in the
solid state shows solvent responsive self-inclusion in solution.[4]
Other examples of self-inclusion include cyclodextrins, which
exhibit self-inclusion properties when the inclusion of bulky end
groups leads to the formation of pseudo[1]rotaxane,[5]
cucurbiturils with a pendant residue acting as hydrophobic
guest,[11] and monosubstituted resorcinarenes[26,27] and
covalently linked dimeric resorcinarene capsules[28] displaying
intramolecular self-inclusion properties in solution.
Self-inclusion can also be undesired for the designed function of
a molecular receptor or the self-assembly of supramolecular
aggregates. In the case of a bambusuril anion receptor, solid
state intramolecular self-inclusion was observed, and it was
expected to lower its affinity for guest binding, even though self-
inclusion could not be detected in solution.[29] Intramolecular self-
inclusion of a b-cyclodextrin was prevented by attaching a bridge
across the narrow rim thus effectively promoting the competitive
supramolecular polymerization.[30]
Introduction
Inclusion complexes, along with control of self-inclusion
behaviour, play an essential role in the preparation and function
of self-assembled nanomaterials.[1–3] In supramolecular
chemistry, macrocyclic hosts are important building blocks
providing cavities for inclusion complexation thereby enabling
the self-assembly of multiple supramolecular architectures.
These supramolecular assemblies range from simple 1:1
inclusion complexes and pseudorotaxanes,[4,5] to polymers[6–9]
and particles[10,11] featuring in some cases gelation properties.[12]
In particular, supramolecular polymers attract increasing interest
[a]
Małgorzata Pamuła, Prof. Maija Nissinen, Dr. Kaisa Helttunen
Department of Chemistry, Nanoscience Center
University of Jyvaskyla
Resorcinarene podands are macrocycles in which the upper rim
hydroxyl groups of the parent resorcinarene have been
functionalized, resulting in conformationally flexible octopus-like
P.O. Box 35, FI-40014 University of Jyvaskyla, Finland
E-mail: kaisa.j.helttunen@jyu.fi
structures. In contrast to resorcinarene cavitands with
a
covalently bridged C4v-symmetrical crown conformation,
resorcinarene podands typically exhibit a C2v-symmetrical boat
conformation in the solid state.† The boat conformation is
Supporting information for this article is given via a link at the end of
the document.
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defined by two resorcinol rings being almost vertically opposed
and the two remaining ones horizontally aligned. Previously,
Results and Discussion
self-inclusion dimers of resorcinarene tetrapodands have been
observed by us
Synthesis
[31,32]
and others.[33–36] Nevertheless,
Resorcinarene podand 1 was prepared by alkylation of the free
hydroxyl groups of racemic tetramethoxyresorcinarene (3)[46]
with four (2-tosyloxy)-ethoxy (6) podand arms according to the
literature procedure.[47] In the podand arm of 1, a sulfonyl group
(-SO3-) connects the tolyl ring with the 1,2-dioxyethane chain. In
order to prepare an analogous compound to the resorcinarene
podand 1 without the sulfonyl group, resorcinarene podand 2
resorcinarene podands tend to display collapsed cavities in the
solid state.[37–45]
Crystal structures of conformationally flexible macrocycles lead
to a question of how to correlate the solid state structure to the
conformation of the molecule in solution. Herein, we have
addressed this problem by synthesizing two resorcinarenes
bearing four p-toluene podand arms connected with sulfonyl or
benzoyloxy linkers to provide similar length but different
hydrogen bond donor/acceptor properties. The single-crystal X-
ray structure of the sulfonyl linked podand 1 displays intriguing
self-inclusion complex, where a cavity forms around the included
arm, which is bound by weak hydrogen bonds. To the best of
our knowledge, this is the first example in which a resorcinarene
podand forms a self-inclusion complex stabilized only by weak
C-H···O hydrogen bonding and C-H···p interactions. We
envision that this structure is rare since it requires inclusion
complexation of a pendant p-tolyl group in a non-preorganized
nor permanent aromatic cavity. Indeed, we found only one
was synthesized by alkylation of
3
with four 2-((4-
methylbenzyl)oxy)ethyl tosylate arms (5) in 44 % yield (Scheme
1). In the podand arm of 2, a -CH2O- group provides similar
length but lacks the hydrogen bond acceptor properties of 1. The
tosylate (5) was prepared in two steps from 4-methyl
benzylchloride and ethylene glycol. The resorcinarene podands
were fully characterized by a complete set of high-resolution
NMR spectra, ESI-MS, and single crystal X-ray diffraction.
X-ray crystallography
Resorcinarene podand
1
was isolated after column
chromatographic purification and formed single crystals suitable
for X-ray diffraction analysis from ethyl acetate/hexane solution.
The crystal structure shows that in this case, the resorcinarene
macrocycle 1 adopts a boat conformation with two opposed
resorcinol rings being almost vertically oriented and the other
two horizontally aligned. This conformation is typical for octa-
functionalized resorcinarenes lacking a permanent aromatic
cavity. Remarkably, in the case of resorcinarene 1 the adopted
boat conformation defines an aromatic cavity, which is filled by
intramolecular self-inclusion of one of the terminal tolyl sulfonyl
substituents of the podand arms.
example
of
a
self-inclusion
complex
of
a
tetramethoxyresorcinarene Schiff base stabilized by NH···O
hydrogen bonds in the CSD database.[23] The electron rich and
electron poor linker moieties (i.e. sulfonyl and -CH2O- groups)
included in the podand arms of resorcinarenes 1 and 2,
respectively, provide opposite tendencies to weak electrostatic
interactions. This allowed us to carry on extensive structural
analysis using computational and experimental methods to
model the relationship of the crystal structures of 1 and 2 with
their conformations in solution. The results show that the
solution structure of 1 correlates well with the conformation
adopted by the molecule in its crystal structure, whereas 2
populates different types of low energy conformations with
possibility to self-inclusion according to computational structures.
X
d
O
b)
g
e
c
Cl
O
HO
O
f
OH
a
b
4
h
i
X
O
j
O
c)
OH
O
O
O
R
R
R
R
5 or 6
s
O
O
O
O
O
a)
l
k
t
u
O
O
S
S
x
HO
O
w
r
5
6
O
v
O
OH
O
p
q
O
O
X
3 R = CH2CH3
O
O
o
n
S
O
1 X = SO2
2 X = CH2
O
O
O
X
m
Scheme 1. Synthesis of tetramethoxy resorcinarene podands 1 and 2 showing NMR assignments. For simplicity, only one enantiomer of the tetramethoxy
resorcinarene is shown. Reaction conditions: a) Cs2CO3, acetonitrile, dibenzo-18-crown-6; b) KOH, ethylene glycol; c) p-toluenesulfonyl chloride, triethylamine,
dichloromethane.
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Figure 1. X-ray structure of a,b) self-inclusion complex of resorcinarene podand 1 indicating weak hydrogen bonds with dashed lines and OCCO dihedral angles,
c) an overlay of XRD (cyan) and DFT energy minimized structure of the lowest energy conformation 1-I, d) resorcinarene podand 2 showing weak hydrogen
bonds with dashed lines and C-centroid distances with black arrows, e) an overlay of XRD (cyan) and DFT energy minimized structure (red) of 2-I. Atom colors: C
= grey, H = white, S = yellow, O = red. Distances are in Ångströms.
In particular, the self-inclusion takes place with one of the arms
substituent) and methoxy oxygens of vertical resorcinol rings
attached to one of the two vertically oriented resorcinol rings (Fig. (dH···O = 2.65–2.82 Å). The podand arms connected to the
1). The included tolyl sulfonyl group resides at a distance of
3.10–3.45 Å from the vertical resorcinol walls indicating the
presence of C-H···p interactions between them.
horizontal resorcinol rings extend away from the cavity and form
edge-to-face p···p interactions with tolyl arms of adjacent
molecules. It should be noted that the second molecule in the
asymmetric unit forms a weak intermolecular hydrogen bond
from the p-tolyl group attached to the vertical resorcinol ring to
podand arm oxygen of a neighboring molecule (see SI).
The self-inclusion complex is supported by two intramolecular C-
H···O bonds from the p-methyl group to the resorcinol oxygens
of vertical resorcinol rings (dH···O = 2.57–2.60 Å). In addition, the
oxygen atoms of the sulfonyl group in the included arm reside
within 2.73–2.79 Å distance from the methoxy and ethoxy
protons of one of the two horizontally oriented resorcinol rings.
The two podand arms attached to the horizontal resorcinol rings
are wrapped around the resorcinarene cavity. The arms have,
however, different dihedral angles of the ethoxy groups. In one
arm OCCO torsion of +70° induce an intramolecular 2.62 Å
contact between methoxy proton and sulfonyl oxygen on one
side (Fig. 1a-b) and rotation of the tolyl group towards the lower
rim. On the other side, the OCCO torsion of -72° places the
corresponding sulfonyl oxygen away from the cavity where it
forms weak intermolecular hydrogen bonds. The fourth podand
arm attached to a vertical resorcinol ring folds to the side of the
molecule, engaging in weak intermolecular interactions.
Computational studies
Conformational search and energy minimization with molecular
mechanics was performed using OPLS3e force field and implicit
solvent model to generate potential conformations for the
resorcinarene podands. The search found 460 potential
conformations for podand 1. In all of these, one vertical arm was
folded inside the cavity similarly to the conformation observed in
the crystal structure. The conformations were divided into five
categories depending on the folding of the horizontal podand
arms and in particular, the OCCO dihedral angles (Fig. 2). In
class I) both OCCO torsions are positive giving rise to two Hs···O
contacts, in class II) one OCCO torsion is positive and one is
negative resulting in one Hs···O contact, in class III) both OCCO
torsions are negative, in class IV) horizontal arm proximal to the
included vertical sulfonate group does not fold around the cavity,
and in class V) the horizontal arm distal to the included sulfonate
group does not fold around the cavity.
In order to compare the role of the sulfonyl group in the crystal
structure of the podand 1, single crystals of resorcinarene
podand 2 were grown from an ethanol solution. The crystal
structure of
2 contains two resorcinarene molecules per
The lowest energy examples of each class were submitted to
DFT geometry optimization using an implicit solvent model to
compare the relative energy differences of each representative
conformation (1-I–1-V) at a higher level of theory. The order of
the relative energies remained the same as in the force field
energy minimization (Fig. 2a).
asymmetric unit, both adopting a boat conformation. In contrast
to resorcinarene 1, the vertically aligned resorcinol rings lean
towards each other, reducing the size of the aromatic cavity of
resorcinarene 2 (Fig. 1d). The podand arms attached to the
vertical resorcinol rings fold above the horizontal resorcinol rings
placing the benzylic carbons at 3.38–3.77 Å distance from the
aryl ring centroids. The cavity is sealed with intramolecular C-
H···O contacts between tolyl ring protons (ortho to the CH2O
The conformational search for podand 2 found 495 potential
conformations. In most of these conformations,
a vertical
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podand arm was backfolded towards the cavity. These
structures were divided in three categories. In class 2-I) one
horizontal arm was folded on top of the cavity, in class 2-II) both
horizontal arms were folded on sides of the cavity, and in class
2-III) one horizontal arm was backfolded. In addition, the search
found 16 open conformations (class 2-IV) without self-inclusion
and seven examples of horizontal podand arm folding into the
cavity (class 2-V) indicating more versatile range of potential
conformations than those found for 1. Interestingly, the benzylic
protons of the vertical aryl rings are directed towards the
horizontal resorcinol rings both in open (2-IV) and self-inclusion
conformations (2-I–2-III). This results in a notable difference in
the position and angle of the self-included tolyl group in 2 in
contrast to 1. In 2 the tolyl group is positioned higher relative to
the aromatic cavity and the p-methyl group is pointing up
whereas in 1 it points down.
multiplicity of the proton signals is in agreement with a C4v-
symmetry, which probably results from the chemical exchange
process between two boat conformation being fast on the 1H
NMR chemical shift time scale. The two multiplets observed at δ
= 3.7 and 4.0 ppm, respectively, were assigned to the protons of
the methylene group alpha to the phenol oxygen atom that
resonate as diastereotopic signals (Hf,r). An AB quartet at 4.5
ppm was assigned to the -CH2O- methylene protons alpha to the
toluene ring (Hd,p). In addition to the two singlets of the aromatic
resorcinol protons, two sharp doublets corresponding to the
protons of the tolyl rings (Hb,n and Hc,o) appear in the aromatic
region of the spectrum.
At room temperature, the tetratosylate resorcinarene (1)
exhibited a similar 1H NMR spectrum. The most significant
difference was the observation of the methylene protons alpha
to the sulfonyl group in 1 appearing as a well-defined triplet at δ
= 4.18 ppm. The analogous signal for the methylene protons
alpha to the benzyloxy substituent in 2 resonated slightly upfield
shifted (δ =3.65 ppm) owing to a reduction in the inductive effect
of the substituent.
The order of the relative energies in structures 2-I–2-V changed
after DFT optimization with PBE0-D3 functional, however,
structure 2-I with self-inclusion of
a vertical podand arm
remained as the lowest energy conformation. In order to confirm
the result, optimization was repeated with another popular DFT
functional (B3LYP-D3), which found the same trend in energies
but significant discrepancy in absolute values along with some
structural changes in optimized geometries (SI). This suggests
that the potential energy landscape of podand 2 is more
versatile than that of podand 1 containing many local minima.
1H NMR experiments
The conformational characterization of the resorcinarene
podands 1 and 2 in solution was carried out using NMR
spectroscopy. Our aim was to assess how the crystal structures
of these conformationally flexible molecules reflect into the
Figure 3. 1H NMR spectra of resorcinarene podands in CD2Cl2 a) 1 at 193 K,
b) 1 at 303 K, c) 2 at 193 K and d) 2 at 303 K. Blue letters refer to protons in
the horizontally aligned resorcinol ring and red letters to the vertically opposed
ring and attached groups.
1
conformation adopted in solution. The H NMR spectrum of the
tetratolyl resorcinarene 2 at 303 K in CD2Cl2 (Fig. 3) displays a
single triplet for the methine bridges (Hj,v), a unique singlet for
the methoxy groups (Hg,s) and two separate singlets for aromatic
protons of the resorcinol rings (Hh,t, Hi,u). The number and
Figure 2. Five representative DFT energy minimized conformations a) for 1 with relative B3LYP-D3 energies, and b) for 2. The relative DFT energies for podand 2
follow an order: 2-I < 2-V < 2-III ≈ 2-II < 2-IV.
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1
We also measured the H NMR spectra of 1 and 2 at different
Table 1. Activation energy parameters for the conformational exchange.
temperatures (303–193 K) to investigate the structural and
dynamic properties of the resorcinarene podands at low
temperatures, and to determine energy barriers for the
conformational exchange (Fig. 3 and SI). In addition, samples
with concentrations between 1–12 mM were measured at 193 K
to confirm that no dimerization was observed in this
concentration range used for the variable temperature
experiments.
Molecule
DG‡298 (kJ mol-1)
DH‡ (kJ mol-1)
21.0
DS‡ (J mol-1 K-1)
-116.0
1
2
55.5
52.0
28.0
-80.7
1H-1H COSY and ROESY
In case of 2, there were no significant changes either in the
shape or in the position of the proton signals when temperature
was lowered to 283 K (see SI). As the temperature was further
decreased to 263 K the singlet from the lower rim protons (Hi,u)
broadened significantly due to the slowing of the conformational
exchange of the two boat conformations close to the NMR
chemical shift timescale. At 243 K, coalescence for the upper
rim protons (Hh,t) and the methoxy groups was observed.Within
the temperature range of 223–193 K new peaks at the aromatic
region and at 3.3–4.7 ppm appeared and sharpened thereby
giving rise to three singlets for resorcinol rings (fourth signal
overlapping with the tolyl signals at 7.12 ppm based on ROESY,
see SI), one signal for methine bridges (Hj,v) and two singlets for
methoxy groups. In addition, a broad doublet with integrals of 4
protons at 7.26 ppm and a multiplet of 14 protons at 7.12 ppm
were observed for the tolyl groups Hc and Hb/Hn/Ho, respectively.
The number and multiplicity of the proton signals at 193 K
1H-1H COSY and ROESY NMR were used to investigate the
conformation of the resorcinarene podands 1 and 2 in solution,
as well as, to confirm the assignment of the protons at low
temperature. The ROESY measurements of 2 at 193 K showed
strong ROE correlations between aryl protons and methoxy
protons, ArHh to Hg and ArHt to Hs, as well as, between aryl and
ethoxy protons, ArHh to Hf, and ArHt to Hr (Fig. 4) indicating that
these protons were coupled through space. On the other hand,
ROE correlations were observed for aryl protons Hi with
indicates that the podand adopts
conformation.
a C2v-symmetrical boat
The results of the 1H variable temperature experiments of
resorcinarene 1 were very similar to those obtained with 2.
Coalescence was observed at 243 K suggesting similar energy
barrier for the boat-to-boat exchange to the one determined for 2.
However, below coalescence temperature, the lower rim methyl
signal was split into two broad triplets (Hx and Hl) and methine
bridges appeared as two separate signals (Hv and Hj).
Interestingly, half of the tolyl ring protons appeared as sharp
doublets at 193 K (Hc and Hb) originating from two magnetically
equivalent podand arms, whereas two broad signals were
assigned for the tolyl protons of the two remaining podand arms.
These broad aromatic signals resonated upfield shifted at 6.60
and 7.10 ppm (Hn and Ho) relative to their sharply resolved
counterparts. Accordingly, the singlet of the Ar-CH3 (Ha) had an
integral equivalent of six protons, and the remaining protons
(Hm) were most likely located together with the lower rim CH2
signals Hw and Hk based on the integral values of these broad
resonances.
The line shapes of the NMR signals arising from the lower rim
CH3 protons (Hl, Hx) and upper rim resorcinol protons (Hh, Ht) at
different temperatures were modelled with line shape analysis to
obtain the exchange rate constants (k). The exchange rates
were analysed using Eyring plots (SI), which provided Gibbs free
energy of activation (DG‡
) of 55.5 kJ/mol for 1 and 52.0
298
kJ/mol for 2 (Table 1). The values are similar to the value
Figure 4. 1H-1H ROESY of 2 at 193 K in CD2Cl2 and a DFT minimized
obtained for octa-acetylated resorcinarene podands (DG‡
=
298
structure 2-I showing key contacts (<
4 Å distance) present in this
54.7 kJ/mol).[35]
conformation matching with ROESY. Negative ROE cross-peaks are shown in
red and positive exchange cross-peaks in blue. The overlap of b, n, o and u
protons hinder the assignment of ROEs marked with a green circle.
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methine Hj/Hv, which allowed the assignment of Hi as one of the
lower rim protons and ArHh and ArHt as the upper rim protons of
the resorcinol rings.
Positive exchange correlations were also observed between Hd
and Hp, which were assigned as benzylic protons. The signal Hd
resonated as an AB quartet with almost identical chemical shift
for the A and B protons, whereas in Hp the DdAB was 0.12 ppm.
The upfield shift in signal Hp in comparison to Hd suggests that
the proton is magnetically shielded by the proximity of an
aromatic ring. This condition is met for the podand arm attached
to the vertical resorcinol ring in the conformation observed in the
X-ray structure, where the benzylic protons establish C-H···p
interactions with the horizontally aligned resorcinol rings. Based
on the computational models, the shielding of the protons Hp
may result from the inclusion of one of the podand arm into the
resorcinarene cavity and from the above-mentioned C-H···p
interactions with a resorcinol ring. Thus, the protons Hd–Hi arise
from the horizontally aligned resorcinol ring in
a boat
conformation, and the protons Hp–Hu were assigned to the
vertically opposed resorcinol ring.
The ROESY spectra of 1 at 193 K (Fig. 5) revealed a similar
pattern of exchange cross-peaks between methoxy protons Hg
and Hs, and upper rim protons ArHh and ArHt as in the spectrum
of 2. Interestingly, the methoxy group Hg showed a ROE
correlation with proton assigned as Hf/Hq, which was not
observed in 2. Comparison to the DFT energy minimized
structures and the X-ray structure showed that from these two
possibilities the distance between Hg and Hq (horizontal-vertical,
~3 Å) is significantly shorter than Hg and Hf (horizontal-horizontal,
~4 Å), and therefore, a more likely source of this correlation.
This assignment is made assuming that the similar chemical
shifts of the methoxy groups between 1 and 2 allow the
assignment of Hg and Hs to horizontally and vertically oriented
resorcinol rings also in 1.
Figure 5. 1H-1H ROESY of 1 at 193 K in CD2Cl2 and a DFT minimized
ROE correlations from the sharp tolyl group signals Hc and Hb
were observed to the resorcinol lower rim groups. Proton Hb
showed weak correlations with methine Hj and methyl Hl. Proton
Hc had cross-peaks with the same lower rim protons and
interestingly, to the ethoxy He/Hf and methoxy Hs. This
combination of ROEs was best explained if the tolyl group Hc/Hb
is attached to the horizontal resorcinol ring and wraps around
the aromatic cavity thereby approaching the vertical methoxy
group Hs and the lower rim of the podand. This interpretation is
in agreement with our tentative assignment of the vertically and
horizontally aligned resorcinol rings, and with the crystal
structure of 1.
The computational structures 1-I–1-V are in excellent agreement
with the observed ROEs. Especially structure 1-I, which also has
the lowest DFT energy, shows all the expected distances from
horizontal podand arm protons Hb and Hc to the lower rim
protons and the vertical methoxy protons Hs. In the structures 1-
IV and 1-V one of the horizontal arms is folding under the
horizontally aligned resorcinol ring. In this conformation the
distance between the tolyl group protons and lower rim protons
are too long for the observed ROEs, which suggests that these
conformations are less populated in solution in accordance with
their higher DFT energies.
structure of 1-I showing key contacts (<
4 Å distance) present in this
conformation matching with ROESY. Negative ROE cross-peaks are shown in
red and positive exchange cross-peaks in blue.
It is important to note that the tolyl groups of the two podand
arms, which are attached to the vertical resorcinol rings show
broad signals for Hm–Ho at 193 K and appear upfield shifted
relative to the corresponding horizontal Ha–Hc. This indicates
that the tolyl groups are engaged in a dynamic process that is
intermediate on the chemical shift timescale. Based on the most
likely conformation of the molecule 1 in solution (ROEs and DFT
structures), especially the proximity of vertical podand arm
ethoxy protons Hq with the horizontal ring Hg, it can be deduced
that the conformation of the vertical podand arm is favorable for
tolyl group inclusion into the aromatic resorcinol cavity. The
upfield shifted and broadened tolyl group signals are indicative
of a second exchange process of low energy barrier involving
dynamic inclusion of the tolyl groups in the magnetically shielded
environment of the resorcinarene cavity.
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Solvents and reagents were purchased from commercial suppliers
Sigma-Aldrich and VWR. Solvents were dried using an MBraun solvent
purification system. The synthesis was performed with dried glassware
(at 120 °C overnight) and under nitrogen balloon. Flash chromatography
was performed with a Combiflash Companion (Teledyne Isco) and
Redisep Gold silica columns. The mass spectrometry was performed
with a Micromass LCT ESI-TOF or an ABSciex QSTAR Elite ESI-Q-TOF
Conclusions
In summary, we have presented the synthesis of two
conformationally flexible resorcinarene podands without
permanent cavities, which were characterized by a complete set
of high-resolution spectroscopic techniques and X-ray
crystallography. In the solid state, the resorcinarenes exhibit
different conformations of the p-toluene podand arms, where a
sulfonyl linker in 1 promotes self-inclusion complexation of the p-
toluene group, and a benzoyloxy linker in 2 pinching of the
resorcinarene cavity. Variable temperature NMR revealed a 52–
55 kJ/mol energetic barrier of the boat-to-boat exchange for both
podands, which indicates that the different podand arm linkers
do not influence significantly to the exchange rate.
The conformation and intramolecular interactions of the
macrocycles were also investigated by ROESY experiments at
low temperature. Additionally, conformational search and DFT
optimizations were carried out to explore the conformation space
of the podands. The observed ROE correlation between the
podand arms and resorcinol protons suggest that the
tetratosylate resorcinarene podand 1 exhibits similar wrapped
conformation of the horizontal podand arms in solution as
observed in the solid state. Furthermore, the magnetic shielding
of the broad aromatic tolyl group signals assigned to the vertical
podand arms, and computational modelling suggest that self-
inclusion of the p-toluene group is highly favorable conformation
also in solution.
mass spectrometer. Melting points were measured with
Scientific SMP30 and are uncorrected.
a Stuart
NMR
NMR spectra were recorded with a Bruker Avance III 500 (500 MHz for
1H and 126 MHz for 13C) spectrometer. The J values are given in Hz.
Variable temperature measurements of the podands were performed with
a
BBI probe. The 1H NMR was obtained in CD2Cl2 at different
temperatures (303 K, 283 K, 263 K, 243 K, 223 K, 203 K and 193 K) at 9
mM concentration for 1, and 11 mM concentration for 2. The ROESY and
COSY 1H-1H NMR were performed at 193K. Complete lineshape analysis
was performed with Bruker TopSpin v. 3.5.
Synthesis
Resorcinarene 1 was synthesized as described previously.[47] δH(500
MHz, CD2Cl2, +30 °C) 7.67 (d, J = 8.2, 8H, Hc,Ho), 7.18 (d, J = 8.0, 8H,
Hb,Hn), 6.76 (s, 4H, Hi,Hu), 6.11 (s, 4H, d, Hh,Ht), 4.32 (t, J = 7.9, 4H,
Hj,Hv), 4.17 (t, J = 4.8, 8H, He,Hq), 4.01–3.93 (m, 4H, Hf,Hr), 3.73–3.62 (m,
4H, Hf,Hr), 3.51 (s, 12H, Hg,Hs), 2.27 (s, 12H, Ha,Hm), 1.87–1.73 (m, 8H,
Hw,Hk), 0.82 (t, J = 6.7, 12H, Hl,Hx) ppm; δC(126 MHz, CD2Cl2, +30 °C)
156.15, 155.29, 145.65, 133.63, 130.41, 128.29, 127.35, 126.67, 126.45,
98.26, 69.42, 67.54, 55.77, 36.62, 28.82, 21.69, 12.78 ppm.
For tetratolyl resorcinarene 2, 1H NMR and ROESY data
suggest that the benzylic protons of the vertical podand arms
are involved in C-H···p interactions with the horizontally aligned
resorcinol rings in solution, which was also observed in all
computational structures. Computational modelling revealed a
more versatile conformation space for resorcinarene 2 than 1,
where self-inclusion of the vertical or horizontal podand arms
were identified as the lowest energy structures. However,
conformational search also found an open conformation without
self-inclusion, which deviates from the crystal structure most
significantly in a more compact orientation of the horizontal
podand arms. Therefore, it is expected that both types of
conformations are populated in solution, whereas the extended
conformation of the horizontal podand arms is an effect induced
by the favorable p···p interactions in crystal packing.
The role of the sulfonyl linker in self-inclusion complex of
resorcinarene 1 is twofold. On one hand the sulfonyl oxygens
are suitable hydrogen bond acceptors for establishing weak
hydrogen bonds that support the self-inclusion complex, and on
the other hand, the oxygen lone pairs direct the podand arms
away from the horizontally aligned resorcinol rings. Comparison
to resorcinarene 2 clearly indicates that the presence of the
sulfonyl group restricts the range of conformations the podand is
likely to adopt. Ultimately, we can conclude that the
conformation of podand 1 in solution correlates well with its solid
state structure, whereas the conformation of podand 2 changes
between self-inclusion complexes and open conformations.
Resorcinarene 2: Tetramethoxy resorcinarene 3 (0.200 g, 0.304 mmol),
Cs2CO3 (0.804 g, 2.468 mmol) and dibenzo-18-crown-6 (0.010 g, 0.028
mmol) were mixed with 30 ml of dry acetonitrile under nitrogen
atmosphere at 100 ˚C with stirring. A white suspension formed. The
solution was heated at reflux for 15 minutes before the addition of 5
(0.428 g, 1.338 mmol) in 20 ml of dry acetonitrile. The suspension was
refluxed for 24 hours. The solution was filtrated while hot through a pad
of celite, and the precipitate was washed with hot acetonitrile. The
solvent was evaporated with a rotary evaporator, and the residue was
dissolved into 50 ml of dichloromethane and washed with 20 ml of water.
The organic solution was dried with MgSO4, evaporated and purified with
flash chromatography using SiO2 column and a hexane/ethyl acetate
gradient (from 8:2 to 0:1). The product was recovered as a colourless oil
and recrystallized from ethanol as white crystalline solid (44 % yield). mp
92—93 °C; δH(500 MHz, CD2Cl2) 7.24 (d, J = 7.9, 8H, Hc,Ho), 7.14 (d, J
= 7.9, 8H, Hb,Hn), 6.68 (s, 4H, Hi,Hu), 6.33 (s, 4H, Hh,Ht), 4.51 (ABq,
ΔδAB = 0.04, JAB = 11.5, 4H, Hd,Hp), 4.44 (t, J = 7.2, 4H, Hj,Hv), 4.05–3.93
(m, 4H, Hf,Hr), 3.81–3.71 (m, 4H, Hf,Hr), 3.71–3.59 (m, 8H, He,Hq), 3.55
(s, 12H, Hg,Hs), 2.32 (s, 12H, Ha,Hm), 1.93–1.75 (m, 8H, Hw,Hk), 0.89 (t, J
= 7.2, 12H, Hl,Hx) ppm; δC(126 MHz, CD2Cl2) 156.12, 155.67, 137.50,
135.90, 129.17, 128.00, 126.35, 98.24, 73.49, 69.54, 69.40, 55.79, 37.22,
28.12, 21.10, 12.85 ppm; HR-MS (ESI-TOF): m/z calcd. for C80H96O12Na+
1271.6794 [M+Na+]; found 1271.6785.
Compound 4: KOH (0.41 g, 7.37 mmol) was dissolved in anhydrous
ethylene glycol by heating to 90–100 °C under nitrogen atmosphere. 4-
Methylbenzyl chloride (1.00 g, 7.11 mmol) was added dropwise to the
yellow solution and heating was continued at 90 °C for 24 hours. The
yellow mixture was diluted with 30 ml of Millipore water and extracted
three times with 20 ml of dichloromethane. The combined extracts were
washed once with water and dried with MgSO4. The organic solvent was
evaporated, and the residue was purified with flash chromatography
using SiO2 column and a petrol ether/ethyl acetate gradient (from 65:35
to 0:1). The product was isolated as a colourless liquid (0.63 g, 53 %)
Experimental Section
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10.1002/chem.201905211
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and used as such in the next step. δH(500 MHz, CDCl3, +30 °C) 7.24 (d, J
= 8.0, 2H), 7.17 (d, J = 8.0, 2H), 4.52 (s, 2H), 3.77–3.72 (m, 2H), 3.60–
3.56 (m, 2H), 2.35 (s, 3H) ppm; δC(126 MHz, CDCl3) 137.5, 134.9, 129.1,
127.9, 73.1, 71.1, 61.9, 21.1 ppm.
We wish to thank M.Sc. Tiina Virtanen for assistance in the
synthesis, M.Sc. Esa Haapaniemi for assistance with the NMR
experiments, and Prof. Gerrit Groenhof, Dr. Sami Malola and
Prof. Petri Pihko for collaboration with the computational studies.
We acknowledge grants of computer capacity from the Finnish
Compound 5: Tosyl chloride (0.768 g, 4.028 mmol) was dissolved into
30 ml of dry dichloromethane under a nitrogen atmosphere, and the
solution was cooled in an ice bath. Triethylamine (0.56 ml, 4.015 mmol)
was added. 2-O-(4-Methyl benzoyl)ethyl alcohol 4 (0.56 g, 3.363 mmol)
was added in 15 ml of dichloromethane, and stirring was continued for an
hour in the ice bath and 24 hours at room temperature. The resulting
brown solution was washed twice with 40 ml of water. The yellowish
organic solution was dried with MgSO4, and the solution was evaporated
to dryness. The product was purified with flash chromatography using
SiO2 column and a hexane/ethyl acetate gradient (from 8:2 to 0:1). The
product was recovered as a colourless liquid (0.494 g, 46 %) with traces
of ethyl acetate and was used as such in the next step. δH(500 MHz,
CDCl3, 25 °C) 7.79 (d, J = 8.1, 2H, Ts-H), 7.31 (d, J = 8.1, 2H, Ts-H),
7.14 (br s, 4H, Ar-H), 4.44 (br s, 2H, Ar-CH2), 4.18 (t, J = 4.7, 2H, SO3-
CH2), 3.63 (t, J = 4.7, 2H, O-CH2), 2.43 (s, 3H, Ts-CH3), 2.34 (s, 3H, Ar-
CH3) ppm; δC(126 MHz, CDCl3) 144.9, 137.7, 134.6, 133.2, 129.9, 129.2,
128.1, 127.9, 73.3, 69.4, 67.4, 21.8, 21.3 ppm; m/z (ESI-TOF) 343.6 (M +
Na+, 100%).
Grid
and
Cloud
Infrastructure
(persistent
identifier
urn:nbn:fi:research-infras-2016072533) and the CSC-IT center
in Espoo. Academy of Finland (grants 309910, 314287, 257246)
is gratefully acknowledged for funding.
Keywords: Resorcinarene • Self-inclusion • Solution structure •
Supramolecular chemistry • X-ray crystallography
†
CSD version 2019 contained 131 structures identified as O-
functionalized resorcinarenes or pyrogallolarenes (podands) and all but
one were in boat conformation.
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Self-inclusion complex of a flexible
resorcinarene podand stabilized by
weak C-H···O interactions was
observed in crystal structure and in
solution.
Małgorzata Pamuła, Maija Nissinen and
Kaisa Helttunen*
Page No. – Page No.
Correlating solution and solid state
structures of conformationally
flexible resorcinarenes – the
significance of a sulfonyl group in
intramolecular self-inclusion
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