Cooperative Binding of Divalent Diamides by N-Alkyl Ammonium Resorcinarene Chlorides

Article abstract of DOI:10.1002/chem.201406504

N-Alkyl ammonium resorcinarene chlorides, stabilized by an intricate array of hydrogen bonds leading to a cavitand-like structure, bind amides. The molecular recognition occurs through intermolecular hydrogen bonds between the carbonyl oxygen and the amide hydrogen of the guests and the cation-anion circular hydrogen-bonded seam of the hosts, as well as through CH7mellip;π interactions. The N-alkyl ammonium resorcinarene chlorides cooperatively bind a series of di-acetamides of varying spacer lengths ranging from three to seven carbons. Titration data fit either a 1:1 or 2:1 binding isotherm depending on the spacer lengths. Considering all the guests possess similar binding motifs, the first binding constants were similar (K₁:10²M^-1) for each host. The second binding constant was found to depend on the upper rim substituent of the host and the spacer length of the guests, with the optimum binding observed with the six-carbon spacer (K₂:10³M^-2). Short spacer lengths increase steric hindrance, whereas longer spacer lengths increase flexibility thus reducing cooperativity. The host with the rigid cyclohexyl upper rim showed stronger binding than the host with flexible benzyl arms. The cooperative binding of these divalent guests was studied in solution through ¹HNMR titration studies and supplemented by diffusion-ordered spectroscopy (DOSY), X-ray crystallography, and mass spectrometry. Cooperative guests: N-Alkyl ammonium resorcinarene chlorides cooperatively bind a series of homoditopic alkyl diamides of varying spacer lengths (see figure). The highest binding was observed with the six-carbon spacer guest. The cooperative binding of these divalent guests was studied in solution by using ¹HNMR titration studies and supplemented by diffusion-ordered NMR spectroscopy (DOSY), X-ray crystallography, and mass spectrometry.

Full text of DOI:10.1002/chem.201406504

DOI: 10.1002/chem.201406504
Full Paper
&
Cooperative Effects
Cooperative Binding of Divalent Diamides by N-Alkyl Ammonium
Resorcinarene Chlorides
N. Kodiah Beyeh,[a] Altti Ala-Korpi,[a] Fangfang Pan,[a] Hyun Hwa Jo,[b] Eric V. Anslyn,*[b] and
Kari Rissanen*[a]
Abstract: N-Alkyl ammonium resorcinarene chlorides, stabi-
lized by an intricate array of hydrogen bonds leading to
a cavitand-like structure, bind amides. The molecular recog-
nition occurs through intermolecular hydrogen bonds be-
tween the carbonyl oxygen and the amide hydrogen of the
guests and the cation–anion circular hydrogen-bonded
seam of the hosts, as well as through CH···p interactions.
The N-alkyl ammonium resorcinarene chlorides cooperatively
bind a series of di-acetamides of varying spacer lengths
ranging from three to seven carbons. Titration data fit either
a 1:1 or 2:1 binding isotherm depending on the spacer
lengths. Considering all the guests possess similar binding
motifs, the first binding constants were similar (K1: 102 mÀ1)
for each host. The second binding constant was found to
depend on the upper rim substituent of the host and the
spacer length of the guests, with the optimum binding ob-
served with the six-carbon spacer (K2: 103 mÀ2). Short spacer
lengths increase steric hindrance, whereas longer spacer
lengths increase flexibility thus reducing cooperativity. The
host with the rigid cyclohexyl upper rim showed stronger
binding than the host with flexible benzyl arms. The cooper-
ative binding of these divalent guests was studied in solu-
1
tion through H NMR titration studies and supplemented by
diffusion-ordered spectroscopy (DOSY), X-ray crystallography,
and mass spectrometry.
Introduction
one substrate leads to higher affinity of subsequent substrates,
and is a common phenomenon in substrate recognition by en-
zymes.[6] Cooperative effects are known to play an important
role in the binding of ion pairs by heteroditopic receptors.[7–11]
N-Alkyl ammonium resorcinarene halides, obtained from the
ring opening of tetrabenzoxazines in the presence of mineral
acids heated at reflux, are stabilized by a strong circular hydro-
High affinity and/or selectivity of substrate recognition by syn-
thetic receptors is an increasing area of research with applica-
tions in biology, such as the recognition of specific amino-acid
side chains on a protein.[1] Exploiting cooperativity to achieve
such recognition is one of many design options. Cooperativity
arises from the interplay of two or more interactions behaving
differently from expectations based on the properties of the in-
dividual interactions acting in isolation.[2–4] This phenomenon is
usually observed in multivalent processes in which the cou-
pling of interactions can lead to positive or negative coopera-
tivity, depending on whether one process favors or disfavors
another. Cooperativity is vital in systems chemistry because it
can lead to collective properties that are absent in the individ-
ual molecular components, and this phenomenon constitutes
one of the most important properties of molecular systems in
biology.[2,3,5] Positive cooperativity occurs when the binding of
+
gen-bonded cation–anion seam (···NR’R’’H2+···XÀ···NR’R’’H2
···XÀ···)2.[12,13] These organic salts are usually referred to as hy-
drogen bond analogues of covalent cavitands due to their
conformational and cavity size similarities.[12,13] These large or-
ganic salts are versatile receptors that can bind a variety of
neutral guests through several weak interactions utilizing the
electron-rich resorcinarene cavity and the hydrogen-bonded
cation–anion seam.[13,14]
Amides possess both hydrogen-bond-donating and accept-
ing groups, making them suitable guests for these receptors.[14]
This functional group is commonly employed in various tech-
nological applications.[15] In biology, the amide or peptide
bond is the principal structural element in proteins and is es-
sential to a vast number of drugs, with the iconic penicillin
being a good example.[16] Because the use of amides is very
broad, the quest for receptors that can suitably bind them is
an interesting and a continuously developing area of re-
search.[15] Recent results from our group show that N-methyl-
substituted amides possessing hydrogen-bond-donating ÀNH
and accepting ÀC=O groups are particularly good guests for
the resorcinarene salt receptors.[14] Together with strong hydro-
gen bonds, these receptors can also bind amides through sev-
[a] Dr. N. K. Beyeh, M. Sc. A. Ala-Korpi, Dr. F. Pan, Prof. K. Rissanen
University of Jyvaskyla, Department of Chemistry, Nanoscience Center, P.O.
Box. 35, FI-40014 University of Jyvaskyla (Finland)
E-mail: kari.t.rissanen@jyu.fi
[b] M. Sc. H. H. Jo, Prof. E. V. Anslyn
Department of Chemistry
The University of Texas at Austin
Austin, Texas 78712 (USA)
E-mail: anslyn@austin.utexas.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406504.
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eral weak CH···p interactions.[14] The positive results from the
binding of mono-amides prompted the investigation of coop-
erativity in the binding of a series of divalent homoditopic dia-
mides of varying spacer lengths. A similar phenomenon has
been reported, in which negatively charged homoditopic
guests with aromatic end groups template dimeric assembly in
the presence of positively charged calixarenes in aqueous
media.[17]
Solution studies
The binding of the mono-acetamides by analogues of the re-
ceptors show 1:1 binding stoichiometry, as determined by Job
plot experiments.[22,23] Based on this knowledge, one would
expect the receptors to bind both ends of the diamides in
a similar 1:1 binding stoichiometry, hence resulting in an over-
all 2:1 host–guest binding stoichiometry. Job plot experiments
involving the receptor 3 with the guest 4 showed a 1:1 stoichi-
ometry as the predominant species, whereas it was 2:1 with
guests 5–7 (see the Supporting Information, Figure S5). The
determination of the stoichiometry of complex systems other
than 1:1 can be a limitation of the Job method,[24] therefore
the binding stoichiometry could be supported by DOSY NMR
spectroscopy, titration data, and analysis by X-ray crystallogra-
phy.[24]
In the study described herein, we set out to investigate the
cooperative binding of diamides 4–7 by the N-alkyl ammoni-
um resorcinarene chlorides 1–3. In the process, diacetamides
4–7 of varying spacer lengths, ranging from three to seven car-
bons, were synthesized and used as homoditopic guests. Co-
operative binding of these diamides by 1–3 was investigated
in solution through a series of 1H NMR titration studies and
supported by diffusion-ordered NMR spectroscopy (DOSY
NMR), single-crystal X-ray diffraction analyses, and electrospray
ionization mass spectrometry (ESI-MS) in the gas phase.
Diffusion-ordered NMR spectroscopy
Diffusion can be used to determine intermolecular interactions
in solution because the diffusion coefficient of a molecular
species under specific conditions (e.g., concentration, solvent,
temperature, etc.) depends on its molecular weight, size, and
shape.[25,26] These receptors are known to encapsulate solvent
molecules within their cavities,[13,14] as shown from the X-ray
structure of the cyclohexyl analogue 2 obtained from a chloro-
form/diethyl ether mixture. A 1:2 host–guest complex with
two chloroform molecules in the cavity was found.[14] Weak in-
teractions such as N(NH2+)ÀH···Cl(CHCl3), CÀH···Cl(CHCl3), and
C(CHCl3)ÀH···ClÀ hold the chloroform in the cavity of the recep-
tor.[14] The inclusion of the CDCl3 in the cavity of 3 was also
supported by its diffusion coefficient of 1.615”10À5 cm2 sÀ1
(pure CDCl3 =2.226”10À5 cm2 sÀ1).[27]
Results and Discussion
N-Alkyl ammonium resorcinarene chlorides 1–3 (Figure 1), with
cyclohexyl and benzyl substituents, were synthesized accord-
ing to reported procedures.[12,13] The ÀCH2 of the benzyl
groups introduces a degree of flexibility to the upper rim of
Our recent study with the N-propyl analogue showed these
large organic salt compounds to aggregate into dimeric as-
sembly in chloroform.[14] The diffusion coefficient of receptor 3
(30 mm) in CDCl3 at 303 K was (0.378Æ0.03)”10À5 cm2 sÀ1
(Figure 2, Table 1). Reported diffusion coefficients of dimeric
long chain resorcinarenes in chloroform were around 0.35”
10À5 cm2 sÀ1.[28] Based on the DOSY experiments (Figure 2,
Table 1, and the Supporting Information, Figures S6–S9) in
chloroform, the receptor 3 exists as a dimer, presumably inter-
acting through electrostatics (salt aggregation), van der Waals,
and/or hydrogen bond interactions. Methanol is known to
compete with hydrogen bonds and thus disrupts assemblies
held together by hydrogen bonds in solution.[28] To confirm
the host is a dimer in solution, 10 and greater than 2000 equiv-
alents of CD3OD were added to the receptor and the DOSY
was measured under the same conditions. The diffusion coeffi-
cient of the receptor after the addition of 10 equivalents of
Figure 1. N-Alkyl ammonium resorcinarene chlorides 1–3 and the diamide
4–7 guests.
the receptors because of the additional rotatable bond com-
pared with cyclohexyl. The strong circular hydrogen-bonded
cation–anion seam, formed between the spherical chloride
anions and the ammonium cations, results in an extended inte-
rior cavity suitable for the recognition of a variety of guest
molecules.[13,14,18] The resorcinarene receptors 1–3 are C4v sym-
1
metric in solution as observed from their H NMR spectra. We
recently showed that these types of receptors could bind a vari-
ety of small neutral amides such as N-alkyl and N-aryl acet-
amides possessing hydrogen-bond-donating and accepting
groups.[14] From the previous results, the idea of the possible
cooperative binding of diamides was initiated and subsequent-
ly, a series of N-alkyl diacetamides 4–7 were synthesized
through the Schotten–Baumann reaction in which an acid
chloride reacts with amines under basic conditions.[19,21] The
length of the spacer between the ÀNH groups of the diamides
varies from three to seven carbons (Figure 1).
CD3OD
was
(0.510Æ0.07)”10À5
and
(0.593Æ0.12)”
10À5 cm2 sÀ1 after more than 2000 equivalents, clearly showing
the presence of smaller species, that is, the monomeric recep-
tor 3. Because CD3OD can only disrupt a complex or aggre-
gate, the diffusion coefficient in chloroform thus suggests a sol-
vent-filled dimeric aggregate that degrades into monomers in
the presence of CD3OD (Table 1). DOSY experiments for guests
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DOSY results show diffusion co-
efficients of the host 3 to be
(0.575Æ0.17)”10À and (0.571Æ
0.21)”10À5 cm2 sÀ1 for guest 6.
This indicates that CD3OD breaks
up the dimeric assembly into
smaller species. The diffusion
coefficient of 6 still indicates
an assembly larger than the
free guest
10À5 cm2 sÀ1).
6
(1.102Æ0.04”
Considering that the exchange
is fast on the NMR timescale, we
conclude a dimeric assembly dis-
assembles in the presence of
methanol into smaller assem-
blies consisting of the pure mon-
omers and 1:1 monomeric com-
plexes in a dynamic mixture. The
diffusion coefficients from host 3
and guests 4–7 are presented in
Table 1. From these results, the
diffusion coefficient for the six-
carbon spacer 6 is closest to the
host 3 suggesting a best-fit as-
sembly. The DOSY experiments
Figure 2. 2D DOSY NMR spectra (in CDCl3 at 303 K) of a 30 mm sample of: a) 3, b) 3+ten equivalents of CD3OD,
c) 6, and d) the host–guest assembly 3+6, showing the chemical species present in the sample. Chemical shifts
[ppm] are shown on the x axis and the diffusion coefficients [logm2 sÀ1] on the y axis of the 2D plot.
thus confirm a dynamic mixture consisting of 1:1 and 2:1
host–guest assemblies in solution.
Table 1. Average diffusion coefficients D [”10À5 cm2 sÀ1] of the guests 4–
7 and host 3 in different combinations in CDCl3 at 300 K. The molar
[a]
ratios of the host–guest mixtures were 2:1.
D, Guest
D, Host
D, Guest
D, Host
1H NMR Titration
3
–
0.378Æ0.03 3/CD3OD
(10 equiv)
–
0.510Æ0.07
1
Cooperativity was investigated through a series of H NMR ti-
3/CD3OD
(>2000 equiv)
3/4
3/5
3/6
3/6:CD3OD
(10 equiv)
3/7
–
0.593Æ0.12
tration experiments between the hosts 2/3 and the diamides
4–7 in CDCl3 at 303 K. In the experiments, an increasing
amount of each diamide was added to a solution of either of
the resorcinarene hosts 2 and 3. At each stage, 0.2 equivalents
of the guests were added, up to three equivalents. From then,
larger amounts were then added to a maximum of six equiva-
4
5
6
1.272Æ0.01
1.113Æ0.09
1.102Æ0.04
–
–
–
0.699Æ0.30 0.403Æ0.02
0.814Æ0.06 0.412Æ0.01
0.411Æ0.10 0.455Æ0.09
0.571Æ0.21 0.575Æ0.17
7
1.078Æ0.04
–
0.748Æ0.05 0.448Æ0.06
1
lents. Complexation-induced shielding of the H NMR resonan-
[a] Diffusion coefficients of CDCl3 ranges 1.615–2.032”10À5 cm2 sÀ1 for
mixtures containing the hosts, and 2.174–3.069”10À5 cm2 sÀ1 for mixtures
containing only the guests. Diffusion coefficient of CD3OD is ꢀ2.1”
ces corresponding to the guest protons was observed
(Figure 3 and 4; see also the Supporting Information, Figur-
es S10–S15). The observed shifts result from the shielding ef-
fects of the aromatic rings of the bowl-shaped host cavity
upon addition of the guests. The guest exchange is fast on the
NMR timescale. Changes to the proton resonances of the
ÀCOCH3, ÀNH, ÀNCH2, and ÀCH2 signals of the guests, as well
10À5 cm2 sÀ1 [27]
.
4–7 and 2:1 host–guest mixtures were analyzed under the
same conditions (Figure 2, Table 1, the Supporting Information,
Figures S6–S9). The changes in the diffusion coefficients of the
complexed guests from the free species to values nearer those
of the hosts are an indication of a host–guest complex in solu-
tion. Taking the 2:1 host–guest mixture of 3 and 6 as an exam-
ple, the diffusion coefficient of the host 3 was (0.455Æ0.09)”
10À5 and (0.411Æ0.10)”10À5 cm2 sÀ1 for guest 6. This shows
a clear encapsulation of 6 and a subsequent host–guest as-
sembly closer to the host aggregate seen in pure CDCl3. Ten
equivalents of CD3OD were added to this sample and the
+
as the ÀOH and ÀRR’NH2 signals of the hosts, were observed.
These changes are attributed to hydrogen bonds and CH···p
interactions formed upon complexation.
The ÀNH2 and ÀOH signals play a key role in stabilizing the
host structure and are thus affected when the diamide sits in
the cavity of the hosts. The most intense spectral changes
were observed for signals from the methyl protons of the dia-
mide guests. Taking into consideration the shielding effect of
the aromatic rings of the resorcinarene skeleton, the highly
shielded methyl protons indicate that the methyl group of the
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Table 2. Binding constants K1 [mÀ1] and K2 [mÀ1] for hosts 2/3 with guests
4–7.[a]
K1
K2
a=(4K2/K1)[b]
4·2
4·3
5·2
5·3
6·2
6·3
7·2
7·3
469Æ60
238Æ21
370Æ31
267Æ18
423Æ31
222Æ40
482Æ28
144Æ33
–
–
–
–
3407Æ310
2059Æ202
4820Æ273
3990Æ616
4245Æ264
3314Æ438
36.83
30.84
45.57
71.89
35.22
92.05
[a] Obtained from monitoring the guest ÀNCH2 signals in CDCl3 at 303 K.
[b] a represents the interaction parameter used to describe cooperativity.
1
Figure 3. Selected region of the H NMR spectra observed upon the titration
of the diamide guest 6 to the N-cyclohexyl ammonium resorcinarene chlo-
ride host 2 in CDCl3 at 303 K. Stars and black dots show the changes in the
ÀNCH2 and ÀCOCH3 signals, respectively.
The upper rim substituent of the host has a direct effect on
the binding affinity towards the guests. The first binding con-
stants were generally higher for the resorcinarene host 2 over
3 (e.g., 6@22 K1: 423mÀ1 and 6@32 K1: 222mÀ1, Table 2). The
more rigid cyclohexyl groups of host 2 provide a more fixed
cavity, whereas the benzyl groups of 3 introduce a degree of
flexibility, which lowers the binding affinity. The first binding
constants K1 were generally similar for each host. The length of
the spacers between the two binding sites of the guests had
a significant effect on the overall binding as seen from the
second binding constants K2. The guest 4 was too short and
could only fit into the cavity of one host. For the longer guests
5–7, positive cooperativity was indicated by monitoring the
second binding constant K2, which was larger than K1 in all
cases.
The interaction parameter a (used to describe cooperativity
and determined by a=4K2/K1, see the Supporting Information)
was large for guests 5–7.[3,4] The highest K2 was observed with
the six-carbon spacer diamide 6 (6@22: K2: 4820mÀ1, a=45.57
for host 2; 6@32: K2: 3990mÀ1, a=71.89 for host 3, Table 2).
This result shows the six-carbon diamide 6 is of optimum
length and fits perfectly in the cavity of the two receptors at
both ends. This result also compliments the DOSY measure-
ment between the receptor 3 and guest 6 (Figure 2, Table 1).
Strain and repulsion are responsible for the lower binding of
the shorter spacer guest 5, whereas more flexibility can explain
the lower binding of the longer spacer guest 7 (Table 2). The
second binding constant showed a chain length dependent
binding strength.
1
Figure 4. Selected region of the H NMR spectra observed upon the titration
of the diamide guest 6 to the N-benzyl ammonium resorcinarene chloride
host 3 in CDCl3 at 303 K. Stars and black dots show the changes in the À
NCH2 and ÀCOCH3 signals, respectively.
guest [CH3C=O] sits deep in the cavity of the host (Figure 3
and 4; see also the Supporting Information, Figures S10–S15).
Thus, the signal changes of the methyl protons are the most
suitable to follow in calculating the binding constant of the
guests. However, due to severe overlap between d=1–3 ppm,
these signals could not be reliably followed in all cases. For
consistency, the -NCH2 signals of the guests that lie above the
cation–anion belt and thus do not strongly feel the anisotropy
of the resorcinarene phenyl rings were reliably monitored and
the binding constants were obtained from the fit of the non-
linear least squares titration curve of the respective titration
data (Figure 3 and the Supporting Information). The titration
data for guest 4 fit a 1:1 host–guest binding isotherm, whereas
the data for the longer spacer containing guests 5–7 best fit
a 2:1 host–guest binding isotherm. Binding constants (see the
Supporting Information, Figures S16–S23) for the complexes
were determined by using the winEQNMR2 computer pro-
gram[29] and are presented in Table 2. K1 and K2 are defined in
the Supporting Information.
In all cases, the effect of the upper rim substituent of the
hosts influences the binding. Whereas the cyclohexyl groups
are rigid, the phenyl rings of the benzyl groups are known to
be oriented parallel to the plane of the hydrogen bond seam
(···H-(R’)N+(R’’)-H···XÀ···H-(R’)N+(R’’)ÀH···XÀ)2 thus introducing
a degree of flexibility on the upper rim of the host 3.[30] Higher
binding constants were observed for the rigid host 2 over the
more flexible host 3, but for the longer spacer guests 6 and 7,
cooperativity was generally higher for the resorcinarene host 3
over host 2 (Table 2). This implies that it is more favorable for
the second host to bind the other end of the guest after the
first binding process has occurred.
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Solid state analyses
salts. Even at very soft ionization parameters, it is often rare
that these salts can survive the high vacuum of a mass spec-
trometer and be seen fully intact.
To further study these systems in the solid state, single crystals
suitable for X-ray analysis were obtained from a 1:4 mixture of
the resorcinarene host 1 and guest 6 in CHCl3. The structure
clearly shows the 2:1 (6@12) host–guest assembly in the solid
state. Similar to the situation with the resorcinarene and mon-
oamide system,[14] each binding motif (CH3CONHÀ) of diamide
6 is located in the center of the cavity of each resorcinarene
host in the plane of the cation–anion belt, with the amide ni-
trogen atoms involved in N(amide)ÀH···ClÀ hydrogen-bonding
interactions (Figure 5). Simultaneously, the amide methyl
A mixture of the hosts 2/3 and amide guests 4–7 in the pos-
itive-ion mode resulted in 1:1 host–guest complexes. In the
spectrum containing host 2, in the positive-ion mode, progres-
sive loss of HCl resulted in signals corresponding to
[2À2HCl+H]+ (m/z=1341), [2À3HCl+H]+ (m/z=1305), and
[2À4HCl+H]+ (m/z=1269) (Figure 6a). Similarly, a spectrum
Figure 5. X-ray structure of the host–guest dimeric 2:1 capsule 6@12; a) ball
and stick representation with the guest in CPK mode; b) CPK representation
of the capsule.
group hydrogen atoms participate in CÀH···p interactions with
the interior cavity of the host. The ÀNCH2 of the guest is locat-
ed above the cation–anion belt, far from the phenyl rings of
the resorcinarene skeleton and thus explain the smaller shifts
of these protons in solution. In addition, one ClÀ anion is ex-
cluded from each resorcinarene host and this position is occu-
pied by a water molecule with a OÀH(water)···O=C(carbonyl)
hydrogen bond. The missing negative charge is accounted for
by the deprotonation of one of the ammonium groups on the
resorcinarene salts. Interestingly, the conformation of the
methyl carbonyl groups is gauche to the 1,6-hexanediaminyl
chain. This conformation efficiently involves the van der Waals
interactions of the cyclohexyl groups such that the dimeriza-
tion energy of the resorcinarene pair is minimized to the larg-
est extent (for a more detailed description, see the Supporting
Information).
Figure 6. ESI mass spectrum showing several signals corresponding to 1:1
monomeric complexes: a) mixture of host 2 and diamide 6; b) mixture of
host 3 and diamide 6. Inset: experimental and calculated isotope patterns of
selected signals.
containing resorcinarene host 3 in the positive ion mode
shows progressive loss of HCl, resulting in signals correspond-
ing to [3À3HCl+H]+ (m/z=1337), [3À4HCl+H]+ (m/z=1301),
and [3À4HCl+2H]+2 (m/z=651) (Figure 6b). Taking a mixture
of host 2 and the diamide 6 as an example, signals corre-
sponding to 1:1 host–guest complexes, such as
[6@(2ÀHCl+H)]+ (m/z=1578), [6@(2À2HCl+H)]+ (m/z=1542),
and [6@(2À4HCl+2H)]2+ (m/z=635) were observed (Fig-
ure 6a). Similarly, the mass spectrum of a mixture of host 3
and the diamide 6 show signals corresponding to 1:1 host–
guest complexes [6@(3À4HCl+H)]+ (m/z=1501) and
[6@(3À4HCl+2H)]2+ (m/z=751) (Figure 6b). In this sample,
a signal corresponding to 1:2 host–guest assembly [6+
6@(3À4HCl+2H)]2+ (m/z= 851) was also observed, which can
be attributed to aggregation of the diamides. The isotope pat-
terns obtained by experiment agree with those simulated on
Gas phase analyses
The host–guest assemblies were investigated in the gas phase
through a series of ESI-MS analyses.[31,32] A CHCl3/acetonitrile
mixture was used as the spray solvent in which N-alkyl ammo-
nium resorcinarene chlorides can be ionized in the gas
phase.[13,14] These salts are held together by several weak inter-
actions, emphasized by the many species seen in the gas
phase. The loss of multiple hydrogen chlorides (HCl) from the
hosts is a common phenomenon with these large organic
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Sciex) in Concord, Ontario (Canada). Details on the DOSY, NMR ti-
trations, and mass spectrometric studies are presented in the Sup-
porting Information. For X-ray crystallographic analysis, the data
was collected at 123 K on an Agilent Super-Nova Diffractometer
using mirror-monochromatized CuKa (l=1.54184 Š) radiation. All
details about data collection and reduction, as well as structure so-
lution and refinement are given in Supporting Information. CCDC-
1039550 for 6@12 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
the basis of natural abundances. Other minor signals were ob-
served in the mass spectra, assumed to either be unspecific as-
semblies or fragmentation adducts. Samples containing combi-
nations of the resorcinarene hosts 2/3 and the other diamide
guests 4–7 were measured and analyzed with results showing
similar patterns as observed in Figure 6 (see the Supporting In-
formation, Figures S24–29).
Conclusion
The present work reports the cooperative binding of diaceta-
mides 4–7 (homoditopic guests) by monovalent N-alkyl ammo-
nium resorcinarene chloride 1–3 receptors. Job plots, H NMR
Acknowledgements
1
titration, and DOSY NMR experiments revealed 2:1 complexes
The authors gratefully acknowledge financial support from the
Academy of Finland (KR: grant nos. 263256 and 265328; NKB:
grant no. 258653), the University of Jyväskylä, Finland and Na-
tional Science Foundation (EVA: grant CHE-1212971)
1
for the longer spacer guests 5–7. The H NMR titration studies
reveal that the lengths of the carbon-chain spacer have a pro-
found effect on the binding stability and stoichiometry, with
a statistical distribution based on the length of the spacer. The
optimum binding was observed with the six-carbon spacer be-
tween the amides confirming the best-fit scenario of the host.
The three-carbon spacer was too short and led to 1:1 binding.
The second binding constants K2 for the longer guests were
larger than K1 in all cases, confirming a positively cooperative
binding process. A 2:1 binding stoichiometry was unambigu-
ously confirmed in the solid state by X-ray single-crystal deter-
mination, which shows a ClÀ anion excluded from the aggrega-
tion. Despite the tendency for this host to subsequently lose
hydrogen chloride, the observation of the 1:1 host–guest com-
plexes in the gas phase shows the complexes to be stable
enough to resist the high vacuum of the mass spectrometer.
Higher assemblies, such as [6+6@(3À4HCl+2H)]+2 (m/z=
851), resulting from diacetamide aggregation were also ob-
served in some cases. These results show that the N-alkyl am-
monium resorcinarene chloride hosts (i.e., a hydrogen-bonded
analogue of a covalent cavitand) possess a cavity robust
enough to cooperatively bind diacetamides. One intriguing
aspect of this work is the fact that a receptor held together by
multiple weak interactions can utilize the same weak interac-
tions to cooperatively bind guest molecules. This work pro-
vides a useful addition to the library of host–guest assemblies
held together by weak interactions.
Keywords: amides · cooperative phenomena · host–guest
systems · hydrogen bonding · molecular recognition · self-
assembly
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Received: December 16, 2014
Published online on May 27, 2015
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