Halogen Bonding Molecular Capsules

Article abstract of DOI:10.1002/anie.201502960

Molecular capsules based solely on the interaction of halogen bonding (XB) are presented along with their host-guest binding properties in solution. The first example of a well-defined four-point XB supramolecular system is realized by decorating resorcin[4]arene cavitands with polarized halogen atoms for dimerization with tetra(4-pyridyl) resorcin[4]arene cavitands. NMR binding data for the F, Cl, Br, and I cavitands as the XB donor show association constants (K_a) of up to 5370 M^-1 (ΔG_{283 K}=-4.85 kcal mol^-1, for I), even in XB-competitive solvent, such as deuterated benzene/acetone/methanol (70:30:1) at 283 K, where comparable monodentate model systems show no association. The XB capsular geometry is evidenced by two-dimensional HOESY NMR, and the thermodynamic profile shows that capsule formation is enthalpically driven. Either 1,4-dioxane or 1,4-dithiane are encapsulated within each of the two separate cavities within the XB capsule, with of up to K_a=9.0 10⁸ M^-2 (ΔG_{283 K}=-11.6 kcal mol^-1).

Full text of DOI:10.1002/anie.201502960

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201502960
German Edition: DOI: 10.1002/ange.201502960
Host–Guest Systems
Halogen Bonding Molecular Capsules**
Oliver Dumele, Nils Trapp, and FranÅois Diederich*
Dedicated to Prof. Albert Eschenmoser at the occasion of his 90th birthday
Abstract: Molecular capsules based solely on the interaction of
halogen bonding (XB) are presented along with their host–
guest binding properties in solution. The first example of
a well-defined four-point XB supramolecular system is real-
ized by decorating resorcin[4]arene cavitands with polarized
halogen atoms for dimerization with tetra(4-pyridyl) resorcin-
[4]arene cavitands. NMR binding data for the F, Cl, Br, and I
cavitands as the XB donor show association constants (Ka) of
up to 5370mÀ1 (DG283K = À4.85 kcalmolÀ1, for I), even in XB-
competitive solvent, such as deuterated benzene/acetone/meth-
anol (70:30:1) at 283 K, where comparable monodentate
model systems show no association. The XB capsular geometry
is evidenced by two-dimensional HOESY NMR, and the
thermodynamic profile shows that capsule formation is
enthalpically driven. Either 1,4-dioxane or 1,4-dithiane are
encapsulated within each of the two separate cavities within the
Figure 1. Various supramolecular capsules based on different interac-
tions, and the first halogen bonding (XB) molecular capsule.
The importance of weak noncovalent interactions, such as
XB, is emerging in many fields of chemistry. XB has received
attention through its frequent application in crystal engineer-
ing.[7] More recently, it was implemented in complex supra-
molecular architectures[8] such as rotaxanes,[8a] catenanes,[8b]
chiral macrocyclic hosts,[8d] and fluorescent anion sensors.[8e]
In medicinal chemistry, XB has become an important tool in
the development of more active and selective ligands.[9]
Model system studies in solution[9e, 10] revealed that the
establishment of a single XB is largely enthalpy driven, but
this gain in enthalpy is mostly compensated by a large
unfavorable entropic term. The latter arises from the rigorous
XB capsule, with of up to Ka = 9.0 108 mÀ2 (DG283K
À11.6 kcalmolÀ1).
=
T
he first covalently bonded molecular containers, carcer-
ands, were reported by Cram et al. in 1985 and complexation
and reactivity within their unique inner phase were inves-
tigated.[1] In the following years, various types of noncovalent
intermolecular interactions were employed to construct
supramolecular containers.[2–4] The first supramolecular,
tennis-ball-shaped capsule capable of guest encapsulation
was assembled by Rebek and co-workers by taking advantage
of an elegantly designed hydrogen-bonding network between
two hemispheres.[2] This work was followed by the introduc-
tion of metal–ligand coordination cages by Fujita et al.,[3]
ionic capsular assembly by Verboom and co-workers,[4] and
a hybrid of the aforementioned.[5] Herein we report the first
molecular capsules assembled purely based on fourfold
halogen bonding (XB), as well as their guest encapsulation
properties in solution (Figure 1).[6]
À
geometric prerequisites for efficient XB, that is, C X···A
angles (A: XB acceptor atom) close to 1808 and the required
sub-van der Waals contact between the XB donor and
acceptor.[11] We hoped that the entropy loss would be reduced
in the fourfold XB interactions established in our capsular
assembly of highly preorganized components, as is well
established for other multidentate bonding systems.[2e, 8e,10c,12]
We selected top-rim-functionalized resorcin[4]arene cav-
itands as a concave, fourfold symmetric platform to generate
the two hemispheres of the XB capsule. Benzimidazole walls
on the cavitand scaffold, as introduced by Rebek and co-
workers,[13] allowed the installation of various substituents at
the upwards-pointing (2-)position leading to vertical exit
vectors for XB donor and acceptor motifs (Scheme 1).
Additionally, these extended benzimidazole cavitands are
conformationally highly preorganized as they exhibit a persis-
tent vase conformation. That conformation is particularly the
case in the presence of alcohols, which bridge and rigidify the
benzimidazole walls through a cyclic network of hydrogen
bonds, as shown by Rebek and co-workers.[13]
[*] O. Dumele, Dr. N. Trapp, Prof. Dr. F. Diederich
Laboratorium für Organische Chemie, ETH Zurich
Vladimir-Prelog-Weg 3, CH-8093 Zurich (Switzerland)
E-mail: diederich@org.chem.ethz.ch
[**] O.D. was supported by a KekulØ fellowship (Fonds der Chemischen
Industrie) and the Studienstiftung des Deutschen Volkes. We are
grateful for support by the ETH research council. We thank RenØ
Arnold and Dr. Marc-Olivier Ebert for recording 2D NMR spectra,
Michael Solar for measuring X-ray crystal structures, Prof. Dr. Carlo
Thilgen for help with the nomenclature, and Dr. Tristan Reekie and
Dr. Bruno Bernet (all ETHZ) for valuable comments on the
manuscript.
2,3,5,6-Tetrafluoro-4-halophenyl and pyridyl moieties
were chosen for the donor and acceptor motifs, respectively.
À
C X bond polarization through electron-withdrawing groups
on the phenyl ring of halophenyl XB donors is commonly
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502960.
applied to the construction of XB architectures.[7b, 8c,10b,c]
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Scheme 1. Left: Synthesis of the cavitands 1a,b and 2a–e which were subjected to investigation of XB-mediated capsule formation. Reagents and
conditions: a) 558C, 140 min, then FeCl3·6H2O (cat.), 708C, 17 h; b) 258C, 2–18 h, then FeCl3·6H2O (cat.), 25–558C, 20 h (X=F, Cl, Br, I). Right:
X-ray crystal structures of 1b (R=C6H13) and 2d (R=C11H23).[21] Solvent molecules, R-alkyl groups, and hydrogen atoms are omitted for clarity.
Thermal ellipsoids are shown at the 50% probability level at 100 K, and benzene guest molecules are drawn as stick model (see Section S6 in the
Supporting Information). Bottom right: The XB donor 4 serves as monodentate comparison probe.
Pyridine, however, is known to be the weakest XB acceptor
among hitherto quantified nitrogen Lewis bases,[10a,d] but
nevertheless, we expected amplified association for such
motifs to form the targeted capsules because of the cooper-
ativity effect of tetradentate XB.
The synthesis of the XB acceptor hemispheres 1a,b and
XB donor hemispheres 2a–e was based on the catalytic
oxidative benzimidazole formation between the octamine
cavitands 3a,b and benzaldehyde derivatives as developed by
Singh et al.,[14] and later applied to the construction of
extended cavitands by Rebek and co-workers (Scheme 1,
left).[13b]
Undecyl alkyl legs (1a, 2a–d) were installed at the lower
rim of the cavitands for enhanced solubility, whereas hexyl
legs (1b, 2e) were the choice for obtaining single-crystals
suitable for X-ray crystallography. Thereby, the design criteria
of vertical exit vectors of the XB motifs could be confirmed by
the X-ray crystal structures obtained for 1b, 2e, and even the
undecyl-legged cavitand 2d (Scheme 1, right). These crystal
structures also revealed the inclusion of two orthogonally
edge-to-edge stacked benzene molecules in the tetraiodo
cavitands 2d,e and one benzene molecule deep inside the
tetralutidine cavitand 1b, entrapped by the four inwards
converging methyl groups of the lutidyl moieties.
Association constants (Ka) for the complexation of
1a···2a–d were determined by titrations monitored by
19F NMR spectroscopy in an optimized solvent system of
deuterated benzene/acetone/methanol (70:30:1), which fully
solubilizes all components involved in this study. A minimum
amount of alcohol solvent is crucial for the solubility and for
bridging the benzimidazole wall flaps by hydrogen bonding,
as was observed in all X-ray structures (see Figure 2E and
Sections S3.1 and S6 in the Supporting Information).[13b] The
temperature for all binding studies was adjusted to 283 K to
minimize the unfavored TDSXB term upon complexation and
allow the quantification of even very weak XB motifs. When
applying these conditions, fast exchange rates relative to the
NMR time scale of the investigated XB capsule formation
were observed in all cases.
We firstly investigated the tetrapyridine version S6 as an
analogue of 1a in XB capsule association studies (see
Figures S14 and S15 in the Supporting Information). How-
ever, upon formation of the (most likely) capsular assembly
S6···2d, a precipitate formed (298 K). Hence, the eight
additional methyl groups in 1a compared to S6 seem to be
an essential structural modification for successful XB quan-
tification in solution. Complexation experiments with 1a
ensured homogenous titration conditions at all times, and it
was therefore constantly employed as a XB acceptor hemi-
sphere, whereas the XB donor hemisphere was systematically
varied by altering only the upper halogen atoms within the
full range of F, Cl, Br, and I.
Binding isotherms generated from the 19F NMR data
show the greatest change in chemical shifts for 1a···2d
[Ddsat(2d, Fortho) = 1.26 ppm] as compared to all other XB
donor cavitands (2a–c) subjected to titrations with 1a
[Ddsat(2a–c, Fortho) = 0.07 ppm; Figures 2A–C and Table 1].
The free enthalpy of binding, DG, for the XB capsule
formation of 1a···2d is À4.83 kcalmolÀ1 (association constant
Ka = 5370mÀ1) in the aforementioned solvent system at 283 K
(Table 1; for all titration data along with detailed analysis see
Section S3).
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Figure 2. A) 19F NMR binding titrations of tetrabromo (2c) and tetraiodo (2d) cavitands with 1a. 19F NMR signals for Fo and Fm atoms, relative to
the halogen substituent in 2c,d, were observed. Curve-fitting to 1:1 isotherms is shown as a solid line. For titrations of 2a,b with 1a, no
meaningful curve-fitting was possible. B) Zoomed-in view on the grey inset box of A: 19F NMR binding titrations of 2c with 1a. C) Titration
progress observed by 19F NMR spectroscopy of 2d (addition of 1a). D) 1H,19F HOESY NMR spectrum of 1a···2d in [D12]mesitylene (+2% 3,5-
dimethylbenzyl alcohol) at 283 K, which shows a cross-signal for the intermolecular through-space H···F coupling between 1a and 2d with two
1,4-dithianes encapsulated as guest molecules (not shown). Conditions of titrations for figures (A)–(C): C6D6/(CD3)2CO/CD3OD (70:30:1) at
283 K, c0(2c,d)ꢀ1.5 mm. E) Calculated model for XB capsule 1a···2d with two 1,4-dioxane guests and four MeOH bridging units to stabilize
intramolecular hydrogen bonding between imidazole walls (nearly optimized at the DFT:B3LYP/cc-pVDZ-LANL2DZ level of theory; see Section S7
in the Supporting Information).
Table 1: Association constants (Ka) and derived binding free enthalpies
(DG) of the XB donors 2a–d with the XB acceptor 1a, and thermody-
namic profile of the XB capsule 1a···2d.
atoms (relative to to the iodo substituent) in 2d in two
independent experiments (Figure 2D and Section S3.7). The
1:1 stoichiometry of the capsular assembly was confirmed by
continuous variation methods (Job plot analysis), and
MALDI-MS showed signals corresponding to the molecular
[a]
[b]
Complex
X
Ka,283K
[mÀ1
DG283K
Ddsat(Fortho/Fmeta)
[ppm]
]
[kcalmolÀ1
]
ion of the 1:1 XB complex (see Sections S3.5 and S3.6).[16]
A
1a···2d
1a···2c
1a···2b
1a···2a
I
5370
602
À4.83
1.26/0.59
0.05/0.23
0.07/0.02
0.07/0.03[d]
DFT model of 1a···2d supports the complementary geometry
of the cavitands with XB angles close to 18088 (Figure 2E).
Methods to obtain a single-crystal suitable for X-ray crystal-
lography of 1b···2e are currently being developed in our
laboratory.
Br
Cl
F
À3.60
[c]
–
–
–
–
–
[c]
[c]
4···3,5-lutidine
I
<1
–
[e]
DG283K
DH[e]
TDS[e]
By subjecting 2c to binding titrations with 1a, it was even
possible to determine XB association constants for organo-
bromines for the first time. This quantification experiment is
remarkable considering the weak acceptor character of the
pyridyl moieties in 1a and the competitive solvent system.
Although the absolute changes in chemical shift of both
fluorine atoms ortho and meta to the bromo substituent in 2c
were small compared to those in 2d, nonlinear curve-fittings
to the 1:1 isotherms resulted in reliable parameters for Ka and
Ddsat (Figures 2A,B). The association constant for 2c with 1a
is Ka = 602mÀ1, a magnitude lower than observed for the
complexation of the 2d, and is thus reflected in a DDG2c-2d
increment of 1.23 kcalmolÀ1 at 283 K. Hence, the difference
for establishing XB with iodine compared to bromine is
[kcalmolÀ1
]
[kcalmolÀ1
À12.61
]
[kcalmolÀ1
]
1a···2d
I
À4.87
À7.75 (283 K)
[a] In C6D6/(CD3)2CO/CD3OD (70:30:1) at 283 K. Determined by curve-
fitting of 19F NMR titration data to 1:1 binding isotherms. Error
estimated to be ca. 20%. [b] Calculated from Ka. [c] No meaningful curve-
fitting of the titration isotherms was possible. [d] Ddsat(Fipso)=0.13 ppm.
[e] Determined by van’t Hoff analysis of VT-NMR binding titrations (see
Section S3.4).
The binding mode of the capsular geometry was con-
firmed by 1H,19F HOESY NMR methods[15] with a cross-
signal for the intermolecular through-space coupling of the H-
C(2,6) protons of the lutidyl moieties in 1a with only the Fortho
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DDGBr!I = À0.31 kcalmolÀ1 per XB formed in the given
solvent conditions.
Table 2: Host–guest binding data for 1,4-dioxane and 1,2-dithiane
binding to 1a, 2d, and 1a···2d.
[a]
[b]
Titrations of Cl (2b) and F (2a) derivatives with 1a
resulted in minor changes of the 19F NMR chemical shifts, and
no meaningful curve fitting to 1:1 binding isotherms was
obtained for any of the observed 19F signals (fitting errors are
within the range or greater than the resulting parameter
values for Ka and Ddsat; see Figures S7–11 in the Supporting
Information).
Guest
Host
Ka,283K
DG283K
[kcalmolÀ1
]
1a
2d
1a···2d
8.9”102 mÀ1
3.6”102 mÀ1
5.8”105 mÀ2
À3.8
À3.3
À7.5
1,4-dioxane
1a
2d
1a···2d
1.0”104 mÀ1
3.4”104 mÀ1
9.0”108 mÀ2
À5.2
À5.9
1,4-dithiane
For comparison of the four-point XB capsule to a mono-
dentate XB assembly of the same binding motif, the
association constant for 4···3,5-lutidine (Scheme 1, right) was
determined and the binding was found to be too weak to allow
À11.6
[a] Ka value determined by 1H NMR integration ratios of bound and free
guest in [D12]mesitylene (+2% 3,5-dimethylbenzyl alcohol) at 283 K;
errors in Ka are estimated to be ca. 20%. [b] Calculated from Ka, 283K
.
a precise analysis by NMR binding titration (Ka < 1mÀ1
,
aforementioned solvent system, 283 K, see Figures S12 and
S13 in the Supporting Information). Along these lines, earlier
solution studies on monodentate perfluoroiodoalkanes with
pyridine as the XB acceptor showed that not even in apolar
solvents (benzene, CCl4, CDCl3) significant association was
observed (Ka = 1, 0.8, and < 0.5mÀ1, respectively for the listed
solvents).[10a]
1,4-dithiane binds with DG283K of À5.2 and À5.9 kcalmolÀ1,
respectively (Table 2). The cavity volume of 1a is smaller than
that of 2d, and 1,4-dioxane shows higher affinities to 1a,
whereas the larger 1,4-dithiane guest fits better in the cavity
of 2d (for cavity volumes see Section S4.4 in the Supporting
Information).
The substantial amplification of XB association in our
multidentate XB capsules is clearly caused by intrinsically
decreasing the unfavorable TDS term through a multidentate
assembly, as confirmed by vanꢀt Hoff analysis of the complex-
ation of 1a···2d (see Section S3.4). The formation of the XB
capsule is enthalpy-driven by DH = À12.6 kcalmolÀ1 and
entropically disfavored by TDS = À7.8 kcalmolÀ1 at 283 K.
In a simple picture, the complexation entropy is already
compensated upon the formation of three halogen bonds, and
the fourth XB can harvest the enthalpic gain without much
additional loss in entropy. Upon formation of three XBs, the
fourth halogen atom is restricted to undergo XB in such
capsular assemblies.
Host–guest binding studies of 1a···2d with 1,4-dioxane
and 1,4-dithiane reveal the shape, volume, and guest affinity
of the inner space of the XB capsule.[2b,17] To conduct these
studies, we developed a noncompetitive solvent system
consisting of [D12]mesitylene with 2% of 3,5-dimethylbenzyl
alcohol, an alcohol to stabilize the cavitandsꢀ conformations
by circular hydrogen bonding to the imidazole moieties
without occupying the cavity, while providing full solubility
(see Section S4.1).
In the quantification of guest binding to the XB capsule
1a···2d, slow exchange of the guests relative to the NMR time
scale was maintained, while the exchange rate for XB capsule
formation remained fast. The presence of the capsular
geometry during guest binding experiments was confirmed
by 1H,19F HOESY NMR spectroscopy (see Section S3.7).
Significant changes in the 1H NMR chemical shifts of the
guest protons inside the capsule 1a···2d, as compared to the
complexes of the single cavitands 1a and 2d, were observed
(Figure 3). The XB capsule 1a···2d does not feature a single
elongated cavity but two separated compartments in the two
associating cavitands. The four lutidine methyl groups con-
verge into the cavity, thus providing a steric barrier for
communication between the two hemispheres in the capsule
(X-ray in Scheme 1). The 1,4-dioxane or 1,4-dithiane guests
fill both compartments in the capsule, thus resulting in a 1:2
host/guest stoichiometry for the capsular complexes of the
In a first step, we investigated binding to only the single
cavitands 1a and 2d by employing 1,4-dioxane and 1,4-
dithiane as guests. The encapsulated guests show slow
exchange rates relative to the NMR time scale (aforemen-
tioned solvent system, 283 K), and their chemical shifts
appear in the negative ppm region, which is typical for top-
closed cavitands and molecular capsules with aromatic
scaffolds.[13b, 18] Binding constants (Ka) were determined
from the integration ratio between the signals of free and
1
the bound guest. In case the H NMR resonance of the free
guest overlapped with other signals of the sample, hexakis-
[(trimethylsilyl)ethynyl]benzene[19] (S14) was employed as
particularly bulky, yet symmetrical internal standard, to
determine the concentration of the free guest (see Sec-
tion S4.1). The free enthalpy of binding of 1,4-dioxane to 1a
and 2d are À3.8 and À3.3 kcalmolÀ1, respectively, whereas
Figure 3. 1H NMR spectra of 1,4-dioxane encapsulated within 1a···2d,
2d, and 1a, in [D12]mesitylene (+2% 3,5-dimethylbenzyl alcohol) at
1
283 K. Locations of guests were determined by H,19F HOESY NMR
spectra (see Section S4.3). 1H NMR chemical shifts of the correspond-
ing signals are given in ppm.
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Jasat, J. C. Sherman, Chem. Rev. 1999, 99, 931 – 968; e) R.
type [(guest ꢁ 1a)···(guest ꢁ 2d)]. For 1a···2d as the host, the
position of the guests, located either in the 1a or the 2d
hemisphere, was determined by assigning each 1H NMR
Warmuth, J. Yoon, Acc. Chem. Res. 2001, 34, 95 – 105.
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Int. Ed. 2002, 41, 1488 – 1508; Angew. Chem. 2002, 114, 1556 –
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Vysotsky, Aust. J. Chem. 2001, 54, 671 – 677; e) L. J. Prins, D. N.
Reinhoudt, P. Timmerman, Angew. Chem. Int. Ed. 2001, 40,
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1649 – 1650; b) M. Fujita, Chem. Soc. Rev. 1998, 27, 417 – 425; for
selected work by others: c) P. Jacopozzi, E. Dalcanale, Angew.
Chem. Int. Ed. Engl. 1997, 36, 613 – 615; Angew. Chem. 1997,
109, 665 – 667; d) D. L. Caulder, K. N. Raymond, J. Chem. Soc.
Dalton Trans. 1999, 1185 – 1200; e) S. R. Seidel, P. J. Stang, Acc.
Chem. Res. 2002, 35, 972 – 983; f) M. M. J. Smulders, I. A.
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1728 – 1754.
1
signal with H,19F HOESY NMR techniques (Figure 3). In
such experiments, the 1H NMR resonances of the guest show
cross-signals of strong intensity with the Fmeta atoms and cross-
signals of weak intensity with the Fortho atoms of the 2d
hemisphere (see Section S4.3).
The most stable association was observed for the complex
(1,4-dithiane)2 ꢁ 1a···2d (DG = À11.6 kcalmolÀ1). The asso-
ciation of 1,4-dithiane to all three molecular hosts, 1a, 2d, and
1a···2d, is stronger than that of 1,4-dioxane (Table 2), and is
attributed to dispersive S···p interactions and possibly S···N
interactions.[20] Gains in DG of À0.4 kcalmolÀ1 for 1,4-
dioxane and À0.5 kcalmolÀ1 for 1,4-dithiane binding in the
XB capsule are observed when compared to the binding
within the single cavitands. We attribute this insignificant
enhancement of guest affinities in the XB capsule to only
a small change in the geometry and electronic environment of
the inner cavities which remain nearly unchanged upon
formation of the XB capsule.
[4] G. V. Oshovsky, D. N. Reinhoudt, W. Verboom, J. Am. Chem.
Soc. 2006, 128, 5270 – 5278.
[5] M. Yamanaka, N. Toyoda, K. Kobayashi, J. Am. Chem. Soc.
2009, 131, 9880 – 9881.
In summary, we present the first example for the
formation of supramolecular XB capsules in solution by
installing tetrafluorohalophenyl motifs as XB donors and
lutidyl moieties as XB acceptors on resorcin[4]arene cavitand
scaffolds. Multidentate four-point XB was realized for X = I
and Br by fulfilling the 1808 angle as a binding criterion
simultaneously for all four XBs formed in the assembly. No
structured association was found for the F and Cl derivatives
(2a and 2b). The strongest XB capsule was formed with the
tetraiodo cavitand 2d complexed with tetralutidine cavitand
1a in the competitive solvent C6D6/(CD3)2CO/CD3OD
(70:30:1) at 283 K. Vanꢀt Hoff analysis revealed the thermo-
dynamic profile of the XB capsule 1a···2d to be largely
enthalpy driven, from which we conclude that decreasing the
entropic term of XB by multidentate binding is the key to self-
assembly of even weak binding partners such as the pyridine
derivative 1a. We further demonstrated guest binding to the
XB capsule 1a···2d, which features two separated compart-
ments. Binding free enthalpies down to À11.6 kcalmolÀ1 were
measured for the 1:2 complex of 1,4-dithiane in 1a···2d at
283 K in [D12]mesitylene (+ 2% 3,5-dimethylbenzyl alcohol).
With this example of a highly defined noncovalent structure
assembled solely by XB, we clearly expect further applica-
tions of halogen bonding in future supramolecular architec-
tures.
[6] A model for the X-ray structure of a pseudocapsular assembly
consisting of a tetra-(3-pyridyl) cavitand and a flexible iodinated
calix[4]arene showed partly the XB dimer in the solid state
besides other geometries (crystallographic R1-value = 21.4%):
a) C. B. Aakerçy, A. Rajbanshi, P. Metrangolo, G. Resnati, M. F.
Parisi, J. Desper, T. Pilati, CrystEngComm 2012, 14, 6366 – 6368;
An XB donor with quadruple iodoethynyl motifs at the upper
rim of a resorcin[4]arene cavitand has recently been reported,
however, the desired formation of a dimeric capsular assembly
with the pendant XB acceptor cavitand has not been observed:
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Keywords: halogen bonding · host-guest systems ·
self-assembly · supramolecular chemistry · X-ray diffraction
How to cite: Angew. Chem. Int. Ed. 2015, 54, 12339–12344
Angew. Chem. 2015, 127, 12516–12521
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Received: March 31, 2015
Published online: May 26, 2015
12344 www.angewandte.org
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Angew. Chem. Int. Ed. 2015, 54, 12339 –12344