Macrocycle embrace: Encapsulation of fluoroarenes by m-phenylene ethynylene host

Article abstract of DOI:10.1002/chem.201406073

We report structural characterization of a new member of m-phenylene ethynylene ring family. This shape-persistent macrocycle also co-crystallizes with hexafluoro-, 1,2,4,5-tetrafluoro-, 1,3,5-trifluoro, and 1,4-difluorobenzene. The four complexes are alm

Full text of DOI:10.1002/chem.201406073

DOI: 10.1002/chem.201406073
Communication
&
Host–Guest Systems
Macrocycle Embrace: Encapsulation of Fluoroarenes by m-
Phenylene Ethynylene Host
[a]
[a]
[b]
[a]
[c]
Ilya Popov, Teng-Hao Chen, Sergey Belyakov, Olafs Daugulis, Steven E. Wheeler, and
[a]
ˇ
Ognjen S. Miljani c´ *
[6]
their center, have been characterized crystallographically, in-
Abstract: We report structural characterization of a new
member of m-phenylene ethynylene ring family. This
shape-persistent macrocycle also co-crystallizes with hexa-
fluoro-, 1,2,4,5-tetrafluoro-, 1,3,5-trifluoro, and 1,4-difluoro-
benzene. The four complexes are almost isostructural, and
all show the fluoroarene included into the central cavity
of the macrocycle. Characterized by multiple short
CÀH···FÀC contacts, these inclusion complexes further di-
merize in the solid state into a 2+2 assembly, in which
the two macrocycles embrace each other by their large
hydrophobic groups that are rotated by 608 relative to
one another.
[7]
cluding the large shape-persistent systems of Hçger and
[8]
Schlꢀter, but notably in the absence of guests in the central
[8]
cavity. The only exception are Schlꢀter’s very large PEMs,
which have been shown to crystallize with three ordered mole-
cules of benzene.
Herein, we present a series of four X-ray crystal structures
that show that m-phenylene ethynylene macrocycle
1
(Figure 1, top) can bind several fluorinated benzenes in an
Shape-persistent macrocycles constructed from arylene and
[
1]
ethynylene structural motifs are characterized by rigid struc-
tures and modular synthesis, which make them promising
building blocks for the construction of nanoporous solids,
liquid crystals, materials for non-linear optics, molecular elec-
tronic components, sensors, and microscopic reactors. Well-de-
fined central cavities also suggest that phenylene ethynylene
macrocycles (PEMs) should function as versatile supramolecular
hosts. Smaller members of this class, in which connections be-
tween the rings are ortho-positioned, have a cavity, which can
[
2]
typically fit only a single atom. These macrocycles, as well as
their metal complexes (in which the metal atom often resides
[
2b,3]
in the center of the cavity),
and complexes with external
[
4]
solvent molecule guests have all been structurally studied.
Significantly larger para-connected PEMs, often dubbed para-
phenyleneacetylenes or “carbon nanorings” on account of
[5]
their hoop-like nonplanar structures, are less common, but
[
5a]
have been shown to include alkylated benzenes and fuller-
[
5b]
enes
in their central cavities. In contrast, meta-connected
systems, which are close-to-planar and have a sizable cavity in
Figure 1. Macrocyclic m-phenylene ethynylene host 1 (top) and its crystal
structure (bottom). Thermal ellipsoids are shown at 50% probability.
ˇ
´
[
a] Dr. I. Popov, Dr. T.-H. Chen, Prof. O. Daugulis, Prof. O. S. Miljanic
Department of Chemistry, University of Houston
112 Fleming Building, Houston, TX 77204-5003 (USA)
E-mail: miljanic@uh.edu
isostructural fashion and 1:1 stoichiometry. These are among
the first crystallographically characterized complexes of planar
arylene ethynylene macrocycles with included molecular
[
b] Dr. S. Belyakov
Latvian Institute of Organic Synthesis
2
1 Aizkraukles Street, Riga LV 1006 (Latvia)
[9]
guests.
[
c] Prof. S. E. Wheeler
Department of Chemistry, Texas A&M University
College Station, TX 77842 (USA)
Compound 1 was synthesized by using a modification of
a literature procedure, which is presented in the Supporting In-
formation. Its single crystals were grown by the slow evapora-
tion of a biphasic system, in which the top layer was EtOH and
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406073.
Chem. Eur. J. 2014, 20, 1 – 6
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Communication
Figure 2. Crystal structures of complexes of 1 with 1,2-difluorobenzene (A), 1,3,5-trifluorobenzene (B), and 1,2,4,5,-tetrafluorobenzene (C). Top row shows the
structure of the complex (thermal ellipsoids presented at 50% probability), whereas the bottom row shows “embraced” dimers of macrocycle 1, in which
each macrocycle includes a molecule of fluorinated arene guest into its central cavity.
the bottom layer was a solution of 1 in dichloromethane. Com-
This inclusion complex dimerizes in the solid state; as shown
in Figure 2A, bottom, two molecules of 1 stack on top of each
other, but rotated by 608 relative to one another. This rotation
pound 1 crystallizes in P3 21 space group, with six molecules
1
of 1 per unit cell. The structure of the macrocycle (Figure 1,
bottom) is close to planar, with one of the benzene rings nota-
bly distorting from the average plane by 14.7(4)8. Deviations in
the triple bonds are minimal: all CÀCꢀC angles are greater
than 176.7(3)8. Transannular distances, defined as the distances
between the internal hydrogen atoms positioned furthest
away from each other across the macrocycle, are 8.63(1),
allows their bulky COOCEt pendant groups to avoid steric hin-
3
[12]
drance; in effect, the two macrocycles “embrace” each other,
and each one of them includes a 1,4-difluorobenzene guest
molecule.
A very similar structure is observed for the complex of 1 and
1,3,5-trifluorobenzene (1·C H F , shown in Figure 2B), which
6
3 3
[
10]
8.77(1), and 9.02(1) ꢂ.
was crystallized under identical conditions. This couple also
Complex of 1 with 1,4-difluorobenzene (1·C H F ) was crys-
crystallizes in the P2 /n space group with four molecules of
6
4
2
1
tallized by the slow evaporation of a biphasic system, in which
the bottom layer contained a solution of 1 in dichloromethane,
and the top layer was neat 1,4-difluorobenzene. The two co-
each compound in the unit cell. Tilt angle between the aver-
aged planes of the macrocyclic host and the trifluorobenzene
guest is 14.8(4)8, whereas the transannular HÀH distances in
1 are 8.65(1), 8.83(1), and 8.88(1) ꢂ. Fluorine atoms in trifluoro-
benzene again establish short contacts with neighboring hy-
drogens of the macrocycle, characterized by the following H···F
distances and CÀH···F angles: 2.37(1) ꢂ/147.9(1)8, 2.50(1) ꢂ/
146.2(1)8, 2.51(1) ꢂ/138.3(1)8, 2.53(1) ꢂ/145.9(1)8, 2.57(1) ꢂ/
148.8(1)8, and 2.77(1) ꢂ/140.0(1)8. Again, and as shown in Fig-
ure 2B, bottom, the two macrocycles interdigitate through
their hydrophobic group, encapsulating two molecules of
1,3,5-trifluorobenzene, which are in an antiparallel orientation.
Tetrafluorobenzene (1,2,4,5-isomer) co-crystallized with 1 in
an isostructural arrangement as its di- and trifluorinated coun-
terparts (Figure 2C, top). The crystals of this complex grew
after seven days from a system in which 1,2,4,5-tetrafluoroben-
zene was layered onto a solution of 1 in dichloromethane.
Once again, 1 essentially does not deform to accommodate
C H F , with transannular HÀH distances of 8.60(1), 8.80(1), and
crystallize (Figure 2A, top) in P2 /n space group with four mol-
1
ecules of each compound in the unit cell. The structure of the
co-crystal shows difluorobenzene included in the central cavity
of the macrocycle, with the small but noticeable tilt (13.2(4)8)
between the average planes of the guest and the host (exclud-
ing the pendant COOCEt groups). The larger ring does not
3
appear to significantly contract to match difluorobenzene, and
the transannular HÀH distances are rather similar to those ob-
served in the crystal structure of “empty” 1: 8.44(1), 8.93(1),
and 8.98(1) ꢂ. Each of the two fluorine atoms of difluoroben-
zene establishes short contacts with two internal hydrogens
from the benzene rings of 1. These contacts are characterized
by H···F distances of 2.36(1), 2.44(1), 2.57(1), and 2.65(1) ꢂ, and
CÀH···F angles of 155.4(1), 142.7(1), 152.0(1), and 149.6(1)8
(
given in the same order as the distances). Thus, some of them
[11]
could be interpreted as weak CÀH···FÀC hydrogen bonds, as
they are generally shorter than the sum of van der Waals radii
for hydrogen and fluorine (2.67 ꢂ).
6
2 4
8.98(1) ꢂ, and the most distorted benzene ring forming an
angle of 12.8(5)8 with the average plane of the macrocycle.
&
&
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Average planes of the host and its fluorinated guest are at an
angle of 13.0(4)8. Eight short H···F contacts are observed in the
structure of 1·C H F ; they range in their H···F distances be-
tween 2.31(1) and 2.61(1) ꢂ, whereas the corresponding
CÀH···F angles vary in the 137.5(1)–151.2(1)8 region. As with
the previous two superstructures, the embraced dimer is ob-
served in the packing diagram (Figure 2C, bottom).
closely, its supramolecular organization, shown in Figure 3,
bottom, is slightly different: the two macrocycles slip out of
the embrace, bringing their bulky alkyl groups into close con-
tact with each other. It should be noted that this crystal struc-
ture is of somewhat poorer quality than the preceding four:
a problem that persisted even when two separate batches of
crystals were used for data collection and refinement.
6
2 4
Hexafluorobenzene also co-crystallizes with
1
(crystals
Despite extensive efforts, we were not able to grow co-crys-
tals of 1 with benzene and obtain experimental evidence of
the importance of fluorination in this series of self-assembled
structures. Therefore, we turned to computation (at the B97-D/
grown under identical conditions as those of 1·C H F ), but this
6
2 4
complex has a slightly different structure. The space group
here changes to P2 /c, with four molecules of each compound
1
[13]
in the unit cell. Transannular HÀH distances in 1 are similar to
all other complexes (8.74(1), 8.92(1), 8.94(1) ꢂ), but one of the
benzene rings of 1 now puckers out of the average plane sig-
nificantly (by 23.8(2)8), resulting in an overall envelope confor-
mation of 1. Similar to all other fluoroarenes studied, C F is
TZV(2d,2p) level) to make this comparison. To simplify the
computations, 1 was replaced with analogue 2, in which the
three pendant ÀCOOCEt3 groups of 1 were replaced with
ÀCOOMe. Supramolecular gas-phase association of 2 was ex-
amined with both benzene and hexafluorobenzene. Both cases
revealed potential-energy surfaces with numerous, nearly iso-
energetic local minima. In both cases, the structures, in which
the benzene or perfluorobenzene adopts a planar orientation
within the cavity of 2, were saddle points on the energy surfa-
ces, not stable minima. However, for benzene, the energy is
6
6
clearly included in the cavity of 1 (Figure 3, top), with the tilt
angle between the average planes of the host and the guest
measuring 8.2(3)8. Short H···F contacts are once again ob-
served, with H···F distances in the 2.39(1)–2.60(1) ꢂ range, and
CÀH···F angles varying between 135.3(6) and 148.1(6)8. Al-
though the structure of 1·C F resembles its relatives quite
À1
lowered by only <0.3 kcalmol if the ring either tilts slightly
6
6
within the cavity or hovers just outside of the cavity. That is,
for benzene, very little energy is required for complete inser-
tion into the cavity. For perfluorobenzene, there is a 2 kcal
À1
mol drop in energy going from the fully inserted arrange-
ment to either a tilted arrangement or complex in which the
perfluorobenzene hovers 1 ꢂ above the plane of 2. Regardless,
this suggests only a moderate cost for C F adopting a fully
6
6
planar orientation inside of 2. Also, for perfluorobenzene, we
identified two complexes in which the perfluorobenzene en-
gages in [p···p] stacking interactions with one of the phenyl
À1
rings of 2, both of which are approximately 2 kcalmol lower
in energy than complexes in which perfluorobenzene resides
either within or just outside of the cavity (see the Supporting
Information for additional details). Although this is clearly in
contrast with the observed crystal structure, we note that the
complexes, in which C F is positioned outside of the cavity,
6
6
would be much less amenable to close packing of 1 and
would presumably leave a void in the middle of 1. On the
other hand, with perfluorobenzene fully inside the cavity of 1,
the modest loss in interaction energy could easily be compen-
sated for by the more favorable packing of 1 in the solid state.
For benzene, the analogous complexes, in which benzene en-
gages in [CÀH···p] interactions with one of the phenyl rings of
2
are of similar energy to complexes featuring benzene in or
[14]
just outside of the cavity.
We could observe no evidence of similar interactions be-
tween 1 and fluoroarenes in solution. Titration of 1 with C F
6
6
1
19
led to no observable changes in its H and F NMR or UV/Vis
[15]
spectra. This situation is analogous to the case of hydrocar-
bon inclusion in the cavities of cyclic paraphenyleneacetylenes,
[5a]
in which no analogous binding was observed in solution.
In summary, we have shown that the shape-persistent mac-
rocyclic host 1 can accommodate several fluorinated benzenes
in its central cavity, with minimal structural deformations
needed. Our results suggest that host–guest chemistry of
6 6
Figure 3. Top: crystal structure of 1·C F (thermal ellipsoids shown at 50%
probability). Bottom: the slipped-stack arrangement of the two host–guest
complexes within the unit cell.
Chem. Eur. J. 2014, 20, 1 – 6
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&
&
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Communication
planar m-phenylene ethynylene macrocycles may be much
richer than previously thought. The observed binding of fluori-
nated arenes appears to be a combined consequence of crys-
tallographic close packing, an optimal steric match with the
host and, possibly, weak [CÀH···FÀC] hydrogen bonds. It would
be intriguing to examine whether other guests could be in-
cluded in the cavity of 1, possibly using other intramolecular
binding interactions. For example, Flood and co-workers have
10.8228(3) ꢂ, a=90.008, b=28.4588(7) ꢂ, b=98.1522(13)8, c=
3
2
1
1
5
1.9360(6) ꢂ,
g=90.008,
V=6688.1(3) ꢂ ;
Z=4;
1calcd
=
À3
À1
.289Mgm ; m=0.168 mm ; radiation wavelength 0.71073 ꢂ; T=
73(2) K; qmax =27.58; 15099 measured reflections (out of which
453 independent); Rint =0.045; R =0.144; wR =0.366; largest
1
2
À3
diff. peak and hole: 1.371 and À0.611 eꢂ .
CCDC-1015944 (1), CCDC-1015943 (1·C H F ), CCDC-1015946
6
4 2
(
1·C H F ), CCDC-1015945 (1·C H F ), and CCDC-1015947 (1·C F )
6 3 3 6 2 4 6 6
contain 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_re-
quest/cif.
[
16]
amply demonstrated that anions can be bound in macrocy-
clic cavities through the exclusive use of CÀH hydrogen bond
[9,12,17]
donors. In that respect, aza-derivatized analogues of 1
[18]
may sufficiently alter its electronic properties to allow observa-
tion of anion binding, perhaps even in solution. Another area
of interest is looking at fluorinated arenes too large to fit
snugly within the cavity of 1, whether they will instead thread
through it, offering a route to a new class of rigid rotaxanes.
We hope that future work in our laboratories may provide an-
swers to these questions.
All computations utilized Gaussian 09 and were carried out at
[13]
the B97-D/TZV(2d,2p) level of theory. All structures were fully op-
timized and harmonic vibrational frequencies computed to confirm
that the structures located were energy minima.
Additional experimental data can be found in the Supporting Infor-
mation.
Acknowledgements
Experimental Section
We thank Dr. James D. Korp (University of Houston) for collec-
tion and refinement of crystal structure data. We also acknowl-
Crystallographic information for compound 1: C H Cl O , M =
73
69.90
2
3
6.95
r
À1
1
129.29 gmol ; crystal dimensions 0.30ꢃ0.25ꢃ0.25 mm ; trigonal
edge the financial support from the University of Houston (to
O. Sˇ .M.), the Norman Hackerman Advanced Research Program
crystal system; space group P3 21; unit-cell dimensions: a=
1
2
1
1
1
3.6003(3) ꢂ, a=90.008,
b=23.6003(3) ꢂ, b=90.008,
c=
1calcd =
(
to O.D.), the National Science Foundation (award CHE-1151292
3
9.7136(3) ꢂ,
g=120.008,
V=9508.9(2) ꢂ ;
Z=6;
ˇ
to O.S.M. and award CHE-1254897 to S.E.W.), the Welch Foun-
À3
À1
.183Mgm ; m=1.338 mm ; radiation wavelength 1.54178 ꢂ; T=
23(2) K; qmax =66.628; 65395 measured reflections (out of which
ˇ
dation (awards E-1571 to O.D., E-1768 to O.S.M., and A-1775 to
S.E.W.) and the Camille and Henry Dreyfus Foundation (to
O.D.). O. Sˇ .M. is a Cottrell Scholar of the Research Corporation
11107 independent); Rint =0.0242; R =0.0464; wR =0.1296; larg-
1
2
À3
est diff. peak and hole: 0.483 and À0.362 eꢂ .
Crystallographic information for complex 1·C H F : C H F O , M =
for Science Advancement. Work of S.B. was supported by the
European Regional Development Fund Project: ERAF 2DP/
2.1.1.1.0/10/APIA/VIAA/059.
6
4
2
78 70
2
3
6
r
À1
1141.34 gmol ; crystal dimensions 0.40ꢃ0.08ꢃ0.06 mm ; mono-
clinic crystal system; space group P2 /n; unit-cell dimensions: a=
1
7
3
1
1
1
.5333(1) ꢂ, a=90.008, b=22.7598(4) ꢂ, b=93.686(1)8, c=
3
6.8754(7) ꢂ, g=90.008, V=6309. 44(18) ꢂ ; Z=4; 1calcd
=
À3
À1
Keywords: fluoroarenes · inclusion complexes · macrocycles ·
shape persistency · X-ray diffraction
.202Mgm ; m=0.626 mm ; radiation wavelength 1.54178 ꢂ; T=
13(2) K; qmax =66.538; 43173 measured reflections (out of which
0995 independent); Rint =0.0234; R =0.0438; wR =0.1145; larg-
1
2
À3
est diff. peak and hole: 0.366 and À0.408 eꢂ .
Crystallographic information for complex 1·C H F : C H F O , M =
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clinic crystal system; space group P2 /n; unit-cell dimensions: a=
1
7
3
1
1
1
.6480(41) ꢂ, a=90.008, b=22.7572(3) ꢂ, b=94.246(1)8, c=
6.5575(5) ꢂ, g=90.008, V=6345.26(15) ꢂ ; Z=4;
.214Mgm ; m=0.654 mm ; radiation wavelength 1.54178 ꢂ; T=
13(2) K; qmax =62.428; 41749 measured reflections (out of which
0379 independent); Rint =0.1301; R =0.0628; wR =0.1712; larg-
3
1calcd
=
À3
À1
1
2
À3
est diff. peak and hole: 0.433 and À0.409 eꢂ .
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6
2
4
78 68
4
3
6
r
1
2
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3
1
1
.6495(4) ꢂ, a=90.008, b=22.7639(10) ꢂ, b=94.466(2)8, c=
6.5143(16) ꢂ,
.234Mgm ; m=0.686 mm ; radiation wavelength 1.54178 ꢂ; T=
23(2) K; qmax =66.628; 43547 measured reflections (out of which
3
g=90.008,
V=6339.0(5) ꢂ ;
Z=4;
1calcd =
À3
À1
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1027 independent); Rint =0.0237; R =0.0541; wR =0.1532; largest
1
2
À3
diff. peak and hole: 0.456 and À0.621 eꢂ .
Crystallographic information for complex 1·C F : C H Cl F O , M =
1
6
6
79 68
2
6
3
6
r
À1
298.23 gmol ; crystal dimensions 0.38ꢃ0.03ꢃ0.02 mm ; mono-
clinic crystal system; space group P2 /c; unit-cell dimensions: a=
1
&
&
Chem. Eur. J. 2014, 20, 1 – 6
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ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communication
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3
group has been replaced with a hydrogen
atom. Relative energy differences in this case are remarkably similar to
those observed for various con-conformations of 2·C
6
H
6
and 2·C
6
F
6
,
À1
1535–1537.
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Received: November 13, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
5
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Communication
COMMUNICATION
Planar and shape-persistent m-phenyl-
ene ethynylene macrocyclic host crystal-
lizes in 1:1 complexes with a series of
fluoroarene guests, including 1,3,5-tri-
fluorobenzene. The combination of
&
Host–Guest Systems
I. Popov, T.-H. Chen, S. Belyakov,
O. Daugulis, S. E. Wheeler, O. S. Miljani c´ *
ˇ
&
& – &&
(
pseudo)sixfold symmetry of the host
Macrocycle Embrace: Encapsulation of
Fluoroarenes by m-Phenylene
Ethynylene Host
and threefold symmetry of the guest is
reminiscent of the combination of sym-
metries observed in the arabesques of
Alhmabra (see figure).
A shape-persistent macrocycle…
from the m-phenylene ethynylene family is capable
…
of encapsulating 1,2-difluorobenzene, 1,3,5-trifluoro-
benzene, 1,2,4,5-tetrafluorobenzene, and hexafluoro-
benzene, in an almost isostructural arrangement, which
places the fluoroarene into the cavity of the macro-
cycle. Curiously, symmetries of the (pseudo)hexagonal
host and its guests do not have to be matched; this
combination of various symmetries is reminiscent of
the arabesques of Alhambra, as shown in the artist’s
ˇ
rendition. For more details, see the Full Paper by O. S.
Miljani c´ et al. on page && ff.
&
&
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
6
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!