Serendipity in the crystallization of a series of C-alkylcalix[4] resorcinarenes from alcoholic solvents

Article abstract of DOI:10.1021/cg100737c

The crystal structures of a series of C-alkylcalix[4]resorcinarenes (alkyl = methyl, propyl, butyl, pentyl, hexyl, heptyl, and undecyl), each crystallized from tert-butanol, are reported. In all seven of these structures, eight tert-butanol molecules link two resorcinarenes with 16 hydrogen bonds, thereby facilitating the formation of simple dimeric hydrogen-bonded capsules, each of which encapsulates one (disordered) tert-butanol molecule as guest. These structures are compared with, and contrasted to, those obtained from the same series of resorcinarenes recrystallized both from mixed solvents containing tert-butanol and from methanol alone.

Full text of DOI:10.1021/cg100737c

DOI: 10.1021/cg100737c
Serendipity in the Crystallization of a Series
of C-Alkylcalix[4]resorcinarenes from Alcoholic Solvents
2010, Vol. 10
4043–4049
Aaron Alexander Momose and Eric Bosch*
Chemistry Department, Missouri State University, 901 South National Avenue, Springfield, Missouri
65897.
Received June 2, 2010; Revised Manuscript Received July 15, 2010
ABSTRACT: The crystal structures of a series of C-alkylcalix[4]resorcinarenes (alkyl = methyl, propyl, butyl, pentyl, hexyl,
heptyl, and undecyl), each crystallized from tert-butanol, are reported. In all seven of these structures, eight tert-butanol
molecules link two resorcinarenes with 16 hydrogen bonds, thereby facilitating the formation of simple dimeric hydrogen-
bonded capsules, each of which encapsulates one (disordered) tert-butanol molecule as guest. These structures are compared
with, and contrasted to, those obtained from the same series of resorcinarenes recrystallized both from mixed solvents
containing tert-butanol and from methanol alone.
Scheme 1
Introduction
The versatility of C-alkylcalix[4]resorcinarenes and their
derivatives in the formation of hydrogen-bonded capsules, in
which multiple resorcinarenes adopt a bowl-like conforma-
tion, is well established.1 The formation of capsules from
resorcinarenes should not, however, be considered routine, as
the resorcinarenes may adopt one of several nonbowl-like
conformations, as amply demonstrated by a series of elegant
and extensive studies.2 Our early studies with resorcinarenes
concentrated on the formation of chain-link hydrogenbonded
capsules based on the self-assembly of C-methylcalix-
[4]resorcinarene with bipyrimidines.3 During the preparation
of a series of resorcinarenes for an extension of that study, we
encountered some difficulty in obtaining analytically pure
products and found that the organic chemists standby, chro-
matography, was both tedious and largely ineffective.
Furthermore, repeated attempts at recrystallization from a
host of single and mixed solvents were time-consuming and of
limited success until we happened upon tert-butanol. Surpris-
ingly, recrystallization from tert-butanol, or mixtures of tert-
butanol with methanol, ethanol, or isopropanol, yielded clear
colorless crystals even from highly colored solutions of the
crude resorcinarenes for each of the resorcinarenes we pre-
pared (see Scheme 1).
Intrigued by this observation, we subjected a series of
crystals to X-ray crystallographic analysis and present evi-
dence herein that tert-butanol is particularly successful for
crystallization/purification of C-alkylresorcinarenes, since
it not only serves as hydrogen-bond acceptor and donor
linking the resorcinarenes into dimeric capsules but is itself
also included as a guest within the capsules. While other
alcohols are clearly equally efficient as hydrogen bond accep-
tors and donors, it is the size and shape of tert-butanol that is
crucial for its inclusion as guest within the dimeric capsules.
These observations will be compared and contrasted to other
studies of the aggregation of resorcinarenes both in the solid
state and in solution.
Experimental Section
General Procedure for the Synthesis of Calix[4]resorcinarenes.4
Resorcinol (8.81 g, 80 mmol), aldehyde (80 mmol), and 95% ethanol
(80 mL) were added to a 200 mL round-bottom flask equipped with
a magnetic stir bar. The solution was sparged with argon for 15 min,
and concentrated HCl (13 mL) was then added. The solution was
refluxed under an argon atmosphere for 18 h and then cooled to
room temperature and poured into a separatory funnel containing
80 mL of diethyl ether. 360 mL of brine was added, and the organic
layer was separated, dried over magnesium sulfate, and evaporated
under vacuum to produce the crude product as a yellow/brown
solid. The crude product was recrystallized twice from 4:1 tert-
butanol/methanol and dried at 100 °C under vacuum for 30 min to
afford the calix[4]resorcinarene in more than 80% yield.
Preparation of Crystals. Approximately 20 mg of the resorcinar-
ene was placed in a screw cap vial and dissolved in the minimum volume
of hot tert-butanol. Crystals generally began to form after several days
and were harvested for X-ray analysis directly from the mother liquor.
Commercially available C-methylresorcinarene (Aldrich) was also re-
crystallized from tert-butanol with a small amount of methanol as
cosolvent. The resultant complexes of tert-butanol with methyl-, propyl-,
butyl-, pentyl-, hexyl-, heptyl-, and undecylresorcinarene respectively,
are numbered 1-7, respectively. Similar procedures were used for
recrystallization from mixed solvents or from methanol alone. The
C-butylcalix[4]resorcinarene complex was recrystallized from metha-
nol/tert-butanol, and the crystalline complex 8 included both tert-
butanol and methanol in a tert-butanol/methanol/C-butylresorcinarene
ratio of 5:4:2. Recrystallization of purified C-propylresorcinarene and
*To whom correspondence should be addressed. Telephone: 417-823-
8735. Fax: 417-836-5507. E-mail: ericbosch@missouristate.edu. Web: http://
chemistry.missouristate.edu/31072.htm.
r
2010 American Chemical Society
Published on Web 08/02/2010
pubs.acs.org/crystal

4044 Crystal Growth & Design, Vol. 10, No. 9, 2010
Table 1. Crystal Data for C-Alkylresorcinarene Complexes with tert-Butanol
Momose and Bosch
complex
1, C-methyl
2, C-propyl
3, C-butyl
4, C-pentyl
5, C-hexyl
6, C-heptyl
7, C-undecyl
formula
M/(g mol-1
C50H77O12.5
878.12
monoclinic
P21/n
15.2339(8)
18.7155(10)
18.5473(10)
90
98.166(1)
90
5234.4(5)
2
1.115
0.079
1908
C58H93O12.5
990.32
triclinic
P1
15.1886(8)
15.5624(8)
15.97878(14)
104.897(1)
113.095(1)
105.025(1)
3069.5(4)
2
C62H101O12.5
1046.43
triclinic
P1
15.352(2)
15.722(2)
16.276(2)
113.402(2)
106.967(2)
103.345(2)
3167.84(7)
2
C66H109O12.5
1102.53
triclinic
C70H117O12.5
1158.64
triclinic
P1
15.1627(18)
15.7027(18)
17.246(2)
111.205(2)
101.202(2)
102.036(2)
3597.6(7)
2
C76H130O13
1251.80
triclinic
C91H157O13
1467.17
triclinic
P1
15.2237(12)
15.7919(13)
22.0893(18)
106.962(1)
92.499(1)
101.835(1)
4941.1(7)
2
)
crystal system
space group
P1
P1
˚
a/A
15.2253(13)
15.5029(13)
17.3279(14)
104.9870(10)
113.9090(10)
100.9750(10)
3404.1(5)
2
15.2662(12)
15.6752(12)
18.6790(15)
113.1150(10)
100.7750(10)
101.8430(10)
3841.7(5)
2
˚
b/A
˚
c/A
R/deg
β/deg
γ/deg
3
˚
V/A
Z
F
calcd./(g cm-3
)
1.071
0.074
1.097
0.075
1.076
0.072
1.070
0.071
1.082
0.072
0.986
0.064
μ/mm-1
F(0,0,0)
1082
0.63 Â 0.42
1146
0.32 Â 0.18
1210.0
0.22 Â 0.19
1274
0.61 Â 0.27
1380
0.38 Â 0.12
1622
0.20 Â 0.30
crystal size, mm3
0.32 Â 0.28
 0.22
 0.39
 0.09
 0.15
 0.18
 0.12
 0.40
temp/K
θ range/deg
reflections collected
independent reflections
data/restraints/parameters
GOF
100(2)
135(2)
100(2)
100(2)
100(2)
100(2)
140(2)
1.6-27.1
62832
11557
11557/2/712
1.022
0.079
1.5-25.0
33061
10789
10789/2/846
1.023
0.040
1.5-26.4
34383
12894
12894/2/747
0.963
0.097
1.4-26.0
36176
13358
13358/1/781
1.014
0.057
1.3-27.3
45359
15963
15963/0/785
1.012
0.070
1.2-25.0
40308
13519
13519/15/850
1.027
0.091
1.0-25.0
53133
17409
17409/53/939
1.119
0.084
R(int)
final R indices [I > 2σ(I)],
R1/wR2
0.057/0.152
0.053/0.155
0.062/0.141
0.057/0.144
0.065/0.183
0.099/0.196
0.111/0.4000
3
largest diff peak/hole/(e A )
˚
0.50/-0.39
0.62/-0.41
0.39/-0.53
0.76/-0.44
0.43/-0.43
0.76/-0.48
0.59/-0.35
Table 2. Crystal Data for Complexes Formed in the Presence of Methanol
C-pentylresorcinarenes from methanol alone yielded single crystals,
suitable for X-ray analysis, of complexes 9 and 10 respectively.
X-ray Structure Determination. Crystals were mounted on fine
glass fibers using viscous hydrocarbon oil. Data were collected using
a Bruker Apex2 CCD diffractometer equipped with Mo KR radia-
complex
formula
M/(g mol-1
8, C-butyl
9, C-pentyl
10, C-propyl
C56H89O12.5
962.27
C25H36O5
417.54
C45H68O13
816.99
)
crystal system
space group
triclinic
P1
monoclinic
C2/c
13.9179(14)
23.2095(14)
14.575(1)
90
90.126(2)
90
4708.1(6)
4
1.175
0.080
1808
0.53 Â 0.34
 0.15
173(2)
triclinic
P1
ꢀ
˚
tion with λ = 0.71073 A. Data collection at low temperatures was
facilitated by use of a Kryoflex system with an accuracy of (1 K.
Initial data processing was carried out using the Apex II software
suite.5 Structures were solved by direct methods using SHELXS-97
and refined using standard alternating least-squares cycles against
F2 using SHELXL-97.6 The program X-Seed was used as a graphi-
cal interface.7 Methine and methylene hydrogen atoms attached to
carbon were placed in idealized positions and refined with a riding
model. Methyl hydrogen atoms attached to carbon were positioned
according to maximum electron density. Hydrogen atoms attached
to oxygen were generally placed based on electron density peaks in
the difference maps. Crystallographic details for complexes 1-7 and
8-10 are summarized in Tables 1 and 2, respectively.
Special Refinement Details. Since most structure solutions fol-
lowed the same general sequence, the structure determination for
complex 1 will be presented and differences then discussed for each
individual structure. Thus, for complex 1, the asymmetric unit consists
of one C-methylresorcinarene with four tert-butanol molecules hydro-
gen-bonded to the resorcinarene hydroxyl groups as well as one-half of
a tert-butanol molecule inside the bowl-shaped cavity of the resorcinar-
ene. Solution/refinement of the resorcinarene and three of the four
hydrogen bonded tert-butanol molecules was routine. In each of these
molecules the acidic hydroxyl hydrogen atoms were placed based on
peaks in the difference maps. Two of the hydrogen bonded tert-butanol
molecules were disordered over two positions. One was disordered only
with respect to the methyl groups, while all the carbons were disordered
over two positions for the other tert-butanol. Both were therefore
refined over these two positions with free variables which converged to
0.44 and 0.5, respectively. The central tert-butanol was refined as a
disordered molecule over two major positions. The central carbon atom
(C49) was assumed to be at an average position between the two true
positions of the disordered central carbon. The oxygen atom (O13) was
assigned based on the shortest bond to the central carbon atom. The
hydrogen on the oxygen was positioned using a riding model. The
central carbon and the oxygen atom were then set at 50% occupancy
while the other two carbon atoms (C50 and C51) were set at 75%
occupancy. With this assignment this portion of the asymmetric unit
represents half of a tert-butanol molecule (C2H5O0.5). Thermal ellipsoid
parameters and bond lengths were understandably large for this central
˚
a/A
14.6157(11)
15.0200(132)
15.481(2)
106.670(2)
107.365(2)
104.3930(10)
2892.4(5)
2
11.3939(14)
12.9275(16)
15.898(2)
81.150(2)
79.891(2)
79.710(2)
2247.8(5)
2
˚
b/A
˚
c/A
R/deg
β/deg
γ/deg
3
˚
V/A
Z
F
calcd./(g cm-3
)
1.105
0.076
1.206
0.087
μ/mm-1
F(0,0,0)
1050
0.54 Â 0.28
884.0
0.34 Â 0.14
crystal size, mm3
 0.26
 0.14
temp/K
173(2)
1.5-25.0
31441
100(2)
1.31-25.0
12068
θ range/deg
reflections
collected
1.7-25.0
25186
independent
reflections
data/restraints/
parameters
GOF
10192
4158
7636
10192/6/680
4158/16/341
7636/0/584
1.017
0.071
0.060/0.2046
1.030
0.056
0.047/0.128
1.014
0.036
0.053/0.132
R(int)
final R indices
[I > 2σ(I)],
R1/wR2
largest diff peak/
0.35/-0.33
0.33/-0.23
0.44/-0.27
3
hole/(e A )
˚
disordered molecule. Thus, the bond lengths for C49-O13, C49-C50,
and C49-C51 were longer than those expected at 1.487, 1.676, and
ꢀ
˚
1.659 A. The six highest Q peaks (0.3 to 0.5) were, not surprisingly, all
located around the central disordered tert-butanol. These latter results
were fairly typical.
For complex 4 the central tert-butanol was somewhat better
defined. Thus, the central carbon atom (C66) was not disordered
over two positions rather it shared a location with the third methyl
;
group of the second disordered position. C66 was therefore modeled
with an occupancy of 100% while the other two carbon atoms

Article
Crystal Growth & Design, Vol. 10, No. 9, 2010 4045
Figure 1. (A) Side view of the asymmetric unit of the C-methylresorcinarene complex, 1, containing one C-methylresorcinarene and four hydrogen-
bonded peripheral tert-butanols and one-half of an encapsulated tert-butanol. The inner tert-butanol is shown as a space filling model disordered over
two positions: (B) top view of the asymmetric unit; (C) side view of the capsule comprising two hydrogen bonded resorcinarenes.
(C65, C67) were modeled with occupancies of 50%. The oxygen
atom (O13) was assigned based on the shortest bond to the central
carbon atom. The hydrogen on the oxygen was positioned using a
riding model. The thermal ellipsoid for C66 is, as expected, unu-
sually large. Also, the two highest Q peaks (0.7 and 0.42) appear
close to the end of one pentyl chain (C32 and C33 specifically). This
was modeled as disorder with a free variable; however, this minor
component of the terminus of the chain only corresponded to about
10% occupancy for the side chain. Furthermore, the atoms could
not be refined anisotropically. Consequently, this minor disorder
model was not included.
For complex 6 an extra tert-butanol was included in the asym-
metric unit close to one of the resorcinarene O-atoms. The aromatic
portion of the resorcinarene and the tert-butanols and two legs of
the resorcinarene were well-defined; however, the ends of the other
two legs were disordered and restraints were used to solve the
structure. The same general comments apply to the undecyl deriva-
tive, complex 7. Two of the legs of the resorcinarene were modeled
as disordered over two major positions. Restraints were also used
extensively during refinement. The remaining disordered center of
electron density was modeled as a disordered methanol.
For complex 8 the asymmetric unit consists of one resorcinarene
with two tert-butanol molecules and two methanol molecules
bonded to the resorcinarene hydroxyl groups as well as one-half
of a tert-butanol molecule inside the bowl-shaped cavity of the
resorcinarene. The refinement of the resorcinarene and the two
hydrogen bonded tert-butanols and two hydrogen bonded metha-
nols was routine. In each of these molecules the acidic hydroxyl
hydrogen atoms were placed based on peaks in the difference maps.
For complex 9, the asymmetric unit contained half of one
C-pentylresorcinarene molecule and two methanol molecules, each
of which is positioned along an axis of symmetry. Hydrogen atoms
on the resorcinol oxygen atoms were located based on difference
maps and were found to occupy two major positions, each with
approximately 50% occupancy. One pentyl chain, C20-C21-
C22-C23-C24, was disordered and refined over two positions,
with the major component having 80% occupancy. Anisotropic
refinement of the terminal methyl group of the minor conformation
resulted in it going nonpositive definite, and therefore, the ISOR
command was used with this atom.
The series of C-alkyl resorcinarenes shown in Scheme 1 was
prepared by hydrochloric acid catalyzed reaction of the
corresponding aldehyde with resorcinol at reflux in deaerated
ethanol under an argon atmosphere.4 The crude products
were recrystallized from tert-butanol or mixtures of tert-
butanol with one of the alcohols methanol, ethanol, or
isopropanol as cosolvent. Gentle heating under high vacuum
then afforded the C-alkyl calix[4]resorcinarenes as colorless
solids, free of the alcohols, in more than 80% yield.
Crystallization from tert-Butanol. Crystals suitable for
X-ray analysis were prepared in small screw-capped vials
or test tubes by crystallization of the resorcinarene from
warm tert-butanol. Commercially available C-methylresor-
cinarene (Aldrich) was also recrystallized from tert-butanol
with a small amount of methanol as cosolvent.
X-ray Crystallographic Analysis. The crystallographic
data for this series of compounds was collected at 100 K
(see Tables 1 and 2) with the exception of those crystals which
cracked when cooled to 100 K. For complex 1, the asymmetric
unit was found to consist of a C-methylresorcinarene in the bowl
conformation along with four tert-butanol molecules hydrogen-
bonded to four resorcinarene hydroxyl groups and one-half of a
tert-butanol molecule nestled inside the bowl-shaped cavity of
the resorcinarene. The C-methylresorcinarene is held in the bowl
conformation by four phenol-phenol hydrogen bonds with
˚
O bond distances ranging from 2.628 to 2.676 A. The
O
3 3 3
phenol-tert-butanol hydrogen bonds had O O bond dis-
3 3 3
˚
tances ranging from 2.573 to 2.801 A. Two of the hydrogen
bonded tert-butanol molecules were disordered over two posi-
tions with relative occupancies of 50:50 and 44:56, respectively.
One of these was disordered only with respect to the methyl
groups while all the carbons were disordered over two positions
for the other tert-butanol. The inner tert-butanol was disordered
over two major positions with the central carbon atom at an
average position between the two true positions of the disordered
central carbon. The asymmetric unit is shown in Figure 1A and
B, showing one conformation of each of the disordered periph-
eral tert-butanol molecules. The encapsulated tert-butanol is
shown as a space filling model disordered over both positions.
Two of these asymmetric units then combine to form a dimeric
capsule, with each of the eight tert-butanol molecules acting as
both hydrogen bond donor and hydrogen bond acceptor, there-
by stitching the rim of the two resorcinarene molecules together
with a total of 16 hydrogen bonds (Figure 1C).
For complex 10 the asymmetric unit contained one C-propylre-
sorcinarene molecule and five methanol molecules. The refinement
of the resorcinarene and the methanols was routine. In each of these
molecules, the acidic hydroxyl hydrogen atoms were placed based
on peaks in the difference maps.
Results and Discussion
The formation and crystallographic characterization of
dimeric capsules by crystallization of C-alkylresorcinarenes
from tert-butanol solutions will first be described. These
structures will then be compared and contrasted with the
crystal structures of some of the same resorcinarenes crystal-
lized either from mixed solvents including tert-butanol or
from methanol alone.
The X-ray structures of the series of C-alkylresorcinarenes
1-7 are very similar. In each case a hydrogen-bonded capsule is
formed with two bowl-conformation resorcinarenes stitched to-
gether by 16 hydrogen bonds with eight tert-butanol molecules.

4046 Crystal Growth & Design, Vol. 10, No. 9, 2010
Table 3. Structural Characteristics of the tert-Butanol Complexes
Momose and Bosch
complex
arene-arene distancea
1, C-methyl
2, C-propyl
3, C-butyl
4, C-pentyl
5, C-hexyl
6, C-heptyl
7, C-undecyl
6.834, 6.837
9.127
2.628-2.676
2.658
6.658, 6.871
9.014
2.659-2.691
2.679
6.798, 6.870
9.095
2.634-2.689
2.665
6.816, 6.866
9.109
2.651-2.677
2.669
6.809, 6.849
9.061
2.642-2.688
2.677
6.797, 6.893
9.046
2.650-2.703
2.682
6.788, 6.885
9.097
2.677-2.682
2.680
complex heightb
intramolecular H-bond O
O
3 3 3
c
distance (range, avg)/A
ꢀ
˚
tert-butanol H-bond O
distance (range, avg)/A
O
2.573-2.801
2.706
2.615-2.782
2.678
2.592-2.766
2.692
2.613-2.743
2.656
2.606-2.715
2.648
2.659-2.746
2.710
2.592-2.765
2.691
3 3 3
d
ꢀ
˚
a Arene centroid-arene centroid across the resorcinarene. b Distance between planes defined by methine C’s on the resorcinarenes. c Phenol OH
hydrogen bonds. d tert-Butanol-phenol hydrogen bonds.
Figure 2. View of individual capsules formed between each of the C-alkylresorcinarenes and tert-butanol taken from the corresponding X-ray
structures: (A) C-butylcalix[4]resorcinarene, 3; (B) C-methylcalix[4]resorcinarene, 1; (C) C-propylcalix[4]resorcinarene, 2; (D) C-undecylcalix-
[4]resorcinarene, 7; (E) C-hexylcalix[4]resorcinarene, 5; (F) C-heptylcalix[4]resorcinarene, 6; (G) C-pentylcalix[4]resorcinarene, 4.
Within each resorcinarene there are four intramolecular hydro-
gen bonds between adjacent resorcinol units that ensure the
bonded central seam, is effectively hidden by the bulk of the ter-
tiary butyl group, which effectively renders the complete exterior
of the capsule hydrophobic. The tert-butyl groups have close
contact with the methyl groups on the alkyl chains in all five
structures. While there is initially little interaction between alkyl
chains of adjacent capsules, this increases with the hexyl deriva-
tive, where there is significant interdigitation of the alkyl chains of
adjacent resorcinarenes that increases with increasing chain
length.
bowl-like resorcinarene conformation is adopted. The O
O
3 3 3
distances in these phenol-phenol H-bonds range from 2.63 to
˚
2.70 A for all structures (see Table 3). The arene centroid-arene
centroid distance measured diagonally across the bowl-shaped
˚
resorcinarene is consistent varying from 6.66 to 6.87 A for all
;
the structures. Furthermore, the hydrogen-bonding O O dis-
tances for the intermolecular H-bonds between the resorcinarene
3 3 3
˚
and the tert-butanol molecules vary between 2.57 and 2.80 A for
Mixed Alcohol Structures. We occasionally noted that the
solubility of the resorcinarenes was relatively low in pure
tert-butanol, so a cosolvent such as methanol or ethanol was
used. Indeed, on a practical level, crystallization of the crude re-
sorcinarenes from a mixture of methanol and tert-butanol was
often the method of choice. We selected a crystal of the C-butyl-
calix[4]resorcinarene complex recrystallized from methanol/
tert-butanol for X-ray analysis. The structure of this complex,
8, shown in Figure 4 clearly shows the same general structure
observed in complexes 1-7 with the exception that two metha-
nol molecules replace two of the peripheral tert-butanols as the
linking hydrogen bond donor-acceptor. The dimensions of the
capsule are very similar to those reported in Table 2. For
example, the diagonal intramolecular arene-arene distances
all structures. Not unexpectedly then, the “height’”of each cap-
sule, h, measured between the planes defined by the methine
hydrogens on each resorcinarene, is almost the same in all the
˚
capsules, varying between 9.014 and 9.127 A (“h” is shown in
Figure 1C). In all the structures, the encapsulated tert-butanol
molecule is disordered over two positions within the dimeric
assembly. This similarity is demonstrated in Figure 2, which
shows the dimeric capsules 1-7. The inner tert-butanol molecule
is shown as a space filling model disordered over both positions.
We were interested in the packing between adjacent capsules
and show partial packing diagrams for complexes 3-7 in
Figure 3. This figure clearly demonstrates the role of van der
Waals forces. The polar portion of the capsule, the hydrogen

Article
Crystal Growth & Design, Vol. 10, No. 9, 2010 4047
Figure 3. Packing of the complexes formed between C-alkylresorcinarenes and tert-butanol, showing the interaction between the alkyl chains:
(A) C-butylresorcinarene; (B) C-pentylresorcinarene; (C) C-hexylresorcinarene; (D) C-heptylresorcinarene; (E) C-undecylresorcinarene.
Figure 4. (A) View of the asymmetric unit of complex 8 formed between C-butylresorcinarene and tert-butanol and methanol. (B) Side view of
the capsule and (C) top view of the capsule.
˚
are 6.758 and 6.917 A while the “height” of the capsule is
˚
9.196 A.
propyl chains as shown in Figure 5B. The one-dimensional
ribbons are held together by bifurcated hydrogen bonds as
shown in Figure 5C. Thus, O1H is hydrogen bonded to both
O4 of an adjacent resorcinarene as well as O8 in the neighboring
resorcinol moiety. The hydrogen bonds are characterized by
Methanol Structures. Recrystallization of the crude resor-
cinarenes from methanol alone was generally unsuccessful in
terms of growing crystals and, in contrast to the use of tert-
butanol, generally required several successive recrystallizations
to generate colorless material. Nevertheless, we prepared X-ray
quality crystals by crystallization of purified C-propylresorci-
narene and C-pentylresorcinarenes from methanol alone (struc-
tures 9 and 10, respectively). While the resorcinarene adopts the
bowl conformation in both structures, hydrogen bonded net-
work type structures are formed in lieu of capsule formation in
the absence of a suitable guest. In structure 10 the asymmetric unit
consists of a C-propylcalix[4]resorcinarene and five methanol
molecules, as shown in Figure 5A. These components form one-
dimensional bilayered hydrogen-bonded ribbons with the resor-
cinarenes arranged in an off-center head-to-head arrangement.
The ribbons stack on top of each other with interdigitation of the
˚
O-O distances of 3.006 and 2.836 A for O1-O8 and O1-O4,
respectively, and O-H-O angles of 137.71 and 137.86° for
O1-H1O-O8 and O1-H1O-O4, respectively.
Comparison to Previously Published Crystal Structures of
Hydrogen Bonded Dimeric Resorcinarene Capsules. Not sur-
prisingly, many of the resorcinarene structures in the Cambridge
database contain alcohol molecules. For example, when this
manuscript was in preparation, there were 114 structures con-
taining C-methylcalix[4]resorcinarene collected in the Cam-
bridge database and 21 of those structures contained alcohols.
Many of these alcohols combine via hydrogen-bonding either to
the resorcinarene or, in some instances, to the amines that were
cocrystallized with the resorcinarene. In a few isolated cases, the

4048 Crystal Growth & Design, Vol. 10, No. 9, 2010
Momose and Bosch
Figure 5. (A) Asymmetric unit of the complex formed between C-propylresorcinarene and methanol showing the bowl-conformation.
(B) Packing of the complex showing the hydrogen bonded network in one dimension and the interdigitation of the propyl chains. (C) Cross-
linking bifurcated hydrogen bonds between adjacent ribbons of the hydrogen bonded complexes shown in part B.
alcohol facilitates the formation of discrete capsules containing
two resorcinarenes. An early example of the formation of a
discrete dimeric capsule containing two C-phenethylresorcinar-
enes linked by isopropanol molecules was reported by Atwood
and Barbour in 1998.8 In that structure, the authors were unable
to definitively locate solvent molecules within the capsule and
speculated that a mixture of isopropanol and 1,2-dichloroben-
zene may have been included. It is, however, noteworthy that
Rissanen and co-workers reported the encapsulation of small
ammonium cations of similar size to tert-butanol, like diethyl-
dimethylammonium cation, in alcohol bridged dimeric cap-
sules.1b,9,10 In this series of structures, ethanol or methanol mole-
cules, occasionally in combination with water molecules, hydro-
gen bonded the two resorcinarenes together. Also, Aoki had ear-
lier shown that water can bridge two resorcinarenes in the encap-
sulation of the similarly sized tetramethylammonium cation by
two C-ethylresorcinarenes.11 In another report, the encapsula-
tion of triethylammonium cation was also facilitated by water
molecules bridging between two resorcinarenes.12 In other struc-
tures reported,13,9 chloride and bromide were also shown to
bridge two resorcinarenes to encapsulate an ammonium cation
and mixed bridging ligands were observed (i.e., water and halide
or alcohol and halide). Furthermore, Rissanen and co-workers
noted that crystallization of C-ethyl resorcinarene from etha-
nol14 resulted in a hydrogen bonded complex in which the resor-
cinarene adopted the cone- or bowl-like conformation and the
ethanol molecules were involved in multiple hydrogen bonds
without capsule formation. A similar observation can be made of
the structure of the C-benzylresorcinarene crystallized from
methanol reported by Rebek and co-workers.15 These related
studies serve to confirm that dimeric alcohol-bridged resorcinar-
ene capsules may be formed provided that an appropriately sized
guest is available. Consequently, while crystallization from alco-
hols alone is rare and seldom results in the formation of dimeric
capsules, the formation of dimeric capsules occurs in our case
because tert-butanol is an appropriately sized guest.
alcohol was chosen.18 Thus, they noted that crystallization
in the presence of 2-ethylbutanol or 2-butanol yielded di-
meric capsules while crystallization in the presence of
2-ethylhexanol yielded the hexameric capsule with six alco-
hol and two water molecules included in the framework.
These authors also noted that crystallization of resorcinar-
enes in the presence of other alcohols did not yield capsules.
These observations are nicely complimented by Mattay and
co-workers’ elegant study of the nature of resorcinarene
assembly with alcohols in apolar solvents.19 In that study,
diffusion NMR spectroscopy was used to determine the
nature of the resorcinarene assemblies. The authors con-
cluded that dimeric capsules were formed in dry CDCl3
containing 2-butanol while hexameric capsules were formed
in 2-ethylhexanol. Given the results here, it is reasonable to
propose that dimeric capsules would also be formed in dry
CDCl3 solution if tert-butanol were added as cosolvent.
Conclusions
Wehaveclearly demonstratedthat use oftert-butanol in the
crystallization of crude C-alkylresorcinarenes provides good
yields of colorless, crystalline products that comprise dimeric
capsules, formed between two resorinarenes and eight tert-
butanol molecules, that encapsulate a single disordered tert-
butanol molecule. These results are consistent with previously
reported solution and solid state studies of dimeric and
hexameric resorcinarene capsule formation. Furthermore, it
is apparent that dimer capsule formation is favored provided
that an appropriate guest molecule is available in this case
;
tert-butanol.
Acknowledgment. This research was supported by the
National Science Foundation, Grant 0415711. The Missouri
State University Provost Incentive Fund funded the purchase
of the X-ray diffractometer. We thank Dr. Charles L.
“Chuck” Barnes for sage advice.
Comparison to Studies of Resorcinarene Aggregation in
Solution. In addition to the reports of dimeric capsule
formation described in the previous section, there is con-
siderable interest in the significantly larger hexameric cap-
sules reported by MacGillivray and Atwood in 1997.16 The
hexamer included eight water molecules in the framework,
and the superstructure was held together by a total of 60
hydrogen bonds. Mattay and co-workers reported the crys-
tal structure of a similar hexameric superstructure based on
pyrogalloarene.17 More recently, Ogano and Holman re-
ported that six of the water molecules in the superstructure
could be replaced by alcohols provided that the correct
Supporting Information Available: X-ray crystallographic infor-
mation files (CIF) are available for complexes 1-10. This informa-
tion is available free of charge via the Internet at http://pubs.
acs.org/.
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