Self-assembly of daisy chain oligomers from heteroditopic molecules containing secondary ammonium ion and crown ether moieties

Article abstract of DOI:10.1002/pola.23861

MALDI-TOF MS of the heteroditopic compound 2-(benzylammoniomethyl)dibenzo- 24-crown-8 hexafluorophos-phate (4) revealed oligomeric ''daisy chain'' species up to the hexamer. Similar results were obtained for 2-(6′-hydroxy- hexylammoniomethyl)dibenzo-24-cr

Full text of DOI:10.1002/pola.23861

Self-Assembly of Daisy Chain Oligomers from Heteroditopic Molecules
Containing Secondary Ammonium Ion and Crown Ether Moieties
HARRY W. GIBSON,1 NORI YAMAGUCHI,1* ZHENBIN NIU,1 JASON W. JONES,1y CARLA SLEBODNICK,1
ARNOLD L. RHEINGOLD,2z LEV N. ZAKHAROV2§
1Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
2Department of Chemistry, University of Delaware, Newark, Delaware 19716
Received 28 September 2009; accepted 1 December 2009
DOI: 10.1002/pola.23861
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: MALDI-TOF MS of the heteroditopic compound 2-
(benzylammoniomethyl)dibenzo-24-crown-8 hexafluorophos-
phate (4) revealed oligomeric ‘‘daisy chain’’ species up to
the hexamer. Similar results were obtained for 2-(60-hydroxy-
stants are reported for these systems. A crystal structure is
reported for the new dimethylbenzylammonoium pseudoro-
taxane. The trimethoxy analog is shown to be capable of slip-
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page formation of a rotaxane, albeit in low yield.
2010
hexylammoniomethyl)dibenzo-24-crown-8
hexafluorophos-
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48:
975–985, 2010
phate (8). The complexations of two substituted di-
benzylammonium salts, 2,20-dimethyldibenzylammonium hexa-
flurophosphate (9a) and 2,20,5-trimethoxydibenzylammonium
hexafluorophosphate (9b), with dibenzo-24-crown-8 were
examined as models for slippage systems; association con-
KEYWORDS: crown ether; crystal structure; host–guest systems;
macrocycles; mass spectrometry; pseudorotaxane; self-organi-
zation; supramolecular polymer
INTRODUCTION Pseudorotaxanes and rotaxanes1 now are
strong contributors to the realm of supramolecular science.2
These molecular recognition motifs have been applied to the
self-assembly of an ever increasing variety of architectures such
as dendrimers1,3 and polymeric systems.1,4 Efforts continue to-
ward the spontaneous construction of ordered linear pseudoro-
taxane arrays,1,5 so called ‘‘daisy chains,’’ a subclass of supramo-
lecular polymers.6 One approach is to use complementary pairs
of homoditopic molecules5(c,d); another is to use heteroditopic
molecules.5(a,b,d,e) Scheme 1 illustrates the general approach to
create such linear arrays 2 from heteroditopic molecules 1. The
use of a single building block of this type is advantageous
because it guarantees the 1:1 stoichiometry between the com-
plementary units required for effective formation of supramo-
lecular aggregates in an essentially step growth process.
bility considerations noted later led to our essential
abandonment of this project. Subsequently, Stoddart et al.
reported their extensive studies of this system and several
analogs.5(d) Our efforts also explored another avenue for
enhancing both the solubility and self-association in analo-
gous heteroditopic systems and examined potential slippage-
based approaches to their rotaxane analogs; here, we sum-
marize those studies.
Some years ago,7 we pursued this methodology using the
complementary dibenzo-24-crown-8 (DB24C8)/dibenzylam-
monium pair, specifically 4. Then Stoddart and his coworkers
reported the isolation and X-ray structure of the cyclic di-
meric pseudorotaxane complex 3, n ¼ 2 from the trifluoroa-
cetate analog of 4 (Scheme 1).5(a) This coupled with the solu-
RESULTS AND DISCUSSION
Daisy Chain Polymers
The synthesis of heteroditopic (A-B) monomer 4 is outlined
in Scheme 2. Aldehyde 5 was prepared from the previously
reported alcohol5(c) by PCC oxidation and converted to the
Additional Supporting Information may be found in the online version of this article.
*Present address: General Electric Advanced Materials, Strongsville, Ohio 44149, USA.
†Present address: E.I. du Pont de Nemours and Company, Jackson Laboratory, Deepwater, New Jersey 08023, USA.
‡Present address: Department of Chemistry and Biochemistry, University of California, San Diego, California 92023, USA.
§Present address: Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403-1253.
Correspondence to: H. W. Gibson (E-mail: hwgibson@vt.edu)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 975–985 (2010) 2010 Wiley Periodicals, Inc.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 1 Cartoon representations of the
formation of linear daisy chain arrays 2 and
cyclic daisy chain species 3 by self-assem-
bly of heteroditopic 1. The circles and ellip-
ses represent the DB4C8 moiety and the
filled rectangles represent the dibenzylam-
monium moiety.
corresponding Schiff base 6a by condensation with benzyl-
amine. Reduction of 6a (NaBH4) produced the secondary
amine 7a, which was converted to 4 by sequential treatment
with HCl and NH4PF6. The structure of 4 was confirmed by
MS results reported by Stoddart et al.,5(d) who detected only
the dimer.
However, in our positive ion MALDI-TOF spectra,10 there was
also clear evidence of oligomeric aggregates up to the hex-
amer (Fig. 1, Table 1). Although these signals were weak, on
the order of 0.2–4.3% of the base peak, their presence indi-
cates that even in the polar, competitive H-bonding matrix
used (dihydroxybenzoic acid) there is an equilibrium involv-
ing species larger than the dimer, probably of linear architec-
ture. Moreover, MS obtained via FAB also revealed oligomers
up to the pentamer8 (Table 1).
1
high-resolution mass spectrometry and its H NMR spectrum
in DMSO-d6, which as a highly competitive H-bond acceptor
essentially prevents pseudorotaxane formation.5(d),8 This het-
eroditopic compound is essentially insoluble in halogenated
solvents such as chloroform and methylene chloride; the sol-
ubility of 4 was < 50 mM in acetone and acetonitrile.
Stoddart et al. used CD3CN as solvent and indicated that two
stereoisomeric cyclic dimeric species were observed, based
on signals for He at 6.41 ppm and Hc at 6.17 ppm in a ratio
of 5:1 in 20 mM solution at 0 ꢀC, but surprisingly claimed
that no uncomplexed 4 was detected.5(d) Because of the com-
plexity of the spectra, they proceeded to study various deu-
terated and fluorinated analogs; when the unsubstituted
benzo ring of 4 was deuterated, the resulting simplified
spectrum clearly revealed the two isomeric (meso and race-
mic, Scheme 3) cyclic dimers as separate signals for Hc (dou-
blets, J ¼ 1.5 Hz, major @ 6.78, minor @ 6.17 ppm), Hd
(doublets of doublets, J ¼ 1.5, 8 Hz, major @ 6.84, minor @
6.98 ppm), and He (doublets, J ¼ 8 Hz, major @ 6.41, minor
@ 6.89 ppm); these observations confirmed the assignments
made for 4 itself. These conclusions were corroborated in
later work; a rotaxane containing similar units revealed the
same chemical shifts.9 Our spectra of 4 in CD3CN were
essentially identical to those reported by Stoddart et al.
Some years ago Busch et al. proposed and demonstrated
that incorporation of suitable hydrogen bonding terminal
moieties could enhance formation of pseudorotaxane com-
plexes.11 Utilizing this concept to enhance solubility and aug-
ment the threading process and additionally provide a route
to rotaxane systems via reactions on the hydroxyl group, we
prepared the 6-hydroxyhexyl analog, 8 (Scheme 2). The spec-
trum of 8 in CD3SOCD3 (Fig. 2) is similar to that of 4 in the
same solvent in the aromatic and ethyleneoxy regions, but
contains additional signals for the hydroxyhexyl unit. Solu-
tions of 8 in CD3COCD3 were also examined by1H NMR
spectroscopy;12
The MALDI-TOF mass spectrum (MS) of a solid sample pre-
pared by slow evaporation of a chloroform/acetonitrile (1/1
v/v) solution of 4 revealed the base peak at m/z ¼ 1137 g/
mol, indicating dimeric supermolecules (probably mostly the
cyclic dimer) after loss of one PF6 moiety and one HPF6. A
peak of nearly 100% intensity at m/z ¼ 1017 g/mol corre-
sponds to dimer after loss of a benzylamine fragment, one
PF6 moiety and one HPF6. These results agree with the LSI
as can be seen in Figure 3 these spectra are complicated. The
signals assigned to He and Hc are observed as a doublet and a
broad singlet at 6.65 and 6.33, respectively, both ꢁ0.2 ppm
upfield from the signals observed by Stoddart et al. for the
cyclic dimer of 4 in CD3CN.5(d) Interestingly, the ratio of these
SCHEME 2 Synthesis of hetero-
ditopic 2-alkylammoniodibenzo-
24-crown-8
hexafluorophos-
phates 4 and 8.
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SCHEME 3 Representations of
the meso and D,L-diasteromers of
the cyclic dimer from 4.
peaks, assumed to be the major He and minor Hc, was constant
at 13 across the concentration range from 0.60 to 10 mM.
Assuming these signals correspond to the major and minor
isomers of the cyclic dimer as assigned by Stoddart et al.,5(d)
inasmuch as the equilibrium between the cyclic dimers is not
concentration dependent, the higher ratio relative to that in 4
indicates that formation of the cyclic dimer is more selective in
this case, perhaps as a consequence of the greater steric
constraints imposed by the hydroxyhexyl group, but affords
the same predominant diastereomer (perhaps the racemate,
Scheme 35(d)) as 4 in CD3CN. Note that the proportions of these
signals relative to the rest of the aromatic region did not change,
again as in the case of 45(d) very surprisingly suggesting that the
cyclic dimer concentration is essentially constant. However,
other changes are observed: the signal at 3.75 ppm is drastically
reduced in intensity as the concentration increases, while
concomitantly the signal at ꢁ4.5 ppm also changes. Indeed,
these observations mirror those for 4. Unfortunately, we are
unable to analyze these spectra in quantitative fashion.
A feature of this particular system that has not been recog-
nized previously is its tacticity. Just as the diastereomeric
relationships between adjacent repeat units in polystyrene
or polypropylene lead to isotacticity and syndiotacticity, the
present systems also give rise to tacticity as illustrated in
Scheme 4. It may be that tacticity in the oligomeric daisy
chains produces the same level of diastereoselectivity as
observed with the cyclic dimers. It is also possible that the
chemical shifts of protons Hc, Hd, and He are the same in the
linear and cyclic species. Moreover, the equilibrium between
linear and cyclic species could be rapid relative to the NMR
time scale, resulting in time-averaged signals for these pro-
tons. If this is true, it explains the otherwise puzzling fact
that the proportion of major and minor signals is independ-
ent of concentration.
Slippage Studies
Examination of CPK molecular models reveals that the phe-
nyl groups at the termini of dibenzylammonium salts are rel-
atively bulky compared to the cavity size of DB24C8. We
were therefore curious about the effect of substituents pur-
posely placed on the phenyl rings of dibenzylammonium
salts on the formation of pseudorotaxanes. As models for
slippage1,13 systems dialkylammonium salts 9a and 9b were
However, both MALDI-TOF (Fig. 4) and FAB8 MS of 8 (Table
2) provide clear evidence of self-assembly into dimeric, tri-
meric, tetrameric, and pentameric aggregates even in very
polar, competitively hydrogen bonding matrices.
FIGURE 1 The MALDI-TOF mass
spectrum of a solid sample pre-
pared from 4 in dihydroxyben-
zoic acid matrix, showing self-
assembled oligomers. See Table
1 for assignments and details.
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TABLE 1 Supramolecular Formulae, Calculated Supramolecular Exact Masses, and Observed (MS) Mass/Charge Ratios from M 5 4ꢂPF6
Structure
Supramolecular Formula
Calcd.
Obs (m/z)
Intensity (%)
M-HPF6þNa
M2-2PF6-C6H5CH2NHCH2
M2-2PF6-C6H5CH2
M2-2PF6
C32H42NO8Na
590.7
1,016.5
1,045.5
1,137.4
1,137.4
1,283.3
1,283.3
1,450.3
1,507.8
1,596.8
1,596.8
1,729.8
1,848.9
1,848.9
1,996.0
1,996.0
2,443.0
2,709.6
3,568.2
3,992.0
4,136.9
590.6a
1,016.6a
1,046.4a
1,136.7a
1,136b
1,282.7a
1,282b
1,450.6a
1,507.3a
1,597.0a
1,598b
1,731.7a
1,848.5a
1,849b
1,996.2a
1,996b
2,443.7a
2,710.0a
3,564b
34
99
C56H74NO16
C57H77N2O16
79
C64H84N2O16
100
100
79
M2-2PF6
C64H84N2O16
M2-PF6þH
C64H85N2O16PF6
C64H85N2O16PF6
C64H84N2O16P2F12Na
C82H111N2O24
M2-PF6þH
19
M2þNa
2.6
2.6
3.0
1.1
4.3
1.7
0.68
1.7
0.35
1.7
1.1
1.9
0.20
0.27
M3-3PF6-C6H5CH2NHCH2C6H5
M3-3HPF6-C6H5CH2N
M3-3HPF6-C6H5CH2N
M3-2PF6-C6H5CH2NHCH2
M3-PF6-HPF6
M3-PF6-HPF6
M3-PF6
C89H116N2O24
C89H116N2O24
C88H116N2O24PF6
C96H125N3O24PF6
C96H125N3O24PF6
C96H126N3O24P2F12
C96H126N3O24P2F12
C120H158N3O32P2F12
C128H168N4O32P3F18
C160H210N5O40P5F30
C192H252N6O48P4F24
C192H252N6O48P5F30
M3-PF6
M4-2PF6-C6H5CH2NHCH2
M4-PF6
M5
M6-2PF6
3,992a
4,137a
M6-PF6
a
b
MALDI-TOF mass spectrum recorded in the positive ion mode in dihy-
droxybenzoic acid matrix; sample prepared by evaporation of a chloro-
form/acetone (1/1 v/v) solution.
FAB mass spectrum recorded in positive ion mode in nitrobenzyl alco-
hol; sample prepared by evaporation of a chloroform/acetone (1/1 v/v)
solution.
prepared as shown in Scheme 5. Salt 9a is sparingly soluble
in CHCl3 and CH2Cl2; in contrast, 9b showed excellent solu-
bility in CHCl3 and CH2Cl2.
indicating slow association and dissociation between the two
components on the 1H NMR time scale.8 The signals for the
benzylic protons of the corresponding pseudorotaxane
The 1H NMR spectrum of an equimolar solution of 9a and
DB24C8 (10 mM in acetone-d6) exhibits two sets of signals,
FIGURE 3 1H NMR spectra (400 MHz, 22 ꢀC) of heteroditopic 8
in acetone-d6: (a) 0.6 mM, (b) 1.0 mM, (c) 5.0 mM, and (d) 10
mM.
FIGURE 2 1H NMR spectrum (400 MHz, 22 ꢀC) of heteroditopic
8 in DMSO-d6, 20 mM.
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FIGURE
4 MALDI-TOF
mass
spectrum of a solid sample pre-
pared from 8, showing self-as-
sembly of oligomers. See Table
2 for assignments and details.
exhibit significant downfield chemical shifts, indicating that
they are positioned in the deshielding region of the benzo
show that a substantial force is required for the substituted
phenyl ring to penetrate the cavity of DB24C8 to achieve the
1:1 pseudorotaxane geometry; the lower Kaexp for 9a versus
that of dibenzylammonium PF6 (Kaexp ¼ 360 Mꢃ1 under the
same conditions14–16) reflects this steric constraint.
ring of the crown ether. Kaexp ¼ [complex]/{[DB24C8]o
ꢃ
[complex]} {[9a]o ꢃ [complex]} ¼ 90 Mꢃ1 based on the
NCH2 1H NMR signals.14–16 CPK models of 9a and DB24C8
TABLE 2 Supramolecular Formulae, Calculated Supramolecular Exact Masses, and Observed (MS) Mass/Charge Ratios from M 5 8ꢂPF6
Structure
Supramolecular Formula
Calcd.
Obs (m/z)
Intensity (%)
M-PF6þH
C31H49NO9
579.3
601.3
579.6a
601.7a
721.8a
841.5a
1,038b
92
50
M-PF6þNa
C31H48NO9Na
M-H
C31H47NO9PF6
C37H63N2O10PF6
C56H79NO17
722.3
38
M1þNH2(CH2)6OHþH
M2-2HPF6-NH2(CH2)6OH
M2-2PF6
841.4
23
1,037.5
1,156.7
1,157.7
1,240.6
1,254.6
1,301.6
1,324.6
1,446.6
1,617.9
1,717.0
1,797.0
1,799.0
2,023.9
2,748.2
2,377.2
2,440.3
2,522.2
2,983.6
4.8
100
100
75
C62H96N2O18
1,156b
M2-2PF6þH
M2-HPF6-H(CH2)3OH
M2-HPF6-H(CH2)2OH
M2-PF6
C62H97N2O18
1,157.7a
1,241.2a
1,253.4a
1,302b
1,325.9a
1,447b
1,618.2a
1,717.4a
1,796b
1,798.9a
2,024b
C59H87N2O17PF6
C60H89N2O17PF6
C62H96N2O18PF6
C62H96N2O18PF6Na
C62H96N2O18PF6
C87H129N2O26
65
73
M2-PF6þNa
M2
15
3.4
9.2
2.7
1.2
8.3
1.1
0.9
3.3
2.5
15
M3-3PF6-NH2(CH2)6OH
M3-3PF6-H2O
C93H142N3O26
M3-2HPF6-(CH2)3OH-(CH2)2OHþNa
M3-2PF6-(CH2)3OH-(CH2)2OHþNa
M3-HPF6
C88H130N3O25PF6Na
C88H132N3O25PF6Na
C93H144N3O27P2F12
C124H192N4O36P3F18
C119H180N4O34PF6Na
C124H190N4O35PF6
C119H180N4O34P2F12Na
C155H235N5O42PF6
b
M4-PF6
2,749b
M4-3PF6-(CH2)3OH-(CH2)2OHþNa
M4-3PF6-H2O
2,377.0a
2,440.0a
2,521.8a
2,983.4a
M4-2PF6-(CH2)3OH-(CH2)2OHþNa
M5-2PF6-2HPF6-3OH
a
MALDI-TOF mass spectrum recorded in the positive ion mode in dihy-
droxybenzoic acid matrix; sample prepared by evaporation of a chloro-
form/acetone (1/1 v/v) solution.
0.75
FAB mass spectrum recorded in positive ion mode in nitrobenzyl alco-
hol; sample prepared by evaporation of a chloroform/acetone (1/1 v/v)
solution.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 4 Representations of
the isotactic and syndiotactic
dyads resulting from self-assem-
bly of 4 into a supramolecular
polymer.
X-Ray analysis of single crystals prepared by vapor diffusion
of hexane into a 1:1 solution of 9a and DB24C8 in chloro-
form (10 mM each) afforded the solid state structure of the
pseudorotaxane (Fig. 5). Both NHþ2 hydrogen atoms partici-
pate in H-bonds, one with an ethyleneoxy oxygen atom (a)
and one with a phenolic oxygen atom (b) of DB24C8. One of
the NCH protons also is primarily hydrogen bonded to an
ethyleneoxy oxygen atom (c). A somewhat unusual feature of
this complex is the interaction of one of the methyl groups
with one of the benzo rings of the host; one methyl proton
is close to a phenolate oxygen atom of the host: CHAO dis-
tance 2.715 Å (d); this proton is also close to the benzo car-
bon atom, 2.714 Å (e), suggestive of a CAHAp interaction.17
There is additional stabilization by p-stacking interaction
between one of the catechol units of DB24C8 and a phenyl
ring of 9a (f). Overall the structure and packing of this pseu-
dorotaxane are very similar to those of other dibenzylammo-
nium salts with DB24C818 and derivatives.16
In the event, the 1H NMR spectrum of an equimolar (20
mM) solution of 9b and DB24C8 in CDCl3 recorded 5 min af-
ter mixing showed no sign of pseudorotaxane formation;
therefore, the solution was warmed to 53 ꢀC to provide the
thermal energy required for the formation of the rotaxane
by the slippage method.1,13 Equilibrium was established after
11 days at 8% complexation, as estimated by 1H NMR;8
using the NCH2 signals (slow exchange, analyzed as outlined
above) Kaexp ¼ 5 Mꢃ1. For synthesis of the rotaxane high
temperature and high concentration will speed the formation
process, although the equilibrium constant for its formation
will be reduced. 9b and DB24C8 were mixed in the solid
state and heated at 150 ꢀC (melt) for 30 min under argon.
1
As expected, the H NMR spectrum of the yellow chloroform
solution of the heated mixture clearly displayed the peak
(4.7 ppm) corresponding to the complexed benzylic protons.
1
In contrast, the H NMR spectrum of a nearly colorless equi-
molar solution of unheated 9b and DB24C8 in CDCl3
recorded one hour after mixing at room temperature showed
no sign of rotaxane formation. The yellow color in the com-
plexed solution is presumed to arise from p-stacking of the
electron rich methoxyphenyl or dimethoxyphenyl ring of the
guest with a relatively electron poor benzo ring of the host,
assuming a coconformation of the two species similar to that
of DB24C8:9a shown in Figure 5(e). Moreover, mass spec-
trometry (ESI) confirmed the formation of the rotaxane in
the heated mixture (Fig. 7); the signal at m/z 736.3713 cor-
responds to [M ꢃ PF6]þ (M ¼ DB24C8:9b; calculated
736.3696, error 2.3 ppm); the former signal is 0.79 times as
intense as the signal at m/z 467.2315 for [DB24C8 þ H3O]þ.
In contrast the solution of the unheated host and guest after
standing at room temperature for 1 day revealed the rotax-
ane signal to the extent of only 11% of that for [DB24C8 þ
H3O]þ; in other words, the concentration of the rotaxane
The size of the terminal aryl groups of 9b was increased rel-
ative to that of 9a by attaching two methoxy groups on one
end and one on the other. According to CPK models, total
insertion of 9b into DB24C8 is nearly impossible even from
the end with one methoxy substituent. Indeed, a single crys-
tal structure of DB24C8 showed a cavity size of 10.2 ꢄ 4.2
Å19 and a new polymorph isolated in this work possessed a
cavity 10.4 ꢄ 2.6 Å (Fig. 6); the longer dimension is from
one benzo carbon to the diagonally opposed benzo carbon
and the shorter dimension refers to the closest HAH trans-
annular distance. The energy minimized structure20 of the
trimethoxy guest 9b has diameters of about 7.2 (methoxy-
phenyl end) and 9.4 Å (dimethoxyphenyl end), respectively.
The bulky ends of 9b thus seemed large enough to act as
stoppers for the formation of rotaxanes from DB24C8.
SCHEME 5 Synthesis of substi-
tuted dibenzylammonium hexa-
fluorophosphates 9a and 9b.
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FIGURE 5 Two views of stick representations of the pseudorotaxane complex formed from 9a and DB24C8 from X-ray crystallogra-
phy; carbons are shown in black, oxygens in red, nitrogen in blue, hydrogens in pink, phosphorous in gray, and fluorines in green.
ꢀ
˚
˚
Interatomic distances and angles for H-bonding: NAO (ethyleneoxy) 2.996 A, NH(1)AO (ethyleneoxy) 2.026 A (a), NAHAO 168.7 ;
ꢀ
˚
˚
˚
NAO (phenoxy) 3.026, NH(2)AO (phenoxy) 2.060 A (b), NAHAO 175.4 ; CAO (ethyleneoxy) 3.504 A, CHAO (ethyleneoxy) 2.582 A
(c), CAHAO 156.9ꢀ. One of tolyl groups lies at an angle of 83.35ꢀ from one of the benzo rings and its methyl carbon atoms sits
ꢀ
˚
˚
˚
3.46 A above that benzo ring; one of the methyl protons is 2.715 A from the phenolic oxygen atom (d), CAHAO 148.6 and 2.714 A
from the adjacent benzo carbon (e), CAHAC 133.7ꢀ, implying some CAHAp interaction perhaps. The other tolyl ring is nearly paral-
lel (8.39ꢀ out) to the other benzo ring and they are separated by <4 A (f), indicating a p-stacking interaction.
˚
was eightfold higher in the heated sample. The latter experi-
ment demonstrates that threading does occur at room tem-
perature, but relatively slowly. Attempts to isolate single
crystals of this rotaxane failed.
thesis of rotaxanes based on DB24C8, although they reduce
the acidity of the compound, which in turn lowers its affinity
for complexation.18
EXPERIMENTAL
Addition of DMSO to the chloroform solution of the heated
mixture did not result in immediate dethreading as would
be the case for a pseudorotaxane, since DMSO is a highly
competitive hydrogen bond acceptor that essentially pre-
vents pseudorotaxane formation.5(d) This clearly demon-
strates that the complex is a rotaxane. The CDCl3:DMSO-d6
(1:1 v:v) solution was examined over time; the signal for the
complexed benzylic protons of 9b (4.7 ppm) persisted for
several days, but was reduced in intensity relative to the sig-
nal for the uncomplexed benzylic protons and that of the a-
protons of the host at 4.05 ppm.8 The color also became less
intense. The half-life was calculated to be 66 6 10 h at
room temperature in the CDCl3:DMSO-d6 (1:1 v:v) solution.
Solvents were used as received. Melting points are uncor-
rected. The 400 MHz 1H NMR spectra were recorded on a
Varian Unity instrument with tetramethylsilane (TMS) as an
internal standard. The following abbreviations are used to
denote splitting patterns: s (singlet), d (doublet), t (triplet),
and m (multiplet). Elemental analyses were obtained from
Atlantic Microlab, Norcross, GA. With the exception of Figure
7, mass spectra were provided by the Washington University
Mass Spectrometry Resource. The mass spectrum of
DB24C8:9b (Fig. 7) was obtained using an Agilent LC-ESI-
TOF system.
X-Ray Crystallography
From these results, we conclude that the methoxy substitu-
ents of 9b may be adequate for controlled slippage-type syn-
X-Ray diffraction data on complex 9a:DB24C8 were collected
on a Siemens P4/CCD diffractometer (T ¼ 218(2) K, MoKa
FIGURE 6 Single crystal X-ray structure of a new polymorph of dibenzo-24-crown-8: left, top view; right, side view; carbons are
shown in black, oxygens in red, and hydrogens in pink.
SELF-ASSEMBLY OF DAISY CHAIN OLIGOMERS, GIBSON ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 7 High-resolution electrospray ionization mass spectrum of an equimolar mixture of DB24C8 and trimethoxy guest 9b
heated 30 min at 150 ꢀC under argon and then dissolved in methanol. The signal at m/z 736.3713 corresponds to [M ꢃ PF6]þ (cal-
culated m/z 736.3691), that at m/z 467.2315 corresponds to [DB24C8 þ H3O]þ, that at m/z 449 corresponds to [DB24C8 þ H]þ, and
that at m/z 288 corresponds to guest [9b]þ. The inset shows the isotopic M þ 1, M þ 2, and M þ 3 peaks at m/z 737.3725 (47%),
738.3754 (13%), and 739 (3%).
radiation, hmax ¼ 23ꢀ, 6080 independent reflections [Rint
¼
2-Formyldibenzo-24-crown-8 (5)
0.0317]). Crystal data: C24H32O8ꢂC16H20NPF6ꢂ0.5(CHCl3), FW
879.48, monoclinic, space group P21/n, a ¼ 10.4640(1) Å,
b ¼ 11.6037(2) Å, c ¼ 35.1819(3) Å, b ¼ 94.927(1)ꢀ. V ¼
To a stirred mixture of 0.94 g (4.4 mmol) of pyridinium
chlorochromate and 40 mL dry CH2Cl2 was added a solution
of 1.73 g (3.62 mmol) of 4-hydroxymethyldibenzo-24-crown-
85(c) in 10 mL of CH2Cl2 and stirring was continued for 2 h.
The solvent was removed in vacuo and the brown residual
solid was purified by recrystallization from ethanol: a white
4256.04(7) Å3, Z ¼ 4, Dc ¼ 1.373 g/cm3, l ¼ 0.24 cmꢃ1
.
The structure was solved by direct methods. In the crystal
structure, there is an inversion disordered solvate CHCl3
molecule, which was treated by the program SQUEEZE.21
Correction of the X-ray data by SQUEEZE (119 electrons/
cell) is very close to the required value (116 electrons/cell)
ꢀ
ꢀ
solid, 1.32 g (77%), mp 97–99 C (lit. 5(d), 105–107 C).
1H NMR (400 MHz, CDCl3, 22 ꢀC): d ¼ 3.85 (8H, m), 3.93
(8H, m), 4.15 (4H, m), 4.22 (4H, m), 6.88–7.44 (7H, m), and
8.92 (1H, s). Anal. calcd. for C25H32O9: C 63.01, H 6.77;
found: C 62.93, H 6.76.
for
a demisolvate CHCl3 molecule. The structure was
refined with anisotropic thermal parameters for all nonhy-
drogen atoms. The positions of the H atoms involved in
hydrogen bonding were found in the F-map and refined
with isotropic thermal parameters. The positions of other H
atoms were calculated. One of the O atoms (O6) in the
macrocycle is disordered over two positions in ratio 82/18.
General Procedure for Schiff Bases
In a 100 mL round bottom flask equipped with a Dean-Stark
trap, condenser and nitrogen bubbler a solution of 12.5
mmol of aldehyde and 12.5 mmol of amine in toluene (25
mL) was refluxed for 24 h. The solvent was rotary evapo-
rated to give the crude product, which, if a solid, was recrys-
tallized, or used without purification to make the corre-
sponding amine.
Convergence was achieved with R1
¼
0.0531, wR2
¼
0.1386 and maximum residual density þ0.552; ꢃ0.285
eꢂÅꢃ3. All software and source scattering factors are con-
tained in the SHELXTL (5.10) program package (G. Shel-
drick, Bruker XRD, Madison, WI).
General Procedure for Reduction of Schiff Bases
to Amines
Colorless needles of DB24C8 crystallized from acetone
through vapor diffusion of pentane. The chosen crystal
(0.06 ꢄ 0.08 ꢄ 0.40 mm3) was centered on the goniome-
ter of an Oxford Diffraction Gemini A Ultra diffractometer
operating with MoKa radiation. The data collection routine,
unit cell refinement, and data processing were carried out
with the program CrysAlisPro (CrysAlisPro 171.33.34,
Oxford Diffraction: Wroclaw, Poland, 2009). The Laue sym-
metry and systematic absences were consistent with the
monoclinic space group P21/n. The structure was solved
by direct methods and refined using SHELXTL NT.22 The
asymmetric unit of the structure comprises 0.5 crystallo-
graphically independent molecules. The final refinement
model involved anisotropic displacement parameters for
nonhydrogen atoms and a riding model for all hydrogen
atoms.
To a 50 mL round bottom flask equipped with a magnetic
stirrer were added 6.14 mmol of Schiff base and CH3OH (25
mL). NaBH4 (0.465 g, 12.3 mmol) was added slowly in small
portions to the methanol solution, which was then refluxed
for 12 h. The solvent was removed in vacuo to give a white
solid which was suspended in H2O and extracted with CHCl3
twice. The organic layers were combined, dried over MgSO4
and concentrated to afford a clear liquid, used without puri-
fication to prepare the hexafluorophosphate salt.
General Procedure for Hexafluorophosphate Salts of
Secondary Amines
To a 100 mL round bottom flask equipped with a magnetic
stirrer were added 2.5 mmol of amine and 2 M HCl (25 mL).
The mixture was stirred for 30 min and the white precipitate
982
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ARTICLE
1
was filtered and washed with cold H2O. This solid was dis-
solved in hot water and the solution was cooled to 0 ꢀC. To
this was added an aqueous solution of NH4PF6 until no fur-
ther precipitation was observed. The precipitate was filtered
and recrystallized from H2O.
(1H, s), and 8.53 (2H, bs). H NMR (400 MHz, CD3SOCD3, 22
ꢀC).8 LR FAB MS (3-NBA): see Table 1. LR FAB MS (3-NBA/
LiI): m/z ¼ 160.1, [3-NBA þ Li]þ, 100%; 313.1, [(3-NBA)2 þ
Li]þ, 50%; 584.3, [M ꢃ HPF6 þ Li]þ, 38%; 710.4, [M þ Cs]þ,
5%; 1155.4, [M ꢃ PF6 ꢃ HPF6]þ, 1%. HR FAB MS (3-NBA)/
LiI): calcd. for C31H47NO9Li: m/z ¼ 584.3411; found: m/z ¼
584.3401.
2-(Benzylideneaminomethyl)dibenzo-24-crown-8 (6a)
Recrystallized from dry ethanol, an off-white solid, mp 78–
79 C. 1H NMR (400 MHz, CDCl3, 22 ꢀC): d ¼ 3.83 (8H, m),
ꢀ
o-(2-Methylbenzylideneaminomethyl)toluene
Slightly yellow liquid, bp 112–114 ꢀC @ 0.08 Torr, 81%. 1H
3.92 (8H, m), 4.15–4.19 (8H, m), 4.79 (2H, s), 6.85–7.44
(12H, m), and 8.27 (1H, s). LR ESI MS: m/z ¼ 588 [M þ
Na]þ. HR MALDI-TOF MS: calcd for [M þ H]þ C32H40O8N: m/
z ¼ 566.2754; found: m/z ¼ 566.2746.
ꢀ
NMR (400 MHz, CDCl3, 22 C): d ¼ 2.39 (3H, s), 2.50 (3H, s),
4.83 (2H, s), 7.17–7.32 (7H, m), 9.73 (1H, d, J ¼ 7.2 Hz), and
8.67 (1H, s).
2-(N-Benzylaminomethyl)dibenzo-24-crown-8 (7a)
Clear liquid, 82%. 1H NMR (400 MHz, CDCl3, 22 ꢀC): d ¼
3.72 (2H, s), 3.78 (2H, s), 3.83 (8H, m), 3.92 (8H, m), 4.15
(8H, m), and 6.82–7.33 (12H, m).
Bis(2-methylbenzyl)amine
Clear liquid, 99%. 1H NMR (400 MHz, CDCl3, 22 ꢀC): d ¼
2.34 (6H, s), 3.83 (4H, s), 7.16 (6H, m), and 7.33 (2H, m).
Bis(2-methylbenzyl)ammonium Hexafluorophosphate (9a)
White solid, 92%, mp 178–180 ꢀC. 1H NMR (400 MHz,
CDCl3, 22 ꢀC): d ¼ 2.22 (6H, s), 4.25 (4H, s), 6.61 (2H, s),
and 7.24–7.37 (8H, m). LR ESI MS: m/z ¼ 226 [M ꢃ PF6]þ;
HR MALDI-TOF MS: calcd for [M ꢃ PF6]þ C16H20N: m/z ¼
226.1596; found: m/z ¼ 226.1588.
2-(N-Benzylammoniomethyl)dibenzo-24-crown-8
Hexafluorophosphate (4)
An off-white solid, 40%, mp 222–225 ꢀC (lit. 5(d)) mp
1
ꢀ
ꢀ
decomp. >210 C). H NMR (400 MHz, DMSO-d6, 22 C): d ¼
3.32 (8H, m), 3.65 (8H, m), 3.75–3.79 (4H, m), 4.03–4.08
(4H, m and 2H, s), 4.12 (2H, s), 6.85–7.45 (12H, m), and
9.04 (2H, s). LR ESI MS (CH3OH:H2O:CH3COOH, 59:49:1): m/
z ¼ 461.3, [M - PF6 -C6H5CH2NH2]þ, 18%; 568.3, [M - PF6]þ,
54%; 581.2, [M ꢃ PF6 þ Na]þ, base peak. HR ESI MS
(CH3OH:H2O:CH3COOH, 59:49:1): calcd. for C32H42NO8 [M ꢃ
PF6]þ: m/z ¼ 568.2911; [M þ 1 ꢃ PF6], 36%; [M þ 2 ꢃ
PF6], 8%; found: m/z ¼ 568.2904, [M þ 1 ꢃ PF6], 38%; [M
þ 2 ꢃ PF6], 5%. HR MALDI MS: calcd for [M ꢃ HPF6 þ Na]þ
C32H41O8NNa: m/z ¼ 590.2730; found: m/z ¼ 590.2738.
o-(2,5-Dimethoxybenzylideneaminomethyl)anisole
Yellow liquid, 96%. 1H NMR (400 MHz, CDCl3, 22 ꢀC): d ¼
3.80 (3H, s), 3.83 (3H, s), 3.85 (3H, s) 4.83 (2H, s), 6.85–
7.31 (6H, m), 7.58 (1H, d, J ¼ 3.2 Hz), and 8.81 (1H, s).
o-(2,5-Dimethoxybenzylaminomethyl)anisole
Yellow liquid, 92%. 1H NMR (400 MHz, CDCl3, 22 ꢀC): d ¼
3.77 (6H, s), 3.80 (3H, s), 3.82 (4H, s), and 6.72–7.29
(7H, m).
2-(60-Hydroxyhexylideneaminomethyl)dibenzo-24-crown-8
(6b)
84%, colorless solid from toluene-hexane, mp 63–65 ꢀC. 1H
NMR (400 MHz, CDCl3, 22 ꢀC): d ¼ 1.2–1.8 (8H, m), 3.60
(2H, t, J ¼ 7), 3.64 (2H, t, J ¼ 7), 3.83 (8H, m), 3.92 (8H, bs),
4.16 (6H, m), 4.22 (2H, t, J ¼ 4), 6.8–6.9 (6H, m), 7.12 (1H,
d, J ¼ 8), 7.39 (1H, s), and 8.13 (1H, s). Anal. calcd. for
o-(2,5-Dimethoxybenzylammoniomethyl)anisole
Hexafluorophosphate (9b)
Yellow crystals, 94%, mp 145–147 ꢀC. 1H NMR (400 MHz,
CDCl3, 22 ꢀC): d ¼ 3.74 (3H, s), 3.86 (3H, s), 3.90 (3H, s)
4.16 (2H, s), 4.20 (2H, s), and 6.79–7.41 (7H, m). LR ESI MS:
m/z ¼ 288 [M-PF6]þ. HR MALDI-TOF MS: calcd. for [M-PF6]þ
C17H22N: m/z ¼ 288.1600; found: m/z ¼ 288.1612.
C31H45NO9: C 62.71, H 7.98, N 2.54; found: C 62.55, H 7.74,
N 2.40.
2-[N-(60-Hydroxyhexyl)aminomethyl]dibenzo-24-crown-8
(7b)
Slippage Route to the Rotaxane from Dibenzo-24-crown-8
and Guest 9b
1
ꢀ
97%, very viscous oil. H NMR (400 MHz, CDCl3, 22 C): d ¼
1.35 (4H, m), 1.53 (4H, m), 2.60 (2H, t, J ¼ 7), 3.62 (2H, t, J
¼ 7), 3.69 (2H, s), 3.91 (8H, m), 4.15 (8H, m), 6.80 (2H, s),
and 6.9 (5H, m). LR FAB MS [3-nitrobenzyl alcohol (3-NBA)/
LiI]: m/z ¼ 160.1, [3-NBA þ Li]þ, 100%; 313.1, [(3-NBA)2 þ
Li]þ, 48%; 584.3, [M þ Li]þ, 32%; 710.4, [M þ Cs]þ, 8%. HR
The host (24 mg, 54 mmol) and the guest (23.9 mg, 55
mmol) in a 10 mL pear-shaped flask were heated at 150 6
ꢀ
5 C in an oil-bath for 30 min with argon protection. The re-
sultant yellow mixture was cooled to room temperature
under argon. A portion of this solid was dissolved in CDCl3
for NMR analysis.
FAB MS (3-NBA/LiI): calcd. for
C31H47NO9Li: m/z
¼
The authors are grateful to the National Science Foundation for
support of this work through grants CHE-95-21,738 and DMR-
07-04,076. They also thank the Washington University Mass
Spectrometry Resource, an NIH Research Resource (Grant No.
P41RR0954). Furthermore, they acknowledge the National Sci-
ence Foundation’s support for purchase of the X-ray diffracto-
meter (CHE-0131128) and the ESI mass spectrometer (CHE-
0722638) at Virginia Tech.
584.3411; found: m/z ¼ 584.3394.
2-[N-(60-Hydroxyhexylammonio)methyl]dibenzo-24-
crown-8 Hexafluorophosphate (8)
77%, colorless solid from H2O, mp 172–173 ꢀC. 1H NMR
(400 MHz, CD3SOCD3, 22 ꢀC): d ¼ 1.27 (4H, m), 1.39 (2H,
m), 1.57 (2H, m), 2.84 (2H, bs), 3.65 (8H, m), 3.75 (8H, m),
4.05 (10H, m), 4.38 (1H, t, J ¼ 5), 6.82–7.02 (6H, m), 7.08
SELF-ASSEMBLY OF DAISY CHAIN OLIGOMERS, GIBSON ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
400–407; (d) Suzaki, Y.; Murata, S.; Osakada, K. Chem Lett
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4 A selection of recent publications on polypseudorotaxanes
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Sforazzini, G.; Brovelli, S.; Latini, G.; Townsend, E.; Williams, C.
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Wilson, J. S.; Cacialli, F.; Anderson, H. L. Adv Funct Mater
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J.; Rabani, G.; Rotello, V. M.; Woisel, P. Tetrahedron 2008, 65,
10 For a study of the efficacy of MALDI-TOF MS for such pseu-
dorotaxanes: see Lehmann, E.; Salih, B.; Gomez-Lopez, M.; Die-
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11 Kolchinski, A. G.; Busch, D. H.; Alcock, N. W. Chem Com-
mun 1995, 1289–1291.
12 These spectra were recorded immediately after the solu-
tions were made to avoid condensation of the ammonium salt
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ARTICLE
with acetone, yielding an iminium salt. See Jones, J. W.;
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17 (a) Nishio, M.; Umezawa, Y.; Takeuchi, Y. Tetrahedron 1995,
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14 We have demonstrated that Kaexp is not a constant for com-
plexation of DB24C8 and its derivatives with dibenzylammo-
nium salts in nonpolar organic solvents; it can vary by more
than an order of magnitude. The reason is that the guest salt is
predominantly ion paired whereas the pseudorotaxane is not;
hence, the overall process is H þ GX ! HGþ þ Xꢃ. The expres-
sion for Kaexp does not include the concentration of Xꢃ. To
solve for the ionization constant of the salt and the binding
constant of the cation with the host requires extensive data.
18 Ashton, P. R.; Fyfe, M. C. T.; Hickingbottom, S. K.; Stoddart,
J. F.; White, A. J. P.; Williams, D. J. J Chem Soc Perkin Trans
1998, 2, 2117–2128.
19 Colquhoun, H. M.; Doughty, S. M.; Stoddart, J. F.; Slawin,
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253–257.
15 Jones, J. W.; Gibson, H. W. J Am Chem Soc 2003, 125,
20 ChemBio3D Ultra 11.0, MM2 minimization routine.
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21 van der Sluis, P.; Spek, A. L.; Acta Cryst
A
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16 Gibson, H. W.; Wang, H.; Bonrad, K.; Jones, J. W.; Slebod-
nick, C.; Habenicht, B.; Lobue, P. Org Biomol Chem 2005, 3,
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194–201.
22 Sheldrick, G. M. Acta Cryst A 2008, 64, 112–122.
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