Convergent synthesis of alternating fluorene-p-xylene oligomers and delineation of the (silver) cation-induced folding

Article abstract of DOI:10.1021/ja0687522

Convergent synthetic routes for the preparation of hitherto unknown fluorene-p-xylene oligomers (containing up to 10 fluorene moieties) from readily available starting materials are described. The conformationally adaptable monomeric receptor (which is made of a pair of fluorene and one p-xylene ring, i.e., Z1) undergoes a simple C-C bond rotation in the presence of silver cations to produce a π-prismand-like receptor which binds a single silver cation with remarkable efficiency (i.e., K ≈ 15 000 M^-1). The data on ¹H NMR spectroscopic titrations with Ag⁺ together with the density functional theory and AM1 calculations allows us to establish that various oligomers of Z1 (i.e., Z2-Z9) also undergo ready folding into the structures that contain multiple π-prismand-like receptor sites in the presence of silver cations. The multiple cavities in Z3-Z9 accommodate a single silver cation per cavity with efficiency similar to that of Z1.

Full text of DOI:10.1021/ja0687522

Published on Web 06/16/2007
Convergent Synthesis of Alternating Fluorene-p-xylene
Oligomers and Delineation of the (Silver) Cation-Induced
Folding
Vincent J. Chebny and Rajendra Rathore*
Contribution from the Department of Chemistry, Marquette UniVersity, P.O. Box 1881,
Milwaukee, Wisconsin 53201-1881
Received December 6, 2006; E-mail: rajendra.rathore@marquette.edu
Abstract: Convergent synthetic routes for the preparation of hitherto unknown fluorene-p-xylene oligomers
(containing up to 10 fluorene moieties) from readily available starting materials are described. The
conformationally adaptable monomeric receptor (which is made of a pair of fluorene and one p-xylene
ring, i.e., Z1) undergoes a simple C-C bond rotation in the presence of silver cations to produce a
π-prismand-like receptor which binds a single silver cation with remarkable efficiency (i.e., K ≈ 15 000
M-1). The data on 1H NMR spectroscopic titrations with Ag+ together with the density functional theory and
AM1 calculations allows us to establish that various oligomers of Z1 (i.e., Z2-Z9) also undergo ready
folding into the structures that contain multiple π-prismand-like receptor sites in the presence of silver cations.
The multiple cavities in Z3-Z9 accommodate a single silver cation per cavity with efficiency similar to that
of Z1.
evolving areas of molecular electronics and nanotechnology.9
A number of such synthetic materials, generally termed “fol-
Introduction
There are numerous examples in nature where weak inter-
and intramolecular bonding interactions (such as hydrogen
bonding,1 π-stacking,2 Columbic interactions,3 metal-ion bind-
ing,4 etc.) permit structure modulation of the biopolymers (such
as polypeptides, ribonucleic acid, polysaccharides) into well-
defined ‘molecular machines’ that perform complex functions
from enzymatic catalysis to information storage and retrieval.5
Furthermore, the design and syntheses of artificial polymeric
(organic) materials whose structures can be modulated by
external stimuli (such as heat,6 light,7 or metal-ion binding8)
constitute an important area of research owing to the fact that
such materials may hold potential for applications in the ever
damers”, have been prepared and are discussed in detail in a
recently published review article by Moore and co-workers.10
We recently synthesized11 a hydrocarbon ligand 1,4-bis(9-
methyl-9H-fluoren-9-yl)methyl]benzene (1, see structure below)
from readily available fluorene and R,R-dichloro-p-xylene that
possesses an unique molecular structure where a simple C-C
single bond rotation converts it from an extended (“Z”)
conformer to an isoenergetic (folded) delta (“∆”) conformer,
as established by density functional theory (DFT) calculations11
at the B3LYP/6-31G* level. The cavity formed by three
aromatic walls (i.e., two fluoranyl rings and one p-xylyl ring)
in the “∆” conformation of 1 is remarkably similar to that found
in π-prismand (2)12sa well-known and efficient receptor for
the binding of a variety of metal cations12,13sas shown in Figure
1. Although, the energy difference between the two conformers
of 1 is only ∼0.4 kcal/mol, X-ray crystal structure analysis
showed that 1 in the solid state exists exclusively as the extended
(1) (a) Murray, T. J.; Zimmerman, S. C. J. Am. Chem. Soc. 1992, 114, 4010-
4011. (b) Corbin, P. S.; Zimmerman, S. C. J. Am. Chem. Soc. 2000, 122,
3799. (c) Margulis, C. J.; Stern, H. A.; Berne, B. J. J. Phys. Chem. B 2002,
106, 10748.
(2) (a) Han, J. J.; Wang, W.; Li, A. J. Am. Chem. Soc. 2006, 128, 672 and
references therein. (b) Zych, A. J.; Iverson, B. L. J. Am. Chem. Soc. 2000,
122, 8898. (c) Nguyen, J. Q.; Iverson, B. L. J. Am. Chem. Soc. 1999, 121,
2639.
(3) (a) Gabriel, G. J.; Sorey, S.; Iverson, B. L. J. Am. Chem. Soc. 2005, 127,
2637. (b) Shepherd, N. E.; Abbenante, G.; Fairlie, D. P. Angew. Chem.,
Int. Ed. 2004, 43, 2687. (c) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173.
(4) (a) Ghadiri, M. R.; Choi, C. J. Am. Chem. Soc. 1990, 112, 1630. (b) Kelso,
M. J.; Hoang, H. N.; Appleton, T. G.; Fairlie, D. P. J. Am. Chem. Soc.
2000, 122, 10488. (c) Nicoll, A. J.; Miller, D. J.; Futter, K.; Ravelli, R.;
Allemann, R. K. J. Am. Chem. Soc. 2006, 128, 9187.
(8) (a) Chang, K.-J.; Kang, B.-N.; Lee, M.-H.; Jeong, K.-S. J. Am. Chem. Soc.
2005, 127, 12215. (b) Suzuki, Y.; Morozumi, T.; Nakamura, H.; Shimo-
mura, M.; Hayashita, T.; Bartsh, R. A. J. Phys. Chem. B 1998, 102, 7910.
(c) Ghosh, S.; Ramakrishnan, S. Macromolecules 2005, 38, 676. (d) Piguet,
C.; Bernardinelli, G.; Hopfgartner, G. Chem. ReV. 1997, 97, 2005 and
references therein. (e) Zhang, F.; Bai, S.; Yap, G. P. A.; Tarwade, V.; Fox,
J. M. J. Am. Chem. Soc. 2005, 127, 10590.
(5) (a) Ritz, T.; Park, S.; Schulten, K. J. Phys. Chem. B 2001, 105, 8259. (b)
Stubbe, J.; van der Donk, W. A. Chem. ReV. 1998, 98, 705 and references
therein. (c) Rasmussen, P. H.; Ramanujam, P. S.; Hvilsted, S.; Berg, R. H.
J. Am. Chem. Soc. 1999, 121, 4738. (d) Alonso, M.; Reboto, V.; Guiscardo,
L.; San, Martin, A.; Rodriguez-Cabello, J. C. Macromolecules 2000, 33,
9480.
(9) Introduction to Molecular Electronics; Petty, M. C., Bryce, M. R., Bloor,
D., Eds.; Oxford University Press: New York, 1995.
(10) Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem.
ReV. 2001, 101, 3893 and references within. Also see: Schmuck, C. Angew.
Chem., Int. Ed. 2003, 42, 2448. Huc, I. Eur. J. Org. Chem. 2004, 17.
(11) Rathore, R.; Chebny, V. J.; Abdelwahed, S. A. J. Am. Chem. Soc. 2005,
127, 8012.
(6) Margulis, C. J.; Stern, H. A.; Berne, B. J. J. Phys. Chem. B. 2002, 106,
10748 and references therein.
(7) (a) Balbo, Block, M. A.; Hecht, S. Macromolecules 2004, 37, 4761. (b)
Khan, A.; Kaiser, C.; Hecht, S. Angew. Chem., Int. Ed. 2006, 45, 1878. (c)
McQuade, T. D.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537.
(12) (a) Pierre, J. L.; Baret, P.; Chautemps, P.; Armand, M. J. Am. Chem. Soc.
1981, 103, 2986. (b) Pierre, G.; Baret, P.; Chautemps, P.; Pierre, J. L.
Electrochim. Acta 1983, 28, 1269.
9
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10.1021/ja0687522 CCC: $37.00 © 2007 American Chemical Society

Preparation of Fluorene-p-xylene Oligomers
A R T I C L E S
NMR spectroscopy it will be shown that these conformationally
adaptable oligomers readily fold into the structures that contain
multiple π-prismand-like receptor sites (which are made of a
pair of fluorene and one p-xylene ring) in the presence of silver
cations as follows.
Figure 1. Optimized structures of the isoenergetic conformers of 1 by
density functional theory (DFT) calculations at B3LYP/6-31G* level and
structural similarity with tris[2.2.2]-paracyclophane or π-prismand (2).
Results and Discussion
Synthesis of Fluorene-p-xylene Oligomers. The initial
attempt (P1) to polymerize fluorene with R,R-dichloro-p-xylene
in the presence of potassium tert-butoxide as a base in
tetrahydrofuran at 0 °C led to a cream-colored solid which
1
melted at 270-274 °C, and its structure was inferred by H/
13C NMR spectroscopy to be a mixture of cyclic oligomers
(structure B), i.e. Scheme 1, reaction P1.
A similar mixture of cyclic oligomers (structure B) was
obtained upon reactions of various mixtures of fluoranyl and
R,R-dichloro-p-xylyl derivatives (vide infra) in Scheme 1 (i.e.,
reactions P2-P5) in tetrahydrofuran in the presence of potas-
sium tert-butoxide as a base, as confirmed by the observation
of fairly narrow signals in the rather simple 1H/13C NMR spectra,
in each case, as shown in Figure 3.14
The identity of the similar mixtures of cyclic oligomers,
obtained in various reactions in Scheme 1, was further confirmed
by MALDI-TOF mass spectrometry (see Figure S1 in the
Supporting Information).
Thus, an unexpected formation of cyclic oligomers in Scheme
1 necessitated development of a different synthetic approach
for preparation of acyclic oligomers (structure A) as follows.
The syntheses of acyclic oligomers containing up to 10
fluorene units were accomplished by a stepwise (convergent)
synthetic approach. The key to the success of this (controlled)
approach lies in the fact that carbon 9 of fluorene can be
selectively mono- or dialkylated by a simple variation of the
reaction conditions and thus allowed straightforward access to
the fragments required for designing convergent routes to the
various acyclic oligomers Z1-Z9 (Scheme 2).
Figure 2. X-ray structure of 1 showing the extended conformer.
“Z” conformer (Figure 2). Note that the X-ray structure of 1
was completely in accord with the calculated structure shown
in Figure 1.
Moreover, variable-temperature 1H NMR analyses of 1
indicated that its two conformers (i.e., extended, Z, and folded,
∆) cannot be frozen out at lower temperatures (i.e., ca. -90
°C). However, the conformational adaptability of 1, which
hereafter will be referred to as Z1, allows it to bind a single
silver cation into the cavity of the ∆ conformer with an excellent
efficiency (K ≈ 15 000 M-1), i.e., eq 1.11
Thus, a highly selective monolithiation of fluorene in
anhydrous tetrahydrofuran using n-BuLi at -78 °C followed
by reaction with an alkyl halide [such as iodomethane, ethyl-
4-(bromomethyl)benzoate, or R,R-dichloro-p-xylene] led to
quantitative formation of the initial building blocks as shown
in Scheme 2 (bottom). [Note that the combination of notations
F, H, M, E, A, C, and X with numeral subscripts for various
intermediates in Scheme 2 refers to the number of fluorenes,
hydrogens, methyls, ester, alcohol, chloro, and xylyl groups,
respectively.]
It is envisioned that the fluorene-p-xylene-based receptor Z1
(shown above in eq 1) can be easily woven into a hitherto
unknown polymeric structure by linking p-xylene groups at
carbon 9 of the fluorene moieties, i.e., structure A.
The second lithiation of the initial building blocks in Scheme
2, in the same pot or after isolation of the products, using n-BuLi
at -78 °C in THF followed by reaction with ethyl-4-(bromo-
methyl)benzoate afforded the ester building blocks that were
easily converted into their corresponding benzyl chlorides by a
simple two-step procedure. For example, reduction of the esters
using lithium aluminum hydride in refluxing THF followed by
reaction of the resulting alcohol with thionyl chloride in
chloroform at ∼0 °C afforded the corresponding benzyl
chlorides in almost quantitative yields. Various benzyl chlorides
Accordingly, herein we will report the first preparation of
oligomers of structure A containing up to 10 fluorene moieties
1
by a convergent synthetic approach. Furthermore, using H
(13) Kang, H. C.; Hanson, A. W.; Eaton, B.; Boelelheide, V. J. Am. Chem.
Soc. 1985, 107, 1979. Also see: (a) Heirtzler, F. R.; Hopf, H.; Jones, P.
G.; Bubenitachek, P.; Lehne, V. J. Org. Chem. 1993, 58, 2781-2784. (b)
Gano, J. E.; Subramaniam, G.; Birnbaum, R. J. Org. Chem. 1990, 55, 4760.
(c) Lindeman, S. V.; Rathore, R.; Kochi, J. K. Inorg. Chem. 2000, 39,
5707. (d) Prodi, L.; Bolletta, F.; Montalti, M.; Zaccheroni, N. Coord. Chem.
Rev. 2000, 205, 59-83. (e) Ikeda, M.; Tanida, T.; Takeuchi, M.; Shinkai,
S. Org. Lett. 2000, 2, 1803. (f) Munakata, M.; Wu, L. P.; Kuroda-Sowa,
T.; Maekawa, M.; Suenaga, Y.; Sugimoto, K. Inorg. Chem. 1997, 36, 4903.
(g) Munakata, M.; Wu, L. P.; Ning, G. L.; Kuroda-Sowa, T.; Maekawa,
M.; Suenaga, Y.; Macao, N. J. Am. Chem. Soc. 1999, 121, 4968. (h) Yang,
R.-H.; Chan, W.-H.; Lee, A. W. M.; Xia, P.-F.; Zhang, H.-K.; Li, K. J.
Am. Chem. Soc. 2003, 125, 2884.
(14) The mixture of cyclic oligomers in Scheme 1 can also bind silver cation
with efficiency similar to those observed with various Zn’s. We are actively
pursuing the isolation of a pure oligomer to delineate its structure and usage
for designing functional materials. These results will be described in a future
publication.
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A R T I C L E S
Chebny and Rathore
Scheme 1. Polymerization Attempts (P1-P5) Using Various Fluoranyl and p-Dichloroxylyl Derivatives
in Scheme 2 (bottom) served as internal and capping units for
the preparation of various oligomers.
afforded a higher homologue of the capping unit (i.e., F2X2CM).
A similar reaction sequence allowed the preparation of capping
units containing three and four fluorene moieties with either an
electrophilic benzyl chloride group or a precursor to a nucleo-
philic fluoranyl anion, i.e., Scheme 2 (top).
The capping unit FXCM can be further elongated by
repeating the high-yielding reaction sequence depicted in the
Scheme 2 (bottom). Thus, in a one-pot procedure reaction of
FXCM with the fluoranyl anion (generated from fluorene and
n-BuLi in THF at -78 °C) followed by addition of another
equivalent of n-BuLi and ethyl-4-(bromomethyl)benzoate af-
forded F2X2EM in excellent yield. Reduction of the resulting
ester with LiAlH4 followed by reaction with thionyl chloride
The syntheses of linear oligomers Z2-Z9 were finally accom-
plished by piecing together various inner building blocks (i.e.,
fluorene, F2XH2, FX2C2, and F2X3C2) and capping units (FXCM,
F2XHM, F3X2HM, and F4X3HM) in tetrahydrofuran using
Figure 3. 1H/13C NMR spectra of the mixture of cyclic oligomers obtained from Scheme 1 in CDCl3 at 22 °C.
Scheme 2. Synthesis of Various Building Blocks and Coupling To Form Zn Oligomersa
a (a) n-BuLi/THF/-78 °C. (b) n-BuLi/-78 °C/ethyl-4-(bromomethyl)benzote. (c) LiAIH4/THF/reflux. (d) SOCI2/CHCI3/0 °C. (e) Fluorene/n-BuLi/THF/-
78 °C.
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Preparation of Fluorene-p-xylene Oligomers
A R T I C L E S
1
Figure 4. (Left) Partial H NMR spectra of Z1 obtained upon an incremental addition of CF3SO3Ag in CDCl3-CD3OD at 22 °C. (Right) Plot of changes
in the chemical shifts of the xylenic protons (indicated by letter “a” in the structure in the Figure 4A) against the added equivalents of a solution of CF3-
SO3Ag in CDCl3-CD3OD at 22 °C.
potassium tert-butoxide as a base at 0 °C in excellent yields as
exemplified by the preparation of Z9 in Scheme 2 (top).
The acyclic oligomers Z2-Z9 were readily purified by
filtration through a short pad of silica gel using mixtures of
ethyl acetate-hexanes as an eluent and characterized by 1H and
13C NMR spectroscopy as well as by mass spectrometry. The
1H NMR spectra of Z1-Z9 compiled in Figure S2 in the
Supporting Information confirmed that an expected ratio of the
integrations of the signals due to the methyl protons with that
of the methylenic or xylenic protons was observed in each case.
Efficient binding of a single silver cation by the conforma-
tionally adaptable receptor Z1 was confirmed by a competition
experiment with deltaphane 2sa well-known and efficient
receptor for silver cationsas well as by a spectrophotometric
determination of the binding constant K ≈ 15 000 M-1 using
the Benesi-Hildebrand procedure (see Figure 5A/B).15 More-
over, the 1:1 complexation stoichiometry for [Z1, Ag+] was
established by Job’s plot analysis (Figure 5C),16 i.e. eq 2.
1
Moreover, the H NMR spectra of oligomers Z2, Z3, and Z4,
with an increasing number of repeating units, showed the
expected number of signals for various methylene and xylenic
protons. Expectedly, further increase of the repeating units in
Z5-Z9 showed that the 1H NMR signals due to the additional
methylenic and xylenic protons were overlapping with one of
the methylenic (2.95 ppm) and xylenic (6.20 ppm) signals in
the spectrum of Z4. The 13C NMR spectra of various acyclic
oligomers Z1-Z9 are also compared in Figure S3 in the
Supporting Information.
It is also important to note that a model compound FXM2,
containing only one fluoranyl moiety, bound Ag+ with much
less efficiency and required a large excess of Ag+ for complete
utilization of the ligand, and a binding constant K ≈ 10 was
1
estimated using H NMR spectroscopy, i.e., eq 3.
Binding of Silver Cation (Ag+) to Fluorene-p-xylene
Oligomers Z1-Z9. The binding of silver cation to various
1
acyclic oligomers Z1-Z9 was monitored by H NMR spec-
1
troscopy. Thus, the changes in the H NMR spectra of Z1 in
chloroform-d (0.05 mM) by an incremental addition of a
concentrated solution of silver trifluoromethane-sulfonate (0.5
mM) in 1:1 chloroform-d/methanol-d4 showed considerable
shifts of the aromatic signals up to addition of 1 equiv of Ag+,
as shown in Figure 4A. Moreover, the 1H NMR spectrum
remained unchanged upon further addition of Ag+ solution, i.e.,
beyond 1 equiv, as shown in Figure 4B.
Thus, the experiments discussed above demonstrated that Z1
binds a single silver cation with remarkable efficiency due to
the fact that it readily adapts a π-prismand-like conformation
by a simple C-C bond rotation (see also eq 1).
(15) Benesi, H. A.; Hildebrand, J. J. J. Am. Chem. Soc. 1949, 71, 2703-2707.
(16) Connors, K. A. Binding Constants; Wiley: New York, 1987.
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Chebny and Rathore
-
Figure 5. (A) Spectra obtained upon incremental addition of a 15 mM solution of Ag+ CF3SO3 in methanol (black) to a 0.8 mM solution of Z1 (red) in
CH2Cl2 at 22 °C. (B) Benesi-Hildebrand plot of Z1 and Ag+CF3SO3-. (C) Job’s plot of a 1:1 complex of Z1 and Ag+ cation, where the absorption at 313
nm was plotted against the mole fraction of Z1 at an invariant total concentration of 0.02 M in a 19:1 mixture of CH2Cl2/CH3OH (v/v).
Figure 6. Plots of 1H NMR chemical shift changes (of the xylenic protons)
attendant upon incremental addition of a 0.2-0.5 mM solution of Ag+
CF3SO3- in 1:1 CDCl3/CD3OD to a 0.02-0.05 mM solution of oligomers
Z1, Z3, Z5, Z7, and Z9, containing even number of fluoranyl moieties, in
CDCl3 at 22 °C. For the actual 1H NMR spectra obtained upon incremental
addition of Ag+ solution, see Figures S8, S10, S12, and S14 in the
Supporting Information.
Figure 7. Job’s plots of a 1:1 complex of Z2 and Ag+ cation (A), a 1:2
complex of Z3 and Ag+ cation (B), and a 1:3 complex of Z5 and Ag+
cation (C). The Job’s plots were obtained by plotting the growth of
In order to determine the efficiency and binding of silver
absorbance at 313 nm against the mole fraction of Z2, Z3, and Z5,
respectively, at an invariant total concentration of 0.02 M in a 19:1 mixture
cations to the higher homologues of Z1, similar 1H NMR spec-
tral titrations of the chloroform-d solutions of Z3, Z5, Z7, and
Z9, containing an even number of fluorenes (i.e., 4, 6, 8, and
10 fluoranyl moieties, respectively), with a concentrated solution
of Ag+ CF3SO3- in a 1:1 mixture of chloroform-d and methanol-
d4 were carried out at 22 °C; see Figures S8, S10, S12, and
of CH2Cl2/CH3OH (v/v).
S14 in the Supporting Information. The reproducible spectral
titrations thus obtained showed that Z3, Z5, Z7, and Z9 bind
2, 3, 4, and 5 equiv of Ag+, respectively, as shown in Figure 6.
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Preparation of Fluorene-p-xylene Oligomers
A R T I C L E S
Figure 8. Calculated molecular structures of representative examples, i.e., Z8 and Z9, using Spartan (AM1) show formation of multiple π-prismand-like
cavities for binding of Ag+ cations.
Figure 9. Calculated molecular structures of various (almost) isoenergetic conformers of Z3 using density functional theory calculations at the B3LYP/
6-31G* level.
The 1H NMR spectral titrations of homologues Z2, Z4, Z6, and
Z8, containing an odd number of fluorenes (i.e., 3, 5, 7, and 9
fluoranyl moieties, respectively) with Ag+ CF3SO3- showed that
these oligomers bind 1, 2, 3, and 4 equiv of silver cations, respec-
tively, as shown in Figure S4 in the Supporting Information.
Additionally, the binding of multiple silver cations to
representative Zn receptors was also probed by Job’s plot
analyses16 (see Figure 7). For example, Z1 (see Figure 5), Z3,
and Z5 showed that the maximum of the binding interactions
takes place at the mole fraction of 0.5 for Z1, 0.33 for Z3, and
0.25 for Z5, which is consistent with the uptake of 1, 2, and 3
Ag+ cations by Z1, Z3, and Z5, respectively, by NMR
spectroscopy titrations in Figure 6. Moreover, the maximum
interaction with Ag+ and Z2 occurs at 0.5 mol fraction of Z2,
thus confirming that it binds only a single silver cation (compare
Figure S4 in the Supporting Information.).
The oligomers Z2, Z4, Z6, and Z8, containing an odd number
of fluorene moieties form only 1, 2, 3, and 4 deltaphane-like
cavities, respectively, upon exposure to a solution of Ag+ while
leaving a single fluorene/xylene pair in an extended conforma-
tion (see Figure 8 for a representative example).
Note that the receptor site with an extended conformer binds
silver cation with much less efficiency, i.e., K ≈ 10 M-1 (see
eq 3). It should be noted that the dynamic nature of silver-cation
binding to various Zn would indicate that a single fluorene/
xylene pair in an extended conformation is in continuous flux
over the entire chain (vide infra).
The calculated AM1 structures (using Spartan) in Figure 8
1
together with the H NMR spectroscopic titrations data with
Ag+ in Figure 6 and Job’s plots in Figure 7 clearly account for
the number of Ag+ cations captured by various oligomers Z1-
Z9. Moreover, it is important to note that the simplicity of the
1H NMR spectra, obtained in the presence of varying equivalents
of Ag+ (in Figures 4 and S7-S14 in the Supporting Informa-
tion), suggests the dynamic nature of the binding of Ag+ to the
multiple receptor sites of Z2-Z9.
As such, the number of Ag+ cations captured by a given
homologue of Z1 (in Figures 6 and 7) is consistent with the
fact that a pair of fluoranyl moieties is necessary for the effective
capture of a single silver cation. As shown above in eqs 1 and
2, an efficient binding of Ag+ requires formation of a π-pris-
mand-like cavity formed by three aromatic walls (i.e., two
fluoranyl moieties and one xylyl group). The oligomeric Zn
exists in rapidly interconverting extended “Z” and folded “∆”
conformations in the absence of silver cations at 22 °C, as can
Thus, a careful analysis of the changes in the chemical shifts
1
of the xylenic protons in the H NMR spectra of Z3 obtained
upon an incremental addition of Ag+ solution (vide infra)
provides further insight into the dynamic nature of silver cation
binding. As shown in Figure 9, the oligomer Z3 can exist in at
least four (almost) isoenergetic conformers (Z3-A, Z3-B, Z3-
C, and Z3-D), as established by density function theory (DFT)
calculations at the B3LYP/6-31G* level.
1
be seen with the simple H NMR spectra (Figure S1 in the
Supporting Information), and the conformational mobility of
various oligomers (Zn) cannot be frozen at lower temperatures.
However, introduction of Ag+ cations induces the folding of
the various oligomers, containing an even number of fluorene
moieties, by simple C-C bond rotations to produce 1, 2, 3, 4,
and 5 deltaphane-like cavities in Z1, Z3, Z5, Z7, and Z9,
respectively (i.e., see Figure 8 for a representative example).
On the basis of the number of observed (sharp) signals in
the 1H NMR spectrum (see Figure S1 in the Supporting
Information) of Z3 it can be easily concluded that the almost
isoenergetic conformers of Z3 in Figure 9 are rapidly intercon-
1
verting on the NMR time scale. The H NMR spectrum of Z3
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A R T I C L E S
Chebny and Rathore
Figure 10. (Left) Partial H NMR spectra of Z3 showing the changes in chemical shifts of xylenic protons upon incremental addition of CF3SO3-Ag+ in
CDCl3-CD3OD at 22 °C. (Right) Plot of changes in the chemical shifts of xylenic protons (indicated by “a” and “b” in the structure in Figure 10) against
the added equivalents of a solution of CF3SO3-Ag+ in CDCl3-CD3OD at 22 °C.
1
corresponds to a symmetrical structure in which a singlet from
the inner xylenic protons (4H, indicated by ‘a’ in Figure 10) is
readily distinguished from the distorted AB quartet from the
outer xylenic protons (8H, indicated by ‘b’ in Figure 10). Figure
10 shows that both the xylenic signals from Z3 shift upfield
upon an incremental addition of up to 1 equiv of Ag+ cation.
Moreover, the incremental addition of Ag+ beyond 1 equiv
shows the upfield shift of only the outer xylenic protons up to
addition of 2 equiv of silver cations, while the signal due to the
inner xylenic protons remains unchanged (Figure 10).
separated from each other to avoid Columbic repulsion between
the cationic guests, i.e., structure Z3-D in Scheme 3. Such an
analysis is consistent with the fact that the chemical shift of
only the outer xylenic protons should be affected upon binding
of the second silver cation while the protons from the central
xylene ring remain unaffected (see Figure 10). Note that an
alternative structure Z3-E containing two deltaphane-like cavi-
ties is much higher in energy (∼5 kcal/mol) compared to the
structures in Figure 9 (or Scheme 3) as established by DFT
calculations.
1
The H NMR spectral change in Figure 10 can be easily
reconciled with the aid of Scheme 3. Thus, exposure of Z3 to
1 equiv of Ag+ will afford three possible structures containing
a deltaphane-like cavity (i.e., Z3-B, Z3-C, and its mirror image
Z3-C′) with equal probability, thus affecting the chemical shifts
of all xylenic protons equally upon binding to the silver cation
(see Figure 10, right). Note that the observation of only one set
of signals in the 1H NMR spectrum indicates that all three silver-
bound structures (i.e., Z3-B, Z3-C, and Z3-C′) are in dynamic
equilibrium on the NMR time scale. Moreover, accommodation
of a second silver cation in structures Z3-B or Z3-C or Z3-C′
requires their transformation into a structure containing two
deltaphane-like cavities in such a way that they are well
Summary and Conclusions
We developed convergent syntheses of hitherto unknown
fluorene-p-xylene oligomers Z1-Z9 in excellent yields with
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8464 J. AM. CHEM. SOC. VOL. 129, NO. 27, 2007

Preparation of Fluorene-p-xylene Oligomers
A R T I C L E S
Scheme 3. Possible Structures of Z3 upon Exposure to 1 and 2 Equiv of Silver Cations
the aid of four repetitive sequences of reactions, i.e., alkylation,
lithium aluminum hydride reduction of benzoic esters, conver-
sion of benzyl alcohols to benzyl chlorides using thionyl
chloride, followed by alkylation as summarized in Scheme 2.
These conformationally mobile and readily soluble oligomers,
containing multiple receptor sites, were easily characterized by
1H and 13C NMR spectroscopy as well as mass spectrometry.
The binding of multiple silver cations to Z3-Z9 was possible
due to the folding of these oligomers, by simple C-C bond
rotations, to produce structures containing multiple deltaphane-
like receptor sites (see Figure 8) for efficient binding of silver
and Z8) containing odd number of fluorene moieties form 1, 2,
3, and 4 deltaphane-like cavities together with a single fluorene/
xylene pair in an extended conformation. Note that the receptor
site with an extended conformer binds silver cation with much
less efficiency, i.e., K ≈ 10 M-1 (see eq 3).
We are actively exploring the syntheses of the Zn analogues
containing different substituents both on xylene and fluorene
moieties to further modulate the binding and selectivity of
various metal cations to these conformationally adaptable
nanometer-sized materials with multiple metal binding sites.
Acknowledgment. We thank Dr. Ilia A. Guzei (University
of Wisconsin at Madison) for the X-ray crystallography and
the Petroleum Research Funds, administered by the American
Chemical Society, and National Science Foundation (Career
award) for financial support.
1
cations (i.e., K ≈ 15 000 M-1). The reproducible H NMR
spectroscopic titration of the oligomers Z1, Z3, Z5, Z7, and
Z9 containing an even number of fluorene moieties confirmed
that they bind 1, 2, 3, 4, and 5 silver cations, respectively,
whereas oligomers Z2, Z4, Z6, and Z8 containing an odd
number of fluorene moieties bind 1, 2, 3, and 4 silver cations,
respectively (see Figure 6). The number of Ag+ cations captured
by various oligomers Z1-Z9 is readily accounted for by the
fact that introduction of Ag+ cations induces folding of the
oligomers containing even number of fluorene moieties to
produce 1, 2, 3, 4, and 5 deltaphane-like cavities in Z1, Z3,
Z5, Z7, and Z9, respectively, whereas oligomers (Z2, Z4, Z6,
Supporting Information Available: Synthetic details, 1H/13C
NMR data for various intermediates, 1H NMR spectral titration
data with silver cation, and X-ray structural data (PDF, CFIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA0687522
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J. AM. CHEM. SOC. VOL. 129, NO. 27, 2007 8465