Tribenzo[27]crown-9: A new ring for dibenzylammonium rods

Article abstract of DOI:10.1021/ol991205j

(formula presented) Dibenzylammonium (DBA⁺) ions thread through the cavity of tribenzo[27]crown-9 (TB27C9) to generate [2]pseudorotaxanes that are stabilized principally by hydrogen-bonding interactions. The stabilities and complexation kinetic

Full text of DOI:10.1021/ol991205j

ORGANIC
LETTERS
2000
Vol. 2, No. 1
61-64
Tribenzo[27]crown-9: A New Ring for
Dibenzylammonium Rods
,†
Stuart J. Cantrill,† Matthew C. T. Fyfe,† Aaron M. Heiss,† J. Fraser Stoddart,*
Andrew J. P. White,‡ and David J. Williams‡
Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles,
405 Hilgard AVenue, Los Angeles, California 90095-1569,
and Chemical Crystallography Laboratory, Department of Chemistry,
Imperial College, South Kensington, London SW7 2AY, U.K.
stoddart@chem.ucla.edu
Received October 30, 1999
ABSTRACT
Dibenzylammonium (DBA+) ions thread through the cavity of tribenzo[27]crown-9 (TB27C9) to generate [2]pseudorotaxanes that are stabilized
principally by hydrogen-bonding interactions. The stabilities and complexation kinetics associated with these pseudorotaxanes depend markedly
on the nature of the substituents situated on the phenyl rings of the DBA+ ions. For example, the complex formed between TB27C9 and the
DBA+ ion bearing electron-withdrawing p-CO Me substituents is stronger than that obtained from TB27C9 and the “parent”, unsubstituted
2
DBA+ ion itself. Furthermore, the “parent” complex equilibrates much more rapidly with its uncomplexed components than do the complexes
generated from TB27C9 and substituted DBA+ ions.
The so-called pseudorotaxanes1 are supramolecular com-
plexes in which one or more rings are pierced by one or
more rodlike components as a result of noncovalent bonding
interactions (Figure 1). They differ fundamentally from their
mechanically interlocked congeners, the rotaxanes,2 in that
† University of California, Los Angeles.
‡ Imperial College, London.
(1) For recent examples, see: (a) Jeon, Y. M.; Whang, D.; Kim, J.; Kim,
K. Chem. Lett. 1996, 503-504. (b) Mirzoian, A.; Kaifer, A. E. Chem. Eur.
J. 1997, 3, 1052-1058. (c) Sleiman, H.; Baxter, P. N. W.; Lehn, J.-M.;
Figure 1. Schematic representation depicting the formation of a
Airola, K.; Rissanen, K. Inorg. Chem. 1997, 36, 4734-4742. (d) Loeb, S.
threaded 1:1 complex (a pseudorotaxane) between two comple-
mentary species wherein the cavity of a suitably sized ring is
skewered by a linear rod.
J.; Wisner, J. A. Angew. Chem., Int. Ed. 1998, 37, 2838-2840. (e) Smith,
A. C.; Macartney, D. H. J. Org. Chem. 1998, 63, 9243-9251. (f) Asakawa,
M.; Ashton, P. R.; Balzani, V.; Boyd, S. E.; Credi, A.; Mattersteig, G.;
Menzer, S.; Montalti, M.; Raymo, F. M.; Ruffilli, C.; Stoddart, J. F.; Venturi,
M.; Williams, D. J. Eur. J. Org. Chem. 1999, 985-994.
(2) (a) Amabilino, D. B.; Stoddart, J. F. Chem. ReV. 1995, 95, 2725-
2828. (b) Gibson, H. W. In Large Ring Molecules; Semlyen, J. A., Ed.;
Wiley: Chichester, 1996; pp 191-262. (c) Ja¨ger, R.; Vo¨gtle, F. Angew.
Chem., Int. Ed. Engl. 1997, 36, 930-944. (d) Chambron, J.-C.; Sauvage,
J.-P. Chem. Eur. J. 1998, 4, 1362-1366. (e) Leigh, D. A.; Murphy, A.
Chem. Ind. (London) 1999, 178-183. (f) Hubin, T. J.; Kolchinski, A. G.;
Vance, A. L.; Busch, D. H. AdV. Supramol. Chem. 1999, 5, 237-357.
the termini of their rod component(s) do not possess bulky
stopper groups that prevent dissociation of the ring(s).
Pseudorotaxanes and rotaxanes have both been incorporated
into simple molecular-sized machines3 since, in some
instances, the relative positions of their constituent parts can
10.1021/ol991205j CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/14/1999

be altered by external stimuli. We have recently developed
a route to the noncovalent synthesis4 of multicomponent
pseudorotaxanes that relies5 on the interpenetration of crown
ethers, such as dibenzo[24]crown-8 (DB24C8)6 and dibenzo-
[30]crown-10 (DB30C10),7 by dialkylammonium ions, like
been reported6,10 recently. Condensation of commercially
available 3,5-dimethoxybenzaldehyde and 3,5-dimethoxy-
benzylamine, followed by sodium borohydride reduction,
protonation, and counterion exchange, afforded the tet-
ramethoxy-substituted dibenzylammonium salt 3‚PF6 in a
75% overall yield.
The ability of TB27C9 to act as a receptor for DBA+ ions
was first investigated by 1H NMR spectroscopy. Each DBA‚
PF6 salt was dissolved with an equimolar quantity of
TB27C9 in CDCl3/CD3CN (3:1). In the case of 1+, the rate
of exchange of the 1:1 complex with its free components
1
was fast relative to the H NMR time scale, resulting in a
spectrum consisting of time-averaged signals. The stability
constant (Ka) for the [TB27C9‚1][PF6] complex was found
the dibenzylammonium (DBA+) cation, principally as a
consequence of [N+-H‚‚‚O] and [C-H‚‚‚O] hydrogen
bonds. Here, we report, for the first time, that the crown
ether tribenzo[27]crown-9 (TB27C9),8 which bears a mac-
rocyclic cavity that is intermediate in size between those of
DB24C8 and DB30C10, is pierced (Scheme 1) by DBA+
1
to be 270 M-1 (at 300 K) from a H NMR dilution11
experiment which used the CH2 protons in 1+ and the Hâ
1
protons in TB27C9 as probes. By contrast, the H NMR
spectrum obtained from a 1:1 mixture of TB27C9 and 2+
contained three distinct sets of signals arising from (i) “free”
TB27C9, (ii) “free” 2+, and (iii) the [2]pseudorotaxane
[TB27C9‚2]+, indicating that the exchange between the 1:1
complex and its “free” components is slow on the 1H NMR
time scale. Accordingly, a Ka value of 710 M-1 could be
calculated for this process (at 300 K) by employing the
single-point method.12 Initially, no signals which indicate the
formation of a [2]pseudorotaxane are evident in the 1H NMR
spectrum (Figure 2) of a 1:1 mixture of TB27C9 and 3‚PF6.
Scheme 1
ions to produce [2]pseudorotaxanes as a result of hydrogen-
bonding interactions. These pseudorotaxanes have been
1
characterized (i) in solution by H NMR spectroscopy, (ii)
in the gas phase by FAB mass spectrometry, and (iii) in the
solid state by X-ray crystallography.
The synthesis of TB27C9 was first reported by Cram9 in
1977, and the hexafluorophosphate salts of 1+ and 2+ have
(3) (a) Balzani, V.; Go´mez-Lo´pez, M.; Stoddart, J. F. Acc. Chem. Res.
1998, 31, 405-414. (b) Sauvage, J.-P. Acc. Chem. Res. 1998, 31, 611-
619. (c) Niemz, A.; Rotello, V. M. Acc. Chem. Res. 1999, 32, 44-52. (d)
Kaifer, A. E. Acc. Chem. Res. 1999, 32, 62-71.
(4) Fyfe, M. C. T.; Stoddart, J. F. Acc. Chem. Res. 1997, 30, 393-401.
(5) (a) Fyfe, M. C. T.; Stoddart, J. F. AdV. Supramol. Chem. 1999, 5,
1-53. (b) Fyfe, M. C. T.; Stoddart, J. F.; Williams, D. J. Struct. Chem.
1999, 10, 243-259.
Figure 2. Partial 1H NMR spectra (400 MHz, 300 K) of an
equimolar mixture of TB27C9 and 3‚PF6 in CDCl3/CD3CN (3:1)
recorded at various time intervals. Initially (t ) 0 h), no signals
corresponding to the 1:1 complex are observed. However, over a
period of weeks, signals arising from the [2]pseudorotaxane
[TB27C9‚3]+ are observed to “grow”, until equilibrium is finally
reached.
(6) For an in depth study of how DB24C8 interacts with DBA+ cations,
see: Ashton, P. R.; Chrystal, E. J. T.; Glink, P. T.; Menzer. S.; Schiavo,
C.; Spencer, N.; Stoddart, J. F.; Tasker. P. A.; White, A. J. P.; Williams,
D. J. Chem. Eur. J. 1996, 2, 709-728.
(7) For a study of the interaction between DB30C10 and substituted
secondary dibenzylammonium ions, see: Ashton, P. R.; Fyfe, M. C. T.;
Schiavo, C.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. Tetrahedron
Lett. 1998, 39, 5455-5458.
(8) Kyba, E. P.; Helgeson, R. C.; Madan, K.; Gokel, G. W.; Tarnowski,
T. L.; Moore, S. S.; Cram, D. J. J. Am. Chem. Soc. 1977, 99, 2564-2571.
However, it soon became apparent that the kinetics associated
with the complexation process are not only slow on the NMR
62
Org. Lett., Vol. 2, No. 1, 2000

time scale but also on the laboratory time scale! Numerous
1H NMR spectra of an equimolar mixture of TB27C9 and
3‚PF6 were recorded at various intervals over a period of
many weekssselected spectra are shown in Figure 2s
revealing that approximately six weeks are required for this
system to reach equilibrium under the conditions of the
experiment. The Ka value for this equilibrium at 300 K was
determined, by the single-point method,12 to be 270 M-1.
Additionally, by using well-known equations,13 the kinetic
and thermodynamic parametersskon, koff, ∆G‡on, and ∆G‡offs
for this process were calculated to be 6.17 × 10-5 M-1s-1,
2.29 × 10-7 s-1, 23.1 kcal mol-1, and 26.4 kcal mol-1,
respectively.
The “gas-phase” behavior of these systems was also
investigated using fast atom bombardment mass spectrometry
(FAB-MS). In the case of both 1+ and 2+, strong signalss
corresponding to the 1:1 complexes formed with TB27C9s
were observed in the mass spectra with m/z values of 739
and 855, respectively. However, mass spectrometric analysis
of a freshly prepared 3:1 CDCl3/CD3CN solution containing
an equimolar mixture of TB27C9 and 3‚PF6 gave rise to a
spectrum in which no signal could be observed for the 1:1
complex. However, after being allowed to equilibrate (1000
h), this sample was subjected to a repeat analysis, resulting
in a spectrum that did contain a strong peak (m/z ) 859)
corresponding to the desired 1:1 complex, thus demonstrating
the very slow kinetics of complexation. This point was
reinforced by performing a “competition” experiment, in
which a solution containing approximately equimolar amounts
of TB27C9, 1‚PF6, 2‚PF6, and 3‚PF6 was subjected to
repeated FAB-MS analysis (Figure 3) over a period of 2
weeks. Initially (t ) 0 d), peaks arising from [TB27C9‚1]+
and [TB27C9‚2]+ were observed, whereas no signal was
present for [TB27C9‚3]+. However, after 6 days, a peak
corresponding to the 1:1 complex formed between TB27C9
and 3+ was noted, and its intensity was found to have
increased when the analysis was repeated after a total of 14
days.
X-ray quality single crystals14 of the [2]pseudorotaxane
[TB27C9‚2][PF6] were obtained upon layering a CH2Cl2/
CH3CN (6:1) solutionscontaining an equimolar mixture of
TB27C9 and 2‚PF6swith hexane. The X-ray analysis shows
the crown ether component to have a distinctly twisted
conformation, with all three catechol units steeply inclined
to each other and with all nine oxygen atoms directed inwards
toward the macroring center. The 1:1 complex is stabilized
by a combination of (i) [N+-H‚‚‚O] hydrogen bonds (a, b,
and c in Figure 4), (ii) π-π stacking (d) between one of the
Figure 4. Solid-state superstructure of the 1:1 complex formed
between TB27C9 and 2+. The hydrogen-bonding geometries are
as follows {[N+‚‚‚O], [H‚‚‚O] Å, [N+-H‚‚‚O] (deg)}: (a) 2.88,
2.06, 155; (b) 3.17, 2.31, 158; (c) 2.99, 2.36, 127. The geometry
of the π-π stacking interaction (d): centroid-centroid separation
3.76 Å, mean interplanar separation 3.49 Å, the rings are inclined
by 5°. The [H‚‚‚π] distance and [C-H‚‚‚π] angle for the [C-H‚‚‚π]
interaction (e) are 2.86 Å and 170°.
catechol rings of the crown ether and one of the phenyl rings
of the cation, and (iii) a [C-H‚‚‚π] interaction (e) between
one of the OCH2 protons of TB27C9 and the phenyl ring of
(9) For Cram’s original procedure, see ref 8. A slightly modified protocol
was employed in our laboratories. For the details, see the Supporting
Information.
(10) Ashton, P. R.; Bartsch, R. A.; Cantrill, S. J.; Hanes, R. E., Jr.;
Hickingbottom, S. K.; Lowe, J. N.; Preece, J. A.; Stoddart, J. F.; Talanov,
V. S.; Wang, Z.-H. Tetrahedron Lett. 1999, 40, 3661-3664.
(11) Connors, K. A. In Binding Constants; John Wiley & Sons: New
York, 1987; pp 21-50.
(12) For leading references on this method, see: Adrian, J. C.; Wilcox,
C. S. J. Am. Chem. Soc. 1991, 113, 678-680.
Figure 3. FAB mass spectra of an equimolar mixture of TB27C9,
1‚PF6, 2‚PF6, and 3‚PF6, analyzed at various time intervals after
the initial mixing. A peak with an m/z value of 859, corresponding
to a 1:1 aggregate of TB27C9 and 3+, is not present in the first
spectrum (t ) 0 d). However, as the solution equilibrates over time,
mass spectrometric analysis reveals the emergence of an intense
peak for this 1:1 complex.
Org. Lett., Vol. 2, No. 1, 2000
63

the cation that also participates in the π-π stacking. The
1:1 complexes pack to form sheets held together by an array
of [C-H‚‚‚π] interactions (f-h in Figure 5).
demonstrated here that the crown ether TB27C9 is an
effective receptor for DBA+ ions. The size of this macro-
cyclesintermediate between that of the well-studied [24]-
crown-8 and [30]crown-10 systemssintroduces yet another
element of control into the self-assembly16 processes associ-
ated with this recognition motif. As in previous examples,17
the strength of the association between these complementary
subunits can be controlled via judicious substitution of the
phenyl rings of the DBA+ ion with either electron-withdraw-
ing or electron-donating moieties. However, our preliminary
investigations have revealed that the kinetics associated with
the TB27C9 macrocycle forming [2]pseudorotaxanes depend
dramatically upon the size and disposition of substituents
appended to the phenyl rings of the DBA+ ion. Indeed, the
rates of complexation/decomplexation are reflected in the
1
nature of the H NMR spectroscopic experiments required
to determine the stability constants for these systems, i.e.,
dilution or single point methodologies over a range of time
scales. The rate of exchange between “free” and “bound”
species is reduced significantly on going from the parent
DBA+ ion (1+) to the p-CO2Me-substituted system (2+), with
the most dramatic slowing down being observed for the 3,5-
di-OMe ion (3+).
Furthermore, the 3-fold symmetry of TB27C9 and the
potential to functionalize each aromatic ring of this crown
ether offer an attractive route to the formation of interpen-
etrating/interlocked three-dimensional networks. The forma-
tion of hydrogen-bonded linear “one-dimensional” tapes18
and two-dimensional sheets19 has been achieved previously,
in the solid state, with carboxyl-substituted DBA+ ions and
DB24C8. The successful union of this paradigm with that
of TB27C9/dialkylammonium ion binding will allow us to
branch out into the next dimension using this reliable and
versatile supramolecular synthon.20
Figure 5. Part of one of the [C-H‚‚‚π] linked sheets of the 1:1
complexes formed between TB27C9 and 2+. The [H‚‚‚π] distances
(Å) and [C-H‚‚‚π] angles (deg) are (f) 2.91, 164; (g) 2.72, 143;
and (h) 2.92, 124.
As part of our ongoing study15 into the nature of the
dialkylammonium ion/crown ether interaction, we have
(13) Ashton, P. R.; Baxter, I.; Fyfe, M. C. T.; Raymo, F. M.; Spencer,
N.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1998,
120, 2297-2307.
Acknowledgment. We thank UCLA for generous finan-
(14) Crystal data for [TB27C9‚2][PF6]:[C48H56NO13][PF6]‚MeCN, M )
1041.0, monoclinic, space group P21/c (No. 14), a ) 14.241(2), b )
16.425(2), and c ) 22.169(2) Å, â ) 93.94(1)°, V ) 5173(1) Å3, Z ) 4,
Fc ) 1.337 g cm-3, µ(CuKR) ) 12.1 cm-1, F(000) ) 2184, T ) 293 K;
clear blocks, 0.23 × 0.20 × 0.13 mm, Siemens P4/PC diffractometer,
graphite-monochromated Cu KR radiation, ω-scans, 7680 independent
reflections. The structure was solved by direct methods, and the full
occupancy non-hydrogen atoms were refined anisotropically. Disorder was
found in the included MeCN solvent molecule, and this was resolved into
two partial occupancy orientations; the non-hydrogen atoms of the major
occupancy orientation were refined anisotropically (those of the minor
occupancy orientation were refined isotropically). The C-H hydrogen atoms
were placed in calculated positions, assigned isotropic thermal parameters,
U(H) ) 1.2Ueq(C) [U(H) ) 1.5Ueq(C-Me)], and allowed to ride on their
parent atoms. The N-H hydrogen atoms were located from a ∆F map and
allowed to refine isotropically subject to an N-H distance constraint (0.90
Å). Refinements were by full matrix least-squares based on F2 to give R1
) 0.087, wR2 ) 0.215 for 3576 independent observed reflections
[|Fo| > 4σ(|Fo|), 2θ e 120°] and 670 parameters. All computations were
carried out using the SHELXTL PC program system.
cial support.
Supporting Information Available: Synthetic protocols
for TB27C9 and 3‚PF6, crystal data for [TB27C9‚2][PF6].
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL991205J
(16) (a) Whitesides G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 154,
1312-1319. (b) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl.
1996, 35, 1154-1196. (c) Sijbesma, R. P.; Meijer, E. W. Curr. Opin. Colloid
Interface Sci. 1999, 4, 24-32.
(17) 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. 2 1998,
2117-2128.
(18) Ashton, P. R.; Fyfe, M. C. T.; Hickingbottom, S. K.; Menzer, S.;
Stoddart, J. F.; White, A. J. P.; Williams, D. J. Chem. Eur. J. 1998, 4,
577-589.
(15) For the most recent examples of how this recognition motif can be
exploited for the syntheses of interlocked molecules under (a) kinetic and
(b) thermodynamic control, respectively see: (a) Rowan, S. J.; Cantrill, S.
J.; Stoddart, J. F. Org. Lett. 1999, 1, 129-132. (b) Cantrill, S. J.; Rowan,
S. J.; Stoddart, J. F. Org. Lett. 1999, 1, 1363-1366.
(19) Ashton, P. R.; Collins, A. N.; Fyfe, M. C. T.; Menzer, S.; Stoddart,
J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 735-739.
(20) Nangia, A.; Desiraju, G. R. Top. Curr. Chem. 1998, 198, 57-95.
64
Org. Lett., Vol. 2, No. 1, 2000