Suitanes

Article abstract of DOI:10.1002/anie.200602173

Molecules in suits: Supramolecular assistance leads to the self-assembly of a mechanically interlocked compound called a suit[2]ane (see picture). Two bisformyldipyrido[24]crown-8 rings (red) recognize and encircle two secondary ammonium ion centers on a

Full text of DOI:10.1002/anie.200602173

Angewandte
Chemie
Supramolecular Chemistry
DOI: 10.1002/anie.200602173
Suitanes**
Avril R. Williams, Brian H. Northrop, Theresa Chang,
J. Fraser Stoddart,* Andrew J. P. White, and
David J. Williams*
The making of mechanically interlocked molecular com-
pounds[1] has evolved from a statistical beginning[2] to the use
of molecular-recognition-driven, self-assembly protocols[3]
that rely upon the operation of covalent,[4] metal,[5] or
noncovalent[6] templates,[7] prior to the formation of the
mechanical bond(s) by either kinetically or thermodynami-
cally controlled processes, involving either dative[8] or cova-
lent[9] bond formation. Previously, we have speculated[10]
about the existence of another class of interlocked molecule
beyond the archetypal catenanes[11] and rotaxanes,[12] which
are not all that dissimilar from carceplexes[13] and yet are
distinguishable from them.[14] In searching for some simple
term for this class of mechanically interlocked molecule
(Figure 1), which consists of two separate components—a
body with two or more limbs that are rigid and protrude
outwards, the torso of which carries a close-fitting, all-in-one
suit, such that it encompasses the torso of the body—we have
come up with the term suitane, in which a number is inserted
between “suit” and “ane” to indicate the number of protrud-
ing limbs. Thus, the simplest member of the series would be
suit[2]ane, a linear structure in which the suit surrounds the
body with limbs protruding outwards in opposite directions
(1808; Figure 1a), such that there is no easy way by which the
suit can be removed from the body. Likewise, suit[3]ane
(Figure 1b) and suit[4]ane (Figure 1c) will consist of bodies
with three and four limbs, respectively, that are trigonal (1208)
and square (908) planar and ready to be fitted out with the
appropriate suit containing three and four holes, respectively.
Figure 1. Schematic representations of the approach that is being
pursued in the construction of a new class of mechanically interlocked
molecular compounds known as suitanes. The construction of a) suit-
[2]ane, b) suit[3]ane, and c) suit[4]ane. The “+” signs on the protrud-
ing limbs of the templates (blue) represent the -NH2+- centers on the
linear, triangular, and square templates. The (red) rectangles intro-
duced in each case in step A are crown ethers with [24]crown-8
constitutions. In step A, noncovalent bonding leads to complex
formation, in which the rectangles are positioned to be covalently
linked under thermodynamic control in step B before being kinetically
fixed in step C.
A doll dressed up in an all-in-one suit, or a “onesie”, provides
an analogy with a suit[5]ane (Figure 2).
Let us consider the specific example (Scheme 1) of a
linear body template[15] LBT-H2
carrying two -CH2-
2+
+
NH2 CH2- recognition sites separated from each other by a
bulky midriff[16] such that the double linking up of two
matching [24]crown-8 rings in the shape of (CHO)2-DP24C8
units[17] can be achieved reversibly by forming four imine
bonds with a diaminobenzene spacer, such as p-phenylenedi-
amine (PPD). The virtue of employing reversible imine bond
formation[18] to clothe the LBT-H2 dication with two
2+
(CHO)2-DP24C8 units and two PPD spacers is that errors
can be corrected[19] before the final structure is fixed by
[*] Dr. A. R. Williams, B. H. Northrop, Dr. T. Chang, Prof. J. F. Stoddart
California NanoSystems Institute and
Department of Chemistry and Biochemistry
University of California, Los Angeles
405 Hilgard Avenue, Los Angeles, CA 90095-1569 (USA)
Fax: (+1)310-206-5621
E-mail: stoddart@chem.ucla.edu
Dr. A. J. P. White, Prof. D. J. Williams
Department of Chemistry
Imperial College
South Kensington, London SW72AY (UK)
Fax: (+44)20-7594-5835
E-mail: djw@imperial.ac.uk
[**] This work was supported by the NationalScience Foundation (CHE-
9974928 and CHE-0092036). B.H.N. thanks the ACS Division of
Organic Chemistry for a graduate fellowship, sponsored by the
Nelson J. Leonard ACS DOC Fellowship, sponsored by Organic
Synthesis, Inc.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Figure 2. A doll dressed in an all-in-one suit constitutes an example of
a suit[5]ane.
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Scheme 1. Synthetic protocolfor the construction of suit[2]ane from
1.0 equivalent of LBT-H22+ and 2.0 equivalents each of (CHO)2-
DP24C8 and PPD.
reducing the imine functions to amino groups. Herein, we
report the efficient self-assembly of a suit[2]ane from five
components by using a dynamic covalent synthetic protocol[20]
on the back of experimental and computational investigations
Figure 3. a) Ball-and-stick representation of the solid-state superstruc-
+
+
ꢀ
ture of the 2:1 complex. The N H···N hydrogen bonds have N ···N
+
2+
ꢀ
and H···N distances of 2.97 and 2.07 Š, respectively, and N H···N
of the 1:2 complex formed between the LBT-H2 dication
+
+
ꢀ
angles of 1798. The N H···O hydrogen bonds have N ···O and H···O
distances of 3.05 and 2.16 Š, respectively, and N H···O angles of
1688. b) Space-filling representation of the same superstructure.
and two equivalents of (CHO)2-DP24C8 in solution and in
the solid state. The tetraimine, which is formed when two
equivalents of PPD are added to a 2:1 mixture of (CHO)2-
DP24C8 and LBT-H2·2PF6, is a remarkably stable compound
in its own right in solution, from whence it crystallizes. We
describe the X-ray crystal (super)structure analysis of both
the 1:2 complex and the suit[2]ane.
+
ꢀ
Computational force-field modeling[23] predicts the most
stable co-conformation of the [3]pseudorotaxane (Figure 4a)
in solution to be that in which the macrocycle pyridine rings
As a model for the dynamic assembly of a suit[2]ane, the
self-assembly of LBT-H22+ and DP24C8 has been investigated
in solution, in the solid state, and by computational methods.
When stoichiometric quantities of DP24C8 were added to
LBT-H2·2PF6 in CD3NO2 both the 1:1 and 2:1 (macrocycle/
are involved in p–p stacking interactions with both of the
2+
LBT-H2
anthracene moieties in a manner similar to
molecular clips[24] or molecular tweezers.[25] In the structure
obtained from modeling, the separation between the 4-
positions of the DP24C8 rings is 8.8 Š, a full 5.7 Š closer
than in the solid-state superstructure. Such proximity is
ideally suited to form a suit[2]ane upon functionalizing the 4-
positions of each pyridine ring with formyl groups and adding
two equivalents of PPD. The lowest energy co-conformer
obtained from modeling of the fully assembled suit[2]ane
(Figure 4b) bears an almost uncanny structural resemblance
to the modeled [3]pseudorotaxane. In the case of the fully
assembled suit[2]ane, the 4-positions of the DP24C8 rings are
separated by 10.1 Š, only 1.3 Š further apart than in the
modeled 2:1 precursor complex. This observation is a strong
indication that the noncovalent interactions responsible for
the self-assembly of the LBT-H2·2PF6 and the DP24C8
macrocycles will be preserved in the dynamically assembled
suit[2]ane. It may also be anticipated that the p-phenylene
ring of PPD can impart greater stability to the interlocked
suit[2]ane through p–p stacking interactions with the central
p-phenylene unit of LBT-H22+. By comparing the lowest
energy structures obtained for DP24C8, LBT-H22+, the
[3]pseudorotaxane, the suit[2]ane, and the tetraimine host of
suit[2]ane, a semiquantitative evaluation of the binding
energies between the host/guest pairs in both the [3]pseudo-
1
thread) complexes were observed by H NMR spectroscopic
analysis. Single crystals, suitable for X-ray analysis, of the
2+
[3]pseudorotaxane formed between LBT-H2 and 2.0 equiv-
alents of DP24C8 were grown by vapor diffusion of iPr2O into
MeCN. Its solid-state superstructure[21] reveals (Figure 3) the
anticipated threading of the dicationic spindle through the
centers of the two DP24C8 rings. Complex stabilization is
+
+
ꢀ
ꢀ
achieved by pairs of N H···N and N H···O hydrogen bonds,
as illustrated in Figure 3a. The 2:1 complex has crystallo-
graphic inversion symmetry, and the planes of the terminal
3,5-difluorobenzyl and central phenylene rings are oriented
essentially orthogonally to the pair of anthracene ring systems
aligned in parallel. The separations of the ring centroids of the
two 3,5-difluorobenzyl groups are 22.8 Š, and the distance
+
between the two NH2 centers is 16.0 Š. With a view to
constructing the desired macropolycycle in which two
DP24C8 rings are doubly linked through the 4-positions of
their pyridyl rings, thus entrapping the central dication, the
separations of these 4-positions are important in the context
of identifying suitable linking spacer groups. In this 2:1
[22]
complex, these distances are approximately 14.5 Š
.
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Figure 5. Partial 1H NMR spectra recorded at 500 MHz in CD3NO2
showing a) free (CHO)2-DP24C8, b) free LBT-H2·2PF6, c) [3]pseudo-
rotaxane, and d) suit[2]ane after equilibration. For the proton (H)
assignments, see Scheme 1.
Figure 4. a) Profile representation of the lowest energy co-conforma-
tion of the model 2:1 complex obtained from computational modeling.
The solution-phase modeling predicts that p–p stacking interactions
between the DP24C8 and anthracene moieties may, upon functionali-
zation of the DP24C8 rings with formylgroups, faciiltate the formation
of the fully assembled suit[2]ane on the addition of PPD. b) The lowest
energy co-conformation of the fully assembled suit[2]ane obtained
from modeling bears a striking resemblance to the modeled 2:1
complex.
DP24C8 shifts from being a singlet centered at d = 4.7 ppm
when uncomplexed to two doublets (d = 4.6and 4.2 ppm) as a
result of their diastereotopic relationship when complexed.
Furthermore, the anthracene protons HI, which point
“inward” toward the central p-phenylene unit of LBT-H2 ,
rotaxane and the suit[2]ane can be made.[26] Such a compar-
2+
2+
ison leads to the conclusion that the binding of LBT-H2 by
undergo a downfield shift of Dd = 0.4 ppm upon complex-
ation (d = 8.4–8.0 ppm). This observation suggests that p–
p stacking occurs between the (CHO)2-DP24C8 pyridine
rings and the anthracene units of LBT-H22+, similar to that
predicted by computational modeling (Figure 4a).
the tetraimine host is 1.1 kcalmolꢀ1 more favorable than
2+
binding of LBT-H2 by two equivalents of DP24C8. Such a
small energy difference is, however, well within the error of
these force-field calculations, and the two binding energies of
the two compounds should, therefore, be taken to be roughly
identical. These modeling results indicate, at the very least,
that formation of the suit[2]ane and concomitant co-con-
formational change in the [3]pseudorotaxane host/guest
The addition of 2 equivalents of PPD to the [3]pseudo-
rotaxane formed from a solution of LBT-H2·2PF6 and
(CHO)2-DP24C8 (1:2) in CD3NO2 results, initially, in a
mixture of kinetic products. The 1H NMR spectrum recorded
after 29 days (Figure 5d) of equilibration corresponds to the
formation of one dominant, highly symmetric, thermody-
namic product in solution. No significant spectroscopic
changes were observed after 29 days. The disappearance of
the aldehyde peak at d = 9.8 ppm and appearance of a sharp
imine peak at d = 8.2 ppm, which integrates to give four
protons, strongly indicate the formation of the suit[2]ane. X-
ray-quality single crystals were grown by vapor diffusion of
iPr2O into Me2CO, and the solid-state structure,[21,27] corre-
sponding to suit[2]ane, was obtained (Figure 6a,b). The
similarity between the suit[2]ane structure as predicted by
computational modeling (Figure 4b) and that obtained from
X-ray crystallographic analysis is quite remarkable. The solid-
state structure of the suit[2]ane reveals the desired encapsu-
lation of the central aromatic core, containing the two
anthracene rings and their linking phenylene unit, within
the macropolycycle. In addition to the mechanical interlock-
ing of the two components, secondary stabilization is achieved
complex do not disrupt, and perhaps even enhance, the
2+
binding of LBT-H2
.
With these encouraging predictions to hand, the dipyr-
ido[24]crown-8 derivative (CHO)2-DP24C8 was prepared
2+
and added to a solution of LBT-H2 in CD3NO2 in a 2:1
ratio. Nearly quantitative formation of the corresponding
1
[3]pseudorotaxane is observed (Figure 5) by H NMR spec-
troscopy within 5 minutes of mixing. The effects and effi-
ciency of complexation is reflected in shifts of the signals
corresponding to protons on both the (CHO)2-DP24C8
macrocycles as well as on the LBT-H2·2PF6 once the latter
finds itself threaded through two equivalents of the former.
For example, methylene protons HF and HG, which are
+
adjacent to the CH2NH2 CH2 centers of LBT-H22+, appear
as singlets at d = 4.8 and 5.8 ppm, respectively, in the free
template (Figure 5b). Upon complexation, HF and HG appear
(Figure 5c) as multiplets and shift to d = 5.1 and 5.5 ppm,
respectively, as they are involved in hydrogen-bonding
interactions with the (CHO)2-DP24C8 macrocycles. The
signal for the benzylic methylene protons of (CHO)2-
ꢀ
ꢀ
+
+
ꢀ
ꢀ
ꢀ
ꢀ
by a combination of N H···N, N H···O, C H···O, and C
H···p interactions, as illustrated in Figure 6a. The overall
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minor changes were observed in the relative intensities of
1
some of the signals in the H NMR spectrum when a sample
of the compound was heated at 708C in CD3CN for 30 days,
the chemical shifts d remained essentially unchanged. More-
over, there was no evidence, after the 30 days of heating, for
the presence of free LBT-H22+ dications, or indeed of the free
tetraimine host. These observations demonstrate the integrity
and stability of this class of mechanically interlocked molec-
ular compounds.[28]
Our success in the template-directed synthesis of suit-
[2]ane holds out hope for being able to self-assemble
suit[3]anes (Figure 1b) and suit[4]anes (Figure 1c) around
trigonal and square planar templates, respectively. When this
“two-dimensional” goal is realized then there is no reason
why entering the third dimension with tetrahedral and
octahedral scaffolds should not be contemplated. Supra-
molecular assistance to covalent synthesis, in which the
template is consumed stoichiometrically, can be regarded as
a forerunner to template-directed reactions[29] in which the
template keeps expressing or even copying itself.
Received: June 1, 2006
Published online: September 19, 2006
Keywords: dynamic covalent chemistry · interlocked molecules ·
.
Figure 6. a) Ball-and-stick representation of the solid-state structure of
mechanical bonds · supramolecular chemistry ·
template-directed synthesis
+
ꢀ
a suit[2]ane. The X H···X and H···X hydrogen bonds labeled a–d have
+
+
ꢀ
X ···N, H···X distances [Š] and X H···X angles [8], respectively, of
2.94, 2.13, 150 (a); 2.95, 2.28, 130 (b); 2.86, 2.03, 154 (c); and 3.27,
ꢀ
2.35, 160 (d). The C H···p interactions labeled as e–g have H···p [Š]
distances and C-H···p angles [8], respectively, of 2.70, 132 (e); 2.91,
120 (f); and 2.97, 145 (g). b) Space-filling representation of the solid-
state structure of the suit[2]ane.
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dimensions of the dication are changed little from those
observed in the 2:1 complex with a centroid–centroid
separation of the two terminal 3,5-difluorobenzyl ring systems
of 23.2 Š and of the nitrogen centers of 16.2 Š. This linking of
the 4-positions of the DP24C8 rings by PPD spacers causes
decreases of approximately 4.0 Š in the separations of the two
pairs of DP24C8 rings relative to those observed in the 2:1
complex. The parallel-aligned PPD ring systems have an
interplanar separation of approximately 8.2 Š.
[5] V. Aucagne, K. D. Hanni, D. A. Leigh, P. J. Lusby, D. B. Walker,
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The successful template-directed synthesis of this suit-
[2]ane echoes the painstaking efforts that went into designing
its constituent parts with crucial input from computational
2+
chemistry. On reflection, LBT-H2 is a fairly rigid template
´
USA 2002, 99, 4797 – 4800; e) F. Aricó, J. D. Badjic, S. J. Cantrill,
which becomes trapped as the “guest” inside a relatively
flexible “host” built up of a couple of difunctionalized crown
ethers (CHO)2-DP24C8 that can embrace the -CH2-
A. H. Flood, K. C.-F. Leung, Y. Liu, J. F. Stoddart, Top. Curr.
Chem. 2005, 249, 203 – 259.
[8] D. H. Busch, Top. Curr. Chem. 2005, 249, 1 – 6 5.
+
2+
NH2 CH2- recognition sites at both ends of the LBT-H2
[9] a) A. G. Kolchinski, N. W. Alcock, R. A. Roesner, D. H. Busch,
Chem. Commun. 1998, 1437 – 1438; b) T. J. Kidd, D. A. Leigh,
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R. H. Grubbs, Angew. Chem. 2003, 115, 3403 – 3407; Angew.
Chem. Int. Ed. 2003, 42, 3281 – 3285; d) E. Guidry, S. J. Cantrill,
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Aricó, T. Chang, S. J. Cantrill, S. I. Khan, J. F. Stoddart, Chem.
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Cantrill, J. F. Stoddart, J. Am. Chem. Soc. 2005, 127, 5808 – 5810.
dication. In the final analysis, it is the choice—herein it is
PPD—of the linker that matters: it must not be too short nor
too long so that it dictates a successful outcome for this first
stab (Figure 1a) at making suit[2]anes, which employs
dynamic covalent and noncovalent chemistries.
Before we can claim that the compound we call suit[2]ane
is a molecule, we have to gather evidence that it is truly a
mechanically interlocked molecular compound. Although
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[10] T. Chang, A. M. Heiss, S. J. Cantrill, M. C. T. Fyfe, A. R. Pease,
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2943 – 2946.
[11] A catenane comprises two or more interlocked rings: a) C. A.
Hunter, J. Am. Chem. Soc. 1992, 114, 5303 – 5311; b) W.-Q.
Deng, A. H. Flood, J. F. Stoddart, W. A. Goddard III, J. Am.
Chem. Soc. 2005, 127, 15994 – 15995.
Org. Chem. 2005, 70, 7956– 7962; e) C. D. Pentecost, A. J.
Peters, K. S. Chichak, G. W. V. Cave, S. J. Cantrill, J. F. Stoddart,
Angew. Chem. 2006, 118, 4205 – 4210; Angew. Chem. Int. Ed.
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[21] Crystal data for [(DP24C8)2(LBT-H2)]: [C94H98F4N6O12]-
(PF6)2·2MeCN, Mr = 1951.83, monoclinic, P21/n (no. 14), a =
12.6367(14), b = 26.388(3), c = 15.594(2) Š, b = 111.060(8)8,
V= 4852.6(11) Š3, Z = 2 (Ci symmetry), 1calcd = 1.336gcm ꢀ3
,
[12] A rotaxane is a molecule that comprises a dumbbell-shaped
component on which one or more rings are located because of
the bulkiness of the stoppers of the dumbbell: a) P.-L. Anelli, N.
Spencer, J. F. Stoddart, J. Am. Chem. Soc. 1991, 113, 5131 – 5133;
b) J. W. Choi, A. H. Flood, D. W. Steuerman, S. Nygaard, A. B.
Braunschweig, N. N. P. Moonen, B. W. Laursen, Y. Luo, E.
DeIonno, A. J. Peters, J. O. Jeppesen, K. Xe, J. F. Stoddart, J. R.
Heath, Chem. Eur. J. 2006, 12, 261 – 279; c) D. S. Marlin, D.
Gonzµlez Cabrera, D. A. Leigh, A. M. Z. Slawin, Angew. Chem.
2006, 118, 83 – 89; Angew. Chem. Int. Ed. 2006, 45, 77 – 83.
[13] “Carceplexes are comprised of host and guest components that
cannot separate from one another without the breaking of
covalent bonds. Their existence does not depend upon host–
guest interactions nor on other than gross size complementarity.
Rather their existence depends upon physical envelopment of
guests during shell closures leading to carceplexes.” This quote is
taken from D. J. Cram, J. M. Cram in Container Molecules and
Their Guests (Ed.: J. F. Stoddart), The Royal Society of
Chemistry, Cambridge, 1994, p. 147.
[14] The class of mechanically interlocked molecules we propose to
call suitanes differ from carceplexes in so far as their existence is
established by host–guest interactions during their template
formation. These interactions will normally live on in the
molecules afterwards, provided the recognition sites are not
switched off by some means or another. Herein, the recognition
in the form of noncovalent bonds would have their strength
considerably diminished and impaired by the addition of base.
[15] The synthesis of LBT-H2·2PF6 is reported in the Supporting
Information.
m(CuKa) = 1.222 mmꢀ1, T= 293 K, colorless prisms; 7207 inde-
pendent measured reflections, F2 refinement, R1 = 0.133, wR2 =
0.364, 2730 independent observed reflections (j Fo j > 4s(jFoj),
2qmax = 1208), 659 parameters.
[22] It should be noted that, in the solid-state superstructure, two co-
conformations of relative occupancies 0.65:0.35 were observed
for the DP24C8 rings, and the separations of the carbon atoms in
the 4-positions of the pyridine rings in the two different co-
conformations were 14.3 and 14.6Š, respectively; Figure 3
depicts the major conformer.
[23] Computations were performed using the program Maestro
v 3.0.038 with the AMBER* force-field and GB/SA solvent
model for CHCl3. Full details of computational procedures can
be found in the Supporting Information.
[24] a) J. N. K. Reek, A. H. Priem, H. Engelkamp, A. E. Rowan,
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9956– 9964; b) J. A. A. W. Elemans, M. B. Claase, P. P. M. Aarts,
A. E. Rowan, A. P. H. J. Schenning, R. J. M. Nolte, J. Org. Chem.
1999, 64, 7009 – 7016.
[25] a) C. W. Chen, H. W. Whitlock, J. Am. Chem. Soc. 1978, 100,
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[26] The [3]pseudorotaxane binding energy was taken as the differ-
ence in energy between the fully optimized structures of the
[3]pseudorotaxane and that of the fully separated LBT-H22+ and
two equivalents of (CHO)2-DP24C8. The binding energy of
suit[2]ane was taken as the difference in energy between the
fully optimized structures of the suit[2]ane and those of the fully
[16] The formation of the 2:1 complexes—or [3]pseudotoraxane—is
2+
separated LBT-H2 plus the tetraimine host.
+
ꢀ
ꢀ
assisted principally by N H···O hydrogen bonds and C H···O
[27] Crystal data for suit[2]ane: [C110H106F4N10O12](PF6)2·8Me2CO,
+
ꢀ
ꢀ
interactions between the CH2NH2 CH2 recognition sites on
Mr = 2590.61, monoclinic, P21/n (no. 14), a = 16.3400(3),
2+
the LBT-H2 dication and the matching dipyrido[24]crown-8
b = 21.1680(3),
c = 20.7430(4) Š,
b = 110.5770(6)8,
V=
rings in (CHO)2-DP24C8. For reviews on the use of this
particular recognition motif as a template, see: a) P. T. Glink,
C. Schiavo, J. F. Stoddart, Chem. Commun. 1996, 1438 – 1490;
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6717.0(2) Š3, Z = 2, 1calcd = 1.281 gcmꢀ3, m(MoKa) = 0.123 mmꢀ1
T= 123 K, pale yellow plates; 12156independent measured
,
reflections, F2 refinement, R1 = 0.083, wR2 = 0.232, 7171 inde-
pendent observed absorption-corrected reflections (j Fo j > 4s-
(jFoj), 2qmax = 518), 837 parameters. CCDC-608949 ([(DP24C8)2-
(LBT-H2)]) and -608950 (suit[2]ane) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
´
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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