Structural and co-conformational effects of alkyne-derived subunits in charged donor-acceptor [2]catenanes

Article abstract of DOI:10.1021/ja071319n

Four donor-acceptor [2]catenanes with cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) as the π-electron-accepting cyclophane and 1,5-dioxynaphthalene (DNP)-containing macrocyclic polyethers as π-electron donor rings have been synthesized under mild co

Full text of DOI:10.1021/ja071319n

Published on Web 06/09/2007
Structural and Co-conformational Effects of Alkyne-Derived
Subunits in Charged Donor-Acceptor [2]Catenanes
†
†,‡
†
†
Ognjen Sˇ . Miljani c´ , William R. Dichtel, Saeed I. Khan, Shahab Mortezaei,
‡
,†
James R. Heath, and J. Fraser Stoddart*
Contribution from California NanoSystems Institute and Department of Chemistry and
Biochemistry, UniVersity of California, Los Angeles, 405 Hilgard AVenue, Los Angeles,
California 90095, and DiVision of Chemistry and Chemical Engineering, California Institute of
Technology, 1200 East California BouleVard, Pasadena, California 91125
Received February 24, 2007; E-mail: stoddart@chem.ucla.edu
Abstract: Four donor-acceptor [2]catenanes with cyclobis(paraquat-p-phenylene) (CBPQT4+) as the
π-electron-accepting cyclophane and 1,5-dioxynaphthalene (DNP)-containing macrocyclic polyethers as
+
π-electron donor rings have been synthesized under mild conditions, employing Cu -catalyzed Huisgen
2+
1
,3-dipolar cycloaddition and Cu -mediated Eglinton coupling in the final steps of their syntheses. Oligoether
chains carrying terminal alkynes or azides were used as the key structural features in template-directed
cyclizations of [2]pseudorotaxanes to give the [2]catenanes. Both reactions proceed well with precursors
of appropriate oligoether chain lengths but fail when there are only three oxygen atoms in the oligoether
chains between the DNP units and the reactive functional groups. The solid-state structures of the donor-
acceptor [2]catenanes confirm their mechanically interlocked nature, stabilized by [π‚‚‚π], [C-H‚‚‚π], and
[
C-H‚‚‚Ã] interactions, and point to secondary noncovalent contacts between 1,3-butadiyne and 1,2,3-
4+
triazole subunits and one of the bipyridinum units of the CBPQT ring. These contacts are characterized
by the roughly parallel orientation of the inner bipyridinium ring system and the 1,2,3-triazole and 1,3-
butadiyne units, as well as by the short [π‚‚‚π] distances of 3.50 and 3.60 Å, respectively. Variable-
1
temperature H NMR spectroscopy has been used to identify and quantify the barriers to the conformationally
and co-conformationally dynamic processes. The former include the rotations of the phenylene and the
bipyridinium ring systems around their substituent axes, whereas the latter are confined to the circumrotation
4+
of the CBPQT ring around the DNP binding site. The barriers for the three processes were found to be
-
1
successively 14.4, 14.5-17.5, and 13.1-15.8 kcal mol . Within the limitations of the small dataset
investigated, emergent trends in the barrier heights can be recognized: the values decrease with the
increasing size of the π-electron-donating macrocycle and tend to be lower in the sterically less encumbered
series of [2]catenanes containing the 1,3-butadiyne moiety.
and reversible molecular switching.6a,d,i,8 Recently, tristable
donor-acceptor [2]catenanes have been proposed as the color-
Introduction
9
Catenanes, topologically nontrivial molecules possessing two
or more mechanically interlocked rings, have been known for
nearly half a century. The current renaissance in their chemistry,
brought about by the judicious use of molecular recognition
and self-assembly aiding and abetting their template-directed
synthesis,4 has elevated them from exotic (but little used)
synthetic targets to potentially broadly applicable chemical
compounds in the context of artificial molecular machines and
molecular electronic devices. Bi- and multistable [2]catenanes
changing elements of electronic paper and displays.
1
4
We have a long-standing interest in the template-directed
8
,9
synthesis and the potential applications of charged donor-
2
1
0
acceptor catenanes, based on cyclobis(paraquat-p-phenylene)
3
4
+
(
CBPQT in Figure 1) as the π-electron acceptor. Although a
(
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1
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†
University of California.
California Institute of Technology.
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P. Angew. Chem., Int. Ed. 2001, 40, 2382-2426. (k) Grieg, L. M.; Philp,
D. Chem. Soc. ReV. 2001, 30, 287-302. (l) Science 2002, 295, 2400-
2421 (Viewpoint on Supramolecular Chemistry and Self-Assembly). (m)
Collin, J.-P.; Heitz, V.; Sauvage, J.-P. Top. Curr. Chem. 2005, 262, 29-
62. (n) Schalley, C. A.; Weilandt, T.; Br u¨ ggemann, J.; V o¨ gtle, F. Top. Curr.
Chem. 2004, 248, 141-200. (o) Sauvage, J.-P., Dietrich-Buchecker, C.,
Eds. Molecular Catenanes, Rotaxanes and Knots; Wiley-VCH: Weinheim,
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J. AM. CHEM. SOC. 2007, 129, 8236-8246
10.1021/ja071319n CCC: $37.00 © 2007 American Chemical Society

Alkyne-Derived Subunits in Charged D−A [2]Catenanes
A R T I C L E S
cyclization of the acceptor or (ii) threading of the donor through
the preformed acceptor ring and its subsequent cyclization
(Figure 2a, b). A third approach, that of tandem heterocatenation
(
Figure 2c), can be proposedsand has indeed been realized in
16
both neutral and charged donor-acceptor catenanes sin which
the formation of the donor and the acceptor rings happen
concurrently. While undoubtedly elegant, tandem heterocatena-
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c‚4PF6 and 2a-c‚4PF6, and the precursors (3, 4‚2PF6, and 5‚2PF6‚Br) to
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1
1
number of interesting π-donors has been demonstrated to bind
4+
to CBPQT , in the past catenanes have been constructed
(
9) (a) Ikeda, T.; Saha, S.; Aprahamian, I.; Leung, K. C.-F.; Williams, A.;
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1
2
13
primarily with hydroquinone, resorcinol, dihydroxynaphtha-
1
4
15
lene (DNP), and tetrathiafulvalene derivatives as electron-
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could proceed through either (i) threading of the acceptor
component through the preformed donor ring, followed by the
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J. AM. CHEM. SOC.
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A R T I C L E S
Miljani c´ et al.
nistically. Since it has not been strictly proven that both rings
form simultaneously during the course of this reaction, it is
possible that tandem heterocatenation still proceeds via a
cyclization-templation-catenation cascade, but with the dif-
ference that either the donor or the acceptor can take on the
role of the template with the outcome being identical.
In our previous syntheses of CBPQT -based [2]catenanes,
the donor ring components were used exclusively as templates.
Typically, 1,4-bis(bromomethyl)benzene (3 in Figure 1) and
While these disadvantages restrict the synthetic options, they
certainly do not eliminate them entirely. Hence, reactions have
been sought after that employ neutral (or mildly acidic),
oxidative, and nucleophile-free conditions. Both the click
1
8
19
reaction and Eglinton coupling were selected as promising
20
candidates, since they proceed at roughly neutral pH, are
4+
21
essentially nucleophile-free, and in the case of Eglinton
coupling, proceed under oxidative conditions.
16c
In a preliminary communication we have demonstrated the
feasibility of cyclizing [2]pseudorotaxanes based on the CB-
1,1′-[1,4-phenylene bis(methylene)]-bis-4,4′-bipyridinium hexaflu-
4+
orophosphate (4‚2PF6 in Figure 1) cyclize to become a CB-
PQT4 ring, encircled around a suitable macrocyclic donor
template (Figure 2d,e). The intermediacy of a [2]pseudorotaxane
that is formed from the donor macrocycle and the partially
cyclized tricationic acceptor thread (5‚2PF6‚Br in Figure 1) has
been confirmed experimentally,17 although this species is
normally not isolated. The alternative route, wherein the acceptor
ring templates the cyclization of the donor macrocycle, has not
yet been explored to our knowledge. The reason is probably
the chemical sensitivity of CBPQT‚4PF6. Its stability has limited
significantly the synthetic arsenal available for the closure of
the donor macrocycle around the CBPQT4 ring to give a
donor-acceptor [2]catenane.
PQT π-accepting ring and macrocyclic polyethers incorporat-
+
ing DNP as the π-donors into the[2]catenanes 1b‚4PF and
6
+
2b‚4PF6 (Figure 1) using the Cu -catalyzed Huisgen 1,3-
cycloaddition between an alkyne and an azide (“click” reaction)
2
+
and the Cu -mediated Eglinton coupling, respectively. This
paper presents an examination of the scope and limitations of
the revised protocols for the synthesis of catenanes 1b,c‚4PF6
and 2b,c‚4PF and the attempted synthesis of [2]catenanes 1a‚
6
4PF and 2a‚4PF . An X-ray crystal structure of the envisioned
6
6
[2]pseudorotaxane precursor to 2a‚4PF reveals a possible reason
6
4
for the failure of the template-directed synthesis in its case.
+
Crystallographically characterized catenanes 1b‚4PF and 2b,c‚
6
4
+
4PF parallel analogous DNP/CBPQT -based systems in their
6
interlocked nature and the alignment of the donor and acceptor
units but also indicate additional interactions between the 1,2,3-
triazole (1b‚4PF6) and 1,3-butadiyne (2b,c‚4PF6) moieties and
4
+
the inner bipyridinium unit of the CBPQT ring. Finally,
1
dynamic H NMR spectroscopic investigations show the de-
pendence of conformational and co-conformational freedom on
the size and nature of the [2]catenanes and allow us to identify
and quantify the barriers to several dynamic processes occurring
in the new catenated compounds.
Syntheses
The synthetic objectives of the present investigation were,
22
in part, inspired by the demonstrated viability of the synthesis
+
of [2]rotaxanes of different types through the use of the Cu -
2
3
catalyzed Huisgen 1,3-dipolar cycloaddition of azides and
(
18) (a) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew.
Chem., Int. Ed. 2002, 41, 2596-2599. (b) Kolb, H. C.; Finn, M. G.;
Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004-2021. (c) Tornoe,
C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057-3064.
Figure 2. Alternative strategies in the template-directed synthesis of charged
donor-acceptor [2]catenanes containing π-electron-rich crown ethers
incorporating dioxybenzene and dioxynaphthalene units (red) and the
π-electron-deficient tetracationic cyclophane, cyclobis(paraquat-p-phenylene)
(19) For a review of Glaser, Eglinton, and related alkyne couplings, see: (a)
Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000,
39, 2632-2657. For the original report, see: (b) Eglinton, G.; Galbraith,
A. R. Chem. Ind. (London) 1956, 737-738.
4
+
(
CBPQT , blue). Alternative heterocatenation strategies are (i) threading
4+
(
a) of the CBPQT ring by an acyclic precursor of the crown ether, followed
by clipping (b) to form the crown ether itself; (ii) all-in-one tandem
heterocatenation approach (c), and (iii) macrocyclization (d) to give the
(
20) It is worth noting that the vast majority of reported Eglinton’s couplings
uses basic amines as solvents. In our work, we adapted the following two
protocols which report successful reactions in MeCN and DMF, respec-
tively. See: (a) Meissner, U.; Meissner, B.; Staab, H. A. Angew. Chem.,
Int. Ed. Engl. 1973, 12, 916-918. (b) Berscheid, R.; V o¨ gtle, F. Synthesis,
4
+
crown ether, followed by clipping (e) to form the CBPQT ring.
The constitution of CBPQT4+ points to the reaction condi-
1
992, 58-62.
21) Typical reaction conditions for the click reaction involve CuSO
ascorbic acid as catalysts, while Eglinton coupling uses Cu(OAc)
tions being a delicate balance between certain extremes. As a
tetracation, the cyclophane is sensitive to reducing agents. The
strain in the cyclophane facilitates nucleophilic attack at the
benzylic positions, displacing the positively charged bipyri-
dinium unit. Finally, basic conditions appear to be equally
harmful, presumably (but not certainly) because deprotonation
of the benzylic methylene group leads to a number of rearranged
and ring-opened products.
(
4
‚5H
‚H
dianion is virtually nonnucleophilic, while the AcO- anion has
2
O/
2
2
O.
2
-
The SO
4
weak nucleophilic character which did not present a significant problem
in our experiments.
(
22) (a) Dichtel, W. R.; Miljani c´ , O. Sˇ .; Spruell, J. M.; Heath, J. R.; Stoddart,
J. F. J. Am. Chem. Soc. 2006, 128, 10388-10390. (b) Aucagne, V.; H a¨ nni,
K. D.; Leigh, D. A.; Lusby, P. J.; Walker, D. B. J. Am. Chem. Soc. 2006,
1
28, 2186-2187. (c) Mobian, P.; Collin, J.-P.; Sauvage, J.-P. Tetrahedron
Lett. 2006, 47, 4907-4909. (d) Aprahamian, I.; Dichtel, W. R.; Ikeda, T.;
Heath, J. R.; Stoddart, J. F. Org. Lett. 2007, 9, 1287-1290. (e) Braunsch-
weig, A. B.; Dichtel, W. R.; Miljani c´ , O. Sˇ .; Olson, M. A.; Spruell, J. M.,
Khan, S. I.; Heath, J. R.; Stoddart, J. F. Chem. Asian J. 2007, 2, 634-647.
(
17) (a) D’Acerno, C.; Doddi, G.; Ercolani, G.; Mencarelli, P. Chem.-Eur. J.
(23) (a) Huisgen, R. Pure. Appl. Chem. 1989, 61, 613-628. (b) Huisgen, R.;
Szeimies, G.; M o¨ bius, L. Chem. Ber. 1967, 100, 2494-2507. (c) Lwowski,
W. In 1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: New
York, 1984; Vol. 1, Chapter 5. (d) Bastide, J.; Hamelin, J.; Texier, F.;
Ven, V. Q. Bull. Chim. Soc. Fr. 1973, 2555-2579. (e) Bastide, J.; Hamelin,
J.; Texier, F.; Ven, V. Q. Bull. Chim. Soc. Fr. 1973, 2871-2887.
2
000, 6, 3540-3546. (b) Capobianchi, S.; Doddi, G.; Ercolani, G.;
Mencarelli, P. J. Org. Chem. 1998, 63, 8088-8089. (c) Capobianchi, S.;
Doddi, G.; Ercolani, G.; Keyes, J. W.; Mencarelli, P. J. Org. Chem. 1997,
6
2, 7015-7017. (d) Brown, C. L.; Philp, D.; Spencer, N.; Stoddart, J. F.
Isr. J. Chem. 1992, 32, 61-67.
8
238 J. AM. CHEM. SOC. VOL. 129, NO. 26, 2007
9

Alkyne-Derived Subunits in Charged D−A [2]Catenanes
A R T I C L E S
Scheme 1. Synthesis of Precursors to π-Donating Rings in Catenanes 1a-c‚4PF6 and 2a-c‚4PF6
alkynes (“click” reaction ) by our and other22b,c groups. The
tolerance of complex formation under the conditions of the click
reaction is noteworthy, and this mild reaction has found use in
1
8
22a
based on donor-acceptor (Sanders, Gunter), transition metal-
27
2
8
29
ligand (Sauvage), and transient covalent bond (Godt) tem-
plating strategies.
18,24
the construction of many diverse systems.
More specifically,
Preparation of suitable DNP-based precursors for the click
reaction commences (Scheme 1) with 1,5-oligo(ethyleneglycol)-
DNP derivatives 6a-c as the starting materials.30 These
materials can be readily converted into the corresponding
monotosylates which are then subjected to nucleophilic
substitutions with sodium azide to produce the azides 7a-c.
Subsequent deprotonations of the free hydroxyl groups were
followed by alkylation with propargyl bromide to yield the
azidoalkynes 8a-c. Exposure of 8a-c to CBPQT‚4PF6 in either
DMF or MeCN gave (Scheme 2) intensely purple-colored
4
+
22
our work on DNP/CBPQT -based [2]pseudorotaxanes showed
that these 1:1 complexes can be stoppered efficiently by click
reactions between an azide-terminated DNP rod and alkyne-
functionalized bulky stoppers. On a fundamental level, this result
demonstrates that copper acetylides (as the proposed mechanistic
3
1
25
intermediates in the click reaction ) are compatible both with
the [2]pseudorotaxane componentssespecially the sensitive
4
+
CBPQT ringsand the complexation which occurs between
them. This realization readily translated into the challenge to
synthesize [2]catenanes and prompted our investigations of the
utility of the Eglinton oxidative alkyne homocoupling, another
reaction proceeding through intermediate copper acetylides. The
mechanistically related Glaser’s alkyne homocoupling has been
employed previously26 in the syntheses of [2]- and [3]catenanes
19
Scheme 2
(
24) (a) Hawker, C. J.; Wooley, K. L. Science 2005, 309, 1200-1205. (b)
Goodall, G. W.; Hayes, W. Chem. Soc. ReV. 2006, 35, 280-312. (c) Diaz,
D. D.; Punna, S.; Holzer, P.; McPherson, A. K.; Sharpless, K. B; Fokin,
V. V.; Finn, M. G. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 4392-
4
403. (d) Laurent, B. A.; Grayson, S. M. J. Am. Chem. Soc. 2006, 128,
238-4239. (e) Riva, R.; Schmeits, P.; Stoffelbach, F.; Jerome, C.; Jerome,
4
R.; Lecomte, P. Chem. Commun. 2005, 5334-5336. (f) Opsteen, J. A.;
van Hest, J. C. M. Chem. Commun. 2005, 57-59. (g) Lutz, J. F.; Borner,
H. G.; Weichenhan, K. Macromol. Rapid Commun. 2005, 26, 514-518.
(h) Ossipov, D. A.; Hilborn, J. Macromolecules 2006, 39, 2113-2120. (i)
solutions, indicative of the formation of the pseudorotaxanes
Wu, P.; Feldman, A. K.; Nugent, A. K.; Hawker, C. J.; Scheel, A.; Voit,
B.; Pyun, J.; Fr e´ chet, J. M. J.; Sharpless, K. B.; Fokin, V. V. Angew. Chem.,
Int. Ed. 2004, 43, 3928-3932. (j) Wu, P.; Malkoch, M.; Hunt, J. N.;
Vestberg, R.; Kaltgrad, E.; Finn, M. G.; Fokin, V. V.; Sharpless, K. B;
Hawker, C. J. Chem. Commun. 2005, 5775-5777. (k) Malkoch, M.;
Schleicher, K.; Drockenmuller, E.; Hawker, C. J.; Russel, T. P.; Wu, P.;
Fokin, V. V. Macromolecules 2005, 38, 3663-3678. (l) Lee, J. W.; Kim,
B. K.; Kim, H. H.; Han, S. C.; Shin, W. S.; Hin, S. H. Macromolecules
[
8 ⊂ CBPQT]‚4PF6. Subjecting these [2]pseudorotaxanes to
the classical click conditions (CuSO4‚5H2O/ascorbic acid or CuI,
in DMF or MeCN) gave the desired [2]catenanes 1b‚4PF6 and
(27) (a) Hamilton, D. G.; Sanders, J. K. M.; Davies, J. E.; Cl e´ gg, W.; Teat, S.
J. Chem. Commun. 1997, 897-898. (b) Gunter, M. J.; Farquhar, S. M.
Org. Biomol. Chem. 2003, 1, 3450-3457.
2
006, 39, 2418-2422. (m) Lee, J. W.; Kim, J. H.; Kim, B. K.; Shin, W.
S.; Jin, S. H. Tetrahedron 2006, 62, 894-900. (n) Fernandez-Megia, E.;
Correa, J.; Rodriguez-Meizoso, I.; Rigurera, R. Macromolecules 2006, 39,
(28) (a) Dietrich-Buchecker, C. O.; Hemmert, C.; Kh e´ miss, A.-K.; Sauvage,
J.-P. J. Am. Chem. Soc. 1990, 112, 8002-8008. (b) Dietrich-Buchecker,
C. O.; Kh e´ miss, A.; Sauvage, J.-P. J. Chem. Soc., Chem. Commun. 1986,
1376-1378.
2
113-2120. (o) Helms, B.; Mynar, J. L.; Hawker, C. J.; Fr e´ chet, J. M. J.
J. Am. Chem. Soc. 2004, 126, 15020-15021. (p) Mynar, J. L.; Choi, T.
L.; Yoshida, M.; Kim, V.; Hawker, C. J.; Fr e´ chet, J. M. J. Chem. Commun.
2
(29) (a) Godt, A. Eur. J. Org. Chem. 2004, 1639-1645. (b) Duda, S.; Godt, A.
Eur. J. Org. Chem. 2003, 3412-3420. (c) U¨ nsal, O¨ .; Godt, A. Chem.-
Eur. J. 1999, 5, 1728-1733.
005, 5169-5171. (q) Malcoch, M.; Thibault, R. J.; Drockenmuller, E.;
Messerschmidt, M.; Voit, B.; Russel, T. P.; Hawker, C. J. J. Am. Chem.
Soc. 2005, 127, 14942-14949.
(30) For 6a, see: (a) Amabilino, D. B. et al. J. Am. Chem. Soc. 1995, 117,
11142-11170. For 6b and 6c, see: (b) Ashton, P. R.; Huff, J.; Menzer,
S.; Parsons, I. W.; Preece, J. A.; Stoddart, J. F.; Tolley, M. S.; White, A.
J. P.; Williams, D. J. Chem.-Eur. J. 1996, 2, 31-44.
(
25) (a) Bock, V. D.; Hiemstra, H.; van Maarseveen, J. H. Eur. J. Org. Chem.
2
005, 2006, 51-68. (b) Himo, F.; Lovell, T.; Hilgraf, R.; Rostovtsev, V.
V.; Noodleman, L.; Sharpless, K. B.; Fokin, V. V. J. Am. Chem. Soc. 2005,
1
27, 210-216. (c) Rodinov, V. O.; Fokin, V. V.; Finn, M. G. Angew.
(31) The monotosylates of 6a and 6b have been reported previously. See,
respectively: (a) Collier, C. P.; Jeppesen, J. O.; Liu, Y.; Perkins, J.; Wong,
E. W.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc. 2001, 123, 12632-
12641. (b) Liu, Y.; Saha, S.; Vignon, S. A.; Flood, A. H.; Stoddart, J. F.
Synthesis 2005, 19, 3437-3445.
Chem., Int. Ed. 2005, 44, 2210-2215. (d) Nolte, C.; Mayer, P.; Straub, B.
F. Angew. Chem., Int. Ed. 2007, 46, 2101-2103.
26) (a) Glaser, C. Ber. Dtsch. Chem. Ges. 1869, 2, 422-424. (b) Glaser, C.
Ann. Chem. Pharm. 1870, 154, 137-171.
(
J. AM. CHEM. SOC.
9
VOL. 129, NO. 26, 2007 8239

A R T I C L E S
Miljani c´ et al.
1
c‚4PF6 in 41% and 23% yields,16c respectively, after purifica-
crystallographically. Single crystals of these compounds/
complexes suitable for X-ray diffraction were grown by vapor
diffusion of i-Pr2O into solutions of the catenanes/pseudoro-
taxane in MeCN at 23 °C during 3-4 d. Crystal, unit cell,
refinement, and some of the common structural parameters for
the four crystal structures are listed in Table 1, and individual
crystal structures/superstructures are shown in Figures 3-6.
Despite extensive experimentation, no X-ray-quality crystals of
1c‚4PF6 could be obtained.
tion by column chromatography. However, the [2]pseudoro-
taxane [8a ⊂ CBPQT]‚4PF6 failed to react, even after prolonged
reaction times, and only unreacted 8a and CBPQT‚4PF6 could
be recovered, following the dethreading that undoubtedly occurs
during chromatography.
Preparation of the precursors for the Eglinton coupling was
significantly simpler as a consequence of the homocoupling
nature of the reaction that utilizes symmetric starting materials.
Thus, a one-step propargylation of 6a-c (Scheme 1) gave 9a-
c, that is the compounds which can undergo the threading-
followed-by-clipping cascade. Conversion of 9 to the corre-
sponding [2]pseudorotaxanes [9 ⊂ CBPQT]‚4PF6 was followed
by exposure (Scheme 3) of the reaction mixture to 2 equiv of
The pseudorotaxane [9a ⊂ CBPQT]‚4PF6 crystallizes (Figure
3) without solvent incorporation, and the supermolecule has a
crystallographic center of inversion. The interpenetrated super-
structure is stabilized by (a) [π‚‚‚π] stacking between the
naphthalene ring of the DNP unit and the bipyridinum units of
4+
CBPQT ring, (b) [C-H‚‚‚π] interactions between the hydro-
Scheme 3
gen atoms in the 4- and 8-positions of the DNP and the
4
+
phenylene rings of the CBPQT ring, and (c) [C-H‚‚‚Ã]
interactions (see Table 1 for corresponding distances) between
the oxygen atoms in the side chain of 9a and the bipyridinium
4
+
33
R-hydrogen atoms in the CBPQT ring. Two symmetry-
-
related PF6 ions are in close (2.40 Å) contact with the
bipyridinium R-hydrogen atoms in the CBPQT ring through
4
+
the fluorine atoms. Unlike the DNP derivatives whose glycol
3
4
chains were terminated with phenoxy groups, the propargyl-
terminated derivative 9a forms only crystals of [9a ⊂ CBPQT]‚
Cu(OAc)2‚H2O in MeCN. After 2 d at 23 °C, the [2]catenanes,
4PF6 on mixing with CBPQT‚4PF6, regardless of the ratio used
2
b‚4PF6 and 2c‚4PF6, were isolated in 97% and 52% yields,
(varied from 1:10 to 10:1) during the attempted crystallizations.
respectively. The former result is a remarkable improvement
The triple bonds of 9a are positioned over the proximal
over that in our original report1 which utilized classical
6c
4+
pyridinium rings of the CBPQT ring, rather than away from
(
23 °C, 5 d) and microwave-assisted32 (90 °C, 20 min)
them. In the superstructure of [9a ⊂ CBPQT]‚4PF6, the closest
intermolecular contacts are between the triple bonds of adjacent
molecules, with distances between the centroids of the neigh-
conditions to produce 2b‚4PF6 in 14 and 21% yield, respectively.
This outcome can be explained tentatively by a slow nucleo-
4
+
philic attack of the acetate counterion on the CBPQT ring of
the [2]catenane molecule, decreasing the yield of the catenation
as it progresses past its optimal point. As in the earlier click
series of reactions, the [2]pseudorotaxane [9a ⊂ CBPQT]‚4PF6
with the shortest precursor donor template failed to react, and
only starting materials were recovered, albeit not completely,
suggesting that partial polymerization might have occurred. The
structural analysis of [9a ⊂ CBPQT]‚4PF6 reveals a possible
explanation for this unfavorable synthetic outcome and, to some
extent, for the failure of [8a ⊂ CBPQT]‚4PF6 to react. The
X-ray crystal structure (for a detailed discussion of structural
features vide infra) reveals that the -CtC-H units in [9a ⊂
CBPQT]‚4PF6 are pointing away from each other in the solid
state (Figure 3) although this observation may be inconsequential
for the solution-phase reactivity. However, in the hypothetical
conformation of the threaded component in [9a ⊂ CBPQT]‚
35
boring CtC bonds alternating between 5.40 and 4.92 Å.
4+
Figure 3. Solid-state superstructure of [9a ⊂ CBPQT] . Aside from the
4
PF6, in which one of the oligoether chains emanating from
PF - counterions, hydrogen atoms and solvent molecules are omitted for
6
the DNP unit is reflected through the plane of the naphthalene
ring thus bringing the two triple bonds close to each other, the
alkyne hydrogen atoms are still separated by a very long 5.87
Å. The oligoether chains are simply too short to cyclize to give
a polyether loop that would not be unreasonably strained.
clarity. Thermal ellipsoids are shown at 50% probability levels. Acceptor
ring is shown in blue, donor thread in red, and triple bonds in purple.
All three [2]catenanes crystallize in a P 1h space group, with
3
6
a single pair of enantiomeric molecules in the unit cell and
(
33) Asakawa, M. et al. J. Org. Chem. 1997, 62, 26-37.
Structural Investigations
4+
(
34) Depending on the ratio between the CBPQT ring and the diphenoxy-
terminated DNP polyether, crystals of 1:1 and 3:1 complexes could be
The [2]catenanes 1b‚4PF6, 2b‚4PF6, and 2c‚4PF6, as well as
the pseudorotaxane [9a ⊂ CBPQT]‚4PF6, were all characterized
isolated. See: Northrop, B. H.; Khan, S. I.; Stoddart, J. F. Org. Lett. 2006,
8, 2159-2162.
(
6
35) These distances (as well as those in 2b‚4PF ) are comparable to those
observed in supramolecularly organized polymerizable diiodobutadiynes
in the solid state: (a) Sun, A.; Lauher, J. W.; Goroff, N. S. Science 2006,
312, 1030-1034. (b) Goroff, N. S.; Curtis, S. M.; Webb, J. A.; Fowler, F.
W.; Lauher, J. W. Org. Lett. 2005, 7, 1891-1893.
(
32) For reviews of microwave-assisted reactions, see: (a) Kappe, O. C. Angew.
Chem., Int. Ed. 2004, 43, 6250-6284. (b) Lindstrom, P.; Tierney, J.;
Wathey, B.; Westman, J. Tetrahedron Lett. 2001, 57, 9225-9283.
8240 J. AM. CHEM. SOC.
9
VOL. 129, NO. 26, 2007

Alkyne-Derived Subunits in Charged D−A [2]Catenanes
A R T I C L E S
Table 1. Crystal, Unit Cell, Refinement, and Selected Structural Parameters of [9a ⊂ CBPQT]‚4PF6,1b‚4PF6, 2b‚4PF6, and 2c‚4PF6
cmpd
[9a
⊂CBPQT]
‚
4PF
6
1b
‚
4PF
6
2b
‚
4PF
6
2c‚4PF
6
molecular formula
crystal description
C60H60N4O6P4F24
red, rod-shaped
triclinic
C73H83N13O7P4F24
red, platelet
triclinic
C76H84N10O8P4F24
light red, block
triclinic
C80H92N10O10P4F24
red, platelet
triclinic
a ) 10.163(3) Å
b ) 12.669(4) Å
c ) 14.245(5) Å
R ) 99.963(4)°
â ) 107.120(3)°
γ ) 112.522(3)°
a ) 15.304(7) Å
b ) 15.316(7) Å
c ) 21.776(14) Å
R ) 95.059(7)°
â ) 106.489(7)°
γ ) 116.953(5)°
a ) 13.690(2) Å
a ) 14.021(3) Å
b ) 14.121(3) Å
c ) 22.210(5) Å
R ) 94.628(2)°
â ) 93.400(2)°
γ ) 93.554(3)°
b ) 13.848(2) Å
c ) 24.046(4) Å
unit cell parameters
R ) 84.815(2)°
â ) 85.732(2)°
γ ) 67.626(2)°3
3
3
3
V ) 1531.98 Å
V ) 4219.95 Å
V ) 4194.23 Å
V ) 4365.5(17) Å
R-factor (%)
space group
Z
5.95
P1h
1
7.62
P1h
2
7.63
P1h
2
3.23
3.29
0.2
8.45
P1h
2
3.24
3.26
1.2
DNP/CBPQT [π‚‚‚π] distance (Å)a
4
+
3.31
3.24
3
.34
1.6
.4
2.55
.62
DNP/CBPQT4 interplanar angle (deg)
+
2.1
2
0.4
1.0
[
C-H‚‚‚π] distance (Å)
C-H‚‚‚O] distance (Å)
2.57
2.27
2.47
2.58
2.40
2.29
2.56
2.59
2.38
2.68
2
[
2.37
-
a
Defined as the distance between the average plane of the bipyridinium subunit and the centroid of the DNP ring’s fused C-C bond.
six solvent (MeCN) molecules, some of which are disordered.
Disorder is also present in some of the PF6- counterions and,
to a smaller extent, in the π-donor ring in 2c‚4PF6 (vide infra).
pyridinium rings resembles closely the case of 1b‚4PF6, with
an interplanar angle of 2.3° and a distance of 3.62 Å between
the triazole centroid and the pyridinium plane. Alternating
4
+
4+
Apparently, in all three [2]catenanes, the DNP/CBPQT
donor-acceptor stacks, observed in some of CBPQT -based
4+
12a,39
interaction is the dominant [π‚‚‚π] one, since the CBPQT ring
encircles the DNP ring in preference to the triazole rings or
butadiyne units. The stabilizing interactions noted in [9a ⊂
CBPQT]‚4PF6 are only slightly perturbed in 1b‚4PF6 and 2b‚
catenanes,
are notably absent in the supramolecular stacks
of 1b‚4PF6. Instead, [2]catenane molecules pack around the
crystallographic inversion center and are separated by two
-
sandwiched and ordered PF6 counterions and four MeCN
-
4
PF6, a situation which is most likely caused by the desymme-
molecules. The sandwiched PF6 ions were completely refined
4+
trization of the DNP/CBPQT recognition motif and by the
more constricted geometry of the cyclic π-donor.37 Intriguingly,
the newly incorporated 1,2,3-triazole (1b‚4PF6) and 1,3-
butadiyne (2b,c‚4PF6) moieties align themselves with the outside
and showed close F‚‚‚H contacts to both the π-donor (glycolic
4+
chain CH2, 2.47 Å) and π-acceptor (R-CBPQT , 2.35 Å)
hydrogen atoms.
4
+
face of one of the bipyridinium units of the CBPQT ring,
forming almost evenly spaced (outer)bipyridinium-DNP-
(inner)bipyridinium-triazole/butadiyne layers.
In 1b‚4PF6 (Figure 4), the distance between the centroid of
the triazole ring and the average plane of the inner bipyridinium
unit is 3.38 Å, very close to that of the DNP-bipyridinium
separation. Additionally, the two planes are nearly parallel (7.3°)
with the triazole ring being slightly offset relative to one of the
two pyridinium rings. Both of these effects can be interpreted
as caused by the [π‚‚‚π] stacking of the two units. The
Cambridge Structural Database (CSD) reports three structures
that include formally both the pyridinium ring and the 1,2,3-
triazole unit, the former being incorporated into an N-pyridinium
3
8
oxide unit, the latter into a benztriazole system. In one of
them,3 intermolecular stacking between the triazole and
8a
(
36) The ∼45° tilt between the average planes of the π-donor and the π-acceptor
Figure 4. Solid-state structure of 1b4+. Aside from the disordered PF6-
counterions, hydrogen atoms and solvent molecules are omitted for clarity.
Thermal ellipsoids are shown at 50% probability levels. Acceptor ring is
shown in blue, donor thread in red, and triazole ring in purple.
6 6
rings in the catenanes 1b,c‚4PF and 2b,c‚4PF
generates an element of
helical chirality, such that molecules with (P) and (M) helicities can be
identified. Additionally, 1,5-dioxynaphthalene and 1,2,3-triazole rings lose
their symmetry elements once incorporated into a macrocycle. This
desymmetrization imparts planar chirality of either a (pR) or a (pS) sense
to both rings. Overall, these considerations mean that 1b,c‚4PF
6
can exist
3
Catenanes 2b,c‚4PF6 align their butadiyne fragments ap-
proximately parallel with the electron-poor inner bipyridinium
unit, in sharp contrast to the nearly perpendicular orientation
observed in DNP-phthalimide-based butadiyne-containing
as a total of 2 ) 8 diastereisomers, while 2b,c‚4PF
6
could exist as a total
1
of four diastereoisomers. X-ray crystallography, and H NMR spectroscopy
reveal evidence for the existence of only one enantiomeric pair in the solid
and solution states, respectively.
(
6
37) One of the [C-H‚‚‚O] interactions is notably lost in 1b‚4PF (Table 1)
because of the geometry of the glycolic chain in the donor.
(
38) (a) Katritzky, A. R.; Kurz, T.; Zhang, S.; Voronkov, M.; Steel, P. J.
Heterocycles 2001, 55, 1703-1710. (b) Savrda, J.; Ziane, N.; Guihem, J.;
Wakselman, M. J. Chem. Res. 1991, 2, 36-37.
(39) Ortholand, J.-Y.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Williams,
D. J. Angew. Chem., Int. Ed. Engl. 1989, 28, 1394-1395.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 26, 2007 8241

A R T I C L E S
Miljani c´ et al.
donor-acceptor catenanes reported by Sanders and co-
workers.1
6a-b,27a,40
In 2b‚4PF6 (Figure 5), the average distance
between the plane of the inner bipyridinium unit and the atoms
of the CtC-CtC axis is 3.50 ((0.07) Å. The bipyridinium
+
+
N -N vector is roughly coplanar with the CtC-CtC vector,
with a torsional angle of 0.50°. Worthy of note is the virtually
unstrained H2C-CtC-CtC-CH2 unit, with an overall de-
formation of a mere 9.8° spread over six atoms.41 Although the
CSD does not report any structures with both the pyridinium
and butadiyne fragments, analogous interactions between electron-
poor arenes and butadiyne-containing systems are known.
Hexafluorobenzene, for example, stacks efficiently with one of
the butadiyne subunits of dehydrobenz[12]annulene4 with an
average distance between the aromatic plane and the atoms of
the CtC-CtC chain of 3.34 ((0.06) Å. As in the case of
2
1
b‚4PF6, no donor-acceptor stacks have been observed. Curi-
Figure 6. Solid-state structure of 2c4+. Aside from the disordered PF6-
counterions, hydrogen atoms and solvent molecules are omitted for clarity.
Thermal ellipsoids are shown at 50% probability levels. Acceptor ring is
shown in blue, donor ring in red, and butadiyne moiety in purple. Only
one of the two equivalent conformations of the diyne subunit is shown.
ously, and unlike 1b‚4PF6, the two molecules of 2b‚4PF6 are
not separated by counterions. Instead, CtC-CtC subunits are
the closest point of contact, with a centroid-centroid distance
35
of 4.98 Å and the closest contact between carbon atoms being
3
.73 Å.
Figure 5. Solid-state structure of 2b4+. Aside from the disordered PF6-
counterions, hydrogen atoms and solvent molecules are omitted for clarity.
Thermal ellipsoids are shown at 50% probability levels. Acceptor ring is
shown in blue, donor ring in red, and butadiyne moiety in purple.
The crystal structure (Figure 6) of the larger butadiyne-based
[
2]catenane 2c‚4PF6 is analogous to that of 2b‚4PF6, at least
Figure 7. Comparison of the side views of the solid-state structures of
with regard to the alignment of the 1,3-butadiyne unit with the
inner bipyridinium plane and despite the evident disorder in
the diyne subunit and its immediate surroundings. Two equiva-
lent conformations of the π-donor ring exist, and they are both
in close contact with the bipyridinium unit, with average
distances of 3.39 ((0.02) and 3.66 ((0.21) Å, respectively.
Unlike in the case of 2b‚4PF6, the bipyridinium N -N vector
is displaced at an angle of 26.8° (21.8° in the other conforma-
tion) with CtC-CtC vector. An overlay (Figure 7) of the
4+
4+
2
b
(bottom) and 2c (top). Structures were overlaid to maximize the
4+
-
overlap in their CBPQT rings. Aside from the disordered PF6 counte-
rions, hydrogen atoms and solvent molecules were omitted for clarity.
Flattening of the π-donating ring in 2c4 is evident (especially in the glycol
chains) and necessary to bring the inner bipiridinium subunit of the
π-acceptor into close contact with the butadiyne moiety of the π-donor.
+
+
+
structures of 2b‚4PF6 and 2c‚4PF6 shows visible flattening of
the π-donor ring in the latter structure, acting to align the
butadiyne and the bipyridinium units. Interestingly, the CtC-
CtC units in 2c‚4PF6 deform by 15.8 and 18.2° (depending
on the conformation, overall over six atoms) to accommodate
this close contact. These observations are a clear indication that
the close butadiyne-bipyridinium contacts in 2b,c‚4PF6 stem
from a real interaction, rather than just a coincidental arrange-
ment.
(
40) (a) Hamilton, D. G.; Davies, J. E.; Prodi, L.; Sanders, J. K. M. Chem.-
Eur. J. 1998, 4, 608-620. (b) Zhang, Q.; Hamilton, D. G.; Feeder, N.;
Teat, S. J.; Goodman, J. M.; Sanders, J. K. M. New J. Chem. 1999, 23,
8
97-903.
(
41) Alkyne units have been known to deform readily in strained systems, in
extreme cases up to 52° over the four atoms of the X-CtC-X unit. See,
inter alia: (a) Eisler, S.; McDonald, R.; Loppnow, G. R.; Tykwinski, R.
R. J. Am. Chem. Soc. 2000, 122, 6917-6928. (b) Gleiter, R.; Merger, R.
In Modern Acetylene Chemistry; Stang, P. J., Diederich, F., Eds.; VCH:
Weinheim, Germany, 1995; pp 285-319. (c) De Graaff, R. A. G.; Gorter,
S.; Romers, C.; Wong, H. N. C.; Sondheimer, F. J. Chem. Soc., Perkin
Trans. 2 1981, 478-480. (d) Dosa, P. I.; Whitener, G. D.; Vollhardt, K. P.
C.; Bond, A. D.; Teat, S. J. Org. Lett. 2002, 4, 2075-2078.
NMR Spectroscopic Studies
1
The importance of H NMR spectroscopy in the characteriza-
(42) Bunz, U. H. F.; Enkelmann, V. Chem.-Eur. J. 1999, 5, 263-266.
tion of donor-acceptor catenanes is manifold. Apart from the
8242 J. AM. CHEM. SOC.
9
VOL. 129, NO. 26, 2007

Alkyne-Derived Subunits in Charged D−A [2]Catenanes
A R T I C L E S
confirmation of the constitutions of individual rings and the
existence of mechanical interlocking, variable-temperature (VT)
H NMR spectroscopy probes the co-conformational dynamics
of the [2]catenane in addition to the conformational dynamics
within each of the rings. Installation of the 1,2,3-triazole ring
or 1,3-butadiyne moiety into the skeleton of the donor macro-
cycle in catenanes is expected to affect the conformational and
co-conformational flexibility of 1b,c‚4PF6 and 2b,c‚4PF6 as a
result of the specific electronic and steric effects present in their
component parts.
Low-temperature 1H NMR spectra agree well with these
expectations based on symmetry arguments.
1
Although the conformational flexibility of the individual [2]-
catenane components is encoded into the covalent structure of
each ring, it is strongly influenced by the presence of a
mechanically interlocking ring. On a higher hierarchical levels
that of co-conformationsthe interlocked structure itself is
dynamic, as the two rings undergo a change in their geometrical
relationship. The remainder of this section will be devoted to a
discussion of conformational and co-conformational processes44
(in that order) observed in 1b,c‚4PF and 2b,c‚4PF .
1
H NMR spectroscopy confirms the mechanically interlocked
6
6
nature of all four compounds. The CBPQT4+ ring encircles the
DNP unit exclusively, as reflected by the characteristically
shielded DNP protons, giving rise to peaks at δ 6.29, 5.95, and
2
.37 ppm (average values for all four of the [2]catenanes). See
the Supporting Information for the individual values. The
triazole protons in 1b,c‚4PF6 resonate at δ 7.05 and 7.31 (CD3-
CN) ppm, respectively, as sharp singlets over a wide range of
temperatures. These chemical shifts are notably different from
the δ value of 8.60 ppm in a related [2]rotaxane,2 suggesting
shielding by the bipyridinium rings which lie in close proximity
in the [2]catenanes. As a consequence of the absence of any
protons associated with the butadiyne moiety in 2b,c‚4PF6 and
the inherent insensitivity of the 13C NMR chemical shifts to
ring current effects, NMR spectroscopy is not useful for probing
the environments of the triple bonds within these catenanes.
Catenanes 2b,c‚4PF6 contain the butadiyne unit which com-
mutes with the C2 axis of the DNP/CBPQT4 assembly. In 1b,c‚
2a
Figure 9. Conformationally and co-conformationally dynamic processes
in the DNP-containing [2]catenanes can be interpreted as rotations around
several different axes. The π-acceptor ring is shown in blue, the π-donor
ring in red. Highlighted processes are: the phenylene rotation (axis 1), the
bipyridinium rotation (axis 2), the circumrotation of the π-donor ring
(process 3, also equivalent to the rotation of the donor ring around axis 2),
ring rocking (incomplete rotation around axis 4), and the circumrotation of
the π-acceptor ring (rotation of the acceptor ring around axis 5).
+
4
PF6, this symmetry element is no longer retained on account
43
of the unsymmetrical nature of the triazole ring. With these
symmetry considerations in mind, the H NMR spectra of 1b,c‚
1
4
PF6 and 2b,c‚4PF6 are readily predicted. Assuming the
completely “frozen” co-conformation shown in Figure 8, four
In the free CBPQT4+ ring, the rotations of both the phenylene
and the bypiridinum units around their substitution axes (axes
1
1
and 2, respectively, in Figure 9) are fast on the H NMR time
scale. The situation changes in [2]catenanes where the unique
combination of steric hindrance and intramolecular interactions
slows down these conformational changes sufficiently to make
1
them observable by H NMR spectroscopy. The phenylene
rotation (axis 1 in Figure 9) was followed by using the signals
4
+
of the phenylene protons on the CBPQT ring as probes. The
completely conformationally (and co-conformationally) re-
4
+
stricted representation of the CBPQT /DNP assembly shown
in Figure 8 implies four sets of signals for 2b,c‚4PF6 and eight
for 1b,c‚4PF6. The phenylene protons are influenced both by
Figure 8. The DNP/CBPQT4+ assembly in donor-acceptor [2]catenanes.
4+
the phenylene rotation and the circumrotation of the CBPQT
Hydrogen atoms relevant to the discussion are shown.
ring (vide infra); if unrestricted, either of these two dynamic
processes halves the number of observable signals. Ultimately,
in the situation where all dynamic processes are uninhibited,
peaks would be expected for the R-CBPQT4+ ring protons in
2
b,c‚4PF6. One pair of peaks arises from the inequivalence
1
the number of signals for the phenylene units in the H NMR
4+
between the inside and the outside R-CBPQT protons, while
the other pair is generated (Figure 8) by the heterotopicity of
HR and HR′. The remaining four R-protons are related to the
above-mentioned ones by the C2 axis and thus appear to be
equivalent. By contrast, in the triazole series, a total of eight
different signals would be expected, since the C2 axis is absent.
spectra of 1b,c‚4PF6 and 2b,c‚4PF6 reduces to one and two,
respectively. The bipyridinium rotation (axis 2 in Figure 9) was
followed by observing the signals corresponding to the HR and
4
+
HR′ atoms on the CBPQT ring. Free bipyridinium rotation
makes these two sets of hydrogen atoms symmetry related and
thus equivalentsin turn decreasing the number of bipyridinium
(
43) The unsymmetric substitution of the triazole ring in 1b,c‚4PF
potentially give rise to two diastereisomers, depending on the orientation
of the triazole relative to the pseudo-C axis passing through the DNP and
6
can
(44) For a recent review of dynamics and stereochemistry of the CBPQT4+-
based [2]catenanes, see: (a) Vignon, S. A.; Stoddart, J. F. Collect. Czech.
Chem. Commun. 2005, 70, 1493-1576. See also: (b) Tseng, H.-R.; Vignon,
S. A.; Celestre, P. C.; Stoddart, J. F.; White, A. J. P.; Williams, D. J.
Chem.-Eur. J. 2003, 9, 543-556.
2
the CBPQT4 rings. However, H NMR spectra of both catenanes showed
+
1
4
+
just a single set of CBPQT , DNP, and triazole peaks, consistent with the
presence of only one (albeit unsymmetric) species.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 26, 2007 8243

A R T I C L E S
Miljani c´ et al.
Scheme 4. Two Alternative Modes of Achieving the Formal
expected. The net effect of these two processes is the same,
and they are indistinguishable by dynamic H NMR spectros-
copy.
4+
Rotation of the Bipyridinum Moiety in the CBPQT -Containing
2]Catenanes
1
[
In addition to conformational dynamics, Figure 9 shows three
general aspects of co-conformational flexibility that have been
44
4+
previously observed in CBPQT -based [2]catenanes. The first
one is tantamount to the circumrotation of the π-donor macro-
cycle around the inside bypiridinum unit of the CBPQT4 ring
+
(rotation 3 in Figure 9). The second process is the partial rotation
of the π-donor macrocycle around the axis of the mechanical
bond between the two interlocked rings, in a motion reminiscent
of rocking a cradle (rotation around axis 4 in Figure 9). Finally,
4+
in a third co-conformational change, the CBPQT ring cir-
4
5
cumrotates around the DNP unit of the π-donor macrocycle
otherwise viewed as the rotation around axis 5 in Figure 9).
(
Fundamental limitations prevent the observation of the first two
aforementioned processes. The circumrotation of the π-donor
4
6
macrocycle (Scheme 5a) is unobservable because of the
nondegeneracy of the [2]catenanes reported in this investigation.
The ring rocking is also nondegenerate as a result of the presence
of [C-H‚‚‚π] interactions in one co-conformation (Scheme 5b,
left) and the absence of these contacts in the other co-
conformation (Scheme 5b, right). The former co-conformation
is the only species observed over a wide range of temperatures.
Finally, the last co-conformational processscircumrotation of
4
+
the CBPQT ring around the DNP unit (Scheme 5a)swas
monitored by following the signals of HR and HR′ atoms (Figure
4
+
8
) on the CBPQT ring. Rotation around axis 5 maintains the
inequivalence of these protonsssince their chemical environ-
ments remain different at all times during the rotationsbut
equilibrates the “inside” and “outside” moieties of the CBPQT4
ring and accordingly reduces the number of signals for R-bi-
pyridinium protons in the spectra of 1b,c‚4PF6 and 2b,c‚4PF6
in half.
+
1
Dynamic H NMR spectroscopy was performed in DMF-d7
(bp 426 K, mp 212 K). The kinetic and thermodynamic
parameters for the dynamic processes observed in 1b,c‚4PF6
and 2b,c‚4PF6 are summarized in Table 2. In some of the VT
experiments, the predictions were obscured by (i) overlapping
(
45) This process is sometimes referred to as the “pirouetting” of the π-donating
4+
ring around the formally motionless CBPQT ring. We prefer to treat this
motion from a different system of reference as the circumrotation of the
π-accepting ring around the formally motionless π-donating ring. This
semantic distinction emphasizes the geometric equivalence of the above
process with the circumrotation of the π-donating ring (rotation 3 in Figure
9). In both processes, one of the rings formally rotates around the axis
proton signals in half. However, steric considerations make the
direct rotation of bipyridinium moieties impossible while the
going through its center, while the other ring remains motionless (and vice
versa). Hence, the difference in the spectroscopic consequences of the two
processes comes from the difference in the identity of the rotating ring,
and not from different modes of rotation. See ref 44a as well as: (a) Ashton,
P. R.; Preece, J. A.; Stoddart, J. F.; Tolley, M. S.; White, A. J. P.; Williams,
D. J. Synthesis 1994, 1344-1352.
4
+
DNP unit is included inside the cavity of the CBPQT ring.
Two different mechanisms33 which account for the apparent
observation of bipyridinium rotation have been put forward. One
possibility, summarized in Scheme 4a, is the displacement of
the DNP ring from the CBPQT4 cavity, followed by its 180°
rotation around its O‚‚‚O axis and its return into the binding
pocket of the cyclophane. Otherwise (Scheme 4b), the DNP
ring can leave the cyclophane binding site, temporarily allowing
(
46) The circumrotation of the π-donating ring around the inner bipyridinum
unit of the π-accepting ring brings about equilibration between two co-
conformations. In a degenerate system, these co-conformations are of equal
energy and thus are equally populated at any given point. The barriers for
circumrotation in such systems are easily derived from the corresponding
+
coalescence parameters. In nondegenerate systems, such as 1b,c‚4PF
6
and
2b,c‚4PF , two co-conformations are not of equal energy; accordingly, their
6
1
populations and the relative intensities of H NMR signals are different. If
the difference in population is sufficiently large (>95:5), the circumrotation
will be fundamentally unobservable by NMR spectroscopy since the only
observable species will be the more stable co-conformation. This situation
is spectroscopically indistinguishable from an alternative in which circum-
rotation does not occur at all. Although highly unlikely, in light of our
previous studies on related degenerate donor-acceptor [2]catenanes, the
4+
the free rotation of the bipyridinium rings in the CBPQT ring
before returning into the cyclophane cavity (in the original
orientation). Since all the interactions that stabilize the DNP/
4
+
CBPQT ensemble need to be broken for either of these two
mechanisms to act, a relatively high energy penalty can be
6 6
absence of π-donating ring circumrotation in 1b,c‚4PF and 2b,c‚4PF
cannot be completely dismissed.
8244 J. AM. CHEM. SOC.
9
VOL. 129, NO. 26, 2007

Alkyne-Derived Subunits in Charged D−A [2]Catenanes
A R T I C L E S
Table 2. Kinetic and Thermodynamic Parameters for the Dynamic Processes Observed in Catenanes 1b,c‚4PF6 and 2b,c‚4PF6
cmpd
1b
‚
4PF
6
1c‚
4PF
6
2b
‚
4PF
6
2c‚4PF
6
Bipyridinium Rotation (Figure 9, axis 2)
protons observed
-
-
-
-
-
-
-
-
-
-
R-CBPQT4
98
217
362
17.5
+
â-CBPQT4+
28.5
63
288
14.5
∆
c
ν (Hz)
-1
k
(s
)
T
c
(K)a
q
(kcal mol-1
∆
G
c
)
)
CBPQT4 Ring Circumrotation (Figure 9, axis 5)
+
R-CBPQT4
180
400
337
15.8
+
R-CBPQT
240
533
300
4+
R-CBPQT
4+
-
-
-
-
-
protons observed
ν (Hz)
b
∆
c
85 (60)
-
1
b
k
(s
)
190 (134)
270
a
T
c
(K)
q
(kcal mol-1
b
∆G
c
13.8
13.1 (12.9)
a
Calibrated using neat MeOH (T < 295 K) or ethylene glycol (T > 295 K) samples. b Two sets of R-CBPQT4+ protons gave slightly different values for
the peak separation in the slow exchange limit.
between the peaks, (ii) accidentally isochronous peaks, and (iii)
the limited temperature range that could be explored experi-
mentally.
ring collapse into a single peak which continues to sharpen
beyond the coalescence point. A ∆Gc value of 17.5 kcal mol
q
-1
was obtained for the bipyridinium rotation (axis 2 in Figure 9)
1
4+
The VT H NMR spectra of 2b‚4PF6 are the easiest ones
Figure 10) to analyze. The low-temperature (231 K) H NMR
in the CBPQT ring of 2b‚4PF6.
1
(
Although we were not able to obtain equally comprehensive
data for the remaining three [2]catenanes, the information
accumulated agrees with our expectations. In 2c‚4PF6, the donor
chain is longer, which is expected to allow some relaxation of
the [2]catenane skeleton and accordingly lower the correspond-
ing rotation barriers. Indeed, the bipyridinium rotation is
spectrum of 2b‚4PF6 features four doublets in the region
4
+
associated with R-bipyridinium protons on the CBPQT ring.
With increasing temperature, these doublets start broadening
and ultimately coalesce into two signals at 270 K, corresponding
q
-1
to a ∆Gc value of 13.0 kcal mol for the circumrotation of
4
+
47
-1
the CBPQT ring (axis 5 in Figure 9). This value is ∼1.0
facilitated by some 3 kcal mol . The separation of peaks
-
1
33
kcal mol higher than that observed in previously studied
corresponding to the lower-energy process, the presumed halting
4+
4+
DNP/CBPQT -containing [2]catenanes, possibly because of the
smaller size of the π-donating ring in 2b‚4PF6. A further
increase in temperature causes the sharpening of these two peaks
into well-resolved doublets (313 K). Finally, at 362 K, all signals
of the circumrotation of the CBPQT ring, could not be
observed down to 231 K.
In the triazole series, 1b‚4PF6, the smallest catenane prepared
in this investigation, has an expectedly high barrier of 15.8 kcal
4
+
-1
4+
corresponding to the R-bipyridinium protons on the CBPQT
mol for the circumrotation of the CBPQT ring. In its
Scheme 5. Ca o-conformational Processes in Donor-Acceptor [2]Catenanes (A ) DNP, B ) 1,2,3-Triazole or 1,3-Butadiyne, C ) D )
Bipyridinium)
a
(
a) Circumrotation of the π-donating ring (horizontal arrows) and the π-accepting ring (vertical arrows). The unequal population of species with encircled
1
DNP (A) and those with encircled 1,2,3-triazole/1,3-butadyine (B) moieties renders the former process unobservable by H NMR spectroscopy. (b) Ring-
rocking of the π-donating ring, also unobservable as a result of unequal populations of the two states.
J. AM. CHEM. SOC.
9
VOL. 129, NO. 26, 2007 8245

A R T I C L E S
Miljani c´ et al.
emerge, namely (i) the higher rigidity of smaller catenanes as
evidenced by the higher barriers and (ii) greater flexibility of
butadiyne-containing catenanes relative to their triazole-contain-
ing counterparts. Evidence for the occurrence of this final
dynamic process, that of phenylene rotation, was observed only
-1
in 2c‚4PF6, allowing us to estimate a barrier of 14.4 kcal mol .
In all of the other catenanes investigated, the peak overlap was
too extensive to allow barriers to conformational and co-
conformational change to be measured.
Conclusion and Outlook
The results presented in this paper demonstrate that [2]-
pseudorotaxanes, constructed from chemically sensitive CB-
4
+
PQT cyclophane and DNP-based π-donor components, and
held together by noncovalent interactions, can act as synthons
in a classical synthetic organic chemistry sense. Two tolerant
reactions were employed as a proof of this concept, which
appears to be broadly applicable. Extension of these protocols
to other reactions is being pursued actively, with palladium-
catalyzed cross-couplings being one of our primary target
reactions. The Eglinton coupling emerges as a tolerant, simple,
and efficient preparative method for making bipyridinium-based
catenanes. This development, coupled with the rigid linear
geometry and unique reactivity of butadiyne units, may open
up new avenues of research based on this constitution. For
example, higher ordering of catenanes could be achieved using
butadiyne (and triazole) fragments as docks for transition
48
metals. The demonstrated utility of rigid molecular components
4
9
in the construction of nanodevices makes catenanes and
rotaxanes with alkyne-based binding sites an appealing prospect.
Acknowledgment. This work was supported by the Micro-
electronics Advanced Research Corporation (MARCO) and its
focus center on Functional Engineered NanoArchitectonics
1
Figure 10. Partial VT H NMR spectra of 2b‚4PF6.
-1
homologue, 1c‚4PF6, this barrier is lowered to 13.8 kcal mol ,
consistent with the larger, more relaxed constitution of the
π-donor. In 1b,c‚4PF6, we were unable to observe evidence of
(FENA), and also by the Defense Advanced Research Projects
Agency (DARPA) and the Center for Nanoscale Innovation for
Defense (CNID). Dr. Ivan Aprahamian (UCLA) is acknowl-
edged gratefully for insightful discussions.
1
the bipyridinium rotationsi.e., further simplification of H NMR
spectrason account of the extensive overlapping between the
signals associated with CBPQT4 protons. Differences in
observed coalescence temperatures and the chemical composi-
tion of the four catenanes call for caution in making comparisons
between the different systems. Nevertheless, two evident trends
+
Supporting Information Available: Experimental details,
spectroscopic data for all new compounds, packing diagrams
for 1b‚4PF , 2b‚4PF , 2c‚4PF , and [9a ⊂ CBPQT]‚4PF in
6
6
6
6
the solid state, and VT-NMR spectroscopic data; crystal-
lographic information files (CIFs) for 1b‚4PF6, 2b‚4PF6, 2c‚
(
47) (a) Oh ki, M. Applications of Dynamic NMR Spectroscopy to Organic
Chemistry; VCH, Weinheim, 1985. (b) Gasparro, F. P.; Kolodny, N. H. J.
Chem. Educ. 1977, 54, 258-261.
4
3
PF6, and [9a ⊂ CBPQT]‚4PF6; full refs 6j, 11c, 12c, 14, 15c,
0a, 33, 48b, 49a, and 49b. This material is available free of
(48) See, inter alia: (a) Hamilton, D. G.; Sanders, J. K. M. Chem. Commun.
charge via the Internet at http://pubs.acs.org.
JA071319N
1
998, 1749. (b) Ashton, P. R. et al. Chem.-Eur. J. 1998, 4, 590-607.
(49) (a) Liu, Y. et al. J. Am. Chem. Soc. 2005, 127, 9745-9759. (b) Huang, T.
J. et al. Appl. Phys. Lett. 2004, 85, 5391-5393.
8246 J. AM. CHEM. SOC.
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VOL. 129, NO. 26, 2007