Polyyne Rotaxanes: Stabilization by Encapsulation

Article abstract of DOI:10.1021/jacs.5b12049

Active metal template Glaser coupling has been used to synthesize a series of rotaxanes consisting of a polyyne, with up to 24 contiguous sp-hybridized carbon atoms, threaded through a variety of macrocycles. Cadiot-Chodkiewicz cross-coupling affords high

Full text of DOI:10.1021/jacs.5b12049

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Article
Polyyne Rotaxanes: Stabilization by Encapsulation
Levon D. Movsisyan, Michael Franz, Frank Hampel, Amber
L. Thompson, Rik R. Tykwinski, and Harry L. Anderson
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b12049 • Publication Date (Web): 11 Jan 2016
Downloaded from http://pubs.acs.org on January 12, 2016
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Polyyne Rotaxanes: Stabilization by Encapsulation
Levon D. Movsisyan,† Michael Franz,‡ Frank Hampel,‡ Amber L. Thompson,†
Rik R. Tykwinski,*,‡ and Harry L. Anderson*,†
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† Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Oxford, OX1 3TA, United Kingdom.
‡ Department of Chemistry & Pharmacy, and Interdisciplinary Center of Molecular Materials (ICMM), University of
Erlangen-Nuremberg (FAU), Henkestrasse 42, 91054 Erlangen, Germany
KEYWORDS: polyyne, rotaxane, active metal template, alkyne cross-coupling, calorimetry, crystal structure
ABSTRACT: Active metal template Glaser coupling has been used to synthesize a series of rotaxanes consisting of a polyyne,
with up to 24 contiguous sp-hybridized carbon atoms, threaded through a variety of macrocycles. Cadiot-Chodkiewicz
cross-coupling affords higher yields of rotaxanes than homo-coupling. This methodology has been used to prepare
[3]rotaxanes with two polyyne chains locked through the same macrocycle. The crystal structure of one of these
[3]rotaxanes shows that there is extremely close contact between the central carbon atoms of the threaded hexayne chains
(C···C distance 3.29 Å vs. 3.4 Å for the sum of van der Waals radii) and that the bond-length-alternation is perturbed in the
vicinity of this contact. However, despite the close interaction between the hexayne chains, the [3]rotaxane is remarkably
stable under ambient conditions, probably because the two polyynes adopt a crossed geometry. In the solid state, the angle
between the two polyyne chains is 74°, and this crossed geometry appears to be dictated by the bulk of the ‘supertrityl’ end
groups. Several rotaxanes have been synthesized to explore gem-dibromoethene moieties as ‘masked’ polyynes. However,
the reductive Fritsch-Buttenberg-Wiechell rearrangement to form the desired polyyne rotaxanes has not yet been achieved.
X-ray crystallographic analysis on six [2]rotaxanes and two [3]rotaxanes provides insight into the non-covalent interactions
in these systems. Differential scanning calorimetry (DSC) reveals that the longer polyyne rotaxanes (C16, C18 and C24)
decompose at higher temperatures than the corresponding unthreaded polyyne axles. The stability enhancement increases
as the polyyne becomes longer, reaching 60 °C in the C24 rotaxane.
chains,1,8,12,13 but it is only recently that this effect has been
demonstrated experimentally in polyynes14 and cumulenes.15 It
Introduction
Rotaxane formation provides a method for altering the
chemical reactivity of a dumbbellꢀshaped molecule, without
modifying its covalent structure, by locking it through the
has also been demonstrated that rotaxane formation can be
used to modify the photophysical behavior of polyynes.16
Our strategy for the synthesis of polyyne rotaxanes is
based on copperꢀmediated coupling of terminal oligoynes.17
cavity of a macrocycle. The stability and photophysical behavꢀ
Similar chemistry has been employed previously to prepare
ior of many πꢀsystems have been enhanced using this threadꢀ
rotaxanes,18ꢀ20 catenanes,20ꢀ23 knots24 and rotacatenanes,25 all
linked with butadiyne (C4) moieties. Here we present the synꢀ
thesis of rotaxanes with chains of 8–24 contiguous spꢀ
hybridized carbon atoms and ‘supertrityl’ (Tr*) endgroups.26,27
Importantly, the yields of the rotaxanes have been improved
by using CadiotꢀChodkiewicz crossꢀcoupling19,22,28 and we
have achieved remarkable selectivity by optimizing the couꢀ
pling partners and the size of the macrocycle. Furthermore,
rotaxanes with dibromoethene moieties have been synthesized
as ‘masked’ polyynes,29 although we have not yet achieved the
reductive FritschꢀButtenbergꢀWiechell rearrangement of these
rotaxanes to unmask the polyyne axles. Finally, differential
scanning calorimetry confirms the hypothesis that rotaxane
formation can indeed stabilize longer polyynes (C16–C24)
relative to the corresponding naked polyynes.
ing strategy.1ꢀ3 This approach is particularly appropriate for
controlling the reactivity of extended polyynes and cumulenes,
Rꢀ(C≡C)nꢀR and R2C=(C=C)n=CR2, since the only way to
covalently modify these πꢀsystems is to change the endgroups
(R), which becomes increasingly irrelevant as the system
becomes longer, with increasing n. Recently, we4 and others5
reported the synthesis of polyyne rotaxanes using an active
copper(I) template effect6 with a phenanthrolineꢀbased macroꢀ
cycle.7 Here, we present a broad investigation of the synthesis,
structure and properties of this new class of ‘insulated molecuꢀ
lar wires’.
Polyynes have been studied extensively as analogues of
carbyne8,9 and because of their unique electronic properꢀ
ties.10,11 Rotaxane formation has often been suggested as a
strategy for improving the stability of linear carbon
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The crystal structures of the new rotaxanes provide some experiment we tested an excess of both 1c (1.2 equiv.) and 3
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3
4
5
6
7
8
insight into the origins of their thermal stability, in that the
distances between neighboring polyyne chains are longer than
for the corresponding naked polyynes,26 which is a key factor
for solidꢀstate polymerization.30 Xꢀray crystallographic analyꢀ
sis also reveals a wealth of information regarding dispersive
interactions between the polyyne chains and the threaded
(1.2 equiv.), relative to the macrocycle. The reaction was
complete after 4 h and the rotaxane was isolated in 43% yield
(Table 2, entry 2). We repeated this reaction keeping the
amount and concentration of 1c constant (1.2 equiv.) while
varying the amount of 3 (1.3–1.5 equiv.) and always obtained
macrocycles, based on a variety of CH/
The structure of a [3]rotaxane, with two C12 chains threaded
through the same macrocycle, reveals a πsp πsp interaction
between the two hexayne chains, which are arranged in a
crossed geometry.
π and π/π interactions.
Scheme 1. The synthesis of a series of supertrityl end-
capped rotaxanes with M1 macrocycle via the homo-
coupling.
/
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O
This work establishes the synthetic methodology for preꢀ
paring polyyne rotaxanes and provides a platform from which
to create a broad range of functional carbonꢀrich materials.
O
N
N
M1
O
Results and Discussion
Varying the length of the polyyne. Polyyne rotaxanes
2bꢀf·M1, with 8–24 contiguous spꢀhybridized carbon atoms,
were prepared using an active Cu(I)ꢀtemplate homoꢀcoupling
strategy (Scheme 1, Table 1).4 In a typical procedure, the 1:1
complex of CuI and the phenanthrolineꢀbased macrocycle M1
(CuI·M1) reacted with a slight excess of the terminal polyyne
1b–f in THF at 60 °C in the presence of K2CO3 and iodine.
Attempts to prepare the butadiyneꢀlinked rotaxane 2a·M1
from the alkyne 1a were not successful; it seems that the Tr*ꢀ
capped polyyne products need at least 8 spꢀcarbon atoms to
provide space for the macrocycle. Preparation of a rotaxane
with 32 spꢀcarbon atoms was also unsuccessful, and we obꢀ
served complete decomposition of octayne 1g in the reaction
mixture. This result indicates that the limit for the current
coupling conditions has been reached at the stage of 2f·M1
with a C24 dodecayne dumbbell.
The product yield for the series of rotaxanes 2bꢀf·M deꢀ
creased with increasing length of the polyyne (Table 1), probꢀ
ably as a consequence of the lower stability of the longer
terminal polyynes. Use of lower reaction temperatures supꢀ
presses decomposition of the starting materials, but does not
increase the yield of rotaxane. In the synthesis of rotaxane
2f·M1, the reaction was stopped after 16 h, when TLC still
showed traces of unreacted 1f, to minimize decomposition of
the product. In all cases, the concentration of macrocycle M1
was 5–10 mM; increasing the concentration to 30 mM (for
conversion of 1c to 2c·M1) did not noticeably alter the yield.
The copperꢀcatalyzed crossꢀcoupling of acetylenes, first
reported by Cadiot and Chodkiewicz,28 has been successfully
utilized for the synthesis of polyynes,9b,31 rotaxaneꢀbased shutꢀ
tles19 and catenanes,22 but the reaction had not been explored
for the synthesis of polyyne rotaxanes. We chose to test the
synthesis of rotaxanes via CadiotꢀChodkiewicz coupling using
triyne 1c and the corresponding bromotriyne 3 (prepared by
AgNO3ꢀcatalyzed bromination of 1c with NBS).32 Equimolar
amounts of triyne 1c and bromotriyne 3 were added to a soluꢀ
tion of the CuI·M1 complex in THF, and the mixture was
stirred under nitrogen at 60 °C. The reaction was complete
after 4 h (monitored by TLC), and the corresponding rotaxane
2c·M1 was isolated in 38% yield (Table 2, entry 1). The yield
of rotaxane from this crossꢀcoupling reaction is slightly higher
than that from homoꢀcoupling (32%). In the latter case, a
slight excess of 1c (2.5 equiv.) was used, thus in the next
O
2)
Tr*
H
(1a–g)
1) CuI, CH2Cl2, CH3CN
n/2
I2, K2CO3, THF, 60 °C
N
N
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
O
O
n
tꢀBu
tꢀBu
tꢀBu
tꢀBu
O
O
2a–g—M1
a (n = 2)
b (n = 4)
c (n = 6)
d (n = 8)
e (n = 10)
f (n = 12)
g (n = 16)
Table 1. Summary of synthesis of Tr*-(C≡C)n-Tr* rotax-
anes via homo-coupling (via Scheme 1).[a]
Starting
material
1aa
1b
Rotaxane
product
2a·M1
2b⋅M1
2c⋅M1
2d⋅M1
2e⋅M1
2f⋅M1
Reaction
time[b]
48 h
48 h
24 h
40 h
36 h
16 h
48 h
n
2
4
6
8
10
12
16
Yield[a]
0
34%
32%
23%
15%
11%
0
1c
1d
1e
1f
1g
2g·M1
[a]
Yields for isolated rotaxanes based on amount of M1
starting material. Conditions: CuI·M1 (1.0 equiv.; 5–10 mM),
1a–g (2.2 equiv.), K2CO3 (4 equiv.), I2 (1.1 equiv.), THF, 60 °C.
[b] Reaction times were judged by TLC.
the rotaxane in good yields (47–53%, Table 2). Finally, we
carried out the reaction at 20 °C using stoichiometric amounts
of 1c and 3. The starting materials were consumed after 76 h,
giving rotaxane 2c·M1 in 26% yield (Table 2, entry 5), while
at 40 °C the reaction was complete after 24 h and the product
yield was 36% (Table 2, entry 4). The synthesis of hexayne
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rotaxanes via crossꢀcoupling is more effective than homoꢀ
coupling, and proceeds at lower temperatures (e.g. homoꢀ
coupling failed at 40 °C). However, crossꢀcoupling is not
applicable to the synthesis of long polyyne rotaxanes due to
the low stability of halogenated polyyne precursors.
1
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3
4
Scheme 2. Synthesis of porphyrin-polyyne [2] and
[3]rotaxanes.
tꢀBu
tꢀBu
5
6
7
tꢀBu
tꢀBu
tꢀBu
Table 2. Optimization of the synthesis of 2c·M1 rotaxane
via Cadiot-Chodkiewicz cross-coupling.[a]
N
N
N
N
Br
+
Zn
Si(iPr)3
8
9
tꢀBu
tꢀBu
(3)
Tr*
Tr*
Br
H
3
4
CuI—M1
tꢀBu
+
2c—M1
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K2CO3, THF
(1c)
tꢀBu
tꢀBu
tꢀBu
tꢀBu
CuI—M1
K2CO3, THF, PhMe, 60 °C
Entry
1c,
equiv. equiv.
3,
Temp.
Time
Yield[a]
1
2
3
4
5
1.0
1.2
1.2
1.0
1.0
1.0
1.2
1.5
1.0
1.0
60 °C
60 °C
60 °C
40 °C
20 °C
4 h
4 h
4 h
24 h
76 h
38%
43%
53%
36%
26%
N
N
tꢀBu
tꢀBu
tꢀBu
O
O
N
N
Zn
R
N
N
[a]
Yields calculated based on M1 (conc. 10 mM). Reaction
conditions: CuI·M1 (1 equiv.), K2CO3 (4 equiv.), O2-free THF.
tꢀBu
tꢀBu
tꢀBu
O
O
tꢀBu
tꢀBu
Mechanistically, it is believed that CadiotꢀChodkiewicz
coupling proceeds via oxidative addition of the bromoacetyꢀ
lene to a Cuꢀacetylide producing a Cu(III) intermediate, which
undergoes reductive elimination affording the crossꢀcoupled
product.17,22,33 However, competing homoꢀcoupling of the
alkynyl halide is often observed, via halogenꢀmetal exꢀ
change,17,28,34 and it has been difficult to achieve high selecꢀ
tivity in crossꢀcoupling reactions of polyynes. In some cases,
selectivity can be obtained by careful choice of the amine
base, solvent and reagent concentrations,35 or by applying a
polymerꢀsupporting technique.36 To test the selectivity of the
crossꢀcoupling reaction, the supertrityl diyne 1b (1.0 equiv.)
and bromotriyne 3 (1.1 equiv.) were reacted with CuI·M1
complex (1.0 equiv.) under the conditions described above
(THF, 60 °C, 12 h). The reaction gave a mixture of two rotaxꢀ
anes, as confirmed by the H NMR and MALDI spectra (Figꢀ
ure S15, SI). The ratio of hexayne and pentayne rotaxanes
(4:1) was estimated from H NMR spectrum. Thus, homocouꢀ
pling of the bromotriyne 3 is indeed competitive with the
desired heterocoupling reaction.
We envisioned that changing the coupling acetylene partꢀ
ners could provide higher selectivity in crossꢀcoupling,28,34 so
the synthesis of porphyrinꢀpolyyne mixed rotaxanes was inꢀ
vestigated. Porphyrin rotaxanes represent an important class of
molecular machines and photoresponsive assemblies,37 and the
macrocycle M1 has previously been used in the activeꢀmetal
template homoꢀcoupling of mesoꢀethynylꢀporphyrins affording
porphyrinꢀcapped rotaxanes.20 Thus, porphyrin 4 (1.0 equiv.),
supertrityl bromotriyne 3 (1.5 equiv.) and CuI·M1 complex
(1.0 equiv.) were reacted in a toluene/THF (2:1) under nitroꢀ
gen at 60 °C to give rotaxane 5a·M1 in 19% yield (Scheme 2).
To our surprise, the formation of unthreaded hexayne or bisꢀ
5a—M1 R = TIPS
TBAF
5b—M1
R = H
CuI—M1, 3
K2CO3, THF, PhMe, 60 °C
tꢀBu
tꢀBu
N
N
N
N
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
O
O
O
O
N
N
N
N
Zn
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
O
O
O
O
1
tꢀBu
tꢀBu
5c—(M1)2
In rotaxane 5a·M1, the second meso position of the porꢀ
phyrin can serve as a coupling partner after desilylation with
TBAF. Thus, rotaxane 5b·M1 was subjected to conditions
similar to that for 5a·M1 to give [3]rotaxane 5c·(M1)2 in 23%
yield (Scheme 2). Again, the reaction was highly selective,
affording the product without formation of 2c·M1 or
bis(porphyrin)ꢀcapped rotaxanes.
1
Synthesis of ‘masked’ polyyne rotaxanes. One limitation
of preparing rotaxanes by oxidative coupling of terminal and
bromoꢀpolyynes is the instability of the starting materials, and
this encouraged us to explore alternative synthetic strategies.
One approach is based on the use of masked oligoynes, in
which the polyyne framework is assembled in a protected
form.38 In the final step, the linear carbon chain is constructed
via elimination of masking functional groups. Recently, the
FritschꢀButtenbergꢀWiechell (FBW) rearrangement of carꢀ
bene/carbenoid intermediates has evolved into a valuable
synthetic methodology for the preparation of polyynes from
geminal dihaloolefinꢀmasked acetylene precursors.29 The basis
of the FBW method is the treatment of 1,1ꢀdibromoꢀ2,2ꢀ
1
porphyrin rotaxanes was not observed by TLC analysis, H
NMR or MALDI spectra of the crude reaction mixture.
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dialkynylethenes with nꢀBuLi, which leads to the in situ forꢀ
Scheme 5. Attempted synthesis of octayne rotaxane
2d·M1 via FBW reaction of 7b·M1.
1
2
3
4
5
6
7
8
mation of a carbenoid species, followed by 1,2ꢀmigration to
yield the corresponding linear polyyne (Scheme 3).29 We are
interested in utilizing gemꢀdibromoolefins as masked polyynes
in the synthesis of rotaxanes. Compound 6a did not undergo
homoꢀcoupling in the presence of the copper(I) complex of
macrocycle M1. However, the reaction of 6a with the bromoꢀ
derivative 6b under crossꢀcoupling conditions furnished the
rotaxane 7a·M1 in 9% yield (Scheme 4). Increasing the length
of the acetylenic axle dramatically improved the yield of this
synthesis. Homoꢀcoupling of 6c, in the presence of CuI·M1
gave rotaxane 7b·M1 in 78% yield (Scheme 4). However, all
attempts at FBW rearrangement of 7b·M1 were unsuccessful,
despite testing a range of reaction conditions (Scheme 5 and
Table S1); rotaxane 7b·M1 reacts with butyl lithium to give a
complex mixture of products. It is not clear why this reaction
fails but it appears that the proximity of the phenanthroline
macrocycle adversely affects the reactivity of the carbenoid
intermediate, promoting alternative pathways.
tꢀBu
Br
Br
N N
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
O
O
tꢀBu
tꢀBu
tꢀBu
9
tꢀBu
tꢀBu
10
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tꢀBu
Br
Br
O
O
7b—M1
nꢀBuLi
N
N
Scheme 3. General mechanism of FBW rearrangement.
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
Br
Br
O
O
nꢀBuLi
tꢀBu
hexane
– 78 °C
tꢀBu
tꢀBu
tꢀBu
O
O
tꢀBu
2d—M1
tꢀBu
Scheme 4. Synthesis of rotaxanes 7a·M1 and 7b·M1.
tꢀBu
tꢀBu
Br
Br
Br
Br
Varying the size of the macrocycle. The supertrityl endꢀ
group is large enough to prevent slippage of macrocycle M1,
and we were interested to test whether it is an effective stopper
for even larger macrocycles. On the other hand, use of smaller
macrocycles is appealing because it should allow the synthesis
of polyyne rotaxanes with simpler terminal groups. In many
rotaxanes, the macrocycle protects the axle from the external
environment,1,2,39 preventing enzymatic digestion,40 or chemiꢀ
cal degradation,41 and in these cases a ‘tight’ fit between macꢀ
rocycle and thread is desirable. We tested a variety of macroꢀ
cycles for the synthesis of hexayne rotaxanes (Scheme 6 and
Table 3). The triyne 1c and bromotriyne 3 were chosen for
rotaxane synthesis since a hexayne axle is long enough to
eliminate steric interactions with the supertrityl endꢀgroup.
The size of macrocycle M1 can easily be adjusted by changing
the length of the alkyl bridge. Decreasing the size from C6
(M1) to C3 (M2) or C4 (M3) worked well, yielding hexayne
rotaxanes 2c·M2 and 2c·M3 under crossꢀcoupling conditions
in 21% and 43% yields, respectively. Under homoꢀcoupling
conditions, the same macrocycle gave lower yields and the
rotaxane 2c⋅M2 was isolated in only 5% yield. A macrocycle
with a C8 linker (M4)7 gave rotaxane 2c⋅M4 in 41% yield
under crossꢀcoupling (9% from homoꢀcoupling) but no rotaxꢀ
ane was isolated with the larger C10 macrocycle (M5),7 probaꢀ
bly because this macrocycle slips off the dumbbell, as indicatꢀ
ed by inspection of CPK models.
NBS,
AgNO3
tꢀBu
tꢀBu
tꢀBu
tꢀBu
Br
acetone
96%
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
6a
6b
CuI—M1, K2CO3
THF, 60 °C
N
N
Br
Br
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
O
O
tꢀBu
tꢀBu
n
tꢀBu
tꢀBu
tꢀBu
tꢀBu
Br
Br
O
O
7a—M1 n = 2, 9%
7b—M1 n = 4, 78%
CuI—M1, K2CO3
I2, THF, 60 °C
tꢀBu
Br
Br
tꢀBu
tꢀBu
The phenanthroline-based macrocycle M6 with a C10
alkyl strap gave the corresponding rotaxane 2c·M6 in 17%
and 26% yields from homo- and cross-coupling, respec-
tively. The macrocycle M715,42 with a more rigid p-tolyl
ether framework gave 2c·M7 in 23% and 54% yield, for
tꢀBu
tꢀBu
6c
tꢀBu
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homo- and cross-coupling respectively (Table 3). Finally, a
Table 3. Summary of the syntheses of rotaxanes 2c·M2-M8
using different reaction conditions (via Scheme 6).
bipyridine-based macrocycle43 gave rotaxane 2c⋅M8 in
23% and 26% yield, from homo- or cross-coupling respec-
tively. Generally, small macrocycles reduce the yield of
rotaxanes in both homo- and cross-coupling, except in the
case of M7. This contrasts with the results obtained for the
synthesis of rotaxanes via active template copper-
catalyzed azide-alkyne cycloaddition reaction, where
smaller macrocycles afforded better yields.43
1
2
3
4
5
6
7
8
Yield (reaction time)[c]
Macrocy-
cle
Rotax-
ane
homo[a]
cross[b]
M2
M3
M4
M5
M6
M7
M8
5% (50 h)
28% (48 h)
9% (42 h)
0% (48 h)
17% (48 h)
23% (62 h)
23% (48 h)
21% (11 h)
43% (8 h)
41% (18 h)[d]
0% (48 h)
27% (6 h)
54% (12 h)
27% (4 h)
2c⋅M2
2c⋅M3
2c⋅M4
2c⋅M5
2c⋅M6
2c⋅M7
2c⋅M8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 6. Synthesis of hexayne rotaxanes with differ-
ent macrocycles.
[a] Homo-coupling reaction conditions: CuI·M, 1c (2.5 eq),
I2, K2CO3, THF, 60 °C. [b] Cross-coupling reaction conditions:
CuI·M, 1c (1.1 equiv), 3 (1.5 equiv), K2CO3, THF, 60 °C. [c]
Yields are calculated referring to macrocycles (c = 5–10 ꢀM).
[d] Reaction temperature: 50 °C.
N
N
N
N
M6
O
O
O
O
H2C
CH2
O
O
x
x
Synthesis of [3]rotaxanes with two threaded polyyne
chains. Rotaxanes with multiple axles passing through a sinꢀ
gle ring are very rare.44 The challenge in preparing this type of
molecular architecture is to satisfy the structural demands of
both components: The macrocycle may require more than one
template site to assemble multipleꢀthreads, and its cavity must
be large enough to accommodate two axles, yet small enough
to prevent the dethreading. Additionally, the macrocycleꢀ
thread interactions that direct the assembly process, must
overcome steric hindrance between crowded dumbbell units.
So far, singleꢀmacrocycle threaded [3]rotaxanes have been
synthesized utilizing hydrophobic interactions,44a octahedral
metal centers as templates,44b,c hydrogenꢀbond formation beꢀ
tween a thread and axles,44d and an activeꢀmetal templated
acetylene homoꢀcoupling.44e,f The axles can be identical44bꢀe or
different44a and can be assembled using the same or different
type of reactions for each step of the threading.
To test the threading of two identical polyynes, the
CuI·2c·M1 stoichiometric complex (1.0 equiv.) was prepared
and mixed with 1c (1.2 equiv.) and 3 (1.6 equiv.) in THF
(Scheme 7). The oxygenꢀfree reaction mixture was stirred for
36 hours at 60 °C in the dark. After workup followed by silica
and sizeꢀexclusion chromatography, the [3]rotaxane (2c)2·M1
was obtained in 6% yield, and 70% of the 2c·M1 rotaxane
starting material was recovered. The product was characterꢀ
ized by mass spectrometry, NMR and UVꢀvis absorption
spectroscopy, and the structure of the molecule was deterꢀ
mined by Xꢀray crystallography. The [3]rotaxane (2c)2·M1 is
stable as a crystalline solid at 4 °C, while at ambient condiꢀ
tions the yellow solid darkens slowly over weeks. To improve
the yield of the doubleꢀthreaded product, a larger macrocycle
was tested, to reduce steric crowding. We were pleased to find
that under similar reaction conditions, rotaxane 2c·M4, with a
larger macrocycle, yields the corresponding [3]rotaxane
(2c)2·M4 in 18% (Scheme 7). The stability of (2c)2·M4 is
comparable to that of (2c)2·M1, and the solid discolors slowly
under ambient conditions, but is stable indefinitely at –20 °C.
M3, x = 4
M4, x = 8
M5, x = 10
M1, x = 6
M2, x = 3
N
N
N
N
O
O
M8
M7
O
O
O
O
O
1) CuI, CH2Cl2
CH3CN
2) 1c or 1c + 3
THF, K2CO3, 60 °C
tꢀBu
tꢀBu
tꢀBu
tꢀBu
N
N
tꢀBu
tꢀBu
tꢀBu
tꢀBu
O
O
tꢀBu
tꢀBu
tꢀBu
tꢀBu
2c—M1–M8
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the supertrityl endꢀgroup also move to higher chemical shift in
response to polyyne elongation (H1 and H2, Figure 3).
Scheme 7. Synthesis of (2c)2·M1 and (2c)2·M4 polyyne
[3]rotaxanes via cross-coupling.
1
2
3
4
5
6
7
8
In the 1H NMR spectrum of the [3]rotaxane (2c)2·M1, proꢀ
tons labeled Hd, Hb, He, Hh, Hg, Hj and Hi move to lower chemꢀ
ical shift, while the chemical shift of proton Hc increases,
compared with the corresponding [2]rotaxane 2c·M1 (Figure
4). Resonances of the protons H1 and H2 from supertrityl
group are unaffected. The two hexayne chains in (2c)2·M1 are
undistinguishable by 1H and 13C NMR spectroscopy in
CD2Cl2, both at 298 K and at 193 K (Figures S16 and S17, SI).
N
N
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
O
O
9
H2C
CH2
tꢀBu
tꢀBu
tꢀBu
tꢀBu
x
x
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
tꢀBu
O
O
x = 6, 2c—M1
x = 8, 2c—M4
1) CuI, CH2Cl2, CH3CN
2) 1c, 3, K2CO3, THF, 60 °C
tꢀBu
tꢀBu
tꢀBu
tꢀBu
N N
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
O
O
tꢀBu
tꢀBu
tꢀBu
tꢀBu
H2C
CH
2 x
x
O
O
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
tꢀBu
x = 6, (2c)2—M1 (6%)
x = 8, (2c)2—M4 (18%)
Spectroscopic characterization of rotaxanes. The
polyyne rotaxanes were characterized by MALDI mass specꢀ
trometry, UVꢀvis absorption and NMR spectroscopy. Threadꢀ
ing does not significantly perturb the electronic structure of
polyynes and the absorption spectra of the rotaxanes resemble
the sum of the macrocycle and polyyne absorptions.4,26 For
example, the absorption spectra of the rotaxanes 2b–f·M1
(Figure 1a) are essentially the sum of the spectra of their comꢀ
ponents, 2b–f and M1. The polyyne absorption bands in the
rotaxanes are shifted to lower energy by about 4 nm, comꢀ
pared to the unthreaded analogs26 (Figure 1b).
In doubleꢀthreaded [3]rotaxanes, the absorption in the
polyyne region is double that of the parent rotaxanes, as exꢀ
pected. Normalization of the spectra of rotaxanes 2c·M1 and
(2c)2·M1 at the absorption maximum (317 nm) shows that the
second vibronic band at ∼297 nm is redꢀshifted by 1 nm and is
slightly more intense in the [3]rotaxane (Figure 2).
Figure 1. (a) The UV-vis absorption spectra of macrocycle M1
(brown line) and rotaxanes 2b–f·M1 in dichloromethane. (b)
Normalized (at the highest absorption band) absorption spec-
tra of 2c·M1 (orange) and 2f·M1 (blue) rotaxanes with their
corresponding unthreaded polyynes (dashed lines) in di-
chloromethane.
The 1H NMR spectra of rotaxanes 2b–f·M1 reveal that the
interactions between the supertrityl endgroups and the macroꢀ
cycle become weaker as the polyyne becomes longer (Figure
3). Upon threading, the chemical shift of proton Hf of the
macrocycle resorcinol moiety increases, while those of protons
Hh and Hg decrease. As the length of the polyyne chain is
increased, these changes become insignificant. Thus in rotaxꢀ
ane 2f·M1, resonances Hf, Hg and Hh become almost identical
to those from free macrocycle M1. The aromatic protons of
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thermal stability with increasing chain length,26 whereas there
1
is less change in stability for the rotaxanes.
2
3
4
5
6
7
8
9
10
11
12
13
14
Figure 2. The comparison of absorption spectra of rotaxanes
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
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46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2c·M1 and (2c)2·M1 (in dichloromethane) normalized at the
absorption maximum at 317 nm.
1
Figure 4. Comparison of H NMR spectra of rotaxanes 2c·M1
(orange) and (2c)2·M1 (blue). Asterisk denotes the solvent
peak (500 MHz, CD2Cl2, 298 K).
Table 4. Comparison of the decomposition temperature
(peak) of Tr*-(C≡C)n-Tr* polyyne rotaxanes and corre-
sponding free polyynes from DSC analysis.
Decomposition temperature
Rotaxane
2c·M1, 287 °
2d·M1, 291 °C
2e·M1, 234 °
2f·M1, 228
Polyyne26
n
6
8
C
2c, 323 °
2d, 275 °
C
C
10
C
C
2e, 220 °
C
12
°
2f, 168
°
C
Rotaxane 2c·M1 decomposes at a lower temperature than
the bare hexayne 2c,26 which is probably because the rotaxane
melts (melting point: 216 °C; dec. 287 °
C) whereas the dumbꢀ
1
bell decomposes without melting (dec. 323 °
C). Here the
Figure 3. Partial H NMR spectra of rotaxanes 2b–f·M1, com-
threaded macrocycle serves to reduce the symmetry of the
polyyne and to disrupt crystalꢀpacking interactions, reducing
the melting point and consequently reducing the thermal staꢀ
bility. However, with further increase of the polyyne length,
all of the compounds undergo thermal decomposition without
melting and the rotaxanes become more stable than the correꢀ
sponding naked dumbbells. The difference in stability increasꢀ
es with increasing polyyne chain length. The greatest enꢀ
hancement in thermal stability is observed for 2f·M1, as DSC
shows a decomposition peak at 228 °C, while for the free
dumbbell 2f, the decomposition peak occurs at 168 °C (Figure
5). As the polyyne chain gets longer, steric shielding of the
carbon chain by the supertrityl groups decreases, providing an
pared with that of the M1 macrocycle (CD2Cl2, 500 MHz, 298
K).
Thermal stability. It was hypothesized decades ago that
mechanical encapsulation via rotaxination should stabilize
extended polyynes.8,12,13 While remarkable stabilization for
cumulene rotaxanes was demonstrated recently,15 the thermal
stability of polyyne rotaxanes was a key aspect that we sought
to address in this study. The thermal stabilities of rotaxanes
2c–f·M1 and the corresponding polyynes 2c–f were compared
by differential scanning calorimetry (DSC), by heating the
samples to 400 °C at 10 °C/min under an atmosphere of nitroꢀ
gen (Table 4), and observing the exothermic thermal decomꢀ
position. Naked polyyne dumbbells show a sharp decrease in
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2c·M7 exhibits the most bending with φ = 165(1)°, however,
2c·M6 has the smallest average C≡C–C(sp) angle (175(2)°).
All the polyyne [2]rotaxanes have mean BLAs of approxiꢀ
mately 0.14–0.15 Å (Table 5), with the exception of 2c·M6
(BLA = 0.160(8) Å); it is not clear whether the unusually high
BLA in 2c·M6 is a result of conformational adjustment due to
the threaded macrocycle or other effects (such as packing
interactions).
opportunity for the macrocycle to act as an additional shield
and suppress the polyyne degradation.
1
2
3
4
5
6
7
8
The cylindrical
πꢀsystems of polyynes can become inꢀ
volved in CH/πsp interactions. Traditionally, CH/πarene interacꢀ
tions have received more attention due to their abundance in
biology,50 whereas CH/πsp interactions are rarely discussed,51
and the few existing reports focus on terminal alkynes rather
than polyynes. We have analyzed the intermolecular CH/πsp
9
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60
and πarene/πsp interactions in polyynes having four or more
triple bonds from the Cambridge Structural Database (CSD)52
and herein we compare these data with results for polyyne
rotaxanes. The following discussion of CH/πsp interactions
offers, to the best of our knowledge, the first rigorous analysis
of this secondary bonding motif.
Figure 5. DSC traces of dodecayne 2f (dash line) and the
corresponding rotaxane 2f·M1 (solid line). Heating: 10
°C/min.
The polyyne
πꢀsystems and macrocycles interact in the
solid state via CH/Csp and C/Csp short contacts (Figure 6). The
phenoxyl groups of the macrocycles interact with the polyyne
Xꢀray Crystallography. Here we report crystal strucꢀ
tures45 of polyyne rotaxanes 2c·M2, 2c·M6, 2c·M7 and
2d·M1, dibromolefin rotaxane 7a·M1, porphyrin rotaxane
5a·M1, and [3]rotaxanes 5c·(M1)2 and (2c)2·M1. Full crystalꢀ
lographic details regarding crystal growth, data collection,
analysis and crystal packing are given in the SI. Crystalloꢀ
graphic data (excluding structure factors) have been deposited
with the Cambridge Crystallographic Data Centre (CCDC
1437276–1437283) and can be obtained via www.
ccdc.cam.ac.uk/data_request/cif. The crystal structures of
rotaxane 2c·M1 and the free hexayne 2c have been published
previously4,26 and are included in the discussion for compariꢀ
son. Selected parameters are summarized in Table 5.
Xꢀray crystallography studies of polyynes provide insights
into the structures of these molecules and into their nonꢀ
covalent interactions.46 For example, an analysis of the strucꢀ
tures of a series of tꢀbutylꢀcapped polyynes47 indicated that
infinitely long polyynes reach a saturation in the bondꢀlengthꢀ
alternation (BLA), which implies that carbyne has alternating
single and triple bonds.48 This result is in agreement with data
from the electronic absorption spectra of Tr*ꢀcapped polyynes
which predict a finite optical bandgap (Eg = 2.56 eV) for carꢀ
byne.26
It is expected that threading will affect the conformation
and packing of a polyyne. For example, the unthreaded hexꢀ
ayne 2c is centrosymmetric,26 whereas in all hexayne rotaxꢀ
anes the macrocycle breaks the symmetry. Thus, the hexayne
chain in 2c·M2 is slightly bent in a helical shape and the averꢀ
age C≡C–C(sp) angle is 177.6(14)°. Similarly, in rotaxanes
2c·M6 and 2c·M7 the hexayne axles are semiꢀhelical and Sꢀ
shaped, with average C≡C–C(sp) angles of 175(2)° and
176(2)°, respectively. In 2d·M1, the octayne chain is slightly
curved in a helical fashion and the average C≡C–C(sp) angle
is 177.0(19)° (Figure 6). Only three Xꢀray structures of ocꢀ
taynes have been reported previously.9a,47,49
π
ꢀsystem and the CH/Csp distances are within the range of
values found in CSD (2.65–2.75 Å; see SI for details). Addiꢀ
tionally, in rotaxane 2c·M6, the alkyl strap of the macrocycle
makes weak contacts with the polyyne
πꢀsystem (c and d
contacts in Figure 6b). In 2c·M7, the shortest CH/Csp distance
is 2.702 Å, which is significantly shorter than the mean value
found for tetraynes and longer homologs (2.82(7) Å, SI). In
addition to the many CH/πsp interactions, the phenoxyl groups
of the compact macrocycle M7 interact with the hexayne
system through πarene πsp interactions (Figure 6c). A search of
the CSD revealed that πarenel πsp interactions are rare in comꢀ
πꢀ
/
/
parison to CH/πsp interactions (14 vs. 120 observed short conꢀ
tacts, respectively). Interestingly, the closest Carene/Csp contact
in 2c·M7 (Figure 6c: 3.193 Å) is the shortest distance comꢀ
pared to other molecules found in the CSD (mean value for
Carene/Csp contacts: 3.36(4) Å). Once the macrocycle cavity
becomes smaller (M7, M2) the Carene/Csp interactions become
significant, in addition to CH/Csp contacts (Figure 6).
Another consequence of mechanical encapsulation is the
larger distances between neighboring polyynes. For instance,
the interꢀspꢀchain distance for 2c·M7 is about 9.7 Å, for
2c·M6 it is 11.6 Å and for 2c·M2 it is 12.9 Å, much longer
than the shortest distance between neighboring molecules of
free hexayne 2c (8.1 Å).26 These distances are far from that
required for topochemical polymerization of the polyynes (ca.
4 Å).30 While this result is not surprising for rotaxinated
polyynes, the crystallographic data clearly illustrate the protecꢀ
tive role of the threaded macrocycles.
In the [3]rotaxane (2c)2·M1, two hexayne chains are arꢀ
ranged in a crossed geometry (Figure 6e), and the macrocycle
M1 sits around the middle of the two hexayne chains (desigꢀ
nated as A and B). The angle between chains A and B is 74°
(measured as a torsional angle between the quaternary sp3
carbons of the Tr* end groups and the centroids between pairs
of these quaternary carbon atoms). In the solid state, the moleꢀ
cule is chiral, due to the helical arrangement of the two hexꢀ
ayne chains inside the cavity of the macrocycle, but the crystal
The angle between two terminal spꢀcarbon atoms and the
centroid of the central C–C bond of the polyyne chain (φ) is a
useful parameter for quantifying the bending of a polyyne
(Table 5).16 In this family of polyyne rotaxanes, the axle in
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is racemic (Pꢀ1). The closest contact between the two hexayne
chains is 3.290 Å (C6A–C6B), which is less than the sum of
van der Waals radii of two spꢀcarbon atoms (3.4 Å).53 Few
1
2
3
4
5
6
7
8
9
10
11
12
13
14
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41
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Xꢀray crystal structures of rotaxanes with nonꢀcovalent interactions (green lines) between macrocycle and dumbbell. (a) 2d·M1;
d(CH/Csp): a: 2.738 Å; b: 2.839 Å; c: 2.828 Å. (b) 2c·M6; d(CH/Csp): a: 2.813 Å; b: 2.670 Å; c: 2.817 Å; d: 2.827 Å. (c) 2c·M7 with
highlighted Carene/Csp contacts: d(Carene/Csp): a: 3.276 Å; b: 3.193 Å; c: 3.296 Å; d: 3.307 Å; e: 3.373. (d) 2c·M2 with highlighted
C
arene/Csp contacts: a: 3.26 Å; b: 3.40 Å; c: 3.36 Å. (e) (2c)2·M1 [3]rotaxane. (f) Carbon-carbon bond lengths and BLA values of hex-
ayne chains in (2c)2·M1 (A and B) and 2c·M1. Errors are estimated at 3σ probability. Unrelated hydrogen atoms and solvents are
omitted for clarity.
Table 5. Summary of crystallographic data of 2c·M2, 2c·M6, 2c·M7 and 2d·M1 rotaxanes. For comparison data for 2c·M1
rotaxane and free hexayne 2c also are presented
.
Compound
BLA (Å)
0.143(9)
0.148(14)
0.160(8)
0.140(15)
0.143(11)
0.143(8)
Avg. Csp–Csp (Å)
1.357(5)
Ref.
Avg. ∠Csp–C≡C (°)
177.8(11)
∠ φ (°)[a]
171.8(1)
174.1(1)
168.1(1)
164.5(1)
172.0(1)
180[c]
Avg. C≡C (Å)
1.214(5)
1.207(8)
1.219(16)
1.211(7)
2c·M1
2c·M2
2c·M6
2c·M7
2d·M1
2c
4
[b]
177.6(14)
174.7(21)
175.8(20)
177.0(19)
1.358(9)
1.343(15)
1.355(6)
1.356(7)
1.359(5)
[b]
[b]
[b]
1.211(5)
1.208(7)
177.0(14)
26
[a]
[b]
[c]
φ is the angle between two terminal sp-carbons and the centroid of the central C–C bond of polyyne chain. This work. 2c
occupies a position across a crystallographic inversion center resulting in φ = 180°.
crystal structures with πsp/
πsp interaction are known54 and
knowledge, this is the first time that such a long single
bond has been found in the middle of a hexayne chain.
In rotaxane 7a·M1, the macrocycle sits on top of one diꢀ
bromoethene moiety, while the second dibromoethene moiety
points in the opposite direction with a slight twist (torsion
angle 141.85(8)°, Figure 7a). Both central triple bonds have a
similar length (1.202 Å), and the length of the single bond
between them is 1.39 Å. The macrocycle interacts with the
axle through several CH/π short contacts formed between the
in all cases the alkyne chains adopt a crossed alignment,
which may prevent significant orbital overlap between the
two close-lying alkynes, conferring stability to the com-
pounds.
The two hexayne chains in (2c)2·M1 have the same cur-
vature within error: the average ∠C–C≡C angles are
176(2)° and 176(3)° for chains A and B, respectively. In
chain A, the average BLA is 0.150(11) Å and in chain B it is
0.175(44) Å, which is surprisingly high (Figure 6f). As a
general rule, in extended polyynes, BLA gradually decreas-
es towards the middle of the chain.47 For example, in ro-
taxane 2c·M1 the C≡C triple bonds get longer and the sin-
gle C–C bonds get shorter towards the middle of the chain,
as depicted in Figure 6f (red line). In (2c)2·M1 both chains
deviate from this trend, especially C-C single bonds at C6,
the position where polyyne chains form a van der Waals
contact. This aberration is clearly reflected in BLA values
of A and B chains of (2c)2·M1 (Figure 6f). To our
π
ꢀsystem of the axle and alkyl chain of the macrocycle. In
addition, a bromine atom of the second dibromoethene moiety
forms van der Waals contacts with the aromatic
the resorcinol part of the macrocycle (Figure 7a).
π system of
In the porphyrin rotaxanes 5a·M1 and 5c·(M1)2, one molꢀ
ecule of methanol is coordinated to the Zn center (Figure
7b,c). In both cases, the tetrayne chains are slightly arced, with
an average ∠C–C≡C angle of 176.3(13)° in 5a·M1.
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graphic structure determinations, we detail a plethora of inꢀ
1
2
3
4
5
6
termolecular nonꢀcovalent interactions within the mechanical
bond of the rotaxanes. The character of the polyyne and macꢀ
rocycle interaction depends on the structure and the size of the
macrocycle. Interestingly, these intermolecular interactions
significantly perturb the BLA in the polyyne chains of the
[3]rotaxane (2c)2·M1.
7
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ASSOCIATED CONTENT
Supporting Information. Synthesis and characterization of
rotaxanes, DSC and crystallographic data, as well as CIF files
are documented in SI. This material is available free of charge
via the Internet at htpp://pubs.acs.org
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AUTHOR INFORMATION
Corresponding Author
*harry.anderson@chem.ox.ac.uk, rik.tykwinski@fau.de
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
We thank Steve M. Goldup (University of Southampton) for
providing a sample of macrocycle M8, and Fabian Fritze (Uni-
versity of Erlangen) for help with optimization of cross-
coupling conditions. We thank the European Research Council
(grant 320969) for support, and the EPSRC UK Mass Spec-
trometry Facility at Swansea University for mass spectra.
L.D.M. acknowledges an Oxford University Raffy Manoukian
scholarship. Funding is gratefully acknowledged from the
University of Erlangen-Nuremberg, the Deutsche For-
schungsgemeinschaft (DFG – SFB 953, “Synthetic Carbon
Allotropes” and DFG grant “Rotaxane Protected Polyynes and
Cumulenes).
Figure 7. X-ray crystal structure of rotaxanes 7a·M1 (a),
5a·M1 (b) and 5c·(M1)2 (c). Caryl/Br short contacts in 7a·M1
are highlighted with green lines. a: 3.33 Å; b: 3.37 Å.
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(45) In general, low temperature single crystal diffraction
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