Synthesis of tripodal [2]rotaxanes: High concentration principle

Article abstract of DOI:10.1039/b209836a

Symmetrical and non-symmetrical tripodal [2]rotaxanes (1) incorporating 1,1'-disubstituted-4,4′-bipyridinium cations (2) and 34-crown-10 (3) have been prepared directly from 4,4′-bipyridine or from monocationic intermediates in high yields at room tempera

Full text of DOI:10.1039/b209836a

Synthesis of tripodal [2]rotaxanes: high concentration principle†
Kirill Nikitin, Brenda Long and Donald Fitzmaurice*
Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland.
E-mail: donald.fitzmaurice@ucd.ie; Fax: +353 1 7162127; Tel: +353 1 7162107
Received (in Cambridge, UK) 8th October 2002, Accepted 6th December 2002
First published as an Advance Article on the web 20th December 2002
Symmetrical and non-symmetrical tripodal [2]rotaxanes (1)
incorporating 1,1’-disubstituted-4,4A-bipyridinium cations
(2) and 34-crown-10 (3) have been prepared directly from
4,4A-bipyridine or from monocationic intermediates in high
yields at room temperature and atmospheric pressure under
conditions that permit the use of high reagent concentrations
(0.1–0.2 M, 150–200 g l21).
In principle, 1a–c may be synthesised by adopting one of two
approaches. A slippage approach,13 in which the axial group is
terminated with a partial stopper B or C that the crown can ‘slip-
over’ at elevated temparatures. A threading approach,14 in
which the axial group threads the crown to form a pseudorotax-
ane and a full stopper group is subsequently attached to the axial
group.
Assembling 1a using the slippage approach was not expected
to yield the desired product since the full stopper group A, based
on the tris(4-tert-butylpenyl)methyl moiety, is too bulky.13 An
option was to prepare 2b or 2c using a partial stopper (B or C)
and thread the crown 3 at elevated temperatures. The partial
stoppers B and C are sufficiently bulky to prevent dethreading
at room temperature.13
The bottom-up assembly of functional nanoscale architectures
in solution is a goal shared by researchers in many fields.1
Expected benefits include greater control over material proper-
ties and technologies that address unmet needs.2
One approach envisaged is the self-assembly of nanoscale
architectures, consisting of molecular and condensed phase
components, in solution.3
In considering what molecular and condensed phase compo-
nents might be suitable,4–6 one is immediately attracted to high-
information content molecules,7 and nanoparticles whose
properties can be tuned by controlling their size and surface
composition.8
There exists a growing number of examples where high-
information content molecules and nanoparticles have been
self-assembled in solution to yield novel nanoscale archi-
tectures, some with significant technological potential.9–11
It is in this context that we report the synthesis of novel
tripodal [2]rotaxanes (1a–c) (Fig. 1) consisting of tripodal axial
units (2a–c) threaded into bis(p-phenylene-34-crown-10) (3)
which is prevented from dethreading by a full stopper (ST)
group (A) or partial stopper groups (B, C). The tripodal group
(TP) binds (after hydrolysis) to the surface of a metal oxide
nanoparticle and orients the axial unit 2 of the rotaxane normal
to the surface.12
Accordingly, partially stopped axial molecules of the general
formula 2 were reacted with the crown 3 (Fig. 2). The conditions
and yields of 1 are given in Table 1.
Slipping of 3 onto 2c, bearing the partial stopper C, did not
proceed in acetonitrile under usual conditions at 60 °C (Table 1,
run 1).13 The same reaction was repeated using higher reagent
concentrations in benzonitrile (runs 2 and 3). Although
benzonitrile dissolves the reagents at concentrations as high as
0.1 M at the reaction temperature, the highest yield achieved did
not exceed 5% (run 3). The yield can not be improved using 2b,
bearing stopper group B (run 4). The symmetrical axial
compound 2d, incorporating a stopper group C at each end (run
5), also did not react with crown even under high concentration
conditions. It is concluded that the tripodal[2]rotaxanes 1 can
not be synthesised using the slippage approach.
Having been unsuccessful in preparing the tripodal [2]rotax-
anes 1b–d using the slippage approach, we considered the
threading approach. The threading approach would generally14
be justified on the basis of the assumption that the 4,4A-
Fig. 2 Slippage of 3 to form [2]rotaxanes 1.
Table 1 Attempted slippage synthesis of [2]rotaxanes 1b–da
Yield
Run
1
2+ST1/ST2
Solvent [2]/M
T/°C Time (%)
1
2
3
4
5
1c
1c
1c
1b
1d
2c+TP/C
2c+TP/C
2c+TP/C
2b+TP/B
2d+C/C
MeCN
PhCN
PhCN
PhCN
PhCN
0.01
0.05
0.1
0.1
0.1
60
60
60
50
75
12 h
2 d
5 d
2 d
2 d
0
0
5
0
0
Fig. 1 Tripodal [2]rotaxanes 1a–c and their components.
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b2/b209836a/
a All reactions were run at 2+1 ratio of crown 3 to the axial unit 2.
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CHEM. COMMUN., 2003, 282–283
This journal is © The Royal Society of Chemistry 2003

Table 3 High concentration synthesis of 1a (Fig. 4)
bipyridinium dication of the axial group would form a
pseudorotaxane with the crown 3 followed by stoppering of the
pseudorotaxane with a suitable stopper. However, for the
synthesis of 1 that approach would require complexation of 3 to
the 4,4A-bipyridinium monocation before alkylation to form the
dication. It should be noted that rotaxane formation based on the
complexation of the monocatrion under ultra high pressure (30
°C, 12 kbar, 2 days, 26% yileld) has been reported.15 On this
basis more facile conditions were sought.
Yield 1
(%)
Run
Rotaxane
ST1
ST2
Time/days
1
2
3
4
1e
1f
1a
1g
A
C
TP
B
A
B
A
B
10
5
5
40
84
48
90
5
a Reactions were run at room temperature in PhCN at 0.2 M starting
Reactions of 4,4-bipyridyl with a full stopper alkylating
reagent A-Br (Fig. 3) were studied. Concurrent formation of 1e
and 2e under a range of conditions (Table 2) was observed. As
has previously been reported, the self-assembly of rotaxane 1e
practically did not proceed in acetonitrile (run 1).15 Traces of 1e
were found when the reaction was run in DMF (run 2). In
benzonitrile the solubility of all components is high and both
products are formed at relatively good yields (run 3). It is
assumed that the mechanism of rotaxane formation involves the
crown 3 being threaded by the monocation intemediate 4 to
form a weak complex 5 and subsequent alkylation of 5 at the
remaining pyridine nitrogen (Fig. 4).
In short, under mild conditions (room temperature and
atmospheric pressure) and in the presence of a high concentra-
tion of crown 3 in benzonitrile, conditions that favour formation
of a complex 5 between the crown 3 and the interim monocation
4, a good yield of 1e is obtained (36%).
To fully exploit these high concentration conditions, pre-
formed compounds 4 were reacted with the full stopper A or
partial stopper B in the presence of crown 3 (Table 3). These
results show a a dramatic increase in the yields obtained.
The yields are higher for the less symmetrical and thus more
soluble reagents 4 and 6 incorporating partial stoppers B and C
(Table 3, runs 2 and 4). This effect cannot be accounted for by
crown concentration as the same concentration of 3 was used in
all cases.
concentration of 3 and 0.1 M of 4
Table 4 Synthesis of the tripodal rotaxane 1b (stopper B)a
T/°C
Time
Yield 1 (%)
Yield 2b (%)
50
40
30
20
20 h
2 d
2 d
5 d
40
52
62
75
37
40
30
24
a Reactions were run in PhCN at 0.2 M starting concentration of 3 and 0.1
M of 4
circumstances there may not be enough solvent molecules to
completely solvate reagent molecules. Accordingly, the rea-
gents simply solvate each other forming intermolecular com-
plexes. In other words, under high concentration conditions the
extent of pseudorotaxane 5 formation may be higher than
expected on the basis of equilibrium constant as evidenced by
the higher yields.
The approach was also applied for the synthesis of tripodal
[2]rotaxane 1b. For this reaction the temperature dependence of
the rotaxane yield has been studied (Table 4). Rotaxane
formation is favoured at lower temperatures, a finding corre-
spondent with the formation of the interim pseudorotaxane.
To summarise, this work describes a novel and elegant
approach to the synthesis of [2]rotaxanes of the common
formula 1 showing tripodal structures, at room temperature and
atmospheric pressure in high yields. A similar approach may be
useful for the synthesis of known and new rotaxanes, catenanes
and other supramolecular systems.
The greater solubility of unsymmetrical reagents 4 and 6
permits total concentrations of ca. 40% by weight. Under these
Notes and references
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Fig. 3 Direct formation of [2]rotaxane from 4,4A-bipyridyl.
Table 2 Synthesis of 1e at room temperaturea
Yield 1e
Yield 2e
Run
Solvent
Solubility
(%)
(%)
1
2
3
MeCN
DMF
PhCN
Sparing
Good
Complete
2.5
9
36
17
68
35
a Reactions were run at 0.2 M nominal starting concentration of 3 and 0.1
M of 4,4A-bipiridyl.
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Am. Chem. Soc., in press).
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Fig. 4 Possible mechanism for high concentration threading to form 1.
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