Active metal template synthesis of a neutral indolocarbazole-containing [2]rotaxane host system for selective oxoanion recognition

Article abstract of DOI:10.1039/c7ob01040k

An active metal template strategy was used to synthesise a neutral indolocarbazole containing [2]rotaxane anion host system. ¹H NMR anion binding investigations reveal that the [2]rotaxane recognises a range of monoanions in acetone-d₅:D₂O 95: 5 with an unusual interlocked host selectivity for acetate and dihydrogenphosphate oxoanions over halides. The rotaxane displays an overall selectivity for the sulfate dianion, favouring a 2: 1 host: guest binding stoichiometry at low sulfate concentration and a 1: 1 stoichiometry in the presence of excess sulfate. Fluorescence titration demonstrates that the [2]rotaxane is also capable of sensing guest anions via significant changes in its emission spectrum.

Full text of DOI:10.1039/c7ob01040k

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Active metal template synthesis of a neutral
indolocarbazole-containing [2]rotaxane host
Cite this: DOI: 10.1039/c7ob01040k
system for selective oxoanion recognition†
Asha Brown,
Thomas Lang, Kathleen M. Mullen
and Paul D. Beer
*
An active metal template strategy was used to synthesise a neutral indolocarbazole containing [2]rotaxane
1
anion host system. H NMR anion binding investigations reveal that the [2]rotaxane recognises a range of
monoanions in acetone-d5 : D2O 95 : 5 with an unusual interlocked host selectivity for acetate and di-
hydrogenphosphate oxoanions over halides. The rotaxane displays an overall selectivity for the sulfate
dianion, favouring a 2 : 1 host : guest binding stoichiometry at low sulfate concentration and a 1 : 1 stoi-
chiometry in the presence of excess sulfate. Fluorescence titration demonstrates that the [2]rotaxane is
also capable of sensing guest anions via significant changes in its emission spectrum.
Received 28th April 2017,
Accepted 16th May 2017
DOI: 10.1039/c7ob01040k
rsc.li/obc
the desired interlocked product. For example, while the hetero-
cyclic indolo-[2,3a]-carbazole group – a potent bidentate hydro-
Introduction
Mechanically interlocked rotaxane and catenane molecules gen-bond-donor motif with integral fluorescent anion sensing
were recently heralded as key components in a new era of mole- capability8
has been elegantly incorporated into
–
cular robotics1,2 and the exploration of controlled molecular macrocyclic9–12 and foldameric13–18 anion receptors, along
motion in these systems is a topical theme within supramole- with two structurally related homocatenane receptors,19,20 the
cular chemistry.3 In parallel, increasing attention is being anion-templated synthesis of an indolocarbazole-containing
devoted to the exploration of the host–guest chemistry of inter- [2]rotaxane anion receptor is yet to be achieved. Although
locked rotaxane and catenane molecules owing to their poten- sulfate and fluoride anions were shown to template the orthog-
tial to incorporate internal preorganised, three-dimensional onal interpenetrative assembly of an indolocarbazole thread-
binding pockets which can effectively shield an encapsulated ing component and isophthalamide-containing macrocycle,21
guest species from the bulk solvent medium. Since anionic this anion templated pseudorotaxane assembly process has
substrates exhibit a range of three-dimensional geometries not to date been successfully extended to the synthesis of a
and high solvation enthalpies in protic solvents, interlocked permanently interlocked rotaxane species in the solution
molecules are particularly attractive as potential host systems phase.22 The single reported example of an indolocarbazole [2]
for anions. Emulating Sauvage’s classical passive metal tem- rotaxane relied on an alternative template approach which
plate approach,4,5 we have used anions as discrete templates to exploited favourable charge-transfer and electrostatic inter-
direct the assembly of interlocked rotaxane and catenane host actions between an electron-rich indolocarbazole threading
systems. Upon removal of the template, the interlocked component and electron-deficient tetracationic 4,4-bipyridi-
product contains a cavity which is complementary in size and nium macrocycle component; however the inherent instability
shape to the anion which directed its formation. Using this of the dicationic 4,4-bipyridinium motif towards oxoanions
methodology we have developed a variety of rotaxane- and cate- precluded a full investigation of the anion recognition pro-
nane-based anion receptors which selectively bind various perties of this receptor.23
anions in competitive solvent media.6,7 However, there remain
During the last decade the active metal template strategy24
some instances when the use of anion templation fails to yield has emerged as an efficient approach to the construction of
interlocked molecules which has rapidly grown in scope and
popularity. In particular, the copper(I)-catalysed azide–alkyne
cycloaddition (CuAAC) variant25,26 has been elegantly exploited
in the synthesis of a fascinating range of rotaxane, catenane and
Chemistry Research Laboratory, Department of Chemistry, University of Oxford,
Mansfield Road, Oxford, OX1 3TA, UK. E-mail: paul.beer@chem.ox.ac.uk
†Electronic supplementary information (ESI) available: Synthetic procedures
knot molecules.27–34 Herein we describe the use of a CuAAC reac-
and characterisation data, 1H NMR and 13C NMR spectra of all new compounds,
1H NMR and fluorescence titration protocols, spectra and binding curves. See
DOI: 10.1039/c7ob01040k
tion in the active metal template synthesis of a previously inac-
cessible neutral indolocarbazole [2]rotaxane anion host system.
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1H NMR anion binding investigations reveal that the rotaxane template rotaxane synthesis methodology,25,26 the macrocycle
host recognises a range of monoanions in acetone-d5 : D2O 95 : 5 component is also functionalised with an internally directed
with a preference for oxoanions over halides, which is unusual pyridyl group: this motif will endotopically bind the copper(I)
and rare among interlocked anion receptors.35 Strong and selec- catalyst during the CuAAC axle-forming reaction, thereby
tive binding of the sulfate dianion is also demonstrated, and the directing the formation of the axle through the cavity of the
[2]rotaxane is shown to be capable of sensing anions via macrocycle.
changes in its fluorescence emission spectrum.
The [2]rotaxane 4 was prepared as outlined in Scheme 2.
The synthesis of the isophthalamide- and pyridyl-functiona-
lised macrocycle and indolocarbazole thread precursor com-
ponents 1 and 2 is described in the ESI.† Macrocycle 1 was
stirred with 1 equivalent of Cu(CH3CN)4PF6, 3 equivalents of
the bis-azide-terminated threading compound 2 and 6 equiva-
Results and discussion
Design and synthesis of the indolocarbazole [2]rotaxane host
The design and active metal template approach to the syn- lents of the alkyne functionalised stoppering compound 3 25 in
CH2Cl2 for 48 hours. After de-metallation (EDTA/NH4OH(aq)),
analysis of the crude product distribution using 1H NMR spec-
thesis of the target indolocarbazole [2]rotaxane anion host
system is illustrated in Scheme 1. The rotaxane incorporates
bidentate indolocarbazole and isophthalamide hydrogen- troscopy indicated approximately 70% conversion of the
macrocycle 1 to the interlocked [2]rotaxane 4, which was iso-
lated in 52% yield after purification by preparative thin layer
bond-donor motifs in the axle and macrocycle components
respectively, which are designed to convergently bind to a
guest anion within a pseudotetrahedral interlocked binding chromatography.
1H NMR techniques were employed to probe the rotaxane’s
interlocked co-conformation in solution. The 1H NMR spec-
pocket. Taking inspiration from Leigh and co-workers’ elegant
work on the development of the original CuAAC active metal
Scheme 1 Design and active metal template based approach to the synthesis of the target indolocarbazole containing [2]rotaxane anion host
system.
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Scheme 2 Active metal template synthesis of the indolocarbazole-containing [2]rotaxane 4.
poration of the macrocycle into the [2]rotaxane (Δδ
=
0.65–1.14 ppm), suggesting that these protons may be shielded
by the aromatic ring currents of the axle indolocarbazole
group. Further evidence that the macrocycle hydroquinone
groups lie in close proximity to the axle indolocarbazole
groups in the [2]rotaxane was provided by 2-D ROESY spec-
troscopy (Fig. 2), which indicated through-space ROE inter-
actions between protons 6 and 7 and the indolocarbazole aro-
matic protons a and b. Since parallel face-centred aromatic
stacking interactions between the indolocarbazole and hydro-
quinone groups, which are both electron-rich, are not pre-
dicted to be energetically favourable,36,37 it is possible that this
co-conformation is stabilised by a complementary hydrogen-
bonding arrangement, as depicted in Scheme 2, with the indo-
locarbazole and hydroquinone aromatic systems favouring a
parallel offset or perpendicular stacking arrangement. This is
Fig. 1 Stacked partial 1H NMR spectra of the axle S8 (top), [2]rotaxane 4
(middle) and macrocycle 1 (bottom) in CDCl3 at 298 K.
trum of the [2]rotaxane 4 in CDCl3 is compared to those of its
constituent axle and macrocycle components in Fig. 1.‡ A pro-
nounced upfield shift and splitting of the macrocycle hydro-
quinone proton resonances (6 and 7) is observed upon incor-
Fig. 2 Section of the 1H–1H ROESY NMR spectrum of the [2]rotaxane 4
(500 MHz; CDCl3; 298 K) showing ROE correlations between the macro-
cycle hydroquinone and axle indolocarbazole proton resonances.
‡The non-interlocked axle component (compound S8, ESI†) of the [2]rotaxane 4
was isolated as a side-product of the [2]rotaxane synthesis reaction.
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Table 1 Association constants for 1 : 1 complexes of the indolocarba-
zole thread 2 and [2]rotaxane 4 with various anions in acetone-d6 : D2O
95 : 5 at 298 K
supported by the observation that the axle indolocarbazole NH
and macrocycle internal isophthalamide proton signals (e, 10
and 11) shift downfield upon rotaxane formation, consistent
with hydrogen-bond-induced polarisation of the X–H (X = C,
N) bonds (Fig. 1).§
Ka (M−1
)
Anionb
2
4
F−
378 (15)
200 (8)
107 (1)
739 (39)
516 (11)
142 (2)
Anion binding investigations
Cl−
Br−
AcO−
1H NMR titration experiments. The anion binding properties
of the [2]rotaxane 4 were probed using 1H NMR titration
experiments in an organic–aqueous acetone-d6 : D2O 95 : 5
solvent mixture. Prior to this analogous titration experiments
were performed using the indolocarbazole thread 2 and macro-
cycle 1 in order to provide preliminary insight into the anion
binding capabilities of the individual axle indolocarbazole and
macrocycle isophthalamide sub-components which comprise
the rotaxane’s interlocked binding cavity. However, upon
addition of increasing concentrations of the tetrabutyl-
ammonium (TBA) salts of a range of anions to a solution of
the macrocycle 1 in acetone-d6 : D2O 95 : 5 minimal pertur-
bations of the proton shifts were observed, suggesting that the
macrocycle’s bis-amide hydrogen-bonding cleft does not inter-
act with the anions to a significant degree in the competitive
acetone/D2O solvent medium.
1347 (40)
2359 (129)
−
H2PO4
NO3
286 (8)
2049 (127)
−
c
c
a Association constant calculated by analysis of the anion induced
changes in the chemical shift of proton d (for compound 2) or proton
11 (for compound 4) using WinEQNMR38 software. Estimated
expanded uncertainties at the 95% confidence interval level are given
in parentheses. b All anions added as TBA salts. c Chemical shift
changes too small to allow for accurate binding constant determi-
nation over the anion concentration range covered.
indicative of the fast exchange complexation of the anion
guests by the [2]rotaxane host molecule (Fig. 3). Upfield pertur-
bations in the internal macrocycle proton 11 and, in most
cases, the indolocarbazole aromatic proton d imply that both
the axle indolocarbazole and macrocycle isophthalamide
groups participate in C–H–X− as well as N–H–X− hydrogen
bonding interactions with the encapsulated anion. The macro-
cycle hydroquinone proton signals 6 and 7 shift upfield upon
anion addition, suggesting that the anion-induced co-confor-
mational changes lead to increased overlap between the π elec-
tron clouds of the macrocycle hydroquinone and indolocarba-
zole functional groups.
Association constants for 1 : 1 stoichiometric binding were
determined by WinEQNMR38 analysis of the titration data,
monitoring the chemical shift changes of the internal macro-
cycle isophthalamide proton 11 (Table 1 and Fig. S17, ESI†).
The [2]rotaxane displays a higher affinity for all of the monoa-
nions studied compared to the individual macrocycle and axle
components, which can be attributed to the availability of con-
vergent hydrogen-bond-donor groups from both the axle
indolocarbazole and macrocycle isophthalamide groups in the
multivalent [2]rotaxane host system. The anion selectivity
trend observed for the [2]rotaxane 4 generally mirrors that of
the indolocarbazole receptor 2. However, while the rotaxane
retains a marginal overall preference for acetate, the most dra-
matic enhancement is for the dihydrogenphosphate anion,
which intimates that the topological arrangement of the hydro-
gen bond donor groups in the rotaxane’s interlocked binding
cavity most effectively complements the tetrahedral geometry
of the dihydrogenphosphate anion.
In contrast for the more potent indolocarbazole receptor 2
clear 1H NMR evidence of anion recognition was obtained.
Although the indolocarbazole NH resonances are not observed
in this protic solvent medium owing to hydrogen–deuterium
exchange, complexation could be inferred from the observed
downfield perturbations in the aromatic CH proton environ-
ment d (Fig. S16, ESI†). Association constants were determined
by non-linear least squares analysis of the anion-induced
changes in the chemical shift of proton d using WinEQNMR38
software (Table 1 and Fig. S17, ESI†). Compound 2 was found
to bind the monoanions in a 1 : 1 stoichiometric ratio with
moderate affinities. The observed order of selectivity (AcO−
≫
F− > H2PO4 > Cl− > Br− > NO3−) is generally consistent with
the anion recognition behaviour of previously reported acyclic
indolocarbazole anion receptors8,39–43 and broadly correlates
with the relative basicities of the monoanions. The marked
preference for acetate may additionally reflect optimal geo-
metric complementarity between the Y-shaped carboxylate
anion and the spatial arrangement of the indolocarbazole
motif’s two preorganised NH hydrogen-bond-donor groups.
Similarly, addition of anions as their TBA salts to the [2]
rotaxane 4 in acetone-d6 : D2O 95 : 5 led to progressive shifts in
a number of the axle and macrocycle proton environments,
−
§As further evidence for the existence of the proposed stabilising complemen-
tary hydrogen bonding arrangement, spontaneous pseudorotaxane assembly
was observed on addition of the indolocarbazole threading compound 2 to a
solution of macrocycle 1 in CDCl3 at 298 K, evidenced by diagnostic upfield
shifts in the macrocycle hydroquinone proton 1H NMR resonances. Both Cl−
anions and Cu(I) cations, which are expected to compete with the indolocarba-
zole threading compound for binding to the macrocycle’s isophthalamide and
pyridyl sub-units respectively, were qualitatively observed to destabilise the
pseudorotaxane assembly.
Sulfate binding experiments. The apparent complementarity
between the [2]rotaxane’s interlocked binding cavity and the
dihydrogenphosphate anion encouraged us to also investigate
the rotaxane’s binding affinity for the tetrahedral sulfate
dianion, since a number of literature reports have established
a precedent for strong and selective sulfate anion binding by
indolocarbazole-containing receptors.14,17,21 Upon titration of
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Fig. 4 Changes in the 1H NMR chemical shifts of protons 11, c and d as
a function of [(TBA)2SO4] observed during titration of (TBA)2SO4 into an
acetone : D2O 95 : 5 solution of the [2]rotaxane 4 at 298 K. Square points
represent experimental data; continuous lines represent theoretical
binding curves for the calculated equilibrium constant values shown in
Tables 2 and S1 (ESI†). For proton assignments, see Fig. 1 and 3.
Fig. 3 1H NMR spectra of a 1.5 mM solution of the [2]rotaxane 4 in
acetone-d6 : D2O 95 : 5 (500 MHz; 298 K) in the presence of (i) 0, (ii) 1
and (iii) 5 equivalents of TBA·H2PO4.
(TBA)2SO4 into a solution of the [2]rotaxane 4 in acetone-
d6 : D2O 95 : 5 the 1H NMR chemical shifts of several of the
rotaxane proton environments exhibited complex sulfate con-
centration dependencies, indicating a composite binding
mode. For example, the binding curves obtained by following
the indolocarbazole aromatic protons c and d display sharp
minima after addition of approximately 0.5 molar equivalents
of the sulfate dianion (Fig. 4), while a slight sigmoidal feature
in the titration curve for the macrocycle internal isophthala-
mide proton 11 can also be discerned at low sulfate concen-
tration. This is consistent with the initial formation of a 2 : 1
2−
stoichiometric 4 : SO4 complex which is replaced by a 1 : 1
Scheme 3 Equilibria between the free [2]rotaxane 4 and 2 : 1 and 1 : 1
stoichiometric 4 : SO4 complexes observed on titration of (TBA)2SO4
into acetone-d6/D2O solutions of the [2]rotaxane.
2−
stoichiometric complex at higher sulfate concentration
(Scheme 3). The 2 : 1 4 : SO4 complex is presumed to be a
2−
sandwich-type assembly in which the sulfate anion nests
within an enclosed cavity formed by the close approach of two
rotaxane molecules, which allows the anion to participate in a The formation of these higher order aggregates is conceivably
maximum of eight N–H–O hydrogen-bonding interactions driven by the dianion’s strong hydrogen-bond-acceptor charac-
(Scheme 3). There are a number of literature precedents for ter and preference for high coordination numbers of up to
the formation of similar 2 : 1 receptor : SO42− complexes.21,44–46 12.47 Analysis of the titration data using WinEQNMR38 soft-
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Table 2 Association constants for 2 : 1 and 1 : 1 complexes of the [2]
rotaxane 4 with SO4 anions in acetone-d6 : D2O 95 : 5 and acetone-
d6 : D2O 90 : 10 at 298 Ka
Table 3 Association constants, log K and K (M−1), for 1 : 1 complexes of
the [2]rotaxane 4 with various anions in acetone : H2O 95 : 5 at 298 K
and observed change in the maximum wavelength of the emission band,
Δλmax(em)(nm), in the presence of each anion
2−
Solvent
Acetone-d6 : D2O 95 : 5b Acetone-d6 : D2O 90 : 10c
Anionb
log Ka
K (M−1
)
Δλmax(em)c (nm)
K11 = β1 (M−1
)
4518 (872)
48 (12)
216 315 (33 950)
280 (17)
94 (11)
26 366 (2713)
K21 (M−1
)
F−
3.12 (0.01)
2.76 (0.02)
1.65 (0.07)
3.32 (0.01)
3.44 (0.04)
1.47 (0.05)
1322
573
45
2103
2728
29
+40
+15
+10
+35
+40
+<5
K11·K21 = β2 (M−2
)
Cl−
Br−
AcO−
a SO4 added as a TBA salt. b Association constants calculated by ana-
lysis of the changes in the chemical shift of proton 11 using
WinEQNMR38 software. Estimated expanded curve-fitting uncertainties
at the 95% confidence interval level are given in parentheses.
c Association constants calculated by analysis of the changes in the
chemical shift of proton c using WinEQNMR38 software.
2−
−
H2PO4
NO3
−
a log K values calculated by analysis of the anion induced changes in
the fluorescence emission band of the [2]rotaxane 4 using Specfit48
software. Estimated standard deviations are given in parentheses. b All
anions added as TBA salts. c Δλmax(em) value are quoted to the nearest
5 nm. λex = 333 nm. Final anion concentrations: 9.6–10.0 mM.
ware, monitoring the internal macrocycle proton signal 11,
determined overall equilibrium constants of β2 = 216 315
(33 950) M−2 and β1 = 4518 (872) M−1 for formation of the 2 : 1
and 1 : 1 receptor : anion complexes respectively (Table 2).
Pleasingly good numerical agreement with these values was
observed when the equilibrium constants were determined
based on the chemical shift changes of protons a and c
(Table S1, ESI† and Fig. 4).
When the titration experiment was repeated in the more
competitive solvent mixture acetone-d6 : D2O 90 : 10 the calcu-
lated values of β2 and β1 were both found to be suppressed
(Table 2 and Fig. S18, ESI†). However, it is notable that while
the K11 (β1) equilibrium is appreciably disfavoured in the pres-
ence of a higher percentage of D2O, the change in the K21 equi-
librium constant is comparatively insignificant. This may
reflect a greater solvophobic contribution to the K21 equili-
brium process.
Fluorescence anion titration experiments. Taking advantage
of the axle-integrated indolocarbazole fluorophore, fluo-
rescence titration experiments were undertaken to probe the
[2]rotaxane’s anion sensing capabilities. The emission spec-
trum of the [2]rotaxane 4 in acetone : H2O 95 : 5 shows a broad,
featureless emission band with a maximum at 410 nm (λex
=
333 nm). Addition of the TBA salts of AcO−, H2PO4−, F−, Cl−
2−
and SO4 led to substantial (15–40 nm) bathochromic shifts
in the maximum of the emission band, along with significant
quenching (F−, AcO−) or enhancements (Cl−) of the emission
intensity in some cases (Fig. 5a and S19a–f, ESI†).¶ For Br−
−
and NO3 anions comparatively minor spectral changes were
observed, consistent with the 1H NMR evidence for weak inter-
actions between these guest anions and the rotaxane host. For
the monoanions, association constants for 1 : 1 stoichiometric
binding were determined by global analysis of the fluorescence
titration data using Specfit48 software (Table 3, Fig. 5b), and
are in good overall agreement with the 1H NMR-derived
binding data.∥
¶Interestingly, with one exception,14 such substantial anion-induced red shifts
in the emission spectrum have not previously been observed for indolocarba-
zole-based anion receptors.8,17,19,43,49 We are continuing to investigate the cause
of the anion-induced fluorescence responses.
Fig. 5 (a) Changes in the fluorescence emission spectrum of a 15 µM
solution of the [2]rotaxane 4 in acetone : H2O 95 : 5 on addition of an
increasing concentration of TBAH2PO4. (b) Changes in the emission
intensity at 410 nm as a function of [TBAX] on addition of various mono-
anions as their TBA salts. Square points represent experimental data;
continuous lines represent theoretical binding curves for the equilibrium
constant values displayed in Table 3. Temperature: 298 K. λex = 333 nm.
∥Global analysis of the data for the (TBA)2SO4 fluorescence titration experiment
again suggested the operation of 2 : 1 and 1 : 1 stoichiometric host : guest equili-
brium binding processes but we were unable to determine quantitative binding
constants within reasonable error from this experiment.
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13 J. Suk and K.-S. Jeong, J. Am. Chem. Soc., 2008, 130, 11868–
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carbazole-based axle component and an isophthalamide- 15 J. Suk, J. Kim and K.-S. Jeong, Chem. – Asian J., 2011, 6,
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1992–1995.
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acetone-d6 : D2O 95 : 5, with, importantly, an unusual and rare 2013, 25, 46–53.
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anions via significant changes in the wavelength and intensity
of its emission spectrum.
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22 However, sulfate and fluoride anion templation was suc-
cessfully applied to the assembly of a [2] rotaxane compris-
ing indolocarbazole axle and isophthalamide macrocycle
components on a gold surface: L. Zhao, J. J. Davis,
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Acknowledgements
We thank the European Research Council for funding under
the European Union’s 7th Framework programme (FP7/
2007–2013), ERC advanced grant agreement number 267426
and the Engineering and Physical Sciences Research Council
for a post-doctoral fellowship (KMM).
24 For a review of early work on the active metal template syn-
thesis of rotaxane and catenane molecules, see:
J. D. Crowley, S. M. Goldup, A.-L. Lee, D. A. Leigh and
R. T. McBurney, Chem. Soc. Rev., 2009, 38, 1530–1541.
25 V. Aucagne, K. D. Hänni, D. A. Leigh, P. J. Lusby and
D. B. Walker, J. Am. Chem. Soc., 2006, 128, 2186–2187.
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