A meta-xylenediamide macrocycle containing rotaxane anion host system constructed by a new synthetic clipping methodology

Article abstract of DOI:10.1039/c1nj20109c

A novel rotaxane containing a meta-xylenediamide macrocycle is prepared by a new clipping methodology. Upon anion metathesis to the non-coordinating hexafluorophosphate salt, the rotaxane host system was shown to bind chloride and bromide anions more stro

Full text of DOI:10.1039/c1nj20109c

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PAPER
A meta-xylenediamide macrocycle containing rotaxane anion host system
constructed by a new synthetic clipping methodologywz
Nicholas H. Evans, Christopher J. Serpell and Paul D. Beer*
Received (in Montpellier, France) 11th February 2011, Accepted 25th March 2011
DOI: 10.1039/c1nj20109c
A novel rotaxane containing a meta-xylenediamide macrocycle is prepared by a new clipping
methodology. Upon anion metathesis to the non-coordinating hexafluorophosphate salt, the
rotaxane host system was shown to bind chloride and bromide anions more strongly than the
basic oxoanions dihydrogen phosphate and acetate in the competitive solvent system 1 : 1
CDCl3 : CD3OD.
triazolium axle13 and, very recently, we reported the prepara-
Introduction
tion of a dicationic rotaxane able to bind chloride in solvent
mixtures containing 35% water.14 Notably, this rotaxane was
The desire to recognise negatively charged species of
biological, medical and environmental importance is
a
prepared via an alternative clipping synthetic route which
did not use Grubbs’ catalyst.15 The exclusion of Grubbs’
catalyst is attractive for a number of reasons, for example its
incompatibility with certain chemical functionalities, and its
not inconsiderable expense.
major contributory factor in the current intense research
interest being shown in anion supramolecular chemistry.1
Within this very diverse field, the use of interlocked
molecules, such as rotaxanes and catenanes, to achieve anion
recognition is beginning to gather pace. Possessing unique
topologically constrained cavities, these molecules have the
potential to demonstrate selectivity in the binding of
anionic guests,2 and a number of research groups have now
reported on the anion binding properties of such species.3–7
In this paper we report the preparation of a novel [2]rotaxane
host system, containing a meta-xylenediamide macrocycle,
that creates a yet to be investigated interlocked hydrogen
bond donating cavity. The rotaxane is synthesized via a
new clipping condensation methodology, which does not
employ Grubbs’ catalyst, and has been characterized by
Our first anion binding [2]rotaxanes consisted of
a
N-methyl pyridinium axle penetrating through an isophthal-
amide macrocycle. Each of these rotaxanes were prepared
by ‘‘clipping’’
a chloride anion templated orthogonal
assembly of axle and bis-vinyl functionalised macrocycle
precursor components using Grubbs’ catalyst. Upon removal
of the chloride anion template these host systems demon-
strated notable selectivity for chloride over more basic
oxoanions.8
We have since set out to adapt and improve such inter-
locked host systems. Chloride9,10 and sulfate11 selective
catenanes have been constructed, as well as a ferrocene-
appended redox-active rotaxane capable of electrochemical
anion sensing.12 Impressive selectivity for bromide over
chloride has been demonstrated in a rotaxane containing a
Department of Chemistry, Inorganic Chemistry Laboratory,
University of Oxford, South Parks Road, Oxford, UK OX1 3QR.
E-mail: paul.beer@chem.ox.ac.uk
w Dedicated to Prof. Didier Astruc on the occasion of his 65th
birthday.
z Electronic supplementary information (ESI) available: Copies of
NMR and HRMS of novel compounds; crystallographic data for
structure of rotaxane 5+Clꢀ, CCDC reference number 813286, and
the protocol and further data from the 1H NMR titrations. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1nj20109c
Fig. 1 Schematic representation of the preparation of the target
xylene rotaxane anion host system.
c
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NMR spectroscopy, electrospray mass spectrometry, and by
the determination of a crystal structure. Anion recognition
studies on the hexafluorophosphate salt of the rotaxane in the
competitive 1 : 1 CDCl3 : CD3OD solvent system reveal the
interlocked host’s ability to bind chloride and bromide anions
selectively in preference to basic oxoanions.
Scheme 1 Synthesis of rotaxane 5+Xꢀ (Xꢀ = Clꢀ, PF6ꢀ) and xylenediamide macrocycle 7.
c
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Results and discussion
Design, synthesis and characterization
The target rotaxane was prepared via a modification of a
previously reported rotaxane synthesis15 but with the reversal
of amine and activated carboxylic acid functionalities of the
macrocyclic precursor components (Fig. 1).
To an equimolar mixture of an activated dicarboxylic acid
macrocycle precursor and a N-methyl pyridinium chloride
axle, an equivalent of meta-xylene diamine is added, which
results in formation of the macrocyclic component of the
rotaxane.y Exchange of the chloride anion for a non-
coordinating anion affords the anion binding host.
ꢀ
The actual synthesis of rotaxane 5+Xꢀ (Xꢀ = Clꢀ or PF6
)
is provided in Scheme 1. Dicarboxylic acid 2 was prepared by
the alkaline hydrolysis of dinitrile 1.16 This was subsequently
activated for amide formation by preparation of diester 3
utilising DCC and N-hydroxysuccinimide.z Diester 3 was then
dissolved in dry CH2Cl2 with one equivalent of axle 4+Clꢀ.
Upon complete dissolution of the two components, a slight
excess of dry NEt3 was added followed immediately by one
equivalent of meta-xylenediamine.
The resulting crude reaction mixture was purified by thin
layer preparative silica gel chromatography. Repeated running
of plates was required due to the presence of the free macro-
cyclic component 7, which was only marginally less polar than
the rotaxane. The final isolated yield of the xylenediamide macro-
cycle containing rotaxane was 15% (from dicarboxylic acid 2).
Macrocycle 7 was prepared in an equivalent reaction, using
unstoppered thread 6+Clꢀ (see Scheme 1) in a yield of 50%.
Comparison of the 1H NMR spectra of rotaxane 5+Clꢀ
with those of its component macrocycle 7 and axle 4+Clꢀ,
indicate the interlocked nature of 5+Clꢀ (Fig. 2). The upfield
shifts of the axle cavity protons x and w, and the downfield
shifts of the macrocycle cavity protons c and e, in the rotaxane
compared to the free components are indicative of competitive
hydrogen bonding to the chloride anion. The splitting
and upfield shifts of hydroquinone protons g and h are
attributed to favourable p–p stacking between the macrocycle
hydroquinone motifs and the pyridinium group of the axle. In
addition, hydrogen bonding of the axle’s N-methyl pyridinium
group to the polyether oxygens of the macrocycle can be
inferred by the downfield shift of resonance z.
Fig. 2 Partial 1H NMR spectra of (a) macrocycle 7 (b) rotaxane
5+Clꢀ and (c) axle 4+Clꢀ. Significant peak shifts are highlighted.
Solvent: CDCl3, T: 293 K. For atom labels see Scheme 1.
a crystal structure (Fig. 3). Crystals of the rotaxane suitable
for X-ray diffraction structural determination were grown by
slow diffusion of diisopropyl ether into a chloroform solution
of the rotaxane, and data were collected using synchrotron
radiation at Diamond Light Source beamline I19. While both
axle amide protons participate in hydrogen bonding to the
chloride anion (both with N(H)ꢁ ꢁ ꢁCl distances of 3.332(5) A),
only one of the macrocycle amide protons does, with an
N(H)ꢁ ꢁ ꢁCl hydrogen bond length of 3.278(7) A. The other
macrocycle nitrogen resides at distance of 4.395(6) A away,
with an N–Hꢁ ꢁ ꢁCl angle of 138.91. This is in contrast to the
rotaxane’s isophthalamide analogue 8+Clꢀ14 (depicted in
Fig. 4) where hydrogen bonding of both macrocyclic amide
protons to the chloride anion are observed in the solid state.
Comparing the 1H NMR spectra of the two rotaxanes (Fig. 5a
and b) reveals that while the resonances for the axle cavity
Further evidence to support the interlocked nature of the
rotaxane was noted by the presence of intercomponent
through-space peaks in the 2D 1H NMR ROESY spectrum
and a monocationic peak at m/z B1634.86 in the electrospray
HRMS, attributable to the organic fragment 5+.
Incontrovertible proof of the interlocked nature of the
chloride salt of the rotaxane was provided by elucidation of
y It is proposed that after formation of the first amide that the chloride
anion will hydrogen bond to this newly created functionality. By being
also strongly bound to the positively charged axle component, the
chloride anion will thus template the components, such that upon
formation of the second amide, the axle is trapped within the macro-
cycle to produce the target rotaxane.
z Attempts to activate 2 to the bis-acid chloride by use of thionyl
chloride often failed, suggesting the bis-acid chloride can be unstable
under the experimental conditions used.
c
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Fig. 3 X-Ray crystal structure of rotaxane 5+Clꢀ. Hydrogen bonding interactions to chloride anion are highlighted.
shifts of the cavity protons c, e, w and x compared to 5+Clꢀ
consistent with loss of the hydrogen-bond accepting chloride
anion from the rotaxane cavity. Crucially the hydroquinone
resonances g and h remain upfield and split, indicating that
p–p stacking between the pyridinium and hydroquinone rings
has been retained and that the rotaxane has remained inter-
locked upon anion exchange. Further evidence is provided by
the appearance of through-space intercomponent cross-
couplings in the 2D 1H NMR ROESY spectrum and the
molecular ion peak in HRMS (see ESIz).
Anion binding investigations
ꢀ
The anion binding properties of rotaxane 5+PF6 were
investigated by 1H NMR titration experiments in the competi-
tive 1 : 1 CDCl3 : CD3OD solvent system. To samples of the
rotaxane, aliquots of tetrabutylammonium (TBA) salts of
various anions were added and NMR spectra recorded.
Table 1 1 : 1 Anion association constants, K (Mꢀ1), of rotaxane
5
Fig. 4 Isophthalamide rotaxane analogue 8+Clꢀ.14
+PF6
ꢀa
Anion
K/Mꢀ1
protons w and x have almost identical chemical shift in the two
rotaxanes, those for the macrocycle cavity protons c/w and e/d
are dramatically less downfield for the xylenediamide macro-
cycle containing rotaxane.
Clꢀ
Brꢀ
H2PO4
OAcꢀ
810
1070
645
285
ꢀ
a
Washing a chloroform solution of rotaxane 5+Clꢀ with an
aqueous solution of NH4PF6 removed thꢀe halide anion and
afforded the anion host system 5+PF6 (see Scheme 1).
Anions added as TBA salts. Solvent: 1 : 1 CDCl3 :CD3OD. T = 293 K.
Association constants, K, calculated using the chemical shifts of ortho-
pyridinium proton y in conjunction with the computer program
winEQNMR2, errors o10%.
1
Inspection of the H NMR spectrum (Fig. 5c) reveals upfield
c
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Perturbations in the chemical shift of various cavity protons
were observed, indicative of anion binding by the rotaxane
(see Fig. 6). The process of anion association being fast
on the NMR timescale allowed for the calculation of
association constants, K, for each proton by the computer
Experimental
General
Commerically available solvents and chemicals were used
without further purification unless stated. Where dry solvents
were used, they were degassed with nitrogen, dried by passing
through a MBraun MPSP-800 column and then used imme-
diately. Deionised water was used in all cases. Triethylamine
was distilled from and stored over potassium hydroxide.
NMR spectra were recorded on Varian Mercury 300, Varian
Unity Plus 500 and Bruker AVII 500 (with 13C Cryoprobe)
spectrometers. Mass spectra were carried out on Waters
Micromass LCT, Waters GCT, Bruker micrOTOF and
Bruker FT–IR spectrometers. Melting points were recorded
on a Gallenkamp capillary melting point apparatus and are
uncorrected.
program winEQNMR2,17 with data fitting to
a 1 : 1
binding model. Values of K calculated from the chemical shift
titration data of ortho-pyridinium proton y are provided in
Table 1.8
ꢀ 14
As observed in the isophthalamide analogue 8+PF6
,
chloride is bound more strongly than the more basic oxo-
anions H2PO4 and OAcꢀ by rotaxane 5+PF6ꢀ. This is
ꢀ
attributed to only chloride being able to penetrate the inter-
locked cavity, whereas the larger oxoanions associate at the
periphery of the rotaxane. Evidence to support this theory is
provided by the lack of shift in cavity proton c and by the
upfield shift in proton x for the oxoanions, in contrast to the
downfield shifts of these resonances upon the addition of
chloride. The binding of bromide by the rotaxane is intriguing.
It is bound stronger than chloride, which may be rationalized
by consideration of the crystal structure of 5+Clꢀ, which
suggests that a larger monoatomic anion (such as bromide)
may be able to participate in greater hydrogen bonding to the
array of hydrogen bond donors in the rotaxane host’s cavity.
The upfield shift of cavity proton x upon addition of bromide
is difficult to account for.
Synthesis
Compounds 1,16 4+Clꢀ,8a 6+Clꢀ18 and 8+Xꢀ (Xꢀ = Clꢀ,
PF6 )
ꢀ 14 have been reported previously. The activated diester 3
was prepared when needed and used immediately, and was not
fully characterized (1H NMR and low resolution mass spectra
may be found in ESIz).
Dicarboxylic acid 2. To dinitrile 1 (1.00 g, 2.19 mmol)
suspended in CH3OH (20 mL), was added NaOH (5.26 g,
131 mmol) in H2O (20 mL). The reaction mixture was heated
to reflux for 16 h, cooled to 0 1C, then acidified with 3 M
HCl(aq) until pH 2. The CH3OH was then removed in vacuo.
The resulting white precipitate was filtered, washed with H2O
and dried on a high vacuum line to give the product as a white
solid (0.95 g, 88%). Mp = 128–130 1C; dH(300 MHz,
d6-DMSO): 12.94 (2H, s, COOH), 6.80–6.87 (8H, m, ArH),
4.58 (4H, s, OCH2CO), 3.98–4.01 (4H, m, CH2), 3.68–3.71
(4H, m, CH2), 3.52–3.59 (8H, m, 2 ꢂ CH2); dC(75 MHz,
d6-DMSO): 170.4, 152.9, 151.9, 115.3, 115.3 (sic), 69.9, 69.8,
69.0, 67.5, 65.0; m/z: (ES) 512.2134 ([M+NH4]+, C24H34NO11
requires 512.2126).
The association constants for Clꢀ, H2PO4ꢀ and OAcꢀ show
that these anions are bound more weakly by rotaxane 5+PF6
ꢀ
ꢀ
than by the analogous isophthalamide rotaxane 8+PF6
,
especially after considering that binding studies with the latter
host system were carried out in the much more competitive
solvent system 45 : 45 : 10 CDCl3 : CD3OD : D2O.14 In the case
of chloride—which is bound within the cavity—this may be
rationalized by the consideration of the crystal structures,
where greater hydrogen bonding is seen in the isophthalamide
rotaxane 8+Clꢀ than in the xylenediamide rotaxane 5+Clꢀ.
More generally, the macrocyclic amides of 5+PF6 are not
ꢀ
conjugated to the aromatic ring meaning they are less acidic
and so are intrinsically poorer hydrogen bond donors than
ꢀ
those in rotaxane 8+PF6
.
Rotaxane 5+Clꢀ. To a suspension of diacid 2 (113 mg,
0.228 mmol) in dry CH3CN (10 mL) was added N-hydroxy-
succinimide (63 mg, 0.546 mmol) and DCC (103 mg, 0.501 mmol),
with the resulting reaction mixture being stirred for 16 h
under N2. The resulting DCC urea was filtered, the solvent
removed in vacuo and the activated diester 3 taken on
assuming to be quantitatively formed. The activated diester
3 (157 mg, 0.228 mmol) and axle 4+Clꢀ (245 mg, 0.228 mmol)
were added to dry CH2Cl2 (50 mL) and stirred for 30 min
under a N2 atmosphere. To the resulting solution NEt3 (58 mg,
79 mL, 0.569 mmol) and meta-xylenediamine (31 mg, 30 mL,
0.228 mmol) were added, and the reaction stirred under N2 for
16 h. The reaction mixture was then washed with 3 M HCl(aq)
(2 ꢂ 50 mL) and H2O (1 ꢂ 50 mL), the organic layer dried
(MgSO4) and solvent removed in vacuo. The crude material
was purified by silica gel prep TLC plates (CH2Cl2 : CH3OH
97 : 3 then repeatedly 98 : 2) to give the product as a yellow
solid (56 mg, 15% from 2). Mp = 190–200 1C (phase transition);
dH(300 MHz, CDCl3): 10.58 (2H, s, pyridinium NH), 9.83
(1H, br s, pyridinium ArH4), 9.15 (2H, s, pyridinium ArH2 & H5),
Conclusions
We have successfully synthesized a novel meta-xylenediamide
macrocycle containing rotaxane via a new clipping methodo-
logy. Upon exchange to the hexafluorophosphate salt, the
rotaxane was shown to bind a range of anions in the competi-
tive solvent mix 1 : 1 CDCl3 : CD3OD. Chloride and bromide
are bound more strongly than the more basic oxoanions
dihydrogen phosphate and acetate. This is attributed to only
the monoatomic halides being able to penetrate the cavity of
the rotaxane, while the polyatomic oxoanions hydrogen bond
peripherally. Work towards constructing further anion
binding interlocked structures with alternative motifs is in
progress in our laboratories.
8 For each anion reasonable agreement was observed in the values of
K calculated from the chemical shifts of protons y, x and c, provided
there was a significant shift in the proton signal. See the ESI for a full
set of calculated association constants.w
c
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Fig. 5 Partial 1H NMR spectra of (a) isophthalamide rotaxane
8+Clꢀ14 (b) xylenediamide rotaxane 5+Clꢀ and (c) xylenediamide
rotaxane 5+PF6ꢀ. Significant peak shifts are highlighted. Solvent:
CDCl3, T: 293 K. For atom labels see Scheme 1.
7.92 (2H, app s, xylyl NH), 7.84 (4H, d, 3J = 8.5 Hz, stopper
ArH), 7.79 (1H, s, xylyl ArH2), 7.03–7.32 (30H, m, stopper
ArH), 6.92 (2H, d, 3J = 7.9 Hz, xylyl ArH4 & H6), 6.73
(1H, t, 3J = 7.9 Hz, xylyl H2), 6.48 (4H, d, 3J = 8.8 Hz,
hydroquinone ArH), 6.21 (4H, d, 3J = 8.8 Hz, hydroquinone
ArH), 4.57 (3H, s, N+CH3), 4.39 (4H, d, 3J = 5.9 Hz,
CH2NH), 4.18 (4H, s, C(O)CH2), 3.67–3.74 (16H, m, 4 ꢂ
OCH2), 1.34 (36H, s, (CH3)3); dC(125.8 MHz, CDCl3): 167.8,
158.4, 152.6, 152.3, 148.4, 147.0, 145.3, 144.3, 143.6, 137.9,
135.4, 133.5, 131.8, 131.2, 130.8, 128.4, 127.7, 127.4, 125.8,
124.3, 119.5, 116.0, 114.6, 70.7, 70.7 (sic), 70.1, 68.7, 68.0, 63.9,
49.0, 43.0, 34.3, 31.4 (1 ArC missing); m/z: (ES) 1634.8687
([M–Cl]+, C106H116N5O11 requires 1634.8666).
Fig. 6 Plots of chemical shift of (a) ortho-pyridinium proton y and (b)
para-pyridiniumꢀproton x and (c) internal xylyl cavity proton c of
rotaxane 5+PF6 versus equivalents of TBA salt added. Solvent: 1 : 1
CDCl3 : CD3OD, T: 293 K.
layer was dried (MgSO4) and then the solvent removed to give
the product as a yellow solid. (54 mg, 90%). Mp = 120–140 1C
(phase transition), at 240 1C decomposition); dH(300 MHz,
CDCl3): 8.98 (2H, br s, pyridinium NH), 8.89 (2H, s,
pyridinium ArH2 & H5), 8.18 (1H, s, pyridinium ArH4), 7.57
3
(4H, d, J = 8.8 Hz, stopper ArH), 7.05–7.32 (34H, m, xylyl
ArH2, H4, H5 & H6 xylyl NH & stopper ArH), 6.61 (4H, d,
3J = 8.8 Hz, hydroq ArH), 6.34 (4H, d, 3J = 8.8 Hz,
Rotaxane 5+PF6ꢀ. A solution of rotaxane 5+Clꢀ (56 mg,
0.0335 mmol) in CHCl3 (15 mL) was washed with 0.1 M
NH4PF6(aq) (10 ꢂ 10 mL), then H2O (3 ꢂ 10 mL). The organic
3
hydroquione ArH), 4.40 (4H, d, J = 5.9 Hz, CH2NH), 4.35
(3H, s, N+CH3), 4.25 (4H, s, C(O)CH2), 3.70–3.73 (12H, m,
3 ꢂ OCH2), 3.49–3.51 (4H, m, OCH2), 1.33 (36H, s, (CH3)3);
c
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dC(125.8 MHz, CDCl3): 168.2, 158.6, 152.3, 152.3 (sic),
148.6, 146.9, 144.7, 144.5, 143.5, 138.6, 134.9, 134.1, 132.0,
131.1, 130.7, 128.9, 127.7, 127.4, 126.6, 125.9, 124.3, 119.0,
116.5, 114.7, 70.8, 70.5, 70.2, 68.6, 67.3, 63.8, 48.9, 42.8, 34.3,
(in conjunction with Johnson Matthey) and for post-doctoral
funding as part of the PhD Plus program. We are grateful to
Diamond Light Source for the award of beamtime on I19
(MT1858) and to the beamline scientists for help and support.
The generous provision of the 1H NMR spectrum of rotaxane
8+Clꢀ (Fig. 5a) by Laura Hancock (University of Oxford) is
acknowledged.
31.4. dF (282.4 MHz, CDCl3): ꢀ70.6 (d, J = 713 Hz, PF6ꢀ);
1
dP(121.5 MHz, CDCl3) d: ꢀ144.1 (septet, 1J = 713 Hz, PF6ꢀ).
m/z: (ES) 1634.8726 ([M–PF6]+, C106H116N5O11 requires
1634.8666).
Notes and references
Macrocycle 7. To a suspension of diacid 2 (54 mg,
0.101 mmol) in dry CH3CN (5 mL) was added N-hydroxy-
succinimide (30 mg, 0.243 mmol) and DCC (50 mg, 0.222
mmol), with the resulting reaction mixture being stirred for
16 h under N2. The resulting DCC urea was filtered, the solvent
removed in vacuo and the activated diester 3 taken on assuming
to be quantitatively formed. A solution of activated diester 3
(70 mg, 0.101 mmol) and template 7+Clꢀ (39 mg, 0.110 mmol)
were added to dry CH2Cl2 (20 mL) and stirred for 30 min
under a N2 atmosphere. To this solution, NEt3 (26 mg, 35 mL,
0.253 mmol) and meta-xylenediamine (14 mg, 13 mL,
0.101 mmol) were added, and the reaction stirred under N2 for
16 h. The reaction mixture was then washed with 3 M HCl(aq)
(2 ꢂ 20 mL), NaHCO3(aq) (2 ꢂ 20 mL) and H2O (1 ꢂ 20 mL),
the organic layer dried (MgSO4) and solvent removed in vacuo.
The crude material was purified by silica gel column chromato-
graphy (CHCl3 : CH3OH 98 : 2 to 97 : 3) to give the product
as a white solid (30 mg, 50% from 2). Mp = 176–178 1C;
dH(300 MHz, CDCl3): 7.19–7.34 (4H, m, xylyl ArH), 6.91
(2H, br s, NH), 6.72–6.78 (8H, m, hydroquinone ArH), 4.53
(4H, d, 3J = 5.9 Hz, NHCH2), 4.46 (4H, s, C(O)CH2),
4.00–4.04 (4H, m, ArOCH2CH2O), 3.84–3.87 (4H, m,
ArOCH2CH2O), 3.68–3.73 (8H, m, 2 ꢂ OCH2); dC(75 MHz,
CDCl3): 168.2, 153.9, 151.2, 138.5, 129.0, 127.4, 125.9, 115.7,
115.4, 70.8, 70.6, 69.7, 68.1, 67.8, 42.7; m/z: (ES) 617.2485
([M+Na]+, C32H38N2NaO9 requires 617.2470).
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Acknowledgements
N. H. E. wishes to thank the EPSRC for the provision of DTA
funding. C. J. S. would also wish to express gratitude to the
EPSRC for
a CASE sponsored doctoral studentship
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 2047–2053 2053