Urea and thiourea based anion receptors in solution and on polymer supports

Article abstract of DOI:10.1080/10610278.2017.1394462

Herein we report the development of a new series of surface bound anion sensors exploiting the urea or thiourea motif capable of binding anions through hydrogen bonding interactions. The use of high resolution magic angle spinning ¹H NMR allows the direct comparison of the anion binding properties of these receptors in solution versus those tethered to polymer resins. Some intramolecular hydrogen bonding and solvent effects were observed at the solution:surface interface however in general the anion binding properties of the polymer bound urea and thiourea receptors were maintained.

Full text of DOI:10.1080/10610278.2017.1394462

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Urea and thiourea based anion receptors in
solution and on polymer supports
S. Byrne & K. M. Mullen
To cite this article: S. Byrne & K. M. Mullen (2017): Urea and thiourea based anion
receptors in solution and on polymer supports, Supramolecular Chemistry, DOI:
10.1080/10610278.2017.1394462
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Date: 02 November 2017, At: 14:39

Supramolecular chemiStry, 2017
https://doi.org/10.1080/10610278.2017.1394462
Urea and thiourea based anion receptors in solution and on polymer supports
S. Byrne and K. M. Mullen
School of chemistry, physics and mechanical engineering, Queensland university of technology, Brisbane, australia
ARTICLE HISTORY
ABSTRACT
received 30 June 2017
accepted 17 September 2017
Herein we report the development of a new series of surface bound anion sensors exploiting the
urea or thiourea motif capable of binding anions through hydrogen bonding interactions. The
use of high resolution magic angle spinning 1H NMR allows the direct comparison of the anion
binding properties of these receptors in solution versus those tethered to polymer resins. Some
intramolecular hydrogen bonding and solvent effects were observed at the solution:surface
interface however in general the anion binding properties of the polymer bound urea and thiourea
receptors were maintained.
KEYWORDS
anion receptors;
supramolecular chemistry;
hr maS Nmr; hydrogen
bonding
The development of a new series of surface bound anion sensors exploiting the urea or thiourea
1
motif capable of binding anions is reported. The use of high resolution magic angle spinning H
NMR allows the direct comparison of the anion binding properties of these receptors in solution
versus those tethered to polymer resins.
be overcome when binding anions in competitive solvent
environments (2). Supramolecular chemists have success-
fully designed and synthesised numerous acyclic, macro-
cyclic and interlocked anion receptors capable of binding
anions by taking advantage of the coorperative effect of
multiple ion–ion, ion–dipole, hydrogen bonding, or π-anion
intermolecular interactions (1f, 3). Initial work was domi-
nated by the use of electrostatic interactions to bind anions
to a variety of charged receptors incorporating ammonium
(4), imidizolium (5), guanidinium (6), thiouronium (7) and
more recently triazolium (8) motifs. However given the ease
of synthesis and handling of neutral receptors, and when
strategically positioned within receptors, modern examples
of neutral systems have been found to bind anions with the
same strength and selectivity as many charged systems (9).
Introduction
Research into the development of molecular receptors
that accurately and selectively detect the presence of neg-
atively charged species, or anions, has grown rapidly. This
is largely due to the essential role such negatively charged
species have on a range of environmentally, chemically
and biologically important processes becoming more
apparent (1). The overall objective of this work is the devel-
opment of a new range of functionalised surfaces capable
of sensing anions in a step towards the development of
functional anion sensing devices.
The design of anion receptors is challenging owing in
part to their relatively low charge density (as compared to
their isoelectronic cation counterparts), their pH depend-
ence and large free energies of solvation, which need to
CONTACT K. m. mullen
kathleen.mullen@qut.edu.au
Supplemental data for this article can be accessed at https://doi.org/10.1080/10610278.2017.1394462
© 2017 informa uK limited, trading as taylor & Francis Group

2
S. BYRNE AND K. M. MULLEN
Since the first reports of anion binding to urea deriva-
between the binding properties of urea based anion recep-
tors in solution versus those attached to polymer resins
could be made (see Scheme 2, Scheme S1 and Scheme S2).
While 1-aminopyrene 4a and 1-aminoanthracene 4b were
commercially available, the porphyrin amine 4c was pre-
pared by adapting Lindsey’s salt/BF3 co-catalytic porphyrin
reaction procedures (21). Each of these amines was then
reacted with triphosgene to produce the corresponding
isocyanates 1a–c, which due to their high reactivity, were
filtered and used immediately in the next step without
further purification. The unpurified isocyanate products
were then reacted with n-dodecylamine to produce the
pyrene, anthracene and porphyrin urea receptors 5a–c in
reasonable yields.
tives were made by Wilcox et al. in 1992 (10), substantial
research into the anion binding properties of (thio)urea
derivatives has been done by Gunnlaugsson (11), Gale (12),
Fabbrizzi (13) and many others (14). The urea motif which
consists of two parallel NH groups either side of a carbonyl
in a planar topology, is particularly effective in binding
trigonal planar anions such as carboxylates or oxoanions
(15). The synthetic ease at which they can be incorporated
into more complex structures is evident in their use for a
range of applications not only in anion sensing but also in
transmembrane transportation of anions (16), as well as in
the formation of self-assembled cages (17).
Whilst there has been much work in the development of
molecular receptors capable of sensing molecular guests
including anions in solution, it can be argued that in order
for these systems to be integrated into functional sensory
devices, they need to be incorporated into solid materials
or deposited onto solid surfaces. To date there have been
very few reported examples of anion receptors or sensors
on surfaces (18). This has been attributed not just to chal-
lenging surface attachment chemistry but also to charac-
terisation difficulties which can limit the detailed analysis
that is otherwise possible with solution analogues. TentaGel
polymer resins are particularly useful surfaces for this work
given the number of differently functionalised TentaGel
resins that are commercially available, and perhaps more
importantly the fact that they can be easily characterised
using 1H High Resolution Magic Angle Spinning (HR MAS)
NMR (19). This ability to directly compare the binding of
anions on surfaces with solution analogues is vital as pre-
vious studies have established that the kinetic and thermo-
dynamic factors that underpin molecular interactions in
supramolecular systems are not always directly translated
from solution, to the solution:surface interface (19f, 20).
The work outlined here describes the development of
a range of urea and thiourea based anion sensors both
in solution and attached to polymer TentaGel resins (see
Scheme 1). The initial synthetic strategy incorporates
anthracene, pyrene or porphyrin sensing motifs to trans-
late the anion binding at the urea receptor into an optical
response. TentaGel-NH2 resins were chosen as the solid
support, and in a concerted synthetic effort the urea bond
forming reaction is also used for the surface attachment.
Full characterisation of the resulting functionalised resins
was performed using 1H HR MAS NMR.
After synthesising a range of soluble urea functional-
ised fluorophores, the potential of these compounds as
anion receptors was investigated. Initially the binding of
chloride to the pyrene urea receptor 5a in CDCl3 was exam-
ined by 1H NMR titration experiments. As shown in Figure
1, upon addition of one equivalent of TBA Cl to the pyrene
urea 5a a significant downfield shift of both urea protons a
and b (Δδ 1.21 and 1.33 ppm, respectively) was observed,
indicative of anion binding. Further titration experiments
were then carried out with other anions (Br−, I−, AcO−,
Results and discussion
Synthesis and anion binding properties of solution
based urea anion receptors
Scheme 1. (colour online) Synthesis of the target surface
attached urea receptors 3a–c. reagents and conditions: (i) ch2cl2,
rt, 7 days.
The synthesis of a series of solution based model anion sen-
sors was initially undertaken so that future comparisons

SUPRAMOLECULAR CHEMISTRY
3
respectively) indicative of complimentary C–H binding,
however these shifts were not as pronounced with the
addition of acetate, suggesting that secondary halide -π
interactions are also contributing to this deviation from
typical anion binding trends.
Fluorescence studies were then carried out to deter-
mine sensing potential of receptors 5a–c. Unfortunately
photodegradation problems were encountered for the
pyrene and anthracene receptors 5a and 5b, as evidenced
by a linear decrease in fluorescence emission as a function
of the number of scans (see Figure S3). Whilst the pho-
tostability of the porphyrin urea receptor 5c was higher,
addition of anions did not result in significant changes in
the fluorescence spectrum.
Synthesis and anion binding properties of solution
based thiourea anion receptors
Scheme 2. Synthesis of the pyrene urea receptor 5a. reagents
and conditions: (i) triphosgene, et3N, toluene, 80 °c, 5 h; (ii)
n-dodecylamine, ch2cl2, rt, 48 h.
In an effort to improve the strength of anion binding from
that observed for the urea receptors 5a–c an analogous
series of thiourea receptors were then prepared. Given the
lower pKa of thioureas compared to ureas, e.g. 18.7 for
diphenylurea and 13.4 for diphenylthiourea (23), stonger
anion binding is expected. The synthesis of thioureas is
similar to that for ureas, with the difference being the
preparation of an isothiocyanate instead of the isocyanate.
Initial work used the thiophosgene derivative,
bis(1-benzotriazolyl)methanethione 6 (24) to prepare the
target isothiocyanates 7a–c. Reaction of 1-(trimethylsi-
lyl)benzotriazole and thiophosgene in CH2Cl2 produced
bis(1-benzotriazolyl)methanethione in 90% yield. This
thiophosgene derivative was then reacted with amine
derivatives 4a–c to give the desired isothiocyanates 7a–c
in high yields (75–98%). Unlike the corresponding isocy-
anates these were stable enough to be purified by column
chromatography. These isothiocyanates 7a–c were then
reacted with dodecylamine to give the desired thiourea
receptors 8a–c (Scheme 3).
With the thiourea compounds prepared, the anion
binding properties of this series were investigated using
1H NMR titration experiments following the same proto-
cols used for the urea compounds. No significant binding
of anions to the pyrene urea receptor 8a was observed
in CDCl3 (see Figure S4). Unfortunately solubility issues
prevented the anion binding properties of this thiourea
receptor to be examined in different solvents. Anion bind-
ing studies were then carried out for the anthracene 8b
and porphyrin thiourea receptors 8c in acetone-d6 as the
equivalent urea receptors discussed above showed higher
−
H2PO4 ) and the respective 1:1 binding constants were
calculated using WinEQNMR (see Figure S1 and Table 1).
Similar downfield shifts in the urea protons were observed
during anion binding titrations with the porphyrin recep-
tor 5c in CDCl3 indicative of anion binding (see Figure S2
and Table 1).
However, much stronger anion binding was observed
when the titrations were repeated in acetone-d6 (22).
Interestingly, prior to any anion addition, the chemical
shifts of the urea protons a and b were found to have a
significant downfield shift in acetone-d6 when compared
to the same spectrum in CDCl3, presumably due to hydro-
gen bonding interactions between the urea moiety and
the solvent acetone. The addition of TBA Cl to a solution
of anthracene urea 5b in acetone-d6 resulted in significant
downfield shifts of both urea protons a and b (Δδ 1.76
and 2.17 ppm, respectively), indicative of anion binding
(Figure 2). Similar downfield shifts were observed upon
the addition of other anions (Br−, I−, AcO−) to both the por-
phyrin and anthracene receptors and the resulting anion
binding affinities calculated using WinEQNMR can be seen
in Table 1.
Two important observations were noted. Firstly the
calculated association constants are significantly higher
in acetone-d6 as compared to CDCl3. This is perhaps unsur-
prising given the competition between the neutral recep-
tor and the ion-pairing of the tetrabutylammonium cation
in non-polar solvents. Secondly, the binding of chloride
anions to the porphyrin and anthracene receptors was
stronger than for acetate anions. Significant shifts of the
neighbouring anthracene proton c were observed upon
additions of chloride and acetate (Δδ 1.454 and 1.047 ppm,
anion binding in this solvent. Upon addition of one equiv
-
alent of TBA Cl to the anthracene thiourea 8b significant
downfield shifts of the thiourea protons a and b were
observed (Δδ1.53 and 1.96 ppm, respectively) along with

4
S. BYRNE AND K. M. MULLEN
Figure 1. (colour online) 1h Nmr spectrum in cDcl3 of: (a) pyrene dodecyl urea 5a with 5 equivalents of tBa cl; (B) pyrene dodecyl urea
5a with 1 equivalent of tBa cl; (c) pyrene dodecyl urea 5a.
Figure 2. (colour online) 1h Nmr spectrum in acetone-d6 of: (a) anthracene dodecyl urea 5b with 5 equivalents of tBa cl; (B) anthracene
dodecyl urea 5b with 1 equivalent of tBa cl; (c) anthracene dodecyl urea 5b; (D) anthracene dodecyl urea 5b in cDcl3.
Table 1. calculated association constants (Ka m−1) for the binding
of various anions to urea receptors 5a–c at 293 K.
Anthracene Porphyrin
Pyrene 5a
(CDCl3)
Porphyrin
5c (CDCl3)
5b (ace-
5c (ace-
Anion
chloride
Bromide
iodide
acetate
Dihydrogen
phosphate
tone-d6)
tone-d6)
450
160
50
790
550
810
370
170
1150
590
12,000
1090
67
2440
–
9300
1600
140
6600
–
smaller shifts of the neighbouring protons on the anthra-
cene group (see Figure 3). Similar downfield shifts were
observed upon addition of a variety of anions (Br−, I−, AcO−)
for both the anthracene and porphyrin thiourea receptors
8b and 8c and the resulting anion binding affinities and
Scheme 3. (colour online) Synthesis of the target thiourea anion
receptors 8a–c. reagents and conditions: (i) ch2cl2, rt, 2 h; (ii)
ch2cl2, rt, 12 h; (iii) ch2cl2, rt, 16 h.

SUPRAMOLECULAR CHEMISTRY
5
Figure 3. 1h Nmr spectrum in acetone-d6 of: (a) anthracene thiourea 8b with 5 equivalents of tBa cl; (B) anthracene thiourea 8b with 1
equivalent of tBa cl; (c) anthracene thiourea 8b.
Figure 5. (colour online) Qualitative evidence of bead
functionalisation of (a) 3a, (b) 3b, and (c) 3c.
Figure 4. (colour online) change in the chemical shift of thiourea
proton aupon addition of anions toa2 mm solution of anthracene
thiourea 8b in acetone-d6 at 293 K.
Notes: Symbols represent experimental data points; continuous lines
represent calculated curves. all anions were added as their tBa salts.
Table 2. calculated association constants (Kam−1) for the binding
of various anions to thiourea receptors 8b and 8c in acetone-d6
at 293 K.
Anion
Anthracene thiourea 8b
Porphyrin thiourea 8c
chloride
Bromide
iodide
1900
430
31
23,000
2920
160
acetate
9200
38,700
Note: errors < 25%.
associated errors were calculated using WinEQNMR (see
Figure 4, and S5 and Table 2) (25). As expected a significant
increase in the binding affinity of the thiourea receptors
Scheme 4. (colour online) preparation of surface attached
thiourea anion receptors 9a–c. reagents and conditions: (i)
ch2cl2, rt, 7 d.

6
S. BYRNE AND K. M. MULLEN
1
Figure 6. hr maS h Nmr spectra in cDcl3 of: (a) pyrene urea functionalised tentaGel resins 3a plus tBa cl, with 32 cpmG loops; (B)
1
pyrene urea functionalised tentaGel resins 3a. h Nmr spectra in cDcl3 of: (c) pyrene urea 3a; (D) pyrene urea 3a plus 10 equivalents
tBa cl.
It was proposed that commercially available TentaGel-NH2
resins could be reacted with the previously prepared iso-
cyanate compounds 1a–c to produce the target surface
attached urea anion receptors. In this design, synthetic
efficiencies are achieved by using the surface attachment
chemistry as the method to also introduce the urea anion
binding receptor to the system.
Table 3. change in chemical shift of urea Nh proton a of the sur-
face and solution anion receptors upon addition of an excess (10
equivalents for solution, at least 10 equivalents for surface) of
various anions.
Anthracene 3b
(Acetone-d6)
Porphyrin 3c
(Acetone-d6)
Pyrene 3a (CDCl3)
Solu-
tion
Solu-
tion
Solu-
tion
Surface
Surface
Surface
To this end urea functionalised resins 3a–c were
obtained by reacting freshly prepared pyrene, anthracene
or porphyrin isocyanate 1a–c with TentaGel-NH2 resins 2.
Whereas the amine functionalised beads are colourless,
the pyrene, anthracene and porphyrin beads 3a–c are yel-
low, orange and purple, respectively, indicating at least
qualitatively successful formation of the surface bound
urea receptors (see Figure 5). IR analysis of the urea func-
tionalised resins 3a–c showed the presence of additional
absorption bands between 1625 and 1645 cm−1 attributed
to the distinctive urea C = O stretch, C–N–H bend frequen-
cies between 1560 and 1530 cm−1 as well as a more prom-
inent NH signal at 3500 cm−1 when compared to the initial
TentaGel-NH2 resins (see Figure S6–S8) (27).
Thiourea functionalised TentaGel resins were prepared
by reaction of the isothiocyanate functionalised reporter
groups 7a–c with the TentaGel-NH2 resins 2 using the
same protocols as were used for the urea resins 3a–c (see
Scheme 4). These resins exhibited the same changes in
colouration as was seen for the urea resins and additional
qualitative evidence was again provided by IR spectros-
copy, with the appearance of a more prominent NH sig-
nal at 3500 cm−1, unfortunately any contribution of the
C = S absorptions was not visible due to overlap with the
absorptions of the resin’s polymer core (see Figure S9–S11).
chloride
Bromide
iodide
3.02
2.20
1.18
4.08
1.53
0.59
0.13 *
2.81
2.15
1.45
0.47
2.79
1.97
1.16
0.33
2.68
2.70
2.01
0.88
3.93
2.41
1.40
0.27
3.87
acetate
*Shift estimated due to broadening and overlapping pyrene resonance.
towards anions as compared to the urea receptors were
observed.
Synthesis and anion binding properties of surface
bound urea and thiourea anion receptors
Having established the anion binding properties of a range
of solution based urea and thiourea receptors the attach-
ment of these receptors to solid supports was investigated.
We have previously demonstrated that 1H HR MAS NMR is
an invaluable technique in monitoring the anion binding
properties of receptors attached to polymer resins (26).
Furthermore we have recently shown that surface func-
tionalization can be extended to resins with lower func-
tional group loading (0.24 mmol/g) (26a). The commercial
availability of such resins with a far greater diversity of end
group functionalization (i.e. amines, carboxylic acids, thi-
ols) than the higher loading resins opens up a wide range
in the attachment chemistry that could be used in the
development of future polymer bound anion receptors.

SUPRAMOLECULAR CHEMISTRY
7
1
Figure 7. h hr maS Nmr spectra in acetone-d6, of: (a) anthracene thiourea functionalised tentaGel resins 9b plus tBa acetate with
8 cpmG loops; (B) anthracene thiourea functionalised tentaGel resins 9b plus tBacl with 8 cpmG loops; (c) anthracene thiourea
functionalised tentaGel resins 9b with 8 cpmG loops; (D) 1h Nmr spectrum in acetone-d6 of anthracene thiourea 8b.
urea functionalised resin 3a resulted in significant downfield
shifts in the urea N–H protons a and b with the greatest
change seen with upon addition of acetate (see Figure 6
and Table 3). Similar downfield shifts in the urea protons a
and b were observed during anion binding titrations with
the surface bound anthracene and porphyrin urea receptors
3b and 3c with the largest change in chemical shift being
observed upon addition of acetate (Δδ2.81 for 3a, Δδ2.68 for
3b and Δδ3.87 for 3c; see Figure S12 and S13 and Table 3).
Although practical considerations prevent quantitative
anion NMR titrations for the resins, the magnitude of these
changes in chemical shift upon anion addition can be at
least qualitatively compared to the solution analogues
5a–c. In general good agreement between the change in
chemical shift upon anion addition for the surface bound
porphyrin and anthracene receptors as compared to the
solution analogues was observed. For the pyrene func-
tionalised beads 3a however, the change in chemical shift
upon addition of anions was lower than that observed
for the solution analogue 5a. This is not due to weakened
anion interaction but rather is a result of the fact that when
compared to the solution based pyrene urea receptor 5a
there is a noticeable difference seen in the chemical shift
of the urea protons a and b for 3a as discussed above.
Overall these results show that attachment of urea based
anion binding systems to TentaGel resins is possible with
minimal impact on anion binding properties.
Table 4. change in chemical shift of thiourea Nh proton a of the
surface and solution anion receptors upon addition of an excess
(10 equivalents for solution, at least 10 equivalents for surface) of
various anions.
Anthracene thiourea (ace-
tone-d6)
Porphyrin thiourea (ace-
tone-d6)
Solution 8b
Surface 9b
Solution 8c
Surface 9c
chloride
Bromide
iodide
2.50
1.69
0.36
4.00
1.96
1.16
0.18
3.57
3.00
2.28
1.06
4.26
2.2
1.4
0.7
3.3
acetate
Elemental analysis was used on both the urea and thiourea
functionalised resins which determined the loading of
the various receptors to be between 0.1 and 0.18 mmol/g
which is between 35 and 65% of the loading quoted by
the manufacturer (see Table S1).
1H HR MAS NMR analysis provided more conclusive evi-
dence for the successful surface attachment of the urea
and thiourea receptors 3a–c and 9a–c. For the pyrene urea
functionalised resins 3a, distinctive peaks for the urea pro-
tons a and b can be seen at 8.18 and 6.37 ppm, respectively
along with the various peaks associated with the pyrene
protons between 8.55 and 7.96 ppm, indicative of success-
ful bead functionalization (see Figure 6). However, when
compared to the solution based pyrene urea receptor 5a
there is a noticeable difference in the chemical shift of the
urea protons a and b, Δδ 1.53 and 1.77, respectively. This is
thought to be primarily due to the close proximity of the
polyethylene glycol chains of the resins leading to intramo-
lecular hydrogen bonding between the urea N–H protons
and the oxygen of the polyethylene glycol. As observed for
the solution analogue 5a, addition of anions to the pyrene
Similar results were also obtained for the thiourea func-
1
tionalised resins 9a–c, as shown in Figure 7, the H NMR
spectrum of the anthracene thiourea beads 9b clearly
shows two distinctive peaks for the thiourea NH protons
a and b (δ 9.18 and 7.73 ppm, respectively) as well as the

8
S. BYRNE AND K. M. MULLEN
various resonances of the anthracene moiety between δ
7.56 and 8.78 ppm, indicating successful bead function-
alization. As observed for solution analogue 8b, addition
of TBA chloride to the anthracene thiourea beads 9b in
acetone-d6 resulted in large downfield shifts for both of
the thiourea NH protons a and b (Δδ 2.33 and 2.41, respec-
tively, see Figure 7). Similar downfield shifts were observed
upon addition of an excess of a variety of anions to the
anthracene and porphyrin thiourea functionalised resins
9b–c (see Table 4). Although practical considerations pre-
vent a quantitative analysis of anion binding strength, the
magnitude of the changes in chemical shift upon binding
is similar to that observed for solution analogues (seeTable
4) (28).
As shown in Table 4 the change in chemical shift upon
addition of an excess of anions to the surface bound
thiourea receptors 9b and 9c is slightly lower than the
change observed upon addition of 10 equivalents to
the solution analogues 8b and 8c. This difference can be
attributed to surface effects, as the initial position of the
thiourea NH proton a and b is also shifted downfield when
compared to the solution analogues (for 8b proton a Δδ
0.24 ppm and proton b Δδ 0.71 ppm).
Again 1H HR MAS NMR analysis of the thiourea function-
alised resins demonstrated similar anion binding trends as
compared to solution analogues. Two challenges that will
be the focus of future work are firstly, the development of
better methodology to allow the strength of anion binding
interactions on the surface to be quantified, and secondly,
the better incorporation of reporter groups into the anion
receptor framework to allow communication of the anion
binding event through optical or electrochemical means.
Experimental
General remarks
Unless otherwise stated, reagents were purchased from
commercial sources and used without further purifica-
tion. All solvents were dried before use over type 3 Å or
4 Å molecular sieves according to standard procedures.
Triethylamine was dried over KOH. Silica gel column
chromatography was carried out using Merck silica gel 60
(grade 9835, 230–400 mesh). Analytical TLC was carried
out on Merck silica gel F254 precoated aluminium sheets.
Solution NMR spectra were recorded on a Bruker Avance
or Varian INOVA 400 MHz spectrometer and referenced to
the relevant solvent peak. TentaGel™ S–NH2 resins were
purchased from Peptides International with a quoted load-
ing of 0.27 mmolg−1 and a particle size of approximately
90 μm. HR MAS NMR Spectra were acquired on a Bruker
Avance 400 spectrometer at room temperature using a
Bruker HR MAS probe. Rotors containing a suspension of
the beads in CDCl3 or Acetone-d6 were spun at 4 or 5 kHz.
One-dimensional HR MAS spectra were obtained with 64
scans. Unless otherwise stated, the CPMG pulse sequence
contained 0, 32 or 128 π-pulses with a repetition time of
30 ms. ESI high-resolution mass spectra were obtained
using a QTOF LC mass spectrometer which utilised electro-
spray ionization. Melting points were measured on a vari-
able-temperature apparatus by the capillary method and
are uncorrected. IR spectra were obtained using a Thermo
Nicolet Nexus 870 esp spectrometer at 4 cm−1 resolution
using 64 scan averaging.
Conclusions
A series of solution based and surface attached urea and
thiourea anion receptors incorporating a range of fluo-
rescent reporter groups has been successfully prepared
and their anion binding properties analysed. For the urea
receptors 5a–c, the strength of anion association was sig-
nificantly higher in acetone-d6 when compared to CDCl3,
with the highest association constants with these recep-
tors being calculated upon the addition of chloride anions.
Successful functionalization of TentaGel resins with urea
anion receptors was achieved and 1H HR MAS NMR anal-
ysis allowed the anion binding potential of these resins
to be qualitatively assessed. In contrast to the solution
analogues, some intramolecular and solvent effects for
the surface attached urea receptors 3a–c were observed.
However in general the changes in chemical shift of the
urea protons upon addition of anions was comparable to
solution analogues indicating the anion binding proper-
ties of these receptors is maintained at the solution:sur-
face interface. The‘reporting’of this anion binding through
changes in fluorescence was not observed largely due to
photodegredation issues. In an attempt to increase the
strength of anion binding, a second series of thiourea func-
tionalised resins and associated solution analogues were
Synthetic procedures
For specific synthetic procedures please see the support-
ing information.
General procedure for the synthesis of urea receptors
5a–c
Amine 4a, 4b or 4c (1 equiv.) and triphosgene (0.5 equiv)
dissolved in dry, degassed toluene (50 mL) under argon.
Triethylamine (0.1 mL) was then added and reaction heated
to 70 °C and left to stir for five hours. The reaction mixture
synthesised. This was successful with increased anion bind
-
ing affinities calculated for the thiourea receptors 8b–c,
with the exception of halides for 8b, however in this series
acetate was found to have the higher binding affinity.

SUPRAMOLECULAR CHEMISTRY
9
was allowed to cool, filtered and the solvent removed. The
crude isocyanate was then dissolved in dry CH2Cl2 under
argon. Dodecylamine (1.5 equiv) was then added and reac-
tion stirred for 48 h. The solvent was evaporated and the
crude product purified by silica gel column to give the pure
products in 36–82% yield.
ORCID
K. M. Mullen
http://orcid.org/0000-0003-4480-0985
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Disclosure statement
No potential conflict of interest was reported by the authors.

10
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