Anion binding by calix[4]arene ferrocene ureas

Article abstract of DOI:10.1039/b309288g

A ferrocene reporting unit has been successfully incorporated onto the upper-rim of calix[4]arenes through either urea or amide hydrogen bonding units for the preparation of three novel receptors which can sense the binding of anions electrochemically. ¹H NMR studies indicate that the receptors have a general preference for binding more basic anions, and in the case of the tetra-urea derivative, a marked selectivity for dihydrogen phosphate. A single crystal X-ray structure of the di-urea derivative reveals the role of hydrogen bonding interactions in the binding of benzoate. Cyclic and square wave voltammetric studies demonstrate that all receptors can electrochemically sense the binding of anions with dihydrogen phosphate inducing the largest shifts.

Full text of DOI:10.1039/b309288g

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Anion binding by calix[4]arene ferrocene ureas
Andrew J. Evans,a Susan E. Matthews,b Andrew R. Cowleyc and Paul D. Beer*a
a Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford,
South Parks Road, Oxford, UK OX1 3QR. E-mail: paul.beer@chem.ox.ac.uk
b School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich, Norfolk,
UK NR4 7TJ. E-mail: susan.matthews@uea.ac.uk
c Chemical Crystallography Laboratory, University of Oxford, 9 Parks Road, Oxford,
UK OX1 3PD. E-mail: andrew.cowley@chem.ox.ac.uk
Received 5th August 2003, Accepted 2nd October 2003
First published as an Advance Article on the web 20th October 2003
A ferrocene reporting unit has been successfully incorporated onto the upper-rim of calix[4]arenes through either
urea or amide hydrogen bonding units for the preparation of three novel receptors which can sense the binding of
anions electrochemically. 1H NMR studies indicate that the receptors have a general preference for binding more
basic anions, and in the case of the tetra-urea derivative, a marked selectivity for dihydrogen phosphate. A single
crystal X-ray structure of the di-urea derivative reveals the role of hydrogen bonding interactions in the binding of
benzoate. Cyclic and square wave voltammetric studies demonstrate that all receptors can electrochemically sense
the binding of anions with dihydrogen phosphate inducing the largest shifts.
Introduction
Anions play a number of crucial biological and environmental
roles and their selective binding and sensing is an area of con-
siderable current interest.1 A variety of approaches to the bind-
ing of anions have been investigated since the early work of
Simmons and Park on polyammonium cryptands,2 with par-
ticular interest being focused on neutral receptors invoking
Scheme 1 Reagents and conditions: (i) p-nitrophenol chloroformate,
hydrogen bond interactions (for example, amides and ureas).
The importance of spatial pre-organisation of the complexing
groups has been recognised and calixarenes in particular have
been developed as scaffolds for the preparation of selective
receptors.3 In recent years both charged4 and neutral5 calix-
[4]arene based receptors have been prepared which show a high
degree of tunability for selective anion complexation. We are
particularly interested in combining hydrogen bonding inter-
actions with sensing moieties to enable detection of anion bind-
ing via optical and electrochemical methods and to this end
have prepared a number of such calixarene based receptors
incorporating transition metal complexes of 2,2Ј-bipyridyl,6
ferrocene7 or cobaltocenium8 as sensors. In this paper we
describe the synthesis of two new calix[4]arene based receptors
incorporating ferrocene urea moieties and contrast their anion
complexation behaviour with that of a tetra-amidoferrocene
calix[4]arene derivative using a combination of 1H NMR
spectroscopy and voltammetry.
ethyldiisopropylamine, CH2Cl2.
readily with the tetra-amino 313 and 1,3-di-amino 410a deriv-
atives of tetrapropylcalix[4]arene (Schemes 2 and 3) in the pres-
ence of Hünig’s base to yield the desired urea receptors 5 and 6
in 48 and 68% yields, respectively.
The tetra-amidoferrocene derivative 7 was prepared in excel-
lent yield (90%) through treatment of 3 with chlorocarbonyl
ferrocene14 in the presence of triethylamine base (Scheme 2).
X-ray crystal structure of 7
X-ray quality single crystals of receptor 7 were grown by slow
evaporation from chloroform–methanol. As Fig. 1 illustrates,
the calixarene core adopts a flattened cone conformation in
which opposed phenyl rings have parallel (interplanar angle
0.6Њ) or sharply inclined (interplanar angle 100.7Њ) positions.
Associated with each receptor are three methanol molecules,
the hydroxyl groups of which are hydrogen-bonded to the carb-
onyl oxygen atoms of the inclined fragments of the tetramer.
Two of these occupy sites within the ‘cusps’ between the
inclined and parallel fragments (O(9)–O(1) = 2.781(3) Å,
O(10)–O(3) = 2.764(3) Å). A further hydrogen bond is formed
by the interaction of the NH group of one of the adjacent
parallel fragments with the oxygen atom of each (N(4)–O(9) =
2.912(4) Å, N(2)–O(10) = 2.888(4) Å). The remaining methanol
molecule (not shown in Fig. 1) lies alongside the receptor, to
which it is linked by a single hydrogen bond (O(11)–O(1) =
2.833(6) Å). In addition, the NH groups of the inclined frag-
ments form intermolecular hydrogen bonds to carbonyl oxygen
atoms of parallel fragments of adjacent tetramers (N(1)–O(3Ј)
= 2.779(4) Å, N(3)–O(7Љ) = 2.775(4) Å).
Results and discussion
Synthesis
A variety of approaches to the preparation of calixarene ureas
have been investigated, the majority based on the use of com-
mercially available isocyanates9 or isocyanate derivatives of
calixarenes.10 In this study the introduction of ferrocene ureas
onto the calixarene skeleton has been achieved through reac-
tion with an isocyanate synthon.11 This method allows the
isolation and purification of a less toxic reagent, enables a con-
vergent approach based on amino functionalised calixarenes to
the synthesis of both amide and urea derivatives, and is readily
adaptable to the introduction of other more complex urea
substituents.
Solution behaviour of 5
Ferrocenemethylamine 1 was prepared following literature
procedures12 and converted to the nitrophenyl carbamate 2
(52% yield after chromatography) through reaction with
p-nitrophenyl chloroformate (Scheme 1). This synthon reacted
The solution behaviour of potential calixarene based anion
receptors is important when considering their binding inter-
actions with anions. The existence of inter- and intra-molecular
D a l t o n T r a n s . , 2 0 0 3 , 4 6 4 4 – 4 6 5 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Scheme 2 Reagents and conditions: (ii) 2, ethyldiisopropylamine, DMAP, CH2Cl2, (iii) chlorocarbonyl ferrocene, triethylamine, CH2Cl2.
ing in the peaks for HA and HB appearing well downfield at 7.46
and 8.04 ppm, respectively. These dimers are kinetically labile
and the rate of association/dissociation (6 sϪ1 at 60 ЊC for
1
5ؒC6D6) determined by H EXSY NMR experiments15 is in
agreement with previously reported values.16 In contrast, with
more competitive solvents such as d6-DMSO (Fig. 2b) there is
disruption of the dimeric structures through competition for
the urea hydrogen bonds and a monomer spectrum is observed
featuring a single signal for the aromatic protons (HC and HD).
The spectral simplification from dimer to monomer is also
observed on the addition of anions to chloroform solutions of
receptor 5 which may be attributed to disruption of the
hydrogen bonding array by the anion binding processes.
In contrast, both the 1,3-di-urea 6 and tetraamide 7 deriv-
atives show no evidence of hydrogen bonding or association
behaviour in solution, both adopting rapidly interconverting
flattened cone conformations.
1H NMR anion binding studies
The solution-phase anion co-ordination behaviour of receptors
5, 6 and 7 was investigated by 1H NMR titrations in a competi-
tive solvent mixture of acetonitrile and DMSO (1 : 1). The
anions chosen for these studies (chloride, benzoate and di-
hydrogen phosphate, added as their tetrabutylammonium salts)
are of contrasting geometries and charge densities.
Addition of anions to the receptors resulted in significant
downfield perturbations (of up to 2.32 ppm for 6 with benz-
oate) of the urea or amide protons indicating that the anions
are bound by the hydrogen bonding moieties. Additional
evidence for the binding of the anions in the vicinity of the
upper-rim was provided by the downfield shifts of the signals
for the aromatic protons ortho to the point of substitution.
Stepwise addition of anions to the receptors enabled the
determination of association constants, as a measure of bind-
ing strength, from EQNMR17 analysis of the titration curves
(Table 1). Importantly, Job plots18 revealed that the anion :
receptor binding stoichiometry was 1 : 1 in all cases (for
example, Fig. 3).
Scheme 3 Reagents and conditions: (ii) 2, ethyldiisopropylamine,
DMAP, CH2Cl2.
hydrogen bonding interactions which must first be broken down
before an anion binding event can take place are of particular
relevance to calix[4]arene ureas. As has been previously noted
for other derivatives,9 the tetra-urea receptor 5 forms stable,
solvent containing hydrogen-bonded dimers in non-polar sol-
vents. The 1H NMR spectrum of receptor 5 in C6D6 is shown in
Fig. 2a. Two peaks are observed for the aromatic protons at 6.69
ppm (HD) and 8.18 ppm (HC), both are doublets with a typical
meta-coupling of 2.5 Hz. In addition, the strong hydrogen
bonds in the dimer cause deshielding of the urea protons result-
D a l t o n T r a n s . , 2 0 0 3 , 4 6 4 4 – 4 6 5 0
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Fig. 1 General structural view of 7 showing the inclusion of two MeOH molecules (most of the hydrogen atoms have been omitted for clarity).
Fig. 2 Aromatic region of the 1H NMR spectrum of receptor 5 in (a) C6D6 and (b) d6-DMSO.
Table 1 Stability constant data for receptors 5, 6 and 7 in 1 : 1
CD3CN : d6-DMSO
Ka/dm3 molϪ1
Anion
5
6
7
ClϪ
PhCO2
H2PO4
15
30
150
15
40
25
30
150
120
Ϫ
Ϫ
a Determined at 298 K; errors estimated to be ≤ 5%.
Fig. 3 Job plot for 6 and benzoate in 1 : 1 CD3CN : d6-DMSO
It is evident that the degree of upper-rim substitution has a
marked effect on the binding properties of the receptors. The
di-substituted receptor 6 exhibits modest association constants
for all tested anions thus showing limited topological dis-
crimination. This behaviour may be a consequence of the flexi-
bility of the receptor which allows the binding cavity to distort
sufficiently to accommodate the anion whilst retaining the max-
imum binding interactions. In contrast, the tetra-urea 5 and
tetra-amide 7 receptors, in which the binding cavity is more
revealing a 1 : 1 anion : receptor binding stoichiometry.
defined and additional hydrogen bond donor sites are available,
show significantly larger association constants, in particular for
the more basic dihydrogen phosphate anion. Interestingly, the
introduction of ureas does not result in markedly larger associ-
ation constants compared to the amide receptor despite the
higher acidity and increased anion binding ability of the urea
NH protons.
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Fig. 4 General structural view of 6ؒ2[N(C4H9)4][PhCO2] (the propyl groups and most of the hydrogen atoms have been omitted for clarity).
Table 2 Electrochemical data for receptors 5, 6 and 7a
Table 3 Electrochemical anion recognition data for receptors 5, 6 and
7a
b
Receptor
Epa
Epc
∆E
Ipa/Ipc
∆Epa/mVb
5
6
7
82
83
265
Ϫ14
Ϫ2
165
96
85
100
1.05
1.05
1.10
Anion
5
6
7
ClϪ
PhCO2
H2PO4
40
50
40
70
50
20
a Conditions: 5 × 10Ϫ4 M solutions of receptors in 0.1 M NBu4BF4 in
1 : 1 CH2Cl2 : CH3CN, glassy carbon working electrode, Pt auxilliary
electrode, Ag/AgNO3 reference electrode. CVs recorded with a scan rate
Ϫ
Ϫ
220
180
200
of 100 mV sϪ1
.
b relative to the Ag/AgNO3 reference, errors 5 mV. E
a Conditions: 5 × 10Ϫ4 M solutions of receptors in 0.1 M NBu4BF4 in
1 : 1 CH2Cl2 : CH3CN, glassy carbon working electrode, Pt auxilliary
electrode, Ag/AgNO3 reference electrode. CVs recorded with a scan rate
of 100 mVsϪ1. SWVs recorded with a scan increment of 2 mV and a
frequency of 25 Hz. b Cathodic shift of oxidation potential produced
by the prescence of 5 equivalents of anion added as the tetrabutyl-
ammonium salt. Errors 10 mV.
½
ferrocene = 55 5 mV vs. Ag/AgNO3 reference electrode.
X-ray crystal structure of 6
Single crystals of 6ؒ2[N(C4H9)4][PhCO2] were obtained from a
1 : 1 acetonitrile : DMSO solution of the compound contain-
ing excess tetrabutylammonium benzoate. In contrast to the
solution studies (Fig. 3), in the solid state the ferrocene urea
groups act independently, binding benzoate in a 2 : 1 anion :
receptor stoichiometry (Fig. 4). The calixarene molecule is situ-
ated on a crystallographic twofold axis of rotation and adopts a
flattened cone conformation in which the two aromatic units
substituted with ureas are flattened (the interplanar angle
between the best planes of the attached phenyl groups is
113.2Њ). The other two phenyl groups taper inward slightly
(interplanar angle 17.1Њ), effectively preventing the formation
of any ‘pocket’. Each urea is hydrogen-bonded through both
NH groups to a benzoate anion (N(1)–O(4) = 2.949(2) Å, N(2)–
O(5) = 2.813(2) Å), the urea and carboxylate groups not being
coplanar (the interplanar angle between the NCN and OCO
planes is 36.3Њ).
interaction is disfavouring reduction back to ferrocene or that
an EC mechanism is in operation.
It is noteworthy that in all cases on addition of dihydrogen
phosphate the appearance of a new Epa wave is seen which
increases to the detriment of the original wave (Fig. 5), whereas
a single shifting wave is observed with chloride and benzoate
(Fig. 6). This redox responsive behaviour with dihydrogen
phosphate suggests the binding of this anion is kinetically slow
on the CV or SWV timescale. Interestingly, for all receptors the
largest magnitude of cathodic shift was noted with dihydrogen
phosphate by up to ∆Epa = 220 mV with 5.
Electrochemical anion sensing studies
In order to evaluate the potential electrochemical sensor cap-
abilities of receptors 5, 6 and 7, their voltammetric behaviour in
the presence and absence of anions was investigated. Cyclic and
square wave voltammograms were recorded for the receptors in
a 1 : 1 CH2Cl2 : CH3CN solvent mixture using NBu4BF4 as
the supporting electrolyte. All receptors display a single quasi-
reversible oxidation for the ferrocene/ferrocenium redox couple
indicating that the ferrocene moieties act independently and are
all oxidised at the same potential (Table 2).
On progressive addition of stoichiometric equivalents of
anionic guest solutions, significant cathodic perturbations of
the ferrocenes’ oxidation potentials Epa were observed for all
receptors (Table 3), concomitant with the disappearance of the
reduction wave Epc. As reported previously with simple acyclic
amide substituted ferrocene derivatives,19 these cathodic shifts
can be attributed to the binding of an anionic guest by the NH
protons of the urea and amide groups in close proximity to the
ferrocene redox centres, facilitating oxidation to ferrocenium.
The disappearance of the reduction wave on anion addition
indicates either that the complexed anion–ferrrocenium cation
Fig. 5 Square wave voltammogram of 5 in the absence and presence
of dihydrogen phosphate in 1 : 1 CH2Cl2 : CH3CN.
Conclusion
New calixarene based anion receptors featuring di- or tetra-
urea binding sites and ferrocene reporter units have been readily
prepared, in acceptable yield, from a novel isocyanate synthon.
1H NMR solution binding studies reveal these receptors and an
D a l t o n T r a n s . , 2 0 0 3 , 4 6 4 4 – 4 6 5 0
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washed repeatedly with sodium carbonate solution (10%, 4 ×
50 mL) then the organic fraction was dried over anhydrous
magnesium sulfate, filtered, and the solvent removed under
vacuum. The residue was purified by column chromatography
(silica, CHCl3 : MeOH 10 : 1). The product was obtained as a
pale yellow powder after precipitation from a CHCl3–hexane
solution (590 mg, 48%). 1H NMR (500 MHz, d6-DMSO): δ 8.19
(s, ArNH, 4H), 6.77 (s, ArH, 8H), 5.90 (t, J = 5.5 Hz, CH2NH,
4H), 4.30 (d, J = 13.0 Hz, ArCHaxAr, 4H), 4.16 (s, C5H5, 20H),
4.13 (s, C5H4, 8H), 4.07 (s, C5H4, 8H), 3.93 (d, J = 5.5 Hz,
CH2NH, 8H), 3.74 (t, J = 7.5 Hz, OCH2CH2CH3, 8H), 3.02 (d,
J = 13.0 Hz, ArCHeqAr, 4H), 1.89 (sext, J = 7.5 Hz, OCH2-
1
CH2CH3, 8H), 0.93 (t, J = 7.5 Hz, OCH2CH2CH3, 12H). H
NMR (500 MHz, C6D6): δ 8.18 (d, J = 2.5 Hz, ArH, 4H),
8.04 (s, ArNH, 4H), 7.46 (t, J = 5.5 Hz, CH2NH, 4H), 6.69 (d,
J = 2.5 Hz, ArH, 4H), 4.76 (d, J = 12.0 Hz, ArCHaxAr, 4H), 4.51
(dd, J = 14.5 & 5.5 Hz, CH2NH, 4H), 4.45 (dd, J = 14.5 & 5.5
Hz, CH2NH, 4H), 4.41 (s, C5H4, 4H), 4.22 (s, C5H4, 4H), 4.11 (s,
C5H5, 20H), 3.93 (s, C5H4, 4H), 3.89 (t, J = 7.5 Hz, OCH2-
CH2CH3, 8H), 3.85 (s, C5H4, 4H), 3.43 (d, J = 12.0 Hz,
ArCHeqAr, 4H), 2.07 (sext, J = 7.5 Hz, OCH2CH2CH3, 8H),
0.89 (t, J = 7.5 Hz, OCH2CH2CH3, 12H). ES-MS m/z: 1617.5
(M ϩ Hϩ), 1656.4 (M ϩ Kϩ). Elemental analysis, found: C,
63.5; H, 5.8; N, 6.8%. C88H96Fe4N8O8ؒ½CHCl3 requires C, 63.4;
H, 5.8; N, 6.7%.
Fig. 6 Square wave voltammogram of 5 in the absence and presence
of benzoate in 1 : 1 CH2Cl2 : CH3CN.
analogous amide derivative bind anions in a 1 : 1 stoichiometry.
Whilst the di-urea receptor 6 binds anions weakly and non-
discriminatorily, both the tetra-urea 5 and tetra-amide 7 recep-
tors generally show stronger binding and selectivity for more
basic anions. X-ray crystallographic studies of 6 with benzoate
confirm the involvement of hydrogen bonding interactions
from the urea moieties in anion binding. All receptors show
potential as electrochemical sensors for anions, with dihydrogen
phosphate displaying markedly larger cathodic shifts than
chloride and benzoate.
1,3-Di-ferrocene-urea functionalised calix[4]arene receptor 6.
2 (400 mg, 1.06 mmol), ethyldiisopropylamine (1 ml, excess)
and DMAP (catalytic) were added to a solution of 1,3-
diaminocalix[4]arene derivative 4 (300 mg, 0.48 mmol) in
CH2Cl2 (10 ml). The solution was stirred for 18 h and moni-
tored by TLC. The mixture was washed repeatedly with 10%
Na2CO3, dried and the solvent evaporated. The residue was
purified by column chromatography (CH2Cl2 : MeOH 9 : 1)
Experimental
General
All chemicals were commercial grade and used without further
purification unless otherwise stated. Solvents were pre-dried
and purified by distillation and stored under nitrogen where
appropriate; dichloromethane and acetonitrile were dried by
distillation over calcium hydride; triethylamine was distilled
from potassium hydroxide. Tetrabutylammonium salts of
chloride, benzoate and dihydrogen phosphate were stored in a
dessicator under vacuum containing self-indicating silica.
1
to yield the product as a pale orange foam (360 mg, 68%). H
NMR (500 MHz, d6-DMSO): δ 8.23 (s, ArNH, 2H), 6.96 (s,
ArH, 4H), 6.41 (m, ArH, 6H), 6.02 (t, J = 5.5 Hz, CH2NH, 2H),
4.30 (d, J = 13.0 Hz, ArCHaxAr, 4H), 4.17 (m, C5H4 & C5H5,
14H), 4.09 (s, C5H4, 4H), 3.97 (d, J = 5.5 Hz, CH2NH, 4H), 3.80
(t, J = 7.5 Hz, OCH2CH2CH3, 4H), 3.68 (t, J = 7.5 Hz, OCH2-
CH2CH3, 4H), 3.06 (d, J = 13.0 Hz, ArCHeqAr, 4H), 1.88 (sext,
J = 7.5 Hz, OCH2CH2CH3, 4H), 1.84 (sext, J = 7.5 Hz, OCH2-
CH2CH3, 4H), 0.99 (t, J = 7.5 Hz, OCH2CH2CH3, 6H), 0.88
(t, J = 7.5 Hz, OCH2CH2CH3, 6H). 1H NMR (300 MHz, C6D6):
δ 7.10 (d, J = 7.5 Hz, ArH, 4H), 6.95 (t, J = 7.5 Hz, ArH, 2H),
6.70 (s, ArNH, 2H), 6.34 (s, ArH, 4H), 5.47 (t, CH2NH, 2H),
4.47 (d, J = 13.5 Hz, ArCHaxAr, 4H), 4.16 (d, J = 5.0 Hz,
CH2NH, 4H), 4.13 (s, C5H4, 4H), 4.00 (s, C5H5, 10H), 3.93
(s, C5H4, 4H), 4.00 & 3.55 (both t, J = 7.5 Hz, OCH2CH2CH3,
2 × 4H), 3.09 (d, J = 13.5 Hz, ArCHeqAr, 4H), 1.90 (sext,
J = 7.5 Hz, OCH2CH2CH3, 4H), 1.66 (sext, J = 7.5 Hz, OCH2-
CH2CH3, 4H), 0.91 (t, J = 7.5 Hz, OCH2CH2CH3, 6H), 0.77 (t,
J = 7.5 Hz, OCH2CH2CH3, 6H). ES-MS m/z: 1105.6 (M ϩ Hϩ).
Elemental analysis, found: C, 68.7; H, 6.4; N, 4.7%. C64H72-
Fe2N4O6 requires C, 69.6; H, 6.6; N, 5.1%.
Ferrocenemethylamine 1,12 tetra-aminocalix[4]arene derivative
10a
3,13 1,3-di-aminocalix[4]arene derivative 4
and chloro-
carbonyl ferrocene14 were prepared according to literature
procedures.
Nuclear magnetic resonance spectra were recorded using
either a 300 MHz Varian VXWorks spectrometer or a 500 MHz
Varian Unity spectrometer. Electrospray mass spectra were
recorded using Micromass LCT equipment and microanalyses
were obtained from an elementar vario EL.
Syntheses
Ferrocene mononitrophenolate 2. p-Nitrophenol chloro-
formate (432 mg, 2.14 mmol) and ethyldiisopropylamine (1 ml,
excess) were added to a solution of ferrocenemethylamine
(460 mg, 2.14 mmol) in CH2Cl2 (15 ml) and the resulting
mixture stirred for 24 h. After evaporation of the solvent, the
residue was purified by column chromatography (ethylacetate :
hexane 4 : 1) to yield the desired product as an orange
Tetra-ferrocene-amide functionalised calix[4]arene receptor 7.
Tetra-aminocalix[4]arene derivative 3 (500 mg, 0.77 mmol) in
CH2Cl2 (50 ml) was added dropwise to a stirred solution of
chlorocarbonyl ferrocene (1150 mg, 4.63 mmol) and triethyl-
amine (5 ml, excess) in CH2Cl2 (50 ml) and the mixture was
stirred for 48 h under nitrogen. Water (100 ml) was added and
the mixture stirred for 30 min. The organic fraction was separ-
ated, washed with water (100 ml), dried over anhydrous mag-
nesium sulfate, filtered, and the solvent removed under vacuum.
The product was obtained as a pale yellow/orange powder after
precipitation from a CHCl3–hexane solution, followed by fil-
1
solid (420 mg, 52%). H NMR (300 MHz, CDCl3): δ 8.23 (d,
J = 9 Hz, ONp, 2H), 7.28 (d, J = 9 Hz, ONp, 2H), 5.25 (br t,
NH, 1H), 4.23 (s, FeCp, 2H), 4.21 (s, CH2NH & FeCp, 9H).
FAB MS m/z: 380 (M), 403 (M ϩ Na).
Tetra-ferrocene-urea functionalised calix[4]arene receptor 5.
Tetra-aminocalix[4]arene derivative 3 (500 mg, 0.77 mmol) in
CH2Cl2 (20 ml) was added dropwise to a stirred solution of 2
(1750 mg, 4.61 mmol), ethyldiisopropylamine (2 ml, excess) and
DMAP (catalytic) in CH2Cl2 (20 ml) and the mixture was
stirred for 24 h under nitrogen. The reaction mixture was
1
tration and washing with hexane (1040 mg, 90%). H NMR
(300 MHz, d6-DMSO): δ 9.18 (s, NH, 4H), 7.18 (s, ArH, 8H),
4.86 (s, C5H4, 8H), 4.43 (d, J = 12.6 Hz, ArCHaxAr, 4H), 4.31 (s,
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C5H4, 8H), 4.14 (s, C5H5, 20H), 3.85 (t, J = 7.5 Hz, OCH2-
CH2CH3, 8H), 3.20 (d, J = 12.6 Hz, ArCHeqAr, 4H), 1.98 (sext,
J = 7.5 Hz, OCH2CH2CH3, 8H), 1.00 (t, J = 7.5 Hz, OCH2-
CH2CH3, 12H). FAB-MS m/z: 1501.5 (M), 1524.5 (M ϩ Na).
Elemental analysis, found: C, 66.5; H, 5.1; N, 3.7%. C84H84-
Fe4N4O8 requires C, 67.2; H, 5.6; N, 3.7%.
auxiliary electrode and a silver/silver nitrate reference electrode
(silver wire in 10 mM silver nitrate in electrolyte solution). In all
experiments, the electrolyte solution was 0.1 M tetrabutyl-
ammonium tetrafluoroborate in 1 : 1 dichloromethane :
acetonitrile. Ferrocene was employed as an internal standard
(E ferrocene = 55 5 mV vs. Ag/AgNO3 reference electrode).
½
CVs were typically recorded with a 5 s equilibration time, a scan
increment of 2 mV and a scan rate of 100 mV sϪ1; two cycles
were recorded and the second one used for analysis. SWVs were
typically recorded with a 5 s equilibration time, a scan incre-
ment of 2 mV and a frequency of 25 Hz. The working electrode
was cleaned between scans by polishing on a solvent-soaked
cloth. Anion sensing studies were performed with the receptor
(5 × 10Ϫ4 M in electrolyte solution) by recording a CV and a
SWV after the addition of 0, 0.5, 1, 2, 5, 10 and 20 equivalents
of anion (tetrabutylammonium chloride, benzoate or di-
hydrogen phosphate, 0.125 M in electrolyte solution, 20 µL is 1
equivalent) to a 5 mL aliquot of the receptor solution.
X-ray crystallography
Typically a single crystal was mounted on a glass fibre using
perfluoropolyether oil and cooled rapidly to 150K in a stream
of cold N2 using an Oxford Cryosystems CRYOSTREAM unit.
Diffraction data were measured using an Enraf-Nonius Kappa
CCD diffractometer (graphite-monochromated Mo Kα radi-
ation, λ = 0.71073 Å). Intensity data were processed using the
DENZO-SMN package.20 The structure was solved using the
direct-methods program SIR92,21 which located all non-hydro-
gen atoms. Subsequent full-matrix least-squares refinement was
carried out using the CRYSTALS program suite.22 Coordinates
and anisotropic thermal parameters of all non-hydrogen atoms
were refined. All hydrogen atoms were positioned geometrically
after each cycle of refinement. A four-term Chebychev poly-
nomial weighting scheme was applied.
Acknowledgements
We thank the EPSRC for a project studentship (A. J. E.) and for
the use of the mass spectrometry service of the University of
Wales, Swansea.
CCDC reference numbers 216792 and 216793.
See http://www.rsc.org/suppdata/dt/b3/b309288g/ for crystal-
lographic data in CIF or other electronic format.
References
Crystal data for 7ؒ3MeOHؒ3CHCl3. Crystals were grown by
slow evaporation from chloroform–methanol. C90H98Cl9Fe4-
N4O11, M = 1954.26, triclinic, a = 14.1072(1), b = 17.9519(1),
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c = 20.8413(2) Å, α = 70.9519(4), β = 82.2956(4), γ = 68.8153(4)Њ,
3
¯
U = 4533.4 Å , T = 150 K, space group P1, Z = 2, µ(Mo Kα) =
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M
=
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D a l t o n T r a n s . , 2 0 0 3 , 4 6 4 4 – 4 6 5 0
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