Mono- and bis-ferrocene 2,5-diamidopyrrole clefts: Solid-state assembly, anion binding and electrochemical properties

Article abstract of DOI:10.1016/S0277-5387(02)01401-8

Four amido-pyrrole cleft anion receptors bearing one or two ferrocene reporter groups have been synthesised and crystallographically characterised. The receptors contain either a non-conjugated (1 and 3) or conjugated (2 and 4) link between the anion binding amido-pyrrole unit and the ferrocene reporter groups. The anion binding affinities and electrochemical behaviour of the receptors in the absence and presence of anions have been studied by ¹H NMR titration techniques and cyclic voltammetry using a Pt microdisc working electrode, respectively.

Full text of DOI:10.1016/S0277-5387(02)01401-8

Polyhedron 22 (2003) 699Á709
/
www.elsevier.com/locate/poly
Mono- and bis-ferrocene 2,5-diamidopyrrole clefts: solid-state
assembly, anion binding and electrochemical properties
Simon J. Coles, Guy Denuault, Philip A. Gale *, Peter N. Horton,
Michael B. Hursthouse, Mark E. Light, Colin N. Warriner
School of Chemistry, University of Southampton, Southampton, SO17 1BJ, UK
Received 17 July 2002; accepted 9 October 2002
Abstract
Four amido-pyrrole cleft anion receptors bearing one or two ferrocene reporter groups have been synthesised and
crystallographically characterised. The receptors contain either a non-conjugated (1 and 3) or conjugated (2 and 4) link between
the anion binding amido-pyrrole unit and the ferrocene reporter groups. The anion binding affinities and electrochemical behaviour
1
of the receptors in the absence and presence of anions have been studied by H NMR titration techniques and cyclic voltammetry
using a Pt microdisc working electrode, respectively.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Anion binding; Pyrrole; Ferrocene; Self-assembly; Electrochemical sensor
1. Introduction
turbations in their electrochemical properties in the
presence of anionic guests [8], whilst Gale, Sessler and
co-workers have recently reported a ferrocene funtiona-
lised calix [4]pyrrole in which a ferrocene CH group is
believed to form a hydrogen bond to a bound anion [9].
In this paper, we report the synthesis of four ferrocene
appended 2,5-diamido-pyrrole clefts and their anion
binding and electrochemical properties in the absence
and presence of anions [10].
Pyrrolic anion receptors have been shown to be both
effective and selective in their complexation properties.
Early systems developed by Sessler et al. were based
upon expanded porphyrins (e.g.: sapphyrin) that bound
small anions such as fluoride within the protonated core
of the macrocycle [1]. Later systems based on non-
aromatic macrocycles such as the calixpyrroles also
showed selectivity for halides [2] whilst complex and
elegant receptors such as the bipyrrole based catenane
2. Experimental
reported by Sessler, Vogtle and co-workers exhibits high
¨
affinity for phosphates [3]. More recently, attention has
been directed to acyclic systems such as the dipyrrolyl-
quinoxalines [4], guanidinium functionalised pyrroles [5]
and amido-pyrrole clefts [6]. A number of groups have
synthesised a variety of electrochemical sensors for
anions [7]. Electrochemical sensors for anions based
upon pyrrole are fairly rare. Sessler et al. have reported
ansa-ferrocene type systems that show significant per-
2.1. Synthesis
2.1.1. 3,4-Diphenyl-1H-pyrrole-2,5-dicarboxylic acid
diferrocenylmethyl-amide 1
3,4-Diphenyl-1H-pyrrole-2,5-dicarbonyl
dichloride
[10] (1.2 g, 0.0032 mol) was dissolved in dry dichlor-
omethane (50 ml). The solution was stirred under a
nitrogen atmosphere and triethylamine (0.711 g, 0.0070
mol), DMAP (catalytic quantity) and ferrocenemethyl-
amine [11] (1.51 g, 0.0070 mol) were added. The reaction
mixture was stirred overnight under a nitrogen atmo-
sphere. The dichloromethane solvent was removed in
* Corresponding author. Tel.:
80596805.
E-mail address: philip.gale@soton.ac.uk (P.A. Gale).
ꢀ
/
44-23-80593332; fax:
ꢀ/44-23-
0277-5387/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0277-5387(02)01401-8

700
S.J. Coles et al. / Polyhedron 22 (2003) 699Á709
/
vacuo and the residue dissolved in acetonitrile (50 ml).
The resultant precipitate was filtered, washed with
C38H31N3O2Fe2 Calc. 673.1110 Found: 673.1107 Dꢂ
/
0.4 ppm.
acetonitrile (3ꢁ
/
15 ml) and water (3ꢁ15 ml) and dried
/
under high vacuum for 2 h. The precipitate was purified
by preparative layer chromatography on silica plates
eluted with dichloromethane/methanol 2% affording the
final product 1 in 31% yield (0.69 g) as an orange
2.1.3. 3,4-Diphenyl-1H-pyrrole-2,5-dicarboxylic acid
monomethyl ester 6
3,4-Diphenyl-1H-pyrrole-2,5-dicarboxylic acid di-
methyl ester 5 [12] (4 g, 0.0119 mol) was suspended in
methanol (200 ml). The suspension was magnetically
stirred and heated to reflux forming a solution. Potas-
sium hydroxide (0.67 g, 0.119 mol) was added in water
(50 ml). After refluxing overnight, the solution was left
to cool to room temperature. The solution was acidified
to pH 1 using concentrated hydrochloric acid, forming a
white precipitate. This was removed by filtration and
1
powder. H NMR in CD2Cl2 300 MHz, d (ppm): 3.89
(m, 4H, FcH), 3.96 (m, 10H, FcH), 4.05 (m, 4H, FcH),
4.10 (m, 4H, CH2), 5.75 (br, s, 2H, amide NH), 7.23 (m,
4H, ArH), 7.30 (m, 6H, ArH), 10.21 (s, 1H, pyrrole NH)
13C NMR in CDCl3 400 MHz d (ppm): 39.2, 67.7, 68.3,
68.8, 84.8, 124.2, 126.1, 128.4, 129.3, 131.2, 133.6, 160.2.
ESꢀ mass spectrum, m/z, 701 (Mꢀ) HRES MS
C40H35N3O2Fe2 Calc. 701.1423 Found: 701.1437 Dꢂ
/
washed with water (3ꢁ50 ml). The white solid was
/
2.0 ppm.
dissolved in ether, and the aqueous layer that formed
was removed. The organic layer was dried with magne-
sium sulfate, filtered and then the solvent removed in
vacuo. The product 6 was obtained in a 78% yield (3 g)
2.1.2. 3,4-Diphenyl-1H-pyrrole-2,5-dicarboxylic acid
bis-ferrocenylamide 2
1
as a white powder. H NMR in DMSO, 300 MHz, d
(ppm): 3.74 (s, 3H, CH3), 7.13Á7.26 (m, 10H, Ar), 12.31
/
3,4-Diphenyl-1H-pyrrole-2,5-dicarbonyl
dichloride
(s, 1H, NH), 12.86 (s, 1H, OH). 13C NMR in DMSO,
400 MHz, d (ppm): 51.73, 51.82, 121.57, 122.15, 123.35,
126.87, 126.96, 127.06, 127.56, 127.59, 127.64, 130.31,
130.88, 131.02, 131.14, 133.76, 133.94, 134.16, 160.77,
[10] (0.83 g, 0.00241 mol) was dissolved in dry dichlor-
omethane (50 ml). The solution was stirred under a
nitrogen atmosphere and triethylamine (0.487 g, 0.00481
mol), DMAP (catalytic quantity) and ferrocenylamine
[11] (0.9 g, 0.00481 mol) were added. The reaction
mixture was stirred overnight under a nitrogen atmo-
sphere. The dichloromethane solvent was removed in
vacuo and the residue purified by column chromato-
graphy on silica gel eluted with dichloromethane/metha-
nol 2% affording the final product 2 in 13% yield (0.21
161.81. ESꢃ mass spectrum, m/z: 320 (MÁ
/
Hꢀ)ꢃ, 641
(2MÁ
/
Hꢀ)ꢃ.
2.1.4. 5-(Ferrocenylmethyl-carbamoyl)-3,4-diphenyl-
1H-pyrrole-2-carboxylic acid methyl ester 7
3,4-Diphenyl-1H-pyrrole-2,5-dicarboxylic acid mono-
methyl ester 6 (1.49 g, 0.0047 mol), ferrocenemethyla-
mine (1 g, 0.0047 mol) were magnetically stirred under a
nitrogen atmosphere in dimethylformamide (50 ml).
Benzotriazol-1-yloxytripyrrolidinophosphonium hexa-
fluorophosphate (2.45 g, 0.0047 mol), triethylamine
(0.47 g, 0.0047 mol) and a catalytic quantity of 1-
hydroxybenzotriazole hydrate were added and the
1
g) as an orange powder. H NMR in CD2Cl2 300 MHz
d (ppm): 3.95 (m, 4H, FcH), 4.02 (m, 10H, FcH), 4.31
(m, 4H, FcH), 6.69 (br, s, 2H, amide NH), 7.35 (m, 4H,
ArH), 7.7.46 (m, 6H, ArH), 10.29 (s, 1H, pyrrole NH)
13C NMR in CDCl3 300 MHz d (ppm): 62.5, 65.3, 69.5,
93.3, 124.7, 126.4, 128.9, 129.5, 131.3, 133.6, 158.6 ESꢀ
mass spectrum, m/z, 673 (Mꢀ) HRES MS

S.J. Coles et al. / Polyhedron 22 (2003) 699Á
/709
701
solution stirred in the dark. After 72 h the dimethylfor-
mamide was removed under high vacuum. Acetonitrile
(50 ml) was added and a precipitate formed. The orange
solid was collected by filtration and washed with
134.34, 159.48, 162.28. ESꢀ mass spectrum, m/z: 504
(Mꢀ), 527 (MꢀNaꢀ). HRES MS: C29H24FeN2O3
Calc. 504.1130, Found: 504.1127 Dꢂ0.9 ppm.
/
/
acetonitrile (3ꢁ
/
15 ml). Purification was achieved by
2.1.6. 3,4-Diphenyl-1H-pyrrole-2,5-dicarboxylic acid 2-
ferrocenylmethyl-amide 5-phenylamide 3
column chromatography on silica gel eluting with
dichloromethane-2% methanol. Crystallisation was
achieved by slow evaporation of dichloromethane:-
methanol (2:1) solution of the product. After drying
under high vacuum the product 7 was obtained in 69%
yield (1.66 g). Elemental analysis: Found: C, 69.06; H,
4.81; N, 5.12. C30H26FeN2O3 requires C, 69.51; H, 5.06;
N, 5.40%. 1H NMR in CDCl3, 300 MHz, d (ppm): 3.77
(s, 3H, CH3), 3.87 (s, 2H, FcH), 3.96 (s, 5H, FcH), 4.04
5-(Ferrocenylmethyl-carbamoyl)-3,4-diphenyl-1H-
pyrrole-2-carboxylic acid 9 (0.45 g, 0.0009 mol) and
aniline (0.083 g, 0.0009 mol) were magnetically stirred
under nitrogen in dimethylformamide (30 ml). Benzo-
triazol-1-yloxytripyrrolidinophosphonium hexafluoro-
phosphate (0.494 g, 0.00095 mol), triethylamine (0.09
g, 0.0009 mol) and a catalytic quantity of 1-hydroxy-
benzotriazole hydrate were added and the flask stirred
in the dark. After 72 h the dimethylformamide was
removed under high vacuum. Acetonitrile (50 ml) was
added and a precipitate formed. The orange solid was
(s, 2H, FcH), 5.89 (s, 1H, amide NH), 7.09Á7.31 (m,
/
10H, Ar), 10.10 (s, 1H, pyrrole NH). 13C NMR in
CD2Cl2, 400 MHz, d (ppm): 37.92, 50.64, 66.49, 67.10,
67.70, 83.71, 119.23, 124.40, 125.04, 126.07, 126.47,
127.37, 128.16, 129.92, 130.16, 130.56, 132.50, 132.57,
158.68, 159.80. ESꢀ mass spectrum, m/z: 518 (Mꢀ).
HRES MS: C30H26FeN2O3 Calc. 518.1287, Found:
collected by filtration and washed with acetonitrile (3ꢁ
/
15 ml). The solid was purified by column chromato-
graphy on silica gel eluting with dichloromethane-2%
methanol. Crystallisation was achieved by slow evapora-
tion of an acetonitrile:dichloromethane (2:1) solution of
the receptor. After drying under high vacuum the
product was obtained in 14% yield (0.07 g). Elemental
analysis: Found: C, 71.28; H, 4.94; N, 7.06.
518.1277 Dꢂ/2.0 ppm.
2.1.5. 5-(Ferrocenylmethyl-carbamoyl)-3,4-diphenyl-
1H-pyrrole-2-carboxylic acid 9
5-(Ferrocenylmethyl-carbamoyl)-3,4-diphenyl-1H-
pyrrole-2-carboxylic acid methyl ester 7 (1.2 g, 0.0023
mol) was suspended in methanol (75 ml), and heated to
reflux under a nitrogen atmosphere. Potassium hydrox-
ide (0.13 g, 0.0023 mol) was added in water (20 ml).
After refluxing overnight the solution was left to cool.
Hydrochloric acid was added to the solution until it
reached pH 1. The resulting solid was collected by
filtration. The solid was dissolved in chloroform (100
ml) and the aqueous layer that formed residual water in
the solid was separated. The organic layer was dried
with magnesium sulfate and then filtered. The solvent
was removed in vacuo. Thin layer chromatography on
silica plates eluting with dichloromethane-2% methanol
revealed that the residue contained some starting
material. Trituration with acetonitrile (200 ml) gave a
yellow solid which was collected by filtration. After
drying this was shown to be the pure acid (0.36 g). The
solution was reduced in vacuo, and the resulting solid
was purified by column chromatography on silica gel
eluting with dichloromethane-2% methanol. This gave a
second crop of material (0.2 g) giving a total yield of
48% (0.56 g). Elemental analysis: Found: C, 64.18; H,
C35H29FeN3O2ꢀ0.16CH2Cl2 requires C, 71.35; H,
/
4.99; N, 7.10%. 1H NMR in CD2Cl2, 300 MHz, d
(ppm): 3.88 (s, 2H, FcH), 3.95 (s, 5H, FcH), 4.04 (s, 2H,
FcH), 4.13 (d, 2H, Jꢂ/5.46, CH2), 5.80 (s, 1H, ferrocene
amide NH), 7.00Á7.43 (multiple overlapping peaks,
/
16H, ArH and NH), 10.31 (s, 1H, pyrrole NH). 13C
NMR in DMSO-d6, 400 MHz, d (ppm): 39.03, 68.45,
68.53, 69.38, 86.19, 120.44, 124.51, 125.25, 125.47,
127.33, 127.63, 128.09, 128.62, 128.90, 129.14, 129.76,
131.59, 131.69, 134.88, 134.98, 139.75, 159.60, 160.74.
ESꢀ mass spectrum, m/z: 579 (Mꢀ). HRES MS:
C35H29FeN3O2 Calc. 579.1604, Found: 579.1610 Dꢂ
/
1.1 ppm.
2.1.7. 5-(Ferrocenyl-carbamoyl)-3,4-diphenyl-1H-
pyrrole-2-carboxylic acid methyl ester 8
3,4-Diphenyl-1H-pyrrole-2,5-dicarboxylic acid mono-
methyl ester 6 (1.76 g, 0.0055 mol), ferrocenylamine (1.1
g, 0.0055 mol) were magnetically stirred under nitrogen
in dimethylformamide (50 ml). Benzotriazol-1-yloxytri-
pyrrolidinophosphonium hexafluorophosphate (2.86 g,
0.0055 mol), triethylamine (0.55 g, 0.0055 mol) and a
catalytic quantity of 1-hydroxybenzotriazole hydrate
were added before wrapping the flask in foil. After 72
h the dimethylformamide was removed under high
4.50; N, 5.12. C29H24FeN2O3ꢀ0.56CH2Cl2 requires C,
/
64.36; H, 4.59; N, 5.08%. 1H NMR in CDCl3, 300 MHz,
d (ppm): 3.86 (s, 2H, FcH), 3.96 (s, 5H, FcH), 4.04 (s,
vacuum. Diethylether (7ꢁ50 ml) was added to the
/
2H, FcH), 4.14 (d, 2H, Jꢂ
/
5.46, CH2), 5.91 (s, 1H,
resultant oil, resulting in a precipitate that was collected
and dried (0.19 g). The remaining solution was reduced
in vacuo leaving another oil, which was purified using
column chromatography on silica gel eluting with
dichloromethane-2% methanol. After drying under
amide NH), 7.12Á
/
7.31 (m, 10H, Ar), 10.41 (s, 1H,
pyrrole NH). 13C NMR in CD2Cl2, 400 MHz, d (ppm):
37.87, 48.57, 67.34, 67.91, 68.31, 85.35, 121.57, 124.84,
126.02, 126.23, 126.90, 127.36, 129.08, 130.64, 130.76,

702
S.J. Coles et al. / Polyhedron 22 (2003) 699Á709
/
high vacuum the product was obtained as a foam (2.3 g)
a combined yield of 79% (2.49 g). Crystallisation was
achieved by slow evaporation of ether:dichloromethane
(4:1) Elemental analysis: Found: C, 69.18; H, 4.64; N,
6.12. C29H24FeN2O3 requires C, 69.06; H, 4.80; N,
5.55%. 1H NMR in CDCl3, 300 MHz, d (ppm): 3.79 (s,
3H, CH3), 3.96 (s, 2H, FcH), 4.02 (s, 5H, FcH), 4.31 (s,
quantity of 1-hydroxybenzotriazole hydrate were added
before wrapping the flask in foil. After 72 h the
dimethylformamide was removed under high vacuum.
The residue was dissolved in acetonitrile (50 ml). Water
(50 ml) and dichloromethane (300 ml) were added to the
solution. The organic layer was then separated and dried
over magnesium sulfate and the solvent removed in
vacuo. The resultant oil was purified using column
chromatography on silica gel eluting with dichloro-
methane-2% methanol. Crystallisation was achieved
from a methanol/dichloromethane solution of the re-
ceptor. After drying under high vacuum the product was
obtained in 38% yield (0.35 g). Elemental analysis:
2H, FcH), 6.75 (s, 1H, amide NH), 7.21Á7.46 (m, 10H,
/
Ar), 10.08 (s, 1H, pyrrole NH). 13C NMR in CD2Cl2,
300 MHz, d (ppm): 29.82, 51.81, 62.24, 65.11, 69.30,
120.32, 127.19, 127.54, 127.64, 127.72, 127.93, 128.67,
128.95, 129.30, 130.68, 130.82, 131.14, 131.71, 132.88,
133.43, 158.28, 160.84. ESꢀ mass spectrum, m/z: 504
(Mꢀ). HRES MS: C29H24FeN2O3 Calc. 504.1131,
Found: C, 69.36; H, 5.07; N, 7.00. C34H27FeN3O2ꢀ
/
1
0.33CH2Cl2 requires C, 69.45; H, 4.70; N, 7.08%. H
Found: 504.1123 Dꢂ
/
1.5 ppm.
NMR in CDCl3, 300 MHz, d (ppm): 3.95 (s, 2H, FcH),
4.03 (s, 5H, FcH), 4.32 (s, 2H, FcH), 6.71 (s, 1H,
2.1.8. 5-(Ferrocenyl-carbamoyl)-3,4-diphenyl-1H-
pyrrole-2-carboxylic acid 10
ferrocene amide NH), 7.04Á7.47 (multiple overlapping
/
5-(Ferrocenyl-carbamoyl)-3,4-diphenyl-1H-pyrrole-2-
carboxylic acid methyl ester 8 (2.4 g, 0.0048 mol) was
dissolved in methanol (125 ml), and heated to reflux
under a nitrogen atmosphere.
peaks, 16H, ArH and NH), 10.36 (s, 1H, pyrrole NH).
13C NMR in CD2Cl2, 400 MHz, d (ppm): 53.42, 62.15,
64.92, 69.13, 92.88, 119.21, 124.15, 124.36, 124.69,
126.03, 126.31, 128.26, 128.59, 128.66, 129.00, 129.16,
130.86, 131.02, 132.90, 133.09, 137.49, 157.91, 158.22.
Sodium hydroxide (6 g, 0.15 mol) was added in water
(120 ml). The solution was kept at 95 8C for 2 h then
left to cool. Hydrochloric acid was added to the solution
until it reached pH 1 resulting in a precipitate. The
resulting solid was collected by filtration and dissolved
in dichloromethane (200 ml) and the aqueous layer
separated. The organic layer was dried with magnesium
sulfate and then filtered. The solvent was removed in
vacuo and the product dried under high vacuum. The
compound needed no further purification and was
obtained in 49% yield (1.15 g). Crystallisation was
achieved by slow evaporation of a chloroform:dichlor-
omethane (1:1) solution of the receptor. Elemental
analysis: Found: C, 65.36; H, 4.67; N, 5.28.
ESꢀ mass spectrum, m/z: 565 (Mꢀ), 1153 (2Mꢀ
HRES MS: C34H27FeN3O2 Calc. 566.1525, Found:
566.1513 Dꢂ2.2 ppm.
/
Naꢀ).
/
2.2. Electrochemistry
Steady state voltammograms were recorded at room
temperature, at 20 mV sꢃ1, with a 25 mm diameter Pt
microdisc prepared as reported previously [13] and a
silver wire counter-reference electrode located in a
separate compartment with 0.1 M AgNO3. The poten-
tial was controlled with a HiTek PPR1 waveform
generator and the current measured with a homemade
current follower. The microdisc was cleaned with 0.3 mm
alumina on a polishing microcloth. Before each voltam-
C28H22FeN2O3ꢀ0.33CH2Cl2 requires C, 65.61; H,
/
4.41; N, 5.40%. 1H NMR in CDCl3, 300 MHz, d
(ppm): 3.96 (s, 2H, FcH), 4.01 (s, 5H, FcH), 4.29 (s, 2H,
mogram the electrode was held at ꢃ
/
2.5 V for 12 s then
FcH), 6.81 (s, 1H, amide NH), 7.21Á
/
7.47 (m, 10H, Ar),
at ꢃ0.175 V for 12 s. This pre-treatment was found
/
10.80 (s, 1H, pyrrole NH). 13C NMR in CD2Cl2, 400
MHz, d (ppm): 62.16, 65.05, 69.22, 120.33, 125.74,
126.77, 127.13, 127.49, 127.81, 128.23, 128.57, 129.19,
130.60, 130.94, 131.02, 132.39, 132.61, 133.19, 158.46,
164.42. ESꢀ mass spectrum, m/z: 490 (Mꢀ). HRES
MS: C28H22FeN2O3 Calc. 490.0974, Found: 490.0972
necessary to obtain reproducible voltammograms. Di-
chloromethane solutions with 0.5 mM ferrocene com-
pound, 1.5 mM tetrabutylammonium salt of the anion
of interest and 0.1 M tetrabutylammonium tetrafluor-
oborate supporting electrolyte were purged with argon
between recordings and kept under an argon blanket
during recordings. A series of voltammograms was
recorded for each solution.
Dꢂ
/
0.5 ppm.
2.1.9. 3,4-Diphenyl-1H-pyrrole-2,5-dicarboxylic acid 2-
ferrocenylamide 5-phenylamide 4
5-(Ferrocenyl-carbamoyl)-3,4-diphenyl-1H-pyrrole-2-
carboxylic acid 10 (0.8 g, 0.0016 mol) and aniline (0.15 g,
0.0016 mol) were magnetically stirred under nitrogen in
dimethylformamide (50 ml). Benzotriazol-1-yloxytripyr-
rolidinophosphonium hexafluorophosphate (0.85 g, 0.02
mol), triethylamine (0.17 g, 0.0016 mol) and a catalytic
3. Synthesis
Receptors 1 and 2 were synthesised by reaction of
either ferrocenylmethylamine [11] of ferrocenylamine
[11]
dichloride [10] in dichloromethane in the presence of
with
3,4-diphenyl-1H-pyrrole-2,5-dicarbonyl

S.J. Coles et al. / Polyhedron 22 (2003) 699Á
/709
703
triethylamine and a catalytic quantity of DMAP fol-
lowed by purification by column chromatography.
Receptors 3 and 4 were synthesised by mono-de-
NMR and high-resolution mass spectrometry (Section
2).
esterifying
3,4-diphenyl-1H-pyrrole-2,5-dicarboxylic
acid dimethyl ester 5 to produce a mono-ester mono-
acid 6. This was then coupled to either ferrocenylmethy-
lamine or ferrocenylamine to afford the mono-esters 7
and 8, respectively which were subsequently de-esterified
affording the mono-acids 9 and 10 and then coupled to
aniline affording receptors 3 and 4 (Scheme 1). The
compounds were fully characterised by 1H and 13C
4. Crystal structures of 1
/
4 and intermediates
Á
The crystal structures of the intermediates and final
products show a number of interesting hydrogen bond-
ing interactions [14]. Data were collected on a Bruker
Nonius Kappa CCD area detector diffractometer with a
Scheme 1.

Table 1
Crystallographic data and CCDC deposition numbers
Compound number
1
2
3
4
7
8
10
CCDC number
Empirical formula
Formula weight
Crystal system
Space group
See Ref. [10]
C40H35N3O2Fe2
701.41
See Ref. [10]
C38H31N3O2Fe2
673.36
189 976
189 980
189 978
C30H26N2O3Fe2
518.38
189 977
189 979
C35H29N3O2Fe
579.46
triclinic
C35H31N3O3Fe
597.48
triclinic
C29H24N2O3Fe
504.35
triclinic
C30H27N2O5Cl3Fe
657.74
triclinic
monoclinic
C2/c
triclinic
¯
orthorhombic
Pbca
¯
P/1
¯
P/1
¯
1
¯
P/1
P/
1
P/
˚
a (A)
16.1333(5)
10.3709(3)
37.6445(12)
12.7602(3)
14.1906(4)
17.3431(5)
87.721(1)
83.137(1)
75.782(2)
4
9.3589(7)
16.076(2)
17.627(3)
86.856(12)
89.616(12)
84.448(9)
4
10.6570(5)
10.9323(6)
13.7118(10)
88.074(2)
82.029(2)
65.031(2)
2
6.0816(4)
24.850(2)
31.120(3)
10.4568(4)
11.6216(5)
11.78872(6)
61.172(2)
70.776(2)
67.863(2)
2
11.0968(4)
11.1517(4)
11.9917(5)
102.233(2)
95.688(2)
92.374(3)
2
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
97.982(1)
Z
8
0.973
Yellow plate
8
0.678
Orange needle
m (mmꢃ1
Crystal
)
1.001
Red prism
0.612
Orange block
0.568
Orange blade
0.696
Orange plate
0.845
Orange block
Crystal size (mm)
Reflections collected
0.15ꢁ
11 432
/
0.10ꢁ
/
0.03
0.15ꢁ
36 068
/
0.10ꢁ
/
0.03
0.25ꢁ
21 893
/0.15ꢁ/0.10
0.25ꢁ
19 514
/0.20ꢁ/0.03
0.20ꢁ
3244
/
0.03ꢁ
/
0.03
0.14ꢁ
16 609
/
0.10ꢁ
/
0.02
0.16ꢁ
21 381
/
0.12ꢁ0.06
/
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit on F2
)
5096 (0.0706)
5096/0/424
0.984
10 432 (0.0867)
10 432/360/909
0.955
21 862 (0.0104)
21 862/0/740
1.042
4086 (0.0787)
4086/0/382
1.016
3244 (0.0000)
3244/0/326
0.970
5111 (0.0439)
5111/0/326
1.040
6433 (0.0476)
6433/6/382
1.029
Final R indices [F2ꢀ
/
2s(F2)]
R1ꢂ
wR2ꢂ
R1ꢂ0.0929,
wR2ꢂ0.0938
0.472 and ꢃ0.434
/0.0474,
R1ꢂ
wR2ꢂ
R1ꢂ0.0968,
wR2ꢂ0.1016
0.325 and ꢃ0.380
/
0.0456,
R1ꢂ
wR2ꢂ
R1ꢂ0.0833,
wR2ꢂ0.1541
0.534 and ꢃ0.375
/0.0591,
R1ꢂ
wR2ꢂ
R1ꢂ0.0986,
wR2ꢂ0.1782
0.514 and ꢃ0.479
/0.0622,
R1ꢂ
wR2ꢂ
R1ꢂ0.1092,
wR2ꢂ0.1289
0.489 and ꢃ0.455
/0.0549,
R1ꢂ
wR2ꢂ
R1ꢂ0.0651,
wR2ꢂ0.1048
0.461 and ꢃ0.505
/
0.0435,
R1ꢂ
wR2ꢂ
R1ꢂ0.0746,
wR2ꢂ0.1542
1.553 and ꢃ0.584
/0.0551,
/0.0817
/0.0862
/0.1380
/
0.1560
/0.1098
/0.0963
/0.1407
R indices (all data)
/
/
/
/
/
/
/
/
/
/
/
/
/
/
Largest difference peak and
ꢃ3
/
/
/
/
/
/
/
˚
hole (e A
)

S.J. Coles et al. / Polyhedron 22 (2003) 699Á
/709
705
˚
O 2.961(3) A).
Fig. 1. The X-ray crystal structure of 1 showing dimer formation in the solid-state via NHÃ
/
O hydrogen bonds (N/
Á Á Á
/
rotating anode generator following standard proce-
dures. Full crystallographic data for the structure
analyses have been deposited with the Cambridge
Crystallographic Data Centre. Crystal data and CCDC
deposition numbers or references for the seven struc-
tures are shown in Table 1.
Fig. 2. X-ray crystal structure of 2 showing dimer formation in the solid-state via NHÃ
/
O and CHÃ
/
O hydrogen bonds (N
/
Á Á Á
/
O 2.795(4) and 3.020(3)
˚
A C
˚
O 3.2245(5) A).
/
Á Á Á
/

706
S.J. Coles et al. / Polyhedron 22 (2003) 699Á709
/
The crystal structures of 1 (Fig. 1) and 2 (Fig. 2)
reveal the formation of solid-state dimers (a motif that
has been observed for other 2-substituted pyrrole
species) [6].
Compound 7 forms infinite chains along the b glide
˚
plane via NHÃ
shown in Fig. 3(b).
Compound 8 crystallised from chloroform/dichloro-
methane forms centrosymmetric dimers via CHÃ
/
O hydrogen bonds (N
/
Á Á Á
/
Oꢂ
/
2.906(5) A)
/
O
˚
hydrogen bonds (C
/
Á Á Á
/
Oꢂ3.432(6) A) from the ferro-
/
cene CÃH to the amide carbonyl oxygen. These dimers
/
form a hexagonal close packed array when viewed down
the c axis as shown in Fig. 4(b).
Compound 10 crystallised from chloroform/dichlor-
omethane forms centrosymmetric dimers via typical
OHÃ
2.648(4) A) (Fig. 5). In addition a chloroform molecule
is bound to the pyrrolic nitrogen via an NHÃCl
/
O carboxylic acid type hydrogen bonds (O
/
Á Á Á
/
Oꢂ
/
˚
/
˚
Clꢂ3.431(6) A).
The crystal structure of compound 3 crystallised from
acetonitrile/dichloromethane reveals two molecules in
the asymmetric unit differing in the orientation of the
interaction (N
/
Á Á Á
/
/
ferrocene (Fig. 6). Centrosymmetric dimers are formed
˚
via NHÃ
/
O (N/
Á Á Á
/
Oꢂ
/
2.900(4) A) and CHÃ
/
O (C
/
Á Á Á
/
Oꢂ
/
Fig. 4. Crystal structure of compound 8 (a) and packing diagram
viewed down c axis (b).
˚
3.473(5) A) hydrogen bonds from the methylene spacer
group.
Compound 4 crystallised from methanol/dichloro-
methane does not form dimers or chains but instead
binds a methanol molecule via NHÃ
/
O (N
/
Á Á Á
/
Oꢂ
/
˚
˚
2.696(6) A) hydrogen
2.930(6) A) and OÃ
/
HO (O
/
Á Á Á
/
Oꢂ
/
bonds (Fig. 7).
5. Anion binding studies and electrochemistry
Association constants for receptors 1Á4 with a variety
/
Fig. 3. Crystal structure of compound 7 (a) and packing diagram
showing formation of infinite chains along the b-glide plane (b) (phenyl
and ferrocenyl groups omitted for clarity).
of anionic guests were determined by 1H NMR titration
techniques (Table 2) [15]. For solubility reasons, di-

S.J. Coles et al. / Polyhedron 22 (2003) 699Á
/709
707
Fig. 5. Crystal structure of compound 10.
chloromethane was used in both the 1H NMR and
electrochemical studies. We have recently observed
unusual 1H NMR titration behaviour upon addition
of fluoride to other 2,5-diamidopyrrole cleft species due
to fluoride acting as a base [16]. Whilst in these studies,
the titration data could be adequately fitted to a 1:1
receptor:anion binding model, we believe, in-light of the
electrochemical behaviour reported below, that this data
should still be treated with caution. Excluding the
fluoride data, it was observed that receptors 1 and 3
are poor anion receptors with receptor 1 binding all the
other putative anionic guests with association constants
lower than 50 Mꢃ1. Compounds 2 and 4, containing the
directly conjugated ferrocene moiety, have moderate to
high affinities for benzoate (as has been observed with
similar receptors [6]) and also show a significant
interaction with dihydrogenphosphate. The 1H NMR
titration profile of compound 2 and benzoate is shown
in Fig. 8(a).
Fig. 6. Crystal structure of compound 3.
different. With the non-conjugated link, the ferrocene
reporter groups are unaffected by the rest of the
molecule and yield identical voltammetric responses.
With the conjugated link, the redox activity of the
ferrocene groups is clearly affected by the rest of the
molecule. An example CV (obtained upon addition of
benzoate to receptor 2) is shown in Fig. 8(b).
With some anions, the voltammetric wave is seriously
distorted as the product of the electrochemical reaction
ꢃ
passivates the electrode. For example H2PO4 affects
the voltammetry of all receptors; the voltammograms do
not reach the expected plateaus, a significant hysteresis
appears between the forward and reverse scans and it
becomes difficult to determine the half-wave potential.
To obtain a clean electrode surface, the electrode was
held at a large negative potential for a few seconds
before the start of each scan. This pre-treatment
removed the passivating material sufficiently to alleviate
the need for polishing the electrode between scans.
The voltammetric wave of the host-guest complexes
was found to shift negatively with all anions except with
Dilution studies were performed and showed no
evidence of self-association in solution. For example in
the case of compound 2, over the concentration range
1
2.6ꢁ
/
10ꢃ4Á
/
3.5ꢁ
/
10ꢃ2 M no significant shift in the H
NMR proton resonances was observed.
The steady state voltammetric response of the four
receptors in presence and absence of guest anions was
recorded with a platinum microdisc electrode. 1 and 3
1
were found to oxidise at very similar potentials (E/ ꢂ
/
/2
0.182 and 0.170 V, respectively). Although the oxidation
of 2 and 4 was found to be easier than that of 1 and 2,
1
their half-wave potentials are significantly different (E/
/2
ꢂ0.080 and 0.058 V, respectively). In all cases the
/
3
1
oxidation wave appears to be reversible, E/ꢃ
/
E/ ꢂ55, 48,
/
/4
/4
55 and 63 mV, respectively. Interestingly, the limiting
currents (4.4, 3.7, 2.0 and 1.8 nA, respectively) reflect
the number of ferrocene reporter groups. Except for the
doubling of the limiting current, the voltammetric
responses of 1 and 3 are almost identical. In contrast
the voltammetric responses of 2 and 4 are significantly
Fig. 7. Crystal structure of compound 4.

708
S.J. Coles et al. / Polyhedron 22 (2003) 699Á709
/
Table 2
Association constants of receptors 1Á/4 with various anions (added as their tetrabutylammonium salts) in dichloromethane-d2 and voltammetric
shifts in ferrocene/ferrocenium redox couple of receptors in the presence of three equivalents of the anion in dichloromethane
Compound 1
Compound 2
Compound 3
Compound 4
Ka (Mꢃ1
)
DE (mV)
131
Ka (Mꢃ1
)
DE (mV)
123 and ꢃ
Ka (Mꢃ1
)
DE (mV)
Ka (Mꢃ1
)
DE (mV)
28 and ꢃ
Fꢃ
Clꢃ
170
ꢃ/
704
68
ꢃ/
/
255b
1518
ꢃ
ꢃ/
/
55
30
0
1582
54
ꢃ/
/
232b
26
B
B/
/
20
20
44
47
34
ꢃ
/
76
0
ꢃ
ꢃ
/53
B
B/
/
20
20
35
69
ꢃ
/
Brꢃ
H2PO4
B
/
20
143
/
12
a
B/20
ꢀ
/
7
130a
296
47
ꢃ
ꢃ
ꢃ
/
248a
29
150
ꢃ
a
ꢃ/
ꢃ
a
HSO4
Benzoate
76
1827
ꢃ
ꢃ
/37
ꢃ
/3
/
ꢃ
/
60
/122
213
ꢃ
/
80
807
/
a
With some anions, the voltammetric wave is seriously distorted as the product of the electrochemical reaction passivates the electrode.
Two waves are observed.
b
bromide which did not affect the electrochemistry of the
2,5-diamidopyrrole clefts show interesting solid-state
assembly properties via a variety of classical and non-
classical hydrogen bonds, as well as functioning as
prototypical electrochemical anion sensors. The electro-
chemistry of these systems is complicated by the
passivation of the electrode surface in the case of
dihydrogenphosphate whilst fluoride anions cause com-
plex two-wave behaviour which we believe may be due
to deprotonation of the pyrrole NH group. However,
large shifts observed upon addition of benzoate to
electrochemical solutions of the receptors 2 and 4 reveal
significant negative shifts of the ferrocene/ferrocenium
couple which are attributed to complex formation with
this oxo-anion and subsequent facile oxidation of the
complex.
hosts. In an ideal situation, these negative shifts should
reflect the increasing ease of oxidation of anion com-
plexes with respect to the free receptors. However, other
effects such as the possible deprotonation of the host
(e.g. upon addition of fluoride) or passivation of the
electrode surface by the product of the electrochemical
oxidation can complicate the results.
Compounds 2 and 4 show a moderate to strong
affinity for benzoate by 1H NMR titration experiments.
The large shifts observed for the ferrocene/ferrocenium
couple in these cases reflect the affinity for this anion
and also the presence of a conjugated bond pathway
between the anion binding site and the ferrocene
reporter groups.
6. Concluding remarks
7. Supplementary material
This paper has demonstrated that in addition to
functioning as an oxo-anion selective anion binding
motif, when appended with ferrocene reporter group(s),
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Centre (Table 1 for deposition numbers). Copies
Fig. 8. (a) 1H NMR titration of compound 2 with tetrabutylammonium benzoate in dichloromethane-d2 and (b) microelectrode cyclic
voltammogram of 2 in the presence of three equivalents of tetrabutylammonium benzoate.

S.J. Coles et al. / Polyhedron 22 (2003) 699Á
/709
709
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