Anion complexation of a pentafluorophenyl-substituted tripodal urea receptor in solution and the solid state: selectivity toward phosphate

Article abstract of DOI:10.1039/b820322a

The binding and selectivity of halides (spherical) and oxyanions (tetrahedral) toward a recently reported pentafluorophenyl-substituted tripodal urea-based receptor L¹ are examined thoroughly in the solid state by single-crystal X-ray crystallo

Full text of DOI:10.1039/b820322a

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Anion complexation of a pentafluorophenyl-substituted tripodal urea receptor
in solution and the solid state: selectivity toward phosphate†
I. Ravikumar,a P. S. Lakshminarayanan,a M. Arunachalam,a E. Sureshb and Pradyut Ghosh*a
Received 13th November 2008, Accepted 19th March 2009
First published as an Advance Article on the web 20th April 2009
DOI: 10.1039/b820322a
The binding and selectivity of halides (spherical) and oxyanions (tetrahedral) toward a recently
reported pentafluorophenyl-substituted tripodal urea-based receptor L1 are examined thoroughly in the
solid state by single-crystal X-ray crystallography as well as in solution by multinuclear NMR
techniques. Crystallographic results show proof of a fluoride encapsulation in the cavity of L1 in
complex [L1(F)][Bu4N], 1. Fluoride encapsulation inside the C3v symmetric cleft is observed via six
hydrogen bonds to all six urea protons of the receptor. In case of complex 2 crystallographic results
show encapsulation of sulfate ion inside a supramolecular cage formed upon 1 : 2 (guest–host) complex
formation between sulfate and L1. Sulfate encapsulation is observed via fourteen hydrogen bonding
interactions from all six urea moieties of two L1 units. Our effort to isolate single crystal of
halides/oxyanions complexes of L2 always yield single crystals of free L2 though literature shows anion
binding with this receptor in solution. Solution state binding studies of L1 are carried out by 1H-NMR
-
titration to calculate binding constants, which show the following anion binding sequence H2PO4
>
SO42-> CH3COO- > F- > Cl- >> Br- whereas there is no binding with I-, NO3 and ClO4 guests.
Comparison of phosphate and sulfate binding in L1 and L2, show higher binding with the
pentafluorophenyl substituted receptor, L1. Further 19F and 31P-NMR experiments in solution are also
-
-
carried out to probe the binding of F- and H2PO4 with L1, respectively. Extensive 1H-NMR
-
experiments in solution and crystallization in the presence of multiple anions are also undertaken to
-
evaluate the selectivity of H2PO4 over other anions.
charge halides is achieved by the use of receptors with high
Introduction
positive charge and bearing a pre-organized arrangement of
appropriate groups.15 Therefore, tren has been extensively used as
a building block for the synthesis of polyamine cage receptors,
which display encapsulation of halides in the receptor cavity
upon protonation,15 among which polyammonium cage receptors
for fluoride is also known.15 Moreover, tren based polyamide
cryptands cage,16 calix pyrrole and silsesquioxane cage,17 have also
shown fluoride encapsulation. Mascal et al. have recently reported
fluoride inclusion complex with triprotonated cyanuric acid based
cylindrophane by a combination of anion-p interactions and ion-
pair-reinforced hydrogen bonding.18
The development of receptors for anions is of considerable current
interest in molecular recognition study.1,2 Tris(2-aminoethyl)-
amine, tren, is an important building block in tripodal receptor
systems for anions that has been studied by different groups.3–12
The binding ability of tren-based acyclic tripodal receptors to-
wards anions varies with the attached moiety to the tren (N4) unit,
since functional groups modify the hydrogen bonding capability,10
as well as the conformation of the receptor.12 Among halides,
recognition of the smallest anion, fluoride, is of special interest
due to its applications in medical and biological fields and also
in drinking water purification process.13 Similarly, tetrahedral
In case of tetrahedral oxyanions a very few receptor sys-
tems are known that have shown crystallographic evidence
2-
-
oxyanions, SO4 and H2PO4 recognition are of current interest
due to their biological and environmental importance.14 In general,
the binding of spherical shaped and carrying single negative
for SO4 encapsulation,9,19 which also includes tripodal urea-
2-
based receptors.9 In our recent study, we have shown encap-
-
sulation of dimers of H2PO4 inside a pseudo dimeric cage
of a pentafluorophenyl-substituted tripodal urea receptor L1
(Scheme 1),12b whereas Bowman-James et al. have shown pocket
binding of this anion in a cyclophane receptor.20 In 1995, Mora´n
et al. demonstrated binding of phosphate and sulfate with the
L2 (Scheme 1) in solution.5 Whereas the binding of guests within
pre-organized macrocyclic systems are relatively straightforward
to understand but the binding processes of flexible podand
receptors remain more illusive.21 We report herein, the structural
evidence of encapsulated F- in the C3v symmetric cleft of L1
aDepartment of Inorganic Chemistry, Indian Association for the Cultivation
of Science, 2A and 2B Raja S. C. Mullick Road, Kolkata 700032, India.
E-mail: icpg@iacs.res.in; Fax: +91 33 2473 2805
bAnalytical Science Discipline, Central Salt and Marine Chemicals Research
Institute, G. B. Marg, Bhavnagar 364002, India
† Electronic supplementary information (ESI) available: 1H-NMR, 13C-
NMR and HRMS of L1, complex 1 and 2. 19F-NMR spectra of L1, n-
Bu4N+F- and L1 + F-. 1H-NMR titration curves of L1 with SO42-. Job plots
-
for L1 with F-, Cl-, Br-, HSO4 and SO42-. ORTEP diagram of complexes
1 and 2, and packing diagram of complex 1. Scatter plot for complexes
H2PO4- and fluoride (1). CCDC reference numbers 679393 (1) and 697104
(2). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b820322a
2-
via 1 : 1 complex formation, and SO4 encapsulation inside a
supramolecular capsule upon 1 : 2 (SO42-:L1) complex formation.
4160 | Dalton Trans., 2009, 4160–4168
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
temperature and slightly warmed for 10 min. The resulting solu-
tion was filtered using a filter paper. Filtrate was allowed to evap-
orate under room temperature. Colourless crystals of the fluoride
complex of L1, [L1(F-)]·n-Bu4N+ (1), suitable for X-ray diffraction
was obtained after a week. Isolated Yield of 1 is 80%.1H NMR
(300 MHz, DMSO-d6): d 1.26 (t, 3 H, NCH2CH2CH2CH3), 1.54
(m, 2 H, NCH2CH2CH2CH3), 1.57 (m, 2 H, NCH2CH2CH2CH3),
2.51 (s, 6 H, NCH2), 3.13 (s, 6 H, NCH2CH2), 3.21 (m, 2 H,
NCH2CH2CH2CH3), 7.50 (br, 3 H, NHa), 10.05 (br, 3 H, NHb). 13
C
NMR (200 MHz, DMSO-d6): d 13.5 (NCH2CH2CH2CH3), 19.3
(NCH2CH2CH2CH3), 23.2 (NCH2CH2CH2CH3) 36.4 (NCH2),
51.4 (NCH2CH2), 57.6 (NCH2CH2CH2CH3), 115.9 (m of s, Ar,
CCF), 137.1 (m, Ar, CF JCF = 326 Hz, JCCF = 18 Hz), 142.4 (m
Scheme 1 Tris(2-aminoethyl)amine based pentafluorophenyl and phenyl
based tripodal urea receptors L1 and L2.
In this work, we also show that L1 has the highest selectivity toward
H2PO4 in solution over other anions like F-, Cl-, Br-, I-, SO4
,
-
2-
=
of d, Ar, CF, JCF = 318 Hz), 155.2 (s, C O). HRMS (+ESI) calcd
for [C43H54F16N8O3]+, 1034.9151, found 774.2715 [L1 + H+].
-
-
ClO4 , AcO- and NO3 .
Synthesis of complex 2
Experimental
Complex 2 was synthesized by reacting L1 and (n-Bu4N)2SO4/
Solvents and starting materials
n-Bu4N+HSO4 . In both the cases, 75 mg of L1 was dissolved
-
Compounds L1 and L2 were prepared as previously described.5,12b
Tetrabutylammonium salts of fluoride, chloride, bromide, iodide,
hydrogensulfate, sulfate (50 wt% in water), dihydrogenphosphate,
nitrate, acetate and perchlorate were purchased from commercial
sources and used without further purification. The solvents were
purified by usual methods prior to use.
in 10 mL of DMSO in a 25 mL beaker. In the case of the
(n-Bu4N)2SO4 salt, 3 mL of (n-Bu4N)2SO4 (50 wt% in water) was
added to the 10 mL of L1 solution whereas, in other case 25 mg
of n-Bu4N+HSO4 was added in one shot to the 10 mL of L1
-
solution. Then in both cases, the mixtures were stirred for 5 min
at room temperature and warmed at 60 ◦C for 10 min. After
cooling to room temperature, both the resulting solutions were
filtered using a filter paper. Filtrates were collected in 25 mL
beaker and allowed to crystallize at room temperature. From
both the solutions colourless crystals of the sulfate complex of
L1, [2L1(SO42-)]·2(n-Bu)4N+ (2), suitable for X-ray diffraction was
obtained after three days. Isolated Yield of 2 is 80%.1H NMR
(300 MHz, DMSO-d6): d 0.88 (t, 3 H, NCH2CH2CH2CH3), 1.25
(m, 2 H, NCH2CH2CH2CH3), 1.51 (m, 2 H, NCH2CH2CH2CH3),
2.32 (s, 6 H, NCH2), 2.95 (s, 6 H, NCH2CH2), 3.11 (m, 2 H,
Physical methods
HRMS were recorded on a Qtof Micro YA263 mass spectrometer.
1H, 13C, 19F and 31P-NMR spectra were recorded on Bruker Avance
300, 75 or 500 MHz spectrometers.
Syntheses of tripodal receptors L1 and L2
The tripodal receptor L2 was synthesized following literature
procedures and characterization data were matched with the
literature data.5 L1 was synthesized following our recent report.12b
2.6 mL (20 mmol) of pentafluorophenyl isocyanate was dissolved
in 25 mL of dry DCM at room temperature in a 100 mL 2-neck
round bottomed flask equipped with a dropping funnel. Then
tren (1.0 mL, 6.5 mmol) was dissolved in 25 mL of dry DCM
and was added drop-wise using a dropping funnel with constant
stirring. The resulting solution was stirred for another 1 h at room
temperature in N2 atmosphere. The colourless precipitate formed
was filtered off and washed with DCM twice. The precipitate
collected was dried in air. Yield of L1 is 98%. 1H NMR (300 MHz,
DMSO-d6): d 2.55 (t, 6 H, NCH2), 3.15 (t, 6 H, NCH2CH2),
6.55 (s, 3 H, NHa), 8.352 (s, 3 H, NHb). 13C NMR (75.47 MHz,
DMSO-d6): d 38.9 (NCH2), 54.5 (NCH2CH2), 115.7 (m of s, Ar,
CCF, JCCF = 15 Hz), 137.9 (m of d, Ar, CF, JCF = 247 Hz), 138.8 (m
of d, Ar, CF JCF = 247 Hz), 143.6 (m of d, Ar, CF, JCF = 246 Hz),
NCH2CH2CH2CH3), 7.42 (br, 3 H, NHa), 8.86 (br, 3 H, NHb). 13
C
NMR (75.47 MHz, DMSO-d6): d 13.9 (NCH2CH2CH2CH3), 19.7
(NCH2CH2CH2CH3), 23.5 (NCH2CH2CH2CH3) 38.3 (NCH2),
54.9 (NCH2CH2), 58.0 (NCH2CH2CH2CH3), 115.8 (m of s, Ar,
CCF, JCCF = 15 Hz), 137.3 (m of d, Ar, CF, JCF = 244 Hz),
138.0 (m of d, Ar, CF JCF = 246 Hz), 143.2 (m of d, Ar,
=
CF, JCF = 246 Hz), 155.1 (s, C O). HRMS (+ESI) calcd for
[C59H90F15N9O7S]+, 1354.4441, found 774.0027[L1 + H+].
1H-NMR studies
Binding constants were obtained by 1H NMR (300 MHz Bruker)
-
titrations of L1 with [n-Bu]4N+A- (A-: F-, Cl-, Br-, SO42-, HSO4 ,
H2PO4- and AcO-) in DMSO-d6 at 25 ◦C. The initial concentration
of L1 was 20mM. Aliquots of anions were addedfrom two different
stock solutions 25 mM and 50 mM of anions (host–guest = up
to 1 : 1, 25 mM stock solution was used, and above 1 : 1 ratio
higher concentration anion was used) for all the anions except
+
1
=
155.4 (s, C O). HRMS (+ESI) calcd for [C27H19F15N7O3] , [L +
H]+ 774.4609, found 774.2267.
-
SO42- and HSO4 . For these two anions initial concentration of L1
-
was 20 mM and aliquots of either SO42-/HSO4 was added from
Synthesis of complex 1
a stock solution of 5 mM. Tetramethylsilane (TMS) in DMSO-d6
was used as an internal reference, and each titration was performed
by 15 measurements at room temperature. All proton signals
were referred to TMS. The association constant,23 K values were
75 mg of L1 was dissolved in 5 mL DMF–MeCN (1 : 1 v/v) binary
solvent in a 25 mL beaker. In to this solution, 25 mg of n-Bu4N+F-
was added in one shot and mixture was stirred for 5 min at room
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 4160–4168 | 4161
©

View Article Online
calculated by fitting the change in the N–H chemical shift with a
1 : 1 association model with non-linear least square analysis for
acetonitrile (MeCN) (1 : 1 v/v) binary solvent and resulting
solution yielded crystals suitable for single-crystal X-ray analysis.
Complex 2 is isolated following two different approaches by
treating L1 with bis-(tetrabutylammonium) sulfate (50 wt% in
water) and tetrabutylammonium hydrogensulfate. Irrespective of
tetrabutylammonium salts, complex 2 is isolated as crystals suit-
able for single-crystal X-ray studies. Syntheses of the receptor L1,
the fluoride complex 1, and sulfate complex 2 are straightforward,
resulting high yield.
F-, Cl-, Br-, H2PO4 and AcO- whereas 1 : 2 association model
-
was fitted for sulfate.
The equation Dd = {([A]0 + [L1]0 + 1/K) (([A]0 + [L1]0 +
1/K)2 - 4[L1]0[A]0)1/2}Ddmax/2[L1]0 was used to determine the K
values. 31P-NMR spectra were recorded at 500 MHz on a Bruker.
Chemical shifts in ppm were ◦relative to an external reference of
85% H3PO4 in DMSO-d6 at 25 C. 19F-NMR spectra were recorded
at Bruker 500 MHz spectrometer. Chemical shifts in ppm were
relative to an internal standard of trifluoro-toulene in DMSO-d6
at 25 ◦C.
Single-crystal X-ray structural studies of 1
We attempted to grow single crystal of complexes formed with L1
and n-Bu4N+X- (X = F, Cl, Br, I). But single crystals suitable
for crystallographic analysis are only obtained in case of the
fluoride complex [L1(F)][n-Bu4N], 1 from a DMF/MeCN binary
solvent system upon slow evaporation. Complex 1 crystallizes in
Single-crystal X-ray studies
The crystallographic data and details of data collection and
refinement for complexes 1 and 2 are listed below.‡ In each case,
a single crystal of suitable size was selected from the mother
liquor and immersed in Paratone oil and then mounted on the
tip of a glass fibre and cemented using epoxy resin. Intensity
data for these two crystals were collected using Mo Ka (l =
¯
the triclinic space group P1. The crystal structure of compound 1
reveals that the fluoride anion is encapsulated inside the tripodal
cavity, with hydrogen bonds to all six urea protons.22 Structural
analysis shows that encapsulation of F- inside the receptor cavity
is governed by six intramolecular N–H ◊ ◊ ◊ F- interactions from
three urea moieties of the tripodal receptor (Table 1 and Fig. 1a).
A correlation of the N–H ◊ ◊ ◊ F angle vs. H ◊ ◊ ◊ F distance (Fig. 2
and Table 1) shows that all are in the strong hydrogen bonding
˚
0.7107 A) radiation on a Bruker SMART APEX diffractometer
equipped with CCD area detector at 100 K. Reflections were
measured from a hemisphere of data collected with each frame
covering 0.5◦ in w. The data integration, reduction and structure
solutions/refinements were carried out using the software pack-
age of Bruker SMART APEX. Graphics were generated using
PLATON24 and MERCURY 1.3.25 In complexes 1 and 2, the non-
hydrogen atoms were refined anisotropically until convergence.
Even though the data for complex 2 was collected at 100 K, two
fluorine atoms, F6 and F8 were found to be disorder. However, we
were unable to model the disorder, as the thermal parameters for
the rest of the phenyl ring were in agreement with the model.
As a result, the F6 and F8 were considered non-disordered
(and planar to the ring) and were refined anisotropically till
convergence. Hydrogen atoms attached to the urea nitrogen atoms
of complex 1 and 2 were located from the difference Fourier map
and refined isotropically. The remainder of the hydrogen atoms in
these complexes were geometrically fixed at idealized positions.
˚
˚
interaction region of dH ◊ ◊ ◊ F < 2.2 A and dN ◊ ◊ ◊ F < 3.0 A. The
geometry around fluoride is distorted trigonal prismatic (Fig. 1b
and c), with a twist angle averaging 4.28◦ from the regular
trigonal prism, which is quite uncommon in anion complexes.1e,16
In cases of polyamide cryptands, Bowman-James et al. have
also reported this uncommon geometry around fluoride in two
◦
complexes with a larger twist angles of 36.6 ,16a and 35◦
A
16b
further two units of fluoride encapsulated L1 are held together via
two intermolecular C–F ◊ ◊ ◊ F–C contacts (Fig. 1d). Details of these
C–F ◊ ◊ ◊ F–C interaction is the interaction of C8–F4 ◊ ◊ ◊ F14–C26
˚
where F4/F14 ◊ ◊ ◊ F14¢/F4¢ = 2.811 A, ∠C8–F4 ◊ ◊ ◊ F14¢/∠C8¢–
F4¢ ◊ ◊ ◊ F14 = 165.82◦ and ∠C26–F14 ◊ ◊ ◊ F4¢/∠C26¢–F14¢ ◊ ◊ ◊ F4¢=
123.84◦. In the dimeric pseudo cage of L1 two fluoride ions are
˚
separated at a distance of 13.242 A whereas distance between the
˚
two bridgehead nitrogen centres is 19.824 A.
Results and discussion
The packing diagram of the compound viewed down b-axis
with various hydrogen bonding interactions is depicted in the
Fig. S22 of the ESI.† The 21 screw related tripodal ligands are
arranged along c-axis with opposite orientation along with screw
related tetrabutylammonium cations between the tripodal ligand
involving in C-H ◊ ◊ ◊ O and C–H ◊ ◊ ◊ F interactions generating
H-bonded layers along ac-plane. Details of these H-bonding
interactions are given in Table 2.
The tripodal urea-based receptors L1 and L2 are obtained by
reaction between tris(2-aminoethyl)amine and three equiv. of
pentafluorophenyl isocyanate and phenyl isocyanate, respectively,
in dry CH2Cl2.12b,5 In the case of complex 1 preparation, L1 is
treated with n-Bu4N+F- in N,N-dimethyl formamide (DMF)–
‡ Crystal data for 1: C43H54F16N8O3, Mr = 1034.94, triclinic, space group
¯
˚
P1, a = 12.9366(13), b = 13.3131(13), c = 15.2703(15) A, a = 108.729(2),
◦
-3
˚
b = 92.717(2), g = 108.518(2) , V = 2328.8(4) A , Z = 2, rcalcd
=
Table 1 H-bonding interactions of F- in complex 1
1.476 g cm-3, m = 0.138 mm-1, T = 100(2) K, 16 364 reflections measured,
6755 observed (I > 2s(I)) 659 parameters; Rint = 0.0292, R1 = 0.0551;
wR2 = 0.1268 (I > 2s(I)), R1 = 0.0680; wR2 = 0.1329 (all data) with
˚
˚
D–H ◊ ◊ ◊ A
d(H ◊ ◊ ◊ A)/A
d(D ◊ ◊ ◊ A)/A
∠D–H–A/◦
-3
-3
˚
˚
GOF = 1.095 I > 2s(I), Drmin/e A = -0.242 and Drmax/e A = 0.324.
Crystal data for 2: C86H108F30N16O10S, Mr = 2127.94, monoclinic, space
N2–H2C ◊ ◊ ◊ F16
N3–H3C ◊ ◊ ◊ F16
N4–H4C ◊ ◊ ◊ F16
N5–H5C ◊ ◊ ◊ F16
N6–H6C ◊ ◊ ◊ F16
N7–H7C ◊ ◊ ◊ F16
2.10(3)
2.02(3)
2.15(3)
1.90(3)
2.05(3)
2.01(4)
2.865(3)
2.793(3)
2.884(3)
2.700(3)
2.835(3)
2.790(3)
149(3)
160(3)
150(3)
165(3)
153(3)
153(3)
˚
group C2/c, a = 21.2559(15), b = 19.5053(13), c = 23.6564(17) A, a =
◦
-3
˚
90.00, b = 95.377(10), g = 90.00 , V = 9764.8(12) A , Z = 4, rcalcd
=
1.447 g cm-3, m = 0.155 mm-1, T = 100(2) K, 24 105 reflections measured,
7052 observed (I > 2s (I)) 691 parameters; Rint = 0.0356, R1 = 0.0787;
wR2 = 0.1798 (I > 2s(I)), R1 = 0.0942; wR2 = 0.1893 (all data) with
-3
-3
˚
˚
GOF = 1.065 I > 2s(I), Drmin/e A = -1.035 and Drmax/e A = 1.171.
4162 | Dalton Trans., 2009, 4160–4168
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Table 2 Intermolecular C–H ◊ ◊ ◊ O and C–H ◊ ◊ ◊ F interactions in 1 with
symmetry code
∠D–H–A/◦
˚
˚
D–H ◊ ◊ ◊ A
d(H ◊ ◊ ◊ A)/A
d(D ◊ ◊ ◊ A)/A
C28–H28A ◊ ◊ ◊ O3b
C28–H28B ◊ ◊ ◊ O2a
C32–H32A ◊ ◊ ◊ O1c
C32–H32B ◊ ◊ ◊ F5d
2.19
2.23
2.35
2.51
3.116(3)
3.162(3)
3.305(3)
3.185(3)
158
161
167
126
a x, y, z. b -x, 1 - y, 1 - z. c -x, -y, 1 - z. d x, y, 1 + z.
crystallographic analysis are obtained in cases of sulfate/bisulfate
and dihydrogenphosphate complexes of L1 as [2·L1(SO4)][n-Bu4N]2
and [L1(H2PO4)][n-Bu4N]·DMF from DMSO and DMF/MeCN
solvent systems respectively upon slow evaporation. In case of
AcO- crystallization process, a semisolid is isolated upon repeated
attempt. In our previous communication we have shown structural
details of dihydrogenphosphate complex of L1.12b The complex
[2·L1(SO4)][n-Bu4N]2, 2 crystallizes in monoclinic system with
C2/c space group. Two inversion-symmetric molecules of L1 form
a cavity that encapsulates a sulfate anion (disordered) in its centre
via hydrogen bonding to the six urea groups (Fig. 3a). There are
total fourteen hydrogen bonding interactions between the twelve
NH groups of two L1 moieties and four O atoms of SO42- (Fig. 3b).
Two of the oxygen atoms O5 and O5A accepts four hydrogen
bonds each and the other two O atoms (O4 and O6) form three
N–H ◊ ◊ ◊ O contacts each with the urea protons of L1 (Table 3).
A correlation of N–H ◊ ◊ ◊ O angle vs. H ◊ ◊ ◊ O distance (Fig. 5)
shows that in the strong hydrogen bonding interaction region of
˚
˚
dH ◊ ◊ ◊ O < 2.5 A and dN ◊ ◊ ◊ O < 3.2 A there are twelve contacts.
Out of twelve contacts only one contact has an N–H ◊ ◊ ◊ O angle
smaller than 140◦, which is N7–H7C ◊ ◊ ◊ O4 with a N7–H7–O4
angle 135◦ but dN ◊ ◊ ◊ O is 2.852 A. One hydrogen bonding interaction
fall in the weak interaction zone (2.5 < dH ◊ ◊ ◊ O < 2.8 A, and 3.2 <
dN ◊ ◊ ◊ O < 3.5 A) where the angle is not smaller than 140 (Table 4).
Similar correlation has been done by Wu et al. in case of sulfate
binding with the tripodal pyridyl urea based metal–organic frame
work, which shows eleven strong and six weak hydrogen bonding
interactions with the encapsulated sulfate.9d
˚
Fig. 1 (a) Perspective of [L1(F)]- showing encapsulation of F- ion (green,
ball and stick) inside the tripodal cavity where dotted lines represent the
(N–H) ◊ ◊ ◊ F- interactions green F, red O, blue N. The tetrabutylammonium
cation and the hydrogens other than urea groups are omitted for clarity,
(b) and (c) showing distorted trigonal prismatic geometry around fluoride.
(d) Dimer of complex 1 via C–F ◊ ◊ ◊ F–C interaction.
˚
◦
˚
Among the reported three examples of sulfate encapsulation
with similar tris(urea) receptors, two are based on metal–organic
framework,9a,c,d and third one is purely organic based.9b Encapsu-
lation of sulfate in our work (i.e. receptor to anion binding 2 : 1)
Table 3 Hydrogen bonding parameters in complex 2 with symmetry code
∠D–H–A/◦
˚
˚
D–H ◊ ◊ ◊ A
d(H ◊ ◊ ◊ A)/A
d(D ◊ ◊ ◊ A)/A
N2¢–H2C ◊ ◊ ◊ O5
N2–H2C ◊ ◊ ◊ O4a
N3–H3C ◊ ◊ ◊ O6
N3¢–H3C ◊ ◊ ◊ O6a
N4¢–H4C ◊ ◊ ◊ O5
N4–H4C ◊ ◊ ◊ O5A
N5¢–H5C ◊ ◊ ◊ O4
N5–H5C ◊ ◊ ◊ O6
N6¢–H6C ◊ ◊ ◊ O5
N6–H6C ◊ ◊ ◊ O5A
N7–H7C ◊ ◊ ◊ O4a
N7¢–H7C ◊ ◊ ◊ O5A
N7¢–H7C ◊ ◊ ◊ O5Aa
2.21 (5)
2.26 (5)
2.13 (5)
2.12 (5)
2.16 (5)
2.44 (5)
2.07 (5)
2.32 (5)
2.25 (4)
2.14 (4)
2.25 (5)
2.52 (5)
2.13 (5)
2.892 (7)
3.034 (6)
2.905 (6)
2.880 (6)
2.958 (6)
3.165 (6)
2.778 (6)
3.001 (6)
3.062 (6)
2.962 (6)
2.852 (6)
3.217 (6)
2.851 (7)
143 (5)
163 (5)
168 (4)
164 (4)
171 (4)
152 (4)
158 (5)
153 (5)
163 (4)
164 (4)
135 (4)
151 (4)
156 (4)
Fig. 2 The scatter plot of N–H ◊ ◊ ◊ F angle vs. H ◊ ◊ ◊ F distance of the
hydrogen bonds in complex 1.
Single-crystal X-ray studies of complex 2
Efforts have been made to grow single crystal of complexes of L1
with the n-Bu4N+X- (X = AcO, HSO4, H2PO4) as well as (n-
Bu4N+)2SO42- (50 wt% in water). But the single crystal suitable for
a Atoms related by the symmetry code: -x, y, 1/2 - z.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 4160–4168 | 4163
©

View Article Online
Fig. 4 A slice of the hydrogen-bonded framework obtained by the self
assembly of anionic capsules along b-axis.
Fig. 3 (a) Crystal structure of 2, showing two inversion related L1
molecules and encapsulated disordered sulfate ion. (b) Depiction of sulfate
encapsulation by 14 hydrogen bonds (dashed lines) from six urea groups;
yellow S, red O, and blue, N. The tetrabutylammonium cations and
hydrogens other than urea groups are omitted for clarity.
Fig. 5 The scatter plot of N–H ◊ ◊ ◊ O angle vs. H ◊ ◊ ◊ O distance of the
hydrogen bonds in complex 2.
Table 4 Intermolecular C–H ◊ ◊ ◊ O and C–H ◊ ◊ ◊ F interactions in complex
2 with symmetry code
to the reported metal–organic frameworks but packing of sulfate
capsules in the crystal lattice are via C–H ◊ ◊ ◊ O and C–H ◊ ◊ ◊ F
interactions generating H-bonded layers along ac-plane (Fig. 4
and Table 4). Table 4 also shows that there are three strong and
two weak C–H ◊ ◊ ◊ O interactions between urea oxygen atoms and
methylene hydrogen of tetrabutylammonium cation along with
one C–H ◊ ◊ ◊ F interaction.
˚
˚
D–H ◊ ◊ ◊ A
d(H ◊ ◊ ◊ A)/A
d(D ◊ ◊ ◊ A)/A
∠D–H–A/◦
C28–H28A ◊ ◊ ◊ O1a
C36–H36A ◊ ◊ ◊ O3b
C36–H36B ◊ ◊ ◊ O1a
C41–H41B ◊ ◊ ◊ O1a
C32–H32A ◊ ◊ ◊ O2c
C28–H28B ◊ ◊ ◊ O3b
C1–H1A ◊ ◊ ◊ F11d
2.548
2.386
2.387
2.405
2.361
2.574
2.473
3.395 (4)
3.296 (5)
3.304 (4)
3.267 (4)
3.284 (5)
3.473 (5)
3.275 (5)
146.0
156.2
157.4
147.8
158.7
154.1
139.9
Halide binding studies in solution
The binding properties of L1 with halides in solution state are
investigated by 1H NMR experiments in DMSO-d6 in the presence
of various halides as their n-Bu4N+X- salts (where X- = F-, Cl-,
Br- and I-). Fig. 6 shows the chemical shift changes found by the
addition of different halides to the urea ligand, L1 in DMSO-d6.
The most substantial changes are observed for the urea protons
(–NHa and –NHb), indicating that this urea –NH provides the
sites of interaction between the ligand and the anions. The large
b
a x, 1-y, -1/2 + z. -x, -1 + y, 1/2 - z. c 1/2 - x, -1/2 + y, 1/2 - z.
d
-x, 2 - y, 1 - z.
is also purely organic based but pattern is quite different from the
reported work, which shows sulfate–water–sulfate encapsulation
with the capsule of two receptor units (i.e. receptor to anion
binding 1 : 1).9b Whereas, this encapsulation pattern is quite similar
4164 | Dalton Trans., 2009, 4160–4168
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Therefore, with the increasing size/decreasing basicity of halides
the association constant regularly diminishes. Our previous study
on protonated pentafluorophenyl substituted tripodal amine
receptor having tetrafluoroborate counter ions shows log K
values 2.68 and 2.15 M-1 for Cl- and Br-, respectively, in same
solvent system,12a indicative of relatively flexible nature of tripodal
ammonium receptor as well as its functionality, which can assist
to bind with larger guest, bromide with a higher binding constant
compared to tripodal urea receptor where difference in binding
constants with chloride and bromide is relatively large.
The 19F-NMR spectra in DMSO-d6 revealed that the encap-
sulation of fluoride anion might persist in solution too (Fig. 8).
A free fluoride resonance for n-tetrabutylammonium fluoride in
DMSO-d6 appears at -98.04 ppm. The addition of L1 to the
solution of tetrabutylammonium fluoride in DMSO-d6 results in
the downfield shift of ~5 ppm of free fluoride resonance indicating
the participation of anion in hydrogen bonding with –NH of
the receptor L1. The downfield shift observed here may be due
to the deshielding effect caused by the ring currents of in-plane
pentafluorophenyl units. In case of macrobicyclic amide receptor
Bowman-James et al. have reported a larger down field shift of
free fluoride signal about 8 ppm.16b Moreover, 19F-NMR data of
L1 shows three peaks at -163.024, -159.302 and -145.217 ppm
corresponding to the fluorine signals of –C6F5 unit whereas L1
in presence of tetrabutylammonium fluoride shows appreciable
changes in the –C6F5 signal pattern (peaks at -164.182, -146.187
and -141.502).† This may be attributed due to the existence of
C–F ◊ ◊ ◊ F–C interaction in the solution state as observed in the
crystal structure.
Fig. 6 Partial 1H NMR spectra (300 MHz, DMSO-d6, 298 K) of L1 and
downfield shift of urea NH groups upon addition of F-, Cl-, Br- and I- as
their tetrabutylammonium salts.
downfield shift of the urea protons Dd –NHa 0.681 and –NHb
1.186 in the case of fluoride; Dd –NHa 0.307 and –NHb 0.206
for chloride; Dd –NHa 0.106 and –NHb 0.047 in the case of
bromide are observed. There are no considerable changes in the
chemical shift of the urea protons with iodide suggesting that the
interaction of L1 with iodide is energetically unfavourable. Based
on the relationship between the Dd values for urea protons and
the ionic radius of halides supports the most significant change is
observed for the smallest anion, fluoride.
To evaluate the halide binding, 1H-NMR titration is carried out
with F-, Cl- and Br- (Fig. 7). The titration curve gives the best fit
for 1 : 1 binding model for host to guest, in agreement with Job
plots indicating a maximum Dd at 0.5 = [L1]/([L1]+[A-]),† and
the association constants are calculated using EQNMR.23 The
stability constant results evaluated are summarized in Table 5.
The stability constant and change in free energy data show that L1
binds very strongly towards F- than any other halides having log
K > 4.0 M-1 (Table 5). However Cl- also displays significant
binding but Br- has much weaker binding with L1 in DMSO-d6.
Oxyanion binding studies in solution
In our earlier communication, solution state binding properties of
L1 with H2PO4 and other oxyanions such as CH3COO-, NO3 ,
-
-
-
ClO4 are reported by 1H-NMR titration experiments in DMSO-
d6.12b The binding properties of L1 with sulfate in solution state is
investigated by 1H NMR experiments in DMSO-d6 in the presence
of (n-Bu4N+)2SO42-. Fig. 9 shows the chemical shift changes found
by the addition of different oxyanions to the urea ligand, L1 in
DMSO-d6. The large downfield shift of the urea protons Dd –NHa
1.256 and –NHb 1.943 ppm in the case of acetate; Dd –NHa 0.870
and –NHb 0.508 ppm for sulfate; in the case of phosphate both the
–NH peaks are broad and there are upfield shift of –NHa resonance
by 0.441 ppm and –NHb resonance show 0.611 ppm downfield
shifts, whereas no shift is observed for NO3- and ClO4- anions. The
opposite nature of –NHa resonance in case of phosphate suggests
that this anion bind with the receptor predominatly by the –NHb,
the upfield shift of the –NHa protons caused presumably due to
either desolvation effect as DMSO is displaced from the cavity
by the anion or a conformational change in the receptor.26 The
Fig. 7 Plot of change in chemical shift of the urea NH groups of L1 with
increasing amounts of [n-Bu4N+X-] in DMSO-d6 at 298 K (X- = F, Cl,
and Br).
Table 5 Binding constants and free energy change of L1 with different
–NH peaks shifts by AcO-, SO4 and H2PO4- are characteristic
2-
halides in DMSO-d6 at 25 ◦
C
to differentiate each others in solution.
1H-NMR titrations for these three oxyanions are carried out to
evaluate the binding constants and stoichiometry. The addition
Anions
log K/M-1
DG/kcal mol-1
-
of aliquots of n-tetrabutylammonium salts of SO42-, H2PO4 and
F-
4.06
3.42
1.27
—
-5.5
-4.7
-1.7
—
Cl-
Br-
I-
CH3COO- to the solutions of the receptor led to the best fit for
1 : 1 stoichiometry of host to guest,12b in agreement with the Job
-
plots in cases of H2PO4 and CH3COO- whereas best fit for 1 : 2
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 4160–4168 | 4165
©

View Article Online
Fig. 8 Partial 19F NMR spectra (500 MHz, DMSO-d6, 298 K) of n-Bu4N+F- and downfield shift of F- resonance upon addition of L1.
respectively. Since complex 2 is also obtained upon treating L1
with tetrabutylammonim bisulfate, we have carried out 1H-NMR
titration experiments with this salt too. The mode of binding
(1 : 2) and binding constant (4.77 M-1) calculated using bisulfate
salt are almost same as in the case of SO42- binding.† These results
indicate that in solution state also bisulfate converts to sulfate in
our experimental conditions, support the solid state formation of
complex 2 when L1 is treated either with tetrabutylammonium salt
2-
-
of SO4 or HSO4
The binding of H2PO4 to L1 is also followed by changes in
-
the chemical shifts of 31P NMR signals of n-Bu4N+H2PO4 . The
-
addition of L1 to a solution of n-Bu4N+H2PO4 in DMSO-d6
-
showed a significant down field shift (Dd = 8.33 ppm) in the
-
31P resonances of the n-Bu4N+H2PO4 with respect to anion in
Fig. 9 Partial 1H NMR spectra (300 MHz, DMSO-d6, 298 K) of L1 and
downfield shift of urea NH groups upon addition of NO3-, ClO4-, AcO-,
the absence of the receptor (Fig. 11), indicating the formation of
a strong complex between the anionic dihydrogenphosphate and
the urea groups of the receptor.
-
SO42-, and H2PO4 as their tetrabutylammonium salts.
stoichiometry of guest to host, is observed in case of SO42- binding
(Fig. 10).†
Fig. 11 Partial 31P NMR spectra (500 MHz, DMSO-d6, 298 K) of
-
n-Bu4N+H2PO4 and downfield shift of 31P resonance upon addition
of L1.
Fig. 10 Plot of change in chemical shift of the urea NH groups of L1 with
Competitive binding studies of different anions
increasing amounts of [n-Bu4N+]2SO4 in DMSO-d6 at 298 K.
2-
In our previous communication we have showed that selectivity
of dihydrogenphosphate in presence of other oxyanions like
The association constants (log K in M-1) of L1 with SO4
,
2-
-
-
H2PO4 and CH3COO- are 4.73 M-1, 5.52 M-1 and 4.45 M-1,
NO3 , AcO-, and ClO4- in DMSO-d6.12b Herein we have accounted
4166 | Dalton Trans., 2009, 4160–4168
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
a large spectrum of anions like all halides, above oxyanions along
with sulfate for selectivity studies of L1 in solution. Detailed 1H-
NMR titration experiments showed the binding order of different
anions towards L1 is H2PO4- > SO42-> CH3COO- > F- > Cl- >>
Br- which supports Hofmeister-like response whereas there is no
of acetate, nitrate, perchlorate and dihydrogenphosphate, which
also results dihydrogenphosphate complex.12b Therefore, both
solution and solid state data confirms selectivity of L1 toward
dihydrogenphosphate over halides and other oxyanions.
Comparative studies of L1 and L2
binding with I-, NO3 and ClO4 guests. To probe the highest
-
-
selectivity of L1 towards H2PO4 over F-, Cl-, Br-, I-, AcO-,
-
Literature report shows the binding constants for L2 with
-
-
2-
1
ClO4 , NO3 and SO4 we have carried out a series of H-NMR
experiments in the presence of multiple anions (Fig. 12). Fig. 12
shows the chemical shift changes in the urea protons (–NHa and
–NHb) found by the addition of several combination of anions to
the urea ligand, L1 in DMSO-d6. In case of the spectrum showed in
Fig. 12b, the experiment is carried out upon addition of equivalent
amount of tetrabutylammonium fluoride to the solution of L1
containing chloride in DMSO-d6. When fluoride salt is added to
the L1 solution in presence of chloride (with chemical shifts of
urea protons Dd –NHa 0.307 and –NHb 0.206 corresponds to
chloride complex) shows the chemical shifts which matches with
the complex 1. This experiment indeed suggests the selectivity
of L1 to smallest anion, fluoride over chloride as well as other
halides in the solution phase. Whereas, chemical shifts of urea
protons of L1 in presence of F- or AcO- changes to the value
2-
3-
SO4
as bis(tetramethylammonium)sulfate and PO4
as
tris(tetramethylammonium)phosphate by 1H-NMR titration,
which are 3.48 M-1 and 4.04 M-1 in DMSO-d6.5 These data clearly
indicate that L1 binds to the phosphate or sulfate more strongly
than L2 in DMSO. The enhanced binding of these oxyanions in
the case of L1 may be attributed to the significantly more acidic
nature of –NH in L1, due to the electron withdrawing character
of the –C6F5 units compared to –C6H5. In case of the anionic
complexes of L1 we have X-ray crystallographically shown the
complete encapsulation of anionic guest inside the C3v symmetric
cleft where all the amide protons are directed towards the cavity.
But no structural evidence of anion complex of L2 is known
till date. At this juncture, we tried to isolate single crystal of
anion complexes of L2. In our repeated attempt to grow single
crystals of L2 receptor with above oxyanions or halides complexes
is unsuccessful. We always isolated free L2 as single crystal upon
treating L2 with tetrabutylammonium salt of different anions. This
indicate in solution state L2 might be in equilibrium with anion
complex, which shows the shift in –NH resonance but in solid
state complexes are not stable enough, resulting single crystals of
L2. Both solution state and/or solid state studies also indicate
the involvement of intermolecular C–F ◊ ◊ ◊ F–C and C–F ◊ ◊ ◊ H–C
interactions between L1 moieties in complexes 1, 2 and reported
dihydrogenphosphate complex.12b Further, –C6F5 moieties, which
are arranged at the open side of the bowl shaped receptor, could
facilitate the anion to encapsulate in the C3v symmetric cavity of L1
via electrostatic attraction. In case of non-fluorinated receptor, L2,
all the above interactions, electronegative effect, as well as positive
cloud effect are not operative which justify our isolation of L2 from
its solution state anion complexes.
2-
corresponds to that of SO4 when tetrabutylammonium sulfate
is added to the respective solutions (Fig. 12c and d) indicate
the selectivity of sulfate over fluoride and acetate. Finally, when
H2PO4 is added to the L1 solution containing F- and SO4 or
-
2-
AcO- and SO4 or F-, AcO- and SO4 chemical shifts of urea
2-
2-
protons (Fig. 12e) are similar as observed in case of L1 in presence
of H2PO4 (Fig. 8) supports the highest selectivity of L1 toward
-
this particular oxyanion.
Conclusions
In conclusion, solution state studies on tren-based tripodal urea
receptor L1 shows strong binding affinity towards fluoride among
-
2-
halides. In cases of oxyanions binding order is H2PO4 > SO4
>
AcO- and no binding is observed with ClO4 and NO3 . In
solution SO4 shows 1 : 2 guest to host binding whereas all other
-
-
2-
1
anions indicate 1 : 1 complex formation. Competitive H-NMR
experiments as well as 1H-NMR titration data suggest selectivity
order of L1 towards different anions which follows the Hofmeister
Fig. 12 Partial 1H NMR spectra (300 MHz, DMSO-d6, 298 K) of L1
(a) and shift of urea NH groups upon addition of different combinations
of anions as their tetrabutylammonium salts as mentioned in the individual
spectra.
series like H2PO4 > SO42-> CH3COO- > F- > Cl- >> Br-
-
-
and highly selective receptor for H2PO4 . The pentafluorophenyl
substituted tripodal receptor L1 shows higher binding toward
oxyanions than its non-fluorinated analogue L2. Further, single-
crystal X-ray study confirms complete encapsulation of the
fluoride ion in L1 and supports 1 : 1 complex formation. This
also represents first structural report on encapsulation of this
ion in a tripodal system. In case of sulfate complex 2, structural
analysis shows the formation of a capsule, which confirms the
We have also performed crystallization from a mixture of anions
to probe the selectivity in solid state too. We tried three different
combinations which are: (i) L1 in presence of tetrabutylammonium
salt of fluoride and chloride which results fluoride complex
1, (ii) L1 in presence of tetrabutylammonium salt of fluoride
and dihydrogenphosphate which results dihydrogenphosphate
complex,12b and (iii) L1 in presence of tetrabutylammonium salt
solution state finding of 1 : 2 complex formations (SO4 : L1).
2-
Anion encapsulation by L1 tripodal receptor represents another
Dalton Trans., 2009, 4160–4168 | 4167
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
class of neutral system for recognition studies of spherical as well
as oxyanions.
12 (a) P. S. Lakshminarayanan, I. Ravikumar, E. Suresh and P. Ghosh,
Inorg. Chem., 2007, 46, 4769–4771; (b) P. S. Lakshminarayanan, I.
Ravikumar, E. Suresh and P. Ghosh, Chem. Commun., 2007, 5214–
5216.
13 (a) Biochemistry of Halogens and Inorganic Halides, ed. K. L. Krik,
Plenum Press, New York, 1991; (b) M. Kleerekoper, Endocrinol Metab.
Clin. North Am., 1998, 27, 441.
Acknowledgements
P. G. gratefully acknowledges the Department of Science and
Technology (DST), New Delhi, India for financial support.
Crystallography of complex 2 is performed at the DST-funded
National Single-Crystal X-Ray Diffraction Facility at the Depart-
ment of Inorganic Chemistry, IACS. The authors would like to
acknowledge the NMR Research Centre, IISc Bangalore, India,
for the 19F-NMR analysis. PSL and MA would like to acknowledge
CSIR, New Delhi, India for Senior Research Fellowship.
14 (a) J. W. Pflugrath and F. A. Quiocho, Nature, 1985, 314, 257–260;
(b) B. L. Jacobson and F. A. Quniocho, J. Mol. Biol., 1988, 204, 783–
787; (c) T. H. Milby and R. C. Baselt, Am. J. Ind. Med., 1999, 35,
192–195; (d) A. K. H. Hirsch, F. R. Fischer and F. Diederich, Angew.
Chem., Int. Ed., 2007, 46, 338–352 and references cited therein.
15 (a) S. Mason, J. M. Llinares, M. Morton, T. Clifford and K. Bowman-
James, J. Am. Chem. Soc., 2000, 122, 1814–1815; (b) Md. A. Hossain,
J. M. Llinares, C. A. Miller, L. Seib and K. Bowman-James, Chem.
Commun., 2000, 2269–2270; (c) J. A. Aguilar, T. Clifford, A. Danby,
J. M. Llinares, S. Mason, E. Garc´ıa-Espan˜a and K. Bowman-James,
Supramol. Chem., 2001, 13, 405–417; (d) Md. A. Hossain, J. M.
Llinares, S. Mason, P. Morehouse, D. Powell and K. Bowman-James,
Angew. Chem., Int. Ed., 2002, 41, 2335–2338; (e) C. A. Ilioudis, D. A.
Tocher and J. W. Steed, J. Am. Chem. Soc., 2004, 126, 12395–12402;
(f) Md. A. Hossain, P. Morehouse, D. Powell and K. Bowman-James,
Inorg. Chem., 2005, 44, 2143–2149; (g) P. S. Lakshminarayanan, D. K.
Kumar and P. Ghosh, Inorg. Chem., 2005, 44, 7540–7546; (h) P. S.
Lakshminarayanan, E. Suresh and P. Ghosh, Angew. Chem., Int. Ed.,
2006, 45, 3807–3811; (i) M. Arunachalam, E. Suresh and P. Ghosh,
Tetrahedron, 2007, 63, 11371–11376.
16 (a) S. O. Kang, J. M. Llinares, D. Powell, D. VanderVelde and K.
Bowman-James, J. Am. Chem. Soc., 2003, 125, 10152–10153; (b) S. O.
Kang, D. VanderVelde, D. Powell and K. Bowman-James, J. Am. Chem.
Soc., 2004, 126, 12272–12273.
17 (a) A. R. Bassindale, M. Pourny, P. G. Taylor, M. B. Hursthouse and
M. E. Light, Angew. Chem., Int. Ed., 2003, 42, 3488–3490; (b) A. F.
Danil de Namor and M. Shehab, J. Phys. Chem. B, 2003, 107, 6462–
6468; (c) G. Cafeo, F. H. Kohnke, A. J. P. White, D. Garozzo and A.
Messina, Chem.–Eur. J., 2007, 13, 649–656; (d) J. L. Sessler, D. An,
W.-S. Cho and V. Lynch, Angew. Chem., Int. Ed., 2003, 42, 2278–2281.
18 M. Mascal, I. Yakovlev, E. B. Nikitin and J. C. Fettinger, Angew. Chem.,
Int. Ed., 2007, 46, 8782–8784.
Notes and references
1 (a) Supramolecular Chemistry of Anions, ed. A. Bianchi, K. Bowman-
James and E. Garc´ıa-Espan˜a, Wiley-VCH, New York, 1997; (b) P. D.
Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486–516; (c) J. M.
Llinares, D. Powell and K. Bowman-James, Coord. Chem. Rev., 2003,
240, 57–75; (d) The Encyclopedia of Supramolecular Chemistry, ed.
J. L. Atwood and J. W. Steed, Dekker, New York, 2004; (e) K. Bowman-
James, Acc. Chem. Res., 2005, 38, 671–678; (f) E. Garc´ıa-Espan˜a, P.
D´ıaz, J. M. Llinares and A. Bianchi, Coord. Chem. Rev., 2006, 250,
2952–2986; (g) K. Wichmann, B. Antonioli, T. So¨hnel, M. Wenzel, K.
Gloe, K. Gloe, J. R. Price, L. F. Lindoy, A. J. Blake and A. Schro¨der,
Coord. Chem. Rev., 2006, 250, 2987–3003; (h) B. L. Schottel, H. T.
Chifotides and K. R. Dunbar, Chem. Soc. Rev., 2008, 37, 68–83.
2 Representative recent articles on anion recognition: (a) S. S. Zhu, H.
Staats, K. Brandhorst, J. Grunenberg, F. Gruppi, E. Dalcanale, A.
Lu¨tzen, K. Rissanen and C. A. Schalley, Angew. Chem., Int. Ed., 2008,
47, 788–792; (b) M. Albrecht, C. Wessel, M. de Groot, K. Rissanen
and A. Lu¨chow, J. Am. Chem. Soc., 2008, 130, 4600–4601; (c) Y. Li and
A. H. Flood, Angew. Chem., Int. Ed., 2008, 47, 2649–2652; (d) R. J.
Go¨tz, A. Robertazzi, I. Mutikainen, U. Turpeinen, P. Gamez and J.
Reedijk, Chem. Commun., 2008, 3384–3386; (e) C. Caltagirone, P. A.
Gale, J. R. Hiscock, S. J. Brooks, M. B. Hursthouse and M. E. Light,
Chem. Commun., 2008, 3007–3009; (f) R. B. Bedford, M. Betham, C. P.
Butts, S. J. Coles, M. B. Hursthouse, P. N. Scully, J. H. R. Tucker, J.
Wilkie and Y. Willener, Chem. Commun., 2008, 2429–2431; (g) D. R.
Turner, M. J. Paterson and J. W. Steed, Chem. Commun., 2008, 1395–
1397; (h) K.-Y. Ng, V. Felix, S. M. Santos, N. H. Rees and P. D. Beer,
Chem. Commun., 2008, 1281–1283; (i) C. Caltagirone, G. W. Bates, P. A.
Gale and M. E. Light, Chem. Commun., 2008, 61–63; (j) C. Schmuck
and V. Bickert, J. Org. Chem., 2007, 72, 6832–6839; (k) O. B. Berryman,
A. C. Sather, B. P. Hay, J. S. Meisner and D. W. Johnson, J. Am. Chem.
Soc., 2008, 130, 10895–10897.
19 (a) R. Menif, J. Reibenspies and A. E. Martell, Inorg. Chem., 1991,
30, 3446–3454; (b) S. Mason, T. Clifford, L. Seib, K. Kuczera and K.
Bowman-James, J. Am. Chem. Soc., 1998, 120, 8899–8900; (c) M. J.
Hynes, B. Maubert, V. McKee, R. M. Town and J. Nelson, J. Chem.
Soc., Dalton Trans., 2000, 2853–2859; (d) B. M. Maubert, J. Nelson,
V. McKee, R. M. Town and I. Pa´l, J. Chem. Soc., Dalton Trans., 2001,
1395–1397; (e) J. Nelson, M. Nieuwenhuyzen, I. Pa´l and R. M. Town,
Chem. Commun., 2002, 2266–2267; (f) D. Farrell, K. Gloe, K. Gloe,
G. Goretzki, V. McKee, J. Nelson, M. Nieuwenhuyzen, I. Pa´l, H.
Stephan, R. M. Town and K. Wichmann, Dalton Trans., 2003, 1961–
1968; (g) S. O. Kang, Md. A. Hossain, D. Powell and K. Bowman-
James, Chem. Commun., 2005, 328–330; (h) S. Kubik, R. Kirchner, D.
Nolting and J. Seide, J. Am. Chem. Soc., 2002, 124, 12752–12760; (i) Z.
Rodriguez-Docampo, S. I. Pascu, S. Kubik and S. Otto, J. Am. Chem.
Soc., 2006, 128, 11206–11210; (j) S. Otto and S. Kubik, J. Am. Chem.
Soc., 2003, 125, 7804–7805.
3 S. Valiyaveettil, J. F. J. Engbersen, W. Verboom and D. N. Reinhoudt,
Angew. Chem., Int. Ed. Engl., 1993, 32, 900–901.
4 (a) P. D. Beer, Z. Chen, A. J. Goulden, A. Graydon, S. E. Stokes and T.
Wear, J. Chem. Soc., Chem. Commun., 1993, 1834–1836; (b) P. D. Beer,
P. K. Hopkins and J. D. McKinney, Chem. Commun., 1999, 1253–1254.
5 C. Raposo, M. Almaraz, M. Mart´ın, V. Weinrich, Ma. L. Musso´ns, V.
Alca´zar, M. C. Caballero and J. R. Mora´n, Chem. Lett., 1995, 759–760.
6 (a) A. Danby, L. Seib, K. Bowman-James and N. W. Alcock, Chem.
Commun., 2000, 973–974; (b) C. Bazzicalupi, A. Bencini, E. Berni, A.
Bianchi, S. Ciattini, C. Giorgi, S. Maoggi, P. Paoletti and B. Valtancoli,
J. Org. Chem., 2002, 67, 9107–9110.
7 K. Kavallieratos, A. Danby, G. J. Van Berkel, M. A. Kelly, R. A.
Sachleben, B. A. Moyer and K. Bowman-James, Anal. Chem., 2000,
72, 5258–5264.
8 P. S. Lakshminarayanan, E. Suresh and P. Ghosh, Inorg. Chem., 2006,
45, 4372–4380.
9 (a) R. Custelcean, B. A. Moyer and B. P. Hay, Chem. Commun., 2005,
5971–5973; (b) D. A. Jose, D. K. Kumar, B. Ganguly and A. Das, Inorg.
Chem., 2007, 46, 5817–5819; (c) R. Custelcean, P. Remy, P. V. Bonnesen,
De-en. Jiang and B. A. Moyer, Angew. Chem., Int. Ed., 2008, 47, 1866–
1870; (d) B. Wu, J. Liang, J. Yang, C. Jia, X.-J. Yang, H. Zhang, N.
Tang and C. Janiak, Chem. Commun., 2008, 1762–1764.
10 V. S. Bryantsev and B. P. Hay, Org. Lett., 2005, 7, 5031–5034.
11 M. A. Hossain, J. A. Liljegren, D. Powell and K. Bowman-James, Inorg.
Chem., 2004, 43, 3751–3755.
20 S. O. Kang, V. W. Day and K. Bowman-James, Org. Lett., 2008, 10,
2677–2680.
21 D. R. Turner, M. J. Paterson and J. W. Steed, J. Org. Chem., 2006, 71,
1598–1608.
22 A CSD (version 5.29, November 2007) search was performed with the
different nitrogen bridge head moieties N{(CH2)mNHX}3 where m =
1, 2, and 3. In systems where m = 1 and 3 there were no hit for fluoride
though 31 hits were obtained when m = 2, but none of these structures
represented fluoride encapsulation in a tripodal system. Similar search
on corresponding 1,3,5-substituted aryl moieties showed no hit on
fluoride encapsulation. In another search with urea and substituted
urea moieties showed three hits with fluoride but none of them is related
to tripodal receptor system.
23 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311–312.
24 A. L. Spek, PLATON-97, University of Utrecht, Utrecht, The Nether-
lands, 1997.
25 Mercury 1.4 Supplied with Cambridge Structural Database, CCDC,
Cambridge, U.K., 2003–2004.
26 S. J. Brooks, P. A. Gale and M. E. Light, Chem. Commun., 2006, 4344–
4346.
4168 | Dalton Trans., 2009, 4160–4168
This journal is
The Royal Society of Chemistry 2009
©