Anion binding studies of tris(2-aminoethyl)amine based amide receptors with nitro functionalized aryl substitutions: A positional isomeric effect

Article abstract of DOI:10.1016/j.ica.2010.02.030

Syntheses and crystal structures of tren-based amide, L¹, N,N′,N″-tris[(2-amino-ethyl)-4-nitro-benzamide] and L², N,N′,N″-tris[(2-amino-ethyl)-2-nitro-benzamide] are reported and compared with previously published tripodal amide rece

Full text of DOI:10.1016/j.ica.2010.02.030

Inorganica Chimica Acta 363 (2010) 2886–2895
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Inorganica Chimica Acta
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Anion binding studies of tris(2-aminoethyl)amine based amide receptors with
nitro functionalized aryl substitutions: A positional isomeric effect
*
I. Ravikumar, P.S. Lakshminarayanan, Pradyut Ghosh
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A&2B Raja S.C. Mullick Road, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Syntheses and crystal structures of tren-based amide, L1, N,N0,N00-tris[(2-amino-ethyl)-4-nitro-benzam-
ide] and L2, N,N0,N00-tris[(2-amino-ethyl)-2-nitro-benzamide] are reported and compared with previously
published tripodal amide receptor L3, N,N0,N00-tris[(2-amino-ethyl)-3-nitro-benzamide]. The crystallo-
graphic results show intramolecular and intermolecular hydrogen-bonding interactions between two
arms of the tripodal receptor and two other adjacent molecules in cases of L1 and L2 whereas in addition
Article history:
Received 14 January 2010
Accepted 22 February 2010
Available online 25 February 2010
Dedicated to Animesh Chakravorty
to the above interactions an aromatic
p p
stacking among tripodal arms is also observed in L3. Receptors
ꢀꢀꢀ
L1, L2 and L3 having electron withdrawing –NO2 substituted (para, ortho and meta, respectively) phenyl
moieties are explored toward their solution state anion binding properties and selectivity studies. The
substantial changes in chemical shifts are observed for the amide protons (–NH) and aromatic protons
(–CH) with Fꢁ and Clꢁ in cases of L1 and L3, and only with Fꢁ for L2, indicating the participation of –
NH and –CH protons in the solution state binding events. Binding constants for the above cases are cal-
culated by 1H NMR titration upon monitoring the –NH signal. Receptor L2 shows exꢁclusive selectivity
Keywords:
Anion recognition
Neutral receptor
Amide
Hydrogen bonds
X-ray crystallography
Supramolecular chemistry
toward Fꢁ in dimethyl sulfoxide (DMSO). The structural aspects of binding Iꢁ, ClO4 and SiF6 with
2ꢁ
the monoprotonated L1, L1H+ꢀIꢁꢀDMF (1), L1H+ꢀClO4ꢁꢀDMF (2) and L1H+ꢀ0.5SiF62ꢁꢀH2O (3), respectively
are examined crystallographically. Anion binding with multiple receptor units is observed via amide
N–Hꢀꢀꢀanion as well as aryl C–Hꢀꢀꢀanion hydrogen-bonding interactions in all the complexes as observed
in cases of previously reported crystal structures of anionic complexes of protonated L3. The aryl group
having nitro functionality that contributes to solution state anion binding with the neutral receptor
and solid state coordination in complexes 1–3 through CHꢀꢀꢀanion interactions is noteworthy.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction and background
by Reinhoudt and Beer are explained by hydrogen-bond formation
between the amide functionality of receptor and the anion. Struc-
The design and syntheses of receptors capable of binding anio-
nic guests is of crucial importance due to their potential applica-
tions in environmental, biological processes and molecular
recognition studies [1–20]. In 1993 Reinhoudt and co-workers
introduced a series of tris(2-aminoethyl)-amine, tren-based tripo-
dal ligands containing amide or sulfonamide groups as anion
receptors, where a conductivity study showed binding preference
in the order H2PO4ꢁ > Clꢁ > HSO4ꢁ [21]. Later tren has been studied
as an important building block in tripodal receptor systems having
amine, amide and urea functionality for anions by us [28,35–37],
and different groups [21–27,29–34]. Anion receptors in nature of-
ten involve amide linkages as hydrogen-bond donors; hence,
amide based receptors are important for anion binding study [1–
9,21]. Beer et al. have shown halide and ReO4ꢁ binding in the tripo-
dal amide by 1H NMR titration studies [22]. These early reports of
anion binding with tren-based tripodal amide receptors in solution
tural information can provide insight on the proper binding topol-
ogy of anions with these tripodal amide receptors. In this regard,
Bowman-James et al. showed that the nitrate salt of a monoproto-
nated tripodal lipophilic amide receptor, where anion is not encap-
sulated in the cavity of a tren unit. This is simply because one of the
amide carbonyl oxygen atom points into the cavity to hydrogen-
bond with the endo oriented part of the apical amine as observed
from the structural investigation [25]. Theoretical investigation
by Hay et al. showed that the effect of electron withdrawing sub-
stituents on the aryl moiety significantly enhances the stability
of anion complexes [33]. The binding ability of tren-based acyclic
tripodal receptors toward anions varies with the attached moiety
to the tren (N4) unit, since functional groups modify the hydrogen
bonding capability [33], as well as the conformation of the receptor
[35–37]. We have reported the coordination of a monoprotonated
tren-based triamide, L3, having nitro functionality at the meta po-
sition with anionic guests of different shapes and geometry [28].
Given our interest in both solution and structural aspects of
anion binding, herein, we report three tren-based tripodal amide
* Corresponding author. Tel.: +91 33 2473 4971; fax: +91 33 2473 2805.
E-mail address: icpg@iacs.res.in (P. Ghosh).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.02.030

I. Ravikumar et al. / Inorganica Chimica Acta 363 (2010) 2886–2895
2887
receptors L1–L3 (see Chart 1) and their solution state anion binding
studies in neutral form via detailed 1H NMR studies along with
structures of L1, L2, and binding of Iꢁ (spherical), 1, ClO4ꢁ (tetrahe-
at room temperature, which yielded suitable crystals for X-ray
analysis in 4 days. Single crystals of L2 were also obtained from
DMF.
2ꢁ
dral), 2, and SiF6 (octahedral), 3, with [HL1]+ and their detailed
2.2.1. L1
molecular interactions. We also demonstrate the effect of posi-
tional isomers toward the selective binding of Fꢁ with L2 in
solution.
Yield: 95%, 1H NMR (300 MHz, DMSO-d6): d 3.31 (m, 6H,
NCH2CH2), 3.83 (m, 6H, NCH2CH2), 7.95 (d, 6H, ArH, J = 7.0 Hz),
8.16 (d, 6H, ArH, J = 7. 0 Hz), 8.78 (br, 3H, ꢁNH). 13C NMR
(75 MHz, DMSO-d6): d 43.40 (NCH2), 53.55 (NCH2CH2), 127.99,
135.03, 140.03, and 151.68 (Ar), 168.94 (C@O). HRMS (ESI): m/z
594.2132 [L1]+.
2. Experimental section
2.1. Materials
2.2.2. L2
Tris(2-aminoethyl) amine, 2-nitrobenzoyl chloride, 3-nitro-
benzoyl chloride, and 4-nitrobenzoyl chloride and tetrabutyl-
ammonium salts of fluoride, chloride, bromide, iodide,
dihydrogenphosphate, hydrogensulfate, perchlorate and nitrate
were purchased from Aldrich chemicals and used as received.
Hydroiodic acid, and perchloric acid were received from SD Fine
Chemicals, India, and hydrofluoric acid was received from Merck,
India. All the solvents and triethylamine were purchased from SD
Fine Chemicals, India, and were purified prior to use.
Yield: 95%, 1H NMR (300 MHz, DMSO-d6): d 2.72 (t, 6H,
NCH2CH2), 3.39 (t, 6H, NCH2CH2), 7.55 (d, 3H, ArH), 7.57 (m, 6H,
ArH), 7.99 (d, 3H, ArH), 8.60 (t, 3H, ꢁNH). 13C NMR (75 MHz,
DMSO-d6): d 45.40 (NCH2), 56.20 (NCH2CH2), 126.79, 128.41,
130.22, 134.88, 136.41, and 148.72 (Ar), 167.65 (C@O). HRMS
(ESI): m/z 594.3674 [L2]+.
2.2.3. L1H+IꢁꢀDMF, 1
Yield: 75%, 1H NMR (300 MHz, DMSO-d6): d 3.26 (m, 6H,
NCH2CH2), 3.71 (m, 6H, NCH2CH2), 8.10 (d, 6H, ArH, J = 7. 0 Hz),
8.18 (d, 6H, ArH, J = 7. 0 Hz), 8.72 (br, 3H, –NH). 13C NMR
(75 MHz, DMSO-d6): d 43.44 (NCH2), 53.14 (NCH2CH2), 128.02,
134.98, 139.68, and 152.09 (Ar), 169.14 (C@O).
2.2. Syntheses
The tripodal amide receptors L1–L3 were synthesized following
our literature procedure [28]. A representative synthesis of L1 is
presented here. Reaction of tren with 4-nitrobenzoyl chloride in
a 1:3 molar ratio at room temperature yielded L1 in high yield.
0.146 g, 1 mmol of tren, was dissolved in 30 mL of dry tetrahydro-
furan (THF) in a 150 mL two neck round bottomed flask. 0.354 g
(3.5 mmol) of dry triethylamine (Et3N) was added to the reaction
mixture and stirred at room temperature under nitrogen atmo-
sphere for 30 min. A solution of 4-nitrobenzoyl chloride (0.557 g,
3 mmol) in 50 mL of dry THF was added drop-wise to the reaction
mixture over a period of 1 h under nitrogen atmosphere with con-
stant stirring at room temperature. After the addition was com-
pleted, a pale yellow precipitate is formed and the reaction
mixture was allowed to stir at room temperature for overnight.
The light yellow precipitate formed was filtered through a filter pa-
per and washed three times with (3 ꢂ 100 mL) of water, two times
with (2 ꢂ 10) mL of cold THF and dried in air. The yellow solid was
re-dissolved in DMF and allowed to evaporate slowly at room tem-
perature. Single crystals of L1 suitable for X-ray diffraction studies
were obtained after two days in 95% yield. Complexes 1, 2, and 3
were obtained by dissolving L1 (50 mg) in 50 mL of DMF and add-
ing 1.5 equiv of 37% HI, 70% HClO4, and 40% HF, respectively. The
respective solutions were stirred at room temperature for 30 min
and filtered in a 100 mL beaker. Filtrates were allowed to evaporate
2.2.4. L1H+ClO4ꢁꢀDMF, 2
Yield: 60%, 1H NMR (300 MHz, DMSO-d6): d 3.33 (m, 6H,
NCH2CH2), 3.71 (m, 6H, NCH2CH2), 8.00 (d, 6H, ArH, J = 7. 0 Hz),
8.21 (d, 6H, ArH, J = 7. 0 Hz), 8.79 (br, 3H, -NH). 13C NMR
(75 MHz, DMSO-d6): d 42.92 (NCH2), 53.89 (NCH2CH2), 126.98,
134.99, 140.12, and 152.03 (Ar), 168.94 (C@O).
2.2.5. L1H+0.5SiF62ꢁꢀH2O, 3
Yield: 55%, 1H NMR (300 MHz, DMSO-d6): d 3.31 (m, 6H,
NCH2CH2), 3.71 (m, 6H, NCH2CH2), 7.98 (d, 6H, ArH, J = 7. 0 Hz),
8.22 (d, 6H, ArH, J = 7.0 Hz), 8.78 (br, 3H, –NH). 13C NMR
(75 MHz, DMSO-d6): d 42.69 (NCH2), 53.57 (NCH2CH2), 126.87,
134.87, 139.99, and 150.98 (Ar), 168.94 (C@O).
3. Methods
1H and 13C NMR spectra were recorded on a Bruker 300 MHz
and a 75 MHz FT-NMR spectrometer (model: Avance-DPX300),
respectively. HRMS (+ESI) measurements were carried out on
Waters QTof-Micro instruments.
Chart 1. General synthesis of L1–L3.

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3.1. 1H NMR titration experiments
cally. Hydrogen atoms attached to all carbon atoms were geomet-
rically fixed while the hydrogen atoms of amide, tertiary amino
nitrogen of the salts, were located from the difference Fourier
map, and the positional and temperature factors were refined iso-
tropically. In complex 3, the hydrogen atoms attached to lattice
water molecule could be located from difference Fourier map.
Binding constants were obtained by 1H NMR (300 MHz Bruker)
titrations of L1–L3 with tetrabutylammonium salts of respective
halides in DMSO-d6 at 25 °C. The initial concentration of corre-
sponding receptor was 20 mM. Aliquots of anions were added from
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). Tetramethylsilane
(TMS) in DMSO-d6 was used as an internal reference, and each
titration was performed by 15 measurements at room tempera-
ture. All the proton signals were referred to TMS. The association
constants, K, were calculated by fitting the change in the N–H
chemical shift with a 1:1 association model with non-linear least
square analysis. WINEQNMR 2.0 was employed in the calculation
of association constants [38]. The error limit in K was less than
10%.
4. Results and discussion
4.1. Syntheses
Acycilic tripodal receptors L1–L3 have been synthesized in high
yield, and the single-crystal X-ray studies are performed to under-
stand the binding capacity of L1 with different anions in monoprot-
onated state. Syntheses of L1–L3 are straightforward and involve a
simple addition of respective acid chlorides to the solution of tren
in presence of Et3N. Single crystals of L1 and L2 suitable for X-ray
studies are obtained by slow evaporation of DMF solution in high
yield. Complexes 1–3 are obtained by titrating L1 with respective
acids in DMF solvent, and crystallization is obtained by slow evap-
oration. Syntheses of these salts are also straightforward, resulting
in high yields. Isolation of any salt of L2 was unsuccessful in this
experimental conditions.
Following equation was used to determine the K values.
D
d ¼ fð½Aꢃ0 þ ½Lꢃ0 þ 1=KÞ ꢄ ðð½Aꢃ0 þ ½Lꢃ0
2
1=2
þ 1=LÞ ꢁ 4½Lꢃ0½Aꢃ0Þ
gD
d
max=2½Lꢃ0
3.2. X-ray crystallography
The crystallographic data and details of data collection for L1, L2
and salts 1–3 are given in Table 1. In each case, a crystal of suitable
size was selected from the mother liquor and immersed in partone
oil, and then it was mounted on the tip of a glass fiber and cemen-
ted using epoxy resin. Intensity data for all crystals were collected
4.2. Solution studies
The host-guest chemistry between receptors and anions in solu-
tion is immediately detected by a dramatic increase in the solubil-
ity of the receptor in nonpolar solvents like CH2Cl2, CHCl3, and
CH3COOC2H5 upon the addition of tetrabutylammonium fluoride/
chloride whereas L1–L3 have solubility only in polar solvents like
DMSO or DMF. To study the positional isomeric effect, we have
synthesized L1, L2 and L3 where nitro group is placed at the para,
ortho and meta positions, respectively with respect to the amide
group of receptors. Solution binding properties of these receptors
with different halides, oxyanions are investigated by 1H-NMR
experiments in DMSO-d6 at 25 °C. Based on the effect of positional
isomers toward anion selectivity we describe the solution state
binding data in following order i.e L3, followed by L1 and L2. The
using Mo Ka (0.71073 Å) radiation on a Bruker SMART APEX dif-
fractometer equipped with a CCD area detector at 100 K (L1, L2, 1
and 2) and at 273 K for complex 3. The data integration and reduc-
tion were processed with SAINT [39] software. An empirical absorp-
tion correction was applied to the collected reflections with SADABS
[40] using XPREP [39]. The structures were solved by direct methods
using SHELXTL [41] and were refined on F2 by the full-matrix least-
squares technique using the SHELXL-97 [42] program package.
Graphics were generated using PLATON [43] and MERCURY 2.3 [44]. In
all five compounds, non-hydrogen atoms were refined anisotropi-
Table 1
Crystal data and structure refinement for L1, L2, complexes [HL1]ꢀIꢀDMF (1), [HL1]ꢀClO4ꢀDMF (2) and [HL1]ꢀ0.5SiF6ꢀH2O (3).
L1
L2
1
2
3
CCDC number
Empirical formula
Fw
Crystal syst.
Space group
a (Å)
760009
760008
C27H27N7O9
593.56
760005
C36H20IN8O0.50
699.50
monoclinic
P21/n
13.1080(8)
11.8907(7)
21.3416(13)
90.00
93.9650(10)
90.00
3318.4(3)
4
760006
C30H35ClN8O14
767.11
monoclinic
P21/n
13.0898(10)
11.7073(9)
21.8748(17)
90.00
93.6820(10)
90.00
3345.3(4)
4
760007
C
54H54N14O18
C54H58F6N14O19.25Si
1353.23
monoclinic
P21/c
13.0590(10)
10.9712(8)
40.322(3)
90.00
92.696(2)
90.00
5770.6(7)
4
1187.11
triclinic
triclinic
ꢀ
ꢀ
P1
P1
12.8485(8)
13.7087(8)
16.6556(10)
81.6730(10)
78.1930(10)
75.8920(10)
2771.0(3)
2
8.7155(10)
11.7684(13)
14.0949(16)
88.566(2)
85.999(2)
68.624(2)
1342.9(3)
2
b Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g/cm3)
Crystal size (mm3)
F(0 0 0)
1.423
1.468
1.400
1.523
1.558
0.44 ꢂ 0.35 ꢂ 0.29
0.22 ꢂ 0.12 ꢂ 0.06
0.48 ꢂ 0.44 ꢂ 0.36
0.46 ꢂ 0.36 ꢂ 0.32
0.48 ꢂ 0.38 ꢂ 0.32
1240
620
1396
1600
2808
l
Mo K (mmꢁ1
)
0.109
0.113
1.004
0.198
0.151
T (K)
100(2)
100(2)
100(2)
100(2)
273(2)
h Range
1.54–28.29
23872
12545
2.36–28.30
11478
6056
1.77–28.28
17248
7052
1.76–28.29
19155
7712
1.56–25.00
28290
10153
Reflections collected
Independent reflections
R(int)
0.0241
0.0427
0.0467
0.0306
0.0837
Data/restraints/parameters
R1; wR2
12545/0/991
0.0494; 0.1192
1.023
6056/0/480
0.0659; 0.1245
1.000
7052/0/528
0.0467; 0.1375
1.111
7712/0/618
0.0521; 0.1139
1.075
10153/0/888
0.0565; 0.1096
0.941
GOF (F2)

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addition of tetrabutylammonium halide, n-Bu4N+Aꢁ salts (where
Aꢁ = Fꢁ, Clꢁ, Brꢁ and Iꢁ) to the solution of L3 in DMSO-d6, a down-
field shift in the N–H resonance is observed in cases of Fꢁ, Clꢁ and
change in the chemical shift of the amide -NH proton. The down-
field shift of the amide proton observed with d of 1.052 ppm
for this ion where as there is no considerable shift observed for
Clꢁ, Brꢁ, and Iꢁ (Fig. 3) or any oxy-anions. This solution 1H NMR
study clearly indicates, L2 as a selective and exclusive receptor
for Fꢁ in DMSO.
D
Brꢁ (Fig. 1). The downfield shifts of the amide proton (
Dd –NH) ob-
served in cases of Fꢁ and Clꢁ are 0.1405 and 0.2515 ppm, respec-
tively. There is negligible shift in the N–H resonance of L3 in case
of Brꢁ
(D
d = 0.0356 ppm) whereas no shift is observed with Iꢁ, sug-
It is also evident from Figs. 1–3, that concomitant downfield
shifts of aromatic hydrogens are observed upon addition of Fꢁ or
Clꢁ to L1 or L3, indicating the participation of aromatic –CH protons
in the halide binding event. It is also noticed that in case of L1 upon
addition of Fꢁ both the aromatic hydrogen Ha and Hb are shifted
(Fig. 2), whereas on addition of Clꢁ to the receptor shows down-
field shift of only the Hb protons (meta to nitro group) which is
in close proximity to the amide –NH and hence could bind to the
Clꢁ more effectively. There is no significant change in chemical
shift of aromatic protons upon addition of other anions (Brꢁ, Iꢁ,
gesting non interacting nature of this ion and very weak interac-
tion with Brꢁ toward L3. In cases of oxy-anions such as NO3
,
ꢁ
ClO4ꢁ, HSO4 no change in chemical shift of the N–H resonance
of L3 is observed. This solution study indicates that the binding
of Iꢁ as well as any oxy-anion with the L3 are energetically unfavor-
able though theoreticaꢁl investigation suggested that L3 could be a
ꢁ
good receptor for NO3 [33]. Instead of NO3 binding, L3 showed
ꢁ
binding toward spherical anions like Fꢁ, Clꢁ, and Brꢁ which is
clearly evident from the amide N–H peak shift of the neutral recep-
tor (Fig. 1).
ꢁ
H2PO4ꢁ, HSO4ꢁ, ClO4 and NO3ꢁ). An appreciable change in chem-
In case of L1 addition of n-Bu4N+Fꢁ led to the enormous down-
ical shift of –CH resonances is noticed for L2 only in case of Fꢁ (as
observed in the case of chemical shift of –NH proton) whereas no
considerable shift is noticed upon addition of any other anions, fur-
ther showing the selectivity of L2 toward Fꢁ. The disturbance in the
aromatic –CH protons is also observed in cases of Fꢁ/Clꢁ/Brꢁ bind-
ing to L3, indicating the participation of –CH protons in the anion
binding event. Thus, binding of halides to the respective receptor
in solution state is clearly evident from 1H NMR analyses where
both N–HꢀꢀꢀXꢁ and C–HꢀꢀꢀXꢁ interactions are present.
field shift of –NH resonance with high
Dd value of 1.894 ppm in
DMSO-d6 (Fig. 2). Addition of n-Bu4N+Clꢁ to the solution of L1 also
shows change in the chemical shift of amide –NH resonance with
D
d value of 0.2724 ppm whereas no considerable change in chem-
ical shift observed for Brꢁ/Iꢁ (Fig. 2) or other oxy-anions salts in
similar conditions, indicating their non-interactive nature toward
this particular receptor having para substituted –NO2 functionality.
It is important to note that the shift in the N–H peak with Fꢁ here is
considerably higher than that observed in the case of L3 whereas
this difference is not prominent in case of Clꢁ. This result indeed
shows a major influence of positional isomers toward the binding
of halides in tripodal amide receptors which is strengthen by our
further study with the ortho isomer i.e. L2. Fig. 3 shows the chem-
ical shift change observed by the addition of halides to L2 in DMSO-
d6 at RT. Interestingly in this case only Fꢁ shows a substantial
To evaluate the halide binding constants in solution, 1H NMR
titration experiments are performed with Fꢁ/Clꢁ as their tetrabu-
tylammonium salts in DMSO-d6 with receptors where ever notice-
able change in chemical shift is observed. Fig. 4a and b show the
change in chemical shift of –NH resonances of L3 upon addition
of aliquots of Fꢁ and Clꢁ, respectively. The titration curve gives
the best fit for 1:1 binding model for host to guest, in agreement
Fig. 1. Partial 1H NMR spectra (300 MHz) of the receptor L3 in DMSO-d6 with n-Bu4N+Aꢁ (where Aꢁ = Fꢁ, Clꢁ, Brꢁ and Iꢁ) at 298 K.

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I. Ravikumar et al. / Inorganica Chimica Acta 363 (2010) 2886–2895
Fig. 2. Partial 1H NMR (300 MHz) spectra of L1 in DMSO-d6 with n-Bu4N+Aꢁ (where Aꢁ = Fꢁ, Clꢁ, Brꢁ and Iꢁ) at 298 K.
Fig. 3. Partial 1H NMR (300 MHz) spectra of L2 in DMSO-d6 with n-Bu4N+Aꢁ (where Aꢁ = Fꢁ, Clꢁ, Brꢁ and Iꢁ) at 298 K.
with Job’s plots indicating a maximum
D
d at 0.5 = [L1]/([L1] + [Aꢁ])
constant data (Table 2) show that L1 binds very strongly towards
Fꢁ than chloride having log K > 4.0. However Clꢁ also displays sig-
nificant binding but other halides has no binding with L1. There-
fore, with the increasing size i.e decreasing basicity of halides the
association constant regularly diminishes which is clearly evident
in this case.
and the binding constants are calculated using WINEQNMR 2 [38].
Binding constant data are summarized in Table 2. It is clear from
Table 2 that binding constants of L3 toward Fꢁ and Clꢁ are
comparable.
Fig. 5a and b show the solution binding of L1 with Fꢁ and Clꢁ,
respectively. The 1H NMR titration curve in these cases also gives
the best fit for 1:1 binding model for host to guest. The binding
This receptor L2 is showing selective solution state binding with
Fꢁ with a large shift of amide –NH peak. Fig. 6 shows the titration

I. Ravikumar et al. / Inorganica Chimica Acta 363 (2010) 2886–2895
2891
Fig. 4. Change in chemical shift of –NH resonance of L3 (20 mM) with increasing amounts of (a) n-Bu4N+Fꢁ and (b) n-Bu4N+Clꢁ in DMSO-d6 at 298 K.
of L2 with the aliquots of n-Bu4N+Fꢁ in DMSO-d6 at room temper-
ature. Titration data again gives the best fit for a 1:1 association of
host to guest as observed in the earlier cases. Binding constant cal-
culated for L2 with Fꢁ is maximum in this series with the value of
log K 5.63 Mꢁ1 (Table 2).
ing scheme is depicted in Fig. 5S. The same atom numbering for L1
is retained in all the structures 1–3 presented in this investigation.
The single crystal X-ray structure of L1 shows strong intramolecu-
lar and intermolecular hydrogen-bonding interactions between
two arms of the tripodal receptor and two other adjacent mole-
cules, respectively.
Amide oxygen O1 of one arm acts as an acceptor and is involved
in strong intramolecular N–HꢀꢀꢀO interaction with the donor amide
hydrogen H6C (N6–H5CꢀꢀꢀO1; N6ꢀꢀꢀO1 = 2.946(3) Å, H5CꢀꢀꢀO1 =
2.18(3) Å, and <N6–H5CꢀꢀꢀO1 = 150(2)°) (Fig. 7). Further, the recep-
tor is also involved in four strong intermolecular N–HꢀꢀꢀO and six
C–HꢀꢀꢀO hydrogen-bonding interactions with two adjacent L1 mol-
ecules. Details of these hydrogen-bonding interactions are pre-
sented in Table 3.
5. Solid state studies
5.1. Crystallographic studies
5.1.1. L1
1
ꢀ
The receptor L crystallizes in triclinic space group P1 (Table 1),
and the ORTEP diagram of the receptor moiety with atom number-
Solid state crystal structure of the receptor L1 shows a C3v sym-
metric cavity which could be suitable for guest encapsulation
(NꢀꢀꢀN distances are 4.284, 4. 290 and 4.484 Å). Of course, the pres-
ence of strong intramolecular hydrogen bonding might resists the
opening of the tripodal amide receptor cavity. The torsion angles
involving N1apicalCCNamide are in folded conformation with angles
70.72, 54.97, and 48.81° for three arms composed of the amide
nitrogen atoms N2, N4, and N6, respectively, whereas torsion an-
gles involving the carbon atoms connecting the terminal phenyl
Table 2
Association constants for L1–L3 with different halides in DMSO-d6 at 298 K.
Receptor
L1
Anions
log K (Mꢁ1
)
Fꢁ
4.06
2.29
5.63
–
3.76
3.32
Clꢁ
Fꢁ
L2
L3
Clꢁ
Fꢁ
Clꢁ
Fig. 5. Change in chemical shift of –NH resonance of L1 (20 mM) with increasing amounts of (a) n-Bu4N+Fꢁ and (b) n-Bu4N+Clꢁ in DMSO-d6 at 298 K.

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I. Ravikumar et al. / Inorganica Chimica Acta 363 (2010) 2886–2895
These intramolecular and intermolecular hydrogen bonds resist
the formation of C3v symmetric like cavity, which is also evident
from the NꢀꢀꢀN distances of the three arms of the receptor. The tor-
sion angles involving N1apicalCCNamide are in folded conformation
with angles 73.31, 76.93, and 49.41° for three arms composed of
the amide nitrogen N2, N4, and N6, respectively, whereas torsion
angles involving the carbon atoms connecting the terminal phenyl
rings in each arm are almost in extended conformation with angles
ꢁ179.15, 179.81, and 176.06°, respectively.
5.2.2. L1H+IꢁꢀDMF, 1
The complex 1 crystallizes in monoclinic space group P21/n (Ta-
ble 1), and the tertiary nitrogen of the tripodal amide receptor is
protonated and turns out to be the mono iodide salt of the L1 with
one molecule of DMF as lattice solvent. The endo oriented proton
H1C of the apical amine is in N–HꢀꢀꢀO intramolecular hydrogen
bonding with one of the amide oxygen (O7) of the receptor without
encapsulation of iodide anion within the C3v cavity of the receptor.
It is evident from Fig. 9, that the one iodide anion is surrounded by
three protonated L1 moieties having four hydrogen-bonding con-
tacts (Fig. 9 and Table 5). These are one N–HꢀꢀꢀIꢁ interactions of
the amide nitrogen atom, N6 and three contacts via C–HꢀꢀꢀIꢁ inter-
actions from the meta, ortho hydrogens with respect to the nitro
group of the phenyl ring and methlyenic protons of the tren archi-
tecture. The tetracoordinated Iꢁ is in distorted square planar geom-
etry. The NꢀꢀꢀIꢁ distances is 3.56 Å with N–HꢀꢀꢀIꢁ 169.7°, which is in
good agreement with the reported values [45,46]. H8 (ortho) and
H23 (meta) with respect to nitro of aryl moiety interact with the
Iꢁ with CꢀꢀꢀIꢁ distances of 4.034 and 4.018 Å and C–HꢀꢀꢀIꢁ angles
of 156.1 and 160.8°, respectively, whereas H19A of methylene car-
bon (C19) interacts with I1 with CꢀꢀꢀIꢁ distances of 4.051 Å and C–
HꢀꢀꢀI angle of 156.5°. Thus, the four-point contact via N–HꢀꢀꢀIꢁ and
C–HꢀꢀꢀIꢁ interactions is responsible for the binding of the iodide
ion with the protonated L1 receptor outside the tren cavity.
Fig. 6. Change in chemical shift of –NH resonance of L2 (20 mM) with increasing
amounts of n-Bu4N+Fꢁ in DMSO-d6 at 298 K.
rings in each arm are almost in extended conformation with angles
170.10, ꢁ175.29, and 179.46°, respectively.
5.2. Crystallographic studies
5.2.1. L2
The neutral triamide receptor L2 crystallizes in triclinic space
ꢀ
group P1 (Table 1) with two asymmetric units, and the ORTEP dia-
gram of the receptor moiety with atom numbering scheme is de-
picted in Fig. 6S. The crystal structure of L2 also shows strong
intramolecular and intermolecular hydrogen-bonding interactions
between two arms of the tripodal receptor and other two adjacent
molecules, respectively (Fig. 8). Amide oxygen O1 and O16 of
two different molecules in the asymmetric unit is involved in
strong N–HꢀꢀꢀO interactions with the donor amide hydrogen atoms
H6C (N6–H6CꢀꢀꢀO1; N6ꢀꢀꢀO1 = 2.8496(18) Å and <N6–H6CꢀꢀꢀO1 =
150(2)°) and H9C (N9–H9CꢀꢀꢀO16; N9ꢀꢀꢀO16 = 2.8665(17) Å and
<N6–H6CꢀꢀꢀO1 = 168(2)°), respectively. Further, the receptor is also
involved in four strong intermolecular N–HꢀꢀꢀO hydrogen-bonding
interactions with two adjacent L2 molecules. Details of these
hydrogen-bonding interactions are shown in Table 4.
Table 3
Intermolecular hydrogen-bonding interactions of L1.
D–HꢀꢀꢀA
d(HꢀꢀꢀA) Å
d(DꢀꢀꢀA) Å
<DHA (°)
N2–H2CꢀꢀꢀO4
N4–H4CꢀꢀꢀO7
C9–H9ꢀꢀꢀO4
2.01(3)
2.05(3)
2.41(3)
2.38(3)
2.65(2)
2.911(3)
2.902(3)
3.21(3)
3.258(3)
3.595(3)
171(3)
161(3)
141(2)
156(2)
172(2)
C14–H14ꢀꢀꢀO7
C18–H18ꢀꢀꢀO4
Fig. 7. L1 showing (a) intramolecular and (b) intermolecular interactions via N–HꢀꢀꢀO and N–HꢀꢀꢀO hydrogen bonds. Non-acidic hydrogens are omitted for clarity.

I. Ravikumar et al. / Inorganica Chimica Acta 363 (2010) 2886–2895
2893
Fig. 8. L2 showing (a) intramolecular and (b) intermolecular interactions via N–HꢀꢀꢀO hydrogen bonds. Non-acidic hydrogens are omitted for clarity.
Table 4
Intermolecular hydrogen-bonding interactions of L2.
Table 5
Hydrogen-bonding interactions between iodide and surrounding L1H+ in complex 1.
D–HꢀꢀꢀA
d(HꢀꢀꢀA) Å
d(DꢀꢀꢀA) Å
<DHA (°)
D–HꢀꢀꢀA
d(HꢀꢀꢀA) Å
d(DꢀꢀꢀA) Å
<DHA (°)
N2–H2CꢀꢀꢀO13
N4–H4CꢀꢀꢀO10
N11–H11CꢀꢀꢀO7
N13–H13CꢀꢀꢀO4
1.95(2)
1.89(2)
1.92(2)
2.04(2)
2.8252(18)
2.720(2)
2.7122(18)
2.8767(19)
176(2)
158(2)
154.8(18)
172.4(19)
N6–H6CꢀꢀꢀI1
C8–H8ꢀꢀꢀI1
2.96(6)
3.14(8)
3.15(9)
3.04(5)
3.562(4)
4.034(4)
4.051(4)
4.018(4)
170(4)
156(2)
157(3)
161(3)
C19–H19AꢀꢀꢀI1
C23–H23ꢀꢀꢀI1
5.2.3. L2H+ClO4ꢁꢀDMF, 2
Complex 2 crystallizes in a monoclinic P21/n space group with
one molecule of DMF as lattice solvent. In complex 2, similar intra-
molecular N–HꢀꢀꢀO hydrogen bonding exists as observed in 1. In
complex 1, a similar bilayer arrangement of the receptor moiety
is retained via various C–HꢀꢀꢀO and N–HꢀꢀꢀO interactions as depicted
in the packing diagram of 2 (Fig. 11S). Each perchlorate ion is encir-
cled by five monoprotonated L1 and one lattice DMF molecule hav-
ing 12 contacts (Fig. 10a and Table 6). A close-up view of
perchlorate binding with the receptor is shown in Fig. 10b for clar-
ity. O10 of the perchlorate ion is making two C–HꢀꢀꢀO (ortho hydro-
gen H17 and methylene proton adjacent to protonated bridgehead
nitrogen, H1B) interaction. O11 of perchlorate anion is making four
contacts with two receptor units via one N–HꢀꢀꢀO and three C–HꢀꢀꢀO
interactions with amide hydrogen (H6C) and methylenic hydro-
gens adjacent to the protonated bridgehead nitrogen H10A, H19A
and H20B, respectively. O12 is involved in three weak C–HꢀꢀꢀO con-
tacts with two receptor units (methylenic hydrogen, H11A and H6
of aryl unit, ortho to nitro group) and one lattice DMF molecule
(H29B). O13 is in three points with two receptor units via two C–
HꢀꢀꢀO and one N–HꢀꢀꢀO. Aryl protons H23 (meta to nitro group)
and H24 (ortho to nitro group) are involved in strong C–HꢀꢀꢀO inter-
actions with O13, whereas O13 is also hydrogen bonded to amide
hydrogen (H6C) via strong N–HꢀꢀꢀO interaction. In overall the per-
chlorate anion is encircled via ten C–HꢀꢀꢀO and two N–HꢀꢀꢀO
contacts.
The packing diagram of complex 1 (See supporting information,
Fig. 10S) viewed down the b-axis shows that the cationic array of
the receptor is arranged diagonal to the ac-plane with chloride be-
tween the adjacent bilayers. The receptor moieties are organized
via intermolecular C–HꢀꢀꢀO interactions between the alkyl hydro-
gen from all three arms of the L1 with oxygen atoms from each ni-
tro group. The lattice DMF molecules interact with receptor
molecules via strong N–HꢀꢀꢀO and C–HꢀꢀꢀO interactions.
5.2.4. L2H+0.5ꢀSIF62ꢁꢀH2O, 3
Hexafluorosilicate salt 3 was obtained on reaction of the tripo-
dal receptor L1 with HF, presumably as a result of glass corrosion.
The asymmetric unit of the salt contains two protonated tripodal
2ꢁ
cations with one SiF6 and two water molecules (O19 and O20)
as solvent of crystallization. In an attempt to understand the bind-
ing of polyatomic anion (SiF62ꢁ) by the protonated receptor L1H+
and lattice water molecule, we have analyzed the interaction of
Fig. 9. MERCURY diagram depicting the interactions of the iodide (pink with dotted
black hydrogen bonds) with three surrounding L1H+ via four 4 hydrogen bonds (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.).
2ꢁ
SiF6 with the surrounding receptor units (Fig. 11). The fluoride

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I. Ravikumar et al. / Inorganica Chimica Acta 363 (2010) 2886–2895
Fig. 10. MERCURY diagram depicting (a) the interactions of the perchlorate anion with five surrounding L1H+ units and one lattice DMF molecule via C–HꢀꢀꢀO and N–HꢀꢀꢀO
hydrogen bonds. (b) Close-up view of perchlorate anion binding. Non-acidic hydrogens are omitted for clarity.
in 18 hydrogen-bonding contacts on SiF62ꢁ. F1 and F2 is involved in
Table 6
Hydrogen-bonding interactions between perchlorate anion and surrounding L1H+ in
complex 1.
the two N–HꢀꢀꢀF and one C–HꢀꢀꢀF interactions each with receptor
units. F3 is involved in three weak C–HꢀꢀꢀF contact with the meth-
ylene hydrogens, whereas F4 is in bonding via three C–HꢀꢀꢀF (alkyl
hydrogens H28B, H37A and H37B) and one O–HꢀꢀꢀF (lattice water
hydrogen H19C) (Table 7). F5 is in three point contact with sur-
rounding receptor units via two C–HꢀꢀꢀF (methylene hydrogens
H46A and H47B) and one N–HꢀꢀꢀF (amide hydrogen H6C) interac-
D–HꢀꢀꢀA
d(HꢀꢀꢀA) Å
d(DꢀꢀꢀA) Å
<DHA (°)
C1–H1BꢀꢀꢀO10
C17–H17ꢀꢀꢀO10
C20–H20BꢀꢀꢀO11
C10–H10AꢀꢀꢀO11
C19–H19AꢀꢀꢀO11
N6–H6CꢀꢀꢀO11
C6–H6ꢀꢀꢀO12
2.60(2)
2.41(2)
2.57(2)
2.64(2)
2.66(2)
2.60(2)
2.59(2)
2.52(2)
2.59(2)
2.41(2)
2.50(2)
2.59(3)
3.317(3)
3.320(3)
3.313(3)
3.567(3)
3.501(3)
3.199(3)
3.109(3)
3.232(3)
3.231(3)
3.320(3)
3.306(3)
3.385(3)
134(3)
160.4(18)
133.9(19)
160.8(17)
147.2(17)
131.2(17)
114.3(17)
128.4(17)
126.2(19)
170.2(19)
142.8(19)
165(2)
2ꢁ
tions. The F6 of SIF6 is weakly hydrogen bonded to one aryl
C–H and one O–H of the receptor and lattice water molecules,
respectively. The observed C–HꢀꢀꢀF and N–HꢀꢀꢀF interaction distance
and angles are within the range reported in the literature [45].
C11–H11AꢀꢀꢀO12
C29–H29BꢀꢀꢀO12
C23–H23ꢀꢀꢀO13
C24–H24ꢀꢀꢀO13
N6–H6CꢀꢀꢀO13
6. Conclusion
The solution-state 1H NMR study of anion binding with tripodal
triamide receptors, L1–L3 shows that positional isomers have an
important role toward the selectivity. All three neutral receptors
bind selectively with halides whereas no solution-state binding is
atoms (F1, F2, F3 and F5) of SiF62ꢁ are each making three contacts,
whereas F4 and F6 is making four and two contacts, respectively
with the surrounding receptor moieties and lattice water resulting
Fig. 11. MERCURY diagram depicting (a) the interactions of the hexafluorosilicate anion with four surrounding L1H+ units and one lattice water molecule via C–HꢀꢀꢀF, N–HꢀꢀꢀF
2ꢁ
and O–HꢀꢀꢀF hydrogen bonds. (b) Close-up view of SiF6 anion binding. Non-acidic hydrogens are omitted for clarity.

I. Ravikumar et al. / Inorganica Chimica Acta 363 (2010) 2886–2895
2895
Table 7
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.ica.2010.02.030.
Hydrogen-bonding interactions between hexafluorosilicate anion and surrounding
L1H+ in complex 3.
D–HꢀꢀꢀA
d(HꢀꢀꢀA) Å
d(DꢀꢀꢀA) Å
<DHA (°)
N4–H4CꢀꢀꢀF1
1.926
2.460
2.401
2.071
1.922
2.447
2.479
2.353
2.395
2.245
2.584
2.638
1.707
2.225
2.612
2.420
2.343
2.589
2.796(4)
2.833(4)
3.226(4)
2.796(5)
2.733(5)
3.221(4)
3.389(5)
3.172(4)
3.332(4)
3.129(5)
3.325(4)
3.420(5)
2.735(4)
2.897(4)
3.520(5)
3.022(5)
3.211(4)
3.060(4)
167.1
111.4
147.8
145.4
156.9
140.6
156.4
141.8
162.2
151.2
133.2
137.8
158.0
146.1
155.6
119.8
155.1
165.6
N6–H6CꢀꢀꢀF1
References
C18–H18ꢀꢀꢀF1
N9–H9CꢀꢀꢀF2
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N11–H11CꢀꢀꢀF2
C41–H41ꢀꢀꢀF2
C19–H19AꢀꢀꢀF3
C29–H29BꢀꢀꢀF3
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O19–H19CꢀꢀꢀF6
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Acknowledgements
P.G. Gratefully acknowledges the Council for Scientific and
Industrial Research (CSIR), New Delhi, India (Grant No. 01(2225)/
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Appendix A. Supplementary material
1H NMR, and HRMS of L1 and L2. ORTEP diagrams of L1, L2 and
complexes 1–3; packing diagrams of complexes 1 and 2. CCDC
760005, 760006, 760007, 760008 and 760009 contain the supple-
mentary crystallographic data for this paper. These data can be