Anion specificity induced conformational changes in cresol-based tripodal podands controlled by weak interactions: Structural and Hirshfeld surface analysis

Article abstract of DOI:10.1039/c0ce00518e

N-bridged tripodal receptors have shown a distinct behaviour on their assembly and binding ability towards complexation with inorganic and organic anions. All supramolecular complexes have been structurally authenticated using X-ray diffraction with a det

Full text of DOI:10.1039/c0ce00518e

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Anion specificity induced conformational changes in cresol-based tripodal
podands controlled by weak interactions: structural and Hirshfeld surface
analysis†
*
Sandeep Kumar Dey, Avijit Pramanik and Gopal Das
Received 10th August 2010, Accepted 4th November 2010
DOI: 10.1039/c0ce00518e
N-bridged tripodal receptors have shown a distinct behaviour on their assembly and binding ability
towards complexation with inorganic and organic anions. All supramolecular complexes have been
structurally authenticated using X-ray diffraction with a detailed analysis of the Hirshfeld surfaces
facilitating an understanding of the type and nature of intermolecular interactions present in the
complexes and extended structures. The p-substituted podand (L1) crystallized in the symmetric
rhombohedral R3c space group to form hemicarcerand in the solid state via intermolecular C–H/p
interactions. Protonation of L1–3 in presence of inorganic anions results in conformational locking of
tripodal cavity by N–H/Oether trifurcated hydrogen bond formation (1–4) due to the endo-orientation
of the bridgehead hydrogen whereas use of organic anions as template leads to the exo-orientation of
the apical proton (5–7) and thereby, results into conformational opening of the tripodal arms via the
formation of N–H/Oanion and p/p interactions with the anion. This study also establishes bilayer
assembly formation in inorganic anion complexes (2–4) guided primarily by interligand C–H/p
interactions and multiple C–H/anion hydrogen bonds. NMR studies further establish the different
orientations of flexible tripodal arms in presence of organic and inorganic counter anions in solution.
that are anion specific can lead us to the challenging development
Introduction
of new selective systems with industrial, ecological, and
biomedical applications.4 Systems that show great oxoanions
selectivity are of special interest in the area of nuclear and toxic
waste management.5
The use of anionic templates for the synthesis of superstructures
has become an attractive strategy in recent years.1 Anions have
shown to be very versatile templates for the synthesis of a wide
range of organic and metal–organic molecules and supramole-
cular assemblies.1,2 Template-induced association of molecular
species represents one of the main approaches in the control of
supramolecular assembly formation.2 The challenges encoun-
tered in the use of anion templating can be attributed to the
properties of anions, such as low charge to radius ratio, various
geometries, high solvation energies, and pH-dependent charge.3
Hydrogen bonds formed by anions are weaker and more difficult
to control as compared to metal cation coordinative bonds.
Although numerous synthetic molecular capsules have been
achieved, the challenges still exist to control the assembly and
disassembly processes when a guest acts as a template. When
anions are an integral part of supramolecular aggregates, it is
expected that if the templating anion is changed with other
anions it might collapse or re-orient the assembly. Therefore, by
varying the geometry of the anions involved in a self-assembly
process, it should in principle be possible to re-orient or rupture
the self-assembled architectures. Template-directed processes
Most hydrogen bonding anion receptors utilize N–H/anion
or O–H/anion hydrogen bond and C–H/anion hydrogen
bonds are rarely utilized for anion binding even though C–H/
anion hydrogen bonds play an important role in nature and thus,
are drawing increasing attention among researchers.6 Although
not typically considered to be significant donors, there is
increasing evidence that C–H groups can participate in hydrogen
bonding and lead to enhanced anion-binding affinity. This
evidence comes in the form of direct observation of close contacts
in crystallographic structures,7 anion-induced chemical shifts of
C–H protons in NMR spectra8 and theoretical calculations.9 The
binding ability of tripodal receptors for anions varies with the
attached functionality to the tripodal unit, since functional
groups modify the hydrogen-bonding capability. Whereas the
coordination chemistry of anionic guests within pre-organized
macrocyclic systems are relatively straightforward to understand
but the binding processes of flexible podand receptors remain
more elusive.
In this study we have chosen cresol based tripodal podands L1–3
(Scheme 1a) to investigate the various possible structural
topologies of supramolecular complexes formed between
protonated podands and different inorganic and organic
anions. The tertiary bridgehead nitrogen of the podands is
more susceptible to undergo protonation and the hydrogen of
the protonated nitrogen could either become exo- or endo-
oriented with respect to tripodal side arms, depending upon
the nature of the anion. Thus, counter anion(s) could play an
Department of Chemistry, Indian Institute of Technology Guwahati,
Assam, 781 03, India. E-mail: gdas@iitg.ernet.in; Fax: +91-361-258-
2349; Tel: +91-361-258-2313
† Electronic supplementary information (ESI) available: X-ray
crystallographic file of the structures in CIF format, ORTEPs,
competitive crystallization experiments, crystal packing networks, 1H,
13C NMR and FT-IR Spectra. CCDC reference numbers
766465–766472. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0ce00518e
1664 | CrystEngComm, 2011, 13, 1664–1675
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range 4000–450 cmꢀ1. Elemental analyses were carried out on
a Perkin-Elmer 2400 automatic carbon, hydrogen and nitrogen
analyzer. Hirshfeld surfaces and fingerprint plots were produced
with CrystalExplorer13 with bond lengths to hydrogen atoms set
to standard values.14
Synthesis and characterization
The acyclic tripodal receptor L1–3 have been synthesized by
modification of our recent literature procedure.15 To a solution
of cresol (5 g, 46 mmol) in ethanol (30 mL) was added crushed
NaOH (2.27 g, 55 mmol), and the solution was stirred at room
temperature for 1 h. To the resulting suspension, tris(2-chloro-
ethyl)amine hydrochloride (3.71 g, 15 mmol) was added at once,
and the mixture was stirred for another 1 h at room temperature.
For completion of the reaction, the mixture was refluxed for 8 h
followed by removal of the solvents under reduced pressure and
addition of 20 mL of cold water. The expected product was
extracted from this mixture, with 3 ꢁ 20 mL of CHCl3. The
organic layer was washed several times with water and dried over
anhydrous Na2SO4 and solvents were removed under reduced
pressure. The crude product was purified by column chroma-
tography using ethyl acetate and hexane (1 : 9) as eluent. The
purity of the compounds was monitored by TLC.
Scheme 1 (a) Molecular structure of tris-[(methylphenyloxy)-ethyl]-
amine (L1–3); (b) schematic representation depicting the conformational
locking of tripodal cavity in acyclic podands (middle) via trifurcated N–
H/Oether hydrogen bond formation in the presence of templating inor-
ganic anion (left) and conformational opening of the tripodal arms due to
the formation of N–H/ Oanion and p/p interactions with the planar
organic anion (right).
important role in assembling and controlling the structural
topologies of the podands in the solid state. Herein we report
a solid-state study of some supramolecular complexes assembled
by an anion-templated reaction of flexible ligands L1–3 with
inorganic anions of spherical (Brꢀ) and tetrahedral geometry
(ClO4ꢀ) as well as planar organic anions (picrate and pyro-
mellitate). In the proof-of-concept experiments described here,
we demonstrate the formation of inorganic anion (bromide and
perchlorate) assisted conformational locking of flexible tripodal
molecules by the formation of intramolecular N–H/Oether tri-
furcated hydrogen bonds and conformational opening in pres-
ence of organic anions by intermolecular N–H/Oanion hydrogen
bond formation resulting into flat and extended orientation of
the flexible tripodal arms (Scheme 1b). The tripodal assembly is
highly anion specific, and the occurrence of the perchlorate
complexes of L1–3 in systems with equivalent concentrations of
competing anions has also been explored.
All supramolecular complexes 1–7 were isolated as white or
yellow solids upon addition of a methanolic solution of 1.1 equiv
of the corresponding acids to the respective ligand solution, L1–3
in methanol (50 mg in 20 mL MeOH). After a constant stirring
for about an hour a solid precipitate was formed which was then
filtered, washed with ether and dried under vacuum. Colourless/
yellow crystals suitable for single-crystal X-ray analysis were
grown by slow evaporation of CH3OH–CH3CN binary solution
mixture of the solids at room temperature within 1–2 weeks. All
complexes have been characterized by 1H-NMR and FT-IR
spectroscopy (ESI).†
ꢂ
1
L1. White crystalline solid; yield: 78%; mp: 65 C; H-NMR
(400 MHz, CDCl3): d ¼ 2.29 (s, 9 H, CH3), 3.12–3.15 (t, 6 H,
NCH2), 4.06–4.09 (t, 6 H, OCH2), 6.79–6.81 (d, 6 H, ArH), 7.06–
7.08 (d, 6 H, ArH); 13C-NMR (100 MHz, CDCl3): d ¼ 20.64
(ꢁ3 C, CH3), 54.59 (ꢁ3 C, NCH2), 66.98 (ꢁ3 C, OCH2), 114.58
(ꢁ3 C, Ar), 130.04 (ꢁ6 C, Ar), 156.81 (ꢁ6 C, Ar). Anal. calcd (%)
for C27H33NO3: C 77.29, H 7.93, N 3.34%. Found: C 78.21, H
6.24, N 2.85%.
We also report a detailed analysis of the various interactions
using Hirshfeld surfaces.10 Hirshfeld surfaces are very informa-
tive in mapping out the interactions between molecules within
the molecular crystal in a visual manner allowing for a rapid
understanding of the nature of such interactions, even for
complex supramolecular systems.11 The related two-dimensional
fingerprint plots give a quantitative analysis of the nature and
type of interaction, which is of particular importance in struc-
tures that contain the same or structurally similar molecules.12
1
L2. Brown viscous liquid; yield: 66%; H-NMR (400 MHz,
CDCl3): d ¼ 2.33 (s, 9 H, CH3), 3.17–3.20 (t, 6 H, NCH2), 4.11–
4.14 (t, 6 H, OCH2), 6.71–6.79 (m, 9 H, ArH), 7.15–7.18 (t, 3 H,
ArH); 13C-NMR (100 MHz, CDCl3): d ¼ 21.45 (ꢁ3 C, CH3),
54.29 (ꢁ3 C, NCH2), 66.26 (ꢁ3 C, OCH2), 111.36 (ꢁ3 C, Ar),
115.42 (ꢁ3 C, Ar), 121.63 (ꢁ3 C, Ar), 129.17 (ꢁ3 C, Ar), 139.35
(ꢁ3 C, Ar), 158.65 (ꢁ3 C, Ar). Anal. calcd (%) for C27H33NO3: C
77.29, H 7.93, N 3.34%. Found: C 78.93, H 5.79, N 2.67%.
Experimental
1
L3. Brown viscous liquid; yield: 73%; H-NMR (400 MHz,
Materials and methods
CDCl3): d ¼ 2.31 (s, 9 H, CH3), 3.28–3.31 (t, 6 H, NCH2), 4.17–
4.20 (t, 6 H, OCH2), 6.87–6.96 (m, 9 H, ArH), 7.20–7.22 (d, 3 H,
ArH); 13C-NMR (100 MHz, CDCl3): d ¼ 16.67 (ꢁ3 C, CH3),
54.96 (ꢁ3 C, NCH2), 63.06 (ꢁ3 C, OCH2), 110.91 (ꢁ3 C, Ar),
122.04 (ꢁ3 C, Ar), 126.93 (ꢁ3 C, Ar), 127.46 (ꢁ3 C, Ar), 131.33
(ꢁ3 C, Ar), 155.59 (ꢁ3 C, Ar). Anal. calcd (%) for C27H33NO3: C
77.29, H 7.93, N 3.34%. Found: C 78.55, H 5.98, N 2.59%.
All reagents and solvents were obtained from commercial sour-
ces and used as received. NMR spectra were recorded on a Var-
ian FT-400 MHz instrument. The chemical shifts were recorded
in parts per million (ppm) on the scale using tetramethylsilane
(TMS) as a reference. The IR spectra were recorded on a Perkin
Elmer-Spectrum One FT-IR spectrometer with KBr disks in the
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X-Ray crystallography‡
provide insight into the anion templated supramolecular
assembly formation and proper binding topology of anions with
the cationic tripodal podand receptors. The neutral p-cresol
based tripodal podand L1 crystallizes in the highly symmetric
rhombohedral space group R3c with C3v axis of symmetry
passing through the bridgehead N-atom (Fig. 1a). The torsion
involving Namino–C–C–Oether with an ideal value of 89.70(3)ꢂ
(ꢃ 90ꢂ) implies that the tripodal arms are oriented neither in
a folded nor in an open conformation with an average distance of
The intensity data were collected using a Bruker SMART APEX-
II CCD diffractometer, equipped with a fine focus 1.75 kW
ꢀ
sealed tube Mo-Ka radiation (l ¼ 0.71073 A) at 298(2) K, with
increasing u (width of 0.3ꢂ per frame) at a scan speed of 6 s per
frame. The SMART software was used for data acquisition.
Data integration and reduction were undertaken with SAINT
and XPREP16 software. Multi-scan empirical absorption
corrections were applied to the data using the program
SADABS.17 Structures were solved by direct methods using
SHELXS-97 and were refined by full-matrix least squares on F2
using SHELXL-97 in Sheldrick’s program package.18 In all the
six compounds, non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms attached to all carbon atoms were
geometrically fixed while the hydrogen of protonated amino
nitrogen of the salts and lattice water molecules were located
from the difference Fourier map, and the positional and
temperature factors are refined isotropically. Structural illustra-
tions have been generated using ORTEP-319 and MERCURY
1.320 for Windows.
15,21
ꢀ
5.037 A between the identical ethereal oxygen atoms.
The
crystal packing diagram of the podand molecule shows that the
structure is stabilized by weak intermolecular C–H/p interac-
tions forming hemicarcerand through self-assembly in the solid
state. Each tripodal unit is surrounded by six other molecules
occupying a chair conformation with a distance measuring
ꢀ
13.43 A between the apical nitrogen atoms of two adjacent
tripodal units (ESI).† Each arm of the tripodal unit is linked with
its neighbouring unit by aromatic C–H/p interactions involving
the hydrogen H5 and electron-p cloud of the phenyl rings (C5/
ꢀ
Cg ¼ 3.648 A) generating a highly symmetrical 2D sheet like
network (Fig. 1b). Overall non-covalent interactions resulted in
the formation of a honeycomb like structure when viewed along
crystallographic c-axis (ESI).†
Results and discussion
In all anion complexes, independently of the geometry and
nature of the anion, the L1–3:anion stoichiometry is 1 : 1.
Crystal structure studies
Structural information obtained from single-crystal X-ray
analysis of podand L1 and anion complexes (1–7) of L1–3 can
‡ Crystal data for L1: FW ¼ C27H33NO3, M ¼ 419.54, CCDC ¼ 766465,
T ¼ 298(2) K, rhombohedral, space group R3c, a ¼ 20.4711(17), b ¼
3
ꢀ
ꢀ
20.4711(17), c ¼ 9.5827(14) A, V ¼ 3477.8(7) A , Z ¼ 6, m ¼ 0.077
mmꢀ1, 1907 unique reflections, 1278 observed (Rint ¼ 0.1102), R(F) ¼
0.0547 (I > 2s(I), wR(F2) ¼ 0.1295 (all data). 1: FW ¼ C27H34ClNO7,
M ¼ 520.00, CCDC ¼ 766466, T ¼ 298(2) K, rhombohedral, space
ꢀ
group R3c, a ¼ 12.58630(10), b ¼ 12.58630(10), c ¼ 28.7108(5) A, V ¼
3
3938.87(8) A , Z ¼ 6, m ¼ 0.191 mmꢀ1, 2175 unique reflections, 1675
ꢀ
observed (Rint ¼ 0.0631), R(F) ¼ 0.0326 (I > 2s(I), wR(F2) ¼ 0.0906
(all data). 2: FW ¼ C27H34ClNO7, M ¼ 520.00, CCDC ¼ 766468, T ¼
ꢁ
298(2) K, triclinic, space group P1, a ¼ 13.5714(2), b ¼ 14.9166(2), c ¼
ꢂ
ꢂ
ꢂ
ꢀ
15.3649(3) A, a ¼ 93.2570(11) , b ¼ 96.0490(10) , g ¼ 116.2500(11) ,
V ¼ 2755.43(8) A , Z ¼ 4, m ¼ 0.181 mmꢀ1, 13 120 unique reflections,
3
ꢀ
12 127 observed (Rint ¼ 0.0209), R(F) ¼ 0.0654 (I > 2s(I), wR(F2) ¼
0.2320 (all data). 3: FW ¼ C27H34ClNO7, M ¼ 520.00, CCDC ¼
ꢁ
766467, T ¼ 298(2) K, triclinic, space group P1, a ¼ 10.5293(2), b ¼
ꢂ
ꢂ
ꢀ
10.7210(1), c ¼ 13.2388(2) A, a ¼ 98.5670(12) , b ¼ 109.2240(10) , g ¼
ꢂ
3
97.0280(11) , V ¼ 1371.66(4) A , Z ¼ 2, m ¼ 0.183 mmꢀ1, 6551 unique
reflections, 6070 observed (Rint ¼ 0.0205), R(F) ¼ 0.0795 (I > 2s(I),
wR(F2) ¼ 0.2108 (all data). 4: FW ¼ C27H38BrNO5, M ¼ 536.48,
CCDC ¼ 766469, T ¼ 298(2) K, monoclinic, space group C2/c, a ¼
ꢀ
ꢂ
ꢀ
43.2178(12), b ¼ 7.6050(2), c ¼ 17.6312(4) A, b ¼ 105.385(3) , V ¼
5587.2(3) A , Z ¼ 8, m ¼ 1.506 mmꢀ1, 6964 unique reflections, 6598
3
ꢀ
observed (Rint ¼ 0.0938), R(F) ¼ 0.0352 (I > 2s(I), wR(F2) ¼ 0.1094
(all data). 5: FW ¼ C33H36N4O10, M ¼ 648.61, CCDC ¼ 766470, T ¼
ꢁ
298(2) K, triclinic, space group P1, a ¼ 10.0888(3), b ¼ 12.3654(4), c ¼
ꢂ
ꢂ
ꢂ
ꢀ
ꢀ
13.7716(4) A, a ¼ 81.260(2) , b ¼ 76.965(2) , g ¼ 83.641(1) , V ¼
1649.10(9) A , Z ¼ 2, m ¼ 0.097 mmꢀ1, 7968 unique reflections, 7776
3
observed (Rint ¼ 0.0643), R(F) ¼ 0.0529 (I > 2s(I), wR(F2) ¼ 0.1646
(all data). 6: FW ¼ C33H36N4O10, M ¼ 648.66, CCDC ¼ 766471, T ¼
298(2) K, monoclinic, space group P21/n, a ¼ 9.890(2), b ¼ 28.388(6), c ¼
ꢂ
3
ꢀ1
ꢀ
ꢀ
11.602(3) A, b ¼ 92.896(9) , V ¼ 3253.2(12) A , Z ¼ 4, m ¼ 0.099 mm
,
Fig. 1 (a) Crystal structure of C3-symmetrical tripodal podand L1
showing the C3v axis of symmetry passing through the bridgehead N-
atom Thermal ellipsoids are scaled to the 30% probability level.
(b) Symmetrical crystal packing in L1 along c-axis showing hexagonal
arrangement of tripodal units around each ligand, occupying a cyclo-
hexane conformation formed as a result of C–H/p interactions (blue
dotted lines).
8066 unique reflections, 7814 observed (Rint ¼ 0.1158), R(F) ¼ 0.0551 (I >
2s(I), wR(F2) ¼ 0.1537 (all data). 7: FW ¼ C37H39NO11, M ¼ 673.69,
CCDC ¼ 766472, T ¼ 298(2) K, monoclinic, space group P21/c, a ¼
ꢂ
ꢀ
13.4682(13), b ¼ 13.6433(13), c ¼ 20.291(2) A, b ¼ 104.935(3) , V ¼
3602.5(6) A , Z ¼ 4, m ¼ 0.091 mmꢀ1, 7600 unique reflections, 7498
3
ꢀ
observed (Rint ¼ 0.1090), R(F) ¼ 0.0615 (I > 2s(I), wR(F2) ¼ 0.1611 (all
data).
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Crystallographic studies reveal that the bridgehead amino group
of L1–3 is protonated in presence of different organic and inor-
ganic acids and participate in different modes of intramolecular
hydrogen bonding as hydrogen-bond donors depending upon the
nature of the counter anions. Upon protonation of L1 with
HClO4 in methanol we isolated complex 1, [HL1+$ClO4ꢀ] where
the hydrogen of the protonated apical nitrogen is endo-oriented
with respect to the tripodal arms and crystallizes in rhombohe-
dral space group R3c retaining the C3v symmetry similar to L1.
The endo-oriented hydrogen of the apical nitrogen is involved in
intramolecular trifurcated N–H/O hydrogen bonding with the
involving atoms Namino–C–C–Oether and the distance between the
ꢀ
identical ethereal oxygen reduces to 3.667 A. Similar to L1, each
tripodal cation is surrounded by six other units occupying a chair
ꢀ
conformation but with a distance measuring 12.01 A between the
apical nitrogen atoms of two adjacent tripodal cations involved
in the cyclohexane class of chair conformation (ESI).† A
ꢀ
considerable decrease of 1.42 A in the cyclohexane ring size is
possibly, due to the immediate outcome of conformational
locking of the podand cavity which allows a closer approach of
tripodal units in the crystals of 1. The protonated podand does
not have any kind of supramolecular interactions with the
perchlorate ion in the solid-state. However, each arm of
a cationic tripodal unit is linked with two neighbouring units by
ꢀ
three identical ethereal oxygen atoms (N1/O1 ¼ 2.781(2) A)
dropping the size of the flexible tripodal cavity when compared
to L1 (Fig. 2a) and thereby, locks up the cavity well known for
cation binding and anionic guest encapsulation.22 As a conse-
quence of trifurcated hydrogen bond formation, the tripodal
arms are now oriented in a folded conformation which is
reflected in the torsional value of ꢃ63.30(2)ꢂ for each arm
ꢀ
aliphatic C–H/p interaction (C1/Cg ¼ 3.981 A) and C–H/O
hydrogen bond formed between a methyl hydrogen with an
ꢀ
ethereal oxygen (C9/O1 ¼ 3.467(2) A). Expansion of 1 through
hydrogen bonds leads to a highly symmetrical 3D organic solid
(Fig. 2b).
Similar modes of conformational adaptability have been
observed for complexes 2 [HL2+$ClO4ꢀ] and 3 [HL3+$ClO4
ꢀ
]
obtained upon protonation of podands L2 and L3 with HClO4.
Both 2 and 3 crystallize in the lower symmetry triclinic space
ꢁ
group P1. The endo-oriented hydrogen forms trifurcated
hydrogen bonds to the ethereal oxygen atoms with an average
ꢀ
ꢀ
hydrogen bond distance (N1/Oether) of 2.730 A and 2.808 A in
complexes 2 and 3, respectively. The gradual increase in the tri-
furcated N–H/O hydrogen bond distances in complexes 1–3 is
apparently due to the increased steric hindrance provided by the
methyl group at meta and ortho positions in podands L2 and L3,
respectively. Unlike 1, perchlorate oxygen atoms in complexes 2
and 3 are engaged in multiple aliphatic C–H/O hydrogen bond
formation with various methylene hydrogen atoms presented
from the neighbouring tripodal cations. In addition, complex 2
also demonstrates the formation of aryl C–H/O hydrogen
bonds with perchlorate anion. Complex 2 has two crystallo-
graphically independent molecules in the asymmetric unit. The
binding of perchlorate by surrounding HL2+ units clearly reveal
that both the symmetrically non-equivalent ClO4ꢀ ions involving
chlorine atoms Cl(1) and Cl(2) are C–H/O hydrogen bonded
with four tripodal cations each but with six-point and four-point
attachments, respectively. Perchlorate oxygen O7 acts as trifur-
cated hydrogen bond acceptor by forming C–H/O hydrogen
bonds with aryl and aliphatic hydrogen H7, H17 and H38A of
three different cations whereas O10 behaves as bifurcated
hydrogen bond acceptor by interacting with aliphatic protons
H11A and H28A of two symmetrically non-equivalent cations.
The six-point contacts on perchlorate anion involving chlorine
Cl(1) is finally satisfied by the interaction between aliphatic
hydrogen H37B with the perchlorate oxygen O9. A close up view
of the binding modes of Cl(1)O4ꢀ anion by four cationic L2 units
along with the relevant contact distances is depicted in Fig. 3a.
The binding of Cl(2)O4ꢀ anion by four encircling cations clearly
reveals that the perchlorate oxygen atoms O11 and O14 are
involved in bifurcated C–H/O hydrogen bond formation with
an aliphatic and an aromatic proton each whereas O12 and O13
are devoid any hydrogen bond contacts. C–H protons H19A and
H52 from two neighbouring cations are making contacts with
perchlorate oxygen O11 while O14 is engaged in C–H/O
interactions with protons H2A and H35 presented from the other
Fig. 2 (a) Crystal structure of C3-symmetrical supramolecular complex
1 depicting the formation of trifurcated N–H/Oether hydrogen bonds
(green dotted lines). Thermal ellipsoids are scaled to the 30% probability
level. (b) Highly symmetrical crystal packing motif in 1 showing hexag-
onal arrangement of tripodal cations around each unit generated as
a result of interligand C–H/p and C–H/O interactions along crystal-
lographic c-axis.
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Fig. 3 (a) Close-up view of the C–H hydrogen bonding contacts on Cl(1)O4ꢀ anion in 2 (green dotted lines) with four encircling L2H+ units (For clarity,
the symmetry independent units have been shown in different colours and binding of Cl(2)O4ꢀ is provided in the ESI).† (b) Crystal packing of complex 2
as viewed down the a-axis showing the hydrophobic bilayer assembly formation of cationic ligand moieties (blue transparent arrows) diagonally along
bc-plane with the perchlorate anions entrapped between the adjacent ligand arrays in a zigzag fashion forming hydrophilic anion chain (pink transparent
shades). (c) Close-up view of the C–H hydrogen bonding contacts on ClO4ꢀ anion in 3 (green dotted lines) with two neighbouring L3H+ units. (d) Crystal
packing motif of complex 3 as viewed down the a-axis showing the hydrophobic bilayer assembly formation of cationic ligand moieties along b-axis with
the perchlorate anions being entrapped between the adjacent bilayers.
two encircling cations (ESI).† However, in complex 3 each
perchlorate ion is involved in a four-point C–H hydrogen
bonding contacts with two neighbouring cations through an
were unsuccessful in obtaining the single crystals of chloride
complex of L1 suitable for XRD analysis. As compared to 1, the
endo-oriented hydrogen forms somewhat weak trifurcated N–
H/O hydrogen bonds to the ethereal oxygen atoms with an
ꢀ
average hydrogen bond distance of 3.375 A (Fig. 3c). Aliphatic
ꢀ
protons H10A and H10B from two tripodal cations interact with
perchlorate oxygen O6 while O5 and O7 make contacts with
H10B and H2A respectively, revealing that the perchlorate
oxygen O6 behaves as bifurcated hydrogen bond acceptor and
methylene hydrogen H10B acts as a bifurcated hydrogen bond
donor (Table 2). Complexes 2 and 3 are further stabilized
through the network of interligand C–H/p hydrogen bonds
formed between the methylene CH2 protons and phenyl rings of
protonated podand. The details of the hydrogen bond interac-
tions are provided in Table 1. The overall non-covalent interac-
tions result in the formation of hydrophobic bilayer assembly of
ligand moieties both in complexes 2 and 3 when viewed down the
crystallographic a-axis with the anions being situated in the
available space between the bilayers in a zigzag fashion stitching
the adjacent cationic arrays by C–H/O hydrogen bonds (Fig. 3b
and 3d).
average distance of 2.758(3) A in complex 4. An average
torsional (Namino–C–C–Oether) value of ꢃ60.14(3)ꢂ implies that
the tripodal arms are all oriented in a folded conformation with
ꢀ
an average distance of 3.569 A between the ethereal oxygen. The
binding of bromide clearly demonstrates that each anion is
involved in a six-point attachment provided by three encircling
L1H+ units and lattice water molecules O4 and O5 (Fig. 4a). The
methylene protons H10B, H20A and H19B from each tripodal
cation is C–H hydrogen bonded to Brꢀ with an average hydrogen
ꢀ
ꢀ
bonding distance of 3.823 A. Hexa-coordination to Br is finally
satisfied by the hydrogen bonded lattice water molecules (O4/
ꢀ
O5 ¼ 2.870(4) A), where one O4 and two O5 molecules make
contacts with each bromide anion via O–H/Br interactions
ꢀ
through an average hydrogen bonding distance of 3.375 A. The
tripodal cations are interlinked among themselves via aromatic
C–H/p interactions involving the phenyl ring (C3g) bonded to
the ethereal oxygen O3 and aryl hydrogen H7 and H14 presented
In our effort to validate the effect of spherical halide anions,
we prepared the bromide and chloride complexes of L1 by
treating a methanolic solution of L1 with HBr and HCl,
respectively. The bromide complex [HL1+$Brꢀ]$2H2O (4) crys-
tallizes in the lower symmetry monoclinic space group C2/c. The
hydrogen of the protonated nitrogen is endo-oriented similar to
perchlorate complex 1 but interestingly, the C3v symmetry of the
podand is lost upon bromide complexation (unlike 1) possibly
due to the presence of two lattice water molecules. However, we
ꢀ
from two neighbouring units (C7/C3g ¼ 3.655 A; C14/C3g ¼
ꢀ
3.922 A). The crystal packing diagram viewed down the c-axis
(Fig. 4b) clearly reveals that the podand molecules beautifully
pack in a bilayer assembly forming a hydrophobic chain of
ligand moieties and the bromide ions are entrapped between
the adjacent bilayers in association with the lattice water
molecules generating a hydrophilic chain parallel along the
b-axis.
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Table 1 Relevent hydrogen bond parameters in Anion complexes 1–7
[HL3+]$[Picrate] (6)
ꢀ
ꢀ
D–H/A
d(H/A)/A
d(D/A)/A
:(DHA)/ꢂ
[HL1+]$[ClO4ꢀ] (1)
N1–H/O4
N1–H/O5
1.80(2)
2.39(2)
2.42(2)
2.61(2)
2.68(3)
2.53(3)
2.70(2)
2.58(2)
2.46(2)
3.29
2.684(3)
150(2)
127(2)
137(2)
122(1)
133(2)
153(2)
121(2)
108(1)
144(3)
154
3.072(3)
3.205(3)
3.244(3)
3.397(4)
3.418(4)
3.310(3)
3.029(3)
3.267(5)
4.197
C2–H2A/O4
C19–H19A/O5
C13–H13/O7
C18–H18B/O7
C2–H2B/O8
C19–H19A/O9
C15–H15/O10
C1–H1B/C1g
C10–H10A/C2g
C3g/Cg(anion)
N1–H/O1
C9–H9/O1
C1–H1B/Cg
2.31(1)
2.70(1)
3.04
2.781(2)
3.467(2)
3.981
114(2)
127(2)
165
[HL2+]$[ClO4ꢀ] (2)
N1–H/O1
N1–H/O2
N1–H/O3
N2–H–O4
N2–H/O5
2.26(3)
2.747(3)
121(2)
117(2)
117(2)
117(2)
117(2)
114(2)
162(2)
159(2)
140(2)
113(2)
140(2)
150(2)
139(2)
176(3)
148(2)
173(2)
138
2.26(3)
2.27(2)
2.26(2)
2.20(3)
2.33(2)
2.65(3)
2.59(3)
2.60(2)
2.63(3)
2.47(3)
2.57(3)
2.47(3)
2.62(4)
2.44(2)
2.64(3)
3.07
2.705(4)
2.718(3)
2.749(3)
2.684(4)
2.778(3)
3.550(5)
3.479(4)
3.408(4)
3.138(5)
3.283(4)
3.448(5)
3.274(5)
3.548(6)
3.310(3)
3.572(4)
3.856
3.25
3.810
3.937
118
[HL1+]$[Pyromellitate] (7)
N2–H/O6
C7–H7/O7
N1–H/O7
C5–H5/O7
C18–H18A/O10
C19–H19A/O4
C1–H1B/O8
O6–H6/O4
1.87(3)
2.70(2)
2.55(3)
2.29(2)
2.46(2)
2.67(2)
1.71(2)
2.71(2)
3.30
2.783(3)
3.594(4)
3.460(5)
3.210(4)
3.334(4)
3.208(3)
2.530(3)
2.576(3)
4.137
149(2)
160(3)
156(2)
157(2)
149(2)
124(2)
176(2)
071(2)
150
C17–H17/O7
C38–H38A/O7
C11–H11A/O10
C28–H28A/O10
C37–H37B/O9
C19–H19A/O11
C52–H52/O11
C2–H2A/O14
C35–H35/O14
C19–H19B/C3g
C28–H28B/C4g
C20–H20B/C6g
O6–H6/O5
O9–H9/O10
C16–H16/C3g
C2g/Cg(anion)
3.931
2.77
3.63
3.696
3.699
158
131
[HL3+]$[ClO4ꢀ] (3)
N1–H/O1
N1–H/O2
N1–H/O3
C2–H2A/O7
C10–H10A/O6
C10–H10B/O5
C10–H10B/O6
C1–H1B/C1g
C11–H11B/C3g
2.44(2)
2.35(3)
2.31(3)
2.68(7)
2.55(5)
2.62(3)
2.46(6)
2.67
2.856(3)
2.768(4)
2.800(3)
3.434(8)
3.411(6)
3.440(5)
3.216(7)
3.527
110(2)
110(2)
116(2)
134(3)
146(3)
141(3)
134(3)
146
3.27
3.967
129
[HL1+]$[Brꢀ]$2H2O (4)
N1–H/O1
N1–H/O2
N1–H/O3
2.23(2)
2.28(3)
2.25(2)
2.96(3)
3.04(4)
3.04(4)
2.51(2)
2.43(2)
2.51(3)
1.97(3)
2.66(3)
2.92
2.755(3)
2.793(3)
2.728(3)
3.816(3)
3.815(4)
3.839(3)
3.396(3)
3.322(3)
3.408(3)
2.870(4)
3.414(4)
3.655
116(2)
115(2)
112(2)
147(2)
137(2)
139(2)
170(3)
171(3)
171(3)
176(3)
134(2)
136
C10–H10B/Br1
C19–H19B/Br1
C20–H20A/Br1
O4–H2O/Br1
O5–H3O/Br1
O5–H4O/Br1
O4–H1O/O5
C11–H11A/O4
C7–H7/C3g
C14–H14/C3g
3.46
3.922
113
[HL1+]$[Picrate] (5)
N1–H/O4
1.81(3)
2.713(3)
3.394(3)
3.386(4)
3.455(5)
3.247(4)
3.263(4)
3.001(4)
3.351(3)
3.476(3)
3.435(3)
4.179
172(3)
133(1)
136(2)
171(2)
148(1)
120(1)
100(1)
148(2)
135(1)
157(1)
128
C2–H2A/O4
C11–H11A/O4
C13–H13/O5
C19–H19A/O10
C1–H1A/O6
C2–H2A/O6
C5–H5/O7
C19–H19B/O7
C26–H26/O9
C22–H22/C3g
C2g/Cg(anion)
Cg(anion)/Cg(anion)
2.65(2)
2.61(2)
2.53(3)
2.37(3)
2.66(3)
2.67(3)
2.52(2)
2.71(2)
2.55(2)
3.53
Fig. 4 (a) Close-up view of bromide binding depicting the C–H and O–
H hydrogen-bonding interactions on bromide (green dotted lines) with
three cationic receptor units and lattice water molecules. (b) Crystal
packing structure of complex 4 as viewed down the c-axis showing the
hydrophobic bilayer assembly formation of cationic ligand moieties
along b-axis and hydrophilic chain of anions between the adjacent
bilayers.
3.839
3.450
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ꢀ
It is worth to be mentioned that in spite of our repeated efforts,
we were unable to crystallize the podands L1–3 in presence of
other oxoacids which finally directed us to carry out the
competitive crystallization experiments in presence of various
protons H2A and H11A (C2/O4 ¼ 3.394(3) A; C11/O4 ¼
ꢀ
3.386(4) A) whereas nitro oxygen atoms O5 and O10 is in
interaction with C–H protons H13 and H19A (C13/O5 ¼
ꢀ
ꢀ
3.455(5) A; C19/O10 ¼ 3.247(4) A), respectively, from the same
+
competing anions to examine the selectivity of HL1–3 for
tripodal cation forming the electrostatic N–H/O4 hydrogen
ꢀ
perchlorate in methanol (ESI).† All the above mentioned
complexes are stabilized by various weak as well as strong
directional hydrogen bonds, which induce rigidity in the formed
cationic podands and thus, serve as the foundation for selective
crystallization of desired salts. Guided by this principle, the
bond in 5 (N1/O4 ¼ 2.713(3) A) as depicted in Fig. 5a. The
nitro oxygen O9 is involved in aromatic C–H/O interaction
with the aryl hydrogen H26 whereas both O6 and O7 acts as
bifurcated hydrogen bond acceptor by interacting with C–H
protons H1A, H2A and H5, H19B, respectively, resulting overall
in nine C–H hydrogen bonding contacts on picrate anion
(Table 1). However in complex 6, O4 is C–H hydrogen bonded to
the aliphatic proton H2A and nitro oxygen O5 is in interaction
with the methylene hydrogen H19A from the same tripodal
cation forming the bifurcated N–H/O hydrogen bonds to O4
+
selectivity of HL1–3 for perchlorate in an experiment of
competitive crystallization has been accomplished.23 High yields
of perchlorate complexes of L1–3 were found even in the presence
of equivalent concentrations of competitive anions. The ligands
expressed high affinity for perchlorate vs. AcOꢀ, BO33ꢀ, CO3
2ꢀ
,
2ꢀ
ꢀ
NO3ꢀ, PO43ꢀ, SO4
and BF4 and the formation of L:
and O5 (N1/O4 ¼ 2.684(3) A; N1/O5 ¼ 3.072(3) A; C2/
ꢀ
ꢀ
ꢀ ꢀ
perchlorate is observed in above 75% yield (ESI, Table S1).†
However, comparatively poorer affinity was observed for
perchlorate in the presence of halides.
O4 ¼ 3.205(3) A; C19/O5 ¼ 3.244(3) A) as depicted in Fig. 5b.
Furthermore, O8, O9 and O10 of picrate are making C–H/O
contacts with hydrogen atoms H2B, H19A and H15, respec-
tively, while O7 is in interaction with C–H protons H13 and
H18B resulting in seven C–H hydrogen bonding contacts on
picrate anion. The details of the hydrogen bonding interactions
are presented in Table 1. The crystal packing motif of complex 5
as viewed down the c-axis, clearly shows that the tripodal cations
are arranged in a face to face fashion diagonally along the ab-
plane with one of the phenyl rings (C2g) bonded to the ethereal
oxygen O2 is involved in intermolecular p/p stacking interac-
tion with the dimeric picrate anion through an interaction
At this juncture we were interested to find out the role of
organic anions as a templating anion in the system that affects
+
the orientation of HL1–3 and we prepared complexes 5
[HL1+$Picrate] and 6 [HL3+$Picrate], which crystallize in triclinic
ꢁ
P1 and monoclinic P21/n space groups, respectively. Single-
crystal X-ray analysis of 5 and 6 revealed the formation of non-
capsular infinite hydrogen bonded network of the cationic units
with picrate anion. In both the complexes, the tripodal arms are
all oriented in three different directions with the hydrogen of the
protonated amine being exo-oriented and forms intermolecular
N–H/O hydrogen bond with the phenolate oxygen O4 of pic-
rate anion. Exo-orientation of the apical hydrogen and lack of
intramolecular noncovalent interactions between the tripodal
arms is perhaps responsible for the flat and extended orientations
of the podands in both the complexes. Interactions of picrate
with the surrounding tripodal units, making various hydrogen
bonds clearly represent that each picrate anion interacts with five
and six tripodal cations in complexes 5 and 6, respectively, where
one or both the oxygen from each nitro group are engaged in
multiple weak C–H/O interactions with different aliphatic and
aromatic hydrogen of the encircling cations (Table 1). The
phenolate oxygen O4 is C–H hydrogen bonded to the aliphatic
ꢀ
strength of 3.839 A along the a-axis (ESI).† However, the crystal
packing of complex 6 reveals that the phenyl ring C3g is in p/p
interaction with two picrate anions situated on either side of the
ꢀ
ring C3g through an interaction strength of 3.937 A along the c-
axis (ESI).†
To examine the conformational adaptability of the podands in
presence of other organic anions, we prepared the PMA (pyro-
mellitic acid) complex of L1, [HL1+$pyromellitate] (7) which
crystallizes in monoclinic space group P21/c. Single crystal
analysis reveals similar kind of conformational behaviour of
+
HL1 unit as observed in complexes 5 and 6, with the tripodal
arms being devoid of any intramolecular interactions are in an
extended conformation and the exo-oriented hydrogen is N–H/
O hydrogen bonded with the carboxylate oxygen O7 of PMA
ꢀ
(N1/O7 ¼ 2.783(3) A) as depicted in Fig. 6a. Interactions of
PMA with the surrounding tripodal units, making various
hydrogen bonds clearly represent that each PMA anion interacts
with four encircling cations in complex 7 where one of the oxygen
from each carboxylate groups are engaged in weak C–H/O
interactions with different aliphatic and aromatic hydrogen of
the encircling cations (Table 1). Carboxylate oxygen atoms O4,
O7, O8 and O10 are making C–H/O contacts with alkyl
hydrogen H19A, H5, H1B and H18A, respectively, from four
different tripodal cations with an average hydrogen bond
ꢀ
distance of 3.399 A. Similar to the picrate complexes, 7 retains
the intermolecular aromatic p/p stacking between one of the
phenyl rings C2g of tripodal cation and electron p-cloud of
ꢀ
pyromellitate anion with an interaction strength of 3.931 A.
Fig. 5 Ball and stick representation of crystal structures of 5 (a) and 6
(b) depicting the open conformation of the tripodal unit and the relevant
H-bond contacts on picrate oxygen atoms (green dotted lines) with the
exo-oriented bridgehead N–H and C–H protons.
PMA units are interlinked among themselves forming a 2D
hydrogen bonded sheet having R4 pattern hydrogen bonded
4
rings along ab plane of the crystal (Fig. 6b).24
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two-dimensional fingerprint plots can also be broken down to
give the relative contribution to the Hirshfeld surface area from
each type of interactions present, quoted as the ‘‘contact
contribution’’.25
Fig. 7(a) displays the Hirshfeld surface mapped with dnorm for
L1 which highlights the interligand C–H/p close contacts
between the tripodal molecules as bright red spots. Two red spots
on the surface of each side arm correspond to the identical C–
H/p interactions formed between the aryl proton H5 and
a phenyl ring of the adjacent podand molecule. The bright red
spot near the edge of the surface of each arm corresponds to the
C5–H5 donor hydrogen atom whereas the spot at the centre of
each identical side arm corresponds to the p acceptor ring. The
corresponding fingerprint plot for the L1 Hirshfeld surface is
shown in Fig. 7(b) with the characteristic ‘‘wings’’ in the upper
left and lower right of the plot that represent the C–H/p close
contacts, with total contact contribution of 27.7% (Table 2).
Fig. 8(a) displays the Hirshfeld surfaces mapped with dnorm for
the tripodal segments in supramolecular complexes of perchlo-
rate anion (1–3) and picrate anion (5 and 6). The dnorm surfaces
highlight the interligand C–H/p close contacts and C–H/O
interactions between the oxygen of perchlorate/picrate anions
and C–H donors of the tripodal unit as red spots. In complex 1,
where there is no involvement of the C–H donors in making
interaction with the perchlorate oxygen atoms, the bright red
spots on the upper half of the surface correspond to the methy-
lene C–H/p donor H1B hydrogen atom whereas the bright
spots in the middle of the surface can be assigned for the p
acceptor ring of the tripodal side arms. Although C9–H9/Oether
contact is present towards the lower part of the Hirshfeld surface,
the area is displayed as faint red to white, meaning contacts
around the van der Waals separation. In perchlorate complexes 2
and 3, the dnorm Hirshfeld surfaces for tripodal segments
primarily highlight the various C–H/Oanion and C–H/p
interactions as bright red to faint red spots on the outer walls of
the tripodal surface, which is in contrast to complex 1, have only
methylene C–H/p and methyl C–H/Oether interactions,
among others. In complex 2, the bright red spots present in the
upper half of Hirshfeld surfaces for the tripodal segments
correspond mostly to methylene C–H/Oanion interactions
except for those marked with stars, which represent C–H/p
interactions. Whereas, the spots towards the edge of the lower
half of the surfaces mostly correspond to C–H/p interactions
and the rest represent aromatic C–H/Oanion contacts. In
complex 3, the spots towards the upper half of the Hirshfeld
surface of the tripodal segment symbolize participation of the
methylene CH2 protons in C–H/Oanion close contacts as
depicted in Table 1. Whereas, the two faint red spots towards the
lower half (marked with stars) of the surface represent aliphatic
C–H/p interactions. The bright spot at the extreme right of the
Fig. 6 (a) Ball and stick representation of the crystal structure of 7
depicting the open conformation of the tripodal unit with relevant H-
bond interactions. (b) 2D hydrogen bonded sheet structure of pyro-
mellitate anions along the ab-plane in complex 7.
Hirshfeld surface analysis
The weak interactions involved in the conformational changes of
tripodal podands in presence of different anions can also be
visualised by the Hirshfeld surfaces, which is a useful tool to
describe the surface characteristics of molecules.10 Hirshfeld
surfaces offer a novel way of visualizing intermolecular interac-
tions by colour-coding short or long contacts, the colour
intensity indicating the relative strength of the interactions. Two-
dimensional fingerprint plots complement these surfaces,
quantitatively summarizing the nature and type of intermole-
cular contacts experienced by the molecules in the crystal. The
Table 2 Summary of the various contact contributions to the tripodal
Hirshfeld surface area in L1 and anion complexes 1–7
L1
1
2
3
4
5
6
7
H/C
H/O
C/C
H/H
27.7
9.0
0.0
62.1
19.3
24.8
0.0
17.1
23.0
0.8
17.1
18.9
1.2
20.5
16.6
0.1
20.3
23.2
1.6
17.9
21.8
4.8
22.3
19.1
2.2
Fig. 7 Hirshfeld surface analysis of the tripodal molecule L1. (a) dnorm
surface. (b) Two-dimensional fingerprint plots with the C–H/p inter-
actions highlighted in colour.
55.6
57.5
61.9
56.6
53.3
52.0
54.5
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surface correspond to the H/H interaction between identical
methyl proton H27A. The corresponding fingerprint plots of the
tripodal Hirshfeld surfaces in complexes 1–3 are shown in
Fig. 8(b) and 8(c) for the C–H/p and C–H/O close contacts,
respectively, highlighted in colour. The contact contributions for
the Hirshfeld surface areas are 19.3, 17.1, and 17.1% for the C–
H/p close contacts and 24.8, 23.0, and 18.9% for the C–H/O
close contacts in complexes 1–3, respectively (Table 2). The
fingerprint plots for the C–H/p close contacts display the
characteristic ‘‘wings’’ in the upper left and lower right of the plot
and show pseudosymmetry on either side of the diagonal where
de ¼ di. A noteworthy feature of the tripodal fingerprint plots of
perchlorate complexes 1–3 is the substantial change in the
characteristic H/C and H/O contact contributions with the
variation in the position of the methyl substituents in L1–3. To
compensate for the decrease in C–H/p and C–H/O close
contacts, the percentage of H/H close contacts increases, and
this effect is more clearly seen in complex 3 due to the steric
hindrance within the tripodal unit offered by the methyl
substituent at ortho-position of the terminal phenyl ring and
thus, the H/H short contacts become more apparent (Table 2).
In picrate complexes of L1 and L3 (5 and 6), the bright red spot
at the centre of the Hirshfeld surfaces mapped with dnorm
(Fig. 8a) correspond to the apical N–H/Oanion interactions with
the phenolate oxygen of picrate anion. Due to the endo-orien-
tation of the hydrogen of the protonated apical nitrogen, similar
type of interaction is absent in the perchlorate complexes of L1–3
and subsequently, no bright red spots could be observed on the
Hirshfeld surfaces near the apical nitrogen of the tripodal
segment. The faint red spots subsequent to the bright spot
correspond to methylene C–H/Oanion contacts both in
complexes 5 and 6. Whereas, spots towards the edge of the
surface correspond mostly to the aromatic C–H/Oanion inter-
actions. Spots that has been marked with a star represent C–H/
p interactions. The fingerprint plots of the tripodal Hirshfeld
surfaces in complexes 5 and 6 are shown in Fig. 8(b) and 8(c) for
the C–H/p and C–H/O close contacts respectively, high-
lighted in colour. The contact contributions for the Hirshfeld
surface areas are 20.3 and 17.9% for the C–H/p close contacts
and 23.2 and 21.8% for the CH/O close contacts in complexes 5
and 6, respectively (Table 2). Similar to the perchlorate
complexes, the fingerprint plots for the C–H/p close contacts
display the characteristic ‘‘wings’’ in the upper left and lower
right of the plot and show pseudosymmetry on either side of the
diagonal where de ¼ di. Whereas, unlike the perchlorate
complexes, the considerable decrease in the H/C and H/O
contact contributions is compensated by an increase in the C/C
close contacts instead of the H/H close contacts (Table 2). The
fingerprint plots for H/O close contacts show a sharp spike at
ꢀ
ꢀ
ꢀ
di ¼ 0.996 A and de ¼ 1.297 A in complex 5 and at di ¼ 0.881 A
ꢀ
and de ¼ 1.207 A in complex 6, which is associated with the apical
N–H/Oanion close contact between the hydrogen of the apical
nitrogen and phenolate oxygen of picrate anion.
Fig. 9 displays the Hirshfeld surfaces mapped with dnorm for
the tripodal segments in bromide and pyromellitate complexes of
L1 (4 and 7) together with the corresponding fingerprint plots for
the C–H/p and C–H/O close contacts. In bromide complex 4,
two bright spots on the upper half of the Hirshfeld surface
Fig. 8 Hirshfeld surface analysis of the tripodal segments in complexes 1, 2, 3, 5 and 6. (a) dnorm surface: spots representing C–H/p interactions have
been marked with stars. (b) Two-dimensional fingerprint plots with the C–H/p interactions highlighted in colour. (c) Two-dimensional fingerprint
plots with the C–H/O interactions highlighted in colour.
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for C–H/p and C–H/O close contacts respectively. Similar to
the picrate complexes, the fingerprint plot for H/O close
ꢀ
ꢀ
contacts shows a spike at di ¼ 0.981 A and de ¼ 1.287 A, which is
associated with the apical N–H/Oanion close contact between
the hydrogen of the apical nitrogen with one of the pyromellitate
oxygen atoms.
Rationalization of C–H hydrogen bond formation
In continuation of our efforts to investigate the anion directed
self assembly of various substituted tripodal molecules24,26 we
discovered that, the cresol based tripodal ether ligands upon
protonation in presence of inorganic anions results into confor-
mational locking of the tripodal cavity via trifurcated N–H/O
hydrogen bond formation due to the endo orientation of the
protonated bridgehead N–H proton. However when organic
anions are used as template, the podand molecules are forced to
adopt a flat and extended orientation of their arms due to
intermolecular face to face interaction involved between one of
the phenyl rings of ligand arms and electron p-cloud of the
planar organic anion. In addition, the hydrogen of the proton-
ated apical nitrogen is exo-oriented by forming electrostatic N–
H/anion hydrogen bond with the organic anion. The existence
of p/p stacking interactions between a phenyl ring of tripodal
arms and aromatic p -cloud of planar organic anion is probably
responsible for exo-orientation of the hydrogen of protonated
bridgehead nitrogen together with other weak supramolecular
interactions with the anions. The sharp spikes present in the two
dimensional fingerprint plots are the characteristic for the
supramolecular complexes of organic anions which represent
apical N–H/Oanion close contact and is absent in the complexes
having inorganic anions. Structural analysis of anion complexes
2–7 reveals the formation of multiple weak aliphatic and
aromatic C–H/anion contacts with the neighbouring tripodal
cations. The unusual and variable coordination modes in
bromide and perchlorate complexes (2–4) suggests that the
interactions with the C–H donors are too weak to impose
a definite coordination structure around the anions, and instead
the CH groups on the flexible arms of the podand embrace the
anion so as to match its size and shape to provide a favourable
electrostatic environment around it. Calculations on C6H6-anion
complexes have shown that the aryl C–H donors are effective
anion binding groups, compared to aliphatic C–H donors.27 The
strength of an aryl C–H donor group can in theory, be adjusted
through the addition of different functional groups on the arene
ring.28 However, protonation at the bridgehead nitrogen render
the methylene C–H hydrogen bond donors sufficiently acidic for
their active participation in C–H/O interactions as observed in
all complexes. Though charge neutralisation in the crystals and
conventional hydrogen bonds are the main driving forces in the
formation of supramolecular complexes,29 yet the weak CH
hydrogen bonds provide added stabilization to the complexes
and thus, satisfy the geometrical necessity of the LH+ units by
providing a favourable electrostatic environment around the
anions. Moreover, interligand C–H/p interactions provide
further stabilization to the supramolecular complexes having
inorganic counter anions (1–4) resulting into the bilayer
assembly formation of ligand moieties in the solid state which has
not been observed in the organic anion complexes (5–7). Detailed
Fig. 9 Hirshfeld surface analysis of the tripodal segments in complexes 4
and 7. (a) dnorm surface. (b) Two-dimensional fingerprint plots with the
C–H/p interactions highlighted in color. (c) Two-dimensional finger-
print plots with the C–H/O interactions highlighted in colour.
correspond to the methylene C–H/Br close contacts (Table 1).
The bright spot towards the upper edge represent the C10–
H10B/Br interaction whereas the spot towards the left (upper
half) stand for C10–H20A/Br interaction. The spots at the
extreme right and left of the lower half of the Hirshfeld surface
represent the C14–H14/p (C3g) interaction for the donor and
the acceptor respectively. The brighter spot at the centre of the
lower half correspond to the C7–H7/ p (C3g) interaction for
the donor atom. Finally, the bright spot at the edge of the lower
half correspond to the repulsive H/H interaction between H9C
and H27B. The contact contributions for the Hirshfeld surface
areas are 6.1 and 20.5% for C–H/Br and C–H/p close
contacts, respectively. Although, there is interaction between the
methylene proton H11A and O4 (lattice water), the area is dis-
played as blue with a contact contribution of 16.6%.
In pyromellitate complex 7, the bright red spot at the middle of
the Hirshfeld surface correspond to the apical N–H/Oanion close
contact with the pyromellitate oxygen atom O7. The next
brighter spot just below to the above mentioned spot represent
the methylene C1–H1B/O8 interaction. The faint red spot at
the edge of upper part of the Hirshfeld surface correspond to the
methyl C18–H18A/O10 contact whereas the faint spot at the
extreme left (lower half) stand for the C5–H5/O7 interaction.
The spot at the centre of the upper half of the surface can be
designated to the C16–H16/p (C3g) close contact. The contact
contributions for the Hirshfeld surface areas are 22.3 and 19.1%
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structural investigation clearly demonstrates that the self-align-
ment, flexibility and orientation of the ligand units, depending
upon the dimensionality and nature of the anions play a crucial
role in making various molecular interactions possible in
complexation of various anions.
Conclusions
We have structurally authenticated for the conformational
changes in cresol-based tripodal molecules, based on anion
specificity and undertaken a detailed analysis of their Hirshfeld
surfaces to gain a greater understanding of the various interac-
tions involved in the crystal packing for assembled arrays con-
taining different anions. To conclude, the present findings
provide evidence for inorganic anion assisted conformational
locking of acyclic tripodal podands L1–3 by the formation of
intramolecular trifurcated N–H/O hydrogen bonds whereas
protonation in presence of organic anions force the podand
molecules to become conformationally open via intermolecular
N–H/anion hydrogen bond formation and p/p interaction.
This is an example of a system in which a simple and flexible
molecule adapts its conformation to respond to the demands of
the specific anion. This conformational movement could prove to
be significant in the field of anion sensing and signal trans-
duction. We have also shown the solid state evidence for the
active participation of both aliphatic and aryl C–H donors in
interaction with different anions, exhibiting aliphatic C–H/
anion hydrogen bond strengths comparable to those of aryl C–
H/anion hydrogen bond. Therefore, due to the interesting
structural and binding properties, the tripodal podand molecules
can provide an excellent case of understanding C–H/anion
hydrogen bonding in its protonated form. Furthermore, the
architecture of the podand molecule could serve as a model for
building novel, highly selective receptors for perchlorate which is
indicated by the impressive selectivity for the anion, even in
highly competitive environment. We are currently investigating
the conformational adaptability of similar class of ligands pos-
sessing amide and urea or thiourea functionalities in the presence
of anions of different dimensionality.
NMR spectroscopy has been used to evaluate and correlate
structural and conformational properties of anion receptors to
their pre-organization and conformational changes for interac-
tion with diverse range of anions.30 The excellent solubility of the
perchlorate and picrate complexes in CHCl3 (4 and 7 are insol-
1
uble in CHCl3) allowed us to perform H NMR experiment in
CDCl3. A comparison of the proton d values of complexes 1 and
5 with L1 showed significant downfield shift of the aliphatic CH2
protons and slight upfield shift of aromatic CH protons whereas
the signal for the CH3 protons remains almost same. Consider-
able downfield shift of the aliphatic CH2 protons indicate the
influence of protonation at the apical nitrogen on the neigh-
bouring methylene protons. However, interestingly, the reso-
nance for the endo-oriented hydrogen of the protonated apical
nitrogen in complex 1 occurs at d ¼ 3.462 ppm and in complex 5
where the hydrogen of the protonated bridgehead nitrogen is
exo-oriented show resonance at d ¼ 3.045 ppm (Fig. 10). The
occurrence of the bridgehead N–H proton resonances at
considerably different d values (Dd ¼ 0.417 ppm) with notable
differences in the CH proton resonances further accounts for the
different orientations of the tripodal side arms in presence of
inorganic and organic counter anions in solution. However, in
DMSO-d6 the resonances for the apical N–H proton could not be
observed.
Acknowledgements
GD acknowledges DST (SR/S1/IC-01/2008) and CSIR (01-2235/
08/EMR-II), New Delhi India for financial support and DST-
FIST for single-crystal X-ray diffraction facility. SKD
acknowledges IIT Guwahati, India for fellowship.
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