A subtle interplay of C-H hydrogen bonds in complexation of anions of varied dimensionality by a nitro functionalized tripodal podand

Article abstract of DOI:10.1039/c0ce00316f

The structural aspects of binding of halides (1 and 2), nitrate (3), perchlorate (4), trifluoroacetate (5) and hexafluorosilicate (6) with the protonated tripodal podand L are examined crystallographically. Anion binding with multiple receptor units is attributable entirely to the (NH) ⁺...anion and multiple C-H...anion hydrogen bonding interactions in all six complexes. Protonation at the apical nitrogen and presence of nitro functionality renders the methylene and aryl hydrogen sufficiently acidic for their active participation in moderate to weak CH...anion interactions are noteworthy. All supramolecular networks of complexes 1-6 are guided by various non-convalent interactions, especially directional CH...O_nitro hydrogen bonds and π-stacking interactions.

Full text of DOI:10.1039/c0ce00316f

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PAPER
A subtle interplay of C–H hydrogen bonds in complexation of anions of varied
dimensionality by a nitro functionalized tripodal podand†
*
Sandeep Kumar Dey, Bimlesh Ojha and Gopal Das
Received 18th June 2010, Accepted 22nd July 2010
DOI: 10.1039/c0ce00316f
The structural aspects of binding of halides (1 and 2), nitrate (3), perchlorate (4), trifluoroacetate (5)
and hexafluorosilicate (6) with the protonated tripodal podand L are examined crystallographically.
Anion binding with multiple receptor units is attributable entirely to the (NH)+/anion and multiple
C–H/anion hydrogen bonding interactions in all six complexes. Protonation at the apical nitrogen
and presence of nitro functionality renders the methylene and aryl hydrogen sufficiently acidic for their
active participation in moderate to weak CH/anion interactions are noteworthy. All supramolecular
networks of complexes 1–6 are guided by various non-convalent interactions, especially directional
CH/Onitro hydrogen bonds and p-stacking interactions.
Asanionsdisplayawiderangeofgeometries,thedirectionalityof
Introduction
hydrogen bonds is frequently utilized to achieve complementarity
between anions and receptors. Most hydrogen bonding anion
receptorsutilize N–H/anion or O–H/anion hydrogen bonds 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.9 Although not typically considered to be significant
donors, there is increasing evidence that C–H groups can partici-
pate in bonding and lead to enhanced anion-binding affinity.10 This
evidence comes in theform of directobservation of close contactsin
crystallographic structures,11 anion-induced chemical shifts of C–H
protons in NMR spectra12 and theoretical calculations.13 Imida-
zolium groups have already been introduced as anion binding
hydrogen bond moieties by forming a (C–H)+/anion ionic
hydrogen bond between C(2)–H in the imidazolium ring and the
guest anion.14 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. Recent
theoretical investigation by Hay et al. showed that the effect of
electron withdrawing substituents on the aryl moiety significantly
enhances the stability of anion complexes.15 A recent review of
anion-arene adducts noted that aryl C–H/anion hydrogen
bonding, rather than interaction with the p system, is by far the
most prevalent motif observed in the solid state for the interaction
of anion with arenes.16 Electronic structure calculations further
probe the structural and energetic aspects of aliphatic C–H/anion
hydrogen bonding confirming that the aliphatic C–H hydrogen
bond is a viable interaction motif for anion host design.17
Anion binding by synthetic receptors is an important and contem-
porary aspect of supramolecular chemistry and has proven its roles in
biological systems, in environmental issues, and in the area of
medicine and catalysis.1,2 The objective of achieving strong anion
binding by hydrogen bonding receptors is an engaging challenge
because of the design difficulties associated with targeting such large
size and variably shaped, relatively diffuse, weakly basic, highly
solvated analytes and interact with receptors only through weak
forces.1a,3 Synthetic receptors have involved either hydrogen bonding
alone (in neutral hosts), or a combination of hydrogen bonding and
electrostatic interactions (in cationic hosts) for anion complexation.4
Recently, Bowman-James has categorized the binding of anions
based on their coordination numbers which is helpful in defining the
notions of complementarity for a given anion and can aid to the
design of optimal anion-binding host structures.5 The most effective
way to bind anions consist in taking advantage of their negative
charge, and accordingly, polyammonium (positively charged)
ligands have been the principal receptor of choice, since they ensure
an adequate electrostatic attraction reinforced by hydrogen-bond
contacts with the coordinated anions.6,7 Whereas the binding of
anionic guests within pre-organized macrocyclic systems are rela-
tively straightforward to understand but the binding processes of
flexible podand receptors remain more elusive.8
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
Some of our main concerns have been to ascertain the
complexation and recognition of anion in the solid-state and the
consequences of weak hydrogen bonding in the intermolecular
network structure.18 Given our interest in structural aspects of
anion binding, we have extensively studied the coordination of
† Electronic supplementary information (ESI) available: X-Ray
crystallographic file of the structures in CIF format; ORTEPs, selected
non-covalent interactions, selected torsion values, crystal packing
networks, 1H and 13C spectra. CCDC reference numbers
761161–761166. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0ce00316f
This journal is ª The Royal Society of Chemistry 2011
CrystEngComm, 2011, 13, 269–278 | 269

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Experimental
Materials and methods
All reagents were obtained from commercial sources and used as
received. Solvents were purified freshly following standard
procedure prior to use. NMR spectra were recorded on a Varian
FT-400 MHz instrument. The chemical shifts were recorded in
parts per million (ppm) on the scale using tetramethylsilane
(TMS) as a reference. Elemental analyses were carried out on
a Perkin-Elmer 2400 automatic carbon, hydrogen and nitrogen
analyzer.
Scheme 1 Structure of tris-[4-(nitrophenyloxy)-ethyl]-amine (L).
X-Ray crystallography
The intensity data were collected using a Bruker SMART APEX-
II CCD diffractometer, equipped with a fine focus 1.75 kW
a cationic nitro functionalized tripodal podand, L (Scheme 1)
with anions of different shapes and geometry. However, we have
ꢀ
sealed tube Mo-Ka radiation (l ¼ 0.71073 A) at 298(2) K, with
recently reported three conformational polymorphs of
L
increasing u (width of 0.3ꢀ per frame) at a scan speed of 3 s per
frame. The SMART software was used for data acquisition.
Data integration and reduction were undertaken with SAINT
and XPREP19 software. Multi-scan empirical absorption
corrections were applied to the data using the program
SADABS.20 Structures were solved by direct methods using
SHELXS-9721a and were refined by full-matrix least squares on
F2 using SHELXL-9721b program package. In all the six
compounds, non-hydrogen atoms were refined anisotropically.
Hydrogen atoms attached to all carbon atoms were geometri-
cally fixed while the hydrogen atom of tertiary amino nitrogen of
the salts was located from the difference Fourier map, and the
positional and temperature factors are refined isotropically.
However, we were unable to locate the hydrogen atoms of the
disordered lattice water molecules (O28, O29 and O30) in
complex 6, from the difference Fourier map. Structural illustra-
tions have been generated using ORTEP-322a and MERCURY
1.322b for Windows.
obtained solely from traditional solvent mediated crystal-
lization.23a Our discovery of the significant binding between
different anions and protonated form of L is attributable entirely
to the electrostatic (N–H)+/anion hydrogen bond and aliphatic
as well as aromatic CH/anion interactions that presented
a means of participating in the re-examination of the role of weak
C–H hydrogen bond donors. Attempts were made to provide
solid-state structural evidence for anion complexation by setting
up a series of crystallization experiments of L with different
anions. Herein, we report the solid state evidence for binding of
chloride, bromide (spherical), nitrate (trigonal planar), perchlo-
rate (tetrahedral), trifluoroacetate and hexafluorosilicate (octa-
hedral) with protonated form of the podand L and their detailed
molecular interactions.‡
‡ Crystal data for 1: FW ¼ C24H25ClN4O9, M ¼ 548.93, CCDC ¼
ꢁ
761161, T ¼ 298(2) K, triclinic, space group P1, a ¼ 7.475(6), b ¼
ꢀ
ꢀ
ꢀ
13.365(10), c ¼ 13.784(10) A, a ¼ 98.90(3) , b ¼ 102.77(3) , g ¼
Ligand design and synthesis
105.84(3) , V ¼ 1257.6(17) A , Z ¼ 2, m ¼ 0.213 mmꢁ1, 3172 unique
ꢀ
3
ꢀ
reflections, 2570 observed (Rint ¼ 0.1018), R(F) ¼ 0.0518 (I > 2s(I),
For a receptor to bind with the anionic guests, it should possess
pre-organized anion binding elements decorated on the suitable
platform/framework. Numerous synthetic anion receptors con-
taining polyammonium, amide, urea/thiourea, pyrrole or indole
groups incorporated in increasingly complicated supramole-
cular skeletons target the efficiency of natural receptors.
However, our idea in designing and synthesizing receptor L for
binding of different anions is principally because of the absence
of conventional N–H or O–H hydrogen bonding functions for
anions so that we could examine the involvement of weak C–H
hydrogen bond donors toward binding of anions with the
protonated form of the podand. Calculations on C6H6-anion
complexes have shown that the aryl C–H donors are effective
anion binding groups compared to aliphatic C–H donors.17 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. It has been well established that the electron-withdrawing
substituent on the benzene ring assist the active participation of
the aryl –CH protons toward anion binding via C–H/anion
interactions. Moreover, protonation at the apical nitrogen in L
could significantly enhance the acidity of the aliphatic –CH2
wR(F2) ¼ 0.1240 (all data). 2: FW ¼ C24H25BrN4O9, M ¼ 593.38,
ꢁ
298(2) K, triclinic, space group P1,
CCDC
¼
761162,
T
¼
ꢀ
ꢀ
a ¼ 7.6818(6), b ¼ 13.3835(11), c ¼ 13.8995(12) A, a ¼ 99.080(1) , b ¼
103.12(5)ꢀ,
g
¼
106.33(3)ꢀ,
V
¼
Z
¼
2,
m
¼
3
1297.45(48) A ,
ꢀ
1.644 mmꢁ1, 6469 unique reflections, 6386 observed (Rint ¼ 0.0188),
R(F) ¼ 0.0344 (I > 2s(I), wR(F2) ¼ 0.0928 (all data). 3: FW ¼
C24H25N5O12
,
M
¼
575.49, CCDC
¼
761163,
T
¼
298(2) K,
monoclinic, space group P21/n, a ¼ 15.2305(7), b ¼ 8.9608(4), c ¼
ꢀ
3
ꢀ
ꢀ
20.0560(10) A, b ¼ 101.073(3) , V ¼ 2686.2(2) A , Z ¼ 4, m ¼
0.116 mmꢁ1, 6659 unique reflections, 6185 observed (Rint ¼ 0.0733),
R(F) ¼ 0.0611 (I > 2s(I), wR(F2) ¼ 0.1162 (all data). 4: FW ¼
C24H25ClN4O13, M ¼ 612.93, CCDC ¼ 761164, T ¼ 298(2) K,
monoclinic, space group P21/c, a ¼ 11.9923(5), b ¼ 12.5750(6), c ¼
ꢀ
3
ꢀ
ꢀ
18.0740(8) A, b ¼ 103.045(2) , V ¼ 2655.3(2) A , Z ¼ 4, m ¼ 0.221
mmꢁ1, 6585 unique reflections, 5805 observed (Rint ¼ 0.0784), R(F) ¼
0.0460 (I > 2s(I), wR(F2) ¼ 0.0969 (all data). 5: FW ¼ C26H25N4O11F3,
ꢁ
M ¼ 626.50, CCDC ¼ 761165, T ¼ 298(2) K, triclinic, space group P1,
ꢀ
ꢀ
a ¼ 6.8340(9), b ¼ 12.5542(11), c ¼ 16.8656(18) A, a ¼ 99.533(5) , b ¼
ꢀ
ꢀ
3
ꢁ1
ꢀ
94.301(6) , g ¼ 99.636(7) , V ¼ 1399.0(3) A , Z ¼ 2, m ¼ 0.129 mm
,
7010 unique reflections, 6711 observed (Rint ¼ 0.101), R(F) ¼ 0.0688
(I > 2s(I), wR(F2) ¼ 0.1081 (all data). 6: FW ¼ C48H50F6N8O20Si,
M ¼ 1201.05, CCDC ¼ 761166, T ¼ 298(2) K, triclinic, space group
ꢁ
ꢀ
P1,
a
¼
ꢀ 15.4544(9),
b
¼
17.9320(9),
c
ꢀ
¼
18.6070(10) A,
a
¼
ꢀ
3
ꢀ
116.352(3) , b ¼ 94.269(5) , g ¼ 106.342(3) , V ¼ 4313.2(4) A , Z ¼ 3,
m ¼ 0.141 mmꢁ1, 24 200 unique reflections, 19 542 observed (Rint
0.0316), R(F) ¼ 0.0935 (I > 2s(I), wR(F2) ¼ 0.2440 (all data).
¼
270 | CrystEngComm, 2011, 13, 269–278
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hydrogen and thereby, could possibly form moderate to weak
C–H/anion hydrogen bonds perhaps of similar strength to aryl
C–H/anion hydrogen bond. Considering the above points, we
have designed and synthesized tripodal podand receptor L
having nitro-substituted aryl terminals for anion binding
studies.
125.96 (ꢂ6C, Ar), 141.23 (ꢂ3C, Ar), 163.14 (ꢂ6C, Ar). Anal.
calcd (%) for C26H25F3N4O11: C 49.84, H 4.02, N 8.94%. Found:
C 50.28, H 4.35, N 8.77%.
[2HL+]$[SiF6ꢁ]$2H2O (6). Pale-yellow precipitate; Yield: 56%
based on ligand L; mp: 196 ꢀC; single-crystals suitable for X-ray
diffraction were obtained from slow evaporation of a CH3CN–
DMF (4 : 1) solution of the compound at RT for twenty days.
1H-NMR (400 MHz, DMSO-d6) d 4.45 (s, 6 H, OCH2), 7.12–7.14
(d, 6 H, ArH), 8.18–8.21 (d, 6 H, ArH); 13C-NMR (100 MHz,
DMSO-d6) d 53.79 (ꢂ3C, NCH2), 67.00 (ꢂ3C, OCH2), 115.35
(ꢂ3C, Ar), 125.57 (ꢂ6C, Ar), 135.12 (ꢂ3C, Ar), 151.37 (ꢂ6C,
Ar). Anal. calcd (%) for C24H27F3N4O10Si0.5: C 47.84, H 4.51, N
9.29%. Found: C 48.42, H 4.36, N 8.83%.
The acyclic tripodal receptor L has been synthesized by
modification of known literature procedure.23 Complexes 1, 2, 3,
4, 5 and 6 were obtained by stirring a solution of L (50 mg) in
20 mL of methanol in a glass beaker and adding 1.2 equiv. of 37%
HCl, 49% HBr, HNO3, 70% HClO4, CF3COOH and 40% HF,
respectively. After a constant stirring for half an hour a solid
precipitate was formed which was then filtered, washed with
ether and dried under vacuum. Single crystals suitable for X-ray
diffraction analysis were obtained from slow evaporation of
CH3OH–CH3CN (1 : 1) binary mixture solution of the
compounds at RT within 1–2 weeks.
Results and discussion
Crystal structure studies
[HL+]$[Clꢀꢁ] (1). White precipitate; Yield: 78% based on ligand
1
L; mp: 185 C; H-NMR (400 MHz, DMSO-d6) d 3.79 (s, 6 H,
Structural information obtained from single-crystal X-ray
analysis of the anionic complexes (1–6) can provide insight into
the proper binding topology of anions with the protonated
tripodal podand receptor. Crystal structure analyses revealed
that the six crystals belong to the lower-symmetry monoclinic
and triclinic crystal systems. Complexation of different anions by
tripodal receptor L is primarily governed by two different types
of supramolecular interactions. In all complexes, the exo-
oriented hydrogen of the protonated tertiary nitrogen of the
ligand is involved in a comparatively strong electrostatic
(N–H)+/anion interaction. In addition to (N–H)+/anion
interaction, the anions are further involved in several C–H/
anion interactions with the alkyl and aryl hydrogen of the
protonated podand satisfying the geometrical necessity of the
flexible LH+ units to respond to the demand of the anions of
different sizes and geometries. Thus, tripodal cations bearing
nitro functions directed the anion assembly formation through
a strong electrostatic hydrogen bond formation along with
weaker noncovalent interactions.
NCH2), 4.54 (s, 6 H, OCH2), 7.15–7.17 (d, 6 H, ArH), 8.21–8.24
(d, 6 H, ArH); 13C-NMR (100 MHz, DMSO-d6) d 53.15 (ꢂ3C,
NCH2), 63.80 (ꢂ3C, OCH2), 115.28 (ꢂ3C, Ar), 125.88 (ꢂ6C,
Ar), 141.38 (ꢂ3C, Ar), 162.77 (ꢂ6C, Ar). Anal. calcd (%) for
C24H25ClN4O9: C 52.51, H 4.59, N 10.20%. Found: C 52.78, H
4.43, N 9.96%.
[HL+]$[Brꢁ] (2). Pale-yellow precipitate; Yield: 75% based on
ligand L; mp: 205 ꢀC; 1H-NMR (400 MHz, DMSO-d6) d 3.84 (s,
6 H, NCH2), 4.56 (s, 6 H, OCH2), 7.16–7.18 (d, 6 H, ArH), 8.22–
8.24 (d, 6 H, ArH); 13C-NMR (100 MHz, DMSO-d6) d 52.97
(ꢂ3C, NCH2), 63.37 (ꢂ3C, OCH2), 115.35 (ꢂ3C, Ar), 125.88
(ꢂ6C, Ar), 141.36 (ꢂ3C, Ar), 162.67 (ꢂ6C, Ar). Anal. calcd (%)
for C24H25BrN4O9: C 48.57, H 4.24, N 9.44%. Found: C 48.69, H
4.38, N 9.05%.
[HL+]$[NO3ꢁ] (3). Pale-yellow precipitate; Yield: 68% based
on ligand L; mp: 176 ꢀC; 1H-NMR (400 MHz, DMSO-d6) d 3.84
(s, 6 H, NCH2), 4.56 (s, 6 H, OCH2), 7.16–7.18 (d, 6 H, ArH),
8.22–8.24 (d, 6 H, ArH); 13C-NMR (100 MHz, DMSO-d6)
d 53.10 (ꢂ3C, NCH2), 63.56 (ꢂ3C, OCH2), 115.28 (ꢂ3C, Ar),
125.89 (ꢂ6C, Ar), 141.38 (ꢂ3C, Ar), 162.74 (ꢂ6C, Ar). Anal.
calcd (%) for C24H25N5O12: C 50.08, H 4.37, N 12.16%. Found:
C 50.56, H 4.75, N 11.88%.
The chloride complex [HL+]$[Clꢁ] (1) crystallizes in triclinic
ꢁ
space group P1, with the tertiary nitrogen of the tripodal podand
is protonated and turns out to be the monochloride salt of the
podand. An ORTEP plot of 1 is shown in Fig. 1a along with the
atom numbering scheme. The binding of chloride with encircling
cations clearly demonstrates that each anion is involved in
a seven-point attachment provided by three LH+ units with six
C–H/Clꢁ contacts through an average C–H hydrogen bonding
[HL+]$[ClO4ꢁ] (4). White precipitate; Yield: 60% based on
ligand L; mp: 162 ꢀC; 1H-NMR (400 MHz, DMSO-d6) d 3.84 (s,
6 H, NCH2), 4.56 (s, 6 H, OCH2), 7.15–7.17 (d, 6 H, ArH), 8.21–
8.23 (d, 6 H, ArH); 13C-NMR (100 MHz, DMSO-d6) d 53.21
(ꢂ3C, NCH2), 63.51 (ꢂ3C, OCH2), 115.38 (ꢂ3C, Ar), 126.02
(ꢂ6C, Ar), 141.49 (ꢂ3C, Ar), 162.76 (ꢂ6C, Ar). Anal. calcd for
C24H25ClN4O13: C 47.02, H 4.11, N 9.14%. Found: C 46.64, H
4.32, N 9.26%.
ꢀ
ꢀ
distance of 3.690 A ranging from 3.601 to 3.821 A and C–H/Cl
angle range from 138 to 173ꢀ (Table 1). The close-up view of the
hydrogen bonding interactions with the chloride ion is shown in
Fig. 1b. The exo-oriented proton H1N of the protonated apical
nitrogen is involved in (N–H)+/Clꢁ interaction with the anion
ꢀ
with a hydrogen bonding distance of 3.013 A and N–H/Cl angle
of 160ꢀ. The methylene hydrogen H18A subsequent to the
protonated nitrogen is C–H hydrogen bonded to Clꢁ with an
[HL+]$[CF3COOꢁ] (5). Reddish-brown precipitate; Yield: 62%
based on ligand L; mp: 154 ꢀC; 1H-NMR (400 MHz, DMSO-d6)
d 3.09 (s, 6 H, NCH2), 4.20 (s, 6 H, OCH2), 7.06–7.08 (d, 6 H,
ArH), 8.12–8.14 (d, 6 H, ArH); 13C-NMR (100 MHz, DMSO-d6)
d 53.15 (ꢂ3C, NCH2), 79.04 (ꢂ3C, OCH2), 115.27 (ꢂ3C, Ar),
interaction length of 3.633 A. In addition, the aliphatic/aryl
ꢀ
protons H2B, H9B and H4 from a different neighbouring cation
make intermolecular C–H/Clꢁ contacts with the anion. Finally,
the seven point-contacts on Clꢁ is fulfilled by the interactions
from the aliphatic protons H9A and H17A of the third cation.
This journal is ª The Royal Society of Chemistry 2011
CrystEngComm, 2011, 13, 269–278 | 271

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Table 1 Relevant (N–H)+/anion and C–H/anion hydrogen bond
data in anionic complexes 1, 2, 3, 4, 5 and 6
:(DHA)/ꢀ
ꢀ
ꢀ
D–H/A
d(H/A)/A
d(D/A)/A
[HL+]$[Clꢁ] (1)
N1–H1N/Cl1
C18–H18A/Cl1
C2–H2B/Cl1
C9–H9B/Cl1
C4–H4/Cl1
1.95
2.85
2.82
2.84
2.89
2.66
2.92
3.013
3.633
3.621
3.684
3.821
3.601
3.780
160
138
140
145
173
162
148
C9–H9A/Cl1
C17–H17A/Cl1
[HL+]$[Brꢁ] (2)
N1–H1N/Br1
C1–H1A/Br1
C1–H1B/Br1
C10–H10A/Br1
C17–H17B/Br1
C18–H18B/Br1
C12–H12/Br1
[HL+]$[NO3ꢁ] (3)
N1–H1N/O10
N1–H1N/O11
C1–H1A/O10
C9–H9A/O11
C10–H10A/O11
C15–H15/O12
[HL+]$[ClO4ꢁ] (4)
N1–H1N/O12
C1–H1A/O10
C1–H1A/O12
C12–H12/O13
C15–H15/O13
C23–H23/O11
[HL+]$[CF3COOꢁ] (5)
N1–H1N/O10
N1–H1N/O11
C1–H1B/F2
2.32
2.81
2.87
2.94
2.99
2.95
2.96
3.194
3.752
3.708
3.730
3.876
3.759
3.894
158
161
145
138
152
141
173
2.07
1.89
2.28
2.46
2.47
2.61
2.894
2.881
3.211
3.184
3.190
3.506
135
160
160
131
130
160
2.12
2.43
2.46
2.45
2.64
2.53
2.974
3.240
3.385
3.266
3.302
3.250
150
140
158
146
128
133
2.60
1.71
2.56
2.43
2.65
2.68
2.51
2.63
2.44
3.248
2.742
3.487
3.194
3.529
3.472
3.055
3.086
3.306
120
174
158
134
150
138
115
110
154
C1–H1B/O10
C2–H2B/F3
C2–H2B/O11
C9–H9B/O10
C4–H4/F2
C7–H7/F1
[2HL+]$[SiF62ꢁ]$2H2O (6)
N1–H1N/F1
N1–H1N/F2
N5–H2N/F8
N5–H2N/F9
N9–H3N/F4
N9–H3N/F6
C1–H1A/F1
C1–H1B/F3
C25–H25B/F2
C25–H25B/F4
C42–H42A/F4
C44–H44/F3
C44–H44/F4
C49–H49A/F5
C49–H49B/F4
C57–H57B/F1
C57–H57B/F6
C58–H58B/F6
C66–H66B/F4
C33–H33B/F7
C9–H9A/F8
C9–H9A/F9
C18–H18A/F9
C20–H20/F9
C33–H33A/F9
2.41
3.118
2.742
2.756
2.997
3.217
2.693
3.081
3.228
3.218
3.375
3.211
3.487
3.441
3.193
3.157
3.328
3.178
3.347
3.340
3.219
3.230
3.544
3.293
3.366
3.025
135
159
168
127
125
166
111
125
134
160
138
147
155
124
114
162
126
133
142
126
146
153
140
159
109
1.88
1.96
2.44
2.55
1.74
2.58
2.56
2.46
2.44
2.42
2.66
2.57
2.54
2.63
2.39
2.51
2.60
2.52
2.55
2.37
2.65
2.49
2.48
2.55
Fig. 1 (a) ORTEP plot (50% ellipsoids) of complex 1; (b) Close-up view
of the seven hydrogen-bonding interactions of chloride anion (blue
dotted lines) with three LH+ units; (c) Crystal packing diagram of
complex 1 along bc-plane depicting the hydrophobic bilayer assembly
formation of ligand moieties and hydrophilic anionic chain along b-axis.
The crystal packing motif viewed down the a-axis (Fig. 1c)
clearly reveals that the receptor molecules beautifully pack in
a bilayer array forming a hydrophobic chain of ligand moieties
and the dimmers of chloride ions are entrapped between the
adjacent bilayers generating a hydrophilic chain parallely along
b-axis. Intermolecular C–H/Onitro hydrogen bonding between
ꢀ
alkyl hydrogen H17B and nitro oxygen O5 (C17/O5 ¼ 3.349 A,
< C17–H17B/O5 ¼ 124ꢀ) is bridging the receptor moieties
along b-axis. The adjacent monolayers of the ligand array are
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interconnected via C–H/p hydrogen bond formed between the
aryl hydrogen H7 and phenyl ring involving carbon C11–C16
ꢀ
(C2g) forming the bilayers along b-axis (C7/C2g ¼ 3.970 A, <
C7–H7/C2g ¼ 157ꢀ). Expansion of a bilayer through hydrogen
bonds suggests that, the bilayers are further interlinked among
themselves via several C–H/Onitro interactions involving
aliphatic/aryl protons (H2B, H10A H8, H16 and H20) and nitro
oxygen atoms (O2, O3 and O6) in association with weak inter-
ꢀ
ligand p/p interactions (C1g/C1g ¼ 4.183 A; C3g/C3g ¼
ꢀ
ꢀ
3.610 A; C1g/C3g ¼ 3.858 A) along a-axis whereas the
hydrophilic anion chain stitches the adjacent bilayers along
c-axis via multiple C–H/Clꢁ interactions generating overall
a 3D hydrogen bonded network (Table S1, ESI).†
The bromide complex [HL+]$[Brꢁ] (2) crystallizes in the lower
ꢁ
symmetry triclinic space group P1 with Z ¼ 2. An ORTEP plot of
2 is shown in Fig. 2a along with the atom numbering scheme. In
addition to ionic (N–H)+/Brꢁ interaction (N/Br ¼ 3.194 A,
ꢀ
<N–H/Br ¼ 158ꢀ) similar to the chloride complex (1), there are
six C–H/Brꢁ hydrogen bonds from different alkyl and aryl
hydrogen of the three coordinating tripodal units with an average
ꢀ
hydrogen bonding distance of 3.786 A ranging from 3.708 to 3.894
ꢀ
ꢀ
A and C–H/Br angle range from 138 to 173 (Table 1). An
unusual hepta-coordination in the halide complexes (1 and 2)
suggests that the interactions with the C–H donors are too weak to
impose a definite coordination structure around the bromide
anions, and instead the CH groups on the flexible arms of the
receptor embrace the anion so as to match its size and shape to
provide a favourable electrostatic environment around it. In
complex 2, the methylene hydrogen H18B subsequent to the
protonated apical nitrogen is C–H hydrogen bonded to Brꢁ while
aliphatic protons H1A, H17B, H1B, H10A and aryl hydrogen
H12 from two adjacent cationic units are making contacts to the
anion intermolecularly via weak C–H/Brꢁ hydrogen bonds. A
close up view of the coordination of bromide anion by four
cationic L units is depicted in Fig. 2b. Thus, the seven-point
attachment via strong (N–H)+/Brꢁ and weak C–H/Brꢁ inter-
actions is responsible for the binding and stabilization of the
bromide ion with the protonated L receptor units. Moreover,
there is marginal difference in the orientation of the tripodal arms
on replacing chloride by bromide which is reflected in the torsion
involving samino (C–Namino–C–C) and sether (Namino–C–C–Oether
)
presumably due to the similar (seven-point contacts) coordination
modes acquired by spherical bromide ion (Table S2, ESI).† The
crystal packing diagram viewed down the a-axis (Fig. 2c) clearly
shows that the receptor molecules beautifully pack in a bilayer
array with two successive tripodal cations are flipped inward
ꢀ
toward each otherin a face to face fashion (dN1/N1 ¼ 11.35 A) and
the bromide ions are trapped between the adjacent monolayers
parallely along b-axis. Similar to 1, the cationic tripodal units are
self assembled via six intermolecular C- H/Onitro interactions
involving different C–H protons with one or both the oxygen from
each nitro group, edge to face interaction and face to face inter-
actions as well (Table S1, ESI).† Intermolecular C–H/Onitro
hydrogen bonding between aryl hydrogen H24 andꢀnitro oxygen
ꢀ
Fig. 2 (a) ORTEP plot (50% ellipsoids) of complex 2; (b) Close-up view
of the seven hydrogen-bonding interactions of bromide anion (blue
dotted lines) with three LH+ units; (c) Crystal packing diagram of
complex 2 along bc-plane depicting the linear and parallel arrangement of
the bromide anions trapped within the cationic tripodal arrays.
O3 (C17/O5 ¼ 3.349 A, < C17–H17B/O5 ¼ 124 ) is bridging
the receptor moieties along b-axis forming monolayers. The
adjacent monolayers of the ligand array are interconnected via
C–H/p hydrogen bond formed between the aryl hydrogen H15
and phenyl ring involving carbon C3–C8 (C1g) forming the
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bilayers along b-axis. Expansion through hydrogen bonds further
reveals that, the bilayers are interlinked among themselves via
multiple C–H/Onitro interactions and interligand p/p stacking
ꢀ
ꢀ
(C3g/C3g ¼ 3.610 A; C2g/C3g ¼ 3.858 A) along a-axis whereas
multiple C–H/Brꢁ interactions stitches the adjacent bilayers
along c-axis.
Complex [HL+]$[NO3ꢁ] (3) crystallizes in monoclinic space
group P21/n with Z ¼ 4. An ORTEP plot of 3 is shown in Fig. 3a
along with the atom numbering scheme. The solid-state structure
of complex 3 shows bifurcated (N–H)+/O interaction between
the hydrogen of the protonated amine with the nitrate oxygen
atoms O11 and O10 through N/O distances of 2.881 and
ꢀ
ꢀ
2.894 A and N–H/O angle of 157 and 133 respectively. In
addition, each nitrate ion is C–H hydrogen bonded with three
encircling podand molecules with an average hydrogen bonding
ꢀ
ꢀ
distance of 3.272 A ranging from 3.184 to 3.506 A and C–H/O
angle range from 130 to 160ꢀ (Table 1). The nitrate oxygen O11 is
involved in moderate aliphatic C–H/O interaction with the
methylene hydrogen H17A and H18A of the same tripodal unit
whereas O12 is in interaction with the methylene hydrogen H1A
suggesting that O11 and O12 behaves as a trifurcated and
bifurcated hydrogen bond acceptors respectively in the anion
complex 3. In addition to the above five interactions, nitrate
oxygen O10 is engaged in a comparatively weaker aryl C–H/O
interaction with the aromatic proton H21 completing the sixth
coordination contact of the nitrate anion. A close up view of the
coordination of nitrate anion by four cationic L units is depicted
in Fig. 3b. Each nitrate anion is held between the tripodal cleft
shaped cavity genereted due to the folded conformation of two
tripodal arms projecting in the similar direction by weak inter-
actions. The packing diagram viewed down the crystallographic
b-axis clearly shows that the cationic array of ligands is arranged
diagonally along the ac-plane with the anions being situated in
a zig-zag fashion between the cationic tripodal arrays (Fig. 3c).
The cationic ligand moieties are self-organized via seven inter-
molecular C–H/Onitro hydrogen bonds between different
aliphatic and aromatic hydrogen atoms with one or both the
oxygen from each nitro group where O3, O5 and O6 behaves as
bifurcated hydrogen bond acceptor (Table S1, ESI).†
Complex [HL+]$[ClO4ꢁ] (4) crystallizes in monoclinic space
group P21/c with Z ¼ 4. An ORTEP plot of 4 is shown in Fig. 4a
along with the atom numbering scheme. Similar to the halide
complexes, there exists (N–H)+/anion interaction between the
hydrogen H1N of the apical nitrogen andꢀO12 of perchlorate
ꢀ
(N1/O12 ¼ 2.974 A, < N1–H/O12 ¼ 150 ). In addition, there
are five intermolecular C–H/O hydrogen bonds formed
between the oxygen atoms of perchlorate anion and different
alkyl and aryl hydrogen of the three neighbouring receptor units
surrounding the anion (Fig. S5, ESI)† with an average C–H/O
Fig. 3 (a) ORTEP plot (50% ellipsoids) of complex 3; (b) Close-up view
of nitrate binding depicting the six hydrogen-bonding interactions of
nitrate anion (green dotted lines) with four cationic receptor units;
(c) Crystal packing diagram of complex 3 along ac-plane depicting the
zigzag arrangement of the nitrate anions diagonally along the ac-plane.
ꢀ
hydrogen bond distance of 3.288 A and C–H/O angle range
from 128 to 158ꢀ. The methylene hydrogen H1A of a tripodal
unit is engaged in bifurcated C–H hydrogen bonding to O10 and
O12 of perchlorate anion and O13 is involved in bifurcated
acceptor C–H/O interaction with the aromatic hydrogen H12
and H15 from two different receptor units. The six-point contact
of perchlorate anion is finally satisfied by the interaction between
aromatic hydrogen H23 with the perchlorate oxygen O11. The
details of these interactions are provided in Table 1. The overall
non-covalent interactions with the anion results in the formation
of a zipper like assembly when viewed down the crystallographic
a-axis with the anions being arranged in a zigzag fashion
stitching the adjacent cationic arrays by C–H/O hydrogen
bonds along b-axis (Fig. 4b). The cationic L moieties are cross
linked among themselves exclusively via six C–H/Onitro inter-
action between different alkyl and aryl hydrogen with one or
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Fig. 4 (a) ORTEP plot of complex 4. (b) Crystal packing diagram of
complex 4 viewed down the a-axis showing the formation of zipper-like
assembly and hydrogen bonding interactions of perchlorate anion (blue
dotted lines) with adjacent cationic receptor molecules.
Fig. 5 (a) ORTEP plot of complex 5; (b) Crystal packing diagram of
complex 5 viewed down the b-axis showing the formation of bilayer
assembly of cationic L moieties along c-axis with trifluoroacetate anions
situated within the tripodal clefts between the adjacent cationic arrays.
both the nitro oxygen atoms among which O3 behaves as
bifurcated hydrogen bond acceptor (Table S1, ESI).†
Complex [HL+]$[CF3COOꢁ] (5) crystallizes in triclinic space
dimeric assembly encapsulating two trifluoroacetate ions within
the dimeric cleft (Fig. S8, ESI).† The cationic tripodal units are
interlinked among themselves via five C–H/Onitro interactions
between alkyl hydrogen with both the oxygen from nitro groups
involving N2 and N3 and also via p/p stacking between iden-
tical phenyl rings involving carbon atoms C11–C16 (C2g) (Table
S1, ESI).† The nitro oxygen O6 acts as bifurcated hydrogen bond
acceptor whereas methylene hydrogen H17B behaves as bifur-
cated hydrogen bond donor.
Silicon hexafluoride salt [2HL+]$[SiF62ꢁ]$2H2O (6) was
obtained on reaction of the tripodal ligand L with HF, presum-
ably as a result of corrosion with the glass surface (beaker). The
overall molecular structure is a 2 : 1 ionic salt of L and H2SiF6
with disordered water molecules present in the crystal lattice as
solvent of crystallization and crystallizes in triclinic space group
ꢁ
group P1 with Z ¼ 2. An ORTEP plot of 5 is shown in Fig. 5a
along with the atom numbering scheme. The binding of
CF3COOꢁ clearly shows that each trifluoroacetate ion is coor-
dinated to five receptor units by a nine point attachment
involving both oxygen and fluorine atoms of the anion. A close-
up view for binding of trifluoroacetate anion has been provided
in the supplementary information (Fig. S7, ESI).† Hydrogen
H1N of the protonated apical nitrogen is in bifurcated (N–H)+/
ꢀ
O interaction with O10 and O11 (N1/ O10 ¼ 3.248 A, <N1–
ꢀ
ꢀ
ꢀ
H/O10 ¼ 124 ; N1/O11 ¼ 2.742 A, <N1–H/O11 ¼ 174 ) as
observed in the nitrate complex. Methylene hydrogen H9B from
the same cation is involved in the formation of moderately strong
aliphatic C–H/O hydrogen bond with O10 of the anion (C9/
ꢁ
P1 with Z ¼ 3. An ORTEP plot of 6 is shown in Fig. S1 (ESI)†
ꢀ
ꢀ
O10 ¼ 3.055 A, <C9–H9B/O10 ¼ 115 ). Aliphatic hydrogen
H1A and H2B from two different receptor units are involved in
bifurcated C–H/anion interaction with O10, F2 and O11, F3,
respectively, whereas aryl hydrogen H4 and H7 from two other
units are in intermolecular C–H/F interaction with F2 and F1,
respectively. The details of these C–H/anion interactions are
provided in Table 1. An average C–H/O/F hydrogen bonding
along with the atom numbering scheme. The asymmetric unit
contains three HL+ units and correspondingly 1.5 SiF62ꢁ units are
present for charge neutralization. In an attempt to understand the
binding of polyatomic anion (SiF62ꢁ) by the tripodal podand
2ꢁ
LH+, we have analyzed the interaction of SiF6 with the
surrounding ligand moieties. Water molecules do not have any
kind of non-covalent interactions either with the cationic tripodal
units or the anionic counterpart in the solid-state.
ꢀ
ꢀ
distance of 3.304 A ranging from 3.055 to 3.529 A indicates the
formation of moderate to weak C–H/anion hydrogen bond.
The crystal packing diagram viewed down the b-axis (Fig. 5b)
along with p/p stacking interaction clearly shows that receptor
molecules beautifully pack in a bilayer array with the trifluoro-
acetates getting sandwiched between the adjacent bilayers. Two
arms of cationic L are projected in one direction to form a cleft
shaped cavity and two such tripodal clefts intercalate to form the
2ꢁ
The binding of SiF6 by multiple protonated L units clearly
reveals that the hexafluorosilicate ions involving silicon atoms
Si(1) and Si(2) interacts with four tripodal units each (Fig. 6a)
but with seventeen-point and sixteen-point coordination
respectively solely via (N–H)+/F and C–H/F interactions. The
hydrogen atoms HN1 and HN9 of the protonated bridgehead
nitrogen atoms from two receptor units on either side of
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surrounding receptor moieties involving aliphatic hydrogen
H25A, H42A, H49B, H66B and aromatic proton H44. F5 is the
only atom that is in a single point contact with the methylene
hydrogen H49A and finally F6 is involved in C–H/F hydrogen
bonding interaction with the methylene hydrogen H57B and
H58B from two different cations completing the 17 hydrogen-
bonding contacts on Si(1)F62ꢁ. Thus, both F1 and F6 forms
trifurcated acceptor hydrogen bonds and H25A, H44 and H57B
are all involved in bifurcated hydrogen bonding in the anion
complex.
Hexafluorosilicate ion involving Si(2) is situated in a symmet-
rical environment with four encircling receptor units providing
a sixteen point contacts to it. A close up view of the coordination
of Si(2)F62ꢁ anion by four cationic L units is depicted in Fig. 6b.
Hydrogen HN5 of the protonated nitrogen forms bifurcated
ꢀ
hydrogen bond with F8 and F9 (N5/F8 ¼ 2.756 A, <N5–H/
ꢀ
ꢀ
ꢀ
F8 ¼ 168 ; N5/F9 ¼ 2.997 A, < N5–H/ F9 ¼ 127 ). F7 and F8
are engaged in moderate C–H/F interaction with alkyl
hydrogen H33B and H9A, respectively, whereas F9 is involved in
a four point C–H/F interaction with H9A, H18A, H20 and
H33A. The details of the C–H/F interaction are given in Table
ꢀ
1. An average hydrogen bond distance of 3.276 A implies active
participation of the alkyl C–H donors towards hexafluorosilicate
binding via moderate C–H/F inteactions. The cationic tripodal
units are cross linked among themselves via multiple interligand
C–H/Onitro interaction and p/p stacking between the phenyl
units (Fig. 6c) generating a 3D supramolecular hydrogen bonded
network with solvent accessible voids measuring a total volume
3
ꢀ
of 434 A in the structure. The presence of only three difference
Fourier peaks in such a large volume shows that most of the
water molecules could not be located due to disorder.
Rationalization of structural features
The hydrogen of the protonated bridgehead nitrogen in L is exo-
oriented in all six complexes and is involved in forming (N–H)+/
anion (1, 2, and 4) or bifurcated (N–H)+/anion hydrogen bonds
(3, 5 and 6) depending upon the dimensionality of the counter
anion. Absence of intramolecular non-covalent interactions
between the receptor arms is likely to be responsible for the flat
and extended orientation of the tripodal arms in LH+ units of all
complexes. The binding of different anions by multiple LH+ units
reveal that nitrate, perchlorate and hexafluorosilicate anions are
coordinated through four tripodal units each whereas coordi-
nation of halides and trifluoroacetate are provided by three and
five LH+ units respectively. Hexafluorosilicate ion involving Si(1)
is located in an unsymmetrical coordinating environment with
Fig. 6 (a) Coordination environment of the two symmetrically non-
ꢁ
equivalent SiF6 anions in complex 6, three LH+ conformers have been
shown in different colours; (b) Close-up view of hexafluorosilicate
ꢁ
binding depicting the sixteen hydrogen-bonding interactions of Si(2)F6
anion (green dotted lines) with four cationic receptor units; (c) Packing
diagram of complex 6 viewed down the a-axis showing the p/p stacking
arrangement of the cationic L units almost diagonal to the bc-plane with
the hexafluorosilicate anions situated between the adjacent cationic
arrays.
Si(1)F62ꢁ are bifurcated (N–H)+/F hydrogen bonded to fluorine
a 17 point contacts whereas SiF6 involving Si(2) is situated in
2ꢁ
ꢀ
atoms F1, F2 and F4, F6, respectively, (N1/F1 ¼ 3.118 A,
a symmetrical environment with a 16 point attachment provided
by four LH+ units in each case (Fig. 6b). The cationic receptor
molecules are self-assembled via multiple C–H/Onitro hydrogen
bonds in all six ionic complexes and p/p stacking interactions
in complexes 1, 2, 5 and 6 generating 3D supramolecular
networks. Intermolecular C–H/p interaction has been observed
only in complex 2. Thus, it can be rationalized that, on proton-
ation of receptor L in presence of anions of varied dimensionality
there is a remarkable change in the orientation of tripodal arms
due to the torsional differences (Table S2, ESI)† and overall
packing as well probably because of the variable coordination
ꢀ
ꢀ
ꢀ
ꢀ
2ꢁ
ꢀ
<N1–H/F1 ¼ 135 ; N1/F2 ¼ 2.742 A, < N1–H/ F2 ¼ 159 ;
ꢀ
N9/F4 ¼ 3.217 A, <N9–H/F4 ¼ 125 ; N9/F6 ¼ 2.693 A,
< N9–H/ F6 ¼ 166ꢀ). In addition, all six F atoms of Si(1)F6
are involved in moderate C–H/F hydrogen bonding with
different alkyl and aryl hydrogen of the encircling LH+ units. F1
is involved in C–H/F interaction with the methylene hydrogen
H1A and H57B of two surrounding ligand moieties and F2 is
hydrogen bonded to H25A. F3 is engaged in bifurcated acceptor
hydrogen bonding with the methylene hydrogen H1B and aryl
hydrogen H44 while F4 forms five C–H/F contacts with the
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modes adopted by different anions through weak C–H/anion
hydrogen bonds to provide a favourable electrostatic environ-
ment around themselves. Moreover, multiple interligand C–H/
Onitro interactions provide further stabilization to the supramo-
lecular complexes 1–6 (Table S1, ESI).†
Finally, cationic tripodal receptors bearing electron withdrawing
substituents appear to be well-suited for fundamental studies of
C–H/anion hydrogen bonding, with the present study clearly
showing the importance of aliphatic as well as aromatic C–H
bond donors in binding of anions when these donors are pre-
organized involving multiple receptor units. Therefore, due to
the interesting structural and binding properties, the tripodal
podand receptor L can provide an excellent case of under-
standing C–H/anion hydrogen bonding in its protonated form.
A systematic survey of complexation and selective recognition of
anions in systems of this type, which involves crystal structure
studies of several substituted tripodal receptor molecules in terms
of careful modelling studies and charge density analyses are
currently being pursued to get further insight into the nature of
their binding modes.
To investigate the solution-state binding of different anions
with the cationic receptor molecule, we have protonated L with
p-toluenesulfonic acid.24 The addition of tetrabutylammonium
ꢁ
(TBA) salts of anions (Brꢁ, NO3 and ClO4ꢁ) separately to
[HL+]$[OTs] in DMSO-d6 showed a downfield chemical shift of
the aromatic C–H resonances (Dd ¼ 0.02–0.03 ppm), which
indicates participation of the receptor in anion binding via weak
hydrogen-bonding interactions of C–H protons (see ESI).†
However, marginal spectral changes have been observed for the
aliphatic C–H resonances. Considerable downfield shift of the
1
aliphatic CH2 protons (Dd ¼ 0.30–0.73 ppm) in the H-NMR
spectra of anion complexes 1–6 indicate the influence of the
protonation at the apical nitrogen on the neighbouring methyl-
ene protons (Fig. S11–S16, ESI).† Since most of the aliphatic
CH/anion contacts are formed with the H atoms on carbons
subsequent to the ammonium ion, it can be argued that these H
atoms are simply in the way due to the close approach of anions
to the positively charged N atom by forming electrostatic (N–
H)+/anion interaction. The other possibility is that protonation
at the apical nitrogen render the methylene CH2 groups suffi-
ciently acidic for their active participation towards anion binding
via weak CH/anion interactions with an average C–H hydrogen
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.
Notes and references
1 (a) A. Bianchi, K. Bowman-James and E. Garcia-Espana,
Supramolecular Chemistry of Anions, Wiley-VCH, New York, 1997;
(b) B. H. M. Snellink-Ruel, M. M. G. Antonisse, J. F. J. Engbersen,
P. Timmerman and D. N. Reinhoudt, Eur. J. Org. Chem., 2000, 165.
2 (a) M. D. Best, S. L. Tobey and E. V. Anslyn, Coord. Chem. Rev.,
2003, 240, 3; (b) V. McKee, J. Nelson and R. M. Town, Chem. Soc.
Rev., 2003, 32, 309; (c) J. L. Sessler, S. Camiolo and P. A. Gale,
Coord. Chem. Rev., 2003, 240, 17; (d) C.-H. Lee, H.-K. Na,
D.-W. Yoon, D.-H. Won, W.-S. Cho, V. M. Lynch, S. V. Shevchuk
and J. L. Sessler, J. Am. Chem. Soc., 2003, 125, 7301; (e)
K. J. Wallace, W. J. Belcher, D. R. Turner, K. F. Syed and
J. W. Steed, J. Am. Chem. Soc., 2003, 125, 9699.
3 (a) P. D. Beer and P. A. Gale, Angew. Chem., Int. Ed., 2001, 40, 486;
(b) J. L. Sessler, P. A. Gale and W. S. Cho, Anion Receptor Chemistry,
The Royal Society of Chemistry, Cambridge, UK, 2006.
4 (a) C. R. Bondy, P. A. Gale and S. J. Loeb, J. Am. Chem. Soc., 2004,
126, 5030; (b) D. R. Turner, E. C. Spencer, J. A. K. Howard,
D. A. Tocher and J. W. Steed, Chem. Commun., 2004, 1352; (c)
S. O. Kang, M. A. Hossain, D. Powell and K. Bowman-James,
Chem. Commun., 2005, 328.
ꢀ
bond distance of 3.436 A. The feeble nature of C–H hydrogen
bonds is also reflected from the marginal chemical shift of C–H
proton resonances in 1H NMR spectra and an average aliphatic
C–H/anion contact angle of 141ꢀ range from 109 to 162ꢀ.
Though charge neutralisation in the crystals and conventional
hydrogen bonds are the main driving forces in the formation of
supramolecular complexes, yet the weak CH hydrogen bonds
provide added stabilization to the complexes and thus, satisfies
the geometrical necessity of the LH+ units by providing
a favourable electrostatic environment around the anions. Thus,
the importance and authenticity of these short CH/anion
contacts can hardly be ignored.
5 K. Bowman-James, Acc. Chem. Res., 2005, 38, 671.
Conclusions
6 (a) H.-J. Schneider and A. K. Yatsimirsky, Chem. Soc. Rev., 2008, 37,
263; (b) E. Garcia-Espana, P. Diaz, J. M. Llinares and A. Bianchi,
Coord. Chem. Rev., 2006, 250, 2952; (c) S. O. Kang,
Md. A. Hossain and K. Bowman-James, Coord. Chem. Rev., 2006,
250, 3038; (d) J. M. Llinares, D. Powell and K. Bowman-James,
Coord. Chem. Rev., 2003, 240, 57.
7 (a) P. S. Lakshminarayanan, I. Ravikumar, E. Suresh and P. Ghosh,
Chem. Commun., 2007, 5214; (b) P. S. Lakshminarayanan, E. Suresh
and P. Ghosh, Inorg. Chem., 2006, 45, 4372.
In summary, we have shown the solid state evidence for the active
participation of both aliphatic CH2 and aryl CH groups in
binding of different anions with the protonated tripodal receptor
L, exhibiting aliphatic C–H/anion hydrogen bond strengths
comparable to those of aryl C–H/anion hydrogen bond. They
are also actively involved in the self assembly of cationic tripodal
moieties by forming C–H/Onitro hydrogen bonds with the
oxygen atoms of terminal nitro groups. Structural studies of
anion binding with protonated L revealed that none of the guests
is encapsulated inside the tripodal arms irrespective of size,
shape, and charge of the anions presumably because of the
absence of conventional anion hydrogen bonding functionalities.
However, detailed structural investigation clearly demonstrates
that the self-alignment and orientation of the multiple ligand
moieties, depending upon the dimensionality of the incoming
anionic guest play a crucial role in making various molecular
interactions possible in the binding of the various anions.
8 D. R. Turner, M. J. Paterson and J. W. Steed, J. Org. Chem., 2006, 71,
1598.
9 (a) S. Metzger and B. Lippert, J. Am. Chem. Soc., 1996, 118, 12467;
(b) P. Auffinger, S. Louise-May and E. Westof, J. Am. Chem. Soc.,
1996, 118, 1181; (c) G. R. Desiraju, Acc. Chem. Res., 1991, 24, 290;
(d) T. Steiner and W. Saenger, J. Am. Chem. Soc., 1992, 114, 10146;
(e) C. V. K. Sharma and G. R. Desiraju, J. Chem. Soc. Perkin
Trans., 1994, 2345; (f) T. Steiner, J. Chem. Soc., Perkin Trans.,
1995, 1315; (g) J. D. Chaney, C. R. Goss, K. Folting,
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