Anion complexation with cyanobenzoyl substituted first and second generation tripodal amide receptors: Crystal structure and solution studies

Article abstract of DOI:10.1039/c5dt00369e

Anion complexation properties of two new tripodal amide receptors have been extensively studied here. Two tripodal receptors have been synthesized from the reaction of cyanobenzoyl acid chloride with two tri-amine building blocks such as (i) tris(2-aminoethyl)amine and (ii) tris(2-(4-aminophenoxy)ethyl)amine, which resulted in the first (L₁) and second (L₂) generation tripodal amides respectively. A detailed comparison of their coordination behavior with anions is also described by crystallographic and solution state experiments. The crystal structure demonstrates various types of spatial orientations of tripodal arms in two receptors and concomitantly interacts with anions distinctively. Intramolecular H-bonding between amide N-H and C=O prevents opening of the receptor cavity in the crystal, which leads to a locked conformation of L₁ having C_3v symmetry and makes amide hydrogen unavailable for the anion which results in side cleft anion binding. However, in L₂ we conveniently shift the anion binding sites to a distant position which increases cavity size as well as rules out any intramolecular H-bonding between amide N-H and C=O. The crystal structure shows a different orientation of the arms in L₂; it adopts a quasi-planar arrangement with C_2v symmetry. In the crystal structure two arms are pointed in the same direction and while extending the contact the third arm is H-bonded with the apical N-atom through a -CN group, making a pseudo capsular cavity where the anion interacts. Most importantly spatial reorientation of the receptor L₂ from a C_2v symmetry to a folded conformation with a C_3v symmetry was observed only in the presence of an octahedral SiF₆^2- anion and forms a sandwich type complex. Receptors L₁ and L₂ are explored for their solution state anion binding abilities. The substantial changes in chemical shifts were observed for the amide (-NH) and aromatic hydrogen (-CH) (especially for F^-), indicating the role of these hydrogens in anion binding. The anion interacts with receptor L₂ more strongly than L₁ as confirmed by ¹H NMR titration upon monitoring the -NH signal.

Full text of DOI:10.1039/c5dt00369e

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Anion complexation with cyanobenzoyl substituted
first and second generation tripodal amide
Cite this: DOI: 10.1039/c5dt00369e
receptors: crystal structure and solution studies†
Md. Najbul Hoque, Abhijit Gogoi and Gopal Das*
Anion complexation properties of two new tripodal amide receptors have been extensively studied here.
Two tripodal receptors have been synthesized from the reaction of cyanobenzoyl acid chloride with two
tri-amine building blocks such as (i) tris(2-aminoethyl)amine and (ii) tris(2-(4-aminophenoxy)ethyl)amine,
which resulted in the first (L1) and second (L2) generation tripodal amides respectively. A detailed com-
parison of their coordination behavior with anions is also described by crystallographic and solution state
experiments. The crystal structure demonstrates various types of spatial orientations of tripodal arms in
two receptors and concomitantly interacts with anions distinctively. Intramolecular H-bonding between
amide N–H and CvO prevents opening of the receptor cavity in the crystal, which leads to a locked con-
formation of L1 having C3v symmetry and makes amide hydrogen unavailable for the anion which results
in side cleft anion binding. However, in L2 we conveniently shift the anion binding sites to a distant posi-
tion which increases cavity size as well as rules out any intramolecular H-bonding between amide N–H
and CvO. The crystal structure shows a different orientation of the arms in L2; it adopts a quasi-planar
arrangement with C2v symmetry. In the crystal structure two arms are pointed in the same direction and
while extending the contact the third arm is H-bonded with the apical N-atom through a –CN group,
making a pseudo capsular cavity where the anion interacts. Most importantly spatial reorientation of the
receptor L2 from a C2v symmetry to a folded conformation with a C3v symmetry was observed only in the
2−
presence of an octahedral SiF6 anion and forms a sandwich type complex. Receptors L1 and L2 are
Received 27th January 2015,
Accepted 23rd April 2015
explored for their solution state anion binding abilities. The substantial changes in chemical shifts were
observed for the amide (–NH) and aromatic hydrogen (–CH) (especially for F−), indicating the role of
these hydrogens in anion binding. The anion interacts with receptor L2 more strongly than L1 as
confirmed by 1H NMR titration upon monitoring the –NH signal.
DOI: 10.1039/c5dt00369e
www.rsc.org/dalton
recognition.15–19 Since the beginning of supramolecular chem-
istry, tris(2-aminoethyl)-amine (tren) has been effectively used
Introduction
Anions are vital species and have widespread use in environ-
mental and biological processes and molecular recognition
studies.1–6 Various artificial systems have been reported to
achieve selective molecular recognition and sensing of anions,
and these systems provide a fascinating control in the for-
mation of supramolecular assemblies.7–9 Recognition of
anionic species is accomplished by electrostatic and
H-bonding interactions.10–14 In addition recent advances also
show that chemical reactions are also very promising for anion
as a primary building block along with amine, amide and urea
functions for recognition of a variety of anions with a high level
of satisfaction in potential application like transport, separation
and extraction of anions.20 Nature often recognizes anions func-
tionalized with amide as H-bond donors. Therefore, a variety of
molecular receptors have been developed containing an amide
platform that performs recognition of anionic targets by estab-
lishing complementary H-bonds.21–23 Few amide based recep-
tors containing tren as the building block recognize anions
through a side cleft or capsular binding.24 Anion complexation
and anion induced supramolecular assembly are more challen-
ging as H-bonds formed by anions are weaker and are strongly
influenced by external stimuli like pH, nature of the guest and
solvent.25 When anions are an inevitable part of supramolecu-
lar aggregates, the assembly and disassembly processes can be
explored nicely by proper tuning. So it was anticipated that if
Department of Chemistry, Indian Institute of Technology Guwahati, Assam 781039,
India. E-mail: gdas@iitg.ernet.in; Fax: +91-361-2582349; Tel: +91-361-2582313
†Electronic supplementary information (ESI) available: Synthetic scheme of L1
and L2, X-ray crystal structures for all complexes, TGA, IR, NMR and mass
spectra of the complexes and NMR titration plots. CCDC 1041356–1041365. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt00369e
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the anion is exchanged 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 to form a completely new assembled system.
A very promising result reported by P. Ghosh et al. shows
the orientation of all six arms of a benzene capped hexa-amide
in the presence of fluoride forming a dimeric capsular
complex.26 The most astonishing fact is that in the presence of
an acetate ion the conformation of the receptor changed com-
pletely and resulted in a non-capsular assembly. Most recently,
we have reported the anion induced conformational changes
of a cresol-based tripodal receptor.27 Our research at this stage
on knowing more about the fascinating structure directing role
of anions is in its infancy. In continuation of our work on
solid state anion recognition,28–40 here we have synthesized
cyanobenzoyl substituted flexible tripodal amides which
include first generation receptor (L1) where tren is used as a
simple building block and elongated second generation tri-
podal (L2) where the tri-amine of a tripodal is used as a building
block. Two receptors having characteristic length and cavity, ele-
gantly exploited for anion complexation and anion binding
studies were also used in solid and solution states. We also
proved that the dimensionality and nature of the anions play a
crucial role in making various molecular interactions possible
in the complexation of various anions in both receptors. Most
importantly we have noticed that the self-alignment and orien-
tation of the flexible second generation tripodal are greatly
influenced by the size and shape of the anion. The X-ray struc-
ture shows that L1 is internally locked and could not welcome
anions into its C3v symmetric cavity, showing a side cleft anion
binding. Positioning of amide functionality to a distant position
with respect to the apical N-atom results in a bigger cavity and
subsequently intimidates the N–H⋯OvC intramolecular
H-bond unlike in L1. The receptor forms a quasi-planar arrange-
ment of the arms, with two arms close to each other while the
third arm points to the opposite direction giving a pseudo cap-
sular complex during H-bonding interactions. Finally we
compare the recognition of anionic guests of different shapes/
geometry and solid state organization of the two kinds of recep-
tors of varying podal lengths. Interestingly anion induced reor-
ientation of the receptor L2 was observed during recognition of
Scheme 1 Molecular structures of the first and second generation tri-
podal amide receptors L1 and L2 respectively.
and their anion complexation have been studied systematically
(Scheme 1). Our rationale for designing the receptors contain-
ing the cyano group is as follows. The strength of an aromatic
C–H donor (effective anion binding group) can be enhanced
through the addition of different functional groups on the
parent phenyl ring. It has been well established that the elec-
tron-withdrawing substituent on the benzene ring assists the
active participation of the aromatic –CH protons toward anion
binding via C–H⋯anion interactions. The –CN group is known
to act as a strong electron withdrawing substituent and there-
fore we have introduced cyano-substituted phenyl terminals to
improve the anion binding affinities. The amide based tripodal
receptors rarely form a capsular complex. Being inspired by
our previous report of the capsular assembly of tripodal
amine30,31 and versatile anion coordination of its elongated
derivatives,32,33 herein we have attempted to obtain a capsular
complex through various modifications like (i) changing the
substituents (ii) creating a larger cavity suitable for guest
inclusion by elongating the receptor’s arm. This is our second
report on tripodal amides (L2) in this category, and given our
interest in main group anions, we decided to carry out further
examinations to achieve the goal with this receptor. In
addition we had the opportunity to examine the effect of arm
length on anion coordination. With the deliberate incorpor-
ation of a phenyl subunit, L2 is capable of creating a specific
spatial arrangement of arms compared to L1 which would sig-
nificantly alter the anion binding fashion. Moreover anion
induced dramatic folding and unfolding of the elongated
receptor L2 is also an important outcome of the study.
2−
the octahedral SiF6 anion. In the solid state the –CN group
provides further stabilization to the supramolecular complexes
through C–H⋯π and anion⋯π interactions. Moreover the solu-
tion state interaction phenomena of two receptors in the neutral
form with the anions of various shapes and sizes like spherical
(F−, Br−, I−), planar (NO3−), tetrahedral (ClO4−) and octahedral
1
(SiF62−) by detailed H NMR studies along with their molecular
Crystal structure studies
binding were thoroughly analyzed.
Anion complexation studies in the solid state with first and
second generation tripodal amides
First generation tripodal amide L1 and its anion complexation.
The receptor L1 crystallizes in a monoclinic space group
C2/c (Table S1†). The receptor has a C3v symmetry and the
Results and discussion
The sequential synthesis of 4-cyanobenzoyl substituted first carbonyl group has a tendency to stay outwards from the cavity.
and second generation tripodal amide receptors (L1 and L2) Among three amide oxygens, O3 of one arm acts as an acceptor
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and is involved in strong intramolecular H-bonding interaction amide CvO, which is now pointed inwards in the C3v cavity of
with the donor amide hydrogen H2N which possibly holds the the receptor. The endo oriented amide CvO also forms a
arms together, while amide oxygen O1 is H-bonded with strong intramolecular H-bonding with amide hydrogen H4N
amide hydrogen H3N and ortho aromatic hydrogen H15 in a (N4⋯O1 = 2.878(3) Å). The other two exo-oriented O2 and O3
fashion which leads to the formation of a centrosymmetric interact with phenyl hydrogen H17 and H15, H3N of the next
dimer (Fig. S1b†). The amide oxygen O2 interacts with H4N of receptors (Fig. S2a†). Though two amide hydrogens are
the neighboring receptor which along with C–H⋯π inter- pointed towards the C3v symmetric cavity, the Br− ion could
actions finally resulted in a 1-D assembly of the receptor where not be encapsulated due to the strong intramolecular H-bonds
they are arranged in an up and down fashion (Fig. S1c†). All which did not allow the receptor to open to welcome the
three amide oxygens form bifurcated H-bonds either with NH bromide ion. Thus, it remains outside the cavity and is sur-
or aromatic CH. The average N–H⋯O and C–H⋯O distances rounded by three L1H+ receptors (Fig. 1b) through one N–
are 2.976 Å and 3.367 Å, which are fairly comparable to pre- H⋯Br− interaction and two C–H⋯Br− interactions. The tri-co-
viously reported values.24 Very interestingly two –CN groups ordinated Br− ion lies outside of the hydrogen bond donor
(N5 and N7) are involved in a Car–H⋯N–H-bond with an aro- plane H2N, H1A and H21B. Packing along the b-axis depicted
matic hydrogen as well as Car–H⋯π by virtue of π-cloud of the the formation of layers which are connected through –CN and
–CN group (Fig. S1d†).
aromatic CH hydrogen (Fig. S2b†). The presence of the Br− ion
The tripodal receptor L1 is enriched with H-bond function- inevitably disrupts the formation of dimeric interactions
alities which enable us to study anion coordination with between receptors as observed in the case of a free receptor. As
anionic guests of different sizes and geometries. Anion com- a course of anion–receptor interaction, we have also isolated
plexation studies under acidic conditions with a protonated the isostructural iodide complex [HL1·I] (3) of the receptor.
receptor are discussed here. With a spherical Br− ion, we The bigger iodide anion behaves very similarly to the bromide
prepared complex [HL1·Br] (2). The complex 2 crystallizes in ion. The crystal structure analysis shows that the inter and
ˉ
the triclinic space group P1. The protonated apical N-atom intramolecular interaction patterns are exactly the same as that
forms a strong H-bond (N1⋯O1 = 2.750(3) Å) with one of of complex 2. The conformation of the protonated receptor
Fig. 1 (a) Conformationally locked orientation of L1. (b) Close-up view of H-bonding interactions with bromide/iodide ions in complex [L1H·Br] (2)
−
and [HL1·I] (3). (c) Simplified sketch of the seven coordinated NO3 ions in complex [(L1H)2·2NO3·H2O] (4). (d) Illustration of H-bonding around the
ClO4− anion in complex [L1H·ClO4] (5) with five encircling L1H+ receptors.
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and coordination environment around an I− ion are also was noticed in the anion coordination mode as expected. In
−
similar to complex 2. The tri-coordinated geometry of the halide complex 4, a seven coordinated NO3 anion is surrounded by
ion is observed with a previously reported tripodal triamide.11 two cationic receptors and water molecules via one N⋯H–O,
The detailed contact distances and angles are given in Table 1.
two Ow⋯H–O and four C⋯H–O contacts (Fig. 1c). Detailed
The interaction of the receptor with oxyanion like planar inspection reveals that O4 and O6 accept two and one methyl-
(nitrate) and tetrahedral (perchlorate) complexes were also ene hydrogen H21A, H22B and H11B respectively. O5 of the
−
studied. Both the nitrate [(L1H)2·2NO3·H2O] (4) and perchlor- NO3 ion makes four contacts via one amide hydrogen H4N,
ate [L1H·ClO4] (5) complexes crystallize in a triclinic system. one ortho H29 and two water O7w molecules. Notably, the crys-
The conformation of the receptor in these cases is similar and talline water molecule acts as a bridging H-bond donor
−
the interaction pattern of amide NH and CvO is also retained between two NO3 ions thus forming a discrete nitrate–water
like the halide complexes. Therefore the protonated receptor cluster [(NO3−)2–(H2O)2]4− (Fig. S3a†). The anion–water con-
interacts with nitrate and perchlorate anions in these complexes tacts in the cluster are O7w⋯O5 = 2.770(2) and 2.880(2) Å. The
in a side cleft binding fashion. However, an obvious difference cluster is surrounded by six L1H+ and is completely encapsu-
lated in the cavity made of these receptors. The other short
contacts originating from the –CN group appear to be similar
to those of the other complexes. The structural aspect of the
Table 1 Non-covalent contacts in L1 and its anion complexes
perchlorate complex 5 is similar to that of the other complexes.
−
The ClO4 ion is surrounded by total five cationic tripodal
D–H⋯A
d(D⋯A)/Å
d(H⋯A)/Å
∠(D–H–A)/°
receptors and is coordinated by ten intermolecular H-bonds
−
L1
(Fig. 1d). The ClO4 ion forms one amide N⋯H–O, three aro-
N2–H2N⋯O3
N3–H3N⋯O1
N4–H4N⋯O2
C15–H15⋯O1
C28–H28⋯O3
C25–H25⋯O2
3.00(2)
3.00(2)
3.012(2)
3.200(2)
3.346(2)
3.557(2)
2.14(1)
2.12(1)
2.23(1)
2.276(1)
2.668(1)
2.707(1)
160.0(9)
165.0(1)
151.0(9)
173.0(1)
130.3(1)
152.3(1)
matic C⋯H–O and six methylene C⋯H–O contacts reaching a
total of ten H-bonds. The oxygen O4 behaves as a trifurcated
H-bond acceptor from H4N amide NH (N4) and two methylene
H21A and H22A. O5 atoms are involved in H-bonding to
one aromatic H26 and two methylene hydrogens H11B
and H22b. The oxygen O6 offers trifurcated H-bonds and
oxygen is connected with one aromatic H16 and two methyl-
ene H1A and H11A. O7 is single point H-bonded to aromatic
H25. The details of the bond distance and angle are given in
Table 1.
[L1H·Br] (2)
N1–H1N⋯O1
N2–H2N⋯Br1
N3–H3N⋯O3
N4–H4N⋯O1
C1–H1A⋯Br1
C21–H21B⋯Br1
2.750(3)
3.343(3)
2.881(3)
2.878(3)
3.895(3)
3.711(2)
2.00(2)
2.51(3)
2.09(2)
2.10(3)
3.00(4)
3.00(4)
141.0(2)
158.0(2)
159.0(2)
156.0(3)
155.0(2)
142.0(1)
Second generation tripodal amide L2 and its anion complexa-
tion. The first generation tripodal amide could not encapsu-
late the anions inside its C3v symmetric cavity, possibly
because the intramolecular H-bond tightly holds the arms,
which resists opening of the cavity to encapsulate the anions.
Concomitantly the conformation became very rigid and un-
suitable for guest encapsulation. Keeping in mind these poss-
ible drawbacks which were a great setback in the case of L1, we
decided to enlarge the length of the arm because of which the
anion binding site shifted to a distant position surely ruling
out the close contact between amide NH and CO and clearly
generating a big space. This design ruled out the possibilities
of intramolecular H-bond formation between amide NH and
CO and helps to open up the structure to accommodate
anionic guests. The real inspiration was the success of anion
encapsulated second generation tripodal amide and urea
receptors from our group.32,40 We extended our study to evalu-
ate substitution and cavity effects for anion recognition and
synthesized several salts of L2 with anions of various dimen-
sionalities like spherical, planar, tetrahedral and octahedral.
Crystallization of the receptor L2 in a neutral medium was
unsuccessful, unlike L1.
[L1H·I] (3)
N1–H1N⋯O1
N2–H2N⋯I1
N3–H3N⋯O3
N4–H4N⋯O1
C1–H1A⋯I1
C21–H21A⋯I1
2.739(8)
3.569(6)
2.858(7)
2.863(7)
4.069(7)
3.886(6)
2.00(8)
2.78(6)
2.08(5)
2.06(5)
3.16(6)
3.05(5)
141.0(2)
158.0(2)
159.0(2)
156.0(3)
157.0(4)
145.0(4)
[(L1H)2·2NO3·H2O] (4)
N1–H1N⋯O3
N2–H2N⋯O2
N3–H3N⋯O3
C21–H21A⋯O4
C22–H22B⋯O4
O7w⋯O5
2.73(4)
2.89(4)
2.88(4)
3.159(7)
3.241(8)
2.880(2)
2.770(2)
3.080(8)
3.255(9)
2.693(7)
1.770(4)
2.110(4)
2.17(3)
2.433(5)
2.276(7)
—
139.0(3)
154.0(3)
149.0(3)
131.3(3)
172.7(3)
—
O7w⋯O5
—
—
N4–H4N⋯O5
C29–H29⋯O5
C11–H11B⋯O6
2.220(4)
2.378(8)
3.637(7)
158.0(4)
157.0(3)
164.6(2)
[L1H·ClO4] (5)
N1–H1N⋯O3
N2–H2N⋯O2
N3–H3N⋯O3
N4–H4N⋯O4
C21–H21A⋯O4
C11–H11B⋯O5
C22–H22B⋯O5
C26–H26⋯O5
C1–H1A⋯O6
C11–H11A⋯O6
C16–H16⋯O6
C25–H25⋯O7
2.73(4)
2.88(4)
2.87(4)
3.04(7)
3.537(9)
3.438(8)
3.369(7)
3.507(8)
3.478(8)
3.515(7)
3.534(6)
3.361(1)
1.980(4)
2.10(3)
2.08(2)
147.0(3)
151.0(2)
152.0(2)
137.0(2)
166.0(3)
158.6(3)
160.4(3)
164.5(3)
141.0(3)
150.5(3)
165.0(3)
162.7(3)
2.35(7)
2.588(7)
2.516(7)
2.439(6)
2.603(7)
2.671(7)
2.639(6)
2.627(5)
2.458(9)
The bifluoride complex [4LH·4HF2·3H2O] (6) was obtained
from reaction of L2 with HF in a plastic vial. The crystal struc-
ture shows a significant change in the conformation of the recep-
tor in contrast to first generation tripodal L1, where one arm of
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the protonated tripodal receptor is directed opposite to the assembly with the help of the cyano group through formation
other two arms (Fig. S4a†). As expected the anion coordination of C–H⋯N and C–H⋯π contacts is shown in Fig. S4c.† An
and the other solid state interaction pattern changed dramati- attempt was made to prepare other halide complexes.
cally compared to L1. The apical N-atom is H-bonded with the However, due to poor crystal quality and corresponding data,
ethereal oxygen atom (O2 and O3) of the two closely connected chloride and iodide complexes cannot be reported, but the
arms. Very interestingly the arm which is directed opposite to structure of the bromide complex [L2H·Br] (7) could be solved
the other two arms is strongly H-bonded with apical NH+ via with satisfaction. Though roughly we could see from these
nitrogen of the –CN group forming a pseudo C3v symmetric structures conformation of the receptor remains the same as
cavity (Fig. S4b†). Very interestingly we have observed the for- in complex 6, the only difference is that they contain solvent
−
mation of HF2 species on treatment with HF acid. Detailed water or DMF molecules as the crystallizing solvent. Very inter-
−
structural analysis shows that HF2 resides around the pseudo estingly we noticed that chloride, bromide and iodide com-
cavity; the F1 ion is strongly H-bonded to one amide hydrogen plexes are isostructural though complex 6 does not belong to
H2 and three aromatic hydrogens H5, H11 and H29 as this category. A sharp difference in the orientation of one
depicted in Fig. 2a. There is no longer a intramolecular H-bond amide hydrogen H3N of the two closely connected arms was
between the amide hydrogen and carbonyl as per our assump- observed compared to complex 6 and it remains outside of the
tion and between the two closely connected arms there was so pseudo cavity (Fig. S5a†). As a consequence solid state inter-
much space that a few water molecules had to come and fill actions changed noticeably. In case of the bromide complex
up the pseudo cavity through various interactions. The other we could not assign the solvent molecules, hence PLATON/
−
F2 of HF2 also encapsulated in the pseudo cavity and SQUEEZE was performed to refine the receptor along with the
H-bonded with amide hydrogen H7A and two aromatic hydro- bromide ion (1.5 Br ion) by excluding the disordered solvent
gens H11 and H46. Stabilization of such a kind of bifluoride electron densities. This calculated amount of 554 electrons per
in the amide based macrocyclic receptor is also reported by unit cell or 98 electrons per molecule may be attributed to the
Bowman-James and coworkers.41 Moreover, a close inspection two water and the two DMF molecules, which is also supported
of the H-bond interaction between carbonyl and CH resulted by thermogravimetric analysis (TGA) (Fig. S10†) of the crystals.
in a dimeric like structure of the receptor where O4 acts as an The distant arm is H-bonded with the protonated apical N-atom
acceptor from H34A and H36. The progress of the 1D dimeric through the cyano group forming a pseudocavity similar to
Fig. 2 (a) Crystal structure of complex [4LH·4HF2·3H2O] (6) depicting the interaction of amide hydrogen with HF2− and formation of a pseudo cap-
sular cavity. (b) Illustration of various H-bonding interactions around bromide in complex [L2H·Br] (7). (c) Illustration of various H-bonding inter-
−
−
actions around NO3 in complex [L2H·NO3·H2O] (8). (d) Partial view of the eight coordinated ClO4 ions in complex [L2H·ClO4] (9). The red contact
implies anion⋯π interaction (π electron of –CN).
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complex 6. Interaction of Br1 in Fig. 2a shows it is tetra co- are H-bonded with six arms of five tripodal receptors and four
ordinated and all four donor atoms lie on a perfect square geo- arms of three tripodal receptors respectively (Fig. 2d and
−
−
metry. Br1 accepts hydrogen from two symmetric amide H3N Fig. S7c†). The Cl(1)O4 and Cl(2)O4 ions show eight and
and methylene H1A. The other half occupied a Br2 ion is seven coordination respectively. Perchlorate oxygen O8 acts as
H-bonded with amide hydrogen H2N and ortho aromatic a bifurcated H-bond acceptor with amide and aromatic hydro-
hydrogen H16. The contacts around the bromide ion fall in gen H3N and H94 including one interesting anion⋯π inter-
−
the range of 3.471(2)–3.808(2) Å. The contact details are given action between Cl(1)O4 and π cloud of the electron rich –CN
in Table 3. The close contact between receptors through cyano group, whereas O9 behaves as a bifurcated H-bond acceptor by
N7 and methylene hydrogen H33A which connects two layers interacting with two aromatic protons H31 and H15. O10
of pseudo capsular assembly is depicted in Fig. S5b.† The con- behaves as a trifurcated H-bond by accepting amide and aro-
tacts around the bromide ion fall in the range of 3.471(2)– matic hydrogens H2N and H12, H15 finally satisfying eight
3.808(2) Å.
coordination. The detailed contact parameters are given in
Further receptor–anion interaction was also investigated for Table 1. The overall non-covalent interactions result in the for-
−
a planar oxyanion i.e. the NO3 ion by analyzing the crystal mation assembly of receptors stitching the adjacent arrays by
structure of the complex [L2H·NO3·H2O] (8). The conformation C–H⋯O/N (carbonyl and –CN group) H-bonds.
of the receptor is similar to complex 6. Structural analysis of
complex 8 revealed that the distant arm is responsible for the generation tripodal amide. The elongated second generation
formation of pseudo capsular cavity; however, unlike tripodal receptor L2 shows a dramatic conformational change
Anion specific conformational adaptability of the second
a
2−
bromide complex 7 amide hydrogen of two closely connected in the presence of only octahedral SiF6 anion in the hexa-
arms is pointed towards the cavity. In this case NO3 is eleven fluorosilicate complex [(L2H)2·SiF6·4H2O·2DMF] (10). The X-ray
−
coordinated arising from two amide N–H⋯O, four aromatic C– structure indicates that the asymmetric unit contains one
H⋯O, one water Ow–H⋯O and one anion⋯π (π cloud of –CN). mono protonated L2H+, one-half hexafluoro silicate anion, two
The nitrate oxygen O6 is involved in a strong H-bond with H4N water molecules and one DMF molecule as the crystallizing
and O9w, and moderate C–H⋯O interaction with the aromatic solvent. The intramolecular H-bonding interactions are
hydrogen H27 and H43 whereas O8 is in interaction with the different than those observed in other complexes of the L2
amide H3N and aromatic H30, H37 and H43 suggesting that receptor. In this case, the endo orientated hydrogen is intern-
each O6 and O8 behaves as a tetrafurcated H-bond acceptor in ally bonded with three ethereal oxygen atoms with an average
the anion complex. In addition to the above eight interactions, N⋯O distance of 2.70 Å. These H-bonding interactions are
nitrate oxygen O7 is engaged in H-bonding interaction with responsible for changes in conformation of the receptor from
amide H4N and the aromatic hydrogen H30 including interest- open to close orientation (Fig. 3a). This structure is very
ing anion⋯π interaction coming from the π enriched –CN similar to our previously reported nitro-phenyl appended
group (N5). The nitrate–water cluster [(NO3−)2–H2O]2− is nearly second generation tripodal amide,33 where we had shown a
2−
planar and confined within the void created by the receptors solvent induced binding discrepancy of the SiF6 anion. In
(Fig. S6b†). The water O9w accepts two amide hydrogen H2N another communication we had structurally authenticated the
giving a tetracoordinated nearly planar geometry. The other conformational changes in a cresol-based tripodal receptor,
water molecule O9Aw shows a similar geometry by interacting based on anion specificity.27 The C2v symmetric tripodal
with methylene hydrogen H2B and carbonyl oxygen O3. One amide changes to a completely folded conformation (C3v) in
2−
carbonyl oxygen O3 of the folded tripodal arm is H-bonded the presence of only SiF6 anion and stabilizes the anion
with aromatic and methylene hydrogen H4 and H2A to form a through formation of a sandwich type complex in a cleft
dimeric interaction between two receptor-like complex 6. The binding mode (Fig. 3b) mainly by N–H⋯F, Ow–H⋯F and C–
dimer is self-organized via C–H⋯O/N (carbonyl and cyano) H⋯F type H-bonds (Fig. 3c). Though all three arms are in the
H-bonds between different aromatic and methylene hydrogen same direction, the protonated receptor is not able to create a
atoms.
suitable cavity to encapsulate the anion. Only two arms from
The anion complex of the receptor with a tetrahedral oxy- each receptor interact with SiF62−, while the third arm forms
anion like perchlorate was also prepared. In this case there H-bonds with the oxygen atom of a carbonyl group of the next
2−
exists a certain amount of DMF–water solvent trapped in the side arm of the same receptor. The binding of SiF6 clearly
crystal lattice in a disordered manner. These electron densities demonstrates that the anion in complex 10 is located outside
were moved during refinement and the count of electrons the tripodal cavity and is stabilized by mainly N–H⋯F and C–
removed in total was 226 per two symmetry independent mole- H⋯F H-bonds from the four receptors with addition of Ow–
cules which account for five DMF and three water molecules H⋯F from two water molecules as depicted in Fig. 3c. The
and was further verified by the TGA experiment (Fig. S11†) of other three fluorine atoms are symmetry equivalent, having
the crystal of complex 9. A similar mode of conformational similar contact points resulting overall in ten H-bonding con-
2−
adaptability has been found in complex 9. The perchlorate tacts on the SiF6 anion. The short contacts around the –CN
anion occupies the pseudo cavity generated by three arms of group are quite different from those observed in the other
two receptors as shown in Fig. S7a.† The symmetrically non- complexes of L2. None of the –CN groups of the π electron
−
equivalent ClO4 ions involving chlorine atoms Cl(1) and Cl(2) cloud participates in H-bonding unlike the other complexes.
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Fig. 3 (a) C3v symmetric folded conformation of L2H+ induced by SiF62− anion in complex [(L2H)2·SiF6·4H2O·2DMF] (10). (b) Sandwich type complex
formation in the solid state. (c) Coordination environment of the SiF62− anion.
Only one –CN (N7) group participates in H-bonding with aro- density on the aromatic ring through bond propagation induce
matic hydrogen H12, while two are reluctant to form any a shielding effect; secondly, the increase of polarization of the
contacts.
C–H bonds through space creates a positive charge on the aro-
Anion binding studies in solution by H NMR spectroscopy. matic hydrogen caused deshielding.42 Thus, it must be the
The interesting solid state chemistry of the acidic amide ortho-CH (CHa), next to the NH which experiences a through
protons also compel us to investigate their solution phase space polarization effect and moves to the downfield region,
interaction with various guest anions (Table 2). For this, the and as the CH(b) is distant from the amide hydrogen, it experi-
respective solution of anions was added sequentially to the ences an upfield shift due to the through bond charge propa-
DMSO solution of the receptors (DMSO is chosen for the solu- gation associated with the hydrogen bonded complex. The
bility purpose), and the changes in the proton frequencies are strong involvement of the amide NH is reflected in the ppm vs.
recorded at room temperature. The maximum shift is observed equiv. plot (Fig. 6a) which recorded a huge downfield shift of
for the fluoride among the halides (Cl−, Br− and I−) while it is 3.014 ppm after 8 equiv. of fluoride solution. At this stage,
the dihydrogen phosphate which dominates among the oxya- CHa is shifted to 0.43 downfield and CHb to 0.117 upfield.
1
nions like NO3 and ClO4−. Again, between the two amide Another basic anion H2PO4−, however interacts weakly with
−
based receptors, L2 interacts in a more effective way than L1 as the receptor L1 and after 3 equiv. of the aforesaid anion, the
a greater shift of NH hydrogen is observed in L2 (Fig. 4), which NHa is shifted 0.183 ppm downfield. Again, the changes with
is probably due to the presence of more acidic amide NH in the anions such as Cl−, Br−, NO3 etc. are negligible (Fig. S38–
−
this receptor. The amide NH of L1 is shifted downfield upon S40†).
addition of fluoride solution while aromatic CHs are split and
Deprotonation of NH protons is observed for L2 in the pres-
move into opposite directions (Fig. 5a). This could be ence of the fluoride anion. However, as shown in Fig. 5b, all
explained on the basis of two different effects of the hydrogen other aromatic CHs are also moved with escalating the concen-
bonding interactions between the guest anions and the recep- tration of the aforesaid anion solution. The Hb and Hc are
tor on the aromatic CH: first, the increases in the electron shifted downfield (Δppm Hb = 0.028, Δppm Hc = 0.126) while
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a
Table 2 Number and nature coordination of anions in L1 and L2
Anions
L1
L2
F−
Br−
I−
NO3
ClO4
—
5 (one NH, one OH and three CH)
4 (two NH and two CH2)
—
11 (two NH, one OH, four CH and one anion⋯π)
8 and 7 (four NH, seven CH, one CH2 and one anion⋯π)
10 (two NH, two OH two CH and four CH2)
3 (one NH and two CH2)
3 (one NH and two CH2)
7 (one NH, two OH, one CH and three CH2)
10 (one NH, three CH and six CH2)
—
−
−
2−
SiF6
a NH = amide, OH = water, CH = aromatic, CH2 = methylene hydrogen.
Table 3 H-bonding contacts in L2 and its anion complexes
D–H⋯A
d(D⋯A)/Å
d(H⋯A)/Å
∠(D–H–A)/°
[4LH·4HF2·3H2O] (6)
N4–H4N⋯O7
N7–H7A⋯F2
N2–H2⋯F1
3.41(6)
2.79(1)
2.85(7)
3.214(7)
3.222(7)
3.390(1)
2.58(1)
2.00(6)
2.01(4)
2.46(3)
2.44(4)
2.48(4)
163.0(4)
153.0(4)
164.0(3)
138.0(4)
142.0(3)
167.0(5)
C5–H5⋯F1
C11–H11⋯F1
C29–H29⋯F1
[L2H·Br] (7)
N2–H2N⋯Br2
N3–H2N⋯Br1
C1–H1A⋯Br1
C15–H16⋯Br2
3.47(2)
3.50(2)
3.81(2)
3.582(3)
2.62(1)
2.68(1)
2.88(1)
2.982(1)
168.0(1)
159.0(1)
160.0(1)
123.7(2)
[L2H·NO3·H2O] (8)
N4–H4N⋯O6
O9w⋯O6
2.59(3)
3.37(4)
—
152.0(2)
—
2.887(3)
3.630(4)
3.247(4)
2.599(3)
3.01(4)
3.162(4)
2.94(3)
3.495(4)
3.228(4)
3.284(4)
C27–H27⋯O6
C43–H43⋯O6
C30–H30⋯O7
N4–H4N⋯O7
O7⋯π (anion⋯π)
N3–H3N⋯O8
C30–H30⋯O8
C37–H37⋯O8
C43–H43⋯O8
2.716(3)
2.457(3)
3.345(5)
2.37(3)
—
2.11(2)
2.589(2)
2.470(2)
2.504(2)
167.6(2)
142.9(2)
132.0(2)
138.0(2)
—
162.0(2)
165.1(2)
138.8(2)
141.6(2)
[L2H·ClO4] (9)
N3–H3N⋯O8
3.36(9)
2.54(7)
—
159.0(4)
—
O8⋯π (anion⋯π)
C94–H94⋯O8
C15–H15⋯O9
C31–H31⋯O9
N2–H2N⋯O10
C7–H7⋯O10
3.160(1)
3.298(8)
3.360(7)
3.467(8)
3.02(6)
3.398(7)
3.437(7)
3.390(1)
3.01(7)
3.348(8)
3.07(6)
3.319(7)
3.279(7)
3.450(8)
2.458(6)
2.571(4)
2.606(5)
2.18(4)
2.605(4)
2.650(4)
2.470(8)
2.19(6)
2.678(6)
2.24(4)
2.534(4)
2.367(4)
2.498(5)
146.4(4)
143.0(3)
154.2(5)
167.0(3)
143.7(3)
142.8(4)
172.6(4)
160.0(3)
129.6(3)
162.0(3)
142.3(4)
166.4(4)
166.5(3)
C12–H12⋯O10
C75–H75⋯O17
N11–H11N⋯O18
C85–H85⋯O18
N9–H9N⋯O20
C53–H53⋯O20
C59–H59⋯O20
C66–H66B⋯O20
Fig. 4 Partial 1H NMR spectra in DMSO-d6 showing the maximum
observable shifts of amide –NH peak upon the addition of excess (5
equiv.) anions in L1 and L2.
[(L2H)2·SIF6·4H2O·2DMF] (10)
N2–H2N⋯O8w
C17–H17B⋯F1
O8w⋯F1
C27–H27⋯F2
N4–H4N⋯F3
C1–H1B⋯F3
N6–H6N⋯O3
3.00(6)
2.17(4)
2.231(2)
2.16(8)
2.525(3)
2.05(2)
2.369(2)
2.12(4)
162.0(3)
142.4(3)
145.0(9)
153.0(4)
146.0(3)
152.9(3)
174.0(3)
3.057(5)
2.875(4)
3.380(7)
2.81(4)
Ha and Hd (Δppm Ha = 0.073, Δppm Hd = 0.08), which are far
from the amide NH, are shifted upfield. Thus the first set of
protons which are pointed in the same direction of the
amide NH are definitely polarized through space interaction
3.263(4)
2.98(6)
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Fig. 6 (a) and (b) 1H NMR titration curves of L1 and L2 with anions in
DMSO-d6 at RT. Net changes in the chemical shifts of amidic –NH are
shown against the increasing amount of anions.
−
changes with the anions such as Cl−, Br−, NO3 etc. are negli-
gible (Fig. S41–S43†).
Conclusions
In summary, we have synthesized cyanobenzoyl substituted
two tripodal amide receptors having different lengths and suc-
cessfully exploited their high potential towards anion reco-
gnition of various dimensionalities. The C3v symmetric first
generation tripodal L1 remains in a locked conformation in
free and protonated states through an intramolecular N–
H⋯O–H-bond. This causes encapsulation of anions irrespec-
tive of size, shape and charge, therefore anion recognition
takes place in a side cleft binding fashion only. Then we
applied a synthesis strategy to cancel the intramolecular
H-bond, where cyanobenzoyl was appended from a bigger plat-
form like triphenyl amine to give second generation tripodal
L2. Such modification resulted in three major advantages
which are (i) the possibility of making the C3v symmetric cavity
for anion encapsulation as the H-bonding between the apical
N-atom and the ethereal O-atom could be able to hold three
arms tightly, (ii) ruling out the intramolecular N–H⋯O–H-
bond which makes amide hydrogen ready for anions and (iii)
producing a larger space for anion recognition. The crystal
structure of the anion complexes shows the close vicinity of two
Fig. 5 Stacking plot showing shift of amide –NH and –CH hydrogen
with gradual addition of TBAF solution to L1 and L2 in DMSO-d6.
by the guest fluoride anion and that is why they are shifted
downfield a. Moreover, due to the formation of the same nega-
tively charged species the second set of protons are upfield
shifted. The first set of protons which are pointed in the same
direction as the amide NH are definitely polarized through
space interaction. However, after excess addition of fluoride we
−
also did not observe the characteristic HF2 peak at the down-
field region. This is probably due to the lower stability of the
−
−
HF2 species in the solution.43 Again with the H2PO4 anion,
the NHa is shifted 0.276 ppm downfield and the aromatic CH
interacts in the same fashion (Δppm Ha = −0.034, Δppm Hb =
0.026, Δppm Hc = 0.039 and Δppm Hd = −0.027). Again, the
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arms, while the other is projected in the opposite direction. In with SADABS.45 The structures were solved by direct methods
the protonated state one arm is H-bonded with the apical using SHELXTL46 and refined on F2 by the full-matrix least-
N-atom through the –CN group forming a pseudo capsular squares technique using the SHELXL-97 program package.47
cavity. The anions remain in the pseudo cavity and are Graphics are generated using MERCURY 3.0.48 In all cases,
H-bonded with one or more amide hydrogens. The most non-hydrogen atoms are treated anisotropically. Wherever
appealing feature of the L2 is that selective anion induced reor- possible, the hydrogen atoms are located on a difference
ganization resulted in change in conformation from open C2v Fourier map and refined. In other cases, the hydrogen atoms
2−
symmetry to folded C3v in the presence of only SiF6 anions are geometrically fixed. PLATON/SQUEEZE49 was performed to
and forms a sandwich type complex. Two neutral receptors refine the host framework in 7 and 9 excluding the disordered
interact with the fluoride ion strongly among the halide ions solvent electron densities, and the detailed calculations are
as a very large chemical shift of amide hydrogen is observed. explained in the respective structure discussion section. These
Both the receptors are reluctant to bind with oxyanions crystallographic calculations were further confirmed by the
strongly, though we observed a comparatively better amide TGA experiment (ESI†).
−
hydrogen shift by H2PO4 anion in both receptors. It is also
clear from solution studies that anions interact with L2 to a
NMR studies
1H NMR titration studies were performed to evaluate the inter-
greater extent than L1. This phenomenon is further validated
by its solid state anion binding as the number of H-bonds
around anions in L2 is apparently higher than in L1.
action of L1 and L2 with anions in DMSO-d6 at room tempera-
ture. Initial concentrations were [ligand]0 = 5 mM, and
[anion]0 = 50 mM. Each titration was performed by 10–
12 measurements at room temperature.
Experimental
Materials and method
Synthesis and characterization
First generation tripodal amide L1. Tripodal amide-based
receptor L1 was synthesized by the reaction of tris(2-aminoethyl)
amine with three equivalents of 4-cyanobenzoyl chloride in the
presence of triethylamine in dry CH2Cl2 at 0 °C.23 After stirring
for 8 h the reaction mixture was filtered off and the precipitate
was washed with plenty of water and methanol to remove the
triethylammonium chloride and dried under vacuum to yield
the colorless solid L1 (yield = 86%). Block shaped crystals of L1
were grown from DMF solution (Scheme S2†).
All reagents were obtained from commercial sources and used
as received. Solvents were distilled freshly following standard
procedures. Tris(2-aminoethyl)amine (tren), triethanol amine
and 4-cyanobenzoyl chloride were purchased from Sigma-
Aldrich and used as received. Solvents for synthesis and crys-
tallization experiments were purchased from Merck, India, and
used as received.
Instrument
Compound L1(1). Yield = 80%, M. P: >300 °C. 1H-NMR
(600 MHz, DMSO-d6) δ (ppm): 8.712 (br, 3H, NH), 7.951 (d, 6H,
ArH, J = 7. 0 Hz), 7.942 (d, 6H, ArH, J = 7.0 Hz), 3.33 (m, 6H,
IR spectra were recorded on a Perkin-Elmer-Spectrum One
FT-IR spectrometer with KBr disks in the range
4000–500 cm−1. NMR spectra were recorded on a Varian
FT-400/600 MHz instrument. Chemical shifts were recorded in
parts per million (ppm) on the scale solvent peak as a refer-
ence. ESI-MS spectra were recorded on a WATERS LC-MS/MS
system, Q-Tof Premier in the Central Instrument Facility (CIF)
of IIT Guwahati. The thermal analyses were performed by
using an SDTA 851 e TGA thermal analyzer (Mettler Toledo) at
a heating rate of 5 °C per min under a N2 atmosphere.
1
NCH2), 2.81 (m, 6H, NHCH2CH2). H-NMR (150 MHz, DMSO-
d6): 164.98, 138.36, 132.30, 127.92, 118.28, 113.47, 52.96 and
37.2. IR spectra (KBr pellet): 3277 cm−1 vs(N–H), 2983 cm−1 vs
(C–H), 2824 cm−1 (C–H), 2228 vs(CuN), 1640 vs(CO), 1538 cm−1
vs(CvC). ESI mass [M + 1] m/z = 534.2230 (calcd 534.590).
Second generation tripodal amide L2. Receptor L2 was syn-
thesized by following the reported procedure.30–33 The nitro
tripodal (B) was obtained by tris(2-chloroethyl)amine hydro-
chloride50 (A) treatment by SN2 substitution with 4-nitrophenol
in EtOH (yield 65%) followed by reduction of the nitro group
to yield the corresponding triamine (C) (yield 72%). Sub-
sequently, triamine (C) solution in CH2Cl2 was slowly added to
a mixture of three equivalents of 4-cyanobenzoyl chloride in
the presence of triethylamine in dry CH2Cl2 and stirred for 8 h
at 0 °C. The reaction mixture is filtered off and the precipitate
is washed with water and methanol to remove the triethyl-
ammonium chloride and dried under vacuum to yield the gray
solid L2 (yield = 80%). See Schemes S1 and S2† for detailed
synthesis.
Crystallographic refinement details
The crystallographic data and details of the data collection for
free receptor L1(1) and anion complexes 2, 3, 4, 5, 6, 7, 8, 9
and 10 are given in Table S1.† In each case, a crystal of suitable
size was selected from the mother liquor and immersed into
silicone oil, then mounted on the tip of a glass fiber and
cemented using epoxy resin. Intensity data for all the crystals
were collected using Mo-Kα radiation (λ = 0.71073 Å) at 298(2)
K, while increasing ω (width of 0.3° per frame) at a scan speed
of 6 s per frame on a Bruker SMART APEX diffractometer
equipped with a CCD area detector. The data integration and
Compound L2. Yield = 85%, M. P: 175–177 °C. 1H-NMR
reduction were processed with SAINT44 software. Empirical (600 MHz, DMSO-d6) δ (ppm): 10.364 (s, 3N–H), 8.079 (d, 6H,
absorption correction was applied to the collected reflections ArH, J = 7.1 Hz), 8.004 (d, 6H, ArH, J = 7.3 Hz), 7.654 (d, 6H,
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ArH, J = 7.0 Hz), 6.940 (d, 6H, ArH, J = 7.2 Hz), 4.087 (t, 6H, spectra (KBr pellet): broad band at 3425 vs(O–H and N–H),
–OCH2, J = 5.4) and 3.05 (t, 6H, –NCH2, J = 5.8). 13C-NMR 3083 cm−1 vs(C–H), 2910 cm−1 (C–H), 2231 vs(CuN), 1648 vs
(150 MHz, DMSO-d6): 163.73, 155.08, 149.27, 139.07, 132.47, (CO), 1518 cm−1 vs(CvC).
[L2H·NO3·H2O](8). Yield = 65%, M.P: 205–207 °C. 1H-NMR
131.87, 128.47, 122.14, 118.40, 115.56, 114.57, 113.73, 66.51
and 53.71. IR spectra (KBr pellet): 3287 cm−1 vs(N–H),
(600 MHz, DMSO-d6) δ (ppm): 10.395 (s, 3N–H), 8.080 (d, 6H,
3074 cm−1 vs(C–H), 2870 cm−1 (C–H), 2240 vs(CuN), 1640 vs
ArH, J = 7.3 Hz), 8.009 (d, 6H, ArH, J = 7.0 Hz), 7.705 (d, 6H,
(CO), 1515 cm−1 vs(CvC). ESI mass [M + 1] m/z = 810.3070
ArH, J = 6.5 Hz), 6.966 (d, 6H, ArH, J = 7.3 Hz), 4.383 (t, –OCH2,
J = 5.3 6H) and 3.819 (t, 6H, –NCH2, J = 5.5). IR spectra (KBr
pellet): broad band at 3435 vs(O–H and N–H), 2983 cm−1 vs(C–
H), 2910 cm−1 (C–H), 2230 vs(CuN), 1657 vs(CO), 1528 cm−1
vs(CvC), 1380 cm−1 vs(NvO).
(calcd. 810.870).
Preparation of the anion complex of the receptors L1 and
L2. All complexes were obtained by stirring L1 or L2 (0.05 mM)
in 5 mL of DMF in a glass beaker (or plastic vial for complex 6)
and 1.2 equiv. of the corresponding acid like 40% HF, 49%
HBr, 55% HI, HNO3 and 70% HClO4. After constant stirring
for half an hour, the clear solution was allowed to slowly evap-
orate. Single crystals suitable for X-ray diffraction analysis were
obtained at RT within 2–3 weeks.
[L2H·ClO4](9). Yield = 70%, M.P: 211–212 °C. 1H-NMR
(600 MHz, DMSO-d6) δ (ppm): 10.396 (s, 3N–H), 8.082 (d, 6H,
ArH, J = 7.0 Hz), 8.010 (d, 6H, ArH, J = 7.1 Hz), 7.946 (s, DMF
solvent), 7.699 (d, 6H, ArH, J = 6.9 Hz), 7.00 (d, 6H, ArH, J = 7.0
Hz), 4.421 (t, –OCH2, J = 5.3 6H) and 3.816 (t, –NCH2, J = 5.5
6H), 2.885 (s, DMF solvent) and 2.726 (s, DMF solvent). IR
spectra (KBr pellet): broad band at 3435 vs(O–H and N–H),
3093 cm−1 vs(C–H), 2962 cm−1 (C–H), 2230 vs(CuN), 1657 vs
(CO), 1520 cm−1 vs(CvC) 1259 and 1240 cm−1 (ClvO).
1
[L1H·Br](2). Yield = 75%, M. P: >300 °C. H-NMR (600 MHz,
DMSO-d6) δ (ppm): 8.995 (br, 3H, NH), 7.939 (d, 6H, ArH, J =
6.8 Hz), 7.889 (d, 6H, ArH, J = 7.1 Hz), 3.733 (m, 6H, NCH2),
3.419 (m, 6H, NHCH2CH2). IR spectra (KBr pellet): 3342 cm−1
vs(N–H), 2983 cm−1 vs(C–H), 2824 cm−1 (C–H), 2231 vs(CuN),
1648 vs(CO), 1541 cm−1 vs(CvC).
[(L2H)2·SIF6·4H2O·2DMF](10). Yield = 60%, M. P: 229–230 °C.
1H-NMR (600 MHz, DMSO-d6) δ (ppm): 10.397 (s, 3N–H), 8.084
(d, 6H, ArH, J = 7.3 Hz), 8.013 (d, 6H, ArH, J = 7.0 Hz), 7.945 (s,
DMF solvent), 7.701 (d, 6H, ArH, J = 6.8 Hz), 7.01 (d, 6H, ArH,
J = 7.0 Hz), 4.396 (t, 6H, –OCH2, J = 5.5) and 3.814 (t, 6H,
–NCH2, J = 5.6), 2.884 (s, DMF solvent) and 2.725 (s, DMF
solvent). IR spectra (KBr pellet): broad band at 3440 vs(O–H
and N–H), 3120 cm−1 vs(C–H), 2963 cm−1 (C–H), 2222 vs
[L1H·I](3). Yield = 70%, M. P: >300 °C. 1H-NMR (600 MHz,
DMSO-d6) δ (ppm): 8.992 (br, 3H, NH), 7.935 (d, 6H, ArH, J =
6.9 Hz), 7.882 (d, 6H, ArH, J = 7.0 Hz), 3.730 (m, 6H, NCH2),
3.413 (m, 6H, NHCH2CH2). IR spectra (KBr pellet): 3277 cm−1
vs(N–H), 2983 cm−1 vs(C–H), 2824 cm−1 (C–H), 2231 vs(CuN),
1640 vs(CO), 1538 cm−1 vs(CvC).
[L1H·NO3]2·H2O(4). Yield = 65%, M. P: >300 °C. 1H-NMR (CuN), 1656 vs(CO), 1518 cm−1 vs(CvC).
(600 MHz, DMSO-d6) δ (ppm): 8.990 (br, 3H, NH), 7.933 (d,
6H, ArH, J = 6.8 Hz), 7.884 (d, 6H, ArH, J = 7.1 Hz), 3.728 (m,
6H, NCH2), 3.409 (m, 6H, NHCH2CH2). IR spectra (KBr pellet):
Acknowledgements
3342 cm−1 vs(N–H), 3014 cm−1 vs(C–H), 2901 cm−1 (C–H),
2231 vs(CuN), 1648 vs(CvO), 1537 cm−1 vs(CvC), 1379 cm−1
vs(NvO).
This work was supported by CSIR and SERB through grant 01-
2235/08/EMR-II and SR/S1/OC-62/2011, New Delhi, India. The
authors thank CIF IITG and DST-FIST for providing instru-
ment facilities. N.H thanks IITG for fellowship.
[L1H·ClO4](5). Yield
=
65%, M. P: >300 °C. 1H-NMR
(600 MHz, DMSO-d6) δ (ppm): 8.990 (br, 3H, NH), 7.930 (d,
6H, ArH, J = 6.9 Hz), 7.883 (d, 6H, ArH, J = 7.0 Hz), 3.725 (m,
6H, NCH2), 3.406 (m, 6H, NHCH2CH2). IR spectra (KBr pellet):
3350 cm−1 vs(N–H), 3046 cm−1 vs(C–H), 2901 cm−1 (C–H),
2231 vs(CuN), 1646 vs(CvO), 1537 cm−1 vs(CvC), 1324 and
1287 cm−1 (ClvO).
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