Hydrogen and halogen bonding in a concerted act of anion recognition: F- induced atmospheric CO2 uptake by an iodophenyl functionalized simple urea receptor

Article abstract of DOI:10.1039/c4dt00940a

Two simple urea based para-halo substituted [Iodo (L₁) and Bromo (L₂)] acyclic receptors have been extensively studied as a receptor for various anions. Receptors L₁ efficiently uptake atmospheric CO₂ and stabilize as air-stable crystals of HCO₃^- dimer (complex 1a) in the presence of n-tetrabutylammonium (n-TBA) fluoride through the simultaneous formation of hydrogen and halogen bonding, yielding a tetrahedrally surrounded non-covalent coordinated complex. However, receptor L₂, in the presence of n-TBA salt of F^-, has been found to form a complex with the octahedral SiF₆^2- anion, where the coordination environment of the anion is merely governed by multiple N-H...F (anion) interactions. The fluoride induces an uptake of aerial CO₂ only for L₁, which is due to the unique ability of L₁ to simultaneously form both hydrogen and halogen bonds with an anionic guest. The most decisive evidence supporting the ability of L₁ to form a halogen bond is obtained via crystallizing the acetate complex of both the receptors. The receptor L₁ stabilizes the acetate anion via both H-bonding and halogen bonding interactions, while the receptor L₂ only forms H-bonding interactions with acetate anion. The solution-state anion binding properties of L₁ and L₂ have been investigated by qualitative and quantitative ¹H NMR titration experiments with halides and oxyanions in DMSO-d₆. Both the receptors showed strong solution-state binding with F^-, HCO₃^- and CH₃COO^-, as observed in the solid-state, whereas both of them have been found to be less interactive with other anions such as Cl^-, Br^-, I^-, NO₃^-, HSO₄^-, and H₂PO₄^-. This journal is

Full text of DOI:10.1039/c4dt00940a

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Hydrogen and halogen bonding in a concerted act
of anion recognition: F− induced atmospheric CO2
uptake by an iodophenyl functionalized simple
urea receptor†
Cite this: Dalton Trans., 2014, 43,
15628
R. Chutia and G. Das*
Two simple urea based para-halo substituted [Iodo (L1) and Bromo (L2)] acyclic receptors have been
extensively studied as a receptor for various anions. Receptors L1 efficiently uptake atmospheric CO2 and
−
stabilize as air-stable crystals of HCO3 dimer (complex 1a) in the presence of n-tetrabutylammonium
(n-TBA) fluoride through the simultaneous formation of hydrogen and halogen bonding, yielding a tetra-
hedrally surrounded non-covalent coordinated complex. However, receptor L2, in the presence of n-TBA
salt of F−, has been found to form a complex with the octahedral SiF6 anion, where the coordination
2−
environment of the anion is merely governed by multiple N–H⋯F (anion) interactions. The fluoride
induces an uptake of aerial CO2 only for L1, which is due to the unique ability of L1 to simultaneously
form both hydrogen and halogen bonds with an anionic guest. The most decisive evidence supporting
the ability of L1 to form a halogen bond is obtained via crystallizing the acetate complex of both the
receptors. The receptor L1 stabilizes the acetate anion via both H-bonding and halogen bonding inter-
actions, while the receptor L2 only forms H-bonding interactions with acetate anion. The solution-state
1
Received 31st March 2014,
Accepted 21st August 2014
anion binding properties of L1 and L2 have been investigated by qualitative and quantitative H NMR titra-
tion experiments with halides and oxyanions in DMSO-d6. Both the receptors showed strong solution-
state binding with F−, HCO3 and CH3COO−, as observed in the solid-state, whereas both of them have
−
DOI: 10.1039/c4dt00940a
−
www.rsc.org/dalton
been found to be less interactive with other anions such as Cl−, Br−, I−, NO3−, HSO4−, and H2PO4
.
for the rational design and construction of molecular
materials with DNA and other biological macromolecules.8 Ho
Introduction
Halogen bonding (XB), the parallel non-covalent world to and co-workers have studied Holliday junctions (four-stranded
hydrogen bonding (HB) is the charge-transfer interaction DNA junctions, the key structural intermediates during the
between Lewis bases and polarizable halogen atoms.1 homologous recombination of DNA) and estimated that a
Halogen-bonding is continuing to expand its horizon2 in a halogen bond that can direct the conformation of a biological
rapid way because of its widespread applicability in the assem- molecule is stronger than an analogous hydrogen bond in the
bly of functional materials (such as liquid crystals and mole- same environment.8c
cular-imprinted polymers),3 conducting and magnetic
The first case of intermolecular donor–acceptor complexes
molecular materials,4 tuning of second-order nonlinear optical was reported by Benesi and Hildebrand that were formed from
responses,5 supramolecular polymers and crystalline assem- iodine and aromatic hydrocarbons.9 However, it was O. Hassel
blies,6 even in medicinal chemistry.7 In the last few years, who introduced this promising non-covalent interaction, namely,
halogen bonding has also been established as a potential tool halogen bonding, to the people as ‘interatomic charge transfer
bonding’ in 197010 and the first use of the term “halogen bond”
was made by Dumas et al. in 1978.11 After a dormant period for
decades, in the 90s by Legon12 with a gas-phase study and
Department of Chemistry, Indian Institute of Technology, Guwahati, Assam 781 039,
specially in the 21st century, halogen bonding research has been
India. E-mail: gdas@iitg.ernet.in; Fax: +91-361-258-2349; Tel: +91-361-258-2313
†Electronic supplementary information (ESI) available: X-ray crystallographic
facilitated by a few people like Resnati, Metrangolo et al.,2 and
file of the structures in CIF format, characterization data including 1H NMR and
Mark S. Taylor and his co-workers.13 Though it is still a challenge
FT-IR spectra, 1H titration spectra and additional crystallographic data. CCDC
to study XB in the solution-phase,14 numerous theoretical
973820–973823, 973825. For ESI and crystallographic data in CIF or other elec-
studies of XB have been reported till date.15
tronic format see DOI: 10.1039/c4dt00940a
15628 | Dalton Trans., 2014, 43, 15628–15637
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While HB is a full-blown tool in molecular recognition as
well as in anion recognition,16 XB is still adolescent from this
Results and discussion
point of view, although XB has been sensibly approached in In 2004, Fabbrizzi et al. have shown the anion recognition pro-
recently developed anion recognition.13,17 Recently, a series of perties of 1,3-bis(4-nitrophenyl)urea, which, due to the pres-
urea-based anion receptors bearing one or two halogen bond ence of electron withdrawing nitro chromophore, resulted in a
donors have been designed to probe the potential for anion fluoride ion induced –NH deprotonation with the subsequent
recognition through the combinations of hydrogen and absorption of atmospheric CO2 in moist THF.25 Encouraged by
halogen bonding by Taylor et al.13e NMR studies revealed that such exciting results, we envisioned that the simple 1,3-bis(4-
two distinct noncovalent interactions act in a concerted halophenyl)urea receptors could turn out to be excellent candi-
manner to achieve the selective binding of halides over oxy- dates for anion complexation via a combined act of hydrogen
anions, a conclusion being further supported by compu- and halogen bonding (Scheme 1).
−
tational studies. Supramolecular anion host systems utilizing
The structural elucidation of Fabbrizzi’s HCO3 complex
−
both these interactions with defined functions are a challen- showed the dimeric association of urea bound HCO3 anions,
ging prospect. Direct F− recognition as well as sensing is an while a crystallized water molecule is hydrogen bonded to a
−
area of immense research interest in supramolecular and bio- HCO3 oxygen atom. However, in the present case, the urea
−
logical chemistry.18,23e,h Interestingly, indirect results obtained bound HCO3 dimer is additionally stabilized by a pair of
by employing F− anion also have developed into an emerging halogen bonding interactions donated from the iodophenyl
field of research. A major environmental issue to be concerned ring of an adjacent receptor to the –OH oxygen atom of each
−
is the significant rise in the CO2 concentration in the HCO3 anion. Further, a distinct directional coordinative
−
atmosphere, which eventually demands the efficient fixation environment has been observed in the CO2 absorbed HCO3
and activation of atmospheric CO2 into green chemicals.19 complex of L1. From the perspective of anion receptor chem-
Microporous aluminosilicates, activated carbons, and metal– istry, crystallization has traditionally been a route to under-
organic frameworks (MOFs) have widely been employed to stand the structural insights of the anion complexes formed,
capture and store CO2 by converting it into green chemicals primarily by single-crystal XRD analysis, which are then related
for the synthesis of specific chemical intermediates.20 to the observed selectivity in solution. Thus, efforts were made
However, in the light of supramolecular chemistry, the to explore the solid-state binding properties of L1 and L2 with
efficient fixation of aerial CO2 as carbonate/bicarbonate can be different anions, by charging excess quaternary ammonium
achieved with artificial H-bonding receptors in the presence of [n-TBA (tetrabutylammonium)/TEA (tetraethylammonium)] salt
hydroxide and fluoride ions.21,23b Gale et al. have also demon- of anions to the individual receptor solutions in aprotic sol-
strated CO2 capture as carbamates (alkylammonium/alkyl- vents such as MeCN or DMSO and allowed to crystallize at
−
carbamate) by a series of urea-based receptors in the presence room temperature. The addition of F−, HCO3 and CH3COO−
of aliphatic amines (CO2 scrubbers) bubbled with CO2 in solubilize the otherwise insoluble receptors L1 and L2 in
dimethyl sulfoxide (DMSO).22
MeCN, indicating a strong receptor–anion interaction. It is
−
Continuing our research in the field of anion recognition,23 interesting to note that the single crystals of HCO3 complex
we report the F− ion induced uptake of atmospheric CO2 that n-TBA[L1·HCO3] (1a) were obtained upon the slow evaporation
−
stabilizes as HCO3 anion (air-stable crystals) by a structurally of the F− containing acetonitrile solution of receptor L1. The
simple acyclic 1,3-bis(4-iodophenyl)urea (L1) receptor. The source of HCO3− is the atmosphere, where hydroxide ions gen-
in situ formed HCO3 complex (1a) is stabilized by a concerted erated in situ from the basic receptor-F− solution dissolve
−
−
act of hydrogen and halogen bonding donated by the aerial CO2 into HCO3
receptors. To the best of our knowledge, 1,3-bis(4-iodophenyl)-
urea is the simplest anion receptor that exhibits CO2 uptake
.
ꢀ
2Fꢀ þ H2O ! OHꢀ þ HF2
−
ꢀ
OHꢀ þ CO2 ! HCO3
and stabilizes the in situ formed HCO3 by a combination of
hydrogen and halogen bonding. Further evidence of
halogen bonding with the 1,3-bis(4-iodophenyl)urea receptor
has been observed in the CH3COO− complex (1b), validating
the interplay of both hydrogen and halogen bonding in the
stabilization of bicarbonate in a receptor-fluoride solution.
Following the trend in the strength of halogen bond
formation viz., –I > –Br > –Cl,2b,24 we have examined the struc-
tural aspects of anion binding with 1,3-bis(4-bromophenyl)-
urea (L2) as a control receptor, where the halogen bond
donating the iodine substituent is replaced by bromine.
Interestingly, halogen bonding was found to be
−
The anion binding topology of the in situ generated HCO3
complex (1a) revealed the involvement of both hydrogen and
halogen bonding in a concerted act of anion recognition. The
potentiality of L1 as a halogen bond donor is unanimous as
completely lacking in both the structurally elucidated anion
complexes 2a and 2b (SiF6
respectively) of L2.
and CH3COO− complexes,
2−
Scheme 1 Molecular structure of the receptors.
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Fig. 1 (a) X-ray structure of L1 shows the urea tape hydrogen bond motif along with π-stacking and I⋯I interactions, and (b) X-ray structure of L2
showing the urea tape hydrogen bond motif along with π-stacking interactions.
confirmed by the single crystal analysis of the n-TBA- an adjacent anion bound receptor molecule. Thus, each
−
[L1·CH3COO] complex (1b), where each –NH group shares one HCO3 anion is coordinated to a receptor by four hydrogen
of the acetate oxygen atoms, which, in turn, is halogen bonded bonds and to another by a halogen bond, which implies that a
−
to the iodophenyl ring of an adjacent receptor. However, the HCO3 dimer is coordinated to four receptor molecules via
anion complexes of L2 (SiF6 and CH3COO− complexes) did two distinct types of noncovalent interactions (Fig. 2a). The
2−
−
not showcase the formation of any halogen bonds with the HCO3 dimer is located below the hydrogen bond donor and
hydrogen bonded anions. Attempts to obtain complexes with above the halogen bond donor platform. The spatial position
other anions (NO3−, HSO4−, H2PO4−, Cl−, Br−, I−) of L1 and L2 of the HCO3 dimer looks as if the dimer is hanging by
−
resulted in the crystallization of the free receptor.
holding the hydrogen bonding threads that are supported by
two halogen bonding pillars from the bottom (Fig. 2b). In
other words, it is the halogen bonds that pulled the HCO3
−
Accounts of crystal structures
dimer out of the more common hydrogen bonded planar struc-
Receptors L1 and L2. Single crystals of L1 and L2 suitable for
XRD analysis were obtained from DMSO and both crystallize
the monoclinic system with the centrosymmetric space group
C2/c. Structural analysis showed weak π-stacking interactions
between the receptor molecules that are N–H⋯O hydrogen
bonded with one another. The π-stacked urea tapes are inter-
linked with one another by halogen–halogen (I⋯I) interactions
in L1, whereas L2, in spite of having similar π-stacked tape
motif, lacks halogen–halogen (Br⋯Br) interactions. Thus, from
the structural features, it can be presumed that L1 has the
possibility to exhibit special noncovalent features (Fig. 1).
Bicarbonate-complex, n-TBA[L1·HCO3], (1a). The in situ gen-
erated bicarbonate complex crystallized in the monoclinic
system with the C2 space group from an acetonitrile solution
−
of L1 containing excess fluoride ions. The source of HCO3 is
from the atmosphere, where hydroxide ions generated in situ
from the basic receptor-fluoride solution dissolves aerial CO2
−
into HCO3 at the air–solvent interface, thereby resulting in
−
the formation of air-stable crystals of dimeric HCO3 complex
stabilized by a combined act of hydrogen and halogen bonds.
Structural elucidation revealed 1 : 1 complex stoichiometry and
−
dimeric association between two receptor coordinated HCO3
anions. Each urea bound HCO3− anion donates and accepts an
O–H⋯O hydrogen bond (1.796 Å) to/from another urea bound
−
HCO3 ion, giving rise to a dimeric anion complex. The
–COO− fragment of a HCO3 anion is hydrogen bonded to the
−
urea–NH groups with a donor–acceptor (N–H⋯O) distance of
1.921(5) and 2.104(4) Å for O2 and O3, respectively. Addition-
ally, HCO3 oxygen O2 and O3 is hydrogen bonded to an aryl
–CH proton (ortho w.r.t. to urea function) with a donor–accep-
−
Fig. 2 (a) Ball-and-stick representation depicting the hydrogen and
halogen bonding contacts on HCO3 dimer in complex 1a as viewed
down the crystallographic b-axis, and (b) ball-and-stick representation
showing the tetrahedral spatial orientation of the bicarbonate dimer in
complex 1a as viewed down the crystallographic c-axis (n-TBA cations
−
tor (C–H⋯O) distance of 2.571(4) and 2.644(4) Å, respectively.
−
Most importantly, the –OH group of HCO3 anion accepts
one strong C–I⋯O halogen bond from the iodophenyl ring of are omitted for clarity of the presentation).
15630 | Dalton Trans., 2014, 43, 15628–15637
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−
ture, as observed in the case of the HCO3 complex of 1,3-bis- shift and concomitant broadening of the urea –NH resonance
(4-nitrophenyl)urea reported by Fabbrizzi et al. Furthermore, with Δδ = 1.84 ppm, indicating the strong solution-state
−
the exposed area at the top and bottom created due to the binding of HCO3 with the urea function. The strong inter-
−
tetrahedral like environment around the bicarbonate dimer action of HCO3 has also been confirmed by monitoring the
are capped by the n-TBA cations via contact ion-pairing differences in the chemical shifts of 13C NMR signals of the
−
(C–H⋯Anion interactions) and C–H⋯π interactions with the TEA salt of HCO3 and complex 1a. TEA(HCO3) in DMSO-d6
phenyl rings to form a compact enclosed system surrounding showed a sharp 13C NMR resonance at 158.91 ppm, whereas,
−
the in situ generated anion.
in complex 1a, the HCO3 resonance originated at 182.12 ppm
The halogen bonding contact (I2⋯O4) in complex 1a has a showing a large downfield shift of 23.27 ppm (Fig. S11, ESI†).
distance of 3.183(4) Å (Fig. 3a), which corresponds to 9% short- Acetate-complex, n-TBA[L1·CH3COO] (1b). Complex 1b crys-
ening of the sum of their van der Waals radii (3.50 Å) (The van tallized in the monoclinic system with the P21/c space group,
der Waals radii of O and I are 1.52 and 1.98 Å, respectively).26 where each acetate oxygen atom is hydrogen bonded to a
Parthasarathy et al. has reported the crystallographic evidence receptor molecule by a pair of N–H⋯O and aryl C–H⋯O
of the directional preferences of intermolecular forces around bonds. A correlation of the N–H⋯O angle versus the N–H⋯O
halogen atoms,27 where they have shown that nucleophiles, in distance shows that both the N–H⋯O hydrogen bonds are in
general, tend to approach the C–X bond in a “head-on” the very strong hydrogen-bonding interaction regions of
fashion with Γ ≈ 165(8)° for the C–I bond. Similar results were d(H⋯O) < 2.5 Å and d(D⋯O) < 3.2 Å (Table S2, ESI†). Further-
obtained by Allen et al.26 and Glaser et al.28b In complex 1a, more, each acetate oxygen atom interacts with a neighbouring
the iodophenyl unit is “head-on” with the HCO3− oxygen at an receptor molecule by accepting a halogen bond each from two
angle ∠(C–I2⋯O4) of Γ = 173.6°(2), which is close to 180° different iodophenyl rings. The halogen bonding contacts
1
[∠(C–O4⋯I2) angle Ω = 110.0(3)°, Fig. 3a]. The H NMR spec- (I⋯O) in this complex were determined to be d(I1⋯O2) =
trum of complex 1a (DMSO-d6) showed appreciable downfield 3.144 Å and d(I2⋯O3) = 3.482 Å (Fig. 4b), which correspond to
11% and 1% shortening of the sum of their van der Waals
radii (3.5 Å). The ∠(C–I⋯O) angles were measured to be Γ =
156.6(8)° and
Γ
=
144.77(8)° for the I1⋯O2 and
I2⋯O3 halogen bonds, respectively. The details of the halogen
bonding contacts and angles in complexes 1a and 1b are listed
in Table 1. In complex 1b, the acetate anion resides in the
plane of the XB donating iodophenyl rings, rather than the HB
donating urea function (Fig. 4d), showcasing the effect of XB
on the H-bonded urea–acetate complex, unlike the most
common urea–acetate cases.29 Recently, Ho and coworkers
have reported that the halogen bonds can adopt an orthogonal
(perpendicular) geometry and they are energetically indepen-
dent of the hydrogen bonds that share a common acceptor
atom.8c Based on their calculations on biomolecules, they
found that in most of the cases, the X⋯O⋯H angle lies in the
range of (85–89)°. Our attempt to analyse the orthogonality of
the halogen bonds in the acetate complex (1b), we find that
the –NH related I⋯O⋯H angles are in the range of 110–115°
and –CH related I⋯O⋯H angles are in the range of 63–66°.
Thus, it may be assumed that the halogen bonds are maintain-
ing an orthogonal relationship to the –NH and –CH hydrogen
bonds with an average angle of 89° (Fig. 4d).
The 1H NMR spectrum of crystalline complex 1b (DMSO-d6)
showed a large downfield shift of the –NH resonance with Δδ =
2.83 ppm, indicating a strong host/guest relationship.
Acetate-complex, n-TBA[L2·CH3COO] (2b). Identical to 1b,
complex 2b crystallized in the monoclinic system with the
P21/c space group with a 1 : 1 complex stoichiometry. However,
there are significant differences in the acetate coordination on
switching from the 1,3-bis(4-iodophenyl)urea to 1,3-bis(4-bromo-
phenyl)urea. In complex 2b, each acetate oxygen atom behaves
as a bifurcated hydrogen bond acceptor, where oxygen O2 is
hydrogen bonded to a –NH proton and an aryl –CH proton,
and oxygen O3 is hydrogen bonded to both urea –NH protons
Fig. 3 (a)
A magnified view of the coordination environment in
complex 1a highlighting halogen bonding distances and angles, and (b)
ball-and-stick (host) and spacefill (guest) representation depicting the
aliphatic C–H⋯O and C–H⋯π interactions from the n-TBA cations to
the dimeric anion and receptors, respectively.
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Fig. 4 Ball-and-stick representation of complex 1b depicting (a) the hydrogen and halogen bonding contacts on CH3COO− anion along the crystallo-
graphic a-axis, (b) halogen bonding distances and angles, (c) orthogonal relationship between hydrogen and halogen bonds, and (d) a magnified
view depicting the planarity of acetate anion with the halogen bond donating iodophenyl rings (light blue colored plane) and not with the hydrogen
bonded urea function (light pink colored plane) (n-TBA cations are omitted for clarity of the presentation).
between two n-TBA cations by forming several C–H⋯O inter-
Table 1 Details of halogen bonding contacts in complexes 1a and 1b
actions, thereby gaining some added solid-state stabilization.
Further, the lack of halogen bonding in 2b contributes to the
planarity of the complex.
Complex
1a
1b
1b
C–I⋯O
C11–I2⋯O4
3.187(4)
5.272(6)
90.8%
173.6(2)
110.0(3)
C1–I1⋯O2
3.144(4)
5.143(4)
89.8%
156.60(8)
132.8(4)
C11–I2⋯O3
3.482(5)
5.339(5)
99%
144.77(8)
133.4(4)
The 1H NMR spectrum of complex 2b (DMSO-d6) also
showed a large downfield shift of the –NH resonance with Δδ =
2.515 ppm, which is similar to complex 1b, indicating a strong
host/guest binding.
d(I⋯O)/Å
d (C–I⋯O)/Å
I⋯O [%]a
∠C–I⋯O/° (Γ)
∠C–O(A−)⋯I/° (Ω)
a [%] of VdW radii.
Solution-state anion-binding study
The solution-state anion binding properties of L1 and L2 were
investigated by qualitative as well as quantitative 1H NMR
experiments in DMSO-d6 using the quaternary ammonium
(n-TBA/TEA) salt of monovalent anions such as F−, Cl−, Br−, I−,
(Fig. 5b). However, in complex 1b, a pair of halogen bonds
(I⋯O) provides added stabilization to the hydrogen bonded
acetate anion. Such a feature showcasing halogen bond for-
mation with the anion was found to be absent in complex 2b.
Moreover, the urea bound acetate complex is sandwiched
HCO3−, CH3COO−, NO3−, H2PO4 and HSO4−. Fig. 6a and 6b
−
show the chemical shift changes observed upon one equi-
valent addition of different anions to the individual solutions
of L1 and L2, respectively, in DMSO-d6. The most significant
change has been observed for the urea –NH proton in the pres-
−
ence of F−, HCO3 and CH3COO−, indicating that the –NH
function acts as the primary site for anion recognition.
The 1H NMR titration of L1 with a standard HCO3 solu-
−
tion, a large downfield shift of urea –NH resonance with Δδ =
2.13 ppm, and a notable upfield shift of ortho-aryl proton with
Δδ = 0.06 ppm (ortho w.r.t. the urea group) were observed.
Tracking the shift of the –NH resonance, the binding constant
−
(log Ka) for HCO3 was (WinEQNMR2) calculated to be 5.16
with 1 : 1 host/guest stoichiometry, which is in agreement with
Job’s plot analysis (Fig. S28 and S29, ESI†). However, the best
fitted curve obtained from WinEQNMR2 was for a mixture of
1 : 1 and 1 : 2 host/guest stoichiometries. Similarly, the titration
Fig. 5 Ball-and-stick representation depicting the H-bonding contacts
of complex 2b on AcO− (n-TBA cations are omitted for clarity of the
presentation).
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Table 2 Association constants in log Ka (M−1) of L1 and L2 with different
anions in DMSO-d6 at 298 K, calculated using WinEQNMR2
Log Ka
Anions
(TBA/TEA
salts)
Log K11
(1 : 1 = host :
guest complex)
Log K12
(1 : 2 = host :
guest complex)
Receptor
L1
L1
L1
L2
L2
L2
F−
4.95
3.69
5.16
4.40
3.66
3.32
8.37
6.34
9.06
8.24
6.53
6.79
AcO−
−
HCO3
F−
AcO−
−
HCO3
3.32 for F−, CH3COO− and HCO3−, respectively (Table 2).
However, in all the cases, WinEQNMR2 has given the best fit
curve for the equilibrium mixture of 1 : 1 and 1 : 2 host/guest
stoichiometries. Other halides (Cl−, Br−, I−) and oxyanions
(NO3−, H2PO4−, HSO4−) hardly had any effect on the urea –NH
resonance, indicating very weak interactions with L1 and L2.
We have also checked the UV/Vis absorption properties of
both the receptors in the presence of all the common anions
in excess. Except for F−, AcO− and HCO3− ions, both the recep-
tors showed no response towards the other anions in a dilute
MeCN solution. With F−, AcO− and HCO3−, both the receptors
get red shifted, supporting the solid-state evidences. We have
checked both the receptors with excess of each of the anions,
where L1 gets red shifted by 10 nm with F− as well as AcO−
and 8 nm with HCO3− (Fig. S51†). Similarly, L2 gets red shifted
by 10 nm with F− and 5 nm with HCO3 as well as AcO−
−
(Fig. S52†).
Conclusion
In the 1,3-bis(4-nitrophenyl)urea compound, electron with-
drawing nitro groups render the urea protons sufficiently
acidic to get deprotonated in the presence of fluoride ions,
Fig. 6 Partial 1H NMR spectra (400 MHz, DMSO-d6) of (a) L1 and (b) L2
with the maximum observable shifts of urea –NH resonance upon the
addition of 1 equivalent of HCO3−, CH3COO−, F−, Cl−, Br−, I−, HSO4
H2PO4− and NO3− as their TEA/n-TBA salts.
,
−
−
and they eventually can capture CO2 as HCO3 hydrogen
bonded to the urea receptor.25 As anticipated, the utilization
of 1,3-bis(4-iodophenyl)urea (L1) decreases the possibility of
fluoride ion induced urea deprotonation due to the less
electronegative character of iodine. However, L1 showcases the
exciting property of fluoride ion induced CO2 capture as
data for F− yielded a log Ka value of 4.95 (Fig. S34, ESI†) for
1 : 1 stoichiometry. The highest downfield shift of –NH reso-
nance has been observed with acetate anion with Δδ =
2.73 ppm. The binding constant (log Ka) for CH3COO− was
calculated to be 3.69 with 1 : 1 host/guest stoichiometry
(Fig. S31 and S32, ESI†).
−
HCO3 complex (1a) stabilized by a combined act of hydrogen
and halogen bonding. However, in a control experiment, 1,3-
bis(4-bromophenyl)urea (L2) crystallized as hydrogen bonded
The titration of L2 with F− and CH3COO− showed a huge
downfield shift of the –NH resonance with Δδ = 2.32 ppm and
2.27 ppm for F− and CH3COO−, respectively. However, the
2−
SiF6
complex in the presence of excess fluoride ions,
suggesting its impotency to act as a CO2 scrubber. The profi-
ciency of L1 as a halogen bond donor has also been authenti-
cated in the crystal structures of the free receptor and acetate
complex 1b. The inability of L2 to form halogen bonds with
anions has also been confirmed by the structural elucidation
of its hydrogen bonded acetate complex 2b, suggesting that it
is the combined act of hydrogen and halogen bonding that
prompted the CO2 uptake from a fluoride containing solution
−
titration with HCO3 resulted in a comparatively lesser shift of
Δδ = 1.14 ppm for L2 –NH resonance, which is ∼1.00 ppm less
than that of L1. In all the three cases, the host/guest stoichio-
metry was found to be 1 : 1, which is in agreement with the
Job’s plot analyses (Fig. S44, S47 and S50, ESI†) and the
binding constants (log K) were calculated to be 4.40, 3.66 and
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of 1,3-bis(4-iodophenyl)urea. Overall, we have shown that a are refined by restraints −1.2 or −1.5Uiso (C), although the iso-
subtle variation in the electronic properties of 1,3-bis(4-halo- tropic free refinement is also acceptable. Structural illus-
phenyl)urea receptors resulted in a drastic change in their trations have been drawn with MERCURY-2.3 35 for Windows.
anion recognition properties in solution as well as solid-state.
The parameters for data collection and the crystallographic
refinement details of isolated anion complexes 1a–b and 2b
are summarized in ESI, Table S1.† However, we were not able
to publish the data for complex 2a due to the poor quality of
the obtained crystal.
Experimental section
Materials, instruments and methods
Synthesis and characterizations
All reagents and solvents were obtained from commercial
sources and used as received without further purification.
Phenyl-isothiocyanate, 4-iodo and 4-bromophenylisothio-
cyanate, tetraalkylammonium salts and 4-iodo and 4-bromo-
aniline were purchased from Sigma-Aldrich and used as
received. Solvents for synthesis and crystallization experiments
were purchased from Merck India, and used as received.
L1 and L2. Symmetric receptors L1 and L2 have been syn-
thesised in quantitative yield by the equimolar reaction of the
aromatic amine (4-iodoaniline and 4-bromoaniline) with the
corresponding phenylisocyanate in tetrahydrofuran (THF), and
the colourless product obtained in both the cases was charac-
terized by NMR, FT-IR, and single-crystal XRD analyses.
L1: 1H NMR (DMSO-d6, 400 MHz): δ (ppm) at 298 K, 7.28 (d,
4H, ArH), 7.59 (d, 4H, ArH), 8.832 (s, 2H, –NH). 13C NMR
(150 MHz, DMSO-d6): δ (ppm) 84.98 (2C, ArH), 120.67 (4C,
ArH), 137.43 (4C, ArH), 139.49 (2C, ArH), 152.30 (1C, CvO).
ESI-Mass: m/z = 463.07 [M]+. FT-IR (ν, cm−1): 1005 (C–I), 1236
(C–N), 1549 (CvC), 1637 (–CvO), 3301 (N–H).
The FT-IR spectra of air dried samples were recorded on a
Perkin-Elmer-Spectrum One FT-IR spectrometer with KBr
disks in the range 4000–400 cm−1 1H NMR spectra were
.
recorded on a Varian FT-400 MHz and Bruker 600 MHz instru-
ment, and the chemical shifts were recorded in parts per
million (ppm) on the scale using tetramethylsilane (TMS) or
residual solvent peak as a reference and 13C NMR spectra were
obtained at 100 MHz and 150 MHz.
1
L2: H NMR (DMSO-d6, 600 MHz): δ (ppm) at 298 K, 7.417
(d, 4H, ArH), 7.435 (d, 4H, ArH), 8.829 (s, 2H, –NH).13C NMR
(150 MHz, DMSO-d6): δ (ppm) 113.609 (2C, ArH), 120.457 (4C,
ArH), 131.675 (4C, ArH), 139.027 (2C, ArH), 152.434 (1C,
CvO). ESI-Mass: m/z = 369.06 [M]+. FT-IR (ν, cm−1): 1070
(C–Br), 1236 (C–N), 1555 (CvC), 1641 (–CvO), 3299 (N–H).
Complex 1a, n-TBA[L1·HCO3−]. Colorless block-shaped crys-
tals bicarbonate complex, 1a, suitable for single-crystal X-ray
diffraction analysis were obtained by charging an excess (10
equiv.) of n-tetrabutylammonium fluoride (n-TBAF) into a
5 mL MeCN solution of L1 (46.4 mg, 0.1 mmol). After the
addition of n-TBAF, the initially insoluble L1 gets dissolved in
MeCN and the solution was stirred for about 30 min at room
temperature and filtered in a test tube for slow evaporation.
Association constants were obtained by 1H NMR (Varian-
400 MHz) titrations of the ligands with tetraethyl ammonium
(TEA)/n-tetrabutylammonium (n-TBA) salts of respective
anions in DMSO-d6 at 298 K. The initial concentration of the
corresponding receptor solution was 10 mM. The aliquots of
anions were added from 50 mM stock solutions of anions (up
to 1 : 5 host/guest stoichiometry) and each titration was per-
formed with 15–20 measurements at room temperature.
WinEQNMR2 software was used to calculate the binding con-
stants (K) values.30
X-ray crystallography
In each case, a crystal of suitable size was selected from the After 4–5 days, the isolated yield of 1a was 92%. Mp: 140 °C.
mother liquor and immersed in silicone oil, and it was 1H NMR, DMSO-d6, (Bruker-600 MHz) at 298 K, δ (ppm) 1.011
mounted on the tip of a glass fibre and cemented using epoxy (t, 12H, n-TBA–CH3), 1.28 (q, 8H, n-TBA–CH2), 1.533 (q, 8H,
resin. The intensity data were collected using a Bruker SMART n-TBA–CH2), 3.125 (t, 8H, n-TBA–N+CH2), 7.57 (d, 4H, ArH),
APEX-II CCD diffractometer, equipped with a fine focus 7.520 (d, 4H, ArH), 10.67 (s, 2H, –NH).13C NMR, DMSO-d6
1.75 kW sealed tube Mo-Kα radiation (λ = 0.71073 Å) at 298(3) (Bruker-150 MHz) at 298 K, δ (ppm) 13.59 (4C, n-TBA–CH3),
K, with increasing ω (width of 0.30 per frame) at a scan speed 19.29 (4C, n-TBA–CH2), 23.15 (4C, n-TBA–CH2), 57.64 (4C,
of 5 s per frame. The SMART software was used for data acqui- n-TBA–N+–CH2), 84.12 (2C, ArH), 120.15 (4C, ArH), 137.21 (4C,
sition. Data integration and reduction were undertaken with ArH), 140.46 (2C, ArH), 153.05 (1C, CvO), and 182.12 (1C,
SAINT and XPREP31 software. Multi-scan empirical absorption HCO3 anion), FT-IR (ν, cm−1): 822 (HCO3−1), 1231 (C–N),
−
corrections were applied to the data using the program 1537 (CvC), 1577 (C–O), 1697 (–CvO), 2960 (C–H), 3420
SADABS.32 The structures were solved by direct methods using (N–H), 3511 (O–H).
SHELXS-9733 and refined with full-matrix least-squares on F2
Complex 1b, n-TBA[L1·CH3COO−]. Acetate-complex 1b was
using SHELXL-97.34 All the non-hydrogen atoms were refined obtained by adding an excess of n-tetrabutylammonium
anisotropically and hydrogen atoms attached to all the carbon acetate into a 5 mL MeCN solution of L1 (46.4 mg, 0.1 mmol).
atoms were geometrically fixed and the positional and temp- In the same fashion, after the addition of acetate salt, the
erature factors are refined isotropically. Hydrogen atoms initially insoluble L1 gets dissolved in MeCN and the solution
attached with the urea nitrogen atoms were located from the was stirred for about 30 min at room temperature and filtered
electron Fourier map and refined isotropically. Usually, the in a test tube. The slow evaporation of the filtrate at room
temperature factors of H-atoms attached to the carbon atoms temperature yielded colorless crystals suitable for single crystal
15634 | Dalton Trans., 2014, 43, 15628–15637
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Paper
X-ray crystallographic analysis within 8–10 days. The isolated
yield of 1b was 70%. Mp: 178 °C. 1H NMR, CDCl3, (Varian-
400 MHz) at 298 K, δ (ppm), 0.93 (t, 12H, n-TBA–CH3), 1.294
(q, 8H, n-TBA–CH2), 1.43 (t, 8H, n-TBA–CH2), 2.029 (s, Acetate–
CH3), 2.987 (t, 8H, n-TBA–N+CH2), 7.485 (d, 4H, ArH), 7.53 (d,
4H, ArH), 11.67 (s, 2H, –NH). 13C NMR, DMSO-d6 (Bruker-
150 MHz) at 298 K, δ (ppm) 13.57 (4C, n-TBA–CH3), 19.27 (4C,
n-TBA–CH2), 23.12 (4C, n-TBA–CH2), 24.73 (1C, Acetate–CH3),
57.61 (4C, n-TBA–N+CH2), 84.17 (2C, ArH), 120.59 (4C, ArH),
137.21 (4C, ArH), 140.43 (2C, ArH), 152.97 (1C, CvO) and
176.53 (acetate–COO−). FT-IR (ν, cm−1): 642 (–COO de-
formation), 823 (–COO), 1003 (C–I), 1235 (C–N), 1553 (CvC),
1634 (–CvO), 2961 (C–H), 3301 (N–H).
Notes and references
1 (a) G. A. Jeffrey, An Introduction to Hydrogen Bonding,
Oxford University Press, Oxford, 1997; (b) P. Metrangolo
and G. Resnati, in Halogen Bonding In Encyclopedia of
Supramolecular Chemistry, ed. J. L. Atwood and J. W. Steed,
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2008; (b) P. Metrangolo, H. Neukirch, T. Pilati and
G. Resnati, Acc. Chem. Res., 2005, 38, 386; (c) K. Risannen,
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Eur. J., 2001, 7, 2511.
2−
Complex 2a, 2n-TBA[L2·SiF62−]. The SiF6 complex 2a of L2
was obtained during a similar attempt as in the case of
complex 1a, but with a completely different result. After the
addition of an excess of n-tetrabutylammonium fluoride
(n-TBAF) into a 5 mL MeCN solution of L2 (37 mg, 0.1 mmol)
contained in a glass vowel, the solution was stirred for about
30 min at room temperature and filtered in a test tube. The
slow evaporation of the filtrate at room temperature yielded
colorless crystals within 7–8 days, which were suitable for
single crystal X-ray crystallographic analysis. The isolated yield
of 2a was 65%. Mp: 133 °C. 1H NMR, DMSO-d6, (Bruker-
600 MHz) at 298 K, δ (ppm) 0.922 (t, 12H, n-TBA–CH3), 1.30
(s, 8H, n-TBA–CH2), 1.55 (p, 8H, n-TBA–CH2), 3.143 (s, 8H,
n-TBA–N+CH2), 7.374 (d, 4H, ArH), 7.52 (d, 4H, ArH), 11.043 (s,
2H, –NH).13C NMR, DMSO-d6 (Bruker-150 MHz) at 298 K,
δ (ppm) 13.54 (4C, n-TBA–CH3), 19.27 (4C, n-TBA–CH2), 23.14
(4C, n-TBA–CH2), 57.64 (4C, n-TBA-N+CH2), 112.69 (2C, ArH),
120.14 (4C, ArH), 131.31 (4C, ArH), 140.44 (2C, ArH) and
153.41 (1C, CvO). FT-IR (ν, cm−1): 740 (SiF62−), 1008 (C–Br),
1227 (C–N), 1648 (–CvO), 2962 (C–H), 3422 (N–H).
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Complex 2b n-TBA[L2·CH3COO−]. Acetate-complex 2b was
obtained in the same way as complex 1b. Mp: 118 °C. 1H NMR,
DMSO-d6 (Bruker-600 MHz) at 298 K, δ (ppm), 0.91 (t, 12H,
n-TBA–CH3), 1.293 (q, 8H, n-TBA–CH2), 1.541 (s, 8H, n-TBA–
CH2), 1.799 (s, Acetate–CH3), 3.132 (s, 8H, n-TBA-N+CH2), 7.384
(d, 4H, ArH), 7.547 (d, 4H, ArH), 11.39 (s, 2H, –NH). 13C NMR,
DMSO-d6 (Bruker-150 MHz) at 298 K. δ (ppm) 13.57 (4C,
n-TBA–CH3), 19.29 (4C, n-TBA–CH2), 23.15 (4C, n-TBA–CH2),
24.74 (1C, Acetate–CH3), 57.65 (4C, n-TBA-N+CH2), 112.49 (2C,
ArH), 120.15 (4C, ArH), 131.31 (4C, ArH), 140.46 (2C, ArH),
153.44 (1C, CvO) and 176.53 (Acetate–COO−). FT-IR (ν, cm−1):
642 (–COO deformation), 829 (–COO), 1070 (C–Br), 1236 (C–N),
1564 (CvC), 1634 (–CvO), 2962 (C–H), 3305 (N–H).
Acknowledgements
G.D. acknowledges CSIR (01/2727/13/EMR-II) and SERB (SR/
S1/OC-62/2011) New Delhi, India for financial support, CIF, IIT
Guwahati and Department of chemistry for providing instru-
ment facilities and DST-FIST for single crystal X-ray diffraction
facility. R.C. thanks IIT Guwahati for Senior Research
Fellowship.
9 H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949,
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