Perfluoroalkylated Calix[4]pyrroles: Fluoride Ion Extraction from an Aqueous Medium

Article abstract of DOI:10.1002/asia.201700897

Octaalkenyl calix[4]pyrrole ((CH₂=CH(CH₂)₂)₈C4P) is highly useful for the postfunctionalization of different calix[4]pyrroles with desired functionalities. Functionalization with perfluoroalkyl chains [CF₃(CF₂)_n; R_fn] gave perfluoroalkyl calix[4]pyrroles (R_fn(CH₂)₄)₈C4P; n=6, 8), having >60 % fluorine content, which created a hydrophobic environment inside the calix[4]pyrrole cavity and recognized fluoride and chloride ions in solution as well as in the solid state. The fluoride ion is extracted efficiently from aqueous CsF and TBAF solutions by using (R_f6(CH₂)₄)₈C4P, as droplets. The fluorinated chain generated a hydrophobic environment which broke the hydration shell associated with the anion and separated out fluoride ions as droplets from aqueous medium. Furthermore, the fluoride ions competitively replaced chloride ions from the (R_f6(CH₂)₄)₈C4P cavity.

Full text of DOI:10.1002/asia.201700897

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Accepted Article
Title: Perfluoroalkylated Calix[4]pyrrole: fluoride ion extraction from
aqueous medium
Authors: Sinchan Maji and Debaprasad Mandal
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Perfluoroalkylated Calix[4]pyrrole: Fluoride Ion Extraction from
Aqueous Medium
Sinchan Maji and Debaprasad Mandal*
Abstract: Octaalkenyl calix[4]pyrrole (CH
2
=CH(CH
2
)
2
)
8
C4P) is highly
cavity for specific interaction with different guest molecules. For
example, the incorporation of perfluoroalkyl groups [CF (CF
useful for post functionalization towards design of different
calix[4]pyrrole with desired functionality. Functionalization with
3
2 n-1
)
abbreviated Rfn] at the terminal double bond would create
hydrophobic environment inside the calix[4]pyrrole cavity due to
highly hydrophobic nature of perfluoroalkyl chains. Hofmeister
biased anions like fluoride ion remains highly solvated in aqueous
perfluoroalkyl chains [CF
3
(CF
2
)
n
;
R
fn
]
gives perfluoroalkyl
calix[4]pyrroles (Rfn(CH C4P; n=6, 8), having >60% fluorine
2 4 8
) )
content which creates hydrophobic environment inside the
calix[4]pyrrole cavity and recognizes fluoride and chloride ion from
solution as well as solid. Fluoride ion is extracted efficiently from
–
solution having very high hydration energy. Once the hydrated F
interacts via hydrogen bonding in the calix cavity, the hydration
shell is destroyed by the hydrophobic environment of
perfluoroalkyl chains (Rfn). Now the interaction between calix-NH
2 4 8
aqueous CsF and TBAF solutions by (Rf6(CH ) ) C4P as droplets.
Fluorous chain generates hydrophobic environment which destroys
the hydration shell associated with anion and separates out fluoride
ion as droplets from aqueous medium. Further fluoride ion
–
and F completely depends on charge density of anion without the
hydration shell and overcomes the Hofmeister biasness.
competitively replaces chloride ion from (Rf6(CH
2
)
4
)
8
C4P cavity.
Fluoride ion, owing to its significance in biology and environmental
pollution, has been one of most popular target for recognition and
9
sensing studies. While most neutral receptors recognize fluoride
Anion specific recognition, extraction and transportation play a
ion from tetrabutylammonium fluoride (TBAF) salt in organic
solvents, only a few can detect fluoride ion from TBAF, KF or CsF
1
key role in several chemical and biological processes. Selective
recognition and extraction of anions particularly the charge-dense
small anions like fluoride, sulfate which are highly hydrophilic, and
–
in aqueous solutions or extracted it from water where F received
least preference among several anions due to its high hydration
2
residing far down of Hofmeister series is often challenging owing
7d,10
energy.
to their high hydration energy. Anion receptors with a variety of
hydrogen bond donors like urea, pyrrole and indole based
systems have been developed for specific interactions where
calix[4]pyrrole has proved to be highly effective and a novel anion
Here, we have shown the synthesis of meso-substituted
octaalkenyl calix[4]pyrrole and dipyrromethane derivative
followed by further substitution of their terminal double bonds with
perfluoroalkyl groups. These perfluoroalkylated calix[4]pyrrole
receptors recognize fluoride ion selectively from solution as well
as solids.
3
receptor. Calix[4]pyrroles are extensively used as neutral
receptors for sensing, extraction and transportation of anions, ion-
4
5
pairs, electron rich compounds and biomolecules . Octamethyl
–
–
calix[4]pyrrole binds effectively with small anions like F and Cl
6
owing to its small cavity size in non-aqueous solvents.
Modification of calix[4]pyrrole cavity is essential to recognize
these anions from aqueous phase. Other particular guests are
accommodated often by modulation of cavity with specific
functionalities by substitution at meso-position or β-pyrrole
7
,8
position of calix[4]pyrrole. However, the modifications are often
cumbersome and offer poor yield due to formation of unwanted
oligomeric products. An easily available common meso-
substituted calix[4]pyrrole would be of great importance to
achieve various customized structure for specific interactions with
different guest molecules. Substitution of alkyl chains with
terminal double bond at the meso-position of calix[4]pyrrole as
tolerable functionality would be a common precursor for various
modifications, where post functionalization of the terminal double
bonds will aid in synthesizing different calix[4]pyrrole with tunable
ꢀ
Scheme 1. Synthesis of octaalkenylcalix[4]pyrrole 2b and
dipyrromethane 3
We have synthesized octa(but-3-en-1-yl)calix[4]pyrrole 2b with
2
5% yield as shown in Scheme 1. However, the homologous
compound 2a with one methylene spacer at meso-position could
not be synthesized in several attempts (Scheme 1). When
equimolar 1a and pyrrole were reacted by varying solvents and
acid catalysts, in each case unreacted ketone was recovered and
[a]
Sinchan Maji, Dr. Debaprasad Mandal
Department of Chemistry
Indian Institute of Technology Ropar
Nangal Road, Punjab 140001, India
E-mail: dmandal@iitrpr.ac.in
a
black polymeric pyrrole was obtained. Further, 1a was
converted to dipyrromethane with large excess of pyrrole
1:10) where pyrrole acted as both reactant as well as solvent. It
3
(
Supporting information for this article is given via a link at the end of
the document
was assumed that dimer formation is facile due to its flexibility
whereas ketone 1a was unable to coordinate with pyrrole to give
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Chemistry - An Asian Journal
10.1002/asia.201700897
COMMUNICATION
2
a due to either steric or electronic effect or both. Whereas,
analogous 4-heptanone easily forms octapropylcalix[4]pyrrole (5)
with pyrrole over 30% yield (Scheme S3, SI). Dimer 3 was
obtained as 45% yield after purification by column
chromatography. Calix[4]pyrrole 2b, and 3 were characterized by
1
13
H, C NMR and HRMS. Single crystals of 2b obtained from slow
evaporation of DCM/MeOH mixture (v/v 10/1) was also analyzed
by single crystal X-ray measurement. The crystal structure shows
2
that calix[4]pyrrole 2b possess a C symmetry and two plane of
symmetry (C2v point group) (Figure S1, SI). The pyrrole rings
adopt 1, 3-alternate conformations and alkenyl groups at meso-
positions are directed outward from the calixpyrrole cavity. The
crystal structure does not show any intermolecular interaction in
the solid state.
Scheme 3. Fluoride ion inside the hydrophobic cavity of 4a after
destroying the hydration shell.
Terminal double bonds of octaalkenyl calix[4]pyrrole 2b and
dimer 3 can easily be functionalized with the addition of phosphine,
amine, hydroxyl, borane and perfluoroalkyl group. The addition of
eight perfluoroalkyl groups (perfluorohexyl, Rf6 and perfluorooctyl,
Presence of perfluoroalkyl chain at meso-position of 4a creates
hydrophobic environment which is helpful for extraction of anions
from aqueous medium since kosmotropic anions have an affinity
R
f8) to these double bonds of 2b give perfluoroalkyl calix[4]pyrrole
13
for hydrophobic concavity.
4a undergoes conformational
4
a and 4b according to Scheme 2 as light yellow solids with 65%
change to attain cone conformation when interacts with an
acceptor anion. At the same time, the anion bound cone
conformer calix core will rearrange the meso perfluoroalkyl chains
which make the calix[4]pyrrole cavity hydrophobic as shown in
Scheme 3. Subsequently, the solvated anion coordinated with
pyrrole NH would release the water molecules of hydration shell
from hydrophobic environment and overcomes Hofmeister bias in
anion extraction process.
1
19
yield. 4a and 4b were characterized by H, F NMR and HRMS,
1
wherein H NMR shows doublets for β-pyrrole CH at 5.93 ppm
and a broad singlet for pyrrole NH at 7.0 ppm (Figure S5, S9, SI)
suggesting 1,3-alternate conformation in CDCl
3
like other
3
, C CF ,
1
1
calix[4]pyrrole. 4a and 4b both are soluble in PhCF
3
6
F
5
chloroform, THF, and ether but are insoluble in methanol, water,
acetonitrile, and DMSO. Both fluorous calix[4]pyrroles behave
very similar except 4b is more fluorophilic due to more fluorine
content than 4a. Perfluorohexyl calix[4]pyrrole 4a forms needle
shape crystals upon slow evaporation from ethyl acetate/hexane
mixture (v/v 1:9). However, single crystal X-ray data could not be
solved due to poor data, probably due to long fluorine chains.
Perfluoroalkyl calix[4]pyrroles (fluorine content ≥60 wt%) are
highly hydrophobic due to large hydrophobic surface of
perfluoroalkylchains and low polarizability of fluorine compared to
Dimer
3 is an important precursor to achieve different
calix[4]pyrrole derivatives from the reaction with different
14
ketones. For example, tetramer 6 and 7 were synthesized from
the reaction of
3 with acetone and 4-hydroxy-2-butanone
respectively (Scheme S4, SI). Calix[4]pyrroles 6 and 7 were
1
13
characterized by H, C NMR and HRMS. Compound 6 is a
1
mixture of two isomers, however, we didn’t observe them in H
NMR. Further, 3 was reacted with benzyl phosphine, to form 3°
phosphine bridged dimer 9 as a major product and 2° phosphine
attached with dimer 8 as minor product (Scheme S4, SI).
In a preliminary study anion binding affinity of 4a was analyzed
1
2
hydrogen.
1
19
with tetraalkylammonium halide by H and F NMR titration in
CDCl and in THF-d . Since 4a is insoluble in most of the polar
3
8
solvents, we have analyzed the binding affinity by solid-liquid
extraction of different tetramethylammonium (TMA) salts. It was
–
–
observed that CDCl
3
solution of 4a can extract F and Cl from
O and TMACl salts respectively. In case
excess solid TMAF.4H
2
1
of TMAF, H NMR showed a clear shift in NH protons from 6.99
ppm to 12.33-12.43 ppm and β-pyrrole CH protons were shifted
–
upfield from 5.93 ppm to 5.52 ppm on binding with F as shown in
Figure 1. Further the interaction of fluoride with NH protons of
calix showed splitting into doublet at room temperature due to
coupling between bound fluoride anion and pyrrole-NH protons.
The extraction of TMAF was also supported from the peak at 3.22
Scheme 2. Synthesis of perfluoroalkyl calix[4]pyrroles 4a and 4b
+
19
ppm correspond to the protons of [TMA] . The F NMR spectrum
of perfluorohexyl calix[4]pyrrole-TMAF complex showed
appearance of a new peak at –93.69 ppm (multiplet) which
indicates fluoride anion bound with pyrrole NH of host
calix[4]pyrrole and coupling between them (Figure S17, SI).
Similarly, on addition of excess solid TMACl to host solution,
pyrrole NH proton was shifted to 11.18 ppm and β-CH peaks were
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Chemistry - An Asian Journal
10.1002/asia.201700897
COMMUNICATION
–
shifted to 5.51 ppm (Figure 1). Cl shows a weaker binding and
of TBAF, pyrrole NH undergoes downfield shift to 12.50 ppm and
1
the binding is only partial as observed in H NMR, one part of NH
β-pyrrole proton was shifted upfield to 5.44 ppm in THF-d
8
(Figure
and β-CH protons were shifted while other part remained at the
3). Upon successive addition of TBAF to 4a in THF-d during NMR
8
–
–
–
same position. However, other TMA salts (Br , I , NO
3
and
) do not show any interaction according to NMR studies.
More importantly, solid CsF showed small binding in CDCl as
evidenced from the shift of NH protons and β-CH in H NMR
Figure S18, SI).
titration, both free calix 4a, (pyrrole NH, 9.40 ppm and β-pyrrole
–
–
HSO
4
CH 5.87 ppm) and fluoride bound complex 4a-F (pyrrole NH-F
3
(12.50 ppm and β-pyrrole CH 5.87 ppm) were present together
and no intermediate peak of partial binding could be observed
indicating a fast complexation kinetics between 4a and fluoride
ion (Figure S19, SI). Screening of other anions for 4a in THF didn’t
1
(
–
–
give any colorimetric response except F and Cl (Figure 2a) or
+
4
a + TMACl
–
3
Me N
change in the NMR. It is interesting that 4a-F complex was
protected within the fluorous cavity even in the presence of water.
Upon addition of D
change was observed both in pyrrole NH as well as β-pyrrole CH
peak positions. On further addition of large excess of D O only
2
O (30 mol% of host) to the NMR solution no
4a + TMAF
+
N
Me
3
2
exchange of NH peak took place whereas β-CH peak shifted only
slightly downfield due to the dilution effect (Figure S20, SI).
Further, when aqueous solution of TBAF was added to the THF
4a
solution of 4a, droplets were formed and settled at the bottom
–
(
Figure 2b), as 4a extracts and separates out F from water in the
form of droplets. The isolated droplets gave dark violet colour
1
1
19
8
when dried and the H NMR in THF-d showed H and F NMR
similar to 4a-TBAF complex (Figure 3 and S22, SI). It was
interesting to note that the remaining solution after separation of
droplets showed only excess of TBAF and a negligible trace of
1
3
Figure 1. Changes in H NMR of 4a in CDCl upon addition of
quantities of solid TMAF and TMACl
1
the host in H NMR (Figure S23, SI). This ensures that maximum
–
amount of host separates out as droplets by extracting F from
aqueous TBAF solution. In a separate experiment when excess
A
B
C
D
E
F
G
H
4
a in THF-d
8
was stirred with TBAF/D
2
O solution, no TBAF traces
O solution confirms
could be detected in the remaining D
2
–
complete extraction of F . To understand the extraction ability, 4a
was treated with aqueous CsF where perfluoro calix[4]pyrrole 4a
–
isolates F as droplets (Figure 2b). After encapsulation of fluoride
ion from the aqueous interface, the complex becomes insoluble
either in THF or water, therefore host-guest complex separates
out as droplets from water-THF solvent mixture. These results
suggest that 4a is capable of extracting fluoride ion exclusively
Figure 2a. Colours of THF solutions (A) 4a (B) 4a + TBAF.3H
4
4
2
O (C)
a + Aliquat 336 (D) 4a + TBABr (E) 4a + TBAI (F) 4a + TBAHSO (G)
a + TMAF in MeOH (H) 4a + TMACl in MeOH
4
from their aqueous solutions. Association constant (K
a
) of fluoride
5
-1
ion with 4a was obtained as 1.24 × 10
M
using Cram’s
1
5
extraction method with 65.7% extraction yield of fluoride ion.
A
B
C
Since the alkyl chains also exert hydrophobicity, a similar binding
studied of TBAF with octapropylcalix[4]pyrrole 5 was performed
in chloroform and in THF-H
2
O mixture. According to β-pyrrole shift
1
in H NMR, a weak binding of fluoride with 5 was detected in
–
CDCl
3
with excess of TBAF which releases F upon addition of
D
2
O and the β-pyrrole peaks return to its previous position (Figure
S24 SI). Further, when aqueous TBAF solution was added to a
Figure 2b. (A) 4a in THF; droplets from (B) 4a in THF + TBAF in H
C) 4a in THF + CsF in H
2
O
THF solution of 5, no droplets formation or any changes was
(
2
O
1
observed in H NMR. From these experiments it was clear that
the hydrophobicity of perfluoroalkyl group in calix[4]pyrroles helps
extraction of fluoride ion from aqueous medium overcoming
Hofmeister bias rather than long chain alkyl groups.
Moreover, when TBAF was added to a THF solution of 4a the
colour changed from yellow to deep wine-red (Figure 2a) possibly
due to the pyrrole-F complex formation in polar aprotic solvents
which is now possible for naked eye detection. Similarly, Aliquat
–
3
2
36 (Cl source) gives a very slow colour change over more than
h. Similarly, in UV-vis study appearance of a new peak at 375
–
–
nm for F and Cl complex of 4a was observed in THF (Figure S15,
SI). It was further confirmed by NMR study where upon addition
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Chemistry - An Asian Journal
10.1002/asia.201700897
COMMUNICATION
extracted efficiently and isolated from aqueous CsF and TBAF by
(
2 4 8
(Rf6(CH ) ) C4P) exclusively as droplets.
Droplets
Acknowledgements
We thank Department of Science and Technology (DST), India
4a + TBAF
(
SB/FT/CS-046/2012) for financial support; S. Maji thanks IIT
Ropar for Fellowship.
Keywords: perfluoroalkyl • calix[4]pyrrole • fluoride ion• anion
4a
recognition • Hofmeister bias
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1]
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Figure 3. Comparative H NMR in THF-d
8
of host 4a, 4a-F complex
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–
In order to understand the comparative binding affinity of F and
–
Cl with the host, equimolar quantities (4.50 equiv.) of both anions
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pyrrole NH peak appears at 12.33-12.43 ppm and the β-CH peaks
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2
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Lett. 2014, 16, 6128-6131.
clearly indicates the formation of ((Rf6(CH
2
)
4
)
8
)calix-F complex
rather than chloride bound complex. To get further insight, both
anions were added sequentially in different equivalence. Initially
[
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addition of 1.0 equiv. of TMACl (solid) to host showed a peak at
–
1
1.18 ppm corresponding to Cl ions bound with pyrrole NH
protons. On successive addition of TMAF (0.5 equiv on 1st
addition and 1.0 equiv. on 2nd addition), the NH peak
5
92.
–
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corresponding to bind Cl decreased and a new peak appeared
–
at 12.33-12.43 ppm gradually. On further addition of TMAF, Cl
–
from cavity was completely replaced by F (Figure S26, SI).
However, further addition of excess TMACl to the resulting
[
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–
solution showed no change in NMR which indicates that F ion
–
–
can replace bound Cl ion from host whereas Cl is not able to
–
remove bound F anion. Therefore perfluorohexyl calix[4]pyrrole
is a better fluoride anion receptor than chloride. Fluoride anion
binding capability of the perfluorocalix[4]pyrrole receptor was also
1
9
confirmed from F NMR. Competitive anion binding studies were
also done with TMABr, TBAI, TMANO and TBAHSO but the
3
4
corresponding anions of these salt did not show any binding with
host perfluorohexyl calix[4]pyrrole.
In conclusion we have synthesized terminal double bond
substituted meso-octaalkenyl calix[4]pyrrole and dialkenyl
dipyrromethane as common useful precursors in good yield for
further functionalization for designing different calix[4]pyrrole with
desired functionality. The terminal double bonds were easily
functionalized with perfluoroalkyl chains and phosphine and they
can be further extended to borane, hydroxyl functionalization and
polymerization for different applications. The dipyrromethane
derivative is an important precursor for the synthesis of various
unsymmetrical calix pyrroles. Highly hydrophobic perfluoroalkyl
[8]
[
[
9]
–
–
2
27–233;.
substituted calix[4]pyrrole recognizes hydrated F and Cl anion
from solid as well as from aqueous solution overcoming
Hofmeister bias with colorimetric detection. Fluoride ion was
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Chemistry - An Asian Journal
10.1002/asia.201700897
COMMUNICATION
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Chemistry - An Asian Journal
10.1002/asia.201700897
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Fluoride anion extraction from
Sinchan Maji and Debaprasad Mandal*
water:
Perfluoroalkyl
substituted
calix[4]pyrroles entrap the fluoride ion
in its hydrophobic cavity and separates
out as droplets from aqueous solution
by destroying the hydration shell and
overcome the Hofmeister bias.
Page No. – Page No.
Perfluoroalkylated Calix[4]pyrrole:
Fluoride Ion Extraction from Aqueous
Medium
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