Synthesis and anion binding properties of the smallest meso-expanded calix[4]pyrrole

Article abstract of DOI:10.1007/s12039-018-1480-x

Abstract: The smallest core expanded calix[4]pyrrole derivative (5) was synthesized via incorporation of an additional sp³meso-carbon at the periphery of the macrocycle. Anion binding study of the macrocycle reveals clearly the effect of core s

Full text of DOI:10.1007/s12039-018-1480-x

J. Chem. Sci. (2018) 130:90
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1480-x
REGULAR ARTICLE
Special Issue on Modern Trends in Inorganic Chemistry
Synthesis and anion binding properties of the smallest
meso-expanded calix[4]pyrrole
∗
B SATHISH KUMAR and PRADEEPTA K PANDA
School of Chemistry, University of Hyderabad, Hyderabad, Telangana 500 046, India
E-mail: pkpsc@uohyd.ernet.in; pradeepta.panda@uohyd.ac.in
MS received 14 February 2018; revised 15 March 2018; accepted 19 March 2018
Abstract. The smallest core expanded calix[4]pyrrole derivative (5) was synthesized via incorporation of an
3
additional sp meso-carbon at the periphery of the macrocycle. Anion binding study of the macrocycle reveals
−
−
−
−
−
−
clearly the effect of core size on its affinity towards various tested ions (F , Cl , Br , I , AcO , H2PO ,
4
−
−
−
−
−
−
−
HSO , ClO , PF , NO , NO , N and CN ). The macrocycle displays the highest affinity towards fluoride
4
4
6
3
2
3
and acetate ions, albeit with reduced affinities compared to parent octamethylcalix[4]pyrrole.
Keywords. Calix[4]pyrrole; meso-expanded Calix[4]pyrrole; host–guest complex; hydrogen-bond; anion.
1
. Introduction
we have reported calix[2]bispyrrolylethenes (2); among
these receptors, 2a displayed colorimetric sensing of
fluoride ion in polar aprotic solvents (Figure 1). Sim-
8
The increased attention towards selective recognition
of fluoride ion is driven by its applications in envi-
ronmental and health sectors, especially in dental care
and in the treatment of osteoporosis. As a result, a
large number of synthetic receptors containing H-bond
donor motifs, Lewis acidic sites and electron defi-
cient organic π-systems have been designed. In this
direction, calix[4]pyrrole 1 has emerged as an attrac-
tive neutral host for anions in the last two decades
ilarly, we could also demonstrate fluorometric sensing
of fluoride ion by calix[2]bispyrrolylarenes 3a–b (Fig-
1
9
ure 1). However, the calix[2]bispyrrolylarenes, in spite
of their close resemblance to calix[2]bispyrrolylethene,
9
display significant affinity towards acetate ion. Further,
2
recently we reported a new type of meso-expanded
calix[4]pyrrole 4 i.e., calix[2]bispyrrolylethane, which
stabilizes infinite 1D chain of water molecules in solid
state, whose anion binding studies will be reported sep-
3
(Figure 1). Consequently, a large number of trans-
formations have been effected on the periphery of
this macrocycle, including modification of its binding
core. As calix[4]pyrrole 1 mostly binds to smaller
halide ions, it was anticipated that bigger core size
would be more suitable to bind larger anions. The
core expansion could be achieved by increasing the
number of pyrrolic moieties (increases the number of
binding sites for hydrogen bonding) or by incorpora-
tion of a spacer between the constituent pyrrolic units.
Towards this, in 1997 the first expanded calixpyrrole
10
arately (Figure 1).
4
Herein, we report the synthesis of a novel meso-
expanded calix[4]pyrrole 5, which belongs to a new
class of macrocycle, namely, the smallest expanded
calix[4]pyrrole. Further, we have also investigated the
anion binding properties of this new macrocycle in both
solution and solid states. The objective was to explore
the relation between the core size and the anion affinity
and selectivity via an incremental change in core size
of the parent calix[4]pyrrole 1 and subsequently com-
pare their anion binding behavior. In particular, we were
interested to know whether the expansion of core size
can lead to the specific selectivity of the macrocycle
towards fluoride ion.
5
was reported by Sessler and coworkers, subsequently,
higher calix[n]pyrroles were synthesized. Further, a
new type of expanded calix[4]pyrroles, containing big-
ger core size, were made by using spacer units viz.,
6
7
carbazole and benzene moieties. In this regard, recently
*
For correspondence
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1480-x) contains
supplementary material, which is available to authorized users.

90
Page 2 of 8
J. Chem. Sci. (2018) 130:90
Figure 1. Calix[4]pyrrole 1 and some of its meso-expanded calix[4]pyrrole analogues (2–4).
2
. Experimental
δ in ppm 137.91, 137.84, 137.75, 131.71, 104.92, 103.32,
1
02.82, 35.42, 35.12, 29.16, 28.98, 26.85. HRMS m/z calcu-
+
2.1 Materials and physical measurements
lated for (M + H ) C27H N4: 415.2862, found: 415.2856.
3
5
1
2
10
Starting materials 6 and 7 were synthesized, character-
Chemicals, reagents and solvents were purchased from com- ized according to the procedure available in literature, and the
mercially available sources. Solvents were purified with characterization data of compounds 410 and 1 were matched
appropriate drying procedure.11 All the tetrabutyl ammonium with the available literature data.
salts and analytical grade solvents for NMR and ITC titration
3
experiments were purchased from Sigma-Aldrich and were
directly used in the titration experiments.
2.3 Single crystal X-ray diffraction analysis
Melting points were determined by slides with cover-
slips on MR-Vis visual melting point range apparatus from
Crystallographic data for 5 was collected on a dual source
Oxford Gemini A Ultra diffractometer and Mo-Kα (λ =
+
LABINDIA Instruments Private limited. IR spectra were
recorded on a JASCO FTIR model 5300 and NICOLET 5700
FT-IR spectrometers. LCMS analyses were carried out by
Shimadzu-LCMS-2010 mass spectrometer and HRMS data
were obtained with Bruker Maxis spectrometer. NMR spec-
tra were recorded on Bruker 400 MHz FT-NMR spectrometer
operating at ambient temperature. TMS was used as internal
0
.71073 Å) radiation was used to collect the X-ray reflec-
Pro
tions. Data reduction was performed using CrysAlis
1
13
71.33.55 software. The structure was solved by using
Olex2-1.0 with anisotropic displacement parameters for
non-H atoms and final refinement was done by SHELXL-
14
2
014/7. Empirical absorption correction was done using
spherical harmonics, implemented in SCALE3 ABSPACK
scaling algorithm. Crystallographic data for 5·TBACl was
collected on BRUKER SMART-APEX CCD diffractome-
ter and Mo-Kα (λ = 0.71073 Å) radiation was used to
collect the reflections on the single crystal. Data reduction
was performed using Bruker SAINT15 software. Intensi-
1
standard for H NMR spectra.
2
.2 Synthesis of macrocycle 5
ꢀ
12
5
,5 -Dimethyldipyrromethane 6 (108 mg, 0.625 mmol) and
1
6
1
0
ties for absorption were corrected using SADABS and
refined using SHELXL-2014/714 with anisotropic displace-
ment parameters for non-H atoms. Hydrogen atoms on O and
N were experimentally located in difference electron density
maps. All C-H atoms were fixed geometrically using HFIX
command in SHELX-TL. A check of the final CIF file using
PLATON17 did not show any missed symmetry (Table 1).
bispyrrolylethane (BPE) 7 (100 mg, 0.625 mmol) were dis-
solved in dry dichloromethane (20 mL), to this dry acetone
2.3 mL, 0.312 mmol) was added under nitrogen atmosphere.
(
Then, boron trifluoride diethyl etherate (10 µL, 0.156 mmol)
was added and stirred at room temperature. After 30 min, the
starting material was finished and as anticipated, three new
spots were observed in TLC. Then, the reaction was stopped
by quenching with sat. sodium bicarbonate solution (5 mL),
the organic layer was separated and concentrated in a rotary 2.4 Anion binding study
evaporator, and the crude product was purified by silica gel
column chromatography by slow elution with 5–10% ethyl 1H NMR titration studies were carried out in a 400 MHz NMR
acetate in hexane. The three products were isolated as (5, spectrometer in acetonitrile-d3 (0.5% v/v D2O in case of flu-
◦
34 mg, 13%), (4, 30 mg, 23%) and (1, 17 mg, 13%).
oride ion) solvent at 25 C. All anions were used in the form
Characterization data of compound 5: Melting point: 162– of their tetrabutylammonium (TBA) salts (fluoride salt as its
◦
1
1
7
64 C (dec.); FTIR Data (KBr): 3386.91, 2970.97, 1416.85, trihydrate). The receptor solutions were titrated by adding
−1 1
040.00, 688.23 cm . HNMR(400MHz, CDCl3):δ inppm known quantities of a concentrated solution of the anions
.47 (s, 2H, NH), 7.08 (s, 2H, NH), 5.95–5.89 (m, 6H, pyr- in question. The anion solution used to effect the titrations
role β-CH), 5.82–5.80 (brs, 2H, pyrrole β-CH), 2.81 (brs, 4H, contained the receptor at the same concentration as the recep-
−
CH2), 1.51 (s, 18H, CH3). 13C NMR (100 MHz, CDCl3): tor solutions into which they were being titrated. The data

J. Chem. Sci. (2018) 130:90
Table 1. Crystallographic data and structure refinement information.
Page 3 of 8 90
Compound
5·EtOAc
5·TBACl
Formula
Formula weight
Color
C31 H42 N4 O2
C43 H70 Cl N5
692.49
Colorless
502.69
Colorless
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
293(2) K
0.71073 Å
Triclinic
298(2) K
0.71073 Å
Monoclinic
Cc
a = 11.604(4) Å, α = 90
P − 1
◦
◦
◦
◦
a = 11.031(3) Å, α = 105.86(2)
◦
b = 12.413(3) Å, β = 107.21(2) b = 19.303(6) Å, β = 93.080(6)
◦
c = 12.599(3) Å, ν = 106.45(2)
c = 19.312(6) Å, ν = 90
Volume = 1455.4(6) Å3
2
Volume = 4320(2) Å3
4
Z
3
3
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
1.147 g/cm
1.065 g/cm
−
1
−1
0.072 mm
0.122 mm
544
1520
3
3
0.21 × 0.20 × 0.16 mm
0.28 × 0.27 × 0.25 mm
◦
◦
2.77 to 24.30
2.05 to 26.63
−12 ≤ h ≤ 12, −11 ≤ k ≤ 14,
−14 ≤ h ≤ 14, −24 ≤ k ≤ 24,
−
14 ≤ l ≤ 11
−24 ≤ l ≤ 24
Reflections collected
8599
22473
Independent reflections
Completeness to theta
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
4735 [R(int) = 0.1958]
99.9% (θ 24.30 )
Semi-empirical from equivalents Semi-empirical from equivalents
0.989 and 0.985
Full-matrix least-squares on F2
4735/13/344
8835 [R(int) = 0.1252]
98.7% (θ 25.242 )
◦
◦
0.970 and 0.966
Full-matrix least-squares on F2
8835/10/436
0.917
0.944
R1 = 0.1301, wR2 = 0.3054
R1 = 0.3476, wR2 = 0.4073
R1 = 0.1079, wR2 = 0.2485
R1 = 0.3097, wR2 = 0.3520
−3
−3
Largest diff. peak and hole
0.35 and −0.28 e.Å
0.227 and −0.201 e.Å
were fitted to a 1:1 binding profile in MATLAB 7.0 package is ∼286 µL). Further, we have checked ITC profile of the
1
8
according to the method of Wilcox using the changes in the basic octamethylcalix[4]pyrrole (1) in presence of TBAF, and
1
pyrrolic NH resonances in the H NMR spectra. Job’s plot TBACl to confirm the accuracy and reproducibility of per-
◦
for host vs. anion was performed in acetonitrile-d3 at 25 C. formed methods.
Equal concentration of compound and anion solution were
mixed in ratios from 1:10 to 10:1 and the chemical shift was
monitored and noted vs the residual CD3CN (1.94 ppm) res-
onance. In this experiment β-CH proton (in case of TBAF)
and NH proton (in case of other ions) resonances of pyrrole
are monitored.
Microcalorimetric titrations were performed using an
isothermal titration calorimeter (ITC) purchased from
Microcal Inc., MA. The Origin software provided by
Microcal Inc. was used to calculate the binding constant
2
.5 Computational Calculations
All DFT structure optimization calculations were done in
Gaussian09 software by using B3LYP method 6-31G*(d,p)
basis set in the gas phase. In all these calculations, the host
molecule was set in cone conformation and assumed 1:1 com-
plex with respect to anions.
19
(
Ka) and the enthalpy change (ꢀ H). Host–guest solutions
are prepared in 0.5% water in acetonitrile in case of fluo- 3. Results and Discussion
ride as a guest and only in acetonitrile (HPLC grade with
<
0.05% water) in case of other ions. The host solution 3.1 Synthesis and characterization of macrocycle 5
is about 2–2.5 mL of required concentration prepared and
taken in VPITC cell (volume ∼1.44 mL), and the correspond- Mixed condensation of 5,5 -dimethyldipyrromethane
ing guest solution about 1–2 mL of required concentration
and bispyrrolylethane (BPE) 7 with acetone in
ꢀ
12
10
6
is prepared and taken in VP-ITC syringe (whose volume presence of BF ·OEt or TFA as the acid catalyst
3
2

90
Page 4 of 8
J. Chem. Sci. (2018) 130:90
Scheme 1. Synthesis of macrocycle 5.
Figure 2. ORTEP-POVray diagram of 5, top view (left) and two different side views (right). Solvent
molecule (EtOAc), all hydrogens bound to carbon atom are excluded for clarity. Thermal ellipsoids are
scaled upto 35% probability level. Color code: grey = Carbon, white = Hydrogen and blue = Nitrogen.
(Scheme 1) resulted in the formation of three macro- with their NHs disposed towards the core (because of
cycles as expected (Scheme 1), where the formation the flexible ethylene bridge) (Figure 2).
nd
of macrocycle 5 (2 fraction, 13%) remain unaltered.
Further, ethylacetate molecule resides as solvate in
However, in the presence of BF ·OEt , the reaction the crystal lattice with hydrogen bonding interactions,
3
2
completes rapidly (in 30 min) with higher yield of where the carbonyl oxygen of ethyl acetate molecule is
rd
macrocycle 4 (3 fraction, 23%) and lower yield of H-bonded with one of the pyrrole-NH of the constituent
st
calix[4]pyrrole 1 (1 fraction, 13%); on the other hand, BPE moiety, having N· · · O distance of 2.99(2) Å, with
◦
TFA catalysed reaction needed longer time to complete associated N–H· · · O angle of ∼163 (Figure S5 in Sup-
(
1
∼5 h) and yielded lesser amount of 4 (8%) and more of plementary Information).
(17%). The products formed were rigorously charac-
terized by various spectroscopic tools (SI = Supplemen- 3.3 Anion binding study of macrocycle 5
tary Information).
Preliminary solution phase anion binding studies were
1
carried out for macrocycle 5 by using H NMR studies
3.2 Crystal structure analysis of macrocycle 5
−
−
−
−
in CD CN with various anions viz., F , Cl , Br , I ,
3
−
−
−
−
−
−
−
−
The semi-expanded macrocycle 5 was crystallized AcO , H PO , HSO , ClO , PF , NO , NO , N and
by slow evaporation of 5% ethyl acetate in hexane CN as their tetrabutylammonium (TBA) salts and the
2
4
4
4
6
3
2
3
−
solution, which resulted in an ethyl acetate solvate spectral changes observed indicate moderate to weak
.
(
5 EtOAc). The macrocycle 5 adopts 1,3-alternate con- binding for most of the ions (Figure S6 in Supplemen-
formation (Figure 2), similar to the basic octamethyl- tary Information). On the other hand, perchlorate and
3
calix[4]pyrrole 1, with slightly bigger cavity and hexafluorophosphate ions display no observable inter-
12
smaller than meso-expanded calix[4]pyrrole 4. Fur- action with the host. Subsequent detailed analysis by
1
ther, the two of the opposite pyrrole units (N1 and N3)
H NMR titration and evaluation of association con-
adopts almost orthogonal geometry with regard to the stants were performed by using Wilcox 1:1 binding
18
calixpyrrole core and the alternate NHs are directed in equation (Table 2, Figures S7–S15 in Supplementary
opposite direction with respect to each other [with dihe- Information), and stoichiometry confirmed via Job’s
dral angles between the two pyrrole units (N1 and N3) plot analysis (Figures S16–S19 in Supplementary Infor-
◦
∼
22 ] while, the remaining two pyrroles [with dihedral mation). The data reveal that 5 displays maximum
◦
angles (N2 and N4) is ∼21 to 29 ] are slightly tilted affinity towards fluoride and acetate ions but with no

J. Chem. Sci. (2018) 130:90
Page 5 of 8 90
Table 2. Summary of association constants Ka (M 1) of receptors 1 and
−
1
5
in the presence of different anions obtained from H NMR in acetonitrile-
◦
d3 at 22–25 C.
TBAFa
TBACl
TBAOAc
TBAH2PO4
> 104
b
> 5 × 10
3b
N. D.
1.3 × 10
3b
1
5
3
3
> 104
3
6.33 ± 2.9 × 10
9.5 ± 1.4 × 10
6.46 ± 2.2 × 10
◦
20
‘
a’ = in acetonitrile-d3 (0.5% v/v D2O) at 22 C; ‘b’ = from ref ; ‘N. D.’
=
not determined.
Table 3. Summary of thermodynamic parameters of different anions with receptors 1 and 5 obtained from ITC in acetonitrile
◦
at 30 C.
−1
Host
Guest
ꢀ H (kcal/mol)
Tꢀ S (kcal/mol)
ꢀ G (kcal/mol)
Ka (M
)
5
3
1
5
1
TBAF (0.5% water in Acetonitrile)
−4.3 ± 0.03
−3.52 ± 0.11
2.99
1.85
−7.39
−5.37
1.82 ± 0.13 × 10
7.61 ± 0.54 × 10
a
5
TBACl
TBAH2PO4
TBAOAc
1.4 × 10
4
3
4
5
1
−4.65 ± 0.13
−11.60
0.92
−5.81
−2.08
−4.03
−5.06
−5.57
−5.79
−5.50
−7.60
−6.48
1.04 ± 0.07 × 10
b
4
1.51 ± ×10
5
1
−7.6 ± 0.21
−11.62 ± 0.08
−11.55 ± 0.17
9.26 ± 0.62 × 10
c
5
2.90 × 10
7
4.76 ± 0.41 × 10
2
2 21a
and 23, respectively (recorded in acetonitrile); All the experiments were performed for minimum
a, b and c: from refs. ,
three times and the obtained values are reproducible within ± 15% error.
clear selectivity towards any other anions like the par- compound 5 from its acetonitrile solution, in presence
ent calix[4]pyrrole 1 (Table 2). Interestingly, the distinct of excess TBACl, resulted in the successful isolation
characteristic of the two pyrrolic NH signals of the 5 is of the diffraction grade crystals of chloride complex.
retained even after complexation with anions except in The macrocycle 5 adopts an irregular cone conforma-
the case of dihydrogenphosphate ion (only at very high tion (owing to its inherent structure), with all the four
anion concentration one peak was observed), indicating pyrrolic NHs hydrogen bonded with the chloride ion
the differential strength of interaction of the two sets (Figure 3). As a result of this irregular cone structure,
of NHs with the incoming anions (Figures S6 and S12 one of the pyrrolic NH of the BPE constituent resides
1
in Supplementary Information). Further, H NMR com- closest to the anion (N3-Cl distance 3.26(2) Å), whereas
petition experiment reveals calixpyrrole 5 has superior the other pyrrolic NH of the BPE moiety resides at the
affinity towards fluoride ion over chloride and acetate farthest (N4-Cl distance 3.42(2) Å) and the pyrrole unit
ions (Figures S20–S21 in Supplementary Information) opposite to it (N1-Cl distance 3.31(1) Å) and adjacent to
in identical conditions, similar to the one noticed in case it from the dipyrromethane constituent (N2-Cl distance
of calixpyrrole 1.
3.38(1) Å), residing at intermediate lengths. This also
These complexation processes were further subjected confirmed the slightly bigger and unsymmetrical nature
to isothermal titration calorimetric (ITC) experiments, of the binding domain of calix[4]pyrrole 5 than 1 (N-Cl
21
3b
to evaluate the energetics of the overall binding event
and the corresponding association constants are briefly
summarized in Table 3 (Figures S22–S26 in Supplemen- ence in binding behavior of the semi-expanded calix[4]
tary Information). pyrrole 5 towards fluoride and acetate ions among tested
distance 3.26–3.33 Å).
To further understand the reason behind the prefer-
The above studies clearly reveal that receptor 5 dis- ions, density functional theory (DFT) calculations for
plays a better preference towards fluoride and acetate the structure optimization with respective anions were
ions, albeit with reduced affinity compared to 1, and performed (Figures 4, S27–S29 in Supplementary Infor-
also shows a relatively weaker affinity towards chloride mation), which reveal the favorable binding nature of
and dihydrogen phosphate ions.
different anions (Table S1 in Supplementary Informa-
19
Several attempts to obtain diffraction grade crystals of tion). For example, the hybrid macrocycle 5 displays
the host–guest complexes of the calix[4]pyrrole deriva- good affinity towards acetate ion and the optimized
tive 5 with anions were made. Slow evaporation of structure of 5 with acetate ion supports well and reveals

90
Page 6 of 8
J. Chem. Sci. (2018) 130:90
Figure 3. ORTEP-POVray diagram of 5.TBACl complex: top view (left) and side view
(
right). Tetrabutylammonium cation and all hydrogens bound to carbon atoms are excluded
for clarity, and thermal ellipsoids are scaled up to 35% probability level. Color code: blue:
Nitrogen, grey: Carbon, white: Hydrogen and green: Chloride; hydrogen bonds are shown
as black dashed lines.
−
Figure 4. Space filled diagram of DFT optimized structure of 5.AcO
complex in two different views. Color code: grey: Carbon, white: Hydro-
gen, blue: Nitrogen and red: Oxygen.
its greater fit of binding of both oxygens of anion with Supplementary Information (SI)
pyrrolic NHs of the macrocycle, possibly indicating the
reason behind the greater affinity (Figure 4).
Crystallographic data (including the structure factor files) for
structures 5 and 5·TBACl in this paper have been deposited
in the Cambridge Crystallographic Data Centre as supple-
mentary publication numbers CCDC 1017831 and 1017832,
respectively. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cam-
4
. Conclusions
We have designed and synthesized a new meso-expan- bridge CB2 1EZ, UK (Fax: +44(0)-1223-336033 or e-mail:
1
deposit@ccdc.cam.ac.uk). Compound characterization, H
NMR titration, ITC and computational data are available as
Supplementary Information at www.ias.ac.in/chemsci.
ded calix[4]pyrrole receptor, which belongs to the first
example of a new class of smallest expanded calix[4]
pyrrole. This macrocycle is endowed with a slightly
bigger binding domain, which is also unsymmetric in
nature compared to the parent calix[4]pyrrole moiety. Acknowledgements
This resulted in relatively reduced anion affinities of this
This work was supported by CSIR, India (Project no. 01
2449)/10/EMR-II) and SERB, India (Project no. SB/S1/IC-
6/2012). Financial and infrastructure support from the UGC,
New Delhi (through the UPE and CAS programs) and
macrocycle compared to the latter. However, exclusive
selectivity towards fluoride ion could not be achieved
and at present efforts are directed towards this. The new
(
5
macrocycle displays highest affinity towards fluoride the DST, New Delhi (through the PURSE and FIST pro-
and acetate ions among the tested ions. grams) are gratefully acknowledged. Authors thank Center

J. Chem. Sci. (2018) 130:90
Page 7 of 8 90
6. (a) Turner B, Botoshansky M and Eichen Y
for Modelling, Simulation and Design (CMSD), University
of Hyderabad, Hyderabad, India for computational facility.
BSK thanks CSIR and DST India for the fellowship.
1998 Extended Calixpyrroles: meso-Substituted
Calix[6]pyrroles Angew. Chem. Int. Ed. 37 2475;
(b) Sessler J L, Anzenbacher P Jr, Shriver J A, Jursikova
K, Lynch V M and Marquez M 2000 Direct Synthesis
of Expanded Fluorinated Calix[n]pyrroles: Decafluo-
rocalix[5]pyrrole and Hexadecafluorocalix[8]pyrrole
J. Am. Chem. Soc. 122 12061; (c) Cafeo G, Kohenke
F H, Torre G L La, White A J P and Williams D J
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