A rationally designed perfluorinated host for the extraction of PFOA from water utilising non-covalent interactions

Article abstract of DOI:10.1039/c7nj03026f

Perfluorooctanoic acid (PFOA) is a persistent organic pollutant and widespread in the environment. Three hosts have been synthesized based upon the formation of a fluorous cavity and hydrogen bonding receptors with the aim of extracting PFOA from water into organic solvents. The hosts based upon a calix[4]arene functionalized at the lower rim with amide groups and fluorous ponytails are effective for the quantitative removal of PFOA. Modification to a partial cone or a trisaminoamine framework reduces the conformational rigidity and lowers the extraction efficiency. A comprehensive NMR spectroscopic analysis both in solution and the solid state, along with other characterization techniques, has elucidated the stoichiometry of the host:guest species and the binding constants have been measured. A computational study has given further insight into the binding modes and corroborated the spectroscopic measurements.

Full text of DOI:10.1039/c7nj03026f

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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ARTICLE
A rationally designed perfluorinated host for the extraction of
PFOA from water utilising non-covalent interactions
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
b
Harrison Omorodion , Miguel Palenzuela , Manuel Ruether , Brendan Twamley James A. Platts
*
a
and Robert J. Baker
DOI: 10.1039/x0xx00000x
Perfluorooctanoic acid (PFOA) is a persistent organic pollutant and widespread in the environment. Three hosts have been
synthesized based upon the formation of a fluorous cavity and hydrogen bonding receptors with the aim of extracting
PFOA from water into organic solvents. The hosts based upon a calix[4]arene functionalized at the lower rim with amide
groups and fluorous ponytails are effective for the quantitative removal of PFOA. A modification to the partial cone or a
trisaminoamine framework reduce the conformational rigidity and lowers the extraction efficiency. A comprehensive NMR
spectroscopic analysis both in solution and the solid state, along with other characterization techniques have elucidated
the stoichiometry of the host:guest species and the binding constants have been measured. A computational study has
given further insight into the binding modes and corroborated the spectroscopic measurements.
www.rsc.org/
1
7
18
Introduction
methods such as sorption
onto activated carbon,
20
1
polyaniline nanotubes, carbon nanomaterials, and photo-
9
2
1
22
23
24
Perfluorinated compounds have a multitude of uses, partly degradation especially using Ga
2
O
3
,
In
catalysts have been reported. Reaction with
OH radicals is not very efficient for photo-degradation, but
2
O
3
,
Fe(III) and
2
5
due to their unique hydrophobic and lipophobic character. In modified TiO
2
•
the past 60 years, they have been used as stain, grease and
2
6
water repellents or in the manufacture of products as diverse Fenton-like chemistry using
1
H
2
O
2
has been reported.
as paints to polishes, adhesives and firefighting foams.
Persulfate oxidation under UV-conditions is also reported to be
29
-
However, due to the very strong C—F bond (485 kJ mol ) efficient.
1
27
28
Coagulation techniques,
31
ball milling,
γ-
recent concern over the toxicity and bioaccumulation has irradiation and sonochemistry have also been utilized in
2
3
30
forced the reclassification as persistent organic pollutants this field, but conspicuous in its absence is a solvent extraction
4
POPs). One of the major pollutants is perfluorooctanoic acid process.
(
5
PFOA), which has been found in oceans and rivers across the We have an interest in the synthesis of fluorous ligands and
6
(
7
world, in the blood serum of numerous animal species,
8
occupationally exposed workers and the general public.
have shown that some of these are useful for the extraction of
32
toxic metals from water. On the basis of structural and
9
Perfluorinated compounds are ubiquitous in the Arctic theoretical investigations we have recently shown that C—
11
F…F—C interactions are stabilizing in the solid state, and
1
0
regions, which points to complex transport mechanisms,
and indicates these POPs are globally distributed, with some collectively can add the same amount of stabilizing energy as a
1
2
33
evidence of vaporization of neutral PFOA from water. The hydrogen bond. There are now a growing number of
probable source of PFOA and related compounds are thought examples of F…F interactions being utilized as crystal
3
4
to be from fluorinated alcoholic telomers, although this has engineering for applications in materials chemistry or in
1
been the subject of robust debate. In the context of an catalysis. Interest in weak C—H…F—C bonding is also
3
35
36
environmental pollutant, it is not treated by current waste burgeoning. Bearing in mind the extensive use of non-
37
water technologies so can contaminate drinking water covalent interactions in sensing anions, we reasoned that
1
4
supplies.
As the C-F bond is very strong, selective with a suitably designed ligand we could extract PFOA from
dehydrofluorination methodologies based on transition metal aqueous media, thus offering a remediation strategy. For this
1
5
catalysts have not been used to a great extent, but recent aim to be realized a number of features need to be
1
6
results with non-metals may hold promise. More traditional incorporated into the ligand system. A conformationally rigid
ligand would be thermodynamically favorable in forming
stable host-guest complexes;
a
system based on
a
calix[4]arene would be useful as this has been used
3
prodigiously in supramolecular chemistry. Hydrogen bonding
8
3
9
will be the primary non-covalent interaction, and to increase
the propensity for this to occur, electron withdrawing groups
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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ARTICLE
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4
0
near the donor atoms are required. Whilst for the highly TGA on a Perkin Elmer Pyris 1 TGA. All spectrophotometric
electron-withdrawing fluorous ponytails eight methylene units analysis were measured on a Lambda 1050 UV-Vis NIR, using
4
1
are required for full insulation, in practice two is almost as fused silica cells with a path length of 1 cm. X-ray
effective. It is also known that the fluorous acid is a strong crystallography data were measured on a Bruker Apex
acid, so hydrogen bonding strategies using both of these diffractometer. The structure was solved by direct methods
4
2
2
inductive effects have been described. It should be noted
that the measurement of pKa’s has been controversial, with a
range <0.1–3.8 reported, depending upon the conditions of
and refined by least squares method on F using the SHELXTL
4
8
program package. Crystal data for C HF O (M = 414.09
8
15 2
g/mol): triclinic, space group P-1, a = 9.9639(17) Å, b =
0.7932(18) Å, c = 23.575(4) Å, α = 9.9639(17)°, β = 90.839(4)°,
3
γ = 90.018(4)°, V = 2518.0(7) Å , Z = 8, T = 100 K, μ(MoKα) =
4
3
the measurement and the concentration of PFOA. “Dipole
1
4
4
engineering” also lends itself to this topic as adding a
fluorous chain reverses the dipole compared to a hydrocarbon
framework that can then match the dipole of the fluorous acid.
Furthermore the presence of a long fluorous chain on the host
will also allow a fluorophilic cavity to engender F…F
interactions and add a further stabilizing non-covalent
interaction. It should be noted that host-guest complexes with
PFOA and its derivatives has not been studied in detail,
-1
3
0
.296 mm , Dcalc = 2.185 g/cm , 25077 reflections measured
0.870° ≤ Θ ≤ 25.499°), 9338 unique (Rint = 0.0630) which were
used in all calculations. The final R was 0.0971 (I > 2σ(I)) and
wR was 0.2831 (all data). The structure was refined as a 2
component rotational twin (twin law: -1 -0.001 0 0 1 0 0 -0.505
1) with a refined fractional twin volume of 19.78%. The
(
1
2
-
4
5
structure was refined with restraints (RIGU, ISOR) and to
convergence. The high residuals may be twinning artefacts.
CCDC 1530103 contains the supplementary crystallographic
although some success has been seen with cyclodextrin hosts
6
and a tripodal fluorous amide has been used to form host-
guest complexes with the related perfluorooctane sulfonate
4
(
Chart 1). Here we report on the synthesis of three types of data for this paper. These data can be obtained free of charge
hosts of varying conformational flexibility based upon a from The Cambridge Crystallographic Data Centre via
calix[4]arene or tren framework, and their reactivity with PFOA www.ccdc.cam.ac.uk/data_request/cif.
as characterized by thermal methods, vibrational spectroscopy
and multinuclear NMR spectroscopy, both in solution and the All compounds and solvents were obtained from commercial
solid state. We also show that these hosts can extract PFOA sources and used as received. The synthesis of
1
and its
from water into an organic solvent with excellent efficiencies,
thereby offering a new remediation technique for the removal
of a persistent organic pollutant.
conversion to
spectroscopic data in our hands are in accord with this. The
transition of to has not been reported previously. The
(CF
CH NH
Spectroscopic data are collated in the supporting information
Figures S1-S16).
3 has been reported in the literature, and the
4
9
2
4
5
0
51
acid
CF
CH
3
2
)
5
CH
2
CH
2
2
CO H
and
the
amine
CF (CF
3
2
)
5
2
2
2
were prepared via the literature.
(
2
7
,2',2'',2'''-((15,35,55,75-tetra-tert-butyl-1,3,5(1,3)-tribenzena-
(1,3)-cyclohexanacyclooctaphane-73,75-diene-12,32,52,72-
tetrayl)tetrakis(oxy))tetraacetic acid, 2
3
A solution of NaOH (4.00 g, 100 mmol) in ethanol (60 cm ) and
3
H
2
O (40 cm ) was added to 5,11,17,23-Tetra-tert-butyl-
5,26,27,28-tetracarboxymethoxycalix[4]arene (6 g, 6 mmol)
and refluxed for 24 h. The cooled solution was acidified with
0% sulfuric acid to pH 1 and the resulting precipitate was
isolated and dissolved (partially soluble) in ethanol-water (1:1
v/v) for recrystallization yielding as colourless needles. The
2
46,47
Chart 1. Examples of host-guest complexes.
5
Experimental
1
1
13
1
19
H, C{ H} and F NMR solution spectra were recorded on a undissolved precipitate was collected, washed with water and
0
Bruker AV400 spectrometer operating at 400.23 MHz, 100.61 dried at 40 C to afford the pure
2 (5g, 94%). Spectroscopic
5
MHz and 376.55 MHz respectively, and were referenced to the data of are in accord with the literature. IR ṽ (cm ) = 3610
2
-1
1
1 13
residual H and C resonances of the solvent used or external (b, OH), 2994 (m, C-H), 1730 (s, C=O, carboxylic acid), 1479 (m,
CFCl . Solid state NMR spectra were recorded on a Bruker 400 C-H), 1392 (w), 1362 1259 (w, C-H), 1191, 1127, 1053 (s, C–O),
3
1
3
19
1
HD at 100.61 MHz ( C), 376.50 MHz ( F). IR spectra were 870 (m, C–H), 800, 753 (s, C–H), 634 (w, C-H). H NMR (400
recorded on a Perkin Elmer Spectrum One spectrometer with MHz, CDCl ) δ (ppm) 8.04 (s, OH); 7.13, 7.19, 6.64, 6.60 (s each,
attenuated total reflectance (ATR) accessory. Mass spectra 2 H each, ArH); 4.54 (br, 8 H, ArOCH ,); 4.41, 3.47 and 3.34 (d,
were measured on a MALDI QTOF Premier MS system. DSC 2 H, 4 H, J 13 Hz, ArCHAr); 1.35, 1.28 and 0.89 (s, 18 H, Bu).
C{ H} NMR (400MHz, CDCl ) δ (ppm) 149.9 (COOH); 125.7
3
2
t
=
1
3
1
spectra were recorded on a Perkin Elmer Diamond DSC and
3
2
| J. Name., 2012, 00, 1-3
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NP lee wa s eJ od uo r nn oa tl aod fj uC s ht em ma ri sg it nr ys
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DOI: 10.1039/C7NJ03026F
ARTICLE
3
ArC), 53.2 (ArOC), 30.0 (CH ); 31.0 (ArCH Ar). MS (MALDI) extracted with DCM (3 x 60 cm ). The organic phase was
(
3 2
3
O (3 x 60 cm ), dilute HCl (3 x 60 cm ) and brine
3
m/z: 880.4398 [M+Na, 100%]; HRMS (MALDI) calculated for washed with H
2
3
(60 cm ). The combined organic phases was dried with MgSO ,
C
52
H
64
O12Na: 903.4295 Found: 903.4299.
4
and the solvent was removed under reduced pressure to
obtain the crude brown solid which was purified by column
2
,2',2'',2'''-((15,35,55,75-tetra-tert-butyl-1,3,5(1,3)-tribenzena-
(1,3)-cyclohexanacyclooctaphane-73,75-diene-12,32,52,72-
7
chromatography (hexane/ethyl acetate, 5:2) to afford
6
as a
brown solid. Yield 1.3 g, (67%). IR ṽ (cm ) 3210 (b, N-H), 2970
w, C-H), 1668 (s, C=O, Amide), 1480 (s, C-H), 1365 (s, C-N),
1233, 1189, 1143, 1121 (s, C–F), 1075, 1016 (w, C–F), 803 (m,
-1
tetrayl)tetrakis(oxy))tetraacetyl chloride, 4
The acid (2 g, 2 mmol) was refluxed with excess SOCl
Argon at 120 C for 6 h. The excess SOCl
reduced pressure to afford which was verified by IR
spectroscopy and used without further purification. IR ṽ (cm )
(
2
2
under
was removed under
0
2
1
C-N), 732, 707 (m, C-H), 696 (m, C-H). H NMR (CDCl
8.6, 8.24, 7.9, (bs, 4 H, NH); 7.03, 6.97, 6.86, 6.64 (s, 8 H, ArH);
.82, 4.60, 4.49 and 4.45 (s, 4 H, ArOCH ,); 4.39, 3.29 (d, 8 H,
CF ); 1.35,
) δ (ppm)
3
) δ (ppm)
4
-
1
4
2
=
1
8
2962 (m, C-H), 1808 (s, C=O, acetyl chloride), 1477 (m, C-H),
415 (w), 1362 1333 (w, C-H), 1259, 1192, 1062 (s, C–O), 948,
05, 758 (s, C–H), 691 (w, C-H).
ArCH
2
Ar); 3.70 (m, 8 H, OCH
2
CH
2
); 2.45 (m, 8 H, CH
2
2
tBu 13
1
1
1
1
.28, 1.09 and 0.89 (m, 36 H, ). C{ H} NMR (CDCl
3
69.0 (CO
2
NH); 146, 133, 130, 128, 125 (ArC), 120.4, 118.2,
16.8, 115.3, 110.2, 107.9, (CF
2
3
and CF ) 65.4, 61.8, 57.8
[2,2',2'',2'''-((15,35,55,75-tetra-tert-butyl-1,3,5,7(1,3)-
(
ArOC); 38.1 (ArCH Ar); 33.2 (CH NH); 32.01 (CH CF ); 31.5,
2
2
2
2
tetrabenzenacyclooctaphane-12,32,52,72-
tetrayl)tetrakis(oxy))tetrakis(N-(3,3,4,4,5,5,6,6,7,7,8,8,8-
tridecafluorooctyl)acetamide)], 5
1
9
2
9.9 (CH
CH ); -121.68 (s, CF
25.98 (s, CF ). MS (MALDI) m/z: 2260.5146 [M+Na, 100%];
3
). F NMR (CDCl
3
) δ (ppm) -80.79 (s, CF
3
); -113.89 (s,
CF
2
2
); -122.68 (s, CF ); -123.24 (s, CF ); -
2 2 2
1
2
HRMS (MALDI) calculated for C84
found 2283.5132.
80 4 8
H N O F52Na: 2283.5044,
1
-amino-1H,1H,2H,2H-perfluorooctane (1.6 g, 4.4 mmol) was
added to a solution of tetraacetyl chloride, 3 (0.8 g, 0.80
3
mmol) dissolved in THF. Et N (10 cm ) was added to the
3
2
H,2H-perfluorooctanoyl chloride, 7
reaction mixture and refluxed for 24 h. The solvent was
removed in vacuo and the brown solid was taken up in DCM
3
80 cm ) and washed with H O (3 x 60 cm ), dilute HCl (3 x 60
3
2H,2H-perfluorooctanoic acid (0.70g, 1.8 mmol) was refluxed
3
with freshly distilled thionyl chloride (1 cm ) for 40 h. The
(
2
3
3
cm ) and brine (60 cm ). The combined organic phases was
dried with MgSO , and the solvent was removed under
excess thionyl chloride along with the HCl and SO
produced were evaporated under reduced pressure. IR ṽ (cm )
2963 (w, C-H), 1807 (s, C=O, Acetyl chloride), 1681 (s, C=C),
1363, 1388, 1317 (w, C-H), 1232, 1186, 1142, 1121 (s, C–F),
068, 1077 (w, C–F), 979, 897, 848, 810, 708 (w, C-H), 628, 614
m, C-H).
2
gases
4
-1
reduced pressure to obtain the crude brown solid which was
purified by column chromatography (hexane/ethyl acetate,
=
-
1
5:2) to afford
5. Yield 1.3 g, (67%). IR ṽ (cm ) 3210 (b, N-H),
1
2970 (w, C-H), 1668 (s, C=O, Amide), 1480 (s, C-H), 1365 (s, C-
(
N), 1233, 1189, 1143, 1121 (s, C–F), 1075, 1016 (w, C–F), 803
1
(
m, C-N), 732, 707 (m, C-H), 696 (m, C-H). H NMR (CDCl
ppm) 7.71 (s, 4 H, NH); 6.82 (s, 8 H, ArH); 4.9 (s, 8 H, ArOCH
.44, 3.28 (s, 8H, ArCHAr); 3.56 (t, 8 H, = 7 Hz, NHCH ); 2.43
3
) δ
N,N',N''-(nitrilotris(ethane-2,1-diyl))tris(3,3,4,4,5,5,6,6,7,7,8,8,8-
pentadecafluorooctanamide), 8
(
2
);
4
J
2
1
3
1
(m, 8H, CH
2 2 3 3
CF ); 1.12 (s, 36 H, CH ). C{ H} NMR (CDCl ) δ
3
7 (0.4 g, 2.4 mmol) was refluxed with triethylamine (0.15 cm ,
(
ppm) 170 (OCNH); 152, 148 (ArC); 134-111 (CF); 74 (ArOC);
1
9
6.8 (ArCH Ar); 34.0 (CNH); 30.4 (CH ); 29.8 (CCF ). F NMR 1 mmol) and tris-(2-aminoethyl)-amine (0.13 g, 0.8 mmol) in
3
2
3
2
3
(
CDCl
3
) δ (ppm) -80.79 (s, CF
3
2 2
); -113.89 (s, CF CH ); -121.68 (s, THF (10 cm ) for 24 h and then diluted with 0.1 M HC1 (20
3
3
2 2 2 2
CF ); -122.68 (s, CF ); -123.24 (s, CF ); -125.98 (s, CF ). MS cm ). The product was extracted with ether (3 x10 cm ) and
3
MALDI) m/z: 2260.5146 [M+Na, 100%]; HRMS (MALDI) saturated aqueous NaHCO (10 cm ) was added. The combined
(
3
calculated for C84
H
80
N
4
O
8
F
52Na: 2283.5044, found 2283.5063.
ether extracts were dried over Na
obtain the syrupy crude product which was purified by column
chromatography (Hexane/ethyl acetate, 5:1) to afford as a
2 4
SO and concentrated to
2,2',2'',2'''-((15,35,55,75-tetra-tert-butyl-1,3,5(1,3)-tribenzena-
(1,3)-cyclohexanacyclooctaphane-73,75-diene-12,32,52,72-
8
-1
7
brown viscous liquid. (0.7 g, 57%). IR ṽ (cm ) 3253 (b, N-H),
tetrayl)tetrakis(N-(3,3,4,4,5,5,6,6,7,7,8,8,8-
tridecafluorooctyl)acetamide)], 6
2962 (w, C-H), 1638 (s, C=O, Amide), 1461 (s, C-H), 1353 (s, C-
N), 1231, 1195, 1140, 1112 (s, C–F), 1085, 1061 (w, C–F), 863
1
m, C-N), 805, 746, 721, 708 (m, C-H), 643 (m, C-H). H NMR
(
[
600 MHz, C
2
D
6
CO]: δ (ppm) 8.03 (s, 3 H, HN); 3.34 (m, 6H,
CH and CH CF overlap).
C{ H} NMR [600 MHz, C D CO]: δ 166.6 (, NCO); 118.7 (m,
1
-amino-1H,1H,2H,2H-perfluorooctane (3 g, 8 mmol) was
added to a stirred solution of (2 g, 2 mmol) in the THF (80
N (10 cm ) was added to the reaction mixture and
CH
1
2
NHCO); 2.65 (m, 12 H, NCH
2
2
2
2
4
3
1
3
3
2
6
cm ). Et
3
2 2 2 2 2 2 2
CF ); 117.8 (m, CF CF CH ); 116.9 (m, CF CF CH ); 111.7 (m,
refluxed for 24 h. The solvent was removed in vacuo and
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
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CF CF CF CF ); 110.9 (m, CF CF CF ); 109.9 (m, CF CF ); 82
2
3
2
2
2
3
2
2
3
1
9
(
CH
2
N); 55 (CH
2
NHCO); 39.8 (CH
2
). F NMR [376.6 MHz, C
CH ); 123.22, -123.59 (s,
CF CF ); -126.1 (CF CF ). MS (ESI-) m/z: 1226.0991 [M+H,
6 6
D ]:
δ -81.9 (s, CF
3
); -119.07, 119.75 (s, CF
2
2
2
2
3
2
100%]; HRMS (ESI+) calculated for C30
22 4 3
H N O F39: 1227.1069,
found: 1227.1074.
General Procedure for the Extraction of PFOA: The ligands (1
molar equivalent) were dissolved in 1,3-
bis(trifluoromethyl)benzene (5 cm ). Solutions of PFOA (1.8
3
3
2
mg, 0.004 mmol) in distilled H O (5 cm ) were carefully added
to the ligand solutions and stirred. The concentration of PFOA
in the aqueous layer was periodically monitored
spectrophotometrically using the peak at 206 nm and
referenced to a standard curve. Control experiments were
carried out without the ligands.
General procedure for synthesis of host:guest complexes:
The hosts (25.0 mg, 0.02 mmol for
and ) were dissolved in isopropanol (5 cm ) and added to a 1
and 2 molar equivalents of PFOA (3.3 mg and 6.7 mg for the
8; 25.0 mg, 11 mmol for 5
3
6
8
;
3
4
2
.8 mg and 9.8 mg for the 5 and 6) dissolved in H O (5 cm ).
o
0
The solution were stirred at 80 C for 1 hr and then at 50 C to
completely evaporate all solvents.
Scheme 1. Synthesis of hosts 5, 6 and 8. i) SOCl
CF (CF CH
2 f 2 2 3 f
; ii) R NH ; iii) SOCl ; iv) tren, Et N. R =
Results and Discussion
3
2
)
5
2
.
Interestingly, we have not been able to obtain single crystals
from recrystallizing commercially available PFOA, and to the
best of our knowledge this is the first time that PFOA has been
structurally characterized. It should be noted that the crystals
were twinned and were modelled as such. Nevertheless, the
structure of PFOA and its packing along the b-axis are shown in
Figure 1. The C-F bond lengths range from 1.309(14) –
Ligand synthesis and structure. The syntheses of the three
ligands we have chosen to study are shown in Scheme 1.
Whilst 1 and 3 are known compounds we found that under our
experimental conditions 66% of the ‘partial-cone’ conformer 2
5
3
was formed. We subsequently optimized the synthetic
conditions to form 1 or 2 cleanly and in good yield. The
conversion to the acid chloride is straightforward and reaction
1.371(10) Å, whilst the C-C bonds are between 1.510(16) -
.576(14) Å. The commonly observed twisting of the chain is
3 2 5 2 2 2
with the fluorous amine CF (CF ) CH CH NH allowed a high
1
yielding synthesis of both full (5) and partial-cone (6)
calix[4]arene with substituted amide groups on the lower rim.
The synthesis of 8 was straightforward. The spectroscopic data
of 5, 6, and 8 are in accord with our formulations but despite
repeated attempts, no crystals suitable for single crystal X-ray
diffraction could be isolated. The NMR and vibrational data of
the hosts are discussed further in the host-guest section
below.
also observed so that the F—C—C—F torsion angles are lower
compared to hydrocarbon chains. This is to relieve
electrostatic interactions between two fluorine groups,
55
although other factors may also be involved. As has been
noted in numerous structures there is a multitude of C—F…F—
C interactions: type I interactions between the CF groups (d
3
F..F
=
2.888 Å, ∠C—F…F = 116°), and bifurcated “three-point
o
interactions” (dF..F = 2.869 and 2.874 Å;
present between the ponytails.
∠F…F…F = 100 ) are
During the course of a reaction between CF
3 2 6 2
(CF ) CF I and
NaN in DMF, instead of isolating the expected CF (CF ) CF N ,
3
3
2 6
2 3
Whilst DFT and AIM have been utilized to understand these
weak interactions, this can be rather time-consuming. A
different methodology to quantify intermolecular interactions
a colorless, crystalline product was formed that was shown to
be CF (CF CO H, PFOA, by X-ray diffraction. The mechanism
3
2
)
6
2
5
6
of formation is not clear but careful analysis of the crude
mixture by NMR spectroscopy shows this to be the only
is using Hirschfeld surfaces.
3 2 5 2 2
product. Dissolution of CF (CF ) CH CH I in DMF has been
54
reported to afford a number of products, and perhaps given
the electron withdrawing nature of the fluorous chain this
weakens the C-I bond and is thus more reactive with DMF.
4
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PFOA is, as expected, the greatest but examination of the
other acids show that the major differences are in the C—
H...H—C interactions. It is perhaps coincidental that the F…F
interactions in PFOA are essentially identical to the H…H
interactions in octanoic acid, but work is ongoing in our group
to fully explore this. However it is interesting to compare the
o
melting points of PFOA (58.4 C from TGA measurements,
o
Figure S21) and octanoic acid (16-17 C).
(
b) Extraction of PFOA from water using 5, 6 and 8. Given
Figure 1. X-ray structure of PFOA (top) and packing as viewed along the crystallographic
b-axis (bottom).
that PFOA is a widespread persistent organic pollutant, our
first aim was to find a system that would allow the removal of
Surprisingly this has rarely been used in the analysis of PFOA from water. The hosts have solubility in common
5
7
fluorous interactions so we have employed this approach to organic solvents, but we also measured the partition
look at the interactions in a series of three carboxylic acids that coefficient of PFOA between a number of common organic
5
9
have been structurally characterized by us, viz
.
solvents and water; interestingly at higher concentrations of
and PFOA foaming and the formation of a third phase was
H and the corresponding non-fluorous octanoic observed. The results are shown in Table S1. Inspection of the
3
3a
32c
CF
CF
3
3
(CF
(CF
2
2
)
)
5
6
CH
CO
2
CO
2
H,
3 2 5 2
CF (CF ) CHMeCO H
2
5
8
acid. The results of this analysis are shown in Figure 2 and partition coefficients suggests that most solvents could act as a
more detailed plots can be found in the supporting co-extractant, but under our experimental conditions they did
information (Figs S17-S20). Mapping the dnorm parameter onto not extract PFOA into the organic phase without the addition
the Hirschfeld surface has been shown to give much subtle of the ligand. Screening of the solvents showed that the
information on weak interactions and C—F...F—C interactions amphiphilic 1,3-bistrifluoromethbenzene had the greatest
are clearly observed as red spots, along with the expected extraction ability, so we have chosen this for more detailed
hydrogen bonding in the carboxylic acid functions. Also studies. It should be noted that this solvent would not be
obvious in this analysis are the bifurcated F…F interactions, useful for large scale applications and some ligand redesign is
most notably in Figure 2(c).
necessary. Work is ongoing in our group to improve the large
scale applicability of this idea, and will be reported on in due
Analysis of the 2D fingerprint plots yields the quantitative course. The extraction of PFOA from water into the fluorous
analysis shown in Figure 2. The C—F…F—C interactions in
solvent was monitored by UV-vis spectroscopy, as PFOA has an
6
0
absorption band at 206 nm in water. The results of the
extraction with the three hosts are shown in Figure 3. It can be
seen that the removal of PFOA from water is effective for all
hosts, with the more rigid full cone conformer
5 the most
effective. The host 8 is the slowest at extracting, suggesting
that conformational rigidity might be a key factor; the shape of
the curve also suggests that an intermediate is formed. UV-vis
5
6
8
100
solvent
8
6
4
2
0
0
0
0
0
0
50
100
150
200
250
300
Figure 2. Map of the property
CF (CF CH CO and (c) CF (CF
d
norm onto Hirshfeld surfaces of (a) PFOA, (b)
Time (Mins)
3
)
2 5
2
2
H
3
2
)
5
CH(Me)CO H. (plotted at -0.1-1.0) and the
2
corresponding quantitative analysis, including octanoic acid.
Figure 3. Results of the extraction of PFOA from water using 1 equivalent of the hosts
in 1,3-(CF , as monitored by UV-vis spectroscopy.
3 2 6 4
) C H
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1
and F NMR spectroscopy for host
9
5 shows that no PFOA is (ii) Characterization of the host-guest complexes. Given the
detectable in the aqueous phase at the end of the experiment. stoichiometric ratios determined from the experiments
Whilst these results are pleasing, it is also worth noting that described above, we sought to use standard characterization
the PFOA does slowly leach back into the water phase if these methods to explore the formation of these species, and any
phases are contacted overnight. The implications from these intermediates that could be formed. Thermal methods are
results are that the host-guest complex is not very stable over useful in the characterization of host-guest complexes.
long periods of time. This could be due to the stronger
hydrogen bonding in water over the hydrogen bonding system DSC and TGA measurements were carried out on the hosts and
in the hosts.
the 1:1 and 2:1 compounds. In our hands PFOA shows two
o
endothermic transitions at 60 C, ascribed to melting and 170
o
(c) PFOA interactions with Hosts 5, 6 and 8. In order to
C associated with vaporization (Figure S21). DSC thermograms
understand the host-guest species present during the show the loss of thermal transitions associated to the PFOA
extraction, we have carried out a number of experiments to guest but no other phase transitions in the hosts and and 8
5
6
determine the stoichiometry and probe the nature of the (Figure S22-S24); only a small increase in the thermal stability
bonding. These conclusions have been supported by of the ligand is observed. TGA adds further weight to the
computational investigations.
proposition of an inclusion complex with the same small
increase in thermal stability of the ligands observed. A plot of
(
i) Determination of the stoichiometry. In order to determine the derivative weight % vs temperature shows evidence of the
o
we vaporization of the guest at 197 C for host
the number of PFOA guests that fit into the hosts
o
have utilized a simple acid-base titration and two solution decomposition of the ligand at 225 C (Figure 5). The behavior
5,
6
and
8
8, along with
based methods, namely absorbance spectroscopy via
a
of
6 and 7 are essentially the same and shown in Figure S25-
6
continuous variations plot and H and F NMR titration S29; this thermal behavior is similar to that observed where
1
1
19
β
-
6
2
45
experiments. For the host
titration with 0.010 M NaOH after the addition of 2 guest complexes from complex
equivalents of PFOA, suggesting 1:1 stoichiometry. for are shown in Figure S30. A number of points warrant
Continuous variations plots confirm this stoichiometry (Figure discussion. Firstly the host is likely to have strong
5
, free PFOA is only detected by cyclodextrin is the host. The infrared spectrum of the host-
5
is shown in Figure 6 and that
a
6
1
19
4
). Finally analysis of the H and F NMR titrations using intramolecular C=O…H—N hydrogen bonding between the
6
WINEQNMR2 gave the best fit to a 1:1 complex with a log
3
β
=
chains, as observed in the non-fluorous analogue (amide I
-1
1
-1
2.66
± 0.35 from the H NMR data. Host 6 also displays a 1:1 band at 1654 cm and amide II band at 1545 cm ) and the
6
4
stoichiometry from titration and continuous variations plots shifts in
5
are comparable. The OH stretching frequency of
the guest shifts to lower wavenumbers upon addition of one
0.20 from F NMR titrations). Given that the major equivalent of PFOA and does not change in position after more
difference between and is the reduced number of fluorous equivalents, consistent with the 1:1 stoichiometry determined
ponytails, and a smaller fluorophilic cavity in
the binding constant can be readily rationalized. Host
1
and the H NMR titration gives a logβ = 1.90 ± 0.21 (logβ =
1
9
1.14 ±
5
6
-
1
6, the trends in in solution. The C=O stretching frequencies of
5
(1667 cm ;
-1
8
is more amide I band) and PFOA (1692 cm ) broaden but only shift
-1
complex as a higher stoichiometry species of ~6 appears to be slightly upon addition of 1 equivalent of PFOA (1643 cm ) and
favored from the continuous variations plot. The binding shift to slightly higher wavenumbers upon addition of 2
-1
constants are, however, lower than those reported in the equivalents of PFOA (1659 cm ), whilst the amide II band
-1
(1547 cm ) is not affected.
4
= 7 x 10 M ; K
-1
2 -2 47
= 9 x 10 M ).
literature for cyclodextrin (K
1
2
5
6
8
0.00
0
0
0
0
0
0
.372
.370
.368
.366
.364
.362
0.5
0.6
0.5
0.4
0.3
0.2
0.1
0.4
0.3
0.2
0.1
0.0
-0.01
5
1
2
:1
:1
PFOA
5
0
100 150 200 250 300 350 400 450 500 550
0
Sample Temperature ( C)
0.0
0.2
0.4
0.6
0.8
1.0 0.0
0.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
Mole Fraction
Figure 5. Derivative weight TGA of 5 and the 1:1 and 2:1 inclusion complexes.
Figure 4. Plots of the continuous Variations method for hosts 5, 6 and 8 with PFOA.
6
| J. Name., 2012, 00, 1-3
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PFOA
1
2
8
:1
:1
1000
1500
3000
3500
PFOA
:1
:1
1
2
5
1000
1500
3000
3500
-1
Wavenumber (cm )
Figure 6. Infrared spectra of PFOA, 1:1, 2:1 inclusion complexes and hosts 5 (bottom)
and 8 (top).
The infrared spectra of
8 and the corresponding 1:1 and 2:1
complexes are shown in Figure 6 and follow a different trend.
-1
The band at 1613 cm does not change in position but now
-1
appears almost as a shoulder but the band at 1661 cm shifts
-
to 1666 in the 1:1 and 2:1 complexes and the band at 1691 cm
1
-1
shifts to 1681 cm ; the amide II bands are not affected. This
is consistent with the host acting as a hydrogen bond acceptor.
Interestingly the OH peak is not observable in the IR spectra of
the
8:guest complexes, but the infrared spectra do not support
a proton transfer mechanism, as the ionized carboxylate show
-1
a C=O stretch at ~1650 cm . Conductivity measurements show
that solutions of and PFOA are non-conductive. Therefore
8
19
6
Figure 7. Overlaid F DOSY spectra of 5:1PFOA (top) and 6:1PFOA (bottom) in d -
the thermal characterization methods and infrared acetone at 300 K (Red = host; Purple = PFOA; Blue = 1:1 host-guest complex).
spectroscopy support the stoichiometry developed above that
From this the hydrodynamic radius can be calculated via the
Stokes-Einstein equation; in our examples the molecules are
likely to be more prolate than spherical, so we have used the
both
complex. Moreover the hydrogen bonding network in hosts
and do not appear to be significantly disrupted upon
5 and 6 are 1:1 host-guest complexes whilst 8 is more
5
6
modified Stokes-Einstein equation where n = 4. Interestingly
1
are similar, but the 1D H
addition of the guest, indicating either weak binding or a
different mechanism.
the hydrodynamic radius of
5
and
6
NMR spectra confirm the asymmetric nature of host
1
6
, and
19
variable temperature H and F NMR spectroscopy show no
sign of conformational isomerization as expected.
Solution Phase NMR Spectroscopy. Multinuclear NMR
spectroscopy has been used to explore the solution
conformers of the hosts and the Spin-Lattice relaxation (T1)
measurements shed further light on the inclusion complexes.
DOSY NMR techniques have been increasingly utilized in
Host
hydrodynamic radius suggests that intramolecular hydrogen
bonding holds this host in a relatively rigid C symmetry. From
the F DOSY spectra of the host-guest complexes, there is an
increase in the size of with the guest whilst small changes are
observed in . However the small changes in DCT and the
corresponding hydrodynamic radius of hosts and support
the formation of a compact inclusion complex. In contrast,
host has a large hydrodynamic radius, suggesting that it does
not have a C rigid structure in solution. Upon addition of one
8 is the most conformationally labile, but the small
6
5
supramolecular host-guest complexes, and are also used. As
shown in the NMR titration experiments (Figure S31-46), there
3
1
9
5
is a shift of the C 2
ppm) and the C 2CO H fluorine nucleus on the guest (δF ≈ -120
ppm) which is diagnostic of an inclusion complex; the other
F CH fluorine nucleus on the host (δF ≈ -118
2
6
F
2
5
6
1
9
peaks in the F NMR spectrum are not significantly shifted.
1
The H NMR spectra are also informative as the OH peak of the
8
1
9
guest shifts upon encapsulation. F DOSY spectra of the host,
guest and the 1:1 and 2:1 complexes have been measured and
the diffusion coefficient, DCT, recorded in Table 1 (selected
spectra are shown in Figure 7; others are in Figure S47-50); for
3
equivalent of guest, the hydrodynamic radius significantly
shrinks indicating a conformational change to encapsulate the
guest. A further addition of a guest forces a much larger
1
9
structure. T
1
measurements of the individual F nuclei were
our measurements we have used DCT for the CF groups at
3
F
δ =
1
9
also measured as there are unique F signals to the host (
δ
F
≈ -
-81 ppm, although we note that DCT for the CH CF nuclei are
2 2
6
6
1
18 ppm) and guest (δ ≈ -120 ppm); the data is recorded in
F
slightly different.
Table 1. The fluorine nuclei of the host and guest decreases
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1 6
Table 1. Diffusion Coefficient, calculated hydrodynamic radius and T for selected fluorous nuclei in host and 1:1 and 2:1 host:guest complexes in d -acetone at 300 K.
Host
PFOA
5
D
CT
2
a
H
Host CF α
2
Host CF
3
Guest CF α
2
Guest CF
(s)
3
(
ms )
(nm)
(s)
(s)
(s)
-
9
9
2.51x10
0.43
-
-
0.30 ± 0.007
2.37 ± 0.04
-
1.60x10
0.67
0.83
0.58
0.51
5.1
0.47 ± 0.02
0.09 ± 0.05
0.65 ± 0.05
0.62 ± 0.05
1.56 ± 0.05
0.71 ± 0.07
0.97 ± 0.3
1.70 ± 0.03
1.54 ± 0.03
2.02 ± 0.05
1.91 ± 0.3
-
-
-
9
5
6
:1PFOA
6
1.29 x10
0.65 ± 0.04
-
1.45 ± 0.05
-
-
9
9
1.86x10
-
:1PFOA
8
2.10x10
0.69 ± 0.04
-
1.86 ± 0.04
-
-
9
2.13x10
1.71 ± 0.08
1.75 ± 0.06
1.60 ± 0.05
-
9
8
8
:1PFOA
:2PFOA
1.35x10
0.79
22.2
0.87 ± 0.03
0.69 ± 0.12
1.62 ± 0.05
1.6 ± 0.05
-10
4.85x10
throughout the series, indicative of reduced motion, apart sharpened in the host:guest complex, whilst the guest C=O
from the T of the host CF
in 5. Therefore both 1D & DOSY resonance is likely to overlap with the host C=O amide
NMR spectroscopy and T
1
2
1
9
13
1
measurements indicates the resonance. In the F{ C} MAS NMR spectrum of this 1:1
formation of a host-guest complex that is stable in solution.
complex the only discernible difference is in the linewidths and
position of the CF3 group at ~ -81 ppm; these could be
deconvoluted (Figure 8) to give two components at δF = -81.43
ppm (Peak 1, relative area = 0.52) and -81.64 ppm (Peak 2,
relative area = 1). Whilst it is not possible to fully assign these
Solid State NMR Spectroscopy. To further probe the solid-
1
3
state structures of the inclusion complexes we have utilized
C
1
and F solid-state MAS NMR spectroscopy and CP-MAS
9
1
3
19
spectros-copy. The C and F NMR spectra for the guest are
relatively sharp and consistent with previous results (Fig S51-
peaks, it is clear that there are multiple CF environments in
3
the host that are reduced in the host:guest complex, possibly
due to the reorganization of the fluorous cavity to
accommodate the guest. The linewidths are narrower in the
complex than the free host indicating an increase in dynamic
behavior, at least of this group. For the 1:1 complex with host
4
S52). The hosts show spectra that are relatively broad but
5
can be assigned on the basis of their solution NMR spectra
1
9
(Figures S53-S68). Variable spin rate
F MAS NMR
spectroscopy are essentially identical at all spin rates,
indicating that there is only one phase present (Figure S69-
6
the same spectral features are observed (Figure S64), but the
1
3
1
S71). In the MAS C{ H} NMR spectra of the hosts the amide
carbon can be clearly observed at 169 ppm in both and
whilst the carbons of the fluorous chain resonate at ~127 ppm. 19 13
spectra are broader and in keeping with the less
conformationally rigid structure of the host. However in the
5
6,
F{ C} MAS NMR spectrum a more pronounced shift occurs
and deconvolution of the peak reveals only two components in
both the host (Peak 1: = -81.75 relative area = 0.96; Peak 2:
= - 82.06 relative area = 1) and host:guest complex (Peak 1:
= -83.23 relative area = 0.73; Peak 2: = -82.75, relative
t
Finally the butyl peaks are observed at 30 ppm, but there is a
shoulder at 37 ppm due to the CH O and C CH CF carbons
2
H
2
2
2
δ
F
1
9
13
The F{ C} MAS NMR spectra are well resolved but rather
broad and suggests slow dynamics in the fluorous chain;
δ
δ
F
F
δ
F
deconvolution of the CF
3
peak using Gaussian functions (Figure
= -81.26 ppm (Peak 1, relative area
0.40), -81.64 ppm (Peak 2, relative area = 1) and -81.92 ppm
area =1).
8
) shows three peaks at δ
F
The linewidths increase in this complex compared to the host,
indicating that there is a different dynamic mechanism to the
complex formed from host 5. Further evidence of support for this
=
(
Peak 3, relative area = 0.62). The 1:1 host guest complex of 5
1
was examined and whilst the C{ H} spectrum is generally very
3
1
19
hypothesis comes from inspection of the F T1ρ relaxation times of
similar to the host, the shoulder at 37 ppm is more
t
pronounced, or alternatively the butyl resonance has
the CF
3
(δ
F
~ -82 ppm), CF
3
CF
2
(δ
F
~ -126 ppm), CH
2
CF
2
F
(δ ~ -113
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ppm) and the remaining CF
2
groups (δ
1ρ reduces slightly in the host:guest
complexes of 5 and 8 but there is a very large decrease in the
CF CH group. This could possibly be due to the more constrained
arrangement of the latter in the host:guest complex whereas the
CF group is relatively free to rotate. This has been confirmed by the
F
~ -122 ppm), which are
3
listed in Table 2. The CF T
2
2
3
variable temperature studies on 5:1 PFOA, which show that only
the CF group is influenced by increasing the temperature (Figure
3
S73).
1
9
13
F
→
C CP/MAS NMR Spectroscopy. Polarization transfer
techniques are useful for determining inclusion complexes as
dipolar interactions between the host and guest can be probed
directly. Thus we have measured the CP MAS spectra of the
host, guests and the corresponding complexes in order to
probe the orientation of the guest. The CP/MAS NMR spectra
of PFOA shows magnetization transfer to the C=O resonance at
1
3
1
Figure 9. C{ H} MAS NMR spectrum of 5 at 10 kHz spin rate and 298 K (Top); 5:PFOA
19
13
19
13
F → C CP/MAS NMR spectrum with a contact time = 10 ms (middle);
CP/MAS NMR spectrum with a contact time = 2 ms (bottom).
5
F → C
162 ppm as well as the CF carbon atoms (Figure S73). Host 5
shows the presence of a resonance at 35 ppm after a contact
time of 2 ms (Figure 9) and increases in intensity with
increasing contact time. This indicates that there is
1
9
Table 2. F T1ρ relaxation times (ms) of the various fluorous groups in the host and
host:guest complexes.
2 2 2 2
magnetization transfer to the CH O or CH CH CF groups. Using
the computationally derived structure (vide infra) this is likely
to be the latter due to the strong hydrogen bonding between
the amides of two chains, and consistent with the infrared
data.
Complex
PFOA
5
CF3
CF CF
CF2
CF CH2
3
2
2
287 ± 1.3
178 ± 0.6
260 ± 2.5
10.1 ± 0.02
223 ± 1.5
11.1 ±
219 ± 7.9
403 ± 2.6
0
.02
5
6
:PFOA
6
155 ± 82
36 ± 7.8
42 ± 0.9
22 ± 0.18
24 ± 0.83
8.1 ± 0.06
24 ± 0.5
14.9 ±
5.1 ± 3
23 ± 2.8
:PFOA
2
6 ± 0.05
3.5 ± 0.009
0
.05
8
8 ± 0.39
29.6 ± 3
5.1 ± 1.4
18 ± 0.3
5 ± 1.4
174 ± 0.2
584 ± 1.4
20 ± 2.2
8
:2PFOA
The same resonance is observed in the host:guest complex but
longer contact times are required to observe the resonance
(
10 ms). This indicates that the chains are now further apart
and the lack of resonance of the guest acid suggests that this
group is not close to the hosts CF groups i.e. is close to the
CH portion of the host. For host there is no observable
2
2
6
magnetization transfer (Figure S77) and no definitive
magnetization transfer in the 1:1 complex, consistent with the
less conformationally rigid structure compared to
5.
Interestingly there is magnetization transfer to the peak at ~30
ppm with a contact time of 8 ms in the 2:1 complex. For host
8, there is no magnetization transfer observed indicating that
the solid-state structure is not C symmetry and the fluorous
3
and non-fluorous domains are not close enough for
magnetization transfer. Upon addition of the host, a resonance
at 163 ppm is observed even at 1 ms contact time (Figure S81).
(c) Computational Studies. Despite numerous efforts we were
unable to grow crystals suitable for X-ray diffraction of the
hosts or the host:guest complexes. Therefore we looked at
host 5 and the corresponding 1:1 host:guest complex by
computational means in order to corroborate our
1
9
13
Figure 8. Top: full F{ C} MAS NMR spectrum of 5:PFOA at 20 kHz spin rate and 298 K;
Bottom: Deconvolution analysis of the spectra of the CF
host:guest complexes.
3
groups of the host and experimental
observations.
The
structure
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moments for
5
and PFOA are 10.89 and 2.76 D, respectively,
while that of the host-guest complex is 12.42 D indicating
Table 3. Summary of Intermolecular bond critical points.
PFOA
ρ
bcp
Octanoic acid
H…H (1)
ρ
bcp
F…F (27)
0.009, 0.008x2,
0.002
0
0
0
0
.007x2,
.006x8,
.005x5,
.004x3,
0
.003x2, 0.002,
.001x3
0.008x2,
.002x2
0.008, 0.005,
.004, 0.003
0
F…O (4)
H…F (4)
F…O (4)
0.008x2,
0.006x2
0
Figure 10. Computed structure of host 5 (left) and the host:guest complex (right).
F…H (26)
0.010, 0.009x2,
0.008, 0.007x5,
0
0
.006x3, 0.005,
of host
5 was manually built using MOE, and energy
0
0
.004x2,
.003x3,
minimization followed by stochastic conformational search
using the MMFF94 forcefield carried out, resulting in a cone-
shaped structure (Figure 10 left) containing two short
C=O…H—N hydrogen bonds (dO…N = 2.627 and 2.745 Å)
0
.002x5, 0.001
H…O (5)
O…H (1)
0.013, 0.011,
.008, 0.006,
.005
0.011
H…O (5)
O…H (1)
0.013, 0.010x3,
0.006
0
comparable to that seen in the structure of the calixarene
n
ligand featuring an hexyl substituent on the amide (dO…N
0
=
0.007
6
4
2
.816(1) and 2.869(4) Å). An extended conformations of
PFOA was placed manually within the cone-shaped cavity of
host with the acid group in proximity to host amide groups,
parallel alignment of dipoles in the complex, and consistent
with our design principle of dipole engineering. Repeating this
process with octanoic acid (OA) as the guest results in a
broadly similar structure, with two C=O…H-N hydrogen bonds
5
and the resulting complex energy minimized with MMFF94.
This structure was then subjected to a short (1 ns) NVT
molecular dynamics simulation to remove any artefacts of
manual placement, and the final snapshot of this trajectory re-
minimized. The resulting structure was optimized with
molecular orbital methods, first with semi-empirical PM7 and
then density functional B97-D/def2-SVP methods, both of
which include empirical correction for dispersion effects,
within a continuum model of aqueous solvation. This
methodology treats all non-covalent interactions in the
complex on an equal footing. Coordinates for the resulting
(
(
1.927 and 2.085 Å) within the receptor and one O—H…O
2.490 Å) H-bond from OA to ether oxygen in . In addition,
5
several C—H…O contacts (2.4 to 2.7 Å) are present, as well as
close” C—H…F contacts (2.4 to 2.9 Å). AIM analysis again
“
shows an abundance of non-covalent interactions; the
interactions are dominated by C—F…H—-C but the hydrogen
bonding interaction is the same magnitude as for PFOA. In
keeping with this, the calculated binding energy of octanoic
acid is only very slightly less than that for PFOA at -63.73 kJ
prediction of a 1:1 host:guest complex of
5 with PFOA are
-1
mol (-15.23 kcal/mol).
available in Supporting Information. This structure (Figure 10
right) retains the short C=O…H—N hydrogen bonds (O…H =
1
.887 and 2.020 Å) within the host, as well as numerous (d) Reactions with other guests. Given that the calculations
contacts between host and guest. The most obvious of these is suggest that the host:guest binding is through the ether
a hydrogen bond from O—H of PFOA to an ether oxygen of
oxygen and C—H…O interactions, we have examined the
O…H = 2.310 Å), but in addition there are numerous close C— binding energies of octanoic acid with
and a fluorous guest
H…O contacts in the range 2.4 to 2.9 Å and F…F contacts in the with no capacity for hydrogen bonding to corroborate these
5
(
5
1
19
range 2.8 to 3.3 Å.
Atoms-in-molecules (AIM) has been utilized and there are a measure the binding constants. Octanoic acid showed no
theories. We have only looked at H and F NMR titrations to
1
19
plethora of C—F…F—C and C—F…F—C interactions as changes in the H or F NMR spectrum, indicating that a stable
summarized in Table 3 and typical for these type of inclusion complex is not formed. Given the theory suggests
3
3
interactions. The hydrogen bonding to the ether oxygen is that once in the cavity it should form a stable hydrogen
also reproduced. The binding energy of guest to host is bonded system, we ascribe this inconsistency to the fact that
5
-1
calculated to be -65.16 kJ mol (-15.57 kcal/mol) at the DFT has an intentionally designed fluorophilic cavity so the
level after counterpoise correction indicating substantial octanoic acid does not enter this cavity in solution. Using
stabilization of the host-guest complex. Clearly, a significant {CF (CF ) CH CH } we were able to ascertain the extent of the
3
2 5
2
2 2
proportion of this stabilization stems from specific interactions fluorophilic cavity without the primary hydrogen bonding.
1
19
noted above, but additional stabilization may stem from Again no shifts in the H and F NMR spectra were observed
overall dipole-dipole interactions. DFT predicted dipole upon addition of up to 20 equivalents. These two experiments
1
0 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C7NJ03026F
ARTICLE
show the design features worked as we anticipated and
suggests selectivity for fluorinated hydrogen bond
donor/acceptor, although we have not explored this in detail.
3
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6
7
Calixarenes are known to be emissive so in principle this
spectroscopic technique could be utilised as a detection
method. However the introduction of the fluorous ponytail in
4
5
Stockholm Convention 2005; Decision POPRC-1/7:
Perfluorooctane
5
and 6 quenches this emission.
sulfonate.
http://www.pops.int/documents/meetings/poprc/docs/che
m_review.htm.
Conclusions
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In summary, we have designed a series of hosts for the
removal of a major global pollutant utilizing hydrogen bonding
and a fluorophilic cavity, excellent extraction ability observed.
The drawback is the leaching of the host out of the guest over
time. The stoichiometry of the host-guest complexes have
been determined and fully characterized by extensive solution
and solid-state NMR spectroscopic techniques. For the
calix[4]arene hosts the stoichiometry is 1:1 and the binding
constant is lower in the partial cone compared to the full cone,
due to the reduction in the fluorophilic cavity. The
conformationally labile tren based ligand is not as effective at
extracting PFOA and the stoichiometry is ~ 6 indicating that a
stable host:guest species is not formed; this is corroborated by
the solution DOSY experiments where large hydrodynamic
radii are measured for the 2:1 complex. Solid state NMR
spectroscopy, particularly cross-polarization techniques, has
also been used and substantiates the solution data. A
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the fluorophilic cavity and the hydrogen bond ability of the
hosts appears to be important for the formation of the
inclusion complex with PFOA. We have also, for the first time,
reported the crystal structure of perfluorooctanoic acid.
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This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
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New Journal of Chemistry
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DOI: 10.1039/C7NJ03026F
ARTICLE
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NP lee wa s eJ od uo r nn oa tl aod fj uC s ht em ma ri sg it nr ys
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Table of Contents artwork
Three hosts for the encapsulation of perfluorooctanoic acid have been synthesized. The
host:guest complexes have been characterized by multinuclear NMR spectroscopy in solution
and the solid state.