Isopropylammonium tetrafluorohydrogenphthalate: Structural characterization and comparison to two related salts with different stoichiometric ratios

Article abstract of DOI:10.1016/j.molstruc.2009.06.013

The normal acid salt isopropylammonium tetrafluorohydrogenphthalate (3) was prepared and its structure was determined by X-ray crystallography. This salt is stabilized by N-H^...O, O-H^...O, C-H^...O, and N-H^...F h

Full text of DOI:10.1016/j.molstruc.2009.06.013

Journal of Molecular Structure 934 (2009) 23–27
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Isopropylammonium tetrafluorohydrogenphthalate: Structural characterization
and comparison to two related salts with different stoichiometric ratios
Hans-Georg Häcker a, Gregor Schnakenburg b, Wilfried Hoffbauer b, Jörg Daniels b, Markus Pietsch a,1
,
Michael Gütschow a,
*
a Pharmaceutical Institute, Pharmaceutical Chemistry I, University of Bonn, An der Immenburg 4, D-53121 Bonn, Germany
b Institute of Inorganic Chemistry, University of Bonn, Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2009
Received in revised form 10 June 2009
Accepted 10 June 2009
Available online 16 June 2009
The normal acid salt isopropylammonium tetrafluorohydrogenphthalate (3) was prepared and its struc-
ture was determined by X-ray crystallography. This salt is stabilized by N–H. . .O, O–H. . .O, C–H. . .O, and N–
H. . .F hydrogen bonds. Compound 3 was characterized by means of solution and solid-state NMR. The
ipso-carbons, whose signals are equivalent in solution, could be distinguished in the solid state, thus
reflecting the asymmetric nature of 3. With respect to structural features, 3 was compared with salts
of different stoichiometry, i.e. the neutral salt bis(isopropylammonium) tetrafluorophthalate (1) and
the anomalous salt isopropylammonium tetrafluorohydrogenphthalate  tetrafluorophthalic acid (2).
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Tetrafluorophthalic acid
13C NMR spectra
X-ray crystallography, Fluorine hydrogen
bonds
1. Introduction
plings in 13C NMR spectroscopy. The structural characterization
is additionally complicated as a dibasic acid (H2Y) is able to form
The ability of covalently bound fluorine to accept hydrogen
bonds has been the subject of a scientific debate [1–3]. This issue
has attracted much interest in analyses of crystal structures of fluo-
rine-containing molecules [4–7], particularly in cases of aromatic
carbon-bound fluorine-mediated hydrogen bonds [8]. The intro-
duction of fluorine is a commonly used strategy in medicinal
chemistry to improve properties of bioactive compounds [9,10].
Although a hydrogen–fluorine exchange results in only minor ste-
ric changes, the high electronegativity of fluorine and the resulting
polarization of the C–F bond remarkably alter the physico-chemi-
cal properties of molecules [2,8]. Therefore, the introduction of
fluorine can lead to enhanced binding interactions, increased met-
abolic stability or selective reactivity [11]. Favorable (fluorophilic)
and unfavorable (fluorophobic) environments within target pro-
teins have been determined. Such approaches have been exploited
to develop inhibitors of serine and cysteine proteases [1–3,12–15]
or to design potent inhibitors of angiogenesis by fluorination of
phthalic acid derivatives [16–19]. In the course of studies on salts
of tetrafluorophthalic acid, one has to face limitations by the ab-
sence of aromatic protons in 1H NMR, and fluorine–carbon cou-
salts with a monoacid base (R) in different stoichiometric ratios.
Besides a neutral (R2Y) and a normal acid salt (RHY), various anom-
alous salts (e.g. RH3Y2, R2H4Y3) can be formed [20–22].
Recently, we described the neutral salt bis(isopropylammoni-
um) tetrafluorophthalate (Fig. 1, 1) and the anomalous salt isopro-
pylammonium tetrafluorohydrogenphthalate  tetrafluorophtha-
lic acid (2) [23]. The molecular structures of both salts could be
revealed by X-ray diffraction and were supported by different
spectroscopic methods. Interestingly, the solid-state NMR spec-
trum of 2 showed a splitting into three resonances of the carbons
attached to the carboxylic groups. To further elucidate this aspect,
we prepared the new normal acid salt isopropylammonium tetra-
fluorohydrogenphthalate (3). Structural data of 3 are presented
herein and are discussed with regard to the related salts 1 and 2.
2. Experimental
2.1. Materials and methods
Tetrafluorophthalic acid was obtained from Alfa Aesar (Kar-
lsruhe, Germany) and isopropylamine was from Merck Schuchardt
(Hohenbrunn, Germany). The melting point was determined on a
Boëtius hot stage microscope apparatus (PHMK, VEB Wägetechnik
Rapido, Radebeul, Germany). The infrared spectrum was recorded
* Corresponding author.
E-mail address: guetschow@uni-bonn.de (M. Gütschow).
Present address: School of Chemistry & Physics, The University of Adelaide, North
1
Terrace, Adelaide, SA 5005, Australia.
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.06.013

24
H.-G. Häcker et al. / Journal of Molecular Structure 934 (2009) 23–27
dihedral angles between planes. All non-hydrogens were refined
anisotropically. Hydrogen atoms were localized by difference elec-
tron density determination and were refined isotropically. CCDC-
730532 contains the supplementary crystallographic data for com-
pound 3. It can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
3. Results and discussion
3.1. Crystal structure
The crystallographic data of compound 3, obtained in the course
of this study, are summarized in Table 1. Selected bond lengths and
angles are given in Table 2, hydrogen bonding parameters are pre-
sented in Table 3. The dihedral angle between the planes of the car-
boxylate fragment (O1/C7/O2) and the protonated carboxyl group
(O3/C8/O4) is 56.4(2)°. The angles of both groups relative to the
tetrafluorobenzene ring are 50.8(2)° (O1/C7/O2), 58.6(2)° (O3/C8/
O4), respectively. These two values show the same tendency com-
pared to those observed in the tetrafluorohydrogenphthalate unit
of compound 2 (30°, 76°) [23], and differ from typical orientations
in hydrogenphthalates with angles between carboxylate fragments
and phenyl planes in the range of 65°–85°, but smaller angles (5°–
40°) between carboxyl groups and benzene rings, respectively [27].
Different types of hydrogen bonds were observed in compound
3. Besides N–H. . .O, O–H. . .O, and C–H. . .O bonds, one fluorine of the
aromatic ring was incorporated in a hydrogen bond of type N–H. . .F.
All three ammonium hydrogen atoms are part of hydrogen bonds
with four different tetrafluorohydrogenphthalate molecules. The
hydrogen bond network is shown in Fig. 2. As in structures 1 and
2, two hydrogens are incorporated in two-center N–H. . .O hydrogen
bonds. In the presented structure 3, both contacts are formed with
oxygens of carboxylate fragments. For hydrogen-acceptor dis-
tances in hydrogen bonds between mono-substituted ammonium
Fig. 1. Structures of isopropylammonium tetrafluorophthalates 1–3 with different
stoichiometry.
on a Bruker Tensor 27 FT-IR instrument. Intensities of selected
bands in the range of 3500–1000 cmÀ1 are indicated as s (strong),
m (moderate), and w (weak). 1H NMR (500 MHz) and 13C NMR
spectra (125 MHz) were recorded on a Bruker Avance DRX 500.
Chemical shifts d are given in ppm referring to the signal center
using the DMSO-d6 peaks (2.49 ppm/39.7 ppm) for reference. Spin
multiplicities are indicated by the following symbols: br s (broad
singlet), d (doublet), and sept (septet). Solid-state, magic-angle
spinning (MAS) NMR experiments were carried out on a Varian
Infinity+ spectrometer equipped with a commercial 4 mm MAS-
NMR double-resonance probe. The magnetic field strength was
9.4 T corresponding to a 13C and 1H resonance frequency of
100.98 and 401.52 MHz, respectively. The 13C MAS-NMR spectrum
was acquired with a ramped 13C{1H} cross-polarization (CP) exper-
iment. The spectrum shown was obtained in 1 h with a repetition
delay of 5 s at room temperature. At 12 kHz spinning frequency the
13C pulse lengths was 2.5
ls, 100 kHz spectral width and 800 tran-
sients. A line broadening of 50 Hz was used in data processing. The
13C chemical shifts refer to tetramethylsilane (TMS). Values for the
spectral parameters (integral, full width half maximum) were
achieved from the least square fitting of the experimental spec-
trum by the spectrometer software. Elemental analysis was per-
formed with a Vario EL apparatus.
Table 1
Crystallographic data for 3.
Crystal data
2.2. Isopropylammonium tetrafluorohydrogenphthalate (3)
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal size (mm)
Crystal system
Space group
a (Å)
C11H11F4NO4
297.20
123(2)
0.71073
A solution of isopropylamine (120 mg, 2.00 mmol) in water
(2.0 mL) was added to tetrafluorophthalic acid (714 mg,
2.00 mmol). After the mixture was stirred at room temperature
for 10 min, the solvent was evaporated under reduced pressure.
The crude product was recrystallized from MeCN to give 3
(502 mg, 84%) as colorless rhombs: m.p. 154–156 °C; IR (KBr,
0.34 Â 0.20 Â 0.10
Monoclinic
P21/n
7.8520(2)
16.2123(5)
9.2330(3)
90
98.026(2)
90
1163.84(6)
4
1.696
0.168
608
2.91–30.04
98.6 (hmax = 30.04°)
À10/11, À22/18, À13/12
15278/3345 [R(int) = 0.0479]
Analytical
0.984 and 0.9616
Full-matrix least squares on F2
3345/0/225
b (Å)
c (Å)
cmÀ1
) 3161m, 2942w, 2570w, 1917w, 1718s, 1628s, 1592s,
1475s, 1376s, 1124m, 1068s; 1H NMR (DMSO-d6) d 1.16 (d,
3J = 6.6 Hz, 6H, CH3), 3.26 (sept, 3J = 6.6 Hz, 1H, CH), 7.90 (br s,
3H, NH); 13C NMR (DMSO-d6) d 20.53 (CH3), 43.00 (CH), 122.19
a
(°)
b (°)
c
(°)
V (Å3)
2
1
(d, JC–F = 14 Hz, C1, C2), 139.60 (d, JC–F = 253 Hz, C4, C5), 143.78
Z
Calculated density (g/cm3)
Absorption coefficient (mmÀ1
F(0 0 0)
1
(d, JC–F = 248 Hz, C3, C6), 163.13 (CO). Solid-state 13C NMR d
)
20.1 (CH3), 46.6 (CH), 118.0, 122.9 (C1, C2), 134–149 (C3, C4, C5,
C6), 165.4 (CO). Analytical data for C11H11F4NO4. Calcd: C, 44.45;
H, 3.73; N, 4.71. Found: C, 44.44; H, 3.79; N, 4.65.
h Range for data collection (°)
Completeness to h (%)
Range of h, k, l
Reflections collected/unique
Absorption correction
Max. and min. transmission
Refinement method
2.3. Crystal structure determination of 3
Data were collected on a Nonius KappaCCD diffractometer
equipped with a low-temperature device (Cryostream, Oxford
Data/restraints/parameters
Goodness-of-fit on F2
0.943
Cryosystems) at 123(2) K using graphite monochromated Mo-K
a
Final R indices [I > 2
r
(I)]
R1 = 0.0373, wR2 = 0.0816
R1 = 0.0660, wR2 = 0.0904
0.250 and À0.352
radiation (k = 0.71073 Å). The structure was solved by direct meth-
ods (SHELXS-97) [24] and refined by full-matrix least squares on F2
(SHELXL-97) [25]. PLATON v.230608 [26] was used for calculating
R indices (all data)
Largest diff. peak and hole (e/Å3)

H.-G. Häcker et al. / Journal of Molecular Structure 934 (2009) 23–27
25
Table 2
groups and carboxylate oxygens, a mean value of 1.84 Å was re-
ported [28]. Thus, the HN1B. . .O2 distance (1.87 Å) is in accordance
therewith, but the HN1A. . .O1 bond is somewhat longer (2.02 Å).
In contrast to 1 and 2, a fluorine atom, i.e. F3 in para-position to
the protonated carboxyl group, is involved in the hydrogen bond-
ing network of structure 3. The hydrogen atom HN1C is forming
a three-center (bifurcated) hydrogen bond to acceptors fluorine
F3 and oxygen O4 from two different tetrafluorohydrogenphtha-
late molecules. The major component of this unsymmetrical bifur-
cated hydrogen bond is directed to the oxygen, as it is more linear
(148° vs. 123°) and has a shorter hydrogen-acceptor distance in
comparison to the minor one directed to the fluorine (2.02 Å vs.
2.43 Å). There are structures reported with bifurcated hydrogen
bonds to fluorine and oxygen, but the donors/acceptors do not be-
long to three different molecules in such cases [5,29].
Selected bond lengths (Å) and angles (°) for 3.
C7–O1
C7–O2
C8–O3
C8–O4
O1–C7–O2
O3–C8–O4
C6–C1–C7–O1
C6–C1–C7–O2
C3–C2–C8–O3
C3–C2–C8–O4
1.2636(15)
1.2389(15)
1.3104(16)
1.2130(16)
128.11(12)
125.38(12)
À52.54(16)
126.81(13)
120.54(12)
À56.65(17)
Table 3
Hydrogen bonding parameters for 3.
The protonated oxygen O3 of the carboxyl group is part of an
intermolecular O–H. . .O bond to oxygen O1 of the carboxylate frag-
ment. Such strong contacts (O. . .O distance 2.55 Å) were also ob-
served in 2 as well as crystals of other hydrogenphthalates
D–H. . .
A
D–H (Å)
H. . .A (Å)
D. . .A (Å)
D–H. . .A (°)
O3–HO3. . .O1a
N1–HN1A. . .O1b
N1–HN1B. . .O2c
N1–HN1C. . .O4d
N1–HN1C. . .F3e
C11–H11B. . .O3b
0.99(2)
1.62(2)
2.5512(13)
2.9287(14)
2.7705(15)
2.8590(15)
3.0292(14)
3.3884(17)
155.2(19)
177.7(13)
170.3(15)
148.2(15)
122.6(13)
150.3(14)
0.906(16)
0.914(18)
0.933(19)
0.933(19)
0.983(18)
2.023(16)
1.865(19)
2.023(18)
2.423(17)
2.500(19)
[23,27,30–32]. Moreover, the carboxyl oxygen O3 accepts
a
C–H. . .O hydrogen bond from a methyl group of the isopropyl
ammonium molecule.
In total four N–H. . .O hydrogen bonds are directed toward the
carboxylate oxygens of 1, the carboxylate fragment of 2 accepts
four hydrogen bonds from N–H or C–H donors, whereas three
hydrogen bonds from O–H or N–H are directed toward the
Symmetry transformations used to generate equivalent atoms: ax + 1/2, Ày + 5/2,
z + 1/2; bÀx + 1/2, y À 1/2, Àz + 3/2; cÀx, Ày + 2, Àz + 2; dÀx + 1/2, y À 1/2, Àz + 5/2;
ex + 1/2, Ày + 3/2, z + 1/2.
Fig. 2. Molecular plot of 3 showing the atom-labelling scheme and displacement ellipsoids at the 30% probability level for the non-H atoms. H atoms are depicted as small
circles of arbitrary radii and dashed lines represent hydrogen bonds.

26
H.-G. Häcker et al. / Journal of Molecular Structure 934 (2009) 23–27
carboxylate oxygens of 3. Taken together the three crystal
structures of 1, 2 and 3 show different binding geometries and
hydrogen bonding arrangements. Only in the structure of 3, a
fluorine atom acts as a hydrogen bond acceptor.
ratio, whereas the shifts of the other benzene carbons are inappro-
priate because of carbon–fluorine couplings.
To further characterize the structure of compound 3, a solid-
state 13C NMR spectrum was recorded (Fig. 3). Chemical shifts of
fluorine-substituted carbons appear together as a broad signal be-
tween 134 ppm and 149 ppm. This was also observed in the solid-
state spectra of the two tetrafluorophthalate-derived salts 1 and 2
[23]. The asymmetry of structure 3, induced by the different pro-
tonation, is reflected by two signals for the ipso-carbons. Integra-
tion of the corresponding peak areas gave similar values (3.1 vs.
3.6). Accordingly, when recording the solid-state spectrum of the
anomalous salt 2, we observed three distinct resonances with
AUC ratios of 1:1:2 [23]. Vila et al. studied 1,3-diphenyl-propane-
1,3-dione and also noticed splitting of the ipso-carbon and carbonyl
carbon signals [34]. The solid-state 13C NMR spectra of 2 (and 3) do
not reveal a dissimilarity of the CO carbons, since their signals are
not separated. The occurrence of one common signal for COOꢀand
COOH carbons is in agreement with the relatively low sensitivity
of the 13C isotropic chemical shift to protonation, as can be ex-
plained by relations between main components d22 and d11 of the
13C chemical shift tensor [36]. However, as a matter of principle,
such a differentiation is possible. Ilczyszyn et al. have determined
the structure of a sarcosine–maleic acid (1:1) complex by X-ray
diffraction and distinguished the charged and uncharged car-
boxyl(ate) groups by means of solid-state 13C NMR spectroscopy
[37]. Barry et al. characterized normal and anomalous tetramethyl-
ammonium salts of dicarboxylic acids and observed different 13C
NMR shifts for nonequivalent carboxyl(ate) carbons in solid state
[38]. In the cases of 2 and 3, crystal and bonding forces which influ-
ence bond angles and distances and thus the resulting 13C chemical
shift [21] might be responsible for a coalescence of the CO signals.
3.2. Spectroscopic characteristics
1H NMR and 13C NMR spectra of compound 3 were recorded in
DMSO-d6. The shifts for the aromatic carbons were assigned on the
basis of 13C–19F coupling constants and substituent increments
1
2
[33]. JC–F and JC–F coupling constants were around 250 Hz and
14 Hz, respectively. Only four different signals for the carbons of
the tetrafluorohydrophthalate molecule appear in the spectrum
of 3, thus different positions relative to the protonated or deproto-
nated carboxyl moiety were not reflected. This can be attributed to
a rapid proton migration in solution. Similar observations have
been made for compound 2 [23], as well as related proton transfers
in 1,3-dicarbonyl compounds [34] and dicarboxylic acids [35].
When comparing the salts 1–3, we noticed substantially differ-
ent 13C chemical shifts for the CO carbons and for those bearing the
COO(H) groups (ipso-C). The values correlate with the ratio of car-
boxylate fragments to carboxyl groups. The neutral salt 1 with
exclusively carboxylate fragments provides shifts of 164.5 ppm
(CO) and 125.9 ppm (ipso-C), respectively. The values of the normal
acid salt 3 with COOꢀand COOH in equal amounts are 163.1 ppm
(CO) and 122.2 ppm (ipso-C). A COOꢀ/COOH ratio of 1:3 in the
anomalous salt 2 yields 162.9 ppm (CO) and 120.3 ppm (ipso-C).
In particular, the value for the ipso-position (substituent increment
for COONa: 8.4, for COOH: 2.1 [33]) indicates the stoichiometric
Acknowledgements
G.S. thanks Prof. Dr. Alexander C. Filippou for support. This work
has been funded by the German Research Society (Graduate
College 804).
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