Synthesis and characterization of the first perfluoroaromatic polyimide of the AB-type

Article abstract of DOI:10.1016/j.jfluchem.2011.09.010

Aminodefluorination of tetrafluorophthalic acid by anhydrous ammonia was shown to give 4-amino-3,5,6-trifluorophthalic acid selectively and in a high yield. Under the action of dicyclohexylcarbodiimide this acid was quantitatively converted into 4-amino-3

Full text of DOI:10.1016/j.jfluchem.2011.09.010

Journal of Fluorine Chemistry 135 (2012) 129–136
Contents lists available at SciVerse ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Synthesis and characterization of the first perfluoroaromatic polyimide of the
AB-type
Tamara A. Vaganova, Inna K. Shundrina, Soltan Z. Kusov, Elena V. Karpova,
*
Irina Yu. Bagryanskaya, Evgenij V. Malykhin
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, Lavrentiev Avenue 9, 630090 Novosibirsk, Russian Federation
A R T I C L E I N F O
A B S T R A C T
Article history:
Aminodefluorination of tetrafluorophthalic acid by anhydrous ammonia was shown to give 4-amino-
3,5,6-trifluorophthalic acid selectively and in a high yield. Under the action of dicyclohexylcarbodiimide
this acid was quantitatively converted into 4-amino-3,5,6-trifluorophthalic anhydride, which is the
simplest monomer of the AB-type. The first perfluoroaromatic AB-type polyimide was prepared by the
one-step high-temperature polycondensation in a benzoic acid melt and characterized using FTIR, NMR,
UV–vis, MALDI TOF MS, and TG/DSC methods. The near-IR spectrum of the polyimide exhibits no
absorption of C–H and O–H bond harmonic vibrations, which testifies to considerable promise of AB-
type perfluorinated polyimides for optical communication systems.
Received 4 April 2011
Received in revised form 22 September 2011
Accepted 25 September 2011
Available online 1 October 2011
Keywords:
Fluorinated polyimides
Polyimide properties
Aminotrifluorophthalic anhydride
Near-IR spectra
ß 2011 Elsevier B.V. All rights reserved.
1. Introduction
branched (dendritic) polymers can be obtained (see [7] and Refs.
herein).
Fluorinated aromatic polyimides (PIs) are used as optical
materials for modern telecommunication systems due to the
excellent transparency and low optical transmission losses, as well
as good dielectric properties and hydrophobicity [1]. These
properties improve as fluorine content increases [1,2]. However,
possibilities of preparing completely fluorinated aromatic PIs are
limited by the low availability of hydrogen-free monomers. Up to
the present the only perfluorinated PI was synthesized via
polymerization of 1,4-bis(3,4-dicarboxytrifluorophenoxy)tetra-
fluorobenzene dianhydride (AA-monomer) and tetrafluoro-m-
phenylene diamine (BB-monomer) [3]. In addition, a series of
promising highly fluorinated PIs was obtained using other
perfluoroaromatic diamines [3–6]. No synthesis of fluorinated
aromatic PIs from AB-monomers (the anhydride and amine groups
are both contained in one molecule) was reported. At the same
time, from a synthetic point of view, by using AB-monomers one
can avoid the strict stoichiometric control, which is required in AA/
BB polycondensation systems. The peculiarities of the structure of
AB-type aromatic PIs (the arrangement of imide cycles, the
combination of cyclic fragments and linkage groups) reduce the
interchain interaction in macromolecules as compared with AA/
BB-type PIs, thereby increasing the processability of PIs. Besides,
on the basis of heterofunctional compounds of ABx-type hyper-
Combining the processing advantages of AB-based PIs with the
called-for properties (optical, dielectric, thermal) of the known
highly fluorinated PIs appears to be an advanced task. Thereupon,
the present work aims at (i) the development of a convenient
method for synthesis of the perfluorinated AB-type monomer – 4-
amino-3,5,6-trifluorophthalic anhydride (1), (ii) investigation of its
polymerization, and (iii) characterization of the primary per-
fluorinated polyimide of the AB-type (PI-AB).
2. Results and discussion
2.1. Synthesis of AB-monomer 1 (4-amino-3,5,6-trifluorophthalic
anhydride)
Preparation of 4-amino-3,5,6-trifluorophthalic acid (2) mixed
with isomeric 3-amino-4,5,6-trifluorophthalic acid (9:1) by the
action of aqueous ammonia upon commercially available tetra-
fluorophthalic acid (3) in the presence of alkaline or alkaline earth
metal salts at 150–170 8C was reported in the patent [8]. However,
no method for isolation of the main product was suggested.
Besides, in aqueous ammonia, at high temperatures (>100 8C),
partially hydrodefluorinated compounds are known to be formed
along with desired products [9]. These by-products are practically
inseparable from perfluorinated compounds, but their presence in
monomers impairs the optical characteristics of polymers [1,3]. In
the previous works [10] we developed an effective and selective
method for aminodefluorination of electrophilic polyfluoroarenes
* Corresponding author. Fax: +7 383 330 9752.
E-mail address: malykhin@nioch.nsc.ru (E.V. Malykhin).
0022-1139/$ – see front matter ß 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2011.09.010

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Scheme 1. Synthesis of aminotrifluorophthalic anhydride 1.
in anhydrous ammonia applied as a reagent and a solvent
simultaneously, which ensures the absence of hydrodefluorination
products. In this manner acid 2 is selectively prepared (Scheme 1)
in a high yield (95%) and purity (98%).
point solvents owing to the acid catalysis of the condensation; and
it was successfully applied for obtaining polyimides based on
hexafluoro-2,4-toluenediamine [4].
AB-type monomer 1 was kept in a benzoic acid melt upon
stirring for 20 h at 160 8C and, further, for 20 h at 180 8C,
whereupon the reaction mixture split into two phases. During
the process there appeared three new signals in the 19F NMR
spectrum (Fig. 1a, signals a, b, and c), whose intensity became
predominant with time. According to the known substituent
shielding parameters [17], these signals correspond to the fluorine
atoms of the internal fragment of PI-AB. In the resulted spectrum
along with the aforementioned signals a–c there is a set of signals
of a substantially lower intensity (Fig. 1a, signals d–i), which can be
ascribed to fluorines of the terminal amine and anhydride
fragments of PI; there are also low-intensity signals of the
uncertain origin (j, k, m).
Acid 2 is a direct precursor of the AB-type monomer 1. However,
non-fluorinated compounds of this type undergo rapid oligomeri-
zation even under slight heating, which complicates their
preparation and isolation. For this reason, the synthesis of AB-
monomers should be carried out under the mildest conditions. We
applied N,N0-dicyclohexylcarbodiimide (DCC) as a mild cyclode-
hydrating agent; this compound is widely used for the condensa-
tion of amino acids in peptide synthesis [11]. DCC interacts with
acid 2 to form anhydride 1 in a high yield and without by-products
(Scheme 1); after a single crystallization, it was used as a
monomer.
2.2. Synthesis and structural characterization of PI-AB
To identify the structural fragments and end groups in the
polymer and to determine the repeated unit weight, the obtained
PI-AB was analyzed by the MALDI-TOF MS, which is used for such
purposes [6,19]. The negative-ion MALDI spectrum of PI-AB
contains repeated fragments (groups of signals) with the period
m/z 199 Da equal to the weight of the polymer structural unit
(C8F3NO2). Fig. 2 shows a fragment of the spectrum in the range n
from 4 to 8; Table 1 presents tentative peak mass assignments.
Besides the peaks of PI-AB radical anions (Entry 3, Table 1), there
are peaks of adducts of PI-AB radical anions with two molecules of
NaOH (Entry 4, Table 1), as well as peaks of the anions of
decarboxylated PI-AB molecules (Entry 1 and 2, Table 1). The latter
particles can be formed during both the MS analysis and the
polycondensation process. As a rule, preparation of the PI samples
for analysis is carried out using water–organic solvents and is
followed by the hydrolysis of some of the anhydride end groups
[6,19]. The electron withdrawing effect of the aromatic fluorines is
known to facilitate CO2-elimination from ortho-dicarboxylic
fragments [20], so even a moderate voltage drop can cause the
destruction of the molecule (cf. [21]). On the other hand, during
Synthesis of AB-type PIs from fluorine-free aminophthalic and
aminophenoxyphthalic anhydride was realized in amide solvents
or pyridine [12,13], including the use of condensing agents
[14,15], and in solid phase [16]. Electrophilicity of fluorinated
anhydrides increases as compared with non-fluorinated com-
pounds owing to the high electronegative effect of fluorines in an
aromatic ring. On the contrary, nucleophilicity of the amino group
decreases greatly because of the same effect [1]. Thus, the
reactivity of perfluorinated compound 1 is determined by a
balance of these opposite factors. We tested the behavior of
monomer 1 in amide solvents in the temperature range from 20 to
130 8C. Monitoring the process was carried out using 19F NMR
spectroscopy; this method provides an essential information on
the chemical transformations taking place during polycondensa-
tion of fluorine containing monomers [1,4,6,17]. Under these
conditions no reaction was found to occur between amine and
anhydride fragments of compound 1.
For this reason, to prepare the perfluorinated PI-AB method of
one-pot high-temperature polycondensation in a benzoic acid melt
[18] was chosen (Scheme 2). For less reactive diamines, this
method possesses an advantage over the synthesis in high boiling
high-temperature one-step polycondensation
a part of the
anhydride end groups is hydrolyzed by the released water, which
can be accompanied by their decarboxylation (e.g. see [22]). Taking
Scheme 2. Synthesis of the perfluorinated polyimide of AB-type.

T.A. Vaganova et al. / Journal of Fluorine Chemistry 135 (2012) 129–136
131
Fig. 1. 19F NMR spectra of PI-AB (a) and model compounds (b).
into consideration the presence of three additional signals j, k, and
m in the 19F NMR spectrum of PI-AB (Fig. 1a), the latter variant
appears to be more probable.
terminal fragment has the signals h, i, and signal g, which coincides
with the signal a that belongs to the imide internal fragment. Apart
from these the spectrum contains the signals of lower intensity – j,
k, m – that apparently belong to the carboxyl terminal fragment.
The ratio of anhydride/carboxyl terminal fragments in PI-AB
molecules, evaluated by the ratio of the integrated intensities of
the signals h and k, is ꢀ4/1. The total intensity of the signals g–m
approximates to the intensity of the signals d–f belonging to the
amine terminal fragment. The averaged length of the polymer
chain, evaluated by the number of the internal and terminal
fragments, comes to ꢀ9 structural units. This corresponds to the
The reliable identification of the terminal fragments in PI-AB
was carried out by 19F NMR spectroscopy using data on the model
compounds, i.e. N-pentafluorophenyl-4-amino-3,5,6-trifluor-
ophthalimide (4), 4-acetamido-3,5,6-trifluorophthalic anhydride
(5) and 3-acetamido-2,4,5-trifluorobenzoic acid (7). These com-
pounds were synthesized for the first time (Scheme 3); compre-
hensive spectral data on them are given in Sections 4.3.3–4.3.6. The
signals in 19F NMR spectra of compounds 4, 5, and 7 are presented
in Fig. 1b.
¯
value of number-averaged molecular weight of PI-AB Mn ꢀ 1800.
Differences in the chemical shifts of signals in the spectra of PI-
AB and of model compounds (Fig. 1) amount to 0.1–1.5 ppm. The
amine terminal fragment of PI has the signals d, e, and f; anhydride
The low value of the inherent viscosity of PI-AB (hinh ꢀ 0.1)
conforms to this evaluation of the molecular weight. The results of
the elemental analysis of PI-AB are satisfactory; this is in

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T.A. Vaganova et al. / Journal of Fluorine Chemistry 135 (2012) 129–136
Scheme 3. Synthesis of the model compounds.
Fig. 2. MALDI-TOF mass spectrum of PI-AB, repeated fragments in the range n from
4 to 8; peak assignment is given in Table 1.
accordance with the above conclusion that the fragmentation of PI
is insignificant.
The FTIR spectral data on PI-AB conform to the imide structure
and show characteristic absorptions at 1747 and 1801 cmꢁ1
(asymmetrical and symmetrical stretching of carbonyl group in the
imide cycle). No absorptions of the anhydride and amino end
groups have been recorded in the spectrum. Fig. 3 shows the FTIR
spectra of PI-AB and the initial anhydride 1.
On account of a minor extent of the fragmentation of the
anhydride end groups by CO2-elimination (ꢀ20% according to the
19F NMR data), this process is not the only reason retarding the
growth of the PI chain. The polymerization degree is significantly
affected by the chain and end group mobility along with the end
group reactivity. In the case of the rigid perfluorinated PI-AB both
of these factors are unfavorable for preparing a high-molecular
polymer. Judging by the data [12,13,16], the simplest non-
fluorinated AB-type PI obtained from aminophthalic anhydride
also has a low molecular weight, whereas the use of aminophe-
noxyphthalic anhydrides as monomers enables to prepare PIs with
Fig. 3. FTIR spectra of PI-AB (a) and aminotrifluorophthalic anhydride 1 (b).
higher molecular weights and better film-forming properties. This
is caused by the effect of the oxygen linkage, which imparts more
flexibility to the polymer chain and higher mobility to the end
groups, as well as enlarges the amino group reactivity due to the
electron donating effect of the oxygen atom. This way of the
Table 1
Mass assignments of the peaks displayed in the MALDI-TOF mass spectrum of PI-AB (see also Fig. 2).
Structure
Monoisotopic mass (Da)
n = 2
n = 3
n = 4
n = 5
n = 6
n = 7
n =8
[AB+H2O–2CO2–H]ꢁ
[AB+H2O–CO2–H]ꢁ
ABꢂS
941.82
1140.76
1184.71
1211.74
1291.77
1339.58
1383.63
1410.59
1490.66
985.77
1012.72
1092.83
1582.51
1609.51
1689.52
1781.36
1808.32
1888.36
[AB+2NaOH]ꢂS
893.86
2087.28

T.A. Vaganova et al. / Journal of Fluorine Chemistry 135 (2012) 129–136
133
of highly fluorinated CF3-containing PIs, whose cut-off wave-
lengths lie between 340 and 360 nm [1]. This shift appears to be
caused by the conjugation of chromophore groups in the PI-AB
chain resulting in the formation of an intramolecular charge-
transfer complex [23].
3. Conclusions
Convenient conditions were found for aminodefluorination of
tetrafluorophthalic acid 3 (anhydrous ammonia, 110 8C), which
provide a high efficiency and isomeric selectivity of the reaction.
Owing to the use of a mild dehydrating agent (dicyclohexylcarbo-
Fig. 4. TG and DTG diagrams of PI-AB, inert (a) and oxidative (b) atmosphere.
diimide), 4-aminotrifluorophthalic acid
2 was quantitatively
converted into 4-aminotrifluorophthalic anhydride 1, which is
the simplest monomer of the AB-type. The first perfluorinated PI-
AB was prepared by the one-step high-temperature polyconden-
sation in a benzoic acid melt. The growth of the macromolecules
seems to be limited by low reactivity of the amino group, partial
destruction of the anhydride end groups, and restricted mobility of
the end groups in a rigid polymer chain. The PI-AB structure was
characterized by FTIR, NMR, MALDI-TOF MS and elemental
analysis. The properties of perfluorinated PI-AB were studied:
absorbance and transmittance in the UV–vis and near-IR regions,
solubility, and thermal stability. Note that this polymer exhibits no
absorption of harmonic vibrations of C–H and O–H bonds in the
Fig. 5. Near-IR absorption spectra of PI-AB (a) and 6FTDA/ODPA (b) dissolved in
acetone-d6.
near-IR region (1.0–1.7
mm), which defines AB-type perfluorinated
structural modification appears to be promising for the perfluori-
nated AB-type monomer as well.
PIs as promising materials for optical telecommunication systems.
4. Experimental
2.3. Properties of the PI-AB
4.1. Materials
In spite of the low values of molecular weight and inherent
viscosity (see Section 2.2) the perfluorinated PI-AB obtained has a
satisfactory thermal and thermooxidative stability appropriate to
high molecular weight PIs [2]. PI-AB exhibits no crystallization or
melting transition in DSC measurements. Glass transition was not
detected by DSC either, which is caused by the rigid structure of the
PI chain; this is typical of rod-like PIs containing no flexible
linkages. The short-term thermal stability of PI-AB was estimated
from the temperatures of 10% weight loss (Td10) using TGA (Fig. 4).
Td10 is 528 8C in an inert and 512 8C in an oxidative atmosphere.
The residual weight retention at 700 8C amounts to 57% and 12% in
inert and oxidative atmospheres, respectively. The maximal rate of
degradation in both atmospheres was fixed at 592 8C.
The following commercial products were used: pentafluoroani-
line (99%), anhydrous NH3, N,N0-dicyclohexylcarbodiimide (DCC,
99%), acetic anhydride (95%). Benzoic acid was purified by
sublimation under reduced pressure. Tetrafluorophthalic acid (3)
was prepared by analogy to the method [24].
4.2. Methods
1H and 19F NMR spectra were recorded on NMR spectrometer
Bruker AV-300 (300.13 MHz and 282.36 MHz for 1H and 19F,
respectively) using residual proton signals of the deuterated
solvent and C6F6 (
d
= ꢁ163 ppm from CCl3F) as internal standard;
d
The perfluoroaromatic PI-AB, in spite of the absence of flexible
linkages and pendant groups, displays good solubility in acetone,
cyclohexanone, DMSO, DMF, DMA, and NMP, which is provided by
the presence of fluorines in the PI backbone. PI-AB film is
transparent, glassy and yellow colored. When heated up to
300 8C, the film becomes yellowish brown and does lose solubility.
The key property providing the application of polyfluorinated
PIs as materials for optical waveguides is their transparency at the
are given in ppm relative to TMS and CCl3F, J are given in Hz. PI-AB
samples for registration of NMR spectra were dissolved to
concentration ꢀ5% in DMA; signal/noise ratio ꢃ 40, integration
error ꢄ 4%. FTIR spectra were recorded on Bruker Vector-22
instrument for KBr disks. UV–visible spectra of the polymer films,
spin-coated from cyclohexanone solutions on a quartz substrate,
were measured on Varian spectrophotometer Cary 5000. Purity of
compounds was performed on HP 5890 Series II gas chromato-
graph (thermo conductivity detector); HP5 column (5% of biphenyl
wavelengths of 1.3 and 1.5
mm referred to as telecommunication
windows [1]. Fig. 5a presents the near-IR absorption spectrum of
PI-AB dissolved in acetone-d6. For comparison, Fig. 5b shows the
spectrum of PI, which is prepared from hexafluoro-2,4-toluene-
diamine and 4,40-oxydiphthalic dianhydride (6FTDA/ODPA) [4]
and contains 22.6% of F and only 1.19% of H. This PI exhibits rather
and 95% of dimethylsiloxane), 30 m ꢅ 0.22 mm ꢅ 2.6
mm; with
helium as carrier gas, flow rate 1 mL minꢁ1; column temperature
programming from 90 8C (2 min) at an increment of 10 8C minꢁ1 to
330 8C (5 min); injector temperature 300 8C; detector temperature
320 8C. Precise molecular weights of ions were determined by
HRMS on Thermo Scientific DFS instrument, ionizing energy of
70 eV. Mass spectra were recorded using autoflex III MALDI-TOF
mass spectrometer (Bruker Daltonics, Germany) equipped with a
pulsed N2 laser (337 nm) in a negative reflectron mode. Ions
formed by a laser beam were accelerated to 20 keV kinetic energy.
The final spectra were obtained by accumulating 2500 single laser
shot spectra. The saturated solution of 2,5-dihydroxybenzoic acid
(DHB) in 50% aqueous acetonitrile was used as a matrix. A sample
an intensive absorption at 1.65
mm due to the second harmonic of
the stretching vibration of the C–H bond in non-fluorinated
aromatic rings. On the contrary, perfluorinated PI-AB has a
transparency window throughout the investigated range
(Fig. 5a). Within the UV–vis range the PI-AB films exhibit a cut-
off wavelength
to 73% for the film of the 40
AB is shifted to the long-wave area as compared with the
l
0 of 400 nm. The transmittance at 450 nm is equal
m in thicknesses. The 0 value of PI-
0 values
m
l
l

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T.A. Vaganova et al. / Journal of Fluorine Chemistry 135 (2012) 129–136
of polymer solution (10% in DMAA) was mixed with the same
volume of matrix solution. About 1 L of the resulting solution was
imide 4, yield 81%. Mp 181–182 8C; FTIR (KBr):
1788, 1734 (C55O), 1640 (NH2); 1H NMR (acetone-d6):
NH2); 19F NMR (acetone-d6):
JF,F = 14 Hz, F-3), ꢁ140.2 (dd, 1F, JF,F = 20 Hz, JF,F = 14 Hz, F-6),
ꢁ142.9 (m, 2F, F-20, F-60), ꢁ147.7 (dd, 1F, JF,F = 21 Hz, JF,F = 20 Hz, F-
5), ꢁ151.9 (tt, 1F, JF,F = 21 Hz, JF,F = 3 Hz, F-40), ꢁ162.0 (m, 2F, F-30,
F-50); EIMS, 70 eV, m/z (rel. int.): 382 [M]+ (93), 338 [M–CO2]+ (54),
173 [M–CO–NC6F5]+ (16), 145 [M–2CO–NC6F5]+ (100); HRMS calcd
for C14H2O2F8N2: 381.9983, found: 381.9978.
n
3493, 3350 (NH2),
6.64 (br.s,
ꢁ133.7 (dd, 1F, JF,F = 21 Hz,
m
d
deposited on the 384 ground steel target plate and allowed to dry
before being introduced into the mass spectrometer. External
calibration in positive mode was done using Peptide Calibration
Standard II (Part No. 222570, Bruker Daltonics, Germany). Spectra
obtained were processed using flexAnalysis 2.4 software package
(Bruker Daltonics, Germany). Mass accuracy about 0.1% was
usually achieved. TG and DSC analyses were performed on
NETZSCH STA 409 instrument. The phase transitions of the PI
were examined using DSC at a heating rate of 20 8E minꢁ1 under
He flow. The short-term thermal stability of PIs was estimated
from the 5% and 10% weight loss temperatures Td5 and Td10 using
TGA with a heating rate of 10 8C minꢁ1 in an inert (He) or oxidative
(He:?2 = 80:20) atmosphere. To avoid the influence of absorbed
water and residual solvents, the samples were preheated to 350 8E
and cooled down to room temperature; the second heating scans
were recorded. Elemental analyses were determined on Eurovector
model EA 3000 CHN analyzer. Fluorine content was determined by
spectrophotometric analysis [25]. The values of inherent viscosity
d
4.3.4. 4-Acetamido-3,5,6-trifluorophthalic anhydride (5)
It was prepared in 85% yield through acylation of anhydride 1 by
acetic anhydride in a benzene:acetonitrile mixture (10:1). Mp
217–218 8C; FTIR (KBr):
1871, 1788 (C55Oanhydride) 1695 (C55Oamide), 1553 (NHAc), 1485
(CH3), 1223 cmꢁ1 (C–O–C); 1H NMR (acetone-d6):
2.24 (s, 3H,
CH3), 9.68 (br.s, 1H, NH); 19F NMR (acetone-d6):
ꢁ116.6 (dd, 1F,
n 3248, 3211 (NHAc), 2991, 2956 (EH3),
d
d
JF,F = 19 Hz, JF,F = 9 Hz, F-3), ꢁ124.0 (dd, 1F, JF,F = 20 Hz, JF,F = 9 Hz,
F-5), ꢁ138.0 (dd, 1F, JF,F = 20 Hz, JF,F = 19 Hz, F-6); EIMS, 70 eV, m/z
(rel. int.): 259 [M]+ (2), 217 [M–COCH2]+ (92), 173 [M–COCH2–
CO2]+ (25), 145 [M–COCH2–CO2–CO]+ (42); HRMS calcd for
(hinh) were determined by Ubbelohde viscosimeter at concentra-
tion 0.5 g dLꢁ1 in NMP at 25 8C. Solubility was determined
qualitatively as follows: 50 mg of PI was mixed with 0.5 mL of
solvent and the mixture was mechanically stirred at room
temperature or upon heating.
C10H4O4F3N: 259.0087, found: 259.0087.
4.3.5. 3-Amino-2,4,5-trifluorobenzoic acid (6) (cf. [8])
Acid 2 (0.54 g, 2.3 mmol), NEt3 (1.01 g, 10 mmol), and NH4Cl
(0.44 g, 10 mmol) were solved in water (10 mL) and the necessary
amount of H2SO4 was added to reduce the pH of the mixture to 2.
The solution obtained was heated at 130 8C in a sealed tube for
170 h. The crude product was extracted with diethyl ether and
crystallized from benzene in yield 80%. Mp 140–141 8C; FTIR (KBr):
4.3. Synthesis of the monomer and model compounds
4.3.1. 4-Amino-3,5,6-trifluorophthalic acid (2)
Tetrafluorophthalic acid (3) (4.76 g, 20 mmol) was placed into
an autoclave, anhydrous NH3 (30 mL) was added through a
measuring funnel with back pressure and the autoclave was sealed.
The reaction mixture was heated up to 110 8C upon stirring by
rotation of the autoclave and kept under these conditions for 100 h.
On completion, the autoclave was cooled and NH3 was slowly
vented. The solid residue was dissolved in water, the solution
obtained was acidified with dilute HCl to pH ꢀ 2, and product was
extracted with CH2Cl2 (3 ꢅ 70 mL). The extract was dried over
CaCl2 and solvent was evaporated to give 4.5 g of acid 2, yield 95%,
n
3472, 3416, 3333, 3201 (NH2), 3094, 3002 (CarH), 2400–2700
(OH), 1698 (C55O), 1641 (NH2) cmꢁ1; 1H NMR (acetone-d6):
7.04
(ddd, 1H, JH,F = 11 Hz, JH,F = 9 Hz, JH,F = 6 Hz, H6), 5.29 (br.s, 2H,
NH2); 19F NMR (acetone-d6)
d
d
: ꢁ131.2 (ddd, 1F, JF,F = 18 Hz,
JF,F = 13 Hz, JH,F = 6 Hz, F-2), ꢁ143.3 (ddd, 1F, JF,F = 21 Hz,
JF,F = 13 Hz, JH,F = 11 Hz, F-5), ꢁ149.4 (ddd, 1F, JF,F = 21 Hz,
JF,F = 18 Hz, JH,F = 9 Hz, F-4); EIMS, 70 eV, m/z (rel. int.): 191 [M]+
(100), 174 [M–OH]+ (43), 146 [M–CO2H]+ (35); HRMS calcd for
C7H4O2F3N: 191.0187, found: 191.0192.
purity 98%. Mp 150–151 8C; FTIR (KBr):
(OH, broad), 1712 (C55O), 1637 (NH2) cmꢁ1
4.52 (br.s, NH2); 19F NMR (CDCl3):
JF,F = 11 Hz, F-3), ꢁ140.0 (dd, 1F, JF,F = 20 Hz, JF,F = 11 Hz, F-6),
ꢁ155.0 (dd, 1F, JF,F = 20 Hz, JF,F = 15 Hz, F-5) (cf. [13]); EIMS, 70 eV,
m/z (rel. int.): 235 [M]+ (64), 217 [M–H2O]+ (62), 191 [M–CO2]+
(61); 174 [M–CO2–H2O]+ (63); 145 [M–2CO2H]+ (100); HRMS calcd
for C8H4O4F3N: 235.0092, found: 235.0096.
n
3497, 3398 (NH2), 3000
1H NMR (CDCl3):
ꢁ138.9 (dd, 1F, JF,F = 15 Hz,
;
d
4.3.6. 3-Acetamido-2,4,5-trifluorobenzoic acid (7)
It was prepared through acylation of acid 6 by acetic anhydride
in benzene and crystallized from a benzene:CH3CN mixture (5:1),
d
yield 85%. Mp 203–204 8C; FTIR (KBr):
(CarH), 2912, 2854 (CH3), 2460–2700 (OH), 1695, 1668 (C55O)
cmꢁ1 1H NMR (acetone-d6):
2.18 (s, 3H, CH3), 7.76 (ddd, 1H,
JH,F = 11 Hz, JH,F = 9 Hz, JH,F = 7 Hz, H6), 9.12 (br.s, 1H, NH); 19F NMR
(acetone-d6)
n 3432, 3309 (NH), 3089
;
d
d
: ꢁ117.5 (ddd, 1F, JF,F = 14 Hz, JF,F = 7 Hz, JH,F = 7 Hz,
4.3.2. 4-Amino-3,5,6-trifluorophthalic anhydride (1)
F-2), ꢁ131.0 (ddd, 1F, JF,F = 23 Hz, JH,F = 9 Hz, JF,F = 7 Hz, F-4),
ꢁ141.2 (ddd, 1F, JF,F = 23 Hz, JF,F = 14 Hz, JH,F = 11 Hz, F-5); EIMS,
70 eV, m/z (rel. int.): 233 [M]+ (1), 191 [M–COCH2]+ (100), 174 [M–
COCH2–OH]+ (21); HRMS calcd for C9H6O3F3N: 233.0294, found:
233.0296.
Solution of DCC (3.5 g, 17 mmol) in CH3CN (10 mL) was added
to the solution of acid 2 (4.0 g, 17 mmol) in CH3CN (20 mL) upon
stirring. The mixture obtained was kept under these conditions for
1 h. The precipitate of N,N0-dicyclohexylurea was filtered off and
washed with CH3CN. Solvent from the combined filtrate was
evaporated and the crude product (3.6 g) was crystallized from a
benzene:CH3CN mixture (3:1) to give 3.3 g of anhydride 1, yield
90%, purity 99%, spectral characteristics were identical to those
reported in [26].
4.4. Synthesis of the AB-type polyimide (PI-AB)
A 50 mL round-bottomed flask equipped with a magnetic stirrer
bar was charged with 10 mmol (2.3 g) of the compound 1 and
benzoic acid (10 g) under argon current. The flask was tightly
closed; the mixture was kept upon stirring at 160 8C for 20 h up to
complete conversion of monomer into non-volatile oligomers.
Then the reactor was equipped with an argon gas inlet tube and a
short condenser with a calcium chloride outlet tube; reaction was
kept at 180 8C for 20 h under an argon atmosphere. On completion
of the reaction benzoic acid was extracted with EtOH (3 ꢅ 60 mL),
polymer residue was washed with hot EtOH and dried in vacuo at
4.3.3. N-pentafluorophenyl-4-amino-3,5,6-trifluorophthalimide (4)
(cf. [27])
Solution of anhydride 1 (0.42 g, 1.94 mmol) and pentafluor-
oaniline (0.34 g, 1.94 mmol) in acetic acid (5 mL) was stirred at
100–110 8C for 45 h. The reaction mixture was diluted with
aqueous solution of NaHCO3; the precipitate formed was filtered
off, washed with water, and crystallized from aqueous EtOH to give

T.A. Vaganova et al. / Journal of Fluorine Chemistry 135 (2012) 129–136
135
2u < 558). Crystallographic data for compound 7: C9H6O3F3N,
MW = 233.03, crystal system monoclinic, space group P21/c,
˚
a = 9.4199(9), b = 12.1657(11), c = 7.8719(8) A,
b
= 92.820(4),
3
V = 901.03(15) A , Z = 4, dcalc = 1.719 mg mmꢁ3
,
m
= 0.168 mmꢁ1
,
˚
crystal size 0.25 mm ꢅ 0.35 mm ꢅ 0.35 mm. Absorption correction
was applied using the SADABS program [28] (transmission 0.68–
0.75). The structure was solved by the direct methods (SHELXS97
[29]) and refined by a full matrix least-squares anisotropic–isotropic
(for atom H) procedure using SHELXL97 program [29]. Thehydrogen
atom positions were located from difference Fourier map. The
final indexes are wR2 = 0.1066, S = 1.08 for all 2048 F2 and
R1 = 0.0397 for 1955 F2 > 4
s (F), number of the refinement
parameters is 169. CCDC-817812 contains the crystallographic data
for compound 7.
Molecular structure of compound 7 is shown in Fig. 6. The bond
lengths and bond angles are typical (cf. [30]). It should be noted
that the atom N1 has the planar configuration, its deviation from
˚
the plane of C3, C8 and H atoms is only 0.028(8) A. According to the
Fig. 6. X-ray structure of compound 7 with non-hydrogen atoms numbering.
X-ray diffraction the crystal packing of 7 (Fig. 7) can be described as
3D supramolecular motif sustained by intermolecular H-bonds
(Table 2) and
p-stacking of the polyfluoroarene–polyfluoroarene
˚
type with an interplanar separation of 3.55 A (the corresponding
˚
intercentroid distance is 3.732(1) A). The packing of molecules in
crystalline organic solid by means of intermolecular interactions
such as O–Hꢆ ꢆ ꢆO and N–Hꢆ ꢆ ꢆO is well studied [31]. Interactions C–
Hꢆ ꢆ ꢆF and pFꢆ ꢆ ꢆ
pF are extensively studied in recent years; they are
weaker, but are considered to play a significant role in forming the
molecular assemblies [32].
Supplementary material
Crystallographic data for compound 7 have been deposited with
CCDC No. 817812. Copies of the data may be obtained, free of
charge, from: www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgment
The authors are thankful to Dr. Vladimir V. Koval, Institute of
Chemical Biology and Fundamental Medicine SB RAS, for the
registration and help in the interpretation of the MALDI TOF mass-
spectrum.
Fig. 7. Crystal packing of compound 7.
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d
ꢁ115.9 (6.8F, F-c), ꢁ117.3 (0.8F, F-i), ꢁ117.5
(0.3F, F-m), ꢁ122.0 (6.9F, F-b), ꢁ123.4 (0.8F, F-h), ꢁ131.2 (0.2F, F-
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N(1)–H(1B)ꢆ ꢆ ꢆO(2)
C(6)ꢆ ꢆ ꢆH(6A)ꢆ ꢆ ꢆF(1)
C(9)ꢆ ꢆ ꢆH(9C)ꢆ ꢆ ꢆO(2)
0.89(3)
0.87(2)
1.01(2)
0.95(3)
1.73(3)
2.02(2)
2.53(2)
2.54(3)
2.600(1)
2.867(2)
3.212(2)
3.335(2)
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