Precursors for pyromellit-bridged silica sol-gel hybrid materials

Article abstract of DOI:10.1039/c2nj40538e

Bridged bis(trialkoxysilylalkyl)pyromellitic diimides 3-6 were prepared as single-source precursors for sol-gel derived organic-inorganic hybrid materials. The synthesis route starts with the formation pyromellit diimide 1 from pyromellitic dianhydride and hexamethyldisilazane (HMDS), followed by metallation of the NH groups to give the dipotassium salt 2. The four molecular hybrid precursors 3-6 were obtained according to the first step of the Gabriel synthesis. The reaction rates were studied as a function of the alkyl chain length and the nature of the halide (Cl vs. I). All products 1-6 were comprehensively analysed using FT-IR, ¹H and ¹³C NMR spectroscopy as well as elemental analysis, and - in the case of 3-6 - also with ²⁹Si NMR spectroscopy. For compounds 3 (with propylene groups and methoxy substituents) and 6 (with methylene groups and ethoxy substituents) single crystal X-ray structures were determined and discussed. Hydrolysis and condensation of the alkoxides 3-6 were carefully monitored with solution ²⁹Si and ¹H NMR spectroscopy providing a basis for further studies on the formation of silica-pyromellit organic-inorganic hybrids from precursors 3-6. Finally the formation of flexible and transparent hydride films using precursors 4 and 6 was proved.

Full text of DOI:10.1039/c2nj40538e

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Precursors for pyromellit-bridged silica sol–gel hybrid
materials†
Cite this: NewJ.Chem., 2013,
37, 169
Stefan Pfeifer,ab Anke Schwarzer,a Dana Schmidt,a Erica Brendler,c Michael Veithb
and Edwin Kroke*a
Bridged bis(trialkoxysilylalkyl)pyromellitic diimides 3–6 were prepared as single-source precursors for
sol–gel derived organic–inorganic hybrid materials. The synthesis route starts with the formation
pyromellit diimide 1 from pyromellitic dianhydride and hexamethyldisilazane (HMDS), followed by
metallation of the NH groups to give the dipotassium salt 2. The four molecular hybrid precursors 3–6
were obtained according to the first step of the Gabriel synthesis. The reaction rates were studied as a
function of the alkyl chain length and the nature of the halide (Cl vs. I). All products 1–6 were
comprehensively analysed using FT-IR, 1H and 13C NMR spectroscopy as well as elemental analysis, and –
in the case of 3–6 – also with 29Si NMR spectroscopy. For compounds 3 (with propylene groups and
methoxy substituents) and 6 (with methylene groups and ethoxy substituents) single crystal X-ray
structures were determined and discussed. Hydrolysis and condensation of the alkoxides 3–6 were
carefully monitored with solution 29Si and 1H NMR spectroscopy providing a basis for further studies on
the formation of silica-pyromellit organic–inorganic hybrids from precursors 3–6. Finally the formation
of flexible and transparent hydride films using precursors 4 and 6 was proved.
Received (in Montpellier, France)
24th June 2012,
Accepted 30th August 2012
DOI: 10.1039/c2nj40538e
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Introduction
Polyimides are typically formed by the reaction between aromatic
acid dianhydrides (e.g. pyromellitic dianhydride) and primary dia-
mines, e.g. ethylenediamine,1–3 as shown in Scheme 1. They are well
known as high performance materials with excellent properties like
oxidative stability at high temperatures, good mechanical properties
and resistance against acids and solvents.4–8 These properties of
polyimides make them interesting for a wide field of applications
like coatings for electrical insulations9,10 or membranes.11
In many cases the low solubility of polyimides in organic
solvents causes processing problems. In order to improve the
solubility the imides were functionalized.12–14 Further investigations
were carried out on the thermal stability of imides, which is
generally high, ranging from 200 up to 450 1C. Aromatic anhydrides
a Institute of Inorganic Chemistry, TU Bergakademie Freiberg, Leipziger Str. 29,
D-09596 Freiberg, Germany. E-mail: edwin.kroke@chemie.tu-freiberg.de;
Fax: +49 (0) 3731-39-4058
b Laboratory of Biophysics, Westphalian University of Applied Sciences,
August-Schmidt-Ring 10, D-45665 Recklinghausen, Germany
c Institute of Analytical Chemistry, TU Bergakademie Freiberg, Leipziger Str. 29,
D-09596 Freiberg, Germany
† Electronic supplementary information (ESI) available: CCDC 883128-883129.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2nj40538e
Scheme 1 Synthesis and structure of imides.
c
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precursors such as bridged molecules. A bridged precursor
consists of an organic spacer and two terminal alkoxysilyl groups
as depicted in Scheme 2. These hybrid precursors can easily be
integrated in common sol–gel systems, which may be used for
surface functionalization, fiber drawings or coatings.
Common polyimides are synthesized according to Scheme 1.
This reaction results in the amidocarboxylic acid which can be
converted under thermal influence to the corresponding imide.
The conversion is possible in the solution and in the solid
phase. A by-product of the imid formation is water. To serve as a
bridged precursor for sol–gel chemistry the diamine must be
replaced by aminoalkyltrialkoxysilane. These compounds are
sensitive to moisture, and even small quantities of water which
are formed during the imid formation can lead to hydrolysis of
the alkoxysilane followed by condensation to siloxanes. To
circumvent this problem the trialkoxysilyl group must be
introduced after the completion of imidization and drying of
the product.
Scheme 2 Schematic structure of bridged sol–gel precursors.
In the literature hydrosilylation and transimidisation were
and diamines lead to higher thermal stabilities in comparison reported as possible routes to triethoxysilylalkylimide.22–27 In
to aliphatic derivatives.
the present study an approach based on the well-known Gabriel
Another approach to increase the thermal stability of poly- synthesis of amines is used for the preparation of
imides is to combine them with sol–gel materials to form organic– bis(trialkoxysilylalkyl)imides.
inorganic hybrid materials.15–20,27 These materials can be divided
Several trialkoxysilyl functionalized imides were prepared
into two major classes. Class I hybrid materials consists of two and characterized using different techniques including 1H,
phases with weak interactions (van-der-Waals) between both 13C, 29Si NMR and FT-IR spectroscopy. Selected monomers
phases. Class II hybrids are materials with inorganic moieties were comprehensively analyzed including single crystal X-ray
covalently bonded to the organic moiety.21 A typical method to structure determination, and further studied regarding their
class II hybrid materials is the use of organically modified sol–gel hydrolysis-condensation and film-formation behavior.
Scheme 3 Formation of amidocarboxylic acid followed by formation of insoluble polyimide caused by hydrolysis and condensation.
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Results and discussion
Bis(trialkoxysilylalkyl)pyromellitic diimide may be prepared by
various ways. The simplest variant would be the direct route using
dianhydrides and aminosilanes as starting materials, which are
both commercially available. This reaction leads to the respective
amidocarboxylic acid as shown in Scheme 3. The water formed in
the second reaction step leads very rapidly to hydrolysis and
condensation of the alkoxysilane moieties. The silsesquioxanes
which are formed during condensation are insoluble. Therefore,
with this kind of precursor it is not possible to produce the
corresponding molecular imide without hydrolysis.
To illustrate the sensitivity to water, the gelation times of the
amidocarboxylic acid, as presented in Scheme 3, with various molar
ratios of alkoxy/water (A/W) in ethanol were studied. The reactions
were performed at room temperature. It turned out that at an A/W-
ratio of 6 : 6 after a few minutes a distinct turbidity of the solution
occurred. After about 30 minutes, the samples were completely
gelled. Further experiments showed that all reaction mixtures with
A/W-ratios greater than 6 : 2 had gelled within 24 h. It is apparent
from these attempts that the above-mentioned reaction is not
suitable for synthesizing bis(trialkoxysilylalkyl)imides since the
water liberated during imidization reaction directly leads to hydro-
lysis of Si–OR groups and condensation of the silanol groups,
which again results in a further water elimination.
Methods reported so far for the synthesis of bis(trialkoxy-
silylalkyl)pyromellitic diimide are hydrosilylation22–24 and trans-
imidation.25–27
An alternative route, which has not been reported until now,
is the first step of the Gabriel synthesis, a well-known method for
the preparation of primary amines.28,29 The Gabriel synthesis is
based on the reaction of a potassium imide, generally potassium
phthalimide, with alkyl halides to produce the alkylated imide
and the potassium halide. In this synthetic route the final step is
the hydrazinolysis to get the primary amine. However, this step
is omitted in the present study. In Scheme 4 the complete
synthesis route starting from pyromellitic diianhydride is shown.
Pyromellitic diimide (PMDI)
The synthesis and characteristics of pyromellitic diimide 1 are
described in the literature.30–33 However, we used a different route
to obtain 1. Pyromellitic diimide was prepared via a reaction of
pyromellitic dianhydride (PMDA) with hexamethyldisilazane
(HMDS). Reactions in dry acetone using molar ratios of PMDA :
HMDS of 1 : 2 or 1 : 4 lead to the respective amidocarboxylic acid
after hydrolysis of trimethylsilylgroups with water. The thermal
imidization occurs by holding the temperature at 200 1C for two
hours. Dimethylsulfoxide and dimethylformamide are suitable
solvents to crystallize and purify the diimide 1.33 The purified
product showed 1H NMR signals at 11.82 ppm and 8.02 ppm which
agree with the literature.34 The FT-IR data are also in accordance
with the reported spectra.35
Scheme 4 Synthesis of the diimide 1, its dipotassium salt 2 and the
bis(trialkoxysilylalkyl)imides 3–6.
3–6. It was synthesized from pyromellitic diimide 1. In the literature
it is reported that the potassium salt is formed by reactions of
ethanolic KOH solutions with imides.36 However, our attempts did
not lead to the product. Much better results were obtained when
DMSO was used as solvent. Due to the insolubility of 2 in DMSO a
white precipitate formed immediately, which was examined with
FT-IR, 1H and 13C NMR spectroscopy. 18-Crone-6 was used to
dissolve the product for NMR investigations in d6-DMSO.
Dipotassium pyromellitic diimide (K2PMDI)
Potassium diimide 2 is a starting material for the synthesis of the
Based on the absence of the vibration at 3200 cmÀ1 it can be
target compounds, i.e. bis(trialkoxysilylalkyl)pyromellitic diimides concluded that a complete substitution of the hydrogen atom
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Fig. 1 FT-IR spectra of bis(trialkoxysilylalkyl)pyromellitic diimides 3–6.
of the imide group against potassium occurred (see Fig. S1, N-CH2-CH2-R shifts downfield from 3.60 ppm for both
ESI†). Furthermore the characteristic CQO bands are shifted to bis(trimethoxysilylpropyl)-pyromellitic diimide 3 and bis(triethoxy-
1
smaller wave numbers. In contrast to the H NMR results of 1 silylpropyl)pyromellitic diimide 4 to 3.69 ppm for bis(triethoxy-
the potassium salt 2 shows only one signal at 7.16 ppm. It can silylethyl)pyromellitic diimide 5. In contrast, compound
6
be assumed that the conversion is close to 100% because no 1H (bis(triethoxysilylmethyl)-pyromellitic diimide) comprises an upfield
signals appear caused by imides or amides. Also, the 13C NMR shift to 3.21 ppm. The aromatic protons, which were found at
data show that the signals for C2 and C3 are shifted downfield, 7.16 ppm for 1 and 8.15 ppm for the potassium salt 2, appear again
while the signal for C1 is shifted upfield. All NMR data and at 8.15 ppm for compounds 3–4 and at 8.13 ppm for 5. For
assignments of compounds 1 and 2 are listed in the ESI.†
compound 6 the signal can be observed at 8.22 ppm. The shift of
the signal can be explained as follows. The shortening of the alkyl
group from compound 4 to 6 causes a decreased +I-effect. This is
reflected by the downfield shift of the signal to 3.69 ppm. By further
shortening of the alkyl group from ethylene 5 to methylene 6 the
protons of the methylene bridge appear upfield shifted at 3.22 ppm,
due to the electron-donating properties of silicon.
Bis(trialkoxysilylalkyl)pyromellitic diimides 3–6
The bis(trialkoxysilylalkyl)pyromellitic diimides were prepared
according to Scheme 4 by treatment of 2 with two molar equivalents
of the respective trialkoxyhalogenoalkylsilane. The reactions were
performed in dry dimethylformamide (DMF). The applied reaction
temperature and time differed depending on the nature of the used
silane. In the case of the iodine compounds the reaction was
performed at 100 1C and the transformation was found to be
completed after eight hours. For all chlorine compounds a tempera-
ture of 140 1C and a reaction time of several days were necessary. A
change in colour of the reaction mixture from colourless to dark
brown was noticed. In contrast to the iodine compound, no homo-
geneous solutions were obtained, because in contrast to potassium
iodide potassium chloride is not soluble in DMF. In conclusion the
reaction rate as a function of the studied silane is as follows:
The 29Si NMR data of compounds 3–6 are shown in Fig. 2. An
upfield shift is observed with shortening of the alkylene bridge
from propylene to methylene.
Single crystal structures of compounds 3 and 6
Single crystals of 3 were obtained from methanol at room tempera-
ture while suitable crystals of 6 were grown at 3 1C in hexane
solution. Both crystal structures show disordered moieties.37 In 3,
lying on an inversion centre, the whole Si(OMe3) group occurs in two
different positions (89 : 11). In contrast, compound 6 shows three
ethoxy groups being disordered in two positions independent of
each other (92 : 8, 73 : 29 and 28 : 72). All bond lengths and angles are
iodopropyl c chloromethyl 4 chloroethyl 4 chloropropyl
The isolated imides 3–6 were examined by FT-IR spectro- within the range of the expected values (Fig. 3). The bond lengths of
scopy. The results are shown in Fig. 1. The most important the imide unit indicate a conjugated system, while Calkyl–N bond to
peaks are the CQO vibration at approximately 1768–1765 cmÀ1 the silyl group is a typical single bond (mean value: 1.47 Å).
and 1712–1698 cmÀ1. This position of the CQO bands is
In both crystal structures (3 and 6), the minimal distance
characteristic for five-membered cyclic imides. Specific bands between adjacent aromatic fragments is larger than 5 Å indicat-
due to the Si–O–C units are present at 1100–1065 cmÀ1 for ing no p–p stacking interactions. On the other hand, in 3 the
Si–O–Me in the case of 3 and for Si–O–Et in the case of 4–6.
disordered ethoxy group O9-C22-C21 shows a C–HÁ Á Áp inter-
1
Compounds 3–6 were analyzed by means of H, 13C and 29Si action (d = 2.97 Å, 1471) for the lower occupied position. In fact,
NMR spectroscopy. The 1H NMR results show that the resonance of the crystal structure of 3 is dominated by C–HÁ Á ÁO contacts,
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Fig. 2 29Si NMR spectra of bis(trialkoxysilylalkyl)pyromellitic diimides 3–6.
along the crystallographic a axis. In the case of 6, C16-H16ÁÁÁO10
(H–O: 2.48 Å, C–H–O: 1491) forms a zigzag chain along the crystallo-
graphic b axis. Other C–HÁÁÁO contacts are in the range of 2.5–2.6 Å
indicating a rather weak influence on the crystal structure.
Hydrolysis and condensation of
bis(trialkoxysilylalkyl)pyromellitic diimides 3–6
In sol–gel chemistry the hydrolysis and condensation behaviour of
hydrolysable compounds are of high interest. There are many
studies of pure molecular precursors or mixtures of various alkoxy
silanes and other molecular compounds.38–44 The most important
parameters which control the hydrolysis and condensation, such as
pH, temperature, solvents and nature of catalysts as well as the
amount of water which is available for the reaction were studied.45–50
Most spectroscopic investigations regarding alkoxy silanes rely on
29Si NMR studies.40,41,51–54 1H NMR spectroscopy52,55 was less
frequently used. Investigations which mainly use FT-IR spectro-
scopy38,39,42 as analysis method have also been published.
Especially 29Si NMR spectroscopy allows us to examine the
chemical environments of the individual silicon atoms and to
analyse them. The most frequently investigated sol–gel precursors
are of the type RxSi(OR)3Àx where R is an alkyl group which may be
functionalized and OR stands for methoxy or ethoxy groups.
To describe the degree of hydrolysis and condensation of
alkoxy silanes the well-known M, D, T and Q notation is used.56
In Scheme 5 a T group is shown schematically with its possi-
bilities of hydrolysis and condensation states. The subscript
number indicates the number of silanol groups. The super-
script number indicates how far the condensation has
progressed.
Fig. 3 Molecular structures of compounds 3 and 6. The thermal displacement
ellipsoids of the non-hydrogen atoms are drawn at the 50% probability level.
Hydrogen atoms are omitted for clarity. Compound 3 shows a disordered
Si(OMe)3 group (89 : 11), while in 6 three ethoxy groups are disordered in two
positions (92 : 8, 73 : 29 and 28 : 72).
which are rather weak (H–O: 2.6 Å). Only one contact C11-
H11Á Á ÁO5 (H–O: 2.42 Å, C–H–O: 1621) forms molecular strands
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Scheme 5 Assignment of notation for hydrolyzed T-groups (T00–T03) and condensed/hydrolyzed T-groups (T12–T30) to molecular structures.
Fig. 4 Hydrolysis of 4 observed by 29Si NMR spectroscopy (solvent: d6-DMSO, for further details see experimental part).
The hydrolysis and condensation behaviour of imides 4–6 signals are shown in Scheme 5. T00 completely disappeared after
1
were examined with H and 29Si NMR spectroscopy. The reac- about 50 minutes. Simultaneous to the hydrolysis products,
tions were performed in d6-DMSO. Aqueous HCl was used for upfield shifted signals occur for the condensation products T1
0
catalysis to provide an acidic medium. The reaction has been (À52.58 ppm) and T11 (À51.60 ppm). T12 (À50.53 ppm) appears first
monitored for 4 hours. In Fig. 4 the time dependent measure- after 50 minutes. The signal intensity for hydrolysis products T0 –T0
1
3
ments of substance 4 are shown.
has a maximum intensity after 20 min. The increase in the signal
The 29Si NMR signal for compound 4 is located at a chemical intensity with reaction time is nearly similar for T0 and T0 . Even
2
3
shift of À46.08 ppm. After 25 minutes signals for T01 (À44.58 ppm), after 4 hours both can be noticed but with very low intensity. T0
1
T02 (À43.60 ppm) and T03 (À42.87 ppm) appear downfield shifted disappears after 100 minutes. Considering the signals assigned to
from the original signal of 4. The structure assignments to these the condensation products it can be observed that the relatively
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Scheme 6 Possibilities for T0 group to form condensation products and signal assignment.
1
Fig. 5 Hydrolysis of 5 as observed by 29Si NMR spectroscopy (solvent: d6-DMSO, for further details see experimental part).
broad bands consist of three signals. This splitting can be explained
For compound 5 the time dependent spectra of the hydrolysis-
with the fact that for example a T0 species can condensate with a condensation reactions are shown in Fig. 5. The initial 29Si NMR
1
T0 , T0 or a T0 group. Therefore three possibilities exist for a T0
signal for 5 appears at À48.6 ppm. This signal is shifted 2.52 ppm
group to form condensation products. The options for condensation upfield compared to the signal of compound 4. The signals
1
2
3
1
products just mentioned are shown in Scheme 6. Similar to T1
for hydrolysis products are as observed for 4 downfield shifted
(T0 = À 52.58 ppm, T0 = À 51.60 ppm and T0 = À 50.53 ppm).
0
various condensation products can occur for T11 and T1 .
2
1
2
3
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Fig. 6 Hydrolysis of 6 observed by 29Si NMR spectroscopy (solvent: d6-DMSO, further details see experimental part).
Both compounds 4 and 5 show similar shifts from the initial signal previous experimental procedure d6-DMSO was used as solvent
to T0 of the hydrolysis products. For compound 4 the signal and 0.001 N HCl as catalyst. The amount of water was calcu-
1
appears 1.5 ppm downfield and for 5 1.44 ppm. From T0 to T0
lated that a maximum hydrolysis of 50% can occur (Rw = 2). The
the shift is 1 ppm in both cases. Except from T02 to T03 the different obtained result is shown in Fig. 7.
1
2
shifts vary more. In the case of 4 the value is 0.72 ppm and for 5 0.44
The spectrum marked with 0 min is the spectrum of pure 3 in
ppm. The formation of the hydrolysis product T02 and T03 seems to d6-DMSO. The important signals for the reaction progress are
be preferred in contrast to T0 which is not detectable over the located around 0.5 ppm, 3.2 ppm, 4.1 ppm and in the range of
1
measured time. Hydrolysis products are still detectable even after 5.8 to 6.8 ppm. In the following spectrum (10 min) the signal for
four hours. It is particularly noticeable that the intensity of T02 and the added water is located at 3.3 ppm. After just ten minutes of
T03 is decreasing over the time but nearly no condensation products reaction time, structural changes can be detected. The signal for
occur. The signal for T11 appears at a chemical shift of À55.14 ppm the protons of the methyl group of methanol with a chemical shift
and in the case of T12 at 54.22 ppm.
of 3.17 ppm appears. The protons of the hydroxyl groups can be
The 29Si NMR signal for bis(trialkoxyalkyl)imid 6 appears at detected first after 20 min reaction time at 4.14 ppm. During the
56.87 ppm (Fig. 6). After 25 min all hydrolysis products and hydrolysis progress an increase of both methanol signals can be
condensation products are formed. In contrast to 4 and 5 where observed while the area for the methoxy group is decreasing.
the shift between T0 and T0 is about 1.5 ppm downfield, the Moreover, there is an alteration of the signals for the protons at
0
1
signal is observed at 2.06 ppm. Almost all hydrolysis products the alpha carbon atoms. In addition to the signal of compound 3 at
disappeared after 4 hours due to ongoing condensation reactions. 0.63 ppm appears a new downfield shifted signal at 0.52 ppm. The
The preferred hydrolysis products are T01 and T0 . The latter shows intensity increases with ongoing reaction time. This signal belongs
2
the highest signal intensity during hydrolysis. Considering the to the trimethoxysilyl group which is singly hydrolysed (T0 ). With
1
signal for condensation grades T1 and T1 it can be recognized increasing reaction time two additional signals appear further
1
0
that these two are the major condensation products. The signals for downfield. The signal at 0.44 ppm is assigned to T0 species and
2
T12 is very low over the whole investigated time but not absent. As the signal for the protons on the alpha carbon of a completely
mentioned for 4 (Scheme 6) the different kinds of hydrolysis hydrolysed silicon atom appears at 0.38 ppm. After about 10 min-
products of T10 are also found for compound 6.
utes, three new signals appear. These are located at 6.78 ppm (T0 ),
1
As already pointed out above, there are a few studies which 6.32 ppm (T0 ) and 5.92 ppm (T0 ) and can be assigned to the
2
3
1
make use of H NMR spectroscopy to study the hydrolysis and hydroxyl groups of the different degrees of hydrolysis.
condensation of alkoxy silanes. In this work, the hydrolysis of 3
After 60 min 50% hydrolysis with regard to the Ha signal
was additionally examined by 1H NMR measurements. As in the occurred. With prolonged reaction time the Ha signal and the
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Fig. 7 Hydrolysis of 3 observed by 1H NMR spectroscopy (solvent: d6-DMSO, further details see experimental part).
by heating up above 200 1C due to self-condensation. However,
films which were produced this way had no smooth surface and
showed many cracks. To optimize this process the bisimides
were prehydrolyzed with hydrochloric acid. For compound 3
0.1 N HCl was used and the A/W was one. At least 120 1C were
required to obtain solid films. A maximum film thickness of
0.9 mm was obtained. The obtained films were transparent,
yellow or brown and flexible as shown in Fig. 8(a) and (b).
By using the same conditions as for precursor 4, 5 delivered also
films (Fig. 8a). But in contrast to compound 4 the quality of the
resulting film was low, i.e. the obtained films contained blisters,
which caused swelling of the film and a decreased flexibility was
observed. To solve this problem water and HCl content was varied.
Decreasing water and/or HCl content did not yield any films. Only
raising the HCl concentration from 0.1 to a 1 N HCl gave better
results.
Fig. 8 (a) Films obtained from precursors 4–6, (b) the flexibility of the resulting
films obtained from precursor 4, (c) the flexibility of the resulting films obtained
from precursor 6.
signals 6.78 ppm and 5.92 ppm (T01–T0 ) should disappear due
3
to complete hydrolysis and condensation, accompanied by an
increase in the signal caused by liberated water. The other
peaks which were assigned to hydrolysis products raised from
the Ha signal show decreasing intensities.
All compounds formed transparent gels after a few days. No
precipitation was observed. This observation encouraged us to
prepare coatings and films using the novel hybrid precursors 3–6.
In contrast to compound 4 and 5 bisimid 6 formed no solid
film by using 0.1 N HCl and an A/W ratio of one at 140 1C. A
temperature of at least 200 1C was required to get solid layers
with a non-sticky surface. At temperatures below 200 1C the
Film preparation of bis(trialkoxysilylalkyl)pyromellitic diimides 3–6
The precursors 3–6 can be used for producing flexible and trans- system remained liquid. At room temperature the hybrid poly-
parent hybrid materials. Therefore, the ability of film formation of mer formed from precursor 6 remained liquid but the viscosity
4–6 was examined. As already mentioned common polyimides are raised. Solid films were obtained at a temperature of 140 1C
not soluble in ethanol. In contrast to that the precursors 4–6 using 1 N HCl and an A/W of one. These films showed no
showed good solubility in ethanol. Thus a 20 w% solution of each defects. They are transparent and flexible as shown in Fig. 8(a)
compound in ethanol was used for all coating experiments. The and (c). In contrast to the obtained films from precursor 4 the
bis(trialkoxysilylalkyl)pyromellitic diimides form films simply maximum thickness was 0.2 mm. Attempts to obtain films with
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higher thicknesses were not successful. During the curing kinetic investigations a scan number of 40 was chosen. FT-IR
process these layers started to swell. spectra were recorded on a Nicolet 380 using 32 scans. Elemental
Further results of the ability of precursors 4–6 as coating analyses were performed with an Elementar C, H, N analyser.
material especially for the electrical insulation of copper wire
will be published elsewhere.
Single-crystal X-ray structure determination
Data collection of 3 and 6 was performed on a STOE IPDS-2
diffractometer (image plate) equipped with a low temperature
Conclusion
device (T = 200(2) K), with graphite-monochromatized MoKa radia-
Bis(trialkoxysilylalkyl)pyromellitic diimides 3–6 were synthesized tion (l = 0.71073 A1) using o and j scans. Reflections were corrected
according to the first step of the Gabriel-synthesis. Different silanes for background, Lorentz and polarization effects. Preliminary struc-
including g-iodopropyltrimethoxysilane, g-chloropropyltriethoxy- ture models were derived by application of direct methods57 and the
silane,b-chloroethyltriethoxysilaneanda-chloromethyltriethoxysilane structures were refined by full-matrix least-squares calculation based
and pyromellitic diimide were used as starting materials. The imide on F2 for all reflections using SHELXL.57 All hydrogen atoms were
1 was obtained from a reaction of pyromellitic dianhydride included in the models in calculated positions and were refined as
and hexamethyldisilazane (HMDS) followed by hydrolysis of the constrained to the bonding atoms. Treatment of disorders for 3: the
trimethylsilyl groups and thermal imidization. Since pyromellitic Si(OMe)3 group is disordered in two positions (89 : 11) including
diimide is relatively unreactive towards trialkoxyhalogenoalkyl- SADI restraints on the geometry, EXYZ and EADP restraints. For 6:
silanes the imide was converted to the respective dipotassium salt 2. three ethoxy groups are disordered in two positions (92 : 8; 71 : 29;
The salt 2 and the different tri(alkoxy)halogenoalkylsilanes reacted in 27 : 73) including SADI restraints on the geometry, EXYZ and EADP
DMF to give the respective bis(trialkoxysilylalkyl)pyromellitic diimides restraints. The highest left electron density peak close to Si1 cannot
3–6inverygoodyields. Thedesiredimideformedmostrapidlyusing g- be assigned to a specific disorder.
iodopropyltrimethoxysilane. The reactions with the chlorinated com-
pounds proceeded significantly slower.
Syntheses
All obtained substances were characterized using FT-IR spectro-
Preparation of pyromellitic diimide (1). 0.5 mol pyromellitic
scopy, 1H, 13C and 29Si NMR spectroscopy as well as elemental dianhydride (PMDA) and 500 ml of dry acetone were added in a
analysis. The crystal structures of 3 and 6 were determined by 1000 ml 2-neck flask under stirring at room temperature. Once
single crystal X-ray diffraction. Both compounds exhibit disordered PMDA was dissolved 1 mol of HMDS was added slowly by a
alkoxy substituents, nevertheless indicating the expected molecular dropping funnel. After HMDS was completely added the mixture
structures with typical bond lengths and angles.
was stirred for 15 min at room temperature. Then 2 equivalents
Finally, the hydrolysis and condensation behavior of the of water with respect to HMDS were given very slowly to the
1
imides were studied using 29Si and H NMR spectroscopy. The reaction mixture. A precipitate was formed immediately in an
29Si NMR spectra indicate hydrolysis signals shifted downfield exothermic reaction. This heterogeneous solution was stirred till
compared to the respective imide, while signals due to con- the temperature decreased to 25 1C. Afterwards, the precipitate
densation products are shifted upfield. In all investigated cases was separated from the solvent and washed a few times with dry
within four hours only singly condensated products could be acetone. The precipitate was heated for two hours at 200 1C
observed. These results provide a basis for the use of the title under air in a crystallizing dish. The resulting product was a
precursor molecules for the preparation of novel hybrid mate- beige fine powdered substance. Yield: 87%. mp 4 320 1C.
rials. They were applied for the formation of transparent and
flexible hybrid films with a thickness up to 900 mm. Further-
more, we obtained electrically insulating coatings on copper
wires. Details on these results will be published elsewhere.
1H NMR (d6-DMSO): d (ppm): 11.82 (s, 2H); 8.09 (s, 2H).
13C NMR (d6-DMSO): d (ppm): 167.46, 137.78, 117.09.
IR: n (cmÀ1): 3197, 1771, 1712, 1694, 1306.
Anal. Calcd. for C10H4N2O4 C: 55.57, H: 1.87, N: 12.96%;
found C: 55.64, H: 1.89, N: 12.92%.
Preparation of dipotassium pyromellitic diimide (2). 0.5 mol
of 1 was dissolved in 250 ml dry DMSO in a two neck flask under
stirring and an argon atmosphere at 100 1C. To this mixture, a
Experimental part
Reagents and solvents
All reagents were used as received. All solvents were dried using solution of 1 mol KOH in 500 ml absolute ethanol was added
standard methods. Pyromellitic dianhydride (PMDA) was pur- drop wise over a period of 10 minutes. A white precipitate was
chased from Alfa Aesar. Hexamethyldisilazane (HMDS) and all formed. After the addition was completed the mixture was
silanes were obtained from ABCR. Acetone, dimethylsulfoxide stirred for further 30 minutes at the same temperature. Finally
(DMSO), dimethylformamide (DMF), hexane and deuterated the precipitate was filtered off hot and washed a few times with
DMSO (d6-DMSO) were purchased from Aldrich Chemical Co.
dry ethanol. The product was dried under reduced pressure and
at a temperature of 100 1C. The resulting product was a white
powdered substance. Yield: 96%. mp 4 320 1C.
1H NMR (d6-DMSO): d (ppm): 7.16.
Syntheses and spectroscopic investigations
Syntheses were carried out under an inert atmosphere of dry argon
using standard Schlenk techniques. 1H, 13C and 29Si NMR spectra
(solution) were recorded on a Bruker DPX 400 spectrometer. For
13C NMR (d6-DMSO): d (ppm): 183.93, 142.10, 110.20.
IR: n (cmÀ1): 1699, 1619, 1592, 1288.
c
178 New J. Chem., 2013, 37, 169--180
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NJC
Anal. Calcd. for K2C10H2N2O4 C: 41.08, H: 0.69, N: 9.59%;
Paper
1H NMR (d6-DMSO): d (ppm): 8.22 (s, 2H); 3.81 (q, 12H); 3.21
(s, 4H); 1.13 (t, 18H).
found C: 40.62, H: 0.74, N: 9.31%.
Preparation of bis(trimethoxysilylpropyl)pyromellitic diimide (3).
13C NMR (d6-DMSO): d (ppm): 165.79, 136.95, 116.94, 58.30,
0.5 mol of 2 was placed in a three-necked flask with stirrer and 23.35, 17.94.
reflux condenser. Dry DMF was added and 1 mol of g-iodopropyl-
trimethoxysilane (I-PTMS) was added to the solution. This mixture
was heated to 80–100 1C until no solid was left. After the reaction
29Si NMR (d6-DMSO): d (ppm): À53.82.
IR: n (cmÀ1): 1765, 1712, 1065.
Anal. Calcd. for C28H44N2O10Si2 C: 41.08, H: 0.69, N: 9.59%;
mixture was allowed to cool down to room temperature the solvent found C: 40.62, H: 0.74, N: 9.31%.
was distilled of at 50 1C under reduced pressure. The remaining
solid was washed with methanol under an argon atmosphere
Film preparation
to remove residual KI. The product was crystallized from dry
methanol and then dried under reduced pressure at room tem-
perature. The resulting product was a white powdered substance.
Yield: 92%. mp = 91 1C.
Films obtained with bis(triethoxysilylpropyl)pyromellitic diimide
(4). An ethanolic solution containing 20 w% of 4 was prepared in a
two-necked flask under an argon atmosphere. While continuous
stirring a 0.1 N HCl solution was added dropwise. The amount of
0.1 N HCl was calculated to maintain an A/W of one. This solution
was stirred for an hour under room temperature. Then 10 ml of this
solution were filled into an aluminium dish. The solution was cured
in an oven at 120 1C for 30 min under air. To obtain free-standing
films like in Fig. 8 the cured hybrid polymer can be carefully
removed from the dish.
1H NMR (d6-DMSO): d (ppm): 8.15 (s, 2H); 3.60 (t, 4H); 3.45
(s, 18H); 1.69 (m, 4H); 0.62 (m, 4H).
13C NMR (d6-DMSO): d (ppm): 166.35, 137.07, 117.03, 50.03,
40.41, 21.15, 6.01.
29Si NMR (d6-DMSO): d (ppm): À42.25.
IR: n (cmÀ1): 1768, 1704, 1071.
Anal. Calcd. for C22H32N2O10Si2 C: 48.87, H: 5.97, N: 5.18%;
found C: 48.92, H: 5.95, N: 5.23%.
Films obtained with bis(triethoxysilylethyl)pyromellitic diimide
(5). An ethanolic solution containing 20 w% of 5 was prepared in a
two-necked flask under an argon atmosphere. While continuous
stirring a 1 N HCl solution was added dropwise. The amount of 1 N
HCl was calculated to maintain an A/W of one. This solution was
stirred for an hour under room temperature. Then 10 ml of this
solution were filled into an aluminium dish. The solution was
cured in an oven at 120 1C for 30 min under air.
Films obtained with bis(triethoxysilylmethyl)pyromellitic
diimide (6). An ethanolic solution containing 20 w% of 6 was
prepared in a two-necked flask under argon. While continuous
stirring a 1 N HCl solution was added dropwise. The amount of
1 N HCl was calculated to maintain an A/W of one. This
solution was stirred for an hour under room temperature. Then
4 ml of this solution were filled into an aluminium dish. The
solution was cured in an oven at 140 1C for 30 min under air. To
obtain free-standing films like in Fig. 8 the cured hybrid
polymer can be carefully removed from the dish.
Preparation of bis(triethoxysilylpropyl)pyromellitic diimide (4).
0.5 mol of 2 were placed in a three-necked flask under an argon
atmosphere with stirrer and reflux condenser, 250 ml of dry DMF
and 1 mol of g-chloropropyltriethoxysilane (Cl-PTES) were added.
This mixture was heated to 140 1C until the solution turns almost
black. After the reaction mixture was allowed to cool down to room
temperature the solution was separated from the precipitated KCl
by filtration. Then the solvent was distilled off under reduced
pressure at a temperature of 50 1C. The product was a dark brown
oil which solidified upon storage. Yield: 89%. mp = 56 1C.
1H NMR (d6-DMSO): d (ppm): 8.15 (s, 2H); 3.75 (q, 12H); 3.61
(t, 4H); 1.70 (m, 4H); 1.14 (t, 18H); 0.62 (m, 4H).
13C NMR (d6-DMSO): d (ppm): 166.36, 136.94, 177.07, 57.74,
40.45, 21.42, 18.15, 7.23.
29Si NMR (d6-DMSO): d (ppm): À44.11.
IR: n (cmÀ1): 1768, 1704, 1097, 1071.
Anal. Calcd. for C28H44N2O10Si2 C: 53.82, H: 7.10, N: 4.48%;
found C: 53.62, H: 6.83, N: 4.40%.
Acknowledgements
Preparation of bis(triethoxysilylethyl)pyromellitic diimide
(5). Compound 5 was synthesized as described for compound We would like to thank Dipl.-Chem. Benjamin Gutschank
4. The resulting product was a beige solid obtained in a yield of (University of Essen-Duisburg) for his help in measuring
81%. mp = 58–60 1C.
1H NMR (d6-DMSO): d (ppm): 8.13 (s, 2H); 3.77 (q, 12H); 3.69 Applied Sciences) for his administrative help. This project is
(m, 4H); 1.15 (t, 18H); 1.08 (m, 4H).
supported by a grant (FKZ1752X08) from the FH3 BMBF pro-
13C NMR (d6-DMSO): d (ppm): 165.78, 136.93, 116.86, 52.80, gram in the context of cooperative project ‘‘Entwicklung einer
NMR spectra and Torsten Pieper (Westphalian University of
33.25, 18.08, 10.28.
nanoskaligen Kupferlackdrahtbeschichtung fu
¨r den Einsatz in
der Automobil- und Medizintechnik (NanoCuL)’’.
29Si NMR (d6-DMSO): d (ppm): À47.94.
IR: n (cmÀ1): 1768, 1698, 1100, 1071.
Anal. Calcd. for C26H40N2O10Si2 C: 52.33, H: 6.76, N: 4.69%;
found C: 51.72, H: 6.50, N: 5.02%.
References
Preparation of bis(triethoxysilylmethyl)pyromellitic diimide
(6). Compound 6 was synthesized as described for compound 4.
The resulting product was a white solid obtained in a yield of
93%. mp = 59–60 1C.
1 C. E. Sroog, J. Polym. Sci., Part D: Macromol. Rev., 1976, 11,
161–208.
2 A. V. Reddy, J. Appl. Polym. Sci., 2000, 75, 1721–1727.
3 J. Preston and Y. Tropsha, Polym. Eng. Sci., 1994, 34, 305–307.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem., 2013, 37, 169--180 179

View Article Online
Paper
NJC
4 A. B. Frazier, Trans. Ind. Electron., 1995, 42, 442–448.
5 G. Ragosta, M. Abbate, P. Musto and G. Scarinzi, J. Mater.
Sci., 2012, 47, 2637–2647.
6 S. Mehdipour-Ataei and N. Bahri-Laleh, Iran. Polym. J., 2008,
17, 95–124.
7 D. Scola and J. H. Vontell, Polym. Compos., 1988, 9, 443–452.
8 N. Miyadera, T. Kuroda, T. Takahashi, R. Yamamoto,
M. Yamaguchi, S. Yagi and S. Koibuchi, Mol. Cryst. Liq.
Cryst., 2003, 406, 39–49.
9 C. Calebrese, L. Hui, L. S. Schadler and J. K. Nelson, IEEE
Trans. Dielectr. Electr. Insul., 2011, 18, 938–945.
10 M. Mesaki, Y. Tatematsu and H. Goda, Furukawa Rev., 2002,
22, 1–4.
36 I. Sideridou-Karayannidou and G. P. Karayannidis,
J. Macromol. Sci., Chem., 1987, 24, 689–700.
37 Crystal data for 3: C22H32N2O10Si2, M = 540.68, monoclinic,
a = 7.7341(6) Å, b = 8.1924(4) Å, c = 20.4587(14) Å, b =
98.448(6)1, V = 1282.22(15) Å3, T = 200(2) K, space group
P21/n, Z = 2, m = 0.196 mmÀ1, 11991 reflections measured,
2258 independent reflections (Rint = 0.0604), 194 parameters,
23 restraints, R1 (I 4 2s(I)) = 0.0403, wR(F2) (I 4 2s(I)) =
0.1053, R1 (all data) = 0.0468, wR(F2) (all data) = 0.1090, S =
1.085. CCDC 883128. Crystal data for 6: C24H36N2O10Si2, M =
568.73, monoclinic, a = 14.2971(4) Å, b = 7.7793(3) Å, c =
25.6231(7) Å, b = 99.546(2)1, V = 2810.38(15) Å3, T = 200(2) K,
space group P21/c, Z = 4, m = 0.183 mmÀ1, 36889 reflections
measured, 5229 independent reflections (Rint = 0.1208), 374
parameters, 7 restraints, R1 (I 4 2s(I)) = 0.0577, wR(F2) (I 4
2s(I)) = 0.1557, R1 (all data) = 0.0634, wR(F2) (all data) =
0.1604, S = 1.092. CCDC 883129.
11 P. M. Budd and N. B. McKeown, Polym. Chem., 2010, 1, 63–68.
12 S. V. Vinogradovja, S. Vygodskijv, V. Korsak and
T. N. Spirina, Acta Polym., 1979, 30, 3–14.
13 D. J. Liaw and B. Y. Liaw, Chem. Mater., 1998, 10, 734–739.
14 K. Choi, J. Junga, H. Kim, B. Sohn, W. Zin and M. Ree, 38 G. De, B. Karmakar and D. Ganguli, J. Mater. Chem., 2000,
Polymer, 2004, 45, 1517–1524.
10, 2289–2293.
15 L. Mascia and A. Kioul, Polymer, 1995, 36, 3649–3659.
16 G. Ragosta and P. Musto, Polym. Lett., 2009, 3, 413–428.
39 B. Orel, R. Jese, A. Vilcnik and U. L. Stanger, J. Sol–Gel Sci.
Technol., 2005, 34, 251–265.
17 S. M. Alam, T. Kawauchi and T. Takeichi, High Perform. 40 Y. Sugahara, S. Okada, S. Sato, K. Kuroda and C. Kato,
Polym., 2010, 22, 742–760. J. Non-Cryst. Solids, 1994, 21–28.
18 M. A. Wahab, II Kim, W.-J. Cho and C.-S. Ha, Mol. Cryst. Liq. 41 Y. Sugahara, S. Okada, S. Sato, K. Kuroda and C. Kato,
Cryst., 2004, 417, 127–134. J. Non-Cryst. Solids, 1992, 25–34.
19 S. Pandey and S. B. Mishra, J. Sol–Gel. Sci. Technol., 2011, 59, 42 R. Pena-Alonso, F. Rubio, J. Rubio and J. L. Oteo, J. Mater.
73–94. Sci., 2007, 42, 595–603.
20 J. H. Kang, K. Cho and C. E. Park, Polymer, 2001, 42, 2513–2520. 43 K. J. Shea and D. A. Loy, Acc. Chem. Res., 2001, 34, 707–716.
21 G. Kickelbeck, Hybrid Materials: Synthesis, Characterization, 44 H. Saito, Y. Nishio, M. Kobayashi and Y. Sugahara, J. Sol–Gel
and Applications, Wiley-VCH, 2007.
22 J. H. Wengrovius, V. M. Powell and J. L. Webb, J. Org. Chem., 45 Y. Xu, X. Sun, D. Wu, Y. Sun, Y. Yang, H. Yuan, F. Deng and
1994, 59, 2813–2817. Z. Wu, J. Solution Chem., 2007, 36, 327–344.
23 O. Dautel, G. Wantz, R. Almairac, D. Flot, L. Hirsch, 46 M. Asomoza, M. P. Dominguez, S. Solis, V. H. Lara, P. Bosch
J.-P. Lere-Porte, J.-P. Parneix, F. Serein-Spirau, L. Vignau and T. Lopez, Mater. Lett., 1998, 36, 249–253.
and J. J. E. Moreau, J. Am. Chem. Soc., 2006, 128, 4892–4901. 47 I. Artaki, T. W. Zerda and J. Jonas, Mater. Lett., 1985, 3,
24 S. T. Hobson and K. J. Shea, Chem. Mater., 1997, 9, 616–623. 493–496.
25 L. Bes, A. Rousseau, B. Boutevin, R. Mercier and B. Sillion, 48 I. Artaki, T. W. Zerda and J. Jonas, Mater. Lett., 1986, 81,
Macromol. Chem. Phys., 2001, 202, 933–942. 381–395.
26 C. L. Homrighausen, B. J. Kennedy and E. J. Schutte, 49 A. H. Boonstra, T. N. M. Bernards and J. J. T. Smits,
J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 4922–4932. J. Non-Cryst. Solids, 1989, 109, 141–152.
27 L. Bes, A. Rousseau, B. Boutevin and R. Mercier, J. Polym. 50 A. H. Boonstra and J. M. E. Baken, J. Non-Cryst. Solids, 1990,
Sci., Part A: Polym. Chem., 2001, 39, 2602–2619. 122, 171–182.
28 M. S. Gibson and R. W. Bradshawi, Angew. Chem., 1968, 1, 51 M. Mazur, V. Mlynarik, M. Valko and P. Pelikan, Appl. Magn.
986–996. Reson, 2000, 18, 187–197.
29 K. Soai, A. Ookawa and K. Kato, Bull. Chem. Soc. Jpn., 1982, 52 D. W. Schaefer, J. Sol–Gel Sci. Technol., 2009, 51, 23–31.
Sci. Technol., 2011, 57, 51–56.
55, 1671–1672.
53 Y. Sugahara, T. Inouea and K. Kuroda, J. Mater. Chem., 1997,
7, 53–59.
30 H. Meyer and K. Steiner, Monatsh. Chem., 1913, 35, 391–405.
31 C. Chiriac, M. Nechifor and F. Tanasa, Rev. Roum. Chim., 54 S. A. Myers, R. A. Assink, D. A. Loy and K. J. Shea, J. Chem.
2007, 52, 883–886.
Soc., Perkin Trans., 2000, 2, 545–549.
¨
¨
32 T. Seckin, I. Ozdemir and S. Koytepe, J. Inorg. Organomet.Polym. 55 Y. Dubitsky, A. Zaopo, G. Zannoni and L. Zetta, Mater. Chem.
Mater., 2009, 19, 143–151.
Phys., 2000, 64, 45–53.
33 I. V. Bulgarovskaya, L. A. Novakovskaya, Yu. G. Fedorov and 56 B. D. Ratner, Biomaterials Science: An Introduction to Materials
Z. V. Zvonkova, Kristallografiya, 1976, 21, 515–519.
34 K. Kacprzak, Synth. Commun., 2003, 33, 1499–1507.
35 Y. Hase, Spectrosc. Lett., 1979, 12, 75–81.
in Medicine, Elsevier, 1996.
57 G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr.,
2008, 64, 112–122.
c
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