Synthesis and spectral characteristics of silicon-containing poly(salicylidene azomethines) derived from aromatic di- and tetraamines

Article abstract of DOI:10.1134/S1070427208080156

New silicon-containing poly(salicylidene azomethines) were prepared by polycondensation of di-or tetrasalicylidene derivatives of aromatic di- or tetraamines. The spectral data suggest the occurrence of noncovalent transannular donor-acceptor interaction

Full text of DOI:10.1134/S1070427208080156

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2008, Vol. 81, No. 8, pp. 1398–1402. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © N.M. Geller, A.G. Ivanov, L.B. Nadezhdina, V.V. Shamanin, L.A. Shibaev, A.V. Gribanov, 2008, published in Zhurnal Prikladnoi
Khimii, 2008, Vol. 81, No. 8, pp. 1320–1325.
MACROMOLECULAR CHEMISTRY
AND POLYMERIC MATERIALS
Synthesis and Spectral Characteristics of Silicon-containing
Poly(Salicylidene Azomethines) Derived
from Aromatic Di- and Tetraamines
N. M. Geller, A. G. Ivanov, L. B. Nadezhdina, V. V. Shamanin,
L. A. Shibaev, and A. V. Gribanov
Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia
Received March 25, 2007
Abstract—New silicon-containing poly(salicylidene azomethines) were prepared by polycondensation of di-
or tetrasalicylidene derivatives of aromatic di- or tetraamines. The spectral data suggest the occurrence
of noncovalent transannular donor–acceptor interaction N→Si providing “nonclassical” conjugation.
DOI: 10.1134/S1070427208080156
The goal of this study was to develop a procedure
for preparing a new class of polymers, Si-containing
poly(salicylidene azomethines) containing donor nitro-
gen atoms and acceptor silicon atoms in the chain,
which creates prerequisites for the formation of in-
tramolecular donor–acceptor charge-transfer com-
plexes (CTCs) in the polymer chain. Such kind of
complexation should be accompanied by effective
charge transfer, which will polarize the polymers and
hence impart to them specific electro- and photophysi-
cal properties.
EXPERIMENTAL
As comonomers we used N,N',N'',N'''-tetra-salicyl-
idene-3,3',4,4'-tetraaminodiphenyl ether (M1),
N,N',N'',N'''-tetrasalicylidene-3,3',4,4'-tetraaminodi-
phenyl (M2), 1,3-bis(salicylidenimino)-4-nitrobenzene
(M3), and 2,4-bis(salicylidenimino)azobenzene (M4):
The monomers were prepared by reactions of sali-
cylaldehyde with 3,3',4,4'-tetraaminodiphenyl and
3,3',4,4'-tetraaminodiphenyl ether in 4 : 1 molar ratio or
with 4-nitro-1,3-diaminobenzene and 2,4-diaminoazo-
benzene in 2 : 1 ratio.
The polymers were prepared in a DMF solution
by polycondensation of the corresponding salicylidene
derivatives of tetraamines and diamines with tetrachloro-
silane or tetraethoxysilane (Vekton, bp 57 and 168°C,
respectively). Polycondensation of disalicylidene de-
1398

SYNTHESIS AND SPECTRAL CHARACTERISTICS OF SILICON-CONTAINING POLY(SALICYLIDENE AZOMETHINES) 1399
Characteristics of the polymers obtained
Polymer
P1
Mn
UV, λ, nm
236, 289, 314, 444, 461
230, 295, 315, 430, 455
230, 323, 349, 421
IR, ν, cm–1
NMR, δ, ppm
1610 (С=N), 1597 (Сar–Сar), 1110 (Si–O) 9.3 (С=N), 6.9–7.6 (Сar–Н)
1606 (С=N), 1596 (Сar–Сar), 1110 (Si–O) 9.4 (С=N), 6.9–7.6 (Сar–Н)
1613 (С=N), 1595 (Сar–Сar), 1112 (Si–O) 9.0 (С=N), 6.9–7.5 (Сar–Н)
1603 (С=N), 1582 (Сar–Сar), 1100 (Si–O) 9.3 (С=N), 6.8–7.5 (Сar–Н)
11 000
8000
P2
P3
12 000
9000
P4
242, 270, 347, 428
rivatives of aromatic diamines was performed with
dimethyldiethoxysilane prepared from dimethyldichlo-
rosilane and sodium ethylate. The polymers were iso-
lated by precipitation into diethyl ether. The character-
istics of the polymers are listed in the table.
An elemental analysis of the monomers was per-
formed on a LECO CHNS(O)-932 dewvice, and con-
sistent results were obtained.
1
The H NMR spectra of 4–5 wt% solutions of
the monomers and polymers in DMF-d–7 were re-
corded on a Bruker Avance 200 NMR spectrometer
operating at 200 MHz; the UV spectra, on an SF-2000
spectrophotometer in methanol or isopropanol in the wave-
length range 200–700 nm; and the IR spectra, on a Ver-
tex IR Fourier spectrometer (Bruker) from KBr pellets
in the 400–4000 cm–1 range. The bands were assigned
taking into account published data [1–3] and data ob-
tained for the model compound.
Polymers P1 and P2. To a solution of 0.006 mol
of M1 in 15 ml of DMF, 0.006 mol of tetrachlorosilane
was added dropwise under agitation. The reaction was
performed at room temperature in an inert atmosphere
(dry Ar) for 8 h, after which the reaction mixture was con-
centrated on a rotary evaporator and the dry residue
was dissolved in methanol. Polymer P1 was isolated
by precipitation from methanol solution into diethyl
ether. Yield 94%.
The ESR spectra were measured at 35 GHz with
the magnetic field varied in a wide range.
Polymer P2 was prepared similarly.
The number-average molecular weights and de-
grees of polymerization of the polymers were esti-
mated by NMR spectroscopy from the ratio of the inte-
gral intensity of the signal of terminal hydroxy groups
to the total integral intensity of the signals of aromatic
protons or to the integral intensity of the signal of
the azomethine proton.
Polymers P3 and P4. To a solution of 0.011 mol
of M3 in 20 ml of DMF, we added 0.011 ml of di-
methyldiethoxysilane under agitation. The reaction
was performed in an inert medium (dry Ar) for 6 h at
80°C, after which the mixture was concentrated on a
rotary evaporator and the dry residue was dissolved in
methanol. Polymer P3 was isolated by precipitation
from methanol solution into diethyl ether. Yield 89%.
Donor–acceptor interactions are well understood
for “low-molecular-weight” reactions of silanes with
various organic polydentate ligands. In particular, reac-
tions of dialkyldichlorosilanes with salen-type ligands
[salen stands for N,N'-ethylenebis(salicylidenimine)]
have been reported [4–9]. These data on the synthesis
of low-molecular-weight CTCs give grounds to expect
that replacement of mono- with di- and tetrasalicyli-
dene derivatives will allow synthesis of polymeric
CTCs. In our case, the polycondensation should in-
volve, firstly, nucleophilic substitution at the silicon
atom and, secondly, N→Si donor–acceptor interaction
with the formation of a stable six-membered ring,
which should significantly stabilize the corresponding
segment of the forming polymer chain.
Polymer P4 was prepared similarly.
The model compound necessary for identifying
the spectra of the synthesized polymers was prepared
by condensation of 1,3-bis(salicylidenimino)benzene
with tetraethoxysilane in 2 : 1 molar ratio:
Polymers P1 and P2 were prepared by polycon-
densation of salicylidene derivatives of tetraamines M1
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GELLER et al.
and M2 with tetrachloro- or tetraethoxysilane in a 1: 1
molar ratio by the following scheme:
tuted fragments II, along with the desired tetrasali-
cylidene fragments I, increases at high concentrations :
Because chlorosilanes are highly reactive, to sup-
press side reactions (hydrolysis of chlorosilanes, fol-
lowed by tetracyclization) and raise the probability of
more complete substitution at silicon (i.e., to raise
the relative content of tetrasalicylidene units I), the poly-
condensation should be performed at low temperature
(20–25°C). Another way to suppress the contribution
of undesirable processes, such as hydrolysis of chloro-
silanes, is their replacement with ethoxysilanes, which
are less reactive and less susceptible to hydrolysis.
However, in the latter case, the leaving group in the poly-
condensation is the ethoxy group, and therefore, to
ensure its complete removal, the reaction should be
performed at 85–90°C.
A = –O– (P1), – (P2).
Polymers P3 and P4 were prepared from bis(salicy-
lidenimino) derivatives and dimethyldiethoxysilane, also
in a 1:1 molar ratio:
The main spectral problem in analysis of the poly-
mers obtained is to confirm the structure of the mono-
meric unit, i.e., to confirm the presence in polymer
chians of a six-membered ring formed by N→Si coor-
dination interaction. This interaction results in a shift
of the electron density from the donor to the acceptor,
which, in turn, should affect the spectral characteristics
of the substance.
According to published data, the formation of
low-molecular-weight complexes is accompanied by
pronounced changes in the IR spectra of both donor
and acceptor molecules [10]. The vibration frequencies
of the donor are shifted toward higher frequencies, and
those of the acceptor, toward lower frequencies. The in-
tensities of the absorption bands also change. In the NMR
spectra, the signals of the donor are shifted downfield
[11]. In our cases, the strongest changes should be ex-
pected for the azomethine group, because this group is
directly involved in the coordination bonding. Indeed,
the intensity ratio of the stretching vibration bands of
the azomethine groups (C=N) and aromatic rings
(Car –Car) in the IR spectra of the samples examined
changes upon complexation. Comparison of the inten-
sities of the bands at 1610 and 1597 cm–1 shows that,
The major factor determining the yield and mo-
lecular weight of the product is the strict observance of
the equimolar ratio of the reactants. It is also important
to properly choose the solvent, monomer concentra-
tion, and temperature. As solvent we chose DMF,
which well dissolves the starting monomers and the
forming polymer and can also serve as acceptor for the
hydrogen chloride released in the reation. To prepare
poly-[tetra(salicylidene azomethines)], it is appropriate
to take the starting monomers as dilute (5–10%) solu-
tions, because the probability of formation of disubsti-
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SYNTHESIS AND SPECTRAL CHARACTERISTICS OF SILICON-CONTAINING POLY(SALICYLIDENE AZOMETHINES) 1401
brations of hydroxy groups whould be expected. How-
ever, by contrast, a series of strong bands at 2978,
2780, and 2439 cm–1 appear in the spectrum. These
bands cannot be assigned to absorption of the polymer
and suggest that DMF is involved in side processes
accompanying the polymerization. In polycondensa-
tion of salicylidene derivatives and chlorosilanes, the
release of hydrogen chloride favors decomposition of
the solvent. The decomposition products, primarily
dimethylamine, are capable of binding with the poly-
mer by donor–acceptor interaction with vacant d orbi-
tals of the Si atoms.
In the process, the N atom of the amine competes
Fig. 1. IR spectra of (1) model low-molecular-weight
with the N atoms of azomethine groups, thereby de-
compound, (2) M1, and (3) P1 (1700–1400 cm–1). (I) In-
creasing the effective conjugation length. The forming
tensity and (ν) wavenumber; the same for Fig. 2.
complex is apparently so stable that the amine cannot
be removed by simple reprecipitation. To eliminate
the above-described side processes, eses we replaced
DMF with a more inert dioxane in further synth.
Fig. 2. IR spectra of (1) M2 and (2) P2 (3600–2000 cm–1).
in the spectrum of the monomer, the intensity of
the first band (C=N groups) is considerably higher
than that of the second band (stretching vibrations of
the ring), whereas in the polymer, the intensity of
the first band decreases and for the model compound,
it becomes even lower than that of the Car–Car band
(Fig. 1).
These differences may be due to a decrease in the
extinction coefficient of azomethine groups as a result
of donor–acceptor interactions. Thus, the intensity ra-
tio of these bands can serve as a criterion of the effi-
ciency of N→Si donor–acceptor bonding.
Figures 3a and 3b show the absorption spectra of
the monomers and the synthesized polymers. A con-
siderable bathochromic shift (50–70 nm) accompanied
by a hyperchromic effect and the appearance of addi-
tional long-wave bands in going from the monomers to
the polymer suggest that multiple CTCs are formed in
the polymer structure and the conjugation length in-
creases.
Along with the above features, strong absorption
in the range from 2000 to 4000 cm–1 engages attention
(Fig. 2). According to the previously described polym-
erization scheme, the amount of OH groups directly is
involved in the formation of the polymer chain should
decrease in the course of the synthesis. Thus, a de-
crease in the absorption in the range of stretching vi-
The absorption spectra of 1,3-bis(salicyliden-
imino)-4-phenylazobenzene (M4) and its silicon-con-
taining polymer P4 are markedly different. The strong
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GELLER et al.
analogous transformation in the reaction with the re-
leased hydrogen chloride.
The position of the azomethine proton signal in
the 1H NMR spectrum also suggests a strong coordina-
tion interaction of the lone electron pair of nitrogen
with the d orbitals of the Si atom.
Interesting information is furnished by ESR spec-
tra. In the ESR spectra of polysalicylidene derivatives
(Fig. 4), there is a narrow singlet with the g-factor
close to that of the free electron, which suggests conju-
gation in the polymer chain.
CONCLUSION
In the silicon-containing poly(salicylidene azo-
methines) prepared, there is intramolecular donor–
acceptor interaction resulting in the formation of charge-
transfer complexes in the polymer backbone.
ACKNOWLEDGMENTS
The study was financially supported by the Divi-
sion of Chemistry and Materials Science, Russian
Academy of Sciences, program “Development of Effi-
cient Methods for Chemical Analysis and Structural
Study of Substances and Materials” (coordinator Acad.
Yu.A. Zolotov).
Fig. 3. UV spectra of monomers and polymers derived
from (a) tetra- and (b) di(salicylidene azomethines). (D) Opti-
cal density and (λ) wavelength. (a) (1) M2, (2) P2, (3) M1,
and (4) P1; (b) (1) P3, (2) M3, (3) M4, and (4) P4.
REFERENCES
1. Archer, R.D., Inorganic and Organometallic Polymers,
New York: Wiley, 2001.
2. Chen, H. and Archer, R.D., Macromolecules, 1995, vol. 28,
no. 5, pp. 1609 –1617.
3. Chen, H. and Archer, R.D., Macromolecules, 1996, vol. 29,
no. 6, pp. 1957 –1964.
4. Much, F., Haberecht, J., Bochme, U., and Roewer, G.,
Monatsh. Chem., 1999, vol. 130, no. 1, pp. 117 –132.
5. Kalikhman, I., Krivonos, S., Ellern, A., and Kost, D.,
Organometallics, 1996, vol. 15, no. 24, pp. 5073–5076.
6. Agustin, D., Rima, G., Gornitzka, H., and Barrau, J.,
J. Organomet. Chem., 1999, vol. 592, no. 1, pp. 1–10.
7. Kost, D. and Kalikhman, I., Adv. Organomet. Chem.,
2004, vol. 50, pp. 1–106.
8. Kost, D., Kalikhman, I., and Raban, M., J. Am. Chem.
Soc., 1995, vol. 117, no. 46, pp. 11 512–11 522.
Fig. 4. ESR spectra of polymers (1) P1 and (2) P3. (I) In-
tensity and (H) magnetic field intensity.
9. Kalikhman, I., Gostevskii, B., Sivaramakrishna, A., et al.,
J. Organomet. Chem., 2003, vol. 686, no. 1, pp. 202–214.
10. Gur’yanova, E.N., Gol’dshtein, I.P., and Romm, I.P.,
Donorno-aktseptornaya svyaz’ (Donor–Acceptor Bond),
Moscow: Khimiya, 1973.
11. Andrews, L.J. and Keefer, R.M., Molecular Complexes
in Organic Chemistry, San Francisco: Holden-Day,
1964.
absorption band of the azo group present in the spec-
trum of the monomer (λ = 470 nm, Fig. 3b, curve 3)
disappears in the spectrum of the polymer (Fig. 3b,
curve 4), suggesting this group is involved either in
specific donor–acceptor interaction, or in a polymer-
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