Synthesis of guanidinopropyl triethoxysilane and its homopolymer as a new class of organosilicon antibacterial agents

Article abstract of DOI:10.1016/j.jorganchem.2020.121243

In current work guanidinopropyl triethoxysilane was synthesized for the first time. It was shown that this compound is stable as a salt of hydroiodic acid, however, it easily undergoes condensation when heated in water, forming a water-soluble branched homopolymer. Aqueous solutions of guanidinopropyl triethoxysilane and its branched homopolymer showed moderate antibacterial activity against Staphylococcus aureus and Escherichia coli.

Full text of DOI:10.1016/j.jorganchem.2020.121243

Journal of Organometallic Chemistry 918 (2020) 121243
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of guanidinopropyl triethoxysilane and its homopolymer as
a new class of organosilicon antibacterial agents
,
F.V. Drozdov a *, A.N. Tarasenkov a, M.S. Parshina a, G.V. Cherkaev a, E.N. Strukova c,
,
b
A.M. Muzafarov a
a Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, 117393, Moscow, Russian Federation
b Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991, Moscow, Russian Federation
c Department of Pharmacokinetics & Pharmacodynamics, Gause Institute of New Antibiotics, Moscow, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
In current work guanidinopropyl triethoxysilane was synthesized for the first time. It was shown that
this compound is stable as a salt of hydroiodic acid, however, it easily undergoes condensation when
heated in water, forming a water-soluble branched homopolymer. Aqueous solutions of guanidinopropyl
triethoxysilane and its branched homopolymer showed moderate antibacterial activity against Staphy-
lococcus aureus and Escherichia coli.
Received 9 January 2020
Received in revised form
17 March 2020
Accepted 19 March 2020
Available online 4 April 2020
© 2020 Elsevier B.V. All rights reserved.
Keywords:
Guanidine
Alkoxysilane
Siloxanes
Antimicrobial
Polycondensation
1. Introduction
branch of guanidine-containing organosilicon compounds,
including polymers, is still undeveloped. Obviously, this is related
Guanidine derivatives are known to have high antimicrobial
activity in relation to a large number of both gram-positive and
gram-negative bacteria. 1,10-hexamethylenebis(5-(p-chlorophenyl)
biguanide) or chlorhexidine is one of the most illustrative examples
of guanidine-containing compounds and leader in the market of
household antimicrobials. There are a large number of guanidine-
based polymers. Various modifications of polyhexamethylene
guanidine (PHMG) are well-known as the most popular disinfec-
tants. The advantages of this type of polymer are simplicity of
synthesis, further modifications and commercial availability of
precursors [1e5]. Using PHMG as an active ingredient and poly-
saccharides of various structures as the basis, a wide variety of
materials have been obtained which can be used for fabrication of
dressings, bandages, personal care products, etc [6,7]. There exist
other types of compounds based on guanidine, exhibiting antimi-
crobial properties, both individual and polymer structure [8e10].
Nevertheless, the analysis of literature sources shows that the
with low stability of the siloxane bond in the presence of such a
strong base as guanidine. Therefore, the aim of this work was to
search for approaches to the synthesis of accessible and stable
organosilicon derivatives of guanidine, which can be used to obtain
organosilicon polymers. Moreover, current work shows the po-
tential of such compounds as components for the manufacture of
antimicrobial materials.
2. Material and methods
Solvents were purified under standard conditions and freshly
distilled prior to use. Thiourea, 3-chloropropyltriethoxysiolane, 3-
aminopropyltriethoxysilane, methyl iodide was purchased from
Sigma-Aldrich with purity not less than 95% and used as it is. 1H,
13C, 29Si Nuclear Magnetic Resonance (NMR) spectra and their
nucleus correlations were recorded using a Bruker Avance II 300
spectrometer at 300, 75 and 60 MHz, respectively. IR spectroscopy
was performed using a “Bruker Equinox 55/S00 spectrometer.
Susceptibility testing of water solutions was performed in
duplicate using broth microdilution techniques according to CLSI
methods (CLSI. Performance standards for antimicrobial
* Corresponding author.
E-mail addresses: drozdov@ispm.ru, fedor.drozdov@gmail.com (F.V. Drozdov).
https://doi.org/10.1016/j.jorganchem.2020.121243
0022-328X/© 2020 Elsevier B.V. All rights reserved.

2
F.V. Drozdov et al. / Journal of Organometallic Chemistry 918 (2020) 121243
susceptibility testing. CLSI document M100-S25. Wayne, PA: Clin-
ical and Laboratory Standards Institute; 2015). After a 24-h expo-
sure with the organism grown in Ca2þ- and Mg2þ-supplemented
Mueller-Hinton broth, trays with 2-fold dilutions with known
concentrations were inoculated with S. aureus ATCC 29213 and
E. coli ATCC 25922 for a final bacterial load of approximately
5 ꢀ 105 CFU/mL. The plates were incubated at 37 ꢁC for 20 h. Cip-
rofloxacin was used as positive control.
3-chloropropyl sisesquioxane (1). Solution of 8.83 g (0.04 mol)
3-chloropropyltriethoxysilane in 20 ml acetic acid was refluxed for
8 h until the proton signals of ethoxy groups are absent in the 1H
NMR spectra. Mixture was washed with water till the neutral me-
dium of water and then evaporated in vacuum 0.1 mbar at 80 ꢁC to
obtain 6.51 g of viscous liquid with 86% yield.
oxide nanoparticles by gunidine’s moieties are known by substi-
tution of the chlorine atom of chloropropyltriethoxysilane frag-
ments covalently bonded to the surface [13e15]. However, there is
only one work by M.G. Voronkov, which reports the preparation of
an individual alkoxysilyl guanidine derivative containing one sila-
nol group [16].
Due to low range of development of this topics, first of all, it was
decided to investigate the possibility of obtaining guanidinopropyl
triethoxysilane as a key monomer for the synthesis of a wide va-
riety of guanidine-containing siloxane polymers.
All the attempts to obtain guanidinopropyl triethoxysilane ac-
cording to the method described in Ref. [16] did not lead to the
desired product. During the reaction, a rubbery reaction mass was
formed, apparently, due to the crosslinking of amino-
propyltriethoxysilane with guanidine. It was possible to obtain
branched siloxane polymers with guanidine fragments directly
analogically with the functionalization of nanoparticles described
previously. For this purposes, a branched siloxane polymer (1)a
with chloropropyl substituents was first obtained, and then an
attempt was made to replace chlorine atoms with guanidine
(Scheme 1, route a). Nevertheless, NMR analysis showed that the
shifts in the proton spectrum of the reaction mixture did not
change, which indicates that this reaction does not proceed.
Therefore, it was decided to obtain both the individual guanidi-
nopropyl triethoxysilane and the siloxane polymers based on it
using in principle different method (Scheme 1, route b).
1H NMR (300 MHz, Chloroform-d)
d: 5.90 (m,1.5H, Vin), 3.49 (br.
m, 2H, ClCH2-), 1.83 (br. m, 2H, -CH2-), 0.72 (br. m, 2H, -CH2Si), 0.21
(s, 3H, SiCH3).
2-Methylisothiourea hydroiodide (2). Was obtained according
convenient procedure [18]. Methyl iodide (23.5 g, 0.17 mol) was
added to thiourea (10.5 g, 0.14 mol) in 70 ml EtOH. The reaction was
refluxed for 2 h and then cooled down to room temperature. The
product was recrystallized in ethanol twice. The resulting solid was
precipitated, filtrated and dried to obtain 27.3
isothiourea hydroiodide with 91% yield as white crystals. 1H NMR
(300 MHz, Chloroform-d) : 8.78 (br. s., 1H, NH), 2.54 (s, 3H, CH3).
13C NMR (75 MHz, Chloroform-d)
: 170.98, 13.58.
g 2-methyl-
d
d
1-(2-(triethoxysilyl)propyl)guanidine (3). Was obtained ac-
cording convenient procedure [18]. 15.0 g (0.688 mol) of 1-(meth-
ylthio)methanamine hydroiodide (1) was dissolved in 65 ml of THF
and 15.23 g (0.688 mol) of aminopropyl triethoxysilane was added.
Reaction mixture was stirred for 8 h and after that solvent was
removed by distillation using liquid nitrogen trap in order to hold
volatile methyl mercaptan. The product was dried in vacuum to
obtain 23.1 g of sticky yellow liquid with 86% yield. 1H NMR
It was well-known, that “guanidinylation” with substituted
guanidine derivatives (with such groups as BOC or Cbz) demands
application of some catalytic systems [17]. Nevertheless, if guani-
dine fragment is unsubstituted, guanidinylation proceeds in mild
conditions [18]. We decided to use this approach here for synthesis
of guanidinopropyl triethoxysilane.
In the first stage, the S-methyl thiuronium (2) salt was obtained,
which was isolated in an individual state. Then, in the second stage,
an guanidinopropyl triethoxysilane (3) was obtained by replacing
the methyl mercapto group by the aminogroup, which was ami-
nopropyltriethoxysilane. Compound 3 was isolated and character-
ized. It was shown that the alkoxysilyl derivative of guanidine 3 is
stable both in an individual state and in an aqueous or alcoholic
solution under ambient conditions and did not undergo poly-
condensation. The 1H NMR spectrum of guanidinopropyl trie-
thoxysilane completely corresponds to the declared structure: all
the signals of protons of all groups are present on the spectrum, and
their integrated intensity corresponds to their theoretical amount
(Fig. 1a), which confirms the absence of silanol groups in the
compound. Including the signal intensity of protons of NH groups at
(300 MHz, Chloroform-d) d: 8.08e6.34 (m, 5H, NH), 3.80 (q, 6H,
OCH2, J ¼ 6.4 Hz), 3.44e3.15 (m, 2H, NHCH2), 1.85e1.49 (m, 2H,
CH2), 1.20 (t, 9H, CH3, J ¼ 7.2 Hz), 0.81e0.59 (m, 2H, SiCH2). 13C NMR
(75 MHz, Chloroform-d) d: 156.98, 57.96, 43.76, 39.72, 22.25, 18.35,
7.28. FTIR (cmꢂ1): 3323, 3159 (NH3þ); 2973 (CH3); 1050, 773
(SiOCH2). Calcd: C 30.69%, H 6.70, N 10.74, Si 7.18. Found: C 30.53%,
H 6.70, N 10.74, Si 6.91.
Condensation of 1-(2-(triethoxysilyl)propyl)guanidine (4).
1.16 g (2.96 mmol) of 1-(2-(triethoxysilyl)propyl)guanidine (2) was
dissolved in 10 ml of distilled water. Reaction was stirred for 4 h at
90 ꢁC. After that the reaction mixture was cool down, diluted with
THF and solvent was evaporated with rotary vaporizer. The polymer
was evaporated in vacuum 0.1 mbar at 80 ꢁC. The yield of the
product was 0.6 g (96%) as yellowish glass. 1H NMR (300 MHz, D2O)
d
~7.0 ppm the presence of the salt form of the compound can be
determined. In addition, the data of elemental analysis are in good
agreement with the calculated content of elements.
d
: 6.96 (s, 1H, ¼NH), 6.51 (br. s., 4H, NH2), 3.08 (s, 2H, -CH2-NH), 1.53
(s, 2H, -CH2-), 0.61 (s, 2H, -CH2-Si). 13C NMR (75 MHz, D2O)
d: 156.6,
Further, to obtain a siloxane polymer, it was necessary to select
the appropriate polymerization conditions of the guanidinopropyl
triethoxysilane. It was shown that the polymerization proceeds
directly in water while been heated to 90 ꢁC. Using 1H NMR, it was
found that complete conversion of alkoxy groups occurs after 4 h of
heating of the reaction mixture. The spectrum of the reaction
mixture shows that the proton signals of ethoxy groups differ from
those that were present in the initial ethoxysilyl monomer (Fig. 1b),
which indicates their belonging to the ethanol which released
during the condensation. The 1H NMR spectrum of the isolated
68.1, 57.5, 25.2, 17.2. FTIR (cmꢂ1): 3320, 3161 (NH2, NH3þ); 1080,
1032 (SiOSi).
3. Results and discussion
Guanidine is a strong base (рКb ¼ 0.4), comparable in basicity
with sodium hydroxide (рКb ¼ 0.2) [11]. It is known that a siloxane
bond cleaves in the presence of strong bases such as inorganic al-
kalis, aqueous ammonia, ammonium silaxanolate, tetrabuty-
lammonium hydroxide [12]. Therefore, the synthesis of - siloxane
polymers with guanidine fragments is rather difficult. To the date,
there is no published data concerning the preparation of
guanidine-based siloxane polymers, and, in addition, silicon-
containing guanidine compounds have not yet been practically
described. Only a few examples of the functionalization of iron
a
Such type of representation of branched silsesquioxane structures is common.
See Silicon-Containing Polymers eds. R.G. Jones, W. Ando, J. Chojnowski, Springer
Science, 2000, Silicon-Containing Dendritic Polymerseds. P.R. Dvornic, M.J. Owen,
Springer Science, 2009.

F.V. Drozdov et al. / Journal of Organometallic Chemistry 918 (2020) 121243
3
Scheme 1. Variants of obtaining a guanidine derivative of a branched siloxane polymer by modifying the chloropropyl derivative of polymer (a) and by synthesis, followed by
polycondensation and blocking of alkoxyguanidine derivative 3 (b).
polymer has broader signals compared to the initial monomer
(Fig. 1c), which indicates the polymer nature of this compound,
while the proton signals are completely consistent with the struc-
ture, and the proton signal intensity of the NH groups at d ~7.0 ppm
almost 2 times lower than in the monomer, which indicates the
absence of ammonium salt in the polymer structure after product
was isolated. Isolated polymer differs significantly from the initial
monomer and is a yellowish glassy substance (Fig. 4b), which also
indicates the occurrence of a condensation process with the for-
mation of a polymer structure. The 29Si NMR spectrum of the
polymer contains two signals at
d ~43 and 52 ppm. (Fig. 2), the
values of which indicate the formation of a branched structure.
Сondensation process occurrence are also confirmed by IR
spectroscopy (Fig. 3). The bands
sponding to the alkoxysilyl groups in initial monomer 3 are prac-
tically absent on the IR spectrum of polymer 4. The intensity of
y
¼ 2973 and 773 cm-1 corre-
y
¼ 1050 cm-1 band is also noticeably reduced, and it’s became
broadened and decomposed, which indicates the transition of
alkoxysilyl groups to siloxane ones.
It was found that the polymerization of guanidinopropyl trie-
thoxysilane 3 also takes place in acetic acid media under typical
alkoxysilane conditions [19], however, water/alcohol solubility of
the resulting polymer significantly complicates its isolation in this
case.
The antimicrobial mechanism of polymers containing specific
cation moieties, such as tetraalkylammonium salts and guanidine
fragments has been studied a long time. It is showed that the
polymer’s cationic fragments interact with anionic cell membrane
and will cause the damage of the structure of the phospholipid
bilayer. Finally, it leads to the death of the bacteria. These sugges-
tions were proved in many works. In all the cases AFM was used to
visualize the morphology changes of bacteria during treated with
such type of polymers [6,23e25].
Susceptibility testing revealed antibacterial activity of both
initial monomer 3 and homopolymer 4 against gram positive and
negative bacteria (Table 1). Minimal inhibitory concentrations
(MICs) of 3 and 4 against S. aureus ATCC 29213 were 0.98 ꢀ 103
mg/
Fig. 1. 1H NMR spectra of guanidinopropyl triethoxysilane (a), the reaction mixture
after condensation of guanidinopropyl triethoxysilane in D2O (b) and the isolated
guanidinopropylsilesesquioxane (c).
mL in both cases, the MICs of 3 and 4 against E. coli ATCC 25922
were 7.8 ꢀ 103 and 3.9 ꢀ 103
mg/mL, respectively.

4
F.V. Drozdov et al. / Journal of Organometallic Chemistry 918 (2020) 121243
Fig. 4. Images of the starting guanidinopropyl triethoxysilane before (a) and after (b)
condensation in water.
Table 1
Antimicrobial activity of initial guanidinopropyl triethoxysilane 3 and their homo-
polymer 4.
compound
S. aureus ATCC 29213,
E. coli ATCC 25922, dilution/
MIC (
g/mL∙ 103)
dilutiona/MIC (
m
g/mL∙ 103)
m
1st measure
2nd measure
1st measure
2nd measure
3
4
512/0.98
512/0.98
512/0.98
512/0.98
64/7.8
128/3.9
64/7.8
128/3.9
a
MIC was determined by sequential dilution of initial 50% solution in several
times.
achieve maximum antibacterial activity, due to the low partial
concentration of active guanidine groups compared to the film’s
surface. Nevertheless, the obtained results in aqueous solutions
provide the reason for investigation of antibacterial properties of
siloxane derivatives in surface, as well as investigation the anti-
bacterial activity of different type of their copolymers according to
the principle of structure-property.
Fig. 2. 29Si NMR spectrum of the isolated guanidinopropylsilsesquioxane 4 (in D2O).
4. Conclusions
Thus, in this article, guanidinopropyl triethoxysilane were ob-
tained for the first time by simple synthetic methods from ami-
nopropyltriethoxysilane and fully characterized. It was shown that
the guanidine derivative undergoes condensation and form a
branched silsesquioxane polymer while been heated in water to
90 ꢁC. 1H NMR spectroscopy confirmed complete conversion of
alkoxy groups is achieved after 4 h. The initial guanidine derivative,
as well as its homopolymer obtained from it, is readily soluble in
water, alcohols and polar aprotic solvents. Atibacterial activity tests
showed that both compounds exhibit moderate bacteriostatic ac-
tivity against E. Coli and S. Aureus. Thus, the guanidinopropyl trie-
thoxysilane obtained by simple synthetic methods from available
starting compounds reveals new possibilities for producing
siloxane polymers with bacteriostatic action against gram positive
and negative bacteria.
Fig. 3. FTIR spectra of guanidinopropyl triethoxysilane 3 (black line) and isolated
guanidinopropylsilsesquioxane 4 (blue line). (For interpretation of the references to
colour in this figure legend, the reader is referred to the Web version of this article.)
Declaration of competing interest
All antibacterial measurements were carried out in dilute
aqueous solutions. Attempts to test on films of 3 and 4 were not
successful due to surface erosion during addition of the nutrient
medium.
The authors declare no competing financial interest.
Acknowledgements
Comparing the acquired results of antibacterial tests with the
data obtained for similar polymers [20e22], we can conclude that
both the initial guanidinopropyl triethoxysilane 3 and the homo-
polymer 4 exhibit a moderate antibacterial activity. Measurements,
which were performed in aqueous solutions, seems to be not
The synthesis of monomers was performed by the Grant of the
President of Russian Federation (МК-3780.2019.3). Investigations
of polymerization of initial monomer and antibacterial tests were
made with financial support of Russian-Vietnamese Tropical Center

F.V. Drozdov et al. / Journal of Organometallic Chemistry 918 (2020) 121243
5
[12] M.G. Voronkov, Siloxane Bond: Physical Properties and Chemical Trans-
formations, first ed., Springer, 1978.
[13] H. Rostami, L. Shiri, Russ. J. Org. Chem. 55 (8) (2019) 1204e1211, https://
doi.org/10.1134/S1070428019080207.
[14] F.A. Tameh, J. Safaei-Ghomi, M. Mahmoudi-Hashemia, H. Shahbazi-Alavi, RSC
Adv. 6 (2016) 74802e74811, https://doi.org/10.1039/C6RA08458C.
[15] L. Heidari, L. Shiri, Appl. Organomet. Chem. 33 (3) (2019) 4636, https://
doi.org/10.1002/aoc.4636.
(Grant Ecolan T-2.5), NMR analysis and investigation of molecular-
weight characteristics of polymer was made using the equipment
of the Collaborative Access Center “Сenter for Polymer Research” of
ISPM RAS.
References
[16] M.G. Voronkov, L.I. Belousova, YuN. Pozhidaev, N.N. Vlasova, Russ. J. Gen.
Chem. 73 (8) (2003) 1239e1242, https://doi.org/10.703632/03/73081239.
[17] D.A. Powell, P.D. Ramsden, R.A. Batey, J. Org. Chem. 68 (2003) 2300e2309,
https://doi.org/10.1021/jo0265535.
[18] N. Aoyagi, Y. Furusho, T. Endo, Synthesis 52 (1) (2020) 150e158, https://
doi.org/10.1055/s-0037-1610735.
[1] M.S. Brzezinska, M. Walczak, U. Jankiewicz, M. Pejchalova, J. Polym. Environ.
26 (2018) 589e595, https://doi.org/10.1007/s10924-017-0974-9.
[2] B. Wang, B. Liu, G. Peng, X. Meng, Z. Jiang, H. Chen, J. Appl. Polym. Sci. 130 (5)
(2013) 3489e3497, https://doi.org/10.1002/app.39369.
[3] Z.X. Zhou, D.F. Wei, Y. Guan, A.N. Zheng, J.J. Zhong. J. Appl. Microbiol. 108 (3)
(2010) 898e907, https://doi.org/10.1111/j.1365-2672.2009.04482.x.
[4] Y. Zhang, J. Jiang, Y. Chen, Polymer 40 (1999) 6189e6198, https://doi.org/
10.1016/S0032-3861(98)00828-3.
[19] A.A. Kalinina, E.V. Talalaeva, A.I. Demchenko, N.G. Vasilenko, YuA. Molodtsova,
N.V. Demchenko, А.М. Muzafarov, Russ. Chem. Bull. Int. Ed. 65 (4) (2016) 1e7,
https://doi.org/10.1007/s11172-016-1405-0.
[5] Y. Guan, H. Zhang, A. Zheng, D. Wei, J. Hu, Z. Wang, X. Xu, Polym. Adv. Technol.
(2019) 1e9, https://doi.org/10.1002/pat.4586.
[6] Y. Guan, L. Qian, H. Xiao, A. Zheng, Cellulose 15 (2008) 609e618, https://
doi.org/10.1007/s10570-008-9208-6.
[7] Y.J. Park, M.H. Jeong, I.J. Bang, H.R. Kim, K.H. Chung, Inhal. Toxicol. 31 (4)
(2019) 161e166, https://doi.org/10.1080/08958378.2019.1624896.
[8] K. Peng, T. Zou, W. Ding, R. Wang, J. Guo, J.J. Round, W. Tu, C. Liu, J. Hu, RSC
Adv. 7 (2017) 24903e24913, https://doi.org/10.1039/C7RA02706K.
[9] E. Escamilla-Garcia, A.G. Alcazar-Pizana, J.C. Segoviano-Ramirez, C. Del Angel-
Mosqueda, A.P. Lopez-Lozano, E. Cardenas-Estrada, M.A. De La Garza-Ramos,
C.E. Medina-De La Garza, M. Marquez, Hindawi Int. J. Microbiol. (2017)
5924717, https://doi.org/10.1155/2017/5924717.
[20] K.E.S. Locock, T.D. Michl, J.D.P. Valentin, K. Vasilev, J.D. Hayball, Y. Qu,
A. Traven, H.J. Griesser, L. Meagher, M. Haeussler, Biomacromolecules 14
(2013) 4021e4031, https://doi.org/10.1021/bm401128r.
[21] K.E.S. Locock, T.D. Michl, H.J. Griesser, M. Haeussler, L. Meagher, Pure Appl.
Chem. 86 (8) (2014) 1281e1291, https://doi.org/10.1515/pac-2014-0213.
[22] Y. Pan, Q. Xia, H. Xiao, Polymers 11 (2019) 1283, https://doi.org/10.3390/
polym11081283.
[23] Y. Guan, L. Qian, H. Xiao, A. Zheng, Cellulose 15 (2008) 609e618, https://
doi.org/10.1007/s10570-008-9208-6.
[24] Y. Xue, H. Xiao, Y. Zhang, Int. J. Mol. Sci. 16 (2015) 3626e3655, https://doi.org/
10.3390/ijms16023626.
[25] S. Ghamrawi, J.-P. Bouchara, O. Tarasyuk, S. Rogalsky, L. Lyoshinad, O. Bulko, J.-
F. Bardeau, Mater. Sci. Eng. C 75 (2017) 969e979, https://doi.org/10.1016/
j.msec.2017.03.013.
[10] M. Gorbunova, L. Lemkina, I. Borisova, Eur. Polym. J. 105 (2018) 426e433,
https://doi.org/10.1016/j.eurpolymj.2018.06.014.
[11] W.H. Brown, T. Poon, Introduction to Organic Chemistry, sixth ed., Wiley-VCH,
2016.