Synthesis of 1,3-bis(chlorodiorganosilyl)-cyclodisilazane via dehydro-chlorination reaction of 1,3-dichloro-Tetraorgano-Disilazane in the presence of deacidification agent

Article abstract of DOI:10.1016/j.jorganchem.2020.121414

A novel convenient synthesis process for 1,3-bis(chlorodiorganosilyl)-cyclodisilazanes is developed via an intermolecular dehydrochlorination of 1,3-dichloro-tetraorgano-disilazane, in the presence of a strong organic alkaline deacidification agent 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). This procedure involves a one-step process under mild synthesis condition, with higher production efficiency and product purity as compared to those of the previously reported methods. The formation of 1,3-bis(chlorodiorganosilyl)-tetraorgano-cyclodisilazane occurs via the primary dehydrohalogenation of intermolecular 1,3-dichloro-tetraorgano-disilazanes to trisilylamine structure with a subsequent ring closure. The silicon atoms with different exocyclic or endocyclic substituents are closely related to the steric hindrance of the substituents. Dehydrochlorination reactions occur more readily on the chlorine atoms attached to the silicon atoms with substituents possessing relatively low steric hindrance. Four 1,3-dichloro-tetraorgano-disilazanes for the synthesis of 1,3-bis(chlorodiorganosilyl)-cyclodisilazanes are prepared. The investigation into the reaction mechanism shows that the equilibrium reaction of cyclosilazane with diorgano-dichlorosilane is a more straightforward and efficient method in the preparation of 1,3-dichloro-tetraorgano-disilazanes, as compared to the trans-silylation reaction of hexamethyl-disilazane with diorgano-dichlorosilane.

Full text of DOI:10.1016/j.jorganchem.2020.121414

Journal of Organometallic Chemistry 923 (2020) 121414
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of 1,3-bis(chlorodiorganosilyl)-cyclodisilazane via dehydro-
chlorination reaction of 1,3-dichloro-Tetraorgano-Disilazane in the
presence of deacidification agent
,
Yongxia Tan a b, Ning Li a, Shuxuan Kong a, Xuezhong Zhang a, Shuhao Zhang a,
,
Zhijie Zhang a *, Zemin Xie a
a Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel convenient synthesis process for 1,3-bis(chlorodiorganosilyl)-cyclodisilazanes is developed via
an intermolecular dehydrochlorination of 1,3-dichloro-tetraorgano-disilazane, in the presence of a strong
organic alkaline deacidification agent 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). This procedure involves
a one-step process under mild synthesis condition, with higher production efficiency and product purity
as compared to those of the previously reported methods. The formation of 1,3-bis(chlorodiorganosilyl)-
tetraorgano-cyclodisilazane occurs via the primary dehydrohalogenation of intermolecular 1,3-dichloro-
tetraorgano-disilazanes to trisilylamine structure with a subsequent ring closure. The silicon atoms with
different exocyclic or endocyclic substituents are closely related to the steric hindrance of the sub-
stituents. Dehydrochlorination reactions occur more readily on the chlorine atoms attached to the silicon
atoms with substituents possessing relatively low steric hindrance. Four 1,3-dichloro-tetraorgano-dis-
ilazanes for the synthesis of 1,3-bis(chlorodiorganosilyl)-cyclodisilazanes are prepared. The investigation
into the reaction mechanism shows that the equilibrium reaction of cyclosilazane with diorgano-
dichlorosilane is a more straightforward and efficient method in the preparation of 1,3-dichloro-tet-
raorgano-disilazanes, as compared to the trans-silylation reaction of hexamethyl-disilazane with dio-
rgano-dichlorosilane.
Received 24 April 2020
Received in revised form
13 June 2020
Accepted 17 June 2020
Available online 15 July 2020
Keywords:
1,3-Dichloro-tetraorgano-disilazane
1,3-Bis(chlorodiorganosilyl)-tetraorgano-
cyclodisilazane
Dehydrochlorination reaction
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
cting significant attention as the starting monomers in the prepa-
ration of thermal resistant elastomers, the complicity of the syn-
Cyclodisilazane is a four-membered rings with alternating Si-N
bonds with decomposition temperature of over 400 ꢀC [1,2]. As
such, cyclodisilazane is able to demonstrate excellent thermal sta-
bility [3]. To prepare various functionalized cyclodisilazanes or linear
polymers with cyclodisilazane units in the polymer chain, 1,3-
bis(chloro-diorganosilyl)-tetraorgano-cyclodisilazane is considered
as an excellent starting material [4e9]. This is because of the phenyl
substituent in the 1,3-bis(chloro-diorganosilyl)-tetraorgano-cyclo-
disilazane which exhibits higher hydrolysis and thermal stability. As
a result, they are excellent monomers for the preparation of high-
temperature-resistant elastomers [10e12]. Even though 1,3-
bis(chloro-diorganosilyl)-tetraorgano-cyclodisilazane has been attra
thesis process has severely hampered the mass adoption of
cyclodisilazanes in various applications.
In the field of 1,3-bis(chloro-diorganosilyl)-cyclodisilazane, the
full methyl-substituted compound is the first to be widely studied.
Wannagat [13] prepared 1,3-bis(chloro-dimethylsilyl)-2,2,4,4-
tetramethyl-cyclodisilazane for the first time via equilibrium redis-
tribution reaction [14] of hexamethyl-cyclotrisilazane or octamethyl-
cyclotetrasilazane with a stoichiometric or deficient amount of
dimethyldichlorosilane (Me2SiCl2) [6,14e16]. Based on the quanti-
tative study of the reaction systems for cyclosilazane and chlor-
osilane, it is shown that an appreciable amount of chains are in
equilibrium with the rings [17]. Upon equilibrium, the long-chain
halogen-terminated silazanes tend to rearrange to short chain
compounds and cyclics [15]. At the same time, the reaction of Si-Cl
bond attacking Si-N bond could either lead to self-redistribution or
reaction between each halogen-terminated silazanes to form various
* Corresponding author.
E-mail address: zhangzj@iccas.ac.cn (Z. Zhang).
https://doi.org/10.1016/j.jorganchem.2020.121414
0022-328X/© 2020 Elsevier B.V. All rights reserved.

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Y. Tan et al. / Journal of Organometallic Chemistry 923 (2020) 121414
products. As a result, the reaction was performed at a high temper-
ature and for a long duration (e.g., 150 ꢀC ꢁ 24 h, or 240 ꢀC ꢁ 15 h)
with a low reaction yield (30-60%) [15,16]. Furthermore, it should be
noted that a considerably higher reaction temperature is required
when phenyl substituents are present. Fink [16] prepared 1,3-
bis(chlorodimethylsilyl)-2,2,4,4-tetramethyl-cyclodisilazane with a
high reaction yield of 84% via an intermolecular polycondensation of
1,3-dichloro-1,1,3,3-tetramethyl-disilazane catalyzed by trimethyl-
amine. However, it should be noted that the synthesis procedure was
carried out under pressure.
Breed [18] reported a convenient laboratory synthesis route to
prepare 1,3-bis(chloro-diorganosilyl)-tetraorgano-cyclodisilazanes
by treating 1,3-dichloro-tetraorgano-disilazanes with butyllithium.
This synthesis route is applicable to the synthesis of bis(chloro-
diorganosilyl)-tetraorgano-cyclodisilazanes bearing various sub-
stituents on the silicon atoms. Nevertheless, the reactions are
difficult to operate due to the use of butyllithium. Despite such
challenge, it is still currently considered as an advantageous
method [19,20].
As the starting material in the preparation of 1,3-bis(chloro-
diorganosilyl)-tetraorgano-cyclodisilazanes, 1,3-dichloro-tetraor-
gano-disilazanes can be prepared via an equilibrium reaction of
cyclotrisilazanes or cyclotetrasilazanes with excessive diorganodi-
chlorosilane [15,18,21]. It can also be prepared via the exchange
reaction of silyl groups between hexamethyl-disilazane and
diorgano-dichlorosilane [20,22]. Unfortunately, the reaction
mechanisms and the feasibilities of these two methods have not
been systematically described in the literature.
Herein, we explore a more suitable synthesis method for the
preparation of 1,3-dichloro-tetraorgano-disilazanes with various
silicon-substituted groups. Thus, both equilibrium reaction pro-
cedures and the trans-silylation reaction procedures are examined
in this work in detail. A convenient and effective method is then
developed for the preparation of 1,3-bis-(chloro-diorganosilyl)-
tetraorgano-cyclodisilazanes via an intermolecular deacidification
of 1,3-dichloro-tetraorgano-disilazanes in the presence of a strong
organic base as a HCl-acceptor.
hexaorgano-trisilazanes and diorgano-dichlorosilanes are initially
formed when pure 1,3-dichloro-tetraorgano-disilazanes are heat-
ed. This is also in good agreement with the study on the reaction of
hexamethyl-cyclotrisilazane with dimethyl-dichlorosilane (Me2
SiCl2) reported by Fink [16]. However, Fink only reported the 1H
NMR spectrum of 1,5-dichoro-hexamethyltrisilazane in the reac-
tion mixture.
An excess diorgano-dichlorosilane can inhibit the equilibrium
redistribution of 1,3-dichloro-tetraorgano-disilazanes (eq. 3 in
Scheme 1), which in turn leads to a high reaction yield of 1,3-
dichloro-tetraorgano-disilazanes. Through this method, com-
pound 1 [21] and 3 [18] have been obtained with good reaction
yields. However, treating hexaphenyl-cyclotrisilazane with an
excess diphenyl-dichlorosilane (Ph2SiCl2) can only result in a 21%
reaction yield for compound 2 [23]. In this work, we prepared
compound 2 with an 89% reaction yield by reacting hexaphenyl-
cyclotrisilazane with Ph2SiCl2 (molar ratio ¼ 1:5) for only 5h.
The 29Si NMR spectra chemical shifts of compounds 1e3 and
intermediate mixed compounds containing 1a, 2a, 3a,1b, 2b, and
3b are summarized in Table 1. The 29Si NMR spectra of the above-
mentioned compounds and the bimolecular equilibrium redistri-
bution reactions of 1,3-dichloro-tetraorgano-disilazane are shown
in the supplementary data (Figs. S1-S3).
The isomerism occurs when the two substituents on one silicon
atom are different, e.g., methyl and phenyl groups. Two geometrical
isomers are demonstrated to exist in the cyclic trimers 2,4,6-
trimethyl-2,4,6-triphenyl-cyclotrisilazane [18] and 2,4,6-trim
ethyl-2,4,6-triphenyl-cyclotrisiloxane [24e26], respectively. Two
sets of signals with a peak area ratio of 1:1 in the 29Si NMR spectra
of compound 3 are observed, which indicates that compound 3 has
two stereoisomers. Two phenyl groups on the same side of the
silicon atoms are referred as cis-isomer, while the phenyl groups on
the opposite side of the silicon atoms are referred as trans-isomer.
The chemical shift of the silicon atoms in cis-3 is 2.66 ppm, and that
of trans-3 is shifted downfield [27] to 2.86 ppm. Similarly, the
various isomers of 1,7-dichloro-1,3,5,7-tetramethyl-1,3,5,7-tetra
phenyl-tetrasilazane (3a) and 1,5-dichloro-1,3,5-trimethyl-1,3,5-
triphenyl-trisilazane (3b) can be distinguished by the chemical
shifts in the 29Si NMR spectrum, as shown in Table 1.
2. Result and discussion
1,3-dichloro-tetraorgano-disilazanes with same substituents on
two silicon atoms can also be prepared via the exchange reaction of
silyl groups between hexamethyl-disilazane and diorgano-
dichlorosilane [19,20,22]. The single exchange reaction preferen-
tially occurs to form Me3SiNHSiR1R2Cl (eq. 4 in Scheme 2), and the
subsequent reaction to synthesis 1,3-dichloro-tetraorgano-dis-
ilazane (eq. 5 in Scheme 2) should be carried out at a higher tem-
perature as shown in Scheme 2. The principle is very similar to the
equilibrium reaction. In order to obtain high reaction yield, a large
excess of diorgano-dichlorosilane is required to restrain the equi-
librium redistribution of 1,3-dichloro-tetraorgano-disilazane (eq. 3
in Scheme 1). On the other hand, it is beneficial for the processing of
the trans-silylation reaction when distilling the trimethyl-
chlorosilane by-product continuously from the reaction mixture
during the reaction process.
Compound 2 or 3 can be obtained by treating hexamethyl-
disilazane with Ph2SiCl2 [19,28] or methylphenyl-dichlorosilane
(MePhSiCl2) [20]. However, it is challenging to obtain com-
pound 1 via the reaction of hexamethyldisilazane with Me2SiCl2.
It has also been reported that Me2SiCl2 does not react with
hexamethyl-disilazane [29]. The reaction may not proceed due to
the low reflux temperature of the reaction mixture. In addition,
the boiling point of Me2SiCl2 is close to that of trimethyl-
chlorosilane by-product, which participates in the reaction to
attack the Si-N bond as trimethylchlorosilane is difficult to be
distilled out of the reaction system. As a result, even with the use
2.1. Synthesis of 1,3-dichloro-tetraorgano-disilazanes
2.1.1. 1,3-Dichloro-tetraorgano-disilazanes with same substituents
on two silicon atoms
1,3-dichloro-tetramethyl-disilazane (1), 1,3-dichloro-tetra-
phenyl-disilazane (2), and 1,3-dichloro-1,3-dimethyl-1,3-diphenyl-
disilazane (3), which have same substituents on two silicon atoms,
are synthesized via refluxing cyclotrisilazane with the corre-
sponding diorgano-dichlorosilane in excess. The reaction mecha-
nism is studied by determining the composition of the reaction
mixture during the reaction below reflux temperature. The 29Si
NMR spectra reveal the preferential formation of 1,7-dichloro-
octaorgano-tetrasilazane in the initial stage (eq. 1 in Scheme 1).
Subsequently, 1,7-dichloro-octaorgano-tetrasilazane reacts with
diorgano-dichlorosilane to generate 1,5-dichloro-hexaorgano-tri-
silazane and 1,3-dichloro-tetraorgano-disilazane (eq. 2 in Scheme
1). Elevated temperatures and prolonged reaction durations are
required when the reactants contain phenyl groups. With the
significantly excessive diorgano-dichlorosilane used in the reac-
tion, the reaction yields of these three 1,3-dichloro-tetraorgano-
disilazanes are close to 90%. However, 1,5-dichloro-hexaorgano-
trisilazanes are always present in the final products. A major factor
which contributes to such observation is the bimolecular equilib-
rium redistribution of 1,3-dichloro-tetraorgano-disilazanes (eq. 3
in Scheme 1). This can be proven by the fact that both 1,5-dichloro-

Y. Tan et al. / Journal of Organometallic Chemistry 923 (2020) 121414
3
Scheme 1. The reaction of cyclotrisilazane with the corresponding diorgano-dichlorosilane to synthesize 1,3-dichloro-tetraorgano-disilazanes with same substituents on two
silicon atoms.
Table 1
NMR data for compounds 1e4 and intermediate mixed compounds.
Formula
Compound
Substituents at silicon
29Si NMR spectra chemical shift (ppm)
1
2
3
4
R1~ R4 ¼ Me
a ¼ b: 13.29
R1~R4 ¼ Ph
a ¼ b: 7.9
R1, R3 ¼ Me; R2, R4 ¼ Ph
R1, R2 ¼ Me; R3, R4 ¼ Ph
R1 ¼ Me; R2, R3, R4 ¼ Ph
R1 ~ R8 ¼ Me
cis: a ¼ b: 2.66; trans: a ¼ b: 2.86
a: 14.53; b: 8.60
A
a: 3.26; b: 8.11
1a
2a
3a
a ¼ d: 11.51; b ¼ c: 5.83
a ¼ d: 9.10; b ¼ c: 21.75
a, d: 1.62, 1.57 and 1.41;
c, d: 13.62, ꢂ13.66, ꢂ13.76, and ꢂ13.83
a: 10.93; b: 4.96;
R
1 ~ R8 ¼ Ph
R1, R3, R5, R7 ¼ Me
R2, R4, R6, R8 ¼ Ph
4a
R
1 ~ R6 ¼ Me; R7, R8 ¼ Ph
c: 6.55; d: 9.66
1b
2b
3b
R1~ R6 ¼ Me
a ¼ c: 11.96; b: 4.33
a ¼ c: 8.78; b: 20.92
a:1.89, 1.84, 1.79 and 1.75;
b: 12.66, ꢂ12.72, and-12.91
a: 11.48; b: 3.57; c: 8.95
a ¼ c: 2.04; b: 9.63
R1~ R6 ¼ Ph
R1, R3, R5 ¼ Me;
R2, R4, R6 ¼ Ph
R1~R4 ¼ Me; R5, R6 ¼ Ph
R1, R2, R5, R6 ¼ Ph;
R3, R4 ¼ Me
4b
4c
2c
3c
R1, R2 ¼ Ph
a: 8.61; b: 6.09
a: 1.49; b: 5.36
R1 ¼ Me; R2 ¼ Ph
of a Lewis acid as a catalyst, the reaction yield of compound 1 is
still low [22]. The 29Si NMR spectra chemical shifts of compound
2c and 3c are summarized in Table 1, and the 29Si NMR spectrum
of compound 2c and 3c are shown in the supplementary data
(Fig. S4).
Based on the analysis of the two above-mentioned methods
used in the preparation of 1,3-dichloro-tetraorgano-disilazanes
with same substituents on the two silicon atoms, it is found that the
equilibrium reaction of cyclotri- or cyclotetra-silazane with the
corresponding diorgano-dichlorosilane is relatively simple and
efficient. This is because no formation of by-product is observed
during the reaction process.
2.1.2. 1,3-Dichloro-tetraorgano-disilazanes with different
substituents on two silicon atoms
Researches are rarely conducted on 1,3-dichloro-tetraorgano-
disilazanes with different substituents on two silicon atoms.
Wannagat [23] reported the treatment of Ph2SiCl2 with a stoi-
chiometric amount of octamethylcyclotetrasilazane, which yielded
a 1,3-dichloro-1,1-dimethyl-3,3-diphenyl-disilazane (4) with a poor
reaction yield of 43%. This product is unstable and it tends to be
converted to compound 1 and 2 upon reaching the column during
the distillation process. However, relevant evidences and mecha-
nism are not described.
We investigate the reaction of hexamethyl-cyclotrisilazane with
Ph2SiCl2 using the stoichiometric quantity, i.e., molar ratio ¼ 1:3.

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Y. Tan et al. / Journal of Organometallic Chemistry 923 (2020) 121414
Scheme 2. The exchange reaction of silyl groups between hexamethyldisilazane and diorganodichlorosilane.
The results show that almost all the product formed at 90 ꢀC is 1,7-
dichloro-1,1,3,3,5,5-hexamethyl-7,7-diphenyl-tetrasilazane (4a).
such, only trace amount of compound 1b is detected during the
reaction. The reaction mechanism is illustrated in Scheme 3.
Similarly, these 1,5-dichloro-hexaorgano-trisilazanes can
continue to react with Ph2SiCl2 to yield compound 1, 2, and 4 as
shown in Scheme 4. The reaction could be accelerated at elevated
temperature, and the composition of the final product is relatively
more complex due to the different substituents on the silicon atoms
of the reactants. After a suitable reaction duration under reflux
This is because the 29Si NMR spectrum only shows four peaks with
the same area ratio, except for the starting material. Compound 4a
is further converted to form compounds 4b, 4c, 1, 2, and 4, which
contain short-chain silazanes mixture, at 110 ꢀC. The fully methyl-
substituted intermediate compound 1b, which is unstable, could
immediately react with Ph2SiCl2 to form compounds 1 and 4. As
Scheme 3. The reaction mechanism of hexamethylcyclotrisilazane with Ph2SiCl2.

Y. Tan et al. / Journal of Organometallic Chemistry 923 (2020) 121414
5
Scheme 4. The reaction of 1,5-dichloro-hexaorgano-trisilazanes with Ph2SiCl2 to yield compound 1, 2, and 4.
condition, a relatively high content of compound 4 is obtained. The
4
is converted to compound 4c and Me2SiCl2 via
a
self-
mixture mainly contains three 1,3-dichloro-tetraorgano-dis-
ilazanes, i.e., compound 1, 2, and 4, and the stable compound 4c,
with the contents of these compounds decreasing in the following
order; 4＞1＞4c＞2. Compound 4c contains four phenyl sub-
stituents, and the steric hindrance of the phenyl groups contributes
to the reduced reactivity of compound 4c with Ph2SiCl2 (eq. 5 in
Scheme 4). However, the rearrangement reaction between the
chlorine-terminated disilazanes become more significant at higher
temperature, and, as a result, the reaction yield of compound 4
decreases after a prolonged reflux condition.
Obviously, the reactants cannot be quantitatively converted to
compound 4 due to the large formation of compound 1. Further-
more, there is a significant amount of Ph2SiCl2 that does not
participate in the reaction. The excessive Ph2SiCl2 is always distilled
out of the mixture along with compound 4 during the subsequent
fractionation process. As such, it is challenging to isolate Ph2SiCl2
from compound 4. On the other hand, the bimolecular rearrange-
ment reaction of compound 4 continues during the distillation
process as evidenced by refluxing the pure compound 4. The 29Si
NMR spectra analysis of the reaction process show that compound
redistribution process as shown in Scheme 5. This is in contrast
to the conversion of compound 4 into compound 1 and 2, as
described by Wannagat [23]. Thus, the reaction yield of compound
4 is ultimately reduced during the fractionation process. The 29Si-
NMR spectrum of the reaction of hexamethyl-cyclotrisilazane with
Ph2SiCl2 is shown in the supplementary data (Fig. S5).
Therefore, the products are determined by the ratio of the re-
actants and the reaction conditions used in the preparation of
compound 4 via the reaction of hexamethylcyclotrisilazane with
Ph2SiCl2. In order to achieve a high reaction yield of the target
compound 4, it is necessary to use an appropriate ratio, i.e., less
than the stoichiometric quantity of Ph2SiCl2, and a suitable reaction
temperature and duration. If a large excess of Ph2SiCl2 is used in the
reaction with hexamethylcyclotrisilazane at elevated temperature
for sufficient duration, and the complete distillation of component
1 and Me2SiCl2 with low boiling points from the reaction mixtures,
compound 2 will be the primary product. The 29Si NMR spectra
chemical shifts of compounds 4a, 4b, 4c, and 4 that are formed in
reaction process are shown in Table 1.
1,3-dichloro-tetraorgano-disilazanes with different substituents
Scheme 5. The bimolecular equilibrium redistribution reaction of compound 4.

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Y. Tan et al. / Journal of Organometallic Chemistry 923 (2020) 121414
on two silicon atoms can also be obtained via the exchange reaction
of silyl groups. A single exchanged product, in which only one tri-
methylsilyl group is substituted, can be obtained easily via the
exchange reaction of hexamethyl-disilazane with diorgano-
dichlorosilane. Even though it is theoretically feasible to synthe-
size 1,3-dichloro-tetraorgano-disilazane by replacing another tri-
methylsilyl group of the single exchanged product, the reaction
yield is low [20]. The study on the reaction process of Me3SiNH-
SiRPhCl and R’PhSiCl2 (R ¼ Ph, R’ ¼ Me; or R ¼ Me, R’ ¼ Ph) reveals
the reaction mechanism, as shown in Scheme 6. The 29Si-NMR
spectrum of these two reactions are shown in the supplementary
data (Fig. S6 and Fig. S7).
Rather than replacing the trimethylsilyl group to produce the
compound ClPh2SiNHSiMePhCl (A), the reactant R’PhSiCl2 prefer-
entially attacks the Si-N bond that is attached to the electron-
withdrawing chlorine atom to form Me3SiNHSiR’PhCl and
RPhSiCl2 (eq. 9). As such, there is a competitive reaction between
each of the components in the mixture (eq. 9, 10, 11, and 12), where
the reactants substituted by methyl and phenyl groups are more
reactive than those substituted by diphenyl groups. Thus, com-
pound 3 is firstly formed in this reaction. At elevated temperature,
the formation of compound 2 and compound A are accelerated,
which are accompanied with the rearrangement reaction of various
1,3-dichlorosilazanes. As a result, the product possesses a highly
complex composition.
other that results in a product with a very complex composition.
The component of products can be altered by changing the reaction
temperature or the ratio of the reactants.
2.2. Synthesis of 1,3-bis(chlorodiorganosilyl)-tetraorgano-
cyclodisilazanes
It has been shown that the presence of bases can facilitate the
closure of the cyclodisilazane ring. Geymayer [21] first reported
that preparation of 1,3-bis(chlorodimethylsilyl)-tetramethyl-cyclo-
disilazane via the reaction of compound 1 with sodium bis(-
trimethylsilyl)amide ([(CH3)3Si]2NNa). This can also be achieved by
heating compound 1 with triethylamine under high pressure [16].
Wannagat [1,30] speculated that two 1,3-dichloro-tetramethyl-
disilazanes may undergo bimolecular condensation in the presence
of trialkylamine or sodium bis(trimethylsilyl)amide as a HCl-
acceptor. However, Breed [18] did not achieve the corresponding
cyclodisilazane when treating 1,3-dichloro-1,3-dimethyl-1,3-
diphenyldisilazane with sodium bis(trimethylsilyl)amide. In this
work, we also failed to synthesize 1,3-bis(chlorodimethylsilyl)-
tetramethyl-cyclo-disilazane via the reaction of compound 1 with
trimethylamine at room pressure.
Butyllithium is once considered as a satisfactory organic base for
the cyclization of hexachloro-disilazane [31]. Fink obtained 1,3-
bis(chlorodimethylsilyl)-tetramethyl-cyclo-disilazane by treating
Me2SiCl2 with cyclotrisilazane lithium salt metallized by butyl-
lithium [16]. Breed [18] provided a method to prepare a variety of
cyclodisilazane derivatives by treating 1,3-dichloro-tetraorgano-
disilazane with butyllithium, which was considered as a convenient
laboratory synthesis route. However, the procedure was carried out
through stepwise reactions. The first step involved the metalation
in nitrogen at ꢂ65 ꢀC. This was followed by the bimolecular
condensation to a trisilylamine structure. Finally, ring closure is
achieve during the heating process. In addition, the butyllithium is
inflammable when exposed to air.
We also attempt to synthesize compound A via the exchange
reaction between 1,3-dichloro-tetraorgano-disilazane with same
substituents on the two silicon atoms and diorgano-dichlorosilane
[20]. Herein, the mechanism of this reaction is investigated in
detail, as shown in Scheme 7. It shows that the bimolecular equi-
librium redistribution of 1,3-dichloro-tetraorgano-disilazanes
occur preferentially. Subsequently, the product of the rearrange-
ment reaction, i.e., 1,5-dichloro-hexaorgano-trisilazane, will further
react with diorgano-dichlorosilane. As a result, different 1,3-
dichloro-tetraorgano-disilazanes are formed.
Each of these 1,3-dichloro-tetraorgano-disilazanes tends to
undergo an intermolecular self-redistribution or react with each
In order to facilitate the formation of cyclodisilazanes, we are
initially interested in the addition of deacidification agent which
Scheme 6. The reaction mechanism of Me3SiNHSiRPhCl and R’PhSiCl2 (R ¼ Ph, R’ ¼ Me; or R ¼ Me, R’ ¼ Ph).

Y. Tan et al. / Journal of Organometallic Chemistry 923 (2020) 121414
7
Scheme 7. The reaction mechanism of 1,3-dichloro-tetraorgano-disilazanes with Ph2SiCl2.
may lead to a one-step dehydrohalogenation reaction of 1,3-
dichloro-tetraorgano-disilazane to form ring. The double ring-
system amidine compound 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) is one of the strong base deacidification agents, which
proves satisfactory and, in some way, advantageous effect in the
synthesis of the 1,3-bis(chlorodiorganosilyl)-tetraorgano-cyclo-
disilazanes. When compound 1 is treated with a near-equivalent
quantity of DBU at 80 ꢀC, the reaction proceeded easily. Further-
more, the reaction yield is nearly quantifiable by the 29Si-NMR
spectra inspection of the mixture after 30 min into the reaction.
The cyclization reaction mechanism of 1,3-dichlorotetraorgano-
disilazanes has been substantiated by determining the composition
of the reaction mixture during the reaction via 29Si-NMR spectra. As
shown in Scheme 8, trisilylamine structure is formed initially via
the primary intermolecular dehydrohalogenation of 1,3-dichloro-
tetraorgano-disilazane, followed by the intramolecular dehy-
drohalogenation to produce cyclodisilazanes. The 29Si NMR spectra
of the cyclization reaction of 1,3-dichloro-temethyl-disilazanes is
shown in the supplementary data (Fig. S8).
For the 1,3-bis(chlorodiorganosilyl)-tetraorgano-cyclodisilaza
nes synthesized from 1,3-dichloro-tetraorgano-disilazane with
different substituents on the silicon atoms, exocyclic and endocy-
clic silicon atoms are governed by the steric hindrance effects of the
substituents on the silicon atoms. Due to the less bulky methyl
substituent, dehydrochlorination reaction occurs more easily on
the chlorine atom attached to the Me2SiCl group. Thus, for com-
pound 8, the methyl substituted silicon atoms are endocyclic
instead of exocyclic.
We have successfully prepared 1,3-bis(chlorodimethylsilyl)-
tetramethyl-cyclo-disilazane (5), 1,3-bis(chlorodiphenylsilyl)-tet-
raphenyl-cyclodisilazane (6), 1,3-bis(chloromethyl-phenylsilyl)-
2,4-dimethyl-2,4-diphenyl-cyclodisilazane (7), and 1,3-bis(chlorodi
phenylsilyl)-tetramethyl-cyclodisilazane (8) with good reaction
yields via this method. The products can be purified by the
Scheme 8. The cyclization reaction scheme of 1,3-dichlorotetraorgano-disilazanes in the presence of deacidification agent.

8
Y. Tan et al. / Journal of Organometallic Chemistry 923 (2020) 121414
crystallization and recrystallization from petroleum ether. The 29Si,
1H, and 13C NMR spectra of compound 5e8 are shown in the sup-
plementary data (Fig. S9-12).
The use of a strong organic alkaline deacidification agent in this
procedure provides a new convenient way of synthesizing 1,3-
bis(chlorodiorganosilyl)-cyclo-disilazanes with different struc-
tures. In this procedure, products can be synthesized via a one-step
reaction. This one-step reaction is a simple process which requires
mild synthesis conditions, with short reaction duration and high
yield.
The 29Si NMR spectra chemical shifts and the FT-IR spectra ab-
sorption frequencies of compound 5e8 are summarized in Table 2.
The FT-IR spectra show that the stretching frequency of cyclic Si4N2
skeleton is located at 820e880 cmꢂ1 and 1000e1040 cmꢂ1
[7,19,20], while the stretching frequency of Si-N-Si is located at
676e697 cmꢂ1 and 1012e1115 cmꢂ1, which are consistent with the
absorption frequencies of hexaphenyl-cyclotrisilazane (PN3 ). It
worth noting that PN3 is purified via recrystallization process before
use. The FT-IR spectra of compound 5-8 are shown in the supple-
mentary data (Fig. S13).
According to the NMR analysis results for compound 7, as shown
in Table 2, the 29Si NMR spectrum of compound 7 shows three sets
of signals for the endocyclic and exocyclic silicon atoms, respec-
tively, which validates the existence of various isomeric structures.
This is in contrast to our previous report whereby only endocyclic
silicon atoms are cis/trans-isomers [20], that was determined by
the 300 MHz 29Si NMR spectrum. In order to explain this result, we
compared the 300 MHz 29Si NMR spectrum with 400 MHz 29Si
NMR spectrum of compound 7. The 300 MHz 29Si NMR spectrum
exhibits only two peaks of endocyclic silicon atoms (ꢂ4.39 ppm
and ꢂ3.75 ppm) and one peak of exocyclic silicon atoms
(ꢂ3.73 ppm), which shows that the peaks with a slight chemical
shift cannot be distinguished by using a low field strength 29Si
NMR-300MHz analytical equipment (see Fig. 3).
Fig. 1. X-ray crystal structure of compound 5. Selected bond distances [Å] and angles
[ꢀ]: Si(1)-Cl(1) 2.0867(4), Si(1)-N(1) 1.6924(9), Si(1)-C(1) 1.8455(11), Si(1)-C(2)
1.8420(12), N(1)-Si(2) 1.7456(9), Si(2)-C(3) 1.8466(12), Si(2)-C(4) 1.8535(12), N(1)-
Si(1)-Cl(1) 109.38(3), N(1)-Si(1)-C(1) 111.54(5), N(1)-Si(1)-C(2) 111.25(5), C(1)-Si(1)-
Cl(1) 105.59(4), C(2)-Si(1)-Cl(1) 106.03(5), C(1)-Si(1)-C(2) 112.70(6), Si (1)-N(1)-Si(2)
133.23(5), N(1)-Si(2)-C(3) 113.92(5), N(1)-Si(2)-C(4) 114.58(5), C(3)-Si(2)-C(4)
109.79(7), N(1)-Si(2)-N(1i) 88.77(4), Si (2)-N(1)-Si(2i) 91.23(4).
those of the organocyclodisilazanes determined in the previous
report [20,32] (see Fig. 2).
For compound 7, X-ray diffraction data presents evidence for the
existence of three isomers, which is consistent with the NMR
analysis results.
3. Conclusion
1,3-dichloro-tetraorgano-disilazanes with same substituents on
two silicon atoms can be obtained via a silyl group exchange re-
action of hexamethyldisilazane with diorganodichlorosilane. It can
also be prepared via an equilibrium redistribution reaction of
cyclosilazane with diorganodichlorosilane. However, the reaction
duration is extended because of the generation of trimethyl-
chlorosilane by-products during the exchange reaction. As such,
these by-products need to be removed from the reaction mixture in
time. Thus, the equilibrium redistribution reaction of cyclosilazane
with corresponding diorganodichlorosilane is a more straightfor-
ward and efficient method in the preparation of 1,3-dichloro-
2.3. The crystal and molecular structures of compounds 5-8
Single crystal x-ray analyses for compounds 5-8 are shown in
Fig. 1- Fig. 4. Selected bond and angle parameters are listed in the
captions of these figures. Each of these crystal structures exactly
corresponds to the molecular structure of the compound. The
comparison of N-Si-N and Si-N-Si angle parameters of compounds
5-8 shows that the structures of the four-membered rings in these
compounds are very similar. This result is in good agreement with
Table 2
FT-IR and NMR data for cyclodisilazanes 5e8.
Cyclodisilazanes
29Si NMR spectra
Infrared spectra Absorption frequencies
Chemical shift (ppm)
a
b
Si2N
Si4N2
C-H in phenyl
Ph-Si
Si-Cl
C-H in Methyl
Me-Si
6.03
7.22
1112
676
1037
879
e
e
521
496
3144
3049
2965
2811
e
1406
1258
ꢂ13.13
11.91
1115
697
1003
828
3067
3066
3050
1962
1893
1822
1426
1960
1889
1817
1427
1963
1891
1821
1427
541
556
526
e
ꢂ3.08
ꢂ3.12
ꢂ3.16
ꢂ3.72
ꢂ3.75
ꢂ4.39
1115
691
1029
841
2964
3021
1255
1253
ꢂ13.22
11.49
1115
693
1046
856
2963
2891

Y. Tan et al. / Journal of Organometallic Chemistry 923 (2020) 121414
9
Fig. 2. X-ray crystal structure of compound 6. Selected bond distances [Å] and angles [ꢀ]: Si(1)-Cl(1) 2.0731(4), Si(1)-N(1) 1.6990(9), Si(1)-C(1) 1.8575(11), Si(1)-C(7) 1.8618(12),
Si(2)-N(1) 1.7592(9), Si(2)-C(13) 1.8623(11), Si(2)-C(19) 1.8578(11), N(1)-Si(1)-Cl(1) 106.79(3), N(1)-Si(1)-C(1) 111.59(5), N(1)-Si(1)-C(7) 115.27(5), C(1)-Si(1)-Cl(1) 107.87(4), C(7)-
Si(1)-Cl(1) 106.42(4), C(1)-Si(1)-C(7) 108.54(5), N(1)-Si(2)-C(13) 112.54(5), N(1)-Si(2)-C(19) 113.13(5), C(13)-Si(2)-C(19) 110.26(5), N(1)-Si(2)-N(1i) 88.26(4), Si (2)-N(1)-Si(2i)
91.74(4).
tetraorgano-disilazane with same substituents on two silicon
atoms. The equilibrium redistribution reaction is also a good choice
to prepare 1,3-dichloro-tetraorgano-disilazane with different sub-
stituents on two silicon atoms. However, in this case, certain spe-
cific reaction conditions, such as the ratio of the reactants, reaction
temperature, reaction duration, and the removal of the by-products
are particularly critical.
1,3-bis(chlorodiorganosilyl)-tetraorgano-cyclodisilazane can be
prepared via the intermolecular condensation of two 1,3-dichloro-
tetraorgano-disilazanes, in the presence of a strong organic alkaline
deacidification agent, i.e., DBU. This simple and efficient method
involves mild synthesis conditions, and it is easy to prepare in
batches. It is also a suitable method to prepare 1,3-bis(chlorodi
organosilyl)-tetraorgano-cyclodisilazane from two different 1,3-
dichloro-tetraorgano-disilazanes with a variety of substituent
groups on the silicon atoms.
4.2. Synthesis of 1,3-dichlorotetraorganodisilazane (1e4)
A mixture of 11.2 g (0.05 mol) hexamethylcyclotrisilazane and
32.3 g (0.25 mol) dimethyldichlorosilane is refluxed for 4 h. The
mixture containing 90% of compound 1 and 10% of the 1,5-
dichlorohexamethyltrisilazane are detected by the 29Si NMR spec-
trum. After distillation, 23.0 g of compound 1 is obtained (reaction
yield of 76%).
A
mixture of hexaphenylcyclotrisilazane and diphenyldi-
chlorosilane with a molar ratio of 5:1 is refluxed for 5 h. After the
reaction, 89% of compound
2 and 11% of 1,5-dichlorohex
aphenyltrisilazane are detected in the mixture from the 29Si NMR
spectrum. After cooling the mixture, it is purified via crystallization
process in toluene to obtain white crystal compound 2 (reaction
yield of 78%).
A mixture of 2,4,6-trimethyl-2,4,6-triphenylcyclotrisilazane and
methylphenyl-dichlorosilane with a 5:1 M ratio is refluxed for 4 h.
The product contains 90% of compound 3 and 10% 1,5-dichloro-
1,3,5trimethyl-1,3,5triphenyltrisilazane according to the 29Si NMR
spectrum. The reaction yield of compound 3 is 76% after distillation.
4. Experimental
A
mixture of hexamethylcyclotrisilazane and diphenyldi-
4.1. General
chlorosilane with a 2:1 M ratio is refluxed for 3 h. The product
contains 70% of compound 4, 17% of compound 1, 8% of compound
2, and 5% of compound 4c according to the 29Si NMR spectrum. The
reaction yield of compound 4 is 56% after distillation.
All reaction vessels and solvents are dried adequately and
distilled before use. The reactants are added to the reaction unit
under the protection of an inert nitrogen atmosphere. It is to
eliminate the presence of moisture during the reaction. The NMR
spectra are tested using Bruker AVANCE 400 (1H, 13C, 400 MHz) and
Bruker AVANCE 400 IIþ (29Si, 400 MHz). The chemical shifts of 1H
and 13C NMR spectra are relative to the residual chloroform (dH
7.26) and the central peak of CDCl3 (dC 77.0). The chemical shifts of
the 29Si NMR spectra are relative to the tetramethylsilane (dSi 0.0).
Fourier transfer infrared spectra (FT-IR) are recorded using a Bruker
Tensor 27 IR (KBr) spectrometer. Elemental analysis is performed
using a FlashEA 1112 microanalytical analyzer.
4.3. Synthesis of 1,3-bis(chlorodiorganosilyl)-
tetraorganocyclodisilazane (5e8)
The mixture of 10.1
g (0.05 mol) of 1,3-dichloro-1,1,3,3-
tetramethyldisilazane (1) and 5 mL of hexane is heated at 80 ꢀC
and is stirred well. Then, a solution of 7.98 g (0.0525 mol) DBU in
4 mL of hexane is added dropwise into the mixture. The mixture is
later incubated for 30 min at 80 ꢀC. After which, the salts are

10
Y. Tan et al. / Journal of Organometallic Chemistry 923 (2020) 121414
Fig. 3. X-ray crystal structure of compound 7. Selected bond distances [Å] and angles [ꢀ]: Si(1)-Cl(1) 2.047(3), Si(1)-N(1) 1.6974(12), Si(1)-C(1) 1.8612(15), Si(1)-C(7) 1.865(8), Si(2)-
N(1) 1.7552(12), Si(2)-C(14) 1.8498(16), Si(2)-C(8) 1.8674(15), N(1)-Si(1)-Cl(1) 107.63(11), N(1)-Si(1)-Cl(1A) 113.46(14), N(1)-Si(1)-C(1) 112.93(6), N(1)-Si(1)-C(7) 111.3(5), N(1)-Si(1)-
C(7A) 104.9(7), Si (1)-N(1)-Si(2) 129.38(7), C(1)-Si(1)-Cl(1) 108.47(11), C(1)-Si(1)-C(7) 109.2(5), N(1)-Si(2)-C(8) 113.15(6), N(1)-Si(2)-C(14) 115.42(7), C(14)-Si(2)-C(8) 110.83(7),
N(1)-Si(2)-N(1i) 88.23(6), Si (2)-N(1)-Si(2i) 91.77(6).
immediately filtered off. The liquid portion is distilled in a vacuum to
evaporate the solvent, and the residue is recrystallized from the
petroleum ether to yield 7.28 g white crystal of compound 5 (reac-
N2Si(CH3)], 2.69e2.92 [s, 2C,Si(CH3)Cl], 127.24e137.03 [m,24C,
Si(C6H5)]. Anal. Calcd. for C28H32Si4N2Cl2 (%):C, 58.03; H, 5.53; N,
4.84; Cl,12.26. Found (%): C, 57.45; H, 5.95; N, 4.74.
tion yield of 88%). Mp. 68 ꢀCe69 ꢀC. 1H NMR:
d
0.40 [s, 12H,
d 4.47,[s, 4C,
A mixture of 9.78g (0.03mol) of 1,3-dichloro-1,1-dimethyl-3,3-
diphenyldisilazane (4) and 8 mL methylbenzene is heated at 110 ꢀC,
and a solution of 5.02 g (0.033mol) DBU and 5 mL methylbenzene is
added dropwise. The mixture is then incubated for 1h at 110 ꢀC; After
recrystallization, 6.8 g white crystal of compound 8 is obtained (re-
N2Si(CH3)], 0.378 [s, 12H, Si(CH3)Cl], 13C NMR:
N2Si(CH3)], 4.58 [s, 4C, Si(CH3)Cl]. Anal. Calcd. for C8H24Si4N2Cl2 (%):
C, 29; H, 7.25%; N, 8.46; Cl, 21.45. Found (%): C, 28.58; H, 7.24; N, 8.20.
A
mixture of 4.5 g (0.01 mol) 1,3-dichloro-1,1,3,3-tetraph
enyldisilazane (2), 5 mL xylene, and 5 mL octane is heated at 130 ꢀC,
and a solution of 1.82 g (0.012 mol) DBU,1 mL xylene, and 1 mL octane
is added dropwise. The mixture is then incubated for 3.5 h at 130 ꢀC.
After recrystallization, 3.1 g white crystal of compound 6 is obtained
action yield of 78%). Mp.142.2e143.7 ꢀC. 1H NMR:
[s,12H,N2Si(CH3)],
127.61e134.80 [m,20C,Si(C6H5)]. Anal. Calcd. for C28H32Si4N2Cl 2 (%):
d
0.099
d
7.414e7.346 [s,20H,Si(C6H5)Cl]. 13C NMR:
d
C, 58.03; H, 5.53; N, 4.84; Cl,12.26. Found (%):C, 57.51; H, 5.50; N, 4.84.
(reaction yield of 75%). Mp. 259 ꢀCe260 ꢀC. 1H NMR:
d
7.1e7.22 [m,
6.91e7.53 [m,20H,Si(C6H5)Cl]. 13C NMR:
127.00e135.70 [m,48C,Si(C6H5)]. Anal. Calcd. for C48H40Si4N2Cl2 (%):
20H,N2Si (C6H5)],
d
4.4. Crystal structure determination of compounds 5-8
d
C, 69.65; H, 4.84; N, 3.39; Cl, 8.59. Found (%):C, 69.59; H, 4.83; N, 3.38.
A mixture of 3.3 g (0.01 mol) 1,3-dichloro-1,3-dimethyl-1,3-
diphenyldisilazane (3) and 5 mL methylbenzene is heated at
110 ꢀC, and a solution of 2.28 g (0.015 mol) DBU and 1 mL meth-
ylbenzene is added dropwise. The mixture is then incubated for 1h
at 110 ꢀC. After recrystallization, 2.4 g white crystal of compound 7
is obtained (reaction yield of 83%). Mp.118.7ꢀCe119.3 ꢀC. 1H NMR:
Single crystals of the compounds 5-8 are obtained by slowly
evaporating their respective solutions in diethyl tetrahydrofuran/
heptane in a glove box at room temperature. A suitable crystal is
selected and carried out using a XtaLAB Synergy R, HyPix diffrac-
tometer. The crystal is kept at 169.97(12) K during the data
collection process. Using Olex2 [33], the structure is solved with the
ShelXT [34] structure solution program using Intrinsic Phasing, and
it is refined with the ShelXL [35] refinement package using Least
Squares minimisation.
d
0.31e0.35 [s, 6H, N2Si(CH3)],
d
0.51e0.74 [s, 6H, Si(CH3)Cl],
0.53e1.15 [s, 2C,
7.05e7.58 [m, 20H, Si(C6H5)]. 13C NMR:
d

Y. Tan et al. / Journal of Organometallic Chemistry 923 (2020) 121414
11
Fig. 4. X-ray crystal structure of compound 8. Selected bond distances [Å] and angles [ꢀ]: Si(1)-Cl(1) 2.0776(5), Si(1)-N(1) 1.6901(11), Si(1)-C(1) 1.8616(14), Si(1)-C(7) 1.8574(14),
Si(2)-N(1) 1.7500(11), Si(2)-C(13) 1.8448(15), Si(2)-C(14), 1.8513(15), N(1)-Si(1)-Cl(1) 109.11(4), N(1)-Si(1)-C(1) 112.86(6), N(1)-Si(1)-C(7) 109.45(6), Si(1)-N(1)-Si(2) 135.05(7), C(1)-
Si(1)-Cl(1) 106.56(5), C(7)-Si(1)-Cl(1) 107.80(5), C(1)-Si(1)-C(7) 110.88(6), N(1)-Si(2)-C(13) 114.99(7), N(1)-Si(2)-C(14) 112.92(6), C(13)-Si(2)-C(14) 109.69(7), N(1)-Si(2)-N(1i)
88.40(5), Si(2)-N(1)-Si(2i) 91.60(5).
[3] W. Fink, Helv. Chim. Acta 52 (1969) 1841e1847.
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Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
[11] W. Fink, U. S. Jpn. Outlook (1971), 3575922.
Appendix A. Supplementary data
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Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2020.121414.
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Supporting information
NMR spectra of the synthesized compounds as well as synthetic
procedures.
Author contributions
The manuscript was written through contributions of all au-
thors. All authors have given approval to the final version of the
manuscript.
[28] Y. Zhu, L. Guo, Z. Zhang, Z. Xie, C. Xu, J. Appl. Polym. Sci. 105 (2007) 749e756.
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Funding
[31] U. Wannagat, P. Schmidt, M. Schulze, Angew. Chem. Int. Ed.
6 (1967)
This research did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.
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