Intramolecular rearrangement of organosilyl groups between oxygen and nitrogen in aminosiloxanes: A joint experimental-theoretical study

Article abstract of DOI:10.1016/S0022-328X(03)00551-5

Lithium amino-di- tert -butysilanolate reacts with halosilanes to give 1-silylamino-1,3-siloxanes (1-4). The tetrakis(1-silylamino)siloxane 4 thermally condenses yielding a spiocyclic six-membered ring (5) and NH₃. Lithium salts of animo-disiloxanes form silylamino-silanolates or amido-disiloxanes. The first includes a 1,3-silyl group migration from the oxygen to the nitrogen atonm. The energies of the isomeric lithium salts of model compunds are calculated. Exemplary crystal structures are presented.(SiOSiN)-four- or eight-membered rings (6,7) are thermally obtaines by LiF-elimination from fluoro containing salts. In further reactions of lithiated animo-disiloxanes with halosilanes isomeric 1-silylamino-1,3-disiloxanes can be obtaines (15,16). The 1,3-silyl group migration can sterically (9) or electronically (10-14) be prevented.The most stable lithium salts of 1-silylamino-1,3-disiloxanesform amides. This explains that in further reactions with halosilanes, the new ligand is bonded wo the nitrogen atom (23-25). In results of crystal structure determinations new lithium-1-fluorosilylamio-1,3-disiloxanes (19-21) are presented. 19 crystallizes as tricyclic, 20 as an unknown pentacyclic, and 21 as monomeric compound. In 21, the shortest Si-N bond length (157.9 pm) with four coordinate silicon is found.

Full text of DOI:10.1016/S0022-328X(03)00551-5

Journal of Organometallic Chemistry 686 (2003) 16ꢀ23
/
www.elsevier.com/locate/jorganchem
Intramolecular rearrangement of organosilyl groups between oxygen
and nitrogen in aminosiloxanes: A joint experimentalꢀtheoretical
study
/
Susanne Kliem a, Uwe Klingebiel a,*, Stefan Schmatz b,*
a Institut fur Anorganische Chemie der Universitat Gottingen, Tammannstr. 4, D-37077 Gottingen, Germany
¨
¨
¨
¨
b Institut fur Physikalische Chemie der Universitat Gottingen, Tammannstr. 6, D-37077 Gottingen, Germany
¨
¨
¨
¨
Received 28 March 2003; received in revised form 27 May 2003; accepted 27 May 2003
Abstract
Lithium amino-di-tert-butysilanolate reacts with halosilanes to give 1-silylamino-1,3-siloxanes (1
/
4). The tetrakis(1-silylami-
ꢀ
no)siloxane 4 thermally condenses yielding a spiocyclic six-membered ring (5) and NH3. Lithium salts of animo-disiloxanes form
silylamino-silanolates or amido-disiloxanes. The first includes a 1,3-silyl group migration from the oxygen to the nitrogen atonm.
The energies of the isomeric lithium salts of model compunds are calculated. Exemplary crystal structures are presented.
(SiOSiN)-four- or eight-membered rings (6,7) are thermally obtaines by LiF-elimination from fluoro containing salts. In further
reactions of lithiated animo-disiloxanes with halosilanes isomeric 1-silylamino-1,3-disiloxanes can be obtaines (15,16). The 1,3-silyl
group migration can sterically (9) or electronically (10
The most stable lithium salts of 1-silylamino-1,3-disiloxanesform amides. This explains that in further reactions with halosilanes,
/14) be prevented.
ꢀ
21) are presented. 19 crystallizes as tricyclic, 20 as an unknown pentacyclic, and 21 as monomeric compound. In
the new ligand is bonded wo the nitrogen atom (23
/
25). In results of crystal structure determinations new lithium-1-fluorosilylamio-
ꢀ
1,3-disiloxanes (19
/
ꢀ
21, the shortest Si-N bond length (157.9 pm) with four coordinate silicon is found.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Aminosiloxanes; Lithium salts; Isomerization
forming a Liꢁ/O cubane [6], while the sodium and
1. Introduction
potassium salts are described as hexagonal prisms [7].
These structural elements are stable even in the gas
phase [6,7].
In contrast to carbon chemistry, compounds with two
or three H-acidic electronegative groups like NH2 or
In the present work, we report on recent experimental
results and rationalize them by means of quantum
chemical calculations. All theoretical results given in
this work were obtained from density-functional calcu-
lations employing the variant B3LYP and the large 6-
OH at one silicon atom can be stabilized kinetically [1ꢀ
/
5]. In the 1980s, the first aminosilanol, (Me3C)2Si(N-
H2)OH [6], that crystallizes in ladder-like chains via H-
bridges could be prepared [7]. Its alkaline metal
derivatives form an aminosilanolate anion that is 15.9
kcal molꢁ1 more stable than the isoelectronic amidosi-
lanol anion.
311ꢂG(2d,p) basis set.
/
(Me3C)2Si(NH2)Oꢀ;
(Me3C)2Si(OH)NHꢀ
2. Results and discussion
The lithium salt was characterized as tetramer,
2.1. 1-Aminosiloxanes
* Corresponding authors. Tel.: ꢂ
393373.
E-mail address: uklinge@gwdg.de (U. Klingebiel).
/
49-551-393052; fax: ꢂ49-551-
/
The stability of the aminosilanolate salts explains the
position of substituents, e.g. in reactions with halosi-
0022-328X/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00551-5

S. Kliem et al. / Journal of Organometallic Chemistry 686 (2003) 16ꢀ
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lanes, numerous mono-, bis-, tris- and tetrakis-1-amino-
siloxanes are formed [7ꢀ10]. For example:
In fluorine-containing compounds, lithium often is
coordinated to the fluorine atom. The most stable
/
isomer of the model compound MeSiF2ꢁ
Me2OLi forms a six-membered (LiꢁOꢁSiꢁNꢁ
ring system with LiÁ Á ÁF contact (III).
/
NHꢁ
Siꢁ
/
Si-
/
/
/
/
/
F)
The tetrakis-(1-amino-siloxane) (4) condenses ther-
mally yielding a spirocyclic six-membered ring and NH3
[11].
2.2. Lithium-1-amido-1,3-disiloxanes and 1,3-
disilylamino-1-olates
Starting with 1-amino-siloxanes, further substitutions
turn out to be more complicated. Lithium salts of these
compounds form, depending on the properties and
bulkiness of the silyl groups, 1,3-disilylamino-1-olates
or the less stable 1-amido-1,3-disiloxanes [7ꢀ11]. The
/
second case includes a 1,3-silyl group migration from the
oxygen to the nitrogen atom, as is well-known from
silylhydroxylamine chemistry [12].
Regarding to Diagram (1) lithium salts of aminodisi-
loxanes are usually isolated as 1,3-disilazane-1-olates
(III). The isomeric 1-amido-1,3-disiloxane (IV) can be
formed via the following transition state,
In order to understand the driving force of this
isomerization, we calculated the energies of the isomeric
lithium salts of the model compounds I and II and
found that I is 0.7 kcal molꢁ1 more stable than II (Fig.
1).
which affords 20.8 kcal molꢁ1
The 1,3-(O0N)-silyl group migration can be sup-
.
/
pressed by bulky groups and 1-amido-1,3-disiloxanes
(IV) are stabilized kinetically, e.g. the lithium salt of 1-
amino-1,1-di-tert-butyl-3-fluoro-3,3-bis(isopropyltri-
methylsilyl)amine crystallizes as type IV and forms the
Without an O0
molꢁ1 less stable than I. The Liꢁ
the Me2SiꢁO bond (I: Me2SiꢁOꢃ
Oꢃ192.8 pm) and lengthens the Me2Siꢁ
Me2SiꢁNꢃ184.1 pm; II: Me2SiꢁNꢃ179.8 pm).
The small energetic difference between I an II explains
the formation of structural isomers in reactions with
halosilanes.
/
Li donor bond (IIa) is even 9.0 kcal
O contact in I shortens
159.5 pm; II: Me2Siꢁ
N bond (I:
/
/
/
/
/
/
/
hitherto unknown six-membered (Fꢁ
ring as a dimer via the LiꢁN bond (Fig. 2) [7].
Lithium salts of types V and IV could not be detected
so far.
/
Siꢁ
/
Oꢁ
/
Siꢁ
/
Nꢁ/Li)
/
/
/
/
/

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S. Kliem et al. / Journal of Organometallic Chemistry 686 (2003) 16ꢀ23
/
Fig. 1. Relative energies (kcal molꢁ1) of isomeric lithium salts Iꢀ
II.
/
3. (SiOSiNH)-eight-and-four-membered rings
3.1. 1,5-Diaza-3,7-dioxa-2,4,6,8-tetrasilacyclo-octanes
Experimental results show that LiF elimination from
lithiated 1-fluoro-1,3-disilazane-1-olates (type III) leads
to the formation of eight-membered (SiOSiN)2 rings
[9,11], e.g.
With halosilanes it is possible to substitute the ring by
silyl groups at the nitrogen atom.
The structure determinations of known eight-mem-
bered (Siꢁ
/
Oꢁ
/
Siꢁ
/
NH)2 rings and also of 6 show mostly
effectively planar rings. Such backbones in molecular
4. 1-Silylamino-1,3-disiloxanes
structures are rare. Particularly unusual features of these
compounds are the virtually linear Siꢁ
/
Oꢁ
/
Si units in the
X-ray structures and calculations of lithium salts of
amino-1,3-disiloxanes, types I and II or types III and IV
explain that in further reactions of these compounds
with halosilanes isomeric 1-silylamino-1,3-disiloxanes
can be obtained [8,9,11]. In reactions of I or III with
halosilanes, the new organosilicon group is bonded to
the oxygen, and in the reactions of II and IV to the
nitrogen atom.
ring (Fig. 3).
3.2. 1-Aza-3-oxa-2,4-disilacyclobutanes
Lithium salts of type IV are kinetically stabilized. LiF
elimination of these salts leads to the formation of four-
membered (Siꢁ
/
Oꢁ
/
Siꢁ/NH) rings, e.g.
The lithium salt of 7 (8) crystallizes as a dimer
forming a four-membered (LiꢁN)2 ring system [10].
The SiÁ Á ÁSi nonbonding distance in the (SiꢁOꢁSiꢁN)
ring amounts to only 237.2 pm, which is in the range of
a typical SiꢁSi single bond.
4.1. Retention of the siloxane unit
/
/
/
/
4.1.1. Kinetically prevented rearrangement
Bulky groups kinetically prevent the silyl group
migration from the oxygen to the nitrogen atom and
/

S. Kliem et al. / Journal of Organometallic Chemistry 686 (2003) 16ꢀ
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23
19
The easy formation of N-difluorosilylamino-1,3-dis-
iloxanes with retention of the siloxane unit promoted us
to study the relative energies of model compounds VII
and VIII.
Fig. 2. Crystal structure of
.
hence the isomerization in the lithiation reaction (type
II), e.g.
We found that VIII is 0.8 kcal molꢁ1 more stable
than VII. Though this is not very much, we could not
isolate isomeric compounds of 10ꢀ14.
/
4.2. Formation of isomeric silylamino-1,3-disiloxanes
In the absence of strong steric or electronic restraints,
the lithium salts of 1-amino-1,3-pentaorganyl disilox-
anes (types I and II) react with halosilanes, e.g.
F2SiPhCMe3 to give both isomeric silylamino-1,3-dis-
iloxanes [8], e.g.
4.1.2. Thermodynamically prevented rearrangement
Starting with lithium salts of 1-amino-1,1,3,3,3-pen-
taorganyl-1,3-disiloxanes (type I), reactions with tri-
fluorosilanes lead only to 1-difluorosilylamino-1,3-
disiloxanes with retention of the siloxane unit.
4.3. 1,3-O0
/
N-silyl group migration*formation of the
/
isomeric 1-silylamino-1,3-disiloxane
Only the rearranged isomer of 1-silylamino-1,3-dis-
iloxane could be detected and obtained in reactions of
halotriorganylsilanes with trifluoro(organyl)silanes with
lithiated 1-amino-1,3-disiloxanes. Trifluoro(organyl)si-
lanes react with the lithium salt of 1-amino-1,1-di-tert-
butyl-3,3-difluoro-3-tert-butyl-1,3-disiloxane, e.g.
Fig. 3. Structure of 6.

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/
Fig. 5. Structure of 14; selected bond lengths (pm) and angles (8):
Si(1)ꢁF(1), 158.6; Si(1)ꢁN(1), 168.5; Si(2)ꢁO(1), 160.6; Si(2)ꢁN(1),
173.4. Si(1)ꢁN(1)ꢁSi(2), 135.8; Si(2)ꢁO(1)ꢁSi(3), 177.2.
5. Crystal structure determinations of 1-
difluorosilylamino-1,3-disiloxanes (13, 14)
/
/
/
/
/
/
/
/
The retention of the siloxane unit in the reaction of 1-
amino-1,3-disiloxanes with BuLi and trifluorosilanes
could be proved by X-ray crystallography of com-
pounds 13 and 14. Compounds 13 and 14 crystallize
from n-hexane as monomers in the monoclinic crystal
system space groups P21/c and P21/n, respectively. In
both compounds, NÁ Á ÁH- or FÁ Á ÁH-donor bonds do not
exist (Figs. 4 and 5).
6. Lithium-1-silylamido-1,3-disiloxanes
No silyl group migration is observed in reactions of
lithiated 1-silylamino-1,3-disiloxanes.
Quantum chemical calculations of model compounds
show that in this case the most stable lithium salt is 1-
lithium-silylamido-1,3-disiloxane (IX), which is 19.2
kcal molꢁ1 more stable than the possible olate isomer
X.
Steric effects may be the reason for the wide angle at
the oxygen atom.
Because of the electron-withdrawing effect of the
fluorine atoms, the Siꢁ/N bond lengths in neighborhood
to SiF2 group are shorter than the others. The N atoms
have a planar environment. The bulky groups at the
siloxane unit are the reason for the large Siꢁ
/
Oꢁ/Si angle
(13: 172.378, 14: 177.178).
The N atom in X has an almost planar environment
with a LiÁ Á ÁN contact of 216.5 pm. The LiÁ Á ÁN contact
in IX is only 184.1 pm.
In lithium salts of fluoro-containing 1-silylamino-1,3-
disiloxane, the hard Lewis acid lithium additionally
binds the hard Lewis base fluorine (Diagram (4)).
Again, the silyl group migration affords too much
Fig. 4. Structure of 13; selected bond lengths (pm) and angles (8):
Si(1)ꢁF(1), 156.26; Si(1)ꢁN(1), 169.16; Si(1)ꢁN(2), 166.62; N(1)ꢁ
Si(2), 174.99; N(2)ꢁSi(4), 174.00; Si(4)ꢁO(1), 159.59; O(1)ꢁSi(5),
161.56. Si(1)ꢁN(2)ꢁSi(4), 130.78; Si(4)ꢁO(1)ꢁSi(5), 172.37.
/
/
/
/
/
/
/
/
/
/
/

S. Kliem et al. / Journal of Organometallic Chemistry 686 (2003) 16ꢀ
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23
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energy, so that the new ligand will be bonded at the
nitrogen atom.
The most stable lithium salt of the isomers VII and
VIII forms the bicyclic compound XI of two four-
membered rings connected by the Liꢁ
/
N unit.
Lithium is coordinated to the nitrogen, fluorine and
oxygen atoms. The SiF2Me group is bonded to the
nitrogen atom and consequently the SiMe3 group to the
oxygen atom.
Fig. 6. Crystal structure of 19; selected bond lengths (pm) and angles
N(2),
(8): Si(1)ꢁ
171.1; Si(2)ꢁ
F(1), 181.8; Li(1)ꢁ
152.8; Si(1)ꢁN(1)ꢁ
/
F(2), 159.3; Si(1)ꢁ
N(2), 167.2; Si(2)ꢁ
N(1??), 193.2; Li(1)ꢁ
Si(2), 145.8; Si(1)ꢁF(1)ꢁ
/
F(1), 164.7; Si(1)ꢁ
O(1), 168.7; Si(3)ꢁ
O(1??), 200.7. Si(3)ꢁ
Li(1), 165. 6.
/
N(1), 160.9; Si(1)ꢁ
/
/
/
/
O(1), 164.3; Li(1)ꢁ
/
/
/
/
O(1)ꢁ
/
Si(2),
/
/
/
/
The other four lithium salts form isomeric amides as
six-membered rings (XII, XIV, XV) with LiÁ Á ÁF contact
or a four-membered ring (XIII) with LiÁ Á ÁO contact.
The two most unstable isomers are bis(silylamino)-
silanolates with Oꢁ
contacts, respectively. The stability of the lithium-silyl-
amides (XIꢀXV) explains that in further reactions with
halosilanes the new ligand will be bonded at the nitrogen
atom.
/
LiÁ Á ÁF2 (XIV) and OꢁLiÁ Á ÁF (XV)
/
The crystal structure of 19 (Fig. 6) shows some
irregularities: one SiꢁF bond length is, because of the
LiÁ Á ÁF contact, 5.3 pm longer than the other. The Si(1)ꢁ
N(1) bond length is in the range of a double bond
length. The LiÁ Á ÁO contact lengthens the Si(2)ꢁO(1)
bond. The LiꢁF contact is unusually short and even
shorter than the SiꢁN bond. Lithium is only three-fold
coordinated (a Liꢃ349.78).
/
/
/
/
/
/
o
/
7. Crystal structure determinations of lithium-1-
silylamido-1,3-disiloxanes
7.2. Crystal structures of lithiated 2,4,6-(Me3C)3C6H2-
SiF2NHSi(CMe3)2-O-SiMe3 (20, 21)
In chemical experiments, lithium salts of types XIII
and XIV could be isolated as dimers forming a tricyclic
compound existing as two (SiOSiN) four-membered
rings connected by (LiFSiN)2 eight-membered ring 19
and a spiro-pentacyclic compound 20 existing as two
(SiOSiN), two (SiF2Li) four-membered rings which are
connected by (LiNSiF2)2 eight-membered ring system.
Compound 20 (Fig. 7) crystallizes from n-hexane as a
¯
dimer in the space group P1:
/
Lithium has a tetrahedral coordination: two fluorine,
one oxygen and one amido contact.
This coordination of the lithium ion was unknown so
far. The Siꢁ/F bonds are equal in length (164.7 pm).
Depending on the used solvents, 14 reacts with n-
BuLi to give 20 or 21. 20 is formed in n-hexane and 21 in
thf.
The lithium salt of 14, which is crystallized from thf
(21) (Fig. 8), is monomeric.
7.1. Crystal structure of (Me3Si)2N-SiF2-NLi-
Si(CMe3)2-O-SiMe3 (19)
The lithium compound 19 is obtained in high yields
from the reaction of 13 with n-BuLi, thus confirming the
structure of 19.
Crystallization of 20 from thf (21) leads to a complete
rupture of the LiÁ Á ÁN contact. Lithium now has
migrated to the stronger Lewis base fluorine. The result

22
S. Kliem et al. / Journal of Organometallic Chemistry 686 (2003) 16ꢀ23
/
Fig. 9. Part of structure of 21.
bond length, and the shortest Siꢁ
coordinate silicons, as far as we know. The Siꢁ
angle of 167.28 is typical of an imine. Therefore, the
molecule must be considered as LiꢁF adduct of an
iminosilane, as shown in Fig. 9.
/
N bond with four
/
NꢁSi
/
/
8. Reactions of lithium-1-fluorosilylamido-1,3-disiloxanes
Fig. 7. Structure of 20; selected bond lengths (pm) and angles (8):
Si(1)ꢁ
/
N(1), 160.7; O(1)ꢁ
/
Si(3), 164.2. F(1)ꢁ
/
Li(1a)ꢁF(2), 69.6.
/
is a short LiÁ Á ÁF bond (186.6 pm) and a long Siꢁ
/
F bond
8.1. Formation of cyclosilazanes
(165.9 pm). The SiꢁN bond is with 157.9 pm, a double
/
Lithium salts of 1-fluorosilylamido-1,3-disiloxanes
(XI) thermally lose LiF with formation of siloxane-
substituted cyclodisilazanes, e.g.
8.2. Synthesis of 1,1-bis(silylamino)-1,3-disiloxanes
Regarding these quantum chemical calculations and
the experiments, it was to be expected that lithium salts
of 1-silylamino-1,3-disiloxanes react with further fluor-
osilanes under retention of the configuration to give 1,1-
bis(silylamino)-1,3-disiloxanes.
Fig. 8. Crystal structure of 21; selected bond lengths (pm) and angles
(8): Si(1)ꢁN(2), 157.9; Si(2)ꢁN(2), 163.2; Si(1)ꢁF(1), 165.9; Si(1)ꢁF(2),
162.0. Li(1)ꢁF(1), 186.6.
/
/
/
/
/

S. Kliem et al. / Journal of Organometallic Chemistry 686 (2003) 16ꢀ
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23
23
isomerization reaction can be sterically prevented by
bulky groups and electronically by difluorosilyl groups
as second substituent. LiF elimination from lithium salts
of fluorofunctional amino-1,3-disiloxanes leads to four-
or eight-membered (SiNSiO) ring systems. No silyl
group migration occurs in reactions of 1-silylamino-
1,3-disiloxanes with lithium organyls. Always the
lithium-1-silylamido-1,3-disiloxane is formed. No iso-
merization occurs for that reason and a third silyl group
will be bonded at the nitrogen atom.
The chemical behavior of aminosilanoles is proved by
experiments, crystal structure determinations and quan-
tum chemical calculations.
Fig. 10. Structure of 25.
Acknowledgements
We are grateful to the Deutsche Forschungsge-
meinschaft and Fonds der Chemischen Industrie for
support of this work.
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/
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¨
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