Synthesis and molecular structure of 1-(2-pyridyloxy)silatrane

Article abstract of DOI:10.1134/S1070363208120074

1-(2-Pyridyloxy)silatrane was synthesized by trans-etherification of 1-ethoxysilatrane with 2-hydroxypyridine as well as by the reaction of the latter with tetraethoxysilane and triethanolamine. Its structure was established by the XRD analysis of a single crystal, and in solution using the methods of ¹H, ¹³C, ²⁹Si NMR and IR spectroscopy.

Full text of DOI:10.1134/S1070363208120074

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 12, pp. 2333–2338. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © M.G. Voronkov, E.А. Zelbst, Yu.I. Bolgova, О.М. Trofimova, A.I. Albanov, N.N. Chipanina, T.N. Aksamentova, А.А. Korlyukov,
M.Yu. Antipin, 2008, published in Zhurnal Obshchei Khimii, 2008, Vol. 78, No. 12, pp. 1988–1993.
Synthesis and Molecular Structure
of 1-(2-Pyridyloxy)silatrane
M. G. Voronkova, E. А. Zelbsta, Yu. I. Bolgovaa, О. М. Trofimovaa, A. I. Albanova,
N. N. Chipaninaa, T. N. Aksamentovaa, А. А. Korlyukovb, and M. Yu. Antipinb
a Favorskii Irkutsk Institute of Chemistry, Siberian Branch, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: voronkov@irioch.irk.ru
b Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
ul. Vavilova 28, Moscow, 119991 Russia
Received July 1, 2008
Abstract―1-(2-Pyridyloxy)silatrane was synthesized by trans-etherification of 1-ethoxysilatrane with 2-
hydroxypyridine as well as by the reaction of the latter with tetraethoxysilane and triethanolamine. Its structure
1
was established by the XRD analysis of a single crystal, and in solution using the methods of Н, 13С, 29Si
NMR and IR spectroscopy.
DOI: 10.1134/S1070363208120074
An attempt to prepare the first representative of 1-
(heteryloxy)silatranes, 1-(2-pyridyloxy)silatrane (I) 2-
PyOSa (Sa = Si(OCH2CH2)3N)) was performed in our
laboratory already 30 years ago by a reaction of 2-
pyridone with 1-ethoxysilatrane [1]. On the basis of
2-hydroxypyridine (III) with tetraethoxysilane and
triethanolamine along the earlier described procedure
[2, 3] also failed [4]. However, Iovel et al. succeeded to
synthesize 1-(3-pyridyloxy)silatrane using this route
[4].
1
the data of IR and Н NMR spectroscopy the product
Here we report a successful trans-etherification of
1-ethoxysilatrane with 2-hydroxypyridine that resulted
in the so far unknown 1-(2-pyridyloxy)silatrane (I)
(Scheme 1а).
of the reaction was assigned the structure of 1-(N-2-
pyridonyl)silatrane (II) [1-С5Н4N(=O)Sа]. Later on we
failed to reproduce these data. Lately an attempt to
obtain 1-(2-pyridyloxy)silatrane (I) by the reaction of
NaOEt,
о-xylene
N
OH + EtOSi(OCH2CH2)3N
N
OSi(OCH2CH2)3N + HOEt
(1а)
III
I
mixture of tetraethoxysilane and triethanolamine
according to [2, 3] (Scheme 1b).
The product of reaction (1а) 2-PyOSa (I) is a
colorless crystalline substance with mp 196°С, easily
hydrolyzed with the air moisture.
The structure of 2-PyOSa (I) was established in a
crystal by the method of XRD, and in the solution, by
means of 1Н, 13С, 29Si NMR, and IR spectroscopy.
We have also synthesized 1-(2-pyridyloxy)silatrane
(I) by the reaction of 2-hydroxypyridine (III) with the
KOH
N
OH + Si(OEt)4 + (HOCH2CH2)3N
I + 4 EtOH
(1b)
III
2333

2334
VORONKOV et al.
13
Using 1Н, С, 15N, and 29Si NMR spectroscopy we
established that in the CDCl3 solution, along with 2-
PyOSa (I), a small amount of the products of its
hydrolysis caused by the water adsorbed on the surface
of the glass was present, 2-pyridone (IIIb) and 1-
silatranol (IV) (Table 1, Scheme 2).
2-Hydroxypyridine (III) is known to be capable of
prototropic tautomerism [5]. In the crystal or nonpolar
solvent it exists as a mixture of two tautomeric forms,
OH (IIIа) and NH (IIIb).
The ratio I:IIIb was 1:0.23. After 48 h the ratio
changed to 0.87:0.60, and after 168 h 2-PyOSa (I) was
completely transformed to IIIb, 1-silatranol (IV) and
the hitherto unknown disilatranyloxane (V) in the
molar ratio of 1.5:0.5:0.5.
N
OH
N
H
O
IIIа
IIIb
N
OSi(OCH2CH2)3N + H2O
N
H
O + HOSi(OCH2CH2)3N
(2)
I
IIIb
IV
N(CH2CH2O)3SiOSi(OCH2CH2)3N
V
Quantum-chemical calculations at the DFT level
(B3LYP/6-311G**) using the Gaussian 98 program
package [6] of the isolated molecules of 1-(2-pyridyl-
oxy)silatrane (I) and 1-(N-2-pyridonyl)silatrane (II), as
well as of 2-hydroxypyridine III in its ОН (IIIа) and
NH (IIIb) tautomeric forms led to the following
results. According to calculations, 2-pyridone (IIIb) is
by only 0.85 kcal mol–1 more stable than 2-hydroxy-
pyridine IIIa. On the contrary, 1-(2-pyridyloxy)-
silatrane I is by 9.71 kcal mol–1 more stable than 1-(N-
2-pyridonyl)silatrane II. This thermodynamic differ-
rence determines the course of reactions 1а and 1b.
silatrane skeleton (1050–1120 cm-1) and the bands of
stretching vibrations of the pyridine ring in the range
of 1400–1600 cm–1. The presence of bands νС=O
(1650 cm–1) in the spectra is the result of hydrolytic
instability of compound I (Scheme 2).
We did not find the molecular structure of com-
pounds containing the 2-pyridyloxy group in the
Cambridge Structural Database. The only item found
was the silatrane derivative containing the pyridyl
group in the Si-substituent, 1-(2-pyridylcarboxy-
methyl)silatrane, 2-PyCOOCH2Sa (VI) [7].
The molecular structure of 1-(2-pyridyloxy)-
silatrane I was determined by the X-ray diffraction
method. The coordination polyhedron of the silicon
atom in the silatrane fragment of molecule I is a
In the IR spectra of 2-PyOSa (I) in the crystalline
state and in CHCl3 solution the most characteristic are
the bands of stretching vibrations C–O–Si of the
Table 1. Parameters of 1Н, 13С, 15N and 29Si NMR spectra of compounds I, III–V (СDCl3)
δH, ppm
δC, ppm
δSi,
δNSa,
ppm
ppm
NCH2 OCH2 H-5
H-3
H-4
H-6 NCH2 OCH2
C-5
C-3
C-4
C-6
C-2
2.93
–
3.91
–
6.74 6.77 7.46 8.16 51.45
57.71 116.01 113.94 138.26 147.69 162.87 –100.8
–350.6
I
6.25 6.56 7.45 7.41
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
III
IV
V
2.87
2.73
3.83
3.78
–
–
–
–
–
–
–
–
51.16
51.28
57.53
58.31
–100.2
–96.33 –357.1
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 12 2008

SYNTHESIS AND MOLECULAR STRUCTURE OF 1-(2-PYRIDYLOXY)SILATRANE
2335
С4
С5
slightly distorted trigonal bipyramid [axial angle
С6
N1Si1O4 175.5(1)°]. The nitrogen atom is displaced
from the plane of the three surrounding carbon atoms
towards the silicon atom by 0.39 Å (∆N). Three
oxygen atoms are located in the equatorial plane of the
bipyramid. Deviation of the silicon atom ∆Si from this
plane towards the exocyclic oxygen atom is 0.12 Å.
N1
С2
С1
С3
O2
O1
O3
Si1
Bond lengths in the silatrane fragment of molecule
I are given in Table 2 in comparison with the cor-
responding analogs, the numeration of atoms is shown
in Fig. 1.
O4
The transannular donor-acceptor bond N→Si in
molecule (I) [2.078(1) Å] is shorter than in all 1-(orga-
nyloxy)silatranes ROSa studied by XRD (Table 2) and
increases with the decrease in the inductive constant σI
of substituent R (Fig. 2) [13].
С8
С7
Earlier [14] we found a strong linear relationship
between the length (in nanometers) of the transannular
dative bond in 1-substituted silatranes containing the
N→Si─C fragment and the inductive constant of the
substituent at the silicon atom:
N2
С9
С10
С11
Fig. 1. Molecule of 1-(2-pyridyloxy)silatrane (I).
lSi←N = (216.80 0.19) – (27.22 1.47)σIR (R 0.989).
endocyclic Si–O bonds, that is, to n–σ* interaction
With this, it was found that the length of the
coordination bond N→Si in 1-(organyloxy)silatranes
calculated from the above equation is shorter than the
experimental one. This is indicative of an additional
electron-withdrawing effect of substituent RO on the
Si(OCH2CH2)3N group. Apparently, this is due to an
interaction of the highest occupied molecular orbital
belonging to the p-type LP of the exocyclic oxygen
atom with electron-withdrawing σ-orbitals of the
(anomeric effect) [15, 16].
The length of the axial O4-Si1 bond lying within the
range of 1.697–1.652 Å (Table 2) is somewhat reduced
with elongation of the N→Si bond. The averaged
length of the three endocyclic equatorial O–Si bonds in
molecule I is close to the lengths of these bonds in the
molecule of MeOSa (1.658 Å) [9]. From other ROSa
molecules it deviates no more than by 0.01 Å. Unfor-
Table 2. Lengths of the main bonds (d, Å) in the molecules of 1-(organyloxy)silatranes ROSa
Si–O
O–C
Si–Oа
О–Са
С–Са
С–Nа
Compound
N→Si
σI, R
References
(exocyclic) (exocyclic) (endocyclic) (endocyclic) (endocyclic) (endocyclic)
2.078
2.518
2.107
2.127
2.152
2.178
2.189
1.697
1.678
1.676
1.669
1.658
1.652
1.659
1.341
1.346
1.372
1.420
1.383
1.638
1.445
1.658
1.660
1.649
1.658
1.648
1.669
1.650
1.421
1.414
1.418
1.433
1.426
1.421
1.431
1.522
1.536
1.507
1.552
1.502
1.521
1.509
1.477
1.461
1.552
1.446
1.459
1.484
1.474
0.08
–
This work
This work
[8]
I
Ib
4-MeC6H4
Me
0.09
–0.01
–0.051
–0.10
–0.17
[9]
Et
[10]
Me3Si
Me3C
[11]
[12]
a
b
The mean values are given for the three endocyclic bonds (the errors of the bond length determination are 0.001–0.002 Å). From
calculations (B3LYP/6-311G**).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 12 2008

2336
VORONKOV et al.
The calculated geometrical parameters of the
lN→Si
pyridine ring of the molecule 2-PyOSa (I) (bond
lengths and angles) are in good agreement with those
found experimentally (Tables 3 and 4). This is true
also for the endocyclic silatranyl O–C bonds and the
equatorial O–Si bonds of molecule I. The most sen-
sitive to the transition of molecule I from the
crystalline to the gas state was the transannular dative
bond N→Si whose length increased by 0.44 Å. This
was accompanied by a decrease of the axial bond Si–O
in 2-PyOSa (I) by 0.02 Å.
2.20
5
2.18
2.16
2.14
2.12
2.10
2.08
4
3
2
1
The lengths of both N–C bonds in the planar
pyridine ring of molecule I are almost equal (Table 3).
Noteworthy, the lengths of all С–C bonds are less than
in molecule VI [7]; this may be assigned to an
enhanced electron-donor inductive effect of the
silatranyl group.
–0.2
–0.1
0.0
0.1
0.2
σI
Fig. 2. Plot of the N→Si bond length in ROSa versus the
inductive constant σI of substituent R. (1) PyOSa, (2) MeOSa,
(3) EtOSa, (4) Me3Si, and (5) Me3COSa.
The ССС angles in the pyridine ring of molecule I
are practically equal (118°–119°), as are the ССN
angles (123°–124°), whereas the CNC angle is equal to
117° (Table 4).
tunately, the X-ray structural data available for
silatranes of the type ROSa are very scarce that does
not allow an unambiguous proof of the linear
correlation (Fig. 2) found for silatranes possessing
oxygen atom in the axial position.
The molecules I in the unit cell are packed at van
der Waals distances, no reduced contacts were found.
The length of the endocyclic C–C bonds, as in all
silatranes, is shorter than in aliphatic organic and
organosilicon compounds of tetrahedral silicon [17].
All N–C bonds in the silatrane skeleton of the mole-
cule of 2-PyOSa (I) are of the same length (1.477 Å).
EXPERIMENTAL
IR spectra of compound I in the crystalline state
(KBr pellets or mulls in mineral oil) and in CHCl3
The exocyclic О–С bond in molecule I is by 0.03 Å
shorter than in 1-(4-methylphenoxy)-silatrane [8] and
somewhat shorter than the same bond in ROSa with R =
Me, Et, Ме3С, as well as in aliphatic organic
compounds (1.41 Å). This is due to an enhanced bond
order for the bond between the oxygen atom and the
aromatic or pyridine ring.
solution were recorded on
a
Specord IR-75
1
13
15
spectrometer. H, C, N and 29Si NMR spectra were
taken on a Bruker DРХ-400 spectrometer (400.13,
100.61, 40.56, 376.50 and 79.5 MHz, respectively) in
CDCl3.
The set of reflections from a single crystal of (I)
(0.35 х 0.30 х 0.20 mm) was obtained on a Bruker
Smart APEX II diffractometer at 100(2) K [λ(MoKα
0.71073 Å irradiation]. Parameters of elementary cell
of the rhombic crystal: a 13.5772(8), b 8.7842(5), c
10.2334(6) Å; V 1220.5(1) Å3, space group Рna21, M
The exocyclic angle Si1O4C7 in molecule I is 131.1о
and in the molecule 4-MeC6H4OSa 125.9° [8]. This is
a typical value for bond angles Rπ–O–Si (for example,
in the molecule of СН2=СНОSiF3 it is 133° [18]).
Table 3. Bond lengths (d, Å) in the pyridine fragment of molecules I and VI
Compound
С7–C8
1.400
1.404
1.412
C8–C9
1.382
1.384
1.402
C9–C10
1.394
1.398
1.407
C10–C11
1.373
1.388
1.409
C11–N2
1.348
1.338
1.353
N2–C7
1.336
1.328
1.346
I
Iа
VIb
а From calculations (B3LYP/6-311G**). b [7].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 12 2008

SYNTHESIS AND MOLECULAR STRUCTURE OF 1-(2-PYRIDYLOXY)SILATRANE
2337
Table 4. Bond angles (ω, deg) in the pyridine fragment of molecules I and VI
Compound
N2C7C8
122.9
123.2
124.0
C7C8C9
118.5
118.1
117.0
C8C9C10
C9C10C11
117.7
C10C11N2
124.5
C11N2C7
119.3
117.1
117.8
117.5
I
Iа
119.4
117.7
123.8
VIb
119.0
118.7
118.9
а From calculations (B3LYP/6-311G**). b [7].
268.35, μ(MoKα) 2.02 cm–1, 2θmax 61.0°. The inten-
sities of 13259 reflections were measured, 3678 in-
dependent reflections were used for further refinement.
The structure was solved by the direct method and
refined in anisotropic approximation for nonhydrogen
atoms. Hydrogen atoms were placed geometrically and
refined with fixed С–Н distances. Final deviation
factors R1 0.0304 [for 3403 reflections with I > 2σ(I)],
wR2 0.0724 (for all reflections), GOF 0.999.
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1.6 g (0.017 mol) of 2-hydroxypyridine III and 3.65 g
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