A distorted trigonal antiprismatic cationic silicon complex with ureato ligands: syntheses, crystal structures and solid state29Si NMR properties

Article abstract of DOI:10.1002/ejic.200900784

Insertion of phenyl isocyanate into the Si-N bond of N-(tri methylsilyl)diethylamine yields the N'-silylated N,N-diethylN'-phenylurea 1, which undergoes transsilylation with SiCl₄ to yield the C ₃-symmetric cationic hexacoordinate si

Full text of DOI:10.1002/ejic.200900784

FULL PAPER
DOI: 10.1002/ejic.200900784
A Distorted Trigonal Antiprismatic Cationic Silicon Complex with Ureato
29
Ligands: Syntheses, Crystal Structures and Solid State Si NMR Properties
Dana Schöne,[a] Daniela Gerlach,[a] Conny Wiltzsch,[a] Erica Brendler,[b] Thomas Heine,[c]
Edwin Kroke,*[a] and Jörg Wagler*[a]
Dedicated to Prof. Gerhard Roewer on the occasion of his 70th birthday
Keywords: Silicon / Chelates / Density-functional calculations / Hypercoordination
Insertion of phenyl isocyanate into the Si–N bond of N-(tri-
methylsilyl)diethylamine yields the NЈ-silylated N,N-diethyl-
NЈ-phenylurea 1, which undergoes transsilylation with SiCl4
to yield the C3-symmetric cationic hexacoordinate silicon
complex 3s+ [tris-κ-O,NЈ-(N,N-diethyl-NЈ-phenylureato)sili-
conium] as chloride salt, which was characterized crystallo-
graphically. The cationic complex 3s+ exhibits a distorted tri-
gonal antiprismatic coordination sphere about the silicon
atom with fac arrangement of the three N-atoms (and the
three O-atoms) relative to one another. This C3-symmetric
complex undergoes isomerization into its asymmetric isomer
3a+ (mer arrangement of NNN or OOO relative to one an-
other) in CDCl3 solution. Hence, two 29Si NMR signals ap-
1
pear and four sets of signals emerge in the H and 13C NMR
spectra. Despite its pronounced axial symmetry, the 29Si
NMR shielding tensor of the cation 3s+ in its chloride salt
exhibits an unusually small span (less than 20 ppm), which
was analyzed CP/MAS NMR spectroscopically and by com-
putational methods.
Introduction
Among the class of hypercoordinate silicon complexes[1]
those with a hexacoordinate silicon atom[2] were found to
exhibit more or less distorted octahedral coordination
spheres. Deviation from the ideal octahedral geometry is
mainly caused by different substituents X (thus resulting in
different Si–X bond lengths) and steric strain, either caused
by intramolecular sources such as chelate effects or by inter-
molecular interactions in the crystal packing. The most ob-
vious sign for distortion of an octahedron is the deviation
of the axial angles from 180°. As shown in Scheme 1 for
selected examples of bis- and tris-chelate hexacoordinate
silicon complexes[3] the presence of small chelate rings re-
sults in pronounced distortion.
Scheme 1. Examples for “distorted octahedral” Si complexes. The
three axial angles are given below the respective drawing. For II
only data for one of the two crystallographically independent mole-
cules are given here as representative example (for IV: R =
PhMe2Si).
The presence of different substituents in the octahedral
coordination sphere of silicon was already shown to occa-
sionally provoke a pronounced span of the 29Si chemical
shift anisotropy (CSA) tensor.[2c,4] The transition to a more
axial system, i.e., pentacoordinate silicon complexes with a
trigonal bipyramidal Si-coordination sphere, causes an even
wider span.[5] To the best of our knowledge, the 29Si NMR
spectroscopic role of axial deformation of the octahedron
with respect to the threefold axis, also creating a molecular
system with a unique axis, has not been elucidated yet. Our
recent investigations on silylated urea derivatives provided
[a] Institut für Anorganische Chemie, Technische Universität
Bergakademie Freiberg,
09596 Freiberg, Germany
Fax: +49-3731-39-4058
E-mail: edwin.kroke@chemie.tu-freiberg.de
joerg.wagler@chemie.tu-freiberg.de
[b] Institut für Analytische Chemie, Technische Universität
Bergakademie Freiberg,
09596 Freiberg, Germany
[c] School of Engineering and Science, Theoretical Physics – Theo-
retical Materials Science, Jacobs University Bremen,
28759 Bremen, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900784.
Eur. J. Inorg. Chem. 2010, 461–467
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
461

E. Kroke, J. Wagler et al.
FULL PAPER
access to a cationic hexacoordinate siliconium complex
comprising three four-membered chelate rings, resulting in
a pronounced C3-symmetric deformation of the coordina-
tion sphere from octahedral towards distorted antipris-
matic. Thus, this interesting complex was subjected to X-
ray diffractional, 29Si NMR spectroscopic and computa-
tional analyses.
Results and Discussion
Phenyl isocyanate reacts with N-(trimethylsilyl)dieth-
ylamine under formal insertion into the Si–N bond, thus
providing convenient access to the NЈ-trimethylsilylated
N,N-diethyl-NЈ-phenylurea 1, whereas excess of phenyl iso-
cyanate would lead to the formation of 2 as a by-product
(Scheme 2). Synthesis of 1 in hexane, however, provides a
product of sufficient purity for further reactions since 1 is
obtained as a hexane-soluble colorless oil, whereas com-
pound 2, even though formed as a minor by-product, pre-
cipitates from hexane to yield white crystalline needles suit-
able for X-ray diffraction analysis (Figure 1).
Figure 1. ORTEP drawing of 2 in the crystal (ellipsoids at the 50%
probability level, hydrogen atoms omitted for clarity). Selected
bond lengths [Å] and dihedral angles [°]: Si1–N1 1.783(2), O1–C10
1.225(2), O2–C17 1.222(2), N1–C10 1.374(3), N2–C10 1.401(2),
N2–C17 1.437(3), N3–C17 1.354(2), Si1–N1–C10–N2 177.3(1),
N1–C10–N2–C17 41.9(3), C10–N2–C17–N3 149.0(2), N2–C17–
N3–C18 169.2(2).
Hence, we probed the chelator qualities of compound 1
in a reaction with silicon tetrachloride (Scheme 2, bottom).
The reaction proceeds towards the cationic tris-chelate 3+
(which was found to exist in two diastereomeric forms in
solution, i.e., the enantiomers of the asymmetric 3a+ and
the C3-symmetric 3s+ cations) in a clean manner. In the 29Si
solid state NMR spectrum only one signal was detected at
δ = –165.9 ppm. Attempts to synthesize the bis-chelate
complex 4 by adjusting the molar ratio 1/SiCl4 to 2:1 failed:
The co-existence of 3+ and unreacted SiCl4 was proven 29Si
NMR spectroscopically. Apparently, the mono- and di-sub-
stituted silanes LSiCl3 and L2SiCl2, respectively, (L = N,N-
diethyl-NЈ-phenylureate) exhibit enhanced reactivity relative
to SiCl4, thus furnishing 3+ as the final substitution prod-
uct. In order to explore an alternative route towards com-
pound 4 phenyl isocyanate was treated with bis(diethyl-
amino)dichlorosilane. Again, complex 3+ was formed, thus
hinting at ligand scrambling which should have given rise
to the formation of 3+ from intermediately formed 4 (not
detected; Scheme 3).
Scheme 2. Synthesis of the silylated urea derivative 1 and the hexa-
coordinate silicon compound 3-Cl thereof (representation of one
canonical form).
Deliberate synthesis of 2 by treating N-(trimethylsilyl)di-
ethylamine with two equivalents of phenyl isocyanate
proved a convenient route to generate this interesting ligand
precursor in reasonable yield. In the present work, however,
we want to focus on 1 as a chelating ligand precursor.
As shown in previous reports, trimethylsilylated (O,N)-
chelators proved useful as starting materials in the synthe-
ses of hypercoordinate silicon chelate complexes since the
by-product of the reactions, Me3SiCl, is volatile and may
be easily removed from the product.[1a,1b] So far, a number
of carboxylic acid derivatives such as carboxylates,[6]
amides[3a] and amidinates[3b,3c] has been studied as ligands
capable of setting up coordination spheres with coordina-
tion number 5 or 6 about a silicon atom. Whereas the for-
mer were occasionally found to act as bidentate chelators,
urea moieties, although reported in the context of silicon
coordination chemistry, served the role of a monodentate
donor or were bidentately bound to the Si-atom in a
κ(O,C)-mode.[7]
Scheme 3. Top: schematic representation of the two pairs of dia-
stereomers of the cation 3+ found in chloroform solution (3s+
=
fac, 3a+ = mer). Bottom: attempts to synthesize 4, leading also to
3-Cl.
462
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 461–467

Cationic Silicon Complex with Ureato Ligands
In addition to not delivering the desired product 4, this
alternative route resulted in the formation of a byproduct,
namely trimerized phenyl isocyanate, which crystallizes
with 3s-Cl as 3s-Cl·THF·(PhNCO)3 (structure A) (Fig-
ure 2). From the same reaction further crystals were ob-
tained which consist of 3s-Cl only (structure B) (Figure 3).
The presence of more than one hexacoordinate silicon
compound in the solid obtained was furthermore indi-
cated by 29Si CP/MAS NMR spectra, which produced a
peak at δ = –165.9 ppm (as for the former product) and a
less intense one at δ = –164.1 ppm (with a shoulder). Ex-
cept the special features caused by the additional compo-
nents in the crystal structure [interaction between chloride
and (PhNCO)3 with a separation of 3.028(1) Å between
Figure 2. ORTEP drawing of 3s+ and the (PhNCO)3·Cl– counterion
in the crystal structure of 3s-Cl·THF·(PhNCO)3 (structure A) (view
along the crystallographic c-axis, 30% probability ellipsoids, hydro-
gen atoms, THF molecule and phenyl groups omitted, selected
Cl– and the centroid of the triazine system, as well as a atoms of asymmetric unit labeled). Atoms Si1 and Cl1 are located
on threefold axes.
THF molecule located on a threefold axis with its O-atom
pointing towards the Si-atom of 3s+ from a distance of
5.522(3) Å] the features of the cation 3s+ are very similar
in both solids (see Figure caption 3). Thus, only the molec-
ular structure of 3s-Cl will be discussed. As indicated in
the canonical formula in Scheme 2, bottom, and by the
bond lengths C7–N1 and C7–N2, the diethylamino sub-
stituent donates electron density towards the ligand donor
sites, thus enhancing the double bond character of bond
C7–N2, whereas bonds C7–O1 and C7–N1 represent sin-
gle bonds strengthened by π-conjugation. This additional
donor action of N2 can be considered source of the struc-
tural differences between the ureato ligand in 3s+ and the
carbamido ligand in the complex Scheme 1 I,[3a] which ex-
Figure 3. ORTEP drawing of 3s+ in the crystal structure of 3s-Cl
hibits shorter C–O and C–N bonds (1.299, 1.308 and
(structure B) (30% probability ellipsoids, hydrogen atoms omitted,
1.300, 1.303 Å, respectively). In a related study, Kost et
al. have shown that the transition from hydrazinimides to
semicarbazones is also accompanied by reinforcement of
the ligand donor capabilities by the additional R2N
group,[8] and the same was shown to apply to pyridine li-
atoms of asymmetric unit labeled). Selected bond lengths [Å] and
angles [°] of 3s+ in 3s-Cl: Si1–O1 1.810(2), Si1–N1 1.833(2), O1–
C7 1.323(3), N1–C7 1.344(3), N2–C7 1.318(3), O1–Si1–N1 71.8(1),
C7–N2–C8 121.7(2), C7–N2–C10 119.9(2), C8–N2–C10 118.2(2);
corresponding parameters of 3s+ in 3s-Cl·THF·(PhNCO)3: Si1–O1
1.800(1), Si1–N1 1.840(2), O1–C7 1.315(2), N1–C7 1.346(2), N2–
gands upon substitution with a 4-dimethylamino group.[4a] C7 1.315(2), O1–Si1–N1 71.9(1), C7–N2–C8 121.3(2), C7–N2–C10
Hence, the ureato ligand in 3s+ is setting up Si–N and Si–
119.4(2), C8–N2–C10 119.0(1).
O bonds of similar length, whereas in the carbamide com-
plex A in Scheme 1 the Si–O bonds (1.885, 1.886 Å) are
longer than the Si–N bonds (1.830, 1.835 Å), again ac-
Although the axial angles N–Si–O* [162.8(1)°] are in the
counting for the carbamide character of the ligand utilized same range as the inter-chelate axial angles of the com-
by Yang and Verkade. In addition to the axial angles being pounds depicted in Scheme 1 (in 3s-Cl position O* is gener-
much smaller than 180°, the coordination sphere about the ated from O1 by operation 2 – y, 1 + x – y, z), the three
silicon atom in 3s+ deviates from octahedral by two dis- identical chelates create a particularly high symmetry of this
tinct features: These are (i) the significantly reduced rota- tris-chelate complex. The pronounced C3-symmetric distor-
tion of the two opposite triangular faces NNN and OOO tion of the Si-coordination sphere in 3s+, which should re-
by only 40.7(1)°, which corresponds to a 32.2% rotation sult in pronounced axiality of the 29Si NMR CSA tensor,
towards trigonal prismatic, and (ii), the noticeably low ra- tempted us to analyze this property experimentally and
tio [IPD/AEL(complex)] = 0.666 (IPD = inter-planar dis- computationally. Since the product obtained from the reac-
tance, AEL = average edge length of the triangles, [IPD/ tion of phenyl isocyanate with bis(diethylamino)dichlorosi-
AEL(octahedron)] = 0.816), thus confirming 18.4% compres- lane was contaminated with by-products, the compound
sion of the octahedron along the ideal threefold axis. Thus, obtained from the reaction of 1 with SiCl4 was used for 29Si
one cannot consider this coordination geometry as slightly CP/MAS NMR analysis of the CSA tensor. The 29Si CP/
distorted octahedral any longer, whereas distorted trigonal MAS NMR spectrum of 3s-Cl, depicted in Figure 4, reveals
antiprismatic would hold for any rotation angle between a very narrow span of the chemical shift anisotropy. Al-
30 and 60° regardless the degree of compression or stretch- though the axiality of the CSA tensor can be discerned
ing of the two parallel triangular faces along the threefold from the principal component δ22 being closer to δ33 than
axis.
to δ11, this result was not expected.
Eur. J. Inorg. Chem. 2010, 461–467
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
463

E. Kroke, J. Wagler et al.
FULL PAPER
pair contributions (e.g., O –14.9 and –25.3), the sum of
which, however, is close to the N-atom influence. Due to
the C3 symmetry of the cation, the influences of all O-atoms
(the same holds for the N-atoms) in direction (11) is equal,
whereas the degenerate character of (22) and (33) is re-
flected in the permutational influence of the O- and N-
atoms in these directions.
Table 2. Selected individual contributions to the 29Si shielding ten-
sor of 3s+, determined using DFT-IGLO-III (Pipek–Mezey local-
ization).
Atom[a]
σiso
σ11
σ22
σ33
O
O
–14.9
–25.3
–15.0
–25.2
–14.9
–25.3
–47.9
–48.0
–48.0
–14.8
–22.4
–14.8
–22.5
–14.8
–22.6
–51.6
–51.7
–51.5
–4.3
–25.7
–40.6
–11.8
–9.9
–12.7
–18.5
–43.4
–22.4
–23.8
–52.6
–14.0
–71.9
O*
O*
O**
O**
N
N*
N**
Figure 4. Bottom: 29Si CP/MAS NMR spectrum of 3s-Cl (νspin
=
364 Hz). Top: overlay of the simulated spectrum using the CSA
–7.6
tensor data from the entry “exp.” in Table 1.
–29.5
–39.4
–78.4
–20.5
We analyzed the CSA tensor by means of quantum
chemistry, approximating the solid-state structure by the
formally positively charged cation 3s+ at the frozen low-
temperature XRD structure B. The CSA tensor, computed
at various DFT and HF levels, confirms the narrow span
(Table 1). The skew, which must be –1 in this axial molecu-
lar system according to the system’s C3 symmetry (two de-
generate principal axes in the molecular and crystal envi-
ronment of threefold symmetry), was not reproduced as
such by the experimental methods applied. HF and DFT
approaches with larger basis sets (i.e. calculations 4 and 5)
converge to similar results, which indicates that theory can
describe the model compound rather well, as both methods
contain quite different approximations. In conclusion, all
methods applied proved suitable to describe the narrow
span of the CSA tensor, which is located in the cation as
follows: Component (11) points along the threefold axis,
whereas components (22) and (33) are degenerate in the
plane orthogonal to the threefold axis.
[a] The asterisked atoms represent symmetry equivalents in the
crystal system.
So far, no satisfactory explanation was found for the
skew of the CSA tensor deviating from –1 in the experi-
ment. Thermal motion of the molecules in the crystal at
elevated temperatures was considered to violate the perfect
C3 symmetry, hence influencing the shape of the CSA ten-
sor. In order to probe this hypothesis, the collection of an
X-ray diffraction data set at elevated temperatures was
planned, and a tiny single-crystalline needle (from the sam-
ple which was used for CSA tensor analysis) was chosen for
data collection. Unit cell determination at 250 K revealed
trigonal cell parameters, but different from those of com-
pound 3s-Cl, the structure of which was determined at
150 K (a = 20.109, c = 14.518 Å for the former, a = 12.028,
c = 14.354 Å for the latter). Upon cooling to 150 K and
further to 100 K no phase transition (at least by means of
striking changes in the unit cell parameters) was observed.
Structure solution revealed a further trigonal modification
of 3s-Cl (structure C, space group P3), but with six crystal-
lographically independent Si-environments. The molecular
shapes of the six individual cations 3s+ are similar to those
in structure B (space group P31c), thus rendering further
discussion of bond lengths and angles superfluous. Struc-
ture C (at 100 K) exhibits an average molecular volume of
847 Å3 per unit 3s-Cl. In sharp contrast, the spatial demand
of 3s-Cl in structure B (at 150 K) is much greater with
899 Å3. This notable difference, which does not only origi-
nate from the different temperatures of X-ray data collec-
tion, gives rise to the assumption that the more closely
packed structure C should be the more stable one (rendering
the crystal of structure B a discovery by chance). Indeed,
Table 1. 29Si NMR CSA tensor of 3s+ determined experimentally
and computationally for structure B. The principal components δ11,
δ22 and δ33 as well as the span Ω and skew κ are given according
to the Herzfeld–Berger notation.[9]
Entry[a] δiso
δ11
δ22
δ33
Ω
κ
exp.
–165.9 –154.7 –167.1 –175.8 21.1
–176.2 –168.1 –180.3 –180.3 12.2
–170.1 –162.2 –174.1 –174.1 11.9
–165.2 –155.3 –170.2 –170.2 14.9
–173.3 –163.4 –178.3 –178.3 14.9
–171.9 –164.4 –175.7 –175.7 11.3
–0.17
–1.00
–1.00
–1.00
–1.00
–1.00
calc1
calc2
calc3
calc4
calc5
[a] Exp.: determined from the MAS spectrum at νspin = 364 Hz,
calc1: DFT-IGLO-III (PBE), calc2: B3LYP/6-311G(d,p), calc3:
HF/6-311G(d,p), calc4: HF/6-311G(2d,p), calc5: GIAO-PBE/TZ2P
(Slater basis).[10]
Analysis of the contributions to the overall shielding with repeated crystal picking and unit cell determination of crys-
DFT-IGLO (using Pipek–Mezey localizition) confirmed tals obtained from the reaction of bis(diethylamino)dichlo-
similar influence of the oxygen and nitrogen donor atoms, rosilane and phenyl isocyanate (upon storage of the reac-
as shown in Table 2. Whereas one dominating lone pair tion mixture for 3 weeks) revealed that, in addition to 3s-
contribution was analyzed for the N-atoms (e.g., N –47.9), Cl·THF·(PhNCO)3 (structure A), crystals of 3s-Cl (structure
the oxygen atoms were found to exhibit two distinct lone C) were present in the solid product.
464
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 461–467

Cationic Silicon Complex with Ureato Ligands
Despite similar Si–N and Si–O bond lengths, the trigo- lecular thermal motion at room temperature might also
nal-antiprismatic distortion of the six individual Si-coordi- contribute to the overall shape of the experimental 29Si CP/
nation spheres in structure C varies slightly (with the distor- MAS NMR spectrum of 3s-Cl (structure C).
tion towards trigonal-prismatic ranging from 32.3 to
36.3%, the compression along the threefold axis ranging
from 15.7 to 16.9%). Characteristic parameters of the Si-
coordination polyhedra in structure C are summarized in
Table 3.
Table 3. Geometric parameters of the Si-coordination spheres of
the six individual cations 1–6 in the crystal structure C of 3s-Cl.
[edge lengths N···NЈ and O···OЈ, the average edge length (AEL)
thereof and NNN-OOO inter-planar distance (IPD) of the trigo-
nal-antiprisms in Å, dihedral angle of the O- and N-donor atom
of one chelate about the C3-axis (tors.(O–C3–N)) in °].
Cation
1
2
3
4
5
6
d(N···NЈ)[a]
2.795 2.795 2.801 2.800 2.807 2.779
2.642 2.626 2.621 2.634 2.611 2.641
d(O···OЈ)[a]
tors. (O–C3–N)[b] 40.04 39.97 38.94 40.62 38.20 40.01
% trig. prism.
AEL
IPD
IPD/AEL
% compression
33.3
33.4
35.1
32.3
36.3
33.3
2.7185 2.7105 2.711 2.717 2.709 2.710
1.853 1.852 1.865 1.843 1.863 1.858
0.682 0.683 0.688 0.678 0.688 0.686
Figure 5. Bottom: 29Si CP/MAS NMR spectrum of 3s-Cl (νspin
=
364 Hz). Top: simulated spectrum using the weighted CSA tensor
data (16.7% contribution of each tensor) from Table 4 (upon set-
ting δiso = –165.9 ppm for each of the six CSA tensors).
16.4
16.3
15.7
16.9
15.7
15.9
[a] Standard deviation: 0.004. [b] Standard deviation: 0.15.
These six slightly different coordination spheres might
give rise to six CSA tensors of slightly different span, the
superposition of which would provide an explanation for
the 29Si solid state NMR spectrum observed in the experi-
ment (Figure 4). In order to get an idea of the variability
of the span of the CSA tensor among these six cations their
solid-state structures were, upon optimization of the H-po-
sitions, employed for CSA tensor calculation on the HF/6-
311G(d,p) level. The results, listed in Table 4, indeed offer
an explanation for seemingly lower axiality of the CSA ten-
sor observed in the experiment. Although the isotropic
chemical shift was predicted to be very similar for the six
independent 29Si nuclei, the CSA tensors of cations 3 and
5 (which exhibit pronounced distortion of the coordination
sphere towards trigonal prismatic) can be expected to show
a larger span than the other four cations. Upon removal of
the computational error on the value of δiso (i.e., upon set-
ting δiso = –165.9 ppm for each of the six CSA tensors in
Table 4), the superposition of the six individual spectra cre-
ated therefrom (each of the six individual Si nuclei contrib-
uting with 16.7%) indeed produces a picture which reflects
the experimental data very well (Figure 5). Although a
plausible explanation was found, additional effects of mo-
Conclusions
The insertion of phenyl isocyanate into the Si–N bond
of a trimethylsilylamine was shown to provide convenient
access to an interesting class of chelating ureato systems,
the coordination behavior of which will be addressed in fur-
ther studies. A first example of a hexacoordinate silicon
complex synthesized therefrom already revealed some spe-
cial features of these ligands in exhibiting donor qualities
different from those of related carbamide systems (I,
Scheme 1): the electron releasing diethylamino substituent
supports strong coordination of both the N- and O-donor
site. This pronounced stabilizing effect of the (O,N) chelate
is considered to facilitate the formation of the herein pre-
sented silicon tris-chelate 3+ (in sharp contrast to the bis-
chelates I, II and III in Scheme 1). The special coordination
geometry of this silicon complex, i.e., distorted trigonal
antiprismatic, was found to give rise to a very narrow 29Si
NMR shielding tensor. The pronounced axiality of the Si-
coordination sphere (C3 symmetry) was expected to gener-
ate a 29Si solid state NMR spectrum also exhibiting pro-
nounced axiality. Instead, a less axial CSA tensor was
found in the experiment, hinting at effects like disturbances
of the ideal C3 symmetry in the crystal lattice at ambient
temperatures and at the superposition of various axial CSA
tensors of different span. Whereas the symmetry-disturbing
influence of molecular thermal motion at room temperature
can always be considered a source of error, repeated single
crystal structure analyses proved the latter hypothesis true
and 29Si solid state NMR spectroscopy a valuable tool com-
plementing X-ray diffraction analyses.
Table 4. 29Si NMR CSA tensor of 3s+ for the six independent cat-
ions of structure C, determined computationally using HF/6-
311G(d,p). The principal components δ11, δ22 and δ33 as well as the
span Ω are given according to the Herzfeld–Berger notation.[9]
Cation
1
2
3
4
5
6
δiso
δ11
δ22
δ33
Ω
–162.1 –163.3 –162.8 –162.9 –162.7 –164.7
–152.4 –153.1 –150.9 –154.2 –148.8 –154.2
–166.7 –168.3 –168.8 –166.9 –169.7 –169.9
–167.2 –168.3 –168.8 –166.9 –169.7 –169.9
14.8
15.2
17.9
12.8
20.9
15.7
Eur. J. Inorg. Chem. 2010, 461–467
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
465

E. Kroke, J. Wagler et al.
FULL PAPER
atiles were removed under reduced pressure and the remaining vis-
cous oil was dissolved in THF (50 mL), whereupon crystallization
of 3s-Cl commenced overnight. After 24 h the solid was filtered off,
washed with THF (3ϫ5 mL) and dried in a vacuum. (Yield: 3.33 g,
5.23 mmol, 23%). This compound decomposes without melting
when heated up to 300 °C in a sealed capillary. C33H45ClN6O3Si
(637.29): calcd. C 62.20, H 7.12, N 13.19; found C 61.85, H 7.11,
N 12.99. The NMR spectra of a solution of 3-Cl in CDCl3 exhibit
sets of signals corresponding to a mixture of the isomeric cations
Experimental Section
Synthesis of N,N-Diethyl-NЈ-(trimethylsilyl)-NЈ-phenylurea (1): To
a solution of N-(trimethylsilyl)diethylamine (10.8 g, 74.4 mmol) in
hexane (50 mL), which was stirred in a 15 °C water bath, phenyl
isocyanate (9.32 g, 78.3 mmol) was added dropwise within a few
minutes and stirring was continued for 3 h. Within this time some
white crystalline precipitate formed (1.38 g, 3.6 mmol, 4.8% of 2,
as identified later), which was filtered off. Upon removal of the
volatiles from the filtrate under reduced pressure product 1 remains
in the flask as a viscous yellowish oil. NMR spectroscopy indicated
sufficient purity for further reactions. (Yield: 18.6 g, 70.5 mmol,
95%). NMR spectra of 1 exhibit broadened signals originating
from configurational exchange processes at the urea moiety. 1H
NMR (400.13 MHz, CDCl3, 22 °C): δ = 0.2 [br. s, 9 H, Si(CH3)3],
0.84 (br. s, 6 H, NCH2CH3), 3.11 (br. s, 4 H, CH2), 6.9–7.3 (3m, 4
H, 2 H, 4 H, Ph) ppm. 13C NMR (100.62 MHz, CDCl3, 22 °C): δ =
–0.1 [-Si(CH3)3], 12.2 (NCH2CH3), 41.5 (CH2), 124.2, 127.0, 128.4,
143.4 (Ph), 162.5 (C=O) ppm. 29Si NMR (79.5 MHz, CDCl3,
22 °C): δ = 8.5 ppm (br.).
1
3s+ and 3a+ in solution. The H NMR spectrum exhibits various
superimposed triplets in the range 0.6–1.4 ppm originating from
the ethyl-CH3 groups and quartets in the range 2.8–3.6 ppm for the
CH2 groups. Even less informative is the part of the aromatic pro-
ton signals. The 13C NMR spectroscopic data, however, clearly
show the presence of 3 small signals of equal intensity for 3a+ and,
in the same chemical shift range, one intense signal for 3s+ in dis-
tinct parts of the spectrum, whereas the chemical shift range of the
phenyl-o,m,p-carbon atoms is characterized by various superim-
posed signals (125.0, 125.7, 126.3, 126.6, 127.0, 127.8, 128.4, 129.2,
129.4, 129.7 ppm). For clarity reasons selected 13C NMR spectro-
scopic data are presented in Table 5.
This compound is very sensitive towards hydrolysis. Although
stored in a sealed Schlenk flask, some crystals of the N,N-diethyl-
NЈ-phenylurea, obviously liberated by traces of moisture, formed
upon storage of the remaining product for some weeks. The solid-
state structure of N,N-diethyl-NЈ-phenylurea was confirmed
X-ray crystallographically. CCDC-743688, crystal dimensions
0.40ϫ0.12ϫ 0.04 mm, C11H16N2O, Mr = 192.26, T = 90(2) K,
orthorhombic, space group Pbcn, a = 15.1284(8), b = 8.8584(4), c
Table 5. 13C NMR spectroscopic data of 3-Cl (chemical shift δ in
ppm) in CDCl3 solution at 22 °C.
δ for 3s+
δ for 3a+
Moiety
12.8, 13.7
41.4, 44.1
137.7
12.6, 12.9 (2ϫ), 13.1, 13.4, 14.0
41.3, 41.6, 42.1, 43.5, 44.1 (2ϫ)
137.1, 137.5, 138.2
CH3
CH2
Ph-ipso
C=O
= 16.3355(8) Å, V = 2189.18(19) Å3, Z = 8, ρcalcd. = 1.167 Mgm–3, 160.2
µ(Mo-Kα) = 0.076 mm–1, F(000) = 832, 2θmax = 52.0°, 16958 col-
lected reflections, 2139 unique reflections (Rint = 0.0654), 132 pa-
159.6, 159.7, 160.8
29Si NMR (79.5 MHz, CDCl3, 22 °C): δ = –168.2, –169.7 ppm, sim-
ilar intensity. 29Si NMR (79.5 MHz, solid state, CP, νspin = 4 kHz):
δiso = –165.9 ppm.
rameters, S = 1.004, R1 = 0.0391 [IϾ2σ(I)], wR2(all data) = 0.0951,
max./min. residual electron density +0.188/–0.225 eÅ–3.
Synthesis of Compound 2: To a solution of N-(trimethylsilyl)dieth-
ylamine (3.79 g, 26.1 mmol) in hexane (50 mL), which was stirred
in a 15 °C water bath, phenyl isocyanate (6.21 g, 52.2 mmol) was
added dropwise within a few minutes and stirring was continued
for further 15 min. Within this time precipitation of 2 commenced.
The mixture was stored at room temperature overnight, whereupon
the solid product was filtered off, washed with hexane (20 mL) and
dried in vacuo; yield 7.96 g, 20.8 mmol, 79.7%; m.p. 112 °C.
C21H29N3O2Si (383.56): calcd. C 65.76, H 7.62, N 10.96; found C
X-ray Crystal Structure Analysis of 3s-Cl (structure C): CCDC-
743692,
crystal
dimensions
0.38ϫ0.12ϫ0.08 mm,
C33H45ClN6O3Si, Mr = 637.29, T = 100(2) K, trigonal, space group
P3, a = b = 20.1088(3), c = 14.5184(5) Å, V = 5084.2(2) Å3, Z =
6, ρcalcd. = 1.249 Mgm–3, µ(Mo-Kα) = 0.190 mm–1, F(000) = 2040,
2θmax = 50.0°, 33245 collected reflections, 10598 unique reflections
(Rint = 0.0665), 793 parameters, S = 0.993, Flack parameter
–0.03(5), R1 = 0.0493 [IϾ2σ(I)], wR2(all data) = 0.0966, max./min.
residual electron density +0.448/–0.302 eÅ–3.
1
66.19, H 7.26, N 10.99. H NMR (400.13 MHz, CDCl3, 22 °C): δ
In an alternative attempt, 3s-Cl was obtained from the reaction of
bis(diethylamino)dichlorosilane with phenyl isocyanate: To a solu-
tion of bis(diethylamino)dichlorosilane (2.50 g, 10.3 mmol) in THF
(20 mL) phenyl isocyanate (2.45 g, 20.6 mmol) was added dropwise.
From the clear solution white crystals formed within one day upon
storage at room temperature. The supernatant was decanted and
the crystals were briefly dried in vacuo. Yield: 1.13 g. This product,
however, consisted of various crystalline compounds as recognized
by 29Si solid state NMR spectroscopy (CP/MAS, νspin = 4 kHz),
signals at δiso = –165.9, –164.1 ppm), two of which were identified
crystallographically as 3s-Cl·THF·(PhNCO)3 (structure A) and 3s-
Cl (structure B).
= 0.21 [s, 9 H, Si(CH3)3], 0.54 (br. s, 6 H, NCH2CH3), 2.72 (br. s,
4 H, CH2), 6.9–7.3 (3m, 4 H, 2 H, 4 H, Ph) ppm. 13C NMR
(100.62 MHz, CDCl3, 22 °C):
δ = –0.1 [-Si(CH3)3], 11.7
(NCH2CH3), 41.3 (CH2), 125.8, 126.0, 126.1, 128.3, 129.0, 129.6,
140.6, 141.1 (Ph), 156.3, 161.3 (C=O) ppm. 29Si NMR (79.5 MHz,
CDCl3, 22 °C): δ = 12.4 ppm. X-ray structure analysis of 2. CCDC-
743689, crystal dimensions 0.30ϫ0.10ϫ0.03 mm, C21H29N3O2Si,
Mr = 383.56, T = 100(2) K, monoclinic, space group P21/c, a =
17.3226(7), b = 6.3823(2), c = 20.2088(7) Å, β = 111.396(2)°, V =
2080.26(13) Å3, Z = 4, ρcalcd. = 1.225 Mgm–3, µ(Mo-Kα) =
0.133 mm–1, F(000) = 824, 2θmax = 50.0°, 13391 collected reflec-
tions, 3668 unique reflections (Rint = 0.0563), 249 parameters, S =
0.963, R1 = 0.0426 [IϾ2σ(I)], wR2(all data) = 0.0925, max./min.
residual electron density +0.263/–0.265 eÅ–3.
X-ray Crystal Structure Analysis of 3s-Cl·THF·(PhNCO)3 (struc-
ture A): CCDC-743690, crystal dimensions 0.35ϫ0.30ϫ0.25 mm,
Synthesis of Tris-κ-O,NЈ-(N,N-diethyl-NЈ-phenylureato)siliconium C58H68ClN9O7Si, Mr = 1066.75, T = 150(2) K, trigonal, space
Chloride (3-Cl): To a solution of 1 (17.8 g, 67.4 mmol) in chloro-
form (30 mL), which was stirred in a 15 °C water bath, SiCl4
(3.82 g, 22.5 mmol) was added dropwise within a few minutes and
stirring was continued for further 2 h. The color of the solution
slowly changed from colorless via yellow to brownish-red. The vol-
group R3c,
a
=
b
=
=
11.8203(1),
c
=
70.3118(14) Å,
V
=
=
8507.8(2) Å3,
Z
6, ρcalcd.
=
1.249 Mgm–3, µ(Mo-Kα)
0.148 mm–1, F(000) = 3396, 2θmax = 52.0°, 21647 collected reflec-
tions, 3623 unique reflections (Rint = 0.0293), 237 parameters, S =
1.067, Flack parameter 0.03(7), R1 = 0.0322 [IϾ2σ(I)], wR2(all
466
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 461–467

Cationic Silicon Complex with Ureato Ligands
Böhme, E. Brendler, S. Blaurock, G. Roewer, Z. Anorg. Allg.
Chem. 2005, 631, 2907–2913.
[5] a) D. Gerlach, E. Brendler, T. Heine, J. Wagler, Organometallics
2007, 26, 234–240; b) R. Bertermann, A. Biller, M. Kaupp, M.
Penka, O. Seiler, R. Tacke, Organometallics 2003, 22, 4104–
4110.
[6] a) B. Theis, C. Burschka, R. Tacke, Chem. Eur. J. 2008, 14,
4618–4639; b) O. Seiler, C. Burschka, T. Fenske, T. Troegel,
R. Tacke, Inorg. Chem. 2007, 46, 5419–5424; c) N. Kano, N.
Nakagawa, Y. Shinozaki, T. Kawashima, Y. Sato, Y. Naruse, S.
Inagaki, Organometallics 2005, 24, 2823–2826; d) J. Wagler, U.
Böhme, E. Brendler, B. Thomas, S. Goutal, H. Mayr, B.
Kempf, G. Ya. Remennikov, G. Roewer, Inorg. Chim. Acta
2005, 358, 4270–4286.
[7] a) J. Wagler, A. F. Hill, Organometallics 2008, 27, 6579–6586;
b) J. Wagler, A. F. Hill, Organometallics 2007, 26, 3630–3632;
c) A. R. Bassindale, S. J. Glynn, P. G. Taylor, N. Auner, B.
Herrschaft, J. Organomet. Chem. 2001, 619, 132–140.
[8] I. Kalikhman, B. Gostevskii, M. Botoshansky, M. Kaftory,
C. A. Tessier, M. J. Panzner, W. J. Youngs, D. Kost, Organome-
tallics 2006, 25, 1252–1258.
[9] a) J. Herzfeld, A. E. Berger, J. Chem. Phys. 1980, 73, 6021; b)
J. Mason, Solid State Nucl. Magn. Reson. 1993, 2, 285.
[10] a) Calculation of NMR and EPR parameters: Theory and Appli-
cations (Eds.: M. Kaupp, M. Bühl, V. G. Malkin), Wiley-VCH,
2004; b) A. M. Köster, P. Calaminici, M. E. Casida, R. Flores-
Moreno, G. Geudtner, A. Goursot, T. Heine, A. Ipatov, F.
Janetzko, J. M. del Campo, S. Patchkovskii, J. Ulises Reveles,
D. R. Salahub, A. Vela, deMon2k developers, 2006; c) M. J.
Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tom-
asi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratch-
ian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gom-
perts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C.
Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A.
Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dap-
prich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q.
Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayak-
kara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian 03, Re-
vision C.02, Gaussian, Inc., Wallingford, CT, 2004; d) G. te
Velde, F. M. Bickelhaupt, E. J. Baerends, C. Fonseca Guerra,
S. J. A. van Gisbergen, J. G. Snijders, T. Ziegler, J. Comput.
Chem. 2001, 22, 931; e) ADF 2008.01, ADF User’s Guide,
http://www.scm.com, SCM, Theoretical Chemistry, Vrije Uni-
versiteit, Amsterdam.
data) = 0.0784, max./min. residual electron density +0.288/–
0.238 eÅ–3.
X-ray Crystal Structure Analysis of 3s-Cl (structure B): CCDC-
743691,
crystal
dimensions
0.45ϫ0.30ϫ0.25 mm,
C33H45ClN6O3Si, Mr = 637.29, T = 150(2) K, trigonal, space group
P31c, a = b = 12.0280(6), c = 14.3543(14) Å, V = 1798.5(2) Å3, Z
= 2, ρcalcd. = 1.177 Mgm–3, µ(Mo-Kα) = 0.179 mm–1, F(000) = 680,
2θmax = 50.0°, 6127 collected reflections, 2109 unique reflections
(Rint = 0.0402), 135 parameters, S = 1.023, Flack parameter
0.00(10), R1 = 0.0371 [IϾ2σ(I)], wR2(all data) = 0.0852, max./min.
residual electron density +0.167/–0.180 eÅ–3.
Supporting Information (see also the footnote on the first page of
this article): Color representations of the six independent cations
3s+ of structure C of 3s-Cl.
Acknowledgments
TU Bergakademie Freiberg and the Deutsche Forschungsgemein-
schaft (DFG) are acknowledged for financial support of this work.
[1] Selected reviews on hypercoordinate silicon chemistry: a) D.
Kost, I. Kalikhman, Acc. Chem. Res. 2009, 42, 303–314; b) D.
Kost, I. Kalikhman, Adv. Organomet. Chem. 2004, 50, 1–106;
c) R. Tacke, M. Pülm, B. Wagner, Adv. Organomet. Chem. 1999,
44, 221–273; d) C. Chuit, R. J. P. Corriu, C. Reye, J. C. Young,
Chem. Rev. 1993, 93, 1371–1448; e) R. J. P. Corriu, C. Guerin,
J. J. E. Moreau, in: Chem. Org. Silicon Compd. (Eds: S. Patai,
Z. Rappoport), Wiley, Chichester, 1989, vol. 1, pp. 305–370; f)
R. J. P. Corriu, J. C. Young, in: Chem. Org. Silicon Compd.
(Eds: S. Patai, Z. Rappoport), Wiley, Chichester, 1989, vol. 2,
pp. 1241–1288.
[2] For recent publications on hexacoordinate silicon complexes
see for example: a) A. Kämpfe, E. Kroke, J. Wagler, Eur. J.
Inorg. Chem. 2009, 1027–1035; b) K. Lippe, D. Gerlach, E.
Kroke, J. Wagler, Organometallics 2009, 28, 621–629; c) G. W.
Fester, J. Wagler, E. Brendler, U. Böhme, D. Gerlach, E. Kroke,
J. Am. Chem. Soc. 2009, 131, 6855–6864; d) S. Metz, C.
Burschka, R. Tacke, Chem. Asian J. 2009, 4, 581–586; e) S.
Metz, C. Burschka, R. Tacke, Organometallics 2009, 28, 2311–
2317; f) G. González-García, E. Álvarez, Á. Marcos-
Fernández, J. A. Gutiérrez, Inorg. Chem. 2009, 48, 4231–4238;
g) I. Kalikhman, E. Kertsnus-Banchik, B. Gostevskii, N.
Kocher, D. Stalke, D. Kost, Organometallics 2009, 28, 512–516;
h) S. Yakubovich, B. Gostevskii, I. Kalikhman, D. Kost, Orga-
nometallics 2009, 28, 4126–4132.
[3] a) J. Yang, J. G. Verkade, J. Organomet. Chem. 2002, 651, 15;
b) H. H. Karsch, P. A. Schlüter, M. Reisky, Eur. J. Inorg. Chem.
1998, 433–436; c) C.-W. So, H. W. Roesky, P. M. Gurubasawa-
raj, R. B. Oswald, M. T. Gamer, P. G. Jones, S. Blaurock, J.
Am. Chem. Soc. 2007, 129, 12049–12054; d) H. H. Karsch, R.
Richter, E. Witt, J. Organomet. Chem. 1996, 521, 185.
Received: August 12, 2009
Published Online: December 8, 2009
[4] a) G. W. Fester, J. Wagler, E. Brendler, U. Böhme, G. Roewer,
E. Kroke, Chem. Eur. J. 2008, 14, 3164–3176; b) J. Wagler, U.
Eur. J. Inorg. Chem. 2010, 461–467
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
467