Chlorosilanes and 3,5-dimethylpyrazole: Multinuclear complexes, acetonitrile insertion and 29Si NMR chemical-shift anisotropy studies

Article abstract of DOI:10.1002/ejic.201300109

The multinuclear silicon complexes [{H₂ClSi(μ-pz*) ₂}₂SiH₂] (1) and [Cl₂Si(μ-pz ^RR)₂SiCl₂] (2, R = Me; 3, R = Ph) were formed by the reaction of dichlorosilane (for 1) or hexachlorodisilane with 1-trimethylsilyl-3,5-dimethylpyrazole (for 1 and 2) and the 3,5-diphenyl analogue (for 3). The reaction of trichlorosilane with 3,5-dimethylpyrazole (Hpz*) in MeCN resulted in the formation of two MeCN insertion products of 2 (4, 5). From different samples of synthesis products of this work a preferred hydrolysis product was obtained: [{(μ-pz*)₂SiH} ₃-μ³-O]⁺Cl^- (6). The structural and electronic features of the compounds synthesised in this work were analysed with single-crystal X-ray diffraction and ²⁹Si cross-polarisation (CP)/magic-angle spinning (MAS) NMR spectroscopy combined with quantum chemical computations to investigate their ²⁹Si chemical-shift anisotropy principal tensor components. The reaction of trichlorosilane with 3,5-dimethylpyrazole (Hpz*) in acetonitrile afforded dinuclear silicon compounds. Their outstanding feature is the formal insertion of one or two molecules of MeCN into the former N-Si bond of a Sipz* moiety. These compounds were examined with single-crystal X-ray diffraction, ²⁹Si cross-polarisation (CP)/magic-angle spinning (MAS) NMR spectroscopy and quantum chemical calculations. Copyright

Full text of DOI:10.1002/ejic.201300109

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DOI:10.1002/ejic.201300109
Chlorosilanes and 3,5-Dimethylpyrazole: Multinuclear
29
Complexes, Acetonitrile Insertion and Si NMR
Chemical-Shift Anisotropy Studies
Florian Bitto,[a] Konstantin Kraushaar,[a] Uwe Böhme,[a]
Erica Brendler,[b] Jörg Wagler,[a] and Edwin Kroke*[a]
Dedicated to Prof. Dr. Werner Uhl on the occasion of his 60th birthday
Keywords: Silicon / Hypercoordination / NMR spectroscopy / Insertion / Nitrogen heterocycles
The multinuclear silicon complexes [{H2ClSi(μ-pz*)2}2SiH2]
(1) and [Cl2Si(μ-pzRR)2SiCl2] (2, R = Me; 3, R = Ph) were
formed by the reaction of dichlorosilane (for 1) or hexachloro-
disilane with 1-trimethylsilyl-3,5-dimethylpyrazole (for 1 and
2) and the 3,5-diphenyl analogue (for 3). The reaction of tri-
chlorosilane with 3,5-dimethylpyrazole (Hpz*) in MeCN re-
sulted in the formation of two MeCN insertion products of 2
(4, 5). From different samples of synthesis products of this
work a preferred hydrolysis product was obtained: [{(μ-
pz*)2SiH}3–μ3-O]+Cl– (6). The structural and electronic fea-
tures of the compounds synthesised in this work were ana-
lysed with single-crystal X-ray diffraction and 29Si cross-po-
larisation (CP)/magic-angle spinning (MAS) NMR spec-
troscopy combined with quantum chemical computations to
investigate their 29Si chemical-shift anisotropy principal ten-
sor components.
Introduction
The (hydrido)chlorosilanes tetrachlorosilane (SiCl4), tri-
chlorosilane (HSiCl3) and dichlorosilane are currently the
most important starting materials for the industrial pro-
duction of solar-grade silicon as well as ultrapure silicon
for electronic applications. One possible route to obtain the
silicon is the thermal decomposition of HSiCl3 into silicon,
hydrogen and tetrachlorosilane as byproduct (the
Siemens process). A variation thereof involves dismutation
of HSiCl3 into SiH4 and SiCl4, catalysed by nitrogen-con-
taining organic compounds, followed by thermal cracking
of the resulting SiH4 and deposition of silicon.[1] A main
objective in this research field is the understanding of the
mechanism of the dismutation and the isolation of interme-
diate species (e.g., higher-coordinated silicon complexes).[2]
Our initial studies on complexation of trichlorosilane
and other hydridochlorosilanes with pyridine bases[3] are
[a] Institut für Anorganische Chemie, TU Bergakademie Freiberg,
Leipziger Strasse 29, 09596 Freiberg, Germany
Fax: +49-3731-39-4058
E-mail: kroke@chemie.tu-freiberg.de
Homepage: http://tu-freiberg.de/fakult2/aoch/agsi/
index.html
[b] Institut für Analytische Chemie, TU Bergakademie Freiberg,
Leipziger Strasse 29, 09596 Freiberg, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300109.
Scheme 1. Selected previously reported pyrazolyl and imidazolyl
silicon compounds.
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now extended by a study of 3,5-dimethylpyrazole. Even
though the use of 3,5-dimethylpyrazole (Hpz*) as a base
(deprotonator or ligand) in silicon chemistry is less com-
mon than the use of pyridine bases, there are already some
previous reports on multinuclear silicon complexes that in-
corporate this heterocycle. The compounds of type [(μ-
pz*)2Si2Cl4–n(pz*)n] (I) (n = 0, 2, 4; Scheme 1) were first
described by Hübler et al. (R = RЈ = Cl, 2);[4a] they were
obtained from the reaction of HSiCl3 with Hpz* in the
presence of NEt3. Breher et al. completed this series of disil-
ane complexes with a combined method of trans-silylation
and salt elimination starting from Si2Cl6.[4b]
Only a few examples of reactions in the coordination
sphere of such pyrazolyl-functionalised silicon compounds
have been reported so far. The reaction of MeSi(pz*)3 with
a zirconium imido compound resulted in a new hetero-
scorpionate complex (II; Scheme 1).[5a] The reaction of di-
methyl formamide with MeSi(pz*)3, which afforded a
scorpionate-like functionalised methane derivative, was re-
ported by Schugar et al. (III; Scheme 1).[5b] For another
heterocycle, recently Tacke et al. reported the activation of
nitriles by [2-(dialkylphosphanyl)imidazole-1-yl]trichloro-
silane to yield pentacoordinate silicon compounds (IV).[5c]
Compounds obtained from the reaction of nitriles with
Si/pz* have not been reported yet and are presented in this
work.
Figure 1. ORTEP representation (30% probability ellipsoids) of the
molecular structure of 1 in the crystal structure of 1·THF. Carbon-
bound hydrogen atoms and THF molecules are omitted for clarity.
Scheme 2. Synthesis of 1 by means of the trans-silylation approach.
disordered (by inversion symmetry) owing to its location
Our own work, which initially focused on the examina- close to a centre of inversion. There are one hexacoordinate
tion of 3,5-dimethylpyrazole in the dismutation of tri- (Si1) and two pentacoordinate (Si2) silicon atoms present
chlorosilane, resulted in a variety of unexpected products of in the molecular structure, with the octahedral coordination
a class of multinuclear higher-coordinated silicon com- geometry around Si1 being nearly ideal. The cis-bond
plexes. These compounds revealed some highly interesting angles are close to 90° [i.e., N2–Si1–N4 = 88.61(6)° and
reactivity patterns that involve the insertion of acetonitrile N2–Si1–N4* = 91.39(6)°, N2–Si1–H1 = 91.1(7)° and N2*–
and Si–Si bond activation. A mechanism for the formation Si1–H1 = 88.9(7)°]. The Si1–N2 and Si1–N4 bond lengths
of the acetonitrile-insertion products is proposed.
[1.942(6) and 1.911(9) Å, respectively] are similar to those
observed for the ionic dichlorosilane–Hpz* adduct
[SiH2(Hpz*)4]Cl2, which are 1.936(1) and 1.949(1) Å.[6] The
angle between the least-squares planes of the five-mem-
bered rings of a pair of bridging pz* moieties is 113.93(6)°.
At silicon atom Si2 the coordination geometry corresponds
to a distorted trigonal bipyramid. The equatorial level is
Results and Discussion
Syntheses and Molecular Structures
The reaction of HSiCl3 with equimolar quantities of occupied by N3 and the hydrogen atoms, whereas the axial
Hpz* and NEt3 in THF resulted in the formation of two positions are occupied by N1 and Cl1. The geometry of the
multinuclear complexes with bridging pz* units. The new coordination sphere is distorted from trigonal bipyramidal
compound [{ClSiH2(μ-pz*)2}2SiH2] (1; Figure 1) and the towards square pyramidal (τ = 0.58).[7] Whereas the axial
previously reported compound [Cl2Si(μ-pz*)2SiCl2] (2; for angle N1–Si2–Cl1 [172.7(1)°] is still close to the expected
crystal structure data, see the Supporting Information) were value of 180°, the equatorial angles differ dramatically from
both obtained as crystalline products suitable for single- the ideal 120° [i.e., N3–Si2–H2b = 108.4(7)°, N3–Si2–H2a
crystal X-ray diffractometry. Apparently, the formation of = 137.7(8)° and H2a–Si2–H2b = 113.9(10)°]. The Si–Cl
1 involves steps of the dismutation of HSiCl3, as indicated bond is remarkably longer than that in dichlorosilane
by the three SiH2 moieties present in the molecule. To prove [H2SiCl2: 2.033 Å, 1·THF: 2.250(1) Å].[8] Although the
the feasibility of the synthesis of 1 from dichlorosilane we length of the axial bond Si2–N1 [1.920(6) Å] is similar to
used a trans-silylation approach. The reaction of dichloro- the other Si–N bonds in the molecule, the equatorial bond
silane (H2SiCl2) with Me3Sipz* resulted in the formation of Si2–N3 is clearly shorter.
1 with a yield of 86% (see the Exp. Sect. and Scheme 2).
For further investigations we also employed the trans-
¯
Solvate 1·THF crystallises in the triclinic space group P1. silylation procedure to accomplish the syntheses of com-
The asymmetric unit consists of one half of a formula unit, pound 2[4b] (Scheme 1) and the sterically more hindered 3,5-
because the central silicon atom is located on a crystallo- diphenylpyrazolyl (pzPh ) derivative bis(3,5-diphenylpyr-
2
[Cl2Si(μ-pzPh )2SiCl2]
(3;
2
graphic centre of inversion. The solvent molecule is twofold azol-1-yl)tetrachlorodisilane
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Scheme 3). Crystals of 3 suitable for X-ray structure analy- withdrawal from nitrogen to result in a weaker Si–N bond.
sis were obtained directly from the reaction solution. The As a response to the Si–N bond lengthening, Si–Cl bond
compound crystallises in the monoclinic space group C2/c shortening is observed in 3.
with one half of a molecule in the asymmetric unit (Fig-
ure 2).
One of our approaches towards dismutation of tri-
chlorosilane was the reaction of HSiCl3 and Hpz* in aceto-
nitrile (MeCN) heated to reflux as polar medium. Surpris-
ingly, a green solution formed and upon cooling colourless
crystals formed. These were identified as the two new dinu-
clear silicon complexes 4 and 5 that resulted from the for-
mal insertion of MeCN into the N–Si bonds of Si-bound
pz* moieties (Figure 3).
Scheme 3. Synthesis of 3 by means of the trans-silylation approach.
Figure 2. ORTEP representation (30% ellipsoid probability) of the
molecular structure of 3. Hydrogen atoms are omitted for clarity.
Figure 3. Left: ORTEP representation (30% probability ellipsoids)
of the molecular structures of 4 (modification 4a) and 5. Most car-
bon-bound hydrogen atoms are omitted for clarity. Right: Corre-
sponding Lewis structures according to bond-length analysis of the
molecular structures. Moieties resulting from acetonitrile are high-
lighted in red.
The silicon atoms in 2 and 3 are pentacoordinate and the
coordination polyhedra are intermediate between trigonal
bipyramids and square pyramids (τ = 0.44 and 0.46, respec-
tively).[7] In contrast to the crystal structure of 2 (asymmet-
ric unit = 1/4 molecule, space group C2/m), the molecular
structure of 3 in its crystals has lower symmetry (asymmet-
ric unit = 1/2 molecule, space group C2/c); this is caused by
the arrangement of the phenyl groups, which are noticeably
out of plane with respect to the pyrazolyl units [torsion
angles: N1–C1–C4–C5 37.6(8)°, N2–C3–C10–C15 29.3(8)°].
Comparison with 2 reveals some significantly different
bond lengths in 3 (Table 1). The longer Si–N bond in 3
can be interpreted both as a result of the sterically more
demanding phenyl substituents and as a response to the
change to a substituent with more intense “negative induc-
tive effect” (–I effect), which leads to an electron-density
For compound 4 we obtained crystals of two monoclinic
modifications, 4a and 4b (with space group P21 and P21/c,
respectively); in both cases one molecule was in the asym-
metric unit. The most remarkable feature of this compound
is the moiety that results from formal insertion of two mole-
cules of acetonitrile in a former N–Si bond of compound
1. The atoms C7, C6, N3 and C9, C8, N4 each represent
one molecule of acetonitrile. The former nitrile groups were
transformed into an amine (N3) and an imine (N4) group.
This complex features a penta- and a hexacoordinate silicon
atom. Si1 is surrounded by six ligands in a distorted-octahe-
dral manner. All Si–N bonds in this compound are signifi-
cantly shorter than in the other compounds presented in
this paper (Table 2).
Table 1. Comparison of selected bond lengths in 2 and 3.
In the coordination sphere of the hexacoordinate Si atom
the Si–N bond lengths are short with respect to formal
dative Si–N(sp2) bonds in other higher-coordinated silicon
compounds.[9,10] The coordination sphere of the pentacoor-
dinate silicon atom in this complex (Si2) is slightly distorted
trigonal bipyramidal (τ = 0.85 and 0.89 for 4a and 4b,
respectively).[7] The very short distance between Si2 and N3
is most likely caused by the covalent nature and equatorial
2 [Å]
3 [Å]
Si–Si
Si–N
Si–Cl
N–N
N–C1
C1–C2
2.282(1)
1.922(2)
2.0625(5)
1.386(2)
1.337(3)
1.393(3)
2.281(3)
1.963(4)
2.038(2)
1.384(6)
1.360(7)
1.384(8)
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Table 2. Selected bond lengths in the Si coordination spheres in 4
(4a left, 4b right).
Si1–X, X =
d(Si1–X) [Å]
Si2–X, X =
d(Si2–X) [Å]
N1
N3
N6
Cl1
Cl2
1.879(2), 1.863(2)
1.880(2), 1.877(2)
1.839(2), 1.824(2)
2.228(1), 2.279(1)
2.304(1), 2.297(1)
N3
N4
N5
Cl3
Cl4
1.764(2), 1.762(2)
1.834(2), 1.840(2)
1.891(2), 1.895(2)
2.103(1), 2.097(1)
2.109(1), 2.076(1)
position of this bond and also supported by the rigid struc-
ture of the molecular backbone. Also, the relative short Si2–
N4 bond is remarkable because it represents a formal dative
Si–N(sp2) bond in an axial position. This is rationalised by
the low steric demand of the hydrogen-substituted nitrogen-
donor atom and the rigid molecule backbone. The feature
of a surprisingly short coordinative bond between an NH-
functionalised imine and silicon was also found in a hexaco-
ordinate silicon complex [with Si–N 1.909(2) Å].[10]
Some related features are found in the molecular struc-
ture of 5, which comprises two hexacoordinate silicon
atoms. These are coordinated by two pz* anionic ligands
and one ketimine-substituted pz* moiety (N7–C16–C17,
through formal MeCN insertion). The coordination geome-
try around Si1 is a distorted octahedron with respect to the
ideal value of the angles [e.g., N3–Si1–N1 = 88.72(5)°, N1–
Si1–N7 = 83.62(4)°, N1–Si1–N5 = 96.72(5)]. The Si–N
bond lengths are similar to or longer than those around the
hexacoordinate Si atom in 4 (Table 3).
Scheme 4. Proposed reaction sequence for the formation of the
backbone of 4 and 5; [Si–Si] denotes the silicon complexes (δ+ and
δ– indicate partial charges).
(Scheme 4, A) and insertion. The resulting ketimine (inter-
cepted as 5) is in equilibrium with its tautomeric enamine
(Scheme 4, B) with its sp2-hybridised and Lewis basic ter-
minal carbon atom, thus enabling another insertion of an
MeCN molecule to form the ligand backbone of 4. The
presented mechanism is based on simple reactivity consid-
erations. Clearly more reactions have to be involved in this
process, namely, the coordination of the pz* units to the
silicon compounds and the dismutation of HSiCl3 as well
as the deprotonation of N3. To prove that complex 2 is a
potential starting material to form 4, thus underlining its
role as a potential intermediate, we carried out reactivity
studies with compounds 2 and 3. Treatment of 2 with aceto-
nitrile resulted in the exclusive formation of the double
MeCN insertion product 4, whereas compound 3 failed to
react under the same conditions. The rather inert behaviour
Table 3. Si–N bond lengths in compound 5.
Si1–N[a]
d(Si1–N) [Å]
Si2–N[a]
d(Si2–N) [Å]
N1
N3
N5
N7
1.891(1)
1.859(1)
1.936(1)
1.892(1)
N2
N4
1.933(1)
1.904(1)
N7
1.910(1)
[a] Bond to a specific nitrogen atom in the coordination sphere.
As for Si1, a distorted-octahedral coordination sphere is
present around Si2. The Si2–N distances are longer than
Si1–N, which is most likely caused by the chloro ligands
trans-disposed to the nitrogen atoms of the pz* units. The
bond lengths Si1–N7 and Si2–N7, with a trans-disposed Si–
H bond in both cases, are only slightly different.
On the one hand, the facts that in 5 one molecule of
MeCN is formally inserted and the chlorosilane units are
not affected by H/Cl redistribution suggest that 5 is an in-
termediate in the formation of 4 (Scheme 5). On the other
hand, the formation of 4 can also be rationalised by the
twofold insertion of acetonitrile into a Si–N bond of com-
pound 2, a compound that formed upon dismutation of
trichlorosilane. For the latter we propose the reaction se-
quence as depicted in Scheme 4.
On the basis of the findings of Hübler et al.[4a] we assume
that first compound 2 is formed (Hpz* is acting as a base),
which then reacts in several steps to yield the final products
(Scheme 5). The first of these is a concerted nucleophilic
attack at the electrophilic carbon (δ+) atom of MeCN Scheme 5. Reaction of 2 to 4 and 5.
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of compound 3 towards MeCN can be attributed to the
greater steric hindrance of the phenyl substituents in con-
trast to the methyl groups.
In some samples prepared with different ratios of HSiCl3
and Hpz*, we found crystals of the interesting hydrolysis
products 6. The cation of this ensemble of products was
first reported by Hübler and co-workers.[4a] We now report
three different crystal structures (of different solvates), and
one of these compounds was characterised by NMR (1H,
13C, 29Si) and Raman spectroscopy (see the Supporting In-
formation). The molecular structure of compound 6 con-
sists of a complex monocation and one chloride as anion
(Figure 4). The numbering 6a–c describes differences in the
crystal structure. The cation consists of three pz*-bridged
Si–H units and one μ3-oxo ligand. For a discussion of the
different crystal structures, see the Supporting Information.
Figure 5. 29Si CP/MAS NMR spectra of 1·THF at different MAS
frequencies and assignment of the isotropic chemical shifts.
Quantum chemical calculations were performed to align
the orientation of the CSA tensor principal components in
the molecular structure. The experimental and calculated
values for compound 1 are given in Table 4; the orientation
of the tensor components δ11, δ22 and δ33 is shown in Fig-
ure 6. Calculated and experimental data are in excellent
agreement for the pentacoordinate silicon atom Si2. The
calculated values for the hexacoordinate silicon atom Si1
show larger deviations from the experimental values, which
in principle reflect a systematic upfield shift of all compo-
nents, thus the overall shape of the tensor (represented by
span Ω and skew κ) is retained. Calculation of this 29Si
CSA tensor after optimisation [at the MPW1PW91/6-
311G(d,p) level of theory] of the Si–H bonds did not cure
this systematic upfield shift. The principal tensor compo-
nents δ11, δ22 and δ33 of Si1 are aligned along the molecular
axes N2–Si1–N2#1, N4–Si1–N4#1 and H1a–Si1–H1b,
respectively.
Table 4. Comparison of the experimental[11] and calculated values
for the 29Si CSA tensor principal components of the pentacoordi-
nate (Si2) and hexacoordinate (Si1) silicon atom in 1 (chemical
shifts δ and span Ω in ppm).
Si2 (exp.)
Si2 (calcd.)
Si1 (exp.)
Si1 (calcd.)
Figure 4. Molecular structure of 6 and crystal structure variations
of 6a–c.
δ11
δ22
δ33
δiso
Ω
–25.3
–157.4
–199.1
–127.3
173.8
–21.3
–155.0
–205.6
–127.4
184.6
–175.6
–192.2
–223.7
–197.4
48.1
–191.4
–206.1
–237.7
–211.7
46.2
κ
–0.52
–0.45
0.28
0.37
29Si Cross-Polarisation (CP)/Magic-Angle Spinning (MAS)
NMR Spectroscopy and Chemical-Shift Tensor Analyses
According to the literature, the investigation of other
In the 29Si CP/MAS NMR spectrum of 1, the silicon hexacoordinate silicon complexes showed that substituents
atom Si2 creates a large number of spinning sidebands. (with their coordinating free-electron pairs) influence the
Therefore, the isotropic shift values of Si1 and Si2 (δiso
)
shielding or deshielding in the orthogonal direction.[12] For
were determined from spectra recorded at different MAS the hexacoordinate silicon atom (Si1) in 1, this leads to the
frequencies (Figure 5). Signals for the hexacoordinate and interpretation that the four equatorial nitrogen atoms lead
the pentacoordinate silicon atom were found at δiso(Si1) = to the highest shielding in the perpendicular direction (i.e.,
–197.4 ppm and δiso(Si2) = –127.3 ppm, respectively. Owing along the axis H1a–Si1–H1b), whereas the Si–H bonds
to the large chemical-shift anisotropy (CSA) of the pentaco- cause lower shielding in the plane perpendicular to the
ordinate silicon atom, the principal components of the H1a–Si1–H1b axis, reflected by δ11 and δ22. Despite these
chemical shifts were determined from the sideband inten- apparent differences, the shielding of Si1 shows a small
sities of a spectrum recorded at 1.7 kHz.[11]
span of only 48 ppm.
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Table 5. Experimental[11] and calculated 29Si CSA tensor principal
components of 2 and 3 (chemical shifts δ and span Ω in ppm).
2 (exp.)
2 (calcd.)
3 (exp.)
3 (calcd.)
δ11
δ22
δ33
δiso
Ω
53.3
–111.4
–142.8
–67.5
196.1
–0.68
81.8
–119.7
–140.8
–59.6
222.6
–0.81
55.2
–104.2
–141.0
–63.5
196.2
–0.63
76.6
–107.9
–136.9
–56.1
213.5
–0.73
κ
shows noticeably large deviation from the experimental
value. This might be caused by computational difficulties
that arise from the two chlorine atoms perpendicular to δ11.
Further information about the quantum chemical treatment
of this problem can be found in the Supporting Infor-
mation. The lowest shielding component (δ11) is aligned
perpendicularly to the equatorial plane of the trigonal bi-
pyramid (see Figure 7). The components of essentially
higher shielding, δ22 and δ33, are situated in the equatorial
plane. The direction of highest shielding points along the
silicon–silicon bond.
Figure 6. Orientation of the 29Si CSA tensor principal components
(calculated) for the pentacoordinate (top) and the hexacoordinate
(bottom) silicon atoms in 1.
In contrast to the shielding of Si1, the 29Si CSA tensor
of the pentacoordinate silicon atom Si2 has a large span.
The direction of lowest shielding (δ11 = –25.3 ppm) is nearly
parallel to the axis (Cl1–Si2–N1) of the trigonal bipyramid.
Substantially larger shielding is observed in the equatorial
plane of the coordination polyhedron at Si2.
The principal components δ22 (–157.4 ppm) and δ33
(–199.1 ppm) point along the Si2–H2a bond and intersect
the H2a–Si2–H2b angle, respectively. As for Si1, the shield-
ing in this coordination geometry can be explained by the
ligand influence in the perpendicular direction. The de-
Figure 7. Orientation of the 29Si CSA tensor principal components
of 2.
Furthermore, the tensor components for compound 4
shielding contributions of the three equatorial substituents have been calculated and determined by NMR spec-
(H2a, H2b, N3) superimpose along the axis of the trigonal troscopy. The hexacoordinate silicon atom Si1 shows very
bipyramid, whereas there are only two axial substituents low span of the chemical-shift tensor (Ω = 10.9 ppm;
(Cl1 and N1), which contribute noticeably to the deshield- Table 6). Therefore, the orientation of the principal compo-
ing in the equatorial plane.
nents, which are similar to one another, is less reliable than
For the purpose of further investigations, 2 was synthe- in the previous cases. Tensor components δ11 and δ22 lie
sised after a procedure published by Breher et al. and was roughly in the plane formed by the atoms N1, N6, Cl1 and
also obtained as single crystals (see the Supporting Infor- Cl2 and intersect the bond angles between these atoms (Fig-
mation).[4b] With 29Si CP/MAS NMR spectroscopic tech- ure 8). The direction of highest shielding (component δ33)
niques and quantum mechanical calculations the 29Si CSA nearly matches the axis N3–Si1–H1. As expected for the
tensor principal components for 2 and 3 were determined pentacoordinate Si atom, the magnetic shielding at Si2 is
as well (Table 5).[11]
highly anisotropic. The lowest shielding (δ11) is found along
The orientation of the CSA tensor components in 2 and the axis of the trigonal bipyramid (N4–Si2–N5; see Fig-
3 is essentially the same. This can easily be rationalised by ure 8). The directions of noticeably higher shielding (com-
the very similar coordination environment of the silicon ponents δ22 and δ33) are situated in the equatorial plane of
atoms in both molecules. Therefore, only the tensor compo- the trigonal bipyramid. The experimental 29Si NMR spec-
nents in 2 will be discussed here. Owing to the distorted- troscopic data show good agreement with the calculated
trigonal-bipyramidal coordination geometry at silicon, the data. Similar deviations as for 1 are observed between cal-
tensor shows a large span. The calculated value for δ11 culated and measured values (for discussion, see above).
Eur. J. Inorg. Chem. 2013, 2954–2962
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Table 6. Calculated and experimental[11] values for the 29Si CSA
tensor principal components of the pentacoordinate (Si2) and
hexacoordinate (Si1) silicon atom in 4 (chemical shifts δ and span
Ω in ppm).
with a concentration of O2 and H2O below 5 ppm. Tetrahydrofuran
was distilled from Na/benzophenone and kept in the dark under
argon. Triethylamine and CDCl3 were distilled from CaH2 and
stored under argon. Acetonitrile, toluene and C6D6 were stored
over molecular sieves and under argon; n-hexane was stored over
sodium wire. Dichlorosilane was condensed into a Schlenk flask at
–78 °C and diluted with solvent (toluene) to give a 40% solution,
which was stored under argon in the refrigerator. Tetrachlorosilane,
trichlorosilane, hexachlorodisilane, 3,5-dimethylpyrazole (Hpz*)
Si2 (calcd.)
Si2 (exp.)
Si1 (calcd.)
Si1 (exp.)
δ11
δ22
δ33
δiso
Ω
43.7
13.9
–193.8
–196.7
–204.7
–198.4
10.9
–188.6
–195.7
–203.0
–195.8
14.4
–183.6
–196.4
–112.1
240.1
–171.1
–194.3
–117.2
208.3
and 3,5-diphenylpyrazole (HpzPh ) were used as received. NMR
2
spectra were recorded with a Bruker DPX 400 spectrometer op-
erating at 400.13 MHz (1H). The spectra were referenced internally
to tetramethylsilane or residual (protic) solvent signals. The 29Si
solid-state CP/MAS NMR spectra were recorded with a Bruker
Avance 400 MHz WB spectrometer operating at 79.51 MHz (29Si)
using a 7 mm probe and a spinning frequency of 4 kHz, if not
noted otherwise. The chemical-shift scale was referenced with
Q8M8 relative to TMS. Raman spectra were recorded with a Bruker
Optik RFS 100/S spectrometer equipped with an Nd:YAG laser
(1500 mW, 1064 nm) in the range 3600–90 cm–1. Elemental analyses
were carried out with a Vario Micro Cube analyser from Elementar
(Hanau, Germany).
κ
–0.89
–0.73
0.48
0.02
Trimethyl(3,5-dimethylpyrazol-1-yl)silane (Me3Sipz*): Me3SiCl
(10.9 g, 100 mmol) was slowly added to a stirred solution of Hpz*
(9.61 g, 100 mmol) and NEt3 (10.2 g, 100 mmol) in n-hexane
(360 mL). It immediately formed a colourless suspension. It was
stirred for 2 h and stored at 5 °C for 24 h. The colourless solid was
removed by filtration (over diatomite) and washed with n-hexane
(40 mL). Complete removal of the solvent from the combined fil-
trate and washings afforded a colourless liquid, yield 15.3 g,
91 mmol, 91%. 29Si{1H} NMR (79.5 MHz, C6D6, 20 °C): δ = 12.5
Figure 8. Orientation of the 29Si CSA tensor principal components
for the pentacoordinate and the hexacoordinate silicon atoms in 4.
1
(s, JSi,C = 29 Hz) ppm. 1H NMR (400 MHz, C6D6, 20 °C): δ =
0.40 (s, 9 H, Si–Me3), 2.10 (s, 3 H, pz*–Me), 2.35 (s, 3 H, pz*–
Conclusion
Me), 5.87 (s, 1 H, pz*–H) ppm. 13C{1H} NMR (100.6 MHz, C6D6,
The system HSiCl3/Hpz* exhibits interesting reactivity
patterns that involve the dismutation of trichlorosilane, acti-
vation of nitriles and the formation of multinuclear silicon
complexes.
1
20 °C): δ = 0.16 (s, JSi,C satellites = 29 Hz, Si–Me3), 12.5 (pz*–
Me), 13.6 (pz*–Me), 107.5 (pz*–CH), 145.2 (pz*–CMe), 151.0
(pz*–CMe) ppm. IR (Raman): ν = 100 (w), 196 (w), 257 (vw), 358
˜
(vw), 413 (w), 466 (w), 588 (m), 637 (s), 697 (vw), 764 (vw), 848
(vw), 970 (vw), 1019 (w), 1092 (vw), 1138 (w), 1255 (vw), 1330
(vw), 1380 (vw), 1415 (vw), 1443 (m), 1553 (vw), 2741 (vw), 2902
(vs), 2961 (s), 3088 (vw), 3122 (w) cm–1.
Starting from different chlorosilanes and by utilising a
trans-silylation approach, the multinuclear silicon com-
pounds 1 (H2SiCl2), 2 and 3 (both Si2Cl6) were obtained.
Reacting HSiCl3 and Hpz* in MeCN unexpectedly afforded
the formal insertion products 4 and 5. For the class of
Si/pz* compounds this reactivity pattern is a unique feature,
especially as no transition metal is incorporated. Some light
was shed on the mechanism of formation of 4 by the reac-
tion of 2 with acetonitrile, which gave rise to the formation
of 4 as the exclusive product. Compound 6 resulted from
hydrolyses of different samples in different solvents and is
therefore supposed to represent a preferred hydrolysis prod-
uct, even though the μ3-oxo coordination is a scarcely en-
countered feature in silicon coordination chemistry.
29Si CP/MAS NMR spectroscopy in combination with
quantum chemical calculations was used to determine the
chemical-shift anisotropy characteristics of the silicon
atoms in the compounds under investigation.
Trimethyl(3,5-diphenylpyrazol-1-yl)silane (Me3SipzPh ): Me3SiCl
2
(1.48 g, 13.6 mmol) was slowly added to a stirred solution of
HpzPh (2.00 g, 9.08 mmol) and NEt3 (1.38 g, 13.6 mmol) in THF
2
(25 mL). The immediately formed colourless suspension was stirred
for 24 h. The colourless solid was filtered and washed with THF
(2ϫ10 mL). Complete removal of the solvent from the combined
filtrate and washings afforded a yellow solid, yield 2.5 g, 8.5 mmol,
1
94%. 29Si{1H} NMR (79.5 MHz, CDCl3, 20 °C): δ = 16.7 (s, JSi,C
1
satellites = 29 Hz) ppm. H NMR (400 MHz, CDCl3, 20 °C): δ =
0.32 (s, 9 H, Si–Me3), 6.55 (s, 1 H, pz*–CH), 7.31 (t, 2 H, pz*–
pPh), 7.43 (m, 4 H, pz*–mPh), 7.89 (d, 4 H, pz*–oPh) ppm.
13C{1H} NMR (100.6 MHz, CDCl3, 20 °C): δ = 0.85 (1JSi,C satel-
lites = 29 Hz, Si–Me3), 105.3 (pz*–CH), 125.94 (pz*–C–oPh),
127.61 (pz*–C–pPh), 128.1 (pz*–C–mPh), 128.5 (pz*–C–mPh),
128.6 (pz*–C–pPh), 129.7 (pz*–C–oPh), 133.1 (pz*–C–ipsoPh),
133.9 (pz*–C–ipsoPh), 152.0 (pz*–CPh), 154.2 (pz*–CPh) ppm. IR:
ν = 445 (w), 485 (w), 513 (vw), 536 (vw), 567 (vw), 635 (w), 664
˜
(s), 680 (vs), 698 (vs), 751 (s), 803 (s), 845 (s), 913 (w), 956 (w), 973
(s), 998 (vw), 1024 (vw), 1055 (w), 1066 (m), 1134 (s), 1180 (w),
1216 (vw), 1251 (m), 1270 (w), 1293 (w), 1316 (w), 1475 (s), 1537
(w), 1889 (vw), 1939 (w), 1959 (w), 2957 (w), 3000 (w), 3054 (w),
Experimental Section
General: All manipulations were performed under an inert atmo-
sphere of dry argon using Schlenk techniques or a Braun glovebox
Eur. J. Inorg. Chem. 2013, 2954–2962
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3094 (w) cm–1. C18H20N2Si (292.15): calcd. C 73.92, H 6.89, N
9.58; found C 74.89, H 6.42, N 10.26.
57.80, H 3.68, N 8.64 (the higher values found for carbon and
hydrogen can be assigned to some adherent toluene).
Synthesis of 4: A suspension of 2 (1.58 g, 4.08 mmol) in acetonitrile
(20 mL) was heated at reflux for 8.5 h. The resulting orange reac-
tion mixture was filtered and the colourless solid was washed with
acetonitrile (2ϫ8 mL) and dried under vacuum, yield 1.45 g,
3.08 mmol, 75%. 29Si CP/MAS (79.51 MHz): δ = –117.2 (Si1),
Synthesis of 1: H2SiCl2 (0.76 g, 7.5 mmol, 40% solution in toluene)
was added to a cooled solution (–78 °C) of Me3Sipz* (1.68 g,
10 mmol) in toluene (10 mL). While warming to room temperature
a colourless solution formed. This was stirred at ambient tempera-
ture for 3 h. The colourless solid was filtered, washed with toluene
(2ϫ10 mL) and dried under vacuum, yield 1.2 g, 2.2 mmol, 88%.
Crystals suitable for X-ray diffraction analyses were obtained by
recrystallisation from THF. The analyses were performed with the
crystals of 1·THF. 29Si CP/MAS (79.51 MHz): δ = –127.3 (Si2),
–195.8 (Si2) ppm. IR (Raman): ν = 96 (s), 129 (vs), 156 (w), 185
˜
(w), 223 (w), 252 (w), 271 (w), 289 (w), 311 (w), 368 (m), 404 (m),
435 (vw), 468 (vw), 505 (vw), 586 (w), 596 (m), 628 (vw), 658 (vw),
712 (vw), 761 (w), 806 (vw), 826 (vw), 917 (vw), 952 (vw), 1003
(vw), 1034 (vw), 1062 (vw), 1088 (vw), 1132 (w), 1172 (vw), 1209
(vw), 1297 (w), 1347 (m), 1371 (m), 1383 (w), 1392 (w), 1443 (vs),
1466 (m), 1540 (w), 1556 (w), 1608 (vw), [ν–Si–H] 2182 (vw), 2740
(vw), 2979 (vw), 3064 (vw), 3119 (vw), 3154 (vw), 3262 (vw) cm–1.
C14H20Cl4N6Si2 (470.33): calcd. C 35.75, H 4.29, N 17.87; found C
35.91, H 4.41, N 17.63.
–197.4 (Si1) ppm. Raman: ν = 99 (s), 108 (s), 137 (m), 163 (s), 221
˜
(s), 308 (vw), 335 (w), 363 (vw), 382 (vw), 434 (m), 457 (vw), 591
(s), 654 (vw), 673 (vw), 770 (vw), 804 (vw), 859 (vw), 948 (vw), 976
(vw), 1011 (vw), 1028 (vw), 1066 (w), 1154 (vw), 1178 (vw), 1322
(vw), 1350 (vw), 1375 (vw), 1384 (vw),1441 (s), 1492 (vw), 1533
(w), [ν–Si–H] 2202 (s), [ν–Si–H] 2250 (m), 2749 (vw), 2932 (vs),
2965 (m), 2985 (m), 3049 (vw), 3067 (vw), 3123 (m), 3135 (w) cm–1.
C24H42Cl2N8OSi3 (613.83): calcd. C 46.96, H 6.90, N 18.26; found
C 47.13, H 6.61, N 18.35.
Quantum Chemical Calculations: DFT calculations were carried
out using Gaussian 09.[13] NMR spectroscopic shielding tensors
were calculated with the gauge-independent atomic orbital (GIAO)
method[14] using the B3PW91[15,16] density functionals, in combina-
tion with the 6-311+G(2d,p)[17–19] basis set for all atoms, with the
geometries from X-ray structure analyses. Calculated absolute
shielding values were converted to relative shifts δ with the calcu-
lated shielding for tetramethylsilane at the same level of theory.
Synthesis of 3: Si2Cl6 (1.5 g, 5.6 mmol) was added to a solution of
Me3SipzPh (2.5 g, 8.5 mmol) in toluene (25 mL). After stirring for
2
24 h, the formed colourless solid was filtered, washed with toluene
(2ϫ10 mL) and dried under vacuum, yield 1.23 g, 1.93 mmol,
34.5%. 29Si CP/MAS (79.51 MHz): δ = –62.7 ppm. 13C CP/MAS
(100.63 MHz): δ = 112.0 (pz*–CH), 129.8 (not resolved, pz*–CPh)
X-ray Crystallographic Study: The X-ray diffraction data sets
(Table 7) were collected with a Bruker-Nonius X8 diffractometer
with an APEX2-CCD detector or a Stoe IPDS2 diffractometer
148.6 (pz*–CPh) ppm. Raman: ν = 87 (w), 106 (m), 124(m), 147
˜
(w), 186 (vw), 204 (vw), 250 (vw), 333 (vw), 618 (vw), 663 (vw),
955 (vw), 967 (vw), 1002 (m), 1031 (vw), 1162 (vw), 1196 (vw), using Mo-Kα (λ = 0.71073 Å) radiation. The structures were solved
1228 (vw), 1296 (vw),1376 (m), 1434 (w), 1408 (vw), 1514 (vw), by direct methods (SHELXS-97) and were refined to convergence
1539 (m), 1065 (vs), 3061 (w), 3115 (vw), 3147 (vw) cm–1.
C30H22Cl4N4Si2 (636.51): calcd. C 56.61, H 3.48, N 8.80; found C
on F2 against all of the independent reflections by the full-matrix
least-squares method with the SHELXL-97 program.[20]
Table 7. Crystallographic data for the compounds 1, 2, 3, 4a, 4b and 5.
1
2
3
4a
4b
5
Empirical formula
Mr [gmol–1]
T [K]
C24H42Cl2N8OSi3
613.83
150(2)
C10H14Cl4N4Si2
388.23
150(2)
0.18ϫ0.10ϫ0.02
monoclinic
C2/m
13.3392(7)
7.4536(3)
8.6388(4)
90
108.899(2)
90
812.61(7)
2
1.587
0.870
C30H22Cl4N4Si2
636.50
150(2)
0.25ϫ0.08ϫ0.02
monoclinic
C2/c
26.009(3)
5.9067(5)
20.8503(19)
90
121.836(4)
90
2721.3(4)
4
1.554
0.554
C14H20Cl4N6Si2
470.43
150(2)
0.22ϫ0.18ϫ0.10
monoclinic
P21
8.1679(3)
16.1527(5)
8.3949(3)
90
115.2271(1)
90
1001.94(6)
2
1.559
0.723
C14H20Cl4N6Si2 C17H26Cl3N7Si2
470.43
150(2)
490.98
150(2)
Crystal size [mm3]
Crystal system
Space group
a [Å]
0.16ϫ0.15ϫ0.12
triclinic
0.30ϫ0.15ϫ0.12 0.35ϫ0.22ϫ0.18
monoclinic
P21/c
monoclinic
P21/c
¯
P1
8.1988(2)
8.2711(2)
11.1634(2)
90.273(2)
97.373(1)
99.616(1)
739.95(3)
1
8.1470(2)
12.1125(3)
20.6071(4)
90
11.8960(2)
13.6961(2)
14.87818(3)
90
b [Å]
c [Å]
α [°]
β [°]
96.2780(10)
90
112.286(1)
90
γ [°]
V [Å3]
2021.32(8)
4
2243.00(7)
4
Z
ρcalcd. [gcm–3]
μ [mm–1]
F(000)
1.378
0.375
326
1.546
1.454
0.717
0.536
396
1304
484
968
1024
Θmax [°]
31.99
27.96
23.10
27.89
30.00
31.00
Collected reflections 14785
4898
9408
10408
26282
38794
Unique reflections
Rint
Parameters
S
5097
0.0478
212
1052
0.0294
260
1.030
1813
0.0895
181
1.036
4418
0.0288
248
0.995
5881
0.0443
248
7768
0.0382
277
1.059
1.081
1.066
R1 [IՆ2σ(I)]
ωR2 (all data)
Largest diff. peak/
hole [eÅ–3]
0.0465
0.1143
+0.436/–0.314
0.0273
0.0620
+0.437/–0.243
0.0584
0.1659
+0.497/–0.559
0.0292
0.0652
+0.265/–0.253
0.0428
0.1216
+0.467/–1.028
0.0328
0.0862
+0.472/–0.331
Eur. J. Inorg. Chem. 2013, 2954–2962
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FULL PAPER
Chem. 1999, 38, 132–135; c) K. Junold, C. Burschka, R. Tacke,
Eur. J. Inorg. Chem. 2012, 189–193.
CCDC-919132 (for 1), -919131 (for 2), -919136 (for 3), -919129 (for
4a), -919137 (for 4b) -919130 (for 5), -919133 (for 6a), -919135 (for
6b) and -919134 (for 6c) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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Supporting Information (see footnote on the first page of this arti-
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of 6a, crystal structure picture, data and discussion of 6a–6c and
comments regarding the deviations in the NMR spectroscopic shift
calculations are provided.
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Acknowledgments
The authors thank the German Research Foundation (DFG) and
the TU Bergakademie Freiberg for financial support.
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Received: January 24, 2013
Published Online: April 11, 2013
Eur. J. Inorg. Chem. 2013, 2954–2962
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