Synthesis and structure of stable base-free dialkylsilanimines

Article abstract of DOI:10.1039/c0nj00121j

Four isolable dialkylsilanimines R^H₂SiNR (R = CH ₂Ph (5a), Ph (5b), 1-adamantyl (5c) and SiMe₃ (5d), where R^H₂ = 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl) were synthesized as air-sen

Full text of DOI:10.1039/c0nj00121j

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis and structure of stable base-free dialkylsilanimineswz
Takeaki Iwamoto,*a Nobuyoshi Ohnishi,a Zhenyu Gui,a Shintaro Ishida,a
Hiroyuki Isobe,a Satoshi Maeda,b Koichi Ohnob and Mitsuo Kira*a
Received (in Montpellier, France) 15th February 2010, Accepted 18th March 2010
DOI: 10.1039/c0nj00121j
Four isolable dialkylsilanimines RH2SiQNR (R = CH2Ph (5a), Ph (5b), 1-adamantyl (5c) and
SiMe3 (5d), where RH = 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl) were synthesized as
2
air-sensitive crystals by the reaction of isolable dialkylsilylene 4 with the corresponding azides.
X-ray analysis shows that 5a–5d are base-free silanimines and 5a and 5b have a remarkable bent
SiQN–R structure, while 5c and 5d have an almost linear structure. Distinct p(SiQN) -
p*(SiQN) and n(N) - p*(SiQN) transition bands of silicon–nitrogen double bonds were
observed for 5a–5d in hexane. A mechanism for the formation of silanimine from the reaction of
silylene and azide is discussed using theoretical calculations with the GRRM method for model
reactions.
Recently, we have applied isolable dialkylsilylene 4 (RH2Si;
RH 1,1,4,4-tetrakis(trimethylsilyl)butane-1,4-diyl),11 to
Introduction
=
2
Since the first synthesis of stable silenes (R2SiQCR2) by
Brook1 and of disilenes (R2SiQSiR2) by West,2 various stable
unsaturated silicon compounds have been synthesized using
kinetic and/or electronic stabilization, and the chemistry of
stable unsaturated silicon compounds has become one of the
most exciting fields of organosilicon chemistry.3
the synthesis of various types of stable unsaturated silicon
compounds such as trisilaallene,12a 2-germadisilaallene,12b
silanechalcogenones,12c
fused
tricyclic
disilenes,12d
silaketenimines12e and tetrasiladienes,12f and demonstrated
that silylene 4 is a useful building block for synthesis of stable
unsaturated silicon compounds. In the present paper, we
would like to report the synthesis and properties of base-free
dialkylsilanimines 5 (RH2SiQNR) with a series of substituents
on the nitrogen atom. X-ray analysis shows an interesting
substituent effect of silanimines 5 on the bending of the
SiQN–R moiety. Distinct p(SiQN) - p*(SiQN) and
n(N) - p*(SiQN) transition bands were observed in the
UV-vis spectra. A mechanism for the formation of silanimines
from the corresponding silylene and azides is proposed using
theoretical calculations for model reactions from the global
reaction route mapping (GRRM) method (by Ohno and
Maeda),13 which enables automated and systematic reaction
global mapping on the potential energy surface of a given
chemical formula.
Silanimines (R2SiQNR0) are fascinating compounds because
they are silicon analogs of imines, which are important
functional groups in organic chemistry. Wiberg et al. synthesized
the first stable silanimine (t-Bu)2SiQNSi(t-Bu)3 (1), and
isolated it as the THF-coordinated form in 19854 and the
THF-free form in 1986.5 Klingebiel et al. then showed that
base-free (i-Pr)2SiQN(2,4,6-(i-Pr)3C6H2) (2) can be isolated as
orange crystals in 1986.6 Since then, several stable silanimines
have been investigated.7–9 Nevertheless, most stable silanimines
are stabilized by the coordination to the SiQN moiety, for
instance, with Lewis bases7 or alkali metal halides8 ligated to
the unsaturated silicon atoms, and transition metals ligated
to the unsaturated bond.9 N-Silylated silanimines 15 and
(t-Bu)2SiQNSi(t-Bu)2Ph (3)10 are the only examples of base-
free silanimines whose structure was determined by X-ray
crystallography. Therefore, synthesis and chemical properties
of stable base-free silanimines that should show the intrinsic
structures and properties of the silicon–nitrogen double bond
still remain to be elucidated.
Results and discussion
Synthesis of dialkylsilanimines
Dialkylsilanimines 5a–5d can be synthesized by the reactions
of dialkylsilylene 4 with the corresponding azides.7e,14 Typically,
to a hexane solution of 4, one equivalent of (azidomethyl)-
benzene in hexane was added dropwise at room temperature
(eqn (1)).15 The color of the solution immediately turned from
orange to pale yellow with evolution of gas. After stirring the
mixture for 5 min at room temperature, the volatiles were
removed in vacuo. Recrystallization from hexane gave
analytically pure air-sensitive colorless crystals of (N-benzyl)-
silanimine 5a in 86% yield. Silanimines 5b (R = Ph; pale
yellow crystals, 77%), 5c (R = 1-adamantyl; colorless crystals,
88%) and 5d (R = SiMe3; colorless crystals, 82%) were
synthesized (eqn (1)) in a similar manner. The molecular
structures of silanimines 5a–5d were determined by 1H, 13C
a Department of Chemistry, Graduate School of Science,
Tohoku University, Aoba-ku, Sendai 980-8578, Japan.
E-mail: iwamoto@m.tains.tohoku.ac.jp;
Fax: +81 22 795 7714; Tel: +81 22 795 7714
b Toyota Physical and Chemical Research Institute, Nagakute,
Aichi 480-1192, Japan
w Electronic supplementary information (ESI) available: Details of
theoretical study. CCDC reference numbers 766385–766388. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0nj00121j
z This article is part of a themed issue on Main Group chemistry, and
is dedicated to the late Prof. Pascal Le Floch (NJC co-Editor-in-Chief,
deceased March 2010) in honour of his remarkable contributions to
main group chemistry.
ꢀc
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and 29Si NMR spectroscopy, MS spectrometry, elemental
analysis and X-ray crystallography.
Silylene 4 reacted with the azide in a 1 : 1 manner to give the
corresponding silanimines 5, even when we varied the stoichio-
metry of the reagents: when either excess silylene 4 or the azide
was used, the corresponding silanimine was obtained along
with the recovery of the excess starting materials (4 or azides).
In contrast, West et al. and Lappert et al. have reported that,
when a N-heterocyclic silylene reacted with an excess amount
of the azide, the 1 : 2 adduct (such as tetraazasiloranes or
azidoaminosilanes) was obtained,16b,d and when an excess
amount of N-heterocyclic silylene reacted with the azide, the
2 : 1 adduct (diazasilacyclopropanes) was obtained.7e,16 Severe
steric hindrance of silylene 4 may prevent further addition of 4
or the azides to the resulting silanimines 5.
Fig. 1 ORTEP drawing of silanimine 5a. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (A) and angles (1):
Si(1)–N(1) 1.5823(18), Si(1)–C(8) 1.863(2), Si(1)–C(9) 1.871(2),
N(1)–C(7) 1.446(3), Si(1)–N(1)–C(7) 138.07(15), N(1)–Si(1)–C(8)
123.13(9), N(1)–Si(1)–C(9) 134.84(9), C(8)–Si(1)–C(9) 102.02(9),
Si(1)–N(1)–C(7)–C(6) –161.39(19), N(1)–C(7)–C(6)–C(3) –7.8(3).
Molecular structures of silanimines 5a–5d
Single crystals of silanimines 5a–5d suitable for X-ray single
crystal analysis were obtained by recrystallization from
hexane. The molecular structures of 5a–5d determined by
X-ray analysis are displayed in Fig. 1–4 and the selected
structural parameters of 5a–5d and related silanimines are
summarized in Table 1. Compound 5 is the first example of a
base-free silanimine with a carbon functional-group on the
nitrogen atom.
X-ray analysis shows that silanimines 5a–5d have base-free
silicon–nitrogen double bonds. Thus, silanimines 5a–5d have
planar tricoordinated unsaturated silicon atoms with the sum
of bond angles around the silicon atom (S j(Si)) of 3601,
similar to base-free silanimines 15 and 3.10 The silicon–
nitrogen distances in 5a–5d (1.5496(14)–1.5858(9) A) are much
shorter than the range of silicon–nitrogen single-bond distances
(1.65–1.85 A)17 and close to those of 1 (1.568(3) A) and 3
(1.573(2) A). To the best of our knowledge, the SiQN distance
of silanimine 5c is the shortest (1.5496(14) A) among the
hitherto known silanimines.
Fig. 2 ORTEP drawing of silanimine 5b. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (A) and angles (1):
Si(1)–N(1) 1.5858(9), Si(1)–C(7) 1.8477(10), Si(1)–C(10) 1.8618(11),
Si(1)–N(1)–C(1) 144.42(8), C(7)–Si(1)–N(1) 123.60(5), C(10)–Si(1)–N(1)
133.79(5), C(7)–Si(1)–C(10) 102.61(5), Si(1)–N(1)–C(1)–C(2) 93.83(16).
The most striking structural feature was found in the
geometry around the nitrogen atom. Although silanimines 5c
and 5d have an almost linear structure with Si–N–R bending
angles y of 173.30(15)1 and 177.19(13)1, respectively, similarly
to base-free N-silylated silanimine 1 (y = 177.8(2)1)5 and 3
(y = 168.34(16)1),10 5a and 5b adopt a remarkably bent
structure with y of 138.07(15) and 144.42(8)1. Since theoretical
calculations have predicted that silanimines have quite flat
potential surfaces with respect to the inversion in plane (for
instance, the inversion barrier of the bent silanimine
H2SiQNH with angle y of 126.61 are predicted to be only
25.1 kJ molꢁ1 at the MP4/6-31G(d) level),18 both electronic
and steric effects on N-substituents of 5a–5d could be
responsible for the geometry around the SiQN double bond.
However, the nearly linear SiQN–R geometry of N-silyl-
silanimine 5d would be mainly owing to the electronic rather
than steric effect of the silyl substituents, because similar linear
geometries have been observed in N-silylsilanimines 1 and 3
and well reproduced by the calculation of a model compound
H2SiQN–SiH3 (y = 175.61 at the MP4/6-31G(d) level).18
Similar substituent dependence of the geometry around the
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Table 1 Selected structural parameters of silanimines 5a–5d and
related base-free silanimines
Silanimine
d(SiQN)/A
y(Si–N–R)/1
S j(Si)a
5a (R = CH2Ph)
5b (R = Ph)
1.5823(18)
1.5858(9)
1.5496(14)
1.5685(18)
1.568(3)
138.07(15)
144.42(8)
173.30(15)
177.19(13)
177.8(2)
359.9(1)
360.00(5)
360.00(8)
360.00(8)
359.9(2)
5c (R = 1-Ad)
5d (R = SiMe3)
t-Bu2SiQNSi(t-Bu)3
b
t-Bu2SiQNSi(t-Bu)2Phc
1.573(2)
168.34(16)
359.83(13)
a
Sum of the bond angles around the unsaturated silicon
b
atom. Ref. 5. Ref. 10.
c
silanimines 5a and 5b (1.582(2) and 1.5858(9) A) are slightly
longer than those of linear silanimines 5c and 5d (1.5496(14)
and 1.5685(18) A). The relatively long SiQN distances in 5c
and 5d compared with those of 5a and 5b would be ascribed to
the high p-character of silanimine nitrogen orbitals owing to
the remarkable bent geometry. The structural characteristics
of silanimines 5a–5d will be discussed in detail using DFT
calculations (vide infra).
Fig. 3 ORTEP drawing of silanimine 5c. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (A) and angles (1): Si(1)–N(1)
1.5496(14), Si(1)–C(11) 1.8751(16), Si(1)–C(14) 1.8766(16), N(1)–C(1)
1.426(2), Si(1)–N(1)–C(1) 173.30(15), C(11)–Si(1)–N(1) 127.30(8),
C(14)–Si(1)–N(1) 131.72(8), C(11)–Si(1)–C(14) 100.98(7).
The phenyl ring plane in N-benzylsilanimine 5a is roughly
parallel to the p plane of the SiQN bond with a dihedral angle
between mean plane C(8)–C(9)–Si(1)–N(1) and the mean plane
of the phenyl ring of 15.681. On the other hand, the phenyl
ring in N-phenylsilanimine 5b is almost perpendicular to the
SiQN bond, with a dihedral angle Si(1)–N(1)–C(1)–C(2) of
93.83(16)1, probably because of severe steric interaction
between the phenyl ring and the bulky trimethylsilyl substituents
on the silacyclopentane ring, allowing the conjugation between
n orbital of the silanimine nitrogen atom and p orbitals of
the phenyl ring. Similar perpendicular geometries between
p(silicon) and p(aromatic) systems was observed in stable
disilenes with polycyclic aromatic substituents and the
dialkylsilylene unit (4).12g
In contrast to the molecular structure in the solid state, the
1H NMR spectra of silanimines 5a–5d in benzene-d6 exhibit
1
similar highly symmetric structures: thirty-six H, twelve 13C,
1
and four 29Si nuclei in four trimethylsilyl groups and four H
and two 13C in two methylene groups in silacyclopentane ring
are all equivalent, respectively. This suggests that silanimines
5a–5d adopt a linear averaged structure in solution or a bent
structure with very low inversion barrier of the SiQN–R
moiety. The 29Si resonances of unsaturated silicon atoms
(dSiu) appeared in the low-field region at 75.34 (5a), 59.1
(5b), 19.9 (5c) and 89.9 (5d). Interestingly, the dSiu of 5c
appears at a rather higher field of 19.9 ppm compared with
those of the other silanimines with a carbon functional-group
on the nitrogen atom (5a and 5b). Although the reason for the
high-field shift remains unclear, the linear geometry around
the nitrogen atom of 5c may be responsible. The dSiu of bent
Me2SiNMe (6b) with y = 1351 is calculated to appear at
+74.3 ppm using calculations at the GIAO/B3LYP/
6-311+G(2df,p) level, while that of linear 6b with y = 1801
is calculated to appear at the higher field of ꢁ17.5 ppm.22
Fig. 4 ORTEP drawing of silanimine 5d. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are shown at the 50%
probability level. Selected bond lengths (A) and angles (1):
Si(1)–N(1) 1.5685(18), N(1)–Si(2) 1.6762(19), Si(1)–C(1) 1.8554(19),
Si(1)–C(4) 1.8573(19), Si(1)–N(1)–Si(2) 177.19(13), C(1)–Si(1)–N(1)
129.72(9), C(4)–Si(1)–N(1) 128.69(8), C(1)–Si(1)–C(4) 101.59(9).
carbon–nitrogen double bond have been observed in diphenyl-
imines; the bent angle y of N-silylated imine Ph2CQNSitBu3 is
154.6(3)1,19 while those of N-carbon-substituted imines are
120.641 for Ph2CQNPh20 and 120.8(2)1 for Ph2CQ
NCH(Ph)2.21 The SiQN double bond distances of bent
UV-vis spectra of dialkylsilanimines
Very few studies have previously been devoted to the
investigation of the electronic structure of silanimines.
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Studies of transient silanimines Me3Si(N3)SiQNSiMe3,23
Mes2SiQNMes (Mes = 2,4,6-trimethylphenyl),24 twisted
bridgehead silanimines,25 Ph2SiQNPh26 and Mes2SiQ
27
NSiMe3 in low-temperature matrices have shown that these
silanimines exhibit an intense p(SiQN) -p*(SiQN) transition
band at shorter wavelength and a weak n(N) - p*(SiQN)
transition band at longer wavelength.
UV-vis absorption spectra and major absorption bands of
silanimine 5a–5d in hexane are summarized in Fig. 5–8 and
Table 2, respectively. Dialkylsilanimines 5a, 5c and 5d show
two similar absorption bands, I and II. Band I, with a large
extinction coefficient, is observed at 255, 246 and 245 nm
for 5a, 5c and 5d, while band II, with a small extinction
coefficient, is observed at 315 (as a shoulder), 320 and
306 nm, respectively. According to the assignment of
absorption bands of the transient silanimines, bands I and II
are assignable to the p(SiQN) - p*(SiQN) and
n(N) - p*(SiQN) transitions, respectively. In the case
of N-phenylsilanimine 5b, a medium absorption band III is
observed at 289 nm (sh), together with bands I and II at
251 and 345 nm, similarly to transient Ph2SiQNPh26
(which shows three bands at 404, 292 and 241 nm). The nature
of the bands I–III will be discussed in detail using TD-DFT
calculations (vide infra).
Fig. 6 UV-vis absorption spectrum of silanimine 5b in hexane at
room temperature superimposed with band positions calculated at the
TD-B3LYP/6-311+G(d,p) level. Selected calculated transitions: a.
p(SiQN) - p*(SiQN) transition. b. n(N) - p*(SiQN) transition. c.
p(phenyl) - p*(SiQN) transition.
Theoretical calculations
Fig. 7 UV-vis absorption spectrum of silanimine 5c in hexane at
room temperature superimposed with band positions calculated at the
TD-B3LYP/6-311+G(d,p) level. Selected calculated transitions: a.
assignable to p(SiQN) - p*(SiQN) transition. b. assignable to
n(N) - p*(SiQN) transition.
(1) Molecular structure. To understand the characteristics
of the geometry around the SiQN double bond in dialkyl-
silanimines 5a–5d, DFT calculations of model compounds
Me2SiQNR (R = SiH3 (6a), CH3 (6b), H (6c), F (6d)) were
performed. Selected structural parameters of 6a–6d optimized
at the B3LYP/6-311+G(d,p) level are summarized in Table 3.
All silanimines 6a–6d are calculated to have planar tricoordinated
silicon atoms, with a sum of bond angles around the unsaturated
silicon atom of 3601. The bending angle y of the silanimines
are significantly dependent on the substituent on the nitrogen
atom, as predicted for H2SiQNR (R = H and SiH3) by
Schleyer.18,28 N-Silyldimethylsilanimine 6a adopts an almost
linear geometry, with a bending angle y of 173.831 and
silicon–nitrogen distance d(SiQN) of 1.579 A, similar to those
for H2SiQN–SiH3 (y = 175.61 and d(SiQN) = 1.549 A). It
should be noted that y decreases in the order 6a (173.831), 6b
(135.901), 6c (120.351), 6d (110.521), while distance d(SiQN)
Fig. 8 UV-vis absorption spectrum of silanimine 5d in hexane at
room temperature superimposed by band positions calculated at the
TD-B3LYP/6-311+G(d,p) level. Selected calculated transitions: a.
assignable to p(SiQN) - p*(SiQN) transition. b. assignable to
n(N) - p*(SiQN) transition.
and the inversion barrier DEinv increase in the same order.
As shown in Fig. 9, displaying the relative energy of
silanimines 6a–6d as a function of Si–N–R bending angle y
at the B3LYP/6-311+G(d,p) level, dimethylsilanimines 6a, 6b
and 6c have very shallow potential surfaces with respect to
bending of the silanimine moiety. A small inversion barrier of
silanimines has been already found in H2SiQNH3 by Schleyer
et al. (25.1 kJ molꢁ1 at the MP4/6-31G(d) level)18 and by
Gordon (23.4 kJ molꢁ1 at the MP4/MC-611G**//HF/
6-31G** level).29 These theoretical calculations show that
introduction of a more electropositive group on the nitrogen
Fig. 5 UV-vis absorption spectrum of silanimine 5a in hexane at
room temperature superimposed with band positions calculated at the
TD-B3LYP/6-311+G(d,p) level. Selected calculated transitions: a.
p(SiQN) - p*(SiQN) transition. b. n(N) - p*(SiQN) transition.
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Table 2 Absorption maxima and extinction coefficient of silanimines
5a–5d in hexane at room temperature
transitions
a
(p(SiQN) - p*(SiQN)) and
b
(n(N) -
p*(SiQN)). The additional weak absorption at 289 nm of 5b
(band III) is assigned as transition c (p(phenyl) - p*(SiQN)).
Highest occupied, second highest occupied and lowest
unoccupied Kohn–Sham orbitals of 5a–5d calculated at the
B3LYP/6-311+G(d,p) level are the n orbital of nitrogen, and
the p(SiQN) and p*(SiQN) orbitals of silanimine, respectively
(Fig. 10). Although the calculated energy levels of the n,
p(SiQN) and p*(SiQN) orbitals are strongly dependent on
the substituent on the nitrogen atom, p(SiQN) - p*(SiQN)
transition bands appear at similar wavelength (around
250 nm), probably due to the cancellation of steric and
electronic effects of N-substituents on the energy levels of
p(SiQN) and p*(SiQN) orbitals. The red-shift of the
n(N) - p*(SiQN) transition band II of N-phenylsilanimine
5b (345 nm) and N-1-adamantylsilanimine 5c (320 nm)
compared with those of the other silanimines can be ascribed
to the high-lying n orbital resulting from the n–p(aryl)
interaction being consistent with the perpendicular geometry
of silanimine and phenyl rings in 5b, and resulting from
the n–s(C–C bond in adamantyl moiety) interaction in 5c,
respectively.
l
max/nm (e/(dm3 cmꢁ1 molꢁ1))
Band Ia
Band IIb
Band IIIc
Silanimine
5a (R = CH2Ph)
5b (R = Ph)
5c (R = 1-Ad)
5d (R = SiMe3)
255 (4900)
251 (9700)
246 (6700)
245 (5400)
315 (sh,d 150)
345 (450)
320 (55)
—
289 (1200)
—
—
306 (30)
a
b
Assignable to p(SiQN) - p*(SiQN) transition. Assignable to
c
n(N) - p*(SiQN) transition. Assignable to n(N) - p*(phenyl)
transition. Shoulder.
d
atom would increase the p-character of nitrogen lone-pair
orbital, resulting in the increase of the Si–N–R bending angle
y (from sp2 to sp),30 reduction of the inversion barrier DEinv
and reduction of the d(SiQN) value. Similar substituent effects
of electropositive groups on the geometry around the CQN
double bond in imines have been investigated theoretically.31
Since y and d(SiQN) of bent silanimines 5a (138.07(15)1,
1.5823(18) A) and 5b (144.42(8)1, 1.5858(9) A), and almost
linear silanimines 5d (177.19(13)1, 1.5685(18) A) are in good
accord with those calculated for N-methylsilanimine 6b
(135.901, 1.598 A) and N-silylsilanimine 6a (173.831, 1.579 A),
respectively, the structural characteristics of silanimines,
5a, 5b and 5d can be explained mainly by the electronic effects
of substituents on the dicoordinated nitrogen atom. In contrast,
N-(1-adamantyl)silanimine 5c adopts an almost linear
structure. The severe steric interaction between the bulky
1-adamantyl substituent and trimethylsilyl group as well as
the shallow potentials for bending in silanimines with a carbon
functional-group on the nitrogen atom may be responsible for
the almost linear structure of silanimine 5c. Highly symmetric
structures of 5a–5d in solution may be due to the shallow
bending potential.
(3) Mechanism for formation of silanimines. Although
formation of silanimines by the reaction of silylenes with
azides have been reported,7e,14 the mechanism has not been
elucidated. Singlet silylenes have a vacant electrophilic 3p
orbital and a nucleophilic n orbital, while azides have nucleo-
philic and electrophilic nitrogen atoms at 1- and 3-positions, as
shown in Scheme 1. In accord with this prediction, the GRRM
method that has been developed by Ohno and Maeda13 has
revealed the following two pathways are the most plausible
among possible pathways: (A) electrophilic addition of silylene
to the azide followed by elimination of nitrogen molecule, and
(B) nucleophilic addition of silylene to the azide followed by
elimination of nitrogen molecule, as observed in the reaction
of N-heterocyclic carbene with azides.32,33 Using the GRRM
methods, we explored two model reactions: (i) formation of
parent silanimine 7a, H2Si (8a) + HN3 (9a) - H2SiQNH (7a) +
N2 (10) and (ii) formation of permethylated silanimine 7b,
Me2Si (8b) + MeN3 (9b) - Me2SiQNMe (7b) + 10. The
GRRM method enables an automated and systematic reaction
route search from an equilibrium structure using anharmonic
downward distortion following (ADDF)13 and conventional
intrinsic reaction coordinate (IRC) following,34 as uphill and
downhill methods. The GRRM method has revealed many
unknown reaction routes35 and synthetic routes.36
(2) UV-vis spectra. To elucidate the nature of the absorption
bands, TD-DFT calculations at the B3LYP/6-311++G(d,p)
level were performed for real silanimines 5a–5d, whose struc-
tural parameters were fixed to the experimental values deter-
mined by X-ray analysis. The calculated absorption maxima
and oscillator strengths of 5a–5d exhibit good qualitative
agreement with the corresponding experimental values of
5a–5d: the band positions and oscillator strengths for 5a–5d
are superimposed using vertical bars in Fig. 5–8.
Judging from the experimental and theoretical UV-vis
spectra, we assigned bands I and II in silanimines 5a–5d to
Two reaction routes A and B from 8a and 9a to 7a are
predicted at the B3LYP/6-311G(d) level using the GRRM
method as shown in Scheme 2. Electrophilic addition of
silylene 8a to azide 9a gives initial adduct 11a or 13a (route
A or A0). Adduct 11a isomerizes to 13a through the rotation
around the Si–N bond with a very low activation energy.
Then, elimination of nitrogen molecule from 13a leads to
formation of silanimine 7a.37 On the other hand, nucleophilic
addition of silylene 8a to azide 9a gives initial adduct 15 (route
B), which undergoes cyclization to give triazasilacyclobutene
17a. Elimination of nitrogen molecule from 17a affords
silanimine 7a. Similar two reaction routes for formation of
Fig. 9 Relative energies of dimethylsilanimines 6a–6d as a function of
Si–N–R bending angle y calculated at the B3LYP/6-311+G(d,p) level.
ꢀc
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Table 3 Selected structural parameters and inversion barrier of dimethylsilanimines Me2SiQNR 6a–6da
Silanimine
d(SiQN)/A
y (Si–N–R)/1b
S j(Si)b/1
DEinvc/kJ molꢁ1
6a (R = SiH3)
6b (R = CH3)
6c (R = H)
1.579
1.598
1.610
1.609
1.656
173.83
135.90
120.35
120.5
360
360
360
—
0.9
9.3
19.3
25.1
85.8
6c (R = H)d
6d (R = F)
110.52
360
a
b
c
The geometry was optimized at the B3LYP/6-311+G(d,p) level. Sum of the bond angles around unsaturated silicon atom. DEinv = E(linear
structure)–E(optimized bent structure). Values from ref. 26. The geometry and energy were calculated at the B3LYP/6-31G(d,p) level.
d
Fig. 10 Frontier Kohn–Sham orbitals (B3LYP/6-311+G(d,p) level)
of silanimines 5a–5d (RH2SiQNR) whose structural parameters were
set at the experimental values determined by X-ray analysis.
Scheme 1
methylated silanimine 7b from dimethylsilylene 8b and
methylazide 9b are predicted at the B3LYP/6-31G(d) level.
In both cases (R = H, Me), activation barriers in route A0
(or A) are much smaller than those of route B.
Although route A0 (or A) is predicted to be favored for the
Scheme 2 Possible reaction routes from silylene 8 and azide 9 to the
model reactions, the steric congestion around Si–N bond in
initial adduct 13 (or 11) in route A0 (or A) is much larger than
that in 15 in route B. If the silylene and azide have considerably
bulky substituents such as the substituents of silylene 4 and the
corresponding azide, initial adduct 13 (or 11) in route A0 (or A)
would be destabilized (rather than 15 in route B) owing to the
steric congestion, and route A might be less favored.
corresponding silanimine 7 and nitrogen molecule (10) (a: R = H; b:
R = CH3). The values of relative energies in kJ molꢁ1 for R = H
calculated at the B3LYP/6-311G(d) level are shown in parentheses and
for R = CH3 calculated at the B3LYP/6-31G(d) level in brackets,
respectively.
Conclusions
In the case of reactions of N-heterocyclic silylenes and
azides reported by Denk and West,7e the corresponding
silanimine would form via route B because the low-lying
3p orbital for formation of complex 11 is missing in the
diaminosilylene owing to the intramolecular interaction of
lone pair orbitals of neighboring nitrogen atoms with the 3p
orbital of the silylene center.
We successfully synthesized a series of base-free dialkylsilani-
mines 5a–5d. Molecular structures determined by X-ray crystallo-
graphy have shown that 5a–5d have base-free silicon–nitrogen
double bonds, and that bending of the SiQN–R moiety in
5a–5d is strongly dependent on the electronic and steric effects
on the substituents on the dicoordinated nitrogen atom.
ꢀc
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Distinct p(SiQN) -p*(SiQN) and n(N) -p*(SiQN) transition
bands were observed in the UV-vis spectra. Two plausible
mechanisms for formation of silanimine from the reaction of
silylene and azide were found using theoretical calculations
with the GRRM method for model reactions.
dC(C6D6, SiMe4) 1.66, 14.48, 28.97, 117.06, 119.87, 128.81,
150.77; dSi(C6D6, SiMe4) 2.15 (SiMe3), 59.12 (SiQN);
l
max(hexane, 293 K)/nm 251 (e/dm3 molꢁ1 cmꢁ1 9700), 289
(1200), 345 (450); m/z (EI, 30 eV) 463 (100%, M+), 448 (86.7),
390 (16.2).
(N-1-Adamantyl)dialkylsilanimine 5c
Experimental
According to a procedure similar to the synthesis of 5a,
reaction of silylene 4 (69 mg, 0.19 mmol) with 1-azido-
adamantane (29 mg, 0.16 mmol) in hexane (3 mL) gave
(N-1-adamantyl)silanimine 5c (74 mg, 0.14 mmol, 88%) as
colorless crystals (Found: C, 59.53; H, 10.75; N, 2.74%.
C26H55Si5N1 requires C, 59.69; H, 10.79; N, 2.68%.); mp
218 1C; dH(C6D6, SiMe4) 0.31 (36 H, s, SiMe3), 1.74 (4H, s,
CH2), 1.63–2.10 (15H, m, adamantyl); dC(C6D6, SiMe4) 2.4
(SiMe3), 14.3 (C), 29.5 (CH2), 31.0, 37.3, 50.6 (Adamantyl),
53.5 (C–N); dSi(C6D6, SiMe4) 1.4 (SiMe3), 19.9 (SiQN);
General procedures
All reactions were carried out under argon atmosphere using a
high-vacuum line, standard Schlenk techniques, a glove box,
and dry oxygen-free solvents. H (400 MHz), 13C (100 MHz),
and 29Si (79 MHz) NMR spectra were recorded on a Bruker
Avance400 spectrometer. Mass spectra were obtained on a
JEOL JMS-600W mass spectrometer. UV-vis spectra were
recorded on a JASCO V-660 spectrometer. Sampling of
air-sensitive compounds were carried out by using a VAC
NEXUS 100027 glove box.
1
l
max(hexane, 293 K)/nm 246 (e/dm3 molꢁ1 cmꢁ1 6700), 320
(55); m/z (EI, 70 eV) 521 (13%, M+), 463 (26), 135 (95),
Materials
73 (100).
Hexane and benzene-d6 were dried in a tube covered with a
potassium mirror, and then distilled prior to use by using
vacuum line. Dialkylsilylene 411 and azidobenzene38 were
prepared according to the published procedure. (Azidomethyl)-
benzene, 1-azidoadamantane and azidotrimethylsilane were
purchased from commercial sources and used without further
purification.
(N-Trimethylsilyl)dialkylsilanimine 5d
According to a procedure similar to the synthesis of 5a,
reaction of silylene 4 (62 mg, 0.17 mmol) with azidotrimethyl-
silane (20 mg, 0.17 mmol) in hexane (3 mL) gave (N-trimethyl-
silyl)silanimine 5d (65 mg, 0.14 mmol, 82%) as colorless
crystals (Found: C, 49.22; H, 10.50; N, 2.84. C19H49Si6N1
requires C, 49.59; H, 10.73; N, 3.05); mp 139 1C; dH(C6D6,
SiMe4) 0.24 (36 H, s, SiMe3), 0.36 (9H, s, N-SiMe3), 1.72 (s, 4
H, CH2); dC(C6D6, SiMe4) 2.0 (SiMe3), 4.5 (SiMe3), 17.1(C),
29.5 (CH2); dSi(C6D6, SiMe4) ꢁ15.50 (N–SiMe3), 1.4 (SiMe3),
89.9 (SiQN); lmax(hexane, 293 K)/nm 218 (e/dm3 molꢁ1 cmꢁ1
6300), 245 (5400), 306 (30); m/z (EI, 70 eV) 461 (12%, M+),
275 (21), 231 (28), 73 (100).
(N-Benzyl)dialkylsilanimine 5a
In a vial equipped with a magnetic stir bar, silylene 4 (42.4 mg,
0.113 mmol) was placed, and then hexane (0.3 ml) was added.
To this solution, a hexane solution (1 mL) of (azidomethyl)-
benzene (15.5 mg, 0.116 mmol) was added dropwise. After
stirring the mixture for 5 min, a pale yellow solution was
obtained. After hexane was distilled off under the reduced
pressure, colorless crystals were obtained. Recrystallization
from hexane gave pure (N-benzyl)silanimine 5a (46.9 mg,
0.0981 mmol, 86%) as colorless crystals (Found: C, 57.91;
H, 9.71; N, 2.88%. C23H47Si5N requires C, 57.79; H, 9.91; N,
2.93); mp 68.1–69.9 1C; dH(C6D6, SiMe4) 0.26 (36 H, s,
Si(CH3)3), 1.75 (4 H, s, CH2), 5.20 (2H, s, CH2Ph), 7.33
(2H, t, J 8, CH), 7.65 (2 H, d, J 8, CH), overlapped with
C6D6 peak (1H, CH); dC(C6D6, SiMe4) 1.75 (SiMe3), 14.85
(CH2), 29.42 (C), 52.87 (CH2Ph), 125.61 (Ph), 127.08 (Ph),
127.92 (Ph), 146.86 (Ph); dSi(C6D6, SiMe4) 2.14 (SiMe3), 75.34
(SiQN); lmax(hexane, 293 K)/nm 255 (e/dm3 molꢁ1 cmꢁ1
4900), 315 (sh, 150); m/z (EI, 30 eV) 477 (21%, M+), 480
(29), 404 (54).
Crystal structure determination of silanimines
Single crystals of silanimines 5a–5d suitable for X-ray diffraction
study were obtained by recrystallization from hexane. A single
crystal of 5a–5d coated with apiezon grease was mounted
on a thin glass fiber and transferred to the cold gas stream
of the diffractometer. X-ray diffraction data were collected
on a BrukerAXS APEXII diffractometer with graphite-
monochromated Mo-Ka radiation (l 0.71073 A). The data
were corrected for Lorentz and polarization effects. An
empirical absorption correction based on the multiple
measurement of equivalent reflections was applied using the
program SADABS.39 The structures were solved by direct
methods and refined by full-matrix least-squares against F2
using all data (SHELXL-97).40
(N-Phenyl)dialkylsilanimine 5b
According to a procedure similar to the synthesis of 5a,
reaction of silylene 4 (57.6 mg, 0.154 mmol) with azidobenzene
(18.5 mg, 0.155 mmol) in hexane (2.0 mL) gave (N-phenyl)-
silanimine 5b (55.3 mg, 0.119 mmol, 77%) as pale yellow
crystals (Found: C, 57.00; H, 9.69; N, 2.99%. C22H45Si5N
requires C, 56.94; H, 9.77; N, 3.02); mp 103.4–105.4 1C;
dH(C6D6, SiMe4) 0.22 (36H, s, SiMe3), 1.73 (4H, s, CH2),
6.80 (1H, t, J 8.0), 6.92 (2H, d, J 8.0), 7.27 (2H, dd, J 8.0, 8.0);
Crystal data for 5a. C23H47NSi5, M = 478.07, triclinic,
a = 10.844(2), b = 11.874(3), c = 13.349(3) A, a =
115.304(2), b = 95.635(2), g = 107.048(2)1, V = 1435.1(5) A3,
ꢀ
T = 100 K, space group P1 (no. 2), Z = 2, 13 815 reflections
measured, 5027 unique (Rint = 0.0340) which were used in all
calculations. The final R1 and wR(F2) were 0.0375
(I > 2s(I)) and 0.0975 (all data).
ꢀc
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Crystal data for 5b. C22H45NSi5, M = 464.04, monoclinic,
9.1616(9), 20.601(2), 14.7242(14) A,
References
a
=
b
=
c =
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b = 93.9340(10)1, V = 2772.5(5) A3, T = 100 K, space group
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a = 9.445(3), b = 10.618(3), c = 16.947(5) A, a = 95.035(3),
b = 93.948(3), g = 111.147(3)1, V = 1569.7(8) A3, T = 120 K,
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ꢀ
space group P1 (no. 2), Z = 2, 17 268 reflections measured,
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¨
Crystal data for 5d. C19H49NSi6, M = 460.13, monoclinic,
a = 14.673(5), b = 9.188(3), c = 21.868(8) A, b = 96.864(4)1,
V = 2927.0(17) A3, T = 120 K, space group P21/c (no. 14),
Z = 2, 25 067 reflections measured, 6344 unique (Rint = 0.0531)
which were used in all calculations. The final R1 and wR(F2)
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Theoretical calculations
All theoretical calculations were performed using a Gaussian
0341 program and reaction routes were searched by the
ADDF13 and IRC34 methods available in the GRRM 1.2
program.13 Geometry optimization and potential surface
search of 6a–6d as a function of Si–N–R bending angle y for
model compounds was carried out at the B3LYP/
6-311+G(d,p) level. Optimized atomic coordinates of 6a–6d
are summarized in the ESIw. Absorption band maxima and
oscillator strengths of 5a–5d (whose structural parameters
were fixed to the experimental values determined by X-ray
analysis) were calculated at the TD-B3LYP/6-311+G(d,p)
level. For a reaction route search for formation of silanimines
H2SiQNH (7a), initial adducts 11a and 15a (whose geometries
were optimized at the B3LYP/6-311G(d) level) were used as
initial structures for the GRRM method.13 For exploration of
low-barrier routes to 7a, the large-ADD (lADD) method was
applied.13d Although automatic reaction route search using
GRRM methods explored various reaction routes among
H3SiN isomers including 7a, only the reaction routes to 7a
with the smallest activation barrier, except for the degenerate
rearrangements such as a configuration inversion, are shown
in Fig. 10. For a reaction route search for formation of
silanimines Me2SiQNMe (7b), the corresponding equilibrium
structures (EQ) and transition state structures (TS) 7b–TS18b
were optimized at the B3LYP/6-31G(d) level. Optimized
atomic coordinates of 7, and 11-TS18 are also summarized
in the ESI.
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Acknowledgements
This work was supported by the Ministry of Education,
Culture, Sports, Science, and Technology of Japan [Specially
Promoted Research (No. 17002005, M.K. and T.I.), a
Grant-in-Aid for Scientific Research on Innovative Areas
(No. 21108501, ‘‘pi-Space’’, T.I.), and a Grant-in-Aid for
Scientific Research B (No. 21350007, T.I. and K.O.)].
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¨
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