An ESR study on structures of a series of silylnitrenes

Article abstract of DOI:10.1016/S0009-2614(01)01112-5

Structures of a series of silylnitrenes formed from silyl azides were investigated by means of ESR. γ-Irradiation and photo-illumination of all the silyl azides resulted in the formation of triplet states even for those having two or three Si-N₃ groups in a molecule. ESR spectra of the silylnitrenes exhibited a part of the fine structure at around 820 mT. All the silyl azides studied gave nearly identical D-values (ca. 1.5cm^-1) and much larger than those in phenylnitrenes. The results suggested that electron spins are localized in the nitrogen p-orbitals to a large extent and was interpreted in terms of a mono-silane linkage of nitrene, -Si-N:, i.e., interrupting spin delocalization.

Full text of DOI:10.1016/S0009-2614(01)01112-5

9 November 2001
Chemical Physics Letters 348 12001) 249±254
www.elsevier.com/locate/cplett
An ESR study on structures of a series of silylnitrenes
Yoshiteru Itagaki, Takashi Iseoka, Toshiyuki Iida, Joji Ohshita,
Masaru Shiotani*, Atsutaka Kunai
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama,
Higashi-Hiroshima 739-8527, Japan
Received 3 July 2001; in ®nal form 18 September 2001
Abstract
Structures of a series of silylnitrenes formed from silyl azides were investigated by means of ESR. c-Irradiation and
photo-illumination of all the silyl azides resulted in the formation of triplet states even for those having two or three
Si±N3 groups in a molecule. ESR spectra of the silylnitrenes exhibited a part of the ®ne structure at around 820 mT. All
the silyl azides studied gave nearly identical D-values 1ca. 1:5 cmÀ1) and much larger than those in phenylnitrenes. The
results suggested that electron spins are localized in the nitrogen p-orbitals to a large extent and was interpreted in terms
of a mono-silane linkage of nitrene, ±Si±N:, i.e., interrupting spin delocalization. Ó 2001 Elsevier Science B.V. All
rights reserved.
1. Introduction
expected by polymerizing the paramagnetic mole-
cules [5±7]. Typically, highly conjugated molecules
containing a number of carbenes, such as
High spin states of organic compounds are of
great interest because of potential application to
organic ferromagnets as new materials and have
been extensively investigated by ESR spectros-
copy. So far, many challenges have been made to
produce organic high-spin molecules. Wasserman
et al. reported ESR spectra of carbene and nitrene
derivatives [1±4]. In particular, phenylenedicar-
bene and phenylenedinitrene exhibited ground
state quintet through the ferromagnetic spin±spin
interaction of the two triplet centers [4]. In the
same manner, the organic ferromagnets can be
poly1phenylenecarbenes), have been synthesized
and exhibited ferromagnetic spin±spin interaction
through the p-conjugated chain [7]. Since the sili-
con polymers, such as polysilanes, are known to be
r-conjugated molecules, silylnitrenes can be basic
fragments for the p±r-conjugated organometallic
ferromagnets. Doi et al. reported the high-spin
molecule containing silicon linkage, where two
phenylnitrenes as spin-carrying center linked by a
monosilane or disilane interact through the p±r
conjugation [8]. The spin states of these com-
pounds were found to strongly depend on the
position of the nitrene centers on the phenyl ring.
So far, there are few reports on the high-spin
molecules containing silicon chain as the spin
* Corresponding author. Fax: +81-824-24-7736.
E-mail address: mshiota@hiroshima-u.ac.jp 1M. Shiotani).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 1 0 1 ) 0 1 1 1 2 - 5

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Y. Itagaki et al. / Chemical Physics Letters 348 22001) 249±254
one ±N3 group in the molecules, while the 2 and 3
groups of compounds have two and three, respec-
tively. The ESR line positions are, however, essen-
tially the same for all the compounds, though the
line widths are slightly di€erent from one another.
The spectra consist of intense signals at around 350
mT, approximately g ˆ 2:00 position, doublet lines
from trapped hydrogen atom and those in the high
®eld at ca. 820 mT. The signals at ca. g ˆ 2:00 are
due to neutral radicals as byproducts: Si centered
radical and/or Si±CHÅ2, Si±C6HÅ5, etc. can be candi-
date radicals. The important feature of the spectra is
observed for the signals at around 820 mT. These
Scheme 1. Silyl azides used in the present study.
coupler. It is of importance to investigate a fun-
damental nature of spin±spin interaction, i.e. the
exchange coupling and the dipolar interaction, in
the silicon-contained molecules. For such purpose,
we synthesized various silylnitrene derivatives as
model compounds and their natures were prelim-
inarily investigated by means of ESR.
2. Experiments
Silyl azides used in the present study are shown
in Scheme 1. They were prepared by the reactions
of the corresponding chlorosilanes with sodium
azide in THF. The bulk silyl azides and 1.0 mol%
solution of the silyl azides in methanol-d4
ꢀCD3OD† were prepared on a vacuum line
ꢀ> 10À4 Torr† and sealed in Suprasil quartz tubes.
The c-ray irradiation was performed with 60Co
source at 77 K for 20 min. Approximate does rate
was 3 Mrad hÀ1. While, the photo-illumination
was carried out at 77 and 4 K with a high-pressure
mercury lamp bearing a Pyrex cut-o€ ®lter
ꢀk > 350 nm†. The ESR spectra were recorded
with a Bruker ESP 300E spectrometer at X band.
All measurements were performed in dark. Theo-
retical calculations were carried out using the
density functional theory 1DFT) in the GAUSSIAN
98 program with the B3LYP/6-31G level of theory.
3. Results and discussions
3.1. c-Irradiation to the bulk silyl azides
ESR spectra of the c-irradiated silyl azides are
shown in Fig. 1. The 1 group of compounds have
Fig. 1. ESR spectra of silyl azides irradiated by c-rays and
recorded at 77 K.

Y. Itagaki et al. / Chemical Physics Letters 348 22001) 249±254
251
are characteristics of X- and Y-components 1or
perpendicular component) of the ®ne structure sig-
nals of Dms ˆ ‡1 ꢀms ˆ ‡1 $ ms ˆ 0† transition
owing to a high spin state, S P 1, although
Dms ˆ À1 ꢀms ˆ 0 $ ms ˆ À1† transition could
not be observed; note that here we assume positive
sign for the zero-®eld splitting constants. The
Z-component 1parallel component) was not ob-
servable because of its extremely low intensity. The
origin of the high spin state should be the silylnit-
rene, ±Si±N:, formed by N2 elimination from the ±
SiN3 group. Since compounds 1a±1d, which have
only one ±SiN3 group, showed almost identical
feature of the ESR spectrum with those of com-
pounds 2 and 3, we assigned all the spectra to be due
to the triplet ground state. Thus, it is possible that
mono-nitrene is solely formed also in 2 and 3 groups
of compounds by c-irradiation. In case of 1d and p-
2c, which are solid crystals at room temperature, the
line widths were much smaller than those of the
other compounds and the X- and Y-components of
the ®ne structure together with 14N-hyper®ne split-
ting 11.5 mT) were clearly observed as shown in Fig.
2. The observation of the X- and Y-components
indicates that px and py orbitals of the nitrogen atom
normal to the Si±N: bond direction are not degen-
erate owing to asymmetry along the Si±N: bond
direction. Taking an example for 1d, the compo-
nents would be caused with slightly di€erent orien-
tations of the three phenyl rings with respect to the
Si±N: bond. On the other hand, 1a with three methyl
groups seems to have a three-fold axis along the Si±
N: bond to exhibit axially symmetric ESR pattern.
The X and Y transitions at 820 mT are characterized
by zero-®eld splitting 1ZFS) parameters, D and E.
The spin Hamiltonian for the zero-®eld splitting
with spin quantum number S, is described as
ꢀ
ꢁ
SꢀS ‡ 1†
HZFS ˆ D Sz2 À
‡ EꢀSx2 À Sy2†;
ꢀ1†
3
where, D and E are zero-®eld splitting constants
which depend on magnitude of the spin±spin in-
teraction and the deviation from axial symmetry of
the molecule, respectively. Values of D and E were
evaluated using the following well-established
equations [10], which are applicable also to the
present case of HZFS ꢁ HZeeman, from the reso-
nance positions of Dms ˆ ‡1 transition.
2
Hx2 ˆ ꢀge=gx† ‰ꢀH0 ‡ D À E†ꢀH0 À 2E†Š;
ꢀ2†
2
Hy2 ˆ ꢀge=gy† ‰ꢀH0 ‡ D ‡ E†ꢀH0 ‡ 2E†Š;
ꢀ3†
where Hx and Hy are the resonance ®elds corre-
sponding to the X and Y components of Dms ˆ ‡1
transition, respectively. For 1d, the observed Hx
and Hy ®elds are 813.4 and 824.9 mT, respectively.
Here we assumed an axial symmetric g tensor,
which are close to the free electron value of ge
Fig. 2. ESR spectra of Ph3SiN3, 1d, c-irradiated and recorded
at 77 K. Upper is survey spectrum and lower is expansion of the
X- and Y-components of Dms ˆ ‡1 transition of the ®ne
structure.
12.0023), i.e., gx ꢂ gyꢀꢂ ge† 6ˆ gz. Then ZFS pa-
rameters were evaluated to be D ˆ 1:54 cmÀ1 and
E ˆ 0:0035 cmÀ1 with H0 ˆ 340 mT ꢀm ˆ 9:50

252
Y. Itagaki et al. / Chemical Physics Letters 348 22001) 249±254
GHz†. In the same manner, D and E values for
p-2c were evaluated to be 1:54 cmÀ1 and
0:0051 cmÀ1, respectively. D-values of all com-
pounds investigated were nearly identical. These
D-values are comparable to those of alkyl nitrenes
11:607 cmÀ1 mT for n-propylnitrene [2]) and much
larger than phenyl nitrenes ꢀꢂ1.0 cmÀ1† [11,12].
This indicates that the electron spins are localized
on the nitrogen atom in the ±Si±N: to a large ex-
tent. That is, here the p±r conjugation does not
play an important role, because the mono silylene
linkage between the nitrene and the phenyl groups,
Ph±Si±N:, interrupts the spin delocalization be-
tween these groups. ESR spectrum of the triplet
nitrene with s-triazine skeleton was reported by
Nakai et al. [9] and evaluated D-value
ꢀ1:445 cmÀ1† was very close to the silylnitrenes in
the present study. They also interpreted this large
D-value in terms of the spin localization on the
nitrene center. In order to verify the spin local-
ization on the nitrogen atom in the silylnitrenes,
we carried out the density functional theory 1DFT)
account for the spin localization, which is inde-
pendent of the orientation of the Si±N: bond with
respect to the pz orbitals. Thus, the triplet states in
the c-irradiated silyl azides maybe caused by the
spin-localization in addition to a mono-nitrene
formation. Doi et al. [8] reported that monosilane
linkage for two phenylnitrenes e€ectively isolates
the two triplet centers.
In this case, a possible way of spin exchange, if
existed, would be through the Si±CH3 sp3 r-bond
slightly overlapping with the 2p-orbitals of the
nitrene and the phenyl-carbons 1super-exchange
[8]). According to the DFT calculations, this
mechanism may not be expected in the molecules
used in the present study because of the spin-lo-
calization on the N-atom.
3.2. Photo-illumination to solid solutions
calculations with
a model compound PhSi
ꢀCH3†2N: 1Scheme 2). Two conformers with dif-
ferent Si±N: orientation with respect to the pz-
orbitals on the phenyl carbons were assumed
there; conformers with h ˆ 0° and 90°. As a result,
the conformer with the Si±N: bond parallel to the
pz-orbital of the phenyl carbons ꢀh ˆ 0°† was
suggested to be more stable than that with h ˆ 90°
by 0.016 eV; the stability of the h ˆ 0° conformer
being due to lower repulsion energy between the
methyl-group and the phenyl-ring. Spin densities
of the nitrogen unpaired electron orbitals 1px and
py) for the two conformers are valuated to be 1.79
and 1.80, respectively; 90% of electron spins are
localized on these orbitals. The calculated results
First, the solid solutions were photo-illumi-
nated at 77 K instead of c-ray irradiation. How-
ever, no signals due to the nitrene were obtained
even after 1 h illumination for all the silyl azides
employed. It could be possible that the nitrene
formed abstracts hydrogen from the solvent to
form ±NH2 in THF 1or ±ND2 in CD3OD) even at
77 K in contrast to the experimental result for
phenylnitrenes reported by Doi et al. [8]; they
could observe the associated nitrene even below 77
K. The reason why silylnitrenes were not observed
at 77 K can be their thermal stability due to the
spin localization on the nitrene center. Thus, we
Scheme 2. Two conformers of PhSiꢀCH3†2N3 for the DFT calculations 1B3LYP/6-31G). The h is de®ned as an angle between 2pz
orbital of the phenyl carbon and Si±N: bond direction.

Y. Itagaki et al. / Chemical Physics Letters 348 22001) 249±254
253
carried out photo-illumination at 4 K to suppress
the subsequent reactions of the nitrene and then
succeeded to observe the nitrene signals 1Fig. 3).
For p-2c, the X- and Y-components of Dms ˆ ‡1
transition were observed for the c-irradiated bulk
sample. However, for the photo-illuminated sam-
ple they were observed to be a broad one line
without the hyper®ne splittings. This is probably
due to line broadening enhanced in the solid so-
lutions. The line positions of the ®ne structure
signals were identical to that for the c-irradiated
samples, suggesting that the photo-illuminations
to the solution at 4 K again resulted in the
formation of triplet states. The IR spectrum of
photo-irradiated o-2c after annealing at room
temperature revealed absorptions due to N±H and
respectively. The nitrene produced by the photo-
initiated N2 liberation was presumably converted
to an ±SiNH2 group by hydrogen abstraction from
the solvent.
The present experiments with di€erent illumi-
nation temperature showed that the mono-silyl-
nitrenes with a triplet state are also formed
photochemically from the silyl azides and the nit-
rene formed abstract hydrogen atom from the
solvent molecules even at a low temperature of 40
K. The dinitrene formation was observed in the
phenyl azide system [8] in the analogous experi-
mental conditions. This may suggest that the
photolytic yields of disilylnitrenes from com-
pounds 2 and 3 may be much lower than that of
dinitrene from the phenyl azides. Further investi-
gation on the photoreaction mechanism may be
required however.
N3 stretching frequencies at 3400 and 2070 cmÀ1
,
4. Conclusions
A series of silylnitrenes was investigated by
means of ESR. c-Irradiation to the bulk samples at
77 K was resulted in ground state triplet due to
mono-nitrene in the molecules. The evaluated
D-values are comparable to alkylnitrenes and lar-
ger than that in phenylnitrenes, indicating that the
electron spins are localized on the nitrene center to
a large extent. This was supported by theoretical
calculations. The photo-illumination did not es-
sentially alter the ESR spectra of the products and
resulted in ground state triplet. The spin localiza-
tion seems to prevent the spin±spin interaction
between the two nitrene centers and enhance the
thermal instability of the radical species.
Acknowledgements
The present study was ®nancially supported by
a subsidy for the JSPS; The Japanese Society for
the Promotion in Science 1Grant No. 10555312).
Fig. 3. ESR spectra of the CD3OD solutions of o-, m- and p-2c
compounds recorded at 4 K immediately after photo-illumi-
nation ꢀk > 350 nm† at 4 K.

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Y. Itagaki et al. / Chemical Physics Letters 348 22001) 249±254
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