Trimethylsilylnitrene and Its Surprising Rearrangement to N-(Dimethylsilyl)methanimine via Silaziridine and Silaazomethine Ylide

Article abstract of DOI:10.1002/chem.201803245

Photolysis of trimethylsilyl azide at 254 nm in Ar matrix at 15 K generates the triplet ground state trimethylsilylnitrene 2 aT, observed by ESR spectroscopy (|D/hc|=1.540 cm^?1; |E/hc|=0.0002 cm^?1). Calculations at the CASPT2(14,13) level reveal the open-shell singlet nitrene 2 aS(¹A“) is a discrete intermediate lying ≈38 kcal mol^?1 above the triplet. The normally expected rearrangement of the nitrene 2 aS to dimethylsilanimine 3 a has a high calculated barrier (33 kcal mol^?1), which explains why this product has never been observed. Instead, the singlet nitrene 2 aS inserts into a methyl C?H bond to yield silaziridine 12 via an activation barrier of only 6 kcal mol^?1. Ring opening of 12 generates a 1-silaazomethine ylide 13, in which a facile 1,2-H shift yields N-(dimethylsilyl)methanimine 5, all with barriers well below the energy of the singlet nitrene.

Full text of DOI:10.1002/chem.201803245

A Journal of
Accepted Article
Title: Trimethylsilylnitrene and its Surprising Rearrangement to N-
(Dimethylsilyl)methanimine via Silaziridine and Silaazomethine
Ylide
Authors: Curt Wentrup, Holger Lüerssen, Hugo Santos Silva, Alain
Dargelos, and Didier Bégué
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To be cited as: Chem. Eur. J. 10.1002/chem.201803245
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Chemistry - A European Journal
10.1002/chem.201803245
Trimethylsilylnitrene and its Surprising Rearrangement to
N-(Dimethylsilyl)methanimine via Silaziridine and Silaazomethine Ylide
Curt Wentrup,* Holger Lüerssen, Hugo Santos Silva, Alain Dargelos, and Didier Bégué*
Abstract:
Photolysis of trimethylsilyl azide at 254 nm in Ar matrix at 15 K generates the triplet ground
-1
state trimethylsilylnitrene 2aT, observed by ESR spectroscopy (|D/hc| = 1.540 cm ; |E/hc| =
-1
0
.0002 cm ). Calculations at the CASPT2(14,13) level reveal the open-shell singlet nitrene
1
2
aS( A”) is a discrete intermediate lying ~38 kcal/mol above the triplet. The normally
expected rearrangement of the nitrene 2aS to dimethylsilanimine 3a has a high calculated
barrier (33 kcal/mol), which explains why this product has never been observed. Instead, the
singlet nitrene 2aS inserts into a methyl C-H bond to yield silaziridine 12 via an activation
barrier of only 6 kcal/mol. Ring opening of 12 generates a 1-silaazomethine ylide 13, in
which a facile 1,2-H shift yields N-(dimethylsilyl)methanimine 5, all with barriers well below
the energy of the singlet nitrene.
Prof. Dr. C. Wentrup, Dr. H. Lüerssen, School of Chemistry and Molecular Biosciences, The University of
Queensland, Brisbane, Queensland 4072, Australia. E-mail: wentrup@uq.edu.au;
http://researchers.uq.edu.au/researcher/3606
Prof. Dr. D. Bégué, H. Santos Silva, Prof. Dr. A. Dargelos, CNRS/Université de Pau et des Pays de
l’Adour/E2S UPPA, Institut des Sciences Analytiques et de Physicochimie pour l’Environnement et les
Matériaux, UMR5254, 64000, Pau, France. E-mail: didier.begue@univ-pau.fr
Supporting information and the ORCID identification numbers for the author of this article can be found under
https://doi.org/10.1002/chem….
1
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Introduction
Despite extensive experimental and computational investigations, our understanding of
silylnitrenes 2 (eq 1) is still very limited, especially as regards the existence of a singlet state
1
1
(
open shell 2S( A”) or closed shell 2S( A’)).
R3Si-N
3
R3Si-N:
(1)
-
N2
1
2
a: R = CH3
b: R = Ph
o
Pyrolysis of triphenylsilyl azide 1b in a vacuum system at 680 C yields a cyclic dimer
[
Ph
2
SiNPh]
2
and a linear polymer, suggesting the intermediacy of the silanimine Ph Si=N-Ph
2
b,[1] but pyrolysis of trimethylsilyl azide 1a at 500 C under nitrogen flow was reported to
o
3
yield about 20 compounds with none identified. Evidence for the formation of silanimines
R
2
Si=NR 3 was obtained by co-pyrolysis of trialkyl- and triphenylsilyl azides with trapping
agents in an atmospheric pressure nitrogen flow system at 610 C.[2] The imines underwent
o
Wittig-type reactions with aldehyde and ketones and were also trapped with (Me
Me SiNMe) in co-pyrolysis reactions to form eight-membered ring systems 4 in moderate
yields (eq 2). These products were also obtained by photolysis of the azides.[3]
2
SiO) and
3
(
2
2
The matrix photolysis of trimethylsilyl azide 1a at 17 K was investigated by Ogilvie,
who postulated a rearrangement of an initially formed but unobserved silanimine to a
[
4]
silylimine: 1a  [3a]  5 (eq. 3). Perutz also concluded that the matrix photolysis of
trimethylsilyl azide at 10 K did not result in compounds containing Si=N bonds, but instead
C=N and Si-H bonds ascribed to 5 or, less likely, Me
Si(H)-CH=NH.[5] Formation of 5 was
2
confirmed by Ferrante, who also obtained 2a by microwave discharge in N
and Ar.[6]
2
2
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o
-2
FVP of trimethylsilyl azide 1a at 830 C/10 hPa with online photoelectron
spectroscopic monitoring resulted in ionization peaks at 9.3 and 10.9 eV assigned to 5, but
the putative silanimine 3a was not observed.[7]
Yet, matrix photolysis experiments have clearly revealed the formation of other
silanimines of the type RR’Si=NR” (e.g. 10 and 11, Scheme 1) and silylimine 9 from silyl
azides 6[
8,9]
but again, the N-methylsilanimine 8 was not observed. Silylnitrenes 7 were
postulated, but neither triplets nor singlets were observed.
Scheme 1. Photochemical silyl azide rearrangements
3
Early calculations on trimethylsilylnitrene 2a located the triplet state ( A”), but a
singlet was not mentioned.[10] Calculations on the unsubstituted silyl azide 6a by Nguyen et
al.[11] indicated that the loss of N
is concerted with the rearrangement to silanimine H Si=NH
2
2
1
0, and that a singlet nitrene H
3
Si-N does not appear as a distinct intermediate, although it
1
was mentioned that preliminary data suggested that the open-shell A” singlet state might
play a role in photolysis.
DFT calculations on 6b by Kuhn and Sander confirmed the concerted nature of imine
[8]
formation and the absence of a minimum for the singlet nitrene 7bS. In a study of bicyclic
compounds containing twisted Si=N bonds, Michl et al.[12] also concluded that silanimine
formation from silyl azides was most likely concerted with N loss, and that discrete singlet
2
silylnitrenes were not involved. However, Bertrand et al. concluded that there are two
reaction paths in the photolysis of cyclic Si-azidosilanes in solution, one involving the triplet
nitrene, which abstracts H from solvent and results in dimerization of the resulting free
radicals, the other being the concerted formation of silanimine from the azide without the
occurrence of a singlet silylnitrene.[13]
Triplet silylnitrene 2aT has been generated by microwave discharges in N or Ar and
2
[
6]
investigated by ESR spectrosocopy in N
2
and Ar matrices at 10-20 K. A zero-field splitting
3
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10.1002/chem.201803245
-1
(
ZFS) parameter |D/hc| = 1.57 cm in N was measured, but the ESR signal was destroyed, if
2
the sample was briefly exposed to the radiation of a Hg arc lamp or on annealing the matrix
to temperatures of ~35 K. Therefore, the nitrene was reportedly not observable under UV
irradiation due to rapid destruction.[6]
Silylnitrenes were also generated by -irradiation of trimethylsilyl azide 1a,
triphenylsilyl azide 1b, and mixed methyl(phenyl)silyl azides using a 60Co source and
investigated by ESR spectroscopy in solid solution at 77 K. For 2aT the ZFS parameters
-1
-1
[14]
|
D/hc| = 1.54 cm and |E/hc| = 0.0035 cm were determined. Methyl(phenyl)silyl azides
and diazides PhSiMe(N and PhSi(N , also yielded ESR spectra with |D/hc| close to 1.5
)
3 2
)
3 3
cm-1 when the azides were irradiated in solid solution at 4 K (>350 nm), but
trimethylsilylnitrene 2aT was not observed under these conditions, and no nitrenes were
1
4
observable at 77 K under photolysis conditions. The N hyperfine splitting was observable
under the –irradiation conditions, but not when the azides were photolyzed at 4 K in solid
solution. It was theorized that the silylnitrenes abstract H from solvent even at 40 K.
Experimental
Trimethylsilyl azide was deposited together with Ar (ratio ~1:1000) on a Cu rod at 15 K in a
closed-cycle liquid helium cryostat. No signals were observable by ESR spectroscopy
(9.3285 GHz, Varian X-band spectrometer). Irradiation of the matrix at 254 nm for 2 h
generated the transition shown in Figure 1, X
2
= 8178 G, Y
2
= 8187 G, H = 3374.4 G, |D/hc|
0
-1
-1
o
=
1.540 cm ; |E/hc| = 0.0002 cm . FVP of the azide at 500 C and deposition of the product
with Ar at 10 K did not result in any observable ESR absorption.
Computational Methods
Ground-state geometries and energies were determined at the DFT level using the B3LYP
exchange-correlation functional with the 6-311G(d,p) basis set. The UB3LYP approach was
used for systems capable of existing in open-shell states. In order to obtain reliable energies
of the open-shell singlet (OSS) nitrenes and the transitions states connecting them to isomeric
molecules, calculations were carried out at both the CASPT2(9,8)/6-311+G(d,p) and
CASPT2(14,13)/SP/6-311+G(d,p) levels. Calculations were performed using the Gaussian 09
and the Molpro program packages.[15,16] The transition state optimizations and IRC
calculations are carried out at both DFT and CASPT2 levels. Excited-state calculations were
performed using ORCA.[17] Harmonic and anharmonic frequencies were calculated at the
4
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B3LYP/6-311++G(3df,3pd) level. Natural spin densities calculated at the B3LYP/6-311g(dp)
level for Me Si-N (2aT) were 1.93 on N and -0.049 on Si (see Supporting Information).
3
Spin-spin (SS) and spin-orbit (SO) contributions to the zero-field slitting (ZFS) parameters
were calculated with the BP86 exchange-correlation functional in UKS mode using the
Resolution of the Identity approximation and with the def2-TZVPP basis set. Convergence
criteria were set to very tight. SO coupling was calculated by solving the spin Hamiltonian in
coupled perturbed form, and SS was calculated directly from the canonical orbitals for the
spin density. All calculations of ZFS parameters were performed using the ORCA 3.0.3
software.[17]
Results and Discussion
The Triplet Nitrene.
Although previous researchers had reported that the ESR spectrum of the triplet nitrene 2aT
was not obtainable under matrix UV photolysis conditions, we found that photolysis of
trimethylsilyl azide 1a in Ar matrix at 254 nm for 2 h generated a transition at 8187 G
ascribed to Me
Si-N 2aT with ZFS parameters |D/hc| = 1.540 and |E/hc| = 0.0002 cm-1
3
[
6]
(Figure 1) in excellent agreement with the data from -irradiation.
3
Figure 1. ESR spectrum of trimethylsilylnitrene 2aT ( A) in Ar at 15 K. X = 8178
2
-1
-1
G, Y
2
= 8187 G, |D/hc| = 1.540 cm ; |E/hc| = 0.0002 cm (H
0
= 3374.4 G). Inset: repeated
scan.
5
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We have reported an excellent linear correlation between observed |D/hc| values and
natural spin densities on the nitrene-N for a large number of nitrenes, calculated at the
B3LYP/EPR-III level.[18,19,20] This would require a spin density on N of 1.83 for a D value of
1
.54. The calculated natural spin densities in Me Si-N 2aT are 1.93 on N and -0.049 on Si at
3
the B3LYP/6-311g(dp) level. Although the value for N is overestimated by 5%, which is
below the computational accuracy, the high D value (1.54) demonstrates that there is little or
no delocalization of a nitrene electron onto Si. This is clearly seen in the total calculated
electron density projected on the spin density (Figure 2).
Figure 2. Total electron density of 2aT projected on the spin density. Blue = high spin
density.
-1
The relatively high D value of 1.54 cm is similar to those of alkylnitrenes, which
again indicates little or no delocalization of electron spin onto Si. There is only a modest
-
1
calculated contribution of spin-orbit coupling (SOC) to D by 0.10 cm . Similarly, a SOC
-1
SiN.[21] In contrast, very strong SOC
contribution to D of 0.15 cm was calculated for H
3
-1 [22]
causes high D values in triplet silylenes (D ~1.64 cm ).
Since SOC is inversely
proportional to the energy difference between the mixing states,[23] the large S-T gap in
silylnitrene (~38 kcal/mol) as well as the fact that the singlet state is an open shell may be
responsible for the low level of SOC in silylnitrenes. Other examples of deviation from the
linear correlation of D values with spin densities due to SOC have been summarized.[20]
Ferrante obtained the IR spectrum of 2aT in N matrix at 10 K by depositing the
2
[
6]
product of the microwave discharge. This only produced small amounts of the silylimine 5,
but a better yield of 5 was obtained by matrix photolysis. The silanimine 3a was not
characterized. We have confirmed Ferrante’s assignments by calculating the IR spectra of
2
aT, 3a, and 5 (see Supporting Information) at the B3LYP/6-311++G(d,p) level, which has
given excellent results for a variety of nitrenes.[24,25] The experimental IR spectra ascribed to
2
aT and 5 are in excellent agreement with the calculations.
6
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Reaction Mechanism.
In order to gain more clarity on the nature of the singlet nitrene Me Si-N 2aS and its possible
3
involvement in the pyrolysis and photolysis of trimethylsilyl azide 1a, we carried out
calculations at DFT as well as CASPT2 (10,9) and CASPT2(14,13) levels.
At the DFT level, the lowest energy path to the silanimine Me Si=NMe 3aS1 is
2
clearly the concerted reaction of the azide (Figure 3); the transition state (A) resembles a
distorted silylnitrene, where one Me group is already tilted toward the N atom (Figure 3).
This transition state has a large degree of closed-shell singlet nature. The activation energy,
which is largely the barrier for loss of N from azide 1a is high, ~61 kcal/mol.
2
Figure 3. The mechanisms of formation and rearrangements of triplet and singlet
trimethylsilylnitrenees 2aT and 2aS at the DFT level. The triplet (T) and two different
conformers (S1 and S2) of the singlet silanimine 3a can form. Energies in kcal/mol.
However, there are two non-concerted paths as well, via the singlet and triplet
nitrenes, but these have very much higher activation energies (Figure 3) and would not be
expected to play any role, if the results of the DFT calculations were accepted.
A major difference at the CASPT2 levels is that the open-shell singlet
trimethylsilylnitrene 2aS (A”) is a discrete intermediate (Figure 4). The activation barrier for
7
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its formation by N loss from the azide is 60 kcal/mol, very close to the 61 kcal/mol at the
2
DFT level. Activation energies for decomposition of silyl azides have not been reported, but
1
the azides are said to be surprisingly stable. Azide 1a has been pyrolysed in flow systems at
atmospheric pressure at temperatures of 350-430 C[26] and 500-600 C,
o
o
[1,2]
and in an FVP
o
-2
[7a]
system at 800 C/10 mm.
The calculated singlet-triplet splitting in 2a is high, ~38 kcal/mol (Figure 4). The high
value agrees with previous calculations for H
Si-N 7a, 42.3 kcal/mol (MCSCF/6-3lG(d))[27]
and 39.7 kcal/mol (MP4SDQ),[9] and for Me
Si(H)-N 7b 41 kcal/mol at the B3LYP/6-
1G(dp) level. For consistency, we calculated the S-T splitting in H SiN 7a (see below) at
the CASPT2(8,9) level as 41 kcal/mol, very close to the value for 2a.
3
2
[
6]
3
3
The singlet nitrene 2aS can rearrange to the silanimine 3aS2 (Figure 4), but again the
barrier is high, ca. 33 kcal/mol above the singlet nitrene, or 94 kcal/mol above the azide. It is
not surprising then, that there is no experimental evidence for formation of Me
in the reaction.
2
Si=NMe 3aS2
Figure 4. Formation of singlet Me
3
Si-N S(A”) 2aS and (potentially) silanimine 3aS2 at the
CASPT2(9,8)/6-311+G(d,p) (upright numbers) and CASPT2(14,13)/SP/6-311+G(d,p)
(numbers in italics) levels. Energies in kcal/mol.
However, there is an alternate and much more favourable route to the observed
reaction product Me
2
Si(H)-N=CH
2
5 (Figure 5). Here, the singlet nitrene 2aS inserts into a
8
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methyl group to form a silaziridine[7a] 12 (Scheme 2, Figure 5 and Figure 6) via a transition
state lying only 6 kcal/mol above the nitrene.
Figure 5. Formation of trimethylsilylnitrene 2aS and the silaaziridine 12 at the
CASPT2(9,8)/6-311+G(d,p) (upright numbers) and CASPT2(14,13)/SP/6-311+G(d,p)
(italics) levels. Energies in kcal/mol.
Silaziridine 12 can easily rearrange to the methanimine 5 by ring opening to a
(-)
(+)
(+)
(-)
(+)
zwitterionic intermediate 13, Me
2
Si -NH =CH
2
 Me
2
Si=NH -CH
2
 Me Si -NH-
2
CH
(-), which is in fact a 1-silaazomethine ylide (Scheme 2 and Figure 6). It is well known
2
that aziridines undergo ring opening to azomethine ylides both thermally and
photochemically.[28] This takes place in thermal reactions well below the temperatures
required for decomposition of silyl azides.
9
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Scheme 2. The silazirine 12 – silaazomethine ylide 13 route to N-silylimine 5.
If the postulated, thermally relaxed silanimine 3aS1 were formed, it would need to
overcome a barrier of nearly 60 kcal/mol and undergo two hydrogen shifts to yield 5 (Figure
6
). Activation barriers for direct 1,3-H shifts are usually very high, because they are
thermally forbidden by the Woodward-Hoffmann rules.[29] Therefore, such reactions
commonly proceed by different, lower-energy mechanisms.[30,31] In our case, the formal 1,3-
H shift 3a  5 takes place in the form of two consecutive 1,2-H shifts via the silaazomethine
ylide 13 (Figure 6).
1
0
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Figure 6. Two paths for formation of N-silylmethanimine Me
2
Si(H)-N=CH . Numbers in
2
upright font: CASPT2(10,9); italics: CASPT2(14,13); in parentheses: DFT (kcal/mol). Blue:
triplets; black: singlets; orange: the singlet silaziridine – silaazomethine ylide route.
The silaziridine 12 – silaazomethine ylide 13 path (orange path in Figure 6) is clearly
favored energetically with its maximum transition state lying only ~45 kcal/mol above the
triplet and 6 kcal/mol above the singlet nitrene, 2aT and 2aS, respectively. This route
explains why the silanimine Me Si=NMe 3a has never been observed experimentally.
2
Silanimine 3a may not even be formed as an intermediate; the conformer 3aS1 should be
relatively stable and observable, whereas the unstable conformer 3aS2 is a very transient
species, which can proceed to the product 5S with virtually no activation barrier (Figure 6).
Also the triplet nitrene 2aT can rearrange to the final product, 5aT, which would then
undergo intersystem crossing to the ground state 5aS. Moreover, 3aS2 and 3aT are nearly
degenerate, and their energy profiles cross at a point close to the 3aS2 and 3aT minima,
thereby allowing the reaction to pass easily from the triplet to the singlet energy surface. In
addition, the singlet and triplet profiles also cross close to the transition states for the
rearrangement 2aT  3aT and 2aS  3aS2 (the crossing points are marked in Figure 6; see
also Supporting Information).
In contrast, the simpler silanimines Me Si=NH 10b and PhSi(H)=NPh 11 (Scheme 1)
2
are observable[8,9] – probably because here there is no N-methyl group, which could form a
silaziridine, a silaazomethine ylide, and hence a silylmethanimine. We calculated the
transition state for the simplest 1,2-H shift H
3
Si-N  H Si=NH (7a  10a) which lies only 4
2
kcal/mol above the singlet nitrene, and 41 kcal/mol above the triplet, at the CASPT2(8,9)
level (Figure 7). Notably, the transition states for the singlet and triplet paths are nearly
degenerate, and the reaction can easily pass from one to the other at this point. Once either
1
0aS2 or 10aT is formed, simple rotation or intersystem crossing will generate the ground
state silanimine 10aS1.
1
1
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Figure 7. Reaction paths for the 1,2-H shift in silylnitrene H
3
Si-N 7a to H Si=NH 10a
2
calculated at the CASPT2(8,9)/6-311+G(d,p) level of theory (values in parenthesis are
calculated at the B3LYP/6-311G(d,p) level of theory). Energies in kcal/mol.
Conclusion
Triplet ground state trimethylsilylnitrene 2aT is generated by UV-photolysis of trimethylsilyl
azide in Ar matrix at 10-15 K. CASPT2(14,13) calculations indicate that the open-shell
1
singlet nitrene 2aS( A”) is a discrete intermediate lying 38 kcal/mol above the triplet. The
rearrangement of singlet nitrene 2aS to N-(dimethylsilyl)methanimine 5 takes place by a new
reaction mechanism, whereby nitrene 2aS inserts into a methyl C-H bond to form a
silaziridine 12. Ring opening of 12 generates a 1-silaazomethine ylide 13, in which a facile
1
,2-H shift yields the final product, 5. This process is considerably easier than the usually
expected 1,2-shift of a methyl group to yield a silanimine 3a (6 versus 33 kcal/mol). The
singlet and triplet energy surfaces overlap at the level of 3aS2 and 3aT, thus implying facile
interchange between the singlet and triplet energy surfaces. The mechanism described in
Figure 6 may apply whenever Si-methyl groups are present in silylnitrenes. The 1,2-H shift in
1
2
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the unsubstituted singlet silylnitrene 7a to silanimine 10a also has a very low calculated
barrier of 4 kcal/mol, and again the singlet and triplet energy surfaces overlap near the
transition state.
Acknowledgements
This work was supported by the Queensland Cyber Infrastructure Foundation at The
University of Queensland (QCIF g01) and the Mésocentre de Calcul Intensif Aquitain of the
Université de Bordeaux and the Université de Pau et des Pays de l’Adour.
Keywords: rearrangement  spin crossover  quantum chemistry  EPR spectroscopy
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3
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Chemistry - A European Journal
10.1002/chem.201803245
1
392; b) see also A. Systermans, V. Lefevre, J.-L. Ripoll, T. Pieplu, P. E. Hoggan, S.
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Graphical Abstract and Entry for Table of Contents:
Unusual rearrangement of trimethylsilylnitrene: Rather than the widely assumed 1,2-
methyl migration, the open-shell singlet trimethylsilylnitrene 2a inserts into a methyl C-H
bond to yield silaziridine 12 over an activation barrier of only 6 kcal/mol. Ring opening of 12
generates the 1-silaazomethine ylide 13, in which a facile 1,2-H shift yields the final product,
N-(dimethylsilyl)methanimine 5.
Silylnitrene Rearrangement
C. Wentrup,* H. Lüerssen, H. S. Silva, A. Dargelos, D. Bégué*
Trimethylsilylnitrene and its Surprising Rearrangement to
N-(Dimethylsilyl)methanimine via Silaziridine and Silaazomethine Ylide
1
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