General Synthesis and Optical Properties of N-Aryl- N′-Silyldiazenes

Article abstract of DOI:10.1021/acs.organomet.9b00654

While organo-substituted diazenes (R¹-N=N-R² with R = alkyl, aryl) are a well-studied class of compounds, much less is known about their heavier group 14 analogues, notably because of their perceived instability. We report herein a g

Full text of DOI:10.1021/acs.organomet.9b00654

Article
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Cite This: Organometallics XXXX, XXX, XXX−XXX
General Synthesis and Optical Properties of N‑Aryl‑N′‑Silyldiazenes
,†,‡,§
Lucie Finck,†,§ Stefan Hecht,‡,∥ and Martin Oestreich*,†
́
Clement Chauvier,*
†Institut fu
̈
̈
̈
r Chemie, Technische Universitat Berlin, Straße des 17. Juni 115, 10623 Berlin, Germany
̈
r Chemie, Humboldt-Universitat zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany
‡Institut fu
S
* Supporting Information
ABSTRACT: While organo-substituted diazenes (R1−N
N−R2 with R = alkyl, aryl) are a well-studied class of
compounds, much less is known about their heavier group 14
analogues, notably because of their perceived instability. We
report herein a general oxidative protocol to prepare various
N-aryl-N′-silyldiazenes by a fast and mild dehydrogenation of
the corresponding silylhydrazines using di-tert-butylazodicar-
boxylate as an oxidant. The optical properties of these stable blue to purple silyldiazenes were also studied by UV/visible
absorption spectroscopy and supported by TD-DFT calculations.
Scheme 1. Literature Precedents for the Synthesis of N-
Aryl-N′-Silyldiazenes
INTRODUCTION
■
1,2-Diazenes substituted with alkyl and aryl groups constitute
an important class of compounds that have found various
applications across the chemical sciences because of the
peculiar properties conferred by the NN double bond. As a
leading example, conjugated azobenzenes (Ar1−NN−Ar2)
have long been used as dyes in the textile industry, and the
main current research focus aims at exploiting their ability to
undergo reversible trans−cis isomerization under light
irradiation in various settings.1 Much less is known about the
chemistry of diazenes in which at least one of the substituents
is an electropositive main-group element because of their
anticipated instability toward decomposition or their difficult
preparation. The latter perception proved to be correct for
hydrogen-substituted diazenes, such as diimide HNNH2 as
well as aryldiimides Ar−NN−H,3 which are thermally
unstable reactive intermediates4 readily releasing N2. Con-
versely, a sizable gain in stability was noted with
trimethylsilyldiazenes (R−NN−SiMe3).
In 1968, Wiberg and co-workers isolated and studied the
light blue, thermolabile trans-bis(trimethylsilyl)diazene (BSD,
Me3Si−NN−SiMe3), which is comparatively more stable
than the corresponding diimide (t1/2 = 7.6 h versus a few
seconds at room temperature) but shares common reactivity
patterns with the latter.5 The synthesis and the properties of N-
aryl-N′-silyldiazenes (Ar−NN−SiR3) have been compara-
tively less studied. Seminal contributions from Wannagat and
dimethylazodicarboxylate (MeO2C−NN−CO2Me,
DMAD), respectively (Scheme 1). Although these early
synthetic efforts point out the thermal stability of the silylated
aryldiazenes, they lack generality and possess significant
drawbacks such as low yields, harsh conditions, and safety
concerns, thereby precluding the development of synthetic
applications of this understudied class of molecules.9 Herein
we report on our studies toward the development of a general
and practical synthesis of N-aryl-N′-silyldiazenes from the
corresponding silylhydrazines using safe and easy to handle di-
tert-butylazodicarboxylate (t-BuO2C−NN−CO2-t-Bu,
DBAD) as a dehydrogenating agent.
Kruger showed that deep blue N-phenyl-N′-trimethylsilyldia-
̈
zene (Ph−NN−SiMe3) can be obtained by the low-yielding
oxidation of the corresponding bismetalated hydrazine
derivative with Br2 (Scheme 1).6 The yield and the practicality
of the dehydrogenation approach have been improved by
Watanabe7 and later by Bottaro,8 who demonstrated that
monosilylated phenylhydrazines (Ph−NH−NH−SiR3, R =
Me, Et) can be directly dehydrogenated to the corresponding
diazene in moderate yield with di-tert-butyl peroxide (tBuO)2
at high temperature or in 90% yield with explosive
Received: October 2, 2019
© XXXX American Chemical Society
A
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a
RESULTS AND DISCUSSION
Table 1. Optimization of the Oxidative Diazene Synthesis
■
Preparation of N-Aryl-N′-Silylhydrazines. We initiated
our investigations with the preparation of the N-aryl-N′-
silylhydrazines, as they are the simplest precursors of the
targeted silyldiazenes. While these monosilylated hydrazines
are generally prepared by lithiation of the corresponding
arylhydrazine free base (ArNHNH2) followed by silylation
with a chlorosilane,10 we devised an alternative route starting
directly from the stable and commercially available arylhy-
drazine hydrochloride salts (ArNHNH2·HCl). In the presence
of an excess of a neutral nitrogen base (Et3N or DBU), the free
arylhydrazine is indeed released in situ from the salt and was
found to react smoothly with various chlorosilanes. For
example, the treatment of p-tolylhydrazine hydrochloride
with an excess of Et3N and trimethylsilyl chloride (Me3SiCl)
in diethyl ether for 15 h at reflux affords the corresponding
trimethylsilylhydrazine (1a) in 60% yield after distillation
(Scheme 2).11 This protocol transposes well to other
b
entry
oxidant
solvent
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
THF-d8
CD2Cl2
n-pentane
time
yield (%)
1
2
3
chloranil
20 min
24 h
15 min
5 min
20 min
20 min
20 min
20 min
20 min
0
c
3
3
4
5
6
6
6
6
47
d
e
95 (65)
4
5
6
7
8
9
0
90
92
97
87
Scheme 2. Preparation of Hydrazine 1a by a One-Pot
Deprotonation−Silylation of Tolylhydrazinium Salt 1
e
98 (79)
a
Reaction conditions: hydrazine 1a (0.10 mmol), oxidant (1.00
equiv), and solvent (0.70 mL, 0.14 M) at room temperature.
Determined by H NMR analysis using 1,3,5-trimethoxybenzene as
b
1
c
d
internal standard. 50% conversion. CuBr (1 mol %) used as catalyst.
e
Isolated yield after distillation under reduced pressure.
arylhydrazine derivatives and chlorosilanes with only marginal
modifications required in some instances (see the Supporting
Information for details), and it generally affords the N-aryl-N′-
silylhydrazines in high purity so that they can be engaged in
the next step without further purification.
copper(I) source16 greatly improved the outcome of the
oxidation (Table 1, entry 3). With 1 mol % CuBr as a catalyst,
hydrazine 1a reacted quantitatively with the diaziridinone to
afford diazene 2a in 95% yield after 15 min. This represents the
first catalytic preparation of an N-aryl-N′-silyldiazene and
demonstrates its stability in the presence of a copper salt.
To probe reagents alternative to the diaziridinone, which is
prepared by a three-step synthesis, electron-poor azodicarbox-
ylates known to oxidize hydrazine derivatives to diazenes were
also investigated.17 While the strained azo compound PTAD
(4, 4-phenyl-1,2,4-triazole-3,5-dione), known to dehydrogen-
ate alcohols,18 only led to protodesilylation of 2a at room
temperature (Table 1, entry 4), DIAD (diisopropyl azodi-
carboxylate, 5) and DBAD (di-tert-butyl azodicarboxylate, 6)
efficiently reacted with hydrazine 1a to afford the correspond-
ing diazene in 90% and 92% yields, respectively (Table 1,
entries 5 and 6). This reaction is extremely fast, as less than 10
min were required to reach full conversion of 1a, and it was
found to be only marginally sensitive to the polarity of the
solvent (Table 1, entries 7−9). The scale-up of this reaction to
10 g was conveniently done with 6 in nonpolar solvents (n-
pentane, n-hexane, or cyclohexane), as the diazene 2a could be
isolated in quantitative yield (79% yield after distillation) and
high purity simply by removing the sparingly soluble
hydrazodicarboxylate [(BocNH)2] by filtration. This method-
ology is therefore operationally simple and offers a better safety
profile in comparison to the early report of Bottaro that used
DMAD as an oxidant.8
Preparation of N-Aryl-N′’-Silyldiazenes. The oxidative
dehydrogenation of the N-aryl-N′-silylhydrazines to the
corresponding silyldiazenes required a careful choice of the
oxidant because of the anticipated sensitivity of the NN−Si
moiety to either fully oxidize to elemental N2 and/or to
protodesilylate, affording the unstable aryldiazenes (Ar−N
N−H). This was confirmed when trimethylsilylhydrazine 1a
was reacted with chloranil (OC6Cl4O) in benzene-d6 at
room temperature. While full conversion of 1a was reached
within 5 min, the vigorous release of a gas, presumably N2, was
1
noted, and the analysis of the reaction mixture by H NMR
spectroscopy and GC/MS measurements showed that toluene
and the bissilylated quinone (Me3SiO−C6Cl4−OSiMe3) were
the major products formed (Table 1, entry 1).12 This
distribution of product suggests that the silyldiazene 2a was
initially produced but rapidly reacted with the hydroquinone
(HO−C6Cl4−OH) by protodesilylation to afford the p-
tolyldiazene (p-tolyl−NN−H) that subsequently decom-
posed to toluene and N2.13 As a consequence, we evaluated
other 2e− oxidants whose reduced forms do not contain an
acidic hydroxyl proton and are thus less prone to induce the
protodesilylation of the N(sp2)−Si bond.
The addition of di-tert-butyldiaziridinone (3)14 to a solution
of the hydrazine 1a in C6D6 then afforded after 24 h a
precipitate and a blue solution; this color is diagnostic of the
formation of the corresponding trimethylsilyldiazene 2a.15 1H
NMR analysis revealed the formation of 2a in 47% yield (50%
conversion) along with a urea ((t-BuNH)2CO) that is
sparingly soluble in benzene (Table 1, entry 2). While full
conversion of 1a could not be reached by either extending the
reaction time to 48 h or changing the solvent, the addition of a
The scope of the reaction was explored using DBAD as an
oxidant, as it is a commercially available, safe, and easy to
handle solid that, unlike diaziridinone 3, does not require
kinetic activation by a metal-based catalyst. The influence of
the various silyl groups on the oxidation of the corresponding
silylated p-tolylhydrazine (1b−e → 2b−e) was first probed
(Scheme 3). The triethylsilylhydrazine 1b and dimethyl-
(phenyl)silylhydrazine 1e were oxidized in high yields and
B
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yields ranging from 80% for the chloro derivative 2g to 34% for
volatile 4-fluorophenyldiazene 2j. The low yield for the latter is
mainly the result of its thermal instability during the distillation
in the presence of small quantities of the hydrazodicarboxylate
(BocNH)2, which could not be removed by filtration. Likewise,
the 2-fluorophenyldiazene 2q was obtained in only 39% yield.
Nevertheless, replacing the Me3Si group in hydrazine 1j by
Et3Si allowed the isolation of the diazene 2k with an improved
yield of 57%, thereby suggesting greater thermal stability of 2k
in comparison to 2j. Hydrazines bearing either electron-
deficient or electron-rich aromatic rings with cyano (1m), nitro
(1p), or methoxy (1n) groups were all dehydrogenated
smoothly to afford the corresponding azo compounds in
yields of greater than 69% after distillation. The presence of
two methyl groups in the 2- and 6-positions did not hamper
the oxidation, since the hindered diazene 2o was obtained in a
61% yield. Finally, a less aromatic aryl group was also well
tolerated, as the more π-conjugated naphth-2-yl derivative 2r
was obtained in quantitative yield as a crystalline deep blue
solid that did not require purification.
Collectively, these results not only further demonstrate the
general ability of 6 to dehydrogenate various silylhydrazines
but also suggest that the oxidation is moderately sensitive to
the substitution pattern of these hydrazines in terms of both
steric and electronic properties. As a limitation, the only
substrate that reacted sluggishly with DBAD was the N-(2-
bromophenyl)-N′-trimethylsilylhydrazine (<50% conversion
after 1 week at 25 °C), presumably because of a destabilizing
interaction between the bromine atom and one of the nitrogen
lone pairs of the corresponding diazene.
Color of N-Aryl-N′-Silyldiazenes. All of the aforemen-
tioned trans-silyldiazenes share a deep blue or purple color,
attributed to the symmetry-forbidden n → π* transition (ε <
100 L mol−1 cm−1) located in the visible region. While the
absorption properties of 2f were recorded by Bock and co-
workers more than 40 years ago,20 the influence of the
substitution pattern on the excitation energies of the n → π*
and π → π* transitions has never been studied because the
synthesis of derivatives was elusive at that time. We therefore
recorded the UV/visible absorption spectra of representative
silyldiazenes,21 and the results obtained for the n → π*
transition are presented in Figure 1 and Table 2 (see the the
Supporting Information for the spectrum in the UV region as
well as for the data on the π → π* transition) along with their
comparison against TD-DFT calculations (vide infra).
Scheme 3. Influence of the Silyl Group on the Hydrazine
Oxidation
a
a
Isolated yields after purification. Boc = tert-butoxycarbonyl.
b
Oxidation was performed in cyclohexane instead of n-pentane.
behaved analogously to the Me3Si derivative 1a. Interestingly,
the steric environment around the silicon atom has no
noticeable effect on the outcome of the oxidation, as the
bulky hydrazines 1c,d bearing the Ph3Si and t-BuMe2Si groups
were fully dehydrogenated within 30 min, and the correspond-
ing diazenes were isolated in 78% and 74% yields, respectively.
In stark contrast with the unstable monosubstituted
aryldiazenes, the isolated silyldiazenes 2a−e possess relatively
high thermal and air stability, allowing common laboratory
practice to be employed for their synthesis and isolation (wet
solvent, use of rotary evaporation, and filtration under air). For
example, the Me3Si derivative 2a has a half-life of more than 48
1
h under air (determined by H NMR spectroscopy from a 0.1
M THF solution), while the Ph3Si- and t-BuMe2Si-substituted
p-tolyldiazenes 2c,d are even more robust, as the latter
withstands deactivated silica gel chromatography and the
former wet t-BuOH washings under air.19
The generality of the oxidative dehydrogenation was further
assessed by variation of the substituents on the aromatic ring
(Scheme 4). The absence of a substituent on the aromatic ring
gave results similar to those with a methyl group in the para
position (81% yield for 2f versus 79% for 2a). In addition,
halogenation in the 4-position was tolerated, including iodine
(as in 2i), and the corresponding diazenes were obtained in
Scheme 4. Influence of the Aromatic Substitution Pattern
on the Hydrazine Oxidation
Overall, the nature of the silyl group has a limited effect, as
the replacement of the Me3Si group in 2a by the more
electropositive Ph3Si group in 2c only induces slight
bathochromic shifts amounting to 11 nm (−324 cm−1) and
12 nm (−1447 cm−1) of the n → π* and π → π* transitions,
respectively. Likewise, the bulky t-BuMe2Si group in 2d barely
modifies the position of the absorption maxima, further
suggesting that the optical properties of the silyldiazene
marginally depend on the stereoelectronic effect of the
organosilyl group. Similar conclusions hold when electron-
releasing groups (+M) are introduced on the aromatic ring in
the para-substituted diazenes 2j (fluorine) and 2n (methoxy),
which result in blue-shifted n → π* but red-shifted π → π*
bands in comparison to the unsubstituted diazene 2f, whereas
both transitions are red-shifted for the electron-poor cyano
derivative 2m. Similar trends are observed for monosubstituted
azobenzenes and can be rationalized for the π → π* transition
by molecular orbital (MO) theory.22 Building on these
C
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that, in all the cases considered in Table 2, the n → π*
(HOMO → LUMO) transition contributes more than 95% to
the lowest-energy vertical excitation, thereby suggesting that a
relative decrease in the HOMO−LUMO gap (in the ground
state) approximatively translates into a lower vertical excitation
energy and therefore a bathochromic shift of the correspond-
ing band.26 A good linear correlation was indeed found
between the computed HOMO−LUMO gaps and the
calculated lowest vertical excitation energies or the correspond-
ing wavelengths (see Figure S5).
These important insights from TD-DFT allow us to
rationalize the different colors of group 14 element-substituted
diazenes at a good level of approximation by considering the
relative stereoelectronic factors that affect the energy of their
HOMO and LUMO MOs in the ground state. As a prominent
illustration, the dramatic bathochromic shift of 169 nm for the
n → π* transition upon replacement of the t-Bu group in Ph−
NN−t-Bu (7) by a Me3Si group in 2f (411 versus 580 nm,
respectively) can be traced back to the relative destabilization
of the HOMO of 2f by approximatively 0.6 eV, while its
LUMO energy is lowered by 0.2 eV (Figure 2), thereby leading
Figure 1. Stacked absorption spectra of N-aryl-N′-silyldiazenes in the
visible region recorded in n-hexane (C = 7.5 × 10−3 M).
Table 2. Experimental and Calculated (TD-DFT)
Absorption Maxima for the n → π* Transition of Various
Diazenes
b
compound
λmax (nm) λTD (nm) εmax (L mol−1 cm−1
)
a
2a (4-Me/Me3Si)
2d (4-Me/t-BuMe2Si)
2c (4-Me/Ph3Si)
577
584
588
573
602
570
578
584
586
577
628
563
587
418
582
47
39
72
38
42
a
a
a
a
a
c
2j (4-F/Me3Si)
2m (4-CN/Me3Si)
2n (4-MeO/Me3Si)
Ph−NN−SiMe3 (2f)
Ph−NN−t-Bu (7)
Ph−NN−GeMe3 (8)
43
c
580
29
d
c
411
144
c
c
562
26
a
b
Recorded in n-hexane solution (7.5 × 10−3 M). Determined at the
PCM-TD-PBE0/6-31+G(d,p)//PCM-M062X/cc-pVDZ level of
theory in n-hexane. In n-hexane from ref 20. In n-hexane from ref
c
d
15.
Figure 2. Comparison of the frontier KS-MO energies between of
diazenes Ph−NN−R (R = t-Bu, Me3Si, Me3Ge) calculated at the
PBE0/6-31+G**//M062X/cc-pVDZ level of theory.
absorption data, we also investigated the possibility of trans−
cis photoisomerization, which is well established for diazenes
such as azobenzenes2 and azoalkanes,23 in the case of
b
silyldiazene 2a. However, upon irradiation of an n-hexane
solution of 2a in both the UV (303 nm) and the visible (596
nm) regions of the spectrum, no changes in the absorption
spectra, indicative of possible trans−cis photoisomerization,
could be detected at temperatures ranging from 25 to −80 °C.
The absorption properties of these silyldiazenes were further
studied computationally by TD-DFT at the TD-PBE0/6-
31+G(d,p)//M062X/cc-pVDZ level of theory, with both steps
including PCM solvent corrections (n-hexane).24 The
isoelectronic analogues of 2f, carbon-based 7 and germa-
nium-based Ph−NN−GeMe3 (8), were also included for
comparison. The computed energies of the lowest vertical
excitations (λTD in Table 2) as well as those for the π → π*
transition (Table S3) reproduce well the experimental results
within a few nanometers, including the subtle auxochromic
shifts induced by the different substituents. A higher deviation
was nonetheless observed for the cyano derivative 2m
(|Δλexp‑TD| = 26 and 12 nm for the n → π* and π → π*
transitions, respectively), presumably because of the poor
description of significant long-range effects at the level of
theory employed.25 Importantly, the calculations also indicate
to a reduced HOMO−LUMO gap for 2f in comparison to its
carbon analogue 7. Analogous arguments were invoked to
account for the related optical behavior of other silyl group
containing chromophores such as acylsilanes27 and N-
silylketimines.28 In addition, similar energy shifts are observed
for the frontier MOs of the germanyldiazene Ph−NN−
GeMe3 (8) and these energetic trends involving the HOMO
are globally consistent with the variation of the experimental
first ionization potentials (IE1, assuming the validity of
Koopman’s theorem) determined for these three compounds
by Bock and co-workers (IE1 = 8.35 eV (7), 7.85 eV (2f) and
7.65 eV (8)) in 1976.20
The HOMO-raising effect of the silyl group in silyldiazenes
has been explained in terms of its enhanced electron-donating
properties in comparison to carbon (ΔχP(Si/C) = 0.65), which
according to Bock and co-workers, “considerably destabilize the
nitrogen lone pair electrons of the azo group”.20 Nonetheless, this
destabilizing inductive effect does not account for the greater
HOMO energy (and lower IE1) of germyldiazenes (ΔχP(Ge/
C) = 0.54) in comparison to the silicon-based analogues.
D
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reported in wavenumbers (cm−1). Infrared (IR) spectra of silyl
diazenes were recorded in a glovebox using a Thermo Nicolet Magna-
IR 750 spectrophotometer equipped with an ATR unit, and the
signals are reported in wavenumbers (cm−1). Melting points (mp)
were determined with a Stuart Scientific SMP20 melting point
apparatus and are not corrected. Boiling points (bp) were measured at
the distillation head and are not corrected. Distillation pressure was
determined directly with the pressure gauge (VAP 5, from
Vacuumbrand) connected to the distillation apparatus. High-
resolution mass spectrometry (HRMS) analysis was performed by
While a rationale of this behavior is out of the scope of the
present study, the simplicity of the synthetic procedures
described in this work will certainly facilitate, in conjunction
with theoretical calculations, the preparation of numerous
group 14 element based diazenes to help decipher precisely the
interactions underlying their peculiar optical properties.
CONCLUSION
■
In summary, we have developed a general oxidative protocol to
prepare various aryl-substituted silyldiazenes by dehydrogen-
ation of the corresponding easily accessible silylhydrazines.
The reaction proceeds quickly, operates under mild conditions,
and benefits from an appreciable scope in terms of both the
silyl groups and the substituents on the aromatic ring tolerated,
which collectively underpin the general competency of di-tert-
butyl azodicarboxylate to dehydrogenate hydrazine derivatives.
The obtained silyldiazenes all possess a purple to deep blue
color due to a long-wavelength n → π* transition, which was
characterized experimentally by absorption spectroscopy and
computationally by TD-DFT. Overall, these results are an
incentive for the development of applications of this
understudied class of molecule and the concise rationalization
of their peculiar optical properties, both endeavors being under
study in our laboratory.
the Analytical Facility at the Institut fur Chemie, Technische
̈
̈
Universitat Berlin.
General Procedure for the Preparation of N-Aryl-N′-
Silyldiazenes (GP1). In a glovebox, a single-neck round-bottom
flask equipped with a magnetic stirring bar was charged with the
silylated arylhydrazine (either solid or liquid) followed by dry n-
pentane, n-hexane, or cyclohexane (0.5 M). To the resulting
homogeneous, vigorously stirred solution was added di-tert-butyl
azodicarboxylate (DBAD, 0.95−1.05 equiv) at room temperature
either portionwise as a solid or as a concentrated solution in dry DCM
(made by dissolving the DBAD in a minimum amount of DCM at
room temperature). Unless otherwise noted, an almost instantaneous
color change from colorless to dark blue or dark purple was noted
with the concomitant precipitation of the hydrazine (BocNH)2. The
resulting suspension was further stirred for 30 min to 1 h at room
temperature, and the crude reaction mixture was then filtered over a
fritted funnel. The blue filtrate was concentrated in vacuo to afford the
crude silylated aryldiazene, which was then distilled under reduced
pressure via a short-path distillation to afford the title compound.
(E)-1-(p-Tolyl)-2-(trimethylsilyl)diazene (2a). This compound
was prepared according to GP1 from the corresponding silylated
hydrazine (10.1 g, 52.0 mmol, 1.00 equiv) and DBAD (11.4 g, 49.4
mmol, 0.950 equiv). Purification by short-path distillation (bp: 60−62
°C at 6.6 × 10−1 mbar) afforded the title compound (2a) (7.90 g,
EXPERIMENTAL SECTION
■
General Information. Reactions were performed in flame-dried
glassware using an MBraun glovebox (O2 < 20 ppm, H2O < 1.0 ppm)
or conventional Schlenk techniques under a static pressure of argon
(glovebox) or nitrogen (fume hood) unless otherwise stated.
Glassware for reactions outside a glovebox was dried under vacuum
using a heat gun. Glassware for reactions performed inside a glovebox
was either dried under vacuum using a heat gun or dried overnight in
a 120 °C oven before being transferred into the glovebox. Liquids and
solutions were transferred with syringes. All stated temperatures refer
to external bath temperatures. Dichloromethane (DCM) was dried
over CaH2 and distilled prior to use. Tetrahydrofuran (THF) and
diethyl ether (Et2O) were dried over sodium/benzophenone and
distilled prior to use. Cyclohexane, n-pentane, and n-hexane were
obtained by distillation or from an MBraun solvent purification
system (SPS-800), degassed with three freeze−pump−thaw cycles,
and stored in a glovebox over thermally activated 4 Å molecular
sieves. Triethylamine was purchased from ABCR and used as
received. DBU was obtained from Aldrich and distilled prior to use.
Unless otherwise stated, arylhydrazine hydrochloride salts were
obtained from ABCR, Acros, Alfa Aesar, Merck, Sigma-Aldrich, or
Tokyo Chemical Industry (TCI) and used as received. 4-
Iodophenylhydrazine hydrochloride,29 di-tert-butyldiaziridinone
(3),30 and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD, 4)31 were
prepared according to reported procedures.
1
41.1 mmol, 79%) as a deep blue liquid. H NMR (500 MHz, C6D6):
δ/ppm 0.34 (s, 9H), 2.03 (s, 3H), 7.01 (d, J = 8.3 Hz, 2H), 7.93 (d, J
= 8.3 Hz, 2H). 13C{1H} NMR (126 MHz, C6D6): δ/ppm −2.54
1
(3C), 21.2, 121.5 (2C), 129.8 (2C), 141.5, 155.3. H,29Si HMQC
NMR (500/99 MHz, C6D6, optimized for J = 7 Hz): δ/ppm 0.34/
11.5. HRMS (APCI):: calculated for C10H17N2Si+ [(M + H)+],
193.1156; found, 193.1155. IR (ATR): ν
̃
/cm−1 3028, 2960, 2922,
2901, 1602, 1507, 1475, 1305, 1248, 1130, 869, 839.
(E)-1-(p-Tolyl)-2-(triethylsilyl)diazene (2b). This compound
was prepared according to GP1 from the corresponding silylated
hydrazine (4.98 g, 20.0 mmol, 1.00 equiv) and DBAD (4.93 g, 21.0
mmol, 1.05 equiv). Purification by short-path distillation (bp: 70−72
°C at 7.6 × 10−2 mbar) afforded the title compound (2b) (3.79 g,
1
16.2 mmol, 81%) as a deep blue liquid. H NMR (500 MHz, C6D6):
δ/ppm 0.93 (q, J = 8.3 Hz, 6H), 1.06 (t, J = 8.3 Hz, 9H), 2.03 (s,
3H), 7.01 (d, J = 8.2 Hz, 2H), 7.89 (d, J = 8.2 Hz, 2H). 13C{1H}
NMR (126 MHz, C6D6): δ/ppm 3.32 (3C), 7.13 (3C), 21.2, 121.3
(2C), 129.7 (2C), 141.4, 155.5. 1H/29Si HMQC NMR (500/99
MHz, C6D6, optimized for J = 7 Hz): δ/ppm 0.93, 1.06/12.7. HRMS
(APCI):: calculated for C13H23N2Si+ [(M + H)+], 235.1625; found,
1H, 13C, 29Si, 19F, and 15N NMR spectra were recorded on Bruker
AV400, AV500, and AV700 instruments. CDCl3, C6D6, and CD2Cl2
were degassed and stored in a glovebox over 4 Å molecular sieves.
Chemical shifts are reported in parts per million (ppm) and are
referenced to the residual solvent signals as the internal standard
(CHCl3 δ 7.26 ppm, C6D5H δ 7.16 ppm, CDHCl2 δ 5.32 ppm for 1H
NMR; CDCl3 δ 77.16 ppm, C6D6 δ 128.06 ppm, CD2Cl2 δ 53.84 ppm
for 13C NMR). Data are reported as follows: chemical shift,
multiplicity (s = singlet, bs = broad singlet, d = doublet, dd =
doublet of doublets, dt = doublet of triplets, t = triplet, tt = triplet of
triplets, q = quartet, m = multiplet, mc = centrosymmetric multiplet),
coupling constants (Hz) and integration. 29Si, 19F, and 15N NMR
spectra were calibrated according to the IUPAC recommendation
using a unified chemical shift scale based on the proton resonance of
tetramethylsilane as the primary reference.32 Infrared (IR) spectra of
silyl hydrazines were recorded on an Agilent Technologies Cary 630
FTIR spectrometer equipped with an ATR unit, and the bands are
235.1626. IR (ATR): ν
̃
/cm−1 3027, 2954, 2912, 2875, 1600, 1507,
1471, 1304, 1237, 1131, 1006, 857, 822.
(E)-1-(Triphenylsilyl)-2-(p-tolyl)diazene (2c). This compound
was prepared according to GP1 from the corresponding silylated
hydrazine (2.04 g, 5.10 mmol, 1.00 equiv) and DBAD (1.24 g, 5.10
mmol, 1.00 equiv). The blue and viscous crude oil obtained after
filtration was then taken up in 5 mL of t-BuOH and filtered again. The
light blue solid obtained was further dried under high vacuum (10−2
mbar) overnight, affording the title compound (2c) (1.51 g, 3.99
1
mmol, 78%) as a light blue powder. Mp: 82−84 °C (t-BuOH). H
NMR (500 MHz, CD2Cl2): δ/ppm 2.43 (s, 3H), 7.34 (d, J = 8.1 Hz,
2H), 7.40−7.46 (m, 6H), 7.47−7.53 (m, 3H), 7.68 (dd, J = 1.5, 8.0
Hz, 6H), 7.76 (d, J = 8.1 Hz, 2H). 13C{1H} NMR (101 MHz,
CD2Cl2): δ/ppm 21.6, 121.4 (2C), 128.4 (6C), 129.9 (2C), 130.7
1
(3C), 132.6 (3C), 136.7 (6C), 143.0, 156.0. H/29Si HMQC NMR
(500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm 7.43, 7.50,
E
DOI: 10.1021/acs.organomet.9b00654
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
7.68/−20.4. HRMS (APCI): calculated for C25H23N2Si+ [(M + H)+],
Hz, 2H). 13C{1H} NMR (126 MHz, C6D6): δ/ppm −2.74 (3C),
122.8 (2C), 126.1, 132.4 (2C), 154.8. 1H/29Si HMQC NMR (500/99
MHz, C6D6, optimized for J = 7 Hz): δ/ppm 0.28/12.9. HRMS
(APCI): calculated for C9H14BrN2Si+ [(M + H)+], 257.0104; found,
379.1625; found, 379.1623. IR (ATR): ν/
̃
cm−1 3011, 1505, 1425,
1113, 824, 798, 735, 695.
(E)-1-(tert-Butyldimethylsilyl)-2-(p-tolyl)diazene (2d). This
compound was prepared according to GP1 from the corresponding
silylated hydrazine (6.24 g, 26.4 mmol, 1.00 equiv) and DBAD (6.20
g, 26.4 mmol, 1.00 equiv). Purification by short-path distillation (bp:
70−72 °C at 2.4 × 10−1 mbar) afforded the title compound (2d)
(4.58 g, 19.5 mmol, 74%) as a deep blue liquid that crystallized while
it was stored cold. Crystals suitable for X-ray diffraction were obtained
by slow cooling of neat 2d to −30 °C. Mp: 29−32 °C (n-pentane).
1H NMR (500 MHz, C6D6): δ/ppm 0.30 (s, 6H), 1.08 (s, 9H), 2.03
(s, 3H), 7.01 (d, J = 8.3 Hz, 2H), 7.90 (d, J = 8.3 Hz, 2H). 13C{1H}
NMR (126 MHz, C6D6): δ/ppm −6.95 (2C), 18.1, 21.2, 26.4 (3C),
121.4 (2C), 129.8 (2C), 141.5, 155.4. 1H/29Si HMQC NMR (500/99
MHz, C6D6, optimized for J = 7 Hz): δ/ppm 0.30, 1.08/13.7. HRMS
(APCI): calculated for C13H23N2Si+ [(M + H)+], 235.1625; found
257.0105. IR (ATR): ν
̃
/cm−1 2960, 2899, 1723, 1577, 1495, 1396,
1296, 1248, 1129, 1064, 1006, 837.
(E)-1-(4-Iodophenyl)-2-(trimethylsilyl)diazene (2i). This com-
pound was prepared according to GP1 from the corresponding
silylated hydrazine (7.96 g, 26.0 mmol, 1.00 equiv) and DBAD (5.80
g, 24.7 mmol, 0.950 equiv). Purification by short-path distillation (bp:
79−85 °C at 4.1 × 10−1 mbar) afforded the title compound (2i) (3.60
g, 11.8 mmol, 46%) as a deep blue liquid. 1H NMR (500 MHz,
C6D6): δ/ppm 0.28 (s, 9H), 7.44 (d, J = 8.6 Hz, 2H), 7.51 (d, J = 8.6
Hz, 2H). 13C{1H} NMR (126 MHz, C6D6): δ/ppm −2.74 (3C), 98.6,
122.8 (2C), 138.5 (2C), 155.3. 1H/29Si HMQC NMR (500/99 MHz,
C6D6, optimized for J = 7 Hz): δ/ppm 0.28/13.1. HRMS (APCI):
calculated for C9H14IN2Si+ [(M + H)+], 304.9965; found, 304.9966.
IR (ATR): ν
̃
/cm−1 3068, 2958, 2899, 1723, 1569, 1488, 1392, 1301,
1248, 1132, 1050, 1002, 832.
235.1627. IR (ATR): ν
̃
/cm−1 3027, 2954, 2928, 2884, 2857, 1603,
1507, 1471, 1247, 1131, 1006, 862, 823. The crystallographic data are
available online in the CCDC database under number CCDC
1956764.
(E)-1-(4-Fluorophenyl)-2-(trimethylsilyl)diazene (2j). This
compound was prepared according to GP1 from the corresponding
silylated hydrazine (5.00 g, 25.0 mmol, 1.00 equiv) and DBAD (6.20
g, 26.3 mmol, 1.05 equiv). Purification by short-path distillation (bp:
34−38 °C at 3.0 × 10−1 mbar) afforded the title compound (2j) (1.60
g, 8.15 mmol, 34%) as a deep purple liquid (volatile). 1H NMR (400
MHz, C6D6): δ/ppm 0.30 (s, 9H), 6.80 (mc, 2H), 7.74 (mc, 2H).
13C{1H} NMR (126 MHz, C6D6): δ/ppm −2.67 (3C), 115.9 (d, J =
23.0 Hz, 2C), 123.3 (d, J = 9.4 Hz, 2C), 153.3 (d, J = 2.9 Hz, 2C),
165.0 (d, J = 251.2 Hz). 19F{1H} NMR (471 MHz, C6D6): δ/ppm
(E)-1-(Phenyldimethylsilyl)-2-(p-tolyl)diazene (2e). This com-
pound was prepared according to GP1 from the corresponding
silylated hydrazine (5.00 g, 19.5 mmol, 1.00 equiv) and DBAD (4.49
g, 19.5 mmol, 1.00 equiv). Purification by short-path distillation (bp:
104−106 °C at 1.5 × 10−1 mbar) afforded the title compound (2e)
(3.85 g, 15.1 mmol, 78%) as a deep blue liquid that crystallized on
standing. Mp: 30−33 °C (n-pentane). 1H NMR (500 MHz, CD2Cl2):
δ/ppm 0.63 (s, 6H), 2.41 (s, 3H), 7.30 (d, J = 8.1 Hz, 2H), 7.37−7.46
(m, 3H), 7.65 (d, J = 8.1 Hz, 2H), 7.67−7.72 (m, 2H). 13C{1H}
NMR (126 MHz, CD2Cl2): δ/ppm −3.91 (2C), 21.5, 121.2 (2C),
128.3 (2C), 129.8 (2C), 130.3, 134.7 (2C), 136.0, 142.3, 155.4.
1H/29Si HMQC NMR (500/99 MHz, CD2Cl2, optimized for J = 7
Hz): δ/ppm 0.63, 7.67−7.72/1.80. HRMS (APCI): calculated for
1
−110.0. H/29Si HMQC NMR (500/99 MHz, C6D6, optimized for J
= 7 Hz): δ/ppm 0.30/12.1. HRMS (APCI): calculated for
C9H14FN2Si+ [(M + H)+], 197.0905; found, 197.0906. IR (ATR):
ν/cm−1 3071, 2961, 2903, 1594, 1506, 1413, 1249, 1220, 1123, 1085,
871, 837.
̃
C15H19N2Si+ [(M + H)+], 255.1312; found, 255.1310. IR (ATR): ν
̃
/
(E)-1-(4-Fluorophenyl)-2-(triethylsilyl)diazene (2k). This
compound was prepared according to GP1 from the corresponding
silylated hydrazine (1.20 g, 5.00 mmol, 1.00 equiv) and DBAD (1.20
g, 5.25 mmol, 1.05 equiv). Purification by short-path distillation (bp:
58−60 °C at 1.0 × 10−1 mbar) afforded the title compound (2k) (680
cm−1 3043, 2957, 2921, 1701, 1583, 1503, 1469, 1424, 1401, 1305,
1250, 1113, 1011, 813.
(E)-1-Phenyl-2-(trimethylsilyl)diazene (2f). This compound
was prepared according to GP1 from the corresponding silylated
hydrazine (5.04 g, 28.0 mmol, 1.00 equiv) and DBAD (6.25 g, 26.6
mmol, 0.950 equiv). Purification by short-path distillation (bp: 26−29
°C at 4.3 × 10−2 mbar) afforded the title compound (2f) (4.04 g, 22.7
1
mg, 2.85 mmol, 57%) as a deep blue liquid. H NMR (500 MHz,
CD2Cl2): δ/ppm 0.92 (q, J = 8.0 Hz, 6H), 1.03 (t, J = 8.0 Hz, 9H),
7.17 (mc, 2H), 7.72 (mc, 2H). 13C{1H} NMR (126 MHz, CD2Cl2):
δ/ppm 3.10 (3C), 6.93 (3C), 115.9 (d, J = 22.8 Hz, 2C), 122.9 (d, J =
9.4 Hz, 2C), 153.5 (d, J = 2.35 Hz), 164.8 (d, J = 249.9 Hz). 19F{1H}
1
mmol, 81%) as a deep blue liquid. H NMR (500 MHz, C6D6): δ/
ppm 0.32 (s, 9H), 7.11 (tt, J = 1.7, 7.3 Hz, 1H), 7.19 (dd, J = 7.3, 8.5
Hz, 2H), 7.95 (dd, J = 1.7, 8.5 Hz, 2H). 13C{1H} NMR (101 MHz,
C6D6): δ/ppm −2.65 (3C), 121.4 (2C), 129.1 (2C), 131.3, 156.6.
1H/29Si HMQC NMR (500/99 MHz, C6D6, optimized for J = 7 Hz):
δ/ppm 0.32/12.0. HRMS (APCI): calculated for C9H15N2Si+ [(M +
1
NMR (471 MHz, CD2Cl2): δ/ppm −111.6. H/29Si HMQC NMR
(500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/ppm 0.92, 1.03/
13.9. HRMS (APCI): calculated for C12H20FN2Si+ [(M + H)+],
239.1374; found, 239.1376. IR (ATR): ν
̃
/cm−1 3069, 2955, 2912,
H)+], 179.0999; found, 179.0997. IR (ATR): ν/cm−1 3062, 2960,
̃
2876, 1594, 1505, 1458, 1412, 1221, 1123, 1006, 865, 840.
Methyl (E)-4-((Trimethylsilyl)diazenyl)benzoate (2l). This
compound was prepared according to GP1 from the corresponding
silylated hydrazine (2.38 g, 10.0 mmol, 1.00 equiv) and DBAD (2.20
g, 9.50 mmol, 0.950 equiv). Purification by short-path distillation (bp:
80−82 °C at 1.1 × 10−1 mbar) afforded the title compound (2l) (1.40
g, 5.92 mmol, 59%) as a deep blue liquid that crystallized while stored
2901, 1720, 1602, 1502, 1451, 1306, 1247, 1152, 1128, 868, 837.
(E)-1-(4-Chlorophenyl)-2-(trimethylsilyl)diazene (2g). This
compound was prepared according to GP1 from the corresponding
silylated hydrazine (5.05 g, 23.5 mmol, 1.00 equiv) and DBAD (5.25
g, 22.3 mmol, 0.950 equiv). Purification by short-path distillation (bp:
55−59 °C at 2.5 × 10−1 mbar) afforded the title compound (2g)
(3.99 g, 18.8 mmol, 80%) as a deep blue liquid. 1H NMR (400 MHz,
C6D6): δ/ppm 0.29 (s, 9H), 7.11 (d, J = 8.2 Hz, 2H), 7.66 (d, J = 8.2
Hz, 2H). 13C{1H} NMR (126 MHz, C6D6): δ/ppm −2.72 (3C),
122.6 (2C), 129.4 (2C), 137.5, 154.5. 1H/29Si HMQC NMR (500/99
MHz, C6D6, optimized for J = 7 Hz): δ/ppm 0.29/12.7. HRMS
(APCI): calculated for C9H14ClN2Si+ [(M + H)+], 213.0609; found,
1
cold (−30 °C). Mp: 28−31 °C (n-pentane). H NMR (500 MHz,
C6D6): δ/ppm 0.29 (s, 9H), 3.46 (s, 3H), 7.82 (d, J = 8.5 Hz, 2H),
8.18 (d, J = 8.5 Hz, 2H). 13C{1H} NMR (126 MHz, C6D6): δ/ppm
−2.78 (3C), 51.7, 121.1 (2C), 131.0 (2C), 132.7, 157.4, 166.2.
1H/29Si HMQC NMR (500/99 MHz, C6D6, optimized for J = 7 Hz):
δ/ppm 0.29/13.3. HRMS (APCI): calculated for C11H17N2O2Si+ [(M
+
̃
/cm−1 3325, 2955,
213.0609. IR (ATR): ν
̃
+ H) ], 237.1054; found, 237.1055. IR (ATR): ν
1297, 1248, 1130, 1088, 1009, 836.
2902, 1723, 1602, 1511, 1474, 1274, 1248, 1154, 1108, 840.
(E)-1-(4-Bromophenyl)-2-(trimethylsilyl)diazene (2h). This
compound was prepared according to GP1 from the corresponding
silylated hydrazine (5.05 g, 19.5 mmol, 1.00 equiv) and DBAD (4.35
g, 18.5 mmol, 0.950 equiv). Purification by short-path distillation (bp:
78−82 °C at 3.6 × 10−1 mbar) afforded the title compound (2h)
(3.04 g, 11.8 mmol, 61%) as a deep blue liquid. 1H NMR (500 MHz,
C6D6): δ/ppm 0.28 (s, 9H), 7.29 (d, J = 8.7 Hz, 2H), 7.58 (d, J = 8.7
(E)-4-((Trimethylsilyl)diazenyl)benzonitrile (2m). This com-
pound was prepared according to GP1 from the corresponding
silylated hydrazine (5.02 g, 24.5 mmol, 1.00 equiv) and DBAD (5.47
g, 23.3 mmol, 0.950 equiv). The title compound (2m) (4.05 g, 19.9
mmol, 81%) was obtained as a deep blue solid. Crystals suitable for X-
ray diffraction were obtained by sublimation (3.0 × 10−1 mbar, 70
1
°C). Mp: 24−28 °C (n-pentane). H NMR (400 MHz, C6D6): δ/
F
DOI: 10.1021/acs.organomet.9b00654
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
ppm 0.25 (s, 9H), 7.01 (d, J = 8.4 Hz, 2H), 7.45 (d, J = 8.4 Hz, 2H).
13C{1H} NMR (126 MHz, C6D6): δ/ppm −2.89 (3C), 114.9, 118.4,
121.3 (2C), 133.3 (2C), 155.8. 1H/29Si HMQC NMR (500/99 MHz,
C6D6, optimized for J = 7 Hz): δ/ppm 0.25/14.4. HRMS (APCI):
calculated for C10H14N3Si+ [(M + H)+], 204.0952; found, 204.0953.
13C{1H} NMR (126 MHz, CD2Cl2): δ/ppm −2.58 (3C), 114.8,
1
127.1, 127.5, 127.9, 128.3, 129.2, 129.9, 134.0, 135.3, 154.0. H/29Si
HMQC NMR (500/99 MHz, CD2Cl2, optimized for J = 7 Hz): δ/
ppm 0.41/13.2. HRMS (APCI): calculated for C13H17N2Si+ [(M +
H)+], 229.1156; found, 229.1157. IR (ATR): ν/cm−1 3050, 2955,
̃
IR (ATR): ν/
̃
cm−1 3336, 3070, 2975, 2900, 2231, 1706, 1692, 1513,
2896, 1492, 1435, 1351, 1244, 1148, 1084, 909.
1246, 1156, 841. The crystallographic data are available online in the
CCDC database under number CCDC 1956763.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00654.
■
(E)-1-(4-Methoxyphenyl)-2-(trimethylsilyl)diazene (2n). This
compound was prepared according to GP1 from the corresponding
silylated hydrazine (7.09 g, 33.7 mmol, 1.00 equiv) and DBAD (7.92
g, 33.7 mmol, 1.00 equiv). Purification by short-path distillation (bp:
78−82 °C at 4.1 × 10−1 mbar) afforded the title compound (2n)
(5.09 g, 24.4 mmol, 72%) as a deep purle liquid. 1H NMR (400 MHz,
C6D6): δ/ppm 0.36 (s, 9H), 3.19 (s, 3H), 6.76 (d, J = 8.9 Hz, 2H),
8.02 (d, J = 8.9 Hz, 2H). 13C{1H} NMR (126 MHz, C6D6): δ/ppm
−2.44 (3C), 55.0, 114.1 (2C), 123.3 (2C), 152.2, 162.6. 1H/29Si
HMQC NMR (500/99 MHz, C6D6, optimized for J = 7 Hz): δ/ppm
0.36/11.1. HRMS (APCI): calculated for C10H17N2OSi+ [(M + H)+],
S
Experimental details and data for the synthesis and
characterization of N-aryl-N′-silylhydrazines (1a−r),
NMR spectra of all compounds, X-ray diffraction data,
and computational methods (PDF)
Cartesian coordinates of all the diazenes presented in
Table 2 (XYZ)
209.1105; found, 209.1105. IR (ATR): ν/
̃
cm−1 2958, 2900, 2837,
1732, 1599, 1585, 1503, 1465, 1245, 1126, 1032, 870, 834.
Accession Codes
CCDC 1956763−1956764 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(E)-1-(2,6-Dimethylphenyl)-2-(triethylsilyl)diazene (2o). This
compound was prepared according to GP1 from the corresponding
silylated hydrazine (1.80 g, 7.20 mmol, 1.00 equiv) and DBAD (1.66
g, 7.20 mmol, 1.00 equiv). Purification by short-path distillation (bp:
73−75 °C at 8.4 × 10−2 mbar) afforded the title compound (2o)
(1.10 g, 4.43 mmol, 61%) as a deep blue liquid. 1H NMR (500 MHz,
CD2Cl2): δ/ppm 0.94 (q, J = 7.7 Hz, 6H), 1.07 (t, J = 7.8 Hz, 9H),
2.13 (s, 6H), 6.97−7.16 (m, 3H). 13C{1H} NMR (126 MHz,
CD2Cl2): δ/ppm 3.1 (3C), 7.0 (3C), 18.0 (2C), 127.7, 128.1 (2C),
AUTHOR INFORMATION
Corresponding Authors
*E-mail for C.C.: clement.chauvier@chem.tu-berlin.de.
*E-mail for M.O.: martin.oestreich@tu-berlin.de.
■
1
129.4 (2C), 158.9. H/29Si HMQC NMR (500/99 MHz, CD2Cl2,
optimized for J = 7 Hz): δ/ppm 1.07/12.1. HRMS (APCI): calculated
for C14H25N2Si+ [(M + H)+], 249.1782; found, 249.1782. IR (ATR):
cm−1 2953, 2912, 2875, 1499, 1460, 1235, 1079, 1006, 723.
(E)-1-(3-Nitrophenyl)-2-(trimethylsilyl)diazene (2p). This
ORCID
ν/
̃
́
Clement Chauvier: 0000-0002-3782-8130
compound was prepared according to GP1 from the corresponding
silylated hydrazine (5.07 g, 22.5 mmol, 1.00 equiv) and DBAD (5.02
g, 21.4 mmol, 0.950 equiv). Purification by short-path distillation (bp:
86−88 °C at 1.8 × 10−1 mbar) afforded the title compound (2p)
(3.47 g, 15.5 mmol, 69%) as a deep blue liquid. 1H NMR (400 MHz,
C6D6): δ/ppm 0.27 (s, 9H), 6.76 (t, J = 8.1 Hz, 1H), 7.78 (dd, J =
2.2, 8.1 Hz, 2H), 8.52 (t, J = 2.2 Hz, 1H). 13C{1H} NMR (126 MHz,
C6D6): δ/ppm −2.87 (3C), 114.8, 125.0, 127.2, 129.7, 149.4, 155.5.
1H/29Si HMQC NMR (500/99 MHz, C6D6, optimized for J = 7 Hz):
δ/ppm 0.27/14.5. HRMS (APCI): calculated for C9H14N3O2Si+ [(M
Lucie Finck: 0000-0002-2583-8390
Stefan Hecht: 0000-0002-6124-0222
Martin Oestreich: 0000-0002-1487-9218
Present Address
∥S.H.: DWI-Leibniz Institute for Interactive Materials,
Forckenbeckstr. 50, 52056 Aachen, Germany, and Institute
of Technical and Macromolecular Chemistry, RWTH Aachen
University, Worringer Weg 2, 52074 Aachen, Germany.
+ H)+], 224.0850; found, 224.0851. IR (ATR): ν/cm−1 3096, 2962,
̃
1734, 1610, 1530, 1349, 1248, 1151, 1074, 843.
Author Contributions
§C.C. and L.F. contributed equally.
(E)-1-(2-Fluorophenyl)-2-(trimethylsilyl)diazene (2q). This
compound was prepared according to GP1 from the corresponding
silylated hydrazine (2.00 g, 10.0 mmol, 1.00 equiv) and DBAD (2.20
g, 9.50 mmol, 0.950 equiv). Purification by short-path distillation (bp:
31−33 °C at 4.0 × 10−2 mbar) afforded the title compound (2q) (760
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
C.C. gratefully acknowledges the Alexander von Humboldt
Foundation for a postdoctoral fellowship (2018−2020). M.O.
is indebted to the Einstein Foundation Berlin for an endowed
professorship. We thank Dr. Elisabeth Irran (TU Berlin) for
collecting the X-ray diffraction data presented in the
Supporting Information. Computations presented in this
study were supported by the Cluster of Excellence Unifying
Systems in Catalysis of the Deutsche Forschungsgemeinschaft
(EXC 2008/1).
mg, 3.87 mmol, 39%) as a volatile deep blue liquid. H NMR (400
MHz, C6D6): δ/ppm 0.29 (s, 9H), 6.70 (mc, 1H), 6.84−6.97 (m,
2H), 7.26 (mc, 1H). 13C{1H} NMR (126 MHz, C6D6): δ/ppm −2.70
(3C), 113.1, 117.4 (d, J = 20.2 Hz), 124.0 (d, J = 4.1 Hz), 132.9 (d, J
= 8.1 Hz), 144.2 (d, J = 7.0 Hz), 162.4 (d, J = 259.0 Hz). 19F{1H}
NMR (471 MHz, C6D6): δ/ppm −128.2. 1H/29Si HMQC NMR
(500/99 MHz, C6D6, optimized for J = 7 Hz): δ/ppm 0.29/13.7.
HRMS (APCI): calculated for C9H14FN2Si+ [(M + H)+], 197.0905;
found, 197.0904. IR (ATR): ν
̃
/cm−1 3070, 2963, 2902, 1588, 1500,
1478, 1442, 1263, 1249, 1217, 1142, 1095, 844.
(E)-1-(Naphthalen-2-yl)-2-(trimethylsilyl)diazene (2r). This
compound was prepared according to GP1 from the corresponding
silylated hydrazine (4.49 g, 19.5 mmol, 1.00 equiv) and DBAD (4.49
g, 19.5 mmol, 1.00 equiv). The title compound (2r) (4.42 g, 19.4
mmol, 99%) was obtained as deep blue large needles after solvent
evaporation. Mp: 29−31 °C (n-pentane). 1H NMR (500 MHz,
CD2Cl2): δ/ppm 0.41 (s, 9H), 7.55−7.59 (m, 2H), 7.64−7.68 (m,
1H), 7.87−7.92 (m, 2H), 8.06−8.10 (m, 1H), 8.46−8.49 (m, 1H).
REFERENCES
■
(1) For selected reviews on azobenzenes, see: (a) Beharry, A. A.;
Woolley, G. A. Azobenzene photoswitches for biomolecules. Chem.
Soc. Rev. 2011, 40, 4422−4437. (b) Bandara, H. M. D.; Burdette, S. C.
Photoisomerization in different classes of azobenzene. Chem. Soc. Rev.
2012, 41, 1809−1825. (c) Kathan, M.; Hecht, S. Photoswitchable
molecules as key ingredients to drive systems away from the global
G
DOI: 10.1021/acs.organomet.9b00654
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
thermodynamic minimum. Chem. Soc. Rev. 2017, 46, 5536−5550.
(d) Dong, L.; Feng, Y.; Wang, L.; Feng, W. Azobenzene-based solar
thermal fuels: design, properties, and applications. Chem. Soc. Rev.
2018, 47, 7339−7368.
(20) Bock, H.; Wittel, K.; Veith, M.; Wiberg, N. Photoelectron
spectra and molecular properties. L. On the blue color of
bis(trimethylsilyl)diimine. J. Am. Chem. Soc. 1976, 98, 109−114.
(21) Note that we also studied the emission properties of compound
2a (in n-hexane solution), but no fluorescence emission could be
detected upon excitation of the π,π* band at 303 nm.
(22) Uno, B.; Matsuhisa, Y.; Kano, K.; Kubota, T. New description
of the substituent effect on electronic spectra by means of substituent
constants. II. n→π* and π→π* Transitions. Chem. Pharm. Bull. 1984,
32, 1691−1698.
(23) Engel, P. S. Mechanism of the thermal and photochemical
decomposition of azoalkanes. Chem. Rev. 1980, 80, 99−150.
(24) For a TD-DFT benchmark on azo compounds, see:
(2) Stanbury, D. M. Kinetic behavior of diazene in aqueous solution.
Inorg. Chem. 1991, 30, 1293−1296.
(3) Kosower, E. M. Monosubstituted diazenes (diimides). Surprising
intermediates. Acc. Chem. Res. 1971, 4, 193−198.
(4) For a recent synthetic application involving phenyldiazenes, see:
Fehler, S. K.; Pratsch, G.; Heinrich, M. R. The Trapping of
Phenyldiazenes in Cycloaddition Reactions. Angew. Chem., Int. Ed.
2014, 53, 11361−11365.
(5) (a) Wiberg, N.; Joo, W.-C.; Uhlenbrock, W. Bis(trimethylsilyl)-
diimine and Tetrakis(trimethylsilyl)hydrazine. Angew. Chem., Int. Ed.
Engl. 1968, 7, 640. For a review on BSD chemistry, see: (b) Wiberg,
N. Bis(trimethylsilyl)diimine: Preparation, Structure, and Reactivity.
Angew. Chem., Int. Ed. Engl. 1971, 10, 374−387.
̀
(a) Jacquemin, D.; Perpete, E. A.; Ciofini, I.; Adamo, C. On the
TD-DFT UV/vis spectra accuracy: the azoalkanes. Theor. Chem. Acc.
̀
2008, 120, 405−410. (b) Jacquemin, D.; Perpete, E. A.; Scuseria, G.
E.; Ciofini, I.; Adamo, C. Extensive TD-DFT investigation of the first
electronic transition in substituted azobenzenes. Chem. Phys. Lett.
2008, 465, 226−229.
(6) (a) Wannagat, U.; Kruger, C. Beitrage zur Chemie der Silicium−
̈
̈
Stickstoff−Verbindungen. XXV. Trialkylsilyl-aryl-diazene. I. Darstel-
lung und Isolierung. Z. Anorg. Allg. Chem. 1964, 326, 288−295.
(25) The computed excitation energies of 2m get closer to the
experimental values when the long-range corrected hybrid functional
CAM-B3LYP is used (|Δλexp‑TD| = 16 and 3 nm for the n→π* and
π→π* transitions, respectively).
̈
(b) Wiberg, N.; Veith, M. Uber Derivate des Hydrazins, 3 Darstellung
und Oxydation silylierter, germylierter sowie stannylierter Hydrazine
und Organylhydrazine. Chem. Ber. 1971, 104, 3191−3203.
(7) Watanabe, H.; Inoue, K.; Nagai, Y. A New Synthesis of
Organosilicon Azo Compounds. Bull. Chem. Soc. Jpn. 1970, 43, 2660.
(8) Bottaro, J. C. Novel synthesis of phenyltrimethylsilyldiazene and
generation of the phenyl anion synthon therefrom. J. Chem. Soc.,
Chem. Commun. 1978, 990.
(9) For synthetic applications of silyldiazenes, see: (a) Watanabe,
H.; Cho, Y.; Ide, Y.; Matsumoto, M.; Nagai, Y. Homolytic
decomposition of phenylazotriphenylsilane in carbon tetrachloride.
J. Organomet. Chem. 1974, 78, C4−C6. (b) Watanabe, H.; Awano, K.-
I.; Ohmori, M.; Kodama, N.; Sakamoto, J.-I.; Onodera, Y.; Nagai, Y.
Phenylazotriorganosilanes as silylated phenyldiazenes; a convenient
precursor for phenyldiazene. J. Organomet. Chem. 1980, 186, 7−11.
(c) Reiß, F.; Schulz, A.; Villinger, A. Synthesis, structure, and
reactivity of diazene adducts: isolation of iso-diazene stabilized as a
borane adduct. Chem. - Eur. J. 2014, 20, 11800−11811.
(10) West, R.; Ishikawa, M.; Bailey, R. E. New anionic rearrange-
ments. III. Catalytic rearrangement of organosilylhydrazines. J. Am.
Chem. Soc. 1967, 89, 4068−4072.
(11) Dilworth, J. R.; Liefde Meijer, H. J. de; Teuben, J. H.
Cyclopentadienyl titanium complexes with aryldiazenido- or phos-
phiniminato-ligands. J. Organomet. Chem. 1978, 159, 47−52.
(12) Watanabe, H.; Matsumoto, M.; Cho, Y.; Nagai, Y. The
synthesis of arylazotriphenylsilanes. Org. Prep. Proced. Int. 1974, 6,
25−29.
(13) Kosower, E. M.; Huang, P.-K. C.; Tsuji, T. Diazenes. V.
Aryldiazenes. J. Am. Chem. Soc. 1969, 91, 2325−2329.
(14) Greene, F. D.; Bergmark, W. R.; Pacifici, J. G. Diaziridinones.
III. Reactions with hydrazines. Isomerization of diaziridinones to
aziridinecarboxamides by hydrazine catalysis. J. Org. Chem. 1969, 34,
2263−2269.
(15) Seidl, H.; Bock, H.; Wiberg, N.; Veith, M. The Color of
Trimethylsilyl Derivatives of Diimine. Angew. Chem., Int. Ed. Engl.
1970, 9, 69−70.
(26) A similar argument is invoked in: Mao, Y.; Head-Gordon, M.;
Shao, Y. Unraveling substituent effects on frontier orbitals of
conjugated molecules using an absolutely localized molecular orbital
based analysis. Chem. Sci. 2018, 9, 8598−8607.
(27) Bock, H.; Alt, H.; Seidl, H. d-Orbital effects in silicon-
substituted π-electron systems. XV. Color of silyl ketones. J. Am.
Chem. Soc. 1969, 91, 355−361.
(28) Chan, L.-H.; Rochow, E. G. Syntheses and ultraviolet spectra of
N-organosilyl ketimines. J. Organomet. Chem. 1967, 9, 231−250.
(29) (a) Klotz, E. J. F.; Claridge, T. D. W.; Anderson, H. L. Homo-
and hetero-3rotaxanes with two pi-systems clasped in a single
macrocycle. J. Am. Chem. Soc. 2006, 128, 15374−15375. (b) Atchison,
J.; Kamila, S.; Nesbitt, H.; Logan, K. A.; Nicholas, D. M.; Fowley, C.;
Davis, J.; Callan, B.; McHale, A. P.; Callan, J. F. Iodinated cyanine
dyes: a new class of sensitisers for use in NIR activated photodynamic
therapy (PDT). Chem. Commun. 2017, 53, 2009−2012.
(30) Du, H.; Zhao, B.; Shi, Y. Pd(0)-catalyzed diamination of trans-
1-phenyl-1,3-butadiene with di-tert-butyldiaziridinone as nitrogen
source. Org. Synth. 2009, 86, 315.
(31) Cookson, R. C.; Gupte, S. S.; Stevens, I. D. R.; Watts, C. T. 4-
Phenyl-1,2,4-triazoline-3,5-dione. Org. Synth. 2003, 51, 121.
(32) Harris, R. K.; Becker, E. D.; Cabral de Menezes, S. M.;
Goodfellow, R.; Granger, P. NMR nomenclature. Nuclear spin
properties and conventions for chemical shifts (IUPAC Recom-
mendations 2001). Pure Appl. Chem. 2001, 73, 1795−1818.
(16) Zhu, Y.; Shi, Y. Facile Cu(I)-Catalyzed Oxidative Coupling of
Anilines to Azo Compounds and Hydrazines with Diaziridinone
under Mild Conditions. Org. Lett. 2013, 15, 1942−1945.
(17) Yoneda, F.; Suzuki, K.; Nitta, Y. A New Hydrogen-Abstracting
Reaction with Diethyl Azodicarboxylate. J. Am. Chem. Soc. 1966, 88,
2328−2329.
(18) Cookson, R. C.; Stevens, I. D. R.; Watts, C. T. A new reagent
for oxidation of alcohols to ketones in neutral solution at room
temperature. Chem. Commun. 1966, 744.
(19) The air stability of the silyldiazenes depends on both the silyl
group and the substituent present on the aromatic ring. For example,
the 4-fluorodiazene 2j presented in Scheme 4 displays a reduced half-
life under air of ca. 7 h in comparison to 2a.
H
DOI: 10.1021/acs.organomet.9b00654
Organometallics XXXX, XXX, XXX−XXX