Synthesis, structure, thermal and cis/trans isomerization of 2,2′-(EMe3)2 (E = C, Si, Ge, Sn) Substituted Azobenzenes

Article abstract of DOI:10.3390/molecules24020303

The synthesis of a series of 2,2′-bis(trimethyl)tetrel azobenzenes is reported, evaluating the different synthetic approaches that different group 14 element substituents individually require. The synthetic access to the carbon substituted congener is ver

Full text of DOI:10.3390/molecules24020303

molecules
Article
Synthesis, Structure, Thermal Behavior and cis/trans
Isomerization of 2,20-(EMe3)2 (E = C, Si, Ge, Sn)
Substituted Azobenzenes
Jonas Hoffmann 1,2,3,4, Thomas Josef Kuczmera 1, Enno Lork 5 and Anne Staubitz 1,2,3,
*
1
Institute for Analytical and Organic Chemistry, University of Bremen, Leobener Straße 7,
D-28359 Bremen, Germany; jonas.hoffmann@uni-bremen.de (J.H.); kuczmera@uni-bremen.de (T.J.K.)
MAPEX Center for Materials and Processes, University of Bremen, Bibliothekstraße 1,
D-28359 Bremen, Germany
Otto-Diels-Institute for Organic Chemistry, University of Kiel, Otto-Hahn-Platz 4, D-24098 Kiel, Germany
Université Rennes, CNRS, ISCR-UMR 6226, 263 Av. du Général Leclerc, F-35042 Rennes, France
Institute for Inorganic Chemistry and Crystallography, University of Bremen, Leobener Straße 7,
28359 D-Bremen, Germany; enno.lork@uni-bremen.de
2
3
4
5
*
Correspondence: staubitz@uni-bremen.de; Tel.: +49-421-218-63210
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: J. Derek Woollins
Received: 30 November 2018; Accepted: 11 January 2019; Published: 15 January 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: The synthesis of a series of 2,20-bis(trimethyltetrel) azobenzenes is reported, evaluating
the different synthetic approaches that different group 14 element substituents individually require.
The synthetic access to the carbon substituted congener is very different from the heavier tetrels,
in that the key step is the formation of the N=N bond in azobenzene, rather than the azobenzene-C
bond. Sn could be introduced with a cross-coupling route, whereas the Si and Ge congeners were
prepared by a stannylation-lithiation-electrophilic quenching sequence. Iodo-lithium exchange was
also a possible route to obtain the dilithiated species, which can be attributed to the chelating effect
of the nitrogen atoms. However, the organo-lead species could not be obtained via these routes.
The resulting structures were fully characterized (NMR, FTIR, HRMS and XRD). Furthermore, their
thermal properties (TGA and DSC) and their photoswitching behavior in solution (UV-VIS & NMR
experiments) were investigated and compared for the different tetrels (C, Si, Ge, Sn).
Keywords: azobenzene; tetrels; group 14 elements; transmetalation; molecular switches;
cross-coupling; photo responsiveness
1. Introduction
In main group chemistry, trends within a group play an important role for the understanding
of how particular properties of a molecule can be tuned. By selecting a congener, in which an atom
is replaced by another element from the same group, the geometric shape of the molecules can
be changed. For instance, in organic compounds, carbon atoms can be replaced by heavier tetrels
(Si, Ge, Sn and Pb) to give semi-inorganic hydrocarbons [1]. However, size is not the only change;
optophysical properties can also be influenced substantially. For example, it has been shown that
for cyclopentadiene analogs, in which the sp3 substituted C atom is replaced by a heavier group 14
element, the HOMO-LUMO gap (HOMO = highest occupied molecular orbital; LUMO = lowest
unoccupied molecular orbital) is reduced due to σ*-π* conjugation [2–6]. Due to our interest in both
main group chemistry and photoswitchable molecules, we systematically investigated the influence
that could be exerted by tetrels in the ortho-position of azobenzenes, as well as the structural influences
this substitution might trigger: Azobenzenes are thermally and photochemically stable molecular
switches that can undergo a reversible trans–cis isomerization under illumination. The properties of
Molecules 2019, 24, 303; doi:10.3390/molecules24020303
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azobenzenes, such as absorption maxima and switching behavior, can be tuned by their substitution
pattern [ 11]. Ortho-substituted azobenzenes distinguish themselves from most molecular switches by
their significantly longer thermal relaxation times of the cis-isomer back to their trans-isomers [12 13].
7
–
,
This property makes ortho-substituted azobenzenes suitable for several applications where long half-life
times of the thermal relaxation are required, giving rise to new materials and dyes, e.g., external stimuli
responsive polymers [14,15].
In general, ortho-azobenzenes can be accessed either by using C–H activation methods with late
transition metals like nickel, palladium, platinum or ruthenium [16
bases [18 19], or by direct halogen-lithium exchange [19 20]. The latter has been shown to proceed with
high efficiency in the synthesis of several ortho-substituted organometallic azobenzenes (Scheme 1) [21].
,17], by deprotonation with metal
,
,
Scheme 1. Functionalization of monosubstituted azobenzenes with organotetrels [21].
This methodology is superior to the classical azocoupling approach and could be used with a high
variety of electrophiles. It should also be mentioned that in this study, the ortho-bis(silyl) azobenzene
(compound
was used for mono-desilylation to give the monosilylated azobenzene displayed above [21]. Other
metalation methods were used to introduce pentacoordinated silyl atoms in azobenzene motifs [20 22].
6 in our study) was synthesized by oxidation of the respective amine. This product
,
It was also found that substitution with heavier group 14 elements, which convey a positive inductive
effect on the aromatic ring, resulted in a slight bathochromic shift of the absorption maxima compared
to unsubstituted azobenzene.
Since we developed a method for the lithiation of azobenzenes by halogen-lithium exchange
reactions for para- and meta-substituted azobenzenes [23,24], we were interested to transfer this
methodology towards disubstituted ortho-azobenzenes. This process involved the synthesis of
tin-substituted azobenzenes through a Kelly-Stille cross-coupling reaction, followed by a low temperature
tin-lithium exchange. Functionalization could be achieved by quenching the lithiated species with
a variety of electrophiles at low temperatures. Initial results showed that the structure-reactivity
relationship of the meta-, para- and ortho-azobenzenes in the Kelly-Stille cross-coupling reaction led to
significant differences in yields (para: 93%, meta: 88% and ortho: 43%). An advantage of a functionalization
methodology that involves stannylated groups in the reagents is the good shelf-life and the potential use
of these reagents as nucleophilic components in Stille type cross-coupling reactions. This was already
reported for para-azobenzenes and gives access to substitutions with aromatic or vinylic groups [25].
In this work, we were interested in the detail of the structure/property relationship of different
tetrel-substituted azobenzenes. In contrast to previous studies, the carbon congener is included and
both ortho-positions of the azobenzene are substituted.
2. Results
The investigation started with the synthesis of ortho-substituted azobenzenes, in which the
substituent was always EMe3 with E = C, Si, Ge and Sn (E = Pb proved inaccessible). Therefore, we
prepared a 2,20-substituted difunctional azobenzene through the oxidation of 2-iodoaniline (
1) with
manganese dioxide to give 2,20-diiodoazobenzene (2) in a yield of 82% (Scheme 2) [26].

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Scheme 2. Oxidative azocoupling to give the difunctional azobenzene
electrophilic component in cross-coupling reactions.
2, which can be used as the
It needs to be pointed out that azobenzenes with di-ortho-substituents are more difficult to prepare
than those with the same substituents in meta- and para-position. Oxidizing reagents, like copper(I)
bromide/air [25], copper(II)/silver(I)/tetra-n-butylammonium bromide [27] or diacetoxyiodobenzene [28],
were explored in the synthesis, but in all cases no conversion was observed.
Following the iodo-lithium exchange, which was already reported [20], 2,20-diiodo-azobenzene
(
2
) was treated with n-butyllithium at a low temperature (
−
105 or
−
78◦C) and quenched with a
trimethylsilyl chloride electrophile (Scheme 3). The reactions proceeded very cleanly, which in our
experience is different to meta- and para-iodinated azobenzenes, where the electrophilic N=N group
can be attacked by the nucleophilic n-butyllithium. This suggests a significant stabilization of the
ortho-lithiated species by an N→Li coordination.
Scheme 3. Direct metalation of azobenzene
2 to give the trimethylsilyl-substituted azobenzene 6.
(Experimental details can be found in the SI).
Although this is a very effective route to synthesize these ortho-substituted azobenzenes, we were
interested in obtaining the tetrel-substituted compounds under cross-coupling conditions. Therefore,
the next step to generate the EMe3 ortho-substituted azobenzenes was a palladium catalyzed
cross-coupling procedure with Me3E-EMe3 reagents. Since this reaction has been already reported for
meta- and para-substituted azobenzenes for E = Sn, we adapted to the reaction conditions also for other
tetrels, for which this type of reagent was available (Si, Ge and Sn) (Scheme 4, Table 1).
Scheme 4. Different hexamethyl dimetals were used in a Stille-Kelly type cross-coupling protocol.

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Table 1. Overview of used substrates and reagents for the synthesis of dimetalated azobenzenes.
Entry
M
Additive
Time/h
Yield
1
2
3
4
5
6
Si
Si
Ge
Ge
Sn
Pb
-
1/8
1
1/8
1
1
0%, no conversion a
0%, no conversion a
0%, no conversion b
0%, no conversion b
60% isolated product c
not performed d
2.00 equiv. Cs2CO3
-
2.00 equiv. Cs2CO3
-
-
1
a For a similar procedure, see References [29
used for azobenzenes, see References [24,25]. This toxic substance decomposes at higher temperatures [32].
,
30]. b For a similar procedure, see Reference [31]. c For a similar procedure
d
We initially assumed that the metal-metal bond dissociation energy (BDE) might have a substantial
impact on the reactivity towards the oxidative addition of the catalyst. The bond dissociation energies
for hexamethyl dielement species have been reported: BDE (Me3C-CMe3) = 76.0 kcal/mol [33], BDE
(Me3Si-SiMe3) = 69.1 kcal/mol [34] and 79.7 kcal/mol [35], BDE (Me3Ge-GeMe3) = 69.1 kcal/mol [34],
BDE (Me3Sn-SnMe3) = 61.6 kcal/mol and BDE (Me3Pb-PbMe3) = 54.6 kcal/mol [34]. Since the BDE of
the tin dimer is relatively lower than the other hexamethyl dielement compounds, we assumed that a
more labile tin-tin bond would react more easily, but it was surprising that neither the Me3Si-SiMe3 (
reagent, nor the Me3Ge-GeMe3 ( ) reagent provided any conversion. Steric reasons may be a potential
issue: The C–Sn bond is longer and thus may lead to less congestion in the ortho-positions. Thus,
the Stille-Kelly type cross-coupling reaction with the hexamethylditin ( ) was optimized towards the
3)
4
5
reaction temperature, reaction time and catalyst load (Scheme 5, Table 2): We envisaged that with this
reagent in hand, alternative routes to the other ortho-substituted azobenzenes may be also accessible
via tin-lithium exchange and electrophilic quenching processes (see below).
Scheme 5. Stille-Kelly type coupling of azobenzene 8.
Table 2. Overview of the Stille-Kelly reaction optimization for the di-stannylated azobenzene 8.
Entry
Cat. Load
Temp.
Time/h
Yield
1
2
3
4
5
6
7
8
9
4 mol%
4 mol%
6 mol%
4 mol%
2 mol%
3 mol%
3 mol% a
11 mol%
11 mol% b
130 ◦C, conv. Heating
130 ◦C, conv. Heating
170 ◦C, µW
8
16
1
1
1
1
1
1
1
17%
19%
31%
27%
49%
60%
7%
170 ◦C, µW
170 ◦C, µW
170 ◦C, µW
170 ◦C, µW
170 ◦C, µW
14%
96%
170 ◦C, µW
a
b
Copper(I) chloride (6.0 equiv.) and lithium chloride (12.0 equiv.) were added. 6.70 equiv. Sn2Me6 was used.
It was found that increasing the reaction temperature from 130 ◦C to 170 ◦C, along with changing
from conventional to microwave assisted heating (entries 2 and 4), led to an increase in yield and a
decrease in reaction time. Furthermore, the catalyst to hexamethyldistannane (5) ratio had a crucial
impact on the success of the reaction. Increasing only the catalyst load led to a decrease in the yield
(60% to 14%, entries 6 and 8). Additives like copper (I) halides [36] and lithium chloride [37] also
lowered the yield of the product (entry 7). Interestingly, a high catalyst load in combination with
a high amount of hexymethyldistannane (5) resulted in nearly quantitative yield. Combined with

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the fact that the reaction vessels always contained an insoluble black precipitate after the reaction,
we assumed that the autocatalytic agglomeration of the palladium catalyst that can occur more
easily with higher catalyst loadings may be the cause of this observation [38]. With an excess of
hexamethyldistannane (
6, 8 and 9), presumably because the desired reaction was accelerated compared to the agglomeration
process. Since hexamethyldistannane ( ) is an expensive and toxic compound, a high excess and high
5), agglomeration was not observed (see the high difference in yield for entries
5
amount of organotin waste is not desirable. Therefore, we performed further reactions with only
2.27 equivalents of this organotin compound, accepting the somewhat lower yield. We also tested
whether the stannylated product
8 might react with the iodinated starting material 2 in a standard
Stille reaction, but this was not the case, which excludes this reaction as a cause for the lower yields
from entries 7 to 8 (see SI).
To access the other ortho-substituted azobenzenes for EMe3 (E = C, Si, Ge), we followed a
transmetalation procedure which was already reported for di-stannylated meta- and para-azobenzenes [23].
Initially, the metalation temperature and time were optimized, with trimethylsilyl chloride (
9) being used
as an electrophile (Scheme 6, Table 3).
Scheme 6. Trimethylsilyl chloride
9was used as a test electrophile to obtain optimized reaction parameters.
Table 3. Optimization of the tin-lithium exchange and the reaction with 9.
Entry
T
Lithiation Time
Yield a
1
2
3
4
5
6
7
8
9
−78 ◦C
−60 ◦C
−40 ◦C
−20 ◦C
0 ◦C
60 min
60 min
60 min
60 min
60 min
1 min
3 min
9 min
30 min
90 min
87%
55%
49%
20%
5%
69%
64%
71%
74%
79%
−78 ◦C
−78 ◦C
−78 ◦C
−78 ◦C
−78 ◦C
10
Yields were determined by using 1,3,5-triiso-propylbenzene as an internal reference for 1H NMR analysis.
a
It was found that increasing the temperature for the lithiation reaction led to a decrease in product
1
and an emergence of unassignable signals in the H NMR spectra. However, the metalation time
played only a minor role in the formation of the product (Entries 6–10). The best conditions were found
to when the lithiation temperature was −78 ◦C and a metalation time of 90 min was used. To expand
the variety of electrophiles to less hard organometals, we made use of a transmetalation from the
hard, reactive lithium towards the softer and less reactive cuprate, which was initially reported by
Knochel [39]. This methodology has so far not been applied to azobenzene chemistry (Scheme 7).
Scheme 7. In addition to the tin-lithium exchange, we investigated the subsequent transmetalation to
organocuprates and further quenching with electrophiles.

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We tested this reaction sequence with several electrophiles and different metalation temperatures
(Table 4).
Table 4. To widen the scope of the transmetalation to softer electrophiles, an organocuprate was
introduced by using CuCN 2 LiCl as softer organometallic reagent. The transmetalation temperature
·
varied from −78 ◦C to −40 ◦C.
Entry
E
Metal
Metalation T.
Yield a
1
2
3
4
5
6
7
8
9
Me3SiCl
Me3SiCl
Me3SiCl
Me3SiCl
Me3SiI
Me3SiI
Me3SiI
MeI
[Li]
[Li]
[Cu]
[Cu]
[Li]
[Cu]
[Cu]
[Cu]
[Cu]
[Li]
−78 ◦C
−40 ◦C
−78 ◦C
−40 ◦C
−78 ◦C
−78 ◦C
−40 ◦C
−78 ◦C
−40 ◦C
−78◦C
87%
73%
90%
7%
50% b
17% b
30% b
5%
MeI
MeI
56%
0%
10
Yields were determined by using 1,3,5-triiso-propylbenzene as an internal reference for 1H NMR analysis. As a
a
b
side reaction, THF might have undergone a ring-opening ether cleavage to form 4-iodo-butyloxytrimethylsilane [40].
The yield decreased if the temperature for the lithiated species was changed from
(Entries 1 and 2), which may be an indication of a reductive attack by the organolithium reagent on the
azo group [18]. The transmetalation to copper at
78 ◦C provided largely the same yields (Entry 3),
although at
40 ◦C, using the cuprate gave a very low yield of 7% (Entry 4). For Me3SiI, a decrease in
−
78 ◦C to
−
40 ◦C
−
−
yield after the transmetalation to copper was observed (Entries 5–7), which was unexpected as this
electrophile is softer. With methyl iodide as an electrophile, we could observe an increase in the yield
by increasing the transmetalation temperature from
using the cuprate was essential for the reaction, because the lithiated species did not react with the
electrophiles (Entry 10).
−
78◦C to 40 ◦C (Entries 8 and 9). In that case,
−
With both methods in hand, we incorporated new trimethyltetrel halides. In dependence of the
nature of the respective electrophile, we were interested in the reactivity towards the hard organolithium
or the soft organocuprate (Scheme 8, Table 5).
Scheme 8. Metalation to obtain di-metalated azobenzenes 10, 11 and 6, 7.
Table 5. Transmetalation of di-stannylated azobenzene
8 with methyl lithium or further metalation to
the cuprate and the reactions of these organometallic reagents with electrophilic trimethyltetrel species.
Entry
E
Metal
Yield
1
2
3
4
Me3CCl
Me3SiCl
Me3GeCl
Me3PbBr
[Li] or [Cu]
[Li] or [Cu]
[Li] or [Cu]
[Li] or [Cu]
both 0%
87%/90%
60%/0% a
both 0%
a
The reaction resulted in a complex mixture of products in which 7 could not be isolated.

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Besides the above mentioned trimethylsilyl chloride (
chloride (12) to give the digermanylated azobenzene (
9
), the reaction of trimethylgermanyl
7
) could be performed in a good yield of 60%
(Entry 3). With tert-butyl chloride (13) and trimethyllead bromide (14), neither reaction was successful,
irrespective of whether the lithium or copper species (Entries 1 and 4) were used, but for different
reasons: The nucleophilic substitution of tert-butyl chloride (13) would be highly disfavored for an SN2
reaction due to its steric hindrance, and also because a SN1 reaction is unlikely under these nonpolar
reaction conditions. The reaction with trimethyllead bromide (14) was unsuccessful with both the
lithium or copper nucleophile. It is possible that the trimethyllead bromide did react, but that the
lead(IV)-substituted azobenzene was then too reactive to be isolated; recent examples of this are given
in Reference [41]. This hypothesis is supported by the fact that after the reactions, a shiny insoluble
black precipitate could be observed, which are probably decomposition products from lead.
Since we were interested in comparing the influence of different trimethyltetrel substituents in
ortho-position of the azobenzenes, we obtained the carbon-derivative via oxidation of the respective
aniline (Scheme 9).
Scheme 9. Oxidative azocoupling of aniline 15 with manganese dioxide as an oxidant. The product 10
and by-product 16 could be separated by column chromatography.
The product 10 was isolated in a moderate yield of 46%, but surprisingly, we could also isolate
a side product, which was characterized by NMR, HRMS, FTIR and XRD and was found to be a
phenazine derivative (Scheme 9 and Figure 1).
Figure 1. Molecular structure of the isolated phenazine 16, showing 50% probability ellipsoids.
It is not clear at present how this side product formed, but the literature suggests that elevated
temperature and oxidative conditions favor the formation of the very thermodynamically stable
phenazines [27].
With the series of ortho-tetrel substituted azobenzenes in hand, we were interested in the
comparison of their structures and the effect of the group 14 elements on the physical properties of
the azobenzenes.

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Figure 2. Molecular structures of molecules 10a
,
6,
7
and
8, showing a 50% probability of ellipsoids. 10
crystalizes with two molecules in the asymmetric unit, only one is shown (10a). The Sn-atom of
8
is
disordered over two positions with occupancies of 90% and 10%, respectively. For reasons of clarity
only the major component is shown.
The crystal structures of all molecules were determined (Figure 2). Since the interaction of
tetrels in the ortho-position of azobenzenes has been reported for electron deficient fluorinated silyl
groups [42,43], we were interested in how the structural parameters of our crystals would compare with
the mono-substituted compounds described by Kano. A special interest was the N=N and carbon-tetrel
bond, as well as the possible interaction of the group 14 element with the azo group, which is here
indicated by the distance and the torsion from the phenyl ring and the azo group (Table 6).
Table 6. Overview of the important crystallographic parameters of compounds 10, 6, 7 and 8.
10a 1
10b 1
6
7
8
Crystal system
Space group
Triclinic
P-1
Monoclinic
P21/c
1.2556(17)
1.5348(14)
2.9402(16)
−163.3(1)
1.2509(18)
1.5374(14)
2.909118
1.258(2)
1.8856(14)
3.0151(11)
175.0(1)
1.256(3)
2.0234(18)
3.0441(14)
177.1(2)
1880129
1.2511(19)
2.144(1) 2
3.047(1) 2
−177.3(1)
1880128
N-N bond length (Å)
C1-tetrel bond length (Å)
N1-tetrel distance (Å)
Torsion angle C1-C2-N1-N10 (◦)
CCSD No.
−164.8(1)
1880127
1880130
1
2
The asymmetric unit consists of two crystallographic independent molecules. Distances to main component
are given.
The ortho-substitution had only a marginal effect on the length of the azo group (from 1.2509(18) Å
to 1.258(2) Å). However, as expected, the distance of the carbon-tetrel bond increased from the lighter to
the heavier elements (1.5348(14) Å (For E = C) to 2.144(1) Å (for E = Sn)). The nitrogen-tetrel distance did
not indicate any interaction of the azo group with the respective tetrels (2.9091(18) Å for Si to 3.0471 Å
for Sn). This can be compared with azobenzenes with SiMe2F (2.585(3) Å) and SiF3 (2.371(4) Å) [41]:
These much-shortened distances indicated that the lone pair of the nitrogen atom was able to coordinate
to the Lewis acidic silicon center, which was not observed in our case. Furthermore, the 29Si NMR
shift of this compound was to be found at 29Si{1H} NMR:
δ
= −4.04 ppm, which is a typical shift for

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tetracoordinated silicon species. This is well aligned with Kano’s work [41], who reported a 29Si{1H}
NMR-shift of 3.8 ppm for a trimethylsilyl substituted azobenzene, as compared to 16.8 ppm if one
of the Me groups was replaced by F, or even 57.8 ppm for a SiF3 group in this position. Furthermore,
−
−
−
the crystal structure differs in the fact that in the case of a nitrogen-tetrel interaction, a four-membered
ring would result, whereas the structures from Kano showed interaction with the second nitrogen atom
of the azo group, resulting in a five membered ring; for this to happen, the aromatic rings would need
to rotate by approximately 180◦, which was not observed. Upon substitution with higher congeners
than carbon or silicon, the germanium and tin substituted azobenzene exhibited a carbon-tetrel bond
length of 2.0234(18) Å and 2.144(1) Å, which can be considered as an ordinary organotetrel bond
length. However, the torsion angles for the CMe3 substituted azobenzene were surprisingly far from
planar (by ca. 15–17◦), which was in contrast to Kano’s work on Si and also our results on the SiMe3
substituted compounds, which show almost no distortion. We assumed that the sterical hinderance of
the tert-butyl group, in comparison with the short C(Ph)–C(CMe3) bond, forced the system to undergo
distortion. Due to more flexible and longer C–Si, C–Ge and C–Sn bonds, we observed less planar
distortion (3–6◦).
Since azobenzenes can undergo trans/cis isomerization, we studied the effect of the different
group 14 elements on the general absorption properties, the cis/trans equilibrium in the dark and the
photostationary state (PSS) at 365 nm and 450 nm through UV and NMR spectroscopy (Scheme 10).
Scheme 10. Light/thermally induced isomerization of synthesized azobenzenes 10 and 6–8.
In general, the UV spectra of the azobenzenes exhibit a strong ππ* absorption around 330 nm and
a weak absorption for the nπ* band of the cis-isomer around 475 nm (Figure 3a,b). Upon irradiation of
the azobenzenes with UV light (365 nm), we observed a significant decrease in the absorbance of these
molecules (Figure 3a), which can be assumed to switch to the respective cis-isomer. Blue light (450 nm)
shifts the photostationary state (PSS) to the trans-isomers (Figure 3b).
Figure 3.
(a
) Absorption spectra of azobenzenes and 10 before and after irradiation with 365 nm
6
,
7
,
8
light for 3 min. (b) Absorption spectra of azobenzenes 6, 7, 8 and 10 after irradiation with 365 nm light
for 3 min, followed by irradiation with 450 nm light for 3 min. In each case, the black vertical lines
indicate the irradiation wavelengths.

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In all cases, the switching was totally reversible but the tin-substituted azobenzene
8
showed a
decrease and broadening of the ππ* band (Figure 3b), which was not expected, and may indicate a
photochemical decomposition process.
To obtain the ratio of the cis/trans-isomer in the PSS, we combined irradiation experiments with
1H NMR analysis. The result of this experiment for molecule 6 is shown below (Figure 4).
Figure 4. 1H NMR (300 MHz) spectra of compound
6 before (red) and after (blue) irradiation with
365 nm light (15 min) in CDCl3.
To compare the general trends upon substitution with different tetrels on the azobenzenes,
we compared the collected isomerization information (Table 7).
Table 7. Collected data of the absorption spectra and switching experiments.
ππ* Band ε
PSS (as Synthesized)
PSS (365 nm)
cis:trans
Compound
τ1/2 NMR (h)
τ1/2 UV (h)
(L mol−1 cm−1
)
NMR cis:trans
10 (C)
6 (Si)
7 (Ge)
8 (Sn)
15272 (325 nm)
12907 (331 nm)
17818 (333 nm)
15632 (338 nm)
a
1:99
1:99
1:99
1:99
80:20
83:17
91:9
55.61
62.17
17.07
20.45
65.84
16.75
73:27 a
11.46 a
17.38 a
Compound 8 presumably underwent decomposition upon irradiation.
Upon substitution of these compounds with heavier group 14 elements, we observed a slight
bathochromic shift of the ππ* band (325 nm to 338 nm). This could be explained by the effect of the
decreasing torsion angle due to the bulky trimethyltetrel groups, allowing extended π-conjugation
(Table 8). However, the effect of the heavier tetrels on the extinction coefficient of the azobenzene
system is rather small.
All systems contained only a small amount of the cis-isomer ‘as synthesized’. Upon irradiation
with 365 nm light, the maximum amount of the cis-isomer was found to range from 80% to 91% from
E = C to E = Ge, respectively. Therefore, the PSS is substantially influenced by the tetrel. For E = Sn,
however, the ratio appeared much lower, but this could also be attributed to photodecomposition.
Irradiation with 450 nm light did not result in a complete switching to the trans-isomer, which can be
observed by the existence of the nπ*-band in the UV-VIS spectra. Unfortunately, irradiation with 365 nm
and 450 nm light caused the tin-substituted azobenzene 8 to experience some decomposition, which
was detected by UV and 1H NMR spectroscopy. This may be caused by a small C–Sn bond-dissociation
energy. We further observed the thermal relaxation of the cis-azobenzenes that had been obtained after
irradiation with 365 nm light with UV-VIS and NMR spectroscopy. The half-life times (
experiments were determined to be between 55.61 h to 65.84 h, in which an increasing half-life time
with heavier tetrels trend could be observed. The UV experiments for compounds and 10 revealed
τ) in the NMR
6, 7

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half-life times of 17.07 h to 20.45 h. In this case, no correlation between the tetrel substitution and
the half-life time could be found (for all data see the SI). There is a substantial discrepancy between
the half-life times measured by the two techniques. It needs to be pointed out in this context that the
concentrations in the NMR were higher by approximately 2 orders of magnitude. Although half-life
times should not be concentration dependent in general, they can be different if agglomerations and
the stacking of molecules occur, which is likely to be the case for the compounds under investigation.
These systems were also analyzed by dynamic scanning calorimetry (DSC). To ensure thermal
stability in the temperature range of investigation of these compounds, thermogravimetric analysis
◦
◦
(TGA) was conducted (see SI). The compounds showed mass loss at 208 C (C, 10), 188 C (Si,
6),
196 ◦C (Ge, ) and 248 ◦C (Sn,
7
8
). However, when a TGA experiment on molecule 10 was interrupted
at mass loss and the residue was analyzed by NMR spectroscopy, no decomposition was observed.
Therefore, we assumed that the azobenzene evaporated at the higher temperature, accounting for the
observed loss of mass.
In general, these compounds showed endothermic melting signals at onset temperatures of 87 ◦C
(E = C), 69 ◦C (E = Si
6
), 83 ◦C (E = Ge) and 102 ◦C (E = Sn
8
), and exothermic crystallization peaks
of 16 ◦C (E = C 10), 50 (E = Si), (48 ◦C (E = Ge) and 56 ◦C (E = Sn
) (Figure 5, Table 8)). In general,
the melting temperature increased within the main group of compounds , but compound 10 proved
8
6–8
to be an exception. Although the temperature ramps were slow (0.5 K/min), the melting peaks were
not completely sharp, additionally, the crystallization peaks showed multiple crystallization events
due to spontaneous seeding (for all data see the SI).
Figure 5. Stacked dynamic scanning calorimetry (DSC) spectra of all target compounds. The DSC
experiments were performed with crystals with a heating rate of 0.5 K/min and a flow of nitrogen gas
of 20 mL/min. As an example, compound 10 was analyzed after three cycles by 1H- and 13C{1H} NMR
spectroscopy and showed no degree of decomposition.
Table 8. Results of the thermoanalytic experiments.
10
6
7
8
Tfusion (Onset) (◦C)
Hfusion (KJ/mol)
86.76
23.93
16.26
−17.78
68.59
13.37
50.34
−16.16
82.85
15.13
48.02
−12.39
102.37
15.05
56.33
−18.03
Tsolidif. (Onset) (◦C)
Hsolidif. (KJ/mol)
We observed that with faster DSC measuring modes (10 K/min) there was a high degree of
amorphousness in the structures (see SI). With slower DSC methods (0.5 K/min), the difference between
the fusion and solidification measured with the slower DSC method enthalpies are still existent and
may be related to a non-crystallizing part of the molecules. The fusion enthalpy of compound 10

Molecules 2019, 24, 303
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was 23.93 KJ/mol, which is rather high in comparison of the other azobenzenes (
6: 13.37 KJ/mol, 7:
15.13 KJ/mol, 8: 15.05 KJ/mol).
3. Discussion
We have described the synthesis, characterization (XRD, NMR, HRMS FTIR), switching and
thermal behavior for four azobenzenes in detail. The synthesis for azobenzene was conducted via
Stille-Kelly cross-coupling with optimized reaction conditions and a high yield. Attempts to synthesize
azobenzenes and using the respective Me6M2 (M = Si, Ge) compound and the same method did not
succeed. However, the synthesis for azobenzenes and was performed through tin-lithium exchange
8
6
7
6
7
with further reaction with the respective trimethyltetrel halides. For ortho-halogenated azobenzenes,
ortho-lithiated species can also be obtained by a simple halogen-metal exchange. The lithium atom
ortho-position is particularly stabilized by coordination with the N atoms of the azogroup, which
facilitates the reaction. Also, the first tin-lithium-copper transmetalation on ortho-azobenzenes has been
tested to enhance the reactivity of the nucleophile towards softer electrophiles. Since the reaction with
tert-butyl chloride (13) (whether via the tin-lithium or tin-lithium-copper transmetalation procedure)
was not successful, we decided to obtain azobenzene 10 via the oxidation of the corresponding aniline.
Furthermore, as a byproduct, a phenazine 16 could be isolated and completely characterized. Due to the
nucleophilic character of the three ortho-metalated azobenzenes (Si, Ge, Sn), these molecules could be
prospective molecules for cross-coupling reactions, see for example silicon [43
or tin [48,49].
,44], germanium [45–47]
4. Materials and Methods
4.1. General Information
For inert reactions, a nitrogen filled glovebox from Pure LabHE and standard Schlenk techniques
2
were used. Except for the preparation of the 2,20-diiodoazobenzene ( ) and 2,20-di(tert-butyl)azobenzene
(10), all reactions were carried out under inert conditions.
All glassware for the inert reactions was dried in an oven at 200 ◦C for several hours prior to use.
Additionally, before starting the reaction, the glassware was heated under vacuum (about 5 × 10−2 mbar
)
and flushed at least three times with argon. The NMR tubes were dried at 110 ◦C for several hours.
Microwave irradiated reactions were carried out using an EmrysTM Optimizer instrument
(Biotage, Uppsala, Sweden), in which the temperature was measured with an external IR detector.
MeLi was titrated prior to usage with menthol, using 2,20-bipyridine as an indicator [50].
For chromatographic purification, silica gel 60 (MERCK, Darmstadt, Germany, 0.015–0.40 mm)
was used. If stated, Celite® 503 (Carl Roth GmbH & Co. KG, Karlsruhe, Germany) was used as a
filtration aid. Thin layer chromatography was performed by using thin layer chromatography (TLC)
Silicagel 60 F254 from MERCK. A UV lamp (λ = 254 nm) was used for detection.
NMR spectra were recorded at 300 K on a Bruker AvanceNeo 500 (Bruker, Rheinstetten, Germany)
(500 MHz (1H), 125 MHz (13C{1H}), 187 MHz (119Sn{1H}), 100 MHz (29Si {1H})). Where possible, NMR
1
signals were assigned using 1H COSY, 1H/13C HSQC and 1H/13C HMBC experiments. H and 13C {1H}
NMR spectra (125 MHz) were referenced against the residual solvent signal, CDCl3 (1H:
δ
= 7.26 ppm,
= 77.16 ppm). 119Sn{1H} and 29Si{1H} NMR spectra (187 and 100 MHz) were measured based on
the external reference of the 1H-NMR signal of tetramethylsilane.
13C:
δ
Reaction controls by 1H-NMR and some 13C{1H} spectra were performed on a Bruker Avance
WB 360 instrument (Bruker, Rheinstetten, Germany) (1H: 360 MHz, 13C{1H}: 91 MHz). The switching
experiments were performed on a Bruker ARX300 (Bruker, Rheinstetten, Germany) (300 MHz (1H)) or
Bruker AvanceNeo 500 (Burker, Rheinstetten, Germany) (500 MHz (1H).
Thermal analyses were performed on a standalone Mettler Toledo DSC 3+ STAR (Mettler-Toledo,
Columbus, OH, USA) or a Mettler Toledo TGA/DSC 3+ System (Mettler-Toledo, Columbus, OH, USA),
where 40 µL and 100 µL aluminum crucibles were used. For TGA experiments, no lids were used,

Molecules 2019, 24, 303
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whereas in DSC experiments pierced lids were used. Thermo analytical data was analyzed with the
STARe software (Version 14.01, Mettler-Toledo, Columbus, OH, USA) by Mettler Toledo.
Infrared spectra were recorded on a NICOLET i510 FT-IR spectrometer from Thermo Fisher
SCIENTIFIC (Thermo Fisher SCIENTIFIC, Waltham, MA, USA) with a diamond window in an area
ranging from 500–4000 cm−1 with a resolution of 4 cm−1. All samples were measured 16 times against
a background scan. Melting points were measured by DSC or using a BÜCHI Melting Point M-560
instrument (BÜCHI, Essen, Germany).
Electron impact (EI) mass experiments were measured using the direct inlet or indirect inlet
methods, with a source temperature of 200 ◦C on a MAT95 XL double-focusing mass spectrometer
from Finnigan MAT (Thermo Fisher SCIENTIFIC, Waltham, MA, USA). The ionization energy of the
electron impact ionization was 70 eV. Atmospheric pressure chemical ionization (APCI) experiments
were performed on a Bruker Impact II from Bruker Daltonics (Bruker Daltonics, Bremen, Germany).
For the UV switching experiments, a 365 nm (Ocean Optics USB 4000, Sahlmann Photochemical
Solutions, Bad Segeberg, Germany, full width at half maximum (FWHM) 10 nm, 1.0 W) and a 443 nm
LED lamp (Ocean Optics USB 4000, Sahlmann Photochemical Solutions, Bad Segeberg, Germany,
FWHM 19 nm, 0.9 W) were used, while ensuring a constant distance towards the cuvettes of 1 cm.
The NMR switching experiments were performed with a circular aligned lamp consisting of four
high power UV-LEDs (365 nm) 300 mW in power each, produced by “Sahlmann Photochemichal
Solutions”. For switching experiments, quartz cuvettes from Hellma Analytics (Hellma Analytics,
Muehlheim an der Ruhr, Germany) (10 mm) or Quartz NMR tubes from Deutero (Deutero GmbH,
Kastellaun, Germany) were used.
UV-VIS spectra were recorded at a resolution of 0.1 nm on a UV-2700 spectrometer from Shimadzu
(Shimadzu, Kyoto, Japan) with a double monochromator. In all cases, cyclohexane was used as a solvent.
X-ray measurements were carried out at 100 K on a Bruker Venture D8 diffractometer
(Bruker, Karlsruhe, Germany) with Mo-Kα (0.7107 Å) radiation. All structures were solved by intrinsic
phasing and refined based on F2 by use of the SHELX program package, as implemented in OLex 1.2 [51].
All non-hydrogen atoms were refined using anisotropic displacement parameters. Hydrogen atoms
attached to carbon atoms were included in geometrically calculated positions using a riding model.
All crystals were obtained by slow evaporation of an acetonitrile/dichloromethane mixture at 25 ◦C.
4.2. Syntheses
4.2.1. 2,20-Diiodoazobenzene (2)
Adapted with changes from Takahashi et al. [26]. A solution of 2-iodoaniline (1) (2.00 g, 9.13 mmol)
and MnO2 (20.0 g, 230 mmol) in toluene (200 mL) was stirred for 3 h at 120 ◦C. After the reaction, the
mixture was cooled to 25 ◦C, filtered through Celite®, washed with toluene (200 mL) and the solvent
was removed under reduced pressure. The red solid that was obtained was dissolved in DCM (5 mL)
and purified by filtration through a short plug of silica (eluent: n-pentane). After evaporation of the
solvent, the dark orange solid was dried (4.6
3.76 mmol, 82%).
×
10−2 mbar, 25 ◦C, 4 h) to give a red compound (1.63 g,
1H NMR (500 MHz, CDCl3)
δ
= 8.04 (dd, 3J = 7.9 Hz, 4J = 1.3 Hz, 2H, H-3), 7.77 (dd, 3J = 7.9 Hz
,
3
4
3
4J = 1.6 Hz, 2H, H-6), 7.46 (ddd, J = 7.9, 7.3 Hz, J = 1.3 Hz, 2H, H-5), 7.20 (ddd, J = 7.9, 7.3 Hz,
4J = 1.6 Hz, 2H, H-4) ppm. 13C{1H} NMR (126 MHz, CDCl3)
δ = 150.87 (C-1), 139.94 (C-3), 132.80 (C-4),
129.06 (C-5), 118.26 (C-6), 103.22 (C-2) ppm. IR (ATR):
ν = 3055 (w), 2921 (w), 2851 (w), 1924 (w),
1838 (w), 1806 (w), 1699 (w), 1561 (m), 1455 (m), 1013 (s), 953 (m), 760 (s), 714 (s) cm−1. HRMS (EI, MAT
95 XL): m/z calcd. C12H8N2I2+ 433.87715 found 433.87701. Rf (n-pentane): 0.51. Mp (Büchi): 146 ◦C
4.2.2. 2,20-Bis(trimethylstannyl)azobenzene (8)
Adapted with changes from Strüben et al. [25] In a glovebox, a microwave vial was charged
with 2,20-diiodoazobenzene (
2) (200 mg, 0.46 mmol), hexamethylditin (5) (422 mg, 1.03 mmol),

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tetrakis(triphenyl)phosphinopalladium(0) (15.7 mg, 13.0
µmol), THF (0.5 mL) and toluene (4.0 mL).
The reaction mixture was heated for 1 h at 170 ◦C using microwave irradiation. The solution was cooled
to 25 ◦C, filtered, rinsed with toluene (10.0 mL) and all volatiles were removed under reduced pressure.
Then, the compound was dissolved in DCM (2 mL) and filtered through a short plug of silica (eluent:
n-pentane). After the solvent was removed, an orange solid (142 mg, 0.28 mmol, 61%) was received.
3
4
1H NMR (500 MHz, CDCl3)
4J = 1.4 Hz, 2H, H-3), 7.47 (td, 3J = 7.1 Hz, 4J = 1.4 Hz, 2H, H-5), 7.43 (td, 3J = 7.1 Hz, 4J = 1.4 Hz, 2H,
H-4), 0.32 (s, 18H, CH3) ppm. 13C{1H} NMR (126 MHz, CDCl3)
= 157.12 (C-1), 146.53 (C-2), 136.65
(C-3), 130.33 (C-4), 129.52 (C-2), 117.73 (C-6),
7.33 (C-7) ppm. 119Sn{1H} NMR (187 MHz, CDCl3)
34.36 ppm. IR (ATR):
δ ,
= 7.78 (dd, J = 7.8 Hz, J = 1.4 Hz, 2H, H-6), 7.74 (dd, 3J = 7.1 Hz
δ
−
−
ν = 3050 (w), 2974 (w), 2909 (w), 2609 (w), 2354 (w), 1965 (w), 1932 (w), 1902 (w),
1853 (w), 1820 (w), 1432 (w), 1294 (w), 1188 (m), 1110 (m), 754 (s), 706 (s) cm−1. HRMS (APCI): m/z calcd.
[C18H26N2Sn2 + H]+ 509.02140 found 509.02126. Rf: (n-pentane): 0.84. Mp (DSC; Onset): 102.37 ◦C
4.2.3. 2,20-Bis(trimethylsilyl)azobenzene (6)
In an inert tube 2,20-bis(trimethylstannyl)azobenzene (
under Schlenk conditions in THF (5.00 mL) and cooled to
0.47 mmol) was added within 5 min and after 1 h at this temperature, trimethylsilyl chloride (9) (200 µL,
4
) (80.0 mg, 0.16 mmol) was dissolved
◦
−
78 C. MeLi (1.88 M in THF, 0.25 mL,
171 mg, 1.57 mmol) was added to the black reaction mixture in one portion. The reaction mixture was
warmed to 25 ◦C over 14 h and the solvent was removed under reduced pressure. The brown solid,
dissolved in DCM (3.00 mL), was purified by a short plug of silica (eluent: n-pentane). The first orange
fraction was filtered through a PTFE filter (0.45 µm). From the filtrate, the solvent was removed to
1
obtain an orange solid (43 mg, 0.132 mmol, 82%). H NMR (500 MHz, CDCl3): δ = 7.72
4J = 1.3 Hz, 4H, H-3 and H-6), 7.48 (ddd, 3J = 7.7, 7.2 Hz, 4J = 1.3 Hz, 2H, H-5), 7.44 (ddd, 3J = 7.7, 7.2 Hz,
4J = 1.3 Hz, 2H, H-4), 0.40 (s, 18H, CH3) ppm. 13C{1H} NMR (125 MHz, CDCl3): δ = 157.27 (C-1)
142.95 (C-2), 134.97 (C-3), 130.14/130.11 (C-4 and C-5), 114.68 (C-6), 0.70 (C-7) ppm. 29Si{1H} NMR
(100 MHz, CDCl3): = 4.04 ppm. IR (ATR): = 3059 (w), 2946 (w), 2987 (w), 2853 (w), 1968 (w),
( ,
dd, 3J = 7.7 Hz
,
δ
ν
1937 (w), 1859 (w), 1737 (w), 1581 (w), 1561 (w), 1465 (w), 1424 (w); 1296 (w), 1241 (m), 1119 (m),
+
1075 (w), 831 (s), 778 (s), 747 (m), 720 (s), 676 (m) cm−1. HRMS(EI): m/z calcd. C18H26N2Si2 326.16290
found 326.16245. Rf (n-pentane): 0.63. Mp (DSC; Onset): 68.59 ◦C
4.2.4. 2,20-Bis(trimethylgermanyl)azobenzene (7)
A schlenk tube was filled with 2,20-bis(trimethylstannyl)azobenzene (
THF (5.00 mL) and cooled to
78 ◦C. Then, MeLi (1.88 M in THF, 0.25 mL, 0.47 mmol) was added
within 5 min and after 1 h at this temperature, trimethylgermanium chloride (14) (200 L, 171 mg,
4) (80.0 mg, 0.16 mmol) in
−
µ
1.57 mmol) was added to the dark reaction mixture in one portion. The reaction mixture was warmed
to 25 ◦C over 14 h and the solvent was removed under reduced pressure. The brown solid, dissolved
in DCM (3.00 mL), was purified by column chromatography (silica, n-pentane). From the filtrate,
the solvent was removed to obtain an orange solid (31 mg, 0.09 mmol, 60%). 1H NMR (500 MHz,
CDCl3):
13C{1H} NMR (125 MHz, CDCl3):
114.98 (C-6), 0.32 (C-7) ppm. IR (ATR):
δ
= 7.78–7.64 (m, 4H, H-3 and H-6), 7.49–7.39 (m, 4H, H4 and H5), 0.49 (s, 18H, CH3) ppm.
= 156.59 (C-1), 146.12 (C-2), 134.21 (C-3), 130.11 (C-4), 129.52 (C-2),
= 3057 (w), 2962 (w), 2905 (w), 1563 (w), 1463 (w), 1432 (w),
δ
ν
1407 (w), 1295 (w), 1234 (m), 1114 (m), 1064 (w), 953 (w), 818 (m), 777 (s), 751 (m), 719 (m), 658 (m) cm−1
.
HRMS (APCI): m/z calcd. [C18H26N2Ge2 + H]+ 417.06085 found 417.06072. Rf (n-pentane): 0.78. Mp
(DSC; Onset): 82.85 ◦C
4.2.5. 2,20-Di(tert-butyl)azobenzene (10)
Adapted with changes from Takahashi et al. [26] 2-tert-butylaniline (15) (5.00 g, 33.5 mmol) was
dissolved in toluene (800 mL) and MnO2 (50.0 g, 575 mmol) was added portionwise. The reaction
mixture was heated to 120 ◦C. After 3 h at 120 ◦C, the orange reaction mixture was filtered with the help

Molecules 2019, 24, 303
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of Celite®. The solvent was evaporated, the organic phase was washed with aq. HCl (2 M, 300 mL),
and then the organic phase was dried over MgSO4. After removal of the solvent, the product was dried
in a vacuum (4 mbar, 110 ◦C, 5h) and could be isolated without further purification (2.26 g, 7.68 mmol,
46%, purity > 95%). For even higher purity, the compound was subjected to column chromatography
(silica, n-pentane). The first isolated band was the mentioned yellow byproduct (62 mg, 0.22 mmol,
1.31%). The second band was the orange product (620 mg, 2.11 mmol, 13%). 1H NMR (500 MHz,
CDCl3)
H-4), 1.56 (s, 18 H, CH3).13C{1H} NMR (126 MHz, CDCl3):
126.80 (C-4), 126.76 (C-3), 117.05 (C-6), 36.33 (C(CH3)3, 32.16 (CH3) ppm. IR (ATR): ν = 2948 (w)
δ
= 7.54–7.50 (m, 4H, H-3 and H-6), 7.40 (td, 3J = 7.6 Hz, 4J = 1.4 Hz 2H, H-5), 7.32 (mc, 2H,
= 151.93 (C-1), 148.60 (C-2), 130.37 (C-5),
δ
,
2918 (w), 2904 (w), 2859 (w), 1478 (m), 1465 (w), 1435 (w), 1388 (w), 1354 (w), 1289 (w), 1277 (w),
1250 (w), 1196 (w), 1160 (w), 1086 (w), 1050 (w), 947 (w), 926 (w), 769 (s), 750 (s) cm−1. HRMS (EI): m/z
calcd. C20H26N2+ 294.20905 found 294.20851. Rf (n-pentane): 0.76. Mp (DSC; Onset): 86.76 ◦C
4.2.6. Isolated By-Product: 1,6-Di-(tert-butyl)phenanzine (16)
1H NMR (500 MHz, CDCl3)
CH3) ppm. 13C{1H} NMR (126 MHz, CDCl3)
10a or C-5a,10a), 129.48 (C-3,8), 129.01 (C-4,9), 126.05 (C-2,7), 36.97 (C(CH3)3, 31.22 (CH3) ppm. IR (ATR):
δ
= 8.11 (mc, 2H, H-4,9), 7.70 (mc, 4H, H-3,8 and H-2,7), 1.76 (s, 18H,
= 148.46 (C-1,6), 142.19 (C-4a,10a or C-5a,10), 141.89 (C-4a,
δ
ν
= 2997 (w), 2949 (w), 2902 (w), 2864 (w), 1616 (w), 1532 (w), 1481 (w), 1471 (w), 1383 (w), 1353 (w),
1343 (w), 1271 (w), 1126 (w), 997 (m), 932 (w), 857 (w), 813 (m), 748 (s) cm−1. HRMS (EI): m/z calcd.
C20H24N2 292.19340 found 292.19284, calcd. [M-CH3]+ 277.16993 found 277.16993, calcd. [M-C3H6]+
+
250.14645 found 250.14621, calcd. [M-C4H9]+ 235.12297 found 235.12263. Rf (n-pentane): 0.81. Mp
(DSC; Onset): 193.89 ◦C
5. Conclusions
Four 2,20-bis(trimethylelement)azobenzenes have been synthesized with E = C, Si, Ge and Sn.
The synthetic route that has to be selected is highly dependent on the element: With carbon, substitution
reactions with a nucleophilic azobenzene and a carbon electrophile were unsuccessful because of the
small size of the carbon atom and its low reactivity. With Si and Ge however, a dilithiated azobenzene
or the corresponding cuprate could be used as a precursor, illustrating the higher electrophilic character.
However, this species can only easily be produced from a stannylated precursor, which can be
prepared through a Stille-Kelly cross-coupling reaction. While the latter procedure proceeds very
well for tin, a similar reaction was not possible for the corresponding hexamethyldigermanium and
hexamethyldisilane. The lead azobenzene could not be isolated at all, which we ascribe to the instability
of Pb(IV) compared to Pb(II).
Supplementary Materials: The following are available online at http://www.mdpi.com/1420-3049/24/2/303/s1:
A list of all reagents and solvents, attempted syntheses, images of all NMR spectra, UV-VIS spectra and 1H NMR
spectra of all switching experiments, images of the data of all thermoanalytical experiments (DSC and TGA) and
1H NMR spectra of the samples after the DSC experiments.
Author Contributions: Conceptualization, A.S.; Data curation, J.H. and E.L.; Funding acquisition, A.S.; Investigation,
J.H. T.J.K. E.L. and A.S.; Methodology, J.H. and A.S.; Project administration, A.S.; Resources, A.S.; Supervision, A.S.;
Visualization, J.H. and E.L.; Writing—original draft, J.H. and A.S.; Writing—review & editing, J.H. T.J.K. E.L. and A.S.
Funding: This project was supported by the Special Research Area 677 “Function by Switching” of the
Deutsche Forschungsgemeinschaft (DFG). This research has been supported by the Institutional Strategy of the
University of Bremen, funded by the German Excellence Initiative. Partial support came also from the Deutsche
Forschungsgemeinschaft (DFG) with an Emmy-Noether-Fellowship for A.S. [STA1195/2-1].
Acknowledgments: J.H. and A.S. thank the NMR department of the Otto-Diels-Institute for Organic Chemistry
for measuring NMR samples.
Conflicts of Interest: The authors declare no conflict of interest.

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Appendix A
X-Ray Crystallography
There are only a few crystal structures reported for di-ortho-substituted azobenzenes: A structure
with methyl groups in ortho-position [52], with fluorodimethylsilyl groups [43] and boron derivatives [42
have been reported. Compared with the N=N bond in pure trans-azobenzene (1.249(4) Å) [52], the azo
]
groups in our compounds have very similar N=N bond lengths (10: 1.251(2),
6: 1.257(3),
7: 1.256(3),
8: 1.251(2) Å. The Crystal data and structure refinement for azobenzenes 10, 6–8 are given (Table A1);
therefore, these substituents have hardly any influence on the character of the azo group.
Table A1. Crystal data and structure refinement for azobenzenes 10, 6–8.
10
6
7
8
Chemical formula
C20H26N2
294.43
C18H26N2Si2
326.59
C18H26N2Ge2
415.59
C18H26N2Sn2
507.79
Formula weight (g mol−1
)
Crystal system
Space group
a (Å)
Triclinic
P-1
Monoclinic
P21/c
10.8005(3)
6.9267(2)
13.1555(3)
90
91.0610(10)
90
984.02(5)
2
Monoclinic
P21/c
10.8027(5)
7.0849(3)
13.0359(6)
90
91.1550(10)
90
997.51(8)
2
Monoclinic
P21/c
10.7087(3)
7.1755(2)
13.1481(4)
90
91.1950(10)
90
1010.08(5)
2
8.6142(2)
9.7497(2)
11.5720(3)
70.3510(10)
71.0950(10)
77.6290(10)
859.85(4)
2
b (Å)
c (Å)
α (◦)
β (◦)
γ (◦)
V (Å3)
Z
Dcalc (g cm−3
)
1.137
0.066
1.102
0.179
1.384
3.012
1.670
2.472
µ (mm−1
)
F (000)
320.0
352.0
424.0
496.0
Crystal size (mm3)
0.27 × 0.24 × 0.14
4.468 to 62.148
51512
0.25 × 0.10 × 0.10
6.194 to 56.992
32222
0.32 × 0.25 × 0.19
7.524 to 61.08
9934
0.20 × 0.10 × 0.10
6.198 to 59.982
64977
2Theta range for data collection (◦)
Reflections. collected
Independent Reflections
5512
2500
3018
2937
R1 = 0.0471,
wR2 = 0.1086
R1 = 0.0755,
wR2 = 0.1240
1.015
R1 = 0.0356,
wR2=0.0785
R1 = 0.0560,
wR2 = 0.0889
1.045
R1 = 0.0247,
wR2 = 0.0675
R1 = 0.0278,
wR2 = 0.0699
1.065
R1 = 0.0127,
wR2 = 0.0287
R1 = 0.0156,
wR2 = 0.0299
1.081
Final R indexes [I >= 2simga(I)]
Final R indexes [all data]
GooF (F2)
Largest diff. peak/hole (e Å−3
)
0.39/−0.23
0.29/−0.28
0.76/−0.42
0.48/−0.50
CCSD No.
1880127
1880130
1880129
1880128
So far, only one of the crystal structures of 2,6-dimethoxy substituted phenazines was reported [53].
In the following, the structural parameters for our obtained phenazine are given (Figure A1, Table A2).
Figure A1. Molecular structure of phenazine 16, showing a 50% probability of ellipsoids.

Molecules 2019, 24, 303
17 of 19
Table A2. Crystal data and structure refinement for phenazine 16.
16
Chemical formula
C20H24N2
292.41
Formula weight (g mol−1
)
Temperature (K)
Crystal system
Space group
a (Å)
100
Monoclinic
P21/c
12.4209(4)
6.2653(2)
b (Å)
12.3389(4)
c (Å)
α (◦)
90
β (◦)
119.2690(10)
90
837.63(5)
γ (◦)
V (Å3)
Z
2
Dcalc (g cm−3
)
1.159
0.068
µ (mm−1
)
F (000)
316.0
Crystal size (mm3)
0.25 × 0.17 × 0.15
6.604 to 56.996
20978
2Theta range for data collection (◦)
Reflections. collected
Independent Reflections
Final R indexes [I >= 2simga(I)]
Final R indexes [all data]
GooF (F2)
2132
R1 = 0.0417, wR2 = 0.0948
R1 = 0.0632, wR2 = 0.1082
1.024
Largest diff. peak/hole (e Å−3
)
0.33/−0.19
CCSD No.
1880126
CCDC 1880126-1880130 (16, 10, 8, 7, 6) contain the supplementary crystallographic data for this
paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/structures.
References
1.
Rivard, E. Group 14 inorganic hydrocarbon analogues. Chem. Soc. Rev. 2016, 45, 989–1003. [CrossRef]
[PubMed]
2.
Yamaguchi, S.; Tamao, K. Theoretical Study of the Electronic Structure of 2,20-Bisilole in Comparison with
1,10-Bi-1,3-cyclopentadiene:
σ*-π* Conjugation and a Low-Lying LUMO as the Origin of the Unusual Optical
Properties of 3,30,4,40-Tetraphenyl-2,20-bisilole. Bull. Chem. Soc. Jpn. 1996, 69, 2327–2334. [CrossRef]
Yamaguchi, S.; Itami, Y.; Tamao, K. Group 14 metalloles with thienyl groups on 2,5-positions: Effects of
group 14 elements on their π-electronic structures. Organometallics 1998, 17, 4910–4916. [CrossRef]
Linshoeft, J.; Baum, E.J.; Hussain, A.; Gates, P.J.; Näther, C.; Staubitz, A. Highly tin-selective stille coupling:
synthesis of a polymer containing a stannole in the main chain. Angew. Chem. Int. Ed. 2014, 53, 12916–12920.
[CrossRef] [PubMed]
3.
4.
5.
6.
Urrego-Riveros, S.; Ramirez y Medina, I.-M.; Hoffmann, J.; Heitmann, A.; Staubitz, A. Syntheses and
properties of tin-containing conjugated heterocycles. Chem. Eur. J. 2017, 24, 5680–5696. [CrossRef]
Ramirez y Medina, I.-M.; Rohdenburg, M.; Mostaghimi, F.; Grabowsky, S.; Swiderek, P.; Beckmann, J.;
Hoffmann, J.; Dorcet, V.; Hissler, M.; Staubitz, A. Tuning the optoelectronic properties of stannoles by the
judicious choice of the organic substituents. Inorg. Chem. 2018, 57, 12562–12575. [CrossRef] [PubMed]
7.
8.
9.
Bandara, H.M.D.; Burdette, S.C. Photoisomerization in different classes of azobenzene. Chem. Soc. Rev. 2012
41, 1809–1825. [CrossRef]
Beharry, A.A.; Woolley, G.A. Azobenzene photoswitches for biomolecules. Chem. Soc. Rev. 2011, 40,
4422–4437. [CrossRef]
Dong, M.; Babalhavaeji, A.; Samanta, S.; Beharry, A.A.; Woolley, G.A. Red-shifting azobenzene photoswitches
for in vivo use. Acc. Chem. Res. 2015, 48, 2662–2670. [CrossRef] [PubMed]
,

Molecules 2019, 24, 303
18 of 19
10. Mahimwalla, Z.; Yager, K.G.; Mamiya, J.-I.; Shishido, A.; Priimagi, A.; Barrett, C.J. Azobenzene
photomechanics: Prospects and potential applications. Polym. Bull. 2012, 69, 967–1006. [CrossRef]
11. Merino, E.; Ribagorda, M. Control over molecular motion using the cis-trans photoisomerization of the azo
group. Beilstein J. Org. Chem. 2012, 8, 1071–1090. [CrossRef]
12. Ahmed, Z.; Siiskonen, A.; Virkki, M.; Priimagi, A. Controlling azobenzene photoswitching through combined
ortho-fluorination and -amination. Chem. Commun. 2017, 53, 12520–12523. [CrossRef]
13. Bléger, D.; Schwarz, J.; Brouwer, A.M.; Hecht, S. o-Fluoroazobenzenes as readily synthesized photoswitches
offering nearly quantitative two-way isomerization with visible light. J. Am. Chem. Soc. 2012, 134,
20597–20600. [CrossRef]
14. Yu, Y.; Nakano, M.; Ikeda, T. Directed bending of a polymer film by light. Nature 2003, 425, 145. [CrossRef]
15. Kizilkan, E.; Strueben, J.; Jin, X.; Schaber, C.F.; Adelung, R.; Staubitz, A.; Gorb, S.N. Influence of the porosity
on the photoresponse of a liquid crystal elastomer. Roy. Soc. Open Sci. 2016, 3. [CrossRef]
16. Nguyen, T.H.L.; Gigant, N.; Joseph, D. Advances in direct metal-catalyzed functionalization of azobenzenes.
ACS Catal. 2018, 8, 1546–1579. [CrossRef]
17. Kakiuchi, F.; Matsumoto, M.; Tsuchiya, K.; Igi, K.; Hayamizu, T.; Chatani, N.; Murai, S. The ruthenium-catalyzed
silylation of aromatic C–H bonds with triethylsilane. J. Organomet. Chem. 2003, 686, 134–144. [CrossRef]
18. Nguyen, T.T.T.; Boussonnière, A.; Banaszak, E.; Castanet, A.-S.; Nguyen, K.P.P.; Mortier, J. Chemoselective
deprotonative lithiation of azobenzenes: Reactions and mechanisms. J. Org. Chem. 2014, 79, 2775–2780.
[CrossRef]
19. Li, J.; Cong, W.; Gao, Z.; Zhang, J.; Yang, H.; Jiang, G. Rh(III)-catalyzed regioselective mono- and di-iodination
of azobenzenes using alkyl iodide. Org. Biomol. Chem. 2018, 16, 3479–3486. [CrossRef]
20. Yamamura, M.; Kano, N.; Kawashima, T.; Matsumoto, T.; Harada, J.; Ogawa, K. Crucial role of N···Si
interactions in the solid-state coloration of disilylazobenzenes. J. Org. Chem. 2008, 73, 8244–8249. [CrossRef]
21. Kano, N.; Komatsu, F.; Kawashima, T. Synthesis and structure of azobenzenes bearing silyl, germyl,
and stannyl groups at 2-position. Chem. Lett. 2001, 30, 338–339. [CrossRef]
22. Kano, N.; Komatsu, F.; Yamamura, M.; Kawashima, T. Reversible photoswitching of the coordination
numbers of silicon in organosilicon compounds bearing a 2-(phenylazo)phenyl group. J. Am. Chem. Soc.
2006, 128, 7097–7109. [CrossRef]
23. Strueben, J.; Hoffmann, J.; Naether, C.; Staubitz, A. Crystal structures of 3,3’-bis(hydroxydimethylsilanyl)
azobenzene and 4,4’-bis(hydroxydimethylsilane)azobenzene. Acta Cryst. E 2016, 72, 1590–1594. [CrossRef]
24. Strueben, J.; Lipfert , M.; Springer , J.-O.; Gould, C.A.; Gates, P.J.; Sönnichsen, F.D.; Staubitz, A. High-yield
lithiation of azobenzenes by tin-lithium exchange. Chem. Eur. J. 2015, 21, 11165–11173. [CrossRef]
25. Strueben, J.; Gates, P.J.; Staubitz, A. Tin-functionalized azobenzenes as nucleophiles in stille cross-coupling
reactions. J. Org. Chem. 2014, 79, 1719–1728. [CrossRef]
26. Takahashi, H.; Ishioka, T.; Koiso, Y.; Sodeoka, M.; Hashimoto, Y. Anti-androgenic activity of substituted azo-
and azoxy-benzene derivatives. Bio. Pharm. Bull. 2000, 23, 1387–1390. [CrossRef]
27. Seth, K.; Roy, S.R.; Kumar, A.; Chakraborti, A.K. The palladium and copper contrast: A twist to products of
different chemotypes and altered mechanistic pathways. Catal. Sci. Tech. 2016, 6, 2892–2896. [CrossRef]
28. Monir, K.; Ghosh, M.; Mishra, S.; Majee, A.; Hajra, A. Phenyliodine(III) diacetate (PIDA) mediated synthesis
of aromatic azo compounds through oxidative dehydrogenative coupling of anilines: Scope and mechanism.
Eur. J. Org. Chem. 2013, 2014, 1096–1102. [CrossRef]
29. Gooßen, L.J.; Ferwanah, A.-R.S. A mild and efficient protocol for the catalytic silylation of aryl bromides.
Synlett 2000, 2000, 1801–1803. [CrossRef]
30. McNeill, E.; Barder, T.E.; Buchwald, S.L. Palladium-catalyzed silylation of aryl chlorides with
hexamethyldisilane. Org. Lett. 2007, 9, 3785–3788. [CrossRef]
31. Komami, N.; Matsuoka, K.; Yoshino, T.; Matsunaga, S. Palladium-catalyzed germylation of aryl bromides
and aryl triflates using hexamethyldigermane. Synthesis 2018, 50, 2067–2075. [CrossRef]
32. Arnold, D.P.; Wells, P.R. Hexamethyldilead: I. Preparation, thermal decomposition and methanolysis.
J. Organomet. Chem. 1976, 111, 269–283. [CrossRef]
33. Zavitsas, A.A. The relation between bond lengths and dissociation energies of carbon–carbon bonds.
J. Phys. Chem. 2003, 107, 897–989. [CrossRef]

Molecules 2019, 24, 303
19 of 19
34. Lappert, M.F.; Pedley, J.B.; Simpson, J.; Spalding, T.R. Bonding studies of compounds of boron and the group
IV element: VI. Mas spectrometric studies on compounds Me4M and Me3M–M0Me3 (M and M0 = C, Si, Ge,
Sn, and Pb): Thermochemical data. J. Organomet. Chem. 1971, 29, 195–208. [CrossRef]
35.
Dávalos, J.Z.; Baer, T. Thermochemistry and dissociative photoionization of Si(CH3)4, BrSi(CH3)3, ISi(CH3)3,
and Si2(CH3)6 studied by threshold photoelectron photoion coincidence spectroscopy. J. Phys. Chem. A
−
2006, 110, 8572–8579. [CrossRef]
36. Farina, V.; Kapadia, S.; Krishnan, B.; Wang, C.; Liebeskind, L.S. On the nature of the “copper effect” in the
stille cross-coupling. J. Org. Chem. 1994, 59, 5905–5911. [CrossRef]
37. Casado, A.L.; Espinet, P.; Gallego, A.M. Mechanism of the stille reaction. 2. Couplings of aryl triflates with
vinyltributyltin. Observation of intermediates. A more comprehensive scheme. J. Am. Chem. Soc. 2000, 122,
11771–11782. [CrossRef]
38. Aggarwal, V.K.; Staubitz, A.C.; Owen, M. Optimization of the mizoroki heck reaction using design of
−
experiment (DoE). Org. Process. Res. Dev. 2006, 10, 64–69. [CrossRef]
39. Knochel, P.; Yeh, M.C.P.; Berk, S.C.; Talbert, J. Synthesis and reactivity toward acyl chlorides and enones of
the new highly functionalized copper reagents RCu(CN)ZnI. J. Org. Chem. 1988, 53, 2390–2392. [CrossRef]
40. Krueerke, U. Halogen-austausch an chlorsilanen und die tetrahydrofuran-spaltung durch brom- und
jodsilane. Chem. Ber. 1962, 95, 174–182. [CrossRef]
41. Olaru, M.; Kather, R.; Hupf, E.; Lork, E.; Mebs, S.; Beckmann, J. A monoaryllead trichloride that resists
reductive elimination. Angew. Chem. Int. Ed. 2018, 57, 5917–5920. [CrossRef]
42. Yoshino, J.; Kano, N.; Kawashima, T. Fluorescent azobenzenes and aromatic aldimines featuring an N–B
interaction. Dalton Trans. 2013, 42, 15826–15834. [CrossRef]
43. Kano, N.; Yamamura, M.; Kawashima, T. 2,2’-Disilylazobenzenes featuring double intramolecular
nitrogen···silicon coordination: A photoisomerizable fluorophore. Dalton Trans. 2015, 44, 16256–16265.
[CrossRef]
44. Denmark, S.E.; Regens, C.S. Palladium-catalyzed cross-coupling reactions of organosilanols and their salts:
practical alternatives to boron- and tin-based methods. Acc. Chem. Res. 2008, 41, 1486–1499. [CrossRef]
45. Hatanaka, Y.; Hiyama, T. Cross-coupling of organosilanes with organic halides mediated by a palladium
catalyst and tris(diethylamino)sulfonium difluorotrimethylsilicate. J. Org. Chem. 1988, 53, 918–920.
[CrossRef]
46. Kosugi, M.; Tanji, T.; Tanaka, Y.; Yoshida, A.; Fugami, K.; Kameyama, M.; Migita, T. Palladium-catalyzed
reaction of 1-aza-5-germa-5-organobicyclo[3.3.3]undecane with aryl bromide. J. Organomet. Chem. 1996, 508,
255–257. [CrossRef]
47. Faller, J.W.; Kultyshev, R.G. Palladium-catalyzed cross-coupling reactions of allyl, phenyl, alkenyl, and alkynyl
germatranes with aryl iodides. Organometallics 2002, 21, 5911–5918. [CrossRef]
48. Nakamura, T.; Kinoshita, H.; Shinokubo, H.; Oshima, K. Biaryl synthesis from two different aryl halides with
tri(2-furyl)germane. Org. Lett. 2002, 4, 3165–3167. [CrossRef]
49. Cordovilla, C.; Bartolomé, C.; Martínez-Ilarduya, J.M.; Espinet, P. The stille reaction, 38 years later. ACS Catal.
2015, 5, 3040–3053. [CrossRef]
50. Tanaka, R.; Kawahara, T.; Shinto, Y.; Nakayama, Y.; Shiono, T. An alternative method for the preparation of
trialkylaluminum-depleted modified methylaluminoxane (dMMAO). Macromolecules 2017, 50, 5989–5993.
[CrossRef]
51. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. OLEX2: A complete structure
solution, refinement and analysis program. J. Appl. Crystallogr. 2009, 42, 339–341. [CrossRef]
52. Harada, J.; Ogawa, K.; Tomoda, S. Molecular Motion and Conformational Interconversion of Azobenzenes in
Crystals as Studied by X-ray Diffraction. Acta Cryst. Sec. B 1997, 53, 662–672. [CrossRef]
53. Nawata, Y.; Iwasaki, H.; Saito, Y. The Crystal Structure of Bis(pyridine-2-carboxamido)nickel(II) Dihydrate.
Bull. Chem. Soc. Jpn. 1967, 40, 515–521. [CrossRef]
Sample Availability: Samples of the compounds 6, 7, 8 and 10 are available from the authors.
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2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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