2-Halo- and/or 4-ethoxycarbonyl-substituted asymmetric 1,3-diaryltriazenes and 1,3-diarylamidines as well as N-methylated congeners

Article abstract of DOI:10.1016/j.molstruc.2019.127622

1,3-Diaryltriazenes Ar–N[dbnd]N–N(R)-R’ [R = H; Ar/R’ = C₆H₄-4-COOEt/Ph (1), C₆H₃-2-Br-4-COOEt/Ph (2), CH₂Ph/C₆H₄-4-COOEt (3)] and the N-methylated congeners [R = Me; Ar/R’ = C

Full text of DOI:10.1016/j.molstruc.2019.127622

Journal of Molecular Structure 1205 (2020) 127622
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
2-Halo- and/or 4-ethoxycarbonyl-substituted asymmetric 1,3-
diaryltriazenes and 1,3-diarylamidines as well as N-methylated
congeners
,
1
a
a
a
b
Silvio Preusser a , Paul R.W. Schonherr , Helmar Gorls , Sven Krieck , Wolfgang Imhof ,
€
€
Matthias Westerhausen a
a Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University, Humboldtstraße 8, 07743, Jena, Germany
,
*
b
€
Institute for Integrated Natural Sciences, Department of Chemistry, University Koblenz-Landau, Universitatsstraße 1, D-56070, Koblenz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
1,3-Diaryltriazenes AreN]NeN(R)-R’ [R ¼ H; Ar/R’ ¼ C6H4-4-COOEt/Ph (1), C6H3-2-Br-4-COOEt/Ph (2),
CH2Ph/C6H4-4-COOEt (3)] and the N-methylated congeners [R ¼ Me; Ar/R’ ¼ C6H4-4-COOEt/Ph (9), C6H2-
2,5-Br2-4-NH2/Ph (10), C6H4-4-Br/C6H4-4-COOEt (11), C6H2-2-Br-4-COOEt/Ph (12), C6H2-2-Br-4-COOEt-6-
Me/C6H4-2-Me (13), C6H3-2-Br-4-F/Ph (14), C6H3-2-Br-6-F/Ph (15), C6H3-2-Br-4-Cl/Ph (16), C6H3-2,4-Br2/
Ph (17)] as well as 1,3-diarylformamidines of the type AreN]CHeN(R)-R’ [R ¼ H; Ar/R’ ¼ C6H3-4-COOEt/
Ph (4), C6H3-2-Br-4-COOEt/Ph (5), C6H3-2-Br-4-F/Ph (6)] and 1,3-diaryl-3-methylamidines [R ¼ Me; Ar/
R’ ¼ C6H3-4-COOEt/Ph (18), C6H3-2-Br-4-COOEt/Ph (19), C6H3-2-Br-4-Cl/Ph (20), C6H3-2,4-Br2/Ph (21)]
crystallize preferably with (syn-E) configuration. For comparison reasons sterically crowded Ph-N(R)-C
(R0) ¼ NeC6H3-2-Br-4-COOEt [R ¼ H, R’ ¼ Ph (7), tBu (8); R ¼ Me, R’ ¼ Ph (22), tBu (23)] are included.
Large substituents destabilize this isomeric form due to intramolecular steric repulsion. 1,3-
Diarylformamidines dimerize via two nearly parallel NeH/N hydrogen bridges. This kind of aggrega-
tion is very beneficial because the N]CeN bond angles are slightly larger than 120ꢀ. Narrower N]NeN
bond angles are found for the 1,3-diaryltriazenes and therefore, aggregation occurs via intermolecular N
eH/O hydrogen bridges to O-Lewis bases. The 1,3-diaryl-3-methylformamidines and -triazenes show
very similar bonding parameters. Thus, the bond lengths of the N]C/N]N double bonds are approx. 4
e9 pm shorter than the NeC/NeN single bonds, supporting charge delocalization within the diazaallyl-
and triazaallyl systems regardless of N-methylation.
Received 8 October 2019
Received in revised form
13 December 2019
Accepted 18 December 2019
Available online 23 December 2019
Keywords:
Diazene
Formamidine
Triazene
Benzotriazole
Hydrogen bridges
E/Z isomerism
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
intensively studied due to the fact that these compounds can easily
be prepared [3] and that they provide a plethora of reactivity pat-
Formamidines and triazenes are isoelectronic compounds with
a long tradition and very similar properties in many aspects which
are widely used and intensively studied according to their wide-
spread applications. Due to their tremendous importance in
organic, medicinal and coordination chemistry, diverse aspects of
these compound classes have been summarized in numerous
excellent reviews (Scheme 1).
terns. The enormous importance of these compound classes is
based on early recognized biological and medicinal activity as well
as on promising pharmaceutical and clinical applications [4,5]. In
organic chemistry, these compounds are widely used for the syn-
theses of heterocycles [6,7], as nucleophilic catalysts [8], in
analytical [9] and as ligands in coordination chemistry [10]. During
our studies with respect to the transformation of 1,3-diaryl-3-
methyltriazenes into N-arylbenzotriazoles for pharmaceutical
purposes we became aware of large variation of the yields
depending on substitution patterns of the triazenes and reaction
conditions. These cyclization reactions have to be mediated by
copper or palladium catalysts in the presence of bases (Scheme 2).
Depending on the use of NH-triazenes (Scheme 2, right) or meth-
ylated triazenes (left), the group R0 is bonded at the benzo unit
Amidines [1] and triazenes [2] are well-known for decades and
* Corresponding author.
E-mail address: m.we@uni-jena.de (M. Westerhausen).
URL: http://www.lsac1.uni-jena.de
Present address: S. Preusser Laborchemie Apolda GmbH, Utenbacherstrabe
1
72e74, D-99510, Apolda, Germany.
https://doi.org/10.1016/j.molstruc.2019.127622
0022-2860/© 2019 Elsevier B.V. All rights reserved.

2
S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
formation of intermolecular hydrogen bridges and hence, the
packing of the molecules in the crystalline state should strongly
depend on the nature of the N-bound group R (hydrogen, methyl).
Due to the fact that we were interested in building blocks for the
preparation of pharmazeutically useful N-aryl-benzotriazoles we
limited our investigation on 1,3-diaryltriazenes, 1,3-diaryl-3-
methyltriazenes, and 1,3-diarylamidines with halide functional-
ities and/or ethoxycarbonyl groups in para-position.
2. Experimental
Instrumentation: All reactions were performed under ambient
conditions and monitored by TLC on silica with embedded fluo-
rescence indicator. NMR spectra were recorded by the Bruker NMR
Spectrometers with 250 MHz, 400 MHz or 600 MHz. Highly
resolved masses were obtained by electron spray ionization using a
Bruker microTOF-Q by direct injection. The calibration was per-
formed with sodium formiate cluster at the mass range of m/
z ¼ 50e1000 Da. Elemental analyses were performed using an
EuroVector EuroEA 3000 instrument. Melting points were deter-
mined by a Büchi apparatus. IR spectra were record using a Bruker
Alpha FT-IR ATR spectrometer. UV spectra were obtained at a
Specord UVeVis spectrometer.
Scheme 1. Isoelectronic rows of the basic compounds (top) and their metalated
congeners (bottom) via consecutive replacement of O by NH and finally of CH by N.
Synthesis: O-Ethyl-phenylformimidate was synthesized using a
slightly modified procedure of Roberts et al. and purified by frac-
tionized vacuum distillation [13]. The synthesis of N-tert-butylcar-
bonyl aniline and N-phenylcarbonyl aniline were performed
according to a slightly modified procedure according to a literature
protocol by mono-N-acylation using pivaloyl chloride and benzoyl
chloride, respectively [14]. The triazenes were prepared in analogy
to well-known published procedures [12]. General procedures for
the synthesis of 1,3-diarylformamidines were published earlier
[15].
Scheme 2. Catalytic cyclization of 1,3-diaryltriazenes to N-aryl-benzotriazoles via a
Buchwald-Hartwig-type cycloamination (R ¼ H, right) or via demethylating cyclo-
amination (R ¼ Me, left).
(Buchwald-Hartwig-type cycloamination) [11] or at the 3-aryl
substituent (demethylating cycloamination) [12]. The benchmark
reaction of the palladium-mediated synthesis of N-4-
ethoxycarbonylphenyl-benzotriazole (R’
¼
COOEt) from 1-(2-
For further details on the synthesis and characterization of these
1,3-diaryltriazenes and 1,3-diarylamidines see the Electronic Sup-
porting Information (ESI).
bromo-4-ethoxycarbonylphenyl)-3-phenyl-3-methyltriazene
(Scheme 2, left) showed a strong dependence on the atmosphere
(air, oxygen, argon) and on the reaction conditions (excess of base,
nature of oxidant and Pd catalyst) [12].
Crystal structure determinations: The intensity data for the
compounds were collected on a Nonius KappaCCD diffractometer
using graphite-monochromated Mo-Ka radiation. Data were cor-
rected for Lorentz and polarization effects; absorption was taken
into account on a semi-empirical basis using multiple-scans
[16e18]. The structures were solved by direct methods (SHELXS)
[19] and refined by full-matrix least squares techniques against Fo2
[20]. The hydrogen atoms of 5, 7, and 24 (with the exception of the
hydrogen atoms bonded to the amidine nitrogen atom N3) were
included at calculated positions with fixed thermal parameters. All
other hydrogen atoms were located by difference Fourier synthesis
and refined isotropically. The crystal of 5 was a non-merohedral
twin. The twin laws were determined by PLATON [21] to (ꢁ1.000
0.000 0.000) (ꢁ0.368 1.000e0.169) (0.000 0.000e1.000). The
contribution of the main component was refined to 0.855 (3). All
non-disordered, non-hydrogen atoms were refined anisotropically
These challenges prompted us to elucidate the influence of the
substitution pattern on the molecular structures of 1,3-
diaryltriazenes, 1,3-diaryl-3-methyltriazenes, and of 1,3-
diarylamidines. 1,3-Diaryltriazenes (E
¼
N) and 1,3-
diarylformamidines (E ¼ CH) can adopt different isomeric forms
as depicted in Scheme 3. The aryl groups can be positioned at the
same (syn) or opposite sides (anti isomer) of the molecule.
Furthermore, E/Z isomeric forms can be observed at the E ¼ N
double bond. However, tautomeric equilibria via 1,3-hydrogen
migration can interconvert isomeric forms of triazenes and ami-
dines which is well-known [1,2] and will not be discussed here. In
addition, the rather acidic NeH functionality (R ¼ H) is prone to the
Scheme 3. Differentiation of the isomeric configurations of 1,3-diarylformamidines
(E ¼ CH; Ar ¼ aryl) as well as 1,3-diaryltriazenes (E ¼ N; R ¼ H) and 1,3-diaryl-3-
methyltriazenes (E ¼ N, R ¼ Me).
Scheme 4. Synthesis of R-substituted triazenes via diazotization and subsequent re-
action with aniline or N-methyl-aniline (ACN acetonitrile, R ¼ H, Me).

S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
3
Table 1
Substitution patterns and yields of 1,3-diaryltriazenes and 1,3-diaryl-3-methyltriazenes of the type AreN]NeN(R)-R’ (prepared via diazotization according to Schemes 4 and
6) and of 1,3-diarylamidines and 1,3-diaryl-3-methylamidines (Scheme 8).
E
R
Ar
R0
Yield (%)a
d
(1HMe
)
d( )
13C{1H}Me
1
2
3
4
5
6
7
8
N
N
N
CH
CH
CH
C-Ph
C-tBu
N
N
N
N
N
N
N
N
N
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
C6H4-4-COOEt
C6H3-2-Br-4-COOEt
CH2-Ph
C6H3-4-COOEt
C6H3-2-Br-4-COOEt
C6H3-2-Br-4-F
C6H3-2-Br-4-COOEt
C6H3-2-Br-4-COOEt
C6H4-4-COOEt
C6H2-2,5-Br2-4-NH2
C6H4-4-Br
C6H3-2-Br-4-COOEt
C6H2-2-Br-4-COOEt-6-Me
C6H3-2-Br-4-F
Ph
Ph
71
70
61
37
43
66
56
50
83
69
35
79
67
80
76
78
72
69
79
47
60
46
23
e
e
e
e
e
e
e
e
e
e
C6H4-4-COOEt
Ph
Ph
Ph
Ph
Ph
Ph
Ph
e
e
e
e
e
e
9
3.73
3.64
3.73
3.77
3.58
3.70
3.71
3.72
3.72
3.52
3.49
3.54
3.54
3.53
2.89
33.3
32.6
32.3
33.8
37.7
33.2
32.6
33.4
33.5
34.5
34.5
34.5
34.5
40.4
39.1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
C6H4-4-COOEt
Ph
C6H4-2-Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
C6H3-2-Br-6-F
C6H3-2-Br-4-Cl
C6H3-2,4-Br2
CH
CH
CH
CH
C-Ph
C-tBu
C6H3-4-COOEt
C6H3-2-Br-4-COOEt
C6H3-2-Br-4-Cl
C6H3-2,4-Br2
C6H3-2-Br-4-COOEt
C6H3-2-Br-4-COOEt
a
Yield of isolated crystalline triazenes and amidines.
[20]. The crystal of 7 was extremely thin and of low quality,
resulting in a substandard data set; however, the structure is suf-
ficient to show connectivity and geometry despite the high final R
value. We will only publish the conformation of the molecule and
the crystallographic data. We will not deposit the data in the
Cambridge Crystallographic Data Centre. Crystallographic data as
well as structure solution and refinement details are summarized
in Table S1 (see Supporting Information). XP (SIEMENS Analytical
X-ray Instruments, Inc.) [22] was used for structure
representations.
red side products were occasionally formed with yields below 3%
which were chromatographically isolated and recognized as dia-
ryldiazenes Ar’-N]NeC6H4-4-NHMe due to an attack of the dia-
zonium salt Ar’-Nþ2 at the para-position of N-methyl-arylamine; the
X-ray structure determination of the diazene 2-Br-4-EtOOC-6-
MeC6H2eN]NeC6H3-3-Me-4-NHMe (24) verified the formation
of this side product as shown in Scheme 5.
Molecular structure and atom labeling scheme of the diazene 24
are depicted at the top of Fig. 1. In the solid state, intermolecular
hydrogen bridges between the N3eH3 functionality and the ester
carbonyl group C8¼O1 lead to a strand structure as shown at the
bottom of Fig. 1. The N1¼N2 bond length of 120.0 (6) pm is a typical
double bond value. The N1eC1 and N2-11 distances of 148.7 (6) and
147.9 (7) pm, respectively, are characteristic single bond values
3. Results and discussion
3.1. Synthesis
with negligible interactions between the diazene and aryl
p-sys-
tems. Formation of intermolecular N3eH3N3/O10 hydrogen
bridges leads to strand formation in the crystalline state.
Numerous preparative protocols have been developed to pre-
pare symmetrically and asymmetrically substituted amidines [1]
and triazenes [2]. Due to the fact that our objective was the syn-
thesis of derivatives of benzotriazoles, we studied mainly asym-
metrically aryl-substituted amidines and triazenes with bromo
functionalities in ortho-position. Another precondition is the
tolerance of ester groups in para-position of the aryl substituent
during the synthesis of these compounds.
Diazotization of 1,4-diaminobenzene with two equivalents of
sodium nitrite in water and subsequent addition of a solution of
The preferred procedure starts with the diazonium salt of the
bromo-substituted aniline which was reacted with aniline, ben-
zylamine or N-methyl-aniline according to Scheme 4. During this
reaction, pH values of 6e7 (aniline, benzylamine) or 7e8 (N-
methyl-aniline) have to be maintained in buffered solutions (ace-
tate or carbonate buffers) to suppress side reactions. The pH value is
a crucial factor and at low values in acidic media, diaryldiazenes
were formed as side products.
This method allowed the synthesis of 1,3-diaryltriazenes and
1,3-diaryl-3-methyltriazenes with generally good yields (Table 1).
Halo- and ester groups are tolerated and the yields of isolated
crystalline compounds are larger than 60%. During separation and
isolation of the 1,3-diaryl-3-methyltriazenes, intensively orange-
Scheme 5. Isolated side product during diazotization and reaction with N-methyl-2-
toluidine yielding the diazene 2-Br-4-EtOOC-6-MeC6H2eN]NeC6H3-3-Me-4-NHMe
(24).

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S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
Fig. 1. Molecular structure and labeling scheme of diazene 2-Br-4-EtOOC-6-MeC6H2eN]NeC6H3-3-Me-4-NHMe (24, at the top). The ellipsoids represent a probability of 30%, H
atoms are shown with arbitrary radii. At the bottom, aggregation via intermolecular NeH/O hydrogen bridges is depicted; the hydrogen bridges are symbolized by dotted lines.
Atoms are shown with arbitrary radii, C-bound hydrogen atoms are omitted for the sake of clarity.
aniline in alcohol yielded 54% of 1,4-bis(3-phenyltriazen-1-yl)
benzene [23]. Contrary to this report, 1,4-diamino-2,5-
dibromobenzene did not react in a similar manner. 1,4-Diamino-
2,5-dibromobenzene was combined with HCl and sodium nitrite in
acetonitrile; subsequent addition of N-methyl-aniline yielded only
the monotriazene congener 1-(2,5-dibromo-4-aminophenyl)-3-
methyl-3-phenyltriazene (10) as depicted in Scheme 6. The amino
group in trans-position to the diazo functionality remained un-
changed and doubly bis(triazene) derivatives were not accessible
by this protocol regardless of the applied stoichiometry of the
substrates.
benzyl-3-methyltriazene, 1-benzyl-3-(4-ethoxycarbonylphenyl)-
3-methyltriazene, and (4-ethoxycarbonylphenyl)-benzyl-methyl-
amine, the latter product formed via elimination of dinitrogen.
The beneficial preparative procedure for the synthesis of
asymmetric 1,3-diarylformamidines consisted of the reaction of the
substituted aniline with N-phenyl-O-ethylimidate at room tem-
perature or 50 ꢀC (Scheme 8, right). The syntheses of 1,3-diaryl-3-
methylformamidines succeeded by the reaction of substituted an-
ilines with N-methyl-N-phenylformamide (Scheme 8, left). The
yields are given in Table 1.
As an alternative pathway, arylazides can be reacted with phe-
nyllithium, however, the ester functionality was very reactive to-
ward phenyllithium. Two thirds of the substrate remained
unchanged, if the precise 1:1 stoichiometry was applied, and one
third formed a product which consumed three equivalents of PhLi
yielding triazene 25 (Scheme 7). Thus, the yield was 32% with
respect to the starting arylazide (yield of 96% with respect to PhLi).
This finding showed that the presence of ester functionalities limits
the preparative access to asymmetric triazenes with ester sub-
stituents and only the beneficial diazotization method (Scheme 5)
gave satisfactory yields as depicted in Table 1.
3.2. Molecular structures
The molecular structure and atom labeling scheme of (syn-E)-
PhCH2-N]NeN(H)eC6H4-4-COOEt (3) are depicted at the top of
Methylation of 1,3-diaryltriazenes via potassiation with KH and
subsequent methylation with MeI in THF yielded the methylated
products with the methyl groups at one of the nitrogen bases,
regiocontrol was challenging and depended on the steric shielding.
Hence, the conversion of 1-(4-ethoxycarbonylphenyl)-3-
benzyltriazene led to a mixture of 1-(4-ethoxycarbonylphenyl)-3-
Scheme 6. Reaction of 1,4-diamino-2,5-dibromobenzene with excess of sodium nitrite
and
N-methyl-aniline
yielding
1-(2,5-dibromo-4-aminophenyl)-3-methyl-3-
phenyltriazene regardless of the applied stoichiometry of the components.

S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
5
Scheme 7. Stepwise reaction of 4-ethoxycarbonylphenylazide with three equivalents of phenyllithium and subsequent hydrolytic workup yielded 1-[4-(hydroxydiphenylmethyl)
phenyl]-3-phenyltriazene (25).
Fig. 2. The methylene moiety at N1 prevents an effective interaction
of the -systems of the triazene and benzyl units. This substitution
a six-membered N3H2O cycle. Due to these hydrogen bridges
strand-like structures are formed in the crystalline state.
p
leads to a slight shortening of the N]N bond and a reduction of the
Molecular structure and atom labeling scheme of the for-
mamidine (syn-E)-4-(EtOOC)-2-BrC6H3eN]CHeN(H)-Ph (5) are
depicted in Fig. 4. The isoelectronic substitution of the central N
atom by a CH fragment leads to a significant widening of the NEN
bond angle. As characteristic for amidines [24], dimerization via
N1eH1N1/N2A hydrogen bridges occurs in the solid state leading
NNN bond angle by approx. 2ꢀ in comparison to diaryltriazenes (see
below). In addition, the interaction of the conjugated p-systems of
the N]NeN and aryl moieties leads to a shortening of the N-CR’
bond to 139.0 (2) pm. Repulsion between the free electron pair at
the middle atom N2 and the C13eH bond leads to different
N3eC8eC9 (117.52 (13)ꢀ) and N3eC8eC13 (122.64 (13)ꢀ) bond
angles. In the crystalline state, aggregation occurs via intermolec-
ular N3eH3N3/O2A hydrogen bridges (N3/O2A 285.7 (2),
N3eH3N3 89 (2), O2A$$$H3N3 200 (2) pm; N3eH3N3eO2A 160.2
(18)ꢀ) to the carbonyl group (Fig. 2, bottom), leading to the for-
mation of a strand-like structure.
to
a centrosymmetric aggregate with an eight-membered
Molecular structure and atom labeling scheme of (syn-E)-4-
[Ph2(HO)CeC6H4-]N]NeN(H)-Ph (25) (top) as well as aggregation
in the crystalline state (bottom) are depicted in Fig. 3. The charge
delocalization within the triazene unit of 25 is significantly more
pronounced than in 1, expressed by the difference of the N]N and
NeN bond lengths (3: 9.0 pm, 25: 3.8 pm). The reason for this
increasing convergence is the fact that both nitrogen atoms in 1-
and 3-positions of the triazene are involved in the intermolecular
hydrogen bridges N1eH1N1/O1A and O1A-H1AO1A$$$N3, forming
Scheme 8. Synthesis of asymmetric 1,3-diarylformamidines (R ¼ H, right) and 1,3-
diaryl-3-methylformamidines (R ¼ Me, left).

6
S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
Fig. 2. Molecular structure and atom labeling scheme of (syn-E)-PhCH2-N]NeN(H)eC6H4-4-COOEt (3) (top). The ellipsoids represent a probability of 30%, H atoms are shown with
arbitrary radii. At the bottom, aggregation to a strand-like structure via NeH/O hydrogen bridges is depicted; the hydrogen bridges are shown with dotted lines.

S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
7
Fig. 3. Molecular structure and atom labeling scheme of (syn-E)-4-[Ph2(HO)CeC6H4-]N]NeN(H)-Ph (25) (top). The ellipsoids represent a probability of 30%, H atoms are drawn
with arbitrary radii. At the bottom, aggregation to a strand-like structure via NeH/O hydrogen bridges is depicted; the hydrogen bridges are shown with dotted lines.
(N2eC7eN1eH1N1)2 cycle. Despite the fact that the aryl group with
the bromo substituent in ortho-position is oriented nearly
perpendicular to the diazaallyl fragment due to steric reasons, the
N1eC6 and N2eC8 bond lengths are very similar.
are very similar. Molecular structure and atom labeling scheme of 9
are depicted in Fig. 6, those of (syn-E)-4-BrC6H4eN]NeN (Me)e
C6H4-4-COOEt (11), containing two functionalized aryl groups, are
shown in Fig. 7. Molecular structure and atom labeling scheme of
(syn-E)-4-F-2-BrC6H3eN]NeN (Me)-Ph (14) are represented in
the ESI.
The molecular structures of (syn-E)-4-F-2-BrC6H3eN]NeN
(Me)-Ph (14), (syn-E)-4-Cl-2-BrC6H3eN]CHeN (Me)-Ph (20), and
(syn-E)-2,4-Br2C6H3eN]CHeN (Me)-Ph (21) are very similar.
Therefore, only the molecular structure and atom labeling scheme
of compound 20 is shown in Fig. 8, the structure of derivative 21 is
depicted in the ESI. Additional N-methyl substitution of the tri-
azenes leads to a slight widening of the NNN bond angles, whereas
N-methylation of the formamidines does not affect the central NCN
bond angle.
Selected structural parameters of triazenes and formamidines as
well as N-methylated congeners are summarized in Table 2. Due to
the fact that the number of asymmetrically N,N0-diaryl-substituted
derivatives is quite small, we included selected symmetric 1,3-
diaryltriazenes and -formamidines for comparison reasons. All
triazenes and formamidines crystallized as (syn-E)-isomers
regardless of the presence of methyl substituents at the for-
mamidine (5, 20, and 21) and triazene moieties (3 and 9e14).
Larger substituents like phenyl (7, benzamidinate) or tert-butyl
groups (8, pivalamidinate) at the carbon atom of the NCN fragment
enforce the (anti-E)- and (syn-Z)-isomeric forms, respectively. The
(syn-Z)-isomeric form of the pivalamidine leads to a significant
We included amidine derivatives with phenyl (benzamidines)
and tert-butyl groups (pivalamidines) in 2-position of the 1,3-
diazaallyl systems. This substitution pattern destabilizes the (syn-
E)-isomeric form and the (anti-E)- (E ¼ C-Ph) and (syn-Z)-config-
uration isomers (E ¼ C-tBu) [15] were observed. In these isomers
dimerization via NeH$$N hydrogen bridges is impossible. The
structural motif of (anti-E)-4-(EtOOC)-2-BrC6H3eN]C(Ph)eN(H)e
Ph (7) is shown in Fig. 5, molecular structure and atom labeling
scheme of (syn-Z)-4-(EtOOC)-2-BrC6H3eN]C (tBu)eN(H)ePh (8)
have been reported elsewhere [15]. The NeC bond lengths of the
1,3-diazaallyl moiety of 8 are comparable to compound 5 but the
NeCeN bond angle is significantly widened to nearly 127.8 (3)ꢀ (5:
122.3 (4)ꢀ). Due to steric reasons compound 7 forms N1A-
H1A$$$N20 hydrogen bridges, leading to a strand-like aggregate.
Contrary to this aggregation behavior, (syn-Z)-4-(EtOOC)-2-
BrC6H3eN]C
(tBu)eN(H)ePh
(8)
forms
intermolecular
N1eH1/O10 hydrogen bridges, also leading to the formation of a
one-dimensional aggregation polymer.
N-Methyl substitution prevents aggregation in the crystalline
state and molecular structures were observed in solution and the
solid. Despite the lack of hydrogen bridges as found in NH-tri-
azenes, the structural parameters of (syn-E)-4-(EtOOC)C6H3eN]
NeN (Me)-Ph (9) and (syn-E)-4-F-2-BrC6H3eN]NeN (Me)-Ph (14)

8
S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
Fig. 4. Molecular structure and atom labeling scheme of (syn-E)-4-(EtOOC)-2-BrC6H3eN]CHeN(H)-Ph (5) (top). The ellipsoids represent a probability of 30%, H atoms are drawn
with arbitrary radii. At the bottom, aggregation to dimeric units via NeH/N hydrogen bridges is depicted; the hydrogen bridges are shown with dotted lines.

S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
9
Fig. 5. Structural motif of (anti-E)-4-(EtOOC)-2-BrC6H3eN]C(Ph)eN(H)ePh (7) (top). The atoms are drawn with arbitrary radii, all C-bound H atoms are omitted for clarity reason.
At the bottom, aggregation to a strand structure is depicted; the hydrogen bridges are shown with dotted lines.

10
S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
Lewis bases are advantageous for the formation of intermolecular
hydrogen bridges, leading to strand-like aggregates.
In all these compounds the N ¼ E double bonds are roughly 4e9
pm shorter than the E-N single bonds. Nearly ideal delocalization
and very similar E ¼ N and E-N bonds have been observed for the
symmetric triazene 2-(MeOOC)C6H4eN]NeNHeC6H4-2-COOMe
[25] and the symmetric formamidine F5C6eN]CHeNHeC6F5
[29]. In general, intramolecular steric strain influences configura-
tion and conformation but the bond lengths are barely affected.
Thus, the NeCMe bond lengths to the methyl groups of approx. 146
pm are very similar for all compounds regardless of the nature of E
(N, CH). The N-Caryl distances to the aryl substituents vary between
139 and 143 p.m. In comparison to these values the NEN moieties
with bond lengths around 127e130 pm for E ¼ N double bonds and
around 133e136 pm for E-N single bonds show slight charge
delocalization leading to smaller differences than expected.
Palladium-mediated demethylating cyclohydroamination of
(syn-E)-4-(EtOOC)-2-BrC6H3eN]NeN (Me)-Ph (11) yields the
corresponding N-(4-ethoxycarbonyl)phenyl-benzotriazole (26,
Scheme 2, left, R’ ¼ 4-COOEt) [12c]. Its molecular structure and
numbering scheme are depicted in Fig. 9. The cyclization leads to a
more acute NNN bond angle of 109.2 (2)ꢀ and slightly elongated
N]N and NeN bonds with values of 129.9 (2) and 137.7 (2) pm,
respectively. Nevertheless, these parameters correspond quite well
to those of the unstrained triazenes, explaining the straightforward
palladium-catalyzed formation of the triazole heterocycle. In
contrast to this triazene cyclization, formamidines show no related
reactivity pattern.
Fig. 6. Molecular structure and atom labeling scheme of molecule A of (syn-E)-4-
(EtOOC)C6H3eN]CHeN (Me)-Ph (9). The ellipsoids represent a probability of 30%, H
atoms are drawn with arbitrary radii.
4. Conclusion
Asymmetric 1,3-diarylformamidines (E ¼ CH) and -triazenes
Fig. 7. Molecular structure and atom labeling scheme of (syn-E)-4-(EtOOC)-2-
(E
¼
N) as well as asymmetrically substituted 1,3-diaryl-3-
BrC6H3eN]CHeN (Me)-Ph (11). The ellipsoids represent
a probability of 30%, H
methylformamidines and -triazenes of the type Ar’-N ¼ E-N(R)-
Ar (R ¼ H, Me) represent important substance classes for the syn-
thesis of azaheterocycles of biochemical and medicinal importance.
Despite this application there are only very few structurally
authenticated asymmetric derivatives known. The syntheses of
these compounds is straightforward with moderate to excellent
yields, making these compounds very attractive as substrates for
e.g. Buchwald-Hartwig-type amination reactions and demethylat-
ing cycloamination for the preparation of substituted benzo-
triazoles. However, this protocol via diazotization of substituted
aminobenzenes and reaction with arylamines is not suitable for the
preparation of doubly triazenyl-functionalized benzene.
atoms are drawn with arbitrary radii.
In the crystalline state, the studied formamidines and triazenes
favor the (syn-E) configuration at the central NEN-fragments. Bulky
groups at the central carbon atom (E ¼ CH, formamidines) and/or at
the nitrogen base introduce severe intramolecular strain which
destabilizes this configuration and other isomers are preferred. In
agreement with the chemical NMR shifts, the N-bound methyl
groups show very similar bonding parameters regardless of the
substitution pattern of the aryl groups and of the nature of E
(amidines, triazenes).
Fig. 8. Molecular structure and atom labeling scheme of (syn-E)-4-Cl-2-BrC6H3eN]
CHeN (Me)-Ph (20). The ellipsoids represent a probability of 30%, H atoms are drawn
with arbitrary radii.
widening of the NCN bond angle due to intramolecular steric
repulsion between the aryl groups. Very bulky groups at the N-
atom can also enforce different isomers as has been shown for 1,3-
diaryl-3-triphenylmethyl-formamidine [31].
In the solid state, 1,3-diarylformamidines dimerize via two
nearly colinear NeH/N hydrogen bridges. This kind of aggregation
is impossible for 1,3-diaryltriazenes due to a significantly narrower
NEN bond angle of approx. 113ꢀ whereas the formamidines exhibit
NCN values of around 122ꢀ. Due to this structural difference the
investigated triazenes favor intermolecular NeH/O hydrogen
bridges to oxygen bases, leading to strand-like aggregates (Table 3).
Due to the fact that bulky groups enforce another isomeric form
than (syn-E)-configuration, strand-like structures are formed via
intermolecular NeH/N or NeH/O hydrogen bridges. These weak
hydrogen bridges are highly asymmetric and quasi-linear with DHA
In
1,3-diarylformamidines
and
1,3-diaryl-3-
methylformamidines the NCN bond angles are slightly larger than
120ꢀ, a typical value for sp2 hybridized carbon atoms. This value
allows a straightforward dimerization via two collinear NeH/N
hydrogen bridges. The contraction of the NEN bond angle to approx.
113ꢀ as found in the 1,3-diaryltriazenes and 1,3-diaryl-3-
methyltriazenes destabilizes this kind of aggregation and other

S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
11
Table 2
Comparison of structural parameters (bond lengths (pm) and angles (deg.)) of selected 1,3-diaryltriazenes (R ¼ H) and 1,3-diaryl-3-methyltriazenes (R ¼ Me) of the type
AreN]NeN(R)-R0 as well as of 1,3-diarylamidines (R ¼ H) and 1,3-diaryl-3-methylamidines (R ¼ Me).a
Isomer
E
R
Ar
R0
N-CAr
N ¼ E
E-N
N-CR’
NeCMe
NEN
Ref.
3
Syn-E
Syn-E
Syn-E
Syn-E
Syn-E
Syn-E
Syn-E
Syn-E
Syn-E
Syn-E
Syn-E
Syn-E
Syn-E
Anti-E
Syn-Z
Syn-E
Syn-E
Syn-E
Syn-E
Syn-E
Anti-E
Syn-E
Syn-E
Syn-E
N
N
N
N
N
N
CH
CH
CH
CH
CH
CH
CH
C-Ph
C-tBu
N
N
N
N
CH
CH
CH
CH
CH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
Me
Me
Me
Alkylb
CPh3
Me
Me
Me
C6H4-2-COOMe
C6H3-2,4-Br2
C6H4-2-Cl
C6H4-4-Cl
CH2-Ph
C6H4eC(Ph)2OH
C6H4-3-Br
C6H4-4-F
C6H4-2-COOMe
C6H3-2,4-Br2
C6H4-2-OEt
C6H4-2-OMe
C6H4-4-COOEt
Ph
C6H4-3-Br
C6H4-4-F
C6H3-2,6-F2
C6H2-2,3,5-F3
C6H2-3,4,5-F3
C6F5
141.8 (3)
142.2 (7)
141.7 (3)
141.5 (2)
146.9 (2)
142.9 (2)
141.0 (8)
141.8 (2)
140.9
129.6 (3)
126.7 (7)
126.3 (3)
125.9 (2)
125.9 (2)
128.3 (2)
129.6 (8)
128.3 (3)
128.6
129.3 (3)
133.2 (7)
132.8 (3)
133.0 (2)
134.9 (2)
132.1 (2)
135.6 (8)
134.2 (2)
134.4
140.9 (3)
138.8 (8)
139.7 (3)
139.0 (2)
139.0 (2)
141.1 (2)
139.4 (8)
141.4 (2)
140.6
e
113.6 (2)
111.6 (5)
112.9 (2)
111.4 (1)
111.5 (1)
113.9 (1)
122.4 (6)
122.2 (2)
120.5
[25]
[26]
[27]
[28]
Here
Here
[24]
[24]
[29]
[29]
[29]
[29]
Here
Here
[15]
Here
Here
[30]
Here
[31]
[32]
[33]
Here
Here
e
e
e
e
25
5
e
e
e
C6H3-2,6-F2
e
C6H2-2,3,5-F3
C6H2-3,4,5-F3
C6F5
C6H3-2-Br-4-COOEt
C6H3-2-Br-4-COOEt
C6H3-2-Br-4-COOEt
C6H4-4-COOEt
C6H4-4-Br
C6H3-2-Br-4-COOEt
C6H3-2-Br-4-F
C6H3-2,6-iPr2
C6H3-2,6-Me2
C6H2-2,4,6-Me3
C6H3-2-Br-4-Cl
C6H3-2,4-Br2
141.0
142.2
141.2
129.8
128.0
131.1
133.1
135.1
133.1
139.6
140.1
140.1
e
122.2
121.8
121.0
e
e
Ph
Ph
Ph
Ph
141.1 (5)
141.9 (13)
138.1 (4)
141.8 (2)
142.8 (2)
141.1 (4)
142.2 (3)
141.8 (4)
140.4
128.5 (5)
130.5 (12)
128.0 (4)
127.5 (2)
127.0 (2)
128.1 (3)
127.3 (2)
128.2 (4)
128.1
135.5 (5)
136.3 (13)
136.4 (4)
133.1 (2)
134.0 (2)
132.5 (3)
134.4 (2)
136.1 (4)
136.8
135.5 (5)
139.7 (13)
142.2 (4)
142.2 (2)
141.6 (2)
141.9 (3)
141.8 (2)
145.1 (4)
142.3
e
122.3 (4)
121.4 (9)
127.9 (3)
113.9 (1)
114.5 (2)
113.3 (2)
113.3 (2)
122.3 (3)
122.3
7
8
9
11
12
14
20
e
e
145.9 (2)
145.8 (2)
146.6 (4)
146.2 (3)
146.5 (4)
150.9
146.8 (3)
146.1 (4)
146.4 (3)
C6H4-4-COOEt
Ph
Ph
C6H3-2,6-iPr2
C6H3-2,6-Me2
Ph
143.0 (3)
139.7 (4)
139.6 (3)
126.8 (3)
128.1 (4)
128.6 (3)
136.3 (3)
136.0 (6)
135.3 (3)
141.7 (3)
142.1 (4)
141.9 (3)
123.5 (3)
121.0 (3)
121.1 (2)
Ph
Ph
21
a
Data without estimated standard deviations were taken from the CSD data base.
Alkyl (CH2)4eOeC6H2-3,4,5-F3.
b
Author contributions section
Silvio Preusser: Synthesis of triazenes and formamidines, data
€
collection. Paul R. W. Schonherr: triazene syntheses, data collec-
€
tion. Helmar Gorls: X-ray structure elucidation. Sven Krieck: data
analysis, manuscript conception. Wolfgang Imhof: study concep-
tion, manuscript editing. Matthias Westerhausen: Supervision of
scientific work, study conception, manuscript draft.
Declaration of interest
Fig. 9. Molecular structure and atom labeling scheme of N-(4-ethoxycarbonyl)phenyl-
benzotriazole (26). The ellipsoids represent a probability of 30%, H atoms are drawn
with arbitrary radii.
Amidines and triazenes are an important substance class to
stabilize low valent metal complexes or to protect highly reactive
bonds. Nevertheless, the protonated formes - amidines and tri-
azenes - attracted significantly less attention. We studied these
compounds in more detail because they show a significant differ-
ence in reactivity with respect to cyclization and formation of
benzotriazole from triazenes whereas the formamidines do not
undergo cyclization under similar reaction conditions.
bond angles larger than 160ꢀ. In compound 7 the very large N1A/
N1B/N2B’/N2A0 distances are characteristic for very weak
hydrogen bridges caused by steric hindrance. The N1/N2’ distance
in unstrained derivative 5 is significantly smaller verifying a
stronger hydrogen bridge-supported network.
This journal is prune to present and discuss structural features,
similarities as well as significant differences.
Table 3
Intermolecular hydrogen bridges of the type D-H$$$A (distances (pm) and DHA bond angles (deg.)) leading to aggregation of 1,3-diaryltriazenes and 1,3-diarylamidines.
Comp.
D-H$$$A
Aggregate
D-H
H$$$A
D$$$A
DHA
Ref.
3
5
7
N3eH3N3/O20
Strand
Cyclic dimer
Strand
Strand
Strand
89 (2)
88
88
88
74 (4)
86 (5)
200 (2)
208
247
254
218 (4)
210 (5)
285.7 (2)
295.2 (5)
335 (1)
341 (1)
290.8 (4)
291.2 (5)
160 (2)
170
173
172
167 (4)
156 (5)
N1eH1N1/N20
N1A-H1AN1A$$$N2B0
N1BeH1BN1B/N2A0
N1eH1N1/O10
8
24
[15]
N3eH3N3/O10
Strand
Buchwald-Hartwig-type amination reactions and demethylating cycloamination of triazenes lead to benzotriazoles with a five-membered N3C2 cycle. A comparable reactivity
has not been observed for related isoelectronic formamidines.

12
S. Preusser et al. / Journal of Molecular Structure 1205 (2020) 127622
aryltriazenes in organic synthesis via C-N/N-N bond cleavage, Curr. Org. Chem.
Acknowledgement
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also acknowledge the valuable support of the NMR (www.nmr.uni-
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