1,3,6,8-tetraazapyrenes: Synthesis, solid-state structures, and properties as redox-active materials

Article abstract of DOI:10.1021/jo300894p

A series of new tetraazapyrene (TAPy) derivatives has been synthesized by reducing 1,4,5,8-tetranitronaphthalene to its corresponding tin salt (I) and reacting it with perfluorinated alkyl or aryl anhydrides. The resulting 2,7-disubstituted TAPy molecules and the known parent compound 1,3,6,8-tetraazapyrene (II) have been further derivatized by core chlorination and bromination. The brominated compounds served as starting materials for Suzuki cross-coupling reactions with electron-poor arylboronic acids. Single-crystal X-ray analyses established polymorphism for some TAPy compounds. The ground-state geometries of all new TAPy derivatives were modeled with DFT methods [B3PW91/6-31 g(d,p) and B3PW91/6-311+g(d,p)], especially focusing on the energies of the lowest unoccupied molecular orbital (LUMO) and the electron affinities (EA) of the molecules. The results of the calculations were confirmed experimentally by cyclic voltammetry to evaluate the substitution effects at the 2 and 7 position and the core positions, respectively, and gave LUMO energy levels that range from -3.57 to -4.14 eV. Fabrication of organic field-effect transistors (OFETs) with several of these tetraazapyrenes established their potential as organic n-type semiconductors.

Full text of DOI:10.1021/jo300894p

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Article
1,3,6,8-Tetraazapyrenes: Synthesis, Solid State
Structures and Properties as Redox Active Materials
Sonja Geib, Susanne C. Martens, Ute Zschieschang, Florian
Lombeck, Hubert Wadepohl, Hagen Klauk, and Lutz Hans Gade
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo300894p • Publication Date (Web): 25 Jun 2012
Downloaded from http://pubs.acs.org on July 5, 2012
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1,3,6,8-Tetraazapyrenes: Synthesis, Solid State Structures and Properties as
Redox Active Materials
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Sonja Geib†, Susanne C. Martens†a, Ute Zschieschang‡, Florian Lombeck†, Hubert Wadepohl†,
Hagen Klauk‡ and Lutz H. Gade†*
†Anorganisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270,
D-69120 Heidelberg, Germany
‡Max Planck Institute for Solid State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany
lutz.gade@uni-hd.de
Table of Contents Graphic
a Current address: Department of Physics, University of Basel, Klingelbergstr. 82, CH-4056 Basel, Switzerland
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Abstract
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A series of new tetraazapyrene (TAPy) derivatives has been synthesized by reducing 1,4,5,8-
tetranitronaphthalene to its corresponding tin salt (I) and reacting it with perfluorinated alkyl or
aryl anhydrides. The resulting 2,7-disubstituted TAPy molecules and the known parent
compound 1,3,6,8-tetraazapyrene (II) have been further derivatized by core chlorination and
bromination. The brominated compounds served as starting materials for Suzuki cross-coupling
reactions with electron poor arylboronic acids. Single crystal X-ray analyses established
polymorphism for some TAPy compounds. The ground state geometries of all new TAPy
derivatives were modeled with DFT methods [B3PW91/6-31g(d,p) and B3PW91/6-311+g(d,p)],
especially focusing on the energies of the lowest unoccupied molecular orbital (LUMO) and the
electron affinities (EA) of the molecules. The results of the calculations were confirmed
experimentally by cyclic voltammetry to evaluate the substitution effects at the 2 and 7 position
and the core positions, respectively, and gave LUMO energy levels that range from -3.57 to -4.14
eV. Fabrication of organic field-effect transistors (OFETs) with several of these tetraazapyrenes
established their potential as organic n-type semiconductors.
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Introduction
During the past decade, there has been growing interest in developing new organic, π-conjugated
materials1 for application in electronic and optoelectronic devices, such as light-emitting diodes
(LEDs), field-effect transistors (FETs) and solar cells.2,3 We recently studied a new class of
polyheterocyclic aromatics, 1,3,8,10-tetraazaperopyrenes (TAPPs)4,5 and established their use
either as functional dyes or n-type semiconductors in the field of organic electronics.6 Inspired by
the interesting results obtained with this class of azaaromatic compounds we decided to
investigate a similar class of polycyclic aromatics, 1,3,6,8-tetraazapyrenes (TAPys), which can
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be seen as "contracted" TAPPs, containing one naphthalene core unit less. By reducing the size
of the aromatic core, not only the solubility of the molecules in organic solvents but also the
options of derivatization were thought to be enhanced. By variation of the substituents at the 2
and 7 position as well as at the aromatic core the electronic properties were expected to be
specifically influenced. For example, introduction of perfluorinated substituents as well as
halogens are known to lower the LUMO energy levels significantly, making such tetraazapyrene
derivatives interesting candidates for n-channel semiconducting materials that might be
processable from solution.
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The parent compound, 1,3,6,8-tetraazapyrene, was first described in 1927 by Dimroth and
Roos, but initially its physical properties remained unexplored and no derivatives were prepared.7
It was not until 1964 that the synthesis of two derivatives, 2,7-dimethyl- and 2,7-diphenyl-
1,4,6,8-tetraazapyrene, was published and the EPR spectra of the corresponding radical cation
and anion were studied in detail.8,9 Three patents describe the synthesis of several 2,7-dialkyl
substituted as well as halogenated 1,3,6,8-tetraazapyrenes which were claimed to possess
pharmaceutical activity against protozoan infections.10 Pozharskii and coworkers synthesized 2,7-
dimethyl-1,3,6,8-tetraazapyrene starting from 6,7-dinitro-2-methylperimidine11 and Aksenov et
al. recently published an alternative synthetic way to obtain inter alia 2,7-dimethyl- and 2,7-
diphenyl tetraazapyrenes from pyrimidine ketones.12 Apart from these few examples,
tetraazapyrenes have been unexplored to date.
In this work, we report a general method for the synthesis of 2,7-perfluoroalkyl-
substituted tetraazapyrenes as well as several methods for the core-substitution of different
derivatives, including bromination and chlorination procedures, which then again give access to
versatile building blocks for further conversions of these compounds in Suzuki cross-coupling
reactions. Furthermore, we present a detailed study of the redox properties of the TAPy
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derivatives and a first investigation into the performance of some TAPys as organic
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semiconducting materials in n-channel thin-film transistors (TFTs).
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Results and Discussion
Synthesis
The parent compound, 1,3,6,8-tetraazapyrene (II), which was subsequently used as starting
material, was synthesized according to literature procedure by reducing 1,4,5,8-
tetranitronaphthalene13 with tin dust in fuming hydrochloric acid and subsequent cyclization with
formic acid.7,14 Core substitution of II could be achieved by classical chlorination and
bromination methods. For the introduction of chlorine substituents, II was dissolved in
chlorosulfonic acid and reacted with chlorine gas in the presence of catalytic amounts of iodine to
yield compound 1a (Scheme 1). The corresponding brominated TAPy (1b) was synthesized
accordingly by direct bromination of II in concentrated sulfuric acid (Scheme 1).
SCHEME 1. Synthesis of 1,3,6,8-tetraazapyrene (II) and its tetrahalogenated derivatives
1a and 1b.
N
N
Cl
Cl
Cl
Cl
Cl2
I2 [cat.]
HClSO3
80°C, 3h
N
N
2+
H
N
N
N
N
H2N
H2N
NH2
NH2
1a (78%)
HCOOH
(SnCl6)2-
Br2
H
I2 [cat.]
H2SO4
80°, 3h
N
N
N
I
II
Br
Br
Br
Br
N
1b (87%)
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SCHEME 2.
Synthesis of the a) 2,7-disubstituted 1,3,6,8-tetraazapyrenes 2a-c and 3 and
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8
b) their core-brominated derivatives 4a-c and 5 and the core-chlorinated derivative 6.
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For the introduction of substituents in the positions 2 and 7 a synthetic method, based on
synthetic procedures developed for known TAPy9,10 and TAPP6 derivatives, was employed. The
synthesis
of
the
2,7-bisperfluoroalkyl-1,3,6,8-tetraazapyrenes
(2a-c)
and
2,7-
bispentafluorophenyl-1,3,6,8-tetraazapyrene (3) was achieved by reducing 1,4,5,8-
tetranitronaphthalene13 as described above to the corresponding ammonium/tin(II) salt14 (I),
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followed by the addition of an excess of the corresponding perfluoroalkyl- or perfluorophenyl
carboxanhydrides in refluxing THF in the presence of catalytic amounts of triethylamine to
generate the free tetraamino species (Scheme 2a). Column chromatography and recrystallization
from methanol gave the desired products 2a-c and 3 in moderate yields of 36 – 63 % as light
yellow solids (Scheme 2a). The relatively low yields are due to as yet incompletely understood
side reactions which give rise to several minor components.
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In contrast to the case of the parent compound II, the core-halogenation of the 2,7-
bis(perfluoralkylated) TAPys was not achieved as easily because of the electron withdrawing
nature of the substituents. In order to obtain core-brominated 2,7-bis(perfluoralkyl)-substituted
TAPy derivatives, 2a – c were reacted with dibromoisocyanuric acid (DBI)15 in concentrated
sulfuric acid (Scheme 2b). Remarkably, only the twofold brominated TAPy derivatives 4a – c
(Scheme 2b) were isolated and no traces of higher substituted reaction products were observed
even though a large excess (8 eq) of DBI was employed. Tetraazapyrenes 4a – c were obtained as
colourless to light yellow solids after extraction with dichloromethane and recrystallization from
methanol. For 2,7-bisperfluorophenyl-TAPy 3 the same reaction conditions only gave mixtures
of two- to fourfold brominated derivatives which could not be separated. However, in this
particular case bromination with bromine in concentrated sulfuric acid could be applied
selectively giving the tetrabrominated product 5 (Scheme 2b). Apparently, the deactivation of the
aromatic system caused by the perfluorinated phenyl substituent does not inhibit the classical
bromination method. The yield, however, is significantly lower (18%) than for 1b (87%).
Core-chlorination of the 2,7-disubstituted TAPys turned out to be a greater challenge and
was eventually only successfully carried out for derivative 2a (Scheme 2b). Whilst the fourfold
chlorination of 2,9-perfluoroalkyl-substituted tetraazaperopyrene (TAPP) derivatives is
selectively achieved with dichloroisocyanuric acid (DCI)16, as we have presented recently,6 the
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conversion of 2a with DCI not only results in a mixture of the one-, two-, three- and fourfold
substituted compound with low yields but also requires more drastic conditions and significantly
longer reaction times. However, the fourfold substituted derivative 6 (Scheme 2b) could be
isolated as the main product after reacting 2a with ten equivalents of DCI in concentrated sulfuric
acid at 100 °C for ten days. Even though the yield is with 13% not significantly lower than for
compound 5 (18%), it is important to note that the formation of 6 as the main product could only
be observed when the reaction was carried out on a small scale.
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SCHEME 3. Suzuki Cross-Coupling of the core-brominated TAPy derivatives 1b and 4a
with arylboronic acids.
Further derivatization was achieved by Suzuki cross-coupling. Due to the fact that bromides are
known to be more reactive in these transformations than their chlorinated analogs17 and since
they were easier to obtain, compounds 1b and 4a were chosen as starting materials for the
following conversions. In order to obtain molecules with low LUMO energy levels, two boronic
acids with electron withdrawing groups in their para-position were chosen for the cross-coupling
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reaction: p-cyanophenylboronic acid and p-trifluoromethylphenylboronic acid. Using
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6
Pd(dppf)Cl2 as catalyst gave the desired cross-coupling target molecules 7a,b and 8a,b (Scheme
3).
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Crystal Structures
Since tetraazapyrenes (TAPys) were thought to potentially display semiconducting properties, it
was of interest to study their packing behavior in the solid state which strongly influences charge-
carrier mobilities and stability and thus the performance of organic semiconducting devices.
Recent literature discusses molecular packing motifs that are favorable for achieving high charge-
carrier mobilities.2k,18,19 Among these motifs, especially a 2-D-π-stacking or at least a 1-D-π-
stacking should be present in the solid state in order to enable an efficient charge transport. On
the other hand, herringbone patterns (with or without π-overlap) are commonly associated with
low or no conductivity.2k,18,19
The crystal and molecular structures of several TAPy derivatives have been determined.
The molecular structures are characterized by the planar tetraazapyrene core with almost identical
bond lengths and angles (see Supporting Information) with either the two perfluoroalkyl
substituents pointing in opposite directions above and below (2b-c, 4b) or the phenyl substituents
being rotated out of the plane of the polycyclic molecular core to reduce the steric repulsion (3,
5), respectively.
More importantly, a variety of motifs have been observed for the packing pattern of TAPy
molecules (Table 1). Apart from the obvious influence of substitution, the organic solvent from
which the crystals were grown also has an impact, as has been shown by the crystallization of
different polymorphic solvent-free forms of 2a and 2c and by the formation of crystal solvates,
such as 3 ·THF and 5 ·2 toluene (Table 1).
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The packing pattern in the monoclinic modification of 2a is dominated by two separate
layers of π-π-stacks with the two stacks being rotated by 86° with respect to each other. The
interplanar distance between the polycyclic aromatic cores within the stacks was found to be 3.41
Å. In the triclinic crystalline form of 2a (Figure 1, right), the molecules arrange in flat layers
which are themselves slip-stacked in the perpendicular direction with an interplanar distance of
3.36 Å, a packing pattern which is very similar to graphite (3.37 Å).20 In both modifications,
weak intermolecular hydrogen bonds are found between the nitrogen and the aromatic hydrogen
atoms of the neighboring molecules (2.53 – 2.58 Å).
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Figure 1. Crystal structures of two polymorphs of compound 2a. (a) Molecular packing in the
monoclinic crystals obtained from methanol or chloroform; (b) Molecular packing in the triclinic
crystals obtained from pentane/ethyl acetate (view along the b-axis). Thermal ellipsoids were
drawn at the 50% probability level and weak hydrogen interactions are indicated .
A similar type of polymorphism was observed for compound 2c: single crystals suitable
for X-ray diffraction were obtained from methanol and a mixture of hexane and ethyl acetate. In
both cases, monoclinic single crystals (space group P21/n) were found, but with significantly
different cell data. In the crystals obtained from hexane/ethyl acetate, the crystal packing pattern
displays a herringbone arrangement (Figure 2a) with interplanar distances of 3.48 Å and 3.50 Å,
respectively. On the other hand, in the polymorph obtained from methanol the molecules are
layered with the tetraazapyrene cores essentially being coplanar (Figure 2b). Two types of
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slipped π-stacks are thus formed (π-π-stacking distance 3.46 Å) with the same lateral orientation
of the molecules along the skew stack axis within each stack and the two types of stack rotated by
about 90 ° relative to one another.
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FIGURE 2. Crystal structures of two polymorphs of compound 2c. (a): Molecular packing in the
monoclinic crystals obtained from hexane/ethyl acetate; (b): Molecular packing of the polymorph
obtained from methanol.
FIGURE 3. Crystal packing pattern of compound 3: (a) showing the twisted arrangement of the
aryl substituent and weak hydrogen interactions between neighboring molecules within each
layer and (b) the pairwise arrangement of layers and the π-π interactions between the
perfluorinated phenyl rings in neighboring “sandwich-type layers” (embedded solvent molecules
have been omitted for clarity).
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The pentafluorophenyl-substituted compound 3 crystallized from THF as a solvate in the
form of monoclinic crystals (space group C21/c). Not surprisingly, the molecular packing pattern
observed in this case differs significantly from the ones discussed above (Figure 3a). The
structure appears to be dominated by pairwise π-π interactions between the perfluorinated phenyl
rings (interplanar distance 3.56 Å) which are twisted out of the plane of the tetraazapyrene core
by 52° in opposite directions.
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The pyrene cores of neighboring molecules are thus so strongly shifted with respect to one
another that hardly any π-overlap can occur. In this rather open network channels are formed
along the crystal c axis (Figure 3b), which are filled with strongly disordered solvent molecules
(see experimental section).
FIGURE 4. Intermolecular packing patterns of: (a) 4a brickwork arrangement, (b) 4b ladder-
type arrangement, (c) 5 typical herringbone-like structure and (d) 6 ladder-type arrangement
(embedded solvent molecules have been omitted for clarity).
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In the solid state structure of the dibrominated tetraazapyrene 4a the molecules stack
along the c axis with an interplanar distance of 3.57 Å (Figure 4a). Within the stacks the
molecules are significantly shifted giving rise to a brickwork arrangement which may be
classified as a two-dimensional stack. Molecules of 4b build parallel stacks along the shortest
axis with an interplanar distance of 3.36 Å (Figure 4b).
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The tetrabromo derivative 5 crystallized as a toluene solvate in the form of monoclinic
crystals (space group P21/c). It displays a typical herringbone-like structure with an enhanced
interplanar distance of 4.53 Å (Figure 4c). In this case, the perfluorinated phenyl substituents are
with 74° even further twisted out of the plane of the aromatic core than in the solid state structure
of 3 (52°) and no π-π interactions between adjacent pyrene molecules can be observed.
Finally, in the crystal structure of the fourfold core-chlorinated tetraazapyrene 6, the
molecules build parallel stacks along the shortest cell axis with an interplanar distance of 3.41 Å
(Figure 4d).
TABLE 1. Selected details of crystal structures.
interplanar
compound
crystallization solvent
space group
distance [Å]
3.41
methanol or chloroform
pentane/ethyl acetate
C2/c
2a
2a
3.36
P
methanol
methanol
3.48
3.48/3.50
3.48
2b
2c
2c
3a
P
P21/n
P21/n
C2/c
hexane/ethyl acetate
THF
3.36
THF
3.57
4a
P
chloroform
toluene
3.36
4.53
4b
5a
P
P21/c
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chloroform
P21/n
3.41
6
a
compound crystallized as a solvate, therefore solvent molecules are embedded in the solid state
structure
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To date, polymorphism has been observed for compounds 2a and 2c, but it is very likely
that other TAPy-derivatives display polymorphism as well. Furthermore, different interplanar
distances ranging from 3.36 Å to 4.53 Å and very diverging arrangements of the molecules in the
solid state, ranging from herringbone structures to brickwork arrangements, have been observed.
Apart from the way in which the molecular shapes influence the crystal structures, the conditions
of crystallization, in particular the solvent, play an important role. When processing thin organic
layers from the liquid phase, this aspect therefore needs to be considered.
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Theoretical Modeling and Electrochemistry
The redox properties of organic compounds, which are relevant for potential applications as
semiconductors, are primarily determined by their frontier orbital energies.21 The relative HOMO
and LUMO energetic positions of an organic material indicate which type of charge carriers are
expected to be predominant and furthermore allow a prediction of the stability of this material in
typical organic semiconductors. In the case of organic n-type materials, in which the charge
transport occurs predominantly by hopping through the LUMO energy levels, low-lying LUMOs
are thought to be required for an efficient electron injection into the semiconductor.3d,21
Furthermore, the electron affinities (EA) of the organic material should be within the range of
2.8 eV to 4.0 eV to provide the stability of the radical anion species against ambient oxidants,
such as water and oxygen.21,22,23 The size of the molecular π-system is one factor that determines
the electron affinity: The smaller a π-system is, the less space is available for an injected electron
to delocalize along that system, implying that smaller π-systems (as in naphthalene or pyrene
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derivatives) generally have lower electron affinities than larger π-systems (as in perylenes or
oligoacenes).
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The electronic structure of the tetraazapyrene derivatives reported in this work were
modelled at the B3PW91/6-31g(d,p) level of DFT which has been validated for this type of
systems in previous studies.4,5 Molecular geometries were optimized and the energy minima
verified by frequency analysis. Electron affinities were determined by using a basis set that
includes diffuse functions (6-311+g(d,p)). An energy optimization of the corresponding anionic
species of each compound was carried out and subtraction of the minimum energy of the neutral
form gives the electron affinity.24
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Bond lengths and angles obtained from the calculations were generally in good agreement
with those found in the crystal structure analyses (see Supporting Information). The Kohn-Sham
HOMO and LUMO of compound 2a and 6 displayed in Figure 5 represent the frontier orbitals of
the TAPy derivatives investigated in this work. The longitudinal axis of the molecule, connecting
the carbon atoms at position 2 and 7, lies in a nodal plane of the frontier orbitals. As a
consequence, the substituents in these positions only have a small effect on the frontier orbital
energies.
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FIGURE 5. Kohn-Sham HOMO (left) and LUMO (right) of 2a (a) and its core-chlorinated
equivalent 6 (b).
Table 2 summarizes the calculated HOMO and LUMO energies and the electron affinities
of TAPy derivatives 2a – c and 3 which have no core substituents. Compounds 2a-c display
HOMO energies of about -7.70 eV and LUMO energies of about -3.70 eV. It is not surprising,
however, that with increasing length of the perfluorinated alkyl chain the energies are not
significantly affected because the substituents at the positions 2 and 7 lie on the nodal plane
referred to above. Insertion of pentafluorophenyl substituents to the pyrene core (3) also results in
lowering of the HOMO and LUMO energy levels compared to the parent compound II but the
effect is less prominent than what was observed for the perfluoroalkyl groups (Table 2). This
observation is in accordance with the observed reactivity of derivatives 2a-c and 3 in the
bromination procedures discussed above.
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TABLE 2. Calculated HOMO and LUMO energies and EAs and measured LUMO energies for
the 2,7-substituted TAPy derivatives 2a-c and 3 (compared to the computed values of the parent
compound II).
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Compound
HOMO,
[eV]a
-7.10
LUMO DFT,
[eV]a
LUMO CV,
[eV]
EA,
[eV]b
1.92
2.65
2.72
2.77
2.55
-3.11
-3.38
II
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2b
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3
-7.69
-3.69
-3.75
-7.72
-3.72
-3.77
-7.74
-3.73
-3.78
-7.09
-3.40
-3.61
a determined with B3PW91/6-31g(d,p)
b determined with B3PW91/6-311+g(d,p)
While bromination of TAPy 3 can still be accomplished with elemental bromine in the
presence of a catalyst, the deactivation of the tetraazpyrene which is caused by introduction of the
stronger electron-withdrawing perfluoroalkyl substituents is too strong and therefore bromination
can only be achieved with the use of DBI. As expected, in the same way as the LUMO energies
were lowered the calculated electron affinities of tetraazapyrenes 3 (2.55 eV) and 2a – c (2.65 eV
- 2.77 eV) increased (Table 2).
In order to compare the results obtained from theoretical modeling with experimentally
accessible data, cyclic voltammograms were recorded for all tetraazapyrenes except for
compound 1b, which was found to be insufficiently soluble in THF. It is well established that the
measured reduction potentials may be correlated with the LUMO energies.25 All CVs display two
reversible reduction half waves that correspond to the sequential formation of the mono- and
dianions (Figure 6).
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FIGURE 6. Cyclic voltammograms of the parent compound II, the 2,7-disubstituted derivative
2a and the core-substituted tetraazapyrenes 4a and 8a recorded in THF (sweep rate 50 mVs-1;
supporting electrolyte Bu4NPF6).
Using ferrocene as an internal standard confirmed that all redox waves corresponded to
one-electron reductions and furthermore allowed the estimation of the LUMO energies for all
compounds. The cyclic voltammograms of the parent compound II (dotted line) and compound
2a (grey line) displayed in Figure 6 nicely visualize the effect of the electron-withdrawing
trifluoromethyl group by a significant shift of the two reversible redox waves of 2a to higher
potentials.
TABLE 3.
Calculated HOMO and LUMO energies and EAs and measured LUMO energies
for the core-substituted TAPy derivatives (compared to the values of the parent compound II).
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Compound
HOMO,
[eV]a
-7.10
-7.49
-7.29
-7.73
-7.76
-7.76
-7.23
-7.90
-7.19
-6.96
-7.44
-7.35
LUMO DFT,
[eV]a
LUMO CV,
EA,
[eV]b
1.92
2.56
2.60
3.01
3.07
3.12
3.07
3.71
3.09
2.79
3.25
3.09
[eV]
-3.38
-3.81
-3.11
II
1a
1b
4a
4b
4c
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-3.86
c
-3.62
-
-3.96
-4.00
-4.00
-4.01
-3.98
-4.14
-3.65
-3.57
-3.91
-3.89
-3.98
-3.99
-3.80
-4.16
6
-3.85
7a
7b
8a
8b
-3.50
-4.04
-3.84
a determined with B3PW91/6-31g(d,p)
b determined with B3PW91/6-311+g(d,p)
c solubility of compound 1b in THF was not sufficient for a CV measurement
The LUMO energy levels obtained from the CV data of 2a-c are slightly lower than the
values calculated at the level of DFT. However, for 3, the difference between theoretically and
experimentally determined LUMO energies is greater (0.21 eV), an observation which has been
previously made in our group for aryl-substituted polycyclic aromatics.5 As indicated above, the
introduction of substituents at the aromatic core is expected to influence the electronic properties
of the TAPy derivatives significantly which is evident when comparing the data for the fourfold
core-chlorinated and brominated species 1a and 1b to the parent compound II (Table 3). The
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LUMO energies of 1a and 1b are decreased by > 0.50 eV and the electron affinities of,
respectively, 2.56 eV and 2.60 eV are significantly higher than that of the parent compound II.
As a next step, the introduction of bromine and chlorine atoms at the pyrene core
combined with perfluorinated alkyl chains in position 2 and 7 was studied (compounds 4a-c, 5
and 6) as well as the properties of the Suzuki cross-coupled products 7a, b and 8a, b. Like the
introduction of perfluoroalkyl substituents to the parent compound in positions 2 and 7 (Table 2)
the subsequent addition of further electron-withdrawing groups, such as bromine (4a-c) or
chlorine (6), to the pyrene core led to an even further decrease of LUMO energy levels and an
increase of electron affinities (Table 3). Also in this case, the cyclic voltammograms of 2a (grey
line) and 4a (dashed line) displayed in Figure 6 illustrate the effect of core-substitution.
Substitution of the bromine atoms of 1b and 4a by p-cyanophenyl groups decreases the
theoretically determined LUMO level, the effect being significant only for the Suzuki cross-
coupled product 7a. According to the experimentally obtained values on the other hand, the
LUMO energy level of 8a is slightly higher than that of 4a but still in the same range and the
LUMO energy level of 7a is actually considerably higher than expected (Table 3). However, in
both cases the EAs are increased significantly from 2.60 eV (1b) to 3.09 eV (7a) and 3.01 eV
(4a) to 3.25 eV (8a), respectively. The effect of the p-trifluorophenyl substituent on the electronic
properties of the TAPys is similar but less prominent (Table 3).
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Electron Mobilities in Thin Layers of TAPy Derivatives
The redoxchemical properties of all TAPy derivatives suggest that these molecules might have
semiconducting properties. To this end, thin-film transistors (TFTs) were fabricated by vapour
deposition of the organic material in high vacuum. Source and drain metal contacts were
subsequently installed on the organic layer by vacuum deposition through a shadow mask, giving
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rise to a bottom-gate, top-contact TFT architecture. The TFTs were measured in ambient air with
a humidity of about 50% under yellow laboratory light. All tested substances displayed either n-
type or no semiconducting properties, whilst ambipolar or p-type behavior was not observed,
confirming the notion that the TAPys presented in this work are potential electron conducting
materials.
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Neither the fourfold core-chlorinated and brominated TAPy derivatives 1a and 1b nor the
Suzuki cross-coupling products 7a,b and 8a displayed a measurable field effect, despite the fact
that their calculated LUMO energies and EAs appear favourable for n-channel semiconduction.
For the latter, the non-planar overall molecular geometry due to the bulky phenyl rings may
inhibit an effective π-overlap on the device surface. Moreover, AFM studies of vapour deposited
films of 1b, 7b and 8a revealed a very rough, uneven and amorphous morphology which does not
seem to be suited for conduction (see Supporting Information).
Whereas TAPy derivatives 2a-c proved to be too volatile to be processed by vapour
deposition, a small field effect was measured for TAPys 3, 4a,c and 5. Table 4 provides an
overview of all measured electron mobilities which we found to be low compared to the recently
studied tetraazaperopyrenes.6
TABLE 4. Electron Mobilities Measured in Bottom-Gate, Top-Contact OTFTs for TAPys 3, 4a,
c and 5.
Compound
3
4a
4c
5
Electron Mobility [cm2/Vs]
1
×
10-3
6×10-4
5×10-5
6×10-5
Conclusion
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In summary, we have developed a general way to synthesize 2,7-disubstituted tetraazapyrenes in
good yields. These compounds can be further derivatized by core chlorination and bromination,
creating valuable starting materials for subsequent Suzuki cross-coupling reactions. Single
crystals suited for X-ray diffraction have been grown from different organic solvents, revealing
polymorphism for some compounds. The LUMO energies have been calculated from the
reduction potentials obtained by cyclic voltammetry and compared to the ones computed by DFT.
LUMO energies of -3.57 to -4.14 eV and electron affinities ranging from 2.55 to 3.71 eV clearly
indicate that tetraazapyrenes that bear electron-withdrawing groups are potential n-type
semiconductors. Organic TFTs have been fabricated by vacuum deposition and the largest
electron mobility of 0.001 cm2/Vs was found for compound 3. It is well known that the right
combination of electrochemical properties and crystal packing is required for large electron
mobilities As we have shown in this first study, the tetraazapyrenes offer the potential for further
derivatization and thus may give access to novel organic semiconductors.
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Experimental Section
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1,4,5,8-tetraaminonaphthalene-hexachlorostannate (I)14 and the parent compound 1,3,6,8-
tetraazapyrene (II)7 were prepared according to literature methods.
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4,5,9,10-tetrachloro-1,3,6,8-tetraazapyrene (1a). 420 mg (2.0 mmol) 1,3,6,8-tetraazapyrene
(II) was dissolved in 50 ml chlorosulfonic acid and 86 mg iodine were added. The mixture was
heated to 80 °C and for a period of 3 h, chlorine gas was passed through the solution. The
resulting mixture was allowed to cool to room temperature and stirred for 12 h. Then it was
poured on ice, the precipitate was collected by filtration and extracted with toluene. The solvent
was removed under reduced pressure to yield 530 mg (1.56 mmol, 78%) of 1a as a golden-brown
1
solid. mp decomposition above 250 °C. H NMR (399.89 MHz, CDCl3,) δ = 10.23 (s, 2H, CH).
13C NMR (150.90 MHz, CDCl3) δ = 158.7 (C7H), 150.2 (C1/4), 140.1 (C2/3), 112.0 (C5/6). HRMS
(EI+) m/z calcd for C12H2Cl4N4: 341.9034, found: 341.9024. Anal. Calcd for C12H2Cl4N4: C 41.90
H 0.59 N 16.29; found: C 42.21 H 0.87 N 15.87.
4,5,9,10-tetrabromo-1,3,6,8-tetraazapyrene (1b). 420 mg (2.0 mmol) 1,3,6,8-tetraazapyrene
(II) was dissolved in 50 ml concentrated sulfuric acid, 86 mg iodine and 1 ml bromine (19.5
mmol) were added. The mixture was heated to 80 °C for 3 h and then allowed to cool to room
temperature overnight. The reaction mixture was poured on ice, the precipitate was collected by
filtration, washed thoroughly with sodium hydroxide solution, water, methanol and pentane and
dried in vacuo to yield 920 mg (1.76 mmol, 87%) of 1b as a light brown solid. mp decomposition
1
13
above 260 °C. H NMR (600.13 MHz, CDCl3) δ = 10.18 (s, 2H, CH). C NMR (150.90 MHz,
CDCl3) δ = 158.9 (C7H), 151.6 (C1/4), 136.7 (C2/3), 113.0 (C5/6). HRMS (EI+) m/z calcd for
79
81
C12H2 Br2 Br2N4: 521.6972, found: 521.6960. Anal. Calcd for C12H2Br4N4: C 27.62 H 0.39 N
10.74; found: C 27.82 H 0.35 N 10.67.
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General Procedure for the Preparation of 2,7-disubstituted Tetraazapyrene Derivatives.
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To a suspension of 1.0 g (1.9 mmol) 1,4,5,8-tetraaminonaphthalene-hexachlorostannate (I) in 30
ml abs. THF was added 0.5 ml abs. triethylamine under argon. 7.9 mmol of the corresponding
anhydride was added slowly under ice cooling. The reaction mixture was heated under reflux for
72 h. Then it was allowed to cool to room temperature, the solvent was removed under vacuum
and the residue was purified by column chromatography (silica, n-hexane/ethyl acetate 4:1). The
product was recrystallized from methanol three times.
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2,7-bis-(trifluoromethyl)-1,3,6,8-tetraazapyrene (2a). Anhydride: trifluoroacetic anhydride.
1
Yield: 407 mg (1.19 mmol, 62%) of 2a as a beige solid. mp 179 °C. H NMR (600.13 MHz,
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CDCl3) δ = 8.92 (s, 4H, CH). C NMR (150.92 MHz, CDCl3) δ = 156.1 (q, JCF = 37.3 Hz, C7),
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153.7 (C1/4), 137.4 (C2/3), 120.0 (q, JCF = 277.0 Hz, CF3), 114.0 (C5/6). F NMR (376.27 MHz,
CDCl3) δ = -68.64 (s, CF3). HRMS (EI+) m/z calcd for C14H4F6N4: 342.0340, found: 342.0352.
Anal. Calcd for C14H4F6N4: C 49.14 H 1.18 N 16.37; found: C 49.09 H 1.35 N 16.43.
2,7-bis-(pentafluoroethyl)-1,3,6,8-tetraazapyrene (2b). Anhydride: pentafluoropropionic
1
anhydride. Yield: 290 mg (0.66 mmol, 45%) of 2b as a beige solid. mp 183 °C. H NMR
(399.89 MHz, CDCl3,) δ = 8.90 (s, 4 H, CH). 13C NMR (150.90 MHz, CDCl3) δ = 156.3 (t,
1
2
2JCF = 25.5 Hz, C7), 153.6 (C1/4), 137.4 (C2/3), 128.9 (tq, JCF = 285.3 Hz, JCF = 36.0 Hz, CF3),
113.8 (C5/6), 110.2 (qt, 1JCF = 258.3 Hz, 2JCF = 36.0 Hz, CF2). 19F NMR (79 MHz, CDCl3,) δ = -
3
3
81.7 (t, 6F, JFF = 1.8 Hz, CF3), 115.5 (q, 4F, JFF = 1.8 Hz, CF2). HRMS (EI+) m/z calcd for
C16H4F10N4: 442.0276, found: 442.0270. Anal. Calcd for C16H4F10N4: C 43.46 H 0.91 N 12.67;
found: C 43.67 H 1.15 N 12.66.
2,7-bis-(heptafluoropropyl)-1,3,6,8-tetraazapyrene (2c). Anhydride: heptafluorobutyric
anhydride. Yield: 280 mg (0.52 mmol, 36%) of 2c as a beige solid. mp 185 °C. 1H NMR (399.98
MHz, CDCl3) δ = 8.94 (s, 4H, CH). 13C NMR (150.90 MHz, CDCl3) δ = 156.4 (t,
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2JCF = 25.8 Hz, C7), 153.6 (C1/4), 137.4 (C2/3), 119.1 – 107.8 (m, C3F7), 113.8 (C5/6). F NMR
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(376.27 MHz, CDCl3) δ = -80.1 (t, 6F, JFF = 9.2 Hz, CF3), -113.7 (q, 4F, JFF = 9.1 Hz, CF2), -
125.4 (s, 4F, CF2). HRMS (EI+) m/z calcd for C18H4F14N4: 542.0212, found: 542.0231. Anal.
Calcd for C18H4F14N4: C 39.87 H 0.74 N 10.33; found: C 39.88 H 1.04 N 10.54.
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2,7-bis-(pentafluorophenyl)-1,3,6,8-tetraazapyrene (3). Anhydride: pentafluorophenyl
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anhydride. Yield: 457 mg (0.85 mmol, 44%) of 3 as a beige solid. mp 255 °C. H NMR (399.98
MHz, CDCl3) δ = 8.80 (s, 4H, CH). 13C NMR (150.90 MHz, CDCl3) δ = 156.7 (C7), 153.5 (C1/4),
146.2 (CPhenyl-F), 144.6 (CPhenyl-F), 138.7 (CPhenyl-F), 136.9 (C2/3), 115.2 (CPhenyl - C7), 112.7
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(C5/6). F-NMR (79 MHz, CDCl3) δ = -160.8 (m, 4F), -151.4 (m, 2F), -141.9 (m, 4F). HRMS
(EI+) m/z calcd for C24H4F10N4: 538.0276, found: 538.0260.
General Procedure for the preparation of 4,9-dibrominated 2,7-perfluoroalkylated
Tetraazapyrene Derivatives. 0.5 mmol 2,7-bis-(perfluoroalkyl)-1,3,6,8-tetraazapyrene was
dissolved in 30ml concentrated sulfuric acid. 574 mg (2.0 mmol) dibromoisocyanuric acid was
added and the resulting reaction mixture was stirred under the exclusion of light for 48h at room
temperature. An additional amount of 574 mg (2.0 mmol) dibromoisocyanuric acid was added
and the mixture was stirred under the exclusion of light for 12h at room temperature. Then it was
poured on ice, neutralized with sodium hydroxide and extracted with methylene chloride. The
organic phase was washed with water and brine and dried over sodium sulfate. The solvent was
evaporated in vacuo and the solid residue was recrystallized from methanol twice.
2,7-bis-(trifluoromethyl)-4,9-dibromo-1,3,6,8-tetraazapyrene (4a). Starting compound: 2a.
Yield: 50 mg (0.1 mmol, 20%) of 4a as a beige solid. mp 323 °C. 1H NMR (600.13 MHz, CDCl3)
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δ = 9.26 (s, 2H, CH). C NMR (150.90 MHz, CDCl3) δ = 156.8 (q, JCF = 37.8 Hz, C7), 154.1
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(C4), 152.0 (C1) 139.4 (C3), 135.1 (C2), 119.7 (q, JCF = 276.3 Hz, CF3), 113.5 (C5/6). F NMR
(79 MHz, CDCl3) δ = -68.67 (s, 6F, CF3). HRMS (EI+) m/z calcd for C14H2Br2F6N4: 497.8550,
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found: 497.8555. Anal. Calcd for C14H2Br2F6N4: C 33.63 H 0.40 N 11.21; found: C 33.49 H 0.53
N 11.05.
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2,7-bis-(pentafluoroethyl)-4,9-dibromo-1,3,6,8-tetraazapyrene (4b). Starting compound: 2b.
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Yield: 51 mg (0.09 mmol, 17%) of 4b as a beige solid. mp 303 °C. H NMR (600.13 MHz,
CDCl3) δ = 9.28 (s, 2H, CH). 13C NMR (150.90 MHz, CDCl3) δ = 157.1 (t, 2JCF = 26.1 Hz, C7),
154.1 (C4), 151.9 (C1) 139.4 (C3), 135.1 (C2), 118.8 (tq, JCF = 287.0 Hz, JCF = 36.1 Hz, CF3),
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113.4 (C5/6), 109.9 (qt, JCF = 256.6 Hz, JCF = 37.0 Hz, CF2). F NMR (79 MHz, CDCl3) δ = -
81.56 (m, 6F, CF3), -115.24 (m, 4F, CF2). HRMS (EI+) m/z calcd for C16H2Br2F10N4: 597.8487,
found 597.8489.
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2,7-bis-(heptafluoropropyl)-4,9-dibromo-1,3,6,8-tetraazapyrene (4c). Starting compound: 2c.
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Yield: 56 mg (0.08 mmol, 16%) of 4c as a beige solid. mp 330 °C. H NMR (600.13 MHz,
CDCl3) δ = 9.29 (s, 2H, CH). 13C NMR (150.90 MHz, CDCl3) δ = 157.1 (t, 2JCF = 26.0 Hz, C7),
154.0 (C4), 151.9 (C1) 139.4 (C3), 135.2 (C2), 117.8 (tq, 6F, JCF = 288.5 Hz, JCF = 33.5 Hz,
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CF3), 113.4 (C5/6), 111.5 (tt, 4F, JCF = 259.8 Hz, JCF = 30.6 Hz, CF2), 109.8 (m, 4F, CF2). F
NMR (79 MHz, CDCl3) δ = -80.03 (t, 6F, 3JFF = 8.9 Hz, CF3), -113.57 (m, 4F, CF2), -125.34 (m,
4F, CF2). HRMS (EI+) m/z calcd for C18H2Br2F14N4: 697.8423, found 697.8399. Anal. Calcd for
C18H2Br2F14N4: C 30.88 H 0.29 N 8.00; found: C 30.53 H 0.65 N 7.86.
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2,7-bis-(pentafluorophenyl)-4,5,9,10-tetrabromo-1,3,6,8-tetraazapyrene (5). 100 mg (0.19
mmol) 2,7-bis-(pentafluorophenyl)-1,3,6,8-tetraazapyrene 3 was dissolved in 50 ml concentrated
sulfuric acid, 86 mg iodine and 1.6 ml bromine (31.2 mmol) were added. The mixture was heated
to 80 °C for 3 h and then allowed to cool to room temperature overnight. The reaction mixture
was poured on ice, the precipitate was collected by filtration, washed thoroughly with sodium
hydroxide solution, water, methanol and pentane and dried in vacuo to yield 29 mg (0.03 mmol,
18%) of 5 as a light brown solid. mp 326 °C. C NMR (150.90 MHz, CDCl3) δ = 158.2 (C7),
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152.5 (C1/4), 137.2 (C2/3), 111.3 (C5/6), perfluorinated aryl carbon signals were not observed. F
NMR (79 MHz, CDCl3) δ = -160.8 (m, 4F), -151.4 (m, 2F), -141.9 (m, 4F). HRMS (EI+) m/z
calcd for C24Br4F10N4: 849.6697, found 849.6684.
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2,7-bis-(trifluoromethyl)-4,5,9,10-tetrachloro-1,3,6,8-tetraazapyrene (6). 50 mg (0.15 mmol)
2,7-bis-(trifluoromethyl)-1,3,6,8-tetraazapyrene 2a was dissolved in 10 ml concentrated sulfuric
acid. 300 mg (1.5 mmol) dichloroisocyanuric acid was added and the resulting reaction mixture
was stirred under the exclusion of light for ten days at 100 °C. Then it was poured on ice,
neutralized with sodium hydroxide and extracted with methylene chloride. The organic phase was
washed with water and brine and dried over sodium sulfate. The solvent was evaporated in vacuo
and the solid residue was recrystallized three times from methanol and three times from THF to
yield 10 mg (0.02 mmol, 13%) of 6 in form of colourless crystals. 13C NMR (150.90 MHz,
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CDCl3) δ = 157.0 (q, JCF = 38.7 Hz, C7), 151.6 (C1), 141.5 (C2/3), 119.5 (q, JCF = 275.6 Hz,
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CF3), 111.8 (C5/6). F NMR (376.27 MHz, CDCl3) δ = -68.62 (s, 6F, CF3). HRMS (EI+) m/z
calcd for C14Cl4F6N4: 477.8781, found: 477.8803.
General Procedure for Suzuki cross-coupling reactions: An oven-dried Schlenk tube was
loaded with 1 eq brominated tetraazapyrene 1a or 4a, 4 to 8 eq of the corresponding boronic acid,
6 to 10 eq of caesium carbonate and 0.1 to 0.2 eq of Pd(dppf)Cl2 and evacuated and refilled with
argon for three times. 30 ml abs. 1,4-dioxane were added, the Schlenk tube was sealed with a
glass cap and the reaction mixture was stirred at 100°C for 48h. The mixture was allowed to cool
to room temperature, the solvent was evaporated and the solid residue was extracted with
chloroform. The organic phase was washed with water and brine, dried over sodium sulfate and
evaporated in vacuo.
4,5,9,10-tetrakis-(4-cyanophenyl)-1,3,6,8-tetraazapyrene (7a).
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Starting compounds: 120 mg (0.23 mmol) 4,5,9,10-tetrabromo-1,3,6,8-tetraazapyrene (1b) and
270 mg (1.84 mmol) 4-cyanophenylboronic acid. Yield: 65 mg (0.12 mmol, 46%) of 7a as a light
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brown solid. mp 458 °C. H NMR (600.13 MHz, CDCl3) δ = 10.00 (s, 2H, C7H), 7.72 (d, 8H,
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3JHH = 7.2 Hz), 7.47 (d, 8H, JHH = 8.4 Hz). C NMR (150.90 MHz, CDCl3) δ = 158.0 (C7),
151.6 (C1/4), 144.9 (C2/3), 139.1 (C8), 132.0 (C9), 131.8 (C10), 118.2 (CN), 114.0 (C5/6), 112.7
(C11). HRMS (FAB+) m/z calcd for C40H19N8: 611.1732, found 611.1752.
4,5,9,10-tetrakis-(4-trifluoromethylphenyl)-1,3,6,8-tetraazapyrene (7b).
Starting compounds: 150 mg (0.29 mmol) 4,5,9,10-tetrabromo-1,3,6,8-tetraazapyrene (1b) and
441 mg (2.32 mmol) 4-trifluoromethylphenylboronic acid. Yield 45 mg (0.7 mmol, 24%) of 7b
as a beige solid. mp 395 °C. 1H NMR (600.13 MHz, CDCl3) δ = 10.04 (s, 2H, C7H), 7.67 (d, 8H,
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3JHH = 8.2 Hz), 7.49 (d, 8H, JHH = 8.0 Hz). C NMR (150.90 MHz, CDCl3) δ = 157.9 (C7),
151.9 (C1/4), 145.3 (C2/3), 138.4 (C8), 131.8 (C9), 130.2 (C11), 125.0 (C10), 123.0 (CF3), 113.9
(C5/6). 19F NMR (79 MHz, CDCl3) δ = -62.68 (s, 12F, CF3). HRMS (EI+) m/z calcd for
C40H18F12N4: 782.1340, found 782.1329.
2,7-bis-(trifluoromethyl)-4,9-bis-(4-cyanophenyl)-1,3,6,8-tetraazapyrene
(8a).
Starting
compounds: 100 mg (0.20 mmol) 2,7-bis-(trifluoromethyl)-4,9-dibromo-1,3,6,8-tetraazapyrene
(4a) and 118 mg (0.80 mmol) 4-cyanophenylboronic acid. Yield 65 mg (0.12 mmol, 60%) of 8a
as a light grey solid. mp 420 °C. H NMR (600.13 MHz, CDCl3) δ = 9.02 (s, 2H, C3H), 8.14 (d,
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4H, 3JHH = 8.1 Hz), 7.98 (d, 4H, 3JHH = 8.2 Hz). 13C NMR (150.90 MHz, CDCl3) δ = 156.2 (C7),
153.7 (C4), 151.9 (C1), 147.1 (C2), 138.7 (C8), 136.4 (C3), 132.5 (C10), 131.5 (C9), 118.2 (CN),
114.3 (C5/6), 114.0 (C11). 19F NMR (79 MHz, CDCl3) δ = -68.76 (s, 6F, CF3). HRMS (MALDI+)
m/z calcd for C28H9F6N6: 543.0787, found 543.0767.
2,7-bis-(trifluoromethyl)-4,9-bis-(4-trifluoromethylphenyl)-1,3,6,8-tetraazapyrene
(8b).
Starting compounds: 130 mg (0.26 mmol) 2,7-bis-(trifluoromethyl)-4,9-dibromo-1,3,6,8-
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tetraazapyrene (4a) and 197 mg (1.04 mmol) 4-trifluoromethylphenylboronic acid. Yield 35 mg
(0.06 mmol, 23%) of 8b as a white solid. mp 419 °C. 1H NMR (600.13 MHz, CDCl3) δ = 9.02 (s,
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2H, C3H), 8.20 (d, 4H, JHH = 8.0 Hz), 7.95 (d, 4H, JHH = 8.1 Hz). C NMR (150.90 MHz,
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CDCl3) δ = 156.1 (q, JCF = 37.2 Hz, C7), 153.8 (C4), 152.3 (C1), 147.7 (C2), 137.9 (C8), 136.2
(C3), 132.1 (q, JCF = 33.0 Hz, C11), 131.3 (C9), 125.8 (C10), 124.8 (C12F3), 119.0 (CF3), 114.2
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(C5/6). 19F NMR (79 MHz, CDCl3) δ = -68.76 (s, 6F, CF3), -62.72 (s, 6F, C12F3). HRMS
(MALDI+) m/z calcd for C28H9F12N4: 629.0630, found 629.0610.
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X-ray Crystal Structure Determinations
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Crystal data and details of the structure determinations are given in the Supporting Information.
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Full shells of intensity data were collected at low temperature (T = 100 K, Mo-Kα radiation,
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sealed tube, graphite monochromator, all compounds except 3·thf or Cu-Kα radiation, microfocus
tube, multilayer mirror optics for 3·thf). Data were corrected for air and detector absorption,
Lorentz and polarization effects;26,27 absorption by the crystal was treated numerically (second
monoclinic form of 2c and compound 4b) or with a semiempirical multiscan method (all
others).27,28,29
The structures were solved by direct methods with dual-space recycling (compounds 2a30
and 2c31,32), by the heavy atom method combined with structure expansion by direct methods
applied to difference structure factors33 (compound 4b) or by the charge flip procedure34 (all
others) and refined by full-matrix least squares methods based on F2 against all unique
reflections.32,35 All non-hydrogen atoms were given anisotropic displacement parameters.
Hydrogen atoms were generally input at calculated positions and refined with a riding model.
When justified by the quality of the data the positions of the hydrogen atoms were taken from
difference Fourier syntheses and refined. Due to severe disorder and fractional occupancy,
electron density attributed to solvent of crystallization (thf) was removed from the structure (and
the corresponding Fobs) of 3 with the BYPASS procedure36, as implemented in PLATON
(SQUEEZE)37. Crystals of 5 were twinned; after de-twinning (approx. twin fractions 0.87:0.13)
refinement was carried out against all observations involving domain 1.
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