Peralkynylated Tetraazaacene Derivatives

Article abstract of DOI:10.1002/chem.201904087

The synthesis of a decaethynylated tetraazapentacene is described. It was obtained by a combination of condensation reactions giving the two pyrazine rings and subsequent consecutive Stille-type couplings. This is the first example of any higher (hetero)acene that is peralkynylated. The presence of the four nitrogen atoms removes the peri interaction of the substituted alkyne groups, giving this rock-stable and highly twisted heteroacene.

Full text of DOI:10.1002/chem.201904087

A Journal of
Accepted Article
Title: Peralkynylated Tetraazaacene-Derivatives
Authors: Hilmar Reiss, Frank Rominger, Jan Freudenberg, and Uwe
Heiko Bunz
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To be cited as: Chem. Eur. J. 10.1002/chem.201904087
Link to VoR: http://dx.doi.org/10.1002/chem.201904087
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Peralkynylated Tetraazaacene-Derivatives
Hilmar Reiss, Frank Rominger, Jan Freudenberg and Uwe H. F. Bunz*[a,b]
[
a]
[a]
[a,c]
Dedication ((optional))
Abstract: We demonstrate the synthesis of a decaethynylated
tetraazapentacene. It is obtained by a combination of condensation
reactions furnishing the two pyrazine rings and subsequent
consecutive Stille-type couplings. This is the first example of any
higher (hetero) acene that is peralkynylated. The presence of the four
nitrogen atoms removes the peri interaction of the substituted alkyne
groups, furnishing this rock-stable and highly twisted heteroacene.
The subsequent synthesis of the symmetrical tetraazapentacene
motif was more challenging. The coupling of 4b with a second
mole of tetrabromoorthoquinone gave the compound 7b-H –
2
after reduction with tin chloride – in only 8% yield, while the
reaction with 4a furnished the product 7a (see SI) in 5%, probably
due to the lower stability of the TMS-groups. The (high yielding)
reduction is critical to allow the next fourfold Stille reaction, since
the oxidized form of 7b-H
N,N’-dihydrotetraazacene 8-H
2
oxidizes the Pd-catalyst. The formed
is oxidized easily into the target
2
Here we disclose the synthesis of an octaalkynylated
tetraazatetracene and a decaalkynylated tetraazapentacene.
Peralkynylated representatives are known of some aromatic
compound 8. With this sequence, the first peralkynylated
azapentacene has been prepared as a red-colored crystalline
material.
systems; the first ever
described was
Vollhardt’s
hexaethynylbenzene, prepared from hexabromobenzene and
1
TMS-acetylene in
a
sixfold Sonogashira-reaction.
Later
2
tetraethynylethylenes were prepared by Hopf et al. and by
Diederich et al., 3 followed by tetraalkynylated cyclobutadiene
complexes achieved by Bunz et al. 4 and peralkynylated
5
cyclopentadienyl species either as radical (Rubin) or stabilized
as cymantrene or ferrocene.6
However, for larger aromatic systems, such as naphthalene or
anthracene, peralkynylated species are unknown – attempts of
alkynylation lead to annulation reactions due to severe peri-
interactions of the alkynes – yet a peralkenylated naphthalene has
7
been prepared by de Meijere. To solve this issue, the offending
peri-interactions will have to be removed by substitution of a C-
CC-R-group through a nitrogen. As a consequence, peralky-
8
nylated phenazine and quinoxaline are known. Yet, none of the
larger peralkynylated N-heteroacenes have ever been made.
Here we disclose the synthesis of the higher homologs, a
peralkynylated tetraazatetracene and
tetraazapentacene
derivative.
9
Starting from diamine 1, reaction with tetrabromoorthoquinone
under acidic conditions furnished the thiadiazole 2, which upon
Stille reaction with the stannylated acetylenes followed by MnO
2
oxidation of the intermediate N,N’-dihydro-compound gave the
hexaethynylated phenazinthiadiazoles 3a, and 3b (Scheme 1).
Attempts to open the thiadiazole ring with LiAlH
yields, with zinc in acetic acid in decomposition. However, SmI
smoothly opened the thiadiazole ring to give the diamines 4a or
b quantitatively. 4a was easily coupled with dione 5, resulting in
the first octaalkynylated tetraazatetracene 6 as a turquoise solid.
4
resulted in low
2
4
[a]
M Sc. H. Reiss, Dr. F. Rominger, Dr. J. Freudenberg, Prof. U. H. F.
Bunz
Organisch-Chemisches Institut, Ruprecht-Karls-Universität
Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, FRG
E-mail: uwe.bunz@oci.uni-heidelberg.de
[
[
b]
c]
Prof. U. H. F. Bunz, Centre for Advanced Materials, Im
Neuenheimer Feld 225, 69120 Heidelberg, Germany
Dr. J. Freudenberg, InnovationLab, Speyerer Straße 4, 69115
Heidelberg, FRG
Supporting information for this article is given via a link at the end of
the document.
Scheme 1. Synthesis of the peralkynylated tetraazaacenes 6 and 8 (top) and
structure of TIPSTAP for comparison (bottom).
10
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Calculated frontier molecular orbitals using TURBOMOLE B3LYP/def2-TZVP //
Gaussian 09 B3LYP/6-311++G**.
The enlargement of the π-system from 6 to 8 leads to a significant
red shift in both the absorption and emission spectra (Figure 1),
the former displaying the typical fine structure of acenes. In
comparison with TIPSTAP (cf. Scheme 1), the tert-butylethynyl-
substituents of 8 red-shift the absorption by 1612 cm-1 from 681
nm to 765 nm. The first reduction potential of 8 occurs at -0.77 V
1
0
Figure 2 and 3 show the single-crystal X-ray structures of 6 and
8. Both arrange in a herringbone-like structure without π-π-
contacts to be observed due to the bulky peripheral substituents.
The bond lengths are in excellent agreement with the calculated
values. While the tetracene backbone of 6 is only slightly bent, the
increased steric pressure upon going to 8 leads to a curious
overall geometry, as particularly the central TIPS-ethynyl groups
are highly bent into the opposite directions and the aromatic core
is twisted along the long molecular axis, with a maximum of 19°
for the central ring. In principle, the presence of the pyrazine rings
should have removed all of the peri-interactions, but the
peripheral substituents are too bulky to fit easily. Yet both
compounds are stable and can be stored indefinitely in the solid,
crystalline state.
(
-0.81 TIPSTAP), while the second reduction potential is located
at -1.20 V (-1.24 V TIPSTAP). The similarity of the first and
second reduction potentials combined with the considerable red
shift suggests that the alkynyl groups primarily lead to a
destabilization of the HOMO but not to a stabilization of the LUMO.
This observation is supported by quantum chemical calculations
(
B3LYP/6-311++g(d,p)) which display a 0.16 eV destabilization (!)
of the LUMO and a 0.37 eV destabilization of the HOMO, resulting
in a calculated decrease of the HOMO-LUMO gap by 0.21 eV
(
-1
1694 cm ), which corresponds well to the measured difference
-1
of 1632 cm .
Figure 1. UV-Vis spectra of bis(triisopropylsilyl)tetraazapentacene (TIPSTAP,
black) and the peralkynylated derivatives 6 (blue) and 8 (red) in n-hexane. Inset:
Normalized absorption and emission spectra magnified to the region of the
lowest energy maxima.
Figure 2. Single-crystal X-ray structure of 6. Ellipsoid plot (top), view from the
side (bottom left) and display of the herringbone-like arrangement (bottom right).
Table 1. Photophysical and electrochemical properties of TIPSTAP, 6 and 8.
[
a]
λ
max, abs
E
(0/-1
[V]
Red
)[b]
E
Red
(-1/-2)
[V]
[
c]
[d]
[d]
E
[
EA
E
LUMO
E
HOMO
em)[a]
[b]
Cpd.
(λmax
,
eV]
[eV]
[eV]
[
nm]
TIPS
TAP
681
(694)
-
0.81
-1.24
-1.20
-1.20
-3.99
-4.04
-4.03
-3.84
-3.72
-3.68
-5.58
-5.57
-5.21
651
6
8
-0.76
-0.77
(
(
660)
765
775)
[
a] Lowest energy absorption and emission maxima measured in n-hexane at
room temperature. [b] First reduction potentials measured by cyclic voltammetry
in CH Cl using Bu NPF
as electrolyte and Fc/Fc+ as internal standard
-4.80 eV) at 0.2 V s . [c] Electron affinity [eV] = -4.8 eV - ERed(0/-1). [d]
2
2
4
-
6
1 11
(
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found: C 69.73, H 8.92, N 3.55. UV-Vis: λmax, abs (hexane) = 651 nm.
Fluorescence: λmax, em (hexane) = 660 nm.
1
,2,3,4-Tetrabromo-8,9,10,11-tetrakis(3,3-dimethylbut-1-yn-1-yl)-6,13-
bis((triisopropylsilyl)ethynyl)-5,14-dihydroquinoxalino[2,3-
b]phenazine (7b-H ): In a heat-gun dried Schlenk tube under argon with
2
molecular sieves 4b (50.0 mg, 56.1 µmol, 1.0 equiv.) and
tetrabromoorthoquinone (47.5 mg, 112 µmol, 2.0 equiv.) were dissolved in
2
2
mL of abs. toluene and 0.50 mL AcOH. The reaction mixture stirred for
days at room temperature while it turned from red to brown. After
completion the mixture was filtered through Celite. Water was added and
the phases were separated, DCM was used for extraction. The organic
phases were treated with sat. aqueous sodium bicarbonate and dried over
magnesium sulfate. After evaporation of the solvent, the crude product was
purified by chromatography on silica using PE/DCM 10:1 -> 5:1 as eluent
to give a mixture of the oxidized and reduced form of 7b. It was dissolved
with 5.00 mL THF and stirred with SnCl
2
(106 mg, 561 µmol, 10 equiv.) in
0.50 mL conc. HCl for 30 min. Water was added to the blue solution and
the phases were separated, DCM was used for extraction. The organic
phases were treated with sat. aqueous sodium bicarbonate and dried over
magnesium sulfate. After evaporation of the solvent, the crude product was
purified by chromatography on silica using PE/DCM 10:1 -> 5:1 to give the
product as a purple solid (6.00 mg, 4.69 µmol, 8%). 1H NMR (600 MHz,
3
CDCl ): δ [ppm] = 7.65 (s, 2H), 1.41 (s, 18H), 1.39 (s, 18H), 1.20-1.17 (m,
Figure 3. Single-crystal X-ray structure of 8. Ellipsoid plot (top), view from the
side (bottom left) and display of the herringbone-like arrangement (bottom right).
13
1
42H). C { H} NMR (150 MHz, CDCl
3
): δ [ppm] = 146.1, 139.4, 139.2,
1
3
2
9
28.0, 127.7, 127.4, 124.0, 110.9, 110.4, 106.6, 105.6, 99.0, 97.7, 72.3,
1.5, 31.5, 29.9, 29.8, 29.5, 19.2, 12.2. IR (neat): ν [cm−1] = 3347, 2921,
859, 1678, 1592, 1570, 1444, 1404, 1385, 1238, 1176, 1102, 1049, 1009,
75, 882, 786, 675, 606, 559, 463. HR-MS(MALDI+): m/z calcd. for
In conclusion we have prepared the first peralkynylated
tetraazatetracene and tetraazapentacene. Despite the excised
peri interactions – achieved by strategic placement of the pyrazine
nitrogen atoms, the alkynes in 8 are bent and the aromatic
perimeter is twisted. The materials are stable and easily
processible and should be attractive precursors for pyrolytic
transformations into C,N-type graphene materials. They are
stable under ambient conditions, probably due to the steric
shielding exerted by the alkyne substituents.
64 4 4 2
C H81Br N Si : 1277.2728; found: 1277.2694. UV-Vis: λmax, abs (hexane) =
591 nm.
1,2,3,4,8,9,10,11-Octakis(3,3-dimethylbut-1-yn-1-yl)-6,13-
bis((triisopropylsilyl)ethynyl)-5,14-dihydroquinoxalino[2,3-
b]phenazine 8-H
.61 µmol, 0.28 equiv.) was placed in a Schlenk tube and transferred into
a glove-box under nitrogen atmosphere. 2.00 mL abs. toluene were added
followed by P(t-Bu) (1.06 mg, 5.25 µmol, 0.56 equiv.). The Schlenk tube
was closed and taken outside the glove-box. Then, against a counter-flow
of argon, 7b-H (12.0 mg, 9.37 µmol, 1 equiv.) and (3,3-dimethylbut-1-yn-
-yl)trimethylstannane (27.5 mg, 112.4 µmol, 12 equiv.) were added and
2
: Bis(benzonitrile)palladium(II) dichloride (1.00 mg,
2
3
Experimental Section
2
1
the reaction mixture stirred at 90 °C for 16 h. After adding DI water,
extraction with DCM and drying of the organic phases over magnesium
sulfate, the crude solution was filtered through a pad of silica. The solvent
was evaporated under reduced pressure and the product was purified by
CCDC 1947891 (6) and 1947892 (8) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. The synthetic details for precursors
and remaining materials can be found in the SI.
chromatography on silica using PE/DCM 9:1 -> 6:1 to give the product as
1
a purple-red solid (8.00 mg, 6.22 µmol, 66%). H NMR (600 MHz, CDCl
3
):
δ [ppm] = 7.53 (s, 2H), 1.44 (s, 18H), 1.42 (s, 18H), 1.40 (s, 18H), 1.38 (s,
2
,3,5,12-Tetrakis((triisopropylsilyl)ethynyl)-7,8,9,10-
tetrakis((trimethylsilyl)ethynyl)-1,2-dihydropyrazino[2,3-b]phenazine
6): 4a (43.0 mg, 45.0 µmol, 1 equiv.) and 5 (24.5 mg, 58.5 µmol,
.3 equiv.) were stirred for 6 h in 2.00 mL of DCM / AcOH (1:1) at room
8H), 1.13-1.09 (m, 42H). 13C { H} NMR (150 MHz, CDCl
1
): δ [ppm] =
1
1
1
2
2
3
43.5, 141.1, 138.4, 128.4, 128.3, 124.8, 123.1, 110.1, 109.8, 109.5, 107.4,
04.8, 104.4, 101.4, 98.4, 78.5, 77.1, 76.7, 31.5, 31.5, 31.5, 31.5, 28.8,
8.8, 28.6, 19.2, 11.5. IR (neat): ν [cm−1] = 3354, 2957, 2923, 2861, 2359,
342, 1726, 1571, 1454, 1246, 1017, 798, 658, 556, 452. HR-
(
1
temperature. The mixture was poured into DI water and extracted with
DCM. The organic phases were treated with sat. aqueous sodium
bicarbonate and dried over magnesium sulfate. After evaporation of the
solvent under reduced pressure, the crude product was purified by
chromatography on silica using PE/DCM 9:1 -> 6:1 -> 4:1 as eluent to give
the product as a turquoise solid (37.0 mg, 27.6 µmol, 61%). M.p.: >400 °C.
MS(MALDI+): m/z calcd. for C88
UV-Vis: λmax, abs (hexane) = 587 nm. Fluorescence: λmax, em (hexane) =
96 nm.
117 4 2
H N Si : 1285.8811; found: 1285.8785.
5
1H NMR (600 MHz, CDCl
1,2,3,4,8,9,10,11-Octakis(3,3-dimethylbut-1-yn-1-yl)-6,13-
bis((triisopropylsilyl)ethynyl)-quinoxalino[2,3-b]phenazine (8): 8-H
5.00 mg, 3.89 µmol, 1 equiv.) was dissolved in 3.00 mL DCM and treated
with MnO (3.38 mg, 38.9 µmol, 10 equiv.). The reaction mixture quickly
3
): δ [ppm] = 1.25-1.23 (m, 42H), 1.21-1.18 (m,
2H), 0.37 (s, 18H), 0.37 (s, 18H). 1 C { H} NMR (150 MHz, CDCl
3
1
2
4
[
3
): δ
ppm] = 143.1, 142.2, 142.1, 141.6, 131.9, 126.8, 122.3, 113.4, 110.4,
09.8, 103.9, 102.4, 102.0, 101.9, 100.9, 19.2, 18.9, 11.9, 11.8, 0.6, 0.3.
(
2
1
IR (neat): ν [cm 1] = 2942, 2891, 2863, 1461, 1442, 1384, 1325, 1246,
163, 1108, 1041, 944, 836, 793, 757, 675, 643, 607, 578, 489, 405. HR-
MS(MALDI+): m/z calcd. for C78 Si : 1337.7740; found: 1337.7965.
Elemental analysis (%) calcd. for C78 Si : C 69.99, H 9.04, N 4.19,
−
turned from purple to red-brown. After 30 min and complete conversion of
the starting material the reaction mixture was filtered through Celite and
the solvent was evaporated under reduced pressure. Chromatography on
silica using PE/DCM 5:1 -> 2:1 as eluent gave the product as a red solid
1
H N
121 4
8
H N
120 4
8
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(
4.80 mg, 3.74 µmol, 96%). M.p.: 270 °C decomp. 1H NMR (600 MHz,
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1
CDCl
3
(150 MHz, CDCl
3
): δ [ppm] = 143.5, 142.4, 131.2, 125.8, 122.3, 115.3,
1
11.9, 111.6, 104.9, 78.9, 76.4, 31.4, 31.3, 29.1, 29.0, 19.2, 11.7. IR
(
neat): ν [cm−1] = 2965, 2922, 2862, 2360, 2341, 2214, 1445, 1396, 1361,
238, 1112, 1061, 802, 727, 659, 587. HR-MS(MALDI+): m/z calcd. for
Si : 1283.8655; found: 1283.8691. UV-Vis: λmax, abs (hexane) =
65 nm. Fluorescence: λmax, em (hexane) = 775 nm.
1
C
7
H N
88 115 4
2
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52, 21-30; i) M. Laskoski, G. Roidl, H. L. Ricks, J. G. M. Morton, M. D.
Acknowledgements
6
We thank the SFB 1249 for generous financial support
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Laskoski, W. Steffen, J. G. M. Morton, M. D. Smith, U. H. F. Bunz, J.
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Keywords: N-heteropentacenes • peralkynylation •
stabilization•X-ray diffraction • peri-interactions
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Text for Table of Contents
H. Reiss, F. Rominger, J. Freudenberg
and U. H. F. Bunz*
Page No. – Page No.
Peralkynylated Tetraazaacene-
Derivatives
1
2
ah )o pD fi eC
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Bo le l rh ardt Angew. Chem. 1986, Mohler KPC. Vollhardt, Angew Chem. 1992,
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6
RB u bn i
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7
8
DB ue n
z
Meijere peralkenylnaphthalene
9
10Diamin-Quelle
11 CV-Quelle
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