Pyrazinacenes: Aza analogues of acenes

Article abstract of DOI:10.1021/jo901832n

(Chemical Equation Presented) A series of edge-sharing condensed oligopyrazine analogues of acenes, the pyrazinacenes, were synthesized and characterized. X-ray crystallographic determinations revealed intermolecular interactions that affect the propensit

Full text of DOI:10.1021/jo901832n

pubs.acs.org/joc
Pyrazinacenes: Aza Analogues of Acenes
Gary J. Richards,†,‡ Jonathan P. Hill,*,† Navaneetha K. Subbaiyan,§ Francis D’Souza,*,§
Paul A. Karr,^ Mark R. J. Elsegood, Simon J. Teat,z Toshiyuki Mori,‡ and
Katsuhiko Ariga†
†Supermolecules Group, WPI Center for Materials Nanoarchitectonics, National Institute for Materials
Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, ‡Fuel Cell Materials Group, National Institute for
Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, §Department of Chemistry, Wichita State
University, 1845 Fairmount, Wichita, Kansas 67260-0051, ^Department of Physical Sciences and
Mathematics, Carhart Science 320, Wayne State College, 1111 Main Street, Wayne, Nebraska 68787,
Chemistry Department, Loughborough University, Loughborough, Leicestershire, United Kingdom LE11
3TU, and zALS, Berkeley Lab, 1 Cyclotron Road, MS2-400, Berkeley, California 94720
jonathan.hill@nims.go.jp; francis.dsouza@wichita.edu
Received September 1, 2009
A series of edge-sharing condensed oligopyrazine analogues of acenes, the pyrazinacenes, were
synthesized and characterized. X-ray crystallographic determinations revealed intermolecular inter-
actions that affect the propensity of the molecules to undergo π-π stacking. Increasing heteroatom
substitution of the acene framework induces shorter intermolecular π-π stacking distances (shorter
than for graphite) probably due to lower van der Waals radius of nitrogen atoms. Hydrogen bonding
is also a determining factor in the case of compounds containing reduced pyrazine rings. Combined
electrochemical, electronic absorption, and computational investigations indicate the substantial
electron deficiency of the compounds composed of fused pyrazine rings. The pyrazinacenes are
expected to be good candidates as materials for organic thin film transistors.
Introduction
significant interest in developing pentacene and other or-
ganic molecular semiconducting materials because of their
likely application in various nascent technologies including
organic field effect transistors (OFETs)3 and photovoltaic
cells.4 While pentacene is a p-type semiconductor in the
solid state, N-substituted oligoacenes are expected to be
Acenes are aromatic compounds composed of con-
densed arrays of edge-sharing five- or six-membered rings,
which sometimes contain heteroatoms such as sulfur or
nitrogen.1 Probably the best known of the acenes is penta-
cene (Scheme 1), which has attracted attention because of its
solid state semiconducting properties.2 There is currently
(2) (a) Bailey, W. J.; Madoff, M. J. Am. Chem. Soc. 1953, 20, 5603. (b)
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Vols. 1, 2. (b) Bjorseth, A., Ed. Handbook of Polycyclic Aromatic Hydrocar-
bons; Dekker: New York, 1983. (c) Harvey, R. G. Polycyclic Aromatic Hydro-
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8914 J. Org. Chem. 2009, 74, 8914–8923
Published on Web 11/02/2009
DOI: 10.1021/jo901832n
r
2009 American Chemical Society

Richards et al.
JOCFeatured Article
SCHEME 1. Chemical Structures of the Pyrazinacenes and
Related Compounds
their synthesis and properties. Fluorubine,7 octaazadihydro-
hexacenes,8 and decaazapentacenes9 have been synthesized,
but their semiconducting properties were not investigated,
usually because of inappropriate elaboration at the mole-
€
cules’ periphery. Stockner and co-workers synthesized a
series of oligoazaacene-type compounds, and those mole-
cules present excellent opportunities for studying the proper-
ties of chromophores formed from extended azaacenes.10
Other workers have also prepared aza-substituted acenes
apparently in anticipation of useful n-type semiconducting
properties.11
Recently, computational methods based on density func-
tional theory (DFT) have been used to highlight the potential
importance of the acene family of compounds.5,12 In parti-
cular, a diradical character of the ground state in higher
oligoacenes such as heptacene was predicted by Bendikov
et al.12a For aza-substituted acenes, it was suggested that
incorporation of only five nitrogen atoms (into pentacene) is
required to obtain properties of the molecules suitable for
their use as n-type organic semiconducting materials.13 In-
cidentally, introduction of nitrile groups at the periphery of
oligoazapentacenes was found computationally to improve
electron affinities and reorganizational energies, which are
important for any potential organic electronic applications.
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promising candidates for n-type device applications because
of their high electron affinities.5 They also have other proper-
ties of significant interest.6 The compounds are less suscep-
tible to degradation through oxidation or dimerization than
their non-aza-substituted cousins due to their predicted
electron deficiencies, although preparation of compounds
having multiple edge-sharing pyrazine groups usually in-
volves several synthetic steps.
€
€
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Despite the promise of the N-substituted oligoacenes in
various applications, there are only a few reports regarding
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J. Org. Chem. Vol. 74, No. 23, 2009 8915

Richards et al.
SCHEME 2. Typical Synthesis of Pyrazinacenesa
JOCFeatured Article
Here we present a method for the synthesis, several X-ray
structures, and some properties of acenes composed of fused
pyrazines/dihydropyrazines, compounds which we refer to
as the pyrazinacenes. We investigated the electrochemical
properties of the compounds and assessed their electronic
structures using standard computational methods. The pro-
cedures employed in the synthesis of these compounds will
permit some flexibility in the introduction of functionality as
peripheral groups or by attachment at the aza-substituted
molecular core.14
aConditions: (i) 5,6-diamino-2,3-dicyanopyrazine, acetic acid/tetrahy-
drofuran, reflux 4 days; (ii) diaminomaleonitrile, Na2CO3, DMSO,
100 ꢀC.
(IV) oxide and sodium periodate. The stability of the com-
pounds against oxidation is expected to be a result of the four
ethenamine conjugatons involving the N lone pairs of the
dihydropyrazine moieties together with extra resonance
stability caused by an increase in the number of Clar rings.
These effects more than offset the destabilization caused by
the formally antiaromatic dihydropyrazine groups accord-
ing to calculations by Wu and co-workers.20a Stability is
also likely to be increased by delocalization of the antiar-
omaticity as suggested by nucleus independent chemical shift
(NICS) calculations on related compounds.20 Dicyanodihy-
dropyrazinacene compounds 5 and 8 do not undergo further
nucleophilic substitution of nitrile groups under the condi-
tions used here. However, the oxidized compound 3 was
found to react readily with alcohols, even in the absence of
base at room temperature and could also be reacted with o-
phenylenediamines to give dihydropyrazinacenes in a similar
way to compounds 2 and 4. This reactivity suggests the
intriguing possibility of synthesizing extended ladder-type
pyrazinacenes using a repeated stepwise nucleophilic sub-
stitution followed by oxidation. However, thus far, we have
only been able to carry out a single cycle of this “step-ladder”
approach to extended pyrazinacenes as oxidation of the
larger dihydropyrazinacenes has not yet been achieved.
Despite this, we were able to obtain extended pyrazinacenes
by employing fused polycyclic aromatic diamines. Generally,
the compounds possess good stabilities in air over long
periods. Their stabilities probably arise from greater electron
deficiencies relative to pentacene, which is known to be
susceptible to oxidation under aerobic conditions.21
Synthesis
Our approach to the synthesis of pyrazinacenes involves a
double nucleophilic attack at the 2,3 positions of 2,3-
dicyanopyrazino[2,3-b]pyrazine derivatives, which ulti-
mately results in substitution of both nitrile groups during
the formation of an N,N0-dihydropyrazine ring. We devel-
oped this synthesis in the knowledge that TCNQ undergoes a
similar double nucleophilic substitution of two of its nitrile
groups by amines or alcohols.15,16 This reaction has been
investigated by several groups for preparation of self-assem-
bling or nonlinear optically active derivatives.17,18 Attempts
to prepare phthalocyanines from 1,4,5,8-tetraaza-2,3-
naphthalonitrile result only in substitution of its nitrile
groups by an available nucleophile such as an alcohol to
give aromatic ethers,14a as illustrated by compounds 7 and 8
in Scheme 1, or by amines such as o-phenylenediamines or
diaminomaleonitrile to give dihydropyrazinacenes as exem-
plified by compounds 5 and 8-11 (Scheme 1). We realized
that this reaction (Scheme 2) presents a useful method for
preparation of discrete oligoacenes, especially fused oligo-
pyrazines, and should also be available for application in the
synthesis of other heteroatom-substituted acenes, even in-
cluding substituted 8,80-dicyanoquinodimethane deriva-
tives.
In fact, the reaction (Scheme 2) can be used to prepare a
variety of derivatives. Most dihydropyrazines are readily
oxidized to the corresponding pyrazines either by sublima-
tion under an oxygen atmosphere or simply by standing in
air.19 In contrast, the compounds presented in this work were
found to be resistant to a variety of oxidants including 2,4-
dichloro-5,6-dicyanobenzoquinone (DDQ), hydrogen per-
oxide, pyridinium chlorochromate, and Fremy’s salt. Only
compound 5 could be oxidized to the hexaazaanthracene, 3,
using ruthenium tetroxide prepared in situ from ruthenium-
Results and Discussion
X-ray Crystal Structures (2, 3, 5,14a 8, 9). The X-ray crystal
structures of several of the compounds were measured
(Figure 1) and used to assess trends in the packing of the
molecules in the solid state. Compounds 2, 3, 5, 8, and 9 are
similarly substituted at the “ends”, except that for 5 phenyl
substituents are ring closed forming a diazatriphenylene
unit, and 8 where nitrile groups are replaced by methoxy
groups. Packing of 2 (see Supporting Information) is gov-
erned by hydrogen bonding between nitrile groups or pyr-
azine N atoms and CH groups of the phenyl rings. For 3 (see
Figure 2), molecules are arranged so that the planes of fused
pyrazine rings are all orthogonal and no stacking interac-
tions are observed.
(14) (a) Richards, G. J.; Hill, J. P.; Okamoto, K.; Shundo, A.; Akada, M.;
Elsegood, M. R. J.; Mori, T.; Ariga, K. Langmuir 2009, 25, 8408–8413.
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FIGURE 1. X-ray crystallographic molecular structures of 2, 3,
5,14a 8, and 9.
FIGURE 3. Crystal packing of 8. (a) Viewed along b-axis,
(b) viewed along c-axis. Molecules have an alternating stacked
arrangement.
the nitrile groups and an adjacent C-C bond (see Figure 3).
These short distances are consistent with the smaller van der
Waals radius of nitrogen, which leads to a minimum contact
˚
distance between N and C of around ∼3.20 A. Although in
the case of 3 there is an edge-to-face arrangement of pyr-
azinacene moieties, we note that the introduction of more
nitrogen atoms into the acene framework should result in
closer average intermolecular distances even in π-π stacked
structures. If nitrile groups are replaced with a non-H-
bonding group such as OCH3 as in 8, then a π-π stacked
structure is obtained (Figure 3) with molecules arranged in
an antiparallel fashion. The nitrile groups’ H-bonding with
phenyl methine groups appears to preclude formation of a
stacked structure in 3 (and 2). Conversely, if the phenyl
groups are forced planar by formation of the triphenylene
unit (as in 5; see ref 14a), then all nitriles and methane groups
are coplanar and a stacked structure was obtained. The
˚
shortest intermolecular distance in 8 is 3.1975(20) A, close
to the ideal length for a C-N van der Waals contact.
In the case of 9, an apparent additional H-bonding
between nitriles and dihydropyrazine units in adjacent mo-
lecules results in a partially overlapped motif (Figure 4)
(again with an antiparallel arrangement of adjacent mole-
cules in a single stack). This is significant in that it indicates a
further possible way for influencing the solid state structure
(and hence the properties) of these molecules. C-H-π
interactions are absent in the pyrazinacene portion of the
molecules, although such interactions mediate interstack
organization. Nitrile groups of 9 do not interact with phenyl
CHs most likely because of the greater acidity of the dihy-
dropyrazine protons. Finally, crystals of 9 contain two
molecules of tetrahydrofuran per molecule, which are also
FIGURE 2. Crystal packing of 3. (a) Viewed along a-axis, (b)
viewed along b-axis. Note the orthogonal disposition of the hex-
aazaanthracene moieties.
There are significant close contacts between pyrazine N
atoms and atoms in adjacent hexaazaanthracene moieties of
neighboring molecules and a short contact between one of
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Richards et al.
JOCFeatured Article
FIGURE 4. Crystal packing of 9. (a) Viewed along a-axis,
(b) viewed along b-axis, (c) viewed along c-axis. Antiparallel stack-
ing is governed by hydrogen bonding. Two tetrahydrofuran mole-
cules per 9 (H-bonded at NH groups) are omitted for clarity.
hydrogen bonded at the dihydropyrazine unit. All of the
diphenyl-substituted pyrazinacenes are twisted from planar-
ity, although this can be mostly attributed to the steric
hindrance of the two phenyl groups since this effect is absent
from the diazatriphenylene-type derivatives 4 and 5. Unfor-
tunately, the higher pyrazinacenes 10-12 are relatively
intractable and could not be crystallized in a form suitable
for single-crystal X-ray analysis, although these compounds
can form other structures when appropriately substituted.14a
Perhaps the most intriguing finding of our single-crystal
X-ray analyses comes from the location of the protons in
compound 9. The condensation of 2 with 5,6-diamino-2,3-
dicyanopyrazine should yield a compound reduced at the
adjacent six-membered ring (i.e., ring B rather than ring C,
Scheme 1). Thus, tautomerization has occurred from the
expected product giving 9. Currently, it is unclear at which
point during the preparation and isolation of 9 does this
change occur, although the compound undergoes solvato-
chromism which might be related to tautomerism. Addition-
ally, crystallizations of 9 occasionally yielded mixtures of
differently colored crystals, although this could be due to
oxidative processes since crystals of 3 were actually obtained
during attempts to crystallize 6. It is currently unclear at
FIGURE 5. Electronic absorption spectra (solid lines) and fluores-
cence spectra (dashed line) of selected compounds in benzonitrile:
(a) 6 (black), 8 (blue), and 9 (red); (b) 10 (black) and 11 (red).
(c) Comparison of the solution state spectra (blue line) and solid
state spectra (red line) of compounds 3, 6, 8, 9, 10, and 11.
which point during crystallization does oxidation to 3 occur.
Tautomerization in reduced polyazaacenes22 has been in-
vestigated previously, and these compounds have attracted
attention as models for the study of aromaticity and molec-
ular orbital theory.5,12,13,20
Optical Absorption Studies. As shown in Figure 5, there is
a systematic red shift in their electronic absorption maxima
with increasing multiplicity of fused rings, as expected.
Comparing the fused ring compound 5 with the nonfused
ring analogue, 2, a 20 nm red shift of the absorption maxima
(22) (a) Sawtschenko, L.; Jobst, K.; Neudeck, A.; Dunsch, L. Electro-
chim. Acta 1996, 41, 123–131. (b) Tang, Q.; Liu, J.; Chan, H. S.; Miao, Q.
Chem.;Eur. J. 2009, 15, 3965–3969.
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Richards et al.
JOCFeatured Article
is observed for the fused ring compound. This is expected, as
the phenyl rings of compound 2 are twisted relative to the
plane of the molecule preventing efficient π-electronic over-
lap and causing a reduction in overall effective conjugation
length. A comparison of the dibenzo compound 10 with the
tetrabenzo compound 11 reveals a 23 nm red shift in the
absorption maximum in the case of compound 11, again,
most likely due to an increase in overall conjugation length.
This observation also suggests that the antiaromatic dihy-
dropyrazine rings do not separate the molecules into two
electronically discrete moieties as a simple resonance struc-
ture analysis might suggest. Fine structure emerges in the
electronic absorption spectra and fluorescence spectra of
compounds containing a greater number of fused rings, and
this can be associated with acene-like electronic structure
again despite the presence of the dihydropyrazine ring.
The electronic absorption spectra of 3, 6, 8, 9, 10, and 11 in
the thin film state (red lines) are also shown in Figure 3c in
comparison with their solution state spectra (blue lines). For
the compounds containing fewer fused rings, there is little
difference except for some broadening, which is expected
when measuring from thin solid films. This probably reflects
a lack of any specific aggregation of the compounds in the
solution or solid states. 9 is notable in that it possesses an
extra band at shorter wavelength around 360 nm when
measured in the solid state. Such bands are often observed
in compounds that tend to aggregate as stacks (so-called H-
aggregation), and this is partly consistent with its X-ray
crystal structure. In solution, both 10 and 11 possess weak
bands at 500 and 540 nm, respectively, also suggesting
aggregation. In the solid state, the UV/vis spectra of 10
and 11 are broadened and undergo a relative increase in
absorbance of bands at shorter wavelength, although no
additional absorption bands appear. Acene aggregation has
been studied by Desvergne and co-workers,23 and they also
observed a lower energy band in substituted tetracene deri-
vatives assigning it to a π-π stacking mode. In 10 and 11,
similar bands can be observed, but these are present also in
the solution state. The study of gelated states of suitably
substituted derivatives will lead to a better understanding of
these aggregation processes, which are critical for electronic
coupling in organic chromophoric compounds.
TABLE 1. Electrochemical Properties of Compounds 2-6 and 8-11 in
Benzonitrile Containing 0.1 M n-Bu4NClO4 at a Scan Rate of 100 mV/s
compound
first oxa
first reda
second reda
2
3
4
5
6
8
9
10
11
-
-
-0.76
-0.24
-1.51b
-0.63c
-1.43
-1.2
-1.17
-1.78
-1.92
-1.63
-0.91
-
-
-
0.47
-
0.32
0.20
0.69
-1.40c
-
-1.62
-1.72
-
-
aOxidation and reduction potentials vs ferrocene. bSee ref 11a. cThis
work. Compared to -0.70 (first red) and -1.48 (second red)11a
.
pyrazine rings passing from 2 to 3 (0.52 and 0.72 V for the
first and second reductions, respectively). Conversely, repla-
cing nitrile groups in 3 with methoxy groups (8) results in an
anodic shift of -0.96 and -0.71 V for the first and second
reductions, respectively, although compound 8 remains
stable against oxidization over the voltage range studied. 6
can be considered a reduced form of 3 with the former being
electron-rich compared to 3. Thus, an oxidation wave is
observed, and 6 requires a more negative potential for
reduction. Compound 9 has redox behavior similar to that
of 6, although the additional pyrazine ring has the now
predictable effect of cathodically shifting its first reduction
potential. Compounds 10-12 show some evidence for being
susceptible to oxidation as expected from the presence of the
dihydropyrazine group, but redox waves are poorly defined.
Measurement of these compounds is complicated by the
requirement of a polar solvent for the analysis so that
proton-coupled reactions including tautomerism and depro-
tonation are more likely to occur under such conditions.
From these data, we surmise that both oxidative ring
closure and increased pyrazine multiplicity favor electron
deficiency while compounds containing dihydropyrazine or
alkoxy groups are relatively electron-rich. In semiconductor
material parlance then perhaps we have access to both n- and
p-type molecules within the same compound class. On a
related subject, [6,6]-phenyl-C61 butyric acid methyl ester
(PCBM) is a fullerene C60-based compound commonly used
as an electron-accepting material in organic photovoltaic
(OPV) devices. PCBM has a LUMO of -3.75 eV and a
HOMO of -5.8 eV as determined by cyclic voltammetry.24
With the exception of compounds 10-12, the compounds
presented in this work show LUMO levels comparable with
PCBM, perhaps suggesting that these compounds or, for
example, polymeric derivatives might find application as
electron-accepting materials in OPVs.
Electrochemistry. Cyclic voltammetric (CV) data of com-
pounds 2-6 and 8-12 are summarized in Table 1. Soluble
compounds were studied in dry benzonitrile, while the higher
pyrazinacenes could only be dissolved in tetrahydrofuran or
dimethylsulfoxidefor CVinvestigations. Compoundswithout
reduced rings gave well-defined reduction waves, while the
redox behaviors of those bearing a dihydropyrazine ring are
more complex. Cyclic voltammograms are shown in Figure 6.
In the compounds containing fewer pyrazine rings, it is
possible to observe the effect of some of the structural
variations. Thus, the difference between compounds 2 and
5 is an oxidative ring closure at the phenyl rings of 2 giving 5,
which leads to a cathodic shift of the reduction waves (0.13
and 0.23 V for the first and second reductions, respectively).
Also, a much more significant cathodic shift of the reduction
waves is observed by increasing the number of condensed
Computational Analysis. 2-6 and 8-12 were modeled
using DFT25 at the B3LYP/6-31G(d,p) level in order to
predict the relative HOMO and LUMO energy levels. The
levels could also be determined to some extent experimen-
tally using cyclic voltammetry and UV/vis absorption spec-
troscopy. The relevant data obtained from those analyses are
contained in Table 2.
HOMO and LUMO levels are stabilized with an increase
in the number of pyrazine units and destabilized in the
reduced N,N-dihydropyrazine forms. This can be rationa-
lized in terms of the overall electron deficiency of the
(23) (a) Desvergne, J.-P.; Brotin, T.; Meerschaut, D.; Clavier, G.; Placin,
F.; Pozzo, J.-L.; Bouas-Laurent, H. New J. Chem. 2004, 28, 234. (b)
Desvergne, J.-P.; Del Guerzo, A.; Bouas-Laurent, H.; Belin, C.; Reichwagen,
J.; Hopf, H. Pure Appl. Chem. 2006, 78, 707–719.
(24) Sensfuss, S.; Al-Ibrahim, M. In Organic Photovoltaics, Sun, S.,
Sariciftci, N. S., Eds.; CRC Press: Boca Raton, FL, 2004; Vol. 23, p 529.
J. Org. Chem. Vol. 74, No. 23, 2009 8919

Richards et al.
JOCFeatured Article
FIGURE 6. Electrochemical behaviors of 2, 3, 5,11a 6, 8, and 9 in benzonitrile containing 0.1 M n-Bu4NClO4 at a scan rate 100 mV/s.
system;more pyrazine groups will tend to increase electron
deficiency, causing a stabilization of the HOMO and
LUMO, whereas reduced pyrazines are more electron-rich,
so a destabilization of the HOMO/LUMO levels is expected.
A similar pattern can be observed when comparing the
dicyano- and dimethoxy-hexaazaanthracene compounds 3
and 8, respectively. Due to the electron-deficient nature of
the pyrazine ring system, all of the compounds have low-
lying HOMOs;even in the case of the reduced pyrazine
compounds. This may account for the high stability of the
compounds toward oxidation. Although the phenazine com-
pounds tend to have comparatively destabilized HOMOs,
due to the higher ratio of more electron-rich phenyl rings, we
could not oxidize these compounds using similar conditions
to those used to oxidize 6, although their very low solubility
in mixtures of carbon tetrachloride and acetonitrile neces-
sary for the oxidation reaction complicated matters.26 The
dicyano-tetraazanapthalene compounds 2 and 5 have low-
lying HOMO and LUMO levels, which partly accounts for
their unusual reactivity toward nucleophiles. The dicyano-
hexaazaanthracene compound 3 has even lower-lying fron-
tier orbitals, suggesting even greater reactivity toward nu-
cleophilic substitution, accounting for the high reactivity of
this compound even toward aliphatic alcohols at room
temperature. Modeling of higher pyrazinacenes in their
oxidized form suggests even higher electron affinities and
ionization potentials, and it is likely that similar reactivity
will be observed for those compounds.
As mentioned previously, 2, 3, 5, and 8 exhibit two fully or
quasi-reversible reduction peaks, and with the exception of
compound 12, all of the other compounds exhibit irreversible
first reduction peaks. The first reduction peaks were used to
estimate the LUMO energy level of the molecules, and the
frontier energy levels were also estimated using DFT meth-
ods. The optical band gaps of the molecules were determined
from their UV/vis absorption spectra in dilute solution. The
(25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci,
B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada,
M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.;
Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A.
D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komar-
omi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.;
Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02;
Gaussian, Inc.: Wallingford, CT, 2004.
(26) Zibuck, R; Seebach, D. Helv. Chim. Acta 1988, 71, 237–240.
8920 J. Org. Chem. Vol. 74, No. 23, 2009

Richards et al.
JOCFeatured Article
TABLE 2. Energy Levels and Band Gaps of the Compounds Derived from Experimental and Computational Data
onset
Ered
CV
ELUMO
DFT
CV/OPT
EHOMO
DFT
EHOMO
OPT
EGAP
DFT
EGAP
compound
ELUMO
2
3
5
6
8
-0.69
-0.15
-0.56
-1.26
-1.14
-1.12
-1.72
-1.8
-4.12
-4.65
-4.24
-3.54
-3.67
-3.68
-3.08
-3
-3.6
-4.16
-3.76
-2.8
-2.86
-3.13
-2.05
-1.98
-2.69
-6.75
-6.88
-6.97
-6.82
-5.75
-6.09
-5.74
-5.14
-4.93
-5.49
2.63
3.28
-6.93
-6.92
-6.22
-6.37
-5.38
-5.71
-5.21
-
2.28
2.68
2.68
2.7
2.25
2.63
2.21
2.21
2.81
3.06
2.95
3.23
2.61
3.09
2.95
2.8
9
10
11
12
-
-
FIGURE 7. Energy levels obtained from experimental (optical/electrochemical; dashed line) and computational data (solid line), and HOMO
and LUMO structures of the compounds.
HOMO energy levels were calculated by subtracting the
optical band gap energy from the LUMO energy determined
by cyclic voltammetry. The results are shown in Table 2. The
DFT calculations overestimate the optically determined
band gap by 0.50 ( 0.24 eV, and the LUMO values are
overestimated by 0.71 ( 0.33 eV and the HOMO values by
0.15 ( 0.5 eV. The differences between the DFT calculations
and the experimentally determined values can be easily
FIGURE 8. Acene-like molecular orbital structure in the
LUMOþ1 of 5 and 9.
explained because (i) DFT calculations were based on a
single molecule in vacuum (no attempt was made to model
solvent effects or other intermolecular forces), (ii) imperfec-
tions in the DFT model, and (iii) imperfections in the
techniques used to determine the experimental values (e.g.,
the optical absorption edge may be difficult to determine due
to “tailing” etc.). Taking these factors into account, the DFT
calculations predict the electrochemical properties of these
compounds reasonably well, and overall trends in electro-
chemical behavior are predicted with good precision. Trends
in the electronic properties of the compounds are illustrated
in Figure 7, where energy levels of HOMOs and LUMOs are
plotted for 3, 6, and 8-11 together with the calculated
structures of the respective orbitals. Of these compounds, 9
stands out as having the smallest HOMO-LUMO band gap
obtained from both computational and experimental data.
Finally, we note a tendency for the calculated LUMO (for
6) or LUMOþ1 (for 2, 5, 8, 9, 10, and 11) structures to have a
symmetrical alternating structure (see Figure 8) similar to
that predicted for polyacenes,27,28 which can be seen as
materials containing two weakly coupled polyacetylene
chains possessing special properties,12,29 while HOMO orbi-
tal structures of the pyrazinacenes appear to be largely due to
conjugative delocalization (except for 5 where the HOMO is
localized on its triphenylene unit). This implies that acene-
like properties might be available in the excited states of these
molecules despite the presence of a dihydropyrazine ring.
Thus, we are currently speculating on the possibility of
photolytically induced insulator-semiconductor switching
of these molecules and their higher derivatives.
Protic compounds 9, 10, and 11 all exhibit oxidation peaks
by cyclic voltammetry. For compound 9, the onset of oxida-
tion occurs around 0.17 V, suggesting HOMO level of
around -4.97 eV and a band gap of 1.29 eV. These values
are not consistent with either the DFT calculations or the
(28) Lowe, J. P.; Kafafi, S. A.; LaFemina, J. P. J. Phys. Chem. 1986, 90,
6602–6610.
(27) (a) Kertesz, M.; Hoffmann, R. Solid State Commun. 1983, 47, 97–
~
102. (b) Garcia-Bach, M. A.; Penaranda, A.; Klein, D. J. Phys. Rev. B 1992,
45, 10891.
(29) (a) Houk, K. N.; Lee, P. S.; Nendel, M. J. Org. Chem. 2001, 66, 5517–
5521. (b) Kivelson, S.; Chapman, O. L. Phys. Rev. B 1983, 28, 7236.
J. Org. Chem. Vol. 74, No. 23, 2009 8921

Richards et al.
JOCFeatured Article
optical measurements, and it is likely that, in the case of 9, 10,
and 11, the electrochemical oxidation peaks may be a result
of deprotonation or tautomeric effects. Deprotonation of the
dihydropyrazine-group-containing compounds is a signifi-
cant issue for the azaacenes because of the presence of sites
both for protonation and for deprotonation. Thus, these
compounds should possess properties of acidity and basicity
albeit complicated by underlying tautomerism phenomena
such as in 9. We determined the pKa of 10 as 13.3 ( 0.1 for its
first deprotonation (by titration with DBU in DMSO30),
which is in the region of acidity of azoles, 4-pyridones, and
similar unsaturated heterocyclic amines.31 10 resists further
deprotonation using organic bases so that it is a simple
matter to isolate the monoanion. Similarly, the pyrazina-
cenes resist protonation by organic acids beyond a mono-
cationic species.
and the organic layer was separated, dried over Na2SO4, filtered,
and solvents were evaporated to give a brown-orange solid. The
crude product was purified by dissolving in ethyl acetate and
passing the solution through a pad of silica gel followed by
further washing of the silica pad with ethyl acetate. The solvent
was removed under reduced pressure to give the product as an
1
orange-brown crystalline solid: Yield 90 mg (91%); H NMR
(300 MHz, CD3CN, 70 ꢀC) δ = 7.77 (m, 4H, ArH), 7.58 (m, 2H,
ArH), 7.47 (m, 4H, ArH) ppm; MALDI-TOF-MS (dithranol)
calcd for C22H10N8 m/z = 386.1, found m/z = 389.1 [M þ 3H]þ.
Compound 3 did not afford satisfactory combustion analysis.
2,3-Dicyano-1,4-dihydro-7,8-diphenyl-1,4,5,6,9,10-hexaazaa-
nthracene, 6. A mixture of 2,3-dicyano-6,7-diphenyl-1,4,5,8-
tetraazanapthalene (0.7 g, 2.10 ꢀ 10-3 mol), diaminomaleoni-
trile (0.5 g, 4.63 ꢀ 10-3 mol), and sodium carbonate (0.9 g,
8.50 ꢀ 10-3 mol) in dimethylsulfoxide (25 cm3) was heated at
80 ꢀC for 4 h. The mixture was added to water (300 cm3) and
extracted with dichloromethane (200 cm3) followed by ethyl
acetate (2 ꢀ 400 cm3). The organic extracts were combined,
dried over Na2SO4, filtered, and concentrated to a black solid.
The crude product was purified by column chromatography
three times (silica gel, dichloromethane eluting to 5% methanol
in dichloromethane) to give the product as a yellow-orange solid
after evaporation of solvent under reduced pressure: Yield
Conclusion
In summary, we have prepared a series of nitrogenous
acenes, the pyrazinacenes, which have potential as n-type
organic semiconductors. Our synthesis is facile and allows
several points for elaboration of the compounds. Structural
features include short interplanar distances and long dis-
tance stacking in their crystals. Hydrogen bonding can also
be a determining factor in the case of compounds containing
reduced pyrazine rings. Combined electrochemical, electro-
nic absorption, and computational investigations indicate
the substantial electron deficiency of the compounds com-
posed of fused pyrazine rings, while those with one reduced
ring might exhibit ambipolar behavior.
Overall, this work suggests structural features that are
useful for design of materials with potential as organic
components of photovoltaic or semiconduting devices and
incorporation of these into appropriate aggregated or thin
film structures will be of interest. The absolute structures and
reactivities of the dihydro-substituted compounds described
here are further subject to tautomerism phenomena, and that
will be the subject of shortly forthcoming reports. Also, in this
work, we found that compounds containing up to four
unsubstituted edge-sharing pyrazine/dihydropyrazine rings
are easily accessible in useful quantities while longer acenes
are available but with non-aza-substituted benzene rings
interrupting the pyrazinacene structure. Synthesis of the high-
er pyrazinacenes is currently underway in our laboratories.
Finally, NH tautomerization is potentially a significant fea-
ture of these compounds, and we are currently studying this
phenomenon, also in the higher pyrazinacenes, with respect to
its effect on the acidity and basicity of these compounds.
1
430 mg (53%); H NMR (300 MHz, DMSO-d6, 25 ꢀC) δ =
11.40 (s, 2H, NH), 7.23 (m, 10H, ArH) ppm; IR (BaF2 disk) ν =
3170 (m), 3086 (m), 3021 (m), 2916 (m), 2851 (m), 2759 (m), 2231
(m, CN stretch), 1570 (m, CdC stretch), 1511 (s, CdN stretch),
1492 (s), 1476 (m), 1465 (m), 1445 (s, C-C stretch), 1410 (s),
1316 (m), 1194 (m); UV/vis (λmax) 415 (ε = 26 100 mol-1 cm-1
)
nm; LDI-TOF-MS calcd for C22H12N8 m/z = 388.10, found m/
z = 389.02 [M þ H]þ. Anal. Calcd for C22H12N8 1/2H2O: C,
3
66.49; H, 3.30; N, 28.20. Found: C, 66.32; H, 3.22; N, 27.81.
2,4-Dimethoxy-7,8-diphenyl-1,4,5,6,9,10-hexaazaanthracene,
8. 2,3-Dicyano-7,8-diphenyl-1,4,5,6,9,10-hexaazaanthracene
(50 mg, 1.30 ꢀ 10-4 mol) was dissolved in methanol (5 cm3),
and the solution was allowed to stand at room temperature for
2 h. The solvent was removed under reduced pressure, and the
residue was purified by column chromatography (neutral alu-
mina, 5% methanol in dichloromethane) to give the product as
an orange, crystalline solid: Yield 35 mg (68%); 1H NMR (300
MHz, CDCl3, 22 ꢀC) δ = 7.72 (m, 4H, ArH), 7.40 (m, 6H, ArH),
4.38 (s, 8H, ArH) ppm; IR (KBr pellet) ν = 3049 (w), 2943 (w,
C-H stretch), 1621 (w), 1569 (s, CdN stretch), 1515 (m), 1463
(s), 1399 (s), 1384 (s), 1324 (m), 1266 (m), 1190 (m), 1020 (m), 723
(w, C-H bend), 700 (m), 688 (m), 597 (m) cm-1; UV/vis (λmax
)
416 (ε = 43 600 mol-1 cm-1) nm; MALDI-TOF-MS (dithranol)
calcd for C22H16N6O2 m/z = 396.13, found m/z = 397.07 [M þ
H]þ. Anal. Calcd for C22H16N6O2: C, 66.67; H, 4.04; N, 21.21.
Found: C, 66.38; H, 4.28; N, 20.86.
2,4-Dicyano-5,12-dihydro-8,9-diphenyl-1,4,5,6,7,10,11,12-oc-
taazatetracene, 9. A mixture of 2,3-dicyano-6,7-diphenyl-
1,4,5,8-tetraazanapthalene (200 mg 6.37 ꢀ 10-4 mol), 2,3-
dicyano-5,6-diaminopyrazine (204 mg, 1.28 ꢀ 10-3 mol), and
sodium carbonate (230 mg, 2.17 ꢀ 10-3 mol) in dimethysulf-
oxide (14 cm3) was heated at 90 ꢀC for 2 h. Ammonium chloride
(200 mg) and acetic acid (200 mg) were added, and the solvent
was removed under reduced pressure. The residue was dissolved
in tetrahydrofuran and passed through a pad of silica gel. The
solvent was removed under reduced pressure, and the residue
was purified by column chromatography (silica gel, tetrahydro-
furan: dichloromethane 1:1) to give the product as an orange
solid which was further purified by recrystallization from tetra-
hydrofuran/hexane: Yield 102 mg (36%); 1H NMR (300 MHz,
acetone-d6, 25 ꢀC) δ = 7.43 (m, 10H, ArH) ppm; IR (KBr pellet)
ν = 3414 (NH stretch), 2924 (C-H stretch), 2221 (m, C-N
stretch), 1607 (m, C-C stretch), 1511 (s), 1482 (s), 1448 (s, C-C
stretch), 1421 (s), 1383 (s), 1364 (s), 1206 (s), 766 (m, C-H bend),
Experimental Section
Synthesis. 2,3-Dicyano-7,8-diphenyl-1,4,5,6,9,10-hexaazaan-
thracene, 3. Ruthenium(IV) oxide (21 mg, 1.58 ꢀ 10-4 mol) and
sodium periodate (725 mg, 3.39 ꢀ 10-3 mol) were added to a
mixture of compound 6 (100 mg, 2.57 ꢀ 10-4 mol), carbon
tetrachloride (5 cm3), acetonitrile (5 cm3), and water (2 cm3).
The mixture changed color immediately from yellow to brown/
orange, and it was then stirred at room temperature for 1 h.
Dichloromethane (10 cm3) and water (10 cm3) were added,
(30) Wu, A.; Masland, J.; Swartz, R. D.; Kaminsky, W.; Mayer, J. M.
Inorg. Chem. 2007, 46, 11190–11201.
(31) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456–463.
8922 J. Org. Chem. Vol. 74, No. 23, 2009

Richards et al.
JOCFeatured Article
695 (s) cm-1; UV/vis (λmax) 509.5 (ε = 22 400 mol-1 cm-1),
phy data for 2 (crystallized by diffusion of hexane into a
dichloromethane solution of 2) were collected at 150 K using
graphite-monochromated Mo KR radiation. C20H8N6, M =
334.34 g/mol, crystal size 0.53 ꢀ 0.08 ꢀ 0.02 mm3, yellow needle,
470.5 (ε = 53 100 mol-1 cm-1), 442.5 (ε = 38 000 mol-1 cm-1
)
nm; MALDI-TOF-MS (dithranol) calcd for C24H12N10 m/z =
440.12, found m/z = 441.05 [M þ H]þ. Satisfactory elemental
analysis could not be obtained although X-ray crystal structure
confirms molecular structure.
˚
˚
space group P2(1)/n, a = 9.0585(18) A, b = 6.9906(14) A, c =
3
˚
˚
25.138(5) A, β = 92.14(3)ꢀ, V = 1590.7(5) A , Z = 4, Fcalcd
=
5,14-Dihydro-[l,n]-dibenzo-5,6,7,12,13,14-hexaazapentacene,
10. A mixture of 5 (100 mg, 3.01 ꢀ 10-4 mol), 1,2-phenylene-
diamine (65 mg, 6.02 ꢀ 10-4 mol), and sodium carbonate
(134 mg, 1.24 ꢀ 10-3 mol) in N,N-dimethylformamide (7 mL)
was heated at 150 ꢀC for 2 h. The mixture was allowed to cool
to room temperature overnight, and the resulting precipitate
was filtered, rinsed with dichloromethane (20 mL), hot water
(3 ꢀ 25 mL), and tetrahydrofuran (20 mL) to give the product as
a yellow-brown solid, which was dried under reduced pressure:
Yield 45 mg (39%); 1H NMR (300 MHz, DMSO-d6, 25 ꢀC) δ =
11.51 (s, 2H, NH), 8.77 (m, 4H, 2 ꢀ ArH), 7.71 (m, 4H,
2 ꢀ ArH), 7.39 (m, 2H, ArH), 7.30 (m, 2H, ArH) ppm; IR
(KBr pellet) ν = 3397 (w, N-H str), 3194, (w, phenanthrene
C-H str), 2925 (w, 1,2-phenylene C-H str), 1592 (m, CdC str),
1547 (m, CdN str), 1495 (m), 1481 (m), 1462 (s, C-C str), 1404
(s), 1284 (m), 1229 (m, C-N str), 762 (m), 748 (s, C-H bend),
722 (s, C-H bend), 613 (m) cm-1; UV/vis (λmax) 458 (ε = 70 800
mol-1 cm-1), 446.5 (ε = 42 500 mol-1 cm-1), 431 (59 300 mol-1
cm-1), 407.5 (ε = 29 000 mol-1 cm-1) nm; MALDI-TOF-MS
(dithranol) calcd for C24H14N6 m/z = 386.13, found m/z =
385.97 [M]þ. Anal. Calcd for C24H14N6: C, 74.60; H, 3.65; N,
21.75. Found: C, 73.93; H, 3.83; N, 21.93.
5,14-Dihydro-[a,c,l,n]-tetrabenzo-5,6,7,12,13,14-hexaazapen-
tacene, 11. A mixture of 2,3-dicyano-[h,j]-dibenzo-1,4,5,10-tet-
raazaanthracene (200 mg, 6.02 ꢀ 10-4 mol), 9,10-diaminophe-
nanthrene (83.5 mg, 4.01 ꢀ 10-4 mol), and sodium carbonate (77
mg, 7.22 ꢀ 10-4 mol) in dimethylsulfoxide (10 cm3) was heated
at 150 ꢀC for 2 h. The mixture was allowed to cool to room
temperature and left to stand for 3 days. The resulting precipi-
tate was filtered and washed with dichloromethane (20 cm3), hot
water (50 cm3), and methanol (10 cm3). Residual low molecular
weight impurities were removed by sublimation to give the pure
product as a pale brown solid: Yield 45 mg (23%); 1H NMR (300
MHz, DMSO-d6, 100 ꢀC) δ = 11.10 (s, 2H, NH), 8.83 (m, 4H,
ArH), 8.72 (m, 4H, ArH), 7.70 (m, 8H, ArH) ppm; IR (KBr
pellet) ν = 3408 (NH stretch), 3062 (CH stretch), 2961 (CH
stretch), 1590 (w, CdC stretch), 1506 (w), 1458 (s, C-C stretch),
1394 (s), 1283 (m), 1227 (m), 768 (w, C-H bend), 724 (w, C-H
bend), 619 (w) cm-1; MALDI-TOF-MS (dithranol) calcd for
C32H18N6 m/z = 486.16, found m/z = 486.02 [M]þ. Satisfactory
elemental analysis could not be obtained.
1.396 mg/m3, μ(Mo KR) = 0.089 mm-1, 12 388 reflections were
measured (2θ < 54.3ꢀ) of which 3247 were unique (Rint 0.0308).
Refinement against F2 to wR2: 0.102 (all data), R1 (2447
reflections with I > 2σ(I)) 0.0413, 275 parameters, no restraints;
all non-H atoms were anisotropic refined.
X-ray crystallography data for 3 (crystallized by diffusion of
methanol into a chloroform solution of 6 (autoxidation to 3
occurred)) were collected at 150 K using silicon 111 monochro-
mated synchrotron radiation. C22H10N8, M = 386.37 g/mol,
crystal size 0.30 ꢀ 0.15 ꢀ 0.01 mm3, orange plate, space group
˚
˚
˚
C2/c, a = 19.1050(16) A, b = 38.439(3) A, c = 13.5450(11) A,
3
˚
R = 90.00ꢀ, β = 132.782(2)ꢀ, γ = 90.00ꢀ, V = 7300.6(10) A ,
Z = 16, Fcalcd = 1.406 mg/m3, μ = 0.091 mm-1, 43 122
reflections were measured (2θ < 60.5ꢀ) of which 8365 were
unique (Rint 0.0509). Refinement against F2 to wR2: 0.1025 (all
data), R1 (5597 reflections with I > 2σ(I)) 0.0415, 541 para-
meters, no restraints; all non-H atoms were anisotropically
refined.
X-ray crystallography data for 8 (crystallized by diffusion of
hexane into a dichloromethane solution of 8) were collected at
150 K using silicon 111 monochromated synchrotron radiation.
C22H16N6O2, M = 396.41 g/mol, crystal size 0.08 ꢀ 0.04 ꢀ 0.01
3
˚
mm , yellow plate, space group C2/c, a = 17.6352(9) A, b =
˚
˚
16.8426(8) A, c = 6.6854(3) A, R = 90.00ꢀ, β = 107.786(3)ꢀ,
γ = 90.00ꢀ, V = 1890.81(16) A , Z = 4, Fcalcd = 1.393 mg/m3,
3
˚
μ = 0.094 mm-1, 11 539 reflections were measured (2θ < 60.5ꢀ)
of which 2179 were unique (Rint 0.0311). Refinement against F2
to wR2: 0.0997 (all data), R1 (1496 reflections with I > 2σ(I))
0.0406, 138 parameters, no restraints; all non-H atoms were
anisotropically refined.
X-ray crystallography data for 9 (crystallized serendipitously
from tetrahydrofuran-d8 in an NMR tube) were collected at
150 K using graphite-monochromated Mo KR radiation. C24-
H12N10 2THF, M = 584.64 g/mol, crystal size 0.40 ꢀ 0.35 ꢀ
3 3
˚
0.22 mm , dark red block, space group C2/c, a = 9.3164(17) A,
˚
˚
b = 7.0649(13) A, c = 23.684(4) A, R = 90.00ꢀ, β = 99.945(3)ꢀ,
γ = 90.00ꢀ, V = 1535.4(5) A , Z = 4, Fcalcd = 1.438 mg/m3,
3
˚
μ(Mo KR) = 0.092 mm-1, 11 372 reflections were measured
(2θ < 54.3ꢀ) of which 2896 were unique (Rint 0.0539). Refine-
ment against F2 to wR2: 0.1237 (all data), R1 (1774 reflections
with I > 2σ(I)) 0.0538, 203 parameters, no restraints; all non-H
atoms were anisotropic refined.
7,16-Dihydro-[p,r]-dibenzo-5,7,8,9,14,15,16,18-octaazahepta-
cene, 12. A mixture of 5 (100 mg, 3.01 ꢀ 10-4 mol), 2,3-
diaminophenazine (63 mg, 3.01 ꢀ 10-4 mol), and sodium
carbonate (200 mg, 1.89 ꢀ 10-3 mol) in N,N-dimethylforma-
mide (10 mL) was heated at 150 ꢀC for 2 h. The mixture was
allowed to cool to room temperature overnight, and the result-
ing precipitate was filtered, rinsed with dichloromethane
(20 mL), hot water (3 ꢀ 25 mL), and tetrahydrofuran (20 mL)
to give the product as a dark brown solid: Yield 44 mg (30%); 1H
NMR (300 MHz, TFA-d1, 25 ꢀC) δ = 9.07 (m, 2H, ArH), 8.74
(m, 2H, ArH), 8.35 (m, 2H, ArH), 8.18 (m, 2H, ArH), 8.15
(s, 2H, ArH), 8.01 (m, 2H, ArH), 7.88 (m, 2H, ArH) ppm; IR
(KBr pellet) ν = 3385 (w, N-H str), 3064 (w, C-C str), 2919 (w,
C-C str), 1593 (m, C-C str), 1467 (s), 1463 (s), 1421 (s), 1368 (s),
1319 (m), 1219 (s, C-N str), 1133 (m, C-N str), 757 (m, C-H
bend), 723 (m, C-H bend) cm-1; MALDI-TOF-MS (dithranol)
calcd for C30H16N8 m/z = 488.15, found m/z = 491.30 [M þ
Acknowledgment. This research was supported by the
World Premier International Research Center Initiative (WPI
Initiative) on Materials Nanoarchitectonics, by Grant-in-Aid
for Scientific Research on Priority Area “Super-Hierarchical
Structures” from Ministry of Education, Culture, Sports,
Science and Technology, Japan, the National Science Founda-
tion (Grant 0804015 to F.D.) and NSF-EPSCoR programs.
We are also grateful to Dr. Akira Sato (NIMS) for X-ray
crystallographic data collection on compounds 2 and 9.
Supporting Information Available: Additional experimental
details. Cif files for 2, 3, 8, and 9. ORTEP diagrams of 2, 3, 8, and
9. Cartesian coordinates and projections of the calculated
structures. This material is available free of charge via the
Internet at http://pubs.acs.org.
2H]þ. Anal. calcd for C30H16N8 5/4H2O: C, 70.51; H, 3.65; N,
3
21.93. Found: C, 70.40; H, 3.53; N, 22.20.
X-ray Crystallography. Structure solutions were by direct
(32) Sheldrick G. M. SHELXTL 5.1; Bruker AXS Inc.: Madison, WI,
1997.
methods and refinement with SHELXTL.32 X-ray crystallogra-
J. Org. Chem. Vol. 74, No. 23, 2009 8923