Aerial oxidation of protonated aromatic amines. Isolation, X-ray structure, and redox and spectral characteristics of N- containing dyes

Article abstract of DOI:10.1021/jo3019126

This work reports the results of our investigation on the aerial oxidation of aromatic amines that are promoted by protic acid. While primary aromatic amines produce substituted phenazines as major products, N-phenyl-o- phenylenediamine produces polycycli

Full text of DOI:10.1021/jo3019126

Article
pubs.acs.org/joc
Aerial Oxidation of Protonated Aromatic Amines. Isolation, X‑ray
Structure, and Redox and Spectral Characteristics of N- Containing
Dyes
Suman K Roy, Subhas Samanta, Mominul Sinan, Pradip Ghosh, and Sreebrata Goswami*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India
S
* Supporting Information
ABSTRACT: This work reports the results of our investigation on the
aerial oxidation of aromatic amines that are promoted by protic acid. While
primary aromatic amines produce substituted phenazines as major
products, N-phenyl-o-phenylenediamine produces polycyclic aromatic
heterocycles like azaacene and secondary and tertiary amines give
exclusively the dyes containing a triphenylmethane moiety. Isolation of
the compounds and the effects of substitutions on the aromatic rings have
been investigated. In this context, plausible reaction steps that are involved
have been discussed. Single-crystal X-ray structure analyses of the
representative compounds are solved to authenticate their formation. In
almost every case, a high degree of delocalization of electron was noted.
The compounds have been characterized thoroughly and show rich
spectral properties. For example, the phenazine molecules exhibited
absorption peaks between 475 and 605 nm because of the charge-transfer
transition from the amine and tricyclopyrazine moiety. Their acidochromic and solvatochromic behaviors, which are supported
by theoretical calculations, are investigated. The polycyclic azacene molecule exhibits strong absorption in the visible region and
fluoresces with high quantum yield. The phenazine dyes undergo a quasi-reversible reduction at a low cathodic potential that
varies linearly as a function of Hammett’s constant.
reactions occur under neat conditions and are brought about by
air. It has also been established that the mediator transition
metal ions, in the above reactions, not only hold anilido
substrates in suitable proximity but also directly take part in the
redox events.
INTRODUCTION
■
Oxidation of aniline is a versatile and valued chemical reaction
that has led to the synthesis of many useful organic compounds.
Upon oxidation, aniline usually produces nitroso or nitro
derivatives. It is also known that oxidative N,N bond fusion
reaction of primary aromatic amines leads to formation of
hydrazo/azo/azoxy1 compounds. In contrast, similar oxidation
reactions involving carbon−nitrogen bond formation2a−f are
poorly developed. It is one of the most important areas of
contemporary research because such reactions can produce a
variety of nitrogen-containing, high-value chemicals ranging
from medicines to functional materials.3a−c Polyaniline is one
such prominent example of a chemical that is of paramount
importance4 in material science.
In recent years, we have been interested in transition-metal-
promoted5a−k carbon−nitrogen bond fusion reactions of aniline
or its derivatives. It has been established that coordinatively
unsaturated metal mediators containing oxidizable metal
centers can act as excellent mediators for Carom−N bond-
making reactions in anilido substrates.5a−k,6a−c For example,
Ru3+ and Os4+ metal ions mediate ortho-dimerization of
primary aromatic amines (ArNH2), whereas Fe3+ mediates5d
multiple C−N bond fusion reactions leading to the formation
of phenazine in 4-methoxyaniline. Very recently, we have
shown5j that strongly oxidizing OsO4 can bring about multiple
ortho-Carom−N bond formation reactions in aniline. All of these
With this background, we set out to explore the aerial
oxidation of protonated aniline substrates in neat solution.
Though H+ is known to behave as an oxidizing agent,7 it cannot
act as a template for gathering aniline moieties as it happens
with the transition-metal ion mediators. Thus, a different
reactivity pattern was anticipated. Accordingly, some chosen
protonated anilido substrates heated separately at 373 K in air
which resulted in slow color changes. In the cases of primary
aromatic amines, substituted phenazines (1a−d) were isolated.
This chemical transformation is a case of aerial oxidation
reaction associated with multiple Carom−N bond formations. In
contrast, a secondary aromatic amine like N-methylaniline or a
tertiary aromatic amine like N,N-dimethylaniline followed
totally different courses of reactions and yielded highly colored
triphenylmethane dyes 2a and 2b of the corresponding amines.
These chemical transformations are collected in Scheme 1.
Furthermore, similar chemical reactions of N-phenyl-o-phenyl-
enediamine resulted in an azaacene-like polycyclic heterocycle
Received: September 4, 2012
Published: October 18, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Compounds 1a−d
Scheme 2. Synthesis of the Triphenylmethane Dyes 2a−d
heterocyclic compound 3 as the major product. The schematic
presentation of the reaction is shown in Scheme 3.
Scheme 3. Synthesis of the Compound 3
Experimental details of the above chemical reactions are
available as Supporting Information. Microanalytical, positive-
ion electron spray ionization mass spectra (ESI-MS), together
with NMR spectral data of compounds 1a−e, 2a−d, and 3,
convincingly support their formulations. Characterization data
of the products are available as Supporting Information
(Figures S1−S23). 1H NMR signals of most of the compounds
are generally overlapping, so unambiguous assignment of
individual proton signals was not possible. However, proton
counts in the spectrum of each of the compounds corroborated
with the formulation of products. The characteristic N−H
proton signals appeared in the region 8.63−4.53 ppm. For
example, compound 1a showed two N−H signals at 5.92 and
8.35 ppm. The crystal violet dyes (2b−d) are symmetric and
showed only one N−H signal in the range 4.71−6.74 ppm. 1H
and 13C NMR data of all of the compounds are included in the
Experimental Section.
The complexes were primarily characterized by the
determination single-crystal X-ray structure analysis of the
representative compounds 1a, 1b, 1d, 1e, [2b]ClO4, and
[H3]CF3SO3. Suitable crystals of 1a,b,d,e were obtained by
slow evaporation of the solution of the respective compound in
dichloromethane−hexane mixture. Crystals of [2b]ClO4 were
obtained by slow diffusion of dichloromethane solution into
hexane, and an X-ray quality crystal of the protonated salt
[H3]CF3SO3 was obtained by slow evaporation of its solution
in dichloromethane−hexane solvent mixture. Molecular views
of compounds 1a and 1d are shown in Figure 1, and those of
compounds [2b]ClO4 and [H3]CF3SO3 are shown in Figure 2.
A close examination of the bond angles and lengths of the
compounds 1a,b,d,e reveals that there has been delocalization
of electron in the tricyclo rings [C16−C17−C18−N1−C1−
C2−C3−C4−C5−C6−N2−C13−C14−C15]. However the
bond parameters of the ring C1−C6 with average C−C length
of 1.395 Å are very similar to a benzonoid structure. The
3. Though oxidation of aniline has long been studied, as far as
we are aware, proton-assisted oxidation of aniline and its
derivatives has not been explored systematically. The reactions,
reported herein, have shown some patterns, and many of the
products, obtained thereof, are not achievable following
conventional synthetic protocols.
RESULTS AND DISCUSSION
■
Chemical Reactions. Since we are interested in ortho-C−N
bond fusion reaction of aromatic amines, we purposefully chose
aromatic amines without any ortho-substitution. When a
mixture of 1 g of aniline and 1 M HClO4 was heated at 100
°C in air, the initial light yellow solution slowly turned brown
over 72 h. After the reaction mixture was cooled to room
temperature, repeated rapid crystallization from diethyl ether
resulted in a brown product. The crude mixture was then
loaded on a preparative silica gel TLC plate for purification and
on elution by chloroform to afford the phenazine 1a in 30%
yield. The reactions also occur with other mineral acids like
H2SO4 and HCl. However, the reactions are more facile in
HClO4. Similar reactions with different substituted aromatic
anilines were performed, and the products are shown in
Scheme 1. In the case of p-anisidine, a different positional
isomer of the corresponding phenazine compound was
obtained.
The reactions of secondary and tertiary amines, however,
followed different courses and led to the formation of
triphenylmethane dyes (Scheme 2). Accordingly, the reaction
of N-phenyl-o-phenylenediamine under similar reaction con-
ditions resulted in the formation of an azaacene-like
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Figure 1. Molecular views of 1a and 1d.
Figure 2. Molecular view of [2b]ClO4 and [H3]CF3SO3.
corresponding bond lengths in the other two rings considerably
deviate from benzonoid situation and are more close to a
quinonoid form. A least-squares plane calculated through the
above 14 atoms of the dibenzo pyrazine fragment indicates that
the skeleton is planar with no atom deviating by more than
0.038 Å from the respective plane.
Overall structure solutions have authenticated the formula-
tion of the compounds as shown in Schemes 1−3. Bond
parameters of these compounds indicated substantial contribu-
tions of the possible resonance forms of the molecules.
Furthermore, the middle aryl amine ring in those compounds
is nearly perpendicular to the pyrazine ring with a torsional
angle C13−N2−C7−C12 = 92.8°. This orientation minimizes
the steric interaction between ortho-hydrogen viz H8 and H12
and those on the phenazine, H5 and H14.
two-phenyl carbon atom[(C2_c and C2_d)] is 120°, which
confirms that the central carbon atom is sp2-hybridized.
X-ray structure analysis of [H3]CF3SO3 has revealed that
phenyl substituents are nearly perpendicular to the slightly
twisted hexaazapentacene plane. Systematic shortening and
lengthening of bond lengths C27−N28 = 1.322 (3) Å and
C44−N11 = 1.349 (5) Å and C42−C43 = 1.383 (4) Å, C25−
C26 = 1.359 (6) Å clearly indicate a quinonoid structure of the
central pyrazine core. However, the two terminal phenyl rings
are in perfect benzonoid form (Table 3).
A very limited number of azapentacene compounds have
been reported so far and were synthesized using a totally
different synthetic protocol than ours. Following an identical
synthesis, as described above, we have also isolated
unsubstituted azapentacene8a−d (5,12-dihydrotetraazapenta-
cene 3a) from 1,2-diaminobenzene. ESI-MS and UV−vis
spectra of the isolated compound are identical to the reported
spectra (see S12 and S41 in the Supporting Information).
Furthermore, we note that azapentacenes are important
materials and their properties depend on substitution(s) as
well. However, to the best of our knowledge, the single-crystal
The compound [2b]ClO4 exists as a symmetric discrete unit.
The C−C bond distance between the central carbon atom C1
and the three phenyl rings [(C2, C2_c, C2_d)] is 1.455(6) Å,
and the bond distance between the phenyl ring carbon and
nitrogen atom of the N-methyl group is C5−N1 = 1.346(5) Å.
The bond angle between the central carbon(C1) atom and the
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Scheme 4. Plausible Mechanism of Phenazine by Aerial Oxidation of [ArNH3]+
X-ray structure of this class of compounds is scarce in the
literature.9
on further protonation and ring closure with C−N bond
formation reaction produces the reduced form of the reference
phenazine B. The final products are the results of further aerial
oxidation of the intermediate B. The structure of the
compound 1d is in fact a regioisomeric analogue of the
compounds 1a−c. The proposed mechanisms for the
compounds are shown in Schemes 4 and 5. The regioselectivity
of amination are controlled by the nature of substitution on the
intermediate B. Thus, whereas the chloro, hydrogen, and o-
OMe substitution direct reactivity at carbon C1, the p-OMe-
substituted phenazine under similar reaction conditions shows
reactivity on the C2 atom of the pendent phenyl ring (Scheme
5).
Plausible Reaction Steps. Since we have not been able to
identify or isolate the intermediates of the above reactions, their
mechanisms are not fully understood. However, a few general
comments on the above transformations may be worthwhile.
First the above reactions do not occur under anaerobic
conditions, confirming that the above proton-mediated redox
events are brought about by aerial oxygen. Second, the
reference reactions proceed even in the presence of a radical
scavenger like TEMPO (2,2,6,6-tetramethylpiperidinoxyl radi-
cal) or BHT (2,6-di-tert-butyl-4-methylphenol), thus excluding
the involvement of radicals in the above reaction pathway.
Furthermore, we note that aromatic amines are known to
undergo two-electron oxidation10 under acidic conditions and
the yields and rates of the reactions in the present cases are
dependent11 on the oxidation potentials of the amines. For
example, p-anisidine has the least11 oxidation potential (0.250
V) among the chosen primary amines and, accordingly, the
corresponding phenazine (1d) is obtained with a greater ease
and in highest yield (65%). By contrast, in the case of p-chloro-
substituted aniline (Eo = 0.86 V) the reaction is sluggish (needs
more than 72 h) and the yield of the product 1b is low (25%).
Based on the available literature, we propose that the primary
step for the formation of phenazine is a 2e− oxidation process,
In the cases of the secondary and tertiary amines, it is
proposed that protonation at the nitrogen center followed by
+
demethylation generates CH3 , which in the presence of air
produces13a,b HCHO. Subsequent reaction of HCHO with
N,N-dimethylaniline forms the intermediate compound D,
which under an acidic environment is transformed to the new
intermediate D1. Repetitions of the identical steps lead to the
final product (Scheme 6). Synthesis of the crystal violet from
N,N-dimethylaniline has been documented in the literature.
However, the compounds 2b−d are not available in the
literature.
Formation of compound 3 from N-phenyl-o-phenylenedi-
amine under similar reaction conditions may be explained by
considering the steps as shown in Scheme 7. The following
observations may be put forward in support of our proposition:
(i) it is an oxidation reaction where proton assists its oxidation;
ArNH3 → ArNH+ (nitrinium cation),10 which subsequently
+
undergoes a p-C−N bond fusion reaction producing p-
semidine. Repetition of the above steps and deamination12
lead to the formation of an intermediate compound A, which
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Scheme 5. Probable Mechanism of Regioisomeric Phenazines
(ii) our efforts to convert the compound dimer E to 3 were
unsuccessful. Thus, we presume that before dimerization two
semidine units combine, via deamination and C−N bond
formation,14a,b to produce an intermediate species F. Further
C−N bond formation reaction on F with another semidine
moloecule associated with aerial oxidation results in the
formation of the final compound 3.
Redox Properties. Redox behavior of the newly synthe-
sized compounds were studied by cyclic voltammetry. Detailed
experimental procedures are submitted as Supporting Informa-
tion. The compounds 1a−d showed three oxidation waves
along with one reduction wave. While the first oxidation
process is reversible (Figure 3), the other two responses are
irreversible. The irreversible redox feature is possibly due to the
possibility of involvement of proton-coupled electron-transfer
processes at the amine function. The redox potentials are
sensitive to the nature of substitution(s) and showed parallel
shift following the Hammett’s substitution constants. Reversi-
bility of the least potential oxidation wave allowed us to record
the EPR spectrum of a electrogenerated cation, 1a+. At low
temperature (liquid nitrogen temperature), it showed a sharp
EPR signal at g = 1.998 signifying the formation of an organic
radical (Figure 3). The one-electron oxidized compound 1a+ is
blue (see the Figure S42, Supporting Information) and is
unstable at room temperature. It readily returns to the parent
compound 1a.
The compounds 2a−d showed a reversible reduction and a
reversible oxidation waves in the potential range −1.5 to +1.8
V. One-electron stiochiometry of the couples was confirmed by
constant potential exhaustive electrolysis. Oxidation is believed
to occur at one of the reduced amine moieties, and the
reduction occurs at the oxidized p-quinone moiety. The
pentacene-like polyaromatic heterocyclic compound 3 showed
two irreversible oxidation waves and an irreversible reduction
wave presumably due to oxidation of the reduced amine fuction
and reduction for the diimine function, respectively. Electro-
chemical data of all the compounds have been submitted as
Supporting Information, and the cyclic voltammograms of the
representatative compounds are available as Figures S31−S34
and Table S5 in the Supporting Information.
Spectral Properties and DFT. Spectral properties of all of
the dyes were recorded in acetonitrile solvent. The spectra of
the compounds 1a−d are displayed in Figure 4, and the
corresponding absorption data are collected in Table S3
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Scheme 6. Probable Mechanism of Triarylmethane Dye
at 601 nm while that for 1d (with p-OMe substitution)
appeared at 478 nm. The compounds 1a−d showed acid-
ochromic properties. A representative compound 1a was
studied for this purpose. Color of the compound 1a is bluish-
violet and shows an absorption at 514 nm. Upon addition of 1
equiv of trifluoroacetic acid (TFA), an intense transition at 601
nm appeared which is associated with a shoulder at 514 nm.
Gradual addition of further TFA ultimately produced an ink-
blue solution. The spectral change is shown in Figure 5. Since
the two sites viz. N3 and N1 are suitable for protonation, two-
step processes are also evident in the spectral change. However,
the compound 1e has only one such nitrogen available for
protonation and is reflected in its spectral change (see Figure
S38, Supporting Information). Protonation of the amine
residue triggers a strong intramolecular charge transfer between
the donor amine and the acceptor quinone chromophore
resulting in red shift of absorption.
Scheme 7. Probable Mechanism of the Compound 3
Similarly, compound 3 undergoes protonation, and accord-
ingly its purple color became blue in [H3]+. X-ray structure
determination has confirmed that protonation occurs at only
one imine nitrogen viz. N11.
These types of compounds also showed solvatochromis-
m.15a−c Solvatochromism of the compound 2a has been
reported,16a−c and thus, we will not discuss it further. The
two representative compounds 1a and 3 have been studied for
this purpose. Their spectra were recorded in different solvents
ranging from low-polar toluene to highly polar methanol. In the
case of 1a, the color change is more prominent. Its solution in
toluene is red-violet and it becomes blue in methanol. Spectral
changes of 1a and 3 are shown in the Figures S35−S37, S39,
and S40 (Supporting Information).
(Supporting Information). In general, these compounds display
a moderately intense broad transition in the range 475−605
nm.
To gather information on the origin of these transitions, we
analyzed the B3LYP/6-31G(d)17 optimized geometries of the
compounds 1a and 3 by DFT calculations. The bond lengths
The lowest energy transition in these compounds is sensitive
to the donor strength of the amine residue. Thus, the chloro-
substituted phenazine 1b showed the lowest energy absorption
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Figure 3. Cyclic voltammogram and EPR spectrum of compound 1a.
Table 1. Selected Bond Distances (Å) for 1a, 1d, and 1e
1a
1d
1e
bond
length
exptl
calcd
1.364
exptl
calcd
1.318
exptl
calcd
1.318
N1−
1.377(5)
1.398(5)
1.371(5)
1.386(5)
1.395(6)
1.434(6)
1.436(6)
1.420(6)
1.421(6)
1.419(6)
1.315(3)
1.375(3)
1.306(3)
1.363(3)
1.467(4)
1.364(4)
1.421(3)
1.488(4)
1.356(4)
1.420(3)
1.322(3)
1.390(3)
1.311(3)
1.372(3)
1.464(3)
1.370(3)
1.431(3)
1.496(3)
1.365(3)
1.433(3)
C18
N2−
1.391
1.369
1.382
1.402
1.428
1.436
1.416
1.417
1.429
1.393
1.309
1.366
1.470
1.370
1.430
1.491
1.360
1.425
1.393
1.309
1.366
1.469
1.374
1.438
1.493
1.373
1.430
C13
N3−
C15
N4−
C16
C13−
C18
C13−
C14
C14−
C15
C15−
Figure 4. UV−vis spectra of 1a−1d in CH3CN.
C16
C16−
C17
C17−
C18
Table 2. Selected Bond Distances (Å) for [2b]ClO4
2b
bond length
exptl
calcd
C1−C2
C2−C3
C2−C7
C3−C4
C4−C5
C5−C6
C6−C7
N1−C5
1.455(5)
1.426(7)
1.435(8)
1.374(7)
1.423(8)
1.428(7)
1.362(7)
1.346(6)
1.446
1.425
1.421
1.376
1.423
1.420
1.381
1.357
LUMO is delocalized over the entire molecule. The HOMO is
delocalized over the amine nitrogen (34%) as well as the π
framework of the tricyclo moiety (47%), and the LUMO is
delocalized on the amine nitrogen atom (37%) and extended
over the tricyclo moiety (54%). On the basis of the values of
the oscillatory strength predicted using TD-DFT, the most
intense transition at 530 nm for the compound 1a is ascribed to
the excitation from HOMO to LUMO which may be assigned
as charge transfer, π →π*.
Figure 5. UV−vis spectral change of 1a upon addition of TFA in
CH2Cl2 solution.
and bond angles of the gas-phase optimized structures match
well with the experimental data (Tables 1−3).
The lowest energy transitions of the compounds, assigned
using the TD-DFT analysis, match well with the experimental
ones. The calculated transitions, oscillator strengths, and the
experimentally observed transition are listed in Table S4
(Supporting Information). The analysis of the frontier MOs of
the compound 1a indicates that the distribution of HOMO and
To have insight into the solvatochromism effect by DFT
calculation, we optimized the compound 1a in three different
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Conclusion. In summary, we have disclosed here a novel
synthesis of a variety of nitrogen-containing dyes by simple
acid-promoted oxidation of aromatic amines. An unusual ring-
closure reaction occurs in primary aromatic amine leading to
the formation of phenazines (1a−e) via multiple p-C−N bond
fusion. Some of these phenazines undergo further substitution-
controlled regiospecific C−N bond fusion reactions (1a−d).
Similar reactions of N-phenyl-o-phenylenediamine provide
azaacenes (3) as the major product and phenazine (E) as a
minor product. In contrast, metal-mediated oxidations of
aniline5a−k mostly result in o-semidine or its higher
homologues. Thus, the results of proton-catalyzed oxidation
of primary aromatic amines clearly are different from transition-
metal template-directed oxidation reaction. Secondary and
tertiary amines react in different ways and provide triaryl-
methane dyes via para carbon−carbon bond formation reaction.
Notably, such a simple synthetic protocol for the synthesis of a
variety of such dyes is not available in the literature. Many of
the polycyclic compounds, reported herein, are new, and their
single-crystal X-ray structures have been solved. Moreover,
phenazines are generally biologically active aromatic natural
products, and continual discoveries of new phenazine natural
products indicate that a large resource of metabolites with
biological significance like antibiotic, antitumor, antimalaria,
and antiparasitic19 activities are still to be disclosed in Nature.
Furthermore, the triphenylmethane dyes belong to an
important class of commercial dyes that have been extensively
employed in the medical and related biological sciences.20
Polycyclic aromatic hydrocarbons like azaacene are promising
for functional materials and act as n-type conducting materials
for organic field effect transistor (OFET) devices.21 The search
for practical applications of these materials is underway.
Table 3. Selected Bond Distances (Å) for [H3]CF3SO3
[H3]
bond length
exptl
calcd
N11−C44
N18−C25
C25−C44
C25−C26
C26−C27
C27−C42
C42−C43
C43−C44
N28−C27
N35−C42
1.349(8)
1.375(8)
1.458(8)
1.359(9)
1.399(9)
1.450(8)
1.383(9)
1.393(9)
1.322(9)
1.357(8)
1.355
1.391
1.455
1.376
1.426
1.455
1.399
1.395
1.326
1.367
solvents (toluene, acetonitrile, and methanol) using a
conductor-like polarizable continuum model. The TD-DFT
was done on the solvent-phase optimized geometry to see the
solvent effect on the lowest energy transition. The transition
follows the same trend, but the magnitude of shift of the lowest
energy transition is smaller than the experimentally observed
value (Figure 6).
Emission properties of all of the compounds were studied by
fluorescence spectroscopy. The crystal violet dyes are generally
fluorescent and showed maximum emission at 609 nm upon
exitation at 570 nm in CH3CN solvent at room temperature.
However, in the cases of the substituted compounds 2c and 2d
these appear at 594 and 611 nm in the same solvent. The
phenazine compounds 1a−c are nonemissive. However, their
close analogue 3 exhibits strong fluorescence in CHCl3 solvent
at room temperature. The emission spectrum is structured and
showed maximum emission λem in the range of 590−640 nm
upon excitation at 520 nm. Notably, both the emission and
absorption bands of the compound 3 are sensitive to acids.
Thus, upon protonation with HCl its maximum emission λem
and exitation wavelengths are red-shifted to 650−700 and 584
nm respectively.
EXPERIMENTAL SECTION
■
Materials and Measurements. All of the reagents and chemicals
were purchased from commercial sources and used without further
purifications. Syntheses of the compounds were done in open
atmosphere. CAUTION: Perchlorate salts of the compounds have
to be handled with care and appropriate safety precautions.
The emission and absorption spectra of compounds 3 and
[H3]+ are shown in Figure 7. Compound 3 was characterized
by a good fluorescent quantum efficiency of 0.52, and its
lifetime is 2.12 ns in CHCl3 solvents. Notably, theses values are
comparable to the emission properties18 of Rhodamine 6G.
1H and 13C NMR spectra were recorded on a 300 or 500 MHz
spectrometer, and SiMe4 was used as the internal standard. An
elemental analyzer was used to collect microanalytical data (C, H, N).
ESI mass spectra were recorded on a mass spectrometer. Electronic
Figure 6. Electronic distributions observed for the frontier molecular orbitals of the compounds 1a, [2b]ClO4, 3, and [H3]CF3SO3.
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Figure 7. UV−vis and fluorescence spectra of 3 and [H3]+.
absorption spectra were measured on a spectrometer using the
acetonitrile solutions. Emission spectra were recorded using a
spectrofluorimeter operating at room temperature (32 °C). Emission
quantum yields were obtained by using rhodamin-6 (ΦF = 0.78 in
ethanol) as reference. Cyclic voltammetry experiments were
performed using an electrochemical analyzer with a conventional
three-electrode configuration consisting of a glassy carbon working
electrode, platinum auxiliary electrode, and a nonaqueous Ag/AgNO3
124.4 (CH), 124.0 (CH), 123.9 (CH), 123.6 (CH), 121.5 (CH),
115.4 (CH), 102.5 (C), 96.3 (CH), 90.6 (CH). IR (KBr, cm−1): 2910,
1590, 1523, 1228, 620. ESI-MS: m/z 668 [M + H]+. Anal. Calcd for
C36H23N5Cl4: C, 64.79; H, 3.47; N, 10.49; Cl, 21.25. Found: C, 64.7;
H, 3.50; N, 10.31; Cl, 21.36.
1-Methoxy-N2,N7,5-tris(2-methoxyphenyl)-3-(2-methoxy-
phenylimino)-3,5-dihydrophenazine-2,7-diamine (1c). Yield:
1
55% (506 mg). Mp: 160−161 °C. H NMR (300 MHz, CDCl3): δ
8.63 (s, NH), 7.73−7.70 (m, 1H), 7.37 (dd, 1H, J1 =7.34, J2 = 8.25),
7.26 (s, 2H), 7.17 (d, 1H, J = 7.39), 7.11 (d, 1H, J = 7.6), 7.02−6.97
(m, 4H), 6.91−6.88 (m, 4H), 6.85−6.74 (m, 3H), 6.49 (s, 1H), 5.85
(s, 1H), 5.17 (s, NH), (3.98, 3.85, 3.81, 3.67, 3.63) (−OCH3). 13C
NMR (75 MHz, CDCl3): δ 156.1 (C), 155.3 (C), 153.9 (C), 151.2
(C), 150.8 (C), 148.8 (C), 146.7 (C), 142.9 (C), 142.0 (C), 140.8
(C), 134.2 (C), 133.6 (C), 131.3 (C), 130.2 (CH), 129.9 (CH), 125.7
(C), 121.9 (CH), 116.1 (CH), 112.9 (CH), 110.7 (CH), 101.8 (CH),
95.4 (CH), 93.9 (CH), 91.8 (CH), 56.4−55.6 (OCH3). IR (KBr,
cm−1): 2927, 1629, 1526, 1240, 620. ESI-MS: m/z 680 [M + H]+.
Anal. Calcd for C41H37N5O5: C, 72.44; H, 5.49; N, 10.30; O, 11.77.
Found: C, 72.43; H, 5.48; N, 10.31; O, 11.76.
reference electrode. The E1/2 values were determined as 1/2(Epa
+
Epc), where Epa and Epc are the anodic and cathodic peak potentials,
respectively. All potentials reported are not corrected for the junction
potential. The solvent in all experiments was CH3CN, and the
supporting electrolyte was 0.1 M tetraethylammonium perchlorate.
Synthesis of Phenazine Derivatives (1a−e). These compounds
were synthesized by the following general procedure. A mixture of the
amine (1 g) with HClO4 (1 M) at 100 °C in air for 1−3 days
produced substituted phenazines compound as the major products.
The initial light yellow solution became dark during this period.
After being cooled to room temperature, the mixture was cleaned
with diethyl ether. A dark mass thus obtained was purified on a
preparative silica TLC plate using the solvent mixture toluene/
chloroform/acetonitrile as eluant. The yields and spectral character-
ization of the products are as follows:
7-Methoxy-N-(4-methoxyphenyl)-5-(4-(4-methoxyphenyl-
amino)phenyl)-3-(4-methoxyphenylimino)-3,5-dihydrophena-
zin-2-amine (1d). Yield: 65% (570 mg). Mp: 175−177 °C. 1H NMR
(300 MHz, CDCl3): δ 8.10 (s, NH), 7.64 (d, 1H, J = 8.82), 7.29 (t,
3H, J = 9), 7.1−7.04 (m, 5H), 6.92 (d, 3H, J = 8.91), 6.82−6.72 (m,
7H), 6.05 (s, 1H), 5.52 (s, 1H), 4.53 (s, NH). 13C NMR (75 MHz,
CDCl3): δ 156.2 (C), 155.3 (C), 151.2 (C), 150.9 (C), 134.2 (C),
130.2 (C), 129.9 (C), 129.2 (C), 128.4 (C), 125.8 (C), 125.4 (C),
123.6 (CH), 122.7 (CH), 122.0 (CH), 121.9 (CH), 120.9 (CH),
120.7 (CH), 120.3 (CH), 113.0 (CH), 112.1 (CH), 110.8 (CH),
102.1 (CH), 95.5 (CH), 93.8 (CH), 91.9 (CH), 56.5−55.7 (OCH3).
IR (KBr, cm−1): 2930, 1650, 1535, 1235, 620. ESI-MS: m/z 650 [M +
H]+. Anal. Calcd for C34H30N4O4: C, 73.54; H, 5.01; N, 15.46; O,
11.46. Found: C, 73.43; H, 5.10; N, 15.31; O, 11.36.
N2,N7,5-Triphenyl-3-(phenylimino)-3,5-dihydrophenazine-
1
2,7diamine (1a). Yield: 30% (284 mg). Mp: 218−220 °C. H NMR
(300 MHz, CDCl3): δ 8.35 (s, NH), 7.63 (d, 1H, J = 8.7), 7.54 (d, 2H,
J1 = 7.5), 7.43−7.38 (m, 5H), 7.27−7.10 (m, 8H), 7.01−6.93 (m, 5H),
6.83 (d, 2H, J = 7.8), 6.12 (s, 1H), 5.92 (s, NH), 5.44 (s, 1H). 13C
NMR (75 MHz, CDCl3): δ 141.1 (C), 140.4 (C), 137.3 (C), 133.1
(C), 130.9 (CH), 129.7 (CH), 129.4 (CH), 128.8 (CH), 128.3 (CH),
123.1 (CH), 122.6 (CH), 121.5 (CH), 121.4 (CH), 119.3 (CH), 91.5
(C). IR (KBr, cm−1): 2923, 1627, 1523, 1245, 620. ESI-MS: m/z 530
[M + H]+. Anal. Calcd for C36H27N5: C, 81.64; H, 5.14; N, 13.22.
Found: C, 81.63; H, 5.15; N, 13.24.
7-Methoxy-N,5-bis(4-methoxyphenyl)-3-(4-methoxypheny-
limino)-3,5-dihydrophenazin-2-amine (1e). Yield: 30% (225 mg).
Mp: 138−140 °C. 1H NMR (300 MHz, CDCl3): δ 8.10 (s, NH), 7.64
(d, 1H, J = 8.86), 7.32 (d, 2H, J = 8.86), 7.14−7.01 (m, 4H), 6.92 (d,
2H, J = 8.89), 6.82−6.72 (m, 6H), 6.05 (s, 1H), 5.52 (s, 1H), (3.87,
3.83, 3.77, 3.67) (−OCH3). 13C NMR (75 MHz, CDCl3): δ 160.1
(C), 159.3 (C), 156.4 (C), 155.9 (C), 152.5 (C), 148.7 (C), 144.6
(C), 135.4 (C), 133.1 (C), 132.9 (C), 131.3 (CH), 129.9 (CH), 129.6
(CH), 129.4 (CH), 124.4 (CH), 122.6 (CH), 116.2 (CH), 114.7
(CH), 114.1 (CH), 110.2 (CH), 98.8 (CH), 91.4 (CH), 55.7−55.5
N2,N7,5-Tris(4-chlorophenyl)-3-(4-chlorophenylimino)-3,5-
dihydrophenazine-2,7-diamine (1b). Yield: 25% (218 mg). Mp:
1
186−187 °C. H NMR (500 MHz, DMSO-d6): 8.82 (s, 1H), 8.58 (s,
NH), 7.74 (d, 2H, J = 8.5), 7.61 (d, 1H, J = 8.5), 7.51−7.44 (m, 6H),
7.24 (t, 4H, J = 7.5), 7.06−7.02 (m, 3H), 6.91 (s, 1H), 6.78 (d, 2H, J =
8.5), 6.12 (s, 1H), 5.19 (s, NH). 13C NMR (125 MHz, d6 DMSO): δ
151.7 (C), 145.8 (C), 144.5 (C), 142.2 (C), 139.1 (C), 139.0 (C),
136.0 (C), 135.6 (C), 133.7 (C), 132.5 (C), 132.1 (C), 132.0 (CH),
131.7 (C), 131.1 (CH), 130.6 (C), 130.4 (CH), 129.9 (CH), 128.4
(CH), 127.9 (CH), 127.0 (C), 126.9 (CH), 126.7 (CH), 126.1 (CH),
10257
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The Journal of Organic Chemistry
Article
(−OCH3). IR (KBr, cm−1): 2923, 1645, 1520, 1245, 620. ESI-MS: m/
z 559 [M + H]+. Anal. Calcd for C34H30N4O4: C, 73.10; H, 5.41; N,
10.03; O, 11.46. Found: C, 73.11; H, 5.40; O, 11.45.
compound. The data were collected on a Bruker SMART APEX-II
equipped with graphite-monochromated Mo Kα radiation (λ =
0.71073 Å) and were corrected for Lorentz polarization effects. A total
of 7877 [1a], 11337 [1b], 32727 [1d], 27130 [1e], 9716 [2b]ClO4,
and 14604 [H3]CF3SO3 reflections were collected, out of which 2509
[1a], 4119 [1b], 9235 [1d], 5459 [1e], 999 [2b]ClO4, and 2240
[H3]CF3SO3 were unique [Rint = 0.0472 [1a], 0.068 [1b], 0.0459
[1d], 0.0450 [1e], 0.0677 [2b]ClO4 and 0.0436 [H3]CF3SO3] and
used in subsequent analyses. The structure was solved by employing
the SHELXS-97 program package and refined by full-matrix least-
squares based on F2 (SHELXL-97). All hydrogen atoms were added in
calculated positions. Crystallographic data of 1a,b,d,e, [2b]ClO4, and
[H3]CF3SO3 are collected in Table S1, Supporting Information.
DFT. Full geometry optimizations of the compounds were carried
out using the density functional theory methods at the (R) B3LYP.17
Calculations were performed without any symmetry constraints. H, C,
and N were assigned the 6-31G* basis set. All calculations were
performed with the GAUSSIAN09W program package.22 The
vibrational frequency calculations were performed to ensure that the
optimized geometries represent the local minima and that there are
only positive eigenvalues. Singlet excitation energies based on B3LYP-
optimized geometries were computed using the time-dependent
density functional theory (TD-DFT) formalism23 in acetonitrile
using the conductor-like polarizable continuum24 model. GAUSS-
SUM25 was used to calculate percentage contribution of various
groups to the frontier orbital and the fractional contributions of
various molecular orbitals in the optical spectral transition.
These compounds were synthesized by the following general
procedure. A mixture of the respective amine (1 g) with 1 M HClO4 at
100 °C in air for 1 day produced substituted dyes containing the
triphenylmethane moiety as the major product. The initial light yellow
solution became a bright color during this period. After being cooled
to room temperature, the mixture was cleaned with diethyl ether. The
dark mass was purified on a preparative silica TLC plate using the
solvent mixture chloroform/acetonitrile as eluant. The yields and
spectral characterization of the products are as follows:
Tris(4-(dimethylamino)phenyl)methylium (2a). Yield: 35%
1
(340 mg). Mp: 205−206 °C. H NMR (300 MHz, CDCl3): δ 7.26
(d, 2H, J = 8.5), 6.8 (d, 2H, J = 8.5), 3.20 (s, 6H). IR (KBr, cm−1):
3360, 2918, 1585, 1172, 623, 1085. ESI-MS: m/z 372 [M]+. Anal.
Calcd for C25H30N3ClO4: C, 64.25; H, 6.64; N, 8.65. Found: C, 64.26;
H, 6.63; N, 8.66.
Tris(4-(methylamino)phenyl)methylium (2b). Yield: 40% (400
mg). Mp: 192−194 °C. 1H NMR (300 MHz, CDCl3): δ 7.7 (d, 2H, J
= 6), 6.59 (d, 2H, J = 8.4), 4.71 (s, NH), 2.89 (s, 3H). 13C NMR (75
MHz, CDCl3): δ 207.1 (C), 152.4 (C), 132.5 (CH), 127.6 (C), 111.0
(CH), 31.0 (CH3). IR (KBr, cm−1): 3355, 2916, 1525, 1172, 620,
1085. ESI-MS: m/z 330 [M]+. Anal. Calcd for C22H24N3ClO4: C,
62.23; H, 5.90; N, 9.47. Found: C, 62.24; H, 5.91; N, 9.45.
Tris(4-amino-3,5-diisopropylphenyl)methylium (2c). Yield:
1
20% (180 mg). Mp: >250 °C. H NMR (300 MHz, CDCl3): δ 7.09
(s, 2H), 5.46 (s, NH2), 3.10−2.92 (q, 2H), 1.27 (d, 12H, J = 6.6). 13
C
NMR (75 MHz, CDCl3): δ 182.0 (C), 150.9 (C), 134.8 (CH), 132.2
(C), 128.6 (C), 27.8 (CH), 22.3 (CH3). IR (KBr, cm−1): 3396, 2923,
1579, 620, 1085. ESI-MS: m/z 540 [M]+. Anal. Calcd for
C37H54N3ClO4: C, 69.40; H, 8.50; N, 6.56. Found: C, 69.24; H,
8.31; N, 6.65.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data tables for 1a,b,d,e, [2b]ClO4, and
[H3]CF3SO3; relevant figures for characterization. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Tris(4-amino-3,5-dimethylphenyl)methylium (2d). Yield: 20%
1
(195 mg). Mp: >250 °C. H NMR (500 MHz, DMSO-d6): δ 6.92 (s,
2H), 6.74 (s, NH2), 2.13 (s, 6H). 13C NMR(125 MHz, DMSO-d6): δ
150.7 (C), 134.6 (C), 134.5 (CH), 132.1 (C), 128.6 (C), 22.7 (CH3).
IR (KBr, cm−1): 3365, 2956, 1535, 620, 1085. ESI-MS: m/z 372 [M]+.
Anal. Calcd for C25H30N3ClO4: C, 63.62; H, 6.41; N, 8.90; Found: C,
63.24; H, 6.80; N, 8.60.
AUTHOR INFORMATION
■
Corresponding Author
*Tel: +91-33-24734971. Fax: +91-33-24732805. E-mail: icsg@
iacs.res.in.
Compounds 3 and E were synthesized by the following reaction
between N-phenyl-o-phenylenediamine and perchloric acid at 100 °C
in air for 1 day. The initial dark solution became blue during this
period. After being cooled to room temperature, the mixture was
cleaned with diethyl ether. The crude mass was purified on a
preparative silica TLC plate using the solvent mixture chloroform as
eluant. The yields and spectral characterization of the products are as
follows:
5,12-Diphenyl-5,12-dihydroquinoxalino[2,3-b]phenazine
(3). Yield: 20% (118 mg). 1H NMR (300 MHz, CDCl3): δ 7.61 (t, 3H,
J = 7.2), 7.41−7.16 (m, 7H), 7.07 (t, 2H, J = 7.5), 6.89−6.78 (m, 6H),
5.34 (2H, s). 13C NMR (75 MHz, CDCl3): δ 147.6 (C), 144.3 (C),
131.2 (C), 130.5 (CH), 127.7 (CH), 126.8 (CH), 121.1 (CH), 120.8
(CH), 117.9 (CH), 117.1 (CH). IR (KBr, cm−1): 1620, 620. ESI-MS:
m/z 437 [M + H]+. Anal. Calcd for C30H20N4: C, 82.55; H, 4.62; N,
12.84. Found: C, 82.6; H, 4.51; N, 12.64.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support received from the Department of Science and
Technology (Project SR/S1/IC/0031/2010), New Delhi, is
gratefully acknowledged. S.G. sincerely thanks DST for a J. C.
Bose fellowship. Crystallography was performed at the DST-
funded National Single Crystal Diffractometer Facility at the
Department of Inorganic Chemistry, IACS. S.K.R., S.S., M.S.,
and P.G. are thankful to the Council of Scientific and Industrial
Research for their fellowships.
3,5-Dihydro-5-phenyl-3-(phenylimino)phenazin-2-amine
(E). Yield: 15% (140 mg). 1H NMR (500 MHz, DMSO-d6): δ 7.65 (d,
1H, J = 8), 7.59 (t, 2H, J = 7.5), 7.52 (t, 1H, J = 7.5), 7.35 (d, 2H, J =
7.5), 7.18−7.11 (m, 4H), 6.88 (t, 1H, J = 10), 6.67 (d, 2H, J = 8), 6.58
(s, NH), 6.44 (t, 2H, J = 10), 5.22 (s, 1H). IR (KBr, cm−1): 2930,
1625, 1540, 1250, 620. ESI-MS: m/z 363 [M + H]+. Anal. Calcd for
C24H18N4: C, 79.54; H, 5.01; N, 15.46. Found: C, 79.60; H, 4.91; N,
15.40.
X-ray Crystallography. Suitable X-ray quality crystals (0.11 ×
0.13 × 0.18) of [1a], (0.12 × 0.14 × 0.16) [1b], (0.12 × 0.14 × 0.16)
[1d], (0.11 × 0.14 × 0.16) [1e], and (0.07 × 0.09 × 0.26)
[H3]CF3SO3 were obtained by slow evaporation of a dichloromethane
solution of the compound, and (0.10 × 0.13 × 0.16) [2b]ClO4 was
obtained by slow diffusion of dichloromethane hexane mixture of the
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