Near-infrared absorbing azo dyes: Synthesis and X-ray crystallographic and spectral characterization of monoazopyrroles, bisazopyrroles, and a boron-azopyrrole complex

Article abstract of DOI:10.1021/jo9003019

(Chemical Equation Presented) Symmetric 2,5-bisazopyrroles 2(a-d) were synthesized by a one-step reaction of substituted phenyl diazonium salts [R′(Ph)N₂⁺Cl^-] [a, R′ = 4-N(CH ₃)₂; b, R′ = 2-OH; c, R′

Full text of DOI:10.1021/jo9003019

pubs.acs.org/joc
Near-Infrared Absorbing Azo Dyes: Synthesis and X-ray
Crystallographic and Spectral Characterization of Monoazopyrroles,
Bisazopyrroles, and a Boron-Azopyrrole Complex
Yan Li, Brian O. Patrick, and David Dolphin*
Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, B.C., V6T 1Z1,
Canada
david.dolphin@ubc.ca
Received February 11, 2009
Symmetric 2,5-bisazopyrroles 2(a-d) were synthesized by a one-step reaction of substituted phenyl
þ
diazonium salts [R0(Ph)N2 Cl-] [a, R0=4-N(CH3)2; b, R0=2-OH; c, R0 = 2-CO2H; d, R0=4-NO2]
with pyrrole under basic conditions. Asymmetric 2,5-þbisazopyrroles 3(a-d) were synthesized by
reacting substituted phenyl diazonium salts [R00(Ph)N2 Cl-] (a, R00 =4-OCH3; b, R00 =H; c, R00 =
4-Br; d, R00 =4-NO2) with 2-(4-dimethylaminophenylazo)-1H-pyrrole (1a) under the same condi-
tions. The reactions of 2a with boron trifluoride and iodomethane provided a BF2-azopyrrole
complex of 1H-pyrrolo[2,1-c]-1,2,4,5-boratriazole (4) and 2,5-bisazo-1-methylpyrrole 5. X-ray
crystallographic and spectral analysis of 1a, 2a, 2b, and 4 showed that 1a has three crystal forms:
1a(I), 1a(II), and 1a(III), the latter two bearing a bicyclic ring system formed via intermolecular
hydrogen bonding. Complex 4 was found to be the most planar due to a rigid trans-azo configuration
˚
and has the longest NdN bond distances (1.322 and 1.300 A) and wavelength of maximum
absorption (754 nm). The NdN bond distances increase in the sequence of monoazopyrrole [1a(I):
˚
˚
1.253 A], bisazopyrrole (2a: 1.283 A), bisazopyrrole with intramolecular hydrogen bonding (2b: 1.293
˚
and 1.293 A), and the BF2-azopyrrole complex. Their maximum absorptions shift bathochromically
in the sequence of monoazopyrrole (1a: 443 nm), bisazopyrroles [2(a-d), 3(a-d), 5: 486-615 nm],
and the BF2-azopyrrole complex. These results are important for the design of near-infrared
absorbing azo dyes and suggest an efficient path for the preparation of near-infrared absorbing azo
dyes by effectively enhancing π-electron delocalization.
Introduction
benzene dyes2 and are environmentally friendly.3 Near-
infrared absorbing azo dyes, based on five-membered het-
erocyclic rings, have been made effectively by using three
methods: (1) introducing electron-donating or -accepting
groups to the azo-linked aromatic ring to increase molecular
Azo dyes are a class of compounds containing a NdN
double bond and, due to their ability to absorb visible light,
and ease of synthesis, have been extensively used in the
textile, fiber, leather, paint and printing industries for more
than a century.1 Among the known azo dyes, five-membered
heterocyclic azo dyes, such as azothiazole, azothiophene,
azopyrrole, and azofuran, are important since they have
pronounced bathochromic absorptions compared to azo-
(2) (a) Weaver, M. A.; Shuttleworth, L. Dyes Pigm. 1982, 3, 81. (b) Sekar,
N. Colourage 1994, 41, 38. (c) Towns, A. D. Dyes Pigm. 1999, 42, 3. (d)
Ledoux, I.; Zyss, J.; Barni, E.; Barolo, C.; Diulgheroff, N.; Quagliotto, P.;
Viscardi, G. Synth. Met. 2000, 115, 213. (e) Li, Z.; Zhao, Y.; Zhou, J.; Shen,
Y. Eur. Polym. J. 2000, 36, 2417.
(3) (a) Freeman, H. S.; Hinks, D.; Esancy, J. Physico-Chemical Princi-
ples of Color Chemistry; Peters, A. T., Freeman, H. S., Eds.; Blackie Academic
and Professional: London, UK, 1996; 254 (b) Sekar, N. Colourage 2004, 51, 58.
(1) Herbst, W.; Hunger, K. Industrial Organic Pigments: Production,
Properties, Applications 2nd; VCH: Weinheim, Germany, 1997.
DOI: 10.1021/jo9003019
r
Published on Web 07/02/2009
J. Org. Chem. 2009, 74, 5237–5243 5237
2009 American Chemical Society

Li et al.
JOCArticle
donor-acceptor polarization;4 (2) expanding the azo-linked
aromatic ring from benzene to naphthalene and from thia-
zole to benzothiazole;5 and (3) extending the conjugated
NdN bond from monoazo to bisazo dyes to enhance
molecular π-resonance effects.6 The near-infrared absorbing
azo heterocycles have been found to be useful as optical data
carriers,4e,4f,6b thermal transfer recording media,4c,5a,5b,6c elec-
trophotographic toners,4b,4d,6f and semiconductor lasers.5d,6e
While numerous syntheses and applications of five-mem-
bered heterocyclic azo dyes have been reported, studies on
the relationship between molecular structure and color by
theoretical and experimental methods have also received
much interest. Most of the theoretical work addresses mo-
lecular donor-acceptor polarization,7-9 with relatively less
interest in the donor-acceptor polarization enhanced by
π-resonance effects of expanded aromatic rings and extended
NdN bonds.9 Experimental studies with X-ray crystallogra-
phy have been limited since it has been difficult to compare
differences in crystallographic structures and the resulting
change in color. In 2004, McNelis and co-workers10 success-
fully investigated the relationship between the donor-
acceptor polarization of azothiophenes and their colors
using X-ray crystallography. 2-(4-Diethylaminophenylazo)
significantly improved by the insertion of a vinylene group
between [4-di(hydroxyethyl)aminophenyl]azobenzene and
benzimidazole, the λmax was bathochromically shifted from
466 to 487 nm. This result indicated, not surprisingly, the
importance of π-resonance effects on the bathochromic
absorptions.
Most azo dyes are prepared via coupling of aromatic
diazonium salts with a nucleophilic component. While this
is true for 2,5-bisazathiophenes which are prepared from
thiophen-2-yl diazonium salts,12 the corresponding analo-
gues of 2-aminothiophene13 and 2,5-diaminothiophene14
require multicomponent condensations; pyrrole, on the
other hand, is more reactive than thiophene or even aniline
(which is 1020 times more reactive than benzene). Few 2,5-
bisazopyrroles have been reported15-17 and we report here
the coupling to pyrrole to produce 2,5-bis(arylazo)pyrroles.
A series of symmetric 2,5-bis(arylazo)-1H-pyrroles [2(a-d)]
and asymmetric 2,5-bis(arylazo)-1H-pyrroles [3(a-d)] have
been prepared and their maximum absorptions appear between
486 and 615 nm in dichloromethane, which are longer than that
of monoazopyrrole 1a (447 nm). When 2a was reacted with
iodomethane, symmetric 2,5-bis(arylazo)-1-methylpyrrole (5)
was obtained, which has a similar absorption at 563 nm.
However, when 2a was reacted with boron trifluoride, the
resulting boron-azopyrrole complex 4 displayed a near-infra-
red absorption at 754 nm. Comparisons of the X-ray crystal-
lographic structures of 1a, 2a, 2b, and 4 show that 4 is the most
planar and has the longest NdN bond distance (1.322 and
˚
thiophene has a NdN bond distance of 1.226 A; when the
strong electron-accepting tricyanovinyl group was introduced,
the resulting products, 2-(4-diethylamino)phenylazo-5-tricya-
novinylthiophene and 2-(4-tricyanovinyl)-phenylazo-5-die-
thylaminothiophene, exhibited NdN bond distances of 1.297
˚
˚
and 1.314 A, with maximum absorptions at 719 and 708 nm,
1.300 A), whereas 1a, 2a, and 2b have NdN bond distances at
1.253 [1a(I)], 1.286 [1a(II)], 1.289 [1a(III)], 1.283 (2a), and 1.293
respectively. Regrettably, the relationship between π-reso-
nance, planarity, and colors for five-membered heterocyclic
azo dyes has not been explored theoretically nor via X-ray
crystallography. In 2004, Centore and co-workers11 investi-
gated [4-di(hydroxyethyl)aminophenyl]azobenzenes contain-
ing a benzimidazole group. When molecular planarity was
˚
and 1.293 A (2b).
Results and Discussion
Synthesis of 2,5-Bisazopyrrole and Derivatives. Following
the general procedure to prepare azo dyes, substituted
phenyl diazonium salts were formed by reacting aromatic
amines [R0PhNH2: R0=4-N(CH3)2, 4-OCH3, 2-OH, H, 4-Br,
2-CO2H, 4-NO2] with sodium nitrite/aqueous HCl at tem-
peratures lower than 0 °C. Neutralization with pyridine and
treatment with a half equivalent of pyrrole in methanol gave
symmetric bisazopyrroles 2(a-d) [R0=4-N(CH3)2, 2-OH, 2-
CO2H, 4-NO2] in 61-82% yields (Scheme 1, path 1). Mono-
azopyrrole 1a was the major product when the 4-dimethyla-
minophenyl diazonium salt reacted with excess pyrrole under
the same conditions (Scheme 1, path 2). Further reactions of
1a with substituted phenyl diazonium salts provided asym-
metric bisazopyrroles 3(a-d) (R00=4-OCH3, H, 4-Br, 4-NO2)
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5238 J. Org. Chem. Vol. 74, No. 15, 2009

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JOCArticle
SCHEME 1. Synthesis of Symmetric 2,5-Bisazopyrroles 2(a-
d) and Asymmetric 2,5-Bisazopyrroles 3(a-d)
SCHEME 2. Reactions of 2,5-Bisazopyrrole (2a) with Boron
Trifluoride and Iodomethane
in 62-79% yields (Scheme 1, path 3). 1a can be converted into
2-(4-dimethylaminophenylazo)-5-nitro-1H-pyrrole (1b) by ni-
trition at -50 °C.18
the oxygen of the o-hydroxy groups and the nitrogen of the
azo bond (dN-pyrrole) with O-H N bond distances of
3 3 3
˚
2.629 (O1 N2) and 2.601 A (O2 N4). Intramolecular
3 3 3
3 3 3
When 2a was mixed with boron trifluoride and diisopro-
pylethylamine, under nitrogen, and refluxed in dichloro-
methane for 1 h, the corresponding BF2-azopyrrole
complex 4 was formed in 85% yield (Scheme 2, path 1).
Complex 4 was purified by chromatography and was found
to be stable in nonprotic solvents such as acetone, tetrahy-
drofuran, acetonitrile, dichloromethane, benzene, etc., but
unstable in protic solvents such as methanol. When 2a was
refluxed with iodomethane and sodium carbonate in acetone
for 3 h, 2,5-bisazo-1-methylpyrrole 5 was obtained in 91%
yield (Scheme 2, path 2).
hydrogen bonding has been reported for o-hydroxy mono-
azo dyes, where the azophenol tautomers have H N(azo)
3 3 3
22
˚
bond distances of 1.66-1.92 A and the hydrazoketone
tautomers have H-N(hydrazone) bond distances of 0.84-
1.24 A.22,23 The H N bond distances of 1.839 (H1o N2)
˚
3 3 3
3 3 3
˚
and 1.819 A (H2o N4) indicate that 2b principally exists as
3 3 3
an azophenol tautomer. Intramolecular hydrogen bonding
between N-1 of pyrrole and a nitrogen of the azo bond for 1a
(I), 2a, and 2b should be very weak if it exists at all as the
distances between the N-1 proton of pyrrole and the nitrogen
˚
˚
of the azo bond [1a(I): 2.592 A; 2a: 2.518 and 2.518 A; 2b:
˚
2.458 and 2.568 A] are slightly smaller than the sum of the
While monoazopyrrole 1a is yellow, these new bisazopyr-
roles [2(a-d), 3(a-d), and 5] are red to blue, and the boron-
azopyrrole complex 4 is green in dichloromethane.
˚
˚
van der Waals radii of hydrogen (1.20 A) andnitrogen(1.50 A)
atoms.24 Surprisingly, the N-1 proton of 1a(I) is rotated
away from the nitrogen of the azo bond, since the H9n-N9-
C28 angle (127.7°) is larger than the H9n-N9-C25 angle
(122.8°). For the BF2-azopyrrole complex 4, the nitrogen of
one azo bond (Ar-Nd) is coordinated to the N-1-BF2
substituent at the pyrrole to form the novel structure 1H-
pyrrolo[2,1-c]-1,2,4,5-boratriazole. The boron atom is sp3
hybridized with an F1-B1-F2 angle (112.0°) divided by the
B1/N2/N3/C9/N4 plane. The B-N bond distances are 1.537
X-ray Crystal Structures. The structures of 1a, 2a, 2b, and
4 were determined by X-ray crystallography and are shown
in Figure 1. Their selected bond lengths and angles are listed
in Table 1. Crystal data and structure refinements are
summarized in the Supporting Information. While 2-phenyla-
zo-1-vinylpyrrole (6)19 and a macrocyclic 2,3-bisazopyrrole16
have been reported with X-ray crystal structures, and some
boron-azo complexes have been identified by NMR, IR, and
MS spectra,20,21 1a, 2a, 2b, and 4 are the first examples of
monoazo-1H-pyrroles, 2,5-bisazo heterocycles (thiophene,
pyrrole, and furan), and boron-azo complexes reported with
X-ray crystal structures.
˚
(B1-N4) and 1.646 A (B1-N2).
The structure of 1a(I) is nearly planar, where the dihedral
angle between the pyrrole and phenyl (C29/C30/C31/C32/
C33/C34) planes is only 5.36°. As shown by side-view draw-
ings in Figure 2, bisazopyrrole 2a is less coplanar and
butterfly like, with identical dihedral angles between the
pyrrole and phenyl planes (C3/C4/C5/C6/C7/C8 and C3*/
C4*/C5*/C6*/C7*/C8*) being 16.59°, which are larger than
the dihedral angles of the phenyl planes in bisazo aromatic
dyes (2.3-14.1°).25 Intramolecular hydrogen-bonded 2b is
slightly ruffled, with the angles of the corresponding two
Monoazopyrrole 1a has three forms within a unit cell: 1a
(I) shows no hydrogen bonding while 1a(II) and 1a(III) are
both intermolecularly hydrogen bonded to each other be-
tween the nitrogen-1 of pyrrole and the nitrogen of an azo
bond with N-H N bond distances of 3.020 (N1 N7)
3 3 3
3 3 3
˚
and 3.087 A (N3 N5). Compound 2a has crystallographic
3 3 3
mirror symmetry with identical H4n-N4-C9 and H4n-
N4-C9* angles of 125.8°. Compound 2b is slightly asym-
metric, it has two intramolecular hydrogen bonds between
(22) (a) Olivieri, A. C.; Wilson, R. B.; Paul, I. C.; Curtin, D. Y. J. Am.
Chem. Soc. 1989, 111, 5525. (b) Conner, J. A.; Kennedy, R. J.; Dawes, H. M.;
Hursthouse, M. B.; Walker, N. P. C. J. Chem. Soc., Perkin Trans. 2 1990, 1,
203.
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1998, 39, 3571. (b) You, X.-L.; Lu, C.-R.; Huang, Z.-L.; Zhang, D.-C. Dyes
Pigm. 2004, 63, 217. (c) Luboch, E.; Kravtsov, V. C. J. Mol. Struct. 2004, 699,
9. (d) Gilli, P.; Bertolasi, V.; Pretto, L.; Lycka, A.; Gilli, G. J. Am. Chem. Soc.
2002, 124, 13554.
(18) Morgan, K. J.; Morrey, D. P. Tetrahedron 1966, 22, 57.
(19) Trofimov, B. A.; Schmidt, E. Y.; Mikhaleva, A. I.; Vasil’tsov, A. M.;
Zaitsev, A. B.; Smolyanina, N. S.; Senotrusova, E. Y.; Afonin, A. V.;
Ushakov, I. A.; Petrushenko, K. B.; Kazheva, O. N.; Dyachenko, A. A.;
Smirnov, V. V.; Schmidt, A. F.; Markova, M. V.; Morozova, L. V. Eur. J.
Org. Chem. 2006, 4021.
(20) Hohaus, E.; Wessendorf, K. Z. Naturforsch. B: Anorg. Chem., Org.
Chem. 1980, 35, 319.
(24) Ladd, M. F. C. Structure and Bonding in Solid State Chemistry; Ellis
Horwood Ltd: New York, 1979; p 253.
(21) (a) Das, M. K.; Bandyopadhyay, S. N. J. Chem. Res., Synop. 1991, 4,
83. (b) Dedkov, Y. M.; Lozinskaya, E. F. Izv. Vyssh. Uchebn. Zaved., Khim.
Khim. Tekhnol. 2002, 45, 118.
(25) (a) Gilardi, R. D.; Karle, I. L. Acta Crystallogr. 1972, B28, 1635. (b)
Freeman, H. S.; McIntosh, S. A.; Singh, P. Dyes Pigm. 1997, 35, 11. (c)
Freeman, H. S.; McIntosha, S. A.; Singh, P. Dyes Pigm. 1997, 35, 149.
J. Org. Chem. Vol. 74, No. 15, 2009 5239

Li et al.
JOCArticle
FIGURE 1. ORTEP views of the structures 1a, 2a CH2Cl2, 2b, and 4 (hydrogen atoms of 1a are not shown).
3
˚
TABLE 1. Selected Bond Distances (A) and Angles (deg) of the Structures 1a, 2a CH2Cl2, 2b, and 4
3
2a CH2Cl2
1a(I)
1a(II)
1a(III)
2b
4
3
0.84(3)
H-N(pyrrole)
0.84(2)
0.90(2),
0.88(2)
0.87(2)
B-N(pyrrole,azo)
1.537(2), 1.646(2)
H
N-N
N(azo)
2.142
2.226
1.289(2)
1.390(2)
1.420(2)
113.8(1)
115.0(1)
1.839, 1.819
3 3 3
1.253(2)
1.42692)
1.453(2)
110.9(2)
112.1(2)
1.286(2)
1.386(2)
1.423(2)
114.2(1)
114.8(1)
1.283(2)
1.380(2)
1.404(2)
113.4(1)
113.6(1)
1.293(1), 1.293(1)
1.379(2), 1.380(2)
1.408(2), 1.412(2)
113.1(1), 114.2(1)
116.1(1), 115.0(1)
1.300(2), 1.322(2)
1.361(2), 1.377(2),
1.392(2), 1.398(2)
112.5(1), 106.7(1),
114.4(1), 119.9(1)
N(azo)-C(pyrrole)
N(azo)-C(phenyl)
N-N-C(pyrrole)
N-N-C(phenyl)
X(B,C)-N4-C
143.2(1), 109.8(1), 106.9(1)
113.8(1), 126.3(1), 119.9(1)
X(B,N)-N2-Y(C,N)
planes being 5.66° (pyrrole and C1/C2/C3/C4/C5/C6 plane)
and 6.43° (pyrrole and C11/C12/C13/C14/C15/C16 plane).
Significantly, complexation improves the planarity of bisa-
zopyrrole molecules; complex 4 is the most planar where the
dihedral angles between the pyrrole and phenyl planes are
4.73° (pyrrole and C3/C4/C5/C6/C7/C8 plane) and 3.53°
(pyrrole and C13/C14/C15/C16/C17/C18 plane). The mean
deviations of carbon and nitrogen atoms from the π-deloca-
lized skeletons containing azo, phenyl, and pyrrole groups
between the aromatic planes do not significantly change
[0.048, 0.096, 0.048, 0.087, and 0.044 ppm for HN (pyrrole),
HC (benzene), HC (pyrrole), HC (benzene), and H3C,
respectively; see the Supporting Information].
All configurations for NdN-C(pyrrole) are in a trans-cis
form. 1a(I) exhibits a NdN bond distance of 1.253 A, which
is longer than those of simple monoazo dyes with thiophene
˚
10
(1.226 A) and shorter than those of 6 with enhanced
˚
19
˚
π-resonance effects (1.271 A) and monoazothiophenes
with electron-withdrawing groups (1.297-1.314 A).10 Inter-
˚
˚
˚
˚
˚
are 0.047 A in 1a, 0.203 A in 2a, 0.065 A in 2b, and 0.045 A in
4. The small proton chemical shift differences of 2a between
25 and -80 °C further indicate that the torsion angles
and intramolecular hydrogen bonds and the second azo
˚
bond extend the NdN bond distances to 1.286 A for
5240 J. Org. Chem. Vol. 74, No. 15, 2009

Li et al.
JOCArticle
FIGURE 2. Side views of the structures of 1a(I), 2a, 2b, and 4.
2-(4-nitrophenyl)azo-1-H-pyrrole (420 nm),27 which indicate
that pyrrole acts as an effective acceptor in 2-arylazopyrrole
chromophores. When the donor-acceptor polarization of 1a is
increased by introducing an electron-accepting nitro group into
the 5-position of the pyrrole, the maximum absorption of 1b is
bathochromically shifted to 530 nm in dichloromethane and to
498 nm in methanol. Furthermore, when the π-resonance is
enhanced by extending the monoazopyrrole to bisazopyrroles,
asymmetric 2,5-bisazopyrroles 3a, 3b, 3c, and 3d have their
maximum absorptions at 548, 558, 572, and 615 nm in dichlor-
omethane and at 543, 543, 554, and 589 nm in methanol,
respectively. The maximum absorption shifts bathochromi-
cally along with the increase in the electron-accepting ability
of substituents in the sequence of p-OCH3 (3a) < p-H (3b)
< p-Br (3c) < p-NO2 (3d), consistent with an increase in
molecular donor-acceptor polarization. Note that the differ-
ence of the maximum absorptions of bisazopyrroles from 3b to
3d is smaller than that of monoazopyrroles from 1a to 1b.
Symmetric 2,5-bisazopyrroles 2a, 2b, 2c, 2d, and 5 also exhibit
maximum absorptions at 575, 519, 486, 490, and 563 nm in
dichloromethane and at 574, 507, 466, 487, and 580 nm in
methanol, respectively, larger than those of 1a. However, the
maximum absorption shifts bathochromically along with the
increase of the electron-donating ability of substituents on both
sides in the sequence p-N(CH3)2 (2a) > o-OH (2b) > p-NO2
(2d) >o-CO2H (2c).
TABLE 2. Absorption Data of Azopyrroles in Dichloromethane and
Methanol
compds
λmax (log ε) in CH2Cl2
λmax (log ε) in MeOH
1a
1b
2a
2b
2c
2d
3a
3b
3c
3d
4
447 (4.79)
530
575 (4.77)
519 (4.62)
486
490 (4.60)
548 (4.69)
558 (4.69)
572 (4.68)
615 (4.69)
754 (4.78)
563 (4.81)
443
498
574 (4.79)
507 (4.66)
466 (4.64)
487 (4.57)
543 (4.74)
543 (4.71)
554 (4.71)
589 (4.70)
5
580
˚
1a(II), 1.289 A for 1a(III), 1.283 A for 2a, and 1.293 and
˚
˚
1.293 A for 2b. The boron-azopyrrole complex 4 has
distinguishingly long NdN bond lengths. The free NdN
˚
bond distance is 1.300 A and the rigid NdN bond distance is
˚
1.322 A, longer than the calculated N-N bond distance for
8
˚
2-arylhydrazone-pyrrole structures (1.316 A). In contrast,
1a(I) clearly has long N-C(pyrrole) and N-C(Ar) bond
distances. The N-C(pyrrole) bond distances are 1.426 A for
1a(I), 1.386 A for 1a(II), 1.390 A for 1a(III), 1.380 A for 2a,
˚
1.391 and 1.388 A for 2b, and 1.361 and 1.377 A for 4, and the
N-C(Ar) bond distances are 1.453 A for 1a(I), 1.423 A for 1a-
(II), 1.420 A for 1a(III), 1.404 A for 2a, 1.408 and 1.412 A for
˚
2b, and 1.398 and 1.392 A for 4, decreasing in the sequence of
˚
˚
˚
˚
˚
˚
˚
˚
˚
˚
Typically, as shown in Figure 3 for 3a and 3c, the maxi-
mum absorption peaks for some bisazopyrroles overlap with
a minor absorption peak. Similar spectral characteristics
have been found for o-hydroxy azo dyes and reflect the
azohydrazone tautomeric equilibria in solution,28 where
the absorption peaks to the red belong to the hydrazone
tautomer and the other to the azo tautomer. Similar tauto-
monoazo, bisazo, and boron-azo complex, suggesting that
complex 4 has the best π-electron delocalization.
UV-Vis Absorption Spectra of 2,5-Bisazopyrrole and De-
rivatives. The maximum absorptions of azopyrroles 1(a,b), 2-
(a-d), 3(a-d), 4, and 5 are listed in Table 2. For monoazo dyes,
the visible absorption spectra are dominated by molecular
donor-acceptor polarization. 2-(4-Dimethylaminophenyl)
azo-1-H-pyrrole (1a) has a maximum absorption at 447 nm
in dichloromethane or 443 nm in methanol, longer than
those for (4-dimethylaminophenyl)azobenzene (408 nm)26 and
meric equilibria, through intramolecular N(azo)
H
N
3 3 3 3 3 3
(pyrrole) bonding, should exist for monoazo- and bisazo-
1H-pyrroles as depicted in Scheme 3. The positions of tauto-
meric equilibria are affected by solvents and substituents; in
addition, protic solvents can weaken the intramolecular
(26) Sawicki, E. J. Org. Chem. 1957, 22, 915.
(27) Ahmad, A. K.; Hassan, Y. I.; Bashir, W. A. Analyst 1987, 112, 97.
(28) Isak, S. J.; Eyring, E. M.; Spikes, J. D.; Meekins, P. A. J. Photochem.
Photobiol., A 2000, 134, 77.
J. Org. Chem. Vol. 74, No. 15, 2009 5241

Li et al.
SCHEME 3. Tautomerization Proposed for Monoazo- and
Bisazo-1H-pyrroles
JOCArticle
by complexation of the electron-withdrawing BF2 group.
Since symmetric bisazobenzidines28 and BF2 complexes of
dipyrromethene29 and tetraarylazadipyrromethene30 have
been investigated as photosensitizers for photodynamic
therapy, the BF2-azo complex described here is a potential
photosensitizer in this field.
FIGURE 3. Absorption spectra of 1a (orange line), bisazopyrrole
3a (red line) and 3c (blue line), and boron-azopyrrole complex
4 (green line) in dichloromethane.
hydrogen bonding and shift the equilibrium toward the azo
tautomer.
For dichloromethane solutions of 3c and 3d, minor ab-
sorption peaks appear as shoulders on the blue side of the
maximum absorption peaks, and tend to strengthen with a
decrease in the electron-accepting ability of substituents in
the sequence of 3b (H)>3c (Br)>3d (NO2). In contrast, for
2a and 3a, the minor absorption peaks appear as a shoulder
on the red side of the maximum absorption peaks, and tend
to weaken with an increase in the electron-donating ability of
substituents in the sequence 3a (OCH3)<3b (H). Compared
to those in dichloromethane, the maximum absorptions in
methanol solutions show large hypsochromic shifts of 15 nm
for 3b, 18 nm for 3c, 26 nm for 3d, and 32 nm for 1b, versus
small shifts of 1 nm for 2a, 3 nm for 2d, 4 nm for 1a, and 5 nm
for 3a. The hypsochromic shifts of the maximum absorptions
by 12 nm for 2b and 20 nm for 2c are due to the disruption of
Conclusion
Symmetric 2,5-bisazopyrroles 2(a-d) and 5, asymmetric
2,5-bisazopyrroles 3(a-d), and the novel BF2-azopyrrole
complex of 1H-pyrrolo[2,1-c]-1,2,4,5-boratriazole (4) have
been synthesized and fully characterized. The near-infrared
absorption of BF2-azo complex 4 is dominated by π-reso-
nance effects which was achieved by extending the conjuga-
tion around the NdN bond and forming a rigid azo
configuration. These results are important for the design of
near-infrared absorbing azo dyes and provide an effective
and simple method to synthesize such dyes by enhancing
π-electron delocalization. The synthesis of proton-inert
complexes of 1H-pyrrolo[2,1-c]-1,2,4,5-boratriazoles and
their application as photosensitizers for photodynamic ther-
apy are under progress.
the intramolecular N(azo)
H
O bonding. These spec-
3 3 3 3 3 3
tral characteristics indicate that the hydrazone tautomers
predominate in dichloromethane solution for monoazo- and
unsymmetric 2,5-bisazo-1H-pyrroles with strong donor-
acceptor polarization, such as 1b, 3b, 3c, and 3d, and the
azo tautomers are predominant for monoazo- and 2,5-
bisazo-1H-pyrroles with non- and weak donor-acceptor
polarization, such as 1a, 2a, 2b, 2d, and 3a, which are
consistent with the X-ray crystal structures. An attempt to
capture the hydrazone tautomer by reacting 2a with MeI
failed; only 2,5-bisazo-1-methylpyrrole 5 was isolated in high
yield. The spectrum of 5 in dichloromethane also has a minor
absorption peak appearing as a shoulder on the red side of
the maximum absorption peak, similar to that of 2-arylazo-1-
vinylpyrroles.19
Experimental Section
Preparation of 2,5-Bis(4-dimethylaminophenylazo)-1H-pyr-
role (2a). To a suspension of the aromatic amine (8.0 mmol)
in water (15 mL) was added concentrated hydrochloric acid
(24 mmol, 2 mL) until the mixture was homogeneous. The
solution was cooled and kept at 0-5 °C in an ice bath and
diazotized by addition of a solution of sodium nitrite (8.2 mmol,
0.566 g) in water (5 mL), followed by stirring for 30 min at 0-
5 °C. To a solution of pyrrole (4 mmol, 0.28 mL) and pyridine
(50 mmol, 4 mL) in methanol (150 mL) was slowly added a
solution of the diazonium salt at 0-5 °C. The resulting mixture
was stirred for 5 h and then evaporated under vacuum to
dryness. The residues were purified by column chromatography,
Significantly, the boron-azopyrrole complex 4 has
its maximum absorption at 754 nm in dichloromethane
(Figure 3), bathochromically shifted by 179 nm relative to
that of 2a. This presumably results from significant
π-electron delocalization in the planar structure dominated
eluting with a mixture of dichloromethane and hexane (1:1).
1
Yield 82%. Mp 228-229 °C. Anal. Calcd for C20H23N7
/
3
3
CH2Cl2 (solvent of crystallization determined by NMR): C,
62.66; H, 6.12; N, 25.16. Found: C, 62.41; H, 6.52; N, 25.52. 1H
NMR (CD2Cl2) δ 9.77 (s, 1H), 7.80 (s, 4H), 6.88 (s, 2H), 6.77 (d,
J=8.9 Hz, 4H), 3.10 (s, 12H). 1H NMR (CDCl3) δ 9.77 (s, 1H),
(29) (a) Yogo, T.; Urano, Y.; Ishitsuka, Y.; Maniwa, F.; Nagano, T. J.
Am. Chem. Soc. 2005, 127, 12162. (b) Atilgan, S.; Ekmekci, Z.; Dogan, A. L.;
Guc, D.; Akkaya, E. U. Chem. Commun. 2006, 42, 4398.
(30) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher, W. M.;
O’Shea, D. F. J. Am. Chem. Soc. 2004, 126, 10619.
5242 J. Org. Chem. Vol. 74, No. 15, 2009

Li et al.
JOCArticle
7.78 (d, J=8.3 Hz, 4H), 6.89 (s, 2H), 6.73 (d, J=8.3 Hz, 4H), 3.09
(log ε) 384 (4.20), 548 (4.69). UV-vis (1.24 ꢀ 10-5 M, CH3OH)
1
(s, 12H). H NMR (CD3OD) δ 7.74 (d, J=9.1 Hz, 4H), 6.81
λ
max (log ε) 382 (4.18), 543 (4.74). EI MS m/z 348 (Mþ).
Preparation of 2,5-Bis(4-dimethylaminophenylazo)pyrrole-1-
(s, 2H), 6.80 (d, J = 9.2 Hz, 4H), 3.07 (s, 12H). 13C NMR
(CD2Cl2) δ 152.7, 146.5, 144.5, 124.9, 114.3, 112.3, 40.3. IR ν
(NdN) 1377 cm-1. UV-vis (1.03 ꢀ 10-5 M, CH2Cl2) λmax (log ε)
411 (4.34), 575 (4.77). UV-vis (1.41 ꢀ 10-5 M, CH3OH) λmax
(log ε) 407 (4.22), 574 (4.79). EI MS m/z 361 (Mþ).
boron Difluoride (4). To a solution of 2a (0.28 mmol, 100 mg)
and diisopropylethylamine (4.2 mmol, 0.73 mL) in dry dichlor-
omethane (50 mL) was slowly added boron trifluoride diethyl
etherate (1.4 mmol, 0.18 mL) under nitrogen. The resulting
solution was refluxed for 1 h and then evaporated under vacuum
to dryness. The residue was purified by column chromatography
eluting with dichloromethane and hexane (1:1) to give a green
product. Yield 85%. Mp 205-206 °C. Anal. Calcd for
C20H22BF2N7: C, 58.70; H, 5.42; N, 23.96. Found: C, 58.99;
H, 5.59; N, 24.14. 1H NMR (CDCl3) δ 7.89 (d, J=9.20 Hz, 4H),
6.94 (s, 2H), 6.72 (d, J=9.30 Hz, 4H), 3.10 (s, 12H). 11B NMR
(CDCl3) δ 3.62. IR ν (NdN) 1422 cm-1. UV-vis (1.12 ꢀ 10-5 M,
CH2Cl2) λmax (log ε) 466 (4.26), 754 (4.78). ESI MS m/z 409 (Mþ).
Preparation of 2,5-Bis(4-dimethylaminophenylazo)-1-methyl-
pyrrole (5). To a mixture of 2a (0.28 mmol, 100 mg) and
potassium carbonate (1.68 mmol, 232 mg) in dry acetone
(50 mL) was added iodomethane (0.84 mmol, 52 μL) under
nitrogen. The resulting solution was refluxed for 3 h and then
evaporated under vacuum to dryness. The residue was purified
by column chromatography eluting with a mixture of dichlor-
omethane and hexane (1:1). Yield 91%. Mp 242-243 °C. Anal.
Calcd for C21H25N7: C, 67.18; H, 6.71; N, 26.11. Found: C,
66.87; H, 6.70; N, 26.28. 1H NMR (CDCl3) δ 7.81 (d, J=8.98
Preparation of 2-(4-Dimethylaminophenylazo)-1H-pyrrole
(1a).31 1a was prepared by using the same procedure as for 2a
but with 2 equiv of pyrrole and it was purified by column
chromatography eluting with a mixture of dichloromethane
1
and hexane (1:2). Mp 147-148 °C. H NMR (CDCl3) δ 9.33
(s, 1H), 7.75 (d, J=9.2 Hz, 2H), 6.84 (d, J=3.6 Hz, 1H), 6.79 (s,
1H), 6.73 (d, J=9.2 Hz, 2H), 6.32 (t, J=3.2 Hz, 1H), 3.03 (s, 6H).
13C NMR (CDCl3) δ 151.8, 146.3, 143.6, 124.1, 119.9, 112.1,
112.0, 111.0, 40.6. IR ν (NdN) 1377 cm-1. UV-vis (1.64 ꢀ 10-5
M, CH2Cl2) λmax 447 (4.52). UV-vis (CH3OH) λmax (log ε) 443.
EI MS m/z 214 (Mþ).
Preparation of 2-(4-Dimethylaminophenylazo)-5-nitro-1H-
pyrrole (1b). This compound was prepared according to the
literature18 by nitration of 1a with a mixture of acetic anhydride
and nitric acid at -50 °C. Dark crystals. Yield 32%. Mp 208-
209 °C. 1H NMR (CDCl3) δ 9.93 (s, 1H), 7.80 (d, J=9.2 Hz, 2H),
7.17 (d, J=4.4 Hz, 1H), 6.74 (d, J=4.0 Hz, 1H), 6.20 (d, J = 8.97
Hz, 2H), 3.11 (s, 6H). 13C NMR (CDCl3) 153.5, 146.7, 143.4,
126.1, 114.9, 112.8, 111.9, 110.5, 40.5. IR:ν (NdN) 1376 cm-1
UV-vis (CH2Cl2) λmax 530. UV-vis (CH3OH) λmax 498.
HR-EI MS (Mþ) m/z calcd for C12H13N5O2 259.1069, found
259.1071.
Preparation of 2-(4-Methoxyphenylazo)-5-(4-dimethylamino-
phenylazo)-1H-pyrrole (3a). 3a was prepared by using the same
procedure as for 2a but with 1 equiv of 1a instead of pyrrole and
purified by column chromatography eluting with a mixture of
dichloromethane and hexane (2:3). Yield 79%. Mp 166-167 °C.
Anal. Calcd for C19H20N6O: C, 65.50; H, 5.79; N, 24.12. Found:
.
Hz, 4H), 6.73 (d, J=9.0 Hz, 6H), 4.18 (s, 3H), 3.07 (s, 12H). 13
C
NMR (CDCl3) δ 151.8, 148.4, 145.2, 124.4, 112.0, 101.0, 40.6,
29.5. IR ν (NdN) 1378 cm-1. UV-vis (1.24 ꢀ 10-5 M, CH2Cl2)
λmax (log ε) 563 (4.81), 767 (4.13). UV-vis (CH3OH) λmax 580.
EI MS m/z 375 (Mþ).
Acknowledgment. This work is supported by QLT Inc.,
Vancouver, BC, and the Natural Sciences and Engineer-
ing Council (NSERC) of Canada. We thank the NMR,
Mass Spectroscopy and Elemental Analysis Laboratories of
the Chemistry Department at the University of British
Columbia.
1
C, 65.15; H, 5.85; N, 24.39. H NMR (CDCl3) δ 9.74 (s, 1H),
7.83 (d, J = 8.8 Hz, 2H), 7.80 (d, J=8.2 Hz, 2H), 7.01-6.95 (m,
3H), 6.92 (d, J=4.1 Hz, 1H), 6.76-6.71 (m, 2H), 3.87 (s, 3H),
3.08 (s, 6H). 13C NMR (CDCl3) δ 161.6, 152.4, 147.6, 146.7,
145.5, 144.2, 125.0, 124.3, 116.2, 114.6, 114.0, 112.0, 55.8, 40.5.
IR ν (NdN) 1380 cm-1. UV-vis (1.31 ꢀ 10-5 M, CH2Cl2) λmax
Supporting Information Available: Experimental proce-
dures, NMR and UV-vis spectra, and detailed X-ray crystal-
lographic characterizations. This material is available free of
charge via the Internet at http://pubs.acs.org.
(31) Kalle, A.-G. NL 6407379, 1965, CAN 62:79569.
J. Org. Chem. Vol. 74, No. 15, 2009 5243