Donor-acceptor triazenes: Synthesis, characterization, and study of their electronic and thermal properties

Article abstract of DOI:10.1021/jo070789x

(Graph Presented) A new class of 1,3-disubstituted-triazenes were synthesized by coupling functionalized benzimidazol-2-ylidenes, as their free N-heterocyclic carbenes or generated in situ from their respective benzimidazolium precursors, to various aryl

Full text of DOI:10.1021/jo070789x

VOLUME 72, NUMBER 25
DECEMBER 7, 2007
© Copyright 2007 by the American Chemical Society
Donor-Acceptor Triazenes: Synthesis, Characterization, and Study
of Their Electronic and Thermal Properties
Dimitri M. Khramov and Christopher W. Bielawski*
Department of Chemistry and Biochemistry, UniVersity of Texas at Austin, Austin, Texas 78712
bielawski@cm.utexas.edu
ReceiVed April 15, 2007
A new class of 1,3-disubstituted-triazenes were synthesized by coupling functionalized benzimidazol-
2-ylidenes, as their free N-heterocyclic carbenes or generated in situ from their respective benzimidazolium
precursors, to various aryl azides in modest to excellent isolated yields (36-99%). Electron delocalization
between the two coupled components was studied using UV-vis spectroscopy, NMR spectroscopy, and
X-ray crystallography. Depending on the complementarity of the functional groups on the N-heterocyclic
carbenes and the organic azides, the respective triazenes were found to exhibit λmax values ranging between
364 and 450 nm. X-ray crystallography revealed bond alteration patterns in a series of triazenes
characteristic of donor-acceptor compounds. Triazene thermal stabilities were studied using thermo-
gravimetric analysis and found to be strongly dependent on the sterics of the benzimidazol-2-ylidene
component and the electronics of the azide component. Triazenes possessing bulky N-substituents (e.g.,
neo-pentyl, tert-butyl, etc.) were stable in the solid-state to temperatures exceeding 150 °C, whereas
analogues with small N-substituents (e.g., methyl) were found to slowly decompose at room temperature.
Triazenes featuring electron-rich phenyl azide components decomposed at higher temperatures than their
electron-deficient analogues. Products of the thermally induced triazene decomposition reaction were
identified as molecular nitrogen and the respective guanidine. Using an isotopically labeled triazene, the
mechanism of the decomposition reaction was found to be analogous to the Staudinger reaction.
Introduction
nium salts,5 photoactive substrates,6 and precursors to various
types of compounds of medicinal importance.7 More recently,
triazenes have been used to facilitate coupling of functionalized
arenes to passivated Si surfaces for applications in semiconduc-
tors and nanoelectronics.8
Triazenes are a unique class of polyazo compounds containing
three consecutive nitrogen atoms in an acyclic arrangement.1
Historically, they have found use as DNA alkylating agents in
tumor therapy,2 iodo-masking groups in the synthesis of small3
and macromolecules,4 protecting groups for amines and diazo-
The two most widely utilized methods of synthesizing
triazenes1 are coupling of aryl diazonium salts to amines9 and
addition of organometallic reagents (RMgX, RLi, etc.) to alkyl
azides.10 However, the reagents used in these reactions are
* Author to whom correspondence should be addressed. Phone: 512-232-
3839. Fax: 512-471-8696.
10.1021/jo070789x CCC: $37.00 © 2007 American Chemical Society
Published on Web 07/03/2007
J. Org. Chem. 2007, 72, 9407-9417
9407

Khramov and Bielawski
(1,3,5-trimethylbenzene) groups in 54-94% isolated yields.
Since imidazol-2-ylidenes are typically prepared via deproto-
nation of their more stable and readily accessible imidazolium
precursors,14 we also demonstrated that N-heterocyclic carbenes
(NHCs) can be generated in situ from their respective precursors
prior to azide coupling.
In many ways, the aforementioned NHC/azide coupling
reaction is similar to the Cu-catalyzed [3 + 2] Huisgen15
cycloaddition reaction between alkynes and azides, now com-
monly categorized under “click chemistry.”16 Both reactions:
(1) provide products in excellent yields, (2) are atom-economi-
cal, (3) proceed with relatively high rates, (4) exhibit good
functional group tolerance, and (5) utilize a relatively broad
range of substrates. Over a short time span, click chemistry has
found utility in a multitude of synthetic, biological, and materials
applications.17 Likewise, in an effort to expand this repertoire,
we demonstrated that the NHC/azide coupling reaction was
useful for the postpolymerization modification of polyolefins
containing pendant azido groups.18
We have recently launched a program that utilizes NHCs as
building blocks for polymer synthesis, with an emphasis on
preparing functional materials suitable for use in electronic
applications.19 In addition to the practical advantages discussed
above, the NHC/azide coupling reaction has two unique features
for this purpose: (1) chemical unsaturation is conserved as
reactants are converted to products and (2) the triazeno bridge
formed formally conjugates the two organic components to each
other (i.e., the imidazole ring and the organic fragment stemming
from the azide).
FIGURE 1. Triazene formation via coupling of a N-heterocyclic
carbene with an azide.
extremely reactive, and often flammable, which necessitates the
use of relatively sophisticated equipment and techniques for
safety and success. As a result, the scope of compatible
substrates and the range of associated applications for preparing
triazenes are restrictive. These limitations warrant the develop-
ment of new methods for accessing this important class of
compound.
Recently, we discovered a new, practical method for preparing
triazenes. In our initial communication,11 we reported that
addition of a N-heterocyclic carbene12 (NHC) to an organic azide
afforded a 1,3-disubstituted triazene in excellent yield (see
Figure 1).13 The coupling reaction was found to proceed rapidly
at room temperature (τ1/2 ∼ minutes) and showed good tolerance
toward a broad array of functional groups and structural
variations. For example, alkyl, aryl, acyl, and tosylated azides
were coupled to imidazol-2-ylidenes with N-substituents ranging
in size from relatively small methyl groups to bulky mesityl
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9408 J. Org. Chem., Vol. 72, No. 25, 2007

Donor-Acceptor Triazenes
select crystalline materials.24 It was concluded that electronic
communication across the N-N bond was minimal. Hence,
while 1,3-dienes are known to exhibit extensively delocalized
structures, azines were called effective “conjugation stoppers.”25
However, Clyburne and co-workers demonstrated that azines
prepared via coupling of an imidazol-2-ylidene to 9-diazofluo-
rene were extensively delocalized, and under certain conditions
exhibited nonlinear optical (NLO) responses.26
TABLE 1. Synthesis of Donor-Acceptor Triazenes Bearing Bulky
N-tert-Butyl Groupsa
Results and Discussion
As shown in Table 1, a range of functionalized benzimidazol-
2-ylidenes and aryl azides were chosen as coupling partners.
Our hypothesis was that if the triazo linkage enabled electronic
communication, then bathochromic shifts should be observed
by UV-vis spectroscopy as the pendant electron-donating and
electron-withdrawing substituents in the resulting triazenes
became more complementary. Similarly, successive downfield
shifts were also expected in the NMR spectra for the same series
of compounds. If quality crystals suitable for X-ray diffraction
could be obtained, then distinct changes in the triazo bond length
patterns should be observed as a function of the electronic
character of the peripheral groups in the solid-state as well.
Triazene Nomenclature. In order to simplify the identifica-
tion of the triazenes discussed in this study, the following
convention was applied: Following the triazene numeric label,
which refers to class of benzimidazol-2-ylidene used (with
attention directed toward changes in the N-substituents), the first
suffix identifies the pendant functional group on the NHC
moiety, and the second suffix identifies the pendant functional
group on the aryl azide. For example, 1-(1,3-di-tert-butyl-
benzimidazol-2-ylidene)-3-phenyltriazene was identified as
1-H-H (see Table 1). Likewise, 1-(1,3-dimethyl-5,6-dimethoxy-
benzimidazol-2-ylidene)-3-(p-nitrophenyl)triazene was identified
as 3-OCH3-NO2 (see Table 3).
entry
product
1-H-H
1-H-OCH3
1-H-NO2
1-OCH3-H
1-OCH3-OCH3
1-OCH3-NO2
1-F-H
1-F-OCH3
1-F-NO2
R1
R2
yield (%)b
1
2
3
4
5
6
7
8
9
H
H
H
OCH3
OCH3
OCH3
F
H
99
99
99
99
99
99
99
99
99
OCH3
NO2
H
OCH3
NO2
H
F
F
OCH3
NO2
a General reaction conditions: 1,3-di-tert-butyl-benzimidazol-2-ylidene
(1.0 equiv) and an organic azide (1.0 equiv), concentration of substrates )
0.1 M, solvent ) THF, 23 °C. b Isolated yields.
calized structures and examine their potential in preparing robust
materials with useful electronic properties. In particular, we
describe the synthesis of a variety of “donor-acceptor” triazenes
where various benzimidazol-2-ylidenes20 were coupled to aryl
azides possessing complementary functional groups. The extent
of electron delocalization was studied using UV-vis spectros-
copy, NMR spectroscopy, and X-ray crystallography. The
thermal stabilities of the triazene products formed from the
NHC/azide coupling reaction were also evaluated and included
efforts toward deconvoluting how various steric and electronic
features influence this important physical characteristic. Previ-
ously, we discovered that at elevated temperatures triazenes
obtained from coupling NHCs to organic azides lose molecular
nitrogen and afford the respective guanidines.11 In the well-
known Staudinger reaction,21 the intermediate phosphatriazene
formed upon combination of a phosphine with an azide also
extrudes molecular nitrogen to form a phosphazene; hence, we
also tested whether triazene decomposition proceeded via an
analogous mechanism.
A secondary objective of this study was fundamental. The
triazeno linkage (CdN-NdN) formed in the NHC/azide
reaction bears structural similarities to azine (CdN-NdC) and
1,3-diene (CdC-CdC) linkages, which prompted us to study
the electronic properties of four-electron, conjugated π-systems
containing three consecutive nitrogen atoms. Previously, Glaser,
Euler, and others prepared and studied the electronic and
structural properties of donor-acceptor azines by NMR spec-
troscopy,22 cyclic voltammetry,23 and X-ray crystallography for
Synthesis of Triazenes 1. In general, triazenes 1 were
prepared by coupling 1,3-di-tert-butyl-benzimidazol-2-ylidene
(generated through deprotonation of its corresponding benz-
imidazolium salt27 using potassium tert-butoxide and then
isolated and used as its free NHC) with 1 mol equiv of aryl
azide.28 The reactions were conducted at ambient temperature
using tetrahydrofuran (THF) as solvent at 0.1 M substrate
concentrations. Depending on the electronics of the azide
(24) Chen G. S.; Anthamatten, M.; Barnes, C. L.; Glaser, R. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1081. (b) Kesslen, E. C.; Euler, W. B.
Tetrahedron Lett. 1995, 36, 4725. (c) Kesslen, E. C.; Euler, W. B.; Foxman,
B. M. Chem. Mater. 1999, 11, 336. (d) Euler, W. B.; Cheng, M.; Zhao, C.
Chem. Mater. 1999, 11, 3702.
(25) Chen, G. S.; Anthamatten, M.; Barnes, C. L.; Glaser, R. J. Org.
Chem. 1994, 59, 4336.
(26) (a) Choytun, D. D.; Langlois, L. D.; Johansson, T. P.; Macdonald,
C. L. B.; Leach, G. W.; Weinberg, N.; Clyburne, J. A. C. Chem. Commun.
2004, 16, 1842. (b) Hopkins, J. M.; Bowdridge, M.; Robertson, K. N.;
Cameron, T. S.; Jenkins, H. A.; Clyburne, J. A. C. J. Org. Chem. 2001, 66,
5713.
(27) Hadei, N.; Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Org.
Lett. 2005, 7, 1991.
(28) Alternatively, many of the triazenes shown in Table 1 may be
prepared by generating the free NHC in situ (from its benzimidazolium
salt) in the presence of an equimolar amount of organic azide. However,
this approach appears to be limited to substrates which are stable to base.
For example, 1-azido-4-nitrobenzene was found to rapidly decompose under
strongly basic conditions. Hence, the use of free NHCs (e.g., 1,3-di-tert-
butyl-benzimidazol-2-ylidene-4,5-dimethoxy-1,3-di-tert-butyl-benzimidazol-
2-ylidene, and 1,3-di-tert-butyl-4,5-difluoro-benzimidazol-2-ylidene) were
necessary to form triazenes containing aryl azide moieties (e.g., Table 1:
1-H-NO2, 1-OCH3-NO2, and 1-F-NO2).
(20) (a) Hahn, F. E.; von Fehrena, T.; Froelich, R. Z. Naturforsch. 2004,
59, 348. (b) Boesveld, W. Marco; Gehrhus, B.; Hitchcock, P. B.; Lappert,
M. F.; Schleyer, P. Chem. Commun. 1999, 8, 755.
(21) Staudinger, H.; Meyer, J. HelV. Chim. Acta 1919, 2, 635.
(22) (a) Lewis, M.; Glaser, R. J. Org. Chem. 2002, 67, 1441. (b) Glaser,
R.; Chen, G. S.; Anthamatten, M.; Barnes, C. L. J. Chem. Soc., Perkin
Trans. 2 1995, 1449. (c) Chen, G. S.; Wilbur, J. K.; Barnes, C. L.; Glaser,
R. J. Chem. Soc., Perkin Trans. 2 1995, 2311.
(23) Sauro, V. A.; Workentin, M. S. J. Org. Chem. 2001, 66, 831.
J. Org. Chem, Vol. 72, No. 25, 2007 9409

Khramov and Bielawski
main group elements26b,29 and may be rationalized by the strong
electron-donating character of the nitrogen atoms present in the
imidazole moiety. In contrast, significant effects on λmax were
observed when the terminal functional group on the azide
coupling partner was varied. For example, replacing a methoxy
group with a nitro group resulted in a 60 nm bathochromic shift
in the λmax (i.e., 1-H-OCH3 f 1-H-NO2; entries 4 and 5).
Similar spectroscopic changes were observed with triazenes
containing electron-poor (i.e., 1-F-OCH3 f 1-F-NO2, ∆λmax
) 55 nm; entries 6 and 7) and electron-rich (i.e., 1-OCH3-OCH3
f 1-OCH3-NO2, ∆λmax ) 48 nm; entries 8 and 9) benzimi-
dazol-2-ylidene components. As expected, triazene 1-OCH3-
NO2, prepared by coupling a highly electron-rich benzimidazol-
2-ylidene with a highly electron-deficient azide, exhibited the
longest λmax at 450 nm for any of the triazenes studied (entry
9). Collectively, these results suggested that the triazeno linkages
(CdN-NdN) were effective in delocalizing electronic charge
between complementary NHC and organic azide components.
Characterization of 1: NMR Spectroscopy. Next, we
turned our attention toward studying the donor-acceptor
TABLE 2. Summary of Key UV-vis Absorbance Data for
Triazenes 1a
entry
compound
1-H-H
1-OCH3-H
1-F-H
1-H-OCH3
1-H-NO2
1-F-OCH3
1-F-NO2
R1
R2
λ
max (nm)
log (ꢀ)b
1
2
3
4
5
6
7
8
9
H
OCH3
H
H
H
OCH3
NO2
OCH3
NO2
OCH3
NO2
364
372
368
380
440
383
438
402
450
4.20
4.17
4.16
4.12
4.35
4.28
4.52
4.41
4.49
F
H
H
F
F
1-OCH3-OCH3
1- OCH3-NO2
OCH3
OCH3
a UV-vis spectra were acquired in CH2Cl2 at ambient temperature. b ꢀ
is the molar absorptivity with units of L mol-1 cm-1
.
component, reaction times ranged from minutes (electron-poor
azides) to hours (electron-rich azides), as monitored by 1H NMR
spectroscopy. To obtain more quantitative information on
reaction progress versus time, the bimolecular rate constants of
reactions involving 1,3-di-tert-butyl-benzimidazol-2-ylidene and
a select range of azides with various electronic properties were
measured by 1H NMR spectroscopy (THF-d8, 23 °C). The
coupling reaction between the free NHC and an electron-poor
azide, 1-azido-4-nitrobenzene, was extremely fast and measured
to be 0.215 L‚mol-1‚s-1. In contrast, the rate constants for
coupling the same NHC to relatively more electron-rich azides,
phenyl azide and 1-azido-4-methoxybenzene, were relatively
1
triazenes (1) via H and 13C NMR spectroscopy. In general,
signals attributed to the arene protons on the azide-containing
moieties shifted upfield whereas the benzimidazol-2-ylidene
protons shifted downfield upon coupling. This result was
consistent with charge transfer from the electron-rich component
to the relatively electron-poor component. The magnitude of
these shifts were strongly dependent on the nature of the
functional group on the azide with a smaller dependency on
the pendant functional group on the benzimidazol-2-ylidene,
although overall changes were small for all compounds studied
(1H NMR: < 0.1 ppm; 13C NMR: < 1.0 ppm). The most
pronounced shifts were observed in the series (1H NMR, solvent
) CDCl3): 1-OCH3-NO2 (δ ) 8.11 ppm, Ar-H ortho to NO2;
7.50 ppm Ar-H meta to NO2), 1-H-NO2 (δ ) 8.14 ppm; 7.54
ppm), and 1-F-NO2 (δ ) 8.18 ppm; 7.58 ppm).
X-ray Crystallography Studies of Triazenes. To obtain
additional support for the existence of conjugation through the
triazeno linkage and to help quantify the extent to which a
dipolar triazene can effectively distribute charge, single-crystal
X-ray crystallography was used to study the donor-acceptor
triazenes described above. Shown in Figure 3 (top) are two bona
fide resonance structures (A and B) for triazenes flanked by
donor (D) and acceptor (A) groups. In our initial investigation,
X-ray crystallography revealed that 1-(1,3-dimesityl-imidazol-
2-ylidene)-3-benzyl-triazene (2), prepared via coupling of 1,3-
dimesityl-imidazol-2-ylidene with benzyl azide, exhibited al-
ternating bond lengths consistent with the structure A (see Figure
3, bottom).11 However, we anticipated that the donor-acceptor
triazenes (1) discussed above would adopt resonance structure
B, as the pendant electron-withdrawing group on one component
was matched with an electron-donating group on the other. More
specifically, the single N-N bonds should shorten and the
double NdN bonds should elongate as the complementarity of
the donor-acceptor groups on the triazenes increased.
slow and found to be 0.003 L‚mol-1‚s-1 and 0.001 L‚mol-1‚s-1
,
respectively. Collectively, these observations were consistent
with a coupling mechanism that involves nucleophilic attack
of a NHC at the terminal nitrogen of an aryl azide. Purification
of the products generated from this reaction was straightforward,
requiring only filtration of the reaction media through a PTFE
filter followed by evaporation of solvent. Using this methodol-
ogy, triazene products were consistently obtained in excellent
(>99%) yields and in high purities. The molecular structures
of triazenes 1 shown in Table 1 were confirmed by NMR
spectroscopy, UV-vis spectroscopy, and for select crystalline
compounds, X-ray crystallography (see below). Mass spectrom-
etry and elemental analyses provided additional support for their
structure and purity.
Characterization of 1: UV-vis Absorption Spectroscopy.
After the donor-acceptor triazenes shown in Table 1 were
synthesized, UV-vis absorption spectroscopy was used to
measure the electronic properties of these compounds. As shown
in the UV-vis absorption spectra included in Figure 2 and
summarized in Table 2, a gradual bathochromic shift in the λmax
of the triazene chromophore was observed as the electron-
donating/electron-withdrawing character of its peripheral func-
tional groups became more complementary. Close analysis of
the data suggested that while electronic communication between
the terminal functional group on the benzimidazol-2-ylidene and
the azide was observed, the interaction was relatively minor.
For example, only 4-8 nm bathochromic shifts were observed
in the λmax upon replacing two hydrogens in the benzimidazol-
2-ylidene to either methoxy or fluoro groups (i.e., 1-H-H f
1-OCH3-H or 1-F-H; entries 1-3). These observations were
in accord with the high degree of bond polarization previously
observed in complexes formed between imidazol-2-ylidenes and
Synthesis and Characterization of Triazenes 3. While we
were unable to obtain quality crystals suitable for X-ray analysis
of the donor-acceptor triazenes 1, a related series of crystalline
analogues (3) were obtained by effectively replacing the bulky
N-tert-butyl substituents on the benzimidazol-2-ylidene moieties
with smaller N-methyl groups. Since benzimidazol-2-ylidenes
(29) Arduengo, A. J.; Calabrese, J. C.; Cowley, A. H.; Dias, H. V. R.;
Goerlich, J. R.; Marshall, W. J.; Riegel, B. Inorg. Chem. 1997, 36, 2151.
9410 J. Org. Chem., Vol. 72, No. 25, 2007

Donor-Acceptor Triazenes
TABLE 3. Synthesis and UV-vis Data of Crystalline Triazenes Bearing N-Methyl Groupsa
entry
compound
3-H-H
3-H-NO2
3-OCH3-H
3-OCH3-NO2
R1
R2
yield (%)b
λ
max (nm)c
log (ꢀ)c
µ
calc (D)d
1
2
3
4
H
H
OCH3
OCH3
H
NO2
H
36
56
69
74
372
444
400
467
4.40
4.38
4.39
4.22
4.29
10.6
5.39
11.2
NO2
a General conditions: 1,3-Dimethyl-benzimidazol-2-ylidene (1.00 equiv) was generated in situ in THF (0.2 M) at -30 °C from its respective benzimidazolium
salt using potassium tert-butoxide (1.05 equiv). The noted organic azide (1.00 equiv) was then added to the resulting reaction mixture. b Isolated yield.
c UV-vis spectra were acquired in CH2Cl2 at ambient temperature. ꢀ is the molar absorptivity with units of L mol-1 cm-1 d Calculated using density
.
functional theory; see: ref 33. X = iodide.
FIGURE 2. UV-vis absorbance spectra (left) color coded with their respective compound (right). Spectra were acquired in CH2Cl2 at ambient
temperature. For λmax (nm) values, see Table 2.
(see Figure 4) and summarized in Table 3, the λmax of triazenes
3 in CH2Cl2 were found to be nearly identical to their di-tert-
butyl analogues (1), which indicated that the main chromophore
was only minimally affected by the differences in steric bulk
between these two series of compounds. Notably, the dipole
moments (µ) of these compounds, as calculated using density
functional theory, correlated with the donor-acceptor groups
on their periphery (see Table 3).33 For example, replacement
of the p-hydrogen atom on the azide component with a nitro
group resulted in a large increase in the calculated dipole,
FIGURE 3. (top) Illustration of two resonance structures for triazenes
compare: 3-H-H, 4.29 D vs 3-H-NO2, 10.6 D (entries 1 and
containing donor (D) and acceptor (A) groups. (bottom) Key bond
2). In contrast, incorporation of electron-donating methoxy
lengths for 1-(1,3-dimesityl-imidazol-2-ylidene)-3-benzyl-triazene (2).11
groups into the benzimidazol-2-ylidene fragment (i.e., 3-OCH3-
H) resulted in only a modest increase in the calculated dipole
with N-methyl groups are known to dimerize to their respective
enetetraamines, which oxidize rapidly in the presence of air,30
NHCs were generated in situ under basic conditions from their
known30 benzimidazolium precursors in the presence of an
organic azide. Using this approach, a range of donor-acceptor
triazenes 3 were prepared in moderate to good yields (36-74%,
see Table 3).31,32 As shown in the UV-vis absorption spectra
(31) Since benzimidazol-2-ylidenes with small N-substituents are prone
to exist as equilibrium between free NHC and its oxygen-sensitive dimer,30
these reactions must be conducted with the rigorous exclusion of oxygen.
(32) Unlike their N-tert-butyl analogues, triazenes 3 were found to exhibit
low solubilities in common organic solvents (i.e., THF, toluene, DMF,
DMSO, CH3CN, etc.) and decomposed slowly over several days in solution
or in the solid-state (see main text for a more detailed discussion of this
phenomenon). These characteristics are partially responsible for the relatively
low yields of the reactions summarized in Table 3.
(30) (a) Wanzlick, H. W.; Buchler, J. W. Chem. Ber. 1964, 97, 2447.
(b) Arduengo, A. J., III; Goerlich, J. R.; Marshall, W. J. Leibigs Ann. 1997,
265. (c) Liu, Y.; Lindner, P. E.; Lemal, D. M. J. Am. Chem. Soc. 1999,
121, 10626.
(33) Dipole moments were calculated at the B3LYP (6-31G*) level of
density functional theory, as implemented in the Spartan 2004 software
package (Wavefunction, Irvine, CA 92612).
J. Org. Chem, Vol. 72, No. 25, 2007 9411

Khramov and Bielawski
FIGURE 4. UV-vis absorbance spectra (left) color coded with their respective compound (right). Spectra were acquired in CH2Cl2 at ambient
temperature. For λmax (nm) values, see Table 3.
FIGURE 5. ORTEP diagrams for (a) 3-H-H, (b) 3-H-NO2, (c) 3-OCH3-H, (d) 3-OCH3-NO2. Selected bond lengths (Å) and angles (deg) are
summarized in Table 4. Solvent molecules have been removed for clarity.
TABLE 4. Selected Bond Lengths (Å) and Angles (deg) for Triazenes 3a
entry
compound
3-H-H
3-H-NO2
3-OCH3-H
3-OCH3-NO2
C1-N1 (Å)
N1-C2 (Å)
C2-N2 (Å)
N2-N3 (Å)
N3-N4 (Å)
N4-C3 (Å)
N1C2N2-N4C3C4 (deg)b
1
2
3
4
1.375(4)
1.390(5)
1.387(4)
1.397(4)
1.365(4)
1.360(5)
1.370(4)
1.360(4)
1.329(4)
1.343(5)
1.339(4)
1.354(4)
1.358(3)
1.346(5)
1.367(4)
1.328(4)
1.281(4)
1.285(4)
1.279(3)
1.287(3)
1.423(4)
1.403(5)
1.430(4)
1.410(4)
7.56
0.69
26.48
1.34
a All crystals analyzed were obtained by slow evaporation of concentrated chloroform solutions. b Absolute angles between planes defined by N1-C2-N2
and N4-C3-C4.
(5.39 D) relative to 3-H-H. This result was in accord with the
aforementioned UV-vis spectroscopy data and consistent with
the benzimidazol-2-ylidene component showing high polariza-
tion. As expected, 3-OCH3-NO2 exhibited the calculated largest
dipole of 11.2 D.
Crystals suitable for X-ray analysis were obtained by slow
evaporation of saturated chloroform solutions of triazenes 3.
ORTEP diagrams for these structures are shown in Figure 5
with key crystallographic information summarized in Tables 4
(selected bond lengths and angles) and 5 (selected crystal data).
Analysis of the data sets revealed that as the complementarity
of the pendant electron-donating and electron-withdrawing
groups on the periphery of the triazenes improved (i.e., 3-H-H
vs 3-OCH3-NO2), the C2-N2 and N3-N4 bonds elongated
(1.329(4) f 1.354(4) Å and 1.281(4) f 1.287(3) Å, respec-
tively) while the N2-N3 bond shortened (1.358(3) f 1.328(4)
Å). Further support for electron delocalization was apparent
from the angles between the planes of the two azide and NHC
9412 J. Org. Chem., Vol. 72, No. 25, 2007

Donor-Acceptor Triazenes
TABLE 5. Selected Crystal Data for Triazenes 3
3-H-H
3-H-NO2
643400
C16H15Cl3N6O2
429.69
triclinic
P-1
6.7593(3)
11.8068(5)
12.3651(5)
95.814(3)°
102.092(2)°
100.095(2)°
940.16(7)
153(2)
3-OCH3-H
643398
C17H21N5O3
343.39
triclinic
P-1
7.1500(2)
13.9550(3)
17.4200(5)
105.406(1)°
90.710(1)°
93.4630(13)°
1671.89(8)
153(2)
3-OCH3-NO2
643399
C17H18N6O4
370.37
monoclinic
P21/c
8.9005(4)
7.5544(3)
25.4468(12)
90°
CCDC no.
empirical formula
formula weight
cryst syst
space group
a, Å
643401
C15H17N5O
283.34
tetragonal
I-4
22.4468(7)
22.4468(7)
22.4468(7)
90°
90°
90°
2893.5(2)
153(2)
b, Å
c, Å
R, deg
â, δꢀγ
90.197(2)°
90°
1710.98(13)
153(2)
γ, δꢀγ
V, Å3
T, K
Z
D
8
2
4
4
calc, mg/m3
1.301
1.518
1.364
1.438
cryst size (mm)
reflections
0.40 × 0.15 × 0.13
0.40 × 0.10 × 0.08
0.30 × 0.20 × 0.14
0.36 × 0.24 × 0.15
2665
6116
10827
5428
collected
independent
reflections
1829
4128
5888
2978
R1, wR2 {I > 2σ(I)}
goodness of fit
0.0493, 0.1060
1.164
0.0671, 0.1274
1.121
0.0563, 0.0922
1.059
0.0527, 0.0831
1.063
components, as defined in Table 4. Triazenes 3-H-NO2 and
3-OCH3-NO2 revealed near perfect planarity between the two
components (angles ) 0.7 and 1.3°, respectively), a manifesta-
tion of good orbital overlap between terminal electron-poor and
electron-rich groups. In contrast, triazenes 3-H-H and 3-H-
OCH3 which possessed relatively mismatched functional groups
exhibited angles up to 26.5°. Collectively, these results were
consistent with the UV-vis and NMR spectroscopic data
discussed above and supported the notion that triazenes 3
possessing complementary functional groups exhibited exten-
sively electron delocalized structures. In concert, the solid-state
data suggested structure B (see Figure 3, top) was a major
resonance contributor in electronically matched, donor-acceptor
triazenes.
Evaluation of the Thermal Stabilities of Triazenes: In-
troduction. During our spectroscopic and crystallographic
studies, qualitative observations suggested that triazenes with
small N-substituents were relatively unstable in solution and in
the solid-state. In contrast, triazenes with large N-substituents
were found to be exceptionally stable, even at elevated tem-
peratures (>150 °C) for extended periods of time. In our initial
communication, we found that the major product of 2 decom-
position was guanidine 4 (R ) Mes, R′ ) Bn), which formed
Via extrusion of molecular nitrogen at elevated temperatures
(>120 °C) (see Figure 6a).11 To determine if the triazenes
synthesized in this study were also susceptible to a similar
decomposition process, 3-H-H was heated to 150 °C in toluene
in a pressure vessel. Under these conditions, the respective
guanidine (5) was observed by 1H NMR spectroscopy and
subsequently isolated in 95% yield (see Figure 6b).34 Supple-
mentary support for the decomposition reaction was obtained
from mass spectrometry and elemental analysis. When compared
to the triazene starting material (3-H-H), the decomposition
product (5) was 28 Da lower in molecular weight and exhibited
a compositional difference consistent with the loss of molecular
nitrogen. However, the mechanism for the decomposition
reaction was unknown, and we were interested in deconvoluting
the key steric and electronic factors governing triazene stability.
Evaluation of the Thermal Stabilities of Triazenes: Ther-
mogravimetric Analysis. In addition to the aforementioned
solution-mediated decomposition reactions, triazene stability was
also studied in the solid-state. Particular attention was directed
toward probing thermal stabilities as a function of triazene size
and electronic characteristics. Initial efforts focused on studying
how the size of the N-substituent as well as electronics of the
benzimidazol-2-ylidene component affected triazene stability.
A range of benzimidazol-2-ylidenes with N-substituents ranging
in size from methyl to bulky neo-pentyl groups were synthesized
using known methods27,35 and coupled to phenyl azide under
(34) Similarly, heating toluene solutions of triazenes 6-H-H and 6-H-
NO2 (see Table 6) to >150 °C afforded their respective guanidines (11
and 12, not shown) in g95% yields. This process was conveniently
monitored using 1H NMR spectroscopy. For example, the methylene groups
(NCH2) in triazenes 6 exhibit diagnostic chemical shifts at 4.0-4.5 ppm in
their 1H NMR spectra; chemical shifts corresponding to the same group in
their respective guanidine products were found at 3.5-3.8 ppm. Note that
while the thermally-induced triazene decomposition reactions generally
afforded high yields (>95%) of guanidine products, a side-reaction was
evident. The residual mass was composed of a mixture of products that
eluded NMR spectroscopic identification and could not be purified via
column chromatography.
FIGURE 6. Guanidine formation from triazene Via loss of nitrogen.
Bn ) benzyl, Mes ) 1,3,5-trimethylphenyl, Ph ) phenyl. (a) This
reaction was reported in ref 11. (b) This work.
J. Org. Chem, Vol. 72, No. 25, 2007 9413

Khramov and Bielawski
TABLE 6. Steric and Electronic Effects on Triazene Stability
FIGURE 7. Illustration of the Staudinger reaction. Nucleophilic attack
of the phosphine on the terminal atom of the organic azide affords an
intermediate phosphatriazene. Subsequent intramolecular cyclization
leads to an unstable cyclophosphatriazene intermediate that decomposes
to a phosphazene and molecular nitrogen.
entry
triazene
3-H-H
6-H-H
1-H-H
R1
R2
R3
Mp (°C)a
Td (°C)b
1
2
3
4
5
6
7
Me
iBu
tBu
nPn
tBu
iBu
iBu
H
H
H
H
OCH3
H
H
H
H
H
H
c
c
93
118
148
156
154
174
139
100-105
126-131
140-142
128-131
c
influenced triazene stability. For example, while triazenes
containing N-methyl groups exhibited a Td ) 93 °C (entry 1),
they were found to slowly decompose at ambient temperature
over time. Comparatively, analogous triazenes with N-iso-butyl
(6-H-H), N-tert-butyl (1-H-H), or N-neo-pentyl (7) substituents
exhibited Tds of 118, 148, and 156 °C, respectively (entries
2-4).34 In contrast, the role of benzimidazol-2-ylidene electron-
ics in influencing triazene thermal stability was found to be
relatively minor. For example, 1-H-H and 1-OCH3-H were
found to exhibit decomposition temperatures (Tds) of 148 and
154 °C, respectively (entries 3 and 5), which was consistent
with the aforementioned UV-vis, NMR, and X-ray spectro-
scopic data, indicating that donor groups in these positions have
only moderate influences on triazene electronics.
Next, attention was focused on studying the role of azide
electronics. Variation of azide electronics appeared to signifi-
cantly influence triazene stability. The Td of 6-H-NO2 which
possessed an electron-deficient p-nitro substituent was 35 °C
higher than an analogue containing a p-methoxy substituent (6-
H-OCH3) (Tds ) 174 and 139 °C, respectively; entries 6 and
7). Collectively, these results suggested that coupling benzimi-
dazol-2-ylidenes possessing bulky N-substituents with electron
deficient azides affords triazenes with the highest thermal
stabilities.
Mechanistic Investigation of the Triazene Decomposition
Reaction. It is well-established that NHCs often function as
phosphine analogues.12,37 Indeed, the NHC/azide coupling
reaction to form triazenes and its thermal decomposition
products share many similarities with the Staudinger reaction.
In the Staudinger reaction, addition of a phosphine to an azide
results in formation of a phosphatriazene (see Figure 7).21 This
compound subsequently undergoes intramolecular cyclization
to form an unstable four-membered intermediate that rapidly
decomposes to a phosphazene, extruding molecular nitrogen in
the process. As with the triazenes discussed above, the thermal
stabilities of the triazenophosphoranes have been improved
through the incorporation of bulky groups.38
7-H-H
1-OCH3-H
6-H-NO2
6-H-OCH3
NO2
OCH3
H
a Melting points (Mp) are uncorrected and were determined under an
atmosphere of air. b Decomposition temperatures (Td) determined using
thermogravimetric analysis under an atmosphere of nitrogen and defined
as the temperature at which 5% mass loss occurred. c Melting point was
below ambient temperature.
basic conditions using the in situ methodology described above
(see Table 6).36
After synthesis and characterization, the thermal stabilities
of various triazenes were evaluated in the solid-state using
thermogravimetric analysis (TGA); results are summarized in
Table 6. To ensure that each triazene studied was decomposing
in the same phase, melting points were also determined and
found to be below the decomposition temperature (Td) for all
compounds studied. In accord with the qualitative assessment
noted above, increasing the size of the N-substituent positively
(35) (a) Quast, H.; Schmitt, E. Chem. Ber. 1968, 101, 4012. (b) Huynh,
H. V.; Han, Y.; Ho, J. H. H.; Tan, G. K. Organometallics 2006, 25, 3267.
(c) Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. Org. Lett. 2006, 8,
1831. (d) Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. Angew. Chem.,
Int. Ed. 2006, 45, 6186. (e) Gehrhus, B.; Hitchcock, P. B.; Pongtavornpinyo,
R.; Zhang, L. Dalton Trans. 2006, 1847. (f) Daniele, S.; Drost, C.; Gehrhus,
B.; Hawkins, S. M.; Hitchcock, P. B.; Lappert, M. F.; Merle, P. G.; Bott,
S. G. J. Chem. Soc., Dalton Trans. 2001, 21, 3179. (g) Myes, T. L.; Diver,
S. T.; Richard, J. P.; Rivas, F. M.; Toth, K. J. Am. Chem. Soc. 2004, 126,
4366.
(36) Although the spectroscopic signatures of the resulting triazenes were
in accord with the aforementioned triazenes, one notable exception was
found. Signals attributable to the methylene groups of the N-iso-butyl
substituents (NCH2) in triazene 6-H-H appeared as two doublets at 3.97
1
ppm and 4.16 ppm in its H NMR spectrum (solvent ) CDCl3) and were
tentatively assigned to cis and trans diazo (NdN) isomers, respectively.
While geometric isomers often absorb radiation to differing degrees (see:
Hartley, G. S. J. Chem. Soc. 1938, 633 and El Halabieh, R. H.; Mermut,
O.; Barrett, C. J. Pure Appl. Chem. 2004, 76, 1445), triazene 6-H-H
exhibited a single signal at λmax ) 367 nm that was similar in shape to the
triazenes discussed above (i.e., 1). This result suggested that the absorption
of radiation was concomitant with geometric isomerization (i.e., cis f trans).
A similar observation was observed in benzothiazole-based triazenes
analogous to 6-H-H where it was determined that cis f trans isomerization
was facilitated with λ ) 405 nm radiation (see: Dorsch, H.-T.; Hoffman,
H.; Ha¨nsel, R.; Rasch, G.; Fangha¨nel, E. J. Prakt. Chem. 1976, 318, 671
and Fangha¨nel, E; Ha¨nsel, R.; Hohlfeld, J. J. Prakt. Chem. 1977, 319, 485).
Notably, of all the triazenes reported herein, only 6-H-H and 6-H-Me
exhibited isomers that were clearly resolved at room temperature by 1H
NMR spectroscopy. Considering density functional theory calculations at
the B3LYP (6-31G**) level of theory suggested that trans 6-H-H was
more stable than its cis isomer by 5.1 kcal/mol, the trans isomer was assumed
to dominate in all of the triazenes prepared in this study (DFT calculations
were performed using Spartan 2004, Wavefunction, Irvine, CA 92612).
To confirm that the decomposition mechanism of the triazenes
was similar to the Staudinger reaction, a decomposition reaction
involving an 15N-labeled triazene was performed. Phenyl azide
(37) For a direct comparision of N-heterocyclic carbenes with phosphines
in various applications, see: (a) Rogers, M. M.; Stahl, S. S. Top. Organomet.
Chem. 2007, 21, 21. (b) Shaughnessy, K. H. Eur. J. Org. Chem. 2006, 71,
1827. (c) Zuo, G.; Louie, J. Angew. Chem., Int. Ed. 2004, 43, 2277.
(38) (a) Shalimov, A. A.; Malenko, D. M.; Repina, L. A.; Sinitsa, A. D.
Russ. J. Org. Chem. 2005, 75, 1376. (b) LePichon, L.; Stephan, D. W.
Inorg. Chem. 2001, 40, 3827. (c) Blum, J.; Yona, I.; Tsaroom, S.; Sasson,
Y. J. Org. Chem. 1979, 44, 4178.
9414 J. Org. Chem., Vol. 72, No. 25, 2007

Donor-Acceptor Triazenes
FIGURE 8. Proposed mechanism of thermally induced triazene decomposition based on a 15N-labeling experiment.
containing an isotopically enriched nitrogen atom bonded to the
phenyl ring was synthesized using literature protocols from 15N-
labeled aniline.39 After coupling the 15N-labeled azide with 1,3-
di-iso-butyl-benzimidazol-2-ylidene, the resulting triazene 8 was
heated to 170 °C (closed vessel) in toluene at 0.1 M. Guanidine
Taken together, these results suggest that the azide-NHC
coupling reaction holds tremendous potential for creating mater-
ials with high nitrogen contents, delocalized electronic structures,
and high thermal stabilities. Recently, we reported19,35c,d,40
a
novel class of bis(NHC)s which feature two diametrically
opposed imidazol-2-ylidenes connected to a common arene
linker. Combination of this monomer with ditopic bis(azide)s
(e.g., 1,4-diazidobenzene) may lead to a novel class of conju-
gated polymeric materials. Furthermore, compounds exhibiting
extensively delocalized structures often leads to molecular
polarization, which is a key requirement for creating highly
ordered materials. By judiciously combining functionalized
azides and NHCs, highly polarized triazenes may find new
applications in nonlinear optics. Efforts toward accomplishing
these goals as well as exploring the utility of triazenes obtained
from the NHC/azide coupling reaction in electronic materials
will be reported in due course.
1
9 was found as the major product (>95% conversion)34 by H
NMR spectroscopy and exhibited a isotopic mass distribution
consistent with 15N-labeled azide starting material by mass
spectrometry. On the basis of these results, we believe the
mechanism to proceed as shown in Figure 8. The nitrogen atom
bonded to the phenyl ring attacks the benzimidazol-2-ylidene
carbon to form a four-membered intermediate (10). Subsequent
fragmentation expelled molecular nitrogen to afford the observed
guanidine 9. This mechanism also provides an explanation for
increased stability observed with triazenes possessing bulky
N-substituents and electron-deficient azide components. The
former groups would sterically prevent nucleophilic attack at
the electrophilic benzylidene carbon atom while the latter groups
would be expected to diminish nucleophilicity of aryl nitrogen
atom through resonance.
Experimental Section
General Procedure A: Synthesis of Triazenes Using Free
N-Heterocyclic Carbenes. After a free N-heterocyclic carbene (1
equiv) was dissolved in THF (approximate concn ) 0.3 M), an
organic azide (1 equiv) was added in a single portion and the
resulting reaction mixture was stirred for up to 8 h at ambient
temperature. Solvent was then removed under reduced pressure to
yield the desired triazene.
General Procedure B: Synthesis of Triazenes Using N-
Heterocyclic Carbenes Generated in Situ from Their Respective
Benzimidazolium Salts. Benzimidazolium salt (1 equiv) and
potassium tert-butoxide (1 equiv) were suspended in THF (ap-
proximate concn ) 0.2 M) and stirred for 30 min. Afterward, the
reaction was cooled to -30 °C, and an organic azide (1 equiv)
was added in a single portion. The resulting reaction mixture was
then stirred for 24 h at 0 °C and then filtered through a 0.2 µm
PTFE membrane. Finally, the solvent was removed under pressure
to yield the desired product.
1-Phenyl-3-(1,3-di-tert-butyl-benzimidazol-2-ylidene)triaz-
ene (1-H-H). Following general procedure A, 1,3-di-tert-butyl-
benzimidazol-2-ylidene41 (0.23 g, 1.0 mmol) was treated with
phenyl azide (0.12 g, 1.0 mmol) in 3 mL of THF for 2 h to afford
0.35 g of the titled compound as a yellow powder (99% yield). 1H
NMR (CDCl3): δ 7.57-7.54 (m, 2H), 7.34-7.30 (m, 2H), 7.11-
7.05 (m, 5H), 1.68 (s, 18H). 13C NMR (CDCl3): δ 161.5, 154.2,
132.1, 128.1, 124.3, 121.7, 121.1, 114.1, 62.0, 30.6. HRMS: [M
+ H]+ calcd for C21H27N5: 350.2345; found, 350.2349. Anal. calcd
for C21H27N5: C, 72.17; H 7.79; N, 20.04; found, C 72.24; H, 7.74;
N, 19.95. Mp ) 100-105 °C. Td ) 148 °C.
Conclusions
In summary, we have prepared a novel series of 1,3-
disubstituted-triazenes by coupling N-heterocyclic carbenes
(NHCs) with various aryl azides. The coupling reactions were
accomplished using free NHCs or NHCs generated in situ under
basic conditions from their azolium precursors. Modest to
excellent isolated yields (36-99%) of triazene product were
obtained from these coupling reactions, with typical yields
exceeding 95%. The triazenes prepared from this reaction were
examined using UV-vis spectroscopy, X-ray crystallography,
NMR spectroscopy, and thermogravimetric analysis. Results
obtained from the UV-vis spectroscopy experiments suggested
that the triazenes exhibited electronically delocalized structures
with λmax values ranging between 364 and 450 nm and
dependent on the complementarities of the coupled N-hetero-
cyclic carbene and azide components. The electronic properties
were dominated by the azide, with minor contributions stemming
from the benzimidazol-2-ylidene moiety. Additional support for
electronic communication was obtained by X-ray crystal-
lography which revealed bond alteration patterns in the triazeno
(CdN-NdN) linkages characteristic of donor-acceptor com-
pounds. Triazene stability was found to be governed by the
N-substituents of the benzimidazol-2-ylidene and the electronic
nature of the azide component. Notably, a triazene comprised
of a benzimidazol-2-ylidene with bulky N-iso-butyl groups and
a p-nitrophenylazide was stable up to 174 °C in the solid-state.
The mechanism of decomposition was elucidated using an
isotopically labeled azide and found to afford molecular nitrogen
and the respective guanidine, in analogy with the Staudinger
reaction.
1-(4-Methoxyphenyl)-3-(1,3-di-tert-butyl-benzimidazol-2-ylid-
ene)triazene (1-H-OCH3). Following general procedure A, 1,3-
di-tert-butyl-benzimidazol-2-ylidene41 (0.23 g, 1.0 mmol) was
treated with p-methoxyphenyl azide (0.15 g, 1.0 mmol) in 3 mL of
THF for 3 h to afford 0.38 g of the titled compound as a yellow
1
powder (99% yield). H NMR (CDCl3): δ 7.58-7.53 (m, 4H),
(39) For additional examples of synthesizing 15N-labeled azides see:
Leseticky, L.; Barth, R.; Nemec, I.; Sticha, M.; Tislerova, I. Czech. J. Phys.
2003, 53, A777.
(40) Boydston, A. J.; Bielawski, C. W. Dalton Trans. 2006, 4073.
(41) See supporting information for the synthesis and characterization
of this compound.
J. Org. Chem, Vol. 72, No. 25, 2007 9415

Khramov and Bielawski
7.07-7.04 (m, 2H), 6.90-6.87 (m, 2H), 3.79 (s, 3H), 1.76 (s, 18H).
13C NMR (CDCl3): δ 161.4, 158.1, 145.6, 132.4, 122.3, 120.8,
113.9, 113.8, 61.5, 55.3, 31.0. HRMS: [M + H]+ calcd for
C22H30N5O: 380.2450; found, 380.2445.
Hz), 122.6, 114.0, 102.8 (dd, JF-C ) 8.9, 15.7 Hz), 62.1, 55.4,
30.9. HRMS: [M + H]+ calcd for C22H28F2 N5O: 416.2262; found,
416.2258.
1-(4-Nitrophenyl)-3-(1,3-di-tert-butyl-5,6-difluoro-benzimida-
zol-2-ylidene)triazene (1-F-NO2). Following general procedure A,
1,3-di-tert-butyl-4,5-fluoro-benzimidazol-2-ylidene41 (0.27 g, 1.0
mmol) was treated with p-nitrophenyl azide (0.16 g, 1.0 mmol) in
3 mL of THF for 1 h to afford 0.43 g of the titled compound as a
1-(4-Nitrophenyl)-3-(1,3-di-tert-butyl-benzimidazol-2-ylidene)-
triazene (1-H-NO2). Following general procedure A, 1,3-di-tert-
butyl-benzimidazol-2-ylidene41 (0.23 g, 1.0 mmol) was treated with
p-nitro-phenyl azide (0.16 g, 1.0 mmol) in 3 mL of THF for 30
min to afford 0.39 g of the titled compound as a red powder (99%
yield). 1H NMR (CDCl3): δ 8.14 (d, J ) 9.2 Hz, 2H), 7.73 (dd, J
) 3.2, 6.4 Hz, 2H), 7.54 (d, J ) 9.2 Hz, 2H), 7.22 (dd, J ) 3.2,
6.4 Hz, 2H), 1.77 (s, 18H). 13C NMR (CDCl3): δ 160.9, 158.1,
144.0, 131.5, 127.8, 122.5, 120.3, 115.1, 62.9, 31.4. HRMS: [M
+ H]+ calcd for C21H27N6O2: 395.2195; found, 395.2198.
1
red powder (99% yield). H NMR (CDCl3): δ 8.17 (d, J ) 9.2
Hz, 2H), 7.59-7.53 (m, 4H), 1.76 (s, 18H). 13C NMR (CDCl3): δ
162.2, 157.3, 146.0 (dd, JF-C ) 16.5, 245.7 Hz), 144.7, 127.4, (t,
JF-C ) 5.9 Hz), 124.8, 120.8, 103.7 (m), 63.3, 31.2. HRMS: [M
+ H]+ calcd for C21H25F2N6O2: 431.2007; found, 431.2007.
1-Phenyl-3-(1,3-dimethyl-benzimidazol-2-ylidene)triazene (3-
H-H). Following general procedure B, 1,3-dimethyl-benzimidazo-
lium iodide35g (0.28 g, 1.0 mmol) and potassium tert-butoxide
(0.11 g, 1.0 mmol) were suspended in 5 mL of THF and stirred for
20 min. Afterward, phenyl azide (0.12 g, 1.0 mmol) was added,
and the resulting solution was stirred for 24 h at 0 °C. The solution
was then diluted with an equal volume of hexanes which caused
solids to precipitate. The crude solids were then collected via
filtration and then washed with water. After the solvent was
removed under reduced pressure, the crude product was triturated
with hexanes. Filtration of resulting suspension afforded 0.095 g
of the titled compound as a yellow solid (36% yield). Crystals
suitable for X-ray analysis were obtained by slow evaporation of a
saturated chloroform solution of the titled compound (CCDC
1-Phenyl-3-(1,3-di-tert-butyl-5,6-dimethoxy-benzimidazol-2-
ylidene)triazene (1-OCH3-H). Following general procedure A, 1,3-
di-tert-butyl-4,5-dimethoxy-benzimidazol-2-ylidene41 (0.29 g, 1.0
mmol) was treated with phenyl azide (0.12 g, 1.0 mmol) in 3 mL
of THF for 2 h to afford 0.41 g of the titled compound as a yellow
1
powder (99% yield). H NMR (CDCl3): δ 7.57-7.55 (m, 2H),
7.31 (t, J ) 8 Hz, 2H), 7.17 (s, 2H), 7.14-7.09 (m, 1H), 3.90 (s,
6H), 1.77 (s, 18H). 13C NMR (CDCl3): δ 161.3, 152.1, 144.8,
128.6, 125.9, 125.3, 121.0, 99.6, 61.8, 56.8, 31.3. HRMS: [M +
H]+ calcd for C23H32N5O2: 410.2556; found, 410.2551. Anal. calcd
for C23H31N5O2: C, 67.46; H 7.63; N, 17.10; found, C 67.31; H,
7.64; N, 16.94. Mp ) 140-142 °C. Td ) 154 °C.
1-(4-Methoxyphenyl)-3-(1,3-di-tert-butyl-5,6-dimethoxy-ben-
zimidazol-2-ylidene)triazene (1-OCH3-OCH3). Following general
procedure A, 1,3-di-tert-butyl-4,5-dimethoxy-benzimidazol-2-
ylidene41 (0.29 g, 1.0 mmol) was treated with p-methoxyphenyl
azide (0.15 g, 1.0 mmol) in 3 mL of THF for 2 h to afford 0.44 g
of the titled compound as a yellow powder (99% yield). 1H NMR
(CDCl3): δ 7.38 (d, J ) 8.8 Hz, 2H), 7.14 (s, 2H), 6.72 (d, J )
8.8 Hz, 2H), 3.89 (s, 6H), 3.79 (s, 3H), 1.76 (s, 18H). 13C NMR
(CDCl3): δ 161.4, 157.9, 145.9, 144.5, 126.2, 122.2, 113.9, 99.7,
61.6, 56.8, 55.4, 31.2. HRMS: [M + H]+ calcd for C24H34N5O3:
440.2662; found, 440.2656. Anal. calcd for C24H33N5O3: C, 65.58;
H 7.57; N, 15.93; found, C 65.48; H, 7.68; N, 15.56.
1-(4-Nitrophenyl)-3-(1,3-di-tert-butyl-5,6-dimethoxy-benzimi-
dazol-2-ylidene)triazene (1-OCH3-NO2). Following general pro-
cedure A, 1,3-di-tert-butyl-4,5-dimethoxy-benzimidazol-2-ylidene41
(0.29 g, 1.0 mmol) was treated with p-nitrophenyl azide (0.16 g,
1.0 mmol) in 3 mL of THF for 30 min to afford 0.45 g of the titled
compound as a red powder (99% yield). 1H NMR (CDCl3): δ 8.11
(d, J ) 9.2 Hz, 2H), 7.50 (d, J ) 9.2 Hz, 2H), 7.23 (s, 2H), 3.91
(s, 6H), 1.58 (s, 18H). 13C NMR (CDCl3): δ 159.8, 158.5, 145.9,
143.6, 125.2, 124.8, 119.9, 99.4, 62.7, 56.6, 31.4. HRMS: [M +
H]+ calcd for C23H31N6O4: 455.2407; found, 455.2408.
1
643401). H NMR (CD2Cl2): δ 7.58-7.55 (m, 2H), 7.42-7.38
(m, 2H), 7.26-7.17 (m, 5H), 3.81 (s, 6H). 13C NMR (CD2Cl2): δ
155.2, 152.1, 132.5, 129.2, 127.0, 122.6, 121.8, 108.6, 31.9.
HRMS: [M + H]+ calcd for C15H16N5: 266.1406; found, 266.1400.
1-(4-Nitrophenyl)-3-(1,3-dimethyl-benzimidazol-2-ylidene)-
triazene (3-H-NO2). Following general procedure B, 1,3-dimethyl-
benzimidazolium iodide35g (0.28 g, 1.0 mmol) and potassium tert-
butoxide (0.11 g, 1.0 mmol) were suspended 5 mL of THF and
stirred for 20 min. Afterward, p-nitrophenyl azide (0.16 g, 1.0
mmol) was added and the resulting solution was stirred for 24 h at
0 °C. The solution was then diluted with equal volume of hexanes
which caused solids to precipitate. The crude solids were then
collected via filtration, washed with water, and dried under reduced
pressure to afford 0.17 g of the titled compound as a red solid (56%
yield). Crystals suitable for X-ray analysis were obtained by slow
evaporation of a saturated chloroform solution of the titled
compound (CCDC 643400). 1H NMR (CDCl3): δ 7.24 (d, J ) 9.2
Hz, 2H), 7.65 (d, J ) 9.2 Hz, 2H), 7.31-7.7.25 (m, 4H), 3.89 (s,
6H). 13C NMR (CDCl3): δ 156.5, 145.6, 131.7, 124.9, 123.4, 121.5,
109.1, 31.9. HRMS: [M + H]+ calcd for C15H15N6O2: 311.1256;
found, 311.1260.
1-Phenyl-3-(1,3-dimethyl-5,6-dimethoxy-benzimidazol-2-ylid-
ene)triazene (3-OCH3-H). Following general procedure B, 1,3-
dimethyl-5,6-dimethoxy-benzimidazolium iodide19b (0.34 g, 1.0
mmol) and potassium tert-butoxide (0.11 g, 1.0 mmol) were
suspended in 5 mL of THF and stirred for 20 min. Afterward,
phenyl azide (0.12 g, 1.0 mmol) was added and the resulting
solution was stirred for 24 h at 0 °C. The solution was then diluted
with equal volume of hexanes which caused solids to precipitate.
The crude solids were then collected via filtration, washed with
water, and dried under reduced pressure to afford 0.23 g of the
titled compound as a yellow solid (69% yield). Crystals suitable
for X-ray analysis were obtained by slow evaporation of a saturated
1-Phenyl-3-(1,3-di-tert-butyl-5,6-difluoro-benzimidazol-2-ylid-
ene)triazene (1-F-H). Following general procedure A, 1,3-di-tert-
butyl-4,5-fluoro-benzimidazol-2-ylidene41 (0.27 g, 1.0 mmol) was
treated with phenyl azide (0.12 g, 1.0 mmol) in 3 mL of THF for
6 h to afford 0.39 g of the titled compound as a yellow powder
1
(99% yield). H NMR (CDCl3): δ 7.59-7.56 (m, 2H), 7.39 (t,
JH-F ) 9.2 Hz, 2H), 7.37-7.31 (m, 2H), 7.20-7.15 (m, 1H), 1.73
(s, 18H). 13C NMR (CDCl3): δ 162.3, 151.4, 145.2 (dd, JF-C
)
16.4, 243 Hz), 128.7, 127.9 (t, JF-C ) 5.93 Hz), 126.4, 121.3, 102.3
(m), 62.3, 30.9. HRMS: [M + H]+ calcd for C21H26F2N5:
386.2156; found, 386.2155.
1
chloroform solution of the titled compound (CCDC 643398). H
1-(4-Methoxyphenyl)-3-(1,3-di-tert-butyl-5,6-difluoro-benzimi-
dazol-2-ylidene)triazene (1-F-OCH3). Following general procedure
A, 1,3-di-tert-butyl-4,5-fluoro-benzimidazol-2-ylidene41 (0.27 g, 1.0
mmol) was treated with p-methoxyphenyl azide (0.15 g, 1.0 mmol)
in 3 mL of THF for 8 h to afford 0.42 g of the titled compound as
an orange powder (99% yield). 1H NMR (CDCl3): δ 7.57 (d, J )
8.0 Hz, 2H), 7.35 (t, JH-F ) 9.2 Hz, 2H), 6.89 (d, J ) 8.0 Hz, 2H),
3.80 (s, 2H), 1.73 (s, 18H). 13C NMR (CDCl3): δ 161.9 (m), 158.7,
145.1 (dd, JF-C ) 15.7, 242.7 Hz), 129.3, 128.2 (t, JF-C ) 5.9
NMR (CDCl3): δ 7.56 (m, 2H), 7.35 (m, 2H), 7.18 (m, 1H), 6.70
(s, 2H), 3.91 (s, 6H), 3.79 (s, 6H). 13C NMR (CDCl3): δ 154.3,
151.5, 146.2, 128.8, 126.4, 125.1, 121.4, 93.9, 56.8, 31.7. HRMS:
[M + H]+ calcd for C17H20N5O2: 326.1617; found, 326.1615.
1-(4-Nitrophenyl)-3-(1,3-dimethyl-5,6-dimethoxy-benzimida-
zol-2-ylidene)triazene (3-OCH3-NO2). Following general proce-
dure B, 1,3-dimethyl-5,6-dimethoxy-benzimidazolium iodide19b
(0.21 g, 0.63 mmol) and potassium tert-butoxide (0.07 g, 0.63
9416 J. Org. Chem., Vol. 72, No. 25, 2007

Donor-Acceptor Triazenes
mmol) were suspended in 5 mL of THF and stirred for 20 min.
Afterward, p-nitrophenyl azide (0.1 g, 0.63 mmol) was added and
the resulting mixture was stirred for 24 h at 0 °C. The reaction
mixture was then diluted with an equal volume of hexanes which
caused solids to precipitate. The crude solids were then collected
via filtration, washed with water, and dried under reduced pressure
to afford 0.22 g of the titled compound as a red solid (94% yield).
Crystals suitable for X-ray analysis were obtained by slow
evaporation of a saturated chloroform solution of the titled
with an equal volume of hexanes which caused solids to precipitate.
The crude solids were then collected via filtration, washed with
water, and dried under reduced pressure to afford 0.38 g of the
titled compound as a yellow oil (99% yield). 1H NMR (CDCl3): δ
7.54 (d, J ) 8.8 Hz, 2H), 7.18 (s, 4H), 6.96 (d, J ) 8.8 Hz, 2H),
4.13 (d, J ) 8.0 Hz, 4H), 3.84 (s, 3H), 2.39 (m, J ) 6.4, 6.8 Hz,
2H), 0.97 (d, J ) 6.4 Hz, 12H). 13C NMR (CDCl3): δ 158.5, 153.9,
145.1, 131.8, 122.5, 121.7, 113.7, 108.7, 55.2, 51.0, 27.7, 19.8.
HRMS: [M + H]+ calcd for C22H30N5O: 380.2450; found,
380.2455. Td ) 139 °C.
1
compound (CCDC 643399). H NMR (CD2Cl2): δ 8.20 (d, J )
9.2 Hz, 2H), 7.59 (m, J ) 9.2 Hz, 2H), 6.64 (s, 2H), 6.70 (s, 2H),
3.92 (s, 6H), 3.85 (s, 6H). 13C NMR (CD2Cl2): δ 157.7, 147.6,
145.2, 125.3, 124.2, 125.1, 121.4, 94.5, 57.1, 32.2. HRMS: [M +
H]+ calcd for C17H19N6O4: 371.1468; found, 371.1468.
N-Phenyl-1,3-dimethyl-benziminoimidazoline (5). 1-Phenyl-
3-(1,3-dimethyl-benzimidazol-2-ylidene)triazene (3-H-H) (0.1 g,
0.38 mmol) was dissolved in 4 mL of toluene and heated to
150 °C for 10 h. After removal of solvent under reduced pressure,
0.09 g of the titled compound was obtained as a yellow oil (99%
1-Phenyl-3-(1,3-di-iso-butyl-benzimidazol-2-ylidene)triaz-
ene (6-H-H). Following general procedure B, 1,3-di-iso-butyl-
benzimidazolium tetrafluoroborate42 (0.32 g, 1.0 mmol) and
potassium tert-butoxide (0.11 g, 1.0 mmol) were suspended in
10 mL of THF and stirred for 20 min. Afterward, phenyl azide
(0.12 g, 1.0 mmol) was added and the resulting mixture was stirred
for 24 h at 0 °C. The reaction mixture was then diluted with an
equal volume of hexanes which caused solids to precipitate. The
crude solids were then collected via filtration, washed with water,
and dried under reduced pressure to afford 0.35 g of the titled
compound as a yellow oil (99% yield). 1H NMR (CD2Cl2): δ 7.58-
7.55 (m, 2H), 7.43-7.39 (m, 2H), 7.26-7.20 (m, 5H), 4.16 (d, J
) 7.2 Hz, 4H), 2.40 (n, J ) 6.4, 7.2 Hz, 2H), 0.97 (d, J ) 6.4 Hz,
12H). 13C NMR (CD2Cl2): δ 154.7, 152.2, 132.4, 129.1, 126.9,
122.4, 121.7, 109.5, 51.7, 28.4, 20.1. HRMS: [M + H]+ calcd for
C21H28N5: 350.2345; found, 350.2342. Td ) 118 °C.
1
yield). H NMR (CD2Cl2): δ 7.28-7.24 (m, 2H), 7.04 (dd, J )
3.0, 5.6 Hz, 2H), 6.94-6.92 (m, 3H), 6.88 (dd, J ) 3.0, 5.6 Hz,
2H), 3.22 (s, 6H). 13C NMR (CD2Cl2): δ 150.5, 146.9, 133.2, 129.0,
122.5, 120.9, 120.5, 106.6, 30.4. HRMS: [M + H]+ calcd for
C15H15N3: 238.1344; found, 238.1347.
N-Phenyl-1,3-di-iso-butyl-benziminoimidazoline (11). 1-Phen-
yl-3-(1,3-di-iso-butyl-benzimidazol-2-ylidene)triazene (6-H-H) (0.1
g, 0.29 mmol) was dissolved in 3 mL of toluene and heated to
170 °C for 10 h. After removal of solvent under reduced pressure,
0.09 g of the titled compound was obtained as a yellow oil (99%
1
yield). H NMR (CD2Cl2): δ 7.23-7.20 (m, 2H), 6.99-6.96 (m,
2H), 6.89-6.83 (m, 5H), 3.53 (d, J ) 7.2 Hz, 4H), 2.11-2.08 (m,
2H), 0.77 (d, J ) 6.4 Hz, 12H). 13C NMR (CD2Cl2): δ 149.9,
146.1, 133.0, 128.5, 122.6, 120.5, 120.4, 107.5, 49.9, 27.9, 20.1.
HRMS: [M + H]+ calcd for C21H28N3: 322.2283; found, 322.2280.
1-Phenyl-3-(1,3-di-neo-pentyl-benzimidazol-2-ylidene)triaz-
ene (7-H-H). Following general procedure A, 1,3-di-neo-pentyl-
benzimidazol-2-ylidene43 (0.26 g, 1.0 mmol) was treated with
phenyl azide (0.12 g, 1.0 mmol) in 3 mL of THF for 2 h to afford
0.38 g of the titled compound as a yellow powder (99% yield). 1H
NMR (CD2Cl2): δ 7.61-7.58 (m, 2), 7.43-7.39 (m, 2H), 7.26-
7.16 (m, 5H), 4.35 (br, 4H), 1.04 (s, 18H). 13C NMR (CD2Cl2): δ
156.2, 152.1, 133.1, 129.2, 126.9, 122.1, 121.8, 110.3, 35.5, 28.2.
HRMS: [M + H]+ calcd for C23H32N5: 378.2658; found, 378.2654.
Anal. calcd for C23H31N5: C, 73.17; H 8.28; N, 18.55; found, C,
73.07; H, 8.25; N, 18.51. Mp ) 126-131 °C. Td ) 156 °C.
1-(4-Nitrophenyl)-3-(1,3-di-iso-butyl-benzimidazol-2-ylidene)-
triazene (6-H-NO2). Following general procedure B, 1,3-di-iso-
butyl-benzimidazolium tetrafluoroborate42 (0.32 g, 1.0 mmol) and
potassium tert-butoxide (0.11 g, 1.0 mmol) were suspended in 10
mL of THF and stirred for 20 min. Afterward, p-nitrophenyl azide
(0.16 g, 1.0 mmol) was added and the resulting mixture was stirred
for 24 h at 0 °C. The reaction mixture was then diluted with an
equal volume of hexanes which caused solids to precipitate. The
crude solids were then collected via filtration, washed with water,
and dried under reduced pressure to afford 0.39 g of the titled
N-(4-Nitrophenyl)-1,3-di-iso-butyl-benziminoimidazoline (12).
1-(4-Nitrophenyl)-3-(1,3-di-iso-butyl-benzimidazol-2-ylidene)tria-
zene (6-H-NO2) (0.15 g, 0.38 mmol) was dissolved in 4 mL of
toluene and heated to 180 °C for 10 h. After removal of solvent
under reduced pressure, 0.14 g of the titled compound was obtained
1
as a red solid (99% yield). H NMR (CDCl3): δ 8.11-8.08 (m,
2H), 7.11 (dd, J ) 3.4, 5.6, 2H), 7.03 (dd, J ) 3.4, 5.6 Hz, 2H),
6.77-6.74 (m, 2H), 3.64 (d, 15.6 Hz, 4H), 2.12-2.09 (m, 2H),
0.77 (d, J ) 6.8 Hz, 12H). 13C NMR (CDCl3): δ 157.1, 148.2,
139.2, 131.6, 125.5, 121.7, 120.3, 108.6, 50.1, 27.5, 19.9. HRMS:
[M + H]+ calcd for C21H27N4O2: 367.2134; found, 367.2132. Anal.
calcd for C21H27N4O2: C, 68.64; H 7.41; N, 15.25; found: C, 68.58;
H, 7.20; N, 15.04.
Acknowledgment. We are grateful to U.S. Army Research
Office (W911NF-05-1-0430 and W911NF-06-1-0147), the
National Science Foundation (CHE-0645563), the Petroleum
Research Fund as administered by the American Chemical
Society (44077-G1), the Welch Foundation (F-1621), and the
University of Texas at Austin for their generous financial support
of this work. Dimitri M. Khramov is grateful to the Dorothy
A. Banks Foundation and the Welch Foundation for graduate
fellowships. We also thank Prof. C. Grant Willson and his
research group for allowing us to use their thermogravimetric
analyzer.
1
compound as a red solid (99% yield). H NMR (CDCl3): δ 8.21
(d, J ) 7.2 Hz, 2H), 7.62 (d, J ) 7.2 Hz, 2H), 7.25 (br, 4H), 4.19
(d, J ) 8.4 Hz, 4H), 2.35 (m, 2H), 0.93 (d, J ) 6.5 Hz, 12H). 13
C
NMR (CDCl3): δ 156.7, 154.1, 145.3, 131.6, 124.7, 123.0, 121.4,
109.8, 51.5, 28.1, 20.0. HRMS: [M + H]+ calcd for C21H27N6O2:
395.2195; found, 395.2198. Mp ) 128-131 °C. Td ) 174 °C.
1-(4-Methoxyphenyl)-3-(1,3-di-iso-butyl-benzimidazol-2-ylid-
ene)triazene (6-H-OCH3). Following general procedure B, 1,3-
di-iso-butyl-benzimidazolium tetrafluoroborate42 (0.32 g, 1.0 mmol)
and potassium tert-butoxide (0.11 g, 1.0 mmol) were suspended in
10 mL of THF and stirred for 20 min. Afterward, p-methoxyphenyl
azide (0.15 g, 1.0 mmol) was added and the resulting mixture was
stirred for 24 h at 0 °C. The reaction mixture was then diluted
Supporting Information Available: Additional experimental
procedures, kinetic data, computational data, and NMR spectra for
all new compounds is available free of charge Via the Internet at
http://pubs.acs.org.
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Komarova, T. N.; Lopyrev, V. A. Russ. J. Org. Chem. 2003, 39, 1467.
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