Tautomerism in the fused N-rich triazolotriazole heterocyclic system

Article abstract of DOI:10.1002/ejoc.201201653

Tautomerism in the [1,2,4]triazolo[3,2-c][1,2,4]triazole fused aromatic system has been investigated by single-crystal X-ray analysis, UV/Vis spectroscopy and theoretical calculations on selected new heterobicycle derivatives. The reactions of 3,4-diamino-1,2,4-triazoles with acyl chlorides or acetic anhydride in pyridine at reflux led to ring closure and the fused aromatic system was obtained as the secondary amide at N2 instead of at N5 as previously reported in the literature. For the [1,2,4]triazolo[3,2-c][1,2,4] triazole system, three different tautomers can be present. Crystal structure analysis showed the presence of only one tautomer or, depending on the packing, a mixture of two; theoretical calculations supported this finding, predicting that for all the investigated compounds one tautomer is energetically disfavoured with respect to the other two. The electronic absorption spectra of some compounds show a marked solvent dependence, which has been interpreted, with the help of theoretical calculations, in terms of acid/base equilibria, which are critically dependent upon the substituents at the heterobicycle. [1,2,4]Triazolo[3,2-c][1,2,4]triazole has three tautomers: 2H, 3H and 5H. It is shown that the most stable tautomer is 2H, tautomer 3H has intermediate energy and the 5H has the highest energy. This compound provides a striking counter-example of the generally held view that the organic conjugated structure with the highest number of alternating single and double bonds is the most stable. Copyright

Full text of DOI:10.1002/ejoc.201201653

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FULL PAPER
Triazole Tautomerism
[1,2,4]Triazolo[3,2-c][1,2,4]triazole
has
R. Centore,* S. Fusco, A. Capobianco,
V. Piccialli, S. Zaccaria,
A. Peluso ......................................... 1–10
three tautomers: 2H, 3H and 5H. It is
shown that the most stable tautomer is 2H,
tautomer 3H has intermediate energy and
the 5H has the highest energy. This com-
pound provides a striking counter-example
of the generally held view that the organic
conjugated structure with the highest num-
ber of alternating single and double bonds
is the most stable.
Tautomerism in the Fused N-Rich Tri-
azolotriazole Heterocyclic System
Keywords:
Nitrogen
heterocycles
/
Tautomerism
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R. Centore et al.
FULL PAPER
DOI: 10.1002/ejoc.201201653
Tautomerism in the Fused N-Rich Triazolotriazole Heterocyclic System
Roberto Centore,*[a] Sandra Fusco,[a] Amedeo Capobianco,[b] Vincenzo Piccialli,[a]
Sabrina Zaccaria,[a] and Andrea Peluso[b]
Keywords: Nitrogen heterocycles / Tautomerism
Tautomerism in the [1,2,4]triazolo[3,2-c][1,2,4]triazole fused
aromatic system has been investigated by single-crystal X-
analysis showed the presence of only one tautomer or, de-
pending on the packing, a mixture of two; theoretical calcu-
ray analysis, UV/Vis spectroscopy and theoretical calcula- lations supported this finding, predicting that for all the in-
tions on selected new heterobicycle derivatives. The reac-
tions of 3,4-diamino-1,2,4-triazoles with acyl chlorides or
acetic anhydride in pyridine at reflux led to ring closure and
the fused aromatic system was obtained as the secondary
amide at N2 instead of at N5 as previously reported in the
literature. For the [1,2,4]triazolo[3,2-c][1,2,4]triazole system,
three different tautomers can be present. Crystal structure
vestigated compounds one tautomer is energetically disfav-
oured with respect to the other two. The electronic absorp-
tion spectra of some compounds show a marked solvent de-
pendence, which has been interpreted, with the help of theo-
retical calculations, in terms of acid/base equilibria, which
are critically dependent upon the substituents at the hetero-
bicycle.
can, in principle, be expected: the 2H, 3H and 5H tauto-
mers (chemical diagrams of the tautomers and atomic num-
bering used throughout the paper for the [1,2,4]triazolo[3,2-
c][1,2,4]triazoles are shown in Scheme 1).
Introduction
Heterocycles are key compounds in synthetic chemistry.
In addition to their long-standing and important applica-
tions in drugs and bioactive compounds, aromatic hetero-
cycles play a fundamental role in modern materials chemis-
try as building blocks of conjugated active molecules in
many emerging fields of organic electronics and optoelec-
tronics: conducting polymers,[1] organic field-effect transis-
tors,[2] organic solar cells[3] and non-linear optically active
compounds.[4] Fused aromatic heterocycles can be an inter-
esting alternative to fused aromatic hydrocarbons[5] in many
applications in organic electronics, because the presence of
heteroatoms can help to modulate the energy of the HOMO
and LUMO levels as well as the electron-poor/rich nature
of the compounds. In this context, N-rich fused systems
and, in particular, those containing acidic N–H groups, are
potentially interesting because they are expected to be more
soluble in solvents like pyridine, DMF or NMP. In ad-
dition, they can show tautomerism. Following our interest
in advanced materials[6] and bioactive compounds[7] based
on 10-electron aromatic fused heterocycles, we directed our
attention towards the [1,2,4]triazolo[3,2-c][1,2,4]triazole
fused aromatic system, for which three different tautomers
Scheme 1. Tautomers of [1,2,4]triazolo[3,2-c][1,2,4]triazole.
A comprehensive experimental and theoretical analysis
of this fused heterocycle is missing. One detailed account
of the synthetic procedures was given in the 1960s but with-
out any comment on its tautomerism (only the 5H isomer
was assumed).[8] Subsequently, tautomerism of the system
was formally acknowledged,[9] but only the 3H and 5H tau-
tomers were considered. Two subsequent papers[10] reported
semi-empirical theoretical calculations of the three tauto-
mers for the simple heterobicycle system with R = RЈ =
H, henceforth HH, with measurement of the electric dipole
moments. Finally, in the few, more recent papers in which
the fused heterobicycle appeared, there was no mention of
tautomerism.[11]
[a] Department of Chemical Sciences, University of Naples
“Federico II”,
Via Cinthia, 80126 Napoli, Italy
Fax: +39-081674090
E-mail: roberto.centore@unina.it
In this paper we present the synthesis of several new tri-
azolo[3,2-c]triazoles with a detailed crystallographic, spec-
troscopic and up-to-date theoretical analysis focused on
tautomerism and regiochemistry.
Homepage: https://www.docenti.unina.it/roberto.centore
[b] Dipartimento di Chimica e Biologia, Università di Salerno,
Via Ponte don Melillo, 84084 Fisciano (SA), Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201653.
2
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Tautomerism in Triazolotriazole
Results and Discussion
Regiochemistry
According to the literature,[8] triazolo[3,2-c]triazoles can
be prepared by the reaction of 3,4-diamino-1,2,4-triazoles
with acyl chlorides or acetic anhydride (RЈ = CH3) in pyr-
idine at reflux. By using a large excess of the acylating
agent, the fused system was obtained with the secondary
amide at N5 from which, by basic hydrolysis, the final com-
pound was obtained (the 5H tautomer). However, it must
be acknowledged that the only analytical data reported in
ref.[8] are elementary analyses, which, of course, do not pro-
vide real proof of either the regiochemistry of amide forma-
tion or which tautomer is formed upon hydrolysis of the
secondary amide. In a similar manner, in subsequent re-
ports,[9,11] formation of the 3H or 5H tautomer is claimed,
Figure 1. Ortep diagram of TT1A. Thermal ellipsoids are drawn at
the 50% probability level. Selected bond lengths [Å], bond angles
at least in the chemical diagrams reported, again without [°] and torsion angles [°]: C2–N3 1.366(4), C3–N2 1.374(4), C3–N3
1.332(4), C3–N4 1.343(4), C4–N1 1.323(4), N1–N2 1.391(4), N4–
any experimental proof.
N5 1.367(4), N2–C3–N3 141.7(4), C4–N4–N5 140.9(3), C10–C5–
C4–N1 –177.4(3), O1–C11–N2–N1 –175.4(3).
In this paper we have prepared several new triazolo[3,2-c]-
triazoles starting from the diaminotriazoles reported in
Scheme 2. These, in turn, were prepared according to a pro-
cedure described by us in a preceding paper.[12]
example, treatment of bis(diaminotriazole) 5 with excess
acetic anhydride afforded the diamidated-bis(heterobicycle)
(see the Exp. Sect.).
In the sole case of bitriazole 4, heating at reflux in excess
acetic anhydride did not afford the diamidated-bis(hetero-
bicycle) but the tetra-acetamide of the bitriazole (TAM) in
80% yield (Figure 2).
Scheme 2. 3,4-Diamino-1,2,4-triazoles used in the synthesis of the
new triazolo[3,2-c]triazoles.
In a first experiment we treated, according to ref.[8], di-
aminotriazole 1 with excess acetic anhydride. After appro-
priate workup (see the Exp. Sect.) we isolated a crystalline
1
product in high yield (79%); its H NMR spectrum indi-
Figure 2. X-ray structure of 3,3Ј,4,4Ј-tetraacetamidobi-1,2,4-tri-
azol-5-yl dihydrate (TAM). Thermal ellipsoids are drawn at the
50% probability level. Selected bond lengths [Å], bond angles [°]
and torsion angles [°]: C1–C1Ј 1.447(6), N1–C1–C1Ј 126.3(4), N3–
C1–C1Ј 123.9(4), N1–C1–C1Ј–N1Ј –180.0(4), N3–C2–N5–C3
176.4(3), C2–N3–N4–C5 99.5(4).
cated the formation of the amidated heterobicycle, as ex-
pected. However, the X-ray analysis (Figure 1) revealed that
the amide was formed at the N2 atom, which is in contrast
to ref.[8] and in agreement with the indirect assignment pro-
posed in ref.[10b] This compound will be denoted henceforth
as TT1A. The molecular structure of TT1A is substantially
flat, as indicated by the dihedral angle between the average
planes of the bicycle and the phenyl ring, which is 2.3(1)°.
The regiochemistry of the amide formation can be ex-
plained simply on the grounds that N2 is the least sterically
Solid-State Structures of the Tautomers
Basic hydrolysis of N2-amidated heterobicycles afforded
encumbered position of the heterobicycle (but see below for the N–H heterobicycles. These can also be obtained in two
the discussion on the packing and electronic effects). The steps by reaction of the diaminotriazole with 1 equiv. of the
reactivity of non-conjugated bis(diaminotriazoles) proved acylating agent, obtaining the amino-hydrazide that can be
to be essentially the same as the monodiaminotriazoles. For isolated and cyclized in the presence of a dehydrating agent,
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R. Centore et al.
FULL PAPER
for example, POCl3 (see the Exp. Sect.). N–H heterobicycles 0.8:0.2. Therefore we can conclude that crystals of TT3 con-
are generally soluble in pyridine, DMF and DMSO. In all tain a mixture of 2H and 3H tautomers, with the 2H isomer
1
the cases that we have investigated, the H NMR spectra prevailing over the 3H tautomer.
recorded in [D6]DMSO show the resonance of the N–H
proton as a broad singlet at high δ values, δ = 13–14 ppm,
which indicates that the proton is very acidic. In fact, the
deprotonated form of the heterobicycle is highly resonance-
stabilized (Scheme 3).
Scheme 3. Resonance forms of the deprotonated heterobicycle.
Figure 4. X-ray molecular structure of 7-n-heptyl-4-(4-nitro-
phenyl)-2(3)H-[1,2,4]triazolo[3,2-c][1,2,4]triazole (TT3). Thermal
ellipsoids are drawn at the 30% probability level. Selected bond
lengths [Å], bond angles [°] and torsion angles [°]: C10–N3
1.376(3), C10–N5 1.332(3), C9–N2 1.337(3), C9–N3 1.328(3), C9–
N4 1.342(3), C8–N1 1.307(3), N1–N2 1.391(2), N4–N5 1.370(2),
N2–C9–N3 141.9(2), C8–N4–N5 140.8(2), C3–C4–C5–C6 64.2(3),
C6–C7–C8–N4 67.1(3), C16–C11–C10–N3 174.5(2).
In two cases we were successful in growing single crystals
of the N–H heterobicycle suitable for X-ray analysis. In the
first case, compound TT2 (R = C6F5, RЈ = CH3 in
Scheme 1, Figure 3), the 2H tautomer is obtained unambig-
uously. It is worth noting that in previous experimental in-
vestigations,[8,9,11] the 2H tautomer had never been consid-
ered for this fused heterocycle even though it was proposed
as the most stable in early theoretical investigations.[10]
In TT3, the dihedral angle between the bicycle and the
phenyl ring is 7.6(1)° and the alkyl chain is out of the plane
of the bicycle.
It is possible, in principle, that the relative stability of the
three tautomers of the triazolotriazole system is influenced
by steric factors. However, in the 3H and 5H tautomers of
TT3, only a loose 1,6 contact between hydrogen atoms
would be present if the phenyl ring is coplanar with the
heterobicycle.[13] Analogously, for TT2, the 3H tautomer
would not suffer any apparent steric problem.
Packing factors could also lead to one tautomer being
favoured over the others. In fact, the compounds shown in
Figures 3 and 4 have a bent shape; if the H atom is bonded
to N2 or N3, it is in the convex part of the molecule,
pointing outwards, whereas in the 5H tautomer, the H atom
is in the concave part of the molecule and is more hidden.
These arguments can be relevant because the packing of the
compounds is likely driven by the formation of N–H···N
hydrogen bonds.
Figure 3. X-ray molecular structure of 4-methyl-7-(pentafluoro-
phenyl)-2H-[1,2,4]triazolo[3,2-c][1,2,4]triazole (TT2). Thermal el-
lipsoids are drawn at the 30% probability level. Selected bond
lengths [Å], bond angles [°] and torsion angles [°]: C2–N3 1.377(4),
C2–N5 1.341(4), C3–N2 1.341(4), C3–N3 1.313(4), C3–N4
1.355(4), C4–N1 1.322(4), N1–N2 1.367(4), N4–N5 1.378(3), N2–
C3–N3 141.7(3), C4–N4–N5 142.5(3), C10–C5–C4–N1 –138.5(3).
In the crystal packing of TT3, hydrogen-bonded chains
are formed with both the donor and the acceptor in the
The molecular conformation of TT2 is determined by the convex part of the molecule (N2–H and N3 in the case of
twist around the C4–C5 bond, which produces a dihedral the predominant 2H tautomer and N3–H and N2 in the
angle of 40.3(1)° between the average planes of the bicycle case of the 3H tautomer; Figure 5). It is worth noting that
and the pentafluorophenyl ring; the twist is probably related the crystal packing of TT3 is invariant (metrically) by the
to the close contacts between F5 and N4 and N5 [F5···N4 exchange of the 2H and 3H tautomers; this means that, in
2.876(4) Å, F5···N5 2.868(3) Å].
principle, both can be present in the same crystal.
In the second case, compound TT3 (R = C7H15, RЈ = p-
In the hydrogen-bonded chains formed by TT2 (Fig-
O2N-C6H4 in Scheme 1, Figure 4), the pattern of tautomer- ure 6), the N2–H donor is bonded to the N5 acceptor of
ism emerging from the crystallographic analysis is more another molecule, producing a convex-into-concave ar-
subtle. In fact, the X-ray analysis gives clear evidence of the rangement that, metrically, could also have been produced
hydrogen atom bonded to N2, but there is also evidence, equally well with the hydrogen bonded to N5 and N2 as
from low temperature data collections from different crystal the acceptor (5H tautomer). However, the fact that no trace
specimens, of another hydrogen atom bonded to N3, the of the 5H tautomer was detected in the X-ray analysis of
two hydrogen atoms having a weight ratio of approximately TT2 strongly suggests that the energy of the 5H tautomer
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Tautomerism in Triazolotriazole
and to a lesser extent at N6, possibly due to the fact that
only 5H possesses adjacent nitrogen atoms (N5 and N6)
with formal sp3 hybridization. The computed relative ener-
gies are reported in Table 1 and the calculated dipole mo-
ments are discussed in the Supporting Information.
Table 1. Relative energies of the 2H, 3H and 5H tautomers of HH
predicted at different levels of theory both in the gas phase and in
polar solvent (DMSO, in parentheses).[a]
Erel [kcal/mol]
HF
B3LYP
BMK
MP2[b]
2H
3H
5H
0.0 (0.0)
0.0 (0.0)
9.3 (4.1)
21.6 (12.9)
22.6 (13.0)
0.0 (0.0)
9.4 (4.0)
22.9 (13.5)
0.0 (0.0)
8.5 (3.3)
21.7 (13.1)
10.4 (4.7)
23.2 (14.2)
5Hp[c] 25.8 (14.8)
23.9 (13.5)[d] 23.1 (13.3)
Figure 5. Hydrogen-bonded chains in the crystal packing of TT3.
Only the most abundant 2H tautomer is shown for clarity. Struc-
tural data of the hydrogen bond: N2–H2···N3i 0.90(3), 1.96(3),
2.849(3) Å, 176(3)°; i = 1 – x, 1/2 + y, 1/2 – z.
[a] Relative energies were calculated by using the 6-31+G** basis
set. [b] The frozen core (FC) approximation was used in MP2 com-
putations. [c] Cs symmetry imposed (planar geometry). [d] Equilib-
rium geometry of 5H tautomer is predicted to be planar by PCM
computations at the BMK level.
is considerably higher than 2H and 3H. Thus, the relative
energies of the three tautomers, as inferred by the X-ray
The 2H tautomer is predicted to be much more stable
than the 3H and 5H tautomers in the gas phase, 5H having
an energy more than 20 kcal/mol greater than 2H. The
Hartree–Fock relative energies are in good agreement with
those determined by MP2, which indicates that the stability
of tautomers cannot be ascribed to correlation effects. HH
is a 10-electron aromatic system in which loss of planarity
could reduce conjugation, thus the geometry of 5H was also
optimized with the constraint of planarity to evaluate the
extent to which π conjugation affects the overall stability.
The resulting stationary point 5Hp possesses only one
imaginary frequency corresponding to the normal coordi-
nate for the pyramidalization of N5. Computations predict
a loss of 1–3 kcal/mol, depending on the method used, in
going from pyramidal to planar 5H, a very small contri-
bution compared with the energy difference between 2H
and 5H, which indicates that the high energy of 5H is not
structures, are E5H ϾϾ E3H Ͼ E2H
.
Figure 6. Hydrogen-bonded chains in the crystal packing of TT2.
Structural data of the hydrogen bond: N2–H2···N5i: 0.90(4),
1.93(4), 2.824(4) Å, 178(3)°; i = –1 + x, y, z.
Theoretical and Spectroscopic Analysis
The tautomerism of [1,2,4]triazolo[3,2-c][1,2,4]triazole due to loss of conjugation.
was investigated theoretically early with reference to the
The results in Table 1 show that the relative energies of
most simple HH compound.[10] Semi-empirical computa- both the 3H and 5H tautomers are lower in a polar environ-
tions indicated that the 2H tautomer is the most stable, fol- ment; 3H is stabilized by around 5 kcal/mol with respect to
lowed by 3H and 5H, the latter having a significantly higher the gas phase according to all methods, and 5H by around
energy. We have theoretically investigated the tautomerism 10 kcal/mol. Computations employing more accurate meth-
in TT1 (R = p-O2N-C6H4, RЈ = CH3 in Scheme 1), TT2 ods (coupled cluster) and extended basis sets confirmed the
and TT3, as well as compound HH, which is free from the results in Table 1 (see the Supporting Information).
steric and electronic effects of substituents, for comparison.
The charge distributions predicted by the HF computa-
The geometries of the 2H, 3H and 5H tautomers of HH tions are shown in Scheme 4. An increase in negative charge
were fully optimized at the HF, DFT (B3LYP and BMK on C7 is predicted on going from 2H to 3H and 5H; this
functionals) and MP2 levels of theory. To consider the ef- trend suggests that substitution at C7 with an electron-with-
fects of a polar environment, geometry optimizations were drawing group should result in the enhanced stability of 3H
carried out both in the gas phase and in solution, using in and 5H compared with 2H. In contrast, the charge on C4
the latter case the polarizable continuum model (PCM). (when summed with the charge on H4) is rather insensitive
The standard parameters of DMSO were employed to to tautomerism.
mimic polar environments. The 6-31+G** basis set was
On the basis of the preceding results, TT1, TT2 and TT3
adopted in this first set of computations. Independently of were studied at the DFT/6-31+G** level of theory both in
the method, planar equilibrium geometries were found for the gas phase and in polar solvents, ethanol and water, by
the 2H and 3H tautomers (Cs symmetry), with the 5H tau- using the PCM method. The computed relative energies of
tomer exhibiting strong pyramidalization at the N5 nitrogen the 2H, 3H and 5H tautomers of TT1, TT2 and TT3 are
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Scheme 4. Charge distribution (HF/QZVPP) in the tautomers of HH.
reported in Table 2 and the calculated dipole moments are
discussed in the Supporting Information. The equilibrium
geometries of the 2H tautomers of TT2 and TT3 closely
resemble those obtained by X-ray diffraction. Pyramidali-
zation of the N5 nitrogen was found for the 5H tautomer
in all three compounds in the gas phase as well as in polar
solvent.
Figure 8. UV/Vis spectra of TT2.
Table 2. Computed relative energies, in reference to tautomer 2H,
of tautomers 3H and 5H of TT1, TT2 and TT3.[a]
Erel [kcal/mol]
Gas
EtOH
H2O
3H
5H
3H
5H
3H
5H
TT1
TT2
TT3
6.9
6.8
14.1
21.4
17.3
25.5
1.2
2.3
8.6
13.0
11.2
17.5
0.9
2.1
8.3
12.4
10.8
16.9
[a] Calculated at the (PCM)BMK/6-31+G** level. Relative energies
predicted by B3LYP computations are given in the Supporting In-
formation.
The relative energies in Table 2 suggest that for TT1 and
TT2, a small fraction of the tautomer 3H could be present
in the solution of polar solvents. Indeed, the UV/Vis spectra
of TT1 and TT2 recorded in ethanol and ethanol/water
mixtures (Figure 7 and Figure 8) show a significant solvent
dependence that could be ascribed to the presence of two
tautomers in solution, whereas for TT3 no significant
changes can be observed (Figure 9).
Figure 9. UV/Vis spectra of TT3.
a new band with a maximum at a longer wavelength
(382 nm). By increasing the concentration of water, the
shoulder at 321 nm weakens and the higher wavelength
band gains intensity. The spectra recorded for TT2 (Fig-
ure 8) are similar to those of TT1, but the maxima of the
two bands are closer to each other (269 and 297 nm).
This spectroscopic behaviour could be explained by an
increase in the concentration of the 3H tautomer in going
from ethanol to water/ethanol mixtures, but this hypothesis
is not supported by TD-DFT computations, the results of
which are reported in Table 3. No substantial changes in
absorption wavelengths and intensities are predicted on
going from the 2H to the 3H tautomer for the three com-
pounds. The most intense transition for TT1 in EtOH is
predicted to be at 319 nm for 2H and 337 nm for 3H. No
significant solvatochromic shift is predicted for a change in
solvent from ethanol to water for both tautomers. We have
thus considered the hypothesis that acid/base equilibria
could be responsible for the observed behaviour. TD-DFT
computations were carried out for the conjugate bases of
Figure 7. UV/Vis spectra of TT1.
The absorption spectrum of TT1 (Figure 7) in EtOH ex- the three compounds. An intense transition, centred at
hibits an intense band with a maximum at 321 nm. On ad- 420 nm, is predicted for the conjugate base of TT1, well
dition of water, the band at 321 nm becomes a shoulder of compatible with the wavelength of the most intense band
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Tautomerism in Triazolotriazole
Table 3. Predicted wavelengths (λ) and oscillator strengths (f) of the most intense transitions of tautomers 2H and 3H and of the conjugate
base of TT1, TT2, and TT3 in H2O and EtOH (in parentheses).[a]
2H
3H
Conj. base
λ [nm]
f
λ [nm]
f
λ [nm]
f
TT1
319(319)
234(234)
221(221)
215(215)
260(260)
215(215)
211(211)
304(304)
206(205)
0.60(0.60)
0.05(0.05)
0.06(0.06)
0.10(0.11)
0.30(0.30)
0.06(0.06)
0.12(0.12)
0.73(0.74)
0.19(0.19)
337(337)
221(221)
0.67(0.68)
0.19(0.19)
420(424)
251(252)
249(250)
202(203)
314(317)
215(216)
0.64(0.65)
0.10(0.09)
0.17(0.17)
0.08(0.09)
0.32(0.33)
0.10(0.10)
TT2
TT3
261(261)
217(217)
212(213)
315(316)
295(295)
207(207)
205(205)
0.42(0.43)
0.09(0.09)
0.07(0.07)
0.07(0.07)
0.67(0.68)
0.08(0.06)
0.15(0.17)
340(342)
250(252)
218(218)
0.68(0.69)
0.08(0.08)
0.23(0.23)
[a] Calculated at the PCM/TD-BMK/6-31+G** level.
DMSO (δ = 2.50 ppm), Py (δ = 7.22, 7.58, 8.74 ppm) or CHCl3 (δ
observed in ethanol/water mixtures. Indeed, as is also evi-
denced by the H NMR spectra (see the discussion preced-
=7.26 ppm). 13C NMR chemical shifts are referenced to the solvent
1
([D6]DMSO:
δ = 39.51 ppm; [D5]Py: δ = 123.87, 135.91,
ing Scheme 3 and also Scheme S1 in the Supporting Infor-
mation), TT1 is estimated to be more acidic than the parent
HH owing to both resonance and inductive effects. The lat-
ter argument can be invoked for TT2 for which theoretical
computations predict an intense transition at 260 nm for
both 2H and 3H (both in ethanol and water), which com-
pares well with the λmax (269 nm) observed in ethanol. The
most intense transition for the conjugated base is predicted
at around 315 nm in both solvents and indeed a band
peaked at 297 nm is observed in the spectra recorded in
ethanol/water mixtures.
The fact that the absorption spectrum of TT3 does not
show significant changes in passing from ethanol to eth-
anol/water mixtures can be ascribed to the expected (and
also predicted) lower acidity of TT3 compared with TT1
and TT2.
150.35 ppm). ESI mass spectrometric analyses were recorded with
an Applied Biosystems API 2000 mass spectrometer equipped with
an electrospray source used in the positive mode. UV/Vis absorp-
tion spectra were measured with a Jasco V560 spectrophotometer.
Syntheses: Reactions of the diaminotriazoles with acetic anhydride,
as well as the basic hydrolysis of the amidated products were per-
formed in accord with literature procedures,[8] so they will be de-
scribed in detail for only one system.
2-Acetyl-4-methyl-7-(4-nitrophenyl)-[1,2,4]triazolo[3,2-c][1,2,4]tri-
azole (TT1A): 3,4-Diamino-5-(p-nitrophenyl)-1,2,4-triazole
(0.500 g, 2.28 mmol) was suspended in acetic anhydride (10 mL).
The mixture was heated at reflux for 3 h. The resulting solution
was cooled to room temperature and poured into water (70 mL).
The pale-pink solid formed was recovered by filtration and recrys-
tallized from dimethylformamide/water, yield 0.510 g (79%), m.p.
1
203 °C. H NMR (200 MHz, [D5]Py): δ = 2.63 (s, 3 H), 2.81 (s, 3
H), 8.43 (d, J = 9.2 Hz, 2 H), 8.60 (d, J = 8.6 Hz, 2 H) ppm. MS
(ESI): calcd. for C12H10N6O3 286.2 [M]+; found 309.0 [M + Na]+.
Conclusions
1,12-Bis(2-acetyl-4-methyl-[1,2,4]triazolo[3,2-c][1,2,4]triazol-7-yl)do-
decane: Yield 77%, m.p. 150 °C. 1H NMR (200 MHz, CDCl3): δ =
1.27–1.36 (m, 12 H), 1.65 (m, 4 H), 1.89 (m, 4 H), 2.53 (s, 6 H),
2.70 (s, 6 H), 2.95 (t, 4 H) ppm. 13C NMR (50 MHz, [D5]Py): δ =
15.5 (2 Me), 22.1 (2 acetyl Me), 25.2, 25.7, 29.3, 29.4, 29.7, 29.8
(12 CH2), 139.5, 143.0, 155.4 (6 Ctriaz), 169.6 (2 CO) ppm. MS
(ESI): calcd. for C24H36N10O2 496.6 [M]+; found 519.2 [M +
Na]+.
[1,2,4]Triazolo[3,2-c][1,2,4]triazole is a 10-electron aro-
matic N-rich fused heterocycle that can exist in three dif-
ferent tautomers: 2H, 3H and 5H. This heterocycle provides
a striking counter-example of the generally held view that
the organic conjugated structure with the highest number
of alternating single and double bonds is the most stable.
In fact, although the 5H tautomer would fulfil such a
requirement, it is in fact the least stable tautomer. Elec-
tronic absorption spectra have shown that the acidity of
[1,2,4]triazolo[3,2-c][1,2,4]triazole derivatives is critically de-
pendent upon the substituents on the heterobicycle.
4-Methyl-7-(4-nitrophenyl)-2H-[1,2,4]triazolo[3,2-c][1,2,4]triazole
(TT1): 2-Acetyl-4-methyl-7-(4-nitrophenyl)triazolo[3,2-c]triazole
(0.370 g, 1.29 mmol) was suspended in 5% KOH (20 mL). The mix-
ture was heated at reflux for 1 h to give a red solution. The solution
was cooled to room temperature and acidified to pH 4 with 0.1 m
HCl. A white product was obtained that was recrystallized from
DMF, yield 0.252 g (80%), m.p. 319 °C (decomp.). λmax (EtOH) =
321 nm; εmax = 1.82 ϫ 104 L mol–1 cm–1 1H NMR (200 MHz,
.
Experimental Section
[D6]DMSO): δ = 2.41 (s, 3 H), 8.43 (m, 4 H), 14.2 (br. s, 1 H) ppm.
13C NMR (50 MHz, [D6]DMSO): δ = 15.05 (Me), 124.6, 126.35,
131.1, 135.6 (CPh), 147.9, 157.65, 168.3 (Ctriaz). MS (ESI): calcd.
for C10H8N6O2 244.2 [M]+; found 245.0 [M + H]+, 267.0 [M +
Na]+.
Materials and Methods: All reagents were analytical grade and
used without further purification. Melting points were determined
by temperature-controlled optical microscopy (Zeiss Axioskop pol-
arizing microscope equipped with a Mettler FP90 heating stage).
NMR spectra were recorded with Gemini or Varian spectrometers
1
operating at 200 MHz in [D6]DMSO, [D5]Py or CDCl3. H NMR 4-Methyl-7-(pentafluorophenyl)-2H-[1,2,4]triazolo[3,2-c][1,2,4]tri-
chemical shifts are referenced to the residual signals of the solvent:
azole (TT2): Yield: 75%, m.p. 212 °C. λmax (EtOH) = 269 nm; εmax
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
7

Job/Unit: O21653
/KAP1
Date: 18-04-13 15:51:54
Pages: 9
R. Centore et al.
FULL PAPER
= 8.8 ϫ 103 Lmol–1 cm–1. H NMR (200 MHz, [D6]DMSO): δ = DMSO): δ = 2.01 (s, 6 H), 2.09 (s, 6 H), 10.56 (s, 2 H), 11.47 (s, 2
1
2.36 (s, 3 H), 14.3 (br. s, 1 H) ppm. 13C NMR (50 MHz, [D6]- H) ppm. 13C NMR (50 MHz, [D6]DMSO): δ = 20.6, 22.8 (4 Me),
DMSO): δ = 14.6 (Me), 126.2, 135.1, 140.2, 142.0 (CPh), 147.0, 140.9, 148.9 (4 Ctriaz), 169.2, 169.5 (4 CO) ppm. MS (ESI): calcd.
156.9, 167.2 (Ctriaz) ppm. MS (ESI): calcd. for C10H4N5F5 289.2
for C12H16N10O4 364.3 [M]+; found 387.1 [M + Na]+.
[M]+; found 290.0 [M + H]+, 311.9 [M + Na]+.
X-ray Analysis: All crystal structure determinations were measured
with a Bruker-Nonius KappaCCD diffractometer equipped with
an Oxford Cryostream 700 apparatus using graphite-monochro-
mated MoKa radiation (λ = 0.71073 Å). Reduction of data and
semi-empirical absorption corrections were performed by using the
SADABS program.[14] The structures were solved by direct meth-
ods (SIR97 program[15]) and refined by the full-matrix least-squares
method on F2 using the SHELXL-97 program[16] with the aid of
the program WinGX.[17] Hydrogen atoms bonded to carbon were
generated stereochemically and refined by the riding model, those
bonded to nitrogen in TT2 and TT3 were found in difference Fou-
rier maps and their coordinates were refined. Crystal and refine-
ment data are summarized in Table 4. The crystal packing analysis
was performed by using the Mercury program.[18]
1,12-Bis(4-methyl-2H-[1,2,4]triazolo[3,2-c][1,2,4]triazol-7-yl)dodec-
ane: Yield: 64%, m.p. 236 °C. 1H NMR (200 MHz, [D5]Py): δ =
1.24 (m, 16 H), 1.97 (m, 4 H), 2.59 (s, 6 H), 3.03 (tr, 4 H) ppm.
MS (ESI): calcd. for C20H32N10 412.5 [M]+; found 413.2 [M +
H]+.
3-Amino-4-(4-nitrophenylbenzamido)-5-n-heptyl-1,2,4-triazole: 3,4-
Diamino-5-n-heptyl-1,2,4-triazole (0.500 g, 2.53 mmol) was dis-
solved in pyridine (20 mL). 4-Nitrobenzoyl chloride (0.493 g,
2.66 mmol) was added to the solution. The suspension was heated
at reflux for 3 h under magnetic stirring and then the solution was
cooled to room temperature and poured into water (50 mL). A
white solid was obtained that was recovered by filtration and
recrystallized from DMF, yield 0.389 g (45%), m.p. 270 °C. 1H
NMR (200 MHz, [D6]DMSO): δ = 0.88 (t, 3 H), 1.30 (m, 8 H),
1.70 (quint., 2 H), 2.69 (t, 2 H), 5.78 (s, 2 H), 8.33 (m, 5 H) ppm.
13C NMR (50 MHz, [D6]DMSO): δ = 13.9 (Me), 22.1, 23.7, 25.7,
28.3, 28.4, 31.1 (6 CH2), 123.15, 129.9, 143.4, 148.9, 150.6, 152.2
(CPh and Ctriaz), 169.4 (CO) ppm.
CCDC-904802 (for TAM), -904803 (for TT1A), -904804 (for TT2)
and -904805 (for TT3) contain the supplementary crystallographic
data fort this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
4-Nitrophenyl-7-n-heptyl-2(3)H-[1,2,4]triazolo[3,2-c][1,2,4]triazole
(TT3): 3-Amino-4-(4-nitrophenylbenzamido)-5-n-heptyl-1,2,4-tri-
azole (0.250 g, 0.722 mmol) was suspended in POCl3 (15 mL). The
suspension was heated at reflux whilst stirring for 4 h. The resulting
solution was added dropwise into cold water (50 mL) whilst stir-
ring. A white solid formed, which was recovered by filtration and
recrystallized from DMF/water, yield 0.202 g (85%), m.p. 187 °C.
λmax (EtOH) = 297 nm; εmax = 1.44ϫ104 Lmol–1 cm–1. 1H NMR
(200 MHz, [D6]DMSO): δ = 0.83 (m, 3 H), 1.2–1.7 (m, 8 H), 1.80
(m, 2 H), 2.95 (t, 2 H), 8.32 (m, 4 H), 13.6 (br. s, 1 H) ppm. 13C
NMR (50 MHz, [D6]DMSO): δ = 13.9 (Me), 22.0, 24.3, 25.1, 28.15,
28.25, 31.0 (6 CH2), 124.1, 127.4, 137.6, 139.7 (CPh), 148.0, 157.2,
165.9 (Ctriaz) ppm. MS (ESI): calcd. for C16H20N6O2 328.4 [M]+;
found 329.1 [M + H]+.
Computational Details: Geometry optimizations at the Hartree–
Fock (HF), density functional theory (DFT, B3LYP and BMK hy-
brid functionals were used), and Møller–Plesset perturbation
theory up to second-order (MP2) levels were carried out in con-
junction with the 6-31+G** basis set. The near Hartree–Fock limit
(NHF) of HH was estimated by optimizing the geometries with the
quadruple-ζ QZVPP basis set including up to g functions for C and
N and up to f functions for H atoms. Effects due to polarization
of the solvent were modelled by the polarizable continuum model
(PCM).[19] The nature of all stationary points (both in vacuo and
in the solution phase) was verified by computing the eigenvalues of
the Hessian matrix. To find the real absolute minimum, geometry
optimizations of the TTn compounds were preceded by optimized
potential energy scans in which all the internal coordinates were
3,3Ј,4,4Ј-Diacetamidobi-1,2,4-triazol-5-yl: Finely ground 3,4-di- optimized but the angular variable ω, the dihedral angle formed by
amino-5-(3,4-diamino-1,2,4-triazol-5-yl)-1,2,4-triazole
(0.525 g,
the bicycle and the aryl planes, which was held fixed in every partial
optimization and varied in the range 0–180° in steps of two degrees.
For TT3 the n-C7H15 substituent was replaced by an ethyl group
in all computations. The adopted computational approach was
validated by performing several test computations at the CC2,
2.68 mmol) was suspended in acetic anhydride (20 mL) at reflux
for 4 h. The mixture was cooled to room temperature and the white
solid was filtered and recrystallized from water/ethanol (90:10 v/v),
yield 0.781 g (80%), m.p. 280 °C. 1H NMR (200 MHz, [D6]-
Table 4. Crystallographic data for the studied compounds.
TT1A
TAM
TT2
TT3
Formula
M
Space group
a [Å]
b [Å]
C12H10N6O3
286.26
Pbca
12.606(4)
7.2980(10)
27.778(2)
90
2555.5(9)
8
1.488
(C12H16N10O4)·2(H2O)
400.38
P21/c
9.231(5)
12.215(7)
9.458(6)
119.30(3)
930.0(9)
4
C10H4F5N5
289.18
P21/c
5.991(3)
22.035(7)
8.670(4)
106.83
1095.5(8)
4
C16H20N6O2
328.38
P21/c
18.594(6)
5.910(3)
15.422(6)
101.13(3)
1662.8(12)
4
c [Å]
β [°]
V [Å3]
Z
Density [g/cm3]
T [K]
1.430
293(2)
0.116
1.753
173(2)
0.173
1.312
173(2)
0.091
296(2)
0.112
μ [mm–1]
Reflections collected
Unique reflections (Rint
R1 [IϾ2σ(I)]
wR2 (all data)
15778
4147
1625 (0.086)
0.0642
7837
10112
3652 (0.041)
0.0474
)
2249 (0.11)
0.0617
0.1580
2470 (0.061)
0.0551
0.1063
0.1443
0.1124
8
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O21653
/KAP1
Date: 18-04-13 15:51:54
Pages: 9
Tautomerism in Triazolotriazole
Oliviero, S. D’Errico, N. Borbone, J. Amato, V. Piccialli, G.
CCSD and CCSD(T) levels of computation. The results are re-
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DFT), NHF, HF, DFT and MP2 computations with the 6-31+G**
basis set were carried out by using the Gaussian 09 package.[20]
Single-point computations (using MP2/6-31+G** optimized geo-
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zen core approximation was employed in all post Hartree–Fock
computations. The resolution of identity was employed in MP2 and
coupled cluster computations with the aug-cc-pVTZ basis set.
Atomic charges where computed by fitting the electrostatic poten-
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computational approach in a recent study of the conjugation in
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by using the BMK functional which is known to furnish better
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[13] As a useful comparison we can mention that, in the case of
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Supporting Information (see footnote on the first page of this arti-
1
cle): H and 13C NMR spectra of new products, computed dipole
moments for HH, TT1, TT2 and TT3, relative energies of tauto-
mers of HH evaluated with the NHF, MP2 and coupled cluster
methods using extended basis sets, relative energies of the tauto-
mers of TT1, TT2 and TT3 evaluated at the (PCM)B3LYP/6-
31+G** level in the gas phase, ethanol and water, charge distri-
bution (HF/QZVPP) of the conjugate base of HH.
Acknowledgments
Thanks are due to the CIMCF of the University of Naples “Feder-
ico II” for the X-ray facility. Financial support from the Ministero
dell’Università e della Ricerca (MIUR) (PRIN 08 and PRIN 09)
and the Compagnia di San Paolo (FARO research project) is grate-
fully acknowledged. The authors also thank Dr. L. Palombi for
helpful discussions.
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Received: December 6, 2012
Published Online: ᭿
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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9