Mildness in preparative conditions directly affects the otherwise straightforward syntheses outcome of Schiff-base isoniazid derivatives: Aroylhydrazones and their solvolysis-related dihydrazones

Article abstract of DOI:10.1016/j.molstruc.2020.129437

Aroylhydrazones are versatile compounds with a series of applications, from biological to technological spheres. The simplicity of their preparation allows for a great chemical variability and synthetic manageability. However, the process can be not as st

Full text of DOI:10.1016/j.molstruc.2020.129437

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Journal of Molecular Structure
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Mildness in preparative conditions directly affects the otherwise
straightforward syntheses outcome of Schiff-base isoniazid derivatives:
Aroylhydrazones and their solvolysis-related dihydrazones
a,∗
d
a
b
c
Daphne S. Cukierman , Beatriz N. Evangelista , Carlos Castanho Neto , Chris H.J. Franco ,
a,∗
e
b
Luiz Antônio S. Costa , Renata Diniz , Jones Limberger , Nicolás A. Rey
a
Laboratório de Síntese Orgânica e Química de Coordenação Aplicada a Sistemas Biológicos (LABSO-Bio), Departamento de Química, Pontifícia Universidade
Católica do Rio de Janeiro, Rio de Janeiro, 22451-900, Brazil
b
Laboratório de Síntese Orgânica e Química Fina (LaSOQF), Departamento de Química, Pontifícia Universidade Católica do Rio de Janeiro, Rio de Janeiro,
2
2451-900, Brazil
c
Departamento de Química, Campus Martelos, Universidade Federal de Juiz de Fora, Juiz de Fora, 36036-900, Brazil
d
Núcleo de Estudos em Química Computacional (NEQC), Departamento de Química, ICE, Universidade Federal de Juiz de Fora, Juiz de Fora, 36036-900,
Brazil
e
Grupo de Cristalografia Química (GCQ), Departamento de Química, ICEx, Universidade Federal de Minas Gerais, Belo Horizonte, 31270-901, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Aroylhydrazones are versatile compounds with a series of applications, from biological to technologi-
cal spheres. The simplicity of their preparation allows for a great chemical variability and synthetic
manageability. However, the process can be not as straightforward as one would imagine. Some pa-
rameters such as specific reactants, the amount of acid employed as catalyst and reaction tempera-
ture can have a direct impact on the obtained product. In the present work, we describe two series
of novel isoniazid-derived compounds prepared from a pair of different aldehyde precursors, as well as
the solvolysis, under harsh synthetic conditions, of the initially formed aroylhydrazones, leading to un-
expected dihydrazones. All compounds were unequivocally characterized in solution using 1D and 2D
NMR experiments in DMSO–d6 and, in the solid-state, by other classic techniques. System I is com-
posed by 2-(1H-pyrazol-1-yl)benzaldehyde and its hydrazone derivatives, while system II comprises 2-
Received 28 June 2020
Revised 27 September 2020
Accepted 9 October 2020
Available online xxx
Keywords:
Aroylhydrazones
Dihydrazones
Isoniazid
Solvolysis
Hirshfeld surfaces
(
4-metoxyphenoxy)benzaldehyde and its related Schiff-base products. The first aldehyde was obtained
for the first time via the copper-catalyzed Ullmann C–N coupling between 2-bromobenzaldehyde and
pyrazole. Single crystals of its aroylhydrazone and dihydrazone derivatives were isolated and thoroughly
characterized, including Hirshfeld surfaces and energy frameworks studies. Finally, we describe an NMR
and theoretically-based proposed reaction pathway for the unexpected formation of the dihydrazones in-
volving the solvolysis of the initially formed isonicotinoyl hydrazone followed by attack to a second free
aldehyde molecule.
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
particularly interesting in medicinal chemistry and have been iden-
tified as important hits and lead compounds to act upon differ-
ent molecular targets [1]. They have been employed in a wide
spectrum of pharmacological applications [2], such as analgesics
[3], anti-inflammatories [4], anti-tuberculosis [5], antitumor agents
[6] and antimalarials [7]. Specifically, N-acylhydrazones have also
been the subject of several studies in the inorganic chemistry field
due to the presence of the R R C=N−NH−CO− moiety, which al-
Aldehyde-derived hydrazones compose an organic class of com-
pounds which present, in their structures, the functional group
R HC=N−NR R . In hydrazones, the azomethine C=N double bond
1
2 3
is conjugated with the electron pair of the neighboring nitro-
gen, which makes them more resistant to hydrolysis than com-
mon Schiff bases. Hydrazones, synthesized through the conden-
sation reaction between hydrazides and carbonyl compounds, are
1
2
lows them to perform as bidentate ligands, coordinating metal ions
through the azomethine nitrogen and the carbonyl oxygen. For ex-
ample, these compounds have been investigated as iron chelators
for the control of neurodegenerative disorders such as Friedre-
ich ataxia and other diseases related to the excess of this metal
∗
Corresponding authors:.
E-mail
addresses:
daphcukierman@gmail.com
(D.S.
Cukierman),
nicoarey@puc-rio.br (N.A. Rey).
https://doi.org/10.1016/j.molstruc.2020.129437
0
022-2860/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: D.S. Cukierman, B.N. Evangelista, C.C. Neto et al., Mildness in preparative conditions directly affects the other-
wise straightforward syntheses outcome of schiff-base isoniazid derivatives: Aroylhydrazones and their solvolysis-related dihydrazones,
Journal of Molecular Structure, https://doi.org/10.1016/j.molstruc.2020.129437

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[
8–10]. In this scenario, ortho-pyridine-derived isonicotinoyl hydra-
employed in the Ullmann reactions, which was degassed before
use. Reactions monitored by TLC were performed on Macherey-
Nagel Alugram® Sil 60/UV254 sheets (thickness 0.2 mm). Purifica-
tion of products was carried out by column chromatography using
Macherey-Nagel silica gel (230–400 mesh).
zones are of interest, since they can efficiently bind mitochondrial
iron through the tridentate Npy, N, O-chelating system [11].
Our research group at the Department of Chemistry of the Pon-
tifical Catholic University of Rio de Janeiro specializes in the syn-
thesis, characterization and application of different hydrazones and
their metal complexes, for a series of purposes ranging from tech-
nological to pharmacological applications [12–23]. For example, we
have explored the luminescent properties of a symmetric dihydra-
zone and its possible use as a constituent of the emitting layer for
the fabrication of OLEDs [16]. We have also shown that a series
of dinuclear copper(II) complexes covering different hydrazonic lig-
ands display potent antiproliferative activity against a panel of sev-
eral cancer cell lines [21,22]. Our efforts have been focused to-
wards neurodegenerative disorders too, such as Alzheimer’s and
Parkinson’s diseases, since we have observed that isonicotinoyl hy-
drazones constitute a promising class of Metal-Protein Attenuat-
ing Compounds (MPACs) acting as selective ligands for biometal-
binding according to the metal hypothesis of these disorders
2
.1.1. Synthesis of 2-(1H-pyrazol-1-yl)benzaldehyde (1) via Ullmann
coupling
An oven-dried resealable Schlenk flask was charged with
-bromobenzaldehyde (1.5 mmol), pyrazole (2.25 mmol), CuI
0.15 mmol) and K PO4 (3.0 mmol), evacuated and back-filled with
2
(
3
nitrogen. Then, degassed DMF (8.0 mL) was added and the re-
action mixture was stirred at 75 °C for 24 h. After this period,
the solution was allowed to cool-down to room temperature, di-
luted with 20 mL of ethyl acetate and washed with deionized wa-
ter (3 × 7 mL). The organic layer was dried over Na SO , filtered
2
4
and concentrated in vacuum. After column chromatography (elu-
ent: hexane – ethyl acetate 6:1), the desired product 1 was ob-
tained as a white solid (53% yield). M.p.: 55–57 °C. GC–MS (EI,
70 eV) m/z (%): 171(1), 144(100), 117(25), 104(7), 90(30), 77(35).
[
13,14,18,19,23]. Moreover, we have just demonstrated the ability
of these compounds in preventing the metal-catalyzed oxidation
of a mutant fragment of human prion protein [20].
2
.1.2. Synthesis of 1,2-bis[2-(1H-pyrazol-1-yl)benzylidene]hydrazone
(2)
Our lead compound, i.e., 8-hydroxyquinoline-2-carboxaldehyde
Compound 2, a dihydrazone, was unexpectedly obtained in
isonicotinoyl hydrazone (INHHQ), is capable of competing, in vitro,
the form of single crystals from the slow evaporation of the
mother liquor in an attempt of synthesizing the 2-(1H-pyrazol-1-
yl)benzaldehyde isonicotinoyl hydrazone (inhere called compound
for the interactions of Zn2 and Cu with the Aβ peptide [14]. In
silico pharmacological analyses demonstrated that the compound is
neutral in physiological pH and it is capable of crossing the blood-
brain barrier, which has recently been proved through its detec-
tion, by HPLC, in the brains of Wistar rats intraperitoneally injected
with INHHQ [18]. Despite the presence of the 8-hydroxyquinoline
+
2+
3
). In this synthesis, 0.5 mmol of isoniazid (isonicotinic acid hy-
drazide) in 10.0 mL of ethanol was dropwise added to 5.0 mL of an
ethanolic solution containing 0.5 mmol of 1. The reaction mixture
was kept under reflux for 3 h and no precipitation was observed
upon cooling of the solution. Thus, 3 drops of conc. hydrochloric
acid were added to the mixture, which immediately turned bright
yellow. It was then refluxed for another 3 h. When performed un-
der such conditions, compound 2 was the only isolated product
after slow evaporation of the solvent.
(
8-HQ) moiety, traditionally used in the synthesis of new MPACs
described in recent literature, we suggested that the Zn2 and
+
Cu2 coordination in INHHQ occurs through the aroylhydrazonic
system, an innovation that opens new perspectives regarding the
development of hydrazones as potential MPACs.
+
However, in spite of the apparent simplicity related to the
one-step synthesis of hydrazones, which partially accounts for the
broad range of applications discussed above, some systems are
not so straightforward. In this context, the current work reports
on the preparation of two novel isonicotinoyl (isoniazid-derived)
hydrazones, obtained from 2-(1H-pyrazol-1-yl)benzaldehyde and
To synthesize compound 2 in higher yields, another route
involving hydrazine as one of the starting materials was at-
tempted. Free hydrazine was obtained from hydrazine dihydrochlo-
ride (0.25 mmol in 5 mL of methanol) by its neutralization with
potassium hydroxide (0.25 mol L 1 methanolic solution), after fil-
trating the KCl precipitate. To the resulting solution, 0.5 mmol of 1,
dissolved in 5 mL of methanol, was dropwise added and the yellow
mixture was left stirring for 4 h at room temperature. Compound
−
2
-(4-metoxyphenoxy)benzaldehyde. We observed that, depending
on the “mildness degree” of the experimental conditions em-
ployed (pH, temperature), a dihydrazone solvolysis product can
take over as the main product of the reaction. All the six com-
pounds (aldehydes, aroylhydrazones and dihydrazones) were un-
equivocally characterized in the solid-state, by means of a series
of classic techniques, as well as in solution (using uni- and bidi-
2
was isolated after slow evaporation of the solvent (65% yield).
M.p.: 159–161 °C.
2
.1.3. Synthesis of 2-(1H-pyrazol-1-yl)benzaldehyde isonicotinoyl
hydrazone (3)
mensional NMR experiments in DMSO–d ). Single crystals ade-
6
To a solution of 1 (0.5 mmol in 5.0 mL of ethanol), an ethanolic
solution of isoniazid (0.5 mmol in 10.0 mL) was dropwise added.
Upon addition of 1 drop of conc. hydrochloric acid, the mixture
turned bright yellow. It was then left stirring for 4 h at room tem-
perature. Single crystals of compound 3 suitable for X-ray diffrac-
tion analysis were obtained after slowly concentrating the solution
quate to structural elucidation through XRD were obtained for the
aroylhydrazone and the dihydrazone derived from 2-(1H-pyrazol-
1
-yl)benzaldehyde and their structures are also discussed, along
with their Hirshfeld surfaces description. Also, a reaction path-
way for the conversion of isonicotinoyl hydrazones into dihydra-
zones in strong acidic medium and under reflux is suggested
and has been studied through 1H NMR experiments for the 2-(4-
metoxyphenoxy)benzaldehyde derivatives. Theoretical calculations
were employed to obtain thermodynamic support for this proposi-
tion.
(
51% yield). M.p. 154–157 °C.
2.1.4. Synthesis of 2-(4-metoxyphenoxy)benzaldehyde (4)
mixture of 4-methoxyphenol (2.75 mmol),
fluorobenzaldehyde (2.50 mmol), K CO3 (3.00 mmol) and DMF
A
2-
2
(
5 mL) was stirred in a resealable Schlenk flask at 150 °C for 4 h.
2
. Materials and methods
Then, the reaction mixture was allowed to cool down, diluted with
−
1
2
0 mL of ethyl acetate and washed with 1.0 mol L
hydroxide and water (3 × 7 mL). The organic layer was dried over
Na SO , filtered and concentrated under vacuum, affording the
potassium
2
.1. Syntheses
2
4
Chemicals and solvents were obtained from commercial sources
aldehyde (compound 4) as a pale yellow solid (71% yield) [24,25].
M. p. 63–65 °C.
and used without further purification with the exception of DMF
2

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2
.1.5. Synthesis of 1,2-bis[2-(4-methoxyphenoxy)
2.3. Theoretical calculations
benzylidene]hydrazone (5)
To evaluate whether the reaction between aldehyde 4 and iso-
niazid under harsh synthetic conditions would afford the dihy-
drazonic product as reported for the previous case, 5.0 mL of an
ethanolic solution containing 0.2 mmol of isoniazid were dropwise
added to another 5.0 mL of an equimolar ethanolic solution of 4.
No change in the color of the solution was observed. Thus, 3 drops
of conc. hydrochloric acid were added. The solution immediately
turned bright yellow and precipitation occurred. The system was
kept under reflux for a period of 3 h, which re-dissolved the pre-
cipitate. Upon cooling and slow evaporation of the solvent, a yel-
low solid was isolated.
The optimization and harmonic frequencies of the hydrazonic
derivatives 2, 3, 5 and 6 have been conducted using the Stefan
Grimme’s Hartree-Fock-3c (HF-3c) method, developed as an option
to achieve good and reliable results in reduced, very convenient,
computational time [34]. For all structures, the level of theory ap-
plied was HF MINIX D3BJ GCP(HF/MINIX), i.e., which contemplates
corrections for basis set superposition error (BSSE) by the geo-
metric counterpoise scheme (gCP) [35], atom-pairwise dispersion
correction with the Becke-Johnson damping scheme (D3BJ) [36,37]
and a correction for short-range basis incompleteness (nine empir-
ical parameters are used overall). Also, this method was applied to
provide the energetic pathway for the mechanistic investigation in-
volving compounds 5 and 6. This method was used on ORCA 3.0.3
release [38].
Alternatively, hydrazine dihydrochloride (0.125 mmol in 2.5 mL
−1
of methanol) was reacted with potassium hydroxide (0.25 mol L
methanolic solution) to obtain the free hydrazine, after separat-
ing the KCl precipitate. To the resultant filtrate, 0.25 mmol of 4
in methanol (2.5 mL) was dropwise added resulting in the imme-
diate formation of a light yellow precipitate corresponding to com-
pound 5. Stirring was kept for additional 40 min before filtration
in a glass funnel (60% yield). M.p. 166–167 °C.
The density functional theory (DFT) approach was also em-
ployed with the well-established level of theory CAM-B3LYP/6–
311+G(2d,p) [39, 40] in order to confirm the assigned vibrational
modes. All calculations were performed at 298 K and 1 atm in gas
phase. DFT calculations were carried out on the Gaussian 09 soft-
ware package [41]. All calculations were performed on Dell servers
from NEQC, UFJF.
2
.1.6. Synthesis of 2-(4-metoxyphenoxy)benzaldehyde isonicotinoyl
2.4. 1H NMR measurements to follow the reaction pathway in
system II
hydrazone (6)
This isonicotinoyl hydrazone was prepared similarly to com-
pound 3, in a Schiff base condensation reaction. A solution con-
taining isoniazid (0.25 mmol in 6.0 mL of ethanol) was dropwise
added to an ethanolic solution of 4 (0.25 mmol in 5.0 mL). One
drop of conc. hydrochloric acid was added as catalyst and the so-
lution immediately turned bright yellow. No precipitation was ob-
served and the reaction mixture was kept under stirring for 3 h
at room temperature. Slow evaporation of the solvent afforded a
bright yellow precipitate (compound 6, 83% yield). M.p. 216–218 °C.
Mechanistic insights on the unexpected formation of the dihy-
drazones 2 and 5 under severe synthetic conditions were obtained
from system II, involving the reaction between aldehyde 4 and
isoniazid. Samples (20 μL) from the reaction mixture were taken
in three different synthetic steps: immediately after the mixture
of both reagents (step 1), after the addition of conc. hydrochlo-
ric acid (step 2) and after 3 h of reaction under reflux (step 3).
These aliquots were diluted to 600 μL with DMSO–d6 and added
to 5 mm NMR tubes and a 1H NMR spectrum of each sample was
recorded.
2
.2. Instrumentation and characterization
3. Results and discussion
Melting points were determined either on a Büchi SMP-20 cap-
illary apparatus or in a Fisatom model 431 apparatus. Thermogravi-
metric studies, on the other hand, were conducted in a Perkin-
Elmer analyzer, model Pyris 1 TGA, under nitrogen atmosphere,
in the range 25–900 °C, using a heating rate of 10 °C min−1. FT-
IR spectra were recorded on a Perkin-Elmer 100 FT-IR spectrome-
ter in KBr. Mass spectra were obtained on a GC/MS Shimadzu QP-
3
.1. 2-(1H-pyrazol-1-yl)benzaldehyde-derived compounds (system I)
Two different strategies were employed in the synthesis of
aldehyde 1 (Scheme 1). Initially, 2-fluorobenzaldehyde was reacted
with pyrazole in the presence of K CO , in an aromatic nucle-
2
3
ophilic substitution condition. In this way, 1 was obtained with
1% yield. This result is similar to those previously described for
5
050 (EI, 70 eV) equipped with a 30 meter DB-5MS column. NMR
3
spectra were recorded on a Bruker Avance III HD-400 spectrome-
ter (operating at 400 MHz for 1H and 100 MHz for 13C). Chemi-
this procedure [42,43]. Alternatively, we evaluated the Ullmann
C−N coupling between 2-bromobenzaldehyde and pyrazole. We
applied a ligand-free system composed by CuI and K PO in DMF
cal shifts are expressed in ppm relative to tetramethylsilane (TMS)
and the spectra were calibrated based on the residual solvent sig-
nal (quintet at 2.50 ppm for 1H and septet at 39.52 for 13C).
3
4
and obtained the target intermediate with an improved yield of
3%. It is important to mention that carbonyl protection was not
5
The single crystal X-ray diffraction data for 2 and 3 were col-
lected in an Agilent-Rigaku Supernova diffractometer using MoKα
˚
(
λ = 0.71073 A) radiation at room temperature (293 K). The data
collection, cell refinements and data reduction were performed us-
ing the CRYSALISPRO software [26]. The structures were resolved
by direct methods using SIR [27] program and refined by SHELXL-
2
018/3 [28] using the WinGX system [29]. All non-hydrogen atoms
were refined with anisotropic thermal parameters. H atoms con-
nected to carbon were placed in idealized positions and treated
by rigid model, with Uiso(H) = 1.2 Ueq for aromatic rings and
CH groups. In compound 3, the hydrazone-related NH group was
treated similarly to CH groups. Figures were drawn using ORTEP-3
for Windows [30], Pov-Ray [31] and Mercury [32] programs. The
analysis of Hirshfeld surfaces, fingerprint plots and energy frame-
works were performed through the CrystalExplorer program [33].
Scheme 1. Two strategies employed in the synthesis of the pyrazolylbenzaldehyde
1.
3

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Fig. 1. 1H NMR (400 MHz) spectra of compounds 1 (green), 2 (red) and 3 (blue) in DMSO–d6 at room temperature.
necessary and that this is the first report of an Ullmann reaction
signal at around 6.60 ppm observable in the spectra of all three
compounds. The equivalent hydrogens of the pyridinic ring derived
from isoniazid, on the other hand, can be detected as a pair of dou-
blets of doublets at 8.76 and 7.80 ppm in compound 3, also in ac-
cordance with our previous studies. They are obviously absent in
the spectrum of 2. Tables S1, S2 and S3 present a thorough report
of all the chemical shifts and couplings for 1H and 13C NMR of
compounds 1, 2 and 3.
between these chemicals. Thus, this methodology afforded the de-
sired compound with superior results in comparison to the exper-
imental methods published formerly.
Aldehyde 1 was initially prepared in order to expand the chem-
ical diversity in our compound library of moderate metal-binding
N-acylhydrazones. However, when reacting this product with iso-
niazid in strong acidic medium, under reflux, single crystals of the
dihydrazone
2
{1,2-bis[2-(1H-pyrazol-1-yl)benzylidene]hydrazone}
The aldehyde-derived compounds belonging to system I were
isolated as single crystals suitable for X-ray diffraction studies,
which allowed for their complete solid-state characterization. The
crystal structure investigations of compounds 2 and 3 indicate
that both crystallize in the monoclinic system, although in dif-
were unexpectedly isolated as the reaction’s main product. Isoni-
azid is an anti-tuberculosis drug widely employed as an impor-
tant construction block in the Schiff-base condensation synthesis
of isonicotinoyl hydrazones.
On the other hand, when performed in milder conditions, this
reaction afforded single crystals of the intended N-acylhydrazone
ferent space groups: P2 /c and P2/c, respectively, containing four
1
molecules per unit cell. Fig. 2 exhibits the molecular representa-
tion of the crystal structures and the crystallographic details for
these compounds are listed in Table 1. Apart from the character-
istic aroylhydrazone NH in 3, no protonation, counter-ions or sol-
vent molecules are observed in either structures. The lack of sol-
vent molecules in the crystal networks of 2 and 3 is confirmed by
the thermal decomposition patterns of both compounds (data not
shown), with no mass loss below 200 °C. The solution NMR spec-
tra also show no additional protonation. As expected, the E config-
uration is predominant around the C=N bonds, due to its greater
stability.
3
[2-(1H-pyrazol-1-yl)benzaldehyde isonicotinoyl hydrazone]. Our
proposition is that the isonicotinoyl hydrazone may actually be
formed at first, since imine generation usually occurs immediately
upon the mixture of the reactants, specially under acidic cataly-
sis. However, further protonation on the carbonyl group increases
the carbon’s electrophilicity, making it susceptible to a nucleophilic
attack by a solvent molecule (i.e., ethanol). The subsequent in-
tramolecular electron rearrangement results in breaking of the N–C
bond, thus producing a terminal hydrazone derived from the struc-
ture of aldehyde 1. Since free 1 is still present in the reaction mix-
ture, a new C=N bond is formed, yielding the dihydrazone 2.
In order to obtain the originally desired hydrazone 3, the syn-
thesis was repeated at room temperature and under less acidic
conditions. Additionally, compound 2 was also obtained from the
reaction between two equivalents of 1 and one of hydrazine.
Selected geometrical parameters for these compounds are dis-
played in Table 2.
In the solid-state, the molecule of the dihydrazone 2 is not
symmetric as one would expect, and the main difference between
each phenyl-pyrazole fragment is in the torsion angles amid the
planes defined by N1(a), N2(a), C1(a), C2(a) and C3(a) - pyrazole
ring - and C4(a), C5(a), C6(a), C7(a), C8(a) and C9(a) - phenyl
ring. In one fragment, the angle is equal to 51.5° and, in the
other, to 53.7° The phenyl rings from each fragment are also
not in the same plane, displaying a dihedral angle of 9.3° The
C=N−N=C link between the fragments, showing average C=N dis-
Fig. 1 displays the 1H NMR spectra (DMSO–d ) of pyrazolyl-
6
benzaldehyde (1) and its derived compounds 2 and 3. The alde-
hyde, characterized through its typical de-shielded hydrogen (H10)
at 9.93 ppm, is clearly converted into azomethines, observed at
8
.52 ppm in 2 and 8.39 ppm in 3. In the latter, a characteristic
aroylhydrazone NH singlet appears at 12.16 ppm, in accordance
to previously published isonicotinoyl hydrazones [19,20]. All the
signals in the 13C NMR spectra, as well as those related to aro-
matic hydrogens have been unequivocally assigned using 2D-NMR
experiments COZY, HSQC and HMBC (Supplementary Information,
Figures S1–10). H2, the most shielded hydrogen, displays a triplet
˚
tances of 1.272(2) A, present a torsion angle of 176.1° The crys-
tal stability is guaranteed by non-conventional CH···N hydrogen
bond interactions between the pyrazole rings of two dihydrazone
molecules, giving rise to a dimeric arrangement. A number of
these dimers, on the other hand, are linked by CH···π interac-
4

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Table 1
Crystal data and refinement parameters for compounds 2 and 3.
Compound 2
Compound 3
Formula
Formula weight (g mol 1)
C20H16N6
C16H13N5O
291.31
−
340.39
Crystal system, space group
Monoclinic, P21/c
293
Monoclinic, P2/c
293
Temperature (K)
a ( A˚ )
b ( A˚ )
c ( A˚ )
7.9805(5)
22.589(1)
10.3283(5)
107.618(6)
1774.5(2)
4
12.3523(6)
7.9564(4)
15.3018(8)
96.325(5)
β (°)
V ( A˚ 3
)
1494.7(1)
Z
4
Radiation type
Mo Kα
0.081
Mo Kα
0.086
μ (mm 1)
−
Crystal size (mm)
0.90 × 0.38 × 0.20
37,675/4752
2963/0.0491
238
0.26 × 0.20 × 0.14
38,444/4054
3187/0.0304
200
Measured / Independent reflections
Observed reflections / Rint
Number of parameters
Robs, Rall
0.0540, 0.0909
0.1429, 0.1667, 1.063
0.157, −0.154
0.0505, 0.0654
0.1378, 0.1460, 1.081
0.227, −0.173
wRabs, wRall, S
ρmax, ꢀρmin (e A˚ 3)
−
ꢀ
Table 2
Selected bond distances and angles for compounds 2 and 3.
Compound 2
Compound 3
Bond distances ( A˚ )
N1−N2
1.359(2)
1.407(2)
1.361(2)
1.273(2)
1.270(2)
–
1.357(2)
1.375(1)
–
N3−N3a / N3−N4
N1a−N2a
C10−N3
1.267(2)
1.348(2)
1.211(2)
C10a−N3a / C11−N4
C11−O1
Bond angles (°)
C4−N1−N2
121.7(1)
121.5(1)
96.03(1)
121.5(1)
119.2(1)
115.9(1)
118.4(1)
115.4(1)
–
C5−C4−N1
118.0(2)
123.1(1)
119.8(1)
112.9(1)
111.4(1)
121.5(1)
121.1(1)
121.1(1)
121.6(1)
–
C4−C5−C10
C5−C10−N3
C10−N3−N3a / C10−N3−N4
N3−N3a−Cl0a / N3−N4−C11
N3a−C10a−C5a / N4−C11−C12
C4a−N1a−N2a
C5a−C4a−N1a
–
C4a−C5a−C10a
–
O1−C11−C12
120.6(1)
123.9(1)
N4−C11−O1
–
Intermolecular interactions
D-H···A
D-H ( A˚ )
H···A ( A˚ )
D···A ( A˚ )
D-H···A (°)
Compound 2
C3a-H3a···N2
i
0.93
0.93
2.46
3.16
3.378(2)
4.031(2)
170.0
157.0
ii
C8-H8···π (pyraz.)
Compound 3
i
N4-H4···N2
0.89
0.93
0.93
0.93
2.12
2.43
2.52
2.74
3.005(2)
3.194(2)
3.426(2)
3.494(2)
177.0
140.0
166.0
138.0
ii
C7-H7···O1
iii
C9-H9···N5
iv
C3-H3···N3
Symmetry codes: 2 - i (1 - x, -y, -z); ii (1 + x, y, z).
Fig. 2. Crystal structure representations of (A) compound 2 and (B) compound 3.
3 - i (-x, y, ½ - z); ii (-x, 2 - y, -z); iii (x - 1, 1 - y, z - ½); iv (-x, 1 - y, -z).
The ellipsoids were drawn at the 50% probability level.
tions involving phenyl and pyrazole rings, respectively, which gen-
group which stabilizes the crystal structure of 3, acting as a hydro-
gen donor in weak NH···N hydrogen bonds involving pyrazole N2
atoms as the acceptors. These interactions generate, as was also
observed in the structure of 2, dimers. Non-conventional CH···O
and CH···N contacts involving the heteroatoms O1, N5 and N3 link
the dimers, giving rise to a more complex tri-dimensional network
(Fig. 3B).
erates a one-dimensional network along the crystallographic axis a
(
Fig. 3A).
Concerning compound 3, the torsion angles between aromatic
rings are 28.1° (phenyl-pyridyl) and 47.4° (phenyl-pyrazole). In this
case, the carbon-nitrogen distances in the CNNC group are dif-
˚
ferent [1.267(2) and 1.348(2) A], which indicates that one link-
age has double bond character while the other is a single bond.
For this reason, N4 is protonated, contrary to what is observed in
compound 2. It is precisely the occurrence of the hydrazonic N4H
The Hirshfeld surface (HS) analysis [44] of the compounds
(Fig. 4A) indicates that the most important interaction is CH···N
in 2 and NH···N in 3. Comparing the HS of both derivatives, it is
5

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Fig. 3. Intermolecular interactions in (A) compound 2, where each color represents a dimeric unit linked in a one-dimensional network along the crystallographic axis a and
B) compound 3. The intricate tridimensional network can be observed along axis b (left) and c (right).
(
possible to perceive the difference in their shapes and volumes
˚ 3
senting the NH···N hydrogen bonds, are observed in the [010] di-
rection and the weakest ones, including non-conventional hydro-
gen bonds like CH···N, are in the [100] direction (Fig. 5B). The
energy framework (Figure S13B) shows that, in the aroylhydra-
zone, both the electrostatic and the dispersive contributions are
important for crystal packing, mostly depending on the crystal-
lographic direction considered, being the electrostatic one more
relevant along [010] and the dispersion energy in the direction
[100].
(
434.94 A3 for 2 and 365.71 A for 3), as expected. The finger-
˚
print plot analysis [45] show that the distribution of intermolec-
ular interactions is also unlike in both compounds, as can be seen
in Fig. 4B. The most representative interaction in these solids are
the nondirectional H···H contacts, that represent 50.4% and 42.2%
for 2 and 3, respectively (Figure S11). In both compounds, the N···H
contacts related to CH···N and NH···N hydrogen bonds are the most
relevant interactions in the crystal structures [di + de equal to 2.3
in 2 (Figure S11A) and 2.0 in 3 (Figure S11B)], and these contacts
have similar contribution in both compounds (14.6% in 2 and 17.8%
in 3). For the aroylhydrazone 3, the observed O···H interaction cor-
responds to a contribution of 9.5% (Figure S11B).
The gas phase structures of compounds 2 and 3 were optimized
at the CAM-B3LYP/6–311+G(2d,p) level of theory and are in ex-
cellent agreement with the X-ray measurements, presenting root-
˚
mean-square deviation (RMSD) values of 0.3365 and 0.1852 A, re-
The analysis of energy frameworks [46] for these compounds
indicates that in 2 the strongest interactions are related to CH···N
spectively. Fig. 6 shows the overlapping of the optimized gas phase
structures and those obtained from the crystallographic analyses,
shown in yellow.
−1
(
−66.7 kJ mol ), the stacking arrangement along the [02–1] direc-
−1
−1
tion (−67.0 kJ mol ) and CH···π (−29.1 kJ mol ), in which the
dispersion energy contribution is more important than the elec-
trostatic energy for the total energy frameworks (Figure S12A).
On the other hand, the strongest interaction in 3 is the hydro-
gen bond NH···N (−105.0 kJ mol−1), which contributes with the
highest electrostatic energy for the total (Figure S12B). The en-
ergy frameworks representations for the total interaction energy
of dihydrazone 2 and aroylhydrazone 3 can be observed in Fig. 5.
As expected, the intermolecular interactions show anisotropic 3D
topology for both compounds. In 2, the strongest interactions,
which include CH···N and CH···π contacts, occur in the [101] di-
rection (Figure S13A) and the weakest interactions are observed
in the [010] direction. In this compound, the dispersion contribu-
tion is more relevant for the crystal packing. On the other hand,
in the crystal of compound 3, the strongest interactions, repre-
The mid-infrared (IR) spectra of the aldehyde precursor 1 (Fig-
ure S14A) and its derivatives 2 and 3 (Figure S14B and C, re-
spectively) confirm some of the structural aspects observed in
the crystal structures of the latter two compounds. The spec-
trum of 1 shows the well-known aldehyde ν(C–H) absorptions at
2880 and 2765 cm 1. These modes, however, are completely ab-
−
sent in the spectra of products 2 and 3. Likewise, the lack of
−1
the aldehyde ν(C=O) band at 1685 cm
was observed in the
spectrum of 2. However, it cannot be confirmed for aroylhydra-
zone 3, since this compound presents intense carbonyl coupled
modes [ν(C=O) + β(H−N−C)] in the region. Another important
vibrational mode observed in 3 but not in 2 is the distinguish-
ing ν(N−H) absorption at 3185 cm 1. On the other hand, cou-
pled azomethine modes can be found in the spectra of both
hydrazones, at 1619 [νas(C=N) + β(H−C=N)] for 2 and 1640
−
6

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Fig. 4. (A) Hirshfeld surfaces and (B) fingerprint plots for compounds 2 and 3.
Fig. 5. Energy framework diagrams for the total interaction energy of (A) compound 2 and (B) compound 3. All diagrams were prepared using cylinder scale of 100.
−
1
[
ν(C=N) + β(H−C=N)]/1562 [β(H−N−N) + β(H−C=N)] cm
3.2. 2-(4-metoxyphenoxy)benzaldehyde-derived compounds
(system II)
for the aroylhydrazone 3. These absorptions are in accordance
with our previously published isonicotinoyl hydrazones [12, 13,
19]. Characteristic pyrazole bands were assigned at 1394 (2)/1397
For the synthesis of the second aldehyde,
4
[2-(4-
(
3) cm 1 [ν(N−N)] and at 767 (2) / 768 (3) cm−1 [γ (H−C)]. The
−
metoxyphenoxy)benzaldehyde], a copper-free protocol based on
nucleophilic aromatic substitution on a fluoroarene was applied.
Thus, 2-fluorobenzaldehyde and 4-methoxyphenol were reacted
in presence of K2CO3 and using DMF as solvent (Scheme 2).
Compound 4 was isolated in 71% yield.
main experimental IR absorptions for hydrazones 2 and 3 are
summarized in Table 3, along with the calculated CAM-B3LYP/6–
3
11+G(2d,p) vibrations. In general, the calculated frequencies are
in accordance with the observed ones. Higher deviations (e.g., the
N−H stretching in 3) may be related to groups involved in hydro-
gen bonds in the solid-state, as shown by the X-ray structures. The
effects of intermolecular interactions are obviously neglected in gas
phase calculations.
The isonicotinoyl hydrazone derivative of aldehyde 4, com-
pound 6, was initially proposed as a tridentate ligand for the bind-
ing of physiological metal ions, which would count with the ben-
zoether oxygen, the azomethine nitrogen and the carbonyl oxy-
7

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Fig. 6. Overlap of DFT optimized and X-ray (yellow) structures for compounds (A) 2 and (B) 3.
Table 3
Assignment of the main IR absorptions of compounds 2 (scale factor: 0.985, R2 = 0.9973) and 3 (scale factor: 0.956, R = 0.9975).
2
Compound 2
Compound 3
Experimental
Calculated
Unscaled
Experimental
Calculated
Unscaled
Assignments
Scaled
IR intensity
Scaled
IR intensity
–
–
–
–
–
–
–
3185
3062
1686
1640
–
3558.6
3120.1
1798.6
1727.0
–
3402.0
2982.8
1719.5
1651.0
–
11.30
27.14
314.7
6.591
–
ν(N−H)
–
–
–
ν(C−H)azomethine
–
–
–
ν(C=O) + β(H−N−C)
ν(C=N)azomethine + β(H−C=N)azomethine
–
–
–
1
–
–
1
–
–
1
7
–
619
394
1736.5
1710.5
199.6
ν
as(C=N)azomethine + β(H−C=N)azomethine
–
–
–
1562
1556
1397
1282
1148
1055
768
1578.4
1547.0
1344.8
1263.3
1192.2
999.6
771.8
704.9
1509.0
1478.9
1285.6
1207.7
1139.7
955.6
737.8
673.9
375.6
53.77
7.217
64.64
174.1
5.026
74.39
44.17
β(H−N−N) + β(H−C=N)azomethine
ν(C=C)pyridine + β(H−C=C)pyridine
ν(N−N)pyrazole + β(H−C=C)benzene
–
–
–
1342.2
1322.1
10.89
–
–
–
ν
as(C=N)pyridine + νas(C=C)pyridine
–
–
–
ν(N−N)hydrazone
049
67
1008.1
768.7
–
993.0
757.2
–
13.13
110.0
–
γ (H−C)azomethine + γ (H−C)benzene
γ (H−C)pyrazole
682
ν
sym(C=N)pyridine
ν: stretching; β: in-plane bending; γ : out-of-plane bending; as: asymmetric; sym: symmetric.
In the solid-state, thermogravimetric decomposition of com-
pound 5 occurs in one step between around 250 to 350 °C (data
not shown), evidencing the absence of solvation molecules in the
network. Compound 6, on the other hand, has a more complex
thermal degradation. The presence of one crystallization water
molecule in the structure of 6 was proposed based on the loss of
4
% of its molecular weight between 100 and 130 °C.
Scheme 2. Synthesis of 2-(4-metoxyphenoxy)benzaldehyde 4.
Since no single crystal was obtained during the preparation
of the hydrazonic compounds belonging to system II, theoretical
gas phase calculations were performed to shed light into some
structural aspects of 5 and 6. The same approach previously used
for optimizing compounds 2 and 3 was employed, because good
agreement was found between the calculated and crystallographic
data. Based on NMR experiments performed between 25 and 65 °C
(Figure S25), no isomerization or conformational changes seem to
occur in the structures of aroylhydrazones 3 and 6, in the range of
temperatures evaluated. The only alterations observed in the spec-
tra are related to the expected displacements due to the increase
in temperature (insets in Figure S25A and B). For this reason, and
because of the similarity between systems I and II, the herein op-
timized structures were calculated for the E isomers for both dihy-
drazone 5 and aroylhydrazone 6 (Fig. 8). In the latter, an anti con-
formation was assumed as well. Selected calculated bond distances
and angles are shown in Table 4.
gen as donor atoms. As observed in the previous system, however,
reaction of 4 with isoniazid in strong acidic medium and under
reflux afforded dihydrazone 5 instead of the desired product. On
the other hand, reaction between aldehyde 4 and isoniazid in mild
conditions resulted in the isonicotinoyl hydrazone 6. For the pur-
poses of this study, compound 5 was also obtained in higher yields
by reacting 4 with hydrazine in a 2:1 stoichiometry.
The 1H NMR spectra (DMSO–d ) of the three compounds con-
6
stituting system II are displayed in Fig. 7. Inhere, as opposed to
system I, little change is observed in the aromatic hydrogen atoms
when comparing the spectra. On the other hand, the shielding ob-
served for H7 is due to the change in its chemical environment,
going from aldehyde in 4 (10.44 ppm) to azomethines in 5 and
6
(8.96 and 8.91 ppm, respectively). In the latter compound, the
two isoniazid characteristic doublets of doublets are present at
.89 and 8.06 ppm, and the distinctive hydrazonic NH resonates at
2.32 ppm. The three methoxy hydrogens H14 absorb at 3.77 ppm
8
Confirming the suitability of the computational approach cho-
sen in the absence of crystal structures, characteristic dihydra-
zone and aroylhydrazone theoretical bond distances and angles are
very similar to those obtained both experimentally (X-ray diffrac-
tion) and by DFT calculations for the related compounds 2 and
1
in 4 and at 3.75 ppm in 5 and 6. Once again, a detailed report of
all the absorptions and observed couplings is summarized in Tables
S4, S5 and S6, while 13C and bidimensional spectra are supplied as
Figures S15–24.
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Fig. 7. 1H NMR spectra (400 MHz) of compounds 4 (green), 5 (red) and 6 (blue) in DMSO–d6 at room temperature.
Fig. 8. Gas phase optimized structures of (A) compound 5 and (B) compound 6.
3
, with the exception of some NH-related parameters between
Table 4
Selected calculated HF-3c bond distances and angles for compounds 5 and 6.
the N-acylhydrazones 3 and 6. Besides this, other observed differ-
ences are obviously associated with the aldehyde-derived moieties,
which are not the same in systems I and II.
Compound 5
Compound 6
Bond distances ( A˚ )
Concerning the differences between 5 and 6, hydrazonic N–N
bond is shorter in the aroylhydrazone, which was also noticed in
the data concerning compounds 2 and 3. Related C=N–N angles,
on the other hand, are larger for 3 and 6, when comparing them
to 2 and 5, respectively. The effects are probably due to the par-
ticularities of electron delocalization in aroylhydrazones, in which
imine and amide groups are conjugated.
C2−O1 (C2a−O1a)
O1−C8 (O1a−C8a)
C7−N1 (C7a−N1a)
N1−N1a / N1−N2
N2−C15
1.368
1.375
1.273
1.416
–
1.370
1.375
1.269
1.364
1.404
1.206
C15−O3
–
Bond angles (°)
The IR spectra of the compounds belonging to system II can
be seen in Figure S26A-C. While the characteristic aldehyde ν(C–
C8−O1−C2 (C8a−O1a−C2a)
O1−C2−C1 (O1a−C2a−C1a)
C2−C1−C7 (C2a−C1a−C7a)
C1−C7−N1 (C1a−C7a−N1a)
C7−N1−N1a (C7a−N1a−N1) / C7−N1−N2
N1−N2−C15
121.0
117.2
119.8
120.0
113.1
–
120.7
117.2
119.6
119.7
118.7
118.9
114.5
123.4
122.0
H) absorptions occur at 2890 and 2775 cm 1 in compound 4, its
−
−1
ν(C=O) mode can be observed as a band centered at 1685 cm
.
No major inductive effects derived from the substitution of the
ortho-N-pyrazolyl by the ortho-4-methoxyphenoxyl group in the
benzaldehyde ring were noticed when comparing the –CHO vi-
brational frequencies of 4 with those of 1. Such absorptions are
N2−C15−C16
–
N2−C15−O3
–
O3−C15−C16
–
9

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Fig. 9. (A) 1H NMR spectra (400 MHz) of aliquots from different steps in the synthesis of compounds 5: step 1 (immediately after the dropwise addition of isoniazid to
compound 4 – green), step 2 (after the addition of acid catalyst – red) and step 3 (after 3 h of reflux – blue) in DMSO–d6 at room temperature. (B) High-field region of the
1H NMR spectrum from step 3, highlighting in yellow the signals associated with the formation of ethyl isonicotinate as a by-product.
not present in the spectrum of dihydrazone 5, indicating that
the isolated product is aldehyde-free. The azomethine νas(C=N)
mode is, as previously observed for dihydrazone 2, coupled with
in-plane H−C=N bending movements, occurring at 1620 cm 1.
Aroylhydrazone 6, on the other hand, although also aldehyde-
free [as confirmed by the absence of ν(C–H) bands at 2890 and
for hydrazones 5 and 6 are summarized in Table 5, along with the
calculated CAM-B3LYP/6–311+G(2d,p) vibrations. Once again, the-
oretical frequencies are, as a whole, in good agreement with the
observed ones.
−
Additional 1H NMR experiments were performed in system II to
support the mechanistic proposition for the observed unexpected
formation of the dihydrazones. The dropwise addition of isoniazid
to 4, both in ethanol, produces neither change of color nor precip-
itation. On the other hand, addition of 3 drops of conc. hydrochlo-
ric acid as catalyst immediately turns the solution bright yellow
and some precipitation can be noticed. When reflux begins, the
precipitate is redissolved. Aliquots from the reaction mixture were
taken at three different moments, namely: step 1 (immediately af-
ter the mixture of both reagents), step 2 (after the addition of cata-
lyst) and step 3 (after a 3-hour reflux). The corresponding 1H NMR
spectra, in the 13.0–6.0 ppm range, are displayed in Fig. 9A.
Interestingly, the spectrum related to step 1 (green) shows that
no reaction occurs at room temperature upon mixture of 4 and
775 cm 1], possesses a series of complex, overlapped modes in
−
2
the ν(C=O) region. The main bands associated with this spec-
−
1
tral interval are present at 1682 cm
[ν(C=O) + β(H−N−C)]
and 1637 cm 1 [ν(C=N)
−
+ β(H−C=N)
]. The char-
azomethine
azomethine
acteristic N-acylhydrazone ν(N–H) absorption can be found at
3
175 cm 1. In both compounds, typical methoxyl bands were ob-
−
−1
served at 2832 (5) / 2833 (6) cm [ν(C−H)] and at 1038 (5) / 1034
(
6) cm 1 [νas(C−O−C)]. Additionally, asymmetric aromatic C−O−C
−
ether stretching modes, coupled with in-plane bending H−C=C vi-
brations of the ortho-4-methoxyphenoxyl moiety, were assigned at
230 and 1234 cm 1 for compounds 5 and 6, respectively, with the
−
1
aid of computational tools. The main experimental IR absorptions
10

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Table 5
Assignment of the main IR absorptions of compounds 5 (scale factor: 0.951, R2 = 0.9994) and 6 (scale factor: 0.951, R = 0.9981).
2
Compound 5
Compound 6
Experimental
Calculated
Unscaled
Experimental
Calculated
Unscaled
Assignments
Scaled
IR intensity
Scaled
IR intensity
–
–
–
–
–
3175
3056
2833
1682
1637
–
3543.9
3017.8
3031.0
1798.3
1727.1
–
3370.2
2869.9
2882.5
1710.2
1642.5
10.5
60.6
47.9
322.6
23.3
–
ν(N–H)
–
–
–
ν(C−H)azomethine
2
–
–
1
–
1
1
–
1
–
1
9
7
–
832
620
3026.0
–
2877.7
–
53.08
–
ν(C−H)MeO
ν(C=O) + β(H−N−C)
ν(C=N)azomethine + β(H−C=N)azomethine
–
–
–
1737.2
–
1652.1
–
298.6
–
ν
as(C=N)azomethine + β(H−C=N)azomethine
1555
1508
1504
1371
1234
1148
1034
964
1576.6
1567.0
1546.4
1422.2
1284.2
1183.6
1092.8
918.3
1499.3
1490.2
1470.6
1352.5
1221.3
1125.6
1039.3
873.3
403.5
274.6
227.6
55.1
471.1
135.3
56.4
22.2
48.0
62.3
β(H−N−N) + β(H−C=N)azomethine
506
502
1565.4
1545.3
–
1488.7
1469.6
–
375.5
536.2
–
β(H−C=C)benzene−MeO + β(H−C=C)benzene
β(H−C−C)azomethine + β(H−N−N)
230
1278.3
–
1215.7
–
995.2
–
ν
as(C−O−C)aromatic ether +β(H−C=C)benzene−MeO
ν(N−N)hydrazone
038
61
1072.0
931.8
777.5
–
1019.5
886.1
739.4
–
126.2
29.1
109.4
–
ν
ν
as(C−O−C)MeO
sym(C=C)benzene−MeO + β(C=C−C)benzene
62
765
775.1
737.1
γ (H−C)benzene
686
704.7
670.2
ν
sym(C=N)pyridine
ν: stretching; β: in-plane bending; γ : out-of-plane bending; as: asymmetric; sym: symmetric.
Fig. 10. Calculated HF-3c energetic profile (ꢀG + electronic energy) for the conversion of 6 into 5. The most important steps are labelled in blue. Reaction intermediates
+
+
+
I1 , I2 , I3 and I4 are highlighted in red color and their gas phase optimized structures are also shown.
11

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isoniazid in the absence of catalyst, and the signals referring to
the unmodified aldehyde and isonicotinic acid hydrazide are ob-
served. The absence of any azomethine-associated resonance, to-
gether with the presence of the characteristic aldehyde H7 (dou-
4. Conclusions
Aroylhydrazones are versatile compounds with a series of ap-
plications, from biological to technological spheres. The simplicity
of their preparation allows for a great chemical variability and syn-
thetic manageability. However, the process can be not as straight-
forward as one would imagine. Some parameters such as specific
reactants, the amount of acid employed as catalyst and reaction
temperature can have a direct impact on the obtained product. In
the present work, the solvolysis of the initially formed isonicoti-
noyl hydrazones, leading to two novel dihydrazones, is described.
The novel approach proposed for the synthesis of precursor 2-
(1H-pyrazol-1-yl)benzaldehyde (1), involving the Ullmann reaction,
provided higher yields than the most common aromatic nucle-
ophilic substitution. Reactivity of this aldehyde towards isoniazid
afforded different main products, mostly depending on the syn-
thetic conditions employed: a Schiff-base condensation under mild
pH and temperature results in 2-(1H-pyrazol-1-yl)benzaldehyde
isonicotinoyl hydrazone (3), while the use of a strong acidic
medium and reflux promote hydrazone solvolysis to yield the dihy-
drazone 1,2-bis[2-(1H-pyrazol-1-yl)benzylidene]hydrazone (2). This
compound was also synthesized through the direct reaction be-
tween 1 and hydrazine dihydrochloride. Compounds 2 and 3 were
obtained in the form of single crystals and characterized by XRD.
In the solid-state, the molecules of both products associate to form
dimers, which in turn interact through non-conventional hydrogen
bonds to generate 1D and 3D networks, respectively.
blet at 10.44 ppm) and isoniazid’s −NH− and −NH [10.09 and
2
4
.61 ppm (not shown), respectively] signals are proof of that. Af-
ter the addition of catalytic amounts of conc. hydrochloric acid
red spectrum), however, a typical imine −HC=N− hydrogen at
.89 ppm and the distinctive hydrazonic −NH− absorption at
2.23 ppm arise, although some aldehyde is still present in so-
(
8
1
lution. There is great similarity between this spectrum and that
of compound 6, indicating that the hydrazone is formed immedi-
ately upon addition of the catalyst. When heating is kept for 3 h,
a new equilibrium can be observed (blue spectrum). While some
of the isonicotinoyl hydrazone is still present in solution, a new
azomethine signal is observed at 8.97 ppm, in accordance with
the chemical shift observed for compound 5-related −HC=N− hy-
drogen. Moreover, all the resonances of this dihydrazone can be
identified in the spectrum as well, indicating the presence of a
mixture of derivatives 5 and 6. The simultaneous occurrence of a
quartet at 4.37 ppm and a triplet at 1.34 ppm (Fig. 9B) indicates
the formation of ethyl isonicotinate, which is consistent with the
expected by-product resulting from the attack on the aroylhydra-
zone’s carbonyl by ethanol, depicting thus a classic solvolysis reac-
tion, followed by condensation of the ensuing hydrazone to a sec-
ond free aldehyde molecule. Scheme S1 summarizes this proposed
reaction pathway for dihydrazone formation, which can probably
be also extended to system I since this process does not seem to be
aldehyde-dependent and might be related to the presence of iso-
niazid as a reactant. Nevertheless, a dihydrazone derived from 2-
pyridinecarboxaldehyde was isolated in our laboratory as an unex-
pected solvolysis product from the corresponding p-toluenesulfonyl
hydrazone (personal communication), which indicates a perhaps
broader scope for this pathway.
A very similar outcome was observed for the related sys-
tem composed by 2-(4-metoxyphenoxy)benzaldehyde (4) and
its aroylhydrazone and dihydrazone derivatives. Aldehyde
was prepared with copper-free protocol based on nucle-
ophilic aromatic substitution on fluoroarene. 1,2-bis[2-(4-
methoxyphenoxy)benzylidene]hydrazone (5) was obtained either
from the reaction between and isoniazid at very low pH
and high temperature, or directly by reacting and hy-
4
a
a
4
Insights into the thermodynamics of the conversion 6 → 5 were
obtained through a computational approach. Structures of all the
reactants and reaction intermediates were optimized in the gas
phase and had their individual thermodynamic parameters calcu-
lated. Fig. 10 displays the energetic profile (ꢀG + electronic en-
ergy) for each of the main steps that comprise the process, along
with the structures of key-intermediates.
4
drazine dihydrochloride. On the other hand, the desired 2-(4-
metoxyphenoxy)benzaldehyde isonicotinoyl hydrazone (6) was iso-
lated only when mild synthetic conditions were applied. Theo-
retical calculations suggest that the structures of these products
are similar to the corresponding hydrazonic derivatives 2 and 3.
System II was employed to support the mechanistic proposition
for the unexpected formation of the dihydrazones in the reaction
medium: the solvolysis of the isonicotinoyl hydrazone followed by
Of the main steps involved in the proposed pathway, only
+
+
the lysis of intermediate I3 into I4 and the by-product HEtIso
(
protonated ethyl isonicotinate, whose signals were unequivocally
attack to a second aldehyde molecule, as was suggested from 1
H
identified in the 1H NMR spectrum shown in Fig. 9B) is, as ex-
pected, endergonic (+20.19 kcal mol−1). Amongst the exergonic
steps, the most favorable are the attack by ethanol on the pro-
NMR experiments and supported by thermodynamic calculations.
From a theoretical point of view, the process is spontaneous as
−1
a whole, with a calculated ꢀG of −1.686 kcal mol
(−7.054 kJ
+
−1
−1
tonated carbonyl of intermediate I1 (–99.43 kcal mol ) and
the condensation between I4 and a second unit of aldehyde 4
mol ) at 298 K.
Finally, we believe that the synthetic cases discussed herein,
supported by strong structural and spectroscopic evidences, as well
as by thermodynamic parameters calculated for system II, can be
–29.87 kcal mol 1), to afford the final product 5 and a water
molecule.
The global reaction can be represented by the equation:
−
(
6
(aroylhydrazone) + EtOH + 4(free aldehyde) → 5(dihydrazone) + EtIso + H2O
useful to other chemists dealing with the apparently obvious art of
preparing aroylhydrazones for a multitude of applications.
The process is slightly exergonic as
a whole and, there-
−
1
fore, spontaneous, with a calculated ꢀG of −1.686 kcal mol
−1
(
−7.054 kJ mol ) at 298 K. Although this value is rather small,
Author contribution
it is important to consider that calculations were performed in the
gas phase. Hence, from a strictly thermodynamic point of view, the
proposed reaction pathway is quite reasonable. A complete theo-
retical study employing higher levels of calculation and including
solvent effects is underway and will be the subject of a specific
future report.
JL planned the syntheses of aldehydes 1 and 4; CCN synthesized
and characterized the precursor aldehydes mentioned before; DSC
and BNE synthesized and characterized the hydrazonic compounds
2, 3, 5 and 6 as well as obtained single crystals for 2 and 3; CHFJ
selected the appropriate crystals and collected the X-ray diffrac-
12

ARTICLE IN PRESS
JID: MOLSTR
[m5G;October 15, 2020;22:11]
D.S. Cukierman, B.N. Evangelista, C.C. Neto et al.
Journal of Molecular Structure xxx (xxxx) xxx
tion data; RD solved the structures of compounds 2 and 3 and
performed their Hirshfeld and energy frameworks analyses; LASC
carried out the computational analyses; DSC, LASC, RD, JL and NAR
interpreted the results obtained; DSC and NAR wrote the paper;
DSC, LASC, RD, JL and NAR revised and corrected the manuscript.
[14] R.A. Hauser-Davis, L.V. de Freitas, D.S. Cukierman, W.S. Cruz, M.C. Miotto,
J. Landeira-Fernandez, A.A. Valiente-Gabioud, C.O. Fernández, N.A. Rey, Dis-
ruption of zinc and copper interactions with Aβ(1-40) by a non-toxic, iso-
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N.A. Rey, Structural and spectroscopic investigation on a new potentially bioac-
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Declaration of Competing Interest
[
16] R.S. Moraes, R.E. Aderne, M. Cremona, N.A. Rey, Luminescent properties of a
di-hydrazone derived from the antituberculosis agent isoniazid: potentiality
as an emitting layer constituent for OLED fabrication, Opt. Mater. 52 (2016)
186–191.
The authors declare no conflict of interest relevant in the con-
text of the present work.
[
17] A.C. González-Baró, V. Ferraresi-Curotto, R. Pis-Diez, B.S. Parajón Costa,
J.A.L.C. Resende, F.C.S. de Paula, E.C. Pereira-Maia, N.A. Rey, A novel oxidovana-
dium(V) compound with an isonicotinohydrazide ligand. A combined experi-
mental and theoretical study and cytotoxity against K562 cells, Polyhedron-
Polyhedron 135 (2017) 303–310.
Acknowledgements
This study was financed in part by the Coordenação de Aper-
feiçoamento de Pessoal de Nível Superior - Brazil (CAPES) – Fi-
nance Code 001. DSC is thankful to CAPES, as well as to Conselho
Nacional de Desenvolvimento Científico e Tecnológico – Brazil
[
[
18] D.S. Cukierman, A.B. Pinheiro, S.L. Castiñeiras-Filho, A.S. da Silva, M.C. Miotto,
A. De Falco, T. de, P. Ribeiro, S. Maisonette, A.L. da Cunha, R.A. Hauser-Davis,
J. Landeira-Fernandez, R.Q. Aucélio, T.F. Outeiro, M.D. Pereira, C.O. Fernán-
dez, N.A. Rey,
A moderate metal-binding hydrazone meets the criteria for
a bioinorganic approach towards Parkinson’s disease: therapeutic potential,
blood-brain barrier crossing evaluation and preliminary toxicological studies,
J. Inorg. Biochem. 170 (2017) 160–168.
(CNPq) for the doctorate fellowships granted. NAR wishes to thank
Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do
Rio de Janeiro – Brazil (FAPERJ) and CNPq for the research fel-
lowships awarded. NAR and JL also thank FAPERJ (grant number
E-26/010.001838/2015) for specifically supporting this project. The
authors are grateful for the Laboratório Multiusuário de Difração
de Raios-X (Universidade Federal de Juiz de Fora – Brazil) for the
XRD facilities. RD is thankful to Fundação de Amparo à Pesquisa do
Estado de Minas Gerais – Brazil (FAPEMIG), CNPq and Financiadora
de Estudos e Projetos (Finep). LASC thanks CNPq for the research
grant 307887/2018–9.
19] D.S. Cukierman, E. Accardo, R.G. Gomes, A. De Falco, M.C. Miotto, M.C.R. Fre-
itas, M. Lanznaster, C.O. Fernández, N.A. Rey, Aroylhydrazones constitute a
promising class of ’metal-protein attenuating compounds’ for the treatment
of Alzheimer’s disease: a proof-of-concept based on the study of the interac-
tions between zinc(II) and pyridine-2-carboxaldehyde isonicotinoyl hydrazone,
J. Biol. Inorg. Chem. 23 (8) (2018) 1227–1241.
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Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2020.129437.
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