Multiple dynamics of hydrazone based compounds

Article abstract of DOI:10.5935/0103-5053.20150092

Hydrazone derivatives of 2-quinolinecarboxaldehyde and 6-bromo-2-pyridinecarboxaldehyde were synthesized by sequence reactions with hydrazine derivatives. These compounds exhibited E/Z isomerization upon irradiation using a mercury lamp (250 W). The configurational changes were monitored by 1H nuclear magnetic resonance (NMR), UV-Vis and fluorescence spectroscopy. Data of concentration of the E/Z isomers versus time showed first order kinetics with constants ranging from 0.024 to 0.0799 min-1. The Z isomers were isolated by chromatographic methods and characterized by 1H NMR, UV-Vis and fluorescence spectroscopy and X-ray diffraction. The Z compounds are stable even in solution for several months. Such stability is due to a thermodynamic stabilization by the formation of an intramolecular hydrogen bond in the Z structure, which is not seen in the E configuration. Furthermore, some of the compounds were used as ligands for various metal centers (Zn²⁺, Co²⁺ and Hg²⁺) and their electronic properties were studied including measurements of cyclic voltammetry. The compounds studied herein allow their use as dynamic systems in dynamic combinatorial chemistry as their properties can be modulated by light, heat and the presence of metal centers. Besides, obtaining a metastable state (Z-isomer) allows the use of these compounds as photo-brakes, and therefore they can be implemented as molecular machines.

Full text of DOI:10.5935/0103-5053.20150092

http://dx.doi.org/10.5935/0103-5053.20150092
J. Braz. Chem. Soc., Vol. 26, No. 6, 1265-1273, 2015.
Printed in Brazil - ©2015 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
A
Multiple Dynamics of Hydrazone Based Compounds
Elkin L. Romero,a Richard F. D’Vries,b Fabio Zuluagaa and Manuel N. Chaur*,a
aDepartamento de Química, Universidad del Valle, A.A., 25360 Cali, Colombia
bInstituto de Física de São Carlos, 13566-590 São Carlos-SP, Brazil
Hydrazone derivatives of 2-quinolinecarboxaldehyde and 6-bromo-2-pyridinecarboxaldehyde
were synthesized by sequence reactions with hydrazine derivatives. These compounds exhibited
E/Z isomerization upon irradiation using a mercury lamp (250 W). The configurational changes
were monitored by 1H nuclear magnetic resonance (NMR), UV-Vis and fluorescence spectroscopy.
Data of concentration of the E/Z isomers versus time showed first order kinetics with constants
ranging from 0.024 to 0.0799 min−1. The Z isomers were isolated by chromatographic methods
and characterized by 1H NMR, UV-Vis and fluorescence spectroscopy and X-ray diffraction. The Z
compounds are stable even in solution for several months. Such stability is due to a thermodynamic
stabilization by the formation of an intramolecular hydrogen bond in the Z structure, which is
not seen in the E configuration. Furthermore, some of the compounds were used as ligands for
various metal centers (Zn2+, Co2+ and Hg2+) and their electronic properties were studied including
measurements of cyclic voltammetry. The compounds studied herein allow their use as dynamic
systems in dynamic combinatorial chemistry as their properties can be modulated by light, heat and
the presence of metal centers. Besides, obtaining a metastable state (Z-isomer) allows the use of
these compounds as photo-brakes, and therefore they can be implemented as molecular machines.
Keywords: hydrazone compounds, configurational dynamics, photo-brakes, electrochemistry
Introduction
facilitates isomerization;22 E/Z isomerization could occur by
either of two different mechanisms: rotation and inversion.
The former occurs through a polar transition state and
results in the rotation of the substituents around the C=N
double bond. The second mechanism occurs through
inversion around the imine nitrogen and goes through a non-
polar transition state.12,16 However, more recent studies have
demonstrated that azo-hydrazone tautomerism and rotation
of the C−N bond is the most likely isomerization mechanism
for systems involving an intramolecular hydrogen bond.12
Moreover, the reversibility of the C=N bond, hydrazones can
undergo constitutional changes leading to the formation of
another useful compound.4,16,17 These compounds may exhibit
coordination dynamics by the presence of coordination sites
in the chemical structure, this feature allows the locking
and unlocking, controlled by metal ions, and therefore the
reversible transformation of different states.4,23,24
Molecular machines, systems designed to perform
delicate and precise tasks, are remote from the original
concepts of the dynamic combinatorial chemistry (DCC),
but these systems share a characteristic from the dynamic
combinatorial libraries (DCL) exploiting dynamic covalent
bonds to steer the composition of systems by external
physical/chemical stimuli and using these changes to extract
information to perform work.1 Currently, there has been
a production of molecular systems whose physical and
chemical properties can be reversibly modulated in response
to an external stimulus, either light,2–7 heat,4,8 addition of
metal ions,9,10 pH changes11-13 or electrochemical potentials.14
These systems, also called “molecular switches”15 are of
great interest in the field of nanotechnology.5
The hydrazones have caught the attention for their use
as molecular systems,16-18 capable of undergoing reversible
changes of configuration, E/Z isomerization,4,16-21 which are
favored by the presence of an additional nitrogen atom that
diminishes the double-bond character of the π system and
The aforementioned properties of hydrazones have
enabled them to be used in supramolecular chemistry17 as
molecular switches,2,3 metallo-assemblies19-21 and sensors17,18
of both cations and anions. Likewise other systems including
light driven molecular motors have been developed2 and in
that sense many of these systems can be implemented as
*e-mail: manuel.chaur@correounivalle.edu.co

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Multiple Dynamics of Hydrazone Based Compounds
J. Braz. Chem. Soc.
digital molecular information storage systems due to the
multiple dynamics that they exhibit in a reversible fashion.
Herein we studied the configurational dynamics
of different hydrazone derivatives of 6-bromo-2-
pyridinecarboxaldehyde and 2-quinolinecarboxaldehyde,
which have been designed to show in the Z state, an
intramolecular hydrogen bond formed between the
amine hydrogen and the pyridine or quinoline ring
nitrogen (Scheme 1) in order to stabilize the otherwise
thermodynamically unstable Z isomer. Kinetics of the
spectrometer. UV-Vis and fluorescence spectra were taken in
PharmaSpec Shimadzu UV-Vis UV-1700 spectrophotometer
and Jasco FP-8500 spectrofluorimeter, respectively. Cyclic
voltammograms were recorded in a bipotentiostat model
700 B series electrochemical Analyzer/Workstation from
CHI Instruments coupled to a computer.
General procedure for oxidation lateral chains
A solution of the quinaldine (1.0 equiv.) and selenium
dioxide (1.7 equiv.) in dioxane/water was heated under
reflux monitored by thin-layer chromatography (TLC). The
solid formed was filtered and washed with dichloromethane
(2 × 2 mL). The filtrates were combined and the solvent
removed under reduced pressure and the resulting oily
residue was purified by column chromatography in
dichloromethane.
1
photoisomerization process was followed by H nuclear
magnetic resonance (NMR), UV-Vis and fluorescence
spectroscopy. Additionally, these compounds were used
as ligands to various metal centers (Zn2+, Co2+ and Hg2+)
and their electronic properties were studied including
measurements of cyclic voltammetry.
H
N
R
N
X
N
N
N
h
ν
N
X
General procedure for hydrazone synthesis
R1
R2
R4
R1
H
R3
R
R2
R4
The hydrazine derivative (1.0 equiv.) was added to an
ethanolic solution of the aldehyde (1.0 equiv.), subjected
to reflux during 3 h. The precipitate was collected by
vacuum filtration and washed with cold ethanol to obtain
the pure product.
R3
E isomer
Z isomer
R2
H
N
R3
R4
N
N
X
R1
General procedure for hydrazone isomerization
R
Scheme 1. Dynamic characteristics shown by the compounds studied:
configuration change, E/Z isomerization and coordination with a metallic
center.
Irradiations were carried out with a mercury lamp and
samples were irradiated in NMR tubes; NMR, UV-Vis and
fluorescence spectra were immediately recorded.
Experimental
Results and Discussion
All starting reagents were acquired from Sigma-Aldrich
and were used without additional purification. Infrared
(IR) spectra were recorded on a Shimadzu FTIR 8400
spectrophotometer in KBr disks and films. NMR spectra (1D
and 2D) were recorded on a 400 MHz Bruker Ultrashield
Synthesis of hydrazone precursors (11a-b)
The carbonyl compounds were obtained by oxidation
of the corresponding 2-methylquinoline 10a-b under reflux
Scheme 2. Synthesis of hydrazonic derivatives 1-8. (a) SeO2/dioxane, reflux;25 (b) ethanol/reflux.

Vol. 26, No. 6, 2015
Romero et al.
1267
using selenium dioxide in 1,4-dioxane and few drops of
water (Scheme 2).25 The reaction was monitored byTLC and
purified by column chromatography using dichloromethane
as eluent; the IR spectrum of compound 11a taken in
KBr pellet showed an absorption band at 1712 cm−1
corresponding to the stretching vibration of the carbonyl
group. In the 1H NMR spectrum of compound 11a a singlet
at 10.22 ppm corresponding to the aldehyde proton was
observed. All types of carbon atoms were completely
assigned 2D NMR techniques.
red crystals, with monoclinic space group C2/c. In the
asymmetric unit, one protonated hydrazone (2-E) molecule
and one crystallographically independent chloride
anion (Figure 1b) were observed. Each ionic pair forms
dimeric subunits in which two chloride anions are shared
between two organic molecules (Figure 2a) with distances
N3−H3A···Cl1 = 3.022(2)Å and N1−H50···Cl1=3.180(2)Å.
The dimeric subunits also forms pillars (supramolecular
chains) by π-π stacking interactions with distances between
centroids of 3.845(1) Å (Figure 2b) along the [1,1,0] and
[−1,1,0] directions. The crystalline packing (Figure 2) is
completed by weak C3-H3···π interactions with distances
of 3.965(1) Å between each pillar.
Synthesis of hydrazone compounds
The reaction of the synthesized carbonyl compounds,
11a-b and 13, with hydrazine derivatives 12a-d, under
ethanol reflux, led to the formation of the corresponding
E-hydrazone compounds 1-8 (Scheme 2) with yields
between 74 and 85%. The compounds were fully
characterized by spectroscopic techniques such as IR
and 1D and 2D NMR (see Supplementary Information
(SI) section). The structural elucidation of the 8-E26 and
2-E compounds was confirmed by single crystal X-ray
diffraction (Figure 1).
On the other hand, the reported hydrazones exhibit high
absorption coefficients in methanol, ε ca. 104 L mol−1 cm−1,
which is common for aromatic or heteroaromatic highly-
conjugated systems in the UV region;27-29 whereas, the data
of electronic band gaps around 2.52-3.02 eV (Table 1)
indicate that these compounds have highest occupied
molecular orbital-lowest unoccupied molecular orbital
(HOMO-LUMO) bands similar and the effect of the
phenyl substituent does not exert a great influence on the
relative energy of the molecular orbitals, however, it is
only an estimate, an analysis more rigorous must include
different substituents on both rings and solvent effects.30
Crystals of compound 2-E were obtained by protonation
of the hydrazonic derivative (2-E), as prismatic light
Figure 1. ORTEP drawing of (a) (E)-6-bromo-2-pyridine-carboxaldehyde-phenylhydrazone and (b) the asymmetric units of (E)-2-((2-(pyridin-2-yl)
hydrazono)methyl)quinolin-1-ium chloride. Ellipsoids are displayed at the 50% probability level.
Figure 2. Representation of the (a) N−H···Cl, (b) π···π and (c) C−H···π supramolecular interactions for the compound 2-E (inset: [100] crystalline packing).

1268
Multiple Dynamics of Hydrazone Based Compounds
J. Braz. Chem. Soc.
Table 1. Physical characteristics of the reported compounds
a
Compound
1-E
Aldehyde
11a
Hydrazine
12a
Yield / %
ε × 104 / (L mol−1 cm−1)
∆Eelec
79
85
74
80
83
81
85
81
8.59
10.35
7.89
3.02
2.81
2.83
2.52
2.89
2.93
2.97
2.90
2-E
11a
12b
3-E
11a
12c
4-E
11a
12d
19.11
15.18
8.27
5-E
11a
12e
6-E
11b
13
12a
7-E
12a
13.66
10.69
8-E
13
12b
a∆Eelec = 1240/Spectral onset (nm).
The results make them good candidates for their use in
optical applications.
benzene ring, which reduces the acidity of hydrogen (N−H).
Also, the presence of the bromide atom in the 8th position
of the quinoline ring can produce an inductive effect
decreasing the charge density of the quinoline nitrogen; as
a consequence, the stabilization of the Z state is dismissed,
which is reflected in the isomerization rate.
Response to photophysical stimulus (configurational
dynamics)
Solutions of the hydrazone derivatives 1-E-6-E in
DMSO-d6 were irradiated with a 250 W mercury lamp
causing photochemical isomerization.4,16 Configurational
Likewise, solutions of compounds 7-E and 8-E in
CD3OD were irradiated monitoring the changes by 1H NMR
spectroscopy. In this case, UV irradiation of 7-E leads to
the configurational isomerization towards 7-Z in a 78%
of conversion after a few minutes (Figure 4) thanks to the
formation of an intramolecular hydrogen bond (Figure 4b)
as it has been previously observed.4 Photoisomerization was
also conducted in DMSO-d6, but in this case, isomerization
occurs in a much lower rate, given that after 210 min of
irradiation only 36% of the 7-Z isomer was obtained, which
clearly shows that photochemical induced isomerization
depends on solvent polarity and the capacity of this for
stabilizing the photo-induced transition state.16
Meanwhile, the isomerization rate of compound 8-E
is lower (k = 0.0183 min−1) when compared to 7-E
(0.0799 min−1), however, the yields of conversion allow
the isolation of compound 8-Z by column chromatography
in 65% yield. The 1H NMR spectrum of 8-Z shows a high
field displacement of the N−H proton due to the formation
of the intramolecular hydrogen bond (Figure 3). On the
other hand, the UV-Vis spectrum shows a hypsochromic
shift (20 nm) in the main absorption peak (Figure 6); this
shift is typical of n→π* transitions and it is caused by the
formation of a intramolecular hydrogen bond, where the
pyridine nitrogen donates its pair of electrons to the N−H
proton, therefore, its n orbital energy level is decreased and
the transition energy is increased, as a result, the absorbed
photon energy is of shorter wavelength. The electronic
HOMO-LUMO calculated by using the spectral onset, also
reflects the lower bandgap of the Z state. Fluorescence is
quenched in the Z state (Figure 6) due to a decrease in the
conjugation of the molecule induced by the formation of
1
changes were monitored over time by H NMR, UV-Vis
and fluorescence spectroscopy.As observed in the 1H NMR
spectra shown in Figure 3, irradiation of the 1-E solution in
450 µL of DMSO-d6 and 50 µL of methanol during 100 min
leads to the configurational transformation in 1-Z with a
24% conversion when equilibrium is reached. This change
occurs through a first order kinetics (k = 0.0328 min−1). In
comparison, photoisomerization of the herein reported
quinoline compounds occur at a much slower rate. After
ca. 60-75 min of irradiation some signals start to unfold,
meaning a much slower isomerization rate. This behavior
may be due to the substitution of the pyridine ring by the
t = 100 min
t = 70 min
t = 50 min
t = 30 min
t = 20 min
t = 10 min
t = 0 min
8.5
8.0
7.5
7.0
Chemical shift / ppm
Figure 3. Expansion of the 1H NMR spectrum (400 MHz) of a
41.3 mmol L−1 solution of 1-E in DMSO-d6 taken at different UV
irradiation times.

Vol. 26, No. 6, 2015
Romero et al.
1269
H
N N
Br
N
N
hv
N
N
H
N
Br
Z
E
78%
t = 16 min
t = 13 min
53%
t = 10 min
42%
38%
24%
t = 7 min
t = 5 min
t = 2 min
t = 1 min
t = 0 min
8.0
7.5
7.0
Chem ical shift / ppm
Figure 4. Expansion of the 1H NMR spectrum (400 MHz) of a 48.3 mmol L−1 solution of 1-E in DMSO-d6 taken at different UV irradiation times.
3 y 5
Hm
Hm
4
3
Ci
N-H
4
5
N
N
Hp
N
H
o
p
Br
m
Hi
Ho
Hm
N-H
3
5
4
Ci
H
N N
Br
N
Hp
5
3
o
p
4
m
13.0
12.5
12.0
11.5
11.0
10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
Chemical shift / ppm
Figure 5. Expansion of the 1H NMR spectrum (400 MHz) of 8-E and 8-Z in DMSO-d6.

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Multiple Dynamics of Hydrazone Based Compounds
J. Braz. Chem. Soc.
Figure 6. UV-Vis (left) and fluorescence (right) spectra of 8-E and 8-Z. Excitation wavelength at 362 nm.
the hydrogen bond since the electron pair of the nitrogen of
the pyridine ring is no longer part (at least in some extend)
of the resonance at expense of the intramolecular hydrogen
bond, which diminishes the conjugation in the framework
and therefore decreases the wavelength absorption.31
of the metal complex proceeds, the N−H proton becomes
unprotected displacing the signals by an inductive effect.
Likewise, IR spectra show the characteristic bands for
each hydrazone complex.
Synthesis of hydrazone complexes
7-E
Complexes were obtained through the reaction of
hydrazone compounds 7-E and 8-E with transition metal
salts in +2 oxidation state (M2+) in suitable solvents
producing the precipitation of these compounds with
yields between 59 and 82% (Table 2). These compounds
were purified by washing with appropriated solvents
and then they were characterized by FTIR and NMR
spectroscopy and elemental analysis. When analyzing
the proton NMR spectra of the metal complexes of
compound 7-E, different displacements in some signals
of the compound can be observed; the most marked is
that for the N−H proton, which increases its displacement
depending on the nature of the metal ion. As the formation
Zn[(7-E)]Cl
Hg[(7-E)]Cl
18
16
14
12
10
8
6
Chemical shift / ppm
Figure 7. Expansion of the 1H NMR spectrum (400 MHz) of hydrazone
7-E and hydrazonic complexes M (7-E) in DMSO-d6.
Table 2. Characteristics of the hydrazone complexes
H
N
N
H
N
Br
N
X
N
N
X
Br
a
Compound
Hydrazone
7-E
Metallic salt
ZnCl2
Yield / %
ε × 104 / (L mol−1 cm−1)
∆Eelec
2.77
2.80
2.74
2.59
2.67
2.63
9a
9b
9c
9d
9e
9f
69
82
71
59
67
76
6.87
6.36
9.50
6.29
5.95
7.96
7-E
CoCl2
7-E
HgCl2
8-E
ZnCl2
8-E
CoCl2
8-E
HgCl2
a∆Eelec= 1240/Spectral onset (nm).

Vol. 26, No. 6, 2015
Romero et al.
1271
UV-Vis spectra show a decrease in the maximum
absorption band, which is accompanied by the appearance
of new bands in the visible region corresponding to the
absorption by metal ions, moreover, the spectral onsets
(Table 2) reduce as the ionic radius increases, showing
that the charge transfer interaction takes place within the
molecule.32 The emission spectrum shows, as expected,
a drastic quenching of fluorescence by the addition of
metal center, which promotes cross between systems, and
diminishes the probability of fluorescence in as much as
the coordination is carried out entirely by the unshared
electrons of nitrogen, decreasing the contribution of these
to the resonance of the system.
the cobalt complex 9b, a displacement of the absorption
bands is observed as well as an increase in the absorptivity
coefficient and an increased fluorescence up to pH 8, due
to a stronger interaction between the metal center and the
deprotonated hydrogen of the amine (Figure 9).
Electrochemistry of the hydrazone complexes
In general, the metal complexes exhibit two irreversible
oxidation and reductions steps, which depend on the nature
of the metal, for example, the zinc(II) complex is oxidized
more easily than its counterparts (see Table 3) due to its
lower electronegativity, which facilitates the loss of an
electron in the oxidative process.
Response to pH changes of hydrazone complexes
Conclusions
The behavior of the hydrazone complexes was
determined at different pH values and it was monitored
through UV-Vis and fluorescence spectroscopies. For
The study of the chemical displacement of the hydrogen
imine signal in NMR of the hydrazone as a function of
Figure 8. UV-Vis spectra (left) and fluorescence spectra (right) of hydrazone 7-E and hydrazone complexes M (7-E) in MeOH.
Figure 9. UV-Vis spectra (left) and fluorescence (right) of the [CoII(1-E)] complex at different pH in MeOH.

1272
Multiple Dynamics of Hydrazone Based Compounds
J. Braz. Chem. Soc.
Table 3. Oxidation / reduction potential [in V vs. (Fc+/Fc)] of the hydrazone complexes obtained in THF + 0.1 mol L−1 (n-Bu)4NPF6
a
Compound
Zn(7-E)]Cl2
Co(7-E)]Cl2
Hg(7-E)]Cl2
Eox2
0.90
0.97
1.00
Eox1
0.59
0.73
0.64
Ered1
−
Ered2
−
∆Egap
−
∆Eelec
2.77
2.80
2.74
Xb
1.65
1.88
1.90
−1.04
−0.96
−1.97
−
1.74
1.60
a∆Eelec= 1240/SpectralOnset (nm); bPauling electronegativity.33
the irradiation time reflects the obtention of a metastable
system, which is thermodynamically stabilized due
to the formation of an intramolecular hydrogen bond
restricting the system’s movement around the C−C(imine)
and N−C(hydrazine) bonds, thus, functioning as a molecular
motor in the E form and as a brake in the Z configuration.
Likewise, movement of this bond is blocked by adding
metal centers. The compounds prepared and studied herein
corroborate the inherent dynamics of certain hydrazones
toward physical and chemical stimulus and the results
inspire the implementation of these systems in the design
of molecular motors, photo-switches and cation sensors.
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Supplementary Information
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Supplementary data (full experimental details, 1H and
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at http://jbcs.sbq.org.br as PDF file.
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Acknowledgements
The authors greatly thank the Vicerrectoría de
Investigaciones of the Universidad del Valle, the Banco de
la República, COLCIENCIAS and the Center of Excellence
for Novel Materials (CENM) for the economic support to
conduct this research. R. D. acknowledges a CAPES/PNPD
scholarship from the Brazilian Ministry of Education and
Crystallography group from IFSC-USP.
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Published online: April 17, 2015
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