X-ray crystallographic, spectroscopic and electrochemical properties of a bi-stable di-2-thienyl ketone 2,4-dinitrophenyl hydrazone (dtkdnph)

Article abstract of DOI:10.1016/j.molstruc.2018.07.008

Di-2-thienyl ketone 2,4-dinitrophenyl hydrazone (dtkdnph), obtained from the acid-catalyzed condensation of di-2-thienyl ketone (dtk) with 2,4-dinitrophenylhydrazine, exhibits rich phyisco-chemical properties. X-ray structural analysis on a single crystal

Full text of DOI:10.1016/j.molstruc.2018.07.008

Accepted Manuscript
X-ray crystallographic, spectroscopic and electrochemical properties of a bi-stable
di-2-thienyl ketone 2,4-dinitrophenyl hydrazone (dtkdnph)
Mohammed Bakir
PII:
S0022-2860(18)30827-5
10.1016/j.molstruc.2018.07.008
MOLSTR 25413
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 2 February 2018
Revised Date: 1 July 2018
Accepted Date: 2 July 2018
Please cite this article as: M. Bakir, X-ray crystallographic, spectroscopic and electrochemical properties
of a bi-stable di-2-thienyl ketone 2,4-dinitrophenyl hydrazone (dtkdnph), Journal of Molecular Structure
(2018), doi: 10.1016/j.molstruc.2018.07.008.
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X-ray crystallographic, spectroscopic and electrochemical properties of a bi-stable
di-2-thienyl ketone 2,4-dinitrophenyl hydrazone (dtkdnph)
Mohammed Bakir, Department of Chemistry, The University of the West Indies – Mona
Campus, Kingston 7, JAMAICA, W.I. Fax: 1 + 876-977-1835, e-mail address:
mohammed.bakir@uwimona.edu.jm

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Abstract
Di-2-thienyl ketone 2,4-dinitrophenyl hydrazone (dtkdnph), obtained from the acid-
catalyzed condensation of di-2-thienyl ketone (dtk) with 2,4-dinitrophenylhydrazine,
exhibits rich phyisco-chemical properties. X-ray structural analysis on a single crystal of
dtkdnph grown from a dmso solution confirmed its identity, revealed non-coplanar
thienyl rings, and a web of non-covalent interactions. Spectroscopic measurements in
protophilic solvents, hydrogen bond acceptors, such as dmso or dmf disclosed high
sensitivity of dtkdnph to its surrounding, established an equilibrium between dtkdnph and
dtkdnph.S (S = solvent) and showed substrates in concentrations as low as 1x10-9 M can
be detected using protophilic solutions of dtkdnph. Variable temperature studies
confirmed the facile inter-conversion between dtkdnph and dtkdnph.S and changes in
enthalpy (∆Hø) of -11.77±0.66, 13.55±0.66 kJmol-1, entropy (∆Sø) of -37.21±2.00 and
33.55±2.00 JK-1mol-1 and free energy (∆Gø) of -0.68 and 3.5 kJmol-1 were calculated in
dmf and dmso, respectively. Electrochemical measurements divulged solvent
dependence, and electrochemical decomposition following the first electron transfer.
Keywords: di-2-thienyl ketone 2,4-dinitrophenylhydrazone; X-ray; Spectroscopy;
Electrochemistry; Sensors

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1. Introduction
The development of molecular systems sensitive to their surroundings continues
to attract research activities because of their physical and chemical properties and
applications in several areas [1-5]. Bi-stability, the ability of the system (reagent plus
solvent molecules) to exist in two different electronic, magnetic or conformational states,
is essential for molecular recognition, as the inter-conversion between states allows for
their structural relaxation and interaction with their surroundings [6-10]. Weak non-
covalent interactions such as electrostatic and hydrogen bonding may maximize these
effects [1]. Hydrazonic compounds of the type RR’C=N-NH-(C=X)nY (R or R’ =
heteroaryl, aryl, alky or H; X = O or S, n = 0 or 1 and Y = R, R’ or NR2) possess a
flexible backbone, hydrogen donor-acceptor sites and a variety of functional groups
making them susceptible to a broad range of physico-chemical changes [11-28].
Hydrazonic compounds have been used as sensitive analytic reagents for the detection
and determination of trace amounts of various substrates in environmental and biological
samples [11-16]. Several hydrazonic compounds display efficient non-linear optical and
liquid crystal properties and are used as advanced materials in areas such as laser
technology, optical communication, optical switching, and optical computing [17-21].
Pharmacological evaluation of several hydrazonic compounds revealed potent anti-
inflammatory, anti-cancer and anti-bacterial activities. Several compounds are currently
being used for the treatment of various diseases such as cancer, malaria, iron-overload
and other ailments [22-28]. The excellent chelating properties of hydrazonic compounds
contribute to their unique biological properties, and role in molecular sensing. We have
been interested in the chemistry of heteroaryl ketone hydrazonic derivatives and their

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metal complexes and reported on their synthesis, optical, electrochemical, catalytic and
structural properties [29-37]. Optical and thermodynamic measurements on di-2-pyridyl
ketone hydrazones in protophilic (hydrogen bond acceptor) solvents revealed reversible
inter-conversion between two interlocked charge-transfer electronic states due to solvent-
compound interaction [38,39]. X-ray structural analysis on a series of di-2-heteroaryl
ketone hydrazonic compounds revealed a web of non-covalent interactions that include
solvent-solute, solute-solute, and π-stacking which may account in part for their
intertwined charge-transfer bands and molecular sensitivity [29-33]. Although the
chemistry of di-2-pyridyl ketone hydrazonic compounds have been extensively studied,
the chemistry of di-2-thienyl ketone hydrazonic compounds is scarce. Only four reports
have been published on the chemistry of di-2-thienyl hydrazonic compounds. These
include, di-2-thienylketoxime and its use as a nephelometric reagent for the determination
of microgram quantities of gold and palladium, di-2-thienyl ketone 4-nitrophenyl
hydrazone (dtknph) and use as a spectrophotometric reagent for determination of trace
amounts of acids and bases, and the structure and electrochemistry of di-2-thienyl ketone
thiosemicarbazone (dtktsc) [29, 39-41]. In the quest to develop the chemistry of di-2-
thienyl ketone hydrazonic derivatives, herein, structural, spectroscopic, and
electrochemical properties of a bi-stable di-2-thienyl ketone 2,4-dinitrophenylhydrazone
(dtkdnph) are described and compared to other di-2-thienyl ketone hydrazonic derivatives
(see Scheme I) [29,39].

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2. Experimental
2.1. Reagents
Di-2-thienyl ketone (dtk) and 2,4-dinitrophenhydrazine were purchased from Sigma-
Aldrich, MCl2 (M = Zn, Cd and Hg) were obtained from BDH and all other reagents were
obtained from commercial sources and used as received. Solvents used in spectroscopic
and electrochemical measurements were analytical grade.

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2.2 Synthesis of di-2-thienyl ketone 2,4-dinitrophenyl hydrazone
A mixture of di-2-thienyl ketone (500 mg, 2.57 mmol), 2,4-dinitrophenyl hydrazine (550
mg, 2.77 mmol), ethanol (200 mL) and two drops of concentrated HCl were allowed to
reflux for 5 hours. The reaction mixture was allowed to cool to room temperature. A deep
red precipitate was filtered off, washed with hexanes, diethyl ether and dried in air, yield
850 mg (88%). Anal. Calc. for C15H10N4O4S2 (%): C 48.12; H 2.69; N 14.96. Found: C
48.68; H 2.76; N 14.89. IR (KBr pellet, cm-1):
ν
(N-H) 3270,
ν
(C-H) 3150-3050,
ν
(C=C
(C-S
and C=N) 1617-1550, ν(Ν=Ο) 1540-1450,
500/ M-1 cm-1): CH2Cl2 405 (36,700); dmf 550
(35,600), 405 (8,000); dmso 556 (9,600), 415 (17,100). 1H-NMR (at 298 K,
ν(C-N) 1442, N-O) 1330-1270, and ν
bending) 832 and 720. UV-vis (λ/nm, ε ±
δ
ppm) in d6-
dmso: 11.42 (1H, s, NH), 8.86 (1H, s, H33), 8.52 (1H, d, H35), 8.12 (1H, d, H11), 8.04
(1H, d, H36), 7.82 (1H, d, H13), 7.73 (1H, d, H21), 7.46 (1H, t, H12), 7.24 (1H, d, H23),
7.16 (1H, t, H22); d7-dmf: 11.58 (1H, s, NH), 8.96 (1H, s, H33), 8.57 (1H, d, H35), 8.21
(1H,d, H11), 8.17 (1H, d, H36), 7.86 (1H, d, H13), 7.81 (1H, d, H21), 7.53 (1H, t, H12),
7.32 (1H, d, H23), 7.23 (1H, t, H22); CDCl3: 11.59 (1H, s, NH), 9.12 (1H, s, H33), 8.40
(1H, d, H35), 8.15 (1H, d, H36), 7.76 (1H, d, H11), 7.50 (1H, d, H13), 7.45 (1H, d, H21),
7.40 (1H, t, H12), 7.21 (1H, d, H23), 7.08 (1H, t, H22).
2.3. X-ray Crystallography

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Single crystals of dtkdnph were obtained from a dmso solution of dtkdnph when
allowed to stand at room temperature for several days. A single crystal was selected and
mounted on a glass fiber with epoxy cement. A Bruker AXS with a Mo Kα radiation and
a graphite monochromator was used for data collection. The SHELXTL software package
version 5.1 was used for structure solution [42-44]. Cell parameters and other
crystallographic information are given in Table 1 along with additional details concerning
data collection, and Table S1 gives a list of atomic coordinates. All non-hydrogen atoms
were refined with anisotropic thermal parameters.
2.4. Optical sensing studies
A stock solution of dtkdnph in protophilic solvent and stock solutions of substrate
in protophilic solvent were prepared separately. Optical changes (electronic absorption
spectral changes) were measured on a solution prepared by mixing appropriate volumes
of dtkdnph and substrate solutions to prepare the desired solution (see figure captions for
details). With the exception of the thermodynamic studies, all measurements were made
at room temperature.
2.5. Physical Measurements
Electronic absorption spectra were recorded on an HP-8452A spectrophotometer
and a Perkin-Elmer UV/VIS/NIR spectrometer
λ-19. Baseline corrections on blank
solvents were recorded prior to measurements. A Lauda-Brinkmann RM6 circular bath
1
was used for temperature control. Solution H NMR spectra were recorded on a Bruker

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500 DRX spectrometer and were referenced to the residual proton and carbon in the
incompletely deuterated solvent. All peak assignments were made using the 2D
experiments HSQC, HMBC, and COSY. Electrochemical measurements were performed
with the use of a Princeton Applied Research (PAR) Model 173 potentiostat/galvanostat
and Model 276 interface in conjunction with a 286 PC. Data were acquired with the
EG&G PARC Headstart program and manipulated using Microsoft Excel. Measurements
were performed in solutions that were 0.1
M
in [N(n-Bu) ](PF ).The E , E and E =
p,a p,c
4
6
p
(E
+ E )/2 values were referenced to the potassium chloride saturated calomel
p,c
p,a
electrode, SCE, at room temperature and are uncorrected for junction potentials. The
number of electrons on the redox waves was determined using the oxidative peak current
+
of the reversible one-electron couple of FeCp2/FeCp2 couple as an internal standard.
+
Ep(FeCp2/FeCp2 ) = + 0.45 ± 0.02 and + 0.45 ± 0.02 V 200 mVs-1 vs. SCE in CH3CN and
dmf, respectively. Electrochemical cells were of conventional design based on
scintillation vials or H-cells. A glassy-carbon disc was used as the working electrode and
Pt-wire as a counter electrode.
3. Results and Discussion
When di-2-thienyl ketone (dtk) was allowed to react with 2,4-dinitrophenyl
hydrazine in refluxing acidified ethanol, di-2-thienyl ketone 2,4-dinitrophenylhydrazone
(dtkdnph) was isolated in good yield (see Scheme 2). This reaction is similar to those
reported for the preparation of a variety of di-2-pyridyl ketone hydrazones and di-2-
thienyl ketone nitrophenyl hydrazone and is in contrast to the harsh conditions employed

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in the synthesis of di-2-thienyl ketone thiosemicarbazone [39,29]. This reaction
highlights the importance of electronic effect on the acid catalyzed condensation
reactions between dtk and substituted hydrazine. This is due to the poor electrophilic
character of the carbonyl group of dtk owing its co-planarity with the electron-rich
thienyl rings, hence condensation reaction with electron poor hydrazines are favorable
[45,46].
The authenticity of dtkdnph was established from the results of its elemental
analysis, X-ray crystallography and a number of spectroscopic measurements. A view of
the asymmetric unit of dtkdnph is shown in Figure 1. The 2,4-dinitrophenyl hydrazone
moiety and the S1-thienyl ring are coplanar and out of planarity with the S2-thienyl ring.
The S2-thienyl ring showed flip-disorder, rotation by ca.180° over two positions, about
the C1-C21 bond. Site occupancy factors of 0.58 and 0.42 for S2 and S2’ when allowed
to refine freely divulge that the two orientations are not equivalent. As a result, no
significance was placed on the geometrical parameters within S2-thienyl ring. The flip-
disorder of the thienyl ring is similar to that reported for dtktsc-dtk and is common in

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several compounds containing unsubstituted thienyl rings [31,47]. The bond distances
and angles (see Table 2 and The Supplemental CIF file) are of the same order as those
reported for other related species [31-33]. For example, in dtktsc, the hydrazone C=N, N-
N and N-C bond distances of 1.302(4), 1.373(3), 1.357(3) and bond angle of C-N-N and
N-N-C 116.5(2) and 120.0 of were observed [31]. The non-coplanar thiophene rings in
dtkdnph is contrast to the virtually coplanar thienyl rings in dtk, and are similar to those
reported for dtktsc and dtk.dtktsc [45,31]
The extended structure of dtkdnph reveals anti-parallel stacks (see Figure S1)
interlocked via a web of classical hydrogen bonds of the type (N-H…X) and
intermolecular non-classical hydrogen bonds of the type (C-H-X) where X = O, S. Figure
2 shows a view of intermolecular hydrogen bonds and a list of hydrogen bonds is given in
Table 3. The hydrogen bond distances and angles are of the same order as those reported
for dtktsc, fac-Re(CO)3(dpknph)Cl and other related compounds (see Table S2)
[31,33,48].
Infrared spectra of dtkdnph showed the absence of
thienyl ketone and 2,4-dinitrophenylhydrazide starting compounds and the appearance of
peaks in the (NH), (CH), (NO2), (C=N and C=N), and (C=S) regions consistent
ν(C=O) and ν(NH2) of di-2-
ν
ν
ν
ν
ν
with its formulation. 1H-nmr measurements in different solvents (see Figure 3, Figure S2
and Scheme 2) disclosed solvent dependence as manifested by the chemical shift
variations in the amide and heteroaryl and aryl regions. The amide proton appeared
downfield compared to the aromatic protons pointing to the decrease in its electron
density due to the strong influence of the imine (C=N) group. Eight protons appeared in

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the aromatic region between 9.50-7.00 ppm disclosing non-equivalent thienyl rings,
consistent with the solid state structure.
1
Variable temperature H-nmr measurements in different solvents show changes
consistent with the extent of hydrogen bonding between dtkdnph and solvent molecules.
The temperature dependence of the NH chemical shift (∆δ/∆T) has been used as a tool
for studying hydrogen bonding in various systems [49,50]. A temperature dependent
chemical shift hints to the shielding of the NH proton via hydrogen bonding. In CDCl3,
as the temperature increased, the amide (NH) proton shifted upfield, the residual solvent
water (HOD) proton shifted downfield and the chemical shifts of the aromatic protons
remain the same. In d6-acetone and d6-dmso, as the temperature increased, the NH, HOD
shifted upfield and a slight upfield shift in H21, H22 and H23 of S3 thienyl ring, as well
as H35 of the 2,4-dinitrophenyl moiety and H13 of S1 thienyl ring was also observed.
1
Figure 4 shows H-nmr spectra of dtkdnph measure in d6-dmso at different temperature.
In d7-dmf, downfield shifts were observed for NH, HOD, and H21, H22 and H23 of S2-
thienyl ring. Figure S3 shows plot of δ(ΝΗ) versus T-1x103, K-1 in different solvents and
gave slopes of +236±11, +159±6, -195±8 and +202±5 for CDCl3, d6-acetone, d7-dmf and
d6-dmso, respectively. These results implicate hydrogen bond dissociation in d7-dmf and
the reverse in other solvents. This is due to its strong protophilic (basic) character or
hydrogen bond accepting properties of d7-dmf. Integration of the NH proton to H35 of
1:1 is temperature independent, suggests the absence of proton exchange between the
amide (NH) and residual water (HOD) protons.

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The electronic absorption spectra of dtkdnph are solvent, concentration and
temperature dependent. Figure S4 shows the electronic absorption spectra of dtkdnph in
different solvents and the extinction coefficients in neat solvents are given in the
experimental section. These spectra divulged strong solvent-compound interaction
1
consistent with the H-nmr measurements. In CH2Cl2, a single electronic transition
appeared at 405 nm and in protophilic (hydrogen bond acceptors) solvents two absorption
bands appeared at 550, 556 nm and 405 and 415 nm in dmf and dmso, respectively. The
high values of the extinction coefficients are in accord with intra-ligand charge-transfer
(ILCT) of the donor-acceptor type assigned to π→π* of the thienyl ring mixed with
n
→π* of the hydrazone (C=N-NH) moiety followed by charge-transfer to the 2,4-
dinitrophenyl moiety. Spectral measurements in dmso revealed as the concentration of
dtkdnph decreased the intensity of the absorbance at 405 nm decreased and the intensity
of the absorbance at 550 nm increased. A plot of A550 versus concentration is non-linear
consistent with dtkdnph-solvent interaction. These results confirm the sensitivity of
dtkdnph to changes in its surroundings, and point to an equilibrium between dtkdnph
observed at 405 nm and dtkdnph.dmso observed at 556 nm. Spectrophotometric titrations
of protophilic solutions of dtkdnph in CH2Cl2/dmf and NaBX4 (X = H or F) further
confirmed the solvent-dtkdnph interaction. Figure 5 shows the electronic absorption
spectra of dtkdnph measure in mixed CH2Cl2/dmf, as the volume % of dmf increased, the
intensity of the absorbance at 550 nm increased and the intensity of the electronic
absorption at 405 nm decreased. The opposite was observed when the volume % of
CH2Cl2 was increased in dmf/CH2Cl2 co-solvent. When a dmf solution of dtkdnph was
titrated with a dmf solution of NaBF4, the intensity of the low-energy electronic transition

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observed at 550 nm decreased and the intensity of the electronic transition observed at
410 nm increased as the concentration of NaBF4 increased. Figure 6 shows the
spectrophotometric titration of a mixture of dtkdnph and NaBH4 with NaBF4 in dmf. The
opposite was observed when a stoichiometric mixture of dtkdnph and NaBF4 in dmf was
titrated with NaBH4. In the presence of excess base (NaBH4), the equilibrium shifts
completely to the low-energy electronic transition observed at 550 nm and in the
presence of excess acid (NaBF4) the equilibrium shifts completely to the high-energy
electronic transition observed at 405 nm. These results established that in neat protophilic
solvent, an equilibrium exists between dtkdnph and dtkdnph.solvent allowing for
calculations of the extinction coefficients of the high- and low-energy electronic
transitions in the presence of excess acid or a base (see Table 3). Variable temperature
electronic absorption measurements confirmed the facile inter-conversion between
dtkdnph and dtkdnph.solvent. Figure 7 shows variable temperature electronic absorptions
spectra of dtkdnph in dmso. These spectra show as the temperature increased, the ratio of
A550/A405 increased and the reverse was observed when the temperature was decreased or
when dmf was used in place of dmso. A plot of RLnK versus T-1x103, K-1 (see insert
Figure 7) gave straight lines with slopes of – (13.55±0.66) x103 and + (11.77±0.66) x103
per decade with intercepts of +33.55±2.00 and –37.21±2.00 for dmso and dmf,
respectively that gave thermodynamic parameters as listed in Table 4. The reversible
inter-conversion between dtkdnph and dtkdnph.solvent, their low activation parameters
and high value for the extinction coefficient account in part for their high sensitivity to
their surroundings and possible use as molecular sensors for variety of stimuli.

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Spectrophotometric titrations of dmf solutions of Group 12 metal-chlorides, MCl2
where M = Zn, Cd or Hg, with dmf solutions of dtkdnph showed as the [MCl2] increased
the intensity of the electronic transition at 550 nm decreased and the intensity of the
electronic transition at 405 nm increased. Figure 8 shows the spectrophotometric titration
of dmf solutions of dtkdnph with dmf solutions of ZnCl2 and a plot of absorbance at 550
and 405 nm versus MCl2 is shown in Figure 8 insert. These results showed that MCl2 in
concentrations as low as 1x10-9 M can be detected and determined using dmf solutions of
dtkdnph and the sensitivity of dtkdnph toward MCl2 increases as the size of the metal ion
increased. In previous reports MCl2 in concentrations as low as 1x10-5 M were detected
using protophilic solutions of dpknph [51].
A comparison of the electronic spectra of dtkdnph with those of dtknph in
protophilic solvents shows subtle differences as manifested by the higher intensity and
blue shift (~50 nm) of the electronic transition of dtkdnph.solvent observed at 550 nm
compared to the low-energy electronic transition of protophilic solutions of dtknph [39].
The higher intensity of the low-energy electronic transition of protophilic solutions of
dtkdnph suggests a stronger solvent-compound interaction due to the stronger acidity of
the amide (NH) proton of dtkdnph owing the stronger withdrawing character of 2,4-
dinitrophenyl moiety compared to 4-nitrophenyl moiety. The ILCT electronic transition
of dtkdnph in non-aqueous solvents is red shifted compared to the ILCT transition of di-
2-thienyl ketone thiosemicarbazone (dtktsc) due to the electron donating character of
NH2 in dtktsc compared to the electron-withdrawing properties of 2,4-dinitrophenyl
moiety and higher polarizability of dtkdnph [31]. The electronic absorption spectra of

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dtktsc in non-aqueous solvents showed a highly intense ILCT transition at ~350±5 nm
and the absence of any significant electronic transition due to dtktsc.solvent [31].
3.3. Electrochemical Properties
The electrochemical properties of dtkdnph in non-aqueous solvents were
investigated using voltammetric techniques. Cyclic voltammograms of dtkdnph measured
in CH3CN and dmf are shown in Figure 9. On a reductively-initiated scan on CH3CN
(Figure 9a), an irreversible reductions appeared at Ep,c = -0.80, -1.37 and -1.68 V
followed by electrochemically generated product waves at Ep,a = -1.46, -1.32, -1.00, -
0.82, +0.38 V and an irreversible oxidation at Ep,a = +1.43 V. On oxidatively-initiated
scan in CH3CN (Figure 9b), similar behavior as reductively-initiated scan was observed,
except an oxidatively generated product wave appeared at Ep,c = -0.22 V and the
reductively generated oxidative waves observed between 0.00 to +1.00 V on reductively-
initiated scan disappeared. The oxidative current (ip,a) at Ep,a = +1.42 V increased, and
the reductive current observed at -0.80 and -1.68 V decreased on oxidatively initiated
scan compared to reductively initiated scan. A series of scan rate variations between 0 to
-1.0 V showed no sign of reversibility on the reduction wave at Ep,c = -0.80 V. These
results showed that dtkdnph undergoes redox decomposition on the first reduction or
oxidation. When a Pt-electrode was used in place of GC-electrode, a hysteresis appeared
at -1.90 V following the first and second irreversible reductions observed at Ep,c = -0.83
and -1.60 V on reductively-initiated scan pointing to electro-catalytic decomposition of
dtkdnph. The voltammograms of dtkdnph measured in CH3CN are similar to those
observed for di-2-thienyl ketone thiosemicarbazone (dtktsc) and a variety of di-2-pyridyl
ketone hydrazonic ligands that include the reduction of di-2-pyridyoxime to di-2-

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pyridylmethyl amine in non-aqueous media [31,36]. For example, cyclic voltammogram
of dtktsc measure in CH3CN on reductively-initiated scan showed three irreversible
reductions at Ep,c = -1.45, -1.93 and -2.20 V and one irreversible oxidation at Ep,a = +1.68
V versus Ag. These results suggest that reductive decomposition of dtkdnph in CH3CN
occurs at the first electronic transfer at the imine (C=N) of the hydrazone backbone to
form radical anion (C-N).- followed by a series of proton coupled with electron transfer
(see Scheme 1a). When dmf was used in place of CH3CN, a broader redox waves
appeared pointing to multiple redox transformations due to the overlap of imine (C=N)
and nitro-redox transformations. This is consistent with the reduction of a variety of
nitroaromatic compounds in non-aqueous media that show reduction potentials close to
those of the imine (C=N) reductions [52,53]. On a reductively-initiated scan in dmf
(Figure 9c) a broad irreversible reduction wave appeared between -0.60 to -1.20 V
centered at Ep,c = –0.90 V, a prewave at -1.26 V and an irreversible reduction at Ep,c = -
1.66 V followed by a series of irreversible product waves at Ep,a = -1.12, -0.91, -0.71, -
0.38, +0.41, +0.65 and an irreversible oxidation at Ep,a = +1.41 V. On a reductive scan
between -1.00
↔-3.00 V, two multi-electron irreversible reductions appeared at -2.15 and
-2.44 V in addition to the reduction waves observed in Figure 10c. On an oxidatively-
initiated scan in dmf (Figure 9d), an irreversible oxidation wave appeared at Ep,a = +1.43
V, followed by oxidatively generated product wave at Ep,c = -0.60 V, an irreversible
reduction at -0.94 V, oxidatively generated product wave at -1.10 V, an irreversible
reduction wave at Ep,c = -1.62 V and a series of reductively generated waves appeared
between Ep,a = -1.30 to 0.00 V. On an oxidative scan between +1.00
↔+3.00 V no
additional oxidation waves appeared beside the irreversible oxidation observed. A

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plausible reduction of dtkdnph in dmf that incorporated the reduction of the nitro-groups
is shown in Scheme 3. Details of the mechanistic pathway for the redox transformations
of dtkdnph remain to be explored.
Owing the convenient preparation of heteroaryl hydrazonic compounds, their
favorable phyisco-chemical properties and potential applications in many important
biological, environmental and industrial processes, studies are in progress in our
laboratory to explore the synthesis, physico-chemical properties, coordination chemistry,

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catalytic and molecular sensing behavior of a variety of heterocyclic ketone hydrazonic
compounds.
Conclusion
X-ray structural analysis established the identity of dtkdnph and exposed a flip disordered
thienyl ring and a web of hydrogen bonds in the extended structure. Spectroscopic
measurements showed strong solvent-dtkdnph interaction, high sensitivity of protophilic
solutions of dtkdnph to changes in their surrounding as manifested by an equilibrium
between two intertwined ILCT transitions due to dtkdnph and dtkdnph.substrate
(substrate = solvent, acid, base of MCl2). MCl2 in concentrations as low as 1x10-9 M can
be detected and determined using dmf solutions of dtkdnph. The extent of solvent-
dtkdnph interaction depends of the hydrogen bonding capacity of the solvent.
Electrochemical measurements revealed a series of single and multi-electron transfers
and electrochemical decomposition of dtkdnph following the first electron transfer.
Acknowledgments
The author wishes to acknowledge Mr. Colin Gyles, Mr. Orville Green and Ms. Toni
Johnson for technical assistant and The University of the West Indies for support of this
research.
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O-
O-
O
N+
O
N+
NH2
S
O-
N+
O
NH
HN
N
N
HN
N
S
S
S
S
S
S
dtkdnph
dtknph
dtktsc
Scheme 1. Representations of di-2-thienyl ketone 2,4-dinitrophenyl hydrazone (dtkdnph),
di-2-thienyl ketone 4-nitrophenyl hydrazone (dpknph), and di-2-thienyl ketone
thiosemicarbazone (dtktsc).