Interaction of thiazolidine-2-thione with 2,3,5,6-tetrabromo-1,4- benzoquinone: A set of sequential interactions involving redox and substitution reactions after an initial charge transfer complexation

Article abstract of DOI:10.1016/j.saa.2011.10.032

Interaction between thiazolidine-2-thione (T2T) as an electron donor and 2,3,5,6-tetrabromo-1,4-benzoquinone (p-bromanil, BRL) as a π-electron acceptor has been studied by using several spectral techniques, viz. UV/visible, IR, ¹H NMR and Mass

Full text of DOI:10.1016/j.saa.2011.10.032

Spectrochimica Acta Part A 86 (2012) 252–255
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Interaction of thiazolidine-2-thione with 2,3,5,6-tetrabromo-1,4-benzoquinone:
A set of sequential interactions involving redox and substitution reactions after
an initial charge transfer complexation
Usama M. Rabie∗, Moustafa H.M. Abou-El-Wafa, Heba Nassar
Chemistry Department, Faculty of Science at Qena, South Valley University, Qena 83523, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2011
Received in revised form 5 October 2011
Accepted 14 October 2011
Interaction between thiazolidine-2-thione (T2T) as an electron donor and 2,3,5,6-tetrabromo-1,4-
benzoquinone (p-bromanil, BRL) as a ␲-electron acceptor has been studied by using several spectral
techniques, viz. UV/visible, IR, 1H NMR and Mass spectra. A substitution reaction has been occurred after
an initial formation of a CT complex, meanwhile a redox reaction has been occurred, in situ, too in which
the interacting donor (T2T) has been oxidized to the corresponding thiazole. Thus, the stoichiometric
ratio of this interaction has been found to be 2:1, T2T:BRL. However, the most interesting finding is that
unexpectedly neither the SH group of the thiol form nor the NH group of the thione form of the T2T has
shared in the substitution reaction with BRL. This finding has been confirmed by the different applied
spectral tools, whereas a plausible reaction pathway has been illustrated and discussed.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Charge transfer complex
2-Thiazoline-2-thione
Bromanil
Tautomerism
Substitution reaction
Spectral measurements
1. Introduction
S
NH
S
N
Due to the –(NH)–(C S)– grouping and the electron donor
properties of N and S atoms, thiazolidine-2-thione (T2T) and its
derivatives have several pharmaceutical [1–11] and industrial
applications [12–16]. Interactions of T2T as an electron donor with
iodine as a ␴-acceptor and DDQ as a ␲-acceptor have been reported
[17,18]. However, according to our knowledge, yet to date no pre-
vious work has been reported for the interaction of T2T with other
␲-type acceptors viz. bromanil, chloranil, TCNE, TCNQ, etc. We
have recently reported [17,18] that T2T has two tautomeric forms
namely thione and thiol forms, Structures I and II, respectively.
However, in the solid state, T2T exists [17,18] as a hydrogen-bonded
thioamide complex (Structure III) formed between the NH of the
thione form and the exocyclic sulfur atom.
S
SH
( II )
( I )
Thione
Thiole
Structures (I) and (II): Thione–thiole tautomere
s
S
H
N
S
H
S
N
S
Structure (III): Dimeric hydrogen-bonded thioamide complex
The complexity of this compound (T2T) and the willing to con-
tinue our studies [18–23] concerning the ␲-type acceptors have
motivated us for this piece of work aiming to study the interaction
of T2T with bromanil whether they undergo a CT complexation
and/or a chemical reaction.
2. Experimental
2.1. Materials and solutions
∗
The electron donor, thiazolidine-2-thione (T2T), was pur-
chased from Aldrich Chemical Co. The electron acceptor,
Corresponding author. Tel.: +20 106755160; fax: +20 965213383.
E-mail address: umrabie@yahoo.com (U.M. Rabie).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.10.032

U.M. Rabie et al. / Spectrochimica Acta Part A 86 (2012) 252–255
253
Fig. 1. Electronic absorption spectra of the CH2Cl2 solutions at 25 ◦C. (a) T2T
Fig. 2. Electronic absorption spectra of the CH2Cl2 solution of the synthesized solid
solution; [T2T] = 1.0 × 10−4 mol dm−3, (b) BRL solution; [BRL] = 1.0 × 10−4 mol dm−3
,
,
product (T–B) at 25 ◦C.
(c)
and
(d)
T2T–BRL
mixture
solution;
[T2T] = 1.0 × 10−4 mol dm−3
[BRL] = 1.0 × 10−4 mol dm−3; where (c) blank = solvent; (d) blank = BRL.
acceptor BRL have indicated the formation of a chemical reaction
between T2T and BRL.
2,3,5,6-tetrabromo-1,4-benzoquinone (BRL) (Aldrich Co.), was
recrystallized from both chlorobenzene and dry methylene chlo-
ride. The solvents used were of pure spectral grade (BDH or E.
Merck).
T2T is an electron donor [17,18] and BRL is a well known ␲-
acceptor having 2.44 eV electron affinity [24]. This means that
formation of a T2T–BRL CT complex might be probably occurred
at least initially [17,18,24,25] when the two species (T2T and BRL)
undergo interaction. Several attempts of the spectral measure-
ments by using different concentrations of each of the interacting
species have been done to observe a CT spectral band, but failed.
However, failure of observing the, expected, CT absorption band(s)
for this T2T–BRL interaction in solutions might be due to either
the perturbation of this (expected) CT band with the absorp-
tions of the free donor and/or the free acceptor or the existence
of the compound T2T in the form of dimeric hydrogen bonded
complex in the solid state, whereas dissociation of this com-
plex into its free (interacting) molecules (either in the thione
or thiol forms) may require some energy and/or elapsing of
time.
2.2. Physical measurements
The electronic absorption spectra were recorded on a Perkin-
Elmer Lambda 40 spectrophotometer equipped with a Julabo FP 40
thermostat ( 0.1 ◦C) using 1.0 cm matched quartz cells. IR spectra
of the synthesized solid product were recorded on a Shimadzu IR-
408 spectrophotometer. 1H NMR and Mass spectra measurements
were carried out in the microanalytical laboratory, Cairo University.
2.3. Synthesis of the solid CT complex
The solid product of the interaction of T2T with BRL was syn-
thesized by mixing the required amount of the donor (0.01 mol)
in CH2Cl2 with an appropriate amount of the BRL (0.01 mol) in
CH2Cl2 in the 1:1 mole ratio. The mixture was refluxed for 4 h.
The resulted precipitate was separated by cooling the solution
and it was washed several times with hexane. The synthesized
solid product (T–B) has a faint green colour and its m.p. is
200 ◦C. T–B has been analyzed and investigated by several spectral
techniques.
3.2. Characterization of the product resulted from the interaction
of T2T with BRL
TLC measurements of the synthesized compound (T–B) have
shown a single peak which means that the interaction of T2T with
BRL has produced a single compound.
The electronic absorption spectra of the CH2Cl2 solution of the
solid product (T–B) have shown a main spectral band located at
400 nm, cf. Fig. 2. This spectral band does not belong to either the
individual reacting species: T2T or BRL. In turn, one can suggest
that the interaction of T2T with BRL has led to a new compound
(T–B) having different electronic transitions rather than those of
each individual reacting species.
3. Results and discussion
3.1. Spectral study of the interaction between T2T and the
ꢀ-acceptors
As shown hereafter, all the data which have been collected from
the IR, 1H NMR and Mass spectra have indicated that a substitution
reaction between T2T and BRL has been occurred. This substitu-
tion reaction has been occurred in which two molecules of T2T
have been reacted with one molecule of BRL. Also, our available
data gathered from all the applied analysis tools (IR, NMR and Mass
spectra) have indicated that the product of this chemical reaction
(T–B) has the following two structures (Structures V and VI) exist-
ing together; i.e. coexistence of both the structural forms: thione
and thiol, Structures V and VI, respectively.
Electronic absorption spectra in the wavelength range of
250–750 nm were recorded for the mixture solutions of the T2T
donor with the ␲-acceptor BRL in methylene chloride, cf. Fig. 1.
The electronic spectra for these mixture solutions (T2T–BRL) did
not show any new spectral band rather than those of the free donor
(T2T) or the free acceptor (BRL), meanwhile all attempts to observe
any CT band(s) for this interacting couple (T2T–BRL) were failed, cf.
Fig. 1. However, as shown hereafter, examinations of the prepared
solid product (T–B) resulting from the interactions of T2T with the

254
U.M. Rabie et al. / Spectrochimica Acta Part A 86 (2012) 252–255
S
SH
3.82 (H, s, 2SH, for the thiol structure (St. VI)). Moreover, the same
O
O
O
NH
1H NMR spectra of the compound T–B (cf. Fig. 4) have provided a
set of additional evidences for the formation of the compound T–B,
as a product of the chemical reaction between T2T and BRL, and,
also, evidences for the coexistence of the aforementioned two sug-
gested structural formulae (Structures V and VI) of this compound.
These set of evidences can be manifested in what follows: (1) no
signal(s) characterizing [26] to a proton of a phenolic OH group has
been shown in these 1H NMR spectra (cf. Fig. 4) which means that
the primary interacting bromanil (BRL) did not fully reduced to the
corresponding hydroquinone. (2) No multiplet signals characteriz-
ing [26] to protons of the aliphatic –CH2–CH2– group have been
shown in these 1H NMR spectra (cf. Fig. 4) which means that each
of the primary interacting T2T compound has lost 3 H atoms in the
form of H2 and HBr molecules. Thus, in addition to the replace-
ment reaction, which produced HBr molecules, a dehydrogenation
by the bromanil via a redox reaction has been occurred too. The
–CH2–CH2– group does no longer exist in any form of the final
product (T–B). (3) The observation of a singlet signal (at 8.24 ppm)
in the NMR spectra of the compound T–B (cf. Fig. 4) indicates that
the sites involved in the thione–thiol tautomerization are far from
the protons in the positions 5 and 19, and, in turn, existence of the
product T–B in either of the two forms (Structures V and VI) do
not affect on the resonance (signal position) of the protons in the
5 and 19 positions. This truly confirms that position number 4 of
the compound T2T is the active site by which T2T has been reacted
with BRL via the replacement reaction. Finally, (4) resonance of the
protons in the positions 5 and 19 of the compound T–B (in either of
the two forms) have been appeared in the NMR spectra as a singlet
sharp signal with high integration, cf. Fig. 4. This confirms that the
protons in the positions 5 and 19 are identical having a symmet-
rical atom-to-atom relationship. Moreover, the coexistence of the
two forms (Structures V and VI) has increased and multiplied the
integration of the signal of the protons in the positions 5 and 19.
A convincing evidence for the structure of the synthesized com-
pound (Structures V and/or VI) has been attained by examining the
Mass spectra of this solid product (T–B) resulting from the interac-
tion of T2T with BRL. Before discussing the Mass spectra, it is worthy
to mention that both structures V and VI might have the same spec-
tral peaks; they have the same molecular weight and except for the
sites of the tautomerization (the exocyclic S and the endocyclic N
atoms) the two structures have the same atom-to-atom relation-
ship. Hence, existence of the product (T–B) in either the two forms
(Structures V and VI) or even the coexistence of the two structures
does not dramatically affect the Mass spectra (peaks positions and
the corresponding fragments).
N
Br
Br
S
S
S
S
Br
Br
NH
N
O
S
HS
Structure V
(thione structure)
The IR spectra of the free donor T2T, the free acceptor BRL and
Structure VI
(thiol structure)
the solid product (T–B) synthesized from the interaction of T2T with
BRL have been measured, cf. Fig. 3.
A careful examination of the IR spectral data has indicated the
coexistence of these suggested structures (Structures V and VI) for
the compound T–B. The IR spectra of the synthesized T–B solid com-
pound (cf. Fig. 3) have indicated the existence of the bromanil (in
its quinone form), the thione oxidized form of T2T and the thiol
oxidized form of the T2T. Thus, the IR spectra of the compound
T–B have showed several peaks located at 1715 and 1415 cm−1
which characterize [26] to the stretching vibrations of C O and
C
C groups, respectively, for the bromanil (in the two Structures
V and VI), cf. Fig. 3. Also, the IR spectra of the compound T–B have
showed peaks located at 3200(w) and 1040 cm−1 which are charac-
teristic [26] to the groups NH and C S, respectively, for the thione
form (Structure V). Further, the same IR spectra of this compound
(T–B) have showed peaks at 2750 and 1590(s) cm−1 which are char-
acteristic [26] to the stretching vibrations of SH and C N groups,
respectively, of the thiol form (Structure IV). Meanwhile, the IR
spectra of the synthesized solid product (cf. Fig. 3) do not show
peak(s) in the range of 3200–3600 cm−1 which may characterize
[26] to OH group. This confirms that BRL exists in the quinone form
and it does not exist in the corresponding hydroquinone (BRLH2).
The 1H NMR spectra have provided further evidences for the
aforementioned structural formulae (Structures V and VI) of the
compound T–B resulted from the interaction of T2T with BRL. The
NMR spectra have indicated the coexistence of the previously two
suggested structures (Structures V and VI) for the compound T–B.
Thus, the 1H NMR spectra (200 MHz, in DMSO) of the solid prod-
uct (T–B) (cf. Fig. 4) have displayed distinct signals characterizing
[26] to the suggested structures (Structure V and VI) namely: ı
(ppm); 8.24 (H, s, 2H of C5 and C19, for the structures V and VI), 7.76
and 7.54 (H, s, 2NH, for the thione structure (St. V)) and 4.08 and
Fig. 3. Infrared spectra of: (a) free T2T, (b) free BRL, and (c) T–B (solid product of
the T2T–BRL interaction).
Fig. 4. 1H NMR spectra of the compound T–B.

U.M. Rabie et al. / Spectrochimica Acta Part A 86 (2012) 252–255
255
meanwhile the interacting donor (T2T) has been oxidized to the
corresponding thiazole. Unexpectedly, and interestingly too, nei-
ther the SH group of the thiol form nor the NH group of the thione
form of the T2T has shared in the substitution reaction with BRL.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.10.032.
Fig. 5. Mass spectra of the compound T–B.
References
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action of T2T with BRL (cf. Fig. 5) have displayed a peak M (M+1) at
m/z (%) = 495 (0.9) suggesting the molecular weight of the assigned
product (Structure V or VI) in which the compound T2T has been
first oxidized by dehydrogenation to the form T2T-2H, whereas
two of the oxidized T2T molecules (T2T-2H) have been added to
a molecule of BRL via a substitution reaction accompanied by the
elimination of two HBr groups (Structure V or VI; M.W. = 494).
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for the compound T–B resulted from the interaction of T2T with
BRL.
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has been occurred according to the following steps or reaction path-
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found an adequate explanation by the hypothetic reaction path-
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4. Conclusion
The interaction of T2T with BRL has led to a chemical reac-
tion after an initial formation of a charge transfer complexation,