Synthesis, spectral, and magnetic studies of benzothiazolium tetrachlorocuprate salts: Crystal structure and semiconducting behavior of bis[2-(4-methoxyphenyl)benzothiazolium] tetrachlorocuprate(II)

Article abstract of DOI:10.1080/00958972.2015.1042874

Reaction of Cu(NO₃)₂·3H₂O with substituted 2-phenylbenzothiazoles and HCl in methanolic solution gave (2-PBZ)₂[CuCl₄], [2-PBZ = substituted 2-phenyl benzothiazolium]. Three such compounds, 2-(4-methox

Full text of DOI:10.1080/00958972.2015.1042874

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Synthesis, spectral, and magnetic
studies of benzothiazolium
tetrachlorocuprate salts: crystal
structure and semiconducting
behavior of bis[2-(4-
methoxyphenyl)benzothiazolium]
tetrachlorocuprate(II)
A. Sarau Devia, Reena Ravindranb, Shiji Fazilc & R. Minithad
a Department of Chemistry, Fatima Mata National College, Kollam,
India
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b Department of Chemistry, Sree Narayana College,
Thiruvananthapuram, India
c Department of Chemistry, Mannaniya College of Arts and
Science, Thiruvananthapuram, India
d Department of Chemistry, S.N. College, Kollam, India
Accepted author version posted online: 20 Apr 2015.Published
online: 18 May 2015.
To cite this article: A. Sarau Devi, Reena Ravindran, Shiji Fazil & R. Minitha (2015): Synthesis,
spectral, and magnetic studies of benzothiazolium tetrachlorocuprate salts: crystal structure
and semiconducting behavior of bis[2-(4-methoxyphenyl)benzothiazolium] tetrachlorocuprate(II),
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2015.1042874
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Journal of Coordination Chemistry, 2015
http://dx.doi.org/10.1080/00958972.2015.1042874
Synthesis, spectral, and magnetic studies of benzothiazolium
tetrachlorocuprate salts: crystal structure and semiconducting
behavior of bis[2-(4-methoxyphenyl)benzothiazolium]
tetrachlorocuprate(II)
A. SARAU DEVI*†, REENA RAVINDRAN‡, SHIJI FAZIL§ and R. MINITHA¶
†Department of Chemistry, Fatima Mata National College, Kollam, India
‡Department of Chemistry, Sree Narayana College, Thiruvananthapuram, India
§Department of Chemistry, Mannaniya College of Arts and Science, Thiruvananthapuram, India
¶Department of Chemistry, S.N. College, Kollam, India
(Received 8 November 2014; accepted 1 April 2015)
Reaction of Cu(NO3)2·3H2O with substituted 2-phenylbenzothiazoles and HCl in methanolic
solution gave (2-PBZ)2[CuCl4], [2-PBZ = substituted 2-phenyl benzothiazolium]. Three such
compounds, 2-(4-methoxyphenyl) benzothiazolium tetrachlorocuprate(II), [(mpbH)2CuCl4],
(C14H12NSO)2CuCl4; 2-(3-methoxy-4-hydroxyphenyl) benzothiazolium tetrachlorocuprate(II), [(mh-
pbH)2CuCl4] (C14H12NSO2)2CuCl4 and 2-(2-hydroxyphenyl)benzothiazolium tetrachlorocuprate(II),
[(hpbH)2CuCl4] (C13H9NSO)2CuCl4 were isolated and characterized by spectral and thermal studies.
Single crystal analysis of bis[2-(4-methoxyphenyl) benzothiazolium] tetrachlorocuprate(II)
[(mpbH)2CuCl4] reveals that it crystallizes in P-1 space group with square planar CuCl24ꢀ and almost
planar 2-(4- methoxyphenyl)benzothiazolium cation (mpbH+). The discrete CuCl42ꢀ units are held
between layers of 2-(4-methoxyphenyl) benzothiazolium, (mpbH+) units, mainly by ionic and
hydrogen bonding, resulting in a 3-D crystal lattice. This compound exhibits green-to-yellow
*Corresponding author. Email: sarayu@fatimacollege.net
© 2015 Taylor & Francis

2
A. Sarau Devi et al.
thermochromic transformation on heating. The variable-temperature magnetic susceptibility
measurements of all the compounds show normal paramagnetic behavior with no significant interac-
tions between the paramagnetic Cu(II) centers. The electrical conductivity for crystal salts of
[(mpbH)2CuCl4] was 1.99 × 10−4 S cm−1 at room temperature indicating a semiconductor.
Keywords: Crystal structure; Benzothiazolium tetrachlorocuprate(II); Magnetic susceptibility;
Thermochromism; Semiconductor
1. Introduction
Benzothiazoles are of interest due to their potential pharmacological properties such as
anticancer, antimicrobial, photosensitizing, antidiabetic, antioxidant, and HIV inhibiting
activities [1–6]. The extensive biological activity of benzothiazole derivatives are attributed
primarily to the presence of extended π-delocalized systems, capable of binding to DNA via
π–π interactions [7]; a large number of therapeutic agents are synthesized with the help of
the benzothiazole nucleus [8]. Depending on the efficiency of the chromophore in the aryl
system, some benzothiazole derivatives have interesting luminescence [9] or fluorescence
properties. Fluorescence and UV–visible properties of benzothiazole complexes are con-
trolled by mode of coordination, as the N- and S-coordinating sites are on different sides of
the heterocyclic ring. Soft metal ions will preferentially bind to the S-coordinating site,
while hard ions will prefer the N-coordinating site [10]. Benzothiazole-based metal
complexes of Re and Tc are also excellent cancer radiopharmaceuticals [11].
Recent developments in inorganic–organic hybrid compounds led us to prepare a series
of substituted 2-phenyl benzothiazoles, A2[MX4], where A is an organic cation, M is a
transition metal ion, and X is a halide [12, 13]. These compounds sometimes exhibit low-
dimensional antiferromagnetic exchange interactions [14, 15]. The π/d-cooperative molecu-
lar systems involving conducting π electrons of radical cation salts with copper counter
anion of localized d spins, attain interest due to its electrical conduction/magnetic properties
[16, 17]. Some TTF frameworks, with a backbone of π-conjugated cation radical salt, were
reported as organic semiconductors [18]. In some other cases, organic salts possess low-
dimensional magnetic moments, because of structural phase transitions due to rotation of
CuCl24ꢀ about crystallographic axes mediated by organic cations [19]. In general, the struc-
tures of these compounds can be described as cationic–anionic layers interconnected by
ionic interaction, hydrogen bonding, π–anion, and π–π stacking interactions, which play a
role in building the entire structure in a 3-D crystal lattice and are responsible for the
properties [20, 21]. Here, due to the presence of an active Jahn–Teller effect in Cu(II), the
CuCl24ꢀ species in these compounds becomes stereochemically non-rigid. The geometry
usually varies from distorted tetrahedral with D2d symmetry to centrosymmetric square
planar with D4h symmetry [22, 23]. The colors of these compounds also depend on geome-
try, with green observed when the trans Cl–Cu–Cl angle is 180º as in square planar
complexes and orange to yellow when the angle decreases. Thus, on application of tempera-
ture, most of these compounds exhibit thermochromism due to a possible change on the
trans angle along with a change in the entire hydrogen bonding network in the crystal struc-
ture [24]. This work deals with the synthesis of a series of substituted 2-phenylbenzothia-
zole (figure 1) salts with CuCl24ꢀ, the structures of which were proved by various spectral
techniques like infrared, electronic, NMR, and ESR, along with thermogravimetric studies.

Benzothiazolium tetrachlorocuprate salts
3
N
OCH3
S
2-(4-methoxyphenyl)benzothiazole, mpb
HO
OCH3
OH
N
S
N
S
2-(2-hydroxyphenyl)benzothiazole, hpbH
2-(3-methoxy-4-hydroxyphenyl)benzothiazole, mhpb
Figure 1. Structures of substituted 2-phenylbenzothiazoles: mpb, mhpb, and hpbH.
The magnetic properties of the CuCl24ꢀ salts are discussed. The crystal structure of
[(mpbH)2CuCl4] is discussed, which explains various interactions between cationic and
anionic units. The semiconducting behavior of the crystalline compound has been explored.
2. Experimental
Cu(NO3)2·3H2O, o-aminothiophenol, anisaldehyde, salicylaldehyde, vanillin, and sodium
bisulfite of Analar grade were purchased from Merck and used as received for preparing
benzothiazole ligands. Methanol (Merck) was used without purification.
2.1. Materials and methods
All chemicals used were Analar, or of high grade. All organic compounds were prepared by
literature methods [25]. Elemental analyses were performed on a Vario EL III CHNS ana-
lyzer. Infrared spectra were recorded on a Thermo Nicolet AVATAR 370 DTGS model FT-
IR Spectrometer using KBr pellets from 4000 to 400 cm−1. The thermal analysis (TGA)
was carried out in air with a heating rate of 10 °C min−1 using a Perkin Elmer Diamond
TG/DTG analyzer. Electronic spectra were recorded in acetonitrile solutions on a Spectro
UV–vis Double Beam UVD-3500 Spectrophotometer from 200 to 1500 nm. EPR spectra in
the solid state and in acetonitrile at liquid nitrogen temperature were recorded on a Varian
E-112 spectrometer with X-band, using TCNE as standard with 100 kHz modulation
frequency and 9.1 GHz microwave frequency at SAIF, IIT, Bombay. The DART-mass
spectra of the compounds were recorded on a Jeol AccuTOF DART JMS-T100LC mass
spectrometer having time-of-flight (TOF) mass analyzer and Direct Analysis in Real Time
(DART) source. Magnetic susceptibility data were measured on a Lake Shore Cryotronics,
Inc. Vibrating Sample Magnetometer. The electrical conductivity has been measured using
a Keithley 2182 A nanovoltmeter for probe conductivity meter (6221 dc current source).
2.2. X-ray crystallography
Single crystal of (mpbH)2CuCl4 (green) of dimensions 0.30 × 0.20 × 0.20 mm was selected
and mounted on a Bruker axs Kappa apex 2 CCD diffractometer, equipped with a graphite
crystal incident beam monochromator and a fine focus sealed tube Mo Kα (λ = 0.71073)

4
A. Sarau Devi et al.
X-ray source. The unit cell dimensions and intensity data were recorded at 293 K. The
program SAINT/XPREP was used for data reduction and APEX2/SAINT for cell
refinement [26]. The structure was solved using SIR92 [27], and refinement was carried out
by full-matrix least squares on F2 using SHELXL-97 [28]. Molecular graphics employed
were ORTEP-III [29] and MERCURY 2.4 [30]. The positional parameters for the N–H
hydrogen were refined using the riding model.
2.3. Synthesis of substituted 2-phenylbenzothiazole ligands (mpb, mhpb, and hpbH)
Equimolar mixture of NaHSO3 (1.25 g, 0.012 mol) and corresponding aldehyde, viz.
anisaldehyde (1.63 g, 0.012 mol) for mpb, vanillin (1.82 g, 0.012 mol) for mhpb, and sal-
icylaldehyde (1.47 g, 0.012 mol) for hpbH, were refluxed in ethanol (15 mL) for 15–20
min. To the mixture o-aminothiophenol (1.25 g, 0.01 mol) was added and continued to
reflux for 4–5 h. On slow cooling, colorless crystalline compound formed in each case was
filtered, washed with water, recrystallized from ethanol, and dried over fused CaCl2. The
solids were recrystallised from hot ethanol (scheme 1).
mpb: Yield: 86%. M.P.: 110 °C. Color: Colorless. Anal. Found (Calcd for C14H11NOS,
241): C, 69.51 (69.68); H, 4.75 (4.59); N, 5.63 (5.80); S, 13.42 (13.28).
mhpb: Yield: 85%. M.P.: 171 °C. Color: Colorless. Anal. Found (Calcd for C14H11NO2S,
257): C, 65.42 (65.27); H, 4.21 (4.30); N, 5.62 (5.44); S, 12.63 (12.46).
hpbH: Yield: 82%. M.P.: 129 °C. Color: Colorless. Anal. Found (Calcd for C13H9NOS,
227): C, 68.75 (68.69); H, 3.76 (3.99); N, 6.32 (6.16); S, 14.28 (14.10).
R2
R3
R1
R2
R3
R1
N
S
NaHSO3,EtOH
Reflux
NH2
SH
+
OHC
4-5 h
R1= H, R2 = H,
R1= H, R2 = OCH3, R3 = OH for mhpb
R1= OH, R2 = H, R3 = H for hpbH
R3 = OCH3 for mpb
Scheme 1. Synthesis of substituted 2- phenyl benzothiazoles.
2.4. Spectral characterization of mpb, mhpb, and hpbH
2.4.1. Infrared spectra. Bands in the 1625–1300 cm−1 range are due to overall ring
skeletal (benzene and thiazole ring) stretching mode [31, 32]. The characteristic ν(C=N)
vibrations of thiazole for mpb, mhpb, and hpbH are assigned at 1597, 1592, and
1588 cm−1, respectively, and ν(C–S) at 687, 716, and 700 cm−1, respectively [33]. Bands at
1308–962 cm−1 correspond to the CH– in plane deformation of benzene and thiazole rings
while out of plane deformations are observed at 860–716 cm−1. The ν(O–H) of mhpb
appeared as a broad band at 3200–2800 cm−1 while ν(O–H) of hpbH appeared as a weak
peak at 3057 cm−1. The phenolic C–O stretch for mhpb is observed at 1191 cm−1 and the
same for hpbH is at 1219 cm−1. The O–CH3 stretching mode for mpb and mhpb is ascribed

Benzothiazolium tetrachlorocuprate salts
5
to peaks at 1021 and 1007 cm−1 while bending mode to the peaks at 434 and 438 cm−1,
respectively [34].
1
2.4.2. H NMR spectra. In the ligands, signals between 6.9 and 8.1 ppm are assigned to
aromatic protons of both benzothiazole and phenyl ring [35]. Comparison of the chemical
shift of the protons in the phenyl ring shows very similar values for mpb, mhpb, and hpbH.
Due to the presence of an additional OH and subsequent establishment of hydrogen
bonding in mhpb, the chemical shift values of the protons in the substituted 2- phenyl rings
show slight variation in comparison with those of mpb. The methoxy protons of mpb res-
onate at 3.87 ppm. The doublet at 8.03 ppm corresponds to C(9)H and C(13)H, whereas the
doublet at 6.99 ppm is due to C(10)H and C(12)H in mpb.
In mhpb, the –OCH3 protons show a slight downward shift to 4.01 ppm due to the with-
drawal of electron density by adjacent OH group [36]. The OH of mhpb is observed at
6.05 ppm. Even though there is hydrogen bonding, due to the short range shielding effect
induced by neighboring –OCH3, the –OH protons do not shift downfield [37]. Due to the
combined resonance and inductive effect of –OH and –OCH3 groups in mhpb, C(9) H is
observed as a singlet at 7.72 ppm, and C(12)H and C(13)H are observed as doublets at 7.53
and 6.99 ppm, respectively.
In hpbH, because of intramolecular proton transfer, keto-enol tautomerism exists between
–OH and thiazolic N, but in solution enol form predominates. Therefore, the peak at
12.5 ppm is assigned to –OH. The doublets at 7.68 and 7.1 ppm corresponds to C(10)H
and C(13)H. The C(12)H is a triplet at 6.94 ppm. The C(11)H resonates at the same
frequency as C(6)H, and the corresponding chemical shift is a quartet at 7.39 ppm.
2.5. Synthesis of the complexes
To a methanolic solution of Cu(NO3)2·3H2O (1 mmol, 0.241 g), a methanolic solution of
the corresponding substituted 2-phenylbenzothiazole (2 mmol, 0.482 g, 2-(4-methoxyphe-
nyl)benzothiazole, (mpb); 0.514 g, 2-(3-methoxy-4-hydroxyphenyl) benzothiazole, (mhpb);
0.454 g, 2-(2-hydroxyphenyl)benzothiazole, (hpbH) was added and refluxed for 15 min. To
the resulting solution, 1 mL of 10 M HCl was added, which on slow cooling yielded green
or yellow crystals suitable for X-ray diffraction. Schematic synthesis of [(mpbH)2CuCl4] is
given in scheme 2. The same product is obtained with CuCl2·2H2O under the same
conditions but with lower yield.
(mpbH)2CuCl4: Yield: 90.35%. Color: Dark green. Anal. Found (Calcd for
(C28H24Cl4CuN2O2S2, 689.95): C, 48.14 (48.74); H, 3.89 (3.50): N, 4.60 (4.06); S, 9.68
(9.29); Cu, 9.37(9.20).
(mhpbH)2CuCl4: Yield: 92.35%. Color: Yellow. Anal. Found (Calcd for (C28H24Cl4Cu-
N2O4S2, 721.98):C, 46.11 (46.57); H, 3.90 (3.35): N, 3.58 (3.88); S, 8.20 (8.88); Cu, 8.46
(8.80).
(hpbH)2CuCl4: Yield: 89.78%. Color: Yellow. Anal. Found (Calcd for (C26H20Cl4Cu-
N2O2S2, 661.93): C, 47.54 (47.17); H, 3.33 (3.04): N, 4.10 (4.23); S, 9.17 (9.68); Cu, 9.87
(9.60).
MeOH
4 + 2H+
2 C14H11NOS + Cu2+ + 4HCl
(C14H12NOS)2CuCl
Scheme 2. Synthesis of [(mpbH)2CuCl4].

6
A. Sarau Devi et al.
3. Results and discussion
The reaction of Cu(NO3)2·3H2O with substituted 2-phenylbenzothiazoles and HCl in
methanol gave bis(2-(substituted phenyl)benzothiazolium) tetrachlorocuprates. Dark green
crystals of (mpbH)2CuCl4, suitable for X-ray diffraction, crystallized spontaneously from
the solution on slow cooling. But (mhpbH)2CuCl4 and (hpbH)2CuCl4 precipitated as yellow
powders. All were soluble in chloroform and acetonitrile, but decompose in aqueous med-
ium. The Cu(II) in A2[CuCl4], is in a distorted tetrahedral coordination environment, which
is the common geometry of tetrachlorocuprate anions. The geometry usually varies from
distorted tetrahedral with D2d symmetry to centro-symmetric square planar with D4h
symmetry. Due to the presence of favorable intermolecular interactions and an active Jahn–
Teller effect in Cu(II), the CuCl24ꢀ species in (mpbH)2[CuCl4] has square planar geometry.
3.1. Crystal structure of (mpbH)2CuCl4
The ORTEP view of (mpbH)2CuCl4 with atom labeling scheme is shown in figure 2. The
X-ray crystal data and structure refinement parameters are given in table 1, and selected
bond distances and angles are listed in table 2. The crystal data are in agreement with the
elemental analysis data of the bulk sample. The unit cell packing of (mpbH)2CuCl4 is
shown in figure 3. The asymmetric unit of the crystal structure features half of CuCl24ꢀ and
one mpbH+. The complete structure can be depicted as encapsulation of discrete CuCl24ꢀ
species in organic layers arranged in zigzag fashion to a 3-D supramolecular network
structure (figure 4).
According to the Jahn–Teller theorem, CuCl24ꢀ deviates from regular tetrahedral geometry.
In (mpbH)2CuCl4, CuCl24ꢀ has a square planar coordination geometry formed by Cl1, Cl2,
Table 1. Crystal data and structure refinement parameters for (mpbH)2CuCl4.
Formula weight
Temperature (K)
Crystal system, space group
Unit cell dimensions
a (Ǻ)
689.95
293 (2)
Triclinic, p − 1
8.7117(3); α = 106.280(2)°
9.5207(3); β = 102.002(2)°
9.9947(6); γ = 109.152(2)°
709.39
1,1.615
1.3253
b (Ǻ)
c (Ǻ)
Volume (A3)
Z, DCalc (Mg/mm3)
Absorption coefficient (mm−1
F (000)
)
351
Crystal size (mm)
θ Range for data collection (°)
0.30 × 0.20 × 0.20 mm
2.25–33.42
Limiting indices
−12 ≤ h ≤ 13,
−14 ≤ k ≤ 14,
−15 ≤ l ≤ 15
Reflections collected/unique [Rint
Completeness to θ = 30.74 (%)
Absorption correction
]
19,891/5428 [0.0272]
99.9
Semi-empirical from equivalents
0.772 and 0.652
5428/1/183
Maximum and minimum transmission
Data/restraints/parameters
Goodness of fit on F2
1.039
Final R indices [I > 2σ (I)]
R indices (all data)
R1 = 0.0368, wR2 = 0.0947
R1 = 0.0563, wR2 = 0.1045
0.925 and −0.580
Largest difference in peak and hole (e A−3
)

Benzothiazolium tetrachlorocuprate salts
7
Table 2. Selected bond lengths (Å) and angles (°) for (mpbH)2CuCl4.
Bond lengths
Bond angles
C(1)–S(1)
C(6)–N(1)
C(7)–N(1)
C(7)–C(8)
C(7)–S(1)
Cl(1)–Cu(1)
Cl(2)–Cu(1)
1.7367(18)
1.384(2)
1.318(2)
C(7)–N(1)–C(6)
C(11)–O(1)–C(14)
C(7)–S(1)–C(1)
Cl(1)–Cu(1)–Cl(1)#1
Cl(1)–Cu(1)–Cl(2)
Cl(1)#1–Cu(1)–Cl(2)
Cl(2)–Cu(1)–Cl(2)#1
115.33(13)
118.80(17)
91.02(8)
180.0
90.369(18)
89.631(18)
180.0
1.439(2)
1.7141(15)
2.2374(5)
2.2783(5)
Note: Symmetry transformations used to generate equivalent atoms: #1 −x + 1,−y,−z + 1.
Table 3. Hydrogen bond geometry (Å and °) for (mpbH)2CuCl4.
D–H⋯A
d(D–H)
d(H⋯A)
d(D⋯A)
<(DHA)
C(2)–H(2)⋯Cl(1)#2
C(10)–H(10)⋯O(1)#3
N(1)–H(1 N)⋯Cl(1)#4
N(1)–H(1 N)⋯Cl(2)#5
0.93
0.93
0.825(9)
0.825(9)
2.79
2.60
2.75(2)
2.463(12)
3.482(2)
3.506(2)
3.2767(15)
3.2516(14)
131.9
166.3
123(2)
160(2)
Note: Symmetry transformations used to generate equivalent atoms: #1 −x + 1, −y, −z + 1; #2x, y + 1, z; #3 −x + 2,
−y, −z; #4 −x + 1, −y, −z; #5 x, y, z − 1.
Cl1’, and Cl2’. This type of distortion is to accommodate a symmetrical structure by
multiple non-covalent associations [38, 39].
Discrete unit of CuCl24ꢀ binds with monomeric [mpbH]+ through hydrogen bonds (table
3) which include asymmetric bifurcated hydrogen bonds [20, 40] lying on either side of the
anion, constituting classical hydrogen bonds, N(1)–H(1)⋯Cl(2) with bond length of N(1)
⋯Cl(2) = 3.2516(14) Å, bond angle N(1)–H(1)⋯Cl(2) = 160(2)°; and N(1)–H(1)⋯Cl(1),
with bond length of N(1)⋯Cl(1) = 3.2767(15) Å and a lower bond angle, N(1)–H(1)⋯Cl(1)
= 123(2)o. There exists a non-classical hydrogen bond C9–H9⋯Cl2, with bond length of
C9⋯Cl2 = 3.689 Å on either side of CuCl24ꢀ are comparable with tetrachlorocuprate com-
plexes and all these interactions presumably help provide 3-D stability [24, 41]. Bifurcating
short contacts N1–Cl1 (3.277 Å) and N1–Cl2 (3.251 Å) originating from thiazolium nitro-
gen, N1, further stabilize the anion in the cavity. The two trans Cl1 also hold two other
mpbH+ units in trans positions by non-classical hydrogen bonds C2-H2⋯Cl1, with bond
length of C(2)⋯Cl(1) = 3.482(2) Å and by a short contact involving thiazolium S atom,
Cl1⋯S (3.353 Å) [figure 5(a)]. Thus, the crystal architecture is maintained by classical and
non-classical hydrogen bonds involving the halogens, and the short contacts [20].
The substituted 2-phenylbenzothiazole is essentially planar. In the thiazole ring of ben-
zothiazole, formed by the condensation of the aldehydic group in anisaldehyde with an
amino and thiol of o-aminothiophenol, the C(7)–N(1) bond length of 1.318(2) Ǻ is shorter
than the C(1)–S(1) and C(7)–S(1) of 1.7367(18) Ǻ and 1.7141(15) Ǻ in the aromatic sys-
tem. The other bond lengths are intermediate between ideal values of corresponding single
and double bonds giving evidence for extended π-delocalization throughout the entire mole-
cule [42] (figure 2). The orientation of methoxy group is cis to S(1) of the thiazole ring.
Each organic layer contains mpbH+ monomers held together in a zigzag manner by non-
classical hydrogen bonds and dihydrogen bonds [43] [figure 5(b)]. The mpbH+ monomer
units join end-to-end by parallel non-classical hydrogen bonding of the type C(10)–H(10)
⋯O(1) with C(10)⋯O(1) bond length of 3.506(2) Å and also by a parallel dihydrogen
interaction C(5)–H(5)⋯H(4)–C(4) with C(5)⋯C(4) distance of 3.911 Å, arranged in a nearly

8
A. Sarau Devi et al.
Figure 2. ORTEP diagram of (mpbH)2CuCl4 showing the atom numbering scheme showing interatomic distances
(Å) and important angles (°).
planar supramolecular chain structure. These organic chains, oriented antiparallel, instead of
parallel stacking [40], along the b-cell direction as ABAB layers, undergo face-to-face
stacked orientation [12, 44, 45] with a mpbH+ centroid–centroid distance of 3.784 Å, which
is less than the van der Waal radii of 3.8 Å, forming a supramolecular 3-D structure [figure
5(c)]. Thus, the compound possesses layer structure, with organic cation layers made up of
substituted 2-phenyl benzothiazolium and anion layers constituting CuCl24ꢀ, where the anion
units are not interconnected by bridging chlorides, Cl⋯Cl, but in some cases, their intercon-
nection leads to unique properties [13, 15]. These inorganic and organic layers are intercon-
nected by ionic bonds, hydrogen bonds of N–H⋯Cl type, and the short contacts.
3.2. Infrared spectra
The IR spectrum of mpb, mhpb, and hpbH show close similarity with the spectrum of
their respective complexes; the spectrum of [(mpbH)2CuCl4] compared with the
corresponding ligand is shown in figure 6. No significant variation in frequency is noticed
for C=N and C–S of benzothiazole moiety and O–CH3 or OH substituent of phenyl ring
in the complexes. Some small changes in intensity/frequency occur due to chemical
environment changes of NH+ after they assemble into 3-D networks. The ν(C=N) is at
1597–1588 cm−1 in all complexes. The ν(C–S) stretch of thiazole ring appears between

Benzothiazolium tetrachlorocuprate salts
9
Figure 3. Unit cell packing diagram of (mpbH)2CuCl4.
Figure 4. Supramolecular network structure of (mpbH)2CuCl4. The CuCl24ꢀ units are encapsulated between
antiparallel layers of mpbH+ moieties, which in turn linked by pi–pi stacking interactions.
720 and 685 cm−1 in all complexes. These observations suggest non participation of ring
N or S in coordination. In the hybrid complexes of mhpb and mpb, O–CH3 stretching
bands are found at 1022–1009 cm−1 and bending bands between 440 and 426 cm−1.
Bands at 1308–962 cm−1 correspond to CH– in plane deformations while out of plane
deformations are observed at 860–726 cm−1. These observations indicate that mpb, mhpb,
and hpbH do not coordinate with copper. A new broad band between 3400 and
3300 cm−1 indicates the presence of N–H, confirming protonation of benzothiazole ring

10
A. Sarau Devi et al.
Figure 5. (a) Interactions between CuCl24ꢀ and mpbH+, (b) Zigzag organic layers connected by short contacts,
and (c) Antiparallel cationic ABAB layer structure viewed along the b axis.
nitrogen on acidification, thereby forming benzothiazolium cation. For example, in
(mpbH)2CuCl24ꢀ, the IR spectrum of “mpb” does not have a peak at 3000–3500 cm−1, but
after complexation, it gives a strong peak around 3400 cm−1 due to the asymmetric/sym-
metric N–H stretching vibration of benzothiazolium salt (N–H+). The N–H+ of benzothia-
2−
zolium cation subsequently forms hydrogen bonds with Cl− of MCl4
leading to
formation of complexes. The broad band around 3400 cm−1 also accounts for N–H⋯Cl
hydrogen bonds, the red shift and broadness can be achieved by weakening and subse-
quent lengthening of the N–H bond by polar nature and by hydrogen bonding [46, 47].
In the case of hpbH and mhpb, very broad bands from 3500 to 2900 cm−1 are observed
due to overlapping of O–H vibrations with the N–H+ vibrations of thiazolium cation. In
accord with the spectral observations, the crystal structure of (mpbH)2CuCl4 shows the
existence of bifurcated hydrogen bonds from N–H+ to Cl−.

Benzothiazolium tetrachlorocuprate salts
11
Figure 6. FT-IR spectra of (a) mpb (KBr pellet) and (b) [(mpbH)2CuCl4] (KBr pellet).
3.3. Electronic spectra
The spectral assignments recorded in acetonitrile are given in table 4. For a square planar
Cu(II) complex with a dx2ꢀy2 ground state, the possible transitions, dx2ꢀy2 → dxy, dx2ꢀy2
→
dz2 and dx2ꢀy2 → dxz, dyz (2B2g ← B1g,2A1g ← B1g and Eg ← B1g), cannot be distin-
guished by their energy as the four d orbitals lie very close together [48]. In conformity
with this, the electronic reflectance spectrum of Cu(II) compounds in the solid state, show
poorly resolved bands at 580–780 nm, but the electronic spectrum of Cu(II) complexes in
acetonitrile, the transitions in high energy region are resolved. Intense transitions in the
higher energy region are due to ligand-to-metal and metal-to-ligand charge transfer transi-
tions. Intensities of these transitions are proportional to the overlap of donor and acceptor
orbitals involved in charge transfer process. Therefore, strong band around 350 (ε = 2576)
nm is assigned as Cl → Cu LMCT transitions [49] and a weak broad band with λmax around
440–476 (ε = 407) nm corresponds to Cu → Cl MLCT and intra-ligand CT transitions. The
UV portion of the spectrum is characterized by intense π–π* transitions of substituted ben-
zothiazole at 256 nm (ε = 1806) and 303 nm (ε = 1736) [50, 51]. The UV portion of the
spectrum is characterized by intense π–π* transitions of substituted benzothiazole ligand.
The dissolution of (mpbH)2CuCl4 in acetonitrile is also accompanied by a change in the
green color of the solid complex to yellow. This may be due to a change in the geometry of
solid square planar CuCl24ꢀ, to commonly observed distorted tetrahedral geometry in
solution [52]. The appearance of weak peaks at 1030 (ε = 385) nm and 1265 (ε = 100) nm
2
2
2
2
corresponding to 2B1
←
2B2, 2E ← 2B2 transitions of D2d geometry support this

12
A. Sarau Devi et al.
Figure 7. Mass spectrum of (mpbH)2CuCl4.
Table 4. Electronic spectral assignments (cm−1) of CuCl42ꢀ complexes of benzothiazoles.
Compounds
π → π*
n → π*
LMCT
d–d
(mpbH)2CuCl4
(mhpbH)2CuCl4
(hpbH2)2CuCl4
255
262
212
295
291
286
357
345
340
1030, 1265
960
995
observation. Weak d-d bands seen in the near-IR to red part of the visible spectrum around
2ꢀ
960 and 995 nm point toward D2d geometry for ½CuCl4ꢁ
in (mhpbH)2CuCl4 and
(hpbH)2CuCl4.
3.4. Mass spectral studies
The mass spectra of the complexes have been recorded in the solid state by DART mass
spectrometer using TOF mass analyzer. The analysis was performed in the DART beam of
excited helium atoms, which involves predominantly the formation of ionized water cluster
followed by proton transfer reaction [53].
The molecular ion peak of (mpbH)2CuCl4 is observed at m/z 690.92 due to [MH+] with a
relative intensity of 5%. The base peak at m/z 639.30 is due to [(M–(CH3)HCl)H+]
(figure 7).
Fragmentation of the complex results in a peak at m/z 617.27 due to [(M–H2Cl2)H+] with
a relative intensity of 65%. A peak at m/z 242.28 with an intensity of 75% is attributed to
[(C14H11NOS)H+] and a less intense peak at m/z 243.28 due to [(C14H12NOS)H+] [54].

Benzothiazolium tetrachlorocuprate salts
13
Table 5. Thermal decomposition data for CuCl24ꢀ complexes of benzothiazoles.
Compound
Step
ΔT (°C)
Mass loss % found (Calcd)
Probable species
2HCl
(mpbH)2CuCl4
1
2
3
1
2
3
4
1
2
3
20–260
260–348
348–
20–130
130–260
260–330
330–
20–270
270–360
360–
10.65 (10.57)
56.23 (56.45)
33.2
5.13 (5.04)
15.15 (15.00)
48.65 (48.40)
31.07
10.65 (11.01)
55.09 (54.61)
34.26
2C6H4-OCH3 + 2Cl− + 2C4H4
(mhpbH)2CuCl4
(hpbH2)2CuCl4
HCl
HCl + 2Cl−
2{C6H3-OH(OCH3)} + C4H4
2HCl
2C6H4-OH + 2Cl− + 2C4H4
The molecular ion peaks of (mhpbH)2CuCl4 and (hpbH)2CuCl4 are observed at m/z
722.98 and m/z 573.02 due to [MH+] with a relative intensity of 6 and 8%, respectively.
For (mhpbH)2CuCl4, the base peak at m/z 603.4206 is due to [(M–(OCH3)HCl(Cl)(OH))
H+], and for (hpbH)2CuCl4, it is at m/z 573, due to [(M–2HCl(OH))H+]. A peak at m/z
258.16 with an intensity of 66% is attributed to the ligand of (mhpbH)2CuCl4 while the
same for (hpbH)2CuCl4 is at m/z 228 with relative intensity of 58%. These observations of
fragments in the mass spectrum support the proposed structures of complexes.
3.5. TG analysis
(mpbH)2CuCl4 exhibits thermochromism, a reversible dependence of color on temperature
[55]. In A2[MX4] compounds, the thermochromic color change is due to geometry change
of CuCl24ꢀ by heating and is reported that an orange–yellow–green color progression occurs
as the anion distortion progresses from tetrahedral to square planar [23, 24]. This change is
assisted with a change in hydrogen bonding network pattern that influences the degree of
deformation of the metal halogen polyhedron, which is manifested in the change of the
absorption spectra [22, 39]. In (mpbH)2CuCl4, the thermochromic transition is observed at
180 °C, above this temperature the green color changes to yellow, but on cooling, reverts
back to green. In the green phase, the mpbH+ units participate in formation of a 3-D net-
work of strong H-bonds that stabilize the square planar coordination of the copper ion, but
during thermochromic transition, the N–H⋯Cl hydrogen bond weakens result in disordering
of benzothiazolium cations. This factor increases electrostatic repulsion between chlorides
in CuCl24ꢀ forcing an increased distortion toward tetrahedral geometry. The green-to-yellow
thermochromic transition corresponds to a D4h → D2d distortion of the CuCl24ꢀ ion due to a
change in its trans Cl–Cu–Cl angle, thereby changing the hydrogen bonding network of the
entire crystal [23, 24].
Thermogravimetric experiments in nitrogen at a heating rate of 10 °C min−1 were per-
formed to explore the thermal stabilities of the complexes. The thermal decomposition data
are given in table 5.
Calculations based on the mass loss confirmed the stoichiometry of two mpbH+/mhpbH+/
hpbH+/ units per metal ion in (mpbH)2CuCl4, (mhpbH)2CuCl4, and (hpbH)2CuCl4. Crystal
data show that the nitrogen of thiazole ring in mpb is protonated and chloride is present as
a counter ion around Cu(II). The first stage of decomposition in (mpbH)2CuCl4 therefore
corresponds to the loss of two hydrogen chloride molecules between 20 and 260 °C. For

14
A. Sarau Devi et al.
0.00
12
10
8
-0.01
-0.02
-0.03
-0.04
-0.05
-0.06
-0.07
-0.08
-0.09
-0.10
DTG
6
4
TG
2
100
200
300
400
500
600
700
800
TemperatureoC
Figure 8. TG and DTG curves of (mpbH)2CuCl4.
Figure 9. EPR spectrum of (mpbH)2CuCl4 in polycrystalline state at 298 K.
(mpbH)2CuCl4, the percentage of hydrogen chloride calculated (10.57%) is in agreement
with the value found (10.65%). A mass loss of 56.23% between 260° and 348° corresponds
to simultaneous loss of the methoxyphenyl moiety and two chlorides along with partial
decomposition of benzothiazole (calculated 56.45%) in a single step. Above 348 °C, the
TGA curve shows continuous thermal decomposition and does not stop until heating ends
at 800 °C (figure 8). Thermogram of (mhpbH)2CuCl4 shows four stage decomposition. In
the first stage between 20 and 130 °C, the compound loses one HCl, and between 130 and
260 °C, it loses (HCl + 2Cl) followed by loss of 2OCH3 and partial decomposition of
benzothiazole moiety between 260 and 330 °C. Thereafter, it shows continuous
thermal decomposition until heating ends at 800 °C. The first stage decomposition in
(hpbH)2CuCl4 corresponds to loss of two HCl molecules followed by loss of 2Cl− ions and
partial loss of benzothiazole moiety corresponding to a mass loss of 55.09% (calculated
54.61%).

Benzothiazolium tetrachlorocuprate salts
15
Figure 10. EPR spectrum of (mpbH)2CuCl4 in frozen acetonitrile at 77 K.
3.6. EPR spectra
EPR spectroscopy is a powerful tool to infer the details of complexes formed by paramag-
netic metal ions. Cu(II), with a d9 configuration, has an effective spin of S = 1/2 and is
associated with a spin angular momentum ms = 1/2, leading to a doubly degenerate spin
state in the absence of a magnetic field. In a magnetic field, this degeneracy is lifted and the
energy difference between these states is given by E = hν = gβH where h is the Planck’s
constant, ν is the frequency, g is Landé splitting factor (equal to 2.0023 for a free electron),
β is the Bohr magneton, and H is the magnetic field. The EPR spectra of polycrystalline
samples at 298 K and in frozen acetonitrile in 77 K were recorded in the X-band, using
100 kHz field modulation, and g factors were quoted relative to the standard marker TCNE
(g = 2.0023).
The X-band EPR spectrum of (mpbH)2CuCl4 (figure 9) and (mhpbH)2CuCl4 in the poly-
crystalline state at room temperature shows only one broad signal at g = 2.189 and 2.213,
respectively, with no absorption at half-field region indicating the existence of a mononu-
clear species [56]. Such isotropic spectrum, consisting of a broad signal gives no informa-
tion on the electronic ground state of the Cu(II) ion present in the complex. The spectrum
of (mpbH)2CuCl4 in acetonitrile solution gave non-axial spectra with three g values,
g1 = 2.23, g2 = 2.08, and g3 = 2.05, indicating loss of symmetry of the square planar
2ꢀ
½CuCl4ꢁ unit in solution (figure 10). The three g values with g1 > g2 > g3 > 2.0023 and
(g2 − g3)/(g1 − g2) < 1, suggest a distorted tetrahedral geometry for the complex in solution
[57]. (mhpbH)2CuCl4 in frozen acetonitrile solution at 77 K gave an axial spectrum with
well defined g|| and g┴ values of 2.21 and 2.034, respectively, corresponding to values of
distorted tetrahedral geometry [14]. Similar observation for (hpbH)2CuCl4 in solution,
2ꢀ
g
║ = 2.21, g┴ = 2.034, is also consistent with tetrahedral ½CuCl4ꢁ [58]. Thus, all Cu(II)

16
A. Sarau Devi et al.
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0
20
40
60
80
100
120
140
160
T/K
Figure 11. Temperature (T/K) dependences of magnetic susceptibilities (χ/emu mol−1) measured at H = 4000 Oe
between 20 and 150 K for (mpbH)2CuCl4.
compounds have D2d geometry in solution. These observations are in agreement with the
electronic spectral data and magnetic behavior of the compounds examined.
3.7. Magnetic properties
Magnetic susceptibilities (χ) of the compounds were measured on a powdered sample from
20 to 300 K at 4000 Oe with a 7410 Lake Shore vibrating sample magnetometer. The tem-
perature (T/K) dependences of magnetic susceptibilities (χ/emu mol−1) measured at
H = 4000 Oe between 20 and 150 K for (mpbH)2CuCl24ꢀ is shown in figure 11 [14, 59]. The
effective magnetic moment (μeff) values calculated from the equation μeff = 2.828 (χ × T)1/2
for (mpbH)2CuCl4 and (mhpbH)2CuCl4 at room temperature, 1.73 and 1.93 BM,
respectively, are normal values for a complex with one unpaired electron. Samples exhibit
Curie law behavior from 20 to 150 K, but above this range, there is a deviation from Curie’s
law. For (hpbH)2CuCl4, μeff of 1.65 BM is within experimental error.
3.8. Electrical conductance
The solid-state conductivity (σ) of (mpbH)2CuCl4 at room temperature is 1.99 × 10−4 S cm−1
indicating its semi-conducting nature [16]. The value was checked by repeating of the
measurements.
4. Conclusion
We report three new ionic complexes of CuCl24ꢀ, with benzothiazolium as cation. They
were characterized by X-ray structure analysis, EA, IR, UV, MS, TG, and EPR. The dark
green thermochromic crystals, (mpbH)2CuCl4, spontaneously crystallized during synthesis,

Benzothiazolium tetrachlorocuprate salts
17
is a semiconducting material at room temperature. All these compounds show paramagnetic
behavior. CuCl24ꢀ displays tetrahedral coordination in (mhpbH)2CuCl4 and (hpbH)2CuCl4
while (mpbH)2 CuCl4 is square planar. Multiple non-covalent interactions like bifurcated
hydrogen bonds of the type N–H⋯Cl and weak interactions like C–H⋯Cl and C–H⋯O
along with π–anion and π–π stacking interactions hold CuCl24ꢀ and mpbH+ together to
maintain the square planar structure of CuCl24ꢀ in (mpbH)2CuCl4. These weak bonds are
helpful in forming a 3-D supramolecular framework, while stability is attributed by inter-
play of strong ionic force with weak interactions. All of the compounds show high thermal
stabilities.
Supplementary material
Crystallographic data for the structural analysis has been deposited with the Cambridge
Crystallographic Data Center, CCDC 809459 for compound [(mpbH)2CuCl4]. The data can
be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from Cambridge
Crystallographic Data Center (CCDC), 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44 0 1223-336033; e-mail: deposit@ccdc.cam.ac.uk].
Acknowledgments
We are grateful to Dr Babu Varghese, SAIF, IIT, Chennai, India, for single crystal X-ray
Diffraction studies, IIT Bombay, India, for EPR spectrum and CDRI, Lucknow, India, for
DART Mass spectral data.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the University Grants Commission, New Delhi, India.
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