A facile synthesis, crystallographic, spectral, thermal, and electrochemical investigations of neutral [Cu2(Et2dtc) 4] dimer

Article abstract of DOI:10.1080/15533174.2013.776596

A neutral dimeric complex [Cu₂(Et₂dtc)₄] (dtc = dithiocarbamate) (1) was synthesized in single step under mild conditions. Structure of 1 has been studied using single-crystal XRD to gauge the influence of CH₃ and CH₂ groups of coordinated dithiocarbamate ligands on the association of the molecules in solid state. Evidently 1 features the ability of these groups to form intermolecular (sp ³)CHΠ(chelate ring) and (sp³)CHS stacking interactions leading to fascinating 3D infinite network. Complex has been completely characterized by spectrophotometric methods, optical behavior, and thermal and cyclic voltammetry studies. Cyclic voltammetric study confirmed the binding affinity of 1 toward H₂PO^-₄. Supplemental materials are available for this article. Go to the publisher's online edition of Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry to view the supplemental file. Copyright Taylor & Francis Group, LLC.

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A Facile Synthesis, Crystallographic, Spectral,
Thermal, and Electrochemical Investigations of Neutral
[Cu2(Et2dtc)4] Dimer
Sanjay K. Verma a , Rahul Kadu a & Vinay K. Singh a
a Department of Chemistry , Faculty of Science, The M. S. University of Baroda , Vadodara ,
India
Accepted author version posted online: 22 May 2013.Published online: 06 Nov 2013.
To cite this article: Sanjay K. Verma , Rahul Kadu & Vinay K. Singh (2014) A Facile Synthesis, Crystallographic, Spectral,
Thermal, and Electrochemical Investigations of Neutral [Cu2(Et2dtc)4] Dimer, Synthesis and Reactivity in Inorganic, Metal-
Organic, and Nano-Metal Chemistry, 44:3, 441-448, DOI: 10.1080/15533174.2013.776596
To link to this article: http://dx.doi.org/10.1080/15533174.2013.776596
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:441–448, 2014
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.776596
A Facile Synthesis, Crystallographic, Spectral, Thermal,
and Electrochemical Investigations of Neutral
[Cu2(Et2dtc)4] Dimer
Sanjay K. Verma, Rahul Kadu, and Vinay K. Singh
Department of Chemistry, Faculty of Science, The M. S. University of Baroda, Vadodara, India
atoms associated with the aromatic moieties are generally more
. . .
amenable for C H ꢀ interactions than the hydrogen atoms as-
A neutral dimeric complex [Cu2(Et2dtc)4] (dtc = dithiocarba-
mate) (1) was synthesized in single step under mild conditions.
Structure of 1 has been studied using single-crystal XRD to gauge
the influence of CH3 and CH2 groups of coordinated dithiocar-
bamate ligands on the association of the molecules in solid state.
Evidently 1 features the ability of these groups to form intermolec-
sociated with a sp3 carbon atoms.[5] However, a recent investiga-
tion[4] showed that the directionality and strength of interactions
involving (sp3)C H group are more significant in terms of the
access angle and the shorter distance between the hydrogen atom
. . .
3
3
. . .
. . .
and acceptor. The existence of C H S interactions is less com-
ular (sp )C H ꢀ(chelate ring) and (sp )C H S stacking in-
teractions leading to fascinating 3D infinite network. Complex has
been completely characterized by spectrophotometric methods, op-
tical behavior, and thermal and cyclic voltammetry studies. Cyclic
voltammetric study confirmed the binding affinity of 1 toward
mon[7] than C H O interactions but may be found in many
. . .
molecular crystals with fascinating properties such as electri-
cal conductivity or superconductivity.[8] Recently, metal-chelate
rings of suitable 1,1-dithiolate metal complexes were reported to
act as a C H acceptors. Thus certain complexes were apparently
–
H2PO4 .
sustained by C H ꢀ(chelate ring) interactions.[9] Since “met-
Supplemental materials are available for this article. Go to the
publisher’s online edition of Synthesis and Reactivity in Inorganic,
Metal-Organic, and Nano-Metal Chemistry to view the supplemen-
tal file.
. . .
alloaromaticity” was discovered in 1945,[10] a large number of
transition metal chelate rings feature this metalloaromaticity.[11]
. . .
However, the existence of C H ꢀ(chelate ring) interaction in
both coordination and organometallic compounds is very lim-
ited and definition of structural parameters for such interactions
to occur is still being explored. The hydrogen bond energy
Keywords cyclic voltammetric study, fluorescence, optical band gap,
3
3
. . .
. . .
S
(sp )C H ꢀ(chelate ring) interaction, (sp )C H
interaction, TGA/DTA, UV-visible
for C H ꢀ(chelate ring) interactions, 6–11 kJ mol–1,[12] was
. . .
. . .
found to be comparable with C H ꢀ(organic) interactions,
1–8 kJ mol–1.[13] Thus the reliability on the C H ꢀ(chelate
. . .
INTRODUCTION
There is a growing interest in identifying the nature of non- ring) synthons certainly provides a new directing tool of molec-
conventional intermolecular interactions,[1] because of their in- ular assembly for coordination and organo-metallic compounds.
creasing demand in the situations where conventional hydro-
On the other hand, metal complexes with sulfur rich lig-
gen bonding does not extend in three dimensions or does not ands are very interesting due to their involvement in the growth
exist due to lack of appropriate donors and acceptors. Interac- of thin layers of metal sulfides, by metal–organic chemical va-
[2]
[3–5]
and ꢀ ꢀ,[6] are por deposition techniques, electrochemical properties, electrical
. . .
. . .
. . .
tions of the type C H O, C H ꢀ,
well established in the crystal engineering where the hydrogen properties, and biological properties. In particular, the intro-
duction of metal dithiocarbamate complexes bearing a direct
metal–sulfur bond, as a single source precursor, has proven to
be a very efficient and an alternative route for the synthesis of
Received 21 June 2012; accepted 10 February 2013.
Mr. Verma and Dr. Singh acknowledge CSIR, New Delhi for finan-
high-quality semiconductor nanoparticles.[14] Alternatively, the
metal dithiocarbamate complexes have been employed as anion
cial support (Project No. 01/(2204)/07/EMR-II). Mr. Kadu acknowl-
binding motifs.[15]
edges UGC, New Delhi for the fellowship. The authors are thankful to
the CSMCRI Bhavnagar for the single crystal and Mr. Shardul Bhatt for
Cyclic voltammetric analysis. Professor BVK is greatly acknowledged
for providing laboratory facility and his consistent support.
Address correspondence to V. K. Singh, Department of Chemistry,
Faculty of Science, The M. S. University of Baroda, Vadodara-390 002,
India. E-mail: vks.msu@gmail.com
Although the structural investigations on copper(II) bis(N,N-
diethyldithiocarbamate) complex were previously made by
Bonamico[16] and Jian’s research group,[17] however, we report
herein an efficient single-step synthesis of copper(II) bis(N,N-
diethyldithiocarbamate) complex dimer [Cu2(Et2dtc)4] 1, which
441

442
S. K. VERMA ET AL.
is characterized by microanalysis, IR, UV-visible, fluorescence, several times by a saturated aqueous solution of NaHCO3, fol-
optical, TGA/DTA, cyclic voltammetric, and single-crystal X- lowed by distilled H2O and hexane respectively. Finally, the
ray diffraction techniques. The emphasis has been made on residue was dried in vacuum to yield the product 1 quantita-
. . .
. . .
evaluation of weak CH ꢀ(chelate ring) and CH S inter- tively, Yield: 0.356 g (99%, based on Cu(OAc)2.H2O); m. p.
molecular interactions and the role of CH3 and CH2 groups of 200.7◦C; Anal. Calcd. (%) for C20H40Cu2N4S8: C, 33.32; H,
coordinated N,N-diethyldithiocarbamate ligands on the associ- 5.55; N, 7.78, S, 35.55. Found (%): C, 33.43; H, 5.60; N, 7.83,
ation of the molecules in solid state.
S, 35.65. IR (KBr disc, cm–1): 2981s, 2928s (ν C H), 1513s (ν
C−N), 997s, 913s (ν C−S) stretching frequency, respectively.
UV-vis (DMSO): 270 nm (π→π∗), 287 nm (n→π∗), 438 nm
(CT→M), 636 nm (d–d) transitions, respectively.
EXPERIMENTAL
Materials and Measurement
X-Ray Crystallography
All solvents were purchased from the commercial sources
and were freshly distilled prior to use. Reagents such as
Cu(OAc)2.H2O, and CS2 were purchased from Merck and these
were used without further purification. Elemental analyses (C,
H, N) were performed on Perkin-Elmer 2400 analyzer. FT-IR
(KBr pellets) spectrum was recorded in the 4000–400 cm–1
range using a Perkin-Elmer FT-IR spectrometer. UV-visible
spectra in DMSO solution were recorded on a Perkin Elmer
Lambda 35 UV-vis spectrometer and the optical characteri-
zation in solid state was performed by UV-visible transmit-
tance measurements. Florescence was recorded on JASCO make
specrofluorometer model FP-6300. Thermal stability analysis
was performed on a SII TG/DTA 6300 in flowing N2 with
a heating rate of 10◦C min−1. Electrochemical measurements
were performed on a CH Instruments 600C potentiostat, using a
Pt disk as the working electrode, Ag/AgCl as the reference elec-
trode and a Pt wire as the counter electrode. Cyclic voltammetric
study was carried out using 1.0 mM solution of the complex 1
in 0.1 M tetra-n-butylammonium tetrafluoroborate (Bu4NBF4)
Single crystals of 1 suitable for single-crystal X-ray diffrac-
tion study were grown by slow evaporation of acetone solu-
tion. Intensity data were collected using Mo-Kα X-ray (λ =
0.71073 Å) radiation on Bruker SMART APEX diffractometer
equipped with CCD area detector at 110 (2) K. The data inte-
gration and reduction were processed with SAINT software.[18]
An empirical absorption correction was applied to the collected
reflections with SADABS software programs.[19] The structure
was solved by direct methods using SHELXTL,[20] and refined
on F2 by the full-matrix least-squares technique using SHELXL-
97 program package.[21] All non-hydrogen atoms were refined
anisotropically until convergence was reached. Crystallographic
data and structure refinement parameters for 1 are given in
Table 1.
RESULT AND DISCUSSION
Synthesis
in acetonitrile with a scan rate of 50 mVs−1
.
The alkali metal salts of dialkyl dithiocarbamates are known
to react with different transition/non-transition metal (II) salts
such as M(ClO4)2 or M(OAc)2 at high temperature to yield
the products of the type [M(R2dtc)2]. Using the same strategy,
Synthesis of 1
Synthesis of 1 was achieved as per Scheme 1.
The diethyldithiocarbamate ligand required for the prepara- a polymorph of the titled compound was previously prepared
tion of 1 was synthesized in situ by reacting Et2NH (0.154 g, in two steps and studied by Jian and co-workers[17] in 1999.
2.1 mmol) with CS2 (0.24 g, 3.15 mmol) in 10 mL of Et3N sol- However, we have prepared the complex 1 in a novel single-
vent for 30 min at room temperature. Cu(OAc)2.H2O (0.20 g, step procedure under mild conditions. The synthetic strategy
1.0 mmol) dissolved in minimum amount of distilled water was used herein is found to be superior as other suffers from dis-
added to this reaction mixture followed by continuous stirring advantages such as low yields, lengthy sequences, and drastic
for 2 h. A change in color from yellow to brown was observed. reaction conditions. The complex has been completely charac-
The solvent was removed in vacuum and the residue was washed terized by spectrophotometric methods (UV and fluorescence),
H2
H2
S
S
S
S
CH3
CH3
C
H3C
H3C
C
N
N
H2
C
Cu
H2
C
S
S
H3C
H3C
H3C
H3C
C
CS2, Et3N
Cu(OAc)2
2 h
C
H2
H2
N
C
NH
H2
C
H2
C
30 min
S
S
S
S
CH3
CH3
H3C
C
.
Et3NH
C
H2
H2
N
N
Cu
C
H3C
C
prepared in situ
H2
H2
SCH. 1. Synthesis of copper(II) bis(diethyldithiocarbamate) dimer.

CRYSTAL, SPECTRAL, THERMAL, CV OF Cu(II) COMPLEX
443
TABLE 1
Crystallographic data and structure refinement parameters for 1
Empirical formula
Formula weight
C20H40Cu2N4S8
720.12
Crystal system
Space group
monoclinic
P21/n
a (Å)
9.6805(6)
b (Å)
10.5504(7)
15.5083(10)
90
101.66
90
1551.23(17), 4
1.542
1.927
c (Å)
α (◦)
β (◦)
γ (◦)
V (Å3), Z
Calculated density (mg m−3
)
Absorption coefficient (mm−1
F(000)
)
FIG. 1. ORTEP diagram of the complex 1 with atom numbering scheme (40%
probability factor for the thermal ellipsoids). (color figure available online).
748
θ range for data collection (◦)
Index ranges
2.29 − 28.24
−9 ≤ h ≤ 12,
−14 ≤ k ≤ 9,
−17 ≤ l ≤ 19
8873/ 3608 [Rint = 0.0229]
3608 / 0 / 234
1.081
S(1)−Cu(1)−S(2), 76.679(14)◦, is slightly shorter than simi-
lar bond angle 77.00(3)◦, and S(3)−Cu(1)−S(4), 77.124(14)◦,
◦
. . .
is slightly longer than 76.59(3) ; the Cu Cu separation of
3.500 Å in 1 is significantly shorter than 3.572(1) Å and the
Cu(1)−S(1)#−Cu(1)−S(1)#1 plane is comparatively more or-
thogonal to the S(1)−S(2)−S(3)−S(4) plane with a dihedral
angle of 88.77◦.
Reflections collected/ unique
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I > 2σ (I)]
R1 = 0.0225, wR2 = 0.0631
R1 = 0.0240, wR2 = 0.0637
0.409/−0.272
R indices (all data)
The unprecedented feature of 1 revealed by single-crystal
Largest diff. peak/hole (e A−3
)
X-ray diffraction study is the intermolecular stacking of 1 into
3
. . .
a fascinating 3D motif directed by novel (sp )C H ꢀ(chelate
3
. . .
ring) and (sp )C H S interactions (Figure 2). Overall, the
structural parameters for these interactions are in general accord
with the literature,[9] although at higher level of precision to
support inter molecular stacking of 1 as depicted by black dotted
lines in Figures 3 and 4.
optical behavior, and thermal and cyclic voltammetry studies.
In the crystallographic study, the emphasis has been made in
the search of weak interactions, which includes a comparative
study with the Jian’s complex.
In fact, in the centro-symmetric unit, there is one indepen-
dent molecule which provides two acceptor {ꢀ(chelate ring)}
Description of the Structure
3
3
. . .
and two donor {(sp )C H} sites to form (sp )C H ꢀ(chelate
The copper(II) bis(diethyldithiocarbamate) dimer 1 is built
up of centro-symmetric dimeric entity in which both copper(II)
atoms are bridged by S(1) atoms of trans chelate rings (CuS2C).
Each copper(II) atom acquires distorted square-pyramid stereo-
chemistry. An ORTEP diagram and selected bond distances (Å)
and bond angles (◦) of 1 are shown in Figure 1 and Table 2,
respectively.
ring) interactions with four neighboring molecules. Based on
C8–H8C ꢀ(chelate ring) interactions, the crystal packing of
. . .
1 appears similar to the crystal packing of earlier structure,
TABLE 2
Selected bond lengths (Å) and bond angles (◦) for 1
We have retrieved the crystal data (CCDC no. 111991) of
previously reported polymorph[17] of complex 1 and have com-
pared the data as well as crystal packing patterns. In addi-
tion to significant differences in the cell parameters, selected
bond distances and bond angles, there is a considerable differ-
ence in the intermolecular stacking interactions, which is elab-
orated in our subsequent discussions. The observed differences
in selected bond distances and bond angles of 1 with reported
structure can be summarized as the axial Cu−S bond distance
of 2.7743(4) Å in 1 is significantly shorter than 2.844(1) Å,
whereas the equatorial bond distance of Cu−S(1), 2.3369(4)
Å, is slightly longer than 2.3270(9) Å; the bond angles of
Bond lengths (Å)
Bond angles (◦)
Cu(1)-S(2) 2.3080 (4) S(2)-Cu(1)-S(4)
160.385 (18)
101.715 (15)
77.124 (14)
76.679 (14)
101.592 (15)
171.704 (16)
Cu(1)-S(4)
Cu(1)-S(3)
Cu(1)-S(1)
2.3109 (4) S(2)-Cu(1)-S(3)
2.3233 (4) S(4)-Cu(1)-S(3)
2.3369 (4) S(2)-Cu(1)-S(1)
Cu(1)-S(1)#1 2.7743 (4) S(4)-Cu(1)-S(1)
S(1)-C(1)
S(2)-C(1)
S(3)-C(6)
S(4)-C(6)
1.7301 (15) S(3)-Cu(1)-S(1)
1.7178 (15) S(1)-Cu(1)-S(1)#1 94.023 (13)
1.7232 (15) S(3)-Cu(1)-S(1)#1 94.273 (14)
1.7227 (16) S(4)-Cu(1)-S(1)#1 98.618 (14)

444
S. K. VERMA ET AL.
FIG. 4. View of 1 along b-axis involving (sp3)C
H
ꢀ(chelate ring) inter-
. . .
FIG. 2. 3D view of supramolecular unit of 1 showing nine molecule aggregate
actions showing a part of the 3D structure. (color figure available online).
3
via (sp3)C
available online).
H
. . .
. . .
ꢀ(chelate ring) and (sp )C H S interactions. (color figure
Unlike Jian’s structure where molecule present in the
3
. . .
centro-symmetric unit is connected through four (sp )C H
S
however, a significant differences in the structural parameters interactions, involving CH2 groups only, our structure clearly
3
. . .
. . .
are observed, which can be summarized as both C8 H8C Cg demonstrates the existence of eight (sp )C H S interac-
. . .
and C8 Cg distances of 2.74 Å and 3.672 Å in 1 are shorter tions involving both CH3 and CH2 groups of coordinated di-
than the similar distances of 3.098 Å and 3.715 Å, respectively ethyldithiocarbamate ligands (Figure 2). Thus, the presence of
3
. . .
(Cg represents the centroid of chelate ring C6S3S4Cu1 and H8C more number of (sp )C H S contacts certainly provides ad-
. . .
atoms being from methyl groups of different diethyldithiocar- ditional stability to the structure of 1. Further, C H S interac-
bamate ligands). The favorable α angle (angle between the line tions in our case seems to be stronger (2.836 Å) than the similar
connecting the H8C atom and Cg and the normal to the chelate distances (2.909 Å) appear in the earlier structure.
ring plane; Figure 5) of 8.42◦ (= 90.00–81.58◦) and the beta
Moreover, our data clearly revealed that C3 H3B and
◦
. . .
angle C8 H8C Cg of 127.27 (Figure 6) in 1 are significantly C9−H9B groups of each molecules are directed toward S1 and
larger than the similar angles of 6.89◦ and 125.84◦.
S2 of neighboring molecules, approaching them from the same
FIG. 3. View of 1 along a-axis involving (sp3)C
H
ꢀ(chelate ring) interactions, which built up by symmetry operations for donors x, y, z, –1/2 + x, 1/2 – y,
. . . . . . . . .
. . .
◦
1/2 + z, 1/2 – x, 1/2 + y, 1/2 – z, –x,1 – y, 1 –z. Key geometrical parameters: C8 H8C 0.97(2) Å, H8C Cg 2.74 Å, C8 Cg 3.672 Å, C8 H8C Cg 127.27 .
(color figure available online).

CRYSTAL, SPECTRAL, THERMAL, CV OF Cu(II) COMPLEX
445
. . .
. . .
FIG. 7. View of 1 along a-axis involving C3-H3B S1 and C9 H9B S2
interactions and showing a part of the 3D structure which built up by symmetry
operations for acceptors x, y, z; 1/2 + x, 1.5 – y, 1/2 + z; –1/2 – x, –1/2 + y, 1/2
– z, –x, 1 – y, 1 – z, and x, y, z; 1/2 – x, 1/2 + y, 1.5 – z, –1/2 + x, 1/2 – y, –1/2 +
z, –x, 1 – y, 1 – z, respectively. Key geometrical parameters:◦C3-H3B 0.93(3) Å,
. . .
. . .
. . .
H3B S1 2.928 Å, C3 S1 3.602 Å, C3-H3B S1 130.11 , C9◦ H9B 0.95(2)
. . .
. . .
. . .
Å, H9B S2 2.836 Å, C9 S2 3.749 Å, C9 H9B S2 162.04 . (color figure
available online).
FIG. 5. Angle between the line connecting the H8C and Cg and the chelate
ring plane. (color figure available online).
. . .
. . .
3
. . .
sides and forming four C3 H3B S1 contacts with H3B S1
enough to facilitate these (sp )C H S interactions with a con-
. . .
contacts with distances of 2.928 Å and C3 S1 distances of
comitant linking of molecules of 1 into a 3D architecture (Fig-
. . . . . .
. . .
3.602 Å and four C9−H9B S2 contacts with H9B S2 dis-
ure 7). The strength of C9−H9B S2 interactions involving
. . .
tances of 2.836 Å and C9 S2 distances of 3.749 Å, respec-
methylene H9B atoms appears to be of higher significance in
terms of the shorter distance between the hydrogen atom and
acceptor.
tively, as shown in Figure 6. The H3B and H9B atoms be-
long to methyl and methylene groups, respectively, whereas S1
and S2 belong to the trans chelate rings, not participated in
C H ꢀ(chelate ring) interactions. The angles C3 H3B S1 Fluorescence Spectral Study
and C9−H9B S2 of 130.11 and 162.04 (Figure 6) are large
. . .
. . .
◦
◦
. . .
The fluorescence properties of the free ligand and its complex
1 studied at room temperature (298 K) in the DMSO solution.
Although the free ligand did not give any emissions upon ir-
radiation by ultraviolet light, its complex 1 gave a maximum
emission at 339 and 520 nm upon excitation at 270 and 438 nm,
respectively. Such red-shifting of intramolecular charge-transfer
emissions by coordination of ligands to copper(II) were previ-
ously observed in dialkoxo-bridged dinuclear copper(II)[22] and
Salen-type bisoxime copper(II)[23] complexes. The fluorescence
spectrum of complex 1 is shown in Figure 8. These fluorescent
emissions may be attributed to the intraligand and CT→M flu-
orescence. The enhancement of fluorescence of 1 may be at-
tributed to the increased conformational rigidity and decreased
nonradiative energy loss. The emission spectrum of the com-
plex 1 suggests that it can be applied in the material science in
making light-emitting devices (LED).
Optical Band Gap Measurement
Since complex 1 absorbs in the visible range of the solar
spectrum, which is primary requisite for a compound to act as
a photosensitizer for wide band-gap semiconductors, we have
FIG. 6. Key geometrical parameters are shown for all the contacts viz.
H8C Cg, H3B S1, and H9B S2. (color figure available online).
. . .
. . .
. . .

446
S. K. VERMA ET AL.
FIG. 8. Florescence spectra of complex 1 at room temperature in DMSO solution under UV excitation at (a) 370 and (b) 438 nm. (color figure available online).
further investigated its optical behavior toward wide band-gap (Figure 9). The result indicates that complex 1 exhibits the fea-
semiconductors using UV-vis transmittance measurements. The ture of a direct band gap semiconductor.
optical absorption coefficient, α, can be obtained from the trans-
mittance spectrum of 1 using the formula α = 1d ln[T1 ], where
Thermogravimetric Study
Thermogravimetric study of 1, performed under N2 atmo-
sphere suggests that the complex is stable up to 200.0◦C (Fig-
ure 10). An insignificant mass loss (<1%) was observed on the
TG curve at 200.2◦C (probably due to loss of solvent impuri-
ties). The appearance of a sharp endothermic peak at 200.7◦C on
DTA curve suggests the occurrence of phase change due to its
melting. After this process, a continuous mass loss of 76.6% was
observed on TG curve between 205–305◦C. The appearance of
a broad endothermic peak at 303.0◦C on DTA curve indicates
the total destruction of 1, essentially taking place in one stage.
The remaining mass of 23.4% is stable after 305◦C and is in
good agreement with the CuS being a final product (theoretical
26.5%). These results are consistent with the DTG curve which
shows two peaks at 200.2 and 304.9◦C confirming two stages of
mass loss, first due to loss of solvent impurities and the second
is due to total destruction of complex 1. However, the thermal
data is in contrast to the literature report where a thermal de-
composition of similar dialkyldithiocarbamate transition metal
(II) complexes[26] proceeds in several stages involving many
exothermic processes.
d is thickness of the sample and T is the transmittance. The
relation between the absorption coefficients α and the incident
photon energy (hν) can be determined using the well known
Davis and Mott equation,[24] which is αhν = [D(hν – Eg)]γ ,
where constant D is the edge width parameter. In order to deter-
mine optical band gap of the complex 1, we applied the models
for both direct and indirect transitions. For this, the (αhν)2 (di-
rect transition) and (αhν)1/2 (indirect transitions) versus hν were
plotted. From these results, it was concluded that the absorption
in the sample corresponds to direct energy gap. The band gap
energy (Eg = 1.42 eV) was obtained by extrapolating the lin-
ear part of the Tauc plot[25] (a plot between (αhν)1/γ and hν)
Cyclic Voltammetric Study
Cyclic voltammetric study of 1 displayed a broad
quasireversible Cu(II)/Cu(III) redox couple, (Figure 11)
which appear at more anodic potentials than the copper(II)
bis(diethyldithiocarbamate) monomer. This anodic displace-
ment of the Cu(II)/Cu(III) redox couple is however con-
sistence with some acyclic and macrocyclic copper(II) bis-
(diethyldithiocarbamate) complexes bearing imidazolium moi-
eties.[15] This suggests that a higher potential is required to
oxidize the Cu(II) center to Cu(III) in 1 as compared to
the monomeric copper(II) bis(diethyldithiocarbamate) complex.
The separation between the anodic and cathodic peaks, DE =
Epa – Epc, is 90 mV at 50 mV/s scan rate, which is larger
FIG. 9. A plot of (αE)2(eV2m–2) versus energy (eV) for 1. (color figure avail-
able online).

CRYSTAL, SPECTRAL, THERMAL, CV OF Cu(II) COMPLEX
447
FIG. 10. TG/DTG/DTA curves for [Cu2(Et2dtc)4] 1. (color figure available online).
FIG. 11. Cyclic voltammogram of the Cu(II)/Cu(III) redox couple of 1 (0.1 M TBABF4/MeCN, 50 mV/s, vs. Ag/Ag+). (color figure available online).
than DE = 59/n mV and the ratio of the current intensity of CONCLUSION
the cathodic and anodic peaks is 1.56, which is different from
unity. This clearly suggests a quasireversible process,[27] prob-
ably occurring due to rearrangement of the coordination sphere
on going from the Cu(II) to the Cu(III) oxidation state.[28] On
addition of five equivalents of H2PO−4 anion to electrochemi-
cal solution of 1, the cathodic peak potential for 1 significantly
shifted to a more negative value by 20 mV, which suggests
that the complexed anion stabilizes the Cu(III) redox state and
hence lower potential is required for the oxidation of Cu(II) to
Cu(III).[29]
The study allow us to conclude that the methyl and methy-
lene groups of coordinated diethyldithiocarbamate ligands and
metalloaromaticity of the four membered chelate rings (MS2C)
3
. . .
of 1, generate intermolecular (sp )C H ꢀ(chelate ring) and
3
. . .
(sp )C H S interactions which lead to the formation of a
supramolecular unit consisting of a nine molecule aggregate
(Figure 2). Such supramolecular units can be used for the gen-
eration of a fascinating 3D array. Our comments about metal-
3
. . .
loaromaticity through (sp )C H ꢀ(chelate ring) interactions
can be of potential interest to the researcher working in the

448
S. K. VERMA ET AL.
area of molecular assembly of coordination compounds. The 10. Calvin, M.; Wilson, K. Stability of chelate compounds. J. Am. Chem. Soc.
1945, 67, 2003–2007.
11. Masui, H. Metalloaromaticity. Coord. Chem. Rev. 2001, 219–221, 957–992.
12. Milcic, M. K.; Medakovic, V. B.; Sredojevic, D. N.; Juranic, N. O.; Tomic,
electronic absorption and emission spectrum suggest that 1 can
finds its application in the material science in making LED. Fur-
ther, complex 1 exhibits the feature of a direct band gap (Eg =
Z. D.; Zaric, S. D. Electron delocalization mediates the metal-dependent
capacity for CH/π interactions of acetylacetonato chelates. Inorg. Chem.
2006, 45, 4755–4763.
1.42 eV) semiconductor. Thermal decomposition of 1 suggests
the formation of CuS as a final product and hence it can provide
a single source material for the synthesis of CuS nanoparticles.
A significant cathodic shift on addition of H2PO−4 anion to the
electrochemical solution of 1 clearly demonstrated that 1 can
sense the binding of H2PO−4 electrochemically.
13. Nangia, A. Supramolecular chemistry and crystal engineering. J. Chem.
Sci. 2010, 122, 295–310.
14. Ajibade, P. A.; Onwudiwe, D. C.; Moloto, M. J. Synthesis of hexade-
cylamine capped nanoparticles using group 12 complexes of N-alkyl-N-
phenyl dithiocarbamate as single-source precursors. Polyhedron 2011, 30,
246–252.
15. Wong, W. W. H.; Phipps, D. E.; Beer, P. D. Acyclic and macrocyclic tran-
sition metal dithiocarbamate complexes containing imidazolium moieties
for anion binding. Polyhedron 2004, 23, 2821–2829.
16. Bonamico, M.; Dessy, G.; Mugnoli, A.; Vaciago, A.; Zambonelli, L.
Structural studies of metal dithiocarbamates. II. The crystal and molec-
ular structure of copper diethyldithiocarbamate. Acta Cryst. 1965, 19,
886–897.
17. Jian, F.; Wang, Z.; Bai, Z.; You, X.; Fun, H. K.; Chinnakali, K. I.;
Razak, A. The crystal structure, equilibrium and spectroscopic stud-
ies of bis(dialkyldithiocarbamate) copper(II) complexes [Cu2(R2dtc)4]
(dtc=dithiocarbamate), Polyhedron 1999, 18, 3401–3406.
18. Sheldrick, G. M. SAINT 5.1 ed. Siemens Industrial Automation Inc., Madi-
son, WI, 1982.
SUPPLEMENTARY MATERIALS
CCDC 863877 for 1 contains supplementary crystallographic
data for this article. This data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; Fax: t44-1223/336-033; Email: deposit@
ccdc.cam.ac.uk).
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