Synthesis, spectral and thermal studies of 2,2′-bipyridyl adducts of bis(N-alkyl-N-phenyldithiocarbamato)zinc(II)

Article abstract of DOI:10.1016/j.molstruc.2010.11.060

Three Zn(II) adducts of N-alkyl-N-phenyl dithiocarbamate with 2,2′-bipyridine have been synthesized and characterized by elemental analysis and spectroscopic techniques. The compounds are formulated as [Zn(mpdtc)₂bpy] (1), [Zn(epdtc)₂

Full text of DOI:10.1016/j.molstruc.2010.11.060

Journal of Molecular Structure 987 (2011) 58–66
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectral and thermal studies of 2,20-bipyridyl adducts
of bis(N-alkyl-N-phenyldithiocarbamato)zinc(II)
b
Damian C. Onwudiwe a, Peter A. Ajibade a, , B. Omondi
⇑
a Department of Chemistry, University of Fort Hare, Private Bag X1314, Alice 5700, South Africa
b Department of Chemistry, University of Johannesburg, Auckland Park Kingsway Campus, Auckland Park 2006, South Africa
a r t i c l e i n f o
a b s t r a c t
Three Zn(II) adducts of N-alkyl-N-phenyl dithiocarbamate with 2,20-bipyridine have been synthesized
and characterized by elemental analysis and spectroscopic techniques. The compounds are formulated
as [Zn(mpdtc)2bpy] (1), [Zn(epdtc)2bpy] (2), and [Zn(bpdtc)2bpy] (3) (where m = methyl, e = ethyl,
b = butyl; p = phenyl and bpy = 2,20-bipyridine). The single crystal X-ray structures of compounds (1)
and (2) are also reported. It revealed that compound (1) exists as a five coordinate trigonal bipyramidal
structures in which one dithiocarbamate acts as a chelating ligand and the other occupying the apical
position acts as a monodentate ligand, while compound (2) exists as six coordinate octahedral species.
Each of the two compounds has four molecules in the unit cell with the uncoordinated S atom in complex
1 involved in a weak C–Hꢀ ꢀ ꢀS interaction. The thermal decomposition profiles of the compounds are also
reported.
Article history:
Received 9 October 2010
Received in revised form 17 November 2010
Accepted 17 November 2010
Available online 28 November 2010
Keywords:
Dithiocarbamate
2,20-Bipyridyl
Adducts
Zinc(II)
Ó 2010 Elsevier B.V. All rights reserved.
Crystal structure
1. Introduction
report the synthesis, characterization and thermal studies of
bipyridyl adducts of zinc(II) dithiocarbamate.
The chemistry of dithiocarbamate complexes have been exten-
sively studied and reported for a great number of metal ions be-
cause of their striking structural features, diversified industrial
applications [1,2] and biological applications [3]. The most impor-
tant properties of dithiocarbamates are their strong metal-binding
ability and the ease with which these complexes undergo sequen-
tial one-electron transfer reactions [4]. Group 12 metal complexes
of dithiocarbamate are disposed toward a four coordinate (MS4)
geometry. They also have the tendency of forming a higher coordi-
nation number due to their large ionic size (ꢁ2.8–2.9 Å) and the
small bite angle (ꢁ71.0–79.9 Å) of the dithiocarbamate ligands.
Hence, in the ML2 form (L = dithiocarbamate), they are coordi-
nately unsaturated leading to the formation of compounds of high-
er geometry either by the addition of one or two molecules of a
Lewis base or by polymerization [5,6]. In recent years, the attention
has been on studying the interaction with different types of nitrog-
enous Lewis bases [7,8] and the fascinating structural conforma-
tions which result as the base occupies a higher coordination
position. The nitrogen donor adducts of dithiocarbamate are usu-
ally highly volatile in vacuum and are of practical interest in the
preparations of thin-film semiconductors [9–11] as well as electro-
luminescent [12] films of transition metal sulfides. Herein, we
2. Experimental
2.1. Materials and measurements
All solvents and chemicals were of analytical grade and used
without further purification. Microanalyses were carried out with
a Perkin-Elmer 240 elemental analyzer. The IR spectra were re-
corded as KBr discs on a Perkin Elmer 2000 FTIR spectrophotometer
in the range 4000–370 cmꢂ1 region. 1H and 13C NMR spectra were
recorded on 400 and 101 MHz Bruker NMR spectrometers, respec-
tively. Chemical shifts are given in ppm (d scale) relative to tetra-
methylsilane (for 1H and 13C nuclei). Thermogravimetric analysis
was performed on a Perkin Elmer thermogravimetric analyzer
(TGA 7) fitted with a thermal analysis controller (TAC 7/DX). A flow
of N2 was maintained with a heating rate of 10 °C/min between
ambient temperature and 750 °C. Ten to 12 mg of the sample was
loaded into an alumina cup and weight changes were recorded as
a function of temperature. UV/visible spectra were recorded using
Perkin Elmer 250 UV spectrophotometer in chloroform.
2.2. Synthesis of adducts
The precursor complexes were prepared as reported earlier
[13]. The adducts were prepared by adding a 20 mL hot chloroform
solution of 2,20-bipyridine (0.40 mmol, 0.062 g) into a hot 20 mL
⇑
Corresponding author. Tel.: +27 040 602 2055; fax: +27 040 602 2094.
E-mail address: pajibade@ufh.ac.za (P.A. Ajibade).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.11.060

D.C. Onwudiwe et al. / Journal of Molecular Structure 987 (2011) 58–66
59
0
0
solution of 0.40 mmol ([Zn(mpdt)2], 0.172 g); ([Zn(epdtc)2],
0.183 g) and ([Zn(bpdtc)2], 0.205 g), respectively, in the same
solvent. The resulting yellow solution was refluxed for 20 min,
then concentrated to about 10 mL and filtered. The pale yellow sol-
ids which separated out from the solution, after 48 h, were filtered
and dried over anhydrous calcium chloride. Single crystals suitable
for X-ray analysis were obtained from slow evaporation of
dichloromethane–methanol solvent mixture of [Zn(mpdtc)2bpy]
and chloroform–ethanol (3:1) solvent mixture of [Zn(epdtc)2bpy].
[Zn(mpdtc)2bpy]: (Yield: 0.174 g, 75%; m.p. 200–201 °C). Anal.
Calc. for C26H24N4S4Zn: C, 53.28; H, 4.13; N, 9.56; S, 21.88. Found:
C, 53.78; H, 4.37; N, 10.29; S, 21.97%.
[Zn(epdtc)2bpy]: (Yield: 0.209 g, 85%; m.p.193–194 °C). Anal.
Calc. for C28H28N4S4Zn: C, 54.76; H, 4.60; N, 9.12; S, 20.88. Found:
C, 54.28; H, 4.87; N, 9.53; S, 21.12%.
[Zn(bpdtc)2bpy]: (Yield:0.213 g, 80%; m.p. 160–162 °C). Anal.
Calc. for C28H28N4S4Zn: C, 57.34; H, 5.41; N, 8.36; S, 19.13%. Found:
C, 57.68; H, 5.26; N, 8.65; S 19.69%.
electron-density hole (ꢂ0.35 e ÅAꢂ3) is located 1.01 ÅA from Zn(1),
0
0
and the highest peak (0.64 e ÅAꢂ3) 0.89 ÅA from C(21). Crystal data
and details of data collection and refinement are given in Table 1.
3. Results and discussion
3.1. X-ray crystal structure of [Zn(mpdtc)2bpy] (1) and
[Zn(epdtc)2bpy] (2)
The two compounds [Zn(mpdtc)2bpy] and [Zn(epdtc)2bpy]
crystallize in the monoclinic space groups P21/c. ORTEP3 [17]
drawings of [Zn(mpdtc)2bpy] and [Zn(epdtc)2bpy] with the num-
bering scheme used are shown in Figs. 1 and 2 for compound 1
and 2, respectively. Selected bond distances and angles are pre-
sented in Tables 2.
3.1.1. Crystal structure of [Zn(mpdtc)2bpy] (1)
The crystal structure consists of discrete molecular species in
which the metal atom is coordinated by one methyl dithiocarba-
mate acting as monodentate ligand another dithiocarbamate li-
gand, acting in a bidentate (S, S) manner; and a bipyridine acting
as a bidentate ligand through the nitrogen atoms. The zinc atoms
are in a distorted [ZnS2SN2] trigonal bipyramidal environment.
The distortion of the regular trigonal bipyramidal geometry is
due to the small bite angle of the two chelating ligands; dithiocar-
bamate and bipyridine whose S(2)–Zn(1)–S(1) and N(2)–Zn(1)–
N(1) chelate angles (71.358(17)° and 75.70(6)°, respectively)
deviate significantly from the expected 90°. The bond angles
2.3. X-ray crystallography
A crystal of dimensions 0.12 ꢃ 0.10 ꢃ 0.03 mmꢂ3 was selected
for compound 1 and glued onto the tip of a glass fibre while for
compound 2 a crystal of dimensions 0.38 ꢃ 0.09 ꢃ 0.03 mmꢂ3 was
selected. The crystal was then mounted in a stream of cold nitrogen
at 100(1) K and centered in the X-ray beam by using a video camera.
The crystal evaluation and data collection for the two compounds
were performed on a Bruker APEXII [14] diffractometer with Mo K
a
(k = 0.71073 Å) radiation and diffractometer to crystal distance of
4.00 cm. The initial cell matrix was obtained from three series of
scans at different starting angles. Each series consisted of 12 frames
collected at intervals of 0.5° in a 6° range about with the exposure
time of 10 s per frame. The reflections were successfully indexed by
an automated indexing routine built in the APEXII program suite
[14]. The final cell constants were calculated from a set of 6460
strong reflections from the actual data collection. The data were
collected by using the full sphere data collection routine to survey
the reciprocal space to the extent of a full sphere to a resolution
of 0.75 Å. A total of 7256 data were harvested by collecting 1176
Table 1
Summary of crystal data and structure refinement.
Identification code
1
2
Empirical formula
Formula weight
Temperature
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C
26H24N4S4Zn
C28H28N4S4Zn
614.15
100(2)
0.71073
Monoclinic
P21/c
586.10
100(2)
0.71073
Monoclinic
P21/c
frames at intervals of 0.5° scans in
of 20 s per frame for compound 1 while a total of 6476 data were
harvested by collecting 1792 frames at intervals of 0.5° scans in
and with exposure times of 10 s per frame for compound 2. These
x and u with exposure times
9.2157(19)
16.205(3)
17.851(4)
105.597(9)
90
2567.7(9)
4
1.516
15.3418(9)
9.2970(5)
24.9359(12)
125.914(3)
90
2880.5(3)
4
1.416
b (Å)
c (Å)
b (°)
x
u
c
(°)
highly redundant datasets were corrected for Lorentz and polariza-
tion effects. The absorption correction was based on fitting a func-
tion to the empirical transmission surface as sampled by multiple
equivalent measurements [14].
Volume (A3)
Z
Dcalc (Mg/m3)
Absorption coefficient
1.306
1.167
(mmꢂ1
F(0 0 0)
)
1208
1272
2.4. Structure solution and refinement
Crystal size (mm)
Theta range (°)
Index ranges
0.38 ꢃ 0.09 ꢃ 0.03
0.12 ꢃ 0.10 ꢃ 0.03
1.73–28.49
1.64–28.54
The systematic absences in the diffraction data were uniquely
consistent for the space group P21/c for both complexes that
yielded chemically reasonable and computationally stable results
of refinement. The structures were solved by direct method with
SIR 97 program [15] and refined by full matrix least squares with
SHELXS 97 [16] provided all non-hydrogen atoms from the
E-map. All non-H atoms were refined with anisotropic displace-
ꢂ12 6 h 6 12
ꢂ21 6 k 6 17
ꢂ23 6 l 6 23
46,454
6476 [R(int) = 0.0372]
Full-matrix least-
squares on F2
99.6
ꢂ20 6 h 6 20
ꢂ12 6 k 6 12
ꢂ33 6 l 6 32
32,564
7256 [R(int) = 0.0604]
Full-matrix least-
squares on F2
98.9
Reflections collected
Independent reflection
Refinement method
Completeness to
h = 28.49°
ment parameters. Aromatic H atoms were placed in geometrically
idealized positions [C–H@0.93 ÅA for aromatic protons and 0.96 for
0
Data/restraints/
6476/1/318
1.053
7256/2/336
parameters
0
Goodness-of-fit on F2
1.008
methyl protons (complex 1) and C–H@0.95 ÅA for aromatic protons,
0
Final R indices [I > 2
r
(I)] R1 = 0.0290,
R1 = 0.0346,
wR2 = 0.0687
R1 = 0.0587,
wR2 = 0.0774
0.635 and ꢂ0.351
0.99 ÅA for methylene protons and 0.98 for methyl protons] and
constrained to ride on their parent atoms, with Uiso(H) = 1.2Ueq(C).
wR2 = 0.0697
R1 = 0.0366,
wR2 = 0.0728
0.644 and ꢂ0.327
R indices (all data)
For complex
1 the deepest residual electron-density hole
0
0
(ꢂ0.33 e ÅAꢂ3) is located 0.56 ÅA from S(4), and the highest peak
Largest diff. Peak and
hole e Åꢂ3
0
0
(0.64 e ÅAꢂ3) 0.80 ÅA from S(3), whereas for 2 the deepest residual

60
D.C. Onwudiwe et al. / Journal of Molecular Structure 987 (2011) 58–66
Fig. 1. ORTEP plots of [Zn(mpdtc)2bpy]. Hydrogen atoms have been omitted for clarity and ellipsoids are drawn at 50% probability level.
Fig. 2. ORTEP plot of [Zn(epdtc)2bpy]. Hydrogen atoms have been omitted for clarity and ellipsoids are drawn at 50% probability level.

D.C. Onwudiwe et al. / Journal of Molecular Structure 987 (2011) 58–66
61
Table 2
Selected bond distances (Å) and angles (°) for complexes 1 and 2.
Distance
1
2
Angles
1
2
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–S(1)
Zn(1)–S(2)
Zn(1)–S(3)
Zn(1)–S(4)
C(11)–S(1)
C(11)–S(2)
C(19)–S(3)
C(19)–S(4)
C(20)–S(3)
C(20)–S(4)
2.238(2)
2.087(2)
2.480(7)
2.4230(7)
2.3444(6)
3.427(7)
1.715(2)
1.721(2)
1.746(2)
1.689(2)
2.235(2)
2.180(2)
2.4486(6)
2.5830(6)
2.5389(6)
2.5226(6)
1.717(2)
1.706(2)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–S(1)
N(1)–Zn(1)–S(2)
N(1)–Zn(1)–S(3)
N(1)–Zn(1)–S(4)
N(2)–Zn(1)–S(1)
N(2)–Zn(1)–S(2)
N(2)–Zn(1)–S(3)
N(2)–Zn(1)–S(4)
S(1)–Zn(1)–S(2)
S(3)–Zn(1)–S(4)
S(1)–Zn(1)–S(3)
S(1)–Zn(1)–S(4)
S(2)–Zn(1)–S(3)
S(2)–Zn(1)–S(4)
75.70(6)
73.39(7)
95.15(5)
87.71(4)
90.05(4)
160.44(4)
113.58
113.58(4)
114.48(4)
106.70(5)
111.13(5)
144.98(5)
84.35(5)
152.38(5)
89.10(5)
91.11(5)
109.88(5)
71.358(18)
71.438(19)
110.82(2)
1.717(2)
1.713(2)
105.87(2)
129.62(2)
73.439(17)
99.53(2)
158.71(2)
for N(2)–Zn(1)–S(1) [113.58(4)°], N(2)–Zn(1)–S(1) [113.58(4)°],
N(2)–Zn(1)–S(3) [114.48(4)°], N(2)–Zn(1)–S(4) [114.48(4)°] and
S(3)–Zn(1)–S(4) [105.87(2)°] are significantly different from the
expected 90° while N(1)–Zn(1)–S(3) [90.05(4)°] and N(1)–Zn–
S(2) [87.71(4)°] are very close to the ideal value. The angle
described by the equatorial positions should be close to 120°;
while S(3)–Zn–S(2) [129.62(2)°] is greater than 120°, the other
angles involving S(3) are less than 120°. This may be due to the ste-
ric requirement of the dithiocarbamate and bipyridine chelate but
it also significantly affect the N(1)–Zn(1)–S(4) [160.44(4)°] bond
angle which is far from the ideal value of 180°. The rest of the
angles around the Zn atom range from 73–44(2)–129.62(2)°
significantly from the ideal right angle geometry. The Zn–S bond
length in [Zn(mpdtc)2bpy]; S(3)–Zn(1) [2.3444(6)° and S(4)–Zn(1)
[2.4228(6)°] are greater than the average value of tetracoordinated
complexes and much more shorter than those reported for hexaco-
ordinated complexes but the values agree well with those found in
other pentacoordinated complex containing similar ligands
[18–20]. The Zn–N bond distances are similar in both complexes
and the angle between the planes defined by the phenyl ring atoms
of the bipyridyl ligand [N(1)–C(1)–C(2)–C(3)–C(4)–C(5) and N(2)–
C(6)–C(7)–C(8)–C(9)–C(10)] is smaller in five coordinate complex 1
(6.5(3)°) as compared to the six coordinate complex 2 (15.9(1)°).
The packing diagram of the complex (Fig. 3) consists of four mole-
cules in the crystal lattice. The free S atom in the complex is linked
through intramolecular H-bonding to the chelating S atom in a
weak C–Hꢀ ꢀ ꢀS interaction.
bidentate chelating ligands, and two nitrogen atoms from the
bipyridine molecule in a ZnS4N2 format. The Zn–S distances are
0
0
S(1)–Zn(1) 0 [2.4486(6) ÅA], S(2)–Zn(1) [2.5830(6) ÅA], S(3)–Zn(1)
0
[2.5389(6) ÅA] and S(4)–Zn(1) [2.5226(6) ÅA]. The difference between
the bond length S(1)–Zn(1), in comparison to the other, might be
responsible for S(2)–Zn–(1) bond length being relatively longer
than the rest. The N(2)–Zn(1)–N(1) angle of 73.39(9)° is similar
to the value obtained in other bipyridine adducts of dithiocarba-
mate [19]. The other angles around the Zn atom are S(1)–Zn(1)–
S(2) [71.358(18)° and S(4)–Zn(1)–S(3) [71.438(19)°]. The bite
angles of the chelating dithiocarbamate is less than those observed
in simple dithiocarbamate complexes due to the increase in the
coordination number around the zinc atom. The reduction in the
bite angles might also be responsible for the changes observed in
the Zn–S bond lengths. The C–S bond lengths ranges from
0
0
0
1.706(2) ÅA to 1.717(2) ÅA with a mean value of 1.713(2) ÅA, which
0
is shorter than the 1.81 ÅA observed in a typical C–S single bond.
Thus all the C–S bond in the complex have partial double bond
character common in most dithiocarbamate compounds. The car-
0
bon nitrogen bond distances C(11)–N(3) [1.344(3) ÅA] and C(20)–
0
N(4) [1.342(3) ÅA] confirm the presence of the partial double0 bond.
0
The N(1)–Zn(1) [2.2354(18) ÅA] and N(2)–Zn(1) [2.1801(18) ÅA] dis-
tances are in the range typically observed for six coordinate com-
plexes containing the bipyridine ligand [18–20]. The packing
diagram of the complex (Fig. 4) showed that the complex has four
molecules in the unit cell.
3.2. Infrared spectral studies
3.1.2. Crystal structure of [Zn(epdtc)2bpy] (2)
The compound is monomeric with four molecules per unit cell.
The zinc atom is in a distorted octahedral environment formed by
four sulphur atoms from two dithiocarbamate molecules acting as
The IR spectra of the free ligands, the precursor complexes and
their respective adducts were compared and assigned on careful
comparison. The most significant difference between the com-
pound and their corresponding precursor complexes were observed
Fig. 3. Crystal packing for complex 1.
Fig. 4. Crystal packing for complex 2.

62
D.C. Onwudiwe et al. / Journal of Molecular Structure 987 (2011) 58–66
Fig. 5. Superimposed electronic spectra of [Zn(mpdtc)2bpy], [Zn(epdtc)2bpy] and [Zn(bpdtc)2bpy].
in the position of the
t
(C–N) and
t
(C–S) band. The thiureide band
(6H, t). This reflects the reduction in the deshielding effect of the
electronegative N-atom as the distance between the nitrogen and
the carbon bearing the protons increases. Both [Zn(epdtc)2bpy]
and [Zn(bpdtc)2bpy] show signals at d = 3.82 (4H, q) and 4.17
(4H, t) ppm, respectively, which correspond to the –CH2 protons
on a highly deshielding atom (Nitrogen). The resonance peaks for
the protons in the alkyl group of dithiocarbamates have been com-
pared with that of the complex. The downfield shifts observed in
the complexes are attributed to the lowered electron density.
In the complexes, the –NCS2 moiety is neutral, while it is anionic
in the free dithiocarbamic ligands [26]. Peaks showing as four
hydrogen triplet at d = 1.37 and 1.73 ppm for [Zn(bpdtc)2bpy] are
ascribed to the methylene protons of the butyl group. The phenyl
ring protons appear in the range d = 7.29–7.44 ppm in all the com-
pounds. This indicates a weak deshielding and could be attributed
to the shift of electron density towards the nitrogen of the NRR0
(R = alkyl; R0 = phenyl) thereby enhancing the electron density on
t
(C–N) which appear at 1491, 1456 and 1455 cmꢂ1 in the precursor
complexes is shifted to 1449, 1441, and 1440 cmꢂ1, respectively for
the adducts [Zn(mpdtc)2bpy], [Zn(epdtc)2bpy] and
[Zn(bpdtc)2bpy]. The lowering of the (C–N) values with respect
t
to the parent complexes may be due to two possible reasons; one
being the change in coordination geometry from tetrahedral (four
coordinate in the starting materials?) to octahedral (six coordinate)
[21], which affect the degree of interaction between the dithiocar-
bamate ligands and the zinc ion leading to a reduction in the vibra-
tional frequency. The second reason could be the transfer of
electrons from the nitrogen of the bipyridine ligand to the zinc
ion enriching the metal electron density upon the adduct formation
[22]. The lengthening of alkyl chain has been found to be accompa-
nied by a decrease in the vibrational frequency of
this accounts for the lower vibrational frequency observed in the
(C–N) of adduct which contains the butyl substituent. The (C–
t(C–N) [23] and
t
t
S–S) mode is usually expected around 1000 70 cmꢂ1 region. Based
on Bonati and Ugo criterion [24], the number of bands observed in
this region could be used to determine the binding mode of the
dithiocarbamate ligand. While the spectra of [Zn(epdtc)2bpy] and
[Zn(bpdtc)2bpy] show a single peak at 969 and 965 cmꢂ1, respec-
tively, which is strongly indicative of a bidentate coordination of
the dithiocarbamate moiety [25], the spectra of compound
[Zn(mpdtc)2bpy] displays a different feature in this region. A single
the sulphur via the thioureide
p-system [9]. The peaks around
r
= 7.91, 8.25, 8.27, and 9.06 ppm are due to the ring protons of
2,20-bipyridine. The peak at d = 9.06 ppm is clearly the protons
adjacent to the coordinated nitrogen. The shift downfield observed
for the protons of the 2,20-bipyridine compared to the free 2,20-
bipyridine results from the reduced electron density as a result
of the loss of lone pair of electron into the zinc atom upon coordi-
nation, and the proximity to the –NCS2.
sharp band appeared at 970 cmꢂ1 which is attributed to
t
(C–S)
The 13C NMR spectra of the three compounds show five signals
stretching vibration of a symmetrical bound dithiocarbamate li-
gand. In addition, two other peaks appeared at 1017 and
1002 cmꢂ1 showing the presence of unsymmetrically bonded
dithiocarbamate indicating that in the adduct, [Zn(mpdtc)2bpy],
both the monodentate and bidentately coordinated dithiocarba-
mate are present in the molecular structure (see Fig. 1). A further
confirmation of this was obtained from the single crystal X-ray
structure of the complex.
in the region
matic carbons from the phenyl substituent of the dithiocarbamate
ligand, and also between = 137.00–149.00 ppm that are ascribed
r = 121.00–129.00 ppm which correspond to the aro-
r
to 2,20-bipyridine carbons. The signals for the carbon of the
methylene bound to nitrogen in the dithiocarbamate are observed
between 47.48 and 59.28 ppm. The highest downfield shift ob-
served in [Zn(bpdtc)2bpy] may be due to the high inductive effect
of the butyl substituent compared to the ethyl and methyl group.
The resonance peak for the b- and c-methylene carbons of the bu-
3.3. NMR spectral studies
tyl group appeared at 29.70 and 20.31 ppm, respectively. All three
compounds show signals around 208–209 ppm, with reduced
intensity typical of quaternary carbon, that correspond to the car-
bon of the –NCS2. The down field shift of the NCS2 moiety com-
pared to the parent complexes, observed between 206.00 and
The 1H NMR spectra of all the compounds show signals for the
hydrogen of the methyl, integrated as six protons, in three dis-
tinctly separate positions, at d = 3.78 (6H, s), 1.30 (6H, t) and 0.93

D.C. Onwudiwe et al. / Journal of Molecular Structure 987 (2011) 58–66
63
Fig. 6. Thermograms of [Zn(mpdtc)2bpy], [Zn(epdtc)2bpy] and [Zn(bpdtc)2bpy].
Table 3
Temperature values for decomposition along with corresponding weight loss values and product obtained.
Compounds
Decomposition range (°C)
Peak temperature (°C)
Weight loss (%)
Product obtained
Mass changes
Calc.
Found
[Zn(mpdtc)2bpy]
150–235
240–360
531–621
211
314
580
28
79
84
Zn(mpdtc)
Zn(NCS)2
ZnS
2.64
3.10
1.67
2.11
3.45
1.68
[Zn(epdtc)2bpy]
[Zn(bpdtc)2bpy]
142–244
256–346
496–587
213
316
527
24
72
85
Zn(epdtc)
Zn(NCS)2
ZnS
2.22
3.63
1.95
3.02
3.29
1.93
160–241
242–360
567–642
190
319
617
21
76
84
Zn(bpdtc)
Zn(NCS)2
ZnS
3.01
3.50
1.88
2.79
3.68
1.57
207.00 ppm, may be ascribed to an increase of
p-bond character or
3.5. Thermogravimetric analysis
delocalization of electron along the C–N bond which is enhanced
by the unshared electron pair in the nitrogen atom [27]. The elec-
tron donating effect by the alkyl substituents and the bidentate
nature of the dithiocarbamate ligand might also have a contribu-
tory role [28].
The thermogravimetric analysis (TGA) of the samples was per-
formed under a nitrogen atmosphere with a heating rate of
10 °C minꢂ1, as depicted in Fig. 6. Temperature values for decompo-
sition along with corresponding weight loss values and product ob-
tainedare presented in Table 3. Thethree compoundsdisplay similar
thermogram which indicates that their mechanism of decomposi-
tion is the same when heated. The decomposition started with a
weight loss of 26.4%, 24.8% and 22.4% in a clearly marked step for
[Zn(mpdtc)2bpy], [Zn(epdtc)2bpy] and [Zn(bpdtc)2bpy], respec-
tively. This corresponds to the loss of the 2,20 bipyridine fragment
(Calc. 26.64%, 25.4% and 23.2%). The peak temperature of 2,20-bipyr-
idine expulsion decreased from [Zn(epdtc)2bpy] to [Zn(bpdtc)2bpy]
(213 °C and 190 °C, respectively) indicating a reduction in Zn–N (of
the bipyridine) bond, and a manifestation of the inductive effect of
the alkyl substituents bonded to the sp2-hybridized nitrogen atom
[32]. The increase in the inductive effect of alkyl substituents in
the series CH3, C2H5 and C4H9 is expected to result in the progressive
displacement of electron density in the R2N–C(S)S-moiety from the
nitrogen atom toward the zinc. The high electron density on the zinc
atom of the compound with high alkyl group reduces the interaction
of the zinc atom with the lone pairs of electron from the nitrogen,
3.4. Electronic spectra of the complexes
The UV–Vis spectra of the three compounds are shown in Fig. 5.
The UV spectra of dithiocarbamate complexes are usually charac-
terized by three absorption bands due to chromospheres group
NCS2 and are attributed to
p
?
p
* transition of NCS and SCS moiety;
transition involving the transition of an electron of the
lone pair on the sulphur atom to an antibonding -orbital [29]. In
the present complexes, these bands are overlapped by an intense
band between 294 and 298 cmꢂ1 due to the
* transition of
*
p
and n ?
p
p
? p
the phenyl ring [30]. No splitting of band is observed in
[Zn(epdtc)2bpy] and [Zn(bpdtc)2bpy] while [Zn(mpdtc)2bpy]
undergoes a peak splitting which indicates the non-equivalence
of the C–S bond [31]. There is no evidence of any d–d transition over
the visible region indicating d10 electronic configuration of the d12
metals.

64
D.C. Onwudiwe et al. / Journal of Molecular Structure 987 (2011) 58–66
Fig. 7. Superimposed TG/DTG curves of [Zn(mpdtc)2bpy] and the TG curve of the precursor complex [Zn2(mpdtc)2].
hence extending the bond length. The lower peak temperature
observed in [Zn(mpdtc)2bpy] compared to [Zn(epdtc)2bpy] may be
due to the monodentate binding mode of one of the dithiocarba-
mates in the former. The loss of 2,20-bipyridine was followed
immediately with a huge weight loss caused by the decay of the
dithiocarbamate (72–76%), leading to the formation of Zn(NCS)2 in
the temperature range of approximately 240–360 °C. Further in-
crease in temperature results in the decomposition of Zn(NCS)2 to
ZnS. This is similar to the observations made in the decomposition
of group 12 dithiocarbamate adduct with nitrogenous bases [33].
To compare the thermal stability of the precursor complexes with
their respective adducts, the thermogram of each adduct was super-
imposed with its respective precursor complex (Figs. 7–9). A critical
look shows that there is a shift in the thermal destruction range of
the dithiocarbamate moiety toward lower temperature for the
adducts compared to the parent complexes. This observation has
been explained as the formation of unstable mononuclear com-
plexes [Zn(S2CNRR0)2] {R = CH3, C2H5, C4H9; R0 = C6H5} via the
detachment of the N-donor base by the adducts [34]. The precursor
complexes originally exist in their binuclear form [13,35,36]. EPR
studies have been used for similar compounds to show the
formation of such mononuclear complexes upon desorption of
Fig. 8. Superimposed TG/DTG curves of [Zn(epdtc)2bpy] and the TG curve of the precursor complex [Zn2(epdtc)2].

D.C. Onwudiwe et al. / Journal of Molecular Structure 987 (2011) 58–66
65
Fig. 9. Superimposed TG/DTG curves of [Zn(bpdtc)2bpy] and the TG curve of the precursor complex [Zn2(bpdtc)2].
N-donor bases from adducts [37,38]. The absence of any plateau be-
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