The chemistry of cadmium-thiocarboxylate derivatives: Synthesis, structural features, and application as single source precursors for ternary sulfides

Article abstract of DOI:10.1021/ic200927w

Novel heterobimetallic complexes [(PPh₃)₂Cu(μ- SCOPh)₂Cd(SCOPh)] (2a), [(PPh₃)₂Cu(μ-SCOth) ₂Cd(SCOth)] (2b), [(PPh₃)₂Ag(μ-SCOth) ₂Cd(SCOth)] (3a), [(PPh

Full text of DOI:10.1021/ic200927w

ARTICLE
pubs.acs.org/IC
The Chemistry of CadmiumꢀThiocarboxylate Derivatives: Synthesis,
Structural Features, and Application As Single Source Precursors for
Ternary Sulfides
Jyotsna Chaturvedi,†,§ Suryabhan Singh,† Subrato Bhattacharya,*,† and Heinrich N€oth‡
†Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
‡Department of Chemistry, University of Munich, Butenandtstrasse 5-13, 81377 Munich, Germany
S Supporting Information
b
ABSTRACT: Novel heterobimetallic complexes [(PPh3)2Cu(μ-SCOPh)2Cd-
(SCOPh)] (2a), [(PPh3)2Cu(μ-SCOth)2Cd(SCOth)] (2b), [(PPh3)2Ag(μ-
SCOth)2Cd(SCOth)] (3a), [(PPh3)2Ag(μ-SCOth)2Cd(H2O)(SCOth)] (3b),
[(PPh3)2Ag(μ-SCOPh)2Cd(SCOPh)] (3c), and a bimetallic complex [PPh3Cd-
(μ-SCOth)SCOth]2 CH2Cl2 (5) (th = thiophene) were prepared and
3
characterized by single crystal X-ray diffraction analysis. A coordination
polymer [Cd(SCOPh)2]n (4) has also been characterized structurally that
exhibited metal-like electrical conductivity. The heterobimetallic complexes
on pyrolyzing under controlled conditions yielded ternary sulfides of
composition CuCd7S8, CuCd10S11, Ag2Cd8S9, and Ag2Cd5S6, which have
been characterized by SEM-EDX and X-ray diffractometry. Photophysical
properties and electrical conductivities of the sulfides have also been studied.
’ INTRODUCTION
precursors to deliver complex phases is ever increasing.9 Metal
thiocarboxylates undergo facile thiocarboxylic anhydride elimina-
tion, leaving the metal sulfide.10 Hampden-Smith et al. have shown
that the metal thiocarboxylates can be used as single molecular
precursors for metal sulfide materials.11 During recent years, Vittal
and Ng developed single-precursor routes to synthesize several
sulfide and selenide thin films. They have synthesized a number
of thiocarboxylate complexes and exploited them to prepare cor-
responding metal sulfides. For example, complexes such as
[{M(SC{O}R)2(bpy)}] (M = Zn, Cd; R = Me/Ph) were utilized
to prepare corresponding metal sulfides (cubic ZnS and cubic
CdS), and heterobimetallic complexes such as [(Ph3P)2MM0-
(SC{O}Ph)4] (M = Cu, Ag; M0 = In/Ga) were used to prepare
binary β-In2S3 as well as ternary, MM0S2-type sulfides.12
We report here the synthesis and molecular structures of novel
heterobimetallic thiocarboxylates containing Cd(II)/Cu(I) and
Cd(II)/Ag(I) atoms and the results of studies related to their
thermal decomposition into ternary sulfides. The importance of
such studies lies in the fact that inclusion of transition metals is
known to create intermediate energy states between valence and
conduction bands of the CdS crystallites, resulting in a smaller
band gap and new optical properties.13 A lot of studies have so far
been conducted using Mn as a dopant, while the study of Cu
doping is yet in a nascent state.14 To the best of our knowledge,
a Cd/Ag sulfide is still unknown.
Over the years, metal sulfides in the forms of powders, thin
films, and nanoclusters have generated a great deal of scientific
and technological interest for different reasons. A number of
transition metal sulfides exhibit semiconductivity and other
interesting properties like luminescence, photoconductivity,
chemical sensing, catalysis, superconductivity, etc.1 For exam-
ple, cadmium sulfide is a direct band gap semiconductor (band
gap 2.42 eV), and its conductivity increases when irradiated
with light, leading to uses as a photoconductor.2 Cadmium
sulfide has two crystalline forms: the more stable hexagonal one
has a wurtzite structure, and the cubic form has a zinc blende
structure. Both polymorphs are piezoelectric, while the hex-
agonal form is also pyroelectric.3 When an impurity (p-type semi-
conductor) is added, it forms the core component of a photovoltaic
cell. A CdS/Cu2S solar cell was one of the first efficient cells known.4
Furthermore, CdS luminesces under electron beam excitation when
doped with Cu+ and Al3+ and is used as a phosphor.5
Though a wide variety of methods have been developed to
synthesize these materials,6 the conventional solid state reaction
and homogeneous precipitation methods suffer from difficulties
like high temperature requirements, long reaction times, the
formation of impure products, mixed phases, etc.7 An attractive
method is the single molecular source approach, where the organic
fragments present in the metal complexes of sulfur-containing
ligands are removed and metal sulfides are reassembled at rela-
tively low temperatures under mild conditions.8 With the in-
creased complexity of electronic devices, the demand for elegant
Received: May 3, 2011
Published: September 20, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
ARTICLE
’ EXPERIMENTAL SECTION
Type 1 (3a): Block -shaped yellow crystals. mp 166ꢀ168 °C. Anal.
cald for C51H39O3P2S6AgCd: C, 51.26; H, 3.35. Found: C, 50.87; H,
3.34. IR spectra (KBr, cmꢀ1): 1575, 1545 ν(CO), 1226 ν (th-C), 916,
895 ν(CꢀS). NMR (CDCl3, δ ppm), 1H: 6.89ꢀ7.70 (Ph and th). 13C:
127.28ꢀ145.70 (Ph and th), 197.64 (COS). 31P: 10.40.
All of the reactions were carried out under a dinitrogen atmosphere.
The solvents were purified using standard methods.15 Cadmium
chloride (CdCl2 2.5H2O) (CDH), thiophene-2-carbonyl chloride,
3
and triphenyl phosphine (Sigma-Aldrich) were used as received. Thio-
benzoic acid (98%) (Sigma-Aldrich) was purified by distillation (bp
85ꢀ87 °C/10 Torr) prior to its use. Thiophene-2-thiocarboxylic acid
was synthesized by literature methods.16 The sodium salt of thiophene-
2-thiocarboxylic acid was obtained by reacting the acid with sodium
methoxide in a stoichiometric ratio.
Type 2 (3b): Block-shaped pale yellow crystals. mp 172ꢀ174 °C.
Elemental analysis, anal. calcd for C51H41O4P2S6AgCd: C, 51.37; H,
3.47. Found: C, 51.23; H, 3.45. IR (KBr, cmꢀ1): 1583 ν(CO), 1224
ν(th-C), 905, 870 ν(CꢀS). NMR (CDCl3, δ ppm), 1H: 6.78ꢀ7.55 (Ph
and th), 2.35 (H2O).13C: 127.29ꢀ145.66 (Ph and th), 197.69 (COS).
31P: 10.47.
IR spectra were recorded using a Varian-3100 FTIR instrument.
NMR spectra were obtained using a JEOL AL300 FT NMR spectro-
meter. Elemental analyses were performed using an Exeter model E-440
CHN analyzer, and electronic absorption spectra were recorded using a
Shimazdu UV-1700 PhermaSpec Spectrophotometer.
Synthesis of [PPh4] [Cd(SCOth)3] (1). To a stirred solution of
sodium thiophene-2-thiocarboxylate (0.210 g, 1.26 mmol) in 5 mL of
methanol was added powdered CdCl2 2.5H2O (0.096 g, 0.42 mmol).
Synthesis of [(PPh3)2Ag(μ-SCOPh)2Cd(SCOPh)] (3c). The
same procedure was followed as mentioned in the case of 2a except in
place of (PPh3)2CuNO3, (PPh3)2AgNO3 (0.382 g, 0.55 mmol) was
used. Block-shaped, colorless crystals suitable for single crystal X-ray
diffraction were obtained from a toluene solution. Yield: 0.59 g (93%).
mp 164 ꢀ 166 °C. Anal. calcd for C57H45O3P2S3AgCd: C, 59.20; H,
3.92. Found: C, 58.97; H 3.90. IR (KBr, cmꢀ1): 1569 ν(CO), 1214
ν(PhꢀC), 938 ν(CꢀS). NMR (CDCl3, δ ppm), 1H: 7.11ꢀ7.89 (Ph).
13C: 127.48ꢀ139.12 (Ph), 206.79 (COS). 31P: 9.49.
3
The reaction mixture was stirred for half an hour. To the resulting
yellow solution was added a solution of tetraphenylphosphonium
bromide (0.176 g, 0.42 mmol) in 5 mL of methanol. After 5 min of
stirring, a dark yellow precipitate was formed. The precipitate was
filtered and dried under reduced pressure and redissolved in a mixture
of chloroform and acetonitrile (5:1). After two days, orange crystals
suitable for X-ray diffraction were obtained. Yield: 0.307 g (83%). Anal.
calcd for C39H29O3PS6Cd: C, 51.26;H, 3.35. Found:C, 50.87; H, 3.34. IR
spectra (KBr, cmꢀ1): 1510 ν(CO), 1224 ν(th-C), 927, 889 ν(CꢀS).
NMR (DMSO-d6, δ ppm), 1H: 7.04ꢀ7.98 (Ph and th ring).13C:
117.05ꢀ148.14 (Ph and th ring), 193.88 (COS).
Synthesis of [Cd(SCOPh)2]n (4). Method A (Using TiCl4). To a
stirred solution of NaSCOPh prepared in situ by reacting thiobenzoic acid
(0.414 g, 3 mmol) andsodiummetal(0.069 g, 3 mmol) in MeOH(15 mL)
was added an aqueous solution (5 mL) of CdCl2 2.5H2O (0.228 g,
3
1 mmol) to geta yellow-coloredsolution ofNa[Cd(SCOPh)3]. A solution
of TiCl4((0.047 g, 0.25 mmol) in MeOH (10 mL) was then added to the
reaction mixture with vigorous stirring at 10 °C. Stirring was continued for
1 h at room temperature. The solvent was then evaporated under reduced
pressure, and the residue was extracted with CHCl3 (20 mL). The material
was filtered off, and solvent from the filtrate was evaporated under reduced
pressure. The yellow-colored crystalline product was washed with ethanol
to remove the dibenzoyl disulfide. After recrystallization from a chloroform
and acetonitrile mixture (1:1), the product was dried under vacuum con-
ditions for 1 h. Single crystals of the compound obtained were very
thin, needle like, and of poor quality. Yield, based on CdCl2: 0.368 g
(95.2%). mp 165 °C. Anal. calcd for C14H10O2S2Cd1: C, 43.48; H,
2.61. Found: C, 43.80; H, 2.70. IR data (KBr, cmꢀ1): 1545 ν(CO),
Synthesis of [(PPh3)2Cu(μ-SCOPh)2Cd(SCOPh)] (2a). To a
methanolic solution (5 mL) of PhCOSH (0.414 g, 3 mmol) was added
NaOMe [generated in situ by dissolving Na (0.069 g, 3 mmol) metal in
5 mL of methanol] with stirring in an ice bath. After 5 min, a solution of
CdCl2 2.5H2O (0.228 g, 1.00 mmol) in 5 mL of water was added to the
3
reaction mixture, which was then stirred for ∼20 min. To the yellow
colored solution was added a solution of (PPh3)2CuNO3 (0.650 g, 1.00
mmol) in 5 mL of CH2Cl2, and the solution was stirred for an hour. The
solvent was evaporated under reduced pressure, and the residue was
extracted with chloroform, which on layering with diethyl ether yielded
crystals of 2a after 3 days. Yield: 1.024 g (92%). Anal. calcd for
C57H45O3P2S3CdCu: C, 61.56, H, 4.08. Found: C, 61.31, H, 4.04. IR
spectra (KBr, cmꢀ1): 1591, 1545 ν(CO), 1207 ν(PhꢀC), 933 ν(CꢀS).
NMR (CDCl3, δ ppm): 1H, 7.14ꢀ7.76 (Ph).13C, 127.50ꢀ139.13 (Ph),
206.93 (COS).31P, 2.25.
1
1221 ν (PhꢀC), 943 ν (CꢀS), 632. NMR (CDCl3, δ ppm), H:
7.09ꢀ7.78 (Ph). 13C: 127ꢀ132.6 (Ph), 200.5 (COS).
Method B (Using MgO). To a stirred solution of thiobenzoic acid
(0.414 g, 3.0 mmol) in 5.0 mL of methanol was added a suspension of
MgO (0.060 g, 1.50 mmol) in MeOH (15 mL) followed by an aqueous
solution (5 mL) of CdCl2 2.5H2O (0.228 g, 1 mmol), resulting in a
3
yellow-colored turbid solution, which was stirred further for 1 h at room
temperature. The solvent was then evaporated under reduced pressure,
and the residue was extracted with a methanol and acetonitrile (1:1)
mixture. The insoluble material was filtered off, and on standing, yellow-
Synthesis of [(PPh3)2Cu(μ-SCOth)2Cd(SCOth)] (2b). To a
stirred solution of sodium thiophene-2-thiocarboxylate (0.298 g, 1.80
mmol) in 10 mL of methanol was added powdered CdCl2 2.5H2O
colored crystals were obtained in a few days. Yield (based on CdCl2
3
3
(0.137 g, 0.60 mmol). After stirring the reaction mixture for 15 min, a
solution of (PPh3)2CuNO3 (0.390 g, 0.60 mmol; in 5 mL of CH2Cl2)
was added dropwise into it. The reaction mixture was stirred further for
an hour; then it was dried under reduced pressure. The yellow residue
was then redissolved in toluene, and after removing the NaCl, the
solution was kept for crystallization. After a day, pale yellow crystals
were obtained. Yield: 0.563 g (83%). mp 144ꢀ146 °C. Anal. calcd for
C51H39O3P2S6CdCu: C, 54.20; H, 3.48. Found: C, 54.19; H, 3.46. IR
spectra (KBr, cmꢀ1): 1543, 1510 ν(CO), 1228 ν(th-C), 918, 879
2.5H2O): 0.368 g (95.0%). mp 165 °C. Anal. calcd for C14H10O2S2Cd1:
C, 43.48; H, 2.61. Found: C, 43.60; H, 2.68. IR data (KBr, cmꢀ1): 1545
1
ν(CO), 1221 ν(PhꢀC), 943 ν(CꢀS). NMR (CDCl3, δ ppm), H:
7.09ꢀ7.78 (Ph). 13C: 127ꢀ132.6 (Ph), 200.5 (COS).
Synthesis of [PPh3Cd(μ-SCOth)SCOth]2 CH2Cl2 (5). To a
3
stirred solution of sodium thiophene-2-thiocarboxylate (0.210 g, 1.27
mmol) in 5 mL of methanol was added CdCl2 2.5H2O (0.145 g, 0.63
3
mmol), followed by a CH2Cl2 solution of PPh3 (0.166 g, 0.63 mmol).
The reaction mixture was stirred for 2 h and then was dried under
reduced pressure. The residue was dissolved in chloroform and filtered
off NaCl. The filtrate was layered with petroleum ether (60ꢀ80 °C
fraction), which on standing yielded colorless crystals after a day. Yield:
0.375 g (84%). mp 182ꢀ84 °C. Anal. calcd for C57H44Cl2O4P2S8Cd2:
C, 48.65; H, 3.15. Found: C, 48.45; H, 3.13. NMR (CDCl3, δ ppm),
1H: 6.93ꢀ7.75 (Ph and th). 13C: 127.62ꢀ145.40 (Ph and th), 198.15
(COS).
1
ν(CꢀS). NMR (CDCl3, δ ppm), H: 6.82ꢀ7.57 (Ph and th). 13C:
127.34ꢀ145.57 (Ph and th), 197.69 (COS). 31P: 2.07.
Synthesis of [(PPh3)2Ag(μ-SCOth)2Cd(SCOth)] (3a) and
[(PPh3)2Ag(μ-SCOth)2Cd(SCOth)H2O] (3b). The same procedure
was followed as mentioned for 2b except the fact that (PPh3)2AgNO3 (0.361
g, 0.52 mmol) was used in place of (PPh3)2CuNO3. Yield: 0.476 g (78%). A
closer look into the product revealed the presence of two types of crystals.
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X-Ray Crystallography. Single crystal X-ray data of compounds
1ꢀ5 were collected on an Xcalibur Eos Oxford Diffractometer using
graphite monochromated Mo Kα radiation (λ = 0.7107 Å). Data
collections for 1 were carried out at 100 K, while those for 2a, 2b, 3a,
3b, 3c, 4, and 5 were carried out at 293 K. Absorption corrections were
made using the multiscan method. Data integration and reductions
were processed with CRYSALIS PRO software.17 Structures were
solved by the direct method and then refined on F2 using the full matrix
least-squares technique with SHELX-97 software18 using the WINGX
program package.19 Hydrogen atoms were, however, placed at the
calculated positions using SHELX default parameters. Disordered
atoms were split into two parts and then refined with free variables
using appropriate restraints. A summary of crystallographic data and
structure solutions are given in Table 1.
Scheme 1
Powder X-ray diffraction scans were obtained using a Bruker Powder
X- Ray Diffractometer with a Cu target (λ = 1.54056 Å).
Thermogravimetry and Pyrolysis. Thermogravimetric analyses
were carried out using a Perkin-Elmer Diamond TG/DTA with a
heating rate of 10 °C minꢀ1 under an argon atmosphere. Pyrolyses of
the compounds in the solid state were performed at 350 °C in a furnace
which was continuously purged with dry N2 gas. The residue obtained in
each case was a mixture of different compounds from which metal
sulfides were isolated by washing the residue successively with ethanol,
diethyl ether, and carbon disulfide. The suspended particles were
collected by centrifugation and were then dried under vacuum condi-
tions and 0.1 mmHg/2 h. We have not tried to analyze the soluble
compounds fully. However, elemental sulfur was obtained on drying the
carbon disulfide solution, while the presence of Cu2+/Ag+ ions was
detected qualitatively, and crystals of triphenylphosphine oxide were
obtained on drying the ethanol and diethyl ether solutions respectively.
SEM-EDS Analysis. Scanning electron microscopic images were
taken on a JEOL model JSM-6390LV, while energy dispersive X-ray
spectrometric data were taken on a JEOL model JED-2300.
Photoluminescence (PL) Spectra and Photoluminescence
Excitation Spectra (PLE). Spatially resolvedPL spectra were collected
using a WItec Alpha SNOM (Germany; Scanning Near-field Optical
Microscope) in confocal mode in transmission geometry from different
regions of the sample which was a thin film on a glass substrate. The
samples were excited by an argon laser (λ = 488 nm) through an OIL
immersion objective lens (100ꢁ) with a numerical aperture (NA = 0.95).
The PL spectra were collected using CCD with a spectral grating of
150 g/mm and an integration time of 5 s at excitation powers of 0.1 and
0.3 mW of the 488 nm laser used.
Scheme 2
mentioning here that there are only a few heterobinuclear
Cd(II)/Cu(II) complexes known20 to date, while complexes of
Cd(II)/Cu(I) and Cd(II)/Ag(I) containing a sulfur ligand are
hitherto unknown.
Though the Na+ ion could easily be exchanged with other large
cations (in complexes 1ꢀ3) which are of a soft nature, an attempt
to replace the same with a hard metal ion such as Ti4+ did not
yield the expected heterobinuclear complexes. Instead, the binary
thiocarboxylate of Cd(II) was isolated. A similar result was
obtained when an attempt was made to isolate the magnesium
salt of the complex [Cd(SCOR)3]ꢀ. Very recent studies on
analogous Zn(II) complexes showed migration of a phosphine
ligand from Cu(I) to Zn(II) during the formation of a heterobi-
nuclear complex.21 Such ligand migration was observable in none
of the Cd(II) complexes during the present studies. However,
binding of phosphine is quite facile with Cd(II) in its thiocarbox-
ylate complex, as shown in Scheme 2.
Pressed Pellet Electrical Conductivity. The pressed pellet
electrical conductivity of complex [Cd(SCOPh)2]n and ternary metal
sulfides CuCd7S8, CuCd10S11, and Ag2Cd5S6 were recorded on a
Kiethley-236 source measurement unit by employing a conventional
two-probe technique.
All of the complexes were found to be stable at ambient
temperature for several months.
Crystal and Molecular Structures. Complex 1 crystallized in
the trigonal system with space group P32 as discrete ca-
tionꢀanion pairs. In the anionic part, Cd(II) is hexacoordinated
by three oxygen and three sulfur atoms. The geometry around
Cd(II) is between trigonal prismatic and trigonal antiprismatic
with a twist angle (ϕ) of 27.6° (Figure 1). Though the anion of 1
’ RESULTS AND DISCUSSION
Syntheses. The sodium salt of the complex [Cd(SCOR)3]ꢀ
was generated in situ, which on exchange of the cation yielded the
complexes 1ꢀ4, as shown in Scheme 1. Notably, the reaction of
bis-triphenyl phosphinesilver(I) nitrate with Na[Cd(SCOth)3]
resulted in two different kinds of crystals with different coordina-
tion environments around the Cd(II) centers: one containing
[(PPh3)2Ag(SCOth)2Cd(SCOth)] (3a) formula units in which
Cd(II) is surrounded by three thiocarboxylate units and the other
having the formula [(PPh3)2Ag(SCOth)2Cd(H2O)(SCOth)]
(3b), in which there is a water molecule coordinated to Cd(II).
The analogous reaction with Na[Cd(SCOPh)3] gave only one
product, [(PPh3)2Ag(SCOPh)2Cd(SCOPh)] (3c). It is worth
is structurally comparable to [Cd(SCOPh)3]ꢀ, the Cd
O
3 3 3
distances in 1 are significantly shorter than those found in the
latter complex22 (2.659ꢀ2.828 Å).
The complex (2a) crystallized in the monoclinic system with the
P21/n space group. The thermal ellipsoid plot is shown in Figure 2.
The CdꢀCu distance 3.577 Å is quite larger than the sum of
the covalent radii of the two metals (2.86 Å). The known Cd(II)/
Cu(II) heterodinuclear complexes are [Cd2Cu2I4L4(DMSO)2],
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Inorganic Chemistry
ARTICLE
Figure 1. Thermal ellipsoid plot (at 50% probability level) of 1. The
thiophene rings are disordered. Hydrogen atoms are omitted for clarity.
Selected metric data: CdꢀS(1) 2.547(13), CdꢀS(2) 2.564(13), CdꢀS-
(3) 2.610(13), CdꢀO(1) 2.560(4), CdꢀO(2) 2.487(3), CdꢀO(3)
2.431(3). S(1)ꢀCdꢀS(2) 122.72(4), S(1)ꢀCdꢀS(3) 105.58(4), S-
(2)ꢀCdꢀS(3) 107.09(4), O(1)ꢀCdꢀO(2) 86.84(11), O(1)ꢀCdꢀ
O(3) 92.39(11), O(2)ꢀCdꢀO(3) 85.95(11), S(1)ꢀCdꢀO(1) 60.93(8),
S(1)ꢀCdꢀO(3) 92.48(8), S(1)ꢀCdꢀO(2) 147.67(8), S(2)ꢀCdꢀ
O(1) 100.72(8), S(2)ꢀCdꢀO(2) 62.34(8), S(2)ꢀCdꢀO(3) 144.53(8),
S(3)ꢀCdꢀO(1) 151.87(9), S(3)ꢀCdꢀO(2) 102.17(8), S(3)ꢀCdꢀ
O(3) 62.23(8).
Figure 3. Thermal ellipsoid plot (at 30% probability level) of 2b with
disordered thiophene rings (hydrogen atoms are omitted for clarity).
Selected metric data: CdꢀS(1) 2.608(3), CdꢀS(3) 2.594(2), CdꢀS(5)
2.478(2), CdꢀO(1) 2.618(7), CdꢀO(2) 2.575(6), CuꢀP(1) 2.292(1),
CuꢀP(2) 2.292(1), CuꢀS(1) 2.583(3), CuꢀS(3), 2.380(1). S(3)ꢀ
CdꢀS(1) 90.23(7), S(5)ꢀCdꢀS(1) 139.02(9), S(5)ꢀCdꢀS(3)
120.42(8), O(2)ꢀCdꢀO(1) 91.67(19), S(5)ꢀCdꢀO(2) 99.95(14),
S(3)ꢀCdꢀO(2) 60.47(13), S(1)ꢀCdꢀO(2) 119.65(13), S(5)ꢀCdꢀ
O(1) 115.29(15), S(3)ꢀCdꢀO(1) 120.48(14), S(1)ꢀCdꢀO(1)
57.51(15), P(1)ꢀCuꢀP(2) 120.97(6), P(1)ꢀCuꢀS(3) 115.18(7),
P(1)ꢀCuꢀS(1) 118.76(8), P(2)ꢀCuꢀS(3) 105.35(7), P(2)ꢀCuꢀ
S(1) 96.57(9), S(3)ꢀCuꢀS(1) 95.79(8).
The three thiocarboxylate ligands display three different
bonding modes: bidentate SCO bridging, S-bridging chelating,
and chelating (terminal), as shown in Figure 2. Notably, the
disordered bridging (μ-S,O) ligand makes it difficult to describe
the geometry around the Cd(II) center. The sof of the two parts
of a disordred atom are similar (48 and 52%). It is, however, clear
that Cd(II) is pentacoordinate and bonded with two oxygen and
one sulfur atoms. For pentacoordinate complexes, Addison
et al.23 have given an angular structural parameter, τ, and defined
it as an index of trigonality. The value of τ varies from 0 to 1 when
the geometry changes from perfect square pyramidal to ideal
trigonal bipyramidal. Two τ values were calculated for the
structure, one considering the S1 and the other considering the
S1A atom bonded to Cd(II). The values obtained (0.23 and
0.09) are indicative of distorted square pyramidal geometry
around Cd(II). The CdꢀS(3) bond [2.493(1) Å] is shorter
than the other two CdꢀS bonds in which S atoms are involved in
bridging.
Besides the phosphorus atoms of two triphenylphosphine
ligands, the Cu(II) is bonded to the sulfur and oxygen atoms
of the μ-S and μ-S,O thiobenzoate ligands, respectively. The
CuꢀS bond length is within the range reported for CuꢀS bond
lengths in copper thiobenzoate complexes.24 The bond angles
around the Cu center are, however, quite deviated from the ideal
value of 109.5° due to steric requirements. Yang et al.25 have
introduced a simple four-coordinate metrical parameter τ4 to
objectively describe the molecular geometry, which is neither
close to tetrahedral nor square planar. The τ4 value for 2a was
found to be 0.86, which is indicative of a trigonal pyramidal
geometry.
Figure 2. Thermal ellipsoid plot (at 30% probability level) of 2a.
Hydrogen atoms have been omitted for clarity. Selected metric data:
CdꢀS(3) 2.493(1), CdꢀS(2) 2.572(1), CdꢀS(1) 2.529(3), CdꢀS(1a)
2.498(4), CdꢀO(3) 2.592(4), CdꢀO(2) 2.607(3), CuꢀP(2) 2.264(1),
CuꢀP(1) 2.286(1), CuꢀS(2) 2.353(1), CuꢀO(1) 2.381(9), S(2)ꢀC-
(44) 1.769(5), S(3)ꢀC(51) 1.747(6), S(1)ꢀC(37) 1.673(19), O-
(1)ꢀC(37) 1.265(15), O(2)ꢀC(44) 1.231(6), O(3)ꢀC(51) 1.232(6),
P(1)ꢀC(13) 1.820(5), P(1)ꢀC(1) 1.825(4), P(1)ꢀC(7) 1.826(5),
P(2)ꢀC(19) 1.818(5), P(2)ꢀC(31) 1.821(5), P(2)ꢀC(35) 1.830(5).
S(3)ꢀCdꢀS(2) 116.25(6), S(3)ꢀCdꢀS(1) 144.26(8), S(2)ꢀCdꢀS-
(1) 85.81(7), S(3)ꢀCdꢀO(1) 119.97(8), O(1)ꢀCdꢀS(2) 123.36(8),
O(1)ꢀCdꢀS(1) 51.26(10), S(2)ꢀCdꢀO(3) 93.76(11), O(1)ꢀCdꢀ
O(2) 102.96(12), O(3)ꢀCdꢀS(1) 91.64(11), S(1)ꢀCdꢀO(2)
115.94(10), O(3)ꢀCdꢀO(2) 138.33(14), S(2)ꢀCdꢀO(2) 60.33-
(14), S(3)ꢀCdꢀO(2) 99.68(9), P(2)ꢀCuꢀS(2) 116.95(5), P(2)ꢀ
CuꢀP(1) 121.35(5), P(2)ꢀCuꢀS(1) 115.47(7), P(1)ꢀCuꢀS(2)
107.55(5), S(2)ꢀCuꢀS(1) 90.71(8), P(1)ꢀCuꢀS(1) 99.43(9), C-
(44)ꢀS(2)ꢀCu 109.34(14), CuꢀS(2)ꢀCd 93.02(5).
[Cd2Cu3Br6L4(DMSO)2], and [Cd4Cu2Cl6L6(H2O)2] (where
LH = 2-dimethylaminoethanol),20 in which the MꢀM distances
vary from 3.399 to 4.043 Å and no MꢀM bond has been considered
while describing the structures. As mentioned already, 2a and 2b
are the first Cd(II)/Cu(I) heterodinuclear complexes, and there
are no structural data available for further comparison of the
Cd(II)ꢀCu(I) distance.
The molecular structure of the Cd/Cu-containing thiophene-
2-thiocarboxylate complex is slightly different from that of 2a.
Two of the three thiocarboxylate groups exhibit a μ-S bridging
mode of bonding. As shown in Figure 3, the cadmium atom in 2b
is hexacoordinated and surrounded by three oxygen and three
sulfur atoms. Cd is tipped above the O1O2O3 plane by 0.093 Å.
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Figure 4. Thermal ellipsoid plot (at 50% probability level) of 3a with
disordered thiophene rings (hydrogen atoms are omitted for clarity).
Selected metric data: CdꢀS(1) 2.612(13), CdꢀS(3) 2.474(1), CdꢀS(5)
2.572(1), CdꢀO(1) 2.464(4), CdꢀO(2) 2.650(4), CdꢀO(3) 2.579(4),
AgꢀP(1) 2.485(1), AgꢀP(2) 2.642(12), AgꢀS(1) 2.694(12), AgꢀS(5)
2.816(1). P(1)ꢀAgꢀP(1) 125.31(4), P(1)ꢀAgꢀS(1) 104.02(4),
P(1)ꢀAgꢀS(5) 95.92(4), P(2)ꢀAgꢀS(5) 117.69(5), P(2)ꢀAgꢀS(1)
116.01(4), S(1)ꢀAgꢀS(5) 91.29(4), S(3)ꢀCdꢀS(1) 127.66(5), S(5)ꢀ
CdꢀS(1) 120.80(9), S(3)ꢀCdꢀS(5) 129.57(5), O(1)ꢀCdꢀO(2)
136.39(14), O(1)ꢀCdꢀO(3) 79.54(12), O(3)ꢀCdꢀO(2) 140.51(13).
Figure 6. Thermal ellipsoid plot (at 50% probability level) of 3b with
disordered thiophene rings (hydrogen atoms are omitted for clarity).
Selected metric data: CdꢀS(1) 2.604(3), CdꢀS(3) 2.647(3), CdꢀS(5)
2.481(3), CdꢀO(2) 2.437(7), CdꢀO(4) 2.290(9), AgꢀP(1) 2.437(3),
AgꢀP(2) 2.478(2), AgꢀS(1) 2.708(3), AgꢀS(3) 2.744(3). P(1)ꢀAgꢀ
P(2) 131.51(9), S(1)ꢀAgꢀS(3) 88.10(9), P(1)ꢀAgꢀS(1) 115.70(9),
P(1)ꢀAgꢀS(3) 117.53(9), P(2)ꢀAgꢀS(1) 101.97(9), P(2)ꢀAgꢀ
S(3) 117.53(9), S(1)ꢀCdꢀS(3) 92.42(9), S(1)ꢀCdꢀS(5)
113.47(10), S(3)ꢀCdꢀS(5) 110.00(10), O(4)ꢀCdꢀO(2) 85.4(3),
O(4)ꢀCdꢀS(1) 94.6(2), O(4)ꢀCdꢀS(3) 140.3(3), O(4)ꢀCdꢀ-S(5)
102.8(2), O(2)ꢀCdꢀS(1) 130.5(2), O(2)ꢀCdꢀS(3) 61.02(12),
O(2)ꢀCdꢀS(5) 114.8(2).
A comparison with the reported bimetallic complexes revealed
notable structural differences with 2a and 2b. Earlier reported
anions, [A{Cd(SCOPh)3}2]ꢀ (where A = Na/K), showed
trigonal planar Cd(II) centers bonded to the three sulfur atoms
of the ligands.26 On the other hand, in the Cu(I)-containing
complexes [(Ph3P)CuM(SCOPh)4] (where M = Ga/In), two of
the three thiocarboxylate ligands exhibit a μ-S,O mode of
bridging the two atoms.27
The heterobimetallic complexes 3a and 3c crystallized in a
monoclinic system with space group P21. The thermal ellipsoid
plots of 3a and 3c are given in Figures 4 and 5. A survey of
literature revealed that these are the first examples of Cd/Ag
heterobinuclear complexes. The CdꢀAg distances (3.568 and
3.598 Å) in 3a and 3c are longer than the sum of the van der
Waals radii of Ag and Cd atoms (3.30 Å), and there is no
possibility of any bonding interactions between these two metal
centers.
In both complexes, Ag(I) is bonded with phosphorus atoms
of two triphenylphosphine moieties and two sulfur atoms of the
thiocarboxylate ligands. The bond angles subtended at the
silver atom are indicative of trigonal pyramidal (τ4= 0.83 and
0.81 for 3a and 3c, respectively) coordination geometry. The
AgꢀS(5) bond length [2.816 (1)Å] in case 3a is possibly the
longest silverꢀsulfur bond reported so far (the longest AgꢀS
bond length reported earlier was 2.735 Å in [(AgPPh3)4(μ-
SC{O}Ph-S)4]).28
Figure 5. Thermal ellipsoid plot (at 50% probability level) of 3c.
Hydrogen atoms have been omitted for clarity. Selected metric data:
CdꢀS(1) 2.612(1), CdꢀS(3) 2.474(1), CdꢀS(5) 2.572(15), Cdꢀ
O(1) 2.464(4), CdꢀO(2) 2.650(4), CdꢀO(3) 2.579(4), AgꢀP(1)
2.485(12), AgꢀP(2) 2.464(1), AgꢀS(1) 2.694(1), AgꢀS(5) 2.816(1).
P(1)ꢀAgꢀP(2) 125.93(2), P(1)ꢀAgꢀS(1) 117.00(2), P(1)ꢀAgꢀS-
(2) 119.48(4), P(2)ꢀAgꢀS(1) 98.61(2), P(2)ꢀAgꢀS(2) 98.65 (1),
S(2)ꢀAgꢀS(1) 89.62(1), S(3)ꢀCdꢀO(1) 99.89(4), S(3)ꢀCdꢀO(2)
102.31(4), O(1)ꢀCdꢀO(2) 81.34(6), S(3)ꢀCdꢀS(1) 133.98(2), O-
(1)ꢀCdꢀS(1) 61.02(4), O(2)ꢀCdꢀS(1) 114.04(4), S(3)ꢀCdꢀS(2)
126.12(12), O(1)ꢀCdꢀS(2) 124.12(4), O(2)ꢀCdꢀS(1) 61.14(4),
S(1)ꢀCdꢀS(2) 96.68(2), S(3)ꢀCdꢀO(3) 61.22(4), O(1)ꢀCdꢀO(3)
135.77(6), O(2)ꢀCdꢀO(3) 138.98(6), S(1)ꢀCdꢀO(3) 101.87(4),
S(2)ꢀCdꢀO(3) 96.78(4).
The triangular faces O2O1S5 and S1S3O3 of the approximate
trigonal prisms are almost parallel to each other with an inter-
planar angle of 1.37° with a meridional configuration. The
coordination environment around the Cu atom is comparable
to that observed in 2a (τ4 = 0.85). The thiophene rings of
terminal thiocarboxylate groups (bonded to Cd) of two adjacent
molecules are arranged in a displaced parallel fashion with a
centroidꢀcentroid distance of 3.94 Å, indicating πꢀπ stacking.
In both 3a and 3c, the Cd(II) is hexacoordinated and is
surrounded by three oxygen and three sulfur atoms. The two
triangular faces are almost parallel to each other in both cases
with interplanar angles of 0.74° and 1.03° respectively in 3a and
3c. As observed in 2a and 2b, the terminal CdꢀS distances are
shorter than the bridging CdꢀS bonds.
Complex 3b crystallized in the triclinic system with the P1
space group. The Cd(II) center (Figure 6) is four coordinated
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Figure 8. Helical structure of the polymeric chain of 4 along the a axis
due to C Hπ interactions.
3 3 3
Figure 7. Thermal ellipsoid plot of complex 4 (at 50% probability).
Hydrogen atoms are omitted for the clarity. Stereochemistries of Cd1
and Cd3 have been shown schematically. Selected metric data: Cd-
(1)ꢀO(1) 2.516(4), Cd(1)ꢀO(2) 2.546(5), Cd(1)ꢀS(1) 2.492(2),
Cd(1)ꢀS(2) 2.512(1), Cd(1)ꢀS(3) 2.579(1), Cd(2)ꢀS(3) 2.498(1),
Cd(2)ꢀS(4) 2.595(2), Cd(2)ꢀS(5) 2.593(1), Cd(2)ꢀO(3) 2.609(4),
Cd(2)ꢀO(4) 2.305(5), Cd(3)ꢀS(5) 2.500(16), Cd(3)ꢀS(6) 2.575
(1), Cd(3)ꢀS(7) 2.595(1), Cd(3)ꢀO(5) 2.633(4), Cd(3)ꢀO(6)
2.286(5), Cd(4)ꢀS(7) 2.514(1), Cd(4)ꢀO(7) 2.536(4), Cd(4)ꢀS(8)
2.490(2), Cd(4)ꢀO(8) 2.523(5), Cd(4)ꢀS(2)#1 2.575(17). O(1)ꢀ
Cd(1)ꢀO(2) 155.74(17), O(1)ꢀCd(1)ꢀS(1) 101.43(12), O(1)ꢀCd-
(1)ꢀS(2) 84.78(11), O(1)ꢀCd(1)ꢀS(3) 83.52(11), S(1)ꢀCd(1)ꢀS(2)
122.61(7), S(1)ꢀCd(1)ꢀS(3) 120.39(7), S(2)ꢀCd(1)ꢀS(3) 117.00(5),
O(2)ꢀCd(1)ꢀS(1) 60.24(14), O(2)ꢀCd(1)ꢀS(2) 117.97(13), O(2)ꢀ
Cd(1)ꢀS(3) 92.39(13), O(4)ꢀCd(2)ꢀO(3) 80.0(2), O(4)ꢀCd(2)ꢀ
S(3) 125.81(14), O(4)ꢀCd(2)ꢀS(5)110.45(17), O(4)ꢀCd(2)ꢀS(4)
61.45(17), S(3)ꢀCd(2)ꢀS(5) 118.74(5), S(3)ꢀCd(2)ꢀS(4) 124.13-
(6), O(6)ꢀCd(3)ꢀO(5) 78.72(18), S(6)ꢀCd(3)ꢀO(6) 62.76(15),
O(6)ꢀCd(3)ꢀS(5) 126.39(14), O(6)ꢀCd(3)ꢀS(7) 110.61(6), S(6)ꢀ
Cd(3)ꢀS(5) 124.61(6), S(6)ꢀCd(3)ꢀS(7) 102.33(6), S(5)ꢀCd(3)ꢀ
S(7) 117.31(5), O(5)ꢀCd(3)ꢀS(7) 82.21(11), O(5)ꢀCd(3)ꢀS(6)
140.49(11), O(5)ꢀCd(3)ꢀS(5) 84.63(10), O(8)ꢀCd(4)ꢀO(7) 156.17-
(17), O(8)ꢀCd(4)ꢀS(2)# 92.67(13), O(8)ꢀCd(4)ꢀS(7) 118.04(14),
O(8)ꢀCd(4)ꢀS(8) 60.86(14), S(2)#ꢀCd(4)ꢀS(7) 115.56(5), S(7)ꢀ
Cd(4)ꢀS(8) 123.25(7), S(2)#ꢀCd(4)ꢀS(8) 121.19(7).
Figure 9. Thermal ellipsoid plot (at 30% probability level) of 5 with
disordered thiophene rings (hydrogen atoms are omitted for clarity).
Selected metric data: Cd(1)ꢀS(1) 2.499(1), Cd(1)ꢀS(3)#1 2.552(14),
Cd(1)ꢀP(1) 2.576(1), Cd(1)ꢀS(3) 2.725(1), S(3)ꢀCd(1)#1 2.552(1),
S(1)ꢀCd(1)ꢀS(3)#1 108.72(5), S(1)ꢀCd(1)ꢀP(1) 123.60(4),
S(3)#1ꢀCd(1)ꢀP(1) 121.87(4), 101.83(4), S(3)#1ꢀCd(1)ꢀS(3) 97.44-
(3), P(1)ꢀCd(1)ꢀS(3) 95.11(4), Cd(1)#1ꢀS(3)ꢀCd(1) 82.56(3).
with a S3CdO2 core in which four coordination sites are occupied
by one oxygen atom (O2) and one terminal and two bridging
sulfur atoms (including two bridging ones) of thiophene-2-
thiocarboxylate groups, while the fifth coordination site is
occupied by the oxygen atom (O4) of a water molecule. From
the angles subtended at Cd, τ was calculated to be 0.16, which
suggests a slightly distorted square pyramidal geometry around
the metal. S1, S3, O2, and O4 constitute the square plane, while
S5 occupies the axial site. The Cd atom is tipped above the basal
plane by 0.852 Å.
The CdꢀO(4) bond length is comparable to the sum of the
covalent radii of the two atoms (2.22 Å), while the CdꢀO(2)
bond is slightly longer and is comparable to the CdꢀO distances
observed in complexes 3a and 3c. The silver atom is at the center
of a AgP2S2 core which is similar to those of 3a and 3c.
Complex 4 is a coordination polymer crystallized in a triclinic
system with the space group P1. The thermal ellipsoid plot is
given in Figure 7.
neutral homoleptic thiocarboxylate compound of any group 12
metal has yet been characterized crystallographically.
In a polymeric chain (Figure SI-1; Supporting Information),
each Cd atom is chelated by one thiobenzoate ligand, while one
oxygen and two sulfur atoms from three different ligands
occupy three coordination sites in a five-coordinate environ-
ment. Each carbonyl oxygen of the bridging ligands is also
bonded to another Cd center. A closer look at the coordination
environments around the Cd atoms revealed that there are two
different types of Cd(II) centers. The coordination spheres
around Cd1 and Cd4 are identical, while those around Cd2 and
Cd3 are of a different kind.
The orientations of the chelating ligands around Cd1 and Cd3
are shown schematically in Figure 7. Cd1 possesses a distorted
trigonal bipyramidal environment (τ = 0.72) made by two
oxygen and three sulfur atoms. The sulfur atoms occupy the
vertices of the equatorial plane forming an approximately equi-
lateral triangle. The Cd1 atom lies in the S3 plane. The O1 sits
rather perfectly on one of the axial sites (O1ꢀCd1ꢀS angles
varying between 83.52 and 101.42°), while the other oxygen O2
is highly displaced from the ideal axial position because of the
small bite angle of the SCO unit.
It may be noted that besides the anionic cadmium thiobenzo-
ate complexes there are only two structurally characterized
neutral cadmium thiocarboxylate complexes: [{Cd(SCOPh)2-
(μ-bpy)}n] and [{Cd2(SCOPh)4(μ-bpy)}n].29 These compounds
have one-dimensional polymeric structures. Interestingly, no
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The geometry around Cd3 is different, basically because of the
altered orientation of the chelating thiobenzoate ligand. More-
over, the Cd3ꢀO6 distance is quite short. As a result, the
geometry around Cd3 (τ = 0.23) is closer to that of a square
pyramid. The Cd3 atom is displaced from the S2O plane (by
0.346 Å). The angles between CdꢀS bonds (102.33 and
124.61°) are far away from 90°. In view of these distortions, an
alternative description could also be made considering the
environment around Cd3 to be closer to a tetrahedron having
S7, S5, S6, and O6 at the vertices, while the O5 atom caps the
triangular S7S6O6 face. Expectedly, the Cd3ꢀO5 distance is
larger than the Cd1ꢀO1 distance.
Due to C H π interactions (2.88 Å), the polymeric chain
has a helical structure along the a axis (Figure 8).
3 3 3
Complex 5 is a symmetrical homobimetallic complex with a
PSCd(μ-S)2CdSP core (Figure 9). In this molecule, each Cd
center is primarily bonded to a phosphorus atom of the triphenyl
phosphine ligand and the sulfur atoms of three thiophene-2-
thiocarboxylate ligands acquiring a distorted trigonal pyramidal
geometry (τ4 = 0.81). The Cd atom is displaced from the basal
plane constituted by P1, S1, and one of the bridging S atoms by
0.355 Å. The two MꢀS bonds are nonequivalent, which is a usual
feature observed in complexes with a Cu2S2 core.30 As the oxygen
(O1) of the terminal thiophene thiocarboxylate group is very close
to the Cd atom (at a distance of 2.674 Å), an alternative description
of the structure could be made, including O1 in the coordination
sphere, thus imparting a highly distorted distorted trigonal bipyr-
amidal geometry (τ = 0.60) around the Cd(II) center.
Table 2. Bond-Valence Parameters for 1, 3a, and 3c
ratio of
total bond Cd O/
Total Bond Valence Approach. Vittal et al.22 have calculated
bond-valence parameters31,32 for the anionic zinc and cadmium
thiobenzoate complexes. Similar calculations have been per-
formed on the complexes 1, 3a, and 3c [about CdS3O3 units in
complexes]. In all of these cases, the ratio of contributions of
Cd O to Cd S are higher than those in the earlier reported
3 3 3
CdꢀS
complex
CdꢀS (Å)
Cd O (Å)
3 3 3
valency (Vi)
1
CdꢀS1 2.548 CdꢀO1 2.560
CdꢀS2 2.565 CdꢀO2 2.487
CdꢀS3 2.610 CdꢀO3 2.431
CdꢀS1 2.613 CdꢀO1 2.464
CdꢀS3 2.475 CdꢀO2 2.650
CdꢀS5 2.572 CdꢀO3 2.579
CdꢀS1 2.577 CdꢀO1 2.541
CdꢀS2 2.584 CdꢀO2 2.542
CdꢀS3 2.477 CdꢀO3 2.623
2.07
0.43
3 3 3
3 3 3
3a
2.06
2.07
0.33
0.32
anionic complexes, revealing the significance of Cd O bonds.
3 3 3
The calculated bond valence parameters are given in Table 2.
Electronic Absorption Spectra. Electronic absorption spec-
tra of all of the heterobimetallic complexes were recorded as their
chloroform solutions (2 ꢁ 10ꢀ3 M). For complex 1, the
spectrum was recorded as a DMSO solution.
3c
The electronic absorption spectra of the complexes of thio-
phene-2-thiocarboxylate are comparable with one another. Com-
plex 1 showed a broad absorption band at 417 nm and sharp
peaks with comparatively higher intensities in the higher energy
region at 291, 246, and 219 nm (Figure SI2; Supporting
Information). It may be noted that only two strong intensity
peaks (at 311 and 260 nm) were observed in the spectrum of the
sodium salt of the ligand. The appearance of a peak at the lower
energy region (417 nm) in 1 was possibly due to the nfπ*
intraligand and metal to ligand charge transfers.
Complex 2b (Figure SI3; Supporting Information) showed a
broad absorption band at 421 nm due to the metal to ligand
charge transfers and two other intense absorptions at 324 and
250 nm assignable to the intraligand charge transfers. In the case
of 3a (Figure SI 4, Supporting Information), the sharp absorp-
tions at 332, 300, and 253 nm may be assigned as intraligand/
interligand charge transfer bands. However, in 3b (Figure SI 4,
Supporting Information), lower energy absorption bands at 492
and 456 nm possibly arose due to the charge transfers involving
the electron pairs of the oxygen of coordinated water.
Figure 10. Thermal decomposition patterns of complexes 2a, 2b, 3a,
and 3c.
Figure 11. PXRD patterns of 6 and 7.
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Figure 12. PXRD patterns of 8 and 9.
Figure 13. SEM-EDS images of (A) 6 and (B) 7.
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Figure 14. SEM-EDS images of (A) 8 and (B) 9.
In the case of 2a (Figure SI3; Supporting Information), the
electronic absorption spectrum (in solid state) showed peaks at
250 and 299 nm assignable as intraligand charge transfer bands,
whereas the peak at 375 nm was possibly due to the metal to
ligand charge transfer. In solution, a broad peak covering
299ꢀ400 nm is observed beside the one at 250 nm. In the
spectrum of 3c (Figure SI3, Supporting Information), peaks
observed at 338 and 317 nm may be due to the intraligand/
interligand charge transfers, while the peaks at higher energy
absorption bands 244 and 220 nm may be assigned as intraligand
charge transfers.
In the solid state spectrum (Figure SI5, Supporting In-
formation) of 4, strong intensity peaks were observed at 249
and 269 nm, while two peaks with lower intensities were
observed at 395 and 423 nm. The strong intensity peaks were
expected to be due to the intraligand charge transfers, whereas
the lower intensity peaks observed at lower frequencies may have
arisen possibly due to the metal to ligand charge transfers.
Thermogravimetry and Pyrolysis. We have investigated the
thermal decomposition patterns of the Cd/Cu- and Cd/Ag-con-
taining thiocarboxylate complexes 2a, 2b, 3a, and 3c (Figure 10).
Complexes 2a, 2b, 3a, and 3c showed single step decomposition in
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the TGA experiment. In the case of 2a, decomposition occurred in
the temperature range of 221ꢀ369 °C with a 65% weight loss,
whereas for sulfur-rich complex 2b, decomposition was observed
between 200 and 395 °C with a 61% weight loss. Complex 3a
showed a one-step decomposition in the temperature range of
200ꢀ415 °C with a 58% weight loss; however, 3c showed a 74.62%
weight loss in the temperature range 192ꢀ372 °C. From the
residual weights of the pyrolyzed products, it is not possible to
predict their elemental composition. However, the higher
residual weights in the cases of 2b and 3a indicate higher
percentages of sulfur which have originated from the presence
of an extra sulfur atom in each thiophene-2-thiocarboxylate
ligand. Earlier studies on thermal decomposition of a few
thiocarboxylate complexes have shown that metal sulfides are
the end products of pyrolysis.12,21 Our recent experiments on
the pyrolysis of Pb/Cu and Pb/Ag heterobimetallic complexes
resulted in the corresponding bimetallic oxides.33
On the basis of these TGA results, we have performed
pyrolysis experiments. On pyrolyzing the complexes 2a and 2b,
Figure 15. Absorption spectra of 6 and 7 in the solid state (recorded as
Nujol mulls).
Figure 16. (A) Confocal micrographs of the regions from where PL spectra were recorded. (B) Photoluminescence spectra of 6 (λexc = 488 nm at an
excitation power of 0.1 mW).
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(JCPDS No. 00ꢀ02ꢀ0563) and acanthite (Ag2S, JCPDS
No. 01ꢀ003ꢀ0844) are noticeable. As observed in 6 and 7,
the d-spacings in 8 and 9 (Table SI2; Supporting Information)
also showed small changes [the d value of 3.12 Å (for 101 peak of
pure CdS) shifted to 3.15 and 3.17 Å in 8 and 9, respectively],
indicating the formation of homogeneous solid solutions of
metal sulfides Ag2Cd8S9 and Ag2Cd5S6. To the best of our
knowledge, no silverꢀcadmium sulfides have ever been reported.
Surface Morphology. The surface morphologies of 6 and 7
were studied using scanning electron microscopy (Figure 13),
which showed the presence of uniform granular crystallites.
Notably, the earlier reported (CuS)x(CdS)1ꢀx showed distinct
CuS and CdS phases in the scanning electron micrographs.37
Figure 14 shows the SEM topography of 8 and 9. At 10 000
magnifications, the surface image of 9 looks granulated with
irregular sizes and shapes; however, 8 has a uniform blistered
surface which bears no cracks.
Optical Properties. The optical properties of 6 and 7 were
investigated by UVꢀvisible spectra, as shown in Figure 15. In both
cases, due to the presence of copper, the electronic absorption
spectrashowed a shifttowardlonger wavelengthsincomparison to
the CdS, which covers only the ultraviolet absorption range from
310 to 490 nm.
Photoluminescence and Photoluminescence Excitation
Spectra. The PL spectra of 6 and 7 confirmed the compositional
homogeneity of the sample. The spectral pattern remained
unchanged over the whole film of the sample, and the intensities
varied with the film thickness proportionately, which could be
monitored by confocal microscopy. The higher energy band edge
emission with a narrow fwhm (12 nm) was observed at 504 nm,
while the lower energy Cu dopant emission appeared as a broad
and intense peak centered at 745 nm (86 nm fwhm). It may be
mentioned here that the band edge emission of CdS/CdSe
nanocrystallites is usually more intense than the lower energy
deep trap emission band which appears at >700 nm35 The Cu
dopant emission on the other hand has a higher intensity and is
more red-shifted.14 Figure 16a indicates the surface region from
where the PL spectra were recorded.
Figure 17. (A) Confocal micrograph of surface area used for data
collection. (B) PL spectra of 7 (λexc = 488 nm at excitation power
0.3 mW).
Intensities of emission peaks in the spectrum of 7 (Figure 17)
were very poor compared to those obtained in 6 under identical
conditions (λexc: 488 nm, 0.1mW, integration time 5 s) due to
the comparatively low copper content in it as compared to 6.
However, when excitation power was increased from 0.1 to
0.3 mW, the number of CCD counts enhanced appreciably. The
band edge emission was observed at 499 nm (fwhm = 2 nm),
while the Cu dopant emission was found at 745 nm (fwhm =
92 nm). Interestingly, the deep trap emission of CdS was also
observable as a weak intensity band centered at 573 nm (fwhm =
175 nm).
The band edge emissions are red-shifted from the band edge
absorption peak by 16 and 11 nm, respectively, in 8 and 9. These
shifts can be corroborated with the quantities of CuS in the CdS
solvent.
Pressed Pellet Electrical Conductivity. For the complex
[Cd(SCOPh)2]n, electrical conductivity data were recorded
from 309 to 353 K, while in case of ternary metal sulfides, the
conductivity data were collected from 303 to 493 K.
ternary sulfide products CuCd7S8 (6) and CuCd10S11 (7) were
isolated, respectively. Similarly, after pyrolysis of the Ag/Cd-
containing complexes 3a and 3c, Ag2Cd8S9 (8) and Ag2Cd5S6 (9)
were isolated, respectively. (The compositions are based on the
ratio of the two metals found by EDX spectral analyses.)
Nature of the Sulfide Particles. The metal sulfides were studied
through their powder X-ray diffraction patterns (Figures 11 and 12).
In both (6 and 7) cases, reflections of CdS (greenockite, JCPDS No.
00ꢀ02ꢀ0563) and CuS (covellite, JCPDS No. 01ꢀ075ꢀ2235)
are clearly observed. On first sight, it appears that the product is a
mixture of CdS and CuS; however, a closer look into the diffraction
pattern revealed small changes in d-spacings with respect to pure
greenockite. Such a pattern is indicative of a solid solution formation
(alloying).34,35 The peak observed at a d-spacing of 3.12 Å in
greenockite (hexagonal CdS) showed a shift to larger values of 3.16
and 3.17 Å, respectively, in 6 (10% Cu) and 7 (14.2% Cu; Table SI 1,
Supporting Information). Variations in lattice parameters are ob-
viously small in these cases.36 Earlier studies on copperꢀcadmium
sulfides prepared by coprecipitation led to the formation of two
separate mineral phases (CuS and CdS), and the X-ray diffraction
spectra showed reflections due to both pure phases.37
At room temperature (36 °C), the [Cd(SCOPh)2]n polymer
showed an electrical conductivity of 1.56 ꢁ 10ꢀ7 S/cm, which
deceased with a rise in temperature, and at 80 °C, it became
nonconducting (σ = 1.75 ꢁ 10ꢀ11 S/cm). The decrease in
electrical conductivity with an increase in temperature indicates
In the X-ray diffraction spectra of the pyrolysis products
of 3a and 3c (Figure 12), reflection peaks of greenockite
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Figure 18. Pressed pellet electrical conductivities of (A) 4, (B) 6, (C) 7, and (D) 9.
that the [Cd(SCOPh)2]n polymer has a metal-like conducting
nature (Figure 18a).
sulfides are the first ones of their kind. Thus, this study provides
a low energy pathway for making copper or silver doped CdS
nanoparticles.
The room temperature electrical conductivity of the ternary
metal sulfides of Cd/Cu are comparable to each other. In the case
of CuCd7S8, the room temperature pressed pellet electrical
conductivity was 1.63 ꢁ 10ꢀ9 S/cm at 36 °C, which gradually
increased to 3.28 ꢁ 10ꢀ7 S/cm at 220 °C (Figure 18b), while in
the case of CuCd10S11, the room temperature electrical con-
ductivity was 2.08 ꢁ 10ꢀ13 S/cm, which continuously increased
with the temperature. Finally, at 220 °C, it became 4.20 ꢁ 10ꢀ9
S/cm. (Figure 18c).
’ ASSOCIATED CONTENT
S
Supporting Information. Additional figures and crystal-
b
lographic information files (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Silver-containing ternary metal sulfide Ag2Cd5S6 showed a
pressed pellet electrical conductivity of 2.03 ꢁ 10ꢀ12 S/cm at
room temperature (36 °C), which gradually increased to 2.70 ꢁ
10ꢀ8 S/cm at 220 °C, indicating its semiconducting nature
(Figure 18d).
Corresponding Author
*E-mail: s_bhatt@bhu.ac.in.
Author Contributions
§J.C. and S.S. have equal contributions in this paper
’ CONCLUSION
’ ACKNOWLEDGMENT
We have synthesized and structurally characterized novel
heterobimetallic Cd(II)/Cu(I) and Cd(II)/Ag(I) thiocarboxy-
late complexes along with a polymeric Cd(II) thiobenzoate. The
compounds of Cd(II)/Ag(I) are unique, being the first ones
containing these two metal atoms in the same molecule. The
heterobimetallic complexes on pyrolysis yielded corresponding
ternary sulfides. Unlike the products of earlier attempts, these
sulfides are solid solutions of CuS in CdS. While the Ag/Cd
The authors are grateful to Prof. T. N. Guru Row, S.S.C.U.,
Indian Institute of Science, for powder X-ray diffraction measure-
ments; Dr. Jaydeep Basu and Mr. Laxmi Narayan Tripathi,
Department of Physics, Indian Institute of Science, Bangalore for
PL measurements; Director, SAIF, Cochin, India for SEM-EDX
studies; and Prof. R. S. Tiwari, Physics Department, Banaras
Hindu University for useful discussions. Financial assistance to
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S.B. in the form of a scheme and fellowships to J.C. and S.S. by the
Council of Scientific and Industrial Research, India are gratefully
acknowledged.
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