Indian Journal of Chemistry
Vol. 44A, March 2005, pp. 504-510
Notes
Synthesis, spectral and kinetic studies of
copper(II) thiocarboxylates as molecular
precursor for metal sulfide
Tarkeshwar Gupta 1 & B P Baranwal *
Department of Chemistry, D.D.U. Gorakhpur University,
Gorakhpur 273 009, India
Email: [email protected]
Received 6 August 2004; revised 4 la11uary 2005
A series of binary and ternary copper(II) complexes of
thiocarboxy lic acids and straight chain fatty acids with general
formula Cu(SOCRh(EtOH) and Cu(SOCR)(OOCR')(EtOH)
[where R = CH3 or C6H5 and R' = c7 Hl5· C IIH2J or CuH27l ha ve
been sy nthesized and characterized. The binary copper(Il)
complexes have been investigated for their potential to act as
precursors for the formation of metal sulfide material s. These
spec ies are expec ted to undergo thiocarboxy lic anhydride
elimination to give stoichi ometric metal sulfides and remove the
organic supporting ligands cleanly. The complexes are
characterized by elemental and thermo gravi metric analyses,
spectra l (IR, UV /Visible and ESR) and molar conductivity
studies, molecular weight and magnetic moment measurements.
The spectroscopic and analytical data have been found to be
consistent with the identified formulae and the compounds are
dimeric with bridging mode of coordination of the ligands.
Magnetic data and ESR studies suggest strong antiferromagnetic
coupling between two copper atoms. Thermal decomposition of
binary copper(II) complexes in the solid state results in the
formation of CuS at low temperature as seen by powder X-ray
diffraction studies. The kinetic parameters of these complexes
ha ve been investigated on the basis of the respective thermal
curves. The values of the activation energy (£,) of thermal
decomposition , reaction order (n), frequency factor (A) and
velocity constant (k) (in the A.rrhenius kinetic equation), have
been establi shed from the thermal data. The preliminary studies
reveal the great potential of this highly tailorable chemical system
as precursors to metal sult.do spec ies.
IPC Code: C07FI /08
Monothiocarboxylates are interesting systems to study
both for their usages as single source precursors 1•3 and
for their versatile coordination behaviour45 because of
thei r 'soft' sulfur donor and ' hard' oxygen donor
si tes. Recently, we have initiated studies o n the
complexation behaviour of monothiocarboxylate
anion and their thermal degradation to prepare metal
sulfide materials 6· 8 . Sulfides of various transition
1
Present Address: Department of Organic Chemistry, Weizmann
Institute of Science, Rehovot 76 I00, Israel.
metals show interesting electrical and optical
properties like semiconductivity, luminescence and
9 10
photoconductivity ' • An attractive method to prepare
these metal sulfides seems to be the degradation of
suitable single source precursor, i.e., metal complexes
of sulfur containing ligands 11 · 13 . Single source
precursors have definite advantages over the
conventional methods, namely, solid-state reaction or
homogeneous
precipitation
techniques.
The
requirement of high temperature (ca. 800°C), long
reaction times and formation of inseparable mixtures
and phase impure compounds handicap the
conventional solid state reaction, while precipitation
methods often lead to the formation of amorphous
materials which require further annealing to render
them crystalline. Single source precurso rs to various
metal sulfides have been synthesized using a wide
range of sulfur donor ligands, viz.., thiolates, mono
14
and dithiocarbamates, dithiocarboxylates, etc •15 •
However, limited reports are available on the uses of
metal monothiocarboxylates compounds to produce
corresponding metal sulfides 16· 17 . Hampden-Smith and
coworkersl. 17 have successfully used a variety of
metal thiocarboxylates as precursors to prepare metal
sulfides that include ZnS and CdS. We report herein
the
synthesis
and
characterization
of
bis(thiocarboxylate) and
their monosubstituted
derivatives of copper(II) with long chain fatty acids.
The thermal decomposition of these compounds under
nitrogen atmosphere followed by TG is also
described.
Experimental
The organic solvents (Qualigens) were purified by
18
standard procedure before use • Cupric chloride
dihydrate (Qualigens), sodium carbonate (CDH),
potassium hydroxide (Qualigens) and fatty acids like
caprylic, lauric and myristic acids (Fluka) were used
as supplied. Thioacetic and thiobenzoic acids (Fluka)
were distilled prior to use. Infrared spectra were
recorded
on
a
Perkin-Elmer
model
125
spectrophotometer using KBr di scs in 300-4000 cm· 1
reg ion. Microanalyses (C and H) were done on Carlo
Erba 1108 analyzer. Electronic spectra were recorded
in their toluene solution of mixed ligand complexes
and in acetonitrile of bis(thiocarboxylato)copper(ll)
NOTES
505
Table !-Analytical and conductance data for the complexes
Compound
(Empirical Formula)
Colour
Yield( %)
Cu(SOCCH 3MEtOH)
(CuC6H 1203S2)
Dark green
Cu(SOCC 6H 5MEtOH)
(CuC 16HI603S2)
Green
(78)
(74)
Cu(SOCCH3)(00CC 7H 15 )(EtOH)
(CuC12H 2404S)
(70)
Cu(SOCCH 3)(00CC 11 H 23 )(EtOH)
(CuC1 6H 32 04S)
(69)
Cu(SOCCH 3 )(00CC 13H 27 )(EtOH)
(CuC1sHJ604S)
Dark green
Bro wnish green
Brownish green
(71)
Cu(SOCC 6H 5)(00CC 7H 15 )(Et0H)
(CuC11H2 604S)
(68)
Cu(SOCC 6H5)(00CC 11 H 23 )(EtOH)
(CuC2 1H3404S)
(70)
Cu(SOCC 6H 5)(00CC 13 H 27 )(Et0H)
(C uC23 H3s04S)
(72)
Yellowish green
Yellowi sh green
Light green
Analz:sis, Found (Calcd)
H
s
(%)
(%)
(%)
c
Conductance
(Q- 1cm 2
mor 1)
Mol.
Wt.
Cu
(%)
512
(259.9)
24.3
(24.5)
27.4
(27.7)
4.7
(4.7)
24.6
(24.7)
17.6
(17.7)
8.12
752
(384.0)
16.4
(16.5)
49.9
(50.0)
4.2
(4.2)
16.5
(16.7)
11.8
(12.0)
9.35
647
(328.0)
19.1
( 19.4)
43.7
(43.9)
7.3
(7.3)
9.6
(9.8)
14.0
( 14.1)
6.71
750
(384.1)
16.3
(16.5)
50.0
(50.0)
8.3
(8.3)
8.0
(8.3)
12.2
( 12.0)
7.11
842
(412.15)
15.4
(15.4)
52.3
(52.5)
8.7
(8.8)
7.9
(7.8)
11.0
(11.2)
6.60
772
(390.0)
16.1
(16.3)
5_2.0
(52 .3)
6.7
(6.7)
8.2
(8.2)
11.4
( 11.8)
5. 11
879
(446.2)
14.3
( 14.2)
56.6
(56.5)
7.7
(7 .7)
7.0
(7.2)
10.1
(10.3)
5.01
980
(474.2)
13.3
(13.4)
58.4
(58.2)
8.1
(8. 1)
6.7
(6.8)
9.6
(9.7)
complexes on a Shimadzu (model-uvmini-1240) UVVisible spectrophotometer. Magnetic susceptibility
measurements were carried out on Gouy balance
using Hg[Co(SCN) 4] as calibrant. Conductance
measurements were carried out on a Century (modelCC-601) co ndu ctivity meter wi th a dip type cell using
2
10- -10-4 molar solut ion of the complexes. Molecular
weights were determined by cryoscopic method using
Beckmann thermometer. Thermal analyses were
performed on Mettler TC lOA TA processor eq uipped
with a four-channel recorder and TGT and DTGT
adapters for titration of gases. The compound was
heated in corundum crucibles, using a Ah0 3
(corundum) as reference. In the gaseous products of
decomposition of the sample, the acid compo nents
were determined by means of adsorption in the TGT
and DTGT adaptor, and then by titration with 0. I M
KOH at pH 9.2. The X-ray powder diffraction
patterns of the residue after TG were recorded with an
X-ray diffractograph (XRD-5 General Electric, USA)
using CuKa radiation . Copper was estimated
gravimetricall y as cuprous thiocyanate, su lfur as
barium sulph ate, acetic acid in the azeotrope by the
19
standard method and alcohol was estimated by an
oxidimetric method as reported earliar20 .
EtOH
(%)
4.88
Synthesis of Cu(SOCCH 3)z.EtOH
Sodium salt of thioacetic acid/thiobenzoic acid was
prepared as reported earli er6 . Bis(thioacetato )copper(II)monoalcoholate was prepared by adding
dropwise ethanoli c solution of sodium thioacetate
(1.02 g; 10.40 mmol) to the ethanolic solution of
CuCI 2.2 H20 (0.89 g; 5.20 mmol). The reaction
mixture was refluxed for 5 h with stirring, cooled at
room temperature and a dark green crystalline product
was obtai ned after evaporating the solvent. This was
washed with ethanol-ether mixture and dried in vacuo
over P40 10 . Bis(thiobenzoato)copper(II)monoalcoholate was also prepared following the same
procedure and the detailed analytical results are
summarized in Table 1.
Synthesis of Cu(SOCCH 3)(00CC 13 H 27 ).EtOH
Myristic acid (1 .2 1 g; 5.30 mmol) was dissolved in
50 mL toluene. This sol uti on was added slowly to a
toluene suspension of Cu(SOCCH 3 h.EtOH (1.38 g;
5.3 1 mmol) with constant stirring. The colour of the
reaction mixture changed from dark green to li ght
green. The contents were refluxed for 20-24 h with
slow and continuous azeotropic fractionati on of the
liberated thioacetic acid with toluene (b.p. I 03°C).
INDIAN J CHEM, SEC A, MARCH 2005
506
After completion of the reaction, the product become
completely soluble in toluene. This product was
precipitated by addition of an excess of ethanol. The
brownish green product was collected by filtration
and drying under vacuum. A few more mixed-ligand
complexes were synthesized following the same
procedure and the analytical results are summarized in
Table l.
Results and discussion
The
thiocarboxylato
complexes
of
type
[Cu(SOCRh.(EtOH)] were synthesized by the
reaction of copper chloride dihydrate and sodium salt
of thiocarboxylic acid:
CuCI 2.2H 20 + 2 NaSOCR ----i~~ Cu(SOCR) 2(Et0H)+2 NaCI
(where R = CH 3 or C 6 Hs)
These compounds were quite stable towards
substitution of thio-ligands but substitution could be
achieved by driving the following reaction in the
forward direction .
~
Cu(SOCR) 2(Et0H) + R'COOH
Cu(SOCR)(OOCR') + RCOSH
Cu(SOCR)(OOCR') + EtOHccxccss)
i
~
Cu(SOCR)(OOCR')(EtOH)
J.
In the process, a nonpolar solvent (toluene) was
used which was found to be the best choice for the
synthesis of these complexes as it forms an azeotrope
with thioacetic/thiobenzoic acid which could be
fractionated out below 11 0°C to push the reaction in
the forward direction. The progress of the reaction
was slow which was checked by estimating the acid
content
in
the
collected
azeotrope.
The
bis(thiocarboxylato )copper(II)
complexes
were
soluble in acetonitrile, chloroform; less soluble in
methanol, ethanol; whereas mono(carboxylato)mono(thiocarboxylato)copper(Il) complexes were soluble
in benzene, toluene and nitrobenzene.
The molar conductivities of the complexes in
acetonitrile/nitrobenzene
at
room
temperature
(Table I) indicate non-electrolytic behavior of the
complexes in respective solvents. The C, H, S and Cu
analyses were close to the calculated values for the
synthesized complexes (Table I) . The molecular
weight o f the complexes was determined in
acetonitrile/toluene by cryoscopic method at room
temperature. The value obtained (Table l) indicates
the dinuclear nature of all the complexes.
In the infrared spectra of the complexes, 0-H
stretching vibrations of carboxylic acids and S-H
stretching vibration of thiocarboxylic acids were
found to be absent in the region, ca. 3200 and 2560
cm· 1• The strong absorptions at 1690 and 950 cm· 1 in
free thiocarboxylic acids are due to D(C·Ol and u (C-S)
vibration, respectively. These were observed in their
1
sodium salts at 1500 and 960 cm· • The decrease in
D(c-o) and slight increase in u (C-Sl indicated that their
sodium salts are ionic in nature 6 . In the complexes,
both D(C-Ol and D(C-Sl decrease compared to their
sodium salts (Table 2), indicating the bridging type of
coordination 21 of thiocarboxylic acids . In the
complexes of type [Cu(SOCR)(OOCR')(EtOH)],
symmetrical and asymmetrical COO stretching
vibrations (u 1 and u 2) due to carboxylate group are
important and their positions and separation (u 2-u 1)
can help in determining the mode of coordination of
22
carboxylate ligand . The ~u value (Table 2) indicated
presence of bridging mode of coordination in all the
complexes. The medium bands observed at ca. 670
1
and 652 cm· could be assigned to 8(oco) and 8(ocs) of
bridging carboxylate and thiocarboxylate ligands,
respectivel/ 3. Absorption near 360 and 520 could be
assigned due to Cu-S and Cu-0 vibration,
4
respectivel/ . The slightly lowering of u (O- H)
frequencies compared to free ethanol indicated the
coordination of 0-H group.
Electronic spectra of all the complexes exhibited a
broad d-d band in visible region , which may be
2
2
assigned to A 1 ~ B 1 g, and two weak C-T bands in
UV region (Table 2). The broadness of d-d band is
expected for copper(II) complexes in a tetragonal
field because of the Jahn-Teller distortion . The
complexes gave no signals in their ESR spectra,
which
indicates the strong antiferromagnetic
interactions between two copper atoms.
Room temperature magnetic moment measurement
values (Table 2) show a strong antiferromagnetic
coupling between the copper centers in a molecule for
all the complexes . The complex [Cu 2 (SOCCH 3) 4
(EtOHh] was subjected to a variable temperature
magnetic susceptibility study in the temperature range
300-17 K. The magnetic moment of 2.51 ).!8 at 300 K
and 0.35 ).! 8 at 17 K per molecule show strong
antiferromagnetic coupling between the copper
T
centers . A plot of XM versus T for [Cu 2 (SOCCH 3 ) 4
NOTES
507
Table 2 - Significant IR and electronic spectral bands* and room temperature magnetic data per molecule for the complexes
d-d band
C-T band
Jlcrr
(Jls)
946 s
672
249,220
2.5
1479 s
948 s
681
245,221
2.4
Cu(SOCCH 3)(00CC 7H 15)(Et0H)
1484 s
946 s
1597 s
1435 s
162
666
269, 224
2.6
Cu(SOCCH 3)(00CC1 1H23)(EtOH)
1486 s
946 s
1600 s
l440m
160
661
266,221
2.5
Cu(SOCCH 3)(00CC 13 H27 )(Et0H)
1482 s
944 s
1599 s
1438
Ill
161
662
270,222
2.5
Cu(SOCC6Hs)(00CC 7 H 15 )(EtOH)
1486
945 s
1605 s
1436 s
169
679
262,220
2.6
Cu(SOCC6H 5)(00CC 11 H23 )(Et0H)
1485 s
946
1604 s
1442
Ill
162
678
274,241
2.6
Cu(SOCC 6H5)(00CC 13H27 )(Et0H)
1486 s
944 s
1600 s
1436 s
164
678
266, 228
2.6
Compound
Dcc-s)
Dec-o)
Cu(SOCCH 3MEtOH)
1480 s
Cu(SOCC 6 H5MEtOH)
VS
Dcc-O)asy
VS
D(C-O)sym
L'iu
*Abbreviations: s, strong; v, very; m, medium; absorption are in cm· 1.
Table 3 -Thermo gravimetric data of the complexes
Mass of th e
sample
( mg)
Temperature
range (°C)
Loss of the
mass(%)
Consumption
ofO.l M KOH
(mL)
Total mass
loss(%) Found
(Calcd)
Metal content in
sintered product
(%)
Cu(SOCCH 3MEtOH)
200
50- !50
170-360
17
46
0
15.2
63.00
(63.21)
66.2
Cu(SOCC6H 5MEtOH)
200
90- 180
235-325
12
63
0
10.3
75.00
(75.10)
66.4
Compound
0.8
~
0.6
-0
E
n
E
0.4
~
1~
:--,
0.2
00
0
50
100
150
200
250
300
Temp {K)
Fig. I -
x~; ve rsus T for the complex [CuiSOCCH 3) 4 .(EtOH)2]
in the temperature ran ge 300-3 17 K.
(EtOH)2] is given in Fig. 1. The nature of coupling
shows that the complex is dinuclear and has a short
Cu-Cu di stance.
Thermogravimetric
analyses
data
for
Cu(SOCCH 3h.EtOH and Cu(SOCC6 H5)2.EtOH are
given in Table 3. Figure 2 shows the thermal curves
of the Cu(SOCRh.EtOH in a dynamic nitrogen
atmosphere. The weight loss and consumption of the
0.1 M KOH for the titration of anhydride gases from
the decomposition in dynamic nitrogen atmosphere
are given in Table 3. The complex decomposes in two
stages. The first stage of weight loss corresponds with
elimination of alcohol and the second with pyrolysis.
The mass loss data (Table 3), elemental analysis
[Found: Cu, 66.2 and 66.4; S, 33.4 and 33.3 from
sintered product of Cu(SOCCH 3h.EtOH and
Cu(SOCC6H 5 h.EtOH, respectively, Calculated for
C uS: Cu, 66.5; S, 33.5] and X-ray diffraction pattern
(Fig. 3) show that the final product was CuS (JCPDS
File No. 36379). The XRD pattern also indicates the
crystalline nature of the sample. The mass loss data
suggest that during the thermal decomposition,
thioanhydride is formed. This has been confirmed by
the consumption of 0.1 M KOH for the titration of the
INDIAN J CHEM, SEC A, MARCH 2005
508
DTG
-\
rv'\ I(
1
I I
D_~A\./
',, It
--DTG
I
l""'v\.'t--r1r-·
\ ,_TbT \ .1
I
I
~-------I
0
't
])!?2<~ --~- ~
' TG
20
f-
40 r
·····. \ .
I\ ,•
DTGT
:~ /:
·.. ;. \I
\'
E
: s..___
<l
- -- -- -- ,~
; .I
.: ;
100
..
~
I
111
·. ;
',
'. ;
'·.J
.
(a)
80
':
0
Fig . 2- Thermal curves
(b) Cu(SOCC 6 H5h,Et0H].
:i..·v __
;
..
60
80 r
I
I
1
600
200
400
Temperature, oc
0
200
---
(b)
400
Temperature.
600
0
(
the dynamic argon atmosphere (V = 20 dm 3/h; Heating rate 5°/min) [(a) Cu(SOCCH 3) 2.EtOH ,
1 .8
1 .6
1 .4
--B
E
'0
Ol
1 .2
1 .0
..Q
(a)
0 .8
2C•
40
50
7(1
20 i degree
~
0 .6
0 .4
Fig. 3 - XRD of the copper sulfide sample
gaseous products. The thermal decomposition of
Cu(SOCRh.EtOH could be described by Eqs (1) and
0 .2
2 .0
2.2
2 .4
2 .6
2.8
3 .0
3 .2
1/T, x 103 [deg- 1]
(2).
Cu(SOCRh.EtOH ---;~~Cu (SOC R h + EtOH
(1)
Cu(SOCR) 2 ----;~~ CuS + (RCOhS
(2)
Fig. 4 - Graphic determination of £" (E" = 2.303.R. tg a) for
Cu(SOCC 6 H5h.EtOH [(a) the desolvation reac ti on; tg a=3xiO\
£.=57.4 kJ/mol ; (b) the thermal decomposition reaction ;
tg a =l xl0 4 , £.=191.5 kJ/mol].
NOTES
509
Table 3 - Thermogravimetric data of the complexes
Mass of the
sample
(mg)
Temperature
range (0 C)
Cu(SOCCH 3h(EtOH)
200
50- 150
170-360
Cu(SOCC6Hsh(EtOH)
200
90- 180
235-325
Compound
Consumption
ofO.l MKOH
(mL)
Total mass
loss(%) Found
(Calcd)
Metal content in
sintered product
(%)
17
46
0
15.2
63.00
(63.21)
66.2
12
63
0
10.3
75.00
(75.10)
66.4
Loss of the .
mass(%)
Table 4 - Kinetic parameters of the thermal decompositions of the complexes
Temperature range (K)
Compound
Cu(SOCCH 3h(EtOH)
Order of
reaction, n
Value of A
Velocity constant,
kat
290 K (s-1)
45.0
0.2
1.2xl08
8.5xl0' 1
57.4
0.2
2.3x108
7.lxl0-3
109.0
0.2
2.8x10 12
1.7xl0- 7
191.5
0.5
3.8x I0 20
9.5Xx10-13
Desolvation reactions
323-423
Cu(SOCC6H5 h(EtOH)
Cu(SOCCH 3)2
Activation energy,
Ea (kJ/mol)
263-453
Decomposition reactions
443-663
508-598
Cu(SOCC6Hsh
2 0 .6
8.4
-o
--
---o
E
E
(b)
0)
0
0)
+i::l
0
+i::l
£l
£l
t:::
.;)
'0
'0
2 0 .4
t:::
8.2
2 0 .2
-0.8
-0.4
-0.2
0.0
-0.6
-0.4
-0 . 2
0 .0
log C
log C
Fig. 5 - Graphic determination of k (log k =log A- £,/2.303 RT) for the Cu(SOCC6H5 h.EtOH: (a) desolvation reaction; n=tg 8=0.2,
A = 2.3x I08 , k = 7 .I x 10' 3 ; (b) the thermal decomposition reaction; n=0.5, A = 3.8x 1020 , k = 9.5x 10· 13 •
Bulk copper sulfide materials were obtained by
pyrolysis of complexes at a temperature 380°C under
a reduced pressure of 0.5 torr. Table 4 gives activation
energy (£,), the reaction order (n), frequency factor
(A) and velocity constant (K) for the thermal
decomposition of the compounds under test. The
calculation technique and equations used were given
Th
.
.
d ata
. 25 ·26 .
ear Iter
_e
th ermogravtmetnc
of
Cu(SOCC 6 H5 hEtOH were used to calculate the
relationship between log dm/dt and liT (Fig. 4) or liT
tg a+ log dm/dt and log C (Fig. 5), where:
The plot of Figs 4 and 5 shows a linear relationship
between Jog dm/dt and liT or liT tg a +log dm/dt and
log C. Error in the determination of Ea in this way is
±3.8%, while for A it is ±6.2%. The velocity constant
INDIAN J CHEM, SEC A, MARCH 2005
510
K was calculated at 290 K. The low values of
activation energy (Ea) for desolvation reaction and
hi gh value for decomposition reaction show that the
decomposition of the complexes is not a
straightforward process. It most probably interferes
with structural rearrangements due to chelate effects
of the bridging ligands 27 . In decomposition reaction ,
Ea for Cu(SOCC6Hsh is higher than Cu(SOCCH3h
which indicates formation of (C 6 H5COhS is slower
th an (CH 3COhS. It is evident from Table 4 that both
desolvation and decomposition reactions followed
fractional or half order degradation kinetics. Velocity
constant and frequency factor (Table 4) indicate the
thioacetato complex degraded faster than the
thiobenzoato complex.
8
9
10
II
12
13
14
15
16
17
Acknowledgement
The authors are thankful to UGC, New Delhi, for
financial assistance. TG is thankful to CSIR, New
Delhi , for Research Fellowship. Further, we are
thankful to CDRI, Lucknow, for doing microanalyses.
20
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2
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