Heterobimetallic complexes containing Cu and Si

Article abstract of DOI:10.1081/SIM-120024322

[Bis(propanediamine)Cu]Cl₂, (1) was allowed to react with SiCl₄ to yield a bimetallic complex (2). Compound (2) was then treated with pyridine to attain the maximum coordination number of silicon (3). The reaction was also followed conductometrically. Complex (3) was reacted with sodium diethyl dithiocarbamate (dtc) to yield (4). The dtc in (4) acts as a bidentate chelating agent as its IR spectrum showed the CS₂ stretching frequency at 1020 cm^-1 which is characteristic of a symmetrically bonded dithiocarbamate group. There is a common peak in the range 246 to 260 nm in the UV-Vis. spectra of (1), (2) and (3) due to L → M charge transfer. The EPR spectra of all species showed different environments for the Cu(II) ion in the various compounds indicating stereochemical changes.

Full text of DOI:10.1081/SIM-120024322

SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 8, pp. 1459–1468, 2003
Heterobimetallic Complexes Containing Cu and Si
*
K. S. Siddiqi, Hina Afaq, Shahab A. A. Nami,
and Ahmad Umar
Department of Chemistry, Aligarh Muslim University, Aligarh, India
ABSTRACT
[Bis(propanediamine)Cu]Cl2, (1) was allowed to react with SiCl4 to
yield a bimetallic complex (2). Compound (2) was then treated with
pyridine to attain the maximum coordination number of silicon (3). The
reaction was also followed conductometrically. Complex (3) was reacted
with sodium diethyl dithiocarbamate (dtc) to yield (4). The dtc in (4) acts
as a bidentate chelating agent as its IR spectrum showed the CS2
stretching frequency at 1020 cm–1 which is characteristic of a
symmetrically bonded dithiocarbamate group. There is a common peak
in the range 246 to 260 nm in the UV-Vis. spectra of (1), (2) and (3) due
to L ! M charge transfer. The EPR spectra of all species showed
different environments for the Cu(II) ion in the various compounds
indicating stereochemical changes.
*
Correspondence: K. S. Siddiqi, Department of Chemistry, Aligarh Muslim
University, Aligarh, 202002, India; E-mail: ks_siddiqi@yahoo.co.in.
1459
DOI: 10.1081/SIM-120024322
0094-5714 (Print); 1532-2440 (Online)
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Copyright D 2003 by Marcel Dekker, Inc.

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INTRODUCTION
In recent years, studies of heteronuclear bimetallic complexes have
been of increasing importance in inorganic and bioinorganic chemistry.[1–5]
In this communication an attempt has been made to synthesize a trinuclear
compound containing three metal atoms and also to study their reactions.
RESULTS AND DISCUSSION
Analytical data, melting points and molar conductance of the
compounds are given in Table 1. The conductometric titration of CuCl2
with 1,3-propanediamine (ppn) in ethanol was done at ambient temperature
and indicated the formation of a 1:2 compound (Figure 1). Further addition
of Cu(II) ion into the propanediamine solution showed an increase in
conductance which then became almost constant due to the increase in
Cu(II) ion concentration, however, precipitation did not occur owing to very
low concentration of the reacting components. The molar conductance of a
one millimolar solution of the compound (1) in DMSO showed it (Table 1)
to be a 1:1 electrolyte as has been noted for potassium thiocyanate[6]
in DMSO.
Compound (1) was allowed to react with SiCl4 according to Eq. 1
resulting in the formation of a trinuclear compound containing one copper
and two silicon atoms (2). Since Group IV elements have the capability
to extend their coordination number from 4 to 6, pyridine was added
to compound (2) to achieve a coordination number 6 at silicon, Eq. 2.
Since Cu(II) is centrally placed in (2) and, being a non-linear molecular
system, it will undergo some type of distortion which would lower the
symmetry. Thus, a weakly held Cu(II) ion would have a distorted octa-
hedral geometry.

Heterobimetallic Complexes
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Siddiqi et al.
Figure 1. Plot of conductometric titration of 0.5 mmol of ppn (dissolved in 10 mL
ethanol) with CuCl2 in mmols (0.05 molar solution in ethanol).

Heterobimetallic Complexes
1463
The reaction of (2) with pyridine was followed conductometrically at
room temperature. When a pyridine solution was added to a solution of the
same concentration of (2), the molar conductance fell sharply until four
moles of pyridine have been added and then became almost constant.
Further addition of pyridine did not show any change in the conductance
because pyridine is a non-conducting species. The molar conductance of a
one millimolar solution of compound (3) was also measured at room
temperature in DMF (Table 1). The value suggested that compound (3) is a
1:1 electrolyte similar to (CH3)4NI for which the molar conductance in
DMF[6] is 88 ohm–1cm2 mol–1. In an attempt to check if the chlorines in (2)
are replaceable and if they may undergo substitution reactions, (2) was
allowed to react with dtc which replaced the chlorine attached to silicon
Eq. 3 and yielded compound (4). It is to be noted that the coordination
number of silicon is thus raised to six.
The formation of compound (4) was also followed conductometrically
(Figure 2). A sodium diethyl dithiocarbamate solution in ethanol was added
to compound (2) in DMSO. The conductivity showed an increase until four
moles of dtc were added and then became almost constant. The molar
conductance of compound (4) increases with further addition of dtc as a
consequence of the replacement of chlorine by dtc. The molar conductance
of a one millimolar solution of compound (4) in DMF indicated that it is a
1:1 electrolyte[6] (Table 1).
Figure 2. Plot of conductometric titration of 0.01 mmol of compound (2) (dissolved
in 10 mL DMSO) with dtc in mmols (0.002 molar solution in ethanol).

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Siddiqi et al.
IR Spectra
The IR spectrum of the primary amine shows two N–H stretching
frequencies in the range 3300–3500 cm–1 attributed to symmetric and
asymmetric modes which are shifted to lower frequency by chelation[7]
(Table 2). In compound (1) there are two sharp bands at 3222 cm–1
(asymmetric) and 3352 cm–1(symmetric), which are due to chelation of
copper with ppn. However, a weak band at about 3430 cm–1 has also been
observed. The N–H deformation band appearing at 1586 cm–1 as a sharp
band is followed by a shoulder at 1657 cm–1. The other sharp bands in the
range 900–650 cm–1 are generally due to C–H absorption, which are
weakened due to hydrogen bonding. A band appearing at 1213 cm–1 has
been assigned to n(C–N).
In compound (2) the two n(N–H) bands (3082 and 3424 cm–1) and two
d(N–H) deformation bands (1482 and 1582 cm–1) have been found to be
shifted to lower wave numbers. This decrease is due to the replacement of
hydrogen from NH2 by silicon [Eq. 1]. A band also appears at 1185 cm–1,
which is due to the C–N stretching frequency.
When pyridine was added to compound (2), the n(N–H) frequency
appears as a broad band at 3431 cm–1 followed by a sharp, medium band at
3040 cm–1, which is split into three bands. There is a common band in the
1657–1582 cm–1 region in compounds (1), (2), (3) and (4). This is a
characteristic band of primary amines, which persists in all the complexes.
Three C=C and C=N absorption bands are known to appear in the 1580–
1485 cm–1 range in free pyridine[7] which have been found to be slightly
shifted towards lower wave numbers in compound (3) due to coordination
to silicon. Ring vibrations due to C–H deformation show two bands in the
range 1000–1200 cm–1 in pyridine.[7] The appearance of these two strong
Table 2. IR spectra of the complexes and their assignments (cm–1).
Compounds
n(N–H)
d(N–H)
n(C–N)
n(C=S)
Primary amines
(1)
3500–3300 b
3352 s
3222 s
1650–1590 m
1586 s
1220–1020 s
1213 w
-
-
(2)
(3)
(4)
3080 vw
3424 vw
3431 b
1582 s
1575 s
1614 s
1185 s
1218 ms
1217 s
-
-
3030 ms
2984 ms
3395 b
1020 s

Heterobimetallic Complexes
1465
bands at 1152 and 1078 cm–1 confirms the presence of pyridine in
compound (3).
Compound (4) results when (2) is allowed to react with dtc. Since dtc
may act as both a monodentate or a bidentate ligand this may be
ascertained by the criteria laid down by Brinkhoff and Grotens based on IR
spectroscopy.[8]
The IR spectrum of a bidentate dtc moiety shows only one n(C=S)
band at around 1000 cm–1, but this band is split into two if it acts as a
monodentate ligand. The IR spectrum of compound (4) showed only one
strong band at 1020 cm–1 attributed to the bidentate dtc moiety. The other
bands are not closely spaced, they are either much below or much above
1000 cm–1. It is, therefore, ascertained that dtc is acting as a bidentate
ligand making silicon six-coordinate.
EPR Spectra
The EPR spectrum of compound (1) at room temperature gave the
values of gx, gy, and gz of 2.194, 2.135 and 2.070, respectively, and gav is
2.132. The structure of compound (1) seems to be distorted octahedral
where the coordination of chlorine is probably not perpendicular to the
plane defined by copper and the four nitrogens of two propanediamines.
The axial positions are very much elongated in order to become stable in
the solid state. The g values of Cu(II) generally ranges between 2.06 to
2.38, and our g values fall in the above range. The gav calculated has been
found to be 2.132 which is also the value generally found for distorted
Cu(II) ions.
The compound (2) has a single line spectrum with g =2.124. However,
a small peak with g =2.08 is shown, the origin of which is not clear
although the presence of an impurity cannot be ruled out. Compound (2) is
further distorted with respect to compound (1) because of the introduction
of a second metal. From its EPR spectrum the two signals in the EPR
spectrum can be interpreted. The bigger signal with g=2.08 corresponds to
a complex structure, where the component of g cannot be separated. The
weaker signal with g= 2.08 is of unknown origin. Alternatively, the larger
component may represent g and the smaller one g . Since g >g , the
?
k
?
k
ground state of the system is d3x
2
2
.
Compound (3) has parallel and perpendicular components with
ꢀ r
g = 2.05, g = 2.07 and gav = 2.06. There is hyperfine structure also, as
k
seen in the g and g components. The average separation between four
?
k
?
hyperfine components is only 3.703 Gauss, which indicates the overlap of
the electron at the nuclear site. Since g >g , therefore, the structure is
suggested to be tetragonally-distorted.
?
k

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Siddiqi et al.
In compound (4) there is a single broad line spectrum with a half width
half maximum at 556 Gauss. This indicates fast relaxation of the Cu(II)
ions in the complex. The width can be attributed either to exchange
interaction or dipolar interaction. But the shape of the spectral line, which
is more of Gaussian than Lorenzian shape, indicates dipolar interaction.
However, there is a small shoulder in the spectral line with g= 2.08, similar
to compound (2), which is of unknown origin.
Electronic Spectra
The electronic spectra of compounds (1), (2) and (3) showed a peak at
252 nm (log e = 2.23 l mol–1 cm–1), 247 nm (log e = 1.21 l mol–1 cm–1) and
262.4 nm (log e = 3.29 l mol–1 cm–1), respectively, which is ascribed to
L ! M charge transfer. The slight variation in the absorption of compound
(2) is due to the addition of SiCl4. A slight variation also occurs in the L ! M
absorption band when pyridine was added to compound (2). The L ! M
charge transfer band could not be detected in compound (4) which might have
been obscured due to the replacement of chloride by the bulky dtc moiety.
In compound (1), the d–d charge transfer band appears as broad band at
584 nm (log e = 2.07 l mol–1 cm–1) in the visible region. The same band
appears[9] at 970 nm in compound (2), at 900 nm in compound (3) and at
about 1000 nm in compound (4). Copper in compound (1), (2), (3) and (4)
appears to be tetragonally-distorted octahedral which splits the e and t2 level.
EXPERIMENTAL
Elemental analyses of the complexes were done with a Carlo Erba 1106
analyser. IR spectra were recorded with a Perkin Elmer model 983
spectrophotometer as KBr discs. UV-Vis. spectra were recorded in the
200–1000 nm range in DMF using a Systronics 117 spectrophotometer at
room temperature. Conductance measurements were made on a CM 180 Elico
conductivity meter. EPR spectra of the solid compound were recorded on a
RE-2X Jeol spectrometer fitted with 100 KHz field modulation. Chloride was
determined as AgCl gravimetrically. Propanediamine (Fluka), CuCl2ꢁ2H2O,
pyridine, sodium diethyl dithiocarbamate (E. Merck), SiCl4 (BDH) were used
as received. Ethyl alcohol was used after distillation over CaCl2. All other
solvents obtained commercially were used without further purification.
Preparation of Compound (1)
CuCl2ꢁ2H2O (10 mmol, 1.70 g) dissolved in ethanol (25 mL) was
added to an ethanolic solution (25 mL) of propanediamine (20 mmol, 1.64

Heterobimetallic Complexes
1467
mL) at room temperature, which afforded a deep blue needle-like
compound. The supernatant was decanted, the crystals washed with ethanol
and diethyl ether and dried over CaCl2 under vacuum. The crystals are
soluble in ethanol, methanol, propanol, water, and DMSO but could not be
crystallized as they become sticky.
Preparation of Compound (2)
To a dry ethanolic solution (25 mL) of compound (1) (5 mmol, 1.41
g) was added SiCl4 (10 mmol, 1.14 g) dissolved in CCl4 (10 mL). The
reaction mixture instantaneously yielded a yellow solid, which was filtered
and purified by washing with dry CCl4 and diethyl ether. Since SiCl4
is hygroscopic all manipulation were done in a dry box to prevent access
of moisture.
Preparation of Compound (3)
Compound (2) (2 mmol, 0.8 g) dissolved in DMSO (25 mL) was added
to an ethanolic solution (10 mL) of pyridine (8 mmol, 0.64 mL). The
resulting mixture was refluxed on a water bath for 8 to 10 h and left
overnight at room temperature to yield a blue, needle-shaped compound.
The compound was washed with cold ethanol and dried in vacuo. It was
recrystallised from a 1:1 mixture of nitromethane and DMF.
Preparation of Compound (4)
A solution of sodium diethyl dithiocarbamate (8 mmol, 1.8 g) in 25 mL
ethanol was slowly added to compound (2) (2 mmol, 0.81 g) dissolved in
25 mL DMSO. The reaction mixture immediately yielded a black solid. The
supernatant was decanted, the solid was washed thrice with cold ethanol
and dried over CaCl2 in a desiccator. The black compound (4) turned
yellow when dried. It was soluble in DMF.
ACKNOWLEDGMENT
The authors are thankful to Prof. S. H. Afaq, Faculty of Medicine, for
recording UV-Vis. spectra, to the Chairman, Department of Chemistry, for
providing the necessary research facilities for this project and to Prof. R. J.
Singh, Department of Physics, Aligarh Muslim University, Aligarh, for
recording the EPR spectra.

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Received December 6, 2001
Accepted May 8, 2003
Referee I: C. H. Yoder
Referee II: J. D. Zubkowski

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