Copper ion mediated selective cleavage of C-S bond in ferrocenylthiosemicarbazone forming mixed geometrical [(PPh3)Cu(μ- S)2Cu(PPh3)2] having Cu2S 2 core: Toward a new avenue in copper-sulfur chemistry

Article abstract of DOI:10.1021/ic2022616

Unprecedented selective cleavage of the carbon-sulfur bond of the ferrocenylthiosemicarbazone moiety has been observed for the first time, resulting in the formation of mixed geometrical binuclear copper complex [(PPh₃)Cu(μ-S)₂Cu(PPh₃)₂]. Upon trying direct synthesis of the title complex, an unusual tetranuclear [Cu ₄(μ₃-Cl)₄(PPh₃)₄] cubane resulted.

Full text of DOI:10.1021/ic2022616

Article
pubs.acs.org/IC
Copper Ion Mediated Selective Cleavage of C−S Bond in
Ferrocenylthiosemicarbazone Forming Mixed Geometrical [(PPh3)
Cu(μ-S)2Cu(PPh3)2] Having Cu2S2 Core: Toward a New Avenue in
Copper−Sulfur Chemistry
Rathinasabapathi Prabhakaran,*,† Palaniappan Kalaivani,† Somanur V. Renukadevi,† Rui Huang,‡
Kittusamy Senthilkumar,§ Ramasamy Karvembu,∥ and Karuppannan Natarajan*,†
†Department of Chemistry, Bharathiar University, Coimbatore 641 046, India
‡Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States
§Department of Physics, Bharathiar University, Coimbatore 641 046, India
∥Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
S
* Supporting Information
ABSTRACT: Unprecedented selective cleavage of the car-
bon−sulfur bond of the ferrocenylthiosemicarbazone moiety
has been observed for the first time, resulting in the formation
of mixed geometrical binuclear copper complex [(PPh3)Cu
(μ-S)2Cu(PPh3)2]. Upon trying direct synthesis of the title
complex, an unusual tetranuclear [Cu4(μ3-Cl)4(PPh3)4]
cubane resulted.
differ from commercial HDS catalysts. Many obstacles will have
to be overcome to make these systems usable in any real
process, including supporting the homogeneous species on a
heterogeneous support.
INTRODUCTION
■
Transition metal and main group metal complexes of
thiosemicarbazones have invited considerable interest for a
variety of reasons, such as variable bonding properties,
structural diversity, and pharmacological properties.1 Copper
chalcogenide compounds including clusters are of continu-
ing interest in structural, synthetic, and materials chemistry,2
while examples of copper/inorganic-sulfur active-site enti-
ties in biology were recently discovered in the case of two
metalloenzymes (cytochrome C oxidase and nitrous oxide
reductase).3 Nitrous oxide reductase (N2OR) has an active-
site {(histidine)7Cu4S} cluster (called CuZ), where nitrous
oxide binds and is reduced in the terminal step in bacterial
denitrification.3 With these findings, there has been a surge of
interest in relevant copper coordination chemistry, with goals
including (1) the generation of discrete and tractable copper
sulfide complexes, (2) the elucidation of their structure and
spectroscopy, and (3) electrochemical study of the copper
sulfide compound, that is, redox and/or atom-transfer reactivity
patterns. Moreover, cleavage of the C−S bond is an important
step in the removal of the sulfur from thiophene in the
hydrodesulphurization (HDS) process.4 Thiophene and its
benzo derivatives represent abundant sulfur-containing impur-
ities in coal and petroleum feedstocks and are among the most
difficult to desulfurize.5 Homogenous transition metal com-
plexes are ideal for probing the mechanism of this process by
allowing analysis of specific steps in the proposed HDS cycle.6
In addition, such complexes can show reactivity patterns that
EXPERIMENTAL SECTION
■
Preparation of [(PPh3)Cu(μ-S)2Cu(PPh3)2]. To a hot solution of
[CuCl2(PPh3)2] (66 mg; 0.1 mmol) in dichloromethane (20 cm3) was
added ferrocenylthiosemicarabzone [HFtsc] (92 mg; 0.2 mmol) in hot
ethanol (20 cm3). The mixture was refluxed for 6 h. The resulting
reaction mixture was concentrated to about 10 cm3, and the solvents
were allowed for partial evaporation and kept at room temperature
for further evaporation to yield two products. One is orange
brown crystals, which could be separated mechanically under a
microscope. Yield: 37% (36.04 mg). Mp 246 °C. FT-IR (KBr): 1430,
1085, 702 cm−1 (for PPh3); 468 cm−1 ν(Cu−S). UV−vis (methanol),
λmax [nm] (ε, M−1 cm−1): 397(2465), 461(1655), 479(1406),
515(929), 554(539), 589(340), 631(140). Elemental analyses calcd.
for C54H45S2P2Cu2: C, 68.48; H, 4.79; S, 6.77%. Found: C, 68.42;
H, 4.80; S, 6.72%. The compound dissolved in common organic
solvents such as dichloromethane, chloroform, benzene, acetone, methanol,
ethanol, DMF, and DMSO. Another product was a dark brown one, which
did not dissolve in any organic solvent (yield: 48 mg). Hence, it was left
without further characterization.
Preparation of [Cu4(μ3-Cl)4(PPh3)4]. [CuCl2(PPh3)2] (132 mg;
0.2 mmol) was refluxed in a 1:1 ethanol (20 cm3)/dichloromethane
(20 cm3) mixture with elemental sulfur (6.5 mg; 0.2 mmol). A black
Received: October 22, 2011
Published: February 29, 2012
© 2012 American Chemical Society
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The formation of a sulfide (S2−) moiety through the thermal
decomposition through C−S bond cleavage of thiolate copper(II)
complexes has been reported.12 It is to be noted that the
metal-sulfido clusters are important components in fundamental
processes such as electron transfer and the reduction of
dinitrogen.13 In addition to this, to the best of our knowledge,
the present work demonstrates the formation of the first
example of a discrete mixed geometrical (trigonal planar and
tetrahedral) dicopper complex having two bridging S2− units
between the copper ions. This is yet another interesting behavior
exhibited by the thiosemicarbazone ligand. In this article, we
report the synthesis, spectral characterization, crystal structure,
DFT study, and electrochemistry of a bis-μ2-sulfido dicopper
complex.
brown copper sulfide precipitated immediately, which was filtered, and
the mother liquid was left for slow evaporation to yield white crystals.
Yield: 9% (26.02 mg). M.p. > 250 °C. FT-IR (KBr): 1431, 1122,
693 cm−1 (for PPh3). UV−vis (methanol), λmax [nm] (ε, M−1 cm−1):
269 (136 000), 274 (60 200), 310 (18 332). (1H CDCl3): δ 6.4−7.7 ppm
(m, aromatic protons of PPh3). Elemental analyses calcd. for
C72H60Cl4P4Cu4: C, 60.11; H, 4.18%. Found: C, 60.18; H, 4.23%.
The compound dissolved in methanol, DMF, and DMSO. The same
product was prepared by refluxing [CuCl2(PPh3)2] in a 1:1 ethanol/
dichloromethane mixture for 5 h.
EXPERIMENTAL METHODS
■
C, H, N, and S analyses were performed using a Vario EL III elemental
analyzer. IR spectra of the ligand and complexes have been recorded
from KBr pellets with a Nicolet instrument in the 4000−400 cm−1
range. Electronic spectra of the complexes were measured in methanol
using a Systronics 119 spectrophotometer in the 800−200 nm range.
Melting points were recorded on a Loba India Melting point
apparatus. The cyclic voltammetric studies were carried out in an
acetonitrile window with a CH Instruments electrochemical analyzer
using a platinum working electrode, and the redox potentials were
referenced to a saturated Ag/AgCl electrode. Suitable single crystals
for X-ray analysis were grown from a ethanol/dichloromethane
mixture. The data were collected at 287 K with a Bruker Smart 1000
CCD Diffractometer using monochromated Mo Kα (λ = 0.71073 Å)
radiation. The data were collected and processed using Saint, and the
structures were solved and refined through full matrix least-squares on
F2 using SHELXTL 6.14. Electron paramagnetic resonance spectra
(EPR) of the powder samples were recorded with a Jeol Tel-100
instrument at X-band frequencies at room and liquid nitrogen
temperatures (77 K) using the 2,2-diphenyl-1-picrylhydrazyl radical
The elemental analysis of the title complex [(PPh3)Cu
(μ-S)2Cu(PPh3)2] agreed very well with the proposed
molecular formula. The calculated value of sulfur (6.77) is
most reliable and relative to the found value (6.72) greatly
supports the formation of the complex. The calculated values
for carbon and hydrogen also matched very well with the found
values. The observed differences between the calculated and
found values are only 0.088%, 0%, and 0.739% for carbon,
hydrogen, and sulfur, respectively. In the IR spectrum of the
title complex, the absence of the bands at 823, 1651, and 3240 cm−1
corresponding to the νCS, νCN, and νNH indicates that the
complex does not have a ferrocenylthiosemicarbazone moiety
in it.7b However, a new band appeared at 468 cm−1 due to the
14
presence of νCu−S
.
The UV−vis absorption spectrum of the
1
(DPPH) as an internal standard. The H NMR and 31P NMR spectra
title complex in methanol at room temperature contains a
number of bands [λmax [nm] (ε, M−1 cm−1)] at 397 (2465),
461 (1655), 479 (1406), 515 (929), 554 (539), 589 (340), and
631 (140). There is a noticeable difference in the charge
transfer energy and intensity for these transitions. The intense
absorptions in the visible region is an indication of the presence
of sulfide bridge complexes. From the elemental analysis and IR
spectrum of the insoluble polymeric compound, there was no
appreciable information found. The EPR spectrum recorded at
room temperature and liquid nitrogen temperature showed the
EPR-silent nature of this compound (Figures S1 and S2,
Supporting Information).
were recorded with a Bruker AMX 400 instrument by using TMS and
ortho-phosphoric acid as references, respectively.
RESULTS AND DISCUSSION
■
As a part of our systematic investigation on the reactions of
thiosemicarbazones with various transition metal complexes,7
a reaction between ferrocenylthiosemicarbazone (H-Ftsc)8
and [CuCl2(PPh3)2]9 was carried out in order to obtain a
hetero-trinuclear complex [Cu(Ftsc)2]. But, a copper ion mediated
selective cleavage of the carbon−sulfur bond of the ferrocenylth-
iosemicarbazone took place, leading to the formation of a mixed
geometrical bis-μ2-sulfido bridged bicopper complex (Scheme 1)
For X-ray single crystal studies, suitable crystals were
obtained from ethanol/dichloromethane mixture by vapor
diffusion method. The crystal structure of the complex is
depicted in Figure 1. The extremely well ordered (R = 0.0394)
unit cell contains a binuclear [(PPh3)Cu(μ-S)2Cu(PPh3)2] in
which Cu(1) is ligated in a tetrahedral P2S2 environment and
Cu(2) is ligated in a trigonal planar PS2 environment. The
crystallographic parameters are given in Table 1. The observed
shorter bond lengths of Cu(2)−S(1), 2.2448(6) Å; Cu(2)−
S(2), 2.3091(7) Å; and Cu(2)−P(2), 2.1742(6) Å in the
Cu(2)S(1)S(2)P(2) core compared to Cu(1)−S(1), 2.4049(7) Å;
Cu(1)−S(2), 2.4503(7) Å; Cu(1)−P(1), 2.2357(7) Å; and
Cu(1)−P(3), 2.2269(10) Å in the Cu(1)P(1)P(3)S(1)S(2)
core may be due to less of a back bonding contribution of
Cu(2) to P(2) phosphine than Cu(1) to P(1) and P(3)
phosphines (Table 2). The variation in Cu−S bond lengths
may be due to variation in the σ bonding contribution of the
sulfide ion and the presence of back bonding triphenylphos-
phine groups in two different geometrical environments.16 The
Cu−P bond lengths have lengthened a little compared to other
reported Cu−P values.17 This lengthening may be due to the
fact that the S2− ligand is in coordination with two extensively
back-bonded copper atoms. Such a kind of S2− binegative
Scheme 1. Preparation of [(PPh3)Cu(μ-S)2Cu(PPh3)2]
along with the insoluble dark brown compound. This unprece-
dented selective cleavage of the C−S bond in the ferrocenylth-
iosemicabazone moiety deserves special attention due to its
relevance to the HDS process.6,10 Bis-μ2-sulfido bridging in the
complex essentially resulted via the selective cleavage of the
carbon−sulfur bond of the ferrocenylthiosemicarbazone ligand.
In this paramagnetic dicopper complex, the copper ions are in
the +2 oxidation state with two different geometries. The
formation of tri-coordinated copper complexes is also very
rarely found in the literature.11 A number of complexes might
appear to be tri-coordinated, but upon close investigation they
have been found to have higher coordination. The formation of
tricoordinated complexes is favored mainly because of steric
considerations over the normal four coordinated complexes.
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Table 2. Selected Structural Parameters (Bond Lengths in Å
and Angles in Degrees) of [(PPh3)Cu(μ-S)2Cu(PPh3)2]
Obtained from X-Ray Crystallography and DFT Calculations
bond lengths
experimental
DFT
Cu1−Cu2
Cu1−S1
Cu1−S2
Cu1−P1
Cu1−P3
Cu2−S1
Cu2−S2
Cu2−P2
2.8988
2.4503
2.4049
2.2357
2.2269
2.2448
2.3091
2.1742
2.6354
2.4036
2.4147
2.4708
2.3943
2.2519
2.2605
2.332
bond angles [deg]
experimental
DFT
P3−Cu1−S1
P3−Cu1−S2
P3−Cu1−P1
P3−Cu1−Cu2
S1−Cu1−S2
S1−Cu1−P1
S1−Cu1−Cu2
S2−Cu1−P1
S2−Cu1−Cu2
P1−Cu1−Cu2
S1−Cu2−S2
S1−Cu2−P2
S1−Cu2−Cu1
S2−Cu2−P2
S2−Cu2−Cu1
P2−Cu2−Cu1
Cu2−S1−Cu1
Cu2−S2−Cu1
102.328
113.84
100.7
101.8
123
Figure 1. Structure of [(PPh3)Cu(μ-S)2Cu(PPh3)2].
131.38
127.674
97.835
125.6
101.4
115.2
52.8
Table 1. Crystal Data and Structure Refinement for
[(PPh3)Cu(μ-S)2Cu(PPh3)2]
102.943
48.743
empirical formula
C54H45Cu2P3S2
978.01
102.94
111.6
52.9
fw
50.574
temp
173 K
100.02
111.2
111.5
128.9
58.2
wavelength
0.71073 Å
monoclinic
P21/n
106.955
134.74
cryst syst
space group
55.142
unit cell dimensions
117.71
119.1
58.5
a
18.999(4) Å
09.771(2) Å
26.359(5) Å
90°
53.563
b
169.458
76.115
162.9
68.8
c
α
75.863
68.5
β
109.56(3)°
90°
torsions angles [deg]
γ
atoms
experimental
theoretical
volume
Z
4610.88 Å3
S2−Cu1−S1−Cu2
P1−Cu1−S1−Cu2
P3−Cu1−S1−Cu2
S1−Cu1−S2−Cu2
P1−Cu1−S2−Cu2
P3−Cu1−S2−Cu2
S2−Cu2−S1−Cu1
P2−Cu2−S1−Cu1
S1−Cu2−S2−Cu1
P2−Cu2−S2−Cu1
13.23(2)
−92.05(2)
129.83(2)
−12.87(2)
92.41(2)
22.283
−98.447
126.899
−22.246
100.977
−125.988
−25.262
161.088
25.199
4
calcd density
abs coeff
F(000)
cryst size
1.409 Mg/m3
1.154 mm−1
2016
1.1 × 0.7 × 0.3 mm
−120.12(2)
−14.29(2)
175.19(2)
14.58(2)
θ range for data collection 1.61−28.36°
limiting indices
−25 ≤ h ≤ 24, −12 ≤ k ≤ 13, −34 ≤ l ≤ 35
reflns collected/unique
completeness to θ
54304/9136 [R(int) = 0.0387]
28.36°
−173.01(2)
−160.451
absorption correction
refinement method
data/restraints/params
Goodness-of-fit on F2
semiempirical from equivalents
full-matrix least-squares on F2
11268/0/550
from the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB2 1EZ, UK [fax: +44-1223/336-033;
e-mail: deposit@ccdc.cam.ac.uk].
0.755
Final R indices [I > 2σ(I)] R1 = 0.0394, wR2 = 0.0997
The source of the sulfido ligand in [(PPh3)Cu(μ-S)2Cu(PPh3)2]
must be from the selective cleavage of the C−S bond of the
thiosemicarbazone ligand, leading to the formation of a μ2-
sulfido bridged unsymmetrical bicopper complex. The expected
mechanism for the formation of the unsymmetrical dicopper
complex is given in Scheme 2. Recently, a few reports have
appeared describing the binuclear copper complexes containing
sulfur bridged thiosemicarbazones, and one among them is a
tetranuclear cluster.15 In our case, it is felt that such species may
be an intermediate, and the carbon sulfur bond of the
intermediate complex might have been cleaved during the
course of the reaction.
R indices (all data)
R1 = 0.0305, wR2 = 0.0951
largest diff. peak and hole
0.414 and −0.864 e.A−3
behavior of sulfur has been observed elsewhere.18 The Cu−Cu
distance was found as 2.8988(5) Å, indicating the absence of
direct metal metal bonding,15 and the two Cu−S−Cu angles
were found as 76.115(16)° and 75.863(15)°, respectively.
S(1)−Cu(1)−S(2) and S(2)−Cu(2)−S(1) bond angles are
106.955(19)° and 97.835(17)°, respectively, indicating tetrahe-
dral and trigonal planar geometry around the copper centers.
Crystallographic data for [(PPh3)Cu(μ-S)2Cu(PPh3)2] and
[Cu4(μ3-Cl)4(PPh3)4] have been deposited at the Cambridge
Crystallographic Data Centre as supplementary publication
(CCDC No. 654122 and 847673). The data can be obtained
free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or
The ground state geometry of the title compound has been
optimized using a density functional theory (DFT) method
employing Becke’s three-parameter hybrid exchange functional
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Scheme 2. Expected Mechanism of the Formation of [(PPh3)Cu(μ-S)2Cu(PPh3) ]
2
(B3)19 combined with the correlation functional of Lee, Yang,
and Parr (LYP),20 together called B3LYP. The Los Alamos
effective core potential (ECP) basis set, LanL2DZ21−23 for the
Cu atom and 6-311G(d.p) basis set for all other atoms have
been used. The vibrational frequency calculation has been
performed at the above level of theory, and the results confirm
that the optimized geometry is at a stationary point of the
potential energy surface without any imaginary frequency. The
selective geometrical parameters obtained from single crystal
X-ray data and DFT calculations are given in Table 2. The
theoretical values are almost consistent with the experimental
values. It has been observed that the root-mean-square
deviation between single crystal X-ray data and optimized
internal coordinates is 0.78 Å only. The density plot has been
made with the isosurface value of 0.05 au. The DFT results
from Mulliken atomic charges show that the Cu atom with two
triphenylphosphines has more of a charge (−0.43 au) than the
Cu with a single triphenylphosphine (−0.12 au). The frontier
molecular orbital diagram shows that both the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) are mainly delocalized on the sulfur atoms and
on the Cu atoms (Figures 2 and 3). There is a small amount of
electron density corresponding to the LUMO being on the P3
atom of triphenylphosphine. The calculated energy gap
between HOMO and LUMO is 3.54 eV (351 nm). This is
comparable with the first intense absorption feature at 397.
Hence, it is concluded that the absorption spectra observed at
397 nm are due to the transition of an electron from HOMO to
LUMO. The DFT calculations were carried out using the
Gaussian 09 program.24 The calculated dipole moment is 2.723
D. The optimized structure of the complex is given in Figure
S3, Supporting Information. The dissociation of one of the
triphenylphosphines in the symmetrical four coordinated
complex is thermodynamically favorable, and this may have
happened because of the steric hindrance of the bulky
triphenylphosphine ligand itself. Moreover, comparatively less
stabilization energy of the unsymmetrical dimeric complex
favors the mixed geometry for the complex. It is to be pointed
out here that tricoordination as such is rare, and only a few such
Cu(II) complexes are known.25 To the best of our knowledge,
the present work demonstrated the first example of discrete
Figure 2. Highest occupied molecular orbital (HOMO) of [(PPh3)-
Cu(μ-S)2Cu(PPh3)2].
unsymmetrical mixed tetrahedral and trigonal planar binuclear
copper(II) complexes having two μ2-S2 units in [(PPh3)Cu
(μ-S)2Cu(PPh3)2] via the selective cleavage of the C−S bond of
the thiosemicarbazone ligand. However, such a simultaneous
occurrence of three- and four-coordinate Cu(I) complexes is
known with the different bridging groups.26
An attempt was made to synthesize [PPh3Cu(μ-S)2Cu-
(PPh3)2] from [CuCl2(PPh3)2] and elemental sulfur under
refluxing conditions in ethanol− dichloromethane, which
resulted in the formation of a CuS precipitate. However, the
evaporation of the mother liquid afforded colorless shiny
crystals. From the analytical, IR, and electronic spectra of
the crystals, no appreciable information could be obtained.
Hence, an X-ray crystallographic investigation was carried
out. Surprisingly, it was found to be a tetranuclear cubane
[Cu4(μ3-Cl)4(PPh3)4] having triply bridged chloride ions.
The structure of the complex is given in Figure 4. The well
ordered (R = 0.0350) unit cell contains a tetranuclear [Cu4
(μ3-Cl)4(PPh3)4] in which Cu(1) and Cu(2) ligated with one
triphenylphosphine and three of the chloride ions by forming a
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Table 3. Crystal Data and Structure Refinement for [Cu4(μ3-
Cl)4(PPh3)4]
empirical formula
C36H30Cl2Cu2P2
722.52
fw
temp
173 K
wavelength
0.71073 Å
orthorhombic
Pbcn
cryst syst
space group
unit cell dimensions
a
17.428(4) Å
b
20.418(4) Å
c
18.156(4)Å
α
90°
β
90°
γ
90°
volume
Z
6461 Å3
8
calcd density
abs coeff
F(000)
cryst size
1.486 Mg/m3
16.06 mm−1
2944
0.45 × 0.25 × 0.25 mm
Figure 3. Lowest unoccupied molecular orbital (LUMO) of
[(PPh3)Cu(μ-S)2Cu(PPh3)2].
θ range for data collection 1.54 to 28.27°
limiting indices
−23 ≤ h ≤ 22, −27 ≤ k ≤ 27, −23 ≤ l ≤ 23
reflns collected/unique
completeness to θ
absorption correction
refinement method
data/restraints/params
goodness-of-fit on F2
69797/6492 [R(int) = 0.0293]
28.27°
semiempirical from equivalents
full-matrix least-squares on F2
7909/0/379
0.762
final R indices [I > 2σ(I)] R1 = 0.0350, wR2 = 0.0959
R indices (all data)
R1 = 0.0237, wR2 = 0.0839
0.293 and −0.516 e.A−3
largest diff. peak and hole
3.16, 2.86, 2.42, and 2.23, indicating the distortion in the
geometry around the metal ions in the binuclear complex.
The appearance of four g values for this complex at 77 K may
be attributed to the presence two different geometries around
the copper centers. This has been shown in its electronic
spectrum also. It could be that the first two g values may be due
to the trigonal copper, and the remaining two g values may be
due to the tetrahedral copper atom in the complex. A g value of
2.23 is indicative of a covalent nature between copper and
phosphorus in the complex.27 However, the observation of such
a kind of EPR splitting pattern is not completely clear to us.
The observed g values are higher than that observed for other
sulfur bridged copper(II) complexes.28
Figure 4. Structure of [Cu4(μ3-Cl)4(PPh3)4]. Phenyl rings were
omitted for clarity.
The magnectic moment value of 3.9 μB for [(PPh3)Cu
(μ-S)2Cu(PPh3)2] showed also the paramagnetic nature of the
complex corresponding to the +2 oxidation state for copper
ions, having one unpaired electron each. Moreover, a Cu−S−Cu
bond angle of 116.5° suggests ferromagnetic exchange in this
complex. A similar behavior has already been reported for μ-OH
bridged Cu(II) complexes.29
tetrahedral core around each metal center. The crystallographic
parameters are given in Table 3. The observed bond lengths
Cu(1)−P(1) = 2.1991 Å and Cu(2)−P(2) = 2.1995 Å are less
than those found in [(PPh3)Cu(μ-S)2Cu(PPh3)2], indicating
a stronger back bonding between copper and triphenylphos-
phine atoms (Table 4). The variation in the copper−chloride
bond lengths indicates a signficant distortion in the cubane
geometry.
EPR spectra of a powdered sample of [(PPh3)Cu(μ-S)2-
Cu(PPh3)2] were recorded at room temperature and at liquid
nitrogen temperature at the X- band frequency. No hyperfine
structure was observed for the complex. The room tempera-
ture spectrum showed a broad signal with the three g values
at 2.903, 2.717, and 2.467, indicating considerable distortion
in the CuS2P and CuS2P2 core (Figure 5a). However, the
spectrum at 77 K (Figure 5b) showed four signals with g values
The 1H NMR spectrum of [Cu4(μ3-Cl)4(PPh3)4] was
recorded in CDCl3, and it exhibited a multiplet at 6.4−7.7
ppm is attributed to the presence of aromatic protons of the
triphenylphosphine ligand (Figure S4, Supporting Information).
31P NMR spectra of [Cu4(μ3-Cl)4(PPh3)4] were recorded at room
temperature and at −80 °C to determine the nature of the
phousphours nuclei of triphenylphosphine. The spectrum at
room temperature showed only one line at 42 ppm, which
may be due to a fast exchange of triphenylphosphine ligands
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Table 4. Selected Bond Lengths (Å) and Angles (deg) for
[Cu4(μ3-Cl)4(PPh3)4]
bond lengths
Cu(1)−P(1)
2.1991(6)
2.4118(6)
2.4243(6)
2.4905(7)
2.4118(6)
2.4458(7)
2.1995(5)
2.3607(6)
2.5160(6)
2.3606(6)
Cu(1)−Cl(1)#1
Cu(1)−Cl(1)
Cu(1)−Cl(2)
Cl(1)−Cu(1)#1
Cl(1)−Cu(2)
Cu(2)−P(2)
Cu(2)−Cl(2)#1
Cu(2)−Cl(2)
Cl(2)−Cu(2)#1
Figure 6. 31{1H}P NMR (a) at 25 °C and (b) at −80 °C in CD2Cl2.
bond angles
The equilibrium constant Kc for comproportionation reaction in
this binuclear complex is defined as
P(1)−Cu(1)−Cl(1)#1
P(1)−Cu(1)−Cl(1)
Cl(1)#1−Cu(1)−Cl(1)
P(1)−Cu(1)−Cl(2)
Cl(1)#1−Cu(1)−Cl(2)
Cl(1)−Cu(1)−Cl(2)
Cu(1)#1−Cl(1)−Cu(1)
Cu(1)#1−Cl(1)−Cu(2)
Cu(1)−Cl(1)−Cu(2)
P(2)−Cu(2)−Cl(2)#1
P(2)−Cu(2)−Cl(1)
124.914(17)
131.24(2)
89.120(19)
112.840(17)
98.861(15)
91.617(14)
90.056(19)
79.186(14)
89.637(15)
130.07(2)
Oxidation
−
−e
Cu(II) − Cu(II) HoooooooooooooooI Cu(III) − Cu(II)
E
(Oxdl)
1/2
−
−e
HooooooooooooooooI Cu(III) − Cu(III)
E
(Oxd2)
1/2
Reduction
118.375(18)
101.568(15)
115.281(17)
91.499(17)
90.506(15)
79.267(14)
87.192(16)
86.576(14)
−
Cl(2)#1−Cu(2)−Cl(1)
P(2)−Cu(2)−Cl(2)
−e
Cu(II) − Cu(II) HooooooooooooooI Cu(I) − Cu(II)
E
(Redl)
Cl(2)#1−Cu(2)−Cl(2)
Cl(1)−Cu(2)−Cl(2)
Cu(2)#1−Cl(2)−Cu(1)
Cu(2)#1−Cl(2)−Cu(2)
Cu(1)−Cl(2)−Cu(2)
1/2
−
−e
HoooooooooooooooI Cu(I) − Cu(I)
E
(Red2)
1/2
2
[Cu(III) − Cu(II)]
K (II − III) =
c
[Cu(II) − Cu(II)][Cu(III) − Cu(III)]
around two copper atoms (Figure 6). Whereas, at −80 °C in
addition to a singlet at 43 ppm, a very weak signal at 29 ppm
was also observed. Probably at low temperatures there is some
restriction on triphenylphosphine exchange, leading to the
observation of two types of triphenylphosphines.
2
[Cu(I) − Cu(II)]
K (II − I) =
c
[Cu(II) − Cu(II)][Cu(I) − Cu(I)]
(F/RT)[E (1)−E (2)]
1/2
1/2
K = e
c
The Kc value for oxidation is 9.403, and for reduction it is 9.098.
These values do indicate a strong electronic coupling between the
metal centers through the bridging sulfur atom. E1/2(1) and E1/2(2)
are the half-wave potentials corresponding to Cu(II)−Cu(II)/
Cu(III)−Cu(II) or Cu(II)−Cu(II)/Cu(I)−Cu(II) and Cu(III)−
Cu(II)/Cu(III)−Cu(III) or Cu(I)−Cu(II)/Cu(I)−Cu(I).30 The
Cyclic voltammogram of the binuclear complex showed a pair of
peaks on both positive and negative potentials corresponding to
two successive one-electron oxidation Cu(II)−Cu(II) → Cu(III)−
Cu(II) → Cu(III)−Cu(III) and similar one-electron reduction
Cu(II)−Cu(II) → Cu(I)−Cu(II) → Cu(I)−Cu(I) (Figure 7).
Figure 5. EPR spectrum of [(PPh3)Cu(μ-S)2Cu(PPh3)2] at RT (a) and LNT (b).
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CONCLUSION
■
In this article, synthesis of [(PPh3)Cu(μ-S)2Cu(PPh3)2] via the
cleavage of the C−S bond of the ferrocenylthiosemicarbazone
ligand has been reported. The structure of this mixed
geometrical binuclear complex has been confirmed both
experimentally (X-ray crystallography) and theoretically (DFT
studies). In addition, a tetranuclear cubane, [Cu4(μ3-Cl)4(PPh3)4],
has been obtained while attempting to synthesize the title
compound directly. A detailed study is underway on the reactions
of various substituted ferrocenylthiosemicarbazones with
[CuCl2(PPh3)2] in order to understand the mechanism for C−S
bond cleavage.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic files in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Tel.: +91 422 2428319. Fax: +91 422 2422387. E-mail:
rpnchemist@gmail.com (R.P.), k_natraj6@yahoo.com (K.N.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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The authors gratefully acknowledged the Council of Science
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Getreidemarkt 9, A-1060 Vienna, Austria is gratefully acknowl-
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and for an enlightening discussion.
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