Studies on ruthenium(III) chalcone thiosemicarbazone complexes as catalysts for carbon-carbon coupling

Article abstract of DOI:10.1080/00958970903373922

New six-coordinate ruthenium(III) complexes [RuX(EPh₃)₂(L)] (X = Cl or Br; E = P or As; L = chalcone thiosemicarbazone) have been prepared by reacting [RuX₃(EPh₃)₃] (X = Cl or Br; E = P or As) with chalcone thiosemicarbazones in benzene under reflux. The new complexes have been characterized by analytical and spectroscopic (IR, electronic, mass, and EPR) data. The redox behavior of the complexes has also been studied. Based on the above data, an octahedral structure has been assigned for all the complexes. The new complexes exhibit catalytic activity for carbon-carbon coupling reactions.

Full text of DOI:10.1080/00958970903373922

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Studies on ruthenium(III) chalcone
thiosemicarbazone complexes as
catalysts for carbon–carbon coupling
M. Muthukumar a , S. Sivakumar a , P. Viswanathamurthi a , R.
Karvembu b , R. Prabhakaran c & K. Natarajan c
a Department of Chemistry , Periyar University , Salem 636 011,
Tamil Nadu, India
b Department of Chemistry , National Institute of Technology ,
Tiruchirappalli 620 015, Tamil Nadu, India
c Department of Chemistry , Bharathiar University , Coimbatore
641 046, Tamil Nadu, India
Published online: 30 Oct 2009.
To cite this article: M. Muthukumar , S. Sivakumar , P. Viswanathamurthi , R. Karvembu ,
R. Prabhakaran & K. Natarajan (2010) Studies on ruthenium(III) chalcone thiosemicarbazone
complexes as catalysts for carbon–carbon coupling, Journal of Coordination Chemistry, 63:2,
296-306, DOI: 10.1080/00958970903373922
To link to this article: http://dx.doi.org/10.1080/00958970903373922
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Journal of Coordination Chemistry
Vol. 63, No. 2, 20 January 2010, 296–306
Studies on ruthenium(III) chalcone thiosemicarbazone
complexes as catalysts for carbon–carbon coupling
M. MUTHUKUMARy, S. SIVAKUMARy, P. VISWANATHAMURTHI*y,
R. KARVEMBUz, R. PRABHAKARANx and K. NATARAJANx
yDepartment of Chemistry, Periyar University, Salem 636 011, Tamil Nadu, India
zDepartment of Chemistry, National Institute of Technology, Tiruchirappalli 620 015,
Tamil Nadu, India
xDepartment of Chemistry, Bharathiar University, Coimbatore 641 046, Tamil Nadu, India
(Received 18 December 2008; in final form 29 July 2009)
New six-coordinate ruthenium(III) complexes [RuX(EPh3)2(L)] (X ¼ Cl or Br; E ¼ P or As;
L ¼ chalcone thiosemicarbazone) have been prepared by reacting [RuX3(EPh3)3] (X ¼ Cl or Br;
E ¼ P or As) with chalcone thiosemicarbazones in benzene under reflux. The new complexes
have been characterized by analytical and spectroscopic (IR, electronic, mass, and EPR) data.
The redox behavior of the complexes has also been studied. Based on the above data, an
octahedral structure has been assigned for all the complexes. The new complexes exhibit
catalytic activity for carbon–carbon coupling reactions.
Keywords: Chalcone thiosemicarbazone; Ruthenium(III) complexes; Spectroscopic studies;
Carbon–carbon coupling reactions
1. Introduction
Transition metal-catalyzed coupling reactions have been extensively studied since they
represent a powerful method for the formation of carbon–carbon bonds. This strategy
has been applied to the synthesis of many organic compounds, especially complex
natural products, in supramolecular chemistry and in materials science [1–3]. The
importance of biaryl units [2] as components of pharmaceuticals, herbicides, and
natural products, as well as in the field of engineering materials such as conducting
polymers, molecular wires, and liquid crystals has attracted enormous interest.
Palladium and nickel-catalyzed Suzuki–Miyaura cross-coupling [4–6] is the most
important and efficient strategy for the construction of unsymmetrical biaryl
compounds. This cross-coupling methodology allows the use of organic solvents and
inorganic bases, tolerates many functional groups, is not affected by steric hindrance of
the substrates, and is suitable for industrial processes [7].
Though several reports are available on catalytic activity of ruthenium complexes,
most of them have been studied for oxidation and reduction reactions [8–14] not for
*Corresponding author. Email: viswanathamurthi@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958970903373922

Ruthenium(III) thiosemicarbazones
297
coupling reactions. Hence, in an attempt to study the catalytic activity of ruthenium
complexes for carbon–carbon coupling reaction, we synthesize a series of six-coordinate
ruthenium(III) chalcone thiosemicarbazone complexes containing PPh3/AsPh3
co-ligands and explore their catalytic activities. Characterizations of the complexes
were accomplished by analytical and spectroscopic (IR, electronic, mass, and EPR)
methods.
2. Experimental
2.1. Materials and methods
All reagents used were of analar or chemically pure grade. Solvents were freshly distilled
by literature methods [15]. Elemental analyses of carbon, hydrogen, and nitrogen were
performed on a Carlo Erba 1108 analyzer at the Central Drug Research Institute
(CDRI), Lucknow, India. MALDI-TOF mass spectral measurements were performed
with a Voyager-De pro instrument. FT-IR spectra were recorded in KBr pellets with a
Nicolet FT-IR spectrophotometer from 400 to 4000 cmꢀ1. Electronic spectra of the
complexes were recorded on a Shimadzu UV-Vis 1650 PC spectrophotometer at 200–
600 nm using CH2Cl2 as solvent. Magnetic susceptibility measurements of the
complexes were made on an EG and G-PARC vibrating sample magnetometer.
Room temperature EPR spectra of powder samples were recorded with a JEOL JES-
FA200 instrument at X-band frequencies using 2,20-diphenyl-1-picrylhydrazyl radical
(DPPH) as internal standard. Cyclic voltammetric measurements were carried out with
a BAS CV-27 electrochemical analyzer in acetonitrile using [NBu4]ClO4 (TBAP) as
supporting electrolyte under nitrogen. A three electrode cell was employed with glassy
carbon working electrode, a platinum wire as counter electrode, and Ag/AgCl as
reference electrode. Melting points were recorded on a Technico micro heating table
and are uncorrected. The starting complexes [RuCl3(PPh3)3], [RuCl3(AsPh3)3], and
[RuBr3(AsPh3)3] were prepared according to literature methods [16, 17].
2.2. Preparation of chalcone thiosemicarbazone ligands
Ligands are prepared by two-step processes. In the first step, the 20-hydroxychalcones
were prepared in 80–90% yield by stirring 2-hydroxyacetophenone (0.6 cm3, 5 mmol)
with corresponding aldehydes (0.531–0.831 g, 5 mmol) in the presence of 50 cm3
alcoholic sodium hydroxide solution (20%). After 24 h stirring, the product was
precipitated by adding concentrated hydrochloric acid, filtered, and recrystallized from
ethanol. In the second step, 20-hydroxychalcones (0.224–0.284 g, 1 mmol) was reacted
with thiosemicarbazide (0.425 g, 4.7 mmol) in ethanol (40 cm3) containing concentrated
hydrochloric acid (0.2 cm3). The reaction mixture was refluxed for 8 h and the solvent
was evaporated under vacuum. The resulting residue (yield 60–68%) was recrystallized
from ethanol.
L1: Yellow, m.p. 120ꢁC, yield 80%. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1604, ꢀ(C–O) 1351,
ꢀ(C–S) 824.

298
M. Muthukumar et al.
OH
S
C
N
NH
C
NH2
CH
CHR
Ligand
L1
R
C6H5
L2
L3
L4
4-(OCH3)C6H4
3, 4-(OCH3)2C6H3
4-(CH3)C6H4
Figure 1. Structure of chalcone thiosemicarbazone ligands.
L2: Yellow, m.p. 128ꢁC, yield 79%. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1598, ꢀ(C–O) 1339,
ꢀ(C–S) 818.
L3: Yellow, m.p. 150ꢁC, yield 75%. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1581, ꢀ(C–O) 1330,
ꢀ(C–S) 829.
L4: Yellow, m.p. 132ꢁC, yield 70%. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1599, ꢀ(C–O) 1324,
ꢀ(C–S) 810.
The general structure of chalcone thiosemicarbazone ligands is given in figure 1.
2.3. Preparation of new ruthenium(III) chalcone thiosemicarbazone complexes
All complexes were prepared by the following procedure. To a solution of
[RuX3(EPh3)3] (X ¼ Cl or Br; E ¼ P or As) (0.1 g, 0.1 mmol) in benzene (20 cm3), the
appropriate chalcone thiosemicarbazone ligands (0.024–0.036 g, 0.1 mmol) were added
in 1 : 1 molar ratio. The mixture was refluxed for 6 h on a water bath. The color of the
reaction mixture gradually deepened during heating. The reaction mixture was
concentrated to around 3 cm3 by removing the solvent under reduced pressure,
cooled, and the product separated by addition of 10 cm3 of petroleum ether
(60ꢁC–80ꢁC). The product was recrystallized from CH2Cl2/petroleum ether mixture,
dried under vacuum, and the purity was checked by TLC.
[RuBr(AsPh3)2(L1)]: Brown, d.p. 169ꢁC, yield 77%, Anal. Calcd for
C52H43N3As2BrOSRu (%): C, 37.5; H, 2.9; N, 3.9. Found (%): C, 37.3; H, 2.4;
N, 3.9. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1606; ꢀ(C–O) 1312; ꢀ(C–S) 739; (PPh3/AsPh3)
1432, 1074, 692. UV (CH2Cl2, nm (", dm3 molꢀ1 cmꢀ1)): 360(10,444), 284(16,666),
240(33,333).
[RuBr(AsPh3)2(L2)]: Brown, d.p. 150ꢁC, yield 74%. Anal. Calcd for
C53H45N3O2As2BrSRu (%): C, 37.5; H, 2.7; N, 3.7. Found (%): C, 36.4; H, 2.1;
N, 3.0. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1607; ꢀ(C–O) 1311; ꢀ(C–S) 739; (PPh3/AsPh3)

Ruthenium(III) thiosemicarbazones
299
1432, 1085, 692. UV (CH2Cl2, nm (", dm3 molꢀ1 cmꢀ1)): 352(17,391), 292(26,086),
242(45,652).
[RuBr(AsPh3)2(L3)]: Brown, d.p. 170ꢁC, yield 72%. Anal. Calcd for
C54H47N3O3As2BrSRu (%): C, 37.5; H, 2.2; N, 3.1. Found (%): C, 37.8; H, 2.4;
N, 3.7. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1603; ꢀ(C–O) 1317; ꢀ(C–S) 741; (PPh3/AsPh3)
1433, 1077, 693. UV (CH2Cl2, nm (", dm3 molꢀ1 cmꢀ1)): 340(14,893), 285(23,404),
250(51,063).
[RuBr(AsPh3)2(L4)]: Brown, d.p. 165ꢁC, yield 78%. Anal. Calcd for
C53H45N3OAs2BrSRu (%): C, 39.2; H, 3.2; N, 4.2. Found (%): C, 38.8; H, 2.8; N, 3.8.
FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1606; ꢀ(C–O) 1314; ꢀ(C–S) 740; (PPh3/AsPh3) 1433, 1075,
693. UV (CH2Cl2, nm (", dm3 molꢀ1 cmꢀ1)): 362(7954), 297(10,227), 248(20,454).
[RuCl(AsPh3)2(L1)]: Brown, d.p. 185ꢁC, yield 73%. Anal. Calcd for
C52H43N3OAs2ClSRu (%): C, 38.1; H, 2.9; N, 3.8. Found (%): C, 38.1; H, 3.5;
N, 3.8. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1608; ꢀ(C–O) 1313; ꢀ(C–S) 739; (PPh3/AsPh3)
1433, 1074, 693. UV (CH2Cl2, nm (", dm3 molꢀ1 cmꢀ1)): 351(14,893), 284(19,361),
243(42,553).
[RuCl(AsPh3)2(L2)]: Brown, d.p. 172ꢁC, yield 75%. Anal. Calcd for
C53H45N3O2As2ClSRu (%): C, 39.1; H, 2.9; N, 3.9. Found (%): C, 40.1; H, 3.5;
N, 3.8. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1608; ꢀ(C–O) 1311; ꢀ(C–S) 740; (PPh3/AsPh3)
1432, 1086, 692. UV (CH2Cl2, nm (", dm3 molꢀ1 cmꢀ1)): 355(13,043), 281(19,782),
240(36,956).
[RuCl(AsPh3)2(L3)]: Brown, d.p. 189ꢁC, yield 75%. Anal. Calcd for
C54H47N3O3As2ClSRu (%): C, 39.1; H, 3.0; N, 2.8. Found (%): C, 40.4; H, 2.9;
N, 1.8. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1603; ꢀ(C–O) 1318; ꢀ(C–S) 741; (PPh3/AsPh3)
1433, 1076, 693. UV (CH2Cl2, nm (", dm3 molꢀ1 cmꢀ1)): 345(11,702), 276(23,404),
243(44,680).
[RuCl(AsPh3)2(L4)]: Brown, d.p. 164ꢁC, yield 80%. Anal. Calcd for
C53H45N3OAs2ClSRu (%): C, 39.7; H, 3.0; N, 4.0. Found (%): C, 38.7; H, 2.5;
N, 3.5. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1606; ꢀ(C–O) 1314; ꢀ(C–S) 741; (PPh3/AsPh3)
1433, 1079, 693. UV (CH2Cl2, nm (", dm3 molꢀ1 cmꢀ1)): 347(23,706), 290(24,568),
242(43,103).
[RuCl(PPh3)2(L1)]: Brown, d.p. 180ꢁC, yield 80%. Anal. Calcd for C52H43N3P2ClOSRu
(%): C, 42.6; H, 3.1; N, 4.2. Found (%): C, 43.1; H, 3.9; N, 4.9. FT-IR (KBr, cmꢀ1):
ꢀ(C¼N) 1605; ꢀ(C–O) 1314; ꢀ(C–S) 748; (PPh3/AsPh3) 1433, 1092, 695. UV (CH2Cl2,
nm (", dm3 molꢀ1 cmꢀ1)): 280(11,363), 235(19,772).
[RuCl(PPh3)2(L2)]: Brown, d.p. 192ꢁC, yield 75%. Anal. Calcd for
C53H45N3O2P2ClSRu (%): C, 42.6; H, 3.2; N, 4.3. Found (%): C, 41.9; H, 3.6;
N, 4.8. FT-IR (KBr, cmꢀ1): ꢀ(C¼N) 1602; ꢀ(C–O) 1313; ꢀ(C–S) 748; (PPh3/AsPh3)
1433, 1092, 695. UV (CH2Cl2, nm (", dm3 molꢀ1 cmꢀ1)): 270(26,595), 238(40,425).
[RuCl(PPh3)2(L3)]: Brown, d.p.179ꢁC, yield 85%. Anal. Calcd for C53H45N3O2P2ClSRu
(%): C, 42.5; H, 3.3; N, 4.5. Found (%): C, 43.2; H, 2.9; N, 4.1. FT-IR (KBr, cmꢀ1):
ꢀ(C¼N) 1600; ꢀ(C–O) 1320; ꢀ(C–S) 748; (PPh3/AsPh3) 1434, 1091, 697. UV (CH2Cl2,
nm (", dm3 molꢀ1 cmꢀ1)): 325(7500), 275(14,500), 238(32,500).

300
M. Muthukumar et al.
[RuCl(PPh3)2(L4)]: Brown, d.p. 170ꢁC, yield 70%. Anal. Calcd for C53H45N3OP2ClSRu
(%): C, 43.3; H, 3.2; N, 4.3. Found (%): C, 42.9; H, 3.9; N, 4.5. FT-IR (KBr, cmꢀ1):
ꢀ(C¼N) 1602; ꢀ(C–O) 1314; ꢀ(C–S) 747; (PPh3/AsPh3) 1433, 1092, 696. UV (CH2Cl2,
nm (", dm3 molꢀ1 cmꢀ1)): 265(11,111), 239(26,666).
2.4. Catalytic activity for carbon–carbon coupling reaction
Magnesium turnings (0.320 g) were placed in a two-necked round-bottomed flask with a
calcium chloride guard tube. A crystal of iodine was added to activate the surface of the
magnesium. Bromobenzene (0.5 cm3 of total 1.275 cm3, 12 mmol) in 5 cm3 of anhydrous
diethyl ether was added with stirring and heated under reflux. The appearance of
turbidity after 5 min indicated the initiation of the reaction. The remaining
bromobenzene in 5 cm3 of ether was added dropwise and the reaction mixture was
refluxed for 40 min. To this reaction mixture, 1.03 cm3 (10 mmol) of the bromobenzene
in 5 cm3 of anhydrous diethyl ether and the ruthenium complex, (0.05 mmol) chosen for
investigation, was added and the mixture was heated under reflux for 6 h. The reaction
mixture was cooled and hydrolyzed with a saturated solution of aqueous ammonium
chloride. The ether extract on evaporation of the solvent gave the crude product which
was chromatographed to get pure biphenyl and compared with an authentic sample
(m.p.: 69–72ꢁC) [18].
3. Results and discussion
New stable ruthenium(III) chalcone thiosemicarbazone complexes of the type
[RuX(EPh3)2(L)] (X ¼ Cl or Br; E ¼ P or As; L ¼ chalcone thiosemicarbazone) are
obtained from the reactions of [RuX3(EPh3)3] (X ¼ Cl or Br; E ¼ P or As) with chalcone
thiosemicarbazone ligands in dry benzene in 1 : 1 molar ratio (scheme 1).
In all the above reactions, the chalcone thiosemicarbazones are binegative tridentate
ligands, replacing one triphenylphosphine/triphenylarsine and two chloride/bromide
ions. Mass spectra of [RuCl(PPh3)2(L4)] displayed molecular ion isotopic cluster at m/z
970, another isotopic cluster due to [RuCl(PPh3)(L4)]þ at m/z 708, and the dominant
fragment ([Ru(PPh3)(L4)]þ) at m/z 673, consistent with the proposed molecular formula
of new ruthenium(III) chalcone thiosemicarbazone complexes.
3.1. Infrared spectroscopic analysis
IR spectra of new complexes, when compared with those of the free ligands, confirm the
coordination of chalcone thiosemicarbazones to ruthenium. The ligands used in the
present study can exhibit thione–thiol tautomerism (scheme 2).
IR spectra of the free ligands display two bands at 3333 and 3514 cmꢀ1 due to ꢀs and
ꢀas of terminal NH2 [19], these bands remain unaltered in the corresponding metal
complexes indicating non-involvement of this group in complexation. A strong band
in the ligands at 1581–1604 cmꢀ1 due to ꢀC¼N shifts to higher wavenumber by
5–27 cmꢀ1 in the complexes [20]. The phenolic ꢀC–O of the free ligands in the region
1351–1324 cmꢀ1 shifts to lower wavenumber due to coordination to ruthenium ion

Ruthenium(III) thiosemicarbazones
301
OH
S
[RuX3(EPh3)3]
+
C
N
NH
C
NH2
CH
CHR
Benzene/
Reflux 6h
EPh3
X
O
Ru
S
C
N
EPh3
CH
CHR
N
C
NH2
[E = P or As; X = Cl or Br; R = C6H5, 4-(OCH3)C6H4, 3, 4-(OCH3)2C6H3 or 4-(CH3)C6H4]
Scheme 1. Formation of ruthenium(III) chalcone thiosemicarbazone complexes.
OH
OH
S
SH
C
C
N
NH
C
NH2
C
N
N
NH2
CH
CH
CHR
CHR
Scheme 2. Tautomerism in chalcone thiosemicarbazone.
through oxygen of the phenol [20]. Absorption due to ꢀO–H was not observed in infrared
spectra of the complexes in the region 3400–3600 cmꢀ1, suggesting deprotonation prior
to coordination to ruthenium. The band due to ꢀC¼S at 830 cmꢀ1 in the free ligand
disappears on complexation and a new band appears around 739–748 cmꢀ1. These
observations may be attributed to the coordination of thiolate sulfur after enolization
followed by deprotonation [21]. Characteristic bands due to triphenylphosphine or
triphenylarsine are also present.
3.2. Electronic spectroscopic analysis
Electronic spectra of the ruthenium(III) chalcone thiosemicarbazone complexes were
recorded in CH2Cl2 from 600 to 200 nm. Most complexes showed two to three bands in
2
the region 362–235 nm. The ground state of ruthenium(III) is T2g and the first excited
2
2
doublet levels in the order of increasing energy are A2g and A1g arising from t42g e1g
configuration. In most of the ruthenium(III) complexes, prominent charge transfer

302
M. Muthukumar et al.
Figure 2. EPR spectrum of [RuCl(PPh3)2(L3)].
bands of the type Lꢁy ! T2g obscure the weaker bands due to d–d transitions [22]. It is
therefore difficult to assign conclusively the bands of ruthenium(III) complexes in the
visible region. However, we assign the bands below 362 nm as being due to charge-
transfer transitions based on the extinction coefficient values, which are characteristic
of ruthenium(III) octahedral complexes [23–26].
3.3. Magnetic moments
The magnetic moment of the complexes has been measured at room temperature using
a vibrating sample magnetometer. The values lie between 1.72 and 2.12 BM, indicating
the presence of one unpaired electron and þ3 oxidation state for ruthenium in all these
complexes [27].
3.4. EPR spectroscopic analysis
Solid state EPR spectra of the complexes (figure 2) were recorded in X-band frequencies
at room temperature and the ‘g’ values are given in table 1. Most of the complexes
showed only one signal with ‘g’ values between 2.01 and 2.45, indicating symmetry
around the ruthenium. Such isotropic lines are usually due to either intermolecular spin
exchange which can broaden the lines or occupancy of the unpaired electron in a
degenerate orbital. However, a few of the complexes, [RuCl(AsPh3)2(L1)] and
[RuCl(AsPh3)2(L2)], showed spectra characteristic of an axially symmetric system

Ruthenium(III) thiosemicarbazones
303
with g around 2.01–2.45 and g around 2.09–2.14. For an octahedral field with
?
k
tetragonal distortion, gx ¼ gy ¼ gz and, hence, two values for ‘g’ indicate tetragonal
distortion in these complexes. Overall, the position of lines and nature of the EPR
spectra of the complexes are characteristic of low-spin ruthenium(III) octahedral
complexes [26, 28, 29].
3.5. Electrochemistry
Electrochemical properties of the new complexes have been examined by cyclic
voltammetry under N2 at glassy carbon working electrode in acetonitrile. [NBu4]ClO4
(0.05 M) as supporting electrolyte and redox potentials are referenced to Ag/AgCl. The
cyclic voltammetric data are given in table 2. As the ligands used in this work are
inactive at the used potential range, we believe that the redox processes observed for
these complexes are metal-centered only.
All the complexes show one reversible reduction couple. [RuBr(AsPh3)2(L2)]
and [RuBr(AsPh3)2(L3)] exhibit one reversible oxidation couple, whereas
[RuBr(AsPh3)2(L4)] and [RuCl(AsPh3)2(L1)] show an irreversible oxidation peak.
Table 1. EPR data of ruthenium(III) chalcone thiosemicarbazone complexes.
Complex
gx
gy
gz
hg*i
[RuBr(AsPh3)2(L1)]
[RuBr(AsPh3)2(L2)]
[RuBr(AsPh3)2(L3)]
[RuBr(AsPh3)2(L4)]
[RuCl(AsPh3)2(L1)]
[RuCl(AsPh3)2(L2)]
[RuCl(AsPh3)2(L3)]
[RuCl(AsPh3)2(L4)]
[RuCl(PPh3)2(L1)]
[RuCl(PPh3)2(L2)]
[RuCl(PPh3)2(L3)]
[RuCl(PPh3)2(L4)]
–
–
–
2.15
2.18
2.16
2.17
2.45
2.45
2.09
2.15
2.04
2.05
2.16
2.01
–
–
–
–
–
–
–
–
–
2.45
2.45
–
–
–
–
–
–
2.14
2.09
–
–
–
–
–
–
2.35
2.33
–
–
–
–
–
–
hg*i ¼ ½1=3g2x þ 1=3gy2 þ 1=3g2z ꢂ1=2.
Table 2. Electrochemical data for some ruthenium(III) chalcone thiosemicarbazone complexes.
Ru(IV)–Ru(III) Ru(III)–Ru(II)
Epa (V) Epc (V) Ef (V) DEp (mV) Epa (V) Epc (V) Ef (V) DEp (mV)
Complex
[RuBr(AsPh3)2(L2)]
[RuBr(AsPh3)2(L3)]
[RuBr(AsPh3)2(L4)]
[RuCl(AsPh3)2(L1)]
[RuCl(AsPh3)2(L3)]
[RuCl(PPh3)2(L1)]
[RuCl(PPh3)2(L4)]
0.95
0.78
0.78
1.02
–
0.73
0.70
0.84
0.74
220
80
–
–
–
ꢀ0.58
ꢀ0.70
ꢀ0.64
ꢀ0.68
ꢀ0.57
ꢀ0.68
ꢀ0.55
ꢀ0.47
ꢀ0.50
ꢀ0.38
ꢀ0.41
ꢀ0.44
ꢀ0.47
ꢀ0.42
ꢀ0.53
ꢀ0.60
ꢀ0.51
ꢀ0.55
ꢀ0.51
ꢀ0.58
ꢀ0.49
110
200
260
270
130
210
130
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Supporting electrolyte, [NBu4]ClO4 (0.05 M); complex, 0.001 M; solvent, CH3CN; DEp ¼ Epa ꢀ Epc, where Epa and Epc are
anodic and cathodic potentials, respectively; Ef ¼ 0.5(Epa þ Epc); scan rate: 100 mV sꢀ1
.

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M. Muthukumar et al.
Some of the complexes showed only reduction couple (table 2). The reduction couples
(RuIII–RuII) are quasi-reversible with peak-to-peak separation values (DEp) ranging
from 110 to 270 mV. The high peak-to-peak separation is attributed to slow electron
transfer and adsorption of the complexes onto the electrode surface [30]. The DEp (80–
220 mV) observed for oxidation (RuIII–RuIV) couple in [RuBr(AsPh3)2(L2)] and
[RuBr(AsPh3)2(L3)] indicate that the oxidation is quasi-reversible. The irreversible
oxidation observed in some complexes is presumably due to the short-lived oxidized
state of the metal ion [31] or due to oxidative degradation [32] followed by electrode
deposition. Based on the analytical and spectroscopic data, an octahedral structure
(scheme 1) has been tentatively proposed for all of the ruthenium(III) chalcone
thiosemicarbazone complexes.
3.6. Catalytic activity
The new ruthenium(III) complexes were tested as catalysts for aryl–aryl coupling. The
system chosen for the study is the coupling of phenylmagnesium bromide with
bromobenzene to give biphenyl as the product. Bromobenzene was first converted to
the corresponding Grignard reagent. Then, bromobenzene followed by the test complex
was added and the mixture was heated under reflux for 6 h. After work up, the reaction
mixture gave biphenyl.
The yields of the biphenyl formed from the coupling reactions varied from 14% to
17% (table 3) depending on the catalyst used. A blank reaction without ruthenium
catalyst produced 1% biphenyl, an insignificant amount compared to the yields of
biphenyl obtained from the reactions catalyzed by ruthenium complexes. This shows
the catalytic role of ruthenium complexes in the coupling reactions. The catalytic
activity of ruthenium(III) complexes towards coupling reactions is less than its activity
for oxidation and transfer hydrogenation reactions [33]. This may be due to the fact
that the active species derived from ruthenium complexes are less stable in the case of
coupling reactions, as the effectiveness of the catalysts is directly related to their ability
to generate the corresponding active species [34]. The product yields are not much
differed for different catalysts used, due to the formation of identical active species in all
reactions. Coupling reactions catalyzed by ruthenium complexes containing PPh3 or
AsPh3 yielded almost equal quantity of biphenyl.
Table 3. Catalytic activity of ruthenium(III) complexes.
Yield of biphenyl
Complex
g
%
[RuBr(AsPh3)2(L1)]
[RuBr(AsPh3)2(L4)]
[RuCl(AsPh3)2(L2)]
[RuCl(AsPh3)2(L3)]
[RuCl(PPh3)2(L3)]
[RuCl(PPh3)2(L4)]
No catalyst
0.221
0.244
0.235
0.217
0.270
0.210
0.021
15
16
14
17
17
14
01

Ruthenium(III) thiosemicarbazones
305
4. Conclusion
Several new ruthenium(III) complexes were synthesized using chalcone thiosemicarba-
zones formed from derivatives of 20-hydroxychalcones and thiosemicarbazide. The new
complexes have been characterized by analytical and spectroscopic data. An octahedral
structure has been tentatively proposed for all the complexes. Some of the
ruthenium(III) chalcone thiosemicarbazone complexes have been tested for C–C
coupling reactions. Though ruthenium(III) complexes with Schiff-base ligands have
been reported as catalysts for aryl–aryl coupling reactions [35], ruthenium(III)-
thiosemicarbazone complexes have been rarely seen as catalysts for this kind of
reaction. Several transition metal thiosemicarbazone complexes have been found in
recent literature [36–42], but with triphenylphosphine as co-ligand is rare [43]. In our
system, the presence of labile triphenylphosphine ligand makes the complexes
catalytically active and the activity may be tuned in the future by changing the
electronic environment around the metal by altering the substituents in the
thiosemicarbazone ligand.
Supplementary material
Representative mass spectrum, electronic spectra, and cyclic voltammogram of
ruthenium(III) chalcone thiosemicarbazone complexes have been provided as supple-
mentary materials.
Acknowledgments
The authors express their sincere thanks to Council of Scientific and Industrial
Research (CSIR), New Delhi [Grant No. 01(2065)/06/EMR-II] for financial support.
M. Muthukumar thanks CSIR for the award of Senior Research Fellowship.
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