Ruthenium(III) chalconate complexes containing PPh3/AsPh3 and their use as catalysts

Article abstract of DOI:10.1080/00958972.2010.520707

Ruthenium(III) complexes of the type [RuX(EPh₃)(L)₂] (X = Cl or Br; E = P or As; L = 2-hydroxychalcone) have been prepared by reacting [RuX₃(EPh₃)₃] with 2-hydroxychalcones in benzene under reflux. The new complexes have been characterized by analytical and spectroscopic (infrared, electronic, electron paramagnetic resonance, and mass) methods. Redox potential studies of the complexes have been carried out to elucidate the electronic structure, geometry, and electrochemical features. On the basis of data obtained, an octahedral structure has been assigned for all the complexes. The new complexes exhibit catalytic activity for the oxidation of primary and secondary alcohols into their corresponding aldehydes and ketones in the presence of N-methylmorpholine-N-oxide as co-oxidant and they were also found to be efficient catalyst for the transfer hydrogenation of ketones.

Full text of DOI:10.1080/00958972.2010.520707

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Ruthenium(III) chalconate complexes
containing PPh3/AsPh3 and their use as
catalysts
M. Muthukumar a , P. Viswanathamurthi a , R. Prabhakaran b & K.
Natarajan b
a Department of Chemistry , Periyar University , Salem 636011,
Tamil Nadu, India
b Department of Chemistry , Bharathiar University , Coimbatore
641046, Tamil Nadu, India
Published online: 01 Oct 2010.
To cite this article: M. Muthukumar , P. Viswanathamurthi , R. Prabhakaran & K. Natarajan (2010)
Ruthenium(III) chalconate complexes containing PPh3/AsPh3 and their use as catalysts, Journal of
Coordination Chemistry, 63:21, 3833-3848, DOI: 10.1080/00958972.2010.520707
To link to this article: http://dx.doi.org/10.1080/00958972.2010.520707
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Journal of Coordination Chemistry
Vol. 63, No. 21, 10 November 2010, 3833–3848
Ruthenium(III) chalconate complexes containing PPh3/AsPh3
and their use as catalysts
M. MUTHUKUMARy, P. VISWANATHAMURTHI*y, R. PRABHAKARANz
and K. NATARAJANz
yDepartment of Chemistry, Periyar University, Salem 636011, Tamil Nadu, India
zDepartment of Chemistry, Bharathiar University, Coimbatore 641046, Tamil Nadu, India
(Received 9 March 2010; in final form 23 July 2010)
Ruthenium(III) complexes of the type [RuX(EPh3)(L)2] (X ¼ Cl or Br; E ¼ P or As;
L ¼ 2-hydroxychalcone) have been prepared by reacting [RuX3(EPh3)3] with 2-hydroxychal-
cones in benzene under reflux. The new complexes have been characterized by analytical and
spectroscopic (infrared, electronic, electron paramagnetic resonance, and mass) methods.
Redox potential studies of the complexes have been carried out to elucidate the electronic
structure, geometry, and electrochemical features. On the basis of data obtained, an octahedral
structure has been assigned for all the complexes. The new complexes exhibit catalytic activity
for the oxidation of primary and secondary alcohols into their corresponding aldehydes and
ketones in the presence of N-methylmorpholine-N-oxide as co-oxidant and they were also
found to be efficient catalyst for the transfer hydrogenation of ketones.
Keywords: Ruthenium(III) complexes; Spectroscopic characterization; Cyclic voltammetry;
Catalytic activity
1. Introduction
Among the platinum group metals, ruthenium has been extensively studied in terms of
its coordination and organometallic chemistry due to their stability, structural novelty
[1, 2], and catalytic applications [3, 4]. As coordination environment imparts properties
of complexes, complexation of ruthenium by diverse ligands is of importance [5, 6].
Organometallic compounds containing phosphine ligands have catalytic applications in
both small-scale organic synthesis and bulk industrial production [7–10].
Selective oxidation of alcohols is one of the most important reactions in organic
synthesis [11]. Several metal-based oxidants and halooxoacids have been used for
alcohol oxidation [12]. Although these reagents are quite general in scope, the oxidation
requires them in stoichiometric amount and produces problematic byproducts. Hence,
the development of efficient catalysts for alcohol oxidation is the ongoing research
interest [13].
*Corresponding author. Email: viswanathamurthi@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.520707

3834
M. Muthukumar et al.
Ruthenium-mediated transfer hydrogenation (TH) of ketones under basic conditions
employing isopropyl alcohol or other hydrogen-donor solvents has emerged as an
atom-economical method for the synthesis of secondary alcohols [14]. The reduction
of carbonyl compounds to the corresponding alcohols is a fundamental transformation
in synthetic organic chemistry. Hydrogen-transfer reactions [15] are advantageous
with respect to other reduction methods for several reasons: (a) the hydrogen donor is
easy to handle (no gas containment or pressure vessels are necessary), cheap, and
environmental friendly (e.g., isopropyl alcohol), (b) possible hazards are minimized, (c)
the mild reaction conditions applied can afford enhanced selectivity [16].
To prepare a simple catalyst system for TH as well as oxidation, we, herein, report
the synthesis of a series of six-coordinate ruthenium(III) chalconate complexes
containing PPh3/AsPh3. The characterizations of the complexes were accomplished
by analytical and spectroscopic (infrared (IR), electronic, mass, electron paramagnetic
resonance (EPR), and cyclic voltammetry) methods. Further, some of the synthesized
complexes have been effectively used as catalysts in the oxidation of alcohols in the
presence of N-methylmorpholine-N-oxide (NMO) in TH of ketones in the presence of
isopropyl alcohol and KOH as the promoter.
2. Experimental
2.1. Materials and methods
All reagents were chemically pure and AR grade. The solvents were purified and dried
according to standard procedures [17]. RuCl3 ꢀ 3H2O was purchased from Loba Chemie
Pvt. Ltd. and was used without purification. CHN analyses were performed by using a
Carlo Erba 1108 analyzer at Central Drug Research Institute (CDRI), Lucknow, India.
MALDI-TOF Mass spectral measurements were performed with a Voyager-De pro
instrument. IR spectra of the ligands and complexes were recorded in KBr pellets with a
Nicolet FT-IR spectrophotometer from 400 to 4000 cmꢁ1. Electronic spectra of the
complexes have been recorded in CH2Cl2 using a Shimadzu UV-Vis 1650 PC
spectrophotometer from 200 to 800 nm. NMR spectra (1H and 13C) of the ligands
were recorded on a Jeol GSX-400 instrument in CDCl3. NMR spectra were obtained
using TMS as the internal standard. EPR spectra of the powder samples of the
complexes were recorded with a JEOL JES-FA200 instrument with X-band frequencies
at room temperature using 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) as
internal standard at Pondicherry University, Pondicherry, India. Cyclic voltammetric
measurements of the complexes were carried out with a BAS CV-27 electrochemical
analyzer. Results were obtained using acetonitrile solution of [(n-C4H9)4N](ClO4)
(TBAP) as supporting electrolyte under nitrogen. A three-electrode cell was employed
with a glassy carbon working electrode, a platinum wire counter electrode, and a
Ag/AgCl reference electrode. Melting points were recorded on a Technico micro
heating table and are uncorrected. The catalytic yields were determined using an ACME
6000 series GC-FID with a DP-5 column of 30 m length, 0.53 mm diameter, and
5.00 mm film thickness. The starting complexes [RuCl3(PPh3)3] [18], [RuCl3(AsPh3)3]
[19], [RuBr3(AsPh3)3] [20], and 2-hydroxychalcone ligands [21] were prepared according

Ruthenium(III) chalconate complexes
3835
OH
O
C
CH
RHC
Ligand
R
1
L
C H
6 5
2
L
4-(OCH )C H
3
6
4
3
L
4-(Cl)C H
6
4
4
L
4-(CH )C H
6 4
3
Figure 1. Structure of 2-hydroxychalcones.
to the literature methods. The general structure of the 2-hydroxychalcones used in this
study is given in figure 1.
2.2. Synthesis of ruthenium(III) chalconate complexes
All the reactions were carried out under anhydrous conditions by following common
procedure. To a solution of [RuX3(EPh3)3] (X ¼ Cl or Br; E ¼ P or As) (0.1 mmol) in
benzene (20 cm3), the appropriate 2-hydroxychalcone (0.2 mmol) was added in 1 : 2
molar ratio. The mixture was heated under reflux for 6 h on a water bath, during which
the reaction mixture gradually changed to deep color. After the reaction time, the
contents were concentrated to around 3 cm3 by removing the solvent under reduced
pressure. The contents are cooled and then the product was separated by the addition of
10 cm3 of petroleum ether (60–80ꢂC). The product was recrystallized from CH2Cl2/
petroleum ether. The compounds were dried under vacuum over fused calcium chloride
and the purity of the complexes was checked by TLC.
2.3. Procedure for catalytic oxidation of alcohols
Catalytic oxidation of primary alcohols to corresponding aldehydes and secondary
alcohols to ketones by ruthenium(III) chalconate complexes were studied in the
presence of NMO as the co-oxidant. A typical reaction using the complex as a catalyst
and primary or secondary alcohol, as substrates at 1 : 100 molar ratio was described as
follows. A solution of ruthenium complex (0.01 mmol) in CH2Cl2 (20 cm3) was added to
the mixture containing substrate (1 mmol), NMO (3 mmol), and molecular sieves. The
solution mixture was refluxed for 3 h and the solvent was evaporated from the mother
liquor under reduced pressure. The solid residue was then extracted with diethyl
ether (20 cm3) and analyzed by GC. The oxidation products were identified by GC
co-injection with authentic samples.

3836
M. Muthukumar et al.
2.4. Procedure for catalytic TH of ketones
The catalytic TH reactions were also studied using ruthenium(III) chalconate complexes
as catalysts, ketone as substrate, and KOH as promoter at 1 : 300 : 2.5 molar ratios.
The procedure is as follows. A mixture containing ketone (3.75 mmol), ruthenium
complex (0.0125 mmol), and KOH (0.03 mmol) was heated to reflux in 10 cm3 of
isopropyl alcohol for 2 h. After completion of reaction the catalyst was removed from
the reaction mixture by the addition of diethyl ether followed by filtration and
subsequent neutralization with 1 mol Lꢁ1 HCl. The ether layer was filtered through a
short path of silica gel by column chromatography. The filtrate was subjected to GC
analyses and the hydrogenated product was identified and determined with authentic
samples.
3. Results and discussion
Paramagnetic, six-coordinate, low-spin ruthenium(III) complexes of general formula
[RuX(EPh3)(L)] (X ¼ Cl or Br; E ¼ P or As; L ¼ 2-hydroxychalcone) were synthesized
in good yields from the reaction of [RuX3(EPh3)3] (X ¼ Cl or Br; E ¼ P or As) with
2-hydroxychalcone in dry benzene in 1 : 2 molar ratio (scheme 1).
In all these reactions, 2-hydroxychalcones behave as monobasic bidentate chelating
ligands by replacing two triphenylphosphine/arsine and chloride/bromide from the
starting complexes.
All the complexes are stable in air at room temperature, brown in color, non-
hygroscopic, and highly soluble in common organic solvents such as dichloromethane,
acetonitrile, chloroform, and DMSO. The analytical data are listed in table 1 and are in
good agreement with the general molecular formula proposed for all the complexes.
Mass spectrum of the representative complex [RuCl(AsPh3)(L1)2] displayed molecular
ion isotopic cluster [RuCl(AsPh3)(L1)2]þ at m/z 889, another isotopic dominant
fragment ([Ru(L1)]þ) at m/z 322. This is consistent with the proposed molecular
formulas of new ruthenium(III) chalconate complexes.
3.1. IR spectroscopic analysis
The important IR absorption bands for the ligands and complexes are shown in table 2.
The free chalcones showed a strong band at 1634–1640 cmꢁ1 due to ꢀC¼O. This band
has been shifted to lower wave number 1599–1626 cmꢁ1 in the ruthenium complexes
indicating coordination of the ligands to ruthenium through carbonyl oxygen [22].
A strong band at 1303–1316 cmꢁ1 in the free chalcone has been assigned to
phenolic ꢀC–O. This band shifts to higher wave number 1310–1319 cmꢁ1 in spectra of
the complexes due to its coordination to ruthenium ion through oxygen [23]. This has
been further supported by the disappearance of the broad ꢀOH band around
3400–3600 cmꢁ1 in the complexes, indicating deprotonation of the phenol prior to
coordination to ruthenium metal. Hence, from the IR spectroscopic data, it is inferred
that both the carbonyl and phenolic oxygens are involved in the coordination of
chalcones to ruthenium ion in all the complexes.

Ruthenium(III) chalconate complexes
3837
HO
[RuX (EPh ) ]
+
3
3 3
O
C
HC
CHR
Benzene/
Reflux 6h
EPh
3
O
O
O
Ru
O
C
C
X
CH
HC
RHC
CHR
(X = Cl or Br; E = P or As; R = C H , 4-(OCH )C H , 4-Cl(C H ) or 4-CH (C H )
6
5
3
6
4
6
4
3
6 4
Scheme 1. Formation of ruthenium(III) chalconate complexes.
Table 1. Analytical data of ruthenium(III) chalconate complexes
Calculated (found) (%)
Complex
Formula
Yield (%)
D.Pt (ꢂC)
C
H
[RuCl(PPh3)(L1)2]
[RuCl(PPh3)(L2)2]
[RuCl(PPh3)(L3)2]
[RuCl(PPh3)(L4)2]
[RuCl(AsPh3)(L1)2]
[RuCl(AsPh3)(L2)2]
[RuCl(AsPh3)(L3)2]
[RuCl(AsPh3)(L4)2]
[RuBr(AsPh3)(L1)2]
[RuBr(AsPh3)(L2)2]
[RuBr(AsPh3)(L3)2]
[RuBr(AsPh3)(L4)2]
C48H37O4PClRu
C50H41O6PClRu
C48H35O4PCl3Ru
C50H41O4PClRu
C48H37O4AsClRu
C50H41O6AsClRu
C48H35O4AsCl3Ru
C50H41O4AsClRu
C48H37O4AsBrRu
C50H41O6AsBrRu
C48H35O4Cl2AsBrRu
C50H41O4AsBrRu
56
62
59
64
58
55
62
65
68
71
63
68
108
162
168
120
146
141
140
150
138
142
133
125
68.20(67.98)
66.33(66.10)
63.06(63.16)
68.76(68.62)
64.83(64.82)
63.46(63.48)
60.17(60.09)
65.47(65.41)
61.74(61.64)
60.43(60.48)
57.50(57.56)
62.44(62.36)
4.41(4.42)
4.56(4.60)
3.86(3.78)
4.73(4.72)
4.19(4.21)
4.35(4.28)
3.68(3.67)
4.50(4.49)
3.99(4.00)
4.16(4.18)
3.52(3.46)
4.30(4.27)

3838
M. Muthukumar et al.
Table 2. IR absorption frequencies (cmꢁ1) and electronic spectroscopic dataa (nm) of free ligands and their
ruthenium(III) chalconate complexes
Complex
ꢀC¼O
ꢀC–O
ꢀC¼C
ꢂmax (") (dm3 molꢁ1 cmꢁ1
)
L1
1639
1639
1640
1634
1626
1308
1303
1316
1305
1313
1572
1564
1585
1562
1544
234(31,468), 223(30,240)
L2
236(30,856), 224(30,210)
233(31,040), 222(30,240)
238(30,460), 225(30,380)
L3
L4
[RuCl(PPh3)(L1)2]
341(23,750), 231(31,040), 219(31,653), 216(33,845)
[RuCl(PPh3)(L2)2]
[RuCl(PPh3)(L3)2]
[RuCl(PPh3)(L4)2]
[RuCl(AsPh3)(L1)2]
[RuCl(AsPh3)(L2)2]
[RuCl(AsPh3)(L3)2]
[RuCl(AsPh3)(L4)2]
[RuBr(AsPh3)(L1)2]
[RuBr(AsPh3)(L2)2]
[RuBr(AsPh3)(L3)2]
[RuBr(AsPh3)(L4)2]
1599
1618
1609
1622
1602
1621
1617
1619
1600
1620
1616
1311
1319
1318
1312
1311
1312
1311
1312
1310
1312
1312
1542
1544
1543
1543
1540
1542
1541
1542
1539
1541
1541
380(21,958), 231(31,040), 215(32,985)
341(23,750), 231(31,040), 215(32,985)
357(22,562), 231(31,040), 220(31,964)
341(23,750), 231(31,040), 215(32,985)
371(21,496), 341(23,750), 231(31,040), 221(31,867)
441(20,385), 341(23,750), 231(31,040), 215(32,985)
341(23,750), 231(31,040), 216(33,845)
441(20,385), 341(23,750), 231(31,040), 224(31,238)
341(23,750), 232(31,180), 220(31,964), 214(33,653)
341(23,750), 231(31,040), 220(31,964), 210(32,179)
441(20,385), 231(31,040), 219(31,653), 215(32,985)
aSolvent used: CH2Cl2.
The ꢀC–C of the free ligands appeared as a separate band in their IR spectra around
1600 cmꢁ1, but could not be identified in spectra of the ruthenium complexes because of
their possible merging with ꢀC¼O [24]. In the complexes, the absorption due to the
phenyl alkene vibration appeared at 1539–1544 cmꢁ1, slightly lower than that observed
in the spectra of the free ligands [25]. In addition, characteristic bands due to
triphenylphosphine or triphenylarsine (around 1440, 1090, and 700 cmꢁ1) were also
present in the spectra of all the complexes [26].
3.2. Electronic spectroscopic analysis
Electronic spectra of the ligands and their complexes were recorded in CH2Cl2 from 200
to 800 nm. For a low-spin d5 system in an octahedral environment, the ground state is
2T2g arising from a t52g configuration. The next higher energy configuration t24g e1g gives
rise to four states, namely 2A2g, 2A1g, 2T1g, and 2T2g, and so four bands are expected. In
most of the ruthenium(III) complexes, the absorption spectra show only charge transfer
(CT) bands [27]. In a d5 system, and especially in ruthenium(III) which is a relatively
good oxidizing agent, the CT bands of the type Lꢁy ! T2g are prominent in the low-
energy region and the weaker bands are obscure due to d–d transitions. It, therefore,
becomes difficult to conclusively assign bands of ruthenium(III) complexes which
appear in the visible region [28].
The electronic spectroscopic data of the free ligands and their complexes are listed
in table 2. The spectra of the free ligands showed two types of transitions. The peak at
225–222 nm was assigned to ꢁ–ꢁ* transitions involving molecular orbitals located on
the phenol. These peaks shift in the spectra of the complexes due to the donation of a
lone pair of electrons by oxygen of the phenoxy to ruthenium [25]. The second type of

Ruthenium(III) chalconate complexes
3839
Table 3. 1H NMR and 13C NMR data (ꢃ in ppm) of 2-hydroxychalcone ligands
Ligand
L1
1H NMR
13C NMR
12.79 (s, –OH), 7.94–6.92
(m, aromatic)
12.92 (s, –OH), 7.91–6.88
(m, aromatic), 3.85 (s, –OCH3)
12.81 (s, –OH), 7.89–6.90
(m, aromatic)
193.74 (s, –C¼O), 163.61 (s, –C–OH),
145.45–118.64 (m, aromatic)
L2
193.65 (s, –C¼O), 163.54 (s, –C–OH),
145.31–114.51 (m, aromatic), 55.41 (s, –OCH3)
193.71 (s, –C¼O), 163.58 (s, –C–OH),
145.42–116.52 (m, aromatic)
L3
L4
12.90 (s, –OH), 7.93–6.94
(m, aromatic), 1.25 (s, –CH3)
193.73 (s, –C¼O), 163.56 (s, –C–OH),
145.40–114.84 (m, aromatic), 20.52 (s, –CH3)
transitions at 238–233 nm are assigned to n–ꢁ* transitions involving molecular orbitals
of the –C¼N and the benzene ring. These bands also shift upon complexation
indicating that the imine nitrogen coordinates to the metal [29].
The spectra of all the complexes show other types of transitions. Broad bands at
441–341 nm can be assigned to CT transitions [23]. The pattern of the electronic spectra
of all the complexes indicate an octahedral environment around ruthenium(III) ion.
3.3. 1H and 13C NMR spectra of 2-hydroxychalcone ligands
1
The H NMR spectra of the 2-hydroxychalcone ligands were recorded in CDCl3 to
confirm the formation of the ligands. The OH signal of all chalcone ligands appeared
as a sharp singlet at 12.79–12.92 ppm (table 3). All the ligands showed peaks at
6.88–7.94 ppm, assigned to protons of phenyl in the 2-hydroxychalcone. The signal due
to two alkene protons appeared at 6.9–7.9 ppm, and hence merged with the multiplet of
aromatic protons. The signal for methoxy proton appeared as a singlet at 3.85 ppm in
L2 and the signal for methyl proton appeared as a singlet at 1.25 ppm in L4.
13C NMR spectra of the ligands exhibit a sharp singlet at 193.65–193.74 ppm
(table 3), assigned to –C¼O carbon. The signal for phenolic –C–OH carbon appeared at
163.54–163.61 ppm. Multiplets around 114.51–145.45 ppm have been assigned to
aromatic carbons. The alkene carbons appeared at 115.45–140.58 ppm, and hence
merged with the aromatic carbons. A sharp singlet at 55.41 ppm was assigned to
methoxy carbon in L2. In addition, a sharp singlet at 20.52 ppm was assigned to methyl
carbon in L4.
3.4. EPR spectroscopic analysis
The solid-state EPR spectra of all the ruthenium(III) chalconate complexes (a repre-
sentative spectrum given in figure 2) were recorded in X-band frequencies at room
temperature.
Most of the complexes showed a single isotropic line with ‘g’ values in the range
2.23–2.43, indicating high symmetry (Oh) around ruthenium. Such isotropic lines are
either due to the intermolecular spin exchange which can broaden the lines or due to the
occupancy of the unpaired electron in a degenerate orbital. Few of the complexes
(table 4, [RuCl(AsPh3)(L1ꢁ3)2]) showed spectra characteristics of an axially symmetric

3840
M. Muthukumar et al.
Figure 2. EPR spectrum of [RuCl(AsPh3)(L4)2].
Table 4. EPR spectroscopic data of ruthenium(III) chalconate complexes
Complex
gx
gy
gz
hgi*
[RuCl(PPh3)(L1)2]
[RuCl(PPh3)(L2)2]
[RuCl(PPh3)(L3)2]
[RuCl(PPh3)(L4)2]
[RuCl(AsPh3)(L1)2]
[RuCl(AsPh3)(L2)2]
[RuCl(AsPh3)(L3)2]
[RuCl(AsPh3)(L4)2]
[RuCl(AsPh3)(L4)2] LNT
[RuBr(AsPh3)(L1)2]
[RuBr(AsPh3)(L2)2]
[RuBr(AsPh3)(L3)2]
[RuBr(AsPh3)(L4)2]
–
–
2.39
2.41
2.43
2.36
2.40
2.28
2.32
2.09
2.12
2.39
2.42
2.23
2.32
–
–
–
–
–
–
–
–
–
–
2.40
2.28
2.32
2.34
2.32
–
1.97
2.03
2.04
1.98
1.96
–
2.26
2.19
2.23
2.14
2.14
–
–
–
–
–
–
–
–
–
–
hgi* ¼ [1/3(gx)2 þ 1/3(gy)2 þ 1/3(gz)2]1/2
.
system with g around 2.28–2.40 and g around 1.97–2.04. 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. However, [RuCl(AsPh3)(L4)2] showed three lines with
three different ‘g’ values (gx ¼ gy ¼ gz), indicating rhombic distortion. The spectra of

Ruthenium(III) chalconate complexes
3841
Table 5. Electrochemical data for some ruthenium(III) chalconate complexes
Ru(IV)–Ru(III)
Ru(III)–Ru(II)
Complex
Epa (V) Epc (V) E1/2 (V) DEp (mV) Epa (V) Epc (V) E1/2 (V) DEp (mV)
[RuCl(PPh3)(L1)2]
[RuCl(PPh3)(L2)2]
[RuCl(AsPh3)(L1)2]
[RuCl(AsPh3)(L3)2]
[RuCl(AsPh3)(L4)2]
[RuBr(AsPh3)(L3)2]
[RuBr(AsPh3)(L4)2]
–
–
–
–
–
–
–
1.08
1.09
1.02
1.05
0.79
0.88
0.77
–
–
–
–
–
–
–
–
–
–
–
–
–
–
ꢁ1.13
ꢁ1.12
ꢁ1.18
ꢁ1.15
ꢁ1.10
ꢁ1.04
ꢁ1.06
ꢁ0.74
ꢁ0.77
ꢁ0.63
ꢁ0.71
ꢁ0.64
ꢁ0.59
ꢁ0.61
ꢁ0.94
ꢁ0.95
ꢁ0.91
ꢁ0.93
ꢁ0.87
ꢁ0.82
ꢁ0.84
390
350
550
440
460
450
450
Supporting electrolyte, NBu4ClO4 (0.05 mol Lꢁ1); complex, 0.001 mol Lꢁ1; solvent, CH3CN; DEp ¼ Epa ꢁ Epc, where Epa
and Epc are anodic and cathodic potentials, respectively; E1/2 ¼ 0.5(Epa þ Epc); scan rate: 100 mV sꢁ1
.
[RuCl(AsPh3)(L4)2] at liquid nitrogen temperature did not show much variation from
that observed at room temperature, indicating rhombic distortion in the complex.
Overall, the position of the lines and the nature of the EPR spectra are characteristic of
the octahedral ruthenium(III) complexes [30, 31].
3.5. Cyclic voltammetry
Redox behavior of some of the complexes was studied by cyclic voltammetry under N2
atmosphere at glassy carbon working electrode in acetonitrile solution (0.05 mol Lꢁ1
:
NBu4ClO4) and the redox potentials are expressed with reference to Ag/AgCl. The
complexes display irreversible oxidative (RuIV/RuIII) and quasireversible reductive
response (RuIII/RuII) at 100 mV sꢁ1. The potentials are summarized in table 5.
The complexes exhibit quasireversible reduction with E1/2 in the range ꢁ0.82 to
ꢁ0.94 V (RuIII/RuII). The reduction observed is quasireversible in nature, character-
ized by large peak-to-peak separation (DEp) of 350–550 mV, suggestive of a single-step
one-electron transfer process [32]. The quasireversible reduction waves with large
peak-to-peak separation values (DEp) are attributed to slow electron transfer and
adsorption of the complexes on the electrode surface [33–35]. The complexes showed an
irreversible oxidation process, which may be ascribed to the short-lived oxidized state
of the metal ion.
The potentials of both the oxidation (RuIV/RuIII) and reduction (RuIII/RuII) have
been found to be sensitive to the nature of the substituent (R) in the chalcone ligands.
The formal potentials E1/2 increase with the increase in electron-withdrawing nature of
the substituent (R). The plot of E1/2 versus ꢄ (ꢄ – the Hammet constant of R:
OCH3 ¼ ꢁ0.27, CH3 ¼ ꢁ0.17, Cl ¼ 0.23, H ¼ 0.00) [36] is linear for reduction (figure 3)
with slopes of 0.33 V (RuIII/RuII) for complexes [RuCl(PPh3)(L1)2], [RuCl(PPh3)(L2)2],
[RuCl(AsPh3)(L3)2], and [RuCl(AsPh3)(L4)2]. The linear correlation of the redox
potentials and electronic nature (ꢄ) of the substituents clearly shows that a single
substituent on the chalcone ligand, which is four bonds away from the metal center, can
influence metal-centered potentials in a predictable manner. Hence, it is clear that the
present ligand system is suitable for stabilizing the higher oxidation state of ruthenium
ions. Further, reduction potentials do not show any systematic variation with the
change in chloride or bromide present in the ruthenium complexes.

3842
M. Muthukumar et al.
–0.86
–0.88
–0.90
–0.92
–0.94
–0.96
–0.3
–0.2
–0.1
0.0
0.1
0.2
0.3
Hammet, s
Figure 3. Least-squares plot of (RuIII/RuII) potentials vs. ꢄ.
3.6. Catalytic oxidation
Catalytic oxidations of primary alcohols and secondary alcohols by the ruthenium(III)
chalconate complexes were carried out in CH2Cl2 in the presence of NMO, and the by-
product water was removed by using molecular sieves. All the complexes oxidize
primary alcohols to the corresponding aldehydes and secondary alcohols to ketones
with high yields (table 6). The aldehydes or ketones formed after 3 h of reflux were
determined by GC and there was no detectable oxidation in the absence of ruthenium
complex.
The oxidation of benzyl alcohol to benzaldehyde resulted in 71–99% yield and
cyclohexanol to cyclohexanone resulted in 47–93% yield. The higher product yield
obtained for the oxidation of benzyl alcohol compared to cyclohexanol is due to the
fact that the ꢅ-CH unit of benzyl alcohol is more acidic than cyclohexanol [37].
Triphenylarsine ruthenium(III) chalconate complexes possess greater catalytic activ-
ity than triphenylphosphine complexes. The catalytic activity of new ruthenium(III)
chalconate complexes is compared with that of corresponding starting complexes
[RuCl3(PPh3)3], [RuCl3(AsPh3)3], and [RuBr3(AsPh3)3]; the activities are comparable
for the oxidation of benzyl alcohol and tremendously improved for the oxidation of
cyclohexanol. The conversion is higher than the conventional catalyst K2Cr2O7
[26, 38]. There is no correlation between the redox potential with catalytic activity of
the complexes. The conversion of primary and secondary alcohols to corresponding
aldehydes and ketones increases with the increase in reaction time up to 3 h
(figure 4).
The complexes are able to react efficiently with NMO to yield high-valent ruthenium-
oxo species [38], capable of oxygen atom transfer to alcohols. This was further
supported by spectroscopic changes that occur by the addition of NMO to a
dichloromethane solution of the ruthenium(III) complexes. The appearance of a peak at
390 nm in UV-Vis spectra is attributed to the formation of RuV ¼ O, in conformity with

Ruthenium(III) chalconate complexes
3843
Table 6. Catalytic oxidation data of ruthenium(III) chalconate complexes
Complex
Substrate
Product
Yield (%)a
TONb
[RuCl(PPh3)(L2)2]
[RuCl(PPh3)(L3)2]
[RuCl(PPh3)(L4)2]
[RuCl(AsPh3)(L2)2]
[RuCl(AsPh3)(L3)2]
[RuCl(AsPh3)(L4)2]
[RuBr(AsPh3)(L2)2]
[RuBr(AsPh3)(L3)2]
[RuBr(AsPh3)(L4)2]
[RuCl3(PPh3)3]
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
88
69
71
61
96
77
98
91
98
87
97
93
98
47
99
55
98
60
68
56
87
77
88
45
07
04
88
69
71
61
96
77
98
91
98
87
97
93
98
47
99
55
98
60
68
56
87
77
88
45
07
04
[RuCl3(AsPh3)3]
[RuBr3(AsPh3)3]
K2Cr2O7/H2SO4
A: benzaldehyde; B: cyclohexanone.
aYield determined by GC and comparing with the analyses of authentic samples.
bRatio of moles of product obtained to the moles of the catalyst used.
100
80
60
40
20
0
A
B
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Time (h)
Figure 4. Catalytic oxidation of benzyl alcohol (A) and cyclohexanol (B) in different time intervals
[RuBr(AsPh3)(L3)2].
other oxo ruthenium(V) complexes [39, 40]. The formation of such species was
identified from the IR spectrum of the solid mass (obtained by evaporation of the
resultant solution to dryness), which showed a band at 860 cmꢁ1, characteristic of
RuV ¼ O species [40, 41].

3844
M. Muthukumar et al.
Table 7. Catalytic TH of ketones by ruthenium(III) chalconate complexes.a
Complex
Substrate
Product
OH
Conversionb (%)
80
TONc
240
[RuCl(PPh3)(L2)2]
O
O
OH
74
222
O
OH
61
94
183
282
O
OH
[RuCl(AsPh3)(L1)2]
33
43
99
O
OH
O
OH
129
O
OH
41
84
123
252
O
OH
[RuBr(AsPh3)(L1)2]
40
52
63
120
156
O
OH
O
OH
O
OH
189
(Continued )

Ruthenium(III) chalconate complexes
3845
Table 7. Continued.
Complex
Substrate
Product
Conversionb (%)
99
TONc
297
O
OH
O
OH
[RuCl3(PPh3)3]
[RuCl3(AsPh3)3]
[RuBr3(AsPh3)3]
42
34
126
102
O
OH
O
OH
38
83
114
249
O
OH
O
OH
23
25
69
75
O
OH
O
OH
33
72
99
O
OH
216
O
OH
30
27
90
81
O
OH
(Continued )

3846
M. Muthukumar et al.
Table 7. Continued.
Complex
Substrate
Product
Conversionb (%)
36
TONc
108
O
OH
O
O
O
OH
OH
OH
86
20
32
258
60
NaBH4
96
O
OH
OH
28
34
84
O
102
aConditions: reactions were carried out heated to reflux using 3.75 mmol of ketone (10 cm3 isopropyl alcohol); catalyst/
ketone/KOH ratio 1 : 300 : 2.5.
bYield of product was determined using a ACME 6000 series GC-FID with a DP-5
column of 30 m length, 0.53 mm diameter, and 5.00 mm film thickness and by comparison with authentic samples.
cTON ¼ ratio of moles of product obtained to the moles of the catalyst used.
3.7. Catalytic TH
The catalytic TH of ketones in the presence of ruthenium chalconate complexes has
been studied in isopropyl alcohol-KOH using a molar ratio 1 : 2.5 : 300 for the catalyst,
KOH, and the ketone in 10 cm3 of isopropyl alcohol. The ruthenium(III) chalconate
complexes showed excellent catalytic activity. Typical results are shown in table 7.
Cyclohexanone converted into cyclohexanol in 84–99% yield. Acetophenone was
converted into 1-phenyl ethanol in 33–80% yield and benzophenone was converted into
benzhydrol in 43–74% yield. Similarly, iso-butylmethylketone underwent TH to afford
the corresponding alcohol in 41–63% yield. The catalytic efficiency of the new
ruthenium(III) chalconate complexes is moderately increased for the conversion of
cyclohexanone to cyclohexanol, but tremendously improved for the reduction of
acetophenone, benzophenone, and isobutyl methyl ketone when compared to the
corresponding starting complexes. The conversion is higher than the conventional
catalyst NaBH4. In the absence of base, no TH of ketone was observed. The base
facilitated the formation of the ruthenium alkoxide by abstracting the proton of the

Ruthenium(III) chalconate complexes
3847
alcohol and subsequently, the alkoxide underwent ꢆ-elimination to give a ruthenium
hydride which is an active species in this reaction [42]. There is no correlation between
the redox potential with catalytic activity of the complexes.
4. Conclusions
Some new ruthenium(III) chalconate complexes were synthesized using chalconate
produced from the derivatives of benzaldehyde and 2-hydroxyacetophenone. The new
complexes have been characterized by analytical and spectroscopic (IR, electronic,
mass, EPR, and cyclic voltammetry) methods. An octahedral structure has been
tentatively proposed for all the complexes. The complexes showed efficient catalytic
activity for the oxidation of both primary and secondary alcohols with excellent yields
in the presence of NMO, and also for TH of aliphatic and aromatic ketones with high
conversions in the presence of isopropyl alcohol and KOH. Although some ruthenium
complexes have been found in recent literature [43, 44], ruthenium(III) chalconate
complexes have been rarely seen as catalysts for the oxidation and TH. In our system,
the presence of triphenylphosphine/arsine may allow the activity to be tuned by
changing the electronic environment around the metal.
Supplementary material
Representative mass spectrum and cyclic voltammogram of ruthenium(III) chalconate
complexes have been provided as supplementary 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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