New ruthenium(III) complexes with tridentate schiff base and their catalytic activity toward oxidation of alcohols

Article abstract of DOI:10.1080/15533170500301952

The synthesis and characterization of several hexa-coordinated ruthenium(III) complexes of the type [RuX₂(EPh₃)(L)] (E = P or As; X = Cl or Br; L = monobasic tridentate ligand derived by the condensation of o-phenylene diamine or ethylene diamine with salicylaldehyde or o-hydroxyacetophenone) are reported. Elemental analyses, IR, electronic, EPR, and cyclic voltammetric data of the complexes are discussed. An octahedral geometry has been tentatively proposed for all the complexes. The new complexes were found to be effective catalysts for the oxidation of benzyl alcohol and cyclohexanol to benzaldehyde and cyclohexanone, respectively, using N-methylmorpholine-N-oxide as a co-oxidant. Copyright

Full text of DOI:10.1080/15533170500301952

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New Ruthenium(III) Complexes with Tridentate Schiff
Base and Their Catalytic Activity toward Oxidation of
Alcohols
K. Saridha a , R. Karvembu a , P. Viswanathamurthi b & S. Yasodhai a
a Department of Chemistry (PG) , Kongunadu Arts and Science College (Autonomous) ,
Coimbatore, India
b Department of Chemistry , Periyar University , Salem, India
Published online: 15 Feb 2007.
To cite this article: K. Saridha , R. Karvembu , P. Viswanathamurthi & S. Yasodhai (2005) New Ruthenium(III) Complexes with
Tridentate Schiff Base and Their Catalytic Activity toward Oxidation of Alcohols, Synthesis and Reactivity in Inorganic, Metal-
Organic, and Nano-Metal Chemistry, 35:9, 707-711, DOI: 10.1080/15533170500301952
To link to this article: http://dx.doi.org/10.1080/15533170500301952
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Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 35:707–711, 2005
Copyright # 2005 Taylor & Francis, Inc.
ISSN: 1553-3174 print/1553-3182 online
DOI: 10.1080/15533170500301952
New Ruthenium(III) Complexes with Tridentate Schiff Base
and Their Catalytic Activity toward Oxidation of Alcohols
K. Saridha and R. Karvembu
Department of Chemistry (PG), Kongunadu Arts and Science College (Autonomous), Coimbatore, India
P. Viswanathamurthi
Department of Chemistry, Periyar University, Salem, India
S. Yasodhai
Department of Chemistry (PG), Kongunadu Arts and Science College (Autonomous), Coimbatore, India
abbreviated in general as HL, where H stands for the acidic
The synthesis and characterization of several hexa-coordinated proton. Each of these ligands has three potential donor sites,
ruthenium(III) complexes of the type [RuX2(EPh3)(L)] (E 5 P or
As; X 5 Cl or Br; L 5 monobasic tridentate ligand derived by the
condensation of o-phenylene diamine or ethylene diamine with
salicylaldehyde or o-hydroxyacetophenone) are reported. Elemen-
tal analyses, IR, electronic, EPR, and cyclic voltammetric data of
viz., a phenolate oxygen, an azomethine nitrogen, and an
amine nitrogen. The primary objective of the present study
has been to synthesize a family of ruthenium complexes
containing one of these tridentate ligands and triphenylpho-
the complexes are discussed. An octahedral geometry has been sphine/triphenylarsine.
tentatively proposed for all the complexes. The new complexes
were found to be effective catalysts for the oxidation of benzyl
alcohol and cyclohexanol to benzaldehyde and cyclohexanone,
respectively, using N-methylmorpholine-N-oxide as a co-oxidant.
EXPERIMENTAL
.
RuCl3 3H2O was purchased from Loba-Chemie Pvt. Ltd.,
Keywords ruthenium(III), Schiff base, triphenylarsine, triphenyl-
phosphine, catalytic oxidation
Mumbai, India, and was used without further purification. All
of the reagents used were of analytical or chemically pure
grade. Solvents were purified and dried according to standard
procedures. Microanalyses were performed at Regional
Sophisticated Instrumentation Centre, Central Drug Research
Institute, Lucknow. IR spectra were recorded on a Jasco
FTIR-410 and Nicolet FTIR (Avatar Model) spectropho-
tometer using a KBr disk. The electronic spectra were
recorded on a dichloromethane with a Jasco-V-530 UV/Vis
spectrophotometer in the 400–200-nm range. Cyclic voltam-
metric studies were carried out with a BAS CV electrochemical
analyzer in dichloromethane using a glassy carbon working
electrode and the potentials were referenced to a silver–
silver chloride electrode. The supporting electrolyte used was
0.1 M tetrabutylammonium perchlorte [Bu4N]ClO4 in dichloro-
methane solution. Melting points were recorded in a Boetius
micro heating table and are uncorrected.
INTRODUCTION
There has been considerable interest in ruthenium com-
plexes in recent years because of their redox stability,
excited-state reactivities, and excited-state lifetimes (Balzani
et al., 1996). Metal complexes of ruthenium containing
nitrogen and oxygen donor ligands are found to be effective
catalysts for oxidation, reduction, hydrolysis, and other
organic transformations (Karvembu et al., 2003; Ramesh,
2004). In the present work, we have chosen a group of three
tridentate O,N,N donor ligands (Figure 1). The ligands are
Received 28 March 2005; accepted 28 July 2005.
We thank Prof. P. R. Athappan, Department of Inorganic Chem-
istry, Madurai Kamaraj University, for his help in recording cyclic
voltammograms.
Address correspondence to R. Karvembu, Department of Chem-
istry (PG), Kongunadu Arts and Science College (Autonomous),
Coimbatore 641 029, India. E-mail: karvembu@rediffmail.com
The starting complexes [RuCl3(PPh3)3] (Chatt et al., 1968),
[RuCl3(AsPh3)3] (Poddar et al., 1974), [RuBr3(AsPh3)3]
(Natarajan et al., 1977) and the ligands (Boghaei and
Mohebi, 2002) were prepared according to the reported litera-
ture procedures. All of the new complexes were prepared under
anhydrous conditions.
707

708
K. SARIDHA ET AL.
then quantified as their 2,4-dinitrophenylhydrazones (Vogel,
1989).
RESULTS AND DISCUSSION
The tridentate Schiff bases react with the ruthenium(III)
complexes of the general formula [RuX3(EPh3)3] (X ¼ Cl or
Br; E ¼ P or As) in 1 : 1 molar ratio in benzene to yield com-
plexes of the type [RuX2(EPh3)(L)].
Benzene
½RuX3ðEPh3Þ3ꢀ þ HL ꢁꢁꢁꢁ!½RuX2ðEPh3ÞðLÞꢀ þ 2EPh3 þ HX
Reflux;5h
The analytical data obtained for the new complexes are in
good agreement with the proposed molecular formulaes
(Table 1). In all of the above reactions, the Schiff bases
behave as uninegative tridentate ligands.
FIG. 1. Structures of the ligands.
Preparation of Ruthenium(III) Complexes
All the new complexes were prepared by the following
general procedure. To a solution of [RuX3(EPh3)3] (X ¼ Cl
or Br; E ¼ P or As) (0.099–0.126 g, 0.1 mmol) in benzene
(25 mL), the appropriate Schiff base ligand (0.016–0.021 g,
0.1 mmol) was added and the mixture was heated under
reflux for 5 h. The resulting solution was concentrated to ca.
3 mL, cooled to room temperature and a small quantity of
petroleum ether (60–808C) was added. The separated
complexes were removed by filtration, washed with petroleum
ether, recrystallized from CH2Cl2/petroleum ether, and dried
in vacuo. Yields: 65–72%.
IR Spectra
In the IR spectra of Schiff bases, a strong band is observed
around 1,580–1,600 cm21 for the free azomethine (–CH55N–)
group. In the complexes, this band is shifted to the region
1,520–1,540 cm21, indicating the coordination of the Schiff
bases through the azomethine nitrogen (Khera et al., 1983).
A strong band observed at 1,280–1,300 cm21 in the free
Schiff bases has been assigned to phenolic C–O stretching.
Complexation, which shifted this band to a higher frequency
region (1,285–1,320 cm21), shows that the other coordination
is through the phenolic oxygen atom (Karvembu et al., 2003).
The free Schiff bases showed a very strong absorption around
3,450–3,500 cm21, which is characteristic of primary amine
Catalytic Oxidation of Alcohols
To a solution of alcohol (1 mmol) in dichloromethane N–H stretching. In the spectra of the new ruthenium(III) com-
(20 mL), N-methylmorpholine-N-oxide (3 mmol) and the plexes the absorptions due to the 2NH2 group has been
ruthenium complex (0.01 mmol) were added. The solution observed at a lower frequency 3,410–3,470 cm21, indicating
was refluxed for 3 h. The mixture was evaporated to dryness that the other coordination site is amine nitrogen atom to the
and extracted with petroleum ether (60–808C). The metal ion (Boghaei and Mohebi, 2002). The other character-
combined petroleum ether extracts was filtered and evaporated istic bands due to triphenylphosphine/triphenylarsine were
to give the corresponding carbonyl compound, which were also present in the expected region.
TABLE 1
Analytical data for the ruthenium(III) Schiff base complexes
Elemental analysis (%) found (calc.)
Yield
(%)
M.p.
(8C)
Complex
C
H
N
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
70
68
65
70
72
69
68
71
67
103
71
58.25 (57.68)
56.34 (54.01)
49.15 (47.84)
56.42 (54.28)
52.63 (50.56)
47.05 (44.40)
57.21 (54.99)
53.18 (51.31)
47.26 (45.18)
4.62 (4.06)
4.10 (3.83)
3.94 (3.37)
4.95 (4.39)
4.86 (4.09)
4.59 (3.59)
4.99 (4.62)
4.89 (4.31)
4.06 (3.79)
4.46 (4.34)
4.33 (4.06)
3.71 (3.60)
4.76 (4.69)
4.45 (4.37)
3.98 (3.84)
4.67 (4.58)
4.41 (4.27)
3.81 (3.76)
72
110
85
82
123
96
174

NEW RUTHENIUM(III) COMPLEXES WITH TRIDENTATE SCHIFF BASE
709
Electronic Spectra
The ground state of ruthenium(III) is T2g and the first
2
2
excited doublet levels in order of increasing energy are A2g
2
and A1g, which arise from the t2g 4eg1 configuration
(Ballhausen, 1962). In most of the ruthenium(III) complexes,
the UV-visible spectra show only charge transfer bands
(Lever, 1984). In a d5 system, and especially in ruthenium(III),
which has relatively high oxidizing properties, the charge
transfer bands of type Lpy ! t2g are prominent in the low-
energy region. Such bands obscure the weaker bands due to
d–d transition. It therefore becomes difficult to assign conclus-
ively the bands of ruthenium(III) complexes which appear in
the visible region.
The electronic spectra of all the complexes in dichloro-
methane showed two to three bands in the 360–240-nm
region (Table 2). These bands have been assigned to charge
transfer transitions. Similar observations have been made for
other ruthenium(III) octahedral complexes (Prabhakaran
et al., 2004).
FIG. 2. EPR spectrum of 1.
EPR Spectra
Redox Behavior
The solid-state EPR spectra at X-band frequencies for
several of the ruthenium(III) complexes have been recorded
at room temperature. All the complexes showed a single isotro-
pic line with g value in the 2.36–2.45 range (Figure 2). Such
isotropic lines are usually observed either due to intramolecular
spin exchange that can broaden the lines or to the occupancy of
the unpaired electron in a degenerate orbital. The nature and
pattern of the EPR spectra suggest an almost octahedral
environment around ruthenium ion in the complexes (Khan
et al., 1990; Medhi and Agarwala, 1980).
The redox behavior of the complexes has been examined in
dichloromethane at a glassy carbon working electrode using
cyclic voltammetry (Table 3). The redox potentials of the
Schiff base complexes are characterized by well-defined
waves in the range 0.81–1.03 V (oxidation) and 20.32 V to
20.73 V (reduction) versus SCE (Figure 3). As the ligands
are inactive in the potential range concerned (þ2.0 V to
22.0 V), the redox waves are assigned to the metal-centered
RuIII–RuII and RuIV–RuIII couples. In this, the oxidation is
quasi-reversible in nature, with a peak-to-peak separation
(DEp) of 150–400 mV (Heinze, 1984). This is attributed to
slow electron transfer and adsorption of the complexes onto
the electrode surface (Wallace et al., 1989). In general, these
complexes show one quasi-reversible oxidation couple and
an irreversible reduction peak. The reduction processes
(RuIII–RuII) for the complexes show an irreversible cyclic
voltammograms. These observations indicate that the charge
transfer process for the RuIII–RuII couple is not in general as
rapid as in the case for RuIV–RuIII. The reason for the
irreversibility for these complexes may be due to reductive
degradation or short-lived reduced state of the metal ion
(Bond et al., 1990). It has also been observed that there is
not much variation in the redox potentials due to the sub-
stitution of triphenylphosphine by triphenylarsine in the
complexes. Similar behavior has been recently observed for
other ruthenium(III) complexes (Ramesh and Maheswaran,
2003; Ramesh, 2004).
TABLE 2
IR and electronic spectral data for ruthenium(III)
Schiff base complexes
Ligand/
complex
n(C–O) n(C55N) n(N–H)
lmax
HL1
(1)
1,283
1,290
1,290
1,307
1,284
1,294
1,293
1,292
1,292
1,314
1,317
1,317
1,591
1,532
1,532
1,532
1,583
1,524
1,528
1,529
1,581
1,532
1,539
1,532
3,500
3,464
3,459
3,452
3,500
3,420
3,417
3,458
3,484
3,471
3,450
3,466
—
345, 291, 241
343, 295, 240
339, 298, 241
—
(2)
(3)
HL2
(4)
356, 241
349, 241
353, 241
—
(5)
(6)
HL3
(7)
335, 241
345, 241
338, 241
Analytical, spectroscopic (IR, electronic, EPR), and electro-
chemical data reveal that the complexes are hexa-coordinated,
with the donor atoms distributed about the metal center in an
octahedral manner (Figure 4).
(8)
(9)
n in cm21; l in nm.

710
K. SARIDHA ET AL.
TABLE 3
Cyclic voltammetric dataa for ruthenium(III) Schiff base complexes
RuIII–RuIV
RuIII–RuII
Complex
Epa (V)
Epc (V)
DEp (mV)
E1/2 (V)
Epa (V)
Epc (V)
DEp (mV)
E1/2 (V)
(1)
(3)
(5)
(8)
1.0
0.79
0.73
0.87
0.85
210
400
160
230
0.895
0.930
0.950
0.965
—
—
—
—
20.36
20.73
20.50
20.65
—
—
—
—
—
—
—
—
1.13
1.03
1.08
aSupporting electrolyte: [NBu4]ClO4 (0.1 M). Scan rate: 100 mV s21. Reference electrode: Ag/AgCl.
DEp ¼ Epa 2 Epc
E1/2 ¼ 0.5(Epa þ Epc)
where Epa and Epc are the anodic and cathodic peak potentials in V, respectively.
Catalytic Oxidation
The oxidation of benzyl alcohol and cyclohexanol was
carried out with different ruthenium complexes as catalysts
in the presence of N-methylmorpholine-N-oxide (NMO) as
co-oxidant in dichloromethane (Table 4). Benzaldehyde was
formed from benzyl alcohol and cyclohexanol was converted
into cyclohexanone after 3 h of stirring at room temperature.
The aldehydes/ketones formed were quantified as their
2,4-dinitrophenylhydrazone derivatives. In no case was there
any detectable oxidation of alcohols/ketones in the presence
of N-methylmorpholine-N-oxide alone and without the ruthe-
nium complex. All of the synthesized ruthenium complexes
were found to catalyze the oxidation of alcohols to aldehydes/
ketones, but the yields and the turnover were found to vary
with different catalysts. The ruthenium(III) complexes were
found to exhibit yields and turnover comparable to those
reported for similar ruthenium(III) complexes (El-Hendaway
et al., 1997). The relatively high product yield obtained for
the oxidation of benzyl alcohol as compared to cyclohexanol
is due to the fact that the a-CH moiety of benzyl alcohol is
FIG. 3. Cyclic voltammogram of 3.
TABLE 4
Catalytic oxidation of alcoholsa by ruthenium(III) Schiff base
complexes in the presence of N-methylmorpholine-N-oxide
Complex
Substrate
Product Yieldb Turnoverc
(1)
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cylohexanol
A
K
A
K
A
K
55
39
51
23
42
34
76
57
70
34
57
49
(4)
(7)
Benzyl alcohol
Cylohexanol
aA ¼ benzaldehyde, K ¼ cyclohexanone, reaction time ¼ 3 h.
bYields based on substrate.
FIG. 4. Proposed structures of the new complexes.
cMoles of product per mole of catalyst.

NEW RUTHENIUM(III) COMPLEXES WITH TRIDENTATE SCHIFF BASE
711
E1-Hendaway, A. M.; A1-Kubaisi, A. H.; A1-Madfa, H. A.
Ruthenium(II) and ruthenium(III) bipyridine complexes and their
catalytic oxidation properties for organic compounds. Polyhedron
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more acidic than the moiety in cyclohexanol (Chatterjee
et al., 2000).
CONCLUSION
Heinze, J. Cyclic voltammetry—“electrochemical spectroscopy.”
Angew. Chem. Int. Ed. Engl. 1984, 23, 831–847.
Mononuclear complexes of the type [RuX2(EPh3)(L)]
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ligand) were synthesized by a simple procedure. Elemental
analysis (C, H, and N) of the complexes agrees well with
the molecular formula proposed. IR spectral data confirm
the coordination of Schiff base through the azomethine
nitrogen, phenolic oxygen, and amine nitrogen to the metal.
Electronic spectra reveal the charge transfer transitions in the
complexes. EPR studies suggest an octahedral geometry
around ruthenium(III). Cyclic voltammetric data shows that
the complexes are redox-active. Moreover, the complexes
exhibit catalytic activity for the oxidation of alcohols in the
presence of N-methylmorpholine-N-oxide as co-oxidant.
Karvembu, R.; Hemalatha, S.; Prabhakaran, R.; Natarajan, K.
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