Binuclear ruthenium(III) Schiff base complexes bearing N4O4 donors and their catalytic oxidation of alcohols

Article abstract of DOI:10.1016/j.saa.2008.02.006

An interesting series of binuclear ruthenium(III) Schiff base complexes bearing bis-salophen/bis-naphophen units of the general composition [(EPh₃)(X)Ru-L-Ru(X)(EPh₃)] (where E = P or As; X = Cl or Br; L = binucleating dianionic tetradentate ligands) have been synthesized and characterized by analytical (elemental analysis, magnetic susceptibility measurements), spectral (FT-IR, UV-vis and EPR) and electrochemical methods. These ruthenium(III) complexes have two N₂O₂ metal binding sites, which are linked to each other with a biphenyl bridge and acts as potential catalyst for oxidation of wide range of primary and secondary alcohols to corresponding aldehydes or ketones with moderate to high conversion in the presence of N-methylmorpholine-N-oxide (NMO). The formation of high-valent Ru^V = O species as a catalytic active intermediate is proposed for the catalytic processes.

Full text of DOI:10.1016/j.saa.2008.02.006

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 71 (2008) 884–891
Binuclear ruthenium(III) Schiff base complexes bearing N4O4
donors and their catalytic oxidation of alcohols
G. Venkatachalam, N. Raja, D. Pandiarajan, R. Ramesh∗
School of Chemistry, Bharathidasan University,
Tiruchirappalli 620 024, India
Received 4 December 2007; received in revised form 22 January 2008; accepted 4 February 2008
Abstract
An interesting series of binuclear ruthenium(III) Schiff base complexes bearing bis-salophen/bis-naphophen units of the general composition
[(EPh3)(X)Ru-L-Ru(X)(EPh3)] (where E = P or As; X = Cl or Br; L = binucleating dianionic tetradentate ligands) have been synthesized and
characterized by analytical (elemental analysis, magnetic susceptibility measurements), spectral (FT-IR, UV–vis and EPR) and electrochemical
methods. These ruthenium(III) complexes have two N2O2 metal binding sites, which are linked to each other with a biphenyl bridge and acts as
potential catalyst for oxidation of wide range of primary and secondary alcohols to corresponding aldehydes or ketones with moderate to high
conversion in the presence of N-methylmorpholine-N-oxide (NMO). The formation of high-valent RuV = O species as a catalytic active intermediate
is proposed for the catalytic processes.
© 2008 Published by Elsevier B.V.
Keywords: Binucleating ligands; Binuclear ruthenium(III) complexes; Synthesis; Characterization; Catalytic oxidation
1. Introduction
tion of low valent ruthenium complexes with various oxidants
such as t-BuOOH, PhIO and NMO, etc. are widely utilized
[12b,16,17]. Elucidation of the nature of reactive intermediates
responsible for oxygen atom transfer in catalytic oxygenation
of organic substrates by cytochrome P-450 and their iron por-
phyrin models is of continuing interest in the field of biological,
bioinorganic and oxidation chemistry [18,19]. Recently, mono
and binuclear transition metal complexes derived from ligands
other than porphyrins have also been employed as catalysts for
oxidation of organic substrates [20–24]. The incorporation of
binucleating Schiff base ligand into ruthenium tertiary phos-
phine/arsine complexes was initially aimed at promoting activity
of such species towards oxidation of alcohols, because the use
of metal–salen or metal–salophen Schiff base complexes are
known to react with oxidants like NMO, PhIO and t-BuOOH to
form metal-oxo-species.
Herein, we describe the synthesis and characterization of a
series of new class of binuclear ruthenium(III) Schiff base com-
plexes along with their catalytic activity towards the oxidation
of a number of primary and secondary alcohols in the presence
of N-methylmorpholine-N-oxide (NMO). The following binu-
cleating Schiff base ligands (Fig. 1) containing two N2O2 type
tetradentate compartments were used to synthesize a new series
of binuclear ruthenium(III) Schiff base complexes (Fig. 1).
The design, synthesis and structural characterization of Schiff
base complexes are a subject of current interest due to their
structural, magnetic, spectral, catalytic and redox properties
[1–9]. Currently, a considerable effort is being invested in the
development of new chelating ligands; particularly, the binu-
cleating imino ligands are versatile and they exhibit very rich
coordination chemistry, such species occupy an important posi-
tion in modern inorganic chemistry [6,10,11]. Much research
has been published concerning the use of Schiff base ligands,
which incorporate nitrogen imine, and phenolate donors seems
appropriate for synthesizing ruthenium complexes capable of
oxidizing organic substrates [9,12]. Mechanisms invoking oxo-
transfer, electron transfer, hydrogen and hydride transfer have
been proposed for oxidations by OsO4 [13], metalloporphyrins
[14], and other metal oxo-complexes. Ruthenium complexes
used for oxidation of organic substrates generally show mecha-
nisms that involve a higher oxidation state of ruthenium [15]. In
this regard, ruthenium oxo-species generated through the reac-
∗
Corresponding author. Fax: +91 431 2407045/20.
E-mail address: ramesh bdu@yahoo.com (R. Ramesh).
1386-1425/$ – see front matter © 2008 Published by Elsevier B.V.
doi:10.1016/j.saa.2008.02.006

G. Venkatachalam et al. / Spectrochimica Acta Part A 71 (2008) 884–891
885
Fig. 1. Structures of binucleating tetradentate Schiff base ligands.
2. Experimental
1 mmol) was added to salicylaldehyde (0.42 g; 4 mmol)
or 2-hydroxy-1-naphthaldehyde (0.68 g; 4 mmol) in 10 cm3
methanol. The reaction mixture was stirred for 1 h. The result-
ing yellow or red solid product thus obtained was filtered,
recrystallized from THF and dried in vacuo. The ligands were
characterized by analytical and spectral data [30].
2.1. Materials and physical measurements
All the chemicals used were chemically pure and AR
grade. Solvents were purified and dried according to stan-
dard procedures [25]. RuCl3·3H2O was purchased from
Loba Chemie Pvt. Ltd., and was used without further
purification. 3,3ꢀ,4,4ꢀ-tetraminobiphenyl, salicylaldehyde, 2-
hydroxy-1-naphthaldehyde and NMO were obtained from
Aldrich. Other chemicals (used in the catalytic studies) were
purchased from Merck and Aldrich. The elemental analyses
were carried out with a Thermo Finnigan EA 1112 elemental
analyzer. FT-IR spectra were taken on a JASCO 400 plus spec-
trophotometer in the range 4000–400 cm−1. Electronic spectra
were recorded on a Cary 300 Bio UV–vis Varian spectropho-
tometer. Magnetic susceptibility measurement was carried out
with a EG and G model 155 vibrating sample magnetometer,
IIT-Madras, Chennai. EPR spectra were recorded on a Varian
E–112 ESR spectrophotometer at X-band microwave frequen-
cies for powdered samples at room temperature and for solution
at77 K, IIT-Bombay, Mumbai. Thespectrawerecalibratedusing
tetracyanoethylene (TCNE) (g = 2.00037). Cyclic voltammetric
measurements were carried out using a Princeton EG and G-
PARC model potentiostat using glassy carbon working electrode
and all the potentials were referred to Ag/AgCl. Gas chro-
matograms were obtained on HP 6890 series GC-FID with a
DB-5 capillary column. The starting ruthenium(III) precursors
[RuCl3(PPh3)3] [26], [RuCl3(AsPh3)3] [27], [RuBr3(AsPh3)3]
[28] and [RuBr3(PPh3)2(CH3OH)] [29] were prepared accord-
ing to the literature reports.
2.3. Synthesis of binuclear ruthenium(III) Schiff base
complexes
All the reactions were carried out under nitrogen atmosphere
and the complexes were synthesized by the following general
procedure. The precursor complexes [RuX3(EPh3)3] (0.2 mmol)
(E = P, X = Cl; E = As, X = Cl or Br) or [RuBr3(PPh3)2(CH3OH)]
(0.2 mmol) and the potential binucleating Schiff base ligands
bis-salophen (H4L1) or bis–naphophen (H4L2) (0.1 mmol) were
taken in THF (20 cm3) and the mixture was heated to reflux
for 3–4 h. The initial brown color of the solution was gradually
changed to red or violet and the reactions were monitored by
TLC. After completion of the reaction, the solution was removed
under reduced pressure. The solid mass thus obtained was puri-
fied by using a silica gel column, and eluted with CHCl3–acetone
mixture. Evaporation of solvent under reduced pressure afforded
60–70% of the solid ruthenium(III) Schiff base complexes.
2.4. Procedure for catalytic oxidation of alcohols
The oxidation reactions of primary and secondary alcohols
using binuclear ruthenium(III) Schiff base complexes 1 and 5 as
catalysts were carried out. A reaction mixture of organic sub-
strate (2.5 mmol), NMO (1.5 mmol) and catalysts (0.01 mmol)
in CH2Cl2 (5 cm3) was refluxed for different time period. From
the reaction mixture the catalyst was removed by the addition
of hexane, the filterate was separated and dried over anhydrous
Na2SO4 and were analyzed by GC with authentic samples or
quantified as 2,4-dinitrophenylhydrazone derivatives. In case of
benzhydrol and benzoin, the products benzophenone and benzil
2.2. Preparation of binucleating tetradentate Schiff base
ligands
The binucleating tetradentate Schiff base ligands were
prepared according to procedures involving Schiff base con-
densation of tetramine with appropriate hydroxyaldehydes. The
methanolic solution of 3,3ꢀ,4,4ꢀ-tetraminobiphenyl (0.214 g;
1
were isolated and confirmed by melting point, FT-IR and H
NMR.

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G. Venkatachalam et al. / Spectrochimica Acta Part A 71 (2008) 884–891
Scheme 1. Structure of binuclear ruthenium(III) Schiff base complexes.
3. Results and discussion
3.1. IR spectra
A new series of binuclear ruthenium(III) complexes of
the type [Ru2(EPh3)2(X)2(L)] (E = P or As, X = Cl or Br;
L = binucleating Schiff base ligand) (Scheme 1) were achieved
by reacting ruthenium(III) precursors [RuX3(EPh3)3] (E = P,
X = Cl; E = As, X = Cl or Br) or [RuBr3(PPh3)2(CH3OH)]
with binucleating tetradentate Schiff base ligands bis-salophen
(H4L1) or bis-naphophen (H4L2) in 2:1 mole ratio, respectively
in THF.
The synthesized binuclear ruthenium(III) Schiff base com-
plexes are stable in air at room temperature, non-hygroscopic
in nature and highly soluble in common solvents such as
dichloromethane, acetonitrile, benzene, chloroform, THF, etc.
producing intense color in their solutions. The analytical data of
the complexes are in good agreement with the calculated values
thus confirming the proposed binuclear composition for all the
complexes.
The important IR absorption bands for the synthesized com-
plexes are shown in Table 1. The observed bands may be
classified into those originating from the ligands and those aris-
ing from the bands formed between ruthenium(III) metal ion and
the coordinating sites. The azomethine nitrogen ν(C N) stretch-
ing frequency of the free ligands (H4L1 and H4L2) appears
around 1620 cm−1 which has been shifted to lower wavenum-
bers in the range 1598–1603 cm−1 in accordance with the
coordination of the azomethine function to the metal ion for all
the complexes [31] and this lowering of the wavenumber may be
attributed to thedecrease in electrondensity onthenitrogenatom
of the azomethine group. A medium band corresponding to phe-
nolic oxygen ν(C–O) is observed at 1274 and 1280 cm−1 for the
free ligands H4L1 and H4L2, respectively. On complexation this
band is shifted to higher frequency in the range 1298–1311 cm−1
for all the ruthenium(III) Schiff base complexes [32]. This is
Table 1
IR and electronic spectral data of binuclear ruthenium(III) Schiff base complexes
Complexes
νC (cm−1
)
νC–O (cm−1
)
λmax (nm) (ε) (dm3/(mol cm))
N
(1) [Ru2Cl2(PPh3)2(L1)]
(2) [Ru2Cl2(AsPh3)2(L1)]
(3) [Ru2Br2(AsPh3)2(L1)]
(4) [Ru2Br2(PPh3)2(L1)]
(5) [Ru2Cl2(PPh3)2(L2)]
(6) [Ru2Cl2(AsPh3)2(L2)]
(7) [Ru2Br2(AsPh3)2(L2)]
(8) [Ru2Br2(PPh3)2(L2)]
1599
1598
1602
1599
1601
1603
1598
1601
1306
1310
1299
1304
1311
1301
1304
1298
418 (6830)a 348 (9292)b 258 (16056)c
546 (5430)a 408 (9835)a 311 (13048)b 272 (18976)c
580 (6390)a 405 (8000)a 306 (10800)b 285 (15210)c
566 (4630)a 402 (7835)a 308 (12038)b 272 (19046)c
540 (4390)a 409 (9045)a 312 (13530)b 275 (18010)c
559 (3752)a 411 (8460)a 320 (12170)b 271 (21150)c
586 (3930)a 414 (7938)a 302 (11638)b 278 (19874)c
590 (3828)a 414 (9804)a 319 (10504)b 259 (19600)c
a
Charge transfer.
n–␲*.
␲–␲* transitions.
b
c

G. Venkatachalam et al. / Spectrochimica Acta Part A 71 (2008) 884–891
887
further supported by the disappearance of the νOH in the range
3400–3440 cm−1 in all the complexes. Further, the bands in the
region 500–550 cm−1 and 400–480 cm−1, which are probably
due to the formation of M–O and M–N bonds respectively [33]
were observed. In addition, all these binuclear ruthenium(III)
Schiff base complexes show strong vibrations near 690, 745 and
1550 cm−1, which are attributed to triphenylphosphine/arsine
ligands.
3.2. Electronic spectra
The electronic spectra of the complexes were recorded in
acetonitrile solution in the region 800–200 nm and the spectral
data are listed in Table 1. The ground state of ruthenium(III) is
2T2g and the first excited doublet levels in the order of increasing
energy are 2A2g and 2A1g, which arise from t24ge1g configuration
[34]. All the binuclear ruthenium(III) complexes in the visible
region display strong band in the range 590–559 nm followed
by a weak shoulder in the range 418–408 nm and are assigned
to be the LMCT transitions [35]. In a d5 system, especially in
ruthenium(III) which has relatively high oxidizing properties,
the charge transfer bands of the type L␲y → T2g are prominent
inthelowenergyregion, whichobscurestheweakerbandsdueto
d–d transitions. The band observed around 250 nm, which is also
present in the free ligands, is assigned to ␲–␲* transition from
the benzene ring and the double bond of the azomethine group.
The bands in the 348–306 nm regions are due to n–␲* transition
of non-bonding electrons present on the nitrogen of the azome-
thine group in the binuclear ruthenium(III) complexes. The
patterns of the electronic spectra of all the complexes indicate
the presence of an octahedral environment around the ruthenium
ion [35].
Fig. 2. EPR spectrum of the binuclear ruthenium(III) Schiff base complex
[Ru2Cl2(PPh3)2(L1)] (1) in toluene at 77 K.
or the absence of any strong magnetic interactions between the
two-ruthenium centers of the molecule.
The room temperature EPR spectra of powdered samples
were measured at X-band frequencies. All the complexes exhibit
well-defined single isotropic lines with ‘g’ values in the range
2.40–2.45. Isotropic lines are usually the results of either
intermolecular spin exchange, which can broaden the lines or
occupancy of the unpaired electron in a degenerate orbital. How-
ever, the EPR spectra of the complexes [Ru2Cl2(PPh3)2(L1)] (1)
and[Ru2Br2(AsPh3)2(L2)](5)recordedintolueneat77 Kshows
rhombic spectra with three different ‘g’ values (gx =/ gy =/ gz)
gx = 2.76, gy = 2.20, gz = 1.88 for (1) and gx = 2.88, gy = 2.24,
gz = 1.82 for (5), respectively. The selected EPR spectrum is
shown in Fig. 2. The rhombicity of the spectra reflects the asym-
metry of the electronic environment around the ruthenium in
the complexes. The EPR spectra of the present binuclear ruthe-
nium(III) Schiff base complexes are similar in nature to those
of the mononuclear complexes [36] and this indicates that the
two paramagnetic centers are equivalent and there is no super
3.3. Magnetic moment and EPR spectra
The room temperature magnetic susceptibility measurements
for two of the binuclear ruthenium(III) Schiff base com-
plexes, viz. [Ru2Cl2(PPh3)2(L1)] and [Ru2Br2(AsPh3)2(L2)]
were measured. The room temperature μeff values per ruthenium
ion for the complexes are 1.72 and 1.68 BM, respectively. It has
been observed that the μeff values indicate a single unpaired
electron and is consistent with non-interacting d5 metal centers
Table 2
Electrochemical data of binuclear ruthenium(III) Schiff base complexes
Complexes
RuI2V,IV–Ru2III,III
Epa (V)
Epc (V)
E1/2 (V)
ꢀEp (mV)
Epa (V)
Epc (V)
E1/2 (V)
ꢀEp (mV)
1
2
3
4
5
6
7
8
0.68
0.70
0.67
0.66
0.70
0.69
0.71
0.64
0.77
0.81
0.76
0.76
0.52
0.80
0.83
0.55
0.72
0.75
0.71
0.71
0.61
0.74
0.77
0.60
90
110
90
100
180
110
120
90
0.92
0.90
0.89
0.94
0.96
0.93
0.89
0.93
1.03
1.02
0.98
1.05
0.78
1.02
1.00
0.75
0.97
0.96
0.93
0.99
0.87
0.97
0.94
0.84
110
110
90
110
180
90
110
180
Supporting electrolyte: [(NBu4)ClO4] (0.005 M); complexes: 0.001 M; solvent: CH3CN; ꢀEp = Epa − Epc where Epa and Epc are anodic and cathodic potentials,
respectively; E1/2 = 0.5 (Epa + Epc); scan rate: 100 mV s−1
.

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G. Venkatachalam et al. / Spectrochimica Acta Part A 71 (2008) 884–891
Table 3
Catalytic oxidation of alcohols by binuclear ruthenium(III) Schiff base complex [Ru2Cl2(PPh3)2(L1)] (1) and [Ru2Cl2(PPh3)2(L2)] (5) in the presence of NMOa
Entry
Substrate
Product
Yield (%)b
TONf
1
41
39
102
98
2
3
38.4
64.8
96
162
90
64
225
160
4
5
18.5
46
46
115
85c
84c
212
210
74.9d
68.2d
187
171
6
7
46d
115
232
92.7d
84d
75d
210
188
8
9
56d
140
133
53.2d
78.7d
76.8d
197
192
10
11
71d
67d
177
168
12
45.4
31.8
113
80

G. Venkatachalam et al. / Spectrochimica Acta Part A 71 (2008) 884–891
889
Table 3 (Continued )
Entry
Substrate
Product
Yield (%)b
TONf
16e
8e
40
20
13
14
39.1e
38e
98
95
24e
60
41
15
16
16.2e
18e
14e
45
35
17
54
135
109
43.7
a
Reaction conditions: reactions were carried out in CH2Cl2 with catalyst/substrate mol ratio 1:250 for 8 h.
Products were quantified as 2,4-dinitrophenylhydrazone derivative.
Yield determined by comparison with authentic samples using HP 6890 series GC-FID with a DP-5 column.
Isolated yield.
Reaction after 12 h.
TON, moles of product/moles of catalyst.
b
c
d
e
f
ꢀ [(EPh3)(X)–RuIV–Y–RuIV–(X)(EPh3)]
exchange interaction between the two metal centers [20]. How-
ever, these new complexes are dimers with one unpaired electron
on each ruthenium(III) atom, leading to an S-value of 1. The pair-
ing of electrons is prevented by the greater distance between two
ruthenium(III) atoms provided by the bridging bis-tetradentate
Schiff base ligands.
It has been observed that there is not much variation in the
redox potentials due to the replacement of PPh3 by AsPh3 and
nature of bridging ligand present in the complexes.
4. Catalytic activity
3.4. Redox properties
As a part of our continuing interest to study the catalytic activ-
ity of ruthenium complexes [32b,38], herein, we examined the
catalytic activity of binuclear ruthenium(III) Schiff base com-
plexes [Ru2Cl2(PPh3)2(L1) (1) and [Ru2Cl2(PPh3)2(L2)] (5) in
the presence of oxidant NMO (Scheme 2) for the oxidation of
number of primary and secondary alcohols (Table 3). The oxida-
tion was performed in CH2Cl2 from which water was removed
using powdered molecular sieves.
Results of present investigation suggest that the complexes
are able to react efficiently with NMO to yield a high valent
ruthenium-oxo-species which is proposed as the intermediate in
ruthenium complexes catalyzed oxidation using NMO [17,32].
When NMO was added to the solution of binuclear ruthe-
nium(III) Schiff base complexes in CH2Cl2, the color of the
reaction mixture changed from reddish brown to green, indicat-
ing the formation of a new species. The aldehyde or ketones
All the binuclear ruthenium(III) Schiff base complexes do not
show any reduction waves at negative potentials but showed two
successive quasi-reversible (ꢀEp = 90–180 mV) oxidation cou-
ples (RuI2V,IV–Ru2III,III) and are metal centered only (Table 2).
The redox potentials are independent of the various scan rates
supporting, quasi-reversibility. The well defined half wavepo-
tentials (E1/2) in the range 0.60–0.77 V corresponding to first
oxidation couple and the second oxidation couple in the range
0.84–0.99 V were observed for all the complexes.
The first oxidation was attributed to the oxidation of one of
the ruthenium(III) centers to the corresponding mixed valence
complex [37] and the second to the ruthenium(IV).
[(EPh3)(X)–RuIII–Y–RuIII–(X)(EPh3)]
ꢀ [(EPh3)(X)–RuIV–Y–RuIII–(X)(EPh3)]

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G. Venkatachalam et al. / Spectrochimica Acta Part A 71 (2008) 884–891
Scheme 2. Binuclear ruthenium(III) Schiff base complexes/NMO catalyzed oxidation of alcohols.
formed after the appropriate time of reflux were determined
by GC analysis with authentic samples or quantified as their
2,4-dinitrophenylhydrazone derivatives [17b,25,32a]. Control
experiments showed that oxidation of the alcohols did not occur
either in the absence of the binuclear ruthenium(III) Schiff base
complexes or co-oxidant NMO (Scheme 2).
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show much activity for the conversion of benzyl alcohol to
benzaldehyde with traces of benzoic acid. Similarly, cinnamyl
alcohol to cinnamaldehdye with traces of cinnamic acid was
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5. Conclusion
The potential binucleating tetradentate Schiff bases bis-
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Acknowledgements
Financial support from the Council of Scientific and
Industrial Research (CSIR), New Delhi (grant no. 01
(1725)/02/EMR-II) is gratefully acknowledged. One of the
authors (G.V.) thank CSIR for the award of Senior Research
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