Luminescent property and catalytic activity of Ru(II) carbonyl complexes containing N, O donor of 2-hydroxy-1-naphthylideneimines

Article abstract of DOI:10.1016/j.saa.2006.07.010

The reaction of the chelating ligands (obtained by the condensation of 2-hydroxy-1-naphthaldehyde with various primary amines) with [RuHCl(CO)(EPh₃)₂(B)] (where E = P; B = PPh₃, py or pip: E = As; B = AsPh₃) in

Full text of DOI:10.1016/j.saa.2006.07.010

Spectrochimica Acta Part A 67 (2007) 256–262
Luminescent property and catalytic activity of Ru(II) carbonyl complexes
containing N, O donor of 2-hydroxy-1-naphthylideneimines
M. Sivagamasundari, R. Ramesh∗
School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, India
Received 27 January 2006; accepted 5 July 2006
Abstract
The reaction of the chelating ligands (obtained by the condensation of 2-hydroxy-1-naphthaldehyde with various primary amines) with
[RuHCl(CO)(EPh3)2(B)] (where E = P; B = PPh3, py or pip: E = As; B = AsPh3) in benzene afforded new stable ruthenium(II) carbonyl complexes
of the general formula [Ru(Cl)(CO)(EPh3)(B)(L)] (L = anion of bidentate Schiff bases). The structure of the new complexes was investigated using
1
elemental analyses, spectral (FT-IR, UV–vis and H NMR) and electrochemical studies and is found to be octahedral. All the metal complexes
exhibit characteristic MLCT absorption and luminescence bands in the visible region. The luminescence efficiency of the ruthenium(II) complexes
was explained based on the ligand environment around the metal ion. These complexes catalyze oxidation of primary and secondary alcohols into
their corresponding carbonyl compounds in the presence of N-methylmorpholine-N-oxide (NMO) as the source of oxygen. The formation of high
valent RuIV O species as a catalytic intermediate is proposed for the catalytic process.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Ru(II) carbonyl complexes; Synthesis; Characterization; Luminescence; Oxidation
1. Introduction
ruthenium bi or polypyridyl complexes, the luminescent chem-
istry of ruthenium(II) complexes containing Schiff bases is not
well developed.
Multidentate ligands are extensively used in coordination
chemistry, since they can be applied in the construction of
new frameworks with interesting properties [1]. Among these
ligands the linear or cyclic Schiff bases obtained by the conden-
sation of primary amines with carbonyl compounds and their
metal complexes find a variety of applications including bio-
logical, clinical, analytical and industrial, in addition to their
important role in catalysis and organic synthesis [2,3]. The
central metal ion in these complexes act as active sites and
thereby successfully catalyzes chemical reactions [4,5]. Metal-
ligand luminescence probes with microsecond decay time have
numerous potential applications in the biophysical and clinical
sciences. Ruthenium(II) MLCT compounds display long lumi-
nescence life time and are extremely photo stable [6,7]. Recently
light-emitting devices (LED) based on the small molecule like
tris-2,2ꢀ-bipyridyl ruthenium(II) were demonstrated [8]. In con-
trast to the considerable growth of literature on the chemistry of
The catalytic conversion of primary and secondary alcohols
into their corresponding aldehydes and ketones is an essential
reaction in organic synthesis. Traditional methods for perform-
ing such transformation generally involve the use of stoichio-
metric quantities of inorganic oxidants like Cr(IV) and generate
large quantities of waste. The development of effective, greener
catalytic systems that use clean and inexpensive oxidants is
an important challenge [9]. The Schiff base transition metal
complexes are attractive oxidation catalysts because of their
cheap, easy synthesis and their chemical and thermal stability
[10]. Ruthenium complexes have been extensively investigated
as catalysts for alcohol oxidation in combination with various
oxidants such as dioxygen [11], iodosobenzene [12], t-BuOOH
[13], H2O2 [14] and NMO [15]. Further the catalytic activities
of ruthenium complexes containing tertiary phosphine or arsine
ligands are well established [16,17].
Therefore, in continuation of our interest in synthesis of new
ruthenium compounds [18–20], we describe here the synthesis,
characterization, luminescent and redox properties of ruthe-
nium(II) complexes containing bidentate Schiff bases along with
their catalytic activity towards oxidation of organic substrates
∗
Corresponding author. Tel.: +91 431 2407035;
fax: +91 431 2407045/2407020.
E-mail address: ramesh bdu@yahoo.com (R. Ramesh).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.07.010

M. Sivagamasundari, R. Ramesh / Spectrochimica Acta Part A 67 (2007) 256–262
257
2.4. Synthesis of Ru(II) carbonyl complexes
All operations were carried out under strictly anhydrous
conditions. All the new complexes were prepared by the fol-
lowing general procedure. To a preheated benzene (20 cm3)
solution of [RuHCl(CO)(EPh3)3] (where E = P or As) or [RuHCl
(CO)(PPh3)2(B)] (where B = PPh3, py or pip) (76.9–108.4 mg;
0.1 mmol) the Schiff base (16.9–32.6 mg; 0.1 mmol) (HL1–
HL4) was added slowly. The solution was refluxed for 5 h
until the mixture gradually changed to deep color. The solution
mixture was concentrated to ca. 3 cm3 under reduced pressure
and cooled. The complex separated out upon the addition of
petroleum ether (60–80 ◦C). The solid was filtered off washed
with petroleum ether and recrystallised from CH2Cl2/petroleum
ether and dried under vacuum. The purity of the compounds was
checked by TLC.
Scheme 1. Structure of the bidentate Schiff bases.
in the presence of NMO. The bidentate Schiff bases were
derived from the condensation of 2-hydroxy-1-naphthaldehyde
with methylamine, cyclohexylamine, 2-aminopyridine and 4-
bromoaniline. The Schiff bases used to prepare the new ruthe-
nium(II) complexes were shown in Scheme 1.
2. Experimental
2.1. Materials
All the reagents used were chemically pure and AR grade.
Solvents were purified and dried according to standard pro-
cedure. RuCl3·3H2O was purchased from Loba Chemie Pvt.
Ltd., Bombay, India and was used without further purifica-
tion. 2-Hydroxy-1-naphthaldehyde and N-methylmorpholine-
N-oxide were purchased from Aldrich. [RuHCl(CO)(PPh3)3]
[21], [RuHCl(CO)(PPh3)2(B)] [22] (where B = py, pip) and
[RuHCl(CO)(AsPh3)3] [23] were prepared by reported litera-
ture methods.
2.5. Catalytic oxidation of alcohol
Catalytic oxidation of primary and secondary alcohols to cor-
respondingaldehydesandketonesrespectively, byruthenium(II)
carbonyl complexes was studied in the presence of NMO as
co-oxidant. A typical reaction using the ruthenium complexes
as catalyst and benzyl alcohol, cinnamyl alcohol, cyclohexanol
and benzoin as substrates at 1:100 molar ratio is described as fol-
lows. A solution of ruthenium complex (0.01 mmol) in 20 cm3
CH2Cl2 was added to the solution of substrate (1 mmol) and
NMO (3 mmol). The solution mixture was refluxed for 3 h and
the solvent was then evaporated from the mother liquor under
reduced pressure. The solid residue was then extracted with
petroleum ether (60–80 ◦C) (20 cm3) and the ether extracts were
evaporated to give corresponding aldehydes/ketones which were
then isolated and quantified as their 2,4-dinitrophenylhydrazone
derivative [25].
2.2. Physical measurements
The analysis of carbon, hydrogen and nitrogen were
performed on elementar systeme model vario EL111 at
Sophisticated Test and Instrumentation Center, Cochin, India.
IR spectra were recorded in KBr pellets in the range 4000–
400 cm−1 region in Jasco 400 plus spectrophotometer. Elec-
tronic spectra were recorded in CH2Cl2 solution with a Cary
300 Bio UV–vis Varian spectrophotometer in the range of
800–200 nm. 1H NMR spectra were recorded on a Bruker
400 MHZ instrument using TMS as an internal reference. Using
Jasco FP-6500 spectrofluorimeter with 5 nm exit slit the emis-
sion intensity measurements were carried out. Electrochemical
measurements were made using a Princeton EG and G-PARC
model potentiostat using a glassy carbon working electrode and
all the potentials were referenced to Ag/AgCl electrode. Melting
Points were recorded with a Boetius micro-heating table and are
uncorrected.
3. Results and discussion
Diamagnetic, stable hexa-coordinated low spin ruthenium(II)
complexes of general formula [RuCl(CO)(PPh3)(B)(L)] and
[RuCl(CO)(AsPh3)2(L)] (where B = PPh3, py or pip; L =
bidentate Schiff bases) are prepared by reacting [RuHCl
(CO)(PPh3)3], [RuHCl(CO)(py)(PPh3)2], [RuHCl(CO)(pip)
(PPh3)2] and [RuHCl(CO)(AsPh3)3] with the respective Schiff
bases in a 1:1 mole ratio in benzene as shown in Scheme 2.
All the new Schiff base ruthenium(II) complexes are highly
colored, stable in air, non-hygroscopic in nature and highly
soluble in common solvents such as dichloromethane, acetoni-
trile, chloroform and DMSO. The analytical data are listed in
Table 1 and are in good agreement with the general molecu-
lar formula proposed for all the complexes. In all reactions, the
Schiff base replaced one PPh3/AsPh3 group and a hydride from
the starting complexes. The Schiff bases behave as monobasic
bidentate ligand in all cases. The heterocyclic nitrogen bases
(pyridine, piperidine) remained intact because the Ru–P bond
is more labile than the Ru–N bond, because of the better ␴
donating ability of the nitrogen bases compared to triphenyl-
phosphine.
2.3. Preparation of Schiff base ligands
The monobasic bidentate Schiff base ligands were pre-
pared by the condensation of 2-hydroxy-1-naphthaldehyde
(0.43 g; 0.0025 mmol) with primary amines such as methy-
lamine, 2-aminopyridine, cyclohexylamine and p-bromoaniline
(0.08–0.43 g; 0.0025 mmol) in 1:1 molar ratio in methanol
(20 cm3) [24]. The solution was heated under reflux for 2 h and
then concentrated to 5 cm3. On cooling the product was sepa-
rated out which was then recrystallised from hot methanol and
the purity was checked by TLC.

258
M. Sivagamasundari, R. Ramesh / Spectrochimica Acta Part A 67 (2007) 256–262
that of the free ligands ν(C–O) (1324–1314 cm−1) suggesting
that the other coordinating atom is phenolic oxygen [25]. The
binding of the metal to the ligand through nitrogen and oxy-
gen atom is further supported by the appearance of new band
in 460–400 cm−1 and 540–510 cm−1 ranges due to ν(M–N) and
ν(M–O) [27], respectively, in the spectra of all the complexes.
A strong band in the region 1957–1935 cm−1 due to terminally
coordinatedcarbonylgroupathigherfrequencythanintheruthe-
nium starting complexes. In the complexes 2, 3, 6, 7, 10, 11,
14, 15 a medium intensity band is observed in the 1020 cm−1
region characteristic of the coordinated pyridine or piperidine
[27]. In addition Schiff base complexes show strong vibrations
near 520, 695, 740 and 1430 cm−1, which are attributed to the
triphenylphosphine or triphenylarsine fragment [28].
The nuclear magnetic resonance spectra of the diamagnetic
compounds taken in CDCl3 solution confirms the complex for-
mation (Table 3). In the spectra of all complexes a sharp singlet
appeared at around 8.10–9.10 ppm has been assigned to azome-
thine proton (>C N). The positions of azomethine signal in
the complexes are up field compared to that of the free ligands
indicating coordination through the azomethine nitrogen atom
[10]. Multiplets are observed around 6.65–8.06 ppm in all the
complexes and have been assigned to the aromatic protons of
triphenylphosphine, triphenylarsine, pyridine and naphthalene
Schiff base ligands [29]. Methyl protons appear as singlet in
the region 2.17–2.50 ppm in the complexes 1, 2 and 4. A broad
singlet appeared in the region 1.57–1.61 ppm for the methy-
lene protons of the cyclohexyl group for 13, 14, 15 and 16. In
the spectra of 7, 11 and 15 the methylene protons appear as
singlet around 1.59–2.05 ppm and another singlet in the region
10.10–10.31 ppmduetoNHprotons. Theabsenceofaresonance
at 13.25–15.21 ppm in the complexes indicate deprotonation of
the phenolic group of the Schiff bases on complexation and
coordination to ruthenium through the phenolic oxygen [18,19].
The spectra did not show any signal in the up field region for
the hydride ligand, thus proving its substitution by the Schiff
bases.
Scheme 2. Formation of Ru(II) carbonyl Schiff base complexes (where E = P;
B = PPh3, pyorpip;E = As, B = AsPh3;R = –CH3, –C6H11, –C5H4N, –C6H4Br).
3.1. Spectroscopic characterization
The IR spectra of the complexes, in comparison with those
of the free ligands, display certain changes, which give an idea
about the type of coordination and their structure. Significant IR
spectral bands of the complexes are listed in Table 2. Free Schiff
bases show a very strong absorption around 1642–1618 cm−1
characteristic of azomethine (>C N) group [19]. Coordination
of the Schiff bases to the ruthenium ion through the azome-
thine nitrogen atom is expected to reduce the electron density
in the azomethine link and thus, lower the ν(C N) absorption
frequency. Hence this band undergoes a shift to lower frequency
to 1620–1600 cm−1 after complexation, indicating coordination
of the azomethine nitrogen to ruthenium [18,26]. The free lig-
ands exhibit a broad band at 3435–3427 cm−1, which may be
assigned to the ν(OH) and this band is absent in the spectra of all
the complexes, conferring the deprotonation of the Schiff base
prior to coordination. The hydroxy protons are displaced by the
metal leading to higher ν(C–O) (1350–1329 cm−1) compared to
Table 1
Analytical data of Ru(II) carbonyl complexes containing N, O donors of 2-hydroxy-1-napthylideneimines
S. No.
Complexes
Formula
Color
m.p. (◦C)
Found (calculated) (%)
C
H
N
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
[Ru(Cl)(CO) (PPh3)2(L1)]
[Ru(Cl)(CO)(PPh3)(py)(L1)]
[Ru(Cl)(CO)(PPh3)(pip)(L1)]
[Ru(Cl)(CO)(AsPh3)2(L1)]
[Ru(Cl)(CO)(PPh3)2(L2)]
[Ru(Cl)(CO)(PPh3)(py)(L2)]
[Ru(Cl)(CO)(PPh3)(pip)(L2)]
[Ru(Cl)(CO)(AsPh3)2(L2)]
[Ru(Cl)(CO)(PPh3)2(L3)]
[Ru(Cl)(CO)(PPh3)(py)(L3)]
[Ru(Cl)(CO)(PPh3)(pip)(L3)]
[Ru(Cl)(CO)(AsPh3)2(L3)]
[Ru(Cl)(CO)(PPh3)2(L4)]
[Ru(Cl)(CO)(PPh3)(py)(L4)]
[Ru(Cl)(CO)(PPh3)(pip)(L4)]
[Ru(Cl)(CO)(AsPh3)2(L4)]
C49H10NO2P2ClRu
C36H30N2O2PClRu
C36H35N2O2PClRu
C49H40NO2As2ClRu
C54H47NO2P2ClRu
C41H37N2O2PClRu
C41H42N2O2PClRu
C54H48NO2As2ClRu
C53H41N2O2P2ClRu
C40H32N3O2PClRu
C40H36N3O2PClRu
C53H41N2O2As2ClRu
C54H41NO2P2ClBrRu
C41H31N2O2PClBrRu
C41H36N2O2PClBrRu
C54H41NO2As2ClBrRu
Green
Green
Green
Green
Green
Green
Green
Brown
Brown
Brown
Brown
Brown
Green
Green
Brown
Brown
222
154
160
176
147
137
148
148
217
180
131
135
167
150
148
132
67.39 (67.61)
62.65 (62.33)
62.20 (62.46)
61.20 (61.08)
69.00 (69.37)
65.00 (65.41)
64.60 (64.29)
63.20 (63.48)
69.80 (69.57)
63.70 (63.49)
63.36 (63.60)
62.14 (62.41)
63.95 (63.63)
56.80 (56.51)
58.91 (58.67)
58.84 (58.51)
4.61 (4.72)
4.38 (4.25)
5.07 (5.32)
4.19 (4.02)
5.10 (5.33)
4.91 (4.46)
5.50 (5.25)
4.71 (4.89)
4.40 (4.19)
4.20 (4.29)
4.78 (4.54)
4.03 (4.31)
4.07 (4.19)
3.60 (3.87)
4.30 (4.16)
3.75 (3.82)
1.61 (1.88)
4.05 (4.25)
4.02 (4.31)
1.45 (1.60)
1.42 (1.43)
3.60 (3.81)
3.61 (3.34)
1.36 (1.51)
2.99 (2.73)
5.56 (5.36)
5.50 (5.39)
2.71 (2.85)
1.30 (1.46)
3.22 (3.09)
3.30 (3.12)
1.72 (1.49)

M. Sivagamasundari, R. Ramesh / Spectrochimica Acta Part A 67 (2007) 256–262
259
Table 2
Important IR, electronic and fluorescencea spectral data of the Ru(II) carbonyl Schiff base complexes
Complexes
γC (cm−1
)
γC–O (cm−1
)
γC (cm−1
)
λmax, nm (ε, dm3 mol−1 cm−1
)
Fluorescence
N
O
data λmax (nm)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1617
1617
1616
1617
1616
1613
1616
1620
1615
1615
1617
1616
1599
1600
1601
1600
1318
1319
1338
1316
1339
1338
1338
1336
1336
1341
1364
1335
1338
1337
1362
1335
1946
1944
1942
1935
1942
1935
1941
1954
1950
1949
1950
1950
1946
1945
1948
1957
694 (380)b, 456 (3404)c, 321 (9520)d, 266 (15296)e
679 (270)b, 436 (3324)c, 319 (9900)d, 260 (13460)e
653 (386)b, 440 (4580)c, 319 (10032)d, 258 (15052)e
670 (224)b, 427 (4744)c, 323 (12048)d, 259 (16432)e
661 (536)b, 419 (4192)c, 314 (11024)d, 262 (15448)e
657 (448)b, 426 (5388)c, 330 (13644)d, 272 (17340)e
631 (504)b, 417 (4491)c, 310 (12024)d, 261 (17168)e
680 (364)b, 416 (3712)c, 314 (10836)d, 264 (15980)e
464 (3240)c, 321 (12812)d, 259 (17024)e
512
510
516
503
501
502
508
496
538
540
543
518
502
497
506
–
694 (228)b, 456 (6000)c, 326 (13952)d, 259 (16064)e
464 (3044)c, 327 (11512)d, 257 (17624)e
719 (358)b, 460 (5060)c, 321 (11940)d, 271 (17752)e
697 (472)b, 420 (4678)c, 320 (10928)d, 254 (15128)e
690 (380)b, 431 (4056)c, 323 (11003)d, 258 (166632)e
431 (3582)c, 324 (9888)d, 267 (15948)e
688 (140)b, 435 (3054)c, 319 (9892)d, 271 (15852)e
a
Excitation at MLCT.
b
c
d
e
1
1A1g → T1g
.
Charge transfer.
n–␲*.
␲–␲*.
Electronic spectra of all the complexes in chloroform showed
octahedral environment around the ruthenium(II) ion similar to
that of other ruthenium octahedral complexes [18,19,31,32].
three to four bands in the region 719–254 nm. All the com-
plexes are diamagnetic, indicating the presence of ruthenium
in the +2 oxidation state. The ground state of ruthenium(II)
3.2. Luminescence property
1
6
in an octahedral environment is A1g, arising from the t2g
3
3
1
configuration. The excited state terms are T1g, T2g, T1g
Emission properties of the ruthenium(II) Schiff base com-
plexes were examined in chloroform at room temperature. The
excitation of these complexes was made at their charge transfer
band. The emission maxima fall in the range 496–543 nm and
the emission maxima of the complexes are listed in Table 2. The
emission maxima for all the complexes have experienced a pos-
itive shift of the order of 56–84 nm, when compared to those of
excitation maxima. A representative spectrum is shown in Fig. 1.
The observed CT luminescence in these complexes may be due
to the presence of the imine functional group. It is likely that
the emission originates from the lowest energy metal to ligand
1
and T2g. Hence four bands corresponding to the transition
1A1g → T1g, 1A1g → T2g, 1A1g → T1gand 1A1g → T2g are
3
3
1
1
possible in order of increasing energy. The electronic spec-
1
1
tral bands at around 719–653 nm are assigned to A1g → T1g
(Table 2). The other high intensity band in the visible region
around 464–419 nm has been assigned to charge transfer transi-
tions arising from the metal t2g level to the unfilled ␲* molecular
orbital of the ligand [18,30–32]. The high intensity bands around
330–310 nm and 272–254 nm has been designated as n–␲* and
␲–␲* transitions respectively, for the electrons localized on the
azomethine group of the Schiff bases. The pattern of the elec-
tronic spectra for all the complexes indicate the presence of an
Table 3
1H NMR data of Ru(II) carbonyl Schiff base complexes
Complexes
1H NMR data (ppm)
1
2
3
4
5
6
7
8
6.65–7.19 (Ar, m), 9.12 (CH N, s), 2.25 (CH3, s)
6.90–7.81 (Ar, m), 8.82 (CH N, s), 2.17 (CH3, s)
6.58–7.72 (Ar, m), 8.11 (CH N, s), 2.50 (CH3, s)
6.51–8.00 (Ar, m), 8.72 (CH N, s), 1.30 (CH2, s)
6.80–7.81 (Ar, m), 8.70 (CH N, s), 1.41 (CH2, s)
6.91–7.80 (Ar, m), 8.36 (CH N, s), 1.30 (CH2, s), 9.50 (NH, s)
6.90–7.70 (Ar, m), 8.30 (CH N, s), 1.35 (CH2, s)
6.54–7.72 (Ar, m), 8.41 (CH N, s)
9
6.75–7.80 (Ar, m), 8.36 (CH N, s), 2.05 (CH2, s), 9.65 (NH, s)
6.80–7.91 (Ar, m), 8.36 (CH N, s)
6.91–7.92 (Ar, m), 8.80 (CH N, s)
6.81–7.74 (Ar, m), 8.63 (CH N, s), 1.3 (CH2, s), 9.72 (NH, s)
6.90–7.80 (Ar, m), 8.25 (CH N, s)
10
11
12
13
Fig. 1. Absorption (—), fluorescence (- - -) and excitation (· · ·) spectra of
[Ru(Cl)(CO)(PPh3)(py)(L2)] in chloroform.

260
M. Sivagamasundari, R. Ramesh / Spectrochimica Acta Part A 67 (2007) 256–262
charge transfer (MLCT) state, probably derived from the exci-
tation involving a d␲(Ru) → ␲*(imine) MLCT transition [33].
The present results show that the emission maxima are
comparatively lower than those observed for the ruthenium(II)
bipyridyl complexes due to smaller ligand field splitting in the
Schiff base complexes and thus reduces the activation energy
for the radiation less process to such an extent that the emission
is quenched [34]. However, the emission maxima are influenced
by the substituents on the coordinating imines and it decreases
slightly with increasing number of substituents at the coordi-
nated nitrogen [35]. Hence complexes 5–8 have lower emission
maxima [34] than 1–4, where methyl is the substituent. Further
the presence of this electron rich methyl group in 1–4 makes
them have higher emission maxima compared to the complexes
13–16 where the substituent is electron-withdrawing bromine.
Complexes 9–12 having the amino pyridine substituent has the
greater emission maxima compared to other complexes and this
may be attributed to the presence of an additional chromophoric
group >C N of the pyridine [36]. Among triphenylphosphine
and triphenylarsine complexes, theemission maxima areslightly
reduced in the arsine complexes where the heavier atom As
induces the spin forbidden transition.
Fig. 2. Cyclic voltammogram of [Ru(Cl)(CO)(PPh3)(py)(L4)].
separation (ꢀE) of 110–220 mV and the cathodic peak current
(ipc) is less than the anodic peak current (ipa). The one electron
nature of this oxidation has been verified by comparing its cur-
rent height (ipa) with that of the ferrocene–ferrocenium couple
under identical experimental conditions [37]
[RuIICl(CO)(EPh3)(B)(L)]
ꢀ [RuIIICl(CO)(EPh3)(B)(L)]+ + e−
3.3. Electrochemical study
The reason for irreversibility observed in the reduction of
all the complexes may be due to short lived reduced state of
the metal ion [38] or due to oxidative degradation [39] of the
ligands. It has been observed from the electrochemical data that
there is little variation in the redox potentials due to replacement
of triphenylphosphine by pyridine or piperidine or by tripheny-
larsine [19]. Hence it is inferred from the electrochemical data
that the present ligand system is ideally suitable for stabilizing
the higher oxidation state of ruthenium ion.
The redox properties of all the complexes were investigated
in acetonitrile solution by cyclic voltammetry and the redox
potentials are expressed with reference to Ag/AgCl. The voltam-
metric data are presented in Table 4 and a representative cyclic
voltammogram of 13 is shown in Fig. 2. Cyclic voltammo-
gram of the complexes (1 × 10−3) exhibit a quasi-reversible
oxidation with E1/2 value +0.61 to +0.82 V (RuIII/RuII) and an
irreversiblereductionintherange−0.72to−0.95withrespectto
the Ag/AgCl at scan rate 100 mV s−1. The oxidation observed is
quasi-reversible in nature characterized by a large peak-to-peak
3.4. Catalytic activity
Table 4
The primary and secondary alcohols (1 mmol) were effi-
ciently oxidized by using 0.01 mmol of the catalyst in the coexis-
tence of 3 mmol of NMO in dichloromethane (Table 5). All the
complexes oxidize primary alcohols to corresponding aldehy-
des and secondary alcohols to corresponding ketones with high
yield. This reaction provides an environmentally friendly route
to the conversion of alcoholic functions to carbonyl groups and
water is the only by product during the course of the reaction
which was removed by using molecular sieves. The aldehydes
and ketones formed after 3 h of reflux were isolated and quanti-
fied as their 2,4-dinitrophenylhydrazone derivatives. In the case
of benzoin, the oxidation product was isolated and characterized
by melting point, IR, 1H NMR. Control experiments were car-
ried out without the ruthenium catalyst under the same reaction
conditions and in no case was there any detectable oxidation of
alcohols
Cyclic voltammetry data of Ru(II) carbonyl Schiff base complexes
Complexes Metal-based oxidation RuIII/RuII
Metal-based
reduction
RuII/RuI Epc (V)
Epa (V) Epc (V) ꢀEp (mV) E1/2 (V)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0.89
0.71
0.93
0.86
0.92
0.90
0.78
0.81
0.85
0.86
0.88
0.93
0.88
0.89
0.93
0.69
0.51
0.72
0.72
0.71
0.70
0.67
0.68
0.68
0.69
0.72
0.71
0.72
0.70
0.71
0.79
0.61
0.84
0.79
0.81
0.80
0.72
0.74
0.76
0.77
0.80
0.82
0.80
0.79
0.82
200
200
210
140
210
192
110
130
170
180
160
220
160
190
220
−0.95
−0.94
−0.89
−0.91
−0.72
−0.85
−0.78
−0.89
−0.77
−0.88
−0.99
−0.82
−0.84
−0.93
−0.87
Supporting electrolyte: NBu4ClO4 (0.005 M); complex: 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
.

M. Sivagamasundari, R. Ramesh / Spectrochimica Acta Part A 67 (2007) 256–262
261
Table 5
Hence, it had been suggested that catalytic oxidation proceeds
throughmetal–oxointermediate. Furthermore, itisrelevantfrom
the cyclic voltammetric data that the oxidation effected by cat-
alysts is likely to occur via higher ruthenium oxidation states,
which is easily accessible through chemical oxidation with co-
oxidatants like NMO, t-BuOOH, PhIO, etc. [17,42].
Catalytic oxidation of alcohols by ruthenium complexes/NMO
Complex
Substrate
Product
Yielda (%)
TONb
Benzyl alcohol
Cinnamyl alcohol
Cyclohexanol
Benzoin
A
A
K
K
55.8c
91.0c
30.0c
32.3d
55.8
91.0
30.0
32.3
2
Benzyl alcohol
Cinnamyl alcohol
Cyclohexanol
Benzoin
A
A
K
K
59.6c
85.4c
35.3c
37.1d
59.6
85.4
35.3
37.1
4. Conclusion
5
The synthesis, characterization and redox properties of
new ruthenium(II) Schiff base complexes of the type [Ru(Cl)
(CO)(EPh3)(B)(L)] are described. These complexes are found
to have luminescent properties and attempts will be made for fur-
ther investigation on this study. The results of the present studies
demonstrate the catalytic ability of the ruthenium(II) complexes
to oxidize primary and secondary alcohols efficiently to their
corresponding carbonyl compounds. A high-valent Ru(IV)–oxo
species is proposed as the active intermediate in the catalytic
process.
Benzyl alcohol
Cinnamyl alcohol
Cyclohexanol
Benzoin
A
A
K
K
52.7c
90.3c
33.6c
40.0d
52.7
90.3
33.6
40.0
12
15
Benzyl alcohol
Cinnamyl alcohol
Cyclohexanol
Benzoin
A
A
K
K
60.9c
93.5c
28.5c
35.0d
60.9
93.5
28.5
35.0
A or K, corresponding aldehyde or ketone.
a
Yield based on substrate: alcohol (1 mmol); NMO (3 mmol); catalyst
(0.01 mmol).
b
Acknowledgements
Ratio of moles of product obtained to the moles of the catalyst used.
Isolated as 2,4-dinitrophenylhydrazone derivative, characterized by 1H
c
NMR and IR.
The authors express their sincere thanks to University Grants
Commission (UGC), New Delhi [Ref. no. F12-45/2003(SR)] for
financial support. One of the authors (M.S.) thanks UGC for the
award of Project Fellow.
d
Isolated yield, characterized by melting point, 1H NMR and IR.
The relatively higher product yield obtained for oxidation of
cinnamyl alcohol is due to the presence of ␣-CH unit in cin-
namyl alcohol, which is more acidic than in benzyl alcohol,
cyclohexanol or benzoin. Also, the oxidation of cinnamyl alco-
hol to cinnamaldehyde takes place with retention of C C double
bond, which is an important characteristic of ruthenium/NMO
system. In the case of oxidation of secondary alcohols, it has
been observed that the oxidation of cyclohexanol is substantially
lower than that of benzoin. This may be due to the fact that the
saturated cyclic ketones are generally prone to undergo dehydro-
genation faster than acyclic ketones [40]. These ruthenium(II)
Schiff base complexes have better catalytic activity compared
to similar other ruthenium complexes as catalyst in the presence
of NMO/t-BuOOH [41–43].
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