Synthesis, luminescent, redox and catalytic properties of Ru(II) carbonyl complexes containing 2N2O donors

Article abstract of DOI:10.1016/j.poly.2005.05.020

Diamagentic ruthenium(II) complexes [Ru(CO)(B)(L)] (where B = PPh ₃, AsPh₃, py (or) pip; L = dianion of tetradentate Schiff bases) were synthesized from the reaction of tetradentate Schiff bases derived from 2-hydroxy-1-naphthaldehyde and the appropriate diamines with monomeric metal precursors [RuHCl(CO)(EPh₃)₂ (B)] (where E = As, B = AsPh₃; E = P, B = PPh₃, py (or) pip). Elemental analyses and spectral (FT-IR, UV-Vis and ¹H NMR) studies of all the new synthesized complexes suggest the presence of an octahedral environment around the Ru(II) ion. 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. Cyclic voltammograms of all the complexes display quasireversible oxidations (Ru^III/Ru^II) in the range 0.76-1.04 V and irreversible reduction peaks (Ru^II/Ru^I) in the range -0.70 to -0.95 V. The observed redox potentials vary with respect to the size of the chelate ring of Schiff base ligands. Further, the catalytic activity of complex 1 has been found to be high towards the oxidation of primary and secondary alcohols into their corresponding aldehydes and ketones in the presence of N-methylmorpholine-N-oxide (NMO) as co-oxidant. The formation of high valent Ru^IVO species as a catalytic intermediate is proposed for the catalytic process.

Full text of DOI:10.1016/j.poly.2005.05.020

Polyhedron 24 (2005) 1885–1892
www.elsevier.com/locate/poly
Synthesis, luminescent, redox and catalytic properties of
Ru(II) carbonyl complexes containing 2N2O donors
*
K. Naresh Kumar, R. Ramesh
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India
Received 25 February 2005; accepted 2 May 2005
Available online 3 August 2005
Abstract
Diamagentic ruthenium(II) complexes [Ru(CO)(B)(L)] (where B = PPh3, AsPh3, py (or) pip; L = dianion of tetradentate Schiff
bases) were synthesized from the reaction of tetradentate Schiff bases derived from 2-hydroxy-1-naphthaldehyde and the appropri-
ate diamines with monomeric metal precursors [RuHCl(CO)(EPh3)2 (B)] (where E = As, B = AsPh3; E = P, B = PPh3, py (or) pip).
Elemental analyses and spectral (FT-IR, UV–Vis and 1H NMR) studies of all the new synthesized complexes suggest the presence of
an octahedral environment around the Ru(II) ion. All the metal complexes exhibit characteristic MLCT absorption and lumines-
cence bands in the visible region. The luminescence efficiency of the ruthenium(II) complexes was explained based on the ligand
environment around the metal ion. Cyclic voltammograms of all the complexes display quasireversible oxidations (RuIII/RuII) in
the range 0.76–1.04 V and irreversible reduction peaks (RuII/RuI) in the range ꢀ0.70 to ꢀ0.95 V. The observed redox potentials
vary with respect to the size of the chelate ring of Schiff base ligands. Further, the catalytic activity of complex 1 has been found
to be high towards the oxidation of primary and secondary alcohols into their corresponding aldehydes and ketones in the presence
of N-methylmorpholine-N-oxide (NMO) as co-oxidant. The formation of high valent RuIV@O species as a catalytic intermediate is
proposed for the catalytic process.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium(II) carbonyl complexes; 2N2O donors; Luminescence; Redox potentials; Catalytic activity
1. Introduction
enhancing solubility and stability, either homogenous
or heterogenous catalysts [3–5]. Further, luminescent
metal complexes are a fascinating class of molecules that
have found applications in many areas such as light
emitting devices (LED) [6], among which luminescent
Ru(II) bipyridyl compounds have been extensively stud-
ied as potential sensing molecules [7–9]. Ruthenium(II)
metal to ligand charge transfer (MLCT) compounds
are known to display long lived luminescence life times
[10] and are extremely photo stable, in particular ruthe-
nium polypyridyl complexes have been researched
extensively for decades as photochemical molecular
devices due to their excellent chemical stability, facial
electron transfer, strong luminescence and emission
and relatively long-lived excited states [11,12]. In con-
trast to the considerable growth of the literature on
The coordination chemistry of ruthenium in the oxi-
dation state +2 is dominated by nitrogen and oxygen
donors [1]. The coordination environment around ruthe-
nium plays a key role in stabilizing its different oxidation
states and hence dictates the redox properties of the cen-
tral metal ion. In addition, the chemistry of chelating
tetradentate Schiff base ligands with ruthenium has also
been extensively studied [2]. This is due to the fact that
Schiff bases offer opportunities for inducing substrate
chirality, tuning metal centered electronic factors,
*
Corresponding author. Tel.: +91 431 2407053; fax: +91 431
2407045/2407020.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.05.020

1886
K.N. Kumar, R. Ramesh / Polyhedron 24 (2005) 1885–1892
W
Where W = - (CH2)2 -
- (CH(CH3)CH2) -
H2 L1 = H2 – Naphen
H2 L2 = H2 – Naphpn
H2 L3 = H2 – Naphprn
H2L 4= H2 – Naphoph
N
N
- (CH2-CH2-CH2) -
- (C6H4) -
OH
HO
Scheme 1. Structure of the tetradentate Schiff bases.
the chemistry of ruthenium bi- or polypyridyl com-
plexes, the luminescent chemistry of ruthenium(II) com-
plexes containing Schiff bases is not well developed.
Furthermore, metal complexes of ruthenium containing
triphenylphosphine or arsine, nitrogen and oxygen do-
nor ligands are found to be effective catalysts for oxida-
tion [13,14]. In particular, the selective oxidation of
alcohols into carbonyl compounds by metal complexes
is well documented in the literature [15–17]. Sharpless
et al. [18] carried out a yield oriented study of oxidation
of cholesterol, geranial, etc., catalyzed by ruthenium
complexes in the presence of N-methylmorpholine-N-
oxide (NMO) and N-N-dimethylaniline-N-oxide, and
only a very few reports are available on the oxidation
chemistry using NMO as co-oxidant.
In view of the rich photophysical and catalytic prop-
erties of organometallic ruthenium complexes, the intro-
duction of Schiff bases with a 2N2O donor set to
ruthenium(II) carbonyl complexes containing triphenyl-
phosphine/triphenylarsine and heterocyclic bases has
been carried out. As an extension of our preliminary
communication [19–21] and to provide further insight
on the catalytic and emission behaviors, an extensive
series of ruthenium(II) carbonyl complexes containing
tetradentate Schiff bases have been synthesized and
characterized. Further, the redox and luminescent prop-
erties are also reported along with the catalytic activity
of ruthenium(II) complexes towards oxidation of pri-
mary and secondary alcohols. The tetradentate Schiff
bases were derived from condensation of 2-hydroxy-1-
naphthaldehyde with the appropriate diamines. The
following Schiff bases were used to prepare the new
ruthenium(II) complexes shown in Scheme 1.
B = py (or) pip) and [RuHCl(CO)(AsPh3)3] [25] were
prepared by reported literature methods.
The analysis of carbon, hydrogen and nitrogen were
performed on a Carlo Erba 1160 and a model 240 Per-
kin–Elmer CHN analyzer at the Central Drug Research
Institute, Lucknow, India. IR spectra were recorded in
KBR pellets in the 4000–200 cmꢀ1 region in a Jasco 400
plus spectrophotometer. Electronic 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 instru-
ment using TMS as an internal reference. Emission
intensity measurements were carried out by using a Jasco
FP-6500 spectrofluorimeter with 5 nm exit slit. Cyclic
voltammetric measurements were carried out on a Bio
Analytical System (BAS) model CV-50W electrochemical
analyzer using acetonitrile as solvent. Melting points were
recorded with a Boetius micro-heating table and are
uncorrected. The catalytic yields were determined using
HP 6890 series GC-FID with a DP-5 column of 30 m
length, 0.32 mm diameter and 0.25 lm film thickness.
2.2. Synthesis of Ru(II) carbonyl Schiff base complexes
All the new complexes were prepared by the follow-
ing general procedure. To a solution of [RuHCl(CO)-
(EPh3)2 (B)] (where E = P; B = PPh3, py (or) pip) and
[RuHCl(CO)(AsPh3)3] (100 mg; 0.10–0.13 mmol) in
benzene the appropriate Schiff base (39–53 mg; 0.10–
0.13 mmol) was added and heated under reflux for 5 h.
The resulting solution was concentrated to ca. 3 cm3
and cooled. Light petroleum (60–80 ꢁC) (5 cm3) was
then added whereupon the complex separated out. The
solid was filtered off, washed and recrystallized from
CH2Cl2/light petroleum ether mixture and dried in
vacuo. The purity of the compounds were checked by
TLC (yield: 65–70%).
2. Experimental
2.1. Material and physical measurements
2.3. Catalytic oxidation
The Schiff bases were prepared in 80–90% yield by
condensation of two mole equivalents of 2-hydroxy-1-
naphthaldehyde and one mole equivalent of the corre-
sponding diamine by following the reported procedure
[22]. All the reagents used were chemically pure or ana-
lytical reagent grade. Solvents were purified and dried
according to standard procedures. RuCl3 Æ 3H2O was
purchased from Loba chemie pvt Ltd. Bombay, India
and was used without further purification. [RuHCl-
(CO)(PPh3)3] [23], [RuHCl(CO)(PPh3)2 (B)] [24] (where
Catalytic oxidation of primary alcohols to the corre-
sponding aldehydes and secondary alcohols to ketones
by ruthenium(II) carbonyl complexes was studied in
the presence of NMO as co-oxidant. A typical reaction
using the complex [Ru(CO)(PPh3)(L1)] as a catalyst,
and primary or secondary alcohol as substrates at a
1:100 molar ratio is described as follows. A solution of
ruthenium complex 1 (0.01 mmol) in 20 cm3CH2Cl2
was added to the solution of substrate (1 mmol) and

K.N. Kumar, R. Ramesh / Polyhedron 24 (2005) 1885–1892
1887
NMO (3 mmol). The solution mixture was refluxed for
3.1. Spectroscopic characterization
3 h and the solvent was then evaporated from the
mother liquor under reduced pressure. The residue was
then extracted with pet. ether (60–80 ꢁC) (20 cm3) and
was analyzed by GC. The oxidation products are known
and are commercially available, and were identified by
GC co-injection with authentic commercial samples.
The important IR absorption bands for the synthe-
sized complexes are shown in (Table 2). The observed
bands may be classified into those originating from the
ligands and those arising from the bonds formed be-
tween Ru(II) and the coordinating sites. The free Schiff
base ligands showed a strong band in the region 1628–
1640 cmꢀ1, which is characteristic of the azomethine
(>C@N) group [19]. Coordination of the Schiff bases
to the metal through the nitrogen atom is expected to re-
duce the electron density in the azomethine link and
lower the cC@N absorption frequency. The band due to
3. Results and discussions
Complexes of the general formula [Ru(CO)(B)(L)]-
(where B = PPh3, AsPh3, py (or) pip; L = dibasic tetrad-
entate Schiff bases) were synthesized by the reactions of
[RuHCl(CO)(PPh3)3], [RuHCl(CO)(PPh3)2(py)], [RuH-
Cl(CO)(PPh3)2(pip)] (or) [RuHCl(CO)(AsPh3)3] with
the respective tetradentate Schiff bases in a 1:1 molar
ratio in benzene as shown in Scheme 2.
The analytical data for the new complexes agree well
with the proposed molecular formula (Table 1). In all
the reactions it has been observed that the Schiff bases
behave as dibasic tetradentate ligands by substituting
for the chloride ion, hydride ion and two triphenylphos-
phine/arsine ligands from each mole of the starting com-
plexes to form the mononuclear complexes. These
observations do indicate a more labile nature of the
Ru–P bond compared to the Ru–N bond of the hetero-
cyclic nitrogen bases in these complexes. The difference
in the strength of Ru–P/As and R–N bonds may be ex-
plained as being due to the better r donating ability of
the nitrogen bases compared to that of triphenylphos-
phine/arsine. The Schiff base ruthenium(II) complexes
are highly colored, stable to air and light, and soluble
in chloroform, methylene chloride, benzene, DMF and
DMSO.
c
C@N is shifted to lower frequencies and appears around
1615–1625 cmꢀ1, indicating coordination of the azome-
thine nitrogen to the ruthenium metal [26]. A strong
band observed at 1310–1330 cmꢀ1 in the free Schiff
bases has been assigned to phenolic C–O stretching.
On complexation this band is shifted to a higher fre-
quency range 1321–1360 cmꢀ1, indicating coordination
through the phenolic oxygen. This has been further sup-
ported by the disappearance of the broad cOH band
around 3395–3410 cmꢀ1 in the complexes, indicating
deprotonation of the phenolic proton prior to coordina-
tion [27]. The coordination of the azomethine nitrogen
and phenolic oxygen are further supported by the
appearance of two bands at 510–540 and 400–
460 cmꢀ1 due to cM–O [27] and cM–N [28], respectively.
The spectra of all the complexes are dominated by a
sharp band between 1925 and 1950 cmꢀ1 due to a termi-
nal cC„O group. In addition, other characteristic bands
due to PPh3 and AsPh3 are also present around
1435 cmꢀ1 in the spectra of all the Schiff base complexes.
A medium intensity band is observed in the 1020 cmꢀ1
region, characteristic of the coordinated pyridine or
piperidine [28].
1
The H NMR spectra of all the complexes were re-
corded to confirm the binding of the Schiff base to the
ruthenium ion (Table 3). The spectra of all the com-
plexes showed a singlet in the region d 8.90–9.54 ppm,
which has been assigned to the azomethine proton
(>C@N). The position of the azomethine signal in the
complexes is downfield in comparison with that of the
free ligands, suggesting deshielding of the azomethine
proton due to its coordination to ruthenium through
the azomethine nitrogen. Multiplets are observed
around d 6.45–8.50 ppm in all the complexes and have
been assigned to the aromatic protons of triphenylphos-
phine, triphenylarsine, pyridine and naphthalene Schiff
base ligands [29]. Two methylene protons appear as
one triplet in the region d 3.53–3.83 ppm in 1, 5, 9 and
13. For the complexes 3, 7, 11 and 15, the signal for
the methylene protons appear as one triplet and one
quintet in the region 3.65–3.85 ppm and d 2.17–
2.44 ppm, respectively [30]. A broad singlet appears in
the region d 1.2–1.4 ppm for the methylene protons
W
N
N
[RuHCl(CO)(EPh ) (B)]
+
3 2
OH
HO
Benzene
reflux/5hr
W
CO
N
N
Ru
O
O
B
Scheme 2. Formation of Ru(II) carbonyl Schiff base complexes (where
E = P; B = PPh3, py or pip; E = As, B = AsPh3 W = –(CH2)2–,
–(CH2)3–, –(CH(CH3)CH2)–, –(C6H4)–).

1888
K.N. Kumar, R. Ramesh / Polyhedron 24 (2005) 1885–1892
Table 1
Physico-chemical and analytical data of Ru(II) carbonyl Schiff base complexes
S. No.
Complex
Formula
Colour
M.p. (ꢁC)
Found (calculated) (%)
C
H
N
1
2
[Ru(CO)(PPh3)(L1)]
[Ru(CO)(PPh3)(L2)]
[Ru(CO)(PPh3)(L3)]
[Ru(CO)(PPh3)(L4)]
[Ru(CO)(py)(L1)]
[Ru(CO)(py)(L2)]
[Ru(CO)(py)(L3)]
[Ru(CO)(py)(L4)]
[Ru(CO)(pip)(L1)]
[Ru(CO)(pip)(L2)]
[Ru(CO)(pip)(L3)]
[Ru(CO)(pip)(L4)]
[Ru(CO)(AsPh3)(L1)]
[Ru(CO)(AsPh3)(L2)]
[Ru(CO)(AsPh3)(L3)]
[Ru(CO)(AsPh3)(L4)]
C43H33N2O3PRu
C44H35N2O3PRu
C44H35N2O3PRu
C47H33N2O3PRu
C30H23N3O3Ru
C31H25N3O3Ru
C31H25N3O3Ru
C34H24N3O3Ru
C30H28N3O3Ru
C31H30N3O3Ru
Green
Green
Green
Brown
Green
Green
Green
Brown
Green
Green
Green
Brown
Green
Green
Green
Brown
202
185
164
215
235
195
157
216
217
182
150
185
182
164
162
152
67.95(68.12)
69.90(70.30)
69.98(70.30)
71.10(71.30)
62.47(62.70)
63.01(63.26)
62.94(63.26)
65.74(65.48)
61.84(62.16)
60.42(60.70)
60.51(60.70)
64.46(64.96)
64.65(64.42)
64.38(64.78)
64.47(64.78)
66.62(66.43)
3.97(4.39)
1.87(2.02)
2.45(2.02)
3.87(4.20)
3.90(4.20)
3.89(4.28)
4.03(4.28)
3.57(3.88)
4.55(4.89)
4.87(5.12)
4.95(5.12)
4.26(4.65)
3.89(4.15)
4.45(4.33)
4.04(4.33)
3.71(3.91)
3.47(3.70)
3.53(3.72)
3.87(3.72)
1.54(1.77)
7.02(7.37)
6.98(7.14)
7.34(7.14)
6.42(6.74)
7.57(7.25)
7.29(7.07)
6.87(7.07)
6.28(6.68)
3.05(3.49)
2.07(1.72)
1.53(1.72)
3.12(3.30)
3
4
5
6
7
8
9
10
11
12
13
14
15
16
C
31H30N3O3Ru
C34H34N3O3Ru
C43H33N2O3AsRu
C44H35N2O3AsRu
C44H35N2O3AsRu
C47H33N2O3AsRu
Table 2
Important IR, electronic spectral datae and fluorescence datae,f of the Ru(II) carbonyl Schiff base complexes
Complex
cC@N (cmꢀ1
)
cC–O (cmꢀ1
)
cC„O (cmꢀ1
)
kmax (nm) (e) (dm3 molꢀ1 cmꢀ1
)
Fluorescence data kmax (nm)
1
2
1617
1620
1617
1617
1615
1617
1625
1619
1616
1617
1617
1617
1616
1617
1617
1625
1351
1338
1360
1321
1349
1339
1335
1321
1327
1358
1360
1325
1321
1321
1338
1321
1934
1931
1933
1935
1936
1933
1935
1942
1941
1937
1931
1950
1925
1918
1924
1947
751(515)a, 415(16688)b, 324(27680)d, 281(39040)c
718(166)a, 409(14170)b, 332(26190)d, 280(34830)c
680(112)a, 419(9880)b, 331(19010)d, 263(38940)c
673(227)a, 487(15000)b, 341(21890)d, 271(39040)c
429(13820)b, 335(14080)d, 291(17510)c
671(210)a, 436(37480)b, 342(41730)d, 283(55390)c
658(107)a, 413(37690)b, 344(41880)d, 281(43130)c
779(118)a, 454(10630)b, 329(20200)d, 270(36900)c
589(248)a, 406(909)b, 325(16890)d, 287(21120)c
718(276)a, 426(35860)b, 343(40840)d, 297(49310)c
716(212)a, 408(13670)b, 334(29390)d, 270(37790)c
485(6210)b, 335(11510)d, 269(31360)c
474
474
499
552
471
516
3
4
5
6
7
8
534
466
490
502
542
9
10
11
12
13
14
15
16
a
669(148)a, 419(4280)b, 318(11190)d, 269(23680)c
691(196)a, 439(14250)b, 334(20340)d, 272(38080)c
688(606)a, 428(35520)b, 344(40090)d, 290(49940)c
457(5720)b, 336(8560)d, 270(30100)c
478
527
1A1g ! T1g
.
1
b
c
d
e
f
Charge transfer.
p–p*.
n–p*.
Solvent – dichloromethane.
Excitation at the higher wavelength absorption peak.
3
3
and another singlet in the region d 10.10–10.30 ppm for
the NH protons in 9, 11 and 12. A sharp singlet ob-
served for the OH protons of all the ligands in the region
d 14.0 ppm disappeared in all the complexes. A repre-
sentative spectrum of the complex [Ru(CO)(PPh3)(L4)]
is shown in Fig. 1.
corresponding to the t52geg1 configuration are T1g, T2g,
1T1g and T2g. Hence, four bands corresponding to the
1
1
3
1
3
1
1
transitions A1g ! T1g, A1g ! T2g, A1g ! T1g and
1A1g ! T2g are possible in order of the increasing
1
energy.
The bands around 779–589 nm and 485–406 nm are
1
1
The electronic absorption spectra of all the complexes
in dichloromethane showed three to four bands in the
region 263–779 nm. All the Schiff base ruthenium com-
plexes are diamagnetic, indicating the presence of ruthe-
nium in the +2 oxidation state. The ground state of
assigned to A1g ! T1g [20,31] and charge transfer
(CT) transitions respectively are listed in Table 2. The
charge transfer bands observed in all the complexes
due to M ! L transitions are possible in the visible
region [31–33]. Moreover, the presence of carbonyl, tri-
phenylphospine/arsine and heterocyclic bases as ligands,
which are capable of producing strong ligand field egꢁ
1
ruthenium(II) in an octahedral environment is A1g,
arising from the t62g configuration, and the excited states

K.N. Kumar, R. Ramesh / Polyhedron 24 (2005) 1885–1892
1889
Table 3
transfer bands and the emission maxima of the com-
plexes have experienced a positive shift of the order of
50–90 nm. The emission maxima fall in the range 471–
552 nm and a representative spectrum is shown in
Fig. 2. The observed CT luminescence in these com-
plexes may be due to the presence of di-imine functions.
It is likely that the emission originates from the lowest
energy metal to ligand charge transfer (MLCT) state,
probably derived from the excitation involving a
dp(Ru) ! p* (di-imine) MLCT transition, and similar
MLCT transitions are observed in ruthenium(II) bipyr-
idyl complexes [34,35]. The slightly lower MLCT transi-
tion energy is probably due to the chelate ring of the
complexes. In the complexes 4, 8, 12 and 16 it is ob-
served that an increase in the rigidity of the ligand re-
sults in an increase in the emission maxima. If
increased rigidity of the complexes is solely responsible
for potential emission changes then it would be expected
that this is because of the ligands present in the com-
plexes [36].
1H NMR data of Ru(II) carbonyl Schiff base complexes
Complex
1H NMR data (ppm)
1
3
6.80–8.10(Ar, m), 8.9(CH@N, s), 3.73(N–(CH2)2, t)
6.95–8.10(Ar, m), 9.15(CH@N, s), 3.85(CH2, t),
2.34(CH2, q)
6.85–8.15(Ar, m), 9.34(CH@N, s).
6.75–8.15(Ar, m), 8.85(CH@N, s), 3.83(N–(CH2)2, t).
6.84–8.1(Ar, m), 9.25(CH@N, s), 3.83(CH2, t),
2.17(CH2, q)
6.81–8.30(Ar, m), 9.42(CH@N, s).
6.90–7.90(Ar, m), 9.34(CH@N, s), 3.65(N–(CH2)2, t),
1.24(CH2, s), 10.13(NH, s).
6.70–8.10(Ar, m), 9.3(CH@N, s), 3.84(CH2, t),
2.30(CH2, q), 1.40(CH2, s), 10.30(NH, s)
6.57–8.40(Ar, m), 9.14(CH@N, s),
1.35(CH2, s), 10.1(NH, s)
4
5
7
8
9
11
12
13
15
6.70–8.35(Ar, m), 9.15(CH@N, s), 3.53(N–(CH2)2, t)
6.95–8.45(Ar, m), 9.21(CH@N, s),
3.65(CH2, t), 2.44(CH2, q)
16
6.45–8.32(Ar, m), 9.0(CH@N, s)
levels place these levels relatively high in energy. There-
fore, the lowest charge bands due to excitation of an
electron from the metal t2g level to an unfilled molecular
orbital derived from the p* level of the ligands should
appear in the relatively high energy region compared
to those due to t2g ! eꢁg transitions [32–34]. The other
high intensity bands in the 344–269 nm region were
characterized by ligand-centered (LC) bands and have
been designated as p–p* and n–p* transitions for the
electrons localized on the azomethine group of the Schiff
bases [33]. The pattern of the electronic spectra of all the
complexes indicated the presence of an octahedral envi-
ronment around the ruthenium(II) ion, similar to that of
other ruthenium(II) octahedral complexes [21].
3.2. Electrochemistry of the complexes
The redox properties of all the complexes were inves-
tigated in acetonitrile solution by cyclic voltammetry
and the redox potentials are expressed with reference to
Ag/AgCl. The cyclic voltammograms of all the com-
plexes (1 · 10ꢀ3) display one quasi-reversible oxidation
with E1/2 values 0.76–1.04 V and one irreversible reduc-
tion, with respect to the SCE. In general, the changes in
oxidation and reduction potential are probably due to
the relative stabilization of the ruthenium(II) state over
ruthenium(III) by a combination of r and p effects exhib-
ited by the ligand. A representative voltammogram of
[Ru(CO)(AsPh3)(L4)] is shown in Fig. 3 and voltammet-
ric data are presented in Table 4. The oxidation observed
is quasi-reversible in nature, characterized by a rather
Emission properties of the ruthenium(II) Schiff base
complexes were examined in dichloromethane solution
at room temperature, and excitation and emission max-
ima are listed in Table 2. In the present series, the exci-
tation of all the complexes was made at their charge
Fig. 2. Absorption (—), fluorescence (–––) and excitation (ꢂ ꢂ ꢂ) spectra
Fig. 1. 1H NMR spectrum of [Ru(CO)(PPh3)(L4)].
of [Ru (CO)(pip)(L4)] in dichloromethane.

1890
K.N. Kumar, R. Ramesh / Polyhedron 24 (2005) 1885–1892
observed in the range ꢀ0.96 to ꢀ1.18 V, which corre-
sponds to reduction of the azomethine(AC@N) moiety
of the Schiff base ligands, and the reduction occurred
at more negative values.
It has also been observed from the electrochemical
data that there is not much variation in the redox poten-
tial due to triphenylphosphine, triphenylarsine, pyridine
or piperidine [21]. Hence, it is inferred from the electro-
chemical data that the present ligand system is ideally
suitable for stabilizing the higher oxidation state of
ruthenium.
3.3. Catalytic activity of the complexes
Catalytic oxidation of primary alcohols and second-
ary alcohols by the synthesized ruthenium(II) carbonyl
Schiff base complex [Ru(CO)(PPh3)(L1)] was carried
out in CH2Cl2 in the presence of NMO, and the by
product water was removed by using molecular sieves.
Complex 1 oxidizes primary alcohols to the correspond-
ing aldehydes and secondary alcohols to ketones with
high yield and the results are listed in Table 5 (entries
1–8). The aldehydes or ketones formed after 3 h of
refluxing were determined by GC and there was no
detectable oxidation in the absence of ruthenium
complex.
Fig. 3. Cyclic voltammogram of [Ru(CO)(AsPh3)(L4)].
large peak-to-peak separation (DEp) 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 current height (ipa) with
that of the standard ferrocene–ferrocenium couple under
identical experimental conditions [37].
The reason for the irreversibility observed for reduc-
tive response of all the complexes may be due to a short
lived reduced state of the metal ion [38] or due to oxida-
tive degradation of the ligands [39]. The reduction po-
tential of Ru(II) complexes are affected by chelate
rings of the ligands and it is observed that the reduction
potential of complexes with a larger chelate ring is more
than that of complexes with a smaller chelate ring. Fur-
ther, an irreversible ligand based reduction peak is
CO
N
N
O
Ru
O
PPh3
O
(1)
0.01mmol
OH
H2O
NMO/CH2Cl2
reflux/ 3 hr
R
R'
R
R'
R,R'=alkyl (or) aryl (or) H
Table 4
Cyclic voltammetry data of Ru(II) carbonyl Schiff base complexes
Complex
Metal based oxidation
RuIII/RuII
Ligand based reduction
RuII/RuI
Epa(V)
Epc(V)
DEp(mV)
E1/2(V)
Epc(V)
Epc(V)
1
2
0.90
0.88
0.86
0.83
0.86
1.06
0.89
0.86
0.85
0.89
0.96
0.85
0.96
1.09
0.90
0.87
0.70
0.71
0.69
0.70
1.02
0.93
0.70
0.96
0.97
0.67
0.76
0.98
1.07
0.98
1.08
1.01
200
170
170
130
160
130
190
100
120
220
200
130
110
110
180
140
0.80
0.80
0.76
0.77
0.94
1.00
0.80
0.91
0.91
0.78
0.86
0.92
1.01
1.04
0.99
0.94
ꢀ0.95
ꢀ0.70
ꢀ0.73
ꢀ0.88
ꢀ0.91
ꢀ0.86
ꢀ0.83
ꢀ0.79
ꢀ0.94
ꢀ0.85
ꢀ0.77
ꢀ0.70
ꢀ0.94
ꢀ0.85
ꢀ0.79
ꢀ0.88
ꢀ0.98
ꢀ0.96
ꢀ1.18
3
4
5
6
7
ꢀ1.01
ꢀ1.12
8
9
10
11
12
13
14
15
16
ꢀ0.98
ꢀ1.06
ꢀ0.97
ꢀ1.02
ꢀ0.97
ꢀ1.11
Supporting electrolyte: NBu4ClO4 (0.005 M); complex: 0.001 M; solvent: CH3CN; DEp = Epa ꢀ Epc where Epa and Epc are anodic and cathodic
potentials, respectively; E1/2 = 0.5(Epa + Epc); scan rate: 100 mV sꢀ1
.

K.N. Kumar, R. Ramesh / Polyhedron 24 (2005) 1885–1892
1891
the oxidation of aliphatic alcohols such as butane-2-ol,
methylpropylcarbinol and isobutylmethylcarbinol to
the corresponding ketones with high conversions. In
the case of benzoin the oxidation product was isolated
Results of the present investigation suggest that the
complexes are able to react efficiently with NMO to
yield a high valent ruthenium-oxo species [2,40] capable
of oxygen atom transfer to alcohols. This was further
supported by spectral changes that occur by addition
of NMO to a dichloromethane solution of the ruthe-
nium(II) complexes. The appearance of a peak at
390 nm is attributed to the formation of RuIV@O spe-
cies, which is in conformity with other oxo ruthe-
nium(IV) complexes [2,41,42]. Further support in favor
of the formation of such species was identified from
the IR spectrum of the solid mass (obtained by evapora-
tion of the resultant solution to dryness), which showed
a band at 860 cmꢀ1, characteristic of RuIV@O species
[2,40,41].
1
and characterized by melting point, IR and H NMR.
Moreover, the complex efficiently catalyzes the oxida-
tion of five and six membered cyclic alcohols to the cor-
responding ketones with conversions of 99% and 92%,
respectively. This reaction provides a new environmen-
tally friendly route to the conversion of alcoholic func-
tions to carbonyl group and water is the only by
product during the course of the reaction. It has been
observed that complex 1 has a better catalytic efficiency
in the case of oxidation of primary and secondary alco-
hols when compared to an earlier report [42–44] on
similar ruthenium complexes as catalysts in the presence
of NMO/t-BuOOH. Hence, it is relevant from the cyclic
voltammetric data that the oxidation effected by catalysts
is likely to occur via higher ruthenium oxidation states,
which are easily accessible through chemical oxidation
with co-oxidants like NMO, t-BuOOH, PhIO etc. [42].
The oxidation of benzylalcohol to benzaldehyde re-
sulted in 89% conversion. Unsaturated cinnamylalcohol
was oxidized to cinannamaldehyde with 95% conver-
sion, without cleavage of carbon–carbon double bonds.
This is an important characteristic of the ruthenium/
NMO system. Further, the complex effectively catalyzes
4. Conclusion
Table 5
Catalytic oxidation of alcohols to the corresponding aldehydes and
ketones with complex 1 in the presence of NMO
A new family of ruthenium(II) carbonyl complexes
containing tetradentate Schiff bases were synthesized
and characterized. These complexes are found to have
luminescent properties and attempts will be made for
further investigation on this study. The oxidation of
alcohols to carbonyl compounds has been studied with
NMO in the presence of catalyst 1 and high yields were
obtained.
Conversion
Entry
Substrate
Product
(%)b
Turnoverc
CHO
OH
OH
1
2
3
89
89
95
CHO
95
OH
O
99
99
Acknowledgment
O
OH
O
We express sincere thanks to the Sophisticated
Instrument Facility, Indian Institute of Science,
Bangalore for providing the NMR facility.
4
5
6
7
8
92.2
46a
92.2
46
O
OH
OH
References
O
O
[1] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 4th
ed., Interscience, New York, 1980 (Chapter 22-F).
[2] M.M.T. Khan, Ch. Sreelatha, S.A. Mirza, G. Ramachandraiah,
S.H.R. Abdi, Inorg. Chim. Acta 154 (1988) 103.
[3] B. De Clercq, F. Verpoort, Macromolecules 35 (2002) 8943.
[4] T. Opstal, F. Verpoort, Synletters 6 (2002) 935.
[5] T. Opstal, F. Verpoort, Angew. Chem., Int. Ed. 42 (2003) 2876.
[6] (a) L. De cola, P. Elser, Coord. Chem. Rev. 177 (1998) 301;
(b) T.E. keyes, B. Evrard, J.G. Vos, C. Brady, J.J. McGarvey, P.
Yayaweera, J. Chem. Soc., Dalton Trans. (2004) 2341.
[7] V. Balzani, F. Bolletta, M.T. Gandolfi, Top. Curr. Chem. 75
(1978) 1.
[8] K. Kalyanasundaram, Coord. Chem. Rev. 46 (1982) 85.
[9] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A.
Von Zelewsky, Coord. Chem. Rev. 84 (1988) 85.
88
99
88
99
O
OH
O
OH
99
99
Substrate (1 mmol); NMO (3 mmol); complex (0.01 mmol): solvent
dichloromethane.
a
Isolated yield, characterized by melting point, 1H NMR and IR.
Yield determined by GC and comparing with the analyses of
b
authentic samples.
c
Moles of product per mole of catalyst.

1892
K.N. Kumar, R. Ramesh / Polyhedron 24 (2005) 1885–1892
[10] E.M. Kober, J. Carper, R.S. Lumpkin, T.J. Meyar, J. Phys.
Chem. 90 (1986) 3722.
[27] R.C. Maurya, P. Patel, S. Rajput, Synth. React. Inorg. Met-org.
Chem. 23 (2003) 817.
[11] V.W.W. Yam, B.W.K. Chu, C.C. Ko, K.K. Cheung, J. Chem.
Soc., Dalton Trans. (2001) 1911.
[12] H. Wolpher, O. Johansson, M. Abrahamson, M. Kritikos, L.
Sun, B. Akermark, Inorg. Chem. Commun. 7 (2004) 337.
[13] R.S. Sdrawn, M. Zamakani, J.C. Coho, J. Am. Chem. Soc. 108
(1986) 3510.
[28] K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Wiley-Interscience, New York, 1971.
[29] J.R. Dyer, Application of Absoption Spectroscopy of Organic
Compounds, Prentice-Hall, New Jersy, 1978.
[30] B.G. Jeong, C.P. Rim, H.N. Chae, K.H. Chjo, K.C. Nam, Y.K.
Choi, Bull. Korean Chem. Soc. 17 (1996) 688.
[14] R.I. Kureshy, N.H. Khan, S.H.R. Abdi, S.T. Patel, P. Iyer, J.
Mol. Catal. 150 (1999) 175.
[31] K. Natarajan, R.K. Poddar, C. Agarwala, J. Inorg. Nucl. Chem.
39 (1977) 431.
[15] S.K. Maiti, K.M. Abdul Malik, R. Bhattacharyya, Inorg. Chem.
Commun. 7 (2004) 823.
[32] A.B.P. Lever, A.B.P. Lever, Inorganic Electronic Spectroscopy,
2nd ed., Elsevier, New York, 1984.
[16] R.A. Sheldon, J.K. Kochi, Metal-Catalyzed Oxidation of Organic
Compounds, Academic Press, New York, 1981.
[17] G.J. Brink, I.W.C.E. Arends, R.A. Sheldon, Science 287 (2000)
1636.
[18] K.B. Sharpless, K. Akashi, K. Oshima, Tetrahedron Lett. 29
(1976) 2503.
[19] R. Ramesh, S. Maheswaran, J. Inorg. Biochem. 96 (2003) 457.
[20] R. Ramesh, M. Sivagamasundari, Synth. React. Inorg. Met-Org.
Chem. 33 (2003) 899.
[21] K. Naresh Kumar, R. Ramesh, Spectrochim. Acta A 60 (2004)
2913.
[33] K. Chichak, U. Jacquenard, N.R. Branda, Eur. J. Inorg. Chem.
(2002) 357.
[34] V.W.W. Yam, B.W.K. Chu, C.C. Ko, K.K. Cheung, J. Chem.
Soc., Dalton Trans. (2001) 1911, and references there in.
[35] B.K. Panda, K.G. Ghosh, S. Chattoppadhyay, A. Chkravoty, J.
Organomet. Chem. 674 (2003) 107, and references there in.
[36] T. Duff, A. Grubing, J.C. Thaomas, J.G. Vos, Polyhedron 22
(2003) 775.
[37] S. Dutta, S.M. Peng, S. Bhattacharya, J. Chem. Soc., Dalton
Trans. (2000) 4623.
[38] A.M. Bond, R. Colton, D.R. Mann, Inorg. Chem. 29 (1990) 4665.
[39] A. Basu, T.G. Kasan, N.Y. Sapre, Inorg. Chem. 27 (1988) 4639.
[40] W.H. Leung, C.M. Che, Inorg. Chem. 28 (1989) 4619.
[41] A.S. Goldstein, R.S. Drago, J. Chem. Soc., Chem. Commun.
(1991) 21.
[22] B. Furniss, A.J. Hannaford, P.W.G. Smith, A.R. Tachell, Vogelꢀs
Textbook of Practical Organic Chemistry, 5th edn. 1982.
[23] N. Ahmed, J.J. Lewison, S.D. Robinson, M.F. Uttley, Inorg.
Synth. 15 (1974) 48.
[24] S. Gopinathan, I.R. Unny, S.S. Deshpande, C. Gobinathan, Ind.
J. Chem. 25A (1986) 1015.
[42] A.M. El-Hendawy, A.H. Alkubaisi, A.E. Kourashy, M.M.
Shanab, Polyhedron 12 (1993) 2343.
[25] R.A Sanchez-pelgado, W.Y. Lee, S.R. Choi, Y. Cho, M.J. Jun,
Trans. Met. Chem. 16 (1991) 241.
[43] D. Chattergee, A. Mitra, B.C. Roy, J. Mol. Catal. A: Chem. 161
(2000) 17.
[26] S.A. Ali, A.A. Soliman, M.M. Aboaly, R.M. Ramadan, J. Coord.
Chem. 55 (2002) 1161.
[44] M.S. El- Shahawi, A.F. Shoair, Spectrochim. Acta, Part A 60
(2004) 121.