Ruthenium(II) hydrazone Schiff base complexes: Synthesis, spectral study and catalytic applications

Article abstract of DOI:10.1016/j.saa.2011.08.033

Ruthenium(II) hydrazone Schiff base complexes of the type [RuCl(CO)(B)(L)] (were B = PPh₃, AsPh₃ or Py; L = hydrazone Schiff base ligands) were synthesized from the reactions of hydrazone Schiff base ligand (obtained from isonicotinoylhydrazide and different hydroxy aldehydes) with [RuHCl(CO)(EPh₃)₂(B)] (where E = P or As; B = PPh ₃, AsPh₃ or Py) in 1:1 molar ratio. All the new complexes have been characterized by analytical and spectral (FT-IR, electronic, ¹H, ¹³C and ³¹P NMR) data. They have been tentatively assigned an octahedral structure. The synthesized complexes have exhibited catalytic activity for oxidation of benzyl alcohol to benzaldehyde and cyclohexanol to cyclohexanone in the presence of N-methyl morpholine N-oxide (NMO) as co-oxidant. They were also found to catalyze the transfer hydrogenation of aliphatic and aromatic ketones to alcohols in KOH/Isopropanol.

Full text of DOI:10.1016/j.saa.2011.08.033

Spectrochimica Acta Part A 83 (2011) 297–303
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Ruthenium(II) hydrazone Schiff base complexes: Synthesis, spectral study and
catalytic applications
R. Manikandan, P. Viswanathamurthi∗, M. Muthukumar
Department of Chemistry, Periyar university, Salem 636011, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 May 2011
Received in revised form 17 August 2011
Accepted 22 August 2011
Ruthenium(II) hydrazone Schiff base complexes of the type [RuCl(CO)(B)(L)] (were B = PPh3, AsPh3 or Py;
L = hydrazone Schiff base ligands) were synthesized from the reactions of hydrazone Schiff base ligand
(obtained from isonicotinoylhydrazide and different hydroxy aldehydes) with [RuHCl(CO)(EPh3)2(B)]
(where E = P or As; B = PPh3, AsPh3 or Py) in 1:1 molar ratio. All the new complexes have been character-
ized by analytical and spectral (FT-IR, electronic, 1H, 13C and 31P NMR) data. They have been tentatively
assigned an octahedral structure. The synthesized complexes have exhibited catalytic activity for oxida-
tion of benzyl alcohol to benzaldehyde and cyclohexanol to cyclohexanone in the presence of N-methyl
morpholine N-oxide (NMO) as co-oxidant. They were also found to catalyze the transfer hydrogenation
of aliphatic and aromatic ketones to alcohols in KOH/Isopropanol.
Keywords:
Ruthenium(II) complexes
Spectroscopic studies
Catalytic oxidation
Catalytic transfer hydrogenation
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
secondary alcohols into their corresponding aldehydes and ketones
plays a central role in organic synthesis [9]. From both an eco-
The effect of ligand on the structure and reactivity of transition
metal complexes is an important topic of research in coordina-
tion and organometallic chemistry as well as in catalysis. The large
impact of the use of Schiff base ligands in metal complexes is
evident from their utility in various catalytic reactions such as,
hydrogenation of olefins and carbonyl groups, transfer of an amino
group, photochromic properties, complexing ability towards some
toxic metals, etc. [1–4]. Hydrazones a special group of compounds
in Schiff base family, characterized by the presence of R1C N–NR2.
The hydrazone Schiff bases of acyl, aroyl and heteroacroyl com-
pounds have an additional donor sites like C O. The additional
donor sites make them more flexible and versatile. This versatil-
ity has made hydrazones as good polydentate chelating agents that
can form a verity of complexes with number of transition metals.
Various studies have also shown that the azomethine group hav-
ing a lone pair of electrons in either p or sp2 hybridized orbital on
nitrogen has considerable biological and catalytic importance [5,6].
Among the transition metals, ruthenium complexes have been
focused the attention of many research groups [7] since, ruthe-
nium complexes have been used as catalyst precursors for variety of
purposes including hydrogenation, oxidation, polymerization and
carbon–carbon bond formation [8]. The oxidation of primary and
nomic and environmental point of view, the quest for effective
catalytic systems that use clean, inexpensive primary oxidants such
as molecular oxygen or hydrogen peroxide, i.e., a green method
for converting alcohols to carbonyls compounds on an industrial
scale remains an important challenge [10]. Most studies of alcohol
oxidation using both homogeneous and heterogeneous catalysts
involve the use of group VIII metal complexes. Ruthenium com-
pounds such as RuCl3 and some other high valent oxoruthenium
complexes have been extensively investigated as catalyst for alco-
hol oxidation using variety of primary oxidants like iodobenzene,
NMO [11], tert-hydroperoxide [12], hypochloride [13], bromated
or a combination of oxygen and an aldehyde.
Ruthenium complexes have a long pedigree as catalysts for
transfer hydrogenation reactions in the presence of isopropanol as
hydrogen source [14,15]. Several recent examples of ruthenium
N-heterocyclic carbene complexes [16–18], Ru(arene)-(diamine)
[19], ruthenium(III) amine-bis(phenolate) tripodal complexes [20]
and Ru(BINAP)(diamine) [21] have become the most prominent
members for the reduction of ketones in high yields. Pincer-type
arylruthenium(II) complexes containing the monoanionic tri-
dentate NCN and PCP ligands have been reported by Koten as
active catalysts for the transfer hydrogenation of ketones in the
presence of isopropanol and KOH [22]. Even though there are
number of reports available on the transfer hydrogenation of
ketones by ruthenium complexes, only limited reports are avail-
able for catalytic transfer hydrogenation of ketones by ruthenium
hydrazone Schiff base complexes. Hence, synthesis of new
∗
Corresponding author. Fax: +91 427 2345124.
E-mail addresses: viswanathamurthi@rediffmail.com,
viswanathamurthi@yahoo.com (P. Viswanathamurthi).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.08.033

298
R. Manikandan et al. / Spectrochimica Acta Part A 83 (2011) 297–303
Fig. 1. Structure of hydrazone Schiff base ligands.
ruthenium
complexes
containing
triphenylphos-
1 mmol), 5-chlorosalicylaldehyde (0.1566 g, 1 mmol), salicy-
phine/triphenylarsine with hydrazone Schiff base ligands is
of greater importance among various transition metal complexes.
laldehyde (0.11 cm3, 1 mmol) or 2-hydroxy-1-naphthaldehyde
(0.1722 g, 1 mmol)} with isonicotinoylhydrazide (0.1371 g,
1 mmol) in methanol (20 ml) under reflux for 2 h. The resulting
yellow solid was separated, filtered, washed with methanol and
dried in vacuo [27]. The general structures of the hydrazone Schiff
base ligands used in this study are given below (Fig. 1) (73–80%
yield).
We, herein report the synthesis of
a series of hexa-
coordinated ruthenium(II) hydrazone Schiff base complexes
containing PPh3/AsPh3 and other co-ligands. The characterization
of the complexes was accomplished by analytical and spectral (IR,
electronic, 1H, 13C and 31P NMR) methods. Further, some of the
synthesized complexes have been effectively used as catalyst in
oxidation of alcohols in the presence of NMO and transfer hydro-
genation of ketones in isopropanol and KOH as base.
2.4. Synthesis of new ruthenium(II) hydrazone Schiff base
complexes
All the new metal complexes were prepared according to the fol-
lowing general procedure. To a solution of [RuHCl(CO)(EPh3)2(B)]
(E = P or As; B = PPh3, AsPh3 or Py) (0.1 g; 0.1 mmol) in ben-
zene (20 ml), the appropriate hydrazone Schiff base ligand
(0.022–0.0378 g, 0.1 mmol) was added in 1:1 molar ratio. The
mixture was heated under reflux for 5 h in water bath. Then
the resulting solution was concentrated to 3 ml and the product
precipitated by the addition of petroleum ether (60–80 ◦C) was
recrystallised using CH2Cl2. The compounds were dried under vac-
uum and the purity of the complexes was checked by TLC.
2. Experimental
2.1. Materials and reagents
All the reagents used were chemically pure and AR grade.
The solvents were purified and dried according to stan-
dard procedures [23]. RuCl3·H2O was purchased from Loba
Chemie Pvt. Ltd. The starting complexes [RuHCl(CO)(PPh3)3] [24],
[RuHCl(CO)(Py)(PPh3)2] [25] and [RuHCl(CO)(AsPh3)3] [26] were
prepared according to the literature reports.
2.5. Catalytic oxidation
2.2. Physical measurements
Catalytic oxidation of primary alcohols to corresponding aldehy-
des and secondary alcohols to ketones by ruthenium(II) hydrazone
Schiff base complexes were studied in the presence of NMO as co-
oxidant. In a typical reaction, ruthenium(II) complexes as a catalyst
and primary or secondary alcohol, as substrates at 1:100 molar
ratio was described as follows. A solution of ruthenium complexes
(0.01 mmol) in CH2Cl2 (20 cm3) was added to the mixture con-
taining substrate (1 mmol), NMO (3 mmol) and molecular sieves.
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 extracted with petroleum ether (60–80 ◦C)
(20 ml) concentrated to ∼1 ml and was analyzed by GC. The oxi-
dation products were identified by GC co-injection with authentic
samples.
Microanalysis of carbon, hydrogen and nitrogen was carried out
using Vario EL III Elemental analyzer at SAIF – Cochin, India. The
IR spectra of the ligand and their complexes were recorded as KBr
pellets on a Nicolet Avatar model in 4000–400 cm−1 range. Elec-
tronic spectra of the ligand and their complexes have been recorded
in dichloromethane using a Shimadzu UV-1650 PC spectropho-
tometer in 200–800 nm range. 1H, 13C and 31P NMR spectra were
recorded in Jeol GSX-400 instrument using DMSO as the solvent.
1H NMR and 13C NMR spectra were obtained at room temperature
using TMS as the internal standard. 31P NMR spectra of the com-
plexes were obtained at room temperature using o-phosphoric acid
as a reference. Melting points were recorded on a Technico micro
heating table and are uncorrected. The catalytic yields were deter-
mined using ACME 6000 series GC-FID with DP-5 column of 30 m
length, 0.53 mm diameter and 5.00 ␮m film thickness.
2.6. Catalytic transfer hydrogenation of ketones
2.3. Synthesis of hydrazone Schiff base ligands
The catalytic transfer hydrogenation reactions were also studied
using ruthenium(II) hydrazone Schiff base complexes as a catalyst,
ketone as substrate and KOH as base at 1:300:2.5 molar ratios.
The hydrazone Schiff base ligands were prepared by the con-
densing 2-hydroxy aldehyde {3-methoxysalicylaldehyde (0.1522 g,

R. Manikandan et al. / Spectrochimica Acta Part A 83 (2011) 297–303
299
ligands by replacing two molecules of triphenylphosphine or
triphenylarsine and one molecule of hydride from the starting
complexes.
All the complexes are stable in air at room temperature, brown
in color, non-hygroscopic in nature and highly soluble in common
organic solvents such as dichloromethane, benzene, acetonitrile,
chloroform and DMSO. The analytical data are listed in Table 1
and are in good agreement with the general molecular formula
proposed for all the complexes.
R1
HO
O
C
[RuHCl(CO)(EPh3)2(B)]
N
N
H
C
H
R2
N
Benzene/
Reflux 5h
3.1. Infrared spectroscopic analysis
N
CO
O
A comparison between the IR spectral data of the free ligand
and the metal complexes was made to study the binding mode of
the ligand to the ruthenium ion in the new metal complexes. Some
of the important IR absorption bands of synthesized ligands and
complexes are shown in Table 2.
Cl
C
N
Ru
B
H
N
C
O
R1
H
The broad band centered at 3436–3444 cm−1 in the free lig-
ands [28] can be assigned to ꢀN–H, which is a remains unaltered
in the complexes. This suggests that the non-participation of NH
group in bonding. A strong band at 1327–1331 cm−1 in the free
ligands is assigned to phenolic C–O stretching. This band shifted
to 1350–1387 cm−1 in complexes, showing that the coordination
through the phenolic oxygen [29]. This is further supported by
the disappearance of ꢀO–H band [28] around 3200–3222 cm−1 and
R2
(B = PPh3, AsPh3 or Py; R1 = OCH3 or H; R2 = Cl or H)
Scheme 1. Formation of Ru(II) hydrazone Schiff base complexes.
ıO–H peak [30] around 1340–1362 cm−1. The ꢀC
observed at
The procedure was described as follows. A mixture containing
ketone (3.75 mmol), the ruthenium complex (0.0125 mmol) and
KOH (0.03 mmol) was heated to reflux in 10 ml of i-PrOH for 3 h.
After completion of reaction the catalyst was removed from the
reaction mixture by the addition of petroleum ether followed by
filtration and subsequent neutralization with 1 M HCl. The ether
layer was filtered through a short path of silica gel by column
chromatography. The filtrate was subjected to GC analysis and the
hydrogenated product was identified and determined with authen-
tic samples.
O
1680–1686 cm−1 in the spectra of the ligands showed a down-
ward shift to 1642–1679 cm−1 in all the complexes indicating
the coordination through the carbonyl oxygen [31]. The band at
1620–1626 cm−1 is assigned to ꢀC N in the spectra of ligands which
is shifted to 1595–1615 cm−1 in the spectra of the complexes indi-
cated that the other coordination is through azomethine nitrogen
[32]. Hence, from the infrared spectroscopic data, it is inferred
that both azomethine, carbonyl and phenolic oxygen atoms are
involved in the coordination of the tridentate Schiff bases to ruthe-
nium ion in all the complexes. Further the strong absorption around
the 1950–1957 cm−1 has been assigned to the terminally coordi-
nated carbonyl group in the new ruthenium complexes [33]. In
the case of complexes containing coordinated heterocyclic nitro-
gen bases [34], a medium intensity band was observed in the region
1027–1031 cm−1. In addition, the other characteristic bands due
to triphenylphosphine and triphenylarsine (around 700, 1090 and
1440 cm−1) were also present in the spectra of all the complexes
[35]. The observed bands in the region 460–475 cm−1 in the mono
nuclear complexes were tentatively assigned to the ꢀRu–Cl vibra-
tions [36].
3. Result and discussion
Diamagnetic, hexa-coordinated low spin ruthenium(II) hydra-
zone Schiff base complexes of general formula [RuCl(CO)(B)(L)]
(B = PPh3, AsPh3 or Py; L = hydrazone Schiff base ligand)
were synthesized in quantitative yield from the reaction of
[RuHCl(CO)(EPh3)2(B)] (E = P or As; B = PPh3, AsPh3 or Py) with
hydrazone Schiff base ligands in dry benzene in 1:1 molar
ratio (Scheme 1). In all these reactions, it was observed that
the hydrazone Schiff base behave as mononegative tridentate
Table 1
Analytical data of free ligands and their ruthenium(II) hydrazone Schiff base complexes.
Compound
Formula
M.Pt (◦C)
Calculated (found) (%)
C
H
N
L1
L2
L3
L4
C14H13N3O3
C13H10N3O2Cl
C13H11N3O2
C17H13N3O2
218
232
229
238
230
160
185
138
175
180
183
203
210
150
120
200
61.98(61.13)
56.64(56.66)
64.72(64.54)
70.09(70.23)
56.85(56.58)
54.79(54.38)
57.62(57.65)
60.29(68.09)
46.74(46.48)
44.03(44.52)
47.16(46.97)
51.74(51.65)
53.48(53.77)
51.56(51.57)
54.06(53.82)
56.81(56.62)
4.83(4.11)
3.66(3.49)
4.59(4.43)
4.49(4.86)
3.90(3.46)
3.44(2.96)
3.75(3.79)
3.79(3.38)
3.31(3.12)
2.72(2.54)
3.12(3.62)
3.21(3.43)
3.67(2.99)
3.25(3.55)
3.54(3.12)
3.5(3.72)
15.49(15.18)
15.24(15.15)
17.42(17.55)
14.42(14.22)
06.03(6.16)
05.99(5.26)
06.28(6.25)
05.86(5.74)
10.90(10.60)
10.81(10.62)
11.58(11.96)
10.49(10.42)
05.67(5.29)
05.64(5.11)
05.91(5.81)
05.52(4.95)
[RuCl(CO)(PPh3)(L1)]
[RuCl(CO)(PPh3)(L2)]
[RuCl(CO)(PPh3)(L3)]
[RuCl(CO)(PPh3)(L4)]
[RuCl(CO)(Py)(L1)]
[RuCl(CO)(Py)(L2)]
[RuCl(CO)(Py)(L3)]
[RuCl(CO)(Py)(L4)]
[RuCl(CO)(AsPh3)(L1)]
[RuCl(CO)(AsPh3)(L2)]
[RuCl(CO)(AsPh3)(L3)]
[RuCl(CO)(AsPh3)(L4)]
C33H27N3O4ClPRu
C32H24N3O3Cl2PRu
C32H25N3O3ClPRu
C36H27N3O3ClPRu
C20H17N4O4ClRu
C19H14N4O3Cl2Ru
C19H15N4O3ClRu
C23H17N4O3ClRu
C33H27N3O4ClAsRu
C32H24N3O3Cl2AsRu
C32H25N3O3ClAsRu
C36H27N3O3ClAsRu

300
R. Manikandan et al. / Spectrochimica Acta Part A 83 (2011) 297–303
Table 2
IR absorption frequencies (cm−1) and electronic spectral data (nm) of free ligands and their ruthenium(II) hydazone Schiff base complexes.
Compound
ꢁN–H
ꢁO–H
ꢁC
O
ꢁC
O
ꢁC
N
ıO–H
ꢁC–O
ꢂmax
L1
L2
L3
L4
3444
3436
3436
3440
3446
3445
3436
3444
3446
3445
3436
3447
3446
3445
3434
3434
3202
3200
3220
3222
–
–
1686
1681
1682
1680
1678
1668
1679
1678
1663
1672
1654
1674
1642
1666
1678
1650
1626
1620
1624
1624
1609
1610
1611
1615
1609
1610
1610
1615
1595
1598
1598
1615
1348
1340
1355
1362
–
1327
1330
1331
1330
1375
1375
1370
1383
1375
1374
1371
1375
1377
1350
1376
1387
297, 234
241, 273, 341
238, 279, 329
–
–
–
229, 258, 323, 347, 364
240, 302, 347, 394, 470
247, 347, 371, 464
252, 347, 358, 470
247, 344, 382, 476
252, 312, 352, 364
241, 347, 371, 464
247, 341, 361, 462
247, 326, 350, 370, 476
237, 304, 352, 437
258, 329, 370, 461
238, 270, 347, 355, 452
261, 320, 347, 379, 485
[RuCl(CO)(PPh3)(L1)]
[RuCl(CO)(PPh3)(L2)]
[RuCl(CO)(PPh3)(L3)]
[RuCl(CO)(PPh3)(L4)]
[RuCl(CO)(Py)(L1)]
[RuCl(CO)(Py)(L2)]
[RuCl(CO)(Py)(L3)]
[RuCl(CO)(Py)(L4)]
[RuCl(CO)(AsPh3)(L1)]
[RuCl(CO)(AsPh3)(L2)]
[RuCl(CO)(AsPh3)(L3)]
[RuCl(CO)(AsPh3)(L4)]
1955
1953
1954
1953
1955
1956
1957
1954
1952
1952
1952
1950
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Table 3
1H NMR data (ppm) of free ligands and their ruthenium(II) hydrazone Schiff base complexes.
Compound
OH (s)
NH (s)
2, 6-Pyridine H (m)
N
CH (s)
3, 5-Pyridine H (m)
Aromatic H (m)
OCH3 (s)
L1
L2
L3
L4
11.91
12.14
11.06
12.96
10.37
10.16
9.98
9.90
9.98
9.87
9.78
9.49
9.88
9.74
9.76
9.44
9.80
9.73
9.75
9.48
8.60–8.79
8.72–8.79
8.77–8.79
8.60–8.64
8.10–8.18
7.81–7.88
7.82–7.90
8.20–8.34
8.20–8.23
7.82–7.90
7.82–7.84
8.20–8.30
8.20–8.29
7.90–8.28
8.23–8.29
8.00–8.13
8.25
8.30
8.35
8.23
8.65
8.52
8.72
8.83
8.70
8.62
8.75
8.80
8.70
8.65
8.84
8.85
7.56–7.85
7.93–8.08
7.82–7.84
8.10–8.18
7.43–7.49
7.38–7.46
7.55–7.57
7.84–7.89
7.45–7.48
7.62–7.68
7.54–7.57
7.83–7.92
7.51–7.58
7.43–7.48
7.83–7.85
7.72–7.78
6.84–7.45
6.72–7.38
6.89–7.60
6.83–7.66
6.64–7.28
6.75–7.36
6.93–7.44
6.88–7.60
6.62–7.14
6.75–7.36
6.83–7.41
6.93–7.58
6.44–7.03
6.76–7.38
6.78–7.25
6.54–7.48
3.90
–
–
–
3.92
–
–
–
3.90
–
–
–
3.93
–
–
[RuCl(CO)(PPh3)(L1)]
[RuCl(CO)(PPh3)(L2)]
[RuCl(CO)(PPh3)(L3)]
[RuCl(CO)(PPh3)(L4)]
[RuCl(CO)(Py)(L1)]
[RuCl(CO)(Py)(L2)]
[RuCl(CO)(Py)(L3)]
[RuCl(CO)(Py)(L4)]
[RuCl(CO)(AsPh3)(L1)]
[RuCl(CO)(AsPh3)(L2)]
[RuCl(CO)(AsPh3)(L3)]
[RuCl(CO)(AsPh3)(L4)]
–
–
–
–
–
–
–
–
–
–
–
–
–
s, singlet; m, multiplet.
Table 4
13C NMR data (ppm) of ruthenium(II) hydrazone Schiff base complexes.
Compound
C
O
C
O
C
N
Aromatic C
OCH3
[RuCl(CO)(PPh3)(L1)]
[RuCl(CO)(PPh3)(L2)]
[RuCl(CO)(PPh3)(L3)]
[RuCl(CO)(PPh3)(L4)]
[RuCl(CO)(Py)(L1)]
[RuCl(CO)(Py)(L3)]
[RuCl(CO)(AsPh3)(L1)]
[RuCl(CO)(AsPh3)(L4)]
205.35
206.15
203.10
204.30
205.25
203.37
205.25
205.32
180.62
182.33
178.95
180.56
181.12
178.36
180.53
182.54
163.73
161.06
164.21
162.57
164.33
163.58
163.27
160.80
124.85–135.48
122.34–135.07
127.86–136.98
122.85–138.43
124.45–132.37
127.66–133.48
124.55–133.62
126.23–136.04
53.60
–
–
–
55.34
–
53.20
–
Table 5
Catalytic oxidation data of ruthenium(II) hydrazone Schiff base complexes.
Complex
Substrate
Product
Benzaldehyde
Yield(%)a
[RuCl(CO)(PPh3)(L1)]
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
Benzyl alcohol
Cyclohexanol
99
83
93
87
97
79
99
76
96
74
97
70
Cyclohexanone
Benzaldehyde
Cyclohexanone
Benzaldehyde
Cyclohexanone
Benzaldehyde
Cyclohexanone
Benzaldehyde
Cyclohexanone
Benzaldehyde
Cyclohexanone
[RuCl(CO)(PPh3)(L2)]
[RuCl(CO)(PPh3)(L3)]
[RuCl(CO)(PPh3)(L4)]
[RuCl(CO)(Py)(L2)]
[RuCl(CO)(AsPh3)(L2)]
a
Yield determined by GC and comparing with the analysis authentic samples.

R. Manikandan et al. / Spectrochimica Acta Part A 83 (2011) 297–303
301
Scheme 3. Catalytic transfer hydrogenation of ketone.
Scheme 2. Catalytic oxidation of alcohols.
ligand coordinated through the keto form. A singlet observed at ı
8.23–8.35 ppm in the spectra of the free ligands assigned to azome-
thine proton which undergo downfield shift to ı 8.52–8.85 ppm in
the complexes indicating the coordination of azomethine nitrogen
to ruthenium metal [42–44]. The signals at ı 7.38–8.34 ppm as mul-
tiplets for all complexes are assigned to the protons of pyridinyl
group of ligand [46], which is shifting slightly to the upfield com-
pared with the free ligands. The multiplets at ı 6.44–7.60 ppm in
the spectra of ligands and complexes are assigned to aromatic pro-
tons. The methoxy protons appeared at ı 3.90–3.93 ppm. The new
complexes did not show any signal in the upfield region at ı −5
to −12 ppm for the hydride ligand, thus proving its substitution by
the mono negative hydrazone Schiff base ligands.
3.2. Electronic spectroscopic analysis
All the new ruthenium(II) hydrazone Schiff base complexes are
diamagnetic indicating the presence of ruthenium in the +2 oxida-
1
tion state. The ground state of ruthenium(II) is A1g arising from
the t6 configuration in an octahedral environment. The excited
2g
state corresponding to the t5 e1 configuration are T1g
,
3T2g
,
3
g
2g
1
1T1g and T2g. Hence, four bands corresponding to the transitions
1A1g → T1g
,
1A1g → T2g
,
1A1g → T1g
,
1A1g → T2g and possible in
3
3
1
1
the order of increasing energy.
The electronic spectra of all the complexes in CH2Cl2 showed
four to five bands in the region 234–485 nm (Table 2). The band
around 437–485 nm range has been assigned to the spin allowed
3.4. 13C NMR spectroscopic analysis
1
1A1g → T1g transition. The other high intensity bands around
The 13C NMR spectra of some of the complexes have showed
(Table 4) a peak at 203.10–206.15 ppm region is due to C O carbon.
The presence of a peak at 178.36–182.54 ppm region is assigned to
carbonyl carbon. In addition, the azomethine (C N) carbon exhibit
its peak in the region of 160.80–164.33 ppm. The multiplets appear
around 122.34–138.43 ppm region are assigned to aromatic and
pyridine carbons. A sharp singlet at 53.20–55.34 ppm are assigned
to methoxy carbon. This confirms the formation of new ruthe-
nium(II) hydrazone Schiff base complexes.
312–394 nm have been assigned to charge transfer transitions aris-
ing from the metal t2g level to the unfilled molecular orbitals
derived from the ␲* level of the ligands [37–40] based on their
extinction coefficient values. The bands below 300 nm were char-
acterized by intra-ligand charge transfer. The electronic spectra
are similar to those observed for other octahedral ruthenium(II)
complexes [41].
3.3. 1H NMR spectroscopic analysis
The 1H NMR spectra of the ligand and the corresponding ruthe-
nium(II) hydrazone Schiff base complexes were recorded in DMSO
to confirm the presence of coordinated ligand in the complexes. The
spectral data and their assignments are listed in Table 3. The spectra
of the free ligands showed a signal at ı 11.06–12.96 ppm charac-
teristics of phenolic OH proton which is absent in the complexes,
suggesting that the coordination is through deprotonated pheno-
lic oxygen [42–44]. Signal due to NH proton at ı 9.90–10.37 ppm
in free ligands is shifted to ı 9.44–9.98 in the complexes [45].
The presence of NH proton in the complexes indicating that the
3.5. 31P NMR spectroscopic analysis
31P NMR spectra of some of the complexes were recorded
to confirm the presence of triphenylphosphine groups in the
N
CO
O
N
O
N
O
Cl
C
N
Ru
B
O
O
H
N
C
O
R1
H
i-PrOH +
KOH
-KCl, H2O
R2
CO
H
C
R'
R
OH
O
N
O
O
H
Ru
B
O
HO
R
H
CO
CO
O
N
H
O
N
O
R'
H
Ru
B
Ru
B
O
O
R
O
CO
O
N
H
O
R'
R
R'
Ru
B
O
B = PPh3, AsPh3 (or) Py; R, R' = alkyl (or) aryl (or) H
Scheme 4. Mechanism of catalytic transfer hydrogenation reactions.
Fig. 2. Catalytic oxidation of benzylalcohol (B) and cyclohexanol (C) in different
time intervals.

302
R. Manikandan et al. / Spectrochimica Acta Part A 83 (2011) 297–303
Table 6
Catalytic transfer hydrogenation of ketones by ruthenium(II) hydrazone Schiff base complexes.
Complex
Substrate
Product
Conversion (%)
[RuCl(CO)(PPh3)(L2)]
83
76
93
[RuCl(CO)(Py)(L2)]
84
81
90
[RuCl(CO)(AsPh3)(L2)]
80
80
86
complexes. A sharp singlet was observed around 22.47–22.62 ppm
due to presence of triphenylphosphine ligand in the complexes.
On the basis of analytical and spectral (IR, electronic, 1H, 13C
and 31P NMR) data, an octahedral structure has been tentatively
proposed for all the new ruthenium(II) hydrazone Schiff base com-
plexes.
addition of NMO to dichloromethane solution of the ruthenium(II)
complexes. The appearance of a peak at 390 nm is attributed to the
formation of RuIV O species, which is in confirmed with other oxo
ruthenium(IV) complexes [48,49]. Further support in favour of the
formation of such species was identified from the IR spectrum of
the solid mass (obtained by evaporation of the resultant solution
to dryness), which showed a band at 860 cm−1, characteristic of
RuIV O species [50].
3.6. Catalytic oxidation
3.7. Catalytic transfer hydrogenation
Catalytic oxidation of primary alcohols and secondary alcohols
by the synthesized ruthenium(II) hydrazone Schiff base complexes
were carried out in CH2Cl2 in the presence of NMO, and the by
product water was removed by using molecular sieves. All the com-
plexes oxidized primary alcohols to corresponding aldehydes and
secondary alcohols to ketones (Scheme 2 and Fig. 2) with high
yields and the results are listed in Table 5. The aldehydes or ketones
formed after 3 h of reflux were determined by GC and there was no
detectable oxidation in the absence of ruthenium complexes.
The oxidation of benzylalcohol to benzaldehyde resulted in
93–99% yield and cyclohexanol to cyclohexanone resulted in
70–87% yield. The relatively higher yield obtained for the oxidation
of benzylalcohol as compared to cyclohexanol is due to the fact that
the ␣-CH unit of benzylalcohol is more acidic than cyclohexanol
[47].
The catalytic transfer hydrogenation of ketones in the presence
of ruthenium(II) hydrazone Schiff base complexes has been studied
in isopropanol-KOH medium using a molar ratio 1:2.5:300 for the
catalyst, KOH and the ketone in 10 cm3 of isopropanol (Scheme 3).
Typical results are shown in Table 6. The catalyst performed
efficiently for both aliphatic and aromatic ketones with high con-
version, acetophenone was converted into corresponding alcohol
upto 80–84% yield. Isobutyl methyl ketone underwent transfer
hydrogenation to afford the corresponding alcohol upto 76–81%
yield. Similarly, cyclohexanone was converted to cyclohexanol
upto 86–93% yield. The role of KOH is to generate the catalyst
from the chloro precursor and the reaction mediates through the
hydride species [51]. The base facilitated the formation of the
ruthenium alkoxide by abstracting the proton of the alcohol and
subsequently, the alkoxide underwent ␤-elimination to give a
ruthenium hydride which is the active species in this reaction [52]
(Scheme 4).
Results of the present investigations suggest that the com-
plexes are able to react efficiently with NMO to yield a high valent
ruthenium-oxo species capable of oxygen atom transfer to alco-
hols. This was further supported by spectral changes that occur by

R. Manikandan et al. / Spectrochimica Acta Part A 83 (2011) 297–303
303
4. Conclusions
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Several ruthenium(II) hydrazone Schiff base complexes of
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L = hydrazone Schiff base ligand) have been prepared by reacting
ruthenium(II) starting complexes [RuHCl(CO)(EPh3)2(B)] (where
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