Synthesis, spectral, electrochemical and catalytic properties of Ru(III) Schiff base complexes containing N, O donors

Article abstract of DOI:10.1016/j.molstruc.2013.12.048

A series of new hexa coordinated ruthenium(III) complexes of the type [RuY₂(EPh₃)₂(X-DPMP)] (where Y = Br or Cl; E = P or As; DPMP = 2-[(2,6-Diisopropyl-phenylimino)-methyl]-phenol, X = H, Br, Cl, I and Ph) have been synthesized by equimolar [RuY₃(EPh ₃)₃] and the Schiff base ligands in benzene. The bidentate Schiff base ligands (X-DPMP) have been derived from condensation of 2,6-diisopropylaniline with mono and multisubstituted salicylaldehyde derivatives. The complexes have been characterized by elemental analysis, magnetic susceptibility, UV-Vis., IR and EPR spectral and electrochemical measurements. All the ruthenium(III) complexes are found to be stable, paramagnetic, low spin, redox active and display either quasi reversible or irreversible redox couples based on metal centre. They have exhibited catalytic activity for the 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) as co-oxidant.

Full text of DOI:10.1016/j.molstruc.2013.12.048

Journal of Molecular Structure 1060 (2014) 49–57
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectral, electrochemical and catalytic properties of Ru(III)
Schiff base complexes containing N, O donors
b
d
e
a,
K. Kanmani Raja c, , N. Indra Gandhi , L. Lekha , D. Easwaramoorthy , G. Rajagopal
⇑
⇑
a Department of Chemistry, Madras Medical College, Chennai 600 003, Tamil Nadu, India
b PG & Research Department of Chemistry, Presidency College (Autonomous), Chennai 600 005, India
c Department of Chemistry, Government Arts College (Men), Nandanam, Chennai 600 035, Tamil Nadu, India
d Department of Chemistry, Saveetha School of Engineering, Thandalam, Chennai 602 105, Tamil Nadu, India
e Department of Chemistry, B.S. Abdur Rahman University, Vandalur, Chennai 600 048, Tamil Nadu, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
ꢀ
Synthesis, characterization of a new
Schiff base and its Ru(III) complexes.
Ru(III) complexes are paramagnetic
with one unpaired electron and
octahedral.
ꢀ
ꢀ
Complexes are redox active based on
metal centre.
New complexes are effective catalyst
for the oxidation of alcohols.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 October 2013
Received in revised form 28 November 2013
Accepted 4 December 2013
Available online 22 December 2013
A series of new hexa coordinated ruthenium(III) complexes of the type [RuY2(EPh3)2(X-DPMP)] (where
Y = Br or Cl; E = P or As; DPMP = 2-[(2,6-Diisopropyl-phenylimino)-methyl]-phenol, X = H, Br, Cl, I and
Ph) have been synthesized by equimolar [RuY3(EPh3)3] and the Schiff base ligands in benzene. The biden-
tate Schiff base ligands (X-DPMP) have been derived from condensation of 2,6-diisopropylaniline with
mono and multisubstituted salicylaldehyde derivatives. The complexes have been characterized by ele-
mental analysis, magnetic susceptibility, UV–Vis., IR and EPR spectral and electrochemical measure-
ments. All the ruthenium(III) complexes are found to be stable, paramagnetic, low spin, redox active
and display either quasi reversible or irreversible redox couples based on metal centre. They have exhib-
ited catalytic activity for the 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) as co-oxidant.
Keywords:
Schiff base
Ruthenium(III)
Catalytic oxidation
Alcohol
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
The interest in the synthesis and characterization of transition
metal complexes containing a Schiff base with nitrogen and oxy-
gen donor atoms lies in their extensive applications in the fields
of catalysis [1,2] and transformations [3,4], which can modify the
structural and electronic properties of transition metal centers
[5]. Transition metal complexes are powerful catalyst for organic
⇑
Corresponding authors. Address: Department of Chemistry, Government Arts
College (Men), Nandanam, Chennai 600 035, Tamil Nadu, India. Tel.: +91
9840712397 (K.K. Raja). Address: Department of Chemistry, Madras Medical
College, Chennai 600 003, Tamil Nadu, India. Tel.: +91 9444644661 (G. Rajagopal).
E-mail addresses: kkanmaniraja@gmail.com (K.K. Raja), rajagopal18@yahoo.com
(G. Rajagopal).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.12.048

50
K.K. Raja et al. / Journal of Molecular Structure 1060 (2014) 49–57
transformations both in homogeneous and heterogeneous reac-
tions and the activity of these complexes varied with the type of
ligand, coordination site and the metal center [6]. Based on it, they
can offer chemo, regio or stereo selectivity with the enhancement
of product yield under mild conditions [7]. Transition metal based
catalytic conversion of primary and secondary alcohols into their
corresponding aldehydes and ketones are essential reaction in or-
ganic synthesis [8–10]. Traditional methods for performing such
transformation generally involve the use of stoichiometric quanti-
ties of inorganic oxidants like Cr (IV) and generate large quantities
of waste [11]. The development of effective, greener catalytic sys-
tem that uses clean and inexpensive oxidants such as NMO, molec-
ular oxygen or hydrogen peroxide for converting alcohols to
carbonyl compounds on an industrial scale remains an important
challenge [12]. Ruthenium complexes containing triphenylphos-
phine or triphenylarsine ligands have been extensively investi-
gated and well established [13] as catalyst for alcohol oxidation
in combination with various oxidants such as dioxygen [14,15],
iodosobenzene [16], t-BuOOH [17], H2O2 [18], NaIO4 [4] and
NMO [19,20].
In contrast to the considerable growth of literature on the
chemistry of Schiff base complexes of first row transition metal,
the chemistry of ruthenium complexes with multi substituted
ligands systems is less well developed. With this in view and
continuing interest of our study [21–23], the present report ac-
counts for the synthesis, characterization of new multi substituted
Schiff bases and their ruthenium(III) complexes with a special
impetus on their spectral and electrochemical investigations. The
general structure of Schiff base ligands used in the present work
and newly synthesized ruthenium(III) complexes are given in
Fig. 1.
complexes in KBr (4000–400 cmꢂ1) were recorded on a Perkin
Elmer 577 grating spectrophotometer. The electronic spectra in
MeCN were obtained on a Shimadzu-160 UV–Vis. Spectrophotom-
eter. Microanalyses for the carbon, hydrogen and nitrogen content
of the new complexes were carried out by the CDRI, Lucknow, In-
dia. The metal contents of the complexes were estimated by incin-
erating them to their oxides in the presence of ammonium oxalate.
1H NMR spectra were recorded in CDCl3 with TMS as an internal
standard on a Brucker 300 MHz spectrometer. X-band EPR spectra
were recorded on a Varian-E-12 spectrometer with a quartz Dewar
for measurements at the liquid N2 temperature and the spectra
were calibrated with DPPH. Cyclic voltammetric measurements
were made in MeCN (HPLC grade) using BAS-CV50 electrochemical
analyzer. The three electrode cell comprised a reference Ag/AgCl,
auxillary Pt and the working glassy carbon electrodes. Bu4NClO4
was used as supporting electrolyte. The single crystal XRD data col-
lection have been obtained using APEX2 (Bruker, 2004), IIT Madras,
Chennai, India.
2.3. Synthesis of ligands
The monobasic bidentate Schiff base ligands were prepared by
condensation of 2,6-diisopropylaniline [2 mmol] with salicylalde-
hyde and substituted salicyaldehydes [2 mmol] in 1:1 M ratio in
ethanol. The reaction mixture was then refluxed for 3 h. Upon cool-
ing to 0 °C, the product separated out as yellow solid precipitated
which was filtered, washed with ice cold ethanol and dried in vac-
uum over anhydrous CaCl2. Yellow crystals suitable for X-ray dif-
fraction were obtained directly from the reaction mixture, by
slow evaporation of the solvent at room temperature.
2. Experimental
2.3.1. 2-[(2,6-diisopropylphenylimino)methyl] phenol [DPMP]
Overall yield 70%. Anal. Calc. C19H23ON: C, 81.10, H, 8.24; N,
4.98. Found: C, 81.20%; H, 8.21%; N, 4.88%. d ppm: 15.23 (s, 1H),
9.07 (s, 1H), 7.96–7.99 (d, 1H), 7.84–7.87 (d, 1H), 7.75–7.77
(d, 1H), 7.46–7.51 (m, 2H), 7.31–7.36 (m, 2H), 7.17–7.24 (m, 2H),
3.05 (m, 2H), 1.21–1.23 (d, 12H). The expanded 1H NMR spectrum
is shown in Fig. 2.
2.1. Materials
All the reagents used were chemically pure and AR grade. Sol-
vents were purified and dried according to standard procedure.
RuCl3ꢁ3H2O was purchased from Loba Chemical Pvt. Ltd., Bombay,
India and was used without further purification. RuCl3(PPh3)3 [24]
RuCl3(AsPh3)3 [25] RuBr3(AsPh3)3 [26] and the Schiff bases were
prepared according to literature procedures. The supporting elec-
trolyte, tertiary-butyl ammonium perchlorate (TBAP) was dried
in vacuum before use.
2.3.2. 4-Chloro-2-[(2,6-diisopropyl-phenylimino)-methyl]-phenol
[Cl-DPMP]
1H NMR (CDCl3) 410 MHz: d ppm 13.12 (s, 1H), 8.27 (s, 1H), 7.12
(d, 1H), 7.22–7.49 (m, 5H), 2.98 (m, 2H), 1.19–1.23 (d, 12H).
13C NMR (CDCl3): d 165.33 (Azomethine), 159.64 (ArACAOH),
145.62 (ArACAAzomethineAN), 138.45 ArACAIsopropyl), 132.95
(Ar), 131.13 (Ar), 125.67 (ArACACl), 123.61 (Ar), 123.25 (Ar),
119.29 (ArACAAzomethineAC), 118.86 (Ar), 28.22 (Aliphatic
CH3), 23.58 (Aliphatic CH).
2.2. Physical measurements
The magnetic susceptibilities of the complexes in the solid state
were measured on a Gouy balance at room temperature using
Hg[Co(SCN)4] as calibrant. The IR spectra of the ligands and
2.3.3. 1-[(2,6-diisopropylphenylimino)methyl] naphthalen-2-ol
[Ph-DPMP]
X
Overall yield 75%. Anal. Calc. C23H25ON: C, 83.34, H, 7.60; N,
4.23. Found: C, 83.23%; H, 7.64%; N, 4.21%. d ppm: 13.11 (s, 1H),
8.30 (s, 1H), 7.34–7.44 (m, 2H), 7.18 (s, 3H), 7.05–7.08 (d, 1H),
6.94–6.99 (m, 2H), 1.16–1.19 (d, 12H).
EPh3
Y
N
O
Ru
CH
N
CH
Y
EPh3
OH
X
2.3.4. 4-Bromo-2-[(2,6-diisopropyl-phenylimino)-methyl]-phenol
[Br-DPMP]
Overall yield 70%. Anal. Calc. C19H22BrON: C, 63.35; H, 6.11; N,
3.89. Found: C, 63.25%; H, 6.25%; N, 3.99%. d ppm: 12.95 (s,1H),
8.28 (s, 1H), 7.03 (d, 1H), 7.23–7.41 (m, 5H), 2.95–3.02 (m, 2H),
1.20–1.23 (d, 12H).
(a)
(b)
E = P or As; Y = Br or Cl;
X = H, Br, Cl, I, Ph for Ruthenium (III) complexes
Fig. 1. The chemical structure of (a) substituted Schiff base ligand [X-DPMP] (b)
Ru(III) – Schiff base complexes.

K.K. Raja et al. / Journal of Molecular Structure 1060 (2014) 49–57
51
Fig. 2. Expanded 1H NMR spectrum of Schiff base ligand 2-[2,6-diisopropylphenylimino)-methyl]-phenol [DPMP].
2.3.5. 2-[(2,6-Diisopropyl-phenylimino)-methyl]-4-iodo-phenol
[I-DPMP]
the representative RuCl2(PPh3)2(Cl-DPMP) complex shows m/z at
1012 along with various fragments and isotopic peaks.
Overall yield 70%. Anal. Calc. C19H22ION: C, 56.03, H, 5.41; N,
3.44. Found: C, 56.23%; H, 5.60%; N, 3.63%. d ppm: 13.19 (s, 1H),
8.25 (s, 1H), 7.68–7.71 (m, 2H), 7.28 (s, 3H), 6.86–6.91 (d, 1H),
2.90–3.04 (m, 2H), 1.18–1.22 (d, 12H).
3.1. Spectral Characterization
All the Ru(III) complexes are uniformly paramagnetic with mag-
netic moments in the range of 1.78–2.15 B.M which corresponds to
one unpaired electron with non-interacting low-spin d5 configura-
tion at 298 K in an octahedral environment. These values are
consistent with the +3 oxidation state of the metal ions [28].
The electronic spectra of all the Ru(III) – Schiff base complexes
in CHCl3 solution showed two or four bands in the region 250–
400 nm. The assortment of intense bands exhibited by the com-
plexes in the region 250–360 nm are believed to be ligand centered
transitions. In addition, the other bands at 400 and 460 nm have
been assigned to charge transfer bands. This is in conformity with
the assignments made for other similar Ru (III) octahedral
complexes [29].
The IR spectrum of the free Schiff base ligands is shown in Fig. 3
and the spectral bands of the Schiff base ligand and the complexes
are listed in Table 2. The IR spectra of the free Schiff bases showed a
very strong absorption band around 1624 cmꢂ1 characteristic of
azomethine (˜C@N) group [30]. Coordination of the Schiff bases
to the ruthenium ion through the azomethine nitrogen atom is ex-
pected to reduce the electron density in the azomethine frequency.
2.4. Synthesis of Ru(III) complexes
All the complexes were prepared under anhydrous conditions
using the following general procedure. To a solution of [RuY3
(EPh3)3] (where Y = Br or Cl; E = P or As) (0.1 mmol) in benzene
(20 ml) was purged the nitrogen gas for 15 min. and the appropri-
ate Schiff base ligand (0.1 mmol) in benzene was added. The
solution was refluxed for 4–5 h and the resulting solution was con-
centrated to 3 ml 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 recrys-
tallised from 1:1 benzene–petroleum ether mixture and dried
under vacuum.
2.5. Typical procedure for alcohol oxidation
The reaction mixture of Primary or secondary alcohol (1 mmol),
RuIIICl2 (AsPh3)2 (Ph-DPMP) Schiff base complex (0.1 mmol), NMO
(1.1 mmol) and dichloromethane (2 mL) was stirred at room tem-
perature. The filtrate obtained was evaporated under reduced pres-
sure and the residual mass was dissolved in a mixture of ethyl
acetate/hexane (1:4) and then passed through a short column of
silica gel using hexane/ethyl acetate (4:1) as eluent. Removal of
solvent and usual workup gave the corresponding aldehydes and
ketones, which were identified by comparing their physical and
spectral data with those of authentic compounds reported in liter-
ature [27].
The band due to azomethine nitrogen m(C@N) showed a modest de-
crease in the stretching frequency for the complexes and is shifted
to lower frequencies, appearing around 1618–1524 cmꢂ1 after
complexation [bathochromic shift], indicating the coordination of
the azomethine nitrogen [31–33]. The free ligands exhibit a broad
band at 3444–3435 cmꢂ1, which may be assigned to the m(OH) and
this band is absent in the spectra of all the complexes. This fact is
further supported by an upward shift in the stretching frequency of
phenolic oxygen in the complex m
(C@O) = 1320–1276 cmꢂ1 [32,33],
indicating the subsequent deprotonation of the phenolic proton
prior to the coordination [34]. This is further substantiated by
the absence of a 1H NMR signal for the OH group around d
11.3 ppm for all the complexes [35]. The binding of the metal to
the ligand through azomethine nitrogen and oxygen atom is fur-
ther supported by the appearance of new band in 623–522 cmꢂ1
and 460–400 cmꢂ1 ranges due to t(MAN) and t(MAO) respectively,
in the spectra of all the complexes [36]. In addition, the Schiff base
complexes show strong vibrations band near 690, 745 and
1550 cmꢂ1, which are attributed to the triphenylphosphine or
arsine fragments [37].
3. Results and discussion
The Ruthenium(III) complexes having general formula
[RuY2(EPh3)2(X-DPMP)] (where Y = Br or Cl; E = P or As;
DPMP = 2-[(2,6-Diisopropyl-phenylimino)-methyl]-phenol, X = H,
Br, Cl, I and Ph) are paramagnetic and the metal to ligand ratio of
all the complexes were 1:1 in benzene. All the new Schiff base
ruthenium(III) complexes are highly coloured, stable in air, non-
hygroscopic in nature and highly soluble in common solvents such
as CH3CN, DMSO and CHCl3 but insoluble in water. The analytical
data listed in Table 1, are in good agreement with the calculated
values thus confirming the general molecular formula proposed
for all the complexes. Mass spectrometry chemical ionization of
The representative EPR spectra of newly synthesized ruthenium
complex in Liquid Nitrogen Temperature is shown in Fig. 4 and the
spectral data are listed in Table 3. All the Ru(III) – Schiff base

52
K.K. Raja et al. / Journal of Molecular Structure 1060 (2014) 49–57
Table 1
Colour, magnetic moment and absorption maxima of Ruthenium(III) complexes.
Complexes
Colour
Mol. wt.
Found (calculated) (%)
leff. (B.M.)
kmax. (cmꢂ1
)
C
H
N
M
1. RuBr2(AsPh3)2 (DPMP)
Brown
991.7
1.99
46,511
38,461
21,739
18,018
52.03
52.15
3.53
3.65
1.41
1.55
10.19
10.30
2. RuBr2(AsPh3)2 (Br-DPMP)
3. RuBr2(AsPh3)2 (Cl-DPMP)
Green
Black
1070.6
2.01
1.97
46,948
38,910
31,152
20,325
17,889
48.19
48.29
3.18
3.30
1.31
1.42
9.44
9.55
1026.15
46,511
36,231
20,576
18,018
50.29
50.39
3.31
3.20
1.36
1.48
9.85
9.72
4. RuBr2(AsPh3)2 (I-DPMP)
5. RuBr2(AsPh3)2 (Ph-DPMP)
Brown
Brown
1117.6
1041.7
2.02
2.01
46,728
37,313
20,242
18,115
46,082
38,461
20,325
17,953
46.17
46.29
3.04
3.24
1.25
1.36
9.04
9.20
54.14
54.26
3.55
3.65
1.34
1.44
9.70
9.82
6. RuCl2(PPh3)2 (DPMP)
Brown
Brown
976.9
1055
63.32
63.52
4.29
4.49
1.72
1.86
12.40
12.53
1.87
1.90
46,296
24,691
7. RuCl2(PPh3)2 (Br-DPMP)
45,662
36,900
27,932
25,188
57.67
57.77
3.79
3.89
1.56
1.75
11.29
11.39
8. RuCl2(PPh3)2 (Cl-DPMP)
9. RuCl2(PPh3)2 (I-DPMP)
Brown
Black
1011.5
1102
1.95
1.99
46,511
28,248
24,630
60.68
60.79
3.99
3.79
1.65
1.85
11.89
11.79
48,076
24,449
54.79
54.69
3.61
3.71
1.49
1.59
10.73
10.86
10. RuCl2(PPh3)2 (Ph-DPMP)
11. RuCl2(AsPh3)2 (DPMP)
Black
1026.9
902.8
65.13
65.26
4.27
4.38
1.62
1.73
11.67
11.79
2.00
2.03
47,169
24,630
Brown
45,045
31,545
25,000
21,645
57.16
57.39
3.77
3.86
1.55
1.45
11.19
11.09
12. RuCl2(AsPh3)2 (Br-DPMP)
13. RuBr2(PPh3)2 (Cl-DPMP)
Black
981.7
1.87
1.89
46,082
31,545
25,062
52.56
52.46
3.46
3.36
1.43
1.56
10.29
10.39
Brown
937.25
46,082
32,573
25,062
55.05
55.25
3.63
3.73
1.49
1.59
10.78
10.89
14. RuBr2(PPh3)2 (I-DPMP)
15. RuBr2(PPh3)2 (Ph-DPMP)
Black
1028.7
952.8
50.16
50.26
3.31
3.52
1.36
1.22
9.83
9.70
1.95
1.99
44,444
24,813
Brown
46,082
37,313
32,573
24,570
13,888
59.19
59.09
3.88
3.78
1.47
1.57
10.61
10.83
complexes are paramagnetic, exhibiting a low-spin d5 system, the
2T2g (octahedral) ground state [38]. All the complexes exhibit three
lines with different ‘g’ values, indicating the presence of magnetic
anisotropy with axial distortion. The average ‘g’ values lie in the
1.93–2.35 range, which indicate the rhombic distortion
[4,31,39,40]. These values are in the range that are obtained for
similar Ru(III) complexes [41].
rings is 76.17 (14)° and an intramolecular OAHꢁ ꢁ ꢁN hydrogen bond
with an S(6) graph-set motif is present. One methyl group is disor-
dered over two sets of sites with site occupancies of 0.66 (3) and
0.34 (3). A weak intermolecular CAHꢁ ꢁ ꢁ
p interaction is observed
in the crystal structure [42].
3.3. Electrochemical studies
3.2. X-ray crystallography
The redox properties of all the Ru(III) complexes were investi-
gated 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
The representative crystal structure of the ligand Br-DPMP is
shown in Fig. 5 and it has the dihedral angle between the benzene

K.K. Raja et al. / Journal of Molecular Structure 1060 (2014) 49–57
53
Fig. 3. IR spectrum of the Schiff base ligand DPMP.
voltammogram Ru(III) complexes are shown in Fig. 6. Cyclic Vol-
tammogram of the RuIIIY2(EPh3)2 (X-DPMP) complexes exhibit
the half wave potentials in ꢂ688 to ꢂ1036 mV range. The corre-
sponding separations between the reduction and oxidation peaks
Fig. 4. Representative ESR spectra of RuIIICl2(AsPh3)2(IDPMP) in LNT.
There is no much variation in the redox potentials due to the
replacement of triphenylphosphine by triphenylarsine [4,18]. The
redox behaviour of Ru(III) complexes at glassy carbon electrode
are in the 228–1143 mV range for scanning rates of 100 mV sꢂ1
.
The Ru(III)/Ru(II) reductions in every case suggest the irreversible
one electron transfer process with respect to the central metal
atom [31,43].
surface follows a diffusion-controlled process which is evident
p
from the plot of SR vs peak current (Fig. 7). A straight line was
RuIIIY2ðEPh3Þ ðCl-DPMPÞ þ eꢂ $ ½RuIIY2ðEPh3Þ ðCl-DPMPÞꢃ
obtained for lower scanning rates (25–200 mV sꢂ1).
2
2
Table 2
IR spectral data of ligands and their Ruthenium(III) complexes.
S.No.
Ligands and complexes
t
(OH)
t
(CH@N)
t
(CAO)
t
(MAN)/t(MAO)
Free ligand
1
2
3
4
5
DPMP
3444
3444
3444
3435
3444
1625
1624
1624
1623
1623
1271
1272
1277
1272
1172
–
–
–
–
–
Br-DPMP
Cl-DPMP
I-DPMP
Ph-DPMP
Ruthenium (III) complex
1
RuBr2(AsPh3)2
(DPMP)
RuBr2(AsPh3)2
(Br-DPMP)
RuBr2(AsPh3)2
(Cl-DPMP)
RuBr2(AsPh3)2
(I-DPMP)
RuBr2(AsPh3)2
(Ph-DPMP)
RuCl2(PPh3)2
(DPMP)
RuCl2(PPh3)2
(Br-DPMP)
RuCl2(PPh3)2
(Cl-DPMP)
RuCl2(PPh3)2
(I-DPMP)
RuCl2(PPh3)2
(Ph-DPMP)
RuCl2(AsPh3)2
(DPMP)
RuCl2(AsPh3)2
(Br-DPMP)
RuCl2(AsPh3)2
(Cl-DPMP)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1574
1572
1576
1572
1583
1524
1568
1577
1618
1574
1574
1572
1574
1572
1582
1311
1311
1334
1313
1309
1323
1373
1285
1335
1398
1309
1315
1311
1309
1375
567
474
600
475
565
474
544
474
563
474
536
466
534
467
532
468
522
461
521
474
617
474
542
474
623
474
621
473
565
474
2
3
4
5
6
7
8
9
10
11
12
13
14
15
RuCl2(AsPh3)2
(I-DPMP)
RuCl2(AsPh3)2
(Ph-DPMP)

54
K.K. Raja et al. / Journal of Molecular Structure 1060 (2014) 49–57
Table 3
ESR spectral data of ruthenium complexes.
S. No.
Ruthenium(III) complexes
g11
g12
g13
hgavei
1
2
3
4
5
6
7
RuBr2(AsPh3)2 (DPMP)
RuBr2(AsPh3)2 (Br-DPMP)
RuBr2(AsPh3)2 (Cl-DPMP)
RuBr2(AsPh3)2 (I-DPMP)
RuBr2(AsPh3)2 (Ph-DPMP)
RuCl2(AsPh3)2 (I-DPMP)
RuCl2(AsPh3)2 (Ph-DPMP)
2.8257
2.7957
2.8023
2.8051
2.7725
2.3959
2.3508
2.1841
2.2011
2.2083
2.1799
2.3452
2.2938
2.0235
1.9613
2.0102
2.0150
1.9120
1.9732
2.0629
1.6808
2.2317
2.3595
2.2442
2.2096
2.2636
2.1394
1.9342
Fig. 6. Cyclic voltamogram of RuBr2 (AsPh3)2 (Np-DPMP)complexes.
Fig. 5. Representative crystal structure of Br-DPMP.
Table 4
Electrochemical data of Ruthenium(III) complexes.
Eap (mV)
Ecp (mV)
S. No.
Ruthenium (III) complexes
D
Ep
E1/2
p
Fig. 7. Scan rate vs Peak current for RuCl2 (AsPh3)2 (Cl-DPMP) complex.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
RuBr2(AsPh3)2 (DPMP)
RuBr2(AsPh3)2 (Br-DPMP)
RuBr2(AsPh3)2 (Cl-DPMP)
RuBr2(AsPh3)2 (I-DPMP)
RuBr2(AsPh3)2 (Ph-DPMP)
RuCl2(PPh3)2 (DPMP)
RuCl2(PPh3)2 (Br-DPMP)
RuCl2(PPh3)2 (Cl-DPMP)
RuCl2(PPh3)2 (I-DPMP)
RuCl2(PPh3)2 (Ph-DPMP)
RuCl2(AsPh3)2 (DPMP)
RuCl2(AsPh3)2 (Br-DPMP)
RuCl2(AsPh3)2 (Cl-DPMP)
RuCl2(AsPh3)2 (I-DPMP)
RuCl2(AsPh3)2 (Ph-DPMP)
ꢂ548
ꢂ488
ꢂ555
ꢂ566
ꢂ546
ꢂ487
ꢂ592
ꢂ383
ꢂ494
ꢂ537
ꢂ520
ꢂ478
ꢂ396
ꢂ494
ꢂ500
ꢂ991
ꢂ1191
ꢂ978
443
703
423
470
487
781
228
953
528
301
1032
908
1143
535
843
ꢂ770
ꢂ840
ꢂ767
ꢂ801
ꢂ790
ꢂ878
ꢂ706
ꢂ860
ꢂ758
ꢂ688
ꢂ1036
ꢂ932
ꢂ968
ꢂ762
ꢂ922
ꢂ1036
ꢂ1033
ꢂ1268
ꢂ820
Table 5
Oxidation of benzyl alcohol (1 mmol) under various conditions by using RuCl2
(AsPh3)2(Ph-DPMP)/NMO system in CH2Cl2.
Yield (%)a
ꢂ1336
ꢂ1022
ꢂ838
Entry
Catalyst (mol%)
Oxidant
Solvent
Time (min)
1
2
3
4
5
5
NMO
NMO
NMO
NaIO4
NMO
CH2Cl2
CH2Cl2
CH3CN
CH2Cl2
CH2Cl2
75
45
120
90
80
87
70
78
10
10
10
–
ꢂ1552
ꢂ1386
ꢂ1539
ꢂ1029
ꢂ1343
120
Trace
a
Isolated yield.
Table 6
3.4. Oxidation of alcohols by Ru(III) Schiff base complex
Oxidation of sec-phenylethanol under various conditions by using RuCl2(AsPh3)2
(Ph-DPMP)/NMO system in CH2Cl2.
RuCl2(AsPh3)2(Ph-DPMP) was tested with or without additive
for the oxidative process of primary and secondary alcohols. Re-
sults of Tables 5 and 6 indicate that the yield of isolated product
increases with the increase of catalytic load (entry 1 and 2). Fur-
ther, the nature of the terminal oxidant plays an important role
in the oxidation process (entry 2 and 4). The solvent system also
affects the reaction time and yields (entry 2 and 3). The observed
results show that NMO is the best oxidant when compare with
Entry
Catalyst (mol%)
Oxidant
Solvent
Time (min)
Yield (%)a
1
2
3
4
5
5
NMO
NMO
NMO
PPNO
NMO
CH2Cl2
CH2Cl2
CH3CN
CH2Cl2
CH2Cl2
120
70
150
110
180
65
82
60
70
10
10
10
–
Trace
a
Isolated yield.

K.K. Raja et al. / Journal of Molecular Structure 1060 (2014) 49–57
55
Table 7
Oxidation of 1° and 2° alcohol to corresponding carbonyl compounds
OH
O
RuCl2(AsPh3)2(PhDPMP)
/ NMO
R2
R1
R2
R1
CH2Cl2(2 ml), r.t.
.
Entry
1
Substrate
Product
Time (min.)
70
Yield (%)a
82
OH
O
2
3
4
5
6
70
65
75
75
75
75
80
78
75
70
OH
O
Me
MeO
Br
Me
OH
O
MeO
OH
OH
O
Br
O
Cl
Cl
OH
O
7
8
85
80
85
85
45
65
75
68
73
87
OH
OH
OH
OH
O
O
O
O
9
10
11
OH
O
(continued on next page)

56
K.K. Raja et al. / Journal of Molecular Structure 1060 (2014) 49–57
Table 7 (continued)
Entry
12
Substrate
Product
Time (min.)
45
Yield (%)a
85
OH
O
H3CO
O2N
H3CO
O2N
13
60
80
OH
O
14
15
60
55
75
75
OH
OH
O
O
a
Isolated yield.
Scheme 1. Proposed scheme for the oxidation of 1° and 2° alcohol to corresponding carbonylcompounds.
NaIO4. Control experiments were carried out without ruthenium
catalyst under the same reaction conditions and there is no detect-
able oxidation of alcohols (entry 5). An efficient oxidative condition
is described with entry 2.
Investigations of oxidation of various phenols under optimized
condition are given in Table 7. Unsubstituted and substituted sec-
phenylethanols (entries 1–6) were efficiently oxidized to corre-
sponding ketones with excellent yield. The change of methyl group
of 1-phenylethanol into larger groups prolongs the reaction time
(entries 7–10).
Simple benzyl alcohols are smoothly converted into their corre-
sponding aldehydes (entry 11, 12 and 13). Allylic alcohols could be
vulnerable to oxidation through the formation of carbonyl com-
pound or epoxide. In the present system allylic alcohols (entry
10 and 11) undergo oxidation to give carbonyl compounds in good
yield for relatively shorter reaction period. The formed product has
been confirmed through NMR spectral study.
4. Conclusions
Ruthenium (III) complexes of multisubstituted Schiff base li-
gands have been prepared and characterized. The magnetic and
spectral evidences show that the complexes are formed through
the coordination of phenolic oxygen and the azomethine nitrogen
atoms. Newly synthesized complexes are found to be octahedral.
The complexes are redox active and display either quasi revers-
ible or irreversible redox couples based on metal centre. In com-
parison, the ruthenium(III) complexes were found to be effective
catalyst for the oxidation of alcohols under mild conditions.
Further, the yield of carbonyl compounds obtained is significantly
higher than those obtained from similar reported catalytic
system.
Acknowledgements
In comparison, the results of the present investigation suggest
that the ruthenium(III) Schiff base complexes are able to react effi-
ciently with NMO to yield a high valent ruthenium species [19,44].
Further, the catalytic oxidation could occur through hydrogen
extraction. The yield of carbonyl compounds obtained is signifi-
cantly higher than those obtained from similar reported Ru(III)-
PPh3/NMO catalytic system [44]. On the basis of previous oxidation
studies [6,19,44], the following mechanism has been proposed as
indicated in Scheme 1.
The authors thank The Dean, Madras Medical College, Rajiv-
gandhi Government General Hospital, Chennai – 3, The Man-
agement, Correspondent and the Principal of BSA Crescent
Engineering College, Chennai 600048, The Principal and the
Staff members of Saveetha University, Chennai and the staff
members, Department of Chemistry, Government Arts College,
Nandanam, Chennai – 35, for providing research facilities and
support.

K.K. Raja et al. / Journal of Molecular Structure 1060 (2014) 49–57
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