Spectral, redox and catalytic studies of triphenylphosphine/triphenylarsine complexes of Ru(III) with N, O donor ligands derived from 2-hydroxy-1-naphthaldehyde and primary amines

Article abstract of DOI:10.1016/j.saa.2005.08.018

A series of new mixed ligand hexacoordinated ruthenium(III) Schiff base complexes of the type [RuX₂(EPh₃)₂(LL′)] (X = Cl, E = P; X = Cl or Br, E = As and LL′ = anion of the Schiff bases derived from the condensation of 2-hydroxy-1-naphthaldehyde with aniline, 4-chloroaniline, 2-methyl aniline and 4-methoxy aniline) are reported. All the complexes have been characterized by analytical and spectral (IR, electronic and EPR) data. The redox behavior of the complexes has also been studied. The complexes exhibit catalytic activity in the oxidation of benzyl alcohol to benzaldehyde in the presence of N-methyl morpholine-N-oxide (NMO). An octahedral structure has been proposed for all of the complexes.

Full text of DOI:10.1016/j.saa.2005.08.018

Spectrochimica Acta Part A 64 (2006) 886–890
Spectral, redox and catalytic studies of triphenylphosphine/triphenylarsine
complexes of Ru(III) with N, O donor ligands derived from
2-hydroxy-1-naphthaldehyde and primary amines
V. Mahalingama, R. Karvembub, V. Chinnusamyc, K. Natarajana,∗
a Department of Chemistry, Bharathiar University, Coimbatore 641046, India
b Department of Science and Humanities, Sri Krishna College of Engineering and Technology, Coimbatore 641008, India
c Department of Chemistry, Ramakrishna Mission Vidyalaya College of Arts & Science, Coimbatore 641020, India
Received 8 February 2005; received in revised form 29 August 2005; accepted 29 August 2005
Abstract
A series of new mixed ligand hexacoordinated ruthenium(III) Schiff base complexes of the type [RuX2(EPh3)2(LLꢀ)] (X = Cl, E = P; X = Cl
or Br, E = As and LLꢀ = anion of the Schiff bases derived from the condensation of 2-hydroxy-1-naphthaldehyde with aniline, 4-chloroaniline,
2-methyl aniline and 4-methoxy aniline) are reported. All the complexes have been characterized by analytical and spectral (IR, electronic and
EPR) data. The redox behavior of the complexes has also been studied. The complexes exhibit catalytic activity in the oxidation of benzyl alcohol
to benzaldehyde in the presence of N-methyl morpholine-N-oxide (NMO). An octahedral structure has been proposed for all of the complexes.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Ruthenium(III); Spectroscopic characterization; Cyclic voltammetry; Catalytic oxidation
1. Introduction
gations on the reactions of ruthenium(III) complexes with Schiff
base ligands, we intended to study the effect of introducing an
additional phenyl ring in the aldehyde part of the Schiff base
in terms of catalytic activity and electrochemistry. Thus, the
present paper deals with the synthesis and spectral characteriza-
tion of some new ruthenium(III) Schiff base complexes obtained
from the reactions of [RuCl3(PPh3)3], [RuCl3(AsPh3)3] and
[RuBr3(AsPh3)3] with Schiff bases derived from 2-hydroxy-
1-naphthaldehyde and primary amines such as aniline, 4-
chloroaniline, 2-methylaniline and 4-methoxyaniline. The gen-
eral structure of the Schiff base ligands used in this study is
depicted below.
The oxidation of primary and secondary alcohols into their
corresponding aldehydes and ketones, respectively, plays a cen-
tral role in organic synthesis [1]. Traditionally, such transfor-
mations have been performed with stoichiometric quantities of
inorganic oxidants, such as chromium (VI) compounds [2]. The
quest for effective catalytic systems that use clean, inexpen-
sive primary oxidants, such as N-methyl morpholine-N-oxide,
molecular oxygen or hydrogen peroxide, i.e. a ‘green method’
for converting alcohols to carbonyl compounds on an indus-
trial scale remains an important challenge [3]. Ruthenium(III)
complexes containing triphenylphosphine or triphenyl arsine
have been extensively investigated as catalyst [4] for alco-
hol oxidations using a variety of primary oxidants, e.g. N-
methyl morpholine-N-oxide [5], tert-butyl hydroperoxide [6],
iodosobenzene [7], hypochlorite [8], bromate [9] or a combina-
tion of oxygen and an aldehyde [10].
The synthesis and characterization of several hexacoordi-
nated ruthenium(III) complexes containing Schiff bases derived
from salicylaldehyde and primary amines have been reported by
our research group earlier [11]. As part of our systematic investi-
∗
Corresponding author. Tel.: +91 4222424655; fax: +91 4222422387.
E-mail address: k natraj6@yahoo.com (K. Natarajan).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.08.018

V. Mahalingam et al. / Spectrochimica Acta Part A 64 (2006) 886–890
887
2. Experimental
2.3.2. Catalytic oxidation [17]
To a solution of benzyl alcohol (1 mmol) in CH2Cl2 (20 cm3),
N-methyl-morpholine-N-oxide (3 mmol) and ruthenium com-
plex (0.01 mmol) were added. The solution was stirred at room
temperature for 3 h. The mixture was evaporated to dryness
and extracted with petroleum ether (bp 60–80 ◦C) (3 × 10 cm3).
The combined petroleum ether mixture was filtered and evap-
orated to give benzaldehyde, which was then quantified as its
2,4-dinitrophenyl hydrazone derivative.
2.1. Reagents and materials
All the chemicals used are of analytical reagent grade
or chemically pure grade. RuCl3·3H2O purchased from
Himedia was used without further purification. Solvents
were purified following standard procedures. The starting
complexes [RuCl3(PPh3)3] [12], [RuCl3(AsPh3)3] [13] and
[RuBr3(AsPh3)3] [14] and the Schiff bases [15,16] were pre-
pared according to literature procedures. The supporting elec-
trolyte, tertiary-butyl ammonium perchlorate (TBAP) was dried
in vacuum before use.
3. Results and discussion
Reactions of the various Schiff bases with [RuX3(EPh3)3]
(X = Cl, E = P; X = Cl or Br, E = As) yielded complexes of the
general formula [RuX2(EPh3)2(LLꢀ)] (X = Cl, E = P; X = Cl or
Br, E = As and LLꢀ = anion of the Schiff bases derived from
the condensation of 2-hydroxy-1-napthaldehyde with aniline,
4-chloroaniline, 2-methylaniline and 4-methoxyaniline). The
complexes are green in colour, non-hygroscopic and are solu-
ble in common organic solvents such as CH2Cl2, DMSO and
DMF. In all the reactions, it has been found that the Schiff
bases behave as uninegative bidentate ligands replacing a triph-
enylphosphine/arsine and a halide ligand from the starting com-
plexes.
2.2. Physical measurements
Infrared spectra were recorded as KBr discs on a Nicolet
Avatar Model FT-IR spectrophotometer in the 4000–400 cm−1
range at room temperature. Electronic spectra of the complexes
were recorded in CH2Cl2 solution with a Hitachi U-3210 spec-
trophotometer in the 800–200 nm range. Room temperature
EPR spectra of powdered samples were recorded on a E-112
Varian model instrument in X-band frequencies using DPPH
The analytical data obtained for the new ruthenium(III) com-
plexes are given in Table 1 and agree very well with the proposed
molecular formula. Magnetic measurements show that all the
complexes are paramagnetic and confirm that ruthenium is in
+3 oxidation state.
as internal standard. Magnetic moments were measured on a
EG & G-PARC vibrating sample magnetometer at 25 ◦C. Cyclic
voltammograms were recorded on a BAS CV-50 electrochemi-
cal analyzer in CH3CN using a glassy-carbon working electrode
and the potentials were referenced to a standard calomel elec-
trode. All microanalyses (C, H and N) were performed using a
Vario EL III Elementar analyzer. Melting points were recorded
with a Raaga apparatus and are uncorrected.
3.1. Spectroscopic studies
The characteristic IR frequencies of the new ruthenium(III)
complexes and their assignments are given in Table 2. In the
IR spectra of the free Schiff bases, a band of medium intensity
appears around 3400–3300 cm−1 due to ν(O–H). This band dis-
appears on complexation with the metal ion, which indicates
the deprotonation of the ligands prior to coordination through
oxygen atom [18]. A strong band around 1628–1618 cm−1 due
to azomethine group of the ligands undergoes a shift to lower
frequency after complexation indicating the coordination of
azomethine nitrogen to ruthenium [19]. A high intensity band
at 1303–1296 cm−1 in the free ligands may be assigned to the
phenolic C–O stretching [20]. In the complexes, C–O stretch-
2.3. Recommended procedures
2.3.1. Synthesis of new ruthenium(III) complexes
All the new complexes were prepared by the follow-
ing general procedure. The Schiff base (0.020–0.028 g,
0.079–0.100 mmol) was added to a solution of [RuX3(EPh3)3]
(X = Cl, E = P; X = Cl or Br, E = As) (0.1 g, 0.079–0.100 mmol)
in benzene (20 cm3). The metal to ligand ratio was 1:1. The
mixture was heated under reflux for 5–6 h. A green colour solu-
tion was obtained, which was concentrated to about 3 cm3.
The complex was separated by the addition of 5 cm3 of
petroleum ether (bp 60–80 ◦C). It was then recystallised from
CH2Cl2/petroleum ether (1:5) and dried in vacuum (yield
60–75%).
ing vibrations are shifted to higher frequency by 20–50 cm−1
.
This shift to higher frequency indicates the bonding of pheno-
lic oxygen to ruthenium [21]. Further, a medium intensity band
which appeared around 530–470 cm−1 assignable for ν(Ru–O)

888
V. Mahalingam et al. / Spectrochimica Acta Part A 64 (2006) 886–890
Table 1
Analytical data and mp s of new Ru(III) complexes
Complexa
mp (◦C)
Calculated (found) %
C
H
N
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
[RuCl2(PPh3)2(NA)]
158
153
130
159
173
144
142
147
135
143
141
125
67.52 (67.50)
65.14 (65.21)
67.78 (67.72)
66.67 (66.63)
61.76 (61.72)
59.76 (59.62)
62.08 (62.03)
61.14 (61.06)
56.86 (56.83)
55.16 (54.43)
57.21 (57.18)
56.42 (56.22)
4.49 (4.59)
4.23 (4.19)
4.24 (4.21)
4.56 (4.48)
4.11 (4.11)
3.88 (3.64)
4.25 (4.23)
4.18 (4.04)
3.78 (3.76)
3.58 (4.08)
3.91 (3.87)
3.86 (3.42)
1.49 (1.41)
1.43 (1.54)
1.46 (1.43)
1.44 (1.43)
1.36 (1.31)
1.31 (1.22)
1.34 (1.31)
1.32 (1.28)
1.25 (1.22)
1.21 (1.63)
1.24 (1.22)
1.22 (1.18)
[RuCl2(PPh3)2(NCA)]
[RuCl2(PPh3)2(NMA)]
[RuCl2(PPh3)2(NMOA)]
[RuCl2(AsPh3)2(NA)]
[RuCl2(AsPh3)2(NCA)]
[RuCl2(AsPh3)2(NMA)]
[RuCl2(AsPh3)2(NMOA)]
[RuBr2(AsPh3)2(NA)]
[RuBr2(AsPh3)2(NCA)]
[RuBr2(AsPh3)2(NMA)]
[RuBr2(AsPh3)2(NMOA)]
(10)
(11)
(12)
a
Green.
Table 2
IR and electronic spectra of the Ru(III) complexes
Complex
ν(C N), ν(C–O) (Phenolic) (cm−1
)
PPh3 or AsPh3 bands (cm−1
)
λmax (nm)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
1614, 1354
1604, 1320
1616, 1376
1597, 1338
1616, 1316
1597, 1330
1614, 1330
1615, 1336
1614, 1335
1623, 1336
1614, 1335
1597, 1335
1426, 1092, 694
1439, 1072, 692
1434, 1092, 693
1437, 1094, 694
1435, 1094, 692
1435, 1095, 692
1435, 1072, 692
1435, 1095, 692
1435, 1075, 691
1435, 1087, 692
1435, 1083, 691
1436, 1085, 691
305, 386, 506
300, 360, 506
380, 530
400, 530
305, 340, 403, 495
305, 344, 381, 485
305, 338, 390, 488
270, 305, 486
306, 330, 440, 525
302, 337, 440, 528
302, 340, 416, 456, 524
304, 344, 388, 480, 525
(10)
(11)
(12)
corresponding to single unpaired electron in a low-spin 4d5 con-
figuration and confirms that ruthenium is in +3 oxidation state
in all the complexes [29,30].
also supports oxygen coordination [22]. These facts prove the
uninegative bidentate nature of the Schiff base ligands. The other
characteristic bands due to triphenylphosphine or triphenylar-
sine (around 1440, 1100 and 700 cm−1) are also present in the
spectra of all the complexes [23].
The solid state EPR spectra of the powdered samples were
recorded at room temperature in X-band frequencies using
DPPH as internal standard. The spectra showed no indication of
any hyperfine splitting. All the complexes exhibit spectra with
g in the range 2.52–2.59 and g in the range 2.38–2.47 (Table 3;
The UV–vis spectra of all the complexes in CH2Cl2 show
two to five bands in the region 270–530 nm and their assign-
ments are given in Table 2. The ground state of ruthenium(III)
⊥
||
(t5 configuration) is 2T2g, while the first excited doublet lev-
2g
els in the order of increasing energy are 2A2g and 2T1g, which
4
1
Table 3
arise from t2g eg configuration [24]. In most of the ruthe-
nium(III) complexes, the electronic spectra show only charge
transfer bands [11,25]. Since in a d5 system, and especially in
ruthenium(III) that has relatively high oxidising properties, the
charge transfer bands of the type L␲y → t2g are prominent in
the low energy region obscuring the weaker bands due to d–d
transition. Therefore, it becomes difficult to assign conclusively
the bands of ruthenium(III) complexes appearing in the visible
region. Hence, all the bands that appear in this region have been
assigned to charge transfer transitions, which are in conformity
with the assignments made for similar ruthenium(III) complexes
[26–28].
EPR spectra data of the Ru(III) complexes
Complex
gx
gy
gz
<g>*
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
2.53
2.52
2.53
2.52
2.56
2.56
2.56
2.56
2.54
2.57
2.55
2.59
2.53
2.52
2.53
2.52
2.56
2.56
2.56
2.56
2.54
2.57
2.55
2.59
2.42
2.41
2.42
2.43
2.47
2.40
2.47
2.38
2.48
2.47
2.47
2.41
2.49
2.48
2.49
2.49
2.53
2.50
2.53
2.50
2.52
2.53
2.52
2.53
(10)
(11)
(12)
The effective magnetic moments (µeff) of some of the com-
plexes were measured at room temperature using vibrating sam-
ple magnetometer. These values range from 1.92 to 2.02 BM
2
1/2
<g>* = [1/3 gx2 + 1/3 gy2 + 1/3 gz
]
.

V. Mahalingam et al. / Spectrochimica Acta Part A 64 (2006) 886–890
889
Table 4
Cyclic voltammetric dataa for some Ru(III) complexes
Complex
Ru(IV)–Ru(III)
Ru(III)–Ru(II)
Epa (V)
Epc (V)
Ef (V)
ꢀEp (mV)
Epa (V)
Epc (V)
Ef (V)
ꢀEp (mV)
(2)
(3)
(6)
(7)
(8)
(9)
1.06
1.13
1.46
1.23
0.78
1.30
–
–
1.26
–
–
–
–
1.36
–
–
–
–
200
–
–
−0.97
−0.89
−0.97
−0.96
−1.00
−0.93
−0.73
−0.67
−0.74
−0.74
−0.74
−0.63
−0.85
−0.78
−0.85
−0.85
−0.87
−0.78
240
220
230
220
260
300
0.92
1.11
380
a
Supporting electrolyte: [NBu4]ClO4 (0.05 M); concentration of the complex: 0.001 M; scan rate: 100 mV s−1; all the potentials were referenced to s.c.e.
Fig. 1). The presence of two different ‘g’ values indicates a
tetragonal distortion in an octahedral field, where gx = gy = gz
[31,32].
3.2. Redox properties
The redox behavior of some of the complexes was studied
by cyclic voltammetry in acetonitrile containing 0.05 M
[NBu4]ClO4. The electrochemical data are compiled in Table 4.
Since the ligands used in this study are not reversibly oxidized
or reduced in the applied potential range, it is assumed
that all redox processes are metal centered only. The cyclic
voltammograms (Fig. 2) of the complexes exhibit a reversible
oxidation peak at a scan rate of 100 mVs−1. The oxidation and
reduction of each complex are characterized by well-defined
waves with Ef values in the range +1.11 to +1.36 V and −0.78
Fig. 2. Cyclic voltammogram of [RuCl2(AsPh3)2(NCA)].
to −0.87 V, respectively. The peak to peak separation values
(ꢀEp) for the redox processes [Ru(IV)–Ru(III)/Ru(III)–Ru(II)]
range from 200 to 380 mV, suggestive of a quasi-reversible
single step one electron transfer process [33,27]. The
complexes [RuCl2(PPh3)2(NCA)], [RuCl2(PPh3)2(NMA)],
[RuCl2(AsPh3)(NMA)] and [RuCl2(AsPh3)2(NMOA)] showed
an irreversible oxidation peak, which may be ascribed as due to
the short-lived oxidized state of the metal ion. Further, a close
examination on the data reveals that the redox potentials do
not vary much due to replacement of triphenylphosphine by
triphenylarsine.
The Ef (oxidation) values of the complexes [11] containing
one phenyl ring in the aldehyde part of the Schiff base ligands
range from 0.56 to 0.79 V and Ef (reduction) ranges from −0.22
to −0.31 V [11]. When these values are compared with that of
new complexes, it has been observed that with the addition of
another phenyl ring in the ligand system, there is a positive shift
in the Ef values (both oxidation and reduction). This can be
explained by the fact that the additional phenyl ring, by its elec-
tron withdrawing nature decreases the electron density around
the metal centre [34].
3.3. Catalytic oxidation
A few of the new ruthenium(III) complexes were tested for
their catalytic activity in the oxidation of benzyl alcohol using
NMO as oxidant (Table 5, Entry 2–5). Benzyl alcohol was
converted to benzaldehyde. The product was quantified as its
2,4-dinitrophenyl hydrazone derivative [35]. In no case, there
was any detectable yield of oxidized product in the absence of
catalyst. From the comparative yield, it is evident that triph-
enylphosphine complexes possess more catalytic activity than
triphenylarsine complexes because Ru–P bond is more labile
than Ru–As bond. It has already been established that the cat-
alytic cycle passes through a RuV O species [36]. The char-
Fig. 1. EPR spectrum of [RuCl2(AsPh3)2(NMOA)].

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V. Mahalingam et al. / Spectrochimica Acta Part A 64 (2006) 886–890
Table 5
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Entry
1
Complex
Yielda of benzaldehyde (%)
39.86
Turnoverb
109.54
[RuCl2(PPh3)2
(sal-p-toluidine)]
2
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4
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(1)
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(7)
(12)
65.18
42.61
46.73
40.34
179.10
117.08
128.39
110.86
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Some new ruthenium(III) complexes have been synthesized
using the Schiff bases formed by condensing some derivatives
of aniline with 2-hydroxy-1-napthaldehyde. The new complexes
have been characterized by analytical, spectral (IR, electronic
and EPR) and magnetic studies. The redox behavior of the com-
plexes has been studied by cyclic voltammetry. An octahedral
structure has been tentatively proposed for all the complexes.
The catalytic activity of some of the complexes (in the oxida-
tion of benzaldehyde) has also been evaluated.