Ruthenium(II) carbonyl complexes containing pyridine carboxamide ligands and PPh3/AsPh3/Py coligands: Synthesis, spectral characterization, catalytic and antioxidant studies

Article abstract of DOI:10.1016/j.saa.2012.10.072

New ruthenium(II) carbonyl complexes bearing pyridine carboxamide and triphenylphosphine/triphenylarsine/pyridine have been prepared by direct reaction of ruthenium(II) precursors with some pyridine carboxamide ligands, N,N-bis(2-pyridinecarboxamide)-1,2-ethane (H₂L¹), N,N-bis(2-pyridinecarboxamide)-1,2-benzene (H₂L²) and N,N-bis(2-pyridinecarboxamide)-trans-1,2-cyclohexane (H₂L ³). The organic ligands offering two N_amide and two N _pyridine donor sites to the metal centre. They have been characterized by elemental analyses, FT-IR, UV-Visible, NMR (¹H, ¹³C and ³¹P) and ESI-MS techniques. Based on the above data, an octahedral structure has been assigned for all the complexes. The catalytic efficiency of the complexes in transfer hydrogenation of ketones in the presence of iPrOH/KOH and N-alkylation of amine in the presence of tBuOK was examined. Furthermore, the antioxidant activity of the ligands and its ruthenium(II) complexes were determined by DPPH radical, nitric oxide radical, hydroxyl radical and hydrogen peroxide scavenging methods, which indicates that the ruthenium(II) complexes exhibit more effective antioxidant activity than the ligands alone.

Full text of DOI:10.1016/j.saa.2012.10.072

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 53–61
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Ruthenium(II) carbonyl complexes containing pyridine carboxamide ligands and
PPh3/AsPh3/Py coligands: Synthesis, spectral characterization, catalytic and
antioxidant studies
⇑
Rangasamy Ramachandran, Periasamy Viswanathamurthi
Department of Chemistry, Periyar University, Salem 636 011, 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
Ruthenium(II) pyridine carboxamide complexes were synthesized and characterized. They have been
assigned an octahedral structure. The new complexes were found to be efficient catalyst for transfer
hydrogenation of ketones and N-alkylation of amine. The complexes also exhibited excellent antioxidant
(DPPH radical, NO radical OH radical and H2O2 scavengers) activities.
"
"
"
"
Ruthenium(II) pyridine carboxamide
complexes were synthesized and
characterized.
They have proved to be an efficient
catalyst for the transfer
hydrogenation of ketones.
The catalytic efficiency of the
complexes in N-alkylation of amine
was examined.
Ruthenium(II) complexes also
exhibit good antioxidant activity.
R
H
NH2
N
OH ruthenium(II) catalyst
tBuOK, 110°C, 6 h
+
R
(R = H or OCH3)
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 August 2012
Received in revised form 25 October 2012
Accepted 30 October 2012
Available online 7 November 2012
New ruthenium(II) carbonyl complexes bearing pyridine carboxamide and triphenylphosphine/triphen-
ylarsine/pyridine have been prepared by direct reaction of ruthenium(II) precursors with some pyridine
carboxamide ligands, N,N-bis(2-pyridinecarboxamide)-1,2-ethane (H2L1), N,N-bis(2-pyridinecarboxa-
mide)-1,2-benzene (H2L2) and N,N-bis(2-pyridinecarboxamide)-trans-1,2-cyclohexane (H2L3). The
organic ligands offering two Namide and two Npyridine donor sites to the metal centre. They have been char-
acterized by elemental analyses, FT-IR, UV–Visible, NMR (1H, 13C and 31P) and ESI-MS techniques. Based
on the above data, an octahedral structure has been assigned for all the complexes. The catalytic effi-
ciency of the complexes in transfer hydrogenation of ketones in the presence of iPrOH/KOH and N-alkyl-
ation of amine in the presence of tBuOK was examined. Furthermore, the antioxidant activity of the
ligands and its ruthenium(II) complexes were determined by DPPH radical, nitric oxide radical, hydroxyl
radical and hydrogen peroxide scavenging methods, which indicates that the ruthenium(II) complexes
exhibit more effective antioxidant activity than the ligands alone.
Keywords:
Pyridine carboxamide ligands
Ruthenium(II) complexes
Catalytic transfer hydrogenation
N-alkylation
Antioxidant activities
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
binding [6] and the preparation of platinum(II) complexes with anti-
tumor properties [7]. One factor which makes the ligands attractive
In the last few decades pyridine carboxamide ligands have been
the subject of intense research in the field of co-ordination chemis-
try, because of their extraordinary properties they have found access
to a great variety of catalytic and biological process, which include
asymmetric allylic alkylation [1], epoxidation [2], cyclopropana-
tions [3], norbornene polymerization [4], hydroxylation [5], DNA-
for catalytic applications is the simplicity whereby the structure of
the ligands can be modified by a modular approach. The electronic
and steric properties can conveniently be modified by altering the
diamine backbone by the introduction of suitable substituent in
the pyridine nuclei [8]. Since the pyridine nitrogen atom resembles
imidazole nitrogen, metal complexes of bispyridylamides have also
been employed as models for metalloenzymes [9].
The transfer hydrogenation of ketones represents an useful
means of producing value-added alcohols under relatively benign
⇑
Corresponding author. Fax: +91 427 2345124.
E-mail address: viswanathamurthi@rediffmail.com (P. Viswanathamurthi).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.10.072

54
R. Ramachandran, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 53–61
conditions, and many effective systems involving phosphorus- and
nitrogen-ligated ruthenium complexes have been developed for
this purpose [10]. The N-alkylation of amines is an important
transformation for the preparation of pharmaceuticals, agrochem-
icals, polymers, dyes, and other fine chemicals [11]. Traditionally,
the alkylation of amines is achieved using conventional alkylating
agents, such as alkyl halides. Many alkyl halides have toxic or even
mutagenic properties and an alternative to using such reagents is
therefore advantageous [12]. In recent years, a number of reports
on the hydroamination [13] and hydroamino-methylation of ole-
fins or alkynes [14] for the synthesis of amines have been reported.
Compared to the frequently applied N-alkylations with alkyl ha-
lides and reductive aminations methods, an economically and
environmentally attractive method is the N-alkylation of amines
using primary and secondary alcohols. Since the early 1980, transi-
tion metal complexes [15], in particular ruthenium complexes
have been shown to be active for this transformation. Recently,
Williams, Beller and co-workers [16] were able to obtain good
yields in the N-alkylation of indoles by using 0.2–0.5 mol% of the
dimeric Shvo’s ruthenium catalyst (0.4–1 mol% Ru) at 110 °C. Ma-
tute also reported that small loadings of ruthenium pincer complex
(1 mol% Ru) afford excellent results in the selective alkylation of
(hetero) aromatic amines with alcohols [17].
have been recorded in dichloromethane using a Shimadzu UV-
1650 PC spectrophotometer in 800–200 nm range. 1H, 13C and
31P NMR spectra were recorded in Jeol GSX-400 instrument using
DMSO as the solvent. 1H and 13C NMR spectra were obtained at
room temperature using TMS as the internal standard. 31P NMR
spectra of the complexes were obtained at room temperature using
o- phosphoric acid as a reference. The ESI–MS spectra were re-
corded by using LC–MS Q–ToF Micro-Analyzer (Shimadzu) in the
SAIF, Panjab University, Chandigarh. Melting points were recorded
on a Technico micro-heating table and are uncorrected. The cata-
lytic yields were determined using ACME 6000 series GC-FID with
DP-5 column of 30 m length, 0.53 mm diameter and 5.00
thickness.
lm film
Synthesis of new ruthenium(II) pyridine carboxamide complexes (1–9)
All the new mononuclear ruthenium(II) complexes were pre-
pared by the following general procedure. An ethanolic solution
of [RuHCl(CO)(EPh3)2(B)] (E = P or As; B = PPh3, AsPh3 or Py) and
the appropriate ligand in 1:1 mol ratio was heated under reflux
for 4–6 h. The solution was filtered while hot, reduced to half of
its volume and left for slow evaporation. The resulting solid ob-
tained was filtered, washed with diethyl ether and dried in vacuo.
The purity of the complexes was checked by TLC.
Oxidation is highly important for many living organisms to pro-
duce their metabolic energy by using biological processes. How-
ever, oxygen-centred free radicals and other reactive oxygen
species, that are continuously produced in cell result death and tis-
sue damage. Atherosclerosis, ageing, cancer, diabetes mellitus,
inflammation, AIDS, etc. may be related to oxidative damage [18].
So, ascorbic acid and tocopherols or superoxide dismutase and cat-
alase are well known antioxidant compounds or enzymes, respec-
tively, defend the organisms against free radical damage [19]. In
the conditions that these defense systems may not be sufficient
to prevent the damage, antioxidant supplements or foods contain-
ing antioxidants may be used to help the human body to reduce oxi-
dative damage [20]. Transition metals are known to participate in
reversible redox reactions. Several metal complexes, especially,
ruthenium complexes have been investigated extensively [21]. In
connection with our ongoing interest in this field of research, we
have already investigated several ruthenium(II) complexes [22].
Herein, we describe the synthesis and characterization of a series
of new class of ruthenium(II) pyridine carboxamide complexes
along with their catalytic activity towards the transfer hydrogena-
tion of ketones and N-alkylation of amine with alcohols in the pres-
ence of tBuOK. Further, the ligands and their ruthenium(II)
complexes were also tested for antioxidant activity.
[Ru(CO)(PPh3)(L1)] (1)
Brown solid, yield: 85%, m.p. 201 °C, elemental analysis calcd.
(%) for C33H27N4PO3Ru: C, 60.09; H, 4.13; N, 8.49. Found: C,
60.18; H, 4.18; N, 8.40. IR (KBr Pellets, cmÀ1): 1941 (s, C„O),
1624 (s, C@O), 1347 (m, CAN), 653 (m, C@N). UV–Visible (CH2Cl2):
kmax (nm): 219, 244, 347, 493. 1H NMR (CDCl3): (d ppm): 9.18 (d,
2H, H-1), 8.37 (t, 2H, H-3), 8.18 (d, 2H, H-4), 7.94 (t, 2H, H-2),
7.85–7.19 (m, 15H, H-PPh3), 4.04, 3.93 (m, 4H, H-6 and H-7). 13C
NMR (CDCl3): (d ppm): 204.73 (C„O), 166.51 (C@O), 157.70 (C-
5), 152.47 (C-1), 141.41 (C-3), 137.23–128.39 (CAPPh3), 127.74
(C-2), 125.58 (C-4), 51.67 (C-6 and C-7). 31P NMR (DMSO): (d
ppm): 28.15 (s, PPh3), ESI–MS (m/z): 659 (M+).
[Ru(CO)(PPh3)(L2)] (2)
Green solid, yield: 74%, m.p. 198 °C, elemental analysis calcd. (%)
for C37H27N4PO3Ru: C, 62.80; H, 3.85; N, 7.92. Found: C, 62.75; H,
3.84; N, 7.40. IR (KBr Pellets, cmÀ1): 1937 (s, C„O), 1637 (s,
C@O), 1340 (m, CAN), 651 (w, C@N). UV–Visible (CH2Cl2): kmax
(nm): 237, 261, 322, 412. 1H NMR (CDCl3): (d ppm): 9.23 (d, 2H,
H-1), 8.54 (m, 2H, H-7), 8.40 (t, 2H, H-3), 8.20 (d, 2H, H-4), 7.94
(t, 2H, H-2) 7.68–7.19 (m, 15H, HAPPh3), 7.09 (m, 2H, H-8). 13C
NMR (CDCl3): (d ppm): 204.13 (C„O), 163.24 (C@O), 158.23 (C-
5), 152.46 (C-1), 143.69 (C-6), 141.87 (C-3), 137.67–128.45
(CAPPh3), 128.07 (C-2), 126.43 (C-4), 123.65 (C-8), 121.20 (C-7).
31P NMR (DMSO): (d ppm): 28.75 (s, PPh3). ESI–MS (m/z): 707 (M +).
Experimental
Materials and reagents
[Ru(CO)(PPh3)(L3)] (3)
Brown solid, yield: 54%, m.p. 272 °C, elemental analysis calcd.
(%) for C37H33N4PO3Ru: C, 62.26; H, 4.66; N, 7.85. Found: C,
62.52; H, 4.60; N, 7.98. IR (KBr Pellets, cmÀ1): 1941 (s, C„O),
Commercially available RuCl3Á3H2O (Loba) was used without
further purification. Solvent were purified and dried according to
standard procedure. The pyridine carboxamide ligands [23] (H2L1,
H2L2 and H2L3) and the starting complexes [RuHCl(CO)(PPh3)3]
[24], [RuHCl(CO)(AsPh3)3] [25] and [RuHCl(CO)(Py)(PPh3)2] [26]
were prepared as reported earlier. The structures of the ligands
are given Fig. 1.
N
N
N
N
Physical measurements
N
N
O
O
O
NH HN
O
NH
HN
Elemental analyses (C, H, N) were performed on Vario EL III
Elemental analyzer at SAIF-Cochin, India. IR spectra (4000–
400 cmÀ1) for KBr pellets were recorded on a Nicolet Avatar model
FT–IR spectrophotometer. The electronic spectra of the complexes
O
O
NH
HN
Fig. 1. Structure of pyridine carboxamide ligands.

R. Ramachandran, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 53–61
55
1627 (s, C@O), 1354 (s, CAN), 653 (w, C@N). UV–Visible (CH2Cl2):
kmax (nm): 233, 247, 371, 457. 1H NMR (CDCl3): (d ppm): 9.27 (d,
2H, H-1), 8.29 (t, 2H, H-3), 8.09 (d, 2H, H-4), 7.94 (t, 2H, H-2),
7.73–7.25 (m, 15H, H-PPh3), 4.26 (m, 2H, H-6), 2.40, 1.83 (m, 8H,
H-7 and H-8). 13C NMR (CDCl3): (d ppm): 204.42 (C„O), 164.14
(C@O), 157.81 (C-5), 152.31 (C-1), 141.82 (C-3), 137.67–128.58
(CAPPh3), 127.63 (C-2), 125.31 (C-4), 52.32 (C-6), 32.68, 24.85
(C-7 and C-8). 31P NMR (DMSO): (d ppm): 28.51 (s, PPh3).
(d, 2H, H-1), 8.64 (d, 2H, HAPy), 8.53 (m, 2H, H-7), 8.39 (t, 2H, H-3),
8.21 (d, 2H, H-4), 7.96 (t, 2H, H-2) 7.82 (t, 2H, HAPy), 7.61 (t, 2H,
HAPy), 7.02 (m, 2H, H-8). 13C NMR (CDCl3): (d ppm): 204.16
(C„O), 163.25 (C@O), 158.17 (C-5), 152.49 (C-1), 152.33 (CAPy),
143.76 (C-6) 141.83 (C-3), 139.21 (CAPy), 128.01 (C-2), 126.41
(C-4), 126.41 (C-4), 125.91 (CAPy), 123.69 (C-8), 121.28 (C-7).
[Ru(CO)(Py)(L3)] (9)
Brown solid, yield: 48%, m.p. 205 °C, elemental analysis calcd.
(%) for C24H23N5O3Ru: C, 54.33; H, 4.37; N, 13.20. Found: C,
54.52; H, 4.60; N, 13.98. IR (KBr Pellets, cmÀ1): 1938 (s, C„O),
1619 (s, C@O), 1341 (s, CAN), 652 (w, C@N). UV–Visible (CH2Cl2):
kmax (nm): 233, 247, 350, 457. 1H NMR (CDCl3): (d ppm): 9.24 (d,
2H, H-1), 8.63 (d, 2H, HAPy), 8.24 (t, 2H, H-3), 8.07 (d, 2H, H-4),
7.96 (t, 2H, H-2,2), 7.81 (t, 2H, HAPy), 7.65 (t, 2H, HAPy), 4.44
(m, 2H, H-6), 2.43, 1.92 (m, 8H, H-7 and H-8). 13C NMR (CDCl3):
(d ppm): 204.51 (C„O), 164.01 (C@O), 157.80 (C-5), 152.37 (C-
1), 152.24 (CAPy), 141.80 (C-3), 139.27 (CAPy), 127.61 (C-2),
125.29 (C-4), 125.09 (CAPy), 54.01 (C-6), 32.71, 24.83 (C-7 and
C-8).
[Ru(CO)(AsPh3)(L1)] (4)
Pale brown solid, yield: 78%, mp. 243 °C, elemental analysis
calcd. (%) for C33H27N4AsO3Ru: C, 56.33; H, 3.87; N, 7.96. Found:
C, 56.53; H, 4.05; N, 7.64. IR (KBr Pellets, cmÀ1): 1941 (s, C„O),
1628 (s, C@O), 1352 (m, CAN), 654 (m, C@N). UV–Visible (CH2Cl2):
kmax (nm): 237, 244, 383, 430. 1H NMR (CDCl3): (d ppm): 9.24 (d,
2H, H-1), 8.35 (t, 2H, H-3), 8.07 (d, 2H, H-4), 7.90 (t, 2H, H-2),
7.88–7.17 (m, 15H, HAAsPh3), 4.05, 3.90 (m, 4H, H-6 and H-7).
13C NMR (CDCl3): (d ppm): 205.01 (C„O), 166.54 (C@O), 157.72
(C-5), 152.41 (C-1), 141.44 (C-3), 137.82–127.87 (CAAsPh3),
127.70 (C-2), 125.51 (C-4), 51.63 (C-6 and C-7).
[Ru(CO)(AsPh3)(L2)] (5)
General procedure for the ruthenium-catalyzed transfer
hydrogenation of ketones
Pale brown solid, yield: 69%, m.p. 248 °C, elemental analysis
calcd. (%) for C37H27N4AsO3Ru: C, 59.12; H, 3.62; N, 7.45. Found:
C, 59.75; H, 3.64; N, 7.48. IR (KBr Pellets, cmÀ1): 1945 (s, C„O),
1625 (s, C@O), 1342 (m, CAN), 652 (w, C@N). UV–Visible (CH2Cl2):
kmax (nm): 207, 245, 352, 447. 1H NMR (CDCl3): (d ppm): 9.26 (d,
2H, H-1), 8.57 (m, 2H, H-7), 8.38 (t, 2H, H-3), 8.19 (d, 2H, H-4),
7.93 (t, 2H, H-2) 7.85–7.20 (m, 15H, HAAsPh3), 7.06 (m, 2H, H-8).
13C NMR (CDCl3): (d ppm): 204.17 (C„O), 163.27 (C@O), 158.27
(C-5), 152.45 (C-1), 143.71 (C-6), 141.88 (C-3), 137.68–128.47
(CAAsPh3), 128.09 (C-2), 126.45 (C-4), 123.63 (C-8), 121.22 (C-7).
ESI–MS (m/z): 751 (M+).
The catalytic transfer hydrogenation reactions were conducted
at a substrate/catalyst/base(S/C/base) molar ratio of 1:0.005:4.
The procedure was described as follows. A mixture containing ke-
tone (1 mmol), the ruthenium complex (1–9) (0.005 mmol) and
KOH (4 mmol) in 10 ml of iPrOH was heated to reflux for 2 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 chromatogra-
phy. The filtrate was subjected to GC analysis and the hydroge-
nated product was identified and determined with authentic
samples.
[Ru(CO)(AsPh3)(L3)] (6)
Brown solid, yield: 45%, m.p. 258 °C, elemental analysis calcd.
(%) for C37H33N4AsO3Ru: C, 58.65; H, 4.39; N, 7.39. Found: C,
58.92; H, 4.65; N, 7.98. IR (KBr Pellets, cmÀ1): 1940 (s, C„O),
1625 (s, C@O), 1354 (s, CAN), 653 (m, C@N). UV–Visible (CH2Cl2):
kmax (nm): 217, 222, 237, 382, 443. 1H NMR (CDCl3): (d ppm): 9.29
(d, 2H, H-1), 8.28 (t, 2H, H-3), 8.23 (d, 2H, H-4), 7.90 (t, 2H, H-2),
7.83–7.16 (m, 15H, HAAsPh3), 4.21 (m, 2H, H-6), 2.46, 1.87 (m,
8H, H-7 and H-8). 13C NMR (CDCl3): (d ppm): 204.47 (C„O),
164.11 (C@O), 157.79 (C-5), 152.37 (C-1), 141.78 (C-3), 137.41–
128.51 (CAAsPh3), 127.61 (C-2), 125.37 (C-4), 53.37 (C-6), 32.72,
24.84 (C-7 and C-8).
General procedure for the ruthenium-catalyzed N-alkylation of amine
A mixture of amine (0.3 mmol), ruthenium(II) complex (1, 2, 4
and 5) (3 Â 10–3 mmol), tBuOK (0.12 mmol) in benzyl alcohol or
p-methoxybenzyl alcohol (0.9 mmol) was placed in a flask under
atmospheric pressure of nitrogen and heated by an oil bath at
110 °C for 6 h. Then brine (3 ml) and CH2Cl2 (5 ml) were added.
The organic layer was separated and the aqueous layer was ex-
tracted into CH2Cl2. The combined organic extracts were dried with
magnesium sulfate and concentrated. The compounds were puri-
fied by chromatography and characterized by NMR spectroscopy.
The product yields were obtained by the 1H NMR integration com-
pared to the internal standard.
[Ru(CO)(Py)(L1)](7)
Brown solid, yield: 54%, m.p. 185 °C, elemental analysis calcd.
(%) for C20H17N5O3Ru: C, 50.42; H, 3.60; N, 14.70. Found: C,
50.58; H, 3.80; N, 14.72. IR (KBr Pellets, cmÀ1): 1941 (s, C„O),
1624 (s, C@O), 1347 (s, CAN), 649 (w, C@N). UV–Visible (CH2Cl2):
kmax (nm): 202, 221, 242, 382, 495. 1H NMR (CDCl3): (d ppm): 9.25
(d, 2H, H-1), 8.60 (d, 2H, Py), 8.38 (t, 2H, H-3), 8.06 (d, 2H, H-4),
7.93 (d, 2H, H-2), 7.84 (t, 2H, HAPy), 7.65 (t, 2H, HAPy), 4.03–
3.97 (4H, H-6 and H-7). 13C NMR (CDCl3): (d ppm): 205.12
(C„O), 166.57 (C@O), 157.71 (C-5), 152.47 (C-1), 151.27 (CAPy),
141.43 (C-1), 138.82 (CAPy), 127.77 (C-2), 125.94 (CAPy), 125.53
(C-4), 51.68 (C-6 and C-7).
N-phenylbenzylamine
1H NMR (300 MHz, CDCl3): d (ppm): 7.34–7.01 (m, 5H, phenyl),
7.11 (m, 2H, phenyl), 6.72 (t, 1H, phenyl), 6.58 (d, 2H, phenyl), 4.31
(s, 2H, ACH2NHA), 3.92 (br, 1H, ANHA).
N-phenyl-(p-methoxybenzyl) amine
1H NMR (300 MHz, CDCl3): d (ppm): 7.26 (d, 2H, phenyl), 7.17
(m, 2H, phenyl), 6.89 (d, 2H, phenyl), 6.65 (d, 2H, phenyl), 6.21
(d, 1H, phenyl), 4.21 (s, 2H, ACH2NHA), 3.95 (br, 1H, ANHA),
3.83 (s, 3H, AOMe).
[Ru(CO)(Py)(L2)] (8)
Green solid, yield: 64%, m.p. 196 °C, elemental analysis calcd.
(%) for C24H17N5O3Ru: C, 55.96; H, 3.27; N, 13.35. Found: C,
55.98; H, 3.84; N, 13.40. IR (KBr Pellets, cmÀ1): 1941 (s, C„O),
1618 (s, C@O), 1338 (s, CAN), 652 (m, C@N). UV–Visible (CH2Cl2):
kmax (nm): 202, 211, 242, 320, 434. 1H NMR (CDCl3): (d ppm): 9.34
Antioxidant assays
The ability of ruthenium complexes to act as hydrogen donors
or free radical scavengers was explored by conducting a series of

56
R. Ramachandran, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 53–61
in vitro antioxidant assays involving DPPH radical, nitric oxide
radical, hydroxyl radical, hydrogen peroxide and comparing the re-
sults with standard antioxidants, including the natural antioxidant
vitamin C and the synthetic antioxidant BHT.
Absorbance of hydrogen peroxide at 230 nm was determined after
10 min against the blank (phosphate buffer).
For the above four assays, all the tests were run in various con-
centrations of the compounds were used to fix a concentration at
which compounds showed in and around 50% of activity. In addi-
tion, the percentage of activity was calculated using the formula,
% of activity = [(A0–AC)/A0] Â 100 (A0 and AC are the absorbance in
the absence and presence of the tested complex respectively).
The 50% of activity (IC50) can be calculated using the percentage
of activity results.
DPPHÅ scavenging assay
The DPPH radical scavenging activity of the compounds was
measured according to the method of Blios [27]. The DPPH radical
is a stable free radical. Because of the odd electron, DPPH shows a
strong absorption band at 517 nm in the visible spectrum. As this
electron becomes paired off in the presence of a free radical scav-
enger, the absorption vanishes and the resulting decolorization is
stoichiometric with respect to the number of electrons taken up.
Various concentrations of the experimental complexes were taken
Results and discussion
Diamagnetic, hexa-coordinated low spin ruthenium(II) pyridine
carboxamide complexes of general formula [Ru(CO)(B)(L)]
(B = PPh3, AsPh3 or Py; L = pyridine carboxamide 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 pyr-
idine carboxamide ligands in ethanol in 1:1 M ratio. In all these
reactions, it was observed that the pyridine carboxamide behave
as dianionic tetradentate ligands by replacing two molecules of tri-
phenylphosphine or triphenylarsine, one molecule of hydride and
one moles of chlorine from the starting complexes.
The analytical data (C, H, N) of all the pyridine carboxamide li-
gands and their ruthenium(II) complexes are in good agreement
with the calculated values, thus confirming the proposed mononu-
clear composition for all the complexes. The complexes were ob-
tained in powder form. All the ligands and their complexes are
stable at room temperature, non-hygroscopic and insoluble in
water and soluble in dichloromethane, chloroform, benzene, aceto-
nitrile, ethanol, methanol, dimethylformamide and dimethylsulf-
oxide. Various attempts have been made to obtain the single
crystals of the complexes but it has been unsuccessful. In order
to confirm the composition of the new complexes, ESI–MS spectra
were recorded in positive mode. The molecular ion peaks (M+) ob-
served at m/z = 659, 707 and 751 (complexes 1, 2 and 5) supporting
the molecular formulae. In addition, few representative fragment
peaks were seen in the spectra of the three complexes. These frag-
ments indicated the coordination of pyridine carboxamide ligands
to the ruthenium metal atom.
and the volume was adjusted to 100 ll with methanol. About 5 ml
of a 0.1 mM methanolic solution of DPPH was added to the aliquots
of samples and standards (BHT and vitamin C) and shaken vigor-
ously. A negative control was prepared by adding 100 ml of meth-
anol in 5 ml of 0.1 mM methanolic solution DPPH. The tubes were
allowed to stand for 20 min at 27 °C. The absorbance of the sample
was measured at 517 nm against the blank (methanol).
NOÅ scavenging assay
The assay of nitric oxide (NOÅ) scavenging activity is based on a
method [28] where sodium nitroprusside in aqueous solution at
physiological pH spontaneously generates nitric oxide, which
interacts with oxygen to produce nitrite ions. These can be esti-
mated using the Greiss reagent. Scavengers of nitric oxide compete
with oxygen leading to reduced production of nitrite ions. For the
experiment, sodium nitroprusside (10 mM) in phosphate buffered
saline was mixed with a fixed concentration of the complex and
standards and incubated at room temperature for 150 min. After
the incubation period, 0.5 ml of Griess reagent containing 1% sulfa-
nilamide, 2% H3PO4 and 0.1% N-(1-naphthyl) ethylenediamine
dihydrochloride was added. The absorbance of the chromophore
formed was measured at 546 nm.
OHÅ scavenging assay
The hydroxyl radical scavenging activity of the complex has
been investigated by using the Nash method [29]. In vitro hydroxyl
radicals were generated by an Fe3+/ascorbic acid system. The
detection of hydroxyl radicals was carried out by measuring the
amount of formaldehyde formed from the oxidation reaction with
DMSO. The formaldehyde produced was detected spectrophoto-
metrically at 412 nm. A mixture of 1.0 ml of iron-EDTA solution
(0.13% ferrous ammonium sulfate and 0.26% EDTA), 0.5 ml of EDTA
solution (0.018%), and 1.0 ml of DMSO (0.85% DMSO (v/v) in 0.1 M
phosphate buffer, pH 7.4) were sequentially added in the test
tubes. The reaction was initiated by adding 0.5 ml of ascorbic acid
(0.22%) and was incubated at 80–90 °C for 15 min in a water bath.
After incubation, the reaction was terminated by the addition of
1.0 ml of ice-cold trichloroacetic acid (17.5% w/v). Subsequently,
3.0 ml of Nash reagent was added to each tube and left at room
temperature for 15 min. The intensity of the color formed was
measured spectrophotometrically at 412 nm against reagent blank.
Infrared spectroscopic analysis
The FT–IR data of the complexes are given in the experimental
section. Meaningful information regarding the bonding sites of the
ligand molecules can be obtained by comparing the IR spectra of
ruthenium(II) complexes with the uncomplexed ligands. The IR
spectra of the ligands exhibit a band at 3316–3330 cmÀ1 due to
NH group. The absence of tN-H in the IR spectra of the complexes
confirms that the ligands are coordinated in their deprotonated
form [31]. A further indication of the formation of deprotonated
complexes is the rather large decrease (red shifted) in the carbonyl
(amide I) stretching frequency exhibited by the ligands upon com-
plex formation. A strong band at 1653–1678 cmÀ1 in the free li-
gands is assigned to C@O stretching. This band shifted to 1618–
1637 cmÀ1 in complexes, which is in accordance with the data re-
ported for the related complexes [32]. The medium bands at 1518–
1536 cmÀ1 and 1250–1278 cmÀ1 in the free ligands are assigned to
amide II and amide III modes. The amide II and amide III bands,
combination of tCAN and dNAH modes, are replaced by a medium
to strong band at 1338–1354 cmÀ1. This replacement is to be ex-
pected, as the removal of an amide proton produces a pure CAN
stretch [33]. The band at 618–624 cmÀ1 in the free ligands, attrib-
utable to the in-plane deformation of the pyridine ring, shifts to
higher frequency at 649–654 cmÀ1 in its ruthenium complexes,
H2O2 scavenging assay
The ability of the complexes to scavenge hydrogen peroxide
was determined using the method of Ruch et al. [30]. A solution
of hydrogen peroxide (2.0 mM) was prepared in phosphate buffer
(0.2 M, pH 7.4) and its concentration was determined spectropho-
tometrically from absorption at 230 nm with molar absorptivity
81 MÀ1 cmÀ1. The complexes (100
(100
l
g mlÀ1), BHT and vitamin C
l
g mlÀ1) were added to 3.4 ml of phosphate buffer together
with hydrogen peroxide solution (0.6 ml). An identical reaction
mixture without the sample was taken as negative control.

R. Ramachandran, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 53–61
57
thus indicating ligation of the pyridine nitrogen atoms [34] of the
ligands to ruthenium. Further the strong absorption around
1945–1937 cmÀ1 have been assigned to the terminally coordinated
carbonyl group in the new ruthenium complexes [35]. In the case
of complexes (7, 8 and 9) containing coordinated heterocyclic
nitrogen bases, a medium intensity band was observed in the re-
gion 1026–1031 cmÀ1. In addition, the other characteristic bands
due to triphenylphosphine and triphenylarsine (around 700,
1090 and 1440 cmÀ1) were also present in the spectra of all the
complexes [36].
multiplets at 7.16–7.73 ppm in the spectra of the complexes 1–6
are assigned to PPh3/AsPh3 protons.
13C NMR spectroscopic analysis
13C NMR spectra of complexes were recorded in CDCl3, and their
assignments are given in the experimental section. A peak at
204.13–205.12 ppm region is due to C„O carbon. The signals at
163.24–166.57 ppm and 157.71–158.27 ppm in the spectrum of
the complexes are assigned to the carbonyl carbon (C@O) and the
pyridyl carbon C-5, respectively [41]. The most significant down-
field shifts correspond to the carbons (C-1), ortho to the pyridyl
nitrogen atom, exhibit its peak in the region of d 152.31–
152.49 ppm. The peaks at 141.41–141.87 ppm are assigned to C-3
carbon. The appearance of a peak at 127.61–128.09 ppm region in
the spectrum arises due to C-2 carbon. The carbons C-4 exhibit its
peak in the region of 125.58–126.45 ppm. The signals at 51.63–
51.68 (complexes 1, 4 and 7) ppm are assigned to C-6 and C-7
respectively. The spectrum of 2, 5 and 8 showed three peaks around
143.71–143.87, 123.63–123.69, 121.20–121.28 ppm corresponding
to the C-6, C-7 and C-8 carbon respectively. The complexes 3, 6 and
9 exhibited three signals around 24.83–24.85 ppm, 32.68–32.72
and 53.37–54.01 ppm corresponding to the C-6, C-7 and C-8 car-
bons. The multiplets appear around 127.87–137.23 ppm region
are assigned to PPh3/AsPh3 carbons. The data confirm the formation
of new ruthenium(II) pyridine carboxamide complexes.
Electronic spectroscopic analysis
The electronic spectra of all the new complexes have been re-
corded in CH2Cl2 solution. The spectral data are given in the exper-
imental section. The band around 412–495 nm range has been
assigned to the spin allowed 1A1g ? 1T1g transition. The other high
intensity bands around 320–383 nm have been assigned to charge
transfer transitions arising from the metal t2g level to the unfilled
molecular orbitals derived from the pà level of the ligands [37]
based on their extinction coefficient values. The bands below
300 nm were characterized by intra-ligand charge transfer. The
electronic spectra are similar to those observed for other octahe-
dral ruthenium(II) complexes [38].
1H NMR spectroscopic analysis
31P NMR spectroscopic analysis
The 1H NMR spectra of the ligands and the corresponding ruthe-
nium(II) pyridine carboxamide complexes were recorded in CDCl3 to
confirm the presence of coordinated ligand in the complexes. The
spectral data and their assignments are given in the experimental
section. The spectra of the free ligands showed a signal at 8.6–
10.7 ppm characteristics of amide [AC(O)NHA] proton [39], which
is absent in the complexes, suggesting that the coordination is
through deprotonated amide nitrogen. As expected, the most signif-
icant downfield shifts correspond to the protons (H-1), ortho to the
pyridyl nitrogen atom, exhibit its peak in the region of 9.18–
9.34 ppm [40]. The triplets at 8.24–8.40 ppm for all complexes are
assigned to the H-3 protons of ligands, which is shifting to the down-
field compared with the free ligands. The signal for H-4 protons was
observed as a doublet at 8.06–8.23 ppm region and H-2 protons
were appeared as a triplet at 7.90–7.96 ppm for all the complexes.
In the spectra of the complexes 2, 5 and 8, two multiplets were ob-
served around 8.53–8.57 and 7.02–7.09 ppm due to the H-7 and
H-8 protons of the ligands. The spectra of the complexes 1, 4 and
7, two multiplets were observed around 3.90–3.97 and 4.03–
4.05 ppm due to the H-6 and H-7 protons respectively. The spectrum
of the complexes 3, 6 and 9 exhibited three signals around 1.83–
1.92 ppm, 2.40–2.46 ppm and 4.21–4.44 ppm, corresponding to
the H-6, H-7 and H-8 protons. The spectra (complexes 7, 8 and 9)
showed two triplets and a doublet for pyridine protons that ap-
peared in the range 7.61–7.65, 7.81–7.84 and 8.60–8.64 ppm. The
31P NMR spectra of some of the complexes were recorded to
confirm the presence of triphenylphosphine group in the com-
plexes. A sharp singlet was observed around 28.15–28.75 ppm
due to presence of triphenylphosphine ligand in the complexes.
On the basis of analytical and spectral IR, electronic, 1H, 13C, 31
P
NMR and ESI-MS data, an octahedral structure (Fig. 2) has been
tentatively proposed for all the new ruthenium(II) pyridine carbox-
amide complexes.
Catalytic transfer hydrogenation (TH) of ketones
TH reaction in which hydrogen is transferred from one organic
molecule to another is of great importance in organic synthesis
since one can avoid the use of molecular hydrogen [42]. The ruthe-
nium(II) complexes (1–9) catalyzed the reduction of ketones to the
corresponding alcohols via hydrogen transfer from iPrOH with
KOH as the promoter. The reaction was conducted at a substrate,
catalyst and base in molar ratio 1:0.005:4 respectively. Complex
5 was selected as a model catalyst for optimization of the reaction
conditions. In order to study the effect of time on the activity, the
product analysis was done at regular intervals of time under sim-
ilar reaction conditions (Fig. 3). In order to optimize the reaction
conditions, different substrate/catalyst/base (S/C/base) ratios were
tested and the results are summarized in Table 1. When increasing
2
2
3
4
1
N
3
4
1
B
B
N
2
N
N
5
3
4
1
5
Ru
B
Ru
N
N
O
O
O
O
N
N
N
N
CO
5
CO
Ru
6
6
7
O
O
N
N
CO
7
8
8
6
7
(B= PPh3, AsPh3 or Py)
Fig. 2. Structure of new ruthenium(II) pyridine carboxamide complexes.

58
R. Ramachandran, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 53–61
conversion of the reaction decreased dramatically by decreasing
the base quantity (Entry 6). In the absence of a base no transfer
hydrogenation of the ketones was observed (Entry 7).
100
90
80
70
60
50
40
30
A variety of ketones were transformed into the corresponding
secondary alcohols using the complexes (1–9) as the catalyst. Typ-
ical results are shown in Table 2. The most efficient conversions are
found in the case of acetophenone with all catalysts (up to 99%),
while in the case of aliphatic secondary ketone the conversions
were up to 84%. The catalysts efficiently catalyzed the reduction
of benzophenone into benzhydrol up to 84–97% conversions. Inter-
estingly, the catalysts show excellent activity for the conversions of
five- and six-membered cyclic ketones to their corresponding alco-
hols with 67–95% conversions in the case of cyclopentanone and
87–98% conversions in case of cyclohexanone. The catalytic effi-
ciency varies in the order of AC6H4A > ACH2CH2A > AC6H10A,
which may be tailored by a bulky group present on diamine back-
bone of the pyridine carboxamide ligand. The complex catalysts 2
and 5 are the most efficient catalyst among all, because of the pres-
ence of the bulky pyridine carboxamide moiety (AC6H4A), which
appears to lead to an improvement in activity. It is further inferred
from the results that the triphenylphosphine, triphenylarsine, pyr-
dine ligands may also influence the catalytic efficiency by their
A
B
C
0.5
1.0
1.5
2.0
2.5
3.0
Time (h)
Fig. 3. Catalytic transfer hydrogenation of: (A) Acetophenone, (B) benzophenone,
(C) cyclohexanone in different time intervals.
Table 1
electron donor–acceptor nature. The presence of
a catalytic
Catalytic transfer hydrogenation of acetophenone by complex (5).
amount of base is necessary for the transfer hydrogenation of ke-
tones. Acetone was identified as only byproduct in all the cases.
As the catalyst is stable in all organic solvents and it can be recov-
ered and the work up process is also very simple for this catalytic
system. The catalytic activity of the present complex is low when
compared to ruthenium complexes containing Schiff base or pincer
ligands [43,44]. However, the catalytic efficiency of the complex is
better than the other ruthenium complexes in the transfer hydro-
genation of ketones with respect to substrate/catalyst ratio
(1:0.005) [45,46].
Entry
Substrate/catalyst/base ratios
Time (h)
Conversion (%)a
1
2
3
4
5
6
7
1
1
1
1
1
1
1
0.1
4
4
4
4
4
2
–
2
2
2
2
4
2
2
64
78
88
99
0.05
0.01
0.005
–
0.005
0.005
Not detected
80
Not detected
a
Conversion of the product determined by GC.
the S/C/base ratio to 1:0.01:4, 1:0.05:4, 1:0.1:4 in (Entry 1, 2 and 3)
iPrOH, the reaction still proceeds with reasonable conversions.
Thus, it was concluded that S/C/base ratio of 1:0.005:4 is the best
compromise between optimum reaction rates (Entry 4). A blank
experiment carried out in the absence of the catalyst gave no
hydrogenation of acetophenone at all (Entry 5). As expected, the
Catalytic N-alkylation of amine
The ruthenium(II) complexes 1, 2, 4 and 5 catalyzed the N-alkyl-
ation of amine with alcohols in the presence of tBuOK (scheme 1). In
a typical experiment, a mixture of aniline, benzyl alcohol or p-
methoxybenzyl alcohol, tBuOK and ruthenium(II) complex
Table 2
Catalytic transfer hydrogenation of ketones using complexes (1–9) as catalyst.a
Substrate
Product
Conversion (%)b
1
2
3
4
5
6
7
8
9
95
98
94
97
99
96
91
92
89
O
OH
OH
94
96
95
97
87
93
95
97
97
98
92
91
90
88
91
95
84
87
O
Ph
Ph
O
O
OH
OH
92
74
93
78
87
72
92
79
95
84
88
73
69
63
72
64
67
61
O
OH
a
Reaction conditions:1.0 mmol substrate, catalyst (0.005 mmol), base (4 mmol) in iPrOH (10 ml) reflux at 80 °C for 2 h.
Conversion of the product determined by GC.
b

R. Ramachandran, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 53–61
59
R
H
NH2
N
OH ruthenium(II) catalyst
BuOK, 110 °C, 6 h
+
t
R
(R = H or OCH3)
Scheme 1. N-alkylation of amine.
Table 3
N-alkylation of amine catalyzed by ruthenium(II) complexes (1, 2, 4 and 5).a
Complex
1
Amine
Alcohol
Product
Yield (%)b
81
NH2
NH2
NH2
NH2
OH
OH
OH
OH
H
N
62
OCH3
OCH3
OCH3
OCH3
OH
OH
OH
OH
H
N
H3CO
2
4
5
86
65
H
N
H
N
H3CO
H3CO
H3CO
71
63
H
N
H
N
88
71
H
N
H
N
a
Reaction conditions: amine (0.3 mmol), catalyst (3 Â 10À3 mmol), tBuOK (0.12 mmol) in benzyl alcohol or p-methoxybenzyl alcohol (0.9 mmol) reflux at 110 °C for 6 h.
b
Yield based on NMR integration.
(1:3:0.4:0.01) was placed in a flask. Excess benzyl alcohol or p-
methoxybenzyl alcohol was used as the solvent, and the mixture
was heated to 110 °C for 6 h. The desired amine product was ex-
tracted from the reaction mixture and analyzed by 1H NMR spec-
troscopy. Product yields were obtained by the 1H NMR integration
compared to the internal standard. As shown in the Table 3, benzyl
alcohols reacted smoothly to give the corresponding products in
good yields (71–88%) whereas, p-methoxybenzyl alcohol gave the
corresponding product in moderate yield (up to 71%). As can be
seen in Table 3, complex 5 is a significantly better catalyst than
other complexes. Comparison with protocols previously developed
for this reaction [47], the catalyst loadings are very low and the
reaction conditions are very mild in the present catalytic system.
the chain of reactions initiated by excess generation of radicals that
is detrimental to human health. Among all free radicals, the hydro-
xyl radical is by far the most potent and therefore the most danger-
ous oxygen metabolite, elimination of this radical is one of the
major aims of antioxidant administration [48]. Hydrogen peroxide
itself is not very reactive, but sometimes it is toxic to cells because
it may give rise to hydroxyl radicals in the cells [49]. According to
relevant reports in the literature [50,51], some transition metal
complexes may exhibit antioxidant activity. We therefore con-
ducted an investigation to explore whether the ligands and com-
plexes have the antioxidant activities.
The antioxidant potential of free pyridine carboxamide ligands
(H2L1, H2L2 and H2L3) and corresponding ruthenium(II) complexes
(1, 2, 3, 4 and 5) against DPPH radical, NO radical, OH radical and
H2O2 assay were investigated with respect to different concentra-
Antioxidant activity
tions of the test compounds varying from 0 to 50 lM and the results
were shown in Table 4. It was observed that, the 50% inhibitory con-
centration (IC50) value of ligands and complexes varies from
139.8 lM to 182.1 lM and from 24.0 lM to 59.7 lM, respectively,
The DPPH, NO and OH radicals have been widely used to test
the ability of compounds as free radical scavengers to evaluate
the antioxidant activity. The scavenging activity may help to arrest

60
R. Ramachandran, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 53–61
Table 4
Acknowledgements
Antioxidant activity of the ligands, complexes (1–5), vitamin C and BHT against
various radicals.
Instrumental facilities providing by SAIF, Cochin and SAIF, Pan-
jab University, Chandigarh, are gratefully acknowledged.
Compound
IC50 (lM)
DPPHÅ
NOÅ
OHÅ
H2O2
H2L1
171.4
139.8
182.1
43.7
30.8
59.7
30.9
24.0
147.6
86.2
95.8
88.4
104.6
11
7.8
37.8
10
6.7
215.8
154.3
76.1
72.6
89.9
35.4
24.9
36
25.9
22.5
232.8
163.4
48
H2L2
31.2
54.7
27.8
22.4
33.4
25.2
19.7
238.5
149.8
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