New ruthenium(II) carbonyl complexes bearing disulfide Schiff base ligands and their applications as catalyst for some organic transformations

Article abstract of DOI:10.1016/j.saa.2014.03.086

Schiff base disulfide ligands (H₂L^1-6) were synthesized from the condensation of cystamine with salicylaldehyde(H ₂L¹), 5-chlorosalicylaldehyde(H₂L²), o-vanillin(H₂L³), 2-hydroxyacetophenone(H ₂L⁴), 3-methyl-2-hydroxyacetophenone(H₂L ⁵), and 2-hydroxy-1-naphthaldehyde(H₂L⁶). H₂L^1-6 reacts with the ruthenium precursor complex [RuHCl(CO)(PPh₃)₃] in benzene giving rise to six new ruthenium(II) complexes of general formula [Ru(CO)L^1-6]. Characterization of the new complexes was carried out by using elemental and spectral (IR, UV-Vis, NMR (¹H and ¹³C) and Mass) techniques. An octahedral geometry was assigned for all the complexes based on the spectral data obtained. The catalytic efficiency of the new complexes in aldehyde to amide conversion in the presence of NaHCO₃, N-alkylation of aniline in the presence of t-BuOK, and transfer hydrogenation of ketones in the presence of iPrOH/KOH reactions were studied. Furthermore, the effect of solvents and catalyst/substrate ratio on the catalytic aldehyde to amide conversion were also discussed.

Full text of DOI:10.1016/j.saa.2014.03.086

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 352–358
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
New ruthenium(II) carbonyl complexes bearing disulfide
Schiff base ligands and their applications as catalyst
for some organic transformations
⇑
Govindan Prakash, Periasamy Viswanathamurthi
Department of Chemistry, Periyar University, Salem 636 011, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A series of new ruthenium(II) disulfide bearing Schiff base complexes based on cystamine and 2-hydroxy-
benzaldehydes/2-acetylbenzaldehydes were synthesized and characterized. The catalytic applications of
the new complexes were studied towards the aldehyde to amide conversion, N-alkylation and transfer
hydrogenation reactions.
ꢀ
ꢀ
A series of ruthenium (II) carbonyl
complexes were synthesized.
The new complexes were
characterized using elemental and
spectral methods.
ꢀ
ꢀ
ꢀ
Ru(II) complexes admirably catalyze
the aldehydes to amide conversion.
New complexes also effectively
catalyze the N-alkylation of amines.
Catalytic transfer hydrogenation was
also done using the new complexes.
a r t i c l e i n f o
a b s t r a c t
Schiff base disulfide ligands (H2L1ꢁ6) were synthesized from the condensation of cystamine with
salicylaldehyde(H2L1), 5-chlorosalicylaldehyde(H2L2), o-vanillin(H2L3), 2-hydroxyacetophenone(H2L4),
3-methyl-2-hydroxyacetophenone(H2L5), and 2-hydroxy-1-naphthaldehyde(H2L6). H2L1ꢁ6 reacts with
the ruthenium precursor complex [RuHCl(CO)(PPh3)3] in benzene giving rise to six new ruthenium(II)
complexes of general formula [Ru(CO)L1ꢁ6]. Characterization of the new complexes was carried out by
using elemental and spectral (IR, UV–Vis, NMR (1H and 13C) and Mass) techniques. An octahedral geom-
etry was assigned for all the complexes based on the spectral data obtained. The catalytic efficiency of the
new complexes in aldehyde to amide conversion in the presence of NaHCO3, N-alkylation of aniline in the
presence of t-BuOK, and transfer hydrogenation of ketones in the presence of iPrOH/KOH reactions were
studied. Furthermore, the effect of solvents and catalyst/substrate ratio on the catalytic aldehyde to
amide conversion were also discussed.
Article history:
Received 30 December 2013
Received in revised form 3 March 2014
Accepted 21 March 2014
Available online 2 April 2014
Keywords:
Disulfide Schiff base ligands
Ruthenium(II) complexes
Spectral studies
Aldehyde to amide conversion
N-alkylation
Transfer hydrogenation
Ó 2014 Elsevier B.V. All rights reserved.
⇑
Corresponding author. Tel.: +91 9443775719; fax: +91 427 2345124.
E-mail address: viswanathamurthi72@gmail.com (P. Viswanathamurthi).
http://dx.doi.org/10.1016/j.saa.2014.03.086
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

G. Prakash, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 352–358
353
Introduction
Materials and methods
The disulfide (RS-SR) group is an important constituent in many
metalloproteins and bioenzymes which plays a vital role in their
thermal stability and biocatalytic activities. It has also been recog-
nized as a potential binding site for metal ions in biological systems
[1]. Introduction of disulfide bonds into the bioenzyme called gluco-
amylase improved the catalytic activity of the enzyme in Aspergillus
awamori [2]. Similar kind of observations were also reported for sul-
fotransferase [3], mannosidase [4], etc. The essential role of the
disulfide group in biocatalytic processes, increased our interest to
study its influence on the catalytic activity of transition metal com-
plexes. Several complexes of nickel [5], cobalt [6], copper [7], rhe-
nium [8], iridium [9], iron [10], etc., bearing disulfide groups have
been synthesized and structurally determined using crystallogra-
phy. The existence of metal–sulfide bond has been well demon-
strated in all the cases. Further, the electronic and steric properties
of salicylaldimine based disulfide Schiff base ligands can be conve-
niently modified by changing the substituents on the phenyl ring.
The relevance of M–S(disulfide) bond on the redox processes of me-
tal centers receiving much importance in the catalytic properties.
Because, the reversible electronic properties of the metal ions play
an important role in the catalytic activity of the complexes.
Amides are valuable intermediates in organic synthesis and are
used as raw materials in many industries like detergents, lubri-
cants and pharmaceuticals [11]. The common preparation of
amides from the rection of amines with acylchlorides or acid anhy-
drides [12] involves highly toxic wastes. Hence the development of
new catalytic processes with more atom economy and less toxic
wastes getting importance. The rearrangement of aldoximes to
amides using transition metal based catalysts is one such impor-
tant method and metals like Ni, Cu, Pd, Ir [13–16], etc. have been
reported in these conversions. [RuCl2(DMSO)4] catalyzed one pot
aldehyde to amide conversion via aldoximes was reported by Hull
et al. [17].
N-alkylation of amines is an important transformation for the
preparation of pharmaceuticals, agrochemicals, polymers, dyes,
and other fine chemicals [18]. A number of reports on the hydro-
amination and hydroamino-methylation of olefins or alkynes [19]
for the synthesis of amines have been reported. Compared to the
frequently applied N-alkylations with alkyl halides and reductive
amination methods, an economically and environmentally attrac-
tive method is the N-alkylation of amines using primary and sec-
ondary alcohols. Recently, Shvo’s ruthenium catalyst and
ruthenium pincer complexes were reported in N-alkylation reac-
tion with good yields [20]. Reduction of carbonyl functionalities
is another essential reaction in organic synthesis because reduction
of ketones is a straightforward route to alcohols. Transfer hydroge-
nation of ketones using isopropyl alcohol as hydrogen donors has
been considered as a reliable method under mild conditions. Trans-
fer hydrogenation using ruthenium(II) complexes of N-tosylethy-
lenediamine [21], NNC ligands containing NH2 group [22] and
NNN ligands [23] have been reported. To the best of our knowledge
no reports are available for the above three useful catalytic conver-
sions i.e., aldehyde to amide conversion, N-alkylation and transfer
hydrogenation, using ruthenium(II) carbonyl complexes bearing
disulfide Schiff base ligands.
Reagents and materials
All the reagents used were chemically pure and AR grade. The
solvents were purified and dried according to standard procedures
[24]. RuCl3ꢂ3H2O was purchased from Loba Chemie Pvt. Ltd., and
was used without further purification. The precursor complex
[RuHCl(CO)(PPh3)3] was prepared according to the literature meth-
ods [25–27].
Physical measurements
The elemental analyses were performed by using Vario EL III Ele-
mental analyzer. FT-IR spectra were recorded as KBr pellets with a
Nicolet FT-IR spectrophotometer over the range 400–4000 cmꢁ1
.
Electronic spectra were taken in CH2Cl2 solution with 1 cm quartz
cells using Shimadzu UV-1650 PC spectrophotometer over the
range 200–800 nm at room temperature and pressure. All the
NMR spectra (1H and 13C) were recorded at room temperature in
Jeol GSX-400 instrument using DMSO-D6 and CDCl3 as solvents.
1H NMR and 13C NMR spectra were recorded using TMS as the inter-
nal standard. Melting points were recorded using Technico melting
point apparatus and were uncorrected. Gas chromatographic (GC)
analyses were conducted on an ACME 6000 series instrument
equipped with a flame ionization detector (FID) using a DP-5 col-
umn of 30 m length, 0.53 mm diameter and 5.00
lm film thickness.
Preparation of 2,20-di(2-hydroxybenzaliminoethyl) disulfides (H2L1ꢁ6
)
A solution of cystamine (0.5 g, 2 mmol) in ethanol was added to
an ethanolic solution of potassium hydroxide (0.1 g, 2 mmol) and
the reaction mixture was stirred under reflux for half an hour.
On cooling to 20 °C a flaky white precipitate formed was filtered
off. To the resulting solution, corresponding 2-hydroxy aldehyde/
ketone {salicylaldehyde (0.42 ml, 4 mmol), 5-chloro salicylalde-
hyde (0.62 g, 4 mmol), 2-hydroxyacetophenone (0.48 ml, 4 mmol)
and 2-hydroxy-1-naphthaldehyde (0.069 g, 4 mmol)} was added
with stirring. The reaction mixture was allowed to stir for 5 h
and then cooled to ꢁ10 °C. Needle like yellow crystals formed were
filtered, washed with minimum amount of ethanol and dried in air.
Elemental composition and properties
2, 20-Di(2-hydroxybenzyliminoethyl)disulfide (H2L1)
Yellow 62%; m.p.73 °C; UV–Vis (CH2Cl2, kmax, nm) 262, 236; IR
(KBr)(mmax, cmꢁ1) 3415 (OH); 1631 (C@N), 1333 (CAO), 751
(CAS); 1H NMR (CDCl3, ppm) d 12.33 (s, 2H, OH); 9.02 (s, 2H,
N@CH); 6.96 (dt, 2H, Ar, J = 7.43), 7.35 (dt, 2H, Ar, J = 7.26), 7.70
(dd, 2H, Ar, J = 7.56), 7.89 (dd, 2H, Ar, J = 7.22), 3.40, 2.52 (both t,
4H each, CH2, J = 6.32); Anal. [found (calc)%] for C18H20N2O2S2 C:
59.97 (59.80), H: 5.59 (5.30), N: 7.77 (7.89), S: 17.79 (17.57).
2, 20-Di(2-hydroxy-5-chlorobenzyliminoethyl)disulfide (H2L2)
Yellow 71%; m.p.112 °C; UV–Vis (CH2Cl2, kmax, nm) 256, 230; IR
(KBr)(m
max, cmꢁ1) 3424 (OH); 1629 (C@N), 1373 (CAO), 763 (CAS);
1H NMR (CDCl3, ppm) d 13.17 (s, 2H, OH); 8.34 (s, 2H, N@CH); 6.91
(d, 2H, Ar, J = 7.46), 7.29 (d, 2H, Ar, J = 7.46), 7.62 (s, 2H, Ar), 3.96,
3.02 (both t, 4H each, CH2, J = 6.86); Anal. [found (calc)%] for
Hence, as an incorporation of our ongoing research towards the
development of new catalytic systems, a series of ruthenium(II)
complexes of N2SO2 type disulfide Schiff base ligands have been
synthesized and characterized by various spectroscopic tech-
niques. Synthesized complexes have been employed as catalysts,
with a view to understand and compare the influence of pentaden-
tate ligands on the conversion of aldehydes to amides, N-alkylation
of aniline and transfer hydrogenation of ketones.
C18H18N2O2S2Cl2 C: 50.47 (50.35), H: 4.41 (4.23), N: 6.68 (6.52),
S: 14.70 (14.94).
2, 20-Di(2-hydroxy-3-methoxybenzyliminoethyl)disulfide (H2L3)
Yellow 56%; m.p.116 °C; UV–Vis (CH2Cl2, kmax, nm) 251, 233; IR
(KBr) (m
max, cmꢁ1) 3417 (OH); 1638 (C@N), 1351 (CAO), 782 (CAS);

354
G. Prakash, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 352–358
1H NMR (CDCl3, ppm) d 13.30 (s, 2H, OH); 8.58 (s, 2H, N@CH); 6.94
(d, 2H, Ar, J = 7.88), 7.35 (dd, 2H, Ar, J = 7.26), 7.50 (d, 2H, Ar,
J = 8.22), 3.12, 2.60 (both t, 4H each, CH2, J = 7.02), 3.92 (s, 6H,
OCH3); Anal. [found (calc)%] for C20H24N2O4S2 C: 57.34 (57.12),
H: 5.51 (5.75), N: 6.78 (6.66), S: 15.46 (15.25).
(C@N), 124.5–136.0 (Aromatic C), 32.1 (SACH2), 26.5 (NACH2).
MS (ESI): m/z = 556.0 [M+]. Anal. [Found (calc) (%)] for C19H16N2O3-
S2Cl2Ru: C, 41.36 (41.01); H, 2.58 (2.90); N, 4.84 (5.03); S, 11.27
(11.52).
[Ru(CO)L3] (3)
2, 2’-Di(2-acetylbenzyliminoethyl)disulfide (H2L4)
Green 66%; m.p. 172 °C; UV–Vis (CH2Cl2, kmax, nm): 474, 352,
287, 227; FT-IR (KBr) (m
max, cmꢁ1) 1628 (C@N), 1376 (CAO), 1973
Yellow 74%; m.p. 83 °C; UV–Vis (CH2Cl2, kmax, nm) 260, 228; IR
(KBr)(m
max, cmꢁ1) 3372 (OH); 1618 (C@N), 1328 (CAO), 758 (CAS);
(C@O); 741 (CAS). 1H NMR (CDCl3, ppm): 9.34 (s, 2H, N@CH),
6.76–7.83 (m, 8H, Ar), 2.54 (t, 4H, NACH2, J = 5.76), 2.04 (t, 4H,
SACH2, J = 5.76), 3.52 (s, 6H, OCH3). 13C NMR (CDCl3, ppm):
210.07(C„O), 165.0 (C@N), 126.5–135.0(Aromatic C), 58.2
(AOCH3), 32.5 (SACH2), 28.5 (NACH2). MS (ESI): m/z = 546.6
[M+]. Anal. [Found (calc) (%)] for C12H22N2O5S2Ru: C, 44.18
(46.06); H, 4.25 (4.05); N, 5.01 (5.12); S, 11.59 (11.72).
1H NMR (CDCl3, ppm) d 11.10 (s, 2H, OH); 8.82 (s, 2H, N@CH); 6.96
(d, 2H, Ar, J = 7.28), 7.42 (dd, 2H, Ar, J = 7.56), 6.62 (dd, 2H, Ar,
J = 8.22), 7.72 (d, 2H, Ar, J = 7.96), 3.52, 3.2 (both t, 4H each, CH2,
J = 7.76), 2.50 (s, 6H, CH3); Anal. [found (calc)%] for C20H24N2O2S2
C: 61.66 (61.82), H: 5.51 (6.23), N: 6.78 (7.21), S: 15.46 (16.51).
2, 2’-Di(2-hydroxy-5-methylbenzyliminoethyl)disulfide (H2L5)
Yellow 70%; m.p. 96 °C; UV–Vis (CH2Cl2, kmax, nm) 248, 224; IR
[Ru(CO)L4] (4)
(KBr) (m
max, cmꢁ1) 3382 (OH); 1632 (C@N), 1314 (CAO), 752 (CAS);
Green 80%; m.p. 162 °C;. UV–Vis (CH2Cl2, kmax, nm): 478, 382,
1H NMR (CDCl3, ppm) d 12.56 (s, 2H, OH); 8.32 (s, 2H, N@CH); 6.82
(d, 2H, Ar, J = 7.16), 7.23 (d, 2H, Ar, J = 7.16), 7.62 (s, 2H, Ar, J = 8.06),
3.85, 2.86 (both t, 4H each, CH2, J = 6.84), 2.22 (s, 6H, Ar-CH3),
2.62(s, 6H, CH3)Anal. [found (calc)%] for C22H28N2O2S2 C: 63.73
(63.43), H: 6.50 (6.77), N: 6.61 (6.72), S: 15.59 (15.39).
268, 240; FT-IR (KBr) (m
max, cmꢁ1) 1615 (C@N), 1362 (CAO), 1906
(C@O); 744 (CAS). 1H NMR (CDCl3, ppm): 9.89 (s, 2H, N@CH),
6.80–7.82 (m, 8H, Ar), 3.10 (t, 4H, NACH2, J = 5.76), 2.78 (t, 4H,
SACH2, J = 5.76), 1.86 (s, 6H, CH3). 13C NMR (CDCl3, ppm): 202.2
(C„O), 160.5 (C@N), 121.5–133.2 (Aromatic C), 31.1 (SACH2),
25.2 (NACH2), 20.1 (ACH3). MS (ESI): m/z = 515.1 [M+]. Anal.
[Found (calc) (%)] for C21H22N2O3S2Ru: C, 49.22 (48.92); H, 4.18
(4.30); N, 5.33 (5.43); S, 13.57 (12.44).
2, 20-Di(2-hydroxynaphthyliminoethyl)disulfide (H2L6)
Yellow 62%; m.p. 84 °C; UV–Vis (CH2Cl2, kmax, nm) 262, 243; IR
(KBr)(m
max, cmꢁ1) 3439 (OH); 1641 (C@N), 1353 (CAO), 753 (CAS);
1H NMR (CDCl3, ppm) d 12.35 (s, 2H, OH); 8.90 (s, 2H, N@CH);
6.68–7.90 (m, 12H, Ar), 3.57, 2.63 (both t, 4H each, CH2, J = 6.42);
Anal. [found (calc)%] for C26H24N2O2S2 C: 63.73 (67.80), H: 6.50
(5.25), N: 6.61 (6.08), S: 15.59 (13.92).
[Ru(CO)L5] (5)
Green 78%; m.p. 168 °C;. UV–Vis (CH2Cl2, kmax, nm): 492, 386,
274, 237; FT-IR (KBr) (m
max, cmꢁ1) 1622 (C@N), 1368 (CAO), 1923
(C@O); 734 (CAS). 1H NMR (CDCl3, ppm): 9.36 (s, 2H, N@CH),
6.72–8.11 (m, 8H, Ar), 3.23 (t, 4H, NACH2, J = 5.76), 2.97 (t, 4H,
SACH2, J = 5.76), 1.57 (s, 6H, CH3), 2.62 (s, 6H, ArACH3). 13C NMR
(CDCl3, ppm): 204.7 (C„O), 161.3 (C@N), 125.1–132.7 (Aromatic
C), 32.3 (SACH2), 26.0 (NACH2), 22.4 (ArACH3), 19.1 (ACH3). MS
(ESI): m/z = 543.3 [M+]. Anal. [Found (calc) (%)] for C23H26N2O3S2-
Ru: C, 50.62 (50.81); H, 4.78 (4.82); N, 5.57 (5.15); S, 11.61 (11.80).
General method for the synthesis of new ruthenium(II) Schiff base
complexes (1–6)
All the complexes were synthesized by the following common
procedure (S2). To a solution of [RuHCl(CO)(PPh3)3] (1 mmol) in
benzene (20 ml), the appropriate Schiff base ligand (1 mmol) was
added in a 100 ml round-bottom flask. The resulting mixture was
heated under reflux for 5 h in water bath. The reaction mixture
gradually changed to a deep color during heating. After the reac-
tion time, the contents were concentrated to 3 ml and then cooled.
Light petroleum ether (60–80 °C) (10 ml) was then added, where
upon the product separated out. The product was recrystallized
from CH2Cl2/light petroleum ether mixture (60–80 °C) and dried
in vacuum. The purity of the complexes was checked by TLC.
[Ru(CO)L6] (6)
Green 76%; m.p. 173 °C;. UV–Vis (CH2Cl2, kmax, nm): 472, 370,
266, 238; FT-IR (KBr) (m
max, cmꢁ1) 1628 (C@N), 1365 (CAO), 1948
(C@O); 743 (CAS). 1H NMR (CDCl3, ppm): 9.46 (s, 2H, N@CH),
6.70–7.94 (m, 8H, Ar), 3.42 (t, 4H, NACH2, J = 6.06), 2.53 (t, 4H,
SACH2, J = 6.06). 13C NMR (CDCl3, ppm): 203.5 (C„O), 165.0
(C@N), 122.5–132.2 (Aromatic C), 32.5 (SACH2), 26.3 (NACH2).
MS (ESI): m/z = 586.4 [M+]. Anal. [Found (calc) (%)] for C27H22N2O3-
S2Ru: C, 55.47 (55.18); H, 3.36 (3.77); N, 4.41 (4.77); S, 10.67
(10.91).
Properties and composition of the new complexes
[Ru(CO)L1] (1)
Green 76%; m.p. 172 °C;. UV–Vis (CH2Cl2, kmax, nm): 452, 338,
Catalytic conversion of aldehydes to amides
272, 236; FT-IR (KBr) (m
max, cmꢁ1) 1618 (C@N), 1360 (CAO), 1958
(C@O); 747 (C-S). 1H NMR (CDCl3, ppm): 9.44 (s, 2H, N@CH),
6.22–7.76 (m, 8H, Ar), 3.15 (t, 4H, NACH2, J = 6.02), 2.50 (t, 4H,
SACH2, J = 6.02). 13C NMR (CDCl3, ppm): 206.5(C„O), 166.5
(C@N), 127.5–134.2(Aromatic C), 33.0 (SACH2), 28.5 (NACH2).
MS (ESI): m/z = 486.8 [M+]. Anal. [Found (calc) (%)] for
Catalytic conversion of aldehydes into their corresponding
amides were carried out using ruthenium(II) complexes as catalyst
in the following general procedure. The reaction vessel was
charged with aldehyde (2 mmol), NH2OH.HCl (2 mmol), NaHCO3
(2 mmol) and ruthenium catalyst (0.01 mmol) and the mixture
was placed under an atmosphere of nitrogen. About 4 ml of dry
and degassed toluene was added and the mixture was stirred for
15 min at room temperature followed by reflux for 18 h. On com-
pletion of the reaction, 2–3 ml methanol was added to the mixture
followed by filtration through Celite to remove the catalyst and
NaHCO3. The crude product was then purified by column chroma-
tography (MeOH/CH2Cl2) and the formation of pure amide was
confirmed using 1H NMR spectral analyses. The catalytic efficiency
in various solvents was also studied.
C
19H18N2O3 S2Ru: C, 46.96 (46.81); H, 3.55 (3.72); N, 5.48 (5.75);
S, 13.47 (13.15).
[Ru(CO)L2] (2)
Green 72%; m.p. 176 °C;. UV–Vis (CH2Cl2, kmax, nm): 446, 360,
270, 237. FT-IR (KBr) (m
max, cmꢁ1) 1614 (C@N), 1385 (CAO), 1963
(C@O); 740 (CAS). 1H NMR (CDCl3, ppm): 9.89 (s, 2H, N@CH),
6.80–7.82 (m, 8H, Ar), 3.10 (t, 4H, NACH2, J = 5.76), 2.78 (t, 4H,
SACH2, J = 5.76). 13C NMR (CDCl3, ppm): 206.0(C„O), 164.5

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355
Catalytic N-alkylation of amines
mixture of amine (0.3 mmol), ruthenium(II) complex
through two azomethine nitrogens, a phenolic oxygen and with a
sulfur atom of disulfide group.
A
(3 ꢃ 10–3 mmol), KOtBu (0.12 mmol) in alcohol (0.9 mmol) was
placed in a flask under atmospheric pressure of nitrogen and re-
fluxed at 110–150 °C for 6 h. On completion of the reaction, brine
(3 ml) and CH2Cl2 (5 ml) were added. The organic layer was
separated and the aqueous layer was extracted into CH2Cl2. The
combined organic extracts were dried with magnesium sulfate
and concentrated. The compound was purified by column chro-
matography (pentane/diethylether, 1:1) using silica (200 mesh)
as solid phase and characterized by 1H NMR analyses and the
yields were obtained by the 1H NMR integration compared to 1,
3, 5-trimethoxybenzene is an internal standard.
Electronic spectroscopic analysis
All the new ruthenium Schiff base complexes are diamagnetic
in nature which indicating the +2 oxidation state of ruthenium
ion. The electronic spectra of ligands in methanol have shown
two bands in the regions 248–268 nm and 228–243 nm which cor-
responds to k-kꢄ and n-kꢄ transitions respectively. Whereas the
spectra of the complexes were recorded in CH2Cl2 showed four
bands in the region 227–492 nm. The bands below 300 nm can
be attributed to intra ligand transitions. The high intensity bands
appeared at 338–386 nm region have been assigned to charge
transfer transitions arising from the metal T2g level to the unfilled
pꢄ molecular orbital of the ligand [30–33]. A weak band observed
above 400 nm in the spectra of all the complexes can be attributed
to the d–d transitions [34].
Catalytic transfer hydrogenation
The catalytic transfer hydrogenation reactions were studied
using the synthesized ruthenium(II) Schiff base complexes as cata-
lyst, ketones as substrate and KOH as base at 1:600:2.5 M ratios
respectively. The procedure was described as follows. A mixture
containing ketone (3.75 mmol), ruthenium complex (0.006 mmol)
and KOH (0.0125 mmol) was heated to reflux in 10 ml of i-PrOH
for 3 h. After completion of the reaction, the catalyst was removed
from the reaction mixture by the addition of petroleum ether fol-
lowed 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
authentic samples.
1H NMR spectroscopic analysis
Coordination of the ligands with the metal atom was further
proved by using 1H NMR spectral analyses. The spectra of the free
ligands have showed a signal at 12.3–14.1 ppm characteristics of
phenolic AOH proton which have disappeared in their complexes,
suggesting the coordination of deprotonated phenolic oxygen
[35,36]. A singlet observed at 8.3–9.1 ppm in the spectra of the free
ligands has been assigned to azomethine proton which has under-
gone a downfield shift around 9.4–9.8 ppm in the complexes indi-
cating the coordination of azomethine nitrogen to ruthenium
metal. The aromatic protons of the phenolic group of ligands have
exhibited their signal in the region 6.7–8.1 ppm. The complexes
have shown their aromatic signals slightly upfield to that of the li-
gands and appeared around 6.2–7.9 ppm. The two methylene pro-
tons (NACH2 and SACH2) have shown their signals as triplet
around 3.1–3.9 and 2.4–2.8 ppm respectively in both ligands and
complexes. The above observations made it clear that the ligands
have coordinated to the ruthenium(II) ion.
Results and discussion
The ruthenium(II) Schiff base complexes of general formula
[Ru(CO)L1ꢁ6] (L = Schiff base ligand) were synthesized from the
reaction of [RuHCl(CO)(PPh3)] with Schiff base ligands in dry ben-
zene with 1:1 M ratio (S1). In these reactions, the Schiff bases be-
have as binegative pentadentate ligands and replacing three PPh3
groups, an hydride ion and a chloride ion from the starting
complexes.
13C NMR spectroscopic analysis
All the complexes were obtained in good yield and stable in
both solid state and in solution. The analytical data were in good
agreement with the general molecular formulae proposed. In addi-
tion, ESI-Mass spectra of the complexes were confirmed the molec-
ular masses of their proposed structures.
13C NMR spectra have been recorded for some of the complexes.
The signal at 203.5–210.07 ppm region corresponds to C„O car-
bon. The azomethine (C@N) carbons have exhibited their signals
in the region 159.5–166.5 ppm. The signals in the region 24.5–
28.5 and 32.0–35.0 ppm can be assigned to the NACH2 and SACH2
methylene carbons respectively. Further the signals of aromatic
carbons have found in the region of 120.0–138.5 ppm. The results
obtained have shown the retention of carbonyl group after the for-
mation of new complexes with Schiff bases.
Infrared spectroscopic analysis
A strong band at 1333–1373 cmꢁ1 in the free ligands can be as-
signed to phenolic CAO stretching. This band shifted to 1351–
1397 cmꢁ1 in complexes, showing that the coordination through
the phenolic oxygen via deprotonation [28]. This is supported by
Mass spectral analysis
the disappearance of the tOAH band in all the complexes which ob-
served in free ligands around 3415–3439 cmꢁ1. The band around
1611–1638 cmꢁ1 in the spectra of ligands can be attributed to tC@N
which is shifted to 1595–1629 cmꢁ1 in complexes indicating the
coordination of azomethine nitrogen with central metal ion [29].
The peak appeared around 751–782 cmꢁ1 in the spectra of ligands
were assigned to C-S stretching. In the new ruthenium(II) com-
plexes, a peak observed in the same region was assigned to unco-
ordinated C-S bond. A new peak appeared around 741–752 cmꢁ1 in
all the complexes was assigned to coordinated CAS bond. This
showed the binding of one of the sulfur atoms in the disulfide link-
age with ruthenium metal. Hence, from the infrared spectral data,
it was inferred that the Schiff bases bound with ruthenium(II) ions
ESI-Mass spectral analyses of the new complexes were studied
in order to confirm the molecular mass of the complexes. The m/z
values of the molecular ion peaks were in good agreement with the
proposed mononuclear structure of all the complexes.
Our effort to obtain single crystals of the complexes suitable for
X-ray analysis went unsuccessful. However, the elemental and
mass spectral data obtained were in good agreement with the
mononuclear ruthenium complex in which the ligand act as bineg-
ative pentadentate ligand with N2SO2 donor atoms. Therefore,
based on the analytical and spectroscopic results, an octahedral
structure (S2) has been proposed for all the new ruthenium(II)
Schiff base complexes.

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G. Prakash, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 352–358
Table 1
Catalytic aldehyde to amide conversion
One pot conversion of benzaldehyde to benzamide using ruthenium(II) complexes;
catalyst optimization.a
Optimization of catalyst and conditions
Entry
Complex
Yield (%)b
To investigate a promising catalytic system, a screen was per-
formed for a model reaction of benzaldehyde to benzamide conver-
sion (S2). The representative results are shown in Table 1. A series
of blank and control experiments were carried out under a set of
conditions which suggest that none of the ruthenium(II) starting
complex or disulfide bearing Schiff base ligand alone or as a mix-
ture causes this conversions under the same reaction conditions.
Among the catalyst tested, the complex [Ru(CO)L3] catalyze the
rearrangement of benzaldehyde with high yields (Table 1) which
may due to the activation of phenyl ring by the AOCH3 group.
Different catalyst/substrate ratios were also tested in order to
optimize the favorable ratio. Benzaldehyde was used as substrate
for these initial experiments and [Ru(CO)L3] used as catalyst in
presence of NH2OH.HCl/NaHCO3. On screening different catalyst/
substrate ratios (1:100, 1:200, 1:500, 1:1000), 1:100 was found
to be the optimum ratio of catalyst/substrate, with the maximum
yield of 92% (Table 2). The effect of solvents was studied under
the optimum conditions, using the solvents toluene, benzene,
acetonitril, dichloromethane and chloroform for the conversion of
benzaldehyde to benzamide. Toluene is found to be the best
solvent with 92% yield among the above solvents (S3).
1
2
3
4
5
6
[Ru(CO)(L1)]
[Ru(CO)(L2)]
[Ru(CO)(L3)]
[Ru(CO)(L4)]
[Ru(CO)(L5)]
[Ru(CO)(L6)]
80
81
92
78
72
84
a
Aldehyde 2 mmol, NH2OH.HCl 2 mmol and catalyst (1 mol%) were refluxed
for18 h in toluene.
b
Isolated yield after column chromatography.
Table 2
Effect of catalyst/substrate ratio on benzaldehyde to benzamide conversion using
[Ru(CO)(L3)].a
Entry
Ratio
Time
Yield (%)b
1
2
3
4
1:1000
1:500
1:200
1:100
18
18
18
18
<20
30
67
92
a
Reaction was carried out with 1–10 mmol of substrate and corresponding
catalyst with NH2OHꢂHCl in toluene under 18 h of reflux.
b
Isolated yield after chromatography.
Table 3
One pot conversion of aldehydes to amides using [Ru(CO)(L3)].a
Entry Aldehydes
Amides
Yields
(%)b
1
2
92
78
3
4
73
86
5
6
88
91
a
Aldehyde 2 mmol, NH2OH.HCl 2 mmol and catalyst (1 mol%) were refluxed for18 h
in toluene.
b
Isolated yield after column chromatography.

G. Prakash, P. Viswanathamurthi / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 352–358
357
Table 4
N-alkylation of benzylamine catalyzed by new ruthenium(II) complexes.
Complex
Medium
Product
Yield (%)a
1
Benzyl alcohol
p-methoxybenzyl alcohol
Benzyl alcohol
p-methoxybenzyl alcohol
Benzyl alcohol
p-methoxybenzyl alcohol
Benzyl alcohol
p-methoxybenzyl alcohol
Benzyl alcohol
p-methoxybenzyl alcohol
Benzyl alcohol
N-phenylbenzylamine
N-phenyl-(p-methoxybenzyl) amine
N-phenylbenzylamine
N-phenyl-(p-methoxybenzyl) amine
N-phenylbenzylamine
N-phenyl-(p-methoxybenzyl) amine
N-phenylbenzylamine
N-phenyl-(p-methoxybenzyl) amine
N-phenylbenzylamine
N-phenyl-(p-methoxybenzyl) amine
N-phenylbenzylamine
79
77
81
72
87
73
82
70
81
64
73
65
2
3
4
5
6
p-methoxybenzyl alcohol
N-phenyl-(p-methoxybenzyl) amine
a
Yield based on 1H NMR integration.
Conversion of benzaldehyde derivatives to their amides
Since our aim is to develop a new catalytic system which is
applicable to a wide range of aromatic, substituted, conjugated
and heterocyclic aromatic aldehydes, various aldehydes were used
in presence of NaHCO3 and the results are summarized in Table 3.
Using [Ru(CO)L3], an admirable yield of 92% was obtained for benz-
aldehyde to benzamide (entry 1). All the derivatives of benzalde-
hydes were also smoothly converted to the corresponding
amides in good yields. The presence of electron donating groups
like ACH3 and AOCH3 (entries 2 and 3) in the substrates alters
the reactions and the corresponding amides were obtained in good
yields 78% and 73% respectively. On the other hand, electron with-
drawing groups in substrates, such as the ANO2 and ACl substitu-
ents (entries 4 and 5) offering better yields of 88%, and 90%
respectively, when compared to substrate containing electron
donating group. The new Ru(II) disulfide catalyst shows good
catalytic activity for the conjugated aldehyde such as, cinnamalde-
hyde which was converted to the conjugated amides (entry 6) in
91% yield. Our observation in this catalytic aldehyde to amide
conversion is comparable with that of a recent observation using
[RuCl2(DMSO)4] complex as catalyst [17].
Table 5
Catalytic transfer hydrogenation of ketones by ruthenium(II) Schiff base complexes.a
Complex Substrate
Product
Conversion (%)b
1
2
3
4
5
6
Benzaldehyde
Isopropylmethyl ketone 4-Methyl-2-pentanol 79
Cyclohexanone
Benzaldehyde
Isopropylmethyl ketone 4-Methyl-2-pentanol 80
Cyclohexanone
Benzaldehyde
Isopropylmethyl ketone 4-Methyl-2-pentanol 82
Cyclohexanone
Benzaldehyde
Isopropylmethyl ketone 4-Methyl-2-pentanol 70
Cyclohexanone
Benzaldehyde
Isopropylmethyl ketone 4-Methyl-2-pentanol 76
Cyclohexanone
Benzaldehyde
1-Phenyl-ethanol
92
Cyclohexanol
1-Phenyl-ethanol
83
92
Cyclohexanol
1-Phenyl-ethanol
88
93
Cyclohexanol
1-Phenyl-ethanol
78
89
Cyclohexanol
1-Phenyl-ethanol
81
90
Cyclohexanol
1-Phenyl-ethanol
83
84
Isopropylmethyl ketone 4-Methyl-2-pentanol 73
Cyclohexanone Cyclohexanol 78
a
Catalyst/substrate/KOH ratio was fixed as 1:600:2.5.and reaction was carried
out in i-prOH under 3 h reflux.
b
The% conversion of product was determined using GC by comparing with
authentic samples.
Catalytic N-alkylation of amine
The ruthenium(II) complexes (1–6) catalyzed the N-alkylation
of amine with alcohols in the presence of tBuOK (S4). In a typical
experiment, a mixture of aniline, benzyl alcohol or p-methoxyben-
zyl alcohol, tBuOK and ruthenium(II) complex (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 extracted from the reaction
mixture and analyzed by 1H NMR spectroscopy. Product yields
were obtained by the 1H NMR integration compared to the internal
standard.
As shown in Table 4, benzyl alcohols reacted smoothly to give
the corresponding products in good yields (73–89%) whereas,
p-methoxybenzyl alcohol gave the corresponding product in mod-
erate yield (64–75%). Nature of substitution on Ru(II)catalysts has
shown considerable variation in their catalytic activities. Electron
withdrawing groups such as ACl and AOCH3 substitution on the
phenyl rings of the catalysts (complexes 2 and 3) afforded higher
yields for both benzyl alcohol and p-methoxybenzyl alcohol substi-
tutions. This may be due to the activation of phenyl rings of the
catalysts. Whereas, ACH3 substituted Ru(II) complexes (complexes
4 and 5) afforded slightly lesser yields. Complex 6 with naphthyl
ring showed comparatively lower yield among all the catalysts.
This may due to the steric hindrance of the naphthyl ring. Compar-
ison with protocols previously developed for this reaction [37], the
catalyst loadings are very low and the reaction conditions are very
mild in the present catalytic system.
Catalytic transfer hydrogenation
The ruthenium(II) disulfide complexes (1–6) catalyzed the
reduction of ketones to the corresponding alcohols via hydrogen
transfer from iPrOH with KOH as the promoter (S5). The reaction
was conducted at a substrate, catalyst and base in molar ratio
1:0.006:2.5 respectively. Typical results have shown in Table 5.
The catalyst performed efficiently for both aliphatic and aromatic
ketones with high conversion. Acetophenone was converted into
corresponding alcohol up to 89–93% yield. Isobutyl methyl ketone
underwent transfer hydrogenation to afford the corresponding
alcohol up to 70–79% yield. Similarly, cyclohexanone was con-
verted to cyclohexanol up to 78–83% yield. The role of KOH is to
generate the catalyst from the chloro precursor and the reaction
mediates through the hydride species. The base facilitated the for-
mation of the ruthenium alkoxide by abstracting the proton of the
alcohol and subsequently, the alkoxide underwent b-elimination to
give a ruthenium hydride which is the active species in this reac-
tion [38].
Conclusions
A new series of ruthenium(II) Schiff base complexes bearing
disulfide group, [Ru(CO)L1ꢁ6] were reported. The new complexes
were characterized by using elemental and spectral methods. From

358
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