Ruthenium(II) 9,10-phenanthrenequinone thiosemicarbazone complexes: Synthesis, characterization, and catalytic activity towards the reduction as well as condensation of nitriles

Article abstract of DOI:10.1080/00958972.2014.977269

The ligands 9,10-phenanthrenequinone-N⁴-substituted thiosemicarbazones (HL1-₃) and their ruthenium(II) complexes were synthesized and characterized by elemental and spectroscopic methods. The ligands are tridentate, monobasic chelating ligands with O, N, and S as the donor sites and are in the thiol form in all the complexes. Catalytic studies showed that all the complexes displayed good catalytic activity towards the reduction of nitriles and also the condensation of nitriles with 2-aminoalcohol under solvent-free conditions.

Full text of DOI:10.1080/00958972.2014.977269

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Ruthenium(II) 9,10-
phenanthrenequinone
thiosemicarbazone complexes:
synthesis, characterization, and
catalytic activity towards the reduction
as well as condensation of nitriles
Panneerselvam Anithaa, Periasamy Viswanathamurthia, Devarayan
Kesavana & Ray Jay Butcherb
a Department of Chemistry, Periyar University, Salem, India
b Department of Chemistry, Howard University, Washington, DC,
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USA
Accepted author version posted online: 15 Oct 2014.Published
online: 10 Nov 2014.
To cite this article: Panneerselvam Anitha, Periasamy Viswanathamurthi, Devarayan Kesavan &
Ray Jay Butcher (2015) Ruthenium(II) 9,10-phenanthrenequinone thiosemicarbazone complexes:
synthesis, characterization, and catalytic activity towards the reduction as well as condensation of
nitriles, Journal of Coordination Chemistry, 68:2, 321-334, DOI: 10.1080/00958972.2014.977269
To link to this article: http://dx.doi.org/10.1080/00958972.2014.977269
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Journal of Coordination Chemistry, 2015
Vol. 68, No. 2, 321–334, http://dx.doi.org/10.1080/00958972.2014.977269
Ruthenium(II) 9,10-phenanthrenequinone thiosemicarbazone
complexes: synthesis, characterization, and catalytic activity
towards the reduction as well as condensation of nitriles
PANNEERSELVAM ANITHA†, PERIASAMY VISWANATHAMURTHI*†,
DEVARAYAN KESAVAN†1 and RAY JAY BUTCHER‡
†Department of Chemistry, Periyar University, Salem, India
‡Department of Chemistry, Howard University, Washington, DC, USA
(Received 8 May 2014; accepted 29 September 2014)
NH2
Reduction
2-Butanol
N
R
Ru(II) Catalyst
O
R
N
Condensation
2-Aminoalcohol
R
The ligands 9,10-phenanthrenequinone-N4-substituted thiosemicarbazones (HL1–3) and their ruthe-
nium(II) complexes were synthesized and characterized by elemental and spectroscopic methods.
The ligands are tridentate, monobasic chelating ligands with O, N, and S as the donor sites and are
in the thiol form in all the complexes. Catalytic studies showed that all the complexes displayed
good catalytic activity towards the reduction of nitriles and also the condensation of nitriles with
2-aminoalcohol under solvent-free conditions.
Keywords: Ruthenium(II) complexes; Thiosemicarbazones; Catalytic activity; Reduction of nitriles;
2-Oxazolines
1. Introduction
Transition metal complexes containing mixed ligands with N and S or N, S, and O donors
exhibit interesting stereochemical, electrochemical, and electronic properties [1,2].
Derivatives of semicarbazones and thiosemicarbazones are amongst the most widely studied
nitrogen and oxygen/sulfur ligands [3, 4]. Particularly, thiosemicarbazones have emerged as
an important class of sulfur donor ligands for transition metal ions because of their mixed
*Corresponding author. Email: viswanathamurthi72@gmail.com
1Present address: Department of BIN Fusion Technology, Chonbuk National University, Jeonju-si, Republic of Korea.
© 2014 Taylor & Francis

322
P. Anitha et al.
hard–soft donor character and versatile coordination behavior [5]. Thiosemicarbazones of
aromatic aldehydes or ketones as tridentate ligands can yield cyclometalated complexes
having two fused five-membered chelate rings at the metal center [6].
There has been much interest in the chemistry of half-sandwich arene ruthenium(II) com-
plexes [7, 8], since syntheses of new and highly active transition metal-based catalysts
derived from arene ligands are used in different catalytic reactions including addition of
carboxylic acids to alkynes [9], transfer hydrogenation of ketones [10], polymerization of
norbornene and methyl methacrylate [11, 12], isomerization of alkenes [13], and dimeriza-
tion of alkynes [14]. Among the various arene ligands, the indenyl anion C9HÀ7 is a typical
ligand in organometallic chemistry. In contrast to the extensively used cyclopentadienyl
metal fragments [M(η5-C5H5)Ln], the analogous indenyl derivatives [M(η5-C9H7)Ln] have
attracted less attention.
The formation of carbon–nitrogen bonds is an important task for organic synthesis as a
number of nitrogen-containing molecules are used industrially for preparation of both bulk
and fine chemicals and for pharmaceuticals. A plethora of naturally occurring compounds
such as alkaloids, amino acids, and nucleotides which contain amino groups are involved in
biological processes [15]. Catalytic methods offer efficient and versatile strategies towards
the synthesis of amines and represent a key technology for the advancement of green chem-
istry, specifically in terms of waste prevention, reducing energy consumption, achieving
high atom efficiency, and generating advantageous economics [16]. In this respect, several
interesting methods have been developed, such as palladium-catalyzed amination of aryl
halides [17], hydroamination of olefins and alkynes [18], hydroaminomethylation of olefins
[19], and reductive amination of carbonyl compounds [20]. Catalytic hydrogenation of
nitriles represents an atom economic and valuable route to amines. However, compared to
reductions of C=C, C=O, and C=N bonds, the hydrogenation of nitriles has been less
investigated.
2-Oxazolines are found in a variety of biologically active natural products and enzyme
inhibitors. They also contribute to the flavors of foods [21]. Substituted oxazolines are an
important class of intermediates in modern organic synthesis [22]. Chiral mono- and bis-ox-
azoline compounds are useful auxiliaries and ligands in asymmetric reactions [23]. They
are also found in the structure of optically active polymers which attracted attention due to
their unique functions [24]. A number of methods have been developed for the preparation
of 2-oxazolines from carboxylic acids [25], carboxylic esters [26], nitriles [27, 28], alde-
hydes [29], hydroxyamides [30], and olefins [31]. Various reaction conditions and a variety
of homogeneous and heterogeneous catalysts have been applied for this purpose. Although
these methods are valuable, most of them involve one or more disadvantages including
harsh reaction conditions, long reaction times, low yields of products, the use of stoichiom-
etric amounts of catalysts, and toxic solvents. So, the development of an efficient, simple,
and environmentally benign catalytic procedure for the synthesis of this heterocycle is still
in high demand.
Based on the above facts and our continuous effort in developing efficient transition
metal catalysts, herein, we have reported the synthesis and spectral characterization of
ruthenium(II) complexes containing 9,10-phenanthrenequinone-N4-substituted thiosemicar-
bazone ligands with indenyl as co-ligand. In addition, the catalytic performance of the
ruthenium(II) complexes were tested for simple organic conversions such as transfer
hydrogenation of nitrile and synthesis of 2-oxazoline.

Ruthenium(II) complexes
323
2. Experimental
2.1. Materials and methods
All reagents used were chemically pure and AR grade. The solvents were purified and dried
according to standard procedures. RuCl3·3H2O was purchased from Loba Chemie Pvt Ltd.
HL1–3 and [RuCl(PPh3)2(η5-C9H7)] were prepared according to literature procedures
[32, 33]. The ORTEP representation of HL1–3 is shown in figures S1–S3 (see online supple-
mental material at http://dx.doi.org/10.1080/00958972.2014.977269). Relevant data collec-
tion and details of the structure refinement are summarized in table S1. Selected bond
lengths and angles for HL1–3 are given in table S2. Microanalyses of carbon, hydrogen,
nitrogen, and sulfur were carried out using a Vario EL III Elemental analyzer at SAIF –
Cochin, India. IR spectra of the ligands and their complexes were recorded as KBr pellets
on a Nicolet Avatar model spectrophotometer from 4000 to 400 cm−1. Electronic spectra of
the complexes have been recorded in dichloromethane using a Shimadzu UV-1650 PC spec-
trophotometer from 800 to 200 nm. 1H and 13C NMR spectra were recorded in a Jeol
GSX-400 instrument using DMSO-d6 as the solvent at room temperature with TMS as the
internal standard. Mass spectra were recorded under HRMS (FAB) using a JEOL JMS600H
mass spectrometer. Melting points were recorded on a Technico micro heating table and are
uncorrected.
2.2. Synthesis of new ruthenium(II) complexes
2.2.1. Synthesis of [Ru(η5-indenyl)(L1)] (1). An ethanolic solution (10 mL) containing
HL1 (0.1 mM) was added to [RuCl(PPh3)2(η5-C9H7)] (0.1 mM) in benzene (10 mL). The
resulting green solution was refluxed for 4 h and then cooled to room temperature, resulting
in a green precipitate. It was filtered off, washed with ethanol, and recrystallized from
CH2Cl2/petroleum ether mixture in order to remove the triphenylphosphine. The purity of
the complex was checked by TLC. Our efforts to obtain single crystals of the complexes
were unsuccessful. Yield 87%, m.p. 198 °C. Anal. Calcd for C24H17N3ORuS (%): C,
58.05; H, 3.45; N, 8.46; S, 6.46. Found: C, 58.69; H, 3.01; N, 8.87; S, 6.98. IR (KBr,
cm−1): 1612 (quinone C=O), 1584 (C=N), 1574 (C=N), 758 (C–S). UV–Vis (λmax/nm):
1
476, 355, 275, 227. H NMR (DMSO-d6, ppm): 4.45 (d, 2H, indenyl), 5.41 (t, 1H, inde-
nyl), 6.95–8.26 (m, 12H, aromatic protons of six-membered ring of indenyl group and
ligand), 9.48 (s, 2H, NH2). 13C NMR (DMSO-d6, ppm): 181.3 (quinone C=O), 172.4
(C–S), 161.8 (C=N), 123.5–138.4 (aromatic carbons of ligand and six-membered ring of
indenyl), 75.2, 78.6, 92.4, 107.2, 109.1 (carbons of five-membered ring of indenyl).
FAB-MS (m/z) = 497.05 [M + H]+.
2.2.2. Synthesis of [Ru(η5-indenyl)(L2)] (2). 2 was prepared using the same procedure as
described for 1 with HL2 (0.1 mM) and [RuCl(PPh3)2(η5-C9H7)] (0.1 mM). Green powder
was obtained. Yield 84%, m.p. 210 °C. Anal. Calcd for C25H19N3ORuS (%): C, 58.81; H,
3.75; N, 8.23; S, 6.28. Found: C, 58.29; H, 3.06; N, 8.87; S, 6.68. IR (KBr, cm−1): 1623
(quinone C=O), 1588 (C=N), 1560 (C=N), 758 (C–S). UV–Vis (λmax/nm): 448, 321, 252,
1
216. H NMR (DMSO-d6, ppm): 3.09 (s, 3H, CH3), 4.49 (d, 2H, indenyl), 5.46 (t, 1H,
indenyl), 7.01–8.02 (m, 12H, aromatic protons of six membered ring of indenyl group and
ligand), 8.46 (s, 1H, NH–CH3). 13C NMR (DMSO-d6, ppm): 181.1(quinone C=O), 173.2

324
P. Anitha et al.
(C–S), 162.3 (C=N), 123.6–135.9 (aromatic carbons of ligand and six-membered ring of
indenyl), 74.7, 78.1, 91.8, 107.1, 109.5 (carbons of five-membered ring of indenyl).
FAB-MS (m/z) = 511.92 [M + H]+.
2.2.3. Synthesis of [Ru(η5-indenyl)(L3)] (3). 3 was prepared using the same procedure as
described for 1 with HL3 (0.1 mM) and [RuCl(PPh3)2(η5-C9H7)] (0.1 mM). Green powder
was obtained. Yield 79%, m.p. 201 °C. Anal. Calcd for C30H21N3ORuS: C, 62.92; H, 3.70;
N, 7.34; S, 5.60. Found: C, 63.29; H, 3.06; N, 7.89; S, 5.16. IR (KBr, cm−1): 1614 (quinone
1
C=O), 1580 (C=N), 1558 (C=N), 746 (C–S). UV–Vis (λmax/nm): 468, 348, 250, 217. H
NMR (DMSO-d6, ppm): 4.44 (d, 2H, indenyl), 5.43 (t, 1H, indenyl), 6.81–8.21 (m, 17H, aro-
matic protons of six-membered ring of indenyl group and ligand), 11.42 (s, 1H, NH–C6H5).
13C NMR (DMSO-d6, ppm): 182.1 (quinone C=O), 172.2 (C–S), 163.1 (C=N), 123.8–136.1
(aromatic carbons of ligand and six-membered ring of indenyl), 74.8, 77.2, 92.1, 107.8,
109.3 (carbons of five-membered ring of indenyl). FAB-MS (m/z) = 573.53 [M + H]+.
2.3. Transfer hydrogenation of nitriles
A flask (25 mL) containing ruthenium(II) complex (1 M%) and 2-butanol (5 mL) was stir-
red for 5 min under an argon atmosphere at room temperature. Afterwards, KOtBu
(0.05 mM) was added and the mixture was stirred for another 5 min. Then, the nitrile
(0.5 mM) was added and placed on a hot plate at 120 °C for 30 min. After completion of
the reaction, the catalyst was removed from the reaction mixture by addition of petroleum
ether followed by filtration and subsequent neutralization with 1 M HCl. The ether layer
was filtered through a short path of silica gel by column chromatography. To the filtrate,
hexadecane was added as a standard and the yield was determined by GC.
2.4. Condensation of nitriles with aminoalcohol
Nitrile (1 mM), amino alcohol (3 mM), and ruthenium(II) complex (10 M%) were mixed
and stirred for 6 h at 80 °C. After completion of the reaction, the mixture was cooled to
room temperature, diluted with ethyl acetate (10 mL), and filtered. The filtrate was
concentrated in vacuo, and the resulting residue was purified by column chromatography to
provide the desired product.
2.4.1. 2-Phenyl-4,5-dihydrooxazole. 1H NMR (DMSO-d6, ppm): 7.19–7.96 (m, 5H,
aromatic CH), 4.40–4.51 (t, 2H, CH2), 3.71–3.82 (t, 2H, CH2).
2.4.2. 2-(4-Nitrophenyl)-4,5-dihydrooxazole. 1H NMR (DMSO-d6, ppm): 8.36–8.40
(d, 2H, aromatic CH), 8.09–8.12 (d, 2H, aromatic CH), 4.48–4.54 (t, 2H, CH2), 4.31–4.38
(t, 2H, CH2).
2.4.3. 2-(4-Chlorophenyl)-4,5-dihydrooxazole. 1H NMR (DMSO-d6, ppm): 7.62–7.78
(d, 2H, aromatic CH), 6.66–6.81 (d, 2H, aromatic CH), 4.41–4.48 (t, 2H, CH2), 4.02–4.10
(t, 2H, CH2).

Ruthenium(II) complexes
325
2.4.4. 2-(p-Tolyl)-4,5-dihydrooxazole. 1H NMR (DMSO-d6, ppm): 7.68–7.74 (d, 2H,
aromatic CH), 7.14–7.21 (d, 2H, aromatic CH), 4.51–4.68 (t, 2H, CH2), 4.12–4.19 (t, 2H,
CH2), 2.41 (s, 3H, CH3).
2.4.5. 4-(4,5-Dihydrooxazol-2-yl)anisole. 1H NMR (DMSO-d6, ppm): 8.38–8.50 (d, 2H,
aromatic CH), 8.02–8.14 (d, 2H, aromatic CH), 4.70–4.88 (t, 2H, CH2), 4.36–4.44 (t, 2H,
CH2), 4.08 (s, 3H, OCH3).
2.4.6. 4-(4,5-Dihydrooxazol-2-yl)aniline. 1H NMR (DMSO-d6, ppm): 7.02–7.18 (d, 2H,
aromatic CH), 6.88–6.92 (d, 2H, aromatic CH), 4.41–4.52 (t, 2H, CH2), 4.10–4.21 (t, 2H,
CH2), 3.80 (s, 2H, NH2).
2.4.7. 1,4-bis(4,5-Dihydrooxazol-2-yl)benzene. 1H NMR (DMSO-d6, ppm): 7.30–7.69
(m, 4H, aromatic CH), 4.52–4.59 (t, 2H, CH2), 4.22–4.30 (t, 2H, CH2).
2.4.8. 2-(Pyridin-4-yl)-4,5-dihydrooxazole. 1H NMR (DMSO-d6, ppm): 8.56–8.82
(d, 2H, aromatic CH), 8.08–8.18 (d, 2H, aromatic CH), 4.54–4.72 (t, 2H, CH2), 4.18–4.29
(t, 2H, CH2).
2.4.9. 2-(Naphthalen-2-yl)-4,5-dihydrooxazole. 1H NMR (DMSO-d6, ppm): 7.33–7.92
(m, 7H, aromatic CH), 4.46–4.53 (t, 2H, CH2), 4.18–4.24 (t, 2H, CH2).
3. Results and discussion
The synthetic route for ruthenium(II) complexes is shown in scheme 1. All the complexes
are stable at room temperature, non-hygroscopic, and highly soluble in common organic
solvents such as dichloromethane, benzene, acetonitrile, chloroform, and DMSO. The
analytical data are in agreement with proposed molecular formula of the complexes. In
addition, FAB-MS spectra were also employed to check the composition of the complexes.
Complexes 1–3 displayed molecular ion isotopic clusters at m/z = 497.05, 511.92 and
573.53 [M + H]+ (figures S4–S6), respectively, confirming the stoichiometry of the
complexes.
3.1. IR spectra
The ligands are monoanionic tridentate, forming two five-membered chelate rings around
ruthenium through a donor set comprising quinone carbonyl oxygen, imine nitrogen, and
thiolate sulfur as revealed from the corresponding shifts in IR frequencies of the respective
vibrations [34]. The bands assigned to azomethine (C=N) and quinone carbonyl (C=O)
vibrations appeared at 1596–1598 and 1630–1634 cm−1, respectively, in spectra of free
ligands are shifted to lower wavenumbers, while the bands at 3111–3148 and 807–
843 cm−1 ascribed to the ν(N–H) and ν(C=S) stretches, respectively, disappeared on metal

326
P. Anitha et al.
O
N
Ph3P
Ru
+
S
N
Ph3P
Cl
HN
C
H
R
Ethanol/Benzene
Reflux 4 h
O
Ru
N
S
N
N
C
H
R
(R = H, CH3 or C6H5)
Scheme 1. Synthetic route of ruthenium(II) complexes.
complexation confirming the thio enolization nature of the ligands and subsequent coordina-
tion through the deprotonated sulfur [35–37]. This was further confirmed by the appearance
of two new bands at 1580–1588 cm−1 and 746–758 cm−1 corresponding to ν(C=N–N=C) and
ν(C–S) stretches, respectively [38].
3.2. UV–Vis absorption spectra
Electronic absorption spectra of the complexes have been recorded in dichloromethane solu-
tion. The complexes showed four intense absorptions in the ultraviolet and visible region
209–476 nm. The less intense absorption at 321–476 nm is probably due to metal-to-ligand
charge transfer transition [39]. The high intensity bands below 300 nm were ligand-centered
transitions, likely due to the chelating, tridentate ligands. The pattern of the electronic
spectra of all the complexes indicated the presence of an octahedral environment around
ruthenium(II), similar to that of other ruthenium complexes [40].
3.3. NMR spectra
1H NMR spectra of the ligands and the corresponding ruthenium(II) complexes were
recorded in DMSO to confirm the presence of coordinated ligand in the complexes.

Ruthenium(II) complexes
327
The singlet at 14.41–14.81 ppm assigned to hydrazinic N–H proton indicates that the
ligands exist in thionic forms. These peaks are not found in the spectra of complexes, con-
sistent with deprotonation of these ligands upon metal complexation [41]. The terminal
NH2 protons in HL1 are magnetically non-equivalent, showing two singlets at 9.07 and
9.36 ppm. These protons became equivalent upon formation of ruthenium complexes as a
singlet at 9.48 ppm [42]. HL2, HL3, and their corresponding complexes showed a singlet at
8.35–11.42 ppm assigned to NH methyl and NH phenyl protons. In complexes, the doublet
and triplet present in the region 4.44–4.49 and 5.41–5.46 ppm, respectively, are assigned to
five-membered ring of indenyl protons. In spectra of all the complexes, the multiplet at
6.81–8.81 ppm is assigned to aromatic protons of ligand and six-membered ring of indenyl.
Further, the methyl protons are at 2.98–3.09 ppm (figures S7–S9).
The 13C NMR spectra of the complexes (figures S10–S12) have a peak at
181.1–182.1 ppm region assigned to quinone carbonyl (C=O) carbon. The azomethine
(C=N) carbon exhibits a peak at 161.8–163.1 ppm. A sharp peak at 31.7 ppm is expected
to be methyl carbon. Resonance at 172.2–173.2 ppm is assigned to C–S of thiosemicarba-
zone. The aromatic carbons of ligands and six-membered ring of indenyl showed peaks at
123.5–138.4 ppm. The resonance at 74.7–109.5 ppm is assigned to five-membered ring of
indenyl carbons.
3.4. Catalytic transfer hydrogenation of nitriles
In order to find optimal reaction conditions, the influence of time, temperature, base, and
the catalyst concentration on the yield were investigated (table 1). Our initial studies on the
development of new ruthenium(II) complexes for nitrile hydrogenations were carried out
Table 1. Screening of reaction time, temperature, base, and catalyst concentration.a
N
NH2
Catalyst
Base/ 2-butanol
Entry
Catalyst (mM)
Time (min)
Temp. (°C)
Base
Conversion (%)b
1
2
3
4
5
6
7
8
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.0025
0.00125
10
20
30
40
50
30
30
30
30
30
30
30
30
30
30
30
80
80
80
80
80
100
120
120
120
120
120
120
120
120
120
120
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
KOtBu
Cs2CO3
Na2CO3
K2CO3
KOH
38
54
66
67
67
74
85
93
78
51
30
62
–
9
10
11
12
13
14
15
16
Py
NEt3
KOtBu
KOtBu
–
52
17
aReaction conditions: benzonitrile (0.5 mM), base (10 M%), and 2-butanol (5 mL).
bYield determined by GC analysis with hexadecane as the internal standard.

328
P. Anitha et al.
Table 2. Reduction of nitriles using ruthenium(II) complexes as catalysts.
Catalyst (1 M%)
KOtBu (10 M%)
R
N
R
NH2
2-butanol
30 min, 120 oC
Yield (%)a
Entry
1
Substrate
1
2
3
N
93
97
90
N
2
89
92
82
H2N
H3C
N
3
4
88
89
90
93
80
81
N
H3CO
O2N
Cl
N
5
6
86
87
89
90
78
79
N
N
7
8
87
68
91
75
83
62
Br
N
N
(Continued)

Ruthenium(II) complexes
329
Table 2. (Continued).
Yield (%)a
Entry
9
Substrate
1
2
3
66
74
60
N
S
N
10
40
45
37
aYield determined by GC analysis with hexadecane as the internal standard.
with benzonitrile as standard substrate using 1 as catalyst. The first set of reactions was run
at constant concentration of catalyst at various time intervals at 80 °C using NaOH as base
(entries 1–5). The yield increased with reaction time and total reaction time of 30 min gave
a constant conversion of 66%. Next, we focused on the effect of reaction temperature on
the catalyst activity (entries 6 and 7). The highest yield of benzylamine was obtained at
120 °C. Next, the influence of different bases on the yield of benzylamine was examined
(entries 8–14). The best result was observed using potassium tert-butoxide with conversion
of 93%. In the presence of organic bases such as pyridine and triethylamine, no product
was observed. Furthermore, the reaction was carried out at different concentrations of cata-
lyst. An excellent yield was obtained for 1 M% of catalyst and it can be observed that even
at very low catalyst loading of 0.0025 mM (entry 15), moderate yield was obtained. The
yield decreased with decrease in catalyst loading and reaches the lowest value of 17% with
0.00125 mM of catalyst (entry 16). No primary amine was observed without catalyst under
similar reaction conditions (entry 17).
Finally, we performed the reduction of different nitriles under optimized conditions such
as 1 M% of catalyst with a reaction time of 30 min at 120 °C using potassium tert-butoxide
as base to demonstrate the scope and limitations of substrate in presence of ruthenium(II)
complexes (table 2). Aromatic nitriles substituted with functional groups such as NH2, CH3,
OCH3, NO2, Cl, and Br were tolerated without eroding the product yields (entries 2–7).
The reduction of hetero aromatic nitrile gave the respective product with moderate yield of
60–75% (entries 8 and 9), whereas 1-naphthonitrile could be converted into primary amine
in low yield of 37–45% (entry 10). Furthermore, the catalytic activity of complexes in trans-
fer hydrogenation of nitriles is different from one another due to the presence of different
substitutions on the terminal part of thiosemicarbazone moiety and the order of reactivity of
ruthenium(II) complexes with respect to the different substitutions on ligands is given by
CH3 > H > C6H5. Gratifyingly, side products are not formed under these catalytic condi-
tions. The catalyst was regenerated after the completion of catalytic reaction and was sub-
jected to IR spectral analysis. The characteristic peaks of the complex remained, indicating
survival of our complexes under the catalytic conditions. Moreover, the present catalytic
system works at mild reaction conditions with low reaction time, low catalyst loading, and
the efficiency in terms of the yield of products without side products is higher than the
existing catalytic systems [43–50].

330
P. Anitha et al.
NH2
R
II
HN
H
H
C
HO
R
O
Ru
N
S
N
N
C
H
O
R
O
H
O
N
N
N
I
N
H
C
N
SH
H
IV
N
C
H
C
N
R
R
SH
R
O
O
O
N
N
H
N
N
H
N
C
C
H
H
III
C
II
SH
N
R
N
SH
R
R
N
C
R
Scheme 2. Possible mechanistic pathway for ruthenium(II)-catalyzed transfer hydrogenation of nitriles.
A proposed mechanism for the hydrogenation of nitrile using ruthenium(II) complexes is
shown in scheme 2. Ruthenium(II) complexes are hydrogen transfer agents in the presence
of a base which can efficiently transfer the proton from the solvent (2-butanol) to nitrile and
convert it into the corresponding amine in two consecutive hydrogen transfer cycles as
shown in the mechanism. The cleavage of the Ru–S bond under catalytic condition opens
up the coordination sphere for 2-butanol which leads to formation of a thiol in I. It is evi-
dent by the presence of C–SH peak at 2576 cm−1 in the IR spectrum of the reaction mass
before the addition of nitrile [51]. Then I undergo β-elimination to give a ruthenium hydride
II which is the active catalyst; this mechanism is proposed by several workers on the stud-
ies of ruthenium complex catalyzed transfer hydrogenation reactions [52, 53]. Ruthenium
hydride complex may form an intermediate III due to the interaction of substrate (nitrile).
Then III results in the formation of IV by insertion of hydride, from which unstable imine

Ruthenium(II) complexes
331
intermediate ejects as shown. Further, the imine intermediate must then immediately
undergo another hydrogen transfer cycle with II and thus, forms the final product, primary
amine [43].
3.5. Synthesis of 2-oxazolines
Benzonitrile and 2-aminoalcohol were chosen as model substrates to optimize the reaction
conditions including reaction temperature, time, catalyst loading, and the ratio of benzoni-
trile to 2-aminoalcohol (table 3). The reaction temperature usually impacts such a reaction.
At 30–50 °C, the reaction did not work (entries 1–3). When the reaction temperature was
raised from 60 to 80 °C, the product yields increased from 42 to 76% (entries 4–6), while
the yield of the product is decreased as the temperature was raised up to 90 °C (entry 7). To
optimize the molar ratio of benzonitrile to 2-aminoalcohol, several ratios (1 : 1, 1 : 2, 1 : 3,
and 1 : 4) were employed. As shown in table 3, the ratio of benzonitrile to 2-aminoalcohol
imposed important effects on the yield of 2-phenyloxazoline. The yield of product was
gradually increased from 31 to 88% as the ratio of benzonitrile to 2-aminoalcohol was
raised from 1 : 1 to 1 : 3 (entries 6, 8, and 9) at 80 °C. Furthermore, the catalyst loading
may also affect the catalytic activity to some extent. The product yield was gradually
increased from 32 to 88% as the catalyst concentration was raised from 4 to 10 M%, with
respect to benzonitrile concentration (entries 11–14). The reaction gave only a low yield
(entry 15) without using a catalyst. Finally to optimize the reaction time, several different
time intervals (4–7 h) were employed. The yield increased with reaction time and total
reaction time of 6 h at 80 °C gave a higher yield (entries 14 and 16–18).
Table 3. Optimization of reaction conditions with respect to temperature, substrate ratio, catalyst concentration,
and time.
O
HO
NH2
Catalyst
CN
+
Solvent free
N
Entry
Temp (°C)
Substrates ratio
Catalyst loading (M%)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
30
40
50
60
70
80
90
80
80
80
80
80
80
80
80
80
80
80
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 2
1 : 1
1 : 3
1 : 4
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
12
12
12
12
12
12
12
12
12
12
4
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
4
5
7
–
–
–
42
68
76
52
31
88
86
32
53
74
88
24
59
70
88
9
10
11
12
13
14
15
16
17
18
6
8
10
–
10
10
10
aIsolated yield.

332
P. Anitha et al.
Table 4. Synthesis of 2-oxazolines catalyzed by ruthenium(II) complexes.a
Yield (%)b
Entry
1
Nitrile
Product
1
2
3
O
87
80
78
CN
N
O
2
3
4
93
89
85
90
85
79
88
81
78
O2N
Cl
O2N
Cl
CN
N
O
CN
N
O
H3C
H3C
H3CO
H2N
NC
CN
CN
CN
N
O
5
6
83
72
78
65
76
60
H3CO
N
O
H2N
N
O
O
7
8
74
88
68
83
64
80
CN
N
N
O
N
N
N
CN
CN
9
78
71
69
O
N
aReaction conditions: nitrile (1 mM), 2-aminoalcohol (3 mM), and catalyst loading = 10 M% at 80 °C for 6 h.
bIsolated yield.
The optimal reaction conditions were identified as: solvent free, 1 : 3 molar ratio of
nitriles to 2-aminoalcohol, 10 M% of catalyst, and with a reaction time of 6 h at 80 °C.
With the optimized reaction conditions, a variety of substituted nitrile derivatives were cho-
sen as the substrates in this tandem reaction (table 4). The condensation reactions were per-
formed and the desired products were isolated in moderate to excellent yields. As shown in
table 4, aromatic nitriles-containing electron-withdrawing substituents proceed in higher
yields than those with electron-donating substituents. For example, higher yields (entries 2
and 3) were obtained for aromatic benzonitrile-bearing electron-withdrawing group relative
to those of electron-donating ones (entries 4–6). The reactions of isonicotinonitrile and
2-naphthonitrile performed well to give good yields (entries 8 and 9). As shown in table 4,
1 exhibited more efficient catalytic performance for condensation of nitriles with 2-aminoal-
cohol than 2 and 3, which may be due to the steric hindrance caused by bulky methyl and
phenyl groups on the terminal part of the thiosemicarbazone ligands in 2 and 3,
respectively. In comparison with other reported catalytic systems, our catalysts exhibit the

Ruthenium(II) complexes
333
best activity in terms of low catalyst loading [54–56], low reaction time [57–60], and high
yield without any side products [60, 61]. Moreover, the present catalytic system works
under solvent-free conditions and prevent the problems which many associate with use of
solvent such as cost, handling, safety, and pollution.
4. Conclusion
The ligands 9,10-phenanthrenequinone-N4-substituted thiosemicarbazones and its ruthenium
(II) complexes were synthesized and characterized by elemental and spectroscopic methods.
The complexes showed efficient catalysis for the transfer hydrogenation of nitriles with high
conversions. Complexes also catalyze the syntheses of 2-oxazolines from condensation of
aromatic nitrile with 2-aminoalcohol. The change in catalytic activities of the complexes
can be reasonably explained by the presence of different substituents on the terminal part of
the thiosemicarbazone moiety. The present approach is simple, convenient, and efficient
compared to previous methods.
Supplementary material
CCDC 979369, 999390, and 999391 contain the supplementary crystallographic data for
the compounds HL1–HL3. These data can be obtained free of charge via http://www.ccdc.
cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Center, 12
Union Road, Cambridge CB2 1EZ, UK; Fax: (+44) 1223 336 033; or E-mail: deposit@
ccdc.cam.ac.uk.
Acknowledgements
The authors express their sincere thanks to Council of Scientific and Industrial Research
(CSIR), New Delhi [grant number 01(2437)/10/EMR-II] for financial support. One of the
authors (PA) thanks CSIR for the award of Senior Research Fellowship.
References
[1] S. Chandra, M. Tyagi. J. Coord. Chem., 184, 778 (2009).
[2] R. Prabhakaran, R. Karvembu, T. Hashimoto, K. Shimizu, K. Natarajan. Inorg. Chim. Acta., 358, 2093
(2005).
[3] U.E. Ayaan. J. Coord. Chem., 65, 629 (2012).
[4] R.F.F. Costa, A.P. Rebolledo, T. Matencio, H.D.R. Calado, J.D. Ardisson, M.E. Cortés, B.L. Rodrigues, H.
Beraldo. J. Coord. Chem., 58, 1307 (2005).
[5] F. Basuli, S.M. Peng, S. Bhattacharya. Inorg. Chem., 39, 1120 (2000).
[6] D.X. West, S.B. Padhye, P.B. Sonawane. Struct. Bond., 76, 1 (1991).
[7] (a) V. Cadierno, J. Dıez, M.P. Gamasa, J. Gimeno, E. Lastra. Coord. Chem. Rev., 193–195, 147 (1999); (b) B.
́
Therrien. Coord. Chem. Rev., 253, 493 (2009); (c) G. Pampaloni. Coord. Chem. Rev., 254, 402 (2010).
[8] (a) P. Govindaswamy, G.S. Fink, B. Therrien. Organometallics, 26, 915 (2007); (b) F.A. Beckford, G.
Leblanc, J. Thessing, M. Shaloski, B.J. Frost, L. Li, N.P. Seeram. Inorg. Chem. Commun., 12, 1094 (2009);
(c) A.M. Pizarro, M. Melchart, A. Habtemariam, L. Salassa, F.P.A. Fabbiani, S. Parsons, P.J. Sadler. Inorg.
Chem., 49, 3310 (2010).
[9] D. Devanne, C. Ruppin, P.H. Dixneuf. J. Org. Chem., 53, 925 (1988).

334
P. Anitha et al.
[10] R.K. Rath, M. Nethaji, A.R. Chakravarty. Polyhedron, 20, 2735 (2001).
[11] P. Alvarez, J. Gimeno, E. Lastra. Organometallics, 21, 5678 (2002).
[12] X. Zhao, X. Luo, B. Li, H. Song, S. Xu, B. Wang. Eur. Polym. J., 44, 3264 (2008).
[13] H. Brunner, T. Zwack, M. Zabel, W. Beck, A. Böhm. Organometallics, 22, 1741 (2003).
[14] M. Bassetti, S. Marini. Organometallics, 21, 4815 (2002).
[15] (a) S.A. Lawrence. Amines: Synthesis, Properties, and Application, Cambridge University Press, Cambridge
(2004); (b) J.F. Hartwig. In Handbook of Organopalladium Chemistry for Organic Synthesis, E.I. Negishi
(Ed.), Vol. 1, p. 1051–1096, Wiley, New York (2002).
[16] (a) P.T. Anastas, M.M. Kirchhoff. Acc. Chem. Res., 35, 686 (2002); (b) P.T. Anastas, M.M. Kirchhoff, T.C.
Williamson. Appl. Catal., A: Gen., 221, 3 (2001).
[17] J.F. Hartwig. Synlett., 9, 1283 (2006).
[18] K.C. Hultzsch, D.V. Gribkov, F. Hampel. J. Organomet. Chem., 690, 4441 (2005).
[19] L. Routaboul, C. Buch, H. Klein, R. Jackstell, M. Beller. Tetrahedron Lett., 46, 7401 (2005).
[20] R.I. Storer, D.E. Carrera, Y. Ni, D.W.C. MacMillan. J. Am. Chem. Soc., 128, 84 (2006).
[21] J.A. Maga. CRC Crit. Rev. Food Sci. Nutr., 14, 295 (1981).
[22] T.G. Gant, A.I. Meyers. Tetrahedron, 50, 2297 (1994).
[23] G. Desimoni, G. Faita, K.A. Jørgensen. Chem. Rev., 106, 3561 (2006).
[24] X. Xi, L. Jiang, W. Sun, Z. Shen. Eur. Polym. J., 41, 2592 (2005).
[25] A. Cwik, Z. Hell, A. Hegedüs, Z. Finta, Z. Horváth. Tetrahedron Lett., 43, 3985 (2002).
[26] P. Zhou, J.E. Blubaum, C.T. Burns, N.R. Natale. Tetrahedron Lett., 38, 7019 (1997).
[27] D.S. Clarke, R. Wood. Synth. Commun., 26, 1335 (1996).
[28] I. Mohammadpoor-Baltork, A.R. Khosropour, S.F. Hojati. Synlett., 18, 2747 (2005).
[29] J.G. Badiang, J. Aubé. J. Org. Chem., 61, 2484 (1996).
[30] E.J. Corey, K. Ishihara. Tetrahedron Lett., 33, 6807 (1992).
[31] S. Minakata, M. Nishimura, T. Takahashi, Y. Oderaotoshi, M. Komatsu. Tetrahedron Lett., 42, 9019 (2001).
[32] P. Anitha, N. Chitrapriya, Y.J. Jang, P. Viswanathamurthi. J. Photochem. Photobiol. B, 129, 17 (2013).
[33] L.A. Oro, M.A. Ciriano, M. Campo, C. Foces-Foces, F.H. Cano. J. Organomet. Chem., 289, 117 (1985).
[34] D. Kovala-Demertzi, M.A. Demertzis, J.R. Miller, C. Papadopoulou, C. Dodorou, G. Filousis. J. Inorg.
Biochem., 86, 555 (2001).
[35] L.J. Bellamy. The Infrared Spectra of Complex Molecules, Vol. 353, Methuen, London (1985).
[36] M. Ulaganatha Raja, N. Gowri, R. Ramesh. Polyhedron, 29, 1175 (2010).
[37] E.M. Jouad, A. Riou, M. Allain, M.A. Khan, G.M. Bouet. Polyhedron, 20, 67 (2001).
[38] A. Castineiras, D.X. West, H. Gebremedhin, T.J. Romack. Inorg. Chim. Acta, 216, 229 (1994).
[39] A. Ceulemans, L.G. Vanquickenborne. J. Am. Chem. Soc., 103, 2238 (1981).
[40] A.B.P. Lever. Inorganic Electronic Spectroscopy, 2nd Edn, Elsevier, New York (1984).
[41] Z. Afrasiabi, E. Sinn, S. Padhye, S. Dutta, S. Padhye, C. Newton, C.E. Anson, A.K. Powell. J. Inorg.
Biochem., 95, 306 (2003).
[42] Z. Afrasiabi, E. Sinn, W. Lin, Y. Ma, C. Campana, S. Padhye. J. Inorg. Biochem., 99, 1526 (2005).
[43] T. Li, I. Bergner, F. Nipa Haque, M. Zimmer-De Iuliis, D. Song, R.H. Morris. Organometallics, 26, 5940 (2007).
[44] D. Addis, S. Enthaler, K. Junge, B. Wendt, M. Beller. Tetrahedron Lett., 50, 3654 (2009).
[45] C. Gunanathan, M. Holscher, W. Leitner. Eur. J. Inorg. Chem., 22, 3381 (2011).
[46] S. Enthaler, D. Addis, K. Junge, G. Erre, M. Beller. Chem. Eur. J., 14, 9491 (2008).
[47] S. Takemoto, H. Kawamura, Y. Yamada, T. Okada, A. Ono, E. Yoshikawa, Y. Mizobe, M. Hidai.
Organometallics, 21, 3897 (2002).
[48] C. Bianchini, V.D. Santo, A. Meli, W. Oberhauser, R. Psaro, F. Vizza. Organometallics, 19, 2433 (2000).
[49] M. Vilches-Herrera, S. Werkmeister, K. Junge, A. Borner, M. Beller. Catal. Sci. Technol., 4, 629 (2014).
[50] S. Chakraborty, H. Berke. ACS Catal., 4, 2191 (2014).
[51] K.H. Reddy, P.S. Reddy, P.R. Babu. Transition Met. Chem., 25, 154 (2000).
[52] (a) R. Noyori. Asymmetric Catalysis in Organic Synthesis, Wiley, New York (1994); (b) I. Ojima. Catalytic
Asymmetric Synthesis, 2nd Edn, Wiley, New York (2000); (c) R. Noyori, S. Hashiguchi. Acc. Chem. Res., 30,
97 (1997).
[53] (a) E.P. Kelson, P.P. Phengsy. J. Chem. Soc., Dalton Trans., 4023 (2000); (b) H. Zhang, B.C. Yang, Y.Y. Li,
Z.R. Donga, J.X. Gao, H. Nakamura, K. Murata, T. Ikariya. Chem. Commun., 1, 142 (2003); (c) J.
Hannedouche, G.J. Clarkson, M. Wills. J. Am. Chem. Soc., 126, 986 (2004).
[54] X. Fernandez, R. Fellous, E. Duñach. Tetrahedron Lett., 41, 3381 (2000).
[55] I. Mohammadpoor-Baltork, A.R. Khosropour, S.F. Hojati. Catal. Commun., 8, 200 (2007).
[56] A. Shaabani, M. Seyyedhamzeh, A. Maleki, F. Rezazadeh. Appl. Catal. A, 358, 146 (2009).
[57] S. Kumar, K. Kandasamy, H.B. Singh, R.J. Butcher. New J. Chem., 28, 640 (2004).
[58] T. Bodner, L. Ellmaier, V. Schenk, J. Albering, F. Wiesbrock. Polym. Int., 60, 1173 (2011).
[59] G.K. Jnaneshwara, V.H. Deshpande, M. Lalithambika, T. Ravindranathan, A.V. Bedekar. Tetrahedron Lett.,
39, 459 (1998).
[60] H. Witte, W. Seeliger. Angew. Chem. Int. Ed., 11, 287 (1972).
[61] J.A. Seijas, M.P. Vázquez-Tato, J. Crecente-Campo. Tetrahedron, 64, 9280 (2008).