Ruthenium(II) bis(hydrazone) complexes derived from 1,3,4-oxadiazoles: Synthesis, crystal structure and catalytic application in N-alkylation reactions

Article abstract of DOI:10.1016/j.ica.2015.01.002

1,3,4-Oxadiazoles (A-C) were derived via a series of reactions between isoniazid and salicylaldehydes. While reacting the oxadiazoles with [RuHCl(CO)(PPh₃)₃] in the presence of NaOH, mononuclear ruthenium(II) complexes bearing 'salen' type N,N′-bis(salicylidene)hydrazone ligands (1-3) were obtained. The oxadiazoles and ruthenium(II) complexes were characterized by analytical and spectral methods. The single crystal XRD analyses of complexes 1 and 2 suggested an octahedral geometry around ruthenium(II) ions in which the bis(hydrazone) act as mononegative bidentate ligands. It was also observed that the presence of an intramolecular hydrogen bonding between the hydroxyl proton and one of the azomethine nitrogens in all the complexes. Further, the complexes were proved as versatile catalysts for the N-alkylation of amines with alcohols under optimized reaction conditions.

Full text of DOI:10.1016/j.ica.2015.01.002

Inorganica Chimica Acta 427 (2015) 203–210
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Ruthenium(II) bis(hydrazone) complexes derived
from 1,3,4-oxadiazoles: Synthesis, crystal structure
and catalytic application in N-alkylation reactions
Govindan Prakash a, Rangasamy Ramachandran a, Muthukumaran Nirmala a,
Periasamy Viswanathamurthi a, , Jesus Sanmartin b
⇑
a Department of Chemistry, Periyar University, Salem 636 011, India
b Department of Inorganic Chemistry, University of Santigo de Compostela, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
1,3,4-Oxadiazoles (A–C) were derived via a series of reactions between isoniazid and salicylaldehydes.
While reacting the oxadiazoles with [RuHCl(CO)(PPh3)3] in the presence of NaOH, mononuclear ruthe-
nium(II) complexes bearing ‘salen’ type N,N0-bis(salicylidene)hydrazone ligands (1–3) were obtained.
The oxadiazoles and ruthenium(II) complexes were characterized by analytical and spectral methods.
The single crystal XRD analyses of complexes 1 and 2 suggested an octahedral geometry around ruthe-
nium(II) ions in which the bis(hydrazone) act as mononegative bidentate ligands. It was also observed
that the presence of an intramolecular hydrogen bonding between the hydroxyl proton and one of the
azomethine nitrogens in all the complexes. Further, the complexes were proved as versatile catalysts
for the N-alkylation of amines with alcohols under optimized reaction conditions.
Received 2 December 2014
Received in revised form 27 December 2014
Accepted 1 January 2015
Available online 7 January 2015
Keywords:
Ru(II) complexes
Bis(hydrazone) ligands
1,3,4-Oxadiazoles
XRD studies
Ó 2015 Elsevier B.V. All rights reserved.
N-alkylation
1. Introduction
the requirement of high temperature, pressure and long reaction
times up to few days. Further the scope of substrates is also lim-
Synthesis of secondary and tertiary amines through the N-alkyl-
ation of primary amine is one of the most important and essential
reactions in synthetic chemistry. N-alkylation using alkyl halides is
a traditional method for this synthesis [1]. However, it is not a
desirable method as it produced harmful salts as waste by-prod-
ucts. The other method, reductive amination of aldehydes or
ketones requires very strong reducing agents [2]. The industrial
importance of several alkylated amines and the disadvantages
found in conventional N-alkylation methods triggered the research
in this field to produce better catalytic system for N-alkylation. The
method of N-alkylation of amines with alcohols using transition
metal catalysts has been reported as an attractive alternate for
the traditional methods, because of its interesting features such
as, it does not create any dangerous by-products, alcohol is more
common and readily available compared to alkyl halides [3]. More
importantly, the usage of transition metal complexes considerably
increases the yield. Metal complexes of ruthenium [4], platinum
[5], rhodium [6] and iridium [7] are some of the important cata-
lysts reported so far that have been proven to give good yield.
However, most of the reported methods have some difficulties like
ited. Hence, research on producing suitable and versatile transition
metal catalysts getting much attention. In the last few decades,
research has been focused largely on N-alkylation reactions cata-
lyzed by ruthenium complexes of various ligands and to extend
the applicability of this reaction towards a broad range of sub-
strates [8]. Our research group has also reported some interesting
ruthenium catalysts for N-alkylation which could be applicable for
a wide range of substrates [9].
Schiff bases derived from hydrazines known as hydrazones are
a class of ligand which has been reported for their excellent ligat-
ing properties [10]. Many hydrazone Schiff base complexes have
been reported as excellent biological and catalytic active molecules
[11,12]. N,N0-bishydrazones known as azines, are formed by the
condensation of both NH2 groups of hydrazine with aldehydes or
ketones. The diazine linkage(N-N) in its structure offers enormous
possibilities for mono and binucleating modes of coordination
because of the flexibility of N–N single bond [13]. However, the
direct synthesis of N,N0-bis(salicylidene) product from hydrazine
has results in very low yield [14] since, the Schiff base is generally
formed by the condensation of any one of the NH2 groups. In our
literature survey, we found only a very few reports on N2O2 donor
bis(salicylidene) hydrazones which may due to their synthetic dif-
ficulty [15].
⇑
Corresponding author. Fax: +91 427 2345124.
E-mail address: viswanathamurthi72@gmail.com (P. Viswanathamurthi).
http://dx.doi.org/10.1016/j.ica.2015.01.002
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

204
G. Prakash et al. / Inorganica Chimica Acta 427 (2015) 203–210
In the present work, we reacted the 1,3,4-oxadiazoles (A–C)
(Calc.)] for C14H11N3O3: C, 62.17 (67.45); H, 4.39 (4.12); N, 15.52
(15.61)%.
derived from isoniazid and salicylaldehydes with ruthenium(II)
precursors in the presence of NaOH, with the aim of producing
Ru(II) complexes bearing oxadiazoles. Surprisingly, we observed
the formation of Ru(II) N,N0-bishydrazone complexes (1–3). As a
part of our continuous research on utilizing new transition metal
complexes for various catalytic applications, herein we have
reported an alternative pathway for the synthesis of ruthenium(II)
bis(salicylidene) complexes (1–3) and their versatile catalytic
activity in the N-alkylation of amines with alcohols.
2.3. Synthesis and characterization of Ru(II) complexes
The oxadiazole compound (A–C) was dissolved in methanol and
added 1 mL of methanolic NaOH solution and stirred at room tem-
perature for 30 min. To this, an equimolar quantity of precursor
complex [RuHCl(CO)(PPh3)3] dissolved in methanol was added
and refluxed with stirring for 3 h. Then the reaction mixture was
filtered and kept at room temperature for 48 h to obtain reddish
crystals of Ru(II) hydrazone complexes. The spectral and elemental
data obtained for complex 1 are in good agreement with that of
similar reported complex, which has been synthesized by the reac-
tion of [RuHCl(CO)(PPh3)3] with N,N0-bis(salicylidene)hydrazine in
methanol in presence of KOH [15c].
2. Experimental
2.1. Materials and methods
The chemicals of AR grade were purchased commercially and
used without further purification. The precursor complex
[RuHCl(CO)(PPh3)3] was prepared based on the literature proce-
dure [16]. Elemental analyses of carbon, hydrogen and nitrogen
were carried out using a Vario EL III elemental analyzer. The IR
spectra were obtained as KBr pellets on a Nicolet Avatar model
spectrophotometer in the range of 4000–400 cmꢀ1. Electronic
spectra were recorded using a Shimadzu UV-1650 PC spectropho-
tometer in 800–200 nm range. The 1H and 13C NMR spectral anal-
yses were performed on Bruker Avance III instrument using TMS as
internal standards. GC-Mass spectral studies were carried out on
JEOL GCMATE II mass spectrometer. Melting points were checked
using the Technico micro heating table and are uncorrected.
2.3.1. Ruthenium(II) bis(5-chlorosalicylidene)hydrazone complex (2)
Reddish orange; Yield: 76%. M.p.: 241 °C. UV–Vis (CH2Cl2,
kmax/nm,
(1636), 452 (22,824). FT-IR (KBr,
e
max/ dm3molꢀ1 cmꢀ1): 249 (36,282), 270 (28,462), 340
m
cmꢀ1): 3339 (OAH), 1672
(C@N1), 1597 (C@N2), 1512 (CAO), 1540 (C@C). 1H NMR (CDCl3,
ppm): 12.03 (s, 1H, OH), 8.74 (s, H, CH@N1), 9.31 (s, 1H, CH@N2),
7.11–7.48 (m, 8H, Ar), 7.73–7.96 (m, PPh3-H, 30H), ꢀ6.82 (d, Ru-
H, 1H). 13C NMR (DMSO-d6, ppm): 206.32 (C„O), 168.83
(C@NARu), 163.76 (C@N), 159.00 (CAO), 135.12 (Ar-C), 134.65
(Ar-C), 134.58 (Ar-C), 134.36 (Ar-C), 134.28 (Ar-C), 134.20 (Ar-C),
133.65 (Ar-C), 132.46 (Ar-C), 132.16 (Ar-C), 129.52 (Ar-C), 128.01
(Ar-C), 127.95 (Ar-C), 127.89 (Ar-C), 124.18 (Ar-C), 119.32 (Ar-C),
118.28 (Ar-C), 117.38 (Ar-C); Anal. [Found (Calc.)] for C51H40N2O3
P2Cl2Ru: C, 63.77 (63.62); H, 4.36 (4.19); N, 2.74 (2.91)%.
2.2. Preparation of oxadiazole compounds (A–C)
The oxadiazole compounds A–C were prepared from isoniazid
and salicylaldehydes in a three stage reaction according to the lit-
erature methods [17]. The products were recrystallized from etha-
nol at 60 °C. The detailed general procedure for the preparation of
oxadiazoles is given in Scheme S1, ESI.
2.3.2. Ruthenium(II) bis(3-methoxysalicylidene)hydrazone complex
(3)
Red; Yield: 73%. M.p.: 268 °C. UV–Vis (CH2Cl2, kmax/nm,
e
max/dm3 molꢀ1 cmꢀ1): 251 (32772), 284 (26123), 342 (1232),
465 (18365). FT-IR (KBr,
m
cmꢀ1): 3271 (OH), 1646 (C@N1), 1604
(C@N2), 1496 (CAO), 1550 (C@C). 1H NMR (CDCl3, ppm): 11.34
(s, 1H, OH), 8.55 (s, H, CH@N1), 8.79 (s, 1H, CH@N2), 6.91–7.24
(m, 8H, Ar), 7.40–7.86 (m, PPh3-H, 30H), 3.30, 2.62 (s, 1H each,
OCH3), ꢀ6.04 (d, Ru-H, 1H). 13C NMR (DMSO-d6, ppm): 203.90
(C„O), 167.23 (C@N-Ru), 158.51 (C@N), 149.86 (ArCAO), 134.30
(Ar-C), 133.24 (Ar-C), 133.16 (Ar-C), 132.71 (Ar-C), 130.40 (Ar-C),
130.17 (Ar-C), 130.07 (Ar-C), 129.63 (Ar-C), 128.91 (Ar-C), 128.79
(Ar-C), 128.56 (Ar-C); Anal. [Found (Calc.)] for C53H46N2O5P2Ru:
C, 66.92 (66.73); H, 4.97 (4.86); N, 2.68 (2.94)%.
2.2.1. 2-(5-Pyridine-4-yl-[1,3,4]oxadiazol-2-yl)-phenol (A)
Compound A was prepared from isonicotinic acid hydrazide
(1.4 g, 10 mmol) and salicylaldehyde (1.6 mL, 11 mmol). Yellow;
Yield: 78%. M.p.: 220 °C. UV–Vis (CH3OH, kmax, nm): 257, 291. IR
(KBr,
m
cmꢀ1): 3399 (OH), 1678 (C@N), 1384 (CAO), 1546 (Aro-
matic C@C). 1H NMR (CDCl3, ppm): 10.92 (s, 1H, OH), 6.98–7.71
(m, 4H, Ar), 7.82 (d, 2H, Py), 8.10 (d, 2H, Py). GC–MS: 239.40
[M]+. Anal. [Found (Calc.)] for C13H9N3O2: C, 65.04 (65.27); H,
3.84 (3.79); N, 17.71 (17.56)%.
2.4. X-ray crystallography
2.2.2. 4-Chloro-2-(5-pyridine-4-yl-[1,3,4]oxadiazol-2-yl)-phenol (B)
Compound B was synthesized from isonicotinic acid hydrazide
(1.4 g, 10 mmol) and 5-chlorosalicylaldehyde (1.7 g, 11 mmol).
Crystals of the complexes 1 and 2 were mounted on glass fibers
and used for data collection. Crystal data were collected at 295(2) K
using a Gemini A Ultra Oxford Diffraction automatic diffractome-
Yellow; Yield: 83%. M.p.: 178 °C. UV–Vis (CH2Cl2, kmax/nm, emax
/
dm3 molꢀ1 cmꢀ1): 242 (26424), 288 (22236). (KBr,
m
cmꢀ1): 3457
ter. Graphite monochromated Mo K
a radiation (k = 0.71073 Å)
(OH), 1680 (C@N), 1345 (CAO), 1445 (C@C). 1H NMR (CDCl3,
ppm): 11.05 (s, 1H, OH), 6.82–7.52 (m, 3H, Ar), 7.80 (d, 2H, Py),
8.10 (d, 2H, Py). GC–MS: 275.04 [M+H]+. Anal. [Found (Calc.)] for
was used throughout. The absorption corrections were performed
by the multi-scan method. Corrections were made for Lorentz
and polarization effects. The structures were solved by direct
methods using the program SHELXS. Refinement and all further cal-
culations were carried out using SHELXL-97 [18]. The H atoms were
included in calculated positions and treated as riding atoms using
the SHELXL default parameters. The non-hydrogen atoms were
refined anisotropically, using weighted full-matrix least squares
on F2. Atomic scattering factors were incorporated in the computer
programs. In the solid state of complex 2, a disorder is observed
within the two different enantiomeric forms in such a manner that
only the CAO group and hydrogen atom shared the ligand position
mutually.
C13H8ClN3O2: C, 56.97 (57.05); H, 2.87 (2.95); N, 15.52 (5.35)%.
2.2.3. 2-Methoxy-6-(5-pyridine-4-yl-[1,3,4]oxadiazol-2-yl)-phenol (C)
Compound C was synthesized from isonicotinic acid hydrazide
(1.4 g, 10 mmol) and o-vanillin (1.67 g, 11 mmol). Yellow; Yield:
68%. M.p.: 276 °C. UV–Vis (CH2Cl2, kmax/nm,
e
max/dm3 molꢀ1
m
cmꢀ1): 3458 (OH),
-
cmꢀ1): 246 (28256), 306 (19743). IR (KBr,
1674 (C@N), 1371 (CAO), 1514 (C@C).1H NMR (CDCl3, ppm):
10.25 (s, 1H, OH), 6.65–7.13 (m, 3H, Ar), 7.20 (d, 2H, Py), 7.72 (d,
2H, Py), 3.35 (s, 3H, OCH3). GC–MS: 269.13 [M]+. Anal. [Found

G. Prakash et al. / Inorganica Chimica Acta 427 (2015) 203–210
205
2.5. General procedure for the N-alkylation of amines with alcohols
bases were isolated (Scheme 1). The exact mechanism of this reac-
tion has not been found yet. However, we have suggested a tenta-
tive pathway as exhibited in Scheme 2. According to which, the
oxadiazoles undergo a ring cleavage in presence of the starting
complex under strong basic condition, thereby producing a new
series of Ru(II) complexes bearing bis(salicylidene)hydrazones
(1–3). All the complexes synthesized were stable at room temper-
ature and highly soluble in organic solvents like chloroform,
dichloromethane and DMSO. Elemental analyses data obtained
for both the oxadiazole compounds and the Ru(II) complexes are
in good agreement with the corresponding molecular formulae.
0.5 mol% ruthenium(II) catalyst was stirred with 4 mmol of
KOH in toluene. To this mixture, 2 mmol of alcohol and 2 mmol
of amine were added and the temperature was raised up to
120 °C. The progress of the reactions was monitored using TLC.
As soon as the reaction was completed, the mixture was cooled
to room temperature and added 3 mL of distilled water. The com-
bined mixture was extracted with CH2Cl2 and dried by adding
magnesium sulfate. The crude product was purified by column
chromatography (n-hexane/EtOAc) and characterized by 1H NMR
spectral analyses. The 1H NMR data obtained for the catalytic prod-
ucts were compared with literature [19].
3.2. X-ray crystallography
3. Results and discussion
Complexes 1 and 2 were crystallized in methanol as orthorhom-
bic (P212121) and triclinic P1 space groups, respectively. The crystal
ꢀ
3.1. Synthesis of new complexes
structure and parameters of complex 1 (Table S1 and S2, ESI) are
similar to the previous report of Trivedi et al. [15c] in which the
complex was synthesized directly from the hydrazone ligand.
Hence, here we discussed only the crystal structure of complex 2
in detail. Table 1 summarizes the crystal data and structure refine-
ment parameters of complex 2 and the selected bond lengths and
bond angles of the same have been given in Table 2. The crystal
structure analysis shows that the complex 2 has been exhibited a
distorted octahedral geometry of coordination around the metal
center. The octahedral geometry has been completed by the
The oxadiazole compounds A–C were synthesized from a series
of reactions starting from the reaction between isoniazid and 2-
hydroxybenzaldehydes as per the literature methods. The obtained
analytical and spectral data of compounds have confirmed the for-
mation of oxadiazole compounds. The oxadiazoles A–C were
expected to act as bidentate NO donor ligands with Ru(II) precur-
sor [RuHCl(CO)(PPh3)3]. However, interestingly, a new series of
ruthenium(II) complexes bearing bis(salicylidene)hydrazone Schiff
H
O
N
NH2
R2
CHO
N
N
OH
i) EtOH / Reflux 2 h
ii) Ac O / Reflux 12 h
iii) MeOH / 10% H SO
2
N
OH
R1
2
O
R1
4
N
A - C
R2
NaOH / [RuHCl(CO)(PPh3)2]
/
MeOH / Reflux 3 h
N
PPh3
OC
H
Ru
O
H
N
N
O
N
O
PPh3
PPh3
N
H
R1
Ru
O
R2
CO
1 - 3
PPh3
R1 = H, R2 = H (1)
R1 = H, R2 = Cl (2)
R2
R1
R1 = OCH3, R2 = H (3)
Scheme 1. General synthesis of ruthenium(II) complexes 1–3.
N
O
H O
HO
N
OH
N
N
N
N
N
NaOH/Methanol
PPh3
H
O
N
N
N
O
[RuHCl(CO)(PPh3)3]
Ru
N
H
CO
O
Ru
PPh3
O
Scheme 2. Tentative mechanism for the formation of hydrazone complex from oxadiazole.

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G. Prakash et al. / Inorganica Chimica Acta 427 (2015) 203–210
Table 1
Crystal data and structure refinement parameters of complexes 2.
Complex 2
Empirical formula
Formula weight
Color
C51H40Cl2N2O3P2Ru
962.76
orange
Crystal dimensions (mm3)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
0.40x0.17x0.06
295(2)
0.71073
triclinic
ꢀ
P1
12.6926(7)
b (Å)
17.8892(9)
c (Å)
22.2870(12)
101.974(2)
100.745(2)
109.686(2)
4476.92(60)
4
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
Dcalc (Mg mꢀ3
)
1.428
0.586
Absorption coefficient (mmꢀ1
)
F(000)
1968
Theta range for data collection (°)
Absorption correction
Refinement method
2.70–26.555
multi-scan
full-matrix least squares on F2
18288/0/1127
1.031
Data/restraints/parameters
Goodness-of-fit on F2
R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0453, wR2 = 0.1110
R1 = 0.0870, wR2 = 0.1110
Fig. 1. The ORTEP diagram of complex 1. Ellipsoids are drawn at the 35% probability
level.
Table 2
Selected geometrical parameters of complex 2.
Complex 2
carbonyl complexes [20]. The bond lengths of metal hydrogen
bonds (Ru–H) has been obtained 1.5415 Å. Intra molecular hydro-
gen bonding was observed between the unbound azomethine
nitrogen and OH proton which causes a slight increased length of
O–H bond. The bond length O(22)–H(22) in complex 2 has been
observed as 0.80(6) Å. The ORTEP views of the complexes 1 and 2
have been depicted in Fig. 1 and Fig. 2, respectively. Crystal packing
diagrams of complexes 1 and 2 have been provided in Figs. S1 and
S2, ESI. Concisely, in both the complexes 1 and 2, the ligands
bis(salicylidene)hydrazones bind with metal center in a mono neg-
ative bidentate fashion along with two triphenylphosphine (trans
to each other), one carbonyl and hydride co-ligands with a coordi-
nation geometry of distorted octahedron.
Interatomic distances
Ru(1)–C(1)
1.844 (5)
2.106 (3)
2.170 (3)
2.3655 (10)
2.3699 (10)
1.5485
Ru(1)–O(11)
Ru(1)–N(11)
Ru(1)–P(11)
Ru(1)–P(12)
Ru(1)–H(1)
Bond angles (°)
C(1)–Ru(1)–O(2)
C(1)–Ru(1)–N(11)
C(10)–Ru(1)–N(11)
C(1)–Ru(1)–P(11)
C(10)–Ru(1)–P(11)
N(11)–Ru(1)–P(11)
C(1)–Ru(1)–P(12)
C(10)–Ru(1)–P(12)
N(11)–Ru(1)–P(12)
P(11)–Ru(1)–P(12)
C(1)–Ru(1)–H(1)
C(10)–Ru(1)–C(1)
N(11)–Ru(1)–H(1)
P(11)–Ru(1)–H(1)
P(12)–Ru(1)–H(1)
179 (3)
99.29 (17)
176.4 (7)
92.01 (14)
92.8 (7)
90.60 (8)
94.54 (14)
84.6 (7)
92.06 (8)
172.44 (4)
79.1
3.3. IR and UV–Vis spectroscopy
Free oxadiazole compounds (A–C) exhibit a broad peak around
3400 cmꢀ1 which corresponds to –OH stretching vibration. These
peaks have shifted to lower frequencies in the IR spectra of corre-
sponding Ru(II) hydrazone complexes around 3236–3339 cmꢀ1
region. This confirms the presence of a free OH group in all the
complexes. The ring C@N bonds of oxadiazoles have shown a sharp
peak at the region 1657–1680 cmꢀ1 [21]. While the IR spectra of
complexes exhibit two azomethine frequencies in the regions
1597–1672 cmꢀ1. These frequencies can be attributed to the
unbound (C@N1) and bound (C@N2) azomethine nitrogens of com-
plexes. The IR spectral data are well explained the obtained crystal
structures of complexes 1 and 2. Further, the same kind of observa-
tions for complex 3 suggested that the complex 3 could have same
modes of coordinations that of 1 and 2.
79.7 (8)
178.3
89.0
88.5
coordination of donor atoms, N(11) and O(11) from bis(salicylid-
ene)hydrazone ligand, P(11) and P(12) from triphenylphosphine
ligands, C(1) from carbonyl ligand and an hydride ligand H(1).
The molecular structure of complex 2 has shown that the interac-
tion of metal center is through only one of the O, N donor sites of
bishydrazone ligand. The P(11)–Ru–P(12) bond angle in complex 2
is 172.44(4)°. This observation confirmed the ‘trans’ arrangement
of triphenylphosphine donors in the complex. The two triphenyl
ligands are slightly bent towards the hydrogen to due to the steric
requirements of the bishydrazone ligand. The Ru–C(1) bond dis-
tances in complex 2 has been observed as 1.844(5) which is com-
parable with the Ru–C bond lengths of previously reported
Electronic spectra of oxadiazole compounds and the complexes
were recorded at room temperature in methanol and dichloro-
methane, respectively. The spectra of A–C have revealed two bands
in the regions 242–257 and 288–306 nm. These bands were attrib-
uted to p–
p⁄ and n–p⁄ transitions. Whereas, Ru(II) complexes have

G. Prakash et al. / Inorganica Chimica Acta 427 (2015) 203–210
207
Fig. 2. ORTEP view of complex 2. The disordered carbonyl and hydrogen ligand have been omitted for clarity. Ellipsoids are drawn at the 35% probability level.
shown four bands in the region 249–465 nm. The two bands below
300 nm region observed in all the complexes are due to intra ligand
transitions [22]. The weak bands around 340 and 342 nm obtained
for complexes 2 and 3, respectively, have been assigned to d–d
transition. These weak bands supported the distorted octahedral
geometry of the complexes. The strong bands obtained above
400 nm can be assigned to metal-to-ligand (M–L) charge transfer
[23].
168.83, 167.23 ppm for complexes 2, and 3, respectively, can be
attributed to the azomethine having nitrogen with bound metal
center and the other two azomethine groups are the unbound
azomethine.
3.5. Mass spectroscopy
The GC–MS analyses of oxadiazoles A, B and C revealed the
molecular ion peak (m/z) at 239.40 ([M]+), 275.04 ([MꢀH]+) and
269.13 ([M]+), respectively, which are in good agreement with
their calculated molecular masses. A representative GC mass spec-
trum has been depicted in Fig. S3, ESI. These observations con-
firmed the formation of oxadiazole compounds as found in
literature.
3.4. NMR spectroscopy
1H NMR spectra of the oxadiazole compounds (A–C) and com-
plexes (2, 3) were recorded in DMSO-d6/CDCl3. Oxadiazole com-
pounds have exhibited a singlet corresponding to the hydroxyl
group at 10.25–11.20 ppm region. The aromatic proton signals of
the same obtained at 6.65–8.10 ppm, in which the signals of pyrid-
inyl protons having slight higher chemical shift values due to the
ring nitrogen. The spectra of complexes (2, 3) have exhibited a sin-
glet at 12.03 and 11.34 ppm, respectively which suggested the
presence of a highly deshielded –OH proton in both the complexes.
The hydride ligands signals have been observed in the negative
region at ꢀ6.82 and ꢀ6.04 ppm for complexes 2 and 3, respec-
tively. The presence of two azomethine protons in complexes has
shown by the couple of chemical shifts at 8.74, 9.31 and 8.55,
8.79 ppm regions corresponding to complexes 2 and 3, respec-
tively. The higher chemical shift value of one of the azomethine
protons is due to the binding of its nitrogen (C@N2) with metal
center [24]. The aromatic protons of hydrazone ligands as well as
that of triphenylphospine coligands have exhibited their chemical
shifts in their corresponding regions.
3.6. Catalytic studies
The N-alkylation of aromatic amines with alcohol was catalyzed
by the synthesized complexes in the presence of a strong base. The
catalytic conditions were initially optimized and then the best cat-
alyst among the three complexes under these optimized conditions
was found. Using the optimized catalyst, the N-alkylations of dif-
ferent amines with three different alcohols were carried out to
investigate the versatility of the same. The results were discussed
below in detail.
3.6.1. Optimization of reaction conditions
A set of test reactions between benzyl alcohol and aniline using
complex 1 as catalyst were carried out based on literature [25] in
order to optimize the various reaction conditions such as base,
time and catalyst loading (Table 3). These preliminary studies
had also aimed to investigate the influence of substituents in cata-
lysts on the catalytic reaction in order to find the complex with
better catalytic activity. First, the screening of different bases
revealed the importance of selected base. In particular, strong
bases like NaOH, KOH and KOtBu have shown very high yield
(Entries 1, 2 and 5) while, K2CO3 and NaHCO3 (Entries 3 and 4)
13C NMR spectra of the complexes (2 and 3) have shown the
chemical shift values of each carbon atoms as expected. The car-
bonyl carbons (C„O) present in complexes 2 and 3 have shown
a peak at 206.32 and 203.90 ppm, respectively. Two azomethine
carbons with a difference in chemical shift values around 5–
10 ppm have been observed in each complex. Among which the
azomethine carbon with higher chemical shift values, viz.,

208
G. Prakash et al. / Inorganica Chimica Acta 427 (2015) 203–210
Table 3
have shown yield of 12% and 17%, respectively. It has to be noted
that there was no reaction observed in the absence of base (Entry
15). The reaction time was then optimized using KOH base. As
expected, the reaction time showed a great influence over the yield
and every 2 h raise in time significantly increase the %yield of the
product. The optimal time was fixed as 14 h (Entry 9), after which
considerable increment in yield was not observed. The influence of
catalyst loading was studied by varying the mol% of the catalyst
from 0.3 to 0.6 by the increase of 0.1 mol% for each entry. These
studies have clearly shown that the 0.5 mol% would be the best
catalyst loading (Entry 9) for our catalyst under given set of
conditions.
Having optimized reaction parameters like time, base, catalyst
loading, etc., it was great important and interesting to find the best
catalyst among the complexes. Hence, all the three complexes
were screened for their catalytic ability against a representative
reaction between aniline and benzyl alcohol under optimized con-
ditions. Complexes 1, 2 and 3 have shown the yields of 85%, 90%
and 84% (Entries 9, 13 and 14), respectively. This shows that the
electron withdrawing chloro- substituent on catalyst favors the
activity of the catalyst (Entry 13) compared to unsubstituted cata-
lyst (Entry 9). This is further confirmed by the observation of neg-
ative impact of electron donating substituent –OCH3 on catalyst
(Entry 14). Reaction did not proceed at all in the absence of catalyst
(Entry 16). Some test reactions were also carried out using the
Optimization of catalytic conditions for the N-alkylation of aniline using benzyl
alcohol.a
OH
NH2
Catalyst
NH
Base,Toluene/Reflux
Entry
Catalyst
Base
Time (h)
Catalyst amt. (mol%)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1
1
1
1
1
1
1
1
1
1
1
1
2
3
2
–
NaOH
KOH
K2CO3
NaHCO3
KOtBu
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
–
16
16
16
16
16
8
10
12
14
14
14
14
14
14
10
10
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.3
0.4
0.6
0.5
0.5
0.5
–
73
84
12
17
77
38
63
72
85
68
79
91
90
84
–
KOH
–
a
Reaction conditions: Aniline (2 mmol)/benzyl alcohol (2 mmol)/catalyst/base
(4 mmol) in toluene at 120 °C.
b
Isolated yield.
Table 4
Catalytic N-alkylation of amines with alcohols using catalyst 2a,b
.
OH
R
X
0.5 mol% (2)
X
NH
R1
KOH, Toluene
R
X
X
NH2
120oC, 14 h
R1
Cl
NH
NH
NH
R1
R1
R1
2a (R1=H),
90%
84%
94%
1a (R1=H),
90%
84%
92%
3a (R1=H),
93%
85%
94%
2b (R1=CH3),
2c (R1=Cl),
1b (R1=CH3),
1c (R1=Cl),
3b (R1=CH3),
3c (R1=Cl),
Cl
NC
NH
NH
NH
N
R1
R1
R1
4a (R1=H),
80%
76%
84%
5a (R1=H),
5b (R1=CH3),
5c (R1=Cl),
63%
58%
66%
6a (R1=H),
6b (R1=CH3),
6c (R1=Cl),
92%
90%
94%
4b (R1=CH3),
4c (R1=Cl),
N
R1
NH
N
R1
NH
N
H
R1
9a (R1=H),
9b (R1=CH3),
9c (R1=Cl),
0 %
0 %
0 %
7a (R1=H),
95%
8a (R1=H),
8b (R1=CH3),
8c (R1=Cl),
81%
77%
80%
7b (R1=CH3), 90%
7c (R1=Cl),
95%
a
Reaction conditions: amine/alcohol/base mole ratio of 1:1:2, 0.4 mol% of catalyst 2 in toluene at 120 °C for 14 h.
Isolated yields.
b

G. Prakash et al. / Inorganica Chimica Acta 427 (2015) 203–210
209
Ru(II)-H
4. Conclusion
A novel route to prepare ruthenium(II) bis(salicylidene)hydra-
zone complexes (1–3) with good yield has been reported, in which
1,3,5-oxadiazoles (A–C) were reacted with [RuHCl(CO)(PPh3)3].
Single crystal XRD studies of complexes revealed distorted octahe-
dral geometry around the metal center in which the ‘salen’ type
bis(hydrazone) ligands performed in mononegative bidentate ON
fashion. The complexes were subjected to catalytic studies over
N-alkylation of amines. The catalytic parameters like temperature,
time, base and catalyst loading were optimized. Complex 2 has
been found to be the best among the three catalysts and which
has been tested for its versatility towards various substrates. The
yields were obtained up to 94% in toluene using KOH as base.
The effect substituents on substrates were also discussed. The
new complexes have been proven to be efficient catalysts for the
N-alkylation of amines.
R1
H
OH
+
KOH
KH
R2
HN
H
H
H
O
Ru(II)
R1
R1
X
O
R1
OH
R2
N
R1
H
H
H
Ru(II)
Ru(II)-H
R1
R2NH2
Z
Y
R2
N
H2O
R1
H
Acknowledgement
Scheme 3. Plausible mechanism for the catalytic N-alkylation using Ru(II) catalyst.
We are thankful to the Department of Chemistry, SAIF, Punjab
University, Chandigarh, and SAIF, IIT Chennai for providing assis-
tance in various spectral studies. One of the authors (G. Prakash)
thanks Periyar University, Salem, for the financial support in the
form of University Research Fellowship.
oxadiazole compound (A–C) as catalyst. However, these com-
pounds had not shown any catalytic activity.
3.6.2. N-alkylation of substituted anilines and 2-aminopyridines
In order to know the general applicability and versatility of the
synthesized catalysts, we have conducted a series of catalytic N-
alkylation reactions using the complex 2 as catalyst under opti-
mized conditions in toluene. Various substituted anilines were
reacted with benzyl alcohols under catalytic conditions and the%
yields were tabulated (Table 4). Electron donating (2a–c & 8a–c)
as well as electron withdrawing groups (3a–c, 4a–c & 5a–c) on
both ortho, meta and para positions were well tolerated by the cat-
alytic system and showed good yields under optimized conditions.
Further, the 2-aminopyridine/pyrimidine (6a–c & 7a–c) were also
perfectly adopted and underwent N-alkylation with high yield.
Both the nature of substituents and their positions have shown
influence over the reaction. In general, electron withdrawing
groups at para position of aniline like p-Cl, p-CN significantly
reduced the yield (5a–b). Particularly, the strong electron with-
drawing nitrile substitution on aniline showed the yield up to
66%. In contrast to which chloro group at meta position enhanced
the yield to 94% (3a–c). 2-aminopyrine and 2-amino pyrimidine
have shown very higher yield (6a–c & 7a–c) compared to aniline
derivatives which may due to the basic nitrogen atom of the aro-
matic ring. With respect to alcohol, p-chlorobenzyl alcohol exhibits
comparatively high yield as expected. When the reaction was car-
ried out using benzyl amine (Ar-alkyl amine), no characteristic
reaction was observed (9a–c). Over all, in the optimized condition
the catalytic system using Ru(II) catalyst have afforded very good
yield with low catalyst load. The only demerit found in this cata-
lytic system is that the base sensitive substituents like ester and
anhydride functionalities on substrates could not be studied using
this method.
Appendix A. Supplementary material
CCDC 1016413 and 1016414 contains the supplementary crys-
tallographic data for complex 1 and 2, respectively. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-
tary data associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.ica.2015.01.002.
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