Ruthenium(II) carbonyl complexes designed with arsine and PNO/PNS ligands as catalysts for N-alkylation of amines via hydrogen autotransfer process

Article abstract of DOI:10.1016/j.jorganchem.2015.05.054

A series of phosphine-functionalized hydrazone/thiosemicarbazone ligands and their corresponding ruthenium(II) carbonyl complexes of the type [RuCl(CO)(AsPh₃)(L)] (1-5) [L = 2-(2-(diphenylphosphino)benzylidene)benzoic acid hydrazone (PNO-BHy), 2-(2-(diphenylphosphino)benzylidene)nicotinic acid hydrazone (PNO-NHy), 2-(2-(diphenylphosphino)benzylidene)-2-furoic hydrazone (PNO-FHy), 2-(2-(diphenylphosphino)benzylidene)-4-ethyl-3-thiosemicarbazone (PNS-EtTs), 2-(2-(diphenylphosphino)benzylidene)-4-cyclohexyl-3-thiosemicarbazone (PNS-CyTs)] have been synthesized based on the ligands with different electronic and steric effects. These complexes were characterized by elemental analyses and various spectral methods. The solid-state structure of the complex 4 was determined by single-crystal X-ray diffraction method. In all of the complexes, the ligand was bound to the Ru(II) center via the PNO/PNS donor atoms. All the ruthenium(II) complexes were demonstrated as highly efficient catalysts for the synthesis of secondary amines/amides by the coupling of primary amines/amides with alcohols at low catalyst loading, and the maximum yield was obtained up to 98%. The N-alkylation reaction can be readily carried out under moderate conditions, and release of water is the sole byproduct. In addition, the effects of substituents on the ligand, solvents, base and catalyst loading on the catalytic activity of the complexes have been investigated. Advantageously, only one equivalent of the alcohol was consumed in the process.

Full text of DOI:10.1016/j.jorganchem.2015.05.054

Journal of Organometallic Chemistry 791 (2015) 130e140
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Ruthenium(II) carbonyl complexes designed with arsine and PNO/PNS
ligands as catalysts for N-alkylation of amines via hydrogen
autotransfer process
Rangasamy Ramachandran a, Govindan Prakash a, Muthukumaran Nirmala a,
,
Periasamy Viswanathamurthi a *, Jan Grzegorz Malecki b
a Department of Chemistry, Periyar University, Salem 636 011, India
b Department of Crystallography, Silesian University, Szkolna 9, 40-006 Katowice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of phosphine-functionalized hydrazone/thiosemicarbazone ligands and their corresponding
ruthenium(II) carbonyl complexes of the type [RuCl(CO)(AsPh3)(L)] (1-5) [L ¼ 2-(2-(diphenylphosphino)
benzylidene)benzoic acid hydrazone (PNO-BHy), 2-(2-(diphenylphosphino)benzylidene)nicotinic acid
hydrazone (PNO-NHy), 2-(2-(diphenylphosphino)benzylidene)-2-furoic hydrazone (PNO-FHy), 2-(2-
(diphenylphosphino)benzylidene)-4-ethyl-3-thiosemicarbazone (PNS-EtTs), 2-(2-(diphenylphosphino)
benzylidene)-4-cyclohexyl-3-thiosemicarbazone (PNS-CyTs)] have been synthesized based on the li-
gands with different electronic and steric effects. These complexes were characterized by elemental
analyses and various spectral methods. The solid-state structure of the complex 4 was determined by
single-crystal X-ray diffraction method. In all of the complexes, the ligand was bound to the Ru(II) center
via the PNO/PNS donor atoms. All the ruthenium(II) complexes were demonstrated as highly efficient
catalysts for the synthesis of secondary amines/amides by the coupling of primary amines/amides with
alcohols at low catalyst loading, and the maximum yield was obtained up to 98%. The N-alkylation re-
action can be readily carried out under moderate conditions, and release of water is the sole byproduct.
In addition, the effects of substituents on the ligand, solvents, base and catalyst loading on the catalytic
activity of the complexes have been investigated. Advantageously, only one equivalent of the alcohol was
consumed in the process.
Received 23 January 2015
Received in revised form
25 May 2015
Accepted 27 May 2015
Available online 30 May 2015
Keywords:
PNS/PNO ligands
Ruthenium
Carbonyl complexes
Hydrogen autotransfer process
N-alkylation
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
they can easily enable coordination sites and, at the same time,
protect the coordination sites by a dynamic “on and off” chelating
Multidentate chelate ligands which can offer a selection of hard-
soft donor atoms that are capable of adapting to the character of the
metal center continue to excite interest [1]. Special attention has
been directed to the use of hetero-multidentate hydrazone/thio-
semicarbazone ligands having both hard and soft donor atoms (e.g.
XNS or XNO, X]O, N and P), since the resulting complexes often
show fascinating coordination chemistry [2] or remarkable prop-
erties in catalytic [3] and biological applications [4]. Hydrazones/
thiosemicarbazones functionalized with an additional donor group
have become important ligands due to the potential hemilability of
the new donor group, which can play a dual role in a catalyst since
effect [5,6] (I-III, Scheme 1). This is an attractive feature for catalytic
systems, as it allows for the systematic development of more active
catalysts utilizing rational catalyst design. In this context, several
donors such as O, N, C, S and P have been reported to functionalize
hydrazones/thiosemicarbazones and showed great catalytic activ-
ities in CꢀX (X¼ C and N) coupling [7], transfer hydrogenation [8]
and aldehyde to amide conversion [9]. Nevertheless, the
phosphine-functionalized hydrazone and thiosemicarbazone
hybrid ligands are relatively not as much investigated.
Alkylated amines are important intermediates in the bulk and
fine chemical industries for the production of agrochemicals,
polymers, dyes, surfactants, pharmaceuticals and biologically active
compounds [10]. A variety of synthetic methods for the preparation
of desired amines have been reported, such as metal-catalyzed
amination of aryl halides (BuchwaldeHartwig amination) [11],
* Corresponding author.
E-mail address: viswanathamurthi72@gmail.com (P. Viswanathamurthi).
http://dx.doi.org/10.1016/j.jorganchem.2015.05.054
0022-328X/© 2015 Elsevier B.V. All rights reserved.

R. Ramachandran et al. / Journal of Organometallic Chemistry 791 (2015) 130e140
131
acceleration effect on the transfer hydrogenation or N-alkylation
amines [30]. Here, we report a series of ruthenium(II) PNO/PNS
complexes (Fig. 1) in which the ligands have been varied system-
atically to study the influence of the oxygen/sulfur donor sub-
stituent as well as the alkyl/aryl substituent's at the terminal
position on the steric and electronic properties of the metal center.
The new ruthenium complexes are active toward N-alkylation of a
wide variety of amines and amides under moderate conditions
with almost quantitative conversions and short reaction times.
Scheme 1. Hemilability of hydrazone/thiosemicarbazone ligands in catalysis.
2. Experimental section
hydroamination [12] and hydroamino-methylation of unsaturated
compounds [13], reductive amination of carbonyl compounds or
reduction of imines, nitriles and nitro compounds [14]. However,
these methods were associated with the side reactions, tedious
work-up procedures, use of toxic reagents and formation of large
amounts of inorganic salts as waste. Hence, developing more effi-
cient and green alkylation methodologies utilizing nonhazardous
and easily available starting materials would be most desirable for
future sustainable processes. Recently, less toxic and more readily
available alcohols were used as the alkylating agents in a greener
approach using a metal catalyzed borrowing-hydrogen strategy or
hydrogen autotransfer process [15]. This approach is atom eco-
nomic, thermodynamically favored, proceeds with only water as
by-product and follows a cascade redox type pathway involving in-
situ alcohol oxidation/imine formation/imine hydrogenation steps.
Although both heterogeneous and homogeneous catalysts have
been known to promote the reaction [16e20], iridium [17] and
ruthenium [18e20] complexes have constituted a vast majority of
the homogeneous catalysts because of their high catalytic perfor-
mance with high product selectivity [21,22]. Several iridium and
ruthenium catalytic systems bearing phosphine ligands [21e27]
have been reported to complete the N-alkylation of amines with
alcohols by means of good yields and selectivity (A-D; Fig. 1). Some
transition-metal-free systems have been reported for the N-alkyl-
ation of amines with alcohols which worked under mild conditions
for shorter reaction times and also provide good yields [28].
However, certain demerits of these systems such as availability,
requirement of additives, poor selectivity, economy, TON etc can be
overcome or improved by transition metal catalysts as alternatives.
During our ongoing exploration of highly active transition
metal catalysts for homogeneous catalysis [29], we have disclosed
that thiosemicarbazone with ONS, NNS, PNS and O2N2 function-
2.1. Physical measurements
Unless otherwise noted, all reactions were performed under an
atmosphere of air. Thin-layer chromatography (TLC) was carried
out on Merck 1.05554 aluminum sheets precoated with silica gel 60
F254, and the spots were visualized with UV light at 254 nm or
under iodine. Column chromatography purifications were per-
formed using Merck silica gel 60 (0.063e0.200 mm). The 1H, 13C
and 31P NMR spectra were measured on a Bruker AV400 instrument
with chemical shifts relative to tetramethylsilane (1H, 13C) and o-
phosphoric acid (31P) at 400, 100, and 162 MHz, respectively. The C,
H, and N analyses were carried out with a Vario EL III Elemental
analyzer. Infrared spectra of the ligands and the metal complexes
were recorded as KBr discs in the range of 4000ꢀ400 cmꢀ1 using a
Nicolet Avatar model FT-IR spectrophotometer. Mass spectra were
measured on a LCꢀMS QꢀToF Micro Analyzer (Shimadzu), using
electrospray ionization (ESI) mode. The melting points were
checked with a Lab India melting point apparatus.
Crystals of 4 were mounted on glass fibers and used for data
collection. Crystal data were collected at 295 K using a Gemini An
Ultra Oxford Diffraction automatic diffractometer. Graphite mon-
ochromated Mo-K
a
radiation (
l
¼ 0.71073 Å) was used throughout.
The absorption corrections were performed by the multi-scan
method. Corrections were made for Lorentz and polarization ef-
fects. The structures were solved by direct methods using the
program SHELXS [31]. Refinement and all further calculations were
carried out using SHELXL. 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. In the solid state, a
disorder is observed within the two different enantiomeric forms in
such a manner that only the carbonyl group and the chlorine atom
share the ligand positions mutually.
alities in Ru(II) complex catalyst can show
a remarkable
Fig. 1. Metal-phosphine based active catalysts for amine/amide synthesis. from alcohols (A-C) and present work.

132
R. Ramachandran et al. / Journal of Organometallic Chemistry 791 (2015) 130e140
2.2. Reagents and materials
2.4.1. 2-(2-(diphenylphosphino)benzylidene)-4-ethyl-3-
thiosemicarbazone (PNS-EtTs)
All solvents were dried and distilled before use by standard
procedures. The common reagents and chemicals available
commercially within India were used. Ruthenium(III) trichloride
hydrate, triphenylarsine, nicotinic acid hydrazide, 2-furoic hydra-
zide and 4-ethyl-3-thiosemicarbazide were procured from Sigma-
eAldrich and used as received. The reported methods were used for
the synthesis of 2-(diphenylphosphino)benzaldehyde [32], 2-(2-
(diphenylphosphino)benzylidene)nicotinic acid hydrazone (PNO-
BHy) [33], 4-cyclohexyl-3-thiosemicarbazide [34] and [RuHCl(-
CO)(AsPh3)3] [35].
Yield: 97% (0.38 g). Mp: 198 ꢁC. Anal. Calcd for C22H22N3PS: C,
67.50; H, 5.66; N,10.73; S, 8.19%. Found: C, 67.72; H, 5.62; N,10.74; S,
8.26%. IR (KBr disks, cmꢀ1): 3326 (m, nNH); 1584 þ 1478 (s,
nC]N þ nCeN); 744 (s,
n d 1.07
C]S). 1H NMR (400 MHz, DMSO-d6):
(t, 3H, J ¼ 7.2 Hz, eCH3), 3.46e3.53 (m, 2H, eCH2), 6.75e6.78 (m,
1H, Ar H), 7.16e7.21 (m, 4H, Ar H), 7.31 (t, 1H, J ¼ 7.2 Hz, Ar H),
7.39e7.45 (m, 6H, Ar H), 7.51e7.72 (m, 2H, Ar H), 8.14 (q,
1H, ꢀNHterminal), 8.64 (d, IH, J ¼ 4.8 Hz, eCH]N), 11.53 (s,
IH, ꢀNHhydrazinic). 13C NMR (100 MHz, DMSO-d6):
d
14.47 (ꢀCH3),
38.14 (ꢀCH2), 127.24 (Ar C), 127.28 (Ar C), 128.64 (Ar C), 128.79 (Ar
C), 128.86 (Ar C), 128.99 (Ar C), 129.09 (Ar C), 129.59 (Ar C), 131.25
(Ar C),131.35 (Ar C),131.75 (Ar C),131.92 (Ar C),133.03 (Ar C),133.35
(Ar C), 133.55 (Ar C), 133.59 (Ar C), 133.86 (Ar C), 135.65 (Ar C),
135.83 (Ar C), 135.94 (Ar C), 137.75 (Ar C), 140.25 (ꢀCH]N), 176.62
2.3. Synthesis of PNO type hydrazone ligands
(C]S). 31P NMR (162 MHz, DMSO-d6):
d
ꢀ14.02 (s, PPh2).
To a solution of 2-(diphenylphosphino)benzaldehyde (0.290 g,
1 mmol) and nicotinic acid hydrazide (0.137 g, 1 mmol)/2-furoic
hydrazide (0.126 g, 1 mmol) in ethanol (20 mL) were added 2ꢀ3
drops of glacial acetic acid. The resulting solution was heated under
reflux over a 3 h period, and then concentrated to ca. 3 mL. The
white crystalline precipitate was filtered off, washed with diethyl
ether (2 x 5 mL), and dried under vacuo.
2.4.2. 2-(2-(diphenylphosphino)benzylidene)-4-cyclohexyl-3-
thiosemicarbazone (PNS-CyTs)
Yield: 93% (0.41 g). Mp: 197e198 ꢁC. Anal. Calcd for C26H28N3PS:
C, 70.09; H, 6.33; N, 9.43; S, 7.20%. Found: C, 70.24; H, 6.21; N, 9.52;
S, 7.26%. IR (KBr disks, cmꢀ1): 3343 (m, nNH); 1569 þ 1475 (s,
nC
]
þ
nCeN); 748 (s, nCeS). 1H NMR (400 MHz, DMSO-d6):
N
d
1.32e1.23 (m, 5H, eCH2ꢀ), 1.58 (d, 1H, J ¼ 12 Hz, eCH2ꢀ), 1.70 (d,
2.3.1. 2-(2-(diphenylphosphino)benzylidene)nicotinic acid
hydrazone (PNO-NAHy)
Yield: 95% (0.39 g). Mp: 165e166 ꢁC. Anal. Calcd for C25H20N2OP:
C, 73.34; H, 4.92; N, 10.26%. Found: C, 73.46; H, 4.89; N, 10.34%. IR
2H, J ¼ 12 Hz, eCH2ꢀ), 1.82 (d, 2H, J ¼ 9.2 Hz, eCH2ꢀ), 4.12 (s, 1H,
eCHꢀ), 6.77 (dq, 1H, J ¼ 1.2, 3.2 Hz, Ar H), 7.14e7.21 (m, 4H, Ar H),
7.32 (td, 1H, J ¼ 1.2, 6.4 Hz, Ar H), 7.38e7.40 (m, 6H, Ar H), 7.43 (td,
1H, J ¼ 1.2, 6.8 Hz, Ar H), 7.79 (td,1H, J ¼ 8.4 Hz, Ar H), 8.09e8.06 (m,
1H, ꢀNHterminal), 8.62 (d, IH, J ¼ 4 Hz, eCH]N), 11.51 (s,
(KBr disks, cmꢀ1): 3295 (m, nNH); 1699 (s,
n
C]O); 1588 þ 1478 (s,
nC
]
N þ nCeN). 1H NMR (400 MHz, DMSO-d6):
d
6.37 (td, 1H, J ¼ 1.2,
IH, ꢀNHhydrazinic). 13C NMR (100 MHz, DMSO-d6):
d
24.82 (ꢀCH2ꢀ),
7.2 Hz, Ar H), 6.85 (td, 1H, J ¼ 2.4, 4.8 Hz, Ar H), 7.1e7.58 (m, 11H, Ar
H), 8.08 (dd, 1H, J ¼ 3.2, 4 Hz, Ar H), 8.21 (d, 1H, J ¼ 8 Hz, Ar H), 8.73
(d, IH, J ¼ 3.6 Hz, eCH]N), 9.01 (s, 1H, Ar H), 9.18 (d, 1H, J ¼ 14 Hz,
25.03 (ꢀCH2ꢀ), 31.76 (ꢀCH2ꢀ), 52.49 (ꢀCHꢀ), 127.62 (Ar C), 127.67
(Ar C), 128.77 (Ar C), 128.84 (Ar C), 129.04 (Ar C), 129.09 (Ar C),
129.64 (Ar C), 133.05 (Ar C), 133.37 (Ar C), 133.57 (Ar C), 135.71 (Ar
C), 135.81 (Ar C), 135.91 (Ar C), 137.36 (Ar C), 137.55 (Ar C), 140.81
Ar H), 12.12 (s, 1H, eNH). 13C NMR (100 MHz, DMSO-d6):
d 123.45
(Ar C), 125.83 (Ar C), 128.85 (Ar C), 128.91 (Ar C), 129.14 (Ar C),
129.46 (Ar C), 130.28 (Ar C), 133.25 (Ar C), 133.45 (Ar C), 135.42 (Ar
(ꢀCH]N), 175.57 (C]S). 31P NMR (162 MHz, DMSO-d6):
(t, J ¼ 9.4 Hz, PPh2).
d
ꢀ13.36
C), 140.48 (Ar C), 145.99 (Ar C), 148.59 (ꢀCH]N), 152.24 (C]O). 31
P
NMR (162 MHz, DMSO-d6):
d
ꢀ17.06 (s, PPh2).
2.5. Synthesis of the complexes 1ꢀ3
A suspension of [RuHCl(CO)(AsPh3)3] (0.100 g, 0.092 mmol) in
ethanol (20 mL) was treated with 2-(2-(diphenylphosphino)ben-
zylidene)benzoic acid hydrazone (0.037 g, 0.092 mmol) or 2-(2-
2.3.2. 2-(2-(diphenylphosphino)benzylidene)-2-furoic hydrazone
(PNO-FHy)
Yield: 87% (0.36 g). Mp: 211e212 ꢁC. Anal. Calcd for
(diphenylphosphino)benzylidene)nicotinic
acid
hydrazone
C
24H19N2O2P: C, 72.35; H, 4.81; N, 7.03%. Found: C, 72.47; H, 4.83; N,
7.12%. IR (KBr disks, cmꢀ1): 3195 (m, nNH); 1685 (s,
n
C]O);
nCeN). 1H NMR (400 MHz, DMSO-d6):
6.83e7.50 (m, 14H, Ar H), 7.57 (t, 1H, J ¼ 6.24 Hz Ar H), 8.05 (s, 1H,
Ar H), 8.15 (s, 1H, Ar H), 8.85 (d, IH, J ¼ 3.6 Hz, eCH]N), 9.13 (s, 1H,
Ar H), 11.98 (s, IH, eNH). 13C NMR (100 MHz, DMSO-d6):
125.78
(0.038 g, 0.092 mmol) or 2-(2-(diphenylphosphino)benzylidene)-
2-furoic hydrazone (0.036 g, 0.092 mmol) and the mixture was
gently refluxed for 8 h. During this time the color changed to or-
ange. The solvent was reduced to half of the volume on a rotary
evaporator, and the suspension was filtered, washed thoroughly
with cold ethanol (10 mL) and diethyl ether (2 х 20 mL). The
product was finally dried under vacuo.
1583 þ 1474 (s, nC
]
N
þ
d
d
(Ar C), 128.91 (Ar C), 129.15 (Ar C), 129.47 (Ar C), 130.07 (Ar C),
133.30 (Ar C), 133.49 (Ar C), 134.12 (Ar C), 135.62 (Ar C), 138.25 (Ar
C), 149.52 (ꢀCH]N), 158.12 (C]O). 31P NMR (162 MHz, DMSO-d6):
2.5.1. [(PNO-BHy)RuCl(CO)(AsPh3)] (1)
d
ꢀ16.84 (d, J ¼ 97.67 Hz, PPh2).
Yield 64% (0.051 g), Mp: 247e248 ꢁC. Anal. Calcd for
C
45H35AsClN2O2PRu: C, 61.54; H, 4.02; N, 3.19%. Found: C, 61.69; H,
2.4. Synthesis of PNS type thiosemicarbazone ligands
3.97; N, 3.22%. IR (KBr disks, cmꢀ1): 1943 (s, nC≡≡O); 1584 þ 1473 (s,
nC
]
N
þ
nCeN); 1276 (m, nCeO); 1433, 1093, 694 (s, for AsPh3). 1H
To a solution of 2-(diphenylphosphino)benzaldehyde (0.290 g,
1 mmol) and 4-ethyl-3-thiosemicarbazide (0.119 g, 1 mmol)/4-
cyclohexyl-3-thiosemicarbazide (0.173 g, 1 mmol) in ethanol
(20 mL) were added 2ꢀ3 drops of glacial acetic acid. The resulting
solution was heated under reflux over a 1 h period, then concen-
trated to ca. 3 mL and cooled to 0 ꢁC for overnight. The pale yellow
precipitate was filtered off, washed with diethyl ether (2 x 5 mL)
and dried under vacuo.
NMR (400 MHz, DMSO-d6):
d
6.56 (t, 2H, J ¼ 8.8 Hz, Ar H),
6.72e6.60 (m, 1H, Ar H), 6.89 (t, 1H, J ¼ 7.2 Hz, Ar H), 7.63e7.10 (m,
27H, Ar H), 7.74 (d, 2H, J ¼ 7.6 Hz, Ar H), 7.98e7.85 (m, 1H, Ar H),
8.47 (d, 1H, J ¼ 7.2 Hz, eCH]N). 13C NMR (100 MHz, DMSO-d6):
d
115.82 (Ar C), 116.81 (Ar C), 119.02 (Ar C), 120.45 (Ar C), 120.73 (Ar
C), 121.11 (Ar C), 121.20 (Ar C), 121.53 (Ar C), 121.89 (Ar C), 122.11 (Ar
C), 122.36 (Ar C), 122.65 (Ar C), 123.27 (Ar C), 123.38 (Ar C), 123.55
(Ar C), 123.71 (Ar C), 124.40 (Ar C), 129.95 (Ar C), 131.17 (Ar C),

R. Ramachandran et al. / Journal of Organometallic Chemistry 791 (2015) 130e140
133
132.35 (Ar C), 133.84 (Ar C), 135.27 (Ar C), 143.34 (C]N), 148.73
(Ar C), 133.82 (Ar C), 138.10 (ꢀCH]N), 170.86 (C]S), 200.23 (C^O).
(CeO), 205.45 (C^O). 31P NMR (162 MHz, DMSO-d6):
PPh2). ESIþ-MS: m/z ¼ 842.72 [MꢀCl]þ.
d
39.68 (s,
31P NMR (162 MHz, DMSO-d6): 29.43 (s, PPh2). ESIþ-MS: m/
d
z ¼ 825.72 [MꢀCl]þ. Crystals suitable for X-ray diffraction were
obtained by slow evaporation of a concentrated solution of 4 in
CH2Cl2/C2H5OH.
2.5.2. [(PNO-NHy)RuCl(CO)(AsPh3)] (2)
Yield 76% (0.061 g), Mp: 243e244 ꢁC. Anal. Calcd for
C
44H34AsClN3O2PRu: C, 60.11; H, 3.90; N, 4.78%. Found: C, 60.29; H,
2.5.6. [(PNS-CyTs)RuCl(CO)(PPh3)] (5)
3.98; N, 4.78%. IR (KBr disks, cmꢀ1): 1944 (s, nC≡≡O); 1572 þ 1480
Yield 72% (0.060 g), Mp: 266e267 ꢁC. Anal. Calcd for
(m, nC
]
N þ nCeN); 1247 (s, nCeO); 1432, 1091, 695 (m, for AsPh3). 1H
C
45H42AsClN3OPRuS: C, 59.05; H, 4.63; N, 4.59; S, 3.50%. Found: C,
57.29; H, 4.14; N, 4.86; S, 3.84%. IR (KBr disks, cmꢀ1): 3395, 3046 (m,
nNH); 1946 (s, nC≡≡O); 1582 þ 1480 (s, nC nCeN); 747 (s, nCeS);
1432, 1091, 695 (s, for AsPh3). 1H NMR (400 MHz, DMSO-d6):
NMR (400 MHz, DMSO-d6):
d
6.44 (t, 1H, J ¼ 8.4 Hz, Ar H), 6.67 (t,
1H, J ¼ 7.2 Hz, Ar H), 7.04 (td, 1H, J ¼ 1.6, 6 Hz, Ar H), 7.17 (td, 2H,
J ¼ 1.6, 6 Hz, Ar H), 7.26 (dd, 4H, J ¼ 1.6, 7.2 Hz, Ar H), 7.32 (t, 1H,
J ¼ 7.6 Hz, Ar H), 7.36e7.56 (m, 16H, Ar H), 7.62 (t, 1H, J ¼ 7.2 Hz, Ar
H), 7.69e7.82 (m, 4H, Ar H), 7.87 (td, 1H, J ¼ 1.2, 6.0 Hz, Ar H), 8.62
(d, 1H, J ¼ 7.0 Hz, eCH]N), 9.24 (d, 1H, J ¼ 1.5 Hz, Ar H). 13NMR
]
N
þ
d
1.06e1.22 (m, 3H, eCH2ꢀ), 1.54 (d, 1H, J ¼ 11.2 Hz, eCH2ꢀ), 1.69 (s,
2H, eCH2ꢀ), 1.87 (s, 2H, eCH2ꢀ), 3.62 (s, 1H, eCHꢀ), 6.43 (t, 1H,
J ¼ 8.8 Hz, Ar H), 6.65 (t, 1H, J ¼ 8.8 Hz, Ar H), 7.03 (td, 2H, J ¼ 2.4,
6 Hz, Ar H), 7.19 (td, 2H, J ¼ 2.4, 6 Hz, Ar H), 7.25e7.28 (m,13H, Ar H),
7.31 (t, 1H, J ¼ 8 Hz, Ar H), 7.35e7.50 (m, 2H, Ar H), 8.14 (s,
1H, ꢀNHterminal), 8.62 (s, 1H, eCH]N). 13NMR (100 MHz, DMSO-
(100 MHz, DMSO-d6):
d 125.83 (Ar C), 127.20 (Ar C), 128.94 (Ar C),
129.39 (Ar C), 129.80 (Ar C), 129.98 (Ar C), 131.38 (Ar C), 131.48 (Ar
C), 131.83 (Ar C), 131.91 (Ar C), 132.18 (Ar C), 133.11 (Ar C), 133.22 (Ar
C), 133.47 (Ar C), 133.56 (Ar C), 133.82 (Ar C) 144.40 (C]N), 149.20
d6):
d
24.59 (ꢀCH2ꢀ), 25.65 (ꢀCH2ꢀ), 32.24 (ꢀCH2ꢀ), 54.52
(CeO), 200.21 (C^O). 31P NMR (162 MHz, DMSO-d6):
PPh2). ESIþ-MS: m/z ¼ 843.70 [MꢀCl]þ.
d
40.16 (s,
(ꢀCHꢀ), 124.81 (Ar C), 125.14 (Ar C), 127.99 (Ar C), 128.10 (Ar C),
128.41 (Ar C), 128.67 (Ar C), 128.85 (Ar C), 129.02 (Ar C), 129.52 (Ar
C), 129.89 (Ar C), 130.15 (Ar C), 130.22 (Ar C), 130.47 (Ar C), 130.87
(Ar C), 131.34 (Ar C), 131.48 (Ar C), 131.82 (Ar C), 133.15 (Ar C), 133.20
(Ar C), 133.42 (Ar C), 133.87 (Ar C), 133.97 (Ar C), 134.55 (Ar C),
138.91 (ꢀCH]N), 167.52 (CeS), 200.21 (C^O). 31P NMR (162 MHz,
2.5.3. [(PNO-FHy)RuCl(CO)(AsPh3)] (3)
Yield 81% (0.163 g), Mp: 120e122 ꢁC. Anal. Calcd for
43H33AsClN2O3PRu: C, 59.49; H, 3.83; N, 3.23%. Found: C, 59.62; H,
C
3.82; N, 3.21%. IR (KBr disks, cmꢀ1): 3323 (m, nNH); 1946 (s, nC≡≡O);
DMSO-d6):
d
32.74 (s, PPh2). ESIþ-MS: m/z ¼ 879.87 [MꢀCl]þ.
1592 þ 1480 (s, nC]
AsPh3). 1H NMR (400 MHz, DMSO-d6):
N þ nCeN); 1267 (s, nCeO); 1432, 1091, 695 (s, for
d
6.37 (td, 1H, J ¼ 7.2, 1.2 Hz,
2.6. Typical procedure for catalytic N-alkylation of amines/
sulfonamides with alcohols
Ar H), 7.10e7.52 (m, 29H, Ar H), 7.67 (dd, 1H, J ¼ 3.6, 1.2 Hz, Ar H),
8.12 (d, IH, J ¼ 3.6 Hz, eCH]N), 8.93 (s, 1H, Ar H). 13NMR (100 MHz,
DMSO-d6):
d
124.10 (Ar C), 128.50 (Ar C), 128.67 (Ar C), 129.92 (Ar
Amine/sulfonamide (1 mmol), alcohol (1 mmol), catalyst
(0.5 mol %), KOH (50 mol %) and toluene (2 mL) were placed in a
25 mL round bottomed flask and stirred on a preheated oil bath
(100 ꢁC) for 12 h. Upon completion (as monitored by TLC), the re-
action mixture was cooled at ambient temperature, H2O (3 mL) was
added and the organic layer was extracted with CH2Cl2. The organic
extract was separated, dried, and concentrated. The desired prod-
uct was purified by column chromatography with n-hexane/EtOAc
as eluent.
C), 130.5 (Ar C), 133.80 (Ar C), 133.20 (Ar C), 133.41 (Ar C), 133.50 (Ar
C), 133.92 (Ar C), 143.40 (C]N), 149.20 (CeO), 200.12 (C^O). 31P
NMR (162 MHz, DMSO-d6):
d
45.19 (s, PPh2). ESIþ-MS: m/z ¼ 832.74
[MꢀCl]þ.
2.5.4. Synthesis of the complexes 4 and 5
A suspension of [RuHCl(CO)(AsPh3)3] (0.100 g, 0.092 mmol) was
treated with 2-(2-(diphenylphosphino)benzylidene)-4-ethyl-3-
thiosemicarbazone (0.036 g, 0.092 mmol) or 2-(2-(diphenylphos-
phino)benzylidene)-4-cyclohexyl-3-thiosemicarbazone (0.041 g,
0.092 mmol) in ethanol (20 mL) and the mixture was gently
refluxed for 6 h. During this time the color changed to orange. The
solvent was reduced to half of the volume on a rotary evaporator,
and the suspension was filtered, washed thoroughly with cold
ethanol (5 mL) and diethyl ether (2 х 5 mL). The product was finally
dried under vacuum, affording an orange crystalline solid.
Representative spectral data for N-Benzylbenzo[d]thiazol-2-
amine (3a): 1H NMR (400 MHz, CDCl3):
d
¼ 4.59 (s, 2H, eCH2-),
7.07 (t, J ¼ 7.5 Hz, 1H, ArH), 7.21e7.28 (m, 2H, ArH), 7.33e7.40 (m,
5H, ArH), 7.89 (d, J ¼ 7.9, 1H, ArH), 8.51 (t, J ¼ 5.6 Hz, 1H, eNH). 13
C
NMR (100 MHz, CDCl3):
128.96, 127.95, 127.72, 137.71, 152.81, 167.79. Assignment of signals
d
¼ 49.49, 118.97, 120.81, 121.63, 126.26,
was further confirmed by DEPT-135 and HSQC-NMR studies.
2.7. Typical procedure for catalytic N,N’-alkylation of diamines with
2.5.5. [(PNS-EtTs)RuCl(CO)(AsPh3)] (4)
alcohols
Yield 85% (0.067 g), Mp: 255e256 ꢁC. Anal. Calcd for
C
41H36AsClN3OPRuS: C, 57.18; H, 4.21; N, 4.88; S, 3.72%. Found: C,
57.29; H, 4.14; N, 4.86; S, 3.84%. IR (KBr disks, cmꢀ1): 3346 (m, nNH);
1943 (s, nC≡≡O); 1580 þ 1482 (s, nC N þ nCeN); 741 (s, nCeS); 1432,
1091, 692 (s, for AsPh3). 1H NMR (400 MHz, DMSO-d6):
1.10 (t, 3H,
Diamine (1 mmol), alcohol (2 mmol), catalyst (1 mol %), KOH
(50 mol %) and toluene (2 mL) were placed in a 25 mL round
bottomed flask and stirred on a preheated oil bath (100 ꢁC) for 12 h.
Upon completion (as monitored by TLC), the reaction mixture was
cooled at ambient temperature, H2O (3 mL) was added and the
organic layer was extracted with ethyl acetate. The organic extract
was separated, dried, and concentrated. The desired product was
purified by column chromatography with CH2Cl2/EtOAc as eluent.
Representative spectral data for N,N0-Dibenzylpyridine-2,6-
]
d
J ¼ 7.2 Hz, eCH3), 3.29 (d, 2H, J ¼ 7.2 Hz, eCH2ꢀ), 6.42 (t, 1H,
J ¼ 8.4 Hz, Ar H), 6.66 (t, 1H, J ¼ 8.8 Hz, Ar H), 7.04 (td, 1H, J ¼ 1.6,
6 Hz, Ar H), 7.18 (td, 2H, J ¼ 1.6, 6 Hz, Ar H), 7.26 (dd, 4H, J ¼ 1.6,
7.2 Hz, Ar H), 7.32 (t, 1H, J ¼ 7.6 Hz, Ar H), 7.36e7.39 (m, 6H, Ar H),
7.47e7.56 (m, 6H, Ar H), 7.62 (t, 1H, J ¼ 7.2 Hz, Ar H), 7.69e7.83 (m,
4H, Ar H), 7.87 (td, 1H, J ¼ 1.2, 6.0 Hz, Ar H), 8.43 (s, 1H, ꢀNHterminal),
diamine (3j): 1H NMR (400 MHz, CDCl3):
d
¼ 4.43 (d, J ¼ 5.7 Hz,
8.67 (s, IH, eCH]N). 13NMR (100 MHz, DMSO-d6):
d
14.37 (ꢀCH3),
4H), 4.63 (bs, 2H), 5.74 (d, J ¼ 7.6 Hz, 2H), 7.36e7.15 (m, 11H). 13
C
24.16 (ꢀCH2ꢀ), 127.56 (Ar C), 127.65 (Ar C), 128.04 (Ar C), 128.64 (Ar
C), 128.69 (Ar C), 128.76 (Ar C), 128.94 (Ar C), 129.39 (Ar C), 129.80
(Ar C),129.98 (Ar C),131.38 (Ar C),131.48 (Ar C),131.96 (Ar C),131.99
(Ar C),132.18 (Ar C),133.10 (Ar C),133.29 (Ar C),133.47 (Ar C),133.56
NMR (100 MHz, CDCl3):
d
¼ 46.4, 95.0, 127.2, 128.8, 139.4, 139.5,
158.2. Assignment of signals was further confirmed by DEPT-135
and HSQC-NMR studies.
The catalytic reactions given in Tables 3e6 were similarly

134
R. Ramachandran et al. / Journal of Organometallic Chemistry 791 (2015) 130e140
Table 1
NMR and ESI-MS) and by X-ray crystallography for 4 as described
further below. The analytical data of the complexes agreed well
with the proposed molecular formulae. Positive ion ESI-MS analysis
of the isolated products 1e5 showed an intense peak for [MꢀCl]þ
(m/z 842 for 1, m/z 843 for 2, m/z 832 for 3, m/z 825 for 4 and m/z
879 for 5), which confirming the proposed nature of these
complexes.
Crystal measurement and refinement data for complex 4.
[(PNSeEt)RuCl(CO)(AsPh3)] (4)
Formula
Fw
Cryst syst
Space group
a, Å
C41H36AsClN3OPRuS
861.20
monoclinic
P21/c
14.47210(10)
16.75920(10)
15.2758(2)
90
96.5563(9)
90
3680.77(6)
4
1744
1.554
1.529
2.947 to 25.049
ꢀ16 ꢂ h ꢂ 17
ꢀ19 ꢂ k ꢂ 19
ꢀ18 ꢂ l ꢂ 15
47586/6507,0.0257
0.998
6507/0/480
0.997
R1 ¼ 0.0239,
wR2 ¼ 0.0574
R1 ¼ 0.0338,
wR2 ¼ 0.0590
The IR spectra of the ligands and the corresponding complexes
provided significant informations about the metal ligand bonding.
A strong vibration appeared at 1588ꢀ1569 cmꢀ1 in the ligands
B, Å
C, Å
a
, deg
, deg
corresponding to nC
]
was shifted to 1592ꢀ1572 cmꢀ1 in all the
b
N
g
, deg
complexes indicating the participation of azomethine nitrogen in
Vol, Å3
Z
bonding [36]. A sharp band was observed at 1699ꢀ1685 cmꢀ1 and
748ꢀ744 cmꢀ1 was ascribed to nC
]O/nC]S in the ligands which has
F(000)
D
calcd, g cmꢀ3
been completely disappeared in the spectra of all the new Ru
Absorption coefficient mmꢀ1
Scan range for data collection (deg)
Index ranges
complexes and the appearance of a new band at 1276ꢀ1247 cmꢀ1
and 747ꢀ741 cmꢀ1 due to nCeO
/nCeS indicate the coordination of
the oxygen/sulfur atom after enolization followed by deprotonation
[37]. All the complexes displayed a medium to strong band in the
region 1946e1943 cmꢀ1, which was attributed to the terminally
coordinated carbonyl group and was observed at a slightly higher
frequency than in the precursor complexes. The IR spectra of all the
complexes therefore confirms the coordination mode of the
phosphino-hydrazone/thiosemicarbazone ligand to the ruth-
enium(II) ion via the azomethine nitrogen, imidolate oxygen/thio-
late sulfur and the phosphorus.
Reflections collected/unique, Rint
Completeness to thetamax
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2
s
(I)]a
R indices (all data)
The 1H NMR spectra of the ligands and their complexes (1ꢀ5)
have shown the signals in the expected regions. The singlets that
appeared for the NeNHeC]X (X]O or S) proton of the free ligands
at 12.12e11.51 ppm were absent in the complexes, supporting the
enolization and coordination of the imidolate oxygen or thiolate
sulphur to the Ru(II) ion. The doublet due to azomethine proton
(8.67e8.12 ppm) in the complexes are slightly downfield compared
to the free ligands (8.85e8.62 ppm), suggesting deshielding upon
coordination to Ru(II) ion. The spectra of the complexes 4 and 5
showed a singlet at 8.43e8.14 ppm, which has been assigned to
NHꢀR protons. The 13C NMR spectra have shown the expected
signals in the appropriate regions. For the uncoordinated ligands,
the C]N and C]O/C]S signals appeared in the regions around
149.52e140.25 ppm and 158.12e152.24/176.62e175.57 ppm. Upon
coordination and formation of the new Ru complexes, a downfield
shift was observed for the signals of the C]N (around 5 ppm),
while the C]O/C]S carbon atom signals are observed in the up-
field region between 149.70 and 148.73 and 170.86e167.52 ppm.
This is consistent with the PNO/PNS coordination and enolization of
the C]O/C]S of hydrazone and thiosemicarbazone moieties. The
peak for C^O carbon appeared at 205.45e200.23 ppm is compa-
rable with earlier observations [38]. In addition, a group of signal
appeared around 32.24e14.37 ppm for complexes 4 and 5 corre-
sponding to the terminal ethyl and cyclohexyl group carbon. The
presence of a residual phosphine coordinated to Ru(II) ion was
confirmed by 31P NMR (see Supporting Information Fig. S1eS10).
The singlet appeared at 45.19e32.55 ppm in complexes 1e5, sug-
gested the presence of phosphine heads in the hydrazone and
thiosemicarbazone chains.
conducted. The resulting amines and amides were identified by
comparison of the 1H and 13C NMR data with those previously re-
ported (see Supporting Information).
3. Results and discussion
3.1. Synthesis of PNO/PNS type ligands and ruthenium(II) complexes
The ligand precursors (Fig. 2) were synthesized by the
condensation of 2-diphenylphosphinebenzaldehyde with benzoic
acid hydrazide, nicotinic acid hydrazide, 2-thiophenecarboxylic
acid hydrazide, 4-ethyl-3-thiosemicarbazide and 4-cyclohexyl-3-
thiosemicarbazide, following the published procedure [32,33].
The new Ru complexes of the type [RuCl(CO)(AsPh3)(L)] were
synthesized by reacting PNO-BHy, PNO-INHy, PNO-FHy, PNS-EtTs
and PNS-CyTs with one equivalent of [RuHCl(CO)(AsPh3)3] in
ethanol under reflux for 6e8 h (Scheme 2). These air stable Ru
complexes, were obtained in good yields (64e85%), and were
characterized by analytical, spectroscopic methods (IR, 1H, 13C, 31P
Table 2
Selected bond lengths and bond angles for complex 4.
Bond length (Å)
Bond angle (ꢁ)
Ru(1)-C(1)
Ru(1)-N(1)
Ru(1)-P(1)
Ru(1)-S(1)
Ru(1)-As(1)
Ru(1)-Cl(1)
C(1)-O(1)
N(1)-N(2)
C(4)-S(1)
C(11)-P(1)
C(30)-As(1)
C(5)-N(1)
1.852(9)
2.1060(18)
2.3303(6)
2.3856(7)
2.4840(3)
2.415(2)
1.132(16)
1.380(3)
1.734(3)
1.824(2)
1.961(2)
1.295(3)
C(1)-Ru(1)-N(1)
C(1)-Ru(1)-P(1)
N(1)-Ru(1)-P(1)
C(1)-Ru(1)-S(1)
N(1)-Ru(1)-S(1)
P(1)-Ru(1)-S(1)
C(1)-Ru(1)-Cl(1)
N(1)-Ru(1)-Cl(1)
C(1)-Ru(1)-As(1)
N(1)-Ru(1)-As(1)
P(1)-Ru(1)-As(1)
S(1)-Ru(1)-As(1)
S(1)-Ru(1)-Cl(1)
P(1)-Ru(1)-Cl(1)
As(1)-Ru(1)-Cl(1)
86.5(3)
90.2(3)
91.45(5)
89.0(3)
The structure of the complex [(PNS-EtTs)RuCl(CO) (AsPh3)] (4)
was further confirmed by single-crystal X-ray diffraction analysis of
suitable crystals of 4, grown by slow evaporation of solution of 4 in
an ethanoledichloromethane (2: 1) mixture. The crystal data and
structure refinement parameters of the complex 4 have been sum-
marized in Table 1 and the selected bond lengths and bond angles
were depicted in Table 2. The ORTEP view of the complex 4 along
with the atomic numbering scheme has been given in Fig. 3. The
single-crystal X-ray study revealed that complex 4 is crystallized in a
monoclinic system with the space group P21/c. The phosphino-
81.49(6)
172.92(2)
172.7(3)
86.67(7)
96.4(3)
169.63(5)
98.466(16)
88.611(18)
87.45(5)
92.54(5)
89.82(5)

R. Ramachandran et al. / Journal of Organometallic Chemistry 791 (2015) 130e140
135
Table 3
Effect of bases on N-alkylation of 2-aminobenzothiazole with benzyl alcohol catalyzed by 1.a
Entry
Base
Amount of base (mol %)
Yield (%)b
1
2
3
4
5
6
7
8
9
e
e
e
Na2CO3
K2CO3
KOAc
NaOH
NaO(t-Bu)
KO(t-Bu)
KOH
50
50
50
50
50
50
20
50
100
e
>2
>5
64
58
71
86
94
92
KOH
KOH
10
a
Reactions were carried out at 100 ꢁC for 12 h using 2-aminobenzothiazole (1 mmol), PhCH2OH (1 mmol), 1 (0.5 mol %), and base in toluene (2 mL).
Isolated yields.
b
Table 4
Effect of solvents on N-alkylation of 2-aminobenzothiazole with benzyl alcohol catalyzed by 1.a
Entry
Solvent
Yield (%)b
1
2
3
4
5
6
7
DMF
10
18
25
54
96
0
Ethanol
Acetonitrile
Benzene
Toluene
DMSO
THF
15
a
Reactions were carried out using 2-aminobenzothiazole (1 mmol), PhCH2OH (1 mmol), 1 (0.5 mol %) at 100 ꢁC (for toluene, DMSO)/boiling temperature (for other
solvents) (2 mL) and KOH (50 mol%).
b
Isolated yields.
thiosemicarbazone ligand coordinated to ruthenium in PNS fashion
by utilizing its phosphorous, imine nitrogen and thiolate sulfur
atoms, with the formation of one six membered and another five
membered ring with a N(1)eRu(1)eS(1) bite angle of 81.49(6)ꢁ
(Fig. 2). The other three sites have been occupied by arsine atom of
triphenylarsine with Ru(1)ꢀAs(1) distances of 2.484(3) and one
chloride and a carbonyl group with Ru(1)ꢀCl(1) and Ru(1)ꢀC(1)
distances of 2.415(2) and 1.852(9) Å, respectively. The observed
bond distances are comparable with those found in other reported
ruthenium complexes [9,30a]. The cis angles C(1)ꢀRu(1)ꢀ
Table 5
Effect of substitution and catalyst loading on N-alkylation of 2-aminobenzothiazole with benzyl alcohol.a
Entry
Catalyst
Amount of catalyst (mol%)
Time (h)
TONb
Yield(%)c
1
2
3
4
5
6
7
8
1
1
1
1
2
3
4
5
1.0
0.5
0.3
0.2
0.5
0.5
0.5
0.5
12
12
24
24
12
12
12
12
94
186
287
370
180
178
196
192
94
93
86
74
90
89
97
96
a
Reactions were carried out at 100 ꢁC for 12 h using 2-aminobenzothiazole (1 mmol), PhCH2OH (1 mmol), catalyst, and base (50 mol %) in toluene (2 mL).
b
c
Turnover number (TON) ¼ (mmol of product)/(mmol of catalyst) after time t.
Isolated yields.

136
R. Ramachandran et al. / Journal of Organometallic Chemistry 791 (2015) 130e140
Table 6
N-Alkylation of various amines with alcohols.a,b,c
N(1)
¼
86.5(3)ꢁ, C(1)ꢀRu(1)ꢀS(1)
¼
89.0(3)ꢁ, N(1)ꢀRu(1)ꢀ
source has become an efficient method in organic synthesis as
illustrated by several useful applications reported in the recent
years [39]. The reaction conditions for this important process are
relatively mild and environment friendly. The complexes 1e5
catalyzed the alkylation of heteroaromatic amines to the corre-
sponding N-alkylated products via hydrogen autotransfer processes
with KOH as the promoter. At the start of our studies, we investi-
gated the N-alkylation of 2-aminobenzothiazole with benzyl
alcohol in the presence of various bases, and the results have been
summarized in Table 3. The catalytic reaction did not proceed well
in the absence of a base or in the presence of weak bases (Entries
1ꢀ4). This is probably because the formation of PhCH2Oꢀ ion is
essential for the displacement of the Cl ligand in 1 to form the
(benzyloxy)ruthenium species as a key intermediate. Thus, the
S(1) ¼ 81.49(6)ꢁ, N(1)ꢀRu(1)ꢀCl(1) ¼ 86.87(8)ꢁ and S(1)ꢀRu(1)ꢀ
As(1) ¼ 88.61(18)ꢁ are more acute than 90ꢁ, whereas the other cis
angles C(1)ꢀRu(1)ꢀP(1) 90.2(3)ꢁ, N(1)ꢀRu(1)ꢀP(1) ¼ 91.45(5)ꢁ,
C(1)ꢀRu(1)ꢀAs(1) ¼ 96.4(3)ꢁ and P(1)ꢀRu(1)ꢀAs(1) ¼ 98.46(16)ꢁ,
are more obtuse than 90ꢁ. The trans angles P(1)ꢀRu(1)ꢀ
S(1) ¼ 172.92(2)ꢁ, C(1)ꢀRueCl(1) ¼ 172.7(3)ꢁ and N(1)ꢀRu(1)ꢀ
As(1) ¼ 169.63(5)ꢁ deviate from linearity. The variations in bond
lengths and angles lead to a significant distortion from an ideal
octahedral geometry for the complex.
3.2. Catalytic studies
Ruthenium catalyzed N-alkylation using alcohol as an alkyl

R. Ramachandran et al. / Journal of Organometallic Chemistry 791 (2015) 130e140
137
Fig. 2. Structure of the ligands designated by hydrazone and thiosemicarbazone used in this work.
Scheme 2. Synthetic strategy for the preparation of Ru(II) complexes with hydrazone/thiosemicarbazone ligands.
catalytic reaction successfully proceeded in the presence of strong
bases (Entries 5ꢀ10), and benzothiozol-2-yl-benzylamine was ob-
tained in 96% yield in the presence of a catalytic amount of KOH
(50 mol %) (Entry 9).
The reaction conditions were further optimized through
different solvents and the results have been given in Table 4.
Toluene was found as the best solvent for the N-alkylation reaction
(Entry 5). No reaction occurred in the case of DMSO as solvent
(Entry 6), while, DMF, ethanol, acetonitrile, benzene and THF
resulted in much lower yields (Entries 1ꢀ4 and 7).
We continued the N-alkylation reaction optimization process
after finding the need for a strong base to activate the ruthenium
complex 1. The following step was performed to study the influence
of the substituents and catalyst loadings on the catalytic activity of
ruthenium complexes (1e6). The results have been exhibited in
Table 5 which indicate that lower catalyst loadings lead to mod-
erate yields and longer reaction times are required to achieve
maximum TONs (Entries 1ꢀ8). Also, as expected, higher catalyst
loadings led to higher yields and higher amine content in the
product distribution (Entries 1ꢀ8). Furthermore, considering the
results when 0.5 mol % of catalyst was used, it is clear that ruthe-
nium complexes containing ethyl group (Entry 7) as a terminal
substituent led to higher yields than those containing phenyl/pyr-
idyl/furyl or cyclohexyl (Entries 2, 5, 6, 7 and 8) and significantly
shorter reaction times were needed to complete the N-alkylation
process. This behavior reveals that the steric and electronic effects
play an important role in terms of catalyst efficiency or in terms of
generation of the catalytically active species. The results indicate
Fig. 3. Molecular structure of [(PNSeEt)RuCl(CO)(AsPh3)] (4). Ellipsoids are shown at
the 35% probability level. The disordered chloride and carbonyl ligand of the structure
have been omitted for clarity.

138
R. Ramachandran et al. / Journal of Organometallic Chemistry 791 (2015) 130e140
that the catalyst 4 is the most efficient catalyst among all, because
of the presence of the least bulky moiety (þI effect), which appears
to lead an improved activity. It is further inferred from the results
that the sulfur containing thiosemicarbazone ligand (PNS-EtTs) in
catalyst 4 may also influence the catalytic efficiency by their higher
electronegativity nature.
3.3. Substrate scope
After establishing the optimized reaction conditions, we inves-
tigated the substrate scope for this methodology. On the basis of
results obtained, the complex 4 shows relatively better catalytic
activity among the five complexes. Hence, the complex 4 was
selected as the model catalyst for the coupling of heteroaromatic
amines and diamines with substituted benzyl alcohols to afford the
corresponding secondary amines; the results have been given in
Table 6. When an equimolar solution of benzyl alcohol and 2-
aminobenzothiazole with 0.5 mol % of catalyst 4 in toluene was
used, the reaction went smoothly to afford the expected benzo-
thiozol-2-yl-benzylamine (3a) in excellent isolated yield (97%). The
reaction of 4-methylbenzyl alcohol with 2-aminobenzothiazole
gave 3b in 94% isolated yield. Another two new coupling part-
ners, namely, 4-chlorobenzyl alcohol and 4-bromobenzyl alcohol,
also furnished the corresponding secondary amines in 98% (3c) and
96% (3d) yields when combined with 2-aminobenzothiazole.
Further substrate scope was examined under identical conditions
when 4-methoxybenzyl alcohol and 4-chlorobenzyl alcohol suc-
cessfully furnished respective N-alkylated products with 2-
aminopyridine in 87% (3e) and 94% (3f) yields. Similarly, the
coupling of benzyl alcohol and 2-aminopyrimidine yielded N-
benzylpyrimidin-2-amine (3g) in 92% isolated yield. Both electron-
donating and electron-withdrawing 4-methylbenzyl alcohol and 4-
chlorobenzyl alcohol were converted into the corresponding
amines in good isolated yields of 92 and 97% (3h and 3i), respec-
tively. One of the outstanding properties of the present catalyst is
its high selectivity for monoalkylation of heteroaromatic amines.
Hence, it was of interest to determine whether this selectivity for
the monoalkylation of primary aromatic functions could be used for
the N,N’-dialkylation of diamines. Primarily, 2,6-diaminopyridine
was reacted with benzyl alcohol, which afforded N,N’-dialkylated
product (3j) with 89% yield. The N,N’-dialkylation with benzyl al-
cohols bearing a halogen atom (-Cl) proceeded to give the corre-
sponding product 3k with excellent yield 92%. Encouraged by the
promising results, we further looked at extending the above
methods to other diamines and alcohols. The dialkylation of m-
phenylenediamines was successfully carried out with benzylic
alcohol in 85% (3l) yield. Similar to m-phenylenediamines, p-phe-
nylenediamines also reacted with benzyl alcohol smoothly to give
the corresponding product 3m (91%) in excellent yields. Instead of
N,N’-dilkylation, reaction of o-phenylenediamine with benzyl
Scheme 4. Plausible mechanism for N-alkylation.
alcohol gave 2-phenyl-1H-benzo[d]imidazole (3n) as the exclusive
product (79%) under the catalytic conditions described for other
alkylations. Reaction of o-phenylenediamine with 4-chlorobenzyl
alcohol afforded the desired 2-substituted benzimidazole in 81%
(3o) isolated yield. 4-Methoxybenzyl alcohol could also be alky-
lated, revealing the formation of solely 2-(4-methoxyphenyl)-1H-
benzimidazole (3p) in 87% yield after 12 h of reaction time. The
formation of 2-arylbenzimidazole product is due to the N-alkyl-
ation of o-phenylenediamine to corresponding imines, followed by
in-situ cyclization and oxidation (Scheme 3). The formation of
cyclized product is due to the intramolecular attack of an amino
group at the imine functionality (Scheme 3). N-Alkylated sulfon-
amide derivatives are pharmaceutically active compounds and a
difficult class of substrate and remain as an essential research topic
in organic synthesis. The compatibility of the catalytic system with
sulfonamide was demonstrated, and very good results (3q) were
obtained for the coupling of sulfonamide with benzyl alcohols us-
ing the catalyst 4. Thus, reaction of 4-methylbenzenesulfonamide
and 4-chlorobenzenesulfonamide with benzyl alcohol gave the
corresponding amides and were isolated in 94 and 98% yields (3r
and 3s), respectively.
A possible catalytic cycle is proposed in Scheme 4 on the basis of
the results obtained and available literature on similar ruthenium-
catalyzed transformations [40]. This catalysis is considered to
proceed via the (benzyloxy)ruthenium intermediate A, which
Scheme 3. N-alkylation of o-phenylendiamine and in-situ cyclization to 2-arylbenzimidazole.

R. Ramachandran et al. / Journal of Organometallic Chemistry 791 (2015) 130e140
317e324;
139
undergoes b-hydrogen elimination to give the ruthenium hydride B
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(benzyloxy)ruthenium intermediate A and completes the catalytic
cycle. The effective formation of the intermediates C may depend
on the steric and electronic properties of the imine or iminium ion.
In comparison with previously reported ruthenium(II) complexes
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