Hydrazone complexes of ruthenium(II): Synthesis, crystal structures and catalytic applications in N-alkylation reactions

Article abstract of DOI:10.1016/j.jorganchem.2020.121411

A series of new Ru(II) complexes of 8-hydroxy quinoline-2-carboxyaldehyde hydrazone of the general formula [RuH(CO)(EPh₃)₂L] (1–6) (E = P or As, L = N’-((8-hydroxyquinolin-2-yl)methylene)thiophene-2-carbohydrazide (HQ-THy), N’-((8-hydroxyquinolin-2-yl)methylene)isonicotinohydrazide (HQ-IHy), N’-((8-hydroxyquinolin-2-yl)methylene) benzohydrazide (HQ-BHy)) have been synthesized. They have been characterized by elemental analysis, IR, NMR (¹H, ¹³C & ³¹P) and ESI-MS spectral methods. Further, structures of two of the complexes have been determined by single crystal X-ray diffraction technique which revealed a pseudo octahedral geometry with the coordination of the quinoline nitrogen and quinoline oxygen atoms of the ligand. All the new complexes have been employed as efficient catalysts in N-alkylation reactions for the synthesis of tertiary amines by the coupling of secondary amines with aromatic primary alcohols at low catalyst loading with maximum yields. In addition, the effects of substituents on the ligands, different solvents as well as bases and amounts of catalyst loading on the catalytic activity of the complexes have been thoroughly investigated. Complex 1 was found to be efficient catalyst towards N-alkylation of alcohols with the amine. Further, a variety of secondary amines and aromatic (hetero) primary alcohols with various functional groups have also been successfully used in the N-alkylation reactions and it has been found that only one equivalent of the alcohol was consumed in the process.

Full text of DOI:10.1016/j.jorganchem.2020.121411

Journal of Organometallic Chemistry 923 (2020) 121411
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Hydrazone complexes of ruthenium(II): Synthesis, crystal structures
and catalytic applications in N-alkylation reactions
Kaliyappan Murugan a, Subbarayan Vijayapritha a, Muthukumaran Nirmala a,
,
Periasamy Viswanathamurthi a *, Karuppannan Natarajan b
a Department of Chemistry, Periyar University, Salem, 636 011, India
b Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore, 64120, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new Ru(II) complexes of 8-hydroxy quinoline-2-carboxyaldehyde hydrazone of the general
formula [RuH(CO)(EPh3)2L] (1e6) (E ¼ P or As, L ¼ N’-((8-hydroxyquinolin-2-yl)methylene)thiophene-2-
carbohydrazide (HQ-THy), N’-((8-hydroxyquinolin-2-yl)methylene)isonicotinohydrazide (HQ-IHy), N’-
((8-hydroxyquinolin-2-yl)methylene) benzohydrazide (HQ-BHy)) have been synthesized. They have
been characterized by elemental analysis, IR, NMR (1H, 13C & 31P) and ESI-MS spectral methods. Further,
structures of two of the complexes have been determined by single crystal X-ray diffraction technique
which revealed a pseudo octahedral geometry with the coordination of the quinoline nitrogen and
quinoline oxygen atoms of the ligand. All the new complexes have been employed as efficient catalysts in
N-alkylation reactions for the synthesis of tertiary amines by the coupling of secondary amines with
aromatic primary alcohols at low catalyst loading with maximum yields. In addition, the effects of
substituents on the ligands, different solvents as well as bases and amounts of catalyst loading on the
catalytic activity of the complexes have been thoroughly investigated. Complex 1 was found to be effi-
cient catalyst towards N-alkylation of alcohols with the amine. Further, a variety of secondary amines and
aromatic (hetero) primary alcohols with various functional groups have also been successfully used in
the N-alkylation reactions and it has been found that only one equivalent of the alcohol was consumed in
the process.
Received 22 January 2020
Received in revised form
26 May 2020
Accepted 15 June 2020
Available online 12 July 2020
Keywords:
8-hydroxy quinoline-2-carboxyaldehyde
hydrazide
Ru(II) complexes
Crystal structure
N-alkylation
N,C3-dialkylation
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
toxicity, easily storable and very low in cost. Besides, the reaction falls
in green chemistry and environmentally friendly since water is the
Since the compounds with nitrogen functionalities are not only
abundantly present in important biologically active molecules but
also find extensive applications inpharmaceuticals, there has been an
upsurge inresearchleadingtothesynthesis ofcompoundscontaining
CeN bond. Various methods have been employed in the synthesis of
compounds containing the CeN bond including amination of aryl
halides [1], reductive amination of carbonyl compounds [2] hydro-
amination [3], hydro amino-methylation of CeC multiple bonds [4] or
N-alkylation [5] and typical compounds thus synthesized are shown
in Fig. 1. Among the methods that have been used for the synthesis of
new compounds, N-alkylation of amines/amides with alcohol is a
significant one because it is not only a simple direct method but also
due tothefacts that alcoholsare readilyavailable, highlystable, low in
sole byproduct [6,7]. In spite of the fact that both heterogeneous and
homogeneous catalysts have been used for the effective N-alkylation
of amine with alcohol, homogeneous catalysts have gained much
prominence due to their high catalytic performance and greater
product selectivity. In this connection, several iridium and ruthenium
complexes containing phosphine ligands have been employed in the
N-alkylation of amines with alcohols in good yields and selectivity
[8e14]. Grigg [15] and Watanabe [16] independently reported the N-
alkylation of amines using [RuCl2(PPh3)3] and [RhH(PPh3)4] for the
first time. Catalysts derived from other transition metals such as Fe
[17], Os [18], Cu [19] and Rh [20] have also shown to be efficient in
catalyzing the reaction with a good yield. However, they are not
trouble free as most of the methods reported have some difficulties
like the requirement of high temperature, pressure and prolonged
reaction timesuptofew days. Thedrawbacks tothereportedcatalysts
necessitate the synthesis of new catalysts.
* Corresponding author.
E-mail address: viswanathamurthi@gmail.com (P. Viswanathamurthi).
https://doi.org/10.1016/j.jorganchem.2020.121411
0022-328X/© 2020 Elsevier B.V. All rights reserved.

2
K. Murugan et al. / Journal of Organometallic Chemistry 923 (2020) 121411
Fig. 1. Examples of important drugs with alkylated amine.
Generally, in the area of development of transition metal catalysts
CDCl3 as solvent and tetramethyl silane or o-phosphoric acid as
reference. Electrospray ionization mass spectra were measured by
liquid chromatographyemass spectrometry quadrupole time-of-
flight Micro Analyzer (Shimadzu) at Indian Institute of Technol-
ogy Madras, Chennai. Melting points were recorded on a Technico
micro heating table and are uncorrected. The catalytic yields were
determined by gas chromatographyeflame ionization detection
(GC-FID) using an ACME 6000 series instrument with a DP-5 col-
for a specific process, besides choosing the right metal ion, the design
of ligand is also very important. Since nature of the ligand has a
profoundeffectonthecoordinationchemistryofametalcomplex,the
designing and “tailoring” of ligand properties can lead to complexes
with efficient homogeneous catalytic activities. In this connection,
specialinteresthasbeenshownontheuseofhydrazoneasligandsnot
only because of their various coordination modes but also due to their
remarkable applications in catalytic and biological activities [21].
Moreover, hydrazones with an additional donor groups have become
an important class of ligands due to the potential hemilability of the
new donor groups can play a dual role in a catalyst since they can
easily enable coordination sites in addition to at the same time, pro-
tect the coordination sites by a dynamic “on and off” chelating effect.
Among the several donor groups that have been reported to func-
tionalize hydrazones [22e24] hydroxy quinoline with the coincident
presence of soft and hard donors received special interest.
Inspired by the high activity of Ru(II) complexes for N-alkylation
reactions and from out of our systematic investigation on finding
effective homogeneous catalysts [25], here in we report the design
and synthesis of hydrazone ligands functionalized with 8-hydroxy
quinoline-2-carboxyaldehyde group and their ruthenium(II) com-
plexes. The coordination modes of the ligand with ruthenium metal
were investigated using FT-IR, 1H, 13C & 31P NMR spectroscopy and
mass spectrometry. The solid-state structures of the catalysts were
established by single-crystal X-ray diffraction. The impact of the
newly synthesized complexes was studied in N-alkylation of sec-
ondary amine with aromatic and heteroaromatic alcohols. The role
of co-ligands in determining the catalytic activity for N-alkylation
was also investigated.
umn of 30 m length, 0.53 mm diameter and 5.00 mm film thickness.
The precursor complexes [RuHCl(CO)(PPh3)3] [27], [RuHCl(-
CO)(AsPh3)3] [28] and 8-hydroxyquinoline-2-carbaldehyde [29]
were prepared according to literature procedures.
2.2. Synthesis of 8-hydroxyquinoline-2-carboxyaldehyde hydrazide
ligands
8-hydroxyquinoline-2-carbaldehyde (1 mmol) was dissolved in
ethanol (20 mL) and reacted with an equivalent of substituted hy-
drazide (1 mmol) in a mixture of ethanol (20 mL) and H2O (1 mL).
The reaction mixture was subsequently refluxed for 6 h. On cooling
the resultant solution, a solid compound separated which was
filtered and washed with ether and dried. All the three 8-
hydroxyquinoline-2-carboxy aldehyde hydrazide ligands were ob-
tained as yellow crystalline precipitate in good yield.
2.2.1. N’-((8-hydroxyquinolin-2-yl)methylene) thiophene-2-
carbohydrazide (HQ-THy) ligand
It was prepared from 8-hydroxyquinoline-2-carbaldehyde
(0.172 g, 1 mmol) and thiophene carboxylic acid hydrazide
(0.142 g, 1 mmol). Yield: 77%. Yellow color solid. M.P. 240e244 ꢁC.
Anal. Calc. for C15H11N3SO2: C, 60.59; H, 3.73; N, 14.13; S, 10.78.
Found: C, 60.67; H, 3.78; N, 14.19; S, 10.85. IR (ATR,
n
cmꢀ1): 3401
2. Experimental section
(OH), 3113 (NH), 1613 (C]O), 1593 (C]N), 1560 (C¼Nring).1H NMR
(300.13 MHZ, CDCl3, ppm): 10.24 (s, OH), 9.54 (s, NH), 8.10 (s, CH]
N), 7.92e7.12 (m, AreH). 13C NMR (75.47 MHZ, CDCl3, ppm): 166.01
(C]O), 153.87 (C]N), 153.87 (C]N), 153.74 (CeO), 129.83 (AreC),
128.78 (AreC), 127.93 (AreC), 127.01 (AreC), 126.91 (AreC), 126.42
(AreC), 126.35 (AreC), 125.88 (AreC), 123.50 (AreC), 110.43 (AreC).
2.1. Starting materials and measurements
Chemically pure and AR grade reagents were used for all the
reactions. The solvents were distilled and dried according to liter-
ature procedures [26]. RuCl3.3H2O was purchased from Sigma
Aldrich. Microanalyses of carbon, hydrogen, nitrogen and sulfur
were carried out using a Vario EL III elemental analyzer. IR spectra
were obtained with Bruker Alpha model FT-IR spectrophotometer
in the 4000e600 cmꢀ1 range. 1H, 13C and 31P NMR spectra were
recorded on a Bruker Advance 400 MHz FT-NMR instrument using
2.2.2. N’-((8-hydroxyquinolin-2-yl) methylene)
isonicotinohydrazide (HQ-IHy) ligand
It was prepared from 8-hydroxyquinoline-2-carbaldehyde
(0.170 g, 1 mmol) and isonicotinic hydrazide (0.137 g, 1 mmol).

K. Murugan et al. / Journal of Organometallic Chemistry 923 (2020) 121411
3
Yield: 75%. Yellow color solid. M.P. 228e230 ꢁC. Anal. Calc. for
16H12N4O2: C: 65.75; H, 4.39; N, 19.17. Found: C, 65.73; H, 4.42; N,
19.25. IR (ATR,
cmꢀ1): 3396 (OH), 3041 (NH), 1680 (C]O), 1647
2.3.3. [RuH(CO)(PPh3)2(HQ-BHy)] (3)
C
The ligand HQ-BHy (0.029 g, 0.1 mmol) was reacted with
[RuHCl(CO)(PPh3)3] (0.095 g, 0.1 mmol) to form complex 3. Yield:
84%. Color: Reddish brown. M.P: 178e181 ꢁC. Anal. Calc. for
n
(C]N), 1545 (C¼Nring). 1H NMR (400 MHZ, CDCl3, ppm): 12.42 (s,
NH), 9.86 (s, OH), 8.68 (s, CH]N), 7.96e7.14 (m, AreH). 13C NMR
(75.47 MHz, CDCl3, ppm): 162.10 (C]O), 153.57 (C]N), 153.45 (C]
N), 150.40 (CeO), 129.16 (AreC), 121.82 (AreC), 121.95 (AreC),
121.63 (AreC), 120.95 (AreC), 120.95 (AreC), 113.04 (AreC), 113.04
(AreC), 111.97 (AreC).
C
54H44N3O3P2Ru: C: 68.56; H, 4.69; N, 4.44. Found: C, 68.64; H,
4.73; N, 4.54. IR (ATR,
n
cmꢀ1): 3048 (NH), 1908 (CO), 1730 (C]O),
1675 (C]N), 1543 (C¼Nring). 1H NMR (400 MHZ, CDCl3,
ppm): ꢀ10.56 (s-Ru-H),10.70 (s, NH), 8.61 (s, HC]N), 7.88e6.33 (m,
AreH).13C NMR (100 MHZ, CDCl3, ppm): 204.49 (CO), 178.99 (C]O),
151.22 (C]N), 148.89 (C¼Nring), 132.82e126.81 (AreC). 31P NMR
(162 MHz, CDCl3, ppm): 29.57 (s, PPh3).
2.2.3. N’-((8-hydroxyquinolin-2-yl)methylene)benzohydrazide
(HQ-BHy) ligand
It was prepared from 8-hydroxyquinoline-2-carbaldehyde
(0.170 g, 1 mmol) and benz hydrazide (0.136 g, 1 mmol). Yield:
78%. Yellow color solid. M.P. 236e239 ꢁC. Anal. Calc. for C17H13N3O2:
C, 70.09, H, 4.50; N, 14.42. Found: C, 70.15; H, 4.52; N, 14.47. IR (ATR,
2.3.4. [RuH(CO)(AsPh3)2(HQ-Thy)] (4)
The ligand HQ-Thy (0.029 g, 0.1 mmol) was reacted with
[RuHCl(CO)(AsPh3)3] (1.085 g, 0.1 mmol) to form complex 4. Yield:
78%. Color: Reddish brown. M.P. 126e129 ꢁC. Anal. Calc. for
n
cmꢀ1): 3396 (OH), 3211 (NH), 1689 (C]O), 1642 (C]N), 1545
C52H42N3SO3As2Ru: C, 60.06; H, 4.07; N, 4.04; S, 3.08. Found: C,
(C¼Nring). 1H NMR (300.13 MHZ, CDCl3, ppm): 11.92 (s, OH), 9.58 (s,
NH), 8.35 (s, CH]N), 7.41e6.92 (m, AreH). 13C NMR (75.47 MHZ,
CDCl3, ppm): 166.50 (C]O), 153.58 (C]N), 154.10 (C]N), 153.74
(C]O), 139.81 (AreC), 136.77 (AreC), 136.18 (AreC), 131.05 (AreC),
127.46 (AreC), 127.26 (AreC), 126.68 (AreC), 121.40 (AreC), 108.17
(AreC).
60.12; H, 4.14; N, 4.09; S, 3.13. IR (ATR,
n
cmꢀ1): 3055 (NH), 1905
(CO), 1730 (C]O), 1627 (C]N), 1572 (C¼Nring). 1H NMR (400 MHZ,
CDCl3, ppm): ꢀ11.30 (s, RueH), 10.74 (s, NH), 8.30 (s, HC]N),
8.29e6.51 (m, AreH). 13C NMR (100 MHZ, CDCl3, ppm): 201.62 (CO),
176.91 (C]O), 157.85 (C]N), 154.20 (C¼Nring), 134.07e121.87
(AreC). ESI-MS: m/z ¼ 1040.0 [M]þ. Crystals suitable for X-ray
diffraction were obtained by slow evaporation of dichloromethane-
petroleum ether solution of complex 4.
2.3. Synthesis of new ruthenium(II) hydroxyquinoline hydrazone
complexes (1e6)
[RuHCl(CO)(EPh3)3] (E
¼
P
or As) (0.1 mmol) and 8-
2.3.5. [RuH(CO)(AsPh3)2(HQ-IHy)] (5)
hydroxyquinolinehydrazide ligand (0.1 mmol) were taken in
ethanol - chloroform mixture (20 mL, 1 : 1, v/v) and heated under
reflux for 8 h during which the solution turned from pale yellow to
reddish brown. After completion of reaction which was confirmed
from thin layer chromatography (TLC), the solution was cooled to
room temperature, a solid compound separated which was filtered
and washed several times with ether and dried in vacuo. All the
new ruthenium(II) hydroxyquinoline hydrazone complexes are
obtained in good yields.
The ligand HQ-IHy (0.029 g, 0.1 mmol) was reacted with
[RuHCl(CO)(AsPh3)3] (1.085 g, 0.1 mmol) to form complex 5. Yield:
65%. Color: Reddish brown. M.P.132e134 ꢁC. Anal. Calc. for
C
53H43N4O3As2Ru: C, 61.54; H, 4.19; N, 5.41. Found: C, 61.63; H, 4.24;
N, 5.49. IR (ATR,
n
cmꢀ1): 3046 (NH), 1904 (CO), 1735 (C]O), 1630
(C]N), 1578 (C¼Nring). 1H NMR (400 MHZ, CDCl3, ppm): -10.68 (s,
RueH), 10.60 (s, NH), 8.20 (s, HC]N), 7.39e6.48 (m, AreH). 13C
NMR (100 MHZ, CDCl3, ppm): 201.80 (CO), 175.57 (C]O), 155.31
(C]N), 152.04 (C¼Nring), 135.49e127.81 (AreC).
2.3.1. [RuH(CO)(PPh3)2(HQ-Thy)] (1)
2.3.6. [RuH(CO)(AsPh3)2(HQ-BHy)] (6)
The ligand HQ-Thy (0.029 g, 0.1 mmol) was reacted with
[RuHCl(CO)(PPh3)3] (0.095 g, 0.1 mmol) to form complex 1. Yield:
87%. Color: Reddish brown. M.P. 158e161 ꢁC. Anal. Calc. for
The ligand HQ-BHy (0.029 g, 0.1 mmol) was reacted with
[RuHCl(CO)(AsPh3)3] (1.085 g, 0.1 mmol) to form complex 6. Yield:
77%. Color: Reddish brown. M.P. 110e112 ꢁC. Anal. Calc. for
C
52H41N3SO3P2Ru: C, 65.67; H, 4.35; N, 4.42; S, 3.37. Found: C,
C
54H44N3O3P2As2Ru: C, 59.19; H, 4.05; N, 3.83. Found: C, 59.26; H,
65.72; H, 4.39; N, 4.49; S, 3.43. IR (ATR,
n
cmꢀ1): 3051 (NH), 1909
4.12; N, 3.89. IR (ATR,
n
cmꢀ1): 3052 (NH), 1907 (CO), 1732 (C]O),
(CO), 1736 (C]O), 1630 (C]N), 1586 (C¼Nring). 1H NMR (400 MHZ,
CDCl3, ppm): ꢀ10.67 (RueH), 10.73 (s, NH), 8.24 (s, HC]N),
8.11e6.26 (m, AreH).13C NMR (100 MHZ, CDCl3, ppm): 204.71 (CO),
174.62 (C]O), 151.67 (C]N), 148.62 (C¼Nring), 133.77e127.62
(AreC). 31P NMR (162 MHz, CDCl3, ppm): 29.41 (s, PPh3). ESI-MS: m/
z ¼ 950.0 [M ꢀ H]þ. Crystals suitable for X-ray diffraction were
obtained by slow evaporation of dichloromethane-ethanol solution
of complex 1.
1632 (C]N), 1572 (C¼Nring). 1H NMR (400 MHZ, CDCl3,
ppm): ꢀ11.09 (s, RueH), 10.62 (s, NH), 8.61 (s, HC]N), 8.18e6.45
(m, AreH). 13C NMR (100 MHZ, CDCl3, ppm): 201.62 (CO), 175.81
(C]O), 156.49 (C]N), 151.12 (C¼Nring), 134.41e123.33 (AreC).
2.4. X-ray crystallographic study
Crystal data were collected for the complexes (1 and 4) at 296 K
2.3.2. [RuH(CO)(PPh3)2(HQ-IHy)] (2)
using a Gemini A Ultra Oxford Diffraction automatic diffractometer.
The ligand HQ-IHy (0.029 g, 0.1 mmol) was reacted with
[RuHCl(CO)(PPh3)3] (0.095 g, 0.1 mmol) to form complex 2. Yield:
82%. Color: Reddish brown. M.P. 125e128 ꢁC. Anal. Calc. for
Graphite monochromated Mo-Ka radiation (
l
¼ 0.71073 Å) was
used throughout. The absorption corrections were performed by
multi-scan method. Corrections were made for Lorentz and polar-
ization effects. The structure was solved by direct method using the
program SHELXS 2018 [30]. 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 aniso-
tropically using weighted full-matrix least squares on F2. Atomic
scattering factors were incorporated in the computer programs.
C
53H43N4O3P2Ru: C, 67.22; H, 4.58; N, 5.92. Found: C, 67.29; H, 4.63;
N, 5.96. IR (ATR,
n
cmꢀ1): 3044 (NH), 1900 (CO), 1735 (C]O), 1623
(C]N), 1573 (C¼Nring). 1H NMR (400 MHZ, CDCl3, ppm): ꢀ10.63 (s,
RueH), 10.80 (s, NH), 8.66 (s, HC]N), 7.70e6.31 (m, AreH). 13C
NMR (100 MHZ, CDCl3, ppm): 202.23 (CO), 175.19 (C]O), 155.94
(C]N),153.97 (C¼Nring),139.18e125.99 (AreC). 31P NMR (162 MHz,
CDCl3, ppm): 29.74 (s, PPh3).

4
K. Murugan et al. / Journal of Organometallic Chemistry 923 (2020) 121411
2.5. Typical procedure for catalytic N-alkylation of hetro cyclic
secondary amines with primary alcohol
the two triphenyl phosphine ligands (PPh3) in 1 and triphenyl
arsine (AsPh3) ligands in 4 are present in trans position of distorted
octahedral Ru(II) complexes. The two PPh3 are slightly bent to-
wards the hydride ligand in complex 1 due to the steric re-
quirements of the 8-hydroxy quinoline hydrazone ligand. The
RueC(1) bond distances in complex 1 and 4 have been observed as
1.812(3) and 1.798(7) Å which is comparable with the RueC bond
lengths of previously reported carbonyl complexes [31]. The bond
lengths of metal hydrogen bonds in complexes 1 and 4 have been
obtained as 1.344 and 1.879 Å. Moreover, intra molecular hydrogen
bonding was observed in complexes 1 and 4.
KOH (40 mol %) was added to a stirred solution of secondary
amine (1 mmol) in 2 mL of toluene. After 1 min of stirring, the
primary alcohol (2.5 mmol) and Ru(II) complex (1 mmol %) were
added and the mixture was stirred at 110 ꢁC for 12 h. The reaction
mixture was cooled to room temperature and dichloromethane
(6 mL) was added. The resulting suspension was filtered through
Celite. The filtrate was subjected to GC analysis using dodecane as
an internal standard. All of the data reported are averages of at least
two runs.
3.2. Infrared spectroscopic analysis
3. Results and discussion
In order to confirm the coordination of ligands to ruthenium
metal, the IR spectra of free ligands were compared with that of
new ruthenium complexes. In the spectra of free ligands, a strong
band has been observed in the region 1647e1593 cmꢀ1 which is
characteristics of the azomethine group and this band has been
found in same frequency in the complexes indicates that the azo-
methine nitrogen of the ligand does not involve in the coordination
with central metal ion. The bands due to NH and C]O which were
observed around 3373e3305 and 1736e1630 cmꢀ1 respectively in
the free ligands have not shifted appreciably in the complexes
indicate that neither N of NH nor O of C]O in the hydrazone li-
gands do not participate in binding of the ligand. A broad band
appeared around 3214e3184 cmꢀ1for OH of the free ligands has
completely disappeared in complexes shows the coordination of
phenolic O atom after deprotonation. The band appeared around
1572e1551 cmꢀ1 in the ligands for the ring C]N has got shifted to
higher frequencies (1586-1556 cmꢀ1) in metal complexes reveals
that the other coordination is through quinoline nitrogen. A band
which appeared around 1909e1900 cmꢀ1 in all the complexes
clearly shows the presence of a terminal carbonyl ligand. Hence,
from the infrared spectroscopic data, it has inferred that the
quinoline nitrogen and phenolic oxygen atom of the hydrazone li-
gands coordinated to the metal ion in all the complexes.
The 8-hydroxyquinoline-2-carboxyaldehyde hydrazide ligands
(HQ-THy, HQ-IHy, HQ-BHy) were synthesized from the condensa-
tion reaction of 8-hydroxyquinoline -2-carbaldehyde with thio-
phenecarbohydrazide, isonicotinohydrazide or benzhydrazide
(Scheme 1). The reaction between these hydrazides with [RuHCl(-
CO)(EPh3)3] (E ¼ P or As) yielded new Ru(II) complexes of the type
[RuH(CO)(EPh3)2L] (E ¼ P or As; L ¼ HQ-THy, HQ-IHy or HQ-BHy)
(Scheme 2). All the ruthenium hydrazone complexes have been
found to be non-hygroscopic solids and soluble in all organic sol-
vents such as dichloromethane, chloroform, THF, acetone, benzene,
toluene, N,N-dimethylformamide (DMF) and dimethyl sulfoxide
(DMSO). All the new complexes were characterized by elemental
analyses, spectral techniques (IR, 1H, 13C & 31P NMR and ESI-mass)
and single-crystal XRD studies (1 and 4). The ESI mass spectra of
complexes 1 and 4 show molecular ion peak with m/z value 950.0
([M ꢀ H]þ) and 1040.0 ([M]þ) respectively which are in agreement
with calculated molecular masses 951.1 and 1039.8. (Figs. S1e2).
3.1. X-ray crystallography
Though we attempted to grow crystals for all the six complexes,
we could obtain good quality crystals suitable for X-ray diffraction
for only two of them, namely [RuH(CO)(PPh3)2 (HQ-Thy)] (1) and
[RuCl(CO)(AsPh3)2 (HQ-Thy)] (4) and hence, we determined their
structures by X-ray diffraction techniques. Crystals of suitable size
of complexes 1 and 4 were obtained from dichloromethane-
ethanol and dichloromethane-petroleum ether mixture and they
both crystallized in monoclinic system with P21/c space group. The
crystallographic data and structural refinement parameters as well
as selected bond length and bond angle are shown in Tables 1 and 2.
The ORTEP views of the complexes 1 and 4 have been depicted in
3.3. 1H NMR spectroscopic analysis
1H NMR spectra of ligands and ruthenium complexes were
recorded in CDCl3 solvent to confirm the binding of hydrazide li-
gands with metal ion and they are shown in Fig. S3- S8. The spectra
of free ligands show a singlet in the region 12.42e10.24 ppm,
characteristic of phenolic OH group. This peak disappeared in their
corresponding complexes confirming the binding of ligands with
metal ion via phenolic O atom after deprotonation of phenolic
hydroxyl group. The NH and azomethine peaks appear in the region
10.80e10.60 and 8.66e8.20 ppm respectively in the spectra of li-
gands do not change their position in the spectra of complexes
indicate that they do not involve in the coordination to the metal
ion. Multiplets were observed around 8.18e6.26 ppm in all the li-
gands and complexes due to aromatic protons. In addition, a singlet
appeared at ꢀ11.30e10.56 ppm due to the RueH in all the
complexes.
Figs.
2 and 3. In both the complexes 1 and 4, the 8-
hydroxyquinoline-2-carboxyaldehyde hydrazide ligand behaves
as monobasic bidentate N, O donors and binds to ruthenium ion via
8-hydroxyquinoline oxygen and nitrogen, form a five-membered
chelate ring with a bite angle of O(1)eRu(2)eN(1) of 77.14(7) and
O(1)eRu(2)eN(1) of 77.07(19) for complexes 1 and 4 respectively.
Ru(2)eN(1) bond distance is 2.240(2) Å and Ru(2)eO(2) bond
distance is 2.0876(17) Å for the complex 1 and the corresponding
bond lengths for complex 4 are 2.239(5) and 2.096(5) Å. Further,
Scheme 1. Synthesis of 8-hydroxyquinoline-2-carboxyaldehyde hydrazide ligands.

K. Murugan et al. / Journal of Organometallic Chemistry 923 (2020) 121411
5
Scheme 2. Synthesis of Ru(II) complexes.
Table 1
Crystal data and structure refinement of complexes 1 and 4.
[RuH(CO)(PPh3)2(HQ-Thy)]
[RuH(CO)(AsPh3)2(HQ-Thy)]
CCDC Number
1967465
1967467
Empirical formula
Formula weight
C52H41N3SO3ClP2Ru
986.40
C52H41N3SO3ClAs2Ru
1074.30
Temperature
296(2) K
296(2) K
Wavelength
Crystal system
Space group
0.71073 Å
Monoclinic
P 21/n
0.71073 Å
Monoclinic
P 21/n
Unit cell dimensions
a
b
c
a
b
g
15.1624(5) Å
18.9080(6) Å
16.8310(6) Å
90ꢁ
15.4143(4) Å
18.8694(4) Å
17.0411(4) Å
90ꢁ.
95.7640(10)ꢁ
90ꢁ
97.1850(10)ꢁ
90ꢁ
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
4800.9(3) Å3
4
4917.6(2) Å3
4
1.420 Mg/m3
0.542 mmꢀ1
2100
1.450 Mg/m3
1.795 mmꢀ1
2160
Crystal size
Theta range for data collection
Index ranges
0.288 ꢂ 0.277 ꢂ 0.162 mm3
2.542e28.288ꢁ.
ꢀ20 ꢃ h<¼20
ꢀ18 ꢃ k<¼25
ꢀ20 ꢃ l<¼22
44854
0.22 ꢂ 0.19 ꢂ 0.17 mm3
2.640e28.263ꢁ.
20 ꢃ h<¼20
ꢀ25 ꢃ k<¼25
ꢀ22 ꢃ l<¼22
45611
Reflections collected
Independent reflections
Completeness to theta ¼ 25.242ꢁ
Absorption correction
Refinement method
11843 [R(int) ¼ 0.0333]
12184 [R(int) ¼ 0.0346]
99.5%
99.8%
None
None
Full-matrix least-squares on F2
11843/0/599
1.013
Full-matrix least-squares on F2
12146/0/556
1.066
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
R1 ¼ 0.0407, wR2 ¼ 0.1074
R1 ¼ 0.0500, wR2 ¼ 0.2458
R1 ¼ 0.0592, wR2 ¼ 0.1168
R1 ¼ 0.1027, wR2 ¼ 0.1643
3.4. 13C NMR spectroscopic analysis
around ruthenium atom. Appearance of peak around
29.74e29.41 ppm indicates the presence of triphenylphosphine
group in the new complexes. The presence of single peak confirms
the coordination of two triphenylphosphine groups in trans posi-
tion (Figs. S15eS17).
The 13C NMR spectra of complexes were recorded in order to
confirm the formation of new ruthenium(II) with 8-
hydroxyquinoline-2-carboxyaldehyde hydrazone (Figs. S9eS14).
Peaks appear around at 139.18e121.87 ppm can be assigned to ar-
omatic carbons. Azomethine carbon (>C]N) and amide group
(NHeC]O) have shown their signals around at 157.85e151.22 and
178.99e175.11 ppm respectively. The appearance of peak at
204.71e201.62 ppm region is due to the terminal carbonyl carbon
of the coordinated carbonyl.
3.6. Catalytic N-alkylation and N, C3-dialkylation of cyclic amines
with alcohols
It is very well known that ruthenium complexes have been used
extensively as precatalysts for N-alkylation of amines with alcohols
[32]. Though several enough examples are known in which
hydrogen borrowing strategy has been employed for the con-
struction of N-alkylation and N,C-dialkylated products arising from
alcohols, no single step process for the simultaneous alkylation of
both the N and the unactivated C3 carbon of secondary amines
3.5. 31P NMR spectroscopic analysis
31P NMR spectra were recorded for complexes 1e3 to confirm
the coordination of triphenylphosphine group and its configuration

6
K. Murugan et al. / Journal of Organometallic Chemistry 923 (2020) 121411
Table 2
Selected bond lengths [Å] and bond angles [ꢁ] for complexes 1 and 4.
Complex 1
Complex 4
Ru(1)-C (16)
Ru(1)-O (1)
Ru(1)-N (1)
Ru(1)-P (1)
Ru(1)-P (2)
Ru(1)eH(1)
1.812(3)
2.0876(17)
2.240(2)
2.3635(7)
2.3683(7)
1.344
Ru (1)-C (16)
Ru (1)-O (1)
Ru (1)-N (1)
Ru (1)-As (1)
Ru (1)-As (2)
Ru(1)eH(1)
1.798(7)
2.096(5)
2.239(5)
2.4414(9)
2.4423(8)
1.879
bond Angles [ꢁ]
C(16)-Ru(1)-O(1)
C(16)-Ru(1)-N(1)
O(1)-Ru(1)-N(1)
C(16)-Ru(1)-P(1)
O(1)-Ru(1)-P(1)
N(1)-Ru(1)-P(1)
C(16)-Ru(1)-P(2)
O(1)-Ru(1)-P(2)
N(1)-Ru(1)-P(2)
P(1)-Ru(1)-P(2)
N(1)-Ru(1)-H(1)
O(1)-Ru(1)-H(1)
H(1)-Ru(1)-P(1)
H(1)-Ru(1)-P(2)
173.54(10)
109.18(11)
77.14(7)
92.06(9)
86.54(6)
89.75(6)
90.73(9)
90.08(6)
94.48(6)
173.87(2)
168.44
C(16)-Ru(1)-O(1)
C(16)-Ru(1)-N(1)
O(1)-Ru(1)-N(1)
C(16)-Ru(1)-As(1)
O(1)-Ru(1)-As(1)
N(1)-Ru(1)-As(1)
C(16)-Ru(1)-As(2)
O(1)-Ru(1)-As(2)
N(1)-Ru(1)-As(2)
As(1)-Ru(1)-As(2)
C(1)-O(1)-Ru(1)
C(6)-N(1)-Ru(1)
H(1)-Ru(1)-As(1)
H(1)-Ru(1)-As(2)
173.4(3)
109.4(3)
77.07(19)
92.4(3)
86.46(15)
88.27(14)
91.6(3)
89.00(15)
94.62(14)
173.95(3)
116.0(4)
109.4(4)
87.14
92.13
85.27
89.15
89.32
Fig. 3. Molecular structure of [RuH(CO)(AsPh3)2(HQ-Thy)] (4). Ellipsoids are shown at
the 50% probability level omitting the hydrogen and chlorine atoms.
Fig. 2. Molecular structure of [RuH(CO)(PPh3)2(HQ-Thy)] (1). Ellipsoids are shown at
the 50% probability level omitting the hydrogen, chlorine atoms and solvent molecule.
However, the use of strong bases such as NaOH, NaOtBu and KOH
lead to good yields of the desired product (Table 3, entries 6, 8e10).
It is evident that with the use of KOH, the yield of the product was
the highest indicate KOH as the best base (Table 3, entry 6). To
determine the amount of KOH (mol %) required for effective
transformation, the amount of KOH was increased from 20 mol % up
to 40 mol % in the model reaction (Table 3, entries 4e6). The results
show that the use of 40 mol % base has resulted not only the
maximum yield for the N-alkylated product but also for N,C3-dia-
lkylated product.
corresponding to a formal C (sp3)-H activation was known until
Bruneau and co-workers reported the N-alkylation and N,C3-dia-
lkylation of saturated cyclic amines in the presence of ruthenium
precatalysts [33]. Hence, we decided to test the catalytic activities
of our new complexes in the N-alkylation and N,C3-dialkylation of
secondary amine with alcohols.
To start with, optimization of base and temperature were
investigated for the N-alkylation and N,C3-dialkylation of pyrroli-
dine with benzyl alcohol as a model reaction and obtained results
are given in Table 3. It is to be noted that weak bases such as
Na2CO3, K2CO3 and Cs2CO3 were not effective (Table 3 entries 1e3).
After finding the need for a strong base to activate the cata-
lyst, we were involved to examine the solvent dependent dif-
ferences in activity of catalyst 1 by carry out the screening

K. Murugan et al. / Journal of Organometallic Chemistry 923 (2020) 121411
7
Table 3
Table 6
Screening of bases for N-alkylation and N,C3-dialkylation of pyrrolidine with benzyl
Catalytic N-alkylation and N,C3-dialkylation of pyrrolidine with benzyl alcohol
alcohol.a.
derivatives.a.
.
Entry
Base
Amount of base (mol%)
A (%)b
B(%)b
1
2
3
4
Na2CO3
K2CO3
Cs2CO3
KOH
KOH
KOH
50
50
50
20
30
40
40
50
20
50
e
e
e
e
21
˂10
22
24
e
e
Entry
Alcohol
A (%)b
24
B (%)b
76
41
70
76
e
5
1
6
7c
8
KOH
NaOH
NaOtBu
NaOtBu
18
20
22
44
38
52
9
10
2
3
36
74
64
26
a
Reagents and conditions: Pyrrolidine (1 mmol), alcohol (2.5 mmol), base
(20e50 mol %), catalyst 1 (1 mol %), toluene (2 mL), reflux at 110 ꢁC for 12 h.
b
The yields of A and B were determined by GC using dodecane as an internal
standard.
c
Room temperature.
4
42
58
Table 4
a
Screening of solvents for N-alkylation and N,C3-dialkylation of pyrrolidine with
Reagents and conditions: Pyrrolidine (1 mmol), alcohol (2.5 mmol), KOH
benzyl alcohol.a.
(40 mol %), catalyst 1 (1 mol %), toluene (2 mL), reflux at 110 ꢁC for 12 h.
b
The yields of A and B were determined by GC using dodecane as an internal
standard.
Table 7
N-alkylation and N,C3-dialkylation of morpholine with benzyl alcohol derivatives.a.
Entry
Solvent
T
oC
Time
A (%)b
B (%)b
1
2
3
4
5
6
7
8
CH3CN
DMF
DMSO
DMA
1, 4 dioxane
THF
80
12
12
12
12
12
12
12
12
18
16
15
13
14
17
20
24
45
42
39
32
38
43
65
76
150
152
162
100
65
Entry
Primary alcohol
A (%)b
93
B (%)b
7
Benzene
Toluene
81
110
1
a
Reagents and conditions: Pyrrolidine (1 mmol), alcohol (2.5 mmol), KOH
(40 mol %), catalyst 1 (1 mol %), solvent (2 mL), reflux for 12 h.
b
The yields of A and B were determined by GC using dodecane as an internal
2
3
91
89
9
standard.
11
Table 5
Catalyst screening for N-alkylation and N,C3-dialkylation of pyrrolidine with benzyl
alcohol.a
Entry
Catalyst
Amount of catalyst (mol %)
A (%)b
B (%)b
4
95
5
1
2
3
4
5
6
1
2
3
4
5
6
0.15/0.25/0.5/1
0.15/0.25/0.5/1
0.15/0.25/0.5/1
0.15/0.25/0.5/1
0.15/0.25/0.5/1
0.15/0.25/0.5/1
8/11/18/24
10/14/21/28
0/15/21/27
0/15/19/29
0/15/20/26
0/15/21/25
0/15/33/76
0/12/18/72
0/8/14/73
0/8/14/71
0/8/14/74
0/8/14/75
a
Reagents and conditions: Morpholine (1 mmol), alcohol (2.5 mmol), KOH
(40 mol %), catalyst 1 (1 mol %), toluene (2 mL), reflux at 110 ꢁC for 12 h.
b
The yields of A and B were determined by GC using dodecane as an internal
standard.
a
Reagents and conditions: Pyrrolidine (1 mmol), alcohol (2.5 mmol), KOH
(40 mol %), catalyst (mol %), toluene (2 mL), reflux at 110 ꢁC for 12 h.
b
The yields of A and B were determined by GC using dodecane as an internal
standard.
After finding out need for a strong base and suitable solvent for
the reaction, we took up the investigation on finding the influence of
the catalyst loading on the catalytic activity. The results indicate that
lower catalyst loadings lead to moderate yields, higher catalyst
loadings lead to higher yields and higher amine content in the
product distribution (Table 5). It is clear from the results that the
complex containing thiophene as terminal substituent in the
hydrazone ligand and triphenylphosphine as coligand (Table 5, entry
1) lead to higher yields than those containing phenyl or pyridine as
terminal substituent and triphenyl arsine as coligand (Table 5, en-
tries 2e6). Hence, complex 1 is selected for further studies.
reaction in different solvents used such as CH3CN, DMF, DMSO,
THF, 1,4 dioxane, toluene, benzene and DMA (Table 4). Aromatic
solvents such as benzene and toluene (Table 4, entries 7 and 8)
were found to be good reaction media than polar aprotic solvents
(Table 4, entries 1e4). Ether like solvents 1,4 dioxane and THF
lead the N-alkylation and N,C3-dialkylation reactions moderately
(Table 4, entries 5 and 6). The results reveal that toluene was the
solvent of choice for the N-alkylation and N,C3-dialkylation
reactions.

8
K. Murugan et al. / Journal of Organometallic Chemistry 923 (2020) 121411
Table 8
entry 3).
N-alkylation and N, C3-dialkylation of piperidine with benzylalcohol derivatives.a.
Taking complex 1 as the catalyst, we wanted to see whether
there is any change in yields in the reaction if it is carried out with
morpholine as the amine since morpholine moiety has played a
significant role in pharmaceutical molecules besides being an
active intermediate in many chemical reactions. Hence, the N-
alkylation and N,C3-dialkylation reactions have been carried out
with a number of benzyl alcohol consisting of different sub-
stituents. Table 7 summarizes the results obtained on the catalytic
activity of 1 with toluene/KOH for this CeN coupling reaction. It is
to be noted that the there is only a very low yield of the dialkyl
product in all the cases indicating that there is no effect of the
change of amine in these reactions.
Having checked the dialkylation with pyrrolidine and morpho-
line with substituted alcohols, we thought of looking at the yield of
the product if we use piperidine as the amine and the results ob-
tained are shown in Table 8. It is seen that the N,C3-dialkylation
products were observed only as a minor product in all of the re-
actions were carried out using substituted benzyl alcohol and
piperidine.
Entry
Primary alcohol
A (%)b
91
B (%)b
9
1
2
3
88
85
12
15
4
93
7
a
Reagents and conditions: Piperidine (1 mmol), alcohol (2.5 mmol), KOH (40 mol
%), catalyst 1 (1 mol %), toluene (2 mL), reflux at 110 ꢁC for 12 h.
b
The yields of A and B were determined by GC using dodecane as an internal
standard.
Finally, we carried out similar catalytic reactions using various
secondary amines with different types of heteroaromatic alcohols
and the results obtained are given in Table 9. It has been observed
that only the mono alkylated product formed in high yields and
dialkylated product formed in very low yields. Hence, it is to be
pointed out from our studies that best result for the synthesis of N-
alkylation and N, C3-dialkylated product could be obtained using
complex 1 as the catalyst taking pyrrolidine and benzyl alcohol.
When compared to previously reported catalysts for N alkylation
reactions [33e35], the present catalyst required lower temperature
and shorter time for effective formation of alkylated products.
Moreover, the catalyst can be applied to a wide range of substrates
including heterocyclic secondary amines and primary alcohols that
were scarcely addressed by previously reported catalysts for N
alkylation reactions.
To expand the scope of the present homogeneous catalyst sys-
tem, the N-alkylation and N, C3-dialkylation reaction have been
carried out with a number of benzyl alcohol consisting of different
substituents. Table 6 summarizes the results obtained on the cat-
alytic activity of 1 for the reaction of pyrrolidine with various
benzyl alcohols in toluene/KOH. The unsubstituted alcohol gives
N,C3-dialkylation product as major product with 76% yield (Table 6,
entry 1). When compared to unsubstituted benzyl alcohol, the yield
of N,C3-dialkylation product is decreased (64%) whereas N-alkyl-
ation product is increased (36%) for methyl substituted benzyl
alcohol (Table 6, entry 2). The electron with drawing groups in
substituted benzyl alcohols such as p-chloro, the N-alkylation
product is predominant over N,C3-dialkylation product (Table 6,
Table 9
N-alkylation and N, C3-dialkylation of secondary amines with heteroaromatic alcohol.a.
b The yields of A and B were determined by GC using dodecane as an internal standard.
a
Reagents and conditions: Secondary amines (1 mmol), primary alcohol (2.5 mmol), KOH (40%), catalyst 1 (1 mol %), toluene (2 mL), reflux at 110 ꢁC for 12 h.

K. Murugan et al. / Journal of Organometallic Chemistry 923 (2020) 121411
9
Scheme 3. Plausible reaction mechanism for the catalytic alkylation reactions.
A plausible mechanism for alkylation reactions based on an
Declaration of competing interest
autotandem catalytic cycle [36] is depicted in Scheme 3. Benzyl
alcohol is converted into benzaldehyde along with the generation
of the ruthenium hydride species mechanism according to previous
literature [37]. The in situ generated benzaldehyde reacts with the
secondary amine to form the iminium ion I which, in the absence of
base undergoes reduction and gives 1-benzylpyrrolidine (A). The
iminium ion I might move into the iminium ion II via a [1,3] hydride
shift. Deprotonation of II might lead to the formation of enamine III
as key intermediate. Finally, enamine III, would lead via nucleo-
philic substitution of benzyl alcohol or aldol-type reaction on
benzaldehyde [38] to the iminium species IV which, in turn, would
afford the expected dialkylated amine 1,3-dibenzylpyrrolidine (B)
after reduction in the presence of the ruthenium hydride species.
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgements
The authors thank CIMF, Periyar University, Salem for the help to
record single crystal X-ray studies.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2020.121411.
4. Conclusions
References
A simple route to the synthesis of novel ruthenium(II) com-
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(1 and 4) were established by single crystal X-ray diffraction study.
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