Synthesis and structural characterization of facile ruthenium(II) hydrazone complexes: Efficient catalysts in α-alkylation of ketones with primary alcohols via hydrogen auto transfer

Article abstract of DOI:10.1016/j.ica.2020.119887

As a immersion for development of new complexes, new Ru(II) complexes (1–3) supported by benzothiazole hydrazine Schiff bases of the type [Ru(SAL-HBT)(CO)(AsPh₃)₂], [Ru(VAN-HBT)(CO)(AsPh₃)₂] and [Ru(NAP-HBT)(CO)Cl(AsPh₃)₂] [SAL-HBT = (salicyl((2-(benzothiazol-2yl)hydrazono)methylphenol)), VAN-HBT = 2-((2-(benzothiazol-2-yl)hydrazono)methyl)-6 methoxyphenol) and NAP-HBT = naphtyl-2-((2-(benzothiazol-2-yl)hydrazono)methyl phenol)] were synthesized. Their identities have been established by satisfactory elemental analyses, various spectroscopic techniques (IR, (¹H, ¹³C) NMR) and also mass spectrometry. The ruthenium(II) ion exhibits a hexa coordination with distorted octahedral geometry. In complexes 1 and 2, the ligand coordinated as dianionic tridentate fashion by forming N^N donor five member and N^O donor six member chelate rings. However, in complex 3, the ligand coordinated as monoanionic bidentate fashion by forming N^N donor five-membered ring. The new ruthenium(II) carbonyl complexes were successfully applied as catalysts in α -alkylation of aliphatic and aromatic ketones with alcohols via borrowing hydrogen strategy. Various parameters such as base, solvent, temperature, time and catalyst loading on the catalytic activity were analyzed. From the results, the catalyst 1 was found to be the best catalyst for α-alkylation reaction to obtain excellent yield. The catalytic system has a broad substrate scope, which allows the synthesis of α-alkylated ketones in mild reaction conditions with low catalyst loading under air atmosphere.

Full text of DOI:10.1016/j.ica.2020.119887

InorganicaChimicaActa512(2020)119887
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Inorganica Chimica Acta
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Research paper
Synthesis and structural characterization of facile ruthenium(II) hydrazone
complexes: Efficient catalysts in α-alkylation of ketones with primary
alcohols via hydrogen auto transfer
Subbarayan Vijayaprithaa, Kaliyappan Murugana, Periasamy Viswanathamurthia,⁎
,
Paranthaman Vijayana, Chinnasamy Kalaiarasib
a Department of Chemistry, Periyar University, Salem 636 011, India
b Department of Physics, Periyar University, Salem 636 011, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
As a immersion for development of new complexes, new Ru(II) complexes (1–3) supported by benzothiazole
hydrazine Schiff bases of the type [Ru(SAL-HBT)(CO)(AsPh3)2], [Ru(VAN-HBT)(CO)(AsPh3)2] and [Ru(NAP-
HBT)(CO)Cl(AsPh3)2] [SAL-HBT = (salicyl((2-(benzothiazol-2yl)hydrazono)methylphenol)), VAN-HBT = 2-((2-
(benzothiazol-2-yl)hydrazono)methyl)-6 methoxyphenol) and NAP-HBT = naphtyl-2-((2-(benzothiazol-2-yl)
hydrazono)methyl phenol)] were synthesized. Their identities have been established by satisfactory elemental
analyses, various spectroscopic techniques (IR, (1H, 13C) NMR) and also mass spectrometry. The ruthenium(II)
ion exhibits a hexa coordination with distorted octahedral geometry. In complexes 1 and 2, the ligand co-
ordinated as dianionic tridentate fashion by forming N^N donor five member and N^O donor six member chelate
rings. However, in complex 3, the ligand coordinated as monoanionic bidentate fashion by forming N^N donor
five-membered ring. The new ruthenium(II) carbonyl complexes were successfully applied as catalysts in α
-alkylation of aliphatic and aromatic ketones with alcohols via borrowing hydrogen strategy. Various parameters
such as base, solvent, temperature, time and catalyst loading on the catalytic activity were analyzed. From the
results, the catalyst 1 was found to be the best catalyst for α-alkylation reaction to obtain excellent yield. The
catalytic system has a broad substrate scope, which allows the synthesis of α-alkylated ketones in mild reaction
conditions with low catalyst loading under air atmosphere.
Ruthenium(II) catalyst
Hydrazino benzothiazole
Crystal structure
α-alkylation
Borrowing hydrogen
1. Introduction
and biological sciences [9]. Such types of ligands were synthesized by
appropriate functionalization of precursor carbonyl compounds. The
The design of ligand can lead to pioneering metal chemistry since
the nature of the multidentate ligands has a profound effect on the
coordination chemistry of a metal complex. In this connection, many
research groups have focused their attention on Schiff base ligands due
to their rich coordination chemistry [1–3]. Particularly, Schiff base li-
gands own their main advantages like easily accessible under smooth
conditions, very much facilitative to adjust their structures and ability
to change chemo-physical properties through the incorporation of some
additional functional groups. The resulting complexes exhibit numerous
applications in catalysts [4], pharmaceuticals [5] and magnetic mate-
rials [6]. Generally special attention has been directed to the use of
multidentate Schiff base ligands including hydrazones having both hard
and soft donor atoms (e.g. N2S2 or XNS, X = N, O or S) because of their
coordination versatility [7] and remarkable applications in catalytic [8]
recent report shows that hydrazone Schiff bases designed from het-
erocyclic hydrazine framework attract considerable attention as ver-
satile ligands with respect to their variable coordination behavior [10].
Apart from the ligand, the properties of the coordination compounds,
particularly catalytic activities depend on the central metal also. Sev-
eral transition metal complexes containing Schiff bases reported as
catalysts in many organic transformations [8,11].
On the other hand, transition metal complexes catalyzed construc-
tion of carbon–carbon bond by alkylation of ketones has attracted
prominent place in organic chemistry [12]. Generally, such an alkyla-
tion can be realized by the reactions of enolates or enamines derived
from ketones with carbon electrophiles, such as alkyl halides [13,14].
However, usage of carcinogenic halogen substrates, necessary pre
functionalization procedure and the formation of stochiometric amount
⁎ Corresponding author.
E-mail address: viswanathamurthi@gmail.com (P. Viswanathamurthi).
https://doi.org/10.1016/j.ica.2020.119887
Received 18 May 2020; Received in revised form 10 July 2020; Accepted 10 July 2020
Availableonline14July2020
0020-1693/©2020ElsevierB.V.Allrightsreserved.

S. Vijayapritha, et al.
InorganicaChimicaActa512(2020)119887
of unwanted salts as waste at the end of the reaction minimizes the
utility of conventional synthesis [15]. To circumvent the above draw-
backs, development of an alternative protocol is essential to achieve the
related objective. In recent years advancement in homogeneous cata-
lysis and catalyst design has led to the development of an alternative
protocol referred as “borrowing hydrogen” and also as “hydrogen au-
totransfer” method in which environmentally benign, inexpensive and
readily available alcohols were used as alkylating partners for alkyla-
tion reactions [16]. The alkylation reactions are in accordance with
sustainable development in views of economy and environment since
such reaction release of water as the only byproduct in addition to
highly atom economical [17,18].
Infrared spectra of the compounds were obtained in the range of
4000–400 cm−1 using Bruker alpha FT-IR spectrophotometer in ATR
mode. 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded in
DMSO‑d6 or CDCl3 at room temperature with a Bruker AV400 instru-
ment using internal standard tetramethylsilane. Quadrupole time-of-
flight Micro Analyzer (Shimadzu) mass spectrometry was used to
measure electrospray ionization mass spectra (ESI) of compounds at
SAIF, Panjab University, Chandigarh. Elemental analysis (C, H, N and S)
were done on a Vario EL III elemental analyzer. The hydrazone ligands
were synthesized according to a previously published method with
slight modifications [28]. The ruthenium precursor [RuHCl(CO)
(AsPh3)3] was synthesized based on the standard procedure [29].
Following on from the pioneering work conducted by Grigg and co-
workers [19], hydrogen borrowing has been investigated extensively by
numerous research groups, and several reviews have been published
pertaining to alkylation reactions involving auto transfer hydrogenation
using alcohols [15]. A variety of ruthenium and iridium metal catalysts,
including RuHCl(CO)(PPh3)3, RuCl2(PPh3)3, [Ir(cod)Cl2]2/PPh3, Pd/C,
and RuCl2(DMSO)4, have been designed for the α-alkylation of carbonyl
compounds using alcohols and these reactions were carried out in the
presence of a base and/or hydrogen acceptor system [20]. Kaneda and
co-workers developed hydrotalcite-supported ruthenium heterogeneous
catalysts, which required prolonged heating at high temperature
(180 °C, 20–30 h) in addition to the use of alkylating alcohols as sol-
vents [21]. Ruthenium(II) half sandwich complexes developed for this
transformation exhibited poor catalytic activity and provided partial
hydrogenation, resulting in the presence of unsaturated vinyl nitrile
predominantly in the mixture of products [22]. Min Zhang and co
workers [23] employed an easily available [Ru(p-cymene)Cl2]2/Xant-
phos/t-BuOK catalyst system for α-alkylation of ketones using pyridyl
methanol as the alkylating reagents. The synthetic protocol allows
synthesizing a wide range of α-pyridyl methylated ketones in reason-
able to excellent isolated yields with high atom-efficiency. Zhu group
[24] reported NNN pincer Ru(II) complex as catalyst for α-alkylation of
ketones with alcohols including (hetero)aryl- or alkyl-ketones and al-
cohols. This protocol enables the facile synthesis of donepezil drug in
83% yield. Glorius and co workers [25] reported ruthenium(II) NHC
catalyst for α-alkylation of methylene ketones including direct one-step
synthesis of donepezil drug. Ruthenium pyridonate catalysts developed
by Chen’s group [26] show TOF values up to 3680 h−1 for α-alkylation
of ketones. Our own group has also successful in applying the ruthe-
nium carbonyl complexes [27] to couple a wide range of amines, dia-
mines and alcohols together.
2.2. Synthesis of ligands
The following common procedure were used to synthesis the li-
gands. Typically an ethanol solution (20 mL) of substituted aldehyde
(1 mmol) was added to 2-hydrazino benzothiazole (1 mmol) and the
resulting mixture was stirred at room temperature for 5 h. The com-
pletion of the reaction was checked by thin layer chromatography (TLC)
and then the resulting white precipitate was filtered, washed using
diethyl ether and air dried.
2.2.1. Synthesis of salicyl((2-(benzothiazol-2-yl)hydrazono)methyl phenol
(SAL-HBT)
The reaction of salicylaldehyde (0.122 g, 1 mmol) and 2-hydrazino
benzothiazole (0.165 g, 1 mmol) was carried out for synthesis of the
ligand SAL-HBT. Yield: 88%. M.p.: 243 °C. Anal. Calcd for C14H11ON3S:
C, 62.43; H, 4.12; N, 15.60; S, 11.91%. Found: C, 62.56; H, 4.23; N,
15.79; S, 12.03%. IR (KBr, ν cm−1): 3352 (br, νOH), 3123 (w, νNH),
1573 + 1467 (s, νC=N + νC−N), 747 (s, νC-S). 1H NMR (400 MHz,
CDCl3, ppm): 10.86 (s, 1H, OH), 8.38 (s, 1H, −CH=N), 7.59–6.89 (m,
4H, Ar H), 4.25 (s, 1H, NH). 13C NMR (100 MHz, CDCl3, ppm): 167.1
(S-C=N), 148.7 (C=N), 133.6 (Ar C), 131.3 (Ar C), 126.7 (Ar C), 124.1
(Ar C), 122.4 (Ar C), 121.8 (Ar C), 119.8 (Ar C), 116.5 (Ar C). ESI mass
(m/z) calcd. for C14H11N3OS, 269.3; Found, 270.4 [M+H]+
.
2.2.2. Synthesis
of
2-((2-(benzothiazol-2-yl)hydrazono)methyl)-6-
methoxyphenol (VAN-HBT)
The reaction of o-vanillin (0.152 g, 1 mmol) and 2-hydrazino ben-
zothiazole (0.165 g, 1 mmol) was carried out for synthesis of the ligand
VAN-HBT. Yield: 87%. M.p.: 210 °C. Anal. Calcd. for C15H13O2N3S: C,
60.18; H, 4.38; N, 14.04; S, 10.71%. Found: C, 60.27; H, 4.54; N, 14.16;
S, 10.83. IR (KBr, ν cm−1): 3574 (br, νOH), 3145 (w, νNH), 1609 + 1469
(s, νC=N + νC−N), 773 (s, νC-S). 1H NMR (400 MHz, CDCl3, ppm): 11.04
(s, 1H, OH), 8.39 (s, 1H, −CH=N), 7.51–6.84 (m, 7H, Ar H), 4.51 (s,
1H, NH), 3.93 (s, 3H, −OCH3). 13C NMR (100 MHz, CDCl3, ppm):
167.1 (S-C=N), 153.3 (Ar C), 151.0 (Ar C), 148.4 (Ar C), 146.7 (C=N),
126.3 (Ar C), 125.1 (Ar C), 124.1 (Ar C), 122.0 (Ar C), 120.8 (Ar C),
119.4 (Ar C), 117.5 (Ar C), 56.3 (Ar C); ESI mass (m/z) calcd. for
Based on the above facts and in continuing our efforts to study the
coordination behavior of complexes and exploit new catalytic system,
this work describes synthesis of new Schiff base from hydrazino ben-
zothiazole and salicylaldehyde/o-vanillin/2-hydroxy-1-naphthaldehyde
and their ruthenium(II) complexes (1–3). The chelating behavior of
ligands with ruthenium ion was extensively investigated using various
spectroscopy techniques and their solid-state structures were estab-
lished by single-crystal XRD. The catalytic properties of the new Ru(II)
complexes (1–3) have been screened in α-alkylation of ketones with
alcohols.
C
15H13N3O2S, 299.3; Found, 300.3 [M + H]+
.
2.2.3. Synthesis of naphtyl-2-((2-(benzothiazol-2-yl)hydrazono)methyl
phenol (NAP-HBT)
2. Experimental
The reaction of 2-hydroxy-1-naphthaldehyde (0.172 g, 1 mmol) and
2-hydrazino benzothiazole (0.165 g, 1 mmol) was carried out for
synthesis of the ligand NAP-HBT. Yield: 84%. M.p.: 230 °C. Anal. Calcd
for C18H13ON3S: C, 67.69; H, 4.10; N, 13.16; S, 10.04%. Found: C,
67.81; H, 4.26; N, 13.29; S, 10.19%. IR (KBr, ν cm−1): 3630 (br, νOH),
3066 (w, νNH), 1582 + 1471 (s, νC=N + νC−N), 774 (s, νC-S). 1H NMR
(400 MHz, CDCl3, ppm): 10.65 (s, 1H, OH), 8.10 (s, 1H, −CH=N),
7.96–7.22 (m, 10H, Ar H), 4.35 (s, 1H, NH). 13C NMR (100 MHz, CDCl3,
ppm): 168.4 (S-C=N), 156.2 (Ar C), 148.4 (Ar C), 142.4 (C=N), 132.6
(Ar C), 129.2 (Ar C), 128.5 (Ar C), 128.1 (Ar C), 126.0 (Ar C), 125.8 (Ar
C), 123.9 (Ar C), 122.1 (Ar C), 121.1 (Ar C), 108.6 (Ar C), 105.6 (Ar C).
2.1. General strategy
Commonly available RuCl3·3H2O was used as supplied from Sigma
Aldrich. All chemicals and solvents were acquired from Merck or
Aldrich. Thin-layer chromatography (Merck 1.05554 aluminum sheets
precoated with silica gel 60 F254) was used for reaction observance and
the spots were visualized with UV light at 254 nm or under iodine.
Column chromatography refinement was done for the complexes using
silica gel (200–400 mesh). Melting points were checked in open capil-
lary tubes on a Technico micro heating table and are uncorrected.
ESI mass (m/z) calcd. for C18H13N3OS, 319.3; Found, 320.3 [M+H]+
.
2

S. Vijayapritha, et al.
InorganicaChimicaActa512(2020)119887
2.2.4. Synthesis of new ruthenium(II) complexes
complex 3 at room temperature.
The following common method has been used to synthesis all the
complexes. The starting complex [RuHCl(CO)(AsPh3)3] (0.1 mmol) was
added to ethanol-chloroform (20 mL, 1:1 v/v) solution of corresponding
ligand (0.1 mmol) and refluxed for 24 h in air. During the course of
reaction, color of the reaction mixture was changed. The thin layer
chromatography (TLC) was used to check progress of the reaction.
When the reaction completes, the solvents were removed using a rotary
evaporator and the resulting crude product was thoroughly washed
with cold ethanol and diethyl ether then purified by recrystallization
using solvents chloroform and petroleum ether.
2.2.8. X-ray crystallographic study
To collect the data of crystals obtained for complexes 1 and 3, it
should be mounted on glass fiber and then used for further analysis
using monochromatic Mo–Kα radiation (λ = 0.71073 Å) in Bruker AXS
Kappa APEX II single crystal X-ray diffractometer at 296°(2) K. By using
SADABS software, the absorption corrections were done due to Lorentz
and polarization effects. To solve the structures of the crystal the least
square on F2 full-matrix was refined by SHELXS-2018 method [30]. The
anisotropic property having least squares on F2 in the weighted full
matrix was used to refine the non-hydrogen atoms. The computer
programming was already incorporated by the atomic scattering fac-
tors.
2.2.5. Synthesis of [Ru(SAL-HBT)(CO)(AsPh3)2] (1)
The ligand SAL-HBT (0.026 g, 0.1 mmol) was reacted with [RuHCl
(CO)(AsPh3)3] (0.105 g, 0.1 mmol) for the synthesis of complex 1.
Yield: 80%. Color: Yellow. M.p.: 185 °C. Anal. Calcd. for
2.2.9. Common procedure for the α‐alkylation of ketone reactions
Ketones (1 mmol), alcohols (1 mmol), catalyst 1 (0.5 mol %), NaOH
(5 mol %) and toluene (2 mL) were added into a round bottom flask
under air atmosphere. The reaction mixture was stirred with reflux in
an oil bath at 105 °C for 8 h. The progress of the reaction was monitored
by TLC. After completion of the reaction, the reaction mixture was
cooled to room temperature. The catalyst was separated by addition of
dichloromethane and petroleum ether solvents mixture (1:1 v/v,
10 mL) followed by filtration through celite. The filtrate was con-
centrated under reduced pressure and the resulting crude product was
purified by column chromatography over silica gel (100–200 mesh)
using eluents petroleum ether and ethyl acetate (95:5, v/v) to afford the
corresponding ketone. The resulting products were identified by com-
parison of the 1H & 13C NMR data of previous reports.
C
51H39As2O2N3RuS: C, 60.72; H, 3.90; N, 4.17; S, 3.18%. Found: C,
60.88; H, 4.06; N, 4.31; S, 3.29%. IR (ATR, cm−1): 1561 + 1459 (m,
νC=N + νC−N); 741 (s, νC-S); 1948 (s, νCO), 1H NMR (400 MHz, CDCl3,
ppm): 8.16 (s, 1H, −CH=N), 7.82 (d, J = 8 Hz, 1H, Ar H), 7.70–7.61
(m, 6H, Ar H), 7.52–7.40 (m, 9H, Ar H), 7.33–6.99 (m, 16H, Ar H),
6.88–6.79 (td, J = 4 Hz, 2H, Ar H), 6.29 (d, J = 8 Hz, 1H, Ar H), 6.12
(t, J = 4 Hz, 2H, Ar H). 13C NMR (100 MHz, CDCl3, ppm): 205.2 (CO),
164.4 (–HC=N), 158.6 (N=C-N), 147.7 (Ar C), 139.5 (Ar C), 133.8 (Ar
C), 131.4 (Ar C), 130.6 (Ar C), 128.8 (Ar C), 125.9 (Ar C), 123.2 (Ar C),
121.9 (Ar C), 118.3 (Ar C), 117.0 (Ar C), 113.9 (Ar C), 127.6–117.8 (Ar
C of AsPh3); ESI: m/z calcd. for C51H39 As2O2N3RuS, 1008.8; Found,
1010.0 [M+2H]+. Single crystals of suitable for X-ray determination
were grown by slow evaporation of chloroform-ethanol (1:1) solution of
complex 1 at room temperature.
3. Results and discussion
2.2.6. Synthesis of [Ru(VAN-HBT)(CO)(AsPh3)2] (2)
The ligand VAN-HBT (0.029 g, 0.1 mmol) was reacted with [RuHCl
(CO)(AsPh3)3] (0.105 g, 0.1 mmol) for the synthesis of complex 2.
Yield: 78%. Color: Yellow. M.p.: 179 °C. Anal. Calcd. for
3.1. Synthesis of ruthenium(II) complexes
The Schiff base ligands SAL-HBT, VAN-HBT or NAP-HBT were ob-
tained from the reaction of 2-hydrazino benzothiazole with salicy-
laldehyde, o-vanillin or 2-hydroxy-1-naphthaldehyde in ethanol. The
Schiff base ligands were reacted with precursor complex [RuHCl(CO)
(AsPh3)3] in ethanol-chloroform (20 mL, 1:1 v/v) mixture for the
synthesis of new complexes (1–3) as shown in Scheme 1. In solid state
all the complexes are air stable, non-hygroscopic and soluble in
common organic solvents such as acetone, benzene, chloroform, di-
chloromethane, dimethylsulfoxide, dimethyl formamide, ethanol, me-
thanol and insoluble in diethyl ether, hexane and petroleum ether. The
formation of the complexes have been confirmed by satisfactory ele-
mental analyses, IR, (1H, 13C) NMR and ESI-mass spectral studies.
Moreover, single crystal X- ray analysis was used to confirm the solid-
state structure of the complexes (1 and 3).
C
52H41As2O3N3RuS: C, 60.12; H, 3.98; N, 4.04; S, 3.09%. Found: C,
60.21; H, 4.03; N, 4.12; S, 3.21%. IR (ATR, cm−1): 1596 + 1458 (m,
νC=N + νC−N); 762 (s, νC-S); 1951 (s, νCO), 1H NMR (400 MHz, CDCl3,
ppm): 8.26 (s, 1H, −CH=N), 7.84 (t, J = 4 Hz, 3H, Ar H), 7.45–7.25
(m, 7H, Ar H), 7.21–6.93 (m, 21H, Ar H), 6.79 (d, J = 8 Hz, 1H, Ar H),
6.44(d, J = 4 Hz, 2H, Ar H), 6.02 (d, J = 4 Hz, 3H, Ar H), 3.86 (s, 3H,
−OCH3). 13C NMR (100 MHz, CDCl3, ppm): 205.2 (CO), 164.4
(–HC=N), 155.3 (N=C-N), 147.7 (Ar C), 143.7 (Ar C), 133.1 (Ar C),
130.7 (Ar C), 128.4 (Ar C), 125.6 (Ar C), 123.0 (Ar C), 122.0 (Ar C),
118.3 (Ar C), 116.8 (Ar C), 112.6 (Ar C), 56.1 (–OCH3), 139.8–116.8
(Ar C of AsPh3); ESI: m/z calcd. for C52H41As2O3N3RuS, 1038.8; Found,
1040.0 [M+2H]+
.
2.2.7. Synthesis of [Ru(NAP-HBT)(CO)Cl(AsPh3)2] (3)
The results of micro analyses (C, H, N and S) of the new complexes
(1–3) were in fine agreement with the expected structure of the ru-
thenium hydrazone complexes. The ESI mass spectra of ligands SAL-
HBT, VAN-HBT or NAP-HBT and complexes (1–3) (Figs. S1-S6 in the
supporting information) show the observed molecular ion peaks are in
good agreement with their expected molecular masses. Moreover, for-
mation of the intended compounds was strongly supported by the ob-
served fragmentation patterns in their mass spectra.
The ligand NAP-HBT (0.031 g, 0.1 mmol) was reacted with [RuHCl
(CO)(AsPh3)3] (0.105 g, 0.1 mmol) for the synthesis of complex 3.
Yield: 73%. Color: Yellow. M.p.: 193 °C. Anal. Calcd for
C
55H42As2O2N3RuSCl: C, 57.65; H, 4.84; N, 6.96; S, 5.31%. Found: C,
57.77; H, 4.94; N, 7.09; S, 5.47%. IR (ATR, cm−1): 3621 (br, νOH);
1587 + 1478 (m, νC=N + νC−N); 782 (s, νC-S); 1956 (s, νCO), 1H NMR
(400 MHz, CDCl3, ppm): 10.76 (s, 1H, OH), 9.13 (s, 1H, −CH=N), 7.80
(t, J = 4 Hz, 3H, Ar H), 7.46–7.26 (m, 23H, Ar H), 7.21 (d, J = 4 Hz,
4H, Ar H), 7.03 (d, J = 4 Hz, 2H, Ar H), 6.96 (t, J = 4 Hz, 6H, Ar H),
6.76 (d, J = 8 Hz, 2H, Ar H), 13C NMR (100 MHz, CDCl3, ppm): 205.5
(CO), 166.5 (–HC=N), 157.9 (N=C-N), 148.0 (Ar C), 133.9 (Ar C),
132.8 (Ar C), 130.2 (Ar C), 129.5 (Ar C), 128.2 (Ar C), 127.6 (Ar C),
126.0 (Ar C), 125.8 (Ar C), 125.6 (Ar C), 122.0 (Ar C), 120.2 (Ar C),
118.2 (Ar C), 116.6 (Ar C); 107.3 (Ar C); 142.7–116.6 (Ar C of AsPh3);
ESI: m/z calcd. for C55H42As2O2N3RuSCl, 1095.0; Found, 1096.0 [M
+H]+. Single crystals of suitable for X-ray determination were grown
by slow evaporation of chloroform-acetonitrile (1:1) solution of
3.2. IR spectra
The information about the bonding between ligands and the ru-
thenium metal in the synthesized complexes were predicted using IR
spectra. A broad band corresponding to a νOH vibration that appeared at
3349–3635 cm−1 in the spectra ligands was completely disappeared in
the spectra of complex 1 and 2, indicates the involvement of phenolic
oxygen in coordination with ruthenium metal. However, in the spec-
trum of complex 3, –OH peak was observed at 3621 cm−1 reveals the
3

S. Vijayapritha, et al.
InorganicaChimicaActa512(2020)119887
Scheme 1. Synthesis of ruthenium(II) complexes (1–3).
non participation of the phenolic oxygen atom in coordination [31].
The spectra of ligands show a peak at 3066–3145 cm−1 corresponding
to νNH. On complexation, the νNH peak disappeared indicate –NH-C=N-
↔ –N=C-NH– tautomerism followed by deprotonation at benzothia-
zole ring nitrogen and coordination with metal. The presence of strong
peak at 1573–1609 cm−1 in the spectra of ligands was assigned to
imine group. However, in the spectra of complexes, the peak moved to
lower frequency and appeared at 1561–1596 cm−1 shows the partici-
pation of azomethine nitrogen in bonding with metal [32]. Further, all
coordinated terminal carbonyl group. The peak for azomethine carbon
appears around 164.4–166.5 ppm in the spectra of complexes (1–3).
The peak due to N=C-N carbon appears at 155.3–165.1 ppm. The
methoxy (OCH3) carbon signal was observed around 56.1 ppm for
complex 2. The aromatic carbons show signals in the region
112.6–148.0 ppm [36].
3.4. Molecular structures of complexes 1 and 3
the complexes display
a medium to strong band in the region
Structures of the newly synthesized complexes 1 and 3 have been
established by single crystal X-ray diffraction method and the ORTEP
drawings are shown in Figs. 1 and 2. The details concerning the data
collection and structure refinement of the complexes are summarized in
1948–1956 cm−1 corresponding to terminally coordinated carbonyl
group which is observed at a slightly higher frequency than in the
precursor complexes [33].
3.3. NMR spectra
The formation of compounds as well as the coordination of ligands
with the ruthenium metal was established by 1H NMR spectroscopy
(Figs. S7-S12 in the supporting information). 1H NMR analyses of all the
compounds were done in deuterated solvent at room temperature. The
spectra of the free ligands show a singlet at 10.65–11.04 ppm for –OH
proton. In the spectra of complexes 1 and 2, –OH proton peak com-
pletely disappeared indicates the involvement of phenolic oxygen in
coordination with metal. On the other hand, the –OH peak not dis-
appeared in the spectrum of complex 3. The complex displays peak in
the region 10.76 ppm shows the non coordination of phenolic –OH
group with metal in complex 3 [34]. The imine proton of ligands ex-
hibits a singlet at around 8.10–8.39 ppm which is shifted to downfield
and appears at 8.16–9.13 ppm in the spectra of all complexes reveals
the coordination of imine nitrogen with ruthenium metal. The ap-
pearance of peak at 4.25–4.51 ppm in the ligands spectra was assigned
to NH proton. The disappearance of NH proton signal in the spectra of
complexes (1–3) indicates –NH-C=N- ↔ –N=C-NH– tautomerism fol-
lowed by deprotonation at benzothiazole ring nitrogen during the co-
ordination with metal. The methoxy (OCH3) protons signal of ligand
and complex 2 were observed at 3.93 and 3.86 ppm respectively.
Aromatic protons appeared in the region 6.09–7.87 ppm as multiplets
in the spectra of complexes and ligands [35].
The 13C NMR spectra of complexes 1–3 further support the forma-
tion of proposed complexes and the spectra show the expected signals
in the appropriate regions (Figs. S13-S18 in the supporting informa-
tion). The appearance of peak at 205.2–205.5 ppm reveals the
Fig. 1. Perspective view (10% probability ellipsoids) of complex 1.
4

S. Vijayapritha, et al.
InorganicaChimicaActa512(2020)119887
Table 2
Selected bond length and bond angle in complexes 1 and 3.
1
3
Inter atomic distance (Å)
Ru(1)-N(3)
Ru(1)-N(1)
Ru(1)-C(51)
Ru(1)-O(1)
Ru(1)-As(1)
Ru(1)-As(2)
As(2)-C(39)
As(2)-C(45)
As(2)-C(33)
As(1)-C(15)
As(1)-C(27)
As(1)-C(21)
S(1)-C(14)
S(1)-C(8)
O(2)-C(51)
N(3)-C(9)
N(1)-N(2)
O(1)-C(1)
N(2)-C(8)
N(1)-C(7)
2.0656(17)
Ru(1)-N(1)
Ru(1)-N(3)
Ru(1)-C(55)
Ru(1)-Cl(1)
Ru(1)-As(1)
Ru(1)-As(2)
As(2)-C(25)
As(2)-C(31)
As(2)-C(19)
As(1)-C(43)
As(1)-C(49)
As(1)-C(37)
S(1)-C(18)
S(1)-C(12)
C(55)-O(2)
N(1)-C(11)
N(1)-N(2)
2.083(3)
2.121(3)
2.148(11)
2.4180(19)
2.4724(5)
2.4849(5)
1.943(4)
1.947(4)
1.951(4)
1.941(4)
1.936(4)
1.950(4)
1.734(6)
1.738(4)
0.577(15)
1.297(5)
1.396(5)
1.339(6)
1.332(6)
1.327(6)
1.397(5)
2.0571(18)
1.863(2)
2.0705(14)
2.4633(3)
2.4800(3)
1.935(2)
1.948(2)
1.950(2)
1.937(2)
1.942(2)
1.954(2)
1.738(3)
1.759(2)
1.141(3)
1.395(3)
1.401(2)
1.309(3)
1.311(3)
1.287(3)
1.346(3)
O(1)-C(9)
N(2)-C(12)
N(3)-C(12)
N(3)-C(13)
Fig. 2. Perspective view (10% probability ellipsoids) of complex 3.
N(3)-C(8)
Table 1
Bond angles (°)
C(51)-Ru(1)-N(1)
C(51)-Ru(1)-N(3)
N(1)-Ru(1)-N(3)
C(51)-Ru(1)-O(1)
N(1)-Ru(1)-O(1)
N(3)-Ru(1)-O(1)
C(51)-Ru(1)-As(2)
N(1)-Ru(1)-As(2)
N(3)-Ru(1)-As(2)
O(1)-Ru(1)-As(2)
C(51)-Ru(1)-As(1)
N(1)-Ru(1)-As(1)
N(3)-Ru(1)-As(1)
O(1)-Ru(1)-As(1)
N(2)-N(1)-Ru(1)
As(1)-Ru(1)-As(2)
Crystal data and structural refinement parameters for complexes 1 and 3.
176.66(8)
99.27(9)
77.49(7)
92.34(8)
90.92(7)
168.36(7)
91.85(7)
88.94(5)
89.44(5)
89.27(4)
91.41(7)
87.85(5)
90.98(5)
89.64(4)
116.99(14)
176.597(11)
C(55)-Ru(1)-N(3)
C(55)-Ru(1)-N(1)
N(3)-Ru(1)-N(1)
C(55)-Ru(1)-Cl(1)
N(3)-Ru(1)-Cl(1)
N(1)-Ru(1)-Cl(1)
C(55)-Ru(1)-As(2)
N(3)-Ru(1)-As(2)
N(1)-Ru(1)-As(2)
Cl(1)-Ru(1)-As(2)
C(55)-Ru(1)-As(1)
N(3)-Ru(1)-As(1)
N(1)-Ru(1)-As(1)
Cl(1)-Ru(1)-As(1)
N(2)-N(1)-Ru(1)
As(2)-Ru(1)-As(1)
170.2(3)
93.3(3)
77.17(13)
93.0(3)
96.53(11)
173.70(10)
88.4(3)
88.92(10)
87.89(10)
92.03(4)
91.9(3)
90.44(10)
90.07(10)
89.98(4)
115.4(2)
177.951(19)
1
3
CCDC Number
Identification code
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
1999571
shelx
C
1008.82
296(2)
0.71073
Monoclinic
P21/n
1995980
shelx
C55H42As2ClN3O2RuS
1095.34
296(2)
0.71073
51H39As2N3O2RuS
Monoclinic
P21/n
11.8308(2)
25.8399(5)
15.0943(3)
90
108.415(10)
90
12.2867(4)
20.5946(7)
18.8494(6)
90
91.844(2)
90
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
4378.14(14)
4
4767.2(3)
4
Table 1. Selected bond distances and bond angles with geometrical
parameters that are essential for discussion are depicted in Table 2. The
single-crystal X-ray studies revealed that complex 1 is crystallized in a
monoclinic system with the space group P21/n whereas complex 3
crystallized in the monoclinic system belongs to the P21/n space group
with 4 independent molecules within the unit cell. In complex 1, the
hydrazone ligand coordinated to ruthenium in an ONN fashion by
utilizing its phenolic oxygen, benzothiazole ring nitrogen and imine
nitrogen with the formation of one six-membered ring and another five
membered ring with a N(1)-Ru(1)-N(3) bite angle of 77.49(7)° (Fig. 1).
However, in comparison with complex 3, the angular distortion ob-
served in 1 is significantly less, which may be due to the strong ONN
chelation of the ligand. The Ru(1)-N(1) and Ru(1)-N(3) bond distances
were found to be 2.0571(18) Å and 2.0656(17) Å, respectively. The
other three sites are occupied by triphenylarsine with Ru(1)-As(1) and
Ru(1)-As(2) distances of 2.4633(3) and 2.4800(3) Å, respectively, and
one carbonyl group with a Ru(1)-C(51) distance of 1.863(2) Å. These
values are comparable with those found for complexes reported pre-
viously [37]. N(1)-Ru(1)-As(2) 88.94(5)°, N(3)-Ru(1)-As(2) 89.44(5)°,
O(1)-Ru(1)-As(2) 89.27(4)°, N(1)-Ru(1)-As(1) 87.85(5)°, O(1)-Ru(1)-As
(1) 89.64(4)° cis angles are more acute than 90°, whereas the other cis
angles, C(51)-Ru(1)-N(3) 99.27(9)°, C(51)-Ru(1)-O(1) 92.34(8)°, N(1)-
Ru(1)-O(1) 90.92(7)°, C(51)-Ru(1)-As(2) 91.85(7)°, C(51)-Ru(1)-As(1)
91.41(7)°, N(3)-Ru(1)-As(1) 90.98(5)° are more obtuse than 90°. The
trans angles O(1)-Ru(1)- N(3) 168.36(7)°, C(51)-Ru(1)-N(1) 176.66(8)°
and As(1)-Ru-As(2) 176.597(11)° show some degree of deviation from
the ideal geometry. The deviation in bond angles and the variations in
Density (calculated) Mg/
1.531
1.526
m3
Absorption coefficient
1.950
1.852
(mm−1
)
F(000)
2032
2208
Crystal size (mm3)
Theta range for data
collection
0.22 × 0.20 × 0.18
2.952 to 28.290°.
0.21 × 0.11 × 0.06
1.054 to 29.077°
Index ranges
−15≤h≤15,
−33≤k≤33,
−18≤l≤18
24,188
10,877
[R(int) = 0.0197]
98.7%
−16≤h≤16,
−28≤k≤28,
−25≤l≤25
80,783
12,776 [R(int) = 0.0391]
Reflections collected
Independent reflections
Completeness to
99.9%
theta = 25.242°
Absorption correction
Refinement method
None
None
Full-matrix least-squares
Full-matrix least squares
on F2
on F2
Data/restraints/
parameters
10,747/0/541
12,702/0/586
Goodness-of-fit on F2
Final R indices
0.846
0.824
R1 = 0.0292,
wR2 = 0.0704
R1 = 0.0446,
wR2 = 0.0865
0.553 and −0.444
R1 = 0.0446,
wR2 = 0.1191
R1 = 0.0748,
wR2 = 0.1615
1.268 and −1.295
[I > 2sigma(I)]
R indices (all data)
Largest diff. peak and
hole e.Å−3
5

S. Vijayapritha, et al.
InorganicaChimicaActa512(2020)119887
bond lengths indicate distortion in the octahedral symmetry of the
complexes.
Table 3
Optimization of catalytic conditions for the α-alkylation of acetophenone using
benzyl alcohol.a
In complex 3, the ruthenium(II) ion exhibits a hexa coordination
with an distorted octahedral geometry where equatorial coordination
comes from benzothiazole ring nitrogen and imine nitrogen of bi-
dentate chelating ligand, a chloride and a carbonyl carbon. A pair of
triphenylarsines completes the axial coordination (Fig. 2). The hy-
drazone ligand coordinated to ruthenium and form a five membered
ring with a N(1)–Ru(1)–N(3) bite angle of 77.17(13)°. The Ru(1)–N(1)
and Ru(1)–N(3) bond distances are 2.083(3) Å and 2.121(3) Å respec-
tively. The other four sites are occupied by two triphenylarsine ligands,
which are mutually trans to each other with Ru(1)–As(1) and Ru(1)–As
(2) distances of 2.4724(5) and 2.4849(5) Å, and one chloride and a
carbonyl group with Ru(1)–Cl(1) and Ru(1)–C(55) distances of
2.4180(19) and 2.148(11) Å, respectively. The N(1)–Ru(1)–As(2)
87.89(10)°, N(3)-Ru(1)-As(2) 88.92(10)°, C(55)-Ru(1)-As(2) 88.4(3)°,
Cl(1)-Ru(1)-As(1) 89.98(4)° cis angles are more acute than 90°, whereas
the other cis angles, C(55)-Ru(1)-N(1) 93.3(3)°, C(55)-Ru(1)-Cl(1)
93.0(3)°, N(3)-Ru(1)-Cl(1) 96.53(11)°, Cl(1)-Ru(1)-As(2) 92.03(4)°,
C(55)-Ru(1)-As(1) 91.9(3)°, N(3)-Ru(1)-As(1) 90.44(10)°, N(1)-Ru(1)-
As(1) 90.07(10)°, N(2)-N(1)-Ru(1) 115.4(2)° are more obtuse than 90°.
The C(55)-Ru(1)-N(3) 170.2(3)°, N(1)-Ru(1)-Cl(1) 173.70(10)°, As(2)-
Ru(1)-As(1) 177.951(19)° trans angles significantly deviate from the
linearity [38–40]. Generally the AsPh3 ligands usually prefer to occupy
mutually cis position for better π-interaction, but in the reported
complexes the presence of CO, a stronger π -acidic ligand, might have
forced the bulky AsPh3 ligands to take trans position for steric reasons
[41,42].
O
O
Ru(II) Catalyst
OH
H2O
Base, Solvent
Reflux
Entry
Base
Solvent
Toluene
T (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
NaHCO3
K2CO3
Na2CO3
Cs2CO3
KOtBu
KOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
__
105
105
105
105
105
105
105
105
90
24
24
24
24
24
24
24
8
8
8
8
8
8
8
8
24
24
24
37
42
50
74
65
78
95
93
68
85
80
72
65
19
25
NR
NR
NR
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Benzene
CH3CN
Dioxane
THF
9
10
11
12
13
14
15
16
17c
18d
100
85
80
70
DMSO
DMF
Toluene
Toluene
Toluene
130
120
RT
105
105
NaOH
a
Acetophenone (1 mmol), benzyl alcohol (1 mmol), base (5 mol %), catalyst
1 (0.5 mol%), refluxed in solvent (2 mL).
b
Isolated yield after column chromatography.
c
3.5. Catalytic studies
The reaction was carried out without base. No reaction.
The reaction was carried out without catalyst. No reaction.
d
3.5.1. Optimization of reaction conditions
Recently, the core moiety of the pharmaceutically active inter-
mediate was successively synthesized through hydrogen auto-transfer
process via alkylation using homogeneous catalysts [24,43]. Hence, we
are interested to assess the catalytic efficiency of Ru(II) hydrazone
complexes in the synthesis of pharmaceutical compounds with C–C
bond by cross coupling of ketones and alcohols known as α-alkylation
reactions. To optimize the best catalytic reaction conditions, we con-
sider the formation of benzylated acetophenone as a benchmark sub-
strate from the reaction of acetophenone with benzyl alcohol in the
presence of Ru(II) hydrazone catalyst 1 (0.5 mol%). The effects of the
base, temperature, solvent and time were investigated. The results of
the preliminary reactions were shown in Table 3. The base plays a key
role in this reaction. To determine the role of base, the alkylation re-
action was carried out with different bases like NaHCO3, Na2CO3,
K2CO3, KOtBu, Cs2CO3, KOH and NaOH. The results reveal weak bases
such as NaHCO3, K2CO3, Na2CO3, were not effective to accelerate the
reaction (Table 3, entries 1, 2 and 3). The bases KOtBu and Cs2CO3 led
to moderate yields (Table 3, entries 4 and 5). Especially, strong base
like KOH and NaOH have shown good to excellent yields (Table 3,
entries 6 and 7). Among the bases enlisted, NaOH (5 mol %) was dis-
covered to be the best base and led to excellent yield in shorter reaction
time for this reaction. (Table 3, entry 7, yield 95%).
It was well known that the solvent can have authentic effect on α-
alkylation reaction. Accordingly, the solvent-dependent differences in
activity of catalyst 1 were studied on the model reaction with fre-
quently used solvents such as toluene, benzene, acetonitrile, 1,4 di-
oxane, tetrahydrofuran, DMF and DMSO (Table 3, entries 1–18).
Among them, polar aprotic solvents like DMSO and DMF invariably
give poor yields (Table 3, entries 14 and 15). Ether like solvents such as
dioxane, THF afford better yield than polar aprotic solvents (Table 3,
entries 12 and 13). Acetonitrile and benzene have shown moderate
yields of 80 and 85% respectively in the alkylation reaction (Table 3,
Entries 10 and 11). However, toluene was found to be the solvent of
choice, bestow excellent yield of 93% in 8 h (Table 3, Entry 8). In
Table 4
Selection of catalyst and influence of catalyst loading on α-alkylation of acet-
ophenone with benzyl alcohol.a
O
O
Ru(II) Catalyst
OH
H2O
NaOH (5 mol%)
Toluene, Reflux
Entry
Catalyst
Amount of catalyst (Mol %)
Yieldb (%)
1
2
3
4
5
6
7
1
2
3
1
1
1
1
0.5
0.5
0.5
0.1
0.2
0.3
1
93
82
88
47
65
74
95
a
Acetophenone (1 mmol), benzyl alcohol (1 mmol), catalyst, NaOH (5 mol
%) in toluene (2 mL) refluxed at 105 °C for 8 h.
b
Isolated yield after column chromatography.
addition, when the reactions were conducted in the absence of base, no
product was formed (Table 3, entry 17). As expected, no considerable
conversion was observed without catalyst under similar reaction con-
ditions after a prolonged reaction time up to 24 h (Table 3, entry 18).
Also, decreasing the reaction temperature from 105 to 90 °C led to
decrease in yield from 93 to 68% (Table 3, entries 8 and 9). Therefore,
105 °C was considered to be suitable temperature which proved that
our catalytic system could achieve the reaction well relatively lower
temperature compared to reported work (140 °C) [44]. When the re-
action was monitored in shorter time, there is no significant change in
yield of the product was observed (Table 3, entry 8). Therefore, the
reaction time of 8 h was considered as optimal reaction time to
6

S. Vijayapritha, et al.
InorganicaChimicaActa512(2020)119887
Table 5
Scope of the reaction in α-alkylation of ketones.a
O
O
Ru(II) Catalyst 1
OH
H2O
R
NaOH (5 mol%)
Toluene, Reflux, 8h
R1
R1
R
Entry
Ketone
Alcohol
Product
Yieldb
1
93
O
O
O
O
O
O
OH
1a
1b
2
3
90
91
OH
OH
H3CO
OCH3
1c
1d
4
5
87
79
O
O
OH
O
O
OH
OH
O
OH
O
1e
1f
6
74
76
70
92
91
OH
OH
O
OH
O
7
OH
H3CO
OH
O
OCH3
OH
O
1g
1h
8
OH
OH
OH
9
O
O
OH
H3CO
H3CO
OH
O
H3CO
OH
O
1i
10
OH
OH
O
H3CO
OH
1j
11
12
94
88
O
OH
H3CO
H3CO
OH
OCH3
H3CO
H3CO
OH
O
1k
OH
O
OH
H3CO
OH
1l
(continued on next page)
7

S. Vijayapritha, et al.
InorganicaChimicaActa512(2020)119887
Table 5 (continued)
Entry
Ketone
Alcohol
Product
Yieldb
13
86
O
O
OH
H3CO
H3CO
OH
O
OH
O
1m
14
82
84
OH
H3CO
H3CO
H3CO
OH
O
OH
1n
15
16
O
OH
H3CO
H3CO
OH
OCH3
OH
O
1o
80
OH
O
H3CO
H3CO
OH
OH
1p
1q
17
18
81
87
O
O
OH
O
O
OH
1r
1s
1t
19
20
83
80
O
O
O
OH
O
OH
a
Ketone (1 mmol), alcohol (1 mmol), catalyst 1 (0.5 mol %) and NaOH (5 mol %) in toluene (2 mL) refluxed at 105 °C for 8 h.
Isolated yield after column chromatography.
b
complete the alkylation reaction.
the α-alkylated products in higher yields (Table 5, entries 1–3). How-
ever, marginal decrease in yield was observed for the reaction of aro-
matic acetophenone with electron-rich 1-phenylethanol (Table 5, entry
4). The steric effect due to the presence of hydroxy group in the ortho
position of acetophenone affects the product yield and hence the yields
are moderate (Table 5, entries 5–8; Yield 70–79%). The reactions of 4-
methoxy-2-hydroxy acetophenone with alcohols such as benzyl alcohol,
4-methyl benzyl alcohol, 4- methoxy benzyl alcohol and 1 phenyl
ethanol offer corresponding pharmaceutically important C–C coupling
products in good to excellent yields (Table 5, entries 9–12; Yield
88–94%). Encouraged by the result, coupling of 5-methoxy-2-hydroxy
acetophenone with various alcohols containing electron donating
groups were done. The reactions offer the desired products in good
yields, however, the yield are lesser than the 4-methoxy-2-hydroxy
acetophenone coupling products (Table 5, entries 13–16). The aliphatic
cyclic ketones, such as cyclohexanone and cyclopentanone were suc-
cessfully transformed into the corresponding α-alkylated ketones in
good yields (Table 5, entries 17–20). The results show that catalyst 1
was effective in the alkylation of not only aromatic but also aliphatic
ketones (Table 5, entries 1–20).
The screening reaction was then extended to selection of catalyst
and catalytic loading in the alkylation reaction and the results were
collated in Table 4. To study the effect of amount of the catalyst, the
reactions were carried out at different amount of catalyst ranging from
0.1 to 1.0 mol %. The catalyst loading increased from 0.1 to 0.5 mol%
of catalyst 1, the product yield also increased and reaches 93% in 8 h.
Doubling the loading of catalyst 1 (1 mol%) led to little difference in
yield (Table 4, entry 6). Hence, the optimum catalyst loading was fixed
to be 0.5 mol % (Table 4, entry 1). Additionally, catalyst 1 was found to
be the best catalyst for α-alkylation reaction to obtain excellent yield
(Table 4, entry 1). Other catalysts such as 2 and 3 show lower activity
than the catalyst 1 (Table 4, entry 1 and 2). From the optimization
results, it is concluded that 0.5 mol % of catalyst 1 in toluene was
sufficient to push this reaction at 105⁰C in presence of NaOH.
3.5.2. Substrate scope
With the optimized reaction conditions in hand, various primary
alcohols were reacted with different types of ketones to examine the
scope of the reaction. All reactions resulted in complete conversion of
starting materials to afford selective formation of the corresponding
ketone (Table 5). Initially, investigation was focused on the α-alkyla-
tion of aromatic acetophenone with alcohols like benzyl alcohol, 4-
methyl benzyl alcohol, 4-methoxy benzyl alcohol and the reactions give
On the basis of the above experimental observations, the complex 1
was proved as effective catalyst for α-alkylation of variety of ketones
with primary alcohols under air via environmentally friendly borrowing
hydrogen pathway. A plausible mechanism for α-alkylation of ketones
8

S. Vijayapritha, et al.
InorganicaChimicaActa512(2020)119887
Scheme 2. Proposed plausible mechanism for α-alkylation of ketones with alcohols.
with alcohols catalyzed by complex 1 is shown in Scheme 2. Initially,
the reaction between Ru catalyst and alcohol in the presence of base
generates Ru-alkoxide species I. Subsequently, β-hydrogen elimination
of alkoxo ruthenium species I gives rise to Ru-H species II and the al-
dehyde intermediate. Although we were unable to isolate R-H species II,
the generation of R-H complexes was well documented [45,46] and has
been verified by the Yu group [47] as the active catalyst for transfer
hydrogenation. Next, the condensation reaction between intermediate
aldehyde and ketone in the presence of NaOH results in α,β-unsaturated
ketone. Finally, the coordination and addition of Ru-H species II into
the double bond of α,β-unsaturated ketone affords Ru species III, which
combines with the alcohol to release the ketone product and regenerate
Ru-alkoxide species I.
effective formation of products, Ru NHC catalyst necessitates [25]
2 mol % catalyst loading and 24 h reaction time at 100 °C. Similarly
[RuCl2(η6-p-cymene)]2 and P,N ligand catalyst [50] was shown efficient
catalytic activity at 120 °C in 18 h. Moreover, NNN pincer Ru(II)
complexes and ruthenium pyridonate complexes [26,51] were effective
only under inert atmosphere. However, using the present catalyst, the
α-alkylation reactions were carried out at relatively low temperature
(105 °C) in shorter time (8 h) using minimal catalyst load (0.5 mol %)
under air. The results offer new possibilities for developing versatile
catalysts for α-alkylation of ketones.
4. Conclusions
The results of present catalyst were compared with the recent cat-
alysts explored for α-alkylation to reveal the better performance. The
previously reported Mn catalyzed [48,49] α-alkylation reactions re-
quired high catalyst loading (1–2 mol %), high temperature (125-
140 °C) and longer time (18–24 h) for completion of reactions. For
A new series of facile Ru(II) complexes (1–3) were synthesized by
the reaction of [RuHCl(CO)(AsPh3)3] with hydrazone ligands (SAL-
HBT, VAN-HBT or NAP-HBT). The formation of the complexes was
established by analytical and spectral (IR, NMR, and ESI-MS) methods.
The solid-state structures reveal the ruthenium(II) ion exhibits a hexa
9

S. Vijayapritha, et al.
InorganicaChimicaActa512(2020)119887
coordination with distorted octahedral geometry. In complex 1 and 2,
the hydrazone ligand coordinated to ruthenium in a dianionic tri-
dentate ONN fashion by utilizing its phenolic oxygen, benzothiazole
ring nitrogen and imine nitrogen with the formation of one six-mem-
bered ring and another five membered ring. However, in complex 3, the
ligand coordinated in a monoanionic bidentate NN fashion by forming a
five-membered ring. The catalytic effectiveness of the titled complexes
was well explored for α-alkylation of a wide range of ketones with
primary alcohols via hydrogen borrowing technique with excellent
yields. Catalytic reactions were carried out at relatively low tempera-
ture in short time with minimal catalyst load (0.5 mol %) under air
atmosphere. The results offer new possibilities for developing versatile
catalysts for α-alkylation reaction of ketones using alcohols.
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Declaration of Competing Interest
[13] (a) G. Casiraghi, L. Battistini, C. Curti, G. Rassu, F. Zanardi, Chem. Rev. 111 (2011)
3076;
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
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Acknowledgements
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We thank SAIF, Punjab University, Chandigarh, Department of
Chemistry, Gandhigram Rural Institiue, Gandhigram (spectral studies)
and CIMF, Periyar University, Salem (single crystal X-Ray) for their
help in characterization studies.
Appendix A. Supplementary data
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M. Yus, Mol. Diversity 14 (2010) 411;
CCDC 1999571 and 1995980 contain the supplementary crystal-
lographic data for complexes 1 and 3. This data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or email: deposit@
ccdc.cam.ac.uk. Supplementary data to this article can be found online
at https://doi.org/10.1016/j.ica.2020.119887.
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