Half-sandwich Ru (II) complexes containing (N, O) Schiff base ligands: Catalysts for base-free transfer hydrogenation of ketones

Article abstract of DOI:10.1002/aoc.5111

Two new half-sandwich Ru (II)(p-cymene) complexes (1 and 2) containing dopamine-based (N, O) Schiff base ligands (L¹H and L²H) were synthesized and characterized by FT-IR, UV–Visible and ¹H & ¹³C NMR spectral techniques, and elemental analyses. The spectroscopic and analytical data revealed monobasic bidentate coordination of the ligands with Ru ion. The molecular structures of L¹H, L²H and 2 were further confirmed by single crystal X-ray diffraction study. Complexes 1 and 2?have been employed as catalysts in the transfer hydrogenation of ketones using 2-propanol as a hydrogen source at 85?°C under base-free condition. Good to the excellent yield of secondary alcohols, gram scale synthesis, and high TON and TOF made this catalytic system interesting.

Full text of DOI:10.1002/aoc.5111

Received: 6 May 2019
DOI: 10.1002/aoc.5111
Revised: 19 June 2019
Accepted: 21 June 2019
F U L L P A P E R
Half‐sandwich Ru (II) complexes containing (N, O) Schiff
base ligands: Catalysts for base‐free transfer hydrogenation
of ketones
C.E. Satheesh1 | Pushpanathan N. Sathish Kumar2 | P. Raghavendra Kumara1
Ramasamy Karvembu2 | Amar Hosamani3 | M. Nethaji4
|
1 Department of Chemistry, UCS, Tumkur
Two new half‐sandwich Ru (II)(p‐cymene) complexes (1 and 2) containing
dopamine‐based (N, O) Schiff base ligands (L1H and L2H) were synthesized
University, Tumakuru 572103, Karnataka,
India
2 Department of Chemistry, National
Institute of Technology Tiruchirappalli,
Tamilanadu, India
3 Solid State and Structural Chemistry
Unit, Indian Institute of Science,
Bangalore 560012, Karnataka, India
4 Department of Inorganic and Physical
Chemistry, Indian Institute of Science,
Bangalore 560012, Karnataka, India
1
and characterized by FT‐IR, UV–Visible and H & 13C NMR spectral tech-
niques, and elemental analyses. The spectroscopic and analytical data revealed
monobasic bidentate coordination of the ligands with Ru ion. The molecular
structures of L1H, L2H and 2 were further confirmed by single crystal X‐ray
diffraction study. Complexes 1 and 2 have been employed as catalysts in the
transfer hydrogenation of ketones using 2‐propanol as a hydrogen source at
85 °C under base‐free condition. Good to the excellent yield of secondary alco-
hols, gram scale synthesis, and high TON and TOF made this catalytic system
interesting.
Correspondence
P. Raghavendra Kumar, Assistant
Professor in Chemistry, Department of PG
Studies and Research in Chemistry,
University College of Science, Tumkur
University, B H Road, Tumakuru 572103,
Karnataka, IN
KEYWORDS
base free, NO ligands, ruthenium, Schiff base, transfer hydrogenation
Email: raghukp1@gmail.com
Funding information
Council for Scientific and Industrial
Research, New Delhi, India, Grant/Award
Number: 01(2701)/12/EMR‐II
1 | INTRODUCTION
friendly, sustainable and phosphine‐free metal complexes
as hydrogenation catalysts has been one of the upfront
Catalytic hydrogenation is a key step in many organic
transformations. The significance of hydrogenation is
due to the potential applications of hydrogenated prod-
ucts in vast areas. The most common reducing agents like
LiAlH4, NaBH4, N2H4, and H2 gas in presence of metals
or metal complexes of phosphine‐based ligands are effi-
cient, but most of them are highly specific, expensive,
toxic, non‐recyclable, air and moisture sensitive, hence
not efficient at industrial level applications. Because of
these practical, environmental and societal demerits, the
development of highly efficient, cost‐effective, eco‐
challenges of catalysis research.[1] Transfer hydrogena-
tion (TH) reactions catalyzed by transition metal com-
plexes are one of the environmentally benign
procedures to replace conventional hydrogenation reac-
tions. Transition metal complexes, particularly of noble
metals have been explored,[2] among which Ru com-
plexes[3] are efficient TH catalysts and more economical
than Pd, Pt, Rh, and Ir catalysts. Mainly, Ru (II)‐arene
complexes are known to be a well‐versed platform for
catalysing TH reactions as well as bioinorganic applica-
tions due to their inherent properties. These types of
Appl Organometal Chem. 2019;e5111.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5111

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SATHEESH ET AL.
catalysts have been explored quite well for the reduction
of various organic functional groups such as carbonyl,
nitrile, nitro, alkene, and an alkyne. The inert arene moi-
ety stabilizes the oxidation state of Ru in the catalytic
cycle. Ligand(s) also have a major role in the transition
metal catalyzed TH reactions especially in tuning the
reactivity and selectivity. In the recent past, Schiff base
ligands have grown into a special class of ligands due to
the few simple and green synthesis using inexpensive
and readily available substrates and exhibit excellent
coordination chemistry.[4,5] Schiff bases prepared from
o‐hydroxy aldehydes/ketones are stabilized by intramo-
lecular hydrogen bonding and many of them are crystal-
line solids, which favour monobasic bidentate (N, O)
coordination mode to form stable complexes with most
of the transition metals.[6] Combination of Schiff base
ligands (LH) and Ru (II)(p‐cymene) precursors led to
the formation of half‐sandwich, piano‐stool complexes
of the type [Ru (II)(p‐cymene)(L)Cl]. These special fea-
tures of [Ru (II)(η6‐p‐cymene)(L)Cl] complexes made
them excel as catalysts in TH reactions.[6,7]
Most of the Ru (II)‐arene catalysts used for TH reac-
tions were active only in the basic medium.[8] But the lim-
itations in using base along with the catalyst are a)
corrosion of reaction vessels in the industries, b) base sen-
sitive ketones cannot be used and c) stereoselectivity is
also affected. Only very few reports are available on base‐
free transition metal catalyzed TH of ketones.3c, 9 To over-
come the limitations caused by a base, the current interest
lies in designing a catalyst for TH under base‐free condi-
tions. Inspired by the potential applications of Schiff base
ligands and their Ru complexes in catalysis, we report
new Ru (II)(p‐cymene) complexes (1 and 2) of two simple
(N, O) Schiff base ligands (L1–2H) as excellent catalysts for
TH of various ketones. The ligands were derived from
2‐(3,4‐dimethoxyphenyl)ethanamine, a dimethyl deriva-
tive of dopamine (DA).
2 | RESULTS AND DISCUSSION
2.1 | Synthesis of ligands (L1–2H) and their
complexes (1 and 2)
The Schiff base ligands, L1H and L2H were synthesized
in high yields (95%) as per the reported method.[10]The
new Ru (II) complexes (1 and 2) were obtained from
direct reaction of the ligands with [Ru(p‐cymene)Cl2]2
in a 2:1 molar ratio in CH2Cl2 at room temperature. On
completion of the reaction, the solvent was removed
and orange‐red solids of the desired complexes (1 and 2)
were obtained in high yields (80–85%) (Scheme 1).
2.2 | Characterization
Composition of the Ru (II) complexes (1 and 2) deter-
mined by elemental analysis (C, H, and N) was in agree-
ment with their expected molecular formulae [Ru(p‐
cymene)Cl(L1–2)]. Structure of 1 and 2 was confirmed
1
by various methods such as UV–Visible, FT‐IR, H and
13C{1H} NMR spectroscopy and mass spectrometry. The
molecular structures of L1H, L2H and 2 were further con-
firmed by single crystal X‐ray diffraction.
2.3 | UV–visible spectroscopy
The electronic spectra of complexes 1 and 2 have been
recorded in CH3OH at 2 × 10−5 M concentration. In the
electronic spectra of complexes 1 and 2, the intense
high‐energy absorption bands observed around λmax, 204
and 230 nm were attributed to intra‐ligand π → π* tran-
sitions which were blue shifted of about 40 nm when
compared to the free ligands.[10] The bands observed
around λmax, 286 nm were also due to intra‐ligand
n → π* transitions. The bands obtained around λmax
,
SCHEME 1 Synthesis of the ligands
(L1H and L2H) and their Ru (II)
complexes (1 and 2)

SATHEESH ET AL.
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390 nm were due to the ligand to metal charge transfer
(LMCT) transitions.[11] A band at λmax, 487 nm due to
metal to ligand charge transfer (MLCT) transition has
appeared only when the spectra were recorded at higher
concentration (10−3 M).
1–2 ppm when compared to those carbons in the
ligands.[10] The NCH2 carbon peak appeared at 71 and
65 ppm in the spectra of complexes 1 and 2 respectively,
which showed a downfield shift of 10–11 ppm as these
carbons appeared at 60 and 55 ppm respectively in the
spectra of ligands L1H and L2H. The peaks for p‐cymene
carbons in 1 and 2 appeared in the range 81–85 ppm,
indicating that the arene ligand coordinated to the Ru
ion. The other aliphatic and aromatic carbons showed
chemical shifts at characteristic δ values. The observa-
tions from the IR and proton NMR spectra indicated that
the ligands coordinate with Ru by monobasic (NO)
donors.
2.4 | FT‐IR spectroscopy
A strong band observed for O‐H stretching at 3418 (L1H)
and 3392 (L2H) cm−1 in the spectra of ligands[10] disap-
peared in the spectra of complexes 1 and 2. Similarly,
the C=N stretching frequency of the ligands (1634/
1607 cm−1) decreased (1610/1595 cm−1) on complexation.
The phenolic C‐O stretching frequency of the ligands was
shifted to a lower wave number upon coordination.[10]
These changes indicated the monobasic bidentate coordi-
nation of the ligands through (N, O) donors with ruthe-
nium. The bands for C=C, C‐N, etc. were observed in
the characteristic region of the IR spectra of 1 and 2.
2.6 | Single crystal X‐ray diffraction
The single crystals of L1H, L2H and 2 were obtained by
recrystallization of the respective compounds in a suit-
able solvent system and their structures were determined
by single crystal X‐ray diffraction. A suitable crystal of
each compound was selected, then mounted on X‐ray dif-
fractometer using MoKα radiation (λ = 0.71073) and col-
lected the data. The crystal data and structure refinement
parameters of L1H, L2H and 2 are given in Table S1. The
ligands L1H and L2H crystallized in monoclinic (P21/c)
and orthorhombic (P212121) crystal systems respectively.
The molecular structures of ligands L1H and L2H are
shown in Figures 1 and 2 respectively. The C=N
(1.2756(16) and 1.294 (3) Å), C‐N (1.4603(15) and 1.466
(2) Å) and ArC‐OH (1.3521(15) and 1.349 (2) Å) bond
lengths in ligands L1H and L2H respectively were found
in agreement with those reported for similar Schiff
bases.[14] The selected bond lengths and bond angles are
given in Table S2. There exist O‐H⋯N intramolecular
hydrogen bonding in L1H (1.843 Å) and L2H (1.768 Å)
as observed generally for Schiff bases derived from 2‐
hydroxy aldehydes or ketones.[12,14] There exist strong
CH⋯O secondary interactions in the solids of ligands
L1H (2.55, 2.56 and 2.59 Å) and L2H (2.59 and 2.47 Å)
(Figure S10 and Figure S11).
Ru complex 2 crystallized in the orthorhombic crystal
system with the space group Pca21. The molecular struc-
ture of 2 is given in Figure 3. Both C(9)‐N (1) (1.559(17)
Å) and O(1)‐C (1) (1.508(19) Å) bonds in 2 were found
lengthened when compared to these bonds in ligand
L2H. The Ru‐O, Ru‐N and Ru‐Cl bond lengths were
2.019 (12), 2.103 (13) and 2.441 (4) Å respectively. These
Ru‐atom (ligand) bond lengths were in agreement with
the values reported in the literature for similar Ru‐Schiff
base complexes.7b The complex 2 has pseudo‐octahedral
geometry and attained piano stool also known as “half‐
sandwich” structure. The complex contained CH⋯O
2.5 | NMR spectroscopy
In the proton NMR spectra of the complexes, the methyl
protons of isopropyl (iPr) group of p‐cymene appeared as
two independent doublets in the range δ, 1.03–1.26 ppm.
The p‐cymene protons in both 1 and 2 were appeared as
four doublets in the range 5.172–5.767 ppm, indicating
the presence of p‐cymene ligand in the complexes.[12]
The signal for phenolic OH proton observed in the spec-
tra of ligands disappeared in the spectra of complexes 1
and 2. The imine (CH=N) proton in the complexes
appeared at 7.77 ppm which was shielded by about
0.5 ppm when compared to the corresponding proton in
the ligands. The NCH2 protons were appeared as two sig-
nals due to their diastereotopic nature in the range 4.47–
4.62 and 4.126–4.261 ppm; these protons showed a down-
field shift of about 0.5–0.7 ppm when compared to these
signals in the respective ligands. But there was no signif-
icant change in the chemical shift of ArCH2 protons in
both the complexes compared to the free ligands. The
methoxy protons in the ligands appeared as two singlets
at 3.79 and 3.74 ppm as reported earlier,[13] which were
slightly shielded on complex formation. These observa-
tions indicated that the ligands L1H and L2H coordinated
to the Ru center in 1 and 2 respectively in deprotonated
(L1 and L2) monoanionic bidentate (N, O) fashion.
In the carbon NMR spectra, the PhC‐O− carbons
appeared at 164.33 and 165.01 ppm in complexes 1 and
2 respectively. These carbons showed the down‐field shift
of about 4–5 ppm whereas the signals for C=N carbons
observed at 168.68 and 171.33 ppm respectively in 1 and
2 but these carbons showed up‐field shift only of about

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SATHEESH ET AL.
FIGURE 1 ORTEP diagram of L1H
with 50% probability ellipsoids
FIGURE 2 ORTEP diagram of L2H
with 50% probability ellipsoids
FIGURE 3 ORTEP diagram of [Ru(p‐
cymene)(L2)Cl].2H2O (2) with 30%
probability ellipsoids (H atoms areomitted
for clarity).
[2.663 (2) Å] secondary interactions resulting in the
supramolecular structure as shown in Figure S12.
catalytic efficiency towards the TH of ketones into sec-
ondary alcohols. The results of optimization studies are
tabulated in Table S4. Acetophenone was employed as a
substrate for the optimization studies. Interestingly, with
this catalytic system, the TH reaction was successful even
in the absence of a base, which was a rare scenario in TH
catalysis. Various hydrogen sources such as methanol, eth-
ylene glycol, glycerol, formic acid, 2‐propanol (IPA), and a
2.7 | Base‐free TH of ketones
The p‐cymene based half‐sandwich Ru (II) complexes (1
and 2) of (N, O) Schiff base ligands were assessed for their

SATHEESH ET AL.
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TABLE 1 Base‐free TH of ketonesa
Entry
Substrate
Product
Time (h)
Yield (%)c
TONd
TOFe (h−1
)
1
7.0
99
990
141
2b
2.0
99
198
99
3
4
5.0
6.0
99
99
990
990
198
165
5
6.0
3.0
99
99
990
198
165
66
6b
7b
3.0
99
198
66
8b
9b
3.0
3.0
99
97
198
194
66
64
10b
11b
3.0
5.0
99
99
198
198
66
40
12b
13b
14b
15b
2.0
6.0
6.0
3.0
99
97
96
99
198
194
192
198
99
32
32
66
(Continues)

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SATHEESH ET AL.
TABLE 1 (Continued)
Entry
Substrate
Product
Time (h)
Yield (%)c
TONd
TOFe (h−1
)
16b
8.0
77
154
19.25
aReaction conditions: Substrate (1 mmol), catalyst 1 (0.1 mol%), 2‐propanol (2 ml) and T = 85 °C.
bCatalyst 1 (0.5 mol%).
cYield (%) was analyzed by GC–MS.
dTurn over number (TON) = No. of moles of product formed per mole of catalyst.
eTurn over frequency (TOF) = TON/Time (h).
mixture of triethylamine and formic acid were tried but
IPA was found to be compatible and effective hydrogen
source (Table S4: entries 1–5 & 18). One mL of IPA was
sufficient for the base‐free TH of 1 mmol of ketone but
two mL of IPA was used in some cases due to the solubility
issues (Table S4: entries 6–9). The temperature of the reac-
tion was optimized as 85 °C, below that there was a signif-
icant decrease in the conversion of acetophenone (Table
S4: entries 11–15). The optimization reactions were carried
out under aerobic condition. The presence of 0.1 mol% of
the catalyst was sufficient for the complete conversion of
acetophenone to 1‐phenylethanol in 7 hr under base‐free
condition but when the catalyst amount was increased to
0.5 mol%, the same conversion was achieved within 2 hr.
This shows the influence of catalyst amount over the reac-
tion time (Table S4: entries 15–18). Both catalysts 1 and 2
were efficient for base‐free TH but catalyst 1 was compar-
atively more efficient than catalyst 2. This slight difference
in the catalytic activity was expected due to the +I effect of
methyl group attached to the imine carbon of catalyst 2,
which reduces the electron deficiency of the metal that
leads to lesser reactivity to some extent, and steric factors
might also decrease the activity of 2, which were absent
in catalyst 1. Hence catalyst 1 was used for the extension
of substrate scope (Table S4: entries 18–19). Even when
NaOH was used as a base in the presence of 0.1 mol % of
the catalyst, it took the same time (7h) to yield 97% of 1‐
phenylethanol (Table S4: entry 10).
higher when 0.1 mol% of catalyst was used compared to
0.5 mol% of catalyst (Table 1: entries 1 and 2). Entries
3–5 (Table 1) showed more time due to the low catalyst
loading. From the literature, it was observed that the
halo‐substituted ketones had poor yield due to the lability
of halogens in basic medium.2a Since this catalytic system
was base‐free, halogens were not labile and hence 100%
selectivity was achieved in halo substituted ketones
(Table 1: entries 7–10 and 15). Generally, o‐substituted
ketones produce significantly lower yield than the p‐
substituted ketones due to the steric effect. But in this
system, no significant difference was observed (Table 1:
entries 7 and 8). The trend followed in terms of electronic
effect was that the ketones substituted with electron
donating groups were readily reduced compared to the
those with electron withdrawing groups (Table 1: entries
11 and 16). Interestingly, 4‐chromanone was successfully
reduced to its corresponding alcohol with an excellent
yield in less time (Table 1: entry 13). This confirmed that
the catalyst was effective even for heterocyclic ketones.
These results revealed the catalytic potential of ruthe-
nium complexes in TH of ketones under base‐free condi-
tion. The high catalytic activity of the complexes may be
due to the long side chain amine moiety used for the for-
mation of the ligands, which provided room for the easy
attack of substrate ketone on the metal center and facili-
tated the reduction. Few Ru (II) complexes of (N, O)
Schiff base ligands have been reported in the literature
as catalysts for TH of ketones, but these catalysts possess
limitations that include use of a base or salts of organic
acids along with suitable hydrogen source and higher
yields of product alcohols resulted only at higher temper-
atures and longer reaction times.8d, 15
Scope of catalyst 1 has been extended for the TH of
various substituted ketones into their corresponding sec-
ondary alcohols. The experimental conditions, yields of
the products, TON and TOF are given in Table 1. The
reactions were carried out under the optimized condi-
tions until the ketone was completely reduced. The reac-
tion was monitored at regular intervals by collecting an
aliquot of the reaction mixture, passing it through the
short pad of silica bed to remove any inorganic impurities
and then analyzed by GC/GC–MS. TON and TOF were
Gram scale synthesis of 2‐adamantanol, an important
intermediate for several commercial drugs,[16] from 2‐
adamantanone was achieved with 97% of isolated yield
(Figure 4). This established that the catalyst was compat-
ible with the bulk scale synthesis under base‐free

SATHEESH ET AL.
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The carbonyl compounds were converted into their corre-
sponding alcohols within 2–30 min. Carmen Carrión
et al.3c described Ru (II)(p‐cymene) complexes containing
bis (pyrazol‐1‐yl)methane ligands as catalysts for base‐free
TH of carbonyl compounds. This system required 0.2 mol%
of the catalyst and IPA as hydrogen source but the time
taken for the complete conversion was 24 hr. Andrew Ruff
et al.3d reported [Cp*Ir (pyridinesulfonamide)Cl] pre‐
catalyst for base‐free TH of ketones. But 1 mol% of Ir pre‐
catalyst was used and time taken for 88% conversion of
acetophenone to 1‐phenylethanol was 3 h when refluxed
in IPA. Pooja Dubey et al.3e reported η5‐Cp*Ir (III) com-
plexes containing Schiff base ligands for base‐free TH of
carbonyl compounds with IPA. This system needed
0.5 mol% of the Ir catalyst and achieved a 93% conversion
of acetophenone to 1‐phenylethanol in 4 hr (Figure 5).
When compared to the previously reported base‐free TH
catalysts, the current catalytic system is cost‐effective,
FIGURE 4 Gram scale synthesis of 2‐adamantanol from 2‐
adamantanone
condition, which paved the way for the industrial applica-
tion of the catalyst.
Sommer et al. 3a reported azocarboxamide based Ru (II)
(p‐cymene) complexes as catalysts for base‐free TH. The
yield of 1‐phenylethanol from acetophenone was 75% in
6 hr at 100 °C with 0.5 mol% of the catalyst under base‐free
condition using IPA as a hydrogen source. Farrar‐Tobar
et al.3b reported commercially available phosphine based
Ru‐MACHOTM‐BH catalyst for selective base‐free TH of
α,β‐unsaturated carbonyl compounds. This catalytic
system was efficient even with 0.1 mol% of the catalyst.
FIGURE 5 Reported TH catalysts for
the reduction of ketones to alcohols under
base‐free conditions

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SATHEESH ET AL.
efficient, phosphine‐free, air stable, tolerant to gram scale
synthesis, and simple and easy methodology in terms of
catalyst preparation and catalysis.
wave number range of 4000–500 cm−1 with an Agilent
FT‐IR spectrometer. UV–Visible spectra were recorded
on a Shimadzu UV‐2600 spectrophotometer using CH3OH
as a solvent. Melting points were determined with a
Gallenkamp melting point apparatus in capillary tubes
closed at one end and are uncorrected. Single crystal X‐
ray diffraction data of L1H, L2H and 2 were collected on
a Bruker SMART APEX CCD‐based X‐ray diffracto-
meter with Mo Kα radiation (λ = 0.71073 Å, T = 193 K).
The molecular structures of L1H, L2H and 2 were
solved by direct methods and refinement was carried out
with SHELXL‐97[18] package and empirical absorption
correction has been applied by SADABS. The refinement
was made by full‐matrix least‐squares on F2 with aniso-
tropic refinement in calculated positions. The reactions
involving the synthesis of complexes were monitored by
thin layer chromatography (TLC) using pre‐coated silica
gel aluminum plates. The catalytic TH reactions were
monitored by gas chromatography (GC) and purity of the
products obtained in TH reactions was confirmed by gas
chromatography–mass spectrometry (GC–MS).
3 | CONCLUSIONS
The new Ru (II)(p‐cymene) complexes of simple (N, O)
Schiff base ligands were synthesized and characterized
by spectral and analytical techniques. The molecular
structures of L1H, L2H and 2 were confirmed by single
crystal X‐ray diffraction studies. There exist an intramo-
lecular OH…… N (L1H and L2H) and intermolecular
CH…..O (2) secondary bonding interactions. Complex
2 has a pseudo‐octahedral piano stool structure. Both
the Ru complexes were explored as catalysts in TH of
ketones. These new complexes 1 and 2 served as excellent
catalysts in aerobic, mild and base‐free conditions for the
TH of various types of ketones; achieved excellent TON,
TOF and selectivity. The complexes were synthesized by
simple methodology using easily available starting
materials and hence the catalysts are cost effective, air‐
stable, and phosphine‐free. The catalysts were compatible
even for gram scale synthesis. Hence, these complexes
would be very promising than the previously reported
base‐free TH catalysts and can be extended for industrial
applications.
4.3 | General procedure for the synthesis
of Ru complexes (1‐2)
[Ru(p‐cymene)Cl2]2 (100 mg, 0.163 mmol) was dissolved
in 20 ml of CH2Cl2 and the resulting solution was stirred
for 10 min. A solution of ligand L1H/L2H (93/98 mg,
0.326 mmol) made in 20 ml of CH2Cl2 was added slowly
to the above solution with vigorous stirring. The stirring
was continued further for 30 min. The progress of the reac-
tion was monitored on TLC. After completion of the reac-
tion, the red color solution was concentrated. The resulted
orange‐red solid was washed with n‐hexane (15 mL × 2)
followed by diethyl ether (15 mL × 2) and then recrystal-
lized in 1:1 mixture of CH2Cl2 and n‐hexane.
4 | EXPERIMENTAL
4.1 | Materials
2‐(3,4‐Dimethoxyphenyl)ethanamine (97%) and RuCl3.
xH2O were purchased from Sigma Aldrich, Bangalore,
India. 2‐Hydroxybenzaldehyde (99.5%) and 2′‐hydroxy
acetophenone (99%) were purchased from Merck Spe-
cialties India Pvt. Ltd. [Ru(p‐cymene)Cl2]2 was prepared
according to the reported literature procedure.[17] Sol-
vents such as C2H5OH, CH3OH, 2‐propanol, petroleum
ether, hexane, CHCl3, and CH2Cl2 were of reagent
grade, purchased from Merck India Pvt. Ltd. and used
without further purification. The organic substrates were
purchased from Sigma Aldrich or Alfa Aesar.
[Ru(p‐cymene)Cl(L1)] (1): Yield: 150 mg (82.9%). M.P.:
188–189 °C. Element. Anal. Calcd. (Found) for
C27H32ClNRuO3: C, 58.42 (58.40); H, 5.81 (5.80); N, 2.52
(2.51). FT‐IR (ν, cm−1): 2962, 2931, 2873, 2833, 2336,
1737, 1610, 1535, 1506, 1463, 1448, 1409, 1330, 1265,
1232, 1201, 1148, 1118, 1030, 1015, 933, 910, 887, 865,
760, 734, 500, 435; UV–Visible (Methanol), λmax in nm (ε,
3
dm mol−1 cm−1): 204 (44650), 227 (28350), 286 (8700)
1
and 393 (2450); H NMR (500 MHz, DMSO‐d6) δ 7.77 (s,
4.2 | Analytical methods
1H), 7.14 (t, J = 7.5 Hz, 1H), 7.04 (s, 1H), 6.97–6.90 (m,
3H), 6.72 (d, J = 8.4 Hz, 1H), 6.36 (t, J = 7.2 Hz, 1H), 5.76
(d, J = 5.6 Hz, 1H), 5.63 (d, J = 6.0 Hz, 1H), 5.58 (d,
J = 6.1 Hz, 1H), 5.32 (d, J = 5.6 Hz, 1H), 4.62–4.51 (m,
1H), 4.26 (dt, J = 12.1, 8.0 Hz, 1H), 3.79 (s, 3H), 3.74 (s,
3H), 2.75–2.64 (m, 1H), 2.18 (s, 3H), 1.24 (dd, J = 11.3,
7.1 Hz, 5H), 1.13 (d, J = 6.8 Hz, 3H); 13C NMR (126 MHz,
Elemental analyses (C, H, and N) were performed on a
1
LECO–CHSNO–9320 elemental analyzer. H and 13C{1H}
NMR spectra of Ru (II) complexes (1 and 2) were recorded
on a Bruker 500 MHz and 126 MHz spectrometers respec-
tively using TMS as an internal standard. The FT‐IR
spectra of 1 and 2 were recorded by scan method in the

SATHEESH ET AL.
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DMSO‐d6) δ 165.01, 164.33, 149.02, 147.77, 135.00, 134.37,
132.08, 121.58, 121.28, 119.62, 113.63, 113.29, 112.41,
100.34, 97.43, 86.95, 83.15, 82.18, 81.04, 71.00, 56.01,
55.85, 46.00, 36.63, 30.56, 22.86, 21.87, 18.61, 9.14; HR‐
MS: m/z found: 520.15 [(M–Cl) = 520.14)] and calcd for
C27H32ClNRuO3 is 555.07.
ORCID
P. Raghavendra Kumara
6522-6148
https://orcid.org/0000-0001-
REFERENCES
[Ru(p‐cymene)Cl(L2)].2H2O (2): Yield: 149 mg (80%).
M.P.: 190–192 °C. Element. Anal. Calcd. (Found) for
C28H34ClNRuO3: C, 59.09 (59.02); H, 6.02 (5.99); N, 2.46
(2.42). FT‐IR (ν, cm−1): 3574, 3375, 3275, 2958, 2933,
2904, 2872, 2837, 1595, 1512, 1466, 1439, 1315, 1255,
1230, 1153, 1140, 1022, 850, 764, 744, 624,528. UV–Visible
(Methanol), λmax in nm (ε, dm 3 mol−1 cm−1): 204 (42350),
233 (23250), 285 (7550) and 382 (2400); 1H NMR (500 MHz,
DMSO‐d6) δ 7.38 (dd, J = 8.0, 1.6 Hz, 1H), 7.10–7.05 (m,
1H), 6.94 (d, J = 1.6 Hz, 1H), 6.87–6.84 (m, 2H), 6.75 (d,
J = 8.2 Hz, 1H), 6.46–6.41 (m, 1H), 5.43 (d, J = 5.8 Hz,
1H), 5.36 (d, J = 5.9 Hz, 1H), 5.33 (d, J = 5.8 Hz, 1H), 5.17
(d, J = 5.8 Hz, 1H), 4.47 (td, J = 12.4, 5.5 Hz, 1H), 4.12
(td, J = 12.4, 4.9 Hz, 1H), 3.73 (s, 3H), 3.70 (s, 3H), 3.15
(ddd, J = 12.4, 8.7, 3.8 Hz, 1H), 2.89 (td, J = 12.6, 5.6 Hz,
1H), 2.60–2.53 (m, 1H), 2.48 (s, 3H), 2.05 (s, 1H), 1.77 (s,
3H), 1.06 (d, J = 6.9 Hz, 3H), 1.03 (d, J = 6.9 Hz, 3H); 13C
NMR (126 MHz, DMSO‐d6) δ 171.33, 168.68, 149.20,
147.92, 132.41, 132.15, 130.74, 128.75, 123.17, 121.10,
114.42, 113.14, 112.47, 103.04, 95.36, 83.52, 82.15, 81.20,
80.01, 65.34, 56.06, 55.96, 34.70, 31.24, 30.69, 22.82, 21.82,
19.13, 18.35; HR‐MS: m/z found: 534.16 [(M‐Cl) = 534.16]
calcd for C28H34ClNRuO3 is 569.09.
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of the
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How to cite this article: Satheesh CE, Sathish
Kumar PN, Kumara PR, Karvembu R, Hosamani A,
Nethaji M. Half‐sandwich Ru (II) complexes
containing (N, O) Schiff base ligands: Catalysts for
base‐free transfer hydrogenation of ketones. Appl
Organometal Chem. 2019;e5111;e5111. https://doi.
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