Transfer hydrogenation via generation of hydride intermediate and base-free alcohol oxidation activity studies on designed ruthenium complexes derived from NNN pincer type ligands

Article abstract of DOI:10.1002/aoc.5615

Ruthenium complexes(1–3) have been synthesized using pincer-type ligands L¹ = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)pyridine, L² = (E)-2-(1-phenyl-2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)pyridine, L³ = (E)-2-(phenyl(2-phenyl-2-(pyridin-2-yl)hydrazono) methyl)pyridine. The molecular structures of all the complexes 1, 2 and 3 were determined by using single crystal X-ray diffraction. These complexes showed excellent catalytic activities such as transfer hydrogenation and alcohol oxidation. Theoretical calculations have been performed to understand the electronic properties of all the complexes using B3LYP as a function and LANL2DZ as a basis set.

Full text of DOI:10.1002/aoc.5615

Received: 11 January 2019
DOI: 10.1002/aoc.5615
Revised: 11 November 2019
Accepted: 22 November 2019
F U L L P A P E R
Transfer hydrogenation via generation of hydride
intermediate and base-free alcohol oxidation activity
studies on designed ruthenium complexes derived from
NNN pincer type ligands
Prasoon Raj Singh | Ankur Maji | Ovender Singh | Udai P. Singh |
Kaushik Ghosh
Department of Chemistry, IIT Roorkee,
Abstract
Roorkee, Uttarakhand, 247667, India
Ruthenium complexes(1–3) have been synthesized using pincer-type ligands
L1 = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)pyridine, L2 = (E)-
Correspondence
Kaushik Ghosh, Department of
2-(1-phenyl-2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)pyridine, L3
=
(E)-
Chemistry, IIT Roorkee, Roorkee –
247667, Uttarakhand, India.
Email: ghoshfcy@iitr.ernet.in
2-(phenyl(2-phenyl-2-(pyridin-2-yl)hydrazono) methyl)pyridine. The molecular
structures of all the complexes 1, 2 and 3 were determined by using single crys-
tal X-ray diffraction. These complexes showed excellent catalytic activities such
as transfer hydrogenation and alcohol oxidation. Theoretical calculations have
been performed to understand the electronic properties of all the complexes
using B3LYP as a function and LANL2DZ as a basis set.
Funding information
Council of Scientific and Industrial
Research (CSIR) New Delhi, India, Grant/
Award Number: 01(2942)/18/EMR–II
K E Y W O R D S
dual catalysis, Electronic properties, pincer-type ligand, ruthenium complexes
1 | INTRODUCTION
workers, Yu and co-workers,[7] Noyori and co-workers[8]
and Szymczak and co-workers.[9] This present study
In the recent years, there has been a great interest in
transition metal-catalyzed hydrogenation reactions of
polar bonds because these reactions are utilized for spe-
cific organic transformations in pharmaceutical and
fine chemical industries.[1–4] Transfer hydrogenation,
i.e. addition of hydrogen to a molecule from a source
other than gaseous hydrogen, exhibits several advantages
over other reduction methods. First, there is no use of
pressurized hydrogen gas, which is inflammable in the
air. Second, the reactant and by-products may be less
toxic, inexpensive and benign to the environment. Third,
operational setup and the procedure are simple and harsh
conditions are not involved.[5] Although NaBH4 reduction
is routinely exploited in industries, hydrogenation of
ketones to their corresponding alcohols was achieved by
ruthenium-based catalysts reported by Morris[6] and co-
originated from our interest in transfer hydrogenation
reaction of ketones to their corresponding alcohols.
On the other hand, oxidation of alcohols to their
corresponding aldehydes or ketones also exhibits enor-
mous importance in industries and pharmaceutical
companies.[10] Chromium (VI) oxide,[11] manganese
(IV) oxide[12] ruthenium (VIII) oxide are well-known
catalysts for alcohol oxidation,[13] however, these catalysts
are not benign for the environment. Development of new
catalyst for controlled oxidation of alcohols to carbonyl
compounds is an essential area of chemical research. In
fact researchers are trying to understand the catalytic activ-
ity of galactose oxidase enzyme which was found to be an
efficient catalyst for mild oxidation of primary alcohol to
their corresponding aldehyde (D-galactose to D-galacto-
hexodialdose).[14] Recently, Zhao and co-workers reported
Appl Organometal Chem. 2020;e5615.
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© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5615

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SINGH ET AL.
ruthenium bis (benzimidazole)(pyridinedicarboxylate)
complex for oxidation of alcohols with hydrogen peroxide
as an oxidizing agent.[15] To the best of our knowledge,
there is a single report, by Yu and co-workers[7d] where
ruthenium complexes derived from tridentate meridional
ligands having NNN donors exhibited transfer hydrogena-
tion as well as alcohol oxidations.
Hence, with this viewpoint, we have synthesized
three ruthenium complexes (1–3) derived from tridentate
pyridyl-based ligands. All the complexes (1–3) were char-
acterized by different spectroscopic methods like UV–vis-
ible, IR, NMR, ESI-MS analysis. The molecular structure
(1–3) of all the complexes were determined by X-ray crys-
tallography. Catalytic transfer hydrogenation and oxida-
tion of different alcohols were examined in the presence
of catalysts.
m/z peak 935.1673, 949.1828 and 1011.1968 due to 1+ (1-
PF6 ), 2+ (2-PF6 ) and 3+ (3-PF6−) was observed respec-
tively. These values authenticated the formation of com-
plexes 1–3 (shown in ESI Figure S13-S24). The solid-state
crystal structure of complexes [Ru(PPh3)2(L1)(Cl)]PF6 (1),
[Ru(PPh3)2(L2)(Cl)]PF6 (2) and [Ru(PPh3)2(L3)(Cl)]PF6
(3) were determined by X-ray crystallography. Structures
were shown in Figures 1, 2 and 3 and selected bond
parameters were deposited in ESI Table S1-S7.
−
−
The structural analysis confirmed that the ligands L1,
L2 and L3 are coordinated to the ruthenium center in the
targeted NNN fashion. Two triphenylphosphines and
chloride ions were also coordinated to the metal center.
The meridionally disposed NNN ligand and the chlorine
group defined the equatorial plane with ruthenium at the
center of this six-coordinate complex; the two PPh3
ligands occupy two axial positions and are situated in
trans orientation. Ru—Nim bond distances in complexes
1, 2 and 3 were found to be lower than our reported
results; however, Ru—P distances were comparable.[17]
We have observed that Ru—NPy distances in all three
structures varied from 2.050–2.109 Å. This range was
consistent with the recent report by Kundu and co-
workers.[19] All the bond distances and bond angles were
summarized in ESI (Table S2-S7).
All complexes were diamagnetic at room temperature
indicating their singlet ground state. The geometry opti-
mization of the complexes 1, 2 and 3 were optimized
using their crystallographic coordinates at (R)B3LYP
levels. For ruthenium metal we have used LANL2DZ
and for C, H, N, P, Cl we used 6-31 g(d) basis set in
solution phase CPCM model in dichloromethane
solvent in their singlet spin state without any ligand sim-
plification. For the theoretical study, we used Gaussian
package 09.[20]
2 | RESULTS AND DISCUSSION
2.1 | Synthesis and characterization of
ligands and complexes
Ligands L1 and L2 were synthesized by reported proce-
dures.[16] Ligands L1, L2 and L3 were synthesized
refluxing of phenyl (pyridin-2-yl)methanamine with
picolinaldehyde, 1-(pyridin-2-yl)ethanone and phenyl
(pyridin-2-yl)methanone respectively in equimolar ratio in
methanol. After 3 hr yellow colored precipitate of ligands
L1, L2 and L3 were obtained and characterized by several
spectroscopic techniques (shown in ESI Figure S9–11).
Complexes 1, 2 and 3 were synthesized by the refluxing of
ruthenium starting material Ru(PPh3)3Cl2 with ligands L1,
L2 and L3 respectively in the equimolar ratio (1:1) in etha-
nol for 3 hr and after that the reaction mixture was
followed by addition of one equivalent of ammonium
hexafluorophosphate (shown in Scheme 1). Complexes
(1–3) were characterized by different spectroscopic tech-
niques. In IR spectra, a shift of 1588 cm−1 to 1636 cm−1
was observed from ligand to complex 1 for azomethine
stretching frequency (νC=N). The same was found to be
1580 cm−1 to 1594 cm−1 for complex 2 and 1580 cm−1 to
1644 cm−1 for complex 3[17] (shown in ESI Figure S1-S4).
Further, bands around at 838 cm−1 and 556 cm−1
were assigned to be the stretching and bending modes
In the geometry optimization of metal complexes the
PPh3 group wereexchanged with PH3 derivatives for sim-
plifying the calculations.[21] The optimized geometry of
the synthesized complexes 1, 2 and 3 were shown in ESI
Figure S25.
2.2 | Electrochemical properties
The electrochemistry of all complexes (1–3) has been
investigated under N2 atmosphere in acetonitrile solvent
by using 0.1 TBAP as the supporting electrolyte within
range 0 V to +1.4 V at scan rate 0.1 V/s using Ag/AgCl
electrode as a reference electrode. Cyclic voltammetry
curves for all metal complexes are shown in Figure 4, ESI
Figure S12).
respectively for PF6 counter anion (1, 2 and 3).[18] The
−
UV–visible spectra were recorded in dichloromethane
and the broad bands were recorded near 468 nm, 474 nm
and 480 nm in complexes 1, 2 and 3 respectively and
these were probably due to metal to ligand charge trans-
fer (MLCT)[17] (shown in ESI Figure S5-S8).
To further authenticate the complexes, we have per-
formed NMR and ESI-MS analysis in acetonitrile. The
Complexes 1–3 were redox-active in acetonitrile sol-
vent, however, the ligands were not. These complexes

TRANSFER HYDROGENATION AND BASE-FREE ALCOHOL OXIDATION ACTIVITY
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SCHEME 1 The synthetic
procedure for the metal complexes
[Ru(PPh3)2(L1)(Cl)]PF6 (1),
[Ru(PPh3)2(L2)(Cl)]PF6(2) and
[Ru(PPh3)2(L3)(Cl)]PF6(3)
FIGURE 1 The ORTEP diagram (30%
probability level) of complex 1, [Ru(PPh3)2(L1)
(Cl)]PF6; (all of the hydrogen atoms and
anions are omitted for clarity). P = PPh3

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SINGH ET AL.
FIGURE 2 The ORTEP diagram
(30% probability level) of complex
2, [Ru(PPh3)2(L2)(Cl)]PF6; (all of the
hydrogen atoms and anions are omitted
for clarity). P = PPh3
FIGURE 3 The ORTEP diagram
(30% probability level) of complex
3 [Ru(PPh3)2(L3)(Cl)]PF6; (all of the
hydrogen atoms and anions are omitted
for clarity). P = PPh3

TRANSFER HYDROGENATION AND BASE-FREE ALCOHOL OXIDATION ACTIVITY
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FIGURE 4 Cyclic voltammograms of a
10–3 M solution of complexes 1–3 in
acetonitrile in presence of 0.1 M
tetrabutylammonium perchlorate (TBAP),
using working electrode: glassy-carbon,
reference electrode Ag/AgCl; auxiliary
electrode: platinum wire, scan rate 0.1 Vs-1
demonstrate quasi-reversible one electron oxidative
response with E1/2 = 1.04 V (complex 1), E1/2 = 1.029 V
(complex 2) and E1/2 = 0.94 V (complex 3) reference to
Ag/AgCl. (Figure 4 and Table 1). These responses were
believed to be of Ru(II) – Ru(III) redox couple. During
oxidation, electron was released from HOMO of complex
1 (HOMO – 75% dxy (Ru), complex 2 (HOMO – 70% dxy
(Ru) and complex 3 (HOMO – 69% dxy (Ru) (Shown in
ESI Figure S26, Table S8, S10, S12). Hence oxidative
response due to metal center involving oxidation Ru(II) –
Ru(III).[17]
were depicted in the supporting information along with
their respective theoretical spectra. To gain a more
insight into the electronic properties of the transitions
involved in the optical absorption processes, we have
investigated time-dependent density functional theory
(TD-DFT) in CH2Cl2 solvent using CPCM model. The
most relevant transitions involved, along with their
energy, character, oscillator strengths of all the com-
plexes, are listed in ESI Table S8-S13 for metal complexes
1, 2 and 3 respectively. To analyze the nature of absorp-
tion, we performed natural transition orbital (NTO) anal-
ysis based on the calculated transition density matrices.
Those are listed in ESI Figure S27-S32 for complexes 1,
2 and 3 respectively. This method offers the most com-
pact representation of the transition density between the
ground and excited states in terms of an expansion into
the single-particle transition model (hole and electron
states for each given excitation).
2.3 | Comparison between theoretical
and experimental UV–visible spectrum
Electronic spectra of complexes 1, 2 and 3 were recorded
in dichloromethane solution at room temperature and
Here we refer to the unoccupied and occupied NTOs
as “electron” and “hole” transition orbitals respectively.
Based on TD-DFT NTO analysis for complex 1 (ESI
Figure S28), a lower energy transition around 463 nm is
computed at 454 nm (2.7276 eV, f = 0.0446) as the dxz
TABLE 1
Electrochemical data for Ru(II)/Ru(III) redox
couple at 298 K[a] vs Ag/AgCl
Complexes
E
pa/V
Epc/V
1.01
E1/2b(V) (ΔEpc, mV)
1.04(70)
(Ru) + 3p(Cl)
π*(SBpy+py) 1MLCT transition.[17]
1
2
3
1.08
1.05
0.86
(Figure 5) For complex 2 (ESI Figure S28), a lower energy
transition around 445 nm is computed at 460 nm
(2.6915 eV, f = 0.0402) as the dxz (Ru) + 3p(Cl) π*(SBpy
0.99
1.029(60)
1.03
0.94(170)
1
+py) MLCT transition.[17] The transition near 375 nm
aElectrochemical data measured in acetonitrile with 0.1 M
tetrabutylammonium perchlorate (TBAP);
bData from cyclic voltammetric measurements; E1/2 is calculated as average
of Epc at scan- anodic (Epa) and cathodic (Epc) peak potentials
(3.0479 eV, f = 0.0834, λtheo = 462 nm) is computed as
the dyz (Ru)
+
π
(SBpy+py)
π*(SBpy+py)
1MLCT/1ILCT/1LLCT transitions. The transition around
324 nm is computed at 346 nm (3.5812 eV, f = 0.1983) as
E
1/2 = 1/2(Epa + Epc);
cΔEp = Epa rate 0.1 Vs−1

6 of 15
SINGH ET AL.
1
the dxz (Ru) + 3p (Cl) π*(SBpy+py) LLCT transition.[17]
A similar pattern was inspected in the other two com-
plexes. (Minor differences were observed in complex 3).
transfer hydrogenation catalyst. In the transfer hydroge-
nation of acetophenones, we used KOH and NaOH bases
because of their cost-effectiveness and easy to handling.
Transfer hydrogenation of p-bromo acetophenones never
occurred only in the presence of base (Table 2, entry 1).
Optimized conditions for transfer hydrogenation were
acetophenone (ketone) (1.0 mmol), base (KOH)
(0.1 mmol), solvent (2- Propanol,ꢀ 3 ml), N2 atmosphere
and catalyst (0.004 mmol) at 80 C for 3 min (Table 2,
Entry 6). The catalytic activities of complexes 1, 2 and
3 were reasonably investigated utilizing the transfer
hydrogenation (TH) reactions of ketones (Table 3, ESI
2.4 | Transfer hydrogenation of ketones
Transfer hydrogenation of ketones is an important reac-
tion because of the environmentally benign nature of
reagents.[22] Our main target was to get exclusively
desired product in optimum conditions. Table 2 provides
us the screening of the experimental condition for
FIGURE 5 Theoretical and experimental
UV-vis spectra of complex 2
TABLE 2
Screening of the experimental conditions for the catalytic transfer hydrogenation [a]
Entry
Catalyst (mmol)
None
Base (10 mol %)
KOH
Time (min)
Yield[b] (%)
1
2
3
4
5
6
7
8
60
60
35
7
0
1(0.001)
None
0
1(0.003)
KOH
60
98
67
100
98
0
1(0.004)
KOH
2(0.003)
NaOH
KOH
30
3
2(0.004)
3(0.004)
KOH
10
60
Ru(PPh3)3Cl2
KOH
aReaction Conditions: p-bromo acetophenone (1.0 mmol), Base (0.1 mmol), Solvent (2- Propanol, 3 ml), 80 ꢀC, N2 atmosphere
bYield determined by GC–MS.

TRANSFER HYDROGENATION AND BASE-FREE ALCOHOL OXIDATION ACTIVITY
7 of 15
TABLE 3
Catalytic transfer hydrogenation of aryl ketones from Ru (II) catalysts 1, 2, 3[a]
Entry
Substrates
Ru (II)cat.
Time (min)
Conversion (%)[b]
TON[c]
245
TOF[d] (h−1
)
1
1
2
3
7
3
7
98
100
98
2227
5000
1531
250
245
2
3
4
5
6
1
2
3
10
5
92
99
90
230
247
225
1437
3093
1406
10
1
2
3
8
5
95
98
91
237
245
227
1826
3062
1263
11
1
2
3
8
4
93
97
91
232
242
227
1788
4041
1421
10
1
2
3
8
4
98
99
96
245
247
240
1884
4125
1500
10
1
2
3
10
8
15
17
10
37
42
25
234
326
100
15
7
8
1
2
3
19
15
30
96
97
90
240
242
225
774
968
450
1
2
3
10
6
77
80
76
192
200
190
1203
2000
904
13
9
1
2
3
8
5
93
95
89
232
238
222
1784
1830
1110
12
(Continues)

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SINGH ET AL.
TABLE 3 (Continued)
10
1
2
3
45
45
45
61
65
57
153
163
143
204
217
190
aReaction Conditions: Ketone (1.0 mmol), KOH (0.1 mmol), iPrOH (3 ml) 80 ꢀC, under N2 atmosphere.
bYield determined by GC–MS.
cTON = [(mmole of product)/ (mmole of catalyst)].
dTOF = TON/Time (h).
Figure S33). Under the typical conditions, transfer hydro-
genation of ketones was performed with 1 mmol of
ketone in 2-propanol at 80oC temperature. Alcohols were
produced as the exclusive products in the presence of
KOH base and 0.004 mmol of catalysts (complexes 1–3)
upon refluxing 2-propanol under nitrogen atmosphere.
To evaluate the scope of the transfer hydrogenation
reaction, tridentate NNN ligands were derived from
different carbonyl compounds and we investigated the
catalytic activity of three different catalysts.
Acetophenones and substituted ketones (R = Cl, OH, Br,
F) were demonstrated transfer hydrogenation in their
corresponding alcohols with good yield (Table 3, Entry
1–5, 7).
Acetophenone contains electron-donating substitu-
ents like p-methyl acetophenone demonstrated higher
conversion in more extended time than others
(Table 3, Entry 7). Although m–nitro acetophenone
showed lowest conversion of ketone to alcohol because
of the electron-withdrawing nature of the NO2 at meta
position. In the case of benzoyl pyridine and benzo-
phenone, we have also observed good conversion.
(Table 3, Entry 8, 9). Investigation of aliphatic ketone
like cyclohexanone gives rise to moderated conversion
(Table 3, Entry 10).
2.5 | Reaction mechanism of transfer
hydrogenation
Herein, we propose a probable mechanism for the trans-
fer hydrogenation reaction catalyzed by complexes 1–
3.[8c, 21] The catalytic process starts after the addition of
isopropyl alcohol via dehydrochlorination of complexes
in the presence of base KOH and the formation of an alk-
oxide intermediate (I). β-Hydride elimination of interme-
diate (I) gave rise to stable and catalytically active
intermediate (II).[7a, 24]
We have investigated NMR spectral studies to support
the mechanism. NMR spectral data of intermediate
(II) was deposited in Figure S37. A triplet signal in the
range of −6.0 ppm to −7.0 ppm peak clearly showed the
formation of the hydride intermediate. In fact, this is con-
sistent with the observation by Yu and co-workers.[25]
The Complex 1—H intermediate also authenticated by
ESI-MS. The m/z peak 901.2147 corresponding to the
[Complex 1—H-Cl−] was observed (shown in ESI
Figure S38). This hydride intermediate reacted with the
ketones giving rise to secondary alcohols via hydride
transfer to the carbonyl carbon. The probable reaction
mechanism of transfer hydrogenation is shown in
Scheme 2.
As a result of this, remarkable substituent effect
was observed on Ru (II) catalysts the electron-
donating capability of complex 2 rendered complex
more catalytically active than 1 and 3.[7a, 23] (Shown in
ESI Table S14) A variety of substituents on aryl
ketones catalyzed to give their corresponding alcohols
as a product. In most of the cases, the reaction
reached ≥90% conversions for a ketone into secondary
alcohols.
2.6 | Alcohol oxidation
Alcohol oxidation by using transition metal complexes as
a catalyst was found an excellent method for oxidation of
alcohols into ketones from base free oxidation method[26]
in the presence of a stoichiometric amount of oxidants.
Table 4 provides the optimization conditions for the

TRANSFER HYDROGENATION AND BASE-FREE ALCOHOL OXIDATION ACTIVITY
9 of 15
SCHEME 2 The proposed
reaction mechanism for transfer
hydrogenation of ketones
oxidation of alcohols. In the absence of a catalyst and oxi-
dizing agent, benzyl alcohol demonstrated no conversion
into benzaldehyde (Shown in ESI Figure S34). In the
presence of atmospheric oxygen, the transformation of
benzyl alcohol into benzaldehyde was negligible (Table 4,
Entry 3). The negligible yield was also obtained in pres-
ence of hydrogen peroxide in the same reaction (Table 4,
Entry 4).
The tert-butyl hydrogen peroxide has given an excel-
lent yield (Table 4, Entry 6–7). After the analysis, we
got tert-butyl hydrogen peroxide (TBHP) is good and
environmentally benign oxidizing agent for alcohol oxi-
dation. So, the best results of alcohol oxidation occurred
under the typical condition, alcohol (1 mmol), catalyst
(0.0025 mmol), oxidizing agent tert-butyl hydrogen per-
oxide (5 mmol) and solvent acetonitrile (3 ml). The oxi-
dation reactions of primary as well as secondary
alcohols were operated by using complex 3 (0.0025 mmol)
as a catalyst and tert-butyl hydrogen peroxide as an
oxidizing agent refluxing in acetonitrile at 40 ꢀC for
3 hr (Table 5). 1-phenyl ethanol and benzyl alcohol
were reacted to give exclusive products as their
corresponding ketones and aldehydes respectively
(Table 5, Entry 1, 2). p-nitrobenzaldehyde showed
comparatively less conversion because of its electron-
withdrawing group (Table 5, Entry 4). Study of aliphatic
alcohol cyclohexanol gives rise to moderated conversion
(Table 5, Entry 5).

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SINGH ET AL.
TABLE 4
Screening of the experimental conditions for the alcohols oxidation[a]
Entry
Catalyst
Solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3OH
Oxident[b]
None
Time (hr)
Yield (%)[c]
1
2
3
4
5
6
7
8
None
5
3
3
5
3
3
3
3
0
RuCl3
TBHP
TBHP
O2
43
1
2
2
2
3
3
90
1.5
12
H2O2
TBHP
TBHP
TBHP
92.5
95
53.3
aReaction Conditions: 1- Phenyl ethanol (1 mmol), Ru (II) catalyst (0.0025 mmol), oxidizing agent (5 mmol), 40 ꢀC temperature, solvent 3 ml.
bO2 at 1 atm.
cYield determined by GC–MS.
TABLE 5
Catalytic alcohol oxidation[a] of primary and secondary alcohols with Ru (II) 3 catalyst
Entry
Substrate
Product
Time (hr)
%Conversion[b]
1
2.5
90
2
2.5
95
3
4
3
3
64
10
(Continues)

TRANSFER HYDROGENATION AND BASE-FREE ALCOHOL OXIDATION ACTIVITY
11 of 15
TABLE 5 (Continued)
Entry
Substrate
Product
Time (hr)
%Conversion[b]
47
5
6
aReaction Conditions: 1- Phenyl ethanol (1 mmol), Ru (II) catalyst (0.0025 mmol), oxidizing agent (5 mmol), 40 ꢀC temperature, solvent 3 ml
bYield determined by GC–MS.
SCHEME 3 Proposed mechanism
for alcohol oxidation

12 of 15
SINGH ET AL.
2.7 | Mechanism of alcohol oxidation
speculate the formation of Ru (IV) = O intermediate
during the catalytic cycle.
We have proposed a probable mechanism for the oxida-
tion of primary as well as secondary alcohol oxidation
based on previously reported literature[27] shown in
Scheme 3. Ru (II) complex will have labile chloride ion
and TBHP will react with ruthenium center and will give
rise to intermediate I, i.e. Ru (III)–O–OC (CH3)3 species.
In the next step,the homolytic cleavage of O–O bond will
give rise to the generation of Ru (IV) = O species and
could generate intermediate II.
Further applications of these complexes for other cat-
alytic activities are under progress.
4 | EXPERIMENTAL SECTION
4.1 | Materials
Intermediate II Ru (IV) = O as well as (CH3)3CO• are
All the reagents were of analytical grade and were used
as obtained. RuCl3Á3H2O was purchased from Loba
Chemie Pvt. Ltd, Mumbai, India.
28]
capable of H• atom abstraction (HAT).[26a,
and H•
atom abstraction from -CH2 as well as -OH groups may
lead to the formation of benzaldehyde.
2.8 | Gram scale synthesis of
4.2 | Physical measurements
1-phenylethan-1-ol
Infrared spectra were obtained on KBr pellets with a
Thermo Nicolet Nexus FTIR spectrometer, using 16 scans
and were reported in cm−1. Electronic absorption spectra
of all the complexes were recorded in dichloromethane
and acetonitrile solvents with an Evolution 600, Thermo
Importantly, the current methodology could be utilized
to directly prepare a gram-scale synthesis of the
1-phenylethan-1-ol. The reaction of acetophenone
(12 mmol) give rise to 1.13 g of 1-phenylethan-1-ol with
93% yield in the presence of 0.25 mol% of catalyst 2.
1
Scientific UV–visible spectrophotometer. H, 13C and 31P
NMR spectra were recorded on a JEOL, 400 MHz spec-
trometer in the deuterated solvents. Cyclic voltammetric
studies were performed on a CH-600 electroanalyser in
dichloromethane with 0.1 M tetrabutylammonium per-
chlorate (TBAP) as the supporting electrolyte. The work-
ing electrode, reference electrode and auxiliary electrode
were a glassy carbon electrode, an Ag/AgCl electrode and
a Pt wire respectively. The concentration of the com-
pounds was in the order of 10−3 M. The fer-
rocene/ferrocenium couple occurred at E1/2 = + 0.55
(70) Vs−1. Ag/AgCl (scan rate 0.1 V s−1) in acetonitrile
under the same experimental conditions. The ESI-MS
experiments were performed on a Brüker micrOTOFTM-
Q-II mass spectrometer.
3 | CONCLUSIONS
The following are the principle findings and conclusions
of our present study:
i. Ruthenium complexes (1–3) derived from pincer-
type ligands containing NNN donors were isolated
and characterized by spectroscopic methods.
ii. Molecular structures of complexes 1, 2 and 3 were
determined by X-ray crystallography. Crystal struc-
tures depicted meridional binding of the pincer type
ligands to the metal center.
iii. These complexes (1–3) were utilized to investigate
the transfer hydrogenation reactions a total of 9
carbonyl compounds were converted to their
corresponding alcohols.
4.3 | X-ray crystallography
iv. A probable mechanism was proposed for this transfer
hydrogenation reaction on the basis of the literature
survey. The hydride intermediate, which was sup-
posed to be the active species for this catalytic pro-
cess was characterized by NMR spectral studies and
ESI-MS analysis.
v. These complexes were also investigated for the oxida-
tion of alcohols. A total of 4 substrates were utilized
to examine catalytic activity.
Orange-red crystals of complexes 1–3 were obtained by
slow evaporation of solution from the complexes in a
CH2Cl2–hexane mixture. The X-ray data collection and
processing for complexes were performed on a Bruker
Kappa Apex-II CCD diffractometer by using graphite
monochromated Mo-Kα radiation (λ = 0.71073 Å) at
273 K for 1–3. Crystal structures were solved by direct
methods. Structure solutions, refinement and data output
were carried out with the SHELXTL program.[29–31]All
non-hydrogen atoms were refined anisotropically. Hydro-
gen atoms were placed in geometrically calculated
vi. We also proposed a mechanism for the alcohol oxida-
tion reaction. Based on literature reviews, we

TRANSFER HYDROGENATION AND BASE-FREE ALCOHOL OXIDATION ACTIVITY
13 of 15
positions and refined using a riding model. Images were
created with the DIAMOND program.[32]
129.017, 129.260, 129.324, 129.511, 129.810, 129.991,
130.230, 130.592, 130.901, 133.444, 133.539, 133.658,
133.768, 133.849 137.085, 137.194, 141.147, 152.148,
154.280, 159.734, 161.291. 31P NMR δ 19.85, −137.2 to
−154.9 (sept). m/z = 935.1673 for 1+ (1-PF¯6).
4.4 | Synthesis of ligands
L1 and L2 were synthesized according to the procedure
reported earlier[16] The precursor complex [Ru(PPh3)3Cl2]
was prepared by following the reported procedure.[33]
4.5.2 | Synthesis of [Ru(PPh3)2(L2)(Cl)]
PF6 (2)
In the similar fashion to the synthesis of 1 [Ru(PPh3)3Cl2]
(192 mg, 0.2 mmol) reacted with ligand L2 (57.6 mg,
0.2 mmol) under N2 atmosphere for 3 hr. After that, cool
reaction mixture at room temperature and add a batch of
ammonium hexafluorophosphate (33 mg, 0.2 mmol) as a
counter anion. Red color solid was obtained after 10 min,
filtered out and dried under vacuum. Recrystallization in
chloroform/n-hexane (v/v = 1:3) gave complex 2 as dark
red crystals. Yield 83%. UV–visible [CH2Cl2, λmax/nm[ɛ/
M−1 cm−1]: 480 (4,022), 382 (3,365), 323 (17,782),
4.4.1 | Synthesis of L3
Benzoyl pyridine (183 mg, 1.00 mmol) and
2-(1-phenylhydrazinyl)pyridine(185.0 mg, 1.00 mmol)
were dissolved in methanol at room temperature starring,
after 4 hr one drop of HCl pour in mixture. Subsequent
to 12 hr a yellow color solid precipitated out. The precipi-
tate is filtered out and dried. 79% yield of ligand L3 has
been obtained. UV–visible [CH2Cl2,
λmax/nm [ɛ/
M−1 cm−1]: 347 (50,772), 255 (11,210).1H NMR
(400 MHz, CDCl3) δ 8.19 (d, J = 24.3 Hz, 1H), 8.12–8.01
(m, 1H), 7.61–7.53 (m, 2H), 7.50–7.41 (m, 1H), 7.38–7.30
(m, 2H), 7.26 (dd, J = 11.3, 8.5 Hz, 1H), 7.15–7.02 (m,
5H), 7.00–6.94 (m, 2H), 6.90 (dd, J = 8.2, 7.0 Hz, 2H),
6.81 (dd, J = 11.8, 6.8 Hz, 1H). 13C NMR (126 MHz,
CDCl3) δ 149.36, 149.23, 147.49, 145.17, 135.68, 131.00,
129.82, 128.67, 128.23, 128.0, 127.72, 127.08, 126.59,
125.50,125.41, 124.76. 124.62, 123.73, 123.04, 122.55,
116.58, 111.91. m/z = 351.1595 for [L3 + H+].
270 (29,777), 229(38,573). H NMR (d6-DMSO, 400 MHz,
1
23 ꢀC) δ 1.169 (s, 3H), 6.997–7.030 (t, J = 7.2, 6.1H),
7.157–7.199 (m, 3H),7.216–7.259 (m, 6H), 7.285–7.322 (t,
J = 7.6,7.2), 7.388–7.443 (m, 22H), 7.564–7.628 (m, 6H),
7.909–7.948 (t, J = 7.6,8 1H), 8.397–8.411 (d, J = 5.6, 1H),
8.860–8.873 (d, J = 5.2, 1H). 13C NMR (DMSO, 400 MHz,
23 ꢀC) δ 16.290, 109.906, 119.822, 128.515, 128.609,
128.788, 128.855, 129.046, 129.403, 129.604, 129.809,
130.172, 130.453, 133.024, 133.119, 133.186, 133.267
133.379, 136.605, 136.716, 140.938, 152.07, 154.28, 159.73
161.089, 162.507. 31P NMR δ 19.9, −133.7 to −154.8
(sept). m/z = 949.1828 for 2+ (2-PF6 ).
−
4.5 | Synthesis of the metal complexes
4.5.1 | Synthesis of [Ru(PPh3)2(L1)(Cl)]
PF6 (1)
4.5.3 | Synthesis of [Ru(PPh3)2(L3)(Cl)]
PF6 (3)
A batch of ligand L1 (55mg, 0.2 mmol) was refluxed in
ethanolic solution of [Ru(PPh3)3Cl2] (192 mg, 0.2 mmol)
under N2 atmosphere for 3 hr. After that, add ammonium
hexafluorophosphate (33 mg, 0.2 mmol) as a counter
anion. Dark red color solid was obtained, filtered out and
dried under vacuum. Recrystallization in chloroform/n-
hexane (v/v = 1:3) gave complex 2 red color crystals.
Yield 80%. UV–visible [CH2Cl2, λmax/nm[ɛ/M−1 cm−1]:
In the similar fashion to the synthesis of 1 [Ru(PPh3)3Cl2]
(192 mg, 0.2 mmol) reacted with ligand L3 (70mg,
0.2 mmol) under N2 atmosphere for 3 hr. After that, add
ammonium hexafluorophosphate (33 mg, 0.2 mmol) as a
counter anion. Red color solid was obtained, filtered out
and dried under vacuum. Recrystallization in dic-
hloromethane/n-hexane (v/v
= 1:3) gave complex
3 brown-black color crystals. Yield 75%. UV–visible
463(2,924),
377(3,473),
322(14,584),
266(24,490),
[CH2Cl2, λmax/nm [ɛ/M−1 cm−1]: 478(2,482), 323(14,369),
288(26,686). 1H NMR (DMSO, 400 MHz, 23 ꢀC) δ
7.07–7.149(m, 5H), 7.211–7.254(m, 6H), 7.279–7.294(d,
J = 6.0, 3H), 7.349–7.441(m, 22H), 7.464–7.517(m, 2H),
7.611–7.630(d, J = 7.6, 1H), 7.662–7.701(t, J = 7.2, 8.4,
3H), 7.813–7.852(t, J = 8, 7.6, 1H), 8.320–8.333(d, J = 5.2,
1H), 8.527–8.541(d, J = 5.6, 1H). 13C NMR (d6-DMSO,
400 MHz, 23 ꢀC) δ 109.712, 120.506, 127.219, 128.916,
268(24,490), 229(25,695) H NMR (d6-DMSO, 400 MHz,
1
ꢀ
23 C) δ 5.942–5.995 (m, 2H), 6.550–6.570(d, J = 8, 1H),
6.847–6.866 (d, J = 8.4, 1H), 6.999–7.070(m, 4H),
7.144–7.287(m,
16H),
7.388–7.440(m,
16H),
7.473–7.542(m, 5H), 7.644–7.685(t, J = 7.6, 8.8, 1H),
7.791–7.827(t, J = 7.6, 6.8, 1H), 8.225–8.237(d, 1H),
8.476–8.509(d, 1H). 13C NMR (DMSO, 400 MHz, 23 ꢀC) δ

14 of 15
SINGH ET AL.
110.07, 119.98, 126.42, 127.29, 127.71, 127.97, 128.45,
128.50, 128.57, 128.54, 128.57, 128.76, 128.82, 129.52,
129.93, 130.11, 130.27, 130.52, 132.85, 132.91, 132.95,
133.11, 136.35, 136.46, 137.99, 139.80, 153.16, 160.93,
163.43. 31P NMR δ 30.3, −6.2, −134.8 to −152.3 (sept).
m/z = 1011.1968. for 2+ (2-PF6−).
ORCID
Kaushik Ghosh https://orcid.org/0000-0002-3792-2848
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There are no conflicts to declare.

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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Singh PR, Maji A,
Singh O, Singh UP, Ghosh K. Transfer
hydrogenation via generation of hydride
intermediate and base-free alcohol oxidation
activity studies on designed ruthenium complexes
derived from NNN pincer type ligands. Appl
Organometal Chem. 2020;e5615. https://doi.org/10.
1002/aoc.5615
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