Metal-ligand cooperativity in a ruthenium(II) complex of bis-azoaromatic ligand for catalytic dehydrogenation of alcohols

Article abstract of DOI:10.1016/j.ica.2018.09.038

Herein a new Ru-phosphine complex (1) with molecular formula [RuL(PPh₃)Cl₂] is reported where L is a redox active pincer ligand 2,6-bis(phenylazo)pyridine. The isolated complex has been characterized by usual spectroscopic techniques including single crystal X-ray crystallographic analysis. Complex 1 efficiently catalyzes aerobic oxidation of a wide range of primary and secondary benzylic, allylic, heterocyclic, alicyclic alcohols under mild conditions and is found to be superior over several other Ru (0, +2 and +3), Ru-H and Ru-PPh₃ catalysts. Mechanistic studies indicate that a transient Ru-H intermediate is formed in the catalytic cycle which gets switched into a Ru-hydrazo intermediate via hydrogen-walking mechanism. The catalyst is regenerated by aerial oxidation producing H₂O₂ as a by-product.

Full text of DOI:10.1016/j.ica.2018.09.038

Accepted Manuscript
Research paper
Metal-ligand cooperativity in a ruthenium(II) complex of bis-azoaromatic li-
gand for catalytic dehydrogenation of alcohols
Tanushri Saha, Rajib Pramanick, Debabrata Sengupta, Sreebrata Goswami
PII:
S0020-1693(18)31138-1
https://doi.org/10.1016/j.ica.2018.09.038
ICA 18496
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
23 July 2018
13 September 2018
13 September 2018
Please cite this article as: T. Saha, R. Pramanick, D. Sengupta, S. Goswami, Metal-ligand cooperativity in a
ruthenium(II) complex of bis-azoaromatic ligand for catalytic dehydrogenation of alcohols, Inorganica Chimica
Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.09.038
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Metal-ligand cooperativity in a ruthenium(II) complex of bis-azoaromatic ligand for catalytic
dehydrogenation of alcohols
Tanushri Saha, Rajib Pramanick, Debabrata Sengupta and Sreebrata Goswami*
School of Chemical Sciences, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata 700 032, India
Email: icsg@iacs.res.in
1

ABSTRACT
Herein a new Ru-phosphine complex (1) with molecular formula [RuL(PPh3)Cl2] is reported
where L is a redox active pincer ligand 2,6-bis(phenylazo)pyridine. The isolated complex has
been characterized by usual spectroscopic techniques including single crystal X-ray
crystallographic analysis. Complex 1 efficiently catalyzes aerobic oxidation of a wide range
of primary and secondary benzylic, allylic, heterocyclic, alicyclic alcohols under mild
conditions and is found to be superior over several other Ru (0, +2 and +3), Ru-H and Ru-
PPh3 catalysts. Mechanistic studies indicate that a transient Ru-H intermediate is formed in
the catalytic cycle which gets switched into a Ru-hydrazo intermediate via hydrogen-walking
mechanism. The catalyst is regenerated by aerial oxidation producing H2O2 as a by-product.
2

1. Introduction
Advanced civilization demands increasing requirement of energy. Development of
energy storage materials can fulfil such issues by producing hydrogen from liquid organic
hydrogen carriers [1-4] and also by activating various small molecules like O2 and CO2 [5, 6].
In recent time growth of various metals, metal oxides and organometallic complexes as
catalysts [7-9] are vastly used in energy-related research. In this respect, ruthenium based
catalysts [7, 8, 10, 11] are known to demonstrate promising catalytic activities in various
oxidation reactions. Most of such systems, however, turn out to be extremely substrate
specific and usually require hydrogen or electron-transfer co-catalyst as additives [8]. As a
consequence, recent studies are focused on incorporation of redox active or proton responsive
ligands to the metal centre which in principle can enhance the catalytic efficiency and reduce
the dependency on additional additives [3, 4]. These ligand frameworks are capable of
participating synergistically with the metal centre [9] to ensure high sustainability and
efficiency which is not achievable otherwise.
As ligands, pyridyl azo-aromatics have recently drawn considerable attention due to
their prosperous redox events [12-14] in the complexes. Recently, we have introduced a
pincer azo-aromatic ligand, 2,6-bis(phenylazo)pyridine (L) [15] which can accommodate
multiple electrons and protons by the use of azo/hydrazo redox couple. Involvement of azo
functionality in alcohol oxidation process was initially proposed by Marko et al. in 1996 [16]
and was corroborated by us [17, 18] and others [19, 20] during recent years. For example,
nickel(II) and zinc(II) complexes of the ligand (L) are shown to dehydrogenate alcohols
under aerobic conditions using exclusive ligand redox, wherein the metal centre acts only as a
template. The reactions, however, require necessary additive, for example zinc dust, which
activates the respective catalyst by single electron reduction of the ligand centre. Galactose
Oxidase (GO) pathway involving hydrogen atom transfer is shown to be operative in the
3

above examples. The above mechanistic understanding prompted us to design suitable metal
complexes of L which would not require prior reduction for catalyst activation for the similar
oxidation reaction and consequently will occur even in the absence of any additive. Recently
we have demonstrated that the properties of the coordinated L can be substantially tuned by
the incorporation of suitable ancillary ligands (Q). The ancillary ligand with strongest π-
acceptor ability induces maximum electron deficiency by effectively participating in the π
system of L, thus causing considerable modification in their physico-chemical properties
[21].
In this report we discuss our success story of using a ruthenium-phosphine complex of
the ligand L, which is capable of bringing out aerobic oxidation of alcohol under additive free
conditions. The ruthenium catalyst, reported herein, is suitable to oxidize a wide variety of 1o
and 2o alcohols using air as the terminal oxidant. In passing it is noted that the examples of
suitable catalyst for aerobic oxidation of alcohols are very few in the literature. The above
catalytic reaction is followed by controlled reactions and spectroscopic analysis which
indicate metal-ligand cooperativity in the catalytic cycle. Finally, based on the mechanistic
understanding a hydrogen-walking mechanism [10, 22] is proposed for the above oxidation
reaction.
2. Experimental
2.1 Materials
Ru(PPh3)3Cl2 was purchased from Sigma-Aldrich. Different alcohols and deuterated
alcohols, used for catalysis, were purchased from either Sigma-Aldrich or Alfa Aesar.
Solvents were purified and dried prior to use. Tetrabutylammonium perchlorate was prepared
and recrystallized following reported procedure [23]. Caution! Perchlorate salts have to be
handled with care and appropriate safety precautions.
2.2 Physical Measurements
4

UV–vis spectra were recorded using a Perkin-Elmer Lambda 950 spectrophotometer.
1
A Perkin-Elmer 783 spectrophotometer was used for obtaining Infrared spectra. H NMR
spectra were recorded using either a Bruker Avance 400 MHz or 500 MHz spectrometers
where SiMe4 was used as the internal standard. A Perkin-Elmer 240C elemental analyzer was
used to collect micro analytical data (C, H, N). All electrochemical measurements were
performed using a PC-controlled PAR model 273A electrochemistry system. Cyclic
voltammetric experiments were performed in acetonitrile solvent containing supporting
electrolyte, 0.1 M Bu4NClO4 under the nitrogen atmosphere. Three electrodes configuration
is comprised of Ag/AgCl as the reference electrode, a Pt disk as working electrode and Pt
wire as the auxiliary electrode. E1/2 for the ferrocenium-ferrocene couple under our
experimental conditions was 0.39 V.
2.3 X-Ray Crystallography
Crystallographic data for complex 1 is collected in Table S1. Suitable crystals of
complex 1 for X-ray diffraction analysis were grown by the slow diffusion of a
dichloromethane solution of the complex into hexane. Crystallographic data were collected
on a Bruker SMART APEX-II diffractometer, equipped with graphite-monochromatic Mo
Kα radiation (λ = 0.71073 Å), and was corrected for Lorentz polarization effects. Data for 1:
a total of 64247 reflections were collected, of which 7088 were unique (Rint = 0.100). These
satisfy the I >2σ(I) criterion and was used in subsequent analysis. The structure was solved
by employing the SHELXS-2014 program package [24] and was refined by full-matrix least
squares based on F2 (SHELXL-2014) [25]. All hydrogen atoms were added at calculated
positions.
2.4 Synthesis of the complex
The ligand (L) [L = 2,6-bis(phenylazo)pyridine] was prepared following the reported
procedure [15]. To a solution of 144 mg of ligand, L (0.50 mmol) in toluene (20 mL), 480 mg
5

Ru(PPh3)3Cl2 (0.50 mmol) was added and the mixture was stirred for 12 h at room
temperature. During this time the color of the solution slowly changed from red to green. The
resulting green solution was evaporated to dryness. The green compound was then purified
by rapid crystallization from dichloromethane and hexane solvent mixture. Suitable crystals
were grown by the slow diffusion of a dichloromethane solution of the complex into hexane.
Yield: 250 mg (70%, based on Ru). IR (KBr, cm−1): 1401 (νN=N). Anal. Calc. for
C35H28Cl2N5PRu (%): C, 58.26; H, 3.91; N, 9.71. Found: C, 57.56; H, 3.97; N, 9.45. 1H NMR
(500 MHz, CDCl3): δ (ppm) 8.35-8.33 (m, 4H), 8.14 (d, J=8 Hz, 2H), 7.83 (t, J=8 Hz, 1H),
7.57-7.54 (m, 2H), 7.44 (t, J=8 Hz, 4H) 7.13-7.10 (m, 3H), 7.00 (t, J=8.5 Hz, 6H), 6.85-6.81
(m, 6H). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) 169.0, 155.1, 132.9, 131.8, 131.2,
129.9, 129.1, 128.7, 127.9, 126.9, 122.1. 31P{1H} NMR (162 MHz, CDCl3): δ 20.63 ppm.
2.5 Procedure for catalytic alcohol oxidation reaction
The catalytic oxidations of primary alcohols to aldehydes and secondary alcohols to
the corresponding ketones were performed using the complex 1 as catalyst in presence of a
base following a general procedure. In a round-bottom flask, 1 mmol of substrate, 0.005
mmol of catalyst (3.6 mg) and 0.010 mmol of tBuOK (1.1 mg) were mixed in 10 mL toluene.
The reaction mixture was heated at 50 oC in aerobic condition with continuous stirring for 6 h
and then the solvent was evaporated with a rotary evaporator under reduced pressure. The
crude product, thus obtained, was purified on preparative silica gel GF-254 TLC plate using
ethyl acetate: hexane (1:10) solvent mixture as eluent. The oxidized products were
characterized by NMR spectroscopy (See Fig. S4-S32).
2.6 Mechanistic analysis
1H NMR analysis of the reaction mixture: In a sealed NMR tube 0.05 mmol 4-chlorobenzyl
alcohol (7.1 mg) was mixed with 0.05 mmol of catalyst 1 (36.0 mg) and 0.01 mmol of tBuOK
(1.2 mg) in 0.8 ml THF-d8. The colour of the solution instantly changed from green to brown
6

1
and the resulting solution was subjected to H NMR spectral analysis. Two distinct NMR
resonances of Ru-H and -CHO group of 4-chlorobenzaldehyde were observed at δ= - 3.51
and 9.95 ppm respectively (Fig. S36).
Isotope labelling experiment: A mixture of 0.25 mmol cyclohexanol (25.0 mg), 0.25 mmol
t
catalyst 1 (180.0 mg), 0.05 mmol BuOK (5.6 mg) in 10 ml dry and deoxygenated toluene
was taken in a sealed schlenk tube. The stirring was continued for 30 min. A portion of the
solution was collected and subjected to IR spectral analysis which showed two distinct
stretching at 2881 and 2935 cm-1 characteristic of N-H doublets corresponding [17, 18] to
the hydrazo intermediate (Fig. S37a). A similar reaction was also carried out in presence of
cyclohexanol-d12. Two distinct IR bands appeared due to N-D stretching at 2108 and 2206
cm-1 [17, 18] (Fig. S37b).
Detection of hydrogen peroxide during the catalytic cycle: In the catalytic alcohol oxidation
reaction apart from the aldehydes or ketone, H2O2 is formed as a by-product which was
characterized by UV-vis spectroscopy [17, 18] through scrutinizing the characteristic peak of
-
I3 at λmax= 350 nm; ε = 26000 M-1 cm-1 upon reaction with KI (Fig. S38). Following a
reported procedure, 1 mmol of 4-chlorobenzylalcohol (142.5 mg) in 10 mL of dry toluene
t
was mixed with 0.005 mmol catalyst 1 (3.6 mg), 0.010 mmol BuOK (1.1 mg) in a round
bottom flask and the mixture was stirred at room temperature for 3h. To the reaction mixture,
an equal volume of water was then added and the aqueous soluble part was extracted using a
separating funnel. The aqueous layer was then acidified with dilute H2SO4 to pH= 2 to stop
further oxidation. To it, 1 mL of a 10% solution of KI and three drops of a 3% solution of
ammonium molybdate were added. Hydrogen peroxide oxidizes I- to I2 which reacts with an
-
excess of I- to form I3 . The chemical reactions are as follows: (i) H2O2 + 2I− + 2H+→ 2H2O
–
–
+I2, (ii) I2 (aqueous) + I−→ I3 (Fig. S38). The absorbance for I3 increases with increasing
concentrations of acid. Ammonium molybdate catalyzes the oxidation of I− to I2.
7

2.7 Computational details
All the calculations were performed using Gaussian 09 program package [26]. 6-
31G* (C, H ,N), 6-311G*(P, Cl), SDD (Ru) level of theory [27] were employed for the
structural optimization and vibrational analysis. Spin restricted approaches were employed
for ground state singlet state. Singlet excitation energies based on the solvent-phase
(acetonitrile) optimized geometry of the complex was computed using Time Dependent
Density Functional Theory (TDDFT) formalism [28, 29] in acetonitrile solvent using
Conductor-like Polarisable Continuum Model (CPCM) [29-31].
3. Results and Discussion
Synthesis and characterization of the Ru-complex (1): In an attempt to engender an efficient
catalyst for aerobic alcohol oxidation reaction in absence of any co-catalyst as additive, we
have synthesized a new ruthenium complex bearing a bis-azoaromatic ligand (L). The ligand,
L was chosen because of its acknowledged ability [15, 17, 18, 32] to participate
synergistically in redox events during alcohol oxidation reaction. The stoichiometric
chemical reaction between Ru(PPh3)3Cl2 with equimolar 2,6-bis(phenylazo)pyridine ligand
(L) in toluene under stirring for 12 h afforded the catalyst RuL(PPh3)Cl2 (1) (Fig. 1). The
Electrospray Ionization Mass Spectrum (ESI-MS) of the isolated complex (1) displayed an
intense peak at m/z 686 amu, which corroborates with the formulation of [RuL(PPh3)Cl]+
(Fig. S1). The complex (1) is diamagnetic as evidenced by room temperature magnetic
moment measurement. It also exhibited a resolved aromatic proton resonances of ligand
moiety of complex 1 in the range of δ = 8.34 – 7.41 ppm (Fig. S2a). The fifteen aromatic
proton resonances of PPh3 group appeared between δ = 7.13 – 6.81 ppm (Fig. S2a) whereas
31
in its P {1H} NMR, the signal for PPh3 ligand appeared at δ = 20.63 ppm (with respect to
free PPh3 as standard, δ = - 4.80 ppm) (Fig. S2c).
8

The isolated complex, 1 is highly crystalline and suitable crystals were obtained by
slow diffusion of its dichloromethane solution into hexane. Single crystal X-ray diffraction
analysis of 1 revealed its distorted octahedral geometry where the central ruthenium ion is
coordinated to a tridentate N,N,N donor ligand in a meridional fashion; the remaining sites
are occupied by two chloride ions in cis position to each other and one PPh3 group (Fig. 1).
The dN-N bond lengths in the isolated complex are 1.313(6) and 1.298(5) Å which are
considerably longer than that of the free ligand (dN-N = 1.225(8) and 1.241(8) Å). Notably, the
ligand L is a strong π-acceptor and the elongation of dN-N bond lengths is due to the metal to
ligand [Ru(II)dπ – π*(azo)] back bonding which is common in the cases of ruthenium(II)
complexes of azo aromatic ligands [23, 33-36]. In the IR spectrum, the N=N stretching
frequency appeared at 1401 cm-1 which also points to the elongation of the azo-chromophore
of the coordinated ligand. The crystallographic details along with the selected bond
parameters were collected in supplementary materials (Table S1-S2).
Fig. 1. (a) Synthesis of complex 1, (b) ORTEP view of complex 1 with 30% probability
ellipsoid. Hydrogen atoms are omitted for clarity.
Cyclic voltammogram of the complex 1 exhibits three single electron waves at 1.19, -
0.27 and -0.51 V vs. Ag/AgCl in acetonitrile (Fig. 2a). The oxidative wave is reversible while
9

the reductive waves are quasi-reversible. To elucidate the locus of plausible electron transfer
we have performed density functional theory (DFT) calculations. The highest occupied
molecular orbital (HOMO) of the isolated complex 1 is localized on the ruthenium centre
(Fig. 2b) whereas the lowest unoccupied orbital (LUMO) is localized on the azo-aromatic
ligand backbone (Fig. 2c). Thus the reversible oxidation is assumed to be metal centered i.e.
Ru(II) → Ru(III), whereas the irreversible waves at cathodic potentials are ascribed to the
reduction of the coordinated ligand, L.
(a)
(b)
(c)
Fig. 2. (a) Cyclic voltammogram of complex 1. Contour plots of complex 1: (b) HOMO (c)
LUMO
Catalysis: To explore the catalytic efficiency, 4-methylbenzyl alcohol was chosen as the
model substrate. In a typical catalytic reaction, 1 mmol of 4-methylbenzyl alcohol was
subjected to react with 0.5 mol% of catalyst 1 (5 x 10-3 mmol) and 1.0 mol% of tBuOK (1 x
o
10-2 mmol) in 10 mL toluene at 50 C for 6 h. The corresponding oxidized product i.e. 4-
methylbenzaldehyde was purified by preparative thin-layer chromatography (TLC) technique
(UV-active silica (GF-254)) using 1:10 ethyl acetate and hexane solvent mixture as eluent.
Initial optimization under argon atmosphere has led to the trace amount of conversion of the
alcohol. Even increase of catalyst loading up to 10 mol% did not improve the yield to a
reasonable amount (Table 1, entry 1). However, a significant improvement of the catalytic
efficiency was observed under aerobic conditions indicating a decisive role of air in the
oxidation process (Table 1, entry 2). Additionally systematic monitoring of solvents revealed
that toluene is the suitable solvent for the desired transformation (Table 1, entry 2). Finally,
10

an efficient catalytic condition was optimized with a catalyst loading of 0.5 mol% under
aerobic condition, resulting in the formation of 91% of 4-methylbenzaldehyde after 6 h in
toluene solvent (Table 1, entry 7).
Table 1: Optimization by varying reaction atmosphere, catalyst loading and screening of
solvents for catalytic alcohol oxidation of 4-methylbenzyl alcohola.
Entry
Catalyst-1
Atmosphere
Solvent
Isolated Yield
(mol %)
(%)
trace
95
1
2
10
10
10
10
5
argon
air
Toluene
Toluene
Acetonitrile
THF
3
air
72
4
air
84
5
air
Toluene
Toluene
Toluene
94
6
2
air
92
7
8b
0.5
0.5
air
91
air
Toluene
62
aReaction conditions: 4-methylbenzyl alcohol (1 mmol), BuOK (1.0 mol%), 10 mL solvent,
t
6 h, heated at 50 oC, bbase free condition.
It is worth mentioning that Ru(II) complexes especially several Ru(II)-PPh3
complexes are known [37] for their potential ability towards alcohol oxidation reaction
(Table 2). In this context, a comparison between various Ru-precursors with the catalyst 1 is
noteworthy to establish the activities as well as participation of ligand redox [9] in the
reaction cycle. Accordingly several Ru (0, +2 and +3) metal precursors were screened and
their catalytic efficiencies are compared with that of complex 1. It was found that our
catalyst, 1 is superior over the chosen Ru(0), Ru(III), Ru(II)-PPh3 and Ru-H precursors under
identical reaction conditions (Table 2).
Table 2: Scrutinizing various ruthenium complexes for catalytic oxidation of 4-methylbenzyl
alcohola.
Isolated yield (%)
Entry
Catalyst
Ru3(CO)12
time (h) Temp. (oC)
1
2
3
4
5
12
12
12
12
12
90
90
90
90
90
trace
trace
28
RuCl3.3H2O
RuCl2(PPh3)3
RuH(CO)Cl(PPh3)3
RuHCl(PPh3)3
39
31
11

6
RuL(PPh3)Cl2 [1]
6
50
91
t
aReaction conditions: 4-methylbenzyl alcohol (1.0 mmol), Ru-catalyst (0.5 mol%), BuOK (1
mol%), 10 mL toluene, in air.
Afterwards, we have scrutinized the scope of this protocol using a variety of primary
alcohols (Table 3). Benzyl alcohols and its various substituted ones selectively yielded their
corresponding aldehydes in high isolated yields. We have examined ortho-, meta- and para-
substituted methoxybenzyl alcohols which successfully afforded the corresponding
benzaldehydes with > 70% yield. It is noteworthy that o-methoxy benzyl alcohol has a
tendency [37] to form chelate ring with the metal centre. However, in our case 74% of the
desired aldehyde was observed after 6 h. The catalytic reaction is also suitable for the
alcohols having unsaturated double bond e.g. the catalyst 1 oxidizes cinnamyl alcohol to
cinnamaldehyde with 80% isolated yield without interference intra-molecular hydrogen
transfer. This observation undoubtedly highlights the outstanding group tolerance. Oxidation
of different sterically congested poly-aryl alcohols (1-naphthylmethanol, 9-
anthracenemethanol or 1-pyrenemethanol) were also achievable (Table 3, entries 3m-3o),
which are known staring materials of many useful compounds [38, 39]. The reaction is also
operative towards the catalytic oxidation of diols like 1,3-benzenedimethanol (Table 3. entry
3l). Catalytic activity of 1 was then explored towards heterocyclic primary alcohols which are
only few [37] in the literature. The catalyst brings about the conversion of 2-
pyridinemethanol and 2,6-pyridinedimethanol to the respective aldehydes (Table 3, entries
3q-3r) in moderate yields (isolated). The reason behind this comparative low yields of these
may be due to the partial coordination [37] of the hetero atoms (here, N) to the Ru-center.
12

Table 3. Aerobic oxidation of primary alcohols by catalyst 1a
t
aReaction conditions: alcohol (1 mmol), catalyst (0.5 mol%), BuOK (1.0 mol%) toluene (10
mL), heating at 50 oC for 6 h. Reported yields refer to isolated compounds.
bhere catalyst (1.0 mol%), tBuOK (5.0 mol%).
cproduct characterized by NMR spectroscopy from reaction mixture.
Subsequently, we screened various 2o alcohols towards aerobic oxidation reaction by
catalyst 1. A large substrate scope was observed in this class of alcohols; both aromatic and
alicyclic ones underwent smooth oxidation to the respective ketones (Table 4, entries 5a-5k)
in good yields.
13

Table 4. Aerobic oxidation of secondary alcohols by catalyst 1a
t
aReaction conditions: alcohol (1 mmol), catalyst (0.5 mol%), BuOK (1.0 mol%) toluene (10
mL), heating at 50 oC for 6 h. Reported yields refer to isolated compounds.
bhere catalyst (1.0 mol%), tBuOK (5.0 mol%).
Moreover, primary aromatic diol like 1,2-benzenedimethanol (6) undergoes oxidation
freely to result phthalide (7) by aerobic lactonization process in 88% yield (isolated). This
phthalide is a lactone that serves the core structure for various useful chemicals like dyes,
natural oil (e.g. butylphthalide) etc [40, 41].
Scheme 1. Oxidative lactonization of 1,2-benzenedimethanol to phthalide
After enlightening the high efficiency of the catalyst 1, we have performed a series of
controlled reactions including isotope labeling experiments to elucidate the plausible reaction
pathway. The first step is assumed to be the replacement of chloride ion by the alkoxide ion
t
in the presence of BuOK. Such substitution is well documented [17, 18, 42-44] in the
14

literature. To prove the formation of the alkoxide-adduct, a mixture of equimolar amount of
t
o
catalyst 1 and BuOK in toluene was stirred at 50 C for 30 min. The color of the solution
changed from green to brown and this brown solution was subjected to IR spectral analysis.
A sharp band at 727 cm-1 (Fig. S34) corresponding to Ru-O stretching frequency points
towards the formation of the Ru-alkoxide intermediate [45]. In the following step, we have
strategically scrutinized the nature of migratory hydrogen atom viz. (a) hydrogen atom
transfer or (b) hydride ion transfer as this is a crucial step for alcohol oxidation reaction. At
first, the catalytic oxidations of various radical clock substrates viz. cyclopropyl methanol,
cyclopropyl phenylmethanol were performed using 1 as the catalyst and the reactions
exclusively yielded cyclopropanecarboxaldehyde (entry 3p) and cyclopropyl phenyl ketone
(entry 5i) respectively (Fig. S35 and S29). The non-existence of any ring opening product
indicates [37] that the second pathway i.e. the hydride ion transfer is operative for the
alcohol oxidation reactions. In addition to the previous experiments, we have also tried to
trap the metal-hydride intermediate using NMR spectroscopy. In a NMR tube, when catalyst
t
1 is mixed with equimolar 4-chlorobenzyl alcohol and 0.2 equivalent BuOK in THF-d8
1
solvent, the color of the solution instantaneously changed from green to brown. In its H
NMR spectrum, a characteristic Ru-H resonance [46] is observed at δ = -3.51 ppm along with
a distinctive ―CHO resonance of 4-chlorobenzaldehyde at δ = 9.95 ppm (Fig. S36). This
observation implies that the reaction proceeds through a hydride transfer to the Ru(II)-centre
from the coordinated alkoxide ion. Interestingly, when a separate chemical reaction
t
containing equimolar cyclohexanol and catalyst 1 (in the presence of 0.2 equivalent BuOK)
was subjected to IR spectral analysis after 30 min, two characteristic peaks [17, 18] of N-H
stretching frequencies at 2881 and 2935 cm-1 were observed. To confirm the identity of N-H
stretching vibrations an identical reaction mixture using cyclohexanol- d12 as substrate was
subjected for IR spectral analysis. Notably, the N-D peaks [17, 18] were observed at 2108
15

and 2206 cm-1 indicating the formation of hydrazo intermediate (Fig. S37). Based on the
above experimental evidences we propose that the reaction proceeds through hydride
migration from coordinated alkoxide to the Ru(II) centre. Thereafter the Ru-H intermediate is
converted to a Ru-hydrazo intermediate via a hydrogen-walking mechanism as proposed by
Grützmacher [10] and Hall [22] in the Ru-diazadiene complex. For comparison, we note here
that the electroprotic storage ability of azo-aromatic ligand is expected to be superior over the
diazadiene ligand due to lower lying π* (acceptor) orbital of the former [15]. Thus hydrogen-
walking in the present case sets in instantaneously causing transient existence of the Ru-H
intermediate.
Lastly the catalyst is regenerated by the aerobic oxidation of hydrazo intermediate
with the formation of H2O2. The generation of H2O2 during the catalytic cycle was confirmed
-
by its chemical detection [17, 18] using I3 as indicator and ammonium molybdate as catalyst
(Fig. S38). Detailed procedure is collected in the experimental section. Based on the above
controlled studies a probable mechanistic pathway is proposed in Scheme 2.
16

Scheme 2. Proposed catalytic cycle
In conclusion, we have designed a simple and sustainable Ru(II)-PPh3 complex
containing a bis azo-aromatic ligand for aerobic alcohol oxidation catalysis. The catalyst is
superior over many of the Ru-metal catalysts in different oxidation states and has the ability
to oxidize a wide variety of alcohols including that containing heterocyclic rings. Mechanistic
investigation revealed that a hydrogen-walking mechanism is operative under the catalytic
conditions where metal-ligand participated synergistically. Overall, the catalytic system
provides an alternative to customary approach for aerial oxidation of alcohol which is scarce
in the literature [37]. Mechanistic pathway may be further useful to generate more efficient
and substrate selective catalyst by tuning the coordinated ligand. Aerobic oxidations of other
17

liquid hydrogen carriers and various transfer hydrogenation reactions by similar systems with
better selectivity are now under active search.
Supplementary material
Supplementary Information contains crystallographic details, tables and figures of
experimental details and NMR spectra of aldehydes/ketones obtained. Crystallographic data
for catalyst 1 has been deposited with Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 1855693. Copies of the data can be obtained free of
charge on application to CCDC.
Acknowledgement
The research was supported by the department of science and technology (DST), India,
SR/S2/JCB-09/2011, EMR/2014/000502. S.G. sincerely thanks DST-SERB for J. C. Bose
fellowship. T.S. and R.P. are thankful to the Council of Scientific and Industrial Research
(CSIR), India for their fellowship support.
References
[1] S. Ramakrishnan, S. Chakraborty, W.W. Brennessel, C.E.D. Chidsey, W.D. Jones, Chem.
Sci., 7 (2016) 117-127.
[2] P.J. Bonitatibus, Jr., S. Chakraborty, M.D. Doherty, O. Siclovan, W.D. Jones, G.L.
Soloveichik, Proc. Natl. Acad. Sci. U. S. A., 112 (2015) 1687-1692.
[3] K. Sordakis, C. Tang, L.K. Vogt, H. Junge, P.J. Dyson, M. Beller, G. Laurenczy, Chem. Rev.
(Washington, DC, U. S.), 118 (2018) 372-433.
[4] C. Gunanathan, Y. Ben-David, D. Milstein, Science (Washington, DC, U. S.), 317 (2007)
790-792.
[5] W. Zhang, W. Lai, R. Cao, Chem. Rev. (Washington, DC, U. S.), 117 (2017) 3717-3797.
[6] C. Federsel, R. Jackstell, M. Beller, Angew. Chem., Int. Ed., 49 (2010) 6254-6257.
[7] T. Mallat, A. Baiker, Chem. Rev. (Washington, DC, U. S.), 104 (2004) 3037-3058.
[8] M. Pagliaro, S. Campestrini, R. Ciriminna, Chem. Soc. Rev., 34 (2005) 837-845.
[9] R.H. Crabtree, Chem. Rev. (Washington, DC, U. S.), 117 (2017) 9228-9246.
[10] R.E. Rodriguez-Lugo, M. Trincado, M. Vogt, F. Tewes, G. Santiso-Quinones, H.
Gruetzmacher, Nat. Chem., 5 (2013) 342-347.
[11] J.L. Boyer, J. Rochford, M.-K. Tsai, J.T. Muckerman, E. Fujita, Coord. Chem. Rev., 254
(2010) 309-330.
[12] S. Joy, T. Kramer, N.D. Paul, P. Banerjee, J.E. McGrady, S. Goswami, Inorg. Chem., 50
(2011) 9993-10004.
[13] A. Das, T.M. Scherer, A.D. Chowdhury, S.M. Mobin, W. Kaim, G.K. Lahiri, Inorg. Chem.,
51 (2012) 1675-1684.
18

[14] N. Van Damme, A.J. Lough, S.I. Gorelsky, M.T. Lemaire, Inorg. Chem., 52 (2013) 13021-
13028.
[15] P. Ghosh, S. Samanta, S.K. Roy, S. Demeshko, F. Meyer, S. Goswami, Inorg. Chem., 53
(2014) 4678-4686.
[16] I.E. Marko, P.R. Giles, M. Tsukazaki, S.M. Brown, C.J. Urch, Science (Washington, D. C.),
274 (1996) 2044-2046.
[17] D. Sengupta, R. Bhattacharjee, R. Pramanick, S.P. Rath, N. Saha Chowdhury, A. Datta, S.
Goswami, Inorg. Chem., 55 (2016) 9602-9610.
[18] R. Pramanick, R. Bhattacharjee, D. Sengupta, A. Datta, S. Goswami, Inorg. Chem., 57
(2018) 6816-6824.
[19] S.D. McCann, S.S. Stahl, J. Am. Chem. Soc., 138 (2016) 199-206.
[20] M. Hayashi, M. Shibuya, Y. Iwabuchi, J. Org. Chem., 77 (2012) 3005-3009.
[21] S.P. Rath, D. Sengupta, P. Ghosh, R. Bhattacharjee, M. Chakraborty, S. Samanta, A.
Datta, S. Goswami, Inorg. Chem., (2018) (DOI: 10.1021/acs.inorgchem.8b01558).
[22] H. Li, M.B. Hall, J. Am. Chem. Soc., 137 (2015) 12330-12342.
[23] S. Goswami, R. Mukherjee, A. Chakravorty, Inorg. Chem., 22 (1983) 2825-2832.
[24] G. Sheldrick, Acta Crystallographica Section C, 71 (2015) 3-8.
[25] G.M. Sheldrick, in, 1997.
[26] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G.
Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P.
Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A.
Montgomery Jr., J.E. Peralta, F. Ogliaro, M.J. Bearpark, J. Heyd, E.N. Brothers, K.N. Kudin,
V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A.P. Rendell, J.C. Burant, S.S.
Iyengar, J. Tomasi, M. Cossi, N. Rega, N.J. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken,
C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.
Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador,
J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski,
D.J. Fox, in, Gaussian, Inc., Wallingford, CT, USA, 2009.
[27] H.F. Schaefer, III, Editor, Modern Theoretical Chemistry, Vol. 4: Applications of
Electronic Structure Theory, Plenum, 1977.
[28] R. Bauernschmitt, R. Ahlrichs, Chem. Phys. Lett., 256 (1996) 454-464.
[29] M. Cossi, V. Barone, J. Chem. Phys., 115 (2001) 4708-4717.
[30] M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput. Chem., 24 (2003) 669-681.
[31] V. Barone, M. Cossi, J. Phys. Chem. A, 102 (1998) 1995-2001.
[32] M. Chakraborty, D. Sengupta, T. Saha, S. Goswami, J. Org. Chem., 83 (2018) 7771-7778.
[33] M.A. Ansari, A. Mandal, A. Paretzki, K. Beyer, J. Fiedler, W. Kaim, G.K. Lahiri, Inorg.
Chem., 55 (2016) 5655-5670.
[34] A. Das, P. Ghosh, S. Plebst, B. Schwederski, S.M. Mobin, W. Kaim, G.K. Lahiri, Inorg.
Chem., 54 (2015) 3376-3386.
[35] S. Chatterjee, S. Mandal, S. Joy, C.-H. Hung, S. Goswami, Inorg. Chim. Acta, 374 (2011)
366-372.
[36] S. Goswami, A.R. Chakravarty, A. Chakravorty, Inorg. Chem., 22 (1983) 602-609.
[37] R. Ray, S. Chandra, D. Maiti, G.K. Lahiri, Chem. - Eur. J., 22 (2016) 8814-8822.
[38] M. Christwardana, Y. Chung, Y. Kwon, NPG Asia Mater., 9 (2017) e386.
[39] F. Bizet, M. Ipuy, Y. Bernhard, V. Lioret, P. Winckler, C. Goze, J.-M. Perrier-Cornet, R.A.
Decreau, Bioorg. Med. Chem., 26 (2018) 413-420.
19

[40] X. Yang, Z. Zhang, Y. Zhao, W. Cui, J. Liu, Hebei Yike Daxue Xuebao, 34 (2013) 1545-
1548.
[41] Z. Liu, H. Zhang, D. Shen, X. Feng, R. Wang, J. Sun, in, Shijiazhuang Pharma. Group
Zhongqi Pharmaceutical Technology Shijiazhuang Co., Ltd., Peop. Rep. China . 2005, pp. 20
pp.
[42] M. Vogt, A. Nerush, M.A. Iron, G. Leitus, Y. Diskin-Posner, L.J.W. Shimon, Y. Ben-David,
D. Milstein, J. Am. Chem. Soc., 135 (2013) 17004-17018.
[43] K.M. Waldie, K.R. Flajslik, E. McLoughlin, C.E.D. Chidsey, R.M. Waymouth, J. Am. Chem.
Soc., 139 (2017) 738-748.
[44] W. Baratta, K. Siega, P. Rigo, Chem. - Eur. J., 13 (2007) 7479-7486.
[45] C.G. Barraclough, D.C. Bradley, J. Lewis, I.M. Thomas, J. Chem. Soc., (1961) 2601-2605.
[46] G. Jia, D.W. Meek, J.C. Gallucci, Organometallics, 9 (1990) 2549-2555.
20

Table of Contents (TOC) only:
The work demonstrates the role of a Ru(II) complex of 2,6-bis(phenylazo)pyridine ligand
containing two chloride and one triphenyl phosphine coligand as an effective catalyst in
aerobic alcohol oxidation reaction. In the catalytic cycle, initial hydride transfer followed by
hydrogen-walking to the coordinated azo functional group forms the hydrazo intermediate.
The hydrazo function of the intermediate reproduces the catalyst by aerial oxidation.
21

Highlights:
 Ruthenium complex of a pincer bis azo-aromatic ligand.
 Metal-ligand cooperativity in catalytic reaction.
 Aerobic alcohol oxidation.
22