Synthesis and characterisation of cycloruthenated benzhydrazone complexes: Catalytic application to selective oxidative cleavage of olefins to aldehydes

Article abstract of DOI:10.1039/c6ra19044h

A simple and convenient method to synthesise new air-stable cyclometalated ruthenium(ii) complexes from the reaction of p-substituted acetophenone benzhydrazide ligands and [RuHCl(CO)(AsPh₃)₃] has been described. The complexes have been characterised by various methods, including elemental analysis, FT-IR, NMR (¹H and ¹³C) and UV-vis. The molecular structures of complexes 3 and 4 have been determined by singlecrystal X-ray diffraction analysis which indicates the tridentate coordination mode of the ligand and the presence of a distorted octahedral geometry around the ruthenium ion. The complexes have been found to be a highly active catalytic system in the selective oxidative cleavage of a wide range of alkenes and alkynes in the presence of NaIO₄ as an oxidant. Notably, the desired aldehyde products were obtained with high conversions and yields using 0.5 mol% catalyst loading within 30 min at room temperature. Furthermore, the influence of reaction parameters such as solvents, oxidants, temperature and catalyst loading was also investigated.

Full text of DOI:10.1039/c6ra19044h

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Synthesis and characterisation of cycloruthenated
benzhydrazone complexes: catalytic application to
selective oxidative cleavage of olefins to
aldehydes†
Cite this: RSC Adv., 2016, 6, 97107
a
a
Thimma Sambamoorthy Manikandan, Rengan Ramesh and David Semerilb
*
A simple and convenient method to synthesise new air-stable cyclometalated ruthenium(II) complexes from
the reaction of p-substituted acetophenone benzhydrazide ligands and [RuHCl(CO)(AsPh3)3] has been
described. The complexes have been characterised by various methods, including elemental analysis, FT-
IR, NMR (1H and 13C) and UV-vis. The molecular structures of complexes 3 and 4 have been determined
by singlecrystal X-ray diffraction analysis which indicates the tridentate coordination mode of the ligand
and the presence of a distorted octahedral geometry around the ruthenium ion. The complexes have
been found to be a highly active catalytic system in the selective oxidative cleavage of a wide range of
alkenes and alkynes in the presence of NaIO4 as an oxidant. Notably, the desired aldehyde products
were obtained with high conversions and yields using 0.5 mol% catalyst loading within 30 min at room
temperature. Furthermore, the influence of reaction parameters such as solvents, oxidants, temperature
and catalyst loading was also investigated.
Received 27th July 2016
Accepted 28th September 2016
DOI: 10.1039/c6ra19044h
www.rsc.org/advances
bond to form aldehydes (Scheme 1), whereas over-oxidation of
aldehydes may produce carboxylic acids. Among the various
Introduction
oxidants used, such as H2O2, O2, oxone, PhI(OAc)2, TBHP etc.,
the choice of NaIO4 as an oxidant due to it being easily soluble
in water, the cheapest one, and easy to handle leads to extensive
applications in most of the fundamental organic trans-
formations.3 Various methodologies have been developed with
great interest for the oxidative cleavage reaction. In earlier years,
this reaction was achieved by ozonolysis with oxidative work-up
which was used widely in industry but it has some drawbacks
like being hazardous, safety risks with explosions, the required
expensive equipment,4 and a large amount of waste being
generated.
Oxidative cleavage of carbon–carbon double bonds has
emanated as one of the most challenging and attractive
processes and has tremendous potential in synthetic applica-
tions of modern organic chemistry,1 facilitating the formation
of carbonyl compounds that widely occur in natural and
synthetic bioactive molecules.2 The oxidative cleavage reaction
of C]C double bonds consists of cleavage of the double bond
associated with the insertion of oxygen into the C]C double
As a consequence of these drawbacks, research efforts have
been directed towards the development of transition metal-
catalyzed oxidative cleavage reactions of olens. In past
years, the scission of the C]C double bond in alkenes, which
is oxidised by various transition metal complexes such as
osmium,5,6 cobalt,7,8 gold,9 palladium,10 copper,11 and iron12–17
catalysts with different oxidants, has been reported. However,
it is still required to resolve the drawbacks such as the low
selectivity, low catalytic activity, harm to the environment,
and severe reaction conditions. These reactions were per-
formed using the simple and high-valent oxo-ruthenium
metal compounds such as RuCl3$nH2O,18 RuO2, and RuO4,19
which have the ability to cleave olen double bonds to
produce aldehydes and carboxylic acids using various co-
oxidants.
Scheme 1 General scheme of transition metal-catalyzed oxidative
cleavage of olefins.
aCentre for Organometallic Chemistry, School of Chemistry, Bharathidasan University,
Tiruchirappalli – 620 024, Tamilnadu, India. E-mail: rrameshbdu@gmail.com; Fax:
+91-431-2407045
bLaboratoire de Chimie Inorganique et Catalyse, Institut de Chimie, Universite de
Strasbourg, UMR 7177, CNRS, France
† Electronic supplementary information (ESI) available: 1H and 13C NMR of the
complexes, crystallographic tables, selected bond lengths, bond angles,
oxidative cleavage of olenic product determination by GC-MS chromatography
and NMR. CCDC no. for complexes 3 and 4: 1453377 and 1452124. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c6ra19044h
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Despite the signicant developments on oxidative cleavage important classes of Schiff base ligands as well as nitrogen-
of olen reactions, the substrate scope and catalyst loadings are oxygen donor ligands which exhibit amide–imidol tautom-
still limited. Therefore, revealing new catalysts with less erism with a unique coordination mode. Benzhydrazone
expensive ruthenium complexes is highly desirable. ligands are coordinated to metal ions usually in a bidentate
Ruthenium-based oxidation catalysis has received much atten- fashion (N,O donor) forming a ve-membered chelate ring.
tion because of the well-developed ruthenium chemistry and However, we explore the tridentate nature of these ligands via
the easy access of various redox states of ruthenium. The use of C–H activation by transition metal ions.27,28
catalytic amounts of ruthenium complexes in the oxidative
In continuation of our ongoing research on the synthesis,
cleavage of olenic double bonds improves the selectivity characterization and catalytic applications of ruthenium(II)
towards the cleavage products while preventing side reactions complexes29,30 and in view of the interesting coordination
such as epoxidation, dihydroxylation, and allylic oxidation. In modes of benzhydrazone ligands, we herein portray the
particular, highly selective oxidative cleavage of styrene by O2 synthesis of four air-stable cyclometalated ruthenium(II)
has been achieved with catalytic amounts of [RuCl(DPP)2]ClO4, complexes via C–H activation of benzhydrazone ligands. The
or trans-[RuCl2(DPP)2] (DPP
¼
1,3-bis(diphenylphosphino) molecular structures of the two complexes were determined by
propane) under mild conditions.20 Peris et al. documented the single-crystal X-ray crystallography. Furthermore, we have
catalytic performance of a Ru(CNC)(CO)Br2 (A) complex towards explored the catalytic efficiency of the Ru(II) benzhydrazone
the oxidative cleavage of olens with 1 mol% catalyst loading complexes towards the oxidative cleavage of olens. To the best
for 24 h.21 Shoair and co-workers described the oxidative of our knowledge, this is the rst study on the catalytic utility of
cleavage of alkenes to acids with IO(OH)5 as the co-oxidant Ru(II) benzhydrazone complexes as catalysts in the selective
using cis-[RuCl2(bipy)2]$2H2O as a catalyst.22 The oxidative oxidative cleavage reaction of olens. The assessment of cata-
cleavage of olens by [RuII(dmp)2(H2O)2]2+ (B) (dmp ¼ 2,9- lysts and optimization of the reaction for the cleavage of a wide
dimethylphenanthroline) using H2O2 as an oxidant in CH3CN at range of substrates including aromatic, aliphatic, heterocyclic,
55 C was developed by Neumann and co-workers.23 Recently, cyclic alkenes, and alkynes have been carried out.
ꢀ
Friedrich et al. reported new (h5-cyclopentadienyl) dicarbonyl
ruthenium(II) amine complexes24a (C) and [(h6-C6H6)RuCl(C5-
Results and discussion
Synthesis of cyclometalated Ru(II) benzhydrazone complexes
H4N-2-C]H]N-R)] PF6 as catalysts24b (D) (2.5 mol%) for styrene
oxidation using CH3CN/H2O as a solvent system at 60 ꢀC.
Condensation of acetophenone with p-substituted benzhy-
drazide in methanol as a solvent afforded ligands 1–4 in good
yields as described in the literature.31 Furthermore, the reaction
of an equimolar amount of p-substituted acetophenone benz-
hydrazone ligands with the ruthenium(II) precursor,
[RuHCl(CO)(AsPh3)3], in reuxing benzene medium in the
presence of triethylamine as a base gave cyclometalated ruth-
enium(II) complexes of the general formula [Ru(L)(CO)(AsPh3)2],
as depicted in Scheme 3. It was noticed that the ligand behaves
as a dianionic tridentate donor and the oxidation state of
ruthenium remained unchanged during the formation of the
complexes. The complexes are generally soluble in polar
solvents such as chloroform, dichloromethane, acetonitrile,
dimethyl sulfoxide etc. The elemental analysis results for the
complexes conrm their composition. Unambiguously, spectral
data of FT-IR, NMR, UV-vis and single-crystal X-ray diffraction
grant evidence for the probable mode of coordination of the
ligand to the ruthenium ion and also provide the geometry of
the complexes.
Although several ruthenium complexes have been shown to
promote this transformation, a new catalyst is still required to
rectify the drawbacks such as high temperature, long reaction
time and high catalyst loadings (Scheme 2).
Interestingly, hydrazone-based transition metal complexes
have signicant features of their synthesis, chelating ability,
structural exibility, electrical and magnetic properties, as well
as potential biological activities.25,26 These characteristics of
hydrazone are considered as the key to the success of the
versatility of hydrazone ligands. Moreover, it is one of the most
Scheme 2 Reported ruthenium(II) catalysts for oxidative cleavage of Scheme 3 Synthesis of cyclometalated Ru(II) carbonyl benzhydrazone
olefin reactions.
complexes.
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Spectral characterization
Single-crystal structure determination
The FT-IR spectra of the ligands and the corresponding ruth- The solid-state structures of cyclometalated ruthenium benz-
enium(II) complexes provided signicant information about the hydrazone complexes 3 and 4 were resolved by single-crystal X-
metal–ligand bonding. The FT-IR spectra of the free ligands ray diffraction. Details on crystallographic data with renement
showed a broad band in the region 3173–3185 cmꢁ1 which parameters, selected bond lengths, and bond angles are gath-
corresponds to nN–H. The ligands also display nC]N and nC]O ered in the ESI (Tables S1–S3†). Both complexes 3 and 4 belong
absorptions in the region of 1610–1639 cmꢁ1 and 1543–1568 to the monoclinic system with the space group P21/n, which
cmꢁ1 respectively, which indicate that the ligands exist in the contains four molecules in the unit cell. The ORTEP view of the
amide form in the solid state. A noteworthy feature here is the complexes is shown in Fig. 1 and 2, and it clearly indicates that
(C]N)imine stretching frequencies for the complexes shied to the benzhydrazone ligand coordinates in a tridentate manner to
lower frequencies (1586–1607 cmꢁ1) when compared to the the ruthenium ion via the aryl carbon, the imine nitrogen and
ligand, suggesting coordination of imine nitrogen to the the imidolate oxygen forming two ve-membered chelate rings.
ruthenium centre. The nC]O absorption was not observed in the In addition, the two triphenylarsine ligands and one carbonyl
complexes indicating that the ligands underwent tautomeriza- group (trans to the imine nitrogen) are coordinated to the
tion and subsequent coordination of the imidolate form during ruthenium centre. In complex 3, the Ru–C1(aryl) bond distance is
complexation. This is further supported by the appearance of almost similar to other reported cyclometalated Ru(II)
new bands in the range 1240–1253 cmꢁ1 which may be attrib- complexes.34 The As1–Ru–As2 angle in complex 3 is 176.34ꢀ,
uted to the C–O band.31 A strong absorption band appeared at revealing that the two AsPh3 ligands are almost vertically situ-
1931–1948 cmꢁ1 and was attributed to the terminally coordi- ated at the two sides of the ligand plane (trans to each other).
nated carbonyl group, and this band shied by 25–53 cmꢁ1 to The ruthenium(II) ion is therefore sitting in a core RuCNOAs2
a higher frequency than that in the precursor complexes. In coordination environment, which is distorted octahedral in
addition, other characteristic bands due to ruthenium-bound nature as reected in all the bond parameters around ruthe-
triphenylarsine around 536, 693, 726 and 1433 cmꢁ1 are also nium. It was observed that complex 4 also adopts a similar
present in the spectra of all the complexes.32
geometry as that in complex 3 with slight changes in the bond
NMR spectroscopy techniques were employed to probe the angles and bond lengths. The X-ray determinations conrm the
bonding arrangement between the ligand and metal ion. In the structure proposed on the basis of spectral data, consistent with
spectra of all the complexes, the multiplet observed at around metal bivalency and the tridentate nature of the ligands.
7.7–6.6 ppm is assigned to the aromatic protons of the phenyl
group of triphenylarsine and benzhydrazone ligands. In addi-
tion, a sharp singlet was observed between 2.3 ppm and 2.5 ppm
due to methyl protons of the azomethine group. The absence of
–NH proton signals (9.1–8.9 ppm) in the complexes supports
Catalytic oxidative cleavage of olens
Our own interest has been centred on exploring the new type of
organic transformations catalysed by ruthenium complexes.
Inspired by the effective catalytic performance of various
enolization, and establishes that the imidolate oxygen is coor-
transformations using ruthenium(II) complexes, we are inter-
dinated to the Ru(II) ion. Furthermore, the spectrum of complex
ested in studying the catalytic transformation of olens into
3 shows a sharp singlet at 3.8 ppm and corresponds to the
aldehydes by employing ruthenium catalysts under milder
methoxy signal of the benzhydrazone ligand (ESI, Fig. S1–S4†).
13C NMR of the Ru(II) complexes showed resonance in the ex-
reaction conditions. These ruthenium(II) complexes have the
pected region (ESI, Fig. S4–S8†). The complexes (142 ppm)
revealed a downeld shi of the imine carbon (–C]N–) relative
to the free ligands (145 ppm) indicating coordination of the
imine nitrogen to the metal centre. Also, the signal assigned to
the imidolate carbon (–NH–C]O), which moves upeld from
169 ppm in the free ligand to 164 ppm in the Ru(II) complexes,
results from the reduced bond order (–N]C–O) upon
coordination.
The absorption spectra of all the complexes showed three
bands in the region 240–465 nm. The complexes exhibit ligand-
based p–p* and n–p* bands at 243 nm and 271 nm and also
relatively intense bands in the region of 453–465 nm assigned to
dp–p* MLCT transitions. Notably, there is no variation in the
energy of the MLCT band suggesting that the energy gap
between the metal–dp and the ligand–p* levels remains
constant even with variation of the substituent in the
complexes. The electronic spectra of all the complexes are
consistent with an octahedral geometry, and also similar to
other octahedral ruthenium(II) complexes.33
Fig. 1 The ORTEP diagram of complex 3 with 30% probability; all H
atoms and one CHCl3 solvent molecule are omitted for clarity.
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H2O with NaIO4 has the ability to produce the maximum
conversion of styrene to benzaldehyde (96%) even at room
temperature as compared to other solvent systems.
We continued our investigation on the reaction to nd out
a suitable oxidant to improve the conversion. Accordingly,
t
various oxidants such as NaIO4, H2O2, m-CPBA, and BuOOH
with the CH3CN/EtOAc/H2O system were used under identical
conditions (styrene (0.2 mmol), complex 1 (1 mol%), oxidant,
CH3CN/EtOAc/H2O (5 mL)). Screening of oxidants indicated
that NaIO4 has the best oxidising ability towards oxidative
cleavage reactions (Table 1). A striking difference in catalyst
performance was observed using NaIO4 compared to that with
other oxidants, where the formation of benzaldehyde was
decreased even with high temperature (Table 1, entries 8–13).
Catalyst screening is essential when embarking on a new
type of catalytic organic transformation. Encouraged by these
results, next we turned our attention to investigate the effects of
the substituent in the complexes using the catalyst loading of
about 1 mol% under the optimized conditions. To this purpose,
we have studied the catalytic activities of all the complexes 1–4
in the oxidative cleavage of the carbon–carbon double bond in
olens (Table 2). All the complexes showed efficient catalytic
performance and selectivity towards the oxidative cleavage of
olen reactions. It was observed that complex 3 shows a rela-
tively higher conversion (100%) and higher yields of benzalde-
hyde (98%) among the four and this may be due to the presence
of an electron-donating substituent (–OCH3).24a Based on this,
complex 3 was selected as the model catalyst for further inves-
tigation with a wide range of alkenes.
With suitable catalysts and solvents established, we sought
to further prove the catalytic efficiency of catalyst 3. A low
catalyst loading (1.0–0.05 mol%) test was carried out under
identical conditions (Table 3). The catalyst loading tests of
complex 3 revealed the best performance at 0.5 mol%. On
reducing the catalyst loading to 0.2, or 0.1 mol%, the reaction
still proceeds accompanied by a drop in the conversion. From
these observations, it was thus concluded that a catalyst loading
of 0.5 mol% of the ruthenium(II) catalyst is the best compromise
for the optimal reaction.
Fig. 2 The ORTEP diagram of complex 4 with 30% probability; all H
atoms and one CHCl3 solvent molecule are omitted for clarity.
potential to cleave the C–C double bond in alkenes, which
inspired us to implement this type of reaction. The reaction
conditions were optimized by utilizing styrene as a model
substrate in aqueous–organic media. The optimization process
led us toward the determination of the best reaction conditions
to analyse the substrate scope. In order to assess the best
reaction conditions, screening of the solvent, oxidants,
temperature, and catalyst loading has been performed.
Solvent choice is extremely important for the efficiency of the
reaction and during the isolation stages. To evaluate the inu-
ence of solvents in our catalytic system, several solvents were
used in the oxidative cleavage reaction of olens. At the outset,
we have chosen the reaction of styrene (0.2 mmol), complex 1 (1
mol%) as the catalyst precursor in the presence of various
solvents, and NaIO4 as an oxidant at room temperature, and the
results are summarized in Table 1. The efficiency of the Ru(II)
catalyst was proved by testing a control experiment without
a catalyst, and it furnished 6% product (Table 1, entry 7). It is
worth noting here that the presence of a catalyst was found to be
essential for efficient conversion of benzaldehyde. As seen in
Table 1, different solvent systems such as DCM/H2O, CH3CN/
H2O, and EtOAc/H2O were investigated, and nally the CH3CN/
ethyl acetate/H2O system was found to be the best choice of
solvent system and affords 100% conversion of styrene to
benzaldehyde within 30 min at room temperature. On the other
hand, DCM/H2O and CH3CN/H2O furnished moderate conver-
sions up to 76% at room temperature. The lowest conversion
(37%) was achieved with the EtOAc/H2O solvent system.
Furthermore, we have studied the inuence of reaction
temperature on the catalytic performance of the oxidative
cleavage reactionsꢀ. When increasing the temperature of the
reaction up to 80 C using DCM/H2O and CH3CN/H2O as the
solvent systems, the conversion of benzaldehyde was also
increased to 95% and 91% respectively (Table 1, entries 1–4).
When using EtOAc/H2O as the solvent system, moderate
conversion (55%) was achieved even at 77 ꢀC (Table 1, entries 5
and 6). It should be pointed out here that CH3CN/ethyl acetate/
Substrate scope for oxidative cleavage reaction of olens
The effective catalytic activity of the ruthenium(II) benzhy-
drazone complexes motivated us to evaluate the substrate scope
of various alkenes towards the oxidative cleavage reaction of
olens under the optimized conditions. The conversion of
aldehyde products was analysed by GC-MS and the corre-
sponding yields were determined by NMR (Table 4, ESI, Fig. S9–
S53†). The selectivity and catalytic activity of complex 3 for
oxidative cleavage of alkenes are summarized in Table 4. Most
of the substrates reached excellent conversions up to 100% with
higher TON and TOF values of 198 and 396 respectively, within
30 min. In particular, both 4-chlorostyrene and 4-uorostyrene
were oxidised to the corresponding aldehydes with 100%
conversion and 99% yield, and achieved the highest nal TON
value of 198 (Table 4, entries 2 and 3). The electron-rich styrene
derivative furnished less conversion of the corresponding
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Table 1 Effect of solvent, oxidant and temperaturea
Entry
Solvent
Oxidant
Temp. (ꢀC)
Conv.b (%)
Yieldd (%)
1
2
3
4
5
6
7
8
DCM/H2O
DCM/H2O
CH3CN/H2O
CH3CN/H2O
EtOAc/H2O
EtOAc/H2O
CH3CN/EtOAc/H2O
CH3CN/CCl4/H2O
CH3CN/EtOAc/H2O
CH3CN/EtOAc/H2O
CH3CN/EtOAc/H2O
CH3CN/EtOAc/H2O
CH3CN/EtOAc/H2O
CH3CN/EtOAc/H2O
NaIO4
NaIO4
NaIO4
NaIO4
NaIO4
NaIO4
NaIO4
NaIO4
NaIO4
m-CPBA
H2O2
25
80
25
82
25
77
25
25
25
25
25
80
25
80
69
95
76
91
37
55
6c
83
96
75
52
89
19
45
57
91
70
89
28
51
4
81
92
71
48
85
13
38
9
10
11
12
13
14
H2O2
tBuOOH
tBuOOH
a
Conditions: styrene (0.2 mmol), catalyst 1 (1 mol%), oxidant (2.0 mmol) in solvents (5 mL, 2/2/1) for 30 min. b Conversion was determined by GC-
MS analysis and is the average of two runs using m-xylene as an internal standard. c Reaction was performed in the absence of catalyst 1. d Isolated
yield.
Table 2 Effect of substituent of catalysta
Table 3 Effect of low catalyst loadinga
Conv.b (%)
Yieldc (%)
Mol%
of Ru catalyst
Entry
Complex
Entry
Conv.b (%)
Yieldc (%)
TONd
1
2
3
4
(1) [Ru(CO)(AsPh3)2(L1)]
(2) [Ru(CO)(AsPh3)2(L2)]
(3) [Ru(CO)(AsPh3)2(L3)]
(4) [Ru(CO)(AsPh3)2(L4)]
96
91
100
89
92
87
98
81
1
2
3
4
5
1.0
0.5
0.2
0.1
0.05
100
96
79
27
12
98
93
72
21
8
98
186
360
210
160
a
Reaction conditions: styrene (0.2 mmol), catalyst 1–4 (1 mol%), NaIO4
(2 mmol) in CH3CN/EtOAc/H2O (2/2/1) (5 mL) at RT for 30 min.
b
a
Conversion was determined by GC-MS analysis and is the average of
Reaction conditions: styrene (0.2–3.8 mmol), catalyst (0.05–1.0 mol%),
two runs using m-xylene as an internal standard. c Isolated yield.
NaIO4 (2 mmol) in CH3CN/EtOAc/H2O (2/2/1) at RT for 30 min.
b
Conversion was determined by GC-MS analysis and is the average of
c
two runs using m-xylene as an internal standard. Isolated yield.
d
TON ¼ ratio of moles of product formed to moles of catalyst used.
aldehyde than the electron-decient styrene derivative did. In
particular, 4-methylstyrene afforded 4-methylbenzaldehyde
(96%) with a lower conversion than that of the electron-decient
styrenes, such as 4-chloro and 4-uorostyrene, which furnished
higher conversions up to 100% (Table 4, entries 2 and 3) under
identical conditions. We have extended the catalytic activity of
complex 3 to internal alkenes and substituted alkene deriva-
tives. Accordingly, cis-stilbene, trans-stilbene and trans-b-
methylstyrene afforded 100% conversion to benzaldehyde
(Table 4, entries 5–7) without any undesired side products.
The possibility of expanding the scope of the new reaction to
achieve a new product was also explored. In this respect, 2-vinyl
naphthalene was employed under identical conditions and
afforded 100% conversion of the corresponding aldehydes
(Table 4, entry 8), in which the benzyl positions remained
unaffected. Interestingly, the introduction of a heteroaromatic
into this system made this methodology more useful for the
preparation of pharmaceuticals and functional materials.
Accordingly, this catalytic system also successfully cleaved the
olenic bond in heterocyclic alkenes such as 2-vinylpyridine
and afforded the expected product in good isolated yields
without affecting the heterocyclic ring (Table 4, entry 9).
Furthermore, this protocol was extended to other cyclic
substrates; cyclohexene was readily cleaved to the correspond-
ing dialdehyde (Table 4, entry 10). In the case of allylcyclo-
pentane and cis-2-heptene (Table 4, entries 11 and 13), the
corresponding acids are formed due to over-oxidation.
However, when the reaction was carried out using 1 mmol of
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Table 4 Oxidative cleavage of various olefins with NaIO4 catalysed by complex 3a
Entry
Substrate
Product
Conv.b (%)
Yieldc (%)
TONf/TOFg
186/372
1
2
96
93
99
100
198/396
3
4
100
96
99
94
198/396
188/376
5
100
99
198/396
6
7
8
9
100
100
100
85
99
99
98
82
198/396
198/396
196/392
164/328
10
94
91
182/364
11
12
100d,e
100
99
98
198/396
196/392
13
100d,e
99
198/396
14
15
Phenylacetylene
Phenyl glyoxal
Benzil
100
100
97
99
194/388
198/396
a
Reaction conditions: styrene (0.4 mmol), complex 3 (0.5 mol%), NaIO4 (2.0 mmol), CH3CN/EtOAc/H2O (2/2/1) (5 mL) at RT for 30 min.
b
c
d
Conversion was determined by GC-MS analysis and is the average of two runs using m-xylene as an internal standard. Isolated yield. Over-
e
f
oxidation of product was observed. Aldehyde products were formed using 1 mmol of NaIO4. TON ¼ ratio of moles of product formed to
moles of catalyst used. g TOF ¼ TON/h (min).
NaIO4, only aldehydes are obtained as the sole products. Hence,
To demonstrate further the utility of the present catalyst,
we feel that the over-oxidation for these substrates may be due we explored the oxidative cleavage reactions to internal
to a higher concentration of oxidant.24a Furthermore, the alkynes, which is rare for Ru(II) complexes. It is gratifying to
1
formation of aldehydes is conrmed by the H and 13C NMR note that the complex efficiently catalysed the oxidation of
spectra (ESI, S46, S47, S52 and S53†). Moreover, a high phenyl acetylene and diphenyl acetylene to their correspond-
conversion of heptanal was obtained for the long chain octene ing aldehydes with high conversions (Table 4, entries 14
(Table 4, entry 12).
and 15).
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At this point, a comparison in terms of catalytic efficiency Furthermore, the catalysts exhibit
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a
signicant catalytic
and scope of application can be established between our performance with a low catalyst loading (0.5 mol%) and show
complex 3 and other reported ruthenium catalysts. The catalytic high turnover numbers at room temperature. The generality of
oxidation studies of alkenes performed by Yang and co-workers this catalyst was demonstrated for a wide range of substrates
using the combination of 3.5 mol% of RuCl3/NaIO4 afforded such as aromatic, aliphatic, cyclic, heteroaromatic and also
moderate yields.18a Peris and co-workers reported ruthenium(II) alkynes under mild conditions.
CNC-pincer bis(carbene) complexes for oxidative cleavage
reactions of olens with low conversions using 1 mol%.21 The
Conclusions
catalyst [RuII(dmp)2(H2O)2]2+ (dmp ¼ 2,9-dimethyl phenan-
throline) has demonstrated a good catalytic activity in the
oxidation of alkenes using H2O2 for a longer reaction time, as
described by Neumann et al.23 Recently, Friedrich and co-
workers developed (h5-cyclopentadienyl) dicarbonyl ruth-
enium(II) amine complexes24a and [(h6-C6H6)RuCl(C5H4N-2-C]
H]N-R)] PF6 as catalysts24b for the oxidative cleavage of olens
with a lower substrate scope using a high catalyst loading. It is
worth mentioning here that the present ruthenium(II) benzhy-
drazone complexes have proven to have an excellent catalytic
activity compared to other ruthenium(II) complexes in terms of
a low catalyst loading (0.5 mol%), higher conversions, as well as
a higher yield at room temperature.
The mechanism of oxidative cleavage reactions of olens
using ruthenium metal complexes is proposed in several works
which involve the formation of a Ru(VI)-dioxo intermediate.23,24
On this basis, we have proposed a plausible reaction mecha-
nism for the oxidative cleavage of olens catalyzed by the
present cyclometalated ruthenium benzhydrazone complex
(Scheme 4). The catalyst reacts with the terminal oxidant NaIO4
to afford the Ru(VI)-cis dioxo intermediate. Then, electrophilic
attack on the C]C bond by the high-valent dioxo intermediate
forms a Ru(IV) cyclo adduct via a concerted [3 + 2] cycloaddition
reaction followed by cleavage of the carbon–carbon double
bond to give the carbonyl product. Hence, the titled complexes
have efficiently cleaved the carbon–carbon double bonds and
selectively produced the carbonyl compounds without any
undesired side reactions under mild reaction conditions.
Moreover, the present catalytic system overcomes the limita-
tions such as toxicity, long reaction times, harsh reaction
conditions, tedious work-up and low product selectivity.
In conclusion, we have successfully demonstrated the synthesis
and characterization of a series of air-stable cyclometalated
Ru(II) complexes bearing tridentate benzhydrazone ligands via
C– H activation. Analytical, spectral and single crystal X-ray
diffraction studies of the complexes evidenced that the
ligands coordinate to ruthenium through C, N and O by disso-
ciation of the imidolate proton and the phenyl proton at the
ortho position of the phenyl ring and revealed the presence of
a distorted octahedral geometry around the Ru(II) ion. The
catalysts were found to be active for the selective oxidative
cleavage reaction of olens with challenging substrates like
aromatic, aliphatic, cyclic and heteroaromatic alkenes in good
to excellent yields as well as conversions in aqueous/organic
media. The catalysts work well at 0.5 mol% and exhibit appre-
ciable higher TON and TOF values within 30 min. Detailed
studies of the reaction mechanism will be undertaken in our
future work.
Experimental section
General considerations
Commercially available RuCl3$3H2O (Loba) and tripheny-
larsine, acetophenone, benzhydrazide derivatives, and CDCl3-
d were purchased from Aldrich and were used as received.
Solvents were freshly distilled before use following the standard
procedures.35 The ligands31 and the ruthenium(II) precursor
[RuHCl(CO)(AsPh3)3]36 were prepared as described in the liter-
ature. Melting points were recorded in the Boetius micro heat-
ing table and are uncorrected. The C, H and N analyses were
performed on a Vario EL III CHNS elemental analyser at the
Sophisticated Test and Instrumentation Centre (STIC), Cochin
University, Cochin. The FT-IR spectra of the complexes were
recorded with KBr pellets using a Perkin-Elmer 597 spectro-
photometer in the range 4000–400 cmꢁ1. The NMR spectra were
recorded on a Bruker 400 MHz spectrometer in CDCl3 solvent
using TMS as an internal standard. The electronic spectra of the
complexes in CH3CN solution were recorded using a Cary 300
Bio UV-vis Varian spectrophotometer in the range 800–200 nm.
Aldehydes were identied by gas chromatography-mass spec-
trometry (GC-MS) using a Shimadzu QP 2010SE with a capillary
column (5% diphenyl/95% dimethyl poly siloxane) (30 m to 0.25
mm to 0.25 mm) and high purity helium as a carrier gas.
Synthetic procedure for new ruthenium(II) complexes
To a solution of [RuHCl(CO)(AsPh3)3] (100 mg, 0.0922 mmol) in
benzene (15 mL) was added p-substituted acetophenone benz-
hydrazone ligand [p-H (L1), p-Cl (L2), p-OCH3 (L3), p-Br (L4)]
Scheme 4 Plausible mechanism for ruthenium-catalysed oxidative
cleavage of olefins.
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(22.0–29.2 mg, 0.0922 mmol) and triethylamine (1.0 mL) as above work-up process. For calculation of the isolated yield, the
a base. Aer the addition, the resultant solution was heated to reaction mixture was extracted by dichloromethane and the
reux temperature for 3 h and the progress of the reaction was solution was evaporated in vacuo. Then the crude product was
monitored using TLC. At the end of the reaction, the solution puried via standard silica gel chromatography using hexane/
was cooled to room temperature. Then the volume of the ethyl acetate as the eluent to give the desired aldehyde
solvent was reduced to 3 mL under reduced pressure, and light product. To ensure the general synthetic utility of our catalytic
petroleum ether (60–80 ꢀC) (10 mL) was added. A yellow-brown system, isolated products were characterized by NMR. Yields are
solid was formed which was dried under vacuum.
calculated based on the isolated products.
Characterization data: [Ru(L1)(CO)(AsPh3)2] (1). Yield: 70.43
mg, 78%. Mp: 220 ꢀC (with decomposition). Anal. found (calcd)
for C52H42N2O2As2Ru: C, 63.85 (63.88); H, 4.39 (4.33); N, 2.81
X-ray crystallography
1
(2.87). FT-IR (KBr, cmꢁ1): 1586 (s), 1240 (m), 1931 (s). H NMR
Crystals of complexes 3 and 4 suitable for single-crystal X-ray
diffraction studies were grown by the slow evaporation of
a chloroform–petroleum ether solution at room temperature. A
single crystal of a suitable size was covered with Paratone oil,
mounted on the top of a glass bre, and transferred to a Bruker
SMART APEX II single-crystal X-ray diffractometer using mon-
(400 MHz, CDCl3, ppm): 2.4 (s, 3H, CH3), 6.9–7.7 (m, 39H, Ar,
AsPh3). 13C NMR (300 MHz, CDCl3, ppm): 21.31 (CH3), 142.75
(C]N), 164.82 (–N]C–O), 191.30 (C^O), 126.38, 127.34,
128.67, 128.81, 129.05, 129.98, 130.06, 131.87, 132.15, 133.18,
134.05, 134.59. UV-vis (CH3CN, l, nm): 463, 270, 242.
ꢀ
˚
ochromated Mo Ka radiation (kI ¼ 0.71073 A). Data were
[Ru(L2)(CO)(AsPh3)2] (2). Yield: 67.15 mg, 72%. Mp: 235 C
collected at 296 K. The absorption corrections were performed
by the multi-scan method using SADABS soware.37 Structures
were solved by direct methods (SHELXS 97) and rened using
SHELXL 97 38 with full-matrix least squares on F2. All non-
hydrogen atoms were rened with anisotropy thermal param-
eters and the hydrogen atoms were constrained to the ideal
positions in the renement procedure. The unit cell parameters
were determined by the method of difference vectors using
reections scanned from three different zones of the reciprocal
lattice. The intensity data were measured using a u and f scan
with a frame width of 0.5ꢀ. Frame integration and data reduc-
tion were performed using Bruker SAINT-Plus (Version 7.06a)
soware.39
(with decomposition). Anal. found (calcd) for C52H41ClN2O2-
As2Ru: C, 62.09 (62.01); H, 4.13 (4.10); N, 2.76 (2.78). FT-IR (KBr,
1
cmꢁ1): 1601 (s), 1245 (m), 1947 (s). H NMR (400 MHz, CDCl3,
ppm): 2.3 (s, 3H, CH3), 6.8–7.65 (m, 38H, Ar, AsPh3). 13C NMR
(300 MHz, CDCl3, ppm): 21.27 (CH3), 142.05 (C]N), 164.52
(–N]C–O), 191.12 (C^O), 126.19, 127.36, 128.35, 128.77,
129.11, 129.94, 130.11, 131.44, 132.25, 133.51, 134.10, 134.60.
UV-vis (CH3CN, l, nm): 457, 268, 245.
[Ru(L3)(CO)(AsPh3)2] (3). Yield: 68.79 mg, 73.5%. Mp: 244 ꢀC
(with decomposition). Anal. found (calcd) for C53H44N2O2As2-
Ru: C, 63.41 (63.48); H, 4.47 (4.42); N, 2.73 (2.79). FT-IR (KBr,
1
cmꢁ1): 1591 (s), 1249 (m), 1935 (s). H NMR (400 MHz, CDCl3,
ppm): 2.4 (s, 3H, CH3), 3.8 (s, 3H, OCH3), 6.6–7.7 (m, 38H, Ar,
AsPh3). 13C NMR (300 MHz, CDCl3, ppm): 21.29 (CH3), 55.64
(OCH3), 142.31 (C]N), 164.24 (–N]C–O), 191.83 (C^O),
123.42, 124.30, 126.30, 127.79, 128.52, 129.70, 130.30, 131.73,
132.66, 133.36, 134.23, 134.55. UV-vis (CH3CN, l, nm): 465, 271,
241.
Acknowledgements
The authors gratefully acknowledge the nancial support from
Science and Engineering Research Board (SERB) (Scheme No.
SR/S1/IC-48/2012) and for Junior Research Fellow to T. S. M. We
thank CEFIPRA, India for the use of GC-MS through the
research project (Ref. No. IFC/5005-1). We also thank DST-India
(FIST programme) for the use of the Bruker Avance DPX 400
MHz NMR at the School of Chemistry, Bharathidasan Univer-
sity, India.
[Ru(L4)(CO)(AsPh3)2] (4). Yield: 64.51 mg, 66.4%. Mp: 241 ꢀC
(with decomposition). Anal. found (calcd) for C52H41BrN2O2-
As2Ru: C, 59.36 (59.39); H, 3.99 (3.93); N, 2.64 (2.66). FT-IR (KBr,
1
cmꢁ1): 1607 (s), 1253 (m), 1943 (s). H NMR (400 MHz, CDCl3,
ppm): 2.4 (s, 3H, CH3), 6.9–7.7 (m, 38H, Ar, AsPh3). 13C NMR
(300 MHz, CDCl3, ppm): 21.63 (CH3), 142.84 (C]N), 164.16
(–N]C–O), 191.56 (C^O), 126.26, 127.42, 128.55, 128.86,
129.06, 129.81, 130.05, 131.68, 132.40, 133.33, 134.11, 134.54.
UV-vis (CH3CN, l, nm): 461, 269, 243.
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