Half-sandwich Ruthenium(II) Schiff base complexes: Synthesis, characterization and effective catalysts for one-pot conversion of aldehydes to amides

Article abstract of DOI:10.1016/j.jorganchem.2019.120964

Five new Schiff base ligands and conformationally rigid half-sandwich organo ruthenium(II) Schiff base complexes (1–5) with the general formula [Ru(η⁶?p?cymene)(Cl)(L_1-5)] (where, L = mono anionic Schiff base ligands) have been synthesized from the reaction of [{(η⁶?p?cymene)RuCl}₂(μ?Cl)₂] with a bidentate Schiff bases ligands. These ruthenium(II) Schiff base complexes were fully characterized by elemental analysis, FT?IR, UV–Vis, ¹H & ¹³C NMR and mass spectroscopy studies. In chloroform solution, all the metal complexes exhibit characteristic metal to ligand charge transfer bands (MLCT) and emission bands in the visible region. The crystal structure of the complexes [Ru(η⁶?p?cymene)(Cl)(L₁)] (1) and [Ru(η⁶?p?cymene)(Cl)(L₃)] (3) were determined by single crystal X?ray crystallography. The complexes exhibited good catalytic activity for aldehydes to amides by one-pot conversion process in the presence of NaHCO₃/NH₂OH·HCl.

Full text of DOI:10.1016/j.jorganchem.2019.120964

Journal Pre-proof
Half-sandwich Ruthenium(II) Schiff base complexes: Synthesis, characterization and
effective catalysts for one-pot conversion of aldehydes to amides
Muniyappan Premkumar, Paranthaman Vijayan, Galmari Venkatachalam
PII:
S0022-328X(19)30407-3
DOI:
https://doi.org/10.1016/j.jorganchem.2019.120964
JOM 120964
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 3 June 2019
Revised Date: 27 September 2019
Accepted Date: 28 September 2019
Please cite this article as: M. Premkumar, P. Vijayan, G. Venkatachalam, Half-sandwich Ruthenium(II)
Schiff base complexes: Synthesis, characterization and effective catalysts for one-pot conversion
of aldehydes to amides, Journal of Organometallic Chemistry (2019), doi: https://doi.org/10.1016/
j.jorganchem.2019.120964.
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Half-Sandwich Ruthenium(II) Schiff base Complexes: Synthesis,
Characterization and Effective Catalysts for One-pot Conversion of
Aldehydes to Amides
a
b
a
Muniyappan Premkumar , Paranthaman Vijayan , Galmari Venkatachalam*
a
PG & Research Department of Chemistry, Government Arts College, Dharmapuri- 636 705,
Tamilnadu, India.
b
School of Chemistry & Physics, University of KwaZulu-Natal, Pietermaritzburg Campus,
Scottsville 3209, South Africa.
Corresponding Author: *E-mail address: gvchem@gmail.com [G. Venkatachalam]
Abstract
Five new Schiff base ligands and conformationally rigid half-sandwich organo
6
ruthenium(II) Schiff base complexes (1‒5) with the general formula [Ru(η ‒p‒cymene)(Cl)(L1-
5)] (where, L = mono anionic Schiff base ligands) have been synthesized from the reaction of
6
[
{(η ‒p‒cymene)RuCl} (µ‒Cl) ] with a bidentate Schiff bases ligands. These ruthenium(II)
2
2
1
13
Schiff base complexes were fully characterized by elemental analysis, FT‒IR, UV–Vis, H & C
NMR and mass spectroscopy studies. In chloroform solution, all the metal complexes exhibit
characteristic metal to ligand charge transfer bands (MLCT) and emission bands in the visible
6
6
region. The crystal structure of the complexes [Ru(η ‒p‒cymene)(Cl)(L
1
)] (1) and [Ru(η ‒p‒
cymene)(Cl)(L )] (3) were determined by single crystal X‒ray crystallography. The complexes
3
exhibited good catalytic activity for aldehydes to amides by one-pot conversion process in the
presence of NaHCO /NH OH·HCl.
3
2
Keywords: Ruthenium(II) complexes; Schiff base; Synthesis; Crystal structure; Aldehyde to
amide
1

1
. Introduction
The amide bond is one of the most fundamental groups in organic chemistry, and it plays
a major role in the elaboration and composition of biological and chemical systems [1-6]. The
amides are an important class of chemicals that have been widely used as chemical intermediates
in organic synthesis and prolific functional groups, with a great importance in both research and
industrial chemistry owing to their prevalence in detergents, engineering plastics, lubricants,
biologically active compounds and pharmaceuticals [7]. The common preparation of amides
from the rection of amines with acid anhydrides or acylchlorides [8] involves highly toxic
wastes. Hence the development of new catalytic processes with more atom economy and less
toxic wastes getting importance. The Beckmann rearrangement is commonly used to transform
oximes into the conversion of corresponding amides [9], and this rearrangement is commonly
used to transform ketoximes into the corresponding N-substituted amides requiring the use of
strong acids [10].
Further, the synthesis of primary amides from aldoximes is very difficult and reactive
reagents have to be used in stoichiometric amounts for the transformation to occur. In addition,
the selectivities for the desired amides are often very low with reactive stoichiometric reagents
because undesired carboxylic acids, aldehydes and nitriles are formed in some cases. While the
migrating group can be aryl or alkyl, it is rarely hydrogen atom that migrates, and so the
Beckmann rearrangement is not a general process for the conversion of aldoximes into
corresponding primary amides [11‒13]. The rearrangement of aldoximes into amides has been
reported using transition metal catalysts containing iridium [14,15], copper [16], nickel [17],
palladium [18], rhodium [19] and zinc [20].
2

Among the transition metal catalysts, ruthenium complexes have been extensively
studied due to their catalytic significances in variety of organic conversions, such as the
hydrogenation of esters to alcohols [21], synthesis of imines from amines and alcohols [22], the
synthesis of amides from amines and esters [23], and the direct synthesis of polyamides from
diamines and diols [24].
In the Schiff base metal complexes, the environment at coordination center can be
modified by attaching different substituents to the ligand, which provides a useful range of
electronic properties and steric essential for the fine-tuning of structure and reactivity [25].
Furthermore, transition metal complexes of ruthenium have attracted much interest mainly
because of their vital applications in several organic syntheses [7,26]. The Schiff base ligands are
among the most fundamental chelating systems particularly for transition metals [27]. The Schiff
base ligands proved to be very attractive for creating new active and selective sites in ruthenium
catalytic species [28,29]. As a special, N, O-bidentate Schiff base ligands allow the fine tuning of
the steric and electronic environment at the ruthenium atom through an appropriate selection of
bulky and/or electron -withdrawing or -donating substituents. The two donor atoms (N and O) of
the chelated Schiff base exhibit opposite features: the phenolate oxygen atom is a hard donor and
will stabilize a higher oxidation state of ruthenium whereas the imine nitrogen is a softer one
and, accordingly, will rather stabilize the lower oxidation state of ruthenium. They have been
used as catalyst precursors for a variety of purposes, including hydrogenation, carbon–carbon
bond formation, polymerization, oxidation and aldehyde to amide conversion [30]. To the best of
6
our knowledge, there are no reports available on catalytic aldehyde to amide conversion by η ‒
p‒cymene ruthenium(II) Schiff base complexes incorporating N, O-donors in presence of
3
NaHCO .
3

6
In the present study, the synthesis of the half‒sandwich η ‒p‒cymene ruthenium(II)
complexes containing bidentate Schiff base ligands were performed. All the synthesized
complexes have been characterized by elemental analysis, IR, UV‒Vis, NMR and Mass
spectroscopic techniques. The molecular structure of complexes 1 and 3 were confirmed through
single crystal X-ray diffraction. The catalytic activity of the ruthenium(II) Schiff base complexes
are investigated in the case of aldehyde to amide in the presence of NaHCO3.
R
Ligands
R
X
X
L H
1
H
OH
H
N
L H
CH3
NO2
Cl
OH
OH
2
L H
3
L H
OH
SH
4
L H
H
5
Fig. 1. General structure of the Schiff base ligands (L H−L H)
1
5
2
. Results and discussion
2
.1. Synthesis and characterization
The half‒sandwich p-cymene ruthenium(II) Schiff base complexes 1−5 were readily
prepared in high yield by reacting the dimeric [(p–cymene)RuCl ] precursor with the
2
2
appropriate Schiff base ligands (L H − L H) in CH Cl at room temperature in 1:2 molar ratio as
1
5
2
2
shown in Scheme 1. The complexes 1−5 were stable in air both solid and in solution state; they
are highly soluble in polar solvents such as CH COCH , CH CN, HCON(CH , (CH SO,
CH Cl and CHCl solvents and partially soluble in methanol, ethanol and insoluble in non-polar
3
3
3
3
)
2
3 2
)
2
2
3
solvents such as hexane and pentane. These ruthenium(II) Schiff base complexes were obtained
4

as reddish-brown solids in yields up to 90%. The analytical data of all the ruthenium(II) Schiff
base complexes are in good agreement with the molecular formula proposed.
Cl
Cl
Cl
Ru
Ru
Ru
Cl
N
X
Cl
N
R
X
H
H
CH Cl / Et N / stirred / 5 h
2
2
3
R
Complex L1; 1
L2; 2
O
CH3
L3; 3
O
NO2
L4; 4
O
Cl
L5; 5
S
H
X
R
O
H
6
Scheme 1. Synthesis of new η ‒p‒cymene Ru(II) Schiff base complexes (1
The infrared spectra of the metal-free ligands (L H‒L H) show absorption bands in the
region 1261–1298 cm can be assigned to phenolic C–O stretching. This bands shifted to 1300–
‒5)
1
5
-1
-1
1
321 cm in ruthenium(II) complexes, showing that the coordination through the phenolic
oxygen via deprotonation [31,32]. This is supported by the disappearance of the phenolic ν(O–H)
and (S–H) absorption bands in all the complexes which observed in free Schiff base ligands
around 3050–3367 cm . The band around 1615–1662 cm in the spectra of ligands can be
ν
-1
-1
-1
attributed to ν(C=N) which is shifted to 1571–1604 cm in ruthenium(II) complexes (1-5)
indicating the coordination of azomethine nitrogen (C=N) with ruthenium(II) metal ion [33,34].
Coordination of the ligand to the ruthenium(II) ion through an azomethine nitrogen is expected
to reduce the electron density in the azomethine link and thus lower the absorption frequency
upon complexation. The band corresponding to ν(S–H) also disappears in the complex containing
−
1
−1
L H ligand. Moreover the absorption due to C–S of L H at 1252 cm is shifted to 1276 cm in
5
5
5

the complex 5, indicating that the second coordination site is a thiophenolic sulfur atom [35].
The IR spectra of all the complexes therefore confirm the mode of coordination of the Schiff
base ligand to the ruthenium(II) metal ion via the azomethine nitrogen and phenolic oxygen and
sulfur atom of thiol group [36–38].
The electronic transition spectra for all the new complexes have been recorded in CHCl3
6
solvent in the region 800–200 nm. These (η –p–cymene) ruthenium(II) Schiff base complexes
show three to four bands in the visible and ultraviolet region 460–208 nm. The absorption
spectra of ruthenium(II) Schiff base complexes in the high-intensity bands in the region 208–260
nm that are very similar and are attributable to the ligand–centered transitions (π–π*, n–π*)
taking place in the aromatic group of Schiff base ligands. The lowest energy absorption bands in
the electronic spectra of the complexes in the visible region 375–460 nm are ascribed to metal to
ligand charge transfer transitions (MLCT). The representative spectra of complex 1-5 were
shown in Fig. 2. Based on the pattern of the electronic spectra of all the complexes indicated the
presence of an octahedral environment around the ruthenium(II) ion has been proposed similar to
that of other ruthenium(II) Schiff base octahedral complexes [39,40].
6

1
2
3
4
5
4
3
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
.5
.0
200
300
400
500
600
Wavelength (nm)
6
Fig. 2. UV-Vis Spectra of [Ru(η –p–cymene)(Cl)(L1-5)] (1-5)
1
3
The H NMR spectra of ruthenium(II) complexes 1-5 were recorded in CDCl which
confirms the bonding of the Schiff base ligand to the ruthenium(II) metal ion. Multiplets
observed in the region δ 6.3–8.1 ppm in the complexes have been assigned to the aromatic
protons of Schiff base ligands. The signal due to the azomethine proton (CH=N) appears at δ
8
.4–9.8 ppm. The position of azomethine signal in the complexes is slightly downfield in
comparison with that of the free ligands, suggesting deshielding of azomethine proton due to its
coordination to ruthenium(II) ion. A sharp singlet due to the –OH and ‒SH proton of the free
Schiff base ligands in the region δ 11.8–13.15 ppm was disappeared in all the complexes, further
supporting coordination of the phenolic oxygen and thiol oxygen to the ruthenium(II) ion [41–
7

1
4
4]. Therefore, the H NMR spectra of the all complexes confirm the bidentate coordination
mode of the schiff base ligands to ruthenium(II) ions. In all the complexes, the cymene protons
appear in the region of δ 4.0–5.6 ppm [45–47]. The isopropyl methyl protons of the p-cymene
appeared as two doublets in the region of δ 0.8–1.62 ppm and the isopropyl methine (CH)
protons appear as a septet in the region of δ 2.0–3.1 ppm. Further, the methyl proton of the p-
cymene appears as a singlet around the range of δ 1.5–2.3 ppm. Additionally, methyl (CH3)
protons appeared as singlet for complex 2 at δ 2.45 ppm. The above observations made it clear
that the Schiff base ligands have coordinated to the ruthenium(II) ion (see supporting
information S1-S10).
2
.2. X-ray structural studies
The crystal structure of the complexes 1 and 3 consist of neutral arene ring bonded to the
ruthenium metal along with chloride and NO-donors of Schiff base ligands. The Schiff base
ligands acts in NO bidentate fashion, around the metal center at forming a five membered ring
with bond angles of 78.7(1) Å (O(1)‒Ru(1)‒N(1)) for complex 1 and 78.4(3) Å (O(1)‒Ru(1)‒
N(1)) for complex 3. Crystallographic refinement data are summarized in Table 1 and the
selected bond distances and angles are presented in Table 2. ORTEP diagram of the molecules 1
and 3 are shown in Fig. 3 and Fig. 4. The compounds 1 and 3 crystallized in the monoclinic
crystal system with the C2/c space group and triclinic with the Pī space group respectively. The
molecular structures of complexes 1 and 3 shows clearly that the Schiff base ligands coordinate
in a bidentate manner to ruthenium ions via the Schiff base nitrogen atom and phenolate oxygen
atom in addition to one arene group and one chlorine atom. The complexes adopt as commonly
observed three-legged piano stool geometry with O, N and Cl atoms as the legs.
8

6
Fig. 3. ORTEP diagram of the complex [Ru(η ‒p‒cymene)(Cl)(L
1
)] (1).
6
Fig. 4. ORTEP diagram of the complex [Ru(η ‒p‒cymene)(Cl)(L
3
)] (3).
9

Table 1
Selected crystal data and structure refinement summary of complexes 1 and 3
1
3
Empirical formula
Formula weight
Temperature
C
31
H
28ClNORu
31 2 3
C H27ClN O Ru
567.06
612.06
293(2) K
0.71073 Å
Triclinic
Pī
296(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Monoclinic
C2/c
Unit cell dimensions
a = 16.5797(6) Å
b = 18.8979(6) Å
c = 16.3631(5) Å
α= 90°
a = 11.086(5) Å
b = 11.138(5) Å
c = 13.250(6) Å
α = 95.454(13)°
β = 99.165(9)°
γ = 93.586(11)°
β= 95.2540(10)°
γ = 90°
3
Volume(Å )
5105.4(3)
8
1602.8(13)
2
Z
3
Density (calculated) (Mg/m )
1.475
1.268
-
1
Absorption coefficient (mm )
0.743
2320
0.602
624
F(000)
3
Crystal size (mm )
0.24 x 0.17 x 0.07
1.638 to 29.050°
0.240 x 0.180 x 0.110
1.842 to 24.998°
Theta range for data collection
Index ranges
-14<=h<=22, -25<=k<=25, -22<=l<=22
21404
-13<=h<=13, -13<=k<=12, -15<=l<=15
7854
Reflections collected
Independent reflections
6831 [R(int) = 0.0302]
5497 [R(int) = 0.0683]
1
0

Completeness to theta = 25.242° 99.6 %
97.4 %
None
Absorption correction
None
2
2
Refinement method
Full-matrix least-squares on F
Full-matrix least-squares on F
Data / restraints / parameters
6781 / 0 / 316
5497 / 0 / 323
2
Goodness-of-fit on F
1.061
1.017
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0387, wR2 = 0.1052
R1 = 0.0620, wR2 = 0.1284
R1 = 0.0966, wR2 = 0.2528
R1 = 0.1319, wR2 = 0.2917
-3
0.647 and -0.631 e.Å
-3
1.972 and -1.814 e.Å
Largest diff. peak and hole
1
1

Table 2
Selected bond distances (Å) and bond angles (º) of complexes 1 and 3
Bond distances/angles
1
Bond distances/angles
3
Ru(1)‒Cl(1)
2.419(8)
2.052(2)
2.142(2)
1.281(4)
2.222(4)
1.435(4)
1.314(3)
1.403(4)
2.175(3)
78.73(9)
83.05(7)
86.08(7)
126.1(3)
130.8(2)
114.92(1)
109.85(1)
114.4(2)
Ru(1)‒Cl(1)
2.399(3)
2.068(7)
2.152(7)
1.281(12)
2.216(10)
1.442(12)
1.290(12)
1.226(14)
1.161(12)
78.9(3)
Ru(1)‒O(1)
Ru(1)‒O(1)
Ru(1)‒N(1)
Ru(1)‒N(1)
N(1)‒C(6)
N(1)‒C(7)
Ru(1)‒C(23)
Ru(1)‒C(23)
N(1)‒C(5)
N(1)‒C(6)
O(1)‒C(1)
O(1)‒C(1)
C(5)‒C(1)
O(2)‒N(2)
Ru(1)‒C(28)
O(3)‒N(2)
O(1)‒Ru(1)‒N(1)
N(1)‒Ru(1)‒Cl(1)
Cl(1)‒Ru(1)‒O(1)
N(1)‒C(6)‒C(7)
C(6)‒N(1)‒Ru(1)
C(1)‒O(1)‒Ru(1)
C(5)‒N(1)‒Ru(1)
N(1)‒C(5)‒C(1)
O(1)‒Ru(1)‒N(1)
N(1)‒Ru(1)‒Cl(1)
Cl(1)‒Ru(1)‒O(1)
O(2)‒N(2)‒O(3)
C(7)‒N(1)‒Ru(1)
C(1)‒O(1)‒Ru(1)
C(6)‒N(1)‒Ru(1)
O(2)‒N(2)‒C(4)
84.4(2)
85.2(2)
120.2(11)
129.6(7)
114.7(5)
109.4(5)
117.1(11)
The Schiff base ligands bind to the ruthenium metal center at O and N forming the five
membered chelate ring with bite angles Cl(1)‒Ru(1)‒O(1) 86.08(7)º, O(1)‒Ru(1)‒N(1) 78.73(9)º
for complex 1 and Cl(1)‒Ru(1)‒O(1) 85.2(2)º, O(1)‒Ru(1)‒N(1) 78.9(3)º for complex 3. The
bond distances of Ru(1)‒O(1) 2.052(2) Å, Ru(1)‒N(1) 2.142(2) Å for the complex 1 and Ru(1)‒
O(1) 2.068(6) Å, Ru(1)‒N(1) 2.151(7) Å for complex 3 are 2.052(2) Å, 2.142(2) Å, 2.069(7) Å
and 2.153(7) Å respectively. The Ru-Cl bond distance is found to be 2.419(8) Å and 2.399(3) Å.
1
2

As all the compounds display similar spectral properties, the other three complexes are assumed
to have similar structure to that of complexes 1 and 3.
2
.3. Catalytic conversion of aldehydes to amides
The catalytic organic conversion of aldehydes to amides by the synthesized η ‒
6
p
‒cymene
ruthenium(II) Schiff base complexes have been studied in toluene in the presence of
NaHCO /NH OH·HCl. Our catalytic system is suitable for aromatic, conjugated and heterocyclic
3
2
aromatic aldehydes. The ruthenium(II) Schiff base complexes efficiently catalyzes the
conversion of aldehydes to their corresponding amides in good yield at one pot process. The
effects of base, solvents and catalyst/substrate ratios were investigated using benzaldehyde which
was selected as a test-substrate (Table 3 and 4). No considerable reaction was observed in the
absence of base or catalyst. The conversion of benzaldehyde to benzamide was performed in
different solvent media such as benzene, dichloromethane, acetonitrile, chloroform, toluene.
Among these, toluene was found to be best, with high conversion. To optimize, and study the
effects of different bases in our catalytic system, we have chosen the reaction between
benzaldehyde in the presence of various bases KHCO
3
3 3 3
, NaHCO , Et N and CH COONa. Among
the different bases, NaHCO
3
was selected as base with given maximum yield [48].
Table 3
Effect of base and solvent in the conversion of benzaldehyde to benzamide using catalyst
6
a
complex [Ru(η –
p
–cymene)(Cl)(L )] (1)
1
O
O
Ru(II) Complex 1, (1 mol%)
.
+
NH OH HCl
2
Base / Solvent
reflux, 18 h
NH2
H
_
_
_____________________________________________________________________________
b
Entry
Base
Solvent
Yield (%)
_____________________________________________________________________________
1
NaHCO Benzene 55
3
1
3

2
3
4
5
6
7
8
NaHCO
NaHCO
NaHCO3
NaHCO3
KHCO3
3
Dichloromethane
Acetonitrile
Chloroform
Toluene
Toluene
Toluene
37
72
45
89
85
70
81
3
Et N
3
CH COONa
Toluene
3
a
2
Benzaldehyde (1 mmol), NH OH·HCl (1 mmol), base (1 mmol), catalyst 1 (1 mol%) and 3 mL
solvent, reflux 18 h.
b
Isolated yield after column chromatography.
The optimized reaction conditions were 18 h and the different catalyst: substrate ratio
1:100, 1:200, 1:300, 1:500, 1:1000) was tested and the results are summarized in Table 4. We
(
started the catalyst/substrate ratios of 1:100, 1:200, 1:300, 1:500 and 1:1000, the reaction
proceeds with good conversion. When increasing the catalyst/substrate ratio from 1:500 and
1
:1000 in toluene, the reaction still proceeds by a downfall in yield of amide. When decreasing
the catalyst/substrate ratio1:300 and 1:200, the reaction proceeds by a moderate yield. Then, it
was concluded that catalyst/substrate ratio of 1:100 is the best compromise between optimal
reaction rates in toluene and we obtained 89% yield of amide.
Table 4
Effect of catalyst/substrate ratio (C/S) on the conversion of benzaldehyde to benzamide using
6
a
complex [Ru(η –
p
–cymene)(Cl)(L
1
)] (1)
O
O
Ru(II) Complex 1, (1 mol%)
.
+
NH OH HCl
2
NaHCO / Toluene
3
NH2
H
reflux, 18 h
_
_
_____________________________________________________________________________
b
Entry
Ratio (C:S)
Time (h)
Yield (%)
_____________________________________________________________________________
1
2
3
4
5
1:1000
1:500
1:300
1:200
1:100
18
18
18
18
18
35
41
58
73
89
1
4

6
without catalyst
18
No product
_
__________________________________________________________________________
a
Conditions: reaction were carried out with substrate (1-10 mmol), ruthenium catalyst (1 mol%),
NH OH·HCl (1-10 mmol) and 3 mL of toluene were refluxed for 18 h.
Isolated yield after column chromatography.
2
b
The above optimized conditions, the catalytic transformation of aldehydes to
corresponding amides in the presence of NaHCO and NH OH·HCl, using all the synthesized
3
2
ruthenium(II) Schiff base complexes as catalysts were tested and the results are summarized in
Table 5. A proposed mechanism is given in scheme 2. oxidative addition of the aldoxime N–OH
bond to Ru(II), followed by nucleophilic attack on the co-ordinated imine in 2, then b-
elimination of cyclometallated 3, and finally reductive elimination to give the amide. It has been
observed that the aromatic aldehydes bearing an electron-withdrawing substance exhibit slightly
higher reactivities (higher yields) as compared to that of benzaldehyde. Electron donating
moietie on benzaldehyde gave slightly lower yields compared with benzaldehyde [49]. The
presence of electron donating groups like –OH and –OCH in the substrates alters the reactions
3
and corresponding amides were obtained in good yields (87-79%) respectively. On the other
hand, electron withdrawing groups, such as the –Cl and –NO substituents offering better yields
2
1
(
98-82%) when compared to substrate containing electron donating group. The representative H
NMR spectra are showed in supporting information S16-S19.
If Ru(II) acted merely as Lewis acid we would expect to see product from the classical
Beckmann rearrangement, such as secondary amides from ketoximes. If Ru(II)/(IV) conversion
is possible in this system, an alternative mechanism (Scheme 2) can be proposed.
1
5

1
HO
R
H
Cl Ru
R
N
NH2
R
N
O
X
oxidative Addition
Reductive Elimination
HO Ru
R
2
H
Ru
R
H
N
R
NH
N
4
R
N
O
X
X
Beta Elimination
Nucleophilic Attack
O
Ru
R
H
R
N
N
H
X
3
Scheme 2. Proposed mechanism for the Ru(II) catalyzed rearrangement of aldoximes.
1
6

Table 5
Catalytic aldehyde to amide conversion using ruthenium(II) arene Schiff base complexes
6
a
[
Ru(η –p–cymene)(Cl)(L )] (1-5)
1-5
O
O
Ru(II) Complex, (1 mol%)
.
+
NH OH HCl
2
R
H
R
NH
2
NaHCO / Toluene
3
reflux, 18 h
R= Ph, 4-NOC H , 2,4-ClC H , 4-OMeC H , 4-OHC H
6 4
6
4
6
3
6
4
_
_
_____________________________________________________________________________
b
Complex
Aldehyde
Amide
Yield (%)
_____________________________________________________________________________
1
2
3
4
5
Benzaldehyde
Benzamide
89
98
94
87
86
85
93
90
81
84
88
89
90
84
80
83
85
82
79
81
92
95
91
83
87
4
2
4
4
-nitrobenzaldehyde
4-nitrobenzamide
2,4-dichlorobenzamide
4-methoxybenzamide
4-hydroxybenzamide
Benzamide
4-nitrobenzamide
2,4-dichlorobenzamide
4-methoxybenzamide
4-hydroxybenzamide
Benzamide
4-nitrobenzamide
2,4-dichlorobenzamide
4-methoxybenzamide
4-hydroxybenzamide
Benzamide
4-nitrobenzamide
2,4-dichlorobenzamide
4-methoxybenzamide
4-hydroxybenzamide
Benzamide
,4-dichlorobenzaldehyde
-methoxybenzaldehyde
-hydroxybenzaldehyde
Benzaldehyde
4
2
4
4
-nitrobenzaldehyde
,4-dichlorobenzaldehyde
-methoxybenzaldehyde
-hydroxybenzaldehyde
Benzaldehyde
4
2
4
4
-nitrobenzaldehyde
,4-dichlorobenzaldehyde
-methoxybenzaldehyde
-hydroxybenzaldehyde
Benzaldehyde
4
2
4
4
-nitrobenzaldehyde
,4-dichlorobenzaldehyde
-methoxybenzaldehyde
-hydroxybenzaldehyde
Benzaldehyde
4
2
4
4
-nitrobenzaldehyde
4-nitrobenzamide
2,4-dichlorobenzamide
4-methoxybenzamide
4-hydroxybenzamide
,4-dichlorobenzaldehyde
-methoxybenzaldehyde
-hydroxybenzaldehyde
a
Aldehyde 1 mmol, NaHCO (1 mmol), NH OH·HCl (1 mmol), catalyst (1 mol%) and toluene 3
3
2
mL were refluxed for 18 h.
b
Isolated yield after column chromatography.
1
7

3
. Conclusion
In conclusion, we report here the synthesis of five new Schiff base ligands and η ‒
6
p‒
cymene ruthenium(II) Schiff base complexes containing bidentate O and N chelating 9-
anthraldehyde phenolate ligands. All the complexes have been fully characterized by spectral
1
13
methods (IR, UV-vis, H & C NMR and Mass). The X-ray crystallographic molecular
structures 1 and 3 by a X-ray diffraction study which reveals that the Schiff base ligand
coordinates to ruthenium(II) ion via azomethine nitrogen and phenolic oxygen and Cl atoms as
the legs, witnessed a typical three-legged “piano-stool” geometry. The catalytic ability of all the
3
complexes were screened for aldehyde to amide conversion reaction presence of NaHCO and
NH OH·HCl and the conversion are found to be good.
2
4
. Experimental procedures
4
.1. Materials
The synthetic work was performed in air and room temperature. All the chemicals in the
current study were purchased from commercial sources and utilized without further purifications.
Solvents were purified and dried according to standard procedures [50]. Commercial
RuCl ·3H O was purchased from Himedia. 9-anthraldehyde, o-aminophenol, thiophenol, 2-
3
2
amino-4-nitrophenol, 2-amino-4-methylphenol and 2-amino-4-chlorophenol were purchased
6
from S.D. Fine Chem Limited, India. The ruthenium(II) precursor starting material [{(η –
p–
cymene)RuCl}
2
2
(µ-Cl) ] was prepared according to literature method [51].
4
.2. Physical measurements
The solid FT-IR spectra of the complexes and the free ligands were recorded in CARY
3
60, Agilent resolution pro spectrophotometer. UV–Vis spectra of the complexes were obtained
1
on a JASCO-630 UV-Vis Varian spectrophotometer. H NMR spectra of the complexes were
1
8

recorded on a Bruker 400 MHz instrument using TMS as an internal reference. Melting points
were recorded with a Boetius micro-heating table and are uncorrected.
4
.3. Preparation of the Schiff base ligands (L H
–
L5H)
1
A mixture of substituted various amines (1 mmol; 0.1091‒0.1541 mg) and 9-
anthraldehyde (1 mmol; 0.2062 mg) in ethanol (10 mL) containing a drop of glacial acetic acid
was refluxed for 1 h. The separated precipitated was filtered and dried in air. Ligands were
further purified by recrystallization from methanol. Yield: 75-90%.
‒
1
1
L
1
H: m.p. 110 °C. Yield: 80%. Selected IR bands (cm ): 1655 (HC=N), 3277 (OH), H‒
NMR (CDCl ) δ (ppm): 6.6-7.8 (m, Ar), 8.7 (1H, s, HC=N), 12.8 (1H, s, OH).
3
−
1
1
L H: m.p. 102 °C. Yield: 90%. Selected IR bands (cm ): 1618 (HC=N), 3339 (OH), H‒
2
NMR (CDCl ) δ (ppm): 6.9-7.8 (m, Ar), 8.7 (1H, s, HC=N), 2.1 (s, 3H), 12.3 (1H, s, OH).
3
−
1
1
L H: m.p. 168 °C. Yield: 85%. Selected IR bands (cm ): 1662 (HC=N), 3367 (OH), H‒
3
NMR (CDCl ) δ (ppm): 6.6-8.2 (m, Ar), 9.6 (1H, s, HC=N), 14.2 (1H, s, OH).
3
−
1
1
L H: m.p. 146 °C. Yield: 75%. Selected IR bands (cm ): 1638 (HC=N), 3050 (OH), H‒
4
NMR (CDCl
H: m.p. 130 °C. Yield: 90%. Selected IR bands (cm ): 1615 (HC=N), 3339 (SH), H‒
NMR (CDCl ) δ (ppm): 6.8-7.9 (m, Ar), 9.0 (1H, s, HC=N), 11.7 (1H, s, SH).
3
) δ (ppm): 6.8-8.0 (m, Ar), 9.3 (1H, s, HC=N), 12.7 (1H, s, OH).
−
1
1
L
5
3
4
.4. Procedure for the synthesis of ruthenium(II) Schiff base complexes (1
–
5)
6
A mixture containing the ruthenium(II) precursor material [(η –
p
–cymene)RuCl2]2
(
1 mmol; 0.06 mg), Schiff base ligands (L H – L H) (0.2 mmol; 0.594‒0.684 mg) and
1 5
triethylamine (0.3 ml) in dichloromethane (20 ml) was taken in a clean 100 ml round bottom
flask. This mixture was allowed to react under stirring at room temperature for 5 h. A color
change of the solution from dark red to red brown was observed. The solution was concentrated
1
9

to 1 mL, and hexane was added to initiate the precipitation of the complexes. The reaction
progress was monitored through TLC.
6
[
Ru(η ‒p‒cymene)(Cl)(L )] (1). Brown solid. Yield = 88%; m.p. 196 °C; Elemental
1
−
1
Anal. Calc: C, 65.66; H, 4.94; N, 2.47; Found: C, 65.72; H, 4.91; N, 2.53 (%). IR (cm ): 1582
1
ν(C=N), 1320 ν(C–O). UV-Vis (CHCl , λmax/nm): 460, 390, 250. H NMR (400 MHz, CDCl ) (δ
3
3
ppm): 6.6-7.8 (m, 13H, aromatic), 8.4 (s, 1H, HC=N), 4.0-5.4 (4H, p-cym-H), 2.2 (s, 3H, CCH3),
13
2
1
3 2 3 2 3
.7-2.8 (m, 1H, p-cym CH(CH ) , 1.1-1.2 (d, 3H, p-cym CH(CH ) . C NMR (CDCl ) δ (ppm):
63 (HC=N), 117-137 (m, Ar). ESI mass m/z =566.
6
[
Ru(η ‒p‒cymene)(Cl)(L
2
)] (2). Brown solid. Yield = 85%; m.p. 283 °C; Elemental
−
1
Anal. Calc: C, 66.20; H, 5.17; N, 2.41; Found: C, 66.88; H, 5.09; N, 2.37 (%). IR (cm ): 1594
1
ν(C=N), 1321 ν(C–O). UV-Vis (CHCl , λmax/nm): 410, 260, 215. H NMR (400 MHz, CDCl ) (δ
3
3
ppm): 6.9-7.8 (m, 12H, aromatic), 8.7 (s, 1H, HC=N), 4.1-5.4 (d, 4H, p-cym-H), 3.1 (s, 3H,
1
3
CCH ), 1.1-1.2 (m, 1H, p-cym CH(CH ) , 2.1 (d, 3H, p-cym CH(CH ) . C NMR (CDCl ) δ
3
3 2
3 2
3
(ppm): 158 (HC=N), 114-130 (m, Ar), 46 (CH3).
6
[
Ru(η ‒p‒cymene)(Cl)(L
3
)] (3). Brown solid. Yield = 71%; m.p. 238 °C; Elemental
−
1
Anal. Calc: C, 60.83; H, 4.41; N, 4.57; Found: C, 60.85; H, 4.40; N, 4.53 (%). IR (cm ): 1604
1
ν(C=N), 1304 ν(C–O). UV-Vis (CHCl , λmax/nm): 390, 265, 210. H NMR (400 MHz, CDCl ) (δ
3
3
ppm): 6.8-7.9 (m, 12H, aromatic), 8.5 (s, 1H, HC=N), 3.8-5.4 (d, 4H, p-cym-H), 2.2 (s, 3H,
CCH ), 1.1-1.3 (m, 1H, p-cym CH(CH ) , 0.8-0.9 (d, 3H, p-cym CH(CH ) . ESI mass m/z =
3
3 2
3 2
6
10.7
6
[
Ru(η ‒p‒cymene)(Cl)(L )] (4). Brown solid. Yield = 83%; m.p. 182 °C; Elemental
4
−
1
Anal. Calc: C, 61.89; H, 4.49; N, 2.32; Found: C, 61.95; H, 4.60; N, 2.30 (%). IR (cm ): 1571
1
ν
(C=N), 1300 ν(C–O). UV-Vis (CHCl
3
, λmax/nm): 375, 260, 208. H NMR (400 MHz, CDCl
3
) (δ
2
0

ppm): 6.9-7.8 (m, 12H, aromatic), 8.8 (s, 1H, HC=N), 4.0-5.5 (4H, p-cym-H), 1.5 (s, 3H, CCH
.0-2.2 (m, 1H, p-cym CH(CH ) , 0.8-1.0 (d, 3H, p-cym CH(CH ) . ESI mass m/z = 600.6
3
),
2
3
2
3 2
6
[
Ru(η ‒p‒cymene)(Cl)(L )] (5). Brown solid. Yield = 87%; m.p. 260 °C; Elemental
5
−
1
Anal. Calc: C, 63.85; H, 4.80; N, 2.40; Found: C, 63.92; H, 4.99; N, 2.46 (%). IR (cm ): 1577
1
ν(C=N), 1276 ν(C–S). UV-Vis (CHCl , λmax/nm): 405, 263, 210. H NMR (400 MHz, CDCl ) (δ
3
3
ppm): 6.9-7.9 (m, 13H, aromatic), 8.7 (s, 1H, HC=N), 3.8-5.6 (4H, p-cym-H), 2.1 (s, 3H, CCH3),
2
3 2 3 2
.7-3.0 (m, 1H, p-cym CH(CH ) , 0.8-1.4 (d, 3H, p-cym CH(CH ) .
4
.5. General procedure for the conversion of aldehyde to amide
Under a nitrogen atmosphere, the reaction vessel was charged with aldehyde (1 mmol),
.
NaHCO (1 mmol), NH OH HCl (1 mmol) and ruthenium catalyst (0.01 mmol) and the mixture
3
2
was refluxed in toluene for 18h. The reaction mixture was cooled to room temperature. On
completion of the reaction, 2 mL of methanol was added to the mixture, followed by filtration
through celite to remove the NaHCO and ruthenium catalyst. The crude product was then
3
2 2
purified by column chromatography (CH Cl /MeOH, 1:1 ratio) over silica gel using as solid
phase, providing the amide in good yield. The identity of the resulting amides was assessed by
1
comparison of their H NMR spectroscopic data with those reported in the literature [52].
4
.6. X-ray crystallographic studies
6
6
The single crystals of [Ru(η –
p
–cymene)(Cl)(L )] (1) and [Ru(η –
p
–cymene)(Cl)(L )] (3)
1
3
were grown from mixture of chloroform–hexane solution at room temperature. A single crystal
of suitable size was covered with Paratone oil, mounted on the top of a glass fiber, and
transferred to a Bruker APEX-II CCD single crystal X-ray diffractometer using graphite
monochromated MoKα radiation (λ = 0.71073). Data were collected at 120 K. The structure was
2
1

solved by direct methods using SIR-97 and the final refinement of the structure was carried out
2
using full matrix least-squares method on F with SHELXL-2018 [53]. All non-hydrogen atoms
were refined with anisotropy thermal parameters. All the hydrogen atoms were geometrically
fixed and collected to refine using a riding model. Cell parameters were retrieved using Bruker
SMART [54‒56] software and refined using Bruker SAINT Plus (Version 7.06a) on all the
observed reflections. In addition, some disordered electron density in complex 3 could not be
modelled accurately and thus was corrected by using the SQUEEZE command of PLATON.30.
3
The electrons recovered corresponded to CHCl solvent molecule.
Acknowledgments
The authors express sincere thanks to Single Crystal X‒ray Diffraction Facility, SAIF
Guwahati University, for providing X‒ray data collection.
Appendix A. Supplementary data
The structural data have been deposited at the Cambridge Crystallographic Data Centre:
CCDC 1890718 and 1901664 for complexes 1 and 3. These data can be obtained free of charge
from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ UK (fax: +44 1223 336033;
email: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
2
2

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2
6

Highlights
6
•
New family of η ‒p‒cymene Ru(II) Schiff base complexes containing mono anionic
bidentate O, N donors of ligands have been synthesized.
•
•
Characterization of complexes was performed by analytical and spectral methods.
Structure of the complexes (1 & 3) was confirmed by single crystal X-ray
crystallography.
•
These complexes proved efficiently catalyze (1 mol%) the one pot conversion aldehyde
to amides.