Ruthenium(II) NNO pincer type catalyst for the conversion of aldehydes to amides

Article abstract of DOI:10.1016/j.inoche.2012.01.035

The reaction of [RuHCl(CO)(PPh₃)₃] and 2-(2-thiazolylazo) cresol ligand (TAC) in benzene under reflux afforded a new pincer type ruthenium(II) carbonyl complex containing thiazolylazo cresol of general formula [RuCl(CO)(PPh₃)(TAC)] (TAC = mono anionic NNO donor). The complex has been characterized by elemental analyses, FT-IR, UV-Vis, and ¹H NMR spectral methods. The crystal structure of the complex [RuCl(CO)(PPh₃)(TAC)] has been determined by single crystal X-ray diffraction which reveals the presence of a distorted octahedral geometry in the complex. The complex has been shown as an efficient catalyst for the conversion of aldehydes to amides in a one-pot process in the presence of NH ₂OH?HCl/NaHCO₃.

Full text of DOI:10.1016/j.inoche.2012.01.035

Inorganic Chemistry Communications 19 (2012) 51–54
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Inorganic Chemistry Communications
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Ruthenium(II) NNO pincer type catalyst for the conversion of aldehydes to amides
⁎
Nandhagopal Raja, Mathiyazhagan Ulaganatha Raja, Rengan Ramesh
School of Chemistry, Bharathidasan University, Tiruchirappalli — 620 024, Tamilnadu State, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of [RuHCl(CO)(PPh3)3] and 2-(2-thiazolylazo) cresol ligand (TAC) in benzene under reflux
afforded a new pincer type ruthenium(II) carbonyl complex containing thiazolylazo cresol of general formula
[RuCl(CO)(PPh3)(TAC)] (TAC=mono anionic NNO donor). The complex has been characterized by elemental
analyses, FT-IR, UV–Vis, and 1H NMR spectral methods. The crystal structure of the complex
[RuCl(CO)(PPh3)(TAC)] has been determined by single crystal X-ray diffraction which reveals the presence
of a distorted octahedral geometry in the complex. The complex has been shown as an efficient catalyst for
the conversion of aldehydes to amides in a one-pot process in the presence of NH2OH·HCl/NaHCO3.
© 2012 Elsevier B.V. All rights reserved.
Received 17 December 2011
Accepted 28 January 2012
Available online 12 February 2012
Keywords:
Thiazolylazo Ru(II) complex
Synthesis
Crystal structure
Aldehyde to amide
The chemistry of ruthenium by different types of ligands is of signifi-
cant importance [1,2] because of the fascinating reactivities exhibited by
the resultant complexes and the nature of the ligand that dictates the
property of those complexes. Particularly, N-heterocyclic o-hydroxy
substituted ligands represent an interesting class of highly sensitive com-
plexing reagents. A novel property of this azomethine (\N_C\N_N\)
nitrogen donor ligands is its ability to stabilize low oxidation state metal
centers through its strong π-acidity and weak δ donor character [3,4]. Be-
cause hetero atoms, ring size and the presence of substituents in the het-
erocyclic ring significantly change the π-acidity of the ligands and
influence the physical and chemical properties of the metal complexes.
For example, thiazolylazo reagents are popular as metal complexing li-
gands in spectrophotometry, high performance ligands chromatography
[5,6] and capillary electrophoresis [7] due to the advantages that they
can form highly sensitive metal complexes.
Amides are an important class of chemicals that have been widely
used as chemical intermediates in organic synthesis, raw materials for
engineering plastics, detergents, and lubricants. The conversion of
carbonyl compounds, such as aldehydes, ketones, and oximes, is a
good candidate for the synthesis of amides [8–10]. Beckmann rear-
rangement is commonly used to transform oximes into the corre-
sponding amides [11]. This rearrangement is commonly used to
transform ketoximes into the corresponding N-substituted amides re-
quiring the use of strong acids [12]. 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 nitriles, car-
boxylic acids, and aldehydes are formed in some cases. While the
migrating group can be alkyl or aryl, it is rarely hydrogen that mi-
grates, and so the Beckmann rearrangement is not a general process
for the conversion of aldoximes into primary amides [13–15].
The rearrangement of aldoximes into amides has been reported
using transition metal catalysts containing nickel [16], copper [17],
palladium [18], iridium [19,20], rhodium [21] and zinc [22]. In a re-
cent advance, Williams and coworkers reported a ruthenium catalyst
for this reaction with high selectivity, excellent yield and low cata-
lyst loading with p-toluenesulfonic acid as an additive [23]. Further,
Crabtree et al. reported ruthenium terpyridine complexes as an effi-
cient catalyst to achieve high yields without any additive [24]. In
addition, [RuCl2(DMSO)4] complex was used as a catalyst for the
aldehydes to primary amide reactions [25]. In contrast to the con-
siderable growth of literature on the chemistry of azo complexes,
to the best of our knowledge there are no reports available for
catalytic transformation of aldehydes to amides by ruthenium(II)
carbonyl complex containing 2-(2-thiazolylazo) cresol incorporat-
ing triphenyl phosphine.
The ligand was prepared by modification of the reported proce-
dure [26]. The general procedure for the synthesis of ruthenium(II)
NNO pincer type complex is as follows. To a benzene solution of
[RuHCl(CO)(PPh3)3] [27] (100 mg, 0.105 mmol) was added to the 2-
(2-thiazolylazo) cresol ligand (23.02 mg, 0.105 mmol) and the mix-
ture was refluxed for 6 h (Scheme 1). The solvent was then removed
under high vacuum at low temperature followed by dichloromethane
and petroleum ether workup gave blue colored solids. The complex is
recrystallized from CH2Cl2/pet. ether and dried under vacuum. Yield
is 70%.
The complex is found to be air stable and soluble in common or-
ganic solvents, such as CHCl3, CH2Cl2, CH3CN, etc. The elemental ana-
lyses are in good agreement with the molecular formula proposed for
the complex. The IR spectrum of the complex confirmed the coordi-
⁎
Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045/2407020.
E-mail address: ramesh_bdu@yahoo.com (R. Ramesh).
nation of the azo group (ν(N
_
1391 cm−1) and phenolic oxygen
N)
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.01.035

52
N. Raja et al. / Inorganic Chemistry Communications 19 (2012) 51–54
Scheme 1. Synthesis of pincer type complex [RuCl(CO)(PPh3)(TAC)].
ruthenium(II) ion with the O, N, N donor atom forming two five
membered chelate rings as in the case of typical pincer complex
within the bite angles of N(3)–Ru(1)–N(1)=76.58(8) and N(1)–
Ru(1)–O(2)=79.53(7). The bond length of Ru(1)–N(1)=2.066(2)
(A°), Ru(1)–N(3)=2.020(2) (A°), Ru(1)–C(1)=1.881(2) (A°) and
Ru(1)–O(2)=2.1040(2) (A°). Similarly, the bond distances of the
Ru(1)–P(1) and Ru(1)–Cl(1) are 2.3151(6) (A°) and 2.4282(7) (A°)
respectively. It has been observed that the PPh3 is mutually trans to
chlorine atom. The Ru\C bond length 1.8807 (A°) in Ru(1)–C(1),
fragment is quite normal like that in the structurally characterized
carbonyl complexes of ruthenium [31]. The overall C1N2O1P1Cl1 co-
ordination sphere around the ruthenium is distorted octahedral in
nature, which is reflected in all the bond parameters around
ruthenium.
Further, the catalytic organic transformation of aldehydes to am-
ides by the synthesized ruthenium(II) thiazolylazo cresol complex
has been studied in toluene in the presence of NH2OH·HCl/NaHCO3
(Scheme 2) and the products were passed through the celites and sil-
ica gel to get pure compounds [32]. The catalytic reaction was carried
out under a set of conditions and a series of blank or control experiments
suggests that none of ruthenium(II) precursors or 2-(2-thiazolylazo) cre-
sol alone or as a mixture causes these transformations under identical re-
action conditions. Complex [RuCl(CO)(PPh3)(TAC)] efficiently catalyzes
the rearrangement of aldehydes into amides with high yields, according
to the proposed mechanism [25].
Fig. 1. The X-ray crystal structure of the complex [RuCl(CO)(PPh3)(TAC)].
In order to optimize the reaction conditions, different catalyst:
substrate ratios were tested and the results are summarized in
Table 1. For these initial experiments benzaldehyde was selected as
a test-substrate and allowed it to react in toluene with catalytic quan-
tities of complex [RuCl(CO)(PPh3)(TAC)] in the presence of NaHCO3
as an additive. In the reaction optimization, we started the C/S ratios
of 1:100, 1:200 and 1:500, the reaction proceeds with excellent con-
versions. When increasing the C:S ratio from 1:500 and 1:1000 in tol-
uene, the reaction still proceeds by a drop in yield of amide. When
decreasing the C:S ratio to 1:200, the reaction proceeds by a moderate
yield. Thus, it was concluded that catalyst:substrate ratio of 1:100 is
the best compromise between optimal reaction rates in toluene and
we obtained 95% yield of amide (entry 4).
To study the effects of different solvents in our catalytic system,
we have chosen the reaction between benzaldehyde with NH2-
OH·HCl and NaHCO3 in the presence of various solvents (Table 2).
Benzene, toluene, xylene, acetonitrile, dichloromethane and chloro-
form are taken for our solvent variation study. Toluene gives an excel-
lent yield of 95% (entry 2) and acetonitrile yields 82% (entry 4).
(ν(C
\
1289 cm−1) to the metal atoms. The spectrum of complex
O)
shows a strong band in the region 1978 cm−1 due to the coordinated
νC`O group [28]. In addition, this complex showed new bands near
530, 695, 740 and 1557 cm−1 due to the triphenyl phosphine ligands.
The electronic spectrum of the complex has been recorded in chloro-
form solution. The spectrum of the complex shows a visible region
with an intense absorption at 446 nm (ε=1020–2051 dm3/mol/cm)
region due to MLCT in addition to the π–π* and n–π* ligand centered
transitions (263–242 nm) [29]. In 1H NMR spectrum of the complex,
the aromatic protons appeared as a multiplet in the region of
6.6–7.3 ppm. The absence of broad singlet at δ 10.4–11.5 ppm indi-
cates the coordination of phenolic oxygen to ruthenium. A singlet
appeared at 1.8 ppm is due to the presence of methyl proton in the
complex [30].
To understand the coordination mode of the 2-(2-thiazolylazo)
cresol ligand and the geometry of the ruthenium(II) complex, the
X-ray crystal structure of the complex [RuCl(CO)(PPh3)(TAC)] was
determined and is shown in Fig. 1. The ligand coordinates to the
Scheme 2. Catalytic organic transformation of aldehydes to amides.

N. Raja et al. / Inorganic Chemistry Communications 19 (2012) 51–54
53
Table 1
The scope of our catalytic system is applicable to a wide range
of aromatic, substituted, conjugated and heterocyclic aromatic alde-
hydes. These aldehydes were converted to their corresponding
amides in one pot process using NaHCO3 and the results are summa-
rized in Table 3 (entries 1–9).
Effect of catalyst/substrate ratio for benzaldehyde to benzamide using complex
[RuCl(CO)(PPh3)(TAC)].a
Entry
Ratio
Time
Yieldb (%)
1
2
3
4
1:1000
1:500
1:200
1:100
12
12
12
12
b25
25
56
Using [RuCl(CO)(PPh3)(TAC)], an excellent yield was obtained for
benzaldehyde to benzamide (entry 1) in 95%. Benzaldehydes with its
derivatives were all smoothly converted to the corresponding amides
in good yields. The presence of electron donating groups like \CH3
and \OCH3 (entries 2 and 3) in the substrates alters the reactions
and the corresponding amides were obtained in good yields 80%
and 75% respectively. On the other hand, electron withdrawing groups,
such as the \NO2, \Cl and \Br substituents (entries 4, 5 and 6) offering
better yields (88%, 90% and 91%) when compared to substrate containing
electron donating group. Recently, a similar observation has been made
on [RuCl2(DMSO)4] complex as catalyst in toluene medium [25]. The pre-
sent catalyst shows good catalytic activity for the conjugated aldehyde
such as, cinnamaldehyde which was converted to the conjugated amides
(entry 7) in 90% yield. Further, the complex efficiently catalyzes the thio-
phenealdehyde to thiophenecarboxamide (entry 8) with yields up to 91%.
In conclusion, new air stable ruthenium(II) carbonyl com-
plex containing pincer type NNO donor of general formula
[RuCl(CO)(PPh3)(TAC)] has been synthesized and characterized. X-
ray diffraction study of the complex confirms the N, N and O coordi-
nation of the ligand and reveals the presence of a distorted octahe-
dral geometry. The catalytic ability of the present complex for the
conversion of aldehydes to amides has been studied and the conver-
sions are found to be excellent.
95
a
Conditions: reactions were carried out. Substrate (1–10 mmol), NH2OH·HCl
(1–10 mmol), catalyst (1 mol%) and 2 ml of toluene were refluxed for 12 h.
b
Isolated yield after column chromatography.
Table 2
Effect
of
solvents
for
benzaldehyde
to
benzamide
Time
using
complex
[RuCl(CO)(PPh3)(TAC)].a
Entry
Solvents
Yieldb (%)
1
2
3
4
5
6
Benzene
Toluene
Xylene
Acetonitrile
Dichloromethane
Chloroform
24
12
24
12
24
24
32
95
37
82
30
24
a
Aldehydes (1 mmol), NH2OH·HCl (1 mmol), NaHCO3 (1 mmol), catalyst (1 mol%)
and 2 ml solvent.
b
Isolated yield after column chromatography.
Further benzene and xylene gave yields such as 32% and 37% respec-
tively. Lowest yield was obtained with dichloromethane and chloro-
form (entries 5 and 6) 30% and 24% respectively.
Table 3
One pot conversion of aldehydes to amides by complex [RuCl(CO)(PPh3)(TAC)].a
S. no
1
Aldehydes
Amides
Yieldb (%)
95
O
O
H
NH2
O
O
NH2
H
2
3
4
80
75
88
H3C
H3C
H3C
O
O
H
NH2
O
O
H3C
O
NH2
O2N
O
O
5
6
7
90
91
H
H
NH2
Cl
Br
Cl
Br
O
O
NH2
H2N
O
O
H
90
91
O
O
S
S
8
NH2
H
a
Aldehydes (1 mmol), NH2OH·HCl (1 mmol), NaHCO3 (1 mmol), catalyst (1 mol%) and toluene (2 ml) were refluxed for 12 h.
Isolated yield after column chromatography.
b

54
N. Raja et al. / Inorganic Chemistry Communications 19 (2012) 51–54
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Acknowledgments
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One of the authors (N.R.) thanks University Grants Commission
(UGC), New Delhi for the award of UGC-SAP RFSMS Scholarship. We
thank D.S.T-India (FIST program) for the use of Bruker Smart APEX
II diffractometer and 1H NMR at the School of Chemistry, Bharathidasan
University.
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Crystallographic data for the structural analysis have been deposited
with Cambridge crystallographic center, CCDC No. 825814. Copies of this
information may be obtained free of charge from The Director, CCDC, 12
Union roads, Cambridge.CB2 1EZ, UK (email: deposit@ccdc.cam.ac.uk).
Complete sets of refined positional coordinates as well as anisotropic
thermal parameters and complete tabulations of bond lengths and
bond angles.
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2012.01.035.
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[32] Typical procedures for the conversion of aldehydes to amides: The reaction vessel
was charged with aldehyde (1 mmol), NH2OH·HCl (1 mmol), NaHCO3 (69.5 mg,
1 mmol), and ruthenium catalyst (6.5 mg, 0.01 mmol) and the mixture was
placed under an atmosphere of N2. About 2 ml of dry and degassed toluene was
added and the mixture was stirred for 15 min at room temperature and followed
by the reflux for 12 h. On completion of the reaction, 2–3 ml MeOH was added to
the mixture followed by filtration through celite to remove catalyst and NaHCO3.
The crude product was then purified using column chromatography (MeOH/CH2Cl2)
and the pure amide was obtained in good yield.