Synthesis, characterization, and catalytic applications of Ru(III) complexes containing N-[di(alkyl/aryl)carbamothioyl]benzamide derivatives and triphenylphosphine/triphenylarsine

Article abstract of DOI:10.1016/j.inoche.2010.05.004

Six coordinated ruthenium(III) complexes of the type [RuX₂(L)(PPh₃)₂] or [RuX₂(L)(AsPh₃)₂] {where L = N-[di(alkyl/aryl)carbamothioyl]benzamide derivatives, X = Cl or Br} have been prepared by the reaction between [RuX₃(PPh₃)₃] or [RuX₃(AsPh₃)₃] (where X = Cl or Br) and N-[di(alkyl/aryl)carbamothioyl]benzamide derivatives in toluene and characterized by elemental analysis, spectral data (electronic, IR and EPR) and magnetic moment studies. The complexes act as efficient catalysts for the oxidation of alcohols in presence of N-methylmorpholine-N-oxide as oxidant at room temperature.

Full text of DOI:10.1016/j.inoche.2010.05.004

Inorganic Chemistry Communications 13 (2010) 952–955
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization, and catalytic applications of Ru(III) complexes
containing N-[di(alkyl/aryl)carbamothioyl]benzamide derivatives and
triphenylphosphine/triphenylarsine
⁎
N. Gunasekaran, R. Karvembu
Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Six coordinated ruthenium(III) complexes of the type [RuX2(L)(PPh3)2] or [RuX2(L)(AsPh3)2] {where L = N-
[di(alkyl/aryl)carbamothioyl]benzamide derivatives, X = Cl or Br} have been prepared by the reaction
between [RuX3(PPh3)3] or [RuX3(AsPh3)3] (where X = Cl or Br) and N-[di(alkyl/aryl)carbamothioyl]
benzamide derivatives in toluene and characterized by elemental analysis, spectral data (electronic, IR and
EPR) and magnetic moment studies. The complexes act as efficient catalysts for the oxidation of alcohols in
presence of N-methylmorpholine-N-oxide as oxidant at room temperature.
Received 10 February 2010
Accepted 19 May 2010
Available online 25 May 2010
Keywords:
Ruthenium(III)
Triphenylphosphine
OS donor ligand
Catalytic oxidation
NMO
© 2010 Elsevier B.V. All rights reserved.
Many ruthenium mixed ligand complexes of β-diketones or thio-
β-diketones and phosphines or arsines have been synthesized and
characterized [1,2]. Some of these complexes exhibit good catalytic
properties [3] and remarkable biological activities [4]. N-[Di(alkyl/
aryl)carbamothioyl]benzamides (Fig. 1) possess same core structure
as that of monothio-β-diketones except that CH2 is replaced by NH.
These ligands provide immense opportunities for altering electronic
and steric effects in its complexes. This might help in designing
effective catalysts. Ruthenium triphenylphosphine or triphenylarsine
complexes are already known as versatile catalysts for various organic
transformations [5]. Oxidation of alcohols to carbonyl compounds is
one of the fundamental transformations in organic chemistry. Hence
we intend to study the synthesis, characterization and catalytic
oxidation properties of ruthenium(III) complexes with N-[di(alkyl/
aryl)carbamothioyl]benzamides and triphenylphosphine or triphe-
nylarsine and our results are reported herein.
The bidentate ligands (HL1–HL4) were prepared from benzoyl
isothiocyanate and secondary amines in dry acetone [6]. These ligands
on reaction with [RuX3(EPh3)3] (X = Cl or Br; E = P or As) [7–9] in
toluene (1:1 molar ratio) yielded new stable ruthenium(III) com-
plexes of the type [RuX2(L)(PPh3)2] or [RuX2(L)(AsPh3)2] {where L =
N-[di(alkyl/aryl)carbamothioyl]benzamide derivatives, X = Cl or Br}
[10]. In all the reactions, the ligands replace one triphenylphosphine/
triphenylarsine and one chloride/bromide ion from starting com-
plexes (Scheme 1). The N-[di(alkyl/aryl)carbamothioyl]benzamides
in all these complexes behave as monobasic bidentate ligands. All the
new complexes are soluble in CH2Cl2, CH3CN, C6H6, DMSO, DMF and
C2H5OH. The analytical data obtained are in good agreement with the
proposed molecular formulae.
The free carbamothioyl ligand shows a very strong absorption
band around 3200 cm−1 in its FT-IR spectrum which is characteristic
of N–H group. In complexes, this absorption has disappeared thus
indicating deprotonation of the ligands prior to coordination through
enolization. A strong band in the region of 1652–1692 cm−1 due to
C=O group of the ligands undergoes a shift to lower frequency
(1474–1484 cm−1) after complexation, indicating coordination of
C
O to ruthenium ion. A medium intensity band in the region of
1243–1314 cm−1 in the free ligands may be assigned to C=S
stretching. In the complexes, C S stretching vibrations are shifted
to lower frequency by 30–125 cm−1. This shift to lower frequency
indicates bonding of ligand to the metal through sulphur atom [6]. The
characteristic bands due to PPh3/AsPh3 (around 1410, 1100 and
750 cm−1) are also present in the spectra of the complexes [11].
Electronic spectra of all the complexes in ethanol showed two to three
bands in the region of 266–211 nm. The ground state of ruthenium
2
(III) is T2g and the first excited doublet levels in the order of
2
2
increasing energy are A2g and T1g which arise from the t42g e1g
configuration. In most ruthenium(III) complexes, the electronic
spectra show only charge transfer bands [12]. The bands observed
for the new complexes are assigned due to charge transfer transitions
⁎
Corresponding author. Tel.: +91 4312503636.
E-mail address: kar@nitt.edu (R. Karvembu).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.05.004

N. Gunasekaran, R. Karvembu / Inorganic Chemistry Communications 13 (2010) 952–955
953
Fig. 1. Structure of ligands.
Scheme 1. Synthesis of Ru(III) complexes.
of the type Lπy →t2g based on the extinction coefficient values
(ε=65,051–10,000 dm3 mol−1 cm−1) which are characteristics of
ruthenium(III) octahedral complexes [13].
acetophenone was higher when 5 was used as a catalyst. This contains
AsPh3 and chloride ligands in addition to L2. The higher activity of
triphenylarsine complex (Table 1, entries 2 and 3) compared to that of
The magnetic moments for the ruthenium(III) complexes have been
measured at room temperature and µeff values are in the range of 1.7–
1.9 BM. The room temperature EPR spectra of powdered samples were
recorded at X-band frequencies. Most of the complexes exhibit spectra
with g⊥ in the range of 1.42–1.80 and g|| in the range of 1.70–2.03. For an
octahedral field with tetragonal distortion gx=gy≠gz and, hence, the two
different ‘g’ values indicate a tetragonal distortion in these complexes [14].
The presence of two ‘g’ values also indicates an axial symmetry for these
complexes and, hence, trans positions are assigned for PPh3 or AsPh3
groups [15]. The average ‘g’ value is in the range of 1.53–1.94 and these
values fit very well with the values obtained for other ruthenium(III)
complexes [16,17]. On the basis of analytical and spectral data, an
octahedral structure has been proposed for all the complexes (Scheme 1).
In order to explore the catalytic activity of ruthenium(III) complexes,
complexes formed from HL2 (4–6) were tested as catalyst for the
oxidation of 1-phenylethanol in the presence of N-methylmorpholine-N-
oxide (NMO) as oxidant and dichloromethane as solvent at 27 °C [18].
Though all the complexes exhibited good catalytic activity, the yield of
Table 1
Effect of complexes on oxidation of 1-phenylethanol.
Entry
Complex
Yield (%)a
1
2
3
4
5
6
4
5
6
2
8
11
91
98
94
96
96
97
a
Yield is determined by GC with area normalization; GC conditions: RTX-5 column,
60 m×0.32 mm, 250 °C; FID detector, 280 °C; injector, 250 °C; carrier gas: N2;rate:2 mL/min.

954
N. Gunasekaran, R. Karvembu / Inorganic Chemistry Communications 13 (2010) 952–955
Table 2
these complexes have shown remarkable activity for the oxidation of 1-
Oxidation of alcoholsa by 5.
phenylethanol (Table 1, entries 4, 5 and 6). Complex 5 was chosen as a
catalyst to extend the scope of substrates. The reaction conditions were
optimized. A 0.01 mmol of catalyst was sufficient for oxidation;
dichloromethane was a better solvent and no further oxidation observed
after 12 h. Table 2 shows the results for the oxidation of various alcohols
using 5 as a catalyst. Benzylic primary and secondary alcohols (Table 2,
entries 1–4 and 6) oxidize smoothly to give aldehydes and ketones re-
spectively. Oxidation of allylic alcohols to α,β-unsaturated carbonyl
compounds has long been of interest. It was found that cinnamaldehyde
was obtained from cinnamylalcohol in 92% yield after stirring 12 h at 27 °C
(Table 2, entry 7) whereas [Ru(acac)2(CH3CN)2]PF6/H5IO6 system gives
only 65% of aldehyde from the same substrate [19]. 1-Cyclohexylethanol
was readily converted into corresponding ketone in 82% yield (Table 2,
entry 9). Conversion of 1-indanol to 1-indanone was carried out at 27 °C
(Table 2, entry 8) and the yield is comparable with K3[Fe(CN)6] catalyzed
oxidation of the same substrate with TEMPO [20]. Cyclohexanol was
converted into cyclohexanone in 51% yield after 12 h of stirring at 70 °C.
Catalytic oxidation of 1-phenoxy-2-propanol proceeded smoothly
(Table 2, entry 5). Ruthenium(III) catalyzed oxidation of 2-octanol
(Table 2, entry 11) did not give satisfactory result even at elevated
temperature. In general, these synthesized ruthenium(III) complexes
work as catalysts at room temperature for the oxidation of various
primary and secondary alcohols. The yield of products obtained from the
present catalytic system is relatively higher compared to other room
temperature catalytic systems [20,21]. The relatively lower product yield
obtained for the oxidation of cyclohexanol (Table 2, entry 10) and 2-
octanol (Table 2, entry 11) compared with most of the aromatic substrates
may be due to the fact that α-CH unit of cyclohexanol and 2-octanol is less
acidic than aromatic substrates [22]. A characteristic peak at 850 cm−1 in
the IR spectrum of reaction mixture supports the fact that the catalytic
cycle passes through a Ru(V)=O species [22]. The precursor complexes
[RuCl3(PPh3)3], [RuCl3(AsPh3)3] and [RuBr3(AsPh3)3] exhibited very less
catalytic activities compared to newly formed ruthenium(III) complexes
which contains N-[di(alkyl/aryl)carbamothioyl]benzamide ligands.
In conclusion, we have prepared and characterized ruthenium(III)
complexes with N-[di(alkyl/aryl)carbamothioyl]benzamide (L1–L4),
PPh3 or AsPh3 and Cl− or Br− ligands. Ruthenium complex which
contains L2, AsPh3 and Cl− is found to be a better catalyst for the
oxidation of alcohols with NMO as oxidant and the scope of substrates
is extended to various alcohols. Interestingly, the ligands as well as
complexes were synthesized at room temperature. In addition, most
of the catalytic reactions were carried out at room temperature.
Entry
1
Substrate
Product
Yieldb (%)
98
2
3
4
5
6
7
8
88
55c
58c
75c
86
92
63
Acknowledgements
N.G. thanks NITT for fellowship and seed money. R.K. acknowl-
edges DST, Government of India, for financial assistance under SERC
Fast Track scheme for Young Scientists (No. SR/FTP/CS-80/2006).
9
82
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2010.05.004.
10
51c
28
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N. Gunasekaran, R. Karvembu / Inorganic Chemistry Communications 13 (2010) 952–955
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[10] Preparation of complexes: All the ruthenium(III) complexes are prepared by the
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