Synthesis, spectral and catalytic dehydrogenation studies of ruthenium complexes containing NO bidentate ligands

Article abstract of DOI:10.1016/j.saa.2014.04.132

The synthesis and characterization of ruthenium mononuclear complexes containing NO bidentate ligands are reported. The complexes cis-[Ru ^II(bpy)₂L](PF₆)_n (1a-c), [Ru ^IIICl(L)₂(H₂O)] (2a-b) and [Ru ^IIICl₂(L)₂]Cl (2c) were prepared by the reaction of cis-[Ru^IICl₂(bpy)₂]·2H ₂O (bpy = 2,2′-bipyridine) and/or RuCl₃· nH₂O with the Ligands: 2-aminophenol (2-aph), 8-hydroxyquinoline (8-hq) and 4-aminoantipyrine (4-apy). These complexes were characterized by elemental analysis, spectroscopic (IR, UV-Vis, ¹H NMR, ESR) and magnetic susceptibility measurements. The ligand field parameters, Δ_o (splitting parameter), B (Racah parameter of interelectronic repulsion), and β (nephelauxetic ratio) were calculated. The redox properties were also investigated electrochemically by cyclic voltammetry. The complexes cis-[Ru^II(bpy)₂(8-hq)](PF₆) ₂ (1b) and [Ru^IIICl(8-hq)₂(H₂O)] (2b) have been investigated in conjunction with N-methylmorpholine-N-oxide (NMO) as co-oxidant for the catalytic dehydrogenation of benzyl amine, p-methyl benzylamine and p-nitrobenzylamine to their respective nitriles.

Full text of DOI:10.1016/j.saa.2014.04.132

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 490–496
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectral and catalytic dehydrogenation studies of ruthenium
complexes containing NO bidentate ligands
⇑
A.F. Shoair, A.A. El-Bindary
Chemistry Department, Faculty of Science, Damietta University, 34517 Damietta, Egypt
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ESR spectrum of [RuIIICl(2-aph)
2 2
(H O)] (2a) in DMF/toluene at 77 K.
ꢀ
ꢀ
ꢀ
Ruthenium mononuclear complexes
containing NO bidentate ligands.
The complexes are characterized by
different spectroscopic techniques.
Ruthenium complexes are used as
catalysts dehydrogenation of different
amines to their respective nitriles.
The relative stabilities of the oxidation
and reduction states are examined
electrochemically.
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of ruthenium mononuclear complexes containing NO bidentate
Received 21 February 2014
Received in revised form 3 April 2014
Accepted 14 April 2014
II
III
III
ligands are reported. The complexes cis-[Ru (bpy)
L)
RuCl
4-apy). These complexes were characterized by elemental analysis, spectroscopic (IR, UV–Vis, H NMR,
ESR) and magnetic susceptibility measurements. The ligand field parameters, (splitting parameter), B
Racah parameter of interelectronic repulsion), and b (nephelauxetic ratio) were calculated. The redox
2
L](PF
6
II
)
n
(1a–c), [Ru Cl(L)
2
(H
2
O)] (2a–b) and [Ru Cl
2
0
(
2
]Cl (2c) were prepared by the reaction of cis-[Ru Cl
2
(bpy)
2
]ꢁ2H
2
O (bpy = 2,2 -bipyridine) and/or
3
ꢁnH
2
O with the Ligands: 2-aminophenol (2-aph), 8-hydroxyquinoline (8-hq) and 4-aminoantipyrine
Available online 30 April 2014
1
(
D
o
Keywords:
Ruthenium
NO bidentate ligands
Catalytic dehydrogenation
Electrochemistry
(
II
properties were also investigated electrochemically by cyclic voltammetry. The complexes cis-[Ru
(
III
2 6 2 2 2
bpy) (8-hq)](PF ) (1b) and [Ru Cl(8-hq) (H O)] (2b) have been investigated in conjunction with
N-methylmorpholine-N-oxide (NMO) as co-oxidant for the catalytic dehydrogenation of benzyl amine,
p-methyl benzylamine and p-nitrobenzylamine to their respective nitriles.
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
their favorable photophysical properties, excited state reactivities
and chemical stabilities [1–4]. In addition, ruthenium complexes
Ruthenium complexes have been extensively studied, particu-
larly those containing 2,2 -bipyridine or NO bidentate ligands. These
complexes have attracted great attention in fields of solar energy
conversion, artificial photosynthesis and optical sensing, due to
have been proven to be a good catalyst for the oxidation of organic
compounds [5–8]. The dehydrogenation of primary amines plays an
important role in the synthesis of nitriles. However, the develop-
ment of new oxidative processes continues to draw an attention
in spite of the availability of numerous oxidizing agents [9,10].
Nitriles are useful intermediates in the synthesis of biologically
active compounds and pharmaceutical substances [11]. Therefore,
0
⇑
Corresponding author. Tel.: +20 1114266996; fax: +20 572403868.
E-mail address: abindary@yahoo.com (A.A. El-Bindary).
http://dx.doi.org/10.1016/j.saa.2014.04.132
386-1425/Ó 2014 Elsevier B.V. All rights reserved.
1

A.F. Shoair, A.A. El-Bindary / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 490–496
491
the development of efficient catalytic systems for the oxidative
dehydrogenation of primary aromatic amines to nitriles has
attracted much attention [12,13]. As a part of our efforts to develop
new ruthenium(II)/(III) complexes containing inexpensive ligands
Gouy’s method, employing Hg[Co(SCN)
4
] for calibration purpose
and were corrected for diamagnetism by using Pascal’s constant.
1
6
H NMR spectra in d -DMSO were recorded on a Varian Unit plus
300 MHz model using TMS as an internal standard. ESR measure-
ments of the polycrystalline samples were recorded on a Bruker
ECS 106 EPR spectrometer using a quartz Dewar vessel at room
temperature (College of Science, Department of Chemistry, Tamk-
ang University, Taiwan). All spectra were calibrated with DPPH
(g = 2.0027). The electrochemical behavior of the complexes was
studied using an electrochemical analyzer CH1 610A (HCH Instru-
ment). The electrochemical cell assembly consists of Pt wire as
working electrode, a Pt wire auxiliary electrode and Ag/AgCl, KCl
[
14–17] and their applications as catalysts [18,19], we report herein
the synthesis and characterization of ruthenium complexes;
II
III
cis-[Ru (bpy)
2
L](PF
6
)
n
2 2
(1a–c), [Ru Cl(L) (H O)] (2a–b) and
III
II
[
Ru Cl
2
(L)
2
]Cl (2c) by the reaction of cis-[Ru Cl
2
(bpy)
2
]ꢁ2H
2
O
0
(
bpy = 2,2 -bipyridine) and/or RuCl
nophenol (2-aph), 8-hydroxyquinoline (8-hq) and 4-aminoantipy-
rine (4-apy). The catalytic activities of the characterized
3
ꢁnH
2
O with the ligands 2-ami-
II
III
complexes; cis-[Ru (bpy)
2 6 2 2
(8-hq)](PF ) (1b) and [Ru Cl(8-hq)
ꢂ1
(
2
H O)] (2b) towards dehydrogenation of benzyl amine, p-methyl-
(3.0 mol L ) as a reference electrode. The reference electrode
benzylamine and p-nitrobenzylamine to their respective nitriles in
the presence of N-methylmorpholine-N-oxide (NMO) as co-oxidant
were investigated at room temperature. The relative stabilities of
the oxidation and reduction states are examined electrochemically.
was separated from the bulk solution by a fitted-glass bridge filled
with the solvent/supporting electrolyte mixture. Tetrabutylammo-
nium chloride (TBAC) was used as the supporting electrolyte and
ꢂ1
the scan rate used was 50 mV s
atmosphere.
in dry DMF under nitrogen
Experimental
Preparation of cis-[RuCl
2
(bpy)
2
]ꢁ2H
2
O complex
Chemicals and physical measurements
II
0
The complex cis-[Ru Cl
2
(bpy)
2
]ꢁ2H
2
O (where bpy = 2,2 -bipyridine)
All chemicals and solvents were of analytical grade and used
without further purification. Ruthenium chloride, 2-aminophenol,
was prepared using the method suggested by Sullivan et al. [20].
8
-hydroxyquinoline and 4-aminoantipyrine were purchased from
Preparation of ruthenium(II) complexes (1a–c)
Sigma–Aldrich Chemicals Company (USA). Microanalyses were
performed using a Perkin–Elmer CHN 2400 microanalysis unit at
Cairo University (Cairo, Egypt). The IR spectra were recorded as
II
) (1a) and cis-[RuII
(4-apy)](PF (1c)
The complexes cis-[Ru (bpy)
2
(2-aph)](PF
6
II
(bpy)
2
(8-hq)](PF
6
)
(1b) and cis-[Ru (bpy)
(Scheme 1) were prepared by adding an methanolic triethylamine
(10%, 0.21 mmol, 0.29 mL) followed by the ligand (1.2 mmol) in
suspension of cis-[RuCl
(104 mg, 0.2 mmol) in aqueous methanol (5 mL). The reaction
2
6 2
)
ꢂ1
KBr discs in the 4000–400 cm range on a JASCO 410 spectropho-
tometer. The electronic absorption spectra were obtained in DMF
solution on a Perkin–Elmer Lambada 2 spectrophotometer. Mag-
netic measurements were carried out at room temperature using
methanol (5 mL) to
a
2 2 2
(bpy) ]ꢁ2H O
II
L]+ (1a–b), cis-[Ru (bpy) L]2+ (1c) and [RuIIICl(L) O)] (2a–b) [RuIIICl
II
Scheme 1. The proposed structures of cis-[Ru (bpy)
-aminophenolate (2-aph), 8-hydroxyquinolate (8-hq) and 4-aminoantipyrine (4-apy)].
2 2 2 2 2 2
(H (L) ]Cl (2c) complexes [ligands (L) =
2

4
92
A.F. Shoair, A.A. El-Bindary / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 490–496
III
mixture was refluxed under nitrogen for 3 h during which the ini-
tial dark brown color turned to dark red. The reaction was cooled
to room temperature and methanol was evaporated under reduced
pressure and the aqueous solution filtered through a sintered glass.
The complexes were precipitated by addition of saturated aqueous
(8-hq)](PF
6
) (1b) or [Ru Cl(8-hq)
(H
2 2
O)] (2b) complexes as a cata-
lyst and N-methylmorpholine-N-oxide (NMO) (3 mmol, DMF) as
co-oxidant in refluxing and stirring condition for 3 h at room
temperature [7,20]. The mixture was taken up in diethylether,
and filtered through a bed of silica gel. The produced benzonitrile
was weighed and identified by comparing the IR and melting
points of the produced benzonitriles with an authentic sample.
solution of NH
plexes were collected by filtration, washed by water and dried in
vacuuo over anhydrous CaCl . The yield was (70–75%).
Elemental analysis for cis-[Ru (bpy)
Calcd (%) for C26 OPF Ru (MW = 666.49): C, 46.81; H, 3.30;
4 6 2
PF (0.1 mg in 3 mL H O). The resulting solid com-
2
II
2
6
(2-aph)](PF ) (1a):
Results and discussion
H
22
N
5
6
N, 10.5; Found: C, 46.41; H, 3.40; N, 10.2. Selected IR bands (KBr,
All the complexes were air stable, non-hygroscopic solids,
which decomposed at high temperature and were found to be
insoluble in common organic solvents. The stoichiometries of the
complexes have been deduced from their elemental analysis
results. The ligands are bidentate and coordinate through oxygen
ꢂ1
cm ): 3430b
t
(OH), 3285m
(RuAN).
Elemental analysis for cis-[Ru (bpy)
Calcd (%) for C29 OPF
t
s
(NH
2
), 3325m
t
as(NH
2
), 465
t(RuAO), 437 t
II
2
(8-hq)](PF
6
) (1b):
H
22
N
5
6
Ru (MW = 703.52): C, 49.46; H, 3.12;
N, 9.94; Found: C, 49.92; H, 3.10; N, 9.75. Selected IR bands (KBr,
atom of deprotonated hydroxyl group and nitrogen atom of the
ꢂ1
II
cm ): 3390b
t
(OH), 455w
t
(RuAO), 435
t
(RuAN).
(4-apy)](PF
2
F12Ru (MW = 905.60): C, 41.07; H,
amino group. However, the complexes cis-[Ru (bpy)
2
L](PF
6
)
II
II
Elemental analysis for cis-[Ru (bpy)
Calcd (%) for C32 OP
2
6 2
) (1c):
(
1a–b) and cis-[Ru (bpy)
2
L](PF
6
)
2
(1c) have C
1
-symmetry while
(L) ]Cl (2c) have
III
III
H
29
N
7
the complexes, [Ru Cl(L)
2
2
(H O)] (2a–b) [Ru Cl
2
2
3
.20; N, 10.82; Found: C, 41.27; H, 3.40; N, 11.22. Selected IR bands
a D2h-symmetry. The magnetic susceptibility measurements show
ꢂ1
6
(
KBr, cm ): 3260m
t
s
2 2
(NH ), 3320m tas(NH ), 1660m t(C@O), 459
that the complexes (1a–c) are diamagnetic (low spin d S = 0)
t(RuAO), 442
t(RuAN).
while the complexes (2a–c) are paramagnetic (leff = 1.82–1.96
5
B.M., low spin d S = ½), as is normal for ruthenium(II) and
Preparation of ruthenium(III) complexes (2a–c)
ruthenium(III) complexes, respectively [21] (Table 1).
III
III
1
II
The complexes [Ru Cl(2-aph)
2
(H
]Cl (2c) (Scheme 1) were prepared by dis-
O (100 mg, 0.4 mmol) in ethanol (10 mL) under
2
O)] (2a) [Ru Cl(8-hq)
(H
2 2
O)]
H NMR spectra of cis-[Ru (bipy)
2
L](PF
6
) (1a–b) and
III
II
(
2b) and [Ru Cl
2
(4-apy)
2
cis-[Ru (bipy)
L](PF )
2 6 2
(1c) complexes
solving RuCl
3
ꢁnH
2
1
reflux until the initial black color turned into green and an etha-
nolic solution of the ligand (2 mmol) was added. The reaction mix-
ture was refluxed for 3 h. The resulting olive-green solid complexes
were collected by filtration, washed by hot ethanol and dried in
The H NMR spectra is a good tool to check the purity of organic
1
compounds. The H NMR spectra of the ruthenium(II) complexes
(1a–c) were recorded in DMSO-d . Due to the similarity of the aro-
6
matic protons signals they all appear in a narrow range. The signals
of the aromatic protons appeared as a multiple in the spectra of the
ligands and complexes within the range 8.20–7.00 ppm. However,
the intensity measurements correspond accurately to the total
number of aromatic protons in these complexes. An isolated signal
was observed within the range 5.20–4.80 ppm and this signal was
assigned to the amino group protons of the ligands.
2
vacuuo over anhydrous CaCl . The yield was (60–70%).
III
Elemental analysis for [Ru Cl(2-aph)
2 2
(H O)] (2a):
Calcd (%) for C12 ClRu (MW = 370.76): C, 38.84; H, 3.77;
14 2 3
H N O
N, 7.55; Found: C, 39.54; H, 3.97; N, 7.75. Selected IR bands (KBr,
ꢂ1
cm ): 3440b
t
(OH), 3280m
(RuAO), 450 (RuAN).
Elemental analysis for [Ru Cl(8-hq)
Calcd (%) for C18 ClRu (MW = 442.82): C, 48.78; H, 3.16;
s 2 2
t (NH ), 3320m tas(NH ), 329vs
(
RuACl), 455
t
t
III
2
2
(H O)] (2b):
H
14
N
2
O
3
Infrared spectra
N, 6.32; Found: C, 49.68; H, 3.46; N, 6.72. Selected IR bands (KBr,
ꢂ1
cm ): 3460b
t
(OH), 324vs
t
(RuACl), 460
t
(RuAO), 444
]Cl (2c):
3
Ru (MW = 613.98): C, 42.99; H, 4.23;
t
(RuAN).
The IR spectral data of the organic ligands and their Ru com-
plexes are compared and the following features can be pointed out:
III
Elemental analysis for [Ru Cl
Calcd (%) for C22 Cl
2 2
(4-apy)
H
26
N
6
O
2
N, 13.68; Found: C, 43.11; H, 4.45; N, 13.78. Selected IR bands (KBr,
(1) The broad and medium intensity band due to t(OH) group
ꢂ1
cm ): 3260m
s 2
t (NH ), 3315m
2
tas(NH ), 1670m
t(C@O), 330vs
which appear in the spectra of the free ligands 2-aminophe-
ꢂ1
t(RuACl), 455w
t(RuAO), 445
t(RuAN).
nol and 8-hydroxyquinoline at 3430–3390 cm region were
ꢂ1
shifted to lower wavenumbers by 5–12 cm upon complex-
ation denoting its coordination to Ru center through
deprotonated hydroxyl group of 2-aminophenol and
Catalytic dehydrogenation of benzylamine to benzonitrile
8
-hydroxyquinoline [16].
Catalytic dehydrogenation of benzylamine (2 mmol) was car-
II
ried out using (0.01 mmol, 10 mL DMF) of cis-[Ru (bpy)
2
Table 1
a
ꢂ1
Electronic spectral data and ligand field parameters (cm ) for the prepared complexes in DMF.
ꢃ
ꢃ
ꢃ
Ru(II) complexes
m
1
(1A1g ? 1
T
1g
)
m
2
(1A1g ? 1
T
2g
)
m
3
(1A1g ? 1
E
1g
)
p
–
p
, n–
p
p
m
2
/
m
1
D
o
B
C
b
1
1
1
a
b
c
17,825
17,825
17,141
25,595
25,706
25,316
40,322
30,387
35,087
1.43
1.44
1.47
17,800
18,564
18,080
712
714
630
3166
3175
2802
0.73
0.75
0.65
ꢃ
Ru(III) complexes
m
1
(2T2g ? 2
A
1g, 2
T
1g
)
m
2
(2T2g ? 2
E
g
)
m
3
(2T2g ? 2
A
1g
)
p
–
p
, n–
m
2
/
m
1
D
o
B
C
b
2
2
2
a
b
c
18,519
17,649
17,035
23,095
23,529
22,222
30,030
29,070
25,445
1.25
1.33
1.30
20,820
23,397
20,820
694
709
694
3086
3153
3086
0.72
0.73
0.72
a
The ligand field parameters (
o
D , B, C and b) were calculated by data obtained from the experimental UV–visible spectra of the complexes and Tanabe–Sugano diagrams for
d6 and d5 octahedral geometry [24].

A.F. Shoair, A.A. El-Bindary / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 490–496
493
ꢂ1
(
(
2) The two bands appearing at 3435 and 3334 cm
spectrum of 2-aminophenol which correspond to
in the
as(NH
electron; thus the bands observed can assigned to ligand field tran-
sitions for ruthenium(III) with a (t2g) configuration. The electronic
5
t
2
)
and
and 3285 cm ) and (3320 and 3280 cm ) in the Ru com-
plexes (1a) and (2a), respectively, indicating that the NH
groups are taking part in coordination to the Ru center [17].
3) In complexes 1c and 2c the bands due to as(NH ) and (-
t (NH
s
2
) were shifted to lower wavenumbers at (3325
spectra of these complexes are similar and displayed three
mean bands. According to Tanabe–Sugano energy matrices [25]
ꢂ1
ꢂ1
5
2
for d -octahedral geometry, The bands observed in the range of
ꢂ1
2
2
17,035–18,519 and 22,222–23,529 cm are assigned to T2g ? A1g
2
2
2
t
2
t
s
or
T1g and
T
2g ? E
g
transitions, respectively. Bands of high
intensity in the range of 25,445–30,864 cm are also observed,
these bands are attributed to p–p and n–p intra-ligand charge
ꢂ1
NH
2
) and
t
(CO) were shifted to lower wavenumbers at
and (3315, 3242 and
670 cm ), respectively, indicating their coordination to
ꢂ1
ꢃ
ꢃ
3315, 3242 and 1660 cm
)
ꢂ1
1
transfer. The ligand field parameters can be estimated by using
2
2
2
2
2
the Ru center. These results are similar to the recently
reported bands in the complex [Mn(NCS) (4-apy) ] [22].
the energies
T
2g ? A1g and/or
T1g and
T
2g ? E
g
transitions,
5
2
2
and the Tanabe–Sugano energy matrices for d -octahedral geome-
(
(
4) For the Ru(III) complexes (2a–c), a strong single band was
try [25]. We found that the splitting parameter,
D
o
in the range of
ꢂ1
ꢂ1
observed at 330 cm assignable to the RuACl group and
absent in the Ru(II) complexes (1a–c) [23].
20,820–23,397 cm
,
the Racah parameter of interelectronic
ꢂ1
repulsion, B in the range of 685–709 cm , and the nephelauxetic
ratio, b in the range 0.65–0.75. These values are close to those
reported for similar octahedral ruthenium(III) complexes [26].
The electronic spectra of the ruthenium(II) complexes are sim-
ilar and display absorption bands in the regions 17,141–17,825,
5) Another weak bands were observed in the 470–455 and
ꢂ1
450–435 cm
regions can be assigned to t(RuAO) and
t(RuAN), respectively, confirming the coordination of both
oxygen and nitrogen atoms to Ru center [23].
ꢂ1
ꢂ1
(
6) The two bands appearing near 845 and 830 cm
in the
25,316–25,706 and 30,387–40,322 cm . The first two bands are
1
1
1
1
Ru(II) complexes (1a–c) corresponding to
t(PAF) stretching
attributable to
A1g ? T1g and
A1g ? T2g transitions, respec-
vibration [24].
tively, while the third high energy band is assigned to
A
1
1g ? 1E1g transition [26,27]. Similarly, the ligand field parame-
ters for ruthenium(II) complexes can be estimated. However, the
values of ligand field parameters ( , B and b) are in the range of
Electronic spectra of complexes
D
o
The electronic spectra of all complexes are recorded in DMF sol-
vent in the range 300–700 nm. The spectral data are listed in
17,800–19,080 630–714 and 0.72–0.73 respectively. The values
are slightly close to those reported [28]. These values of the split-
ting parameters placed the ligands in the middle range of the
spectrochemical series and provide reassurance that these ligands
are coordinated to the ruthenium(II) ions through the oxygen atom
of deprotonated hydroxyl group and nitrogen atom of the amino
group. The value of B for ruthenium(II) complexes also indicates
Table 1 and a representative spectrum is shown in Fig. 1. The
ground state of ruthenium(III) ion (t 25 g configuration) is
2
T1g and
the first excited doublet levels in order of increasing energy are
2
2
1
A
2g and
5
ruthenium(III) complexes are low spin (t2g) with one unpaired
a considerable overlap with strongly covalent metal–ligand bond
II
[
29,30]. Electronic spectrum of cis-[Ru (bpy)
2 6
(2-aph)](PF ) (1a) is
shown in Fig. 1 as representative example.
2
1
1
1
1
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
.8
.6
.4
.2
.0
III
III
ESR spectra of [Ru Cl(L)
complexes
(H
2 2
O)] (2a–b) and [Ru Cl
2
2
(L) ]Cl (2c)
III
III
The complexes [Ru Cl(L)
(H
2 2
O)] (2a–b) and [Ru Cl
2
(L)
2
]Cl (2c)
5
are one electron paramagnetic species (low spin d , S = ½). This
confirms the +3 oxidation state of ruthenium. The ESR spectral data
are listed in Table 2 and one representative example of their spec-
tra (Fig. 2). The spectra of these solid complexes were similar and
recorded in DMF/Toluene at 77 K. The spectra of all the complexes
showed no hyperfine splitting. By ignoring the non-planarity of the
carbon atoms, relative to the coordinated oxygen and nitrogen
atoms of the ligands, a D2h-symmetry for the complexes is sug-
gested in which the coordinated chlorides are passing through
the Z-axis while the coordinated oxygen and nitrogen atoms of
the ligands pass through X and Y axes as shown in Scheme 2. In
300
400
500
600
700
800
900
Wavelength (nm)
II
Fig. 1. Electronic spectrum of cis-[Ru (bpy)
2
6
(2-aph)](PF ) (1a) in DMF.
Table 2
a
II
L]+ (1a–b), cis-[Ru (bpy)
II
L]2+ (1c) and [RuIIICl(L)
O)] (2a–b) [RuIIICl
Magnetic, ESR and cyclic voltammetric data for cis-[Ru (bpy)
2
2
(H
2 2
2
(L)
2
]Cl (2c) complexes (L = 2-aph, 8-hq and
4
-apy).
Complex
l
eff
g
||
g
\
E
pa(mV)
E
pc
D
(mV)
E
Assignment
(reduction)
E
pa
E
pc
D
(mV)
E
Assignment
(oxidation)
Scan rate
ꢂ1
(B.M.)
(mV)
(mV)
(mV)
(mV s
)
1
a
0
0
0
1.93
1.82
2.02
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
80
75
70
–
–
–
550
380
390
660
400
950
440
310
320
–
–
–
110
70
70
–
–
–
Ru(II)/Ru(III)
Ru(II)/Ru(III)
Ru(II)/Ru(III)
Ru(III)/Ru(IV)
Ru(III)/Ru(IV)
Ru(III)/Ru(IV)
50
50
50
50
50
50
1
b
1c
2a
2b
2c
2.35 2.35 ꢂ740
2.38 2.32 ꢂ600
2.40 2.36 ꢂ290
ꢂ820
ꢂ675
ꢂ360
Ru(III)/Ru(II)
Ru(III)/Ru(II)
Ru(III)/Ru(II)
a
Conditions: supporting electrolyte (0.1 M tetrabutylammonium chloride in DMF solvent), concentration of the complex (6 ꢄ 10ꢂ3 M),
D
E = Epa ꢂ Epc and
E
1/2 = 0.5(Epa + Epc), where Epa and Epc are the anodic and cathodic cyclic voltammetric peak potentials, respectively.

4
94
A.F. Shoair, A.A. El-Bindary / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 490–496
symmetry for the complexes and hence trans positions are
III
assigned for the two chlorides ligand in the case of [Ru Cl
2c) complex (Fig. 2). This assignment is in conformity with the
assignment made for similar octahedral ruthenium(III) complexes
31].
2 2
(L) ]Cl
(
[
Electrochemical studies
The electrochemical properties of all complexes have been
ꢂ3
investigated by using cyclic voltammetry in DMF (6 ꢄ 10 M)
and supporting electrolyte (0.1 M) tetrabutylammonium chloride
(
TBAC). Voltammetric data are presented in Table 2 and selective
voltammograms are shown in Fig. 3. All ruthenium(II) complexes
show one reversible oxidation wave ( E = 60–110 mV) on the
positive side and this wave assigned to Ru(II)/Ru(III) oxidation.
The anodic peak current (ipa) and the cathodic peak current (ipc
are almost equal. Under the same conditions ruthenium(III) com-
plexes show one reversible reduction wave ( E = 70–80 mV) on
the negative side against Ag/AgCl and irreversible oxidation peak
pa = 380–950 mV) on the positive side. The former is assigned
D
)
Fig. 2. ESR spectrum of [RuIIICl(2-aph)
D
2
(H
2
O)] (2a) in DMF/toluene at 77 K.
(E
NMO
Benzonitrile
to Ru(III)/Ru(II) reduction and the latter is assigned to Ru(III)/
Ru(IV) oxidation. The one electron nature of these waves was con-
firmed by comparing the peak heights for each wave with that of
the ferrocene–ferrocenium couple under the same conditions
Co-oxidant
Catalyst
Substrate
(Fig. 3).
NMM
Benzylamine
Scheme 2. Catalytic cycle for dehydrogenation of benzylamine to benzonitrile.
Catalytic dehydrogenation of aromatic amines to nitriles
Attempts to dehydrogenate benzyl amine, p-methyl benzyl-
II
view of the suggested stereochemistry of these complexes, it was
found that they showed tetragonal ESR spectra with two distinct
g-values. The first signal was found near 2.40–2.35 (g|| in the axial
amine and p-nitrobenzylamine by cis-[Ru Cl (bpy) ]ꢁ2H O/N-
2
2
2
methylmorpholine-N-oxide (NMO) catalyst system to their corre-
sponding nitriles were all unsuccessful since an unidentified prod-
uct was found at the end, probably due to the degradation of
case) and the second near 1.98–1.78 (rhombic component of g
The presence of two g values is an indication of an octahedral field
with tetragonal distortion (g –g –g ) and also points out an axial
\
).
II
aromatic ring by cis-[RuCl (bpy) ]ꢁ2H O complex [32]. The two
2
2
2
II
III
x
y
z
complexes, cis-[Ru (bpy) (8-hq)](PF ) (1b) and [Ru Cl(8-hq)
2
6
2
2 6
cis-[Ru(bpy) (8-hq)](PF ) (1b)
1
0 µA
Ru(II)/Ru(III)
2 2
[RuCl(8-hq) (H O)] (2b)
R(III)/Ru(IV)
Ru(III)/Ru(II)
II
) (1b) and [RuIIICl(8-hq)
O)] (2b). Concentration = 6 ꢄ 10ꢂ3 M in 0.1 M tetrabutylammonium
Fig. 3. Cyclic voltammograms of the complexes: cis-[Ru (bpy)
2
(8-hq)](PF
6
2
(H
2
ꢂ1
chloride DMF solution) at scan rate 50 mV s
.

A.F. Shoair, A.A. El-Bindary / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 490–496
495
Table 3
Optimum conditions for catalytic dehydrogenation of RAPhCH
2
ANH
2
3 2
to RAPhCN (R = H, p-CH and p-NO ).
Entry
Solvent
Temp (°C)
Time (H)
Yield (%)
cis-[Ru (bipy)
Yield (%)
[Ru Cl(8-hq)
II
III
2
(8-hq)](PF
6
) (1b)
2
(H
2
O)] (2b)
H
p-CH
3
p-NO
2
H
p-CH
3
p-NO
2
1
2
3
4
5
Dioxane
THF
CH Cl
2 2
DMF
DMF
25
25
25
25
40
5
5
5
3
2
25
22
20
63
88
24
33
22
63
85
30
36
25
60
86
31
27
26
70
95
36
35
27
68
91
35
36
33
71
88
II
(
(
H
2
O)] (2b) were examined as catalysts instead of cis-[Ru Cl
2
mechanistic study was suggested for dehydrogenation of benzyl-
0
bpy)
The catalytic dehydrogenation reaction occurs according the
following equation:
2
]ꢁ2H
2
O for catalytic dehydrogenation of these substrates.
amine by [RuCl(ppy)(tpy)] (ppy = 2-phenylpyridine and tpy = 2,2 :
0
00
6 ,2 -terpyridine) with molecular oxygen as co-oxidant [33]. In con-
clusion, we have developed an efficient method for catalytic oxida-
tion of these amines to nitriles by these two complexes. Attempts to
dehydrogenate some aliphatic amines are in progress.
Ru-catalyst; DMF
RAPhACH
2
NH
2
þ2NMOƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!RAPhACNþ2NMMþ2H
2
O
R@H, p-Methyl and p-Nitro
Conclusion
Our preliminary results are summarized in Table 3. In order to
establish the optimum conditions for catalytic dehydrogenation
of these substrates we studied the influence of solvent on these
II
Two series of ruthenium complexes, cis-[Ru (bpy)
2
L](PF
6
)
II
III
(
[
1a–b), cis-[Ru (bpy)
2
L](PF
6
)
2
(1c) and [Ru Cl(L)
2
(H
2
O)] (2a–b)
oxidations was investigated. In CH
low yields of nitriles were obtained with CH
THF (Table 3, entries 1–3) and dramatic increase in the yields from
2
Cl
2
, dioxane, THF and DMF,
III
Ru Cl
2
(L)
2
]Cl (2c) have been synthesized and characterized by
2
Cl , dioxane, and
2
several physicochemical techniques. The spectroscopic analysis
1
(
IR, UV–Vis, H NMR, ESR) of these complexes indicates an octahe-
3
4
6% to 71%, however, were obtained in DMF (Table 3, entry 3 and
) which was therefore used as the solvent in all other experi-
ments. Next, the effect of temperature was examined. Increasing
the temperature from 25 to 40 °C resulted in a considerable
increase in the yield and shorter reaction times (Table 3, entry
dral geometry for all the complexes. Cyclic voltammograms of
ruthenium(II) complexes exhibit one reversible oxidation wave
on the positive side attributable to Ru(II)/Ru(III) reduction, while
ruthenium(III) complexes exhibit one reversible reduction wave
on the negative side against Ag/AgCl, and irreversible oxidation
peak on the positive side. The former is assigned to Ru(III)/Ru(II)
reduction and the latter is assigned to Ru(III)/Ru(IV) oxidation.
5
). It was found that yields obtained with the ruthenium(III) com-
plex are slightly higher than whose obtained with the ruthe-
nium(II) complex, probably due to the +3 oxidation state of the
ruthenium(III) enhances the oxidation reactions more the +2 oxi-
dation state of ruthenium(II) complex.
In a typical experiment, a DMF solution of an excess amount of
NMO (3 mmol) was added to a stirred solution of benzylamine
II
III
The complexes, cis-[Ru (bpy)
2 6
(8-hq)](PF ) (1b) and [Ru Cl
(
8-hq) (H O)] (2b) dehydrogenate benzylamine, p-methylbenzyl-
2
2
amine and p-nitrobenzylamine to their respective nitriles in DMF
as a solvent and in the presence of N-methylmorpholine-N-oxide
(
NMO) as co-oxidant in good yields and turnover.
(
(
2 mmol) and a catalytic amount of the complex (1b or 2b)
0.01 mmol). After stirring for 15 min the reaction mixture turned
to dark brown, probably due to the coordination of benzylamine to
ruthenium center as had been reported by Griffith and co-workers
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