Synthesis, structural characterization and catalytic activity study of Mn(II), Fe(III), Ni(II), Cu(II) and Zn(II) complexes of quinoxaline-2- carboxalidine-2-amino-5-methylphenol: Crystal structure of the nickel(II) complex

Article abstract of DOI:10.1016/j.poly.2010.08.016

Five new transition metal complexes [MnL(OAc)]·H₂O (1), [FeLCl₂] (2), [NiL₂]·H₂O (3), [CuLCl] (4) and [ZnL₂]·2H₂O (5) have been synthesized using a tridentate Schiff base ligand, HL (quinoxaline-2-carboxalidine-2-amino-5- methylphenol) and the complexes have been characterized by physicochemical and spectroscopic techniques. The spectral analyses reveal an octahedral geometry for 3, square pyramidal structure for 2 and square planar structure for 4. Analytical and physicochemical data indicate tetrahedral structure for 1 and octahedral structure for 5. The crystallographic study reveals that [NiL ₂]·H₂O shows distorted octahedral geometry with a cis arrangement of N₄O₂ donor set of the bis Schiff base and exhibits a two-dimensional polymeric structure parallel to [0 1 0] plane. The complexes were screened for catalytic phenol hydroxylation reaction. Coordinatively unsaturated manganese(II), iron(III) and copper(II) complexes were found to be active catalysts. The poor catalytic activity of the nickel(II) complex is due to coordinatively saturated octahedral nature of the complex. Maximum conversion of phenol was observed for the copper(II) complex and the major product was catechol.

Full text of DOI:10.1016/j.poly.2010.08.016

Polyhedron 29 (2010) 3014–3020
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, structural characterization and catalytic activity study of Mn(II),
Fe(III), Ni(II), Cu(II) and Zn(II) complexes of quinoxaline-2-carboxalidine-2-
amino-5-methylphenol: Crystal structure of the nickel(II) complex
a
a
a
a
a
Manju Sebastian , V. Arun , P.P. Robinson , Annu Anna Varghese , Rani Abraham ,
b
a,
⇑
E. Suresh , K.K. Mohammed Yusuff
a
Department of Applied Chemistry, Cochin University of Science and Technology, Cochin 682 022, Kerala, India
Analytical Sciences Division, Central Salt and Marine Chemicals Research Institute, Bhavnagar, Gujarat 364 002, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Five new transition metal complexes [MnL(OAc)]ꢀH O (1), [FeLCl ] (2), [NiL ]ꢀH O (3), [CuLCl] (4) and
2
2
2
2
Received 7 May 2010
Accepted 12 August 2010
Available online 20 August 2010
[ZnL
2
]ꢀ2H
2
O (5) have been synthesized using a tridentate Schiff base ligand, HL (quinoxaline-2-carboxali-
dine-2-amino-5-methylphenol) and the complexes have been characterized by physicochemical and
spectroscopic techniques. The spectral analyses reveal an octahedral geometry for 3, square pyramidal
structure for 2 and square planar structure for 4. Analytical and physicochemical data indicate tetrahe-
Keywords:
Schiff base
Quinoxaline-2-carboxaldehyde
dral structure for 1 and octahedral structure for 5. The crystallographic study reveals that [NiL
2
]ꢀH
2
O
shows distorted octahedral geometry with a cis arrangement of N donor set of the bis Schiff base
4 2
O
and exhibits a two-dimensional polymeric structure parallel to [0 1 0] plane. The complexes were
screened for catalytic phenol hydroxylation reaction. Coordinatively unsaturated manganese(II), iron(III)
and copper(II) complexes were found to be active catalysts. The poor catalytic activity of the nickel(II)
complex is due to coordinatively saturated octahedral nature of the complex. Maximum conversion of
phenol was observed for the copper(II) complex and the major product was catechol.
Ó 2010 Elsevier Ltd. All rights reserved.
2
-Amino-5-methylphenol
Crystal structure
Phenol hydroxylation
1
. Introduction
Transition metal complexes containing nitrogen and phenolic
catalytic hydroxylation of phenols in heterogenized systems has
been reported in many cases [15–18].
We have recently reported the Schiff base, quinoxaline-2-car-
boxalidine-2-amino-5-methylphenol, and its cobalt(II) complex
[19]. In continuation of our work, we present here the synthesis
and characterization of manganese(II), iron(III), nickel(II), cop-
per(II) and zinc(II) complexes of the Schiff base HL. The crystal
oxygen donor atoms are of considerable interest due to their
potential application in catalysis, medicine and material science
[
1–4]. They are often used to construct extended network struc-
tures and coordination polymers with high structural stability
and special topologies [5–8]. Some of the metal complexes find
wide applications as oxidation catalysts in the production of bulk
and fine chemicals and in the elimination of organic pollutants like
phenol. Liquid phase hydroxylation of phenol to catechol and
structure of [NiL
2
]ꢀH
2
O is studied. All the prepared complexes are
screened for their catalytic activity for the liquid phase hydroxyl-
ation of phenol. Suitable reaction conditions have been optimized
to give maximum hydroxylation.
2 2 2
hydroquinone using simple oxidants like O and H O is a reaction
of industrial importance. There is a continuing interest in the
production of dihydroxybenzenes by the direct hydroxylation of
phenol with the environmentally benign oxidant, hydrogen perox-
2
. Experimental
.1. Materials
-Amino-5-methylphenol (Aldrich), manganese(II) acetate tet-
2
2 2
ide (H O ) [9]. Catechol and hydroquinone are two of the many
phenolic derivatives of high value. They are widely used as photo-
graphic chemicals, antioxidants [10,11], polymerization inhibitors,
flavoring agents, and medicine [12,13]. A number of novel estrane
skeletons have been synthesized using catechol estrogens [14]. The
2
rahydrate (Merck), ferric chloride anhydrous (Merck), nickel(II)
acetate tetrahydrate (Qualigens), copper(II) chloride dihydrate
(
Merck) and zinc(II) acetate dihydrate (Merck) were used as sup-
plied. The synthesis of quinoxaline-2-carboxaldehyde was carried
out as reported in the literature [20,21]. The Schiff base HL was
prepared by the reported procedure [19] by stirring methanolic
⇑
Corresponding author. Tel./fax: +91 484 2575804.
E-mail address: yusuff@cusat.ac.in (K.K.M. Yusuff).
0
277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.08.016

M. Sebastian et al. / Polyhedron 29 (2010) 3014–3020
3015
solution (60 mL) of quinoxaline-2-carboxaldehyde (1.58 g,
0.0 mmol) and 2-amino-5-methylphenol (1.23 g, 10.0 mmol).
Structure of the Schiff base is given in Fig. 1.
complex was recorded on a Varian E-112 spectrometer using TCNE
as the standard. TG-DTA analyses were carried out on a Perkin
Elmer Pyris Diamond TG/DTA analyser. The products of the
catalytic reaction were analyzed using a Chemito 8510 gas chro-
matograph fitted with FID detector.
1
2.2. Preparation of complexes
A solution of the Schiff base (2.63 g, 10.0 mmol) in 1:1 chloro-
2.4. Catalytic activity study: oxidation of phenol
form-methanol (50 mL) was added to a solution of manganese(II)
acetate tetrahydrate (2.45 g, 10.0 mmol), ferric chloride (1.62 g,
The catalytic activity study towards the hydroxylation of phenol
was carried out in a 50 mL two necked RB flask fitted with a water
cooled condenser. In a typical reaction, an aqueous solution of 30%
H O (2.67 mL, 25 mmol) and phenol (2.2 mL, 25 mmol) were
mixed in acetonitrile (10 mL) and the reaction mixture was heated
at 343 K with continuous stirring in an oil bath. The complex
(0.015 mmol) was added to the reaction mixture and the reaction
was considered to be started at this time. During the reaction,
the products were analyzed using a gas chromatograph after spe-
cific interval of time by withdrawing small aliquots. The effects
of various parameters such as amount of oxidant, catalyst, temper-
ature and time of the reaction were studied to see their effect on
the reaction product pattern.
0
.01 mol), nickel(II) acetate tetrahydrate (1.24 g, 5.0 mmol),
copper(II) chloride dihydrate (1.71 g, 10.0 mmol) or zinc(II) acetate
dihydrate (1.10 g, 5.0 mmol) in methanol (15 mL). The solution
was then refluxed for 3 h and left to cool at room temperature
2
2
(
27 ± 2 °C) overnight. The crystalline complexes that separated
out were filtered, washed with ether and dried in vacuum over
anhydrous calcium chloride.
[
MnL(OAc)]ꢀH
2
O (1). Yield: 65%, color: red, M.P: 295–297 °C, k
m
ꢁ
1
2
ꢁ1
(
DMF): 15 ohm cm mol
,
leff (B.M): 5.94. Elemental Anal. Calc.
for C18
5
H
17
N
3
O
4
Mn: C, 54.83; H, 4.35; N, 10.66; Mn, 13.93. Found: C,
4.78; H, 3.98; N, 10.69; Mn, 13.96%.
FeLCl ] (2). Yield: 70%, color: black, M.P: 260–263 °C, k
[
2
m
(DMF):
eff (B.M):4.01. Elemental Anal. Calc. for
OFe: C, 49.40; H, 3.11; N, 10.80; Fe, 14.35. Found: C,
9.67; H, 3.15; N, 10.69; Fe, 14.31%.
NiL O (3). Yield: 80%, color: blue, M.P: 300–302 °C, k
]ꢀH
DMF): 12 ohm cm mol
for C32
3.70; H, 3.99; N, 13.93; Ni, 9.74%. Blue crystals suitable for X-
ꢁ
1
2
ꢁ1
9
C
4
ohm cm mol , l
H12Cl N
16 2 3
2.5. X-ray crystallography
[
2
2
m
X-ray crystal structure determination was performed with a
ꢁ1
2
ꢁ1
(
,
leff (B.M): 3.2. Elemental Anal. Calc.
Bruker SMART APEX CCD X-ray diffractometer, using graphite
monochromated MoK radiation (k = 0.71073 Å, and scans).
26 6 3
H N O Ni: C, 63.92; H, 4.36; N, 13.98; Ni, 9.76. Found: C,
a
u
x
6
The data was reduced using SAINTPLUS [22] and a multiscan absorp-
tion correction using SADABS was performed [23]. The structure was
solved using SHELXS-97, and full matrix least squares refinement
ray analysis of species (3) were obtained by slow diffusion of meth-
anolic solution of the complex.
[
CuLCl] (4). Yield: 70%, color: violet, M.P: 240 °C, k
m
(DMF):
eff (B.M): 1.95. Elemental Anal. Calc. for
ClOCu: C, 53.19; H, 3.35; N, 11.63; Cu, 17.59. Found: C,
3.16; H, 3.36; N, 11.67; Cu, 17.64%.
ZnL O (5). Yield: 85%, color: violet, M.P: 245–247 °C, k
DMF): 8 ohm cm mol
2
against F was carried out using SHELXL-97 in anisotropic approxi-
ꢁ1
2
ꢁ1
1
C
5
3 ohm cm mol , l
mation for non-hydrogen atoms [24]. The hydrogen atoms of the
ligands were positioned geometrically, and allowed to ride on their
parent carbon atoms. The hydrogen atoms of the water molecules
were located in the difference Fourier map and refined isotropi-
cally. All hydrogen atoms were assigned using fixed Uiso values,
equal to 1.2 Ueq of the parent atom for the ligand carbons and
16
12 3
H N
[
2
]ꢀ2H
2
m
ꢁ1
2
ꢁ1
(
,
l
eff (B.M): 0. Elemental Anal. Calc. for
C
32
H
28
N
6
O
4
Zn: C, 61.40; H, 4.51; N, 13.42; Zn, 10.45. Found: C,
6
1.75; H, 4.12; N, 13.79; Zn, 10.42%.
1
.5 for hydrogen atoms of water. A summary of crystallographic
data and refinement parameters are given in Table 1.
2.3. Physical measurements
3
. Results and discussion
Elemental analysis of the Schiff base and complexes were done
on a Vario EL III CHNS analyzer. The estimation of metal ions was
carried out on a Thermo Electron Corporation, M series Atomic
Absorption Spectrophotometer. Far-IR spectra of the complexes
All the five complexes are soluble in polar organic solvents like
3
methanol, CH CN, DMF and DMSO. The molar conductivity mea-
ꢁ3
ꢁ1
surements in 10 M DMF solutions revealed the non-electrolytic
were recorded using polyethylene pellets in the 500–100 cm
region on a Nicolet Mega 550 FTIR Instrument. The molar
conductivities of the complexes were measured using a Systronic
conductivity bridge type 305 in DMF. 1H NMR spectra were
Table 1
Crystal data summary for [NiL ]ꢀH O.
2
2
Formula
Formula weight
Crystal system
Space group
a (Å)
32 6 3
C H26 Ni N O
601.30
Monoclinic
1
P2 /n
13.677(4)
13.760(3)
14.572(4)
96.445(5)
2725.1(12)
4
3 6
recorded in CDCl or DMSO-d on a Bruker AVAVCE III 400 MHz -
NMR spectrometer using TMS as the internal standard. The IR spec-
tra were recorded on a JASCO FTIR-4100 spectrophotometer using
ꢁ
1
KBr pellets in the range 4000–400 cm . The electronic spectra of
the complexes were recorded on a Thermoelectron Nicolet evolu-
tion 300 UVꢁVis spectrophotometer. Magnetic susceptibility mea-
surements were done at room temperature on a Magway MSB Mk
b (Å)
c (Å)
b (°)3
V (Å )
Z
1
Magnetic Susceptibility Balance. The EPR spectrum of the copper
T (K)
298
1.466
0.76
Dc (mg mꢁ3)
l
(Mo K
a
) (mmꢁ1)
F(0 0 0)
1248
h range for data collection (°)
Reflections collected/unique
Goodness-of-fit (GOF) on F2
1.94–25.00
12261/4797
1.194
R
R
1
, wR
, wR
2
[I > 2
(all data)
r
(I)]
0.0694, 0.1579
0.1007, 0.1650
1
2
2
o
2
c
2
o
Fig. 1. Structure of quinoxaline-2-carboxalidine-2-amino-5-methylphenol.
R
1
=
R
(|F
o
| ꢁ|F
c
|)/R
|F
o
|, wR
2
= [R
w(F
ꢁF
)2/R
w(F
)2]0.5.

3
016
M. Sebastian et al. / Polyhedron 29 (2010) 3014–3020
nature of the complexes [25]. The analytical data clearly give the
evidence of formation of 1:2 complexes in the case of nickel(II)
and zinc(II) complexes while others are formed in 1:1 ratio of metal
to ligand. The 1:2 metal ligand ratio itself suggests an octahedral
geometry for the Ni(II) and Zn(II) complexes, while the 1:1 com-
plexes may form four, five or six coordinated complexes depending
upon the solvent molecules and anions in the coordination sphere.
maxima are listed in the Table 2. The UV–Vis spectrum of the Schiff
ꢁ1
base shows two strong bands at 42 550 and 25 970 cm due to
* transitions. [30]. For all the complexes, the * bands are not
altered to a greater extent on coordination.
The spectrum of the Mn(II) complex shows three shoulders at
p–
p
p–p
ꢁ
1
6
4
6
16 645, 22 765 and 27 765 cm assignable to
A
1
? T
l
1 1
, A ?
4
6
4
2
T (G) and A
1
? T
2
(D) transitions, respectively. The eff value of
the Mn(II) complex was found to be 5.94 B.M. at room temperature
as expected for high spin d system [31,32].
1
5
3
.1. H NMR spectrum of [ZnL
2
]ꢀ2H
2
O
The electronic spectrum of the iron(III) complex shows a weak
ꢁ1
The NMR spectrum of the zinc(II) complex is taken in DMSO-d
6
.
broad band in the near IR region at 6890 cm indicating a square
pyramidal structure. Similar observations have been made by Mar-
tin and White for the five coordinated iron(III) dithiocarbamate
complex, which shows characteristic band in the near IR region
due to low lying spin forbidden states [33,34]. The charge transfer
The phenolic OH signal at 9.70 ppm observed in the spectrum of
the ligand is not seen in the spectrum of the Zn(II) complex indicat-
ing the participation of the phenolic OH group in chelation with
proton displacement. The signal due to azomethine proton gets
shifted upon complexation, which might probably be due to the
donation of the lone pair of electrons by the nitrogen to the central
metal atom, resulting in the formation of a coordinate linkage
ꢁ1
band observed at 29 000 cm obscure the rest of the d–d transi-
tions. The iron(III) complex shows a magnetic moment of 4.01
B.M., which confirms that iron(III) complex is five coordinated.
ꢁ1
(
M
N). The aromatic and quinoxaline protons resonate as a com-
For the nickel(II) complex, the band at 17 450 cm is due to the
3A2g ? T1g transition of nickel(II) in octahedral environment. The
intensity of this d–d band is considerably increased possibly due
to the influence of nearby intense charge-transfer transitions
[33]. The complex has a room temperature magnetic moment va-
lue of 3.1 B.M., which is in the normal range observed for octahe-
dral Ni(II) complexes.
3
plex multiplet in the region d 7.46–8.20 ppm. The signal due to
methyl group attached to the phenolic ring is observed almost at
the same chemical shift for that in spectrum of the ligand. A broad
singlet appeared in the region 3.91–4.26 indicates the presence of
water molecules in the complex.
3
.2. Infrared spectra
The spectrum of [CuLCl] shows two bands in the visible region,
ꢁ1
2
1g ? A1g and 2
2
at 22 220 and 15 950 cm assignable to
B
B
1g
?
2
The most important IR spectral bands of the ligand and its
g
E transitions, respectively [33,35] indicating the square planar
metal complexes are listed in Table 2. The
v
(C@N) band at
geometry of the copper(II) complex. The magnetic moment of the
complex is found to be 1.95 B.M. The zinc(II) complex exhibits a
charge transfer transition at 18315 cm .
ꢁ1
1
626 cm of the Schiff base shifts to lower frequencies in com-
ꢁ1
plexes indicating weakening of the azomethine bond due to coor-
dination to the metal [26]. For the Schiff base, the C@N– stretching
ꢁ
1
of quinoxaline ring is observed as a strong band at 1549 cm . This
3.4. Thermal analysis
ꢁ1
band undergoes a small shift (ꢂ5 cm ) on complexation; such
small shifts have been reported when ring nitrogens are involved
Thermal stability of the Schiff base and the complexes were
investigated using TG-DTA under nitrogen atmosphere with a
heating rate of 10 °C min
in coordination to the metal [19]. The m(C–O) band for all the com-
ꢁ1
plexes appears at lower frequencies compared to that for the free
ligand, which is due to the participation of phenolic oxygen on che-
lation [27]. Thus the Schiff base behaves as a monobasic tridentate
over a temperature range of 40–
1000 °C. The Schiff base HL exhibits a one stage decomposition
which begins at 160 °C. The weight loss below 200 °C observed
in complexes 1, 3 and 5 indicate the presence of lattice water. All
the complexes are found to be thermally stable and exhibit multi
stage decomposition pattern. The thermal data upto 200 °C of the
complexes is given in Table 3. The decomposition of the organic
part occurs above 200 °C for all complexes. TG results show good
agreement with the formula of the complexes arrived from the
analytical data.
ligand. In the spectra of complexes [MnL(OAc)]ꢀH
2
O, [NiL
2
]ꢀH
2
O
ꢁ1
and [ZnL O the broad band observed at 3300–3400 cm is
due to the presence of water in the lattice. Appearance of new
bands in the spectra of all complexes in the regions 490–450 and
has been attributed to m(M–O) and m(M–N) [2],
respectively. The IR spectra of the chloro complexes, (2) and (4), re-
2
]ꢀ2H
2
ꢁ
1
4
50–400 cm
ꢁ
1
veal an additional new band at 346 and 332 cm which can be as-
signed to
displays
m(M–Cl) [28]. The IR spectrum of the acetato complex, (1),
ꢁ
ꢁ
ꢁ1
m
asym (COO ) and
msym (COO ) at 1577 and 1390 cm
3.5. EPR spectral studies
which is consistent with monodentate acetato coordination [29].
The X band EPR spectrum (Fig. 2) of the copper(II) complex is
taken in DMF at 77 K. The ESR spectrum of the copper(II) complex
3.3. Magnetic susceptibility and electronic spectra measurements
gives a spectrum with g|| and g
tively. The trend g|| > g > g (g
observed for the copper(II) complex suggests that the unpaired
\
values at 2.28 and 2.12, respec-
is the g value of free electron)
Electronic spectra of the Schiff base and its complexes were ta-
ken in methanol in the range 50 000–10 000 cm . The absorption
\
e
e
ꢁ1
Table 2
Infrared and electronic spectra of the Schiff base HL and its complexes.
Compound
IR data (cmꢁ1)
(O–H)
Electronic spectral dataa
m
m
(C@N)
m
(C–O)
m
(M–O)
m
(M–N)
Absorption maxima (log e)
HL
3365
3317
1626
1605
1609
1579
1612
1587
1262
1257
1254
1253
1250
1251
42550 (3.96), 25970 (3.81)
[MnL(OAc)]ꢀH
[FeLCl ] (2)
[NiL O (3)
]ꢀH
[CuLCl] (4)
[ZnL O (5)
]ꢀ2H
2
O (1)
465
490
457
425
465
413
424
432
409
418
41270 (3.56), 32540 (3.25), 27765 (3.30), 22765 (3.27), 16645 (1.67)
42110 (4.12), 35730 (3.96), 29000 (3.70), 6890 (1.24)
41150 (4.13), 27855 (3.88), 17450 (3.83)
40325 (3.74), 37735 (3.65), 22220 (2.65), 15950 (1.27)
42195 (3.72), 30395 (3.30), 25840 (3.31), 18315 (3.36)
2
2
2
3373
2
2
3381
a
ꢁ1
ꢁ1
ꢁ1
Absorption maxima in cm and
e in L mol cm .

M. Sebastian et al. / Polyhedron 29 (2010) 3014–3020
3017
Table 3
Thermal decomposition data (below 200 °C) for complexes.
Compound
TG temperature
range/°C
% Weight loss
(fragment lost)
[
[
[
MnL(OAc)]ꢀH
NiL O (3)
]ꢀH
ZnL O (5)
]ꢀ2H
2
O (1)
46–101
50–130
46–101
4.5 (1 H
3.1 (1 H
2.9 (2 H
2
O)
2
O)
2
O)
2
2
2
2
For complexes 2 and 4, there is no weight loss below 200 °C.
Fig. 3. Ortep representation of the complex 3, with 50% probability ellipsoid
(
hydrogen atoms are omitted for clarity).
Table 4
Selected bond lengths (Å) and angles (°) for [NiL
2
]ꢀH
2
O.
Ni(1)–N(3)
Ni(1)–N(6)
Ni(1)–O(2)
Ni(1)–O(1)
N(3)–Ni(1)–N(6)
O(2)–Ni(1)–N(4)
1.988(4)
1.992(4)
2.033(4)
2.044(4)
176.00(18)
159.61(17)
Ni(1)–N(4)
Ni(1)–N(1)
N(3)–C(9)
N(3)–C(10)
O(1)–Ni(1)–N(1)
C(9)–N(3)–C(10)
2.226(4)
2.290(4)
1.273(6)
1.391(6)
157.68(16)
127.6(5)
Fig. 2. ESR spectrum of the copper(II) complex.
2
2
electron is most likely localized in d ꢁd orbital of copper(II) ion
x
y
and the complex has an axial symmetry [36,37].
3.6. Crystal structure of [Ni(L)
2 2
]ꢀH O
Single crystal X-ray diffraction analysis reveals that complex 3
forms a two-dimensional polymeric chain due to the aggregation
of the discrete monomeric entities through classical intermolecular
O–HꢀꢀꢀO and O–HꢀꢀꢀN hydrogen bonds. The asymmetric unit as
illustrated in the ORTEP diagram (Fig. 3), consists of a mono-
3 6 4 8 5 2 2
meric Ni{(CH )C H (O)N@CH–C H N } unit with a solvent water
molecule present in the lattice. In the complex, the ligand quinoxa-
line-2-carboxalidine-2-amino-5-methyl phenol behaves as a mono-
anionic, tridentate Schiff base with its NNO donor set which
coordinates to the metal center through one imine nitrogen, one
quinoxaline ring nitrogen atom and a deprotonated phenolic oxygen
atom in a cis arrangement. The nickel(II) center adopts a distorted
octahedral geometry being chelated by two units of such tridentate
Schiff base ligands.
Selected bond lengths and angles are listed in Table 4. The Ni–N
(
imine) distances are comparable to those observed for the similar
kind of complexes present in the literature [38–40]. The three
trans-angles at nickel(II) are 176.00(18), 157.68(16) and
1
59.61(17)° for N(3)–Ni(1)–N(6), O(1)–Ni(1)–N(1) and O(2)–
Ni(1)–N(4), and the cis angles are in the range 76.64(18)–
06.03(17). The bite angles show deviation from ideal value due
Fig. 4. Packing diagram of the complex 3, view along b axis.
1
to the rigid structure of the Schiff base.
cell, as shown in Fig. 4, since the crystal belongs to a centrosymmet-
ric space group. Both hydrogens of the uncoordinated water
molecule are involved in the intermolecular hydrogen bonding
with the neighbouring phenolic oxygen atom and uncoordinated
A further insight into the structure of 3 reveals the presence of a
C2 symmetry axis, and the complex is expected to exhibit optical
isomerism. However both the enantiomers co-exist in the unit

3
018
M. Sebastian et al. / Polyhedron 29 (2010) 3014–3020
quinoxaline nitrogen atom through O(3)–H(1w)ꢀꢀꢀN(5) and O(3)–
H(2w) ꢀꢀꢀO(1) interactions (H(1w) and H(2w) are the hydrogens
of water molecule). Similarly there are some weak intermolecular
3.7. Oxidation of phenol
Liquid phase hydroxylation of phenol catalyzed by the Schiff
and intramolecular hydrogen bondings in the lattice. There is
p–p
2 2
base complexes using H O as an oxidant has been studied in
stacking interaction between phenolic ring and symmetry related
quinoxaline. A few significant hydrogen bonding parameters are
listed in Table 5. All these interactions afford an infinite two-dimen-
sional chain propagating along [0 1 0] direction as shown in Fig. 5.
Based on the above physicochemical, spectral and crystallo-
graphic analyses, the suggested structures of the complexes are gi-
ven in Fig. 6.
CH CN. As the hydroxyl group present on phenol is ortho and para
3
directing, the hydroxylation of phenol is expected to give two
products viz. catechol and hydroquinone. Table 6 presents the
results for phenol hydroxylation reaction with the manganese(II),
iron(III) and copper(II) complexes of HL. The Schiff base and the
nickel(II) and zinc(II) complexes do not catalyze the reaction. In
order to acquire suitable reaction conditions for maximum trans-
formation of phenol, [CuLCl] (the complex which showed highest
activity during the screening studies) was studied in detail by vary-
ing the following reaction conditions: the amount of catalyst (the
Table 5
Selected hydrogen bond interactions in the complex.
2 2
copper complex), temperature, concentration of H O and time.
D–HꢀꢀꢀA
Symmetry
Distances (Å)
Angles (°) D–HꢀꢀꢀA
DꢀꢀꢀA
3.015(7) 2.33
HꢀꢀꢀA
3
.7.1. Effect of the amount of catalyst
Effect of variation of concentration of the catalyst on the rate of
reaction is illustrated in Table 7. Five different concentrations (5,
0, 15, 20 and 25 mmol) of the catalyst [CuLCl] were used for the
O(3)–
1/2 + x, 3/2 ꢁy,
135
144
H(1w)ꢀꢀꢀN(5) ꢁ1/2 + z
O(3)–
H(2w)ꢀꢀꢀO(1)
1 ꢁx, 1 ꢁy, 1 ꢁz 2.862(7) 2.02
1
fixed amount of phenol (2.2 mL, 25 mmol) and oxidant (2.67 mL,
Table 6
Result of phenol hydroxylation reaction catalyzed by complexes of the Schiff base HL.
Catalyst
Phenol conversion (%) Product distribution
Catechol Hydroquinone
TONb
[
[
[
MnL(OAc)]ꢀH
FeLCl
CuLCl]
2
O
11.70
9.29
16.10
10.30
7.95
14.48
1.40
1.34
1.62
200
167
268
2
]
Reaction time, 2 h; reaction temperature, 343 K; phenol:H
2
O
2
(molar ratio) = 1:1;
acetonitrile = 10 mL; catalyst, 0.015 mol.
a
Fig. 5. Two-dimensional polymeric structure of [NiL
2
]ꢀH
2
O.
TON: total turnover number, moles of products formed per mole of catalyst.
Fig. 6. Suggested structures of the complexes 1, 2, 3, 4 and 5.

M. Sebastian et al. / Polyhedron 29 (2010) 3014–3020
3019
Table 7
3.9. Effect of H
2 2
O concentration
The influence of the amount of catalyst.
Catalyst weight,
mmol)
Phenol
conversion (%)
Product distribution
Table 10 presents the effect of H
2
O
2
concentration on reaction
(
rate. The influence of oxidant on reaction was monitored using five
different concentrations of aqueous 30% H viz. 15, 20, 25, 30
and 35 mmol for a fixed amount of phenol (2.2 mL) and catalyst
(5 mg) in 10 mL CH CN. The lowest H concentration results in
only ca. 3% phenol hydroxylation. Other two concentrations gave
comparable results. This information suggests that H /phenol
Catechol
Hydroquinone
2 2
O
5
1
1
2
2
3.30
3.27
0.03
0.84
1.62
1.46
2.69
0
5
0
5
14.38
16.10
14.36
13.01
13.54
14.48
12.90
10.32
3
2 2
O
2 2
O
ratio of 1:1 is ideal for the maximum conversion as well as maxi-
mum efficiency. Thus, excess oxidant is not an essential condition
to maximize phenol conversion.
Reaction time, 2 h; reaction temperature, 343 K; phenol:H
acetonitrile = 10 mL.
2
O
2
(molar ratio) = 1:1;
The proposed mechanism of the phenol hydroxylation reaction
for the copper(II) complex is given in Scheme 1. The copper(II)
complex is square planar and it interacts with H O and produces
2 2
2
3
5 mmol) in 10 mL CH CN. An amount of 10–20 mmol for the cat-
alyst gave comparable results in both cases with ca. 14–16% phenol
conversion in 2 h of reaction time at 343 K. Lowering the amount of
catalyst resulted in the poor conversion [17]. Thus, 15 mmol of the
catalyst was considered to be sufficient enough to give good
performance.
the active species (Cu–OOH ꢀ) [42]. This active species subsequently
interacts with phenol and produces the intermediate (Cu–phenol–
OOH ꢀ) as shown in Scheme 1. The intermediate facilitates the attack
of OOHꢀat the ortho and para position of phenol producing catechol
and hydroquinone. The intermolecular oxidation–reduction reac-
tion between the hydrogen peroxide and phenol seems to be an
intramolecular electron transfer reaction in the intermediate
species (phenol–Cu–OOH ꢀ) , which might decrease the activation
energy for the oxidation of phenol resulting in significant enhance-
ment of the rate. The proposed Scheme can correlate with geome-
try of the complex. The complexes 1, 2 and 4 which contain vacant
coordination positions are found to catalyze phenol hydroxylation
3.7.2. Effect of temperature
Table 8 presents the data with regard to variation of reaction
temperature on hydroxylation of phenol. Five different tempera-
tures (303, 323, 333, 343 and 353 K) were considered, while keep-
ing other parameters constant (i.e., 2.2 mL phenol, 2.67 mL H
and 15 mmol catalyst), for the catalytic performance in 10 mL of
CH CN. Below 343 K, conversion of phenol was very poor. Above
43 K the decomposition of H would be accelerated, which is
2 2
O
3
3
2 2
O
not beneficial to phenol hydroxylation [41]. Thus, 343 K is consid-
ered as, the minimum required temperature to supply sufficient
energy to reach the energy barrier of phenol transformation.
Table 10
The influence of H
2
O
2
concentration.
Phenol conversion
Hydrogen peroxide
mmol)
Product distribution
(
(%)
Catechol Hydroquinone
3.8. Effect of time
1
2
2
5
0
5
3.27
7.57
16.10
13.37
15.00
3.27
0
7.44
0.13
1.62
1.56
1.83
The appropriate reaction time is the main assurance for the per-
14.48
11.81
13.17
fect reaction. Too long a time appears to be unfavorable for phenol
hydroxylation [41]. Table 9 shows that the optimum reaction time
is 2 h. There was no significant change in the conversion and selec-
tive formation of catechol beyond 3 h.
30
35
Reaction time, 3 h; reaction temperature, 343 K; catalyst, 0.015 mol; aceto-
nitrile = 10 mL.
Table 8
The influence of temperature.
Temperature (K)
Phenol conversion (%)
Product distribution
Catechol
Hydroquinone
3
3
3
3
3
03
23
33
43
53
0
0
0
0
0.97
1.62
2.17
1.23
8.20
16.10
16.57
1.23
7.23
14.48
14.40
2 2
Reaction time, 2 h; catalyst, 0.015 mol; phenol:H O (molar ratio) = 1:1; aceto-
nitrile = 10 mL.
Table 9
The influence of reaction time.
Time (h)
Phenol conversion (%)
Product distribution
Catechol
Hydroquinone
1
2
3
4
5
11.66
16.10
14.19
15.88
17.31
11.25
14.48
12.00
13.01
13.51
0.41
1.62
2.19
2.87
3.80
Reaction temperature, 343 K; catalyst, 0.015 mol; phenol:H
acetonitrile = 10 mL.
2
O
2
(molar ratio) = 1:1;
Scheme 1. Proposed mechanism for hydroxylation of phenol.

3
020
M. Sebastian et al. / Polyhedron 29 (2010) 3014–3020
while complexes 3 and 5 which do not contain coordinatively
unsaturated locations are inactive.
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. Conclusion
[
(
In summary we have successfully prepared five complexes of
6
the Schiff base HL and demonstrated that manganese(II), iron(III)
and copper(II) complexes can serve as catalyst for phenol hydrox-
ylation reaction. Only the manganese(II), iron(III) and copper(II)
complexes with vacant coordination sites show good activity in
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catalytic activity of Zn(II) complex might be due to its octahedral
structure and its incapability to have variable oxidation states.
The molecular structure of the nickel(II) complex was confirmed
by X-ray crystallography. The crystal is a racemic mixture of two
crystallographically independent enantiomers of the complex.
The molecules exhibit a two-dimensional polymeric structure par-
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