Cu(II) and Ni(II) complexes based on monofunctional and dendrimeric pyrrole-imine ligands: Applications in catalytic liquid phase hydroxylation of phenol

Article abstract of DOI:10.1016/j.ica.2010.05.010

New bis(pyrrolide-imine) copper(II) and Ni(II) complexes Cl and C2 [{(C₃H₇)-N=CH(C₄H₃N)} ₂Cu], [{(C₃H₇)-N=CH-(C₄H ₃N)}₂Ni] as well as the bimetallic dendrimeric (pyrrolide-imine) copper(II) and nickel(II) complexes C3 and C4, [DAB-{(N=CH-C₄H₃N)₄}Cu₂], [DAB-{(N=CH-C₄H₃N)₂}Ni₂] (DAB-G1-polypropyleneimine dendrimer with a diaminobutane core) were prepared in good yields. The structure and composition of the complexes were confirmed by a combination of analytical techniques. These complexes were investigated as catalysts in the hydroxylation of phenol in aqueous media in the pH range of 2-6 for the mononuclear complexes, C1 and C2 while the bimetallic systems, C3 and C4 were studied over the pH range 2-8. H₂O₂ was used as the oxidant under an oxygen atmosphere. The copper systems generally showed higher activity as compared to their nickel analogues. Catechol was the predominant product followed by hydroquinone with small amounts of para-benzoquinone. The nickel complexes showed better selectivity for catechol. The pH of the reaction medium also plays a role in both activity and selectivity with pH 3 being optimal for activity and pH 6 for selectivity to catechol.

Full text of DOI:10.1016/j.ica.2010.05.010

Inorganica Chimica Acta 363 (2010) 2643–2651
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Inorganica Chimica Acta
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Cu(II) and Ni(II) complexes based on monofunctional and dendrimeric
pyrrole-imine ligands: Applications in catalytic liquid phase
hydroxylation of phenol
Jane N. Mugo , Selwyn F. Mapolie a,*, Juanita L. van Wyk b
a
a
Chemistry Department, Stellenbosch University, Private Bag X1 Matieland, 7602 Stellenbosch, South Africa
Chemistry Department, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 May 2009
Received in revised form 27 April 2010
Accepted 4 May 2010
Available online 19 May 2010
New bis(pyrrolide-imine) copper(II) and Ni(II) complexes C1 and C2 [{ (C H )-N@CH (C H N)} Cu],
3
7
4
3
2
[
{(C
3
H
7
)-N@CH-(C
4
H
3
N)}
2
Ni] as well as the bimetallic dendrimeric (pyrrolide-imine) copper(II) and
N) }Cu ], [DAB-{(N@CH-C N) }Ni ] (DAB = G1-
nickel(II) complexes C3 and C4, [DAB-{(N@CH-C
H
4 3
4
2
H
4 3
4
2
polypropyleneimine dendrimer with a diaminobutane core) were prepared in good yields. The structure
and composition of the complexes were confirmed by a combination of analytical techniques. These com-
plexes were investigated as catalysts in the hydroxylation of phenol in aqueous media in the pH range of
Keywords:
Hydroxylation
Phenol
2–6 for the mononuclear complexes, C1 and C2 while the bimetallic systems, C3 and C4 were studied
over the pH range 2–8. H was used as the oxidant under an oxygen atmosphere. The copper systems
2 2
O
generally showed higher activity as compared to their nickel analogues. Catechol was the predominant
product followed by hydroquinone with small amounts of para-benzoquinone. The nickel complexes
showed better selectivity for catechol. The pH of the reaction medium also plays a role in both activity
and selectivity with pH 3 being optimal for activity and pH 6 for selectivity to catechol.
Ó 2010 Elsevier B.V. All rights reserved.
Pyrrolylaldiminate
Pyrrolide-imine
Dendrimer
Metallodendrimers
1
. Introduction
Hydroxylation of phenol is a process that involves the conversion
precursors supported on (or intercalated in) oxides, clay, polymers
and mesoporous silica have been proposed as catalysts but few have
shown significant activity or satisfactory product selectivity in
aqueous media. Some of these materials also require very compli-
cated syntheses [5–7]. Schiff base complexes containing transition
of this aromatic alcohol to dihydroxybenzenes and it has attracted
increased attention since the 1970s [1]. Several approaches have
been used to carry out the hydroxylation process included amongst
which are electrochemical oxidation, photo-catalytic oxidation,
photo-electrochemical oxidation, supercritical oxidation and wet
oxidation. The latter is achieved using an oxidant such hydrogen
peroxide, ozone, or oxygen in aqueous solution. In the presence of
2
+
3+
3+
2+
2+
metal ions such as Fe , Fe , Mn , Co and Cu have been used
as mimetic peroxidases in the catalytic hydroxylation of phenol by
hydrogen peroxide [8,9]. Supported homogeneous catalysts have
also been reported using different types of supports e.g. polymeric
coordinated complexes [10] and metal complexes immobilized on
inorganic supports or encapsulated in zeolites [11–13].
a catalyst, the process is referred to as catalytic wet oxidation
and a Fe2 homogeneous catalyst
+
The primary products for phenol hydroxylation are catechol
(CT) and hydroquinone (HQ) when hydrogen peroxide is used as
oxidant. However, under favourable conditions, hydroquinone
can be further oxidized to para-benzoquinone (para-BQ) while cat-
echol yields ortho-benzoquinone (ortho-BQ). These products are
widely used in various applications such as starting materials for
medicines, perfumes and agrochemicals [14].
Mono-anionic bidentate nitrogen ligands are of current interest
particularly for the preparation of low valent, low coordinate metal
complexes of hard transition and main group elements [15]. Pyrrol-
ylaldiminate ligands are bidentate nitrogen chelates that form
five-membered ring systems with metal ions [16]. They have re-
ceived much less attention as compared to other Schiff bases such
as salicylaldimines with only few examples being reported. Those
(
CWO). CWO of phenol using H
2
O
2
is a well-known route giving good yields and high selectivity of the
products. It can be accomplished under relatively mild conditions
(
130 °C, 0.5–1 MPa) [2–4]. Aqueous hydrogen peroxide and oxygen
are viewed as the most attractive oxidants from both an ecological
and an economical viewpoint. H is a relatively strong oxidant;
2 2
O
it is stable and produces water as a by-product. Natural peroxidases
have also been studied in the oxidation of phenol but the instability
and the high cost of natural enzymes have stimulated a search for
alternatives that could be used in a catalytic manner. A number of
different materials containing copper, iron, molybdenum or zinc
*
Corresponding author. Tel.: +27 21 8082722; fax: +27 21 8083849.
E-mail address: smapolie@sun.ac.za (S.F. Mapolie).
0
020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.05.010

2
644
J.N. Mugo et al. / Inorganica Chimica Acta 363 (2010) 2643–2651
previously reported are largely based on main group metals [15],
with a few early and late transition metals [17–19]. The main appli-
cation for the known pyrrolylaldiminato metal complexes has been
as catalysts in the polymerization and oligomerization of olefins
(0.86 mL, 10.53 mmol) via a syringe. Few drops of glacial acetic
acid were added as a catalyst. The solution was stirred at room
temperature for 6 h. The solvent was removed from the reaction
mixture using a rotary evaporator yielding an orange oil. The prod-
[
20–22]. In contrast various salicylaldiminato complexes have been
uct was purified by dissolving it in CH
with water (5 ꢀ 25 mL). The organic layer was collected, dried over
anhydrous MgSO and filtered by gravity. Removal of CH Cl
yielded an orange oil. Yield 0.80 g, 56%. H NMR (CDCl ) d (in
HH = 7.0 Hz), 1.66 (q, 2H, CH CH
HH = 7.0 Hz), 6.24 (t, 1H,
2 2
Cl (25 mL) and washing it
reported to be active in the hydroxylation of phenol [23–25]. How-
ever, to our knowledge there are no reports of the employment of
pyrrolylaldiminato complexes in the hydroxylation of phenol.
In this paper the synthesis of monomeric and bimetallic dendri-
meric of copper(II) and nickel(II) pyrrolylaldiminato complexes is
described. Their evaluation as catalysts in the aqueous hydroxyl-
ation of phenol under an oxygen atmosphere in the presence
4
2
2
1
3
3
ppm) 0.93 (t, 3H, CH
HH = 7.0 Hz), 3.46 (t, 2H, NCH
3
,
J
3
2
,
4
3
J
2
,
J
3
2
C
4
H
3
NH,
(s, br, 1H, C
d (in ppm) 11.7 (aliphatic CH
CH ), 109.5; 113.7; 121.4, (pyrrole CH), 151.4 (HC-N), FT-IR
J
HH = 2.8 Hz); 6.51 (d, 1H, C
NH) and 8.03 (s, 1H, HC@N). { H} C NMR (CDCl
) 24.3 (CH CH ), 62.7 (NCH ) and
4 3
H NH, JHH = 2.8 Hz); 6.91
1
13
4
H
3
3
)
H
2
O
2
is also discussed.
3
3
2
2
2
ꢁ
1
(
[
cm , neat between NaCl plates)
m
C
@
N
= 1640 and GC–MS m/z
2
. Experimental
+
M+H] 137 calculated for
C
H
8 12
N
2
.
Microanalysis Calc. for
C
8
H
12
N
2
ꢂ0.25H
2
O; C, 68.29; H, 8.95; N, 19.91. Found: C, 68.11; H,
2.1. Materials
8
.48; N, 19.41%.
Pyrrole-2-carboxyaldehyde, propyl amine, copper acetate, nick-
2
.3.2. Synthesis of the 1st generation poly (propylene-imine) pyrrole-
el acetate, sodium hydride, phenol, catechol, hydroquinone, benzo-
quinone and formic acid were purchased from Sigma–Aldrich Ltd.
The polypropylene imine dendrimer with a diaminobutane core,
DAB-(NH ) was purchased from Symo-Chem, the Netherlands.
2 4
Acetonitrile was obtained from BDH Chemicals Ltd., sodium
hydroxide and hydrochloric acid (32%) were from Kimix Chemicals,
imine ligand, L2 (Scheme 2)
Into a Schlenk tube containing dry diethyl ether (20 mL) and
anhydrous MgSO
.05 mmol) were added yielding an orange–yellow mixture. A
solution of DAB-(NH (0.4 g, 1.26 mmol) dissolved in dry diethyl
4
(ca. 2 g) pyrrole-2-carboxylaldehyde (0.48 g,
5
2 4
)
ether (5 mL) was added to the mixture. A catalytic amount of
glacial acetic acid was added and the mixture stirred at r.t. for
2 2
South Africa, while H O (30 wt%) was from Merck Chemicals Ltd.
All chemicals were used without further purification. All solvents
used were of analytical grade and were dried and distilled prior
to use employing the appropriate drying agents.
4
8 h after which the Et
ing an orange oil. The oil was dissolved in CH
and the filtrate washed with water (5 ꢀ 25 mL). The organic layer
was collected, dried over anhydrous MgSO and filtered by gravity.
Removal of CH Cl yielded an orange oil. This method was adapted
2
O was removed from reaction mixture giv-
2
2
Cl (25 mL), filtered
4
2
.2. General methods and analysis
2
2
from the procedure for the preparation of dendrimeric pyridine–
diimine ligands with slight modification [26]. Yield 0.32 g, 41%.
Ligands and metal complexes were synthesized using standard
Schlenk techniques under nitrogen using a dual vacuum/nitrogen
Schlenk line. The NMR spectra were recorded on a Varian Gemini
1
H NMR (CDCl
3
) d (in ppm) 1.36 (s, br, 4H, –NCH
2
CH
HH = 7.0 Hz), 2.42 (t, br, 12H,
HH = 7.2 Hz), 3.53 (t, 8H, C@NCH
2
core),
3
1
.74 (t, 8H, –NCH
NCH CH core and branches,
HH = 6.2 Hz); 6.22 (t, 8H, C
2 2
CH branches, J
1
13
2
000 spectrometer ( H at 200 MHz, C at 50.3 MHz) at room tem-
3
–
2
2
J
2
,
perature using tetramethylsilane as an internal standard. The
chemical shifts are reported in d (ppm) and referenced relative to
residual proton signals for the NMR solvent. Infrared spectra were
recorded on a Perkin–Elmer Paragon 1000 PC FT-IR spectropho-
tometer as KBr pellets for solids or between NaCl plates for oils.
GC–MS analysis was performed using a Finnigan-Matt GCQ-Gas
chromatograph equipped with an electron impact ionization
source at 70 eV and a 30 m HP–MS capillary column with a station-
ary phase based on 5% phenylmethylpolysiloxane. ESI-MS spectra
were obtained on a Waters API Q-TOF Ultima spectrometer cali-
brated with NaF. UV–Vis spectra were recorded on a GBC UV/VIS
3
3
J
4
H
3
NH,
J
HH = 2.4 Hz), 6.46 (d, 4H,
NH); 8.03 (s, 4H, HC@N).
) { H} d (in ppm), 25.0, 28.3, 51.4, 53.8, 58.8, (den-
2
C
H
4 3
NH,
C NMR (CDCl
drimer framework CH
JHH = 2.2 Hz); 6.8 (s, 4H, C H
4 3
13
1
3
2
) 109.5, 114.3, 121.9, 130.1, (pyrrole CH) and
ꢁ1
1
52.1 (HC@N), FT-IR (cm
neat between NaCl plates);
+
m
C
@
N
= 1636. ESI-MS m/z [M+H] 625 calculated for C36
Microanalysis Calc. for C36 Cl ; C, 65.69, H, 8.01, N,
10ꢂ0.5CH
0.90. Found: C, 65.32; H, 7.84, N, 20.80%.
52 10
H N .
H
52
N
2
2
2
2
.3.3. Synthesis of [{(C
C1 was prepared using a reported method for the preparation of
(2,6-i Pr )-N@CH-(C N)] }Cr, with slight modifications
3 7 4 3 2
H )-N@CH-C H N} Cu], C1
9
20 spectrophotometer as dichloromethane solutions. Microanaly-
ses were performed at the University of Cape Town’s micro analyt-
ical laboratory.
{
C
2 6
H
3
H
4 3
2
[
18]. To a mixture of NaH (0.022 g, 0.92 mmol) and dry THF
(
15 mL) in a Schlenk tube, a solution of L1 (0.118 g, 0.87 mmol)
Oxidation reactions were carried out on a 12 place Radley’s Dis-
covery Technologies parallel reactor equipped with glass reaction
vessels and an inert gas distribution system and reflux unit.
High-pressure liquid chromatography (HPLC) was performed using
a HP 1090 liquid chromatograph equipped with a ZORBAXÒ C18
column of dimensions 4.6 ꢀ 150 mm and a UV–Vis spectropho-
tometer. The pH of the buffers was measured at 25 °C using a Metr-
ohm 744 pH meter.
in THF (5 mL) was added via a syringe giving a pink colour. Imme-
diate evolution of a gas was also observed. The mixture was stirred
for 6 h at r.t. over which time it gradually changed colour to red-
brown. After 6 h the solution was filtered under nitrogen into an-
other Schlenk tube and Cu(OAc)
The resulting mixture was then stirred at room temperature for
8 h. The solution gradually acquired a deep green colour. Removal
of the solvent in vacuo gave a deep green solid. The solid was then
dissolved in CH Cl and vacuum filtered. A deep green residue was
obtained and dried under vacuum. Recrystallization was per-
formed using mixture of CH Cl
/EtOH (1:2) at ꢁ4 °C. Yield 0.09 g,
65%. Melting point 123–124 °C, FT-IR (cm
2
2
ꢂH O (0.088 g, 0.43 mmol) added.
4
2
2
2
2
.3. Synthesis of ligands and complexes
2
2
ꢁ1
.3.1. Synthesis of propyl-(1-H pyrrol-2-ylmethylene) imine, L1
,
KBr pellet):
+
(
Scheme 1)
To yellow solution of pyrrole-2-carboxylaldehyde (1 g,
0.53 mmol) in dry diethyl ether (20 mL) was added propylamine
m
(C@N) = 1590. ESI-MS m/z [M+H] 335 calculated for C16 Cu.
22 4
H N
a
Microanalysis Calc.: C, 57.55; H, 6.69; N, 16.78. Found: C, 57.00; H,
6.76; N, 17.08%.
1

J.N. Mugo et al. / Inorganica Chimica Acta 363 (2010) 2643–2651
2645
Et O
2
O
N
H
+
H N
6 h, r. t.
N
2
N
H
L1
Scheme 1. Synthesis for propyl-(1-H-pyrrol-2-ylmethylene)-imine, L1.
N
NH
NH
N
H
2
N
H N
2
N
H
Et O
2
N
N
+
O
N
N
N
H
48h
H
N
H
N
N
2
N
H N
2
Scheme 2. Synthetic route to dendrimeric ligand, [DAB-PPI-(N@CH(C
4
H
3 4
NH) ], L2.
2.3.4. Synthesis of [{(C
H )-N@CH-C H N}
3 7 4 3 2
Ni], C2
2.3.6. Preparation of dendrimeric Ni(II) complex, C4
A mixture of NaOH (0.1 g, 2.206 mmol), L1 (0.3 g, 2.206 mmol)
and Ni(OAc) O (0.28 g, 1.103 mmol) in dry methanol (20 mL)
was refluxed for 4 h at 68 °C, Scheme 3. After 4 h the mixture
The dendrimeric Ni(II) analogue, C4, was prepared using a sim-
ilar method as employed for C2 at the ratio of 1:2:4 for L2:Ni(OA-
c) .4H O:NaOH. The product was precipitated out of the CH Cl
2 2 2 2
2
ꢂ4H
2
was filtered and the solvent removed in vacuo giving a black solid.
solution using hexane and obtained as a brown amorphous solid.
ꢁ1
This solid was dissolved in (20 mL) CH
2
2
Cl , filtered again and con-
Yield 0.29 g, 70%. FT-IR (cm
)
m
(C@N) = 1584. ESI-MS m/z
2
+
centrated to ꢃ10 mL. Hexane (30 mL) was then added precipitating
[M+H] 369 calculated for C36
10Ni
ꢂ0.25H
57.22; H, 6.35; N, 18.15%.
H
48 2
N10Ni . Microanalysis Calc. for
out a deep green almost black solid. Yield 0.21 g, 63%. Melting
C
36
48
H N
2
2
O; C, 57.33; H, 6.44; N, 18.44. Found: C,
ꢁ
1
point 174–176 °C, FT-IR (cm , KBr pellet):
m
(C@N) = 1590. ESI-
Ni. Microanalysis Calc.
O; C, 56.84; H, 6.86; N, 16.57. Found: C,
6.29; H, 6.39; N, 13.89%.
+
MS m/z [M+H] 330 calculated for C16
22 4
H N
for C16 Ni 0.5 H
H
22
N
4
2
2.4. Catalytic hydroxylation of phenol
5
Phenol hydroxylation was performed using a 12 place Radley’s
2
.3.5. Preparation of dendrimeric Cu(II) complex, C3
C3 was prepared using a similar method as employed for C1 at a
Discovery Technologies Heated Carousel Reaction Station fitted
with a reflux unit as well as a gas distribution system. The reac-
tions were carried out in 45 mL glass reaction vessels under an
oxygen atmosphere in deionised water buffered at the appropriate
pH. The prerequisite amounts of each catalyst corresponding to
0.01 mmol of metal, 1 mmol of phenol and 10 mL of buffer solution
were added to the reaction vessel. The metal concentration was
kept constant irrespective of the nature of the catalyst precursor
ratio of 1:2:4 for L3:Cu(OAc)
was obtained as a green amorphous solid after recrystallization
from CH
2
2
ꢂH O:NaH (Scheme 4). The product
ꢁ
1
2
Cl
2
/hexane. Yield 0.20 g, 62%, FT-IR (cm )
2+
m
(C@N) = 1584. ESI m/z [M+H] 375 calculated for C36H N10Cu .
48 2
Microanalysis Calc.: C, 57.83; H, 6.47, N, 18.73. Found: C, 57.82;
H, 6.57; N, 17.33%.
CH3
N
N
Cu(OAc) .H O
2
2
Cu
N
2
THF
48 h, r. t.
N
H
N
N
H C
3
Ni(OAc) .4H O
C1
THF
8 h, r. t.
2
2
4
CH3
N
N
Ni
N
N
H C
3
C2
Scheme 3. Synthetic route to monomeric complexes.

2
646
J.N. Mugo et al. / Inorganica Chimica Acta 363 (2010) 2643–2651
N
N
N
H
N
H
N
N
H
N
H
N
N
N
M(OAc) .nH O
2
2
N
N
N
N
N
M
M
N
N
N
N
N
C3: M=Cu
C4: M=Ni
Scheme 4. Preparation of dendrimeric metal complexes.
used. The resulting mixture was the stirred at 110 °C for 15 min.
The hydroxylation reaction was initiated by addition of 1 mmol
onance at d 8.04 ppm for the imine proton with the corresponding
1
3
carbon signal at 151.4 ppm in the C NMR spectrum. The signal for
the protons of the CH group adjacent to the imino group is ob-
2
H O
2
using a 30% (w/w) solution and allowed to continue refluxing
2
1
3
for 6 h under an oxygen atmosphere. After 6 h, the reaction mix-
ture was cooled to r.t. and 1 mL portions removed and diluted to
served at d 3.50 ppm while in the C NMR this signal was observed
at 62.7 ppm. There is a significant down field move as compared to
1
0 mL and filtered through a nylon syringe membrane filter.
The products were analyzed using a HP 1100 liquid chromatograph
equipped with ZORBAX SB-C18 column of dimensions
.6 ꢀ 150 mm at ambient temperature. The analytes were eluted
using a two-component gradient programme starting with 0.1%
formic acid. The percentage of the second solvent, CH CN, was in-
2
the signal for the corresponding CH group in the propyl amine
starting material. A similar trend is observed for the dendrimeric
ligand, L2. In this case the shift is from d 2.67 ppm in the DAB-
a
4
2 4
(NH ) to d 3.53 ppm in L2. The signal due to the N@CH group oc-
1
3
curs at d 8.03 ppm. The C NMR spectrum of L2 shows a signal at
152.1 ppm for the N@CH group. The pyrrole ring carbon signals are
3
creased linearly from 0% to 10% during the first 5 min, then to 40%
during the next 10 min and then reduced to 0 in the last 10 min.
Detection of the products was performed with a dual wavelength
UV detector (254 and 275 nm).
2
at 114.3, 121.9 and 130.1 ppm and that of the CH group of the
dendrimeric framework are observed between 25.0 ppm and
58.8 ppm.
3
. Results and discussion
3.1.2. FT-IR spectroscopy
The FT-IR spectra show characteristic
t
(C@N) bands at
3
.1. Synthesis and characterization of ligands
ꢁ1
ꢁ1
1
640 cm and 1636 cm for L1 and L2, respectively. The t(N–
ꢁ1
H) band was observed at ca. 3100 cm for both ligands. The broad-
ness of the latter can be associated to intermolecular hydrogen
bonding of the NH groups [27].
The ligands were prepared by Schiff base condensation of pyr-
role-2-carboxyaldehyde with propyl amine to yield L1 and with
generation 1 polypropylene imine dendrimer [DAB-(NH ] to pro-
duce L2 using the ratios of 1:1 and 4:1, respectively, as shown in
Schemes 1 and 2. Reactions were performed in dry Et O and glacial
2 4
)
2
acetic acid was added as catalyst. The two ligands were obtained as
orange oils. Both ligands showed good solubility in common sol-
vents such as dichloromethane, diethyl ether, chloroform, tetrahy-
drofuran as well as in toluene. Characterization of the ligands was
via H, C NMR and FT-IR spectroscopy, mass spectrometry, UV–
Vis spectroscopy and microanalysis. The ligands were stored at
3
.1.3. Mass spectrometry
Electron impact mass spectrometry was also used to character-
ize L1. The spectrum obtained shows a singly charged ion at m/z
37 which corresponds to the molecular ion of L1. The spectrum
1
1
13
shows a fragmentation pattern involving initial loss of the propyl
chain followed by the loss of the imine group to produce a frag-
ment at m/z 69 corresponding to pyrrole group. L2 showed a
molecular ion peak at m/z 779 obtained by electron spray ioniza-
tion mass spectrometry (ESI-MS). This corresponds to the calcu-
lated molecular weight of 624 with a potassium unit associated
with each of the four pyrrole groups on the periphery of the ligand.
4
°C and were stable at this temperature.
3
.1.1. NMR characterization
The H NMR data showed clear evidence for the condensation of
1
the aldehyde with the amine. For L1, the spectrum exhibits a res-

J.N. Mugo et al. / Inorganica Chimica Acta 363 (2010) 2643–2651
2647
3
3
.2. Synthesis and characterization of complexes
copper complex shows bands at 272, 333, 420 and 659 nm. The
*
*
first two bands can be assigned to intraligand
p ? p and n ? p
.2.1. Copper complexes (C1 and C3)
The two copper complexes were prepared by the treatment of
transitions within the ligand. These are very similar to what is ob-
served in the spectrum of the free ligand although there are shifts
to slightly higher wavelengths in the complexes. The band at
420 nm can be assigned to a charge transfer band while that at
the ligand with sodium hydride resulting in the formation of the
pyrrolide-imine sodium salt. Deprotonation of the two ligands
proceeded cleanly affording red-brown Na salts. The formation of
the pyrrolide-imine salt was confirmed using 1H NMR spectros-
copy and a up field shift of all signals was observed. After filtration
of the excess NaH under nitrogen, the requisite amount of
659 nm can be assigned to d–d transitions (dxy ! dx
2ꢁy2
and
dz
2
! dx
2ꢁy2
) [28]. The spectra obtained are similar to what is nor-
mally observed for square planar complexes of complexes of cop-
per [29].
Cu(OAc)
2
ꢂH
2
O was added. The acetate dissolved giving a blue col-
The analogous Ni complex also shows 4 bands with these
appearing at 264, 321, 406 and 612 nm. Similarly the multinuclear
complexes based on the dendrimeric ligand shows similar spectra
to the mononuclear analogues although the intensities of the peaks
are much weaker in this case.
our that changed gradually to green over the reaction time. Analyt-
ically pure compounds were obtained after recrystallization. C1
was obtained as a deep green crystalline material while C3 was iso-
lated as a green amorphous solid.
Attempts to measure EPR spectra for the complexes were only
successful for the two copper complexes. X-band (9.5 GHz) EPR
3
.2.2. Nickel complexes (C2 and C4)
Initial attempts to prepare the nickel complex using the method
spectra were record in CH
2
Cl
2
solution at 298 K (Fig. 1). The mono-
= 2.107 ± 0.02. The
nuclear copper complex give a spectrum with g
0
described for copper were unsuccessful. Instead the nickel ana-
spectrum exhibits hyperfine coupling as a result of the unpaired
electron being delocalized onto the N atoms of the N,N chelating
ligand. The hyperfine coupling constant was found to be
logues were prepared by in situ deprotonation of the ligand using
NaOH in the presence of Ni(OAc)
C2, was isolated as a deep green solid while C4, the dendrimeric
2
ꢂ4H
2
O. The monomeric complex,
ꢁ
4
ꢁ1
A
0
= 65.4 ꢀ 10 cm . The dendrimeric copper complex C3 shows
analogue, was obtained as a brown amorphous solid. Recrystalliza-
a broad band with a g value at 2.07 ± 0.02. These g values are typ-
0
2 2
tion of the two solids from CH Cl /hexane gave analytically pure
ical of distorted square planar complexes are similar to those of
N,O chelate complexes of Cu reported by Barberá et al. [30].
The unresolved broad band in the ESR spectrum of the copper
metallodendrimer can be possibly attributed to metal–metal inter-
action in the binuclear complex. Similar results have been reported
for other Schiff base complexes of copper [31].
The magnetic moment values of the Cu(II) and Ni (II) complexes
are shown in Table 1. The observed values for Cu(II) complexes fall
in the range of other Cu(II) complex with no appreciable spin cou-
pling between unpaired electrons of the copper atom. These values
suggest a square planar structure as suggested by the EPR data
products.
Both the copper and nickel complexes are fairly stable at room
temperature in the solid state but show signs of decomposition in
solution.
3.2.3. FT-IR of complexes
According to the FT-IR spectra, the mononuclear Cu(II), and
Ni(II) complexes show a significant shift of the m(C@N) stretching
frequencies from 1640 to 1590 and 1596 cm , respectively. This
indicates coordination through the azomethine nitrogen. The ab-
ꢁ
1
ꢁ1
sence of the broad
m(N–H) band at ca. 3200–3084 cm signify
[
31,32]. Ni(II) complexes with coordination number 4 normally
the deprotonation of the pyrrole nitrogen. The elemental analysis
results confirm that C1 and C2 are bis(pyrrolide-imine) complexes.
These results are further supported by the ESI-MS where singly
charged species were observed at m/z 335 for C1 and at m/z 330
for C2. These complexes are fairly stable at room temperature in
the solid state but show signs of decomposition in solution.
The FT-IR spectra for Cu(II) and Ni(II) dendrimeric complexes
have magnetic moments of 2.9–3.2 BM. The mononuclear Ni(II)
complex, (C2), has a magnetic moment value of 2.05. The reduction
of the leff could be due to intermolecular antiferromagnetic inter-
action between neighbouring molecules or by the existence of both
square planar and tetrahedral forms in the solid state. The dendri-
meric Ni(II) complex has tetrahedral geometry with a
leff value of
3
.31. This value is similar to that reported by Kasumor as well as by
showed a significant shift of the m(C@N) band of the ligand from
ꢁ
1
ꢁ1
Holm for other bis salicylaldimine complexes of nickel with tetra-
hedral geometries [33–35].
1
638 cm to 1586 and 1600 cm for C3 and C4, respectively.
The composition of the dendrimeric complexes was also estab-
lished by mass spectrometry and elemental analysis. The elemen-
tal analysis showed that both complexes are bimetallic systems.
The ESI-MS spectra showed doubly charged species of these com-
plexes at m/z 375 and 369 for C3 and C4, respectively.
3.3. Phenol hydroxylation
The complexes prepared were evaluated as catalysts in the li-
quid phase hydroxylation of phenol. The reactions were performed
in deionized water over the pH range 2–6 for the mononuclear
complexes and 2–8 for the dendrimeric complexes using appropri-
ate buffer solutions to maintain the pH at a constant value. The me-
3.2.4. UV–Vis spectra
The UV–Vis spectroscopic data for the ligands and their Cu(II)
and Ni (II) complexes are presented in Table 1. The mononuclear
ꢁ3
ꢁ1
tal concentration of 10 mol L was also kept constant in all the
catalytic runs irrespective of the catalyst used. In all cases a phe-
2 2
nol:metal ratio of 100:1 and 1:1 phenol to H O ratio was main-
Table 1
ESR data, magnetic moments and UV–Vis data of the free ligands and their metal
complexes.
tained unless otherwise stated. The results for the catalytic
activity and product selectivity for the various catalysts are shown
in Table 2.
(10ꢁ6)
Compound
X
g
leff (BM)
kmax (nm)
It was found that all the catalysts investigated were active for
phenol hydroxylation. Under the conditions used, the mononuclear
Cu(II) catalyst was found to be the most active over a wide pH
range with the highest conversion (82%) being obtained at pH 3.
All the other catalysts with the exception of complex C2 also
showed their highest phenol conversion at this pH. The mononu-
clear Ni(II) catalyst was observed to be the least active catalyst
L1
C1
C2
L2
C3
C4
–
–
280, 330
270, 335 , 429, 659
280, 330 , 406, 612
280, 331
235, 352 , 426, 652
276, 334, 401, 610
a
6.13
2.55
–
5.74
6.73
2.12
2.05
–
2.16
3.31
a
a
a
Broad.

2
648
J.N. Mugo et al. / Inorganica Chimica Acta 363 (2010) 2643–2651
Fig. 1. (a) EPR spectrum of complex, C1. (b) EPR spectrum of C3.
Table 2
Effect of pH on catalytic activity of mononuclear catalysts.
a
Catalyst
Metal
pH
% Selectivity (w/w)b
CT
% Phenol conversion
TOFc
HQ
p-BQ
C1
Cu
Cu
Cu
Cu
Cu
2
3
4
5
6
54.37
66.97
62.40
67.55
75.48
44.37
32.23
37.42
32.18
24.21
0.69
0.80
0.18
0.27
0.31
52.46
82.00
75.21
63.13
58.75
8.74
13.67
12.54
10.52
9.79
C2
C3
Ni
Ni
Ni
Ni
Ni
2
3
4
5
6
95.04
72.37
75.31
93.23
99.92
4.95
0.22
5.90
46.77
20.55
21.40
30.79
0.98
7.80
3.43
3.57
5.13
27.41
24.69
6.62
0.15
0.08
Cu
Cu
Cu
Cu
Cu
Cu
Cu
2
3
4
5
6
7
8
55.33
73.63
83.76
52.59
63.67
74.64
61.19
42.23
25.15
15.96
47.24
35.81
24.41
37.90
2.44
1.22
0.28
0.47
0.51
0.95
0.91
51.26
64.90
58.35
39.53
44.31
61.64
62.82
8.54
10.82
9.73
6.59
7.39
10.27
10.47
C4
Ni
Ni
Ni
Ni
Ni
Ni
Ni
2
3
4
5
6
7
8
96.18
92.62
80.00
89.96
99.89
85.91
68.88
3.82
0.69
0.27
0
0.11
0.35
0.45
9.75
41.90
21.37
30.27
31.53
36.74
56.93
1.63
6.98
5.33
5.05
5.26
5.05
9.49
6.69
19.73
10.04
13.74
30.67
a
b
c
ꢁ4
ꢁ1
Ten milliliters buffer solution, [M] = 1 ꢀ 10 mol L , phenol:M = 100:1, 30% H
Products, HQ, CT, and BQ, distribution is given on a tar free basis.
TOF = moles phenol consumed/moles metal/h.
2 2 2 2 2
O as oxidant; phenol:H O = 1:1, 110 °C at 1 atm O .
overall. Fig. 2 shows a graphical representation of the activities of
the four catalysts. The catalysts are ranked as follows in terms of its
activity for phenol hydroxylation: C1 > C3 > C4 > C2. From this it
can be seen that copper-based catalysts are more active than the
nickel analogues at all the pH values studied.
to the right. At pH 2, all the catalysts showed the lowest activity.
It can be concluded that at low pH, step 1 in Scheme 5 is being sup-
pressed resulting in less of the species, M Ln being formed. This in
+
turn leads to step 2 being slowed down with low concomitant for-
Å
mation of the OH radical. This lowers the overall rate of the
These results can be explained in terms of the generally ac-
cepted radical reaction mechanism for the hydroxylation of aro-
matic hydrocarbons [36–38]. Scheme 5 outlines in a step-wise
fashion the proposed mechanism for the hydroxylation reaction
mediated by a metal catalyst [39]. This mechanism is expected to
also apply to the bis(pyrrolide-imine) Cu(II) complexes as well as
to the nickel analogues. As can be seen from Scheme 5 the hydroxyl
radical is readily formed via metal mediated decomposition of
hydroxylation process [24].
Catechol and hydroquinone are the expected products in the
initial step since the OH group of phenol is ortho and para directing
group. Several researchers have reported catechol and hydroqui-
none as the only products observed under similar conditions. These
include Maurya et al. [39], Salavati-Niasari and Bazarganipour [40],
who observed only these two products. In previous work, we also
obtained catechol and hydroquinone as the major products while
using salicylaldimine based metal complexes with hydrogen per-
oxide as oxidant [24,25].
2 2
H O in acidic media. The latter facilitates the removal of the
ꢁ
OH produced in step 2; Scheme 5 thus shifting the equilibrium

J.N. Mugo et al. / Inorganica Chimica Acta 363 (2010) 2643–2651
2649
1
1
1
4
2
0
8
6
4
2
0
C1
C2
C3
C4
2
3
4
5
6
7
8
pH
Fig. 2. Effect of pH on the TOF values for all the four catalysts.
+
2
+
+1
+2
HO2.
+
H
+
M
Ln
Ln
(1)
(2)
H O
+
M
Ln
2
2
+1
-
M
Ln
+
H O
HO
+
+
HO
M
2
2
OH
OH
H
OH
.
(3)
+
HO
OH
.
HO
H
OH
OH
(4a)
OH
H
OH
H
.
OH
OH
H
.
(4b)
HO
H
OH
+2
+
+1
H O
M
Ln
HO
H
2
2
+
+
H O
+
M
Ln
(5)
2
+
+
-
H
HO
+
(6)
H O
2
M = Cu or Ni
Scheme 5. Proposed radical mechanism for the hydroxylation of phenol to catechol and hydroquinone.

2
650
J.N. Mugo et al. / Inorganica Chimica Acta 363 (2010) 2643–2651
3
.3.1. The effect of pH on phenol conversion
The copper-based catalysts showed better activity when com-
pared to the nickel analogues over the pH range evaluated (Table
). The highest conversion was generally obtained at pH 3 for the
of cases. The predominance of catechol is not unexpected for me-
tal-mediated hydroxylation occurring via a pathway, which in-
volves initial weak coordination of both phenol and H O to the
2 2
2
active site. The anchoring of the two reactants in close proximity
to each other leads predominantly to a cis arrangement, which in
turn results in ortho-substitution of the phenol [24]. From our re-
sults it can be seen that the copper complexes produce much less
catechol than the corresponding nickel analogues. This is the case
for both the mononuclear as well as the dendrimer systems. Thus it
would appear that nickel favours ortho-hydroxylation over para-
hydroxylation. There is no significant difference between the selec-
tivity of the monometallic and dendrimeric complexes of the same
metal. However, it appears that the pH of the reaction medium af-
fects the product selectivity.
At some pH values, the nickel complexes form exclusively cate-
chol. Thus for example both nickel catalysts C2 and C4 produce
only catechol at pH 6. The ratio of catechol to hydroquinone is
greatly affected by the pH of the reaction medium, (Figs. 3–6).
Overall, there appears to be an increase in catechol production as
the pH increases. This is fairly dramatic in the case of the nickel
systems with hydroquinone production fairly low at higher pH val-
ues with pH 6 being the optimal for catechol formation.
different catalyst systems. Although the metallodendrimers are
bimetallic in nature, the catalytic reactions were done in such a
way as to ensure that the metal concentration remained the same
as that used for the mononuclear catalyst systems. The nickel sys-
tems, C2 and C4, showed an increase in activity in going from pH 2
to 3, followed by a decline over the pH range 3–5 and a slight in-
crease at pH 6. However, for the dendrimeric catalyst, C4 the
reduction in activity with increasing pH is not so dramatic as is
the case for C2. The mononuclear copper analogue, C1 showed
the highest activity at pH 3 after which the activity shows a steady
decline up to pH 6. The copper dendrimeric complex C3 also shows
its highest activity at pH 3 but the decline in activity is not so dra-
matic with increasing pH. In fact at pH 6 we actually see an in-
crease in activity. In addition, the activity continues to increase
at both pH 7 and pH 8 for the dendrimeric complexes. This is unlike
the mononuclear complexes which are inactive in neutral and
alkaline media. This increase in the activity for Ni(II) and Cu(II)
dendrimeric systems at pH values 6–8 is thought to be due to
the fact that the tertiary amine groups in the dendrimer backbone
may be protonated in acidic medium. This would lead to retarda-
tion of the catalytic process. In this case the protonation of the
amine functionalities within the dendrimer framework would low-
The significant effect of pH on product selectivity is also ob-
served in the case of the copper systems. In the case of the mono-
nuclear Cu(II) system, the CT: HQ ratios increases from ca. 1:1 at
pH 2 to 2:1 at pH 3–5. At pH 6 the amount of catechol produced
increases even further giving a CT:HQ ratio of 3:1 for the mononu-
clear copper system. In the case of the dendrimeric copper system
+
er the concentration of the free H ions in solution. As the concen-
tration of free H+ ions is lowered; the equilibrium reaction
represented by step 2 (Scheme 5) is shifted to the left thus slowing
down the formation of the hydroxyl radical which in turn pro-
motes the hydroxylation process. In a neutral or slightly basic
medium, the protonation of internal N atoms of the dendrimer is
Selectivity catalyst C1
8
7
6
5
0
0
0
0
ꢁ
prevented by the high concentration of the HO ion. Step 2
catechol
hydroquinone
p-benzoquinone
(Scheme 5) is now less impeded which results in increased hydrox-
ylation activity.
The higher activity of the copper complexes C1 and C3 com-
pared to the nickel analogues could be due to the fact that the cop-
per systems have a greater ability to mediate the decomposition of
40
Å
2
H O
2
to form the HO radical. The enhanced performance of copper
30
as compared to nickel has previously been seen for salicylaldimine
complexes [25].
20
It appears that pH 3 is the optimal pH for the decomposition of
H O to the hydroxyl radical mediated by pyrrolylaldiminato based
2 2
1
0
0
metal complexes. This has also been reported as the optimum pH
for homogeneous Fenton processes [3]. Tatibouëta et al. [41] re-
ported that both homogeneous and heterogeneous systems of
commercially available Greek bentonite (mixture of Al–Fe species
at atomic ratio Al:Fe equal to 9:1) were very sensitive to pH
changes. In these systems the homogeneous catalysts gave maxi-
mum phenol conversion in the pH range of 3–3.7. Dramatic reduc-
tion in conversion levels from ꢄ80% at pH 3 to less than 5% at pH
2
3
4
5
6
pH
Fig. 3. Effect of pH on product selectivity for catalyst C1.
Selectivity catalyst C2
120
2
.5 and 10% at pH 4 was observed. However, this contrasts with
catechol
hydroquinone
p-benzoquinone
what was observed in the case of salicylaldiminato complexes
which were more active at less acidic conditions [24,25]. Thus, it
can be concluded that our systems do in fact exhibit Fenton-type
reactions. Previously the optimum pH for hydroxyl radical forma-
tion was found to be pH 3.8 under acidic conditions [28].
100
80
60
Control experiment at pH 3 in the absence of metal catalyst
shows extremely low conversion of phenol (<5%). It is thus clear
that the metal complex is required to accelerate the hydroxylation
process and for it to proceed at acceptable levels of conversion.
4
2
0
0
0
3.3.2. Product selectivity
2
3
4
pH
5
6
The major products observed in the phenol hydroxylation using
the pyrroliminato complexes as catalysts are catechol and hydro-
quinone with catechol formed in larger quantities in the majority
Fig. 4. Effect of pH on product selectivity for catalyst C2.

J.N. Mugo et al. / Inorganica Chimica Acta 363 (2010) 2643–2651
2651
was found to be the most active. At pH lower than 3, all the cata-
lysts showed dramatic reduction in activity while at higher pH the
activity reduced then leveled out as the pH was further increased.
With regard to selectivity, the catalysts were more selective for
catechol over hydroquinone.
Selectivity catalyst C3
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
catechol
hydroquinone
p-benzoquinone
Acknowledgements
Acknowledgements go to National Research Foundation and the
Water Research Commission South Africa for financial support.
This work was also supported by Research Committees of the Uni-
versity of the Western Cape and Stellenbosch University.
References
2
3
4
5
6
7
8
pH
[
[
[
[
[
[
[
[
1] H. Tang, Y. Ren, B. Yue, S. Yan, H. He, J. Mol. Catal. 260 (2006) 121.
2] F. Luck, Catal. Today 27 (1996) 195.
3] W. Zhao, Y. Luo, P. Deng, Q. Li, Catal. Lett. 73 (2001) 199.
4] J.G. Mei, S.M. Yu, J. Cheng, Catal. Commun. 5 (2004) 437.
5] A. Sorokin, B. Meunier, Chem. Commun. (1994) 1799.
6] K. Zhu, L. Chibiao, W. Yue, Appl. Catal. B 16 (1998) 365.
7] S. Jiang, Y. Kong, J. Wang, X. Ren, Q. Yan, J. Porous Mater. 13 (2006) 341.
8] J. Barrault, J.-M. Tatibouët, N. Papayannakos, C.R. Acad. Sci., Ser. IIc: Chim. 3
Fig. 5. Effect of pH on product selectivity for catalyst C3.
Selectivity of catalyst C4
1
1
20
00
catechol
(
2000) 777.
9] J.-Q. Xie, J.-Z. Li, X.-G. Meng, C.-W. Hu, X.-C. Zeng, Transition Met. Chem. 29
2004) 388.
10] M.R. Maurya, S. Sikarwar, J. Mol. Catal. A: Chem. 263 (2007) 175.
[11] M.R. Maurya, I. Jain, S.J.J. Titinchi, Appl. Catal. A 249 (2003) 139.
hydroquinone
p-benzoquinone
[
(
[
8
6
4
2
0
0
0
0
0
[
12] C.R. Jacobs, S.P. Varkey, P. Ratnasamy, Micropor. Mesopor. Mater. 22 (1998)
65.
13] M.R. Maurya, M. Kumar, S.J.J. Titinchi, H.S. Abbo, S. Chand, Catal. Lett. 86 (2003)
7.
[14] M.R. Maurya, U. Kumar, P. Manikandan, J. Chem. Soc., Dalton Trans. (2006)
561.
15] L.-C. Liang, C.-W. Yang, M.Y. Chiang, C.-H. Hung, P.-Y. Lee, J. Organomet. Chem.
679 (2003) 135.
16] H. Hao, S. Bhandari, Y. Ding, H.W. Roesky, J. Magull, H.-G. Schmidt, M.
Noltemeyer, C. Cui, Eur. J. Inorg. Chem. (2002) 1060.
4
[
9
3
[
[
[
17] S.D. Drouin, H.M. Foucault, G.P.A. Yap, D.E. Fogg, Can. J. Chem. 83 (2005)
2
3
4
5
6
7
8
748.
pH
[18] V.C. Gibson, C. Newton, C. Redshaw, G.A. Solan, A.J.P. White, D.J. Williams, J.
Chem. Soc., Dalton Trans. (2002) 4017.
Fig. 6. Effect of pH on product selectivity for catalyst C4.
[19] Y.-S. Li, Y.-R. Li, X.-F. Li, J. Organomet. Chem. 667 (2003) 185.
[
20] Y. Yoshida, S. Matsui, Y. Takagi, M. Mitani, T. Nakano, H. Tanaka, N. Kashiwa, T.
Fujita, Organometallics 20 (2001) 4793.
[
[
21] R.M. Bellabarba, P.T. Gomes, S.I. Pascu, J. Chem. Soc., Dalton Trans. (2003) 4431.
22] S. Matsui, T.P. Spaniol, Y. Takagi, Y. Yoshida, J. Chem. Soc., Dalton Trans. (2002)
4529.
a similar trend of increasing catechol production with increasing
pH is also observed although the highest catechol formation is ob-
served at pH 4.
The formation of some p-benzoquinone is also observed in
some of the reactions. This indicates that under our conditions,
hydroquinone undergoes secondary oxidation but catechol does
not. Higher amounts of benzoquinone were observed in cases
where the TOF values were lower.
[
[
23] J.L. van Wyk, S.F. Mapolie, A. Lennartson, S. Jagner, Inorg. Chim. Acta 361 (7)
(
2008) 2094.
24] J.L. van Wyk, S. Mapolie, A. Lennartson, M. Håkansson, S. Jagner, Z. Naturfosch.,
B: Chem. Sci. 62b (2007) 331.
[
[
[
25] S. Ray, S.F. Mapolie, J. Darkwa, J. Mol. Catal. A: Chem. 267 (2007) 143.
26] G. Smith, R. Chen, S.F. Mapolie, J. Organomet. Chem. 673 (2003) 111.
27] H. Günzeler, H.-U. Gremlich, An Introduction to IR Spectroscopy, Translated by
M.-J. Blümich, Wiley-VCH Verlag GmbH, 69469 Weinheim (Federal Republic of
Germany), 2002, p. 176.
[
[
28] W.M. Coleman, L.T. Taylor, Inorg. Chem. 10 (1971) 2195.
29] V.T. Kasumov, F. Koksal, Spectrochim. Acta, Part A: Mol. Biomol. Spectrosc. 58A
4
. Conclusion
(
10) (2002) 2199.
[
[
30] J. Barberá, M. Marcos, A. Omenat, J.-L. Serrano, Liq. Cryst. 27 (2000) 255.
31] N. Domracheva, A. Mirea, M. Schwoerer, L. Torre-Lorente, G. Lattermann,
Chem. Phys. Chem. 6 (2005) 110.
Monofunctional and multifunctional pyrrole-imine ligands have
been successfully synthesized and characterized. Bis(pyrrolide-
imine) Cu(II) and Ni(II) complexes were obtained from the mono-
functional ligand while the dendrimeric ligand gave bimetallic
metallodendrimers.
[32] H.M. El-Tabl, A. El-Saied, M.I. Ayad, Inorg. Met. Org. Chem. 32 (2002) 1247.
[
33] N.M. El-Metwally, I.M. Gabr, A.M. Shallaby, A. El-Asmy, J. Coord. Chem. 58
2005) 1145.
34] V.T. Kasumor, Transition Met. Chem. 27 (2002) 228.
(
[
These complexes were evaluated as catalysts in the hydroxyl-
ation of phenol. It is clear that the catalytic activity of these com-
plexes in the hydroxylation of phenol to the dihydroxylbenzenes
[35] G.W. Everett, R.H. Holm, J. Am. Chem. Soc. 87 (1965) 2117.
[
[
[
36] A. Santos, E. Barroso, F. García-Ochoa, Catal. Today 48 (1999) 109.
37] A. Pintar, J. Levec, J. Catal. 135 (1992) 345.
38] A. Pintar, J. Levec, Chem. Eng. Sci. 47 (1992) 2395.
(
catechol and hydroquinone), is dependent on the pH of the reac-
[39] M.R. Maurya, S.J.J. Titinchi, S. Chand, J. Mol. Catal. A: Chem. 193 (2003) 165.
[40] M. Salavati-Niasari, M. Bazarganipour, Catal. Commun. 7 (2006) 336.
tion medium. The optimal pH was established to be between 3
and 4 for all the catalysts. The mononuclear copper complex, C1,
[
41] J.-M. Tatibouëta, E. Guéloua, J. Fournier, Top. Catal. 33 (2000) 225.