The synthesis of copper(II) salicylaldiminato complexes and their catalytic activity in the hydroxylation of phenol

Article abstract of DOI:10.1515/znb-2007-0305

The synthesis of copper(II) salicylaldiminato complexes and their application in the catalytic hydroxylation of phenol is reported. Tetracoordinated copper complexes (Cu(Lⁿ)₂) were obtained by reacting the N-phenylsalicylaldimine ligands (HL¹ -HL⁷) with copper acetate in a 2:1 mole ratio. The reaction of N-(2,6-diisopropyl) phenyl-3,5-di-tert-butylsalicylaldimine (HL⁷) with copper acetate in a 1:1 mole ratio afforded a dinuclear complex, which was not obtained with the other ligand systems. All complexes were characterized using FT-IR and elemental analysis. X-Ray crystal structures of complexes 2, 5 and 8 have also been obtained. The catalytic activity of these complexes was evaluated in the hydroxylation of phenol using oxygen and hydrogen peroxide as co-oxidants in aqueous media in the pH range 3-6. AU complexes studied were found to be active for the hydroxylation process over the pH range studied with higher selectivity for catechol formation.

Full text of DOI:10.1515/znb-2007-0305

The Synthesis of Copper(II) Salicylaldiminato Complexes and their
Catalytic Activity in the Hydroxylation of Phenol
Juanita L. van Wyka, Selwyn Mapoliea, Anders Lennartsonb, Mikael Ha˚kanssonb, and
Susan Jagnerc
a Chemistry Department, University of the Western Cape, Private Bag X17, Bellville, 7535,
South Africa
b Department of Chemistry, Go¨teborg University, SE-412 96 Go¨teborg, Sweden
c Department of Chemical and Biological Engineering, Chalmers University of Technology,
SE-412 96 Go¨teborg, Sweden
Reprint requests to S. Mapolie. E-mail: smapolie@uwc.ac.za
Z. Naturforsch. 2007, 62b, 331 – 338; received November 13, 2006
Dedicated to Prof. Helgard G. Raubenheimer on the occasion of his 65th birthday
The synthesis of copper(II) salicylaldiminato complexes and their application in the catalytic hy-
droxylation of phenol is reported. Tetracoordinated copper complexes (Cu(Ln)2) were obtained by re-
acting the N-phenylsalicylaldimine ligands (HL1 – HL7) with copper acetate in a 2 : 1 mole ratio. The
reaction of N-(2,6-diisopropyl)phenyl-3,5-di-tert-butylsalicylaldimine (HL7) with copper acetate in a
1 : 1 mole ratio afforded a dinuclear complex, which was not obtained with the other ligand systems.
All complexes were characterized using FT-IR and elemental analysis. X-Ray crystal structures of
complexes 2, 5 and 8 have also been obtained. The catalytic activity of these complexes was evalu-
ated in the hydroxylation of phenol using oxygen and hydrogen peroxide as co-oxidants in aqueous
media in the pH range 3 – 6. All complexes studied were found to be active for the hydroxylation
process over the pH range studied with higher selectivity for catechol formation.
Key words: Salicylaldimine, Tetracoordinated Copper(II) Complexes, Hydroxylation, Phenol
Introduction
catalytic systems employed are environmentally haz-
ardous. Poor selectivities are also obtained and a sig-
nificant amount of tar material is often produced. Re-
cently catalysts based on zeolites [4, 5] have been de-
veloped to improve the selectivies and the economical
viability of this process. Although a commercial pro-
cess has been developed employing these zeolite cat-
alytic systems [6], the utilization of these catalysts in-
dustrially has been limited by their complicated syn-
thetic route and the associated high cost. In this pa-
per the synthesis of copper(II) salicylaldiminato com-
plexes 1 – 8 is described and the catalytic activity of
these complexes evaluated in the hydroxylation of phe-
nol to HQ and CT using oxygen and hydrogen peroxide
as co-oxidants in aqueous media.
Due to environmental concerns there has been an
increase in the demand by industry to introduce pro-
cesses which are environmentally and economically
sustainable. As a result much attention has been fo-
cused on developing catalytic processes, which uti-
lize cheap, abundant and non-toxic oxidants such as
dioxygen and hydrogen peroxide. Over the past decade
there has been an increase in the interest in transi-
tion metal complexes based on porphyrins, phthalocya-
nines and Schiff bases. These complexes are widely
used as peroxidase mimics [1 – 3] and have been em-
ployed to study the oxidation of phenol using hydrogen
peroxide as oxidant. Currently the oxidation of phe-
nol with hydrogen peroxide is of significance since the
process provides an environmentally friendly means of
obtaining the dihydroxybenzenes, hydroquinone (HQ)
and catechol (CT), which are widely used as raw ma-
Results and Discussion
All Schiff base ligands were synthesized accord-
terials in the agrochemical and fine chemical indus- ing to a well-established synthetic procedure which in-
tries. There are a number of industrial processes for the volved the condensation of a substituted salicylalde-
production of these dihydroxybenzenes, however the hyde with the appropriate aromatic amine [7]. Ligands
0932–0776 / 07 / 0300–0331 $ 06.00 © 2007 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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332
J. L. van Wyk et al. · Synthesis of Copper(II) Salicylaldiminato Complexes
Table 1. Characterization data for mono-functional salicylaldimine ligands HL1 – HL7.
Ligand
NMR spectra
Anal. found
(calcd.)
IR spectra
b
(ppm)a
(cm−1
)
HL1 (R = H; R1 = H; R2 = H),
1H NMR: δ = 1.25 (d, 12H), 3.07 (sept, 2H), 6.9 – 7.15 (m, 2H),
C13H11NO:
ν(C=N) 1617
m. p. 49 – 50 ◦C
7.4 – 7.46 (m, 2H), 8.38 (s, 1H)
C 78.79 (79.16), ν(C–O) 1273
13C NMR: δ = 23.48, 28.11, 117.32, 118.68, 118.98, 123.23, H 5.58 (5.62),
125.43, 132.17, 133.19, 138.66, 146.19, 161.23, 166.57
N 7.74 (7.10)
HL2 (R = Pr; R1 = H; R2 = H), 1H NMR: δ = 1.25 (d, 12H), 3.07 (sept, 2H), 6.9 – 7.15 (m, 2H), C19H23NO:
ν(C=N) 1625
C 81.24 (81.10), ν(C–O) 1279
13C NMR: δ = 23.48, 28.11, 117.32, 118.68, 118.98, 123.23, H 8.70 (8.24),
125.43, 132.17, 133.19, 138.66, 146.19, 161.23, 166.57 N 5.22 (4.98)
21H27NO:
(m, 3H), 8.60 (s, 1H), 13.69 (s, 1H) C 81.50 (81.51), ν(C–O) 1272
i
m. p. 55 – 57 ◦C
7.4 – 7.46 (m, 2H), 8.38 (s, 1H)
HL3 (R = H; R1 = t Bu; R2 = tBu), 1H NMR: δ = 1.29 (s, 9H), 1.45 (s, 9H), 6.9 – 7.2 (m, 4H), 7.4
C
ν(C=N) 1613
m. p. 108 – 110 ◦C
13C NMR: δ = 29.46, 31.49, 34.19, 35.13, 118.34, 121.16, H 8.69 (7.89),
126.50, 126.8, 127.99, 129.32, 137.02, 140.57, 148.79, 158.28, N 4.15 (4.53)
163.81
HL4 (R = Pr; R1 = H; R2 = Cl), 1H NMR: δ = 1.24 (d, 12H), 3.02 (sept, 2H), 7.05 (d, 1H), 7.3 (s,
C19H22ClNO:
ν(C=N) 1625
i
m. p. 114 – 115 ◦C
3H), 7.4 (s, 2H), 8.31 (s, 1H)
C 72.19 (72.25), ν(C–O) 1273
13C NMR: δ = 23.51, 28.19, 118.96, 119.40, 123.35, 123.73, H 6.98 (7.02),
125.77, 131.23, 133.05, 138.59, 145.77, 159.82, 159.82, 165.48 N 4.02 (4.48)
HL5 (R = H; R1 = H; R2 = Cl), 1H NMR: δ = 6.92 (m, 1H), 7.2 – 7.4 (m, 7H), 8.51 (s, 1H), 13.26 C13H10ClNO:
ν(C=N) 1614
C 67.32 (67.40), ν(C–O) 1276
13C NMR: δ = 118.84, 119.97, 123.66, 127.32, 129.48, 131.20, H 4.26 (4.35),
132.89, 148.01, 159.72, 161.26 N 5.60 (6.05)
23H31NO:
C 81.43 (81.85), ν(C–O) 1269
m. p. 107 – 108 ◦C
(br s, 1H)
HL6 (R = iPr; R1 = tBu; R2 = H), 1H NMR: δ = 1.26 (d, 12H), 1.57 (s 9H), 3.07 (sept, 2H), 6.89
C
ν(C=N) 1618
m. p. (66 – 68 ◦C
(t, 1H), 7.26 (s, 3H), 7.6 (dd, 2H), 8.37 (s, 1H)
13C NMR: δ = 23.60, 28.14, 29.46, 34.99, 118.21, 118.58, H 9.24 (9.26),
123.25, 125.36, 130.4, 130.58, 137.86, 138.91, 146.35, 160.72, N 3.95 (4.15)
167.28
HL7(R = Pr; R1 tBu; R2 = Bu), 1H NMR: δ = 1.26 (d, 12H), 1.42 (s 9H), 1.58 (s 9H), 3.11 (sept,
C
27H39NO:
ν(C=N) 1620
i
t
m. p. 79 – 81◦C
2H), 7.24 (m, 4H), 7.59 (d, 1H), 8.38 (s, 1H)
C 82.52 (82.39), ν(C–O) 1270
13C NMR: δ = 23.62, 28.06, 29.52, 31.49, 34.21, 35.2, 117.77, H 10.13 (9.99),
123.17, 125.23, 126.65, 128.10, 137.18, 138.88, 140.48, 146.43, N 3.92 (3.56)
158.46, 167.53
a
b
Recorded in CDCl3; recorded as nujol mulls between NaCl plates.
Scheme 1. Synthesis of sal-
icylaldimine ligands HLn
and copper(II) complexes
1 – 7.
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J. L. van Wyk et al. · Synthesis of Copper(II) Salicylaldiminato Complexes
Complex Formula Anal found (calcd.)
333
a
IR spectra (cm−1
)
Table 2. Characterization
data for copper(II) sali-
cylalidminato complexes
1 – 8.
C
H
N
ν(C=N) ν(C–O)
1
1
2
3
4
5
6
7
8
C26H20CuN2O2 · /2H2O
67.85 (67.17) 4.17 (4.42) 6.01 (6.14) 1606
72.90 (73.11) 7.11 (7.10) 4.45 (4.49) 1602
74.57 (74.14) 7.73 (7.70) 4.13 (4.12) 1609
65.46 (65.84) 6.12 (6.11) 3.77 (4.04) 1610
1327
1323
1317
1322
1322
1318
1324
1325
C38H44CuN2O2
C
C
C
C
C
42H52CuN2O2
a
38H42Cl2CuN2O2
Recorded as nujol mulls be-
1
26H18Cl2CuN2O2 · /2H2O 58.41 (58.49) 3.23 (3.46) 5.54 (5.34) 1606
tween NaCl plates.
46H60CuN2O2
74.81 (75.01) 8.17 (8.21) 3.78 (3.80) 1602
73.62 (73.32) 8.73 (9.06) 2.53 (2.97) 1610
66.50 (66.84) 8.13 (8.21) 2.75 (2.78) 1614
54H76CuN2O2 ·2H2O
C56H80Cu2N2O5 H2O
Table 3. Crystallographic data
for 2, 5 and 8.
Compound
Empirical formula
Mr
2
5
8
C38H44CuN2O2 C26H18Cl2CuN2O2 C56H80Cu2N2O5
624.3
524.9
988.3
Crystal system
Space group
triclinic
P1
monoclinic
C2/c
23.634(7)
9.337(2)
15.087(5)
90
137.343(7)
90
2256(1)
4
monoclinic
P21/c
14.944(2)
24.968(3)
14.736(2)
90
94.539(4)
90
5481(1)
4
R1 = ΣꢀFo|−|Fcꢀ/Σ|Fo|;
¯
2
wR2 = {Σ[w(Fo −Fc2)2/
Σ[w(Fo2)2)]}
.
1/2
˚
a, A
8.0725(16)
10.2332(9)
11.7022(3)
60.62(2)
76.39(3)
81.64(4)
818.2(1)
1
˚
b, A
˚
c, A
α, deg.
β, deg.
γ, deg.
3
˚
V, A
Z
F(000), e
331
1068
2112
Dcalc, g cm−3
µ (MoKα ), cm−1
Temperature, ◦C
2θ range, deg.
Absorption correction
Data / refined parameters
R1 [I ≥ 2σ(I)]
wR2 (all reflections)
1.27
7.0
20(2)
1.54
12.32
20(2)
1.20
8.21
20(2)
3.6 < 2θ < 52.0
empirical
9953 / 589
0.064
0.167
1.33 / −0.40
4.0 < 2θ < 54.0 7.6 < 2θ < 54.0
empirical
3230 / 206
0.055
0.119
0.45 / −0.86
empirical
2235 / 151
0.032
0.091
0.27 / −0.35
−3
˚
Max/min residual electr. density, e A
Scheme 2. Synthesis of the
dinuclear complex 8.
were obtained as pale yellow to orange yellow solids rates [8]. Alkoxy bridged dimers containing iron [9]
in good yields. Structural data for the ligand systems and zinc [10] have also been reported recently.
HL1 – HL7 and copper complexes 1 – 8 are provided in
Tables 1 and 2, respectively. The mononuclear tetraco-
ordinated copper(II) complexes 1 – 7 were quite read-
ily obtained via a 2 : 1 mole equivalent reaction of the
appropriate ligand with copper acetate. The reaction of
HL7 in a 1 : 1 mole ratio with copper acetate afforded
the dinuclear complex 8. Similar bimetallic species
were not obtained with the other ligand systems. The
formation of bimetallic species has been reported in the
cases where bi- and tridentate salicylaldimine ligands
were reacted with metal halides, nitrates and perchlo-
IR Spectra
A broad band is observed in the range 2800–
3000 cm−1 in the IR spectra of the free ligands un-
der investigation. This band is attributed to the O–
H stretching vibration. The occurrence of the band at
such a low wavenumber is characteristic of these lig-
ands and is an indication of fairly extensive hydrogen
bonding [11]. The C=N band is observed in the region
1613 – 1625 cm−1 for the free ligands, but is shifted
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334
J. L. van Wyk et al. · Synthesis of Copper(II) Salicylaldiminato Complexes
˚
˚
Table 5. Selected bond lengths (A) and angles (deg) for the
Table 4. Selected bond lengths (A) and angles (deg) for the
core of 5.
core of 2.
Cu(1)–N(1)
Cu(1)–O(1)
1.985(1)
1.901(1)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(1)i
O(1)–Cu(1)–O(1)i
N(1)–Cu(1)–N(1)i
93.56(6)
151.86(6)
88.07(8)
97.93(8)
Cu(1)–N(1)
Cu(1)–O(1)
1.999(2)
1.869(2)
O(1)–Cu(1)–N(1)
O(1)–Cu(1)–N(1)i
91.39(9)
88.61(9)
Symmetry code: i −x, −y, 1−z.
Symmetry code: i −x, y, 1.5−z.
Fig. 2. The molecular structure of 5 and atomic numbering
scheme adopted. Cu(1) is situated on a crystallographic two-
fold axis.
Fig. 1. The molecular structure of 2 and atomic numbering
scheme adopted. Cu(1) is situated on a crystallographic cen-
tre of symmetry.
˚
Table 6. Selected bond lengths (A) and angles (deg) for the
core of 8.
to 1602 – 1614 cm−1 in the spectra of the copper com-
plexes, which indicates coordination of the azomethine
nitrogen atom to the metal [12]. Coordination of the
ligand to the metal is further confirmed by the shift
in the phenoxy C–O stretching vibration from 1269–
1279 cm−1 to a higher frequency upon M–O coordina-
tion.
Cu(1)–N(1)
Cu(1)–O(1)
Cu(1)–O(3)
Cu(1)–O(5)
1.951(3)
1.884(3)
1.960(3)
1.880(3)
Cu(2)–N(2)
Cu(2)–O(2)
Cu(2)–O(4)
Cu(2)–O(5)
1.952(3)
1.881(3)
1.956(4)
1.880(3)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–O(3)
N(1)–Cu(1)–O(5)
O(1)–Cu(1)–O(3)
O(1)–Cu(1)–O(5)
O(3)–Cu(1)–O(5)
93.6(1)
91.9(1)
166.5(1)
166.4(1)
89.6(1)
91.9(1)
N(2)–Cu(2)–O(2)
N(2)–Cu(2)–O(4)
N(2)–Cu(2)–O(5)
O(2)–Cu(2)–O(4)
O(2)–Cu(2)–O(5)
O(4)–Cu(2)–O(5)
93.1(1)
88.1(1)
166.8(1)
165.8(1)
90.1(1)
92.0(1)
X-Ray crystallography
The crystal structures of complexes 2, 5 and 8 are
shown in Figs. 1 – 3, respectively. X-Ray crystallogra-
phy revealed the Schiff base ligands to coordinate in
a bidentate fashion with two ligands coordinating to
one copper centre for both complexes 2 and 5. How-
ever, the ligands of complex 2, which contain iPr sub-
stituents on the imino-bound aromatic ring, are coor-
dinated in a trans arrangement whereas the ligands of
complex 5, which do not contain substituents on the
imino-bound aromatic ring coordinate in a cis arrange-
ment. Similar results have been obtained with cobalt
analogues of this type of ligand systems [13]. The crys-
tal structure of complex 8 revealed coordination of one
ligand per copper centre with an acetoxy and a hydroxy
group bridging between the two copper centres. Note:
In 2, Cu is situated on a centre of symmetry so that
Fig. 3. The molecular structure of 8 and atomic numbering
scheme adopted.
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J. L. van Wyk et al. · Synthesis of Copper(II) Salicylaldiminato Complexes
335
N(1)–Cu(1)–N(1)i and O(1)–Cu(1)–O(1)i are both ex-
actly 180◦. In 5, Cu(1) is situated on a twofold axis.
The phenyl rings attached to N(1) and N(1)i are al-
most co-planar [dihedral angle 6.3(1)◦], the distance
˚
between the ring centroids being 3.62 A, indicative of
weak intramolecular π ···π interactions. Crystal and
experimental data are summarized in Tables 3 – 6.
Catalyst evaluation
The hydroxylation of phenol catalyzed by com-
plexes 1 – 4 and complex 8 was studied in deion-
ized water in the pH range 3 – 6 using appropriately
buffered solutions. All reactions were performed under
an oxygen atmosphere using 6 % H2O2 as co-oxidant.
A substrate to metal ratio of 100 : 1 and a 1 : 1 mole
ratio of phenol to H2O2 was used for all reactions.
Results for the hydroxylation reactions performed are
shown in Table 7.
Catalyst activity
All the complexes evaluated were found to be ac-
tive for phenol hydroxylation. The catalyst activity was
tested over a range of pH values in order to evaluate to
what extent the acidity of the medium affects phenol
conversion. It was found that the complexes perform
better at moderately acidic pH values. The highest phe-
nol conversion (77 %) was obtained with complex 8
at pH 6. This however is only slightly higher than the
Scheme 3. Proposed reaction pathway for phenol hydroxyla-
tion.
conversion obtained for the other complexes at simi- of OH− anions from the reaction mixture. Conversely,
lar pH values. Reactions were also attempted in alka- at alkaline pH, the high OH− concentration shifts the
line media, but these showed no phenol consumption. equilibrium in step 2 to the left, thus lowering the hy-
The results obtained are in agreement with what has droxyl radical concentration. This is what we observe,
as all the catalysts are inactive at pH values above pH 7.
been reported in the literature for other metal-mediated
phenol hydroxylation processes [14]. The results can We however found that too low a pH also has a detri-
be explained in terms of the generally accepted reac- mental effect on catalyst activity. Thus the catalysts are
tion mechanism for the oxidation of aromatic hydro- ineffective at pH 2 and below, with no phenol hydroxy-
carbons [15].
lation being observed. At such low pH values we have
the possibility of step 1 in Scheme 3 being retarded.
This ultimately has a negative effect on the production
of the hydroxyl ions (step 2). In several of our exper-
iments we found that the optimum pH was around 5
or 6.
The accepted reaction pathway can basically be
summarized by the steps outlined in Scheme 3.
Our results are in general agreement with what has
been reported in the literature. Thus the effectiveness
of the hydroxylation process in acidic media can be
explained in terms of some of the steps outlined in
We also attempted to perform the reactions in ace-
Scheme 3. Acidic media enhance the metal-mediated tonitrile as solvent. In this case no oxidation products
decomposition of H2O2 to the hydroxyl radical, which were observed for the catalyst systems evaluated. A
is an important species in the hydroxylation of phe- possible reason for this is that in acetonitrile the gen-
nol [16 – 17]. The decomposition of the peroxide is eration of the hydroxyl radical is retarded as a result
shown in step 2 (Scheme 3). This step can clearly be of coordination of the solvent to the metal centre. This
enhanced in moderately acidic media by the removal prevents H2O2 interaction with the active site and hin-
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J. L. van Wyk et al. · Synthesis of Copper(II) Salicylaldiminato Complexes
Table 7. Effect of pH on the catalytic activity of com-
plexes 1 – 4 and 8a.
Catalyst
pH
%W
%PhOH
Conversion
TOFc
Dihydroxybenzenesb
HQ
CT
1
3
4
5
6
37.18 62.82
40.99 59.01
42.01 57.99
37.02 62.98
56
69
72
71
9.4
11.5
12.0
11.9
2
3
4
3
4
5
6
38.13 61.87
31.99 68.01
37.86 62.14
32.69 67.31
49
73
74
65
8.2
12.2
12.4
10.8
Chart 2. Effect of pH on hydroquinone : catechol ratio for
catalyst 2.
3
4
5
6
32.67 67.33
34.39 65.61
36.53 63.47
35.77 64.23
62
68
62
73
10.4
11.4
10.3
12.2
3
4
5
6
37.45 62.55
28.74 71.26
33.63 66.37
30.40 69.60
66
66
63
72
11.0
11.0
10.5
12.0
8
3
4
5
6
31.79 68.21
28.24 71.76
32.32 67.68
39.11 60.89
65
64
73
77
10.9
10.7
12.1
12.8
a
◦
Reaction performed in 10 mL H2O, Phenol : Cu = 100 : 1; 10 C
b
at 1atm O2; 6 % H2O2 as co-oxidant; Phenol : H2O2 = 1 : 1; HQ
= Hydroquinone; CT = Catechol; product distribution given on a tar
Chart 3. Effect of pH on hydroquinone : catechol ratio for
catalyst 3.
free basis; c TOF = (mol phenol consumed/mol Cu)/hour.
Chart 1. Effect of pH on hydroquinone : catechol ratio for
catalyst 1.
Chart 4. Effect of pH on hydroquinone : catechol ratio for
catalyst 4.
ders the decomposition of hydrogen peroxide to the hy-
droxyl radical. Another potential problem could be the
reaction of the organic solvent with any HO• radical,
if it should form in the first place. Similar observations
have previously been reported in the literature for pro-
cesses in which organic solvents react with the active
hydroxy radical [17].
In some instances minute traces of benzoquinones
(BQ) were also detected. In almost all cases the cat-
echol to hydroquinone ratio was around 2 : 1, with
the exception of complex 1, which produces slightly
higher levels of hydroquinone than the other catalysts.
In the latter case the CT : HQ ratios are around 1.5 : 1.
The predominance of catechol over hydroquinone is
not unexpected for copper-mediated hydroxylations
occurring via the free radical process. It has previously
Catalyst selectivity
The major products observed under the conditions been reported that the hydroxylation occurs via a path-
described were hydroquinone (HQ) and catechol (CT). way which involves initial weak coordination of both
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J. L. van Wyk et al. · Synthesis of Copper(II) Salicylaldiminato Complexes
337
Nujol mulls using NaCl windows. Microanalyses were per-
formed by the University of Cape Town Microanalytical Lab-
oratory.
1
Synthesis of salicylaldimine ligands HL – HL7
To a methanol solution (15 mL) of the appropriately
substituted 2-hydroxybenzaldehyde (12 mmol) were added
formic acid (0.5 mL) and the corresponding aromatic amine
(16 mmol). The resulting reaction mixture was stirred at r. t.
for approximately 15 h. During this time a solid precipitated
from the solution and was isolated by filtration, washed with
cold methanol and dried under vacuum. The salicylaldimine
ligands were obtained as light yellow to orange yellow solids
in yields of 65 – 93 %.
Chart 5. Effect of pH on hydroquinone : catechol ratio for
catalyst 8.
phenol and H2O2 to the active site. The anchoring of
the two reaction partners in close proximity leads pre-
dominantly to a cis arrangement, which in turn results
in ortho-substitution of the phenol [18].
Synthesis of copper complexes 1 – 8
Copper acetate monohydrate (0.5 mmol) and the appro-
priate Schiff base ligand (1 mmol) were refluxed in methanol
(20 mL) for 4 h. During this time a solid precipitated from
solution. The reaction mixture was cooled to 0 ◦C for approx-
imately 15 min and the solid was isolated by vacuum filtra-
tion. Complex 8 was prepared using a 1 : 1 mole ratio of lig-
and HL7 to copper acetate. Complexes were recrystallized by
slow diffusion of ethanol into concentrated dichloromethane
solutions of the complexes.
Conclusions
Copper(II) complexes of N-phenylsalicylaldimine
ligands were synthesized and the molecular structure
of complexes 2, 5 and 8 were determined using single
crystal X-ray crystallography. The crystal structure of
complex 8 revealed the formation of a dinuclear cop-
per complex while both 2 and 5 are mononuclear com-
plexes with coordination of the ligands trans and cis,
respectively. The catalytic activity of complexes 1 – 4
and 8 were evaluated in the hydroxylation of phenol
in aqueous media under pH-controlled conditions. For
all catalysts the major products detected were HQ and
CT although the catalyst appeared to be more selec-
tive towards the formation of CT. There was no signif-
icant pH effect in terms of catalyst activity and only a
slight pH effect was observed favouring the formation
of CT for all catalysts. For complexes containing sub-
stituents on the aromatic ring attached to the imino ni-
trogen atom, a slight steric effect was observed which
favored the formation of CT.
General procedure for the hydroxylation of phenol
A 12 place RADLEYS Heated Carousel Reaction Station
fitted with a reflux unit as well as a gas distribution system
was used to perform the hydroxylation reactions. In a typ-
ical reaction, phenol (1 mmol) and the appropriate catalyst
(0.1 mmol) were placed in a 50 mL glass reaction vessel
followed by the appropriately buffered solution which was
saturated with oxygen (10 mL). The temperature of the reac-
◦
tion mixture was brought to 110 C under an oxygen atmo-
sphere and the mixture stirred at this temperature for 15 min.
A 6 % H2O2 (w/w) solution (1 mmol) was added and the
◦
reaction mixture was stirred at 110 C under an oxygen at-
mosphere for a further 6 h. The reaction mixture was cooled
to r. t. and a 1 mL sample withdrawn, filtered through a sy-
ringe filter and diluted 20 times. The consumption of phenol
and the oxidation products obtained were analyzed with a HP
1090 Liquid Chromatograph unit equipped with a ZORBAX
SB-C18 column of dimension 4.6 × 150 mm. The mobile
phase used was a mixture of 0.1 % formic acid solution and
acetonitrile.
Experimental Section
All chemicals were of reagent grade and used as received.
All solvents were dried over the appropriate drying agent
and distilled prior to use. Reactions and other manipulations
were carried out using a dual vacuum/nitrogen line and stan-
dard Schlenk techniques unless stated otherwise. 1H NMR
X-Ray crystallography
Suitable crystals for X-ray analysis were obtained by
(200 MHz) and 13C NMR (50 MHz) spectra were recorded slow diffusion of ethanol into dichloromethane solutions
on a Varian XR200 spectrometer, using tetramethylsilane as of the complexes 2, 5 and 8. Crystals of the complexes
an internal standard. Infrared spectra were recorded on a were mounted on glass capillaries and transferred to a
Perkin Elmer Paragon 1000PC FT-IR spectrophotometer as Rigaku R-AXIS IIc diffractometer. Diffracted intensities
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338
J. L. van Wyk et al. · Synthesis of Copper(II) Salicylaldiminato Complexes
were measured at ambient temperature, using graphite-
˚
Complex 5: A dark-green crystal fragment with approx-
monochromated MoKα radiation (λ = 0.71073 A) from a imate dimensions 0.25 × 0.25 × 0.40 mm. Refinement of
RU-H3R rotating anode operated at 50 kV and 90 mA. 151 parameters based on all 2235 reflections yielded R1
=
Ninety oscillation photographs with a rotation angle of 2◦ 0.032 and wR2 = 0.090 for I ≥ 2σ(I) (2129 reflections) and
were collected and processed using the CrystalClear soft- R1 = 0.032 and wR2 = 0.091 for all reflections; maximum
−3
˚
ware package. Empirical corrections were applied for the ef- and minimum residual electron density: 0.27; −0.35 e A
.
fects of absorption using the REQAB program under Crys- Complex 8: A needle-shaped greyish-green crystal with
talClear. The structures were solved by Direct Methods [19] approximate dimensions 0.15 × 0.15 × 0.40 mm. The hy-
and refined using full-matrix least-squares calculations on F2 droxyl hydrogen atom on O(5) was located from a difference
(SHELXL-97) [20] on all reflections, both programs oper- map and refined with a fixed isotropic thermal parameter =
3
˚
ating under the WinGX program package [21]. Anisotropic 0.05 A . Refinement of 589 parameters based on all 9953 re-
thermal displacement parameters were refined for all non- flections yielded R1 = 0.064 and wR2 = 0.147 for I ≥ 2σ(I)
hydrogen atoms; the hydrogen atoms were included as a (7185 reflections) and R1 = 0.106 and wR2 = 0.167 for all re-
riding contribution. Structural illustrations have been drawn flections; maximum and minimum residual electron density:
−3
˚
with ORTEP-3 for Windows [22] under WinGX [21] (Figs. 1, 1.33; −0.40 e A
.
2 and 3).
Supplementary crystallographic data for this paper have
been deposited at The Cambridge Crystallographic Data
Centre [23].
Acknowledgements
We would like to acknowledge the National Research
Foundation and the WRC, South Africa for financial sup-
port. This work was also supported by the Swedish Re-
search Council (Natural and Engineering Sciences, VR-NT),
by Sida/NRF (Sweden – South Africa Research Partnership
Programme, Project SRP-2001-040) and by The Royal Soci-
ety of Arts and Sciences in Gothenburg (KVVS).
Complex 2: A needle-shaped brownish-green crystal with
approximate dimensions 0.15 × 0.15 × 0.3 mm. Refinement
of 206 parameters based on all 3230 reflections yielded R1 =
0.055 and wR2 = 0.114 for I ≥ 2σ(I) (2307 reflections) and
R1 = 0.069 and wR2 = 0.119 for all reflections; maximum
−3
˚
and minimum residual electron density: 0.45; −0.86 e A
.
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