A new vanadium (III) complex of 2,6-bis(3,5-diphenylpyrazol-1-ylmethyl) pyridine as a catalyst for ethylene polymerization

Article abstract of DOI:10.3390/molecules18044728

The tridentate ligand 2,6-bis(3,5-diphenylpyrazol-1-ylmethyl)pyridine, abbreviated as 2,6-[(3,5-ph₂pz-CH₂)₂-py], a new pyridine-pyrazole derivative, was prepared from 2,6-bis(bromomethyl)pyridine and 3,5-diphenylpyrazole. The ligand was characterized by means of elemental analyses, ATR-IR, ¹H- and ¹³C-NMR spectroscopy and single crystal X-ray crystallography. Using this ligand, a new mononuclear vanadium (III) complex, {2,6-[(3,5-ph₂pz)CH₂]₂py} VCl₃ was prepared and characterized by elemental analysis, ATR-IR and HR-MS. The complex was investigated as a catalyst for ethylene polymerization and compared with 2,6-[(3,5-Me₂pz)CH₂]₂py} VCl₃ to explore the effect of the substituent on the pyrazolyl rings on ethylene polymerization. High catalytic activity was observed at ambient temperature. After treatment with AlEtCl2, these complexes showed high activity for ethylene polymerization converting ethylene to highly linear polyethylene and affording high molecular weight polymers (up to 1.0 × 10⁶ g/mol) with unimodal molecular weight distributions.

Full text of DOI:10.3390/molecules18044728

Molecules 2013, 18, 4728-4738; doi:10.3390/molecules18044728
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
A New Vanadium (III) Complex of 2,6-Bis(3,5-diphenylpyrazol-
1-ylmethyl)pyridine as a Catalyst for Ethylene Polymerization
Hanna S. Abbo * and Salam J. J. Titinchi
Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535,
Cape Town, South Africa; E-Mail: stitinchi@uwc.ac.za
* Author to whom correspondence should be addressed; E-Mail: habbo@uwc.ac.za;
Tel.: +27-21-959-3078; Fax: +27-21-959-3055.
Received: 11 March 2013; in revised form: 17 April 2013 / Accepted: 18 April 2013 /
Published: 22 April 2013
Abstract: The tridentate ligand 2,6-bis(3,5-diphenylpyrazol-1-ylmethyl)pyridine,
abbreviated as 2,6-[(3,5-ph2pz-CH2)2-py], a new pyridine-pyrazole derivative, was
prepared from 2,6-bis(bromomethyl)pyridine and 3,5-diphenylpyrazole. The ligand was
1
13
characterized by means of elemental analyses, ATR-IR, H- and C-NMR spectroscopy
and single crystal X-ray crystallography. Using this ligand, a new mononuclear vanadium
(III) complex, {2,6-[(3,5-ph2pz)CH2]2py}VCl3 was prepared and characterized by
elemental analysis, ATR-IR and HR-MS. The complex was investigated as a catalyst for
ethylene polymerization and compared with 2,6-[(3,5-Me2pz)CH2]2py}VCl3 to explore the
effect of the substituent on the pyrazolyl rings on ethylene polymerization. High catalytic
activity was observed at ambient temperature. After treatment with AlEtCl2, these
complexes showed high activity for ethylene polymerization converting ethylene to highly
linear polyethylene and affording high molecular weight polymers (up to 1.0 × 106 g/mol)
with unimodal molecular weight distributions.
Keywords: pyrazole; 2,6-bis(pyrazol-1-ylmethyl)pyridine; vanadium complex; ethylene
polymerization; high density polyethylene
1. Introduction
Pyrazole-based ligands are widely used in syntheses of transition metals coordination compounds,
which has been reviewed [1]. They can act as terminal or bridging ligands and have tridentate

Molecules 2013, 18
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chelation at each metal centre [2,3]. Despite the numerous reports on the synthesis and properties of
late transition metal complexes containing pyrazolyl based ligands, very little is known about their
catalytic activity in olefin oligomerization and polymerization [4–9].
An interesting fact arises when a six-membered heterocycle such as pyridine and a five-membered
heterocycle such as pyrazole are combined in a single ligand unit. The substitution at the 2 and
6-positions of the pyridine ring gives tridentate ligands such as 2,6-bis(pyrazol-1-yl)pyridine and
2,6-bis(pyrazol-1-ylmethyl)pyridine. The coordination chemistry of pyrazolyl-pyridine-based chelating
ligands has been studied extensively [10–14]. The possibility of introducing groups in the 3,5-positions
of the pyrazole ring causes tuning of the electronic and steric properties, depending on the substituents
used, which modify the catalytic response of the metal center and define the polymerization reaction.
The structure of these tridentate ligands is closely related to the 2,6-diiminopyridine [15], which have
been ubiquitously used as ligands for post-metallocene olefin polymerization catalysts.
In recent years, there has been considerable interest in designing new catalysts of late transition
metal complexes consisting of tridentate nitrogen-based ligands as non-metallocene olefin
polymerization catalysts [15,16]. These catalysts enhance the both activity and microstructure of the
resultant polymers.
To expand our research work [6,9], which aims to synthesize new pyrazolyl-pyridine complexes
with high catalytic activity towards ethylene polymerization, herein we describe the synthesis of
the ligand 2,6-[(3,5-ph2pz-CH2)2-py] and its mononuclear vanadium (III) complex, {2,6-[(3,5-
ph2pz)CH2]2py}VCl3. We have also studied the influence of polymerization conditions, such as
temperature, pressure, reaction time and optimized Al/V molar ratios on the productivity of
polyethylenes and on the polyethylene properties. To our knowledge the vanadium complex reported
herein is amongst the first examples of a vanadium pyrazolyl-pyridine catalyst bearing phenyl groups
as a precursor for ethylene polymerization which shows long lifetime during the catalytic cycle.
2. Results and Discussion
2.1. Synthesis of 2,6-Bis(3,5-diphenylpyrazol-1-ylmethyl)pyridine and Its Vanadium Complex
2,6-Bis(3,5-diphenylpyrazol-1-ylmethyl)pyridine was synthesized by the reaction of two
equivalents of 3,5-diphenylpyrazole and one equivalent of 2,6-bis(bromomethyl)pyridine in benzene in
the presence of 40% aqueous NaOH and 40% aqueous tetrabutylammonium bromide (Scheme 1).
Analytically pure compounds were obtained in 78% yield after purification by chromatography on
silica gel. This compound is very stable in air and can be stored at room temperature for a long period
of time. It was characterized by NMR and IR spectroscopy, as well as by microanalysis and X-ray
crystallography. The proton and 13C-NMR and the IR data, along with elemental analysis for the ligand
correspond very well with its crystal structure. Crystallographic data for the structure reported
here has been deposited with CCDC (Deposition No. CCDC-688192; CCDC 688192 contains the
supplementary crystallographic data for this paper. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail:
deposit@ccdc.cam.ac.uk). The crystal data and structural refinement parameters of the title compound

Molecules 2013, 18
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are summarized in Table 1. The molecular structure and the numbering for the ligand atoms is shown
in Figure 1.
Scheme 1. Synthesis of 2,6-[(3,5-ph2pz-CH2)2-py] and its vanadium complex 1.
N
N
N
N
N
N
40% NaOH
V
VCl3
THF
N
N
N
N
+
2
N
HN
40% TBAB
Benzene
Cl
Cl
Cl
N
Br
Br
Table 1. Crystal data and structural refinement parameters of 2,6-[(3,5-ph2pz-CH2)2-py].
CCDC deposition
Empirical formula
Formula weight
Temperature
688192
C37H29N5
543.65
298(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Orthorhombic
Pbca
Unit cell dimensions
a = 16.5531(4) Å
b = 9.5085(2) Å
c = 37.3606(8) Å
5880.4(2) Å3
= 90°.
= 90°.
 = 90°.
Volume
Z
8
Density (calculated)
Absorption coefficient
F(000)
1.228 Mg/m3
0.074 mm−1
2288
Crystal size
0.32 × 0.14 × 0.12 mm3
1.09 to 28.00°.
−20<=h<=21, −12<=k<=11, −48<=l<=49
68614
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta = 28.00°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
7105 [R(int) = 0.1090]
100.0%
None
0.9912 and 0.9768
Full-matrix least-squares on F2
7105/0/380
0.670
R1 = 0.0400, wR2 = 0.0928
R1 = 0.1564, wR2 = 0.1467
0.0021(3)
0.229 and −0.180 e−3
Å

Molecules 2013, 18
4731
Figure 1. Molecular structure of 2,6-[(3,5-ph2pz-CH2)2-py].
1
The results of the H-NMR, IR and elemental analysis were in good agreement with the proposed
structure of the title compound. The ORTEP structure shows that the two pyrazolyl units are not
coplanar with the pyridine ring and form dihedral angles N(2)-C(16)-C(17)-N(3) of −33.3(2)° and
N(4)-C(22)-C(21)-N(3) of −41.6(2)°. It can be found that the carbon–nitrogen bond lengths on the
molecular skeleton are basically intermediate between typical C-N single (1.47 Å) and C=N double
bonds (1.27 Å). The two substituted pyrazolyl units and their phenyl groups are pointed outwards from
the pyridine ring. The phenyl rings attached to carbon C7 and C23 are not parallel but not quite
coplanar with the pyrazolyl ring, with dihedral angles C(1)-C(6)-C(7)-N(1) of 6.5(3)° and
C(25)-C(24)-C(23)-N(5) of −15.3(3)°, respectively. On the other hand, the phenyl rings attached to C9
[C(10)-C(15)] and C31 [C(32)-C(37)] are tilted by 131.3(2)° and −46.7(3)°, with dihedral angles of
N(2)-C(9)-C(10)-C(11) and N(4)-C(31)-C(32)-C(33), respectively, with respect to the pyrazolyl rings.
The vanadium complex, {2,6-[(3,5-ph2pz)CH2]2py}VCl3, was prepared in good yield (65%) by the
reaction of this tridentate ligand with VCl3 (Scheme 1). The complex was characterized by a
combination of ATR-IR spectroscopy, high resolution mass spectrometry and micro analysis.
A strong band at 1574 cm−1 in the spectrum of the ligand assigned to ν(C=N) was lowered by
20 cm−1 upon complexation due to the coordination of the azomethine nitrogen with the metal ion. The
complex exhibited band at 378 cm−1 due to ν(V-Cl) stretching mode [17].
The mass spectrum of the complex showed the molecular ion and the base peak at m/z 698.15
[M−H] and 544.25 [M-VCl3], respectively, confirming its formula. The mass fragmentation pattern
supported the suggested structure of the complex. The mass and microanalytical data of the complexes
correspond well with the proposed structure. The coordination between the N-pyridine moiety and the
metal ion were confirmed by crystal structures for related systems [8,15,18]. No suitable NMR data
were obtained due to the paramagnetism of the metal used. Repeated attempts to grow an X-ray crystal
of the complex were unsuccessful.
2.2. Polymerization of Ethylene
On the basis of the encouraging preliminary reported results using 2,6-[(3,5-Me2pz-CH2)2-py]VCl3,
(2), as an active catalyst for ethylene polymerization [6], it was selected for further optimization,
investigating the influence of polymerization conditions on the productivity of the catalyst system

Molecules 2013, 18
4732
employed and the nature of the polymer produced. These results promote us to prepare
2,6-[(3,5-ph2pz-CH2)2-py]VCl3 (1) as a new catalyst for ethylene polymerization and to explore the
effect of the substituent on the pyrazolyl rings on the ethylene polymerization outcome.
Ethylene polymerization studies were conducted in toluene using a 300 mL stainless steel Parr
reactor. To study the influence of the catalytic polymerization conditions on the productivity of the
catalysts employed, we carried out a series of experiments under different reaction times, temperature,
pressure, as well as the ratio of Al/V. Table 2 shows the optimization results for the ethylene
polymerization using catalysts 1 and 2 and its influence on the polyethylene properties produced.
Table 2. Polymerization of ethylene using precatalysts 1 and 2.
Cat
(mol)
1(2.5)
1(2.5)
1(1.5)
1(1.5)
1(5)
Temp Time P C2H4 Yield
Activity
Mw
Run No.
Al/V ratio
PD
(°C) (min) (bar)
PE/g (Kg mol−1·h−1) ×10−6
1
2
1500
1500
1500
1500
1500
1000
2000
1500
500
25
50
25
25
25
25
25
25
25
25
25
25
50
25
25
25
25
25
25
25
30
30
15
30
60
30
15
15
10
60
10
30
30
60
30
60
60
120
10
60
10
10
10
10
10
10
10
1
1.25
0.38
0.59
0.99
1.27
0.47
0.21
0.17
2.0
1000
304
0.89 2.7
0.73 4.3
3
1570
1320
270
1.0
3.1
4
0.92 2.8
5
--
--
--
--
--
--
6
1(1.5)
1(1.5)
1(2.5)
2(5)
626
7
560
8
272
0.74 4.3
9
10
10
10
10
10
10
10
10
10
10
1
2400
860
0.6
---
0.8
--
2.9
---
2.2
---
--
10
11
12
13
14
15
16
17
18
19
20
2(5)
500
4.3
2(5)
250
2.5
3010
1240
292
2(5)
250
3.1
2(5)
250
0.73
4.7
--
2(5)
250
940
0.68 2.5
-- ---
2(2.5)
2(2.5)
2(5)
500
1.7
1360
1400
840
500
3.5
0.58 2.4
0.59 2.3
1000
1000
250
4.2
2(5)
5.6
560
--
0.75 3.2
0.6 2.5
--
2(5)
0.35
2.1
422
2(5)
250
1
410
Upon activation with a small amount of AlEtCl2, both complexes show good catalytic activity for
ethylene polymerization under 1 and 10 bar of ethylene. The polyethylenes produced with these
systems have a high molecular weight (Mw) between 0.6 − 1.0 × 106 g/mol and relatively narrow
molecular weight distributions (Mw/Mn = 2.2 − 4.3) in all cases, depending on reaction conditions. This
might suggest that the polymerization proceeds via a single-site.
Both catalysts are most active at 25 °C, with maximum productivity of 1,570 Kg·mol−1·h−1 for
catalyst 1 and 3,010 Kg·mol−1·h−1 for catalyst 2. At 50 °C the overall productivity drops substantially
for both catalysts after 30 min reaction time (entries 2 and 13) due to partial decomposition of the
catalytically active species at higher temperatures. This observation promises advantages for industrial
use of these catalysts, as commercial ethylene polymerization is mainly done at high temperature and

Molecules 2013, 18
4733
pressure. The polymer molecular weight tends to decrease with the increasing temperature of
polymerization, which could be attributed to the much faster β-hydride elimination over the propagation.
To investigate the effect of co-catalyst to catalyst ratio, the Al/V ratio was varied from 1,000:1 to
2,000:1 for catalyst 1 and the optimum ratio was found to be 1,500:1 (entry 3), while for catalyst 2, the
optimum ratio of Al/V ratio is 250:1 (entry 11). The possible reason is that an optimal amount of
AlEtCl2 is needed to effectively activate the catalyst, while an excess amount will create too many
alkylaluminum impurities and may cause the degradation of the catalyst system.
High catalyst concentration results in a decrease of the catalytic activity for both catalysts, which
could be interpreted as a mass-transport effect, moreover the reaction mixture becomes denser at
higher concentration, which inhibits ethylene from reaching the active center.
The observed activity decreased at lower ethylene pressure 1 bar and increasing the pressure to (10 bar)
increased the activity under same conditions for both catalysts (entries 8, 19 and 20), probably as an
effect of increased ethylene concentration in solution.
The reaction time has a great effect on productivity for vanadium-based polymerization catalysts as
they are generally considered to deactivate through reduction of the metal centre to V (II) during the
catalytic cycle [19–23]. A series of experiments were carried out at 25 °C for range of time 10–60 min,
thus, increasing the reaction time from 15 min to 1 h, the activity for 1 was observed to decrease
dramatically (entries 3 and 5). The activity for 2 on the other hand, decreased from 3,010 to
940 Kg·mol−1·h−1 when the reaction time increased from 10 min to 1 h (entries 11 and 14). This may be
attributed to the embedding of catalyst sites in the solid polyethylene produced during the first
minutes. In comparison to the earlier reports for known vanadium systems which usually deactivated
after short time under polymerization conditions, our catalysts are more stable and still have moderate
catalytic activity after 30 min. [19–23]. It appears that the ligand system that we are employing is
capable of stabilizing the activated metal centre.
The polymerization activity of a catalyst can be influenced by the steric and electronic
characteristics of the ligand. Catalyst 2 bearing electron-donating methyl groups on the pyrazolyl ring
exhibited higher activity than the corresponding catalyst 1 bearing electron-withdrawing phenyl
groups. A possible reason for decreasing in activity with increased steric bulk could be reduced
accessibility of the monomer to the vacant metal center. On the other hand, it is commonly believed
that electron-donating groups increased the stability of the active species while electron-withdrawing
groups reduce the stability of the active species.
Gel permeation chromatography (GPC) analyses (Figure 2) of the polyethylenes obtained indicate
that they possess high molecular weights with narrow molecular weight distributions in all cases as
expected for single-site catalysts. The high–temperature NMR spectra verified that all polyethylenes
produced by both catalysts are of high order linearity, which based upon the observation of only signal
from the methylene group (CH2) in 13C-NMR spectra (Figure 1S). Differential scanning calorimetry
(DSC) analyses demonstrated that the polyethylenes are essentially linear and show melting points of
ca. 135 °C (Figure 3).

Molecules 2013, 18
Figure 2. GPC diagram of polyethylene provided by runs 4 and 14.
4734
1e8
1e7
Low
Limit
High
Limit
1e6
1e5
1e4
1e3
1e2
1e1
Run 4
Run 14
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
Retention Time
Figure 3. DSC profile of polyethylene sample produced by catalyst 1 (entry 1).
6
117.50°C
4
119.75°C
202.1J/g
2
0
128.03°C
285.7J/g
-2
-4
-6
135.04°C
-50
0
50
100
Temperature (°C)
150
200
250

Molecules 2013, 18
4735
3. Experimental
3.1. General
All reagents were of reagent grade and were used without further purification. The NMR spectra
13
were recorded on a Varian Gemini 2000 instrument (1H at 200 MHz and C at 50.1 MHz) at room
temperature for the ligand in CDCl3, while those of polyethylene were recorded in 1,2,4-
trichlorobenzene/benzene-d6 at 120 °C. Elemental analysis was performed by the micro analytical
laboratory at the University of Cape Town, South Africa. FT-IR spectra were carried out on a
Perkin-Elmer Spectrum 100 FTIR spectrometer. High resolution mass spectra were determined on a
Waters Synapt G2 mass spectrometer (ASAP probe and ESI) at the Central Analytical Facility, University
of Stellenbosch (South Africa). Polymer molecular weights were performed by high temperature GPC
(1,2,4-trichlorobenzene, 160 °C, rate = 1.0 mL/min) on a Polymer Laboratories GPC220 instrument
using polystyrene standards at the Institute of Polymer Science at the University of Stellenbosch
(South Africa). Crystallographic data was collected on a Bruker APEX II CCD area detector
diffractometer with graphite monochromated Mo K radiation (50kV, 30mA) using the APEX 2 [24]
data collection software. The collection method involved -scans of width 0.5 and 512 × 512 bit data
frames. Data reduction was carried out using the program SAINT+ and absorption corrections were
made using XPREP [25]. The crystal structure was solved by direct methods using SHELXTL [26].
Non-hydrogen atoms were first refined isotropically followed by anisotropic refinement by full matrix
least-squares calculations based on F2 using SHELXTL. Hydrogen atoms were first located in the
difference map then positioned geometrically and allowed to ride on their respective parent atoms.
Diagrams and publication material were generated using SHELXTL, PLATON [27] and ORTEP-3 [28].
3.2. Synthesis of 3,5-Diphenylpyrazole
This compound was synthesized following a procedure described in the literature [29] with slight
modifications. To a well stirred solution of dibenzoylmethane (5 g, 22.32 mmol) in (50 mL) ethanol,
hydrazine monohydrate (1.74 mL, 35.57 mmol) was added dropwise. The reaction mixture was heated
at 50 °C for ½ h, and then refluxed overnight and white crystals were obtained, filtered, washed with a
small volume of ethanol and dried in air.
3.3. Synthesis of 2,6-[(3,5-ph2pz-CH2)2-py]
To a well stirred solution mixture of 2,6-bis(bromomethyl)pyridine (0.5 g, 1.89 mmol) and
3,5-diphenylpyrazole (0.83 g, 3.77 mmol) in benzene (40 mL) was added dropwise 40% aqueous
NaOH (12 mL) and 40% aqueous tetrabutylammonium bromide (12 drops) (Scheme 1). The reaction
mixture was heated under reflux for 18 h. The organic layer was separated, dried over MgSO4, filtered
and concentrated by distillation under reduced pressure. Pure product (0.8 g, 78%) was obtained by
column chromatography over silica gel (CH2Cl2/ether, 8:1) as a white solid. Anal. Calc. for C37H29N5
(543): C, 81.76; H, 5.34; N, 12.89. Found: C, 81.06; H, 5.25; N, 12.23%. 1H-NMR (CDCl3): 5.47 (4H,
s, CH2); 6.69 (2H, s, 4-pz); 6.88 (2H, d, 3,5-py); 7.34–7.45 (16H, m, 2,3,5,6-ph); 7.58 (1H, t, 4-py);
13
7.86 (4H, dd, 4-ph); C-NMR (CDCl3): δ 54.33; 103.75; 120.46; 125.82; 127.98; 128.64; 128.78;

Molecules 2013, 18
4736
130.01; 146.32; 151.35; 156.76. IR: (KBr cm−1): ν(C=N) 1574 cm−1. High-quality off-white single
crystals suitable for X-ray diffraction were obtained by recrystallization from ethanol.
3.4. Synthesis of {2,6-[(3,5-ph2pz)CH2]2py}VCl3 (1)
A suspension of VCl3 (0.062 g, 0.34 mmol) in anhydrous THF (10 mL) was added dropwise to a
stirred solution of 2,6-[(3,5-ph2pz-CH2)2-py] (0.213 g, 0.34 mmol) (Scheme 1). Upon stirring and
heating to 65 °C the colour of the turbid reaction mixture changed to greenish gray after 2 h. After 24 h
and having cooling down the reaction medium to room temperature, it was concentrated to ca. 2 mL by
rotary evaporation. Addition of hexane at this stage precipitated a dark greenish gray powder from the
solution. It was filtered, washed with hexane and ether, and dried under vacuum. Yield: 0.11 g (46%),
M.p. > 300 °C. Anal. (C37H29N5Cl3V) Calcd: C, 63.40; H, 4.14; N, 9.99. Found: C, 62.50; H, 4.55; N,
9.93%. IR (KBr cm−1): (C=N) 1554 cm−1. HRMS-TOF (ESI+) (ASAP): m/z 698.15 [M-H]; 628.16 [M-
Cl2]; 544.25 [M-VCl3] (base peak); 219.09 [3,5-diphenylprazolyl, C15H11N2] (Figure 2S).
4. Conclusions
The vanadium complex reported herein is amongst the first example of a vanadium pyrazolyl-
pyridine catalyst bearing phenyl groups useful as a catalyst precursor for ethylene polymerization.
Moreover, further studies on this catalytic system have been explored, including the effect of reaction
parameters and the effect of substituents on the pyrazolyl ring on the catalyst activity performance.
In the presence of AlEtCl2, pyrazolyl-pyridine vanadium complexes were highly active catalysts for
ethylene polymerization at 25 °C, producing high molecular weight polyethylenes with narrow
molecular weight distributions under all types of polymerization conditions. These results indicate that
the pyrazolyl-pyridine ligand system is capable of stabilizing the active species and maintaining
single-site catalytic behaviors of the vanadium catalysts for ethylene polymerization. In addition, these
catalysts show extended catalyst life times compared with known vanadium systems which are usually
deactivated after a short time under the typical polymerization conditions.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/18/4/4728/s1.
Acknowledgments
The authors are grateful to the National Research Foundation (NRF), South Africa, Eskom and the
research committee of the University of the Western Cape for financial support.
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Molecules 2013, 18
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