New iron bis(imino)pyridyl complexes containing dendritic wedges for alkene oligomerisation

Article abstract of DOI:10.1039/b411884g

The synthesis, characterisation and catalytic behaviour of new iron bis(imino)pyridyl complexes containing dendritic wedges, as well as the synthesis of bis(para-hydroxyphenylimino)pyridines is described. The hydroxyl functionality of the bis(para-hydroxy

Full text of DOI:10.1039/b411884g

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F U L L P A P E R
New iron bis(imino)pyridyl complexes containing dendritic wedges
for alkene oligomerisation†
Matthew J. Overett,‡ Reinout Meijboom§ and John R. Moss*
Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa.
E-mail: jrm@science.uct.ac.za; Fax: +27 (0)21 689 7499; Tel: +27 (0)21 689 2535
Received 2nd August 2004, Accepted 19th November 2004
First published as an Advance Article on the web 21st December 2004
The synthesis, characterisation and catalytic behaviour of new iron bis(imino)pyridyl complexes containing dendritic
wedges, as well as the synthesis of bis(para-hydroxyphenylimino)pyridines is described. The hydroxyl functionality of
the bis(para-hydroxyphenylimino)pyridines was used to attach dendritic wedges of the carbosilane type as well as the
benzylphenyl ether type. After attachment of the dendritic wedges, complexation of these ligands to iron(II) chloride
was achieved. The resulting dendritically functionalised bis(imino)pyridyl iron complexes were tested in the catalytic
oligomerisation of ethene.
Introduction
The oligomerisation of ethene is one of the primary industrial
processes for the production of linear 1-alkenes.1 Oligomers in
the range C6–C20 are used as co-monomers in the polymerisation
of ethene to give linear low-density polyethene (LLDPE), or for
the preparation of detergents and synthetic lubricants. Catalysts
currently used in industry for the Shell Higher Olefin Process
(SHOP)2 contain Ni(II) complexes bearing bidentate mono-
Fig. 1 Iron bis(imino)pyridyl precatalyst for the oligomerisation of
1-alkenes.
anionic ligands.1,2 Cationic Ni(II) a-diimine complexes were also
reported to be effective ethene oligomerisation catalysts,3 while
applications in catalysis and as new materials, led us to consider
the possibility of functionalising late transition metal oligomeri-
sation catalysts with dendritic components. This could take the
form of attaching dendritic wedges to the catalysts, thus creating
a catalyst at the core of a dendrimer. A catalytic site at the core of
a dendrimer makes it possible to control the microenvironment
around the catalytic centre and thus allows modifications of the
catalytic selectivity.12 Incorporation of a catalytically active site
in a dendritic macromolecule has the added potential advantage
of enabling one to separate the catalyst from the product stream
by means of ultra-filtration methods.
Functionalising a bis(imino)pyridyl ligand with dendritic
wedges to create a catalytic ligand at the core of a dendrimer is
likely to be synthetically achievable. The aryl components of the
ligand are particularly amenable to dendritic functionalisation,
as a wide range of anilines with different functionalities for
substitution are commercially available. These may be reacted in
standard Schiff-base type reactions with 2,6-diacetylpyridine to
form the bis(imino)pyridyl framework with aryl rings suitable
for attachment of dendritic wedges. Dendritic functionalisation
at the para-position is most favourable. The direct steric control
around the catalytic centre may be controlled by appropriate
ortho-substituents (R1, Fig. 1), thereby ensuring an oligomeri-
sation catalyst. The heteroatomic functionality, for attachment
of the dendritic wedges, is introduced on the para-position (R2,
Fig. 1) and thus removed from the active catalytic centre in order
to minimise unfavourable interactions.
iron-based bis(imino)pyridyl complexes were described as highly
active compounds in the catalytic oligomerisation of ethene in
combination with the co-catalyst MAO or MMAO.4,5
Several modifications of the bis(imino)pyridine backbone,
which have already been described in the literature,6 mostly lead
to a decrease in catalytic activity. Recently, fluoro-substituted
bis(imino)pyridine complexes and their catalytic reactivity in
ethene oligomerisations were described,7 as were bis(imino)-
pyridine complexes containing methoxy and CF3 groups.8
Since the independent reports by Gibson4,6 and Brookhart9,10
on iron and cobalt bis(imino)pyridyl catalysts and nickel
and palladium a-diimine catalysts for the polymerisation and
oligomerisation of 1-alkenes, much attention has been focused
on these late transition metal catalysts as alternatives to estab-
lished technologies.
It is known that the position and steric bulk of the substituents
on the aryl rings of the bis(imino)pyridyl catalysts (Fig. 1) play
a crucial role in determining the selectivity of the catalyst. The
ortho substituents (R1) have been identified as being particularly
important. If both R1 groups on each ring are non-hydrogen
substituents, and preferably bulky alkyl substituents such as iso-
propyl groups, then the catalysts are selective for the production
of high molecular weight polymers from ethene. If only one
of the ortho substituents R1 on each ring is an alkyl group,
and the other a hydrogen, then the catalyst is selective for the
oligomerisation of ethene to linear 1-alkenes with a Schultz–
Flory chain length distribution, as well as for the dimerisation
of longer chain 1-alkenes (such as 1-hexene).
The effect of substituents in the para-position (R2) has
Results and discussion
been less studied. Our interest in dendritic molecules,11 their
Preparation of ligands
Appropriate ligands would be bis(imino)pyridyl ligands with
either both ortho-substituents (R1) being hydrogen, or one of
the substituents R1 on each ring being a methyl group and the
other a hydrogen. Substituent R2 was chosen to be a hydroxyl
group for ease of attachment of alkylbromide functionalised
dendritic wedges, using the Williamson ether synthesis. Thus
† Electronic supplementary information (ESI) available: Synthetic de-
tails. See http://www.rsc.org/suppdata/dt/b4/b411884g/
‡ Current address: Sasol Technology R&D, P.O. Box 1, Sasolburg 1947,
South Africa.
§ Current address: Department of Chemistry and Biochemistry, Rand
Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa.
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 5 5 1 – 5 5 5
5 5 1
©

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Scheme 1 Synthesis of para-hydroxy functionalised bis(imino)pyridyl ligands 1a and 1b.
Scheme 2 Synthesis of 2–6.
compounds 1a and 1b were synthesized using simple Schiff-
base condensation reactions of 2,6-diacetylpyridine and two
molar equivalents of either 4-aminophenol or 4-amino-m-cresol
(Scheme 1).
FeCl2·4H2O in THF (Scheme 3), as reported in the literature.17
The literature precatalysts 1210 (see Fig. 1; R1 = Me; R2 = H)
and eight new iron complexes with dendritic substituents ( 7a–
11) were prepared. After completion of the reaction (judged by
the absence of solid particles of FeCl2), the intensely coloured
complexes were precipitated out of solution by the addition of
diethyl ether. All new complexes were obtained as dark green
powders (except 8a and 10a) in near quantitative yields. The
compounds 8a and 10a may have a different structure as they are
bright purple and exhibit no catalytic activity. Attempts to grow
crystals suitable for X-ray crystallography were unsuccessful for
all of the new compounds.
Full characterisation of the complexes proved to be difficult.
High spin d6 iron complexes are paramagnetic, and only in some
instances have 1H-NMR data been reported.18 In our experience,
obtaining interpretable spectra proved impossible. Many of the
complexes are not soluble in readily available NMR solvents
(CDCl3, benzene-d6), and the line broadening and unpredictable
shifts of the peaks rendered assignment speculative at best. IR
spectroscopy is of little value as a diagnostic technique for these
compounds. Useful characterisation techniques were therefore
limited to elemental analysis and mass spectrometry. The larger
dendritic complexes are amorphous solids or oils, and may
retain some solvent even after long periods of evacuation. The
carbosilane substituted complexes appeared to be moderately
air-sensitive, and therefore were stored under inert conditions
before use in catalytic reactions.
The para-hydroxyl group allows attachments of various den-
dritic wedges containing an alkylbromide functional group at the
focal point. Using the Williamson ether synthesis, a quantitative
yield of dendritically functionalised bis(imino)pyridyl ligands
is expected, as in the synthesis of poly(benzylphenylether)
dendrimers.13 Two equivalents of the appropriate bromoalkyl
wedge (see Scheme 2) were reacted with either 1a or 1b in
acetone heated under reflux, using K2CO3 as the base and
a sub-stoichiometric amount of 18-crown-6 (Scheme 2). The
reactions typically were allowed to proceed for between 48 and
72 h. Compounds 2 to 6 were then extracted by means of
an aqueous work-up. Analytically pure samples were obtained
by recrystallisation from an appropriate solvent. Fre´chet’s
poly(benzylphenylether) dendritic wedges13 were chosen because
they are easily prepared and react readily with hydroxyaryl com-
pounds. They have previously been used as dendritic catalysts
for anionic ring opening polymerisation reactions.14 Carbosilane
dendritic wedges with a bromoalkyl focal point have previously
been reported,15 and similar compounds have been prepared
for this work.16 The carbosilane wedges have the advantage of
being inert and non-polar, so the influence of the ether linkages
in the poly(benzylphenylether) could be evaluated. Details on
the wedges employed are given in Scheme 2.
Reasonable elemental analyses were generally obtained for the
poly(benzylphenylether) substituted complexes. The FAB mass
spectra were more instructive, and in all cases the parent ion and
fragments involving the loss of chlorine were observed. For some
of the larger complexes, the observed molecular ion was fairly
Preparation of the iron complexes
The iron precatalyst complexes, 7 to 11, were prepared by stirring
a slight excess of the bis(imino)pyridyl ligands 2 to 6 with
5 5 2
D a l t o n T r a n s . , 2 0 0 5 , 5 5 1 – 5 5 5

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Scheme 3 Synthesis of the iron complexes 7–11.
low in intensity, but the isotopic patterns corresponded well with
the theoretical patterns for the calculated empirical formulae.
substituted analogues 8b and 10b. The dendritically substituted
catalysts 8b, 9 and 11 displayed higher activity than their non-
dendritic analogues 7b and 10b, and the highest activities were
obtained with the G1 poly(benzylphenylether) and carbosilane
substituents (entries 6 and 10). This increased activity might
be ascribed to either increased solubility of the catalysts or
to a beneficial interaction between the dendritic wedges and
the catalytic centre, in the case of the poly(benzylphenylether)
functionalised catalysts. To test this second hypothesis, runs
were carried out with 12 (entries 11 and 12) and 8b (entry
13), in which 2 equivalents of G1 poly(benzylphenylether)
wedge were added as a catalyst promoter. The activity of the
runs with 12 and 2 equivalents of G1 was significantly higher
than 12 alone, whereas the a-value remained unchanged. It
appears that the poly(benzylphenylether) wedges might act as
promoters in this type of catalysis, probably by stabilising
the active catalyst against decomposition. The addition of
two equivalents of G1 caused no additional enhancement of
catalytic activity with 8b, which is already functionalised with
a poly(benzylphenylether) wedge. It has previously been noted
that aryl ether compounds can enhance catalytic activity and
selectivity in ethene oligomerisation systems.19
Catalytic runs
Complexes 7 to 12 were evaluated as catalysts for the oligomeri-
sation of ethene to higher 1-alkenes. The complexes were
activated using methylaluminium oxane (MAO; [Al]/[Fe] = 400)
in toluene, and the active catalysts were subsequently exposed
to 1 bar ethene at 30 ◦C. Analysis by gas chromotography
was used to determine the Shultz–Flory constant a. The total
yield and turnover number (T.O.N.) was calculated by removing
the volatiles in vacuo and measuring the residual mass. The
lost volatile oligomers were determined approximately by GC
analysis of the involatile oligomers and extrapolation using
the a-value, as described previously.5 The catalytic data are
presented in Table 1.
In order to establish reproducibility, and draw useful com-
parisons between the new catalysts and literature catalysts, the
first two runs (entry 1 and 2) were performed using 2,6-bis-[1-
(2-methylphenylimino)ethyl]pyridine iron dichloride (12).5 All
para-substituted complexes 7a–11 were active oligomerisation
catalysts, with the exception of 8a and 10a. The effect of the
para-ether linkage itself may be deduced from the catalytic
behaviour of 7b (entry 4). Under the conditions used in this
study, both the activity and a-values were higher than that for
unsubstituted analogue 12 (entries 1 and 2). The increase in
a-value, relative to that for 12, may be observed in all runs
using the dendritically substituted precatalysts. This appears
to be due to the para-ether functionality alone, as no effect
of dendrimer size on the a-value is apparent. The presence
of an ortho-methyl group made little difference to activity or
selectivity in the case of 7a and 7b. Surprisingly, complexes 8a
and 10a showed no catalytic activity, unlike their ortho-methyl
Conclusions
A range of bis(imino)pyridyl iron complexes containing large
dendritic wedges was successfully synthesised and characterised.
The attached wedges are of both the poly(benzylphenylether)
and the carbosilane type. These complexes constitute a new
and exciting family of oligomerisation catalysts. It was shown
that these bis(imino)pyridyl iron compounds are active in the
oligomerisation of ethene, using MAO as a co-catalyst. The ac-
tivity of these new catalysts is not related to the type of dendritic
Table 1 Results of the catalytic runsa
Entry
Precatalyst
Time/min
Yield/g
1-Alkenes (%)
a
T.O.N. (×103)
1
2
3
4
5
6
7
8
9
10
11
12
13
12
60
60
60
60
60
60
60
60
60
60
60
60
60
2.98
3.48
4.47
4.56
—
5.85
4.83
—
3.82
6.29
6.62
6.50
5.88
85
85
89
82
—
80
83
—
91
90
88
86
82
0.71
0.68
0.75
0.77
—
0.77
0.76
—
0.72
0.75
0.68
0.71
0.75
5.32
6.21
7.98
12
7a
7b
8.13
8a
Inactive
10.45
8.62
Inactive
6.82
11.23
11.82
11.61
10.50
8b
9
10a
10b
11
12b
12b
8bb
a Ethene pressure: 1 bar; catalyst loading: 20 lmol; [Al]/[Fe] = 400; initial temperature: 30 ◦C; solvent: toluene; total volume: 30 cm3; time: 60 min;
b Two equivalents of G1 wedge added.
D a l t o n T r a n s . , 2 0 0 5 , 5 5 1 – 5 5 5
5 5 3

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wedge, however the activity of the complexes shows small varia-
tions related to the size of the dendritic wedge. This opens up the
possibility of using even larger dendritic wedges as substituents,
without interfering with the catalytic activity. Larger dendritic
substituents would make it feasible to separate the catalyst from
the product stream by means of ultra-filtration methods.
in anhydrous methanol (15 cm3) was heated to 60 ◦C for 4 days.
After cooling, the dark yellow solution was concentrated in
vacuo until a yellow precipitate formed. Further precipitation
was allowed at −20 ◦C for 15 h. The precipitate was recovered by
filtration and washed with cold methanol. After recrystallisation
from hot methanol, 1b was obtained as a yellow, crystalline
◦
solid. (0.690 g, 1.85 mmol, 81%); mp 212–214 C; (Found: C,
74.0; H, 6.2; N, 11.3. C23H23N3O2 requires: C, 74.0; H, 6.2;
Experimental
N, 11.2%); mmax/cm−1 (THF) 3314 (OH), 1635 (C N), 1569,
=
3
1494; dH (400 MHz; acetone-d6) 8.40 (2H, d, J(HH) 8.0 Hz,
General remarks
m-Hpyr), 7.99 (1H, t, 3J(HH), p-Hpyr), 7.98 (2H, s, aryl-OH),
All manipulations were carried out under purified nitrogen,
using glovebox (MBraun Unilab) or standard Schlenk line
techniques under pre-purified argon.20 Diethyl ether, pentane,
tetrahydrofuran (THF) and toluene were dried by passage
through a column containing alumina (neutral, Brockmann
grade I) and distilled from sodium/benzophenone ketyl prior
to use.21 Dichloromethane (DCM) was distilled from CaH2;
methanol and ethanol were distilled from Mg activated by I2; and
acetone was distilled from Drierite prior to use.21 All reagents
were stored under pre-purified argon. Poly(benzylphenylether)
wedges G0, G1, and G2 were prepared according to the
method of Hawker and Fre´chet.13 Carbosilane wedges csG0
and csG1 were prepared using a method similar to that of Van
Heerbeek et al.15 and were reported previously.16 Compound
1217 was prepared according to literature methods. All other
reagents were purchased from Sigma Aldrich. NMR spectra
were recorded on either a Varian Unity-400 (1H, 400 MHz;
13C, 100.6 MHz; 29Si, 79.5 MHz) spectrometer or a Varian
Mercury-300 (1H, 300 MHz; 13C, 75.5 MHz) spectrometer
at ambient temperature. Chemical shifts were referenced to
tetramethylsilane (TMS), using the residual protio impurities
in the solvent (1H NMR), the solvent resonances (13C NMR) or
external TMS (29Si NMR). Infrared spectra were recorded on
a Perkin-Elmer Paragon 1000 FT-IR spectrometer in the range
450–4400 cm−1. Spectra were recorded on solutions of samples
between NaCl plates. Mass spectra were determined by Dr
P. Boshoff of the mass spectrometry unit at the Cape Technikon.
The selected m/z values given refer to the most abundant
isotopes. In all cases, the isotopic distribution patterns were
checked against the theoretical distribution. GC analyses were
performed on a HP 5890 series II GC fitted with a 50 m PONA
column with a film thickness of 0.5 mm. Elemental analyses were
performed using a Carlo Erba EA1108 elemental analyser in the
microanalytical laboratory of the University of Cape Town.
6.78 (2H, m, Haryl), 6.72 (2H, m, Haryl), 6.57 (2H, d, 3J(HH)
=
8.4 Hz, Haryl), 2.37 (6H, s, N CMe), 2.08 (6H, s, aryl-Me); dC{H}
=
(100 MHz; CDCl3) 166.3 (N CMe), 156.1 (4-Caryl), 154.1 (2,6-
Cpyr), 142.4 (1-Caryl), 137.0 (4-Cpy), 129.2, 122.0, 119.4, 117.2,
+
=
113.0, 17.4 (aryl-Me), 15.6 (N CMe); m/z (FAB) 374 (M
+
H, 100), 358 (M+ − Me, 16%).
Attachment of the wedges to the ligands
The various wedges were attached to the 2,6-bis-[1-(4-
hydroxyphenylimino)ethyl]pyridines in essentially the same
manner. The synthesis of a representative compound is given
(for further details, see ESI†).
Synthesis of 2,6-bis-[1-(4-benzyloxyphenylimino)ethyl]pyridine,
2a
A mixture of 1a (0.271 g, 0.784 mmol), benzyl bromide (0.72 g,
4.2 mmol), K2CO3 (0.325 g, 2.35 mmol), 18-crown-6 (40 mg,
0.15 mmol) and acetone (20 cm3) was heated under reflux for
72 h. The volatiles were removed in vacuo, and the residue par-
titioned between dichloromethane and slightly alkaline water.
The aqueous phase was extracted with dichloromethane (3 ×
20 cm3), and the combined organics were then washed with
slightly alkaline water (3 × 20 cm3) and slightly alkaline brine
(1 × 20 cm3). After drying over Na2SO4 and filtration, the
volatiles were removed in vacuo, leaving a yellow, waxy solid.
Recrystallisation from dichloromethane/methanol gave 2a as a
yellow, crystalline solid. (0.309 g, 0.588 mmol, 75%), mp 228–
229 ◦C; (Found: C, 80.1; H, 5.9; N, 7.9. C35H31N3O2 requires
C, 80.0; H, 5.9; N, 8.0%); mmax/cm−1 (DCM) 1636 (C N), 1604,
=
1569, 1502 (Ar), 1455, 1366, 1211 (Ar–O), 1167, 1121, 1024
(CH2–O); dH (300 MHz; CDCl3) 8.33 (2H, d, 3J(HH) 8 Hz, m-
3
Hpy), 7.84 (1H, t, J(HH) 8 Hz, p-Hpy), 7.48–6.80 (18H, m,
=
Haryl and HPh), 5.09 (4H, s, OCH2Ph), 2.44 (6H, s, N CMe);
dC{H} (75 MHz; CDCl3) 167.3 (N CMe), 155.9 (4-Caryl), 155.7
=
Synthesis of 2,6-bis-[1-(4-hydroxyphenylimino)ethyl]pyridine, 1a
(2,6-Cpy), 144.9 (1-Caryl), 137.4, 136.7, 128.6, 127.9, 127.5,
=
122.2, 120.9, 115.6, 70.7 (OCH2Ph), 16.1 (N CMe); m/z (EI)
A mixture of 2,6-diacetylpyridine (0.908 g, 5.56 mmol), 4-
aminophenol (1.275 g, 11.68 mmol), toluene (30 cm3) and a few
crystals of p-toluenesulfonic acid was heated under reflux for
72 h, collecting the formed water in a Dean–Stark condenser.
A thick, light yellow precipitate formed. After cooling, the
precipitate was collected by filtration, and washed successively
with dichloromethane, ether and pentane. After recrystallisation
from hot methanol, 1a was recovered as a light yellow powder.
(1.812 g, 5.24 mmol, 84%); mp 245–247 ◦C; (Found: C, 73.1; H,
5.6; N, 12.0. C21H19N3O2 requires C, 73.0; H, 5.5; N, 12.1%);
525 (M+, 41), 434 (M+ − bz, 100) 342 (M+ − ArObz, 14%).
Synthesis of the iron complexes
The complexation of iron(II)chloride to the ligands (7–12) is
performed in essentially the same way. The synthesis of a
representative compound is given (For full details, see ESI†).
Synthesis of [2,6-bis-[1-(4-benzyloxyphenylimino)ethyl]-
pyridine]iron dichloride, 7a
mmax/cm−1 (THF) 3300 (OH), 1633 (C N), 1568, 1506; dH
=
A mixture of 2a (163 mg, 0.310 mmol) and FeCl2·4H2O (53 mg,
0.27 mmol) was stirred in THF (15 cm3). A dark purple colour
formed immediately. The mixture was stirred for 6 h, after
which time diethyl ether (20 cm3) was added, resulting in the
precipitation of a dark purple solid. After centrifuging the
mixture, the supernatant was removed, and the powder was
washed with 40% THF in ether (5 × 15 cm3) and ether (1 ×
15 cm3), centrifuging and removing the supernatant each time.
After drying in vacuo, 7a was obtained as a dark purple powder
(154 mg, 0.236 mmol, 89%); mp >200 ◦C (dec); (Found: C, 63.5;
H, 4.8; N, 6.2. C35H31Cl2N3O2Fe requires C, 64.4; H, 4.8; N,
(300 MHz; DMSO-d6) 9.11 (2H, br s, aryl-OH), 9.29 (2H, d,
3J(HH) 8 Hz, 3,5-Hpy), 7.99 (1H, t, 3J(HH) 8 Hz, 4-Hpy), 6.82–
=
6.76 (8H, m, Haryl), 2.38 (N CMe); dC{H} (75 MHz; DMSO-d6)
=
166.4 (N CMe), 156.1 (4-Caryl), 154.7 (2,6-Cpy), 143.7 (1-
=
Caryl), 137.8 (4-Cpy), 122.4, 121.7, 116.2, 16.4 (N CMe); m/z
(EI) 345 (M+, 100), 330 (M+ − Me, 5%).
Synthesis of 2,6-bis-[1-(4-hydroxy-2-methylphenylimino)ethyl]-
pyridine, 1b
A solution of 2,6-diacetylpyridine (0.374 g, 2.29 mmol), 4-
amino-m-cresol (1.127 g, 9.15 mmol) and formic acid (5 drops)
6.4%); mmax/cm−1 (DCM) 1582 (C N), 1503 (Ph), 1465, 1454,
=
5 5 4
D a l t o n T r a n s . , 2 0 0 5 , 5 5 1 – 5 5 5

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1379, 1238, 1171, 1110, 1014; m/z (FAB) 652 (M+ + H, 3), 616
(M+ − Cl, 29%).
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14 I. Gitsov, P. T. Ivanova and J. M. J. Fre´chet, Macromol. Rapid
Commun., 1994, 15, 387.
A mixture of toluene (22.7 cm3) and MAO (5.3 cm3 of a 10%
solution in toluene) was thermostatted at 30 ◦C. The vigorously
stirred solution was saturated with ethene at 1 atm. A suspension
of the catalyst precursor (20 lmol) in toluene (2.0 cm3) was
added via syringe. An immediate colour change occurred. The
reaction was quenched after 60 min by adding 2-propanol
(2 cm3) followed by aq. HCl (0.1 M, 2 cm3). Hexane (20 cm3) and
water (20 cm3) were added and the organic phase was separated.
The aqueous phase was extracted with hexane (2 × 20 cm3)
and the combined organic phases were washed with water (3 ×
20 cm3) and brine (3 × 20 cm3), dried over MgSO4 and filtered.
The volatiles were evaporated in vacuo to leave the non-volatile
oligomers.
Acknowledgements
We would like to thank the University of Cape Town and
Sasol for financial support. Sasol Technology is gratefully
acknowledged for a bursary for M.J.O.
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