New palladium α-diimine complexes containing dendritic wedges for ethene oligomerisation

Article abstract of DOI:10.1016/j.ica.2005.04.013

The synthesis of new α-diimine ligands containing zero and first generation dendritic wedges is described. These ligands were reacted with (COD)PdCl₂ to give the corresponding palladium α-diimine complexes. The resulting palladium complexes are active in the oligomerisation of ethene and the activity was compared to a related nickel complex.

Full text of DOI:10.1016/j.ica.2005.04.013

Inorganica Chimica Acta 358 (2005) 3491–3496
www.elsevier.com/locate/ica
New palladium a-diimine complexes containing dendritic wedges
for ethene oligomerisation
1
Burgert Blom, Matthew J. Overett , Reinout Meijboom , John R. Moss
2
*
Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa
Received 30 March 2005; accepted 11 April 2005
Available online 2 June 2005
Abstract
The synthesis of new a-diimine ligands containing zero and first generation dendritic wedges is described. These ligands were
reacted with (COD)PdCl2 to give the corresponding palladium a-diimine complexes. The resulting palladium complexes are active
in the oligomerisation of ethene and the activity was compared to a related nickel complex.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Palladium; a-Diimine ligand; Dendritic wedge; Oligomerisation, Ethene
1. Introduction
decrease in catalytic activity. Recently, related fluoro-
substituted bis(imino)pyridine complexes of iron and
their catalytic reactivity in ethene oligomerisations were
described [6], as were bis(imino)pyridine complexes
containing methoxy and CF3 groups [7]. Recently we
described iron bis(imino)pyridine complexes containing
dendritic wedges for ethene oligomerisation [8]. Here
we wish to report the related palladium a-diimine systems
containing wedges of the poly(benzylphenyl ether) type.
Since the independent reports by Gibson and Brook-
hart [9] on nickel and palladium a-diimine catalysts and
iron and cobalt bis(imino)pyridyl catalysts for the poly-
merisation and oligomerisation of 1-alkenes [9], much
attention has been focused on these late transition metal
catalysts as alternatives to established technologies.
It is known that the position and steric bulk of the
substituents on the aryl rings of the a-diimine catalysts
(Fig. 1) plays a crucial role in determining the selectivity
of the catalyst. In particular, the ortho substituents (R1)
have been identified as important. If R1 is a non-hydro-
gen substituent, and preferably a bulky substituent such
as iso-propyl groups, then the resulting catalysts are
selective for the production of high molecular weight
polymers from ethene. If only one of the two ortho
The oligomerisation of ethene is one of the major
industrial processes for the production of linear 1-alkenes
[1]. Oligomers in the range C6–C20 are used as co-mono-
mers in the polymerisation of ethene to give linear
low-density polyethene (LLDPE) or for the preparation
of detergents and synthetic lubricants. Catalysts cur-
rently used in industry for the Shell Higher Olefin Process
(SHOP) [2] contain Ni(II) complexes bearing bidentate
mono-anionic ligands [1,2]. Cationic Ni(II) a-diimine
complexes were also reported to be effective ethene olig-
omerisation catalysts [3], while iron-based bis(imino)pyr-
idine catalysts were described as highly active compounds
in the oligomerisation of ethene in combination with the
co-catalyst MAO or MMAO [4]. Several modifications of
the bis(imino)pyridine backbone which have already
been described in the literature [5] mostly lead to a
*
Corresponding author. Tel.: +27 21 6502535; fax: +27 21 6897499
E-mail address: jrm@science.uct.ac.za (J.R. Moss).
1
Present address: Sasol Technology R & D, P.O. Box 1, Sasolburg
1947, South Africa.
2
Present address: Department of Chemistry, University of the Free
State, P.O. Box 339, Bloemfontein 9300, South Africa.
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.04.013

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B. Blom et al. / Inorganica Chimica Acta 358 (2005) 3491–3496
2. Results and discussion
R1
R1
2.1. Preparation of ligands
R2
N
N
R2
M
Appropriate ligands would be a-diimine ligands with
either both ortho-substituents (R1, Fig. 1) being hydro-
gen, or one of the substituents R1 on each ring being a
methyl group and the other a hydrogen. We chose to
prepare ligands with one methyl group on the ortho-
position for this study. Substituent R2 was chosen to
be a hydroxyl group for ease of attachment of alkylbro-
mide functionalised dendritic wedges, using the standard
Williamson ether synthesis. Thus compound 1 was syn-
thesised using the simple Schiff-base condensation reac-
tion of 2,3-butadione and two molar equivalents of
amino-m-cresol (Scheme 1).
X
X
M = Ni, Pd, X = Cl, Br
Fig. 1. Palladium and nickel a-diimine catalyst for the oligomerisation
of 1-alkenes.
positions R1 on the aryl rings is an alkyl substituent, the
catalyst is selective for the oligomerisation of ethene to
linear 1-alkenes with a Schulz-Flory [10] chain length
distribution, and for the dimerisation of longer chain
olefins (such as 1-hexene). The effect of substituents in
the para-position (R2) has been less studied. Our interest
in dendritic molecules [11], their applications in catalysis
and as new materials, led us to consider the possibility of
functionalising late transition metal oligomerisation cat-
alysts 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. By having
a catalytic site at the core of a dendrimer, it should be
possible to control the micro-environment around the
catalytic centre and thus allow modifications of the cat-
alytic selectivity [12]. In addition to the latter, dendritic-
ally substituted homogeneous catalysts should be easily
separated from the product stream by means of ultra-
filtration methods.
Functionalising an a-diimine ligand with dendritic
wedges to create a catalytic ligand at the core of a den-
drimer 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 com-
mercially available. These may be reacted in standard
Schiff-base type reactions with 2,3-butadione to form
the a-diimine framework with aryl rings suitable for
attachment of dendritic wedges. Dendritic functionalisa-
tion at the para-position is most favourable. The direct
steric control around the catalytic centre may be con-
trolled by appropriate ortho substitution (R1, Fig. 1),
thereby ensuring an oligomerisation catalyst. The het-
eroatomic 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 or-
der to minimise unfavourable interactions.
The para-hydroxyl group allows attachments of vari-
ous dendritic wedges containing an alkylbromide func-
tionality on the focal point. Using the Williamson
ether synthesis, a quantitative yield of dendritically func-
´
tionalised a-diimine ligands was expected. Frechet and
co-workers reported the quantitative build-up of
poly(benzylphenyl ether) dendrimers [13] when applying
mild reaction conditions. Applying similar conditions,
we reacted two equivalents of the appropriate bro-
moalkyl wedge with 1 in acetone, heated under reflux,
using K2CO3 as the base and a sub-stoichiometric
amount of 18-crown-6 as phase transfer catalyst
(Scheme 2). The reactions typically were allowed to pro-
ceed for between 48 and 72 h. The compounds 2 and 3
were separated from the inorganic salts using an aque-
ous work-up. Analytically pure samples were obtained
´
by recrystallisation from an appropriate solvent. Fre-
chets poly(benzylphenyl ether) dendritic wedges [13]
were chosen because they are easily prepared and react
readily with hydroxyaryl compounds. These wedges
have previously been used as dendritic catalysts for an-
ionic ring opening polymerisation reactions [14]. We re-
ported the use of these wedges connected to iron
bis(imino)pyridyl complexes [8] in alkene oligomeri-
sation previously. Details on the wedges employed are
given in Scheme 2.
1
The ligands 2 and 3 were characterised by H, 13C
NMR spectroscopy, IR spectroscopy and satisfactory
elemental analysis. For ligand 2, satisfactory mass spec-
trometric was obtained and the molecular ion was ob-
served. Ligand 3 unfortunately decomposed under MS
MeOH
Formic acid
+ 2 H2N
OH
HO
N
N
OH
O
O
1
Scheme 1. Synthesis of 1.

B. Blom et al. / Inorganica Chimica Acta 358 (2005) 3491–3496
3493
O
O
Br-Rwedge
=
Br
Br
G0
G1
K2CO3
18-crown-6
acetone
∆
Rwedge
+ 2 Br-Rwedge
OH
N
N
Rwedge
HO
N
N
1
2: Rwedge = G0
3: Rwedge = G1
Scheme 2. Synthesis of 2 and 3.
(COD)PdCl2
CH2Cl2
Rwedge
Rwedge
N
N
Rwedge
N
N
Rwedge
Pd
Cl Cl
2: Rwedge = G0
3: Rwedge = G1
4: Rwedge = G0
5: Rwedge = G1
Scheme 3. Synthesis of 4 and 5.
conditions and only fragments could be identified, but
not the molecular ion.
by addition of iso-propanol, followed by addition of
1 M HCl. An aqueous work-up was used to remove alu-
minium salts, and the organic phase was dried. An inter-
nal standard of undecene (C11 1-alkene) was added in
order to determine the absolute amounts of each 1-
alkene formed. The volatiles were removed in vacuo,
leaving behind the involatile oligomers (some C10, most
C12, all C14+), along with a certain amount of residual
toluene. After determining the mass of the involatile
oligomers (recovered yield), they were taken up in hex-
ane, and GC analysis was used to determine the contri-
bution to the mass by residual toluene and the amounts
of C10 and C12 still present. Detection of higher weight
oligomers (C20+) and polymers is not possible using GC.
2.2. Preparation of the metal complexes
Palladium complexes of the a-diimine ligands 2 and 3
were prepared by treating a slight excess of the ligand
with (COD)PdCl2 in dichloromethane (Scheme 3). The
resulting yellow complexes (4 and 5) were characterised
by NMR and IR spectroscopy and elemental analysis.
Preliminary attempts to prepare the analogous nickel
complexes were successful [15]. Treatment of the substi-
tuted a-diimine ligands with (DME)NiBr2 in dichloro-
methane [16] led to the formation of the corresponding
nickel complexes [15]. We will report on the formation
of these nickel complexes in a separate communication.
Table 1
Yields of non-volatile ethene-oligomers
2.3. Catalytic ethene oligomerisation
Entrya
Catalyst precursor
Yield (mg)
Complexes 4 and 5 were used to investigate the cata-
lytic behaviour in catalytic ethene oligomerisation. The
catalytic runs were repeated with the literature com-
pound 6 (see Fig. 1; R1 = Me; R2 = H; M = Pd,
X = Cl) [17] as well as the corresponding NiBr2 complex
of ligand 2. The conditions and yields of oligomers from
the catalytic runs were performed are given in Table 1.
The catalytic oligomerisation reactions were quenched
1b
2
6
25
148
112
66
6
3
4
5c
6
5
2–NiBr2 complex
1605
a
Conditions: 1 bar ethylene, [cat] = 20 lmol, [Al]/[cat] = 400, T:
40 ꢁC, solvent: toluene, volume: 30 cm3, t: 60 min.
b
Run at 25 ꢁC.
Run conducted using 50% of normal amounts.
c

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B. Blom et al. / Inorganica Chimica Acta 358 (2005) 3491–3496
From the data in Table 1, there appears to be no real
from Mg activated by I2, and acetone was distilled from
Drierite prior to use [19]. All reagents were stored under
pre-purified argon. Poly(benzylphenylether) wedge G1
was prepared according to the method of Hawker and
correlation between the activity of the complexes and
the size (i.e. generation) of the dendritic wedges on the
complexes. It is therefore concluded that the dendritic
wedges do not sterically interfere with the cationic metal
centre. Since even the large MAO oligomers can activate
the metal centre, it is suggested that dendritic wedges on
the 4-position of the phenyl rings do not constitute any
steric hindrance. Similar conclusions were previously
drawn for the related iron bis(imino)pyridyl complexes
[8]. The nickel complex of ligand 2 [15] is clearly more
active than the corresponding palladium complexes –
which was expected from literature analogues [17]. We
suggest that the present palladium a-diimine complexes
can be used as models for the more active (but harder to
characterise) nickel complexes.
´
Frechet [13]. Compound 6, [2,3-bis(2-methylphenyli-
mino)butane]palladium dichloride, was prepared
according to the method of Brookhart and Svejda [17].
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) 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 either
the residual protio impurities in the solvent (1H NMR)
or the solvent resonances (13C NMR). Infrared spectra
were recorded on a Perkin–Elmer Paragon 1000 FT-IR
spectrometer in the range 450–4400 cmÀ1. Spectra were
recorded on solutions of samples between NaCl plates.
Mass spectra were recorded on a Finnigan MAT GCQ
GC/MS. The selected m/z values given refer to the most
abundant isotopes. In all cases the isotopic distribution
pattern was checked against the theoretical distribution.
Oligomer samples were analysed by GC, using a Carlo-
Erba 4200 machine equipped with a Zebron ZB-1 col-
umn and an hydrogen flame ionization detector. The
GC was coupled to a Columbia Supergrator 3A integra-
tor. Elemental analyses were performed using a Carlo-
Erba EA1108 elemental analyser in the microanalytical
laboratory of the University of Cape Town.
Due to the nature of the work-up, low molecular
weight oligomers in the C4–C10 range were lost. Subse-
quent GC analysis was therefore only performed on
the higher molecular weight oligomers. Although a
Schulz–Flory [10] distribution of oligomers was ex-
pected, GC analysis did not show such a distribution.
3. Conclusions
New a-diimine ligands containing dendritic wedges
were successfully synthesised and characterised. The at-
tached wedges are of the poly(benzylphenyl ether) type.
These ligands were used successfully to synthesise new
a-diimine palladium complexes, and the resulting com-
plexes were fully characterised. These complexes consti-
tute a new and exciting family of oligomerisation
catalysts, which have the potential of being separated
by ultra-filtration methods. It was shown that these a-
diimine palladium complexes are active in the oligomer-
isation of ethene when using MAO as a co-catalyst. The
activity of the complexes is not related to the size (gen-
eration) of the dendritic wedges. This opens up the pos-
sibility for using even larger dendritic wedges as
substituents, without interfering with the catalytic activ-
ity. Larger dendritic substituents could make feasible the
separation of the catalyst from the product stream by
means of ultra-filtration methods.
4.2. Synthesis of 2,3-bis(2-methyl-4-hydroxylphenylimino)
butane (1)
A mixture of 2,3-butadione (912 mg, 10.6 mmol), 2-
methyl-4-hydroxyaniline (3.25 g, 26.4 mmol) and formic
acid (a few drops) was stirred in MeOH (35 ml) at room
temperature for 24 h. The mixture was concentrated un-
til a yellow precipitate formed. After allowing further
precipitation at À20 ꢁC for 16 h, the precipitate was
recovered by filtration. After recrystallisation from hot
MeOH, 1 was obtained as a yellow powder. (1.56 g,
5.26 mmol, 50%); m.p. 122–126 ꢁC; mmax/cmÀ1 (THF):
3310 vs (O–H), 1636 (C@N), 1607 m, 1583 m; dH
(300 MHz, acetone-d6): 8.06 (2H, broad s, aryl-OH),
6.70–6.50 (6H, m, Haryl), 2.12 (6H, s, N@CMe), 2.05
(6H, s, aryl-Me); dC{H} (75 MHz, acetone-d6): 167.6
(N@C Me), 154.3 (4-Caryl), 142.1 (1-Caryl), 129.1 (Caryl),
119.1 (Caryl), 117.2 (Caryl), 113.0 (Caryl), 17.3 (aryl-Me),
14.8 (N@CMe); m/z (FAB) 296 (M+ + H, 93), 281
(M+ À Me, 31%).
4. Experimental
4.1. General remarks
All manipulations were carried out under purified
nitrogen, using glovebox (MBraun Unilab) or standard
Schlenk line techniques under pre-purified argon [18].
Dichloromethane was dried by passage through a col-
umn containing alumina (neutral, Brockmann grade I)
and distilled from CaH2 [19]. Methanol was distilled
4.3. Synthesis of 2,3-bis(2-methyl-4-benzyloxyphenylimino)
butane (2)
A mixture of 1 (490 mg, 1.66 mmol), 18-crown-6 (90
mg, 0.34 mmol), K2CO3 (580 mg, 4.20 mmol), acetone

B. Blom et al. / Inorganica Chimica Acta 358 (2005) 3491–3496
3495
(30 ml) and benzyl bromide (0.50 ml, 4.2 mmol) was
heated under reflux for 48 h. The volatiles were removed
in vacuo and the residue was partitioned between
CH2Cl2 and slightly alkaline (pH ꢀ 7.5) water. The
aqueous phase was extracted with CH2Cl2 (3 · 20 ml),
and the combined organics were washed with slightly
alkaline water (3 · 20 ml) and slightly alkaline brine
(3 · 20 ml). After drying over Na2SO4 and filtration,
the volatiles were removed in vacuo, leaving a yellow
crystalline material. This was recrystallised from
CH2Cl2/MeOH to give 2 as a yellow powder. (341 mg,
0.715 mmol, 43%); m.p. 130–134 ꢁC; (Found: C, 80.1;
H, 6.9; N, 6.0%. C32H32N2O2 requires C, 80.6; H, 6.8;
N, 5.9%); mmax/cmÀ1 (CH2Cl2): 2922 w, 2869 w, 1640 s
(C@N), 1604 m, 1576 w, 1490 vs (Ar), 1379 w, 1362
m, 1241 s, 1200 vs (Ar–O), 1162 m, 1119 s, 1028 s
(CH2–O); dH (300 MHz, CDCl3): 7.46–6.58 (16H, m,
Haryl), 5.07 (4H, s, OCH2Ph), 2.16 (6H, s, Me), 2.14
(6H, s, Me); dC{H} (75 MHz, CDCl3): 168.0 (N@CMe),
155.6 (4-Caryl), 143.1 (1-Caryl), 137.3 (1-CPh), 129.0,
128.5, 127.9, 127.5, 118.8, 117.1, 112.4, 70.3 (OCH2Ph),
18.1 (aryl-Me) 15.5 (N@CMe).
for 24 h. The thick yellow precipitate that had formed
was recovered by filtration and washed with CH2Cl2
and hexanes to give 4 as a yellow–orange powder.
(142 mg, 0.217 mmol, 55%); m.p. (dec) > 220 ꢁC;
(Found: C, 58.6; H, 4.8; N, 4.2%. C32H32Cl2N2O2Pd re-
quires C, 58.8; H, 4.9; N, 4.3); mmax/cmÀ1 (CH2Cl2): 2929
w, 2856 w, 1602 s, 1579 w (C@N), 1495 s (Ar), 1352 m,
1233 m (Ar–O), 1170 m; dH(300 MHz, DMSO-d6): 7.45–
6.91 (16H, m, Haryl), 5.09 4H, s, OCH2Ph), 2.30 (6H, s,
N@CMe), 2.06 (6H, s, aryl-Me);
dC{H}(75 MHz,
DMSO-d6): 181.2 (N@CMe), 157.2 (6-Caryl), 138.1 (1-
Caryl), 136.8 (1-CPh), 131.6, 128.2, 127.6, 127.5, 122.7,
115.9, 111.9, 69.4 (OCH2Ph), 20.0 (Me), 17.9 (Me).
4.6. Synthesis of {2,3-bis[2-methyl-4-(3,5-dibenzyloxy)
benzyloxyphenylimino]butane} palladium dichloride (5)
A mixture of (COD)PdCl2 (12 mg, 0.042 mmol) and 3
(50 mg, 0.055 mmol) was stirred in CH2Cl2 (10 ml) for
24 h. The volatiles were removed in vacuo and the resi-
due was recrystallised from CH2Cl2/hexanes to give 5 as
a glassy red solid. (30 mg, 0.028 mmol, 66%); m.p. 170–
173 ꢁC (dec.); (Found: C, 66.5; H, 5.1; N, 2.6%.
PdC60H56Cl2N2O6 requires C, 66.8; H, 5.2; N, 2.6%);
4.4. Synthesis of 2,3-bis[2-methyl-4-(3,5-dibenzyloxy)
benzyloxyphenylimino]butane (3)
m
max/cmÀ1 (KBr): 2360 w, 1670 w, 1570 w (C@N),
1474 s (Ar), 1445 m, 1360 m, 1022 m (CH2–O);
dH(300 MHz, CDCl3): 7.34–6.81 (32H, m, Haryl), 4.97
(8H, s, OCH2Ph), 4.83 (4H, s, OCH2-aryl wedge), 2.38
(6H, s, N@CMe), 2.09 (6H, s, aryl-Me); dC{H}(75 MHz,
CDCl3): 181.0 (N@CMe), 160.1 (3,5-Caryl wedge), 158.1
(4-Caryl core), 139.3, 138.5, 136.9, 131.7, 128.5, 127.9,
127.6, 123.0, 116.9, 106.5, 101.7, 70.1(OCH2-aryl
wedge), 69.9(OCH2Ph), 20.3 (N@CMe), 18.0 (aryl-Me).
A mixture of 1 (219 mg, 0.739 mmol), G1 (559 mg,
1.46), 18-crown-6 (40 mg, 0.15 mmol) and K2CO3
(261 mg, 1.90 mmol) in acetone (20 ml) was heated un-
der reflux for 48 h. The volatiles were removed in vacuo
and the residue was partitioned between CH2Cl2 and
slightly alkaline water. The aqueous phase was extracted
with CH2Cl2 (3 · 20 ml), and the combined organics
were washed with slightly alkaline water (3 · 20 ml)
and slightly alkaline brine (3 · 20 ml). After drying over
Na2SO4 and filtration, the volatiles were removed in va-
cuo, leaving a yellow solid. Recrystallisation from
CH2Cl2/MeOH gave 3 as a yellow powder. (380 mg,
0.422 mmol, 57%); m.p. 143–146 ꢁC; (Found: C, 79.6;
H, 6.0; N, 3.5%. C60H56N2O6 requires C, 80.0; H, 6.3;
N, 3.1); mmax/cmÀ1 (CH2Cl2): 2922 w, 2876 w, 1639 m
(C@N), 1597 vs (Ar), 1490 s (Ar), 1452 m, 1376 m,
1202 s (Ar–O), 1160 vs, 1119 m, 1028 m (CH2–O); dH
(300 MHz, CDCl3): 7.42–6.64 (32H, m, Haryl), 5.05
(8H, s, OCH2Ph), 5.00 (4H, s, OCH2-aryl wedge),
2.14 (6H, s, Me), 2.12 (6H, s, Me); dC{H} (100 MHz,
CDCl3): 167.9 (N@CMe), 160.2 (3,5-Caryl wedge), 155.5
(4-Caryl core), 143.1, 139.8, 136.9, 136.7, 128.9, 128.5,
127.9, 127.4, 118.7, 117.8, 112.4, 106.4, 70.2 (OCH2-aryl
wedge), 70.1 (OCH2Ph), 17.9 (aryl-Me), 15.4 (N@CMe).
4.7. Catalytic ethene oligomerisation
A mixture of toluene (22.7 cm3) and MAO (5.3 cm3 of
a 10% solution in toluene) was thermostatted at 40 ꢁC.
The vigorously stirred solution was saturated with ethene
at 1 atm for 30 min. A suspension of the catalyst precur-
sor (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) fol-
lowed 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). 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.
4.5. Synthesis of [2,3-bis(2-methyl-4-benzyloxyphenyli-
mino)butane]palladium dichloride (4)
Acknowledgements
A mixture of (COD)PdCl2 (113 mg, 0.396 mmol) and
2 (261 mg, 0.548 mmol) was stirred in CH2Cl2 (15 ml)
Financial assistance from Sasol Technology, Sasol
Polymers and the Research Fund of the University of

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B. Blom et al. / Inorganica Chimica Acta 358 (2005) 3491–3496
[8] M.J. Overett, R. Meijboom, J.R. Moss, Dalton Trans. (2005) 551.
[9] (a) L.K. Johnson, C.M. Killian, M. Brookhart, J. Am. Chem.
Soc. 117 (1995) 6414;
Cape Town is gratefully acknowledged. Sasol Technol-
ogy is gratefully acknowledged for a bursary for
M.J.O. Part of this material is based on work supported
by the South African National Research Foundation.
Any opinions, findings and conclusions or recommenda-
tions expressed in this material are those of the authors
and do not necessarily reflect the views of the NRF.
(b) C.M. Killian, L.K. Johnson, M. Brookhart, Organometallics
16 (1997) 2005;
(c) B.L. Small, M. Brookhart, A.M.A. Bennet, J. Am. Chem. Soc.
120 (1998) 4049.
[10] P.J. Flory, J. Am. Chem. Soc. 62 (1940) 1661.
[11] (a) M.A. Hearshaw, J.R. Moss, Chem. Commun. (1999) 1;
(b) M.A. Hearshaw, A.T. Hutton, J.R. Moss, K.J. Naidoo, Adv.
Dendritic Macromol. 4 (1999) 1;
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