A novel nickel (II) complex based on a cyclam-cored generation-one dendrimeric salicylaldimine ligand and its application as a catalyst precursor in norbornene polymerization: Comparative study with some other first generation DAB-polypropyleneimine metallodendrimers

Article abstract of DOI:10.1016/j.poly.2012.08.015

A new cyclam based first generation metallodendrimer with nickel centres on the periphery as well as at the core was synthesized by reacting a novel tetrakis(salicylaldimine) cyclam ligand with nickel acetate. The trinuclear complex was evaluated as a catalyst precursor in the vinyl polymerization of norbornene using methylaluminoxane as co-catalyst. The activity of this cyclam-based complex was compared with that of first generation DAB-PPI-salicylaldimine and DAB-PPI-pyridine-imine complexes (DAB = diaminobutane, PPI = polypropyleneimine). The cyclam based complex was found to have superior activity than both the DAB-PPI analogues as well as analogous mononuclear complexes.

Full text of DOI:10.1016/j.poly.2012.08.015

Polyhedron 47 (2012) 87–93
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
A novel nickel (II) complex based on a cyclam-cored generation-one dendrimeric
salicylaldimine ligand and its application as a catalyst precursor in
norbornene polymerization: Comparative study with some other first
generation DAB-polypropyleneimine metallodendrimers
⇑
Rehana Malgas-Enus, Selwyn F. Mapolie
Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa
NRF/DST Centre of Excellence in Catalysis (c⁄change), Private Bag X1, Matieland 7602, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
A new cyclam based first generation metallodendrimer with nickel centres on the periphery as well as at
the core was synthesized by reacting a novel tetrakis(salicylaldimine) cyclam ligand with nickel acetate.
The trinuclear complex was evaluated as a catalyst precursor in the vinyl polymerization of norbornene
using methylaluminoxane as co-catalyst. The activity of this cyclam-based complex was compared with
that of first generation DAB-PPI-salicylaldimine and DAB-PPI-pyridine-imine complexes (DAB = diamin-
obutane, PPI = polypropyleneimine). The cyclam based complex was found to have superior activity than
both the DAB-PPI analogues as well as analogous mononuclear complexes.
Received 22 March 2012
Accepted 6 August 2012
Available online 17 August 2012
Keywords:
Schiff base ligands
Cyclam
Ó 2012 Elsevier Ltd. All rights reserved.
DAB-PPI
Metallodendritic catalysts
Norbornene polymerization
Nickel metallodendrimers
1. Introduction
the double bond of the p-component is opened. Of the three types
of polynorbornene, only the vinyl addition polymerization provides
a completely saturated polymer with no rearranged norbornene
units [6]. The vinyl polymerization of norbornene is usually acti-
vated by transition metal complexes such as palladium, chromium,
cobalt, titanium, iridium, rhodium, copper, zinc and nickel [7].
Dendrimers are highly branched macromolecules which are
monodispersed having well defined architectures. These macro-
molecules can be designed in such a way that they have discrete
peripheral functionalities. The definitive structures and the associ-
ated chemical and physical properties result in dendrimers being
explored in a wide range of applications. These include drug deliv-
ery [8], biosensors [9], MRI agents [10], light emitting diodes [11]
and use as bio-conjugates [12]. The incorporation of metals into
the dendrimer frame-work leads to the formation of metalloden-
drimers. These metal containing dendrimers have been employed
as multinuclear catalyst precursors [13].
We previously reported the synthesis of dendritic nickel
complexes derived from generation 1 (G1) and generation 2 (G2)
dendrimeric salicylaldimine ligands based poly(propyleneimine)
dendrimer scaffolds of the type, DAB-(NH2)n (n = 4 or 8, DAB = diam-
inobutane). These catalysts were found to be active for norbornene
polymerization giving polymers with moderate to high molecu-
lar weights and low polydispersity indices [14]. In this paper
we now report on the evaluation of three nickel generation 1
Norbornene can typically be polymerized via three routes lead-
ing to different polymer structures. These routes are: (a) ring-
opening metathesis polymerization (ROMP), (b) radical polymeri-
zation and (c) vinyl or addition polymerization of norbornene as
shown in Fig. 1 [1].
The type of polynorbornene formed depends on the catalyst
employed. Ring-opening metathesis polymerization (ROMP) is
the most studied route to produce polynorbornene [2]. The unique
properties of ROMP polynorbornene are due to the presence of the
double bond within the polymer backbone. This makes cross-link-
ing and vulcanization of the polymer possible [3].
In the radical polymerization of norbornene, the framework is
rearranged to produce poly (2,7-bicyclo[2,2,1] hept-2-ene) oligo-
mers [4]. The polymer obtained via this route has low molecular
weights (molecular weight <1000 Da) and is usually obtained in
low yields because of rearrangements and chain transfer reactions.
The following activators typically initiate radical polymerization;
azoisobutronitrile (AIBN) and tert-butyl perpivilate [5].
Vinyl or addition polymerization yields a 2,3-connected polymer,
which occurs when the bicyclic structural unit remains intact and
⇑
Corresponding author.
E-mail address: smapolie@sun.ac.za (S.F. Mapolie).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.08.015

88
R. Malgas-Enus, S.F. Mapolie / Polyhedron 47 (2012) 87–93
2.2.2. Synthesis of DL1, DAB-PPI G1 salicylaldimine modified
dendrimer
Ligand DL1 was prepared as previously reported in the litera-
ture [14] and was obtained in a yield of 90%. m.p. 66–68 °C. IR
(cmꢀ1): (C–O) 1284 (s); 13C
(O–H) 2924 (m); (C@N) 1632 (s);
NMR in CDCl3 (d ppm): 25.1, 28.4, 51.5, 54.0, 57.4, 117.0, 118.4,
118.8, 131.1, 132.1, 161.3, 164.9. Anal. Calc. for C44H56N6O4: C,
72.10; H, 7.70; N, 11.47. Found: C, 71.90; H, 7.90; N, 11.49%. MS
(MALDI-TOF) Calcd. (C44H56N6O4) [M]+ at m/z = 733.
a)
ROMP
n
n
m
m
m
n
b) cationic / radical
2.2.3. Synthesis of DL2 cyclam-based G1 salicylaldimine modified
dendrimer
G1 cyclam-propyl dendrimer (0.31 g, 0.72 mmol) was added to
dry toluene (10 ml) in a Schlenk tube, under nitrogen. Salicylalde-
hyde (0.35 ml, 2.9 mmol) was added to the solution. The mixture
was allowed to stir for 72 h at room temperature. The solvent
was evaporated on a rotary evaporator and a red oil was obtained.
Dichloromethane (20 ml) was added to the oil and the solution
washed with water (5 ꢁ 30 ml). The dichloromethane layer was
dried over potassium carbonate after which the latter was filtered
off. The filtrate was evaporated to give a red oil. Yield: 78%. IR
c) vinyl / addition
n
Fig. 1. Three different routes to polynorbornene formation [1].
metallodendrimer complexes based on dendrimeric Schiff base li-
gands as catalyst precursors in the vinyl addition polymerization
of norbornene. Two of the complexes are based on diaminobutane-
cored polypropylene imine (DAB-PPI) dendrimers while the third
complex is based on a cyclam cored PPI dendrimer. Cyclam-based
dendrimers is a relatively new concept, with not many examples
reported in literature. However, those reported show favourable
properties especially since the cyclam core is able to coordinate to
metal centres. Both types of dendrimers were modified via Schiff
base condensation reactions after which the functionalized dendri-
mers were complexed to nickel salts to form metallodendrimers.
The DAB-based salicylaldimine complex, C1 has been reported
before [14] but the DAB-pyridinyl-imine complex C3 and the cyclam
based complex, C2 are both novel.
(cmꢀ1): (C–O) 1276 (s). 1H
m(O–H) 3058 (m); m(C@N) 1662 (s); m
NMR in CDCl3 (d ppm) 1.59 (m, 4H, NCH2CH2CH2N in cyclam ring),
1.80 (br m, 8H, NCH2CH2, side arm), 2.42 (br t, 8H, NCH2CH2, side
arm), 2.48–2.52 (m, 16H, NCH2CH2CH2N in cyclam ring), 3.58
(8H, t, CH2N@CH), 6.93 (t, 4H, Ar), 7.02 (d, 4H, Ar), 7.21 (t, 4H,
Ar), 7.56 (d, 4H, Ar), 7.56 (d, 4H, Ar), 8.33(s, 4H, N@CH) 13C NMR
in CDCl3 (d ppm): 29.91; 31.31; 48.05; 51.68; 117.84; 120.06;
133.94; 137.27; 141.14; 161.85; 169.09. Anal. Calc. for
C
50H68N8O4: C, 63.63; H, 7.30; N, 11.53. Found: C, 64.46; H, 7.35;
N, 11.27%. ESI–MS Calc. (C50H68N8O4) [M+H]+ at m/z = 846.
2.2.4. Synthesis of DL3 DAB-PPI G1 iminopyridyl modified dendrimer
DL3 was prepared employing the method previously reported
by us [17]. The spectroscopic data of the product compares favour-
ably with that reported.
2. Experimental
2.1. General
2.2.5. Synthesis of C1 DAB-PPI G1 salicylaldimine nickel
metallodendrimer
C1 was previously reported by us and was thus prepared using
the method previously employed [14]. Characterization data
obtained correspond well with that reported.
All reactions were carried using standard Schlenk procedures
under an inert atmosphere (nitrogen or argon) or in a nitrogen-
filled glovebox. Solvents were dried by distillation prior to use
and all other reagents were employed as obtained. NMR (1H: 300
and 400 MHz; 13C: 75 and 100 MHz) spectra were recorded on Var-
ian VNMRS 300 MHz; Varian Unity Inova 400 MHz spectrometers
and chemical shifts are reported in ppm, referenced to the residual
protons of the deuterated solvents and tetramethylsilane (TMS) as
internal standard.
ESI–MS (positive and negative modes) analyses were performed
on either Waters API Quattro Micro or Waters API Q-TOF Ultima
instruments by direct injection of sample (capillary voltage:
3.5 kV; cone voltage: 15 RF1:40; source: 100 °C; desolvation temp.:
400 °C; desolvation gas: 500 L/h; cone gas: 50 L/h). FT-IR analysis
was performed on a Thermo Nicolet AVATAR 330 instrument,
and was recorded as neat spectra (ATR) unless otherwise specified.
Melting point determinations were performed on a Stuart Scien-
tific SMP3 melting point apparatus and are reported as uncor-
rected. Elemental analyses were performed by the micro
analytical laboratory at the University of Cape Town.
2.2.6. Synthesis of C2, cyclam-PPI G1 salicylaldimine nickel
metallodendrimer
To the first generation cyclam-based salicylaldimine dendrimer
ligand, DL2 (0.125 g, 0.15 mmol) in ethanol (10 ml) in a round
bottom flask was added nickel acetate tetrahydrate (0.11 g,
0.44 mmol) and the reaction mixture was allowed to stir under re-
flux for 24 h forming a lime green solution. The solvent was evap-
orated to give
a lime green oily residue. The residue was
recrystallized from dichloromethane (2 ml) and diethyl ether
(10 ml) to afford C2 as a lime green solid. Yield = 63%. m.p.: 260–
263 °C. IR (cmꢀ1):
m(C@N) 1623 (s) and m(C–O) 1319 (s). Anal. Calc.
for C54H70N8Ni3O8ꢂ2.5CH2Cl2: C, 50.67; H, 5.74; N, 8.29. Found: C,
50.19; H, 5.58; N, 8.16%. ESI–MS calc for (C50H64N8Ni3O4) (cationic
complex, [M+K]+ at m/z = 1057.
The complexes C4 and C5 were prepared using standard litera-
ture procedures [15].
2.2.7. Synthesis of C3, DAB-PPI G1 iminopyridyl nickel
metallodendrimer
2.2. Synthesis
Ni(DME)Br2 (0.18 g, 0.6 mmol) was added to a solution of DL3
(0.10 g, 0.15 mmol) in dry ethanol (15 ml). An immediate colour
change from orange to brown is observed, and all reagents dis-
solved to give a clear brown solution. The reaction mixture was re-
fluxed for 24 h after which the solvent was evaporated to give a
brown solid residue. Dichloromethane (5 ml) was added to the
2.2.1. Synthesis of L1 and L2
L1 [16a] and L2 [16b] were obtained using literature proce-
dures. Characterization by 1H NMR and IR spectroscopy confirms
the identity of the compound.

R. Malgas-Enus, S.F. Mapolie / Polyhedron 47 (2012) 87–93
89
and KBH4 but none of these gave complete reduction to the amine.
We finally opted for a method involving the use of Na/toluene/eth-
anol as a reducing agent. This is based on a modification of a pro-
cedure reported in the literature [18]. The reduction process was
monitored using IR spectroscopy by following the disappearance
R
R
N
N
N
N
of the
with a concomitant appearance of the
m
(C@N) band at 2244 cmꢀ1 of the nitrile starting material
m(N–H) band of the amine
group of the product at 3352 cmꢀ1. The 1H NMR spectrum of L2,
also shows the appearance of a peak at d3.59 ppm, due to the
methylene protons of the carbon adjacent to the amino group.
R
R
L1: R = CN
3.2. Synthesis of Schiff base ligands
L2: R = CH2NH2
3.2.1. Preparation of DAB cored salicylaldimine ligand, DL1
Ligand DL1 was synthesized as previously reported via Schiff
base condensation of the G1-DAB-(NH2)4 dendrimer with salicylal-
dehyde [14]. The ligand was isolated as a yellow solid in high yield,
85%. DL1 was characterized using IR and NMR spectroscopy and
the data corresponds well with that reported previously.
Fig. 2. Tetracyanoethylcyclam ligands L1 and its reduced analogue L2 reported by
Wainwright [16].
residue which was then triturated, leading to the formation of a
brown precipitate. The solid was filtered and washed with CH2Cl2
(3 ꢁ 5 ml) to give C3 as a brown solid. Yield = 85%. m.p.: 249–
3.2.2. Preparation and characterization of the generation 1 cyclam-
based propyl-imine Schiff base ligands, DL2
The G1 cyclam-tetrakis(aminopropyl) dendrimer, L2, was re-
acted with salicylaldehyde to obtain the novel modified dendrimer
ligand, DL2 as shown in Fig. 3. The reaction time was 72 h, and the
251 °C. IR (cmꢀ1):
m(C@N) at 1639 (s); m(C@C) pyr at 1596 (s) and
m
(C@N) pyr at 1567 (s). ESI–MS calc for (C40H52Br8N10Ni4),
[M+2Na]+ at m/z = 1592. Due to the hygroscopic nature of the
material we were unable to obtain suitable microanalysis.
product was isolated as a red oil. The m(C@N) band in the FT-IR
spectrum for DL2 is observed at 1662 cmꢀ1 and the
m
(C–O) stretch-
2.3. Typical polymerization procedure
1
ing frequency at 1276 cmꢀ1. The
H NMR spectrum of the
compound shows the signal for the imine proton at 8.33 ppm while
in the 13C NMR spectum, the imine carbon is observed at
161.85 ppm.
The amount of catalyst corresponding to 5 lmol of nickel was
added to an appropriate amount of dry toluene in a Schlenk tube,
under nitrogen. 5 ml (25 mmol) of a 5 M norbornene in toluene
solution was added to the reaction vessel. The required amount
of MAO (1.7 M, 10 %) was then added to the reaction mixture, to
initiate the polymerization. The mixture was allowed to stir for
30 min at room temperature. The polymerization was stopped by
adding the reaction solution to 200 ml of acidic methanol (95:5).
A white solid precipitated out of solution. The polymer was filtered
and then dried in the oven for 24 h. The norbornene:nickel ratio
was 5000:1 and the total volume of the reaction mixture was
25 ml.
3.2.3. Preparation of DAB cored pyridinyl-imine ligand, DL3
This was prepared as described by Smith et al. [17]. The identity
of the compound was confirmed by comparing its IR and 1H NMR
spectra with that previously reported by us.
3.3. Synthesis of DAB and cyclam-cored imine based nickel
metallodendrimers
3.3.1. Synthesis and characterization of the generation 1 DAB
salicylaldimine nickel metallodendrimer complex, C1
This complex was previously reported by us and we therefore
prepared the complex in a similar manner [14]. The identity of
the complex was confirmed by IR and UV spectroscopy and
elemental analysis. The structure is shown below in Fig. 4.
3. Results and discussion
3.1. Synthesis of generation 0.5 and generation 1 cyclam-cored
polypropylene imine dendrimers, L1 and L2
Wainwright [16] previously described the synthesis of
N,N0,N00,Ns000-tetrakis(2-cyanoethyl)-1,4,8,11-tetra-azacyclotetrade-
cane which is referred to as the tetrakis(cyanoethyl)cyclam ligand,
L1 (Fig. 2), by reacting cyclam with excess acrylonitrile under
reflux in the absence of solvent via a Michael addition reaction.
Subsequent recrystalization of the oily residue leads to a white
crystalline solid. We found that the optimum conditions to prepare
this were simply the addition of the reagents employed, at room
temperature, using methanol as a solvent. This gave an improved
yield of the compound.
3.3.2. Synthesis and characterization of the generation 1 cyclam
salicylaldimine nickel metallodendrimer complex, C2
The G1 cyclam-propyl salicylaldimine dendrimer, DL2, was re-
acted with Ni(OAc)2ꢂ4H2O employing similar reaction conditions
N
N
N
N
N
N
N
N
The FT-IR spectrum of L1, shows an intense band at 2244 cmꢀ1
HO
HO
OH
OH
which is indicative of the
m
(C@N) stretching frequency. The 1H
NMR spectrum of L1 shows two peaks at 1.67 and 2.48 ppm, which
is due to the cyclam protons and another two peaks at 2.63 and
2.79 ppm for the cyanoethyl protons. L1 was further characterized
by 13C NMR spectroscopy, ESI–mass spectrometry and elemental
analysis.
We attempted several approaches to reduce the nitrile to the
primary amine. This included the use of LiAlH4, NaBH4, H2/Pd–C
Fig. 3. Generation 1 cyclam-based propyl imine Schiff base ligand, DL2.

90
R. Malgas-Enus, S.F. Mapolie / Polyhedron 47 (2012) 87–93
be assigned to
p ?
p⁄ transitions. On complexation of the periph-
eral salicylaldimine groups to the metal ion, the UV–Vis spectrum
of the cyclam based salicylaldimine nickel complex shows a third
band around 340 nm (complex spectrum) which is assigned to a
metal-to-ligand charge transfer (MLCT) transition. The weak fourth
band at 400 nm is assigned to forbidden d–d transitions, Ni(II)
coordinated to an unsubstituted cyclam core usually absorbs at
around 445 nm as reported by Hancock and co-workers [20]. When
comparing the UV–Vis spectrum of the G1 DAB salicylaldimine
nickel complex, C1, and the G1 cyclam salicylaldimine nickel
complex, C2, we observe that the latter has an extra band in its
spectrum at 400 nm, and this is thought to be due to the complex-
ation of the nickel to the cyclam core.
O
O
N
N
N
N
O
O
Ni
N
Ni
N
Fig. 4. The G1 DAB-core salicylaldimine modified nickel metallodendrimer, C1.
3.3.3. Synthesis and characterization of the generation 1 DAB
iminopyridyl nickel metallodendrimer complex, C3
to those used for the synthesis of C1 to give the cyclam-based
salicylaldimine nickel complex C2 as shown in Fig. 5.
The G1-DAB iminopyridyl nickel complex, C3 which is also no-
vel, (Fig. 6) was synthesized by reacting the iminopyridyl modified
dendritic ligand, DL3, with Ni(DME)Br2 as a metal precursor. The
reaction conditions were similar to those used for the synthesis
of C1–C2. The reaction of ligand, DL3 with Ni(DME)Br2, resulted
in a brown solid being isolated as product (yield, 85%). The imino-
pyridyl complex appears to be hygroscopic and consequently had
to be stored in a nitrogen purged glove-box. This is in contrast to
the salicylaldimine complexes which are quite air stable, as well
as stable in solution. A major difference between the salicylaldi-
mine complexes, C1–C2 and the iminopyridyl complex C3, is that
the latter has double the amount of nickel centres at the periphery
compared to the former. Thus the G1 iminopyridyl complex, C3,
has four nickel centres whereas the G1 salicylaldimine complex,
C1, has only 2 nickel centres.
The synthesized G1 cyclam-based salicylaldimine metalloden-
drimer, C2, is similar to the G1 DAB-based salicylaldimine metallo-
dendrimer complex, C1, with two metal centres on the dendrimer
periphery. However, the cyclam metallodendrimers also has an
extra nickel centre coordinated to the cyclam core. This was con-
firmed by ICP-OES, which gave a nickel content corresponding to
three nickel centres per mole of ligand. The complex is cationic
with two acetate groups as counter ions. The complex was isolated
as a lime green solid in a yield of 63%. The most significant bands
monitored by FT-IR is the shift in the
m(C@N) and the m(C–O)
stretching frequencies, which indicates that metal coordination be-
tween the nickel and the dendrimer ligand had occurred. The
m
(C@N) band shifted from 1662 to 1623 cmꢀ1 and the
m(C–O) band
from 1276 to 1319 cmꢀ1. The magnetic moment for C2 was found
to be 3.19 BM. The magnetic moment of C2 is slightly higher than
its G1 DAB analogue, C1, which has a magnetic moment of
2.95 BM. These values are within the range normally obtained for
tetrahedral Ni(II) systems [19].
From the IR spectrum, we observe a shift in the
ing frequency from around 1647 to 1639 cmꢀ1 when comparing
the ligand and complex spectra. The pyridine ring (C@N) stretch-
m(C@N) stretch-
m
ing frequency also shifted from around 1586 cmꢀ1 to around
1596 cmꢀ1 for complex, C3. The magnetic moment of this nickel
iminopyridyl metallodendrimer also gives information regarding
the geometry of the complex. The magnetic moment of the imino-
pyridyl complex, C3 is in a similar region to that of the salicylaldi-
mine complexes, being 3.64 BM. This is within the range of
magnetic moments expected for a Ni(II) centre with tetrahedral
geometry. This value is higher than those for C1 and C2 indicating
that the latter two complexes possibly have a more distorted tetra-
hedral geometry.
The ESI mass spectrum (positive ion mode) for C2 shows a high
mass peak at m/z = 1057 which can be assigned to a potassium
adduct of the molecular ion of the cationic species. A peak assigned
to the pure ligand is also observed at m/z = 846.
The UV–Vis spectrum of the G1 cyclam ligand, DL2, in the solu-
tion phase is similar to that of the DAB salicylaldimine ligand,
showing three absorption bands. Two of the bands viz. at 260 nm
and 225 nm are similar for both the DAB salicylaldimine and the
cyclam salicylaldimine ligands. The third band however differs
and occurs at 330 nm in the spectrum for the DAB ligand while
in the cyclam ligand spectrum it occurs at 320 nm. These bands
The iminopyridyl complex is less thermally stable than the
salicylaldimine complexes, C1–C2. It is also fairly sensitive to
moisture.
are assigned to
ligands.
p ?
p⁄ transitions of the aromatic units to the
The UV–Vis spectrum of the G1 cyclam complex, C2, shows four
absorption bands. The two bands observed at 260 and 225 nm are
similar to what is observed in the spectrum of the ligand and can
N
N
Ni
Br
N
N
(OAc)2
Br
Br
Ni
Br
N
N
Br
Br
N
N
N
N
N
N
N
O
O
O
O
N
2+
Ni
Ni
Ni
Br
Ni
Br
Ni
N
N
N
N
C2
Fig. 5. The G1 cyclam-based salicylaldimine modified nickel metallodendrimer, C2.
Fig. 6. Structure of DAB G1 iminopyridyl nickel metallodendrimer, C3.

R. Malgas-Enus, S.F. Mapolie / Polyhedron 47 (2012) 87–93
91
Table 1
Norbornene polymerization activity for the various catalyst precursors.a
Al/Ni
G1 DAB- Sal
TOFb
G1 Cyclam–Sal
G1-DAB Py-Im
Sal-model
TOFb
Im-model
TOFb
TOFb
TOFb
C1
C2
C3
C4
C5
1000
2000
3000
4000
5000
3.76 ꢁ 105
1.12 ꢁ 105
2.52 ꢁ 105
3.28 ꢁ 105
2.52 ꢁ 105
0
0.74 ꢁ 105
3. 00 ꢁ 105
3.29 ꢁ 105
3.60 ꢁ 105
2.37 ꢁ 105
0.75 ꢁ 105
2.59 ꢁ 105
2.26 ꢁ 105
1.86 ꢁ 105
–
0.82 ꢁ 105
2.83 ꢁ 105
2.50 ꢁ 105
2.09 ꢁ 105
–
1.98 ꢁ 105
4.68 ꢁ 105
3.96 ꢁ 105
2.04 ꢁ 105
The bold figures indicate the optimum activity for each of the catalysts.
a
Reaction conditions: catalyst: 5 lmol Ni; time: 30 min, solvent: toluene; total volume: 25 ml; temperature: room temperature; monomer: Ni = 5000.
TOF: g of polymer produced per mol of Ni per hour.
b
The ESI mass spectrum for C3 shows a high mass peak at m/
z = 1592 which can be assigned to an adduct of the parent ion con-
taining two sodium units.
In addition to the multinuclear complexes prepared we also
synthesized the known mononuclear complexes, C4 and C5. This
involved the reaction of the Schiff base ligand with either the
monofunctional salicyaldimine ligand (for C4) or the monofunc-
tional pydinyl-imine ligand (for C5).
of the pyridine-imine system relative to the salicylaldimine
catalyst, C1 could be due to the fact that the nickel centres in the
imino-pyridyl catalyst are more accessible than is the case for
the salicylaldimine catalysts. This should facilitate the interaction
of the substrate with the active centres of the catalyst.
It should however be noted that when carrying out the compar-
ative study between the DAB and the cyclam nickel precursors, the
total nickel concentration was maintained at 5 lmol.
The norbornene polymerization activities of the multinuclear
complexes were also compared to that of two mononuclear model
compounds, C4 and C5 (Figs. 7 and 8). Both the mononuclear com-
plexes displayed optimum activity at an Al:Ni ratio of 2000:1. This
was at an Al:Ni ratio lower than that for all three multinuclear
complexes. As indicated above the complexes based on dendritic
ligands have internal tertiary amine functionalities which may
interact with the Lewis acidic aluminium co-catalyst. Despite
requiring higher levels of co-catalyst to reach optimum activity,
all three multinuclear catalysts, C1–C3, show enhanced activity
relative to the two model compounds. Thus a positive dendritic
enhancement is observed for all three complexes.
3.4. Norbornene polymerization
3.4.1. Evaluation of generation 1 DAB and cyclam cored nickel
metallodendrimers, C1–C3, as norbornene polymerization pre-
catalysts
In this paper we compare the behaviour of the three generation
1 (G1) metallodendrimers C1–C3 as catalysts precursors in the
polymerization of norbornene. The catalysts were activated using
methylaluminoxane as co-catalyst. A range of Al:Ni ratios were
evaluated in an effort to obtain the optimum activity. It should
be noted that in all cases the amount of Ni in the reaction mixture
was kept constant irrespective of the precatalyst employed. The
activity results are summarized in Table 1. When comparing
the activities of the three multinuclear systems it is evident that
the cyclam cored system is the most active of the three multinu-
clear catalysts tested followed by the DAB-pyridine-imine system.
The polymerization was conducted using a range of Al:Ni ratios.
The cyclam based catalyst, C2, shows an optimum activity of
4.68 ꢁ 105 g PNB/molNi/h which is much higher than that of the
DAB-PPI based catalysts, C1 (3.28 ꢁ 105 g PNB/molNi/h) and C3
((3.60 ꢁ 105 g PNB/molNi/h). These activities compare well with
those reported for other Ni salicyaldimine complexes [21].
The optimum activity for both DAB cored systems was found to
be at an Al:Ni ratio of 4000:1 while that for the cyclam cored sys-
tem, C2, was 3000:1. It thought that the reason for C1 and C3
requiring a higher amount of Al co-catalysts is that the MAO being
Lewis acidic first coordinates to the tertiary amine functionalities
within the frame-work of the DAB based dendrimers. We have
seen similar behaviour in other catalytic processes using other
PPI dendrimers [14,22]. These tertiary amines are absent in the cy-
clam cored dendrimer. The latter is thus not able to form an acid-
base adduct with the aluminium alkyl co-catalyst. The enhanced
activity of the catalyst based on the cyclam cored dendrimer, C2,
is most likely due to the fact that in this case we are dealing with
a cationic species. It should be noted that the central metal centre
is coordinated by the tetradentate cyclam frame-work which is
neutral. The net result is that the overall complex is cationic. The
cationic centre is thought to also reduce the electron density of
the outer nickel centres which are bonded to the salicylaldimine
units. This therefore makes the metal centres to be highly electro-
philic which in turn will promote monomer coordination ulti-
mately leading to a more effective catalyst. The enhanced activity
3.4.2. Characterization of polynorbornene produced
The polymers obtained in the reactions described in the previ-
ous section, are not soluble at room temperature in common
organic solvents such as dichloromethane, THF, chloroform and
methanol. This is characteristic of vinyl norbornene polymers.
These types of polymers are reported in literature to be soluble
O
N
CH
3
Ni
H C
3
N
O
Fig. 7. Structure of mononuclear model complex, C4.
N
N
CH
3
Ni
Br
Br
Fig. 8. Structure of mononuclear imino-pyridyl model complex, C5.

92
R. Malgas-Enus, S.F. Mapolie / Polyhedron 47 (2012) 87–93
Table 2
GPC data for the polymers obtained using catalysts C1–C5.a
Al:Ni ratio
C1
C2
C3
C4
C5
Mw ꢁ 105 (Da)
PDI
Mw ꢁ 105 (Da)
PDI
Mw ꢁ 105 (Da)
PDI
Mw ꢁ 105 (Da)
PDI
Mw ꢁ 105 (Da)
PDI
1000
2000
3000
4000
5000
6.39
7.66
5.12
7.12
8.34
2.60
2.02
2.51
2.2
–
–
–
–
4.08
4.90
4.42
4.84
–
2.87
3.05
2.92
2.83
–
5.12
5.17
4.56
5.34
–
3.12
2.00
2.12
2.23
–
10.3
7.61
9.66
11.07
2.80
2.30
2.10
2.00
11.17
9.56
10.84
11.36
2.00
2.12
2.23
2.01
2.02
a
Measured at 160 °C against polystyrene standards.
in warm cyclohexane, chlorobenzene and trichlorobenzene, and
this was also the case for our polymers.
11.36 ꢁ 105 Da. For all three catalyst systems there is an overall in-
crease in molecular weight with increasing amount of co-catalyst
employed. The polymers obtained using the pyridinyl-imine pre-
catalyst generally have higher molecular weights compared to
those polymers obtained from the DAB and cyclam cored dendritic
salicylaldimine catalysts. The molecular weights obtained with
these dendritic catalysts are much higher than those obtained
using mononuclear salicylaldimine Ni complexes [21].
All polymers obtained exhibit extremely good polydispersity
indices ranging between 2.01 and 2.80. This indicates that the
molecular weights are reasonably uniform which points to the uni-
form nature of the catalytically active sites. We are thus dealing
with ‘‘single site’’ catalysts. The PDI values compare favourably
with those reported for other polynorbornene systems produced
via addition polymerization and in many cases we have PDI’s
which are superior [21,26]. Once again the pyridinyl-imine cata-
lysts, C3 performs slightly better than the other two analogues in
terms of the molecular weight distribution. It is also observed that
in all cases the PDI values tend to decrease slightly with increasing
Al:Ni ratios employed during the polymerization processes. It thus
appears that lower polydispersity indices are obtained for the
higher molecular weight polymers. This is slightly unusual since
one would expect that the longer chain growth the more the like-
lihood of chain transfer processes occurring.
3.4.2.1. FT-IR spectroscopy of isolated polymers. The polymers
obtained were characterized by FT-IR spectroscopy. The IR spectra
revealed no traces of any C@C double bonds, which often appear in
the region of 1620–1680 cmꢀ1. This eliminates the possibility of
ROMP polynorbornene. The ROMP polynorbornene would still
have the double bond intact in the polymer backbone. As men-
tioned previously, complexes of nickel tend to polymerize nor-
bornene by the vinyl addition mechanism. The IR spectrum
shows strong
m .
(C–H) absorption bands around 2943 cmꢀ1
Medium intensity bands are also observed around 1452 and
1373 cmꢀ1 clearly indicative of a saturated hydrocarbon species.
3.4.2.2. 1H NMR spectroscopy of isolated polymers. The polymers ob-
tained are soluble in warm cyclohexane or warm trichlorobenzene.
The proton NMR spectra were recorded in trichlorobenzene spiked
with C6D6 at 50 °C. The sample preparation involves heating the
polymer in a 9:1 mixture of trichlorobenzene and deuterated ben-
zene at 110 °C. After cooling, a viscous liquid was observed, for
which a proton NMR spectrum could be obtained at 50 °C. From
the 1H NMR the signals for the HC@CH protons around 5.1 ppm
are absent in the spectrum. The presence of the C@C bond usually
indicates the formation of polynorbornene by ring opening
metathesis polymerization (ROMP). Therefore we can conclude
that either vinyl addition polymerization or radical polymerization
is occurring. This spectrum also resembles that reported by Yang
et al. who produced polynorbornene via the vinyl polymerization
mechanism [23].
From the GPC data it can be seen that a similar type of polynor-
bornene is formed irrespective of the catalyst precursor used. All
the polymers obtained had molecular weights far exceeding the
1000 Da limit usually observed for polynorbornene produced via
free radical polymerization. In addition the 1H NMR spectra of
the polymers show no indication of unsaturated carbons in the
polymer backbone thus ruling out a ROMP process. It can thus be
concluded that norbornene polymerization using the dendritic
catalysts proceeds via a vinyl addition mechanism.
3.4.2.3. TGA and DSC of polymers prepared. The polymers obtained
were also studied via thermal analysis. The TGA plot and its deriv-
ative show that the polymers obtained are very stable up to 450 °C.
There’s an initial weight loss of 5% from 25 to 425 °C, and then a
90% weight loss from 425 to 475 °C. Complete decomposition
occurs after 600 °C. The DTGA plot shows an average decomposi-
tion temperature of around 450 °C. The TGA curve is similar to
that reported by Wang et al. for other polynorbornene samples
prepared via addition polymerization [24].
The DSC curve for our polynorbornene does not show an endo-
thermic peak upon heating to 400 °C. Vinyl norbornene polymers
are known to decompose before melting. The polynorbornene ob-
tained using titanium catalysts, reported by Mi et al., exhibited
glass transition temperatures (Tg) in range of 330–400 °C, and
decomposes without melting [25].
4. Conclusion
NMR and IR spectra of the obtained polynorbornene show that
no ROMP occurs in the polymerization processes but that the reac-
tion proceeds via a 2,3-vinyl addition mechanism. The molecular
weights of the obtained polynorbornenes are relatively high, rang-
ing from 105 to 106 g/mol. This means that the polymerization is
not a typical cationic or radical polymerization, which usually re-
sults in low molecular weights (molecular weight <1000 g/mol)
and low yields because of rearrangements and chain transfer
reactions. We can thus conclude that these types of nickel metallo-
dendrimer catalysts polymerize norbornene through the vinyl-
addition mechanism.
3.4.2.4. Gel permeation chromatography (GPC). Gel permeation
chromatography was conducted on the polymers obtained from
the catalytic polymerization using 1,2,4-trichlorobenzene as
solvent at 160 °C. GPC data are summarized in Table 2. All the cat-
alysts systems employed gave moderate to high molecular weight
polymers with molecular weights ranging from 5.12 ꢁ 105 to
Three different types of Ni catalysts were tested, (a) the DAB
G1–G3 salicylaldimine complexes, (b) the DAB G1 iminopyridyl
complex and (c) the cyclam G1 salicylaldimine complex.
The cyclam cored salicylaldimine complex, C2 shows the high-
est overall activity of the three catalysts tested. Its optimum activ-
ity occurs at an Al:Ni ratio of 3000:1 whereas the optimum activity

R. Malgas-Enus, S.F. Mapolie / Polyhedron 47 (2012) 87–93
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Acknowledgements
We would like to thank the NRF-DST Centre of Excellence in
Catalysis (c⁄change) and the Research Committee of Stellenbosch
University for financial support. We also thank Dr. Nyambeni
Luruli at SASOL R&D for performing the GPC analysis.
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