Introduction
1 Introduction
The catalytic polymerisation of olefins by well-defined organometallic catalysts has
traditionally been the exclusive field of the early transition metals (groups 3 – 5).
Metallocene catalysts (with a ligand set based on two η5-cyclopentadienyl moieties),
which often contain a group 4 metal in its highest oxidation state, can achieve
tremendous activities. These catalysts can readily be fine-tuned by ligand
modifications and continue to be a subject of research, both in academia and in
industry.1
Early transition metal catalysts are usually hard Lewis acids which implies formation
of strong complexes with hard Lewis bases. Olefins, however, are relatively weakly
bound since they are soft Lewis bases, binding through their π-cloud to the metal.
The combination of a hard Lewis acidic active site and a soft substrate makes these
catalysts extremely sensitive to the presence of polar molecules (containing e.g. O or
N) in the reaction medium (either as impurities, or as part of the substrate).
Therefore, they are often designated ‘oxophilic’. Polymerisation may be inhibited
because the olefin has to complete for the metal centre with Lewis bases in the
reaction medium. In the case of a hard Lewis base, coordination may even lead to
irreversible poisoning of the catalyst. In addition, early transition metal catalysts
contain highly polar Mδ+–Cδ– bonds, which are vulnerable to protonolysis (e.g. by
adventitious H2O in the medium). As a consequence, polymerisations with these
catalysts often require the use of a large excess of cocatalyst to scavenge impurities
from the reaction medium.
An associated disadvantage of the early transition metal catalysts is that they cannot
be employed for the copolymerisation of monomers with polar functionalities. The late
transition metals (groups 9 – 11) are much less oxophilic and coordination of polar
molecules is often readily reversible. The increased heteroatom tolerance allows
polymerisations to be run at lower cocatalyst concentrations. The late metals also
offer much better chances for copolymerisation of ethene or propene with
comonomers containing polar functionalities.2 The reduced oxophilicity of the late
transition metals is illustrated by the SHOP (Shell Higher Olefin Process) system for
oligomerisation of ethene, which utilises a homogeneous Ni(II) catalyst in an alcoholic
solvent.3-11 Heteroatom tolerance also enables the alternating copolymerisation of
ethene
and
carbon
monoxide
by
Pd(II)
catalysts.12
Nevertheless,
homopolymerisation of simple olefins by late transition metals has been largely
unknown for a long time, although polyethene formation had occasionally been
observed with certain SHOP catalysts.6,10,11,13,14 The failure of most Ni catalysts to
polymerise ethene to high molecular weight polyethene is related to the fact that β-H
elimination is fast, resulting in the formation of dimers15 or oligomers at most.
1.1 Late transition metals
A major breakthrough was accomplished in 1995 when Brookhart introduced Ni(II)
and Pd(II) catalysts based on α-diimine ligands with o-substituted aromatic Nsubstituents. In the presence of methylalumoxane (MAO), these compounds are
active in the polymerisation of ethene and α-olefins.
1
Chapter 1
Ar
N Ar
N
M
Br
Br
M = Ni, Pd, Ar = 2,6-iPr2C6H3
Figure 1.1
In principle, the transfer of a β-H atom from the growing polymer chain to the metal
center alone does not necessarily lead to chain termination: simple dissociation of the
olefinic polymer resulting from β-H-elimination is significantly endothermic. If the
coordinated olefinic polymer remains coordinated to the metal centre, reinsertion will
take place enabling further chain growth (Scheme 1.1). However, the olefinic polymer
can be displaced from the metal by associative coordination of a new monomer
molecule. Brookhart recognised that chain termination can thus be retarded by
blocking associative displacement of the chain through well-positioned steric
hindrance applied by the ancillary ligand.
In the Ni and Pd catalysts, the aryl rings of the ligand are forced into an orientation
perpendicular to the diimine backbone. The o-substituents effectively shield the axial
positions of the square coordination plane, thus preventing associative displacement
of the coordinated polyolefin chain by the monomer. This retardation of chain transfer
with respect to chain growth, enables the formation of high molecular weight
polymers. If aryl groups with sterically less demanding o-substituents are employed,
the catalysts produce oligomers (α-olefins).16,17 Besides their influence on the ratio of
rate of chain transfer to the rate of chain growth, the large aryl substituents also
hamper the formation of inactive [(diimine)2M] complexes.18,19
P
N
P
N
M
P
β-H elim.
M
N
N
M
reinsertion
N
H
N
C2H4
C2H4
N
N
P
+
M
M
N
H
H
N
P
Scheme 1.1 Associative chain displacement. M = Ni, Pd, P = growing polymer chain
Since Brookhart’s original findings, the use of appropriate steric protection on the
ancillary ligand to block associative chain displacement has led to many new Ni and
Pd polymerisation and oligomerisation catalysts. Many of these contain an α-diimine
2
Introduction
as the key ligand structure,18,20-26 although other neutral (N,N)-,27,28 (N,O)-,29 (P,O)30,31
(P,P)-,32-35 and (P,N)-ligands36-43 have yielded active systems as well.
The principle of shielding axial positions has also revived the development of
monoanionic ligands for neutral Ni(II) catalysts. Neutral catalysts are expected to
have a higher heteroatom tolerance than the cationic catalysts. Grubbs introduced a
family of catalysts based on sterically hindered salicylaldiminato ligands (Figure
1.2A).44,45 In the presence of B(C6F5)3 or Ni(COD)2 (COD = cis-1,5-cyclooctadiene),
these complexes polymerise ethene with high activity. Many related Ni polymerisation
catalysts with monoanionic (N,O)-ligands have been reported, some of which do not
require a cocatalyst at all (Figure 1.2B,C).19,46-53
With respect to the charge and denticity of the ligand, the Grubbs-type catalysts can
be compared to the SHOP oligomerisation catalysts. Not surprisingly, SHOP-type
Ni(II) (P,O)-chelates equipped with well-positioned steric bulk have now been
reported to produce polyolefins.54,55 Anionic (N,N)-ligands equipped with sterically
demanding groups have also been applied in Ni polymerisation catalysts.56,57
R
Ar
N
O
Ar
Ni
Ph
PPh3
A
Ar = 2,6-iPr2C6H3
R = 9-Anthracenyl
N
O
Ni
Ar
PPh3
Ph
B
Ar = 2,6-iPr2C6H3
N
O
Ni
PPh3
Ph
C
Ar = 2,6-iPr2C6H3
Figure 1.2 Neutral Ni(II) olefin polymerisation catalysts
1.2 Iron-based catalysts
In 1998, the groups of Gibson and Brookhart reported highly active catalysts based
on divalent Fe and Co producing highly linear polyethylene in the presence of MAO
cocatalyst.58,59 The catalyst precursor complexes contain a pyridine-2,6-diimine (PDI)
ligand with sterically demanding aryl groups on the imine nitrogens. In general, the
Fe derivatives are much more active than the Co derivatives. These catalysts again
demonstrate the principle of steric protection of the metal centre. Reduction of the
steric bulk on the nitrogen aryl substituents, converts the polymerisation catalysts into
oligomerisation catalysts.60-64 If the steric bulk is reduced too drastically, however,
inactive [(PDI)2Fe]2+ complexes are formed.65
3
Chapter 1
N
Ar
Cl
N M
Cl
N
Ar
[(PDI]MCl2]
M = Fe, Co, Ar = 2,6iPr2C6H3
Figure 1.3
Several attempts have been made to develop new Fe or Co based catalysts by
modification of the basic structure of the PDI ligand. Instead of aryl substituents, NR2
groups have been introduced in the PDI ligand as steric protection. The
corresponding Fe and Co complexes produce oligomers or low molecular weight
polyethene.66. Related to these derivatives are pyrrolyl-substituted PDI ligands.66,67
Replacement of one or both imine donors by amine functionalities strongly decreases
the activity of the iron complexes.68 Replacement by N-heterocyclic carbene donors
results in loss of activity with cobalt.69 Bis(iminophophoranyl)pyridine ligands have
been
employed
by
Al-Benna
et
al.
to
prepare
the
complexes
[{C5H3N(R2P=NR’)2}MX2] (M = Fe, Co). In spite of their structural similarity to the PDI
ligand, the Fe and Co complexes showed only low (Fe) to moderate (Co) activity in
ethene polymerisation in the presence of MAO.70 Variation of the central donor by
changing the pyridine for thiophene, furan, carbazole, pyrimidine or triazine moieties
also leads to significantly less active catalysts.67,71 If the pyridine donor is replaced by
the anionic pyrrolato group, the ligand coordinates to Fe(II) in bidentate mode,
forming bis(ligand)Fe complexes which are inactive upon activation with MAO.72 In
general, any modification of the basic PDI ligand structure leads to loss of activity.
Attempts to develop alternative olefin polymerisation catalysts based on Fe have met
with very limited success. In the patent literature examples have appeared of Fe(II)
precatalysts with tridentate monoanionic ligands (Figure 1.4A–C). Their activities
however, are very much lower than those achieved by the Gibson/Brookhart
system.73 Phosphinimide Fe(III) complexes (Figure 1.4D) have been found to form
modestly active systems with MAO under rather demanding conditions.74
PPh2
Me2Si
Cl
N
Fe
N
Cl
Me2Si
PPh2
A
NMe2
NAr
Cl
Fe
NAr
B
Fe
Cl
Br
Cl
Br
NMe2
C
P(tBu)3
Cl
Fe
N
N
Br
Fe
Br
P(tBu)3
D
Figure 1.4
It appears that design of new polymerisation catalysts based on Fe or Co is less
straightforward than in the case of Ni or Pd. One of the main reasons is that for the
former systems there is still uncertainty about the nature active species (see section
1.6). Possibly, the electron configuration on the metal forms a special combination
4
Introduction
with the electronic structure of the ligand. It may well be that the known redox-activity
of the PDI ligand contributes to the success of the PDI/Fe- or PDI/Co-combination.75
1.3 Activation of precatalysts
As seen from the examples in the preceding sections, active olefin polymerisation
catalysts are now available for a range of late as well as early transition metals. In
many cases, the actual active catalyst species is not isolated and used as a welldefined organometallic complex, but is generated in situ from a catalyst precursor or
precatalyst (that itself is not active) and a cocatalyst. In order to discuss the nature
and properties of the active catalyst species, the various methods used to generate
such active species are briefly reviewed.
1.3.1 Activation by alumoxanes
Many transition metal precatalysts can be activated by MAO (MethylAlumOxane)
cocatalyst.76-78 The composition and structure of MAO are not well-understood, and
therefore MAO is often represented by the formula [AlOMe]n. MAO is obtained by
controlled hydrolysis of AlMe3 and constitutes a complex mixture involving many
species and equilibria.79 Residual AlMe3 (either free or associated) present in MAO
participates in many of these equilibria and a formulation as [AlOMe]n·(AlMe3)x is
therefore more appropriate. MAO is thought to serve three important purposes in
homogeneous catalytic olefin polymerisation. Firstly, when transition-metal halide
complexes are used as catalyst precursor, MAO will methylate this precatalyst to
generate species with metal-methyl bonds. Secondly, MAO acts as a Lewis acid
species in abstracting an alkyl (or halide) anion R– from the transition metal,
generating a cationic metal-alkyl species with a weakly nucleophilic [R-MAO]– anion
as counterion. Thirdly, the excess MAO used in activating the catalyst can
conveniently scavenge impurities from the feed/medium, thus protecting the active
catalyst species. Nevertheless, there are some disadvantages associated with the
use of MAO as activator. It appears that the alkyl abstraction process and the
concomitant generation of the weakly nucleophilic anion requires a substantial
excess of MAO to be at its most effective (at least several hundreds of equivalents,
frequently more). This, combined with the relatively high cost of MAO, makes it less
attractive for industrial application. In addition, the poorly defined nature of MAO
makes it more difficult to establish the true nature of the active species in MAO
activated systems.
1.3.2 Activation by abstraction of halide or alkyl ligands
Due to the disadvantages attached to the use of MAO as activator (described above),
alternative activation strategies have been actively pursued (Scheme 1.2). Activators,
other than MAO, have been developed which react with catalyst precursors in a
stoichiometric ratio to give active catalysts with a well-defined composition and
structure. These so-called ‘single-component’ systems provide an opportunity to
study the catalytic species in much more detail. The alternative activators however,
often require alkylated metal centres in the precatalyst. In most cases, such
precatalysts are conveniently obtained by alkylation of dihalide complexes using
alkylaluminum, alkyllithium or Grignard reagents.
5
Chapter 1
An alkyl halide precursor can be dehalogenated by treatment with an alkali or silver
salt resulting in the precipitation of the alkali or silver halide. The resulting cationic
transition metal complex is left with a vacant site and its charge is balanced by the
anion provided by the abstracting agent. Alternatively, the vacancy can be created by
treatment of a dialkyl metal complex with a Brønsted acid. Protonation eliminates an
alkane and generates an alkyl cation with a vacancy. The proton is usually
introduced in the form of an oxonium or ammonium salt. The conjugate base of the
Brønsted acid (e.g. Et2O or C6H5NMe2) may coordinate to the metal centre, however
this is not necessarily problematic as these are quite weak Lewis bases, in particular
with the less electrophlic late transition metals. Nevertheless, it is mandatory that the
complementary anion displays very weak nucleophilicity (vide infra).
A third route to cationic single component catalysts involves abstraction of an alkyl
anion from a dialkyl precursor by a strong Lewis acid. Reaction of a metal dialkyl
complex [LnMR2] with triphenylcarbenium (trityl) salts provides an electron deficient
monoalkyl
cation
[LnMR]+
with
concomitant
formation
of
RCPh3.
Tris(pentafluorphenyl)borane,80 B(C6F5)3 is also frequently used as alkyl abstracting
agent. In this case, the cationic species will be accompanied by the borate anion
[RB(C6F5)3]– which incorporates the alkyl group formerly attached to the metal centre
of the catalyst precursor. A fourth method for dealkylation consists of oxidative
cleavage of the M–R bond using e.g. ferrocinium of silver salts.81
R
R
alkyl
abstraction
(Ligand)M
R
Counterion
(Ligand)M
halide
abstraction
R
(Ligand)M
X
MAO
X
dialkylation
monoalkylation
(Ligand)M
X
Scheme 1.2 Generation of cationic active species
1.4 Basic demands for catalytic activity
Although active catalytic systems described in section 1.3 are obtained from different
precursors in combination with various activators, they are believed to produce the
same cationic active species (Scheme 1.2). In all cases, at least one alkyl group
remains bound to the cationic transition metal complex. The activity of [(Cp)2ZrX2],
[(α-diimine)NiX2] or [(PDI)FeX2] precatalysts can be rationalised by invoking the
electron deficient cations [(Cp2)Zr(IV)R]+, [(α-diimine)]Ni(II)R]+ and [(PDI)Fe(II)R]+,
respectively as the catalytically active species. These cationic species [(Ligand)MR]+
all have valence electron counts of 14 (or less).
6
Introduction
For the early transition metals, it has been shown that the ionic species
[(Cp’)2ZrR][XB(C6F5)3] (R = Me, CH2Ph, X= R, C6F5) are active for olefin
polymerisation without the need for an additional cocatalyst.82-86 Brookhart and others
have undertaken detailed mechanistic studies to establish the nature and reactivity of
the late transition metal α-diimine systems. Single-component catalysts were
generated by alkyl abstraction from the alkylated precursors [(α-diimine)MR2] (M= Ni,
Pd). It was found that the electron deficient cations [(α-diimine)MR]+ are indeed
responsible for the polymerisation activity.87-91 Many theoretical studies on the αdiimine Ni and Pd catalysts have been based upon these cations as the active
species.92-102 For the Fe and Co catalysts, however, the presumed active species
[(PDI)MR]+, has not been isolated thus far (see section 1.6).
One common property of the active species is the presence of a metal carbon σbond. This is involved in the migratory insertion process with the alkene substrate,
leading to the C–C bond formation. Secondly, their low electron counts (≤ 14 VE)
suggest that at least two empty valence orbitals are required for high activity. For
insertion to take place, an empty coordination site (vacancy) is needed cis to the
metal carbon σ-bond. These three seem to be the minimal criteria for polymerisation
activity. For late transition metals, associative displacement of the growing polymer
chain has to be prevented (otherwise only dimers or low oligomers are formed). This
requires specific steric protection of the active centre. The examples of catalysts that
are mentioned in this paragraph all involve cationic active species. Although
positively charged metal alkyl catalysts indeed generally show higher activities in
olefin polymerisation than isoelectronic neutral species, the cationic nature is not a
prerequisite for activity. This is demonstrated by the highly active neutral Ni catalysts
discussed in section 1.1.
1.5 Cation-anion interactions
For those catalysts which react with an activator to give a cationic species, the
conditions of coordinative and electronic unsaturation demand that the interaction
between cation and anion is absent or weak. Therefore, the charge of the cation
must be counterbalanced by a weakly coordinating anion, since more nucleophilic
anions will occupy the free coordination site essential for polymerisation activity.
Coordination of the anion is usually a reversible process and the monomer competes
with the anion for the active site.
However, since polymerisation reactions are often performed in solvents of low
polarity, the active catalyst cannot be considered as an entirely free cation
accompanied by a ‘spectator’ anion. Due to the Coulombic attraction, the anion is
always in close proximity to the active site. However, adequate steric protection of
the metal centre may preclude close association of the ion pair. In fact, the activity of
a cationic polymerisation catalyst turns out to be directly dependent on the degree of
association between the cationic metal centre and the counterion.103,104 In cases
where the steric protection of the metal centre is insufficient, coordination of the anion
can even lead to irreversible degradation of the anion by the highly electrophilic
cation, which results in deactivation of the catalyst system. Thus, the interactions
between cation and anion determine to a large extent the thermal stability103,105 (and
consequently on catalyst lifetime) of ionic catalyst systems. Cation-anion interactions
can also have a marked influence on chain transfer characteristics and, with some
special anions, on the stereoregulation of α-olefin polymerisation.106,107 The well7
Chapter 1
known fluorinated tetraaryl borate anions [B(C6F5)4]– and [B(3,5-(CF3)-C6H3)4]– are
among the most weakly coordinating anions.103,108,109 The electronegative fluorine
atoms dissipate the negative charge on the boron atom, thus reducing the overall
nucleophilicity of the anion.
Since single-component cationic systems are well-defined ionic organometallic
compounds, these catalysts allow detailed study of cation and anion as well as of
their interactions. When B(C6F5)3 is used as abstracting agent, the presence of the R
group in the [RB(C6F5)3]– anion renders it more coordinating than e.g. [B(C6F5)4]- and,
depending on the nature of the cation, the alkyl moiety may bridge between the
boron and the metal centre. In case of a methyl bridge, the methyl hydrogens are
bent away from B and oriented toward the metal centre.82-84,110,111 If a benzyl group is
abstracted, the boron-bound benzyl is often found to coordinate to the metal by its πelectrons (Figure 1.5B).84,112,113
R
(Ligand)M
F
(Ligand)M
Me
F
B
R
F
F
A
F
F
F
F
B
3
F
F
3
B
Figure 1.5 Cation-anion interactions after methyl (A) and benzyl (B) abstraction by
B(C6F5)3
Horton et al. have correlated the 19F NMR shifts of alkyltris(pentafluorophenyl) anions
to their degree of association with cationic d0 metal cations in the ion pairs
[LnMR][RB(C6F5)3] (R = Me, CH2Ph). It was observed that a chemical shift difference
between the meta and para fluorine resonances (∆δ(m,p)) of 3 – 6 ppm is
characteristic for coordination of the alkyl moiety in the anion to the cation, whereas a
smaller value (≤ 3 ppm) for ∆δ(m.p) indicates noncoordination.114 19F NMR
spectroscopy can thus be used to differentiate between contact ion pairs and
(solvent) separated ion pairs.115 Two dimensional 19F{1H} HOESY116 (heteronuclear
NOESY) NMR spectroscopy has been applied to study the interactions between the
tetrafluoroborate anion and the electrophilic α-diimine complexes [(αdiimine)PtMe(η2-olefin)]+, which are catalytically inactive model systems for the
analogous Ni(II) and Pd(II) polymerisation catalysts. The model system confirms that
the iPr groups on the aryl nitrogen substituents effectively shield the axial positions of
the metal centre, as H···F-interactions between the anion and the Pt-methyl and
olefinic protons were not observed.117
Many new boranes and borate anions containing fluorinated aryl groups have been
synthesised with the aim of generating ever more weakly coordinating or chemically
more inert counterions for cationic olefin polymerisation catalysts.79,118-122 The central
atom of the anion is no longer the exclusive realm of the group XIII elements.
Recently, the scope of weakly coordinating anions has been widened by investigation
8
Introduction
of perfluoraryloxymetalates as counterions, which contain a transition metal as the
central atom.123
1.6 Active species in iron and cobalt pyridine-2,6-diimine
systems
From section 1.4, it becomes clear that the major criteria for homogeneous transition
metal catalysts for olefin polymerisation are electronic unsaturation and the presence
of a metal carbon bond cis to a vacant coordination site. In contrast to the welldocumented early transition metal and Ni(II) α-diimine systems, the nature of active
species in the Fe(II) and Co(II) PDI systems is not very well established. Despite
intensive research efforts, many questions about the mechanism still remain
unanswered, especially those with regard to the nature of the active species.
In analogy to the single component systems known for early transition metals and the
Ni and Pd α-diimine systems, an electron deficient, cationic alkyl species [(PDI)MR]+
(M = Fe, Co) is generally proposed to be responsible for the polymerisation activity.
Although experimental evidence for the involvement of such species is lacking,
several theoretical studies have been based on this assumption. For the cobalt
catalyst, a low spin doublet (S = 1) ground state is proposed for the 15-valence
electron alkyl cation.124 For the iron catalyst, ab initio calculations by Griffith et al. as
well as density functional calculations by Deng et al., propose a singlet (S = 0)
ground state for the 14 valence electron alkyl cation.125,126 In contrast, Khoroshun et
al., using a different functional, conclude that it must have a triplet or quintet ground
state.127
The iron catalyst precursors are usually activated with aluminoxane cocatalysts and
polymerisation results are strongly dependent on the type or activator.128,129 On the
basis of NMR data, Bryliakov has proposed dinuclear Fe-Al complexes as the active
catalyst, however none of these species has actually been isolated.130 In contrast to
the early transition metals and to the Ni and Pd catalysts, a single-component
catalyst based on the (PDI)M system (M = Fe, Co) has not been reported. This is due
to the fact that dialkylation of the dihalide precatalysts to obtain [(PDI)MR2] is not
straightforward. This hampers studies directed towards alkyl abstraction, leading to
the supposed cationic monoalkyl species. Furthermore, experimental work on the Fe
and Co systems is complicated by the paramagnetic nature of the complexes.
For the iron dihalide precursor, dialkylation appears to lead to decomposition with
concomitant formation of the dialkyl coupling product, suggesting reductive
elimination from an intrinsic dialkyl species [(PDI)FeR2].131,132 Cationic PDI Fe(II)
precatalysts without iron-alkyl bonds have been prepared by halide abstraction from
the dihalide complexes using AgSbF6. The resulting monohalide species are
stablised by MeCN or THF and have [BPh4]– as the counterion. Halide-free cations
are obtained in the presence of acetylacetonate. All these ionic systems form active
polymerisation catalysts when activated with MAO or when treated with AlMe3 (10
equiv.) in combination with B(C6F5)3 (1 equiv.).131 On the basis of EPR and
Mössbauer studies, Gibson et al. have suggested that the active species obtained
from activation with MAO contains iron in the +3 oxidation state.133
9
Chapter 1
In the case of cobalt, attempted dialkylation leads to formal reduction to the
diamagnetic cobalt(I) monoalkyl [(PDI)CoR].* The monoalkyl itself is unreactive
towards ethene, but in the presence of strong Lewis acids like MAO or
Al2Me6/B(C6F5)3 it was found to form an active polymerisation catalyst. The ionic
dinitrogen adduct [(PDI)Co(N2)][MeB(C6F5)3] is obtained upon methyl abstraction
from the monomethyl Co(I) species [(PDI)CoMe]. 1H NMR studies show that the ionic
species
slowly
polymerises
ethene
leaving
the
ethene
adduct
[(PDI)Co(C2H4)][MeB(C6F5)3] after consumption of the monomer. Note that these
precursors do not contain a cobalt-carbon σ-bond and it is still unclear how
polymerisation is initiated from the cationic species. Speculation on possible
pathways may include reductive coupling of two ethene molecules, C-H activation of
ethene or reaction of bound ethene with B(C6F5)3 to form a zwitterionic species, all of
which involve the Co(III) oxidation state.134-137 Thus far, it is not clear whether similar
redox processes as those found for Co, are also operational in the Fe case. In this
context, it is interesting that identical polymerisation activities are found irrespective
of the oxidation state of the halide precursor, i.e. [(PDI)FeCl2] or [(PDI)FeCl3]. This
probably reflects initial reduction of the Fe(III) precursor to Fe(II) by the Al
cocatalyst.59,138
The synthetic inaccessibility of the Fe(II) and Co(II) dialkyl PDI derivatives has thus
far prevented isolation of the monoalkyl cations by alkyl abstraction routes. This has
prompted the Brookhart group to study the Ru(II) and Rh(III) analogues. For these
second row transition metals, the diamagnetic target alkyl cations have been
prepared as their ethene adducts [(PDI)RuMe(C2H4)]+ and [(PDI)RhMe(C2H4)]2+,
respectively. These, however showed little to no polymerisation activity.139 Also the
Rh(I) monoalkyls [(PDI)RhR] produced no polymer upon activation with MAO.140
1.7 Electron deficient iron hydrocarbyls
If iron-based homogeneous olefin polymerisation is to be understood in full detail,
fundamental knowledge about electron deficient iron alkyls and their reactivity is
indispensable. Synthesis methodology and reactivity studies of electronically
unsaturated iron alkyls may be extrapolated to existing catalytic systems and can
possibly lead to the development of new catalysts.
Iron is found in oxidation states ranging from –II to +VI.141 However, organoiron
compounds have mainly been reported for Fe(II). Even for the very common +III
oxidation state, organometallic species are scarce in literature. The rich
organometallic chemistry of Fe(II) deals for the larger part with diamagnetic,
electronically saturated, 18-valence electron compounds. In most cases the valence
shell of iron is completed by strong-field ligands like Cp or CO.142
Literature reports on isolable, electronically unsaturated, (i.e. 14 valence electrons or
less) iron hydrocarbyls, are rather scarce. As a consequence, reactivity studies
involving such complexes are limited as well. Cationic, electron deficient iron
hydrocarbyls are completely unknown. Iron hydrocarbyls are usually obtained by
treatment of halide precursors with organolithium or Grignard reagents.142 In general,
stable compounds are isolated only if alkyl ligands without β-H atoms are employed.
*
Alkylation of the first row Mn(II) dihalide (inactive for polymerisation) has also resulted in
reduction yielding inactive formally Mn(I) monoalkyl species.194
10
Introduction
Furthermore, in order to obtain low electron counts, sterically hindered alkyls or aryls
are preferred, since these limit coordination numbers.
1.7.1 Homoleptic aryl and alkyl complexes
Homoleptic iron(II) aryl complexes143 include the neutral tetramesityldiiron [FeMes2]2
(Figure 1.6A) and the structurally similar [Fe(2,4,6-iPr3-C6H2)]2 (10 valence
electrons)144-147 in which two of the mesityls bridge the iron centres in the dimer.148-151
Only for the extraordinary bulky 2,4,6-tBu3-C6H2 (Mes*), a mononuclear, twocoordinate complex [Fe(Mes*)2] is obtained.152,153 Anionic, homoleptic aryls are
represented by [Li(OEt2)]4[FePh4] (16 valence electrons, Fe(0))154 and the
tetranaphthyl ferrate [Li(OEt2)]2 [Fe(naphthyl)4] (14 valence electrons, Fe(II)).155
The only isolated homoleptic iron alkyls are the Fe(IV) complex tetrakis(1norbornyl)iron156 (12 VE) (Figure 1.6B) and the dialkyl [Fe(C(SiMe3)3)] (10 VE).157-159
The former complex owes its stability to the fact that the iron is bonded to the
bridgehead carbons, which precludes decomposition by β-H elimination, the latter
lacks β-hydrogens. Due to steric crowding, these complexes are inert towards Lewis
bases.156 The dialkyl [Fe(CH2SiMe3)2] has been mentioned as the product of the
reaction between FeCl2 and LiCH2SiMe3, but experimental or characterisation data
are unavailable.160 Similarly, preparation of [Fe(CH2Ph)2] has been claimed, but its
isolation is doubtful.161 Anionic, homoleptic Fe(II) akyl complexes are represented by
the tetraalkylferrates [FeR4]2– which result from exhaustive alkylation of FeCl2 or
FeCl3 and can be isolated as their dilithium salts.162,163 The intermediate FeMe2
cannot be isolated and decomposes in THF at ca. –10 °C.163 Homoleptic Fe(III) aryl
or alkyl complexes are unknown.
Fe
Fe
4
Fe
A
B
Figure 1.6 Homoleptic iron hydrocarbyls
1.7.2 Diaryl and dialkyl complexes with stabilising ligands
Reaction of the homoleptic diaryls [FeMes2]2 or [Fe(2,4,6-iPr3C6H2)]2 with Lewis
bases gives rise to 3- or 4-coordinated diaryl complexes of the type [LnFeMes2] with
valence electron counts of 12 or 14 (L = phosphane, aminophosphane, phosphite,
chelating diphosphane, (o-disubstituted) pyridine, chelating diamine, α-diimines,
nitrile, n = 1,2 depending on the steric demands of L).145,147,149,151,164 A systematic
11
Chapter 1
investigation of the geometries and electronic structures of [L2FeMes2] compounds
was reported very recently by Chirik and coworkers.165 High-spin, S = 2 tetrahedral
complexes are observed with monodentate or chelating amines. Monodentate
phosphanes and phosphites form planar, S = 1 complexes. With chelating
phosphanes, both geometries are possible, depending on the substituents on the
chelating P atoms. Aryl substituents yield planar structures, while alkyl substituents
furnish tetrahedral compounds. Alternatively, stabilised iron diaryls can be obtained
by arylation of halide precursor complexes. Thus, Chatt and Shaw prepared the
planar, S = 1 bisaryl trans-[(PEt2Ph)2FeAr2] (Ar = C6Cl6) by reaction of the
corresponding dichloride with C6Cl6MgBr.165,166 The use of o-disubstituted aryls
seems crucial for stabilisation of the planar S = 1 configuration.165,166 The doubly
orthometallated
complex
[κ4C,C,N,N{(C6H4CH2)(CH3)NCH2CH2N(CH3)(CH2C6H4)}Fe] is stabilised by coordination of the
diamine moiety in the bisaryl ligand.167 Similar stabilisation by a Lewis basic side arm
is found for the bisaryl complex [{κ2C,N-(C6H4P(Ph)2=NSiMe3)}2Fe].168
Electron deficient Fe(II) dialkyls have been stabilised by chelating diphosphane
ligands with sterically demanding substituents. The complexes [(dippe)FeR2] (Figure
1.7A) (dippe = 1,2-bis(diisopropylphosphino)ethane) were obtained by treatment of
the corresponding dibromides with dialkyl magnesium reagents (dippe = 1,2bis(diisopropylphosphino)ethane).169 With smaller alkyls (e.g. Et or tBu),
decomposition in benzene or toluene takes place above 0 °C, producing
[(dippe)Fe(η6-arene)] which is assumed to result from solvent trapping by intrinsic
decomposition products.170 Nitrogen donor ligands have also been reported to
stabilise 14 VE iron alkyls. The complex [(TMEDA)FeCH2Ph)2], (TMEDA = N,N’tetramethylethylenediamine) was only observed by 1H NMR spectroscopy.171,172 Alkyl
ligands bearing a pyridine donor have successfully been employed in the tetrahedral
complexes [Fe(R2C(2-py))2] (Figure 1.7B) and [(TMEDA)Fe(R2C(2-py))] (14 VE).173175
Chirik recently reported a series of Fe(II) dialkyls supported by diimine or diamine
ligands (Figure 1.7C–E).176 Alkyl ligands with considerable sterical demands like
benzyl, (trimethylsilyl)methyl or neopentyl are required. Attempts to introduce smaller
alkyls, e.g. methyl, ethyl or benzyl, resulted in reduction to Fe(0) and dissociation of
the ancillary ligand.
12
Introduction
iPr
P
SiMe3
Me3Si
iPr
N
R
Fe
Fe
P
iPr
N
R
SiMe3
SiMe3
iPr
C
R = CH2SiMe3, CH2CiMe3, CMe3,
CH2CMe2Ph, CH2Ph, CH2C6H4Me
iPr
D
O
iPr
N
N
Fe
iPr
R
C
R = CH2SiMe3
Me3C
Fe
iPr
R
N
N
N
R
O
D
R = CH2SiMe3, CMe3, CH2Ph
CMe3
Fe
R
R
N
R
E
R = CH2SiMe3, CH2Ph
Figure 1.7 Stabilised Fe(II) dialkyls
The only Fe(III) dialkyl complex was reported very recently. The dimethyl complex is
stabilised by a monoanionic 1,8-bis(iminocarbazolide) ligand (Figure 1.8).177
Ph
N
N
Fe
Me
Me
N
Ph
Figure 1.8
1.7.3 Monoalkyl complexes with stabilising ligands
Iron(II) monoalkyls with 14 valence electrons have been prepared using substituted
derivatives of hydrotris(pyrazolyl)hydroborate.178 In spite of its formal electron
deficiency, the ethyl complex [(TpiPr)FeEt] (Figure 1.9A) is resistant to β-H
elimination.179-182 A monoanionic mixed P,N-donor ligand was used by Fryzuk to
stabilise the 14 valence electron iron monoalkyls [{N(SiMe2CH2PPh2)2}FeR] (R =
CH2SiMe3, CH(SiMe3)2).183 Gibson has reported a 14 VE methyl Fe(II) complex
supported by a monoanionic 1,8-bis(iminocarbazolide) ligand (Figure 1.9B).177 As a
part of our own research strategy (section 1.8), we have employed the β-diketiminate
ligand to synthesise Fe monoalkyls with a formal electron count of 12VE (Figure
1.9C). In the course of this research the group of Holland has also intensively studied
this family of Fe(II) monoalkyls with a wide variety of hydrocarbyls.184-187 A more
detailed discussion can be found in Chapter 4.
13
Chapter 1
Fe(III) monoalkyls and monoaryls with 15 VE have been isolated with diananionic
tetradentate ligands that enforce square pyramidal geometry, e.g. porphyrin or salen
(N,N’-ethylenebis(salicylaldiminato)).188-192 The iron-carbon bonds in these
complexes however, are of limited stability due to rapid homolysis.193
H
iPr
iPr
iPr
B
R'
N
N
N
N
N
N
N
N
iPr Fe
iPr
R'
Ar
iPr
Fe
Me
N
Ar
Ar
N
N
Fe
Ar
R
CH3
A
B
Ar = 2,4,6-Me3C6H2
C
Ar = 2,6-iPr2C6H3,
R’= tBu, Me
Figure 1.9
1.8 Research objectives and strategy
Active species in homogeneously catalysed olefin polymerisation are generally
cationic, electron deficient (i.e. formal electron count ≤ 14) transition metal alkyls, but
for the (PDI)Fe-system the nature of the active species is still unclear. Therefore,
fundamental research directed at synthesis and characterisation of new, electron
deficient iron alkyls can be valuable for the design of new Fe-based polymerisation
catalysts. Such compounds are still relatively rare in literature and consequently, the
generation of cationic iron derivatives through alkyl abstractions has remained an
unexplored field of research. Alkyl abstraction studies may yield useful information on
interactions of highly electrophilic cations with substrates, solvents and anions. This
information can provide further insight into mechanisms of propagation, inhibition and
deactivation of Fe-based catalysts. Thus far, only one type of iron catalyst, based on
the PDI ligand framework, has proved to be highly active for olefin polymerisation and
it is not clear why Fe(II) and the PDI ligand form an apparently uniquely effective
combination. This forms an incentive to develop new ligands for use with Fe as
polymerisation catalysts.
As the iron(II) pyridine-2,6-diimine system has so far not proved to be very suitable
for studying organoiron derivatives, it is not the ligand of choice for this research.
However, some of its properties will be conserved in the ligands to be used in this
research. The ligands must provide adequate steric protection to the Fe centre. This
limits the number of ligands around Fe(II), thus ensuring complexes with low electron
counts. For the same reason, ligands should donate no more than 6 electrons to the
metal. One approach is to change the ligand system from a meridional N3 system to
a facial one while keeping the number and type of donor atoms unaffected. One
intrinsic difficulty with neutral ligands is the stability of iron dialkyls which in general
14
Introduction
suffer from reductive elimination. Therefore, monoanionic ligands form an attractive
alternative since they will lead to neutral, monoalkyl Fe(II) species.
Characterisation of the iron halides and hydrocarbyls can be complicated due to the
expected paramagnetism of these compounds. NMR spectroscopy may not always
reveal structural information, although it can still be useful to provide ‘fingerprints’ of
the compounds. X-ray diffraction will therefore be an important complementary
characterisation method. The targeted Fe(II) alkyls will be screened for insertion
chemistry with olefins. Although no reports have appeared to date on neutral Fe(II)
monoalkyls with high catalytic activity olefin polymerisation, it cannot be excluded a
priori that highly electrophilic Fe(II) monoalkyls may be active.
In order to further increase the electrophilicity of the iron centre, generation of
cationic species will be investigated using perfluoroarylborane and -borate alkyl
abstracting agents. In the case of Fe(II) monoalkyls, alkyl abstraction will lead to
cations without an Fe–C bond required for olefin insertion, and therefore catalytic
activity may not be expected. However, the lack of a reactive Fe–C bond also permits
study of interactions of the cations with alkenes, solvents, Lewis bases and
associated anions without interference of reactions involving the Fe–C bond. 19F
NMR spectroscopy can be a valuable tool to investigate these interactions.
1.9 Outline
The four subsequent chapters (2 – 5) deal with the synthesis and characterisation of
electron deficient iron alkyls supported by anionic and neutral nitrogen donor ligands.
Chapter 2 starts with an overview of the rich coordination chemistry of the amidinate
ligand as well as its applications and synthesis. The sterically demanding
benzamidinate ligand [PhC(NAr)2]– (Ar = 2,6-iPr2C6H3) is chosen for stabilisation of
electron deficient iron alkyls. Complexation to Fe(II) is described as well as the role of
added Lewis bases in attempts to obtain monoalkyl derivatives.
Chapter 3 describes the Fe(II) coordination chemistry of two functionalised
benzamidinate ligands [RNC(Ph)NCH2CH2NMe2]– (R = Ar, SiMe3). These ligands
have an N,N-dimethylaminoethyl substituent on one of the nitrogen atoms, making
the ligand a potential tridentate donor. The preparation and characterisation of
dinuclear chloro complexes and their reactivity towards alkylating agents will be
discussed.
In Chapter 4, the chelate ring of the monoanionic bidentate ligand is expanded by 2
carbon atoms by the use of β-diketiminates, [HC{C(Me)NAr}2]–, which are expected
to bind more strongly to Fe(II) than the amidinates. First, this type of ligand is briefly
introduced by discussing its synthesis, coordination chemistry and applications.
Synthesis and characterisation of Fe(II) monoalkyls and some exploratory reactivity
studies will be reported.
In Chapter 5, an approach is chosen that differs from that of the previous three
chapters. Here, a neutral tridentate nitrogen donor ligand is introduced. The pyridylsubstituted β-diketimine ligand [HC(2-py)(C(Me)NAr)2], contains the same number
and type of donor atoms as the PDI ligand used by Brookhart and Gibson. In contrast
to the PDI ligand, this ligand can only coordinate facially to a metal centre. Ligand
synthesis is reported, as well as its introduction onto Fe(II) and alkylation of the iron
complex.
15
Chapter 1
In Chapter 6, the reactivity of the Fe(II) alkyls isolated in Chapters 2 – 4 towards
several borane and borate alkyl abstraction reagents is investigated. The fluoroarylsubstituted borate anions make 19F NMR spectroscopy a valuable tool to probe the
association of the generated ion pairs. Very weak interactions can be revealed due to
the paramagnetism of the d6 Fe(II) cations, which induces large shifts and significant
line broadening in the 19F NMR spectra.
The work described in the chapters 2 and 3 revealed that amidinates are kinetically
quite labile ligands. This resulted in ligand redistribution reactions, yielding Fe(II)
bis(amidinate) complexes that apparently are thermodynamically favoured. Due to
this stability, bis(amidinate) complexes offer good perspectives for an in-depth study
of the coordination chemistry of sterically hindered amidinates. In Chapter 7, a series
of bis(amidinate) complexes of the 3d metals with sterically demanding
benzamidinate ligands is described. Structural parameters, spectroscopic and
magnetic properties of the bis(amidinate) complexes are compared to assess the
effect of metal and ligand variation. Some exploratory reactivity studies are included
as well.
16
Introduction
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