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Ancillary ligand effects in organoyttrium chemistryDuchateau, Robbert

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1

General Introduction.

1-1. Organometallics in Catalysis.

Although the development of organometallic chemistry and homogeneous catalysis started

separately in the early fifties, these fields of research have become firmly intertwined during

the last decade. This stems from the fact that transition metals play a key role in many C-X (X

= C, H, hetero-atom) bond transformations. Industrially important catalytic processes such as

isomerization, polymerization, hydrogenation, hydroformylation and oxidation of olefins as well

as (oxidative) coupling of alkynes involve transition metal-carbon or transition metal-hydrogen

σ-bonds.1 Even when the metal complexes do not act as catalysts, their template function can

tune or facilitate metal ‘mediated’ reactions.

The research in coordination and organometallic chemistry, recently strongly supported by

theoretical studies,2 has provided much information for the understanding of the fundamentals

of homogeneous catalysis. To obtain more insight into the mechanisms of catalytic processes

involving M-C or M-H bonds, detailed studies of the properties and behavior of transition metal

complexes containing these functions is required. The use of ancillary ligands such as

cyclopentadienyls, phosphines and alkoxides resulted in well-defined metal carbyl and hydrido

complexes which cleared the way for detailed reactivity studies. Nowadays, several examples

of rational catalyst design, based on ‘fine tuning’ of the ancillary ligand system, are known. In

the rhodium-catalyzed hydroformylation of olefins1,3 and in the palladium-catalyzed

methoxycarbonylation of propylene (selective production of methyl methacrylate)1,4 and

copolymerization of ethylene/CO,1,5 high reaction rates and selectivity were obtained by

adjusting the steric and electronic properties of the ligand system. In early transition metal

chemistry (group 4, 5), a major break-through was achieved with the introduction of the

cyclopentadienyl (Cp) group as a stabilizing ligand,6 whereas the

bis(pentamethylcyclopentadienyl) set has become the predominant coordination environment

in organolanthanide and group 3 metal chemistry.7 Optimalization of the catalyst performance

by varying the bis(cyclopentadienyl) ligand system or the metal (group 3, 4 metals or

lanthanides) has led to the development of ‘single-site’ catalysts, capable of polymerizing a

1

Chapter 1.

2

wide variety of α-olefins in a highly stereoselective (and even enantioselective) fashion, with

an activity up to three times higher than that of conventional Ziegler-Natta catalysts.1,8

+

Zr R Zr R

+

Zr R

+

Zr R

+

Si

1980 1985 1986 1988

Figure 1. Development of cationic zirconocene systems used in α-olefin polymerization.

In addition to α-olefin polymerization, these metallocenes are capable of catalyzing a wide

range of other reactions such as olefin hydrogenation,9a-c hydroboration,9d-f hydrosilylation,9g-h

amino-olefin hydroamination/cyclization,9i-j acetylene oligomerization and polymerization,9k-n

amino-acetylene hydroamination/cyclization,9o dehydrogenative coupling of silanes9p-q and a

variety of nucleophilic addition reactions of substrates containing carbonyl groups.9r-t

1-2. Group 3 Metals and Lanthanides.

The chemistry of group 3 metals and lanthanides has developed from a minor to a very

large and important area in organometallic chemistry.10 In the past, the bonding in

organolanthanide complexes was considered to be essentially ionic. Hence, the chemistry of

these systems was assumed to be more related to that of group 1 and 2 metals than to the

early transition metals. However, the discovery that organolanthanide carbyl and hydrido

complexes can activate small molecules such as CH4, N2, CO, olefins and acetylenes, in a

similar way as their early transition metal congeners, has changed this view dramatically. In

the recent literature, a variety of examples can be found demonstrating that the performance

of lanthanide (or group 3 metal) based catalysts approaches or even exceeds that of early

transition metal analogues.7-9

General Introduction.

3

Properties of Group 3 Metals and Lanthanides. Although scandium and yttrium are

formally not f-elements, their chemical behavior and structural properties are very similar to

those of the lanthanides. Apart from a few exceptions,10b,11 the most common oxidation state

for lanthanides is trivalent. This is caused by their low ionization energies and results in the

characteristic high Lewis acidity of these metals.12 The high similarity between the electronic

properties of group 3 metals and lanthanides results from the low energy lanthanide 4f orbitals.

Calculations indicate that the lanthanide 4f valence orbitals do not extend beyond the electron

density of the filled 5s and 5p orbitals.13 As a consequence, interactions with the 4f orbitals are

of little or no importance. Since small differences in size of the coordination gap can have a

dramatic effect on the reaction rate and selectivity,7-9 with ionic radii ranging from 0.745 Å

(Sc3+) to 1.032 Å (La3+), the desired reactivity of the electronically very similar group 3 metals

and lanthanides can be tuned by varying the metal.

Group 3 metal and lanthanide compounds are strong Lewis acids. The need for electron

density is often satisfied by the formation of ‘ate’ compounds, or by coordination of Lewis

bases.14 If this is not possible, even interactions with the electron density of C-H or Si-C bonds

(agostic interaction) can serve to reduce the electron deficiency of the metal center.7e,f,l, 15 Due

to the strong Lewis acidity of these metals, M-X (X = C, H, hetero-atom) bonds are highly

polar. Therefore, these bonds are liable to react with polar substrates, while sometimes the

polarity of the M-X bond is even sufficient to polarize otherwise inert C-H bonds, resulting in

hydrogen transfer, often referred to as ‘C-H bond activation’.7-9,15d,16

+RLnLn +XLnLn

X

H

R

LnLnδ+ δ+

δ-

δ-

H2C C

R'

H+RLnLn LnLn

R'R

C

C

R

LnLn

H

H

H

R'

δ+

δ-

δ+

δ-

X H R H

Chapter 1.

4

Figure 2. σ-Bond metathesis reactions: X-H (X = H, C, hetero-atom) activation and migratory

insertion of an olefin.

General Introduction.

5

Compared with middle and late transition metals, the bond dissociation energies of the Ln-

X bonds are high.17 Therefore, homolytic bond breakage or formation is generally not

observed and the chemistry of early transition metals and lanthanides is dominated by

concerted σ-bond metathesis reactions such as C-X bond activation and migratory insertion of

unsaturated substrates (olefins, alkynes, ketones, nitriles), which involve polar four-centered

transition states (Figure 2).

Another important property of these metals is the small difference in bond dissociation

enthalpy between Ln-C and Ln-H bonds.17 As a consequence, β- and γ-hydrogen elimination

are frequently observed. As an example, while {Cp*2Ln(µ-H)}2 complexes are excellent

catalysts for the polymerization of ethylene, they fail to polymerize propylene due to

competitive γ-hydrogen transfer resulting in inactive η3-allyl species, Cp*2Ln(η3-C3H5).7d

The electronic properties of neutral group 3 and lanthanide species are comparable to

those of their isoelectronic cationic group 4 metal analogues and exactly this feature conceals

the power of group 3 metals and lanthanides. Neutral group 3 or lanthanide single component

catalysts are directly accessible, whereas group 4 metal species have to be activated by a

cocatalyst (borates, MAO) before they show catalytic activity.6,8,9 Consequently, isolation and

characterization of the latter is complicated by their multi-component composition and, not

seldomly, by only a small percentage of the transition metal centers being active. Although it

has to be stated that for many catalytic reactions, the cationic group 4 metal systems are

catalytically more active than their neutral group 3 or lanthanide congeners, it is clear that the

latter are very useful for fundamental, reactivity and mechanistic studies.

1-3. Ancillary Ligands.

As already mentioned above, ancillary ligands are crucial for the stability and reactivity of

organometallic compounds. As with the early transition metals, lanthanide (and group 3 metal)

chemistry has historically been dominated by cyclopentadienyl ligands. The organometallic

chemistry started in 1954 with Cp3Ln compounds.18 Much of the chemistry that followed

involved bis(cyclopentadienyl) systems, Cp2LnX(L) (X = functional group, L = Lewis base).19

Modest effort, by a limited number of research groups, was put into the development of this

area and, for a long time, this chemistry remained in the shadow of that of the (early and late)

transition metals. A major break-through was achieved in 1980 with the introduction of the

pentamethylcyclopentadienyl (Cp*) group.20 Due to its special combination of steric and

Chapter 1.

6

electronic properties, the bis(pentamethylcyclopentadienyl) ligand system has become the

predominant coordination environment in group 3 and lanthanide chemistry.7-9 Due to the

many similarities with the cationic group 4 metal metallocenes which are well known for their

catalytic activity in α-olefin polymerization reactions,6,8a-i the chemistry of Cp*2LnR complexes

has developed rapidly to become the most extensively studied organolanthanide system.

Of course, different ligand systems have been studied, but with the ‘never change a

winning team’ philosophy in mind, most modifications to influence the reactivity of these

compounds have been restricted to variations in the cyclopentadienyl substituents.8,9,15d,21

Although it seems certain that the dominance of the Cp* and related ligands will continue for

some time, there must be much more group 3 metal and lanthanide chemistry beyond that of

Cp*.

Recently, a number of papers appeared describing the application of mono-Cp* or non-

cyclopentadienyl ancillary ligand environments. Some examples of alternative ligands used in

group 3 metal and lanthanide chemistry are carboranes,22a,b cyclopentadienyls with a pendant

amido function,22c,d alkoxides/aryloxides,22e-g porphyrinogens,22h porphyrins,22i,j

pyrazolylborates,22k,l amidodiphosphines,22m-o cyclooctatetraene,22p diiminophosphines22q and

diphosphinomethanides.22r Remarkably, most papers only report the synthesis of the

complexes. The only complexes for which their potential in catalysis has been investigated are

[Cp*[C2B9H11]ScR]-,22b [η5:σ-(C5Me4)Me2SiN(CMe3)]ScR,22c,d Cp*[ArO]YR,22e [OEP]YR22i

and {[(R2PCH2SiMe2)2N]Y(µ-Cl)(η3-C3H5)}2 (Figure 3).22o It appears that replacing

cyclopenta-dienyl ligands by hetero-atom functionalities shows divergent chemistry. The most

successful modification so far was introduced by Bercaw et al. , who replaced the

pentamethylcyclopentadienyl ligands in Cp*2ScR by a tetramethylcyclopentadienyl group

containing a pendant amido functionality. The resulting, ‘single component, single site,

constrained geometry’ catalyst, [η5:σ-(C5Me4)Me2SiN(CMe3)]ScR, showed an increased α-

olefin insertion rate and α-olefin tolerance compared to Cp*2ScR.22c,d Subsequently, other

academic and industrial groups have expanded this idea, introducing modifications in the

bridging unit and/or the metal.23 The fact that Dow Chemical Co. has commercialized α-olefin

polymerization processes based on group 4 metal catalysts stabilized by this type of ligand,

clearly emphasizes its success.23g Replacement of cyclopentadienyl ligands by other hetero-

atom containing substituents generally resulted in an overall decrease in reactivity of the

system,22 compared with that of the corresponding Cp*2LnR compounds.

General Introduction.

7

O

OM

R'

R'

R

BC

B

CB

B BB

M RN

M

tBu

Me2SiR O

MR

MNN

N N

RN

NN

N

MR N

N N

BH N N M R

2

N M

Me2Si

Me2SiPR2

PR2

R

2

A B C D

E F G

H I J

2

P

N

M

N

Ph

Ph

SiMe3

SiMe3

R

2

C

P

M

P

RR'

Figure 3. Several examples of alternative ligand systems used in group 3 metal and lanthanide

chemistry: (A) Cp*(carborane), (B) cyclopentadienyl with pendant amido functionality, (C)

Cp*(aryloxo), (D) bis(naphtholate), (E) porphyrinogen (OEPG), (F) porphyrin (OEP), (G)

tris(pyrazolyl)borate, (H) amidodiphosphine, (I) diiminophosphine, (J) diphosphinomethanide.

What Makes the Cp* Ligands so Special? Cp* ligands are sufficiently electron donating

to satisfy very electrophilic metal centers, whereas their steric bulk is large enough to prevent

oligomerization of the compounds which often leads to catalytically inactive species. The

methylated ring also provides extra solubility to the compounds. Due to the charge

delocalization within the η5-bonded ring, the Brønsted basicity of the ligand is low and

protolysis is essentially not observed. The main disadvantage of pentamethyl-cyclopentadienyl

Chapter 1.

8

complexes is the facile intramolecular metalation upon thermolysis, yielding fulvene species.7e-

h However, for compounds with M-R (R = alkyl, hydride) bonds so reactive that they even

attack alkanes, this is not so surprising.7-9,15d,16

1-4. Goal and Survey of this Thesis.

Our interest in this field of chemistry is aimed at obtaining a better understanding of how,

and to what extent, electronic and steric properties of ancillary ligands influence the reactivity

of a complex. When these effects are understood, these insights can be used rationally to

design and tune (new) catalyst systems.

Ln

L

L

RLn R Ln

L

R

Figure 4. Stepwise replacement of Cp* ligands by alternatives (L).

When deciding in which direction to proceed, we argued that stepwise replacement of Cp*

ligands in the bis(pentamethylcyclopentadienyl) ancillary ligand environment by alternative

hard Lewis basic functionalities would gradually render the metal more electron deficient

(Figure 4). This increase in electrophilicity of the metal center is expected to increase the

polymerization activity for α-olefins, since chain termination by β-hydrogen transfer will be

suppressed due to the thermodynamic instability of the hydride formed.24 Furthermore, a

decrease in electron density at the metal center in cationic ethylene bridged bis(indenyl)

zirconium systems has been found to lead to an increase in stereoselectivity of propylene

insertion.21

General Introduction.

9

Ligands that could serve as alternatives for Cp*, suited for our purpose, should satisfy several

demands:

� They have to be monoanionic hard Lewis base ligands.

� They have to be inert ‘spectator’ ligands, that show no competetition with the Ln-R

bond.

� They should have sufficient steric bulk to prevent oligomerization of the compounds,

which normally leads to poorly soluble, catalytically inactive species.

� They should formally be 6 or less electron donors.

� Their synthesis should be easy, with the possibility to modify the steric and electronic

properties.

Choice of Alternative Ancillary Ligands. The general type of ligands shown in Figure 5

was chosen because it allows full freedom of adjusting steric and electronic properties. Many

varieties are known to be excellent supporting ligands and they are easy to synthesize or even

commercially available. Molecular Mechanics studies were used to determine the size of the

X, Y and Z substituents required to approach the steric bulk of the bis-Cp* ligand set.

X, Z = C, N, O, P, S; Y = C, Si, N, S.

XY

Z

-

N

C

N

R

R"R' -R = R" = CMe3R = Ar, Me, H

R', R" = SiMe3, CMe3, C6H11, Ph

N

Si

O R"R -

Figure 5. Left: general type of the chosen ligands. Right: ligands of choice: amidinates and

alkoxysilylamides.

Eventually, two different types of ligands were selected for further investigation: amidinates

and alkoxysilylamides (Figure 5). The most extensively studied amidinate is the N,N’-

bis(trimethylsilyl)benzamidinato ligand. Pioneering work of Dehnicke,25 Roesky,26 and

Chapter 1.

10

Edelmann27 demonstrated the stabilizing ability of this ligand in the chemistry of main group

metals, transition metals and f-elements. The first report of the N,O-bis(tert -

butyl)alkoxydimethylsilylamide used as a supporting ligand was by Veith et al. who showed

that this chelate is ideally suited for the stabilization of coordinatively unsaturated main group

metal complexes.28 Subsequently, Edelmann et al . extended this work to lanthanide

chemistry.29

When we started our work, papers exclusively dealing with synthetic aspects of simple

coordination chemistry involving these ligands had appeared. For lanthanides, the metals of

our interest, exclusively homoleptic or halide derivatives were known and no effort was made

to use these ligands as stabilizing ancillaries in catalysis. We considered the possibility that the

amidinato and alkoxysilylamido ligands can stabilize alkyl and hydrido derivatives, which could

be active as catalysts in C-X bond activation or insertion reactions. Since the steric bulk of

these particular ancillary ligands can easily be tuned and their electron donating capacity is

expected to be lower than that of Cp*, they were thought to be ideal to probe the influence of

different steric and electronic properties of ancillary ligands on the reactivity of the compounds.

Selection of Metal and Ligands. Although a variety of electronically and sterically different

amidinato and alkoxysilylamido ligands have been applied as supporting ligands for several

metals (group 3, 4 metals, lanthanides),30 in this thesis we have limited ourselves to the N,N’-

bis(trimethylsilyl)benzamidinate and the N,O-bis(tert -butyl)alkoxydimethylsilylamide as

supporting ligands in the chemistry of yttrium. Both ligands were chosen because of their

proven stabilizing ability in main group metal and lanthanide complexes. Furthermore,

Molecular Mechanics calculations showed that the steric bulk of these ligands is comparable

to that of Cp* (for details see Chapter 7), which is necessary to prevent extensive

oligomerization of the compounds formed. Since we wanted to investigate the influence of the

ligand environment on the reactivity of a system and compare the results with known

chemistry, yttrium was the metal of choice. Of all the group 3 metals and lanthanides, yttrium

has been subject of the majority of any reactivity studies, directed towards the influence of

ligand variation on the reactivity. Systems that have been investigated for this purpose are

[C5H4R’]2YR (R’ = H, Me),19 Cp*2YR,7f,h Cp*[ArO]YR22d and [OEP]YR.22h Another advantage

of investigating yttrium along with the structurally and chemically similar lanthanides is that Y3+

is diamagnetic, which allows straightforward characterization of the derivatives by 1H and 13C

NMR spectroscopy. A particular attractive feature of yttrium is that 89Y is a 100 % natural

abundance I = ½ element. As a consequence, it can provide valuable structural information via

General Introduction.

11

Y-C and Y-H coupling. An additional advantage of this I = ½ nucleus is that it can be studied

by 89Y NMR spectroscopy.31

Survey of the Thesis. The aim of the work presented in this thesis is to get a general

picture of how the character and reactivity of Y-R (R = alkyl, hydride) bonds change as a result

of variations in the steric and electronic properties of the ancillary ligand system.

Several ancillary ligand systems were introduced to develop suitable coordination

environments that could function as alternatives for the Cp*2YR system. All synthesized

complexes were extensively characterized. When possible, alkyl and hydrido complexes,

potential catalysts or catalyst precursors, were structurally characterized. Furthermore, prior to

the investigation of the reactivity of these compounds, their thermal stability and behavior in

common solvents was tested. Subsequent studies were directed towards similarities and

differences in reactivity of these systems, compared with known chemistry. Therefore, the

choice of substrates was limited to a selection of those previously used to test the reactivity of

[C5H4R’]2YR (R’ = H, Me), Cp*2YR, Cp*[ArO]YR, [OEP]YR and [Cp2ZrR]+ (R = carbyl,

hydride). These include olefins, alkynes, and hetero-atom containing unsaturated substrates

such as carbon monoxide, nitriles and pyridines.

In Chapter 2 and 3, the attention is focussed on the chemistry of the bis(N,N’-

bis(trimethylsilyl)benzamidinato) yttrium system. In Chapter 2, the synthesis and physical

characterization of a wide variety of bis(benzamidinato) yttrium complexes is described. A

suitable starting material to develop this chemistry was found to be

[C6H5C(NSiMe3)2]2YCl.THF (2.2). Chloride metathesis of 2.2 appeared to be facile and

provided a variety of derivatives. Treatment with alkylating reagents resulted in the formation

of neutral and anionic carbyl species. Subsequent hydrogenolysis of the neutral carbyls is a

convenient route to the first non-cyclopentadienyl yttrium hydrido derivatives [p-X-

C6H4C(NSiMe3)2]2Y(µ-H)}2 (X = H, MeO). In Chapter 3, the potential of the bis(N,N’-

bis(trimethylsilyl)benzamidinato) yttrium alkyl and hydrido complexes in C-X (X = H,

heteroatom) bond activation and migratory insertion reactions was tested. The different ligand

environment, when compared with the corresponding Cp*2YR complexes, appears to have a

pronounced influence on the reactivity; the chemistry of the bis(benzamidinato) yttrium system

is dominated by the formation of catalytically inactive dimers. Chapter 4 and 5 deal with the

chemistry of the bis(N,O-bis(tert -butyl)alkoxydimethylsilylamido) yttrium system. In Chapter 4,

the synthesis and characterization of several bis(alkoxysilylamido) yttrium derivatives is

described. As with the corresponding bis(benzamidinato) yttrium mono chloro THF adduct,

Chapter 1.

12

[Me2Si(NCMe3)(OCMe3)]2YCl.THF (4.2) is a useful starting material for a variety of

complexes. In a separate section, a survey of the reactivity of [Me2Si(NCMe3)(OCMe3)]2Y-

CH(SiMe3)2 (4.6) towards unsaturated substrates and dihydrogen is described. A further

extension of this chemistry can be found in Chapter 5 where the synthesis of the bis(N,O-

bis(tert -butyl)alkoxydimethylsilylamido) yttrium pyridyl and α-picolyl derivatives and their

reactivity towards a variety of unsaturated substrates is examined. These reactivity studies

clearly indicate the limited applicability of alkoxysilylamides as supporting ligands. Their high

Brønsted base character results in undesired protonation and loss of these ligands in the

presence of acidic protons or upon heating. In Chapter 6 some preliminary results concerning

the synthesis of mixed Cp*-benzamidinato yttrium complexes are reported. Although this work

is not complete, some interesting results with respect to stability and reactivity have been

observed. Finally, in Chapter 7 the steric and electronic properties of the various ligand

systems are described and compared with the bis(pentamethyl-cyclopentadienyl) and

bis(cyclopentadienyl) ligand sets. Although distinguishing between steric or electronic effects is

often difficult, much of the reactivity observed for the various systems is in good agreement

with both their steric and electronic properties.

References and Notes.

1 (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis , Wiley-Interscience: New York,

1992. (b) Moulijn, J. A.; Sheldon, R. A.; van Bekkum, H.; van Leeuwen, P. W. N. M.; In

Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial

Catalysis , Moulijn, J. A.; van Leeuwen, P. W. N. M.; van Santen, R. A., Eds., Elsevier:

Amsterdam, 1993.

2 For example see: (a) Kawamura-Kuribayashi, H.; Koga, N.; Morokuma, K. J. Am. Chem. Soc.

1992, 114, 2359-2366. (b) Kawamura-Kuribayashi, H.; Koga, N.; Morokuma, K. J. Am. Chem.

Soc. 1992, 114, 8687-8694. (c) Prosenc, M.-H.; Janiak, C.; Brintzinger, H.-H.

Organometallics 1992, 11, 4036-4041. (d) Castonguay, L. A.; Rappé, A. K. J. Am. Chem.

Soc. 1992, 114, 5832-5842. (e) Ziegler, T.; Folga, E.; Berces, A. J. Am. Chem. Soc. 1993,

115, 636-637. (f) Siegbahn, P. E. M. J. Am. Chem. Soc. 1993, 115, 5803-5812. (g) Sini, G.;

Macgregor, S. A.; Eisenstein, O.; Teuben, J. H. Organometallics 1994, 13, 1049-1051.

3 van Rooy, A.; Orij, E. N.; Kamer, P. C. J.; van der Aardweg, F.; van Leeuwen, P. W. N. M. J.

Chem. Soc., Chem. Commun. 1991, 1096-1097.

4 (a) Drent, E. Eur. Patent. Appl. EP-A-271144 , 1988. (b) Drent, E.; Budzelaar, P. H. M.;

Jager, W. W. Eur. Patent Appl. EP-A-386833 , 1990. (c) Drent, E.; Budzelaar, P. H. M. Eur.

Patent Appl. EP-A-386834 , 1990. (d) Doyle, M. J.; van Gogh, J.; van Ravenswaay Claasen,

General Introduction.

13

J. C. Eur. Patent Appl. EP-A-392601 , 1990. (e) Drent, E.; Budzelaar, P. H. M.; Jager, W.

W.; Stapersma, J. Eur. Patent Appl. EP-A-441447 , 1991. (f) Drent, E.; Arnoldy, P.;

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6 (a) Ewen, J. A. J. Am. Chem. Soc. 1984, 106, 6355-6364. (b) Clawson, L.; Soto, J.;

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7 For example see: (a) Watson, P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 104, 6471-6473. (b)

Watson, P. L. J. Chem. Soc., Chem. Commun. 1983, 276-277. (c) Watson, P. L. J. Am.

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Winter, A.; Rohrmann, J.; Bachmann, B.; Antberg, M.; Dolle, V.; Paulus, E. F.

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9 (a) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. J.

Am. Chem. Soc. 1985, 107, 8111-8118. (b) Conticello, V. P.; Brard, L.; Giardello, M. A.;

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12 Rothwell, I. P.; Watson, P. L. In Selective Hydrocarbon Activation: Principles and

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General Introduction.

15

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18 Wilkinson, G.; Birmingham, J. M. J. Am. Chem. Soc. 1954, 76, 6210.

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30 Duchateau, R.; Teuben, J. H. To be published.

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