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University of Groningen
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.;
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3 van Rooy, A.; Orij, E. N.; Kamer, P. C. J.; van der Aardweg, F.; van Leeuwen, P. W. N. M. J.
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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.
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Organomet. Chem. 1991, 417, 235-251.
6 (a) Ewen, J. A. J. Am. Chem. Soc. 1984, 106, 6355-6364. (b) Clawson, L.; Soto, J.;
Buchwald, S. L.; Steigerwald, M. L.; Grubbs, R. H. J. Am. Chem. Soc. 1985, 107, 3377-3378.
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Jordan, R. F.; Taylor, D. F.; Baenziger, N. C. Organometallics 1990, 9, 1546-1557. (h)
Borowsky, S. L.; Baenziger, N. C.; Jordan, R. F. Organometallics 1993, 12, 486-495.
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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N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091-8103. (e) Thompson, M.
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