Rare-Earth-Metal Pentadienyl Half-Sandwich and Sandwich Tetramethylaluminates–Synthesis, Structure, Reactivity, and Performance in Isoprene Polymerization

Article abstract of DOI:10.1002/chem.201900108

Targeting the synthesis of rare-earth-metal pentadienyl half-sandwich tetramethylaluminate complexes, homoleptic [Ln(AlMe₄)₃] (Ln=Y, La, Ce, Pr, Nd, Lu) were treated with equimolar amounts of the potassium salts K(2,4-dmp) (2,4-dmp=2,4-dimethylpentadienyl), K(2,4-dipp) (2,4-dipp=2,4-diisopropylpentadienyl), and K(2,4-dtbp) (2,4-dtbp=2,4-di-tert-butylpentadienyl). The reactions involving the larger rare-earth-metal centers lanthanum, cerium, praseodymium, and neodymium gave selectively the desired half-sandwich complexes [(2,4-dmp)La(AlMe₄)₂], [(2,4-dipp)La(AlMe₄)₂], and [(2,4-dtbp)Ln(AlMe₄)₂] (Ln=La, Ce, Pr, Nd) in high crystalline yields. Smaller rare-earth-metal centers yielded preferentially the sandwich complexes [(2,4-dmp)₂Ln(AlMe₄)] (Ln=Y, Lu) and [(2,4-dipp)₂Y(AlMe₄)]. Activation with fluorinated borate/borane co-catalysts gave highly active catalyst systems for the fabrication of polyisoprene, displaying molecular weight distributions as low as M_w/M_n=1.09 and a maximum cis-1,4 selectivity of 90.4 %. The equimolar reaction of half-sandwich complex [(2,4-dtbp)La(AlMe₄)₂] with B(C₆F₅)₃ led to the isolation and full characterization of the single-component catalyst {{(2,4-dtbp)La[(μ-Me)₂AlMe(C₆F₅)]}[Me₂Al(C₆F₅)₂]}₂. The reaction of the latter complex with 10 equivalents of isoprene could be monitored by ¹H NMR spectroscopy. Also, a donor-induced aluminato/gallato exchange was achieved with [(2,4-dtbp)La(AlMe₄)₂] and GaMe₃(OEt₂) leading to [(2,4-dtbp)La(GaMe₄)₂].

Full text of DOI:10.1002/chem.201900108

A Journal of
Accepted Article
Title: Rare-Earth-Metal Pentadienyl Half-Sandwich and Sandwich
Tetramethylaluminates – Synthesis, Structure, Reactivity, and
Performance in Isoprene Polymerization
Authors: Reiner Anwander, Damir Barisic, Dennis Buschmann, David
Schneider, and Cäcilia Maichle-Mössmer
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To be cited as: Chem. Eur. J. 10.1002/chem.201900108
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Rare-Earth-Metal Pentadienyl Half-Sandwich and Sandwich
Tetramethylaluminates – Synthesis, Structure, Reactivity, and
Performance in Isoprene Polymerization
Damir Barisic, Dennis A. Buschmann, David Schneider, Cäcilia Maichle-Mössmer, and Reiner
Anwander*
Abstract: Targeting the synthesis of rare-earth metal pentadienyl
half-sandwich tetramethylaluminate complexes, homoleptic
Ln(AlMe4)3 (Ln = Y, La, Ce, Pr, Nd, Lu) were treated with equimolar
amounts of the potassium salts K(2,4-dmp) (2,4-dmp 2,4-
substituted[9] pentadienyl complexes have been probed as well.
Moreover, initial reactivity studies revealed that
homoleptic/supported Ti(II), V(II), and Cr(II) pentadienyl
complexes M(2,4-dmp)2 display catalysts for ethylene
polymerization.[10] Rare-earth metal pentadienyl complexes have
been described almost exclusively for the ligands 2,4-
dimethylpentadienyl and 2,4-di-tert-butylpentadienyl. Examples
include the homoleptic complexes Ln(2,4-dmp)3 (Ln = Y, La, Nd,
Sm, Gd, Dy, Tb, Er, Lu)[11] and Ln(2,4-dtbp)2 (Ln = Sm, Yb)[12] as
well as the divalent donor adducts [(2,4-dmp)2Yb(dme)][11d] and
[Yb{1,5-(Me3Si)2C5H5}2{diglyme}].[13] Heteroleptic complexes are
represented by [(2,4-dmp)4Ln2X2] (Ln = Y; X = Br, Ln = La; X = Br,
Ln = Nd; X = Cl, Br, I),[11g] half-open metallocenes such as
[(C8H8)Ln(2,4-dmp)(thf)] (Ln = Nd, Sm, Er)[14] and a halogenido
cluster compound [Nd6(2,4-dmp)6Cl12(thf)2].[15] The complexes
Ln(2,4-dmp)3 (Ln = Y, La, Nd), [(2,4-dmp)LnX]2 (Ln = Y; X = Br,
Ln = La; X = Br, Ln = Nd; X = Cl, Br, I) and [Nd6(2,4-dmp)6X12(thf)2]
(X = Cl, Br) have been scrutinized by Taube and co-workers
regarding their performance in butadiene polymerization and
proved to be highly efficient precatalysts for the stereoselective
fabrication of cis-1,4-polybutadiene.[16] The latter study also
=
dimethylpentadienyl),
K(2,4-dipp)
(2,4-dipp
=
2,4-di-iso-
2,4-di-tert-
propylpentadienyl), and K(2,4-dtbp) (2,4-dtbp
=
butylpentadienyl). The reactions involving the larger rare-earth metal
centers lanthanum, cerium, praseodymium, and neodymium gave
selectively
the
desired
half-sandwich
complexes
[(2,4-
dmp)La(AlMe4)2], [(2,4-dipp)La(AlMe4)2], and [(2,4-dtbp)Ln(AlMe4)2]
(Ln = La, Ce, Pr, Nd) in high crystalline yields. Smaller-sized rare-
earth metal centers yielded preferentially the sandwich complexes
[(2,4-dmp)2Ln(AlMe4)] (Ln
= Y, Lu) and [(2,4-dipp)2Y(AlMe4)].
Activation with fluorinated borate/borane cocatalysts gave highly
active catalyst systems for the fabrication of polyisoprene, displaying
molecular weight distributions as low as Mw/Mn = 1.09 and a maximum
cis-1,4 selectivity of 90.4%. The equimolar reaction of half-sandwich
complex [(2,4-dtbp)La(AlMe4)2] with B(C6F5)3 led to the isolation and
full characterization of the single-component catalyst {{(2,4-
dtbp)La[(µ-Me)2AlMe(C6F5)]}[Me2Al(C6F5)2]}2. The reaction of the
latter complex with 10 equiv. of isoprene could be monitored by 1H
NMR spectroscopy. Also,
a
donor-induced aluminato/gallato
outlined a mechanism for the activation and subsequent
exchange was achieved with [(2,4-dtbp)La(AlMe4)2] and GaMe3
leading to [(2,4-dtbp)La(GaMe4)2].
polymerization involving these complexes, proposing halogenido-
containing rare-earth metal half-sandwich alkylaluminate
complexes as active species (Scheme 1).
+ AlRR’2
[Nd(2,4-dmp)(AlRR’3)2]
Nd(2,4-dmp)3
Introduction
Following the report on the first transition metal pentadienyl
complex by Pettit and co-workers in 1962,[1] such open variants of
the ubiquitous cyclopentadienyl (Cp) congeners have drawn
considerable interest throughout the 1980s and 1990s in
alkaline,[2] alkaline-earth,[3] main group,[3a] and transition metal
chemistry.[4] Most commonly, the unsubstituted pentadienyl and
+ MAO
+
+
[Nd(2,4-dmp)]2+[MAO]2–
cis-1,4-polybutadiene
2,4-dimethylpentadienyl
ligands
were
employed,
but
Scheme 1. Activation mechanism of Nd(2,4-dmp)3 with MAO (=
methylalumoxane) or AlRR’2 as proposed by Taube et al. (R = halogenido, R’ =
alkyl).[16]
tert-butyl-,[3c,d,5] phenyl-,[6] CF3-,[7] trialkylsilyl-,[8] and silyloxy-
[*]
Damir Barisic, Dennis A. Buschmann, Dr. David Schneider, Dr.
Cäcilia Maichle-Mössmer, Prof. Dr. Reiner Anwander
Institut für Anorganische Chemie
In the meantime, similar half-sandwich and non-cyclopentadienyl
dialkyl complexes have been accessed for a wide range of rare-
earth metals, showing excellent performances as precatalysts in
butadiene and isoprene polymerization.[17] The rare-earth metal
catalyzed coordinative 1,3-diene polymerization draws major
interest in scientific and industrial research, as it provides the
Auf der Morgenstelle 18, 72076 Tübingen
E-mail: reiner.anwander@uni-tuebingen.de
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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most efficient initiators with high activity and highly stereoregular
polymers.[18] As natural resources cannot satisfy the ever more
increasing worldwide demand for polyisoprene, a great deal of
research is conducted into highly stereospecific synthesis routes
toward synthetic rubber.[17a,18,19] Intrigued and encouraged by the
findings of Taube and others, we set out to target rare-earth metal
pentadienyl half-sandwich and sandwich tetramethylaluminate
chemistry using alternative synthesis approaches. The present
study gives a full account of the synthesis of such precatalyst
systems as well as their performance in isoprene polymerization
including the mechanisms of catalyst activation. We also report
on a rare example of a single-component catalyst and a feasible
AlMe3/GaMe3 Lewis acid switch. This study is also an extension
of our existing bis(tetramethylaluminate) library.[17c-e,20]
temperature (–40 °C) and a slight stoichiometric deficiency were
crucial for the synthesis of half-sandwich complexes 1La, 2La, 3Ln
(Ln = La, Ce, Pr, Nd) as reactions at ambient temperature and/or
an equimolar amount of potassium salt led to ligand scrambling
and product mixtures. Even dissolution of the half-sandwich
complexes in aliphatic or aromatic solvents led to a substantial
amount of ligand scrambling over time, with the neodymium
compound 3Nd being the least stable. When dissolving 3Nd in
[D6]benzene for NMR spectroscopic measurements, immediate
ligand scrambling to form the sandwich complex [(2,4-
dtbp)2Nd(AlMe4)] (6) and Nd(AlMe4)3 could be observed (Figure
S9). This dynamic behavior is most likely attributable to the
neodymium metal size, as ligand redistribution is much less
pronounced in solutions of complexes of the larger metal centers
lanthanum, cerium, and praseodymium and the fact that the
medium-sized yttrium and the small lutetium cations do not form
half-sandwich complexes at all. For an unambiguous
interpretation of the NMR spectra, the neodymium sandwich
complex 6 was synthesized independently from Nd(AlMe4)3 and
two equivalents of potassium salt K(2,4-dtbp) (Scheme 2).
Results and Discussion
Synthesis of Ln(III) Pentadienyl Complexes. In contrast to Cp
derivatives such as (C5Me5)Ln(AlMe4)2, (C5Me4H)Ln(AlMe4)2,
[1,3-(Me3Si)2C5H3]Ln(AlMe4)2,
[1,2,4-(Me3C)3C5H2]Ln(AlMe4)2, which are readily available via
protonolysis of Ln(AlMe4)3 with the corresponding
(C5Me4SiMe3)Ln(AlMe4)2,
or
Ln(AlMe4)3
+ 0.9 KL
–40 °C
+ KL
+ 2 KL
r.t.
– 2 K(AlMe4)
– K(AlMe4)
cyclopentadienes,[17d,21] this protocol was not applicable for Ln(III)
pentadienyl half-sandwich complexes. The homoleptic complexes
Ln(AlMe4)3 (Ln = Y, La, Lu) did not react with 2,4-dimethyl-1,3-
pentadiene, 2,4-di-iso-propyl-1,3-pentadiene, or 2,4-di-tert-butyl-
1,3-pentadiene, not even at elevated temperatures. Utilizing the
salt metathesis route[22] by reacting La(AlMe4)3 with 0.9 equiv. of
the corresponding potassium salts K(2,4-dmp) (2,4-dmp = 2,4-
r.t. or –40 °C
R
R
tBu
R
Me
Me
Al
Me
Me
Me
Me
R
Al
tBu
Ln
Nd
Ln
Me
Me
Me
Me
Me
Me
Me
Me
Me
R
tBu
Al
Al
Me
R
tBu
dimethylpentadienyl), K(2,4-dipp) (2,4-dipp
propylpentadienyl), and K(2,4-dtbp) (2,4-dtbp
=
2,4-di-iso-
2,4-di-tert-
1La (R = Me)
2La (R = iPr)
3Ln (Ln = La, Ce, Pr, Nd; R = tBu)
4Ln (Ln = Y, Lu; R = Me)
5Y (R = iPr)
6
=
butylpentadienyl) at low temperature, on the other hand, led to the
formation of half-sandwich complexes [(2,4-dmp)La(AlMe4)2] (1La),
[(2,4-dipp)La(AlMe4)2] (2La), and [(2,4-dtbp)La(AlMe4)2] (3La),
respectively (Scheme 2). Unfortunately, complex 1La could not be
obtained in pure form as a substantial amount of starting material,
La(AlMe4)3, was constantly detected in the proton NMR spectrum.
The remainder of the large Ln(III) centers showed a similar
reactivity toward K(2,4-dtbp), leading to half-sandwich complexes
[(2,4-dtbp)Ln(AlMe4)2] (Ln = Ce (3Ce), Pr (3Pr), Nd (3Nd)). Contrary
to our expectations, the reaction of one equivalent of K(2,4-dmp)
with the smaller lanthanide(III) centers yttrium and lutetium
selectively produced the sandwich complexes [(2,4-
dmp)2Y(AlMe4)] (4Y), and [(2,4-dmp)2Lu(AlMe4] (4Lu), regardless
of the reaction temperature. These compounds could also be
obtained in higher yields using two equivalents of potassium salt.
The same reactivity could be observed for Y(AlMe4)3 and newly
synthesized K(2,4-dipp), leading to sandwich complex [(2,4-
dipp)2Y(AlMe4)] (5), but the Lu(AlMe4)3–K(2,4-dipp) reaction led
only to a mixture of compounds that could not be separated. The
reactivity of K(2,4-dtbp) toward the smaller rare-earth metal
centers was described recently.[24] A similar size-dependent
product distribution in salt metathesis reactions could recently be
observed by our group when investigating the formation of rare-
Scheme 2. Synthesis of pentadienyl (L)-supported half-sandwich complexes 1-
3 and sandwich complexes 4-6.
NMR Spectroscopic Investigation of Ln(III) Pentadienyl
Complexes. The 1H NMR spectra obtained from the diamagnetic
lanthanum (1La, 2La, 3La), yttrium (4Y, 5), and lutetium (4Lu
)
complexes show signal ratios of 1:2:2:6 (2,4-dmp), 1:2:2:2:6:6
(2,4-dipp), and 1:2:2:18 (2,4-dtbp), in accordance to the already
known homoleptic rare-earth metal pentadienyl complexes.[3d,11g]
All complexes show distinct signals for Hc, Hexo, and Hendo protons
(Figure 1) with clear signal splitting for the larger lanthanum(III)
complexes except 1La, which is supported by the sterically least
demanding pentadienyl ligand. This indicates a relatively rigid
pentadienyl
ligand
coordination
in
complexes
[(2,4-
dipp)La(AlMe4)2] and [(2,4-dtbp)La(AlMe4)2]. On the other hand,
complex 1La and the ones with the smaller yttrium(III) and
lutetium(III) centers (4Y, 4Lu) show only singlets for the pentadienyl
protons, pointing to a more dynamic behavior (4Lu gave singlets
at even -80 °C). This behavior could originate from an h5–h1–h5
shift of the ligand, or ligand “flipping”, as previously described by
Ernst in 1985.[4a] Only one signal in the range of –0.36 to –0.04
ppm was observed for the [AlMe4] moiety in all complexes except
earth metal fluorenyl half-sandwich complexes.[20e]
A low
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density located on the latter. When comparing complexes 1La, 2La,
and 3La, it should be noted that the metal-centroid distance
decreases with increasing substituent size (1La > 2La > 3La),
attributable to opposing effects, the ability of the sterically more
demanding substituents iPr and tBu to interact with the metal
center via peripheral methyl groups, and a concomitant h3/h2-
coordination switch of the aluminato ligand.
for the sandwich complex 4Lu with the smallest Ln(III) cation,
where two signals – for bridging and terminal methyl groups – are
present. Furthermore, the 2,4-di-iso-propylpentadienyl ligand in
complexes 2La and 5 shows two distinct signals for the methyl
groups of the isopropyl substituent, also indicating a restricted
2
mobility. In addition, the yttrium(III) complexes show a JH,Y
coupling of 1.78 Hz (4Y, 5) for the [AlMe4] groups, respectively,
characteristic of yttrium tetramethylaluminate complexes.[19b,20e]
X-Ray Structure Analyses of Ln(III) Pentadienyl Sandwich
Complexes. In the solid state, complexes 4Y and 4Lu are
isostructural (Figures 3/top and S41) with a gauche conformation
of the two pentadienyl ligands toward each other (conformation
angle c×= 46°).[3d] Complex 5 shows a similar structure in which
the pentadienyl ligands adopt an eclipsed conformation (c×= 1°),
however, with the iPr groups pointing away from each other
(Figure 3/bottom). The even more bulky 2,4-di-tert-butyl-
pentadienyl ligand in complex 6 conversely has a conformation
angle of 46° (Figure S42). The different conformation in complex
5 involves a possible interaction between the CH protons H17 and
H20 of the iPr groups with the negatively charged carbon atoms
Figure 1. Proton assignment in pentadienyl ligands.
X-Ray Structure Analyses of Ln(III) Pentadienyl Half-
Sandwich Complexes. All isolated half-sandwich complexes
were examined by X-ray structure crystallography. Single crystals
were obtained from saturated n-hexane/TMS solutions at –40 °C.
The molecular structures of [(2,4-dmp)La(AlMe4)2] (1La), [(2,4-
dipp)La(AlMe4)2] (2La), and [(2,4-dtbp)La(AlMe4)2] (3La) are
depicted in Figure 2; those of complexes 3Ce and 3Pr are
isostructural to 3La, and shown in the supporting information
(Figures S38 and S39). Complex 3Nd crystallized in a different
space group, but is isostructural to the other half-sandwich
complexes (Figure S40). Selected bond lengths for the lanthanum
half-sandwich complexes are given in Table 1. All half-sandwich
complexes show the structural motif already observed for
…
…
C1 and C5 (C1 H20 = 2.860(9) Å; C5 H17 = 2.881(2) Å),
“locking” the ligands in place as a result (Figure 3). The [AlMe4]
groups in these complexes show an h2-planar coordination mode
(6),
which
is
characteristic
of
lanthanidocene
tetramethylaluminate complexes[25] and an h2-bent coordination
mode (4Y, 4Lu, and 5), most likely due to the Me and iPr groups of
complexes 4 and 5 being sterically less demanding than the tBu
groups of 6, thus allowing approach of an additional CH3 group to
the rare-earth-metal center (e.g., C25 in complex 5).
Table 1. Selected interatomic distances [Å] of 1La, 2La, and 3La
1La
2La
3La
cyclopentadienyl-
and
fluorenyl-supported
half-sandwich
complexes.[17d,e,20e] The pentadienyl ligand coordinates in a U-
shaped h5-fashion to the metal center. This U-shape is typical for
pentadienyl ligands and was also observed previously for homo-
and heteroleptic rare-earth metal pentadienyl complexes such as
Ln(2,4-dmp)3 (Ln = Y, La, Nd, Gd, Dy, Tb, Er, Lu)[11a-g], [Ln(2,4-
dtbp)2(do)] (Ln = Sm, Yb; do = thf, dme)[12], or [(2,4-dmp)2LnX]2
(Ln = Y, Nd, Gd; X = Cl, Br, I).[11g,24] At first glance, all pentadienyl
half-sandwich complexes under study seem to feature 7-
coordinate rare-earth metal centers. However, an h3-coordination
by one [AlMe4] ligand likely compensates for enhanced steric
…
La Ct
2.416
2.410
2.386
La–C1
La–C2
La–C3
La–C4
La–C5
2.818(3)
2.874(3)
2.783(3)
2.924(3)
2.880(4)
3.020(4)
3.246(2)
2.841(4)
2.853(4)
3.083(4)
2.715(4)
2.722(4)
2.856(4)
2.894(5)
2.734(9)
2.890(0)
2.874(8)
3.029(8)
3.225(6)
2.875(5)
2.847(0)
3.106(7)
2.680(6)
2.680(5)
2.906(2)
2.872(2)
2.668(2)
2.852(2)
2.844(2)
3.289(4)
3.274(2)
2.713(2)
2.764(2)
4.625(2)
2.716(2)
2.726(2)
unsaturation of the lanthanum centers in complexes 1La (La C10
…
…
La Al1
= 3.083(4) Å; torsion angles: La–C8–Al1–C9 = 50.62 °; La–C12–
Al2–C13 = 8.96 °) and 2La (La C14 = 3.106(7) Å; torsion angles:
…
…
La Al2
La–C12–Al1–C13 = –51.59 °; La–C16–Al2–C17 = –9.41 °). This
La–C8/13/14
La–C9/12/15
La–C10/14/16
La–C12/17/19
La–C13/16/18
interaction is only observed in the solid state.[20b] In complexes 3Ln
the bulkier 2,4-dtbp ligand apparently prevents such an h3-
coordination, causing both [AlMe4] groups to adopt an h2-
coordination (torsion angles for 3La: La–C14–Al1–C15 = –6.32 °;
La–C18–Al2–C19 = 14.44 °). Compared to the Cp ligand in
,
[(C5Me5)La(AlMe4)2] (La C(C5Me5)), 2.753(2)–2.801(3) Å),[21a] the
–
pentadienyl ligand in complexes 1La, 2La, and 3La seems
positioned further away from the metal center. In particular, the
central C3 atom in 3La is found comparatively close to the
lanthanum center– the probable cause being the higher charge
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Figure 2. Molecular structures of [(2,4-dmp)La(AlMe4)2] (1La, left), [(2,4-dipp)La(AlMe4)2] (2La, middle), and [(2,4-dtbp)La(AlMe4)2] (3La, right); complexes 3Ce, 3Pr
,
and 3Nd are isostructural (Figures S38, S39, and S40). Hydrogen atoms are omitted for clarity. Atomic displacement parameters are set at the 50% probability level.
Selected bond lengths are given in Table 1.
1.975(2); C1–C2 1.370(3); C2–C3 1.430(3); C3–C4 1.429(3); C4–C5 1.377(3);
C8–C9 1.377(4); C9–C10 1.421(4); C10–C11 1.432(3); C11–C12 1.355(4);
Ct1–Y–Ct2 133.4(4); Y–C15–Al 82.4(6); Y–C16–Al 80.4(2); C15–Y–C16
81.6(4); C15–Al–C16 112.9(9). 5Y: Y–C1 2.659(3); Y–C2 2.719(3); Y–C3
2.599(3); Y–C4 2.732(3); Y–C5 2.704(3); Y–C12 2.772(3); Y–C13 2.738(3); Y–
C14 2.579(3); Y–C15 2.730(3); Y–C16 2.717(4); Y–C23 2.641(4); Y–C24
…
…
…
…
2.660(4); Y C25 4.138(8); Y H17 3.902(1); Y H20 3.849(3); Y Al 3.102(7);
Al–C23 2.061(4); Al–C24 2.065(4); Al–C25 1.994(4); Al–C26 1.975(4); C1–C2
1.382(5); C2–C3 1.425(4); C3–C4 1.439(4); C4–C5 1.378(5); C12–C13
1.368(5); C13–C14 1.440(4); C14–C15 1.428(4); C15–C16 1.368(5); Ct1–Y–
Ct2 128.4(2); Y–C23–Al 81.5(8); Y–C24–Al 81.0(4); C23–Y–C24 77.7(8); C23–
Al–C24 107.5(3).
Reactivity of [(2,4-dtbp)La(AlMe4)2] (3La) toward GaMe3. To
examine
the
feasibility
of
a
donor-assisted
tetramethylaluminato/gallato exchange, the half-sandwich
pentadienyl complex 3La was treated with an excess of GaMe3
and three equivalents of diethyl ether. This reaction protocol was
utilized earlier by our group to gain access to tetramethylgallate
complexes of the early rare-earth metals, demonstrating that
diethyl ether binds more strongly to trimethylaluminum than
trimethylgallium.[26] Indeed, the reaction proceeded as expected
to generate the lanthanum half-sandwich pentadienyl
tetramethylgallate complex [(2,4-dtbp)La(GaMe4)2] (7) (Scheme
3).
However, despite several recrystallization attempts, complex 7
remained contaminated with a significant amount of an unknown
by-product, as evident from distinct methyl signals in the variable
temperature 1H NMR spectrum (Figures S19, S20, and S37). X-
ray structure analysis of complex 7 revealed a similar structural
motif as observed for the corresponding tetramethylaluminate
complex 3La (Figure 4). However, one of the [GaMe4] groups in 7
exhibits an h2-bent coordination, owing to the lower Lewis acidity
of gallium(III) compared to aluminum(III) and, as a result, less
repulsion between the two metal centers. This distinct
tetramethylgallato coordination also causes the pentadienyl
ligand to be located further away from the lanthanum(III) center
than in parent complex 3La while retaining its typical U-shaped h5-
Figure 3. Molecular Structure of 4Y and 5Y with atomic displacement parameters
set at the 50% level. For 5Y hydrogen atoms except for H17 and H20 and iPr
groups of one pentadienyl ligand are omitted for clarity. Selected interatomic
distances [Å] and angles [°]: 4Y: Y–C1 2.709(3); Y–C2 2.702(2); Y–C3 2.599(2);
Y–C4 2.702(2); Y–C5 2.704(2); Y–C8 2.727(3); Y–C9 2.698(2); Y–C10
2.594(2); Y–C11 2.703(3); Y–C12 2.808(3); Y–C15 2.593(3); Y–C16 2.677(3);
…
Y
Al 3.097(1); Al–C15 2.067(3); Al–C16 2.065(2); Al–C17 1.975(3); Al–C18
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…
…
…
2.833(3); La–C19 2.743(3); La C20 3.257(3) La Ga1 3.254(1); La Ga2
3.027(1); Ga1–C14 2.101(3); Ga1–C15 2.096(3); Ga1–C16 1.978(3); Ga1–C17
1.985(3); Ga2–C18 2.074(3); Ga2–C19 2.090(3); Ga2–C20 2.018(3); Ga2–C21
1.967(3); C1–C2 1.364(3); C2–C3 1.449(3); C3–C4 1.428(3); C4–C5 1.379(3);
Ct–La–Ga1 110.6(6); Ct–La–Ga2 136.7(5); La–C14–Ga1 83.5(6); La–C15–
Ga1 83.9(3); C14–La–C15 79.4(8); C14–Ga1–C15 112.3(9); La–C18–Ga2
76.2(1); La–C19–Ga2 74.3(7); C18–La–C19 73.7(2); C18–Ga2–C19 106.9(3).
coordination.
A similar tetramethylgallato coordination was
already observed for [(C5Me5)La(GaMe4)2].[26]
tBu
tBu
3 GaMe3
3 GaMe3 Et2O
La
Me
Ga
– 2 AlMe3 Et2O
Me
Me
Me
Polymerization of Isoprene and Investigation of Active
Species. Earlier studies conducted by our group revealed that
half-sandwich rare-earth metal tetramethylaluminate complexes
[CpRLn(AlMe4)2] (Ln = Y, La, Nd, Lu) polymerize isoprene
Me
Me
Ga
Al
Me
Al
Me
7
tBu
[B(C6F5)4]
tBu
[Ph3C][B(C6F5)4] (A)
Me
Me
Me
Me
Me
Me
Me
Me
efficiently
when
cationized
with
borate
cocatalysts
La
La
+
Me
Me
– Ph3C–(2,4-dtbp)
– Ph3C–Me
– AlMe3
[B(C6F5)4]
Me
[CPh3][B(C6F5)4] (A), [PhNMe2H][B(C6F5)4] (B), and B(C6F5)3
(C).[17c-e,20e] Since homoleptic 2,4-dimethylpentadienyl complexes
of the rare-earth metals (Ln = La, Nd, Y) were investigated by
Taube et al. regarding the polymerization of butadiene,[16] we
were interested in studying the polymerization performance of the
half-sandwich pentadienyl complexes 2La, 3La, 3Ce, and 3Nd. We
refrained from probing complex 1La in this regard, as it could not
be isolated in pure form. Also, direct addition of isoprene to a
solution of the half-sandwich complexes or activation with
chloride-based cocatalyst such as Me2AlCl or Et2AlCl did not lead
to a polymerization reaction (catalyst preformation times up to 30
minutes). An 1H NMR spectroscopic study of an equimolar
reaction mixture of 3La and Me2AlCl indicated the formation of a
mixture of unreacted half-sandwich complex, metallocene
complex [(2,4-dtbp)2La(AlMe4)], [(2,4-dtbp)2AlMe], AlMe3, and
LaCl3 (elemental analysis/Figure S25). Therefore, as a general
procedure, the complexes were activated by addition of one or
two equivalents of borate/borane cocatalysts A, B, or C to the
respective toluene solutions prior to testing for the
homopolymerization of isoprene. The polymerization results are
summarized in Tables 2, S4 and S5.
Al
major product
Me
minor product
3La
tBu
tBu
[B(C6F5)4]
[PhNMe2H][B(C6F5)4] (B)
Me
Me
Me
Me
Me
Me
Me
+
La
La
Al
Al
Me
Me
– 2,4-dtbpH
– PhNMe2–AlMe3
– CH4
Me
Me
[B(C6F5)4]
Me
minor product
Al
major product
F4
F5
tBu
Al Me
Me
F5
F
tBu
Me
B(C6F5)3 (C)
Al
Al
Me
Me
Me
Me
La
La
F
– BMe3
Me
Me
tBu
Al
Me
F5
F5
tBu
8
F4
Scheme 3. Synthesis of Bis(gallate) [(2,4-dtbp)La(GaMe4)2] (7) and equimolar
activation reactions of 3La with perfluorinated borates A and B as well as borane
C, including the formation of {{(2,4-dtbp)La[(µ-Me)2AlMe(C6F5)]}[Me2Al(C6F5)2]}2
(8). For reactions 3La/A and 3La/B proposed species only.
To investigate if the active species correspond to those revealed
for cyclopentadienyl-supported catalysts,[17c,e] NMR-scale
reactions were conducted with compound 3La and an equimolar
and double-molar amount of cocatalysts A-C. Activation of 3La
with cocatalyst A led to abstraction of the pentadienyl ligand and
formation of Ph3C–(2,4-dtbp) as the main product along with the
same active species that can be found when activating La(AlMe4)3
with cocatalyst A (Scheme 3).[19e] The solid-state structure of
Ph3C–(2,4-dtbp) could be determined by X-ray crystallographic
analysis and is shown in Figure S43. Another activation by-
product, Ph3C–Me, derived from the abstraction of one [AlMe4]
methyl group could also be detected as a minor product as could
be a third species in the 19F NMR spectrum. Such abstraction of
supposedly/putative ancillary ligands was previously described
for the activation of half-sandwich fluorenyl and indenyl
complexes.[27] Interestingly, the polymer microstructure derived
from catalyst system 3La/A was almost identical to the one derived
from La(AlMe4)3/A[19e] although at least three different species are
present in the reaction mixture (Table 2, entry 7). The addition of
another equivalent of borate cocatalyst
A
led to the
Figure 4. Molecular Structure of 7 with atomic displacement parameters set at
the 50% level. Hydrogen atoms are omitted for clarity. Selected interatomic
distances [Å] and angles [°]: La–C1 2.936(3); La–C2 2.896(2); La–C3 2.709(3);
La–C4 2.879(3); La–C5 2.856(3); La–C14 2.717(3); La–C15 2.736(3); La–C18
disappearance of the signals of the active species mentioned
above and a large AlMe3 signal appeared in the proton NMR
spectrum. Signals of Ph3C–(2,4-dtbp) and Ph3C–Me were still
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present, with the amount of Ph3C–Me being significantly larger
than in the equimolar catalyst system, caused by an additional
abstraction of a methyl group by the cocatalyst. The 19F NMR
spectrum of the catalyst system 3La/2A shows only one set of
signals and thus only one active species as opposed to the
equimolar catalyst system.
A structurally similar complex could be isolated earlier by us from
the reaction of [(C5Me5)La(AlMe4)2] and B(C6F5)3.[17c] In the solid
state, complex
8 features a dimeric structure with two
[(C6F5)2AlMe2] units bridging between the lanthanum centers
forming a contact ion pair. A single Me/[C6F5] exchange occurred
also within the terminal alkyl aluminate moiety, which exhibits one
ortho-fluorine atom interaction with the lanthanum center. The
2,4-dtbp ligand retained its U-shaped h5-coordination.
Activation of 3La with cocatalyst B gave a significantly different
activation pattern with the main products detectable in the 1H
NMR
spectrum
being
the
and
ionic
the
species
trimethylaluminum
[(2,4-
In contrast to the C5Me5-supported complex, 8 shows dynamic
behavior in solution with a monomer-dimer equilibrium at ambient
temperature. The signals of the pentadienyl backbone appear
much broader in the dimer than those of the monomer as the large
[C6F5] groups and the resulting steric pressure probably facilitate
a dynamic behavior of the ligand. The dynamic behavior and
hence less tightly bonded contact ion pairs are also reflected in
the substantially weaker interactions between the lanthanum
center and the terminal and bridging heteroaluminato ligands
[(C6F5)AlMe3] and [(C6F5)2AlMe2], respectively. The La–C(methyl)
dtbp)La(AlMe4)][B(C6F5)4]
dimethylanilinium adduct PhNMe2–AlMe3 (Scheme 3). As a minor
side product, the proligand 2,4-di-tert-butyl-1,3-pentadiene (2,4-
dtbpH) could also be observed and three different signal sets are
present in the 19F NMR spectrum (cf., equimolar activation with
A). Addition of another equivalent of cocatalyst B resulted in
increasing amounts of 2,4-dtbpH in the 1H NMR spectrum,
corresponding to further protonation of the active species [(2,4-
dtbp)La(AlMe4)][B(C6F5)4] and leading to the formation of a
species similar to that in system 3La/2A. The 19F NMR spectrum
of the mixture 3La/2B shows two different signal sets, indicative of
the co-existence of two distinct species.
…
(2.673(7)–2.929(7) Å) and La F distances (2.957(4) Å) are
significantly longer than in the corresponding cyclopentadienyl
complex (2.62(1)–2.79(1) Å and 2.62(1) Å).[17c] The La
F
…
interaction is not retained in solution as the 19F NMR spectrum
Equimolar treatment of 3La with borane C led to the formation of
shows only two signal sets for ortho, para, and meta-fluorine
isolable
compound
{{(2,4-dtbp)La[(µ-
atoms. VT NMR spectroscopic measurements indicated a
Me)2AlMe(C6F5)]}[Me2Al(C6F5)2]}2 (8), which was further
characterized by NMR spectroscopy and X-ray crystallography
(Scheme 3 and Figure 5).
decrease of and subsequent disappearance of the monomer
signals corroborating that the dimeric structure is the predominant
species at lower temperatures (Figure S36). Complex
8
polymerizes isoprene without addition of any activating agent and
thus features a rare single-molecule/component catalyst (cf., vide
infra). A 1H NMR-scale experiment with 10 equivalents of
isoprene added to a [D8]toluene solution of 8 over the course of 5
h (Figure 6) revealed a very slow polymerization initiation rate,
with polyisoprene signals starting to appear after 45 min and
subsequent disappearance of the terminal methyl group signal at
0.56 ppm, implicating that isoprene is inserted into a La–Me bond
rather than in between the La(III) center and the pentadienyl
ligand.[17c]
Figure 5. Molecular Structure of 8 with atomic displacement parameters set at
the 50% level. Hydrogen atoms and crystal lattice toluene are omitted for clarity.
Selected interatomic distances [Å] and angles [°]: La–C1 2.890(6); La–C2
2.858(6); La–C3 2.676(6); La–C4 2.837(6); La–C5 2.805(6); La–C14 2.673(7);
…
La–C15 2.725(8); La–C23 2.886(6); La–C24 2.929(7); La F1 2.957(4);
…
La Al1 3.252(2); F1–C18 1.385(7); Al1–C14 2.050(8); Al1–C15 2.031(9); Al1–
…
C16 1.979(7); Al1–C17 2.019(7); La1 La1’ 7.692; Ct–La1–Al1 127.6(9); Ct–
Figure 6. 1H NMR spectra of the reaction of 10 equiv. isoprene with 8 monitored
over the course of 5 h. Asterisk for the methyl groups of 8, # for isoprene, + for
polyisoprene.
La1–F1 171.2(9); La1–C14–Al1 85.9(8); La–C15–Al1 84.9(5); C14–Al1–C15
110.6(3); C14–La1–C15 76.8(7); C23–Al2–C24’ 105.7(7); La1–C23–Al2
171.9(1); C24–La1–C23 78.6(2)
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On the other hand, multiple signals in the tBu region between 0.97
and 1.21 ppm emerged when polyisoprene started to form. These
findings might be explained by either 1) that at some point, the
polyisoprene chain does insert in between the metal center and
the pentadienyl ligand after all or 2) decomposition of the catalyst.
cis- and lowest trans-polyisoprene contents with the highest cis-
polyisoprene content of 90% for the system 2La/2C (entry 6).
Complex 8 features a single-component catalyst, producing a
polyisoprene with a higher trans-content, as opposed to that of
the binary system 3La/C (entries 11 versus 13). For comparison,
cyclopentadienyl-supported bis(tetramethylaluminate) complexes
of the larger rare-earth metal centers when combined with
cocatalysts B and C afford mainly trans-polyisoprene.[17c-e] The
higher cis-polyisoprene contents in the present catalyst systems
are yet another hint to the importance of the supporting ligand in
isoprene polymerization. As the pentadienyl ligands can be
regarded as “open” cyclopentadienyl systems, they allow for more
Catalyst system 3La/2C afforded polyisoprenes with the highest
cis-1,4 content, pointing to
a
coordinatively unsaturated
lanthanum center catalyst (cf., vide supra). The nature of the
active catalyst present in the reaction mixture could not be
determined but 1H NMR spectroscopy revealed again the
formation of BMe3 (d = 0.67 ppm, Figure S34). Furthermore, at
least three individual tBu signals are also detected in the spectrum, space above the metal center, thus facilitating an anti-
indicating a mixture of compounds. This is also underlined by the
presence of three signal sets in the 19F NMR spectrum.
coordination of the polymer chain and a cis-1,4-insertion of
isoprene into the growing chain.[29]
Effect of the Cocatalyst. In general, all tested complexes 2La, 3La, Effect of the Pentadienyl Ligand. A change in the substitution
3Ce, and 3Nd afforded high polymer yields (>92%) and relatively
narrow molecular weight distributions Mw/Mn (<2.00) with all three
cocatalysts A, B, and C (Table 2). Equimolar amounts of
cocatalysts A, B, and C gave polymers with a relatively equal cis-
and trans-content distribution (Table 2, entries 1, 3, 5, 7, 9, and
11; Table S4, entries 14, 16, 18, 20, 22, and 24), whereas an
additional equivalent of cocatalyst led to higher cis and 3,4-
selectivities (Table 2, entries 2, 4, 6, 8, 10, and 12, Table S4,
entries 15, 17, 19, 21, 23, and 25), corresponding to increasingly
unsaturated rare-earth metal centers. Entries 2 (2La/2A) and 21
(3Nd/2A) stand out as they show a broader polydispersity index
Mw/Mn of 2.24 and 2.19, respectively. A double-molar amount of
cocatalyst C (entries 6, 12, 19, and 25) gave rise to the highest
pattern on positions C2 and C4 of the pentadienyl ligand from tBu
to the sterically less demanding iPr substituent (Table 2, entries
1-6) did not result in a significant change of the polyisoprene
microstructure. The only catalyst system that showed a marked
difference when changing from tBu to iPr was 2La/2A, which
exhibited a considerably larger Mw/Mn value (2.24) compared to
the corresponding catalyst system 3La/2A (1.23). Of note should
be the slightly minor yields of 2La (92-98%) in comparison with 3La
(99%). The largely non-existent influence of the substitution
pattern on the pentadienyl ligand on the polymerization outcome
could be expected as the substituents on C2 and C4 point away
from the active site of the catalyst.
Table 2. Polymerization of isoprene by lanthanum pentadienyl complexes 2La, 3La, and 8
d
entrya
precatalyst
cocatalystb
yield [%]
95
trans-1,4–c
cis-1,4–c
46.1
3,4–c
3.8
Mn (x 103)d
Mw/Mn
1.16
Tg [°C]e
–64.2
eff.f
1.33
1
2La
2La
2La
2La
2La
2La
3La
3La
3La
3La
3La
3La
8
A
50.1
5.0
51.3
137.4
43.5
40.4
75.4
68.0
47.0
61.6
43.8
55.9
66.9
78.5
38.6
2
2A
B
98
79.4
41.0
67.7
55.6
90.4
44.1
76.8
43.6
66.7
55.4
88.1
46.0
15.6
3.7
11.0
2.6
6.5
3.9
14.3
4.7
9.9
2.9
6.6
2.6
2.24
1.21
1.14
1.15
1.09
1.12
1.23
1.10
1.10
1.15
1.14
1.10
–51.4
–60.1
–56.4
–63.6
–58.7
–61.9
–49.9
–61.8
–55.4
–61.4
–57.8
–64.2
0.50
1.56
1.68
0.90
1.00
1.45
1.10
1.55
1.22
1.02
0.87
1.76
3
92
55.3
21.3
41.8
3.1
4
2B
C
97
5
95
6
2C
A
95
7
>99
>99
>99
>99
>99
99
52.0
8.9
8
2A
B
9
51.7
23.4
41.7
5.4
10
11
12
13
2B
C
2C
-
>99
51.4
aGeneral polymerization procedure: 0.02 mmol of precatalyst, 8 mL of toluene, 20 mmol of isoprene, 2 h, 40 °C; bA = [CPh3][B(C6F5)4]; B = [PhNMe2H][B(C6F5)4]; C
= B(C6F5)3; catalyst preformation: 30 min. cDetermined by 1H and 13C NMR spectroscopy in CDCl3. dDetermined by GPC against polystyrene standards (Mn,theory
68.0 x 103 molL-1). Assuming a living polymerization: 68e Determined by DSC at 20 K/min. f Initiation efficiency = Mn(calculated)/Mn(measured).
=
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Effect of the Rare-Earth Metal Center in Complexes 3La-3Nd
The decrease in metal center size from lanthanum (3La) over
cerium (3Ce) to neodymium (3Nd) in equimolar catalyst systems
(cocatalysts B and C) resulted in an increase of the cis-content
.
obtained. Such monomeric half-sandwich complexes feature an
h5-U-shaped coordination of the pentadienyl ligand and distinct
tetramethylaluminato coordination in the solid state. Like the
cyclopentadienyl-supported half-sandwich complexes, the
pentadienyl-supported complexes can be cationized by treatment
with perfluorinated borate or borane compounds to form highly
active catalyst systems for the homopolymerization of isoprene.
Notably the “open” pentadienyl coordination implies in general
and
a subsequent decrease of the trans-content in the
polyisoprenes (entries 9, 11, 16, 18, 22, and 24). This is in good
agreement with results of earlier isoprene polymerization studies
of CpR-supported half-sandwich complexes, in which the even
smaller rare-earth metal centers yttrium and lutetium showed a
significant cis-1,4-polyisoprene selectivity.[17c,20e] Interestingly, for
the systems with two equivalents of cocatalyst, the cis-content
decreased for the catalyst systems 3Ce/2B (entry 17) and 3Ce/2C
(entry 19) in comparison to the lanthanum and neodymium
analogues (entries 10, 12, 23, and 25). Also, lower polymer yields
were found for the cerium containing catalyst systems compared
to lanthanum and neodymium.
higher cis-1,4-contents than
a
cyclopentadienyl ancillary
environment. Such cationization can be achieved by abstraction
of the pentadienyl ligand (with trityl borate [CPh3][B(C6F5)4]) or
one methyl group of the [AlMe4] ligand (with [PhNMe2H][B(C6F5)4])
as evidenced by NMR spectroscopy. Furthermore, a single-
component catalyst could be isolated from the equimolar reaction
of half-sandwich complex [(2,4-dtbp)La(AlMe4)2] with B(C6F5)3 via
Me/[C6F5] exchange. Compared to the C5Me5 congener,
compound {{(2,4-dtbp)La[(µ-Me)2AlMe(C6F5)]}+[Me2Al(C6F5)2]–}2
Performance of of Ln(III) Pentadienyl Metallocene Complexes. features less tightly bonded contact ion pairs which markedly
Representative polymerization reactions were also run with the
open sandwich complexes 4Y and 4Lu (Table S5). Only a few
examples of such “monoalkyl” species were previously shown to
be capable of polymerizing 1,3-dienes.[20d] These rare examples
are limited to the dimeric complexes [(C5Me4)2Ln(µ-
affects the polymerization performance. Overall, the choice of the
rare-earth metal and cocatalyst greatly influences the
microstructure of the obtained polyisoprenes whereas choice of
the pentadienyl ligand seems a less determining factor. Also of
note should be that aluminato/gallato exchange of [(2,4-
dtbp)La(AlMe4)2] with the Lewis acid GaMe3 and Et2O as a donor
led to the corresponding gallate complex [(2,4-dtbp)La[GaMe4)2]
showing markedly different solid-state features compared to its
aluminate precursor.
[29]
[12]
Me)2AlMe2]2
and [(C5Me4)SiMe2P(Cy)Ln(CH2SiMe3)]2
(Ln =
Y, Lu; Cy = cyclohexyl),[30] which however require careful
activation with one equivalent of co-catalyst per dimeric complex.
Activation of monomeric complexes 4Y and 4Lu with borate/borane
cocatalysts could either produce cationized species devoid of any
alkylaluminate moiety or mixed pentadienyl/tetramethylaluminate
complexes by abstraction of one pentadienyl ligand (cf., vide
supra). Since the generated catalysts are similarly active as the
half-sandwich complexes (polymer yields >92%), the 2,4-
dimethylpentadienyl ligand (2,4-dmp) is proposed to engage in
the polymerization process via a h5/h1- coordination switch. The
polymers produced by complexes 4Ln display comparatively
broad molecular weight distributions Mw/Mn (>1.70) with all three
cocatalysts A, B, and C (Table S5). Moreover, relatively high
molecular weights (Mn,max = 160000) and reduced catalyst
efficiencies suggest the formation of a smaller number of active
species compared to the half-sandwich precatalysts. The highest
cis 1,4-selectivities were observed for catalyst system 4/C (max.
89%). In case of precatalyst 4Y the use of double-molar amounts
of cocatalyst A produced similar polymers as for the equimolar
catalyst mixtures.
Experimental Section
General Considerations. All manipulations were performed under
rigorous exclusion of air and moisture using standard Schlenk, high-
vacuum, and glovebox techniques (MBraun UNIlabpro ECO); <0.5 ppm O2,
<0.5 ppm H2O, argon atmosphere). The solvents n-hexane, toluene,
diethyl ether, and tetrahydrofuran (thf) were purified using Grubbs-type
columns (MBraun SPS, solvent purification system). C6D6 (99.6%, Sigma-
Aldrich), and toluene-d8 (99.6%, Sigma-Aldrich) were dried by letting the
solvents stand over Na/K-alloy for at least 24 h and subsequently filtrated.
CDCl3 (99.8%, Sigma-Aldrich) was used as received. All solvents were
stored inside a glovebox. Ln(AlMe4)3 (Ln = Y, La, Ce, Pr, Nd, Lu) were
synthesized according to literature procedures.[19b,e,31] K(2,4-dmp) was
prepared from 2,4-dimethyl-1,3-pentadiene (98%, Sigma-Aldrich) and K/
NEt3 according to a procedure published by Yasuda and co-workers.[2]
K(2,4-dtbp) was prepared from 2,4-tert-butyl-1,3-pentadiene and
Schlosser’s base.[3d] Tetramethylsilane was purchased from Sigma-Aldrich,
dried over potassium, distilled and stored in a glovebox prior to use. GaMe3
(optoelectronic grade) was purchased from Dockweiler Chemicals and
used as received. [CPh3][B(C6F5)4], [PhNMe2H][B(C6F5)4], and B(C6F5)3
were obtained from Boulder Scientific and used without further purification.
NaH, Ph3P–CH3Br, n-BuLi (2.5 M in n-hexane), and DMSO (≥99%) were
purchased from Sigma-Aldrich and used as received. 2,5,6-Trimethylhept-
4-en-3-one was synthesized according to a literature procedure.[32] NMR
spectra of air and moisture sensitive compounds were recorded by using
J. Young valve NMR tubes at ambient temperature on either a Bruker
AVII+400 (1H, 13C, 19F) or a Bruker AVII+500 (low temperature spectra).
NMR chemical shifts are referenced to internal solvent resonances and
reported in parts per million relative to tetramethylsilane (TMS), and CCl3F.
Coupling constants are given in Hertz. Elemental analyses were
performed on an Elementar Vario Micro Cube. Size-exclusion
Conclusions
The formation of rare-earth metal pentadienyl half-sandwich
complexes could be achieved via a salt metathesis route and is
mainly dependent of the rare-earth metal size and the reaction
temperature employed. Rather unexpectedly, the largest rare-
earth metal centers selectively formed half-sandwich complexes
[(2,4-dtbp)Ln(AlMe4)2] (Ln
=
La, Ce, Pr, Nd), [(2,4-
dipp)La(AlMe4)2], and [(2,4-dmp)La(AlMe4)2], whereas with
smaller-sized Ln(III) centers only sandwich complexes [(2,4-
dipp)2Y(AlMe4)] and [(2,4-dmp)2Ln(AlMe4)] (Ln = Y, Lu) could be
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10.1002/chem.201900108
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chromatography (SEC) was performed on a Viscotek GPCmax consisting
of a GPCmax apparatus and a model TDA 302 triple detector array.
Sample solutions (1.0 mg polymer per mL THF) were filtered through a
0.45 µm syringe filter prior to injection. The flow rate was 1 mL/min. dn/dc
and dA/dc data were determined by means of the integrated OmniSec
software. The microstructure of the polyisoprenes was determined on a
Bruker AVBII+400 spectrometer in CDCl3 at ambient temperatures. Glass
transition temperatures of the polyisoprenes (Tg) were recorded on a
Perkin-Elmer DSC 8000, calibrated with cyclohexane and indium
standards, and by scanning from –100 °C up to + 100 °C with heating rates
of 20 K/min and cooling rates of 60 K/min in N2 atmosphere.
added, which led to the formation of a colorless precipitate. The latter was
separated via filtration and the process repeated until no more precipitate
could be observed. After re-cooling the remaining oil to –40 °C, 1 formed
as orange crystals overnight suitable for an X-ray diffraction study.
Unfortunately, no crystalline yield can be given due to co-crystallization of
unreacted La(AlMe4)3. Yield (determined by 1H NMR): 74%; 1H NMR (500
MHz, [D8]toluene, –40 °C): d = 4.65 (s, 1H; –CH=), 3.86 (s, 2H; =CHexo),
3.37 (s, 2H; =CHendo), 1.63 (s, 6H; –CH3), –0.21 ppm (s, 24H; Al(CH3)4);
13C{1H} NMR (126 MHz, [D8]toluene, 26 °C): d = 153.0 (s, 2C; CCH3), 95.7
(s, 1C; –CH=), 94.2 (s, 2C; =CH2), 29.2 (s, 2C; CCH3), 1.9 ppm (s, 8C;
Al(CH3)4). An elemental analysis cannot be provided due to contamination
with La(AlMe4)3.
2,4-Di-iso-propyl-1,3-pentadiene. Sodium hydride (2.43 g, 0.10 mol) was
placed inside a three-necked flask equipped with a nitrogen inlet and
dropping funnel. DMSO (50 mL) was slowly added under vigorous stirring
and subsequently heated to 80 °C. After hydrogen evolution had stopped
(approx. 0.5 h), the reaction mixture was cooled to ambient temperature
and a solution of Ph3P–CH3Br (36.1 g, 0.10 mol) in 100 mL of DMSO was
added slowly under vigorous stirring. After 10 min 2,5,6-trimethylhept-4-
en-3-one (15.4 g, 0.10 mol) was added and the solution heated to 60 °C
for 2 h and stirred for another 18 h at ambient temperature. Afterwards, the
reaction mixture was poured onto 150 mL of distilled water and extracted
3 times with 50 mL of n-hexane. The combined organic phases were
washed with water (50 mL), dried over Na2SO4 and fractionated under
reduced pressure to obtain 2,4-di-iso-propyl-1,3-pentadiene as a colorless
liquid (9.35 g, 0.06 mol, 61%). B.p. 35 °C (5 mbar); 1H NMR (400 MHz,
CDCl3, 26 °C): d = 5.65 (d, 4J(H,H) = 1.18 Hz, 1H; –CH=), 4.96 (dd, 2J(H,H)
= 2.27 Hz, 4J(H,H) = 1.12 Hz, 2H; =CHendo), 4.67 (dd, 2J(H,H) = 2.31 Hz,
4J(H,H) = 1.74 Hz, 2H; =CHexo), 2.31 (m, 1H; –CH(CH3)2), 2.29 (m, 1H; –
CH(CH3)2), 1.70 (s, 3H; –CH3), 1.04 (d, 3J(H,H) = 6.88 Hz, 6H; –CH(CH3)2),
1.00 ppm (d, 3J(H,H) = 6.88 Hz, 6H; –CH(CH3)2); 13C{1H} NMR (101 MHz,
CDCl3, 26 °C): d = 152.6 (s, 1C; CCH(CH3)2), 144.4 (s, 1C; CCH(CH3)2),
122.6 (s, 1C; –CH=), 109.8 (s, 1C; =CH2), 37.4 (s, 1C; CCH(CH3)2), 35.4
(s, 1C; CCH(CH3)2), 21.7 (s, 2C; CCH(CH3)2), 21.6 (s, 2C; CCH(CH3)2),
15.2 ppm (s, 1C; –CH3); elemental analysis calcd (%) for C11H20: C 86.76,
H 13.24; found: C 84.26, H 12.51. The low carbon and hydrogen values
are caused by residual Et2O.
[2,4-dipp)La(AlMe4)2] (2). To a cooled (–40 °C) and stirred suspension of
K(2,4-dipp) (85.6 mg, 0.45 mmol) in 2.5 mL of toluene, an equally cold
solution of La(AlMe4)3 (200 mg, 0.50 mmol) in 2.5 mL of toluene was added.
The reaction mixture was stirred at –40 °C for 18 h and afterwards the
solvent was removed in vacuo. The solid residue was washed 3 times with
n-hexane (3 mL) and the combined organic phases concentrated to yield
an orange oil. After addition of tetramethylsilane (0.3 mL), orange crystals
of 2 (156 mg, 0.34 mmol, 75%) formed overnight at –40 °C. 1H NMR (400
MHz, [D6]benzene, 26 °C): d = 4.71 (t, 4J(H,H) = 2.20 Hz, 1H; –CH=), 4.22
(dd, 2J(H,H) = 2.62 Hz, 4J(H,H) = 2.09 Hz, 2H; =CHexo), 3.62 (d, 2J(H,H) =
1.92 Hz, 2H; =CHendo), 2.18 (m, 2H; –CH(CH3)2), 1.02 (d, 3J(H,H) = 6.87
Hz, 6H; –CH(CH3)2), 0.86 (d, 3J(H,H) = 6.73 Hz, 6H; –CH(CH3)2), –0.16
ppm (s, 24H; Al(CH3)4); 13C{1H} NMR (101 MHz, [D6]benzene, 26 °C): d =
163.8 (s, 2C; CCH(CH3)2), 94.8 (s, 1C; –CH=), 90.8 (s, 2C; =CH2), 39.3 (s,
2C; CCH(CH3)2), 25.5 (s, 2C; CCH(CH3)2), 20.1 (s, 2C; CCH(CH3)2), 1.9
ppm (s, 8C; Al(CH3)4); elemental analysis calcd (%) for C19H43Al2La: C
49.14, H 9.33; found: C 48.13, H 8.51. The low carbon and hydrogen
values can be explained by residual tetramethylsilane.
[2,4-dtbp)La(AlMe4)2] (3La). To a cooled (–40 °C) and stirred suspension
of K(2,4-dtbp) (98.0 mg, 0.45 mmol) in 5 mL of n-hexane, an equally cold
solution of La(AlMe4)3 (200 mg, 0.50 mmol) in 5 mL of n-hexane was added.
The reaction mixture was stirred at –40 °C for 2 h and the remaining
precipitate was removed via centrifugation and washed 3 times with n-
hexane (3 mL). The combined organic phases were concentrated to give
an orange oil. After addition of tetramethylsilane (0.3 mL), orange crystals
of 3La (172 mg, 0.35 mmol, 78%) formed overnight at –40 °C. 1H NMR (400
MHz, [D6]benzene, 26 °C): d = 4.99 (t, 4J(H,H) = 2.33 Hz, 1H; –CH=), 4.33
(dd, 2J(H,H) = 2.92 Hz, 4J(H,H) = 2.44 Hz, 2H; =CHexo), 4.15 (d, 2J(H,H) =
2.52 Hz, 2H; =CHendo), 0.95 (s, 18H; –C(CH3)3), –0.04 ppm (s, 24H;
Al(CH3)4); 13C{1H} NMR (101 MHz, [D6]benzene, 26 °C): d = 167.2 (s, 2C;
CC(CH3)3), 92.0 (s, 1C; –CH=), 90.2 (s, 2C; =CH2), 39.4 (s, 2C; CC(CH3)3),
30.4 (s, 6C; CC(CH3)3), 4.6 ppm (s, 8C; Al(CH3)4); elemental analysis calcd
(%) for C21H47Al2La: C 51.22, H 9.62; found: C 51.90, H 9.32.
K(2,4-dipp). KOtBu (6.89 g, 0.06 mol) was suspended in 100 mL of n-
hexane and the suspension was cooled to –78 °C. To the chilled and
stirred suspension, 2,4-di-iso-propyl-1,3-pentadiene (9.35 g, 0.06 mol) and
subsequently n-BuLi (26 mL, 0.07 mol, 2.5 M in n-hexane) were added
slowly. After complete addition, the reaction mixture was warmed slowly to
ambient temperature and stirred for another 18 h. The precipitate was
separated off and washed 3 times with 25 mL of n-hexane and once with
20 mL of cold THF. After drying in vacuo, K(2,4-dipp) was obtained as an
off-white solid (8.20 g, 0.04 mol, 70%). 1H NMR (400 MHz, [D8]THF,
26 °C): d = 3.48 (dd, 4J(H,H) = 2.21 Hz, 4J(H,H) = 1.84 Hz, 1H; –CH=),
3.44 (dd, 2J(H,H) = 2.50 Hz, 4J(H,H) = 1.86 Hz, 2H; CHexo), 3.09 (d, 2J(H,H)
= 1.99 Hz, 2H; CHendo), 2.13 (m, 2H; –CH(CH3)2), 1.08 ppm (d, 3J(H,H) =
6.90 Hz, 12H; –CH(CH3)2); 13C{1H} NMR (101 MHz, [D8]THF, 26 °C): d =
156.2 (s, 2C; CCH(CH3)2), 79.5 (s, 1C; –CH=), 73.3 (s, 2C; =CH2), 40.7 (s,
2C; CCH(CH3)2), 25.0 ppm (s, 2C; CCH(CH3)2); elemental analysis calcd
(%) for C11H19K: C 69.40, H 10.06; found: C 66.76, H 8.93. Although these
results are outside the range viewed as establishing analytical purity, they
are provided to illustrate the best values obtained to date.
[2,4-dtbp)Ce(AlMe4)2] (3Ce). To a cooled (–40 °C) and stirred suspension
of K(2,4-dtbp) (98.0 mg, 0.45 mmol) in 5 mL of n-hexane, an equally cold
solution of Ce(AlMe4)3 (200 mg, 0.50 mmol) in 5 mL of n-hexane was
added. The reaction mixture was stirred at –40 °C for 2 h and the remaining
precipitate was removed via centrifugation and washed 3 times with n-
hexane (3 mL). The combined organic phases were concentrated to give
a red oil. After addition of tetramethylsilane (0.3 mL), dark orange crystals
of 3Ce (148 mg, 0.30 mmol, 67%) formed overnight at –40 °C. 1H NMR
(400 MHz, [D6]benzene, 26 °C): d = 42.26 (s, 1H; –CH=), 3.21 (s, 24H;
Al(CH3)4), 3.03 (s, 18H; –C(CH3)3), –16.77 (s, 2H; =CH2), –32.85 ppm (s,
2H; =CH2); elemental analysis calcd (%) for C21H47Al2Ce: C 51.09, H 9.60;
found: C 51.02, H 9.45.
[2,4-dmp)La(AlMe4)2] (1). To a cooled (–40 °C) and stirred suspension of
K(2,4-dmp) (75.5 mg, 0.56 mmol) in 2.5 mL of toluene, an equally cold
solution of La(AlMe4)3 (250 mg, 0.63 mmol) in 2.5 mL of toluene was added.
The reaction mixture was stirred at –40 °C for 18 h and afterwards the
solvent was removed in vacuo. The solid residue was washed 3 times with
n-hexane (3 mL) and the combined organic phases concentrated to give
an orange oil. Th oil was cooled to –40 °C and 0.1 mL of n-hexane was
[2,4-dtbp)Pr(AlMe4)2] (3Pr). To a cooled (–40 °C) and stirred suspension
of K(2,4-dtbp) (97.7 mg, 0.45 mmol) in 5 mL of n-hexane, an equally cold
solution of Pr(AlMe4)3 (200 mg, 0.50 mmol) in 5 mL of n-hexane was added.
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The reaction mixture was stirred at –40 °C for 2 h and the remaining
precipitate was removed via centrifugation and washed 3 times with n-
hexane (3 mL). The combined organic phases were concentrated to give
a red oil. After addition of tetramethylsilane (0.3 mL), dark orange crystals
of 3Pr (108 mg, 0.22 mmol, 45%) formed overnight at –40 °C. 1H NMR (400
MHz, [D6]benzene, 26 °C): d = 96.48 (s, 1H; –CH=), 8.22 (s, 24H; Al(CH3)4),
6.29 (s, 18H; –C(CH3)3), –48.10 (s, 2H; =CH2), –78.99 ppm (s, 2H; =CH2);
elemental analysis calcd (%) for C21H47Al2Pr: C 51.01, H 9.58; found: C
50.23, H 9.01.
diffraction analysis could be obtained from a concentrated n-hexane
solution. 1H NMR (400 MHz, [D6]benzene, 26 °C): d = 4.86 (t, 4J(H,H) =
2.00 Hz, 2H; –CH=), 4.09 (dd, 2J(H,H) = 2.64 Hz, 4J(H,H) = 2.05 Hz, 4H;
=CHexo), 3.38 (d, 2J(H,H) = 1.96 Hz, 4H; =CHendo), 2.24 (m, 4H; –CH(CH3)2),
1.13 (d, 2J(H,H) = 6.88 Hz, 12H; –CH(CH3)2), 0.97 (d, 2J(H,H) = 6.66 Hz,
12H; –CH(CH3)2), –0.30 ppm (d, 2J(H,Y) = 1.78 Hz, 12H; Al(CH3)4); 13C{1H}
NMR (101 MHz, [D6]Benzene, 26 °C): d = 159.5 (s, 4C; CCH(CH3)2), 87.4
(s, 2C; –CH=), 82.6 (s, 4C; =CH2), 38.9 (s, 4C; CCH(CH3)2), 25.3 (s, 4C;
CCH(CH3)2), 20.4 (s, 4C; CCH(CH3)2); 13C signals attributable to the
[AlMe4] ligand could not be resolved; elemental analysis calcd (%) for
C26H50AlY: C 65.25, H 10.53; found: C 65.25, H 10.36.
[2,4-dtbp)Nd(AlMe4)2] (3Nd). To a cooled (–40 °C) and stirred suspension
of K(2,4-dtbp) (96.9 mg, 0.44 mmol) in 5 mL of n-hexane, an equally cold
solution of Nd(AlMe4)3 (200 mg, 0.49 mmol) in 5 mL of n-hexane was
added. The reaction mixture was stirred at –40 °C for 2 h and the remaining
precipitate was removed via centrifugation and washed 3 times with n-
hexane (3 mL). The combined organic phases were concentrated to give
a dark red oil. After addition of tetramethylsilane (0.3 mL), red crystals of
[2,4-dtbp)2Nd(AlMe4)] (6). To a stirred suspension of K(2,4-dtbp) (100 mg,
0.46 mmol) in 5 mL of n-hexane, a solution of Nd(AlMe4)3 (100 mg, 0.57
mmol) in 5 mL of n-hexane was added. The reaction mixture was stirred
at ambient temperature for 18 h and afterwards the solvent was removed
in vacuo. The solid residue was washed 3 times with n-hexane (3 mL) and
the combined organic phases dried under reduced pressure to obtain 6 as
a green solid (123 mg, 0.21 mmol, 85%). Crystals suitable for X-ray
diffraction analysis could be obtained from a concentrated n-hexane
solution. 1H NMR (400 MHz, [D6]benzene, 26 °C): d = 34.50 (s, 2H; –CH=),
4.10 (s, 36H; –C(CH3)3), –12.40 (br s, 12H; Al(CH3)4), –27.20 (s, 4H; =CH2),
–34.50 ppm (s, 4H; =CH2); elemental analysis calcd (%) for C30H58AlNd: C
61.07, H 9.91; found: C 60.42, H 9.21.
1
3Nd (126 mg, 0.25 mmol, 57%) formed overnight at –40 °C. H NMR (400
MHz, [D6]benzene, 26 °C): d = 44.78 (s, 1H; –CH=), 7.99 (s, 24H; Al(CH3)4),
3.81 (s, 18H; –C(CH3)3), –31.39 (s, 2H; =CH2), –45.92 ppm (s, 2H; =CH2);
elemental analysis calcd (%) for C21H47Al2Nd: C 50.67, H 9.52; found: C
50.48, H 9.58.
[2,4-dmp)2Y(AlMe4)] (4Y). To a stirred suspension of K(2,4-dmp) (153 mg,
1.14 mmol) in 5 mL of toluene, a solution of Y(AlMe4)3 (200 mg, 0.57 mmol)
in 5 mL of toluene was added. The reaction mixture was stirred at ambient
temperature for 18 h and afterwards the solvent was removed in vacuo.
The solid residue was washed 3 x with n-hexane (3 mL) and the combined
organic phases concentrated. After standing overnight at –40 °C, yellow
crystals of 4Y (109 mg, 0.30 mmol, 52%) could be obtained. 1H NMR (400
MHz, [D6]benzene, 26 °C): d = 4.89 (t, 4J(H,H) = 1.98 Hz, 2H; –CH=), 3.98
(s, 4H; =CHexo), 3.21 (s, 4H; =CHendo), 1.69 (s, 12H; –CH3), –0.36 ppm (d,
2J(H,Y) = 1.78 Hz, 12H; Al(CH3)4); 13C{1H} NMR (101 MHz, [D6]benzene,
26 °C): d = 148.2 (s, 4C; C(CH3)), 90.4 (s, 2C; –CH=), 86.3 (s, 4C; =CH2),
29.2 (s, 4C; C(CH3)), 8.8 ppm (s, 4C; Al(CH3)4); 13C signals attributable to
the [AlMe4] ligand could be resolved with an 1H-13C HSQC NMR
experiment; elemental analysis calcd (%) for C18H34AlY: C 59.01, H 9.35;
found: C 58.34, H 9.36. The same outcome was observed when the
reaction was performed at -40 °C.
[2,4-dtbp)La(GaMe4)2] (7). To a stirred solution of 3La (100 mg, 0.20 mmol)
in n-hexane (2 mL), GaMe3 (70.0 mg, 0.61 mmol) was added under
vigorous stirring at ambient temperature. Subsequently, a solution of
GaMe3∙Et2O (154 mg; 0.81 mmol) in 1 mL of n-hexane was also added
and the reaction mixture was stirred for 30 min. at r.t. After concentrating
the solution under reduced pressure, cooling to –40 °C, and addition of 0.1
mL of tetramethylsilane, orange crystals of 7 (94.0 mg, 0.16 mmol, 80%)
could be obtained. 1H NMR (500 MHz, [D6]benzene, 26 °C): d = 4.98 (t,
4J(H,H) = 2.04 Hz, 1H; –CH=), 4.33 (dd, 2J(H,H) = 3.15 Hz, 4J(H,H) = 2.39
Hz, 2H; =CHexo), 4.16 (d, 2J(H,H) = 2.34 Hz, 2H; =CHendo), 0.97 (s, 18H; –
C(CH3)3), 0.10 (s, 24H; Ga(CH3)4); 13C{1H} NMR (126 MHz, [D6]benzene,
26 °C): d = 166.3 (s, 2C; CC(CH3)3), 90.9 (s, 1C; –CH=), 99.3 (s, 2C; =CH2),
39.0 (s, 2C; CC(CH3)3), 30.2 (s, 6C; CC(CH3)3), 6.0 ppm (s, 8C; Ga(CH3)4);
Elemental analysis cannot be provided due to contamination with an
unknown side-product.
[2,4-dmp)2Lu(AlMe4)] (4Lu). To a stirred suspension of K(2,4-dmp) (123
mg, 0.92 mmol) in 5 mL of toluene, a solution of Lu(AlMe4)3 (200 mg, 0.46
mmol) in 5 mL of toluene was added. The reaction mixture was stirred at
ambient temperature for 18 h and afterwards the solvent was removed in
vacuo. The solid residue was washed 3 times with n-hexane (3 mL) and
the combined organic phases concentrated. After standing overnight at –
40 °C, yellow crystals of 4Lu (114 mg, 0.25 mmol, 55%) could be obtained.
1H NMR (400 MHz, [D6]benzene, 26 °C): d = 4.94 (t, 4J(H,H) = 1.95 Hz,
2H; –CH=), 3.92 (s, 4H; =CHexo), 3.08 (s, 4H; =CHendo), 1.76 (s, 12H; –
CH3), –0.16 ppm (br s, 6H; (µ–Me2AlMe2)); –0.34 (br s, 6H; (µ–Me2AlMe2));
13C{1H} NMR (101 MHz, [D6]Benzene, 26 °C): d = 147.9 (s, 4C; C(CH3)),
90.1 (s, 2C; –CH=), 84.5 (s, 4C; =CH2), 29.2 (s, 4C; C(CH3)), 11.6 (s, 2C;
(µ–Me2AlMe2)); 11.4 ppm (s, 2C; (µ–Me2AlMe2)); 13C-signals attributable
to the [AlMe4] ligand could be resolved with an 1H-13C HSQC NMR
experiment; elemental analysis calcd (%) for C18H34AlLu: C 47.79, H 7.57;
found: C 47.31, H 7.05.
{{(2,4-dtbp)La[( -Me)2AlMe(C6F5)]}[Me2Al(C6F5)2]}2∙(toluene)2 (8). To a
µ
stirred solution of 3La (50.0 mg, 0.10 mmol) in toluene (1.5 mL), a solution
of B(C6F5)3 (52.0 mg, 0.10 mmol) in toluene (1.5 mL) was added. The
reaction mixture was stirred for 30 min and then concentrated in vacuo.
After cooling to –40 °C, orange crystals of 8 (70 mg, 0.03 mmol, 70%) were
obtained. Caution! Owing to the formation of thermal and shock-sensitive
[Me2Al(C6F5)2]-, extra care should be taken when handling the dry solid.
Crystals have to be dried at normal pressure as drying under reduced
pressure causes toluene to be removed from the compound, resulting in
unpredictable behavior of the product. 1H NMR (400 MHz, [D6]benzene,
26 °C): d = 5.08 (s, 2H; –CH=), 4.34 (br s, 8H; CH2), 0.86 (s, 36H; –
C(CH3)3), 0.56 (s, 6H; –AlMe(C6F5)), 0.19 (s, 24H; –(µ-Me)2Al); 13C{1H}
NMR (126 MHz, [D8]Toluene, 26 °C): d = 93.1 (s, 2C; –CH=), 91.4 (s, 4C;
=CH2), 39.3 (s, 4C; CC(CH3)3), 29.6 (s, 12C; CC(CH3)3), 9.6 (s, 2C; –
AlMe(C6F5)), 6.4 (s, 8C; –(µ-Me)2Al); 13C signals not visible in the 13C{1H}
NMR spectrum could be resolved with an 1H-13C HSQC NMR experiment;
19F{1H} NMR (376 MHz, [D6]benzene, 26 °C): d = –121.8 (br s, 6F; o-F), –
122.3 (br s, 6F; o-F), –153.3 (br s, 3F; p-F), –154.0 (br s, 3F; p-F), –160.6
(br s, 6F; m-F), –161.1 (br s, 6F; m-F); elemental analysis calcd (%) for
C86H92F30Al4La2: C 49.63, H 4.46; found: C 49.77, H 4.16. The discrepancy
in toluene content between crystal structure (three molecules of toluene)
and elemental analysis (two molecules of toluene) can be attributed to loss
of one molecule of toluene upon drying.
[2,4-dipp)2Y(AlMe4)] (5). To a stirred suspension of K(2,4-dipp) (207 mg,
1.08 mmol) in 5 mL of n-hexane, a solution of Y(AlMe4)3 (200 mg, 0.57
mmol) in 5 mL of n-hexane was added. The reaction mixture was stirred
at ambient temperature for 18 h and afterwards the solvent was removed
in vacuo. The solid residue was washed 3 times with n-hexane (3 mL) and
the combined organic phases dried under reduced pressure to obtain 5 as
a yellow-green solid (195 mg, 0.41 mmol, 71%). Crystals suitable for X-ray
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Kulsomphob, G. C. Turpin, K.-C. Lam, C. Youngkin, W. Trakarnpruk, P.
Carroll, A. L. Rheingold, R. D. Ernst, J. Chem. Soc., Dalton Trans. 2000,
3086-3093.
Polymerization of Isoprene. A detailed polymerization procedure is
described as a typical example. A solution of [CPh3][B(C6F5)4] (A) (1 equiv.,
18.4 mg, 0.02 mmol) in toluene (3.5 mL) was added to a solution of pre-
catalyst 3La (9.98 mg, 0.02 mmol) in toluene (3.5 mL) and the mixture was
stirred at ambient temperature for 30 min. After addition of isoprene (2.00
mL, 20 mmol), the polymerization was carried out at ambient temperature
for 2 h. The polymerization mixture was quenched in a large quantity (50
mL) of methanol containing 0.1% (w/w) 2,6-di-tert-butyl-4-methylphenol as
a stabilizer. After washing with methanol, the polymer was dried in vacuo
at ambient temperature to constant weight. The monomer conversion was
determined gravimetrically. The microstructure of the polymer was
examined by 1H and 13C NMR spectroscopy in CDCl3.
[7] W. Trakarnpruk, A. M. Arif, R. D. Ernst, Organometallics 1992, 11, 1686-
1692.
[8] a) W. A. Donaldson, P. T. Bell, M.-J. Jin, J. Organomet. Chem. 1992, 441,
449-456; b) R. W. Gedridge, A. M. Arif, R. D. Ernst, J. Organomet. Chem.
1995, 501, 95-100.
[9] a) W. Trakarnpruk, A. L. Rheingold, B. S. Haggerty, R. D. Ernst,
Organometallics 1994, 13, 3914-3920; b) V. Kulsomphob, K. A. Ernst, K.-
C. Lam, A. L. Rheingold, R. D. Ernst, Inorg. Chim. Acta 1999, 296, 170-
175.
[10] a) E. A. Benham, P. D. Smith, E. T. Hsieh, M. P. McDaniel, J. Macromol.
Sci. A. 1988, 25, 259-283; b) P. D. Smith, M. P. McDaniel, J. Polym. Sci. A.
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X-Ray Crystallography and Crystal Structure Determinations. Crystals
of 1, 2, 3La, 3Ce, 3Pr, 3Nd, 4Y, 4Lu, 5, 6, 7, 8, and Ph3C–(2,4-dtbp) were
grown by standard techniques using saturated solutions of n-hexane/TMS
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MoKa (l = 0.71073 Å) and CuKa (l = 1.54184 Å) radiation. The data
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S3 and in the CIF files. CCDC depositions 1886707-1886718 contain all
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
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Acknowledgements
We are grateful to the German Science Foundation (An 238/14-
2) and Bridgestone Japan for generous support.
Keywords: lanthanides • pentadienyl ligand • isoprene
polymerization • half-sandwich complex •
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Entry for the Table of Contents
FULL PAPER
Equimolar treatment of open half-
sandwich complex [(2,4-
Damir Barisic, Dennis A. Buschmann,
David Schneider, Cäcilia Maichle-
Mössmer, and Reiner Anwander*
dtbp)Ln(AlMe4)2] with B(C6F5)3 gives
isolable compound {{(2,4-dtbp)La[(µ-
Me)2AlMe(C6F5)]}[Me2Al(C6F5)2]}2 with
less tightly bonded contact ion pairs
compared to the C5Me5 congener,
which markedly affects the
F4
F5
tBu
Page No. – Page No.
Al Me
Me
F5
F
tBu
Me
Al
Al
Rare-Earth-Metal Pentadienyl Half-
Sandwich and Sandwich
Tetramethylaluminates – Synthesis,
Structure, Reactivity, and
Performance in Isoprene
Polymerization
Me
Me
Me
Me
La
La
F
Me
Me
tBu
Al
Me
F5
polymerization performance.
F5
tBu
F4
single-component catalyst
This article is protected by copyright. All rights reserved.