Lanthanide(II) complexes bearing mixed linked and unlinked cyclopentadienyl-monodentate-anionic ligands

Article abstract of DOI:10.1016/S0022-328X(01)01237-2

This paper is intended to provide an overview on recent progress in the chemistry of divalent lanthanide complexes bearing mixed (linked and unlinked) cyclopentadienyl-monodentate-anionic ligands. The synthesis, structures, and one-electron-transfer and p

Full text of DOI:10.1016/S0022-328X(01)01237-2

Journal of Organometallic Chemistry 647 (2002) 61–70
www.elsevier.com/locate/jorganchem
Mini review
Lanthanide(II) complexes bearing mixed linked and unlinked
cyclopentadienyl–monodentate-anionic ligands
Zhaomin Hou *, Yasuo Wakatsuki
Organometallic Chemistry Laboratory, RIKEN (The Institute of Physical and Chemical Research), Hirosawa 2-1, Wako, Saitama 351-0198,
Japan
Received 2 August 2001; accepted 3 September 2001
Abstract
This paper is intended to provide an overview on recent progress in the chemistry of divalent lanthanide complexes bearing
mixed (linked and unlinked) cyclopentadienyl–monodentate-anionic ligands. The synthesis, structures, and one-electron-transfer
and polymerization reactions of these complexes are described. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Divalent lanthanides; Metallocene complex; Mixed ligands; Monodentate-anion ligand; Polymerization; Reduction
1. Introduction
sterically demanding substrates. Thus (C5Me5)2Sm(thf)x
(x=0 or 2) can polymerize ethylene, but cannot poly-
merize styrene or dienes owing to steric hindrance [6].
In contrast, heteroatom-containing monodentate-an-
ionic ligands such as aryloxide, silylamide, and thiolate
are less sterically demanding and more flexible than
C5Me5, but the electron-donating power of these lig-
ands is not strong enough to enable the Ln(II) center to
transfer an electron to an unactivated olefin monomer.
In principle, the mixed C5Me5–monodentate-anionic
ligand systems should offer a sterically and electroni-
cally unique environment for the lanthanide(II) center.
The combination of the strong electron-donating ability
of C5Me5 (which is important for a Ln(II) complex to
initiate a reaction) and the good flexibility of a
monodentate-anionic ligand (which is important for a
catalytic process) would create a new (catalytic) system
that is different from the homoleptic analogs. Toward
this end, several types of such mixed-ligand-supported
lanthanide(II) complexes have been recently synthesized
and have had their reactivity studied. Here we wish to
make an overview on the progress in this field.
The organometallic chemistry of the lanthanides in
low oxidation states has long been dominated by the
metallocene complexes that bear two pentamethylcy-
clopentadienyl (C5Me5) ligands [1]. Attempts to extend
the lanthanide organometallic chemistry beyond the
traditional realm of the metallocene complexes have led
to the synthesis and structural characterization of a
number of lanthanide(II) complexes bearing various
ligand systems, most of which are analogous to the
metallocenes in that they all bear two identical ancillary
ligands [2]. Reactivity comparison studies on these com-
plexes have shown that the reactivity of a lanthanide(II)
complex is strongly influenced by both the steric and
the electronic properties of the ancillary ligands. The
sterically demanding and electronically donating bis(-
pentamethylcyclopentadienyl) ligand set (C5Me5)2 can
offer excellent solubility, reducing power, and protec-
tion for the metal center, which, for example, has
enabled the samarocene(II) complex (C5Me5)2Sm(thf)x
(x=0 or 2) to activate small molecules such as N2 [3],
CO [4], or CO2 [5]. However, the steric bulkiness of the
(C5Me5)2 ligand set could, on the other hand, become a
drawback particularly in catalytic reactions involving
2. Synthesis and structures of lanthanide(II) complexes
with mixed C5Me5–monodentate-anionic ligands
The mono(pentamethylcyclopentadienyl)lanthanide-
* Corresponding author. Fax: +81-48-4624665.
E-mail address: houz@postman.riken.go.jp (Z. Hou).
(II) iodide complexes [(C5Me5)Ln(m-I)(thf)2]2 (Ln=Sm
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01237-2

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Z. Hou, Y. Wakatsuki / Journal of Organometallic Chemistry 647 (2002) 61–70
The C5Me5–OAr-ligated samarium(II) complex 1a
was first obtained by partial protonation of
(C5Me5)2Sm(thf)2 with one equivalent of HOAr (Ar=
C6Ht2Bu2-2,6-Me-4) in toluene (Scheme 1) [9]. This
complex adopts a dimeric structure via m-OAr bridges
as shown by X-ray analysis. Analogous protonation of
(C5Me5)2Sm(thf)2 with HOSi(OtBu)3 yielded the unsym-
metrical binuclear Sm(II) complex 2, which contains
one terminal C5Me5 and three bridging OSi(OtBu)3
ligands (Scheme 1) [10]. Complex 2 could formally be
viewed as a combination of the mixed ligand complex
‘(C5Me5)Sm{OSi(OtBu)3}’ and the bis(siloxide) complex
‘Sm{OSi(OtBu)3}2’.
Complex 1a could also be synthesized by reaction of
(C5Me5)2Sm(thf)2 with one equivalent of Sm(OAr)2-
(thf)3 in toluene (Scheme 2) [9]. Similar reaction of
(C5Me5)2Sm(thf)2 with Sm{N(SiMe3)2}2(thf)2 afforded
the N(SiMe3)2 analog 1b [11]. These heteroleptic,
dimeric complexes 1a,b were stable in toluene, but
existed in an equilibrium with (C5Me5)2Sm(thf)2 and
Sm(ER)2(thf)x (ER=OAr or N(SiMe3)2) in THF. Ad-
dition of two equivalents of hexamethylphosphoric tri-
amide (hmpa) to 1a,b in THF, however, afforded in
high yield the hmpa-coordinated, monomeric com-
plexes 3a,b, respectively (Scheme 2) [9,12].
[7a] or Yb [7b]) could be obtained by reaction of LnI2
with one equivalent of MC5Me5 (M=Na or K) or
(C5Me5)2Ln(thf)2. These complexes usually adopt a
dimeric structure when in solid state, but exist in equi-
librium with other species in THF solution [7]. In
contrast, the reaction of the more sterically demanding
LiC5Me4SiMet2Bu with one equivalent of YbI2 afforded
the hexanuclear Yb(II) species {Yb6(C5Me4SiMet2Bu)6-
I8}{Li(thf)4}2 [7b]. Derivation of other mono(cyclopen-
tadienyl)lanthanide(II) complexes from these iodide
complexes has not been reported. Formation of the
(pentamethylcyclopentadienyl)ytterbium(II)
stannyl
complex (C5Me5)Yb{Sn(CHt2Bu)3}(thf)2 was observed
in an NMR tube reaction between Yb{Sn(CHt2Bu)3}2-
(thf)2 and one equivalent of C5Me5H, but it was not
clear whether this complex was isolable [8].
The siloxide complex 2 was stable both in toluene
and in THF, but reacted with hmpa in THF to give the
monomeric complex (C5Me5)Sm{OSi(OtBu)3}(hmpa)2
as the only isolable product [10]. It is also noteworthy
that 2 could act as a neutral coordination ligand
through the C5Me5 unit. The reaction of 2 with
Sm{OSi(OtBu)3}3(thf)2 yielded the inverse sandwich
Sm(III)–Sm(II) trinuclear complex 4, showing that the
coordination of 2 to the Sm(III) center is stronger than
that of THF (Scheme 3).
Scheme 1.
An attempt to synthesize 1a by the metathetical
reaction of Sm(OAr)2(thf)3 with one equivalent of
KC5Me5 yielded the unexpected product 5a, which
could be viewed as a C5Me5–OAr-ligated Sm(II) com-
plex coordinated by the neutral ‘(C5Me5)K(thf)2’ ligand
(Scheme 4) [9]. In fact, addition of two equivalents of
KC5Me5 to a THF solution of 1a did give 5a in almost
quantitative yield.
Scheme 2.
The most efficient route to 5a is the reaction of
(C5Me5)2Sm(thf)2 with one equivalent of KOAr. This
route could be successfully extended to the synthesis of
the analogous thiolate (5c) [12], amide (5e–f) [12], silyl
(5g–i) [13], and alkyl (5j–l) [14] lanthanide(II) com-
plexes as shown in Scheme 5. Complexes 5a–l all adopt
a similar polymeric structure via ‘intermolecular’ inter-
actions between the K atom and a C5Me5 ligand.
Reaction of 5a,e with two equivalents of hmpa (per Sm)
in THF afforded the corresponding hmpa-coordinated,
monomeric complexes 3a,b, respectively, showing that
the ‘C5Me5K’ ligand in this type of complexes could be
replaced by an appropriate coordinative ligand.
Scheme 3.

Z. Hou, Y. Wakatsuki / Journal of Organometallic Chemistry 647 (2002) 61–70
63
probably owing to the stronger electron-donating abil-
ity of the phosphide ligand.
‘C5Me5Na’ could also act as a stabilizing ligand for
the C5Me5–N(SiMe3)2-ligated lanthanide(II) complexes.
The use of NaN(SiMe3)2 in place of KN(SiMe3)2 in the
reactions with (C5Me5)2Ln(thf)2 afforded the
‘C5Me5Na(thf)3’-coordinated Ln(II) complexes 7a
(Ln=Sm) and 7b (Ln=Yb), respectively (Scheme 7)
[12]. In contrast with the ‘C5Me5K’-coordinated com-
plexes 5a–l, which adopt a polymeric structure through
‘intermolecular’ K···C5Me5 interactions, the ‘C5Me5-
Na’-coordinated complexes 7a,b adopt a ‘monomeric’
form owing to coordination of more (three) THF lig-
ands to the Na atom.
Scheme 4.
The reaction of (C5Me5)2Yb(OEt2) with one equiva-
lent of Li{Si(SiMe3)3}(thf)3 in toluene gave the
monomeric, mixed ligand complex 8, with release of
LiC5Me5 (Scheme 8) [15]. This reaction contrasts with
those described above for the formation of the
‘C5Me5M’-coordinated complexes 5–7 (M=K or Na).
3. Synthesis and structures of lanthanide(II) complexes
with silylene-linked C5Me4-amido or -siloxo ligands
Scheme 5.
As described above, lanthanide(II) complexes bearing
mixed cyclopentadienyl–monodentae-anionic ligands
often suffer from ligand redistribution problem. One
way to solve this problem could be linking of the two
ligands. It is well known that the silylene-linked cy-
clopentadienyl-amido unit [Me2Si(C5Me4)NtBu]2− acts
as a useful ancillary ligand for Group 4 metals and the
lanthanides in +3 oxidation state [16]. Attempts to
apply this ligand to the synthesis of analogous lan-
thanide(II) complexes were, however, unsuccessful. The
reaction between SmI2(thf)2 and Li2[Me2Si(C5Me4)-
NtBu] yielded an unidentified yellow, possibly Sm(III)
compound, while the similar reaction with YbI2(thf)2
led to incorporation of LiI into the Yb(II) product (9)
(Scheme 9) [17,18]. The acid–base reactions between
Ln{N(SiMe3)2}2(thf)2 (Ln=Sm, Yb) and (C5Me4H)-
SiMe2NHtBu resulted in metallation only at the cy-
clopentadienyl unit, affording the metallocene com-
plexes 10a and 10b, respectively (Scheme 10) [17,18].
The use of the more protonic aniline derivatives such
as (C5Me4H)SiMe2NHPh to react with Ln{N(SiMe3)2}2-
Scheme 6.
Scheme 7.
Scheme 8.
The analogous reaction of (C5Me5)2Sm(thf)2 with one
equivalent of KPHAr (Ar=C6Ht2Bu3-2,4,6) afforded
the phosphide complex 6 (Scheme 6) [12]. In contrast
with 5a–l, in which the ‘C5Me5K’ unit is bonded to the
Ln(II) center with the C5Me5 part, the ‘C5Me5K’ unit in
6 is bonded to the phosphide site with its K atom,
Scheme 9.

64
Z. Hou, Y. Wakatsuki / Journal of Organometallic Chemistry 647 (2002) 61–70
Scheme 13.
m(II) naphthalenide (C10H8)Yb(thf)2 with (C5Me4H)-
SiMe2OSiMe2OH as shown in Scheme 12 [19].
4. Synthesis and structures of lanthanide(II) complexes
with silylene-linked C5Me4-phosphido ligands
Scheme 10.
In sharp contrast with the plenty of linked cyclopen-
tadienyl–amido complexes of various metals [16], the
analogous phosphido compounds remain very rare.
Previous attempts to obtain silylene-linked cyclopenta-
dienyl–phosphido zirconium complexes by reactions of
Li2[Me2Si(C5Me4)(PR)] (R=cyclohexyl or C6H2Me3-
2,4,6) with ZrCl4 did not give the expected cyclopenta-
dienyl–phosphido complexes, but instead led to PꢀSi
bond cleavage of the ligands [20].
Scheme 11.
Lanthanide complexes bearing silylene-linked cy-
clopentadienyl–phosphido ligands seemed to be more
stable than the Group 4 metal analogs. The metatheti-
cal reaction between [K2{Me2Si(C5Me4)(PAr)}(thf)4]2
and two equivalents of SmI2(thf)2 in THF easily af-
forded the corresponding cyclopentadienyl–phosphido
samarium(II) complex 13a (Scheme 13) [21]. Analogous
reaction with YbI2(thf)2 gave the Yb(II) analog 13c.
Two of the three THF ligands in 13a could be removed
under vacuum to give the corresponding mono-solvated
Sm(II) complex 13b, while dissolving of 13c in toluene
followed by evaporation of the solvent under vacuum
yielded the unsolvated Yb(II) complex 13d. Treatment
of 13a with hmpa or dimethoxyethane (dme) afforded
the corresponding hmpa- or dme-coordinated Sm(II)
complex Me2Si(C5Me4)(PAr)Sm(L)2 (L=hmpa (13e) or
dme (13f)), respectively [21]. Complexes 13a–f, together
with the potassium complex [K2{Me2Si(C5Me4)-
(PAr)}(thf)4]2, represent the first examples of struc-
turally characterized, silylene-linked cyclopentadienyl–
phosphido complexes. These complexes were stable in
solid state at room temperature under an inert atmo-
sphere, but the lanthanide(II) complexes 13a–f gradu-
ally decomposed into yet unidentified products
Scheme 12.
(thf)2 successfully afforded the corresponding cyclopen-
tadienyl–anilido lanthanide(II) complexes 11a,b
(Scheme 10) [18]. Recrystallization of 11b from tolu-
ene–hexane removed two of its three THF ligands to
give 11c (Scheme 11). Complex 11c adopts a dimeric
structure via ‘intermolecular’ interaction between the
Yb atom and the Ph group.
The linked cyclopentadienyl–siloxo ytterbium(II)
complex 12, which forms a dimeric structure via the
siloxo-bridges, was obtained by reaction of the ytterbiu-

Z. Hou, Y. Wakatsuki / Journal of Organometallic Chemistry 647 (2002) 61–70
65
Analogous reaction of the silylene-linked cyclopenta-
dienyl–anilido ytterbium(II) complex 11b with one
equivalent of fluorenone afforded the Yb(III) ketyl
complex 15 (Scheme 15) [18]. Treatment of 15 with
hexane–ether removed one of its two THF ligands, and
led to dimerization of the radical unit to give the
pinacolate complex 16. Dissolving 16 in THF cleaved
the central CꢀC bond of the pinacolate unit and regen-
erated the ketyl species 15 quantitatively. These reac-
tions are analogous to those reported for the
bis(aryloxide)- or bis(silylamido)-supported lan-
thanide(III) ketyl–pinacolate complexes (cf. Schemes
16 and 17), but are in contrast with those reported for
the lanthanidocene(III) ketyl complexes 21a,b, which
remained unchanged under the similar conditions
(Scheme 18) [22]. It is also noteworthy that the pinaco-
late unit in 16 is arranged in a gauche conformation,
which is in sharp contrast with the trans form found in
the bis(aryloxide)- and bis(silylamido)-supported pina-
colate complexes 18 and 20.
Scheme 14.
in THF solution. Nevertheless, their reduction chem-
istry could be examined in THF or toluene.
5. One-electron transfer reactions
The reaction of the C5Me5–OAr-ligated Sm(II) com-
plex 3a with one equivalent of fluorenone yielded the
corresponding structurally characterizable Sm(III) ketyl
complex 14 (Scheme 14) [22a], showing that the mixed
C5Me5–OAr ligand system is able to support both
Sm(II) and Sm(III) species. Complex 14 represents a
rare example of a lanthanide complex bearing all differ-
ent ligands.
The reaction of the silylene-linked cyclopentadienyl–
phosphido samarium(II) complex 13a with benzophe-
none gave the corresponding disamarium(III)
benzophenone–dianion complex 22 as the only isolable
Scheme 15.
Scheme 16.
Scheme 17.

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Z. Hou, Y. Wakatsuki / Journal of Organometallic Chemistry 647 (2002) 61–70
tively yielded the corresponding ketyl complexes [24].
Complex 22 represents the first example of a struc-
turally characterized lanthanide(III) ketone–dianion
complex. The only other structurally characterized lan-
thanide ketone–dianion complex is the Yb(II) complex
[Yb(m-h1:h2ꢀOCPh2)(hmpa)2]2 [25–27].
Reduction of azobenzene with either one or two
equivalent of 11b in toluene afforded the same 1:2
product 23 (Scheme 20) [18], which is in contrast with
the previously reported analogous reactions of lan-
thanide(II) complexes bearing other ligand systems such
as (C5Me5)2Sm(thf)2 [28a], (C5H5)2Yb(thf) [28a], and
Sm(Tp%)2 (Tp%=BH(3,5-dimethylpyrazolyl)3) [28b]. The
two Yb atoms in 23 are unsymmetrically bridged by a
cis-oriented azobenzene–dianion unit, which is bonded
in a h3 fashion to one Yb atom and in a h2 fashion to
the other.
Scheme 18.
The binuclear C5Me5–siloxo Sm(II) complex 2 could
act as a two-electron reducing agent, which upon reac-
tion with one equivalent of azobenzene gave the corre-
sponding binuclear Sm(III) azobenzene–dianion
complex 24 (Scheme 21) [10]. The azobenzene unit
bridges the two Sm atoms in a m2-h1 fashion, with each
N atom bonding to one Sm.
Treatment of 13a with ICH2CH2I in toluene easily
produced the corresponding samarium(III) iodide com-
plex 25 (Scheme 22) [21]. The iodide complex 25 could
be used as a starting material for the synthesis of other
derivatives such as aryloxide and silylamide analogs
[29]. The silylene-linked cyclopentadienyl–phosphido
lanthanide(III) complexes seemed to be more stable
than the analogous lanthanide(II) complexes in THF
solution.
Scheme 19.
When the ‘C5Me5K’-coordinated, (pentamethylcy-
clopentadienyl)samarium(II) alkyl complex 5j was
stirred with two or more equivalents of H3SiPh in THF,
the polyhydrido Sm(III)–K cluster complex 26 was
obtained (Scheme 23) [14]. Complex 26 consists for-
mally of six ‘(C5Me5)SmH2’ and three ‘KH(thf)2’ units
(Fig. 1). The six Sm atoms form a trigonal prism whose
three squares are each capped by one K atom. There
are 15 hydrido ligands in this molecule, one being
body-centered in m6-H–Sm6 fashion and others each
capping a metal triangle in either m3-H–Sm3 or m3-H–
Sm2K form. This complex represents the first example
Scheme 20.
product no matter whether one or two equivalents of
13a were employed (Scheme 19) [21]. These results
suggest that the formation of a benzophenone–dianion
complex is easier than that of a ketyl one in the present
reaction. This is in contrast with the reactions of the
homoleptic Sm(II) complexes, such as Sm(Tp%)2 (Tp%=
BH(3,5-dimethylpyrazolyl)3)
[2b],
Sm{C(SiMe3)2-
(SiMe2OMe)}2(thf) [23], (C5Me5)2Sm(thf)2 [22], and
Sm(OAr)2(thf)3 (Ar=C6Ht2Bu2-2,6-Me-4) [22], with one
equivalent of benzophenone or fluorenone, which selec-
Scheme 21.

Z. Hou, Y. Wakatsuki / Journal of Organometallic Chemistry 647 (2002) 61–70
67
no activity for the polymerization of ethylene under the
same conditions [18,21], suggesting that one-electron
transfer from the Ln(II) center to an ethylene monomer
plays a very important role in the initiation of the
present polymerization reactions.
The C5Me5–ER-ligated Sm(II) complexes 5a–e,g,j, 6,
and 7a, which bear the neutral ‘C5Me5M’ ligand (M=
K or Na), all showed high activity for the polymeriza-
tion of ethylene at 25 °C under 1 atm, yielding linear
polyethylene with Mn up to 3×106 (against polystyrene
standard) [12–14,31]. These unlinked, mixed-ligand-
supported complexes are generally more active than the
samarocene(II) complexes (C5Me5)2Sm(thf)n (n=0–2)
and also than the silylene-linked cyclopentadienyl–
anilido and –phosphido analogs 11a and 13b, probably
owing to easy dissociation of the ‘C5Me5M’ ligand to
generate a coordinatively/sterically unsaturated, more
flexible C5Me5–ER-ligated-Sm(II) center. The ER-lig-
and dependence of the reactivity in this series of com-
plexes was also observed. The 4-Me-2,6-tBu2C6H2O-
Scheme 22.
Scheme 23.
of a structurally characterized dihydrido lanthanide
complex and also the first example of a structurally
characterized polyhydrido lanthanide cluster with an
interstitial hydrogen atom.
6. Polymerization and other catalytic reactions
The silylene-linked cyclopentadienyl–anilido samari-
um(II) complex 11a showed a moderate activity (44.8
kg-polymer (mol-Sm)−1 −1) for the polymerization of
h
ethylene in toluene at room temperature under 1 atm,
which afforded linear polyethylene with high molecular
weight (Mn=7.26×105) and narrow polydispersity
(Mw/Mn=1.58) [18]. The mixed-ligand effect is obvious
in the present reaction on comparison of these results
with those reported for the samarocene(II) complex
(C5Me5)2Sm(thf) (5.06 kg-polymer (mol-Sm)−1 h−1
,
MnB2.5×104, Mw/Mn=2.28) [12,30] and the bis(sily-
lamido) samarium(II) complex Sm{N(SiMe3)2}2(thf)2,
which was inert for ethylene polymerization [12]. The
mono(thf)-coordinated,
samarium(II) complex 13b was also active for ethylene
polymerization (13.6 kg-polymer (mol-Sm)−1 −1),
cyclopentadienyl–phosphido
h
which yielded an extremely high molecular-weight poly-
mer that was insoluble in ortho-dichlorobenzene at
135 °C (Mn\4×106) [21]. In contrast, the tris(thf)-,
hmpa-, or dme-coordinated Sm(II) analogs 13a,e,f did
not show an activity for ethylene polymerization under
the same conditions. It is evident from these results that
creation of a sterically (or coordinatively) unsaturated
metal center is essential for a polymerization reaction.
On the other hand, the THF-free or mono(thf)-coordi-
nated, less reducing Yb(II) complex 13d or 11c showed
Fig. 1. X-ray structure of 26. Top: a general view. Bottom: a view
along the threefold axis. The SmꢀC5Me5 bonds are omitted for
clarity.

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Z. Hou, Y. Wakatsuki / Journal of Organometallic Chemistry 647 (2002) 61–70
7. Conclusion
It is clear from the above studies that lanthanide(II)
complexes bearing mixed cyclopentadienyl–monoden-
tate-anionic ligands can serve as a new class of one-
electron transfer agents and polymerization catalysts.
In many cases these complexes behave differently than
ordinary metallocene complexes, as a result of the
interplay between the steric and electronic effects of the
mixed ligands. The reactivity of such a complex can be
tuned by changing the monodentate anion part, as
shown in the 5a–l series.
On the other hand, since the mixed ligand systems
usually offer a more open ligand sphere for the metal
center than the metallocenes, such mixed-ligand-sup-
ported complexes need to adopt a dimeric structure, or
to bear more additional neutral, coordinative ligands to
compensate the steric unsaturation. Therefore, the use
of an easily dissociable, neutral stabilization ligand
becomes more critical for creation of a highly reactive
system in this case. In this regard, the ‘C5Me5K’ unit is
more suitable than the conventional coordinative lig-
ands such as THF or hmpa for the C5Me5–ER-ligated
Ln(II) species (e.g. 5a–l). Further search for a good
neutral ligand for the complexes bearing the linked
ligands will be of great importance. Compared to the
homoleptic analogs, lanthanide complexes bearing
mixed ligand systems still remain much less extensively
explored. As the expertise in this field increases and
more new ligand systems are developed, the diverse
chemistry available from the lanthanide elements will
be further extended.
Scheme 24.
ligated complex 5a showed the highest activity (194.4
kg-polymer (mol-Sm)−1 −1), while the (Me3Si)2N-lig-
h
ated complex 5e gave the highest molecular weight of
polyethylene (Mn=3.10×106).
Complexes 5a–e,g,j showed high activity also for the
polymerization of styrene at room temperature [12–
14,31]. In most cases, a quantitative conversion was
achieved in less than 1 h, affording atactic polystyrenes
with Mn ranged from 8.2×104 to 3.5×105 and Mw/
Mn=1.45–2.45. These results are in striking contrast
with what was observed in the case of the sa-
marocene(II) complexes (C5Me5)2Sm(thf)n (n=0 or 2),
which were inactive for styrene polymerization even
under more severe conditions [6a]. More remarkably,
these complexes, especially the thiolate complex 5c,
showed a unique reactivity for the copolymerization of
styrene and ethylene, which yielded the corresponding
block copolymers under the presence of both
monomers (Scheme 24). The polystyrene content in the
resulting copolymers could be adjusted simply by
changing the feeding amount of styrene monomer un-
der an atmosphere of ethylene. Although extensive
studies on styrene–ethylene copolymerization reactions
have been carried out by use of various catalytic sys-
tems [16b,32], this is the first example of selective block
copolymerization of styrene and ethylene.
The ring-opening polymerization of o-caprolactone
(CL) occurred vary rapidly, when the C5Me5–OAr-lig-
ated dimeric Sm(II) complex 1a [33] or the silylene-
linked cyclopentadienyl–phosphido Sm(II) complex
13b [21] was used as a catalyst. These complexes,
together with the samarium(II) bis(aryloxide) complex
Sm(OAr)2(thf)3, are among the most active catalysts
ever reported for the ring-opening polymerization of
lactones [33,34]. Complex 13b was active also for 1,4-cis
selective polymerization of 1,3-butadiene (1,4-cis–1,4-
trans–1,2-=92.8:4.0:3.2; Mn=3.07×105, Mw/Mn=
1.99) in the presence of MMAO (MMAO=modified-
methylaluminoxane which contains isobutylaluminox-
ane) as a cocatalyst, although its activity and stereose-
lectivity were not as high as those of the
samarocene-based systems [35].
Acknowledgements
This work was supported by the President’s Special
Research Grant of RIKEN and grant-in-aid from the
Ministry of Education Culture, Sports, Science and
Technology of Japan. We are grateful to all our
coworkers who have effectively participated in the work
described here and whose names are shown in the list of
references.
References
[1] Reviews: (a) F.T. Edelmann, in: E.W. Abel, F.G.A. Stone, G.
Wilkinson, M.F. Lappert (Eds.), Comprehensive Organometallic
Chemistry II, vol. 4, Pergamon, Oxford, 1995, p. 11;
(b) C.J. Schaverien, Adv. Organomet. Chem. 36 (1994) 283;
(c) W.J. Evans, Adv. Organomet. Chem. 24 (1985) 131;
(d) W.J. Evans, Polyhedron 6 (1987) 803;
The (pentamethylcyclopentadienyl)samarium(II) al-
kyl complex 5j showed good activity and selectivity for
the hydrosilylation of several types of olefins [14]. Some
of these reactions are in contrast with those reported
for the lanthanide metallocene catalysts [36].
(e) T.J. Marks, R.D. Ernst, in: G. Wilkinson, F.G.A. Stone,
E.W. Abel, Comprehensive Organometallic Chemistry, vol. 3,
Pergamon, Oxford, 1982, p. 173.

Z. Hou, Y. Wakatsuki / Journal of Organometallic Chemistry 647 (2002) 61–70
69
(b) Z. Hou, T. Miyano, H. Yamazaki, Y. Wakatsuki, J. Am.
Chem. Soc. 117 (1995) 4421.
[2] For examples, see: (a) F.T. Edelmann, Angew. Chem. Int. Ed.
Engl. 34 (1995) 2466;
[23] W. Clegg, C. Eaborn, K. Izod, P. O’Shaughnessy, J.D. Smith,
Angew. Chem. Int. Ed. Engl. 36 (1997) 2815.
[24] Structurally characterized alkali and alkaline-earth metal ketyl
complexes have also been reported. For examples, see: (a) Z.
Hou, Y. Wakatsuki, Chem. Eur. J. 3 (1997) 1005;
(b) Z. Hou, X. Jia, A. Fujita, H. Tezuka, H. Yamazaki, Y.
Wakatsuki, Chem. Eur. J. 6 (2000) 2994;
(b) J. Takats, J. Alloys Compd. 249 (1997) 52;
(c) K. Mashima, A. Nakamura, J. Chem. Soc. Dalton Trans.
(1999) 3899;
(d) F. Nief, Eur. J. Inorg. Chem. (2001) 891;
(e) W.J. Evans, D.K. Drummond, H. Zhang, J.L. Atwood,
Inorg. Chem. 27 (1988) 575;
(f) J.R. van den Hende, P.B. Hitchcock, S.A. Holmes, M.F.
Lappert, W.-P. Leung, T.C.W. Mak, S. Prashar, J. Chem. Soc.
Dalton Trans. (1995) 1427;
(c) Z. Hou, X. Jia, Y. Wakatsuki, Angew. Chem. Int. Ed. Engl.
36 (1997) 1292;
(d) Z. Hou, A. Fujita, H. Yamazaki, Y. Wakatsuki, J. Am.
Chem. Soc. 118 (1996) 2503.
(g) G. Rabek, G.P.A. Yap, A.L. Rheingold, Inorg. Chem. 34
(1995) 4521;
[25] For structural characterization of a lanthanide(II) ketone dian-
ion complex, see: (a) Z. Hou, H. Yamazaki, K. Kobayashi, Y.
Fujiwara, H. Taniguchi, J. Chem. Soc. Chem. Commun. (1992)
722;
(h) Z. Hou, A. Fujita, T. Yoshimura, A. Jesorka, Y. Zhang, H.
Yamazaki, Y. Wakatsuki, Inorg. Chem. 35 (1996) 7190;
(i) C. Eaborn, P.B. Hitchcock, K. Izod, Z.-R. Lu, J.D. Smith,
Organometallics 15 (1996) 4783;
(j) C.M. Forsyth, G.B. Deacon, Organometallics 19 (2000) 1205;
(k) G. Heckmann, M. Niemeyer, J. Am. Chem. Soc. 122 (2000)
4227.
(b) Z. Hou, H. Yamazaki, Y. Fujiwara, H. Taniguchi,
Organometallics 11 (1992) 2711.
[26] For examples of generation and reactions of lanthanide(II) ke-
tone–dianion species, see: (a) Z. Hou, Y. Wakatsuki, Top.
Organomet. Chem. 2 (1999) 233;
[3] W.J. Evans, T.A. Ulibarri, J.W. Ziller, J. Am. Chem. Soc. 110
(1988) 6877.
[4] W.J. Evans, J.W. Grate, L.A. Hughes, H. Zhang, J.L. Atwood,
J. Am. Chem. Soc. 107 (1985) 3728.
(b) Z. Hou, K. Takamine, O. Aoki, H. Shiraishi, Y. Fujiwara, H.
Taniguchi, J. Org. Chem. 53 (1988) 6077;
[5] W.J. Evans, C.A. Seibel, J.W. Ziller, Inorg. Chem. 37 (1998) 770.
[6] (a) Y. Zhang, Z. Hou, Y. Wakatsuki, Macromolecules 32 (1999)
939;
(c) Z. Hou, T. Yoshimura, Y. Wakatsuki, J. Am. Chem. Soc.
116 (1994) 11169;
(d) T. Yoshimura, Z. Hou, Y. Wakatsuki, Organometallics 14
(1995) 5382.
(b) W.J. Evans, D.M. DeCoster, J. Greaves, Macromolecules 28
(1995) 7929;
(c) W.J. Evans, T.A. Ulibarri, J.W. Ziller, J. Am. Chem. Soc.
112 (1990) 2314;
[27] For structurally characterized alkali-metal ketone–dianion com-
plexes, see: (a) Z. Hou, A. Fujita, H. Yamazaki, Y. Wakatsuki,
Chem. Commun. (1998) 669;
(d) W.J. Evans, T.A. Ulibarri, J.W. Ziller, J. Am. Chem. Soc.
112 (1990) 219.
(b) B. Bogdanovic, C. Kruger, B. Wermeckes, Angew. Chem.
Int. Ed. Engl. 19 (1980) 817.
[7] (a) W.J. Evans, J.W. Grate, H.W. Choi, I. Bloom, W.E. Hunter,
J.L. Atwood, J. Am. Chem. Soc. 107 (1985) 941;
(b) S.P. Constantine, G.M.D. Lima, P.B. Hitchcock, J.M.
Keates, G.A. Lawless, Chem. Commun. (1996) 2421.
[8] F.G.N. Cloke, C.I. Dalby, P.B. Hitchcock, H. Karamallakis,
G.A. Lawless, J. Chem. Soc. Chem. Commun. (1991) 779.
[9] Z. Hou, Y. Zhang, T. Yoshimura, Y. Wakatsuki, Organometal-
lics 16 (1997) 2963.
[10] M. Nishiura, Z. Hou, Y. Wakatsuki, unpublished result.
[11] Z. Hou, Y. Wakatsuki, unpublished result.
[12] Z. Hou, Y. Zhang, H. Tezuka, P. Xie, O. Tardif, T. Koizumi, H.
Yamazaki, Y. Wakatsuki, J. Am. Chem. Soc. 122 (2000) 10533.
[13] Z. Hou, Y. Zhang, Y. Wakatsuki, unpublished result.
[14] Z. Hou, Y. Zhang, O. Tardif, Y. Wakatsuki, J. Am. Chem. Soc.
123 (2001) 9216.
[28] (a) W.J. Evans, D.K. Drummond, L.R. Chamberlain, R.J. Doe-
mons, S.G. Bott, H. Zhang, J.L. Atwood, J. Am. Chem. Soc.
110 (1988) 4983;
(b) J. Takats, X.W. Zhang, V.W. Day, T.A. Eberspacher,
Organometallics 12 (1993) 4286.
[29] Z. Hou, O. Tardif, Y. Wakatsuki, unpublished result.
[30] (a) E. Ihara, M. Nodono, K. Katsura, Y. Adachi, H. Yasuda,
M. Yamagashira, H. Hashimoto, N. Kanehisa, Y. Kai,
Organometallics 17 (1998) 3975;
(b) W.J. Evans, R.A. Keyer, J.W. Ziller, J. Organomet. Chem.
394 (1990) 87.
[31] (a) Z. Hou, S. Kaita, Y. Wakatsuki, Pure Appl. Chem. 73 (2001)
291;
(b) Z. Hou, J. Synth. Org. Chem. Jpn. 59 (2001) 82;
(c) Z. Hou, Y. Wakatsuki, J. Alloys Compd. 303–304 (2000) 75;
(d) Z. Hou, H. Tezuka, Y. Zhang, H. Yamazaki, Y. Wakatsuki,
Macromolecules 31 (1998) 8650.
[15] M.M. Corradi, A.D. Frankland, P.B. Hitchcock, M.F. Lappert,
G.A. Lawless, Chem. Commun. (1996) 2323.
[16] (a) J. Okuda, T. Eberle, in: A. Togni, R.L. Halterman (Eds.),
Metallocenes, vol. 1, Wiley-VCH, Weinheim, Germany, 1998, p.
415;
[32] For examples, see: (a) G. Xu, Macromolecules 31 (1998) 2395;
(b) T. Arai, T. Ohtsu, S. Suzuki, Macromol. Rapid Commun. 19
(1998) 327;
(b) A.L. McKnight, R.M. Waymouth, Chem. Rev. 98 (1998)
2587.
[17] Z. Hou, T. Koizumi, Y. Wakatsuki, unpublished result.
[18] Z. Hou, T. Koizumi, M. Nishiura, Y. Wakatsuki, Organometal-
lics 20 (2001) 3323.
(c) G. Xu, S. Lin, Macromolecules 30 (1997) 685;
(d) L. Oliva, S. Mazza, P. Longo, Macromol. Chem. Phys. 197
(1996) 3115;
(e) C. Pellecchia, D. Pappalarrdo, M. D’Arco, A. Zambelli,
Macromolecules 29 (1996) 1158;
[19] A.A. Trifonov, E.N. Kirillov, A. Fisher, F.T. Edelmann, M.N.
Bochkarev, Chem. Commun. (1999) 2203.
[20] T. Koch, S. Blaurock, F.B. Somoza, A. Voigt, R. Kirmse, E.
Hey-Hawkins, Organometallics 19 (2000) 2556.
[21] O. Tardif, Z. Hou, M. Nishiura, T. Koizumi, Y. Wakatsuki,
Organometallics (2001) 20, in press.
(f) S. Bogaert, J.-F. Carpentier, T. Chenal, A. Mortreux, G.
Ricart, Macromol. Chem. Phys. 201 (2000) 1813.
[33] M. Nishiura, Z. Hou, T. Koizumi, T. Imamoto, Y. Wakatsuki,
Macromolecules 32 (1999) 8245.
[34] For examples of ring-opening polymerization of lactones by
other lanthanide complexes, see: (a) K.C. Hultzsch, T.P. Spaniol,
J. Okuda, Organometallics 16 (1997) 4845;
[22] (a) Z. Hou, A. Fujita, Y. Zhang, T. Miyano, H. Yamazaki, Y.
Wakatsuki, J. Am. Chem. Soc. 120 (1998) 754;
(b) L.S. Boffa, B.M. Novak, Macromolecules 30 (1997) 3494;

70
Z. Hou, Y. Wakatsuki / Journal of Organometallic Chemistry 647 (2002) 61–70
(c) M. Yamashita, Y. Takemoto, E. Ihara, H. Yasuda, Macro- (b) S. Kaita, Z. Hou, Y. Wakatsuki, Macromolecules 34 (2001)
molecules 29 (1996) 1798;
1538.
(d) W.M. Stevels, M.J.K. Ankone´, P.J. Dijkstra, J. Feijen,
Macromolecules 29 (1996) 3332;
(e) W.M. Stevels, M.J.K. Ankone´, P.J. Dijkstra, J. Feijen,
Macromolecules 29 (1996) 8296;
(f) W.M. Stevels, M.J.K. Ankone´, P.J. Dijkstra, J. Feijen,
Macromol. Chem. Phys. 196 (1995) 1153;
(g) W.J. Evans, H. Katsumata, Macromolecules 27 (1994)
2330.
[36] For examples of lanthanidocene-catalyzed hydrosilylation reac-
tions, see: (a) G.A. Molander, E.C. Dowdy, Top. Organomet.
Chem. 2 (1999) 119;
(b) T. Sakakura, H.-J. Lautenschlager, M. Tanaka, J. Chem.
Soc. Chem. Commun. (1991) 40;
(c) G.A. Molander, M. Julius, J. Org. Chem. 57 (1992) 6347;
(d) P.-F. Fu, L. Brard, Y. Li, T.J. Marks, J. Am. Chem. Soc. 117
(1995) 7157;
[35] (a) S. Kaita, Z. Hou, Y. Wakatsuki, Macromolecules 32 (1999)
9078;
(e) G.A. Molander, E.D. Dowdy, B.C. Noll, Organometallics 17
(1998) 3754.