Synthesis of the first rare earth metal bis(alkyl)s bearing an indenyl functionalized N-heterocyclic carbene

Article abstract of DOI:10.1021/om0700922

Treatment of indenyl-modified imidazolium bromide [C₉H ₇CH₂CH₂(NCHCHN(C₆H ₂Me₃-2,4,6)CH)-Br] ((IndH-NHC-H)Br) with rare earth metal tetra(alkyl) lithium (Ln(CH₂SiMe₃)₄Li(THF) ₄) or with (trimethylsilylmethyl)lithium (LiCH₂SiMe ₃) and rare earth metal tris(alkyl)s (Ln(CH₂SiMe ₃)₃(THF)₂) sequentially afforded the first NHC-stabilized monomeric rare earth metal bis(alkyl) complexes (Ind-NHC)Ln(CH₂SiMe₃)₂ (1, Ln -Y; 2, Ln = Lu; 3, Ln = Sc) via double-deprotonation reactions. Complexes 1-3 are THF-free isostructural monomers. The monoanionic Ind-NHC species bond to the central metal ion in a η⁵:κ¹ constrained geometry configuration (CGC) mode, which combine with the two cis-located alkyl moieties to form a tetrahedron ligand core, leading to the chirality of the complexes. Under the presence of activators AlEt₃ and [Ph₃C][B(C ₆F₅)₄], complex 2 showed catalytic activity toward the polymerization of isoprene to afford 3,4-regulated polyisoprene (91%).

Full text of DOI:10.1021/om0700922

Organometallics 2007, 26, 3167-3172
3167
Synthesis of the First Rare Earth Metal Bis(alkyl)s Bearing an
Indenyl Functionalized N-Heterocyclic Carbene
Baoli Wang,†,‡ Dun Wang,†,‡ Dongmei Cui,*,† Wei Gao,† Tao Tang,† Xuesi Chen,† and
Xiabin Jing†
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and Graduate School of
the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
ReceiVed January 30, 2007
Treatment of indenyl-modified imidazolium bromide [C9H7CH2CH2(NCHCHN(C6H2Me3-2,4,6)CH)-
Br] ((IndH-NHC-H)Br) with rare earth metal tetra(alkyl) lithium (Ln(CH2SiMe3)4Li(THF)4) or with
(trimethylsilylmethyl)lithium (LiCH2SiMe3) and rare earth metal tris(alkyl)s (Ln(CH2SiMe3)3(THF)2)
sequentially afforded the first NHC-stabilized monomeric rare earth metal bis(alkyl) complexes (Ind-
NHC)Ln(CH2SiMe3)2 (1, Ln ) Y; 2, Ln ) Lu; 3, Ln ) Sc) via double-deprotonation reactions. Complexes
1-3 are THF-free isostructural monomers. The monoanionic Ind-NHC species bond to the central metal
ion in a η5:κ1 constrained geometry configuration (CGC) mode, which combine with the two cis-located
alkyl moieties to form a tetrahedron ligand core, leading to the chirality of the complexes. Under the
presence of activators AlEt3 and [Ph3C][B(C6F5)4], complex 2 showed catalytic activity toward the
polymerization of isoprene to afford 3,4-regulated polyisoprene (91%).
modified NHC was used to stabilize Ni bromide species.6
Recently, indenyl- or fluorenyl-functionalized NHC moiety-
supported Ti and V mixed amide/halogen complexes and Ni
bromide have been reported.7 Comparatively, rare earth metal
complexes bearing functionalized NHC ligands have remained
less explored, although the strongly nucleophilic NHC ligands
of two-electron donors are anticipated to stabilize the highly
unsaturated Lewis acidic lanthanide ions. Breakthroughs have
been achieved by the Arnold group, who have successful
isolated carbene-amine-supported lanthanide bis(amido) com-
plexes via a unique reaction pathway, the transamination of the
lithium carbene-amine with homoleptic lanthanide tris(amide)s.8
However, NHC-supported rare earth metal bis(alkyl) complexes
have not been known to date.9 On the other hand, rare earth
metal bis(alkyl) complexes are highly active single-component
catalysts or crucial precursors of the cationic counterparts after
being activated by MAO or borates and have shown tremendous
catalytic activities toward polymerizations of olefin,10 conjugated
monomers,11 and polar monomers.12 Therefore, exploration of
new lanthanide bis(alkyl) complexes has been a research
Introduction
Thermally stable N-heterocyclic carbenes (NHCs) originated
by Arduengo in 1991 have garnered an upsurge in interest in
the past decade and become versatile ligands to stabilize and
activate metal centers in quite different key catalytic steps of
organic syntheses, for example, C-C coupling, aryl and amide
amination, hydrosilylation, and olefin metathesis.1 In these
complexes, the NHC mainly plays the role of organophosphane
as a solvate albeit with more robust coordination.2 Functional-
ization of the imidazolium salt with alkoxide or alkylamides
leads to polydentate ligands in combination with NHC with a
pendant anionic functionality. The resulting NHCs are hemili-
able and covalently bond to the metal center, anticipated to tune
the coordination sphere and rigidity and chirality of a complex,
and have attracted more attention due to their potential in the
area of homogeneous catalysts.3 The tripodal and “pincer”
systems of bisphenolate- or amido-functionalized NHC allowed
the isolation of Ti, Zr, and Hf complexes;4,5 the phenoxy-
* Corresponding author. E-mail: dmcui@ciac.jl.cn. Fax: +86 431
85262773. Tel: +86 431 85262773.
† Changchun Institute of Applied Chemistry.
(4) (a) Aihara, H.; Matsuo, T.; Kawaguchi, H. Chem. Commun. 2003,
2204. (b) Jones, N. A.; Liddle, S. T.; Wilson, C.; Anold, P. L. Organo-
metallics 2007, 26, 755.
(5) (a) Spencer, L. P.; Winston, S.; Fryzuk, M. D. Organometallics 2004,
23, 3372. (b) Spencer, L. P.; Fryzuk, M. D. J. Organomet. Chem. 2005,
690, 5788.
(6) (a) Wang, Z.; Sun, H.; Yao, H.; Shen, Q.; Zhang, Y. Organometallics
2006, 25, 4436. (b) Li, W.; Sun, H.; Chen, M.; Wang, Z.; Hu, D.; Shen,
Q.; Zhang, Y. Organometallics 2005, 24, 5925.
(7) (a) Downing, S. P.; Danopoulos, A. A. Organometallics 2006, 25,
1337. (b) Sun, H.; Hu, D.; Wang, Y.; Shen, Q.; Zhang, Y. J. Organomet.
Chem. 2007, 692, 903.
(8) (a) Arnold, P.; Mungur, S.; Blake, A.; Wilson, C. Angew. Chem.,
Int. Ed. 2003, 42, 5981. (b) Liddle, S. T.; Arnold, P. L. Organometallics
2005, 24, 2597. (c) Arnold, P.; Liddle, S. Chem. Commun. 2005, 5638. (d)
Arnold, P.; Liddle, S. Chem. Commun. 2006, 3959.
(9) Functionalized NHC-stabilized bis(alkyl) complexes based on metals
of the entire range of the periodic table are also scarce. The bisphenolate-
functionalized NHC-ligated titanium bis(alkyl) complex is one of the rare
examples reported to date; see ref 3.
‡ Graduate School of the Chinese Academy of Sciences.
(1) (a) Arduengo, A.; Harlow, R.; Kline, M. J. Am. Chem. Soc. 1991,
113, 361. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290. (c)
Garrison, J. C.; Youngs, W. J. Chem. ReV. 2005, 105, 3978. (d) Bourissou,
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references therein.
(2) (a) Coygreave, J. H.; Colclough, D.; Kociok-ko¨hn, G.; Ruggiero, G.;
Frost, C.; Weller, A. Dalton Trans. 2004, 1519. (b) Corbera´n, R.; Sanau´,
M.; Peris, E. Organometallics 2006, 25, 4002. (c) Herrmann, W. A.; Munck,
F. C.; Artus, G. R. J.; Runte, O.; Anwander, R. Organometallics 1997, 16,
682. (d) Tulloch, A.; Danopoulos, A.; Tooze, R.; Cafferkey, S.; Kleinhenz,
S.; Hursthouse, M. Chem. Commun. 2000, 1247. (e) Wright, J.; Danopoulos,
A.; Motherwell, W.; Carroll, R.; Ellwood, S.; Sassmannshausen, J. Eur. J.
Inorg. Chem. 2006, 4857. (f) Wang, X.; Liu, S.; Weng, L.; Jin, G. Chem.-
Eur. J. 2007, 13, 188. (g) Tulloch, A.; Winston, S.; Danopoulos, A.;
Eastham, G.; Hursthouse, M. Dalton Trans. 2003, 699. (h) Mercs, L.; Labat,
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10.1021/om0700922 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 05/23/2007

3168 Organometallics, Vol. 26, No. 13, 2007
Scheme 1. Synthesis of N-Indenyl Imidazolium Bromide, (IndH-NHC-H)Br
Wang et al.
Scheme 2. Synthesis of Rare Earth Metal Tetra(alkyl)
concern. Unfortunately, preparation of rare earth metal bis(alkyl)
complexes bearing monoanionic ligands has been hindered due
to the highly active character of lanthanide alkyl species and
the relatively less crowded environment of the metal center,
which always result in ligand redistribution, dimerization of the
molecule, and the formation of salt or solvent adducts.13 Thus,
investigation of rare earth metal bis(alkyl) complexes chelated
by NHC ligands is, obviously, a challenge and attractive. Here
we report the preparation of rare earth metal bis(alkyl) com-
plexes bearing N-heterocyclic carbene ligands via new synthetic
pathways. The preliminary isoprene polymerization data using
the lutetium complex are also given.
Lithium
(THF)215 at room temperature (Scheme 2). Reaction of (IndH-
NHC-H)Br and Y(CH2SiMe3)4Li(THF)4 was carried out at room
temperature in toluene for 40 min under vigorous stirring, and
then concentration, filtration, and cooling the resultant solution
to -30 °C afforded the colorless crystalline complex (Ind-
NHC)Y(CH2SiMe3)2 (1) in modest yield (38%) (one step).
Alternatively, (IndH-NHC-H)Br reacted with LiCH2SiMe3 for
40 min at room temperature, and then the reaction mixture was
in situ added to a Y(CH2SiMe3)3(THF)2 solution and maintained
for another 20 min, to afford complex 1 in high yield (62.5%)
(two steps). The lutetium (2) and scandium (3) analogues were
synthesized following a two-step procedure albeit with longer
reaction time at the second step (2 h).
Results and Discussion
The indenyl-modified imidazolium bromide, (IndH-NHC-H)-
Br, was synthesized according to a modified literature proce-
dure.7 Indene was treated with butyllithium at -78 °C, and then
the reaction mixture was warmed to room temperature and kept
stirring overnight. Addition of the above mixture to bromoet-
hylindene gave bromoethyl indene, which reacted with N-
mesitylimidazole in dioxane under refluxing for 5 days to
1
generate (IndH-NHC-H)Br in high yield (Scheme 1). The H
NMR spectrum was indicative of the formation of the desired
product. The two discrete triplet resonances could be attributed
to the ethylene protons, and the signal appearing in the upfield
region, δ 3.32, was assignable to the indenyl nonconjugated
protons. The ylidene proton NHC-H displayed a typical singlet
resonance in the downfield region, δ 10.24. The yttrium tetra-
(alkyl) lithium salt (Y(CH2SiMe3)4Li(THF)4) was obtained by
treatment of yttrium trichlorides with 4 equiv of tris(methylsi-
lylmethyl)lithium (LiCH2SiMe3) at room temperature in THF
for 1 h,14 which could also be in situ generated by simply mixing
a THF solution of LiCH2SiMe3 with equimolar Y(CH2SiMe3)3-
The two-step procedure could be explained as shown in
Scheme 3. (IndH-NHC-H)Br was deprotonated by LiCH2SiMe3
with the release of tetramethylsilane (TMS) and LiBr to give
the neutral intermediate IndH-NHC. Monitoring the reaction by
1H NMR technique demonstrated that the characteristic reso-
nance of the ylidene proton disappeared and the TMS signal
showed up (the deuterated C6H6 contains no TMS standard).
The reaction was rapid, and almost no lithium alkyl signal at δ
-1.76 could be observed after 40 min. The following depro-
tonation of the neutral intermediate IndH-NHC took place
immediately upon addition to the Ln(CH2SiMe3)3(THF)2 solu-
tion, evidenced by the loss of the resonance for a nonconjugated
proton of the indenyl moiety (δ 3.32, Vide supra). The resultant
monoanionic Ind-NHC fragment covalently bonded to the rare
earth metal bis(alkyl) unit, which was confirmed by NMR
spectrum analysis. The ethylene spacer between the indenyl and
imidazolyl groups showed four discrete multiple resonances
compared with two triplets in the free ligand, indicating that
both carbon atoms are chiral (Figure 1). The rotation about the
N-Cmesityl bond was restricted, as the ortho-methyl of the
mesityl fragment gave two discrete resonances at δ 1.79 and
2.21 compared to the sharp singlet at δ 1.94 in the free ligand.
The methylene protons of LnCH2SiMe3 species were diaste-
reotopic, displaying interesting and complicated resonances of
two independent AB spins in the upfield regions: δ -2.09/-
1.81 and -0.86/-0.57 for 1, δ -2.44/-2.09 and -1.08/-0.78
for 2 (Figure S7), and δ -1.82/-1.58 and -0.29/-0.09 for 3
(Figure S10). Further splitting of the AB spins into doublets
due to coupling with the Y atom (I ) 1/2) was observed for
complex 1. The ylidene carbon in all complexes gave the
(10) See reviews: (a) Hou, Z.; Wakatsuki, Y. Coord. Chem. ReV. 2002,
231, 1. (b) Gromada, J.; Carpentier, J. F.; Mortreux, A. Coord. Chem. ReV.
2004, 248, 397. (c) Hyeon, J. Y.; Gottfriendsen, J.; Edelmann, F. T. Coord.
Chem. ReV. 2005, 249, 2787. (d) Bambirra, S.; Bouwkamp, M. W.;
Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2004, 126, 9182. (e)
Kretschmer, W.; Meetsma, A.; Hessen, B.; Schmalz, T.; Qayyum, S.;
Kempe, R. Chem.-Eur. J. 2006, 12, 8969. (f) Zeimentz, P. M.; Arndt, S.;
Elvidge, B. R.; Okuda, J. Chem. ReV. 2006, 106, 2404, and the references
therein.
(11) (a) Hultzsch, K. C.; Voth, P.; Beckerle, K.; Spaniol, T. P.; Okuda,
J. Organometallics 2000, 19, 228. (b) Luo, Y.; Yao, Y.; Shen, Q.
Macromolecules 2002, 35, 8670. (c) Luo, Y.; Baldamus, J.; Hou, Z. J. Am.
Chem. Soc. 2004, 126, 13910. (d) Kirillov, E.; Lehmann, C. W.; Razavi,
A.; Carpentier, J.-F. J. Am. Chem. Soc. 2004, 126, 12240. (e) Li, X.; Hou,
Z. Macromolecules 2005, 38, 6767. (f) Li, X.; Baldamus, J.; Hou, Z. Angew.
Chem., Int. Ed. 2005, 44, 962.
(12) (a) Yasuda, H.; Yamamoto, H.; Yokota, K.; Miyake, S.; Nakamura,
A. J. Am. Chem. Soc. 1992, 114, 4908. (b) Piers, W. E.; Emslie, D. J. H.
Coord. Chem. ReV. 2002, 233-234, 131. (c) Cabaret, O. D.; Vaca, B. M.;
Bourissou, D. Chem. ReV. 2004, 104, 6147.
(13) Cyclopentadienyl complexes with a furyl or amino functionality have
been reported to support rare earth metal bis(alkyl) species; see: (a) Arndt,
S.; Spaniol, T.; Okuda, J. Organometallics 2003, 22, 775. (b) Hultzsch,
K.; Spaniol, T.; Okuda, J. Angew. Chem., Int. Ed. 1999, 38, 227.
(14) Atwood, J.; Hunter, W.; Rogers, R.; Holton, J.; McMeeking, J.;
Pearce, R.; Lappert, M. Chem. Commun. 1978, 140. For details for the
preparation of yttrium, scandium, and lutetium tetra(alkyl) lithium and
characterization, see the Supporting Information.
(15) Lappert, M. F.; Pearce, R. Chem. Commun. 1973, 126.

Rare Earth Metal Bis(alkyl)s
Organometallics, Vol. 26, No. 13, 2007 3169
Table 1. Crystallographic Data and Refinement for 1, 2, and 3
1
2
3
formula
cryst size, mm
fw
C31H45YN2Si2
0.37 × 0.24 × 0.13
590.78
C31H45LuN2Si2
0.17 × 0.16 × 0.10
676.84
C31H45ScN2Si2
0.24 × 0.14 × 0.08
546.83
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P2(1)/c
10.5665(7)
23.2486(16)
13.9158(10)
90
107.2620(10)
90
3264.5(4)
4
monoclinic
P2(1)/c
10.5025(6)
23.2946(13)
13.8088(7)
90
106.8380(10)
90
3233.5(3)
4
monoclinic
P2(1)/c
10.3718(10)
23.321(2)
13.6852(13)
90
106.253(2)
90
3177.9(5)
4
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D
calcd (g/cm3)
1.202
1.390
1.143
2θ max (deg)
52.08
18.80
1248
52.10
31.48
1376
52.14
3.28
1176
µ (cm-1
F(000)
)
no. of obsd reflns
no. of params refnd
GOOF
6430
325
1.026
6374
334
0.956
6277
334
0.852
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
0.0381
0.0878
0.0260
0.0598
0.0518
0.0845
Scheme 3. Synthesis of NHC-Ligated Rare Earth Metal Bis(alkyl) Complexes
characteristic signal at δ 191.23 (doublet) for 1, 199.92 for 2,
and 188.01 for 3 (Figures S6, S8, and S11), comparable to δ
186.0, 190.3, and 190.4 found in the NHC-containing complexes
(BuNCH2CH2[C{N(CHCH)NBu}])Y[N(SiMe3)2]2,8aY[N(SiHMe2)2]3-
[C{MeN(CHCH)NMe}], and Y[N(SiHMe2)2]3[C{MeN(CHCH)-
NMe}]2,2c respectively, suggesting the formation of a direct Ln-
Ccarbene linkage. There was no doubt that the chirality of
complexes 1-3 could be significantly influenced by the
covalently bonded NHC ligand.
It is noteworthy that all complexes 1-3 could also be
accessed by treatment of (IndH-NHC-H)Br with butyllithium,
nBuLi (20 min), or potassium amide, KN(SiMe3)2 (12 h),
respectively, followed by addition to the Ln(CH2SiMe3)3(THF)2
solution.16 This confirmed further that the neutral IndH-NHC
was the intermediate of these reactions, as indenyl-functionalized
imdazolyl bromide deprotonated by KN(SiMe3)2 to yield a
neutral carbene compound is an established mechanism.7a
However, the yields of the complexes were very low in the cases
using KN(SiMe3)2, because the deprotonation reaction was slow
and the resultant neutral carbene compound was unstable at
room temperature.
t
t
Obviously, the one-step route could also be explained as a
double-deprotonation process because the reagent Y(CH2-
SiMe3)4Li(THF)4 was the LiCH2SiMe3 adduct of Ln(CH2-
SiMe3)3(THF)2 bearing two extra THF molecules. The difference
from the two-step procedure was that the resulting neutral
compound IndH-NHC was not observable, as it was deproto-
nated by Ln-CH2SiMe3 species in the system as soon as it was
generated. This might be the reason that the one-step procedure
was rapid. However, the yield was low owing to the purity of
the tetra(alkyl) lithium salt if it was synthesized via treatment
of LnCl3 by LiCH2SiMe3.
The molecular structures of complexes 1-3 in solution were
fairly consistent with those in the solid state, illustrated by X-ray
diffraction analysis as shown in Figure 2 (complex 1). All
(16) When (IndH-NHC-H)Br was deprotonated by using nBuLi, the
resultant reaction mixture was added in situ to the Ln(CH2SiMe3)3(THF)2
solution, whereas by using KN(SiMe3)2 (12 h), the reaction mixture was
vacuumed thoroughly and washed with hexane to extrude the resulting HN-
(SiMe3)2 before addition to the Ln(CH2SiMe3)3(THF)2 solution.

3170 Organometallics, Vol. 26, No. 13, 2007
Wang et al.
Figure 1. 1H NMR spectrum of complex 1.
Table 2. Selected Bond Lengths [Å] and Angles [deg] for
Complexes 1-3
Ln ) Y
Ln ) Lu
Ln ) Sc
Ln-C1
Ln-C2
Ln-C3
Ln-C4
Ln-C9
Ln-C24
Ln-C25
Ln-C12
Ln-Ccent
2.709(3)
2.672(3)
2.640(3)
2.686(3)
2.700(3)
2.370(3)
2.370(3)
2.501(3)
2.396(2)
2.661(3)
2.607(4)
2.584(3)
2.653(3)
2.675(3)
2.319(3)
2.319(3)
2.443(3)
2.344(2)
2.531(3)
2.487(3)
2.482(3)
2.572(3)
2.579(3)
2.209(3)
2.208(3)
2.350(3)
2.227(2)
C24-Ln-C25
Ccent-Ln-C12
C24-Ln-C12
C25-Ln-C12
C12-N2-C15
107.88(10)
100.10(2)
95.06(9)
107.11(9)
123.9(2)
106.60(12)
101.60(4)
94.98(11)
106.35(11)
124.0(3)
106.56(10)
104.60(5)
94.04(9)
105.67(10)
124.9(2)
formed by the metal ion and the alkyl carbon (2.208-2.370 Å)
(Table 2). The Ln-Cpcent bond lengths averaging 2.322 Å are
similar to that in a CGC-type furyl-indenyl yttrium bis(alkyl)
complex (Lu-Cpcent ) 2.367(4) Å).13a However, the bond
angles of C(24)-Ln-C(25) (106.56(10)-107.88(10)°) are much
smaller than 124.4° in the latter complex due to the different
locating mode for the bis(alkyl) species, but they are larger than
the average 101.5° for the other reported C-Ln-C bond angles
due to the combined influence of the constrained chelating
ligands and noncoordination of the THF solvent.11c,18 Complexes
1-3 represent the first examples of anionic NHC-ligated rare
earth metal bis(alkyl)s, as far as we are aware. In addition, they
are also rare examples of such solvent-free complexes,19
indicating the strong electron-donating nature of the NHC
moiety. The number of coordinated solvent molecules, in some
cases, is crucial in governing the catalytic activity of the
complex; for instance, the two-THF-solvated benzamidinate
yttrium bis(alkyl) complex loses its catalytic activity toward
Figure 2. X-ray structure of 1 with 40% probability thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
complexes are isomeric solvent-free monomers, adopting tet-
rahedral geometries (Table 1). THF is extruded from the central
metal coordination sphere owing to the strong electron-donating
effect of the ylidene carbon. The NHC ligand coordinates to
the Ln ion in a η5/k1 constrained geometry configuration (CGC)
mode. The two alkyl species are located in cis-positions with
one endo and the other exo with respect to the imidazole ring.
The bond length of Ln-Ccarbene is 2.501(3) Å for 1, 2.443(3) Å
for 2, and 2.350(3) Å for 3, consistent with the order of Ln
ionic radius,17 comparable to Y-Ccarbene, 2.501(5) Å, in the
complex (tBuNCH2CH2[C{N(CHCH)NtBu}])Y[N(SiMe3)2]2;8a
however, it is slightly longer than the bond length of Ln-C
(18) (a) Bambirra, S.; Boot, S. J.; van Leusen, D.; Meetsma, A.; Hessen, B.
Organometallics 2004, 23, 1891. (b) Bambirra, S.; Meetsma, A.; Hessen,
B.; Bruins, A. P. Organometallics 2006, 25, 3486. (c) Hayes, P. G.; Welch,
G. C.; Emslie, D. J. H.; Noack, C. L.; Piers, W. E.; Parvez, M.
Organometallics 2003, 22, 1577.
(19) (a) Bambirra, S.; van Leusen, D.; Meetsman, A.; Hessen, B.; Teuben,
J. Chem. Commun. 2001, 637. (b) Cameron, T.; Gordon, J.; Michalczyk,
R.; Scott, B. Chem. Commun. 2003, 2282.
(17) Shannon, R. D. Acta Crystallogr. 1976, A 32, 751.

Rare Earth Metal Bis(alkyl)s
Organometallics, Vol. 26, No. 13, 2007 3171
Synthesis of Indenyl Imidazolium Bromide (IndH-NHC-H)-
Br. At -78 °C, 54 mL (1.60 mol/L) of n-butyllithium was dropwise
added to the hexane solution (50 mL) of indene (9.960 g, 85.74
mmol), and the mixture was warmed to room temperature slowly
and kept stirring overnight. Filtration and washing with hexane
afforded white solids of indenyllithium. The ether suspension of
indenyllithium was cooled to -78 °C, which was added dropwise
to bromoethylidene (32.216 g, 171.49 mmol) within 2 h. Warming
to room temperature slowly, reaction for more than 12 h, and
removal of volatiles in Vacuo gave oily residues, which were passed
through a silica gel column with hexane as eluent to afford pure
bromoethylindene (Ind-EtBr) in 66.1% yield (12.640 g). Treatment
of Ind-EtBr (6.330 g, 28.37 mmol) with 1-(mesityl)imidazole22
(5.285 g, 28.37 mmol) in 1,4-dioxane (60 mL) under refluxing for
5 days gave sticky oils after removal of volatiles. The sticky oils
were dissolved in CH2Cl2 and precipitated with ether. The resulting
viscous products were passed through a silica gel column with acetyl
acetate as eluent to wash off the movable parts. The silica gel
column was washed with methanol to collect the nonmovable part.
Driving of methanol gave the anticipated compound (IndH-NHC-
ethylene polymerization compared to the highly active coun-
terpart of one THF adduct.20
Recent reports show that rare earth metal bis(alkyl)complexes
are active for isoprene polymerization.21 Thus, complexes 1, 2,
and 3 were tested for this activity. The preliminary results
showed that they were inert if used alone. Upon activation with
equimolar [Ph3C][(C6F5)4B] and AlEt3, moderate activities but
high selectivity were observed. Under the condition of isoprene/
2/[Ph3C][(C6F5)4B]/AlEt3 ) 1000:1:1:1, the polymerization was
carried out in toluene for 12 h, affording an 80% yield of
polyisoprene with a molecular weight (Mn) of 37 000 (PDI )
1.44) and 91% 3,4-regulation. A detailed investigation of the
catalytic performances of the complexes is in process.
Conclusion
In summary, we have demonstrated several efficient routes
for the preparation of rare earth metal bis(alkyl) complexes
bearing indenyl-functionalized NHC ligands via the deproto-
nation of an indenyl imidazolium bromide by a lithium alkyl
followed by deprotonation of the in situ-generated intermediate
of the neutral indenyl N-heterocyclic carbene by a rare earth
metal alkyl species. The complexes represent the first examples
of functionalized NHC covalently bonding to rare earth metal
alkyl units, which display rare monomeric and solvent-free
structures, having potential as homogeneous catalysts.
1
H)Br (7.572 g, 65.2%). H NMR (400 MHz, CDCl3, 25 °C): δ
1.94 (s, 6H, C6H2Me3), 2.31 (s, 3H, C6H2Me3), 3.32 (s, 2H, indene),
3.35 (t, JH-H ) 6.4 Hz, 2H, CH2CH2), 5.14 (t, JH-H ) 6.4 Hz, 2H,
CH2CH2), 6.47 (s, 1H, indene), 6.95 (s, 2H, C6H2Me3), 6.99 (s,
1H, NCH), 7.15-7.46 (multi, 4H, indene), 7.58 (s, 1H, NCH), 10.24
(s, 1H, imidazolium-H). 13C NMR (100 MHz, CDCl3, 25 °C): δ
17.36 (s, 2C, C6H2Me3), 21.02 (s, 1C, C6H2Me3), 28.65 (s, 1C, CH2-
CH2), 38.05 (s, 1C, indene), 49.25 (s, 1C, CH2CH2), 118.78 (s,
1C, indene), 122.53, 123.24 (s, s, 2C, NCH), 124.05, 125.24, 126.45
(multi, s, 3C, indene), 129.60 (s, 1C, ipso-C6H2Me3), 129.76 (s,
2C, C6H2Me3), 132.04 (s, 1C, indene), 134.13 (s, 1C, ipso-indene),
138.00 (s, 1C, Cylidene), 138.77 (s, 1C, ipso-C6H2Me3), 141.25 (s,
2C, ipso-C6H2Me3), 143.86, 144.11 (s, s, 2C, ipso-indene). IR (KBr
pellets): ν 3426 (w), 3039 (s), 2953 (s), 2921 (s), 1755 (w), 1711
(m), 1608 (m), 1564 (m), 1547 (s), 1486 (m), 1459 (s), 1382 (m),
1330 (w), 1264 (w), 1205 (s), 1162 (m), 1108 (w), 1070 (m), 856
(m), 774 (m) cm-1. Anal. Calcd for C23H25BrN2 (%): C, 67.48; H,
6.16; N, 6.84. Found: C, 66.89; H, 6.03; N, 7.16.
Experimental Section
General Methods. All reactions were carried out under a dry
and oxygen-free argon atmosphere by using Schlenk techniques or
under a nitrogen atmosphere in an MBraun glovebox. All solvents
were purified from an MBraun SPS system. Organometallic samples
for NMR spectroscopic measurements were prepared in the
1
glovebox by use of NMR tubes sealed with paraffin film. H and
13C NMR spectra were recorded on a Bruker AV400 (FT, 400 MHz
1
for H; 100 MHz for 13C) spectrometer. NMR assignments were
Synthesis of Complex (Ind-NHC)Y(CH2SiMe3)2 (1). (a) In a
glovebox, (IndH-NHC-H)Br (0.150 g, 0.366 mmol) and Y(CH2-
SiMe3)4Li(THF)4 (0.269 g, 0.366 mmol) were added to a 25 mL
flask, and the reaction took place immediately upon the addition
of toluene (15 mL). The reaction mixture turned from a suspension
to clear solution gradually and the color changed to deep red. The
solution was maintained for 40 min and then concentrated until it
became cloudy. The precipitated LiBr was removed by filtration.
The filtrate was concentrated under reduced pressure and then
cooled to -30 °C. Colorless crystalline complex 1 was isolated
within several days (0.082 g, 37.9%). (b) To a flask were added
(IndH-NHC-H)Br (0.150 g, 0.366 mmol), LiCH2SiMe3 (0.035 g,
0.366 mmol), and 10 mL of toluene. After reacting for 20 min under
vigorous stirring, the reaction mixture was added to a toluene
solution (10 mL) of Y(CH2SiMe3)3(THF)2 (0.181 g, 0.366 mmol),
which was maintained for another 20 min. The phenomena and
the process of workup were similar to that described previously in
procedure (a). A high yield of complex 1 was isolated (0.135 g,
62.4%). 1H NMR (400 MHz, C6D6, 25 °C): δ -2.09, -1.81 (ABX,
confirmed by 1H-1H COSY and 1H-13C HMQC experiments when
necessary. IR spectra were recorded on a VERTEX 70 FT-IR.
Polymer molecular weight was measured on a TOSOH HLC 8220
GPC with THF as eluent. Elemental analyses were performed at
National Analytical Research Centre of Changchun Institute of
Applied Chemistry (CIAC). Indene was obtained from Aldrich and
purified by distillation before use. Bromoethylidene, mesitylamine,
glyoxal, para-formaldehyde, etc., were purchased from the National
Medicine Company (China) and were used without further purifica-
tion.
X-ray Crystallographic Studies. Crystals for X-ray analysis
were obtained as described in the preparations. The crystals were
manipulated in a glovebox. Data collections were performed at
-86.5 °C on a Bruker SMART APEX diffractometer with a CCD
area detector, using graphite-monochromated Mo KR radiation (λ
) 0.71073 Å). The determination of crystal class and unit cell
parameters was carried out by the SMART program package. The
raw frame data were processed using SAINT and SADABS to yield
the reflection data file. The structures were solved by using the
SHELXTL program. Refinement was performed on F2 anisotro-
pically for all non-hydrogen atoms by the full-matrix least-squares
method. The hydrogen atoms were placed at calculated positions
and were included in the structure calculation without further
refinement of the parameters.
2
2
dd, JH-H ) 10.8 Hz, JY-H ) 2.8 Hz, 2H, Y-CH2SiMe3), -0.86,
2
2
-0.57 (ABX, dd, JH-H ) 10.8 Hz, JY-H ) 2.8 Hz, 2H, Y-CH2-
SiMe3), 0.28 (s, 9H, Y-CH2SiMe3), 0.44 (s, 9H, Y-CH2SiMe3), 1.79
(s, 3H, C6H2Me3), 1.92 (s, 3H, C6H2Me3), 2.21 (s, 3H, C6H2Me3),
2.72-2.77 (multi, 1H, CH2CH2), 2.81-2.87 (multi, 1H, CH2CH2),
3.57-3.62 (multi, 1H, CH2CH2), 3.81-3.87 (multi, 1H, CH2CH2),
5.88 (d, 3JH-H ) 1.2 Hz, 1H, NCH), 6.07 (d, 3JH-H ) 1.2 Hz, 1H,
NCH), 6.70 (s, 1H, C6H2Me3), 6.77, 6.81(AB, 3JHH ) 3.2 Hz, 2H,
(20) Bambirra, S.; van Leusen, D.; Meetsma, A.; Hessen, B.; Teuben, J.
Chem. Commun. 2003, 522.
(21) (a) Zhang, L.; Luo, Y.; Hou, Z. J. Am. Chem. Soc. 2005, 127, 14562.
(b) Zhang, L.; Suzuki, T.; Luo, Y.; Nishiura, M.; Hou, Z. Angew. Chem.,
Int. Ed. 2007, 46, 1909.
(22) Gardiner, M.; Herrmann, W.; Reisinger, C.; Schwarz, J.; Spiegler,
M. J. Organomet. Chem. 1999, 572, 239.

3172 Organometallics, Vol. 26, No. 13, 2007
Wang et al.
indene), 6.80 (s, 1H, C6H2Me3), 7.08-7.19 (multi, 3H, indene),
7.94 (d, 3JH-H ) 8.4 Hz, 1H, indene). 13C NMR (100 MHz, C6D6,
25 °C): δ 4.82 (s, 3C, Y-CH2SiMe3), 5.03 (s, 3C, Y-CH2SiMe3),
18.15 (s, 1C, C6H2Me3), 18.38 (s, 1C, C6H2Me3), 21.39 (s, 1C,
C6H2Me3), 28.95 (s, 1C, CH2CH2), 39.52 (d, 1C, JY-C ) 39.7 Hz,
Y-CH2SiMe3), 42.77 (d, 1C, JY-C ) 37.9 Hz, Y-CH2SiMe3), 54.03
(s, 1C, CH2CH2), 98.28 (s, 1C, indene), 109.24 (s, 2C, ipso-indene),
118.85 (s, 1C, indene), 119.40 (s, 1C, indene), 120.89(s, 1C, NCH),
121.58 (s, 1C, indene), 121.83 (s, 1C, NCH), 122.04 (s, 1C, indene),
124.97 (s, 1C, indene), 130.06, 130.27 (s,s, 2C, C6H2Me3), 135.25
(s, 1C, ipso-indene), 135.54 (s, 1C, ipso-C6H2Me3), 135.86 (s, 2C,
ipso-C6H2Me3), 140.23 (s, 1C, ipso-C6H2Me3), 191.23 ppm (d, JY-C
) 46.0 Hz, 1C, Y-Cylidene). IR (KBr pellets): ν 3159 (w), 3133
(m), 2947 (s), 1678 (w), 1608 (w), 1555 (w), 1487 (m), 1458 (m),
1403 (m), 1381 (w), 1350 (m), 1333 (m), 1247 (s), 1234 (s), 1209
mmol). The mixture was kept stirring for another 2 h. The
phenomena and the process of workup were similar to those
described previously, and a high yield of complex 3 was isolated
1
(0.130 g, 64.9%). H NMR (400 MHz, C6D6, 25 °C): δ -1.82,
2
-1.58 (AB, JH-H ) 10.8 Hz, 2H, Sc-CH2SiMe3), -0.29, -0.09
2
(AB, JH-H ) 10.8 Hz, 2H, Sc-CH2SiMe3), 0.25 (s, 9H, Sc-CH2-
SiMe3), 0.40 (s, 9H, Sc-CH2SiMe3), 1.76 (s, 3H, C6H2Me3), 1.97
(s, 3H, C6H2Me3), 2.20 (s, 3H, C6H2Me3), 2.76-2.79 (multi, 2H,
CH2CH2), 3.51-3.57 (multi, 1H, CH2CH2), 3.89-3.95 (multi, 1H,
3
3
CH2CH2), 5.90 (d, JH-H ) 1.6 Hz, 1H, NCH), 6.09 (d, JH-H
)
1.6 Hz, 1H, NCH), 6.67 (s, 1H, C6H2Me3), 6.80 (s, 1H, C6H2Me3),
6.85, 6.89 (AB, JHH ) 3.2 Hz, 2H, indene), 7.05-7.10 (multi,
3
2H, indene), 7.12-7.18 (multi, 1H, indene), 7.96 (d, JH-H ) 8.4
Hz, 1H, indene). 13C NMR (100 MHz, C6D6, 25 °C): δ 4.48 (s,
3C, Sc-CH2SiMe3), 4.62 (s, 3C, Sc-CH2SiMe3), 18.27 (s, 1C,
C6H2Me3), 18.51 (s, 1C, C6H2Me3), 21.38 (s, 1C, C6H2Me3), 28.44
(s, 1C, CH2CH2), 46.13 (br, 1C, Sc-CH2SiMe3), 50.48 (br, 1C, Sc-
CH2SiMe3), 53.48 (s, 1C, CH2CH2), 100.95 (s, 1C, indene), 110.15-
(s, 2C, ipso-indene), 118.52 (s, 1C, indene), 120.15 (s, 1C, indene),
120.60 (s, 1C, NCH), 121.57 (s, 1C, indene), 121.72 (s, 1C, NCH),
122.28 (s, 1C, indene), 126.29 (s, 1C, indene), 129.75, 129.90 (s,s,
2C, C6H2Me3), 135.48 (s, 1C, ipso-indene), 135.80 (s, 1C, ipso-
C6H2Me3), 136.54 (s, 2C, ipso-C6H2Me3), 139.85 (s, 1C, ipso-
C6H2Me3), 188.01 ppm (br, 1C, Sc-Cylidene). IR (KBr pellets): ν
3160 (m), 3133 (m), 3007 (w), 2948 (s), 2892 (m), 2845 (m), 2809
(m), 1679 (w), 1609 (w), 1557 (m), 1488 (m), 1458 (s), 1404 (m),
1381 (w), 1353 (m), 1336 (m), 1248 (s), 1237 (s), 1208 (w), 1160
(w), 1159 (w), 1106 (m), 1030 (w), 862 (s), 753 (s), 740 (s) cm-1
.
Anal. Calcd for C31H45YN2Si2 (%): C, 63.02; H, 7.68; N, 4.74.
Found: C, 62.91; H, 7.56; N, 4.69.
Synthesis of Complex (Ind-NHC)Lu(CH2SiMe3)2 (2). Follow-
ing procedure (b) described above, (IndH-NHC-H)Br (0.150 g,
0.366 mmol) was first reacted with LiCH2SiMe3 (0.035 g, 0.366
mmol) for 20 min in a 25 mL sample tube, and then a toluene
solution (10 mL) of Lu(CH2SiMe3)3(THF)2 (0.213 g, 0.366 mmol)
was added in situ. The mixture was kept stirring for another 2 h.
The phenomena and the process of workup were similar to those
described previously. Complex 2 was isolated in a 68.9% yield
1
(0.171 g). H NMR (400 MHz, C6D6, 25 °C): δ -2.44, -2.09
2
(AB, JH-H ) 10.8 Hz, 2H, Lu-CH2SiMe3), -1.08, -0.78 (AB,
(w), 1109 (m), 1031 (m), 862 (s), 817 (s), 751 (s), 742 (s) cm-1
.
Anal. Calcd for C31H45ScN2Si2 (%): C, 68.09; H, 8.29; N, 5.12.
Found: C, 68.02; H, 8.13; N, 5.20.
2JH-H ) 10.8 Hz, 2H, Lu-CH2SiMe3), 0.30 (s, 9H, Lu-CH2SiMe3),
0.45 (s, 9H, Lu-CH2SiMe3), 1.77 (s, 3H, C6H2Me3), 1.92 (s, 3H,
C6H2Me3), 2.21 (s, 3H, C6H2Me3), 2.70-2.76 (multi, 1H, CH2CH2),
2.79-2.86 (multi, 1H, CH2CH2), 3.54-3.59 (multi, 1H, CH2CH2),
Isoprene Polymerization. In a glovebox, isoprene (1.0 mL, 10
mmol) was added into a 25 mL flask. Then 10 µmol of complex 2
and 1 equiv of AlEt3 (0.01 mmol) and 1 equiv of [Ph3C][B(C6F5)4]
(9.6 mg, 0.01 mmol) were added to initiate the polymerization.
After stirring for 12 h, methanol was injected into the system to
terminate the polymerization. The reaction mixture was poured into
a large quantity of methanol to precipitate the white solid of
polyisoprene, which was filtered and dried under vacuum at ambient
temperature to constant weight, yielding 0.544 g of polymer.
3
3.83-3.90 (multi, 1H, CH2CH2), 5.87 (d, JH-H ) 1.6 Hz, 1H,
3
NCH), 6.06 (d, JH-H ) 1.6 Hz, 1H, NCH), 6.69 (s, 1H, C6H2-
Me3), 6.75, 6.76 (AB, 3J ) 3.2 Hz, 2H, indene), 6.79 (s, 1H, C6H2-
Me3), 7.10 (s, 1H, indene), 7.11 (s, 1H, indene), 7.16-7.21 (multi,
1H, indene), 7.94 (d, 3JH-H ) 8.4 Hz, 1H, indene). 13C NMR (100
MHz, C6D6, 25 °C): δ 4.99 (s, 3C, Lu-CH2SiMe3), 5.18 (s, 3C,
Lu-CH2SiMe3), 18.16 (s, 1C, C6H2Me3), 18.41 (s, 1C, C6H2Me3),
21.38 (s, 1C, C6H2Me3), 28.74 (s, 1C, CH2CH2), 44.98 (s, 1C, Lu-
CH2SiMe3), 49.61 (s, 1C, Lu-CH2SiMe3), 54.31 (s, 1C, CH2CH2),
97.99 (s, 1C, indene), 108.44 (s, 2C, ipso-indene), 118.62 (s, 1C,
indene), 119.48 (s, 1C, indene), 121.11 (s, 1C, NCH), 121.50 (s,
1C, indene), 121.89 (s, 1C, NCH), 122.11 (s, 1C, indene), 125.44
(s, 1C, indene), 129.93, 130.15 (s,s, 2C, C6H2Me3), 135.29 (s, 1C,
ipso-indene), 135.61 (s, 1C, ipso-C6H2Me3), 135.95 (s, 2C, ipso-
C6H2Me3), 140.16 (s, 1C, ipso-C6H2Me3), 199.92 ppm (s, 1C, Lu-
Acknowledgment. We are thankful for financial support
from National Natural Science Foundation of China for Project
Nos. 20571072 and 20674081; The Ministry of Science and
Technology of China for Project No. 2005CB623802; and
“Hundred Talent Program” of CAS. We are indebted to Prof J.
G. Zhang for discussions on NMR analysis.
C
ylidene). IR (KBr pellets): ν 3160 (w), 3134 (w), 3060 (w), 3028
Supporting Information Available: Experimental details and
1H NMR data for rare earth metal tetra(alkyl) lithium, 1H, 13C, and
1H-13C HMQC NMR spectra for (IndH-NHC-H)Br, 13C NMR
(w), 2947 (s), 2892 (m), 2841 (w), 2804 (w), 1679 (w), 1608 (w),
1556 (w), 1487 (m), 1458 (m), 1404 (m), 1381(w), 1351 (m), 1334
(m), 1248 (s), 1236 (s), 1209 (w), 1159 (w), 1108 (m), 1030 (m),
857 (s), 816 (m), 753 (s), 740 (s) cm-1. Anal. Calcd for C31H45-
LuN2Si2 (%): C, 55.01; H, 6.70; N, 4.14. Found: C, 54.49; H,
6.11; N, 3.97.
Synthesis of Complex (Ind-NHC)Sc(CH2SiMe3)2 (3). Follow-
ing the procedure (b) described above, (IndH-NHC-H)Br (0.150
g, 0.366 mmol) was first reacted with LiCH2SiMe3 (0.035 g, 0.366
mmol) for 20 min in a 25 mL flask and then added in situ to a 10
mL toluene solution of Sc(CH2SiMe3)3(THF)2 (0.165 g, 0.366
1
spectrum for complex 1, H and 13C NMR spectra for complexes
2 and 3, 1H-13C HMQC NMR spectrum for complex 2, 1H NMR
spectrum for polyisoprene, ORTEP drawings of molecular structures
for complexes 2 and 3, and CIF files and crystallographic data for
complexes 1-3, including atomic coordinates, full bond distances,
and bond angles as well as anisotropic thermal parameters, are
available free of charge via the Internet at http://pubs.acs.org.
OM0700922