1,2-Azaborolyl-ligated half-sandwich complexes of scandium(IΠ) and Lutetium(iπ): Synthesis, structures, and syndiotactic polymerization of styrene

Article abstract of DOI:10.1021/om800734v

Half-sandwich Sc(III) and Lu(III) complexes with ancillary trisubstituted 1,2-azaborolyl (Ab) ligands have been conveniently prepared via the reaction of the corresponding anionic Ab ligands with Cationic Sc(III) and Lu(III) dialkyl species in good yields

Full text of DOI:10.1021/om800734v

Organometallics 2009, 28, 517–522
517
1,2-Azaborolyl-Ligated Half-Sandwich Complexes of Scandium(III)
and Lutetium(III): Synthesis, Structures, and Syndiotactic
Polymerization of Styrene†
Xiangdong Fang,*,‡ Xiaofang Li,§ Zhaomin Hou,*,§ Jalil Assoud,‡, and Rui Zhao‡
Department of Chemistry, UniVersity of Waterloo, Ontario N2L 3G1, Canada, and Organometallic
Chemistry Laboratory, RIKEN (The Institute of Physical and Chemical Research), Hirosawa 2-1,
Wako, Saitama 351-0198, Japan
ReceiVed July 31, 2008
Half-sandwich Sc(III) and Lu(III) complexes with ancillary trisubstituted 1,2-azaborolyl (Ab) ligands
have been conveniently prepared via the reaction of the corresponding anionic Ab ligands with cationic
Sc(III) and Lu(III) dialkyl species in good yields and characterized crystallographically. In the solid-
state molecular structures of the half-sandwich mono-Ab Sc(III) complexes 5 and 6, the interaction between
the Sc metal center and Ab ligand is strongly influenced by the exocyclic B substituent, whereas in the
analogous Lu(III) system 8 this interaction becomes much less prominent, as indicated by a relatively
shorter Lu-B bond distance along with an attenuated exocyclic B-N bond interaction. Upon activation,
the Sc(III) complexes 5 and 7 were found to be highly active in the syndiospecific polymerization of
styrene.
available Cp derivatives.2 In particular, as the isoelectronic and
isolobal Cp analogues,3,4,6,7 1,2-azaborolyl (Ab) ligands have
been observed to be more electron-donating than the Cp
counterparts by Fu and co-workers,4,7c which is also supported
by the DFT calculation results.5 One additional advantage of
Ab over Cp ligands has been noted for easy modulation of the
electronic nature of the Ab ligand in the complexes being
studied.4 Ab ligands also merit from their unique ring geometry,
which may provide a coordination environment different from
that of Cp.6 Surprisingly, in sharp contrast to the rapid
development of Cp-containing half-sandwich group III metal
complexes, no example even exists in the literature concerning
analogous Ab-ligated half-sandwich transition-metal alkyl
complexes.7,8 We have recently reported the syntheses and
characterization of multiply substituted Ab ligands and dem-
onstrated that these Ab ligands can serve as good supporting
ancillary ligands in group IV transition-metal complexes.6 These
results provide further incentives for the study of half-sandwich
transition-metal complexes based on Ab ligands in a broad sense.
In this contribution we describe a novel synthesis and structural
comparison of mono-Ab dialkyl complexes of Sc(III) and
Lu(III), the first examples of Ab-ligated half-sandwich group
Introduction
Cyclopentadienyl (Cp)-based half-sandwich group III rare-
earth metal dialkyl complexes (Figure 1, A) have attracted a
tremendous amount of current research interest in selective
organic transformations and homogeneous polymerization ca-
talysis, due to their unique bonding and structural features.1 For
example, the coordination site in half-sandwich group III metal
Cp complexes becomes generally more accessible to the
substrate and the metal center is more electronically unsaturated,
which furnishes the complexes with significantly higher reactiv-
ity with respect to their metallocene analogues (Figure 1, B).
On the other hand, this sterically and electronically unsaturated
character of A also hampers its synthesis, mainly because of
the thermodynamics-driven ligand redistribution reaction that
prefers to yield relatively more stabilized metallocene analogues
B as the product. Thus, the synthetic strategies used to stabilize
these considerably labile half-sandwich group III metal com-
plexes and to avoid the occurrence of ligand redistribution
involve either sterically demanding Cp ligands such as pen-
tamethylcyclopentadienyl (Cp*) or Cp derivatives bearing a
chelate structure, as utilized in constrained geometry catalysts
(CGC). Despite the great success in the Cp chemistry, a myriad
of useful Cp surrogate ligands have been synthesized and studied
with an aim to provide new possibilities relative to more readily
(2) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Chem. ReV.
2002, 102, 1851.
(3) Ashe, A. J., III; Fang, X. Org. Lett. 2000, 2, 2089.
(4) Liu, S.-Y.; Lo, M. M. C.; Fu, G. C. Angew. Chem., Int. Ed. 2002,
41, 174.
* To whom correspondence should be addressed. Tel: 01-519-888-4567,
ext. 36229 (X.F.). Fax: 01-519-746-0435 (X.F.). E-mail: xdfang@uwaterloo.ca
(X.F.); houz@riken.jp (Z.H.).
(5) Orian, L. ReV. Roum. Chim. 2007, 52, 551.
(6) Fang, X.; Assoud, J. Organometallics 2008, 27, 2408.
(7) There are a few scattered examples of Ab-ligated half-sandwich late-
transition-metal non-alkylated complexes (e.g. Cr/Co/Rh); see: (a) Schmid,
G. In ComprehensiVe Heterocyclic Chemistry II; Shinkai, I., Ed.; Elsevier:
Oxford, U.K., 1996; Chapter 3.17. (b) Schmid, G.; Schu¨tz, M. Organome-
tallics 1992, 11, 1789. (c) Liu, S.-Y.; Hills, I. D.; Fu, G. C. Organometallics
2002, 21, 4323.
† The design, syntheses, and structural determination of Sc complexes
5-7, Lu complex 8, and initial polymerization study were carried out
independently at the University of Waterloo; the polymerization study of
5-7 and elemental analyses of 5-7 were performed at RIKEN.
‡ University of Waterloo.
§ RIKEN.
X-ray crystallographer.
(8) Half-sandwich rare-earth-metal dialkyl complexes bearing pyrollyl
and phospholyl ligands were reported recently. See: (a) Nishiura, M.;
Mashiko, T.; Hou, Z. Chem. Commun. 2008, 2019. (b) Jaroschik, F.; Shima,
T.; Li, X.; Mori, K.; Ricard, L.; Le Goff, X.-F.; Nief, F.; Hou, Z.
Organometallics 2007, 26, 5654. (c) Roux, E. L.; Nief, F.; Jaroschik, F.;
To¨rnroos, K. W.; Anwander, R. Dalton Trans. 2007, 4866.
(1) For reviews on Cp-ligated half-sandwich rare-earth-metal complexes,
see: (a) Arndt, S.; Okuda, J. Chem. ReV. 2002, 102, 1953. (b) Hou, Z.;
Luo, Y.; Li, X. J. Organomet. Chem. 2006, 691, 3114. (c) Okuda, J. Dalton
Trans. 2003, 2367.
10.1021/om800734v CCC: $40.75
2009 American Chemical Society
Publication on Web 12/18/2008

518 Organometallics, Vol. 28, No. 2, 2009
Fang et al.
Scheme 2
Figure 1. Cp-based rare-earth-metal complexes (M ) Sc; Y;
lanthanide metals. R ) alkyl group).
Scheme 1
through a salt elimination pathway. We further envisioned that
Okuda’s cationic species can serve as the precursors for the
direct preparation of THF-coordinated half-sandwich rare-earth-
metal dialkyl complexes such that the synthetic difficulty
encountered above can be easily resolved (Scheme 2). Thus, a
simple reaction of [M(CH2SiMe3)2(THF)3]+[BPh4]- (M ) Sc,
Lu) with the corresponding trisubstituted Ab ligands 2-46 in
THF afforded the desired half-sandwich Sc/Lu complexes as
air- and moisture-sensitive colorless crystals in good yields,
which appeared to be much more soluble in hydrocarbon
solvents such as pentane and hexane. The colorless block single
crystals of complexes 5-8 suitable for X-ray diffraction
experiments were grown slowly in pentane at -35 °C. In
contrast, the reaction of less substituted Ab ligands such as
1-tert-butyl-2-phenyl-1,2-azaborolyl with the above Sc/Lu cation
species yielded uncharacterizable products, and attempts to
prepare a yttrium analogue of complexes 5-8 with a disubsti-
tuted Ab ligand afforded only [(Ab)2Y(CH2SiMe3)(THF)],
presumably as a result of facile ligand redistribution.11 These
observations underline the importance of multiply substituted
Ab ligands in the synthesis of Ab-ligated half-sandwich metal
complexes.
III rare-earth metal complexes. Upon activation with trityl cation
and AliBu3, these complexes show interesting catalytic activity
in the syndiospecific polymerization of styrene.
Results and Discussion
In analogy to the Cp chemistry,9 our initial synthetic approach
involved the reaction of M(CH2SiMe3)3(THF)2 (M ) Sc, Lu)
with the conjugate acids of the anionic Ab ligands, as shown
in Scheme 1. Unfortunately, the expected Sc or Lu adducts such
as complexes 5-8 were not observed, as monitored by 1H NMR
spectroscopy, even after prolonged reaction time at 70 °C.
Admittedly, this observation was completely consistent with the
fact that the conjugate acids of Ab ligands were less acidic than
those of Cp ligands, due to the less aromatic nature of the Ab
ligands, relative to their Cp analogues. Therefore, the chemistry
of Ab ligands did not necessarily follow that of Cp congeners,
which prompted us to find other synthetic routes for the
preparation of the intended mono-Ab group III metal dialkyl
complexes 5-8.
In 2005, Okuda et al. reported that some THF-coordinated
rare-earth-metal (Sc/Y/Lu) dialkyl cations and monoalkyl di-
cations can be conveniently prepared in excellent yields via the
reaction of the corresponding trialkyl species with a weak protic
acid such as [Ph2NMeH]+[BPh4]-.10 We reasoned that, unlike
lithium halides that might form “ate” types of bimetallic
complexes by bridging two electron-deficient metal centers (rare-
earth metal and lithium) via a halide group, noncoordinating
salts such as Li+[BPh4]- had no lone pair of electrons to share
with the metal centers and thus were ideal candidates for the
preparation of salt-free half-sandwich group III metal complexes
The solid-state molecular structures of Sc (5 and 6) and Lu
(8) complexes along with the selected bond lengths and angles
are summarized in Figures 2-4, respectively. With the exception
of Sc complex 7, whose structural parameters could not be
refined, the other two Sc complexes (5, B-Ph; 6, B-NiPr2)
similarly show a typical three-legged piano-stool coordination
geometry around the Sc center. In both cases, the Ab ligands
coordinate to the Sc centers in an unsymmetric way such that
Sc atoms tend to slip away from boron (Sc-B (Å): 5, 2.664(2);
6, 2.715(2))12 and become more tightly affiliated with intraring
C (Sc-C (Å): 5, 2.443(2)-2.512(2); 6, 2.411(2)-2.453(2)) and
N (Sc-N (Å): 5, 2.505(2); 6, 2.459(2)), while the Ab ligands
adopt a slight envelope-like structure. Therefore, the Sc
coordination modes can be best depicted as η4. Interestingly,
the exocyclic B substituent seems to dictate the coordination
mode of the metal center, because this metal slippage away from
boron becomes much more intensified when a more electron-
donating group such as -NiPr2 is utilized, as seen in complex
6. Notably, in the case of complex 6, this coordination slippage
makes the Sc atom even more tightly bound to an intraring N
that bears a more sterically demanding tert-butyl group. Thus,
it is likely that the above slippage is induced electronically by
the exocyclic B substituent rather than the steric effect. As a
consequence, the Sc-C(Ab) and Sc-N(Ab) bond lengths in 6
are much shorter than those observed in 5 (vide supra). In
(9) (a) Evans, W. J.; Brady, J. C.; Ziller, J. W. J. Am. Chem. Soc. 2001,
123, 7711. (b) Hultzsch, K. C.; Spaniol, T. P.; Okuda, J. Angew. Chem.,
Int. Ed. 1999, 38, 227. (c) Cameron, T. M.; Gordon, J. C.; Scott, B. L.
Organometallics 2004, 23, 2995. (d) Cui, D. M.; Nishiura, M.; Hou, Z. M.
Macromolecules 2005, 38, 4089.
(11) Fang, X.; Deng, Y.; Xie, Q.; Moingeon, F. Organometallics 2008,
27, 2892.
(10) (a) Elvidge, B. R.; Arndt, S.; Zeimentz, P. M.; Spaniol, T. P.; Okuda,
J. Inorg. Chem. 2005, 44, 6777. (b) Arndt, S.; Okuda, J. AdV. Synth. Catal.
2005, 347, 339.
(12) Shorter Sc-B bond distances (1.50-1.55 Å) were recorded in a
dinuclear [{Cp*(C2B9H11)ScH}2]2 dianionic species; see: Bazan, G. C.;
Schaefer, W. P.; Bercaw, J. E. Organometallics 1993, 12, 2126.

Half-Sandwich Complexes of Sc(III) and Lu(III)
Organometallics, Vol. 28, No. 2, 2009 519
Figure 4. ORTEP drawing of Lu complex 8. Hydrogen atoms are
omitted for clarity, and thermal ellipsoids are drawn at the 30%
probability level. Selected bond lengths (Å) and bond angles (deg):
Lu1-B1 ) 2.775(3), Lu1-N1 ) 2.643(2), Lu1-C1 ) 2.601(3),
Lu1-C2 ) 2.592(3), Lu1-C3 ) 2.567(3), Lu1-O1 ) 2.269(2),
Lu1-C15 ) 2.331(3), Lu1-C19 ) 2.323(4), B1-N2 ) 1.471(4),
B1-C1 ) 1.506(4), B1-N1 ) 1.498(4), C1-C2 ) 1.422(4),
C2-C3 ) 1.363(4), C3-N1 ) 1.414(3); B1-N2-C9 ) 116.1(2),
B1-N2-C12)119.5(2), C9-N2-C12)116.2(2), O1-Lu1-C15
) 97.59(10), O1-Lu1-C19 ) 94.29(10), C15-Lu1-C19 )
103.09(11).
Figure 2. ORTEP drawing of Sc complex 5. Hydrogen atoms are
omitted for clarity, and thermal ellipsoids are drawn at the 30%
probability level. Selected bond lengths (Å) and bond angles (deg):
Sc1-B1 ) 2.664(2), Sc1-C1 ) 2.512(2), Sc1-C2 ) 2.490(2),
Sc1-C3 ) 2.443(2), Sc1-N1 ) 2.505(2), B1-N1 ) 1.480(3),
B1-C3 ) 1.503(3); O1-Sc1-C15 ) 98.1(1), O1-Sc1-C16
) 98.2(1), C15-Sc1-C16 ) 103.6(1).
denced by a relatively shorter average Sc-C(Ab) bond length
(6: 2.45 Å) vs average Sc-C(Cp*) bond length (1: 2.50 Å).
It is of great interest to compare the X-ray structures of 6
and 8, as they only differ in the metal atoms. First, complexes
6 and 8 share some structural similarities, as the metal centers
in both cases are pseudotetrahedrally coordinated to give the
complexes a typical three-legged piano-stool geometry, in which
only one Ab ring is allowed to ligate the η4-metal center that is
shifted away from B (6, vide supra; 8, Lu-B ) 2.775(3) Å,
Lu-C/N ) 2.567(3)-2.643(3) Å). Second, both 6 and 8 differ
from each other. In a closer examination, one can tell that the
slippage away from B becomes relatively less prominent in the
Lu complex 8, as the observed metrical difference between
Lu-B and Sc-B bond lengths (2.77 vs 2.71 Å) is noticeably
smaller than that of normal Lu-B and Sc-B covalent bonds
(2.42 vs 2.26 Å).14 In addition, the exocyclic B-N bond of the
Ab ligand in 8 is attenuated with less double bond character.
For example, the elongation of the exocyclic B-N bond length
in 8 (6, 1.441(3) Å; 8, 1.471(3) Å) has been noted, and the
diisopropylamino group is twisted away from being coplanar
with the Ab ring such that the diisopropylamino N in 8 turns
pyramidal (sum of the three bond angles around N (in deg): 6,
360; 8, 352). Another notable structural difference is that upon
switching the metal center from Sc to Lu, the metrical values
of intraring bond lengths of the Ab ligand in the Lu complex 8
fall short of those of Sc analogue 6, and the Ab ring in complex
8 is almost planar, in contrast to an η4-coordinated envelope-
like Ab structure of complex 6. Perhaps this structural discrep-
ancy of complexes 6 and 8 cannot be simply ascribed to the
different ionic radii of Sc (0.88 Å) and Lu (1.00 Å),14 because
Figure 3. ORTEP drawing of Sc complex 6. Hydrogen atoms are
omitted for clarity, and thermal ellipsoids are drawn at the 30%
probability level. Selected bond lengths (Å) and bond angles (deg):
Sc1-B1 ) 2.715(2), Sc1-C1 ) 2.453(2), Sc1-C2 ) 2.441(2),
Sc1-C3 ) 2.411(2), Sc1-N1 ) 2.459(2), B1-N1 ) 1.525(3),
B1-N2 ) 1.440(3), B1-C1 ) 1.532(3); O1-Sc1-C15 )
96.5(1), O1-Sc1-C16 ) 97.0(1), C15-Sc1-C16 ) 108.0(1).
addition, the Sc-O(THF) and Sc-C(alkyl) bond distances in
complex 6 are all relatively longer than the corresponding
bond distances in complex 5, consistent with a weaker
interaction between Sc and non-Ab groups in 6, as a result
of the stronger electron-donating character of the B-diiso-
propylamino Ab ligand vs that of the B-phenyl Ab ligand. It
is also worth noting that the bonding interaction between Sc
and the Ab ligand in 6 may be even stronger than that in the
comparable Cp*Sc(CH2SiMe3)2(THF) complex 1,13 as evi-
(13) Li, X.; Baldamus, J.; Hou, Z. Angew. Chem., Int. Ed. 2005, 44,
962.
(14) Lange’s Handbook of Chemistry, 13th ed.; Dean, J. A., Ed.;
McGraw-Hill: New York, 1985.

520 Organometallics, Vol. 28, No. 2, 2009
Fang et al.
Table 1. Syndiospecific Polymerization of Styrenea
in analogous bis-Ab yttrium alkyl complexes, the Ab ligands
were observed to coordinate to the Y atom with a larger ionic
radius (1.04 Å)14 in a way much similar to that in complex 6,15
which clearly does not sustain the above ionic-radius assump-
tion. In view of the electropositive nature and limited radial
extension of the 4f orbital configuration generally associated
with lanthanide metals,16 we expect that the electrostatic
interaction between the lanthanide metal center (e.g., Lu) and
ligand (e.g., Ab) becomes important. As a result, the Lu-Ab
interaction possesses more ionic character, which drives the Ab
ligand to keep its original aromatic identity. Nonetheless, further
studies are needed to provide a conclusive explanation for the
above observations.
amt of
AliBu3
yield sPSb
Tm
Mw/Mn (°C)
d
c
c
entry cat. (equiv) [M]/[Sc] (%) (%) 105Mn
1
2
3
4
5
6
7
8
5
5
5
6
7
7
7
7
7
8
8
8
8
2
2
2
2
2
2
2
5
10
1
2000
2500
3000
2000
2000
2500
3000
3000
3000
500
100 100 6.8
100 100 9.1
94 100 13.5
2.2
2.1
1.9
270
270
270
27 100 31.2/2.2 1.5/2.5 271
100 100 5.7
100 100 10.5
95 100 16.3
100 100 4.1
100 100 1.9
20 100 0.38
16 100 0.30
10 100 0.10
22 100 0.36
2.5
1.7
1.5
3.1
3.5
2.6
1.8
1.5
2.6
271
271
271
271
270
270
271
270
270
The 1H and 13C NMR spectroscopic investigations were
carried out in benzene-d6 or toluene-d8 to gain insight into their
solution-phase behavior. The NMR data of all Sc and Lu
complexes in hydrocarbon solution are consistent with their
monomeric structures. For example, the ring H resonances
appeared as two doublets ranging from 3.9 to 6.8 ppm in all
four cases. In particular, the ambient-temperature NMR spectrum
suggests the slow B-N rotation in complexes 6 and 8, as
indicated by diastereotopic iPr groups. Upon heating up to 100
°C in toluene-d8, no coalescence of two signals corresponding
9
10
11
12
13
2
5
1
500
500
1000
a Conditions: Sc, 25 µmol; [Sc]/[B] ) 1/1 (mol/mol); 30 mL of
toluene; t ) 10 min, T ) 25 °C; Lu, 25 µmol; [Lu]/[B] ) 1/1 (mol/
mol); 30 mL of toluene; t ) 10 h, T ) 25 °C. b Determined by 13C
NMR. Solvent fractionation showed no presence of atactic polystyrene.
c Determined by Tosoh HLC-8121GPC/HT in 1,2-dichlorobenzene or
Varian PL GPC-220 with Wyatt HELEOS MALS in 1,2,4-trichlorobenzene
at 145 °C against polystyrene standard. d Determined by DSC.
i
to the Pr methyl groups (6, δ 25.0, 24.8 ppm; 8, 24.8, 24.7
ppm) in the 13C NMR spectra has been observed, which implies
a fairly strong B-N π bond with a rotational barrier of ∆Gq >
19.2 ( 0.5 kcal mol-1 in both cases.17 This result agrees well
with the strongly electron deficient nature of the metal centers
that effectively withdraws electron density away from the Ab
ligand and makes boron interact with its electron-rich exocyclic
substituent, as evidenced by short exocyclic B-N bond lengths
(6, 1.440(3) Å; 8, 1.471(4) Å). It should be pointed out that the
presence of a bulky N-tBu group also helps to increase the free
energy of the B-N bond rotational barrier.
At the beginning of the preliminary polymerization study,
we were intrigued by the role of Ab ligand as a strong electron
donor in stabilizing electron-deficient cationic metal species,
the catalytically active species responsible for the chain growth
in syndiospecific styrene polymerization processes.1b,18 It is
expected that this relatively more stabilized cationic Ab-ligated
metal species, relative to the Cp analogues, may have an
increased life span and essentially contribute to a more efficient
catalyst system. In fact, when activated by 1 equiv of
[Ph3C]+[B(C6F5)4]- in the presence of AliBu3, Sc complexes
5-7 showed high activity for the syndiospecific polymerization
of styrene to yield polystyrene with syndioselectivity of >99%
(rrrr). Some representative results are summarized in Table 1.
In the case of complexes 5 and 7, 2000 equiv of styrene
monomer could be quantitatively polymerized within 10 min,
yielding unimodal syndiotactic polystyrenes with moderate
molecular weight distributions (Mw/Mn ) 2.2-2.5), consistent
with the predominance of a single homogeneous catalytic species
(Table 1, entries 1 and 5). Under the same conditions ([M]/
[Sc] ) 2000), Sc complex 6 showed lower activity and afforded
polystyrene with a bimodal molecular weight distribution,
suggesting the presence of two active species in this system
(Table 1, entry 4). Similarly, Lu complex 8 showed lower yet
steady activity that could last at least for 10 h, and its
polymerization was unaffected by the relative monomer-to-
catalyst ratio (Table 1, entries 10 and 13) but was significantly
influenced by the amount of AliBu3 added in the polymerization
process, as the increase of the AliBu3 addition resulted in lower
molecular weight and smaller Mw/Mn (Table 1, entries 11 and
12). Complexes 5 and 7 were then further examined under
various monomer-to-catalyst ratios (2000-3000) (Table 1,
entries 1-3 and 5-7). In both systems, the molecular weight
of the resultant polymers has been significantly increased with
decreased polydispersity as the monomer-to-catalyst ratio is
raised. In the case of [M]/[Sc] ) 3000, syndiotactic polystyrene
with an Mn value as high as 1.6 × 106 and Mw/Mn controlled at
1.5 can be obtained (Table 1, entries 3 and 7). The addition of
AliBu3 also showed great influence on the present catalyst
system.19 In the case of 7/[Ph3C]+[B(C6F5)4]-, the use of more
AliBu3 led to a higher yield of polystyrene (Table 1, entries 7
and 8). On the other hand, an increase of the [Al]/[Sc] ratio
resulted in lower molecular weight and broader molecular weight
distribution of the resulting polymers (Table 1, entries 7-9),
suggesting that the chain-transfer reaction to AliBu3 could occur
more frequently at a higher AliBu3 feed. Nevertheless, the
present catalyst systems yield syndiotactic polystyrene with
molecular weight much higher than that produced in the case
of the Cp-ligated analogous systems, albeit in the presence of
AliBu3.
Conclusions
In summary, we have developed an efficient synthesis for
novel half-sandwich Sc and Lu dialkyl complexes 5-8, the first
examples of Ab-ligated half-sandwich group III metal com-
plexes. Our synthesis has the advantage of being general. The
interaction between the Sc metal center and Ab ligand is strongly
(15) Two bis-Ab yttrium alkyl complexes with B-substituted diisopro-
pylamino groups have been prepared and crystallographically characterized.
In both cases, the diisopropylamino N atoms are sp2 hybridized and the
exocyclic B-N bond lengths are in an appropriate range for a moiety with
substantial double-bond character (unpublished work).
(16) Evans, W. J. Inorg. Chem. 2007, 46, 3435.
(17) Friebolin, H. Basic One-and Two-Dimentional NMR Spectroscopy,
3rd ed.; Wiley-VCH: New York, 1998; p 307.
(18) (a) Luo, Y.; Baldamus, J.; Hou, Z. J. Am. Chem. Soc. 2004, 126,
13910. (b) Zhang, H.; Luo, Y.; Hou, Z. Macromolecules 2008, 41, 1064.
(19) In the absence of AliBu3, 7/[Ph3C][B(C6F5)4] alone showed much
lower activity and gave a bimodal polymer under the same conditions, which
was in contrast with case for the Cp-ligated analogues.

Half-Sandwich Complexes of Sc(III) and Lu(III)
Organometallics, Vol. 28, No. 2, 2009 521
6 and 7 can be prepared analogously. 1H NMR (300 MHz, C6D6):
δ 7.85 (d, J ) 2 Hz, 2H, ArH), 7.31 (t, J ) 2 Hz, 2H, ArH), 7.19
(t, J ) 2 Hz, 1H, ArH), 6.76 (d, J ) 2.6 Hz, 1H), 4.50 (d, J ) 2.6
Hz, 1H), 3.65 (br, 4H, THF), 2.00 (s, 3H, AbMe), 1.43 (s, 9H,
tBu), 1.07 (br, 4H, THF), 0.33 (s, 18 H, SiMe3), 0.16 (dd, J ) 27.2
and 11.5 Hz, 2H, CH2Si), -0.13 (s, 2H, CH2Si). 13C NMR (75.5
MHz, C6D6): δ 134.5, 132.7, 127.3, 126.7, 109.9, 72.3, 57.3, 32.4,
24.7, 16.5, 4.2, 4.0. 11B NMR (96.3 MHz, C6D6): δ 33.0. Anal.
Calcd for C26H49BNOScSi2: C, 62.01; H, 9.81; N, 2.78. Found: C,
62.37; H, 9.68; N, 2.59.
influenced by the exocyclic B substituent, whereas in the
analogous Lu(III) system, this interaction becomes much less
prominent, as revealed by the X-ray diffraction experiments.
When treated with 1 equiv of [Ph3C]+[B(C6F5)4]- in the presence
of AliBu3, these Ab-ligated half-sandwich metal complexes
demonstrate moderate-to-high catalytic activity in producing
syndiotactic polystyrene with molecular weight higher than that
for the corresponding Cp systems. Further studies toward the
preparation of the analogous half-sandwich complexes based
on other group III rare-earth metals and the copolymerization
of CO2 and epoxides are in progress.
Synthesis and Characterization of Scandium Complex 6. A
solution of lithium salt 3 (0.385, 1.59 mmol) in 5 mL of THF was
treated with a solution of [Sc(CH2SiMe3)2(THF)3]+[BPh4]- (1.20
g, 1.59 mmol) in 7 mL of THF to yield complex 6 as colorless
Experimental Section
1
block crystals (0.56 g, yield 67%). H NMR (300 MHz, C6D6): δ
General Method and Instrumentation. All operations involving
organometallic compounds were carried out under an atmosphere
of pure dinitrogen by means of standard Schlenk techniques and
an MBraun Unilab 1200/780 glovebox. The oxygen and moisture
in the atmosphere of the MBraun Unilab glovebox were constantly
monitored by both oxygen and moisture analyzers to ensure O2/
H2O concentrations below 0.1 ppm. Et2O and THF were distilled
under nitrogen prior to use from sodium/benzophenone ketyl and
CH2Cl2 from CaH2. Pentane, hexane, and toluene were distilled
from Na/K alloy. n-BuLi (2.5 M in hexanes) and MeMgBr (3.0 M
in Et2O) were purchased from Sigma-Aldrich and used as received.
The deuterated NMR solvents were obtained from Cambridge
Isotope Laboratories. THF-d8 was degassed and dried over Na/K
alloy. C6D6 and CDCl3 were distilled from CaH2 and degassed prior
to use. 1,2-Azaborolyl ligands6 and [M(CH2SiMe3)2(THF)3]+[BPh4]-
(M ) Sc; Lu)10 were prepared using the literature procedures.
Styrene (99%, Sigma-Aldrich) was dried by stirring with CaH2 for
2 days and was purified by distillation and vacuum transfer prior
to use. All other materials were commercially available and used
without further purification. All B-containing samples for NMR
spectroscopic measurements were prepared in the glovebox. NMR
6.40 (d, J ) 2.8 Hz, 1H, AbH), 3.93 (d, J ) 2.8 Hz, 1H, AbH),
3.75 (br, 4H, THF), 3.70 (sept, J ) 6.6 Hz, 2H, NCH), 1.87 (s,
3H, C(4)Me), 1.59 (s, 9H, tBu), 1.27 (d, J ) 6.6 Hz, 6H, NCHCH3),
1.17 (br, 4H, THF), 0.33 (s, 18 H, SiMe3), 0.24 (d, J ) 11.2 Hz,
1H, CH2SiMe3), -0.20 (dd, J ) 26.2, 11.5 Hz, 2H, CH2SiMe3),
-0.32 (s, J ) 11.2 Hz, 1H, CH2SiMe3). 13C NMR (75.5 MHz,
C6D6):
δ 131.0, 125.6, 106.1, 72.4, 55.7, 47.8, 32.2,
24.8(CH(CH3)2), 24.7(CH(CH3)2), 23.9, 16.1, 4.31, 4.08. 11B NMR
(96.3 MHz, C6D6): δ 31.8. Anal. Calcd for C26H58BN2OScSi2: C,
59.29; H, 11.10; N, 5.32. Found: C, 59.54; H, 10.90; N, 5.68.
Synthesis and Characterization of Scandium Complex 7. A
solution of lithium salt 4 (0.250, 1.59 mmol) in 5 mL of THF was
treated with a solution of [Sc(CH2SiMe3)2(THF)3]+[BPh4]- (1.20
g, 1.59 mmol) in 7 mL of THF to yield complex 7 as colorless
1
block crystals (0.50 g, yield 72%). H NMR (300 MHz, C6D6): δ
6.65 (d, J ) 2.9 Hz, 1H, AbH), 4.41 (d, J ) 2.9 Hz, 1H, AbH),
t
3.67 (br, 4H, THF), 1.91 (s, 3H, C(4)Me), 1.43 (s, 9H, Bu), 1.12
(br, 4H, THF), 1.07 (s, 3H, BMe), 0.30 (s, 18H, SiMe3), 0.12 (d, J
) 10.2 Hz, 1H, CH2SiMe3), -0.14 (d, J ) 11.2 Hz, 1H, CH2SiMe3),
-0.32 (dd, J ) 11.2 and 10.2 Hz, 2H, CH2SiMe3). 13C NMR (75.5
MHz, C6D6): δ 130.9, 109.4, 71.9, 56.4, 31.7, 24.8, 15.9, 4.3, 4.1.
1
spectra were recorded on a Bruker Avance 300 spectrometer. H
11B NMR (96.3 MHz, C6D6):
δ 33.7. Anal. Calcd for
and 13C NMR spectra were calibrated using the signals from the
solvents referenced to Me4Si. The 11B NMR spectra were referenced
to external BF3 · OEt2. Chemical shifts (δ) are reported in parts per
million (ppm). The combustion, differential scanning calorimeter
(DSC), and gel permeation chromatography (GPC) analyses were
performed at the facility of Organometallic Chemistry Laboratory
of RIKEN (Japan), the University of Michigan, or the University
of Waterloo. The X-ray diffraction experiments were carried out
at the X-ray facility center, Department of Chemistry, University
of Waterloo (Canada).
C21H47BNOScSi2: C, 57.12; H, 10.73; N, 3.17. Found: C, 56.72;
H, 10.38; N, 3.31.
Synthesis and Characetrization of Lutetium Complex 8. A
solution of lithium salt 3 (0.22 g, 0.90 mmol) in 5 mL of THF was
treated with a solution of [Lu(CH2SiMe3)2(THF)3]+[BPh4]- (0.80
g, 0.90 mmol) in 7 mL of THF to yield complex 8 as colorless
1
block crystals (0.37 g, yield 52%). H NMR (300 MHz, C6D6): δ
6.39 (d, J ) 3.0 Hz, 1H, AbH), 3.93 (d, J ) 3.0 Hz, 1H, AbH),
3.79 (b, 4H, THF), 3.74 (sep, J ) 6.0 Hz, 2H, NCHMe2), 1.86 (s,
3H, CH3(Ab)), 1.58 (s, 9H, tBu), 1.33 (d, J ) 6.0 Hz, 6H, CH3(iPr)),
1.27 (d, J ) 6.0 Hz, 6H, CH3(iPr)), 1.18 (b, 4H, THF), 0.31 (s,
18H, SiMe3), 0.22 (d, J ) 12 Hz, 1H, CH2Si), -0.16 (d, J ) 12
Hz, 1H, CH2Si), -0.25 (d, J ) 12 Hz, 1H, CH2Si), -0.32 (d, J )
12 Hz, 1H, CH2Si).13C NMR (75.5 MHz, C6D6): δ 131.0 (C4(Ab)),
106.1 (C5(Ab)), 96.1 (b, C3(Ab)), 72.4, 55.7, 47.8, 41.1 (b), 37.6
(b), 32.2, 24.8 (CH(CH3)2), 24.7 (CH(CH3)2), 23.9, 16.7, 4.3, 4.1,
-2.5. 11B NMR (96.3 MHz, C6D6): δ 31.6. Anal. Calcd for
C26H58BN2OSi2Lu: C, 47.55; H, 8.90; N, 4.27. Found: C, 47.91;
H, 9.12; N, 4.20.
Polymerization Procedures. A typical procedure for styrene
polymerization using 7/[Ph3C][B(C6F5)4]/AliBu3 (Table 1, entry 5)
is as follows. In a glovebox, a solution of [Ph3C][B(C6F5)4] (23
mg, 25 µmol) in 15 mL of toluene was added to a solution of 7
(11 mg, 25 µmol) in 15 mL of toluene in a 100 mL flask. The
mixture was stirred at room temperature for a few minutes, and 2
equiv of AliBu3 (50 µL, 50 µmol) was added into the above mixture
with stirring, followed by styrene (5.2 g, 50 mmol) within 2 min.
The magnetic stirring was stopped within a few seconds due to the
increased viscosity. After ca. 10 min, methanol (2 mL) was added
to terminate the polymerization. The flask was then taken out from
The GPC analyses were performed using Tosoh HLC-8121GPC/
HT or Varian PL GPC-220 with Wyatt HELEOS MALS systems
maintained at 145 °C. 1,2-Dichlorobenzene (Tosoh) or 1,2,4-
trichlorobenzene (Varian) was used as an eluant with a flow rate
at 1.0 mL/min. Mw, Mn, and Mw/Mn values were obtained against
polystyrene standard or with MALS. The DSC measurement for
the melting temperatures (Tm) of polystyrene samples were obtained
with an SII Nanotechnology EXSTAR6220 instrument at a heating
rate of 10 °C/min under a helium atmosphere.
Synthesis and Characterization of Scandium Complex 5. A
typical procedure for the synthesis of Ab-ligated half-sandwich
scandium(III) and lutetium(III) dialkyl complexes is given as
follows. A solution of lithium salt 2 (0.350 g, 1.59 mmol) in 5 mL
of THF was added dropwise into a colorless solution of
[Sc(CH2SiMe3)2(THF)3]+[BPh4]- (1.20 g, 1.59 mmol) in 7 mL of
THF at -78 °C. The mixture was stirred for 2 h at -78 °C and
then for 20 min at 25 °C. After solvent removal, the pale yellow
residue was extracted with 20 mL of hexane. The mixture was
filtered to remove LiBPh4. The filtrate was further concentrated and
crystallized at -35 °C to give compound 5 as colorless block
crystals (0.60 g, 1.18 mmol, 75% yield). Other complexes such as

522 Organometallics, Vol. 28, No. 2, 2009
Fang et al.
the glovebox and the mixture was poured into methanol (400 mL)
to precipitate the polymer. The white polymer powder was collected
by filtration, washed with methanol, and dried under vacuum at 60
°C to a constant weight (5.2 g, 100%).
g/cm3, Z ) 4, R1 ) 0.0449, wR2 ) 0.0979, for 301 parameters
and 7567 reflections (I > 2σ(I)).
Crystal data for 6: C26H58BN2OScSi2, Mw ) 526.69, triclinic,
j
P1, a ) 10.4194(7) Å, b ) 12.5772(8) Å, c ) 14.1792(9) Å, R )
Crystallographic Data. The single crystals of complexes 5, 6
and 8 were immersed in FOMBLIN Y oil (HVAC 140/13, Sigma-
Aldrich), mounted on a glass fiber, and examined on a Bruker AXS
SMART-CCD 1K detector diffractometer equipped with a Cryos-
tream N2 cooling device using graphite-monochromated Mo KR
radiation (λ ) 0.710 73 Å) at 173 K, respectively. The determination
of crystal class and unit cell was carried out by the SMART program
package.20 The raw frame data were processed using SAINT21 and
SADABS22 to yield the reflection data file. The structures were
solved using the SHELXTL program.23 Refinement was performed
on F2 anistropically for all the non-hydrogen atoms by full-matrix
least-squares methods. Analytical scattering factors for neutral atoms
were used throughout the analysis.
75.5220(10)°, ꢀ ) 76.1870(10)°, γ ) 66.6320(10)°, V ) 1631.07(18)
Å3, Dc ) 1.072 g/cm3, Z ) 2, R1 ) 0.0555, wR2 ) 0.0999, for
308 parameters and 9170 reflections (I > 2σ(I)).
Crystal data for 8: C26H58BLuN2OSi2; Mw ) 656.70, triclinic,
j
P1, a ) 9.2953(6) Å, b ) 10.4744(7) Å, c ) 19.1047(12) Å, R )
94.948(1)o, ꢀ ) 97.502(1)o, γ ) 111.509(1)o, V ) 1697.5(2) Å3, T
) 296(2) K, Z ) 2, Dc ) 1.285, 22 005 measured reflections, 9799
unique reflection (Rint ) 0.0186), 298 refined parameters, R1(I >
2σ(I)) ) 0.0288, wR2(F2) ) 0.0611.
Acknowledgment. Financial support to X.F. from the
Natural Sciences and Engineering Council of Canada
(NSERC), the Canadian Foundation for Innovation (CFI),
the Ontario Innovation Trust (OIT), and the University of
Waterloo is gratefully acknowledged.
Crystal data for 5: C26H49BNOScSi2, Mw ) 503.61, monoclinic,
P21/n, a ) 11.3970(5) Å, b ) 18.2738(8) Å, c ) 15.0873(7) Å, R
) 90°, ꢀ ) 95.8040(10)°, γ ) 90°, V ) 3126.1(2) Å3, Dc ) 1.070
(20) SMART Software Users Guide, Version 4.21; Bruker AXS, Inc.:
Madison, WI, 1997.
(21) SAINT+, Version 6.02; Bruker AXS, Inc., Madison, WI, 1999.
(22) Sheldrick, G. M. SADABS; Bruker AXS, Inc., Madison, WI, 1998.
(23) Sheldrick, G. M. SHELXTL, Version 5.1; Bruker AXS, Inc.,
Madison, WI, 1998.
Supporting Information Available: CIF files giving crystal-
lographic data for complexes 5, 6, and 8. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM800734V