A bis(1,2-azaborolyl)yttrium alkyl complex: Synthesis, structure, and polymerization study

Article abstract of DOI:10.1021/om800073s

Yttrium complex 3, the first example of a group III metal alkyl complex with an ancillary 1,2-azaborolyl ligand, was prepared and characterized; complex 3 initiates methyl methacrylate (MMA) polymerization with living character to afford syndiotactic-rich poly(methyl methacrylate) (PMMA).

Full text of DOI:10.1021/om800073s

2892
Organometallics 2008, 27, 2892–2895
Communications
A Bis(1,2-Azaborolyl)yttrium Alkyl Complex: Synthesis, Structure,
and Polymerization Study
Xiangdong Fang,* Yuanfu Deng, Qinxing Xie, and Firmin Moingeon
Department of Chemistry, UniVersity of Waterloo, Waterloo, Ontario, Canada N2L 3G1
ReceiVed January 26, 2008
environment around the metal center.6 As we are interested in
Summary: Yttrium complex 3, the first example of a group III
metal alkyl complex with an ancillary 1,2-azaborolyl ligand,
was prepared and characterized; complex 3 initiates methyl
methacrylate (MMA) polymerization with liVing character to
afford syndiotactic-rich poly(methyl methacrylate) (PMMA).
their prominent features, it is to our major surprise to find
that group III rare-earth-metal complexes based on the Ab
ligand have never been reported, since Schmid’s pioneering
synthesis of the anionic 1,2-azaborolyl ligand.4a In this
communication, we present the first synthesis and charac-
terization of a THF-coordinated bis-Ab yttrium alkyl complex
(YL2(CH2SiMe3)(THF) (3), L ) 1-methyl-2-phenyl-1,2-
azaborolyl). In addition, we have found that this novel yttrium
complex can serve as a single-component initiator for the
polymerization of methyl methacrylate (MMA) in a living
fashion.
The ominipresence of the cyclopentadienyl (Cp) group in
organometallic chemistry is best demonstrated by its successful
utilization in various metal catalysts or catalyst precursors for
olefin polymerization1 and selective organic synthesis.2 The
participation of the Cp ligand in metal catalysis is mainly as a
spectator ligand that remains tightly bound to the metal center
during the entire catalytic process. Thus, by interacting sterically
and electronically with the metal atom, the Cp ligand can both
control the number of open coordination sites around the metal
center and influence the reactivity of the metal center. It is not
surprising to see that one of the major themes in organometallic
chemistry is to modulate the reactivity and selectivity of the
catalysts, which in turn has spurred the design and syntheses
of numerous Cp analogues.3 In this regard, the 1,2-azaboroyl
(Ab) group has been investigated as a potential isoelectronic
and isolobal surrogate for the Cp ligand in many metal
complexes such as the Ab derivatives of group IV and VIII
transition metals.4 One of the advantages offered by the Ab
systems is that the electronic nature of the metal center can be
easily modulated by judicious choices of the exocyclic sub-
stituent at boron, as demonstrated in the electrochemical study
of the iron complexes by Fu and co-workers.5 Another merit
of the Ab ligands lies in their different ring geometry relative
to the Cp counterparts, which might provide a unique steric
The synthesis of complex 3 is outlined in Scheme 1. Reaction
of YCl3(THF)3 with 2 equiv of the Ab lithium species 14c in
THF affords complex 2, a THF-coordinated bis(1-methyl-2-
phenyl-1,2-azaborolyl)yttrium chloride complex, which has been
determined as a mixture of meso and racemic isomers in a 1:3
1
molar ratio by H NMR spectroscopy. Attempts to isolate the
racemic isomer from the above mixture in toluene were
unsuccessful, due to the tenacious formation of insoluble white
solids, presumably oligomeric or polymeric yttrium chloride
compounds. The results of elemental analysis and the 7Li NMR
spectrum are consistent with the formation of the expected
yttrium chloride complex 2 and hence exclude the possibility
of 2 forming the “ate”-type complexes with LiCl.7 The
subsequent alkylation step was carried out by treating yttrium
compound 2 with 1 equiv of LiCH2Si(CH3)3 at -78 °C, which
gave rise to complex 3 (meso:racemic ) 1:3) as the final
product. In contrast to 2, a pure racemic isomer of 3 can be
(6) Fang, X. D.; Assoud, J. Organometallics 2008, 27, 2408.
(7) Preparation of 2: the Ab lithium species 1 (0.98 g, 6.0 mmol) and
YCl3(THF)3 (1.23 g, 3.0 mmol) in a Schlenk flask with a stir bar were
treated with 20 mL of THF at 0 °C. The mixture was refluxed at 70 °C for
3 h. After solvent removal, the oily residue was dissolved in 15 mL of
toluene to give a clear solution, which was concentrated and crystallized
at-30 °C. After it was rinsed with pentane and dried in vacuo, the product
was obtained as white crystals (0.94 g, 60% yield). 1H NMR (300 MHz,
C6D6): δ 7.83 (d, J ) 8.0 Hz), 7.73 (d, J ) 8.0 Hz), 7.26 (t, J ) 7.6 Hz),
7.20-7.03 (m), 6.60(dd, J ) 5.4, 2.3 Hz), 6.30 (dd, J ) 5.3, 2.2 Hz), 5.95-
6.00 (m), 5.18 (dd, J ) 5.3, 2.2 Hz), 5.10 (dd, J ) 5.4, 2.3 Hz), 3.44 (s),
3.38 (s), 3.17 (b, THF), 0.64 (b, THF). 13C NMR (75.45 MHz, C6D6): δ
134.3, 127.7, 127.4, 127.2, 120.7, 120.3, 118.6, 96.2 (b), 71.8, 37.3, 37.0,
24.7. 11B NMR (96.3 MHz, C6D6): δ 31.3. 7Li NMR (116.6 MHz, C6H6):
no signal. Anal. Calcd for C24H30B2ClN2OY: C, 56.69; H, 5.95; N, 5.51.
Found: C, 56.82; H, 5.75; N, 5.49. On the basis of the 1H NMR spectrum,
the product 2 has been determined as a mixture of meso and racemic isomers
(meso:racemic ) 1:3; see the Supporting Information). Structural deter-
mination by X-ray diffraction was performed, but the structural parameters
could not be refined.
* To whom correspondence should be addressed. E-mail: xdfang@
uwaterloo.ca. Tel: 01-519-888-4567, ext. 36229. Fax: 01-519-746-0435.
(1) (a) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (b)
McKnight, A. L.; Waymouth, R. M. Chem. ReV. 1998, 98, 2587.
(2) (a) Titanium and Zirconium in Organic Synthesis; Marek, I., Ed.;
Wiley: New York, 2002. (b) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004,
37, 673.
(3) (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
Int. Ed. 1999, 38, 428. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV.
2003, 103, 283. (c) Bolton, P. D.; Mountford, P. AdV. Synth. Catal. 2005,
347, 355.
(4) (a) Schmid, G. In ComprehensiVe Heterocyclic Chemistry II; Shinkai,
I., Ed.; Elsevier: Oxford, U.K., 1996; Vol. 3, Chapter 3.17. (b) Liu, S. Y.;
Hills, I. D.; Fu, G. C. Organometallics 2002, 21, 4323. (c) Ashe, A. J. III.;
Fang, X. D. Org. Lett. 2000, 2, 2089.
(5) Liu, S. Y.; Lo, M. M. C.; Fu, G. C. Angew. Chem., Int. Ed. 2002,
41, 174.
10.1021/om800073s CCC: $40.75
2008 American Chemical Society
Publication on Web 06/14/2008

Communications
Organometallics, Vol. 27, No. 13, 2008 2893
Scheme 1. Synthesis of Racemic 3
obtained by repeated recrystallization in a hydrocarbon solvent
such as toluene or hexane.8
Similarly, complex 3 from the above reaction was a mixture of
meso and racemic isomers (meso:racemic ) 1:4), and racemic
3 could be separated from the above mixture by repetitive
recrystallization. Clearly, the facile formation of the thermo-
dynamically more stabilized 3, relative to the less stabilized 4,
is the result of an intermolecular ligand redistribution reaction,
also commonly observed in group III metal and lanthanide Cp
chemistry.10 This result suggests that ligand 1 is not sizable
enough to prevent the occurrence of ligand redistribution in the
above reaction. Therefore, we expect that using more sterically
demanding Ab ligands may be helpful in isolating the corre-
sponding half-sandwich yttrium Ab complex by making the
system kinetically sufficiently inert with respect to ligand
distribution.
Another route for the preparation of 3 was discovered
unexpectedly in the synthesis of the half-sandwich yttrium
bis(alkyl) complex with supporting ancillary Ab ligand (Scheme
1). Okuda et al. have reported a facile synthesis of an interesting
monocationic THF-coordinated yttrium bis(alkyl) species and
its crown ether adduct.9 We anticipated that the reaction of this
monocationic yttrium species with lithium salt 1 would lead to
the half-sandwich yttrium bis(alkyl) complex 4. However,
instead of the much desired 4, only the bis-Ab yttrium complex
3 and Y(CH2SiMe3)3(THF)3 were isolated as the final products.
(8) Preparation of racemic 3: a solution of LiCH2Si(CH3)3 (96 mg, 1.0
mmol) in 5 mL of THF was added dropwise into a solution of 2 (0.52 g,
1.0 mmol) in 5 mL of THF at-78 °C. The mixture was stirred for 2 h
at-78 °C and then 20 min at 25 °C. After solvent removal, the oily residue
was extracted with 25 mL of toluene and the mixture was filtered. The
filtrate was concentrated and layered with hexane. The mixture was
crystallized at-30 °C for 2 days to afford pale yellow solids, which could
be recrystallized by slow diffusion of hexane into its toluene solution. The
product 3 was obtained as white crystals (0.30 g, 53% yield). On the basis
of the 1H NMR spectrum, the product 3 has been determined to be a mixture
of meso and racemic isomers (meso:racemic ) 1:3; see the Supporting
Information). Pure racemic 3 was obtained as colorless crystals by
performing recrystallization (3×) in hexane/toluene (0.15 g, 27% yield).
Attempts to isolate meso 3 were unsuccessful. Another route leading to 3:
a solution of the Ab lithium species 1 (0.11 g, 1.1 mmol) in 10 mL of THF
was added dropwise into a solution of [Y(CH2SiMe3)2(THF)4]+[BPh4]-
(1.0 g, 1.1 mmol) in 7 mL of THF at-78 °C. The clear yellow solution
gradually turned cloudy, and the mixture was warmed to 25 °C and stirred
for 15 min. After solvent removal, the residue was extracted with pentane
(20 mL) and the mixture was filtered. The filtrate was further concentrated
and crystallized at-35 °C to remove Y(CH2SiMe3)3(THF)3 which came
out first. Complex 3 (meso:racemic ) 1:4) was obtained as white crystalline
solids after recrystallization. (0.31 g, 50% yield). 1H NMR (300 MHz, C6D6):
δ 7.82 (d, J ) 7.0 Hz, 2H, Ar H), 7.58 (d, J ) 6.8 Hz, 2H, Ar H), 7.30 (t,
J ) 7.3 Hz, 2H, Ar H), 7.20-7.06 (m, 4H, Ar H), 6.66 (d, J ) 3.0 Hz, 1H,
Ab H), 6.31 (m, 1H, Ab H), 6.16-6.13 (m, 2H, Ab H), 5.22 (d, J ) 3.0
Hz, 1H, Ab H), 5.11 (d, J ) 3.0 Hz, 1H, Ab H), 3.31 (s, 3H, NCH3), 3.19
(s, 3H, NCH3), 2.91 (b, 4H, THF), 0.57 (b, 4H, THF), 0.40 (s, 9H, SiMe3),-
0.71 (dd, 2JY-C-H ) 3.24 Hz, 2JH-C-H ) 10.4 Hz, 1H, CH2SiMe3),-0.89
A single crystal of racemic 3 suitable for X-ray diffractometry
was grown by slow diffusion of hexanes into the saturated
toluene solution of 3 at -30 °C. The solid-state molecular
structure of 3, as illustrated in Figure 1,11 shows a typical bent
metallocene sandwich complex, in which the Y atom has a
pseudotetrahedral coordination geometry. Two Ab rings are
tiltedwithadihedralangleof53.5°,andthecentroid-yttrium-centroid
(Ct01-Y1-Ct02) angle is 128.8(1)o, comparable with those
observed in the analogous bis-Cp systems.12 Notably, the large
Y1-C11-Si1 bond angle of 137.5(1)o deviates greatly from
the ideal tetrahedral angle (109.5°), which may reflect the spatial
requirement of the sterically demanding trimethylsilyl group and
other ligand groups attached to the Y atom.13 As a matter of
fact, the Y-C(alkyl) bond distance (2.397(2) Å) is consistent
(9) (a) Arndt, S.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2003,
42, 5075. (b) Zeimentz, P. M.; Arndt, S.; Elvidge, B. R.; Okuda, J. Chem.
ReV. 2006, 106, 2404.
(10) (a) Booij, M.; Kiers, N. H.; Meetsma, A.; Teuben, J. H. Organo-
metallics 1989, 8, 2454. (b) Xu, G. X.; Ren, J. Q.; Huang, C. H.; Wu, J. G.
Pure Appl. Chem. 1988, 60, 1145.
2
2
(dd, JY-C-H ) 3.24 Hz, JH-C-H ) 10.4 Hz, 1H, CH2SiMe3). 13C NMR
j
(75.45 MHz, C6D6): δ 134.0, 133.9, 127.2, 126.9, 120.2, 119.1, 116.7, 115.3,
(11) Crystal data for 3: C28H41B2N2OSiY, M ) 560.25, triclinic, P1, a
71.7, 36.9, 30.3 (d, JY-C ) 43.4 Hz), 24.4, 5.4. 11B NMR (96.3 MHz,
) 9.8648(5) Å, b ) 12.5348(6) Å, c ) 13.7472(6) Å, R ) 102.128(1)°, ꢀ
) 107.478(1)°, γ ) 108.072(1)o, V ) 1452.6(2) Å3, Dc ) 1.281 g/cm3, Z
) 2, R1 ) 0.037, wR2 ) 0.089, for 361 parameters and 7939 reflections
(I > 2σ(I)).
1
C6D6): δ 30.4. Anal. Calcd for C28H41B2N2OSiY: C, 59.97; H, 7.37; N,
5.00. Found: C, 58.08; H, 7.33; N, 4.90. The combustion carbon analysis
for organolanthanide complexes is difficult, due to their extreme sensitivity
toward air and moisture, which has been repeatedly observed in the literature.
For example, see: (a) Arndt, S.; Beckerle, K.; Zeimentz, P. M.; Spaniol,
T. P.; Okuda, J. Angew. Chem., Int. Ed. 2005, 44, 7473. (b) Cameron, T. M.;
Gordon, J. C.; Scott, B. L. Organometallics 2004, 23, 2995.
(12) Schumann, H.; Genthe, W.; Bruncks, N.; Pickardt, J. Organome-
tallics 1982, 1, 1194.
(13) Haan, K. H. D.; Bore, J. L. D.; Teuben, J. H. J. Organomet. Chem.
1987, 327, 31.

2894 Organometallics, Vol. 27, No. 13, 2008
Communications
Figure 1. An ORTEP diagram of racemic yttrium complex 3.
Hydrogen atoms are omitted for clarity. Thermal ellipsoids are
drawn at the 30% probability level. Selected bond lengths (in Å)
and bond angles (in deg): Y1-C1 ) 2.656(3), Y1-C2 ) 2.622(3),
Y1-C3 ) 2.633(3), Y1-N1 ) 2.699(2), Y1-B1 ) 2.795(3),
Y1-C11 ) 2.394(3), Y1-O1 ) 2.395(2), Y1--Ct01 ) 2.389(3),
Y1-Ct02 ) 2.402(3), B1-N1 ) 1.466(3), B1-C1 ) 1.509(3),
B1-C5 ) 1.577(3), C1-C2 ) 1.428(3), C2-C3 ) 1.378(3),
C3-N1 ) 1.399(3), N1-C4 ) 1.462(3); Ct01-Y1-Ct02 )
128.8(1), O1-Y1-C11 ) 89.0(1), C11-Y1-Ct01 ) 111.2(1),
C11-Y1-Ct02 ) 128.9(1), Y1-C11-Si1 ) 137.5(1).
Figure 3. Time course plots of Mn, Mw/Mn, and f vs polymer yields
in MMA polymerization by initiator 3 at -20 °C (17.8 µmol of 3,
solvent toluene, [sol]/[M] ) 20:1 (v/v), 20 min (9, runs 1-4); Mn
) number-average molecular weight, Mw ) weight-average mo-
lecular weight, Mw/Mn ) molecular weight distribution (polydis-
persity, PDI) (*); f ) initiator efficiency (4)).
carbon atom has been confirmed by the presence of two sets of
signals at -0.71 and -0.89 ppm (2JY-C-H ) 3.2 Hz,13,15
2JH-C-H ) 10.4 Hz14a), as illustrated in Figure 2. Similarly,
two Ab rings give rise to six distinctive H signals in racemic 3,
indicating a chiral environment lack of symmetry around the Y
metal center. In addition, a doublet signal at 30.3 ppm with
1JY-C ) 43.4 Hz is attributed to the methylene carbon atom in
the 13C NMR spectrum of racemic 3, which is comparable with
the values found for the corresponding bis-Cp yttrium alkyl THF
complexes.13 Therefore, it is highly possible that the racemic
complex 3 exists monomerically in solution. The conversion
between the meso and racemic isomers of 3 must not take place
appreciably in solution, because racemic 3 did not transform
1
Figure 2. H NMR spectrum of racemic yttrium complex 3 (300
MHz, C6D6, 20 °C).
1
into its meso isomer, as monitored by H NMR spectroscopy
at 25 °C for 24 h.
with a single covalent bond length between Y and C atoms.14
The two Ab rings are almost planar (0.018 and 0.024 Å
deviation out of the mean planes, respectively) but are unsym-
metrically coordinated to the Y atom in such a manner that Y
is closer to C and N (2.62-2.70 Å) and farther away from B
(2.80 Å). Thus, the coordination of the Ab rings to the Y atom
can be best described as η4.
Lanthanide complexes, in particular Cp derivatives, have been
utilized as highly efficient initiators to prepare poly(methyl
methacrylate) (PMMA).16,17 Among these lanthanide Cp deriva-
tives, living systems are of the greatest interest, as they confer
control over molecular weight and polydispersity (PDI) as well
The NMR spectral data of racemic 3 in the solution phase
are in agreement with its monomeric solid-state molecular
structure. At the beginning of the study, we anticipated that the
(trimethylsilyl)methyl group attached to the Y atom could serve
as an indicator for the relative ratio of the meso and racemic
isomers in the final product. In racemic 3, two H groups (Ha
and Hb) of CH2SiMe3 are expected to be prochiral and
inequivalent, while those of meso 3 will become identical. In
fact, this inequivalence of two hydrogen groups at the methylene
(15) Evans, W. J.; Dominguez, R.; Hanusa, T. P. Organometallics 1986,
5, 263.
(16) Because there are extensive literature precedents concerning
organolanthanide-initialized MMA polymerization, only some representative
examples are given here: (a) Yasuda, H.; Yamamoto, H.; Yokota, K.;
Miyake, M.; Nakamura, A J. Am. Chem. Soc. 1992, 114, 4908. (b) Yasuda,
H; Yamamoto, H.; Yamashita, M.; Yokota, K.; Nakamura, A.; Miyake, S.;
Kai, Y.; Kanehisa, N. Macromolecules 1993, 26, 7134. (c) Giardello, M. A.;
Yamamoto, Y.; Brard, L.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 3276.
(d) Boffa, L. S.; Novak, M. Macromolecules 1994, 27, 6993. (e) Qian, Y. L.;
Bala, M.; Yousef, M.; Zhang, H.; Huang, J. L.; Sun, J. Q.; Liang, C. F. J.
Organomet. Chem. 2002, 188, 1. (f) Woodman, T. J.; Schormann, M.;
Hughes, D. L.; Bochmann, M. Organometallics 2004, 23, 2972. (g) Gamer,
M. T.; Rasta¨tter, M.; Roesky, P. W.; Steffens, A.; Glanz, M. Chem. Eur. J.
2005, 11, 3165. (h) Cui, P.; Chen, Y. F.; Zeng, X.; Sun, J.; Li, G.; Xia, W.
Organometallics 2007, 26, 6519.
(14) (a) Lange’s Handbook of Chemistry, 13th ed.; Dean, J. A., Ed.;
McGraw-Hill: New York, 1985. (b) Sum of the covalent radii: r(Y) + r(C)
) 1.62 + 0.77 ) 2.39 Å.

Communications
Organometallics, Vol. 27, No. 13, 2008 2895
as microstructure in the polymerization process.17 However,
examples of yttrium Cp complexes that could initiate living
MMA polymerization are few in number.16b,17b,e Fortunately,
our preliminary results indicate that yttrium complex 3 initiates
MMA polymerization in a living manner in toluene at -20 °C,
as evidenced by the fact that the number-average molecular
weight (Mn) of PMMA grows linearly with the amount of
monomer consumed (Figure 3). In all runs, over 97% yields of
PMMA have been obtained in 20 min using a small load of
initiator (17.8 µmol of 3) and the Mn values of PMMA reach
up to 3.5 × 105 with relatively low PDI values (1.2-1.5).
Notably, all gel permeation chromatography (GPC) curves
possess bimodal molecular weight distributions, plagued with
a small peak attributed to a higher molecular weight fraction
of PMMA (see the Supporting Information). It is unlikely that
the small fraction of high-molecular-weight PMMA is generated
via O2-mediated coupling of two polymer chains,18 because this
small high-molecular-weight fraction remained in the GPC
chromatogram even after the O2-free methanol had been used
to terminate the growing centers at the end of the polymerization
process. Therefore, it is our current thinking that this small
fraction peak more likely originates from a second species
formed during the polymerization with a low concentration.16e
Low-molecular-weight tailing responsible for increasing the
polydispersity is also observed in the main GPC peak. The exact
origin of the tailing is still uncertain, but it seems likely that
the tailing may be the result of slow initiation and/or chain
termination by a small amount of impurity at the early stage of
the polymerization process. Microstructural analysis indicates
that the PMMA produced with racemic 3 is a syndiotactic-rich
polymer (mm:mr:rr ) 10.0:27.4:62.6),19 which is more con-
sistent with a site-control mechanism as predicted by the
Bernoulli model (2[mm]/[mr] ) 1).20 It is worth noting that
this triad ratio is somewhat unexpected and is different from
the chain-end control of tacticity commonly observed in the
MMA polymerization initiated by group III metal and lanthanide
Cp derivatives.16 In comparison, blank MMA polymerization
experiments using the corresponding Ab lithium salt as initiator
did not afford PMMA, which rules out the possibility that the
observed polymerization is initiated by a free anionic Ab ligand.
In summary, the THF-coordinated yttrium alkyl complex 3
is the first example of a group III metal complex with a
supporting ancillary Ab ligand. X-ray diffraction revealed η4-
coordinated Ab rings in the racemic yttrium complex 3.
Complex 3 serves as an efficient single-component initiator to
initiate MMA polymerization with living character, and the
polymerization is more consistent with a site-control mechanism.
Further effort will be directed to extend this synthetic approach
to other lanthanide complexes based on 1,2-azaborolyls, in
particular multisubstituted Ab ligands, and to prepare their THF-
free lanthanide analogues by ligand modification that will
promote rapid initiation and exhibit higher stereospecificity in
the polymerization process.
Acknowledgment. We are grateful to the Natural Sciences
and Engineering Research Council of Canada (NSERC), the
Canadian Foundation for Innovation (CFI), and the Ontario
Innovation Trust (OIT) for their generous financial support.
We also thank Prof. Dr. Mario Gauthier (University of
Waterloo) for a helpful discussion.
Supporting Information Available: Text, figures, and a table
providing general remarks on the experiments and characterization
data for all complexes and PMMA, NMR spectroscopic data for 2
and 3, GPC chromatograms of the PMMA prepared, and polym-
erization results and a CIF file giving X-ray crystallographic data
for yttrium complex 3. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM800073S
(17) For reviews, see: (a) Hou, Z. M.; Wakatsuki, Y. Coord. Chem.
ReV. 2002, 231, 1. (b) Yasuda, H. J. Organomet. Chem. 2002, 647, 128.
(c) Okuda, J. Dalton Trans. 2003, 2367. (d) Edelmann, F. T.; Lorenz, V.
Coord. Chem. ReV. 2000, 209, 99. (e) Nakayama, Y.; Yasuda, H. J.
Organomet. Chem. 2004, 689, 4489.
(20) Typical procedure for MMA polymerization using 3 as the initiator:
toluene (3.0 mL) and MMA (0.210 g, 118 equiv) were added in a Schlenk
flask in the glovebox. The temperature of the flask was kept at-20 °C, and
a toluene solution (1.0 mL) containing complex 3 (10.0 mg, 17.8 µmol)
was then added into the flask via a syringe. The mixture was magnetically
stirred for 20 min and was then poured into excess MeOH. The resultant
polymer was collected on a filter paper by filtration, washed with MeOH,
and then dried in vacuo. MeOH was obtained by purging commercial MeOH
with N2 for 30 min to ensure that it was O2-free prior to use.
(18) (a) The Chemistry of Organolithium Compounds; Wakefield, B. J.,
Ed.; Pergamon Press: New York, 1974. (b) Panek, E. J.; Whitesides, G. M.
J. Am. Chem. Soc. 1972, 94, 8768.
(19) (a) Bovey, F. A. J. Polym. Sci. 1960, 46, 59. (b) Bovey, F. A.;
Tiers, G. V. D. J. Polym. Sci. 1960, 44, 173.