Pyrrolide-ligated organoyttrium complexes. Synthesis, characterization, and lactide polymerization behavior

Article abstract of DOI:10.1021/om060781y

The N,N-bidentate ligand 2-{(N-2,6-diisopropylphenyl)iminomethyl)}pyrrole (L¹) and the N,N,P-tridentate ligand 2-{(N-2-diphenylphosphinopnenyl) iminomethyl)}pyrrole (L²) have been prepared. Their reactions with homoleptic yttrium tris(alkyl) compound Y(CH₂SiMe₃) ₃(THF)₂ have been investigated. Treatment of Y(CH ₂SiMe₃)₃(THF)₂ with 1 equiv of L¹ generated a THF-solvated bimetallic (pyrrolylaldiminato)yttrium mono(alkyl) complex (1) of central symmetry. In this process, L¹ is deprotonated by metal alkyl and its imino C=N group is reduced to C-N by intramolecular alkylation, generating dianionic species that bridge two yttrium alkyl units in a unique η⁵/η¹:κ¹ mode. The pyrrolyl ring behaves as a heterocyclopentadienyl ligand. Reaction of Y(CH₂SiMe₃)₃(THF)₂ with 2 equiv of L¹ afforded the monomeric bis(pyrrolylaldiminato)yttrium mono(alkyl) complex (2), selectively. Amination of 2 with 2,6-diisopropylaniline gave the corresponding yttrium amido complex (3). In 3 the pyrrolide ligand is monoanionic and bonds to the yttrium atom in a η¹: κ¹ mode. The homoleptic tris(η¹κ ¹-pyrrolylaldiminato)yttrium complex (4) was isolated when the molar ratio of L¹ to Y(CH₂SiMe₃)₃(THF) ₂ increases to 3:1. Reaction of L² with equimolar Y(CH₂SiMe₃)₃(THF)₂ afforded an asymmetric binuclear complex (5). The dianionic N,N,P tridentate moieties derived from L² coordinate to yttrium atoms in η⁵/ η¹:κ² modes, generating tetrahedron and trigonal/pyramidal geometry around the two metal centers, respectively. Both alkylation of the imino C-N group of ligand L² and the pyrrole's ability to act similar to a heterocyclopentadienyl moiety were also found in complex 5. Complexes 1-3 and 5 initiated polymerizations of D,L-lactide to give atactic polylactides with high molecular weights and narrow molecular weight distributions.

Full text of DOI:10.1021/om060781y

Organometallics 2007, 26, 671-678
671
Pyrrolide-Ligated Organoyttrium Complexes. Synthesis,
Characterization, and Lactide Polymerization Behavior
Yi Yang,†,‡ Shihui Li,†,‡ Dongmei Cui,*,† 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 August 28, 2006
The N,N-bidentate ligand 2-{(N-2,6-diisopropylphenyl)iminomethyl)}pyrrole (L1) and the N,N,P-
tridentate ligand 2-{(N-2-diphenylphosphinophenyl)iminomethyl)}pyrrole (L2) have been prepared. Their
reactions with homoleptic yttrium tris(alkyl) compound Y(CH2SiMe3)3(THF)2 have been investigated.
Treatment of Y(CH2SiMe3)3(THF)2 with 1 equiv of L1 generated a THF-solvated bimetallic (pyrrolyla-
ldiminato)yttrium mono(alkyl) complex (1) of central symmetry. In this process, L1 is deprotonated by
metal alkyl and its imino CdN group is reduced to C-N by intramolecular alkylation, generating dianionic
species that bridge two yttrium alkyl units in a unique η5/η1:κ1 mode. The pyrrolyl ring behaves as a
heterocyclopentadienyl ligand. Reaction of Y(CH2SiMe3)3(THF)2 with 2 equiv of L1 afforded the
monomeric bis(pyrrolylaldiminato)yttrium mono(alkyl) complex (2), selectively. Amination of 2 with
2,6-diisopropylaniline gave the corresponding yttrium amido complex (3). In 3 the pyrrolide ligand is
monoanionic and bonds to the yttrium atom in a η1:κ1 mode. The homoleptic tris(η1:κ1-pyrrolylaldiminato)-
yttrium complex (4) was isolated when the molar ratio of L1 to Y(CH2SiMe3)3(THF)2 increases to 3:1.
Reaction of L2 with equimolar Y(CH2SiMe3)3(THF)2 afforded an asymmetric binuclear complex (5).
The dianionic N,N,P tridentate moieties derived from L2 coordinate to yttrium atoms in η5/η1:κ2 modes,
generating tetrahedron and trigonal/pyramidal geometry around the two metal centers, respectively. Both
alkylation of the imino CdN group of ligand L2 and the pyrrole’s ability to act similar to a
heterocyclopentadienyl moiety were also found in complex 5. Complexes 1-3 and 5 initiated
polymerizations of D,L-lactide to give atactic polylactides with high molecular weights and narrow
molecular weight distributions.
originated by Gambarotta have also been demonstrated to
possess unique reactivity such as N2 reduction.4 In contrast, these
ligands have received scant attention in trivalent lanthanide
chemistry. Some related examples are rare earth metal com-
plexes supported by dipyrrolide5 or porphyrinogen ligands,
which usually aggregate to less predictable macrocyclic
structures.2a,6 Recently, Arnold reported a bis(pyrrolide)sa-
marium complex where the pyrrolyl ligands coordinate to the
central metal in η1-modes, which showed interesting stereo-
control of the polymerization of methyl methacrylate.7 Thus,
synthesis of rare earth metal complexes bearing a η5-coordinated
pyrrolide moiety is therefore of obvious interest. On the other
hand, rare earth metal alkyl complexes are highly active single-
site catalysts or crucial precursors of the cationic counterparts
after being activated by MAO or borates, having shown
Introduction
The pyrrolide compounds have gathered particular research
interest recently as spectator ligands by virtue of their strong
metal-ligand bonds and exceptional and tunable steric and
electronic features required for compensating coordinative
unsaturation of metal centers and catalytic activity toward
polymerization.1 Additionally, they can act as a versatile ligand
due to η5/κ1 bonding capability of the pyrrolyl moiety, antici-
pated to be an alternative of cyclopentadiene.2 Thus, quite a
few complexes based on transition metals have been developed,
which have exhibited high activity for olefin polymerization.3
The pyrrolide-attached low-valent rare earth metal complexes
* Corresponding author. E-mail: dmcui@ciac.jl.cn. Fax: (+86) 431
5262773. Tel: (+86) 431 5262773.
† Changchun Institute of Applied Chemistry.
(3) (a) Yoshida, Y.; Matsui, S.; Takagi, Y.; Mitani, M.; Nakano, T.;
Tanaka, H.; Kashiwa, N.; Fujita, T. Organometallics 2001, 20, 4793. (b)
Yoshida, Y.; Saito, J.; Mitani, M.; Takagi, Y.; Matsui, S.; Ishii, S.; Nakano,
T.; Kashiwa, N.; Fujita, T. Chem. Commun. 2002, 1298. (c) Yoshida, Y.;
Mohri, J.; Ishii, S.; Mitani, M.; Saito, J.; Matsui, S.; Makio, H.; Nakano,
T.; Tanaka, H.; Onda, M.; Yamamoto, Y.; Mizuno, A.; Fujita, T. J. Am.
Chem. Soc. 2004, 126, 12023.
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G. Angew. Chem., Int. Ed. 1999, 38, 3657. (b) Ganesan, M.; Gambarotta,
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‡ Graduate School of the Chinese Academy of Sciences.
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Gade, L. H. Chem. Commun. 2000, 173. (c) Dawson, D. M.; Walker, D.
A.; Thornton-Pett, M.; Bochmann, M. J. Chem. Soc., Dalton Trans. 2000,
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P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 2002, 4017. (e) Mutsui,
S.; Spaniol, T. P.; Takagi, Y.; Yoshida, Y.; Okuda, J. J. Chem. Soc., Dalton
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10.1021/om060781y CCC: $37.00 © 2007 American Chemical Society
Publication on Web 12/22/2006

672 Organometallics, Vol. 26, No. 3, 2007
Yang et al.
tremendous catalytic activities toward olefin polymerizations8
and highly regio- or stereoselective polymerizations of conju-
gated monomers9 and polar monomers.10 However, the unstable
property hinders the isolation and application of such complexes.
Many ancillary ligands have been used in an attempt to stabilize
metal alkyls. Up to date, they are dominated by cyclopentadienyl
derivatives, including mono-, bis-, and ansa-auxiliaries.11 Only
recently have compounds containing heteroatoms such as
bidentate amidinates,12 guanidinates,13 â-diketiminates,14 and
salicylaldiminates15 been explored to support rare earth metal
alkyls. The pyrolide ligands for such usage have been less
investigated. Here we wish to report the synthesis, molecular
structures, and coordination modes of the pyrrolide-ligated
yttrium alkyl complexes and the amido derivative. The catalytic
behavior of these complexes toward ring-opening polymerization
of lactide is also presented.
Scheme 1 . Synthetic Pathway for Preparation of Complexes
1-4a
Results and Discussion
a Reagents and condition: Ar ) 2,6-iPr2-C6H3, toluene, rt, 24 h; (i)
1 equiv of Y(CH2SiMe3)3(THF)2; (ii) 1/2 equiv of Y(CH2SiMe3)3(THF)2;
(iii) 2,6-iPr2-C6H3NH2, 6 h; (iv) 1/3 equiv of Y(CH2SiMe3)3(THF)2.
Synthesis and Characterization of Complexes 1-4. The
N,N-bidentate ligand 2-{(N-2,6-diisopropylphenyl)iminomethyl)}-
pyrrole (L1) was prepared by condensation reaction of pyrrole-
2-carboxyaldehyde with 1 equiv of 2,6-diisopropylaniline. The
reaction of L1 with 1 equiv of yttrium alkyls, Y(CH2SiMe3)3-
(THF)2, took place immediately upon addition at room tem-
perature. The reaction mixture was kept stirring for 24 h to
afford pyrrolylaldiminato yttrium mono(alkyl) complex 1 as the
major product (55%) (Scheme 1). The neutral ligand L1 was
first deprotonated by one metal alkyl of Y(CH2SiMe3)3(THF)2
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Figure 1. X-ray structure of 1 with 35% probability thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
with the release of tetramethylsilane, and then, the imino CdN
group of which was alkylated via intramolecular migration of
the metal alkyl, to generate dianionic species. 1H NMR analysis
of complex 1 displayed that the pyrrolyl protons showed a high-
field shift at δ 6.25 compared to that at δ 6.54 of the η1-Ν
coordination mode,5 suggesting that the pyrrolyl moiety coor-
dinates to the yttrium atom in a η5-fashion. The methylene
protons of Y-CH2SiMe3 gave an AB spin around δ -0.4
(JY-C-H ) 8 Hz), different from the singlet resonance at δ -0.6
in the yttrium tris(alkyl)s. The intramolecular alkyl migration
was confirmed by the formation of a C(H)(CH2SiMe3)N group
that exhibited discrete doublet resonances assigned to the
methylene protons. The solid-state structure of complex 1, as
identified by X-ray analysis, is shown in Figure 1. Each yttrium
atom bonds to an alkyl and a THF molecule to form a unit;
dimerization via the dianionic pyrrolylaldiminate bridges forms
a structure of central symmetry. The pyrrolide moiety coordi-
nates to yttrium atoms in unique η5/η1:κ1 modes like a
cyclopantadienyl ligand to generate a distorted trigonal bipy-
ramidal core. The pyrrolyl nitrogen N(2), THF oxygen, and alkyl
carbon C(25) occupy the equatorial positions, while the center

Pyrrolide-Ligated Organoyttrium Complexes
Organometallics, Vol. 26, No. 3, 2007 673
Table 1. Selected Bond Distances (Å) and Angles (deg) for 1
Y-N(1)
2.256(4)
2.382(5)
2.646(5)
2.631(6)
70.15(17)
76.27(17)
107.7(5)
115.4(4)
Y#-C(20)
2.768(6)
2.851(6)
2.800(6)
1.503(8)
103.73(17)
121.5(3)
119.3(5)
100.75(16)
Y-N(2)
Y#-C(19)
Y#-N(2)
Y#-C(21)
N(1)-Y-N(2)
N(2)-Y-N(2)#
N(1)-C(13)-C(18)
C(18)-N(2)-Y
Y#-C(18)
N(1)-C(13)
Y-N(2)-Y#
C(13)-N(1)-Y
N(2)-C(18)-C(13)
N(1)-Y-N(2)#
of the pyrrolyl ring and the anilido nitrogen N(1) are axial. The
bond distances between yttrium and the pyrrolyl carbon, Y-η5-
C(pyr ring), ranging from 2.631(6) to 2.851(6) Å (Table 1), are
comparable to Sm-η5-C(pyr ring) (2.850(4)-2.928(4) Å) in a
samarium dipyrrolide complex6a if the difference of the ionic
radii is considered.16 It is noteworthy that Y#-N(2) (2.646(5)
Å) and Y#-C(21) (2.631(6) Å) are much shorter than Y#-
C(18) (2.800(6) Å), Y#-C(19) (2.851(6) Å), and Y#-C(20)
(2.768(6) Å). This is different from the lanthanocene complexes,
where all Y-η5-C(Cp ring) bond lengths are close, varying in
a narrow range of 2.646(3)-2.674(4) Å. This could be due to
the much stronger yttrium and nitrogen bond.17 The bond
distance of Y#-N(2) (2.646(5) Å) is longer than Y-N(2)
(2.382(5) Å) and Y-N(1) (2.256(4) Å), well consistent with
the η5/η1:κ1 coordination mode of the pyrrolide ligand. The bond
length of C(13)-N(1), 1.503(8) Å, is close to the normal C-N
single bond (1.4904(17) Å),18 indicating that the CdN bond of
the ligand has been reduced to a single bond by intramolecular
alkylation. The intramolecular alkylation has been found in N,O
bidentate Salen titanium or zirconium bis(benzyl) complexes,
which results in decomposition of the complexes and loss of
catalytic activity for ethylene polymerization.19 For N,N biden-
tate dipyrrolylimino or dipyridylimino compounds, the CdN
bond is reduced to C-N by intermolecular alkyl migration to
form a monanionic moiety that stabilizes lutetium bis(alkyl)s
or even cationic alkyl species,20 whereas alkylation of the Cd
N group of the pyrrolylaldiminato ligand affording dianionic
species to support a complex such as 1 has not been known, as
far as we are aware. Complex 1 also represents the first example
of a rare earth metal bis(alkyl) complex bearing a η5-coordinated
pyrrolide, namely, a hetero-cyclopentadienyl ligand.
Figure 2. X-ray structure of 3 with 35% probability thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 3
and 4
3
4
Y-N(1)
2.347(2)
2.457(2)
2.367(2)
2.491(2)
2.260(2)
2.493(2)
2.326(2)
2.487(2)
2.334(2)
2.471(2)
2.315(2)
71.35(8)
95.17(8)
71.26(8)
96.99(8)
71.11(8)
93.67(8)
105.08(8)
Y-N(2)
Y-N(3)
Y-N(4)
Y-N(5)
Y-N(6)
N(1)-Y-N(2)
N(2)-Y-N(3)
N(3)-Y-N(4)
N(4)-Y-N(5)
N(5)-Y-N(6)
N(6)-Y-N(1)
N(5)-Y-N(1)
O(1)-Y-N(2)
71.52(8)
85.56(8)
69.46(8)
85.93(8)
119.43(8)
151.96(8)
sponding amido complex 3 (80.3%) (Scheme 1). The molecular
structure of 3 was resolved by X-ray diffraction analysis to be
a monomer, adopting a six-coordinate octahedral geometry
(Figure 2). The two pyrolide ligands located in almost perpen-
dicular positions coordinate to the yttrium atom in a η1:κ1-N,N
bidentate fashion, leaving the THF molecule and anilido moiety
in a cis-arrangement. No η5-bonding mode of the pyrrolyl ring
has been observed. The bond distances between yttrium and
the chelate nitrogen of the pyrrolyl ring Y-N(pyr) (Y-N(1),
2.347(2) Å; Y-N(3), 2.367(2) Å) are shorter than those of
yttrium and the imino nitrogen Y-Nimino (Y-N(2), 2.457(2)
Å; Y-N(4), 2.491(2) Å) (Table 2). The bond length of yttrium
and amino nitrogen Y-N(5) (2.260(2) Å) is the shortest, which
is comparable to those found in bis(amido) yttrium complexes
Y{N(SiMe3)2}2{OSitBu(2-C6H4(CH2NMe2))2} (2.237(9) Å)21
and Y{N(SiMe3)2}2{N-isopropyl-2-(isopropylamino)troponimi-
nato} (2.236(3) Å).22 The acute N-Y-N angles (71.52(8)° and
69.46(8)°) and obtuse O-Y-N(2) angle (151.96(8)°) are
comparable to those found in a related samarium alkyl complex
mentioned previously (N-Sm-N, average 69.12°; O(1)-Sm1-
N(5), 160.37(9)°).7
Following a similar procedure, treatment of Y(CH2SiMe3)3-
(THF)2 with 2 equiv of L1 generated complex 2 in high yield
1
(82.3%) via abstractions of metal alkyls (Scheme 1). The H
NMR spectrum displayed that the pyrrolyl protons gave
resonances in the olefinic region at δ 6.54, shifted downfield
compared to δ 6.25 in complex 1, indicating an η1-Ν coordina-
tion mode of the pyrrolylaldiminato ligand. The singlet reso-
nance at δ -0.50 was assignable to the equivalent methylene
protons of YCH2SiMe3. The presence of a signal for the imino
proton CHdN (δ 7.82) suggested the absence of alkylation.
1
Although without X-ray analysis, the H NMR spectrum is
informative to figure out the molecular structure of complex 2,
being a bis(pyrrolylaldiminato)yttrium monoalkyl, analogous
with samarium alkyl [2-(CHdNC6H3-2,6-iPr2)-5-tBuC4H2N]2-
Sm(CH2SiMe3)(THF) reported by Anorld, albeit isolated by a
different synthetic pathway.7 This was further proved by
amination of 2 with 2,6-diisopropylaniline to afford the corre-
(16) Shannon. R. D. Acta Crystallogr. A 1976, 32, 751.
(17) William, J. E.; Fujimoto, C. H.; Matthew, A. J. Organometallics
2002, 21, 1825.
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tallics 2004, 23, 2797.
(19) Knight, P. D.; Clarke, A. J.; Kimberley, B. S.; Jackson, R. A.; Scott,
P. Chem. Commun. 2002, 352.
The homoleptic tris(η1:κ1-pyrrolylaldiminato)yttrium complex
4 was quantitatively isolated when 3 equiv of L1 reacted with
(20) Cameron, T. M.; Gordon, J. C.; Michalczyk, R.; Scott, B. L. Chem.
Commun. 2003, 2282.
(21) Shao, P.; Berg, D. J.; Bushnell, G. W. Inorg. Chem. 1994, 33, 6334.
(22) Roesky, P. W. Eur. J. Inorg. Chem. 1998, 593.

674 Organometallics, Vol. 26, No. 3, 2007
Yang et al.
Figure 3. X-ray structure of 4 with 35% probability thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
Figure 4. X-ray structure of 5 with 35% probability thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
Scheme 2 . Synthetic Pathway for Preparation of Complex 5
(alkyl) intermediate B. B reacts with another molecule of neutral
ligand L2 to yield complex 5 via metal alkyl abstraction and
intramolecular alkylation as described previously for the forma-
1
tion of 1 (Scheme 2). The H NMR spectrum of complex 5
was complicated and difficult to assign until the resolution of
the solid structure. The four discrete doublet-doublet signals
in the upfield region δ -1.07 to -0.17 were attributed to the
diastereotopic protons of the metal alkyls. The methylene
protons of the newly formed C(H)(CH2SiMe3)N species were
also nonequivalent, giving separate resonances at δ 0.27, 0.77,
and 1.67. This indicated the structurally asymmetric character
of complex 5 in solution. Moreover, complex 5 belongs to the
rare examples of bis(alkyl) complexes based on rare earth
metals, as the highly active character of the two metal alkyls
and the relatively less steric environment make the isolation of
such complexes difficult.23 The solid structure of complex 5 is
far from those reported previously. For instance, a lutetium bis-
(alkyl) complex bearing a monoanionic N,N,N tridentate anilido-
pyridine-amido ligand is monomeric. The two alkyl moieties
are arranged in trans-positions. The geometry about the metal
center is described as twisted square pyramidal. The base of
the pyramid is defined by the nitrogen atoms, which contribute
to stabilize the complex,20 while complex 5 is an asymmetric
dimer (Figure 4). The two N,N,P tridentate pyrrolide ligands,
bearing dianionic species, coordinate to Y(1) in a η1:κ2/η1:κ2
fashion along with a solvated THF moiety to generate seven-
coordinate trigonal/pyramidal geometry around Y(1), whereas
the pyrrolyl rings bond to Y(2) in a η5/η5 mode to form a
reverse-sandwich geometry between the Y(1) and Y(2) atoms.
The two alkyl moieties coordinate to Y(2) in cis-positions to
the two pyrrolyl rings. The pyrrolyl rings and alkyl carbon atoms
form an eight-coordinate tetrahedron geometry around Y(2). It
seems at first glance that the Y(2) unit is a metallocene bis-
(alkyl). In fact, it is noteworthy that one pyrrolyl ring coordinates
to Y(2) as a neutral ligand, while another bears one negative
charge. Thus, the resulting bond lengths formed by Y(2) with
η5-C and η5-N of the two rings are different. The bond lengths
of the C and N atoms of one pyrrolyl ring to the Y(2) ion are
Y(CH2SiMe3)3(THF)2 at room temperature for 24 h (Scheme
1). Complex 4 was reported previously to be prepared by
treatment of yttrium tris(amido) complex Y{N(SiMe3)2}3 with
3 equiv of L1; however, no crystallographic information was
provided.5 We isolated single crystals of 4 and characterized
its molecular structure by X-ray analysis, as shown in Figure
3. All three pyrrolide ligands coordinate to the yttrium in a η1:
κ1-N,N bidentate mode. No coordination of THF is found. The
bond lengths of Y-Npyr and Y-Nimino are comparable to the
corresponding ones found in complex 3 and do not need to be
discussed further (Table 2).
Synthesis and Characterization of Complexes 5. The N,N,P
tridentateligand2-{(N-2-diphenylphosphinophenyl)iminomethyl)}-
pyrrole, L2, was prepared by condensation reaction of pyrrole-
2-carboxyaldehyde with 2-diphenylphosphinophenylamine in the
presence of a catalytic amount of formic acid. Treatment of L2
by equimolar Y(CH2SiMe3)3(THF)2 afforded yellow powders.
Crystallization from a mixture of toluene and hexane gave
yellow crystals of complex 5 in quantitative yield. The probable
mechanistic pathway was described as Scheme 2. First, neutral
ligand L2 protonates one metal alkyl species of Y(CH2SiMe3)3-
(THF)2 to give a bis(alkyl) intermediate A. Alkylation of the
imino CdN group in A by the incoming Y(CH2SiMe3)3(THF)2
generates C(H)(CH2SiMe3)N- species attaching to Y(1), while
the pyrrolyl ring η5-coordinates to Y(2), to afford binuclear tetra-
(23) The first monocyclopentadienyl rare metal bis(alkyl) complexes were
reported by Lappert 30 years ago; see: Lappert, M. F.; Pearce, R. Chem.
Commun. 1973, 126. Recently, rare earth alkyl complexes have demonstrated
unique catalytic activities; see refs 9h,i.

Pyrrolide-Ligated Organoyttrium Complexes
Organometallics, Vol. 26, No. 3, 2007 675
Table 3. Selected Bond Distances (Å) and Angles (deg) for 5
metal alkyls upon the monomer approach and insertion during
polymerization. Thus, although each metal center bears one
reaction site (Y-σ-C), complex 1 was not a real “single-site”
catalyst. When using complex 2 as initiator, high catalytic
activity and much more controllable polymerization were found.
The molecular weight of the resultant PLA increased with the
increase of monomer-to-initiator ratio that was close to the
theoretic value (calculated on the assumption that each metal
alkyl initiates the polymerization). The deviation was obvious
at ratios higher than 800. Nevertheless, in all cases the molecular
weight distribution was very narrow over the entire monomer-
to-initiator ratio range, indicating a single-site catalyst system
(Table 4, entries 2-6). However, the polymerization performed
in CH2Cl2 afforded PLA with a much broader molecular weight
distribution (Table 4, entries 7, 8). This might be due to the
coordination of CH2Cl2, which changed the steric environment
and the Lewis acidity of the central metal, which was consistent
with the previously reported bis(phenolate)yttrium amide sys-
tem.25 The amido complex 3 was comparable to its precursor 2
in terms of catalytic activity. Due to the above-mentioned reason,
the polydispersity of the isolated PLA was broadened (Table
4, entries 9, 10).25a In contrast, no obvious polymerization at
room temperature was observed by using homoleptic complex
4, which might be due to the chelate Y-η1-N initiator in 4 being
less active than the Y-σ-C in 1 and 2 or Y-σ-N in 3 (Table
4, entry 11). Bis(alkyl) complex 5, as expected, demonstrated
to be as efficient an initiator as the mono(alkyl) counterparts.
Also, it was not surprising that the resultant PLA had a lower
molecular weight and broader molecular weight distribution,
as both alkyl species participated in the initiation, resulting in
multiple sites in the complex 5 system (Table 4, entries 12,
13).
Y(1)-N(1)
2.419(4)
2.369(4)
2.8901(15)
2.632(4)
2.686(5)
2.792(6)
2.690(5)
2.738(6)
2.402(5)
96.4(2)
Y(1)-N(2)
2.292(4)
2.359(5)
3.0289(15)
2.701(4)
2.795(5)
2.692(5)
2.726(6)
2.731(5)
2.381(6)
104.90(16)
Y(1)-N(3)
Y(1)-N(4)
Y(1)-P(1)
Y(2)-N(3)
Y(1)-P(2)
Y(2)-N(4)
Y(2)-C(51)
Y(2)-C(53)
Y(2)-C(55)
Y(2)-C(57)
Y(2)-C(59)
C(59)-Y(2)-C(63)
N(4)-Y(1)-N(3)
Y(2)-C(52)
Y(2)-C(54)
Y(2)-C(56)
Y(2)-C(58)
Y(2)-C(63)
Y(1)-N(3)-Y(2)
81.39(15)
Table 4. Ring-Opening Polymerization of D,L-Lactide by
Complexes 1-5a
Mcalcd
Mn
entry cat. [LA]/[Ln] solvent conv (%) (×104)b (×104)c PDI
1
2
3
4
5
6
7
8
9
10
11
12
13
1
2
2
2
2
2
2
2
3
3
4
5
5
300
150
300
500
800
1000
300
800
300
800
300
300
800
THF
THF
THF
THF
THF
THF
CH2Cl2
CH2Cl2
THF
THF
THF
THF
THF
100
100
93
91
92
100
100
85
96
96
4.32
2.16
4.02
6.56
10.6
14.4
4.32
5.21 1.53
2.21 1.31
3.82 1.26
5.31 1.18
6.46 1.12
9.00 1.12
2.66 1.75
9.04 1.63
3.32 1.56
9.80
4.15
11.1
12.8
n.d.
1.63
n.d.
trace
93
93
n.d.d
4.02
2.44 1.49
7.62 1.49
10.7
b
a General conditions: 20 °C, 1 h, [LA] ) 1.0 mol/L. Mcal ) ([LA]/
c
[Ln]) × 144.13 × X (X ) conversion). Measured by GPC calibrated with
d
standard polystyrene samples. n.d. ) not determined.
Experimental Section
unequal, varying in a wide range of 2.632(4)-2.795(5) Å (Table
3), which are comparable to those in 1. Comparatively, the
distances between Y(2) and the atoms N(4), C(55), C(56), C(57),
and C(58) from the other pyrrolyl ring are uniform, varying
within a narrow range from 2.690(5) to 2.738(6) Å. These bond
distances are strongly reminiscent of those Dy-η5-C(Cp ring),
ranging from 2.638(2) to 2.704(3) Å) in complex (C5Me4-
SiMe3)Dy(CH2SiMe3)2(THF),10e suggesting that the pyrrolyl ring
can be an ideal alternative to the cyclopentadienyl ancillary
ligand. The bond distances between Y(1) and nitrogen atoms
N(1), N(2), N(3), and N(4), averaging 2.360 Å, are comparable
to Y-N (2.204(4)-2.449(5) Å) reported by Mashima.5 The
bond distances Y(2)-C(59) (2.402(5) Å) and Y(2)-C(63)
(2.381(6) Å) and the corresponding bond angle C(59)-Y(2)-
C(63) (96.4(2)°) have been found to be not obviously different
from those in the previously reported yttrium bis(alkyl) com-
plexes (Y-C, av 2.40 Å; C-Y-C, av 101.5°).9h,10e,24
Polymerization of D,L-Lactide Initiated by Complexes 1-5.
Complexes 1-5 were used to initiate the ring-opening poly-
merizations (ROP) of D,L-lactide (D,L-LA), respectively. Rep-
resentative results are summarized in Table 4. The polymeri-
zation initiated by complex 1 was carried out at room temperature
smoothly to reach almost complete conversion in 1 h in THF
media (Table 4, entry 1). However, the molecular weight
distribution of the resultant polylactide (PLA) was relatively
broad. This could be attributed to the mutual effects of the two
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 by 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
confirmed by the 1H-1H COSY and 1H-13C HMQC experiments
when necessary. IR spectra were recorded on a Vertex 70 FT-IR.
The molecular weight and molecular weight distribution of the
polymers were measured by a GPC Waters 410. Elemental analyses
were performed at National Analytical Research Centre of Chang-
chun Institute of Applied Chemistry (CIAC). 2,6-Diisopropylaniline
was obtained from Aldrich and purified by distillation before use.
Pyrrole-2-carboxyaldehyde and 2-diphenylphosphanylphenylamine
were prepared according to the literature.26,27
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 -80
°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
(25) (a) Ma, H.; Okuda. J. Macromolecules 2005, 38, 2665. (b) Ma, H.;
Spaniol, T.; Okuda, J. Dalton Trans. 2003, 4770.
(24) (a) Bambirra, S.; Boot, S. J.; Leusen, D.; Meetsma, A.; Hessen, B.
Organometallics 2004, 23, 1898. (b) Bambirra, S.; Meetsma, A.; Hessen,
B. Organometallics 2006, 25, 3486. (c) Hayes, P. G.; Welch, G. C.; Emslie,
D. J. H.; Noack, C. L.; Piers, W. E. Organometallics 2003, 22, 1577.
(26) David, M. W.; Leung, S. H.; Senge, M. O.; Smith, K. M. J. Org.
Chem. 1993, 58, 7245.
(27) (a) Dai, X.; Wong, A.; Virgil, S. C. J. Org. Chem. 1998, 63, 259.
(b) Cooper, M. K. Inorg. Synth. 1989, 25, 129.

676 Organometallics, Vol. 26, No. 3, 2007
Table 5. Summary of Crystallographic Data for 1, 3, 4, and 5
Yang et al.
1‚0.5C6H14
3‚C7H8
4
5
formula
cryst size, mm
fw
cryst syst
space group
a (Å)
C
30.5H47N2OSi2Y
C57H76N5OY
0.33 × 0.24 × 0.15
936.14
monoclinic
P21/c
11.5198(8)
12.3059(9)
38.706(3)
90
106.323(2)
90
C51H63N6Y
0.26 × 0.23 × 0.16
848.98
triclinic
P21/c
11.6880(7)
16.6699(10)
24.4478(15)
90
100.5320(10)
90
C66H88N4OP2Si4Y2
0.47 × 0.23 × 0.18
1305.52
monoclinic
P21/c
20.3821(8)
19.3542(7)
22.0003(9)
90
116.1680(10)
90
7789.1(5)
4
0.25 × 0.19 × 0.09
602.79
monoclinic
P21/c
10.1563(12)
20.818(2)
17.4417(19)
90
99.432(2)
90
3638.0(7)
4
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
5265.9(7)
4
1.181
4683.1(5)
4
1.204
D
calcd (g/cm3)
1.101
1.113
radiation (λ), Å
Mo KR (0.71073)
Mo KR (0.71073)
Mo KR (0.71073)
Mo KR (0.71073)
2θ max, deg
52.14
52.14
52.12
52.06
µ (cm-1
F(000)
)
16.90
1276
11.50
2000
12.85
1800
16.22
2736
no. of obsd reflns
7159
10 370
9231
15 283
no. of params refnd
353
590
535
724
GOF
R1
0.974
0.0742
0.975
0.0508
0.965
0.0496
1.092
0.0602
wR2
0.2064
0.1252
0.1128
0.2253
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 the calculated positions
and were included in the structure calculation without further
refinement of the parameters. The crystallographic data and the
refinement of complexes 1, 3, 4, and 5 are summarized in Table 5.
2-(2,6-iPr2C6H3NdCH)C4H3NH (L1). To a dried MeOH solu-
tion (20 mL) of pyrrole-2-carboxyaldehyde (1.90 g, 20 mmol) were
added 2,6-diisopropylaniline (3.6 g, 20 mmol) and a catalytic
amount of formic acid (0.25 mL) under stirring. The reaction
mixture was stirred for 4 h at room temperature. Subsequently, the
white precipitate was separated by filtration and then washed with
IR (KBr pellets): ν 3241, 3070, 3049, 2896, 1620, 1575, 1560,
1478, 1463, 1433, 1418, 1339, 1311, 1264, 1198, 1159, 1128, 1093,
1062, 1033, 999, 970, 942, 880, 853, 816, 786, 741, 693, 602, 586,
521, 503, 492, 476, 435, 407 cm-1. Anal. Calcd for C23H19N2P
(%): C, 77.95; H, 5.40; N, 7.90. Found: C, 77.90; H, 5.07; N,
7.76.
{[2-(2,6-iPr2C6H3NC(H)(CH2SiMe3))C4H3N]Y(CH2SiMe3)-
(THF)}2 (1). To a toluene solution (5.0 mL) of Y(CH2SiMe3)3-
(THF)2 (0.383 g, 0.775 mmol) was added dropwise equivalent L1
(0.197 g, 0.776 mmol in 2 mL of toluene) at room temperature.
The mixture was then stirred for 24 h. Removal of the volatiles
gave a deep yellowish, oily residue. The residue was dissolved with
3 mL of hexane and then cooled to -30 °C for 12 h to give yellow
solids. The solids were washed carefully by a small amount of
hexane (0.5 mL) and then dried in vacuum to afford complex 1 in
55% yield (0.257 g). Colorless single crystals for X-ray analysis
grew from a mixture of toluene and hexane at -30 °C within a
week. 1H NMR (400 MHz, C6D6, 25 °C): δ -0.42 (m, 4H, Y-CH2-
SiMe3), 0.32 (s, 18H, Y-CH2SiMe3), 0.51 (s, 18H, CH-CH2-
SiMe3), 0.95 (m, 1H, CH-CH2SiMe3), 1.35 (br, 8H, THF), 1.38
(d, JH-H ) 8.0 Hz, 6H, CHMe2), 1.41 (m, 2H, CH-CH2SiMe3),
1.45 (d, JH-H ) 8.0 Hz, 6H, CHMe2), 1.63 (d, JH-H ) 8.0 Hz, 6H,
CHMe2), 1.70 (d, JH-H ) 8.0 Hz, 1H, CH-CH2SiMe3), 1.77 (d,
JH-H ) 8.0 Hz, 6H, CHMe2), 3.38 (br, 8H, THF), 4.30 (m, 4H,
CHMe2), 4.61 (m, 1H, CH-CH2SiMe3), 5.69 (d, JH-H ) 8.0 Hz,
1H, CH-CH2SiMe3), 6.25 (s, 2H, 4-pyr), 6.68 (s, 2H, 3-pyr), 7.16
(m, 6H, C6H3), 7.82 ppm (s, 2H, 5-pyr). 13C NMR (100 MHz, C6D6,
25 °C): δ 1.62 (s, 6C, CH-CH2SiMe3), 5.36 (s, 6C, Y-CH2SiMe3),
25.61 (br, 4C, THF), 25.81, 25.96, 26.48, 27.38 (s, 8C, CHMe2),
26.64, 26.90, 27.74, 28.33 (s, 4C, CHMe2), 27.94 (s, 2C, CH-
CH2SiMe3), 28.60 (s, 2C, Y-CH2SiMe3), 64.44, 64.82 (s, 2C, CH-
CH2SiMe3), 70.13 (s, 4C, THF), 108.24 (s, 2C, 3-pyr), 114.30 (s,
2C, 4-pyr), 123.74 (s, 2C, 5-pyr), 124.75 (s, 4C, m-C6H3), 125.25
(s, 2C, p-C6H3), 148.69, 148.88 (s, 4C, o-C6H3), 150.00 (s, 2C,
ipso-pyr), 158.61 ppm (s, 2C, ipso-C6H3). IR (KBr pellets): ν 2960,
2869, 1579, 1460, 1424, 1382, 1362, 1321, 1297, 1248, 1232, 1191,
1106, 1036, 1016, 861, 804, 780, 742, 712, 619 cm-1. Anal. Calcd
for C61H94N4O2Si4Y2 (%): C, 60.77; H, 7.86; N, 9.29. Found: C,
60.22; H, 7.23; N, 9.50.
cold methanol. Removal of the solvents afforded L1 in 73.8% yield
1
(3.75 g). H NMR (300 MHz, CDCl3, 25 °C): δ 1.12 (d, JH-H
)
6.9 Hz, 12H, CHMe2), 3.06 (hepta, JH-H ) 6.9 Hz, 2H, CHMe2),
6.20 (m, 1H, 3-pyr), 6.49 (s, 1H, 4-pyr), 6.60 (t, 1H, 5-pyr), 7.15
(m, 3H, m,p-C6H3), 7.95 (s, 1H, NdC-H), 10.39 ppm (br, 1H,
N-H). 13C NMR (300 MHz, CDCl3, 25 °C): δ 24.03 (4C, CHMe3),
28.35 (2C, CHMe3), 110.31 (1C, 4-pyr), 117.14 (1C, 3-pyr), 123.65
(1C, 5-pyr), 124.61 (2C, m-C6H3), 124.97 (1C, p-C6H3), 130.33
(1C, ipso-pyr), 139.43 (2C, o-C6H3), 148.92 (1C, ipso-C6H3),
153.16 ppm (1C, NdCH). IR (KBr pellets):ν 3235, 3060, 2964,
2926, 2869, 1628, 1586, 1554, 1458, 1418, 1384, 1363, 1338, 1311,
1253, 1181, 1133, 1108, 1089, 1056, 1033, 932, 883, 860, 832,
803, 781, 760, 746, 607 cm-1. Anal. Calcd for C17H22N2 (%): C,
80.27; H, 8.72; N, 11.01. Found: C, 80.54; H, 8.07; N, 10.93.
2-(2-Ph2PC6H4NdCH)C4H3NH (L2). To a dried MeOH solution
(15 mL) of pyrrole-2-carboxyaldehyde (0.34 g, 3.6 mmol) were
added 2-diphenylphosphinophenylamine (1.00 g, 3.6 mmol) and a
catalytic amount of formic acid (0.25 mL) under stirring. The
reaction mixture was stirred for 24 h at 40 °C. The viscous oil
residues were isolated after driving off the solvent under reduced
pressure. Recrystallization from ethyl ether yielded L2 as colorless
1
crystals in 35% yield (0.45 g). H NMR (300 MHz, CDCl3, 25
°C): δ 6.12 (m, 1H, 3-pyr), 6.50 (s, 1H, 4-pyr), 6.67 (dd, 1H, JH-H
) 7.4 Hz, C6H4), 6.96 (s, 1H, 5-pyr), 7.02-7.06 (dd, 1H, JH-H
7.4 Hz, C6H4), 7.11(t, 1H, JH-H ) 7.4 Hz, C6H4), 7.14-7.19 (m,
4H, o-C6H5), 7.32-7.34 (m, 6H, m,p-C6H5), 7.40 (t, 1H, JH-H
)
)
7.4 Hz, C6H4), 7.97 (s, 1H, NdC-H), 11.35 ppm (s, 1H, N-H).
13C NMR (300 MHz, CDCl3, 25 °C): δ 110.65 (1C, 4-pyr), 116.23
(1C, 3-pyr), 117.38 (1C, 5-pyr), 123.18 (1C, ipso-PC6H4), 125.91
(2C, m-PC6H4), 128.80 (6C, m,p-C6H5), 130.23 (1C, p-PC6H4),
132.07 (1C, ipso-pyr), 133.12 (1C, o-PC6H4), 134.40, 134.66 (6C,
o,ipso-C6H5), 137.76 (1C, ipso-NC6H4), 148.67 ppm (1C, NdCH).
[2-(2,6-iPr2C6H3NdCH)C4H3N]2Y(CH2SiMe3)(THF) (2). Fol-
lowing a procedure similar to that described for the preparation of
1, treatment of a toluene solution (5.0 mL) of Y(CH2SiMe3)3(THF)2
(0.283 g, 0.572 mmol) with 2 equiv of L1 (0.294 g, 1.148 mmol in
1
2 mL of toluene) afforded complex 2 (0.355 g, 82.3%). H NMR
(400 MHz, C6D6, 25 °C): δ -0.52 (s, 2H, CH2SiMe3), 0.18 (s,

Pyrrolide-Ligated Organoyttrium Complexes
Organometallics, Vol. 26, No. 3, 2007 677
9H, CH2SiMe3), 1.12 (s, 6H, CHMe2), 1.14 (s, 6H, CHMe2), 1.20
(s, 12H, CHMe2), 1.34 (s, 4H, THF), 3.14 (br, 4H, CHMe2), 3.81
(s, 4H, THF), 6.54 (s, 2H, 3-pyr), 6.92 (s, 2H, 4-pyr), 7.06 (s, 2H,
5-pyr), 7.16 (b, 6H, m,p-C6H3), 7.82 ppm (s, 2H, NdC-H). 13C
NMR (100 MHz, C6D6, 25 °C): δ 4.80 (3C, SiMe3), 23.17 (1C,
CHMe2), 23.60 (2C, CHMe2), 25.98 (2C, CHMe2), 26.32 (1C,
CHMe2), 26.64 (2C, THF), 26.64 (2C, CHMe2), 28.76 (1C,
CHMe2), 29.04 (2C, CHMe2), 29.32 (1C, CHMe2), 34.63 (d, JY-C
) 39.3 Hz, 1C, CH2SiMe3), 71.37 (2C, THF), 113.73 (4C, 3,4-
pyr), 123.23 (2C, 5-pyr), 124.21 (4C, m-C6H3), 126.76 (2C, p-C6H3),
137.40 (2C, ipso-pyr), 138.83 (2C, o-C6H3), 142.36 (2C, o-C6H3),
148.27 (2C, ipso-C6H3), 164.48 ppm (2C, NdCH). IR (KBr
pellets): ν 2961, 2868, 1627, 1577, 1443, 1392, 1301, 1171, 1036,
981, 861, 747 cm-1. Anal. Calcd for C42H61N4OSiY (%): C, 66.82;
H, 8.14; N, 7.42. Found: C, 66.57; H, 8.25; N, 7.24.
[2-(2,6-iPr2C6H3NdCH)C4H3N]2Y(NH-C6H3-iPr2-2,6)(THF) (3).
Reaction of complex 2 (0.306 g, 0.405 mmol) and 2,6-diisopro-
pylaniline (0.0071 g, 0.400 mmol) in toluene (7 mL) at room
temperature took place immediately upon addition. The reaction
mixture was kept stirring for 6 h. The volatiles were removed under
reduced pressure to leave a brown residue, which was dissolved
with hexane (3 mL) and then cooled to -30 °C. After 12 h,
crystalline solids precipitated on the bottom of the flask. The solids
were separated by filtration and washed with a small amount of
hexane (3 × 0.5 mL) and then dried in vacuum to afford yellow
crystals of 3 (0.274 g, 80.3%). Recrystallization from toluene at
-30 °C for a couple of days gave crystals suitable for X-ray
analysis. 1H NMR (400 MHz, C6D6, 25 °C): δ 0.79-1.39 (m, 36H,
CHMe2), 1.20 (br, 4H, THF), 2.63-2.71 (m, 2H, CHMe2), 2.72-
2.79 (m, 2H, CHMe2), 3.13-3.17 (m, 1H, CHMe2), 3.50-3.53 (m,
1H, CHMe2), 3.73, 3.98 (br, 4H, THF), 4.78 (s, 1H, NH), 6.40 (s,
1H, 3-pyr), 6.65 (s, 1H, 3-pyr), 6.78 (s, 1H, 4-pyr), 6.80 (d, JH-H
) 3.2 Hz, 1H, 4-pyr), 6.89 (d, JH-H ) 7.6 Hz, 1H, 5-pyr), 7.05 (d,
JH-H ) 3.6 Hz, 1H, 5-pyr), 7.10-7.32 (m, 9H, m, p-C6H3), 7.66
(s, 1H, CHdN), 7.88 (s, 1H, CHdN). 13C NMR (100 MHz, C6D6,
25 °C): δ 22.54, 23.05, 23.30, 23.60, 23.89, 24.17, 24.51, 25.50,
26.61, 26.90, 27.20, 27.61 (12C, CHMe2), 28.27 (1C, CHMe2),
28.62 (2C, CHMe2), 29.03, 29.60, 33.37 (3C, CHMe2), 25.94 (2C,
THF), 72.83 (2C, THF), 113.33, 113.64 (2C, 3-pyr), 116.04, 119.39
(2C, 4-pyr), 122.81, 123.02 (2C, 5-pyr), 123.36, 124.38 (2C,
p-NC6H4), 123.57, 124.79 (4C, m-NC6H4), 126.87 (3C, m,p-
NHC6H4), 132.75, 133.02, 134.82, 137.79, 140.81, 142.14 (6C,
o-C6H4), 137.07, 140.34 (2C, ipso-pyr), 148.12, 148.77 (2C, ipso-
NC6H4), 152.60 (1C, ipso-NHC6H4), 164.12, 164.94 ppm (2C, CHd
N). IR (KBr pellets): ν 3678, 3231, 3064, 2962, 2868, 1627, 1596,
1578, 1461, 1391, 1340, 1302, 1257, 1171, 1091, 1035, 981, 883,
782, 746 cm-1. Anal. Calcd for C57H76N5OY (%): C, 73.13; H,
8.18; N, 7.48. Found: C, 73.37; H, 8.29; N, 7.58.
Y[2-(2,6-iPr2C6H3NdCH)C4H3N]3 (4). To a toluene solution
(3.0 mL) of Y(CH2SiMe3)3(THF)2 (0.192 g, 0.386 mmol) was added
dropwise 3 equiv of L1 (0.295 g, 1.160 mmol in 2 mL of toluene)
under stirring. The mixture was reacted for 24 h at room
temperature, then the volatiles were removed under reduced
pressure. The residue was dissolved with 3 mL of hexane. The
hexane solution was cooled to -30 °C and kept at this temperature
overnight to afford white solids deposited on the bottom of the
flask. The solids were collected by filtration and then dried under
reduced pressure to generate 4 as a white powder (0.263 g, 80.2%).
Colorless crystals for X-ray analysis grew from the mixture of
toluene and hexane at -30 °C within several days. 1H NMR (400
MHz, C6D6, 25 °C): δ 0.68 (d, JH-H ) 6.8 Hz, 9H, CHMe2), 0.96,
0.97, 0.99, 1.01, 1.02 (m, 27H, CHMe2), 2.52 (hepta, JH-H ) 6.8
Hz, 3H, CHMe2), 2.91 (hepta, JH-H ) 6.8 Hz, 3H, CHMe2), 6.46,
6.47 (dd, JH-H ) 3.6 Hz, 3H, 4-pyr), 6.91 (d, JH-H ) 3.6 Hz, 3H,
3-pyr), 6.96 (s, 3H, 5-pyr), 7.05 (m, 3H, p-C6H3), 7.13 (m, 6H,
m-C6H3), 7.69 ppm (s, 3H, NdCH). 13C NMR (100 MHz, C6D6,
25 °C): δ 23.18, 23.69, 26.33, 26.69 (12C, CHMe2), 28.75, 29.31
(6C, CHMe2), 114.24 (3C, 4-pyr), 123.96 (3C, 3-pyr), 124.21 (3C,
5-pyr), 124.71 (3C, m-C6H3), 126.82 (3C, p-C6H3), 137.08 (3C,
ipso-pyr), 140.80 (3C, m-C6H3), 142.14 (3C, o-C6H3), 143.55 (3C,
o-C6H3), 147.66 (3C, ipso-C6H3), 164.83 ppm (3C, NdCH). IR
(KBr): ν 3585, 2964, 2867, 1627, 1595, 1573, 1490, 1389, 1291,
1170, 1035, 981, 749 cm-1. Anal. Calcd for C51H63N6Y (%): C,
72.15; H, 7.48; N, 9.90. Found: C, 71.68; H, 7.22; N, 10.35.
[2-(2-Ph2PC6H3NC(H)(CH2SiMe3))C4H3N]2Y2(CH2SiMe3)2-
(THF) (5). To a stirred solution of Y(CH2SiMe3)3 (THF)2 (0.206
g, 0.416 mmol) in 4.0 mL of toluene was added dropwise L2 (0.147
g, 0.414 mmol) in 2 mL of toluene. After stirring for 12 h at room
temperature, the solvent was stripped off and 3 mL of hexane was
added. The mixture was then cooled to -30 °C and kept at this
temperature overnight to afford reddish-yellow solids, which were
washed with a small amount of hexane to remove impurities and
then dried in vacuum to afford 5 as a yellow powder in 40.5%
yield (0.11 g). Suitable yellow crystals for X-ray analysis grew
1
from a mixture of toluene/hexane at -30 °C in a week. H NMR
(400 MHz, C6D6, 25 °C): δ -1.07 (dd, JY-C-H ) 12.0 Hz, 2JH-H
) 4.0 Hz, 1H, Y-CH2SiMe3), -0.60 (dd, JY-C-H ) 12.0 Hz, 2JH-H
) 4.0 Hz, 1H, Y-CH2SiMe3), -0.25 (dd, JY-C-H ) 12.0 Hz, 2JH-H
) 4.0 Hz, 1H, Y-CH2SiMe3), -0.17 (dd, JY-C-H ) 12.0 Hz, 2JH-H
) 4.0 Hz, 1H, Y-CH2SiMe3), 0.20 (s, 9H, CH-CH2SiMe3), 0.27
(m, 1H, CH-CH2SiMe3), 0.43 (s, 9H, Y-CH2SiMe3), 0.47 (s, 9H,
CH-CH2SiMe3), 0.61 (s, 9H, Y-CH2SiMe3), 0.77 (m, 1H, CH-
CH2SiMe3), 1.09 (br, 4H, THF), 1.67 (m, 2H, CH-CH2SiMe3),
3.53 (br, 4H, THF), 5.11 (d, JH-H ) 8.0 Hz, 1H, CH-CH2SiMe3),
5.65 (d, JH-H ) 8.0 Hz, 1H, CH-CH2SiMe3), 6.35 (s, 1H, 4-pyr),
6.55 (t, JH-H ) 7.2 Hz, 1H, m-PC6H4), 6.62 (t, JH-H ) 7.2 Hz, 1H,
m-PC6H4), 6.67 (s, 1H, 3-pyr), 6.70 (d, JH-H ) 7.2 Hz, 1H,
o-PC6H4), 6.76 (s, 1H, 4-pyr), 6.81 (m, 1H, o-PPh2), 6.83 (s, 1H,
5-pyr), 6.98 (t, JH-H ) 8.0 Hz, 2H, m-PPh2), 7.04-7.18 (m, 1H,
3-pyr, 1H, 4-pyr, 11H, PPh2), 7.22 (t, JH-H ) 7.6 Hz, 4H, p-PPh2),
7.30 (m, 1H, o-PC6H4), 7.39 (t, JH-H ) 8.0 Hz, 2H, m-PPh2), 7.44-
7.49 (m, 2H, m-NC6H4). 13C NMR (100 MHz, C6D6, 25 °C): δ
0.87, 1.14 (s, 6C, CH-CH2SiMe3), 4.94, 5.14 (s, 6C, Y-CH2-
SiMe3), 25.41 (br, 2C, THF), 26.09 (s, 1C, CH-CH2SiMe3), 28.42
(d, JY-C ) 41.1 Hz, 1C, Y-CH2SiMe3), 29.39 (d, JY-C ) 40.3
Hz, 1C, Y-CH2SiMe3), 31.08 (s, 1C, CH-CH2SiMe3), 56.43, 58.07
(s, 2C, CH-CH2SiMe3), 70.77 (br, 2C, THF), 110.78, 111.37 (s,
2C, 3-pyr), 114.19 (d, J ) 6.0 Hz, 2C, o-NC6H4), 115.13 (d, J )
6.0 Hz, 2C, m-PC6H4), 116.82, 117.15 (s, 2C, 4-pyr), 122.96, 124.49
(s, 2C, ipso-PC6H4), 128.97-129.96 (m, 14C, p-PC6H4, m,p-C6H5),
133.77-135.35 (m, 14C, o-PC6H4, o,ipso-C6H5), 130.63, 130.83
(s, 2C, 5-pyr), 134.46, 134.90 (s, 2C, ipso-pyr), 151.70 ppm (d, J
) 32 Hz, 2C, ipso-NC6H4). IR (KBr pellets): ν 3054, 2950, 2891,
1575, 1480, 1450, 1434, 1285, 1248, 1166, 1129, 1095, 1034, 861,
786, 743, 695 cm-1. Anal. Calcd for C66H88N4OP2Si4Y2 (%): C,
60.72; H, 6.79; N, 4.29. Found: C, 60.37; H, 6.65; N, 4.30.
Polymerization of D,L-Lactide. A typical procedure for polym-
erization of D,L-LA was performed in a 25 mL round-bottom flask
in a glovebox. To a stirred solution of D,L-LA (0.572 g, 3.97 mmol)
in 3.0 mL of THF was added a THF solution (1.0 mL) of complex
2 (0.010 g, 0.0132 mmol, [LA]/[Cat] ) 300:1). The reaction mixture
was stirred for 1 h at room temperature and then was terminated
by 1.0 mL of a mixture of HCl/CH3OH/CHCl3 (1:100:600 v/v).
The viscous solution was quenched by ethanol (1:6 v/v) to give
white solids, which were filtered, washed with ethanol, and then
dried at 40 °C for 24 h in vacuo to give PLA (0.534 g, 93.3%).
The molecular weight and the molecular weight distribution of the
resulting polymer were determined by GPC. The tacticity of the
PLA was calculated according to the methine region homonuclear
1
decoupling H NMR spectrum.
Conclusion
We have demonstrated a synthetic pathway for preparing a
series of pyrrolide-stabilized yttrium mono- or bis-alkyl and

678 Organometallics, Vol. 26, No. 3, 2007
Yang et al.
homoleptic complexes by treatment of rare earth metal tris-
(alkyl)s with neutral pyrrolide compounds. The amido derivative
was isolated by amination of the corresponding alkyl complex
with aniline. The protonolysis reaction along with intra- or
intermolecular alkylation of the CdN bond of the ligands was
observed in the process. The molecular structures and the
coordination mode of the ligand are strongly dependent on
ligand frameworks and reaction conditions. Complexes 1 and
5 represent rare examples of rare earth metal alkyl complexes
stabilized by pyrrolide species in η5-coordination mode, indicat-
ing that the pyrrolyl ring is an alternative to heterocyclopenta-
diene under certain conditions. All complexes except 4 are
highly efficient initiators for controllable polymerization of D,L-
LA to afford atactic polylactide with high molecular weight and
narrow molecular weight distribution.
Acknowledgment. We acknowledge financial support from
Jilin Provincial Science and Technology Bureau for project no.
20050555, The 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.
Supporting Information Available: CIF files and crystal-
lographic data for 1, 3, 4, and 5 including atomic coordinates, full
bond distances and bond angles, and anisotropic thermal parameters,
as well as GPC curves of the PLA samples are available free of
charge via the Internet at http://pubs.acs.org.
OM060781Y