Synthesis, structure, and catalytic activity of organolanthanides with chiral biphenyl-based tridentate amidate ligands

Article abstract of DOI:10.1016/j.jorganchem.2010.11.032

Four new chiral organolanthanide amidate complexes have been readily prepared in good yields via silylamine elimination reaction between Ln[N(SiMe₃)₂]₃ (Ln = Sm, Y, Yb) and chiral amidate ligands, (R)-2-(mesitoylamino)-2′-methoxy-6,6′-dimethyl-1, 1′-biphenyl (1H) and (R)-2-(mesitoylamino)-2′-dimethylamino-6, 6′-dimethyl-1,1′-biphenyl (2H). The steric effect of the ligand coupled with the size effect of the lanthanide ion plays an important role in complex formation. For example, treatment of 1H with half equiv of Sm[N(SiMe₃)₂]₃ gives the C₂- symmetric bis-ligated amidate complex (σOMe:κO:κN-1) ₂SmN(SiMe₃)₂ (3) in 75% yield, while reaction of 1H or 2H with half equiv of Ln[N(SiMe₃)₂]₃ (Ln = Y, Yb) affords the C₁-symmetric bis-ligated amidate complexes [(κO:κN-1)(σOMe:κO:κN-1)]LnN(SiMe ₃)₂ (Ln = Y (4), Yb (5) and the C₁-symmetric mono-ligated amidate complex (σNMe₂:κO:κN-2) Y[N(SiMe₃)₂]₂ (6), respectively, in good yields. These organolanthanide amidate complexes have been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analyses. They are active catalysts for asymmetric hydroamination/cyclization of aminoalkenes and ring-opening polymerization of rac-lactide, affording cyclic amines in excellent conversions with good ee values and isotactic-rich polylactides, respectively.

Full text of DOI:10.1016/j.jorganchem.2010.11.032

Journal of Organometallic Chemistry 696 (2011) 2186e2192
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, structure, and catalytic activity of organolanthanides with chiral
biphenyl-based tridentate amidate ligands
,
Qiuwen Wang a, Furen Zhang a, Haibin Song b, Guofu Zi a
*
a Department of Chemistry, Beijing Normal University, Beijing 100875, China
b State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new chiral organolanthanide amidate complexes have been readily prepared in good yields via
silylamine elimination reaction between Ln[N(SiMe3)2]3 (Ln ¼ Sm, Y, Yb) and chiral amidate ligands, (R)-
2-(mesitoylamino)-20-methoxy-6,60-dimethyl-1,10-biphenyl (1H) and (R)-2-(mesitoylamino)-20-dime-
thylamino-6,60-dimethyl-1,10-biphenyl (2H). The steric effect of the ligand coupled with the size effect of
the lanthanide ion plays an important role in complex formation. For example, treatment of 1H with half
Received 5 September 2010
Received in revised form
14 November 2010
Accepted 18 November 2010
equiv of Sm[N(SiMe3)2]3 gives the C2-symmetric bis-ligated amidate complex (
(SiMe3)2 (3) in 75% yield, while reaction of 1H or 2H with half equiv of Ln[N(SiMe3)2]3 (Ln ¼ Y, Yb) affords
the C1-symmetric bis-ligated amidate complexes [( O: N-1)( OMe: O:
N-1)]LnN(SiMe3)2 (Ln ¼ Y (4), Yb
(5) and the C1-symmetric mono-ligated amidate complex ( NMe2: O: N-2)Y[N(SiMe3)2]2 (6), respec-
tively, in good yields. These organolanthanide amidate complexes have been characterized by various
spectroscopic techniques, elemental analyses, and X-ray diffraction analyses. They are active catalysts for
asymmetric hydroamination/cyclization of aminoalkenes and ring-opening polymerization of rac-lactide,
affording cyclic amines in excellent conversions with good ee values and isotactic-rich polylactides,
respectively.
sOMe:kO:kN-1)2SmN
Keywords:
Chiral amidate ligands
Organolanthanide complexes
Asymmetric hydroamination/cyclization
Polymerization of lactide
k
k
s
k
k
s
k
k
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
Ligand modification plays a key role in developing new catalyst
precursors for asymmetric synthesis. In recent years, we have
Chiral organolanthanide complexes based on non-Cp multi-
dentate ligands have received growing attention in the past two
decades [1e13]. One of the initial driving forces for this work is the
longstanding interest in the development of catalysts for intra-
molecular asymmetric alkene hydroamination [6e13], since the
hydroamination is a highly atom-economical process in which an
amine NeH bond is added to an unsaturated carbonecarbon bond
leading to the formation of nitrogen-containing heterocycles that
are prevalent in naturally occurring and/or biologically and phar-
macologically active compounds [14e24]. Up to date, many non-Cp
chiral organolanthanide catalysts for asymmetric alkene hydro-
amination have been studied [25e50], however, only a small
number of successful catalysts have been reported affording signif-
icant enantioselectivity (>90% ee) [36], and those highly enantio-
selective inductions are observed only for one or two substrates.
Thus, the development of new lanthanide catalysts for asymmetric
alkene hydroamination is a desirable and challenging goal.
developed a series of chiral non-Cp multidentate ligands, and their
Ag(I), Cu(II), Rh(I), Ta(V), Ti(IV), Zn(II), Zr(IV) and lanthanide metal
complexes are useful catalysts for a wide range of transformations
[51e73]. Among these, group 4 metal complexes with chiral biaryl-
based amidate ligands are useful chiral catalysts for the hydro-
amination/cyclization [66,69,70], which is also observed by Schafer
and others [74e77]. For example, the bis-ligated zirconium
complexes (1)2Zr(NMe2)2 and (2)2Zr(NMe2)2, derived from amidate
ligands (R)-2-(mesitoylamino)-20-methoxy-6,60-dimethyl-1,10-bip-
henyl (1H) and (R)-2-(mesitoylamino)-20-dimethylamino-6,60-
dimethyl-1,10-biphenyl (2H), are effective chiral catalysts for
hydroamination/cyclization of aminoalkenes, in which excellent
conversions (up to 100%) and moderate enantioselectivities (up to
57% ee) have been obtained [66,70]. In our ongoing research, we are
now focusing on the preparation of this type of bis-ligated catalysts
coordinated by chiral C1-symmetric tridentate ligands, and to
further explore the coordination chemistry of the chiral amidate
ligands 1H and 2H, we have recently extended our work to
lanthanide chemistry. Herein, we report on some observations
concerning the chemistry of amidate ligands 1H and 2H with
* Corresponding author. Tel.: þ86 10 5880 6051; fax: þ86 10 5880 2075.
E-mail address: gzi@bnu.edu.cn (G. Zi).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.11.032

Q. Wang et al. / Journal of Organometallic Chemistry 696 (2011) 2186e2192
2187
lanthanide amides, and the applications of the resulting complexes
as catalysts for the asymmetric hydroamination/cyclization of
aminoalkenes and the polymerization of rac-lactide.
n 2961 (m),1612 (s),1577 (s),1498 (s),1414 (s),1259 (s),1086 (s),1016
(s), 958 (s), 818 (s), 796 (s). Anal. Calcd forC56H70N3O4Si2Y: C, 67.65;H,
7.10; N, 4.23. Found: C, 67.75; H, 7.15; N, 4.21%.
2. Experimental section
2.4. Preparation of complex [(kO:kN-1)(sOMe:kO:kN-1)]YbN
(SiMe3)2 (5)
2.1. General methods
This compound was prepared as colorless crystals from the
reaction of 1H (0.37 g, 1.0 mmol) with Yb[N(SiMe3)2]3 (0.33 g,
0.5 mmol) in toluene (20 mL) and recrystallization from a toluene
solution by a similar procedure as in the synthesis of 3. Yield: 0.39 g
All experiments were performed under an atmosphere of dry
dinitrogen with rigid exclusion of air and moisture using standard
Schlenk or cannula techniques, or in a glovebox. All organic solvents
were freshly distilled from sodium benzophenone ketyl immedi-
ately prior to use. Racemic lactide was recrystallized twice from dry
toluene and then sublimed under vacuum prior to use. (R)-2-
(mesitoylamino)-20-methoxy-6,60-dimethyl-1,10-biphenyl (1H)[70],
(R)-2-(mesitoylamino)-20-dimethylamino-6,60-dimethyl-1,10-biph-
enyl (2H) [66], Ln[N(SiMe3)2]3 [78], 2,2-dimethylpent-4-enylamine
[31], 2,2-dimethylhex-5-enylamine [31], and 1-(aminomethyl)-1-
allylcyclohexane [42] were prepared according to literature
methods. All chemicals were purchased from Aldrich Chemical Co.
and Beijing Chemical Co. used as received unless otherwise noted.
Infrared spectra were obtained from KBr pellets on an Avatar 360
Fourier transform spectrometer. Molecular weights of the polymer
were estimated by gel permeation chromatography (GPC) using
a PL-GPC 50 apparatus. 1H and 13C NMR spectra were recorded on
a Bruker AV 500 spectrometer at 500 and 125 MHz, respectively. All
(73%). M.p.: 237e239 ꢀC (dec.). IR (KBr, cmꢁ1):
n 2962 (m), 1608 (s),
1575 (s), 1443 (s), 1259 (s), 1085 (s), 1017 (s), 950 (s), 796 (s). Anal.
Calcd for C56H70N3O4Si2Yb: C, 62.37; H, 6.54; N, 3.90. Found: C,
62.42; H, 6.52; N, 3.94%.
2.5. Preparation of complex (sNMe2:kO:kN-2)Y[N(SiMe3)2]2 (6)
This compound was prepared as colorless crystals from the
reaction of 2H (0.38 g, 1.0 mmol) with Y[N(SiMe3)2]3 (0.28 g,
0.5 mmol) in toluene (20 mL) and recrystallization from a toluene
solution by a similar procedure as in the synthesis of 3. Yield: 0.32 g
(81%). M.p.: 132e134 ꢀC (dec.). 1H NMR (C6D6):
d 6.74 (s, 2H, aryl),
6.68 (m, 2H, aryl), 6.59 (s, 2H, aryl), 6.57 (s, 1H, aryl), 6.45 (s, 1H,
aryl), 2.68 (s, 3H, CH3), 2.61 (s, 6H, CH3), 2.01 (s, 3H, CH3), 1.89 (s, 3H,
CH3), 1.84 (s, 3H, CH3), 1.67 (s, 3H, CH3), 0.41 (s, 18H, Si(CH3)3), 0.28
chemical shifts are reported in
d
units with reference to the residual
(s, 18H, Si(CH3)3). 13C NMR (C6D6):
d
181.6, 146.9, 142.4, 139.9, 138.4,
137.2, 136.5, 134.2, 133.2, 133.0, 132.9, 132.4, 125.9, 121.5, 118.6, 47.1,
27.2, 25.8, 21.0, 20.8, 6.4, 5.6. IR (KBr, cmꢁ1):
2962 (m), 2859 (w),
protons of the deuterated solvents for proton and carbon chemical
shifts. Melting points were measured on an X-6 melting point
apparatus and were uncorrected. Elemental analyses were per-
formed on a Vario EL elemental analyzer.
n
1613 (m),1570 (m),1495 (s),1427 (s),1260 (s),1238 (s),1096 (s), 938
(s), 812 (s). Anal. Calcd for C38H65N4OSi4Y: C, 57.40; H, 8.24; N, 7.05.
Found: C, 57.53; H, 8.09; N, 6.83%.
2.2. Preparation of complex (sOMe:kO:kN-1)2SmN(SiMe3)2 (3)
2.6. General procedure for asymmetric hydroamination/cyclization
A toluene solution (10 mL) of 1H (0.37 g, 1.0 mmol) was slowly
added to a toluene solution (10 mL) of Sm[N(SiMe3)2]3 (0.32 g,
0.5 mmol) with stirring at room temperature. The resulting solu-
tion was refluxed overnight to give a light yellow solution. The
solution was filtered, and the filtrate was concentrated to about
2 mL. Complex 3 was isolated as colorless crystals after this solution
stood at room temperature for three days. Yield: 0.40 g (75%). M.p.:
In a nitrogen-filled glove box, precatalyst (0.016 mmol), C6D6
(0.7 mL), and aminoalkene (0.32 mmol) were introduced sequentially
into a J. Young NMR tube equipped with a Teflon screw cap. The
reaction mixture was subsequently kept at room temperature, at
60 ꢀC or 120 ꢀC to achieve hydroamination, and the reaction was
monitored periodically by 1H NMR spectroscopy. The cyclic amine
was vacuum transferred from the J. Young NMR tube into a 25 mL
Schlenk flask which contained 62 mg (0.32 mmol) of (S)-(þ)-O-ace-
tylmandelic acid. The resulting mixture was stirred at room temper-
ature for 2 h and the volatiles were removed in vacuo. The resulting
diastereomeric salt was then dissolved in CDCl3 and the enantiomeric
excesses were determined by 1H NMR spectroscopy [31].
140e142 ꢀC (dec.). 1H NMR (C6D6):
d
9.41 (d, J ¼ 6.8 Hz, 2H, aryl),
8.40 (d, J ¼ 6.8 Hz, 2H, aryl), 6.75 (m, 8H, aryl), 5.80 (s, 4H, aryl), 5.11
(s, 6H, OCH3), 1.97 (s, 6H, CH3), 1.78 (s, 6H, CH3), 1.42 (s, 6H, CH3),
0.31 (s, 12H, CH3), ꢁ0.69 (s, 18H, Si(CH3)3). 13C NMR (C6D6):
d 183.4,
153.9, 141.7, 137.6, 137.2, 137.1, 136.8, 136.2, 132.0, 131.3, 129.6, 129.1,
126.0, 124.7, 122.1, 67.9, 31.7, 27.0, 22.8, 16.6, 3.2. IR (KBr, cmꢁ1):
n
2960 (m), 1612 (s), 1577 (s), 1502 (s), 1462 (m), 1372 (s), 1259 (s),
1091 (s), 1017 (s), 972 (s), 796 (s). Anal. Calcd for C56H70N3O4Si2Sm:
C, 63.71; H, 6.68; N, 3.98. Found: C, 63.88; H, 6.72; N, 4.02%.
2.7. General procedure for polymerization of rac-lactide
Under nitrogen gas, a Schlenk flask was charged with a solution
of the complex (typically 0.005 mmol) in toluene (0.2 mL) or THF
(0.2 mL). To this solution was added rapidly a toluene or THF
solution (5.0 mL) of rac-lactide (5.0 mmol), and the reaction
mixture was vigorously stirred for 1 h at room temperature. The
polymerization was quenched by the addition of acidified meth-
anol. The resulting precipitated polylactide was collected, washed
with methanol several times, and dried in vacuum at 50 ꢀC over-
night. The microstructure of polymers was determined by homo-
decoupled 1H NMR experiments [79e81].
2.3. Preparation of complex [(kO:kN-1)(sOMe:kO:kN-1)]YN
(SiMe3)2 (4)
This compound was prepared as colorless crystals from the reac-
tion of 1H (0.37 g,1.0 mmol) with Y[N(SiMe3)2]3 (0.28 g, 0.5 mmol) in
toluene (20 mL) and recrystallization from a toluene solution by
a similar procedure as in the synthesis of 3. Yield: 0.41 g (83%). M.p.:
210e212 ꢀC (dec.). 1H NMR (C6D6):
d
7.74 (d, J ¼ 7.7 Hz, 2H, aryl), 7.29
(d, J ¼ 7.7 Hz, 2H, aryl), 7.02 (m, 2H, aryl), 6.87 (m, 2H, aryl), 6.81 (m,
2H, aryl), 6.72 (m, 4H, aryl), 6.49 (m, 2H, aryl), 3.80 (s, 3H, OCH3), 3.56
(s, 3H, OCH3), 2.22 (s, 6H, CH3), 2.04 (s,12H, CH3),1.98 (s, 6H, CH3),1.91
2.8. X-ray crystallography
(s, 6H, CH3), 0.44 (s, 18H, Si(CH3)3). 13C NMR (C6D6):
d 180.4, 156.9,
144.1, 138.6, 137.6, 137.4, 136.2, 133.8, 133.4, 132.5, 130.6, 128.8, 127.2,
Single-crystal X-ray diffraction measurements were carried out
on a Rigaku Saturn CCD diffractometer at 113(2) K using graphite
124.8,122.3,122.1, 58.2, 55.8, 22.3, 20.8,19.8,19.6, 4.8. IR (KBr, cmꢁ1):

2188
Q. Wang et al. / Journal of Organometallic Chemistry 696 (2011) 2186e2192
Table 2
monochromated Mo K
a
radiation (
l
¼ 0.71070 Å). An empirical
Selected bond distances (Å) and bond angles (deg) for compounds 3-6.
absorption correction was applied using the SADABS program [82].
All structures were solved by direct methods and refined by full-
matrix least squares on F2 using the SHELXL-97 program package
[83]. All the hydrogen atoms were geometrically fixed using the
riding model. The crystal data and experimental data for 3e6 are
summarized in Table 1. Selected bond lengths and angles are listed
in Table 2.
Compound
3 (Sm)
4 (Y)
5 (Yb)
6 (Y)
LnN (av.)
LnN(SiMe3)2
2.448(8)
2.312(6)
2.388(2)
2.240(2)
2.358(3)
2.195(2)
2.380(2)
2.244(2)
2.259(2)
LneO (av.)
LneO(1)
LneO(3)
LneC
2.515(6)
2.668(6)
2.701(6)
2.845(9)
2.867(8)
63.6(3)
2.343(2)
2.513(2)
2.313(2)
2.482(2)
2.279(2)
2.725(3)
69.2(2)
2.749(3)
2.778(3)
83.6(3)
63.6(3)
2.729(3)
2.752(3)
83.5(3)
64.0(3)
3. Results and discussion
Torsion (arylearyl)
61.0(3)
3.1. Synthesis and characterization of complexes
It has been shown that organolanthanide amidate complexes
can be efficiently prepared via silylamine elimination reaction
between Ln[N(SiMe3)2]3 and protic amidate ligands [73,84,85].
Therefore it is rational to propose that the acidic proton in the chiral
amidate ligands 1H and 2H would allow the similar silylamine
elimination to occur between 1H or 2H and metal amides. In fact,
treatment of (R)-2-(mesitoylamino)-20-methoxy-6,60-dimethyl-
1,10-biphenyl (1H) with half equiv of Sm[N(SiMe3)2]3 in toluene
crystal X-ray diffraction analyses. The 1H NMR spectra of 3 and 4
supports the ratio of amino group N(SiMe3)2 and ligand 1 is 1:2, and
the 1H NMR spectrum of 3 shows it is symmetrical on the NMR time
scales, consistent with its C2-symmetric structure; while the 1H
NMR spectrum of 4 shows it is nonsymmetrical on the NMR time
scales due to its C1-symmetric structure. The 1H NMR spectrum of 6
supports the ratio of amino group N(SiMe3)2 and ligand 2 is 2:1, and
shows it is nonsymmetrical on the NMR time scales, consistent
with its C1-symmetric structure. Their 13C NMR spectra are
consistent with the conclusions.
gives the C2-symmetric bis-ligated amidate complex (sOMe:kO:kN-
1)2SmN(SiMe3)2 (3) in 75% yield (Scheme 1), while reaction of 1H
with half equiv of Ln[N(SiMe3)2]3 (Ln ¼ Y, Yb) does not lead to the
The solid-state structure of 3 shows that the Sm3þ is
s-bound to
expected C2-symmetric complexes (
s
OMe:kO:kN-1)2LnN(SiMe3)2,
two nitrogen atoms and four oxygen atoms from two ligands 1 and
one nitrogen atom from the amido N(SiMe3)2 group in a doubly
capped trigonal-bipyramidal geometry (Fig. 1) in which O(1) and O
(3) occupy apical positions and N(1) and N(2) are capping two faces
of trigonal-bipyramidal surface with the average distance of SmeN
(2.448(8) Å), and the average distance of SmeO (2.515(6) Å),
respectively. The distance of SmeN(SiMe3)2 is 2.312(6) Å, which is
close to that found in [(S)-2-Me2NeC20H12-20-(NCHC4H3N)]2SmN
(SiMe3)2 (2.258(2) Å) [56]. The twisting between the phenyl rings of
torsion angles are 63.6(3) and 61.0(3)ꢀ, which are smaller than
those found in (1)2Ti(NMe2)2 (73.3(1) and 69.3(1)ꢀ) and (1)2Zr
(NMe2)2 (69.7(2) and 71.3(2)ꢀ) [70].
instead, the C1-symmetric bis-ligated amidate complexes [(
1)( OMe: O:
k
O: N-
k
s
k
k
N-1)]LnN(SiMe3)2 (Ln ¼ Y (4), Yb (5)) have been
isolated in good yields (Scheme 1), presumably due to the size
effect of the lanthanide ions [86]. However, under similar reaction
conditions, treatment of 2H with half equiv of Y[N(SiMe3)2]3
(Ln ¼ Y, Yb) does not give the expected bis-ligated amidate complex
(sNMe2:kO:kN-2)2YN(SiMe3)2 or [(kO:kN-2)(sNMe2:kO:kN-2)]YN
(SiMe3)2, instead, the C1-symmetric mono-ligated amidate complex
NMe2: O: N-2)Y[N(SiMe3)2]2 (6) has been isolated in 81% yield
(s
k
k
(Scheme 2), presumably due to the steric effect of the ligand.
These complexes are stable in a dry nitrogen atmosphere, while
they are very sensitive to moisture. They are soluble in organic
solvents such as THF, DME, pyridine, toluene, and benzene, and
only slightly soluble in n-hexane. They have been characterized by
various spectroscopic techniques, elemental analyses, and single-
The single-crystal X-ray diffraction analyses confirm that 4 and 5
are isostructural. In each molecule [(kO:kN-1)(sOMe:kO:kN-1)]LnN
(SiMe3)2, the Ln3þ is
s-bound to two nitrogen atoms and three
Table 1
Crystal data and experimental parameters for compounds 3e6.
Compound
3
4
5
6
Formula
C56H70N3O4Si2Sm
1055.68
Monoclinic
P1211
9.388(1)
23.244(3)
12.073(2)
96.87(1)
2615.5(5)
2
C56H70N3O4Si2Y
994.24
Orthorhombic
P212121
9.404(2)
23.396(4)
23.539(4)
90
5178.9(16)
4
C56H70N3O4Si2Yb
1078.37
Orthorhombic
P212121
9.421(2)
23.402(5)
23.412(4)
90
5161.5(17)
4
C38H65N4OSi4Y
795.21
Orthorhombic
P212121
9.101(2)
18.548(4)
26.265(5)
90
4433.9(16)
4
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
b
(deg)
V (Å3)
Z
Dcalc (g/cm3)
1.340
1.216
1.275
1.220
1.388
1.906
1.191
1.455
m(Mo/K
a
)
calc (mmꢁ1
)
Size (mm)
F(000)
0.20 ꢂ 0.20 ꢂ 0.20
0.32 ꢂ 0.30 ꢂ 0.28
0.24 ꢂ 0.22 ꢂ 0.18
0.42 ꢂ 0.18 ꢂ 0.16
1098
2104
2228
1696
2
q
range (deg)
3.40 to 55.88
18,674
2.46 to 55.76
49,892
3.48 to 55.78
44,920
2.68 to 55.72
30,356
No. of reflns, collected
No. of unique reflns
No. of obsd reflns
10,561 (Rint ¼ 0.0368)
12,344 (Rint ¼ 0.0690)
12,286 (Rint ¼ 0.0427)
10,384 (Rint ¼ 0.0443)
9788
10,287
11,568
8314
Abscorr (Tmax, Tmin
R
)
0.79, 0.79
0.074
0.73, 0.70
0.052
0.73, 0.66
0.030
0.80, 0.58
0.037
Rw
Rall
gof
0.193
0.077
1.05
0.087
0.065
0.99
0.060
0.032
0.99
0.073
0.048
0.85

Q. Wang et al. / Journal of Organometallic Chemistry 696 (2011) 2186e2192
2189
O
O
N
O
N
O
+
2HN(SiMe3)2
Sm
N
Me3Si
SiMe3
3 (75% yield)
C
120
12h
C7H8
O
+
Ln[N(SiMe3)2]3
2
N
H
OMe
1H
C7H8
Fig. 1. Molecular structure of 3 (thermal ellipsoids drawn at the 35% probability level).
C
120
12h
MeO group, attributable to a binding (sOMe) and a free MeO group,
respectively. The average distance of LneN is 2.388(2) Å for Y, and
2.358(3) Å for Yb, respectively, and the average distance of LneO is
2.343(2) Å for Y, and 2.313(3) Å for Yb, respectively. The distances of
LneN(SiMe3)2 2.240(2) Å for Y, and 2.195(2) Å) for Yb, are compa-
rable to the corresponding values of 2.208(3) Å for Y, and 2.169(3) Å
for Yb found in [(S)-2-Me2NeC20H12-20-(NCHC4H3N)]2LnN(SiMe3)2
[56]. The twisting between the phenyl rings of torsion angles are
73.4(1) and 74.0(1)ꢀ for Y, and 74.4(2) and 74.7(2)ꢀ for Yb, which are
comparable to those found in 3 (63.6(3) and 61.0(3)ꢀ), (1)2Ti
(NMe2)2 (73.3(1) and 69.3(1)ꢀ) and (1)2Zr(NMe2)2 (69.7(2) and 71.3
(2)ꢀ) [70].
O
O
N
O
N
O
+
2HN(SiMe3)2
Ln
N
Me3Si
SiMe3
Ln = Y (4; 83% yield), Yb (5; 73% yield)
Scheme 1. Synthesis of complexes 3e5.
The solid-state structure of 6 shows that the Y3þ is
s-bound to
two nitrogen atoms and one oxygen atom from ligand 1 and two
nitrogen atoms from two amido N(SiMe3)2 groups in a distorted-
trigonal-bipyramidal geometry (Fig. 4) with the average distance of
YeN 2.380(2) Å, and the distance of YeO 2.279(2) Å, respectively.
The distances of YeN(SiMe3)2 are 2.244(2) and 2.259(2) Å, which
are close to those found in {2-(Me2N)-20-[2,4,6-Me3C6H2)C(O)N]-
oxygen atoms (one oxygen atom from MeO group) from two
ligands 1 and one nitrogen atom from amino group N(SiMe3)2 in
a distorted-pentagonal-pyramidal geometry (Figs. 2 and 3) in
which N(3) occupies apical position, the other MeO group is far
away from the metal center. The solution NMR characterization
data is also consistent with this C1-symmetric binding mode, as the
1H NMR spectrum of 4 shows two signals at 3.80 and 3.56 ppm for
O
N
H
+
Y[N(SiMe3)2]3
NMe2
2H
C
120
12h
C7H8
SiMe3
SiMe3
SiMe3
O
N
N
+
HN(SiMe3)2
Y
N
N
Me2
SiMe3
6 (81% yield)
Scheme 2. Synthesis of complex 6.
Fig. 2. Molecular structure of 4 (thermal ellipsoids drawn at the 35% probability level).

2190
Q. Wang et al. / Journal of Organometallic Chemistry 696 (2011) 2186e2192
Table 3
Enantioselective hydroamination/cyclization of aminoalkenes.a
Entry Precat. Substrate
(M)
Product
Temp. Time Conv. Ee
(ꢀC)
(h)
(%)b
(%)c
1
2
3
4
5
3 (Sm)
3 (Sm)
4 (Y)
5 (Yb)
6 (Y)
60
20
60
60
60
16
48
16
16
16
95
27
97
90
100
66
74
54
50
13
H
N
NH2
7a
7b
6
7
8
9
3 (Sm)
4 (Y)
5 (Yb)
6 (Y)
60
60
60
60
16
16
16
16
98
95
92
60
48
45
12
NH2
NH
100
8b
8a
10
11
12
13
3 (Sm)
4 (Y)
5 (Yb)
6 (Y)
120
120
120
120
24
24
24
24
92
95
90
38
34
28
24
NH2
NH
100
9a
9b
a
Conditions: C6D6 (0.70 mL), aminoalkene (0.32 mmol), catalyst (0.016 mmol).
Determined by 1H NMR based on p-xylene as the internal standard.
b
c
Fig. 3. Molecular structure of 5 (thermal ellipsoids drawn at the 35% probability level).
Determinedby1H NMRofitsdiastereomeric (S)-(þ)-O-acetylmandelic acid salt [31].
1,10-C20H12}Y[N(SiMe3)2]2 (2.265(5) and 2.244(5) Å) [73]. The
twisting between the phenyl rings of torsion angle is 69.2(2)ꢀ,
which is comparable to those found in (2)2Ti(NMe2)2 (70.0(2) and
69.7(2)ꢀ) and (2)2Zr(NMe2)2 (71.1(6) and 68.6(6)ꢀ) [66].
value decreases largely (Table 3, entries 3 and 4). When the mono-
ligated complex 6 is used, the rate increases slightly but the ee
value decreases significantly (Table 3, entry 5), presumably due to
the steric effects. The organolanthanide amidate complexes 3e6
are also effective catalysts for the substrate 8a, giving spiro-pyr-
rolidine products in excellent conversions with moderate enan-
tioselectivity (Table 3, entries 6e9). The data also show that the
formation of six-membered rings can also be performed with our
catalysts at 120 ꢀC (Table 3, entries 10e13), and a moderate enan-
tioselectivity (up to 38%), mediated by the catalyst 3, has been
obtained (Table 3, entry 10). Like those C1-symmetric bis-ligated
complexes [(S)-2-MeO-C20H20-20-(NCHC4H3N)]2LnN(SiMe3)2 (Ln ¼
Y, Yb) [64], complexes 4 and 5 are more chiral effective catalysts for
the hydroamination/cyclization reaction than the C1-symmetric
mono-ligated complex 6, but less than those initiated by C2-
symmetric bis-ligated complexes 3 and [(S)-2-Me2N-C20H12-20-
3.2. Catalytic activity
To evaluate the catalytic ability of the complexes 3e6, the
asymmetric hydroamination/cyclization of aminoalkenes and
polymerization of rac-lactide have been tested under the condi-
tions given in Tables 3 and 4, respectively.
The results (Table 3) clearly show that all substrates are con-
verted to the cyclic product at 60 ꢀC or elevated temperature in
good conversions. The samarium complex 3 is a noticeably good
catalyst at 60 ꢀC, giving excellent conversion and good enantiose-
lectivity (up to 66%; Table 3, entry 1). On decreasing the tempera-
ture from 60 ꢀC to 20 ꢀC, the ee value increases slightly but the rate
falls significantly (Table 3, entry 2). When the complexes 4 (Y3þ
)
and 5 (Yb3þ) are used, the rate does not change much but the ee
Table 4
Polymerization of rac-lactide catalyzed by chiral organolanthanide amides 3e6.a
O
O
O
O
O
O
complex
O
O
O
m
O
+
O n
O
O
O
O
O
rac-Lactide
Isotactic Polylactide
b
Entry Precat. (M) Solvent
Conv. (%) Mnb (kg/mol) Mw/Mn
Pmc (%)
1
2
3
4
5
6
7
8
3 (Sm)
4 (Y)
5 (Yb)
6 (Y)
3 (Sm)
4 (Y)
5 (Yb)
6 (Y)
Toluene
Toluene
Toluene
93
94
84
72.9
66.2
59.3
74.2
71.8
54.1
58.7
73.8
1.24
1.28
1.26
1.31
1.36
1.48
1.32
1.35
72
68
67
58
70
62
64
54
Toluene 100
THF
THF
THF
THF
94
78
85
93
a
Conditions: 20 ꢀC, precat./LA (mol/mol) ¼ 1/1000; polymerization time, 1 h;
solvent, 5 mL; [LA] ¼ 1.0 mol/L.
b
Measured by GPC (using polystyrene standards in THF).
Pm is the probability of meso linkages between monomer units and is deter-
c
mined from the methine region of the homonuclear decouped 1H NMR spectrum in
CDCl3 at 25 ꢀC [79e81].
Fig. 4. Molecular structure of 6 (thermal ellipsoids drawn at the 35% probability level).

Q. Wang et al. / Journal of Organometallic Chemistry 696 (2011) 2186e2192
2191
(NCHC4H3N)]2LnN(SiMe3)2 (Ln ¼ Sm, Y, Yb) [56], due to the coor-
dination effect of the oxygen atom (from MeO group) [64].
The complexes 3e6 are also active catalysts for the ring-opening
polymerization (ROP) of racemic-lactide under mild conditions
(Table 4). Yttrium complex 4 allows the 94% complete conversion of
1000 equiv of lactide within 1 h at room temperature in toluene at
[rac-LA] ¼ 1.0 mol Lꢁ1 (Table 4, entry 2). Polymerizations with
this yttrium initiator/catalyst proceed much more slowly in THF
(Table 4, entry 6), presumably due to the competitive coordination
between the monomer and this donor solvent, as often observed in
this type of ROP reactions promoted by oxophilic meta-based
systems [44,87]. This difference in activity between toluene and
THF solvent is not observed with complexes 3 and 5 (Table 4,
entries 1, 3, 5 and 7); however, the ytterbium complex 5 is less
active than yttrium complex 4 in toluene medium (Table 4, entries 2
and 3). The reasons for the different solvent dependence between
the yttrium complex and ytterbium or samarium complex, are not
clear at this time, however, the nature of the rare earth metal ions
and lanthanide metal ions seems to be a major reason for this
difference. When the mono-ligated complex 6 is used, again, the
difference in activity between toluene and THF solvent is observed,
while the rate increases slightly (Table 4, entries 4 and 8).
The resulting polylactides are all isotactic rich under the cond-
itions examined. Molecular weights and polydispersities of the
polymers produced ranged from 54.1 to 74.2 kg molꢁ1 and 1.24 to
1.48, respectively. The catalytic activities of 3e6 resemble that of
[2-(2,6-iPr2C6H3N]CH)C4H3N]2Y(CH2SiMe3)(THF)2 [88], while the
microstructure of the resulting polylactides are similar to those
initiated by [(S)-2-MeO-C20H12-20-(NCHC4H3N)]2LnN(SiMe3)2}2
[64]. The enantioselectivity of the complexes in the rac-lactide
polymerization process is not clear at this time, however, the
coordination environment around the metal center seems to be
a major factor for this selectivity. Further computational investi-
gation of the ligand architecture for this transformation is still
ongoing.
of the catalyst architecture to improve the enantioselectivity or ster-
eoselectivity for hydroamination/cyclization or rac-lactide polymer-
ization, and on the exploration of these catalysts towards other types
of reactions.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (20972018, 21074013), the Fundamental
Research Funds for the Central Universities, and Beijing Municipal
Commission of Education.
Supplementary material
CCDC 791534, 791535, 791536, and 791537 contain the
supplementary crystallographic data for 3, 4, 5 and 6, respectively.
These data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif.
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