Synthesis, structure, and reactivity of yttrium complexes with chiral biaryldiamine-based N4-ligands

Article abstract of DOI:10.1016/j.inoche.2010.01.008

The chiral biaryl-based N₄-ligands, (R)-5,5′,6,6′,7,7′,8,8′-octahydro-2,2′-bis(pyrrol-2-ylmethyleneamino)-1,1′-binaphthyl (1H₂) and (S)-2,2′-bis(pyrrol-2-ylmethyleneamino)-6,6′-dimethyl-1,1′-biphenyl (2H₂), can effectively

Full text of DOI:10.1016/j.inoche.2010.01.008

Inorganic Chemistry Communications 13 (2010) 445–448
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, structure, and reactivity of yttrium complexes with chiral
biaryldiamine-based N4-ligands
a
a
a
b
Guofu Zi a, , Li Xiang , Xue Liu , Qiuwen Wang , Haibin Song
*
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
The chiral biaryl-based N4-ligands, (R)-5,50,6,60,7,70,8,80-octahydro-2,20-bis(pyrrol-2-ylmethyleneamino)-
1,10-binaphthyl (1H2) and (S)-2,20-bis(pyrrol-2-ylmethyleneamino)-6,60-dimethyl-1,10-biphenyl (2H2),
can effectively stabilize the chiral rare earth metal chloride complexes such as 1-YCl(dme) (3) and 2-
YCl(dme) (4), which offers important intermediates for the preparation of chiral rare earth catalysts con-
taining the M–C or M–X (X = heteroatom) bonds. For example, treatment of 3 with half equiv of 1Na2 in
THF gives the binuclear complex 1-Y(thf)-1-Y(thf)-1 (5) in 70% yield. These complexes have been charac-
terized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analyses. The com-
plex 5 is an active catalyst for the ring-opening polymerization of rac-lactide, affording isotactic-rich
polylactides.
Article history:
Received 17 November 2009
Accepted 14 January 2010
Available online 20 January 2010
Keywords:
Chiral N4-ligands
Yttrium complexes
Synthesis
Structure
Reactivity
Ó 2010 Elsevier B.V. All rights reserved.
Chiral rare earth metal complexes based on non-Cp ligands
have received growing attention in the past decades [1–6], because
the lability of rare earth metal element-ligand bonds and the flex-
ibility of their coordination geometries make these elements
highly suitable for use in catalysis. However, the rare earth metal
complexes supported by non-Cp ligands usually encounter salt
addition, dimerization or ligand redistribution, and these factors
also make it difficult to generate well-defined chiral architectures
that will lead to efficient enantioselective reactions [1]. Thus, chiral
multidentate ligands have attracted considerable interest, which
can provide proper sterics and electronics for the rare earth metal
center to prevent the side-reactions mentioned above via varying
the size and electron affinity of the substituents. Although many
rare earth catalysts based on non-Cp ligands have been reported
[1–6], the development of new chiral rare earth catalysts is a desir-
able and challenging goal. In recent years, we have developed a
series of rare earth complexes based on chiral non-Cp multidentate
ligands, and they have shown that they are useful catalysts for a
range of transformations [7–12]. In our attempt to further explore
the chiral non-Cp ligand system and their application in rare earth
chemistry, we have recently extended our research work to chiral
tetradentate N4-ligands, (R)-5,50,6,60,7,70,8,80-octahydro-2,20-bis
(pyrrol-2-ylmethyleneamino)-1,10-binaphthyl (1H2) and (S)-2,20-
bis(pyrrol-2-ylmethyleneamino)-6,60-dimethyl-1,10-biphenyl (2H2)
[13,14], and found they are useful to stabilize the rare earth metal
chloride complexes, which are important intermediates for the
preparation of chiral rare earth catalysts containing the M–C or
M–X (X = heteroatom) bonds. We report herein on some observa-
tion concerning the ligands 1H2 and 2H2 use in rare earth
chemistry.
Deprotonation of the chiral ligands (R)-5,50,6,60,7,70,8,80-octa-
hydro-2,20-bis(pyrrol-2-ylmethyleneamino)-1,10-binaphthyl (1H2)
and (S)-2,20-bis(pyrrol-2-ylmethyleneamino)-6,60-dimethyl-1,10-
biphenyl (2H2) are achieved by reaction with an excess of
NaH in DME or THF. The resulting disodium salt 1Na2 or 2Na2
thus formed is reacted with 1 equiv of YCl3 in a mixed solution
of DME and toluene or a solution of THF, after recrystallization
from a benzene or DME solution, to give the chiral yttrium chlo-
ride complexes 1-YCl(dme)ꢀ0.5C7H8ꢀ0.5C6H6 (3ꢀ0.5C7H8ꢀ0.5C6H6)
and 2-YCl(dme)ꢀ0.375DMEꢀ0.125THF (4ꢀ0.375DMEꢀ0.125THF) in
good yields, respectively (Schemes 1 and 2). The rare earth me-
tal chloride complexes are important intermediates for the prep-
aration of rare earth catalysts containing the Ln–C or Ln–X
(X = heteroatom) bonds, i.e., the chlorine atom in these rare
earth metal chloride complexes can be replaced by other groups
via metathesis reactions. For example, treatment of 3 with half
equiv of 1Na2 in THF gives, after recrystallization from a toluene
solution, the binuclear complex 1-Y(thf)-1-Y(thf)-1ꢀC7H8 (5ꢀC7H8)
in 70% yield, which can also be prepared in 54% yield by salt
metathesis reaction between YCl3 with 1.5 equiv of 1Na2
(Scheme 1).
These complexes are stable in dry nitrogen atmosphere, while
they are very sensitive to moisture. They are soluble in organic sol-
vents such as THF, DME, pyridine, toluene, and benzene, but only
slightly soluble in n-hexane. They have been characterized by
* Corresponding author. Tel.: +86 10 5880 7843; fax: +86 10 5880 2075.
E-mail address: gzi@bnu.edu.cn (G. Zi).
1387-7003/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.01.008

446
G. Zi et al. / Inorganic Chemistry Communications 13 (2010) 445–448
N
N
N
1) xs NaH
2) YCl3
N
H
dme
Y
Cl
N
H
DME
N
N
N
1
H2
3
1) xs NaH
1Na2
2) 0.67 YCl3
C7H8/THF
THF/C7H8
N
N
N
N
N
N
thf
Y
N
Y
thf
N
N
N
N
N
5
Scheme 1.
3 and 4 support the ratio of DME and ligand 1 or 2 is 1:1, and the 1H
NMR spectrum of 5 supports the ratio of THF, toluene and ligand 1
is 2:1:3. Their IR spectra exhibit a weak typical characteristic N@C
absorption at about 1620 cmꢁ1, and the typical characteristic N–H
absorption of the ligands 1H2 and 2H2 [13,14] in the region of
3200–3400 cmꢁ1 disappear.
N
N
H
1) xs NaH
2) YCl3
N
Y
N
N
dme
Cl
N
H
N
DME
N
The single-crystal X-ray diffraction analyses show that there are
two molecules 1-YCl(dme) and one toluene molecule and one ben-
zene molecule for 3, and eight molecules 2-YCl(dme) and three
DME molecules and one THF molecule for 4, in the lattice. In each
2
H2
4
Scheme 2.
molecule 1-YCl(dme) or 2-YCl(dme), the Y3+ is
r-bound to four
nitrogen atoms of the ligand 1 or 2 and two oxygen atoms from
DME and one chlorine atom in a distorted-pentagonal-bipyramidal
geometry (Figs. 1 and 2) with the average distance of Y–N
(2.422(4) Å) for 3 and (2.410(5) Å) for 4, and the average distance
various spectroscopic techniques, elemental analyses, and X-ray
diffraction analyses (Table 1). Their 1H and 13C NMR spectra indi-
cate that they are symmetric on the NMR time scale, which is con-
sistent with their C2-symmetric structures. The 1H NMR spectra of
Table 1
Crystal data and experimental parameters for compounds 3, 4, and 5.
Compound
3ꢀ0.5C7H8ꢀ0.5C6H6
4ꢀ0.375DMEꢀ0.125THF
5ꢀC7H8
Formula
C81H90N8Cl2O4Y2
1488.34
Orthorhombic
P212121
14.735(1)
15.047(1)
16.451(1)
90
90
90
3647.6(2)
4
1.355
C60H69.5N8Cl2O5.75Y2
1243.46
Monoclinic
P1211
14.462(1)
32.499(2)
14.467(1)
90
104.65(1)
90
6568.1(8)
4
C105H108N12O2Y2
1747.85
Triclinic
P1
11.084(3)
11.095(2)
19.865(5)
76.89(1)
85.72(1)
80.05(1)
2341.8(9)
1
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a
(deg)
b (deg)
c
(deg)
V (Å3)
Z
Dcalc (g/cm3)
1.257
1.890
1.239
1.289
l
(Mo/K
a
)
calc
(mmꢁ1
)
1.712
Size (mm)
F (000)
0.32 ꢂ 0.22 ꢂ 0.20
0.20 ꢂ 0.16 ꢂ 0.14
0.16 ꢂ 0.10 ꢂ 0.10
1552
2574
916
2h range (deg)
3.66–52.02
30177
7173 (Rint = 0.0864)
5534
3.16–54.28
58879
28381 (Rint = 0.0542)
21162
3.74–52.00
23427
15129 (Rint = 0.0448)
10460
No. of reflns, collected
No. of unique reflns
No. of obsd reflns
Abscorr (Tmax, Tmin
R
)
0.73, 0.61
0.054
0.78, 0.70
0.060
0.88, 0.82
0.058
Rw
0.103
0.137
0.141
wR2 (all data)
0.113
0.151
0.148
gof
1.01
1.01
0.93

G. Zi et al. / Inorganic Chemistry Communications 13 (2010) 445–448
447
structural data are close to those found in [(R)-C20H12(NCHC4H3
N)2]YCl(dme) [7]. The torsion angle between two aryl rings is
(73.7(1)°) for 3 and (69.9(6)°) for 4, which are comparable to that
(70.1(3)°) found in [(R)-C20H12(NCHC4H3N)2]YCl(dme) [7].
The single-crystal X-ray diffraction analysis shows that there
are two molecules 1-Y(thf)-1-Y(thf)-1 and two toluene molecules
of solvent in the lattice. Coordination of three ligands 1 around
two Y3+ ions results in the formation of the binuclear complex 1-
Y(thf)-1-Y(thf)-1, in which the two Y3+ ions are linked by one li-
gand anion 1 (Fig. 3a). Fig. 3b shows that each Y3+ ion is
r-bound
to six nitrogen atoms from two ligand anions 1 and one oxygen
atom from THF in a distorted-pentagonal-bipyramidal with the
average distance of Y–N (2.438(6) Å) for Y(1) and Y–N
(2.445(6) Å) for Y(2), and the distance of Y–O (2.339(4) Å) for
Y(1) and (2.355(4) Å) for Y(2). These structural data are close to
those found in 3 and 4 (Table 2). The torsion angles between two
naphthyl rings are 69.7(2)°, 72.8(2)° and 79.9(2)°, which are com-
parable to that found in 3 (73.7(1)°).
The polymerization data show that the complex 5 can initi-
ate the ring-opening polymerization (ROP) of racemic-lactide
under mild conditions (Table 3). It allows the complete conver-
sion of 1000 equiv of lactide within 3 h at room temperature in
toluene at [rac-LA] = 1.0 mol Lꢁ1 (Table 3, entry 1). However,
polymerizations with this yttrium initiator/catalyst proceed
much more slowly in THF (Table 3, entry 2), presumably due
to the competitive coordination between the monomer and this
donor solvent [15]. This difference in activity between toluene
and THF solvent is observed more clearly at a lower tempera-
ture (Table 3, entries 3 and 4) than at higher temperature (Ta-
Fig. 1. Molecular structure of 3 (thermal ellipsoids drawn at the 35% probability
level).
ble 3, entries
5 and 6). The resulting polylactides are all
isotactic rich under the conditions examined. Molecular weights
and polydispersities of the polymers produced ranged from 34.7
to 70.9 kg molꢁ1 and 1.21 to 1.32, respectively. Our results
show that the catalytic activities of 5 resembles that of [(S)-2-
MeO-C20H20-20-(NCHC4H3N)]2YN(SiMe3)2}2 [12]. Under similar
reaction conditions, no detectable polymerization activity is ob-
served for complexes 3 and 4, even stirred at room temperature
for one week.
In conclusion, the chiral biaryl-based N4-ligands 1H2 and 2H2
can effectively stabilize the rare earth metal chloride complexes,
which offers important intermediates for the preparation of chiral
rare earth catalysts containing the M–C or M–X (X = heteroatom)
bonds. For example, the chlorine atom in complex 3 can be re-
placed by sodium amide salt 1Na2 to give a binuclear complex 1-
Y(thf)-1-Y(thf)-1 (5). The complex 5 is an active catalyst for the
ring-opening polymerization of rac-lactide, affording isotactic-rich
polylactides.
Fig. 2. Molecular structure of 4 (thermal ellipsoids drawn at the 35% probability
level).
of Y–O(DME) (2.457(3) Å) for 3 and (2.420(4) Å) for 4, and the
distance of Y–Cl (2.577(2) Å) for 3 and (2.595(2) Å) for 4. These
Fig. 3a. Molecular structure of 5 (thermal ellipsoids drawn at the 35% probability level).

448
G. Zi et al. / Inorganic Chemistry Communications 13 (2010) 445–448
Fig. 3b. Core structure of 5 (thermal ellipsoids drawn at the 35% probability level).
Appendix A. Supplementary material
Table 2
Selected bond distances (Å) and bond angles (deg) for compounds 3, 4, and 5.
CCDC 754817, 754818 and 754819 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.inoche.2010.01.008.
Compound
Y–N (av)
3
4
5
2.422(4)
2.410(5)
Y(1): 2.438(6)
Y(2): 2.445(6)
Y(1): 2.339(4)
Y(2): 2.355(4)
Y–O
2.479(3)
2.434(3)
2.577(2)
73.7(1)
2.421(4)
2.419(4)
2.595(2)
69.9(6)
Y–Cl
Torsion (aryl–aryl)
69.7(2)
72.8(2)
79.9(2)
References
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Table 3
Polymerization of rac-lactide catalyzed by complex 5.a
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O
O
O
O
O
O
complex
O
O
O
m
O
+
O n
O
O
O
O
O
rac-Lactide
Isotactic Polylactide
b
b
c
Entry
T (°C)
Solvent
Conv. (%)
Mn (kg/mol)
Mw/Mn
Pm (%)
1
2
3
4
5
6
20
20
10
10
40
40
Toluene
THF
Toluene
THF
Toluene
THF
100
87
85
45
100
100
70.9
58.2
56.4
34.7
69.2
68.7
1.23
1.27
1.21
1.24
1.32
1.29
72
64
76
67
68
65
[14] M. Kettunen, C. Vedder, F. Schaper, M. Leskelä, I. Mutikainen, H.-H. Brintzinger,
Organometallics 23 (2004) 3800–3807.
[15] A. Amgoune, C.M. Thomas, T. Roisnel, J.-F. Carpentier, Chem.-Eur. J. 12 (2006)
169–179.
a
Conditions: precat./LA (mol/mol) = 1/1000; polymerization time, 3 h; solvent,
5 mL; [LA] = 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 decoupling 1H NMR spectrum
in CDCl3 at 25 °C.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (20972018), and Beijing Municipal Commission of
Education.