Synthesis, structure, and catalytic activity of organoyttrium complexes with chiral binaphthyl-based amidate ligands

Article abstract of DOI:10.1016/j.inoche.2010.09.034

Two new chiral organoyttrium amidate complexes 1-Y[N(SiMe₃) ₂]₂ (3) and 2-Y[N(SiMe₃)₂] ₂?C₆H₁₂ (4?C₆H ₁₂) have been readily prepared in good yields by silylamine elimination reaction between Y[N(SiMe₃)₂]₃ and chiral binaphthyl-based amidate ligands, (R)-2,2′-bis(mesitoylamino)-1, 1′-binaphthyl (1) and (S)-2-(mesitoylamino)-2′-(dimethylamino)-1, 1′-binaphthyl (2), respectively. Complexes 2 and 3 have been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analyses. Complexes 3 and 4 are active catalysts for the polymerization of rac-lactide, leading to the isotactic-rich polylactides.

Full text of DOI:10.1016/j.inoche.2010.09.034

Inorganic Chemistry Communications 14 (2011) 72–74
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, structure, and catalytic activity of organoyttrium complexes with chiral
binaphthyl-based amidate ligands
Furen Zhang a, Jiaxin 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:
Two new chiral organoyttrium amidate complexes 1-Y[N(SiMe3)2]2 (3) and 2-Y[N(SiMe3)2]2·C6H12
(4·C6H12) have been readily prepared in good yields by silylamine elimination reaction between Y[N
(SiMe3)2]3 and chiral binaphthyl-based amidate ligands, (R)-2,2′-bis(mesitoylamino)-1,1′-binaphthyl (1)
and (S)-2-(mesitoylamino)-2′-(dimethylamino)-1,1′-binaphthyl (2), respectively. Complexes 2 and 3 have
been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analyses.
Complexes 3 and 4 are active catalysts for the polymerization of rac-lactide, leading to the isotactic-rich
polylactides.
Received 7 September 2010
Accepted 27 September 2010
Available online 16 October 2010
Keywords:
Chiral amidate ligands
Yttrium complexes
Polymerization of lactide
© 2010 Elsevier B.V. All rights reserved.
Chiral rare earth metal complexes based on non-Cp ligands have
attracted increasing attention in the past decades [1–6]. This interest
is spurred by the lability of rare earth metal element-ligand bonds and
the flexibility of their coordination geometries. These features make
them highly suitable for use in catalysis. However, the rare earth
metal complexes supported by non-Cp ligands usually suffer from 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, the development
of new chiral rare earth catalysts is a desirable and challenging goal. In
recent years, we have developed a series of chiral non-Cp multi-
dentate ligands, and their early transition metal complexes are useful
catalysts for a wide range of transformations [7–15]. For example,
group 4 metal complexes with chiral binaphthylamidate ligands
(R)-2,2′-bis(mesitoylamino)-1,1′-binaphthyl (1) [12] and (S)-2-
(mesitoylamino)-2′-(dimethylamino)-1,1′-binaphthyl (2) [11] are
useful chiral catalysts for the hydroamination/cyclization, in which
good enantioselectivities (up to 86% ee) with excellent conversions
(up to 100%) have been obtained [16]. Encouraged by the attractive
features of the amidate ligand system, and to our knowledge, no chiral
rare earth metal amidate catalyst has been reported. We have recently
started exploring ligands 1 and 2 in rare earth chemistry.
occur between 1 or 2 and metal amides. In fact, treatment of 1 or 2
with 1 equiv of Y[N(SiMe3)2]3 in toluene gives, after recrystallization
from a toluene or cyclohexane solution, the amidate yttrium
complexes 1-Y[N(SiMe3)2]2 (3) and 2-Y[N(SiMe3)2]2·C6H12
(4·C6H12), respectively, in good yields (Schemes 1 and 2) [19].
Complexes 3 and 4 are stable in dry nitrogen atmosphere, while they
are very sensitive to moisture. Complexes 3 and 4 are soluble in
organic solvents such as THF, DME, pyridine, toluene, and benzene,
but only slightly soluble in n-hexane. Complexes 3 and 4 have been
characterized by various spectroscopic techniques, elemental analy-
ses, and X-ray diffraction analyses [19,20].
The solid-state structure of 3 shows that the Y3+ is σ-bound to one
nitrogen atom and two oxygen atoms from ligand 1 and two nitrogen
atoms from two amido N(SiMe3)2 groups in a distorted-trigonal-
bipyramidal geometry (Fig. 1) with the average distance of Y-N (2.330
(3) Å), and the average distance of Y-O (2.254(2) Å), respectively. The
potentially tetradentate amidate ligand adopts a tridentate binding
mode. The solution NMR characterization data are also consistent
with this C1 symmetric binding mode, as the 13C NMR spectra show
two signals at about 181.5 ppm and 172.0 ppm for the carbonyl group,
attributable to a bidentate (κO, κN) binding and a monodentate (κO)
binding, respectively [12]. The distances of Y-N(SiMe3)2 are 2.272(3)
and 2.286(3) Å, which are slightly longer than that found in [(S)-2-
Me2N-C20H12-2′-(NCHC4H3N)]2YN(SiMe3)2 (2.258(2) Å) [7]. The
twisting between two naphthyl rings of torsion angle is 85.9(1)°,
indicating that they are almost perpendicular to each other.
It has been documented that rare earth metal amidate complexes
can be efficiently prepared via silylamine elimination reaction
between Ln[N(SiMe3)2]3 and protic amidate ligands [17,18]. It is
rational to propose that the acidic protons in the chiral amidate
ligands 1 and 2 would allow the similar silylamine elimination to
The solid-state structure of 4·C6H12 shows there is one 4 molecule
and one solvated cyclohexane molecule in the lattice. In the molecule
4, the Y3+ is σ-bound to two nitrogen atoms and one oxygen atom
from ligand 2 and two nitrogen atoms from two amido N(SiMe3)2
groups in a distorted-trigonal-bipyramidal geometry (Fig. 2) with the
⁎
Corresponding author. Tel.: +86 10 5880 6051; fax: +86 10 5880 2075.
E-mail address: gzi@bnu.edu.cn (G. Zi).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.09.034

F. Zhang et al. / Inorganic Chemistry Communications 14 (2011) 72–74
73
Scheme 1. Synthesis of complex 3.
average distance of Y-N 2.388(5) Å, and the distance of Y-O 2.294(4)
Å, respectively. The distances of Y-N(SiMe3)2 are 2.265(5) and 2.244
(5) Å, which are slightly longer than those found in {[2,6-iPr2C6H3NC
(O)]-1-C10H7}Y[N(SiMe3)2]2(THF) (2.214(2) and 2.231(2) Å) [18].
The twisting between two naphthyl rings of torsion angle is 67.9(2)°,
which is comparable to those found in (2)2Ti(NMe2)2 (70.4(2) and
69.9(4)°) and (2)2Zr(NMe2)2 (71.6(5) and 69.2(5)°) [11].
The polymerization data show that the complexes 3 and 4 can
initiate the ring opening polymerization (ROP) of racemic-lactide
(rac-LA) [21] under mild conditions (Table 1). It allows the complete
conversion of 1000 equiv of lactide within 2 h at room temperature in
toluene at [rac-LA]=1.0 mol L–1 (Table 1, entries 1 and 2). However,
polymerizations with these yttrium initiators/catalysts proceed much
more slowly in THF (Table 1, entries 5 and 6), presumably due to the
competitive coordination between the monomer and this donor
solvent [22]. This difference in activity between toluene and THF
solvent is observed more clearly at a lower temperature (Table 1,
entries 1, 2, 5 and 6) than at higher temperature (Table 1, entries 3, 4,
7 and 8). The microstructure of polymers, as determined by homo-
decoupled 1H NMR experiments in the region of 5.30–5.00 ppm [23–
26], shows the resulting polylactides are all isotactic-rich (riched in
the region of 5.16–5.11 ppm) under our conditions examined.
Molecular weights and polydispersities of the polymers produced
ranged from 65.4 to 71.3 kg mol−1 and 1.21 to 1.34, respectively. Our
results show that the catalytic activities of 3 and 4 resemble that of [2-
(2,6-iPr2C6H3N CH)C4H3N]2Y(CH2SiMe3)(THF)2 [27], while the mi-
crostructure of the resulting polylactides are similar to those initiated
by [(S)-2-MeO-C20H12-2′-(NCHC4H3N)]2LnN(SiMe3)2}2 [10].
Fig. 1. Molecular structure of 3. Selected bond lengths (Å) and bond angles (deg): Y(1)-
O(1) 2.340(2), Y(1)-O(2) 2.168(2), Y(1)-N(1) 2.433(2), Y(1)-N(3) 2.272(3), Y(1)-N(4)
2.286(3), Y(1)-C(21) 2.801(3), O(1)-Y(1)-O(2) 141.68(9), O(1)-Y(1)-N(1) 54.68(7), O
(1)-Y(1)-N(3) 107.96(9), O(1)-Y(1)-N(4) 87.71(9), O(2)-Y(1)-N(1) 87.91(8), O(2)-Y
(1)-N(3) 101.49(9), O(2)-Y(1)-N(4) 101.14(10), N(1)-Y(1)-N(3) 127.72(9), N(1)-Y
(1)-N(4) 112.28(10), N(3)-Y(1)-N(4) 115.86(11), torsion (aryl–aryl) 85.9(1).
Commission of Education, and Excellent Doctoral Dissertation Fund
of Beijing Normal University.
Appendix A. Supplementary material
CCDC 791749, and 791750 contain the supplementary crystallo-
graphic data for 3 and 4. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
In conclusion, two new chiral rare earth metal amidate complexes
have been readily prepared via silylamine elimination reaction
between Y[N(SiMe3)2]3 and chiral binaphthyl-based amidate ligands
1 and 2. Both complexes are active catalysts for the polymerization of
rac-lactide, leading to the isotactic-rich polylactides.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (20972018, 21074013), the Fundamental
Research Funds for the Central Universities, Beijing Municipal
Fig. 2. Molecular structure of 4. Selected bond lengths (Å) and bond angles (deg): Y(1)-
N(1) 2.367(4), Y(1)-N(2) 2.674(5), Y(1)-N(3) 2.265(5), Y(1)-N(4) 2.244(5), Y(1)-O(1)
2.294(4), Y(1)-C(21) 2.736(6), N(1)-Y(1)-N(2) 76.7(2), N(1)-Y(1)-N(3) 126.8(2), N
(1)-Y(1)-N(4) 113.6(2), N(2)-Y(1)-N(3) 95.6(2), N(2)-Y(1)-N(4) 109.8(2), N(3)-Y(1)-
N(4) 118.8(2), O(1)-Y(1)-N(1) 56.9(2), O(1)-Y(1)-N(2) 133.08(13), O(1)-Y(1)-N(3)
105.9(2), O(1)-Y(1)-N(4) 95.66(17), torsion (aryl–aryl) 67.9(2).
Scheme 2. Synthesis of complex 4.

74
F. Zhang et al. / Inorganic Chemistry Communications 14 (2011) 72–74
Table 1
Polymerization of rac-lactide (LA) catalyzed by complex 3 and 4a.
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
Precatalyst
T (°C)
Solvent
Conv. (%)
100
100
100d
100d
85
Mn (kg/mol)
Mw/Mn
Pm (%)
1
2
3
4
5
6
7
8
3
4
3
4
3
4
3
4
20
20
40
40
20
20
40
40
Toluene
Toluene
Toluene
Toluene
THF
THF
THF
THF
70.3
69.5
71.3
70.4
65.4
67.8
67.2
68.7
1.28
1.34
1.21
1.24
1.32
1.30
1.34
1.29
68
59
71
62
68
60
66
63
90
95d
98d
a
Conditions: precat./LA (mol/mol)=1/1000; polymerization time, 2 h; solvent, 5 mL; [LA]=1.0 mol/L.
Measured by GPC (using polystyrene standards in THF).
b
c
Pm is the probability of meso linkages between monomer units and is determined from the methine region (5.30–5.00 ppm) of the homonuclear decoupled 1H NMR spectrum in
CDCl3 at 25 °C [23–26].
d
Polymerization time, 0.5 h.
131.4, 131.0, 129.7, 128.3, 128.0, 125.7, 124.9, 124.5, 124.4, 124.3, 123.8, 122.7,
122.2, 24.4, 21.7, 4.1, 3.1. IR (KBr, cm−1): ν 3310 (m), 2936 (m), 1722 (s), 1471
(s), 1389 (s), 1270 (s), 1159 (s), 972 (s), 753 (s). Anal. Calcd for C52H71N4O2Si4Y:
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary material with this article can be found, in the online
version, at doi:10.1016/j.inoche.2010.09.034.
C, 63.38; H, 7.26; N, 5.69. Found: C, 63.49; H, 7.06; N, 5.77%. Preparation of 4·C6H12
.
This compound was prepared as colorless crystals from the reaction of 2 (0.23 g,
0.5 mmol) with Y[N(SiMe3)2]3 (0.28 g, 0.5 mmol) in toluene (20 mL) and
recrystallization from a cyclohexane solution by a similar procedure as in the
synthesis of 3. Yield: 0.40 g (85%). M.p.: 169–171 °C (dec.). 1H NMR (C6D6): δ 7.81
(d, J=9.2 Hz, 1H, aryl), 7.63 (m, 2H, aryl), 7.44 (d, J=8.1 Hz, 1H, aryl), 7.28 (d,
J=8.9 Hz, 1H, aryl), 7.14 (d, J=9.6 Hz, 1H, aryl), 7.05 (m, 2H, aryl), 6.92 (m, 2H,
aryl), 6.85 (s, 1H, aryl), 6.79 (m, 1H, aryl), 6.50 (s, 2H, aryl), 2.81 (s, 3H, CH3), 2.70
(s, 6H, NCH3), 2.09 (s, 3H, CH3), 1.65 (s, 3H, CH3), 1.43 (s, 12H, C6H12), 0.46 (s,
18H, Si(CH3)3), 0.27 (s, 18H, Si(CH3)3). 13C NMR (C6D6): δ 182.0, 145.1, 141.1,
138.5, 136.3, 134.5, 134.3, 133.9, 132.7, 131.6, 130.7, 130.2, 128.9, 128.5, 128.4,
127.3, 126.8, 126.6, 126.1, 125.6, 124.6, 123.4, 119.4, 47.4, 27.0, 22.8, 20.5, 6.1, 5.4.
IR (KBr, cm−1): ν 2961 (m), 2932 (w), 1612 (m), 1455 (s), 1417 (s), 1259 (s),
1093 (s), 1066 (s), 1014 (s), 929 (s), 797 (s). Anal. Calcd for C50H77N4OSi4Y: C,
63.12; H, 8.16; N, 5.89. Found: C, 63.23; H, 8.07; N, 5.73%.
References
[1] H.C. Aspinall, Chem. Rev. 102 (2002) 1807–1850.
[2] F.T. Edelmann, D.M.M. Freckmann, H. Schumann, Chem. Rev. 102 (2002)
1851–1896.
[3] O. Dechy-Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 104 (2004)
6147–6176.
[4] J. Gromada, J.-F. Carpentier, A. Mortreux, Coord. Chem. Rev. 248 (2004) 397–410.
[5] W.E. Piers, D.J.H. Emslie, Coord. Chem. Rev. 233–234 (2002) 131–155.
[6] G. Zi, Dalton Trans. (2009) 9101–9109.
[7] G. Zi, L. Xiang, H. Song, Organometallics 27 (2008) 1242–1246.
[8] Q. Wang, L. Xiang, H. Song, G. Zi, J. Organomet. Chem. 694 (2009) 691–696.
[9] G. Zi, L. Xiang, X. Liu, Q. Wang, H. Song, Inorg. Chem. Commun. 13 (2010) 445–448.
[10] Q. Wang, L. Xiang, H. Song, G. Zi, Dalton Trans. (2008) 5930–5944.
[11] G. Zi, X. Liu, L. Xiang, H. Song, Organometallics 28 (2009) 1127–1137.
[12] G. Zi, F. Zhang, L. Xiang, Y. Chen, W. Fang, H. Song, Dalton Trans. 39 (2010)
4048–4061.
[13] Q. Wang, H. Song, G. Zi, J. Organomet. Chem. 695 (2010) 1583–1591.
[14] G. Zi, F. Zhang, H. Song, Chem. Commun. 46 (2010) 6296–6298.
[15] F. Zhang, H. Song, G. Zi, J. Organomet. Chem. 695 (2010) 1993–1999.
[16] G. Zi, J. Organomet. Chem. (2010), doi:10.1016/j.jorganchem.2010.07.034.
[17] L.J.E. Stanlake, J.D. Beard, L.L. Schafer, Inorg. Chem. 47 (2008) 8062–8068.
[18] L.J.E. Stanlake, L.L. Schafer, Organometallics 28 (2009) 3990–3998.
[19] Preparation of 3. Under nitrogen gas, a toluene solution (10 mL) of 1 (0.29 g,
0.5 mmol) was slowly added to a toluene solution (10 mL) of Y[N(SiMe3)2]3
(0.28 g, 0.5 mmol) with stirring at room temperature. The resulting solution was
refluxed overnight to give a light yellow solution. The solution was filtered, and
the filtrate was concentrated to about 2 mL. 3 was isolated as colorless crystals
after this solution stood at room temperature for three days. Yield: 0.39 g (79%).
M.p.: 218–220 °C (dec.). 1H NMR (C6D6): δ 8.06 (d, J=8.8 Hz, 1H, aryl), 7.91 (d,
J=8.8 Hz, 1H, aryl), 7.64 (d, J=8.1 Hz, 1H, aryl), 7.51 (m, 2H, aryl), 7.43 (m, 2H,
aryl), 6.83 (m, 3H, aryl), 6.65 (m, 2H, aryl), 6.55 (s, 2H, aryl), 6.46 (s, 2H, aryl), 6.36
(s, 1H, NH), 2.67 (s, 3H, CH3), 2.14 (s, 6H, CH3), 2.12 (s, 3H, CH3), 1.80 (s, 3H, CH3),
1.76 (s, 3H, CH3), 0.53 (s, 9H, Si(CH3)3), 0.32 (s, 27H, Si(CH3)3). 13C NMR (C6D6): δ
181.5, 172.0, 139.8, 138.2, 137.8, 137.5, 136.5, 136.1, 134.9, 134.3, 134.2, 132.7,
[20] Crystal data for 3: C52H71N4O2Si4Y, fw=985.40, monoclinic, P1211, a=12.708(2)
Å, b=19.822(3) Å, c=17.621(2) Å, β=104.67(1)°, V=4294.0(9) Å3, Z=2,
R1=0.046 for 14786 (IN2σ(I)), wR2=0.127 (all data). Crystal data for 4·C6H12
:
C
50H77N4OSi4Y, fw=951.43, triclinic, P1, a=9.000(1) Å, b=11.502(1) Å,
c=13.067(2) Å, α=101.60(1), β=95.96(1), γ=91.03(1), V=5963.3(3) Å3,
Z=1, R1=0.048 for 5613 (IN2σ(I)), wR2=0.099 (all data).
[21] General procedure for polymerization of rac-lactide. In a glovebox, 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 2 h at room temperature. The polymerization was quenched
by the addition of acidified methanol. The resulting precipitated polylactide was
collected, washed with methanol several times, and dried in vacuum at 50 °C
overnight.
[22] R. Heck, E. Schulz, J. Collin, J.-F. Carpentier, J. Mol. Catal. A: Chem. 268 (2007)
163–168.
[23] J.E. Kasperczyk, Macromolecules 28 (1995) 3937–3939.
[24] K.A.M. Thakur, R.T. Kean, E.S. Hall, M.A. Doscotch, J.I. Siepmann, E.J. Munson,
Macromolecules 30 (1997) 2422–2428.
[25] X. Pang, X. Chen, H. Du, X. Wang, X. Jing, J. Organomet. Chem. 692 (2007)
5605–5613.
[26] T.M. Ovitt, G.W. Coates, J. Am. Chem. Soc. 124 (2002) 1316–1326.
[27] Y. Yang, S. Li, D. Cui, X. Chen, X. Jing, Organometallics 26 (2007) 671–678.