Synthesis and characterization of rare-earth metal complexes supported by a new pentadentate Schiff base and their application in heteroselective polymerization of rac-lactide

Article abstract of DOI:10.1039/c5cy00322a

A series of neutral rare-earth metal aryloxides and amides supported by a new pentadentate (N₂O₃) salen ligand were synthesized, and their catalytic behaviors for the ring-opening polymerization of rac-lactide (rac-LA) were explored. The protolysis reactions of N,N′-bis(3,5-di-tert-butylsalicylidene)-2,2′-diaminodiphenyl ether (LH₂) with (ArO)₃Ln(THF) (ArO = 2,6-Bu^t₂-4-MeC₆H₂O) and Ln[N(SiMe₃)₂]₃ in a 1:1 molar ratio in THF gave the neutral rare-earth metal aryloxides LLn(OAr)(THF)_n [n = 0, Ln = Sc (1), Yb (2); n = 1, Ln = Y (3), Sm (4) and Nd (5)] and rare-earth metal amides LLnN(SiMe₃)₂ [Ln = Yb (6), Y (7)], respectively. X-ray structural determination showed that complexes 1, 2, 6 and 7 have monomeric structures, in which the coordination geometry around the rare-earth metal atom can be best described as a distorted trigonal prism. Complexes 3 and 5 are THF-solvated monomers and each of the rare-earth metal atoms is seven-coordinated to form a distorted capped trigonal prism. It was found that all of these complexes can efficiently initiate the ring-opening polymerization (ROP) of rac-LA to give heterotactic-rich polylactides (PLAs). The highly heterotactic PLA (P_r up to 0.93) was obtained using complex 2 as the initiator at a polymerization temperature of 0 °C. The observed increasing order of activity of 1 < 2 < 3 < 4 ≈ 5 is in agreement with the order of their ionic radii, whereas the order of stereoselectivity is in the reverse order. The rare-earth metal salen amides can initiate rac-LA polymerization in a controlled manner, while polymerization using the rare-earth metal salen aryloxides is less controlled at room temperature.

Full text of DOI:10.1039/c5cy00322a

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DOI: 10.1039/C5CY00322A
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ARTICLE TYPE
Synthesis and characterization of rare-earth metal
complexes supported by a new pentadentate Schiff base and
their application in the heteroselective polymerization of
rac-lactide
5
Yu Cui, Weikai Gu, Yaorong Wang,* Bei Zhao,* Yingming Yao* and Qi Shen
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 2015
DOI: 10.1039/x000000x
Fax: (86)512ꢀ65880305; Eꢀmail: yrwang@suda.edu.cn; yaoym@suda.edu.cn.
Electronic supplementary information (ESI) available. CCDC 1051829ꢀ1051831 (for 1ꢀ3) and 1051832ꢀ1051834 (for
10 5ꢀ7). For crystallographic data in CIF or other electronic format see DOI: 10.1039/x000000x
Key Laboratory of Organic Synthesis of Jiangsu Province and Suzhou Key Laboratory of Macromolecular Design and
Precision Synthesis, College of Chemistry, Chemical Engineering and Materials Science, Dushu Lake Campus,
Soochow University, Suzhou 215123, People’s Republic of China.
15
A series of neutral rareꢀearth metal aryloxides and amides supported by a new pentadentate (N2O3) Salen
ligand were synthesized, and their catalytic behaviors for the ringꢀopening polymerization of racꢀlactide
(racꢀLA) were explored. The protolysis reactions of N,N’ꢀbis(3,5ꢀdiꢀtertꢀbutylsalicylidene)ꢀ2,2’ꢀ
diaminodiphenyl ether (LH2) with (ArO)3Ln(THF) (ArO = 2,6ꢀBut2ꢀ4ꢀMeC6H2O) and Ln[N(SiMe3)2]3 in
20 a 1:1 molar ratio in THF gave the neutral rareꢀearth metal aryloxides LLn(OAr)(THF)n [n = 0, Ln = Sc
(1), Yb (2); n = 1, Ln = Y (3), Sm (4), Nd (5)] and rareꢀearth metal amides LLnN(SiMe3)2 [Ln = Yb (6),
Y (7)], respectively. Xꢀray structural determination showed that complexes 1, 2, 6 and 7 have a
monomeric structure, in which the coordination geometry around the rareꢀearth metal atom can be best
described as a distorted trigonal prism. Complexes 3 and 5 are a THFꢀsolvated monomer and each of
25 rareꢀearth metal atoms is sevenꢀcoordinated to form a distorted capped trigonal prism. It was found that
all of these complexes can efficiently initiate the ringꢀopening polymerization (ROP) of racꢀLA to give
heterotacticꢀrich polylactides (PLAs). The highly heterotactic PLA (Pr up to 0.93) was obtained using
complex 2 as the initiator at the polymerization temperature of 0 ºC. The observed activityꢀincreasing
order of 1 << 2 < 3 < 4 ≈ 5 is in agreement with the order of their ionic radii, whereas the order for
30 stereoselectivity is in the reverse order. The rareꢀearth metal Salen amides can initiate racꢀLA
polymerization in a controlled manner, while rareꢀearth metal Salen aryloxides are less controlled at
room temperature.
50 heterotactic PLAs.8
Introduction
Tetradentate Salen ligands, which are generally obtained by the
condensation of salicylaldehydes with diverse diamines, are one of
the popular ligands in coordination chemistry of transition and
rareꢀearth metals,11, 12 and the corresponding metal complexes are
Polylactides (PLAs) as biodegradable, biocompatible and
35 renewable materials have attracted intensive attention over the past
two decades because of their potentially wideꢀranging applications
such as packing, fibers, composites and medical devices.1 The most
convenient method of synthesizing PLAs is the ring opening55
polymerization (ROP) of lactide (LA) initiated by metalꢀbased
40 complexes,2 due to the advantages of wellꢀcontrolled molecular
weight and narrow molecular weight distribution. Therefore, a
number of main group,3ꢀ6 transition metal,7 and rareꢀearth metal
complexes8ꢀ10 incorporating a variant of ligands have been60
synthesized for this purpose. Among the variety of initiators,
45 organoꢀrareꢀearth metal complexes have got a great interest due to
their high activity, controllability and stereoselectivity. In
particular, rareꢀearth metal complexes bearing tetradentate
heteroatomꢀbridged bisphenolate ligands are extremely efficient65
initiators for the controlled ROP of racꢀLA to give highly
widely used in various of
metalꢀcatalyzed organic
transformations13 and polymerizations.14 In particular, aluminum
complexes bearing chiral and achiral Salenꢀtype ligands can
promote the ROP of racꢀLA to give highly isotactic stereoblock
PLAs by an enantiomorphic site control or a chainꢀend control
mechanism.3 However, most of the rareꢀearth metal complexes
bearing the same ligands did not affect stereoselectivity in the
polymerization of racꢀLA.3c,3d Exceptions are yttrium complexes
bearing racꢀbinaphthylꢀbridged Salen, which generate heterorich
PLAs with Pr up to 0.91.15 Recently, Williams et al. reported that
rareꢀearth metal phosphasalen (an iminophosphorane derivative of
the Salen ligand) complexes are high isoꢀselective or heteroꢀ
selective initiators for racꢀLA polymerization by changing the
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structure of phosphasalen ligand or the metal size.10 Furthermore, pellet, cmꢀ1): 3413 (w)，2956 (s), 2906 (w), 2868 (w), 1615 (s),
the catalytic activities of phosphasalen rareꢀearth metal complexes 1589 (s), 1532 (s), 1505 (s), 1487 (s), 1461 (w), 1431 (s), 1385 (s),
are higher than those of analogous Salen rareꢀearth metal60 1360 (w), 1312 (w), 1256 (s), 1200 (s), 1163 (s), 1107 (w), 1026
complexes,10, 3c, 16 because phosphasalen ligands are expected to be (w), 983 (w), 928 (w), 871 (s), 833 (s), 791 (s), 750 (s), 628 (w),
more electronꢀdonating than their Salen counterparts. These results 519 (s).
5
imply that the increased activity of rareꢀearth metal Salen
Synthesis of LSc(OAr) (1). A solution of Sc(OAr)3 (10 mL,
complexes might be expected upon introducing electronꢀdonating 0.15 M, 1.50 mmol) in THF was injected quickly to a solution of
substituent on the backbone of the Salen ligand. Therefore, a new65 preligand LH2 (0.95 g, 1.50 mmol) in THF (20 mL). The mixture
pentadentate (N2O3) Salen, N,N’ꢀbis(3,5ꢀdiꢀtertꢀbutylsalicylidene)ꢀ was stirred at room temperature for 12 h, and then THF was
10 2,2’ꢀdiaminodiphenyl ether, was introduced to organoꢀrareꢀearth evaporated completely under reduced pressure. The resultant
metal chemistry, and a series of rareꢀearth metal aryloxo and yellow solid was washed with hexane about three times, and
bis(trimethysily)amido complexes were prepared and wellꢀ crystallized from a mixture of THF (2 ml) and nꢀhexane (4 ml) at
characterized. Furthermore, the catalytic performances of these70 room temperature to afford complex 1 as yellow crystals (0.97 g,
rareꢀearth metal Salen complexes in the heteroselective ROP of 72%). Anal. Calcd. for C57H73N2O4Sc: C, 76.48; H, 8.22; N, 3.13;
15 racꢀLA were also explored.
Found: C, 77.09; H, 8.02; N, 3.05. 1H NMR (400 MHz, C6D6,
25 °C): δ 8.28 (s, 1H, CH=N), 8.11 (s, 1H, CH=N), 7.78 (d, 1H,
JH−H = 2.6 Hz, ArH), 7.60 (d, 1H, JH−H = 2.56 Hz, ArH), 7.19 (m,
2H, ArH), 7.10 (m, 1H, ArH), 7.05 (m, 1H, ArH), 6.97 (m, 3H,
ArH), 6.91 (m, 1H, ArH), 6.71(m, 2H, ArH), 6.64 (m, 1H, ArH),
6.55 (m, 1H, ArH), 3.57 (m, 3H, αꢀCH2, solvate THF), 2.33 (s, 3H,
CH3), 1.45 (s, 27H, overlap with THF signal, C(CH3)3), 1.43 (m,
3H, overlap with But signal, βꢀCH2, solvate THF), 1.38(s, 9H,
C(CH3)3), 1.33 (m, 9H, C(CH3)3), 1.24 (m, 2.6H, CH2, nꢀhexane),
1.19 (s, 9H, C(CH3)3), 0.89 (t, 2H, CH3, nꢀhexane). 13C{1H} NMR
(100 MHz, C6D6, 25 °C): δ 167.9, 165.0 (CH=N), 161.5, 152.2,
148.5, 144.9, 141.0, 140.5, 139.8, 138.5, 137.8, 133.0, 131.8, 130.0,
129.7, 129.4, 127.1, 126.5, 125.9, 125.4, 124.3, 122.4, 121.9, 121.6,
119.9, 114.8 (Ar–C), 68.1 (αꢀCH2, THF), 35.9, 35.8, 35.2, 34.7,
34.5, 34.4 (C(CH3)3), 32.3 (CH2, nꢀhexane), 32.8, 32.0, 31.9, 31.8,
30.6, 30.3 (C(CH3)3), 26.2 (βꢀCH2 THF), 23.4 (CH2, nꢀhexane),
22.0 (CH3), 14.7 (CH3, nꢀhexane). IR (KBr pellet, cmꢀ1): 2955 (s),
2903 (w), 2867 (w), 1616 (s), 1589 (s), 1541 (s), 1488 (s), 1407 (s),
1387 (w), 1359 (w), 1256 (s), 1200 (s), 1163 (s), 860 (w), 835 (s),
745 (s), 699 (w).
Experimental Section
General Methods. The complexes described below are sensitive75
to air and moisture. Therefore, all manipulations were performed
under pure argon with rigorous exclusion of air and moisture using
20 Schlenk techniques. Solvents were dried and freed of oxygen by
refluxing over sodium/benzophenone ketyl and distilled prior to
use.
3,5ꢀDiꢀtertꢀbutylꢀ2ꢀhydroxybenzaldehyde,
2,2’ꢀ80
diaminodiphenyl ether and racꢀlactide (racꢀLA) are commercially
available. racꢀLA was recrystallized twice from dry toluene and
25 then was sublimed under vacuum at 50 ºC. Ln(ArO)3(THF)17 (ArO
= 2,6ꢀBut2ꢀ4ꢀMeC6H2O) and Ln[N(SiMe3)2]3 were prepared
18
according to the procedures. Carbon, hydrogen, and nitrogen85
analyses were performed by direct combustion with a CarloꢀErba
EAꢀ1110 instrument. The IR spectra were recorded with a Nicoletꢀ
30 550 FTIR spectrometer as KBr pellets. The 1H and 13C NMR
spectra were recorded in a C6D6 solution for complexes 1, 3 and 7
with a Unity Varian spectrometer. Because of their paramagnetism,90
no resolvable NMR spectrum for the other complexes was obtained.
The microstructures of PLAs were measured by homodecoupling
35 1H NMR spectroscopy at 20 ºC in CDCl3 on a Unity Varian ACꢀ
400 spectrometer. Molecular weight and molecular weight
distribution (PDI) were determined against a polystyrene standard95
by gel permeation chromatography (GPC) on a PL 50 apparatus,
and THF was used as an eluent at a flow rate of 1.0 mL/min at 40
40 ºC.
Synthesis of LYb(OAr) (2). The synthesis of complex 2 was
carried out in the same way as that described for complex 1, but
Yb(OAr)3(THF) (10.5 mL, 0.143 M, 1.50 mmol) was used instead
of Sc(OAr)3. Yellow crystals were obtained in a THF/toluene/nꢀ
hexane (1:1:2) solution (1.04 g, 68%). Anal. Calcd. for
C57H73N2O4Yb: C, 66.91; H, 7.19; N, 2.14. Found: C, 67.36; H,
7.11; N, 2.17. IR (KBr pellet, cmꢀ1): 2953 (s), 2905 (w), 2867 (w),
1614 (s), 1588 (s), 1531 (s), 1486 (s), 1427 (s), 1387 (s), 1359 (w),
100 1332 (w), 1229 (s), 1163 (s), 1107 (w), 928 (w), 835 (s), 791 (w),
744 (s), 675 (w).
Synthesis of LY(OAr)(THF) (3). The synthesis of complex 3
was carried out in the same way as that described for complex 1,
but Y(OAr)3(THF) (6.4 mL, 0.235 M, 1.50 mmol) was used instead
105 of Sc(OAr)3. Light yellow crystals were obtained in a mixture
solution of THF (3 mL) and nꢀhexane (3 mL) (1.13 g, 69%). Anal.
Calcd. for C61H81N2O5Y: C, 72.45; H, 8.07; N, 2.77. Found: C,
72.66; H, 7.51; N, 2.82. 1H NMR (400 MHz, C6D6, 25 °C) : δ 8.32
(s, 1H, CH=N), 8.14 (s, 1H, CH=N), 7.76 (s, 1H, Ar), 7.68 (s, 1H,
110 Ar), 7.19 (s, 1H, Ar), 7.05 (m, 3H, Ar), 7.01 (d, 1H, JH−H = 7.2 Hz,
Ar), 6.85 (m, 3H, Ar), 6.68 (m, 4H, Ar), 3.51 (m, 8H, αꢀCH2, THF),
2.27 (s, 3H, CH3), 1.60 (s, 18H, C(CH3)3), 1.50 (s, 9H, C(CH3)3),
1.39 (s, 9H, C(CH3)3), 1.38 (m, 8H, βꢀCH2, THF), 1.35 (m, 9H,
C(CH3)3), 1.27 (s, 9H, C(CH3)3), 1.24 (m, 4H, CH2, nꢀhexane),
115 0.89 (t, 3H, CH3, nꢀhexane). 13C{1H} NMR (100 MHz, C6D6,
Synthesis of LH2.
A
solution of 3,5ꢀdiꢀtertꢀbutylꢀ2ꢀ
hydroxybenzaldehyde (11.7 g, 50 mmol) in methanol (30 mL) was
added to a solution of 2,2’ꢀdiaminodiphenyl ether (5.0 g, 25 mmol)
in methanol (60 mL). The mixture was stirred at 90 ºC for 5 h and
45 then was cooled. The precipitate was filtered and washed with
methanol to give the orange product (14.55 g, 92%). Anal. Calcd.
for C42H52N2O3: C, 79.71; H, 8.28; N, 4.43. Found: C, 80.23; H,
1
8.24; N, 4.45. H NMR (400 MHz, CDCl3, 25 °C): δ 13.52 (s, 2H,
ArOH), 8.72 (s, 2H, N=CH), 7.38 (d, 2H, JH−H = 2.4 Hz, ArH),
50 7.27 (s, 1H, overlap with residual signal of CDCl3, ArH), 7.25 (d,
1H, JH−H = 2.2 Hz, ArH), 7.18 (d, 1H, JH−H = 2.0 Hz, ArH), 7.16 (d,
2H, JH−H = 2.12 Hz, ArH), 7.14 (d, 1H, JH−H = 2.4 Hz, ArH), 7.11
(m, 2H, ArH), 6.98 (d, 1H, JH−H = 1.88 Hz, ArH), 6.96 (d, 1H, JH−H
= 2.12 Hz, ArH), 1.39 (s, 18H, C(CH3)3), 1.25 (s, 18H, C(CH3)3).
55 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 165.1 (CH=N), 158.7,
149.9, 140.4, 139.9, 137.1, 128.1, 127.4, 127.0, 124.4, 121.2, 119.5,
118.6(Ar–C), 35.3, 34.3 (C(CH3)3), 31.7, 29.6 (C(CH3)3). IR (KBr
2
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25 °C) : 168.9, 168.4 (CH=N), 160.9, 151.3, 146.4, 144.0, 141.0, as follows: A 20 mL reaction flask, equipped with a magnetic
141.2, 140.4, 138.1, 137.7, 137.3, 136.4, 132.6, 132.1, 130.7, 130.4,60 stirring bar, was charged with racꢀLA (0.50 g, 3.47 mmol) and
126.8, 126.2, 125.4, 124.2, 123.3, 122.6, 121.4, 120.4, 113.3 (Ar– THF (2.78 mL). After the monomer was dissolved, a THF solution
C), 68.2 (αꢀCH2, THF), 36.2, 34.9, 34.7, 34.5 (C(CH3)3), 32.3, 32.0, of complex 2 (0.69 mL, 0.0069 mmol) was added by syringe. The
30.8, 30.6 (C(CH3)3), 31.9 (CH2, nꢀhexane), 26.0 (βꢀCH2, THF), mixture was immediately stirred vigorously for 10 minutes at
23.4 (CH2, nꢀhexane), 22.0 (CH3), 14.7 (CH3, nꢀhexane). IR (KBr 25 °C, during which time an increase in the viscosity was observed.
pellet, cmꢀ1): 2955 (s), 2904 (w), 2866 (w), 1616 (s), 1588 (s), 154265 The reaction mixture was quenched by the addition of 1.0 mL of
(s), 1486 (s), 1474 (s), 1407 (s), 1399 (w), 1255 (s), 1200 (s), 1163 HCl/CH3OH (0.05/10 v/v) and then poured into ethanol to
5
(s), 860 (w), 835 (s), 746 (s), 675 (s).
Synthesis of LSm(OAr)(THF) (4). The synthesis of complex 4 dried at 50°C for 12 h in vacuum, and weighed (0.48 g, 95%).
was carried out in the same way as that described for complex 1, Oligomer Preparation. Oligomerization of racꢀLA was carried
but Sm(OAr)3(THF) (16.3 mL, 0.092 M, 1.50 mmol) was used70 out with complex 2 and 7 in THF at 25 °C in a molar ratio of
instead of Sc(OAr)3. Yellow crystals were obtained in monomer/initiator of 20. The reaction was stirred for 5 min and
THF/toluene/nꢀhexane (1:1:1) solution (1.04 g, 65%). Anal. Calcd. then quenched by adding wet nꢀhexane. The precipitated oligomers
precipitate the polymer. The polymeric product was filtered and
10
a
1
15 for C61H81N2O5Sm: C, 68.30; H, 7.61; N, 2.61. Found: C, 68.64; H, were collected, dried under vacuum, and used for H NMR and
7.80; N, 2.61. IR (KBr pellet,cmꢀ1): 2956 (s), 2905 (w), 2867 (w), MALDIꢀTOF mass spectra measurement.
1699 (s), 1617 (s), 1588 (s), 1538 (s), 1486 (s), 1387 (s), 1359 (w),75 X-Ray Crystallographic Structure Determination. Suitable
1229 (s), 1163 (s), 1107 (w), 835 (s), 744 (s), 680 (s).
single crystals of complexes 1-3 and 5-7 were sealed in a thinꢀ
Synthesis of LNd(OAr)(THF) (5). The synthesis of complex 5 walled glass capillary for determination of the singleꢀcrystal
20 was carried out in the same way as that described for complex 1, structures. Intensity data were collected with a Rigaku Mercury
but Nd(OAr)3(THF) (12.5 mL, 0.12 M, 1.50 mmol) was used CCD area detector in ω scan mode using MoꢀKα radiation (λ =
instead of Sc(OAr)3. Green crystals were obtained in a THF/nꢀ80 0.71073 Å and λ = 0.71075 Å). The diffracted intensities were
hexane solution (1.09 g, 68%). Anal. Calcd. for C61H81N2O5Nd: C, corrected for Lorentz/polarization effects and empirical absorption
68.69; H, 7.65; N, 2.63. Found: C, 68.65; H, 7.62; N, 2.56. IR (KBr corrections. Details of the intensity data collection and crystal data
25 pellet, cmꢀ1): 2953 (s), 2905 (w), 2867 (w), 1614 (s), 1588 (s), 1532 are given in Table S1.
(s), 1486 (s), 1459 (s), 1420 (s), 1387 (w), 1332 (w), 1229 (s), 1163
(s), 1126 (w), 835 (s), 791 (w), 744 (s), 689 (s).
The structures were solved by direct methods and refined by
85 fullꢀmatrix leastꢀsquares procedures based on |F|2. The hydrogen
Synthesis of LYb[N(SiMe3)2] (6).
A
solution of atoms in these complexes were generated geometrically, assigned
Yb[N(SiMe3)2]3 (9.7 mL, 0.154 M, 1.50 mmol) in THF was appropriate isotropic thermal parameters, and allowed to ride on
30 injected quickly to a solution of preligand LH2 (0.95 g, 1.50 mmol) their parent carbon atoms. All of the hydrogen atoms were held
in THF (20 mL). The mixture was stirred overnight at room stationary and included in the structure factor calculation in the
temperature, and then THF was evaporated completely under90 final stage of fullꢀmatrix leastꢀsquares refinement. The structures
reduced pressure. The resultant yellow solid was crystallized from were solved and refined using SHELEXLꢀ97 programs.
a mixture of THF and nꢀhexane at room temperature to afford
35 yellow crystals of 6 after several days (0.94 g, 65%). Anal. Calcd.
for C48H68N3O3Si2Yb: C, 59.79; H, 7.11; N, 4.36; Found: C, 59.79;
H, 7.13; N, 4.35.
Synthesis of LY[N(SiMe3)2] (7). The synthesis of complex 7
Results and discussion
Synthesis and Structure of Rare-earth Metal Aryloxides 1ꢀ5.
The new pentadentate (N2O3) Salen, N,N’ꢀbis(3,5ꢀdiꢀtertꢀ
butylsalicylidene)ꢀ2,2’ꢀdiaminodiphenyl
was carried out in the same way as that described for complex 6,95
40 but Y[N(SiMe3)2]3 (7.4 mL, 0.203 M, 1.50 mmol) was used instead
of Yb[N(SiMe3)2]3. Yellow crystals were obtained in a THF/nꢀ
hexane solution (0.88 g, 67%). Anal. Calcd. for C52H76N3O4Si2Y
(7·THF): C, 65.59; H, 8.04; N, 4.41. Found: C, 65.54; H, 8.00; N,
4.33. 1H NMR (400 MHz, C6D6, 25 °C): δ 8.16 (s, 1H, CH=N),
45 8.03 (s, 1H, CH=N), 7.84 (s, 1H, Ar), 7.64(s, 1H, ArH), 6.98 (m,
5H, ArH), 6.79 (m, 1H, ArH), 6.59 (m, 4H, ArH), 3.54 (m, 6H, αꢀ
CH2, THF), 1.91 (s, 9H, C(CH3)3), 1.63 (s, 9H, C(CH3)3), 1.37 (m,
6H, overlap with But signal, βꢀCH2, THF), 1.36 (s, 9H, overlap
with THF signal, C(CH3)3), 1.27 (s, 9H, C(CH3)3), 0.18 (s,18H,
50 Si(CH3)3). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ 168.0, 166.7
(CH=N), 148.4, 141.2, 136.5, 136.2, 131.5, 130.8, 130.5, 126.6,
126.4, 126.0, 125.4, 123.1, 122.3, 120.6, 120.5, 114.9 (Ar–C), 69.0
(αꢀCH2, THF), 36.6, 36.4, 34.5 (C(CH3)3), 32.3 (CH2, nꢀhexane),
32.1, 30.9, 30.7 (C(CH3)3), 25.7 (βꢀCH2, THF), 23.4 (CH2, nꢀ
55 hexane), 14.7 (CH3, nꢀhexane), 3.8, 3.0, 2.4 (Si(CH3)3).
ether
(LH2),
was
synthesized by the condensation reaction of 3,5ꢀdiꢀtertꢀbutylꢀ2ꢀ
hydroxybenzaldehyde with 2,2’ꢀdiaminodiphenyl ether in 2:1
molar ratio in refluxing methanol, and isolated in 92% yield as
orange powder. It was fully characterized by 1H, 13C{1H} NMR
100 spectroscopy and elemental analysis.
Previous works have shown that the protolysis reaction between
homoleptic rareꢀearth metal aryloxides and preligands is a useful
method for the synthesis of saltꢀfree rareꢀearth metal aryloxo
derivarives19. Therefore, this method was also used to synthesize
105 the desired heteroleptic rareꢀearth metal aryloxides in present case.
Reactions of (ArO)3Ln(THF) (ArO = 2,6ꢀBut2ꢀ4ꢀMeC6H2O) with
the Salen precursor (LH2) in a 1:1 molar ratio in THF, after
workup, gave the desired rareꢀearth metal aryloxides
LLn(OAr)(THF)n [n = 0, Ln = Sc (1), Yb (2); n = 1, Ln = Y (3),
110 Sm (4), Nd (5)] as microcrystals in high isolated yields (65ꢀ72%)
from a mixture solution of THF and nꢀhexane, as shown in Scheme
1. Complexes 1ꢀ5 are fully characterized by element analysis, IR
and NMR spectroscopy in the case of complexes 1 and 3. The IR
spectra of complexes 1ꢀ5 exhibited two strong absorption bands
115 near 1616 and 1588 cm–1, which were characteristic bands of the
Typical Procedure for the Polymerization Reaction. The
procedures for the polymerization of racꢀLA initiated by the
complexes 1-7 were similar. A typical polymerization procedure is
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1
C=N group.12d The H NMR spectra of complexes 1 and 3 clearly lengths in ethyleneꢀbridged bis(βꢀketoiminato) ytterbium aryloxide
showed the peaks of the pentadentate Salen ligand and the aryloxo [CH2NC(Me)CHCOPh]2Yb(OAr)(THF)] (2.090(3) Å)22 and amine
group. All of these complexes are soluble in THF and toluene, but55 bridged
insoluble in nꢀhexane.
bis(phenolate)
ytterbium
aryloxide
complex
[Me2NCH2CH2N{CH2ꢀ(2ꢀOꢀC6H2ꢀBut2ꢀ3,5)}2]Yb(OC6H3ꢀ2,6ꢀ
Pri2)(THF) (2.084(6) Å).8h
5
But
Complexes 3 and 5 are isomorphous and only the molecular
diagram of complex 3 is shown in Fig. 2. Each of the metal centers
60 in complexes 3 and 5 is sevenꢀcoordinated by one aryloxy group,
one THF and the pentadentate (N2O3) Salen ligand. The
coordination geometry around the metal center can be best
described as a distorted capped trigonal prismatic environment, in
which O2, O4 , N1 and O3, O5, N2 can be considered to constitute
65 the two triangular faces of the prism with the dihedral angle of
6.12º for 3 and 7.36º for 5, and O1 to occupy the capping position.
The dihedral angles between the two phenolate groups in
complexes 3 and 5 are 129.3º and 130.7º, respectively, which are
bigger than those in complexes 1 and 2 (101.0º for 1 and 104.3º for
2), probably due to the low coordination number in the later. The
LnꢀO(Salen), LnꢀN(Salen), LnꢀO(Ar) and LnꢀO(ether) bond
lengths in complexes 3 and 5 are comparable to the corresponding
values in complexes 1 and 2 when the difference in metal sizes is
taken in consideration.21
But
O
N
OH
N
N
Ln(OAr)3(THF)
r.t., THF
Ln
O
But
But
+
2 ArOH
O
O
ArO (THF)n
ArO = 2,6-But-4-MeC6H2O
OH
N
But
But
But
Ln = Sc (1), n=0
Ln = Yb (2), n=0
Ln = Y (3), n=1
Ln = Sm (4), n=1
Ln = Nd (5), n=1
But
Scheme 1. Synthesis of Complexes 1-5.
The definitive molecular structures of complexes 1ꢀ5 were
10 further confirmed by singleꢀcrystal structure determination. All of
these complexes have monomeric structures, and their selected
bond lengths and bond angles are provided in Table 1. The bond70
parameters for complex 4 were not provided due to the poor quality
of the crystal data.20 It was found that the ionic radii of the rareꢀ
15 earth metals can influence the solid state structure of the rareꢀearth
metal Salen complexes. In complexes 1 and 2, there is no THF
molecule to coordinate to the scandium and ytterbium ions,75
whereas there is one THF molecule to coordinate to the larger
yttrium, samarium and neodymium ions in complexes 3ꢀ5.
20 Complexes 1 and 2 are isomorphous and only the molecular
diagram of complex 1 is shown in Fig. 1. In complexes 1 and 2,
each of the rareꢀearth metal atoms is sixꢀcoordinated with three
oxygen atoms and two nitrogen atoms from the pentadentate
(N2O3) Salen ligand, and one oxygen atom from the aryloxo group.
25 The coordination geometry around the metal center can be best
described as a distorted trigonal prism, in which the dihedral angles
between the two triangular faces defined by O1, O4, N1 and O2,
O3, N2, respectively, are 8.13º for 1 and 11.6º for 2. The Scꢀ
O(Salen) distances of 1.984(2) and 2.022(2) Å and the ScꢀN(Salen)
30 bond lengths of 2.348(2) and 2.270(2) Å in complex 1 are
comparable with the corresponding bond lengths in
[(Salen’)Sc(THF)2][BPh4] (av. 1.970(2) Å and 2.228(3) Å, Salen’
Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) for
Complexes 1-3 and 5
Bond lengths
1
2
3
5
Ln1ꢀO1
2.578(2)
1.984(2)
2.022(2)
1.955(5)
2.348(2)
2.270(2)
1
2.514(2)
2.104(2)
2.132(2)
2.070(3)
2.432(3)
2.389(3)
2
2.529(2)
2.171(2)
2.207(2)
2.129(3)
2.523(3)
2.519(3)
3
2.587(2)
2.235(2)
2.272(2)
2.207(2)
2.615(3)
2.619(3)
5
Ln1ꢀO2
Ln1ꢀ O3
Ln1ꢀ O4
Ln1ꢀN1
Ln1ꢀN2
Bond angles
O1ꢀLn1ꢀN1
O1ꢀLn1ꢀN2
O1ꢀLn1ꢀO4
N1ꢀLn1ꢀN2
O2ꢀLn1ꢀO3
O4ꢀLn1ꢀO5
63.94(7)
64.26(7)
89.35(7)
108.19(8)
91.34(8)
ꢀ
64.46(9)
64.40(9)
90.06(9)
110.38(10)
91.22(10)
ꢀ
62.13(9)
62.18(9)
87.80(9)
118.24(10)
91.80(9)
168.76(10)
61.18(8)
61.00(8)
89.23(8)
116.10(9)
96.67(8)
169.83(9)
=
N,N’ꢀbis(3,5ꢀdiꢀtertꢀbutylsalicylidene)ethaneꢀ1,2ꢀdiamine),12d
(Salcyc)ScCl(THF) (av. 1.975(5) Å and 2.234(6) Å, Salcyc =
35 (1R,2R)ꢀ(–)ꢀ1,2ꢀcyclohexanediyl)bis(nitrilomethylidyne)bis(2,4ꢀdiꢀ
tertꢀbutyl)phenol)12e and other sixꢀcoordinate scandium hydroxo
complexes with Salen ligands (1.977ꢀ2.028 Å and 2.248ꢀ2.321
Å)12e. It is noteworthy that the ScꢀO(ether) distance of 2.578(2) Å
is much longer than the ScꢀO(THF) bond lengths in
40 [(Salen’)Sc(THF)2][BPh4] (2.177(2) and 2.154(2) Å)12d and
(Salcyc)ScCl(THF) (2.287(5) and 2.284(2) Å),
12e
reflecting that
the interaction between the scandium atom and the oxygen atom of
the diphenyl ether group is very weak. The ScꢀO(Ar) distance of
1.955(2) Å is slightly shorter than the ScꢀO(Salen) distances, but
45 comparable to the ScꢀO(phenolate) bond lengths (1.947(2)ꢀ
1.969(1) Å) in scandium aminoꢀbridged bis(phenolate)
complexes.8l In complex 2, the YbꢀO(Salen) distances of 2.104(2)
and 2.132(2) Å, the YbꢀN(Salen) bond lengths of 2.432(3) and
2.389(3) Å, and the YbꢀO(ether) bond length of 2.514(2) Å
50 compare well with those in complex 1 when the difference in the
ionic radii between Yb and Sc is considered.21 The YbꢀO(Ar)
distance of 2.070(3) Å is consistent with the corresponding bond
4
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yttrium amide.12a Treatment of Ln[N(SiMe3)2]3 with H2L in a 1:1
25 molar ratio in THF for 12 h at room temperature conveniently
afforded the expected ytterbium and yttrium amide LLnN(SiMe3)2
[Ln = Yb (6), Y (7)] as light yellow crystals in good yields (65%
for complex 6, 67% for complex 7) upon crystallization from a
mixture solution of THF and nꢀhexane (Scheme 2). Complexes 6
30 and
7 were characterized by element analysis and NMR
spectroscopy in the case of complex 7, and their molecular
structures were determined by singleꢀcrystal structure analysis.
Both complexes are extremely sensitive to air and moisture, but
they are stable in dry argon atmosphere. Complexes 6 and 7 are
35 freely soluble in THF and toluene, but insoluble in nꢀhexane.
Fig. 1 Molecular structure of complex 1 showing the atom numbering scheme.
Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms
and free solvent molecules are omitted for clarity. Complex 2 is isomorphous
with complex 1.
5
Scheme 2. Synthesis of Complexes 6 and 7
40
Complexes 6 and 7 are isomorphous and only the molecular
diagram of complex 6 is shown in Fig. 3. The selected bond
lengths and angles for complexes 6 and 7 are listed in Table 2. In
complexes 6 and 7, each of the metal centers is sixꢀcoordinated by
one amido group and the pentadentate (N2O3) Schiff base. The
45 coordinate geometry around the metal center can be best described
as a distorted trigonal prism, which is similar to those in complexes
1 and 2. The dihedral angles of the planes of the trigonal faces are
16.0º for 6 and 16.8º for 7. The LnꢀO(Salen) and LnꢀN(Salen) bond
lengths in complexes 6 and 7 are comparable to the corresponding
50 values in complexes 1 and 2 when the difference in metal sizes is
taken in consideration.21 However, the YbꢀO(ether) and YꢀO(ether)
distances of 2.694(4) Å and 2.663(2) Å, respectively, are obviously
longer than the YbꢀO(ether) bond length in complex 2, reflecting
the strong influence of the bulky amido group [N(SiMe3)2] to the
55 metal center. The LnꢀN(amide) distances in complexes 6 and 7
are 2.202(5) and 2.245(2) Å, respectively, which are comparable to
the corresponding bond lengths in βꢀketoiminato ytterbium amides
(2.222ꢀ2.230 Å)22ꢀ23 and yttrium Salen amides, such as
Å),12a
and
Fig. 2 Molecular structure of complex 3 showing the atom numbering scheme.
(Salen’)Y[N(SiHMe2)](THF)
(Salcyc)Y[N(SiHMe2)](THF)
(Salpren)Y[N(SiHMe2)](THF) [2.309 Å, Salpren
dimethylpropandiyl)bis(nitrilomethylidyne)bis(2,4ꢀdiꢀtertꢀ
butyl)phenol ].12d
(2.27
(2.303(2)
Å)24
10 Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms60
and free solvent molecules are omitted for clarity. Complex 5 is isomorphous
with complex 3.
=
(2,2ꢀ
Synthesis and Structure of Ytterbium and Yttrium Amides 6
15 and 7. In order to compare the catalytic behavior of the rareꢀearth
metal Salen aryloxides and amides in the ROP of racꢀLA, the
ytterbium and yttrium amides supported by the pentadentate
(N2O3) Salen ligand were prepared. Generally, amine elimination
reaction is a straightforward method for the synthesis of rareꢀearth
cꢀe
20 metal amides using Ln[N(SiMe3)2]3 as the precursor.12a,
This
synthetic approach did succeed in our case, although the reaction of
Y[N(SiMe3)2]3 with the Salen N,N’ꢀbis(3,5ꢀdiꢀtertꢀ
butylsalicylidene)ethylenediamine did not afford the desired
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30 were 100 and 300, respectively. The high activity of the
pentadentate Salen rareꢀearth metal complexes might be ascribed to
the existence of an additional donor group on the backbone of the
Salen ligand, which is consistent with that found by Williams et al.
in the phosphasalen rareꢀearth metal alkoxide systems.10
35
As shown in Table 3, polymerization media has a great impact
on the catalytic activity, but has no obvious effect on the
stereoselectivity for racꢀLA polymerization. Overall, THF is a
good solvent. For instance, using yttrium aryloxide 3 as the
initiator, a yield of 96% can be reached within 10 min in THF at
40 room temperature when the molar ratio of monomer to initiator is
500, and Pr value of the resulted PLA is 0.81 (Pr = probability of
racemic enchainment; Table 3, entry 9; Fig. S1); whereas the yield
is 90% and Pr is 0.79 in toluene under the same polymerization
conditions (Table 3, entry 10). Furthermore, polymerization
45 performed in CH2Cl2 led to a dramatic decrease in activity, but the
heterotacticity of the resultant PLA dropped slightly (Pr = 0.78,
Table 3, entry 11). The solvent effect on the stereoselectivity in our
cases is quite different from those observed in the amineꢀbridged
bis(phenolate) rareꢀearth metal systems.8b,iꢀk,9a In the latter, the
polymerizations in THF gave apparently higher selectivity than
those in toluene and in CH2Cl2.
Fig. 3 Molecular structure of complex 6 showing the atom numbering scheme.
Thermal ellipsoids are drawn at the 20% probability level, and hydrogen atoms
and free solvent molecules are omitted for clarity. Complex 7 is isomorphous
with complex 6.
5
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for50
Complexes 6 and 7
Bond
The ionic radii of the rareꢀearth metals also affected the catalytic
activity and stereoselectivity for racꢀLA polymerization. The
samarium (4) and neodymium (5) complexes showed the highest
6
7
Bond angles
6
7
lengths
activity among these rareꢀearth metal Salen aryloxides and the
yields exceeded 85% within 5 min when the molar ratio of
monomer to initiator was 500 (Table 3, entries 13 and 16), whereas
the yield was only 71% using the scandium complex (1) as the
initiators even if the molar ratio of monomer to initiator decreased
60 to 100 and the reaction time was extended to 100 min (Table 3,
entry 1). The increasing order of activity Sc << Yb < Y < Sm ≈ Nd
is in agreement with the order of their ionic radii of the central
metals. In contrast, the increasing order in stereoselectivity follows
the decreasing order of the ionic radii, e.g. Nd (Pr = 0.76) < Sm (Pr
= 0.78) < Y (Pr = 0.81) < Yb (Pr = 0.85) ≈ Sc (Pr = 0.85) (Table 3,
entries 1, 2, 8, 13 and 16). The influence of the ionic radii on the
polymerization of racꢀLA in this system is in agreement with the
previous observations: a crowded coordination environment around
the metal center is disadvantageous for polymerization activity, but
is beneficial for stereoselectivity.8b,iꢀk,9a
In order to understand the influence of polymerization
temperature on the heterotacticity of the resultant polymer, the
polymerization of racꢀLA at different temperature was conducted
using ytterbium aryloxide 2 as the initiator (Table 4, entries 3, 5, 6
and 7). The results show that lowering the polymerization
temperature from 25 to 0 ºC resulted in an increase in Pr from 0.85
to 0.93, and a decrease in PDI from 1.30 to 1.06 (Table 3, entries 3
and 7; Fig. S2). A similar phenomenon was also observed in zincꢀ
based polymerization system.25 This change might be attributed to
the depression of transesterification reactions during the racꢀLA
polymerization at 0 ºC. The measured molecular weights (Mn) of
the resulting polymers increased linearly with the monomer
conversion and the molecular weight distribution (PDI) of the
polymers kept almost unchange (1.02ꢀ1.09) (Fig. 4), indicating the
polymerization of racꢀLA initiated by complex 2 proceeds in a
living fashion at 0 ºC.
Ln1ꢀO1
2.694(4)
2.101(4)
2.141(4)
2.471(5)
2.419(5)
2.202(5)
2.663(2)
2.168(2)
2.135(2)
2.459(2)
2.502(2)
2.245(2)
O1ꢀLn1ꢀN1
O1ꢀLn1ꢀN2
O1ꢀLn1ꢀN3
N1ꢀLn1ꢀO3
N1ꢀLn1ꢀN2
O2ꢀLn1ꢀO3
61.38(15)
61.54(16)
97.36(17)
165.96(16)
104.07(17)
93.47(17)
61.66(6)55
61.31(6)
97.76(7)
Ln1ꢀO2
Ln1ꢀO3
Ln1ꢀN1
Ln1ꢀN2
Ln1ꢀN3
106.50(7)
104.92(7)
93.94(6)65
10
ROP of rac-LA Initiated by Rare-earth Metal Salen Complexes
1-7. The catalytic behavior of complexes 1ꢀ7 for the ringꢀopening70
polymerization of racꢀLA was examined and the representative
results are listed in Table 3. It was found that all of these rareꢀearth
15 metal Salen complexes can effectively initiate the ROP of racꢀLA
in THF to give the polymers with high molecular weights and
moderate to narrow molecular weight distributions (PDI = 1.02ꢀ75
1.46). These rareꢀearth metal Salen complexes displayed
a
relatively higher activity in comparison with those reported rareꢀ
20 earth metal Salen complexes.3c,m,15,16 For example, using yttrium
aryloxide 3 as the initiator, polymerization was almost complete in
15 min at room temperature when the molar ratio of monomer to80
initiator was 1000 (Table 3, entry 12). However, using the
binaphthylꢀbridged yttrium Salen complexes as the initiators, it
25 took about 14 h for [(Salbinap)YOCH2CH2NMe2]23c [SalbinapH2 =
2,2’ꢀbis(2ꢀhydroxybenzylideneamino)ꢀ1,1’ꢀbinaphthyl] and 3 h for
(Salbinap’)Y[N(SiHMe2)](THF)15 [Salnap’H2
butylꢀ2ꢀhydroxybenzylideneamino)ꢀ1,1’ꢀbinaphthyl]
complete conversion when the molar ratios of monomer to initiator
=
2,2’ꢀbis(3ꢀtertꢀ85
to reach
6
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Table 3. Polymerization of rac-LA Initiated by Complexes 1-7a
Yield
d
Mcc
(×104)
1.02
Mn
Entry
1
Cat.
1
2
2
2
2
2
2
3
3
3
3
3
4
4
4
5
5
5
6
6
6
7
7
7
7
7
[M]0/[I]0
100
t/min
100
5
(%)b
71
70
95
87
99
91
96
77
96
90
51
92
86
98
96
85
98
98
90
88
95
92
94
85
96
90
(×104)
1.31
PDId
1.07
1.21
1.30
1.23
1.13
1.02
1.06
1.28
1.38
1.35
1.56
1.28
1.34
1.44
1.33
1.38
1.46
1.39
1.19
1.16
1.22
1.23
1.20
1.21
1.24
1.19
Pre
0.85
0.85
0.85
0.85
0.88
0.94
0.93
0.81
0.81
0.79
0.78
0.81
0.78
0.78
0.79
0.76
0.76
0.78
0.89
0.90
0.89
0.89
0.88
0.88
0.86
0.85
2
500
5.05
5.95
3
500
10
15
60
60
75
5
6.85
6.24
4
1000
500
12.54
7.13
11.82
6.91
5f
6g
250
3.28
3.78
7g
250
3.46
4.33
8
500
5.55
5.06
9
500
10
10
10
15
5
6.92
7.21
10h
11i
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
500
6.49
6.05
500
3.68
4.28
1000
500
13.26
6.20
13.29
6.96
500
10
15
5
7.06
7.55
1000
500
13.84
6.13
14.55
6.59
500
10
15
5
7.07
8.21
1000
500
13.56
6.49
14.21
10.81
12.02
21.33
9.22
1000
2000
500
12
60
5
12.68
27.39
6.63
1000
1500
2000
3000
12
20
60
60
13.55
18.38
27.67
38.91
13.47
20.39
25.29
33.67
General polymerization conditions: THF as the solvent, [racꢀLA]0 = 1 mol/L, at 25 °C. Yield: weight of polymer obtained/weight of monomer used. c Mc =
a
b
d
(144.14) ×[M]0/[I]0 × (polymer yield) (%). Measured by GPC calibrated with standard polystyrene samples. e Pr is the probability of racemic linkages between
f
5
monomer units determined from the methane region of the homonuclear decoupled 1H NMR spectrum at 25 °C. At 10 °C. g At 0 °C. h In Toluene . i In CH2Cl2.
earth metal aryloxides, rareꢀearth metal amides showed higher
15 activity for racꢀLA polymerization (Table 3, entries 2, 8, 19, 22).
For example, using ytterbium amide 6 as the initiator, a 90%
yield was achieved in 5 min at room temperature when the molar
ratio of monomer to initiator is 500 (Table 3, entry 19), whereas
the yield was 70% using ytterbium aryloxide 2 as the initiator
20 (Table 3, entry 2). These results are consistent with those
observed in the salalen rareꢀearth metal amide systems9a,i
.
Generally, the organoꢀrareꢀearth metal amides showed worse
controllability for racꢀLA polymerization than the rareꢀearth
metal alkoxides due to the lower nucleophilicity of the silylamide
25 group.8b,9a,i However, rareꢀearth metal amides 6 and 7 displayed
good controllability. The number average molecular weights (Mn)
of the resulting polymers increased with the molar ratio of
monomer to initiator ([M]0/[I]0) (Table 3, entries 19ꢀ26), whereas
the molecular weight distributions (PDI) kept almost unchanged
30 (1.16ꢀ1.24). Furthermore, a linear increase in Mn with monomer
conversion recorded with yttrium amide 7 as initiator, while the
PDIs kept in the range of 1.19ꢀ1.24, as shown in Fig. 5. The
MALDIꢀTOF mass spectrum of the oligomer obtained from the
oligomerization of racꢀLA with complex 7 and quenching with
35 benzyl alcohol (Fig. S3), revealed that almost no
Fig. 4 Plots of Mn and Mw/Mn vs conversion for ROP of racꢀLA initiated by
complex 2. Conditions: THF as solvent, [M]0/[I]0 = 250, [M]0 = 1.0 M, T = 0
10 °C.
The influence of the structure of initiating group on the
catalytic behavior was also investigated. In comparison with rareꢀ
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transesterification side reactions occurred during the
polymerization because the peaks accounting for oligomers with
nonintegral lactide repeat unit are relatively small in
a
comparison with those of oligomers with a integral lactide repeat
unit. These results displayed that the rareꢀearth metal amide
stabilized by pentadentate Salen group can initiate racꢀLA
polymerization in a controlled manner.
5
25 Fig.
7 kapp versus the concentration of complex 2 for the racꢀLA
−1
polymerization at 25°C in THF (kp = 243.7±1.29 L•mol min −1, linear fit,
R2 = 0.996).
To further elucidate the polymerization behavior, the
30 polymerization kinetics of racꢀLA initiated by complex 2 was
measured with different molar ratios of monomer/initiator at 25
ºC in THF. In each case, the conversion increases as the
prolongation of the polymerization time, and a first order kinetics
dependence on the concentration of racꢀLA with a momentary
35 induction time was observed as shown in Fig. 6. This induction
period may be related to the much bulky aryloxide group. To
determine the order in initiator, kapp (the apparent polymerization
rate constant) was plotted versus the concentration of complex 2
(Fig. 7). From this plot, kapp increased linearly with the initiator
40 concentration, manifesting that the order of initiator is firstꢀorder
as well. Therefore, the polymerization of racꢀLA using 2
followed a kinetic equation of the form −d[M]/dt = kapp[M] =
kp[M][I], where kp is the propagation rate constant. To gain some
insights into the ROP mechanism of racꢀLA with these rareꢀearth
45 metal Salen complexes, oligomerization of racꢀLA by complex 2
in a 1:20 molar ratio at room temperature was carried out.
Endgroup analysis of the oligomer by 1H NMR spectroscopy
clearly showed the existence of an aryloxide group (ArOꢀ)
according to the resonances at about 1.56, 2.07, 7.43 ppm, as
50 shown in Fig. S4. Meanwhile, no proton resonance of the Salen
10 Fig. 5 Plots of Mn and Mw/Mn vs conversion for ROP of racꢀLA initiated by
complex 7. Conditions: THF as solvent, [M]0/[I]0 = 2000, [M]0 = 1.0M, T =
25°C.
15 Fig. 6 Kinetics of the racꢀLA polymerization using complex 2 as initiator in
THF at 25 °C, [M]0 = 1 M. (○) [M]0/[I]0 = 1000, kapp = 0.2495 ± 0.0067 minꢀ1
(linear fit, R2 = 0.998); (∆) [M]0/[I]0 = 1500, kapp = 0.1489 ± 0.0038 minꢀ1
(linear fit, R2 = 0.999); (●) [M]0/[I]0 = 2000, kapp = 0.11914 ± 0.00057 minꢀ1
(linear fit, R2 = 0.997); (■) [M]0/[I]0 = 2500, kapp = 0.10434 ± 0.0037 minꢀ1
20 (linear fit, R2 = 0.998).
1
ligand was observed in the H NMR spectrum of the oligomer,
which revealed that the pentadentate (N2O3) Salen group was not
involved in the polymerization process. A MALDIꢀTOF mass
spectrum of the oligomer further revealed that only oligomers
55 with OAr end cap were formed (Fig. 8). These results indicated
that the aryloxide group in complex 2 acted as the initiating group
in the ROP of racꢀLA and the polymerization proceeds via a
common “coordination ꢀinsertion” mechanism. Remarkably,
peaks accounting for linear oligomers with a nonintegral lactide
60 repeat unit also could be found in the MALDIꢀTOF mass
spectrum, which indicating that there are intermolecular
transesterfication reactions during the racꢀLA polymerization and
the polymerization of racꢀLA initiated by the rareꢀearth metal
aryloxides at room temperature is not well controlled.
65
8
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DOI: 10.1039/C5CY00322A
40
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45
50
55
60
4
Fig. 8 MALDIꢀTOF mass spectrum of racꢀLA oligomer initiated by complex 2
([M]0/[I]0 =20:1, in THF, 25 °C; doped with CF3CO2Na).
65
Conclusions
5
In summary, a new pentadentate (N2O3) Salen group was first
introduced into organoꢀrareꢀearth metal chemistry, and a series of
neutral rareꢀearth metal amido and aryloxo complexes stabilized
by this ligand were synthesized via protolysis reactions using
(ArO)3Ln(THF) and Ln[N(SiMe3)2]3 as the precursors,
70
10 respectively, and their structural features have been provided by
an Xꢀray diffraction study. It was found that all of these rareꢀearth
metal Salen complexes are efficient initiators for the ROP of racꢀ
LA and display good activity and hetereoselectivity. The
observed activityꢀincreasing order Sc << Yb < Y < Sm ≈ Nd is in
15 agreement with the order of the ionic radii, but the heterotacticity
follows the opposite sequence. Lowering the polymerization
temperature caused an increase in heterotacticity. The results
presented here indicated that the Salen ligand systems have great
potential application in designing highly active polymerization
20 initiators.
75
80
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85
Acknowledgements
90
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Financial support from the National Natural Science Foundation
of China (Grants 21174095, 21172165 and 21132002), the PAPD,
the Major Research Project of the Natural Science Foundation of
25 the Jiangsu Higher Education Institutions (Project 14KJA150007)
and the Qing Lan Project is gratefully acknowledged.
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