Group 4 metal complexes with new chiral pincer NHC-ligands: Synthesis, structure and catalytic activity

Article abstract of DOI:10.1039/c4dt00510d

Chiral group 4 NHC-metal complexes were prepared in good yields by amine elimination from M(NR₂)₄ (M = Ti, Zr, Hf; R = Me, Et) and chiral pincer NHC-ligands, L4 (L4a and L4b), L5 and L6, which are derived from (S,S)-diphenyl-1,2-ethanediamine. Treatment of M(NR₂)₄ with 1 equiv. of L4 in THF gives, after recrystallization from a benzene solution, the chiral titanium amides (L4)Ti(NMe₂)(Br)(THF) (7) and (L4)Ti(NMe₂)(Cl)(THF) (11), zirconium amides (L4)Zr(NMe ₂)(Br)(THF) (8), (L4)Zr(NEt₂)(Br)(THF) (10), (L4)Zr(NMe₂)(Cl)(THF) (12) and (L4)Zr(NEt₂)(Cl)(THF) (14), and hafnium amides (L4)Hf(NMe₂)(Br)(THF) (9) and (L4)Hf(NMe ₂)(Cl)(THF) (13), respectively. Similarly, the reactions of L5 or L6 with 1 equiv. of M(NR₂)₄ yield the titanium amide (L6)Ti(NMe₂)(Cl)(THF) (16), the zirconium amides (L5)Zr(NMe ₂)(Cl)(THF) (15), (L6)Zr(NMe₂)(Cl)(THF) (17) and (L6)Zr(NEt₂)(Cl)(THF) (19), and the hafnium amide (L6)Hf(NMe ₂)(Cl)(THF) (18), respectively. Complexes 7-19 were characterized by various spectroscopic techniques and elemental analyses. The molecular structures of 10 and 14-19 were also established by X-ray diffraction analyses, which represent the first example of the structurally characterized group 4 chiral NHC-metal complex. Furthermore, 7-19 are active catalysts for the polymerization of rac-lactide in the presence of isopropanol, leading to the heterotactic-rich polylactides. the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00510d

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Group 4 metal complexes with new chiral
pincer NHC-ligands: synthesis, structure
and catalytic activity†
Cite this: Dalton Trans., 2014, 43,
8261
Ning Zhao,a Guohua Hou,a Xuebin Deng,a Guofu Zi*a and Marc D. Walter*b
Chiral group 4 NHC–metal complexes were prepared in good yields by amine elimination from M(NR2)4
(M = Ti, Zr, Hf; R = Me, Et) and chiral pincer NHC-ligands, L4 (L4a and L4b), L5 and L6, which are derived
from (S,S)-diphenyl-1,2-ethanediamine. Treatment of M(NR2)4 with
1 equiv. of L4 in THF gives,
after recrystallization from a benzene solution, the chiral titanium amides (L4)Ti(NMe2)(Br)(THF) (7)
and (L4)Ti(NMe2)(Cl)(THF) (11), zirconium amides (L4)Zr(NMe2)(Br)(THF) (8), (L4)Zr(NEt2)(Br)(THF) (10),
(L4)Zr(NMe2)(Cl)(THF) (12) and (L4)Zr(NEt2)(Cl)(THF) (14), and hafnium amides (L4)Hf(NMe2)(Br)(THF) (9)
and (L4)Hf(NMe2)(Cl)(THF) (13), respectively. Similarly, the reactions of L5 or L6 with 1 equiv. of M(NR2)4
yield the titanium amide (L6)Ti(NMe2)(Cl)(THF) (16), the zirconium amides (L5)Zr(NMe2)(Cl)(THF) (15),
(L6)Zr(NMe2)(Cl)(THF) (17) and (L6)Zr(NEt2)(Cl)(THF) (19), and the hafnium amide (L6)Hf(NMe2)(Cl)(THF) (18),
respectively. Complexes 7–19 were characterized by various spectroscopic techniques and elemental
analyses. The molecular structures of 10 and 14–19 were also established by X-ray diffraction analyses,
which represent the first example of the structurally characterized group 4 chiral NHC–metal complex.
Furthermore, 7–19 are active catalysts for the polymerization of rac-lactide in the presence of
isopropanol, leading to the heterotactic-rich polylactides.
Received 18th February 2014,
Accepted 6th March 2014
DOI: 10.1039/c4dt00510d
www.rsc.org/dalton
magnesium, zinc, calcium, aluminum, lanthanides, tin, group
4 metals, germanium, indium and iron,1d,4 but among these,
Introduction
Biodegradable polymers derived from renewable resources the chiral group 4 catalysts are especially promising.5 Unfortu-
such as polylactides have received much attention over the nately, compared to other metals, structurally well-character-
past decade because of their attractive physical and mechan- ized chiral group 4 complexes that initiate the controlled ring-
ical properties.1 In addition, the chain stereochemistry deter- opening polymerization of lactides are still scarce.5
mines the polymer properties and the rate of degradation.2 For
In recent years, transition-metal complexes with chiral
example, whereas the enantiopure polylactide melts at 180 °C, N-heterocyclic carbene (NHC) ligands have become increas-
a much higher melting temperature (230 °C) is found for ingly popular because of their stability to air and moisture and
stereocomplex polymers formed by an equivalent mixture of their strong σ-donating, but poor π-accepting abilities.6 An
poly(L-lactide) and poly(D-lactide).3 Therefore, the polymeri- additional driving force is the longstanding interest in cata-
zation of rac-lactide with stereoselective catalysts remains a lysts for enantioselective reactions such as olefin metathesis,7
challenge and an opportunity for chemists. To date, numerous conjugate addition of enones,8 allylic alkylations,9 olefin
reviews have covered catalyst systems for the ring-opening hydrogenations10 and hydrosilylations.11 Encouraged by the
polymerization (ROP) of cyclic esters based on metals such as attractive features of chiral NHC-ligands in general, we are now
focusing on the preparation of group 4 metal complexes co-
ordinated by chiral multi-dentate NHC-ligands, and to our
knowledge no chiral group 4 metal NHC-catalyst has yet been
structurally authenticated.6,12 More recently, we have designed
and prepared a new series of tridentate chiral pincer NHC-
ligands L4–L6 from (S,S)-diphenyl-1,2-ethanediamine, and
found them to be useful ligands for group 4 metals, which are
potential catalysts for the polymerization of lactides. Herein,
we report on the synthesis of these chiral NHC-ligands, their
use in group 4 chemistry, and the application of the resulting
aDepartment of Chemistry, Beijing Normal University, Beijing 100875, China.
E-mail: gzi@bnu.edu.cn; Fax: +86-10-58802075; Tel: +86-10-58806051
bInstitut für Anorganische und Analytische Chemie, Technische Universität
Braunschweig, Hagenring 30, 38106 Braunschweig, Germany.
E-mail: mwalter@tu-bs.de; Fax: +49-531-3915387; Tel: +49-531-3915312
†Electronic supplementary information (ESI) available: NMR spectra of
a
representative polymer sample. CCDC 975119–975120 and 975123–975128.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4dt00510d
This journal is © The Royal Society of Chemistry 2014
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complexes as catalysts in the polymerization of rac-lactide (rac- (hexane–ethyl acetate = 10 : 1) by a similar procedure as out-
LA).
lined in the synthesis of 1. Yield: 2.47 g (92%) (Found: C,
80.53; H, 8.30; N, 5.30. C36H44N2O2 requires: C, 80.56; H, 8.26;
1
N, 5.22%). H NMR (C6D6): δ 10.84 (s, 2H, OH), 7.21–7.09 (m,
12H, aryl), 6.62 (m, 4H, aryl), 4.06 (s, 2H, CH), 3.78 (d, J = 13.4
Hz, 2H, CH2), 3.59 (d, J = 13.4 Hz, 2H, CH2), 3.33 (s, 2H, NH),
1.38 (s, 18H, C(CH3)3). 13C NMR (C6D6): δ 156.1, 137.1, 135.7,
127.1, 126.8, 126.7, 126.6, 126.4, 125.7, 125.2, 121.7, 117.6,
Experimental
General methods
Group 4 complexes and catalytic reactions were performed
under an atmosphere of dry dinitrogen with rigid exclusion of
air and moisture using standard Schlenk or cannula tech-
niques, or a glovebox. All organic solvents were freshly distilled
from sodium benzophenone ketyl immediately prior to use.
Racemic lactide (rac-LA) was recrystallized twice from dry
toluene and then sublimed under vacuum prior to use. All
chemicals were purchased from Aldrich Chemical Co. and
Beijing Chemical Co. and 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 chromato-
65.6, 49.4, 33.7, 28.6. IR (KBr, cm−1): ν 3427 (s), 2957 (s), 1590
ˉ
(m), 1437 (s), 1389 (m), 1235 (s), 1087 (s), 749 (s).
Preparation of 3. This compound was prepared as colorless
oil from the reaction of 3,5-di-tert-butyl-salicylaldehyde (2.34 g,
10.0 mmol) with (S,S)-diphenyl-1,2-ethanediamine (1.06 g,
5.0 mmol) in absolute ethanol (30 mL) at room temperature,
followed by reduction with NaBH4 (2.00 g, 52.6 mmol) in
ethanol, and purification by flash column chromatography
(hexane–ethyl acetate = 10 : 1) by a similar procedure as out-
lined in the synthesis of 1. Yield: 2.92 g (90%) (Found: C,
81.40; H, 9.40; N, 4.30. C44H60N2O2 requires: C, 81.43; H, 9.32;
graphy (GPC) using a PL-GPC 50 apparatus. H and 13C NMR
1
1
N, 4.32%). H NMR (C6D6): δ 7.08–7.01 (m, 10H, aryl), 6.97 (d,
spectra were recorded on a Bruker AV 500 spectrometer at 500
and 125 MHz, respectively. All chemical shifts are reported in δ
units with reference to the residual protons of the deuterated
solvents for proton and carbon chemical shifts. Melting points
were measured on X-6 melting point apparatus and were
uncorrected. Elemental analyses were performed on a Vario EL
elemental analyzer.
J = 2.1 Hz, 2H, aryl), 6.55 (d, J = 2.1 Hz, 2H, aryl), 3.84 (d, J =
5.2 Hz, 2H, CH2), 3.72 (s, 2H, CH), 3.42 (d, J = 5.2 Hz, 2H,
CH2), 1.25 (s, 18H, C(CH3)3), 1.08 (s, 18H, C(CH3)3); NH and
OH protons were not observed. 13C NMR (C6D6): δ 154.0, 149.4,
142.3, 142.2, 139.8, 137.5, 135.3, 127.3, 127.1, 127.0, 126.9,
126.8, 66.1, 50.3, 34.4, 33.3, 30.9, 29.1. IR (KBr, cm−1): ν 3428
ˉ
(s), 2960 (s), 1604 (m), 1480 (s), 1233 (s), 875 (s).
Preparation of L4a. Compound 1 (2.00 g, 4.71 mmol),
NH4Br (0.69 g, 7.07 mmol) and triethyl orthoformate (15 mL)
Syntheses
Preparation of 1. Salicylaldehyde (1.22 g, 10.0 mmol) was were heated at 120 °C for one day. After the reaction mixture
mixed with (S,S)-diphenyl-1,2-ethanediamine (1.06 g, was cooled to room temperature, diethyl ether (40 mL) was
5.0 mmol) in absolute ethanol (30 mL) and stirred for 4 h at added to precipitate a colorless solid, which was filtered and
room temperature. NaBH4 (2.00 g, 52.6 mmol) was added in washed with diethyl ether to give L4a as a colorless solid.
small portions at 0 °C, and the solution was then warmed to Yield: 2.23 g (92%) (Found: C, 67.54; H, 5.30; N, 5.42.
50 °C and kept at this temperature for 2 h. The solvent was C29H27N2BrO2 requires: C, 67.58; H, 5.28; N, 5.43%). M.p.:
1
removed, the residue was treated with H2O (20 mL), extracted 273–275 °C. H NMR (DMSO-d6): δ 10.17 (s, 2H, OH), 9.13 (s,
with ethyl acetate (20 mL × 3) and washed with brine (20 mL). 1H, NCHN), 7.40 (m, 6H, aryl), 7.17 (m, 6H, aryl), 6.95 (m, 4H,
The combined organic layers were dried with anhydrous aryl), 6.75 (m, 2H, aryl), 4.83 (d, J = 14.3 Hz, 2H, CH2), 4.70 (s,
Na2SO4 and the solvent was removed under reduced pressure. 2H, CH), 4.10 (d, J = 14.3 Hz, 2H, CH2). 13C NMR (DMSO-d6): δ
The residue was further purified by flash column chromato- 158.3, 156.0, 135.8, 130.8, 130.3, 129.5, 129.4, 127.0, 119.1,
graphy (hexane–ethyl acetate = 10 : 1) to give 1 as a colorless 118.7, 115.6, 71.5, 45.9. IR (KBr, cm−1): ν 3435 (m), 3175 (s),
ˉ
oil. Yield: 2.01 g (95%) (Found: C, 79.12; H, 6.60; N, 6.64. 2927 (m), 1648 (s), 1599 (s), 1461 (s), 1373 (s), 1277 (s), 1106 (s),
C28H28N2O2 requires: C, 79.22; H, 6.65; N, 6.60%). 1H NMR 757 (s).
(C6D6): δ 7.14–7.07 (m, 4H, aryl), 6.91 (m, 6H, aryl), 6.68 (m,
Preparation of L4b. This compound was prepared as a color-
2H, aryl), 6.58 (m, 2H, aryl), 6.52 (m, 4H, aryl), 3.57 (s, 2H, less solid from the reaction of 1 (2.00 g, 4.71 mmol), NH4Cl
CH), 3.51 (d, J = 13.8 Hz, 2H, CH2), 3.19 (d, J = 13.8 Hz, 2H, (0.38 g, 7.07 mmol) and triethyl orthoformate (15 mL) at
CH2); NH and OH protons were not observed. 13C NMR (C6D6): 120 °C, followed by washing with diethyl ether by a similar
δ 158.9, 138.3, 129.4, 129.0, 128.5, 128.3, 128.2, 128.1, 127.9, procedure as described for the synthesis of L4a. Yield: 2.11 g
127.8, 66.7, 50.1. IR (KBr, cm−1): ν 3427 (s), 2853 (w), 1616 (s), (95%) (Found: C, 73.90; H, 5.78; N, 5.93. C29H27N2ClO2
ˉ
1588 (s), 1489 (s), 1454 (s), 1251 (s), 755 (s).
requires: C, 73.95; H, 5.78; N, 5.95%). M.p.: 260–262 °C. 1H
Preparation of 2. This compound was prepared as colorless NMR (DMSO-d6): δ 10.48 (s, 2H, OH), 9.20 (s, 1H, NCHN), 7.38
oil from the reaction of 3-tert-butylsalicylaldehyde (1.78 g, (m, 6H, aryl), 7.17 (m, 6H, aryl), 7.07 (m, 2H, aryl), 6.93 (m,
10.0 mmol) with (S,S)-diphenyl-1,2-ethanediamine (1.06 g, 2H, aryl), 6.72 (m, 2H, aryl), 4.85 (d, J = 14.3 Hz, 2H, CH2), 4.68
5.0 mmol) in absolute ethanol (30 mL) at room temperature, (s, 2H, CH), 4.09 (d, J = 14.3 Hz, 2H, CH2). 13C NMR (DMSO-
followed by reduction with NaBH4 (2.00 g, 52.6 mmol) in d6): δ 158.3, 156.2, 135.9, 130.7, 130.1, 129.5, 129.4, 127.1,
ethanol, and purification by flash column chromatography 119.0, 118.7, 115.8, 71.5, 46.0. IR (KBr, cm−1): ν 3424 (m),
ˉ
8262 | Dalton Trans., 2014, 43, 8261–8272
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3093 (s), 2954 (m), 1648 (s), 1600 (s), 1461 (s), 1374 (s), 1277 (Found: C, 58.27; H, 5.42; N, 5.80. C35H38N3BrO3Zr requires: C,
(s), 1197 (s), 1108 (s), 757 (s).
58.40; H, 5.32; N, 5.84%). M.p.: 177–179 °C (dec.). 1H NMR
Preparation of L5. This compound was prepared as a color- (C6D6): δ 7.20 (m, 2H, aryl), 6.90 (m, 8H, aryl), 6.68 (m, 8H, aryl),
less solid from the reaction of 2 (2.00 g, 3.73 mmol), NH4Cl 4.54 (br s, 2H, CH2), 3.80 (m, 6H, CH2 and THF), 3.38 (m, 8H,
(0.30 g, 5.60 mmol) and triethyl orthoformate (15 mL) at CH and NCH3), 1.36 (m, 4H, THF). 13C NMR (C6D6): δ 209.0,
120 °C, followed by washing with diethyl ether by a similar 162.1, 138.1, 137.8, 137.2, 129.3, 129.2, 128.3, 125.7, 119.5, 119.2,
procedure as described for the synthesis of L4a. Yield: 1.83 g 118.8, 117.9, 75.4, 69.4, 47.0, 45.5, 24.8. IR (KBr, cm−1): ν 2963
ˉ
(84%) (Found: C, 76.21; H, 7.43; N, 4.82. C37H43N2ClO2 (m), 1595 (w), 1384 (m), 1260 (s), 1091 (s), 1018 (s), 798 (s).
requires: C, 76.20; H, 7.43; N, 4.80%). M.p.: 240–242 °C. 1H
Preparation of (L4)Hf(NMe2)(Br)(THF) (9). This compound
NMR (DMSO-d6): δ 9.25 (s, 1H, NCHN), 9.10 (s, 2H, OH), was prepared as colorless microcrystals from the reaction of
7.41–7.32 (m, 6H, aryl), 7.21 (m, 2H, aryl), 7.10 (m, 4H, aryl), L4a (0.26 g, 0.50 mmol) with Hf(NMe2)4 (0.18 g, 0.50 mmol) in
6.70 (m, 4H, aryl), 5.33 (d, J = 14.6 Hz, 2H, CH2), 4.54 (s, 2H, THF (20 mL) and recrystallization from a benzene solution by
CH), 4.17 (d, J = 14.6 Hz, 2H, CH2), 1.40 (s, 18H, C(CH3)3). 13C a similar procedure to the synthesis of 7. Yield: 0.29 g (72%)
NMR (DMSO-d6): δ 166.0, 157.0, 154.3, 139.5, 136.0, 129.5, (Found: C, 52.15; H, 4.67; N, 5.26. C35H38N3BrO3Hf requires:
1
129.3, 129.0, 127.4, 127.0, 121.9, 120.3, 71.2, 47.0, 34.5, 29.7. C, 52.09; H, 4.75; N, 5.21%). M.p.: 230–232 °C (dec.). H NMR
IR (KBr, cm−1): ν 3412 (m), 3090 (s), 2956 (s), 1638 (s), 1589 (C6D6): δ 7.24 (m, 2H, aryl), 6.90 (m, 8H, aryl), 6.70 (m, 8H,
ˉ
(m), 1438 (s), 1378 (s), 1208 (s), 1093 (s), 752 (s). Colorless crys- aryl), 4.53 (br s, 2H, CH2), 3.78 (m, 6H, CH and THF), 3.54 (s,
tals of 2(L5)·C6H6 suitable for X-ray structural analysis were 6H, NCH3), 3.30 (br s, 2H, CH2), 1.36 (m, 4H, THF). 13C NMR
grown from a benzene solution.
(C6D6): δ 213.6, 163.1, 138.7, 137.8, 130.1, 129.6, 128.8, 128.4,
Preparation of L6. This compound was prepared as a color- 127.6, 126.1, 120.5, 120.1, 118.0, 76.0, 69.5, 47.3, 45.2, 25.4. IR
less solid from the reaction of 3 (2.00 g, 3.08 mmol), NH4Cl (KBr, cm−1): ν 2962 (s), 1595 (m), 1451 (m), 1384 (s), 1260 (s),
ˉ
(0.25 g, 4.62 mmol) and triethyl orthoformate (15 mL) at 1090 (s), 1019 (s), 798 (s).
120 °C, followed by washing with diethyl ether by a similar
Preparation of (L4)Zr(NEt2)(Br)(THF) (10). This compound
procedure as in the synthesis of L4a. Yield: 1.84 g (86%) was prepared as pale yellow crystals from the reaction of L4a
(Found: C, 77.69; H, 8.53; N, 4.05. C45H59N2ClO2 requires: C, (0.26 g, 0.50 mmol) with Zr(NEt2)4 (0.19 mg, 0.50 mmol) in
77.72; H, 8.55; N, 4.03%). M.p.: 235–237 °C. 1H NMR (DMSO-d6): THF (20 mL) and recrystallization from a benzene solution by
δ 9.22 (s, 1H, NCHN), 8.84 (s, 2H, OH), 7.34 (m, 2H, aryl), 7.31 a similar procedure to the synthesis of 7. Yield: 0.27 g (73%)
(m, 4H, aryl), 7.18 (d, J = 2.0 Hz, 2H, aryl), 7.04 (m, 4H, aryl), (Found: C, 59.50; H, 5.57; N, 5.64. C37H42N3BrO3Zr requires: C,
6.61 (d, J = 2.0 Hz, 2H, aryl), 5.32 (d, J = 14.5 Hz, 2H, CH2), 4.44 59.42; H, 5.66; N, 5.62%). M.p.: 120–122 °C (dec.). 1H NMR
(s, 2H, CH), 4.17 (d, J = 14.6 Hz, 2H, CH2), 1.38 (s, 18H, C(CH3)3), (C6D6): δ 7.18 (m, 4H, aryl), 6.94 (m, 6H, aryl), 6.81 (m, 2H,
1.14 (s, 18H, C(CH3)3). 13C NMR (DMSO-d6): δ 157.0, 151.8, aryl), 6.66 (m, 6H, aryl), 5.33 (br s, 1H, CH), 4.56 (br s, 3H, CH
141.8, 138.5, 135.9, 129.5, 129.2, 127.2, 125.9, 123.9, 120.8, 71.1, and CH2), 4.26 (br s, 2H, CH2), 3.95 (m, 2H, THF), 3.83 (m, 2H,
47.4, 34.7, 33.7, 31.2, 29.7. IR (KBr, cm−1): ν 3422 (w), 3085 (w), THF), 3.62 (br s, 2H, CH2), 3.35 (br s, 2H, CH2), 1.36 (m, 4H,
ˉ
2957 (s), 1637 (s), 1482 (s), 1362 (m), 1201 (s), 699 (s).
THF), 1.25 (m, 6H, N(CH2CH3)2). 13C NMR (C6D6): δ 209.2,
Preparation of (L4)Ti(NMe2)(Br)(THF) (7). While stirring a 162.1, 137.6, 137.4, 129.6, 129.3, 129.2, 128.7, 128.3, 125.6,
THF solution (5 mL) of Ti(NMe2)4 (0.11 g, 0.50 mmol) was 119.1, 118.2, 117.9, 75.6, 70.0, 47.3, 46.2, 24.7, 14.9. IR (KBr,
slowly added to a THF (15 mL) suspension of L4a (0.26 g, cm−1): ν 2962 (s), 1595 (s), 1480 (s), 1453 (s), 1260 (s), 1090 (s),
ˉ
0.50 mmol) at room temperature. After this mixture was stirred 1019 (s), 797 (s).
overnight at room temperature, the solution was filtered and
Preparation of (L4)Ti(NMe2)(Cl)(THF) (11). This compound
the solvent was removed under reduced pressure. The resulting was prepared as red microcrystals from the reaction of L4b
red solid was recrystallized from a benzene solution to give 7 (0.24 g, 0.50 mmol) with Ti(NMe2)4 (0.11 g, 0.50 mmol) in THF
as red microcrystals. Yield: 0.25 g (74%) (Found: C, 61.96; H, (20 mL) and recrystallization from a benzene solution by a
5.62; N, 6.25. C35H38N3BrO3Ti requires: C, 62.14; H, 5.66; N, similar procedure to the synthesis of 7. Yield: 0.22 g (70%)
6.21%). M.p.: 145–147 °C (dec.). 1H NMR (C6D6): δ 7.19 (m, 2H, (Found: C, 66.58; H, 6.05; N, 6.60. C35H38N3ClO3Ti requires: C,
aryl), 6.93 (m, 2H, aryl), 6.85 (m, 4H, aryl), 6.70 (m, 4H, aryl), 66.51; H, 6.06; N, 6.65%). M.p.: 154–156 °C (dec.). 1H NMR
6.55 (m, 6H, aryl), 4.56 (br s, 1H, CH), 4.38 (m, 3H, CH and (C6D6): δ 7.20 (m, 2H, aryl), 6.95 (m, 4H, aryl), 6.86 (m, 6H,
CH2), 3.74 (s, 6H, NCH3), 3.67 (m, 4H, THF), 3.49 (br s, 2H, aryl), 6.67 (m, 6H, aryl), 4.49 (d, J = 10.1 Hz, 2H, CH2), 4.43 (d,
CH2), 1.41 (m, 4H, THF). 13C NMR (C6D6): δ 208.4, 164.3, 163.8, J = 10.1 Hz, 2H, CH2), 3.86 (br s, 6H, CH and THF), 3.62 (s, 6H,
136.6, 129.4, 128.9, 128.6, 128.2, 127.8, 118.8, 118.1, 117.8, 75.1, NCH3), 1.41 (m, 4H, THF). 13C NMR (C6D6): δ 209.4, 166.1,
68.0, 53.1, 48.1, 25.0. IR (KBr, cm−1): ν 2962 (m), 1593 (s), 137.5, 129.4, 128.9, 128.2, 127.8, 126.0, 120.2, 119.5, 118.5,
ˉ
1482 (s), 1450 (s), 1260 (s), 1108 (s), 1034 (s), 886 (s), 799 (s).
118.3, 117.6, 75.4, 67.4, 52.9, 47.8, 25.0. IR (KBr, cm−1): ν 2962
ˉ
Preparation of (L4)Zr(NMe2)(Br)(THF) (8). This compound (m), 1593 (m), 1480 (m), 1452 (m), 1260 (s), 1090 (s), 1018 (s),
was prepared as pale yellow microcrystals from the reaction of 798 (s).
L4a (0.26 g, 0.50 mmol) with Zr(NMe2)4 (0.14 g, 0.50 mmol) in
Preparation of (L4)Zr(NMe2)(Cl)(THF) (12). This compound
THF (20 mL) and recrystallization from a benzene solution by was prepared as pale yellow crystals from the reaction of L4b
a similar procedure to the synthesis of 7. Yield: 0.24 g (68%) (0.24 g, 0.50 mmol) with Zr(NMe2)4 (0.14 g, 0.50 mmol) in THF
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(20 mL) and recrystallization from a benzene solution by a
Preparation of (L6)Ti(NMe2)(Cl)(THF) (16). This compound
similar procedure to the synthesis of 7. Yield: 0.22 g (66%) was prepared as red crystals from the reaction of L6 (0.35 g,
(Found: C, 62.22; H, 5.66; N, 6.22. C35H38N3ClO3Zr requires: C, 0.50 mmol) with Ti(NMe2)4 (0.11 g, 0.50 mmol) in THF
62.24; H, 5.67; N, 6.22%). M.p.: 163–165 °C (dec.). 1H NMR (20 mL) and recrystallization from a benzene solution by a
(C6D6): δ 7.24 (m, 2H, aryl), 6.95 (m, 8H, aryl), 6.70 (m, 8H, similar procedure to the synthesis of 7. Yield: 0.28 g (65%)
aryl), 4.53 (m, 2H, CH2), 3.83 (s, 2H, CH), 3.63 (br s, 6H, CH2 (Found: C, 71.54; H, 8.23; N, 4.93. C51H70N3ClO3Ti requires: C,
and THF), 3.44 (br s, 3H, NCH3), 3.32 (br s, 3H, NCH3), 1.39 71.52; H, 8.24; N, 4.91%). M.p.: 143–145 °C (dec.). 1H NMR
(m, 4H, THF). 13C NMR (C6D6): δ 210.1, 162.6, 138.5, 137.9, (C6D6): δ 7.63 (d, J = 10.1 Hz, 2H, aryl), 6.97 (m, 4H, aryl), 6.86
137.3, 130.0, 129.6, 128.6, 128.3, 125.4, 119.3, 118.5, 118.0, (m, 6H, aryl), 6.40 (m, 2H, aryl), 4.58 (m, 3H, CH and CH2),
75.7, 68.1, 46.4, 45.5, 25.5. IR (KBr, cm−1): ν 2963 (m), 1593 4.16 (br s, 1H, CH), 3.57 (m, 4H, THF), 3.53 (s, 6H, NCH3), 3.21
ˉ
(w), 1481 (m), 1384 (m), 1260 (s), 1091 (s), 1018 (s), 798 (s).
(br s, 2H, CH2), 1.98 (s, 9H, C(CH3)3), 1.92 (s, 9H, C(CH3)3),
Preparation of (L4)Hf(NMe2)(Cl)(THF) (13). This compound 1.38 (m, 4H, THF), 1.25 (s, 18H, C(CH3)3). 13C NMR (C6D6), δ
was prepared as colorless microcrystals from the reaction of 201.8, 157.3, 152.3, 140.9, 139.1, 136.5, 135.5, 128.2, 127.2,
L4b (0.24 g, 0.50 mmol) with Hf(NMe2)4 (0.18 g, 0.50 mmol) in 126.8, 125.7, 124.3, 120.1, 70.4, 66.4, 48.5, 40.4, 34.3, 32.8,
THF (20 mL) and recrystallization from a benzene solution by 30.2, 29.1, 25.8. IR (KBr, cm−1): ν 2962 (s), 1602 (w), 1443 (m),
ˉ
a similar procedure to the synthesis of 7. Yield: 0.27 g (70%) 1384 (m), 1260 (s), 1091 (s), 1018 (s), 799 (s).
(Found: C, 55.11; H, 5.02; N, 5.56. C35H38N3ClHfO3 requires: C,
Preparation of (L6)Zr(NMe2)(Cl)(THF) (17). This compound
55.12; H, 5.02; N, 5.51%). M.p.: 214–216 °C (dec.). 1H NMR was prepared as pale yellow crystals from the reaction of L6
(C6D6): δ 7.23 (m, 2H, aryl), 6.95 (m, 8H, aryl), 6.65 (m, 8H, (0.35 g, 0.50 mmol) with Zr(NMe2)4 (0.14 g, 0.50 mmol) in THF
aryl), 4.54 (m, 2H, CH2), 3.86 (s, 2H, CH), 3.73–3.24 (m, 12H, (20 mL) and recrystallization from a benzene solution by a
CH2 and THF and NCH3), 1.39 (m, 4H, THF). 13C NMR (C6D6): similar procedure to the synthesis of 7. Yield: 0.28 g (63%)
δ 214.5, 164.3, 138.5, 137.4, 129.9, 129.6, 129.1, 128.5, 127.7, (Found: C, 68.04; H, 7.85; N, 4.66. C51H70N3ClO3Zr requires C,
125.4, 120.1, 119.2, 117.5, 76.1, 67.7, 47.8, 47.0, 25.5. IR (KBr, 68.08; H, 7.84; N, 4.67%). M.p.: 158–160 °C (dec.). 1H NMR
cm−1): ν 2962 (m), 1594 (w), 1450 (m), 1384 (m), 1260 (s), 1091 (C6D6): δ 7.57 (m, 4H, aryl), 7.09 (m, 4H, aryl), 7.02 (m, 2H,
ˉ
(s), 1019 (s), 798 (s).
aryl), 6.92 (m, 4H, aryl), 4.40 (br s, 2H, CH2), 3.63 (br s, 8H,
Preparation of (L4)Zr(NEt2)(Cl)(THF) (14). This compound CH, CH2 and THF), 3.20 (s, 6H, NCH3), 1.91 (s, 9H, C(CH3)3),
was prepared as pale yellow crystals from the reaction of L4b 1.89 (s, 9H, C(CH3)3), 1.22 (s, 13H, C(CH3)3 and THF), 1.20 (s,
(0.24 g, 0.50 mmol) with Zr(NEt2)4 (0.19 g, 0.50 mmol) in THF 9H, C(CH3)3). 13C NMR (C6D6): δ 206.2, 160.2, 139.3, 137.9,
(20 mL) and recrystallization from a benzene solution by a 137.6, 129.0, 128.9, 128.8, 128.7, 125.5, 125.4, 125.0, 124.1,
similar procedure to the synthesis of 7. Yield: 0.26 g (73%) 78.8, 68.1, 45.3, 39.1, 34.0, 33.9, 30.5, 30.4, 25.5. IR (KBr,
(Found: C, 63.21; H, 6.06; N, 5.96. C37H42N3ClO3Zr requires: C, cm−1): ν 2962 (s), 1384 (s), 1260 (s), 1092 (s), 1020 (s), 799 (s).
ˉ
63.18; H, 6.02; N, 5.97%). M.p.: 200–202 °C (dec.). 1H NMR
Preparation of (L6)Hf(NMe2)(Cl)(THF) (18)·2THF (18·2THF).
(C6D6): δ 7.18 (m, 2H, aryl), 6.92 (m, 8H, aryl), 6.81 (m, 4H, This compound was prepared as colorless crystals from the
aryl), 6.73 (m, 4H, aryl), 5.38 (br s, 1H, CH), 4.57 (br s, 4H, reaction of L6 (0.35 g, 0.50 mmol) with Hf(NMe2)4 (0.18 g,
CH2), 4.27 (br s, 1H, CH), 3.81 (m, 2H, THF), 3.73 (m, 2H, 0.50 mmol) in THF (20 mL) and recrystallization from a
THF), 3.58 (br s, 2H, CH2), 3.36 (br s, 2H, CH2), 1.37 (m, 4H, benzene solution by a similar procedure to the synthesis of 7.
THF), 1.26 (m, 6H, N(CH2CH3)2). 13C NMR (C6D6): δ 209.3, Yield: 0.40 g (71%) (Found: C, 62.63; H, 7.67; N, 3.73.
161.5, 136.8, 128.9, 128.5, 128.0, 127.5, 126.5, 125.0, 118.6, C59H86N3ClHfO5 requires: C, 62.64; H, 7.66; N, 3.71%). M.p.:
117.4, 117.1, 74.8, 68.3, 46.6, 45.4, 24.1, 14.4. IR (KBr, cm−1): 110–112 °C (dec.). 1H NMR (C6D6): δ 7.59 (s, 2H, aryl), 7.09 (m,
ν 2962 (m), 1594 (w), 1481 (m), 1384 (m), 1260 (s), 1089 (s), 4H, aryl), 7.02 (m, 4H, aryl), 6.93 (d, J = 6.8 Hz, 2H, aryl), 6.11
ˉ
1017 (s), 797 (s).
(m, 2H, aryl), 5.15 (d, J = 13.0 Hz, 1H, CH), 4.64 (d, J = 11.1 Hz,
Preparation of (L5)Zr(NMe2)(Cl)(THF) (15). This compound 1H, CH), 4.40 (m, 2H, CH2), 3.72 (m, 12H, THF), 3.29 (s, 6H,
was prepared as pale yellow crystals from the reaction of L5 NCH3), 3.15 (m, 2H, CH2), 1.93 (s, 9H, C(CH3)3), 1.88 (s, 9H, C
(0.29 g, 0.50 mmol) with Zr(NMe2)4 (0.14 g, 0.50 mmol) in THF (CH3)3), 1.22 (s, 18H, C(CH3)3), 1.21 (m, 12H, THF). 13C NMR
(20 mL) and recrystallization from a benzene solution by a (C6D6): δ 216.9, 160.7, 139.0, 138.7, 138.4, 138.2, 129.0, 128.9,
similar procedure to the synthesis of 7. Yield: 0.28 g (70%) 126.2, 125.6, 125.3, 124.8, 124.1, 78.9, 69.9, 50.5, 44.7, 35.8,
(Found: C, 65.55; H, 6.89; N, 5.34. C43H54N3ClO3Zr requires: C, 33.9, 31.7, 30.3, 25.4. IR (KBr, cm−1): ν 2962 (s), 1602 (w), 1442
ˉ
65.58; H, 6.91; N, 5.34%). M.p.: 156–158 °C (dec.). 1H NMR (m), 1384 (m), 1260 (s), 1091 (s), 1018 (s), 799 (s).
(C6D6): δ 7.44 (m, 2H, aryl), 6.93 (m, 8H, aryl), 6.69 (m, 6H, aryl),
Preparation of (L6)Zr(NEt2)(Cl)(THF) (19)·3C6H6 (19·3C6H6). This
4.75 (d, J = 14.1 Hz, 2H, CH2), 4.55 (s, 2H, CH), 3.64 (m, 4H, compound was prepared as pale yellow crystals from the reac-
THF), 3.39 (d, J = 14.1 Hz, 2H, CH2), 3.29 (s, 6H, N(CH3)2), tion of L6 (0.35 g, 0.50 mmol) with Zr(NEt2)4 (0.19 g,
1.76 (s, 9H, C(CH3)3), 1.39 (s, 9H, C(CH3)3), 1.37 (m, 4H, THF). 0.50 mmol) in THF (20 mL) and recrystallization from a
13C NMR (C6D6): δ 212.9, 162.7, 139.1, 138.6, 137.8, 137.3, 129.4, benzene solution by a similar procedure to the synthesis
129.2, 128.6, 127.8, 126.4, 125.6, 117.9, 78.4, 69.7, 50.1, 45.2, of 7. Yield: 0.42
g (73%) (Found: C, 73.40; H, 7.97;
35.7, 30.5, 25.6. IR (KBr, cm−1): ν 2961 (s), 1585 (s), 1416 (s), N, 3.64. C71H92N3ClO3Zr requires: C, 73.38; H, 7.98; N, 3.62%).
ˉ
1
1384 (m), 1257 (s), 1092 (s), 1020 (s), 870 (s), 810 (s).
M.p.: 102–104 °C (dec.). H NMR (C6D6): δ 7.57 (m, 2H, aryl),
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7.12 (m, 24H, aryl), 7.08 (m, 2H, aryl), 6.90 (m, 2H, aryl), 6.12 program.15 The crystal data and experimental data for L5, 10
(m, 2H, aryl), 5.48 (br s, 1H, CH), 4.76 (br s, 1H, CH), 4.46 and 14–19 are summarized in Tables 1 and 2. Selected bond
(br s, 2H, CH2), 4.29 (br s, 2H, CH2), 3.75 (m, 4H, THF), 3.28 lengths and angles are listed in Table 3.
(m, 4H, CH2), 1.94 (s, 9H, C(CH3)3), 1.88(s, 9H, C(CH3)3), 1.21
(s, 18H, C(CH3)3), 1.14 (m, 10H, N(CH2CH3)2 and THF).
13C NMR (C6D6): δ 214.5, 160.2, 139.3, 138.4, 138.0, 137.9,
Results and discussion
Synthesis and characterization of pro-ligands
129.0, 128.0, 127.8, 127.5, 125.5, 125.1, 124.2, 79.0, 69.4,
51.0, 44.2, 35.8, 33.9, 31.7, 30.5, 25.4, 14.0. IR (KBr, cm−1): ν
ˉ
Condensation of (S,S)-diphenyl-1,2-ethanediamine with
1
2962 (s), 1603 (w), 1438 (m), 1384 (s), 1260 (s), 1092 (s), 1019
(s), 798 (s).
equiv. of salicylaldehyde, 3-tert-butylsalicylaldehyde or 3,5-di-
tert-butylsalicylaldehyde in absolute ethanol at ambient temp-
erature, followed by reduction with an excess of NaBH4 in
ethanol forms the chiral diamines 1–3 (Schemes 1–3). Sub-
sequent cyclization of 1–3 with triethyl orthoformate in the
presence of NH4Br or NH4Cl at 120 °C gives the imidazolium
salts L4 (L4a and L4b), L5 and L6, respectively, in good yields
(Schemes 1–3). All compounds are air-stable and have been
characterized by various spectroscopic techniques and elemen-
General procedure for polymerization of a rac-lactide
In a glovebox, rac-lactide (rac-LA) (0.360 g, 2.5 mmol), 2-propa-
nol (0.01 mmol, in 0.5 mL of toluene or THF), complex (typi-
cally 0.01 mmol, in 0.5 mL of toluene or THF), and toluene or
THF (4.0 mL) were added sequentially into a Schlenk flask
with stirring. The flask containing the reaction mixture was
subsequently placed in an oil bath and stirred for 0.5 h at
70 °C. The polymerization was quenched by the addition of
cold acidified methanol. The precipitated polylactide was col-
lected, washed with cold methanol several times, and dried in
vacuum at 50 °C overnight.
1
tal analyses. The H and 13C NMR spectra are consistent with
their C2-symmetric structure. In addition, besides aromatic
stretches the IR spectra of L4–L6 also feature the characteristic
O–H (at ca. 3420 cm−1) and strong CvN stretches (at ca.
1640 cm−1). The C2 symmetric structure of L5 was also con-
firmed by X-ray diffraction analysis (Fig. 1).
X-ray crystallography
Synthesis and characterization of complexes
Single-crystal X-ray diffraction measurements were carried out
on a Bruker SMART CCD diffractometer using graphite mono- Amine elimination between M(NMe2)4 and protic reagents is a
chromated Mο Kα radiation (λ = 0.71073 Å). An empirical very efficient way for the synthesis of group 4 metal amide com-
absorption correction was applied using the SADABS plexes.16 Hence, a similar reaction is expected for the acidic
program.13 All structures were solved by direct methods and protons in the ligands L4 (L4a and L4b), L5 and L6 and metal
refined by full-matrix least squares on F2 using the SHELXL-97 amides. In fact, treatment of M(NR2)4 (M = Ti, Zr, Hf; R = Me, Et)
program package.14 All the hydrogen atoms were geometrically with 1 equiv. of L4 in THF gives, after recrystallization from a
fixed using the riding model. Disordered solvents in the voids benzene solution, the chiral titanium amides (L4)Ti(NMe2)(Br)-
of 15, 16 and 17 were modeled or removed using the SQUEEZE (THF) (7) and (L4)Ti(NMe2)(Cl)(THF) (11), zirconium amides (L4)
Table 1 Crystal data and experimental parameters for compounds L5, 10 and 14–15
Compound
2(L5)·C6H6
80H92N2Cl2O4
1244.48
Monoclinic
P21
10.306(1)
31.646(4)
11.070(1)
99.48(1)
3561.2(7)
2
10
14
15
Formula
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C
C37H42N3BrO3Zr
747.87
C37H42N3ClO3Zr
703.41
C43H54N3ClO3Zr
787.56
Orthorhombic
C2221
18.048(1)
24.570(1)
46.164(2)
90
20 471.3(13)
16
1.022
0.300
0.22 × 0.13 × 0.11
6624
Orthorhombic
P212121
11.088(1)
14.680(1)
21.126(1)
90
3438.4(4)
4
1.445
Orthorhombic
P212121
11.160(1)
14.834(2)
20.608(2)
90
3411.5(7)
4
1.370
β (°)
V (Å3)
Z
Dcalc (g cm−3
)
1.161
0.143
0.49 × 0.30 × 0.22
1332
μ(Mo/Kα)calc (mm−1
Size (mm)
)
1.521
0.42 × 0.36 × 0.30
1536
0.440
0.27 × 0.12 × 0.08
1464
F(000)
2θ range (°)
No. of reflns collected
No. of obsd reflns
4.00 to 50.50
17 778
9062
3.86 to 55.24
20 249
7178
4.16 to 55.10
20 265
7149
3.76 to 50.50
18 556
15 790
Abs corr (Tmax, Tmin
R
)
0.97, 0.93
0.055
0.66, 0.57
0.029
0.97, 0.89
0.032
0.97, 0.94
0.051
Rw
0.135
0.146
0.060
0.061
0.063
0.065
0.121
0.125
wR2 (all data)
Gof
CCDC
1.01
975119
1.00
975120
1.02
975123
1.04
975124
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Table 2 Crystal data and experimental parameters for compounds 16–19
Compound
16
17
18·2THF
19·3C6H6
Formula
Formula weight
Crystal system
Space group
a (Å)
C51H70N3ClO3Ti
856.45
C51H70N3ClO3Zr
899.77
C59H86N3ClHfO5
1131.25
Orthorhombic
P212121
12.781(1)
14.952(1)
30.090(2)
5750.3(5)
4
1.307
1.908
0.45 × 0.40 × 0.26
2360
C71H92N3ClO3Zr
1162.15
Orthorhombic
P212121
15.211(1)
18.900(1)
22.591(2)
6494.5(8)
4
1.189
0.257
0.45 × 0.42 × 0.39
2480
Orthorhombic
P212121
12.392(1)
15.178(2)
29.978(3)
5638.4(10)
4
1.009
0.236
0.45 × 0.22 × 0.18
1840
Orthorhombic
P212121
12.729(2)
15.022(2)
30.104(4)
5756.1(12)
4
1.038
0.273
0.60 × 0.18 × 0.13
1912
b (Å)
c (Å)
V (Å3)
Z
Dcalc (g cm−3
)
μ(Mo/Kα)calc (mm−1
Size (mm)
)
F(000)
2θ range (°)
No. of reflns collected
No. of obsd reflns
3.82 to 50.50
10 187
9028
3.84 to 50.50
10 419
9645
3.84 to 50.50
28 673
9927
3.88 to 50.50
31 878
10 522
Abs corr (Tmax, Tmin
R
)
0.96, 0.90
0.040
0.97, 0.85
0.031
0.64, 0.48
0.022
0.91, 0.89
0.041
Rw
0.097
0.101
0.076
0.077
0.048
0.048
0.102
0.108
wR2 (all data)
Gof
CCDC
1.04
975125
1.04
975126
1.01
975127
1.04
975128
Table 3 Selected bond distances (Å) and bond angles (°) for compounds 10 and 14–19
Compound
M–O (av.)
M–O(THF)
M–X
M–C
M–N
Sum angle of N(3)
10 (Zr)
14 (Zr)
15 (Zr)
16 (Ti)
17 (Zr)
18 (Hf)
19 (Zr)
2.136(2)
2.139(2)
2.115(3)
1.993(2)
2.110(2)
2.098(2)
2.122(2)
2.401(2)
2.406(2)
2.332(3)
2.208(2)
2.313(2)
2.290(2)
2.344(2)
Br: 2.661(1)
Cl: 2.498(1)
Cl: 2.499(1)
Cl: 2.410(1)
Cl: 2.501(1)
Cl: 2.478(1)
Cl: 2.515(1)
2.416(2)
2.419(2)
2.397(4)
2.252(2)
2.401(2)
2.370(3)
2.410(4)
2.046(2)
2.046(2)
2.046(4)
1.901(2)
2.044(2)
2.041(3)
2.052(3)
359.3(2)
359.5(2)
359.7(4)
359.9(2)
359.6(2)
359.7(3)
360.0(3)
Zr(NMe2)(Br)(THF) (8), (L4)Zr(NEt2)(Br)(THF) (10), (L4)Zr(NMe2)-
Complexes 16 and 17 are isostructural. The M4+ ion features
(Cl)(THF) (12) and (L4)Zr(NEt2)(Cl)(THF) (14), and hafnium a distorted-octahedral ligand environment in (L4)Zr(NEt2)(Br)-
amides (L4)Hf(NMe2)(Br)(THF) (9) and (L4)Hf(NMe2)(Cl)(THF) (THF) (10), (L4)Zr(NEt2)(Cl)(THF) (14), (L5)Zr(NMe2)(Cl)(THF)
(13), respectively, in good yields (Scheme 1). Similarly, the (15), (L6)Ti(NMe2)(Cl)(THF) (16), (L6)Zr(NMe2)(Cl)(THF) (17),
reactions of L5 or L6 with 1 equiv. of M(NR2)4 (M = Ti, Zr, Hf; (L6)Hf(NMe2)(Cl)(THF) (18) and (L6)Zr(NEt2)(Cl)(THF) (19)
R = Me, Et) also afford the chiral titanium amide (L6)Ti(NMe2)- (Fig. 2–8). Complexes 10 and 14–19 represent, to our knowl-
(Cl)(THF) (16), zirconium amides (L5)Zr(NMe2)(Cl)(THF) (15), edge, the first example of the structurally characterized group
(L6)Zr(NMe2)(Cl)(THF) (17) and (L6)Zr(NEt2)(Cl)(THF) (19), 4 chiral NHC–metal complex. The average M–O distances are
and the hafnium amide (L6)Hf(NMe2)(Cl)(THF) (18), respec- 1.993(2) Å for Ti, 2.110(2) Å to 2.139(2) Å for Zr, and 2.098(2) Å
tively, in good yields (Schemes 2 and 3).
for Hf. The Zr–Br distance is 2.661(1) Å (for 10), and the
Complexes 7–19 are stable in a dry nitrogen atmosphere, M–Cl distance is 2.410(1) Å for Ti, 2.498(1) Å to 2.515(1) Å for
while they are very sensitive to moisture. They are soluble in Zr, and 2.478(1) Å for Hf. The M–C(carbene) distances of 2.252
organic solvents such as THF, DME, pyridine, toluene, and (2) Å for Ti, 2.397(4) Å to 2.419(2) Å for Zr, and 2.370(3) Å
benzene, and only sparingly soluble in aliphatic solvents such for Hf are in the typical range for a M–C σ-bond. These struc-
as n-hexane. They have been characterized by various spectro- tural data can be compared with those found in [η3-O,C,O-
scopic techniques and elemental analyses. The 1 : 1 : 1 ratio {(3,5-(Me3C)2-C6H2O)2N2C3H4}]M(OiPr)(Cl)(THF) (M
=
Ti,
between the NR2 (R = Me, Et) group, the coordinate THF and Zr)17a,b
and
[η3-O,C,O-{(3,5-(Me3C)2-C6H2O)2N2C3H4}]-
the ligand L4, L5 or L6 is established by 1H NMR spectroscopy. MCl2(THF) (M = Ti, Zr).17c,d Furthermore, the M–NR2 (R = Me,
Furthermore, the characteristic O–H and CvN stretches at ca. Et) distances are short with 1.901(2) Å for Ti, 2.044(2) Å
3420 and 1640 cm−1 in L4–L6 disappear upon treatment with to 2.052(3) Å for Zr, and 2.041(3) Å for Hf. This in combina-
M(NR2)4, supporting the formation of complexes 7–19. The tion with the planar geometry around the nitrogen atom
solid-state structures of 10 and 14–19 have further been con- N(3) suggests that the sp2-hybridized N-atom engages in
firmed by X-ray diffraction analyses.
N(pπ)→M(dπ) interactions.16
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Scheme 1 Synthesis of complexes 7–14.
Scheme 3 Synthesis of complexes 16–19.
Polymerization of rac-lactide
Efficient ring opening polymerization (ROP) of rac-lactide (rac-
LA) is achieved by the chiral group 4 NHC–metal complexes
7–19 under the conditions listed in Table 4. With the zirco-
nium and hafnium complexes 8–10 and 12–14 complete con-
version of 250 equiv. of lactide is achieved within 0.5 h at
70 °C in toluene at [rac-LA] = 0.5 mol L−1 (Table 4, entries 2, 7,
8 and 10–12). A more detailed analysis was undertaken for 8
that acts as a single-site initiator for the controlled polymeri-
zation of rac- LA. The formed polylactides have experimental
Mn values (Mn,exp) that are very close to the calculated Mn,calcd
Scheme 2 Synthesis of complex 15.
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Fig. 1 Molecular structure of the cation in L5 (thermal ellipsoids drawn
at the 35% probability level).
Fig. 4 Molecular structure of 15 (thermal ellipsoids drawn at the 35%
probability level).
Fig. 2 Molecular structure of 10 (thermal ellipsoids drawn at the 35%
probability level).
Fig. 5 Molecular structure of 16 (thermal ellipsoids drawn at the 35%
probability level).
values and that the molar mass distributions are very narrow
(Mw/Mn = 1.18–1.21; Table 4, entries 2–6). In addition, for
complex 8 a first order kinetic dependence on the concen-
tration of rac-LA and no induction period were observed
(Fig. 9). The Mn,exp values increase linearly with the monomer
conversion, whereas the Mw/Mn values remain in the narrow
range of 1.17–1.22 (Fig. 10). However, when the more bulky
ligands L5 and L6 are used, the polymerization with the
Fig. 3 Molecular structure of 14 (thermal ellipsoids drawn at the 35%
probability level).
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Fig. 8 Molecular structure of 19 (thermal ellipsoids drawn at the 35%
probability level).
zirconium and hafnium complexes 15 and 17–19 is slightly
slower (Table 4, entries 13, and 15–17), presumably because of
the increased steric hindrance at the metal centers. Although
the zirconium and hafnium complexes are effective catalysts
for the polymerization of rac-LA, the titanium complexes 7, 11
and 16 exhibit only poor catalytic activity (Table 4, entries 1, 9
and 14), consistent with the smaller ionic radius of Ti4+. These
differences also prevail in THF solution (Table 4, entries
18–30), but the polymerization with these group 4 initiators/
catalysts proceeds much more slowly in THF (Table 4, entries
18–30), most likely a consequence of competitive monomer–
solvent coordination to the metal ion. A similar competition
was observed for the organoyttrium and organoaluminum cata-
lysts.16a,18 In the absence of isopropanol, no polymerization
occurs in toluene or THF solution even when heated at 70 °C
for 72 h. The polymer microstructure, as determined by
homo-decoupled 1H NMR experiments,19 shows that the
polylactides are heterotactic-rich polylactides under conditions
examined. The catalytic activities of 7–19 resemble that of [η3-
Fig. 6 Molecular structure of 17 (thermal ellipsoids drawn at the 35%
probability level).
O,C,O-{(3,5-(Me3C)2-C6H2O)2N2C3H4}]M(OiPr)(Cl)-(THF) (M
=
Ti, Zr),17a,b while the microstructures of the resulting polylac-
tides are similar to those initiated by [(R)-(2-O-C6H4)-CHvNCH
(Me)(C6H5)]2Zr(OiPr)2.5c
Conclusions
Chiral group 4 NHC–metal complexes were prepared and struc-
turally characterized. These complexes represent the first
example of the structurally characterized group 4 chiral NHC–
metal complex, and they can initiate the ring-opening polymer-
ization of rac-lactide in the presence of isopropanol, leading to
the heterotactic-rich polylactides. Nevertheless, the reactivity is
Fig. 7 Molecular structure of 18 (thermal ellipsoids drawn at the 35%
probability level).
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Table 4 Polymerization of rac-lactide catalyzed by chiral group 4 NHC-complexes 7–19a
b
c
Mn,exp
Mn,calcd
b
d
Entry
Precat.
Solvent
Conv. (%)
(kg mol−1
)
(kg mol−1
)
Mw/Mn
Pr
1
7 (Ti)
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
THF
65
100
100
100
100
92
100
100
70
22.9
35.4
14.9
21.3
29.2
40.4
36.2
35.4
24.6
36.8
35.3
35.7
34.6
11.2
32.7
32.6
32.8
12.4
22.6
23.2
19.9
13.5
21.6
21.8
22.3
18.5
5.9
23.4
36.0
14.4
21.6
28.8
39.7
36.0
36.0
25.2
36.0
36.0
36.0
34.2
10.8
33.8
33.1
32.4
12.6
22.3
23.0
20.9
13.7
21.2
22.3
21.6
17.3
5.8
1.18
1.21
1.20
1.18
1.19
1.21
1.21
1.23
1.20
1.23
1.21
1.25
1.22
1.16
1.24
1.27
1.22
1.20
1.26
1.31
1.36
1.21
1.32
1.35
1.36
1.32
1.19
1.34
1.30
1.35
0.70
0.74
0.73
0.73
0.74
0.73
0.72
0.67
0.72
0.70
0.66
0.70
0.69
0.69
0.68
0.67
0.71
0.70
0.72
0.70
0.68
0.71
0.72
0.70
0.67
0.70
0.68
0.69
0.71
0.70
2
8 (Zr)
3e
8 (Zr)
4f
5g
6h
7
8 (Zr)
8 (Zr)
8 (Zr)
9 (Hf)
10 (Zr)
11 (Ti)
12 (Zr)
13 (Hf)
14 (Zr)
15 (Zr)
16 (Ti)
17 (Zr)
18 (Hf)
19 (Zr)
7 (Ti)
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
100
100
100
95
30
94
92
90
35
8 (Zr)
THF
62
9 (Hf)
10 (Zr)
11 (Ti)
12 (Zr)
13 (Hf)
14 (Zr)
15 (Zr)
16 (Ti)
17 (Zr)
18 (Hf)
19 (Zr)
THF
64
THF
58
THF
38
THF
59
THF
62
THF
60
THF
48
THF
16
THF
50
17.2
15.8
16.4
18.0
16.2
17.3
THF
45
THF
48
a Conditions: 70 °C, precat./isopropanol/LA (mol/mol/mol) = 1/1/250; precatalyst (0.01 mmol); polymerization time, 0.5 h; solvent, 5 mL; [LA] =
0.5 mol L−1 b Measured by GPC (using polystyrene standards in THF). c Calculated by ([LA]/[precat.]) × 144 × X (X = conv.). d Pr is the probability
.
of forming an r-dyad by insertion and is determined from the methine region of the homonuclear decoupled 1H NMR spectrum in CDCl3 at
25 °C.19 e Precat./isopropanol/LA (mol/mol/mol) = 1/1/100. f Precat./isopropanol/LA (mol/mol/mol) = 1/1/150. g Precat./isopropanol/LA (mol/mol/
mol) = 1/1/200. h Precat./isopropanol/LA (mol/mol/mol) = 1/1/300.
Fig. 9 Ln([LA]0/[LA]) vs. time plot for the ROP of rac-LA initiated by
Fig. 10 Mn and Mw/Mn vs. conversion plots for the ROP of rac-LA
complex 8. Conditions: precat./isopropanol/LA (mol/mol/mol) = 1/1/150, initiated by complex 8. Conditons: precat./isopropanol/LA (mol/mol/mol) =
[LA] = 0.5 mol L−1, solvent = toluene, T = 70 °C. kobs = 3.14 × 10−3 s−1
.
1/1/150, [LA] = 0.5 mol L−1, solvent = toluene, T = 70 °C.
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strongly influenced by the size of the metal ion and the sol-
vents. For example, fast polymerization is observed in toluene,
whereas the conversion is slow in THF because of competitive
monomer–solvent coordination to the metal ions. The zirco-
nium and hafnium complexes are efficient precatalysts for
polymerization of rac-LA, while the titanium complexes exhibit
only poor catalytic activity because of the smaller ionic radius of
Ti4+. Further studies will focus on the application of these com-
plexes towards other asymmetric reactions and the exploration
of new group 4 metal complexes based on chiral ligands.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (grant no. 21172022, 21272026), the
Specialized Research Fund for the Doctoral Program of Higher
Education, the Fundamental Research Funds for the Central
Universities, the Beijing Municipal Commission of Education,
and the Deutsche Forschungsgemeinschaft (DFG) through the
Emmy-Noether program (WA 2513/2-2).
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