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

Abstract

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.

---

Introduction

Biodegradable polymers derived from renewable resources such as polylactides have received much attention over the past decade because of their attractive physical and mechanical properties.¹ In addition, the chain stereochemistry determines the polymer properties and the rate of degradation.² For example, whereas the enantiopure polylactide melts at 180 °C, a much higher melting temperature (230 °C) is found for stereocomplex polymers formed by an equivalent mixture of poly(L-lactide) and poly(D-lactide).³ Therefore, the polymerization of rac-lactide with stereoselective catalysts remains a challenge and an opportunity for chemists. To date, numerous reviews have covered catalyst systems for the ring-opening polymerization (ROP) of cyclic esters based on metals such as magnesium, zinc, calcium, aluminum, lanthanides, tin, group 4 metals, germanium, indium and iron,^1d,4 but among these, the chiral group 4 catalysts are especially promising.⁵ Unfortunately, compared to other metals, structurally well-characterized chiral group 4 complexes that initiate the controlled ring-opening polymerization of lactides are still scarce.⁵

In recent years, transition-metal complexes with chiral N-heterocyclic carbene (NHC) ligands have become increasingly popular because of their stability to air and moisture and their strong σ-donating, but poor π-accepting abilities.⁶ An additional driving force is the longstanding interest in catalysts for enantioselective reactions such as olefin metathesis,⁷ conjugate addition of enones,⁸ allylic alkylations,⁹ olefin hydrogenations¹⁰ and hydrosilylations.¹¹ Encouraged by the attractive features of chiral NHC-ligands in general, we are now focusing on the preparation of group 4 metal complexes coordinated 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 complexes as catalysts in the polymerization of rac-lactide (rac-LA).

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 techniques, 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 chromatography (GPC) using a PL-GPC 50 apparatus. ¹H and ¹³C NMR spectra were recorded on a Bruker AV 500 spectrometer at 500 and 125 MHz, respectively. All 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.

Syntheses

Preparation of 1. Salicylaldehyde (1.22 g, 10.0 mmol) was mixed with (S,S)-diphenyl-1,2-ethanediamine (1.06 g, 5.0 mmol) in absolute ethanol (30 mL) and stirred for 4 h at room temperature. NaBH₄ (2.00 g, 52.6 mmol) was added in small portions at 0 °C, and the solution was then warmed to 50 °C and kept at this temperature for 2 h. The solvent was removed, the residue was treated with H₂O (20 mL), extracted with ethyl acetate (20 mL × 3) and washed with brine (20 mL). The combined organic layers were dried with anhydrous Na₂SO₄ and the solvent was removed under reduced pressure. The residue was further purified by flash column chromatography (hexane–ethyl acetate = 10 : 1) to give 1 as a colorless oil. Yield: 2.01 g (95%) (Found: C, 79.12; H, 6.60; N, 6.64. C₂₈H₂₈N₂O₂ requires: C, 79.22; H, 6.65; N, 6.60%). ¹H NMR (C₆D₆): δ 7.14–7.07 (m, 4H, aryl), 6.91 (m, 6H, aryl), 6.68 (m, 2H, aryl), 6.58 (m, 2H, aryl), 6.52 (m, 4H, aryl), 3.57 (s, 2H, CH), 3.51 (d, J = 13.8 Hz, 2H, CH₂), 3.19 (d, J = 13.8 Hz, 2H, CH₂); NH and OH protons were not observed. ¹³C NMR (C₆D₆): δ 158.9, 138.3, 129.4, 129.0, 128.5, 128.3, 128.2, 128.1, 127.9, 127.8, 66.7, 50.1. IR (KBr, cm⁻¹): [small nu, Greek, macron] 3427 (s), 2853 (w), 1616 (s), 1588 (s), 1489 (s), 1454 (s), 1251 (s), 755 (s).

Preparation of 2. This compound was prepared as colorless oil from the reaction of 3-tert-butylsalicylaldehyde (1.78 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 outlined 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; N, 5.22%). ¹H 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, 65.6, 49.4, 33.7, 28.6. IR (KBr, cm⁻¹): [small nu, Greek, macron] 3427 (s), 2957 (s), 1590 (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 outlined 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; N, 4.32%). ¹H NMR (C₆D₆): δ 7.08–7.01 (m, 10H, aryl), 6.97 (d, 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): [small nu, Greek, macron] 3428 (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) were heated at 120 °C for one day. After the reaction mixture was cooled to room temperature, diethyl ether (40 mL) was added to precipitate a colorless solid, which was filtered and washed with diethyl ether to give L4a as a colorless solid. Yield: 2.23 g (92%) (Found: C, 67.54; H, 5.30; N, 5.42. C₂₉H₂₇N₂BrO₂ requires: C, 67.58; H, 5.28; N, 5.43%). M.p.: 273–275 °C. ¹H NMR (DMSO-d₆): δ 10.17 (s, 2H, OH), 9.13 (s, 1H, NCHN), 7.40 (m, 6H, aryl), 7.17 (m, 6H, aryl), 6.95 (m, 4H, aryl), 6.75 (m, 2H, aryl), 4.83 (d, J = 14.3 Hz, 2H, CH₂), 4.70 (s, 2H, CH), 4.10 (d, J = 14.3 Hz, 2H, CH₂). ¹³C NMR (DMSO-d₆): δ 158.3, 156.0, 135.8, 130.8, 130.3, 129.5, 129.4, 127.0, 119.1, 118.7, 115.6, 71.5, 45.9. IR (KBr, cm⁻¹): [small nu, Greek, macron] 3435 (m), 3175 (s), 2927 (m), 1648 (s), 1599 (s), 1461 (s), 1373 (s), 1277 (s), 1106 (s), 757 (s).

Preparation of L4b. This compound was prepared as a colorless solid from the reaction of 1 (2.00 g, 4.71 mmol), NH₄Cl (0.38 g, 7.07 mmol) and triethyl orthoformate (15 mL) at 120 °C, followed by washing with diethyl ether by a similar procedure as described for the synthesis of L4a. Yield: 2.11 g (95%) (Found: C, 73.90; H, 5.78; N, 5.93. C₂₉H₂₇N₂ClO₂ requires: C, 73.95; H, 5.78; N, 5.95%). M.p.: 260–262 °C. ¹H NMR (DMSO-d₆): δ 10.48 (s, 2H, OH), 9.20 (s, 1H, NCHN), 7.38 (m, 6H, aryl), 7.17 (m, 6H, aryl), 7.07 (m, 2H, aryl), 6.93 (m, 2H, aryl), 6.72 (m, 2H, aryl), 4.85 (d, J = 14.3 Hz, 2H, CH₂), 4.68 (s, 2H, CH), 4.09 (d, J = 14.3 Hz, 2H, CH₂). ¹³C NMR (DMSO-d₆): δ 158.3, 156.2, 135.9, 130.7, 130.1, 129.5, 129.4, 127.1, 119.0, 118.7, 115.8, 71.5, 46.0. IR (KBr, cm⁻¹): [small nu, Greek, macron] 3424 (m), 3093 (s), 2954 (m), 1648 (s), 1600 (s), 1461 (s), 1374 (s), 1277 (s), 1197 (s), 1108 (s), 757 (s).

Preparation of L5. This compound was prepared as a colorless solid from the reaction of 2 (2.00 g, 3.73 mmol), NH₄Cl (0.30 g, 5.60 mmol) and triethyl orthoformate (15 mL) at 120 °C, followed by washing with diethyl ether by a similar procedure as described for the synthesis of L4a. Yield: 1.83 g (84%) (Found: C, 76.21; H, 7.43; N, 4.82. C₃₇H₄₃N₂ClO₂ requires: C, 76.20; H, 7.43; N, 4.80%). M.p.: 240–242 °C. ¹H NMR (DMSO-d₆): δ 9.25 (s, 1H, NCHN), 9.10 (s, 2H, OH), 7.41–7.32 (m, 6H, aryl), 7.21 (m, 2H, aryl), 7.10 (m, 4H, aryl), 6.70 (m, 4H, aryl), 5.33 (d, J = 14.6 Hz, 2H, CH₂), 4.54 (s, 2H, CH), 4.17 (d, J = 14.6 Hz, 2H, CH₂), 1.40 (s, 18H, C(CH₃)₃). ¹³C NMR (DMSO-d₆): δ 166.0, 157.0, 154.3, 139.5, 136.0, 129.5, 129.3, 129.0, 127.4, 127.0, 121.9, 120.3, 71.2, 47.0, 34.5, 29.7. IR (KBr, cm⁻¹): [small nu, Greek, macron] 3412 (m), 3090 (s), 2956 (s), 1638 (s), 1589 (m), 1438 (s), 1378 (s), 1208 (s), 1093 (s), 752 (s). Colorless crystals of 2(L5)·C₆H₆ suitable for X-ray structural analysis were grown from a benzene solution.

Preparation of L6. This compound was prepared as a colorless solid from the reaction of 3 (2.00 g, 3.08 mmol), NH₄Cl (0.25 g, 4.62 mmol) and triethyl orthoformate (15 mL) at 120 °C, followed by washing with diethyl ether by a similar procedure as in the synthesis of L4a. Yield: 1.84 g (86%) (Found: C, 77.69; H, 8.53; N, 4.05. C₄₅H₅₉N₂ClO₂ requires: C, 77.72; H, 8.55; N, 4.03%). M.p.: 235–237 °C. ¹H NMR (DMSO-d₆): δ 9.22 (s, 1H, NCHN), 8.84 (s, 2H, OH), 7.34 (m, 2H, aryl), 7.31 (m, 4H, aryl), 7.18 (d, J = 2.0 Hz, 2H, aryl), 7.04 (m, 4H, aryl), 6.61 (d, J = 2.0 Hz, 2H, aryl), 5.32 (d, J = 14.5 Hz, 2H, CH₂), 4.44 (s, 2H, CH), 4.17 (d, J = 14.6 Hz, 2H, CH₂), 1.38 (s, 18H, C(CH₃)₃), 1.14 (s, 18H, C(CH₃)₃). ¹³C NMR (DMSO-d₆): δ 157.0, 151.8, 141.8, 138.5, 135.9, 129.5, 129.2, 127.2, 125.9, 123.9, 120.8, 71.1, 47.4, 34.7, 33.7, 31.2, 29.7. IR (KBr, cm⁻¹): [small nu, Greek, macron] 3422 (w), 3085 (w), 2957 (s), 1637 (s), 1482 (s), 1362 (m), 1201 (s), 699 (s).

Preparation of (L4)Ti(NMe₂)(Br)(THF) (7). While stirring a THF solution (5 mL) of Ti(NMe₂)₄ (0.11 g, 0.50 mmol) was slowly added to a THF (15 mL) suspension of L4a (0.26 g, 0.50 mmol) at room temperature. After this mixture was stirred overnight at room temperature, the solution was filtered and the solvent was removed under reduced pressure. The resulting red solid was recrystallized from a benzene solution to give 7 as red microcrystals. Yield: 0.25 g (74%) (Found: C, 61.96; H, 5.62; N, 6.25. C₃₅H₃₈N₃BrO₃Ti requires: C, 62.14; H, 5.66; N, 6.21%). M.p.: 145–147 °C (dec.). ¹H NMR (C₆D₆): δ 7.19 (m, 2H, aryl), 6.93 (m, 2H, aryl), 6.85 (m, 4H, aryl), 6.70 (m, 4H, aryl), 6.55 (m, 6H, aryl), 4.56 (br s, 1H, CH), 4.38 (m, 3H, CH and CH₂), 3.74 (s, 6H, NCH₃), 3.67 (m, 4H, THF), 3.49 (br s, 2H, CH₂), 1.41 (m, 4H, THF). ¹³C NMR (C₆D₆): δ 208.4, 164.3, 163.8, 136.6, 129.4, 128.9, 128.6, 128.2, 127.8, 118.8, 118.1, 117.8, 75.1, 68.0, 53.1, 48.1, 25.0. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2962 (m), 1593 (s), 1482 (s), 1450 (s), 1260 (s), 1108 (s), 1034 (s), 886 (s), 799 (s).

Preparation of (L4)Zr(NMe₂)(Br)(THF) (8). This compound was prepared as pale yellow microcrystals from the reaction of L4a (0.26 g, 0.50 mmol) with Zr(NMe₂)₄ (0.14 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.24 g (68%) (Found: C, 58.27; H, 5.42; N, 5.80. C₃₅H₃₈N₃BrO₃Zr requires: C, 58.40; H, 5.32; N, 5.84%). M.p.: 177–179 °C (dec.). ¹H NMR (C₆D₆): δ 7.20 (m, 2H, aryl), 6.90 (m, 8H, aryl), 6.68 (m, 8H, aryl), 4.54 (br s, 2H, CH₂), 3.80 (m, 6H, CH₂ and THF), 3.38 (m, 8H, CH and NCH₃), 1.36 (m, 4H, THF). ¹³C NMR (C₆D₆): δ 209.0, 162.1, 138.1, 137.8, 137.2, 129.3, 129.2, 128.3, 125.7, 119.5, 119.2, 118.8, 117.9, 75.4, 69.4, 47.0, 45.5, 24.8. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2963 (m), 1595 (w), 1384 (m), 1260 (s), 1091 (s), 1018 (s), 798 (s).

Preparation of (L4)Hf(NMe₂)(Br)(THF) (9). This compound was prepared as colorless microcrystals from the reaction of L4a (0.26 g, 0.50 mmol) with Hf(NMe₂)₄ (0.18 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.29 g (72%) (Found: C, 52.15; H, 4.67; N, 5.26. C₃₅H₃₈N₃BrO₃Hf requires: C, 52.09; H, 4.75; N, 5.21%). M.p.: 230–232 °C (dec.). ¹H NMR (C₆D₆): δ 7.24 (m, 2H, aryl), 6.90 (m, 8H, aryl), 6.70 (m, 8H, aryl), 4.53 (br s, 2H, CH₂), 3.78 (m, 6H, CH and THF), 3.54 (s, 6H, NCH₃), 3.30 (br s, 2H, CH₂), 1.36 (m, 4H, THF). ¹³C NMR (C₆D₆): δ 213.6, 163.1, 138.7, 137.8, 130.1, 129.6, 128.8, 128.4, 127.6, 126.1, 120.5, 120.1, 118.0, 76.0, 69.5, 47.3, 45.2, 25.4. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2962 (s), 1595 (m), 1451 (m), 1384 (s), 1260 (s), 1090 (s), 1019 (s), 798 (s).

Preparation of (L4)Zr(NEt₂)(Br)(THF) (10). This compound was prepared as pale yellow crystals from the reaction of L4a (0.26 g, 0.50 mmol) with Zr(NEt₂)₄ (0.19 mg, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.27 g (73%) (Found: C, 59.50; H, 5.57; N, 5.64. C₃₇H₄₂N₃BrO₃Zr requires: C, 59.42; H, 5.66; N, 5.62%). M.p.: 120–122 °C (dec.). ¹H NMR (C₆D₆): δ 7.18 (m, 4H, aryl), 6.94 (m, 6H, aryl), 6.81 (m, 2H, aryl), 6.66 (m, 6H, aryl), 5.33 (br s, 1H, CH), 4.56 (br s, 3H, CH and CH₂), 4.26 (br s, 2H, CH₂), 3.95 (m, 2H, THF), 3.83 (m, 2H, THF), 3.62 (br s, 2H, CH₂), 3.35 (br s, 2H, CH₂), 1.36 (m, 4H, THF), 1.25 (m, 6H, N(CH₂CH₃)₂). ¹³C NMR (C₆D₆): δ 209.2, 162.1, 137.6, 137.4, 129.6, 129.3, 129.2, 128.7, 128.3, 125.6, 119.1, 118.2, 117.9, 75.6, 70.0, 47.3, 46.2, 24.7, 14.9. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2962 (s), 1595 (s), 1480 (s), 1453 (s), 1260 (s), 1090 (s), 1019 (s), 797 (s).

Preparation of (L4)Ti(NMe₂)(Cl)(THF) (11). This compound was prepared as red microcrystals from the reaction of L4b (0.24 g, 0.50 mmol) with Ti(NMe₂)₄ (0.11 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.22 g (70%) (Found: C, 66.58; H, 6.05; N, 6.60. C₃₅H₃₈N₃ClO₃Ti requires: C, 66.51; H, 6.06; N, 6.65%). M.p.: 154–156 °C (dec.). ¹H NMR (C₆D₆): δ 7.20 (m, 2H, aryl), 6.95 (m, 4H, aryl), 6.86 (m, 6H, aryl), 6.67 (m, 6H, aryl), 4.49 (d, J = 10.1 Hz, 2H, CH₂), 4.43 (d, J = 10.1 Hz, 2H, CH₂), 3.86 (br s, 6H, CH and THF), 3.62 (s, 6H, NCH₃), 1.41 (m, 4H, THF). ¹³C NMR (C₆D₆): δ 209.4, 166.1, 137.5, 129.4, 128.9, 128.2, 127.8, 126.0, 120.2, 119.5, 118.5, 118.3, 117.6, 75.4, 67.4, 52.9, 47.8, 25.0. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2962 (m), 1593 (m), 1480 (m), 1452 (m), 1260 (s), 1090 (s), 1018 (s), 798 (s).

Preparation of (L4)Zr(NMe₂)(Cl)(THF) (12). This compound was prepared as pale yellow crystals from the reaction of L4b (0.24 g, 0.50 mmol) with Zr(NMe₂)₄ (0.14 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.22 g (66%) (Found: C, 62.22; H, 5.66; N, 6.22. C₃₅H₃₈N₃ClO₃Zr requires: C, 62.24; H, 5.67; N, 6.22%). M.p.: 163–165 °C (dec.). ¹H NMR (C₆D₆): δ 7.24 (m, 2H, aryl), 6.95 (m, 8H, aryl), 6.70 (m, 8H, aryl), 4.53 (m, 2H, CH₂), 3.83 (s, 2H, CH), 3.63 (br s, 6H, CH₂ and THF), 3.44 (br s, 3H, NCH₃), 3.32 (br s, 3H, NCH₃), 1.39 (m, 4H, THF). ¹³C NMR (C₆D₆): δ 210.1, 162.6, 138.5, 137.9, 137.3, 130.0, 129.6, 128.6, 128.3, 125.4, 119.3, 118.5, 118.0, 75.7, 68.1, 46.4, 45.5, 25.5. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2963 (m), 1593 (w), 1481 (m), 1384 (m), 1260 (s), 1091 (s), 1018 (s), 798 (s).

Preparation of (L4)Hf(NMe₂)(Cl)(THF) (13). This compound was prepared as colorless microcrystals from the reaction of L4b (0.24 g, 0.50 mmol) with Hf(NMe₂)₄ (0.18 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.27 g (70%) (Found: C, 55.11; H, 5.02; N, 5.56. C₃₅H₃₈N₃ClHfO₃ requires: C, 55.12; H, 5.02; N, 5.51%). M.p.: 214–216 °C (dec.). ¹H NMR (C₆D₆): δ 7.23 (m, 2H, aryl), 6.95 (m, 8H, aryl), 6.65 (m, 8H, aryl), 4.54 (m, 2H, CH₂), 3.86 (s, 2H, CH), 3.73–3.24 (m, 12H, CH₂ and THF and NCH₃), 1.39 (m, 4H, THF). ¹³C NMR (C₆D₆): δ 214.5, 164.3, 138.5, 137.4, 129.9, 129.6, 129.1, 128.5, 127.7, 125.4, 120.1, 119.2, 117.5, 76.1, 67.7, 47.8, 47.0, 25.5. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2962 (m), 1594 (w), 1450 (m), 1384 (m), 1260 (s), 1091 (s), 1019 (s), 798 (s).

Preparation of (L4)Zr(NEt₂)(Cl)(THF) (14). This compound was prepared as pale yellow crystals from the reaction of L4b (0.24 g, 0.50 mmol) with Zr(NEt₂)₄ (0.19 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.26 g (73%) (Found: C, 63.21; H, 6.06; N, 5.96. C₃₇H₄₂N₃ClO₃Zr requires: C, 63.18; H, 6.02; N, 5.97%). M.p.: 200–202 °C (dec.). ¹H NMR (C₆D₆): δ 7.18 (m, 2H, aryl), 6.92 (m, 8H, aryl), 6.81 (m, 4H, aryl), 6.73 (m, 4H, aryl), 5.38 (br s, 1H, CH), 4.57 (br s, 4H, CH₂), 4.27 (br s, 1H, CH), 3.81 (m, 2H, THF), 3.73 (m, 2H, THF), 3.58 (br s, 2H, CH₂), 3.36 (br s, 2H, CH₂), 1.37 (m, 4H, THF), 1.26 (m, 6H, N(CH₂CH₃)₂). ¹³C NMR (C₆D₆): δ 209.3, 161.5, 136.8, 128.9, 128.5, 128.0, 127.5, 126.5, 125.0, 118.6, 117.4, 117.1, 74.8, 68.3, 46.6, 45.4, 24.1, 14.4. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2962 (m), 1594 (w), 1481 (m), 1384 (m), 1260 (s), 1089 (s), 1017 (s), 797 (s).

Preparation of (L5)Zr(NMe₂)(Cl)(THF) (15). This compound was prepared as pale yellow crystals from the reaction of L5 (0.29 g, 0.50 mmol) with Zr(NMe₂)₄ (0.14 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.28 g (70%) (Found: C, 65.55; H, 6.89; N, 5.34. C₄₃H₅₄N₃ClO₃Zr requires: C, 65.58; H, 6.91; N, 5.34%). M.p.: 156–158 °C (dec.). ¹H NMR (C₆D₆): δ 7.44 (m, 2H, aryl), 6.93 (m, 8H, aryl), 6.69 (m, 6H, aryl), 4.75 (d, J = 14.1 Hz, 2H, CH₂), 4.55 (s, 2H, CH), 3.64 (m, 4H, THF), 3.39 (d, J = 14.1 Hz, 2H, CH₂), 3.29 (s, 6H, N(CH₃)₂), 1.76 (s, 9H, C(CH₃)₃), 1.39 (s, 9H, C(CH₃)₃), 1.37 (m, 4H, THF). ¹³C NMR (C₆D₆): δ 212.9, 162.7, 139.1, 138.6, 137.8, 137.3, 129.4, 129.2, 128.6, 127.8, 126.4, 125.6, 117.9, 78.4, 69.7, 50.1, 45.2, 35.7, 30.5, 25.6. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2961 (s), 1585 (s), 1416 (s), 1384 (m), 1257 (s), 1092 (s), 1020 (s), 870 (s), 810 (s).

Preparation of (L6)Ti(NMe₂)(Cl)(THF) (16). This compound was prepared as red crystals from the reaction of L6 (0.35 g, 0.50 mmol) with Ti(NMe₂)₄ (0.11 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.28 g (65%) (Found: C, 71.54; H, 8.23; N, 4.93. C₅₁H₇₀N₃ClO₃Ti requires: C, 71.52; H, 8.24; N, 4.91%). M.p.: 143–145 °C (dec.). ¹H NMR (C₆D₆): δ 7.63 (d, J = 10.1 Hz, 2H, aryl), 6.97 (m, 4H, aryl), 6.86 (m, 6H, aryl), 6.40 (m, 2H, aryl), 4.58 (m, 3H, CH and CH₂), 4.16 (br s, 1H, CH), 3.57 (m, 4H, THF), 3.53 (s, 6H, NCH₃), 3.21 (br s, 2H, CH₂), 1.98 (s, 9H, C(CH₃)₃), 1.92 (s, 9H, C(CH₃)₃), 1.38 (m, 4H, THF), 1.25 (s, 18H, C(CH₃)₃). ¹³C NMR (C₆D₆), δ 201.8, 157.3, 152.3, 140.9, 139.1, 136.5, 135.5, 128.2, 127.2, 126.8, 125.7, 124.3, 120.1, 70.4, 66.4, 48.5, 40.4, 34.3, 32.8, 30.2, 29.1, 25.8. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2962 (s), 1602 (w), 1443 (m), 1384 (m), 1260 (s), 1091 (s), 1018 (s), 799 (s).

Preparation of (L6)Zr(NMe₂)(Cl)(THF) (17). This compound was prepared as pale yellow crystals from the reaction of L6 (0.35 g, 0.50 mmol) with Zr(NMe₂)₄ (0.14 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.28 g (63%) (Found: C, 68.04; H, 7.85; N, 4.66. C₅₁H₇₀N₃ClO₃Zr requires C, 68.08; H, 7.84; N, 4.67%). M.p.: 158–160 °C (dec.). ¹H NMR (C₆D₆): δ 7.57 (m, 4H, aryl), 7.09 (m, 4H, aryl), 7.02 (m, 2H, aryl), 6.92 (m, 4H, aryl), 4.40 (br s, 2H, CH₂), 3.63 (br s, 8H, CH, CH₂ and THF), 3.20 (s, 6H, NCH₃), 1.91 (s, 9H, C(CH₃)₃), 1.89 (s, 9H, C(CH₃)₃), 1.22 (s, 13H, C(CH₃)₃ and THF), 1.20 (s, 9H, C(CH₃)₃). ¹³C NMR (C₆D₆): δ 206.2, 160.2, 139.3, 137.9, 137.6, 129.0, 128.9, 128.8, 128.7, 125.5, 125.4, 125.0, 124.1, 78.8, 68.1, 45.3, 39.1, 34.0, 33.9, 30.5, 30.4, 25.5. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2962 (s), 1384 (s), 1260 (s), 1092 (s), 1020 (s), 799 (s).

Preparation of (L6)Hf(NMe₂)(Cl)(THF) (18)·2THF (18·2THF). This compound was prepared as colorless crystals from the reaction of L6 (0.35 g, 0.50 mmol) with Hf(NMe₂)₄ (0.18 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.40 g (71%) (Found: C, 62.63; H, 7.67; N, 3.73. C₅₉H₈₆N₃ClHfO₅ requires: C, 62.64; H, 7.66; N, 3.71%). M.p.: 110–112 °C (dec.). ¹H NMR (C₆D₆): δ 7.59 (s, 2H, aryl), 7.09 (m, 4H, aryl), 7.02 (m, 4H, aryl), 6.93 (d, J = 6.8 Hz, 2H, aryl), 6.11 (m, 2H, aryl), 5.15 (d, J = 13.0 Hz, 1H, CH), 4.64 (d, J = 11.1 Hz, 1H, CH), 4.40 (m, 2H, CH₂), 3.72 (m, 12H, THF), 3.29 (s, 6H, NCH₃), 3.15 (m, 2H, CH₂), 1.93 (s, 9H, C(CH₃)₃), 1.88 (s, 9H, C(CH₃)₃), 1.22 (s, 18H, C(CH₃)₃), 1.21 (m, 12H, THF). ¹³C NMR (C₆D₆): δ 216.9, 160.7, 139.0, 138.7, 138.4, 138.2, 129.0, 128.9, 126.2, 125.6, 125.3, 124.8, 124.1, 78.9, 69.9, 50.5, 44.7, 35.8, 33.9, 31.7, 30.3, 25.4. IR (KBr, cm⁻¹): [small nu, Greek, macron] 2962 (s), 1602 (w), 1442 (m), 1384 (m), 1260 (s), 1091 (s), 1018 (s), 799 (s).

Preparation of (L6)Zr(NEt₂)(Cl)(THF) (19)·3C₆H₆ (19·3C₆H₆). This compound was prepared as pale yellow crystals from the reaction of L6 (0.35 g, 0.50 mmol) with Zr(NEt₂)₄ (0.19 g, 0.50 mmol) in THF (20 mL) and recrystallization from a benzene solution by a similar procedure to the synthesis of 7. Yield: 0.42 g (73%) (Found: C, 73.40; H, 7.97; N, 3.64. C₇₁H₉₂N₃ClO₃Zr requires: C, 73.38; H, 7.98; N, 3.62%). M.p.: 102–104 °C (dec.). ¹H NMR (C₆D₆): δ 7.57 (m, 2H, aryl), 7.12 (m, 24H, aryl), 7.08 (m, 2H, aryl), 6.90 (m, 2H, aryl), 6.12 (m, 2H, aryl), 5.48 (br s, 1H, CH), 4.76 (br s, 1H, CH), 4.46 (br s, 2H, CH₂), 4.29 (br s, 2H, CH₂), 3.75 (m, 4H, THF), 3.28 (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, 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): [small nu, Greek, macron] 2962 (s), 1603 (w), 1438 (m), 1384 (s), 1260 (s), 1092 (s), 1019 (s), 798 (s).

General procedure for polymerization of a rac-lactide

In a glovebox, rac-lactide (rac-LA) (0.360 g, 2.5 mmol), 2-propanol (0.01 mmol, in 0.5 mL of toluene or THF), complex (typically 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 collected, washed with cold methanol several times, and dried in vacuum at 50 °C overnight.

X-ray crystallography

Single-crystal X-ray diffraction measurements were carried out on a Bruker SMART CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). An empirical absorption correction was applied using the SADABS program.¹³ All structures were solved by direct methods and refined by full-matrix least squares on F² using the SHELXL-97 program package.¹⁴ All the hydrogen atoms were geometrically fixed using the riding model. Disordered solvents in the voids of 15, 16 and 17 were modeled or removed using the SQUEEZE program.¹⁵ The crystal data and experimental data for L5, 10 and 14–19 are summarized in Tables 1 and 2. Selected bond lengths and angles are listed in Table 3.

Table 1 Crystal data and experimental parameters for compounds L5, 10 and 14–15

Compound	2(L5)·C6H6	10	14	15
Formula	C80H92N2Cl2O4	C37H42N3BrO3Zr	C37H42N3ClO3Zr	C43H54N3ClO3Zr
Formula weight	1244.48	747.87	703.41	787.56
Crystal system	Monoclinic	Orthorhombic	Orthorhombic	Orthorhombic
Space group	P21	P212121	P212121	C2221
a (Å)	10.306(1)	11.088(1)	11.160(1)	18.048(1)
b (Å)	31.646(4)	14.680(1)	14.834(2)	24.570(1)
c (Å)	11.070(1)	21.126(1)	20.608(2)	46.164(2)
β (°)	99.48(1)	90	90	90
V (Å^3)	3561.2(7)	3438.4(4)	3411.5(7)	20 471.3(13)
Z	2	4	4	16
D calc (g cm^−3)	1.161	1.445	1.370	1.022
μ(Mo/Kα)calc (mm^−1)	0.143	1.521	0.440	0.300
Size (mm)	0.49 × 0.30 × 0.22	0.42 × 0.36 × 0.30	0.27 × 0.12 × 0.08	0.22 × 0.13 × 0.11
F(000)	1332	1536	1464	6624
2θ range (°)	4.00 to 50.50	3.86 to 55.24	4.16 to 55.10	3.76 to 50.50
No. of reflns collected	17 778	20 249	20 265	18 556
No. of obsd reflns	9062	7178	7149	15 790
Abs corr (Tmax, Tmin)	0.97, 0.93	0.66, 0.57	0.97, 0.89	0.97, 0.94
R	0.055	0.029	0.032	0.051
R w	0.135	0.060	0.063	0.121
wR2 (all data)	0.146	0.061	0.065	0.125
Gof	1.01	1.00	1.02	1.04
CCDC	975119	975120	975123	975124

---

Table 2 Crystal data and experimental parameters for compounds 16–19

Compound	16	17	18·2THF	19·3C6H6
Formula	C51H70N3ClO3Ti	C51H70N3ClO3Zr	C59H86N3ClHfO5	C71H92N3ClO3Zr
Formula weight	856.45	899.77	1131.25	1162.15
Crystal system	Orthorhombic	Orthorhombic	Orthorhombic	Orthorhombic
Space group	P212121	P212121	P212121	P212121
a (Å)	12.392(1)	12.729(2)	12.781(1)	15.211(1)
b (Å)	15.178(2)	15.022(2)	14.952(1)	18.900(1)
c (Å)	29.978(3)	30.104(4)	30.090(2)	22.591(2)
V (Å^3)	5638.4(10)	5756.1(12)	5750.3(5)	6494.5(8)
Z	4	4	4	4
D calc (g cm^−3)	1.009	1.038	1.307	1.189
μ(Mo/Kα)calc (mm^−1)	0.236	0.273	1.908	0.257
Size (mm)	0.45 × 0.22 × 0.18	0.60 × 0.18 × 0.13	0.45 × 0.40 × 0.26	0.45 × 0.42 × 0.39
F(000)	1840	1912	2360	2480
2θ range (°)	3.82 to 50.50	3.84 to 50.50	3.84 to 50.50	3.88 to 50.50
No. of reflns collected	10 187	10 419	28 673	31 878
No. of obsd reflns	9028	9645	9927	10 522
Abs corr (Tmax, Tmin)	0.96, 0.90	0.97, 0.85	0.64, 0.48	0.91, 0.89
R	0.040	0.031	0.022	0.041
R w	0.097	0.076	0.048	0.102
wR2 (all data)	0.101	0.077	0.048	0.108
Gof	1.04	1.04	1.01	1.04
CCDC	975125	975126	975127	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)	2.136(2)	2.401(2)	Br: 2.661(1)	2.416(2)	2.046(2)	359.3(2)
14 (Zr)	2.139(2)	2.406(2)	Cl: 2.498(1)	2.419(2)	2.046(2)	359.5(2)
15 (Zr)	2.115(3)	2.332(3)	Cl: 2.499(1)	2.397(4)	2.046(4)	359.7(4)
16 (Ti)	1.993(2)	2.208(2)	Cl: 2.410(1)	2.252(2)	1.901(2)	359.9(2)
17 (Zr)	2.110(2)	2.313(2)	Cl: 2.501(1)	2.401(2)	2.044(2)	359.6(2)
18 (Hf)	2.098(2)	2.290(2)	Cl: 2.478(1)	2.370(3)	2.041(3)	359.7(3)
19 (Zr)	2.122(2)	2.344(2)	Cl: 2.515(1)	2.410(4)	2.052(3)	360.0(3)

---

Results and discussion

Synthesis and characterization of pro-ligands

Condensation of (S,S)-diphenyl-1,2-ethanediamine with 1 equiv. of salicylaldehyde, 3-tert-butylsalicylaldehyde or 3,5-di-tert-butylsalicylaldehyde in absolute ethanol at ambient temperature, followed by reduction with an excess of NaBH₄ in ethanol forms the chiral diamines 1–3 (Schemes 1–3). Subsequent 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 elemental analyses. The ¹H 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 C=N stretches (at ca. 1640 cm⁻¹). The C₂ symmetric structure of L5 was also confirmed by X-ray diffraction analysis (Fig. 1).

Scheme 1 Synthesis of complexes 7–14.

Scheme 2 Synthesis of complex 15.

Scheme 3 Synthesis of complexes 16–19.

Fig. 1 Molecular structure of the cation in L5 (thermal ellipsoids drawn at the 35% probability level).

Synthesis and characterization of complexes

Amine elimination between M(NMe₂)₄ and protic reagents is a very efficient way for the synthesis of group 4 metal amide complexes.¹⁶ Hence, a similar reaction is expected for the acidic protons in the ligands L4 (L4a and L4b), L5 and L6 and metal amides. In fact, treatment of M(NR₂)₄ (M = Ti, Zr, Hf; R = Me, Et) 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, in good yields (Scheme 1). Similarly, the reactions of L5 or L6 with 1 equiv. of M(NR₂)₄ (M = Ti, Zr, Hf; R = Me, Et) also afford the chiral titanium amide (L6)Ti(NMe₂)(Cl)(THF) (16), 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, in good yields (Schemes 2 and 3).

Complexes 7–19 are stable in a dry nitrogen atmosphere, while they are very sensitive to moisture. They are soluble in organic solvents such as THF, DME, pyridine, toluene, and benzene, and only sparingly soluble in aliphatic solvents such as n-hexane. They have been characterized by various spectroscopic techniques and elemental analyses. The 1 : 1 : 1 ratio between the NR₂ (R = Me, Et) group, the coordinate THF and the ligand L4, L5 or L6 is established by ¹H NMR spectroscopy. Furthermore, the characteristic O–H and C=N stretches at ca. 3420 and 1640 cm⁻¹ in L4–L6 disappear upon treatment with M(NR₂)₄, supporting the formation of complexes 7–19. The solid-state structures of 10 and 14–19 have further been confirmed by X-ray diffraction analyses.

Complexes 16 and 17 are isostructural. The M⁴⁺ ion features a distorted-octahedral ligand environment in (L4)Zr(NEt₂)(Br)(THF) (10), (L4)Zr(NEt₂)(Cl)(THF) (14), (L5)Zr(NMe₂)(Cl)(THF) (15), (L6)Ti(NMe₂)(Cl)(THF) (16), (L6)Zr(NMe₂)(Cl)(THF) (17), (L6)Hf(NMe₂)(Cl)(THF) (18) and (L6)Zr(NEt₂)(Cl)(THF) (19) (Fig. 2–8). Complexes 10 and 14–19 represent, to our knowledge, the first example of the structurally characterized group 4 chiral NHC–metal complex. The average M–O distances are 1.993(2) Å for Ti, 2.110(2) Å to 2.139(2) Å for Zr, and 2.098(2) Å for Hf. The Zr–Br distance is 2.661(1) Å (for 10), and the M–Cl distance is 2.410(1) Å for Ti, 2.498(1) Å to 2.515(1) Å for Zr, and 2.478(1) Å for Hf. The M–C(carbene) distances of 2.252(2) Å for Ti, 2.397(4) Å to 2.419(2) Å for Zr, and 2.370(3) Å for Hf are in the typical range for a M–C σ-bond. These structural data can be compared with those found in [η³-O,C,O-{(3,5-(Me₃C)₂-C₆H₂O)₂N₂C₃H₄}]M(OⁱPr)(Cl)(THF) (M = Ti, Zr)^17a,b and [η³-O,C,O-{(3,5-(Me₃C)₂-C₆H₂O)₂N₂C₃H₄}]MCl₂(THF) (M = Ti, Zr).^17c,d Furthermore, the M–NR₂ (R = Me, Et) distances are short with 1.901(2) Å for Ti, 2.044(2) Å to 2.052(3) Å for Zr, and 2.041(3) Å for Hf. This in combination with the planar geometry around the nitrogen atom N(3) suggests that the sp²-hybridized N-atom engages in N(p_π)→M(d_π) interactions.¹⁶

Fig. 2 Molecular structure of 10 (thermal ellipsoids drawn at the 35% probability level).

Fig. 3 Molecular structure of 14 (thermal ellipsoids drawn at the 35% probability level).

Fig. 4 Molecular structure of 15 (thermal ellipsoids drawn at the 35% probability level).

Fig. 5 Molecular structure of 16 (thermal ellipsoids drawn at the 35% probability level).

Fig. 6 Molecular structure of 17 (thermal ellipsoids drawn at the 35% probability level).

Fig. 7 Molecular structure of 18 (thermal ellipsoids drawn at the 35% probability level).

Fig. 8 Molecular structure of 19 (thermal ellipsoids drawn at the 35% probability level).

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 zirconium and hafnium complexes 8–10 and 12–14 complete conversion 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 polymerization of rac- LA. The formed polylactides have experimental M_n values (M_n,exp) that are very close to the calculated M_n,calcd values and that the molar mass distributions are very narrow (M_w/M_n = 1.18–1.21; Table 4, entries 2–6). In addition, for complex 8 a first order kinetic dependence on the concentration of rac-LA and no induction period were observed (Fig. 9). The M_n,exp values increase linearly with the monomer conversion, whereas the M_w/M_n 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 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 catalysts.^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-O,C,O-{(3,5-(Me₃C)₂-C₆H₂O)₂N₂C₃H₄}]M(OⁱPr)(Cl)(THF) (M = Ti, Zr),^17a,b while the microstructures of the resulting polylactides are similar to those initiated by [(R)-(2-O-C₆H₄)CH=NCH(Me)(C₆H₅)]₂Zr(OⁱPr)₂.^5c

Fig. 9 Ln([LA]₀/[LA]) vs. time plot for the ROP of rac-LA initiated by complex 8. Conditions: precat./isopropanol/LA (mol/mol/mol) = 1/1/150, [LA] = 0.5 mol L⁻¹, solvent = toluene, T = 70 °C. k_obs = 3.14 × 10⁻³ s⁻¹.

Fig. 10 M _n and M_w/M_nvs. conversion plots for the ROP of rac-LA initiated by complex 8. Conditons: precat./isopropanol/LA (mol/mol/mol) = 1/1/150, [LA] = 0.5 mol L⁻¹, solvent = toluene, T = 70 °C.

Table 4 Polymerization of rac-lactide catalyzed by chiral group 4 NHC-complexes 7–19^a

Entry	Precat.	Solvent	Conv. (%)	M n,exp ^b (kg mol^−1)	M n,calcd ^c (kg mol^−1)	M w/Mn ^b	P r ^d
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 P r 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.
1	7 (Ti)	Toluene	65	22.9	23.4	1.18	0.70
2	8 (Zr)	Toluene	100	35.4	36.0	1.21	0.74
3^e	8 (Zr)	Toluene	100	14.9	14.4	1.20	0.73
4^f	8 (Zr)	Toluene	100	21.3	21.6	1.18	0.73
5^g	8 (Zr)	Toluene	100	29.2	28.8	1.19	0.74
6^h	8 (Zr)	Toluene	92	40.4	39.7	1.21	0.73
7	9 (Hf)	Toluene	100	36.2	36.0	1.21	0.72
8	10 (Zr)	Toluene	100	35.4	36.0	1.23	0.67
9	11 (Ti)	Toluene	70	24.6	25.2	1.20	0.72
10	12 (Zr)	Toluene	100	36.8	36.0	1.23	0.70
11	13 (Hf)	Toluene	100	35.3	36.0	1.21	0.66
12	14 (Zr)	Toluene	100	35.7	36.0	1.25	0.70
13	15 (Zr)	Toluene	95	34.6	34.2	1.22	0.69
14	16 (Ti)	Toluene	30	11.2	10.8	1.16	0.69
15	17 (Zr)	Toluene	94	32.7	33.8	1.24	0.68
16	18 (Hf)	Toluene	92	32.6	33.1	1.27	0.67
17	19 (Zr)	Toluene	90	32.8	32.4	1.22	0.71
18	7 (Ti)	THF	35	12.4	12.6	1.20	0.70
19	8 (Zr)	THF	62	22.6	22.3	1.26	0.72
20	9 (Hf)	THF	64	23.2	23.0	1.31	0.70
21	10 (Zr)	THF	58	19.9	20.9	1.36	0.68
22	11 (Ti)	THF	38	13.5	13.7	1.21	0.71
23	12 (Zr)	THF	59	21.6	21.2	1.32	0.72
24	13 (Hf)	THF	62	21.8	22.3	1.35	0.70
25	14 (Zr)	THF	60	22.3	21.6	1.36	0.67
26	15 (Zr)	THF	48	18.5	17.3	1.32	0.70
27	16 (Ti)	THF	16	5.9	5.8	1.19	0.68
28	17 (Zr)	THF	50	17.2	18.0	1.34	0.69
29	18 (Hf)	THF	45	15.8	16.2	1.30	0.71
30	19 (Zr)	THF	48	16.4	17.3	1.35	0.70

---

Conclusions

Chiral group 4 NHC–metal complexes were prepared and structurally characterized. These complexes represent the first example of the structurally characterized group 4 chiral NHC–metal complex, and they can initiate the ring-opening polymerization of rac-lactide in the presence of isopropanol, leading to the heterotactic-rich polylactides. Nevertheless, the reactivity is strongly influenced by the size of the metal ion and the solvents. 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 zirconium 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 complexes towards other asymmetric reactions and the exploration of new group 4 metal complexes based on chiral ligands.

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).

Notes and references

1. For selected reviews, see: (a) E. Chiellini and R. Solaro, Adv. Mater., 1996, 8, 305–313; (b) R. E. Drumright, P. R. Gruber and D. E. Henton, Adv. Mater., 2000, 12, 1841–1846; (c) M. J. Stanford and A. P. Dove, Chem. Soc. Rev., 2010, 39, 486–494; (d) R. H. Platel, L. M. Hodgson and C. K. Williams, Polym. Rev., 2008, 48, 11–63; (e) K. E. Uhrich, S. M. Cannizzaro, R. S. Langer and K. M. Shakesheff, Chem. Rev., 1999, 99, 3181–3198.
2. (a) M. S. Reeve, S. P. McCarthy, M. J. Downey and R. A. Gross, Macromolecules, 1994, 27, 825–831; (b) J. R. Sarasua, R. E. Prud'homme, M. Wisniewski, A. LeBorgne and N. Spassky, Macromolecules, 1998, 31, 3895–3905; (c) N. Nomura, J. Hasegawa and R. Ishii, Macromolecules, 2009, 42, 4907–4909.
3. (a) Y. Ikada, K. Jamshidi, H. Tsuji and S. H. Hyon, Macromolecules, 1987, 20, 904–906; (b) H. Tsuji and Y. Ikada, Polymer, 1999, 40, 6699–6708.
4. For selected recent reviews, see: (a) B. J. O'Keefe, M. A. Hillmyer and W. B. Tolman, J. Chem. Soc., Dalton Trans., 2001, 2215–2224; (b) O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou, Chem. Rev., 2004, 104, 6147–6176; (c) N. E. Kamber, W. Jeong, R. M. Waymouth, R. C. Pratt, B. G. G. Lohmeijer and J. L. Hedrick, Chem. Rev., 2007, 107, 5813–5840; (d) C. M. Thomas, Chem. Soc. Rev., 2010, 39, 165–173; (e) A. Sauer, A. Kapelski, C. Fliedel, S. Dagorne, M. Kol and J. Okuda, Dalton Trans., 2013, 42, 9007–9023.
5. (a) F. Zhang, H. Song and G. Zi, J. Organomet. Chem., 2010, 695, 1993–1999; (b) R. H. Howard, C. Alonso-Moreno, L. M. Broomfield, D. L. Hughes, J. A. Wright and M. Bochmann, Dalton Trans., 2009, 8667–8682; (c) A. J. Chmura, D. M. Cousins, M. G. Davidson, M. D. Jones, M. D. Lunn and M. F. Mahon, Dalton Trans., 2008, 1437–1443; (d) J. Lee, Y. Kim and Y. Do, Inorg. Chem., 2007, 46, 7701–7703; (e) M. Hu, M. Wang, P. Zhang, K. Jin, Y. Chen and L. Sun, Polym. Bull., 2012, 68, 1789–1799.
6. For selected reviews, see: (a) M. C. Perry and K. Burgess, Tetrahedron: Asymmetry, 2003, 14, 951–961; (b) V. César, S. Bellemin-Laponnaz and L. H. Gade, Chem. Soc. Rev., 2004, 33, 619–636; (c) S. Roland and P. Mangeney, Top. Organomet. Chem., 2005, 15, 191–229; (d) R. E. Douthwaite, Coord. Chem. Rev., 2007, 251, 702–717; (e) L. H. Gade and S. Bellemin-Laponnaz, Coord. Chem. Rev., 2007, 251, 718–725; (f) D. R. Snead, H. Seo and S. Hong, Curr. Org. Chem., 2008, 12, 1370–1387; (g) L. N. Gu, G. B. Zhu, H. B. Song and G. F. Zi, Chin. J. Org. Chem., 2009, 29, 1499–1507.
7. T. J. Seiders, D. W. Ward and R. H. Grubbs, Org. Lett., 2001, 3, 3225–3228.
8. (a) D. Martin, S. Kehrli, M. d'Augustin, H. Clavier, M. Mauduit and A. Alexakis, J. Am. Chem. Soc., 2006, 128, 8416–8417; (b) K. S. Lee and A. H. Hoveyda, J. Am. Chem. Soc., 2010, 132, 2898–2900.
9. S. Lee and J. F. Hartwig, J. Org. Chem., 2001, 66, 3402–3415.
10. M. C. Perry, X. H. Cui, M. T. Powell, D. R. Hou, J. H. Reibenspies and K. Burgess, J. Am. Chem. Soc., 2003, 125, 113–123.
11. (a) W. A. Herrmann, L. J. Goossen, C. Köcher and G. R. Artus, Angew. Chem., Int. Ed. Engl., 1996, 35, 2805–2807; (b) W.-L. Duan, M. Shi and G.-B. Rong, Chem. Commun., 2003, 2916–2917.
12. B. Cardinal-David, D. E. A. Raup and K. A. Scheidt, J. Am. Chem. Soc., 2010, 132, 5345–5347.
13. G. M. Sheldrick, SADABS, Program for Empirical Absorption Correction of Area Detector Data, University of Göttingen, Göttingen, Germany, 1996.
14. (a) G. M. Sheldrick, SHELXL-97, Program for the Refinement of Crystal Structure from Diffraction Data, University of Göttingen, Göttingen, Germany, 1997; (b) G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 2008, 64, 112–122.
15. SQUEEZE: P. V. D. Sluis and A. L. Spek, Acta Crystallogr., Sect. A: Found. Crystallogr., 1990, 46, 194–201.
16. For selected papers, see: (a) G. Zi, Q. Wang, L. Xiang and H. Song, Dalton Trans., 2008, 5930–5944; (b) L. Xiang, H. Song and G. Zi, Eur. J. Inorg. Chem., 2008, 1135–1140; (c) G. Zi, X. Liu, L. Xiang and H. Song, Organometallics, 2009, 28, 1127–1137; (d) G. Zi, F. Zhang, X. Liu, L. Ai and H. Song, J. Organomet. Chem., 2010, 695, 730–739; (e) G. Zi, F. Zhang, L. Xiang, Y. Chen, W. Fang and H. Song, Dalton Trans., 2010, 39, 4048–4061; (f) Q. Wang, H. Song and G. Zi, J. Organomet. Chem., 2010, 695, 1583–1591; (g) G. Zi, J. Organomet. Chem., 2011, 696, 68–75.
17. (a) C. Romain, B. Heinrich, S. Bellemin-Laponnaz and S. Dagorne, Chem. Commun., 2012, 48, 2213–2215; (b) C. Romain, L. Brelot, S. Bellemin-Laponnaz and S. Dagorne, Organometallics, 2010, 29, 1191–1198; (c) H. Aihara, T. Matsuo and H. Kawaguchi, Chem. Commun., 2003, 2204–2205; (d) D. Zhang, H. Aihara, T. Watanabe, T. Matsuo and H. Kawaguchi, J. Organomet. Chem., 2007, 692, 234–242; (e) S. Barroso, S. R. M. M. de Aguiar, R. F. Munhá and A. M. Martins, J. Organomet. Chem., 2013 DOI:10.1016/j.jorganchem.2013.11.041.
18. (a) Q. Wang, L. Xiang, H. Song and G. Zi, J. Organomet. Chem., 2009, 694, 691–696; (b) Q. Wang, F. Zhang, H. Song and G. Zi, J. Organomet. Chem., 2011, 696, 2186–2192; (c) A. Amgoune, C. M. Thomas, T. Roisnel and J.-F. Carpentier, Chem.–Eur. J., 2006, 12, 169–179; (d) N. Zhao, Q. Wang, G. Hou, H. Song and G. Zi, J. Organomet. Chem., 2014, 754, 51–58; (e) N. Zhao, Q. Wang, G. Hou, H. Song and G. Zi, Inorg. Chim. Acta, 2014, 413, 128–135; (f) W. Ren, L. Chen, N. Zhao, Q. Wang, G. Hou and G. Zi, J. Organomet. Chem., 2014, 758, 65–72.
19. (a) J. E. Kasperczyk, Macromolecules, 1995, 28, 3937–3939; (b) K. A. M. Thakur, R. T. Kean, E. S. Hall, M. A. Doscotch, J. I. Siepmann and E. J. Munson, Macromolecules, 1997, 30, 2422–2428; (c) J. E. Kasperczyk, Polymer, 1999, 40, 5455–5458; (d) T. M. Ovitt and G. W. Coates, J. Am. Chem. Soc., 2002, 124, 1316–1326; (e) M. T. Zell, B. E. Padden, A. J. Paterick, K. A. M. Thakur, R. T. Kean, M. A. Hillmyer and E. J. Munson, Macromolecules, 2002, 35, 7700–7707; (f) F. Drouin, P. O. Oguadinma, T. J. J. Whitehorne, R. E. Prud'homme and F. Schaper, Organometallics, 2010, 29, 2139–2147.

---

Footnote

† 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

---