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Synthesis and Characterization of Yttrium and Ytterbium Complexes Supported by Salen Ligands and Their Catalytic Properties for racLactide Polymerization Weikai Gu,† Pengfei Xu,† Yaorong Wang,*,† Yingming Yao,*,†,‡ Dan Yuan,† and Qi Shen† †

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 ‡ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China S Supporting Information *

ABSTRACT: A series of yttrium and ytterbium complexes supported by Salen ligands with different steric and electronic properties were synthesized, and their catalytic performances for the polymerization of rac-lactide (rac-LA) were explored. The phenol elimination reactions of (ArO)3Ln(THF) (ArO = 2,6-But2-4-MeC6H2O) with a number of Salen ligands (CH3)2C[CH2NCH(C6H2-2-OH-3,5-R2)]2 [L1H2, R = H; L2H2, R = Cl; L3H2, R = But; L4H2, R = CMe2Ph] and CH2[CH2NCH(C6H2-2-OH-3,5-Cl2)]2 (L5H2), in a 1:1 molar ratio gave yttrium and ytterbium Salen aryloxides L1Ln(OAr)(THF)2 (Ln = Y (1), Yb (2)), L2Ln(OAr)(THF)n (Ln = Y (3), n = 2; Ln = Yb (4), n = 1), L3Y(OAr) (5), L4Y(OAr)(THF) (6), and L5Yb(OAr)(THF) (7), respectively. The amine elimination reactions of L3H2 with Y[N(SiMe3)2]3 in a 1:1 molar ratio and then with 1 equiv of phenol ArOH, and alcohols PhCH2OH and PriOH, produced complex 5 and the yttrium Salen alkoxides [L3Y(μ-OCH2Ph)]2 (8) and [L3Y(μ-OPri)]2 (9), respectively. X-ray structural determination showed that complexes 1−4 and 7 have a THF-solvated monomeric structure, and complex 5 has an unsolvated monomeric structure, whereas complex 8 has an unsolvated dimeric structure. All of these complexes can initiate the ring-opening polymerization of rac-LA at 30 °C in THF. It was found for the first time that the overall coordination environment around the metal center has an obvious influence on the stereoselectivity of these lanthanide Salen complexes. Five-coordinated complex 5 with bulky tert-butyl substituent groups on the phenyl rings displayed apparently lower stereoselectivity than seven-coordinated complex 1, although there is no substituent on the ortho-positions of the Salen ligand L1 in the latter. Complex 7 bearing Cl substituents on the Salen ligand showed the highest stereoselectivity among these lanthanide complexes for rac-LA polymerization, and heterotactic polylactides (Pr up to 0.88) can be obtained at 30 °C in THF.

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INTRODUCTION The stereocontrolled ring-opening polymerization (ROP) of rac-lactide (rac-LA) is an important topic in recent years because the mechanical, physical, and degradable properties of polylactides (PLAs) greatly depend on their microstructures.1,2 For example, syndiotactic and isotactic PLAs are crystalline polymers with a high melting temperature, whereas heterotactic PLAs are amorphous and show no melting point. Extensive research has demonstrated that many discrete complexes, including aluminum,1a−c,3,4 indium,5 zinc,6 lanthanide,7−9 and other metals,10 are good stereoselective initiators for the ROP of rac-LA. Among these complexes, lanthanide complexes have attracted considerable attention due to their high activity, high © 2015 American Chemical Society

stereoselectivity, and good controllability for the polymerization. Previously reported results revealed that the structure of the ancillary ligands has a profound influence on the stereoselectivity of rac-LA polymerization. Generally, the selectivity of the initiator increases with increasing the bulkiness of the ancillary ligands. For instance, Carpentier et al. found that the yttrium complex stabilized by an alkoxy-aminobis(phenolate) ligand bearing bulky tert-butyl substituents at the ortho-positions can initiate the stereoselective ROP of racLA to produce heterotactic PLAs with Pr = 0.80, whereas the Received: March 18, 2015 Published: June 9, 2015 2907

DOI: 10.1021/acs.organomet.5b00223 Organometallics 2015, 34, 2907−2916

Article

Organometallics

mL/min at 40 °C. MALDI-TOF mass spectra were recorded using a Bruker Reflex II mass spectrometer. Synthesis of L1Y(OAr)(THF)2 (1). A solution of Y(OAr)3(THF) (0.108 M, 9.2 mL, 1.0 mmol) was added to a THF solution of L1H2 (0.31 g, 1.0 mmol). The mixture was stirred 12 h at room temperature, and then the precipitate formed was removed by centrifugation. Colorless crystals were obtained from a concentrated THF solution at room temperature in several days (0.52 g, 69%). Anal. Calcd for C42H59N2O5Y: C, 66.30; H, 7.82; N, 3.68. Found: C, 65.94; H, 7.92; N, 3.69. 1H NMR (400 MHz, C4D8O, 25 °C): δ 8.05 (s, 2H, CH N), 7.16 (m, 4H, ArH), 6.63 (s, 2H, ArH), 6.54 (d, 2H, J = 8.3 Hz, ArH), 6.45 (m, 2H, ArH), 3.54 (s, 2H, α−CH2, THF), 3.32 (s, 4H, NCH2), 2.01 (s, 3H, CH3), 1.69 (s, 2H, β−CH2, THF), 1.12 (s, 18H, C(CH3)3), 0.88 (s, 6H, C(CH3)2). 13C{1H} NMR (100 MHz, C4D8O, 25 °C): δ 170.9 (CHN), 168.3, 163.0, 138.9, 135.5, 134.7, 125.6, 123.7, 122.0, 121.6, 114.4 (Ar-C), 69.9 (NCH2), 68.4 (α-CH2, THF), 37.1 (C(CH3)3), 35.7 (C(CH3)2), 31.5 (C(CH3)3), 26.5 (β-CH2, THF), 24.8 (C(CH3)2), 21.5 (CH3). IR (KBr pellet, cm−1): 2948(s), 2897(m), 2862(m), 1619(w), 1540(m), 1450(s), 1398(m), 1230(s), 1196(m), 1146(m), 1124(m), 1062(w), 1036(m), 904(m), 890(m), 824(w), 738(m), 668(m), 657(m). Synthesis of L1Yb(OAr)(THF)2 (2). The synthesis of complex 2 was carried out in the same way as that described for complex 1, but Yb(OAr)3(THF) (0.169 M, 5.9 mL, 1.0 mmol) was used instead of Y(OAr)3(THF). Yellow crystals were obtained in concentrated THF solution (0.55 g, 66%). Anal. Calcd for C42H59N2O5Yb: C, 59.70; H, 7.04; N, 3.32. Found: C, 59.49; H, 7.22; N, 3.24. IR (KBr pellet, cm−1): 2950(s), 2899(m), 2865(m), 1611(w), 1541(m), 1474(s), 1397(m), 1262(s), 1189(m), 1147(m), 1123(m), 1062(w), 1036(m), 989(m), 890(m), 860(w), 756(m), 669(m), 658(m). Synthesis of L2 Y(OAr)(THF)2 (3). To a THF solution of Y(OAr)3(THF) (0.108 M, 9.2 mL, 1.0 mmol) was added a THF solution of L2H2 (0.45 g, 1.0 mmol) slowly. The mixture was stirred 12 h at room temperature, and then THF was evaporated completely under reduced pressure. The resulted yellow solid was washed with hexane and crystallized from a mixture of THF (0.5 mL) and toluene (6 mL) solution at room temperature to afford complex 3 as yellow crystals (0.61 g, 68%). Anal. Calcd for C42H55Cl4N2O5Y: C, 56.14; H, 6.17; N, 3.12. Found: C, 56.01; H, 6.12; N, 3.22. 1H NMR (400 MHz, C4D8O, 25 °C): δ 8.10 (s, 2H, CHN), 7.41 (d, 2H, J = 2.8 Hz, ArH), 7.24 (d, 2H, J = 2.8 Hz, ArH), 6.68 (s, 2H, ArH), 3.54 (s, 8H, αCH2, THF), 3.44 (br, 2H, NCH2), 3.22 (br, 2H, NCH2), 2.02 (s, 3H, CH3), 1.69 (s, 8H, β-CH2, THF), 1.11 (s, 18H, C(CH3)3), 0.90 (s, 6H, C(CH3)2). 13C{1H} NMR (100 MHz, C4D8O, 25 °C): δ 170.1 (CH N), 161.6, 138.8, 134.1, 133.4, 126.9, 126.0, 124.9, 122.4, 117.8 (ArC), 73.6 (NCH2), 68.4 (α-CH2, THF), 37.1 (C(CH3)3), 35.7 (C(CH3)2), 31.8 (C(CH3)3), 30.9 (C(CH3)2), 26.6 (β-CH2, THF), 21.5 (CH3). IR (KBr pellet, cm−1): 2956(s), 2904(m), 2869(m), 1613(w), 1525(m), 1414(s), 1391(m), 1331(w), 1247(s), 1210(m), 1166(m), 1062(w), 998(m), 863(w), 755(m), 711(m), 679(m). Synthesis of L2Yb(OAr)(THF) (4). Following the procedure described for the synthesis of complex 3, reaction of L2H2 (0.45 g, 1.0 mmol) with Yb(OAr)3(THF) (0.169 M, 5.9 mL, 1.0 mmol) in THF gave complex 4 as yellow crystals (0.57 g, 63%). Anal. Calcd for C38H47Cl4N2O4Yb: C, 50.12; H, 5.20; N, 3.08. Found: C, 50.23; H, 5.26; N, 2.99. IR (KBr pellet, cm−1): 2953(s), 2913(m), 2868(m), 1619(w), 1524(m), 1455(s), 1391(m), 1296(w), 1230(s), 1211(m), 1167(m), 1055(w), 979(m), 862(w), 755(m), 681(m), 621(m). Synthesis of L3Y(OAr) (5). Method A. The synthesis of complex 5 was carried out in the same way as that described for complex 3, but L3H2 (0.53 g, 1.0 mmol) was used instead of L2H2. Yellow crystals were obtained in n-hexane solution (0.45 g, 54%). Anal. Calcd for C50H75N2O3Y: C, 71.40; H, 8.99; N, 3.33. Found: C, 71.87; H, 9.11; N, 3.35. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.76 (s, 2H, J = 2.6 Hz, CHN), 7.74 (d, 2H, J = 2.6 Hz, ArH), 7.16 (s, 2H, overlap with residual signal of C6D6, ArH), 7.13 (d, 2H, J = 2.6 Hz, ArH), 4.01 (d, 2H, J = 12.5 Hz, NCH2), 2.64 (d, 2H, J = 12.7 Hz, NCH2), 2.34 (s, 3H, CH3), 1.70 (s, 18H, C(CH3)3), 1.51 (s, 18H, C(CH3)3), 1.34 (s, 18H) (C(CH3)3), 0.65 (s, 3H, C(CH3)2), 0.24(s, 3H, C(CH3)2). 13 C{1H} NMR (100 MHz, C6D6, 25 °C): δ 173.6 (CHN), 164.9,

yttrium complex stabilized by a similar ligand with methyl substituents initiates rac-LA polymerization with apparently low selectivity (Pr = 0.56).7b To further explore the relationship between the structure of lanthanide complex and the stereoselectivity of rac-LA polymerization, it is essential to design and synthesize lanthanide complexes with different ancillary ligands. Salen ligands, as one of the alternatives to cyclopentadienyl ligand, have been widely utilized in coordination chemistry of transition metals11 due to their advantages of easy preparation, low cost, and tunable steric and electronic properties. In comparison, the application of these ligands in organolanthanide chemistry is not popular.12 However, it has been found that the lanthanide complexes bearing Salen ligands have great potential in homogeneous catalysis, and a lot of lanthanide Salen complexes can catalyze a number of chemical transformations,13 including asymmetric Aldol−Tishchenko reactions,14 asymmetric nitro-Mannich reactions,15 enantioselective ring-opening reactions of aziridines,16 and ROP of εcaprolactone and L-LA.17 In contrast, the stereocontrolled ROP of rac-LA initiated by Salen lanthanide complexes still remains relatively unexplored.3b,18 Only the yttrium complex with racbinaphthyl-bridged Salen ligand is a hetereoselective initiator for the ROP of rac-LA,18b whereas the other lanthanide Salen complexes show no selectivity. In order to understand the influence of the structure of lanthanide Salen complex on the stereoselectivity for rac-LA polymerization, a series of yttrium and ytterbium complexes stabilized by several different Salen ligands (Figure 1) were synthesized, and their catalytic

Figure 1. Salen ligands.

behaviors for the ROP of rac-LA were explored. It was found that the overall coordination environment around the metal center in these lanthanide Salen complexes can also apparently affect the heteroselectivity. Herein we report these results.

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EXPERIMENTAL SECTION

General Procedures. All the manipulations were performed under a purified argon atmosphere using standard Schlenk techniques. Solvents were degassed and distilled from sodium benzophenone ketyl under argon prior to use. Ln(ArO)3(THF)19 (Ln = Y, Yb; ArO = 2,6But2-4-MeC6H2O), Y[N(SiMe3)2]3,20 and Salen ligands L1H2−L5H221 were prepared according to the procedures reported in the literature. rac-LA was recrystallized twice from ethyl acetate, sublimed under reduced pressure, and then recrystallized from dry toluene prior to use. Carbon, hydrogen, and nitrogen analyses were performed by direct combustion with a Carlo-Erba EA-1110 instrument. The IR spectra were recorded with a Nicolet-550 FTIR spectrometer as KBr pellets. The 1H and 13C NMR spectra were recorded in a C6D6 (or C4D8O) solution for lanthanide complexes, and the microstructures of PLAs were measured by homodecoupling 1H NMR spectroscopy at 20 °C in CDCl3 on a Unity Varian AC-400 spectrometer. Molecular weight and molecular weight distribution (PDI) of PLAs were determined against a polystyrene standard by gel permeation chromatography (GPC) on a PL 50 apparatus, and THF was used as an eluent at a flow rate of 1.0 2908


Article

Organometallics

MHz, C6D6, 25 °C): δ 7.77 (s, 4H, CHN), 7.68 (d, 4H, J = 2.6 Hz, ArH), 7.14 (d, 4H, J = 2.6 Hz, ArH), 4.58 (m, 2H, OCH(CH3)2), 4.06 (d, 4H, J = 11.7 Hz, NCH2), 2.68 (d, 4H, J = 11.7 Hz, NCH2), 1.65 (s, 36H, C(CH3)3), 1.37 (s, 36H, C(CH3)3), 1.31 (d, 12H, J = 6.0 Hz, OCH(CH3)2), 0.78 (s, 6H, C(CH3)2), 0.41 (s, 6H, C(CH3)2). 13 C{1H} NMR (100 MHz, C6D6, 25 °C): δ 170.9 (CHN), 165.4, 140.0, 136.6, 130.2, 129.7, 122.5 (Ar-C), 74.4 (NCH2), 65.9 (OCH(CH3)2), 37.6 (C(CH3)3), 36.2 (C(CH3)3), 34.8, 34.5 (C(CH3)2), 32.1, 30.7 (C(CH3)3), 29.0 (OC(CH3)2), 27.5, 23.1 (C(CH3)2). IR (KBr pellet, cm−1): 2957(s), 2913(s), 2865(s), 1612(w), 1537(m), 1472(s), 1415(s), 1379(m), 1315(w), 1238(s), 1202(m), 1164(m), 1025(w), 1010(m), 869(w), 754(m), 697(m), 628(m). Typical Procedure for the Polymerization Reaction. All polymerizations of rac-LA were performed in the glovebox, and the typical polymerization procedure is as given below. A 20 mL vial, equipped with a magnetic stirring bar, was charged with the desired amount of monomer and solvent. After the monomer was dissolved, a solution of the initiator was added to this solution by pipet. The mixture was stirred to desired time, and then each vial was taken out from the glovebox and quenched by ethanol. The mixture was then poured into hexane to precipitate the polymer, which was dried in a vacuum oven at 50 °C for 12 h and weighed. Oligomer Preparation. The oligomerization of rac-LA was carried out with complex 7 as the initiator in THF at 30 °C, in a molar ratio of [M]0/[I]0 = 10 ([M]0 = 0.5 mol/L). The reaction mixture was stirred for 10 min and then quenched by addition of n-hexane and one drop of water. The precipitated oligomer was collected and dried under vacuum. X-ray Crystallographic Structure Determination. Suitable single crystals of complexes 1−5, 7, and 8 were mounted on glass fibers for determining the single-crystal structures. Intensity data were collected with an Agilent Gemini Atlas CCD (λ = 0.71073 Å) or a Rigaku Mercury CCD (λ = 0.71075 Å) area detector in ω scan mode utilizing Mo Kα radiation. The diffracted intensities were corrected for Lorentz/polarization effects and empirical absorption corrections. Details of the intensity data collection and crystal data are given in the Supporting Information (Table S1). The structures were solved by direct methods and refined by fullmatrix least-squares procedures on |F|2. The hydrogen atoms were generated geometrically, assigned appropriate isotropic thermal parameters, and allowed to ride on their parent carbon atoms. All of the hydrogen atoms were included in the structure factor calculation in the final stage of full-matrix least-squares refinement. The structure solution and refinement were performed by using SHELXL-97.

160.7, 140.2, 138.1, 131.4, 130.5, 125.8, 125.0, 122.6 (Ar-C), 75.3 (NCH2), 36.4 (C(CH3)3), 36.2 (C(CH3)3), 35.0 (C(CH3)2), 34.5 (C(CH3)3), 32.0 (C(CH3)3), 31.2 (C(CH3)3), 30.9 C(CH3)3, 27.8(C(CH3)2), 22.0 (CH3), 21.4 (C(CH3)2). IR (KBr pellet, cm−1): 2950(s), 2860(m), 1612(w), 1530(m), 1410(s), 1373(m), 1316(w), 1240(s), 1204(m), 1165 (m), 1065(w), 1030(m), 872(w), 754(m), 695(m), 622(m). Method B. A solution of L3H2 (0.53 g, 1.0 mmol) in THF (20 mL) was added to a THF solution of Y[N(SiMe3)2]3 (0.368 M, 2.7 mL, 1.0 mmol). The mixture was stirred overnight at room temperature, and then ArOH (0.22 g, 1.0 mmol) was added. The mixture was further stirred for 12 h, and then THF was evaporated completely under reduced pressure. n-Hexane (6 mL) was added to extract the residue. Yellow crystals were obtained at room temperature in several days (0.61 g, 73%). Synthesis of L4Y(OAr)(THF) (6). The synthesis of complex 6 was carried out in the same way as that described for complex 3, but L4H2 (0.78 g, 1.0 mmol) was used instead of L2H2. Yellow crystals were obtained in a mixture solution of toluene (6 mL) and n-hexane (3 mL) (0.76 g, 61%). Anal. Calcd for C74H91N2O4Y: C, 76.53; H, 7.90; N, 2.41. Found: C, 76.28; H, 8.17; N, 2.23. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.61 (s, 2H, CHN), 7.51 (m, 2H, ArH), 7.43 (m, 4H, ArH), 7.35 (m, 3H, ArH), 7.19−7.25 (m, 10H, ArH), 7.09−7.13 (m, 5H, ArH), 6.98−7.01 (m, 2H, ArH), 3.91 (d, J = 12.4 Hz, 2H, NCH2), 3.57 (m, 4H, α-CH2, THF), 2.54 (d, J = 12.6 Hz, 2H, NCH2), 2.40 (s, 3H, CH3), 2.15 (s, 6H, C(CH3)2Ph), 1.85 (s, 6H, C(CH3)2Ph), 1.68 (s, 12H, C(CH3)2Ph), 1.44 (m, 4H, β-CH2, THF), 1.40 (s, 18H, C(CH3)3), 0.63 (s, 3H, C(CH3)2), 0.18 (s, 3H, C(CH3)2). 13C{1H} NMR (101 MHz, C6D6, 25 °C): δ 172.6 (CHN), 164.5, 151.6, 139.7, 137.8, 137.6, 134.6, 132.6, 128.8, 127.5, 127.0, 126.4, 126.0, 125.6, 124.4, 122.9 (Ar-C), 74.2 (NCH2), 68.4 (α-CH2, THF), 44.0 (C(CH3)2Ph), 42.9 (CCH3)2Ph), 36.4 (C(CH3)3), 34.8 (C(CH3)2), 31.5 (C(CH3)3), 30.8 (C(CH3)2Ph), 28.8 (C(CH3)2Ph), 27.3 (C(CH3)2), 26.1 (β-CH2, THF), 22.0 (CH3). IR (KBr pellet, cm−1): 2957(s), 2868(m), 1612(w), 1542(m), 1417(s), 1379(m), 1319(w), 1250(s), 1204(m), 1158(m), 1065(w), 1030(m), 861(w), 766(m), 700(m), 622(m). Synthesis of L5Yb(OAr)(THF) (7). Following the procedure described for the synthesis of complex 3, reaction of L5H2 (0.42 g, 1.0 mmol) with Yb(OAr)3(THF) (0.169 M, 5.9 mL, 1.0 mmol) in THF produced complex 7 as yellow crystals (0.51 g, 58%). Anal. Calcd for C36H43Cl4N2O4Yb: C, 48.99; H, 4.91; N, 3.17. Found: C, 48.74; H, 4.96; N, 3.11. IR (KBr pellet, cm−1): 2956(s), 2906(m), 2867(m), 1619(w), 1523(m), 1465(s), 1397(m), 1311(w), 1237(s), 1210(m), 1167(m), 1048(w), 989(m), 862(w), 765(m), 692(m), 643(m). Synthesis of [L3Y(μ-OCH2Ph)]2 (8). Following the procedure described for the synthesis of complex 5 (Method B), reaction of L3H2 (0.53 g, 1.0 mmol) with Y[N(SiMe3)2]3 (0.368 M, 2.7 mL, 1.0 mmol) and then with benzyl alcohol (0.10 mL, 1.0 mmol), upon extraction with a mixture solvent of toluene (5 mL) and n-hexane (3 mL), gave complex 8 as yellow crystals (0.52 g, 71%). Anal. Calcd for C84H118N4O6Y2: C, 69.21; H, 8.16; N, 3.84. Found: C, 69.54; H, 8.23; N, 3.83. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.67 (d, 4H, J = 2.6 Hz, CHN), 7.51 (s, 4H, ArH), 7.24 (m, 4H, ArH), 7.19 (m, 4H, ArH), 7.02 (m, 6H, ArH), 5.16 (s, 4H,CH2Ph), 3.83 (d, 4H, J = 11.8 Hz, NCH2), 2.48 (d, 4H, J = 12.0 Hz, NCH2), 1.69 (s, 36H, C(CH3)3), 1.35 (s, 36H, C(CH3)3), 0.64 (s, 6H, C(CH3)2), 0.19 (s, 6H, C(CH3)2). 13C{1H} NMR (100 MHz, C6D6, 25 °C): δ 171.9 (CH N), 165.0, 146.1, 139.5, 136.7, 130.7, 130.1, 126.5, 126.3, 122.6 (ArC), 74.6 (NCH2), 68.1 (CH2Ph), 36.7, 36.3 (C(CH3)3), 34.4(C(CH3)2), 32.1, 31.1 (C(CH3)3), 27.6, 23.4, (C(CH3)2). IR (KBr pellet, cm−1): 2957(s), 28(s), 2868(s), 1612(w), 1533(m), 1460(s), 1410(s), 1383(m), 1310(w), 1236(s), 1202(m), 1160 (m), 1025(w), 1010(m), 879(w), 744(m), 695(m), 632(m). Synthesis of [L3Y(μ-OiPr)]2 (9). Following the procedure described for the synthesis of complex 5 (Method B), reaction of L3H2 (0.53 g, 1.0 mmol) with Y[N(SiMe3)2]3 (0.368 M, 2.7 mL, 1.0 mmol) and then with isopropanol (0.07 mL, 1.0 mmol) in THF gave complex 9 as yellow solid (0.49 g, 72%). Anal. Calcd for C78H118N4O6Y2: C, 67.04; H, 8.74; N, 4.11. Found: C, 67.34; H, 8.95; N, 3.95. 1H NMR (400

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RESULTS AND DISCUSSION

Synthesis of Lanthanide Complexes. We previously reported that the protolysis reaction of 4-(2-hydroxy-5methylphenyl)imino-2-pentanone with (ArO) 3 Ln(THF) (ArO = 2,6-But2-4-MeC6H2O) in THF gave the desired lanthanide aryloxides.22 Therefore, a similar method was adopted to synthesize lanthanide aryloxides bearing Salen ligands. It was found that reaction temperature plays a key role in this reaction. When 1 equiv of Y(OAr)3(THF) was added to a THF solution of L1H2 at ambient temperature, the color of the solution changed from yellow to colorless immediately, indicating that the reaction took place. After workup, colorless crystalline solids of yttrium aryloxide L1Y(OAr)(THF)2 (1) were isolated from a concentrated THF solution in 69% yield. However, when the same reaction was conducted in THF at 60 °C, a THF-insoluble product was formed. We postulated that a coordination polymer was formed during the reaction at 60 °C, which is insoluble in THF. A similar phenomenon was also observed by Anwander et al. in the reaction of Salen with yttrium amide.12a At room temperature, treatment of Ln(OAr)3(THF) (Ln = Y, Yb) with a series of Salen ligands 2909


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Organometallics Scheme 1. Synthesis of Complexes 1−7

Scheme 2. Synthesis of Complexes 5, 8, and 9

L1H2−L5H2 in a 1:1 molar ratio in THF afforded the desired lanthanide Salen aryloxides (L1Yb(OAr)(THF)2 (2), L2Ln(OAr)(THF)n [Ln = Y (3), n = 2; Ln = Yb (4), n = 1], L3Y(OAr) (5), L4Y(OAr)(THF) (6), and L5Yb(OAr)(THF) (7) in high isolated yields, as shown in Scheme 1. Generally, lanthanide aryloxo and alkoxo complexes can be obtained conveniently from the corresponding lanthanide amides by the protolysis reactions with various phenols and alcohols.12a,e,23 It is expected that the lanthanide Salen amides, which can be synthesized in situ by the amine elimination reaction of Ln[N(SiMe3)2]3 with Salen ligands, can be converted easily to the lanthanide Salen aryloxo and alkoxo complexes. Indeed, the yttrium aryloxide L3Y(OAr) (5) and yttrium alkoxides [L3Y(μ-OR′)]2 [R′ = CH2Ph (8), Pri (9)] were obtained via the proton exchange reactions of Y[N(SiMe3)2]3 with L3H2 in a 1:l molar ratio in THF and subsequent reaction with 1 equiv of phenol (ArOH) or alcohols (PhCH2OH or PriOH), as shown in Scheme 2. The compositions of complexes 1−9 were confirmed by elemental analysis, IR, and NMR spectroscopy in the case of yttrium Salen complexes. The presence of Salen ligand in these complexes was evidenced by the characteristic NC stretching vibrations at 1611−1619 cm−1 in the IR spectra, as well as by the proton resonances of CHN group at 7.61−8.10 ppm in the 1H NMR spectra of yttrium complexes. In the 1H NMR spectra of complexes 1 and 3, multiple resonances at 3.54 and 1.69 ppm can be attributed to the THF protons, which indicated that complexes 1 and 3 might be a THF-solvated

species. In comparison, no resonance of the THF protons was found in the 1H NMR spectra of complexes 5, 8, and 9, indicating that there are solvent-free species. It is worthy to note that the 1H NMR spectra of complexes 8 and 9 in C6D6 are slightly different from those in C4D8O. For example, The protons of the NCH2− group of complex 8 display one set of resonances at 3.83 and 2.48 ppm in C6D6, whereas two sets of resonance peaks at 3.84 and 2.81 ppm, and 4.08 and 2.97 ppm, respectively, were observed in C4D8O (Figure 2). It is reasonable to postulate that there is an equilibrium between monomeric and dimeric structure in THF solution for complexes 8 and 9. Further structure determination (see below) revealed that complex 8 has dimeric structure, which indicated that only dimeric species exist in benzene or toluene solution for complexes 8 and 9. All the crystals obtained decompose in a few minutes when they are exposed to air, but they are stable in argon. Complexes 1 and 2 are freely soluble in THF and slightly soluble in toluene. Complexes 3, 4, 6, and 7 are soluble in THF and toluene but insoluble in n-hexane, while complexes 5, 8, and 9 are soluble even in n-hexane. Crystal Structures. To provide complete structural information, single-crystal X-ray structural analyses were carried out for complexes 1−5, 7, and 8, and selected bond lengths and angles for these complexes are shown in the Supporting Information, Tables S2 and S3, respectively. Crystals suitable for an X-ray structure determination of complexes 1 and 2 were obtained from a THF solution at room temperature, whereas the crystals of complexes 3, 4, and 7 were grown from a 2910


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Figure 2. 1H NMR spectra of complex 8 in C4D8O (a) and C6D6 (b) solution at 25 °C.

toluene/THF solution, and complexes 5 and 8 were grown from an n-hexane solution. It was found that the bulkiness of the Salen ligands has effect on the solid state structures of these complexes. For the less bulky Salen ligands L1H2 and L2H2, there are one or two coordinated THF molecules bonded to the lanthanide centers in complexes 1−4. For the bulky ligand L3H2, there is no coordinated THF molecule bonded to the metal center in complex 5. X-ray diffraction analyses displayed that complexes 1−3 have solvated monomeric structures and their structure features are similar. Therefore, only the ORTEP diagram of complex 1 is provided in Figure 3. Each of the lanthanide ions is sevencoordinated with two oxygen atoms and two nitrogen atoms from the Salen ligand, one oxygen atom from the aryloxo group, and two oxygen atoms from two THF molecules. The coordination arrangement around the metal center can be best described as a distorted capped trigonal prismatic geometry, in which O(4) from THF molecule occupies the capping position. The dihedral angles between the two phenyl rings of the Salen ligands range from 146.9° to 149.9° in complexes 1−3, whereas the analogous angle in (Salen′)Y(OSiPh3)(THF)(CH3CN) [Salen′ = N,N′-bis(3,5-di-tert-butylsalicylidene)ethane-1,2-diamine] is 161°, indicating the Salen′ ligand adopts a more planar orientation around the yttrium atom.23b The Y−O(Salen) bond lengths in complexes 1 and 3 range from 2.188(2) to 2.221(2) Å, whereas the Y−N(Salen) bond lengths range from 2.494(2) to 2.621(2) Å. These bond lengths are consistent with the corresponding bond lengths in (Salen′)Y(OSiPh3)(THF)(CH3CN).23b The Y−O(Ar) distances of 2.162(2) and 2.136(2) Å in complexes 1 and 3, respectively, are slightly longer than the Y−O(SiPh3) bond length of 2.094(2) in (Salen′)Y(OSiPh3)(THF)(CH3CN).23b The bond distances of Yb−O(Salen) [2.149(4), 2.163(4) Å], Yb−N(Salen)

Figure 3. ORTEP diagram of complex 1 showing the atom-numbering scheme. Thermal ellipsoids are drawn at the 10% probability level, and hydrogen atoms are omitted for clarity.

[2.522(5), 2.480(5) Å], and Yb−O(Ar) [2.125(4) Å] in complex 2 are comparable with the corresponding distances in complexes 1 and 3, when the difference in ionic radii between Y and Yb is considered.24 Complexes 4 and 7 have a THF-solvated monomeric structure in the solid state, and the coordination geometry around the six-coordinated ytterbium atoms can be described as a distorted octahedron for 4 and a distorted trigonal prism for 7 (Supporting Information, Figures S1 and S2). Complex 5 has an unsolvated monomeric structure, and the molecular 2911


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Table 1. Ring-Opening Polymerization of rac-Lactide Initiated by Complexes 1−9

structure is shown in the Supporting Information (Figure S3). The yttrium center in complex 5 is five-coordinated in a distorted square pyramidal fashion by the Salen ligand (L3) and the aryloxy group. The Y−O(Salen) distances [2.125(2) and 2.136(2) Å], the Y−N(Salen) distances [2.402(2) and 2.415(2) Å], and the Y−O(Ar) distance [2.094(2) Å] are comparable to the corresponding values in (Salen′)Sc(OSitBuPh2) [av. 1.972(3), 2.222(2), and 1.901(2) Å], 12c (Salcyc)Sc(OC6H2But2-2,6) [av. 1.978(2), 2.231(2), and 1.946(2) Å] [Salcyc = (1R,2R)-(−)-1,2-cyclohexanediyl)bis(nitrilomethylidyne)bis(2,4-di-tert-butyl)phenol],12e and complexes 1−3 when the difference in metal sizes is considered.24 Complex 8 shows an unsolvated dimeric structure, and the molecular structure is shown in Figure 4. In complex 8, two

entry

cat.

[M]0/ [I]0

time

yield (%)a

Mcb (×104)

Mnc (×104)

Đc

Prd

1 2e 3 4 5 6 7 8 9e 10e 11f 12 13 14 15g 16g 17 18 19

1 1 2 3 3 4 4 5 5 5 5 6 7 7 7 7 7 8 9

200:1 200:1 200:1 200:1 200:1 200:1 200:1 200:1 200:1 200:1 200:1 200:1 200:1 200:1 200:1 200:1 500:1 200:1 200:1

5h 24 h 5h 1h 2h 1h 2.5 h 10 min 10 min 15 min 50 min 5h 2.5 h 3.5 h 5h 7h 9h 10 min 10 min

93 59 84 84 97 61 90 93 78 91 60 93 84 99 80 93 92 82 87

2.6 1.7 2.4 2.4 2.8 1.8 2.5 2.7 2.2 2.6 1.7 2.6 2.4 2.9 2.3 2.7 6.6 2.4 2.5

4.5 3.1 3.5 5.1 5.3 3.6 5.0 8.6 5.83 6.7 8.3 3.6 4.9 4.8 1.9 2.1 7.8 2.9 3.0

1.91 1.80 2.00 1.61 1.71 1.45 1.73 1.89 2.46 2.23 1.72 1.38 1.58 1.74 1.12 1.13 1.65 1.58 1.43

0.80 0.65 0.85 0.80 0.81 0.85 0.84 0.63 0.48 0.51 0.43 0.75 0.88 0.88 0.89 0.88 0.88 0.64 0.63

Polymerization conditions: THF as the solvent, T = 30 °C, [rac-LA] = 1 M. aYield: weight of polymer obtained/weight of monomer used. b Mc = (144.13) × [M]0/[I]0 × polymer yield (%). cMeasured by GPC calibrated with standard polystyrene samples. dMeasured by homodecoupling 1H NMR spectroscopy at 25 °C in CDCl3. eCHCl3 as the solvent. fToluene as the solvent. gThe polymerization was carried out in the presence of 1 equiv of benzyl alcohol. Figure 4. ORTEP diagram of complex 8 showing the atom-numbering scheme. Thermal ellipsoids are drawn at the 10% probability level, and solvent molecules and hydrogen atoms are omitted for clarity.

these lanthanide Salen complexes for rac-LA polymerization. The lanthanide complexes stabilized by ligand L3 bearing tertbutyl substituents on the Salen ligand showed the highest activity among these lanthanide Salen complexes. For example, using complex 5 as the initiator, the yield reached 93% in 10 min in THF when the molar ratio of monomer to initiator was 200 (Table 1, entry 8), whereas it took at least 2 h to reach similar yields under the same polymerization conditions when the other lanthanide Salen complexes were used as the initiators (Table 1, entries 1, 5, and 12). The decreasing activity order of yttrium complexes is 5 > 3 > 1 ≈ 6, reflecting that the influence sequence of the substituents on the Salen ligands is But > Cl > H ≈ CMe2Ph. To our surprise, the lanthanide Salen complexes bearing tert-butyl substituent groups on the phenyl rings (L3) displayed the lowest stereoselectivity among these lanthanide Salen complexes (Table 1, entries 8, 18, and 19). Using complex 5 as the initiator, the Pr value (probability of racemic enchainment) of the resultant PLA is 0.63, whereas the Pr value increases to 0.80 when the initiator has no ortho-substituent group on the phenyl rings of the Salen ligand (complex 1). The decreasing order in heteroselectivity for the substituent group on the Salen ligands is H (Pr = 0.80) ≈ Cl (Pr = 0.81) > CMe2Ph (Pr = 0.75) > But (Pr = 0.63). This order is quite different from that observed in the amine-bridged bis(phenolate) lanthanide systems,7b,c,f in which the heteroselectivity of the corresponding lanthanide complexes increases as the increase of the bulkiness of the ortho-substituent groups on the phenyl rings. In our case, the coordination number (CN) is 7 for complex 1, whereas the CN is 5 for complex 5, which resulted in a more crowded coordination environment around the yttrium atom in complex 1 than that in complex 5. Thus, complex 1 displayed higher stereoselectivity than

OCH2Ph groups bridge the two yttrium centers, forming an Y2O2 core. Each of the yttrium atoms is six-coordinated by two oxygen atoms and two nitrogen atoms from the Salen ligand (L3) and two oxygen atoms from the two OCH2Ph groups to form a distorted trigonal prism. The dihedral angles between the two phenyl rings from the Salen ligand are 82.1 and 77.2°, respectively, which are smaller than those in complexes 1−5 and 7. The average Y−O(Salen) and Y−N(Salen) bond lengths of 2.149(5) and 2.427(6) Å, respectively, are similar to the corresponding values in other Salen yttrium complexes, (Salen′)Y(OC6H2But2-2,6)(THF) (av. 2.172(2), 2.457(3) Å),23a L3Y[N(SiHMe2)2](THF) (av. 2.162(2), 2.457(4) Å),12d and (Salcyc)Y[N(SiHMe2)2](THF) (av. 2.170(3), 2.462(2) Å).13a The average Y−O(CH2Ph) bond length (2.271(5) Å) is in accord with the corresponding values in [(Salan)Y(μ-OiPr)]2 [2.276(2) Å; Salan = MeN(CH2)2MeN{CH2-(2-OC6H2But2-3,5)}2]25 and [(Salalen)Y(μ-OCH2Ph)]2 [2.263(6) Å; Salalen = (2-O-C6H2But2-3,5)CHNCH2CH2N(Me)CH2(2-OC6H2But2-3,5)].8i Ring-Opening Polymerization of rac-LA by Complexes 1−9. To assess the influence of the structures of these lanthanide Salen complexes on the polymerization activity, controllability, and stereoselectivity, the catalytic behavior of complexes 1−9 for the ROP of rac-LA was explored. As illustrated in Table 1, all of these complexes are efficient initiators for rac-LA polymerization in THF at 30 °C to give PLAs with high molecular weights. It can be seen that the structure of the Salen ligand has an obvious effect on the catalytic activity and stereoselectivity of 2912


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4 has a first-order dependence on rac-LA concentration. The apparent rate constant for complex 3 (kapp = 0.0305 ± 0.0009 min−1) is obviously higher than that for complex 4 (kapp = 0.0132 ± 0.0004 min−1) (Figure 5), whereas the Pr value for complex 3 (Pr = 0.81) is slightly lower than that for complex 4 (Pr = 0.84). These results revealed that the catalytic activity for the lanthanide Salen complexes increases as the increase of the ionic radius (ionic radius: Y3+ > Yb3+), whereas the stereoselectivity decreases, which is consistent with those observed in the other lanthanide systems for rac-LA polymerization.7i,k,8i Because complex 7 displayed the highest heteroselectivity for rac-LA polymerization among these lanthanide Salen complexes, the relationship between the number-average molecular weight (Mn) and the molar ratio of monomer to initiator ([M]0/[I]0) was explored using complex 7 as the initiator (Supporting Information, Figure S4). It was found that the number-average molecular weights of PLAs increased linearly with the molar ratios of monomer to initiator, indicating that the polymerization proceeded in somewhat controlled manner. However, the polymer dispersities of the resultant PLAs were relatively larger (Đ = 1.57−1.77), although the polymer dispersity values kept almost unchanged. Generally, the main reasons resulted in large PLA dispersity value are slow initiation and/or transesterification. To understand the reason in our system, the polymerization of rac-LA initiated by complex 7 in the presence of 1 equiv of benzyl alcohol was conducted. It can be seen that the Đ values of the resulting PLAs decreased apparently, and the molecular weights of the PLAs also decreased and were closed to the calculated ones (Table 1, entries 15 and 16). Thus, the relatively larger dispersity value in this system should be mainly attributed to the slow initiation step because of the less nucleophilicity of aryloxo group than alkoxo group. Similar phenomenon was also observed in other lanthanide amide systems.7b,c,8b It is worthy to note that the polymerization in our case is not an immortal polymerization. The polymerization can not proceed when 4 equiv of benzyl alcohol were added. Furthermore, the kinetics of rac-LA polymerization under different monomer/initiator ratios ([M]0/[I]0 = 200−600) were studied at 30 °C using complex 7 as the initiator. In all cases, the plots of ln [M]0/[M]t vs polymerization time at different molar ratios exhibit a good linear relationship (Figure 6), indicating that the polymerization follows first-order kinetics for the monomer concentration. Thus, the rate law of −d[M]/ dt = kapp[M] is suggested, where kapp = kp[I]n and kp is the propagation rate constant. Plotting ln kapp vs ln [I] gives the value of n, the order for the initiator concentration. When ln kapp vs ln [I] is plotted, a straight line with a slope of 0.96 is obtained (Figure 7), which indicates first-order kinetics for the initiator concentration. Therefore, the polymerization of rac-LA initiated by complex 7 follows a kinetic equation of the form −d[M]/dt = kp[M][I]. The effect of polymerization temperature on rac-LA polymerization was also studied using complex 7 as the initiator. It was found that the polymerization rate increased with increasing polymerization temperature (Supporting Information, Figure S5), and the apparent rate constants (kapp) of the polymerization are 0.012, 0.019, 0.030, and 0.038 min−1 when the polymerization was conducted in THF at 30, 40, 50, and 60 °C, respectively. The temperature dependence of kapp is depicted with an Arrhenius plot as shown in the Supporting Information (Figure S6). According to the slop of the plot, the value of the apparent activation energy Eapp is 32.4

complex 5, indicating that the overall coordination environment around the central metal, besides the ancillary ligand, also has an influence on the stereoselectivity of rac-LA polymerization. Furthermore, the structure of the bridge of Salen ligand has no obvious influence on the catalytic behavior. Replacing the 2,2-dimethylpropylene bridge of the Salen ligand with propylene resulted in a slight change in kapp of complex 4 (kapp = 0.0132 ± 0.0004 min−1) and complex 7 (kapp = 0.0120 ± 0.0004 min−1) (vide inf ra; Figure 5), and a slight increase of the

Figure 5. Kinetics of the rac-LA polymerization: complex 3, ▲, kapp = 0.0305 min−1 (linear fit, R2 = 0.9933); complex 4, ◆, kapp = 0.0132 min−1 (linear fit, R2 = 0.9947); and complex 7, ■, kapp = 0.012 min−1 (linear fit, R2 = 0.9969). [M]0/[I]0 = 200, THF as solvent.

heteroselectivity from 0.84 to 0.88 (Table 1, entries 7 and 14). This difference is apparently smaller than that in the tetradentate-diainionic imine-thiobis(phenolate) zirconium systems for rac-LA polymerization.10c The polymerization media affected not only the activity, but also the stereoselectivity. Complex 5 showed higher activity and heteroselectivity in THF than in toluene and CH3Cl (Table 1, entries 8−11). For example, the isolated yield is 93% when polymerization of rac-LA was conducted in THF in 10 min, and the Pr value of the resultant PLA is 0.63, whereas the yield is 60% in toluene in 50 min, and the Pr value of the polymer is 0.43, and the yield is 78% in CHCl3 in 10 min, and the Pr value of the resultant PLA is 0.48. The solvent effect on the stereoselectivity of rac LA polymerization in our cases is consistent with those observed in the amine-bridged bis(phenolate) lanthanide systems.7i−k As expected, the initiating group can affect the catalytic activity, but can not affect the stereoselectivity. Changing the initiating group from aryloxy to benzyloxy group (PhCH2O) or isopropoxy group (PriO) results in somewhat decrease in catalytic activity, but the Pr values of the resultant PLAs are similar (Table 1, entries 8, 18, and 19). The lower catalytic activity of complexes 8 and 9, in comparison with complex 5, may be attributed to the fact that the yttrium alkoxides have dimeric structures, and the cleavage of the dimeric structure by the rac-LA monomer is slow. A similar phenomenon was also observed in the Salalen lanthanide alkoxide systems.8i The ionic radii of lanthanide metals also influence the catalytic activity and stereoselectivity. The heterotacticity of the resulting PLAs with ytterbium complexes 2 and 4 is higher than those initiated by the yttrium complexes 1 and 3, whereas the ytterbium complexes showed relatively lower activity than the yttrium complexes (Table 1, entries 1, 3, 5, and 7). For example, the polymerization of rac-LA initiated by complex 3 or 2913


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CONCLUSION In summary, a series of yttrium and ytterbium aryloxo and alkoxo complexes stabilized by different Salen ligands were synthesized, and their structural features were provided via Xray structure determination. These yttrium and ytterbium complexes are efficient initiators for the ring-opening polymerization of rac-lactide to give heterotactic PLAs. It was found that the structure of the Salen ligand has an obvious effect on the catalytic activity and stereoselectivity of these lanthanide Salen complexes for rac-LA polymerization. Furthermore, it was found for the first time that, besides the ancillary ligand, the overall coordination environment around the central metal also has an influence on the stereoselectivity of rac-LA polymerization. Kinetics study revealed that rac-LA polymerization initiated by these yttrium and ytterbium Salen complexes is first-order for both lactide concentration and initiator concentration. MALDI-TOF mass analysis revealed that the aryloxo or alkoxo group in these complexes is the actual initiating group, and the Salen ligand itself was not involved in the polymerization process. Meanwhile, the existence of intermolecular transesterification reactions during rac-LA polymerization in lanthanide Salen systems was observed. Further studies of the design and synthesis of lanthanide Salen complexes and their application in homogeneous catalysis are in process in our laboratory.

Figure 6. Kinetics of the rac-LA polymerization using complex 7 as initiator in THF at 30 °C, [LA]0 = 1 M: plot 200:1, ◆, kapp = 0.012 min−1 (linear fit, R2 = 0.9959); plot 300:1, ■, kapp = 0.0076 min−1 (linear fit, R2 = 0.9952); plot 400:1, ▲, kapp = 0.0056 min−1 (linear fit, R2 = 0.9942); plot 500:1, ◇, kapp = 0.0046 min−1 (linear fit, R2 = 0.9969); and plot 600:1, □, kapp = 0.0043 min−1 (linear fit, R2 = 0.9798).

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data, selected bond lengths and angles, molecular structures of complexes 4, 5, and 7, kinetic data, MALDI-TOF mass spectrum, and CIF files. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00223.

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AUTHOR INFORMATION

Corresponding Authors

*Fax: (86)512-65880305. Tel.: (86)512-65882806. E-mail: [email protected]. *E-mail: [email protected].

Figure 7. Linear plot of ln kapp vs ln [I] for rac-LA polymerization using complex 7 as the initiator (THF, 30 °C, n = 0. 96, linear fit, R2 = 0.9804).

Notes

The authors declare no competing financial interest.

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kJ mol−1, which is comparable with that of rac-LA polymerization initiated by enolic Schiff base aluminum complex (35.4 kJ mol−1),26a but is apparently lower than that of the polymerization initiated by tin(II) octanoate (70.9 kJ mol−1).26b The initiation mechanism was explored by end group analysis of the oligomer of rac-LA, which was prepared by the oligomerization of rac-LA initiated by complex 7 in a 10:1 molar ratio. A MALDI-TOF mass analysis of the oligomer revealed that only the oligomer bearing OAr end-cap was formed (Supporting Information, Figure S7), indicating that the Salen ligand itself was not involved in the polymerization process. This finding confirms that the aryloxide group in complex 7 is the actual initiating group, and the polymerization occurred via a so-called coordination insertion mechanism. Furthermore, the MALDI-TOF mass analysis revealed the existence of linear oligomers with a nonintegral lactide repeat unit, which means that the existence of intermolecular transesterification reactions during rac-LA polymerization initiated by lanthanide Salen aryloxides.

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grants 21174095, 21132002, and 21372172), the PAPD, the Major Research Project of the Natural Science Foundation of the Jiangsu Higher Education Institutions (Project 14KJA150007) and the Qing Lan Project is gratefully acknowledged.

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REFERENCES

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