Synthesis and characterization of aminophenolate-ligated rare-earth metal amide complexes and their catalytic activity for lactides polymerization

Journal Pre-proof Synthesis and characterization of aminophenolate-ligated rare-earth metal amide complexes and their catalytic activity for lactides polymerization Min Li, Wenyi Li, Yingming Yao, Yunjie Luo PII:

S1002-0721(19)30741-0

DOI:

https://doi.org/10.1016/j.jre.2019.11.010

Reference:

JRE 645

To appear in:

Journal of Rare Earths

Received Date: 5 September 2019 Revised Date:

7 November 2019

Accepted Date: 13 November 2019

Please cite this article as: Li M, Li W, Yao Y, Luo Y, Synthesis and characterization of aminophenolateligated rare-earth metal amide complexes and their catalytic activity for lactides polymerization, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.11.010. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.

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Synthesis and characterization of aminophenolate-ligated rare-earth metal amide complexes and their catalytic activity for lactides polymerization

3

Min Li,a Wenyi Li,b Yingming Yao,b,c,*, Yunjie Luoa,∗

1

4 5 6 7 8

a

School of Material Science and Chemical Engineering, Ningbo University, Ningbo 315211, China College of Chemistry, Chemical Engineering & Materials Science, Soochow University, Suzhou 215123, China

b

c

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

9 11

ABSTRACT: Amine elimination of Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 with aminophenol H[ON] {H[ON] = 2-(CH2NC5H10)-4,6-tBu2-C6H3OH} in 1:2 molar ratio in THF gave the monometallic

12

rare-earth metal amide complexes [ON]2LnN(SiMe3)2 (Ln = Yb (1), Y (2), Gd (3), Sm (4), Nd (5))

13

in 57%-73% isolated yields. All theses complexes were characterized by elemental analysis. The

14

molecular structures of complexes 1-4 were determined by single crystal X-ray diffraction. These

15

complexes were highly active for L-lactide polymerization to give high molecular weight polymers with unimodal molecular weight distributions. In addition, these complexes could also initiate rac-lactide polymerization with high activity to afford heterotactic-rich polylactides.

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16 17 18 19

Keywords: Rare-earth metal complex; Aminophenolate ligand; Polymerization; Lactide

20 21 22 23 24 25 26 27 28

1. Introduction Development of biocompatible and/or biodegradable polymers from renewable resources to reduce our dependence on petroleum has attracted intensive attention in academic and industrial interest. Polylactides (PLAs) can be utilized as one kind of environmentally-benign materials possessing bio-renewability, bio-degradability and bio-compatibility properties.1-2 It is now commonly accepted that the ring-opening polymerization (ROP) of lactides promoted by metal-based complexes is the most convenient and efficient method to access PLAs in terms of precise control of molecular weight, terminal composition, and micro-structure. Upon to date, Foundation item: Project supported by National Natural Science Foundation of China (21572205, 21971130, 21871198, 21674070), the Natural Science Foundation of Zhejiang Province (Y19B040010), the State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, the Natural Science Foundation of Ningbo Municipal, and K. C. Wong Magna Fund in Ningbo University. ∗ Corresponding author. School of Material Science and Chemical Engineering, Ningbo University, Ningbo 315211, P. R. China. E-mail address: [email protected] (Y.J. Luo)

29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46

numerous structurally discrete metal-based complexes have been used for lactide polymerization.3-9 Among them, rare-earth metal complexes have been proved to be effective initiators for lactides polymerization.8-9 Owing to the unique electronic features of rare-earth metal metals, the catalytic performance of rare-earth metal complexes is dependant mainly on the coordination configurations at central metals. As a result, increasing attention has been paid to the preparation of organo rare-earth metal complexes stabilized by various ancillary ligands, as well to explore the structure-reactivity relationship of the resulting rare-earth metal complexes. Recently, rare-earth metal complexes incorporated by aminophenolate ligands have been found to exhibit good activity, well controllability, and high stereo-selectivity toward the ROP of lactides.10-14 We also reported that the rare-earth metal complexes bearing amine bridged bis(phenolate) frameworks were capable of initiating rac-lactide polymerization to give heterotactic PLAs.15-21 To further investigate the influence of phenolate ligand backbones on the catalytic performance, as well to compare the polymerization behaviors of mono- and bimetallic rare-earth metal amide complexes, we prepared a series of rare-earth metal amide complexes [ON]2LnN(SiMe3)2 stabilized by a monoanionic aminophenolate scaffold [ON]– {[ON]–= 2-(CH2NC5H10)-4,6-tBu2-C6H3O}, and employed these rare-earth metal amide complexes as initiators for lactides polymerization. Here we report these results.

47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

2. Materials and methods 2.1. General considerations All the manipulations were carried out under a nitrogen atmosphere using the Schlenk technique and a glovebox. HN(SiMe3)2 was commercially obtained. THF, hexane, and toluene were distilled from sodium benzophenone ketyl. rac-lactide and L-lactide were purchased from Arcos, and recrystallized from reflux toluene. LnCl3 were purchased from Strem Chemicals. C6D6 and CDCl3 were obtained from Cambridge Isotope Laboratories. H[ON] {H[ON] = 2-(CH2NC5H10)-4,6-tBu2-C6H3OH},22 Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 (Ln = Nd, Sm, Gd, Y, Yb)23-25 were prepared according to the literature. Rare-earth metal contents were determined by EDTA titration using an xylenol orange indicator and a hexamine buffer. NMR spectra were recorded on a Unity Varian spectrometer at 25 °C. Carbon, hydrogen, and nitrogen analyses were conducted on a Carlo-Erba EA-1110 instrument by direct combustion. Molecular weights and molecular weight distributions were determined by gel permeation chromatography on a PL 50 apparatus against a polystyrene standard, using THF as an eluent at a flow rate of 1.0 mL/min at 40 °C. Synthesis of [ON] 2YbN(SiMe3)2 (1) A THF solution (15 mL) of H[ON] (1.21 g, 4.0 mmol) was added drop by drop to a THF

67

solution (15 mL) of Yb[N(SiMe3)2]3(µ-Cl)Li(THF)3 (1.82 g, 2.00 mmol) at room temperature. After stirring at room temperature overnight, the volatiles were removed under a reduced pressure. The oily residual was washed by hexane, and dissolved in a mixture of hexane/THF (5:1 v/v). Yellow

68

crystals were collected after several days at – 30 °C (1.37 g, 73%). Anal. Calcd for

65 66

2

71

C46H82N3O3Si2Yb: C, 58.88; H, 8.81; N, 4.48; Yb, 18.44. Found: C, 58.96; H, 8.95; N, 4.52; Yb, 18.32. Synthesis of [ON] 2YN(SiMe3)2 (2)

72

Complex 2 was prepared by a procedure similar to that for complex 1, using

69 70

73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108

Y[N(SiMe3)2]3(µ-Cl)Li(THF)3 (1.65 g, 2.00 mmol) to afford complex 2 as colorless crystals (1.14 g, 67%). 1H NMR (400 MHz, C6D6): 7.59 (d, J = 2.8 Hz, 2H, Ar-H), 7.15 (m, 2H, Ar-H), 4.20 (br, 4H, Ar-CH2), 3.56-2.97 (m, 10H, N(CH2)5), 1.62 (s, 18H, tBu), 1.41 (s, 18H, tBu), 1.40-1.12 (m, 10H, N(CH2)5), 0.30 (s, 18H, N(SiMe3)2). 13C NMR (400 MHz, C6D6): 161.1, 137.91, 136.1, 126.4, 124.6, 123.5 (Ar-C), 67.8 (ArCH2N), 35.5, 34.3, 32.1, 30.4 (C(CH3)3), 25.8, 24.3 (CH2), 5.8 (SiMe3). Anal. Calcd for C46H82N3O3Si2Y: C, 64.68; H, 9.68; N, 4.92; Y, 10.41. Found: C, 64.21; H, 9.69; N, 4.76; Y, 10.45. Synthesis of [ON] 2GdN(SiMe3)2 (3) Complex 3 was prepared as a procedure similar to that for complex 1, using Gd[N(SiMe3)2]3(µ-Cl)Li(THF)3 (1.79 g, 2.00 mmol) to give complex 3 as colorless crystals (1.02 g, 57%). Anal. Calcd for C46H82N3O3Si2Gd: C, 59.89; H, 8.96; N, 4.55; Gd, 17.04. Found: C, 59.64; H, 8.86; N, 4.28; Gd, 17.13. Synthesis of [ON] 2SmN(SiMe3)2 (4) Complex 4 was prepared by a procedure similar to that for complex 1, using Sm[N(SiMe3)2]3(µ-Cl)Li(THF)3 (1.78 g, 2.00 mmol) to give complex 4 as yellow crystals (1.30 g, 72%). Anal. Calcd for C46H82N3O3Si2Sm: C, 60.34; H, 9.03; N, 4.59; Sm, 16.42. Found: C, 60.13; H, 8.96; N, 4.51; Sm, 16.29. Synthesis of [ON] 2NdN(SiMe3)2 (5) Complex 5 was prepared by a procedure similar to that for complex 1, using Nd[N(SiMe3)2]3(µ-Cl)Li(THF)3 (1.77 g, 2.00 mmol) to give complex 5 as blue crystals (1.18 g, 65%). Anal. Calcd for C46H82N3O3Si2Nd: C, 60.74; H, 9.09; N, 4.62; Nd, 15.86. Found: C, 60.54; H, 9.18; N, 4.72; Nd, 15.99. 2.2. General procedures for lactides polymerization The procedures for lactides polymerization initiated by these complexes were similar, only a typical procedure is given. The desired amounts of L-lactide and toluene were added into a 50 mL Schlenk flask equipped with a magnetic stirring bar. The flask was placed in an oil bath at 60 °C until the monomer was dissolved completely. Then a toluene solution of rare-earth metal complex was introduced into the flask via a syringe. The resulting mixture was stirred at 60 °C for the pre-determined time. The polymerization was terminated by the addition of 1 mol/L HCl/EtOH solution, and the polymer was precipitated from a large amount of ethanol. The collected polymer was dried in vacuum at 80 °C. 2.3. X-Ray single crystal structure determinations Single crystals of complexes 1−4 for structural determination were sealed in a thin-walled glass capillary. Intensity data were collected on a Rigaku Mercury CCD area detector using Mo Kα radiation (λ = 0.071070 nm). The diffraction intensities were corrected for Lorentz/polarization effects and empirical absorption corrections. The molecular structures were solved by direct 3

111

methods, and refined by full-matrix least-squares procedures based on |F|2. The non-hydrogen atoms were refined anisotropically. All the hydrogen atoms were generated geometrically, allowed to ride on their parent carbon atoms, and held stationary and included in the structure factor

112

calculation in the final stage of full-matrix least-squares refinement. CCDC 1946984-1946987

113

contains the supplementary crystallographic data for 1-4, respectively. These data can be obtained

114

free of charge from The www.ccdc.cam.ac.uk/data_request/cif.

109 110

115

Cambridge

Crystallographic

Data

Centre

via

116 117

3. Results and discussion

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3.1. Synthesis and characterization of complexes 1-5

119

We reported previously that the anionic bimetallic phenolate rare-earth metal bis(amide) complexes [ONNO]{Ln[N(SiMe3)2]2(µ-Cl)Li(THF)}2 (Ln = Y, Sm, Eu, Er) could be prepared via

120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139

amine elimination between Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 and a piperazidine-bridged bis(phenol) H2[ONNO] {H2[ONNO] = 4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)piperazidine} in 2:1 molar ratio at 60 °C, and these complexes showed good activity toward L-lactide and rac-lactide polymerization.16,20 To compare the polymerization performance of mono- and bimetallic rare-earth metal amide complexes for lactides polymerization, the aminophenol H[ON] {H[ON] = 2-(CH2NC5H10)-4,6-tBu2-C6H3OH}, which could be considered to possess a half unit of the piperazidine-bridged bis(phenol) H2[ONNO],16,20 was prepared and used as the proligand to synthesize the corresponding rare-earth metal amide complexes. Initially, to compare the reaction pattern of the amine-bridged bis(phenolate) rare-earth metal complexes,12,16 we tried to obtain the phenolate-ligated rare-earth metal bis(amide) complexes [ON]Ln[N(SiMe3)2]2. However, NMR monitoring reaction of Y[N(SiMe3)2]3(µ-Cl)Li(THF)3 with one equivalent of H[ON] in C6D6 showed that there were two signals with an integration of ca. 1:2 at 0.30 and 0.11 ppm, assignable to SiMe3 groups for [ON]2YN(SiMe3)2 and HN(SiMe3)2, respectively. Besides, the reaction between Y[N(SiMe3)2]3(µ-Cl)Li(THF)3 and H[ON] in 1:2 molar ratio in C6D6 also produced quantitatively a product assignable for [ON]2YN(SiMe3)2. These results indicated that the formation of the bis(phenolate) yttrium mono(amide) complex [ON]2YN(SiMe3)2 was independent on the molar ratio of the starting materials. It suggested that the ancillary ligand [ON]– was not sterically demanding enough to stabilize the desired rare-earth metal bis(amide) complexes [ON]Ln[N(SiMe3)2]2. Therefore, on a preparative scale, treatment of

141

Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 with H[ON] in 1:2 molar ratio in THF at room temperature, after workup, afforded cleanly the bis(phenolate) rare-earth metal mono(amide) complexes

142

[ON]2LnN(SiMe3)2 (Ln = Yb (1), Y (2), Gd (3), Sm (4), Nd (5)) in 57%-73% isolated yields

143

(Scheme 1). Complexes 1-5 were characterized by elemental analysis, complex 2 was also subjected

140

4

144

to NMR spectroscopy analysis. The molecular structures of complexes 1-4 were determined by

145

single crystal X-ray diffraction. These complexes are soluble in THF and toluene, but insoluble in hexane. Insert Scheme 1 herein

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3.2. Crystal structures

150

Single crystals of complexes 1-4 suitable for structural determination were grown from a

151

mixture solution of THF and hexane at 5 °C. Single crystal X-ray diffraction showed that

152

complexes 1-4 were isomorphous, therefore, only the ORTEP diagram of complex 1 is illustrated in

153

Fig. 1. Details of the crystallographic data, the selected bond lengths and bond angles are provided

154

in Tables 1−3. As shown in Fig. 1, complex 1 is symmetric, unsolvated and mononuclear. The central metal is five-coordinated by two oxygen atoms, two nitrogen atoms from two aminophenotes, and one amide group to adopt a pyramid geometry, in which O(1), O(1A), N(1) and N(1A) occupy equatorial positions, and N(2) occupies an axial position. The bond angles of

155 156 157

162

N(1)-Ln-N(1A) are distorted significantly from the ideal value of 180o to 166.3(3)o for complex 1, 166.0(1)o for complex 2, and 165.2(4)o for complex 3, whereas it is distorted slightly to 171.8(1)o for complex 4. The average Ln–O(Ar) bond lengths are in the range of 0.2165(3) (for Sm) to 0.2089(4) nm (for Yb), which reflects the normal rare-earth metal contraction effect from Sm3+ to Yb3+.26 Similar tendency of the average Ln–N (ring) bond length change is observed in these

163

rare-earth metal complexes. However, the average Ln–O bond lengths in complexes 1-4 are

164

apparently shorter than those in the piperazidine bis(phenolate) rare-earth metal amide complexes.16,20 The Ln-N(SiMe3)2 bond lengths are in agreement with those in bis(phenolate)-liagted rare-earth metal amide complexes.16,20 Insert Fig. 1 herein Insert Table 1, Table 2 and Table 3 herein

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165 166 167 168 169 170

3.3. Ring-opening polymerization of lactides initiated by complexes 1-5

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The catalytic behavior of complexes 1-5 for L-lactide polymerization was tested. The

172

polymerization results are listed in Table 4. These neutral rare-earth metal amide complexes are able to initiate L-lactide polymerization, giving polymers with high molecular weights (> 104) and molecular weight distributions Mw/Mn ranging from 1.64 to 1.96. For example, employing complex

173 174 175 176 177 178

1 as the initiator, 99% conversion reaches within 30 min at 60 oC at 3500:1 molar ratio of monomer to initiator. The polymerization can still give a 59% conversion within 1.5 h even when the molar ratio of monomer to initiator increases to 5000:1 under the same polymerization conditions (Table 4, entries 7 and 8). However, solvent effect is contrast to those observed in the catalyst systems of the 5

179

piperazidine-bridged bis(phenolate) rare-earth metal amide complexes.16 The polymerization

180

performed in toluene is better than that in THF. For example, when complex 1 is used as the initiator, the conversion in toluene reaches upon to 99% in less than 30 min at 60 oC, and gives a high molecular weight polymer. Whereas, the conversion is only 75% in THF under the same polymerization conditions, produces a polymer with a relatively low molecular weight (Table 4,

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entries 2 and 3). As expected, the polymerization proceeds faster at higher polymerization temperature. For example, employing complex 1 as the initiator, a complete conversion occurs at 60 o C for 30 min, while the conversion is only 88% at 25 oC even the polymerization is extended to 3 h (Table 4, entries 1 and 3). Insert Table 4 herein

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The influence of the metal size on the polymerization activity for L-lactide was observed. Small metal radii results in low polymerization activity. For example, using complex 5 (Nd) as the initiator, the conversion is 97% when [M]/[Ln] is 10000 in 30 min, while it is only 59% using complex 1 (Yb) as the initiator at 1.5 h, even M]/[Ln] decreases to 5000 (Table 4, entries 8 and 17). The observed decreasing polymerization activity tendency for these rare-earth metal amide complexes, 5 (Nd) > 4 (Sm) > 3 (Gd) > 2 (Y) > 1 (Yb), is well in agreement with the decreasing order of the ionic radii.26 In comparison with the bimetallic piperazidine-bridged bis(phenolate)

197

rare-earth metal amide complexes, complexes 1-5 are much more active. For example, complex 2

198 200

can promote a complete polymerization of L-lactide in 30 min when [M]/[Ln] is as high as 5000 (Table 4, entry 10), in contrast, only 74% monomer was consumed in 3 h at [M]/[I] of 600 by the piperazidine-bridged bis(phenolato) yttrium amide complex.16

201

Complexes 1-5 can also serve as highly active initiators for rac-lactide polymerization. As

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summarized in Table 5, these rare-earth metal complexes are effective for rac-lactide polymerization. The polymerization proceeds smoothly at 25 oC in THF, affording the polymers

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199

203 204 205 206 207 208 209 210 211 212 213 214 215 216

with high molecular weights (>104) and Mw/Mn ranging from 1.70 to 2.42. The increasing order of the polymerization activity is in accordance to the increasing order of the ionic radii, as observed in L-lactide polymerization with these complexes. NMR spectra show that the Pr values of the resulting polymers fall in the range of 0.64 to 0.73, indicative of a moderate stereo-selectivity control. This stereo-controllability of these rare-earth metal amide complexes is similar to that of the piperazidine-bridged phenolate rare-earth metal amide complexes.16,20 Despite these rare-earth metal amide complexes showed high activity towards L-lactide and rac-lactide polymerization, the polymerization proceeded not in a controlled fashion, which led to the significant deviation of the measured molecular weights from the calculated ones. Besides, the poor control of the polymerization might ascribe to both the phenolate ligand and the amide group in such kind of rare-earth metal complexes would initiate simultaneously the polymerization,27 as well the contamination of impurity from lactides and solvent. Insert Table 5 herein

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218 219 220

4. Conclusions The neutral bis(aminophenolate) rare-earth metal mono(amide) complexes [ON]2LnN(SiMe3)2 (Ln = Yb (1), Y (2), Gd (3), Sm (4), Nd (5)) could be prepared by amine elimination of

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Ln[N(SiMe3)2]3(µ-Cl)Li(THF)3 with aminophenol H[ON] in molar ratio of 1:1 or 1:2. This ligand scaffold is not sterically bulky enough to stabilize mono-aminophenolate rare-earth metal bis(amide)

223

complexes [ON]Ln[N(SiMe3)2]2. Complexes 1-5 were highly active for L-lactide polymerization in

224

227

toluene at 60 oC. They could also promote the polymerization of rac-lactide with high activity to produce heterotactic-rich polylactides. Although the attempt to prepare the aimed mononuclear rare-earth metal bis(amide) complexes was unsuccessful, which disturbed the investigation of polymerization behaviors of mono- and bimetallic rare-earth metal bis(amide) complexes,

228

complexes 1-5 still showed a promising reactivity for lactides polymerization.

221

225 226

229 230 231 232

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polyester microstructures. Chem Soc Rev. 2010;39:165. Sarazin Y, Carpentier JF. Discrete cationic complexes for ring-opening polymerization catalysis of cyclic esters and epoxides. Chem Rev. 2015;115:3564. Lyubov DM, Tolpygin AO, Trifonov AA. Rare-earth metal complexes as catalysts for ring-opening polymerization of cyclic esters. Coord Chem Rev. 2019;392:83. Skinner MEG, Tyrrell BR, Ward BD, Mountford PJ. New N- and O-donor ligand environments in organoscandium chemistry. J Organomet Chem. 2002;647:145. Cai CX, Amgoune A, Lehmann CW, Carpentier JF. Stereoselective ring-opening polymerization of racemic lactide using alkoxy-amino-bis(phenolate) group 3 metal complexes. Chem Commun. 2004;3:330. Liu XL, Shang XM, Tang T, Hu NH, Pei F K, Cui DM, Chen XS, Jing XB. Achiral rare-earth metal alkyl complexes bearing N,O multidentate ligands. Synthesis and catalysis of highly heteroselective ring-opening polymerization of rac-lactide. Organometallics. 2007;26:2747. Carpentier JF. Rare-earth complexes supported by tripodal tetradentate bis(phenolate) ligands: a 7

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privileged class of catalysts for ring-opening polymerization of cyclic esters. Organometallics. 2015;34:4175. 14. Ligny R, Hänninen MM, Guillaume SM, Carpentier JF. Steric vs. electronic stereocontrol in syndio- or iso-selective ROP of functional chiral β-lactones mediated by achiral yttrium-bisphenolate complexes. Chem Commun. 2018;54:8024. 15. Luo YJ, Li WY, Lin D, Yao YM, Zhang Y, Shen Q. Rare-earth metal alkyl complexes supported by a piperazidine-bridged bis(phenolato) ligand: synthesis, structural characterization, and catalysis for the polymerization of L-lactide and rac-lactide. Organometallics. 2010;29:3507. 16. Li WY, Zhang ZJ, Yao YM. Control of conformations of piperazidine-bridged bis(phenolato) groups: syntheses and structures of bimetallic and monometallic rare-earth metal amides and their application in the polymerization of lactides. Organometallics. 2012;31:3499. 17. Nie K, Fang L, Yao YM, Zhang Y, Shen Q, Wang YR. Synthesis and characterization of

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18. Hao OY, Nie K, Yuan D, Zhang Y, Cui DM, Yao YM. A convenient method to prepare random

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bimetallic lanthanide-alkali metal complexes stabilized by aminophenoxy ligands and their catalytic activity for the polymerization of 2,2-dimethyltrimethylene carbonate. Dalton Trans. 2013;42:10179.

301

9

302 tBu t

Bu

But O

N

t

Bu

Ln

THF, r.t.

OH +

N

- HN(SiMe3)2, - LiCl

But

O

N(SiMe3)2

N

1/2 Ln[N(SiMe3)2]3 (µ-Cl)Li(THF) 3 But Ln = Yb (1), Y (2), Gd (3), Sm (4), Nd (5)

303 304 305

Scheme 1. Synthesis of rare-earth metal amide complexes 1−5

10

306 307 308

309 310 311 312

Fig. 1. ORTEP diagram of complex 1 with 20% probability ellipsoids. Hydrogen atoms are omitted for clarity.

313

11

314 315 316

Table 1 Crystallographic data for complexes 1− −5. 1

2

3

4·THF

C46H82N3O2Si2Yb

C46H82N3O2Si2Y

C46H82N3O2Si2Gd

C50H90N3O3Si2Sm

Formula weight

938.37

854.24

922.58

987.78

Temperature (K)

223(2)

223(2)

223(2)

223(2)

Crystal size (mm)

0.20×0.15×0.10

0.45×0.35×0.35

0.30×0.20×0.20

0.35×0.30×0.30

Monoclinic

Monoclinic

Monoclinic

C 2/c

C 2/c

C 2/c

C 2/c

a (nm)

1.6832(1)

1.6816(2)

1.6789(2)

3.6728(2)

b (nm)

1.6170(1)

1.6224(1)

1.6294(2)

1.64028(9)

c (nm)

1.8250(2)

1.8277(2)

1.8306(3)

1.78301(9)

β (°)

98.365(2)

98.654(2)

98.991(4)

93.479(2)

4.9144(7)

4.9295(7)

4.9465(12)

10.7217(11)

4

4

4

8

Dcalc (g/cm )

1.268

1.151

1.239

1.224

Absorption coefficient(mm-1)

1.988

1.268

1.425

1.179

F(000)

1972

1848

1948

4200

θ (°)

25.50

25.50

25.50

25.50

Reflections collected

13568

15800

12331

26819

Unique reflections

4527

4565

4569

9923

4033

4218

2922

8051

237

237

255

526

GOF

1.127

1.045

1.002

1.120

R1

0.0584

0.0553

0.1100

0.0508

wR2

0.1427

0.1469

0.2532

0.1056

1754/–1352

1715/–1085

4932/–4149

1157/–639

Empirical formula

Crystal system Space group

3

V(nm ) Z 3

Monoclinic

Observed reflections [I> 2.0σ (I)] No. of variables

∆ρmax/∆ρmin (e/nm3) 317 318

12

319

Table 2

320

Selected bond lengths (nm) and angles (°) for complexes 1−3. 1

2

3

Ln(1)–O(1)

0.2089(4)

0.2110(2)

0.2157(7)

Ln(1)–N(1)

0.2489(6)

0.2531(3)

0.2567(8)

Ln(1)–N(2)

0.2220(8)

0.2260(4)

0.2328(11)

O(1A)–Ln(1)–O(1)

118.0(3)

117.8(1)

116.1(4)

O(1)–Ln(1)–N(2)

121.0(1)

121.08(6)

122.0(2)

O(1)–Ln(1)–N(1)

79.5(2)

78.52(8)

77.7(3)

N(2)–Ln(1)–N(1)

96.8(1)

96.99(6)

97.4(2)

O(1)–Ln(1)–N(1A)

93.4(2)

94.21(8)

94.4(3)

N(1A)–Ln(1)–N(1)

166.3(3)

166.0(1)

165.2(4)

321

13

322 323

Table 3

324

Selected bond lengths (nm) and angles (°) for complex 4. Sm(1)–O(2) Sm(1)–O(1) 0.2153(3)

0.2165(3)

Sm(1)–N(3)

0.2318(3)

Sm(1)–N(1)

0.2623(3)

Sm(1)–N(2)

0.2627(3)

O(1)–C(1)

0.1341(5)

O(2)–Sm(1)–O(1)

113.2(1)

O(2)–Sm(1)–N(3)

123.3(1)

O(1)–Sm(1)–N(3)

123.5(1)

O(2)–Sm(2)–N(1)

98.3(1)

O(1)–Sm(1)–N(1)

77.1(1)

N(3)–Sm(1)–N(1)

94.0(1)

O(2)–Sm(1)–N(2)

77.1(1)

O(1)–Sm(1)–N(2)

98.4(1)

N(3)–Sm(1)–N(2)

94.2(1)

N(1)–Sm(1)–N(2)

171.8(1)

325

14

326 327

Table 4

328

L-Lactide polymerization initiated by complexes 1−5a. Entry initiator

[M]/[I]

T (°C)

t (h)

conv.b Mcc×10-4 Mnd×10-4 (%)

1

1

300:1

25

3

88

3.80

9.17

1.96

2e

1

300:1

60

0.5

75

3.24

3.83

1.84

3

1

300:1

60

0.5

95

4.10

8.63

1.90

4

1

500:1

60

0.5

99

7.13

12.42

1.89

5

1

1000:1

60

0.5

98

14.11

17.72

1.91

6

1

1500:1

60

0.5

99

21.38

22.16

1.87

7

1

3500:1

60

0.5

99

49.90

29.74

1.84

8

1

5000:1

60

1.5

59

42.48

28.82

1.86

9

2

3500:1

60

0.5

99

49.90

21.92

1.77

10

2

5000:1

60

0.5

99

71.28

22.83

1.67

11

2

6000:1

60

0.5

75

64.80

19.39

1.85

12

2

6000:1

60

1.5 h

88

76.03

10.94

1.98

13

3

6000:1

60

0.5 h

97

83.80

23.09

1.64

14

3

8000:1

60

0.5 h

40

46.08

7.92

1.75

15

4

8000:1

60

0.5 h

95

109.44

21.80

1.67

16

4

10000:1

60

0.5 h

82

118.08

23.42

1.67

17

5

10000:1

60

0.5 h

97

139.68

25.09

1.77

329

a

Polymerization conditions: in toluene, [L-LA]=1 mol/L;

330

b

Conv.: weight of polymer obtained/weight of monomer used;

331

c

Mc =144.13×[M]/[I]×conv.;

332

d

Determined by GPC calibrated with polystyrene standards;

333

e

Mw/Mnd

In THF.

334

15

335 336

Table 5

337

rac-Lactide polymerization with complexes 1−5 .

a

conv. (%)b Mcc×10-4

Mnd×10-4

Mw/Mnd

Pre

6.62

5.71

2.02

0.71

83

17.93

10.28

2.02

0.71

1

79

34.13

9.06

2.42

0.67

4000:1

1

81

46.66

12.67

2.05

0.71

1

500:1

1

50

3.60

11.90

2.04

0.71

6

3

500:1

1

89

6.41

5.70

1.71

0.71

7

4

500:1

1

98

7.06

6.82

1.76

0.65

8

5

500:1

1

90

6.48

6.16

1.71

0.65

9

5

1500:1

1

98

21.17

13.10

1.77

0.64

10

5

3000:1

1

99

42.77

16.85

1.78

0.67

11

5

5000:1

1

94

67.68

17.70

1.74

0.68

12

5

8000:1

1

88

101.38

23.50

1.70

0.73

Entry

Initiator

[M]/[I]

t (h)

1

2

500:1

1

92

2

2

1500:1

1

3

2

3000:1

4

2

5

338

a

Polymerization conditions: in THF, [rac-lactide] = 1 mol/L, 25 °C;

339

b

Conv.: weight of polymer obtained/weight of monomer used;

340

c

Mc = (144.13)×[M]/[I]×yield;

341

d

Determined by GPC calibrated with polystyrene standards;

342

e

Characterized by homodecoupling 1H NMR spectroscopy in CDCl3 at 20 °C.

343 344

16

345 346 347

Graphical abstract

348 349 350 351 352

Aminophenolate-ligated lanthanide mono(amide) complexes [ON]2LnN(TMS)2 (Ln = Yb, Y, Gd, Sm, Nd) were prepared via amine elimination, and served as highly active initiators for the ring-opening polymerization of L-lactide and rac-lactide polymerization. t

Bu

But

N O Ln

O

But

O

N(TMS)2

N O

O n

O

t

O

Bu

O

O

353

O

Lactide

PLA

17

n

Conflicts of interest There are no conflicts to declare.