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Aluminum complexes with bidentate amido ligands: synthesis, structure and performance on ligand-initiated ring-opening polymerization of rac-lactide† Junpeng Liu and Haiyan Ma* A series of mononuclear aluminum dimethyl complexes bearing bidentate N-[2-(1-piperidinyl)benzyl]anilino or N-(2-morpholinobenzyl)anilino ligands were synthesized via reactions of the corresponding proligands with trimethylaluminum upon heating, while at ambient temperature two trimethylaluminum adducts with neutral N-[2-(1-piperidinyl)benzyl]aniline proligands were obtained. These complexes were well characterized by NMR spectroscopy, elemental analysis and occasionally by EI-HRMS. The molecular structures of the typical trimethylaluminum adduct 2b and aluminum dimethyl complex 3b were further confirmed by X-ray diffraction studies. All the aluminum dimethyl complexes could effectively initiate the ring-opening polymerization (ROP) of rac-lactide in a well-controlled manner to afford PLAs with narrow molecular weight distributions (PDI = 1.06–1.18). The polymer samples obtained are systematically end-

Received 3rd February 2014, Accepted 10th March 2014 DOI: 10.1039/c4dt00353e www.rsc.org/dalton

capped with the bidentate ancillary ligands as characterized by 1H NMR and ESI-TOF mass spectroscopy. Moreover, the introduction of substituent(s) at the ortho-position(s) of the anilino moiety in the ligand results in an obvious decrease in the catalytic activity of the corresponding aluminum complex, and complexes with meta-chloro substituted anilino units show higher activities likely due to the enhanced electrophilicity of the metal centers induced by the anilino groups.

Introduction During the past decades, aliphatic polyesters, especially polylactides (PLAs), have attracted considerable attention, not only because of their biodegradable and biocompatible properties,1–3 but also because these materials provide valid alternatives to petrochemical thermoplastics due to their attractive and improving performance characteristics.4–9 The ring-opening polymerization (ROP) of cyclic esters catalyzed by organometallic complexes is the most effective and versatile method for preparing aliphatic polyesters. Among the variety of organometallic catalysts, aluminum complexes, particularly those supported by Salen or Salan ancillary ligands,10–28 have drawn considerable attention due to their inexpensiveness and high stereoselectivity in the polymeriz-

Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of Organometallic Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, People’s Republic of China. E-mail: [email protected]; Fax: +86 21 64253519; Tel: +86 21 64253519 † Electronic supplementary information (ESI) available: CIF files and a table giving crystallographic data for 2b and 3b, figures giving 1H and 13C NMR spectra of 1a–j, 2b, 2d, and 3a–j, and figures giving the typical GPC traces of PLA. CCDC 983597 and 983598 for 2b and 3b. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00353e

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ations of rac-lactide and meso-lactide. Systematic studies have demonstrated that the auxiliary ligands surrounding the aluminum centers in these catalysts significantly influence the polymerization behavior. Owing to their easy preparation, most well-defined aluminum complexes are obtained as aluminum mono-(or bis-) alkyl complexes supported with multidentate ligands, where the Al–alkyl bonds are generally inactive toward the ROP of the cyclic esters.29–31 A co-initiator such as aliphatic or benzylic alcohol has to be adopted in combination with aluminum alkyl complexes, assuming that the aluminum alkyl complexes react with the alcohol to generate the desired Al–OR bonds, on which the ROP should start.32–52 However, for this reason, the end groups of the resultant polymers, which may work as functional groups for further transformations, are limited to the monotonous structures of aliphatic or benzylic esters. Furthermore, the alcoholysis reaction of aluminum alkyl complexes is often not easily anticipated, and the products of the reaction can sometimes be difficult to identify.46,47,53,54 So to synthesize aluminum alkyl catalysts that can initiate the ROP of cyclic esters directly and provide polyesters with versatile functional end groups in a controllable manner, would be meaningful academically and industrially. Up to now, except for some aluminum thiolates acting as initiators to afford polylactide with a thioester end in a rela-

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tively controllable manner, almost none of the aluminum alkyl complexes show analogous catalytic performance in the ROP of cyclic esters.55–57 Complexes bearing β-diketiminate ligands have received much attention in view of their specific coordinating sphere, where the constrained ligand results in a six-membered ring and causes the metal atom to be surrounded by bulky substituents on one side but open on the other side toward the entering monomers.35,54,58–61 Moreover, it has been found that aluminum alkyl complexes of β-diketiminate ligands could catalyze the polymerization of lactones without any co-initiator. Our previous work showed that aluminum ethyl complexes of β-diketiminate ligands could initiate the polymerization of ε-CL as single component catalysts, but affording polymers with broad molecular weight distributions and undefined end groups.54 In 2013, Peng and coworkers reported that aluminum alkyl complexes bearing aliphatic N-substituted β-diketiminate ligands also showed activities toward the polymerization of ε-CL in the absence of alcohol, producing polymers with relatively narrow molecular weight distributions (PDI = 1.15–1.46).61 However, none of these β-diketiminate aluminum complexes are active toward the ROP of lactides. To explore more efficient and well-controlled aluminum catalysts for the polymerization of lactides in the absence of a coinitiator, we synthesized a series of aluminum alkyl complexes bearing β-diketiminate analogues as ligands where a nitrogencontaining ring-structure and a more flexible skeleton were introduced, which proved to be capable of initiating the ROP of rac-lactide with living features and providing narrowly distributed PLAs end-capped systematically with the bidentate amido ligands.

Results and discussion Synthesis and structural characterization of aluminum complexes As shown in Scheme 1, the proligands 1a–1j were prepared by condensation reactions of 2-(1-piperidinyl)benzaldehyde or 2-(4-morpholino)benzaldehyde with the substituted anilines, followed by reduction with sodium borohydride. Different substituents such as methyl, isopropyl and chloro groups were introduced to the aniline moieties of the proligands with the aim of investigating the steric and electronic effects on the catalytic behavior of the target aluminum complexes. Due to the strong hygroscopicity of these compounds and their similar polarity to the starting material aniline and the reduction product of unreacted benzaldehyde, all the aniline proligands 1a–1j could not be isolated in an analytically pure form. The alkane elimination strategy, employed extensively for the synthesis of amido or phenolate aluminum alkyl complexes, was adopted to synthesize the target aluminum methyl complexes. Initially, 2,6-dimethyl-N-[2-(1-piperidinyl)benzyl]aniline (1b) was treated with trimethylaluminum at ambient temperature in a hexane–toluene mixture; during the reaction no instantaneous evolution of any gaseous product could be

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Scheme 1 Synthesis of N-[2-(1-piperidinyl)benzyl]aniline 1a–1f and N-[2-(4-morpholino)benzyl]aniline 1g–1j.

observed. After a normal work-up procedure, a colorless crystalline product could be isolated. Based on the proton resonance integrations in the 1H NMR spectrum of this product, it could be found that the stoichiometric structure consists of three methyl groups bonded to a metal center and one bidentate ligand that possesses chemical shifts distinguishable from those of 1b. The sharp signal at −0.85 ppm indicated that all three methyl groups are equivalent. Furthermore, a broad signal assignable to the amino proton of the ligand could be observed at 5.31 ppm, suggesting that the bidentate ligand is coordinated to the aluminum center in a neutral form and there is no methane elimination reaction taking place between the neutral proligand and trimethylaluminum. Thus, instead of the target aluminum dimethyl complex, the product provided by the reaction of trimethylaluminum with the proligand 1b at ambient temperature is actually an adduct as depicted in Scheme 2. The coordination mode of 2b was further confirmed

Scheme 2

Synthesis of trimethylaluminum adducts 2b and 2d.

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Fig. 1 ORTEP diagram of the molecular structure of 2b. Thermal ellipsoids are presented at 30% probability. Hydrogen atoms except for H2 are omitted for clarity. Selected bond distances (Å) and angles (°): Al1– C21 1.963(2), Al1–C22 1.963(2), Al1–C23 1.963(2), Al1–N2 2.0939(18), N1–C1 1.464(3), N1–C5 1.477(2), N1–C6 1.428(2), N2–C13 1.461(2), C23–Al1–C22 117.12(11), C23–Al1–C21 111.01(11), C22–Al1–C21 113.91(12), C23–Al1–N2 106.15(9), C22–Al1–N2 106.08(9), C21–Al1–N2 100.75(9).

by X-ray single crystal diffraction analysis. As shown in Fig. 1, the central aluminum atom of complex 2b is surrounded by three methyl groups and one amino donor of the ligand, possessing a distorted tetrahedral geometry with the angles of C21–Al1–N2 (100.75(9)°), C22–Al1–C21 (113.91(12)°) and C23– Al1–C21 (111.01(11)°) close to the regular tetrahedral angle of 109.28°. The nitrogen atom coordinating to the center aluminum also takes a distorted tetrahedral geometry as indicated by the angles of C13–N2–Al1 (115.04(12)°), C12–N2–Al1 (116.18(12)°), C13–N2–C12 (115.02(16)°). The nitrogen atom of the piperidinyl unit is not coordinated to the metal center since the heterocyclic ring is located far away from the aluminum atom. The trimethylaluminum adduct of proligand 1d was also obtained as colorless crystals by a similar procedure carried out in a hexane–toluene mixture at room temperature (Scheme 2). The 1H and 13C NMR spectra of the adduct 2d are analogous to those of 2b, except that the proton signal of the amino NH of 2d appears at 7.81 ppm, which is significantly downfield shifted when compared to the corresponding signal of 2b, likely induced by the electron-withdrawing chloro substituent at the meta-position of the phenyl ring. Nevertheless, for the morpholino-anilines 1g–1j, neither the dimethyl aluminum complex nor the trimethylaluminum adduct could be isolated from the reactions of trimethylaluminum with the corresponding proligands at room temperature. When the reaction of trimethylaluminum with proligand 1b was carried out at 90 °C in toluene, continuous evolution of the expected gaseous product from the reaction solution could be observed obviously. Colorless crystals were obtained after a typical work-up procedure. In the 1H NMR spectrum of these

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Dalton Transactions

Scheme 3

Synthesis of aluminum dimethyl complexes 3a–3j.

crystals, the proton signal of the aluminum methyl groups appeared at −0.90 ppm as a singlet and the amino hydrogen of the proligand disappeared completely. From the integration ratios of the Al–CH3 signal with respect to those of the ligand, a stoichiometric structure of the desired dimethyl aluminum complex 3b as illustrated in Scheme 3 could be deduced. Thus, the designed methane elimination reaction did take place at the elevated temperature of 90 °C in toluene. Further experimental studies demonstrated that the trimethylaluminum adduct 2b could also be transformed to the aluminum dimethyl complex 3b by heating in toluene solution at 90 °C. Single crystals of 3b suitable for X-ray diffraction analysis were grown in a hexane–dichloromethane mixture at −20 °C, and the molecular structure was established by X-ray diffraction study. The molecular structure of complex 3b depicted in Fig. 2 shows that the central Al atom is surrounded by both nitrogen donors of the ligand and two methyl groups, adopting a distorted tetrahedral geometry. The N1–All–N2 [93.92(9)°] angle in complex 3b is close to those in related known Al complexes.4 The distance of Al1–N2 (amido) [1.8185(19) Å] in complex 3b is significantly shorter than the corresponding distance in complex 2b [Al1–N2 (amido) = 2.094(2) Å]. In complex 3b, the bond distance of All–N1 (amine) [2.080(2) Å] is longer than the Al1–N2 (amido) distance [1.8185(19) Å], but is close to Al1–N2 (amido) in complex 2b [2.0939(18) Å], showing a coordination characteristic of the Al1–N1 (amine) bonding in 3b. Furthermore, the sum of the three angles [C14–N2– C15 = 113.09(17)°, C14–N2–A1 = 123.07(15)°, C15–N2–Al = 122.76(15)°] around the N2 atom being 358.92(47)° which is very close to 360° might imply an sp2 hybridization state of this nitrogen atom, and in this way an additional pair of electrons could be further donated to the Lewis acidic aluminum center.

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benzylanilino aluminum dimethyl complexes 3g–3j could be obtained for X-ray diffraction analysis. Further attempts to synthesize the corresponding aluminum alkoxy complexes bearing these bidentate amido ligands via alcoholysis reaction failed. The reaction of representative complex 3b with isopropanol led to the production of the free ligand 1b and some indissolvable solids, which hampered further understanding of the reaction. Catalysis of the aluminum complexes in the ROP of rac-lactide

Fig. 2 ORTEP diagram of the molecular structure of 3b. Thermal ellipsoids are presented at 30% probability. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Al1–N1 2.080(2), Al1– N2 1.8185(19), Al1–C1 1.974(3), Al1–C2 1.967(3), N1–C8 1.488(3), N1–C7 1.499(3), N1–C3 1.505(3), N2–C15 1.425(3), N2–Al1–N1 93.92(9), C2– Al1–N1 118.96(12), N2–Al1–C2 111.22(12), N2–Al1–C1 119.99(12), C2– Al1–C1 111.15(14), C1–Al1–N1 100.64(11).

Under heating conditions, the reactions of trimethylaluminum with N-[2-(1-piperidinyl)benzyl]aniline proligands 1a and 1c–1f at 90 °C and with N-(2-morpholinobenzyl)aniline compounds 1g–1j at 75 °C readily afforded the target aluminum dimethyl complexes 3a and 3c–3j, as shown in Scheme 3. The difference in reaction temperature is likely due to the discrepancy of the electro-donating ability between the piperidinylbenzyl and morpholinobenzyl moieties of the ligands. All aluminum alkyl complexes give satisfactory elemental analysis or EI-HRMS results and the 1H and 13C NMR spectra match their structures respectively. In the 1H NMR spectra of these complexes, the amino hydrogen signals of the ligand precursors are not observed and the aluminum methyl signals appear at −0.86 to −1.17 ppm as sharp singlets. The proton resonance of each CH2 unit of the piperidinyl moiety in complexes 3a–3f splits into two integrating sets, due to the inhibited conformational exchange of e-H and a-H arising from the coordination of the N-donor of the piperidinyl to the aluminum center. However, this phenomenon is not observed in the 1 H NMR spectra of N-(2-morpholinobenzyl)anilino aluminum complexes 3g–3j. In their 1H NMR spectra, the proton signals of methylenes adjacent to the N-donors of the morpholino moieties shift to the lower field region significantly. For example, the corresponding signal of ligand 1i appears at 2.99 ppm in the 1H NMR spectrum, and that of complex 3i appears at 3.58 ppm. All these features confirm that the N-donors in the morpholino parts of the ligands take part in the coordination with the central aluminum atoms. Unfortunately, due to the easy efflorescence in an inert gas atmosphere out of solution, no suitable single crystal of the morpholino-

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Polymerizations were carried out in toluene solution at 65 °C with a molar ratio of monomer to initiator of 100 ([LA]0/[Al]0). The two trimethylaluminum adducts 2b and 2d proved to be inactive toward the ring-opening polymerization of rac-lactide. Reports on the catalytic applications of trimethylaluminum adducts are rather rare. Carpentier and co-workers found that amino-bis( pyrazolyl) ligands could afford the adducts by reacting with trimethylaluminum.62 However, no related catalytic data were reported. We tentatively suggest that the inactivity of 2b and 2d for lactide polymerization is likely due to neither the coordinated neutral ligands nor the Al–Me bonds being able to act as an effective initiating group for the polymerization. Nevertheless, aluminum dimethyl complexes 3a–3j proved to be moderately active toward the ROP of rac-LA when used as single component initiators under the conditions employed in this work. From the data compiled in Table 1, it could be observed that the substituents on the anilino moieties of the supporting ligands have different influences on the catalytic activities of these aluminum complexes. For example, no matter what electron-withdrawing or electron-donating substituents are on the ortho-positions of the anilino moieties of the ligands, complexes 3b–3c and 3g–3h all demonstrate relatively lower activities in comparison with those of the other aluminum dimethyl complexes reported in this work (runs 3, 5, 11, 13), which may be due to the steric hindrance of the ortho-substituents, hampering the incorporation of the rac-LA monomer. Hence, the electronic effect is not obvious for the ortho-chloro substitution. However, complexes with chloro- or alkyl substituents on the meta- or para-positions show different catalytic activities toward the ROP of rac-LA. Complex 3c with a metachloro group and complex 3e with a para-chloro group demonstrate slightly higher activities than complex 3f with a para-isopropyl and complex 3a without any substituent (runs 7, 8 versus 1, 9). This fact illustrates that, except for those on the ortho-positions, electron-withdrawing groups help to improve the catalytic activity, while, to a certain extent, electron-donating groups reduce it for this series of catalysts. As shown in Table 1, there is not a clear law between the influence of the piperidinyl and morpholino groups of the ligands on the activities of these complexes toward the ROP of rac-LA, suggesting that the additional donor atom in the morpholino ring has no obvious influence on the coordination/insertion of the monomer during the polymerization process. Furthermore, polylactides with very narrow molecular weight distributions (PDIs, ranging from 1.06 to 1.18) could be provided by aluminum dimethyl complexes 3a–3j. The number average

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Table 1

Dalton Transactions The ring-opening polymerization of rac-lactide initiated by aluminum dimethyl complexesa

Run

Initiator

t/h

Conv.b (%)

Mn, calcd c

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

3a (H)

15 18 18 22 15 20 15 15 15 18 15 22 15 24 15 15

84 94 78 96 74 92 92 88 85 91 60 95 44 94 88 86

12 362 13 802 11 526 14 118 10 956 13 548 13 548 12 972 12 548 13 412 8922 13 962 6632 13 832 12 974 12 686

3b (o-Me2) 3c (o-Cl) 3d (m-Cl) 3e (p-Cl) 3f (p-iPr) 3g (o-Me, O) 3h (o-Me2, O) 3i (m-Cl, O) 3j (p-Cl, O)

M′n d (×104)

Mw/Mn d

1.16

1.11

1.45

1.18

1.63 1.28 1.55

1.14 1.08 1.08

1.56

1.13

0.52

1.55

1.09

0.51

1.40 1.53 1.43

1.11 1.08 1.16

0.53

Pm e 0.53 0.52 0.53 0.54 0.53

a

[rac-LA]0/[Al]0 = 100, [rac-LA]0 = 1.0 M, in toluene, 65 °C. b Determined by the integration ratio of the methine protons in the monomer and polymer. c Mn, calcd = ([rac-LA]0/[Al]0) × 144.13 × conv. % + Mw (molecular mass of ligand). d The number average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were determined by a gel permeation chromatograph, calibrated with polystyrene standards in THF (30 °C), M′n = 0.58 Mn.63 e Pm is the probability of forming a new m-dyad, determined by homonuclear decoupled 1H NMR.

Fig. 3 Plots of M’n (Mw/Mn values indicated in parentheses) vs. monomer conversion for rac-lactide polymerization using complex 3b as initiator (in toluene, 65 °C, [rac-LA]0 = 1 mol L−1, [rac-LA]0/[Al]0 = 100; ■: M’n = 0.58 Mn, Mn and Mw/Mn were determined by GPC, calibrated with polystyrene standards in THF;63 ●: theoretical values based on monomer conversion).

molecular weights of the polymers determined by GPC approximately match those calculated according to the conversions (Table 1). All these features indicate that the polymerizations are well-controlled. But the polymers obtained by these catalysts are atactic, as indicated by homonuclear decoupled 1 H NMR spectroscopic analyses. A linear relationship, between the corrected number average molecular weights (M′n) and the conversions of the racLA monomer under the same polymerization conditions, was observed by using complex 3b as the initiator over the entire conversion range (as shown in Fig. 3), and polymers with narrow molecular weight distributions were provided (PDI < 1.10 until 70–80% conversion). The result implies some living characters of the polymerization process catalyzed by complex

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3b as a single component catalyst. From Fig. 3, it was also noted that, with the increase of monomer conversion in the late stage, the M′n values deviate from the theoretical molecular weights and the PDIs of the polymers broaden slightly. This might be due to a certain degree of transesterification reactions happening during the later stage of the polymerization when the monomer was nearly consumed. Further evidence for this could be found in the ESI-TOF mass spectrum of racLA oligomers obtained by using complex 3a as the catalyst (Fig. 5, vide infra).46,64,65 To understand how these complexes (3a–3j), acting as single component catalysts, initiated the ROP of rac-LA and the role played by the bidentate amido ligands of the complexes during the course of the polymerization, an NMR scale reaction of the most soluble complex 3f with 1.5 equiv. of raclactide was carried out in benzene-d6 at room temperature. The 1H NMR spectrum indicated significant up-field shifts of the signals assignable to the NCH2 protons of the piperidinyl group (from 3.12, 2.93 ppm for 3f to 2.34, 2.20 ppm; see ESI†), suggesting the dissociation of the neutral nitrogen donor from the metal center. Meanwhile the original singlet of the Ar–CH2 protons of the ligand split into two doublets of doublets and three Al–CH3 signals in a 1 : 2 : 1 ratio could be observed, which were accompanied by the appearance of four quartets at 4.97, 4.86, 4.57 and 4.53 ppm assignable to the methine protons of the lactide monomers. Furthermore, the resonances of the rest of the protons of the amido ligand could be clearly located. All these data support the formation of two kinds of monomer-inserted metal species in dimeric form, which are similar to those reported previously.66 Upon addition of another batch of rac-lactide ([3f ] : [rac-LA] = 1 : 5), the signals of one structure with lactide methine resonances at 4.97, 4.52 ppm trailed off. Gradual polymerization occurred when the solution was warmed at elevated temperature, represented by the disappearance of the mentioned four quartets. It was

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Fig. 4

1

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H NMR spectrum (CDCl3, 400 MHz) of rac-lactide oligomer prepared from rac-lactide using 3a as initiator.

also noted that the signals attributable to the Al–CH3 protons remained at the high field region although with some complexity, confirming that the aluminum methyl group did not take part in the initiation. An end-group analysis of the low molecular weight polymer prepared by complex 3a ([3a] : [rac-LA] = 1 : 30) was performed by 1H NMR spectroscopy and ESI-TOF mass spectrometry. In the 1H NMR spectrum, as shown in Fig. 4, except for the typical signal peaks of the PLA main chain such as those at 1.48–1.62 ppm and 5.04–5.24 ppm, the signals relating to the bidentate amido ligand moiety could also be identified at 1.85, 3.20, 4.25 and 6.55–7.40 ppm, which are obviously different from those of the free ligand 1a. This indicates that the polymer might incorporate the bidentate ligand as one of the end groups during the polymerization. This assumption is further confirmed by the ESI-TOF mass spectrum of the same oligomer sample shown in Fig. 5, where a series of peaks differing by 144, namely the mass of one rac-LA molecule, are systematically ended with the same group having a mass of 265 that is exactly the molecular weight of the amido ligand of complex 3a, proving that the linear polymers are terminated by an intact amido ligand on one end and a hydroxyl on the other end. All the results suggest that the bischelating amido ligands in these aluminum dimethyl complexes do act as initiating groups during the polymerization of rac-lactide, that is, the polymerization should be initiated through an insertion of the LA monomer into the Al–N (amido) bond. From the literature, it is well-known that in comparison with alkoxy groups, amido groups such as the silylamido group commonly included in well-defined complexes of rare-earth metals, alkaline earth metals and zinc are relatively poor in initiation when exposed to the catalytic ROP of cyclic esters, and in general badlycontrolled polymerization processes and polyesters with moderate molecular weight distributions are produced.67–81 For example, Mu and coworkers78 reported that, in the absence of alcohol as

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co-initiator, dinuclear aluminum dimethyl complexes supported by anilido-aldimine ligands were inactive toward the polymerization of cyclic esters. Fontaine and Thibault found that, without the addition of alcohol, aluminum methyl complexes bearing phosphine/thiol-amido ligands could show moderate catalytic activities toward the polymerization of ε-caprolactone and low activities toward that of rac-lactide, but the PDIs of the obtained polymers were nevertheless wide, and there was no clear initiation mechanism that could be drawn for the polymerization reaction initiated by those complexes.79 Up to now, a living-type rac-lactide polymerization initiated by an amido group has never been reported previously, it is surprising that the bidentate amido groups involved in the aluminum complexes 3a–3j could efficiently initiate the polymerization of rac-lactide with living features without exception. Probably the suitable steric and electronic factors of this type of ligand enhanced the nucleophilicity of the amido groups considerably, which behaved as effective initiating groups in the polymerization. Based on the above results, a polymerization process via a coordination–insertion mechanism could be assumed. As shown in Scheme 4, at first, the rac-LA monomer coordinates to the metal center by a carbonyl oxygen, and the amido-nitrogen atom of the ligand attacks nucleophilically on the carbonyl carbon of the coordinated lactide. While the acyl–oxygen bond ruptures, the oxygen atom coordinates to the aluminum center to produce an active aluminum–alkoxo bond. This initiation step is followed by continued monomer insertion into the active metal–alkoxo bond until the polymerization is quenched by water in the wet solvent.

Conclusions Aluminum dimethyl complexes 3a–3j supported by bidentate N-[2-(1-piperidinyl)benzyl]anilino or N-(2-morpholinobenzyl)-

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Fig. 5

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ESI-TOF mass spectrum of rac-lactide oligomer obtained from rac-LA using 3a as initiator ([rac-LA]0 : [3a]0 = 30 : 1).

Scheme 4

Proposed mechanism for lactide polymerization initiated by complex 3a.

anilino ligands were synthesized via alkane elimination reactions at elevated temperature. When the reactions of trimethylaluminum and the proligands were performed at room temperature, trimethylaluminum adducts 2b and 2d were formed. The molecular structures of complexes 2b and 3b were further confirmed by X-ray diffraction techniques. Without any co-initiator, the aluminum dimethyl complexes 3a–3j could

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polymerize rac-lactide with living features, producing atactic polylactides with high molecular weights and narrow molecular weight distributions. The substituents on the anilino moieties of the ligands also had some influence on the activities of these aluminum complexes for the ROP of rac-LA. Complexes with ortho-substituents on the anilino unit, no matter whether they are electron-donating or electron-withdrawing,

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show lower activities than the others, while the introduction of an electron-withdrawing substituent on the meta- or parapositions improves the catalytic activity. Based on the endgroup analysis via 1H NMR spectroscopy and ESI-TOF mass spectrometry, a coordination–insertion mechanism via the initiation by monomer insertion into the Al–N (amido) bond could be proposed for the ROP of rac-lactide by these aluminum complexes.

Experimental section General considerations All manipulations involving air-sensitive compounds were carried out under a dry argon atmosphere using a standard Schlenk line or glove box techniques. n-Hexane and toluene were refluxed over sodium benzophenone ketyl under argon prior to use. Dichloromethane was dried over calcium hydride. AlMe3 (2.0 M solution in toluene) was purchased from Aldrich and used as received. 2-(1-Piperidinyl)benzaldehyde and 2-morpholinobenzaldehyde were prepared according to the published procedures.82 rac-Lactide (Aldrich) was recrystallized with dry toluene and then sublimated twice under vacuum at 80 °C. All other chemicals were commercially available and used after appropriate purification. Glassware and vials used in the polymerization were dried in an oven at 120 °C overnight and exposed to a vacuum–argon cycle three times. NMR spectra were recorded on a Bruker AVANCE-400 spectrometer with CDCl3 as solvent (1H: 400 MHz; 13C: 100 MHz). Chemical shifts for 1H and 13C NMR spectra were referenced internally using the residual solvent resonances and reported relative to tetramethylsilane (TMS). Elemental analyses were performed on an EA-1106 instrument. Gel permeation chromatography (GPC) analyses were carried out on a Waters instrument (M515 pump, Optilab Rex Injector) in THF at 25 °C, at a flow rate of 1 mL min−1. Calibration standards were commercially available narrowly distributed linear polystyrene samples that cover a broad range of molar masses. Synthesis of the proligands and complexes N-[2-(1-Piperidinyl)benzyl]aniline (1a). To a solution of 2-(1-piperidinyl)benzaldehyde (3.78 g, 20.0 mmol) in benzene (80 mL), aniline (1.86 g, 20.0 mmol) was added dropwise at room temperature and then para-toluenesulfonic acid monohydrate (2.0 mg) was added under stirring. A Dean–Stark apparatus was attached and the mixture was heated to reflux. After cooling to r.t., all volatiles were removed via rotary evaporation to give a yellow solid which was recrystallized with ethyl acetate; yellow crystals were obtained (4.82 g, 91.3%). The obtained yellow crystals were dissolved in absolute alcohol (200 mL), sodium borohydride (11.4 g, 30.0 mol) was added slowly at room temperature to the yellowish solution and the solution was heated to reflux for 36 h again. After cooling, water was added to the reaction mixture and the organic layer was extracted with dichloromethane (3 × 15 mL). The organic phases were combined and dried over anhydrous MgSO4.

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Removal of the solvent by rotary evaporation gave a yellowish viscous oil, which was purified by column chromatography (silica gel 100, Merck, anhydrous ethanol) to provide a colorless oil (3.44 g, ∼64%, 95% purity), which could not be further purified. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.35 (d, 1H, J = 7.5 Hz, Ar–H), 7.23 (td, 1H, J = 7.5, 1.5 Hz, Ar–H), 7.20–7.14 (m, 2H, Ar–H), 7.11 (d, 1H, J = 7.5 Hz, Ar–H), 7.03 (td, 1H, J = 7.5, 1.0 Hz, Ar–H), 6.73–6.63 (m, 3H, Ar–H), 4.66 (br, 1H, –NH), 4.37 (s, 2H, Ar–CH2), 2.89 (t, 4H, J = 5.2 Hz, –NCH2–), 1.73 (quintet, 4H, J = 5.2 Hz, –CH2–), 1.62–1.48 (m, 2H, –CH2–); 13 C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 152.6, 148.6, 133.7, 129.13, 129.07, 127.9, 123.5, 120.0, 117.2, 113.0 (all Ar–C, Ar– C–N), 54.0 (Ar–CH2), 44.8 (–NCH2–), 26.7 (–CH2–), 24.2 (–CH2–). 2,6-Dimethyl-N-[2-( piperidin-1-yl)benzyl]aniline (1b). The procedure was similar to that of compound 1a except that 2,6dimethylaniline (2.42 g, 20.0 mmol) was used. Purification by column chromatography (silica gel 100, Merck, anhydrous ethanol) provided a colorless oil (3.92 g, ∼60%, 90% purity), which could not be further purified. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.36 (dd, 1H, J = 7.5, 1.5 Hz, Ar–H), 7.23 (td, 1H, J = 7.8, 1.5 Hz, Ar–H), 7.11 (dd, 1H, J = 7.8, 1.0 Hz, Ar–H), 7.03 (td, 1H, J = 7.5, 1.0 Hz, Ar–H), 6.97 (d, 2H, J = 7.5 Hz, Ar–H), 6.80 (t, 1H, J = 7.5 Hz, Ar–H), 4.16 (s, 2H, Ar–CH2–), 3.79–3.57 (br, 1H, –NH), 2.85 (t, 4H, J = 5.4 Hz, –NCH2–), 2.28 (s, 6H, Ar–CH3), 1.69 (quintet, 4H, J = 5.4 Hz, –CH2–), 1.60–1.50 (m, 2H, –CH2–); 13C{1H} NMR (100 Hz, CDCl3, 25 °C): δ 151.9, 145.5, 134.7, 128.7, 128.6, 127.7, 127.0, 122.7, 120.7, 119.2 (all Ar–C, Ar–C–N), 53.6 (Ar–CH2–), 47.8 (–NCH2–), 25.7 (–CH2–), 23.3 (–CH2–), 17.6 (Ar–CH3). 2-Chloro-N-[2-(1-piperidinyl)benzyl]aniline (1c). The procedure was similar to that of compound 1a except that 2-chloroaniline (2.54 g, 20.0 mmol) was used. Purification by column chromatography (silica gel 100, Merck, anhydrous ethanol) provided a colorless oil (3.46 g, ∼50%, 85% purity), which could not be further purified. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.34 (d, 1H, J = 7.5 Hz, Ar–H), 7.27–7.21 (m, 2H, Ar–H), 7.16–7.00 (m, 3H, Ar–H), 6.70 (d, 1H, J = 7.2 Hz, Ar– H), 6.60 (td, 1H, J = 7.7, 1.3 Hz, Ar–H), 5.21 (br t, 1H, J = 5.7 Hz, –NH), 4.44 (d, 2H, J = 5.7 Hz, Ar–CH2), 2.89 (t, 4H, J = 5.2 Hz, –NCH2–), 1.73 (quintet, 4H, J = 5.2 Hz, –CH2–), 1.62–1.50 (m, 2H, –CH2–); 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 151.7, 143.2, 132.2, 128.0, 127.9, 127.0, 126.7, 122.6, 119.1, 118.1, 116.0, 110.5 (all Ar–C, Ar–C–N), 53.1 (Ar–CH2–), 43.2 (–NCH2–), 25.6 (–CH2–), 23.2 (–CH2–). 3-Chloro-N-[2-(1-piperidinyl)benzyl]aniline (1d). The procedure was similar to that of compound 1a except that 3-chloroaniline (2.54 g, 20.0 mmol) was used. Crystallization in ethanol afforded a white solid (4.03 g, ∼50%, 80% purity), which could not be further purified. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.31 (d, 1H, J = 7.6 Hz, Ar–H), 7.25 (td, 1H, J = 7.6, 1.5 Hz, Ar–H), 7.12 (d, 1H, J = 7.2 Hz, Ar–H), 7.08–6.99 (m, 2H, Ar–H), 6.68–6.60 (m, 2H, Ar–H), 6.52 (dd, 1H, J = 8.2, 2.2 Hz, Ar–H), 4.83 (br, 1H, –NH), 4.34 (d, 2H, J = 3.4 Hz, Ar– CH2–), 2.89 (t, 4H, J = 5.3 Hz, –NCH2–), 1.73 (quintet, 4H, J = 5.7 Hz, –CH2–), 1.64–1.53 (m, 2H, –CH2–); 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 152.7, 149.7, 135.0, 133.2, 130.1,

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129.2, 128.2, 123.7, 120.2, 117.1, 112.5, 111.5 (all Ar–C, Ar– C–N), 54.2 (Ar–CH2–), 44.5 (–NCH2–), 26.8 (–CH2–), 24.3 (–CH2–). 4-Chloro-N-[2-(1-piperidinyl)benzyl]aniline (1e). The procedure was similar to that of compound 1a except that 4-chloroaniline (2.54 g, 20.0 mmol) was used. Crystallization in ethanol afforded a white solid (3.82 g, ∼63%, 95% purity), which could not be further purified. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.31 (d, 1H, J = 7.5 Hz), 7.23 (td, 1H, J = 7.5, 1.5 Hz, Ar–H), 7.14–7.07 (m, 3H, Ar–H), 7.03 (td, 1H, J = 7.5, 1.0 Hz, Ar–H), 6.57 (d, 2H, J = 8.8 Hz, Ar–H), 4.76 (br, 1H, –NH), 4.33 (s, 2H, Ar–CH2–), 2.88 (t, 4H, J = 5.3 Hz, –NCH2–), 1.72 (quintet, 4H, J = 5.3 Hz, –CH2–), 1.62–1.51 (m, 2H, –CH2–); 13 C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 152.8, 148.3, 133.6, 129.5, 128.6, 127.1, 123.3, 119.1, 117.4, 112.1 (all Ar–C, Ar– C–N), 54.1 (Ar–CH2–), 44.4 (–NCH2–), 26.5 (–CH2–), 22.8 (–CH2–). 4-Isopropyl-N-[2-(1-piperidinyl)benzyl]aniline (1f ). The procedure was similar to that of compound 1a except that 4-isopropylaniline (2.70 g, 20.0 mmol) was used. Purification by column chromatography (silica gel 100, Merck, anhydrous ethanol) provided a yellow oil (4.15 g, ∼60%, 90% purity), which could not be further purified. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.36 (dd, 1H, J = 7.5, 1.3 Hz, Ar–H), 7.22 (t, 1H, J = 7.5, 1.6 Hz, Ar–H), 7.10 (dd, 1H, J = 7.9, 0.8 Hz, Ar–H), 7.06–7.01 (m, 3H, Ar–H), 6.61 (d, 2H, J = 7.5 Hz, Ar–H), 4.50 (br, 1H, –NH), 4.34 (s, 2H, Ar–CH2), 2.91 (t, 4H, J = 5.3 Hz, –NCH2–), 2.79 (sept, 1H, J = 6.9 Hz, –CH(CH3)2), 1.75 (quintet, 4H, J = 7.5 Hz, –CH2–), 1.61–1.51 (m, 2H, –CH2–), 1.20 [d, J = 6.9 Hz, 6H, –CH(CH3)2]; 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 151.4, 145.6, 136.6, 132.9, 128.1, 126.7, 125.9, 122.5, 118.8, 112.0 (all Ar–C, Ar–C–N), 53.1 (Ar–CH2–), 44.0 (–NCH2–), 32.1 [–CH(CH3)2], 25.7 (–CH2–), 23.0 [–CH(CH3)2, –CH2–]. 2-Methyl-N-[2-(4-morpholino)benzyl]aniline (1g). To a solution of 2-(4-morpholino)benzaldehyde (3.82 g, 20.0 mmol) in benzene (80 mL), o-toluidine (2.14 g, 20.0 mmol) was added dropwise at room temperature and then para-toluenesulfonic acid monohydrate (2.0 mg) was added under stirring. A Dean– Stark apparatus was attached and the mixture was heated to reflux. The reaction mixture was cooled and removal of solvent by rotary evaporation gave a yellow solid which was recrystallized with ethyl acetate. Yellow crystals were obtained (4.56 g, 76.1%). The obtained crystals were dissolved in absolute alcohol (200 mL) and sodium borohydride (11.4 g, 30.0 mol) was added slowly at room temperature to the yellowish solution. The solution was heated to reflux for 36 h again. After cooling, water was added to the reaction mixture and the organic layer was extracted with dichloromethane (3 × 15 mL), the organic phases were combined and dried over anhydrous MgSO4. Removal of the solvent by rotary evaporation gave a viscous oil, which was purified by crystallization in ethanol to afford colorless crystals (3.62 g, ∼55%, 88% purity), which could not be further purified. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.42 (dd, 1H, J = 7.5, 1.2 Hz, Ar–H), 7.31 (td, 1H, J = 7.8, 1.5 Hz, Ar–H), 7.18–7.06 (m, 4H, Ar–H), 6.71–6.63 (m, 2H, Ar–H), 4.46 (s, 2H, Ar–CH2–), 4.17 (br, 1H, –NH), 3.88 (t, 4H, J =

9106 | Dalton Trans., 2014, 43, 9098–9110

Dalton Transactions

4.6 Hz, –OCH2–), 3.01 (t, 4H, J = 4.6 Hz, –NCH2–), 2.19 (s, 3H, Ar–CH3); 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 151.1, 146.3, 134.0, 130.0, 129.3, 128.2, 127.1, 124.3, 121.9, 119.8, 117.0, 109.9 (all Ar–C, Ar–C–N), 67.4 (–OCH2–), 53.0 (Ar–CH2), 44.2 (–NCH2–), 17.6 (Ar–CH3). 2,6-Dimethyl-N-[2-(4-morpholino)benzyl]aniline (1h). The procedure was similar to that of compound 1g except that 2,6dimethylaniline (2.42 g, 20.0 mmol) was used. Purification by column chromatography (silica gel 100, Merck, anhydrous ethanol) provided a colorless oil (3.59 g, ∼54%, 90% purity), which could not be further purified. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.49 (dd, 1H, J = 7.6, 1.2 Hz, Ar–H), 7.32 (td, 1H, J = 8.0, 0.8 Hz, Ar–H), 7.20–7.10 (m, 2H, Ar–H), 7.03 (d, 2H, J = 7.5 Hz, Ar–H), 6.87 (t, 1H, J = 7.5 Hz, Ar–H), 4.23 (s, 2H, Ar– CH2–), 3.85 (t, 4H, J = 4.6 Hz, –OCH2–), 3.52–3.00 (br, 1H, –NH), 2.95 (t, J = 4.6 Hz, 4H, –NCH2–), 2.32 (s, 6H, Ar–CH3); 13 C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 151.4, 146.5, 135.9, 130.1, 129.7, 128.9, 128.3, 124.6, 122.0, 120.4 (all Ar–C, Ar– C–N), 67.5 (–OCH2–), 53.5 (Ar–CH2–), 48.5 (–NCH2–), 18.7 (Ar–CH3). 3-Chloro-N-[2-(4-morpholino)benzyl]aniline (1i). The procedure was similar to that of compound 1g except that 3-chloroaniline (2.54 g, 20.0 mmol) was used. Crystallization in ethanol afforded colorless crystals (3.71 g, ∼58%, 95% purity), which could not be further purified. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.39 (dd, 1H, J = 7.6, 1.3 Hz, Ar–H), 7.32 (td, 1H, J = 7.9, 1.5 Hz, Ar–H), 7.18 (dd, 1H, J = 8.2, 1.0 Hz, Ar–H), 7.13 (td, 1H, J = 7.5, 1.3 Hz, Ar–H), 7.08 (t, 1H, J = 8.2 Hz, Ar– H), 6.70 (m, 1H, Ar–H), 6.68 (d, 1H, J = 1.3 Hz, Ar–H), 6.52 (dd, 1H, J = 7.2, 1.0 Hz, Ar–H), 4.58 (br, 1H, NH), 4.39 (s, 2H, Ar– CH2), 3.89 (t, 4H, J = 4.5 Hz, –OCH2–), 2.99 (t, 4H, J = 4.50 Hz, –NCH2–); 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 150.9, 149.3, 134.9, 133.3, 130.1, 129.2, 128.2, 124.3, 120.0, 117.1, 112.3, 111.2 (all Ar–C, Ar–C–N), 67.3 (–OCH2–), 53.0 (Ar–CH2–), 43.8 (–NCH2–). 4-Chloro-N-[2-(4-morpholino)benzyl]aniline (1j). The procedure was similar to that of compound 1h except that 4-chloroaniline (2.54 g, 20.0 mmol) was used. Crystallization in ethanol afforded colorless crystals (3.99 g, ∼59%, 90% purity), which could not be further purified. 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.38 (dd, 1H, J = 7.6, 1.3 Hz, Ar–H), 7.30 (td, 1H, J = 7.8, 1.6 Hz, Ar–H), 7.16 (dd, 1H, J = 8.0, 1.0 Hz, Ar–H), 7.14–7.07 (m, 3H, Ar–H), 6.57 (d, 2H, J = 8.8 Hz, Ar–H), 4.48 (br, 1H, NH), 4.38 (s, 2H, Ar–CH2–), 3.86 (t, 4H, J = 4.6 Hz, –OCH2–), 2.97 (t, 4H, J = 4.6 Hz, –NCH2–); 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 151.1, 147.0, 133.5, 129.3, 129.1, 128.4, 124.4, 122.0, 120.1, 114.0 (all Ar–C, Ar–C–N), 67.5 (–OCH2–), 53.1 (Ar–CH2), 44.5 (–NCH2–). {2,6-Dimethyl-N-[2-(1-piperidinyl)benzyl]aniline}AlMe3 (2b). A toluene solution of trimethylaluminum (2.0 mL, 4.0 mmol, 2 M in toluene) was added slowly to the solution of proligand 1b (1.176 g, 4.00 mmol) in n-hexane (15 mL) under stirring. No instantaneous evolution of methane could be observed. The solution was stirred for 48 h at room temperature. Upon concentration, a white solid precipitated from the solution, which was further purified by recrystallization with n-hexane several

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times at about −20 °C to afford colorless crystals (0.535 g, 36.5%). Found: C, 74.90; H, 9.89; N, 7.58. Anal. calcd for C23H35AlN2: C, 75.37; H, 9.63; N, 7.64%; 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.22 (td, 1H, J = 8.0, 1.7 Hz, Ar–H), 7.09 (d, 1H, J = 7.6 Hz, Ar–H), 6.96–6.84 (br, 3H, Ar–H), 6.84 (t, 1H, J = 7.3 Hz, Ar–H), 6.78 (d, 1H, J = 7.3 Hz, Ar–H), 5.33 (s, 1H, N–H), 4.43 (br s, 2H, Ar–CH2), 2.80–1.50 (br, 6H, Ar–CH3), 2.76 (m, 4H, –NCH2–), 1.75 (m, 4H, –CH2–), 1.58 (m, 2H, –CH2–), −0.85 (s, 9H, Al–CH3); 13C{1H} NMR (100 Hz, CDCl3, 25 °C): δ 154.5, 139.5, 131.3, 131.0, 130.4, 129.7, 129.5, 125.3, 123.9, 120.5 (all Ar–C, Ar–C–N), 55.5 (Ar–CH2), 48.5 (–NCH2–), 26.3 (–CH2–), 24.1 (–CH2–), 19.0 (br, Ar–CH3), −7.8 (Al–CH3). {3-Chloro-N-[2-(1-piperidinyl)benzyl]aniline}AlMe3 (2d). The procedure was similar to that of complex 2b, except that proligand 1d (1.200 g, 4.00 mmol) was used. Crystallization of the crude product with n-hexane at −20 °C several times afforded colorless crystals (0.553 g, 37.1%). Found: C, 67.46; H, 8.11; N, 7.36. Anal. calcd for C21H30AlClN2: C, 67.64; H, 8.11; N, 7.51%; 1 H NMR (400 MHz, CDCl3, 25 °C): δ 7.82 (br s, 1H, N–H), 7.22 (td, 1H, J = 7.4, 2.0 Hz, Ar–H), 7.17 (dd, 1H, J = 8.0, 1.6 Hz, Ar– H), 7.12 (t, 1H, J = 8.0 Hz, Ar–H), 7.05–6.99 (m, 3H, Ar–H), 6.95 (t, 1H, J = 1.6 Hz, Ar–H), 6.82 (dd, 1H, J = 8.0, 1.6 Hz, Ar–H), 4.84–4.11 (br, 2H, Ar–CH2–), 2.91 (br, 4H, –NCH2–), 1.89 (br, 4H, –CH2–), 1.69 (br, 2H, –CH2–), −0.93 (s, 9H, Al–CH3); 13C {1H} NMR (100 Hz, CDCl3, 25 °C): δ 153.3, 144.0, 135.3, 132.3, 130.6, 130.4, 130.0, 126.1, 125.9, 122.4, 122.1, 120.4 (all Ar–C, Ar–C–N), 54.6 (br, Ar–CH2), 51.7 (–NCH2–), 26.8 (–CH2–), 24.0 (–CH2–), −9.3 (AlCH3). {N-[2-(1-Piperidinyl)benzyl]anilino}AlMe2 (3a). A toluene solution of trimethylaluminum (2.0 mL, 4.0 mmol, 2 M in toluene) was added slowly to the solution of proligand 1a (1.064 g, 4.000 mmol) in n-hexane (15 mL) under stirring. No instantaneous evolution of methane could be observed. The solution was stirred for 6 h at room temperature and then heated to 90 °C for 24 h. Upon concentration, a white solid precipitated from the solution, which was further purified by recrystallization with a mixture of dichloromethane and n-hexane to afford colorless crystals (0.343 g, 26.6%). Found: C, 74.37; H, 8.61; N, 8.70. Anal. calcd for C20H27AlN2: C, 74.50; H, 8.44; N, 8.69%; 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.47 (d, 1H, J = 8.0 Hz, Ar–H), 7.40–7.30 (m, 2H, Ar–H), 7.28–7.16 (m, 3H, Ar–H), 6.73 (d, 2H, J = 7.6 Hz, Ar–H), 6.62 (t, 1H, J = 7.2 Hz, Ar–H), 4.48 (s, 2H, Ar–CH2–), 3.65 (d, 2H, J = 13.6 Hz, –NCH2–), 3.40 (t, 2H, J = 12.4 Hz, –NCH2–), 1.95–1.79 (m, 3H, –CH2–), 1.78–1.66 (m, 2H, –CH2–), 1.64–1.50 (m, 1H, –CH2–), −0.86 (s, 6H, Al–CH3); 13C{1H} NMR (100 MHz, CDCl3): δ 154.1, 142.5, 133.9, 132.2, 128.5, 127.5, 128.4, 121.8, 114.4, 114.3 (all Ar–C, Ar–C–N), 54.4 (Ar–CH2–), 50.3 (–NCH2–), 23.4 (–CH2–), 21.5 (–CH2–), −10.0 (Al–CH3). {2,6-Dimethyl-N-[2-(1-piperidinyl)benzyl]anilino}AlMe2 (3b). The procedure was similar to that of complex 3a, except that proligand 1b (1.176 g, 4.000 mmol) was used. Crystallization of the crude product with dichloromethane and n-hexane at −20 °C several times afforded colorless crystals (0.530 g, 37.9%). Found: C, 74.65; H, 8.82; N, 8.02. Anal. calcd for C22H31AlN2: C, 75.39; H, 8.92; N, 7.99%; 1H NMR (400 MHz,

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CDCl3, 25 °C): δ 7.32 (d, J = 8.1 Hz, 1H, Ar–H), 7.25–7.07 (m, 3H, Ar–H), 6.97 (d, 2H, J = 7.4 Hz, Ar–H), 6.82 (t, 1H, J = 7.4 Hz, Ar–H), 4.26 (s, 2H, Ar–CH2–), 3.59 (m, 2H, –NCH2–), 3.31 (m, 2H, –NCH2–), 2.22 (s, 6H, Ar–CH3), 2.07–1.89 (m, 2H, –CH2–), 1.88–1.71 (m, 2H, –CH2–), 1.71–1.56 (m, 2H, –CH2–), −1.17 (s, 6H, Al–CH3); 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 150.8, 145.1, 138.2, 135.7, 131.2, 128.2, 127.3, 126.4, 123.0, 120.7 (all Ar–C, Ar–C–N), 57.5 (Ar–CH2–), 52.8 (–NCH2–), 23.3 (–CH2–), 22.6 (–CH2–), 19.1 (Ar–CH3), −8.3 (Al–CH3). {2-Chloro-N-[2-(1-piperidinyl)benzyl]anilino}AlMe2 (3c). The procedure was similar to that of complex 3a, except that proligand 1c (1.200 g, 4.000 mmol) was used. Crystallization of the crude product with dichloromethane and n-hexane at −20 °C several times afforded colorless crystals (0.516 g, 36.2%). Found: C, 67.13; H, 7.86; N, 7.37. Anal. calcd for C20H26AlClN2: C, 67.31; H, 7.34; N, 7.85%; 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.46 (d, 1H, J = 8.1 Hz, Ar–H), 7.39–7.30 (m, 2H, Ar–H), 7.29–7.13 (m, 3H, Ar–H), 6.86 (d, 1H, J = 8.2 Hz, Ar–H), 6.59 (t, 1H, J = 7.6 Hz, Ar–H), 4.53 (s, 2H, Ar–CH2–), 3.65 (m, 2H, –NCH2–), 3.33 (m, 2H, –NCH2–), 1.87 (m, 3H, –CH2–), 1.69 (m, 2H, –CH2–), 1.55 (m, 1H, –CH2–), −0.94 (s, 6H, Al–CH3); 13C {1H} NMR (100 MHz, CDCl3, 25 °C): δ 151.9, 143.1, 133.3, 132.4, 129.2, 128.0, 127.6, 126.4, 123.0, 122.3, 116.3, 115.8 (all Ar–C, Ar–C–N), 55.6 (Ar–CH2), 49.6 (–NCH2–), 23.7 (–CH2–), 21.7 (–CH2–), −9.2 (Al–CH3). {3-Chloro-N-[2-(1-piperidinyl)benzyl]anilino}AlMe2 (3d). The procedure was similar to that of complex 3a, except that proligand 1d (1.200 g, 4.000 mmol) was used. Crystallization of the crude product with dichloromethane and n-hexane at −20 °C several times afforded colorless crystals (0.440 g, 30.9%). Found: C, 67.08; H, 7.87; N, 7.41. Anal. calcd for C20H26AlClN2: C, 67.31; H, 7.34; N, 7.85%; 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.42–7.03 (m, 5H, Ar–H), 6.94 (t, 1H, J = 6.7 Hz, Ar–H), 6.64–6.42 (m, 2H, Ar–H), 4.31 (s, 2H, Ar–CH2), 3.50 (d, 2H, J = 13.2 Hz, –NCH2–), 3.28 (t, 2H, J = 12.0 Hz, –NCH2–), 1.78 (m, 3H, –CH2–), 1.64 (m, 2H, –CH2–), 1.49 (m, 1H, –CH2–), −0.97 (s, 6H, Al–CH3); 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 155.5, 142.5, 134.7, 133.4, 132.4, 129.2, 128.0, 126.8, 122.1, 114.5, 114.0, 113.4 (all Ar–C, Ar–C–N), 54.3 (Ar–CH2–), 50.3 (–NCH2–), 23.3 (–CH2–), 21.5 (–CH2–), −10.0 (Al–CH3). {4-Chloro-N-[2-(1-piperidinyl)benzyl]anilino}AlMe2 (3e). The procedure was similar to that of complex 3a, except that proligand 1e (1.20 g, 4.00 mmol) was used. Crystallization of the crude product with dichloromethane and n-hexane at −20 °C several times afforded colorless crystals (0.482 g, 33.8%). Found: C, 67.13; H, 7.81; N, 7.37. Anal. calcd for C20H26AlClN2: C, 67.31; H, 7.34; N, 7.85%; 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.44 (d, 1H, J = 8.1 Hz, Ar–H), 7.37–7.29 (m, 2H, Ar–H), 7.23 (t, 1H, J = 7.1 Hz, Ar–H), 7.09 (d, 2H, J = 7.8 Hz, Ar–H), 6.61 (d, 2H, J = 7.8 Hz, Ar–H), 4.39 (s, 2H, Ar–CH2–), 3.60 (d, 2H, J = 13.6 Hz, –NCH2–), 3.36 (td, J = 13.6, 3.5 Hz, 2H, –NCH2–), 1.93–1.76 (m, 3H, –CH2–), 1.76–1.65 (m, 2H, CH2−), 1.63–1.48 (m, 1H, –CH2–), −0.91 (s, 6H, Al–CH3); 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 152.7, 142.5, 133.6, 132.4, 128.4, 127.9, 126.8, 122.1, 119.2, 115.6 (all Ar–C, Ar–C–N), 54.5

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(Ar–CH2–), 50.3 (–NCH2–), 23.4 (–CH2–), 21.5 (–CH2–), −10.1 (Al–CH3). {4-Isopropyl-N-[2-(1-piperidinyl)benzyl]anilino}AlMe2 (3f ). The procedure was similar to that of complex 3a, except that proligand 1f (1.232 g, 4.000 mmol) was used. Crystallization of the crude product with dichloromethane and n-hexane at −20 °C several times afforded colorless crystals (0.488 g, 33.5%). HRMS calcd for C23H33AlN2: 364.2459. Found: 364.2462; 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.42 (d, 1H, J = 8.2 Hz, Ar–H), 7.35–7.25 (m, 2H, Ar–H), 7.19 (t, 1H, J = 8.8, Ar– H), 7.04 (d, 2H, J = 8.4 Hz, Ar–H), 6.67 (d, 2H, J = 8.4 Hz, Ar–H), 4.44 (s, 2H, Ar–CH2), 3.70–3.51 (m, 2H, –NCH2–), 3.45–3.26 (m, 2H, –NCH2–), 2.80 (Sept, 1H, J = 6.9 Hz, CH(CH3)2), 1.94–1.74 (m, 3H, –CH2–), 1.74–1.60 (m, 2H, –CH2–), 1.60–1.48 (m, 1H, –CH2–), 1.21 [d, 6H, J = 6.9 Hz, CH(CH3)2], −0.90 (s, 6H, Al– CH3); 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 152.1, 142.7, 134.8, 134.2, 132.3, 127.6, 126.6, 126.6, 122.0, 114.2 (all Ar–C, Ar–C–N), 54.6 (Ar–CH2), 50.3 (N–CH2), 33.0 [CH(CH3)2], 24.3 [–CH(CH3)2], 23.4 (CH2), 21.5 (CH2), −9.9 (Al–CH3). {2-Methyl-N-[2-(4-morpholino)benzyl]anilino}AlMe2 (3g). The procedure was similar to that of complex 3a, except that proligand 1g (1.128 g, 4.000 mmol) was used. Crystallization of the crude product with dichloromethane and n-hexane at −20 °C several times afforded colorless crystals (0.444 g, 32.8%). Found: C, 70.63; H, 8.22; N, 8.13. Anal. calcd for C20H27AlN2O: C, 70.98; H, 8.04; N, 8.28%; 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.41–7.28 (m, 3H, Ar–H), 7.26–7.19 (m, 1H, Ar–H), 7.18–7.03 (m, 2H, Ar–H), 6.96 (d, 1H, J = 6.8 Hz, Ar–H), 6.77–6.63 (t, 1H, J = 7.4 Hz, Ar–H), 4.61 (s, 2H, Ar–CH2–), 3.95 (br, 4H, –OCH2–), 3.60 (br, 4H, –NCH2–), 2.30 (s, 3H, Ar–CH3), −1.03 (s, 6H, Al–CH3); 13C{1H} NMR (100 MHz, CDCl3 25 °C): δ 154.0, 142.3, 134.8, 132.4, 130.8, 129.7, 127.8, 127.0, 126.4, 121.6, 119.0, 118.2 (all Ar–C, Ar–C–N), 62.7 (–OCH2–), 56.5 (Ar–CH2–), 49.7 (–NCH2–), 20.3 (Ar–CH3), −9.8 (Al–CH3). {2,6-Dimethyl-N-[2-(4-morpholino)benzyl]anilino}AlMe2 (3h). The procedure was similar to that of complex 3a, except that proligand 1h (1.184 g, 4.00 mmol) was used. Crystallization of the crude product with dichloromethane and n-hexane at −20 °C several times afforded colorless crystals (0.478 g, 33.9%). Found: C, 71.87; H, 8.61; N, 7.65. Anal. calcd for C21H29AlN2O: C, 71.56; H, 8.29; N, 7.95%; 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.34–7.30 (m, 2H, Ar–H), 7.31–7.21 (m, 2H, Ar–H), 7.06 (d, 2H, J = 7.4 Hz, Ar–H), 6.90 (t, 1H, J = 7.4 MHz, Ar–H), 4.33 (s, 2H, Ar–CH2), 4.08 (br, 4H, –OCH2–), 3.79 (br, 2H, –NCH2–), 3.50 (br, 2H, –NCH2–), 2.28 (s, 6H, Ar–CH3), −1.08 (s, 6H, Al–CH3); 13C{1H} NMR (100 MHz, CDCl3, 25 °C): δ 150.3, 144.0, 138.0, 135.5, 131.5, 128.4, 127.5, 126.9, 123.3, 120.5 (all Ar–C, Ar–C–N), 63.3 (–OCH2–), 57.2 (Ar–CH2–), 51.6 (–NCH2–), 19.1 (Ar–CH3), −8.7 (Al–CH3). {3-Chloro-N-[2-(4-morpholino)benzyl]anilino}AlMe2 (3i). The procedure was similar to that of complex 3a, except that proligand 1i (1.208 g, 4.00 mmol) was used. Crystallization of the crude product with dichloromethane and n-hexane at −20 °C several times afforded colorless crystals (0.438 g, 30.6%). Found: C, 64.41; H, 6.92; N, 7.82. Anal. calcd for C19H24AlClN2O·(0.28C6H14): C, 64.86; H, 7.35; N, 7.31%; 1H

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NMR (400 MHz, CDCl3, 25 °C): δ 7.41–7.32 (m, 3H, Ar–H), 7.30–7.22 (m, J = 6.8 Hz, 1H, Ar–H), 7.04 (t, J = 7.9 Hz, 1H, Ar– H), 6.65 (s, 1H, Ar–H), 6.59 (t, J = 8.9 Hz, 2H, Ar–H), 4.40 (s, 2H, Ar–CH2–), 3.93 (br, 4H, –OCH2–), 3.58 (br, 4H, –NCH2–), −0.87 (s, 6H, Al–CH3); 13C{1H} NMR (100 Hz, CDCl3, 25 °C): δ 155.2, 141.8, 134.7, 133.6, 132.6, 129.3, 128.1, 127.3, 121.8, 114.8, 114.1, 113.5 (all Ar–C, Ar–C–N), 62.5 (–OCH2–), 54.0 (Ar–CH2–), 49.6 (–NCH2–), −10.1 (Al–CH3). {4-Chloro-N-[2-(4-morpholino)benzyl]anilino}AlMe2 (3j). The procedure was similar to that of complex 3a, except that proligand 1j (1.208 g, 4.00 mmol) was used. Crystallization of the crude product with dichloromethane and n-hexane at −20 °C several times afforded colorless crystals (0.509 g, 35.5%). Found: C, 63.96; H, 7.08; N, 7.80. Anal. calcd for C19H24AlClN2O: C, 63.59; H, 6.74; N, 7.81%; 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.40–7.32 (m, 2H, Ar–H), 7.30–7.21 (m, 2H, Ar–H), 7.10 (d, 2H, J = 8.5 Hz, Ar–H), 6.61 (d, 2H, J = 8.5 Hz, Ar–H), 4.39 (s, 2H, Ar–CH2), 3.93 (br, 4H, –OCH2–), 3.58 (br, 4H, –NCH2–), −0.88 (s, 6H, Al–CH3); 13C{1H} NMR (100 MHz, CDCl3): δ 152.5, 141.9, 133.8, 132.7, 128.5, 128.1, 127.2, 121.8, 119.6, 115.6 (all Ar–C, Ar–C–N) (Ar–C), 62.6 (–OCH2–), 54.0 (Ar–CH2–), 49.6 (–NCH2–), −10.1 (Al–CH3). X-ray crystallographic study Single crystals suitable for X-ray diffraction studies were obtained from n-hexane for 2b and from a mixture of dichloromethane and n-hexane for 3b at −20 °C. The crystallographic data for complexes 2b and 3b were collected on a Bruker SMART APEX diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 20 °C. All calculations were performed using SHELXL-2013.83 The structures were solved by direct methods and subsequently refined on F2 by fullmatrix least-squares methods. SADABS absorption corrections were applied to the data. All non-hydrogen atoms of 2b and 3b were refined anisotropically.84 Hydrogen atoms of 2b and 3b, except atom H2 in 2b found by a differential Fourier map, were treated using a riding model. Molecule structures were generated using the ORTEP III program.85 Typical procedure for the polymerization reaction To a solution of rac-lactide (216 mg, 1.50 mmol) in toluene (1.0 mL), a solution of amido aluminum complex (0.015 mmol) in toluene (0.5 mL) was added. The total volume was 1.5 mL. The flask was then immersed into an oil bath of 65 °C for polymerization. The polymerization was quenched by the addition of wet petroleum ether. After removal of the volatiles, the residue was subjected to 1H NMR analysis. Monomer conversion was determined by observing the methine resonance integration of monomer vs. polymer in the 1H NMR (CDCl3, 400 MHz) spectrum. The purification of the polymer in each case was managed by dissolving the crude samples in CH2Cl2 and precipitating the polymer solution with methanol. The obtained polymers were further dried in a vacuum oven at 60 °C for 36 h. The polymer samples were, in selected cases, analyzed by GPC.

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Acknowledgements This work was subsidized by the National Natural Science Foundation of China (NNSFC, 20604009, 21074032), the Program for New Century Excellent Talents in University (for H. Ma, NCET-06-0413) and the Fundamental Research Funds for the Central Universities (WD1113011, WK1214048). All financial support is gratefully acknowledged. H. Ma also thanks the very kind donation of a Braun glove-box by AvH foundation.

Notes and references 1 J. C. Wu, T. L. Yu, C. T. Chen and C.-C. Lin, Coord. Chem. Rev., 2006, 250, 602–626. 2 L. C. Palmer, C. J. Newcomb, S. R. Kaltz, E. D. Spoerke and S. I. Stupp, Chem. Rev., 2008, 108, 4754–4783. 3 L. Little, K. E. Healy and D. Schaffer, Chem. Rev., 2008, 108, 1787–1796. 4 Y.-L. Chung, J. V. Olsson, R. J. Li, C. W. Frank, R. M. Waymouth, S. L. Billington and E. S. Sattely, Chem. Eng., 2013, 1, 1231–1238. 5 S. Myungeun, J. Christopher and A. Marc, ACS Macro Lett., 2013, 2, 617–620. 6 M. J. Stanford and A. P. Dove, Chem. Soc. Rev., 2010, 39, 486–494. 7 C. K. Williams and M. A. Hillmyer, Polym. Rev., 2008, 48, 1–10. 8 S. Mecking, Angew. Chem., Int. Ed., 2004, 43, 1078–1085. 9 O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou, Chem. Rev., 2004, 104, 6147–6176. 10 H.-L. Chen, S. Dutta, P.-Y. Huang and C.-C. Lin, Organometallics, 2012, 31, 2016–2025. 11 D. J. Darensbourg, O. Karroonnirun and S. J. Wilson, Inorg. Chem., 2011, 50, 6775–6787. 12 I. Meulen, E. Gubbels, S. Huijser, R. Sablong, C. E. Koning, A. Heise and R. Duchateau, Macromolecules, 2011, 44, 4301–4305. 13 D. J. Darensbourg and O. Karroonnirun, Organometallics, 2010, 29, 5627–5634. 14 M. Normand, T. Roisnel, J.-F. Carpentier and E. Kirillov, Chem. Commun., 2013, 49, 11692–11694. 15 H.-L. Han, Y. Liu, J.-Y. Liu, K. Nomura and Y.-S. Li, Dalton Trans., 2013, 42, 12346–12353. 16 X.-F. Yu and Z.-X. Wang, Dalton Trans., 2013, 42, 3860– 3868. 17 B. Gao, R. Duan, X. Pang, X. Li, Z. Qu, Z. Tang, X. Zhuang and X. Chen, Organometallics, 2013, 32, 5435–5444. 18 M. Normand, V. Dorcet, E. Kirillov and J.-F. Carpentier, Organometallics, 2013, 32, 1694–1709. 19 M. P. F. Pepels, M. Bouyahyi, A. Heise and R. Duchateau, Macromolecules, 2013, 46, 4324–4334. 20 N. Ikpo, S. M. Barbon, M. W. Drover, L. N. Dawe and F. M. Kerton, Organometallics, 2012, 31, 8145–8158.

This journal is © The Royal Society of Chemistry 2014

Paper

21 K. Nie, W. Gu, Y. Yao, Y. Zhang and Q. Shen, Organometallics, 2013, 32, 2608–2617. 22 Y. Wang and H. Ma, Chem. Commun., 2012, 48, 6729–6731. 23 M. H. Chisholm, J. C. Gallucci, K. T. Quisenberry and Z. P. Zhou, Inorg. Chem., 2008, 47, 2613–2624. 24 N. Nomura, R. Ishii, Y. Yamamoto and T. Kondo, Chem. Eur.J., 2007, 13, 4433–4451. 25 R. Ishii, N. Nomura and T. Kondo, Polym. J., 2004, 36, 261– 264. 26 T. M. Ovitt and G. W. Coates, J. Am. Chem. Soc., 2002, 124, 1316–1326. 27 Z. Zhong, P. J. Dijkstra and J. Feijen, J. Am. Chem. Soc., 2003, 125, 11291–11298. 28 S. L. Hancock, M. F. Mahon and M. D. Jones, Dalton Trans., 2013, 42, 9279–9285. 29 Z. Liu, W. Gao, J. Zhang, D. Cui, Q. Wu and Y. Mu, Organometallics, 2010, 29, 5783–5790. 30 F. Hild, N. Neehaul, F. Bier, M. Wirsum, C. Gourlaouen and S. Dagorne, Organometallics, 2013, 32, 587–598. 31 A. Gao, Y. Mu, J. Zhang and W. Yao, Eur. J. Inorg. Chem., 2009, 3613–3621. 32 F. Hild;, L. Brelot and S. Dagorne, Organometallics, 2011, 30, 5457–5462. 33 J. A. Castro-Osma, C. Alonso-Moreno, J. C. García-Martinez, J. Fernández-Baeza, L. F. Sánchez-Barba, A. Lara-Sánchez and A. Otero, Macromolecules, 2013, 46, 6388–6394. 34 J. S. Klitzke, T. Roisnel, E. Kirillov, O. L. Casagrande Jr. and J.-F. Carpentier, Organometallics, 2014, 33, 309–321. 35 K. Bakthavachalam and N. D. Reddy, Organometallics, 2013, 32, 3174–3184. 36 W. Alkarekshi, A. Armitage, O. Boyron, C. J. Davies, M. Govere, A. Gregory, K. Singh and G. A. Solan, Organometallics, 2013, 32, 249–259. 37 G. Li, M. Lamberti, D. Pappalardo and C. Pellecchia, Macromolecules, 2012, 45, 8614–8620. 38 M. Lamberti, I. D’Auria, M. Mazzeo, S. Milione, V. Bertolasi and D. Pappalardo, Organometallics, 2012, 31, 5551–5560. 39 C. Bakewell, R. H. Platel, S. K. Cary, S. M. Hubbard, J. M. Roaf, A. C. Levine, A. J. P. White, N. J. Long, M. Haaf and C. K. Williams, Organometallics, 2012, 31, 4729–4736. 40 M. Bouyahyi, T. Roisnel and J.-F. Carpentier, Organometallics, 2012, 31, 1458–1466. 41 W. Zhang, Y. Wang, J. Cao, L. Wang, Y. Pan, C. Redshaw and W.-H. Sun, Organometallics, 2011, 30, 6253–6261. 42 W.-A. Ma and Z.-X. Wang, Organometallics, 2011, 30, 4364– 4373. 43 S. Tabthong, T. Nanok, P. Kongsaeree, S. Prabpai and P. Hormnirun, Dalton Trans., 2014, 43, 1348–1359. 44 S. Pracha, S. Praban, A. Niewpung, G. Kotpisan, P. Kongsaeree, S. Saithong, T. Khamnaen, P. Phiriyawirut, S. Charoenchaidet and K. Phomphrai, Dalton Trans., 2013, 42, 15191–15198. 45 N. Yang, L. Xin, W. Gao, J. Zhang, X. Luo, X. Liu and Y. Mu, Dalton Trans., 2012, 41, 11454–11463. 46 N. Iwasa, S. Katao, J. Liu, M. Fujiki, Y. Furukawa and K. Nomura, Organometallics, 2009, 28, 2179–2187.

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View Article Online

Published on 12 March 2014. Downloaded by Queens University - Kingston on 26/08/2014 14:15:08.

Paper

47 N. Iwasa, M. Fujiki and K. Nomura, J. Mol. Catal. A: Chem., 2008, 292, 67–75. 48 J. Liu, N. Iwasa and K. Nomura, Dalton Trans., 2008, 3978– 3988. 49 N. Iwasa, J. Liu and K. Nomura, Catal. Commun., 2008, 9, 1148–1152. 50 P. Pappalardo, L. Annunziata and C. Pellecchia, Macromolecules, 2009, 42, 6056–6062. 51 P. Hormnirun, E. L. Marshall, V. C. Gibson, A. J. P. White and D. J. Williams, J. Am. Chem. Soc., 2004, 126, 2688–2689. 52 X. Pang, H. Du, X. Chen, X. Wang and X. Jing, Chem. - Eur. J., 2008, 14, 3126–3136. 53 A. D. Schwarz, Z. Chu and P. Mountford, Organometallics, 2010, 29, 1246–1260. 54 S. Gong and H. Ma, Dalton Trans., 2008, 3345–3357. 55 L. Postigo, M. C. Maestre, M. E. G. Mosquera, T. Cuenca and G. Jiménez, Organometallics, 2013, 32, 2618–2624. 56 C. H. Huang, F. C. Wang, B. T. Ko, T. L. Yu and C.-C. Lin, Macromolecules, 2001, 34, 356–361. 57 P. Dubois, P. Degee, R. Jerome and P. Teyssie, Macromolecules, 1993, 26, 2730–2735. 58 C. J. Jaffredo, J.-F. Carpentier and S. M. Guillaume, Macromolecules, 2013, 46, 6765–6776. 59 P. Brignou, J.-F. Carpentier and S. M. Guillaume, Macromolecules, 2011, 44, 5127–5135. 60 B. M. Chamberlain, M. Cheng, D. R. Moore, T. M. Ovitt, E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 2001, 123, 3229–3238. 61 D. Li, Y. Peng, C. Geng, K. Liu and D. Kong, Dalton Trans., 2013, 42, 11295–11303. 62 B. Lian, C. M. Thomas, O. L. Casagrande, C. W. Lehmann, T. Roisnel and J. F. Carpentier, Inorg. Chem., 2007, 46, 328– 340. 63 I. Barakat, P. Dubois, R. Jerome and P. Teyssie, J. Polym. Sci., Part A: Polym. Chem., 1993, 31, 505–514. 64 R. Hoogenboom, D. Wouters and U. S. Schubert, Macromolecules, 2003, 36, 4743–4749. 65 A. Otero, A. Lara-Sánchez, J. Fernández-Baeza, C. Alonso-Moreno, J. A. Castro-Osma, I. Márquez-Segovia,

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L. F. Sánchez-Barba, A. M. Rodríguez and J. C. GarciaMartinez, Organometallics, 2011, 30, 1507–1522. J. Lewiński, P. Horeglad, K. Wójcik and I. Justyniak, Organometallics, 2005, 24, 4588–4593. W. Yi and H. Ma, Inorg. Chem., 2013, 52, 11821–11835. L. Wang and H. Ma, Macromolecules, 2010, 43, 6535–6537. J. Ejfler, S. Szafert, K. Mierzwicki, L. B. Jerzykiewicz and P. Sobota, Dalton Trans., 2008, 6556–6562. V. Poirier, T. Roisnel, J. F. Carpentier and Y. Sarazin, Dalton Trans., 2009, 9820–9827. H. Y. Chen, B. H. Huang and C. C. Lin, Macromolecules, 2005, 38, 5400–5405. L. F. S. Barba, C. A. Moreno, A. Garces, M. Fajardo, J. F. Baeza, A. Otero, A. L. Sanchez, A. M. Rodriguez and I. L. Solera, Dalton Trans., 2009, 8054–8062. C. E. Willans, M. A. Sinenkov, G. K. Fukin, K. Sheridan, J. M. Lynam, A. A. Trifonov and F. M. Kerton, Dalton Trans., 2008, 3592–3598. H. Ma and J. Okuda, Macromolecules, 2005, 38, 2665–2673. J. Wang, T. Cai, Y. Yao, Y. Zhang and Q. Shen, Dalton Trans., 2007, 5275–5281. W. Gao and D. Cui, Organometallics, 2008, 27, 5889–5893. E. Grunova, E. Kirillov, T. Roisnel and J.-F. Carpentier, Organometallics, 2008, 27, 5691–5698. W. Yao, Y. Mu, A. Gao, W. Gao and L. Ye, Dalton Trans., 2008, 3199–3206. M. H. Thibault and F. G. Fontaine, Dalton Trans., 2010, 39, 5688–5697. D. Chakraborty and E. Y.-X. Chen, Organometallics, 2002, 21, 1438–1442. W. Zhang, Y. Wang, L. Wang, C. Redshaw and W.-H. Sun, J. Organomet. Chem., 2014, 750, 65–73. R. L. Bentley and H. Suschitzky, J. Chem. Soc., Perkin Trans. 1, 1976, 1725–1734. G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 2008, 64, 112–122. SADABS, Bruker Nonius Area Detector Scaling and Absorption Correction-V2.05, Bruker AXS Inc., Madison, WI, 1996. C. K. Johnson, ORTEP-III: A FORTRAN Thermal Ellipsoid Plot Program for Crystal Structure Illustrations, Report ORNL5138, Oak Ridge National Laboratory, Oak Ridge, TN, USA, 1976.

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Aluminum complexes with bidentate amido ligands: synthesis, structure and performance on ligand-initiated ring-opening polymerization of rac-lactide.

A series of mononuclear aluminum dimethyl complexes bearing bidentate N-[2-(1-piperidinyl)benzyl]anilino or N-(2-morpholinobenzyl)anilino ligands were...
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