Stereoselectivity Switch between Zinc and Magnesium Initiators in the

Nov 3, 2014 - The reactions of enantiopure zinc complexes 1 and 2 with (S)-methyl lactate or (R)-tert-butyl lactate resulted in the target lactate com...
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Stereoselectivity Switch between Zinc and Magnesium Initiators in the Polymerization of rac-Lactide: Different Coordination Chemistry, Different Stereocontrol Mechanisms Haobing Wang, Yang Yang, and Haiyan Ma* Shanghai Key Laboratory of Functional Materials Chemistry and Laboratory of Organometallic Chemistry, East China University of Science and Technology, Shanghai, P. R. China S Supporting Information *

ABSTRACT: The preparation and characterization of several zinc and magnesium complexes bearing chiral aminophenolate ligands, [(S)- or (R)-L]ZnN(SiMe3)2 (1, 2), [(S)- or (R)L]ZnOtBu (3, 4) and [(S)- or (R)-L]MgN(SiMe3)2 (5, 6) ((S)- or (R)-L = (S)- or (R)-2-{N-benzyl-N-[(1-nbutyl-2pyrrolidinyl)methyl]}aminomethyl-4-methyl-6-tritylphenolate), have been reported. While the X-ray diffraction studies as well as the NMR spectroscopic data revealed that zinc and magnesium silylamido complexes possess similar structures both in the solid state and in solution, their catalytic performance toward rac-LA polymerization exhibited significant difference. The replacement of zinc center by magnesium realized an interesting stereoselectivity switch from isoselective (Pm = 0.80) to heteroselective (Pr = 0.81). The reactions of enantiopure zinc complexes 1 and 2 with (S)-methyl lactate or (R)-tert-butyl lactate resulted in the target lactate complexes 7−9 as a mixture of two diastereomers, but in different ratios. Two typical zinc lactate isomers 8α and 9β/9β′ were isolated and further characterized by the X-ray diffraction method to have pentacoordinated geometry where the coordination of lactate carbonyl group displays some hemilabile nature. The molecular structures of 8α and 9β/9β′ indicated that the specific configuration of lactate moiety in combination with the aminophenolate ligand of given chirality would favor the formation of one of the two diastereomers in each case. Based on the structures of zinc lactate model complexes and the active propagating species, as well as the preferential enchainment of one specific lactide monomer in the polymerization, a cooperation of enantiomorphic-site control and chain-end control mechanisms involving three types of active species was proposed for the formation of stereoblock PLAs from rac-LA by zinc initiators. Results of apparent rate constants for rac-, D-, and L-LA polymerizations by magnesium initiators are consistent with a chain-end control mechanism. The 13C NMR spectra of magnesium lactate complexes proved that the bonding of carbonyl group to magnesium center is stronger than that in zinc active species. The coordination number of magnesium species can expand easily to six in the presence of coordinative molecules, suggesting that the heteroselective magnesium active species have a hexacoordinated core structure. The difference of coordination geometry between zinc and magnesium active species should be responsible for the stereoselectivity switch in the polymerization of rac-LA.

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transition metals,17 are the most widely studied, and some of them have displayed excellent stereocontrol in the catalytic ROP of rac-LA.2,7a−c,8,9,18,20−22 Especially, discrete aluminum complexes stabilized by a limited number of salen- or salan-type ligands could afford isotactically tapered or stereoblock PLA depending on the polymerization mechanisms (enatiomorphicsite control or chain-end control).8,9 These isotactic polymers obtained from low-cost rac-LA exhibit dramatically improved melting point (190−210 °C) in comparison with the homochiral polymers (170−180 °C) and become highly desirable.7b,8c,p,9e,f To search for more efficient and isoselective organometallic catalysts in addition to the generally less active aluminum initiators, many well-characterized complexes of

INTRODUCTION As a biodegradable aliphatic polyester, polylactide (PLA) which combines both biocompatible nature and sustainable property has been extensively explored in recent years.1 The environmental advantages of this material impel its applications ranging from sustainable drug releasing carriers, medical facilities, to agricultural and commodity packaging materials.1d Since the stereoregularity of polylactide is a crucial factor determining the physical properties of PLA such as crystallinity and the rate of hydrolysis and therefore the potential utility of the material, the catalytically stereocontrolled ring-opening polymerization (ROP) of rac-lactide (rac-LA) which enables the production of PLAs with versatile stereoregularities has gained considerable importance.2−4 By far, metal-based catalysts,2 which combine an appropriate organic ligand with a central metal atom such as alkali metals,5 alkaline earth metals,6 Zn,7 Al,8,9 In,10,11 Ga,12 Ge,13 Sn,14 rare earth metals,15 as well as Ti/Zr16 and other © XXXX American Chemical Society

Received: September 12, 2014 Revised: October 22, 2014

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Scheme 1. Some Stereoselectively Switchable Catalysts in the ROP of rac-Lactide

Scheme 2. Synthesis of Zinc and Magnesium Complexes 1−6

various metals have been synthesized and extensively investigated for the polymerization of rac-LA in the past two decades. However, only a few of them can afford isotactic enriched PLAs (Pm = 0.77−0.91) from rac-LA,7a−c,10b,18a,b,21 which is in contrast to the relatively large number of heteroselective catalysts obtained by varying the central metal and the ancillary ligand architecture.2a,b,i,6b,15q−s,22,23 Therefore, designing suitable organic ligands for a specific metal center to achieve high isoselectivity and activity in the polymerization of rac-LA remains a significant focus in this field.

From the literature reports, we have noticed some interesting stereoselectivity switches in the polymerization of rac-LA obtained by fine-tuning the ancillary ligand structure as well as the metal center of the adopted initiator. For instance, Gibson and co-workers19 reported that by varying the substituents on the phenolate rings aluminum complexes (I) with salan ligands could initiate the polymerization of rac-LA either isoselectively (Pm = 0.79) or heteroselectively (Pr = 0.96). Zirconium alkoxide complexes (II) reported by Kol’s group could produce PLAs with stereotacticity gradually changing from Pr = 0.73 to Pm = 0.67 via a stepwise adjustment B

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of the fluxionality of the ONSO ligand framework.20 Williams and co-workers18a reported an unexpected stereocontrol on the ROP of rac-LA (Pr = 0.90 to Pm = 0.84) by changing the imine linker of the phosphasalen yttrium initiators (III). Very recently, the same group21 further found that a stereoselectivity switch could also be achieved by varying the metal center: the same pentadentate phosphasalen ligand in complexation with lutetium and lanthanide centers forms metal complexes (IV) possessing very closely related coordination geometries but displaying opposite stereoselectivities (Pm = 0.89 to Pr = 0.72). The rationales for these observations are not yet assured, with some hypotheses that the differences in the geometry/ coordination environment around the metal center might be responsible.2i A similar selectivity switch is not yet accessible among complexes of divalent metals. Coates and co-workers22 reported the highly heteroselective polymerization of rac-LA initiated by β-diketiminate zinc complexes (V) via a chain-end control mechanism, whereas their magnesium counterparts produced atactic PLA under the similar conditions. Chisholm proved that β-diketiminate magnesium complexes (VI) could exhibit high heteroselectivity when the polymerization was carried out in a coordinative solvent.23 Based on these facts, the formation of different aggregation structures of zinc and magnesium active species is normally assumed to be the crucial factor that determines the catalytic properties of these systems. Our group has devoted to develop efficient catalysts via the use of biocompatible metals of groups 2 and 12, since these metals hold the most promise for industrial applications owing to their low cost, low toxicity, and remarkably high activities of the corresponding complexes.6b−d,l,7c,f,i Recently, we communicated the enantiopure zinc complex 1 supported by a chiral aminophenolate ligand derived from (S)-2-{N-benzyl-N[(1-nbutyl-2-pyrrolidinyl)methyl]}aminomethyl-4-methyl-6-tritylphenol ((S)-LH), which belongs to one of the few isoselective catalysts for rac-LA polymerization in addition to aluminum complexes and gives isotactic stereoblock PLA.7c In this work, the aminophenol proligand (R)-LH with a reversed configuration was synthesized, and both proligands were used to synthesize the enantiopure zinc silylamido and tert-butoxide complexes as well as magnesium analogues (2−6, Scheme 2), with the aim of understanding the effect of supporting ligand on the stereoselectivity of these complexes for the ROP of racLA. Surprisingly, PLAs with inverse stereoregularities were obtained by zinc (Pm = 0.80) and magnesium (Pr = 0.81) complexes. To the best of our knowledge, this is a very rare example that a dramatic stereoselectivity switch has been observed for zinc/magnesium initiated lactide polymerization system. Magnesium (rion = 0.72 Å) has a very close ionic radius to that of zinc (rion = 0.74 Å),24 so it is conceived that the mechanism of stereoselectivity switch observed for our zinc/ magnesium systems might be different from that suggested by William’s group for their rare earth complexes, where the difference in ionic radius between lutetium (rion = 0.86 Å) and lanthanum (rion = 1.03 Å) is significantly larger. Herein we report our initial discoveries and present a general explanation for the stereoselectivity switch by taking some insight into the coordination structure of the propagating active species as well as the kinetic details.

prepared according to our published procedure.7c Zinc silylamido complex [(R)-L]ZnN(SiMe3)2 (2) was readily synthesized via the reaction of proligand (R)-LH with Zn[N(SiMe3)2]2 in a 1:1 molar ratio in toluene and was isolated as colorless crystalline solids in 71% yield. X-ray quality crystals of complex 2 were grown in benzene and structurally determined to possess a distorted tetrahedral geometry at the metal center as illustrated in Figure 1.7c The molecular

Figure 1. Molecular structure of [(R)-L]ZnN(SiMe3)2 (2) (depicted with ellipsoids at 30% probability and H atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Zn(1)−O(1) 1.952(4), Zn(1)−N(1) 2.137(5), Zn(1)−N(2) 2.121(6), Zn(1)−N(3) 1.931(6); O(1)−Zn(1)−N(1) 97.9(2), O(1)−Zn(1)−N(2) 104.2(2), O(1)−Zn(1)−N(3) 119.1(2), N(1)−Zn(1)−N(2) 84.1(2), N(1)−Zn(1)−N(3) 115.9(2), N(2)−Zn(1)−N(3) 127.1(2).

structure of 2 can be brought into a perfect congruence with the mirror image of [(S)-L]ZnN(SiMe3)2 (1). All the corresponding bond lengths and angles of 1 and 2 exhibit very limited differences. The related chiral atoms in these two enantiomers have reversed configurations, and thus the two complexes can be denoted as (S C S N R N R Z n )-1 and (RCRNSNSZn)-2, respectively. The NMR scale reaction of complex 1 with 2-propanol in C6D6 indicated the formation of the target zinc isopropoxide complex (see Supporting Information Figure S27); the poor crystallinity of the complex however hampered the isolation of analytically pure product in a synthetic scale, whereas zinc tertbutoxide complexes 3 and 4 could be obtained by treating an equimolar amount of zinc silylamido complex 1 or 2 with tertbutanol and isolated in high yields. The three-step reactions of proligand (R)- or (S)-LH with stoichiometric amounts of K[N(SiMe3)2], ZnCl2, and KOtBu in the order could also lead to complex 3 or 4 accompanied by the formation of 2 equiv of KCl (Scheme 2). Both routes are highly diastereoselective, affording enantiopure complexes 3 and 4.25 Magnesium silylamido complexes [(S)-L]MgN(SiMe3)2 (5) and [(R)-L]MgN(SiMe3)2 (6) were prepared similarly by adding a stoichiometric amount of the corresponding proligand to a solution of Mg[N(SiMe3)2]2 in n-hexane and were isolated in moderate yields. Both complexes 5 and 6 were obtained as a mixture of two diastereomers but with high diastereoselectivity (isomer a:isomer b = 7:1, Scheme 2). As suggested for the zinc analogues,7c this optimized combination of substituents in the ligand framework also induced one specific configuration of the complex dominantly. X-ray quality crystals were obtained from a saturated benzene solution of complex 5. The molecule

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RESULTS AND DISCUSSION Synthesis and Characterization of Zinc and Magnesium Complexes. The proligands (R)-LH and (S)-LH were C

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Npyrrolidinyl(1) = 2.137(3) Å vs Zn(1)−Nskeleton(1) = 2.145(3) Å in 1). Based on the molecular structure of 5a and our previous results concerning the diastereomers of similar zinc complexes,7c the stereogenic denotation for 5b, 6a, and 6b could be assigned accordingly (Scheme 2). The synthesis of magnesium alkoxide complexes via the reaction of silylamido complexes with tert-butanol or 2propanol was also attempted and proved to be unsuccessful. For instance, the formation of nearly a half equiv of the free ligand was observed in the 1H NMR spectrum of a reaction mixture of complex 5 with even less than 1 equiv of alcohol, indicating an easy dissociation of the ligand moiety from the metal center in the presence of protonic sources. Polymerization of rac-LA Initiated by Zinc and Magnesium Complexes. Complexes 1−6 were examined for the catalytic ring-opening polymerization of rac-LA ([LA]0/ [M]0) = 200−1000) at ambient temperature in different solvents. As displayed in Table 1, zinc complexes 1−4 showed moderate catalytic activities toward the polymerization and required 90−480 min to reach high monomer conversions. The silylamido group is an inferior initiating group, as the corresponding complexes yielded polymers with broad molecular weight distributions and molecular weights higher than the theoretical values (runs 1, 3, 5, and 7). By contrast, the polymerizations initiated by alkoxide initiator systems (1/iPrOH, 2/iPrOH, 3, 4; runs 2, 4, 6, 8, and 9−13) were well-controlled, yielding isotactic-dyad-enriched PLAs (Pm = ∼0.80) with narrow molecular weight distributions (Mw/Mn = 1.03−1.12) and Mn values close to those predicted from the monomer/initiator ratio. The linear relationship of molecular weight vs conversion plot in conjunction with narrow molecular weight distributions further suggests the living nature of the polymerizations initiated by zinc alkoxides (see Supporting Information Figures S12 and S13). There is basically no difference between the catalytic behaviors of zinc tert-butoxide complexes (3 and 4) and the in situ formed zinc isopropoxides (1/iPrOH, 2/iPrOH). Moreover, with the initiation of zinc

possesses exactly the same configuration as that of zinc analogue 1 and can be denoted as (SCSNRNRMg)-5a. As shown in Figure 2, in 5a one oxygen and two nitrogen donors

Figure 2. Molecular structure of [(S)-L]MgN(SiMe3)2 (5a) (depicted with ellipsoids at 30% probability and H atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Mg(1)−O(1) 1.9301(16), Mg(1)−N(1) 2.1725(19), Mg(1)−N(2) 2.1935(19), Mg(1)−N(3) 2.0182(19), O(1)−Mg(1)−N(1) 95.22(7), O(1)− Mg(1)−N(2) 106.16(7), O(1)−Mg(1)−N(3) 119.85(8), N(1)− Mg(1)−N(2) 82.22(7), N(1)−Mg(1)−N(3) 115.72(8), N(2)− Mg(1)−N(3) 127.10(8).

of the ligand along with the silylamido group coordinate to the magnesium atom, constructing a distorted tetrahedral geometry at the metal center. The difference between N(3)−Mg(1)− N(2) (127.10(8)°) and N(1)−Mg(1)−N(2) (82.22(7)°) illustrates a severe distortion from an ideal tetrahedral topology. The Mg(1)−N(3)amido bond is significantly longer than Zn− N(3) in the zinc counterparts. It is interesting to note that Mg(1)−N(2)pyrrolidinyl (2.1935(19) Å) bond is slightly longer than Mg(1)−N(1)skeleton (2.1725(19) Å), which is opposite to that observed for the zinc counterparts (for instance, Zn(1)−

Table 1. ROPs of rac-LA Initiated by Zinc and Magnesium Complexes 1−6a run

Cat.

feed ratio

solvent

temp (°C)

time (min)

convb (%)

Mcc (×104)

Mnd (×104)

PDId

Pm/Pre

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

1 1 1 1 2 2 2 2 3 3 4 3 + 4 (1:1)f 3 + 4 (1:1)f 5 5 5 5 6

200:1:0 200:1:1 200:1:0 200:1:1 200:1:0 200:1:1 200:1:0 200:1:1 200:1:0 1000:1:0 200:1:0 200:1:0 200:1:0 200:1:0 200:1:0 200:1:0 200:1:0 200:1:0

Tol Tol THF THF Tol Tol THF THF Tol Tol Tol Tol Tol Tol Tol THF pyridine Tol

25 25 25 25 25 25 25 25 25 25 25 25 −38 25 −38 25 25 25

180 90 300 180 240 90 480 180 90 360 90 90 3 days 25 24 h 25 120 25

84 99 71 86 93 97 89 91 94 80 96 95 79 96 85 90 78 98

2.42 2.85 2.05 2.48 2.68 2.78 2.56 2.63 2.71 11.5 2.71 2.74 2.28 2.76 2.45 2.59 2.25 2.82

2.95 3.00 3.80 3.34 3.34 2.98 3.49 2.85 2.81 7.51 3.67 3.06 3.71 4.02 2.91 4.53 4.74 4.45

1.66 1.12 1.54 1.09 1.40 1.04 1.63 1.04 1.06 1.10 1.05 1.06 1.13 1.72 1.57 1.69 1.70 1.58

0.80/0.20 0.80/0.20 0.80/0.20 0.80/0.20 0.80/0.20 0.79/0.21 0.80/0.20 0.80/0.20 0.80/0.20 0.80/0.20 0.79/0.21 0.81/0.19 0.84/0.16 0.22/0.78 0.19/0.81 0.30/0.70 0.41/0.59 0.22/0.78

[rac-LA]0 = 1.0 mol L−1, feed ratio = [LA]0/[M]0/[iPrOH]0. bDetermined by 1H NMR spectroscopy. cMn,calcd = [LA]0/[M]0 × conv % × 144.13 g mol−1. dDetermined by GPC. eDetermined by analysis of all of the tetrad signals in the methine region of the homonuclear-decoupled 1H NMR spectrum.13 fPrepared by mixing equal molars of the homochiral complexes. a

D

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Figure 3. Plots of ln([LA]0/[LA]t) vs time for the ROP of D-LA, L-LA, and rac-LA catalyzed by zinc complexes (NMR tube reactions for 1 and 2 in C6D6, [LA]0/[Zn]0 = 50:1, 20 °C; polymerization scale reactions for 3 and 4 in toluene, [LA]0/[Zn]0 = 200:1, 25 °C).

Mechanism of rac-LA Polymerization with Isoselective Zinc Complexes. Microstructural analysis of PLAs formed from rac-LA revealed that isotactic PLAs were yielded by zinc initiators. In all cases, the relatively small sis signal in comparison with those of sii, iis, and isi in the homonuclear decoupled 1H NMR spectra (see Supporting Information, Figures S14−17) excludes the dominant presence of single insertion stereoerrors of −RRRRSSRRRR−/−SSSSRRSSSS− sequences normally suggested to arise from an enantiomorphicsite control by a chiral initiator (sis:sii:iis:isi = 1:1:1:2). The close intensity ratio of sii:iis:isi to 1:1:1 as well as basically ignorable sis signal indicates that the polymer main chains are essentially stereoblocks (e.g., −RRRRRRSSSS−) (see Supporting Information Figure S25). It has been reported that, in addition to those formed via chain-end controlled polymerizations of rac-LA with achiral initiators,8k,o,p by using a racemic version of the highly isoselective aluminum initiators,9f−h stereoblock PLAs could also be formed through an enantiomorphic-site control mechanism in combination with polymeryl exchange processes. Lamberti9a and Mehrkhodavandi’s groups10b further showed that both of the racemic versions of their aluminum and indium catalysts resulted in a decrease of isotacticity (Pm = 0.80−0.70 for aluminum catalyst; Pm = 0.77− 0.74 for indium catalyst), attributable to the involvement of polymeryl exchange events. Therefore, a racemic version of zinc tert-butoxide catalysts, prepared by mixing equimolar amounts of the homochiral complexes 3 and 4, was employed in rac-LA polymerization (Table 1, runs 12 and 13). Unexpectedly,

complexes 1 and 2, the polymerization runs carried in THF proved to be slower than those performed in toluene (runs 1, 2, 5, 6 vs 3, 4, 7, 8). This phenomenon is in line with the results reported by us and Carpentier’s group utilizing zinc complexes with claw-type aminophenolate ligands7f or bulky multidentate amino−ether phenolate ligands.7g We tentatively attribute this fact to a competitive coordination of the solvent molecular to the zinc center. As shown in Table 1, the nature of solvent however showed no influence on the tacticity of the resultant polymer, isotactic PLAs (Pm = 0.80) were yielded in each case (runs 1−8). Although magnesium complexes 5 and 6 share very similar structures with the isoselective zinc counterparts, to our great surprise, heterotactic-dyad enriched PLAs (Pr = 0.78−0.81) were obtained from rac-LA (runs 14−18). In comparison to the zinc initiators, complexes 5 and 6 are highly active for rac-LA polymerization; nearly full conversion of monomer could be reached within 25 min at 25 °C in toluene (runs 14 and 18). However, gel permeation chromatography analysis (GPC, versus polystyrene standards) revealed that polymers with Mn values deviated from the theoretical data and broad molecular weight distributions (Mw/Mn = 1.57−1.72) were obtained (runs 14−18). Furthermore, the coordinating ability of the reaction solvent also exhibited reasonable influence on the heteroselectivity of complex 5; solvent of stronger coordination ability led to the PLA with lower heterotacticity (Tol > THF > Py; runs 14, 16, and 17). E

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two enantiomeric monomers is consumed (kinetic resolution manner),9c,d,i whereas if the selective polymerization is controlled by a chain-end control mechanism, the consumption of L- and D-LA should be of identical rates.8k In this work, the concurrence of a preferential enchainment of one enantiomeric monomer and the first-order dependence on monomer concentration in the entire conversion range of rac-LA polymerization is inconsistent with either a pure chain-end control mechanism or a pure enantiomorphic-site control mechanism. By integrating all these experimental features, it is conceivable that the isoselective polymerization of rac-LA initiated by complexes 1−4 is more likely a cooperation of both an enantiomorphic-site control mechanism (SCM) and a chainend control mechanism (CEM) and that the latter seems to be even dominant. A similar hypothesis has also been suggested to rac-LA polymerization initiated by aluminum complexes with chiral salalen ligands.9a Then it raises questions: How do these chiral zinc initiators contribute to the formation of stereoblock PLAs from rac-LA via probably a dominant chain-end control mechanism? Since the presence of multiple stereogenic centers in the ligand framework of these zinc complexes seems to have less influence on the polymerization, is the lack of crucial enantiomorphic site control due to a partial dissociation of the two neutral nitrogen donors from the metal center? Structures of Zinc Model Complexes and Active Propagating Species. To have a further understanding on the polymerization mechanism and the structure of active species, we examined a series of NMR tube reactions using zinc complexes 1−4. Complexes 1 and 2 were treated respectively with (S)-methyl lactate in C6D6 to generate in situ the model complexes [(S)-L]Zn[(S)−OCH(Me)CO2Me] (7) and [(R)L]Zn[(S)−OCH(Me)CO2Me] (8) (Scheme 3). To our surprise, the two 1H NMR spectra displayed significant distinction. As depicted in Figure 4, the reaction of 1 and (S)-methyl lactate led to 7 as a mixture of two diastereomers in a 2:1 molar ratio, while that of complex 2 and (S)-methyl lactate afforded one diastereomer dominantly (for convenience, this dominant diastereomer is denoted as 8α, the diastereomer in trace amount is then 8β, and the ratio of two diastereomers is 10:1).26 Crystals of 8α suitable for X-ray diffraction study were obtained successfully from C6D6 in the NMR tube at room temperature (Figure 5). The coordination geometry around the zinc center may be viewed either as a distorted tetrahedron or a trigonal bipyramid. On one hand, the bond lengths and bond angles between the metal center and NNO ligand match closely with those in the tetracoordinated complex 2 except that the angle of O(1)−Zn(1)−N(2) expands from 104.2(2)° to 113.76(17)°, likely due to the replacement of the larger silylamido group by a smaller alkoxide group. The lactate ligand is located relatively away from the aminophenolate ligand, but all the corresponding angles (O(2)−Zn(1)−O(1), O(2)− Zn(1)−N(1), O(2)−Zn(1)−N(2)) are still in a reasonable range of a distorted tetrahedron. Based on such a geometry, the original configurations of all stereogenic centers in complex 2 are reserved in 8α, which could be denoted as RCRNSNSZn-8α. On the other hand, the remarkably long distance of 2.734(5) Å between zinc atom and the carbonyl oxygen O(3) of the lactate ligand in 8α does still lie within the sum of van der Waals radii of zinc and oxygen (2.10 + 1.55 = 3.65 Å),27 indicative of a weak interaction existing in the solid state. Gibson and coworkers witnessed a similar weak metal−carbonyl interaction in

similar catalytic activity as well as nearly the same isoselectivity for rac-LA polymerization to those of the enantiopure complexes 3 and 4 was obtained (runs 9, 10, and 12). These results implied that the isoselective mechanism of zinc complexes 1−4 is quite different from a dominant site control mechanism; meanwhile, the polymeryl exchange events are likely not involved in our cases. It became attractive for us to probe the stereocontrol mechanism of these enantiopure zinc complexes in the polymerization of rac-LA, in particular, the mechanism accounting for the formation of stereoblock PLAs. Kinetic Studies. Previously we communicated that zinc silylamido complex 1 exerts a nearly 4-fold rate preference for D-LA polymerization relative to that of L-LA in C6D6 at 20 °C (kapp(D‑LA)/kapp(L‑LA) ≈ 4).7c In this work, we further investigated the kinetics of rac-, D-, and L-LA polymerizations catalyzed by zinc complexes [(R)-L]ZnN(SiMe3)2 (2), [(S)-L]ZnOtBu (3), and [(R)-L]ZnOtBu (4). As the enantiomer of complex 1, complex 2 was found to exhibit an opposite preference for Dand L-LA polymerizations under the same conditions (kapp(D‑LA)/kapp(L‑LA) ≈ 1/4) (Figure 3a,b; Table 2, Cat. 1 and Table 2. Apparent Rate Constants for the ROPs of D-, L-, and rac-LA Initiated by Zinc and Magnesium Complexesa Cat.

kapp(D‑LA) (min−1)

kapp(L‑LA) (min−1)

kapp(rac‑LA) (min−1)

1 2 3 4 5 6

0.039 0.010 0.111 0.049 0.075 0.027

0.011 0.040 0.043 0.110 0.065 0.029

0.0045 0.0047 0.023 0.022 0.163 0.070

a

Polymerizations catalyzed by 1, 2, and 6 were performed in C6D6 at 20 °C. Monomer and catalyst concentrations were held constant at 0.50 and 0.01 M, respectively. Polymerizations catalyzed by 3−5 were carried out with 200 equiv of LA in toluene at 25 °C and followed to 90% conversion by 1H NMR spectroscopy.

2). Similar trends were also found for the polymerizations of Dand L-LA catalyzed by zinc tert-butoxide complexes 3 and 4 (Figure 3c,d; Table 2, Cat. 3 and 4), except that all the apparent rate constants increased significantly and the preference ratios for D- and L-LA polymerizations slightly decreased (for 3, kapp(D‑LA)/kapp(L‑LA) = 2.58; for 4, kapp(D‑LA)/kapp(L‑LA) = 1/2.24), since different polymerization conditions were adopted (in toluene, at 25 °C). Furthermore, as reported previously,7c during the polymerization of rac-LA with 1 as initiator, the tacticities of the resultant polymers changed to a certain extent with monomer conversion (see Supporting Information Figure S26), with the lowest isotacticity obtained at around 50−60% conversion. These results suggest that an enantiomorphic-site control is likely involved in these systems. It is worthy of noting that, as indicated in Figure 3a,b and Table 2, the apparent rate constants of rac-LA polymerization catalyzed by complexes 1 and 2 are similar, but are significantly lower than those of D- and L-LA polymerizations. A similar situation is also found for the polymerizations initiated by complexes 3 and 4 (Figure 3c,d; Table 2, Cat. 3 and 4). Besides, it is observed that in all cases the polymerization of racLA is first order in monomer concentration even up to 95% monomer conversion. According to literature reports,8k,9c,d,i if the selective polymerization is controlled by an enantiomorphic-site control mechanism, the rate of rac-LA polymerization should decrease apparently when the preferential one of the F

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Scheme 3. Synthesis of Zinc Lactate Complexes 7−9

a β-diketiminate tin complex {BDI}Sn(OCH(Me)CO2iPr) ({BDI} = H2C{C(Me)N-2,6-(iPr)2C6H3}2) realized by forming a loose five-membered chelate (Sn−Ocarbonyl = 2.755(6) Å).14c By taking the mentioned weak interaction into consideration, the coordination geometry in 8α should be better described as a trigonal bipyramid, where O(3)carbonyl, Zn, and N(2)pyrrolidinyl form the axis and O(1)phenolate, O(2)alkoxide, and N(1)skeleton occupy the equatorial positions. Since no ideal crystals could be isolated from the mixture of 7, the reaction of complex 2 with 1 equiv of (R)-tert-butyl lactate which is in high purity of 99% was carried out in toluene to generate the corresponding model complex. [(R)-L]Zn[(R)−OCH(CH3)CO2tBu] (9) was obtained as a pair of diastereomers in 56% yield (1:1.25). From a benzene solution of this mixture, colorless single crystals (denoted as 9β) could be isolated. As depicted in Figure 6, a rarely monomeric pentacoordinated zinc complex chelated by the alkoxide and carbonyl oxygen atoms of tert-butyl lactate unit as well as three donors of NNO ligand was obtained. The bond length of Zn(1)−O(3)carbonyl (2.280(3) Å) is similar to that of the tetracoordinated zinc complex {BDI}Zn(OCH(Me)CO2Me) (2.189(2) Å) reported by Coates’s group22 but is significantly shorter than the one observed in 8α. Two largest angles of N(1)−Zn(1)−O(3) (169.12(10)°) and O(2)−Zn(1)−O(1) (126.17(11)°) give a τ value of 0.72, indicating that in the solid state 9β possesses a distorted trigonal bipyramidal geometry at the metal center. Being different from those of 8α with a (S)methyl lactate terminus, in 9β three equatorial sites are occupied by O(1)phenolate, O(2)alkoxide, and N(2)pyrrolidinyl, while O(3)carbonyl of the lactate ligand and the skeleton N(1) bond to the zinc center in the axial sites. The central zinc atom is 0.088 Å above the plane defined by O(1), O(2), and N(2), proving nearly ideal planarity of these four atoms. Again, the

configuration of the chiral aminophenolate ligand moiety in 9β is the same as that in complex 2. In an effort to isolate the other diastereomer of 9, the mixture was recrystallized with toluene to afford colorless prismatic crystals 9β′. Cell parameters of 9β′ are totally different from those of 9β, and more interestingly two independent molecules are observed in the single cell unit. The structure of one of the molecules is nearly identical to that of 9β; very similar bond lengths and angles are observed, represented by a close matched distance of zinc to carbonyl oxygen atom (Zn(2)− O(7) = 2.212(4) Å in 9β′ vs Zn(1)−O(3) = 2.280(3) in 9β). The distance of zinc to carbonyl oxygen in the second molecule of 9β′ is significantly elongated (Zn(1)−O(3) = 2.525(4) Å) (Figure 7), but in general the coordination geometry of this molecule is still similar to that of 9β, with O(3)carbonyl, Zn, and N(1)skeleton forming the axis and O(1)phenolate, O(2)alkoxide, and N(2)pyrrolidinyl occupying the equatorial positions. Both molecules of 9β′ have the same configuration as those of 9β and 2. This is a rare example that a hemilabile chelating interaction of carbonyl group has been observed in one crystal cell unit. To verify whether these two structures are exactly the two diastereomers observed via the 1H NMR spectroscopic method, variable temperature 1H NMR spectra of 9 in C7D8 were detected. However, no obvious fluxional behavior or dynamic exchange process could be observed in the temperature range of −60 to 100 °C (see Supporting Information Figure S31), which excluded this possibility indubitably, since the coordination of carbonyl to central metal is believed to be hemilabile as evidenced by the X-ray diffraction studies of 8α, 9β, and 9β′. The reactions of enantiopure complexes 1 and 2 with (S)methyl lactate or (R)-tert-butyl lactate all result in a mixture of two diastereomers, although in different ratios (1:2 for 7, 10:1 for 8, and 1:1.25 for 9). Unfortunately, we are not able to G

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Figure 5. Molecular structure of 8α (depicted with ellipsoids at 30% probability and H atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Zn(1)−O(1) 1.901(4), Zn(1)−O(2) 1.860(5), Zn(1)−O(3) 2.734(5), Zn(1)−N(1) 2.108(5), Zn(1)−N(2) 2.144(5), O(2)−Zn(1)−O(1) 125.36(18), O(2)−Zn(1)−N(1) 122.1(2), O(1)−Zn(1)−N(1) 99.31(17), O(2)−Zn(1)−N(2) 104.5(2), O(1)−Zn(1)−N(2) 113.76(17), N(1)−Zn(1)−N(2) 84.9(2), O(3)−Zn(1)−N(2) 163.25(2), O(3)−Zn(1)−N(1) 87.29(17), O(3)−Zn(1)−O(1) 82.11(2), O(3)−Zn(1)−O(2) 67.44(2).

Figure 4. 1H NMR spectra (400 MHz, C6D6) of the reactions of (A) [(S)-L]ZnN(SiMe3)2 (1) and (S)-methyl lactate, (B) [(R)-L]ZnN(SiMe3)2 (2) and (S)-methyl lactate, (C) [(R)-L]ZnOtBu (4) and DLA; and (D) [(R)-L]ZnOtBu (4) and L-LA (for clarity, only the middle parts are displayed).

Figure 6. Molecular structure of 9β (depicted with ellipsoids at 30% probability and H atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Zn(1)−O(1) 1.933(2), Zn(1)−O(2) 1.908(2), Zn(1)−O(3) 2.280(3), Zn(1)−N(1) 2.171(3), Zn(1)−N(2) 2.114(3), O(2)−Zn(1)−O(1) 123.26(11), O(2)−Zn(1)−N(2) 126.17(11), O(1)−Zn(1)−N(2) 109.98(11), O(2)−Zn(1)−N(1) 99.36(11), O(1)−Zn(1)−N(1) 95.17(10), N(2)−Zn(1)−N(1) 82.20(11), O(2)−Zn(1)−O(3) 80.13(11), O(1)−Zn(1)−O(3) 94.14(10), N(2)−Zn(1)−O(3) 89.33(10), N(1)−Zn(1)−O(3) 169.12(10).

obtain single crystals for both diastereomeric products of the same reaction. There arise two possibilities accounting for the formation of a pair of diastereomers: (1) Similar to the situation encountered in the synthesis of zinc silylamido complexes bearing this type of aminophenolate ligands,7c a pair of diastereomers with reversed configurations at both the skeleton nitrogen atom and the zinc center are formed due to a dissociation−association process of the two neutral nitrogen donors of the ligand upon the addition of lactate. In this case, the carbonyl of the lactate ligand in the diastereomers is suggested to be either dissociated from the metal center or in a fluxional state in solution; otherwise, more than two diastereomers should be formed. (2) The configuration of the chiral aminophenolate ligand moiety in the parent complex is reserved during the reaction, and the formation of diastereomers is due to the different coordination mode of the chiral, bidentate lactate ligand. In this regard, the coordination of chiral lactate to zinc center is stereoselective and stable even at relatively high temperature.

Pyridine or 4-(N,N-dimethylamino)pyridine (DMAP) is often used to investigate the lability of the ligand.6a,7n In order to have some insight into the formation mechanism of two diastereomers of each model complex, the chelating ability of the aminophenolate ligand in these zinc complexes was studied by treating complex 2 with 3 equiv of DMAP in C6D6. However, the addition of DMAP did not lead to any change in the chemical shifts of the zinc complex, indicating that the strong coordinating DMAP neither bonded with the zinc center to form a pentacoordinated structure nor replaced any nitrogen donor of the NNO ligand to give a tetracoordinated structure (see Supporting Information Figure S32). We attribute this H

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Figure 7. Molecular structure of 9β′. Zn(1)−O(3) = 2.525(4) Å, Zn(2)−O(7) = 2.212(4) Å; the other relevant bond lengths and angles are given in Figure S1.

result to the intense chelating effect of our NNO tridentate ligand. As mentioned before, in the synthesis of enantiopure zinc tert-butoxide complex 3 or 4 via two different routes, no other diastereomer was detected, which meant that no configuration inversion of the complex took place. It promoted us to check whether the stereogenic center of lactate induced the reassembly of the ligand and therefore the formation of diastereomers of complexes 7−9. Thus, the reactions of complexes 1 and 2 with (S)-isobutanol and (−)-borneol of different steric bulkiness were carried out respectively in C6D6, which however afforded enantiopure zinc alkoxide products exclusively (see Supporting Information Figures S33−S36). In each case, the four doublets assignable to Ar−CH2 units of the resultant zinc alkoxide complex display similar coupling patterns as those of complex 2.28 It is clear that the configuration of the chiral tridentate ligand bonding to metal center is reserved in the alkoxide complexes, and the chiral alkoxy ligand just occupies the coordination site of silylamido group. All these facts suggest that the coordination of the tridentate ligand to zinc center is considerably stable and the formation of diastereomers of lactate model complexes 7−9 might be attributable to the coordination of carbonyl to metal center.14a To verify this hypothesis, complex 1 was further treated with 1 equiv of ethyl 2-hydroxyacetate in C6D6, and as expected the formation of two diastereomers in ca. 1:3 molar ratio was observed (see Supporting Information Figure S37). Furthermore, resonances for the CO moiety in the 13C{1H} NMR spectra of the in situ generated lactate complexes 7−9 in C6D6 were detected at δ (CO) = 185.0−187.6 ppm (see Supporting Information Figures S42−S44). These downfield shifts with respect to the corresponding resonances of (S)methyl lactate (176.1 ppm) and (R)-tert-butyl lactate (178.3 ppm) in the same solvent also demonstrate the coordination of carbonyl to metal center and that the resulting lactate complexes have pentacoordinated structures. Theoretically, six configurational isomers would be expected for a zinc lactate complex where both a tridentate ligand and a bidentate ligand are involved into coordination with the metal center to construct a trigonal bipyramidal geometry, namely with carbonyl oxygen or alkoxide oxygen of the lactate ligand anti to the three donors of NNO ligands, respectively (Scheme 4). Through an observation of the molecular structures of 8α and 9β, the orientation of carbonyl oxygen cis to the donors of NNO ligand in structures D−F are unlikely due to steric

Scheme 4. Proposed Configuration Isomers of TrigonalBipyramidal Zinc Lactate Complex Involving a Tridentate Ligand (Gray Arrows Indicate the Axes)

repulsion between the ester group of the lactate unit and the substituents on N,N,O donors. In structure C, two nitrogen donors of the ligand are located in the equatorial plane; however, the five-membered chelating ring renders the corresponding angle of N(1)−Zn−N(2) (82.20(11)− 84.90(2)° in 8α, 9β, 9β′) to deviate from ideal 120° significantly, which would result in a very unstable configuration. Therefore, structures A and B are most likely and are found just to correspond to the molecular structures of 8α and 9β, respectively. Assuming that the diastereomer 8β detected in trace amount in the reaction of complex 2 and (S)-methyl lactate possesses a configuration similar to structure B, on the basis of molecular structures of 9β and 9β′, significant steric repulsion would be expected between the α-methyl group of the (S)-lactate ligand and the n-butyl group on the pyrrolidinyl nitrogen N(2) (Scheme 5), while in 8α the steric repulsion between this methyl group and n-butyl of the pyrrolidinyl unit as well as the o-trityl group of phenoxide ring is weak or ignorable.29 The remarkable difference of steric repulsion between these two structures is in good consistent with the dominant formation of 8α in the reaction (8α:8β = 10:1). When a similar assumption is adopted for complex 9, then the uncharacterized diastereomer 9α has a configuration of structure A, which becomes less favorable because of certain steric repulsion existing between the α-methyl group of lactate ligand and the I

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diastereomers could be considered to have a configuration similar to 9β or 9α, respectively. By referring the molecular structure of 8α, the coordination site located between the lactate ligand and the rotatory Nbenzyl group in 4−1αL is more accessible for the incoming LA monomer due to the existence of a sterically bulky trityl group and the α-methyl group of (S)-lactate terminus in the opposite coordination site. Based on previous DFT calculation results31 for a β-diketiminate magnesium initiator that a stable transition state features a weak interaction between the Oacyl atom of the approaching monomer and the coordinated Ccarbonyl atom of ring-opened LA, as well as a stronger interaction between the metal−alkoxy oxygen atom and the Ccarbonyl atom of the approaching monomer, the coordination of L-LA will be more favorable since its methyl groups are oriented far away from the metal center as shown in Scheme 6. Just as the formation of 4− 1αL from [(R)-L]ZnOtBu and L-LA, the zinc species formed after this insertion will possess a configuration similar to 4− 1αL, which therefore favors continuous insertion of L-LA to form the −LLLLL− chain sequence. A similar situation is encountered for the diastereomeric species 4−1βD, instead that the accessible site chooses the coordination of D-LA and −DDDD− chain sequence is formed. Because of the existence of n-butyl group on the pyrrolidinyl nitrogen in this site which raises certain steric repulsion, the coordination/insertion of DLA to the metal center of 4−1βD is suggested to be slower than that of L-LA to the zinc center of 4−1αL, which is in accord with the preferential polymerization of L-LA catalyzed by complex 4 (Table 2). As to the minor species 4−1αD, although the α-methyl group of the lactate ligand points toward the site located between the lactate ligand and N-benzyl group, this site is still relatively accessible than the one occupied by the bulky trityl group. The repulsion between the α-methyl group of the lactate ligand and the incoming monomer hinders the facile coordination/insertion of both monomers, with D-LA inferior to L-LA. Thus, a very slow insertion of L-LA to 4−1αD will occur, which then leads to a species similar to 4−1αL favoring the continuous insertion of L-LA. The two diastereomers 4−1αL and 4−1αD finally afford the same intermediate species 4−2αL basically after the selective insertion of L-LA. Once a less favored D-LA is inserted to 4− 2αL, then species 4−3βD and 4−3αD similar to 4−1βD and 4− 1αD, respectively, will be formed. As discussed above, 4−3βD will favor the continuous insertion of D-LA to generate −DDDDLLLLL− stereoblock, while 4−3αD behaves similarly as 4−1αD to allow the insertion of L-LA, therefore leading to a single D-LA-inserted defect (−LLLLDLLLLL−), which was observed in small quantity (the sis signal at 5.23 ppm) in the methine region of the homonuclear decoupled 1H NMR spectrum of the polymer. In the case of the coordination/ insertion of a less favored L-LA monomer to the species 4−2βD, a configuration transformation will take place to generate dominantly the species 4−3αL which favors the continuous insertion of L-LA to form −LLLLLDDDD− stereoblock. Therefore, three types of diastereomeric intermediates with configurations similar to 4−1αL, 4−1αD, and 4−1βD are involved thoroughly in the polymerization of rac-LA and exchange to each other, finally leading to stereoblock PLAs with some isolated defects as characterized by the homonuclear decoupled 1H NMR spectroscopy on the tetrad level. In the proposed polymerization pathways, the favorable insertion of L-LA to species 4−1αL, 4−2αL, 4−3αL, etc., as well as D-LA to species 4−1βD, 4−2βD, 4−1βD, etc., is suggested to

Scheme 5. Repulsive Interaction between Substituents of Equatorial Sites in Zinc Lactate Complexes

revolving benzyl group on the equatorial N atom.30 Compared to the repulsion between the α-methyl group of lactate unit and the rigid pyrrolidinyl group in 8β, the steric repulsion interaction in 9α is relatively weak, which explains well why the disfavored 8β is a trace component (9%) while the disfavored 9α is obtained in minority (45%). On the basis of all the above discussions, we suggest that the chiral combination of zinc complex and a specific lactate as well as the selective orientation of lactate ligand leads to the formation of diastereomers of model complexes 7−9 in different ratios. To compare those lactate complexes with the real polymerization active species, zinc tert-butoxide complex 4 was treated with 1 equiv of D- and L-LA, and the quantified production of zinc species with single lactide insertion was observed spectroscopically. As expected, the formation of two diastereomers (2:1) from mismatched chiral combination of “[(R)L]Zn” fragment and tert-butyl (R,R)-lactidate emerged again, while nearly enantiopure product was formed from the matched combination of [(R)-L]ZnOtBu (4) with (S,S)-lactide (L-LA) (Figure 4). Proposed Isoselective Polymerization Mechanism. The above observations enlighten us to explain the formation of isotactic stereoblock PLAs from rac-LA by an enantiopure zinc initiator, especially when a certain preference for one of the two enantiomeric monomers could still be observed, for instance, kapp(L‑LA)/kapp(D‑LA) = 2.24 for complex 4 in toluene. According to the above NMR reaction studies, the single insertion of racLA monomer into Zn−OtBu bond of complex 4 will lead to mainly three diastereomeric structures as shown in Scheme 6. The diastereomer 4−1αL with a configuration similar to 8α, resulted from the matched combination of [(R)-L]ZnOtBu and L-LA (S,S chirality), is produced in major amount; while the mismatched combination of [(R)-L]ZnOtBu and D-LA (R,R chirality) leads to 4−1βD in relatively major amount (favorable) and 4−1αD (unfavorable) in small amount. The latter two J

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Scheme 6. A Hypothetical Cooperation Mechanism of SEM and CEM in Stereoselective Polymerization of rac-LA with 4

be resulted from the specific chiral environment of the active species induced by the last inserted monomer. The insertion of a mismatched monomer to these species will lead to a transformation of the ligand configuration to generate active centers favoring continuous insertion of monomers with reversed chirality. As a result, stereoblock chain sequences −LLLLLDDDD− and −DDDDLLLLL− are formed. All these processes involve a chain-end control mechanism (CEM). In the case that no ligand configuration transformation takes place after the insertion of a mismatched monomer, the insertion “error” will be corrected by inserting a right monomer afterward; thus a site control mechanism is involved.

Furthermore, the preferential insertion of L-LA to species 4− 1αL, 4−2αL, 4−3αL, etc., relative to D-LA to species 4−1βD, 4− 2βD, 4−1βD, etc., is also a result of site control. Mechanism of rac-LA Polymerization with Heteroselective Magnesium Complexes. To have some insight into the heteroselective polymerization mechanism of rac-LA initiated by magnesium complexes, the apparent rate constants of L-, D-, and rac-LA polymerizations catalyzed by magnesium complexes 5 and 6 were also obtained. As illustrated in Figure 8a,b, magnesium complexes 5 and 6 showed a very small preferences for D-LA or L-LA polymerization (for 5, kapp(L‑LA)/ kapp(D‑LA) = 1.150; for 6 kapp(L‑LA)/kapp(D‑LA) = 0.935, Table 2, Cat. K

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Figure 8. Plots of ln([LA]0/[LA]t) vs time for the ROP of D-LA, L-LA, and rac-LA catalyzed by magnesium complexes (NMR tube reactions for 6 in C6D6, [LA]0/[Mg]0 = 50:1, 20 °C; polymerization scale reactions for 5 in toluene, [LA]0/[Mg]0 = 200:1, 25 °C).

Figure 9. VT 1H NMR spectra (C7D8, 500 MHz) of [(S)-L]MgN(SiMe3)2 (5) and 3 equiv of DMAP.

According to the X-ray diffraction studies of zinc complex 2 and magnesium complex 5a as well as the NMR spectroscopic results, it is clear that magnesium silylamido complexes 5 and 6 possess structures quite similar to those of the zinc counterparts 1 and 2 both in the solid state and in solution. However, inversed stereoselectivities for rac-LA polymerization were observed, which promoted us to explore the potential coordination ability of the magnesium complexes in solution. Thus, 3 equiv of DMAP was added to a solution of complex 5 in C6D6. Interestingly, an adduct ([(S)-L]MgN(SiMe3)2· 2DMAP) with two molecules of coordinated DMAP was formed at ambient temperature. The VT 1H NMR spectra of this adduct in C7D8 were further determined as shown in Figure 9. At ambient temperature, all proton signals of the

5 and 6), which is nearly negligible and indicates the lack of an enantiomorphic-site control. For both complexes, the apparent rate constant of rac-LA polymerization is more than 2 times higher than those of the homochiral monomers either on a polymerization scale (for 5) or on a NMR reaction scale (for 6). Coates and co-workers demonstrated that β-diketiminato zinc isopropoxide species showed 7 times preference for rac-LA polymerization than for L-LA polymerization (krac/kL = 7) and able to mediate highly heterospecific ROP of rac-LA (Pr = 0.90).22 It is widely accepted that highly heterotactic PLA produced by these complexes is a result of chain-end control. Our result is inclined to Coates’ study, with a smaller kapp(rac‑LA)/kapp(L‑or D‑LA) and relatively lower heteroselectivity (Pr = 0.78−0.81) for rac-LA polymerization. L

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coordination/insertion of lactide monomer from two adjacent coordination sites alternatively and for each site the last inserted lactate terminus will choose one specific monomer similar to the situations of zinc species 4−1αL and 4−1βD, therefore leading to the formation of heterotactic PLAs. This hypothesis is further supported by the polymerization results of rac-LA preceded in solvents of different coordinating ability. According to literature reports, most alkaline earth and rare earth metal complexes show enhanced heteroselectivity in THF than in toluene toward rac-LA polymerization.6b,16r,16s,22 In this work, the heterotacticity of PLA obtained in THF by magnesium complex 5 has a slightly decrease relative to that obtained in toluene (from 0.78 in toluene to 0.70, Table 1, entry 16). The most telling experiment is the polymerization initiated by complex 5 in pyridine, the heterotacticity of resulted PLA diminishes to Pr = 0.59 (Table 1, entry 17). Such evidence implies that the solvent molecules take part into competitive coordination to the metal center, blocking one of the coordination sites to a certain degree. The therefore formed magnesium active species would behave more like the zinc active species, which tend to favor the insertion of lactide monomer of the same configuration. The stronger the coordination ability of the solvent molecules is, the higher the isotactic enchainment tendency will be found in the polymerization of rac-LA.

adduct complex are well-resolved and clear coupling modes of ArCH2 protons of the ligand entity can be observed, indicating that both N donors of the aminophenlate ligand are not dissociated from the metal center. Most significantly, two separate resonances accounting for DMAP in different coordination state are visible at 0 °C. Upon increasing the temperature to 50−75 °C, fluxional behavior of the adduct could be detected, which may involve first the dissociation of DMAP molecules and then a fast association/dissociation equilibrium of the neutral nitrogen donors. Therefore, it is conceivable that at ambient temperature magnesium species bearing the chiral aminophenlate ligands reported in this work will tend to have a hexacoordinated metal center when any external coordinative molecules, such as donating solvent molecules and lactide monomers, are present. We attempted to characterize the corresponding magnesium lactate model complexes via the NMR scale reactions of magnesium silylamido complexes 5 and 6 with (S)-methyl lactate, (R)-tert-butyl lactate in C6D6. Disappointingly, the reactions of complexes 5 and 6 with (S)-methyl lactate failed to afford sufficient information owing to the extreme sensitivity of these magnesium complexes toward protonic sources, nearly half equiv of the complex decomposed to the free ligand. By using (R)-tert-butyl lactate, the signals of the target magnesium lactate model complexes could be well-recognized in the 1H NMR spectra, although some decomposition of the complexes to free ligand still occurred (∼17%, see Supporting Information Figures S40 and S41). Similar to those of zinc complexes, both reactions afforded the target model complexes as a mixture of two diastereomers but in different ratios (for 5, 3.3:1; for 6, 1:1.25). Furthermore, two resonances attributable to the CO moiety were detected at 193.04 and 192.20 ppm for the reaction product of 5 and (R)-tert-butyl lactate (see Supporting Information Figure S45). Those are consistent with Chisholm’s recent result6a that a similar chemical shift of CO moiety at 194.1 ppm was observed for a pentacoordinated magnesium compound (BDI)Mg(OCMe2COOEt)(DMAP). In present work, the resonances of CO moiety are obviously downfield shifted with respect to the related resonances of zinc model complex [(R)-L]Zn[(R)−OCH(CH3)CO2tBu] 9 (187.69 and 187.54 ppm) and free (R)-tert-butyl lactate (175.82 ppm), indicating that the coordination interaction of carbonyl to magnesium center is stronger than that in the zinc counterpart. Based on the molecular structures of zinc model complexes 8α and 9β, it is therefore suggested that the diastereomers of the magnesium model complex obtained in each case should also possess a pentacoordinated configuration resembling to those of zinc counterparts, but with stronger carbonyl−metal interaction. Because of the sensitivity of these magnesium silylamido complexes toward protonic sources, we were not able to generate in situ the magnesium isopropoxide or tert-butoxide complex in satisfied purity that could react with D-LA or L-LA to form magnesium active species with single lactide insertion. On the basis of the structure analysis for the model complex and the coordination ability of these magnesium complexes in solution, it is reasonable to suggest that the magnesium active species generated in the polymerization of rac-LA should possess a hexacoordinate geometry where six coordination sites are occupied respectively by the last inserted lactate terminus in a bidentate mode and a coordinated lactide monomer in addition to the tridentate NNO ligand. The additional coordination site in magnesium species may enable the

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CONCLUSIONS We studied the polymerization behavior of zinc silylamido, tertbutoxide complexes, and magnesium silylamido complexes bearing chiral tridentate aminophenolate ligands for the ringopening polymerization of rac-LA. Zinc complexes proved to be good catalysts not only in molecular weight control but also in isoselectivity control. Magnesium complexes exhibited high activity and moderate heteroselectivity for the ROP of rac-LA. The results of kinetic studies for the polymerizations of D-LA, LLA, and rac-LA catalyzed by zinc initiators, and the structures of zinc lactate model complexes in solution and in the solid state helped us to establish a cooperative mechanism accounting for the formation of stereoblock PLAs with some isolated defects by enantiopure zinc initiators. That is, both an enantiomorphicsite control mechanism and a chain-end control mechanism should be involved in the polymerization process, with the former being dominant. The higher coordination number of magnesium active species in solution in comparison to that of zinc species might enable the coordination/insertion of lactide monomer from two adjacent coordination sites by turns that prefer to choose lactide monomer of reversed configuration, therefore leading to the formation of heterotactic PLAs via a chain-end control mechanism. The difference of coordination geometry between zinc and magnesium propagating species, which was evidenced by the X-ray determination and/or NMR spectroscopy, should be responsible for the stereoselectivity switch between zinc and magnesium complexes toward the polymerization of rac-LA.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, including synthetic details, crystal structure data for complexes 2, 5a, 8α, 9β, and 9β′, typical polymerization procedures, rate studies, 1H and 13C NMR spectra of all complexes and related NMR reactions, homonuclear decoupled 1H NMR spectra of typical polymer M

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samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (NNSFC, 21074032) and the Fundamental Research Funds for the Central Universities (WD1113011, WK1214048) is gratefully acknowledged.

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