Efficient and Heteroselective Heteroscorpionate Rare-Earth-Metal

Article pubs.acs.org/Macromolecules

Efficient and Heteroselective Heteroscorpionate Rare-Earth-Metal Zwitterionic Initiators for ROP of rac-Lactide: Role of σ‑Ligand Zehuai Mou,†,‡ Bo Liu,† Xinli Liu,† Hongyan Xie,†,‡ Weifeng Rong,†,‡ Lei Li,†,‡ Shihui Li,† and Dongmei Cui*,† †

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

ABSTRACT: A series of oxophosphine (3,5-Me2Pz)2CHP(R)2O (Pz = pyrazole; R = tBu (HL1), Cy (HL2)) and iminophosphine (3,5-Me2Pz)2CHP(R)2NAr (R = Cy, Ar = Ph (HL3); R = Ph, Ar = Ph (HL4), Ar = 2,6-Me2-phenyl (HL5)) heteroscorpionate ligands were synthesized. Abstraction of the methide proton of these ligands by rare-earth-metal tris(alkyl)s, Ln(CH2SiMe3)3(THF)2, afforded the corresponding zwitterionic bis(alkyl) complexes L1−5Ln(CH2SiMe3)2(THF) (L1, Ln = Y (1a), Lu (1b); L2, Ln = Y (2a), Lu (2b); L3, Ln = Y (3a), Lu (3b); L4, Ln = Y (4a), Lu (4b); L5, Ln = Y (5a), Lu (5b), while metathesis reaction of the lithium salts of LiL3 and LiL4 with YCl3(THF)2 or YBr3(THF)2 followed by treatment with LiCH2SiMe3 and KN(SiHMe2)2, respectively, afforded the first heteroscorpionate yttrium mixed halogen/alkyl or amido complexes L3−4Y(Cl)(CH2SiMe3)(THF) (L3 (6a), L4 (7a)), L3−4Y(Cl)(N(SiHMe2)2)(THF) (L3 (8a), L4 (9a)), L4Y(Br)(CH2SiMe3)(THF) (10a), and L4Y(Br)(N(SiHMe2)2)(THF) (11a). The structures of these complexes were well-defined, and the molecular structures of 1a, 2a, 3b, 4b, 5a, and 7a were further characterized by single crystal X-ray diffraction analysis. Complexes 1−5 showed similar high activity toward the ROP of rac-LA at room temperature, and both the alkyl species participated in initiation, of which the lutetium complexes exhibited slightly higher selectivity than their yttrium analogues (Pr = 0.85−0.89 vs 0.80−0.84) despite the bulkiness of the ligands. Interestingly, the mixed halogen complexes 6a−11a were single-site initiators, where the σ-halogen moiety remaining on the central metal showed, for the first time, facilitating the heteroselectivity up to Pr = 0.98. This result sheds new light on designing specifically selective catalytic precursors.

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INTRODUCTION Biodegradable and biocompatible polylactides (PLAs) have become one of the most promising environmentally benign materials and widely applied in medical and pharmaceutical industries as well as daily used plastics anticipated to partly replace polyolefins.1 The versatile applications significantly rely on the variable microstructures of PLAs that are governed by the initiators employed via ring-opening polymerization (ROP) processes, in particular, when using rac-LA as the monomer. To access the controllable stereoregulated PLAs,2 the major strategy is to synthesize novel complexes by designing various bulky ligands to match the steric demanding of the central metals. For example, high isotactic PLA can be obtained with some Salen (tetradentate phenoxyimine) aluminum alkoxide or alkyl complexes,3 while heterotactic PLA can be synthesized by using Salan (tetradentate phenoxyamine) aluminum initiators,4 β-diiminate (BDI) zinc and magnesium alkoxides,5 scorpionate calcium complexes,6 lithium tert-butoxide,7 and indium,8 etc. complexes.9 Rare-earth-metal complexes are known as highly active initiators in this field10 albeit with low selectivity, which © 2014 American Chemical Society

might be attributed to their large ionic radii that require steric encumbered ligands to compensate the thus coordination unsaturation. The amino-bis(phenolate) ligated yttrium complexes invented by Carpentier,11 Mountford,12 and our group13 can catalyze ROP of rac-LA in a living and even immortal manner with high heterotactic selectivity (Pr > 0.96). Similar high selectivity had been achieved by Okuda et al. with 1,ωdithiaalkanediyl-bridged bis(phenolato) scandium complexes.14 Williams reported the highly active dimeric bis(oxo/ thiophosphinic)diamido ligated yttrium complexes15 and the moderate heteroselective monomeric bis(oxophosphinic)amido and phospha-Salen yttrium complexes by introducing a nitrogen atom on the linking bridge (Pi = 0.84).15e,16,17 Recently, isotactic-enriched PLAs have been realized with rareearth-metal complexes by Carpentier,18 Arnold,19 and Otero.20 In general, the selectivity is governed by the ligand geometry Received: January 27, 2014 Revised: March 17, 2014 Published: March 27, 2014 2233

dx.doi.org/10.1021/ma500209t | Macromolecules 2014, 47, 2233−2241

Macromolecules

Article

Scheme 1. Synthesis of Zwitterionic Rare-Earth-Metal Complexes 1−5

Figure 1. ORTEP diagram of the molecular structures of complexes 1a (left), 4b (middle), and 5a (right). Thermal ellipsoids are drawn at the 35% probability level. All of the hydrogen atoms are omitted for clarity.

around the metal center via the “enantiomorphic-site control”19−21 or “chain-end control”16,17,22 mechanism. To date, however, no reports have been related to the role of the σbonded ligands in controlling the selectivity, which are usually considered to participate in the initiation. Herein, we report rare-earth-metal complexes supported by new heteroscorpionate ligands23 bearing various bulky substituent groups that show high activity and moderate to high hetereoselectivity toward the ROP of rac-LA. More importantly, we demonstrate that introducing an inert σbonded halogen group to these complexes affords the rareearth-metal initiators with excellent hetereoselectivity (Pr = 0.98), which is the first example that the σ-bonded ligand facilitates the selectivity, although the “halogen determines regularity” had been accepted long time ago when using Ziegler−Natta catalysts for propylene and conjugated diene polymerizations.24

compounds (3,5-Me2Pz)2CHPR2 (Pz = pyrazole; R = tBu, Cy, or Ph) were synthesized with a similar method we reported previously.25 Treatment of these compounds with hydrogen peroxide in dichloromethane or with organoazides in THF gave the targeted oxophosphine heteroscorpionate ligands (3,5Me2Pz)2CHP(tBu)2O (HL1) and (3,5-Me2Pz)2CHP(Cy)2O (HL2) and the iminophosphine heteroscorpionate ligands (3,5-Me 2 Pz) 2 CHP(Cy) 2 NPh (HL 3 ), (3,5-Me 2 Pz) 2 CHP(Ph)2NPh (HL4), and (3,5-Me2Pz)2CHP(Ph)2NAr(2,6-Me2) (HL5) (Scheme 1). The ligands HL1−HL5 reacted with rare-earth-metal tris(alkyl)s, Ln(CH2SiMe3)3(THF)2 (Ln = Y, Lu), at −30 °C to give complexes 1−4 and 5a, while at 50 °C to afford complex 5b (Scheme 1). Complexes 1−4 were moisture and air sensitive but stable at room temperature under a nitrogen atmosphere, while complexes 5 gradually changed from colorless to yellow in solution. The 1H NMR spectra of complexes 1−5 revealed that the resonances arising from the methine protons of the ligands disappeared, suggesting the formation of carbanions.26 There are one set of signals for the two metal−alkyl moieties and singlet resonances for the pyrazole protons of 4-H, 3-Me,

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RESULTS AND DISCUSSION Synthesis and Characterization of Complexes 1−5. The phosphine modified bis(3,5-dimethylpyrazolyl)methane 2234


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Scheme 2. Synthesis of Zwitterionic Yttrium Mixed Halogen/Alkyl (Amido) Complexes 6a−11a

5.40, and 5.43 ppm, respectively, which shift downfield as compared to that in homoleptic complex {Y[N(SiHMe2)2]3(THF)} (4.94 ppm),28 suggesting the absence of β(Si−H) agnostic interaction11e observed frequently in the bis(dimethylsilyl)amido rare-earth complexes.20,22a,29 Noteworthy was that the two pyrazole rings are equivalent although there are no symmetric centers in molecule structures, owing to the probable rapid exchange positions between the halogen ligand and the silylalkyl (or silylamido) group. These solutionstate structures were consistent with that established by X-ray diffraction as shown in Figure 2 (7a). The heteroscorpionate moiety adopts the same coordination mode to that in complex 4b. C(11), P(1), N(5), Y(1), and O(1) generated a plane

and 5-Me, indicating that the two alkyl groups and the two pyrazole rings are equivalent, respectively. The molecular structures of 1−5 in solid states were defined by single crystal X-ray diffraction analysis (Figure 1, 1a (left), 4b (middle), and 5a (right); for 2a and 3b see the Supporting Information Figure S33). In general, the metal ion is capped by the tridentate NNO or NNN ligand in a κ3-coordination fashion without direct bond linkage to the apical carbanion and coordinated to a solvated THF and two alkyl groups, adopting a distorted tetragonal bipyramidal geometry close to the previously reported complex (3,5-Me2Pz)2CP(Ph)2OY(CH2SiMe3)2(THF).25 For complexes 1a and 2a, the bond lengths of Y(1)−O(1) between the yttrium ion and the oxygen from the ligand, 2.207(3) Å (1a) and 2.218(2) Å (2a) are consistent with those in the literature,25,27 but significantly shorter than those of Y(1) −O(2) between the yttrium ion and the oxygen from THF, 2.405(3) Å (1a) and 2.404(2) Å (2a), suggesting that the coordination of THF molecule is weak and fluxional. For complexes 3b and 4b, the bond lengths of lutetium and phosphine imodo nitrogen Lu(1)−N(5), 2.298(3) Å (3b) and 2.341(3) Å (4b) are much shorter than those between lutetium to the pyrozolyl nitrogen atoms (average 2.491 Å for 3b and 2.487 Å for 4b). Variation of the substituents of the ligand framework on the phosphorus atom affects the orientation of the phenyl ring on N(5). The phenyl ring is almost parallel to planar NPCO framework in 4b, which deviates from the corresponding plane in 3b but perpendicular to the corresponding plane in 5a owing to the steric repulsion. Synthesis and Characterization of Monohalogen Complexes 6a−11a. Ligands HL3 and HL4 were lithiated by RLi via alkane elimination, which were followed sequentially by metathesis reaction with yttrium chloride YCl3(THF)2 and alkylation with LiCH 2 SiMe 3 or amination with K[N(SiHMe2)2], to give the corresponding mixed chloride/alkyl complexes 6a and 7a and the mixed chloride/amido complexes 8a and 9a, respectively. The yttrium mixed bromide/alkyl and amido complexes 10a and 11a were prepared in the same procedure by using YBr3(THF)2 instead (Scheme 2). In the 1H NMR spectra of these complexes, the integral intensity ratio of the silylalkyl (or silylamido) group to the ligand is 1:1. The silylhydride SiHMe2 in 8a, 9a, and 11a gives a septet at 5.19,

Figure 2. ORTEP diagram of the molecular structure of complex 7a. Thermal ellipsoids are drawn at the 35% probability level. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): N(1)−Y(1) 2.452(3), N(3)−Y(1) 2.565(3), N(5)−Y(1) 2.377(2), O(1)−Y(1) 2.380(2), C(30)−Y(1) 2.408(3), Cl(1)−Y(1) 2.5632(10); N(5)−Y(1)−N(1) 88.36(9), N(5)−Y(1)−N(3) 91.76(8), C(30)−Y(1)−Cl(1) 104.81(9). 2235


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Table 1. ROP of rac-LA Initiated by Heteroscorpionate Rare-Earth Bis(alkyl) Complexesa run

cat.

T (°C)

t (min)

convb (%)

Mn,calcdc (104)

Mn,expd (104)

Mw/Mnd

Pre

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

1a 1b 2a 2b 1a 1b 2a 2b 3a 3b 4a 4b 5a 5b 1b 1b 3b 3b

25 25 25 25 5 5 5 5 25 25 25 25 25 25 25 25 25 25

15 15 15 15 10 30 10 30 15 15 15 15 15 15 10 10 10 10

98 97 99 97 94 42 97 88 97 98 98 97 97 96 98 85 99 57

1.41 1.40 1.43 1.40 1.35 0.60 1.40 1.27 1.40 1.41 1.41 1.40 1.41 1.38 2.82 2.50 2.85 1.64

1.88 3.10 1.70 2.94 2.19 2.69 3.12 2.60 3.56 2.78 3.67 2.43 2.85 2.63 3.60 3.34 3.68 2.31

1.48 1.63 1.80 2.05 1.72 1.72 1.95 2.12 1.15 1.13 1.20 1.12 1.26 1.13 2.07 1.84 1.49 1.19

0.82 0.87 0.83 0.86 0.86 0.91 0.85 0.89 0.84 0.89 0.84 0.85 0.80 0.83 0.78 0.88 0.83 0.89

Conditions: [rac-LA]0 = 0.8 M, solvent (THF), [M]/cat. = 200. bDetermined by 1H NMR spectrum. cMn,calcd = [M]/2cat. × 144 × conv (%). Determined by GPC against polystyrene standard, Mn using a correcting factor for polylactides (0.58). eDetermined by homonuclear decoupled 1H NMR spectrum. fSolvent (5 mL of toluene). [M]/cat. = 400. g[M]/cat. = 400. a

d

catalytic performances of complexes 3−5 were more promising as compared to our previously reported complexes supported by the iminophosphine hetereoscorpionate with a methoxy side group (3,5-Me2Pz)2CP(Ph)2N(C6H4-o-OMe)Ln(CH2SiMe3)2 (conversion 98% in 15 min, Pr = 0.80−0.89 vs conversion 95% in 5 h, Pr = 0.68−0.7025), as the solvated THF molecule coordinates to the metal center is flexible and reversible in complexes 3−5 that facilitates to improve the stereoselectivity of the catalytic systems,15e while in the late case the coordination of the methoxy group to the metal center is strong and irreversible. Whereas, THF was not the optimum medium for the current polymerization systems, given as Table 1 runs 15−18, the polymerization with 3b proceeded faster in toluene to transfer 400 equiv of rac-LA to PLA in 10 min while only 57% conversion could be reached in THF under the same conditions, although the stereoregularity of the resulting PLAs decreased slightly. This was ascribed to the competition between THF and rac-LA for coordinating to the active metal centers (toluene usually does not coordinate to metal center when a polar monomer polymerization performed in it).15a,b,e,22b Noteworthy was that complexes 4 showed a slightly higher heteroselectivity than the more steric bulky complexes 5 (Table 1, runs 13 and 14), which might be explained by the vertical positioned phenyl rings in complexes 4 (the phenyl ring takes perpendicular position to the ligand framework, leaving a more opening sphere to the metal center) that orientates the monomer coordination−insertion during the propagation cycle.6a,11b,22b,31 Generally, monoanionic auxiliary ligand supported rareearth-metal bis(alkyl) complexes are double-site initiators because of the highly active nature of the metal−carbon bonds, although a few bis(alkyl) (or bis(amido)) complexes have been found of single-site where the two alkyl (or amido) groups take endo (less active) or exo (active) positions against the ligands.15b,32 The polymerization data listed in Table 1 show that the molecular weights measured by GPC for most of the resulting PLAs were larger than the theoretical values calculated based on both alkyls participating in the initiation.

adopting the meridional configuration that bisects the angle formed by the chlorine and the silylalkyl groups. The Cl(1)− Y(1) bond length (2.5632 Å) is slightly shorter than the yttrium terminal chloride bond lengths reported in the literature,30 owing to the enhanced Lewis acidity of the metal center. Ring-Opening Polymerization of rac-LA Initiated by Bis(alkyl) Complexes. All the zwitterionic rare-earth-metal bis(alkyl) complexes 1−5 were employed as single-component catalysts to initiate the ROP of rac-LA at room temperature to reach conversions over 94% within 15 min with heteroselectivity Pr ranging from 0.80 to 0.89 depending on the electronics and sterics of ligands as well as central metals (Table 1, runs 1−4 and 9−14). When the polymerization was conducted at a lower temperature (5 °C), the yttrium complexes 1a and 2a maintained superior activity (up to 94% conversion in 10 min) as compared with the lutetium counterparts 1b and 2b, while the polymerization initiated by complexes bearing dicyclohexylphosphine substituents proceeded faster than that initiated by the corresponding analogues bearing di-tert-butylphosphine substituents (2a vs 1a and 2b vs 1b) (Table 1, runs 5−8). In contrast, all the lutetium complexes 1b and 2b showed higher hetereoselectivity than the yttrium counterparts 1a and 2a despite the steric bulkiness of the ligands and polymerization temperature (Table 1, runs 1−8). The oxophosphine heteroscorpionate complexes 1 and 2 were comparable to the analogous (3,5-Me2Pz)2CP(Ph)2OY(CH2SiMe3)2(THF) reported previously by us in the aspect of stereoselectivity but provided much higher catalytic activity (conversion 97−99% in 15 min vs conversion 95% in 1 h25). The iminophosphine hetereoscorpionate ligated complexes 3, 4, and 5 exhibited similar performances as complexes 1 and 2 but provided PLAs with controlled molecular weight and narrow molecular weight distribution (Table 1, runs 9−14), which might be contributed to the more steric bulkiness of the iminophosphine hetereoscorpionate ligands than the more opening coordination sphere around metal center generated by the oxophosphine hetereoscorpionate ligands. In addition, the 2236


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Ring-Opening Polymerization of rac-LA Initiated by Monohalogen Complexes. These novel mixed halogen/alkyl and halogen/amido complexes 6a−11a were subjected to initiate ROP of rac-LA, which showed lower activity by taking 1 h to transfer 200 equiv of monomers into polymers compared with 15 min for the bis(alkyl) analogues 3a and 4a. This might be ascribed to the single active site per metal against the double sites in the later two cases, as the strong σ-bonded chlorine/ bromine did not participated in the initiation, and the presence of which retarded the monomer insertion into the active site via increasing the steric bulkiness around the metal center. Thus, the bromido complexes 10a and 11a exhibited slightly lower activity than their chloride analogues 7a and 9a (Table 3, runs 8, 9 vs runs 3, 6), respectively. The molecular weight of the resultant PLA was in consistence with the theoretical value calculated following the single-site active species. Strikingly, however, this σ-bonded halogen group endowed the metal center excellent heteroselectivity up to Pr = 0.98 (Figure 4), no matter whether the other σ-bonded initiator was alkyl or amido (Table 3, runs 1−6, 8, and 9), which was much higher than the bis(alkyl) complexes 3−4 bearing the same ligands and competitive with the best records in the literature.9c,13a In addition, the distinguished selectivity was maintained even at elevated polymerization temperatures (9a, Pr = 0.90, Tp = 60 °C) (Table 3, run 7). This suggested that the σ-halogen group remained on the active center to orient the monomer coordination and insertion. The 1H NMR and MALDI-TOF analyses of an L-LA oligomer obtained with complex 9a revealed that the molecule chain is clearly capped with −N(SiHMe2)2 group and hydroxyl group at both ends (Figures S35 and S36), suggesting that the ROP of rac-LA in this system was indeed conducted via the coordination insertion mechanism. The thus “halogen-control” selectivity has been accepted with respect to the iso-specifically selective propylene polymerization and cis-1,4 regioselective polymerization of conjugated dienes by using Ziegler−Natta catalysts,24a−d which, regarding the polymerization of polar monomers, has not been reported yet.

Thus, complex 3b was chosen to ascertain single or double sites of the current systems (Table 2). At the lower monomer-toTable 2. Effect of Monomer-to-Initiator Ratioa run

[M]/[I]

convb (%)

Mn,calcdc (104)

Mn,calcdd (104)

Mn,expe (104)

Mw/Mne

1 2 3 4 5 6

100 200 400 600 800 1000

94 96 94 89 98 91

0.68 1.38 2.71 3.84 5.64 6.55

1.35 2.76 5.41 7.69 11.3 13.1

1.30 2.15 3.09 4.53 5.16 7.81

1.21 1.25 1.39 1.27 1.54 1.26

Conditions: 10 μmol of initiator (3b), solvent (THF), T = 25 °C, reaction time (2 h) was not optimized. bDetermined by 1H NMR spectrum. cMn,calcd = [M]/2[I] × 144 × conv (%). dMn,calcd = [M]/[I] × 144 × conv (%). eDetermined by GPC against polystyrene standard, Mn using a correcting factor for polylactides (0.58). a

initiator ratios 100:1−200:1, the molecular weight of the resulting PLA was close to the theoretic value calculated as the single-site initiator, while with the ratio increasing from 400:1 to 1000:1, the molecular weight was comparable to that arising from the double-site initiators. Thus, we suggested in this system that both alkyl groups participated in the initiation, and the deviation of the experimental values under the lower monomer-to-initiator ratios from the theoretic ones was ascribed to the aggregation of the active species under higher concentrations.33 The polymerization kinetic behaviors for complexes 3a, 4a, 5a, and 3b in THF at room temperature were further investigated ([LA]0 = 0.5 M, [LA]/[cat.] = 300). The polymerization data showed linear fits to plots of ln{[M]0/ [M]t} vs time (Figure 3) to establish the first-order kinetics in

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CONCLUSION We report a series of new bis(alkyl) rare-earth-metal zwitterionic complexes supported by oxophosphine and iminophosphine heteroscorpionate ligands that act as doublesite initiators for the polymerization of rac-LA to exhibit significantly enhanced activity and high heteroselectivity. Polymerization kinetic study suggests these systems present the first-order on the monomer concentration, among which the yttrium-based initiators are more active, while the lutetiumbased ones are highly specific selective. Strikingly, the mixed halogen/alkyl and amido complexes bearing the similar heteroscopionate ligands display as single-site initiators, as the σ-bonded halogen group does not participate in the initiation that facilitates to the distinguished heteroselectivity of Pr = 0.98 at room temperature. This work shows for the first time the σbonded ligand controls the selectivity more powerfully than changing the substituents and metal centers, which sheds new light on designing specific-selective initiators.

Figure 3. Semilogarithmic plots of rac-LA conversion versus time (T = 25 °C, in THF, [LA]0 = 0.5 M, [LA]/[cat.] = 300): 3a (black squares, kapp = 4.59 × 10−3 s−1, R2 = 0.993), 4a (red circles, kapp = 2.96 × 10−3 s−1, R2 = 0.992), 5a (blue triangles, kapp = 2.49 × 10−3 s−1, R2 = 0.999), 3b (purple stars, kapp = 1.07 × 10−3 s−1, R2 = 0.998).

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monomer concentration in each case. The slopes clearly indicated the influences of ligands on polymerization rates in an order of 3a > 4a > 5a and a much faster rate for the yttrium complex than its lutetium analogue (kapp = 4.59 × 10−3 s−1 for 3a vs 1.07 × 10−3 s−1 for 3b), which is comparable to that reported by Williams.15e

EXPERIMENTAL SECTION

General Procedures. All reactions were carried out under a dry nitrogen atmosphere using standard Schlenk techniques or in a glovebox filled with dry nitrogen. Hexane and toluene were purified using an SPS Braun system. THF was dried by distillation over sodium 2237


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Table 3. ROP of rac-LA Initiated by Rare-Earth Mono(alkyl/amido) Complexesa run

cat.

M/cat.

T (°C)

t (h)

convb (%)

Mn,calcdc (104)

Mn,expd (104)

Mw/Mnd

Pre

1 2 3 4 5 6 7 8 9

6a 6a 7a 7a 8a 9a 9a 10a 11a

200 200 200 200 200 200 400 200 200

25 0 25 0 25 25 60 25 25

1 7 1 10 2 1 0.5 1 1

95 97 94 97 86 88 90 87 77

2.74 2.79 2.71 2.79 2.48 2.53 5.18 2.51 2.21

2.38 2.96 3.02 3.59 3.17 3.93 6.68 2.99 2.87

1.10 1.14 1.20 1.37 1.30 1.28 1.35 1.31 1.39

0.93 0.96 0.97 0.98 0.93 0.98 0.90 0.98 0.98

Conditions: [rac-LA]0 = 0.8 M, solvent (THF). bDetermined by the 1H NMR spectrum. cMn,calcd = [M]/cat. × 144 × conv (%). dDetermined by GPC against polystyrene standard, Mn using a correcting factor for polylactides (0.58). eDetermined by the homonuclear decoupled 1H NMR spectrum.

a

follows: A solution of complex 1a (14.0 mg, 20 μmol, [LA]/[Y] = 200:1) in THF (2 mL) was added to a stirred solution of rac-LA (0.576 g, 4 mmol) in THF (3 mL). The polymerization took place immediately at room temperature. The system became viscous in a few minutes. It was stirred for 15 min and then quenched by adding several drops acidified ethanol, and a small sample of the viscous solution was separated for the measurement of conversion by 1H NMR. Then polymers were precipitated with abundant ethanol, collected, and dried at 40 °C for 24 h in vacuo. The molecular weight and polydispersity index of the resulting polymers were determined by GPC. The tacticity of the PLA was calculated according to the methine region homonuclear decoupled 1H NMR spectrum. Synthesis of Proligands and Representative Complexes. Synthesis of (3,5-Me 2 Pz) 2 CHP( t Bu) 2 . A solution of bis(3,5dimethylpyzazoly)methane (4.08 g, 20 mmol) in THF in a Schlenk flask was cooled down to −65 °C, followed by slow addition of nBuLi in hexane (12.5 mL, 1.65 mol/L) with a syringe over 25 min under vigorous agitation, which immediately resulted in a white suspension. The mixture was stirred for another 2 h at this temperature. Then a solution of tBu2PCl (3.61 g, 20 mmol) in 10 mL of THF was added dropwise over 20 min, which generated a gray-green solution. The reaction mixture was heated to 50 °C and stirred for an additional 12 h, which turned into dark red solution. Removal of the THF led to brown solids, and toluene (150 mL) was added to extract. The insoluble solid was removed by filtration, and the filtrate was evaporated to give an orange solid (yield: 87%). 1H NMR (400 MHz, C6D6): δ 7.12 (s, 1H; C−H), 5.57 (s, 2H; Pz−H), 2.70 (s, 6H; Pz−CH3), 2.13 (s, 6H; Pz−CH3), 1.07 (s, 18H; tBu−H), 1.05 ppm (s, 18H; tBu−H). 31P NMR (162 MHz, C6D6): δ 44.14 ppm. Anal. Calcd for C19H33N4P: C, 65.49; H, 9.55; N, 16.08. Found: C, 65.41; H, 9.50; N, 16.03. Synthesis of (3,5-Me2Pz)2CHP(Cy)2. The compound was synthesized with the same method described as above using chlorodicyclohexylphosphine (yield: 76%). 1H NMR (400 MHz, CDCl3): δ 6.72 (s, 1H; C−H), 5.76 (s, 2H; Pz−H), 2.36 (s, 6H; Pz−CH3), 2.18 (s, 6H; Pz−CH3), 1.82−1.80 (m, 2H; Cy−H), 1.71−1.64 (m, 4H; Cy−H), 1.59−1.58 (m, 2H; Cy−H), 1.46−1.40 (m, 4H; Cy−H), 1.16−1.01 (m, 8H; Cy−H), 0.89−0.83 ppm (m, 2H; Cy−H). 31P NMR (162 MHz, C6D6): δ 14.60 ppm. Anal. Calcd for C23H37N4P: C, 68.97; H, 9.31; N, 13.99. Found: C, 68.92; H, 9.28; N, 13.65. Synthesis of (3,5-Me2Pz)2CHP(tBu)2O (HL1). Hydrogen peroxide (1.6 g of a 30% w/w solution, 14 mmol) was added dropwise to a stirred solution of (3,5-Me2Pz)2CHP(tBu)2 (2.44 g, 7 mmol) in CH2Cl2 at 0 °C. The mixture was then warmed to room temperature and stirred for 2 h. Sodium thiosulfate (4.74 g, 30 mmol) aqueous solution was added to the mixture slowly and stirred for another 1 h. Separated the aqueous and organic layers, extracted the aqueous layer with CH2Cl2 twice (2 × 20 mL), collected the organic phase, and dried with anhydrous MgSO4 overnight. Removing the solvent under reduced pressure gave a yellow residue. Washed the residue with hexane gave HL1 as a white solid (yield: 61%). 1H NMR (400 MHz, C6D6): δ 6.63 (d, J = 12 Hz, 1H; C−H), 5.62 (s, 2H; Pz−H), 2.23 (s,

Figure 4. Methine region of the homonuclear decoupled 1H NMR spectra (400 MHz, CDCl3, 298 K) of the PLAs obtained from 4a (curve 2, run 11 in Table 1) and 9a (curve 1, run 6 in Table 3) at room temperature. with benzophenone as indicator under a nitrogen atmosphere and was stored over freshly cut sodium in a glovebox. Rare-earth-metal tris(alkyl)s were synthesized according to a previous report.34 Bis(3,5dimethylpyrazolyl)methane was synthesized according to the literature from 3,5-dimethylpyrazole and dichloromethane catalyzed by n Bu4NBr.35 rac-LA was recrystallized with dry ethyl acetate three times. Glassware and flasks using in the polymerization were dried in an oven at 115 °C overnight and exposed to a vacuum−nitrogen cycle three times. The molecular weight and molecular weight distribution of the polymers were measured using a TOSOH HLC 8220 GPC instrument at 40 °C with THF as eluent against polystyrene standards. Organometallic samples for NMR spectroscopic analysis were prepared in a glovebox by the use of NMR spectroscopy tubes and then sealed by paraffin film. 1H, 31P, and 13C NMR spectra were recorded using a Bruker AV400 spectrometer. The MALDI-TOF mass spectrum was obtained with a Bruker Daltonic MicroFlex LT at the National Analytic Research Center of the Changchun Institute of Applied Chemistry (CIAC). X-ray Crystallographic Studies. Crystals for X-ray analysis were obtained as described in the following preparations. The crystals were manipulated in a glovebox. Data collection was performed at −86.5 °C using a Bruker SMART APEX diffractometer with a CCD area detector and graphite monochromated Mo Kα radiation (λ = 0.710 73 Å). The determination of crystal class and unit-cell parameters was carried out by the SMART program package. The raw frame data were processed using SAINT and SADABS to yield the reflection data file. The structures were solved by using the SHELXTL program. ROP of rac-LA. Polymerizations of rac-LA were carried out in a 15 mL flask under a N2 atmosphere. A typical procedure was described as 2238


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6H; Pz−CH3), 2.08 (s, 6H; Pz−CH3), 1.32 (s, 18H; tBu−H), 1.29 ppm (s, 18H; tBu−H). 31P NMR (162 MHz, C6D6): δ 79.63 ppm. Anal. Calcd for C19H33N4OP: C, 62.61; H, 9.13; N, 15.37. Found: C, 62.57; H, 9.09; N, 15.33. Synthesis of (3,5-Me2Pz)2CHP(Cy)2O (HL2). HL2 was synthesized using the same method as HL1 (yield: 65%). 1H NMR (400 MHz, CDCl3): δ 6.37 (d, J = 8 Hz, 1H; C−H), 5.83 (s, 2H; Pz−H), 2.40− 2.29 (m, 2H; Cy−H), 2.20 (s, 6H; Pz−CH3), 2.18 (s, 6H; Pz−CH3), 2.06−0.83 ppm (m, 20H; Cy−H). 31P NMR (162 MHz, C6D6): δ 52.99 ppm. Anal. Calcd for C23H37N4OP: C, 66.32; H, 8.95; N, 13.45. Found: C, 66.27; H, 8.90; N, 13.40. Synthesis of (3,5-Me2Pz)2CHP(Cy)2NPh (HL3). A solution of phenyl azide (0.54 g, 4.5 mmol) in THF (10 mL) was added dropwise to a stirred solution of (3,5-Me2Pz)2CHP(Cy)2 (1.2 g, 3.0 mmol) in THF (40 mL) at room temperature in a glovebox. After 12 h, the solvent was removed in vacuo to give a light yellow solid, which was washed with hexane three times and isolated by filtration, followed by drying under reduced pressure at room temperature (yield: 65%). 1H NMR (400 MHz, C6D6): δ 7.29 (t, J = 8 Hz, 2H; Ph−H), 7.10 (d, J = 8 Hz, 2H; Ph−H), 6.87 (t, J = 8 Hz, 1H; Ph−H), 6.63 (d, J = 8 Hz, 1H; Ph− H), 5.60 (s, 2H; Pz−H), 3.04−2.94 (m, 2H; Cy−H), 2.30−2.26 (m, 2H; Cy−H), 2.12 (s, 6H; Pz−CH3), 1.98 (s, 6H; Pz−CH3), 1.83−0.82 ppm (m, 18H; Cy−H). 31P NMR (162 MHz, C6D6): δ 33.34 ppm. Anal. Calcd for C29H42N5P: C, 70.85; H, 8.61; N, 14.24. Found: C, 70.51; H, 8.50; N, 14.11. Synthesis of (3,5-Me2Pz)2CHP(Ph)2NPh (HL4). To a solution of (3,5-Me2Pz)2CHP(Ph)2 (3.88 g, 10 mmol) in THF, phenyl azide (1.67 g, 14 mmol) diluted with 5 mL of THF was added dropwise in a glovebox under stirring, and then the yellow solution was heated to 70 °C under a nitrogen atmosphere for 2 days. The solvent was removed in vacuo to give a light yellow solid, which was washed with hexane and isolated by filtration as a white solid, followed by drying under reduced pressure at room temperature (yield: 79%).1H NMR (400 MHz, C6D6), δ 7.96 (m, 4H; Ph−H), 7.19 (m 4H; Ph−H), 7.02 (m, 7H; Ph−H), 6.84 (s, 1H; C−H), 5.53 (s, 2H; Pz−H), 1.96 (s, 6H; Pz−CH3), 1.92 ppm (s, 6H; Pz−CH3). Anal. Calcd for C29H30N5P: C, 72.63; H, 6.31; N, 14.60. Found: C, 72.43; H, 6.18; N, 14.51. Synthesis of (3,5-Me2Pz)2CHP(Ph)2NAr(2,6-diMe) (HL5). HL5 was synthesized using the same method as HL4 (yield: 69%). 1H NMR (400 MHz, C6D6), δ 8.20−8.14 (m, 4H; Ph−H), 7.07 (m, 8H; Ph− H); 6.81 (m, 1H; Ph−H), 6.72 (d, J = 12.0 Hz, 1H; C−H), 5.45 (s, 2H; Pz−H), 2.15 (s, 6H; Ph−CH3), 1.97 (s, 6H; Pz−CH3), 1.55 ppm (s, 6H; Pz−CH3). Anal. Calcd for C31H34N5P: C, 73.35; H, 6.75; N, 13.80. Found: C, 73.27; H, 6.61; N, 13.59. Synthesis of Complex 1a. Tris(alkyl)s yttrium complex [Y(CH2SiMe3)3(THF)2] (0.5 mmol) was dissolved in dry hexane and cooled to −30 °C. A THF suspension of HL1 at −30 °C was added into the above solution. After stirring for 4 h at room temperature, the reaction mixture was concentrated to a small portion and was added several drops of hexane. Colorless crystal deposited at the bottom of the bottle from the solution under −30 °C after 3 days. Yield: 75%. 1H NMR (400 MHz, C6D6): δ 5.56 (s, 2H; Pz−H), 3.58 (s, 4H; THF), 2.19 (s, 6H; Pz−CH3), 2.14 (s, 6H; Pz−CH3), 1.19 (s, 4H; THF), 1.04 (s, 9H; tBu−H), 1.01 (s, 9H; tBu−H), 0.60 (s, 18H; Si−Me3), −0.28 ppm (s, 4H; Y−CH2). 31P NMR (162 MHz, C6D6): δ 75.22 ppm. 13C NMR (100 MHz, C6D6): δ 148.29 (C3 or C5), 145.06, 144.99 (C3 or C5), 105.76 (C4), 70.50 (THF), 63.92 (d, J = 67 Hz; P−C), 36.32 (P− C−Me3), 35.59 (P−C−Me3), 31.08 (Y−CH2), 30.71 (Y−CH2), 28.36 (P−CMe3), 25.18 (THF), 14.54 (Pz−CH3), 12.74 (Pz−CH3), 5.08 ppm (Si−Me3). Anal. Calcd for C31H62N4O2PSi2Y: C, 53.27; H, 8.94; N, 8.02. Found: C, 53.20; H, 8.65; N, 7.89. Synthesis of Complex 6a. To a solution of HL3 (0.3932 g, 0.8 mmol) in THF at −30 °C, a THF solution of LiCH2SiMe3 (0.0753 g, 0.8 mmol) was dropwise added under stirring. After 30 min, the mixture was added to a suspension of YCl3 (0.1562 g, 0.8 mmol) in THF, which was prepared 1 day in advance. With the reaction proceeding, the above reaction suspension became clear gradually. After 7 h, the mixture was cooled to −30 °C and a solution of LiCH2SiMe3 (0.0716 g, 0.76 mmol) was added slowly, then being stirred for 3 h at room temperature. The solvent was removed under

reduced pressure, and the residue was extracted with toluene. The insoluble solid was removed by filtration. The filtrate was concentrated to 2 mL and was added several drops of hexane. Complex 6a was obtained as pale yellow solids within 2 days when the solution was kept at −30 °C. Yield: 53%. 1H NMR (400 MHz, C6D6): δ 7.72 (d, J = 8 Hz, 2H; Ph−H), 7.32 (t, J = 8 Hz, 2H; Ph−H), 7.01 (t, J = 8 Hz, 1H; Ph−H), 5.65 (s, 2H; Pz−H), 3.63 (s, 4H; THF), 2.37 (s, 6H; Pz− CH3), 2.22 (s, 6H; Pz−CH3), 1.97, 1.60, 1.48, 1.24, 0.96 (m, 22H; Cy−H), 1.32 (s, 4H; THF), 0.35 (s, 9H; Si−Me3), −0.23 ppm (s, 2H; Y−CH2). 31P NMR (162 MHz, C6D6): δ 39.24 ppm. Anal. Calcd (%) for C37H60ClN5OPSiY: C, 57.39; H, 7.81; N, 9.04. Found: C, 57.33; H, 7.79; N, 8.89. Synthesis of Complex 8a. To a solution of HL3 (0.3932 g, 0.8 mmol) in THF at −30 °C, a THF solution of LiCH2SiMe3 (0.0753 g, 0.8 mmol) was added dropwise. After 30 min, the mixture was added to a suspension of YCl3 (0.1562 g, 0.8 mmol) in THF, which was prepared 1 day in advance. With the reaction proceeding, the above reaction suspension became clear gradually. After 7 h, the mixture was cooled to −30 °C and a solution of K[N(SiHMe2)2] (0.1303 g, 0.76 mmol) was added slowly. Then, the reaction mixture was kept stirring for 5 h at room temperature. The solvent was removed under reduced pressure, and the residue was extracted with toluene. The insoluble solid was removed by filtration. The filtrate was concentrated to 2 mL and was added several drops of hexane. Complex 8a was obtained as white solids within 2 days when the solution was kept at −30 °C. Yield: 60%. 1H NMR (400 MHz, C6D6): δ 7.78 (d, J = 8 Hz, 2H; Ph− H), 7.30 (t, J = 8 Hz, 2H; Ph−H), 7.95 (t, J = 8 Hz, 1H; Ph−H), 5.56 (s, 2H; Pz−H), 5.19 (sept, J = 3.2 Hz, 2H; Si−H), 3.62 (m, 4H; THF), 2.44 (s, 6H; Pz−CH3), 1.96, 1.78, 1.63, 1.45, 1.13, 0.97, 0.59 (m, 22H; Cy−H), 2.15 (s, 6H; Pz−CH3), 1.36 (m, 4H; THF), 0.44 ppm (d, J = 3.2 Hz, 12H; SiH−Me2). 31P NMR (162 MHz, C6D6): δ 38.31 ppm. 13 C NMR (100 MHz, C6D6): δ 149.86, 148.34, 148.24 (C3 or C5), 145.11, 145.05, 129.43, 129.42, 122.57, 122.55 (Ph) 106.13 (C4), 69.14 (THF), 54.21 (d, J = 127 Hz; P−C), 37.91, 37.23, 27.83, 27.71, 27.28, 27.16, 26.68, 25.81, 25.78, 25.61 (Cy), 26.56 (THF), 14.88 (Pz−CH3), 12.21 (Pz−CH3), 3.52 ppm (SiH−Me2). Anal. Calcd (%) for C37H63ClN6OPSi2Y: C, 54.23; H, 7.75; N, 10.26. Found: C, 54.16; H, 7.70; N, 10.20. CCDC-921207 (1a), 921208 (2a), 957722 (3b), 957723 (4b), 957724 (5a), and 965061 (7a) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details for complexes 1b, 2−5, 7a, and 9a−11a; 1 H and 31P NMR spectra of all complexes; table giving selected bond lengths and bond angles; molecular structures of complexes 2a and 3b; the crystallographic data for complexes 1a, 2a, 3b, 4b, 5a, and 7a; 1H NMR and MALDI-TOF mass spectra of an oligomer. 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]; Fax (+86) 431 85262774; Tel +86 431 85262773 (D.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by The National Natural Science Foundation of China for project nos. 51321062, 21274143, and 21361140371. 2239


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