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3,4-Polymerization of Isoprene by Using NSN- and NPN-Ligated Rare

Jul 17, 2014 - The rare-earth metal complexes bearing NSN-bidentate β-diimidosulfonate ... proceeded in these systems were explicated by DFT simulati...
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3,4-Polymerization of Isoprene by Using NSN- and NPN-Ligated Rare Earth Metal Precursors: Switching of Stereo Selectivity and Mechanism Bo Liu,†,‡ Lei Li,†,∥ Guangping Sun,‡ Jingyao Liu,§ Meiyan Wang,*,§ Shihui Li,*,† and Dongmei Cui*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ Key Laboratory of Automobile Materials of Ministry of Education, Department of Materials Science and Engineering, Jilin University, Changchun, 130025, China § Institute of Theoretical Chemistry, State Key Laboratory of Theoretical and Computational Chemistry, Jilin University, Changchun 130022, China ∥ Graduate School of the Chinese Academy of Sciences, Beijing 100039, China S Supporting Information *

ABSTRACT: The rare-earth metal complexes bearing NSN-bidentate β-diimidosulfonate ligands (RNSNdipp)Ln(CH2SiMe3)2(THF)n (R = Ph, Ln = Lu (1a), n = 1, Y (1b), n = 2, Sc (1c), n = 1; R = PhNMe2, Ln = Lu (1d), n = 1) were synthesized by treatment of the ion-pairs [Ln(CH2SiMe3)2(THF)x][BPh4] with equimolar amount of the ligand lithium salts (RNSNdipp)Li(THF)2 (NSNdipp = S(NC6H4iPr2-2,6)2). Addition reaction between lutetium tris(alkyl)s, Ln(Z)3(THF)n and NSNdipp gave the corresponding dialkyl complexes (ZNSNdipp)Lu(Z)2(THF)n (Z = CH2SiMe3, n = 1 (1e); Z = oCH2C6H4NMe2, n = 0 (1f)). Deprotonation of β-imidophosphonamido ligands H−NPNdipp and H−NPNEt (NPNdipp = Ph2P(NC6H3iPr2-2,6)2, NPNEt = PPh2(NC6H3iPr2-2,6)(NC6H4-Et-2)) with Lu(CH2SiMe3)3(THF)2 yielded the corresponding dialkyl complexes (NPNdipp)Lu(CH2SiMe3)2(THF) (2) and (NPNEt)Lu(CH2SiMe3)2(THF) (3). All the complexes had been structurally well-defined, and 1a, 1b, 1e, 2, and 3 were further characterized by X-ray diffraction analysis where the almost planar NSN rare-earth metal unit is Cs (or pseudo Cs) symmetry with the two alkyl groups arranging on both sides and a coordinated THF against it. Upon activation with [PhMe2NH][B(C6F5)4] and AliBu3, all these complexes exhibited high 3,4-regioselectivity (ranging from 91% to >99%) for the polymerization of isoprene. Moreover, the excellent isospecific selectivity up to mmmm > 99% have been achieved with complexes 1 depending on the electronics of the sulfur substituents to give crystalline polyisoprene with the highest Tm (170 °C) reported to date. The NPN-bidentate β-imidophosphonamide ligated rare-earth metal complexes provide both high syndio- and iso- 3,4-selectivities (3,4 > 99%, rr = 66%, mmmm = 96%) depending on the frameworks, steric environment and geometry of the ligands. The regio- and stereo- selective mechanisms proceeded in these systems were explicated by DFT simulation.

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INTRODUCTION

dienes, there exist more complicated regio- and stereospecifically selective polymerization modes.2 For instance, the mostly investigated regioselective polymerizations afford cis-1,4polybutadiene, the most popular rubber, and cis-1,4-polyisoprene, a promising candidate to replace natural rubber for

Precise control of the microstructures of synthetic polymer materials has been a target of macromolecule science, which could provide polymers with versatile properties. For instance, propylene and styrene etc. monomers bearing one prochiral carbon polymerize via specific selective coordination mechanism to highly crystalline isotactic or syndiotactic products with higher melting points and strengths than their atactic analogues.1 Regarding the monomers such as conjugated 1,3© 2014 American Chemical Society

Received: May 23, 2014 Revised: July 9, 2014 Published: July 17, 2014 4971

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tires manufacture.3 Meanwhile some reports concern about the trans-1,4 stereoselective polymerizations to give plastic like polymers.4 Comparatively, the 3,4-regioselective polymerization of isoprene and 1,2-regioselective polymerization of butadiene are difficult to achieve due to the specific steric and electronic demanding of the coordination/insertion modes of the monomers to the metal active species. 1,2-Polymerization of butadiene was achieved with some Ziegler−Natta systems based on limited metals such as Co, Cr, and Fe,5 while the efficient 3,4-polymerization of isoprene became possible only very recently with a few cationic rare-earth metal and transition metal catalytic systems.6 On the basis of this, accessing further stereoselectivity is obviously challenging but promising to both academic and industrial fields,7 as stereoregulated 3,4- or 1,2polydienes might be the important components of green tires or high performance resins.8 Herein we report by using rareearth metal precursors bearing novel NSN-bidentate βdiimidosulfonate and NPN-bidentate β-imidophosphonamide ligands, not only the perfect 3,4-regio-selectivity but also the switching from iso- to syndio- stereoselectivity, were achieved, for the first time. The factors that may influence the activity, selectivity and mechanism were investigated and further elucidated by density functional theory (DFT) simulation.

(NPNdipp = Ph2P(NC6H3iPr2-2,6)2, NPNEt = PPh2(NC6H3iPr2-2,6)(NC6H4-Et-2)) with Lu(CH2SiMe3)3(THF)26c,e yielded the NPN-bidentate β-imidophosphonamido lutetium dialkyl complexes (NPNdipp)Lu(CH2SiMe3)2(THF) (2) and (NPNEt)Lu(CH2SiMe3)2(THF) (3) (Scheme 2). Complexes 1a−1f were structural analogues of Scheme 2. Synthesis of Complexes 2 and 3

THF solvates (two THF for 1b and no THF for 1f) showing similar topology in 1H NMR spectra to give metal-methylene resonances around δ 0.46−0.50 ppm downfield shifting as compared to their tris(alkyl) precursors. The isopropyl substituents and the two metal alkyls give one set resonances in each of complexes 1a−1e (except 1f), respectively, indicative of the fluxional nature as a result of Cs symmetry of these complexes. X-ray crystallographic analysis of 1a, 1b and 1e showed that these complexes were monomeric where the central metal ions chelate to the N,N-bidentate ligands to form slightly twisted LnNSN-planes in a meridional configuration bisecting the two alkyl moieties while the solvated THF molecules positioning against it (Figure 1 for 1a; SFigure 1,

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RESULTS AND DISCUSSION Synthesis and Characterization of Complexes 1−3. Treatment of rare-earth metal alkyl ion-pairs [Ln(CH2SiMe3)2(THF)x][BPh4] with one equivalent of ligand lithium salts (RNSNdipp)Li(THF)2 (NSNdipp = S(NC6H4iPr22,6)2)9 generated from the addition reaction of the sulfur diimide S(NC6H4iPr2-2,6)2 with RLi across the SN double bond, afforded NSN-bidentate β-diimidosulfonate rare-earth metal dialkyl complexes (RNSNdipp)Ln(CH2SiMe3)2(THF)n (R = Ph, Ln = Lu (1a), n = 1, Y (1b), n = 2, Sc (1c), n = 1; R = PhNMe2, Ln = Lu (1d), n = 1). Direct addition of lutetium tris(alkyl)s, Ln(Z)3(THF)n, over the SN double bond of the sulfur diimide, NSNdipp, gave the corresponding dialkyl complexes (ZNSNdipp)Lu(Z)2(THF)n (Z = CH2SiMe3, n = 1 (1e), δS‑CH2 = 2.48 ppm; Z = o-CH2C6H4NMe2, n = 0, (1f), δS‑o‑CH2 = 4.17 ppm) (Scheme 1). Deprotonation of βimidophosphonamido ligands H−NPNdipp and H−NPNEt Scheme 1. Synthesis of Complexes 1a−1f

Figure 1. X-ray structure of complex 1a with thermal ellipsoids at 30% probability. Hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Lu1−N1 2.306(5), Lu1−N2 2.283(5), Lu1−O1 2.313(4), Lu1−C1 2.321(7), Lu1−C5 2.335(6); N2−Lu1−N1 63.57(16), C1−Lu1−C5 112.5(2), O1− Lu1−C1 94.1(2), O1−Lu1−C5 91.7(2), Si1−C1−Lu1 131.5(4), Si2− C5−Lu1 125.9(4), N1−S1−N2 95.4(2).

Supporting Information, for 1b, 1e, 2, and 3). Complexes 1a and 1e adopt trigonal bipyramidal geometry with the two metal alkyl carbon atoms occupying the apexes, while complex 1b takes tetragonal bipyramidal geometry. Complexes 2 and 3 possess very close structures to 1a and 1e, in which the bidentate NPN ligands coordinate to Lu3+ ions while the two alkyl moieties arranging on both sides of the deformed LuNPN 4972

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Table 1. Polymerization of Isoprene by Complexes 1−3a microstructures (%)c run

cat.

AlR3/cat.

Tp (°C)

time (min)

yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20f 21f 22f

1a 1b 1c 1d 1e 1f 2 2 2 2 3 1a 1b 1c 1d 1e 1f 2 3 1a 3 2

AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(2) AliBu3(5) AliBu3(10) AliBu3(20) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10) AliBu3(10)

10 10 10 10 10 10 10 10 10 10 10 −30 −30 −30 −30 −30 −30 −30 −30 −30 −30 −30

10 10 10 60 60 60 10 10 10 10 10 360 360 360 360 360 360 2160 720 720 720 1440

100 100 100 100 100 100 82 86 89 85 100 100 100 100 100 100 100 67 83 100 89 70

Mnb

4

(×10 )

Mw/Mnb

3,4-

mm/mmmm

mr

rr/rrrr

Tg/Tm (°C)d

eff. (%)e

2.3 2.3 2.1 2.1 1.5 1.8 1.7 1.9 1.8 1.6 2.5 2.3 2.1 2.0 2.0 1.9 2.3 1.6 2.3 1.6 2.2 1.7

98.0 97.1 95.0 93.1 91.2 95.0 97.2 95.7 96.0 95.8 97.1 >99 >99 99 >99 98.2 >99 >99 98.3 99.0 98.7 99.0

92/68 90/60 93/58 74/19 58/5 70/18 15 1 2 3 74/20 >99/>99 >99/>99 97/97 80/43 79/23 66/15 0 96/96 55 53 1

5 8 7 24 40 29 29 43 41 46 25

3 2 2 2 2 1 56/27 56/32 57/31 51/29 1

3 20 21 34 41 4 43 42 33

59/32

27 28 28 30 27 28 45 44 45 42 34 30/170 30/165 32/140 31 35 32 46 34/148 40 40 50

46 50 50 59 53 49 71 150 262 385 92 19 19 21 18 20 24 285 45 23 34 125

14.7 13.5 13.7 11.6 12.9 13.9 7.9 3.9 2.3 1.5 7.4 35.0 36.7 32.1 37.1 34.7 28.7 3.1 12.6 30.0 17.8 3.8

2 5 66/36

Conditions: C6H5Cl 5 mL, Cat. 10 μmol, isoprene 1 mL, [Cat]0/[A]0/[IP] = 1:1:1000, A = [PhMe2NH][B(C6F5)4]. bDetermined by means of gel permeation chromatography (GPC) against polystyrene standards; Measured by means of 1H NMR and 13C NMR spectroscopy in CDCl3. d Determined by differential scanning calorimetry (DSC). eCatalyst efficiency = Mn(calculated)/Mn(measured). fPolymerization in toluene. a

in THF, CH2Cl2 or CHCl3 having a melting point Tm = 170 °C (SFigures 14−19), the highest value found for such polymers,6d as far as we are aware, suggesting the long isotactically regulated sequences (wide-angle X-ray diffraction analysis (WARD) is provided as SFigures 20,21). Surprisingly, the NPNEt-ligated complex 3 also gave a distinguished isospecific selectivity (mm/ mmmm = 96%/96%) as compared to the less steric NPNMeanalogues of highly 3,4-regioselective but non stereoselective reported by our group recently (Table 1, runs 19).6c,e More remarkably, switching of the stereoselectivity was realized for the first time by employing the most bulky NPNdipp-ligated complex 2 to give moderate syndiotactic PIP with rr/rrrr = 59%/32% that increased to rr/rrrr = 66%/36% when performing the polymerization in toluene (Table 1, runs 18, 22). This is the highest synditactic PIP obtained to date for lanthanide catalytic systems, to innovate new rare-earth metal catalysts for high syndiotactic (>95%) 3,4-polymerization of 1,3-conjugated dienes is an obviously challenge. The enriched syndiotacticity endowed the polymer a very high glasstransition temperature Tg (50 °C). On the contrary, for complexes 1 and 3 the change of the polymerization medium aroused drastic decrease of isoselectivity although the 3,4regioselectivity was preserved (Table 1, runs 20, 21). DFT calculations were employed to simulate the polymerization process. First, the role of AliBu3 was determined to abstract the coordinated THF molecule in complexes 1−3 (except 1f) and extrude impurities in the systems, as the polymerization could not perform in the absence of AliBu3.11,12 Then, the polymerization of IP catalyzed by (PhNSNdipp)Lu(CH2SiMe3)2(THF) (1a) was taken as the example to elucidate the mechanism. The cationic active species (PhNSNdipp)Lu(CH2SiMe3)+ would be generated when the catalyst precursors were activated by [PhMe2NH][B(C6F5)4].13 (PhNSNdipp)Lu-

planes, except that 2 is Cs symmetry. Noteworthy was that the substituent phenyl or alkyl group on sulfur(IV) atom in 1a, 1b, or 1e stands over the LnNSN plane while the two phenyl rings on the phosphorus(V) atom in 2 or 3 locate on both sides of the LuNPN metallocycle, arising a more steric bulky environment around the metal center. Polymerization of Isoprene. These neutral complexes 1− 3 were inert to the polymerization of isoprene (IP), which upon activation with [PhMe2NH][B(C6F5)4] and AliBu3 showed different catalytic performances in respect of the types of central metal ions, the steric bulkiness and the NSNand NPN-ligand frameworks. All these complexes showed high activities at 10 °C to transfer 1000 equiv of IP completely in short times (10 min to 1 h, Table 1, runs 1−7, 11). Lowering the temperature to −30 °C, the NSN-ligands supported complexes 1 still provided higher activities than 2 and 3 attached to the NPN ligands, which might be attributed to the less steric NSN-ligands. Complexes 1a−1c bearing the phenylsubstituted sulfur ligand PhNSNdipp exhibited superior activity than complexes 1d−1e bearing alkyl- or aminoalkyl-substituted sulfur ZNSN-ligands, as the Z groups are more electron donating than phenyl group, which disfavored isoprene coordination.3d,6a,d,10 Complex 2 with the most bulky NPNdipp ligand displayed the lowest activity (Table 1, runs 7−11). Strikingly, these ternary catalytic systems of Cs symmetry exhibited distinguished 3,4-regio-selectivity over 98.2% up to >99% despite of the ligand frameworks and the central metal ions (Table 1, runs 12−20). Moreover, based on the 3,4-regioselectivity the excellent isospecific stereoselectivity was also achieved with all systems composed of the NSN-ligated precursors 1, among which complexes 1a and 1b behaved perfectly (mm > 99% and mmmm > 99%) (Table 1, runs 12− 13, SFigures 2−13). The resultant PIP is crystalline and soluble 4973

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Figure 2. Free energy profiles calculated for the 3,4-insertion and 4,3-insertion of the first isoprene as well as 3,4-insertion insertion of the second isoprene. The energy reference point is 1Me + 2 trans-IP.

(CH2SiMe3)+ can be simplified to (PhNSNdipp)Lu(CH3)+ (1Me). The energy profiles of the polymerization of IP catalyzed by 1Me are presented in Figure 2.14 The re coordination of η4-cis-IP15 to 1Me gives two complexes 2Mecis34 and 2Mecis43. 3,4-Insertion and 4,3-instertion of η4cis-IP into the Lu−CH3 bond of 2Mecis34 and 2Mecis43 proceed via transition states TS23Mecis34 and TS23Mecis43 (the vinyl group remains coordinated to the central metal ion) to generate a η3-allyl intermediate 3Mecis34 and a σ-alkyl intermediate 3Mecis43, respectively. The energy of the transition state TS23Mecis34 is 12.6 kcal/mol lower than TS23Mecis43,16 indicating that 3,4-insertion is preferred to 4,3-instertion, in contrast to the previous simulation results for the 3,4polymerization of IP in which IP coordinates to the active metal center in the η2-vinyl mode followed by the 4,3insertion.6a−c Thus, the η3-allyl-metal 3Mecis34 rather than the four-membered metallocyle 3Mecis43 is the probable intermediate as commonly accepted in Ziegler−Natta catalytic systems. The NSN−Lu unit in 3Mecis34 is almost planar having a Cs symmetry. Thus, the newly generated allyl group occupies the enantio-site of the first IP took, leaving the other enantio site to adopt the second IP monomer. The second incoming η4cis-IP coordinates to 3Mecis34 using its re or si faces, giving the intermediate 4Mecis34iso or 4Mecis34syn, respectively. The following 3,4-insertion (C4 of IP attacks C3 of the allyl group) via the transition state TS45Mecis34iso or TS45Mecis34syn, affords the η3-allyl intermediate 5Mecis34iso or 5Mecis34syn. This can be elucidated clearly: the coordination and insertion of the former IP monomer to the active metal center (3Mecis34) using its re (or si) face leaves the other enantio site of the Cs symmetric active metal center in 3Mecis34 for the following IP

monomer coordination and insertion using also its re (or si) face via the pathway involving the transition state TS45Mecis34iso , leading to the isospecific enchainment 5Mecis34iso; while the coordination and insertion of the following IP monomer uses its si (or re) face via the pathway involving TS45Mecis34syn, generating syndiotactic enchainment 5Mecis34syn. The energy difference between the isotactic and syndiotactic transition states TS45Mecis34iso and TS45Mecis34syn is 2.9 kcal/mol, suggesting that the isotactic sequences are indeed favored.14 The stereoselectivity of these cationic NSNcomplexes proceeds in the similar mechanism but different profile to the stereoselective polymerizations of α-olefins by using the Cs (or C1) symmetric group-4 metallocene precursors. In the later cases the rapid polymer chain “back-skipping” leaves the “same” enantio-site for the incoming α-olefin monomer, leading to isospecific enchainment, while the alternative coordination−insertion of the α-olefin monomer at the two enantio-sites of the Cs symmetric active metal center generates syndioselectivity.17 The nature behind the excellent 3,4-stereoisoselectivity of the complexes 1 might be attributed to the less bulkiness of the NSN-tridentate ligands (so is the case of the NPNET ligand in complex 3) that facilitates the exoendo arrangement of the coordinating IP monomer against the η3-allyl polymeric moiety (Figure 3). This work proves for the first time by DFT calculations that the exo-endo arrangement provides the isotactic diad while the exoexo configuration gives syndiotactic regularity.18 On the other hand, the moderate syndioselectivity of complex 2 might be attributed to the most bulkiness of NPN ligand that makes isoprene coordination to the active sites more difficulty (low activity as shown in Table 4974

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using a Rigaku D/Max 2500 V PC X-ray diffractometer (Cu Kα, λ = 1.5406 Å). The diffraction patterns were collected during continuous scan at a speed of 10°/min between the angles of 5 and 50°. X-ray Crystallographic Study. Suitable single crystals of complexes were sealed in a thin-walled glass capillary for determining the single-crystal structure. Data collection was performed at −80 °C on a Bruker SMART diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The SMART program package was used to determine the unit-cell parameters. The absorption correction was applied using SADABS. The structures were solved by direct methods and refined on F2 by full-matrix least-squares techniques with anisotropic thermal parameters for non-hydrogen atoms. Hydrogen atoms were placed at calculated positions and were included in the structure calculation without further refinement of the parameters. All calculations were carried out using the SHELXS-97 program. Molecular structures were generated using ORTEP program. Synthesis of (PhNSNdipp)Lu(CH2SiMe3)2(THF) (1a). A diethyl ether solution (10 mL) of phenyllithium (1.7 g, 20 mmol) was added dropwise to 100 mL round-bottom flask containing a hexane (25 mL) solution of NSNdipp at room temperature. Gradually, the color of solution changed from dark-red to yellow green. The reaction mixture was stirred for another 30 min at room temperature. Removal of most solvent gave a yellow green vicious oily liquid, which was recrystallized from Et2O/THF at −30 °C in 85% yield. 1H NMR (400 MHz, C6D6, 25 °C): δ 1.00 (d, 12H, CHMe2), 1.27 (d, 12H, CHMe2), 4.00 (m, 4H, CHMe2), 6.72−6.78 (m, 1H, Ar−H), 6.79−6.86 (m, 2H, Ar−H), 6.84−6.93 (m, 2H, Ar−H), 7.00−7.10 (m, 5H, Ar−H), 7.45−7.6 (s, 1H, Ar−H). 13C NMR (100 MHz, 25 °C, C6D6): δ 15.12 (s, CHMe2), 24.93 (s, CHMe2), 25.29 (s, CHMe2), 25.37 (s, CHMe2), 28.28 (s, CHMe2), 66.14 (s, THF), 68.48 (s, THF), 122.27 (s, Ar−C), 123.65(s, Ar−C), 125.66 (s, Ar−C), 129.44 (s, Ar−C), 145.91 (s, Ar−C), 151.71(s, Ar−C). A THF (5 mL) solution of (PhNSNdipp)Li(THF)2 (0.305 g, 0.5 mmol) was added dropwise into a mixture of Lu(CH2SiMe3)3(THF)2 (0.290 g, 0.5 mmol) and [Et3NH][BPh4] (0.211 g, 0.5 mmol) in THF (5 mL) at room temperature and the mixture was stirred for 15 min. After solvent removal under reduced pressure, the residue was extracted with hexane and the resulting suspension was filtered to remove the inorganic salt. The filtrate was further concentrated and crystallized at −30 °C to give pale yellow crystals 1a in 80% yield. 1H NMR (400 MHz, C6D6, 25 °C): δ −0.40 (s, 4H, CH2SiMe3), 0.34 (s, 18H, SiMe3), 0.48 (d, 6H, CHMe2), 1.11 (br s, 4H, THF-β-CH2), 1.39 (d, 6H, CHMe2), 1.47 (d, 6H, CHMe2), 1.54 (d, 6H, CHMe2), 3.61 (br s, 4H, THF-α−CH2), 3,40 (m, 4H, CHMe2), 6.83−6.88 (m, 3H, Ar−H), 6.93−7.01 (m, 4H, Ar−H), 7.12 (m, 2H, Ar−H), 7.68 (d, JH−H = 6.8 Hz, 2H, o-SC6H5). 13C NMR (100 MHz, C6D6, 25 °C): δ 4.60 (s, CH2SiMe3), 25.45 (s, CHMe2), 25.63 (s, CHMe2), 25.76 (s, CHMe2), 27.24 (s, CHMe2), 27.49 (s, CHMe2), 28.96 (s, CHMe2), 42.61 (br s, CH2SiMe3), 124.58 (s, Ar− C), 125.66 (s, Ar−C), 128.92 (s, Ar−C), 132.02 (s, Ar−C), 140.12 (s, Ar−C), 144.28 (s, Ar−C), 146.63 (s, Ar−C), 147.71 (s, Ar−C). Synthesis of (PhNSNdipp)Y(CH2SiMe3)2(THF)2 (1b). By a procedure similar to that described for the preparation of 1b, treatment of ( P h N S N d i p p ) L i (T H F ) 2 (0 .3 05 g, 0 .5 m m ol ) w i th Y(CH2SiMe3)3(THF)2 (0.250 g, 0.5 mmol) and [Et3NH][BPh4] (0.211 g, 0.5 mmol) gave yellow crystals of 1b in 77% yield. Single crystals suitable for X-ray analysis were obtained from a THF/hexane mixture at −30 °C within 2 days. 1H NMR (400 MHz, C6D6, 25 °C): δ −0.18 (s, 4H, CH2SiMe3), 0.38 (s, 18H, SiMe3), 1.50 (d, 24H, CHMe2), 3.96 (m, 4H, CHMe2), 6.80−6.90 (m, 4H, Ar−H), 6.90− 7.02(m, 4H, Ar−H), 7.07−7.13 (m, 2H, Ar−H), 7.65−7.70 (m, 1H, Ar−H). 13C NMR (100 MHz, C6D6, 25 °C): δ 3.67 (s, CHSiMe3), 24.86 (s, CHMe2), 25.43 (s, CHMe2), 25.81 (s, CHMe2), 27.55 (s, CHMe2), 28.98 (s, CHMe2), 35.99 (s, CH2SiMe3), 124.43 (s, Ar−C), 125.44 (s, Ar−C), 128.22 (s, Ar−C), 131.88 (s, Ar−C), 139.96 (s, Ar−C), 144.01 (s, Ar−C), 146.28 (s, Ar−C), 147.46 (s, Ar−C). Synthesis of (PhNSNdipp)Sc(CH2SiMe3)2(THF) (1c). By a procedure similar to that described for the preparation of 1c, treatment of (PhNSN d i p p )Li(THF) 2 (0.305 g, 0.5 mmol) with Sc(CH2SiMe3)3(THF)2 (0.230 g, 0.50 mmol) and [Et3NH][BPh4] (0.211 g, 0.5 mmol) gave yellow crystals of 1c in 69% yield. 1H NMR

Figure 3. Optimized structures of the transition states TS45Mecis34iso and TS45Mecis34isyn.

1), providing more chance for site-epimerization or chain epimerization.19

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CONCLUSION In conclusion, we have shown that both the β-diimidosulfinato NSN-bidentate and some β-iminophosphonamido NPNbidentate ligands provide proper steric bulkiness for the rareearth metal centers, leading to excellent 3,4-regio selectivity for the polymerization of isoprene. The electronics of the substituted groups on NSN-ligand frameworks affects the catalytic performances that the electron withdrawing groups arouse increase of the activity and facilitates the formation of the isotactic sequences. For the precursors bearing the NPNligands, based on their 3,4-regio-selectivity, those having more opening coordination centers display excellent isospecific stereoselectivity while those bearing the crowded coordination sphere switch to the syndioselectivity. DFT calculation has revealed that isoprene coordinates to the active spieces in η4-cismode followed by 3,4-insertion to give η3-allyl intermediate; the next incoming isoprene molecule and the allyl group form exoendo arrangement toward the active sites, which ensures the isoselective regularity. For the NPN-ligated precursors, the steric bulkiness of the ligands may make exo-exo transition state slightly more stable than the exo-endo transition state leading to the syndiotacticity.

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EXPERIMENTAL SECTION

General Methods. All manipulations were performed under a dry and oxygen-free argon atmosphere using standard high-vacuum Schlenk techniques or in a glovebox. All solvents were purified via a SPS system. ClPPh2, 2,6-diisopropylaniline, thionyl chloride, LinBu and trimethylchlorosilane were purchased from Aldrich and used without further purification. Isoprene was purified by distillation over calcium hydride under a nitrogen atmosphere. Azide, [Et3NH][BPh4], [Ph3C][B(C6F5)4] 3, Lu(CH2SiMe3)3(THF)2, NSNdipp, NPNdippH, complex 3 were prepared according to the published procedures.20 1H and 13C NMR spectra were recorded on a Bruker AV400 (FT, 400 MHz for 1H; 100 MHz for 13C) spectrometer. The stereorerularity of polyisoprene can be determined by 13C NMR. The signals at δ 149.27, 149.07, 148.48, 148.15, 148.00, 147.66, 147.48, 146.72, 146.48, and 146.28 ppm were arising from mmmm, mmmr, rmmr, mmrr, mmrm, rmrr, mrmr, rrrr, mrrr, and mrrm. The molecular weight (Mn) was measured by TOSOH HLC-8220 GPC at 40 °C using THF as eluent (the flow rate is 0.35 mL/min) against polystyrene standards. Differential scanning calorimetry (DSC) analyses were carried out on a Q100 DSC from TA Instruments under a nitrogen atmosphere. Wide-angle X-ray diffraction (WAXD) measurements were performed 4975

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(400 MHz, C6D6, 25 °C): δ 0.50 (s, 4H, CH2SiMe3), 0.31 (s, 18H, SiMe3), 1.50 (d, 24H, CHMe2), 3.96 (m, 4H, CHMe2), 6.81−6.92 (m, 3H, Ar−H), 6.90−7.02 (m, 6H, Ar−H), 7.65−7.70(m, 2H, Ar−H). 13 C NMR (100 MHz, C6D6, 25 °C): δ 4.00 (s, CH2SiMe3), 24.35 (s, CHMe2), 25.68 (s, CHMe2), 25.89 (s, CHMe2), 27.50 (s, CHMe2), 28.90 (s, CHMe2), 35.55 (s, CH2SiMe3), 124.40 (s, Ar−C), 125.08 (s, Ar−C), 128.27 (s, Ar−C), 131.99 (s, Ar−C), 140.98 (s, Ar−C), 144.89 (s, Ar−C), 146.88 (s, Ar−C), 148.46 (s, Ar−C). Synthesis of (C6H4NMe2NSNdipp)Lu(CH2SiMe3)2(THF)2 (1d). By a procedure similar to that described for the preparation of 1d, treatment of (C6H4NMe2NSNdipp)Li(THF)2 (0.312 g, 0.5 mmol) with Lu(CH2SiMe3)3(THF)2 (0.290 g, 0.5 mmol) and [Et3NH][BPh4] (0.211 g, 0.5 mmol) gave yellow crystals of 1d in 76% yield. Single crystals suitable for X-ray analysis were obtained from a THF/hexane mixture at −30 °C within 2 days. 1H NMR (400 MHz, C6D6, 25 °C): δ −0.40 (s, 4H, CH2SiMe3), 0.35 (s, 18H, SiMe3), 0.57 (d, 6H, CHMe2), 1.40 (d, 6H, CHMe2), 1.57 (d, 12H, CHMe2), 1.81 (d, 6H, NMe2), 3.96 (m, 4H, CHMe2), 6.47−6.53 (m, 1H, Ar−H), 6.85−7.10 (m, 6H, Ar−H), 7.20−7.31 (m, 3H, Ar−H). 13C NMR (100 MHz, C6D6, 25 °C): δ 4.63 (s, CH2SiMe3), 25.03 (s, CHMe2), 25.34 (s, CHMe2), 25.86 (s, CHMe2), 26.40 (s, CHMe2), 27.51 (s, CHMe2), 29.05 (s, CHMe2), 42.37 (br s, CH2SiMe3), 45.83(s, NMe2), 120.75 (s, Ar−C), 123.44 (s, Ar−C), 124.46 (s, Ar−C), 125.41 (s, Ar−C), 132.21(s, Ar−C), 136.68 (s, Ar−C), 140.04 (s, Ar−C), 146.98 (s, Ar− C), 147.85 (s, Ar−C), 156.19 (s, Ar−C). Synthesis of (CH2SiMe3NSNdipp)Lu(CH2SiMe3)2(THF) (1e). To a hexane solution (3 mL) of Lu(CH2SiMe3)3(THF)2 (0.580 g, 1 mmol) was added dropwise 1 equiv of a THF solution (5 mL) of NSNdipp (0.381 g, 1 mmol) at ambient temperature. The mixture was stirred for 60 min at ambient temperature, and after removal of all volatiles under reduced pressure, the residue was extracted with hexane. The hexane solution was further concentrated and crystallized at −30 °C to give pale yellow crystalline solids 1e in 86% isolated yield. 1H NMR (400 MHz, C6D6, 25 °C): δ −0.46 (s, 4H, CH2SiMe3), −0.44 (s, 9H, SCH2SiMe3), 0.33 (s, 18H, SiMe3), 1.37 (d, 6H, CHMe2), 1.43 (d, 6H, CHMe2), 1.47 (d, 12H, CHMe2), 2.48 (s, 2H, S−CH2SiMe3), 3.87 (m, 2H, CHMe2), 4.36 (m, 2H, CHMe2), 7.07−7.13 (m, 2H, Ar−H), 7.18−7.21 (m, 4H, Ar−H). 13C NMR (100 MHz, C6D6, 25 °C): δ −1.66 (s, S−CH2SiMe3), 4.43 (s, CH2SiMe3), 24.95 (s, CHMe2), 25.69 (s, CHMe2), 26.51 (s, CHMe2), 27.30 (s, CHMe2), 28.71 (s, CHMe2), 42.16 (br s, CH2SiMe3), 124.63 (s, Ar−C), 124.85 (s, Ar−C), 125.65 (s, Ar−C), 140.57 (s, Ar−C), 146.84 (s, Ar−C), 147.34 (s, Ar−C). Synthesis of (o-CH2C6H4NMe2NSNdipp)Lu(o-CH2C6H4NMe2)2 (1f). To a hexane solution (3 mL) of Lu(o-CH2C6H4NMe2)3 (0.577 g, 1 mmol) was added dropwise 1 equiv of a THF solution (5 mL) of NSNdipp (0.381 g, 1 mmol) at ambient temperature. The mixture was stirred for 60 min at ambient temperature, and after removal of all volatiles under reduced pressure, the residue was extracted with hexane. The hexane solution was further concentrated and crystallized at −30 °C to give pale yellow crystalline solids 1f in 70% isolated yield. 1 H NMR (400 MHz, C6D6, 25 °C): δ 1.01 (d, 6H, CHMe2), 1.33 (d, 6H, CHMe2), 1.34 (d, 6H, CHMe2), 1.39 (d, 6H, CHMe2), 1.81 (s, 4H, Lu-o-CH2C6H4NMe2), 1.93 (s, 6H, NMe2), 2.39(s, 12H, NMe2), 3.86 (m, 2H, CHMe2), 4.38 (m, 2H, CHMe2), 5.68−5.78 (m, 2H, Ar− H), 6.46−6.56 (m, 5H, Ar−H), 7.07−7.13 (m, 1H, Ar−H), 6.75−6.84 (m, 2H, Ar−H), 7.07−7.12 (m, 6H, Ar−H), 7.18−7.21 (m, 2H, Ar− H). 13C NMR (100 MHz, C6D6, 25 °C): δ 25.73 (s, CHMe2), 26.15 (s, CHMe2), 27.40 (s, CHMe2), 28.52 (s, CHMe2), 28.71 (s, CHMe2), 45.10 (s, NMe2), 46.62 (s, NMe2), 50.94 (s, CH2), 57.24(s, CH2), 117.98 (s, Ar−C), 119.99 (s, Ar−C), 120.44 (s, Ar−C), 123.96 (s, Ar−C), 124.90 (s, Ar−C), 125.34 (s, Ar−C), 125.56 (s, Ar−C), 126.51 (s, Ar−C), 129.53 (s, Ar−C), 132.52 (s, Ar−C) 142.23 (s, Ar− C), 145.18 (s, Ar−C), 146.34 (s, Ar−C), 147.23 (s, Ar−C) 147.56 (s, Ar−C), 153.85 (s, Ar−C). Synthesis of (NPNdipp)Lu(CH2SiMe3)2(THF) (2). To a hexane solution (3 mL) of Lu(CH2SiMe3)3(THF)2 (0.580 g, 1 mmol) was added dropwise 1 equiv of a THF solution (5 mL) of NPNdippH (0.536 g, 1 mmol) at ambient temperature. The mixture was stirred for 60 min at ambient temperature, and after removal of all volatiles under reduced pressure, the residue was extracted with hexane. The hexane

solution was further concentrated and crystallized at −30 °C to give pale yellow crystalline solids 2 in 70% isolated yield. 1H NMR (400 MHz, C6D6, 25 °C): δ −0.60 to −0.30, −0.30 to −0.14 (b, 4H, CH2SiMe3), 0.35 (s, 18H, SiMe3), 0.40−0.80 (b, 9H, CHMe2), 0.99− 1.11 (d, 3H, CHMe2), 1.10−1.53 (br, 12H+4H, CHMe2 + THF-β− CH2), 3.43−3.98 (br, 4H + 4H, CHMe2 + THF-α-CH2), 6.50−6.91 (m, 5H, Ar−H), 6.93−7.10 (m, 1H, Ar−H), 7.10−7.14 (m, 7H, Ar− H), 7.20−7.60(m, 3H, Ar−H), 13C NMR (100 MHz, C6D6, 25 °C): δ 4.59 (s, CH2SiMe3), 24.10 (s, CHMe2), 24.69 (s, CHMe2), 24.91 (s, CHMe2), 28.98(s, CHMe2), 43.61 (s, CH2SiMe3), 120.63 (s, Ar−C), 123.48 (s, Ar−C), 124.15 (s, Ar−C), 124.63 (s, Ar−C), 131.10 (s, Ar−C), 131.69 (s, Ar−C), 133.10 (s, Ar−C), 146.20 (s, Ar−C), 31P NMR (162 MHz, C6D6, 25 °C): δ 22.29. Isoprene Polymerization. A typical polymerization procedure (run 4, Table 1) was described as follows. Isoprene (1.0 mL, 10 mmol), 1a (9.6 mg, 10 μmol), a chlorobenzene solution of AliBu3 (0.10 mmol, 0.2 mL × 0.5 M), and a chlorobenzene solution (3 mL) of A (8.0 mg, 10 μmol) were charged into a flask. After 10 min, the viscous reaction solution was poured into ethanol (ca. 40 mL) containing a small amount of hydrochloric acid to terminate the polymerization. The white solid of polyisoprene was precipitated, filtered, washed with ethanol, and dried under vacuum at 30 °C to a constant weight. Computational Studies. Molecular geometries of the complexes were optimized without constraints via DFT calculations using the B3LYP functional.21 Lutetium was represented with Stuttgart− Dresden pseudopotential in combination with its adapted basis set (SDD).22 The relatively small basis set 3-21G was used for the substituents on N, S, and P, and 6-31G(d) was used for all other main group atoms. Frequency calculations were carried out at the same level of theory to identify all of the stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency), and to provide free energies at 298.15 K. Intrinsic reaction coordinates23 (IRC) were calculated for the transition states to confirm that the structures indeed connect two relevant minima. All calculations were performed with the Gaussian 09 software package.24

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ASSOCIATED CONTENT

* Supporting Information S

1

H NMR, 13C NMR, WAXD, and DSC for the selected polyisoprene samples and a summary of crystallographic data and .cif files for complexes 1a, 1b, 1e, 2, and 3. These information are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(D.C.) E-mail: [email protected]. Fax: (+86) 431 85262774. Telephone: +86 431 85262773. *(M.W.) E-mail: [email protected]. *(S.L.) E-mail: [email protected]. 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 projects No. 201104074, 201104072, 21304088, 51321062, 21374112, 21361140371, and 21203073. The authors are grateful to Computing Center of Jilin Province for essential support.

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REFERENCES

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