Highly 3,4-Selective Polymerization of Isoprene with NPN Ligand

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Organometallics 2009, 28, 4814–4822 DOI: 10.1021/om900261n

Highly 3,4-Selective Polymerization of Isoprene with NPN Ligand Stabilized Rare-Earth Metal Bis(alkyl)s. Structures and Performances Shihui Li,†,‡ Dongmei Cui,*,† Danfeng Li,†,‡ and Zhaomin Hou§ †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, ‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China, and §Organometallic Chemistry Laboratory, RIKEN, Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Received April 7, 2009

Deprotonation of Ar1NHPPh2NAr2 (H[NPN]n, n=1-10) by Ln(CH2SiMe3)3(THF)2 (Ln=Lu, Y, Sc, Er) generated a series of rare-earth metal bis(alkyl) complexes [NPN]nLn(CH2SiMe3)2(THF)2 (1-10), which under activation with [Ph3C][B(C6F5)4] and AliBu3 were tested for isoprene polymerization. The correlation between catalytic performances and molecular structures of the complexes has been investigated. Complexes 1-5 and 8, where Ar1 is nonsubstituted or ortho-alkyl-substituted phenyl, adopt trigonal-bipyramidal geometry. The Ar1 and Ar2 rings are perpendicular in 1-4 and 8 but parallel in 5. When Ar1 is pyridyl, the resultant lutetium and yttrium complexes 9a and 9b adopt tetragonal geometry with the ligand coordinating to the metal ions in a N,N,N-tridentate mode, whereas in the scandium analogue 9c, the ligand coordinates to the Sc3þ ion in a N,N-bidentate mode. These structural characteristics endow the complexes with versatile catalytic performances. With increase of the steric bulkiness of the ortho-substituents Ar1 and Ar2, the 3,4-selectivity increased stepwise from 81.6% for lutetium complex 1 to 96.8% for lutetium complex 6 and to 97.8% for lutetium complex 7a. However, further increase of the steric bulk of the ligand led to a slight drop of 3,4-selectivity for the attached complex 5 (95.1%). When the smaller scandium ion was employed, the corresponding complex 7c provided 98.1% 3,4-selectivity, which reached 99.4% when the polymerization was performed at -20 °C, and the polymerization had quasi-living characteristics. Complexes 9a and 9b, containing an electron-donating ligand, gave higher 3,4-selectivities (85.0% vs 85.5%) than those attached to electron-withdrawing ligands 9c (33%) and 10 (77%).

Introduction It is well known that the properties of polymers are significantly dependent on the microstructures of polymer chains, which are mainly governed by the catalysts applied. This is the exact case for the polymerization of a conjugated diene that gives trans-/cis-1,4 or 3,4 (or 1,2) regulated polymer owing to the regio- and stereoselective nature of the catalysts.1 Thus, the optimization of catalyst systems through fine-tuning the steric hindrance and configuration and electronics of ancillary ligand frameworks, which exert a defined coordination environment around a metal center, has attracted increasing research interest. For instance,

Ziegler-Natta catalysts based on Ti, Co, Ni, and Nd metal chlorides, first innovated to initiate the polymerization of butadiene in the early 1960s, are highly active to give polymers with 98% cis-1,4-regularity, but upon heterogenization, gel formation is observed.2 The addition of an electron donor to lanthanide trichlorides improves the catalytic activity and selectivity slightly, whereas the system is still heterogeneous.3 Covalent NCN-pincer type lanthanide chloride complexes form homogeneous catalysts that show an extremely high cis-1,4 selectivity governed by the geometry of the pincer ligand.4 Bis(cyclopentadienyl) (bis-Cp)-ligated f- and d-block metal complexes combined with MAO or AlR3 have established homogeneous

*Corresponding author. Fax: (þ86) 431 85262773. Tel: þ86 431 85262773. E-mail: [email protected]. (1) (a) Baugh. L. S.; Canich. J. A. M. Stereoselective Polymerization with Single-Site Catalysts; Taylor & Francis: New York, 2008; p 447. (b) Porri, L.; Giarrusso, A.; Ricci, G. Prog. Polym. Sci. 1991, 16, 405. (c) Tobisch, S. Acc. Chem. Res. 2002, 35, 96. (d) Kuran, W. Principle of Coordination Polymerization; John Wiley and Sons Ltd.: New York, 2001; pp 275-321. (e) Friebe, L.; Nuyken, O.; Obrecht, W. Adv. Polym. Sci. 2006, 204, 1. (f) Fischbach, A.; Anwander, R. Adv. Polym. Sci. 2006, 204, 155. (g) Arndt, S.; Okuda, J. Adv. Synth. Catal. 2005, 347, 339. (h) Zeimentz, P. M.; Arndt, S.; Elvidge, B. R.; Okuda, J. Chem. Rev. 2006, 106, 2404.

(2) (a) Porri, L.; Giarrusso, A. Conjugated Diene Polymerisation. In Comprehensive Polymer Science; Pergamon Press: Oxford, 1989; Vol. 4, pp 53-108. (b) Hsieh, H.; Yeh, H. Rubber Chem. Technol. 1985, 58, 117. (c) Longiave, C.; Castelli, R. J. Polym. Sci. C 1963, 4, 387. (d) Shen, Z.; Gong, C.; Chung, C.; Ouyang, J. Sci. Sin. 1964, 13, 1339 (Chem. Abstr.: 1964, 61 (10), 12091). (3) (a) Shen, Z.; Ouyang, J.; Wang, F.; Hu, Z.; Yu, F.; Qian, B. J. Polym. Sci., Polym Chem. Ed. 1980, 18, 3345. (b) Yang, J.; Hu, J.; Pan, S.; Xie, D.; Zhong, C.; Ouyang, J. Sci. Sin. 1980, 23, 734 (Chem. Abstr.: 1980, 93 (12), 115054h). (4) Gao, W.; Cui, D. J. Am. Chem. Soc. 2008, 130, 4984.

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single-site catalyst systems of high cis-1,4 selectivity.5 NonCp-ligand-stabilized lanthanide alkylaluminates6 and bis(phosphinophenyl)amido rare-earth metal cations7 allow control over molecular weight, polydispersities, and polymer regularity. Alkyl-bridged lanthanide aluminates in combination with bis(alkyl) aluminum chloride provide the most successfully modified Ziegler-Natta catalyst system for high cis-1,4 polymerization of isoprene,8a-8c which switches to trans-1,4 selectivity when using a Cp0 precursor activated with organoborate.8d,8e Similarly, introducing a Cp ligand into the neodymium boron hydrido complex Nd(BH4)3(THF)3 significantly improves the trans-1,4 selectivity of the resultant half-metallocene Cp*Nd(BH4)2(THF)2.9 It should be noted that the above-mentioned studies focus mainly on the correlation of cis- or trans-1,4 selectivity of the catalysts with their molecular structures. The 3,4-polymerization of isoprene has been explored less, although this polymer is an important component of high-performance rubbers.10 Few catalytic systems can initiate the 3,4-polymerization of isoprene, as isoprene prefers to coordinate to the metal center in a cis-1,4 mode under generation of a thermally stable η3-π-butenyl species. Thus to initiate 3,4-polymerization of isoprene, the steric hindrance at the metal center is requisite. Previously, several catalyst systems have been reported to produce 3,4-polyisoprene units as a major (5) (a) The Ban, H.; Kase, T.; Kawabe, M.; Miyazawa, A.; Ishihara, T.; Hagihara, H.; Tsunogae, Y.; Murata, M.; Shiono, T. Macromolecules 2006, 39, 171. (b) Thuilliez, J.; Monteil, V.; Spitz, R.; Boisson, C. Angew. Chem., Int. Ed. 2005, 44, 2593. (c) Barbotin, F.; Montail, V.; Llauro, M.; Boisson, C.; Spitz, R. Macromolecules 2000, 33, 8521. (d) Montail, V.; Spitz, R.; Boisson, C. Polym. Int. 2004, 53, 576. (e) Kaita, S.; Hou, Z.; Wakatsuki, Y. Macromolecules 1999, 32, 9078. (f) Kaita, S.; Hou, Z.; Wakatsuki, Y. Macromolecules 2001, 34, 1539. (g) Kaita, S.; Doi, Y.; Kaneko, K.; Horiuchi, A. C.; Wakatsuki, Y. Macromolecules 2004, 37, 5860. (h) Kaita, S.; Yamanaka, M.; Horiuchi, A. C.; Wakatsuki, Y. Macromolecules 2006, 39, 1359. (i) Kaita, S.; Hou, Z.; Nishiura, M.; Doi, Y.; Kurazumi, J.; Horiuchi, A. C.; Wakatsuki, Y. Macromol. Rapid Commun. 2003, 24, 179. (6) (a) Fischbach, A.; Klimpel, M. G.; Widenmeyer, M.; Herdtweck, E.; Scherer, W.; Anwander, R. Angew. Chem., Int. Ed. 2004, 43, 2234. (b) Arndt, S.; Beckerle, K.; Zeimentz, P. M.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2005, 44, 7473. (7) Zhang, L.; Suzuki, T.; Luo, Y.; Nishiura, M; Hou, Z. Angew. Chem., Int. Ed. 2007, 46, 1909. (8) (a) Fischbach, A.; Meermann, C.; Eickerling, G.; Scherer, W.; Anwander, R. Macromolecules 2006, 39, 6811. (b) Meermann, C.; T€ ornroos, K.; Nerdal, W.; Anwander, R. Angew. Chem., Int. Ed. 2007, 46, 6508. (c) Fischbach, A.; Perdih, F.; Herdtweck, E.; Anwander, R. Organometallics 2006, 25, 1626. (d) Zimmermann, M.; T€ornroos, K.; Anwander, R. Angew. Chem., Int. Ed. 2008, 47, 775. (e) Zimmermann, M.; T€ ornroos, K.; Sitzmann, H.; Anwander, R. Chem.;Eur. J. 2008, 14, 7266. (9) (a) Bonnet, F.; Visseaux, M.; Pereira, A.; Barbier-Baudry, D. Macromolecules 2005, 38, 3162. (b) Bonnet, F.; Visseaux, M.; Pereira, A.; Bouyer, F.; Barbier-Baudry, D. Macromol. Rapid Commun. 2004, 25, 873. (c) Bonnet, F.; Visseaux, M.; Barbier-Baudry, D. J. Organomet. Chem. 2004, 689, 264. (10) (a) Wolpers, J. U.S. Patent 5,104,941, 1992. (b) Jonny, D. M. U.S. Patent 5,356,997, 1994. (11) (a) Natta, G.; Porri, L.; Carbonaro, A. Makromol. Chem. 1964, 77, 126. (b) Gronski, W.; Murayama, N.; Cantow, H. J.; Miyamoto, T. Polymer 1976, 17, 358. (c) Ricci, G.; Battistella, M.; Porri, L. Macromolecules 2001, 34, 5766. (d) Sun, Q.; Wang, F. Acta Polym. Sin. 1988, 2, 145. (e) Bazzini, C.; Giarrusso, A.; Porri, L. Macromol. Rapid Commun. 2002, 23, 922. (f) Nakayama, Y.; Baba, Y.; Yasuda, H.; Kawakita, K.; Ueyama, N. Macromolecules 2003, 36, 7953. (g) Bazzini, C.; Giarrusso, A.; Porri, L.; Pirozzi, B.; Napolitano, R. Polymer 2004, 45, 2871. (12) (a) Zhang, L.; Luo, Y.; Hou, Z. J. Am. Chem. Soc. 2005, 127, 14562. (b) Wang, B.; Wang, D.; Cui, D.; Gao, W.; Tang, T.; Chen, X.; Jing, X. Organometallics 2007, 26, 3167. (c) Wang, B.; Cui, D.; Lv, K. Macromolecules 2008, 41, 1983. (d) Li, S.; Miao, W.; Tang, T.; Dong, W.; Zhang, X.; Cui, D. Organometallics 2008, 27, 718. (e) Zhang, L.; Nishiura, M.; Yuki, M.; Luo, Y.; Hou, Z. Angew. Chem. Int. Ed. 2008, 47, 2642. (f ) Zhang, H.; Luo, Y.; Hou, Z. Macromolecules 2008, 41, 1064. (g) Yu, N.; Nishiura, M.; Li, X.; Xi, Z.; Hou, Z. Chem. Asian J. 2008, 3, 1406.

Organometallics, Vol. 28, No. 16, 2009

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Scheme 1. Preparation of Complexes 1-10

component,11,12 but few of them showed the 3,4-selectivity higher than 95%, and the stereospecific 3,4-polymerization of isoprene is even more scarce. The isospecific 3,4-polymerization of isoprene was recently achieved by use of a halfsandwich phosphido-bridged binuclear yttrium catalyst12a or an amidinate-ligated yttrium catalyst,12e while the fluorenyl/ N-heterocyclic carbene-ligated rare-earth catalysts yielded syndiotactic rich 3,4-polyisoprene.12c The lack of a precise understanding of factors determining the 3,4-selectivity of a complex hinders the design of highly 3,4-selective catalysts, in particular, those based on rare-earth metals with larger ionic radii and high coordination numbers. Therefore, further exploration of new catalyst systems of high 3,4-selectivity based on rare-earth and transition metals is still of obvious interest and challenge. Here we wish to report that by introducing sterically and electronically tunable ligands, a highly 3,4-selective rareearth metal based catalytic system has been established, and the correlation between the catalytic performances and the molecular structures has been investigated.

Results and Discussion Syntheses and Characterization of NPN-Type Rare-Earth Metal bis(alkyl)s. Iminophosphonamine compounds H[NPN]n (n = 1-10) were prepared via Staudinger reaction by treatment of aminodiphenylphosphine (Ar1NHPPh2: Ar1 = C6H5, 4-MeC6H4, 2-MeC6H4, 2-EtC6H4, 2,6-Et2C6H3, C5NH4, 2-FC6H4) with a slight excess of the corresponding azides (Ar2N3: Ar2 = 2,4,6-Me3C6H2, 2,6-Et2C6H3, 2,6iPr2C6H3, 2-MeC6H4) (Scheme 1). H[NPN]n reacted with an equimolar amount of Ln(CH2SiMe3)3(THF)2 via deprotonation, affording a series of rare-earth metal bis(alkyl) complexes, [NPN]nLn(CH2SiMe3)2(THF) (n = 1, Ln = Lu (1);12d n=2, Ln=Lu (2); n=3, Ln=Lu (3); n=4, Ln=Lu (4); n=5, Ln=Lu (5); n=6, Ln=Lu (6); n=7, Ln=Lu (7a), Y

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Figure 1. X-ray structure of 2 with 35% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

Li et al.


Figure 4. X-ray structure of 8 with 35% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Figure 2. X-ray structure of 3 with 35% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

(7b), Sc (7c), Er (7d); n=8, Ln=Lu (8); n=9, Ln=Lu (9a), Y (9b), Sc (9c);12d n=10, Ln=Lu (10)) (Scheme 1). All the complexes were isolated from a mixture of THF/ hexane at -30 °C in moderate to high yields (56-84%). The NMR spectra for most complexes (7d is paramagnetic) were indicative for the formation of bis(alkyl) species by giving resonances around δ -0.05 to -0.40, assignable to the methylene protons of the metal alkyl species, while the methylene protons of Sc-CH2SiMe3 in 7c shifted to δ 0.45 owing to the stronger Lewis acidity of the Sc3þ ion. X-ray diffraction analyses show that lutetium complexes 2-4 and 8 are isostructural monomers with a solvated THF molecule similar to complex 112d (Figures 1-4). Each NPN ligand chelates η2-N,N bidentate to Lu3þ in a meridional configuration, which bisects the two alkyl groups. The molecules adopt trigonal-bipyramidal geometry. The Ar1 aryl ring is coplanar with the NPN plane and almost perpendicular to the Ar2 aryl ring. The P-N(1) bond lengths are close to the P-N(2) bond lengths, indicating a delocalization of the π electrons within the N-P-N fragments. The

bond angles N-P-N (100.2(2)o in 2, 98.4(6)o in 3, 100.8(2)o in 4, 99.9(2) o in 8) and N-Lu-N (64.90(10)o in 2, 64.45(4)o in 3, 64.79(13)o in 4, 64.92(16)o in 8) fall in the reasonable ranges reported previously.13 The molecular structure of complex 5 is also a THF solvate, adopting a trigonal-bipyramidal geometry with a η2-N,N bidentate ligand. However, in 5 the aryl rings Ar1 and Ar2 are parallel, and both, in order to minimize interaction with the two P-phenyl groups, are vertical rather than coplanar to the NPN plane, like that in 1-4 (Figure 5). This change of the molecular conformation leads to the Luþ ion being further away from the NPN plane with a deviation of 0.3677 A˚, which is obviously larger than the average 0.1304 A˚ in 1-4 and 8. Meanwhile, the average Lu-C (2.334(3) A˚) and Lu-O (2.303(2) A˚) bond distances are shorter than the corresponding analogues (2.356(3) and 2.332(4) A˚) in 3 and 4. These data reflect that the conformational variation (13) (a) Vollmerhaus, R.; Shao, P.; Taylor, N. J.; Collins, S. Organometallics 1999, 18, 2731. (b) Stapleton, R. L.; Chai, J.; Taylor, N. J.; Collins, S. Organometallics 2006, 25, 2514. (c) Vollmerhaus, R.; Tomaszewski, R.; Shao, P.; Taylor, N. J.; Wiacek, K. J.; Lewis, S. P.; Al-Humydi, A.; Collins, S. Organometallics 2005, 24, 494.

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Figure 6. X-ray structure of 9a with 35% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

results in a more open environment around the metal center in 5. This is consistent with the NMR spectral analysis, where the two alkyl groups can exchange positions rapidly within the NMR time scale to show one sharp singlet at δ -0.21. For complexes 9a and 9b, the pyridyl iminophosponamide ligand coordinates to the metal center in a N,N,N-tridentate mode, adopting a tetragonal-bipyramidal geometry (Figures 6 and 7). In contrast, in the scandium analogue 9c, the pyridyl iminophosponamide ligand chelates to the Sc3þ ion in an N,Nbidentate mode, leaving the pyridyl N(3) atom dangling, which is ascribed probably to the smaller Sc3þ ionic radius. This is in agreement with the 1H NMR spectral analysis, where the pyridyl R-H in 9a (δ 8.41) and 9b (δ 8.57) shift downfield compared to that in 9c (δ 7.92). The strong interaction between the central metal ion and the pyridyl N(3) atom leads to the smaller N-P-N and N-Lu-N bond angles of 97.96(14)o and 62.18(9)o in 9a compared to 99.43(13)o and 64.74(9)o in 1, respectively, and the larger C-Lu-C bond angle of 115.40(12)o in 9a versus 112.51(12)o in 1. These differences in molecular structures among complexes 1-9 might influence their selectivity significantly (vide infra). Catalytic Activity for Isoprene Polymerization. All the complexes 1-10 alone could not induce the polymerization of isoprene, while the ternary catalyst systems composed of these complexes, an equimolar amount of [Ph3C][B(C6F5)4], and a slight excess of aluminum alkyls exhibited catalytic activity and even strikingly high 3,4-selectivity for the polymerization of isoprene.


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Figure 7. X-ray structure of 9b with 35% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity.

Figure 8. Plot of the polyisoprene Mn and polydispersity (Mw/ Mn) as a function of isoprene-to-Sc ratio. Using 9c/[(Ph3C)B(C6F5)4]/10AliBu3 as catalyst at 20 °C: b Mn, O Mcalcd, and 2 Mw/Mn.

As seen in Table 1, complexes 1-8 and 10, bearing N-aryl phosphonimido ligands, showed a similar catalytic activity that reaches a complete conversion within 15 min, and no influence of the steric hindrance of the ligands and the type of central metal on the catalytic activity was observed (entries 1-10 and 18). With the ternary catalyst system 7c/[Ph3C][B(C6F5)4]/AliBu3, the polymerization of isoprene was performed at various monomer-to-initiator ratios ranging from 500 to 5000. The result showed that the molecular weights of the obtained polymers increased linearly with the ratios, which were comparable to the theoretical values, while the molecular weight distributions were in a narrow range (PDI = 1.19-1.48), indicating high catalytic efficiencies (78%95%) and a controllable nature of the polymerization (Figure 8). However, for complexes 9, bearing the pyridyl iminophosphonamide moiety [NPN]9, the type of central metal affected the catalytic activity significantly. The yttrium complex 9b showed similar high activity to complexes 1-8 (entry 16). However, with the lutetium counterpart 9a the polymerization was sluggish and a full conversion could be achieved in a much longer time (60 min, entry 15). The scandium analogue 9c displayed the lowest catalytic activity (80% in 360 min) (entry 17). 3,4-Selectivity for Isoprene Polymerization. Effect of Sterics of Ligands. Complexes 1-5, which have identical Ar2 but

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Table 1. Polymerization of Isoprene with Complexes 1-6, 7a-7d, 8, 9a-9c, and 10a microstructuresb entry

cat.

temp (°C)

time (min)

yield (%)

3,4 (%)

cis-1,4 -(%)

trans-1,4- (%)

Mnc 10-4

Mw/Mnc

effd

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

1 2 3 4 5 6 7a 7b 7c 7d 7c 7c 7c 8 9a 9b 9c 10

20 20 20 20 20 20 20 20 20 20 40 0 -20 20 20 20 20 20

15 15 15 15 15 15 15 15 15 15 10 30 360 15 60 15 360 15

100 100 100 100 100 100 100 100 100 100 100 89 81 100 100 100 80 100

81.6 82.0 95.1 96.6 95.1 96.8 97.8 92.3 98.1 93.8 91.6 98.5 99.4 84.4 85.0 85.5 33.3 77.5

17.5 17.3 4.2 3.0 4.3 3.0 2.0 6.9 1.8 5.9 8.2 1.4 0.6 15.1 14.6 14.0 65.9 21.6

0.9 0.7 0.7 0.4 0.6 0.2 0.2 0.8 0.1 0.3 0.2 0.1 0 0.5 0.4 0.5 0.8 0.9

5.44 6.92 7.53 6.50 6.57 6.07 7.43 8.74 6.48 10.97 5.03 11.49 14.90 6.45 4.28 14.60 15.90 3.94

1.8 1.7 1.7 1.8 2.0 1.9 1.9 1.2 1.2 1.3 1.4 1.4 1.5 1.9 1.8 1.8 1.9 1.6

1.25 0.98 0.90 1.05 1.04 1.12 0.92 0.78 1.05 0.62 1.35 0.59 0.46 1.05 1.59 0.47 0.43 1.73

a Conditions: toluene 5 mL, complex 10 μmol, [Ph3C][B(C6F5)4] 10 μmol, AliBu3 100 μmol, [IP]0:[Ln]0 = 1000. b Measured by the means of 1H NMR and 13C NMR spectroscopy in CDCl3. c Measured by GPC calibrated with standard polystyrene samples. d Initiation efficiency = Mn (calcd)/Mn (found).

different Ar1 moieties, exhibited various selectivities in the polymerization of isoprene. Complex 1, containing nonsubstituted Ar1, gave a modest 3,4-selectivity (81.6%). Modifying Ar1 by adding a methyl group in the para-position of Ar1, the resultant complex 2 did not show an improvement in 3,4selectivity (82.0%). Surprisingly, switching the para-methyl to the ortho-position, the resultant complex 3 exhibited a 3,4selectivity as high as 95.1% (entry 3). Replacing the orthomethyl by a bulkier ethyl, the 3,4-selectivity increased further to 96.6% for complex 4 (entry 4). The bulky ortho-substituent provided efficient steric shielding to the metal center, which favored isoprene η2-coordination to generate a four-membered metallocyclic active species, affording 3,4-regularity.12a,12d A DFT calculation has demonstrated that the coordination and insertion of butadiene into the Sm-H bond of the complex [(C5Me5)2SmH] takes place in a highly favored η2fashion because of steric effects.14 However, when both orthopositions of Ar1 were substituted by ethyl groups (H[NPN]5), surprisingly, the resultant complex 5 did not show further improvement in 3,4-selectivity but a slight drop (95.1%) compared to complex 4. X-ray diffraction analysis has revealed that the molecular configuration of 5 differs from 4. The monoethyl-substituted Ar1 ring in 4 is almost coplanar to the NPN plane and perpendicular to the Ar2 ring. However, in 5 both the two-ethyl-substituted Ar1 ring and the Ar2 ring are vertical to the NPN plane, leaving a relatively open metal center (vide supra). Thus introduction of a bulky substituent could not increase the efficient steric shielding around the central metal endlessly but might reduce it via conformation change in response to the repulsion among the intramolecular substituents. Therefore the 3,4-selectivity was depressed to some degree. As expected, the steric bulk of the ortho-subsituent in Ar2 also plays an important role in determining the catalyst’s selectivity. Complex 8, containing mono ortho methyl substituted phenyl only gave a 84.4% 3,4-selectivity, much lower than 95.1% of complex 3, which has three methyl substituents (entries 3 and 14). Complexes 4, 6, and 7a, bearing ortho (14) Kaita, S.; Koga, N.; Hou, Z.; Doi, Y.; Wakatsuki, Y. Organometallics 2003, 22, 3077.

methyl, ethyl, or isopropyl substituents on the Ar2, respectively, showed stepwise increase in 3,4-selectivity from 96.6% for 4 to 96.8% for 6 and to 97.8% for 7a (entries 4, 6, and 7). Electronic Effect of the Ligands. Once the role of the orthosubstituents on the aryl rings was established, the electronic effect of the ligands was investigated. The lutetium and yttrium complexes 9a and 9b, with Ar1 being pyridyl, showed 3,4-selectivity of 85.0% and 85.5%, respectively, which were higher than that of complex 1, where Ar1 is phenyl (81.6%). This might be explained by the coordination of the electron-donating pyridyl N(3) atom in both 9a and 9b to the central metal, which leads to spatial crowding around the metal center and reduces the electrophilicity of the metal ion, which facilitates isoprene η2coordination to afford a better 3,4-selectivity.14 This was proven further by the catalytic performance of the scandium analogue 9c, which gave a 1,4-enriched polyisoprene (66%, entry 17). The pyridyl in 9c did not bind to Sc3þ, which generated a more open sphere around the metal center and behaved as an electron-withdrawing group, which favored isoprene η4-coordination.9 This was the exact case in complex 10, bearing a weak electron-withdrawing fluorinesubstituted ligand ([NPN]10), which increased the Lewis acidity of the Lu3þ ion and increased the chance of isoprene η4-coordination, resulting in a moderate 3,4-selectivity (entry 18). Effects of the Central Metal and the Reaction Temperature. The size of the rare-earth metal ion exerts a great influence on selectivity; thus changing the central metal from scandium to yttrium causes the 3,4-selectivity to decrease dramatically from 88.5% to 43.8%.12d A similar effect was found in the fluorenyl N-heterocyclic carbene systems, where the 3,4selelctivity was improved by decreasing the ionic radius of the central metal.12c In this work we found that when the ligand was bulky enough, the size of the metal ion did not exhibit as obvious an influence as comparing scandium complex 7c with its yttrium analogue 7b, 98.1% vs 92.3%; however, the relationship between 3,4-selectivity and the size of the metal ion still followed the trend Y (92.3%) < Er (93.8%) < Lu (97.8%) < Sc (98.1%), in reverse of the ionic

Article


radius, Y (1.04 A˚) > Er (1.03 A˚) > Lu (1.00 A˚) > Sc (0.89 A˚) (entries 7-10).15 Reaction temperature also affected the catalytic performance. With the decrease of the polymerization temperature, the catalytic activity was depressed; in contrast, the number average molecular weight increased obviously from 6.48 104 at 20 °C to 14.9 104 at -20 °C with a relatively narrow molecular weight distribution (PDI = 1.35-1.53) (entries 9 and 13). The observed increase of the molecular weight may be attributed to reduced numbers of active species generated as well as the chain transfer, etc., side reactions. The 3,4-selectivity was substantially improved from 91.6% at 40 °C and 98.1% at 20 °C to 98.5% at 0 °C and up to 99.4% at -20 °C (entries 9, 11-13). This might be explained as follows: at a higher temperature the steric hindrance arising from the conformation of the ligand aryl rings became smaller owing to their faster rotation. Conclusion. We have demonstrated that rare-earth metal bis(alkyl) complexes supported by iminophosphonamide ligands in combination with aluminum trisalkyls and organoborate are highly catalytically active and 3,4-selective for isoprene polymerization. Complexes bearing alkyl substituents in the aryl rings of the ligands exhibit higher activity than those bearing electron-withdrawing substituents, irrespective of the central metal type. In addition, the sterics and electronics of the ligands play significant roles in governing the 3,4-selectivity of the complexes. With the increase of bulkiness of the ortho-substituents of the aryl rings of the ligands as well as reducing the size of the metal ion, the resultant complexes provide an increasing 3,4-selectivity, as the steric shielding around the metal center leads to isoprene η2-coordination. The complexes bearing electron-donating ligands show higher 3,4-selectivity, as the space-filled and electronegative metal center facilitates isoprene η2-coordination. In contrast an electrophilic metal center attached to ligands with electron-withdrawing groups favors isoprene η4-coordination, resulting in moderate 3,4-regulated or even cis-1,4-enriched polyisoprene. These results elucidate for the first time the correlation of molecule structures of complexes and catalytic behaviors in 3,4-selective isoprene polymerization, which sheds new light on designing catalysts and investigating the mechanism of conjugated diene polymerization.

Experimental Section General Methods. All manipulations were performed under a dry and oxygen-free argon atmosphere using standard highvacuum Schlenk techniques or in a glovebox. All solvents were purified from an MBRAUN SPS system. ClPPh2, AliBu3, LnCl3, and LiCH2SiMe3 were purchased from Aldrich. Mesityl azide, 2-methylphenyl azide, 2,6-diethylphenyl azide, 2,6-diisopropylphenyl azide,16 Ar1NHPPh2, C5H5NNHPPh2dNC6H2Me3-2,4,6,12d,17a [Ph3C][B(C6F5)4], Ln(CH2SiMe3)3(THF)2 (Ln=Sc, Y, Lu, Er),17b and complexes 1 and 9c12d were prepared according to published procedures. Isoprene (99%, Acros) was dried over CaH2 under stirring for 48 h and distilled before use. (15) Effective ionic radius of Ln3þ for coordination number 6: Shannon, R. D. Acta Crystallogr, Sect. A 1976, 32, 751. (16) (a) Murata, S.; Abe, S.; Tomioka, H. J. Org. Chem. 1997, 62, 3055. (b) Pilyugina, T. S.; Schrock, R. R.; Hock, A. S.; M€uller, P. Organometallics 2005, 24, 1929. (c) Al-Benna, S.; Sarsfield, M. J.; Thornton-Pett, M.; Ormsby, D. L.; Maddox, P. J.; Bres, P.; Bochmann, M. J. Chem. Soc., Dalton Trans. 2000, 4247. (17) (a) Contreras, R.; Grevy, M. J.; G-Hernandez, Z.; G-Rodriguez, M.; Wrackmeyer, B. Heteroat. Chem. 2001, 12, 542. (b) Lappert, M.; Pearce, R. Chem. Commun. 1973, 126.

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H and 13C NMR spectra were recorded on a Bruker AV400 (FT, 400 MHz for 1H; 100 MHz for 13C) or Bruker AV300 (FT, 300 MHz for 1H; 75 MHz for 13C). NMR assignments were confirmed by 1H-1H (COSY) and 1H-13C (HMQC) experiments when necessary. Elemental analyses were performed at the National Analytical Research Centre of the Changchun Institute of Applied Chemistry (CIAC). 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. X-ray Crystallographic Studies. Crystals for X-ray analysis were obtained as described in the preparations. The crystals were manipulated in a glovebox. Data collections were performed at -88.6 °C on a Bruker SMART APEX diffractometer with a CCD area detector, using graphite-monochromated Mo KR radiation (λ=0.71073 A˚). 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. Refinement was performed on F2 anisotropically for all nonhydrogen atoms by the full-matrix least-squares method. The hydrogen atoms were placed at the calculated positions and were included in the structure calculation without further refinement of the parameters. Synthesis of 4-MeC6H4NHPPh2dNC6H2Me3-2,4,6 (H[NPN]2). A THF solution (5 mL) of mesityl azide (0.97 g, 6.1 mmol) was added dropwise to a 100 mL Schlenk flask containing a THF solution (30 mL) of 4-methylphenylaminodiphenylphosphine (1.74 g, 6.0 mmol) at 0 °C under nitrogen atmosphere. The reaction mixture was stirred for 12 h at room temperature. Removal of volatiles gave a reddish-black viscous oil, which was further purified by washing with hexane to afford pale gray solids, H[NPN]2 (1.67 g, 65%). 1H NMR (300 MHz, CDCl3, 25 °C): δ 2.07 (s, 6 H, o-NC6H2Me3), 2.19 (s, 3 H, p-NC6H2Me3), 2.21 (s, 3 H, C6H4Me), 6.71 (s, 2 H, NC6H2), 6.87-6.92 (m, 4 H, o,m-NC6H4), 7.38-7.44 (m, 4 H, m-PC6H5), 7.49-7.54 (m, 2 H, p-PC6H5), 7.73-7.80 (m, 4 H, o-PC6H5). 13C NMR (75.5 MHz, CDCl3, 25 °C): δ 20.84 (s, 2 C, o-NC6H2Me3), 21.03 (s, 1 C, p-NC6H2Me3), 21.11 (s, 1 C, NC6H4Me), 118.66 (d, 3JP-C = 6.0 Hz, 2 C, oNC6H4), 129.04 (Ar), 129.22 (Ar), 130.01 (Ar), 131.24 (Ar), 132.17 (Ar), 132.49 (Ar), 132.61 (Ar), 132.70 (Ar), 139.52 (s, 1 C, ipsoNC6H4), 142.98 (s, 1 C, ipso-NC6H2). Synthesis of 2-MeC6H4NHPPh2dNC6H2Me3-2,4,6 (H[NPN]3). Following a similar procedure, treatment of mesityl azide (0.97 g, 6.1 mmol, in 5 mL of THF) with 2-MeC6H4NHPPh2 (1.74 g, 6.0 mmol, in 30 mL of THF) yielded compound H[NPN]3 (1.96 g, 77%). 1H NMR (300 MHz, CDCl3, 25 °C): δ 2.00 (s, 6 H, oNC6H2Me3), 2.13 (s, 3 H, p-NC6H2Me3), 2.18 (s, 3 H, C6H4Me), 5.07 (br, 1 H, NH), 6.67 (s, 2 H, NC6H2), 6.72 (t, 3JH-H=7.2 Hz, 1 H, p-NC6H4), 6.84 (t, 3JH-H =7.2 Hz, 1 H, m-NC6H4), 7.06 (d, 3 JH-H = 7.2 Hz, 2 H, o,m-NC6H4), 7.32-7.38 (m, 4 H, mPC6H5),7.41-7.46 (m, 2 H, p-PC6H5), 7.71-7.78 (m, 4 H, oPC6H5). 13C NMR (75 MHz, C6D6, 25 °C): δ 18.16 (s, 1 C, C6H4Me), 20.66 (s, 2 C, o-NC6H2Me3), 21.03 (s, 1 C, pNC6H2Me3), 118.46 (Ar), 121.38 (Ar), 124.73 (Ar), 127.16 (Ar), 128.24 (Ar), 128.95 (Ar), 129.08 (Ar), 129.25 Ar), 130.79 (Ar), 132.04 (Ar), 132.28 (Ar), 132.40 (Ar), 133.67 (Ar), 140.06 (s, 1 C, ipso-NC6H4), 143.89 (s, 1 C, ipso-NC6H2). Synthesis of 2-EtC6H4NHPPh2dNC6H2Me3-2,4,6 (H[NPN]4). Following a similar procedure, treatment of mesityl azide (0.97 g, 6.1 mmol, in 5 mL of THF) with 2-EtC6H4NHPPh2 (1.83 g, 6.0 mmol, in 30 mL of THF) yielded compound H[NPN]4 (1.78 g, 68%). 1H NMR (300 MHz, CDCl3, 25 °C): δ 1.22 (t, 3JH-H = 7.5 Hz, 3 H, CH2CH3), 2.01 (s, 6 H, o-NC6H2Me3), 2.13 (s, 3 H, p-NC6H2Me3), 2.53 (quart, 3JH-H = 7.5 Hz, 2 H, CH2CH3), 5.21 (br, 1 H, NH), 6.67 (s, 2 H, NC6H2), 6.74-6.85 (m, 2 H, m,p-NC6H4), 7.08 (d, 3JH-H =7.2 Hz, 2 H, o,m-NC6H4), 7.34-7.46 (m, 4 H, o-NC6H4, m,pPC6H5), 7.71-7.78 (m, 4 H, o-PC6H5). 13C NMR (75 MHz,

4820


C6D6, 25 °C): δ 13.88 (s, 1 C, CH2CH3), 20.71 (s, 2 C, oNC6H2Me3), 21.07 (s, 1 C, p-NC6H2Me3), 24.82 (s, 1 C, CH2CH3), 118.94 (Ar), 121.41 (Ar), 126.92 (Ar), 128.78 (Ar), 129.10 (Ar), 129.27 (Ar), 132.07 (Ar), 132.29 (Ar), 132.41(Ar), 133.67 (Ar), 139.52 (s, 1 C, ipso-NC6H4), 143.86 (s, 1 C, ipsoNC6H2). Synthesis of 2,6-Et2C6H3NHPPh2dNC6H2Me3-2,4,6 (H[NPN]5). Following a similar procedure, treatment of mesityl azide (0.97 g, 6.1 mmol, in 5 mL of THF) with 2,6-Et2C6H3NHPPh2 (2.00 g, 6.0 mmol, in 30 mL of THF) yielded compound H[NPN]5 (1.45 g, 52%) .1H NMR (300 MHz, CDCl3, 25 °C): δ 0.88 (t, 3JH-H =7.2 Hz, 6 H, CH2CH3), 2.31 (quart, 3JH-H = 6.9 Hz, 4 H, CH2CH3), 1.97 (s, 6 H, o-C6H2Me3), 2.08 (s, 3 H, pC6H2Me3), 6.59 (s, 2 H, C6H2), 6.75-6.84 (m, 3 H, C6H3), 7.237.37 (m, 6 H, m,p-PC6H5), 7.61-7.67 (m, 4 H, o-PC6H5). 13C NMR (75 MHz, C6D6, 25 °C): δ 14.65 (s, 2 C, CH2CH3), 20.86 (s, 2 C, o-NC6H2Me3), 21.03 (s, 1 C, p-NC6H2Me3), 26.14 (s, 2 C, CH2CH3), 125.87 (Ar), 126.83 (Ar), 128.46 (d, 3 JP-C=12.8 Hz, 4 C, m-PC6H5), 128.68 (Ar), 128.85 (Ar), 129.42 (Ar), 130.16 (Ar), 131.68 (Ar), 132.36 (d, 2JP-C=9.6 Hz, 4C, oPC6H5), 134.83 (Ar), 138.97 (Ar). Synthesis of 2-EtC6H4NHPPh2dNC6H3Et2-2,6 (H[NPN]6). Following a similar procedure, treatment of 2,6-diethylphenylazide (1.07 g, 6.1 mmol, in 5 mL of THF) with 2EtC6H4NHPPh2 (1.83 g, 6.0 mmol, in 30 mL of THF) yielded compound H[NPN]6 (2.03 g, 75%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 0.98 (t, 3JH-H=7.6 Hz, 6 H, C6H3(CH2CH3)2), 1.29 (t, 3JH-H=7.6 Hz, 3 H, C6H4CH2CH3), 2.50-2.61 (m, 6 H, C6H3(CH2CH3)2, C6H4CH2CH3), 5.24 (br, 1 H, NH), 6.80 (t, 3 JH-H=7.2 Hz, 1 H, p-NC6H4), 6.84 (t, 3JH-H=7.2 Hz, 1 H, mNC6H4), 6.90 (t, 3JH-H=7.6 Hz, 1 H, p-NC6H3), 6.95 (d, 3JH-H = 7.6 Hz, 2 H, o-NC6H3), 7.14 (d, 3JH-H = 7.6 Hz, 1 H, oNC6H4), 7.18 (br, 1 H, m-NC6H4), 7.40-7.44 (m, 4 H, mPC6H5), 7.49-7.53 (m, 2 H, p-PC6H5), 7.75-7.80 (m, 4 H, oPC6H5). 13C NMR (75 MHz, C6D6, 25 °C): δ 13.77 (s, 1 C, CH2CH3), 14.94 (s, 2 C, C6H3(CH2CH3)2), 24.68 (s, 1 C, CH2CH3), 25.89 (s, 2 C, C6H3(CH2CH3)2)), 118.64 (Ar), 119.73 (Ar), 121.52 (Ar), 125.87 (Ar), 126.98 (Ar), 128.77 (Ar), 129.22 (d, 3JP-C=13.1 Hz, 4 C, m-PC6H5), 131.78 (Ar), 132.08 (Ar), 132.36 (d, 2JP-C = 9.6 Hz, 4 C, o-PC6H5), 133.53 (Ar), 138.15 (Ar), 139.43 (s, 1 C, ipso-NC6H4), 145.40 (s, 1 C, ipsoNC6H3). Synthesis of 2-EtC6H4NHPPh2dNC6H3iPr2-2,6 (H[NPN]7). Following a similar procedure, treatment of 2,6-diisopropylphenyl azide (1.24 g, 6.1 mmol, in 5 mL of THF) with 2EtC6H4NHPPh2 (1.83 g, 6.0 mmol, in 30 mL of THF) yielded compound H[NPN]7 (1.61 g, 56%). 1H NMR (300 MHz, CDCl3, 25 °C): δ 0.77 (d, 3JH-H = 6.6 Hz, 12 H, CH(CH3)2), 1.18 (t, 3JH-H=7.2 Hz, 3 H, CH2CH3), 2.47 (quart, 3JH-H=7.5 Hz, 2 H, CH2CH3), 3.26 (m, 3JH-H =6.9 Hz, 2 H, CH(CH3)2), 5.12 (br, 1 H, NH), 6.76-6.87 (m, 5 H,), 7.06 (d, 3JH-H=7.2 Hz, 1 H, o-NC6H4), 7.14 (br, 1 H, m-NC6H4), 7.34-7.45 (m, 6 H, m, p-PC6H5), 7.60-7.66 (m, 4 H, o-PC6H5). 13C NMR (75 MHz, C6D6, 25 °C): δ 13.66 (s, 1 C, CH2CH3), 24.12 (s, 4 C, CH(CH3)2), 24.57 (s, 1 C, CH2CH3), 28.48 (s, 2 C, CH(CH3)2), 118.29 (Ar), 120.24 (Ar), 121.39 (Ar), 123.08 (Ar), 126.95 (Ar), 128.60 (Ar), 128.74 (Ar), 129.00 (Ar), 129.09 (Ar), 129.28 (d, 3 JP-C=13.0 Hz, 4C, m-PC6H5), 129.54 (Ar), 131.33 (Ar), 131.45 (Ar), 131.72 (Ar), 132.10 (Ar), 132.38 (d, 2JP-C=9.4 Hz, 4C, oPC6H5), 133.07 (Ar), 139.56 (s, 1C, ipso-NC6H4), 142.90 (s, 1C, ipso-NC6H3). Synthesis of 2-MeC6H4NHPPh2dNC6H4Me-2 (H[NPN]8). Following a similar procedure, treatment of 2-methylphenylazide (0.82 g, 6.1 mmol) with 2-MeC6H4NHPPh2 (1.74 g, 6.0 mmol, in 30 mL of THF) yielded compound H[NPN]8 (1.32 g, 83%). 1H NMR (300 MHz, CDCl3, 25 °C): δ 2.29 (s, 6 H, C6H4Me), 6.68 (t, 3JH-H=6.6 Hz, 2 H, p-NC6H4), 6.80 (t, 3JH-H =7.2 Hz, 2 H, p-MeC6H4), 6.95 (br, 2 H, m-NC6H4), 7.05 (d, 3 JH-H = 7.2 Hz, 2 H, o-MeC6H4), 7.31-7.44 (m, 6 H, m,pPC6H5), 7.79-7.86 (m, 4 H, o-PC6H5). 13C NMR (75.5 MHz,

Li et al. CDCl3, 25 °C): δ 19.00 (s, 2 C, C6H4Me), 120.52 (s, 2 C, pNC6H4), 127.14 (Ar), 129.31 (d, 3Jp-c=12.9 Hz, 4 C, m-PC6H5), 130.64 (Ar), 131.63 (s, 2 C, p-PC6H5), 132.28 (d, 2Jp-c=9.4 Hz, 4 C, o-PC6H5), 133.37 (Ar). Synthesis of 2-FC6H4NHPPh2dNC6H2Me3-2,4,6 (H[NPN]10). Following a similar procedure, treatment of mesityl azide (0.97 g, 6.1 mmol, in 5 mL of THF) with 2-FC6H4NHPPh2 (1.77 g, 6.0 mmol, in 30 mL of THF) yielded compound H[NPN]10 (1.67 g, 65%). 1H NMR (300 MHz, CDCl3, 25 °C): δ 1.97 (s, 6 H, o-NC6H2Me3), 2.09 (s, 3 H, p-NC6H2Me3), 6.61(s, 2 H, NC6H2), 6.70 (t, 3JH-H=6.3 Hz, 1 H, o-NC6H4), 6.78 (t, 3JH-H=7.5 Hz, 1 H, p-NC6H4), 6.84 (t, 3JH-H =8.7 Hz, 1 H, m-NC6H4), 7.25 (t, JH-H=8.1 Hz, 1 H, m-NC6H4), 7.28-7.36 (m, 4 H, m-PC6H5), 7.42-7.47 (m, 2 H, o-NC6H2Me3). 13C NMR (75.5 MHz, CDCl3, 25 °C): δ 20.37 (s, 2 C, o-NC6H2Me3), 21.04 (s, 1 C, pNC6H2Me3), 115.42 (d, 2JC-F=19.9 Hz, o-FC6H4), 119.77 (s, 1 C, m-NC6H4), 122.43 (Ar), 124.92 (s, 2 C, m-NC6H2), 129.12 (d, 3 JP-C = 13.2 Hz, 4 C, m-PC6H5), 129.41 (Ar), 132.29 (d, 2 C, 1 JP-C=10.12 Hz, ipso-PC6H5), 132.73 (d, 2JP-C=9.6 Hz, 4 C, oPC6H5). Synthesis of [NPN]2Lu(CH2SiMe3)2(THF) (2). To a hexane solution (2.0 mL) of Lu(CH2SiMe3)3(THF)2 (0.233 g, 0.4 mmol) was added dropwise 1 equiv of H[NPN]2 (0.170 g, 0.4 mmol in 2 mL of THF) at room temperature. The mixture was stirred for 30 min at room temperature and then concentrated to about 0.5 mL. Addition of 1 mL of hexane and cooling to -30 °C for 2 days afforded crystalline solids, which were washed with a small amount of hexane to remove impurities and dried in vacuo to give pale yellow solids of 2 (0.240 g, 71%). Single crystals suitable for X-ray analysis were obtained from a mixture of THF/hexane at -30 °C within 2 d. 1H NMR (400 MHz, C6D6, 25 °C): δ -0.07 (br, 4 H, CH2SiMe3), 0.59 (s, 18H, SiMe3), 1.30 (br, 4 H, THF), 1.80 (s, 6H, o-NC6H2Me3), 2.14 (s, 3 H, C6H4Me), 2.20 (s, J = 2.4 Hz, 3 H, p-NC6H2Me3), 3.69 (br, 4 H, THF), 6.72 (s, 2 H, NC6H2), 7.04-7.12 (m, 8 H, o-NC6H4, m, p-PC6H5), 7.25 (d, 3JH-H=8 Hz, 2H, m-NC6H4), 7.79-7.84 (m, 4 H, o-PC6H5). 13C NMR (100 MHz, C6D6, 25 °C): δ 5.00 (s, 6 C, SiMe3), 20.82 (s, 2 C, o-NC6H2Me3), 21.02 (s, 1 C, C6H4Me), 21.12 (s, 1 C, p-NC6H2Me3), 25.73 (s, 2 C, THF), 44.31 (s, 2 C, CH2SiMe3), 69.45 (br, 2 C, THF), 122.26 (d, 3JP-C=15 Hz, 2 C, o-NC6H4), 129.02 (d, 3JP-C=11 Hz, 4 C, m-PC6H5), 129.72 (s, 2 C, m-NC6H2), 130.17 (s, 1 C, m-NC6H4Me), 132.00 (s, 2 C, pPC6H5), 132.85 (s, 1 C, p-NC6H4), 133.01 (d, 2JP-C=9 Hz, 4 C, o-PC6H5), 135.37 (d, 1JP-C=5 Hz, 2 C, ipso-PC6H5), 142.00 (s, 1 C, ipso-NC6H2), 146.06 (s, 1 C, ipso-NC6H4). Synthesis of [NPN]3Lu(CH2SiMe3)2(THF) (3). Following a similar procedure described for the synthesis of 2, treatment of Lu(CH2SiMe3)3(THF)2 (0.232 g, 0.4 mmol in 1.0 mL of hexane) with 1 equiv of H[NPN]3 (0.170 g, 0.4 mmol in 3 mL of THF) afforded complex 3 (0.267 g, 79%). Single crystals suitable for X-ray analysis were obtained from a mixture of THF/hexane at -30 °C within 2 d. 1H NMR (300 MHz, C6D6, 25 °C): δ -0.32 (br, 4 H, CH2SiMe3), 0.42 (s, 18 H, SiMe3), 0.98 (br, 4 H, THF), 1.62 (s, 6 H, o-NC6H2Me3), 2.10 (d, J = 2.4 Hz, 3 H, pNC6H2Me3), 3.51 (s, 3 H, C6H4Me), 3.64 (br, 4 H, THF), 6.37-7.40 (m, 1 H, o-NC6H4), 6.58 (s, 2 H, NC6H2), 6.666.69 (m, 2 H, m,p-NC6H4), 6.90-7.02 (m, 6 H, m,p-PC6H5), 7.22-7.24 (m, 1 H, m-NC6H4), 7.61-7.67 (m, 4 H, o-PC6H5). 13 C NMR (100 MHz, C6D6, 25 °C): δ 5.29 (s, 6 C, SiMe3), 20.76 (s, 2 C, o-NC6H2Me3), 21.10 (s, 1 C, p-NC6H2Me3), 25.52 (s, 2 C, THF), 42.50 (d, JF-C =15 Hz, 2 C, CH2SiMe3), 70.47 (br, 2 C, THF), 119.84 (s, 1 C, p-NC6H4), 121.94 (d, 3JP-C=12.1 Hz, 2 C, o-NC6H4), 126.48 (s, 1 C, p-MeC6H4), 128.99 (d, 3JP-C =11.3 Hz, 4 C, m-PC6H5), 129.76 (s, 2 C, m-NC6H2), 131.48 (s, 1 C, mNC6H4), 131.60 (s, 1 C, ipso-MeC6H4), 132.99 (s, 2 C, p-PC6H5), 132.60 (s, 2 C, o-NC6H2), 133.12 (d, 2JP-C = 8.9 Hz, 4 C, oPC6H5), 135.70 (d, 1Jp-c =5.4 Hz, 2 C, ipso-PC6H5), 141.76 (s, 1 C, ipso-NC6H2), 148.71 (s, 1 C, ipso-NC6H4). Synthesis of [NPN]4Lu(CH2SiMe3)2(THF) (4). Following a similar procedure described for the synthesis of 2, treatment of

Article Lu(CH2SiMe3)3(THF)2 (0.232 g, 0.4 mmol in 1.0 mL of hexane) with 1 equiv of H[NPN]4 (0.175 g, 0.4 mmol in 3 mL of THF) afforded complex 4 (0.288 g, 84%). Single crystals suitable for X-ray analysis were obtained from a mixture of THF/hexane at -30 °C within 2 d. 1H NMR (400 MHz, C6D6, 25 °C): δ -0.34 (br, 4 H, CH2SiMe3), 0.38 (s, 18 H, SiMe3), 0.92 (br, 4 H, THF), 1.64 (s, 6 H, o-NC6H2Me3), 1.78 (t, JH-H = 6.9 Hz, 3 H, CH2CH3), 2.11 (d, J = 2.4 Hz, 3 H, p-NC6H2Me3), 3.66 (br, 4 H, THF), 4.02 (quart, 3JH-H=7.2 Hz, 2 H, CH2CH3), 6.42 (d, 3 JH-H =8.1 Hz, 1 H, o-NC6H4), 6.59 (s, 2 H, NC6H2), 6.69 (t, 3 JH-H=7.2 Hz, 1 H, m-NC6H4), 6.78 (t, 3JH-H=7.2 Hz, 1 H, pNC6H4), 6.87-7.00 (m, 6 H, m,p-PC6H5), 7.34 (d, 3JH-H =7.2 Hz, 1 H, m-NC6H4), 7.60-7.67 (m, 4 H, o-PC6H5). 13C NMR (100 MHz, C6D6, 25 °C): δ 5.52 (s, 6 C, SiMe3), 12.65 (s, 1 C, CH2CH3), 20.91 (s, 2 C, o-NC6H2Me3), 21.13 (s, 1 C, pNC6H2Me3), 25.22 (s, 2 C, THF), 26.84 (s, 1 C, CH2CH3), 41.82 (s, 2 C, CH2SiMe3), 71.06 (s, 2 C, THF), 119.95 (s, 1 C, pNC6H4), 121.77 (d, 3JP-C=12.1 Hz, 1 C, o-NC6H4), 126.18 (s, 1 C, m-NC6H4), 127.09 (s, 1 C, m-NC6H4), 129.90 (d, 3JP-C =12 Hz, 4 C, m-PC6H5), 129.79 (s, 2 C, m-NC6H2), 131.65 (s, 1 C, pNC6H2), 131.99 (s, 2 C, p-PC6H5), 132.33 (s, 1 C, o-NC6H4), 133.06 (d, 2JP-C=9.0 Hz, 4 C, o-PC6H5), 134.54 (d, 1JP-C=17 Hz, 2C, ipso-PC6H5), 135.67 (s, 2 C, o-NC6H2), 141.91 (s, 1 C, ipso-NC6H2), 148.49 (s, 1 C, ipso-NC6H4). Synthesis of [NPN]5Lu(CH2SiMe3)2(THF) (5). Following a similar procedure described for the synthesis of 2, treatment of Lu(CH2SiMe3)3(THF)2 (0.232 g, 0.4 mmol in 1.0 mL of hexane) with 1 equiv of H[NPN]5 (0.186 g, 0.4 mmol in 3 mL of THF) afforded complex 5 (0.197 g, 56%). Single crystals suitable for X-ray analysis were obtained from a mixture of THF/hexane at -30 °C within 2 d. 1H NMR (400 MHz, C6D6, 25 °C): δ -0.21 (s, 4 H, CH2SiMe3), 0.45 (s, 18 H, SiMe3), 1.07 (t, JH-H=8 Hz, 6 H, CH2CH3), 1.15 (br, 4 H, THF), 2.26 (d, J = 4 Hz, 3 H, pNC6H2Me3), 2.38 (s, 6 H, o-NC6H2Me3), 2.98 (quart, 3JH-H=8 Hz, 4 H, CH2CH3), 3.77 (br, 4 H, THF), 6.92 (s, 2 H, NC6H2), 6.98-7.07 (m, 6 H, m,p-PC6H5), 7.11-7.15 (m, 1 H, p-NC6H3), 7.19 (d, 3JH-H=7.2 Hz, 2 H, m-NC6H3), 7.50-7.56 (m, 4 H, oPC6H5). 13C NMR (100 MHz, C6D6, 25 °C): δ 5.12 (s, 6 C, SiMe3), 14.57 (s, 2 C, CH2CH3), 21.16 (s, 1 C, p-NC6H2Me3), 22.30 (s, 2 C, o-NC6H2Me3), 25.19 (s, 2 C, THF), 26.54 (s, 2 C, CH2CH3), 44.47 (s, 2 C, CH2SiMe3), 71.06 (s, 2 C, THF), 123.87 (s, 1 C, p-NC6H3), 126.32 (s, 2 C, m-NC6H3), 128.29 (d, 3JP-C= 11 Hz, 4 C, m-PC6H5), 130.48 (s, 2 C, m-NC6H2), 131.50 (s, 2 C, p-PC6H5), 131.49 (s, 1 C, m-NC6H2), 132.51 (d, 2JP-C=8.0 Hz, 4 C, o-PC6H5), 135.56 (s, 2 C, ipso-PC6H5), 136.99 (s, 1 C, oNC6H2), 137.90 (s, 1 C, o-NC6H2), 141.04 (s, 2 C, o-NC6H3), 142.38 (s, 1 C, ipso-NC6H2), 144.58 (s, 1 C, ipso-NC6H3). Synthesis of [NPN]6Lu(CH2SiMe3)2(THF) (6). Following a similar procedure described for the synthesis of 2, treatment of Lu(CH2SiMe3)3(THF)2 (0.232 g, 0.4 mmol in 1.0 mL of hexane) with 1 equiv of H[NPN]6 (0.181 g, 0.4 mmol in 3 mL of THF) afforded complex 6 (0.268 g, 78%). Single crystals suitable for X-ray analysis were obtained from a mixture of THF/hexane at -30 °C within 2 d. 1H NMR (400 MHz, C6D6, 25 °C): δ -0.26 (br, 4 H, CH2SiMe3), 0.47 (s, 18 H, SiMe3), 0.78 (t, 3JH-H=6.9 Hz, 6 H, o-NC6H3(CH2CH3)2), 1.05 (br, 4 H, THF), 1.85 (t, 3 JH-H = 6.9 Hz, 3 H, o-NC6H4CH2CH3), 2.30 (br, 4 H, oNC6H3(CH2CH3)2), 3.4 (br, 4 H, THF), 4.09 (quart, 3JH-H=7.2 Hz, 2 H, NC6H4CH2CH3), 6.50 (d, 3JH-H = 8.0 Hz, 1 H, oNC6H4), 6.77 (t, 3JH-H=7.2 Hz, 1 H, m-NC6H4), 6.86 (t, 3JH-H =7.2 Hz, 1 H, p-NC6H4), 6.97-7.09 (m, 9 H, m,p-PC6H5, m,pNC6H3), 7.41 (d, 3JH-H=7.2 Hz, 1 H, m-NC6H4), 7.67-7.71 (m, 4 H, o-PC6H5). 13C NMR (100 MHz, C6D6, 25 °C): δ 5.20 (s, 6 C, SiMe3), 12.65 (s, 1 C, o-NC6H4CH2CH3), 13.94 (s, 2 C, oNC6H3(CH2CH3)2), 25.41 (s, 2 C, o-NC6H3(CH2CH3)2), 25.22 (s, 2 C, THF), 26.76 (s, 1 C, o-NC6H4CH2CH3), 41.76 (s, 2 C, CH2SiMe3), 71.20 (s, 2 C, THF), 120.16 (s, 1 C, p-NC6H4), 121.74 (d, 3JP-C = 12.1 Hz, 1 C, o-NC6H4), 123.90 (s, 1 C, pNC6H3), 125.47 (s, 2 C, m-NC6H3), 126.16 (s, 1 C, m-NC6H4), 127.14 (s, 1 C, m-NC6H4), 128.92 (d, 3JP-C = 12 Hz, 4 C, m-


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PC6H5), 131.39 (s, 1 C, o-NC6H4), 131.99 (s, 2 C, p-PC6H5), 133.17 (d, 2JP-C =9.0 Hz, 4 C, o-PC6H5), 134.74 (d, 1Jp-c =17 Hz, 2 C, ipso-PC6H5), 140.93 (d, 3JP-C=6.0 Hz, 2 C, o-NC6H3), 143.68 (s, 1 C, ipso-NC6H3), 148.28 (s, 1 C, ipso-NC6H4). Synthesis of [NPN]7Lu(CH2SiMe3)2(THF) (7a). Following a similar procedure described for the synthesis of 2, treatment of Lu(CH2SiMe3)3(THF)2 (0.232 g, 0.4 mmol in 1.0 mL hexane) with 1 equiv of H[NPN]7 (0.192 g, 0.4 mmol in 3 mL of THF) afforded complex 7a (0.29 g, 81%). 1H NMR (300 MHz, C6D6, 25 °C): δ -0.39 (s, 4 H, CH2SiMe3), 0.31-0.38 (m, 24 H, SiMe3, CH(CH3)2), 1.13-1.17 (m, 10 H, THF, CH(CH3)2), 1.67 (t, 3 JH-H =7.2 Hz, 3 H, CH2CH3), 3.48 (m, JH-H =6.6 Hz, 2 H, CH(CH3)2), 3.70 (br, 4 H, THF), 3.88 (quart, 3JH-H=7.2 Hz, 2 H, CH2CH3), 6.45 (d, 3JH-H =8.1 Hz, 1 H, o-NC6H4), 6.69 (t, 3 JH-H=7.2 Hz, 1 H, m-NC6H4), 6.78 (t, 3JH-H=7.2 Hz, 1 H, pNC6H4), 6.89-7.06 (m, 9 H, m,p-PC6H5, m,p-NC6H3,), 7.31 (d, 3 JH-H=7.2 Hz, 1 H, m-NC6H4), 7.51-7.58 (m, 4 H, o-PC6H5). 13 C NMR (75.5 MHz, C6D6, 25 °C): δ 4.69 (s, 6 C, SiMe3), 13.04 (s, 1 C, o-NC6H4CH2CH3), 23.81(s, 2 C, THF), 25.04 (s, 4 C, oNC6H3CH(CH3)2), 25.88 (s, 1 C, o-NC6H4CH2CH3), 28.63 (s, 2 C, NC6H3CH(CH3)2), 41.16 (s, 2 C, CH2SiMe3), 70.22 (s, 2 C, THF), 121.29 (s, 1 C, p-NC6H4), 123.02 (d, 3JP-C=12.1 Hz, 1 C, o-NC6H4), 124.36 (s, 1 C, p-NC6H3), 124.66 (s, 2 C, m-NC6H3), 126.14 (s, 1 C, m-NC6H4), 127.69 (s, 1 C, m-NC6H4), 129.02 (d, 3 JP-C=12 Hz, 4 C, m-PC6H5), 131.73 (s, 1 C, m-NC6H4), 131.97 (s, 2 C, p-PC6H5), 133.58 (d, 2JP-C = 9.0 Hz, 4 C, o-PC6H5), 137.01 (d, 1Jp-c = 17 Hz, 2 C, ipso-PC6H5), 138.34 (s, 2 C, oNC6H3), 146.06 (s, 1 C, ipso-NC6H3), 146.33 (s, 1 C, ipsoNC6H4). Anal. Calcd (%) for C44H66N2OPSi2Lu: C 58.65, H 7.38, N 3.11. Found: C 58.51, H 7.31, N 3.04. Synthesis of [NPN]7Y(CH2SiMe3)2(THF) (7b). Following a similar procedure described for the synthesis of 2, treatment of Y(CH2SiMe3)3(THF)2 (0.198 g, 0.4 mmol in 1.0 mL of hexane) with 1 equiv of H[NPN]7 (0.192 g, 0.4 mmol in 3 mL of THF) afforded complex 7b (0.24 g, 73%). 1H NMR (300 MHz, C6D6, 25 °C): δ -0.29 (d, JY-H=2.7 Hz, 4 H, CH2SiMe3), 0.28-0.41 (m, 24 H, SiMe3, CH(CH3)2), 1.12-1.17 (m, 10 H, THF, CH(CH3)2), 1.76 (t, 3JH-H=7.2 Hz, 3 H, CH2CH3), 3.34 (m, 3JH-H = 6.6 Hz, 2 H, CH(CH3)2), 3.67 (br, 4 H, THF), 4.04 (quart, 3 JH-H=6.9 Hz, 2 H, CH2CH3), 6.36 (d, 3JH-H=7.8 Hz, 1 H, oNC6H4), 6.65 (t, 3JH-H=7.2 Hz, 1 H, m-NC6H4), 6.74 (t, 3JH-H =7.2 Hz, 1 H, p-NC6H4), 6.93-7.02 (m, 9 H, m,p-PC6H5, m,pNC6H3,), 7.29 (d, 3JH-H=7.2 Hz, 1 H, m-NC6H4), 7.561-7.63 (m, 4 H, o-PC6H5). 13C NMR (75.5 MHz, C6D6, 25 °C): δ 5.01 (s, 6 C, SiMe3), 12.53 (s, 1 C, o-NC6H4CH2CH3), 23.68 (s, 2 C, THF), 25.14 (s, 4 C, o-NC6H3CH(CH3)2), 25.86 (s, 2 C, oNC6H3CH(CH3)2), 25.86 (s, 1 C, o-NC6H4CHCH3), 33.36 (d, JY-C=39 Hz, 2 C, CH2SiMe3), 70.19 (s, 2 C, THF), 120.01 (s, 1 C, p-NC6H4), 121.15 (d, 3JP-C =12 Hz, 1 C, o-NC6H4), 123.70 (s, 1 C, p-NC6H3), 125.79 (s, 2 C, m-NC6H3), 126.27 (s, 1 C, mNC6H4), 127.76 (s, 1 C, m-NC6H4), 128.93 (d, 3JP-C=12 Hz, 4 C, m-PC6H5), 131.04 (s, 1 C, o-NC6H4), 131.96 (s, 2 C, pPC6H5), 133.12 (d, 3JP-C = 9 Hz, 4 C, o-PC6H5), 134.45 (d, 1 JP-C=19 Hz, 2 C, ipso-PC6H5), 141.31 (d, 3JP-C=6 Hz, 2 C, oNC6H3), 145.92 (s, 1 C, ipso-NC6H3), 147.42 (s, 1 C, ipsoNC6H4). Anal. Calcd (%) for C44H66N2OPSi2Y: C 64.84, H 8.16, N 3.44. Found: C 64.92, H 8.09, N 3.20. Synthesis of [NPN]7Sc(CH2SiMe3)2(THF) (7c). Following a similar procedure described for the synthesis of 2, treatment of Sc(CH2SiMe3)3(THF)2 (0.180 g, 0.4 mmol in 1.0 mL of hexane) with 1 equiv of H[NPN]7 (0.192 g, 0.4 mmol in 3 mL of THF) afforded complex 7c (0.23 g, 76%). 1H NMR (400 MHz, C6D6, 25 °C): δ 0.45 (s, 18 H, CH2SiMe3), 0.52 (s, 4 H, CH2SiMe3), 0.98-1.02 (m, 9 H, CH(CH3)2)), 1.35 (m, 7 H, THF, CH(CH3)2), 1.67 (t, 3JH-H = 7.6 Hz, 3 H, CH2CH3), 3.71-3.75 (m, 4 H, CH(CH3)2, CH2CH3), 3.98 (br, 4 H, THF), 6.77 (d, 3 JH-H=8.0 Hz, 1 H, o-NC6H4), 6.87 (t, 3JH-H=7.2 Hz, 1 H, mNC6H4), 6.99-7.18 (m, 10 H, m,p-PC6H5, m,p-NC6H3, pNC6H4), 7.47 (d, 3JH-H = 7.6 Hz, 1 H, m-NC6H4), 7.52-7.57 (m, 4 H, o-PC6H5). 13C NMR (100 MHz, C6D6, 25 °C): δ 4.65 (s,

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6 C, SiMe3), 14.29 (s, 4 C, o-NC6H3CH(CH3)2), 14.64 (s, 1 C, oNC6H4CH2CH3), 25.34 (s, 2 C, o-NC6H3CH(CH3)2), 25.80 (s, 2 C, THF), 26.64 (s, 1 C, o-NC6H4CHCH3), 42.66 (s, 2 C, CH2SiMe3), 71.50 (s, 2 C, THF), 122.13 (s, 1 C, p-NC6H4), 123.03 (d, 3JP-C = 10 Hz, 1 C, o-NC6H4), 12413 (s, 1 C, pNC6H3), 126.15 (s, 2 C, m-NC6H3), 126.34 (s, 1 C, m-NC6H4), 128.71 (d, 3JP-C = 12 Hz, 4 C, m-PC6H5), 131.98 (s, 2 C, pPC6H5), 132.85 (s, 1 C, m-NC6H4), 133.77 (s, 1 C, o-NC6H4), 133.22 (d, 3JP-C=9 Hz, 4 C, o-PC6H5), 137.78 (d, 1Jp-c=19 Hz, 2 C, ipso-PC6H5), 140.97 (d, 3JP-C = 6 Hz, 2 C, o-NC6H3), 143.79 (s, 1 C, ipso-NC6H3), 146.84 (s, 1 C, ipso-NC6H4). Anal. Calcd (%) for C44H66N2OPSi2Sc: C 68.53, H 8.63, N 3.63. Found: C 68.82, H 8.60, N 3.55. Synthesis of [NPN]7Er(CH2SiMe3)2(THF) (7d). Following a similar procedure described for the synthesis of 2, treatment of Er(CH2SiMe3)3(THF)2 (0.229 g, 0.4 mmol in 1.0 mL of hexane) with 1 equiv of H[NPN]7 (0.192 g, 0.4 mmol in 3 mL of THF) afforded complex 7d (0.29 g, 83%). Anal. Calcd (%) for C44H66N2OPSi2Er: C 59.15, H 7.45, N 3.14. Found: C 58.17, H 7.55, N 3.10. Synthesis of [NPN]8Lu(CH2SiMe3)2(THF) (8). To a hexane solution (2.0 mL) of Lu(CH2SiMe3)3(THF)2 (0.233 g, 0.4 mmol) was added dropwise 1 equiv of H[NPN]8 (0.158 g, 0.4 mmol in 2 mL of THF) at room temperature. The mixture was stirred for 30 min at room temperature and then concentrated to about 0.5 mL. Addition of 1 mL of hexane and cooling to -30 °C for 2 days afforded crystalline solids, which were washed with a small amount of hexane to remove impurities and dried in vacuo to give pale yellow solids of 8 (0.268 g, 82%). 1H NMR (300 MHz, C6D6, 25 °C): δ -0.36 (s, 4 H, CH2SiMe3), 0.39 (s, 18 H, SiMe3), 1.09 (br, 4 H, THF), 2.52 (br, 6 H, C6H4Me), 3.64 (br, 4 H, THF), 6.54-6.58 (m, 2 H, o-NC6H4), 6.68-6.74 (m, 4 H, m,pNC6H4), 6.68-6.74 (m, 6 H, m,p-PC6H5, m-NC6H4), 6.91-7.02 (m, 4 H, o-PC6H5). 13C NMR (100 MHz, C6D6, 25 °C): δ 5.28 (s, 6 C, SiMe3), 21.42 (s, 2 C, o-NC6H4Me), 25.68 (s, 2 C, THF), 42.19 (s, 2 C, CH2SiMe3), 69.98 (s, 2 C, THF), 121.94 (s, 2 C, pNC6H4), 124.72 (br, 2 C, o-NC6H4), 126.82 (s, 2 C, m-NC6H4), 129.01 (d, 3JP-C = 11 Hz, 4 C, m-PC6H5), 131.38 (s, 2 C, pPC6H5), 132.34 (s, 2 C, m-NC6H4), 133.18 (d, 2JP-C=9 Hz, 4 C, o-PC6H5), 147.17 (s, 2 C, ipso-NC6H4). Synthesis of [NPN]9Lu(CH2SiMe3)2(THF) (9a). To a hexane solution (2.0 mL) of Lu(CH2SiMe3)3(THF)2 (0.233 g, 0.4 mmol) was added dropwise 1 equiv of H[NPN]9 (0.164 g, 0.4 mmol in 2 mL of toluene) at room temperature. The mixture was stirred for 30 min at room temperature and then concentrated to about 0.5 mL. Addition of 3 mL of hexane and cooling to -30 °C for 2 days afforded colorless crystalline solids that were washed with a small amount of hexane to remove impurities and dried under vacuum to give pale yellow solids of 9a (0.216 g, 65%). 1H NMR (400 MHz, C6D6, 25 °C): δ -0.15 (s, 4 H, CH2SiMe3), 0.57 (s, 18 H, SiMe3), 1.22 (br, 4 H, THF), 1.95 (s, 6 H, o-NC6H2Me3), 2.23 (d, JP-H=2.8 Hz, 3 H, p-NC6H2Me3), 3.84 (br, 4 H, THF), 6.36 (d, 3JH-H=8.0 Hz, 1 H, o-NC5H5N), 6.44 (t, 3JH-H=6.0 Hz, 1 H, p-NC5H5N), 6.75 (s, 2 H, C6H2), 7.01 (t, 3JH-H=6.4 Hz, 1 H, m-NC5H5N), 7.04-7.13 (m, 6 H, m,p-PC6H5), 7.70-7.75 (m, 4 H, o-PC6H5), 8.41 (d, 3JH-H = 6.0 Hz, 1 H, m-NC5H5N). 13C NMR (100 MHz, C6D6, 25 °C): δ 5.32 (s, 6 C, SiMe3), 21.15 (s, 2 C, o-NC6H2Me3), 21.34 (s, 1 C, p-NC6H2Me3), 25.78 (s, 2 C, THF), 42.42 (s, 2 C, CH2SiMe3), 69.69 (s, 2 C, THF), 114.93 (s, 1 C, o-C5H5N), 115.92 (s, 1 C, p-C5H5N), 128.80 (d, 3JP-C=12.7 Hz, 4 C, m-PC6H5), 129.66 (s, 2 C, m-NC6H2), 131.56 (s, 1 C, pNC6H2), 131.85 (s, 2 C, p-PC6H5), 132.53 (d, 2JP-C=9.5 Hz, 4 C, o-PC6H5), 135.44 (d, 1Jp-c=5.7 Hz, 2 C, ipso-PC6H5), 138.83 (s, 1 C, m-C5H5N), 142.73 (s, 1 C, ipso-NC6H2), 148.35 (s, 1 C, mC5H5N), 161.59 (s, 1 C, ipso-NC5H5N). Synthesis of [NPN]9Y(CH2SiMe3)2(THF) (9b). To a hexane solution (2.0 mL) of Y(CH2SiMe3)3(THF)2 (0.198 g, 0.4 mmol) was added dropwise 1 equiv of H[NPN]9 (0.164 g, 0.4 mmol in

Li et al. 2 mL of toluene) at room temperature. The mixture was stirred for 30 min at room temperature and then concentrated to about 0.5 mL. Addition of 3 mL of hexane and cooling to -30 °C for 2 d afforded colorless crystalline solids, which were washed with a small amount of hexane to remove impurities and dried in vacuo to give pale yellow solids of 9b (0.209 g, 70%). 1H NMR (400 MHz, C6D6, 25 °C): δ -0.05 (d, JY-H=4 Hz, 4 H, CH2SiMe3), 0.54 (s, 18 H, SiMe3), 1.30 (br, 4 H, THF), 2.00 (s, 6 H, o-NC6H2Me3), 2.23 (d, J = 2.8 Hz, 3 H, p-NC6H2Me3), 3.81 (br, 4 H, THF), 5.82 (d, 3JH-H=8.0 Hz, 1 H, o-NC5H5N), 6.45 (t, 3JH-H=6.0 Hz, 1 H, p-NC5H5N), 6.73 (s, 2 H, C6H2), 6.87 (t, 3 JH-H = 7.6 Hz, 1 H, m-NC5H5N), 7.06-7.14 (m, 6 H, m,pPC6H5), 7.60-7.65 (m, 4 H, o-PC6H5), 8.57 (d, 3JH-H=4.8 Hz, 1 H, m-NC5H5N). 13C NMR (100 MHz, C6D6, 25 °C): δ 5.21 (s, 6 C, SiMe3), 21.23 (s, 2 C, o-NC6H2Me3), 21.33 (s, 1 C, pNC6H2Me3), 25.60 (s, 2 C, THF), 34.80 (d, 1JY-C = 38 Hz, 2 C, CH2SiMe3), 70.15 (s, 2 C, THF), 114.95 (s, 1 C, o-C5H5N), 116.14 (s, 1 C, p-C5H5N), 128.91 (d, 3JP-C = 12 Hz, 4 C, mPC6H5, 129.70 (s, 2 C, m-NC6H2), 131.54 (s, 1 C, p-NC6H2), 131.93 (s, 2 C, p-PC6H5), 132.21 (d, 2JP-C = 10 Hz, 4 C, oPC6H5), 135.18 (d, 1Jp-c=6 Hz, 2 C, ipso-PC6H5), 139.87 (s, 1 C, m-NC5H5N), 143.28 (s, 1 C, ipso-NC6H2), 148.38 (s, 1 C, mNC5H5N), 162.53 (s, 1 C, ipso-NC5H5N). Synthesis of [NPN]10Lu(CH2SiMe3)2(THF) (10). Following a similar procedure described previously, treatment of Lu(CH2SiMe3)3(THF)2 (0.232 g, 0.4 mmol in 1.0 mL of hexane) with 1 equiv of H[NPN]10 (0.171 g, 0.4 mmol in 2 mL of THF) afforded complex 10 (0.244 g, 72%). 1H NMR (400 MHz, C6D6, 25 °C): δ -0.30, -0.40 (s, 4 H, CH2SiMe3), 0.62 (s, 18 H, SiMe3), 1.08 (br, 4 H, THF), 1.82 (s, 6 H, o-NC6H2Me3), 2.20 (d, J=2.4 Hz, 3 H, p-NC6H2Me3), 3.78 (br, 4 H, THF), 6.30 (t, 3JH-H=8.4 Hz, 1 H, o-NC6H4), 6.48-6.52 (m, 1 H, p-NC6H4), 6.57-6.59 (m, 1 H, p-FC6H4) 6.70 (s, 2 H, NC6H2), 6.99-7.04 (m, 4 H, mPC6H5), 7.06-7.08 (m, 2 H, p-PC6H5), 7.18 (t, JH-H=8.4 Hz, 1 H, m-NC6H4), 7.67-7.72 (m, 4 H, o-PC6H5). 13C NMR (100 MHz, C6D6, 25 °C): δ 5.26 (s, 6 C, SiMe3), 20.94 (s, 2 C, oNC6H2Me3), 21.14 (s, 1 C, p-NC6H2Me3), 25.37 (s, 2 C, THF), 41.58 (d, JF-C =15 Hz, 2 C, CH2SiMe3), 71.06 (br, 2 C, THF), 115.56 (d, JF-C=20 Hz, 1 C, m-NC6H4), 119.33 (d, JF-C=3 Hz, 1 C, p-NC6H4), 121.01 (d, JF-C=8 Hz, 1 C, o-NC6H4), 125.23 (s, 2 C, m-NC6H2), 129.13 (d, JP-C=12 Hz, 4 C, m-PC6H5), 129.73 (s, 2 C, m-NC6H2), 131.32 (s, 1 C, p-NC6H2), 132.24 (s, 2 C, pPC6H5), 132.72 (d, JP-C=10 Hz, 4 C, o-PC6H5), 135.52 (d, JP-C =6 Hz, 2 C, ipso-PC6H5), 137.63 (d, JF-C =11 Hz, 1 C, ipsoNC6H4), 141.94 (s, 1 C, ipso-NC6H2), 157.91 (d, JF-C =18 Hz, 1 C, ipso-FC6H4). Isoprene Polymerization. Under a N2 atmosphere, isoprene (1.0 mL, 10 mmol), complex 7c (9.0 mg, 10 μmol), and toluene (3 mL) were added into a 25 mL flask containing a magnetic stir bar. A toluene solution (2 mL) of [Ph3C][B(C6F5)4] (9.6 mg, 10 μmol) was charged into the flask. Then a hexane solution of AliBu3 (100 μmol, 0.2 mL 0.5 M) was added to initiate the polymerization. After 15 min, methanol was injected to terminate the polymerization. The reaction mixture was poured into a large quantity of ethanol to precipitate the white solids of polyisoprene, which were collected by filtration and dried under vacuum at ambient temperature to a constant weight (100%).

Acknowledgment. We thank for financial support The National Natural Science Foundation of China for project No. 20674081, The Ministry of Science and Technology of China for project No. 2005CB623802, and “Hundred Talent Program” of the Chinese Academy of Sciences. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.

Highly 3,4-Selective Polymerization of Isoprene with NPN Ligand

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