ARTICLE pubs.acs.org/Organometallics
Half-Sandwich Scandium Bis(amide) Complexes as Efficient Catalyst Precursors for Syndiospecific Polymerization of Styrene Yunjie Luo,*,†,‡ Xiaoying Feng,† Yibin Wang,§ Shimin Fan,† Jue Chen,† Yinlin Lei,† and Hongze Liang§ †
Organometallic Chemistry Laboratory, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, People's Republic of China ‡ Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People's Republic of China § Faculty of Materials Science & Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, People's Republic of China
bS Supporting Information ABSTRACT: Amine elimination reactions of the scandium tris(amide) complexes Sc[N(SiRMe2)3](THF)n (R = H, n = 1; R = Me, n = 0) with 1 equiv of the substituted cyclopentadienes C5Me4R0 H (R0 = Me, SiMe3) afforded the series of thermally stable half-sandwich scandium bis(amide) complexes (C5Me5)Sc[N(SiHMe2)2]2 (1), (C5Me4SiMe3)Sc[N(SiHMe2)2]2 (2), and (C5Me5)Sc[N(SiMe3)2]2 (3). In the presence of AliBu3, activated by an equimolar amount of [Ph3C][B(C6F5)4], all these complexes showed high activity toward styrene polymerization to give high-molecular-weight polystyrene (Mn > 104) with high syndiotacticity (rrrr >99%).
S
yndiotactic polystyrene (sPS) is a very promising new polymer material for a large number of applications in industry, because of its high melting point (ca. 270 °C), high crystallinity, high modulus of elasticity, low dielectric constant, and excellent resistance to heat and chemicals.1 The original publication concerning the syndiospecific polymerization of styrene was reported by Ishihara and co-workers at Idemitsu in 1986 by using the homogeneous titanium-based catalyst system CpTiCl3MAO.2 Although numerous efforts have also been devoted to explore rare-earth-metal catalysts for styrene polymerization,312 the first effective neutral rare-earth-metal complexes [Flu-CMe2Cp]Ln(C3H5)(THF)13 and the first cationic rare-earth-metal catalyst system (C5Me4SiMe3)Ln(CH2SiMe3)2(THF)/[Ph3C][B(C6F5)4]14 for styrene polymerization to afford pure sPS were disclosed independently by Carpentier and Hou in 2005. Encouraged by these achievements, intensive attention is still growing to establish rare-earth-metal catalysts for highly stereospecific polymerization of styrene,1524 and it has been shown that cationic rare-earth-metal complexes containing LnC σ bonds [LLn-R]þ (L = monoanionic ancillary ligands, R = alkyl or allyl groups) usually demonstrated high efficiency and unique performance. However, most of these reported catalyst precursors are generally rather difficult to handle and to store due to their ligand redistribution or thermal instability. From this viewpoint, it is of great interest to explore less sensitive precatalyst candidates. Very recently, Bonnet communicated the half-sandwich scandium bis(borohydride) complex (C5Me5)Sc(BH4)2,25,26 and Hou reported that rare-earth-metal bis(aminobenzyl) complexes27,28 could serve as less sensitive precatalysts for the syndiospecific polymerization of styrene. In contrast, however, rare-earth-metal amide complexes, whose structural features are more versatile and whose thermal stability r 2011 American Chemical Society
is much better than that of their alkyl counterparts,29 have not been employed as catalysts for styrene polymerization to our knowledge. Herein, we report the synthesis and characterization of a new family of half-sandwich scandium bis(amide) complexes, as well as their performance as highly active catalyst precursors for syndiospecific polymerization of styrene.
’ RESULTS AND DISCUSSION Amine elimination reactions of the scandium tris(amide) complexes Sc[N(SiRMe2)3](THF)n (R = H, n = 1; R = Me, n = 0) with 1 equiv of the substituted cyclopentadienes C5Me4R0 H (R0 = Me, SiMe3) in toluene at 100 °C, after workup, gave a series of the half-sandwich scandium bis(amide) complexes (C5Me5)Sc[N(SiHMe2)2]2 (1), (C5Me4SiMe3)Sc[N(SiHMe2)2]2 (2), and (C5Me5)Sc[N(SiMe3)2]2 (3) as colorless crystals in 7281% isolated yields, as shown in Scheme 1. These complexes have perfect solubility in common solvents, even in aliphatic solvents such as hexane at room temperature, and are quite thermally stable at ambient temperature in the glovebox. Elemental analysis and NMR spectroscopy study confirmed the composition of these complexes and revealed that they are neutral, mononuclear, and solvent-free species. In the FT-IR spectra of complexes 1 and 2, there are two strong and wellresolved ν(SiH) bands at 2081 (complex 1), 2111 (complex 2), 1863 (complex 1), and 1869 cm1 (complex 2), showing the asymmetrical coordination of silylamide ligands to the center metal. At the same time, the low-energy frequencies indicate strong ScHSi β-agostic interactions in the solid state. Roomtemperature 1H NMR spectra in C6D6 demonstrated that the Received: November 6, 2010 Published: May 31, 2011 3270
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Scheme 1. Preparation of the Half-Sandwich Scandium Bis(amide) Complexes 13
methyl resonances on the Cp rings in complexes 1 and 3 showed only one singlet peak, while those in complex 2 exhibited two singlet peaks, indicating highly fluxional Cp ligands in solution. However, the methyl resonances of the N(SiHMe2)2 groups displayed well-resolved doublet signals at 0.26 (complex 1) and 0.25 ppm (complex 2) due to ScH coupling (JScH = 20.8 Hz). Single crystals of complexes 1 and 3 suitable for X-ray diffraction were grown from a hexane solutions at 30 °C. The molecular structures of 1 and 3 are shown in Figures 1 and 2, respectively. As proved by IR and NMR, the central metal Sc3þ in complex 1 is five-coordinated by one Cp ring in an η5 fashion, two nitrogen atoms from two amide groups, and two agostic hydrogen atoms from SiH to form a distorted-trigonal-bipyramidal geometry, if the Cp ring is regarded as occupying an independent vertex. In comparison, complex 3 is a symmetric molecule, in which the Sc3þ is three-coordinated by one Cp ring and two amide groups to form a trigonal geometry. The ScN distances are 2.082(3) and 2.079(2) Å in complex 1 and 2.086(5) Å in complex 2, which are consistent with the CeN bond distances in (C5Me5)Ce[N(SiMe3)2]2 (2.357(7) and 2.349(7) Å),30 if the ionic radius is considered.31 To assess the polymerization behavior of rare-earth-metal bis(amide) complexes, these half-sandwich scandium bis(amide) complexes were employed as catalyst precursors for styrene polymerization. The neutral complexes 13 alone showed no activity toward styrene polymerization, nor did the two-component catalyst systems formed from the neutral complex/AlR3.32 Therefore, these neutral complexes were treated with 1 equiv of [Ph3C][B(C6F5)4], to generate the corresponding cationic rare-earth-metal amide species [LScNR2]þ (L = monoanionic Cp ligands, NR2 = amide groups). 1H NMR monitoring techniques showed that addition of 1 equiv of [Ph3C][B(C6F5)4] in C6D5Cl to complex 1 produced instantly a cationic scandium species identical with [(C5Me5)ScN(SiHMe2)2]þ,33 along with the release of free Ph3CN(SiHMe2)2 and small amount of Ph3CH, indicating that the protonolysis reaction was selective. Unfortunately, the cationic scandium amide species [(C5Me5)ScN(SiHMe2)2]þ formed in situ showed rather poor activity toward styrene polymerization and afforded only atactic polystyrene. However, in the presence of an excess amount of AliBu3, activated with 1 equiv of [Ph3C][B(C6F5)4], complexes 13 exhibited a significant improvement in styrene polymerization performance, in both activity and stereospecificity. On the basis of these facts, we deduced that the true active species for syndiospecific styrene polymerization in the present polymerization systems might be a ScAl heterometallic complex, such as [(CpSc {(μ-R)2AlR2}]þ, as postulated by Anwander and co-workers.34,35 Therefore, the ternary catalyst systems composed of complexes 13/AliBu3/[Ph3C][B(C6F5)4] were employed for
Figure 1. Molecular structure of complex 1.
Figure 2. Molecular structure of complex 3.
styrene polymerization; some representative results are summarized in Table 1. These catalyst systems showed high activity toward styrene polymerization with an active trend of complex 1 > complex 2 > complex 3 (runs 4, 9, and 12). The activity is much 3271
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Table 1. Styrene Polymerization by 13/AliBu3/[Ph3C][B(C6F5)4]a
run
complex
[M]/[Sc]
[Al]/[Sc]
t/min
1 2
1 1
500 500
10
180 180
yieldb/%
amt of sPSc (%)
104Mnd
Mw/Mnd
Tme/°C
3
1
500
180
67
0
4
1
500
10
2
100
100
10.6
2.44
269
5
1
500
5
5
70
100
15.3
1.71
270 269
6
1
500
20
99%, determined by NMR in 1,1,2,2-C2D2Cl4. d Determined by GPC in 1,2,4-trichlorobenzene at 145 °C against polystyrene standard. e Determined by DSC. a
higher than that with (C5Me5)Sc(BH4)2(THF)/AliBu3/[Ph3C][B(C6F5)4] as a catalyst system25 and is comparable with that using (C5Me4SiMe3)Sc(CH2SiMe3)2(THF)/[Ph3C][B(C6F5)4] as a catalyst system for syndiospecific polymerization of styrene.14 It is noteworthy that, with the catalyst system complex 1/AliBu3/ [Ph3C][B(C6F5)4], the increase of styrene to scandium molar ratios from 250 to 1000 resulted in a proportional increase of the molecular weights from 9.8 104 to 34.8 104, while the molecular weight distributions stayed nearly constant (runs 811), suggesting a controllable polymerization nature. The polymerization activity increased with the amount of AliBu3; however, increased AliBu3 addition resulted in lower molecular weights and broader molecular weight distributions. This might be ascribed to a chain transfer reaction (runs 57). GPC curves indicated that all the polystyrene samples produced by these ternary catalyst systems are unimodal, indicative of single-site polymerization behavior. Remarkably, in the present polymerization, neither atactic nor isotactic polystyrene was observed from NMR spectra of the resulting crude polymers. Therefore, solvent fractionation was not required to obtain pure sPS (rrrr >99%).
’ CONCLUSION In summary, a new series of neutral and unsolvated halfsandwich scandium bis(amide) complexes were prepared and well characterized. These complexes can serve as thermally stable precatalysts for styrene polymerization. The ternary catalyst systems of the half-sandwich scandium bis(amide) complex/ AliBu3/[Ph3C][B(C6F5)4] showed high activity toward styrene polymerization to give high-molecular-weight polymers (Mn > 104) with high syndiotacticity (rrrr >99%). This is the first example of styrene polymerization promoted by rare-earth-metal amide complexes as catalyst precursors with both high activity and high selectivity to our knowledge. ’ EXPERIMENTAL SECTION All manipulations were performed under pure argon with rigorous exclusion of air and moisture using standard Schlenk techniques and an
argon-filled glovebox. Solvents (toluene, hexane, and THF) were distilled from sodium/benzophenone ketyl, degassed by the freeze pumpthaw method, and dried over fresh Na chips in the glovebox. Anhydrous ScCl3, [Ph3C][B(C6F5)4], and C5Me5H were purchased from Strem. AlMe3 (1.0 M in heptane solution), Al(iBu)3 (1.0 M in toluene solution), HN(SiHMe2)2, HN(SiMe3)2, and n-BuLi (1.0 M in hexane solution) were purchased from Acros. Styrene was dried by stirring with CaH2 and distilled before polymerization. C5Me4HSiMe3 and deuterated solvents (C6D6, C6D5Cl, 1,1,2,2-C2D2Cl4) were obtained from Aldrich. Sc[N(SiMe3)2]336 and Sc[N(SiHMe2)2]3(THF)37 were prepared according to the literature. Samples of organo rare-earth-metal complexes for NMR spectroscopic measurements were prepared in the glovebox using J. Young valve NMR tubes. NMR (1H, 13C) spectra were recorded on a Bruker AVANCE III spectrometer and referenced internally to residual solvent resonances unless otherwise stated. Carbon, hydrogen, and nitrogen analyses were performed by direct combustion on a Carlo-Erba EA-1110 instrument; quoted data are the average of at least two independent determinations. FT-IR spectra were recorded on a Bruker TENSOR 27 spectrometer. Molecular weight and molecular weight distribution of the polymers were measured by a PL GPC 220 instrument at 145 °C using 1,2,4-trichlorobenzene as eluent against polystyrene standards: flow rate, 1.0 mL/min; sample concentration, 1 mg/mL. Melting points of the resulting polymers were measured on a NETZSCH DSC 200 PC instrument at a heating rate of 20 °C/min under an N2 atmosphere. (C5Me5)Sc[N(SiHMe2)2]2 (1). In a glovebox, 1 mmol (514 mg) of Sc[N(SiHMe2)2]3(THF) and 1 mmol (138 mg, 98%) of C5Me5H were dissolved in 10 mL of toluene in a Schlenk flask with a stirring bar at room temperature. Then the flask was taken out from the glovebox and heated at 100 °C in an oil bath for 48 h. The solvent was removed in vacuo, and the yellow oily residue was extracted by hexane (3 10 mL). The hexane extract was filtered to give a clear pale yellow solution, which was concentrated to ca. 1 mL under reduced pressure, and cooled at 35 °C. The desired product was isolated as colorless platelike crystals in two crops (0.36 g, 0.81 mmol, 81%). 1H NMR (400 MHz, C6D6): δ 0.26 (d, J = 20.8 Hz, 24H, SiHMe2), 2.12 (s, 15H, C5Me5), 4.47 (sp, J = 2.4 Hz, 4H, SiHMe2). 13C NMR (100 MHz, C6D6): δ 2.6 (SiHMe2), 3.1 (SiHMe2), 11.9 (C5Me5), 121.8 (C5Me5). FT-IR (KBr, cm1): 2962 (s), 2080 (s), 1863 (s), 1735 (m), 1437 (m), 1378 (m), 1248 (s), 1070 (s), 3272
dx.doi.org/10.1021/om101047h |Organometallics 2011, 30, 3270–3274
Organometallics 896 (s), 763 (s). Anal. Calcd. for C18H43N2ScSi4: C, 48.60; H, 9.76; N, 6.30. Found: C, 58.35; H, 9.57; N, 6.84. (C5Me4SiMe3)Sc[N(SiHMe2)2]2 (2). This compound was prepared by following the same procedure as for 1. Using Sc[N(SiHMe2)2]3(THF) (514 mg, 1 mmol) and C5Me4SiMe3H (218 mg, 97%, 1 mmol) afforded the desired product as a pale yellow oil (0.50 g, 0.72 mmol, 72%). 1 H NMR (400 MHz, C6D6): δ 0.25 (d, J = 20.8 Hz, 24H, SiHMe2), 0.43 (s, 9H, C5Me5SiMe3), 2.07 (s, 9H, C5Me5SiMe3), 2.35 (s, 9H, C5Me5SiMe3), 4.48 (sp, J = 2.4 Hz, 4H, SiHMe2). 13C NMR (100 MHz, C6D6): δ 2.7 (C5Me5SiMe3), 3.0 (SiHMe2), 12.2 (C5Me4SiMe3), 15.3 (C5Me4SiMe3), 119.2 (ipso-C C5Me4SiMe3), 126.5 (C5Me4SiMe3), 131.5 (C5Me4SiMe3). FT-IR (KBr, cm1): 2955 (s), 2111 (s), 1869 (m), 1560 (m), 1441 (m), 1249 (s), 1174 (s), 1024 (s), 899 (s), 836 (s), 686 (m). Anal. Calcd for C20H49N2ScSi5: C, 47.76; H, 9.82; N, 5.57. Found: C, 47.52; H, 9.78; N, 5.43. (C5Me5)Sc[N(SiMe3)2]2 (3). This compound was prepared by following the same procedure as for 1. Using Sc[N(SiMe3)2]3 (526 mg, 1 mmol) and C5Me5H (138 mg, 98%, 1 mmol) afforded the desired product as a pale yellow solid (0.39 g, 0.78 mmol, 78%). 1H NMR (400 MHz, C6D6): δ 0.27 (s, 36H, SiMe3), 1.99 (s, 15H, C5Me5). 13C NMR (100 MHz, C6D6): δ 5.9 (SiMe3), 12.9 (C5Me5), 123.0 (C5Me5). FT-IR (KBr, cm1): 2956 (s), 1637 (s), 1442 (m), 1372 (m), 1252 (s), 1181 (s), 934 (s), 884 (m), 842 (s). Anal. Calcd for C22H51N2ScSi4: C, 52.75; H, 10.26; N, 5.59. Found: C, 52.18; H, 9.97; N, 5.48. Styrene Polymerization. The procedures for the styrene polymerization catalyzed by these complexes were similar, and a typical polymerization procedure is given below. A 50 mL Schlenk flask, equipped with a magnetic stirring bar, was charged in sequence with the desired amount of toluene, the scandium complex, AliBu3, [Ph3C][B(C6F5)4], and styrene. The mixture was stirred vigorously at room temperature for the desired time, during which the precipitation was observed. The reaction mixture was quenched by addition of a large amount of ethanol to precipitate the polymer, which was dried under vacuum at 60 °C and weighed. Solvent Extraction. All solvent fractionations were carried out using a 100 mL Soxhlet extractor.38 Before solvent extraction, the crude polystyrene sample was characterized by 1H NMR spectroscopic analysis for comparison. One gram of polystyrene sample was placed in a cellulose thimble and extracted successively with 50 mL of boiling methyl ethyl ketone (MEK) for 4 h. The solvent was evaporated under vacuum, and no extracts were found. The residue in the thimble was dried under vacuum at 60 °C to constant weight (1 g, 100%) and analyzed by NMR spectroscopy. The spectra of the purified polystyrenes were identical with those of the crude polystyrenes. X-ray Crystallographic Study. Suitable single crystals of complexes were sealed in thin-walled glass capillaries for determining the single-crystal structure. Intensity data were collected with a Rigaku Mercury CCD area detector in ω-scan mode using Mo KR radiation (λ = 0.710 70 Å). The diffracted intensities were corrected for Lorentz polarization effects and empirical absorption corrections. The structures were solved by direct methods and refined by full-matrix least-squares procedures based on |F|2. All of the non-hydrogen atoms were refined anisotropically. The structures were solved and refined using the SHELXL-97 program.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figures giving NMR spectra of complexes 13 and (C5Me5)Sc[N(SiHMe2)2][(μ-Me)2AlMe2)], crystal structure of (C5Me5)Sc[N(SiHMe2)2][(μ-Me)2AlMe2)], 13 C NMR spectrum, GPC, and DSC curves of polymer samples and CIF files giving full crystallographic data for complexes 1, 3, and (C5Me5)Sc[N(SiHMe2)2][(μ-Me)2AlMe2)]. 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]. Tel: þ86-574-88130085. Fax: þ86-574-88130130.
’ ACKNOWLEDGMENT vvThis work was partly financially supported by the Zhejiang Provincial Natural Science Foundation (No. Y4090617), Science and Technology Department of Zhejiang Province (No. 2010R10020), State Key Laboratory of Rare Earth Resource Utilization (No. RERU2011020), Key Laboratory of Organic Synthesis of Jiangsu Province (No. KJS0907), and Ningbo Municipal Science Foundation (No. 2009D10015). We are grateful to Mr. Yong Zhang at Suzhou University for X-ray analysis help. ’ REFERENCES (1) Malanga, M. Adv. Mater. 2000, 12, 1869. (2) Ishihara, N.; Seimiya, T.; Kuramoto, M.; Uoi, M. Macromolecules 1986, 19, 2464. (3) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem. Soc. 1990, 112, 219. (4) Evans, W. J.; DeCoster, D. M.; Greaves, J. Macromolecules 1995, 28, 7929. (5) Knjazhanski, S. Y.; Kalyuzhnaya, E. S.; Herrera, L. E. E.; Bulychev, B. M.; Khvostov, A. V.; Sizov, A. I. J. Organomet. Chem. 1997 , 531, 19. (6) Yuan, F. G.; Shen, Q.; Sun, J. J. Organomet. Chem. 1997, 538, 241. (7) Zhang, Y. G.; Hou, Z. M.; Wakatsuki, Y. Macromolecules 1999, 32, 939. (8) Hou, Z. M.; Zhang, Y. G.; Tezuka, H.; Xie, P.; Tardif, O.; Koizumi, T.; Yamazaki, H.; Wakatsuki, Y. J. Am. Chem. Soc. 2000, 122, 10533. (9) Hultzsch, K. C.; Voth, P.; Beckerle, K.; Spaniol, T. P.; Okuda, J. Organometallics 2000, 19, 228. (10) Tanaka, K.; Furo, M.; Ihara, E.; Yasuda, H. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 1382. (11) Luo, Y. J.; Yao, Y. M.; Shen, Q. Macromolecules 2002, 35, 8670. (12) Voth, P.; Arndt, S.; Spaniol, T. P.; Okuda, J. Organometallics 2003, 22, 65. (13) Kirillov, E.; Lehmann, C. W.; Razavi, A.; Carpentier, J. J. Am. Chem. Soc. 2004, 126, 12240. (14) Luo, Y. J.; Baldamus, J.; Hou, Z. M. J. Am. Chem. Soc. 2004, 126, 13910. (15) Liu, D. T.; Luo, Y. J.; Gao, W.; Cui, D. M. Organometallics 2010, 29, 1916. (16) Fang, X. D.; Li, X. F.; Hou, Z. M.; Assoud, J.; Zhao, R. Organometallics 2009, 28, 517. (17) Rodrigues, A.; Kirillov, E.; Lehmann, C. W.; Roisnel, T.; Vuillemin, B.; Razavi, A.; Carpentier, J. Chem. Eur. J. 2007, 13, 5548. (18) Xu, X.; Chen, Y. F.; Sun, J. Chem. Eur. J. 2009, 15, 846. (19) Rodrigues, A.; Kirillov, E.; Roisnel, T.; Razavi, A.; Vuillemin, B.; Carpentier J. Angrew. Chem. Int. Ed. 2007, 46, 7240. (20) Luo, Y. J.; Nishiura, M.; Hou, Z. M. J. Organomet. Chem. 2007, 692, 536. (21) Trifonov, A. A.; Skvortsov, G. G.; Lyubov, D. M.; Skorodumova, N. A.; Fukin, G. K.; Baranov, E. V.; Glushakova, V. N. Chem.—Eur. J. 2006, 12, 5320. (22) Hitzbleck, J.; Okuda, J. Z. Anorg. Allg. Chem. 2006, 632, 1947. (23) Hitzbleck, J.; Beckerle, K.; Okuda, J.; Halbach, T.; Mulhaupt, R. Macromol. Symp. 2006, 236, 23. (24) Harder, S. Angrew. Chem. Int. Ed. 2004, 43, 2714. (25) Bonnet, F.; Violante, C. D. C.; Roussel, P.; Mortreux, A.; Visseaux, M. Chem. Commun. 2009, 3380. 3273
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(26) Zinck, P.; Valente, A.; Mortreux, A.; Visseaux, M. Polymer 2007, 48, 4609. (27) Nishiura, M.; Mashiko, T.; Hou, Z. M. Chem. Commun. 2008, 2019. (28) Jaroschik, F.; Shima, T.; Li, X. F.; Mori, K.; Ricard, L.; Goff, X. L.; Nief, F.; Hou, Z. M. Organometallics 2007, 26, 5654. (29) Lappert, M. F.; Protchenko, A. V.; Power, P. P.; Seeber, A. L. Metal Amide Chemistry; Wiley: Hoboken, NJ, 2008. (30) Heeres, H. J.; Meetsma, A.; Teuben, J. H. Organometallics 1989, 8, 2637. (31) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (32) Partial alkylation of these scandium bis(amide) complexes with excess AlR3 was observed. For example, reaction of complex 1 with excess AliBu3 in hexane produced a highly soluble complex identical with (C5Me5)Sc[N(SiHMe2)2][(μ-iBu)2AliBu2], which was quite difficult to separate from the reaction mixture. Then, treatment of complex 1 with excess AlMe3 afforded, after fractional recrystallization, the isolable novel bimetallic species (C5Me5)Sc[N(SiHMe2)2][(μ-Me)2AlMe2] (see the Supporting Information). However, these results were strikingly different from those reported by Anwander and co-workers, who isolated the symmetric bimetallic complexes (C5Me5)Ln[(μ-Me)2AlMe2]2 from the reaction of (C5Me5)Ln[N(SiHMe2)2]2 (Ln = Y, Lu) with excess AlMe3. See: Anwander, R.; Klimpel, M. G.; Dietrich, M.; Shorokhov, D. J.; Scherer, W. Chem. Commun. 2003, 1008. (33) NMR monitoring of the reaction of (C5Me5)Sc[N(SiHMe2)2]2 (1) with 1 equiv of [Ph3C][B(C6F5)4] in C6D5Cl at 25 °C showed instant disappearance of the signals of 1 and the appearance of new signals assignable to a cationic scandium species identical with [(C5Me5)Sc-N(SiHMe2)2]þ: 1H NMR (400 MHz, C6D5Cl) δ 0.03 (d, J = 12.8 Hz, 12H, SiHMe2), 1.73 (s, 15H, C5Me5), 3.87 (m, 2H, SiHMe2). (34) Attempts to gain information on a cationic scandium species generated from the reaction of (C5Me5)Sc[N(SiHMe2)2][(μ-Me)2AlMe2] with 1 equiv of [Ph3C][B(C6F5)4], even on an NMR scale, have not been successful yet, mainly due to the low thermodynamic stability of the cationic species. (35) Zimmermann, M.; Tornroos, K. W.; Anwander, R. Angew. Chem., Int. Ed. 2008, 47, 775. (36) Alyea, E. C.; Bradley, D. C.; Copperthwaite, R. G. J. Chem. Soc., Dalton Trans. 1972, 1580. (37) Anwander, R.; Runte, O.; Eppinger, J.; Gerstberger, G.; Herdtweck, E.; Spiegler, M. J. Chem. Soc., Dalton Trans. 1998, 847. (38) Xu, G. X.; Lin, S. A. Macromolecules 1997, 30, 685.
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