Note pubs.acs.org/Organometallics
Rapid and Efficient Procedure for the Synthesis of Group 3 Metal Tris[bis(dimethylsilyl)amide] THF Complexes Adrian R. Smith and Tom Livinghouse* Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, United States S Supporting Information *
ABSTRACT: An expedient method for the synthesis of group 3 metal tris[bis(dimethylsilyl)amide] THF complexes from the corresponding tris[bis(trimethylsilyl)amide]s is described. Advantages of this technique over previous methods include short reaction times, simple workup, and near-quantitative yields that require only commercially available reagents.
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INTRODUCTION Group 3 metal tris[bis(trimethylsilyl)amide]s, M[N(SiMe3)2]3 (1), have become extremely popular as metalating reagents for the synthesis of a variety of group 3-based catalysts via “amine elimination”.1a−d Advantages associated with these reagents include the ability to effect metalations in noncoordinating solvents, the preclusion of halide contamination, avoidance of “ate”-complex formation, simplicity of purification due to facile removal of HN(SiMe3)2, and their commercial availability.1c A major limitation to the general application of this method is connected to the intrinsic steric dimension of the −N(SiMe3)2 ligand. To obviate this shortcoming, Herrmann and Anwander have developed an extremely useful alternative strategy based on ligand exchange involving less sterically hindered amide complexes of the type M[N(SiHMe2)2]3(THF)n (2).2,3a−h In addition to the enhanced reactivity profile typically observed for these reagents, the presence of the spectroscopically distinctive Si−H signal frequently serves as a useful means to monitor reaction progress.1c Despite the inherent advantages possessed by these precursors, their preparation by the reaction of MCl3(THF)n and LiN(SiHMe2)2 has remained somewhat tedious due to the necessity of LiCl separation and lowtemperature recrystallization from pentane.2 We describe here an exceptionally simple, direct synthetic route to these complexes based on ligand exchange between the commercially available amides M[N(SiMe3)2]3 (1a−e) and HN(SiHMe2)2 in the presence of THF.
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to reach completion at ambient temperature; however additional HN(SiHMe2)2 and heating resulted in a dramatic rate acceleration. Not surprisingly, a ligand exchange reaction using THF as the solvent in the absence of benzene or toluene, although successful, was notably lethargic. This result is consistent with competitive coordination of THF to the metal center, thereby hindering transamination relative to trials containing only a slight excess of THF. A notable advantage of the optimized procedure is that simple removal of volatiles in vacuo is all that is necessary to isolate the product (Table 1). Table 1. Synthesis of Group 3 Metal Complexes 2a−e
a
metal
product
(THF)n
ta
1a 1b 1c 1d 1e
Sc Y Nd Sm Lu
2a 2b 2c 2d 2e
1 2 2 2 2
2.75 h 5 min 5 min 5 min 1.67 h
Time to 95% conversion based on integration in 1H NMR.
The efficacy of this technique for larger scale preparations was demonstrated by the synthesis of 2b and 2d. In these cases toluene was used as solvent and additional THF was used to compensate for the reaction temperature. Additionally, the reaction time for formation of 2b and 2d on this scale was lengthened to 30 min to compensate for the inability to directly monitor the progress of the reaction. In both cases, reactions were nearly quantitative. It is also noteworthy in a preparative context that this method was not found to be extendable to simple aliphatic secondary amines. Accordingly, efforts to utilize diisopropylamine and cis-2,6-dimethylpiperidine resulted in
RESULTS AND DISCUSSION
In an initial study, we attempted to synthesize THF-free amides corresponding to the general formula M[N(SiHMe2)2]3 by the reaction of M[N(SiMe3)2]3 with excess HN(SiHMe2)2 in toluene. These early, and admittedly not exhaustive, trials resulted in incomplete ligand exchange (as monitored by NMR).3i We predicted that the addition of THF would drive the equilibrium favoring ligand exchange since the desired complex would be stabilized by solvation, whereas the starting amide is not. This proved to be the case. Only slight excesses of both HN(SiHMe2)2 and THF were necessary for the reaction © 2012 American Chemical Society
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Special Issue: Recent Advances in Organo-f-element Chemistry Received: July 20, 2012 Published: October 31, 2012 1528
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mmol), and THF (15 mL) in an analogous fashion provided Sc[N(SiMe3)2]3 (1a, 2.20 g, 88%). Typical NMR-Scale Reaction. In a glovebox, M[N(SiMe3)2]3 (0.016 mmol) was added to a J. Young NMR tube and dissolved in 0.5 mL of benzene-d6. Tetrahydrofuran (3.46 mg, 0.048 mmol) and 1,1,3,3-tetramethyldisilazane (12.8 mg, 0.096 mmol) were then added, and the tube was sealed. The reactant mixture was then placed in an oil bath maintained at 90 °C and monitored by 1H NMR. Upon completion of ligand exchange, the volatiles were removed on a Schlenk line (0.02 mmHg) and the product was dissolved in benzened6 to determine 1H NMR data. The yield for 2a was 8.0 mg (97%), 2c was 10.7 mg (96%), and 2e was 11.2 mg (98%). 1H NMR spectra were identical to those described in the literature. 3 , 6 Y[N(SiHMe2)2]3·2THF (2b): 1H NMR (C6D6): δ 0.39 ppm (36 H, d, −SiCH3), 1.32 (8 H, m, THF), 3.82 (8 H, m, THF), 4.99 (6 H, h, SiH). 13C NMR (C6D6): δ 3.31 ppm (12 C, m, −CH3), 25.43 (4 C, m, THF), 70.62 (4 C, m, THF). Sc[N(SiHMe2)2]3·THF (2a): 1H NMR (C6D6): δ 0.43 ppm (36 H, d, −SiCH3), 1.24 (4 H, m, THF), 3.93 (4 H, m, THF), 5.11 (6 H, h, SiH). 13C NMR (C6D6): δ 3.36 ppm (12 C, m, −CH3), 25.50 (2 C, m, THF), 73.00 (2 C, m, THF). Preparative-Scale Reaction. Tris(amide) Y[N(SiMe3)2]3 (1b) (2.85 g, 5 mmol) was added to an oven-dried 100 mL Schlenk flask equipped with a magnetic stirring bar and an argon inlet. Toluene (35 mL), tetrahydrofuran (0.65 g, 50 mmol), and 1,1,3,3-tetramethyldisilazane (0.80 g, 30 mmol) were then added, and the flask was attached to a Schlenk line. The reactant mixture was heated at 90 °C under an argon atmosphere with stirring for 30 min and then cooled to −78 °C, whereupon the volatiles were removed under vacuum (0.02 mmHg). The isolated yield for 2b was 3.05 g (97%). In a variation, using 1d (1 mmol) under identical conditions, an isolated yield of 2d of 0.676 g (98%) was obtained. 1H NMR spectra were identical with those in the literature.3,6 Preparation of Y[OC(t-Bu)3]3·THF (3b). The conventional tris(amide) Y[N(SiMe3)2]3 (1b) (9.12 mg, 0.016 mmol) and tri-tertbutylcarbinol (9.62 mg, 0.048 mmol) were dissolved in 0.5 mL of benzene-d6. Tetrahydrofuran (3.46 mg, 0.048 mmol) and 1,1,3,3tetramethyldisilazane (12.8 mg, 0.096 mmol) were then added. The reactant mixture was stored at ambient temperature and monitored by 1 H NMR, whereupon the formation of 3b was complete (48 h) and 3b (11.8 mg, 97%) was isolated. The 1H and 13C NMR spectra were identical to those described in the literature.2 1H NMR (C6D6): δ 1.23 ppm (4 H, m, THF), 1.49 (81 H, s, −CH3), 4.11 (4 H, m, THF). 13C NMR (C6D6): δ 25.22 ppm (2 C, −CH2−), 34.66 (27 C, −CH3), 45.95 (9 C, -CMe3), 73.36 (2 C, O−CH2), 92.44 (3 C, O−C).
only partial exchange, suggesting that pKa differences may play a large role in this method’s efficiency.7 The conventional tris(amide) Y[N(SiMe3)2]3 (1b) has been shown to be ineffective for the conversion of (t-Bu)3COH (tritox-H) to the corresponding Y(III) alkoxide (3b), whereas Y[N(SiHMe2)2]3(THF)2 (2b) efficiently engages in this transformation.2 We envisioned that 1,1,3,3-tetramethyldisilazane might be used as a reaction partner to lead to ligand exchanges attainable by way of the extended silylamide route. Unfortunately, a catalytic amount of 1,1,3,3-tetramethyldisilazane did not effectively promote the metalation of (t-Bu)3COH by Y[N(SiMe3)2]3 (1b) in the presence or absence of THF. However, when used in excess (6 equiv), HN(SiHMe2)2 with THF (3 equiv) led to the desired product (3b) quantitatively at 48 h at room temperature (Scheme 1).8 This result is most Scheme 1. Synthesis of Yttrium(III) Complex 3b
encouraging, since it suggests that other complexes that have previously been accessed via the extended silylamide route might be prepared by the simple expedient of metalating the proligand of interest with M[N(SiMe3)2]3 (1) in the presence of HN(SiHMe2)2 and THF. In conclusion, we have shown that simple amine exchange in the presence of THF is a highly effective means for the direct synthesis of the group 3 tris(amide) complexes M[N(SiHMe2)2](THF)n. As a synthetic limitation, this reaction protocol was found not to be generally extendable to exchange with simple aliphatic secondary amines.
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EXPERIMENTAL SECTION
Materials and Methods. All manipulations were performed under an argon atmosphere, using standard Schlenk line techniques or in an argon-filled drybox. Benzene-d6 and tetrahydrofuran were distilled from sodium prior to use. Toluene was distilled from calcium hydride prior to use. 1,1,3,3-Tetramethyldisilazane was distilled from potassium prior to use. The complexes Y[N(SiMe3)2]3,4 Nd[N(SiMe 3 ) 2 ] 3 , 4 Lu[N(SiMe 3 ) 2 ] 3 , 4 Sc[N(SiMe 3 ) 2 ] 3 , 5 and Sm[N(SiMe3)2]34 were synthesized by a modification of the procedure reported in the literature. Physical and Analytical Measurements. 1H and 13C NMR spectra were recorded on a Bruker DPX-300 (300 MHz) or DRX-500 (500 MHz) spectrometers. 1H NMR chemical shifts (δ) are reported in parts per million (ppm) downfield of TMS. Chemical shifts for carbon are reported in parts per million downfield of tetramethylsilane. Data are represented as follows: chemical shift, multiplicity (br = broad, s = singlet, d = doublet, t = triplet, m = multiplet), integration, and coupling constants in hertz (Hz). Synthesis of Y[N(TMS)2]3 (1b). In a glovebox, YCl3(THF)3.5 (5.41 g, 12.1 mmol) and lithium bis(trimethylsilyl)amide (6.10 g, 36.5 mmol) were added to an oven-dried 100 mL Schlenk flask equipped with a stir bar. The flask was attached to a Schlenk line. Under a positive argon pressure, tetrahydrofuran (35 mL) was then added through the side arm of the Schlenk flask. The resulting mixture was stirred under argon atmosphere overnight, then cooled to −78 °C to achieve solvent degassing, whereupon the volatiles were removed under vacuum (0.02 mmHg) over 2 h. The resulting mixture of 1b and LiCl was purified via sublimation with a sublimation apparatus (110 °C, 0.001 mmHg) over 72 h. The isolated yield for 1b was 5.62 g (81%). This method is general for the synthesis of all of the M[N(SiMe3)2]3 amides reported here. Accordingly, the use of ScCl3(THF)3 (1.75 g, 4.76 mmol), LiN(SiMe3)2 (2.39 g, 14.30
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ASSOCIATED CONTENT
S Supporting Information *
Selected NMR spectra are available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Generous financial support for this research was provided by the National Science Foundation.
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REFERENCES
(1) (a) Huang, C.-H., Ed. Rare Earth Coordination Chemistry: Fundamentals and Applications; John Wiley & Sons: Singapore, 2010. (b) Gade, L. H. Chem. Commun. 2000, 173−181. (c) Anwander, R. Top. Organomet. Chem. 1999, 2, 1−61. (d) Mishra, S. Coord. Chem. Rev. 2008, 252, 1996−2025. 1529
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(2) Herrmann, W. A.; Anwander, R.; Munck, F. C.; Scherer, W.; Dufaud, V.; Huber, N. W.; Artus, G. R. J. Z. Naturforsch. 1994, 49b, 1789−1797. (3) For examples of rare-earth-based catalysts synthesized from the extended silylamide route precursors, see: (a) Zi, G.; Li, H.; Xie, Z. Organometallics 2002, 21, 1136−1145. (b) O’Shaughnessy, P. N.; Knight, P. D.; Morton, C.; Gillespie, K. M.; Scott, P. Chem. Commun. 2003, 1770−1771. (c) Kerton, F. M.; Whitwood, A. C.; Willans., C. E. Dalton Trans. 2004, 2237−2244. (d) Hultzsch, K. C.; Hampel, F.; Wagner, T. Organometallics 2004, 23, 2601−2612. (e) Westmoreland, I.; Arnold, J. Dalton Trans. 2006, 4155−4163. (f) Heck, R.; Schulz, E.; Collin, J.; Carpantier, J. J. Mol. Catal. A: Chem. 2007, 268, 163−168. (g) Wu, B.; Gallucci, J. C.; Parquette, J. R.; RajanBabu, T. V. Angew. Chem., Int. Ed. 2008, 48, 1126−1129. (h) Li, G.; Lamberti, M.; Mazzeo, M.; Pappalardo, D.; Roviello, G.; Pellecchia, C. Organometallics 2012, 31, 1180−1188. (i) Marks has reported that THF-free La[N(SiHMe2)2]3 can be prepared by the treatment of La[N(SiMe3)2]3 with HN(SiHMe2)2 (11 equiv) overnight followed by recrystallization from pentane at −30 °C. Yuen, H. F.; Marks, T. J. Organometallics 2008, 27, 155−158. (4) Bradley, D. C.; Ghotra, J. S.; Hart, F. A. J. Chem. Soc., Dalton Trans. 1973, 1021−1023. (5) Alyea, E. C.; Bradley, D. C.; Copperthwaite, R. G. J. Chem. Soc., Dalton Trans. 1972, 1580−1584. (6) Anwander, R.; Runte, O.; Eppinger, J.; Gerstberger, G.; Herdtweck, E.; Spiegler, M. J. Chem. Soc., Dalton Trans. 1998, 847− 858. (7) For the synthesis of Ln[NiPr2]3(THF) complexes, see: Aspinall, H. C.; Tillotson, M. R. Polyhedron 1994, 13, 3229−3234. (8) The use of HN(SiHMe2)2 (50 and 10 mol %) led to incomplete conversion. When the preparation of Y[OC(t-Bu)3]3 was attempted in the absence of THF, incomplete ligand exchange was observed.
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