Dehydrocoupling of silanes

 

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Authors:

1. Jingwei Luo (G)

2. zohrab ahmadi (G)

3.

4.

5.

 

Contents 1 Introduction 2 General reaction 3 Reaction types 4 General Mechanism 5 Catalysts classification 6 Limitation and drawbacks 7 References

 

 

 

Introduction

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      Dehydrocoupling of silanes is also called dehydrogenative coupling of silanes, it is a type of chemical reaction involving the breaking of  Si-H bonds and the formation of Si-Si bond, or Si-heteroatoms bonds (exp. C, N, O). The products always involve the formation of oligo or polysilanes.¹ Polysilanes are very important chemicals because of their special optical and electronic properties, and they can also be designed as precursors of silicon carbide.^2,3 All the properties originate from the delocalization of their sigma (σ) electronic back bone. This leads to the absorption of UV light with a specific wavelength in the Si-Si chain.⁴

 

      Historically, Wurtz type coupling of chlorosilanes was used as the major method for the synthesis of polysilanes. This is a radical reaction; where lithium and sodium are generally used metals. The first polysilane of this type  was synthesised in 1921,⁵ but the limitations are obvious; the reaction conditions are strict, the product is a mixture, it is not able to control how many units are there in the final product and the mixture is hard to separate. Usually polysiloxane species are generated and silicon compounds with functional group can not be used as reactants, although researchers are still trying to improve this method.^6,7 New approaches can avoid these drawbacks.^8-11 Dehydrocoupling of silanes was discovered by Professor John F. Harrod in McGill University when he investigated on polymerization of organosilanes by group 4 complexes.^12-15 The reaction is a transition metal involving homogeneous catalysis. The advantages of dehydrocoupling of silane include easy purification (main by-product is H₂), functional silane applicable, stereochemical and chain length controllable.¹⁶

 

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General Reaction

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      Dehydrocoupling  involves two reactants with E-H bonds (E could be an element like Si, N or B)  to give E-E as the targeting molecule and H₂ as the only byproduct. Dehydrocoupling of silanes requires silane as a one of the reactants. The products can be simple molecules or polysilanes.^17,18 Transition metal catalysts are usually needed to speed up the reaction as silicon is not a labile element.

 

 

Figure 1 Simple silane dehydrocoupling reactions^{17, 18}

 

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Reaction types

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1- Homodehydrocoupling

      This type of reaction includes dehydrocpoupling of silanes to form Si-Si bond.¹⁹

 

 

2- Heterodehydrocoupling

      This type of hydrocoupling has been reported for several groups like: SiH/NH,²⁰ SiH/CH,²¹ SiH/OH,²² and SiH/S²³ (C=S) using different kinds of catalysts. Fig 2 shows an example of SiH/OH dehydrocoupling.

 

Figure 2 Si-Si/Si-O dehydrocoupling of silanes and alcohols.²²

 

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General Mechanism

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      The mechanism of deydrocoupling is strongly dependent on the nature of the catalyst. For late transition metal complexes like Ni and Pd  oxidative addition followed with reductive elimination is the dominant mechanism. But, for early transition metals like Ti or Zr the sigma bond metathesis process is probable, because in these cases (d⁰) the oxidative addition is forbidden. The proposed cycles of these two mechanism are shown below.²⁴

 

 

Figure  3 Oxidative addition/reductive elemination mechanism for late transition metal complexes like Pd (X) complexes.

 

 

Figure 4 Sigma bond metathesis mechanism for early transition metal complexes like Ti (IV) complexes.

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Catalysts classification

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      Different kinds of catalysts have been reported to catalyze the formation of Si-Si or Si-heteroatom bonds. Among the variety of catalysts,  transition metal complexes from Group 4, 9 and 10 have been used more than other complexes.

 

Figure 5 Different kinds of catalysts used for dehydrocoupling of silane (Original periodic table was copied from wikipedia.org). 

 

 

      Titanocene and zirconoce complexes have been reported  for both homodehydrocoupling  and cross dehydrocouling of silane with different groups like amines. It is believed that group 4 metallocene complexes are the most active catalysts for dehydrocoupling.¹⁷ Also dehydrocoupling has been reported using the Uranium complex [(Et₂N) ₃U][BPh₄].²⁵  Homodehydrocoupling of silane has been found not to be a competeting reaction in this case. Copper chloride has been found as an active catalyst for coupling silanes with amines to form Si-N bonds. However, the activity is not high enough to form high molecular weight polysilazanes.²⁶  Also CuI has been reported as an useful catalyst for polymerization of silane and alkyne.²⁷

 

      Recently the first reaction of silane with thiol was reported without the use of transition metal complexes by using boron complex (C₆F₅).¹ Using the ferrocene complexes like [CpFe(CO)₂Me] has been reported  as an appropriate method for tertiary silanes where other transition methal catalysts are not able to catalyze tertiary silanes.¹⁸

 

Figure 6 Boron catalyst.¹

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Limitation and drawbacks

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      The conventional method of dehydrosilation like Wurtz-type condensation usually are performed in harsh reaction conditions and in most cases formation of by-products like polysiloxanes are problematic. Also the effect of transition metal complexes like group 4, 9 and 10 catalysts are limited to primary and secondary silane and they are not effective for tertiary silanes.²⁸ Late transition metals are not able to produce high MW polymers and they are limited to short chains.²⁹ Therefore, finding more effective catalyst, especially cheep and environmetally freindly ones still is a need.

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References

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[1] Harrison, D.; Edwards, D.; McDonald, R.; Rosenberg, L. Dalton Transactions. 2008, 3401-3411. 

[2] Chandrasekhar, V.; Krishnan, V.;  Sasikumar, P.; Murthy, V. Journal of Inorganic and Organometallic Polymers and Materials, 2007, 17, 439.

[3] Maddocks, A.; Hook, J.; Stender, H.; Harris, A. Journal of Materials Science, 2008, 43, 2666-2674.

[4] Miller, R. D.; Michl, Chemical Review, 1989, 89, 1359. 

[5] Kipping, F. S.Journal of Chemical Society. 1921, 119, 830. 

[6] Miller, R. D.; Jenkner, P. K. Macromolecules 1994, 27, 5921-5923.

[7] Kawabe, T.; Naito, M.; Fujiki, M. Macromolecules, 2008, 41 (6), 1952–1960. 

[8] Harrod, J. F. In Inorganic and Organometallic Polymers with Special Properties; Laine, R. M., Ed.; Kluwer Academic Publishers: Amsterdam, The Netherlands, 1992; 87. 

[9] Woo, H.-G.; Tilley, T. D. In Ultrastructure Processing of AduancedMaten'als; Uhlmann, D. R., Ulrich, D. R., Eds.; John Wiley and Sons, Inc.: New York, 1992; 651. 

[10] Sakamoto, K.;Obata, K.; Hirata, H.; Nakajima, M.; Sakurai, H. Journal of the American Chemical Society. 1989,111, 7641. 

[11] Cypryk, M.; Gupta, Y.; Matyjaszewski, K. Journal of the American Chemical Society. 1991,113, 1046. 

[12] Forsyth, C.; Nolan, S.; Marks, T. Organometallics. 1991, 10, 2543-2545. 

[13] Harrod, J. F.; Yun, S. S. Organometallics. 1987, 6, 1381-1387. 

[14] Aitken, C. T.; Harrod, J. F.; Samuel, E.Journal of Organometallic Chemistry. 1986, 279, 11-13. 

[15] Aiken, C. T.; Harrod, J. F.; Samuel, E. Journal of Chemical Society. 1986, 108, 4059-4066.

[16] Kennedy, V. Ph.D thesis, Case Western Reserve University, 1993, 4.

[17] Peulecke, N. ; Thomas. D.; Baumann, W.; Fischer, C.; Rosenthal, U. Tetrahedron Letters. 1997, 38, 6655-6656.

[18] Itazaki, M.;  Ueda, K.;  Nakazawa, H. Angewandte Chemie International Edition. 2009, 121, 3363 –3366.

[19]  J, Y, Corey.; Advanced Silicon Chemistry. 1991, 1, 327.

[20] Armor, J, N. Inorganic Chemistry. 1978, 17, 203.

[21] Valdimir, K, D.; Leo, J, P.; Patrick, J, C.; Donal, H, B.;Journal of the American Chemical Society. 2003, 125, 8043-8058.

[22] Bo, H, K.; Myong, S, C.; Mi, A, K.; Hee, G, W. Journal of Organomettalic Chemistry. 2003, 685, 93-98.

[23] Chauhan, B, P,S.; Boudjouk, P. Tetrahedron Letter. 2000, 41, 1127.

[24] Fryzuk, D, M.; Rosenberg, L.; Retting, J, S. Inorganic Chimica Acta, 1994, 222, 345-364.

[25] Jia, X, W.; Aswini, K, D.; Jean, C, B.; Micheal.; Moris, S, E. Journal of Organimetallic Chemistry. 2009, 610, 49-57.

[26] H, Q, Liu.; J, F, Harrod. Canadian Journal of Chemistry. 1992, 70, 107.

[27] H, Q, Liu.; J, F, Harrod. Canadian Journal of Chemistry. 1990, 68, 1100.

[28] Herzing, C, Organosilicon Chemistry. 1994, 253-260.

[29] Rosenberg, L.; Kobus, Danielle, N., Journal of Organomettalic Chemistry, 2003, 685, 107-112.

 

 

 

 

 

 

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Introduction

 

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References

 

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