Polysilanes Obtained by Catalytic Dehydrogenation

Harrod and coworkers have found that metallocene catalysts such as Cp2ZrMe2 and Cp2TiMc2 have been found to catalyze the conversion of primary arylsilanes such as PhSiHs at room temperatures to afford short-chain polysilanes H-(PhHSi)n-H which contain Si-H terminal groups (Fig. 7.16) [56-57]. These Group 4 metallocenes have bent structures in contrast to ferrocene where the iron is sandwiched between the two cyclopentadi-enyl rings. 

The new generation dehydrogenative polymerization catalysts employed include [CpCp Zr(H)2](Cp = pentamethylcyclopentadienyl), CpCp M(Si(SiMe3)3R) (M = Zr, Hf R = CH3, Cl) [19-21]. Combinations of metallocene dichlorides and -butyllithium such as Cp2MCl2/2 BuLi (M = Ti, Zr Hf) have also been employed (Fig.7.17) [21]. 

The mechanism of the catalytic dehydrogenation appears to involve the cleavage of the Si-H bonds which is activated by the catalyst [19-20]. It has been proposed by Tilley that the active catalytic species is a metal hydride of the type L M-H (where L is the other ligand or ligands such as cyclopentadienyl or pentamethylcyclopentadienyl that are present on the metal). The metal hydride interacts with a compound containing a Si-H bond to generate a four-centered transition state (see Eq.7.11) 

Cleavage of Si-H and M-H bonds with the concomitant formation of Si-M bonds and elimination of H2 occurs (see Eq. 7.12). 

The species LnM-Si(H)2Ph interacts with another molecule of silane to 

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