The Wurtz-reductive coupling polymerization of dichlorodiorganosilanes with sodium metal is the standard method for the synthesis of polysilanes. A theoretical study using density functional theory has been carried out giving valuable insights into the mechanism that we compare with experimental results from our group and others. The initial single electron transfer from sodium to dichlorodimethylsilane results in the formation of a sodium cation chlorosilane anion radical pair or a chlorosilane radical; calculations at the B2GPLYP/6-311+G(3df,3p)//B3LYP/6-311+G(d,p) level of theory suggests that the radical anion is the energetically and kinetically preferred intermediate. The subsequent sodium chorodimethylsilane species formed by the subsequent single electron transfer reaction from sodium can generate 3 possible structures namely a silylenoid, a silyl anion and an inverted structure. Again studies at the B2GPLYP/6-311+G(3df,3p)//B3LYP/6-311+G(d,p) level indicate that the silylenoid species is the most thermodynamically stable form. The addition of further dimethylsilane units leads to chains where the α-chloro-ω-sodium cyclosilane is thermodynamically stabilised in preference to the all-trans sodium silyl anionic form. This species is characterised by - Si-Na-Cl-Si- bonding interactions that 'close' the ring and stabilise the cyclic structure.This apparent preference for stable cyclic intermediates in the chain growth of polysilanes by the Wurtz method suggests a ready explanation for cyclic silanes being the majority product in most polysilane syntheses from dichlorosilanes utilising alkali metals.

Theoretical study of the sodium mediated coupling of organchlorosilanes: Stable α-chloro-ω-sodium cyclosilane intermediates in the Wurtz synthesis of polysilanes?

249th National Meeting (2015)

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