Silicon

  Silicon tetraalkyl and tetraaryl derivatives (R₄Si), as well as alkyl or aryl silicon halides (R_nSiCl_{4 - n}, n = 1–3) can be prepared by reaction types 1.1–1.5. Note that variation in stoichiometry provides flexibility in synthesis, although the product specificity may be influenced by steric requirements of the organic substituents. Reaction 1.1 is used industrially (the Rochow process).

                              (1.1)

                                    (1.2)

                     (1.3)

                  (1.4)

  The structures of these compounds are all similar: monomeric, with tetrahedrally sited Si and resembling their C analogues. Silicon_carbon bonds are relatively strong (the bond enthalpy term is 318 kJ mol^-1) and R₄Si derivatives possess high thermal stabilities. The stability of the Si^_C bond is further illustrated by the fact that chlorination of Et₄Si give  ) ClCH₂CH₂)₄Si, in contrast to the chlorination of R₄Ge or R₄S_n which yields R_nGeCl₄­ _{- n} or R_nSnCl_{4- n} (see equation 18.49). An important reaction of Me_nSiCl_{4_n} (n = 1–3) is hydrolysis to produce silicones (e.g. equation 1.6).

                            (1.5)

(1.1)

   The reaction of Me3SiCl with NaCp leads to 1.1, in which the cyclopentadienyl group is η¹. Related η¹- complexes include) η¹-C₅Me₅(₂SiBr₂ which reacts with anthracene/potassium to give the diamagnetic silylene(η⁵- C₅Me₅)₂Si. In the solid state, two independent molecules are present (Figure 1.1a, b) which differ in the relative orientations of the cyclopentadienyl rings. In one molecule, the two C₅-rings are parallel and staggered (compare Cp₂Mg) whereas in the other, they are tilted (Figure 1.1c).  We return to this observation at the end of Section 18.5.  The reactions between R₂SiCl₂ and alkali metals or alkali metal naphthalides give cyclo-(R₂Si)_n by loss of Cl^- and Si^_Si bond formation. Bulky R groups favour small rings (e.g.(2,6-Me₂C₆H₃)₆Si₃ and ^tBu₆Si₃) while smaller R substituents encourage the formation of large rings (e.g. Me₁₂Si₆;Me₁₄Si₇ and Me₃₂Si₁₆).

Fig. 1.1 The solid state structure of (η⁵-C₅Me₅(₂Si contains two independent molecules. (a) In the first molecule, the cyclopentadienyl rings are co-parallel, while (b) in the other molecule they are mutually tilted; (c) the tilt angle is measured as angle α [P. Jutzi et al. (1986) Angew. Chem. Int. Ed. Engl., vol. 25, p. 164]. Hydrogen atoms are omitted for clarity; colour code: Si, pink; C, grey.

   Reaction 1.6 is designed to provide a specific route to a particular ring size.

                       (1.6)

   Silylenes, R₂Si (analogues of carbenes), can be formed by a variety of methods, for example, the photolysis of cyclic or linear organopolysilanes. As expected, R₂Si species are highly reactive, undergoing many reactions analogous to those typical of carbenes. Stabilization of R₂Si can be achieved by using sufficiently bulky substituents, and electron diffraction data confirm the bent structure of {(Me₃Si)₂HC}₂Si (∠C^_Si^_C = 97⁰).    The sterically demanding 2,4,6-ⁱPr₃C₆H₂ group has been used to stabilize 1.2, the first example of a compound containing conjugated Si=Si bonds. An unusual feature of 1.2 is the preference for the s-cis conformation in both solution and the solid state.

(1.2)