Decamethylsilicocene

Kxihler Thorsten and Jutzi Peter, Decamethylsilicocene Synthesis, Structure,... 

The sharply increasing interest in the chemistry of monovalent silyliumylidene ion derivatives of the type RSi stimulated a great number of experimental studies in this field, which recently culminated in the synthesis of the very remarkable (Tl -Me5C5)Si+ cation 25+ TPFPB- (Scheme 2.21). A stable 25+ TPFPB- was obtained by the oxidation of decamethylsilicocene 26 with (Me5C5H2) TPFPB in CH2CI2 (Scheme 2.21). 

In 1989 Jutzi et al. reported the reaction of decamethylsilicocene 50 with tri-n-butylphosphine selenide in benzene at room temperature, leading to almost quantitative formation of a 1,3,2,4-diselenadisiletane derivative 52, a head-to-tail [2+2] cycloaddition reaction product of the initially formed silaneselone 51.35 The intermediacy of silaneselone 51 was experimentally supported by the reaction in the presence of 2,3-dimethyl-1,3-butadiene resulting in the formation of the corresponding [2+4] cycloaddition reaction product 53 (Scheme 14). 

Expectations from an Unusual Compound The Chemistry of Decamethylsilicocene... 

Some years ago we were successful in our attempts to synthesize a Jt-complex with divalent silicon as the central atom. Starting from dihalogeno(pentamethylcyclopentadienyl)silanes, we have been able to prepare decamethylsilicocene (1) by reductive elimination processes [1]. Characteristic data concerning the synthesis, structure, and bonding of 1 have been published elsewhere together with preliminary results concerning the chemistry of this compound [2]. Here we describe some further progress in this field. 

The influence of the acidity of the relevant protic substrates on the final reaction products is further demonstrated by two examples shown in Scheme 2. Decamethylsilicocene (1) reacts with HBF4 to yield the compound [Cp SiF]4, whereas in the reaction with PyH+BF4 (Py=pyridine) the oxidative addition product Cp 2Si(H)F is formed. Reaction of 1 with two equivalents of F3CS03H leads to an ionic species,... 

Decamethylsilicocene (1) reacts with primary and secondary amines to give the corresponding oxidative addition products in good yield [8],... 

Decamethylsilicocene (1) reacts with 2,2 -bipyridyls to give deep violet oxidative addition products... 

We have recently prepared some new and very thermolabile CO- and N2 comPlexes derived from titanocene [1] or decamethyltitanocene [2], and have characterized them by their vibrational spectra. As well as "classical" matrix spectroscopy, we have used spectroscopy in liquid xenon (LXe). The application of chemistry and methodology indicates the decamethylsilicocene structure, which represents the first example of a stable jt-complex of divalent silicon [3]. Reaction with CO or N2 leads to the two title complexes [4] ... 

Fig. 3. Molecular structure of the linear (la) (D5d) and of the bent (lb) (C2v) isomer of decamethylsilicocene in the solid state.

Fig. 5. Schematic MO diagram of decamethylsilicocene (1) (only occupied MO s are shown).

Summarizing the available bonding information, decamethylsilicocene (1) is regarded as an electron-rich silicon(II) compound containing a hypercoordinated silicon atom which is sandwiched between two rather weakly 7i-bonded pentamethylcyclopentadienyl ligands and thus is effectively shielded the lone-pair orbital at silicon is part of the frontier orbitals of the molecule. 

Whereas decamethylferrocene—here regarded as the prototype of a metallocene—can be easily transferred to the corresponding radical cation or anion, this is not the case for decamethylsilicocene (1). CV measurements have already shown that the cation 1+ is only a very short-lived transient species.10 Consequently, chemical experiments to prepare salts containing 1+ have also failed so far. Similar observations have been made concerning... 

After a decade of research the basic principles in the chemistry of decamethylsilicocene (1) seem to be understood. This compound shows the reactivity of a nucleophilic silylene due to the fact that the Tt-bonded pentamethylcyclopentadienyl ligands are easily transferred to a-bonded substituents during the reaction. The steric requirements of these substituents permit reactions with bulky substrates. The migratory aptitude and the leaving-group character of the pentamethylcyclopentadienyl groups... 

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