Silicon cation

Silicon cluster reactions are an example of a newly emerging field of research which is very amenable to study with electronic structure methods. This exercise will examine the potential surface for silicon cation reacting with silane (SiH4). Such reactions are central to the growth of large silicon clusters, which occurs by sequential additions of -SiHj ... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

The four oxygen anions in the tetrahedron are balanced by the -i-4 oxidation state of the silicon cation, while the four oxygen anions connecting the aluminum cation are not balanced. This results in -1 net charge, which should be balanced. Metal cations such as Na", Mg ", or protons (H" ) balance the charge of the alumina tetrahedra. A two-dimensional representation of an H-zeolite tetrahedra is shown ... 

While related to its carbon analogs, the existence of the RsSi species as a free ion in condensed phases had been doubted for a long time. However, NMR characterization using bulky aryl substituents has provided evidence for the triply coordinated silicon cation. " However, definitive evidence was recently reported by the groups of Reed and Lambert with a silyl cation species bound to three mesityl groups and a carborane [HCBnMesBrg] counterion (Eig. 7.5). It was suggested that... 

Uncertainties in understanding the exact mechanistic details of these reactions are sure to stimulate continued work to define the nature of trivalent silicon cations in ionic reductions by organosilicon hydrides. 

With the compound (2-Me2NCH2C6H4)PhSi(H)(03SCF3) 786, it was demonstrated that more than one built-in ligand is needed for the stabilization of trivalent silicon cations as, in this structure, the triflate anion is involved in the coordination to the silicon atoms.810 The related Si-O bond distance of 1.951(1) A is longer than the standard Si-O bond length (approx. 1.6 A) and therefore compound 786 might be best described as contact ion pairs (Figure 5). 

Based on the same strategy, Denmark and coworkers developed a vinylogous aldol reaction using enolate activation with a catalyst derived from SiCl4 and dimeric phosphoramide 47 [24,25]. This strategy relies on the observation that not all Lewis acid - Lewis base interactions diminish the Lewis acidity [26-28]. Due to the formation of a pentacoordinated silicon cation (48), both the enolate and the substrate can be assembled in a closed transition state, giving rise to the observed high selectivities (Scheme 19) [29,30]. 

Dealumination processes are usually used in conjunction with production of the acid form of zeolite Y for many catalytic apphcations, and zeolites A and X are in most cases no longer used in acid catalytic applications because the high amount of aluminum in their frameworks makes them difficult to stabilize using various dealumination techniques. Successful dealumination and at least partial annealing of defects, resulting from movement of silicon cations to the aluminum vacancies, can be assessed by measurement of the reduction of the unit cell size of the zeolite. This unit cell reduction is a consequence of the relative ionic radii of AF (0.54 A) and Si + (0.40A). 

A count of five main-group valence electrons can also be obtained by e.g. adding H to the silicon cation, [CsH Si]" ". 

Further on, Sawamura et al. [37] investigated the influence of different counter anions on the catalytic activity of cationic silicon Lewis acids. In the studies an achiral salt was used. In previous cases [30] acetonitrile was used as a solvent, which is known to coordinate strongly the silicon cation species. Therefore, the application of toluene as a solvent was investigated with a silicon cationic species. Although even toluene coordinates a silicon cation [25, 38], an enhanced activity compared to other solvents, was found. The achiral salt was prepared in situ from triethylsilane and [Ph3C][B(CgFj) ] (17) as depicted in Scheme 10. 

With halogen electrophiles both retention and inversion of stereochemistry have been observed. In this case the addition of the electrophile may lead to the -silicon cation, or a cyclic halonium ion. Scheme 5 shows a generalized mechanism for the reaction of vinylsilanes with electrophilic reagents120. 

C. Reactions of Atomic Silicon Cations with Neutral Molecules. 1117... 

The amount of literature which has accumulated over the last few years concerning the subject of ion-molecule reactions of silicon cations with neutral molecules is tremendous. In a review of the various reactions of Si+ with neutral molecules published in 1990, Bohme44 lists almost 100 reactions studied by selected-ion flow tube (SIFT) techniques alone84,85. In the meantime, this list will have grown to even larger dimensions and we can therefore only confine ourselves to the description of a few exemplary cases45 50. 

The N-protonated form of HNSi, the H2NSi+ cation 69 and its possible isomers have been studied extensively by theoretical methods230-234. Bohme47 suggested that the ion 69, which is formed upon reaction of silicon cations with ammonia and subsequent dissociation of 69 upon electron capture (equation 33)235, is of prime importance for the formation of Si—N bonded species such as 67 in the chemistry of interstellar matter. 

With both the Fu and the Denmark catalysts it can be assumed that catalysis is effected by formation of a highly electrophilic silicon cation D from tetrachlorosi-lane and the nucleophilic catalyst C, i.e. by attack of the pyridine N-oxide or of the phosphoramide O-atom on silicon, followed by ionization (Scheme 13.38). The latter cation can then activate the epoxide toward nucleophilic attack by the chloride ion. Exchange of the product silane for another molecule of tetrachlorosilane completes the catalytic cycle [75],... 

Fig. 9, from Warren s 1929 paper on the crystal structure and chemical composition of amphiboles 48>, is a physical picture of the silicon-oxygen chain in diopside. Large circles represent van der Waals-like domains of oxide ions (r = 1.40 A 2>) smaller, dashed circles represent van der Waals-like domains of silicon cations [r = 0.41 A 2>). 

The reactions of atomic silicon cations with a large variety of neutral molecules have been reported and summarized in reviews focussed on the kinetic, energetic and the mechanistic study of these reactions by selected-ion flow tube (SIFT)10 and guided ion beam techniques11. 

Ion-molecule reactions of silicon cations Simonetta Fornarini 1027... 

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