Cationic silicon complexes

The first few pentacoordinate cationic silicon complexes (153-155) were discussed in an earlier review2. The first two were described as trivalent silicenium ions stabilized by mtermolecularcoordination183 184, while 155 is an intramolecular silicenium ion21a. Since then, a few other compounds were reported. Oxidation of the dihydrido-hexacoordinate chelate 156 by the addition of excess iodine produced 157185. The X-ray structure of 157 showed a slightly distorted TBP for the cation, with a well separated (Si—I distance 5.036 A) I8 2 anion for every two cations. The nitrogens are in the apical positions, and both Si-N distances (2.08 and 2.06 A) are longer than covalent bonds but well within the coordination distance. 

In contrast, syn products are formed through a boat-like transition state, also involving a cationic silicon complex. In this complex, however, the silicon atom is pentacoordinated and one phosphoramide only is bound to the silicon atom. 

However, for silicon to be specifically transported, concentrated, and deposited, it seems logical to suppose that chelate-type compounds are involved. Weiss and Herzog (127) find that in plants the cationic silicon complexes of tropolone derivatives are formed in the pH region 4.0-6.8. Such a complex might be translocated and concentrated and then the silicic acid liberated and deposited if the pH were slightly raised to 7. The energy involved in transport, concentration, and release may be quite small. It would mainly involve the reactions required to change the pH... 

Hexacoordinated silicon complexes form octahedra in most cases, or with different ligands distorted octahedra. Deviations from this rule of thumb are seldom observed. One exception in the last 5 years was the distorted trigonal antiprismatic cationic silicon complex 87 with ureato ligands (Scheme 23) [227]. 

Scheme 23 Cationic silicon complex 87 with ureato ligands (left) and schematic representation of...

Hexamethylphosphoramide (HMPA) adducts of tetrachlorosilane (SiCU) were investigated with NMR spectroscopy in solution and solid-state structures [93]. In solution, the meridional and facial isomers of the hexacoordinated cationic silicon complex [SiCl3(HMPA)3] Cl predominate at all HMPA concentrations and are in equilibrium with the hexacoordinated neutral tram- and cA-[SiCL((HMPA)2] complexes, as well as the pentacoordinated cationic Si-complex cis- [SiCl3(HMPA)2] "Cr [93]. 

Mn quadrupolar broadening. On heating these compounds decompose via hetero-lytic cleavage of the p -H-Si bond, typical behavior for a cationic silane complex because the silicon center becomes electron deficient upon coordination of the H-Si bond to metal and thus activated toward nucleophilic attack. 

A formally pentavalent cationic silicenium complex, with intramolecular base-stabilization, is the silylene-iron 167, with a Fe=Si double bond. The crystal structure of 167 shows that silicon is essentially tetrahedral in this compound, and the solution 29 Si chemical shift is 8 = 118.3 ppm, compatible with the double bond character rather than with pentacoordination at silicon189. 

The displacement of triflate ion from silicon is important as a route to cationic silylene complexes, like the one shown in equation 102. 

A special case of reversible ionization of a hexacoordinate silicon complex has been described as a novel tautomeric equilibrium.41 It differs from the formation of siliconium-ion salts in that the positive charge resides on nitrogen, in a dimethylammonium cation, and not on silicon. The transsilylation of lg with 12 in equimolar concentrations leads to the pentacoordinate zwitterionic complex 13 (Eq. (10), Section II.B.5). However, when the molar ratio was 2 1, respectively, an equilibrium mixture of tautomers (58, 59) was obtained, as shown in Eq. (21). The same mixture was also obtained when a second mole-equivalent of lg was added to 13. 

For several of the siliconium salts crystal structure analyses were obtained, confirming the pentacoordination and the ionic nature of the compounds (well separated cations and anions). The crystal structures for 90a(OTf), 90c(OTf), 91a(OTf), 91a(AlCl4), and 93a(OTf) are depicted in Figs. 47-51, respectively. Further structural support is found in the 29Si NMR chemical shifts (Table XXVI). A remarkable observation in Table XXVI is the nearly equal 29Si chemical shifts of siliconium salts sharing the same silicon complex, but with different anions [e.g. 91a(OTf), 91a(Br), and 91a(AlCl4)] the equal shifts are the evidence that the siliconium cations are essentially independent of... 

As mentioned above, cationic phosphenium is isoelectronic with carbene, silylene, germylene, stannylene, and plumbylene, because the central element has lone pair electrons and a vacant p orbital as well as two substituents. Therefore, their transition metal complexes have attracted considerable attention. It might be of special interest to compare cationic phosphenium complexes with silylene complexes because phosphorus and silicon are both situated in the third row of the periodic table. 

The methyl and ring proton resonances of cationic acetylacetonato complexes of silicon are shifted downfield relative to the resonance of neutral silicon diketonates, whereas the resonances of the anionic complexes are shifted up-... 

The rate of fixation of the monochlorotriazinyl dyes is very much slower. The result is that there is abundant opportunity for the normal acid dyeing mechanism to give uniform distribution of the dye molecules before they become permanently anchored by covalent bond formation. It has been found that, for some unknown reason, cationic surface active compounds tend to reduce skitteriness, if a non-ionic compound is also present to prevent the precipitation of the cation-dye complex. In order to prevent foaming which can become troublesome the addition of a silicone antifoaming product has been recommended. 

U.S. 5332569 (1994) Wood et al. (Alberto-Culver) Silicon oil in an organic solvent-based carrier comprising PEG and an anionic-cationic emulsifier complex formed from anionic (phosphate or sulfate) copolymer of dimethylpolysiloxane (20 -0 units) and polyoxyethylene (3-15 units) and cationic conditioning compound having at least one quaternary nitrogen or amido amine and one hydrophobic aliphatic or silicone polymer chain Improved stability of silicone emulsion... 

Keywords siliconium cations, Hexacoordinate silicon, binuclear silicon complexes... 

The standard group valence is 4. For Si, Ge, and Sn, however, coordination numbers exceeding 4 occur regularly. The central atoms in such species are thus hypervalent, that is, they have more than an octet of valence electrons. For example, 2,2-bipyridyl reacts with triphenylchlorosilane to yield a relatively stable pentacoordinate silicon complex. Observe that, although the Si carries a -1 formal charge, the complex as a whole is cationic ... 

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