Silicon hexacoordinate

Nakajima reported the use of a chiral bipyridine N,N -dioxide 18 in the desym-metrization of acyclic meso epoxides (Figure 7.3). Although the enantioselectivity was not as high as in the method developed by Fu for meso-stilbene oxide (90% ee vs. 94% ee), it was higher for the same aliphatic epoxide (74% ee vs. 50% ee) [57]. Nakajima showed that mono-N-oxide derivatives 19 and 20 were much less effective than 18 in tenns of both yield and enantioselectivity, and accordingly proposed a unique mechanism for 18 involving a hexacoordinate silicon intermediate coordinated to both N-oxides of the catalyst. 

The use of trifluorosilanes permits reactions through hexacoordinate silicon, which presents an opportunity for chelation control. For example, a-hydroxy ketones give syn diols.110... 

Stable hypervalent silicon complexes with chelating diphosphinomethanide ligands similar to II, but with hexacoordinated silicon centers are obtained from 2 and SiCl4 (Eq.(4)). 

Hexacoordinated silicon takes benefit from the favorable octahedral arrangement of ligands around a coordination center. As expected, negatively charged substituents are superior in stabilizing high silicon coordination numbers. Most recently however, compounds with chelate ligands, mono anionic derivatives and compounds with more electropositive donors have been added to the series of penta- and hexacoordinated silicon species. 

The reactivity of penta- and hexacoordinated silicon compounds has been described to be very different from the reactivity of the corresponding tetracoordinated derivatives [ 1], An increase in reactivity towards nucleophiles has been observed in the case of neutral and anionic pentacoordinated silicon compounds as exemplified by the following Schemes [2],... 

In the case of neutral systems the geometry corresponds to a bicapped tetrahedron or, in other words, to a tetrahedron which undergoes two nucleophilic coordinations. They have been observed [5] to be stable in solution. A careful H variable temperature NMR study shows that these compounds undergo an intramolecular isomerization which takes place without cleavage of Si-N bonds. The AGt of this so-called "Baylar Twist" isomerization has been estimated at between 12 and 18 kcal/mole depending on the substituents at the silicon. We can conclude that this work supports strongly hexacoordinated silicon structures as possible intermediates in the usual course of nucleophilic reactions at silicon. 

The reactivity of hexacoordinated silicon species themselves has been considered [7, 8] when we have tried to find a way to obtain organosilanes directly from silica. There is a challenge for chemistry to find a way to obtain silanes avoiding the route through silicon metal, (Scheme 4). 

We have solved this problem taking into account previous German work [9] which describes the preparation of hexacoordinated silicon species directly by reaction of silica with catechol in presence of ammonia. 

The second step is now the study of the reactivity of these hexacoordinated silicon species towards nucleophiles in order to obtain Si-C or SiH bond formation. The nucleophilic attack takes place at silicon with displacement of catechol and formation of organosilanes. 

Going back to mechanistic studies it is not clear if the reactions of nucleophiles with hexacoordinated silicon compounds are pure nucleophilic substitutions or if they take a more complex route. However there is another challenge to find whether the silicon atom can accept being in heptacoordination. Such a possible situation has been observed with a tricapped tetrahedron structure of a silane which has been proved to be isosteric with the corresponding germane of which the X-ray structure determination has been carried out. 

A diaryldihydrosilane with a hexacoordinated silicon center, produced through intramolecular coordination, is reported not to react with benzaldehyde, although the silane is capable of reducing silver ion to silver metal.113 There is also a report of a heptacoordinate silicon hydride species with the ability to transfer hydride to trityl cation while remaining inactive toward methanol.108114... 

Compounds 9-16 were found to undergo an acidic reaction in aqueous solution.21 Possibly, these zwitterionic A5S/-silicates behave as Lewis acids reacting with the Lewis base OH- to generate protons (2 H20 OH- + H30+). This interpretation is supported by the observation that the related compounds 6 and 7 react with [HNMe3]F in aqueous solution to yield the ionic A6S/-silicates 25 and 26, respectively (Scheme 4 the Lewis bases F-and OH- are isoelectronic).24 The identity of these hexacoordinated silicon... 

The oxidative cleavage of the Si—C bond requires the presence of at least one heteroatom on silicon. The proposed mechanism involves formation of a pentacoordinate silicon species as the initial key intermediate and a hexacoordinate silicon species in the transition states (equation 75). 

Si-N bonds in compounds with hexacoordinate silicon and tricoordinate nitrogen atoms... 

All Si—N bonds in compounds where hexacoordinate silicon atom is bonded to tricoordinate nitrogen atoms are dative bonds, where the nitrogen atom provides its lone-pair electrons to the bond. The average Si — N bond was calculated from 31 individual values to be 1.969 A (s.d. 0.05 A and s.m. 0.008 A). 

FIGURE 23. Histogram of Si—F bond lengths in compounds with a hexacoordinate silicon atom... 

The average Si—Cl bond length in compounds with a hexacoordinate silicon atom was determined from 17 experimental values to be 2.184 A (s.d. 0.02 A, s.m. 0.006 A). The results are shown in Figure 25 and Table 29. 

The main methods for the synthesis of hexacoordinate silicon compounds are similar to those for pentacoordinate complexes and were outlined in a recent review6. These methods include (a) addition of nucleophiles (neutral or anionic) to tetracoordinate silanes (b) intermolecular or intramolecular coordination to an organosilane (c) substitution of a bidentate ligand in a tetrafunctional silane. The following discussion focuses mainly on new complexes, reported since the recent reviews6,7 were published. 

VI. NEUTRAL HEXACOORDINATE SILICON COMPLEXES A. Intramolecular Coordination... 

Relatively few ligand types have been used for the formation of neutral hexacoordinate silicon complexes, resulting in several complex types 182-193208 -218. Acetylacetonato (acac) chelates [182, (acac SiXY] were prepared directly from the reaction of... 

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