N-Si coordination

The most widely studied compounds possessing N —Si coordination are silatranes, azasilatranes and their analogs75. This family of chelates warrants a separate discussion, which can be found elsewhere75. 

Stable N—Si coordinated complexes generally contain a chelate ring with a covalent bond to silicon on one side, and a dative N - Si bond on the other. Only few such bidentate ligand types have been used, and the resulting families of related chelates are depicted in structures 90-10076 -95. 

In an attempt to stabilize low-valent silicon compounds, a class of compounds was studied in which N -> Si coordination served to stabilize a double bond between silicon and either sulfur114,115 phosphorus116, nitrogen116, oxygen114,115,117 or a transition metal113,118-121 (e.g. 107). In these compounds the silicon is formally pentavalent, though it is coordinated to only four atoms. This topic belongs more appropriately to the chapter on silylenes, and is summarized here briefly for the sake of completeness. 

TABLE 22. 29Si chemical shifts for bis-catecholato silicon complexes with N—Si coordination (CD2G2)... 

Remarkably facile thermal generation of a silylene has recently been demonstrated for the pentacoordinate alkoxydisilane 92416. Intramolecular N—>-Si coordination in 92 was shown by X-ray crystallography. This disilane underwent thermolysis at 110°C in toluene, or 90 °C in DMF, producing the silylene 93, which was trapped with 2,3-dimethylbutadiene and diphenylacetylene (Scheme 26). The 2 1 adduct with diphenylacetylene was shown to have one nitrogen intramolecularly coordinated to silicon, even though the silicon atom lacks any electronegative substituents. 

Using careful NMR monitoring, the reaction starting with amide or lactams was confirmed to be multistage including a previously non-described 0-[(dimethylchlor-osilyl)methyl]imidate showing a N —> Si coordination. The chelated imidate is formed under kinetic control whereas the chelated amide is formed under thermodynamic control.44... 

The initial intermediate 33 is the normal bis-(N — Si)-coordinated complex, which is obtained in analogy to numerous other neutral hexacoordinate complexes 30-38 (Eq. 15), prepared from SiC and various trichlorosilanes and substituted O-trimethylsilylated hydrazides. The final product 70 is the result of a novel molecular rearrangement, by which a chloride has been displaced by the dimethylamino-nitrogen and has migrated from carbon to silicon, accompanied by ring expansion and conversion of the N -> Si to O -> Si coordination. 

N Si coordination (76, 7776 78a77 ) and most recently also with oxygen (78b,78 7 979), sulfur (78c),78 and phosphorus (78d)78 donor-ligands. The present section describes the chemistry of donor-stabilized silyl cations with hydrazido chelates. 

The first hydrazide-based siliconium-cation salts were obtained during an attempt to prepare neutral, hexacoordinate bis-chelates with O Si coordination (80), in analogy to the extensively studied isomeric N Si coordinated 30-38 (see Section III.A.l). In analogy to Eq. (15), the A-trimethylsilylhydrazides 81 were allowed to react with polyhalosilanes 22-25 (Eq. 33). However, in contrast to Eq. (15), the expected 80 were not... 

In retrospect, we realize that the ability to observe coupling constants through two and three bonds across the dative bond in the present silicon complexes resulted from the favorable near 90° H-Si-N angle. In many of the previously reported N Si coordinated silicon complexes this angle was near 180°, and no coupling could be observed. 

In conclusion, two consecutive ligand-site exchange processes were measured in hexacoordinate complexes 1-4, and were assigned to (X,C1) and (0,0)-1,2-shifts (in order of increasing activation barrier). Possibly also Si-N cleavage follows in these compounds at higher temperature. N H coupling was observed for the first time across the N—>Si coordinative bond. 

Summary Introduction of a chiral ligand to a series of pentacoordinate silicon complexes led to the assignment of two intramolecular rate processes Si-N cleavage and pseudorotation. Linear correlations between and Si chemical shifts of the complexes, as well as between the latter and the N-methyl exchange barriers were attributed to variation in strength of N—>Si coordination. 

In these systems therefore it may be concluded that the major factor determining the aptitude for pentacoordination is the capacity of the Si-X bond to be stretched under the influence of the donor atom. The stronger N -> Si coordination in chlorosilatrane compared with fluorosilatrane24, and the exceptional increase in length of some Si-Cl bonds trans to the donor atom in pentacoordinate systems, as revealed by X-ray analysis and discussed in Section III.B, are in agreement with this interpretation. The sequence above parallels the tendency to inversion in nucleophilic substitution of X at a chiral silicon centre, and the susceptibility to racemization in nucleophilic solvents. 

The two distinct sets of energies shown in Tables 6 and 7, respectively, probably reflect the operation of two quite different dynamic processes. The first, higher, values may be attributed to the energy required to sever the N -> Si coordinate bond and permit inversion at the nitrogen atom, in order to equilibrate the methyl group environments, whereas the second, lower, set corresponds to pseudorotation at the pentacoordinate silicon atom. 

Scheme 17. Evidence for weak intramolecular N-Si coordination revealed by comparison of NMR parameters. ...

We have recently reported the first observation and measurement of spin-spin interactions in neutral hexacoordinate silicon chelates [1] which extend across the N— Si coordinative bond and over two, three, and even four bonds. These coupling constants were highly sensitive to small geometrical modifications in the complex. The major geometrical requirement found for spin-spin coupling over two bonds (N—>Si-F or N—>Si-H) in hexacoordinate complexes la-lc was that the corresponding bond angle be 90° or very close to it. 

Table 2. Coupling constants (Hz) across the N- Si coordinative bond in 4.

This unexpected difference between two apparently similar reactions raises the question of whether the N->Si coordinated octahedral 3 can also, under certain conditions, dissociate to form pentacoordinate siliconium chloride salts. To answer this question we have re-investigated the chemistry of 3 in some detail. We now report on the reversible ionic dissociation of 3 to siliconium salts, and the resulting control of the coordination number of silicon by various means temperature,... 

An osmium-substituted silatrane (226) has been reported in 1998, which bears the transition metal atom as a formal covalently bonded substituent, whereas the silatrane N atom acts as the additional lone pair donor. In this particular compound, the transannular N Si coordination is remarkably weak, which is reflected by the N—Si separation of 3.00 A [373]. The use of electron rich transition metals as lone pair donors themselves to enhance the silicon coordination sphere has also been explored as early as 1994. The Ni(0) and Pd(0) centers in compounds 227 and 228, however, exhibit poor o-donor qualities, thus leading to transannular silicon metal separations of about 4 A [373-378]. 

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