29Si chemical shifts complexes

By this method also the zwitterionic silicates 9-15 were obtained The geometry at silicon in these compounds is TBP, like in anionic and neutral pentacoordinate silicon complexes. A typical crystal structure is shown in Figure 5 for compound 9. This structure apparently also exists in solution (CD3CN), as the 29Si chemical shift for 9 in this solvent (—122.9 ppm) compares well with the solid state CP-MAS shift of -121.0 pm28 31. 

No simple correlation can be found between the solid state calculated percent (TBP — SP) deviation and the 29Si chemical shifts in solution. Table 4 demonstrates this analysis for complexes sharing the same ligand environment at silicon, four oxygens and a phenyl group the crystal structures of four closely related spirosilicates (19, 21, 49, 50) were... 

TABLE 6. Comparison of solid state vs solution 29Si chemical shifts for several zwitterionic complexes... 

TABLE 7. 29Si chemical shifts for dinuclear Si,Si complexes... 

FIGURE 11. Correlation of activation barriers for Si—N cleavage and 29Si chemical shifts (at 300 K) for complexes 100a- 100h95... 

TABLE 16. 29Si chemical shifts and Si—O, Si—Cl bond lengths in complexes with OSiCjCl ligand framework... 

FIGURE 18. The temperature gradient of the 29Si chemical shift in Si—O chelate complexes (in solution) plotted against the deviation (A) of silicon from the equatorial plane (in the solid state). The gradient K is expressed as K = (<529SiTj — <529Six2)/(Ti — T2)... 

A few ionic bis-catecholato N-coordinated complexes 177-181 have been reported192 205 207. Crystallographic studies of 177, 180 and 181 showed all three to have a near-octahedral geometry, with Si—O bond lengths very similar to those in the tris-catecholato complexes, and typical Si—N dative bond lengths of 2.157, 2.085 and 2.173 A, respectively. The 29Si chemical shifts for 177-181, which are in line with hexacoordination, are listed in Table 22. 

TABLE 21. 29Si chemical shifts for ionic hexacoordinate complexes (in DMSO)203... 

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

Table 27 lists the 29Si chemical shifts for the high coordination complexes. Triphenyl-silane serves as a tetracoordinate model for the complexes 197 and 200, for comparison. It can be seen that the increased coordination in the latter complexes effects a small upheld shift, much smaller than is normally observed upon going from tetra- to penta- (Table 11, Section III.A.2.b) and hexacoordination (Table 24, Section VI.A.2). This is in accord with the crystallographic results, which show that no significant change in hybridization takes place in these compounds, in contrast to TBP and octahedral complexes.

Fig. 3. Linear correlation between 15N and 29Si chemical shifts for differently substituted complexes 4.14 Reproduced with permission from the American Chemical Society.

Si Chemical Shifts of Hexacoordinate Mono- and Ditriflato (51-54) Complexes... 

Si Chemical Shifts for Binuclear Complexes 55a,c,g,j (CD2C12 Solution, 180 K AND 300 K)69... 

Evidence that 63 is pentacoordinate also in solution is provided by the typically pentacoordinate 29Si chemical shift, and by its highly characteristic temperature dependence, as follows [5, ppm (T, K)] —41.4 (330), —39.6 (300), —37.2 (275), —35.6 (243).41 The inverse relationship is the evidence for the coordination of the chloride to silicon (Si <— Cl—), rather than O —Si coordination (which was reported to have a regular dependence of 5(29Si) on temperature, i.e., downfield shift as the temperature increases).34 However, the present results do not permit the distinction of a bridged complex in solution from one in which a different chloride is attached to each silicon. 

The unexpected downfield shift in 66a,b, in connection with strong temperature and solvent effects on the 29Si chemical shift (Table XXI), indicates that the dibromo complexes 66, unlike the difluoro- (64) and dichloro- (65) analogs, undergo ionization to form the bromide salts (68) already at room temperature (Eq. 27).20,52... 

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... 

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