Trialkylsilylium ion

Until the year 2002 no experimental data existed on the structures of unperturbed R3E+ cations, the exact analogues of the carbenium ions. Computational data combined with NMR chemical shift calculations, which could be compared to experiment, were the only source of reliable structural information for silylium ions6,7,13,77,121 while for germylium, stannylium and plumbylium ions this combined approach was not attractive due to either the non-existence of the experimental data (Ge) or the complexity of the computational problem (Sn, Pb). On the other hand, a series of excellent experimental studies demonstrated, for example, the high coordination tendency of small trialkylsilylium ions either toward the counteranion38,114,127,138 or toward the solvent.36,37,67,116,127 The solid state structures of these silyl cation salts showed clear indications either of cation/anion or cation/ solvent coordination. Thus, the nature of the observed cation, i.e. the degree of silylium ion character remained disputable.10,11,13... 

In summary, trialkylsilylium ions R3Si+ are stabilized by forming tetracoordinated Si complexes of type III. Pentacoordination at Si will only occur if there are just weak steric interactions between substituents R and solvent S. Mostly, this is parallel to a low internal stability of the silylium ion in question. Accordingly, R2HSi(S)2+ complexes can be observed experimentally while this is not possible for trialkyl silyl cations. [43]... 

The difficulty in interpreting these results, at the time, arose from the lack of reliable points of comparison for the expected Sn chemical shifts of stannylium ions. There was no doubt that values higher than 300 were deshielded to an unprecedented extent, but were these shifts sufficient to demonstrate tricoordination or trivalency Computation in the early 1990s could not provide a reliable answer. Arshadi et al bypassed the calculational problem in 1996 by publishing a remarkable empirical correlation between structurally analogous silicon and tin compounds ( Si chemical shift vs Sn chemical shift). Since reliable calculations were available for the Si chemical shifts of trialkylsilylium ions, this plot could provide at least an indication of the expected Sn chemical shifts of trialkylstannylium ions, which proved to be 5 ca. 1700. The species observed by Birchall, Lambert, and Sakurai thus were very far from the expected chemical shift and hence from the ideal tricoordinate geometry of the stannylium ion. Since the values are deshielded to some extent, pentacoordination could be ruled out. The best description of the structures observed by all these authors, therefore, is the bond-stretched, solvent-coordinated stannyl cation 8. Lambert and Kuhlmann observed high conductivity, so that the neutral anion-coordinated variant 9 could be eliminated. Such structures (8) also apply to those reported in 1992 by Edlund et al as the tetrahedral part of the equilibrium with pentacoordinate species. 

Furthermore, the Si NMR chemical shift (S = 81.8 ppm) measured in toluene differed very mnch from what is expected by qnantum chemical calculation of model compounds for planar trialkylsilylium ions (Mc3Si+, 5 Si = 355.7 ppm EtsSi", 5 Si = 354.6 ppm). For distorted trimethylsilylium ions with C—Si—C angles of 114.0° and 109.5°, even more deshielded Si chemical shifts of 368.2 and 397.0 ppm were calculated [IGLO basis H at HF/6-31 G(d) level]. 

Scheme 40 Hydrodefluorination reaction of alkyl fluorides catalyzed by trialkylsilylium ions in silanes (anion omitted)...

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