Silacyclopentadienyl anion

Siloles, germoles, stannoles, and plumboles have been extensively reviewed.180 The aromaticity of these systems is still under scrutiny. In the study, the cyclopentadienyl anion had the highest aromatic character, with a NICS value of —14 and a Bird index of 100. Structures 138 and 139 also showed high and similar aromaticity indices (Scheme 59). For the planar silacyclopentadienyl anion 138, the NICS value is —10.2 and the Bird index 80, while the values for anion 139 are —10.9 and 77, respectively. 

A natural extension of the above studies is to charged aromatic molecules containing silicon. The potential for aromaticity in the silacyclopentadienyl anion 101 and the silacyclopropenium cation 102, both formally possessing an aromatic (4n + 2) -electron network, was studied by Gordon and coworkers2463. 101 is isoelectronic with the cyclopentadienyl anion and 102 with the cyclopropenium cation, both well established aromatic systems with high delocalization energies. 

FIGURE 43. Optimized structures (6-31G ) of the C2v and Cs silacyclopentadienyl anion (101)259 and of the silacyclopropenium cation (102)246a. Bond lengths are in A, bond angles in deg. Reprinted with permission from J. Org. Chem.y 51,5028 (1986). Copyright (1986) American Chemical Society259. 

The potential aromatic silacyclopentadienyl anion 101246a 259 was discussed in Section VI.C. Despite the potential aromaticity of the planar C2v structure, 101 adopts a pyramidal Cs structure259. Silacyclopentadiene is also by only 2.2 kcal mol 1 more acidic than SiH4259. Significant stabilization is however present in the planar structure, as reflected in the lower inversion barrier of 16.2 kcal mol 1 in Cs 101 compared with an inversion barrier of about 26 kcal mol-1 in SiH3 at a comparable level of theory352. 

Scheme 34 Structure of substituted silacyclopentadienyl anion (X = F, BH2, AIH2, CH3, SiH3, vinyl Y = F, CH3, SiHs, vinyl, phenyl).

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