Planar lithium silolide: aromaticity, with significant contribution of non-classical resonance structures

Abstract

The first planar lithium silolide characterized by X-ray crystallography is reported. The structural data, NMR and DFT results reveal high aromatic character, with the unusual dominance of non-classical resonance structures.The cyclopentadienide ion is a textbook example for a planar aromatic compound alongside benzene. 1a The replacement of a carbon atom by silicon results in the silacyclopentadienyl (silolide) anion, 1b-f for which the planarity and the apparently related aromaticity are far from being trivial. the structural similarity for the isoelectronic silolide anion is also expected. Computational studies have also predicted that silolide is pyramidal; 5 however, even with the non-planar silicone a moderate aromaticity was reported for the parent compound, 5a furthermore the presence of the countercation has a flattening effect on silicon. 5 A noteworthy feature of the calculated planar silolide structure was the equalization of the C-C distances, the Cb-Cb 0 bond being slightly (by 0.008 Å) shorter than Ca-Cb. Accordingly, for the planar silolide the ''b'' resonance structure had larger (30%) weight than the classical ''a'' (17%) according to a natural resonance theory (NRT) study (Fig. 1 Si and upfield in 13 C NMR) with respect to the neutral precursor disilane was attributed to cyclic delocalization. 7 Unfortunately, for this particular silolide, no X-ray structure could be obtained, The situation is further complicated by the fact that for 1-Me-tetraphenylsilolide the dimerization of the anion was reported, yielding tricyclic systems with a central 1,3-disilacyclobutane ring. 8While no planar silolide has been reported to date, Sekiguchi's 1,2,3-trisilacyclopentadienide and 1,2-disila-3-germacyclopentadienide anions have been shown to exhibit planar X-ray structures with equalized bond lengths, and with 29 Si NMR data indicating aromaticity.9 Furthermore, nearly planar transition metal (Zr, Hf or Ru) complexed silolides were also synthetized. 10In a computational search for planar silolide anions we revealed that due to the charge stabilizing effect, p-acceptor SiH 3 groups at the a carbons enhance the planarity and aromaticity of the naked silolide ring considerably. 5d This effect was already noted on the related phosphole with SiH 3 11 and BH 2 12 substituents, and the charge stabilizing effect was also shown in the case of the silyl-substituted stannole dianion. 13Furthermore, the a-silyl groups enhanced the relative weight of the b type resonance structure. 5d Z 5 -Coordination of the Li Fig. 1 Resonance structures for planar heteroaromatic five membered rings, with different heteroelements. E can be S, PR, or SiR À (R:H, alkyl, aryl, etc.). The numbering of the ring atoms is shown on resonance structure a.

Potassium Salts of 2,5‐Bis(trimethylsilyl)‐Germolide: Switching between Aromatic and Non‐Aromatic States

“…Similarly, Kovács and Nyulászi et al. predicted computationally and showed experimentally that the lithium silolide Li[ 4 ] with 2,5‐bis(trimethylsilyl)‐substituents adopts a planar silole mono‐anion structure with a significant high degree of aromaticity, in contrast to previously reported alkyl‐ and aryl‐substituted alkali silolides . Our results for the germolide 5 and related model compounds point into the same direction.…”

“…2,5‐Disilylsubstitution in anionic group 14 heteroles has a remarkable effect on the electronic properties of these compounds . This is indicated by large low‐field shifts of the 29 Si and 119 Sn NMR resonances upon trimethylsilyl substitution in silole dianions 15 a , and stannole dianions 16 a compared to their aryl‐substituted analogues (Figure ).…”

“…The particular substitution pattern of germolides 5 and 6 with two silyl groups in α‐position to the germanium atom clearly facilitate this process, as it decreases significantly the energy difference between the planar ( 5 ) and bent ( 6 ) structure. In this context, it is of interest that NMR investigations on lithium silolide Li[ 4 ] suggest an equilibrium between the planar structure (as found also in the solid state) and a bent structure in solution . Similarly, Lee et al.…”

“…[4,11] pounds. [9][10][11] This is indicated by large low-field shifts of the 29 Si and 119 Sn NMR resonances upon trimethylsilyl substitution in silole dianions 15 a, [11] and stannole dianions 16 a [10] compared to their aryl-substituted analogues (Figure 9). For the silole dianion 15 a,w ea ttributed this effect to efficient hyperconjugation betweent he silyl substituents and the cyclic delocalized p-systemo ft he silole ring and we showedt hat similar effects are also operative for germole dianion 11.…”

“…This group showed that the planar aromatic sila‐ and germacyclopentadienides could be established by η 5 ‐coordination to transition metals, such as Ru, Hf, and Fe ( 1 – 3 , Figure ) . In contrast, all structural data that have been available for simple alkali sila‐ or germacyclopentadienides were obtained for well separated ion pairs with strongly pyramidalized anions until very recently when the group of Kovács and Nyulászi reported the crystal structure of lithium 2,5‐bis(trimethylsilyl)silolide Li[ 4 ] that features a planar aromatic silolyl anion. In this work the authors emphasize the importance of the particular substitution pattern with two silyl groups in the 2,5‐position.…”