Substituent effect on the aromaticity of the silolide anion

The effect of the substituents on the planarity and aromaticity of the silolide anion was studied computationally. It was revealed that pi-electron acceptor groups (e.g., silyl or trimethylsilyl) at the alpha position of the ring reduce substantially the inversion barrier about the central silicon increasing the aromaticity according to isomeric stabilization energy (ISE) and NICS values. In the planar and highly aromatic silolide anions, the mesomeric structures with the largest weight exhibit a Si=C double bond and a negative charge which is located at the alpha or beta carbons based on NRT calculations. 2,5-disilylsilacyclopentadienides coordinated by naked Li+ have planar minima, however, further coordination by THF molecules (as in a solution) reduces somewhat the flattening of the silicon pyramid. NMR calculations were carried out to understand the connection between the Si-29 chemical shift and the aromaticity of the ring. It was revealed that the commonly accepted charge transfer-chemical shift relationship is strongly influenced by the substituents and the counter cation. THF complexation of the Li counter cation has a small influence on the NMR shift.