Octahedral and bicapped-tetrahedral silicon configurations in the solid state and their dynamic coexistence in solution

Hexacoordinate silicon bis chelates with the SiC ₂O ₂N ₂ ligand framework have been prepared, with different nitrogen-donor ligands, NMe ₂ and N=CMe ₂. Crystal diffraction analyses revealed that the former type had a bicapped-tetrahedral geometry and the latter an octahedral molecular geometry in the solid state. The two N→Si dative bonds in the isopropylideneimino-coordinated compound are sufficiently strong to change silicon from its tetrahedral ground-state configuration to the higher energy octahedral configuration. In contrast, the weaker NMe ₂ donor groups do not form strong enough bonds to cause this change, and as a result silicon remains tetrahedral and forms only substantially longer and weaker N→Si dative bonds (2.7-2.8 Å). Remote electron-releasing NMe ₂ substituents on the chelate rings render the ligand NMe ₂ groups stronger donors, resulting in reversal of the solid-state geometry back to octahedral. ²⁹Si, ¹H, and ¹³C NMR spectra at various temperatures provide evidence for the coexistence in equilibrium of the bicapped-tetrahedral and octahedral geometries in solution, with the octahedral to tetrahedral population ratio increasing as the temperature is decreased.