The effect of σ/π, σ and π donors on the basicity of silylene superbases: a density functional theory study

In this study, we examined computationally that the combination of π- and σ-donor substituents results in higher basicity of silylene compounds compared to the usage of π-donor substituents alone. The designed silylenes possess three types of ligands: sigma (σ) donors ( 4–6 ), a combination of π-and σ-donors ( 7–9 ) and π-donors ( 10–12 ). We examined the effect of phosphazene and substituted phosphazene substituents on σ-donor boron atoms to enhance the basicity of the Si( ii ) centre. Gas phase proton affinity (PA) of 337.1 kcal mol −1 was predicted for compound 6 with only σ-donor ligand, which is in the range of hyperbases. The pairing of π- and σ-donor substituents leads to the proton affinity value ∼341.4 kcal mol −1 for compound 9 , which is higher than that of the corresponding π-donor ligand 12 (310.1 kcal mol −1 ). PA values predicted for these silylenes are comparable to the strongest inorganic superbases such as alkali-metal hydroxides, hydrides, and oxides. The optimized geometry of 9 suggests that the smaller distance between the nitrogen atom of the phosphazene substituent and Si (1.97 Å) facilitates donation of the lone pair electron of N to a vacant orbital of Si and subsequently enhances basicity. A combination of π- and σ-donor ( 7–9 ) and only σ-donor ( 4–6 ) ligands are the preferred ligands for enhancing the basicity of silylene compounds over π-donor ligands alone ( 10–12 ). Stability analysis by dimerization energy elucidated that the studied compounds should exist as monomers. The PA values further corroborated well with the ionization potential (IP), electron affinity (EA), absolute electronegativity ( χ ) and absolute hardness ( η ).