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

Abstract

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).